Ullmann's Encyclopedia of Industrial Chemistry || On-Line Monitoring of Chemical Reactions

� 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Article No : c18_c01

On-Line Monitoring of Chemical Reactions

WOLF-DIETER HERGETH, Wacker Polymer Systems, Burghausen, Germany

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . 348

2. Reaction Calorimetry . . . . . . . . . . . . . . . . 350

2.1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . 350

2.2. Heat Flow Balance and Principles of

Measurement . . . . . . . . . . . . . . . . . . . . . . . 350

2.2.1. Heat Flow Balance . . . . . . . . . . . . . . . . . . . 350

2.2.2. Basic Modes of Operation . . . . . . . . . . . . . . 353

2.3. Applications and Instrumentation . . . . . . . 354

3. Ultrasonic Methods . . . . . . . . . . . . . . . . . . 355

3.1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . 355

3.2. Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355

3.3. Applications . . . . . . . . . . . . . . . . . . . . . . . . 356

4. Dielectric Spectroscopy . . . . . . . . . . . . . . . 358

4.1. Theory and Mechanisms . . . . . . . . . . . . . . 358

4.2. Instrumentation and Applications . . . . . . . 360

5. Optical Spectroscopy . . . . . . . . . . . . . . . . . 362

5.1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . 362

5.2. Instrumentation for Reaction Monitoring . 362

5.3. Applications of Optical Spectroscopy. . . . . 364

5.3.1. UV – VIS Spectroscopy. . . . . . . . . . . . . . . . 364

5.3.2. NIR Spectroscopy . . . . . . . . . . . . . . . . . . . . 366

5.3.3. IR Spectroscopy . . . . . . . . . . . . . . . . . . . . . 368

5.3.4. Raman Spectroscopy . . . . . . . . . . . . . . . . . . 370

5.3.5. Fluorescence . . . . . . . . . . . . . . . . . . . . . . . . 373

5.3.6. Other Spectroscopic Techniques. . . . . . . . . . 374

6. Particle Size Analysis. . . . . . . . . . . . . . . . . 374

6.1. Scattering Techniques . . . . . . . . . . . . . . . . 374

6.1.1. Turbidimetry . . . . . . . . . . . . . . . . . . . . . . . . 375

6.1.2. Angular Static Light Scattering . . . . . . . . . . 375

6.1.3. Dynamic Light Scattering . . . . . . . . . . . . . . 375

6.1.4. Other Optical Techniques. . . . . . . . . . . . . . . 376

6.2. Separation Techniques. . . . . . . . . . . . . . . . 377

7. Chromatography . . . . . . . . . . . . . . . . . . . . 378

7.1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . 378

7.2. GC Hardware Components . . . . . . . . . . . . 379

7.3. Applications of Gas Chromatography . . . . 381

7.4. Other Chromatographic Techniques . . . . . 381

8. Electroanalytical Methods . . . . . . . . . . . . . 381

8.1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . 381

8.2. Conductometry . . . . . . . . . . . . . . . . . . . . . 382

8.3. Potentiometry . . . . . . . . . . . . . . . . . . . . . . 383

8.4. Amperometry, Voltammetry, and

Coulometry . . . . . . . . . . . . . . . . . . . . . . . . 384

9. Miscellaneous Methods . . . . . . . . . . . . . . . 384

9.1. Mass Spectrometry . . . . . . . . . . . . . . . . . . 384

9.2. Densimetry and Dilatometry . . . . . . . . . . . 385

9.3. Rheometry . . . . . . . . . . . . . . . . . . . . . . . . . 387

9.4. NMR Spectroscopy . . . . . . . . . . . . . . . . . . 388

References . . . . . . . . . . . . . . . . . . . . . . . . . 389

Article with Color Figures

Abbreviations

AOTF: acousto-optic tunable filter

APACT: Advances in Process Analytics and

Control Technology

APM: acoustic plate mode

ATR: attenuated total reflection

CCD: charge-coupled device

CHDF: capillary hydrodynamic fractionation

CID: charge-injection device

CPAC: Center for Process Analytical

Chemistry

CPACT: Center for Process Analytics and

Control Technology

CRDS: cavity ring-down spectroscopy

CTAC: Control Theory and Applications

Center

CVD: chemical vapor deposition

DGEBA: diglicidyl ether of bisphenol A

DGEBF: diglicidyl ether of bisphenol F

DDM: diaminodiphenylmethane

DLS: dynamic light scattering

DWS: diffusing wave spectroscopy

EWA: evanescent wave absorption

FDA: Food and Drug Administration

FFF: field flow fractionation

FID: free induction decay

DOI: 10.1002/14356007.c18_c01.pub2

FIR: far infrared

FOCS: fiber-optic chemical sensor

FODLS: fiber-optic dynamic light scattering

FOQELS: fiber-optic quasi elastic light

scattering

FRA: frequency response analyzer

FT: Fourier transform

FTIR: Fourier transform infrared

GC: gas chromatography

GDCh: Gesellschaft Deutscher Chemiker

HPCL: high perfomance liquid

chromatography

HT: Hadamard transform

HDC: hydrodynamic chromatography

IC: integrated circuit

IFPAC: International Forum for Process An-

alytical Chemistry

IR: infrared

LEC: liquid exclusion chromatography

LED: light-emitting diode

LIBS: laser-induced breakdown

spectroscopy

LIPS: laser-induced plasma spectroscopy

MACC: McMaster Advanced Control

Consortium

MCEC: Measurement and Control Engineer-

ing Center

NIR: near infrared

NMR: nuclear magnetic resonance

PCS: photon correlation spectroscopy

PLS: partial least squares

PREACH: Process Related Environmental An-

alytical Chemistry

PS: power source

PVC: poly(vinyl chloride)

QELS: quasi-elastic light scattering

QCM: quartz crystal microbalance

RIM: reaction injection molding

SAW: surface acoustic wave

SEC: size exclusion chromatography

SERS: surface-enhanced Raman

scattering

SH: shear-horizontal

SLS: static light scattering

SRS: stimulated Raman scattering

UV: ultraviolet

VIS: visible

XPS: X-ray photoelectron spectroscopy

Symbols

A: absorbance (extinction), instrument

constant

AR: reactor wall heat exchange area

ah: Kuhn – Mark – Houwink exponent

B: instrument constant

c: concentration

C0: equivalent capacitance of free space

ci: concentration of component icp,cool: specific heat capacity of coolant

cp,k: specific heat capacity of kth reactor

feed

cp,liq: specific heat capacity of jacket liquid

Cp,mixture: heat capacity of reaction mixture

Cp,tot: heat capacity of filled reactor

d: distance, path length

D: diffusion coefficient, dielectric

displacement

dp: particle diameter

dstir: stirrer diameter

E: electric field

fres: resonance frequency

f: frequency, particle size distribution

function

g: contraction factor

G: shear modulus, distribution function

g1: electric field autocorrelation function

g2: light intensity autocorrelation

function

G: conductance

h: depth

DHr,j: reaction enthalpy of jth reaction

I: electrical current, light intensity

I0: incident light (radiation) intensity

Id: detected light intensity

Il: light intensity at distance lK: bulk modulus, scattering coefficient

Kh: Kuhn – Mark – Houwink parameter

l: (optical) path length, distance

kQ: heat transfer coefficient

Kb: Boltzmann constant

m: refractive index ratio

mk: mass of component kmcool: mass of coolant

mliq: mass of jacket liquid

Mw: weight-average molecular mass

Mw,crit: critical weight-average molecular

mass

346 On-Line Monitoring of Chemical Reactions Vol. 25

n: refractive index

nm: refractive index of medium

np: refractive index of particle

Nn: stirrer speed

p: pressure

P: polarization

p0: pressure amplitude

pbubble: bubble pressure

pgel: gel point

ph: hydrostatic pressure

px: pressure amplitude at distance xP0: power number

q: scattering vector

Qacc: accumulated heat

Qcal: heat generated by calibration heaters

Qel: heat generated by electrical heaters

Qfeed: heat generated by material feed

Qi: sources and sinks of heat

Qloss: heat losses

Qosc: heat of forced temperature

oscillations

Qr: heat of reaction

Qreflux: heat flow over reflux condensor

Qstir: heat input due to stirring

Qtrans: heat transfer through reactor wall

R: reflection coefficient, tube diameter

r: radius

rh: hydrodynamic radius

Rj: reaction rate of jth reaction

t: time

T: transmission coefficient, temperature

dT: amplitude of temperature oscillations

T1: outer reactor wall temperature

T2: inner reactor wall temperature

Tcool,in: coolant temperature (inflow)

Tcool,out: coolant temperature (outflow)

Tg: glass transition temperature

TJ: reactor jacket temperature

TJ,in: jacket temperature (inflow)

TJ,out: jacket temperature (outflow)

Tk: temperature of kth reactor feed

Tosc: period of tube oscillations

Tosc,cal: period of tube oscillations filled with

calibration liquid

TR: reactor temperature

u: sound velocity

U: electrical voltage

u0: sound velocity of reference

component

U0: initial electrical voltage

(Du/Dc)i: sound velocity-concentration coeffi-

cient iul,liq: longitudinal sound velocity of liquids

ul,sol: longitudinal sound velocity of solids

umix: sound velocity of a mixture

utr,liq: transversal sound velocity of liquids

utr,sol: transversal sound velocity of solids

Ux: electrical voltage at distance xV: volume

Vexcess: excess volume

Vi: volume of component iVr: volume of reaction mixture

Vtotal: total volume

x: distance

xr: monomer conversion

Xr: calorimetric conversion

Y: complex admittance

Z: acoustic impedance, complex

impedance

Z0: real component of complex

impedance

Z00: imaginary component of complex

impedance

a: sound attenuation

aliq: sound attenuation in liquids

b: adiabatic compressibility

d: loss angle

e: decadic extinction coefficient

e*: dielectric permittivity

e0: free space dielectric permittivity

e¥: high frequency dielectric permittivity

e0: storage component of dielectric

permittivity

e00: loss component of dielectric

permittivity

e*dipolar: dipolar reorientation contribution to

e*g: surface tension

G: decay constant

h: (bulk) viscosity

h0: viscosity of dispersion medium

hs: shear viscosity

hv: volume viscosity

j: wave phase, volume fraction

jr: phase shift between temperature

oscillations

L: conductivity

l: wave length

Vol. 25 On-Line Monitoring of Chemical Reactions 347

1. Introduction

The terms ‘‘on-line monitoring’’ and ‘‘processanalytical chemistry’’ are often used synony-mously. They are gaining increasing attentionand importance in both industry and academia.The extent of agreement between the two terms isindeed very broad in that they provide sufficient-ly accurate and immediate information on vari-ables that describe the state of a chemicalreaction.

On-line monitoring of reactions encompasseson-stream and on-reactor application of analyti-calmethods tomonitor the chemical compositionof a reaction mixture, to identify process-relatedchemical species, and to quantify the concentra-tion of reaction ingredients, products and bypro-ducts. In addition to revealing the state of thereactor, on-line analysis of physical parameters(temperature, pressure, level, density, viscosity,etc.) may also reflect the extent of a chemicalreaction.

Process analytical chemistry comprises appli-cations which supply relevant process informa-tion of interest ‘‘in-time’’: The time for samplingand analysis is very short compared to the overallreaction time and thus allows adequate monitor-ing and efficient control of the reaction. Theutilization of the analytical data for control strat-egies makes process analytical chemistry anessential integral part of process engineering andcontrol systems.

Process analytical methods may be classifiedas off-line, at-line, on-line, or in-line with respectto sampling, sample transport, and analysis itself(Table 1) [1]. There is no clear-cut line betweenthe different classes, and the boundaries are evenmoving: ‘‘Some of today’s off-line techniquesmay become tomorrow’s on-line techniques’’[2]. An increasing number of off-line techniqueshave been converted into on-line methods byautomated, robot-assisted withdrawal of samplesfrom the reactor or from bypass or processstreams and feeding them into off-lineinstruments.

On-line monitoring and control of chemicalreactions contributes to:

. Guaranteeing and improving product qualityand consistency (i.e., repeatability of productproperties within narrow specification ranges)

. Increasing the efficiency of the process

. Ensuring safe reactor operation by monitoringprocess and reactor parameters

. Understanding fundamentals of the reactionitself

. Saving time for analysis and sample transport

. Reducing emissions by avoiding sample with-drawal and transport

. Reducing costs for labor, raw materials, off-spec products, and process waste

In most cases, purchasing and installationcosts of on-line and in-line analytical instruments

lexc: excitation wave length

lm: medium light wave length

lQ: heat conductivity

n: frequency (of light), average velocity

q: scattering angle

r: density

rcal: density of calibration liquid

rend: density at reaction completion

rmix: density of reaction mixture

rstart: density at reaction start

sdc: dc conductivity

t: oscillation period, relaxation time,

optical transmittance (turbidity)

w: oscillation (angular) frequency

wmax: frequency at loss maximum

Table 1. Classes of process analyzers (adapted from [1])

Process analyzer Sampling Sample transport Analysis

Off-line manual to remote or centralized laboratory automated/manual

At-line discontinuous/manual to local analytical equipment automated/manual quick check

On-line automated integrated automated

In-line integrated no transport automated

Noninvasive no contact no transport automated


exceed those of off-line equipment, but as soonas the measurements required exceed a certain(relatively small) number per day, on-line anal-ysis becomes superior to off-line analysis(Fig. 1). However, the real savings of on-linereaction monitoring are due to improved processefficiency, lower raw materials consumption andwaste generation, and,most important, the abilityto manufacture high-quality products.

Several issues have to be addressed whendesigning an on-line analysis application [1],[3–8]:

. Information necessary to monitor and controlthe process (physical parameters, chemicalcomposition, etc.)

. Frequency ofmeasurements with respect to thetimescale of the reaction

. Average values, typical fluctuations, dynamicranges and expected extremes of properties

. Type, precision, and response time of sensors

. Robustness of sensors and simplicity ofinstallation

. Full automation and minimummaintenance ofequipment

. Number (combination) of sensors and locationof measurement

. Proper sampling and, if required, sampleconditioning

. Form of data output and further handling ofinformation

. Compatibility with process control system

. Safety precautions and possible hazards

. Costs of instrumentation and availability oftrained personnel

Demands on performance characteristics ofon-line analytical techniques differ for academicresearch applications, process development andpilot plant operation, and monitoring of industri-al manufacturing processes, as summarized inTable 2 (see also! Analytical Chemistry: Pur-pose and Procedures; ! Chemometrics; !Plant and Process Safety; ! PolymerizationProcesses, 1. Fundamentals; ! Process Devel-opment, 1. Fundamentals and Standard Course;! Sampling).

The number of textbooks introducing the fieldof process analytical chemistry is still limited [4],[6–8], [10–15]. Since 1993 a series of reviewarticles on the progress in process analytics havebeen published [16–21]. On-line monitoringtechniques play a major role in the contents ofseveral scientific journals such as Journal ofProcess Analytical Chemistry [22], AppliedSpectroscopy [23], Analytical Chemistry [24],andAnalytical andBioanalytical Chemistry [25].Several international conferences devoted solelyto process analytics have been established overthe years. The most importants ones are IFPAC(International Forum for Process Analytical

Figure 1. Total costs for on-line and off-line process analy-ses (including instrument costs, labor costs, energy, material,etc.)A) Costs for purchase of off-line instruments; B) Costs forpurchase and installation of on-line equipment

Table 2. Relative importance of various performance characteristics

of on-line analyses in academic research, process development, and

industry [9]

Characteristics Academic

research

Process

development

Manufacturing

High accuracy þþþ þþþ þþþGood reproducibility þþþ þþþ þþþGood selectivity þþþ þþþ þþþGood sensitivity þþþ þþþ þþþExtended linearity þþ þþþ þþþGood stability þþ þþþ þþþRobustness þþ þþþ þþþHigh analysis

frequency

þþ þþþ þ/þþþ*

Short time delay

of result

þþ þþ þþþ

Low price þþþ þþ þMulti-analyte analysis þþþ þþþ þEase of use þ þþþ þþþEase of validation þ þþ þþþHigh flexibility þ þþþ þEase of implementation þ þþþ þþLow maintenance þ þþþ þþþ*Process-dependent.þþþVery important,þþ important,þ not

very important.


Chemistry [26]), APACT (Advances in ProcessAnalytics and Control Technology [27]), andPREACH (Process Related Environmental Ana-lytical Chemistry [28]). The reader should also beaware of the process analytical technology initia-tive of the FDA [29] which is of crucial impor-tance to the pharmaceutical industry. Academicas well as academic-industrial consortia drive thedevelopment of new, more reliable and cost-effective techniques for on-line monitoring ofchemical reactions applicable to the industry. Themost active ones are CPAC (Center for ProcessAnalytical Chemistry, Seattle, USA [30]),MCEC(Measurement and Control Engineering Center,Knoxville USA [31]), CPACT (Center for Pro-cess Analytics and Control Technology, New-castle, Strathclyde, Hull, UK [32]), MACCMcMaster Advanced Control Consortium, Ha-milton, Canada [33]), andCTAC (Control Theoryand Applications Center, Coventry, UK [34]). AWorking Group on Process Analytical Chemistryof the Society of German Chemists (GDCh,Frankfurt/M, D [35]) has been founded in 2005.

2. Reaction Calorimetry

2.1. Introduction

Themajority of chemical processes are accompa-nied by temperature changes of the reaction mix-ture owing to release or consumption of heat in thecourse of the reaction.Heat evolution is a definite,reproducible, and directly measurable character-istic of a chemical reaction. This enables one tomonitor the extent of a reaction by measuringtemperature or heat flux changes (see [36–47] and! Plant and Process Safety for introduction,overviews, applications, and related subjects).

Reaction calorimetry is noninvasive, rapid,accurate, robust, and, as it is based on tempera-ture measurements, relatively easy to carry out.Temperatures can be measured directly withinthe reaction mixture (reactor interior), at thereactor/exterior interface (reactor wall), or in theheating/cooling liquid of the reactor jacket.

