Prof. Dr.-Ing. Andreas Pfennig

Department of Chemical Engineering - Products, Environment, and Processes (PEPs)

Thermal Unit Operations (Procédé de Séparation Thermique,

Thermische Trennverfahren)

Andreas Pfennig

Liège, 2017

Prof. Dr.-Ing. Andreas Pfennig

Products, Environment, and Processes

Department of Chemical Engineering

Université de Liège - Sart-Tilman

Quartier Agora

Allée du six Aout 11, Bâtiment B6c

4000 Liège

Belgique

phone: +32 (0) 4 366-3521

andreas.pfennig@ulg.ac.be

www.chimapp.ulg.ac.be

Copyright: Andreas Pfennig, 2014, 2015, 2016, 2017

where not stated otherwise

This is a preliminary version of a document to be published under creative commons

licence in the future. This current version may only be used as lecture manuscript in

my corresponding lectures.

In case you find errors or have additions of content available, I would be happy to

receive it for inclusion in this document: andreas.pfennig@ulg.ac.be

Disclaimer:

This manuscript has been carefully prepared. Nevertheless no guarantee for correct-

ness as well as for absence of errors and typing errors can be given. Especially no

liability can be claimed for equipment or processes designed based on the infor-

mation presented here. The scope of the corresponding exam covers all information

given in the lecture and exercise classes, not on the content of this manuscript.

Foreword

Foreword

This manuscript is a result of teaching two successive lectures on thermal unit opera-

tions for several years. At the same time it was realized that while similar lectures

exist at different universities the content is often differently combined to shape the

individual lectures. Additionally some preliminary introductory lecture on the whole

range of chemical engineering may exist in which e.g. 4 lecture hours are reserved

for presenting a first glimpse of thermal unit operations. It is the goal of this manu-

script to take care of all of these demands. At the same time it was realized that the

didactic presentation of the material in a lecture may significantly differ from a sys-

tematic presentation of the topic.

As a consequence of these different demands this manuscript is arranged primarily

following a systematic approach. The individual chapters on the other hand are de-

signed in a way that they can be relatively freely combined, i.e. the content is some-

times explained with significant repetition to other chapters so that the chapters can

be used independently of each other in the different lectures. Nevertheless it was

tried to keep the content concise so that this text can in principle also be used as a

reference in practice.

In order to facilitate reading derivations that may not be of general interest when us-

ing this manuscript as reference but which may be relevant when teaching the con-

tent is kept in shades of grey.

For an introduction into thermal unit operations I use e.g. the following chapters:

• introduction

• introduction to distillation

• single stage distillation

• rectification with boiling feed

• distillation column internals

For the basic course the basic chapters on distillation, solvent extraction, absorption,

adsorption and chromatography are used including the basic design strategies and

presenting the various possible types of equipment. The general considerations are

introduced along the way wherever appropriate.

Foreword

In an advanced course the reactive separation processes, the advanced design

strategies as well as multicomponent separations are treated.

Of course other sorting of the content is possible, e.g. if the processes deal with sole-

ly fluid phases or contain solids like in adsorption, chromatography, solids extraction,

drying and crystallization.

Finally it is my pleasure and duty to thank all those assisting me in compiling the con-

tent of these lectures in this manuscript. Unfortunately and honestly after all the years

of giving these lectures I don’t know any more who of my Ph.D. students and teach-

ing assistants have contributed to the originally German text and the English transla-

tion – most of them I guess. The following have contributed during the last years, so

their names have not been lost, or their names have been conserved in the data files

used: Enes Aksamija, Jan Bernd Bol, Florian Buchbender, Pornprapa Chuttrakul,

Dirk Delinski, Shabnam Ebrahimi, Marlene Fritz, Nicole Kopriwa, Yang Liu, Leo Mey-

er-Schwickerath, Hannes Noll, Anton Reiter. These and all those unnamed I whole-

heartedly thank for their contribution.

1 Introduction 1

1 Introduction

1.1 Overview

Thermal process engineering (Thermische Verfahrenstechnik) is a sub-discipline of

chemical engineering and deals with the separation of multi-component mixtures into

their components or component groups. It allows e.g. to pick desired components or

to selectively remove undesired or harmful substances from mixtures. Thermal sepa-

ration steps are common and integral part of overall production processes in various

industries, e.g. in

• chemical industry,

• petroleum industry,

• food production,

• healthcare, pharmaceuticals, and

• environmental technology.

