Page 257 - Chemical engineering design

PIPING AND INSTRUMENTATION

Figure 5.21. Ratio control 233

Distillation column control is discussed in detail by Parkins (1959), Bertrand and Jones
(1961) and Shinskey (1984) Buckley et al. (1985).
Column pressure is normally controlled at a constant value. The use of variable pressure
control to conserve energy has been discussed by Shinskey (1976).
The feed flow-rate is often set by the level controller on a preceding column. It can be
independently controlled if the column is fed from a storage or surge tank.
Feed temperature is not normally controlled, unless a feed preheater is used.
Temperature is often used as an indication of composition. The temperature sensor
should be located at the position in the column where the rate of change of temperature
with change in composition of the key component is a maximum; see Parkins (1959).
Near the top and bottom of the column the change is usually small. With multicomponent
systems, temperature is not a unique function of composition.
Top temperatures are usually controlled by varying the reflux ratio, and bottom temper-
atures by varying the boil-up rate. If reliable on-line analysers are available they can be
incorporated in the control loop, but more complex control equipment will be needed.
Differential pressure control is often used on packed columns to ensure that the packing
operates at the correct loading; see Figure 5.22d (see p. 234).
Additional temperature indicating or recording points should be included up the column
for monitoring column performance and for trouble shooting.

5.8.8. Reactor control
The schemes used for reactor control depend on the process and the type of reactor. If a
reliable on-line analyser is available, and the reactor dynamics are suitable, the product
composition can be monitored continuously and the reactor conditions and feed flows
controlled automatically to maintain the desired product composition and yield. More
often, the operator is the final link in the control loop, adjusting the controller set points
to maintain the product within specification, based on periodic laboratory analyses.
Reactor temperature will normally be controlled by regulating the flow of the heating
or cooling medium. Pressure is usually held constant. Material balance control will be
necessary to maintain the correct flow of reactants to the reactor and the flow of products
and unreacted materials from the reactor. A typical reactor control scheme is shown in
Figure 5.23 (see p. 235).