An advantage of reaction calorimetry is thatthe heat of reaction Qr derived from temperaturemeasurements is directly proportional to the rateof reaction. This allows easy access to basickinetic and thermodynamic data of chemicalreactions.

With reaction calorimetry, rates of reaction orconversion of reactants can be determined quasi-instantaneously and continuously with a highdegree of resolution. This makes reaction calo-rimetry an ideal tool for real-time feedback con-trol of chemical composition during the course ofreaction.

Reaction calorimetry not only providesquantitative information on the chemical pro-cess itself (e.g., heat of reaction, reaction rate,conversion) but also on reactor parameters nec-essary for safe reactor operation and processdesign:

. Global reaction kinetics

. Heat production rates

. Necessary cooling power

. Reactant accumulation

. Adiabatic temperature rise to avoid runawayreactions

. Heat-transfer coefficients for scale-up

However, reaction calorimetry is a nonselec-tive method. It is impossible to distinguish be-tween parallel chemical reactions with heat gen-eration and simultaneous enthalpic processeswithin the system such as phase transitions,crystallization, mixing, and dissolution.

2.2. Heat Flow Balance and Principlesof Measurement

2.2.1. Heat Flow Balance

The basis for reaction calorimetry is the energybalance around the reactor. Sources and sinks ofheat Qi which contribute to the overall heat fluxbalance (Eq. 1) are shown schematically in Fig-ure 2:Xi

_Qi ¼ 0 ð1Þ

The quantity of interest in reaction calorimetry isthe heat flux Qr generated by chemical reactionswithin the reactor

_Qr ¼ VR

Xj

rjð�DHr;jÞ ð2Þ

whereVR is the volume of the reactionmixture, rjthe rate of reaction j, and DHr,j the reaction


enthalpy of reaction j. For typical heats of reac-tion, see ! Plant and Process Safety.

Different rates of heat generation within thereactor and heat flow out of or into the reactorlead to temperature changes of the reaction mix-ture. This accumulated heatQacc is a major causeof uncertainty and error in quantitative analysisof reaction calorimetry data because it dependson the derivative of the reactor temperature TR

_Qacc ¼ Cp;tot_TR ð3Þ

where Cp,tot is the heat capacity of the filledreactor (reactor plus reaction mixture).

Feeding of material into the reactor with amass flow rate dmk/dt (and also withdrawal ofmaterial from the reactor) leads to a heat fluxdQfeed/dt

_Qfeed ¼Xk

_mkcp;kðTk�TRÞ ð4Þ

where cp,k is the specific heat capacity of the kthreactor feed and Tk its temperature.

The heat input by stirring the (viscous) reac-tion mixture Qstir is given by

_Qstir ¼ P0N3nd

5stirrmix ð5Þ

where P0 is the power number, Nv the stirrerspeed, dstir the diameter of the stirrer, and rmix thedensity of the reaction mixture.

The energy balance of the reactor is alsoinfluenced by heat losses to the surroundingsQloss (e.g., by conduction or radiation). It isimportant to identify and quantify all possiblesources of heat losses around a reactor becauseheat losses might be significantly larger (2 –5 times) than Qacc, Qstir, or Qfeed. In most cases,heat losses must be determined by separate ex-

periments prior to the reaction or without arunning reaction.

Additional heat can be removed by the inser-tion of reflux condensers into the reactor. Theheat flow over the reflux condenser can be cal-culated according to Equation

Qreflux ¼ mcoolcp;coolðTcool;in�Tcool;outÞ ð6Þ

where m_ cool is the coolant mass flow through thecondenser, cp,cool is the specific heat capacity ofthe coolant, and Tcool,in � Tcool,out the tempera-ture difference across the condenser.

For calibration purposes (see below), heatmay also be generated by additional heaters

_Qcal ¼ UI ð7Þ

where U and I are the voltage and current of theelectrical heater, respectively.

The quantity that is actually measured in acalorimetric experiment is the heat flux throughthe reactor wall to the cooling/heating systemQtrans. There are two main methods to determineQtrans: heat flux calorimetry and heat balancecalorimetry.

Heat Flux Calorimetry. In heat flux calo-rimetry, the generated heat is measured bymeansof the temperature difference between reactorand jacket. The heat transfer between reactorinterior and jacket medium Qtrans depends on theoverall heat transfer coefficient kQ through thewall, the heat exchange area of the reactor wallAR that is actually in contact with the reactionmixture (‘‘wet area’’), and the difference betweenreactor temperature and mean jacket fluid tem-perature _TJ

_Qtrans ¼ kQ�ARð _TJ�TRÞ ð8Þ

Typically, the temperature difference is suffi-ciently large for heat flux calorimetry to be verysensitive.

Advantages of heat flux calorimetry are its (i)high sensitivity because of the (relatively large)temperature difference between reaction medi-um and jacket fluid, (ii) fast response and highaccuracy, and (iii) its independence on the flowrate of the jacket medium.

The reactor area for heat exchange with thejacket AR is almost constant for batch reactions.There can be some variation in AR as a result ofdensity changes of the medium during the course

Figure 2. Schematic of a reaction calorimeter


of the reaction. In semibatch reactions, AR is nolonger a constant. Feeding of reaction compo-nents into the reactor leads to an increase of itsfilling level. If the feeding rates are known theirinfluence on AR can be taken into account.

A major problem in heat flux calorimetry isthe determination of the exact kQ value. Theoverall heat transfer coefficient may change con-siderably during the course of the reaction be-cause it depends on, for example, the mixture’sviscosity and density, the stirring rate (hydrody-namics of the reactor), and heat transfer throughthe reactor/jacket interface (influence of filmformation or reactor fouling). Hence, kQAR mustbe calibrated separately in a typical heat fluxcalorimetry experiment. Several variations areproposed in the literature and offered by instru-ment manufacturers:

Continuous Calibration during the reactioncan be achieved by application of well-definedheat pulses produced by a calibration heater. Thereactor heat measured with these pulses enablesone to quantify the heat of reaction. However, thehot surface of the calibration heater may alsocause problems with temperature-sensitive sam-ples (e.g., product degradation, film formation).

REICHERT [48], [49] developed an elegantmethod to overcome the problem of variationsin kQAR, known as temperature oscillation calo-rimetry. A small sinusoidal temperature changeis added to the overall temperature/time charac-teristic of the reactor via the jacket liquid or acalibration heater. These forced temperature os-cillations

_Qosc ��1þsinðwtÞ

�ð9Þ

create temperature oscillations of the reactorwith amplitude dTR and frequency w ¼ 2p/t,where t is the oscillation period. Decoupling andseparate determination of oscillating and non-oscillating terms in the energy balance equationallows the simultaneous on-line calculation ofthe rate of reaction as well as the heat transfervalue of the reactor kQAR:

kQ�AR ¼ �wCp;mixturedTRsinjr

dTJ�dTRcosjr

ð10Þ

where jr is the phase shift between the oscilla-tions, and dTJ the amplitude of temperatureoscillations of the jacket liquid. Temperature

oscillation calorimetry has been critically re-viewed and compared to conventional calorime-try in [50], [51].

One- or Two-Point Calibration. In the caseof a constant heat transfer coefficient during thereaction the heat transfer value kQAR can bedetermined by calibration before the start of thereaction.

Several commercially available calorimetersapply a two-point calibration. The first calibra-tion must be carried out before the reaction runstarts to determine an initial value of kQAR. Asecond calibration after completion of the reac-tion in combination with the assumption of alinear behavior between start and end calibrationallows one to back-interpolate the reaction dataafter the end of the reaction.

However, the reactor/jacket interlayer isprone to film formation and precipitation ofreaction products or byproducts, and the viscosi-ty of the reaction mixture may change dramati-cally in a short time interval, all ofwhich can alterkQ by orders of magnitude at a certain time (notcontinuously). Hence, interpolation is inaccu-rate, and even pre- and post-calibration of kQ byseparate experiments may lead to erroneousresults.

Absolute Heat Flux Calibration. Absoluteheat flux calorimetry (ChemiSens AB, Lund,Sweden) relies on the fact that the thermal con-ductivity lQ within the reactor wall is constantthroughout the reaction, in contrast to the overallheat transfer coefficient kQ through the wall(wall þ interlayers). Calorimeter calibration canbe carried out by utilizing the temperature gradi-ent (T1 � T2)/d (see Fig. 3). Thus, the heat fluxcan be calculated according to

kQ�ARðTJ�TRÞ ¼ lQdARðT1�T2Þ ð11Þ

Note that the immediate vicinity of the reactorwall to both sides (i.e., reaction mixture andjacket liquid) may behave like an additional thinfilm characterized by other distinct heat transferresistances.

Heat Balance Calorimetry. An alternativeapproach formeasuring the heat generatedwithinthe reactor is to determine the total heat balance


of the cooling/heating liquid circulating throughthe jacket:

_Qtrans ¼ _mliqcp;liqðTJ;in�TJ;outÞ ð12Þwhere cp,liq is the specific heat capacity of thejacket liquid. In this case,Qtrans is independent ofboth heat exchange area and heat transfer coeffi-cient. However, it depends on the circulation ratem_ liq of the jacket liquid. Heat losses at the outerjacket wall directly influence the heat balance ofthe circulating liquid.

Heat balance calorimetry is relatively slowand insensitive compared to heat flux calorime-try because of the small temperature differenceTJ,in � TJ,out. This also leads to stricter require-ments with respect to the accuracy of tempera-ture measurements. To achieve 0.5 W resolu-tion, it is necessary to measure the temperaturewith an accuracy of about 0.001 K (compared to0.05 K in heat flux calorimetry for the sameresolution). Under typical conditions, energyresolution in heat balance calorimetry is on theorder of 2 W, compared with 0.5 W in heat fluxcalorimetry.

Heat Compensation Calorimetry. In heatcompensation calorimetry, both the vessel tem-perature TR and the jacket-fluid temperature TJare fixed with TJ < TR. Heat from an additionalelectrical heater Qel is supplied to the mixture tomaintain the temperature TR of the reactants.After the reaction has been started and a heat Qr

released, the electrical power Qel is reduced tohold TR constant. This reduction inQel is equal tothe reaction heatQr under the condition that kQAR

and the heat losses Qloss are constant throughoutthe reaction.

As in heat flux calorimetry, the unknown andchanging quantity kQAR requires calibration of

the calorimeter. The hot surface of the electricalheater is prone to the formation of films, coat-ings, precipitates, and hot spots.

2.2.2. Basic Modes of Operation

From a thermodynamic point of view, three basicmodes of operation in heat flux calorimetry canbe distinguished: adiabatic, isoperibolic, andisothermal calorimetry.

Adiabatic Reaction Calorimetry. An adi-abatic reaction calorimeter is characterized bythermal insulation of the reaction mixture fromthe surroundings. As a result, the heat releasedduring the reaction is stored within the reactionmixture. Thus, the temperature gradient of thereaction mixture directly reflects the ongoingreaction, and the total heat balance is simplygiven by (cf. Eq. 13):

_Qr ¼ CR;totT_R ð13Þ

In practice, thermal insulation of the reactionmixture can be achieved either by means of aninfinitely large thermal resistance between mix-ture and surroundings (e.g., Dewar calorimeterwith evacuated reactor jacket) or by continuouslymatching the jacket temperature to that of themixture such that there is no heat transfer throughthe reactor/jacket wall (TJ(t) ¼ TR(t)).

A major disadvantage of adiabatic reactioncalorimetry is the interdependence of mass bal-ance and heat balance via the reaction rate, whichrequires an adequate kinetic model for dataanalysis.

Isothermal Reaction Calorimetry. In iso-thermal reaction calorimetry, the reaction mix-ture temperatureTR is held constant, for example,by adjusting the temperature of the jacket fluid TJor by controlling the power of a compensationheater. In practice, it is very difficult to keep thereactor strictly isothermal because of the nonzeroheat transfer resistance of the reactor wall anddeviations and delays in power control. Thus,heat accumulation must be taken into account inthe isothermal heat balance

_Qr ¼ kQARðTR��TJÞþCp;total_TR ð14Þ

Figure 3. Absolute Heat Flux Calorimetry: Schematic ofreactor wall temperature gradient (reproduced by permissionof ChemiSens AB, Lund, Sweden)


Isoperibolic reaction calorimetry is oppo-site to isothermal calorimetry in the sense that thejacket temperature TJ is held constant instead ofthe reactor temperature TR. As a consequence, TRchanges during the reaction. Heat is both accu-mulated in the reactionmixture and transferred tothe jacket liquid. Hence, the heat balance can bedescribed as in isothermal calorimetry (seeEq. 14).

2.3. Applications and Instrumentation

The heat of reaction Qr(t) can be calculatedaccording to Equation 1 if all contributions Qi

to the overall heat balance of the reactor aremeasured or known. If only one exothermicchemical reaction proceeds in the vessel, thereaction rate is given by Equation 2, and theactual relative calorimetric conversion Xr(t) isgiven by Equation 15

XrðtÞ ¼

Rt0

QrðtÞdtRt;final0

QrðtÞdtð15Þ

On-line calorimetry has been used by chemicalcompanies to monitor the extent of reactionsfor several decades [52–57]. Industrial-scalereactors have a relatively low ratio of heat-exchange surface area to volume compared tobench-scale reactors and calorimeters; hence,their heat-removal capacity is limited. Toachieve a maximum production rate in indus-trial-scale reactors, the reaction rate should bematched to the heat-removal capacity through-out the reaction. On-line reaction calorimetryprovides some of the data necessary for adesired reaction run and safe reactor operation(e.g., heat release rates, heat losses, reactionrate, conversion).

Reaction calorimetry has been used to mon-itor polymerization reactions and crystalliza-tion processes [58–60]. Strategies for on-linecontrol and optimization of reactions on thebasis of calorimetric measurements during thecourse of the reaction have been described[61–67]. Reaction calorimetry is the methodof choice for studying thermal runaway reac-tions [68] (see ! Plant and Process Safety).The application of calorimetry to investigating

the kinetics of reactive blending and reactiveextrusion is discussed in [69], and the appli-cation of adiabatic temperature rise for moni-toring reaction injection molding (RIM) in[70]. On-line calorimetric data can be used topredict final product properties by means ofmultivariate statistics [71] or neural networks[72].

Most instrument suppliers (e.g., Mettler-Toledo, ChemiSens, HEL) offer computer-con-trolled calorimeters that can be run in differentmodes (Figure 4). The nonselectivity of reactioncalorimetry in monitoring complex reactions(e.g., reactions in which parallel processes occur,copolymerizations, etc.) can be overcome bycombining calorimetry with other on-line meth-ods to obtain additional information. Thesemethods include density, pressure, gas chroma-tography, dielectric spectroscopy, conductome-try [73], and infrared [74], [75] and Ramanspectroscopy [76] (Fig. 5; ! Infrared and Ra-man Spectroscopy).

Calorimetric data of chemical reactions havealso been determined by differential scanningcalorimetry [77–80] (see ! Plastics, Analysis;! Plastics, Properties and Testing).

Figure 4. Reaction calorimeter RM-2S for heat flow and heatbalance measurements (reproduced by permission of Chemi-Sens AB, Lund, Sweden)


3. Ultrasonic Methods

3.1. Introduction

The propagation of ultrasound in matter is aphysical effect that is receiving increasing atten-tion for on-line monitoring of chemical process-es. Ultrasonic methods are easy to use, safe,nondestructive, and noninvasive. Signal re-sponse times on the order of milliseconds allowreal-time monitoring of fast reactions.

In most applications, ultrasound is generatedby piezoelectric transducers. Because of the re-versibility of the piezoelectric effect, the trans-ducers can act both as emitters and receivers ofultrasonic waves. Depending on the require-ments of the application, a wide variety of readilyavailable and cost effective piezoelectric materi-als can be utilized as transducers (e.g., quartz,lithium niobate, barium titanate, poly(vinylidenefluoride), zinc oxide).

Ultrasonic sensors and ultrasonic systems formaterials characterization, process monitoring,and applications in chemistry are reviewed in[81–88].

3.2. Theory

Aplanar elastic ultrasonic wave can be describedby its complex alternating pressure p(x,t) as afunction of time t and distance x (Eq. 16)

pðx; tÞ ¼ p0exp iw t� xu

� �h i�exp½�ax� ð16Þ

where w ¼ 2pf (f ¼ frequency), p0 is the pres-sure amplitude, and u and a are the velocity andattenuation of sound in matter. The frequency ofultrasound waves is in the range of 20 kHz toabout 1 GHz with corresponding wavelengthson the order of 1.6 cm to 0.3 mm in air (u � 330m/s), 6 cm to 1.2 mm in liquids (u � 1200 m/s),and 20 cm to 4 mm in solids (u � 4000 m/s).

Both u and a strongly depend on the materi-al’s state and properties. In solids, the longitudi-nal and transverse sound velocities ul,sol andutr,sol are related to the bulk modulus K, the shearmodulus G, and the density r according to

u2l;sol ¼Kþ 4

3G

rand u2tr;sol ¼

G

rð17Þ

The longitudinal ultrasonic velocity in liquidsul,liq depends on both the adiabatic compressibil-ity b of the liquid and its density (Eq. 18).

u2l;liq ¼1

brð18Þ

Ultrasonic shear waves in liquids (utr,liq) do notpropagate significantly. They are strongly damp-ened, and therefore not of technical importance.