In these different industrial areas thermal separation is e.g. used to separate the de-

sired product from a reaction mixture, specifically e.g. a pharmaceutical component

from a fermentation broth or alcohol from wine. Also harmful components can be re-

moved from a waste stream before it can be released into the environment or a suc-

cessive treatment step, e.g. nitrous oxide is removed from flue gas or a detrimental

organic component is removed before waste water is treated in a waste-water treat-

ment plant.

In order to examine an entire production process to see where thermal separation

steps find their place, it is useful to divide it into individual process steps. These indi-

vidual process steps, which are connected with each other via material flows, are

usually referred to as “unit operations” (Grundoperationen) in process engineering

and are represented by blocks in the corresponding diagrams. This subdivision into

unit operations allows to break down a variety of processes into a relatively small

number of different types of unit operations that are only connected in different ways

and treat different materials to produce the very wide variety of components generat-

ed e.g. in chemical industry.

1 Introduction 2

Fig. 1-1 shows a block-diagram for a simple chemical process consisting of a pre-

treatment step, one reactor and a separation step, the so-called downstream pro-

cessing. One essential step is obviously the chemical reaction, since there the usual-

ly desired actual change of the components takes place. Since the reagents (Edukte)

often contain components that are catalyst poisons (Katalysatorgift), i.e. harmful for

some catalysts, like in some cases H2S or CO, a first separation is required that re-

moves these components from the reagents. In the reactor the chemical reaction

generally is not complete. Two major non-idealities occur: On the one hand unreact-

ed reagents are leaving the reactor which are often separated and recycled

(Rückfühtung) for economic reasons. Also side reactions often occur that either lead

to by-products (Nebenprodukte) that have a certain value or to by-components that

are of no value and can at most be utilized thermally. Separating these components

from the major product (Hauptprodukt) is a typical task for thermal process engineer-

ing. This separation after a reactive conversion step is usually referred to as down-

stream processing.

recirculation ofunreacted raw materials

reagents

products

by-products

cleaning reactorseparation

process

wastewaste

Fig. 1-1: Schematic representation of a simple chemical process

In the separation steps often up to 80% of the overall investment and operating costs

are consumed. This shows that there is great potential for saving – or wasting –

money in this area by optimal design of the corresponding unit operations. In order to

design optimal separation steps, it is necessary to examine each separation pro-

cesses in depth using scientifically well-founded methods which will be presented in

this lecture.

Process engineering has made significant progress during the last century. While

before 1900 chemical engineering was essentially a craft, it turned into an engineer-

ing science during the first decades of the 20th century. Historically one has to keep

1 Introduction 3

in mind that chemical-engineering thermodynamics developed between 1850 and

1900 and the true nature of polymers had only been realized completely after signifi-

cant debate during the 1930th. Only after these fundamental developments chemical

engineering could develop into a fully developed engineering science. It is the goal of

this lecture to facilitate the systematic and scientifically founded design of separation

steps that eventually lead to plants like the one shown in Fig. 1-2.

Fig. 1-2: Thermal separation plants on an industrial scale (source:

http://commons.wikimedia.org/wiki/File:Oil_refinery_canada.jpg, author:

http://www.flickr.com/photos/keepitsurreal/, The Chevron oil refinery in Burnaby,

B.C., Canada).

1.2 Principles of thermal separation processes

Thermal separation processes rely on the fact that the concentrations of components

in a mixture usually differ in two coexisting phases in equilibrium (Gleichgewicht). In

order to achieve a separation of components in a mixture, this mixture is transformed

into a two-phase state. By doing so, enrichment of one or more of the components in

one of the two phases is achieved and, thus, separation is possible. If such a step is

performed several times in succession, essentially arbitrary purities can be achieved

in principle. The second phase can either be induced by a shift in conditions, e.g.

1 Introduction 4

temperature and/or pressure like in distillation, or by adding an auxiliary second

phase like in extraction and absorption.

