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HarsNet THEMATIC NETWORK ON HAZARD ASSESSMENT OF HIGHLY REACTIVE SYSTEMS

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3. Testing Techniques and Methods

HarsBook has been prepared by a HarsNet working group. The text has been prepared in good faith but the authors accept no responsibility for the consequences of application of the information contained herein. The HarsNet Thematic network is European Community Project BET2-0572 funded in part by contract number BRRT-CT98-5066.

Keywords: thermal, calorimetry, measurement, testing, equipment, scanning, isothermal, adiabatic, reaction, hazard 3.1 Introduction. The aim of this chapter is to present a comparison among various types of thermal assessment techniques. It is divided in six sections. The first section is to establish what is intended when a thermal assessment is performed, and to define the most relevant parameters for thermal safety. Section number two gives a short presentation of the most common techniques and/or instruments used in thermal hazard assessment. A third section presents the principles of measurement. Section four discusses possible operation modes used to study the thermal behaviour of chemicals and reacting mixtures. A tabulated comparison of the different techniques and experimental conditions is presented in section five. Finally, a list of some relevant references is presented in section six. 3.2. Objectives of testing in thermal hazard assessment Testing should provide enough data to predict the behaviour of a process in order to:

Judge the suitability of the proposed process. ��

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Produce data for engineering purposes. Avoid unexpected side reactions or decomposition of chemicals and mixtures. Suggest measures to prevent runaway situations. Design protection systems.

INDUSTRIAL AND MATERIALS TECHNOLOGIES PROGRAMME OF THE EUROPEAN COMMISSION. PROJECT: BET2-0572 COORDINATION: Prof. Dr. R. Nomen Tel. +34-93-267 20 00 Fax. +34-93-205 62 66 http://www.iqs.url.es/harsnet


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Definitions of safety relevant parameters It must be remarked that most of the measured values of the parameters named here may be influenced by the experimental conditions and operation procedures. Thus it must be left to the judgement and evaluation of expert users of each specific method whether the results are reliable or not. Critical test conditions and their potential influence on results must be defined according to the previous knowledge of the system and to the methods that will be used. Parameters are classified in four categories – temperature, pressure, heat or power and time -, depending on the measured variables. Usually, temperature and heat are considered to be an indication of the severity of a possible runaway event but pressure is the process variable that may cause damage to equipment, harm to workers and chemical releases to the environment. The time scale over which a maloperation could develop into a serious event is critical. The higher the temperature, pressure and heat release are, the more catastrophic are the potential consequences. This list is open and should be reviewed periodically.

Temperature Symbol Definition and remarks

Tp Process temperature.

MTSR Maximum Temperature attainable by the Synthesis Reaction. This is the maximum temperature that can be reached due to the desired synthesis reaction when carried out under adiabatic conditions. For a batch process, MTSR can be determined by adiabatic calorimetry or estimated as

MTSR = Tp - �rH/Cp (�rH is itself negative for an exothermic reaction) MTSR can be computed as a function of time considering the actual accumulation of unreacted materials within the reactor. For batch or semi-batch process, MTSR can be estimated considering three scenarios: ��

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Batch: all reactants are mixed at the beginning of the process, and adiabatic conditions are assumed. Interrupted semi-batch: dosing is stopped after a cooling failure. Only the actual accumulation is considered at any time. Continued semi-batch: dosing is not stopped when adiabatic conditions are established. Actual accumulation and remaining heat of reaction should be considered.

Tcf Temperature after a cooling failure. This is a recently introduced term indicating the expected final temperature increase of a reaction mixture after a cooling failure. In many cases, it is equivalent to MTSR.

�adTd Adiabatic temperature increase. This is the adiabatic increase of temperature due to the reaction of the chemicals in the reactive system.

Td

Tonset Temperature of decomposition or onset temperature. This is the lowest temperature at which exothermic reaction is observed. This temperature depends on the sensitivity of the instrument used. The higher the sensitivity the lower this temperature.



SADT Self Accelerating Decomposition Temperature. The temperature of a cooling medium that will

just cause the contents of a particular sized container to reach the Temperature of No Return, i.e. the temperature at which the heat generation rate just equals the capacity to lose heat to the coolant. SADT depends on the size of container (the cooling time constant) as well as the thermokinetics of the reaction.