The sound attenuation a of solids and liquidsdepends on several loss mechanisms. Intrinsic,viscous, and thermal losses may contribute to a,as well as scattering and reflection effects atinterfaces in heterogeneous samples, relaxationprocesses in polymers, electrokinetic losses indisperse systems, and structural losses in con-centrated systems (e.g., pseudoplasticity). Thus,a is influenced by material properties such asdensity, viscosity, thermal conductivity, expan-sion, and capacity; by the morphology of thesample (grain size and shape); and by tempera-ture and pressure. It is difficult to derive analyt-ical expressions for a because of its complexdependence on these parameters. However, insimple homogeneous liquid systems, a can bedescribed as

Figure 5. Reaction calorimeter RC1 equipped with React IRInfrared Spectrometer (ASI Applied Systems) for simulta-neous on-line calorimetry and infrared analysis (courtesy ofMettler-Toledo GmbH, Analytical)

ColorFig


aliq

f 2¼ 2�p2

r�u34

3hsþhv

� �ð19Þ

where hs and hv are the shear and the volumeviscosities of the liquid. Sound is strongly ab-sorbed at high frequencies (f2 dependence) andcannot penetrate deeply into the material.

A third acoustic parameter of importancewithrespect to reaction monitoring is the acousticimpedance Z of the material (Eq. 20).

Z ¼ r�u ð20Þ

It is typically measured in reflection experimentsat interfaces by means of the reflection coeffi-cient R (Eq. 21).

R ¼ Z1�Z2Z1þZ2 ¼ 1�T ð21Þ

where Z1 and Z2 are the acoustic impedances ofthe materials on both sides of an interface, and Tis the transmission coefficient.

A common method for determining acousticproperties is to monitor (compressional) soundpulses travelling over a well-defined distance(Fig. 6). The emitting transducer E generates asound burst of initial amplitude p0 (proportionalto the applied voltage U0) which travels a dis-tance x in a time Dt. In multiple-echo mode, Dt isthe time delay between successive echoes, and xis twice the distance between transducers. Thedampened (and broadened) pulse reaching thereceiving transducer R generates an electricalvoltageUx proportional to the pressure amplitudepx at the receiver. By using this method, soundvelocity and attenuation can simply be calculatedaccording to Equations 22 and 23.

u ¼ x

Dtð22Þ

a ¼ 1

xlnp0px¼ 1

xlnU0

Uxð23Þ

Measurements of pulse travelling time are highlyaccurate and relatively easy to carry out com-pared to sound attenuation measurements.

The frequency of ultrasonic sensors for reac-tion monitoring is generally in the 1 – 10 MHzrange. In principle, the accuracy of sound veloci-tymeasurements is poor for very low frequencies(< 100 kHz) and increases with increasing fre-quency. However, attenuation does also increasefor f � 10 MHz, and the distance between trans-ducers must be shortened for the signal to reachthe receiver. As a result, the accuracy of pulsetravelling timemeasurements also deteriorates atvery high frequencies. Typical demands on theaccuracy of ultrasonic sensors for reaction moni-toring are

Duu� 10�3

Daa� 0:5%

DZZ� 10�3

These conditions imply time measurements inthe nanosecond and picosecond range, and ofamplitude resolutions of 12 bit and higher [81].Additionally, temperature control has to be betterthan � 0.1 K.

In addition to u and a, other sound wavecharacteristics (e.g., wave phase j, resonancefrequency fres and shift Dfres) can be measuredin certain ultrasonic sensor applications for ma-terials characterization and chemical processesmonitoring [81].

3.3. Applications

The basic condition for monitoring chemicalprocesses bymeans of ultrasound is that physicalproperties that determine u,a, orZ change duringthe course of the reaction (see Eqs. 17 – 20). Inpolymer production andmodification, curing andcross-linking reactions, mechanical moduli aswell as the mixture’s density, compressibility,viscosity, thermal conductivity, etc. alter dra-matically, thus influencing sound attenuation andvelocity (see Table 3). Hence, those processesare ideal for application of ultrasound to detect

Figure 6. Schematic of ultrasonic pulse travelling measure-ments. (The transducer arrangement can also be used inmultiple reflection echo mode.)


the extent of reaction. Even electrochemicalprocesses can be monitored because of the sen-sitivity of ultrasound to the type of ion andconcentration.

A major advantage of ultrasound (velocity)measurements in industrial applications isthe capability to plug the sensor directly intothe reacting mixture. They do not necessarilyrequire a sampling loop. Care should be takenbecause plug-in transducers may cause foulingat their edges or lead to formation of coagula andfilms. The disturbing influence of gas bubbles inthe reaction mixture between the transducers isdemonstrated in Figure 7. Monitoring of acous-tic impedance Z via reflection coefficient mea-surements may reduce the bubble disturbances.However, so far, no application of Z determina-tion in reaction monitoring has been describedin the literature.

A simple empirical but useful approach fordescribing the sound velocity ofmulticomponentreaction mixtures is based on the assumption thateach component i of the mixture contributesindependently to the sound velocity of the mix-ture umix according to its concentration ci

umix ¼ u0þXi

DuDc

� �i

ci ð24Þ

where u0 is the sound velocity of a referencecomponent (e.g., solvent, dispersion medium)whose ultrasonic velocity is either constantthroughout the process or changes in a knownmanner, and Du/Dc is the (temperature-depen-dent) sound velocity – concentration coefficient,which itself may depend on concentration. How-ever, higher order terms are neglected in Equa-tion 24. The influence of component i on thesound velocity of component j is also neglected.Equation 24 is the basis for on-line concentrationmeasurements in numerous applications inchemistry, biochemistry, and the foods industry.

Ultrasound has been used tomonitor chemicalreactions since the 1970s [89–92]. Ultrasonic on-line monitoring of polymerization reactions wasreported in [93–99]. Crosslinking reactions (cur-ing, gelation) were investigated with ultrasoundin [92], [100–106]. As an example, the influenceof the extent of reaction of the epoxide ring in asystem consisting of the diglycidyl ether of bi-sphenol A (DGEBA) and 4,40-diaminodiphenyl-methane (DDM) on the ultrasonic velocity andrelative ultrasonic absorption is shown in Fig-ure 8. Viscoelastic properties of polymer meltsin extruders can be monitored by in-line deter-mination of ultrasonic parameters, along withtemperature and pressure measurements [69],[91], [107–111].

Ultrasound is sensitive to the local concentra-tion of oil in emulsions and thus enables moni-toring of creaming and flocculation processes[112], [113]. It is applicable to both dilute andconcentrated emulsions.

Ultrasound is not only a tool for monitoringchemical reactions but also one for initiatingthem. The origin of the enhancement of chemicalreactions by ultrasonication is the intensity of thecavitation bubbles produced in the sonicatedmedium at higher sound intensities [87], [88],[114–116] (see ! Sonochemistry).

The dependence of ultrasonic parameters ongrain sizes in heterogeneous media enables oneto determine particle sizes and particle size dis-tributions in latices, emulsions, suspensions, etc.even at very high concentrations of the dispersephase [117–121]. Thus, acoustic spectroscopy(as well as electroacoustic spectroscopy [119],[122]) is developing rapidly as an alternative to

Figure 7. Influence of gas bubbles on ultrasound velocitymeasurements during emulsion polymerization

Table 3. Ultrasound velocities of monomers and polymers at 20 C

Substance Monomer Polymer

Butyl acrylate 1233 m/s 1375 m/s

Styrene 1354 m/s 2120 m/s

Vinyl acetate 1150 m/s 1853 m/s

Vinyl chloride 897 m/s 2260 m/s


light-scatteringmethods for particle size analysisin dense media [123–127].

Ultrasonic microsensors are used for materi-als characterization and reaction monitoring[81], [83]. Ultrasonic microsensors based on thequartz crystal microbalance (QCM) principle are

very sensitive for a wide variety of chemicalapplications. The high precision and ease ofmeasurement of the QCM frequency has madeit a useful tool for measuring slight changes inmass on the QCM electrode surface. By coatingthe surface with a sensitive layer, the QCM canbe used as selective gas or even under-liquidsensor. The determination of contact angles andsurface tensions with a QCM is described in[128]. Detergency processes have been moni-tored in real-time by using a QCM [129]. The useof shear-horizontal (SH) acoustic plate mode(APM) devices to monitor cross-linking reac-tions in polymer films in real time is described in[130]. Photo-cross-linking of diacetylene thiol-based monolayers on surface acoustic wave sen-sors (SAW) has been monitored in situ [131].Recent developments include the application offiber-optic interferometric ultrasound sensors formonitoring the cure of epoxy resins [132], [133].

In addition to the active ultrasound measure-ments discussed above passive ultrasonic mea-surements can also be utilized to monitor chemi-cal reactions. In active ultrasound, an acousticwave is generated and changes to the wavecaused by the ongoing process are being mea-sured, whereas the acoustic emissions created bythe process itself are the signals of interest in apassive ultrasonic measurement [134]. Acousticemissions have been measured for equipmentfault detection, gas – liquid mixing and flow,bubble formation and monitoring, foam break-up, powder flow and mixing, particle collisionmonitoring in solids and solid – fluid suspen-sions, compaction, and chemical reaction moni-toring. In particular, chemical processes withphase changes during the course of the reactionand processes including granular material areacoustically active.

4. Dielectric Spectroscopy

4.1. Theory and Mechanisms

Dielectric spectroscopy deals with the electricalresponse of the polarizationP (w) ofmatter to theapplication of an oscillating electric field E (w)

DðwÞ ¼ eoEðwÞþPðwÞ ¼ eðwÞe0EðwÞ ð25Þwhere D is the dielectric displacement vector, wis the angular frequency, and e* and e0 are the

Figure 8. Ultrasonic velocity (A) and relative ultrasonicabsorption (B), both measured at 1 MHz, as a function ofthe extent of epoxide ring reaction P for a DGEBA/DDMsystem at 70 C; gel point Pgel ¼ 0.58(reprint from [104], with permission from Elsevier Science)


dielectric permittivity and the permittivity of freespace, respectively. Several atomic and molecu-lar mechanisms contribute to the dielectric be-havior of matter [135–138]:

1. Electronic polarization (i.e., induced dipolemoment due to the displacement of electronsfrom their equilibriumpositionwith respect tothe nucleus)

2. Displacement polarization (atomic, ionic, lat-tice polarization; e.g., induced dipolemomentdue to the deflection of ions from their equi-librium positions in molecules)

3. Orientation polarization of permanent dipolespresent in the system (i.e., alignment to thedirection of the external field)

4. Motion of intrinsic and extrinsic charge car-riers (ions, particles)

5. Charge transfer and polarization at electrodes6. Interfacial polarization and distortion of the

electrical double layers at interfaces in het-erogeneous systems

A certain mechanism contributes to the over-all polarization of the sample if its characteristicrelaxation time is faster than the time scale of theexternal field. Hence, the various polarizationmechanisms can be distinguished on the basisof their different frequency dependencies.

Time scales for resonances of electronic anddisplacement polarization of atoms and ions arein the femtosecond (10�14 to 10�15 s;UV region)and picosecond range (10�12 to 10�13 s; IRregion), respectively. Thus, these induced di-poles contribute instantaneously to the dielectricproperties of matter typically measured in thefrequency range 10�5 to 1011 Hz.

Key phenomena determining the change ofdielectric properties during the course of a chem-ical reaction are orientation polarization of per-manent dipoles, migration of charges, and inter-facial effects in heterogeneous systems.

In contrast to electronic and atomic polariza-tion, the orientation of permanent dipoles is time-dependent and not always instantaneous. It isgoverned by the (micro) viscosity of the imme-diate surroundings. In polymeric systems, it re-quires cooperative motion of chains or chainsequences, too. Dipole relaxation is the stronglytemperature dependent reorientation of dipolesdue to thermal fluctuations after the removal ofthe external field. As the frequency is scanned,

various characteristic dipole relaxations accom-panied by corresponding losses are observed.

The migration of charges mainly influencesthe dielectric losses. The (ionic) conductivitycontribution to the loss factor is inversely pro-portional to the frequency of the external field.Ionic conductivity can be correlated with viscos-ity, since fluidity is indicated by the ease withwhich charge carriers can migrate through thesample.

The complex dielectric permittivity e*(w) iscomposed of two parts, a real (or storage) com-ponent e0(w) in phase, and an imaginary (or loss)component e00(w) out of phase with the appliedoscillating electric field (Eq. 26).

eðwÞ ¼ e0ðwÞþie00 ðwÞ ð26Þwhere i is

ffiffiffiffiffiffiffiffiffiffið�1Þp. e0ðwÞ is proportional to the

capacitance of the sample, and e00(w) is influ-enced both by the material’s conductivity and bydipolar losses. A sigmoidal change in e0(w) andthe appearance of a peak in e00(w) is indicative ofa relaxation process (Fig. 9)

e0ðwÞ ¼ e00�ðe00�e0¥Þw2t2

1þw2t2ð27Þ

e00 ðwÞ ¼ ðe

00�e0¥Þwt1þw2t2

ð28Þ

where t is the relaxation time. The loss factortan d is defined by

tandðwÞ ¼ e00 ðwÞe0ðwÞ ð29Þ

As indicated above, the dielectric permittivity isnot a constant but a function of frequency. Ad-ditionally, e* is strongly influenced by tempera-ture since both the transitional mobility of chargecarriers as well as the reorientational motion ofdipoles are temperature-dependent. For somecases in the absence of chemical reactions, thetemperature dependence of dielectric propertiescan be described in terms of Arrhenius andWilliams – Landel – Ferry (Vogel – Fulcher)type relations, depending on the nature of thedipolar motions (single activation and coopera-tive process, respectively).

In addition to the complex dielectric permit-tivity, there are several other quantities that canbe alternatively derived from dielectric spectra


[139], [140]. These include the impedance andadmittance (or conductivity), the dielectric mod-ulus, and the susceptibility. Admittance spectraY*(w), the inverse of impedance spectra Z*(w)¼Z0(w) � iZ00(w)¼ (Y*(w))�1 for linear response,can be calculated from permittivity data accord-ing to Equation 30.

eðwÞ ¼ YðwÞiwC0

ð30Þ

where C0 is the equivalent capacitance of freespace.

Transformations from one formalism to an-other can help resolve particular aspects of therelaxation processes, but no new information canbe extracted in the linear response regime of thematerial. The impedance spectrum Z*(w) is oftenused for discussion because dielectric data can beanalyzed in terms of equivalent electrical circuitsconsisting of resistors and capacitors connectedin parallel and series. The Z*(w) formalism (e.g.,Z00 vs. Z0 plots) is particularly helpful in separat-ing bulk and surface phenomena (e.g., polariza-tion in the bulk and electrode polarization).

4.2. Instrumentation and Applications

Instrumentation. According to the fre-quency of the external electric field, severalfrequency regimes for dielectric experiments canbe distinguished which require different mea-surement principles [140–142] (Fig. 10).

The common instrumentation in the imped-ance regime from about 10�5 to 108 Hz includes

Figure 9. Dielectric relaxation: Dielectric permittivity and dielectric loss as a function of frequency

Figure 10. Principles of dielectric measurements:A)Frequency response analysis technique;B)Resonance circuit;C) standing wave in hollow conductorPS: Power source


current – voltage measurements in electrode/sample/electrode sandwich arrangements ofelectrical (ratio-arm) bridges, and resonance (os-cillating) circuits. The main techniques for di-electric reaction monitoring are (1) the direct a.c.measurement of amplitude and phase betweenvoltage and current over a certain frequencyrange by using a frequency response analyzer(FRA), and (2) step voltage application (or re-moval) with rapid acquisition of the currentresponse over time and transformation of thesignal into the frequency domain.

Minimum time intervals for dielectric mea-surements are on the order of tenths of seconds,so dielectric spectroscopy in the impedance fre-quency regime is suitable for on-line monitoringof chemical reactions. Typical accuracies ofmeasurements achievable in the impedance fre-quency regime are about � 0.05% for e0,� 0.2% for e00, and better than 10�4 for tan d.

Dielectric cells for liquids are generally sim-ple and consist of two concentric cylinders withthe fluid in the space between them. These mea-surements are of very high precision because ofthe good dimensional stability of the cells. Solidsamples are typically sandwiched between metalplate electrodes as in a capacitor. Compared toliquid cells, solid sample set-ups are quite com-plicated because they must take into account forthe sample’s thermal coefficient of expansion.

The radio regime from about 106 to 1010 Hzrequires resonance circuits (low w) or guided-wave techniques (high w; e.g., standing waves inhollow conductors).

The typical equipment for the microwaveregion from 109 to several 1011 Hz is coaxial orrectangular waveguides.

Propagating wave techniques are utilized inthe IR and UV – VIS regimes above a few1011 Hz with Fourier transform technique as themethod of choice.

Applications. Chemical reactions are ac-cessible to reaction monitoring by dielectricspectroscopy if they lead to changes in the rateof motion of molecules contributing to the di-electric response of the mixture (e.g., rotationalor vibrational motion of dipoles and translationalmotion of charge carriers).

Many applications of dielectric spectroscopypublished so far are for monitoring the cure ofresins. Dielectric spectroscopy can detect

changes in the polymer cross-link density duringthe course of polymerization (Fig. 11). Whenvan der Waals or other weak interactionsbetween molecules are replaced by covalentbonds during the conversion of low-viscositymonomers into polymers, the dipole moments ofmolecules and the mechanisms of their Browniandiffusion change, as does the size of the diffusingmoieties. The restrictions on the diffusion ofdipoles and charge carriers increase [143–148].

Both the real and the imaginary parts of thedielectric permittivity can have dipolar andionic-charge polarization components. At lowerfrequencies, e00 is typically dominated by themobility of ions in the sample, whereas dipolarreorientation mobility contributes to e00 at higherfrequencies (Eq. 31).

eðwÞ ¼ edipolar�isdc

we0ð31Þ

Here sdc is the d.c. conductivity due to ever-present ionic impurities [149], [150]. The ionicmobility (i.e., sdc) is a molecular probe forquantitatively monitoring viscosity changes dur-ing cure. At short curing times, e00 is dominatedby the d.c. conductivity. When gelation occurs,the d.c. conductivity decreases strongly since themobility of the charge carriers decreases due tonetwork formation. Peaks in e00 indicate whendipolar reorientation processes contribute to e00.