Mass transfer, which is necessary for the enrichment, takes place through the inter-

face between the phases approaching equilibrium if enough time is available. The

starting point for a separation thus is always a non-equilibrium state.

Two things are, therefore, important when dealing with thermal separation processes

and describing the processes quantitatively:

• the position of the phase-equilibrium (see lectures on chemical-engineering

thermodynamics), which characterizes the theoretical endpoint to be achieved

by the mass-transfer across an interface and

• the mass-transfer rate leading to the system approaching equilibrium, since

engineers generally don’t want to wait arbitrarily long to achieve their goal.

One of the results obtained from the quantitative description of mass transfer is

ckAn ∆=& , (1-1)

where n& is the flux of substance (Stoffstrom) e.g. in mol/s, k the mass-transfer coef-

ficient (Stofftransportkoeffizient), A the mass-transfer area e.g. the area of the inter-

face in m² and c∆ the driving concentration gradient e.g. in mol/m³, which generally

describes the distance towards equilibrium.

In practice, a high value of n& is desirable. k is mainly determined by the material

system and flow conditions and in a given setup can only minimally be varied, e.g. via

variation of pressure or temperature, since the flow in two-phase systems can often

only be varied in a rather limited range. As a result, the operating conditions (Be-

triebsbedingungen) should be designed such that A and c∆ are maximized. How

this can be achieved depends on the thermal unit operation regarded. Generally the

interface between the two phases is maximized by dispersing one of the phases with-

in the other to form small drops, bubbles or thin films. The concentration gradient can

e.g. be maximized by operating the equipment in a counter-current fashion (Ge-

genstrom). The optimal interconnection of the equilibria as well as the specific

equipment design to maximize the interface is topic of this lecture.

1 Introduction 5

1.3 Concept of a theoretical stage

Since it is usually complicated to describe the separation performance of a unit oper-

ation based on interfacial area and mass-transfer coefficients, as a good approxima-

tion a simplified approach is generally used. This simplification relies on the concept

of theoretical stages (theoretische Trennstufe) which avoids the detailed description

of mass transfer. Today approaches are available though that account for mass-

transfer rates in the so-called rate-based approach, which will also be regarded in

this lecture.

A theoretical stage is defined as that part of an apparatus or equipment where the

leaving streams are in equilibrium. An entire piece of process equipment for separa-

tion can then be regarded as consisting of several theoretical stages which are inter-

connected by appropriate fluxes (Ströme). By introducing this concept, two things can

clearly be separated and distinguished:

• The equilibrium between the phases, knowledge of which is required for de-

scribing the behavior of each individual theoretical stage and which allows to

answer the question for the optimal arrangement and interconnection of the

equilibrium stages by the entering and leaving streams to solve the separation

task and

• the ‘amount of equipment’ required for the material realization of each theoret-

ical stage. This ‘amount of equipment’ thus characterizes the interfacial area

generated in the equipment and the mass-transfer rate.

This distinction is important as it creates a theoretical concept for investigation, which

allows relatively easy design of separation equipment. For the material system to be

separated it is only required that the relevant equilibrium is measured e.g. in a beaker

under optimally controlled conditions. This can usually relatively easily be achieved

even for systems which are not that easy to handle e.g. due to toxicity, stability or

volatility and only small quantities of the material system are required. The amount of

equipment required for the realization of a theoretical stage can then be examined

with any other suitable material system, the relevant equilibria of which are well

1 Introduction 6

known and which can easily be handled under lab conditions also in larger quantities,

where e.g. concentration analysis is simple and the system is non-toxic.

It is obvious that introducing the concept of a theoretical stage assumes that this dis-

tinction can be made in principle. Experience shows that this distinction is valid espe-

cially for distillation, which is the most common separation process, at least under

most of the conditions, and represents a relatively good first approximation for many

other separation processes. Thus large process simulation tools like Aspen or

ChemCAD also use this assumption. In distillation the ‘amount of equipment’ required

for realizing a theoretical stage is to a good degree independent of the specific nature

of the system regarded. Thus theoretical stages will also be used as theoretical basis

for the first design considerations in this manuscript. Nevertheless it should clearly be

stated that the assumptions implied in the concept of theoretical stages may fail,

even for distillation. Larger deviations are e.g. known for the distillation of aqueous

systems. Some rate-based concepts are thus also included here.

