Page 69 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Chapter 2 The Structure and Synthesis of Process Flow Diagrams

When looking at a process flow diagram (PFD) for the first time, it is easy to be confused or
overwhelmed by the complexity of the diagram. The purpose of this chapter is to show that the evolution
of every process follows a similar path. The resulting processes will often be quite different, but the

series of steps that have been followed to produce the final processes are similar. Once the path or
evolution of the structure of processes has been explained and understood, the procedure for
understanding existing PFDs is also made simpler. Another important benefit of this chapter is to provide
a framework to generate alternative PFDs for a given process.

2.1 Hierarchy of Process Design

Before discussing the steps involved in the conceptual design of a process, it should be noted that often
the most important decision in the evolution of a process is the choice of which chemical syntheses or
routes should be investigated to produce a desired product. The identification of alternative process
chemistries should be done at the very beginning of any conceptual design. The conceptual design and
subsequent optimization of a process are “necessary conditions” for any successful new process.
However, the greatest improvements (savings) associated with chemical processes are most often due to
changes, sometimes radical changes, to the chemical pathway used to produce the product. Most often,
there are at least two viable ways to produce a given chemical. These alternative routes may require
different raw materials and may produce different by-products. The cost of the raw materials, the value of

the by-products, the complexity of the synthesis, and the environmental impact of any waste materials and
pollutants produced must be taken into account when evaluating alternative synthesis routes.

Douglas [1,2], among others, has proposed a hierarchical approach to conceptual process design. In this

approach, the design process follows a series of decisions and steps. The order in which these decisions
are made forms the hierarchy of the design process. These decisions are listed as follows.
1. Decide whether the process will be batch or continuous.
2. Identify the input/output structure of the process.
3. Identify and define the recycle structure of the process.
4. Identify and design the general structure of the separation system.
5. Identify and design the heat-exchanger network or process energy recovery system.

In designing a new process, we follow steps 1 through 5 in that order. Alternatively, by looking at an
existing process, we can work backward from step 5 and eliminate or greatly simplify the PFD and,
hence, reveal much about the structure of the underlying process.

Let us start with this five-step design algorithm and see how it can be applied to a chemical process. Each
of the steps is discussed in some detail, and the general philosophy about the decision-making process
will be covered. However, because steps 4 and 5 require extensive discussion, these will be covered in
separate chapters (Chapter 12 for separations, and Chapter 15 for energy recovery).