The ionic conductivity signal of dielectricmeasurement data has been used to study curingof epoxy resins. The conversion was calculated

Figure 11. Real and imaginary impedance as a function oftime with frequency as parameter during the reaction of thediglycidyl ether of bisphenol F (DGEBF) with 4,40 methyle-nedianiline (MDA) at 36 C [146]a) Z0 ¼ 500 Hz; b) Z00 ¼ 500 Hz; c) Z0 ¼ 2000 Hz;d) Z00 ¼ 2000 Hz; e) Z0 ¼ 10 000 Hz; f) Z00 ¼ 10 000 Hz


based on a Williams – Landel – Ferry modelapproach [151]. Dielectric spectroscopy in the 10to 105 Hz frequency range with a ratio-armbridge has been used to monitor the UV poly-merization of a polymer-stabilized liquid crystalcell containing a mixture of nematic and chiralliquid crystals together with DSM monomer andphotoinitiator [152]. The authors discuss theirresults in terms of capacitance units. They haveshown that the decreasing loss serves as anexcellent monitor for changing molecular pro-cesses and the state of photopolymerization,whereas the capacitance itself does not changedramatically during the course of the polymeri-zation. Low-frequency losses observed at thebeginning of the reaction are mainly caused byionic impurities. After polymerization, the liquidcrystal cell has the dielectric characteristic of arelatively low loss dielectric.

An electrical impedance spectroscopymethodhas been developed as a real-time online controltool for thermoset composite manufacturing byexploiting the change of the imaginary imped-ance maximum as a reaction progress indicator[153].

In particular, dielectric spectroscopy is themethod of choice for following the curing ofpolar coatings. In these thin-layer systems, veryhigh signal-to-noise ratios can be achieved be-cause their capacitance is inversely proportionalto their thickness, like the capacitance in a platecapacitor as a function of interplate distance. Aspecial thin-layer dielectric method has beendevised which measures the dissipation factorof an adhesive during the cure of adhesivelybonded joints and converts it into the degree ofcure of the adhesive [154].

Dielectric spectroscopy has been also appliedto disperse systems [155–158]. The relaxationpattern is the dielectric fingerprint of a colloidalsystem [159]. Correlations between relaxationfrequencies and suspension properties such asparticle volume fraction, electrolyte concentra-tion, Zeta potential, pH, surface conductivity,particle radius, Debye screening length, degreeof ionization, and diffusion coefficient have beendeveloped [160], [161]. These characteristicschange during reactions in colloidal systems, andon-linemonitoring is possible.Moreover, dielec-tric spectroscopy is especially valuable for study-ing concentrated and opaque dispersions. For alatex exhibiting a low-frequency relaxation pro-

cess, the relaxation time at the loss maximumt ¼ 1/wmax is proportional to the square of theparticle radius, and inversely proportional to thediffusion coefficient of the electrical double layerof counterions up to 30% total solids [162].

5. Optical Spectroscopy

5.1. Introduction

Optical spectroscopy covers the range of theelectromagnetic spectrum from the ultraviolet(UV, l � 10 nm) to the infrared (IR, l< 1 mm). Experimental equipment for opticalspectroscopy in this wavelength range is con-structed with typical optical elements such asmirrors, lenses, gratings, and optical filters. Theincident energy is delivered by light sources. Thelong-wavelength endof the electromagnetic spec-trum (i.e., millimeter range) requires completelydifferent experimental techniques (e.g., hollowconductors for microwave spectrometry) as doesthe short-wavelength end (l � 100 nm) withspecial X-ray and g-ray equipment.

Electronic absorptions determine the spectralfeatures observable in the UV – VIS region.Molecular vibrations, rotations, and combinedrotational-vibrational transitions of moleculescan be detected in the near (NIR) to the far (FIR)infrared wavelength range. Spectra can either berecorded in absorption (UV, VIS, NIR, IR, FIR)or emission (IR, Raman, fluorescence).

For general reading, fundamentals, instru-mentation and applications, see [12] and! Ul-traviolet and Visible Spectroscopy and! Infra-red and Raman Spectroscopy.

5.2. Instrumentation for ReactionMonitoring

Spectroscopy over the entire spectral range ofoptical wavelengths has been applied to on-lineprocess monitoring for several decades. The vastmajority of applications is based on nondisper-sive instruments, and light absorption or emis-sion is typically measured either at a singlewavelength or by integration over a certain (rath-er small) wavelength range.

In absorption spectroscopy, the light is gener-ated by monochromatic light sources (including


lasers) or polychromatic light sources combinedwith optical filters. Tuning of the wavelength tocharacteristic absorptions of certain moleculesenables one to detect substances and to observethe variation of their concentration as a functionof time with high selectivity and sensitivity.

In transmissionmeasurements, the fundamen-tal relation between the incident light intensity I0,the transmitted light intensity Il, and the concen-tration of the analyte c in a cell of optical length lis given by the Lambert – Beer law:

logI0ðnÞIlðnÞ ¼ log

1

tðnÞ ¼ AðnÞ ¼ eðnÞcl ð32Þ

where e(n) is the (decadic) extinction coefficientof the sample at frequency n, l is the length of themeasurement cell, A is the absorbance (extinc-tion), and t is the transmittance of the sample.Monitoring procedures based on this classicalphotometric method are still extensively usedbecause of its simplicity, ruggedness, sensitivity,and low cost and maintenance requirements. Itworks best for well-characterized continuousmaterial streams but is less advantageous fordynamic processes (e.g., some batch operations).

The number of substances that can be detectedsimultaneously by nondispersive instruments inabsorption and emission modes is limited, anddetailed molecular information on the speciesundergoing a reaction is not available. Theserestrictions can be overcome by parallel installa-tion of several nondispersive instruments oper-ating at different wavelengths, or utilization ofdispersive instruments (spectrometers) to exploitthe whole spectral information.

With classical dispersive wavelength-scanspectrometers, data collection is time-consumingbecause such an instrument records the datapoints of the spectrum consecutively. Therefore,dispersive scanning spectrometers are not suit-able for process monitoring.

However, the substitution of point detectorsby array detectors in dispersive instruments of-fers the opportunity to acquire large amounts ofspectrally and/or spatially resolved data in ex-tremely short periods of time. Semiconductorarray detectors are characterized by high quan-tum efficiency, wide dynamic range, fast res-ponse, low noise, low power requirement, andcontinuously decreasing costs. Array detectorssuffer from either limited resolution or limited

bandwidth. However, the two-dimensionality ofsome types of array detectors in combinationwith cross-dispersion enables the complete spec-trum to be obtained with high resolution. Sincethe whole spectrum is available with one shot oflight on the detector, chemical reaction monitor-ing on the sub-second timescale is possible.Another advantage of two-dimensional arraydetectors is the ability to record several spectrasimultaneously in different lines of the array thusenabling multiplexing. Fundamentals and appli-cations of array detectors [charge-transferdevices (CCD, CID), photodiode arrays, andphotoconductor arrays] in both absorption andemission spectroscopy are reviewed in [163](see ! Ultraviolet and Visible Spectroscopy,Section 3.6.).

With Fourier transform (FT) spectrometers,the spectral information is first recorded in thetime domain as an interferogram and then Fouriertransformed into the frequency (wavenumber)domain. The optical throughput of an FT instru-ment is very high because light of all frequenciesis recorded simultaneously. Depending on thespectral resolution and the signal-to-noise ratiorequired, FT spectrometers are suitable for mon-itoring very fast chemical processes on the time-scale of seconds to minutes. However, they areless rugged than photometers or array-detectorinstruments because the interferometer is suscep-tible to vibrations, for instance, in a production-plant environment.

The introduction of fiber optics has been amajor step forward in reaction monitoring byoptical spectroscopy. Nowadays, most instru-ment suppliers offer their spectrometers withoptional fiber optics. Optical fibers connect theremote instrumentwith the reactor or process linefor on-line monitoring with the followingadvantages:

. Reduced risk of fire or explosion in productionunits

. Reduced interaction of the delicate spectrom-eter with the hostile process environment

. Enhanced accessibility of various measure-ment points in a process stream or reactor

. Simplified multiplexing

Light may be delivered and collected throughseparate single fibers or fiber arrays; alternative-ly, fibers can be arranged in integrated fiber


bundles in which a light source fiber is sur-rounded by detector fibers, or in which sourceand detection fibers are randomly distributedwithin the bundle. A review of the developmentsin fiber optic applications in molecular spectros-copy is given in [164] (see ! Chemical andBiochemical Sensors). Commonly used fibertypes are listed in Table 4.

Recently, the development of fiber-opticchemical sensors (FOCS) based on evanescentwave absorption (EWA) spectroscopy has expe-riencedmuch progress and improvement. FOCSscan be employed for real-time in situ monitoringby simply immersing them in the mixture andestablishing a connection between the fiber, thelight source, and the detection system. They offerfast response times on the order of seconds withall the advantages of attenuated total reflection(ATR) spectroscopy, and considerably improvedsensitivity.

Figure 12 shows various instrument/probe/process vessel configurations for the applicationof optical spectroscopy to the monitoring ofchemical reactions. Spectra can either be re-corded in transmission, reflection, transflection,or attenuated total reflection. Probes can be di-rectly immersed into the reaction mixture, or thespectra can be recorded through windows in thereactor or pipe wall.

The polarization state of reflected light can bedetermined in addition to its intensity with spec-troscopic ellipsometrymethods ranging fromUVto IR wavelengths [165]. Spectroscopic ellipso-metry is a powerful tool to characterize opticalproperties of thin films and near-surface regionsof bulk materials. It has been applied to study thedeposition ofmaterial inCVDprocesses, thermalevaporation or magnetron sputtering [166],[167].

5.3. Applications of OpticalSpectroscopy

5.3.1. UV – VIS Spectroscopy

UV – VIS spectroscopy is highly sensitive, butthe spectra are less informative because theytypically consist of a few broad absorption peaks.Therefore, UV – VIS spectroscopic reactionmonitoring is less commonly used than otherspectroscopic techniques. Typical applicationsof UV – VIS spectroscopy include photometricgas analysis in transmission gas cells for thedetection of sulfur dioxide, nitrogen oxides,chlorine,mercury vapor, toluene, benzene,meth-ane, ethanol, etc. Detection limits are in the orderof tens of ppm in air [8].

A major advantage of the UV – VIS spectralregion is that water does not absorb considerablyin the entire VIS region up to the mid-UV, thusmaking it attractive for water and wastewateranalysis. UV – VIS absorption measurements inaqueous or other fluid process streams are typi-cally performed in transmission or ATR modes.With ATR probes, even highly opaque mixturesand liquid systems containing strongly scatteringparticulate solids are accessible to processanalysis.

In disperse systems, light scattering by parti-cles accounts for much of the attenuation of theincident light. Thus particle size measurement byUV – VIS scattering methods is common prac-tice in colloid laboratories and at productionsites.

The general applicability of UV – VIS spec-troscopic techniques as well as their advantagescompared to engineering process data in order toobtain characteristics of ongoing chemical reac-tions, to control reactions, to predict productproperties, and to determine end points of batchand blending processes has been discussed inconjunction with data handling methods in detailin [168].

The versatility of UV – VIS spectroscopy inprocess monitoring has been extended by thedevelopment of UV array spectrometers [169].

Optical fibers for use in the VIS region arereadily available. Transmission of visible lightthrough optical cables is possible over long dis-tances for remote process monitoring. It has beenshown that fiber-optic UV – VIS spectroscopyand chemometric data treatment (window factor

Table 4. Fiber types and typical optical wavelength range [164]

Fiber type

Approximate

optical range

High-purity fused silica 200 – 2000 nm

Plastics 350 – 1100 nm

Glass 350 – 1800 nm

Low-OH fused silica 250 – 2700 nm

Zirconium fluoride 2 – 5.5 mmChalcogenide 3 – 11 mmSilver halides 5 – 20 mm


Figure 12. Instrument probe process vessel configurations for the application of optical spectroscopy to monitoring chemicalreactionsA) Transmission; B) Transflection; C) Reflection; D) Scattering (90 ); E) Attenuated total reflection (crystal); F) Attenuatedtotal reflection (single-bounce tip); G) Evanescent-wave fiber opticFO ¼ fiber optic; L ¼ light source; D ¼ detector


analysis) enable one to detect reaction intermedi-ates of an industrially significant process thatcould not be detected otherwise (e.g., by HPLC)[170]. UV – VIS fiber optics have also beenapplied to monitor chemical processes in super-critical fluids [171]. In the short-wavelength UV,however, the optical absorption of the fiber coreand/or cladding material restricts light through-put. Recently, evanescent-wave fiber optic ab-sorption sensors for use in the UV and VISregions have been described [172], [173]. UVreflection spectroscopy with a fiber optic acces-sory has been used to characterize the cure of bis(maleimide) resins [174], [175].

5.3.2. NIR Spectroscopy

Absorption bands in the near infrared originatefrom electronic transitions of the heaviest atomsand from overtones and combinations of funda-mental vibrations in the IR. In organic sub-stances, C – H, N – H, and O – H vibrationsgive rise to dominant absorptions in the near-IR.Therefore, the detection of water was one of thefirst on-line applications of NIR spectroscopy.However, water interference may also compli-cate NIR spectroscopy of aqueous samples [176].NIR-active vibration overtones and combina-tions are characterized by low molar absorptiv-ities and high scattering efficiencies, and the NIRbands are broad and overlapped.

The peculiarities of NIR spectrometry lead toimportant consequences in practice:

1. Near IR wavelengths are able to penetrateinto and sometimes even through a sample,thus enabling measurements on real worldsamples in the millimeter or centimeter thick-ness range. Spectra can be recorded on mosttypes of liquid, solid, powder, or gaseoussamples without sample pretreatment. Samplesize may vary from tiny bits to large chunks.

2. NIR spectra contain all the spectral informa-tion necessary for fingerprinting, albeit buriedunder broad and strongly overlapped absorp-tion features. Inmost cases, the spectra cannotbe interpreted directly, unlike the individualpeaks of IR spectra, and calibrations andchemometric methods have to be exploitedfor quantitative analysis. This may cause pro-blems in NIR reaction monitoring of batch

processes with frequently changing recipesand reaction regimes because of the necessityto provide an extensive training data set foreach process [177].

Typical analysis time is on the order of sec-onds or even shorter, with sensitivities down totenths of a percent; hence, NIR is an ideal tool forobserving fast reactions. Spectra of chemicallychanging mixtures can be recorded throughquartz glass windows mounted in reactors andpipes or by fiber optic immersion probes fordiffuse reflectance, ATR, transmission, andtransflection measurements. The application ofcheap silica fibers allows truly remote sensingwith distances up to kilometers between processand spectrometer because of the extremely lowattenuation of quartz in the VIS to NIR range.

NIR equipment suitable for reaction monitor-ing includes both holographic grating/semicon-ductor array detector instruments and Fouriertransform spectrometers. Light-emitting diode(LED) based instruments have no moving partsbecause each wavelength band is produced by adifferent diode with precise wavelength tunabil-ity and monochromaticity, thus making the in-strument rugged to harsh process environments.LED spectrometers are highly sensitive andselective, with fast response and exceptional dy-namic range. So far, they have been applied toon-line monitoring of HF and H2S traces in refin-eries. However, their high costs are prohibitive formost on-line applications. Acousto-optic tunablefilter (AOTF) based instruments have been occa-sionally used for on-line analysis. Hadamard trans-form (HT) spectrometers have not been describedfor NIR on-line applications so far.

Typical problemswithNIRmeasurements arethe need to check the calibration of the instru-ment, since the instrumental response maychange after some time [178], and the sensitivityof NIR spectra to the sample temperature, whichcomplicates training procedures. NIR equipmentand the application ofNIR to reactionmonitoringare reviewed in [179–183] (see ! Infrared andRaman Spectroscopy).

NIR spectroscopy has been widely used forthe characterization of epoxy curing [184, 185]with conventional fiber optics [186], [187],evanescent wave high-index fiber optic sensors[188], and the NIR ATR technique [189]. Otherfiber optic applications include reaction


monitoring in explorative organic synthesis[190], real-time analysis of light alkenes inhigh-pressure reactors at elevated temperatures[191], remote monitoring of styrene emulsionpolymerization in the short-wavelength NIR re-gion [192], and determination of octane in refin-ery processes [193]. A fiber optic coupled AOTFNIR spectrometer has been described for dryereffluent monitoring in a chemical plant [194].The number of papers describing the utilizationof NIR spectroscopy to monitor the progress ofpolymerization reactions of silanes [195] andvinyl monomers [196–202] is continuously in-creasing. One of the major advantages of NIRspectroscopy in polymerizationmonitoring is thecapability to extract molecular mass informationfor reaction control instantaneously that cannotbe measured otherwise in a timely manner[203–205]. NIR spectroscopy has been shownto be suitable for on-line detection of reactioncompletion in a closed-loop hydrogenator inpharmaceutical production [206], for end-pointdetection in pharmaceutical high-shear granula-tion processes [207], and in-line monitoring ofthe moisture content in fluid-bed dryers [208].Diffuse reflectance NIR spectroscopy has beenevaluated as an on-line technique for the moni-toring of powder blending [209–211]. Gas-phasereal-time monitoring by transmission NIR spec-troscopy is described in [212], [213]. NIR spec-troscopy has been utilized to measure major

components in the production of acetic acid[214]. On-line particle size monitoring based onNIR spectroscopy has been described for phar-maceutical nanoparticles [215], [216] as well asfor emulsion polymers [217]. A promising appli-cation of remote-sensing NIR spectroscopy isthe on-line identification and classification ofplastics material [218–220].