1.4 Methods of thermal process design

In order to design an optimal thermal process plant or to gain information about how

to best run a separation equipment, a mathematical model of the separation process

is required. A relatively simple model can be obtained from the so-called MESH

equations. The acronym MESH refers to

• Material balances for each component

• Equilibria between the phases (representing the theoretical stages)

• Summation of mol fractions to unity

• Heat or – more correctly – energy balances

For every separation method these equations are suitable in principle and can be

combined appropriately in order to achieve a quantitative description of the separa-

tion process.

One key ingredient in these MESH equations apparently are balances. Before being

able to set up balances one has to realize that there are quantities for which one can

set up a balance. First one observes that there exist quantities of which one can as-

1 Introduction 7

sume that they don't change, i.e. that these quantities are conserved and that this

conservation also applies across processes and process steps. For these conserved

quantities (Erhaltungsgrößen) it is obvious that setting up balances is appropriate. So

the first question is, which quantities are fundamentally conserved, since they are the

first candidates to appear in balances. Then it will be seen that actually it is sufficient

that it is possible to describe quantitatively how the quantities change at the condi-

tions regarded to be able to set up balances. Thus balances can also be set up for

non-conserved quantities as long as their change can be described.

Conserved quantities are very fundamental, since they determine the very basic

structure of our world. Emmy Noether (*23.03.1882, †14.04.1935, see Fig. 1-3), a

German mathematician, proved that any invariance in the structure of physical laws

directly relates to a conserved quantity. This Noether theorem thus means that the

following invariances in our physical laws directly force the conservation of the corre-

sponding variables:

• time invariance energy conservation,

• place invariance momentum conservation,

• orientation invariance angular-momentum conservation.

This implies in turn that the conserved quantities energy, momentum, and angular

momentum directly relate to the very structure of the physical laws describing our

world. These invariances don't mean that everything is independent of time, place,

and orientation, but that the structure of our physical laws is independent of these

parameters. I.e. a physical law found on earth can be assumed to be also valid on

the moon, on Jupiter and anywhere else in the universe. Since experience shows

that these invariances hold for physical laws, i.e. we also expect them to be valid to-

day as well as in the future thus relating to the time invariance, the corresponding

properties are strictly conserved, i.e. energy, momentum, and angular momentum are

strictly conserved quantities.

1 Introduction 8

Fig. 1-3: Emmy Noether, a German mathematician (*23.03.1882, †14.04.1935),

source: http://commons.wikimedia.org/wiki/File:Noether.jpg,

http://creativecommons.org/licenses/by-sa/3.0/deed.de

In chemical engineering and other sciences often conservation of mass m is as-

sumed, where mass is apparently not directly a conserved quantity in the list above.

How can this be understood? We know since Einstein that mass directly relates to

energy E :

2mcE = , (1-2)

with c being the speed of light. Thus mass conservation can be regarded as a spe-

cial form of energy conservation. In chemical engineering we usually regard mass

and energy conservation separately, even though they are actually linked via Eq. 1-2.

Apparently nature is structured such that matter comes in quantized quantities like

elementary particles, atoms, and molecules that are relatively invariant under typical

chemical-engineering conditions. Thus chemical-engineering processes typically

don't change atoms and elementary particles. As a consequence the energy quanti-

ties associated with them remain constant. In contrast, in nuclear physics atoms and

elementary particles are modified and the corresponding energy needs to be ac-

counted for. Since this does not happen in ordinary chemical-engineering processes,

as chemical engineers we can assume that the units, i.e. atoms and elementary par-

1 Introduction 9

ticles, remain unchanged and thus the energy associated with their mass remains

unchanged as well.

This allows splitting energy conservation into two distinct contributions, one relating

to the mass of the elementary particles and atoms – remaining unchanged in chemi-

cal engineering – and all other energetic contributions like kinetic and potential ener-

gy or the energy associated with the chemical bonds between atoms to form mole-

cules. As a consequence mass and the rest of energy can be considered separately

as conserved quantities in chemical-engineering balances.

