Table Of ContentPHYSICAL PRINCIPLES OF
CHEMICAL ENGINEERING
by
DR. PETER GRASSMANN
Professor at the Swiss Federal Institute of Technology
("ETH") in Zürich
Edited by
DR. H. SAWISTOWSKI
Imperial College, London
Translated by
R. HARDBOTTLE
PERGAMON PRESS
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Copyright © 1971 H. R. Sauerländer
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First German edition 1961
First English edition 1971
Library of Congress Catalog Card No. 71-81317
Printed in Germany
08 012817 3
PREFACE TO THE FIRST GERMAN EDITION
WHEN, more than 7 years ago, I was asked if I was willing to undertake the
compilation of the first volume of the present series of books, I was fully
aware of the attraction and the difficulty of the task. The attraction was the
excitement of dealing comprehensively with a field of knowledge, the technical
and scientific importance of which is indisputable. But the difficulties grew
the more I tried to define the principles underlying chemical engineering and,
on the basis of these, classify the numerous data and material that are now
available. I often felt that the task was beyond my capabilities. But textbooks
should be written by a single individual, first so as to attain uniformity of
presentation, second so as not to discourage the reader: how can he hope to
understand what it takes a whole team of specialists to explain?
It was already difficult to define clearly the limits of the field to be described.
In most cases chemical engineering is not only concerned with open systems;
it is itself such an open system in which everything is in a state of flux. A con
siderable amount of knowledge is continually flowing into it from other
fields, and this it processes and inseminates in its own characteristic fashion,
before passing it on again to a very large number of fields of application.
Demarcation is thus very arbitrary, and in many instances my readers and
critics would probably have placed the lines of demarcation differently. My
object was to provide a foundation on which the mechanical engineer—and
in particular the future chemical engineer—the chemist, the physical chemist,
and the physicist, could all continue to build together.
With this object in view, quite different methods of representation were
adopted for the individual fields. It had to be assumed that the principles of
thermodynamics are known. They simply had to be extended a little. Chemists
and physical chemists mostly work with closed systems and rarely deal with
open ones. On the other hand the mechanical engineer knows little of multi-
component thermodynamics and chemical thermodynamics.
I have treated the kinetic theory of gases in rather more detail, as it forms an
indispensable basis, not only for vacuum engineering but also for a deeper
knowledge of the transfer of momentum, heat, and mass. At the same time I
welcomed the opportunity of demonstrating the close connection between this
field and probability calculations and statistics, fields of science whose impor
tance for chemical engineering cannot be doubted.
No doubt the opening paragraphs of Chapter 4 contain much that will be
already known to the mechanical engineer from the study of applied mechanics.
However, it appeared important to discuss here the main concepts for other
groups of readers for future reference purposes.
la* XI
Preface to the First German Edition
Xll
It hardly needs mentioning what great importance has to be attached to
the ratio between the surface and the volume of the processed particles. This
justified the devotion of a special chapter to the questions connected with
increase in the specific surface (size reduction, surface energy, particle and
drop-size distribution functions, etc.).
Although fluid dynamics has already been dealt with in other works, one
often misses in them an illustration of the many parallels which exist between
the flow of a fluid or of energy, entropy, and, with certain restrictions, also the
flow of electricity. These parallels, however, not only facilitate a comprehen
sive survey of fields which are otherwise usually treated separately, but also
provide the basis for numerous analogy operations.
The importance of model theory to the whole field of chemical engineering,
of transport processes (i.e. transfer of momentum, heat, and mass), of multi
phase flow processes and also of rheology, does not need any special mention.
Finally, in the last chapter I have tried to fit much of what has already been
said into a larger framework, and to introduce new viewpoints which could
not be considered in sufficient detail previously.
Of course many fields which have assumed fundamental importance in
various branches of chemical engineering have been treated very summarily
or not mentioned at all, such as vibration theory, measurement and control
engineering, nuclear engineering, parts of electrical engineering, electronics,
and optics. The reader will understand that we had to restrict ourselves to
those essentials of physical knowledge which are important for the topics
being discussed. In conjunction with the knowledge derived from a study of
physics, the material in this book may provide the minimum knowledge
necessary for tackling a large number of problems.
The equations are written whenever possible as dimensionally homogeneous.
Thus they can be used directly with any consistent system where the mass is
the basic quantity and, in the case of inconsistent systems, after suitable con
version of the units. In accordance with the recommendations of the VDI (Verein
Deutscher Ingenieure—Society of German Engineers), the numerical values are
mostly given in the SI (système international, § 1.6) which is related to the
CGS system. But since the chemical engineer must also be able to work in the
technical and the CGS measuring systems, so that he can converse with mecha
nical engineers of the older generations, on the one hand, and with physicists,
on the other hand, these and other systems have also been used occasionally
in the problems.
