Page 22 - The engineering of chemical reactions

6 Introduction

The chemical engineer almost never encounters a single reaction in an ideal single-
phase isothermal reactor, Real reactors are extremely complex with multiple reactions,
multiple phases, and intricate flow patterns within the reactor and in inlet and outlet streams.
An engineer needs enough information from this course to understand the basic concepts
of reactions, flow, and heat management and how these interact so that she or he can begin
to assemble simple analytical or intuitive models of the process.
The chemical engineer almost never has kinetics for the process she or he is working
on. The problem of solving the batch or continuous reactor mass-balance equations with
known kinetics is much simpler than the problems encountered in practice. We seldom know
reaction rates in useful situations, and even if these data were available, they frequently
would not be particularly useful.
Many industrial processes are mass-transfer limited so that reaction kinetics are
irrelevant or at least thoroughly disguised by the effects of mass and heat transfer. Questions
of catalyst poisons and promoters, activation and deactivation, and heat management
dominate most industrial processes.
Logically, the subject of designing a chemical reactor for a given process might
proceed as shown in the following sequence of steps.
bench-scale batch reactor -+ bench-scale continuous -+ pilot plant --f operating plant
The conversions, selectivities, and kinetics are ideally obtained in a small batch reactor, the
operating conditions and catalyst formulation are determined from a bench-scale continuous
reactor, the process is tested and optimized in a pilot plant, and finally the plant is constructed
and operated. While this is the ideal sequence, it seldom proceeds in this way, and the
chemical engineer must be prepared to consider all aspects simultaneously.
The chemical engineer usually encounters an existing reactor that may have been
built decades ago, has been modified repeatedly, and operates far from the conditions of
initial design. Very seldom does an engineer have the opportunity to design a reactor from
scratch. Basically, the typical tasks of the chemical engineer are to

1. Maintain and operate a process,
2. Fix some perceived problem, or
3. Increase capacity or selectivity at minimum cost.

While no single course could hope to cover all the information necessary for any of these
tasks, we want to get to the stage where we can meaningfully consider some of the key ideas.
Real processes almost invariably involve multiple reactors. These may be simply
reactors in series with different conversions, operating temperatures, or catalysts in each
reactor. However, most industrial processes involve several intermediates prepared and
purified between initial reactants and final product. Thus we must consider the flow diagram
of the overall process along with the details of each reactor.
One example is the production of aspirin from natural gas. Current industrial tech-
nology involves the steps

natural gas --f methane --f syngas + methanol + acetic acid -+ acetylsalicylic acid
Although a gas company would usually purify the natural gas, a chemical company would
buy methane and convert it to acetic acid, and a pharmaceutical company would make and
sell aspirin.