Tdg Onset Temperature for the Production of Decomposition Gases.

AZTt Maximum temperature at which no decomposition takes place after a time t.

Minimum Safe Temp.

Minimum Safe Temperature. The minimum temperature at which the heat transfer may be maintained within operative conditions to control the system. Some physical phenomena such as crystallisation, precipitation or viscosity increase define the value of this temperature.

Maximum Self Heating

Rate

Maximum Self Heating Rate. The maximum rate of increase of temperature under adiabatic conditions.

Heat/Power Symbol Definition and remarks

RQ� Power release. The thermal power released by the system at a certain time.

MAXRQ� Maximum power release. The maximum of RQ�

QR Evolved Heat. The total amount of heat released from the beginning of the studied process (synthesis or decomposition) to a certain time. The total evolved heat corresponds to the integral over the whole process. For a single process and total conversion, it corresponds to the heat of reaction (-�rH) or decomposition (-�dH) at the process conditions.

Accum. Accumulation, Accumulation of Unreacted Materials, or Thermal Potential. It is the amount of heat that can be released by the system after stopping feeds.

Time Symbol Definition and remarks TMRad

TMR

Time to Maximum Rate (Under Adiabatic Conditions). This is the time to achieve the maximum self-heating rate, due to side reactions or decomposition, under adiabatic conditions. It depends on initial temperature. It can be convenient to determine this at the MTSR.

Induction Time

Induction Time, Isothermal Induction Time, Autocatalytic Induction Time, or Induction Period. The time after which exothermic activity will appear under isothermal conditions, e.g. for chain radical reactions, polymerisations and autocatalytic reactions. The induction time is a function of temperature, and concentrations of inhibitors, catalysts and impurities.

3-3 INDUSTRIAL AND MATERIALS TECHNOLOGIES PROGRAMME OF THE EUROPEAN COMMISSION. PROJECT: BET2-0572 COORDINATION: Prof. Dr. R. Nomen Tel. +34-93-267 20 00 Fax. +34-93-205 62 66 http://www.iqs.url.es/harsnet


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3.3. Techniques and instruments.

Symbol Definition and remarks DTA Differential Thermal Analysis. This consists of the measurement of the change of the

difference in temperature between the sample and a reference while they are subjected to a particular temperature regime.

The typical sample size is in the order of mg.

DSC Differential Scanning Calorimetry. DSC involves measurement of the heat released or absorbed by the sample cell in comparison with a reference twin cell while they are subjected to the same temperature regime. DSC may use several principles of measurement, including Heat Flow, Power Compensation, and Calvet.

The typical sample size is of the order of mg.

�-C Micro-Calorimetry. Micro-calorimeters and thermal activity monitors can be considered essentially as large scale, very sensitive DSCs, being able to reproduce some process conditions that are not possible on the smaller scale (e.g. stirring and dosing) or to detect the production of very low heat rates. Micro-Calorimeters use Calvet as the typical principle of measurement.

Samples are usually in the range of 1 to 10 g.

RC Reaction Calorimetry. Reaction calorimetry is the most common method used to fully simulate the process under industrial conditions. It consists of the measurement of heat flow rate which is the consequence of the reactive behaviour, or physical changes, of a sample. The heat generated or absorbed by the system may be measured using one of several principles of measurement, Heat Flow, Heat Balance, Power Compensation, Isoperibolic Balance and Calvet. Some combinations of these are possible. Non-differential instruments are typically used. However, recent developments use twin cells in order to try to improve the measurements. The typical sample size is of the order of g or kg.

AdC Adiabatic Calorimetry. In Adiabatic Calorimetry, heat transfer between the sample and its surroundings is minimised using one of several different procedures. The temperature and pressure of the sample are the characteristics that are measured.

The typical sample size is of the order of g.

Pseudo-adiabatic

Pseudoadiabatic instruments are used for screening purposes, essentially. They can be classified as non-differential thermometry (NDT) techniques. They perform the measurement of the change of the difference in temperature between the sample and its surroundings while it is subjected to a particular temperature regime. In some instruments pressure is also measured. The typical measuring procedure consists of setting a temperature program and recording the difference of temperatures between the sample and its surroundings (e.g. an oven or a bath).

The characteristic range of sample size is of the order of g.



