Page 160 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Chapter 6 Understanding Process Conditions

In previous chapters, process flow diagrams (PFDs) were accepted without evaluating the technical
features of the process. The process topology and process operating conditions were provided but were
not examined. Economic evaluations were carried out, but without confirming that the process would

operate as indicated by the flow diagram.

It is not uncommon to investigate process economics based upon assumed process performance. For
example, in order to justify spending the capital to develop a new catalyst, the economics of a process

using a hypothetical catalyst with assumed characteristics, such as no unwanted side reactions, might be
calculated.

The ability to make an economic analysis of a chemical process based on a PFD is not proof

that the process will actually work.

In this chapter, you will learn how to analyze the reasons why the specific temperatures, pressures, and
compositions selected for important streams and unit operations have been chosen. Stream specifications
and process conditions are influenced by physical processes as well as economic considerations and are
not chosen arbitrarily. The conditions used in a process most often represent an economic compromise
between process performance and the capital and operating costs of the process equipment. Final

selection of operating conditions should not be made prior to the analysis of the process economics. In
this chapter, we concentrate on analyzing process conditions that require special consideration. As an
example, we do not address why a reactor is run at 600°C instead of 580°C, but rather concentrate on the
reasons why the reactor is not run at a much lower temperature, for example, 200°C. This type of analysis
leads us to question how process conditions are chosen and makes us consider the consequences of
changing these conditions.

6.1 Conditions of Special Concern for the Operation of Separation and Reactor

Systems

Process streams are rarely available at conditions most suitable for reactor and separation units.
Temperatures, pressures, and stream compositions must be adjusted to provide conditions that allow
effective process performance. This is discussed in Chapter 2, where the generic BFD was introduced
(see Figure 2.4[a]). This figure showed two feed preparation blocks: one associated with the reactor and
the second with the separation section.

We provide two generalizations to assist you in analyzing and understanding the selection of process
conditions.
• It is usually easier to adjust the temperature and/or pressure of a stream than it is to change its
composition. In fact, often the concentration of a compound in a stream (for a gas) is a
dependent variable and is controlled by the temperature and pressure of the stream.