Page 73 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Figure 2.1 Input/Output Structure of the Process Concept Diagram for the Toluene Hydrodealkylation
Process

1. A single “cloud” is drawn to represent the concept of the process. Within this cloud the
stoichiometry for all reactions that take place in the process is written. The normal convention
of the reactants on the left and products on the right is used.
2. The reactant chemicals are drawn as streams entering from the left. The number of streams
corresponds to the number of reactants (two). Each stream is labeled with the name of the
reactant (toluene and hydrogen).
3. Product chemicals are drawn as streams leaving to the right. The number of streams corresponds
to the number of products (two). Each stream is labeled with the name of the product (benzene

and methane).
4. Seldom does a single reaction occur, and unwanted side reactions must be considered. All
reactions that take place and the reaction stoichiometry must be included. The unwanted
products are treated as by-products and must leave along with the product streams shown on the
right of the diagram.

2.3.2 The Input/Output Structure of the Process Flow Diagram

If the process concept diagram represents the most basic or rudimentary representation of a process, then
the process flow diagram (PFD) represents the other extreme. However, the same input/output structure is
seen in both diagrams. The PFD, by convention, shows the process feed stream(s) entering from the left
and the process product stream(s) leaving to the right.

There are other auxiliary streams shown on the PFD, such as utility streams that are necessary for the
process to operate but that are not part of the basic input/output structure. Ambiguities between process
streams and utility streams may be eliminated by starting the process analysis with an overall input/output
concept diagram.

Figure 2.2 shows the basic input/output structure for the PFD (see Figure 1.3). The input and output
streams for the toluene HDA PFD are shown in bold. Both Figures 2.1 and 2.2 have the same overall
input/output structure. The input streams labeled toluene and hydrogen shown on the left in Figure 2.1
appear in the streams on the left of the PFD in Figure 2.2. In Figure 2.2, these streams contain the reactant
chemicals plus other chemicals that are present in the raw feed materials. These streams are identified as
Streams 1 and 3, respectively. Likewise, the output streams, which contain benzene and methane, must
appear on the right on the PFD. The benzene leaving the process, Stream 15, is clearly labeled, but there
is no clear identification for the methane. However, by referring to Table 1.5 and looking at the entry for
Stream 16, it can be seen that this stream contains a considerable amount of methane. From the