Near-infrared spectroscopy is one of the fewanalytical methods applicable to real-time moni-toring of chemical processes in polymer meltsduring extrusion [221]. Figure 13 shows a ce-ramic transmission NIR probe with variable op-tical pathlength that is adapted to a corotatingtwin-screw extruder and interfaced to an NIRspectrometer via fiber optic cables. On-line NIRspectra, as shown in Figure 14, provide thequantitative composition of a polypropylene/eth-ylene – vinyl acetate copolymer mixture duringblend formation with an accuracy of about 1%.With the help of ATR (Fig. 15) and diffusereflectance probes the formation of styrene –maleimide copolymer (Fig. 16), and the fillerconcentration in molten, opaque polypropyl-ene/chalk powder composites have been moni-tored. In the latter application, the inhomoge-neous distribution of filler particles may lead tosubstantially different spectra during the courseof extrusion. The scattering contribution ofthe particles to the NIR spectra has to be cor-rected before chemometric analysis. Appropriate

Figure 13. Ceramic transmissionNIR probewith variable optical pathlength and diffuse reflectance dipper probe for insertioninto extruder (courtesy of IPF Dresden)


chemometric calibrations models are required inbothcases [222–224].HANSENet al. [225–228]wereeven able to derive rheological properties of ethyl-ene – vinyl acetate copolymers from NIR spectrarecorded in-line in an extruder via fiber optics.

5.3.3. IR Spectroscopy

Infrared spectroscopy has a long tradition of usein chemistry, materials science, physics, andeven reaction monitoring [229]. It is one of thestandard techniques in most analytical laborato-ries. The IR region of the spectrum is highlyspecific and very sensitive to molecular struc-tures. Individual peaks can be assigned unequiv-ocally to chemical species and calibrated foranalyte concentration. The timescale for FTIRspectroscopic analysis is on the order of seconds.All of this makes IR spectroscopy attractive foron-line process monitoring.

High molar absorptivities in the IR are advan-tageous for gas analysis but may present chal-lenges for liquid analysis because sample layerthicknesses (optical pathlengths) would have tobe in the micrometer range to transmit IR radia-tion. This distance is too short for circulation ofthe reaction mixture, so transmission and trans-flection measurements cannot be used for pro-cess monitoring.

The technique of choice is ATR spectroscopy.The short penetration depth of the evanescentwave is both the strength and a limitation of ATRspectroscopy. ATR crystals are prone to surfacefouling and layer build-up, which leads to inter-

color

fig

Figure 14. On-line NIR spectra of a polypropylene/ethylene – vinyl acetate copolymer during blend formation (courtesy ofIPF Dresden)

color

fig

Figure 15. ATR immersion probe adapted to a corotationtwin-screw extruder (courtesy of Carl Hanser Verlag)a) ATR immersion probe; b) ZnSe crystal; c) Intermediateplate; d) Melt in the intermeshing zone; e) screw


ference with the process signals. Additional is-sues to be considered with respect to on-lineapplications of ATR are the scratch resistanceof the crystal; the relatively low light throughput,which requires sensitive detectors; and the lin-earity (or nonlinearity) of calibration.

A major obstacle to industrial IR on-lineapplications is the lack of appropriate fiberstransparent to IR radiation. The chalcogenidefibers currently used are very expensive and haveonly limited light throughput and hence do notenable remote monitoring. Infrared spectro-meters should be installed close to the processto allow the use of automated sampling devices,short fibers, or optical systems that transmit thelight via mirrors and tubes to the analyzer. Thelatter option is shown in Figure 5, in which a‘‘mirror-arm’’ is interfaced to an FTIR spectrom-eter. In Figure 17, a waterfall spectrum and thecorresponding line intensities as a function oftime are shown for the reaction of secondaryamines with paraformaldehyde. In this case, alinear calibration model makes it possible toexamine the reaction kinetics on-line on a quan-titative basis.

In aqueous systems, the strong water absorp-tion in the IR region may interfere with spectralfeatures of interest for reaction characterization.Nevertheless, on-linemonitoring of 2-ethylhexylacrylate/styrene emulsion copolymerizationwith

Figure 16. Formation of styrene – maleimide copolymer monitored with a diffuse reflectance dipper probe (courtesy of CarlHanser Verlag)FA ¼ C-18 fatty amine; PS ¼ polystyrene; SMA ¼ styrene – maleic anhydride copolymer; SMI ¼ styrene – maleimidecopolymer

Figure 17. Waterfall spectrum (a) and the correspondingline intensities (b) as a function of time for the reactionof secondary amines with paraformaldehyde (courtesy ofMettler-Toledo GmbH, Analytical)


an ZnSe ATR fiber optic FTIR device has beendemonstrated successfully [230].

Applications of FTIR spectroscopy to poly-merization reactions include kinetic studies oflaser-induced photopolymerization directly in aspectrometer [231], multiacrylate polymeriza-tion studies with a quartz crystal ATR device[232], homo-, co-, and terpolymerization reac-tion monitoring of acrylates and vinyl acetate[233], [234], and monitoring of carbocationicpolymerization with ATR probes [235], [236].A water-cooled mid-infrared chalcogenide fiber-optic probe with an ZnSe crystal was describedfor in situ monitoring of an acid-catalyzed ester-ification reaction in toluene at 110 C [237].Cross-linking reactions in various resins havebeen studied by IR spectroscopy [238–242], andFTIR imaging with focal-plane array detection[243]. The melt crystallization process of isotac-tic polystyrene was studied by means of in situFTIR spectroscopy [244].

On-line gas process stream analysis was car-ried out by piping the gas through a transmissiongas cell [245] or by means of photoacoustic gassensors [246]. Chemical processes in thin filmsand coatings have been investigated by photo-acoustic FTIR spectroscopy [247] and in situ IRellipsometry [248]. FTIR in situ spectroscopy hasbeen applied to monitor yeast fermentation pro-cesses [249], and the homogeneously catalyzedliquid-phase hydrolysis of sucrose [250]. It hasbeen shown that pharmaceutical batch processescan be monitored by FTIR real-time analysis[251]. On-line measured mid-IR spectra in com-bination with multivariate methods (orthogonalprojection approach, principal component anal-ysis, multivariate curve resolution) enable theoperator to detect the end point of a chemicalanalysis [252]. ATR – FTIR in conjunction withmultivariate quality-control methods has alsobeen explored to control the manufacturing pro-cess of PVC extra-corporal medical disposables[253].

5.3.4. Raman Spectroscopy

Fundamentals. Raman spectroscopy com-bines some of the advantages of (FT)IR and NIRspectroscopy without the limitations associatedwith these techniques. Nowadays, it is one of thefastest growing areas of analytical chemistry.

Raman spectroscopy produces well-resolvedspectra of fundamental vibrations comparable toIR spectroscopy. The interaction probabilities oflight with themolecule for the two techniques arequite different. Infrared absorption is favoredwhen a molecule has a permanent dipole whichis modulated by the vibration. Raman scatteringoccurs when themolecule is polarizable, with thepolarizability modulated by the vibration. Thusthe intensities of IR andRaman bands of the samesubstance are fundamentally different, and thetwo methods provide complementary informa-tion about the molecule. Some of the bands caneven be absent in one or the other spectrum.

The Raman line intensity is directly propor-tional to the number of corresponding oscillatorsin the scattering volume and the intensity of theilluminating radiation. Line intensity changesprimarily reflect concentration changes withinthe sample, thusmaking calibration of the spectrarather simple and straightforward. Raman spec-troscopy does not require extensive sample prep-aration, and Raman spectra can be taken fromalmost all samples without pretreatment.

The Raman effect is an extremely weak,inelastic scattering process. Typically, only10�6 to 10�9 of incident photons undergo Ramanscattering, and low-loss optical devices are re-quired for proper recording of spectra. In Ramanspectroscopy, there is always a competitivemechanism for light emission present, namely,fluorescence of the sample. In some cases, thequantum yield of fluorescence outweighs theRaman scattering efficiency by several orders ofmagnitude, and the fluorescence of the samplecompletely obscures the Raman spectrum. Themost convenient way to avoid sample fluores-cence is excitation at longer wavelengths. How-ever, as the excitation wavelength increases, thelight scattering intensity decreases according to al�4 power law.

Raman signals of some species aremuchmoresensitive than the corresponding IR absorptions.The ability to measure Raman spectra of sym-metrical diatomicmolecules (for instanceO2,N2,H2, F2, Cl2, Br2, I2 are not active in IR absorption)opens up opportunities in gas analysis. Ramanspectroscopy is sensitive to nonpolar molecularvibrations. Hence, double or triple bonds inmonomeric or polymeric molecules are strongRaman scatterers. Vinyl or diene monomers caneasily be identified, and their concentrations


determined by means of their double bonds.Therefore, Raman spectroscopy is an ideal toolfor monitoring the disappearance of monomersduring the course of polymerization, and fordetecting residual monomers in final products.Metal carbonyls in catalysts and cyanides inplating baths give rise to strong Raman signalsdue to their triple bonds. Raman spectroscopycan be used to monitor the quality of diamond ordiamond-like carbon and to distinguish betweenisomers [254].

Water, with a strong molecular dipole, has anintense IR absorption, but a very weak Ramanresponse. Thuswater obscures IR spectra, where-as Raman scattering is more or less oblivious tothe presence of water. Hence, Raman spectros-copy can easily be applied to aqueous solutions,emulsions, latices, and suspensions. Dispersesystems scatter light because of refractive indexdifferences between particles and the surround-ing continuous phase (e.g., elastic Rayleigh orMie scattering of latices, emulsions, aerosols).Multiple elastic light scattering reduces severelythe scattering volume from which the light forRaman spectroscopy is collected. Hence, confo-cal illumination and collection optics are essen-tial for efficient sample excitation and signaldetection. Light scattering affects spectral inten-sities but not band shapes. With confocal optics,interference of both the window material andfilms built up on window surfaces with thespectrum is much reduced, and spectra can beobtained from samples through colored or opa-que sample container walls.

Instruments. Dispersive scanning Ramanspectrometers operating in the UV or VIS regionare not very suitable for process monitoringbecause of the time required to record a spectrum.Additionally, the short wavelength excitationmay cause severe sample fluorescence. Themainadvantage of dispersive scanning instruments isthe very high resolution achievable by usingdouble or triple monochromators.

As in IR spectroscopy, FT Raman spectro-meters are suitable for monitoring even very fastchemical processes on the timescale of seconds.For practical and cost reasons, most instrumentmanufacturers use the same type of interferom-eters in their IR and FT Raman spectrometers.Hence, excitation in FT Raman spectroscopy istypically at 1064 nm with Nd:YAG lasers.

Therefore, sample fluorescence is very muchreduced, but FT Raman spectra of aqueous sam-ples are modified by self-absorption of water intheNIR (i.e., forwavenumbers above 2000 cm�1

with excitation at 1064 nm). Pros and cons of FTRaman spectroscopy are reviewed in [255–257].

The application of holographic transmissiongratings and semiconductor array detectors(CCD or CID) has led to the rediscovery ofdispersive Raman spectrometers. The major ad-vantage of these instruments is they do notcontain any moving parts and are thus ruggedand compact for on-line applications in chemicalplants. However, the insensitivity of a CCD atwavelengths above 1050 nm restricts the wave-length of the incident light. For a CCD-basedinstrument, excitation of the sample has to bedone below 800 nm to obtain the entire spectrumup to 3000 cm�1. Most instruments for reactionmonitoring are equippedwith recently developedinexpensive and compact NIR solid-state diodelasers operating between 780 and 850 nm,whereby 785 nm or 840 nm are commonly used.A review on diode lasers for Raman spectroscopyis given in [258].

In this wavelength range, low-cost silica opti-cal fibers deliver the laser light to the measure-ment site and return the signal efficiently forremote monitoring. Holographic filters integrat-ed into the probe head remove the Raman signalgenerated within the excitation optical fiber andprevent reinjection of elastically scattered laserlight into the signal return fiber. Raman spec-trometrywith fiber-optic sampling is described in[259–262].

For trends in instrumentation and applica-tions, see [263–267].

Applications. Prior to the 1990s, only a fewpublications deal with Raman reaction monitor-ing. The validity of the method has been demon-strated for the suspension polymerization of sty-rene [268] and vinyl chloride [269], the thermalpolymerization of styrene [270–272] and methylmethacrylate [272], the solution polymerizationof methyl methacrylate [273], the g-initiatedpolymerization of diacetylene [274], and themicroemulsion polymerization of styrene andmethyl methacrylate [275].

The decreasing intensity of the n(C¼C) Ra-man lines of the monomer during the course ofthe reaction was monitored as a measure of the


extent of conversion to polymer. The positions ofthe Raman double bond stretching vibration ofseveral monomers are listed in Table 5. Ramanspectra recorded during emulsion copolymeriza-tion of styrene and butyl acrylate are shown inFigure 18. The n(C¼C) double bond peaks ofthe two monomers cannot be resolved since thespectral resolution of the instrument was on thesame order of magnitude as the difference be-tween their peak maxima (ca. 7 cm�1). Thedouble bond peak disappears almost completely

during the reaction. Based on the spectralchanges as a result of polymerization, the con-version of the two monomers as a function oftime can be calculated with an accuracy of about� 1%. The combination of Raman on-line mon-itoring and automated spectra evaluation enableschemical reactions to be controlled efficiently.

Between 1990 and 2005 a wealth of publica-tions appeared on in situRaman investigations, forinstance, of polymerization reactions [276–285],epoxy resin cure [286–288], crystalline-statephotoreactions [289], fast high-pressure de-composition reactions [290], hydrolysis ofacetic anhydride to acetic acid in a hydrother-mal/supercritical water reactor [291], liquid-phase chemistry of aliphatic organic peroxides[292], crystallization of polymer melts [293],a PCl3 production process [294], petroleumdistillate quality in pipelines [295], synthesisand hydrolysis of ethyl acetate [296], the man-ufacture of titanium dioxide [297], the biotrans-formation by yeast of glucose to ethanol [298],homogeneous catalysis (Heck reaction) using asolvent band as internal standard to improvequantification [299], and chlorosilane streamcomposition [300]. Chemical reactions haveeven been monitored in aerosol particles bymeans of Raman spectroscopy [301–304].There is evidence that Raman spectra alsocontain information on the size of particlesundergoing a chemical reaction [305], [306].

Table 5. Double bond Raman lines

Monomer n~(C¼C), cm�1

Vinyl chloride 1607

Acrylonitrile 1610

Vinyl sulfonate 1619

N-Methylacrylamide 1629

Styrene 1631

Methyl acrylate 1635

2-Ethylhexyl acrylate 1637

Butyl acrylate 1638

Diisopropyl fumarate 1638

Ethyl acrylate 1638

Butadiene 1639

Glycidyl methacrylate 1640

Methyl methacrylate 1641

Allyl methacrylate 1641/1648

Veo Va 9/10a 1646

Vinyl acetate 1648

Vinyl propionate 1648

Crotonic acid 1658

*Vinyl esters of versatic acids.

Figure 18. Raman spectra recorded during emulsion copolymerization of styrene and butyl acrylate. The spectrawere recordedwith a fiber-based instrument equippedwith a CCD detector. Laser excitation at 785 nmwith an output power of about 90 mWwas performed through a glass window in the reactor wall. The accumulation time for each spectrum was about 15 s. Spectrawere recorded at regular intervals of 1 min. Hence, the instrument meets the requirements for an on-line process monitoringsystem.


Raman spectrometers have been attached tomicroreactors for reaction monitoring [307].Multiplexing in a chemical production environ-ment can easily be achieved by Raman systemsequipped with fiber optics [308]. The develop-ment of low-cost low-resolution spectrometersmay open new on-line application opportunitiesin the future [309].

Micro-Raman spectroscopy has been success-fully installed for process and growth monitoringand control of GaN, AlGaN, and AlN [310], andfor on-line defect review in IC manufacturingprocesses [311]. Raman compositional mappingof paper coatings enables one to detect the spatialdistribution of pigments, binders, and fillers[312]. Large-spot noncontacting Raman probeshave been developed to on-line inspect pharma-ceutical formulations and tablets [313].

The study of Raman spectra of adsorbedmolecules on surfaces is one of the most promis-ing areas in Raman spectroscopy. Moleculesadsorbed on metal surfaces show enhancementof the scattering efficiency by up to seven ordersof magnitude [314], [315]. This surface-en-hanced Raman scattering (SERS) is ultrasensi-tive for detecting numerous adsorbed compoundsby means of their vibrational spectra. Originally,SERS experiments were carried out with eitherbare metal surfaces or metal colloids mixed withsolutions of the sample. Covering the substrateswith extremely thin modifying layers enables theconstruction of chemical sensors for remotemonitoring [316–319].

A nonlinear Raman technique, stimulatedRaman scattering (SRS), has been used to mon-itor the bulk polymerization of styrene andmethyl methacrylate [320]. In contrast to spon-taneous Raman scattering, most compoundsshow only a few Stokes lines in their SRSspectrum. As a result, spectral interference ofSRS-active compounds in a reaction mixture isminimized.

5.3.5. Fluorescence

Fluorescence spectral signals are quite intensebecause of the efficient electronic excitation andfluorescent emission process. This enables mea-surements down to the nanosecond timescale onsmall samples with remarkable accuracy. Thesensitivity is high over a wide concentration

range. The spectral features are rather broadcompared to those of other spectroscopic meth-ods. Fluorescent techniques provide informationon molecular arrangement and dynamics of mi-cro-heterogeneous systems, and on micropolar-ity or microviscosity [321], [322].

Steady-state fluorescence spectroscopy andfluorescence lifetime measurements have beenused for on-line reaction monitoring. Sources offluorescence can be the reaction componentsthemselves or a fluorescent probe that is addedto enhance the selectivity of the measurement.Reaction monitoring with fiber-optic fluores-cence sensors coated with fluorescent probe mo-lecules is an emerging technology. In contrast,fluorescence signals in Raman spectroscopy are asource of disturbance because they originatefrom largely unknown impurities and mask themore specific Raman signal. The fluorescencetechnique is potentially much cheaper than Ra-man spectroscopy.

