Vacuum Condensers

Outlet Temperature and Pressure. It is important to have proper subcooling in the vent end of the unit to prevent large amounts of process vapors from going to the vacuum system along with the inerts.

Control. It is necessary to have some over-surface and to have a proper baffling to allow for pressure control during process swings, variable leakage of inerts, etc. One designer adds 50% to the calculated length for the oversurface. The condenser must be considered part of the control system (similar to extra trays in a fractionator to allow for process swings not controlled by conventional instrumentation).

CHEMICAL REACTION ENGINEERING

Since before recorded history, we have been using chemical processes to prepare food, ferment grain and grapes for beverages, and refine ores into utensils and weapons. Our ancestors used mostly batch processes because scaleup was not an issue when one just wanted to make products for personal consumption.

The throughput for a given equipment size is far superior in continuous reactors, but problems with transients and maintaining quality in continuous equipment mandate serious analysis of reactors to prevent expensive malfunctions. Large equipment also creates hazards that backyard processes do not have to contend with.

Not until the industrial era did people want to make large quantities of products to sell, and only then did the economies of scale create the need for mass production. Not until the twentieth century was continuous processing practiced on a large scale. The first practical considerations of reactor scaleup originated in England and Germany, where the first large-scale chemical plants were constructed and operated, but these were done in a trial-and-error fashion that today would be unacceptable.

The systematic consideration of chemical reactors in the United States originated in the early twentieth century with DuPont in industry and with Walker and his colleagues at MIT, where the idea of reactor “units” arose. The systematic consideration of chemical reactors was begun in the 1930s and 1940s by Damkohler in Germany (reaction and mass transfer), Van Heerden in Holland (temperature variations in reactors), and by Danckwerts and Denbigh in England (mixing, flow patterns, and multiple steady states). However, until the late 1950s the only texts that described chemical reactors considered them through specific industrial examples. Most influential was the series of texts by Hougen and Watson at Wisconsin, which also examined in detail the analysis of kinetic data and its application in reactor design. The notion of mathematical modeling of chemical reactors and the idea that they can be considered in a systematic fashion were developed in the 1950s and 1960s in a series of papers by Amundson and Aris and their students at the University of Minnesota.

In the United States two major textbooks helped define the subject in the early 1960s. The first was a book by Levenspiel that explained the subject pictorially and included a large range of applications, and the second was two short texts by Aris that concisely described the mathematics of chemical reactors. While Levenspiel had fascinating updates in the Omnibook and the Minibook, the most-used chemical reaction engineering texts in the 1980s were those written by Hill and then Fogler, who modified the initial book of Levenspiel, while keeping most of its material and notation.


The major petroleum and chemical companies have been changing rapidly in the 1980s and 1990s to meet the demands of international competition and changing feedstock supplies and prices. These changes have drastically altered the demand for chemical engineers and the skills required of them. Large chemical companies are now looking for people with greater entrepreneurial skills, and the best job opportunities probably lie in smaller, nontraditional companies in which versatility is essential for evaluating and comparing existing processes and designing new processes. The existing and proposed new chemical processes are too complex to be described by existing chemical reaction engineering texts.


The first intent of this text is to update the fundamental principles of the operation of chemical reactors in a brief and logical way. We also intend to keep the text short and cover the fundamentals of reaction engineering as briefly as possible.


Second, we will attempt to describe the chemical reactors and processes in the chemical industry, not by simply adding homework problems with industrially relevant molecules, but by discussing a number of important industrial reaction processes and the reactors being used to carry them out.



Third, we will add brief historical perspectives to the subject so that students can see the context from which ideas arose in the development of modern technology. Further, since the job markets in chemical engineering are changing rapidly, the student may perhaps also be able to see from its history where chemical reaction engineering might be heading and the causes and steps by which it has evolved and will continue to evolve.




Every student who has just read that this course will involve descriptions of industrial process and the history of the chemical process industry is probably already worried about what will be on the tests. Students usually think that problems with numerical answers (5.2 liters and 95% conversion) are somehow easier than anything where memorization is involved. We assure you that most problems will be of the numerical answer type.
However, by the time students become seniors, they usually start to worry (properly) that their jobs will not just involve simple, well-posed problems but rather examination of messy situations where the boss does not know the answer (and sometimes doesn’t understand the problem). You are employed to think about the big picture, and numerical calculations are only occasionally the best way to find solutions. Our major intent in discussing descriptions of processes and history is to help you see the contexts in which we need to consider chemical reactors. Your instructor may ask you to memorize some facts or use facts discussed here to synthesize a process similar to those here. However, even if your instructor is a total wimp, we hope that reading about what makes the world of chemical reaction engineering operate will be both instructive and interesting.

CHEMICAL REACTORS

The chemical reactor is the heart of any chemical process. Chemical processes turn inexpensive chemicals into valuable ones, and chemical engineers are the only people technically trained to understand and handle them. While separation units are usually the largest components of a chemical process, their purpose is to purify raw materials before they enter the chemical reactor and to purify products after they leave the reactor. Here is a very generic flow diagram of a chemical process.

Enzyme Fermentation

The term "fermentation" can be used in its original strict meaning (to produce
alcohol from sugar-nothing else) or it can be used more or less broadly. We
will use the modern broad definition:

From the simplest to the most complex, biological processes may be classed as fermentations, elementary physiological processes, and the action of living entities. Further, fermentations can be divided into two broad groups: those promoted and catalyzed by microorganisms or microbes (yeasts, bacteria, algae, molds, protozoa) and those promoted by enzymes (chemicals produced by microorganisms). In general, fermentations are reactions wherein a raw organic feed is converted into product by the action of microbes or by the action of enzymes.