A further simplification is possible, if chemical reactions can be excluded, because

then not only the mass remains unchanged but also the number of molecules of any

species, i.e. their amount of substance (Stoffmenge). Thus if chemical reactions can

be excluded the amount of substance of each chemical species is remaining constant

and this can be regarded in a corresponding balances. Also the mass of each indi-

vidual species remains constant under these conditions.

If chemical reactions occur and as long as nuclear reactions can be excluded – which

is the case for the majority of chemical-engineering applications – at least the atoms

remain unchanged. Thus in balances set up for each atomic species, independent of

the molecules the atoms are associated with, they remain unchanged, allowing to set

up element balances.

Summarizing we realize that there exist certain quantities that remain unchangeable

within a process – at least under certain conditions, namely:

• mass (as long as nuclear reactions can be excluded),

• mass of a component in a mixture (if nuclear and chemical reactions can be

excluded),

• amount of substance (if nuclear and chemical reactions can be excluded),

• amount of substance of a component in a mixture (if nuclear and chemical re-

actions can be excluded),

• amount or mass of atoms (if chemical reactions occur but no nuclear reac-

tions)

• energy (i.e. that fraction not associated with mass via Eq. 1-2),

• momentum, and

1 Introduction 10

• angular momentum.

As can be also seen later in this manuscript, balances can of course not only be set

up for conserved quantities, but can be applied more generally. If quantities for which

balances are to be set up change, then the change that occurs has to be known

quantitatively or some type of quantitative characterization is required for solving the

balance. As an example the amount of substance can be balanced even in case

chemical reactions occur requiring reaction rate to be quantifiable. The change due

to the chemical reaction only has to be taken into account correctly. Similarly balanc-

es can also be set up for non-conserved quantities like exergy as long as production

and destruction rates can be characterized quantitatively.

In setting up a balance one of the most essential steps is the definition of the control

volume, i.e. that volume for which the balance is formulated. What is inside this con-

trol volume is sometimes called the system. The system has to be enclosed com-

pletely and without any gap by a mental boundary, which does not belong to the sys-

tem regarded, but rather marks the boundary towards the environment. This closed

boundary thus defines exactly where the system is separated from its environment.

The boundary of the control volume is in principle a virtual boundary but often is cho-

sen as to coincide with real boundaries, i.e. the wall of a tank or a unit operation. The

control volume for a tank is usually fixed in space, but this is not a prerequisite. For

description of flowing media frequently also balances are used that move with the

flow. Thus the control volume can change with time. In this case the surface of the

control volume nevertheless has to be clearly defined – as in any case.

Thus great care should be taken to properly define the boundary of the control vol-

ume. It may happen that by proper choice of this boundary the setting up and solving

of the balances is greatly simplified. As a consequence it is wise to invest some time

for finding a good choice for the control volume, especially if some advanced balanc-

es are to be set up.

As a next step in setting up and solving the balances all changes occurring with re-

spect to this control volume are defined, i.e. changes inside as well as fluxes across

the boundary. The general structure of a balance is always similar: the change within

the control volume is related to what passes across the surface of the control volume

1 Introduction 11

and what may be produced or consumed within the control volume. Thus a balance

can be written in general terms as:

consumed is what-

produced is what

leaving is what-

entering is what

volume control the inside change

+

+

=

. (1-3)

In general it is thus to be expected that more than one contribution to each term may

occur. Thus e.g. all entering and leaving "connections" should be carefully regarded.

Of course depending on the system for which the balance is set up, some terms may

also not be present at all, which generally simplifies matters.

Balances can be set up for the absolute changes of the regarded quantity with re-

spect to the control volume as shown in Eq. 1-3 or for fluxes into or out of the control

volume, where also the so-called source and drain terms have to be formulated as

production and consumption rates. If a balance is set up for fluxes, the general for-

mulation of the balance reads

rate nconsumptio-

rate production

leaving is whatof rate-

entering is whatof rate

volume control the inside rate onaccumulati

+

+

=

. (1-4)

The fluxes in this lecture will be characterized by a dot above the symbol, i.e. a mass

flow rate is characterized by m& .