Fluorescence methods have been demonstrat-ed to be useful in academic research for in situmonitoring of photoinitiated processes [323],curing [324–327] and polymerization[328–334]. Polymer colloids have been charac-terized by fluorescence quenching techniques inorder to draw conclusions about the internalparticle structure, transport phenomena, andparticle flocculation of latices [335], [336]. Atraditional field for the application of variousfluorescence techniques are micellization behav-ior, molecular interactions, aggregation andclouding phenomena in surfactant solutions[337–340]. Optical fluorescence microscopy iscapable of in situ chemical analysis of phase-separating polymer blends [341]. Various as-pects of latex coalescence and film formationwere studied by fluorescence techniques[342–345]. Various monitoring applications offluorescence-based sensors and fluorescentprobes are described in [346–351].

Fluorescence is not commonly used for on-line monitoring of chemical processes outsideacademia. Nevertheless, fuel in the cylinderwall oil film of a combustion engine has beendetected in situ with a fiber-optic laser-inducedfluorescence system [352]. Laboratory analyti-cal work published in the past indicates thatfluorescence methods may become especiallyuseful to monitor biotechnological processes inthe future.


5.3.6. Other Spectroscopic Techniques

In addition to the classical optical spectroscopictechniques discussed above laser-based spectro-scopic methods are gaining increasing impor-tance for process monitoring, gas analysis andtrace element detection. Laser remote sensingtechniques have been reviewed in [353].

In a laser gas sensor, the frequency of atunable laser (diode) or a quantum cascade laseris adjusted to the absorption frequency of aspecified gas component thus enabling the de-tection ofmolecules from the percent down to theppm range in a rather complex matrix [354].

A high-energy laser pulse generates a plasma(a dielectric breakdown) on the material of inter-est in laser-induced breakdown spectroscopy,LIBS (or laser-induced plasma spectroscopy,LIPS). The resulting light emission can be ana-lyzed with a spectrograph [355] to detect gaseouscomponents in air [356], to determine the con-centration and size of particles in water [357], toanalyze steel components in the melt at elevatedtemperatures [358], and to identify plastics in arecycling stream [359].

In cavity ring-down spectroscopy (CRDS) alaser pulse is trapped in an optical cavity betweentwo reflecting mirrors. The light intensity withinthe cavity decays as a function of time [360].Absorbing species inside the cavity increase thetotal cavity losses thus enabling the detection oftrace molecules in gases.

With the development of multivariate techni-ques, 3D spectral image analysis became veryimportant for systems where spatial distributionof components is key to process monitoring,understanding, and control [361]. Applicationsinclude optical spectroscopic imaging from theUV to the IR wavelength range in absorption,fluorescence and Raman spectroscopy as well asmagnetic resonance, X-ray fluorescence or XPSimaging.

6. Particle Size Analysis

An important characteristic of disperse systems(latices, emulsions, aerosols, suspensions, pow-ders) is their particle size distribution. Particlesizes affect almost all properties of a dispersesystem (electrical, optical, rheological, etc.), aswell as its stability. The evolution of the particle

size distribution is a sensitive indicator of theprogress of heterophase reactions, and it deter-mines the application properties of final reactionproducts.

In this chapter, the main focus is on processparticle size measurements in suspensions. Forreviews of particle measurements, including in-process particle size distribution measurementsand classification of microsized particles, see! Air, 5. Emission Measurements, Chap. 7. and! Characterization of a Classification or Sepa-ration Process.

6.1. Scattering Techniques

The scattering of electromagnetic waves (light,X-rays) and neutrons by particulate matter is apowerful tool for studying particle sizes andshapes, internal particle morphologies, particledynamics (even under shear), the structure anddynamics of concentrated disperse systems, andparticle charges. Wave interference yields infor-mation on particle sizes and shapes whenever thewavelength is of a comparable order of magni-tude to the size of the scatterer.

Neutron and X-ray scattering both rely on theavailability of corresponding neutron and radia-tion sources. Hence, their use for on-line sizemonitoring is extremely limited.

Particle size determination by light scatteringtechniques is a well established broad field ofexperimental methods. In addition to inelasti-cally scattered light (Raman and Brillouin scat-tering), there are two basic approaches that ex-ploit light scattering data for on-line particle sizeanalysis: Static light scattering (SLS) and dy-namic light scattering (DLS). SLS methods in-volve measurement of the time-averaged angularscattering aswell as turbiditymeasurements (i.e.,elastically scattered light). DLS measures thevery small frequency shift of scattered lightcaused by translation and rotation of scatterers,or, in the FT time domain, fluctuations of the lightintensity by correlation techniques. Thus, quasi-elastic light scattering (QELS), Rayleigh line-width/spectroscopy, intensity correlation spec-troscopy, and photon correlation spectroscopy(PCS) are synonymous with DLS.

Most of the light scattering techniques requireextensive dilution of the extracted sample toavoid multiple-scattering effects. Automatic


dilution devices are interfaced to the light scat-tering instrument for on-line applications. Directoptical access to the (undiluted) reaction mixturebecame possible with the development of fiber-optic probes and instruments.

For general reading see [362–364].

6.1.1. Turbidimetry

The turbidimetric experiment is rapid, precise,reproducible, and absolute (i.e., no calibration isrequired). Spectrophotometers are readily avail-able. Turbidity gives ameasure of the attenuationof light traversing a suspension or aerosol ofnonabsorbing particles. Turbidimetry may re-quire dilution of the mixture to avoid multiplescattering effects. The wavelength-dependentturbidity t of a (diluted) sample provides infor-mation on particle size and concentration. Tur-bidimetric (and nephelometric) measurements atsingle wavelengths can be used to determine theconcentration of particulate matter in liquid andgaseous process streams. Fraunhofer diffractionis used for particles sizes � 1 mm.

Several publications describe the determina-tion of particle size during emulsion polymeriza-tion by specific turbidity t/j or turbidity ratiomeasurements [365–367]. For a polydispersesuspension, the turbidity is related to the polymervolume fraction j according to

t ¼ 3

2j

R¥0

d2p �K dplm ;

npnm

� �f ðdpÞddp

R¥0

d3p f ðdpÞddpð33Þ

where K, f, lm, and dp are the scattering coeffi-cient, the size distribution function, the wave-length of light in the medium, and the particlediameter, respectively. The quantities K and tare functions of both the refractive indices of theparticles np and of the medium nm, and of the sizeof particles relative to the wavelength of the lightdp/lm. K can be calculated from the general Mietheory. If the size distribution function f isknown, the particle size distribution can be esti-mated from specific turbidity measurements atseveral wavelengths. Despite the complicateddependence of t on K and its direct dependenceon the distributional form f (which is sometimesunknown), the specific turbidity can yield (1) theturbidity average particle diameter and the vol-

ume-surface average (i.e., D3,2) diameter forsmall and large particles, respectively, for anyvalue of m ¼ np/nm, (2) the weight averagediameter for m < 1.15 and particles that aresmaller than the wavelength of the light, and(3) an estimate of the weight average particlediameter in all other (monomodal) cases if a log-normal particle size distribution is assumed[366], [367].

Turbidimetry has been used to study the co-agulation kinetics of aqueous dispersions [368],[369]. Effortsweremade to extend the theoreticalbasis of turbidity to higher concentrations[370–372].

6.1.2. Angular Static Light Scattering

Angle-dependent static light scattering measure-ments with goniometers are not very suitable foron-line monitoring because of the time requiredand the demands discussed in! Plastics, Anal-ysis, Section 5.1.4.. A fast-response multichan-nel photometer capable of on-line monitoringeven at moderate concentrations is described in[373]. The scattered light is simultaneously mea-sured at 168 angles. This static light scatteringinstrument has a response time of 100 ms and anangular resolution of 1.

Static light scattering at a fixed angle is valu-able for studying the formation and aggregationof latex particles during emulsion polymeriza-tions [374].

Several instruments are on the market whichautomatically dilute aqueous dispersions to adesired (extremely low) concentration and per-form static multiple-angle light scattering. Dif-ferentmodes of light scattering (including Fraun-hofer diffraction) are typically combined withinone instrument to enable particle size analysisover a wide range of sizes.

6.1.3. Dynamic Light Scattering

Particles suspended in a continuous mediumundergo random Brownian motion. Hence, thephase of light waves scattered by Brownianparticles also fluctuates randomly in a time-re-solved light scattering experiment. Their mutualinterference leads to a net randomly fluctuatingscattered light intensity I(t) at the detector. The


normalized autocorrelation function g2(t0) of the

light intensity as a function of time delay t0 isgiven by

g2ðt0Þ ¼ hIðtÞ�Iðtþt0Þi

hIi2 ð34Þ

with hI(t) � I(t þ t0)i (t0 ! ¥) ¼ hIi2, thesquare of the average scattered light intensity.The field autocorrelation function g1(t

0) can becalculated if the Siegert relation holds

g2ðt0Þ ¼ AþB�jg1ðt0Þj2 ð35Þ

where A and B are instrument-related constants.For a monodisperse system of Brownian par-

ticles, their diffusion coefficientD can simply bederived from g1(t

0) according to Equation 36

g1ðt0Þ ¼ expð�q2Dt0Þ ¼ expð�Gt0Þ ð36Þ

where G is the decay constant. The scatteringvector q is defined as

q ¼ 4pnl

sinq2

� �ð37Þ

where n, l, and q are the refractive index, thewavelength of light, and the scattering angle,respectively. The Stokes – Einstein equation re-lates the diffusion coefficient D to the hydrody-namic radius of the particles rh

rh ¼ kbT

6phDð38Þ

where kb, T, and h are the Boltzmann constant,temperature, and viscosity of the dispersion me-dium, respectively.

For polydisperse systems, the autocorrelationfunction becomes a sum of exponentials with adistribution function G(G)

g1ðt0Þ ¼Z¥0

GðGÞexpð�Gt0ÞdG ð39Þ

Particle size characteristics and distributionfunctions can be derived frommeasured autocor-relation functions by (1) directly inverting theLaplace integral equation (Eq. 39, a mathemati-cally ill-conditioned problem), or (2) parameterfits, exponential sampling, regularization techni-ques, and histogram analysis.

Some advantages make dynamic light scatter-ing attractive for on-line applications:

1. DLS is an absolute method; no calibrationover time is necessary. With conventionalDLS systems, accuracies of � 1% for themean particle diameter can be achieved ona routine basis within a few minutes of mea-surement time. Care must be taken in reactingsystems, where either temperature or viscosi-ty change during the course of the reaction(see Eq. 38).

2. Particle properties such as density or refrac-tive index do not affect the time behavior ofthe intensity fluctuations. Thus, the derivedparticle sizes are independent of the chemicalcomposition of the particles.

3. In the single-scattering regime, diffusioncoefficients and particles sizes are indepen-dent of particle concentration. This is anadvantage over turbidity measurements, forwhich exact knowledge of the particle con-centration is crucial for size calculations.

4. The use of inexpensive fiber-optics allowsremote monitoring.

Two approaches are possible for on-line DLSmeasurements:

1. A device interfaced with the process line orreactor automatically captures a certainamount of themixture and dilutes it sufficient-ly to avoid multiple scattering and allownoise-free measurements [375].

2. A fiber-optic probe (optode) is directly im-mersed into the mixture and both illuminatesthe colloidal system and collects the scatteredlight (FOQELS: fiber optic quasi elastic lightscattering; FODLS: fiber optic dynamic lightscattering [376–378]. This backscattering ar-rangement shortens the light path within thedispersion and makes the method applicableto turbid and concentrated systems. Mono-mode fibers further suppress multiple scatter-ing effects [379].

Particle growth during emulsion polymeriza-tion has been studied by conventional DLS[380–384] and fiber-optic DLS [385], [386].

6.1.4. Other Optical Techniques

The effect of multiple scattering can be useful forstudying highly concentrated dispersions. The


path of multiply scattered light in the dispersionis similar to the path of particles or moleculesunder the influence of Brownian diffusion[387–390]. The concept of photon migration ordiffusing wave spectroscopy (DWS) has alreadybeen applied to particle size determination.

Colloidal refractometry is an optical methodthat measures the refractive index n of a disper-sion and analyzes it by using Mie theory. Thismethod allows the structure of concentrated dis-persions to be probed in the undiluted state. Themeasured values of n can provide a measure ofthe volume average particle size [391].

Frequency-domain photonmigration has beenemployed to on-line monitor the variation of theactive pharmaceutical ingredient concentrationas well as changes of particle sizes in powders[392]. Chord length distributions of particles inthe micron to millimeter range of emulsions andsuspensions can be on-line determined by meansof focal-beam reflectance measurement methods[393], [394]. Particle sizes ranging from 50 nmup to 3 mm are accessible by means of dynamicextinction spectroscopy [395]. Transmissionfluctuation spectrometry [396], [397] combinedwith the autocorrelation technique covers a par-ticle size range from 1 mm to 1 mm and a particleconcentration of up to 12 % of flowing particlesuspensions [398].

6.2. Separation Techniques

Recently, particle separation techniques (chro-matography, fractionation) have been developedthat are capable of on-line monitoring of particlesize. These methods require fully automatedremoval of samples from the reaction mixture,dilution, and injection into the instrument.

Size exclusion chromatography (SEC) is ap-plicable with porous (liquid exclusion chroma-tography LEC) and nonporous packing of thecolumn (hydrodynamic chromatography HDC).Particles are separated because of the depen-dence of the rate of particle flow through gapsbetween packing beads on particle size. Largerparticles are eluted first since they travel throughlarger gaps with higher flow rates. The mainadvantage of SEC methods is that particle sizedistributions can be obtained directlywithout anyassumption regarding the mathematical form ofthe distribution. Calibration of the instruments

with particle size standards is necessary butsimple. Disadvantages are the limited resolutionbecause of radial dispersion, and relatively longelution times of up to half an hour [399], [400].

Much higher resolution than with SEC meth-ods can be achieved by capillary hydrodynamicfractionation (CHDF) [401] and field-flow frac-tionation (FFF). The fluid in a CHDF capillaryhas a parabolic velocity profile with the greatestfluid velocity at the center of the tube and zerovelocity at the wall. Particles move radially dueto their Brownian motion. Larger particles areunable to approach the wall as closely as smallerparticles. Hence, larger particles travel throughthe tube faster than smaller particles. This sepa-ration effect is exclusively a function of particlesize; it is independent of particle density. Theefficiency of capillaries for separating particlesdepends on the eluent viscosity, the flow rate, andthe capillary diameter. The optimumparticle sizefor CHDF analysis is < 1 mm. The main disad-vantage of this method is the possibility of capil-lary clogging. The CHDF method has been usedto monitor the evolution of the particle sizedistribution during emulsion [402] and minie-mulsion polymerization [403].

Field-flow fractionation (FFF) is a family ofchromatography-like elution techniques basedon influencing the rate of particle flow througha narrow channel by applying an external fieldperpendicular to the flow direction [404–408].The external field separates particles of differentsizes by driving them into different localizedlaminas (< 10 mm thick) of the parabolic veloc-ity profile of the eluent within the fractionationchannel thus causing their separation. In contrastto CHDF, FFF does not necessarily require cali-bration of the instrument. Retention times forparticles of different sizes can be calculateddirectly from first principles.

FFF can be classified according to the type ofapplied field into sedimentation, thermal, elec-trical, cross-flow, and steric. The most suitableexternal fields for particle size analysis are cross-flow and sedimentation fields. The particle sepa-ration in cross-flow occurs according to the hy-drodynamic radius of the particles, whereas theireffective weight separates particles in a sedimen-tation field. The particle size ranges for cross-flow and sedimentation field FFF are 10 nm to100 mm and 50 nm to 100 mm, respectively.Recently, FFF (including thermal FFF) has been


used to characterize the size and composition ofcore/shell latices [409]. On-line coupling of FFFwith multi-angle laser light scattering is de-scribed in [410], [411].

7. Chromatography

7.1. Introduction

Chromatographic methods (especially gas chro-matography) are the most widely applied familyof techniques for determining the chemical com-position of reaction mixtures. For a discussion ofbasic principles, techniques, instrumentation,sample extraction and preparation, applications(including on-line applications), advantages andlimitations see ! Basic Principles of Chroma-tography,! Gas Chromatography, and! Liq-uid Chromatography, 2. Methods, Programmedand Coupling Techniques.

A comprehensive discussion of (gas) chroma-tography for on-line monitoring of chemicalreactions has been published [412]. Process ap-plications can also be found in [413–416].

The term ‘‘process chromatography’’ almostexclusively refers to the application of gas chro-matography (GC) to monitor chemical reactions

or process streams. Advantages of GC are itssimplicity, high sensitivity and selectivity, andease of automation. Liquid chromatography (LC,including HPLC), supercritical fluid chromatog-raphy (SFC), electrophoresis (EP), and ion chro-matography (IC) are less common in processmonitoring because of their higher maintenancerequirements, greater complexity, and highercosts for purchase and maintenance.

Because GC (like other chromatographicmethods) is an inherently discontinuous batch-like analytical technique which is applied tocontinuously monitor a chemical system in anonbenign environment, there are distinct differ-ences between laboratory and process instru-ments, although the basic components of a gaschromatograph can be found in instruments ofboth categories (Fig. 19) and the basic principlesof separation are the same.

In contrast to laboratory instruments, processanalyzers have to meet stringent safety andexplosion-proof requirements because of theirclose vicinity to the product line. The typicalGC process instrument is designed to detect andmonitor only one or a few (previously known)components of the reaction mixture so as tocontrol a chemical reaction or process stream.This requires special injection valves and

Figure 19. Schematic of a process GC analyzer GS: gas supply for carrier, auxiliary, and purge gases; V: valves forpreseparation, column switching, and injection; VP/LP: vapor-phase/liquid-phase sampling; SHC: sample handling andconditioning system; R: reactor; S: separation unit; C: column(s); D: detector; O: thermostatically controlled oven; PC/C/NW:personal computer/controller/network; W: waste gases


column-switching devices. As in conventionalGC, the amount of sample required for processGC is extremely small. Hence, the samplingsystem must ensure that the sample is represen-tative of the process to be monitored. Often, asample conditioning system is necessary to fur-ther process the sample, which may still bereactive, for GC analysis.