This whole classification is shown in Fig. 27.1.
      Enzyme fermentations can be represented by

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Classification of Reactions

There are many ways of classifying chemical reactions. In chemical reaction
engineering probably the most useful scheme is the breakdown according to
the number and types of phases involved, the big division being between the
homogeneous and heterogeneous systems. A reaction is homogeneous if it takes
place in one phase alone. A reaction is heterogeneous if it requires the presence
of at least two phases to proceed at the rate that it does. It is immaterial whether
the reaction takes place in one, two, or more phases; at an interface; or whether
the reactants and products are distributed among the phases or are all contained
within a single phase. All that counts is that at least two phases are necessary
for the reaction to proceed as it does.

Sometimes this classification is not clear-cut as with the large class of biological
reactions, the enzyme-substrate reactions. Here the enzyme acts as a catalyst in
the manufacture of proteins and other products. Since enzymes themselves are
highly complicated large-molecular-weight proteins of colloidal size, 10-100 nm,
enzyme-containing solutions represent a gray region between homogeneous and
heterogeneous systems. Other examples for which the distinction between homogeneous
and heterogeneous systems is not sharp are the very rapid chemical
reactions, such as the burning gas flame. Here large nonhomogeneity in composition
and temperature exist. Strictly speaking, then, we do not have a single phase,
for a phase implies uniform temperature, pressure, and composition throughout.
The answer to the question of how to classify these borderline cases is simple.
It depends on how we choose to treat them, and this in turn depends on which description we think is more useful. Thus, only in the context of a given situation
can we decide how best to treat these borderline cases.



Cutting across this classification is the catalytic reaction whose rate is altered
by materials that are neither reactants nor products. These foreign materials,
called catalysts, need not be present in large amounts. Catalysts act somehow as
go-betweens, either hindering or accelerating the reaction process while being
modified relatively slowly if at all.
Table 1.1 shows the classification of chemical reactions according to our scheme
with a few examples of typical reactions for each type.

MASS AND ENERGY BALANCES

Mass Balances (Sinnott, 1985)

Material balances are the basis of process design. A material balance taken over the complete process will determine the quantities of raw materials required and products produced. Balances over individual process units set the process stream flows and compositions. A good understanding of material balance calculations is essential in process design. Here the fundamentals of the subject are covered, using simple examples to illustrate each topic. Practice is needed to develop expertise in handling what can often become very involved calculations. More examples and a more detailed discussion of the subject can be found in the numerous specialist books written on material and energy balance computations.

Material balances are also useful tools for the study of plant operation and trouble shooting. They can be used to check performance against design, to extend the often limited data available from the plant instrumentation, to check instrument calibrations, and to locate sources of material loss.

The general conservation equation for any process system can be written as:

Material out = Material in + Generation - Consumption - Accumulation

For a steady-state process the accumulation term will be zero. Except in nuclear processes, mass is neither generated nor consumed; but if a chemical reaction takes place a particular chemical species may be formed or consumed in the process. If there is no chemical reaction the steady-state balance reduces to

Material out = Material in

A balance equation can be written for each separately identifiable species present, elements, compounds or radicals; and for the total material.

Example

2000 kg of a 5 per cent slurry of calcium hydroxide in water is to be prepared by diluting a 20 per cent slurry. Calculate the quantities required. The percentages are by weight.

Let the unknown quantities of the 20% solution and water be X and Y respectively. Material balance on Ca(OH)2

eq. a
Balance on water:
eq. 2

From equation (a) X = 500 kg
Substituting into equation (b) gives Y = 1500 kg
Chech material balance on total quantity:

X + Y = 2000
500 + 1500 = 2000, correct

to be continued...

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Overview of Chemical Reaction Engineering

Every industrial chemical process is designed to produce economically a desired
product from a variety of starting materials through a succession of treatment
steps. Figure 1.1 shows a typical situation. The raw materials undergo a number
of physical treatment steps to put them in the form in which they can be reacted
chemically. Then they pass through the reactor. The products of the reaction
must then undergo further physical treatment-separations, purifications, etc.-
for the final desired product to be obtained.
Design of equipment for the physical treatment steps is studied in the unit
operations. In this book we are concerned with the chemical treatment step of
a process. Economically this may be an inconsequential unit, perhaps a simple
mixing tank. Frequently, however, the chemical treatment step is the heart of
the process, the thing that makes or breaks the process economically.

Design of the reactor is no routine matter, and many alternatives can be
proposed for a process. In searching for the optimum it is not just the cost of
the reactor that must be minimized. One design may have low reactor cost, but
the materials leaving the unit may be such that their treatment requires a much
higher cost than alternative designs. Hence, the economics of the overall process
must be considered.
Reactor design uses information, knowledge, and experience from a variety
of areas-thermodynamics, chemical kinetics, fluid mechanics, heat transfer,
mass transfer, and economics. Chemical reaction engineering is the synthesis of
all these factors with the aim of properly designing a chemical reactor.
To find what a reactor is able to do we need to know the kinetics, the contacting
pattern and the performance equation. We show this schematically in Fig. 1.2.

Figure 1.1 Typical chemical process.



Figure 1.2 Information needed to predict what a reactor can do.

Much of this book deals with finding the expression to relate input to output
for various kinetics and various contacting patterns, or

output = f [input, kinetics, contacting]

This is called the performance equation. Why is this important? Because with
this expression we can compare different designs and conditions, find which is
best, and then scale up to larger units.



source: Octave Levenspiel

ACTIVITY COEFFICIENTS OF ACIDS, BASES, AND SALTS

This table gives mean activity coefficients at 25°C for molalities in the range 0.1 to 1.0. See the following table for definitions, references, and data
over a wider concentration range.





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source: Petr Vany´sek