These two versions of balances are relevant, because for several separation pro-

cesses two distinctly different modes of operation are available: as batch process or

as a continuous process (Batch-Prozess, diskontinuierlicher Prozess vs. kontinuierli-

cher Prozess). With a batch process a given quantity of material is processed and

since usually the concentration changes on separation progress the transient behav-

ior needs to be described. After completion of the separation task the equipment is

emptied and the next batch filled in, the batch process starts again. In continuous

operation it is common practice to consider the process to be in steady state (sta-

tionärer Zustand). Thus, the process and state quantities are independent of time.

1 Introduction 12

The development of methods to describe transient processes is gaining importance

since these methods account for aspects such as controllability and optimum opera-

tion mode for reagent or product change which are common in some areas of indus-

try.

If the left side of Eq. 1-4 is zero this means that the amount of the balanced quantity

inside the control volume is constant, which corresponds to steady state as men-

tioned. Steady state in the strict definition means that everything in the process re-

garded is independent of time. Thus if the fluxes are evaluated, the composition or

physical properties at any position inside the system will be found to be constant and

thus independent of time. Thus if a process inside the control volume is in steady

state the left side of Eq. 1-4 vanishes exactly. The reverse conclusion is not strictly

possible, since periodic local fluctuations inside the control volume that for the entire

system compensate each other may nevertheless lead to a zero accumulation term.

I.e. the system is not in steady state but the left-hand side of Eq. 1-4 nevertheless is

zero.

If the left side of Eq. 1-4 is non-zero, the state is definitely not in steady state, i.e. un-

dergoes a transient variation. In that case time-dependent properties and variables

describing the inside of the system will result.

If a balance is set up for a conserved quantity, no generation or consumption of that

quantity can occur. I.e. if the balance is set up for the entire energy for some process

– or some other conserved quantity –, then these source and drain terms are exactly

zero. Nevertheless such contributions can occur with respect also to energy, if only a

specific form of energy is regarded, e.g. heat, where a source term may result be-

cause mechanical energy is converted into heat by friction. This shows also that

when setting up a balance some basic understanding of the fundamental processes

occurring in the control volume may be helpful.

A good example for regarding absolute changes is your purse as shown in Fig. 1-4.

The changes inside are positive, if you enter money, and negative, when you take

out some money, e.g. if you pay for something in a shop. Money in a purse often is a

more or less conserved quantity, i.e. source and drain terms are in most cases negli-

gible. Similar to your bank account you usually don't expect money to come out of the

1 Introduction 13

nowhere or to disappear without reason. But if e.g. you have a goose laying golden

eggs inside your control volume or choose to burn your money, then source and

drain terms may need to be regarded – just for visualization.

Fig. 1-4: A simple example regarding balances for absolute quantities (sources:

www.microsoft.com, www.heinzelmen.de/preise/)

In this context it should be mentioned here in passing that balances for e.g. mass m

and amount of substance n for a specific component of a mixture can be expressed

applying the overall values weighted with the appropriate fractions of that component

in the mixture, i.e.

mwm ii = , (1-5)

nxn ii = , (1-6)

where the quantities with the index i refer to the quantities with respect to component

i and their respective fractions in the mixture, namely mass fraction iw and mole

fraction ix .

1.5 Examples of thermal separation processes

The mixture to be separated usually consists of a single phase in the beginning.

Based on the principal considerations above it is thus required that a second phase

is created either by changing the phase state of the system, which is achieved e.g. by

1 Introduction 14

adding or removing energy, or by adding additional components leading to phase

separation. However, one goal of process design is that there should be as little en-

ergy or additives as possible to run the process in order to minimize process costs.

Based on the type of phases that are used for the separation each thermal separa-

tion process can be characterized as shown in Tab. 1-1 with some examples of ther-

mal separation process and exemplary products.

solid liquid gaseous

solid unusual

because of slow

mass transfer

solution and melt

crystallization

sugar, Si-wafer

adsorption, desorption

cleaning of waste water

solids extraction

brewing coffee or tea

adsorption, desorption

cleaning of exhaust gas

with activated carbon

sublimation,

desublimation

freeze drying of food,

lyophilisation of bacteria

liquid liquid-liquid extraction

pharmaceuticals from fer-

mentation broth

distillation, rectification

gasoline, diesel from

crude oil, cognac, most

used method for separa-

tion in chemical industry

absorption, desorption

cleaning exhaust gas,

gas scrubbing

gaseous no phase separation

at low pressure

Tab. 1-1: Examples of thermal separation processes

This manuscript on thermal unit operations aims at providing detailed insight into the

most common of these thermal unit operations. Based on this insight, quantitative

statements on the separation processes will be possible. Today, thermal separation

processes are often modelled using numerical methods. Nevertheless in this lecture

a strong focus will be on graphical methods, because they allow a deeper insight into