Paragraphs which presuppose a knowledge of higher mathematics are
marked with an asterisk. They can be omitted on first reading.
Problems which one has worked out for oneself remain best in memory.
For this reason, and also to give the reader the pleasure of finding things out
for himself, much of what should have been said in the text is disguised in the
form of problems. Their solution should train the reader to think for himself.
As an aid the solutions are given on pp. 819-60.
Although I have attempted to conform to the current specifications in the
symbols (cf. the list on pp. xxv et seq.) and in the names of the units, I have
Preface to the First German Edition xiii
intentionally departed from this practice at one point. In the case of the tempe
rature difference or of units related to degrees Kelvin, instead of the "degree",
as recommended, I have used the relevant symbol °C or °K. In this way a clear
distinction can be drawn between these two cases, so that, for example, the
difference between the specific heat related to a temperature difference and the
entropy related to the absolute temperature, can be clearly shown in writing, f
In addition, this leads to a more consistent terminology: if a unit related to the
volume is written with m~3 why should not a unit related to the °C be written
9
with^-1?
The judicious reader will realize that only a very small fraction of the
available literature—something in the region of 0-1-1%—could be referred
to. In the subsidiary field of atomization, which is dealt with here in a single
section, about 1300 original publications had appeared up to 1958, and now
about 1500 publications appear annually on the subject of fractionation.
Works which are frequently referred to are given in abbreviated form, for
which pp. xxi-xxiv should be consulted.
Naturally I would write the book in a different way if I had to write it
again today. Unfortunately—and this is hardly avoidable when a new survey
is being made of such a wide field—too many things will have been over
simplified. I shall always be grateful, therefore, for comments and suggestions
for improving the book.
But I am given some confidence by the fact that many of my colleagues have
kindly assisted me. Above all I would like to thank Professor Dr. O. Fuchs
(Konstanz), with whom I had frequent and long discussions on the many prob
lems of this book. The following from among my colleagues have also kindly
gone over various sections of the book and pointed out errors and omissions
in it: W. Barth (TH Karlsruhe), F. Bo§njakovic (TH Stuttgart), E. Cremer
(Innsbruck University), R. Gregorig (Technical University, Berlin), B. L. van
der Waerden (Zürich University), and H. Ziegler (ETH Zürich). I also have to
thank numerous other colleagues for their advice.
The preparation of the book would have taken far longer if I had not had
the help of my assistants in detecting omissions in the text, in checking the
problems, and in producing the drawings. I should like to thank my many
co-workers in this connection.
I was also kindly assisted by firms and publishers who gave permission for
illustrations to be reproduced or supplied photographs. The source of each
of the illustrations is mentioned below the illustration.
The field treated in this book intrudes at one and the same time into the
three well-ordered and well-explored realms: physics, mechanical engineering,
and physical chemistry. Unfortunately, the inhabitants of these three realms
speak a somewhat different language from each other, use different systems of
units, and differ in their opinions as to what is and what is not important. As
a trained physicist, working first for many years in industry and now as a
t As was suggested in the first German edition the 13th Conference on Weights and
Measures has decided to use the symbol K instead of the formerly used °K. Unfortunately
it was too late to use this new standard in the text.
XIV Preface to the First German Edition
teacher in the faculties of mechanical engineering and chemistry at the Swiss
Federal Institute of Technology (ETH) in Zürich, I have tried to learn all
three languages, but I fear that the "native" of each realm will repeatedly
recognize me as a "foreigner", despite my attempt to speak a language which
all three understand.
I hope that this book will contribute to understanding between mechanical
engineers, chemical engineers, chemists, physical chemists, and physicists,
for, as the reader can see for himself from the example of the third problem
on § 3.1, they can achieve more when they work as a team than when they
attempt to solve the problems on their own as individuals. I also hope that
this book will make some contribution to the discovery of the wonderful and
all-pervading network of natural laws in which, though it is woven from a
small number of threads only, all the threads are skilfully interlaced with
each other.
P. GRASSMANN
PREFACE TO THE ENGLISH EDITION
SINCE the first German edition was published in the Spring of 1961, it has
been necessary to revise many sections of this book in order to include the
rapid advancements being made in the field of Chemical Engineering. In
particular, paragraphs 2.8, 5.8, 6.18, 7.23, 9.31, 9.36, 10.15, 10.16, 10.17,
11.4, 12.15 have either been extensively revised or completely rewritten. All
other sections have been carefully examined and improved. The bibliographical
data, in particular, has been extended by the inclusion of over 700 new quota
tions, mostly from the English literature. Despite this extension, however,
it has not been possible to include more than about 1 °/ of the original papers
00
published in the fields discussed in this book. For example, in 1968 more
than 1000 papers were published in the field of two-phase flow alone. In other
fields, such as optimization and heat- and mass-transfer, even the number of
published books is so large that it would be impractical to list them all.
