Page 72 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Finally, it is important to recognize the role of pilot plants in the development of processes. It has been

long understood that what works well in the laboratory often does not work as well on the large scale. Of
course, much of the important preliminary work associated with catalyst development and phase
equilibrium is most efficiently and inexpensively completed in the laboratory. However, problems
associated with trace quantities of unwanted side products, difficult material handling problems, and
multiple reaction steps are not easily scaled up from laboratory-scale experiments. In such cases, specific
unit operations or the entire process may be “piloted” to gain better insight into the proposed full-scale
operation. Often, this pilot plant work is carried out in batch equipment in order to reduce the inventory of
raw materials. Sometimes, the pilot plant serves the dual purpose of testing the process at an intermediate
scale and producing enough material for customers and other interested parties to test. The role and
importance of pilot plants are covered in detail by Lowenstein [5].

2.3 Step 2—The Input/Output Structure of the Process

Although all processes are different, there are common features of each. The purpose of this section is to
investigate the input/output structure of the process. The inputs represent feed streams and the outputs are
product streams, which may be desired or waste streams.

2.3.1 Process Concept Diagram

The first step in evaluating a process route is to construct a process concept diagram. Such a diagram uses
the stoichiometry of the main reaction pathway to identify the feed and product chemicals. The first step to
construct such a diagram is to identify the chemical reaction or reactions taking place within the process.
The balanced chemical reaction(s) form the basis for the overall process concept diagram. Figure 2.1
shows this diagram for the toluene hydrodealkylation process discussed in Chapter 1. It should be noted
that only chemicals taking place in the reaction are identified on this diagram. The steps used to create

this diagram are as follows.