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3.4. Principles of measurement

Name Definition and remarks Heat Flow The power evolved by the sample ( ) is evaluated as . �T

is the variable measured by the instruments. The product of the overall heat transfer coefficient and the transfer area ( U·A ) is obtained through calibration.

FLOWQ� � � TAUQFLOW ��

Heat Balance This is mainly used in Reaction Calorimetry. is evaluated using an energy

balance on the jacket as FLOWQ�

� �outjjFLOW Tm ��

injjp Tc �Q . The fundamental measurements are the inlet ( Tj in ) and outlet ( Tj out ) temperatures of the fluid in the jacket. Its mass flow ( ) is usually monitored. Calibration is required to determine

· cjm�

jm� p j and the heat losses from the jacket.

Calvet The Calvet principle is typical for microcalorimetry ( �-C )but it is also used in DSC, and in some cases in RC. The instruments based on this principle are the most sensitive. However, they are usually limited to low heating rates and moderate temperatures. In a Calvet calorimeter, the sample container is surrounded by a secondary wall. The difference of temperatures between the internal and external surfaces of the secondary wall are measured with extreme accuracy using a set of thermocouples (“thermopile”). The geometry, thickness and thermal conductivity of the secondary wall are known. is evaluated using the balance at the secondary wall.

FLOWQ�

Power Compensation (Heat Flux)

Instruments based on the Power Compensation principle are used both for DSC and RC. In Power Compensation DSC, the sample pan and the reference pan are placed in twin ovens. Any difference of temperature between the external wall of the two crucibles is measured and kept as small as possible by controlling the power supply to the sample oven. In Power Compensation RC, the heat transfer between the sample and its surroundings is kept as constant as possible, and the temperature of the sample is held as close to the set point as possible by controlling the power supplied to an internal electrical heater (compensation heater). In both cases, the power supplied is recorded. is assumed to be equal to the difference between the supplied electrical power and a baseline value that is determined when no reaction is occurring.

FLOWQ�

Isoperibolic Balance This principle is essentially applied to RC. A vessel (ballast) is inserted between the system and an isothermal jacket (surroundings) in order to maintain the system quasi-isothermal. Heat transfer coefficients between the system and the ballast, and between the ballast and the jacket might be high. The heat transfer coefficient between the ballast and the jacket is almost constant, and can be easily determined by calibration using an electrical calibration heater. is determined directly using the heat balance around the ballast.

FLOWQ�

Adiabatic balance This principle of measurement is specific to Adiabatic Calorimetry ( AdC ), but can be approximated by some reaction calorimeters. The adiabatic temperature increase is recorded. Instruments differ in the so called phi factor, which is the ratio between the true adiabatic temperature increase for the sample alone and that recorded for the (sample + cell). The lower the phi factor, the better the results reproduce the large scale thermal behaviour .

Some combination of principles of measurement (e.g. Power Compensation and Heat Flow) have been proposed in order to avoid some of the difficulties or limitations of the above principles of measurement.



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3.5. Operation modes

Name Definition and remarks Temperature Scanning A constant rate temperature ramp is applied to the sample. The main data obtained

from scanning tests are the heat capacity (Cp), the power release as a function of time and/or temperature, the initial temperature of the exotherm (Td or Tonset), the temperature of the peak (highest) reaction rate as a function of the heating rate, heat of reaction/decomposition, and changes of pressure in the sample container, and amount of evolved gases, when available. The temperature scanning mode is traditionally used to obtain reaction kinetics, both using classical procedures (model based) and new approaches which do not require any assumption about the model (“model-free” and NPK – “non parametric kinetics”) The kinetics obtained must be used with care in the context of extrapolation. Users must be advised to use isothermal and/or adiabatic tests to validate the results.

Isothermal The behaviour of the sample is recorded at constant temperature. These methods are mainly used in RC or as a complement to scanning methods. The main data obtained from Isothermal methods are the power release as a function of time, induction time and time of the peak of heat generation. These variables can be obtained as a function of temperature for autocatalytic processes. Changes of pressure in the sample container, and amount of evolved gases can be measured. The shape of the exotherm gives direct information about the kinetics of the reaction.

Isoperibolic The temperature of the environment around the sample is kept constant. With the exception of the quasi-isothermal instruments that use the isoperibolic principle of measurement, the isoperibolic mode produces the most difficult data to analyse. Its applications are usually restricted to simulating industrial procedures.

