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.

Raw materials from another chemical process or purchased externally must usually be purified to a suitable composition for the reactor to handle. After leaving the reactor, the unconverted reactants, any solvents, and all byproducts must be separated from the desired product before it is sold or used as a reactant in another chemical process. The key component in any process is the chemical reactor; if it can handle impure raw materials or not produce impurities in the product, the savings in a process can be far greater than if we simply build better separation units. In typical chemical processes the capital and operating costs of the reactor may be only 10 to 25% of the total, with separation units dominating the size and cost of the process. Yet the performance of the chemical reactor totally controls the costs and modes of operation of these expensive separation units, and thus the chemical reactor largely controls the overall economics of most processes. Improvements in the reactor usually have enormous impact on upstream and downstream separation processes. Design of chemical reactors is also at the forefront of new chemical technologies. The major challenges in chemical engineering involve


1. Searching for alternate processes to replace old ones,
2. Finding ways to make a product from different feedstocks, or
3. Reducing or eliminating a troublesome byproduct

The search for alternate technologies will certainly proceed unabated into the next century as feedstock economics and product demands change. Environmental regulations create continuous demands to alter chemical processes. As an example, we face an urgent need to reduce the use of chlorine in chemical processes. Such processes (propylene
to propylene oxide, for example) typically produce several pounds of salt (containing considerable water and organic impurities) per pound of organic product that must be disposed of in some fashion. Air and water emission limits exhibit a continual tightening that shows no signs of slowing down despite recent conservative political trends.