Each presentation of a given topic, starting with the fundamentals, which
is more than a purely descriptive or pragmatic representation, sets a progres
sively higher standard for the reader. If he has to design, for example, a falling
film absorber, he himself is expected to know that equations for the thickness
of the film will be found under the headings "fluid mechanics", the rate of
absorption will be treated under "mass transfer", and the wall thickness
equations appear under the heading "permissible internal pressure". In this
respect I trust to the reader who is capable of thinking and does not want
to be programmed.
I have tried, in so far as possible, to employ the most common symbols
used in the English literature. One deviation from this is that I have used an
asterisc instead of a point to indicate a flow. This deviation was considered
necessary because a point above the symbol requires a larger space between
the lines, easily gets lost in printing, and may be confused with a derivative
with respect to time. In this way Q indicates a quantity of heat, Q* a heat flow
and q* a heat flux.
I am indebted to my co-workers, Mr. H. Bode and Mr. W. Müller, who
have greatly assisted me in editing this translation, and especially to Mr.
M. McNally who has revised the galley proofs in order to eliminate some
unevennesses in writing. I am sincerely grateful for the advice given to me
by Dr. Sawistowski who has taken care of this English edition.
Zürich
xv
INTRODUCTION
CHEMICAL engineering is the science of the production and processing of
chemical products. Here the terms "production and processing" and "chemi
cal products" are to be taken very broadly. The former includes operations
of a mechanical nature such as grinding, sieving, and screening, but also the
effects of pressure and temperature, of chemical reagents, of heat and light,
and of ultrasound. The only operations excluded are those of shaping (forging,
turning, milling, etc.) and combustion processes in so far as these are carried
out in order to produce mechanical work (combustion engines, steam and gas
turbines, rockets, etc.).
The term "chemical products" should be taken very generally. It includes
stones and soils, glass and textiles, as well as foodstuffs and delicacies, road
surfacings, and effluents.
Generalizing, it can be said that chemical engineering is concerned with
technical operations of changing, not the external shape but the internal nature
of bodies.
There can scarcely be any other field where such great changes in the physical
parameters have to be taken into account. The working field of the chemical
engineer nowadays extends from about 1°K (= -272°C) to about 2·106οΚ,
and from pressures of 10~9 ata to about 10,000 atm. There is probably no other
field where the engineer has to dispense so completely with tradition and re
peatedly find new trails. But the life of an operation in the chemical industry
often amounts to only 7 or 8 years before it is supplanted by a more up-to-date
process.! 40-50% of the turnover of large chemical works comes from products
in fields which are less than 10 years old!
The viewpoint of chemical engineering is so far removed from that of the
chemist or the mechanical engineer, that there is an urgent need for demarcat
ing its boundaries in the general field of science. The chemist aims at determin
ing the molecular structure of matter, its properties, and fields of application.
The apparatus is to him only a means to an end, a shell around the object of
his interest, which is the chemical reaction. The mechanical engineer, on the
other hand, makes design calculations for machines and wants to deal with
things that move and turn rather than equipment through which something
flows. He is accustomed to thinking in terms of flows of forces, not of flows
of materials. J Neither has this need escaped the notice of university authorities.
At present there are departments of chemical engineering at almost all uni
versities in England and the U.S.A., while in German-speaking areas a dis-
t R. S. Söhngen, Chem.-Ing.-Techn. 31, 175-82 (1959).
î K. Riess, Chem.-Ing.-Techn. 30, 696-9 (1958).
xvii
XV111 Introduction
tinction is drawn between the "Verfahrensingenieur" trained at departments
of mechanical engineering, and the "chemical technologist" {Chemieingenieur)
trained in the chemistry department.