Adiabatic In adiabatic mode, loss of cooling is simulated. All of the results are affected by the phi factor of the instrument as indicated above. The main data obtained from the adiabatic mode are the temperature and pressure increases vs. time, the initial temperature of the exotherm ( Td or Tonset), the maximum temperature attainable by the synthesis reaction (MTSR) and/or adiabatic temperature increase (�adTd), and the time to maximum rate under adiabatic conditions (TMRad) as a function of initial temperature. The self-heating and change of pressure rates are calculated directly from the experimental data. The adiabatic mode can be used for kinetic modelling, using procedures similar to the scanning mode.

Pseudo-adiabatic Some instruments apply a constant temperature rate ramp to the sample. However, when the heat evolved by the sample exceeds the net heat losses, the sample is allowed to self-heat. This operation mode is called pseudo-adiabatic because the apparent phi factor and the heat transfer coefficient are very low, but the heater (oven) continues following the ramp. There is a net heat flux between the sample and its surroundings. Consequently, it becomes quite impossible to quantify the amount of heat released or to perform kinetic modelling.

Temperature modulated

A sinusoidal change in the temperature set-point is applied in both scanning and isothermal experiments. Temperature modulated heat-flow RC experiments are useful for continuous monitoring of the value of U·A and Cpr



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3.6. Tabulated comparison for different methods and techniques Pseudoadiabatic instruments are indicated as NDTA.

Thermometry Calorimetry Method DTA NDTA DSC �-C RC AdC

Principle of Measurement Heat flow X X Heat balance X Calvet X X X Power compensation X X Isoperibolic balance X Adiabatic balance (X) X Operation Mode Scanning X X X X (X) Isothermal X X X X X Isoperibolic X Adiabatic (X) X Self Heating X Main Measurements

�T X T X X X Q� X X X P X X (X) X Typical Parameters Sample size mg g Mg g hg - kg dag Sensitivity �K cK �W < �W W cK Operating range: Minimum T (ºC) -200 ambient -200 -20 -80 ambient Maximum T (ºC) 2000 500 600 300 300 500

Table 3.1. Principal features of methods used in thermal hazard assessment of highly reactive chemicals and systems. X indicates a common characteristic, and (X) an option for some instruments.

Considering possible measurements, a general trend is to couple thermal analytical and calorimetric techniques (e.g. DTA-TGA) or to link other instruments to calorimeters



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(e.g. spectroscopic devices) in order to obtain complementary information particularly for kinetic modelling.


Other Possible Measurements1

EG (Evolved Gas) X X X X Determined System Properties2

Cp X2 X2 X2 U (Heat Trans. Coef.) X1 Other Features3

Dosing X X X X Stirring X X X X

Table 3.2. Some complementary features of methods used in thermal hazard assessment of highly reactive chemicals and systems. Subscripts on X indicates the typical mode of operation used to obtain such a property: 1 = scanning, and 2 = isothermal.

Notes to Table 3.2: 1.- Only Evolved Gas is included in the section for Other Measurments. It is considered to be a key parameter to assess the design of protection measures for reactive chemicals and systems. 2.- Depending on the instrument, some other properties of the system could be obtained (e.g. viscosity, vapour pressure, …). 3.- In the Other Features section only Dosing and Stirring are mentioned, because they are considered to be key factors for simulating process conditions.



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Safety Related Obtainable Parameters

MTSR C C C E �adTd C C (C) E Onset temperature E E E E (E) E Tdg E E (E) E AZTt E E (E) E Minimum Safe T E Max. Self Heating Rate M M C Accumulation E TMRad M M M E Induction Time E1 E E1/M2 E1 E1 E

Kinetics M M M M

Table 3.3. Thermal safety relevant parameters obtained by the different methods. E direct experimental measurement, C calculated directly from measurements, and M obtained through a model. Letters in brackets indicate particular possibilities. Subscripts indicate the typical mode of operation used to obtain such a parameter: 1 = scanning, and 2 = isothermal.


Focused Experiments

Influence of Stirring E E E Influence of Chemical Composition

E E E E E

Feed Sequence E E Type of Process E E E

Table 3.4. Specific questions that must be studied to scale up a chemical process are listed. The influence of Chemical Composition includes all the possible influences of impurities, mischarging of reactants, bad use of inhibiting or stabilising agents, etc. Type of process includes batch or semi-batch.



