Page 30 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Chapter 1 Diagrams for Understanding Chemical Processes

The chemical process industry (CPI) is involved in the production of a wide variety of products that
improve the quality of our lives and generate income for companies and their stockholders. In general,
chemical processes are complex, and chemical engineers in industry encounter a variety of chemical

process flow diagrams. These processes often involve substances of high chemical reactivity, high
toxicity, and high corrosivity operating at high pressures and temperatures. These characteristics can lead
to a variety of potentially serious consequences, including explosions, environmental damage, and threats
to people’s health. It is essential that errors or omissions resulting from missed communication between
persons and/or groups involved in the design and operation do not occur when dealing with chemical
processes. Visual information is the clearest way to present material and is least likely to be
misinterpreted. For these reasons, it is essential that chemical engineers be able to formulate appropriate
process diagrams and be skilled in analyzing and interpreting diagrams prepared by others.

The most effective way of communicating information about a process is through the use of
flow diagrams.

This chapter presents and discusses the more common flow diagrams encountered in the chemical process
industry. These diagrams evolve from the time a process is conceived in the laboratory through the
design, construction, and the many years of plant operation. The most important of these diagrams are
described and discussed in this chapter.

The following narrative is taken from Kauffman [1] and describes a representative case history related to
the development of a new chemical process. It shows how teams of engineers work together to provide a
plant design and introduces the types of diagrams that will be explored in this chapter.

The research and development group at ABC Chemicals Company worked out a way to
produce alpha-beta souptol (ABS). Process engineers assigned to work with the
development group have pieced together a continuous process for making ABS in
commercial quantities and have tested key parts of it. This work involved hundreds of

block flow diagrams, some more complex than others. Based on information derived
from these block flow diagrams, a decision was made to proceed with this process.

A process engineering team from ABC’s central office carries out the detailed process
calculations, material and energy balances, equipment sizing, etc. Working with their
drafting department, they produced a series of PFDs (Process Flow Diagrams) for the
process. As problems arise and are solved, the team may revise and redraw the PFDs.
Often the work requires several rounds of drawing, checking, and revising.

Specialists in distillation, process control, kinetics, and heat transfer are brought in to
help the process team in key areas. Some are company employees and others are
consultants.