Page 148 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

multiple output streams is traced in a backward direction. If chemical A is present in
any output stream, it must appear in the single input stream (see Figure 5.1[b]).
Tactic 3: Systems such as distillation columns are composed of multiple unit operations with a
single input or output stream. It is sometimes necessary to consider such equipment
combinations as blocks before implementing Tactics 1 and 2.

Figure 5.1 Tactics for Tracing Chemical Species

When tracing chemicals through a PFD, it is important to remember the following:

Only in reactors are feed chemicals transformed into product chemicals.

You may occasionally encounter situations where both reactions and physical separations take place in a
single piece of equipment. In most cases, this is undesirable but unavoidable. In such situations, it will be
necessary to divide the unit into two imaginary, or phantom, units. The chemical reactions take place in
one phantom unit, and the separation in the second phantom unit. These phantom units are never shown on
the PFD, but we will see that such units are useful when building a flowsheet for a chemical process
simulator (see Chapter 13).

We demonstrate these guidelines in Example 5.1, by determining the paths of the primary chemicals in the
toluene hydrodealkylation process. The only information used is that provided in the skeleton process
flow diagram given in Figure 1.3.

Example 5.1