Page 157 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

benzene represented in the PFD. As a last resort, reference should be made to the flow table to determine
the composition of the streams, but this fails to develop analytical skills that are essential to understand
the process.

5.6 Written Process Description

A process description, like a flow table, is often included with a PFD. When a description is not
included, it is necessary to provide a description based upon the PFD. Based on the techniques developed
in this and Chapter 1, you should be able to write a detailed description of the toluene hydrodealkylation
process. Table 5.1 provides such a description. You should read this description carefully and make sure
you understand it fully. It would be useful, if not essential, to refer to the PFD in Figure 1.5 during your
review. It is a good idea to have the PFD in front of you while you follow the process description.

Table 5.1. Process Description of the Toluene Hydrodealkylation Process (Refer to Figures 5.3 and
1.5)

Fresh toluene, Stream 1, is combined with recycled toluene, Stream 11, in the storage tank, V-101.
Toluene from the storage tank is pumped, via P-101, up to a pressure of 25.8 bar and combined with the
recycled and fresh hydrogen streams, Streams 3 and 5. This two-phase mixture is then fed through the
feed preheater exchanger, E-101, where its temperature is raised to 225°C, and the toluene is completely

vaporized. Further heating is accomplished in the heater, H-101, where the temperature of the stream is
raised to 600°C. The stream leaving the heater, Stream 6, enters the reactor, R-101, at 600°C and 25.0
bar. The reactor consists of a vertical packed bed of catalyst, down through which the hot gas stream
flows. The hydrogen and toluene react catalytically to produce benzene and methane according to the
following exothermic reaction:

The reactor effluent, Stream 9, consisting of benzene and methane produced from the reaction, along with
the unreacted toluene and hydrogen, is quenched in exchanger E-102, where the temperature is reduced to
38°C using cooling water. Most of the benzene and toluene condenses in E-102, and the two-phase
mixture leaving this exchanger is then fed to the high-pressure phase separator, V-102, where the liquid
and vapor streams are allowed to disengage.

The liquid stream leaving V-102 is flashed to a pressure of 2.8 bar and is then fed to the low-pressure
phase separator, V-103. The liquid leaving V-103, Stream 18, contains toluene and benzene with only
trace amounts of dissolved methane and hydrogen. This stream is heated in exchanger E-103 to a
temperature of 90°C prior to being fed to the benzene purification column, T-101. The benzene column,
T-101, contains 42 sieve trays and operates at approximately 2.5 bar. The overhead vapor, Stream 13,
from the column is condensed using cooling water in E-104, and the condensate is collected in the reflux
drum, V-104. Any methane and hydrogen in the column feed accumulates in V-104, and these
noncondensables, Stream 19, are sent to fuel gas. The condensed overhead vapor stream is fed from V-
104 to the reflux pump P-102. The liquid stream leaving P-102, Stream 14, is split into two, one portion