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3.7. References A non-exhaustive and open list of some relevant references is given. 3.7.1. Literature Barton, J.; Rogers, R. Chemical Reaction Hazards. Ed.; IChemE: Rugby, 1993. ISBN 0-85295-284-8. Beruzzi, A.; Zaldívar, J.M. Safety of Chemical Batch Reactors and Storage Tanks. Ed.; Euro Courses, Reliability and Risk Analysis, Volume 1; Commission of the European Communities: Luxembourg, 1991. ISBN: 0-7923-1233-3 Berthold, W. Carrying Out Measurements; Proceedings of the European Conference on Evaluation of Thermic Hazards and Prevention of Runaway Chemical Reactions: Zurich, November 1982. Brogli, F. Screening for Hazards Due to Exothermic Chemical Reactions; Proceedings of the European Conference on Evaluation of Thermic Hazards and Prevention of Runaway Chemical Reactions: Zurich, November 1982. Brown, M. E. Handbook of Thermal Analysis and Calorimetry. Ed.; Volume 1: Principles and Practice; Elsevier Science B.V.: Amsterdam, 1998. ISBN: 0-444-82085-X. Cronin, J.L.; Nolan, P.F.; Barton, J.A. IChemE Symposium Series No. 102, 1987; pp 113-122. DIERS Reports: Bench Scale ERS Sizing Tools: Equipment Details and Test Procedures: FI /84-4; DIERS (Design Institute for Emergency Relief Systems); AIChE, 1984. Fauske, H.K.; Clare, G.H.; Creed, M.T. RSST: Laboratory Tool for Characterising Chemical Systems. International Symposium on Runaway Reactions; CCPS, Cambridge, Massachusetts, March 1989; AIChE: New York; pp 367-371. Grewer, T. Thermal Hazards of Chemical Reactions. Elsevier Science B.V.: Amsterdam, 1994; Industrial Safety Series: vol. 4. ISBN: 0-444-89722-4. Gustin, J.L. J. Loss Prev. Process Ind. 1993, 6, 275-292. INSET Tool L. Chemical Reaction Reactivity – Stability Evaluation. Input: Dossiers of INSET Stage II. Laboratory Tests. INSET, version 1.0; July 1997; Part 2 Tool L.



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Nomen, R.; Sempere, J.; Lerena, P. Heat flow reaction calorimetry under reflux conditions. Thermochimica Acta, 1993, 225, 263-276. Ottaway, M.R. Analytical Proc., 23: 116 Pey, A. RARE Project: Risk Assessment of Runaway Events. TNO-report, TNO-MEP - R 99/184; TNO Institute of Environmental Sciences, Energy Research and Process Innovation: Apeldoorn, 1999. Rasmussen, B. Ph.D. Unwanted Chemical Reactions in the Chemical Process Industry. Risø National Laboratory: DK-4000 Roskilde. ISBN 87-550-1306-6; ISSN 0418-6435. Regenass, W. Exothermicity of the Desired Reaction, Reaction Calorimetry. Proceedings of the European Conference on Evaluation of Thermic Hazards and Prevention of Runaway Chemical Reactions: Zurich, November 1982. Rogers, R.L. IChemE Symposium Series No. 114, 1989, 47-107. Serra. E.; Nomen, R.; Sempere, J. Maximum Temperature attainable by runaway of synthesis reaction in semi-batch processes. J. Loss Prev. Process Ind.,1997, 10, 211-215. Sempere, J.; Nomen, R.; Serra, R.; Cardillo, P. Thermal hazard assessment using closed-cell Adiabatic Calorimetry. J. Loss Prev. Process Ind., 1997, 10, 52-62. Singh, J. PHI-TEC: Enhanced Vent Sizing Calorimeter; International Symposium on Runaway Reactions; CCPS, Cambridge, Massachusetts, March 1989; AIChE: New York; pp 313-330. Singh, J.; Waldram, S.P.; Appelton, M.S.; Proceedings of the 8th International Symposium of Loss Prevention and Safety Promotion in the Process Industries: Antwerp, June 1995; J.J. Mewis; H.J. Pasman; E.E. De Rademaeker. Ed.; Elsevier Science B.V., 1995; Volume II, pp 145-159. ISBN: 0-444-82136-8. Steinbach, J. Safety Assessment for Chemical Processes. Wiley-VCH: Weinheim, 1998. ISBN 3-527-28852-X.