1 Introduction 15

the relations between different design parameters that would not be available only

looking at numerical schemes. Also for a first design of unit operations most often

graphical methods are used in industry even today. And finally even if the solution of

the MESH equations has been achieved numerically, the results are often represent-

ed graphically in exactly the same diagrams. This is often the simplest way to check if

the numerical result obtained really is a solution to the problem posed or if the simu-

lation went wrong, which easily can occur due to the complex and non-linear behav-

ior of the set of determining equations, which have to be solved iteratively in general.

Thus it is important to familiarize with these representations. Also short-cut methods

will be presented which allow the engineer to get a quick approximate idea of wheth-

er a process is feasible, just by evaluating simple equations on a calculator. The ac-

curacy of these short-cut methods is apparently significantly less than that of rigorous

simulations, since idealizing assumptions need to be made e.g. for the equilibrium in

order to obtain simple equations that can be solved directly and analytically. And as

already mentioned previously also some rate-based approaches will be presented in

the following chapters.

In this manuscript the most frequently used thermal unit operations will be introduced

together with typical models for describing the phenomena taking place in the corre-

sponding separation equipment. The models for individual processes are often rela-

tively similar. Thus corresponding general considerations are treated in chapter 2.

Together with the design methods also typical types of equipment will be shown in

detail with a variety of construction options as to give a broad overview over available

apparatuses.

1.6 Process intensification

In industry there is a continuous economic pressure to increase process efficiency.

One concept for reaching this goal is process intensification. The targets are cheaper

and safer processes, lower energy consumption, smaller equipment and plants, a

shorter time from first laboratory experiment to the final production and less waste

and by-products. The definition of process intensification is actually differently used in

the literature. Sometimes it is defined that the space-time-yield is increased signifi-

1 Introduction 16

cantly. Even at least 100-fold increase in space-time-yield is proposed as qualifying

measure as process intensification. Other literature refers to process intensification

already, if two previously separate process steps are combined in one apparatus giv-

ing rise to some limited increase in space-time-yield by utilizing such a so-called hy-

brid process.

Several methods for process intensification have been proposed. On the one hand,

existing processes can be improved. For example, batch processes can be intensi-

fied by change to continuous processes. To increase mixing and heat transfer the

existing conventional stirred tanks can be replaced by more intensive reactors, e.g.

micro-reactors or centrifugal forces can be applied. A higher automation can make a

process also more effective (Heck, 2006). On the other hand, the existing processes

can be replaced by completely new, more effective ones.

Looking at Fig. 1-5, where the technical maturity and the application for different sep-

aration techniques are shown, it can be deduced which areas are most promising for

process intensification. Classical separation process like rectification, solvent extrac-

tion, and absorption are technically mature and widely used in industry. In general

various models to describe their behavior exist and are used to design equipment.

Because of the maturity of these processes, further optimization will be increasingly

difficult. Thus it is more promising to regard especially those processes that are not

yet as mature. E.g. combining reactive steps with separations, like reactive extraction

or reactive distillation, so-called hybrid processes, are most promising. While such

approaches offer significant chances for process intensification also significant re-

search is required to improve maturity. Nevertheless optimizing existing 'mature' pro-

cess steps may be promising as well, because often the original design dates back

many years or even decades. Since then better design methods as well as more effi-

cient equipment design has become available.

In this course several hybrid processes and process-intensification methods are pre-

sented as well. Since the possibilities to combine process steps into one apparatus

are vast, we focus on those that today appear most relevant and promising.

1 Introduction 17

Fig. 1-5: Classical, reactive and bio-reactive separation techniques in chemical indus-

try (after Gorak, Stankiewicz, 2012)

firstapplication

industrialproduction

tech

nic

ala

pplic

atio

n

invention technical maturity optimal process

experiments models

„classical“separations

reactiveseparations

bio-reactiveseparations