The nonspecific character of detection basedexclusively on retention time (i.e., no positiveidentification) makes GC preferably suitable forprocess monitoring of continuous processes withsmall variations in composition. Different mole-cules may have the same retention time, confus-ing quantification of the chromatograms andleading to misinterpretation of overlappingpeaks. Additionally, GC analysis requires timefor separation and detection. This time intervalmust be reasonably short compared to both therun time of the reaction and the time required tofeed the analytical result into a process control-ler, and to influence the reaction.

7.2. GC Hardware Components

Carrier Gas. In principle, the same carrierand auxiliary gases can be used in both laboratoryand process GC analyzers (usually hydrogen,helium, argon, synthetic air, or nitrogen, depend-ing on the application). Most high-purity gasesare supplied in containers (e.g., high-pressure gascylinders). For an on-line operation of the GCanalyzer, the gas storage and delivery systemsshould be equipped with valves that switch au-tomatically between empty and full containers.

Sampling. The quality of a GC analysis of areacting mixture is only as good as the quality ofthe samples withdrawn from it. Because of thevery small amounts of sample necessary for GC,it is crucial to withdraw samples that are repre-sentative of the mixture. The sampling point(sample probe) should be located at a point inthe bulk of the medium where it is homogeneousand well mixed so as to obtain the ‘‘true’’ bulkcomposition of the mixture. Close to the walls,adsorption and deposition phenomena and stag-nant flow may locally change the overallcomposition.

From an analytical point of view, the bestlocation for the analyzer would be right next to

the process line or sampling loop. However, anumber of factors and restrictions have to betaken into consideration for placing the chro-matograph relative to the sampling point, e.g.,1) space available for installation, 2) mainte-nance requirements, 3) safety requirements, 4)accessability of sampling point and chromato-graph, 5) connection to utilities and supplies(carrier gas, electricity, compressed air), 6) opti-mization of instrument lag time versus controlcycle time, 7) minimization of sample transport,8) reactivity/stability of the analyte. In mostapplications, a sample transfer line bridges thedistance between sampling point and analyzer.Length and diameter of the transfer line must beoptimized for (high) analyte flow. Pressure dropand temperature changes along the line mustbe controlled. When the analysis is complete,the sample is returned to the process by a separateline or disposed of along with the carrier/purgegas.

In many processes, GC samples are taken atelevated temperature and/or pressure. Thesesamples may contain unconverted components,other reactive ingredients, water vapor, liquids,or solid impurities. Hence, process samples mustbe appropriately conditioned before feedingthem into the GC analyzer. The sample condi-tioning system performs depressurizing andtemperature reduction as well as filtration andremoval of liquids (e.g., by vaporization). Itsdifferent elements may be close to the samplepoint (pressure reducer, filter) or close to theanalyzer (temperature control, flow control). Theconditioning system ensures that the temperatureof gaseous samples is held above the dew point toavoid condensation and that the temperature ofliquid samples is well below the boiling point toavoid bubble formation in the line.

Sample transfer and conditioning are the mosttime-consuming part of an on-line GC analysis.The lag time between process and analysis is onthe order of a fewminutes and must be taken intoaccount in designing reaction control systemsbased on conventional GC. With microsensorGC systems, timescales on the order of (milli)seconds for on-line analysis are possible.

Analyzer Oven. In contrast to most labora-tory instruments, theactual process chromatograph(i.e., injection system and valves, column(s),detector(s), controls) is placed in a single


temperature-controlled oven. Temperature controlof better than� 0.05 K suppresses fluctuations intemperature-sensitive processes such as gas flow,mobile-phase diffusion, adsorption/desorption,and chemical interactions. Since detection is solelybased on retention time, all processes that caninfluence retention times must proceed unalteredto avoid loss of calibration under automated oper-ating conditions. Constant temperature is usuallyguaranteed by circulating heated air within theprocess analyzer oven or by using high-massmetalheat reservoirs. In some applications, tighter tem-perature control can be circumvented by an inde-pendent delayed injection of a standard or byadding an internal standard to the carrier gas.

Pressure control and programming is domi-nant in process chromatographs, although sometemperature programmable analyzers and gradi-ent systems have been recently described in theliterature.

Valves and Columns. In contrast to labora-tory instruments, a process chromatograph isusually designed to monitor only one or just afew components of the reacting system forcontrol purposes. Exceptions include some pet-rochemical applications, in which up to severalhundreds of components are monitored simul-taneously. Additionally, the process GC analyz-er must operate unattended in a rather harshenvironment over extended periods of time, butstill provide reproducible and stable data. Ofspecial importance is a constant sample volume/mass for injection (and volume of standard, ifnecessary). Therefore, dedicated valve systemsfor sample injection and column switching havebeen developed [412] (see ! Liquid Chroma-tography, 1. Fundamentals, History, Instrumen-tation, Materials, ! Liquid Chromatography,2. Methods, Programmed and Coupling Tech-niques). For gaseous samples, the sample size isnormally defined by the volume of an externalsampling loop connected to different valveports. The injection volume of liquid samplescan be defined by the size of internal grooveswithin the valve or by external sampling loops.

Sliding, rotary, and diaphragm valves witha wide variety of internal pathways and portconfigurations are commonly used in processchromatography of gaseous samples. For liquidsamples, piston (syringe) valves may also beapplied. Valves are heated externally or internal-

ly. Depending on the component(s) of interestwithin the complex reaction mixture, cutting(mainly heart-cut technique) and other column-switching techniques (e.g., backflushing) or mul-ticolumn analysis are often combined with pre-column separation. The main reason for usingcolumn-switching techniques in on-line processGC is to drastically reduce the total analysis timeand to fully separate the components of interestfrom the mixture. In general, the same types ofvalves are used for injection and columnswitching.

Traditionally, the use of packed columns pre-dominates in process GC. As in laboratory GC,packed columns are being replaced by capillariesin an increasing number of on-line applications.With capillary columns, chromatographic sepa-ration is improved, resolution is higher, andanalysis time is shorter thanwith packed columnsbecause of the much simpler flow and diffusionpattern of the analyte. However, capillaries arenormally less sensitive, more expensive, and notso easy to use as packed columns. A majordrawback of capillary columns in process appli-cations is their sensitivity to flow, pressure, andespecially temperature fluctuations.

Columns, their supports, coatings, and pack-ing materials have to withstand possible corro-sive action of ingredients of the reaction gasmixture and should be inert to reactions withsolute molecules (e.g., by silylation of the col-umn packing material or capillary coatings). Thepresence of water vapor and oxygen traces inprocess gases may also be of relevance to columnselection.

Detectors in process chromatographs haveto complywith the overall safety, robustness, andreliability standards of an on-line instrument.The overwhelming majority of GC detectorsutilized in process analyzers are of the thermalconductivity (TCD) and flame ionization (FID)types because of their ease of use and sensitivityto the process components of interest. TCDs aresensitive to changes in the bulk thermal conduc-tivity of the carrier gas in the presence of theanalyte, which gives a concentration-dependentsignal, whereas FIDs sense the number of certainionized molecules entering the flame with aspecific flow rate. In some rare cases, photoioni-zation (PID) and flame photometric (FPD) de-tectors are applied.


Several methods have been utilized in processGC to convert detector output signals (peakareas) into desired analyte concentration values,such as the separate injection of calibration stan-dards, the use of internal standards, and calcula-tions based on relative response for the differentdetector types.

The basic operations of a GC can also beminiaturized and micromachined on a siliconwafer. Micro-GC devices are compact, morerobust, and faster than classical GCs what makesthem very attractive for process monitoring ap-plications [417].

7.3. Applications of GasChromatography

GC has a long tradition of use for controllingprocesses and process streams in industry. On-line GC is one of the most important techniquesformonitoring distillation, rectification, and con-version processes in refineries and chemical andpharmaceutical plants. It is extensively applied incatalysis research, development, and productionto evaluate catalysts and study reaction mechan-isms and kinetics. Another field of use is inmetallurgical smelters for gas and sulfur analysisin metals. It is widely utilized as a part ofworkplace safety systems, e.g., to ensure airquality in production units, pilot plants, andlaboratories.

On-line GC has proved useful for determiningorganic substances in water [418], [419], waste-water analysis [420], and air monitoring[421–423]. Reports have been published on theon-line analysis of petroleum [424], [425] andgasoline [426–428], the determination of octanenumbers with n-alkanes as reference [429], theon-linemonitoring ofmethyl formate conversionto methanol and carbon monoxide [430],Fischer – Tropsch synthesis [431] and thermo-lysis/decomposition of organic compounds[432], [433]. The application of gas chromatog-raphy (among other techniques) for on-line mon-itoring of polymerization reactions is reviewed in[434]. Gas chromatography of the reaction mix-ture [435], [436] and the head space of the reactor[437], [438] can be applied for on-line determi-nation of residual monomer composition inemulsion polymerization.

Head-space GC in liquid reaction mixtures isnot straightforward for direct reaction monitor-ing because it requires knowledge of the parti-tioning of each component between the gas phaseabove the reaction mixture and the liquid mix-ture, and, especially in multiphase reactions,between the different phases of the reactionmixture (e.g., aqueous phase, monomer droplets,and polymer particles in an emulsion polymeri-zation). Additionally, the head space should bein thermodynamical equilibrium with the fluidreaction mixture.

The utilization of (mass) sensors for gas andliquid analysis has been reviewed in [439–441],In-line applications of chemical sensors to moni-tor polymerization reactions has not been de-scribed in the literature so far. Such sensors shouldhave a very high selectivity so as to detect com-ponentswellbelow thepercent range inamatrixofsolventor solventvapor (includingwater)contain-ing a large number of ingredients, some of themwith a chemical structure comparable to that of theanalyte. Additionally, the sensors would have todeal with precipitates on their surfaces.

7.4. Other ChromatographicTechniques

Applications of liquid chromatographic systemsto monitoring chemical and biochemical pro-cesses are described in [442–448]. SAW sensorsare useful gas sensing tools for on-line monitor-ing of the mobile phase of process HPLC withchanging mobile phase composition or gradients[449].On-line ion chromatography has been usedto monitor adipic acid production [450], tracemetal ions [451] and cations [452], and oxides influe gases [453]. Reviews on process ion chro-matography can be found in [454] and [455]. Insome rare cases, capillary electrophoresis [456],[457] and supercritical fluid chromatography[458], [459] have been described as on-linereaction monitoring techniques.

8. Electroanalytical Methods

8.1. Introduction

Electroanalytical methods are based on the inter-action of electrical fieldswith chemical processes


or on chemically induced electrical signals onelectrodes. The major advantage of electroana-lytical methods is the electrical output of themeasurement (i.e., electrical current, potential,resistance, or charge), which can directly beprocessed electronically without any furthertransformation, in contrast to other monitoringtechniques. Although the main application forelectroanalytical techniques is process monitor-ing in aqueous solutions, they can also be appliedto nonaqueous and nonliquid systems.

In most electroanalytical measurement sys-tems, the sensing electrodes are in direct contactwith the reaction medium and all of its compo-nents, not only with the species of interest. Thiscan lead to cross-sensitivity and poisoning, and toabrasion, deactivation, and contamination of theactive electrode surfaces. Additionally, the ana-lytical information of interest must be extractedfrom the integral signal. Calibration of the sens-ing system is typically necessary at the beginningof a measurement cycle and subsequently inregular intervals. Accumulation of static elec-tricity within the sensing system and on its bodymust be avoided.

Compact titration configurations for on-lineprocess analytics are commercially available notonly for electroanalytical applications such asacid – base titrations but also for precipitationand complexometric titrations [460].

8.2. Conductometry

Conductometric measurements rely on the exis-tence of charges in the medium that are mobile inan applied electrical field (e.g., ions in electrolytesolutions or melts, electrons in metals). In con-ductometry, no reaction takes place at the elec-trodes. Electrical conductance of the medium orelectrical capacitance of a measurement cell isthe measured quantity. Conductometry in solu-tions and melts is a nonspecific method becauseall anions and cations contribute to the medium’sconductivity. However, the method is very ro-bust, precise, sensitive, fast, and does not requireextensive maintenance.

Concentration ci and mi of all the ions i presentin a medium determine its conductivity L (andconductance G)

L ¼ kG ¼ kFXi

cinimi ð40Þ

where ni is the ion valency, F is Faraday’sconstant, and k a cell constant. This linear rela-tionship between concentration and conductivityis the basis for concentration measurements inthe lower concentration range (� 0.1 W�1cm�1). For higher electrolyte concentrations,Equation 40 becomes nonlinear because of ionicinteractions. Additionally, polarization effectsmay occur at the electrodes. The temperaturedependence of the ionic mobility m leads to analmost (quasi) linear dependence of L on tem-perature in the temperature range common forconductivity measurements in electrolyte solu-tions (0 – 100 C).

Two different principles of conductivity mea-surements are in industrial use: 1) two- or four-electrode conductive cells with direct electrodecontact to the medium, and 2) inductive, electro-deless, and contactless devices.

Contact electrode cells are applied innonaggressive media. The cell itself is typicallypart of a Wheatstone bridge operating withlow-frequency alternating current to avoidelectrode polarization. Two-electrode cells arein use in the conductance range 10�8 to 10�1

W�1 cm�1, whereas four-electrode cells aresuitable for the higher conductance range of10�3 to 10 W�1 cm�1.

Electrodeless inductive measurements are themethod of choice for highly conductive systems;for aggressive, abrasive, or corrosive media; andfor high-temperature applications. The mediumto be measured is circulated through a loop; twotransformer coils are attached to the inert wallmaterial of the loop pipe (Fig. 20). A generatorsupplies alternating voltage to the primary coil.The induced alternating voltage at the secondarycoil depends on the conductivity of the solutioncirculating through the loop. For contactless mea-surements, themedium’s conductance is typically

Figure 20. Schematic of an electrodeless conductivity mea-surement device G: generator; D: detector; P: primary coil;S: secondary coil; �: alternating current


much higher than 10�1 W�1 cm�1. Inductive de-vices are less sensitive than electrode cells.

The electrical conductivity Ld of a dispersesystem is a function of both the volume fractionof the disperse phasej (j � 1) and the electricaldouble layer properties of the particles (surfaceconductivity Ls, Zeta potential z)

Ld ¼ 1

fL0þLS ð41Þ

where L0 is the conductivity of the dispersionmedium and f a form factor accounting for theshape and the volume of the (nonconducting)particles [461]. Thus, conductivity measure-ments provide information on particle formation,disappearance of particles, phase inversions, andparticle swelling in heterophase reactions such ascrystallization and emulsion and suspension po-lymerization [462–466].

Electrical resistance tomography is a rathernew technique for on-line monitoring of chemi-cal reactions of particulate systems in tank re-actors. The voltage between individual electro-des of a system of electrodes distributed over oneor several layers in the reactor wall is used tocalculate an image of the electrical resistancedistributionwithin the reactor which is indicativeof changes of the dispersion state of multiphasesystems [467].

Most applications of conductometry deal withmonitoring of process streams rather than directreaction monitoring. Typical application fieldsare drinking water purification and tap watermonitoring, wastewater treatment, cleanness ofultrapure water in the electronics industry, dis-tillation processes, neutralization and precipita-tion reactions, concentration monitoring of acids(e.g., in production of concentrated sulfuric ac-id), lyes, and salt solutions, and monitoring ofprocess streams in dairies and breweries.

Electrostatic sensors can be used to on-linemeasure the mass median size of particles in adilute-phase pneumatic suspension. The sensorconfiguration includes a wire-mesh electrode,signal conditioning electronics, and digital signalprocessing algorithms [468].

8.3. Potentiometry

Potentiometric techniques are characterized bythe fact that electrode reactions at equilibrium are

involved in the potential measurement. In poten-tiometry, the actual electrical current through theelectrode is zero. The electrode potential cannotbemeasured directly but only as a differencewithrespect to the potential of another electrode(reference electrode). Electrode potentials andthe potential difference between two electrodescan be calculated according to Nernst’s equation(Eq. 42)

Uelectrode � RT

Fln

aiai;ref

� �ð42Þ

where R is the gas and F is Faraday’s constant.The ion activities ai can be activities of twodifferent ions within the same solution (redoxpair), activities measured with two differentelectrodes, or activities at two different concen-trations or partial pressures.

Ion-Selective Electrodes. The swellingglass layer of ion-selective electrodes is designedto be sensitive to a certain type of ions andinsensitive to others. This can be achieved byusing membranes containing inorganic salts ofthe ions of interest (e.g., Naþ, Kþ, Cu2þ, NH4þ,Cl�, F�, I�). Conditioning of the mixture to acertain conductivity and pH level is a prerequisitefor ensuring precise concentrationmeasurementswith ion-selective electrodes. Hence, the use ofthis type of electrodes for direct (in-line) inser-tion into a reaction mixture is limited [469],[470].

pHMeasurement is themost important andmost widely used potentiometric technique inaqueous solutions or water-containing systems.Measurements of pH are crucial for monitoringprocesses in the chemical and pharmaceuticalindustries, the food and beverages industry,waste-treatment plants, environmental protec-tion, and biochemical operations [471]. In par-ticulate systems, the pH has a dramatic influenceon colloidal stability (e.g., cosmetic products,emulsion and suspension polymerization). Caremust be taken to take into account temperaturevariations of the reacting system because of thestrong temperature dependence of electrodepotentials.

pH measurements are easily performedwith standard glass electrodes. For processapplications, the most common pH-sensitive


membranes consist of lithium glasses. Otheralkali metal glasses are prone to increased hy-drolysis and instability, especially in alkalinesolution. Several types of glasses are availablefor different process environments (standard,highly alkaline, highly acidic, high temperature).Problems may arise with glass electrodes due tocontamination of the surface by precipitation orfilm formation. Cleaning and maintenance ofglass electrodes on a regular basis are criticalfor successful on-line pH measurements.