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3.7.2. ASTM standards E473–99 Standard Terminology Relating to Thermal Analysis E476–87(1993) Standard Test Method for Thermal Instability of Confined

Condensed Phase Systems (Confinement Test) E487–99 Standard Test Method for Constant Temperature Stability Of

Chemical Materials E537–98 Standard Test Method for Assessing the Thermal Stability of

Chemicals By Methods of Thermal Analysis E698–99 Standard Test Method for Arrhenius Kinetic Constants for

Thermally Unstable Materials E794–98 Standard Test Method for Melting And Crystallization

Temperatures By Thermal Analysis E967–97 Standard Practice for Temperature Calibration of Differential

Scanning Calorimeters and Differential Thermal Analyzers E1231–96a Standard Practice for Calculation of Hazard Potential Figures of

Merit for Thermally Unstable Materials E1269–99 Standard Test Method for Determining Specific Heat Capacity by

Differential Scanning Calorimetry E1356–98 Standard Test Method for Assignment of the Glass Transition

Temperatures by Differential Scanning Calorimetry or Differential Thermal Analysis

E1445–98 Standard Terminology Relating to Hazardous Potential of Chemicals

E1623–99 Standard Test Method for Determination of Fire and Thermal Parameters of Materials, Products, and Systems Using an Intermediate Scale Calorimeter (ICAL)

E1641–99 Standard Test Method for Decomposition Kinetics by Thermogravimetry

E1860–97a Standard Test Method for Elapsed Time Calibration Thermal Analyzers

E1952–98 Standard Test Method for Thermal Conductivity and Thermal Diffusivity by Modulated Temperature Differential Scanning Calorimetry

E1953–98 Standard Practice for Description of Thermal Analysis Apparatus E1981–98 Standard Guide for Assessing the Thermal Stability of Materials by

Methods of Accelerating Rate Calorimetry E2041–99 Standard Method for Estimating Kinetic Parameters by Differential

Scanning Calorimeter Using the Borchardt and Daniels Method E2046–99 Standard Test Method for Reaction Induction Time by Thermal

Analysis E2070–00 Standard Test Method for Kinetic Parameters by Differential

Scanning Calorimetry Using Isothermal Methods D4871–00 Standard Guide for Universal Oxidation/Thermal Stability Test

Apparatus



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3.7.3. web sites American Society For Testing and Materials. http://www.astm.org/. 30.04.2002 HarsNet. http://www.harsnet.de. 30.04.2002 Fauske & Associates Inc.. http://www.fauske.com. 30.04.2002 Hazard Evaluation Laboratory. http://www.helgroup.co.uk. 30.04.2002 Thermal Hazard Technology. http://www.science.org.uk. 30.04.2002 Mettler Toledo. www.mt.com. 30.04.2002 Perkin-Elmer. http://www.perkin-elmer.com/ai/ai.nsf/. 30.04.2002 PQAT Group. The NPK (non parametric kinetics). http://harsnet.iqs.url.es/pqat/Research/NPK/NPK01.htm. 30.04.2002 Setaram. http://www.setaram.com. 30.04.2002 Safetynet. http://www.safetynet.de/. 30.04.2002 TA Instruments. http://www.tainst.com. 30.04.2002 Thermal Analysis, Calorimetry and Rheology. Information Central http://www.bkpublishing.com/. 30.04.2002 Universidad de Zaragoza. Departamento de Ingeniería Química y Tecnología del Medio Ambiente. Nuevo Software de Control de un RSST. http://wzar.unizar.es/invest/sai/ins_ele/prestaci/ejemplo4.html. 30.04.2002


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http://www.fauske.com/

http://www.helgroup.co.uk/

http://www.science.org.uk/

http://www.mt.com/

http://www.perkin-elmer.com/ai/ai.nsf/

http://harsnet.iqs.url.es/pqat/Research/NPK/NPK01.htm

http://www.setaram.com/

http://www.safetynet.de/

http://www.tainst.com/

http://www.bkpublishing.com/

http://wzar.unizar.es/invest/sai/ins_ele/prestaci/ejemplo4.html

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