Suppose a batch reactor such as the one in Fig. 22.12 has a gas ingredient added to the liquid feed. As long as the reaction is proceeding normally, the gas is absorbed in the liquid (where it reacts), keeping the pressure low. However, if the reaction slows or the gas feed is greater than can be

Question

Suppose a batch reactor such as the one in Fig. [latex]22.12[/latex] has a gas ingredient added to the liquid feed. As long as the reaction is proceeding normally, the gas is absorbed in the liquid (where it reacts), keeping the pressure low.

However, if the reaction slows or the gas feed is greater than can be absorbed, the pressure will start to rise. The pressure rise can be compensated by an increase in liquid feed, but this may cause the cooling capacity to be exceeded. Describe a solution to this problem using overrides (see Chapter 16).

Accepted Answer

The intention is to run the reactor at the maximum feed rate of the gas to minimize the time cycle, but the reactor is also cooling-limited. Therefore, if the pressure controller calls for a gas flow that exceeds the cooling capability of the reactor, the temperature will start to rise. The reaction temperature is not critical, but it must not exceed some maximum temperature. The temperature controller will then take over control of the feed valve and reduce the feed rate. The output of the selector sets the setpoint of a flow controller. The flow controller minimizes the effects of supply pressure changes on the gas flow rate. So this is a cascade type control system, with the primary controller being an override control system.

In an override control system, one of the controllers is always in a standby condition, which will cause that controller to saturate. Reset windup can be prevented by feeding back the selector relay output to the setpoint of each controller. Because the reset actions of both controllers have the same feedback signal, control will transfer when both controllers have no error. Then the outputs of both controllers will be equal to the signal in the reset sections. Because neither controller has any error, the outputs of both controllers will be the same. Particular attention must be paid to make sure that at least one controller in an override control system will always be in control. If not, then one of the controllers can wind up, and reset windup protection is necessary.

Question 22.11: Suppose a batch reactor such as the one in Fig. 22.12 has a ...

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