Page 147 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Chapter 5 Tracing Chemicals through the Process Flow Diagram

In Chapter 2, we classified the unit operations from a PFD into one of the six blocks of a generic block
flow process diagram. In this chapter, you gain a deeper understanding of a chemical process by learning
how to trace the paths taken by chemical species through a chemical process.

5.1 Guidelines and Tactics for Tracing Chemicals

In this chapter, guidelines and some useful tactics are provided to help you trace chemicals through a

process. Two important operations for tracing chemical pathways in PFDs are the adiabatic mixer and
adiabatic splitter.
Mixer: Two or more input streams are combined to form a single stream. This single output stream
has a well-defined composition, phase(s), pressure, and temperature.
Splitter: A single input stream is split into two or more output streams with the same temperature,
pressure, and composition as the input stream. All streams involved differ only in flowrate.

These operations are found where streams meet or a stream divides on a PFD. They are little more than
tees in pipelines in the plant. These operations involve little design and minimal cost. Hence, they are not
important in estimating the capital cost of a plant and would not appear on a list of major equipment.
However, you will find in Chapter 13 that these units are included in the design of flowsheets for
implementing and using chemical process simulators.

We have highlighted the mixers and splitters as shaded boxes on the flow diagrams presented in this
chapter. They carry an “m” and “s” designation, respectively.

5.2 Tracing Primary Paths Taken by Chemicals in a Chemical Process

Chemical species identified in the overall block flow process diagram (those associated with chemical
reactions) are termed primary chemicals. The paths followed by primary chemicals between the reactor
and the boundaries of the process are termed primary flow paths. Two general guidelines should be
followed when tracing these primary chemicals.
1. Reactants: Start with the feed (left-hand side of the PFD) and trace chemicals forward toward

the reactor.
2. Products: Start with the product (right-hand side of the PFD) and trace chemicals backward
toward the reactor.

The following tactics for tracing chemicals apply to all unit operations except for chemical reactors.
Tactic 1: Any unit operation, or group of operations, that has a single or multiple input streams and a
single output stream is traced in a forward direction. If chemical A is present in any
input stream, it must appear in the single output stream (see Figure 5.1[a]).
Tactic 2: Any unit operation, or group of operations, that has a single input stream and single or