Page 47 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

crucial to the operation of the process. In addition, the reaction is exothermic, and the reactor effluent
temperature must be carefully monitored. For this reason Stream 6 (entering) and Stream 9 (leaving) have
temperature flags.

Example 1.7

The pressures of the streams to and from R-101 in the benzene process are also important. The difference
in pressure between the two streams gives the pressure drop across the reactor. This, in turn, gives an
indication of any maldistribution of gas through the catalyst beds. For this reason, pressure flags are also
included on Streams 6 and 9.

Of secondary importance is the fact that flags are useful in reducing the size of the flow summary table.
For pumps, compressors, and heat exchangers, the mass flows are the same for the input and output
streams, and complete entries in the stream table are not necessary. If the input (or output) stream is
included in the stream table, and a flag is added to provide the temperature (in the case of a heat
exchanger) or the pressure (in the case of a pump) for the other stream, then there is no need to present
this stream in the flow summary table. Example 1.8 illustrates this point.

Example 1.8

Follow Stream 13 leaving the top of the benzene column in the benzene PFD given in Figure 1.5 and in
Table 1.5. This stream passes through the benzene condenser, E-104, into the reflux drum, V-104. The
majority of this stream then flows into the reflux pump, P-102, and leaves as Stream 14, while the
remaining noncondensables leave the reflux drum in Stream 19. The mass flowrate and component
flowrates of all these streams are given in Table 1.5. The stream leaving E-104 is not included in the
stream table. Instead, a flag giving the temperature (112°C) was provided on the diagram (indicating
condensation without subcooling). An additional flag, showing the pressure following the pump, is also
shown. In this case the entry for Stream 14 could be omitted from the stream table, because it is simply the
sum of Streams 12 and 15, and no information would be lost.

More information could be included in Figure 1.5 had space for the diagram not been limited by text
format. It is most important that the PFD remain uncluttered and easy to follow in order to avoid errors
and misunderstandings. Adding additional material to Figure 1.5 risks sacrificing clarity.

The flow table presented in Table 1.5, the equipment summary presented in Table 1.7, and Figure 1.5
taken together constitute all the information contained on a commercially produced PFD.

The PFD is the first comprehensive diagram drawn for any new plant or process. It provides all of the
information needed to understand the chemical process. In addition, sufficient information is given on the
equipment, energy, and material balances to establish process control protocol and to prepare cost
estimates to determine the economic viability of the process.

Many additional drawings are needed to build the plant. All the process information required can be taken
from this PFD. As described in the narrative at the beginning of this chapter, the development of the PFD
is most often carried out by the operating company. Subsequent activities in the design of the plant are