Page 177 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

The purpose of this control valve is to reduce the pressure of the stream entering the fuel gas line from
23.9 bar to 2.5 bar. This reduction in pressure represents a potential loss of useful work due to the
throttling action of the valve. Referring to Table 6.4, we can see that when we throttle a gas, we can
recover work by using a turbine, although this may not be economically attractive. The operation of this
valve is justified because of its control function.

6.4.6 Pressure Control Valve on Stream from V-102 to V-103

The purpose of this valve is to reduce the pressure of the liquid leaving V-102. This reduction in pressure
causes some additional flashing and recovery of dissolved methane and hydrogen from the
toluene/benzene mixture. The flashed gas is separated in V-103 and sent to the fuel gas line. The purpose
of this valve is to control the pressure of the material fed to the distillation column T-101. Because the
stream passing through the valve is essentially all liquid, little useful work could be recovered from this
stream.

This completes our review of the conditions of special concern for the toluene hydrodealkylation process.

6.5 Summary

In this chapter, you learned to identify process conditions that are of special interest or concern in the
analysis of the PFD. A series of tables was presented in which justifications for using process conditions
of special concern were given. We introduced the process conditions matrix (PCM) for the toluene
hydrodealkylation process and identified all the equipment in which process conditions of special
concern existed. Finally, by comparing the process conditions from the PFD to those given in the tables,
we learned to analyze why these conditions were selected for the process and where improvements may
be made.

References

1. Walas, S. M., Chemical Process Equipment: Selection and Design (Stoneham, MA:
Butterworth, 1988).

2. Tarhan, M. O., Catalytic Reactor Design (New York: McGraw-Hill, 1983).

Short Answer Questions

1. State two common criteria for setting the pressure of a distillation column.