Page 35 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

4. Basic control loops, illustrating the control strategy used to operate the process during normal
operations, will be shown.

It is clear that the PFD is a complex diagram requiring a substantial effort to prepare. It is essential that it
should remain uncluttered and be easy to follow, to avoid errors in presentation and interpretation. Often
PFDs are drawn on large sheets of paper (for example, size D: 24″ × 36″), and several connected sheets
may be required for a complex process. Because of the page size limitations associated with this text,
complete PFDs cannot be presented here. Consequently, certain liberties have been taken in the
presentation of the PFDs in this text. Specifically, certain information will be presented in accompanying
tables, and only the essential process information will be included on the PFD. The resulting PFDs will
retain clarity of presentation, but the reader must refer to the flow summary and equipment summary tables
in order to extract all the required information about the process.

Before we discuss the various aspects of the PFD, it should be noted that the PFD and the process that we
describe in this chapter will be used throughout the book. The process is the hydrodealkylation of toluene
to produce benzene. This is a well-studied and well-understood commercial process still used today. The
PFD we present in this chapter for this process is technically feasible but is in no way optimized. In fact,

there are many improvements to the process technology and economic performance that can be made.
Many of these improvements will become evident when the appropriate material is presented. This
allows the techniques provided throughout this text to be applied both to identify technical and economic
problems in the process and to make the necessary process improvements. Therefore, as we proceed
through the text, we will identify weak spots in the design, make improvements, and move toward an
optimized process flow diagram.

The basic information provided by a PFD can be categorized into one of the following:
1. Process topology
2. Stream information
3. Equipment information

We will look at each aspect of the PFD separately. After we have addressed each of the three topics, we
will bring all the information together and present the PFD for the benzene process.

1.2.1 Process Topology

Figure 1.3 is a skeleton process flow diagram for the production of benzene (see also the block flow
process diagram in Figure 1.1). This skeleton diagram illustrates the location of the major pieces of
equipment and the connections that the process streams make between equipment. The location of and
interaction between equipment and process streams are referred to as the process topology.

Figure 1.3 Skeleton Process Flow Diagram (PFD) for the Production of Benzene via the
Hydrodealkylation of Toluene