Glass electrodes with counterpressure are ap-plicable for on-line monitoring of reactions up totemperatures of about 80 C and pressures up to60 bar. Mechanical instability is a major draw-back of glass pH electrodes in high-pressureapplications.

Solid-state ion-sensitive field effect transistor(ISFET) sensors show faster response, loweralkaline errors, and improved mechanical stabil-ity than glass electrodes. However, they alsosuffer from surface contamination. The applica-bility of ISFET pH sensors is limited to pressureswell below 50 bar and temperatures up to 85 C[472]. There is no solid-state sensor on the mar-ket applicable for pressures above 60 bar. An as-yet not commercially available ZrO2 sensor forpH measurements under high pressure (83 bar)and high temperature (285 C) is described in[473]. A number of different optical and fiber-optical pH sensors have been developed[474–477].

Redox Potential. The oxidizing or reducingcharacter of a solution can be measured with(inert) precious metal electrodes (Pt, Au) relativeto a reference electrode. Redox potential mea-surements are highly nonspecific because allredox couples of a mixture contribute to thepotential. The redox potential also strongly de-pends on pH. Thus, the applicability of redoxpotential measurements is mainly limited to de-termining qualitatively, e.g., nitrites in wastewa-ter (denitrification), cyanides, copper, and chro-mium in galvanic wastes, or chlorine in publicswimming pools.

Potentiometric Oxygen Analysis. Solid-state electrolytes that exhibit ionic conductivityat elevated temperatures (> 500 C) can also beused for potentiometric measurements. PorousZrO2 membranes covered with platinum grids

have been used to determine oxygen in gasespotentiometrically. Different oxygen concentra-tions on the two sides lead to a potential differ-ence across the membrane in proportion to thedifference in oxygen partial pressure.

8.4. Amperometry, Voltammetry, andCoulometry

Amperometric, voltammetric, and coulometrictechniques are not so widely used as otherelectroanalytical methods for reaction monitor-ing. They are characterized by electrode reac-tions at polarized electrodes that are detected bycurrent – voltage measurements. Because of theelectrode reactions the electrodes have to becleaned on a regular basis, and a certain fractionof the mixture under investigation is consumedduring the measurement.

Applications include the amperometric deter-mination of chlorine, dissolved oxygen, hydra-zine, and ozone in pulp-bleaching baths; tap,industrial, and wastewater, amperometric moni-toring of hydrogen peroxide in textile bleachingbaths [478], the amperometric monitoring ofgases and air at work places, the voltammetricdetermination of heavy metals in waste streams,and the coulometric analysis of sulfurcompounds.

9. Miscellaneous Methods

9.1. Mass Spectrometry

In the 1990s, mass spectrometry (MS) emergedas technique for on-line analytical purposes. Themethod is highly sensitive and fast with anextraordinary molecular selectivity. The detec-tion principle of MS is based on an ionizationof chemical species followed by a separation ofthe fragments according to their mass-to-chargeratio (see ! Mass Spectrometry). Due to thewide variety of ionization techniques, most typesof sample can be analyzed by MS. Ionizationtechniques for process applications include elec-tron impact (EI), resonance-enhanced multipho-ton ionization (REMPI), microwave plasmaionization, thermal ionization, and electrosprayionization (ES). Direct capillary and membraneinlets are most commonly used for sample


introduction. MS process applications are re-viewed in [479–483].

In an increasing number of process applica-tions MS is interfaced to pre-separation techni-ques such as GC or LC to improve resolution andselectivity in the analysis of complex (reaction)mixtures. Reduced costs, size, and complexity ofthe instruments have even made portable MSdevices possible. Efforts have been undertakenwithin the last couple of years to miniaturize MSinstruments (‘‘MS on a chip’’).

The traditional on-line fields of use of MS areprocess gas stream and environmental gas anal-ysis, environmental and wastewater monitoring,and monitoring of fermentation processes. Traceamounts of aromatics and chlorinated com-pounds in flue gases of industrial incineratorshave been detected by REMPI-MS [484–486].

MS has also been applied to monitoring liq-uid-phase and gas-phase chemical reactions. On-line MS is useful for monitoring alkanes andalkenes in rubber production [487], monomersand solvents in polyethylene production [488],monitoring the photocatalytic degradation ofphenol and trichloroethylene [489] or electro-chemical polymerization of aniline [490].

The modern semiconductor industry reliesheavily on MS process monitoring [491],[492]. MS can be used to monitor gaseous impu-rities in high-purity gases, to study plasma etch-ing processes [493],molecular beamepitaxy, andchemical vapor deposition processes [494].

9.2. Densimetry and Dilatometry

The physical basis of densimetry and dilatometryis the mass density difference (if any) betweeninitial reaction mixture and the final reactionproduct. As the reaction proceeds, this densitydifference leads to an increase or decrease in thedensity of the mixture r(t) and to an overallshrinkage or expansion, respectively, of the vol-ume of the reaction mixture V. Density changescan be measured continuously with responses onthe order of one measurement per second, whilethe repetition time for measurements of volumechanges is in the millisecond range. Therefore,the extent of reaction can be calculated at anytime if the amount of ingredients initiallycharged to the reactor and/or continuously fedto the reaction mixture is known.

The following factors must be considered inapplying these techniques:

1. For a quantitative analysis of density changes,it has to be checked carefully whether thereaction mixture behaves as an ideal mixtureor not; that is, does Equation 43

Vtotal ¼Xi

ViþVexcess ð43Þ

with Vexcess ¼ 0 hold, where Vtotal, Vi, andVexcess are the volume of the reaction mixture,volume of component i, and excess volume,respectively.

2. Accurate densimetric and dilatometric mea-surements rely substantially on the accuracyof temperature control to be better than� 0.1K (in some cases even better than � 0.01 K).

3. As with all other ‘‘integral’’ methods, a singlevariable (density, volume, temperature, pres-sure) is insufficient to describe multi-compo-nent reaction mixtures.

Density measurements are suitable for reac-tion monitoring of polymerizations (in bulk,solution, emulsion, suspension) because ofthe considerable density differences betweenmonomers and polymers (Table 6). In a two-component system, the extent of reaction (e.g.,conversion xr(t) in polymerizations) can simplybe calculated according to Equation 44 byassuming ideal mixing behavior

Table 6. Densities of monomers and polymers at 20 C

Monomer Density, g/cm3

Monomer Polymer

Acrylic acid 1.051 1.37

Acrylonitrile 0.806 1.17

1,3-Butadiene 0.621 0.97

Butyl acrylate 0.899 1.08

n-Butyl methacrylate 0.886 1.06

Ethyl acrylate 0.924 1.12

2-Ethylhexyl acrylate 0.887 0.99

Ethyl methacrylate 0.914 1.13

Methyl acrylate 0.954 1.22

Methyl methacrylate 0.944 1.19

o-Methylstyrene 0.916 1.01

Styrene 0.906 1.04

Vinyl acetate 0.932 1.18

Vinyl chloride 0.911 1.39

Vinyl methyl ether 0.750 1.06


XrðtÞ ¼1

rstart� 1

rðtÞ1

rstart� 1

rend

ð44Þ

where rstart and rend are the density of themixture at the beginning and end of the reaction.

Most of the common principles to performdensity measurements (pyknometer, buoyan-cy) are unsuitable for reaction monitoring.Methods that have been applied to processmonitoring include hydrostatic pressure mea-surement, the maximum bubble pressure tech-nique, oscillations of U-shaped tubes, cylin-ders or plates, and absorption of radioactiveradiation or X-rays.

A rather robust technique is the measurementof the resonance frequency of a U-shaped tube(Fig. 21). This resonance frequency is a verysensitive function of the mass of fluid within thetube, and, hence, of its density. The densimeterhas to be calibrated with a liquid of knowndensity rcal that gives a period Tosc,cal of theU-tube oscillations. The density of the mixturer(t) can be derived from the change of the periodof oscillations Tosc(t) provided the temperature Tof the mixture is held constant (Eq. 45)

rðtÞ�rcal ¼ const:�T2oscðtÞ�T2

osc;cal

�ð45Þ

To obtain noise-free results, gas bubbles shouldbe avoidedwhile circulating the reactionmixturethrough the tube densimeter. In low-viscositymedia, bubble traps can prevent bubbles enteringthe densimeter tube. A prerequisite for the appli-cation of Equation 45 is that the viscosity of thecalibration liquid is roughly equal to the viscosityof themixture because of its influence on the tubeattenuation. The accuracy of density measure-ments with commercially available oscillatingtubes can reach the range 10�6 to 10�7 g/cm3.Applications of U-shaped tube density measure-ments to monitoring reactions can be found in[495–497].

In addition to gas bubbles and temperatureuncertainties, film formation on the inner surfaceof the tube, formation of sediments, and cloggingof the pipemay cause problemswith thismethod.These problems can be partially overcome by theinverse arrangement: a tuning fork inserted intothe mixture. Again, the change of resonancefrequency of the fork is related to the density ofthe surrounding liquid, and the amplitude with

which the fork oscillates (i.e., the attenuation) is afunction of the viscosity of the medium.

The maximum bubble pressure technique isbased on the pressure pbubble of a spherical gasbubble of radius r at a capillary tip immersed inthe reaction mixture at depth h (Fig. 22). Thispressure is the sum of the hydrostatic pressure ofthe liquid above the capillary tip, and the pressuregenerated by the liquid – bubble interfacialtension g

Figure 21. U-shaped tube for density measurements (cour-tesy of Anton Paar KG)


pbubble ¼ rghþ 2gr

ð46Þ

where g is the acceleration due to gravity [498],[499]. Thus, pbubble depends on both the immer-sion depth of the capillary tip into the liquid, andthe capillary and, therefore, bubble diameter.

For density measurements, two capillaries ofequal diameter are inserted into the mixture atdifferent depths hi beneath the surface. This leadsto a difference of pressure readingsDpbubble of thetwo capillaries, and

r ¼ Dpbubbleðh1�h2Þg ð47Þ

The internal bubble pressure due to surface ten-sion is identical for the two capillaries.

In addition, the two-capillary principle canalso be used to measure the surface tension of theliquid. In this case, two capillaries of differentdiameters r1 and r2 are immersed into the liquid atthe same depth and experience the same hydro-static pressure but different internal pressuresdue to different bubble curvatures. Thus,

g ¼ Dpbubble2 1

r1� 1

r2

� � ð48Þ

Corrections must be made to account for theinfluence of capillary forces and nonsphericalbubble shapes [498]. In stirred or turbulent me-dia, bubble shear-off may cause problems as wellas coagulum formation or clogging of capillariesin film-forming media. The accuracy of bubbledensimeters is on the order of 10�3 g/cm3.

The hydrostatic pressure ph of a liquid mea-sured at a depth h is

ph ¼ rgh ð49Þ

The hydrostatic pressure difference Dph betweentwo pressure sensors at different depths of dis-tance Dh¼ h1� h2 is directly proportional to theliquid density

r ¼ DphDhg

ð50Þ

The physical basis of Equations (49) and (50) isessentially identical. In stirred or flowing media,the hydrodynamics may add considerable noiseto the pressure readings of the sensor.

The absorption of radioactive radiation (in-cluding X-rays) depends on the molecular/atom-ic absorption coefficient, the density (concentra-tion) of the ingredients, and the thickness of thematerial throughwhich the radiation passes (pathlength d). After calibration of the absorption ofboth the source/cell/detector arrangement andthemixture, the density can be calculated accord-ing to

Id/I0expð�rdÞ ð51Þwhere I0 and Id are the intensity of the radiationsource and the detected radiation, respectively.

Radiometric density determination is advan-tageous in systems under very high pressure andin corrosive media because neither the sourcenor the detector is in direct contact with themixture.

The application of dilatometry to reactionmonitoring is discussed in detail in [500].Time-resolvedmeasurements of volume changesfor transparent and nontransparent samples witha laser scanning dilatometer show a simple linearrelation between shrinkage and chemical conver-sion during epoxy curing [501]. In general, dila-tometry is a very useful tool for academic re-search but only of limited value for reactionmonitoring in (continuously stirred) industrialreactors.

9.3. Rheometry

The theory of rheology, instrumentation andapplications are extensively described in

Figure 22. Schematic of maximum bubble pressure method


[502–504] (see also ! Rheometry, ! FluidMechanics).

Various processes in the plastics industry areaccompanied by viscosity changes of the reactionmixture:

1. Most of plasticsmanufacturing processes leadto a substantial increase of viscosity becauseof growing chains, branching, and cross-linking. Rheological characterization ofplastics is essential for understanding andinfluencing their processability.

2. Curing reactions transform liquids into gelsand solids. Thus, the resistance of the materialto flow changes by orders of magnitude.

3. Melt flow index is an important parameter formonitoring and controlling reaction extru-sion, reaction blending, and reaction injectionmolding [69], [70].

Increasing viscosity in noncross-linked sys-tems can be related to the molecular mass andconcentration of growing chains. For linear poly-mers, the bulk viscosity h is a function of theweight averagemolecular massMw similar to theKuhn – Mark – Houwink equation for the in-trinsic viscosity [h] of polymers in solution

h ¼ KhMahw ð52Þ

where Kh is a parameter that depends on inter-chain friction. The exponent ah is about 1 – 2 forshort chains, and increases to 3.4 above a criticalmolecular weight Mw,crit [505]. Equation 52holds also for branched polymers if Mw is re-placed by gMw, where the contraction factor g isdefined as the ratio of the radius of gyration of thebranched molecule to that of a linear molecule ofthe same weight average molecular mass.

In cross-linking reactions, the viscosity of themixture increases dramatically while approach-ing the gel point [506], [507]. After reaching thegel point, the cross-linked reaction mixture be-haves like a soft solid. For temperatures abovethe glass transition temperature Tg the mechani-cal properties of the material may be treated byrubber elasticity theory.

The viscosity of disperse systems depends onthe volume fraction j of disperse material, parti-cle shape and size distribution, interparticularinteraction, etc. For monodisperse spherical par-ticles at low j, viscosity is proportional to thevolume fraction of the disperse phase

h ¼ h0ð1þ2:5jÞ ð53Þwhere h0 is the viscosity of the dispersion medi-um. For higher volume fractions and systemswith dominant interparticle interaction, numer-ous theoretically based, semi-empirical, and em-pirical h – j relations can be found in the litera-ture [508], [509]. Thus, viscosity reflects changesof particle number and size [510] in heterophasereactions (e.g. emulsion formation, emulsionpolymerization, suspension polymerization, dis-persion polymerization).

Principles and methods for on-line rheometryinclude rotational and capillary process visc-ometers (see ! Rheometry), as well as the ap-plication of ultrasound (see Chap. 3), and tube orfork oscillations (see Section 9.2).

Widely used techniques for measuring meltviscosity in extruders are capillary viscometryand pressure drop analysis [69], [70], [511],[512]. The pressure drop Dp of melt flow througha nozzle is directly proportional to the meltviscosity according to the Hagen – Poiseuillelaw

Dp ¼ � 8hvIR2

ð54Þ

where v�, l, and R are the average velocity in thepipe, its length, and diameter, respectively.

9.4. NMR Spectroscopy

Nuclear magnetic resonance (NMR) spectrosco-py (see ! Nuclear Magnetic Resonance andElectron Spin Resonance Spectroscopy) is prob-ably the most important spectroscopic techniquefor studying chemical structures. Signals of mo-lecules and sub-units of molecules, and the sig-nals of their various spatial arrangements andconfigurations are much more pronounced inNMR than in any other spectroscopical method.Nevertheless, NMR is still mainly an off-linetechnique because of elaborate sample prepara-tion requirements, the instruments sensibilityto temperature variations, the necessity of instru-ment tuning (magnet shimming, drift compensa-tion), and the time required to run spectra.However, some recent developments in instru-mentation (magnets, radio frequency source anddetector circuitry), and the adaptation to flowcells havemadeNMR spectroscopy applicable toon-line analysis. A major advantage of NMR


spectroscopy is the noninvasive character of thetechnique.

Most of the process measurements so farutilize pulsed NMR in which the free inductiondecay (FID) signal of magnetic-field alignedmolecular spins after the application of a per-pendicular radio frequency pulse is analyzed as afunction of time. Network formation during theradiation-induced polymerization ofN-isopropy-lacrylamide has been monitored by T2 spin –spin relaxation time measurements [513]. Thecure of an epoxy – amine system has been studiedby NMR spectroscopy and compared to theresults of alternative techniques such as ultra-sound and dielectrics [514]. The solids content ofrubber latex can be determined without anyspecial sample preparation with a total measure-ment time of ca. 10 s by quasi-on-line applicationof pulsed NMR [515]. FID signals in combina-tion with a chemometric technique (PLS) enableone to predict the styrene content in a butadiene –styrene resin [516]. Other applications includedetermination of hydrogen in various materials,and several solvents in gasoline.

High resolution NMR spectroscopy has beenrarely applied to on-line analysis. Because of itsnatural abundance, 1H NMR is superior to allother nuclei for process measurements. Applica-tions include the determination of the totalhydrogen content of hydrocarbons, the determi-nation of crystallinity and melt index of polyeth-ylene, cure dynamics, and copolymer formationmonitoring [517], carbamic acid formation insupercritical carbon dioxide [518], the chemicalequilibria and reaction kinetics of the formalde-hyde – water – methanol system, the reactive gasabsorption of CO2 in aqueous solutions of mono-ethanolamine [519], are monitoring processes inrefineries [520] and the emulsion polymerizationof butyl acrylate [521]. Low-field NMR spec-troscopy has been shown to be applicable tomonitor the production of benzene [522].

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