The educational possibilities in various countries have already been re
peatedly described·! The economic importance of chemical engineering springs
among other things from the fact that, e.g. in West Germany, about one-
third of the total turnover of the industry depends on processes which form
part of the field of chemical engineering in the sense in which it is used here.J
No complicated field of science can be broken up into a series of chapters
and paragraphs which follow one another like the links of a chain in one-dimen
sional succession. It is as if we were to try to describe in words a complex piece
of apparatus which can only be represented by drawings or perhaps even by
a spatial model. And in our paper-saturated era the reader will not readily
follow the advice which Schopenhauer gave in the preface to The World as
Will and Image {Die Welt als Wille und Vorstellung), of reading the book
twice, but this is the only way in which the many and various relations which
bind the parts into a whole can be recognized correctly. Thus the present author
will try to acquaint the reader with ideas and facts bordering on what he
already knows, bearing in mind that the arrangement adopted is only one of
several possibilities and at the same time informing the reader of the other
classification principles. In all there are four different ways of surveying the
subject:
1. Describing the methods of making individual materials or groups of
materials, e.g. ammonia, glass, or azo dyes. This approach to chemical
technology is the one which is familiar to the chemist.
2. Dealing with the unit operations. Here operations are grouped together
which serve the same purpose, e.g. crushing a solid material or separating
a mixture. Such unit operations are, for example, sieving, screening,
filtration, evaporation, and distillation.
3. Treatment based on the physical principles. Here, for example, in thermo
dynamics, all the processes are listed which are governed by the unordered
form of energy, heat.
4. Treatment based on the logical and mathematical principles. Here, for
example, the "similarity considerations and model laws" or all phenom
ena which can be treated with the aid of vector analysis are grouped
together.
t England: R. E. Johnstone: A survey of chemical engineering education and practice.
Trans. Inst. Chem. Engrs. 39, 263 (1961). Chemical Engineering, Course Comparison Bulletin,
Careers Research and Advisory Centre, Cambridge, 1967.
North America: O. A. Hougen, Chemical engineering education in the United States of
America, Chem, Engineer, No. 191, CE 222 (1965).
European countries: Achema Yearbook 1968/70, 66-964, published by Dechema, Frank
furt on Main.
West Germany: Verfahrenstechnik—Forschung und Lehre, Verein Deutscher Ingenieure,
Düsseldorf, 1967.
% K. Riess, loc. cit.
Introduction xix
It will easily be seen that, starting from chemical technology, which is
satisfied with the description of individual processing methods, an increasing
level of abstract thinking is demanded in surveys made according to 2, 3, or 4,
while design details recede more and more into the background. On the other
hand, on proceeding from a few basic facts, a progressively larger field can be
studied. Whereas with the 300 or so inorganic and 20,000 or so organic materials
at present produced on a large scale, a complete and general survey of chemical
technology is impossible and the treatment of all the unit operations would
require a considerable quantity of paper. On restricting oneself to the physical
principles much of what is necessary for the understanding and calculation of
a large variety of methods can be given in a short space. Of course, the logical
and mathematical approach would be still more concise, because, for example,
the same partial differential equation describes a heat conduction process, the
flow of an incompressible, in viscid liquid or the electrical field between charged
bodies. But this would lead us too far into abstract concepts. Only occasionally
in what follows shall we adopt this last viewpoint, as in the general treat
ment of the balance equation resulting from any law of conservation (§1.1,
§§ 6.4-6.6), in the survey of dimensional analysis and model theory in Chap
ter 8, and in the summarizing remarks made in the final chapter.
These four principles of classification are compared in the four columns of
Table 1.1, where—as in later similar surveys—for clarity a very large number
of details have had to be omitted. Here the first line gives the name of the
branch of study.
An attempt has been made, by analogy with the unit operations of chemical
engineering, to evolve a science of chemical unit processes. These two, opera
tions and processes, together embrace what is generally understood by chemical
engineering. Thus, depending on the oxidizing agent used, oxygen, ozone,
hydrogen peroxide, permanganate, chromic acid, etc., and the form (gas, liquid,
solid) and chemical nature of the material being oxidized, the unit process of
oxidation embraces such varied processes that the situation soon starts to
resemble once more that of chemical technology of the old style.
Historically, progress took place from chemical technology, i.e. from the
first column on the left. This survey method soon came into danger of collapsing
under the enormous number of individual facts. Moreover, because of its
nature, it remained foreign to the mechanical engineer. It was therefore a step
forward when in 1893 for the first time G. Lunge in Zürich! recognized that
a large number of operations in many production processes resemble each
other. The science of these unit operations was brought to the fore by A. D.
Little in 1915. Even today it is still of great help in classifying the overwhelming
mass of facts, especially in surveying the field. %
Since, meanwhile, the number of unit operations has risen from 30 to 50,
the problem arises as to whether this survey method should not be replaced by
a more comprehensive one. This problem is encountered when we relate these
unit operations to their physical principles.
t Quoted after Brown [T2], p. 1.
% Cf., for example, Perry [H4]; Fortschr. Verfahrenstechnik, 1952-7 (see under R2).
