Page 20 - The engineering of chemical reactions

4 Introduction

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.

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