Control Level with a Cascade

This requires different thinking about master and slave loops.

By Cecil L. Smith, Cecil L. Smith, Inc.

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On switching the filters, an abrupt decrease occurs in the discharge valve opening. On the time frame of the trend in Figure 5, the response of the flow controller is instantaneous. However, there's no effect on discharge flow. (The decreased valve opening merely compensates for the lower pressure drop across the filters.) In the cascade configuration, the flow controller completely isolates the level controller from any consequences from switching the filters.

 Control Performance Figure 5. Flow controller isolates level controller from any consequences when filters are switched.
Control Performance
Figure 5. Flow controller isolates level controller from any consequences when filters are switched.


CONSTANT TOTAL DISCHARGE FLOW

Up until now we've focused on maintaining as constant a discharge flow as possible given the variations in feed flow to the vessel and the capacity of the vessel to smooth these variations. Some applications, though, add a complication. In them:

• Two (or possibly more) discharge flows are present, with the vessel level controller manipulating one.

• Other factors determine the second discharge flow and that flow isn't constant.

• Any change in the second flow must lead as quickly as possible to an equal but opposite change in the flow regulated by the vessel level controller — that is, the total discharge flow must remain constant.

 Distillation Control Figure 6. Manipulating distillate flow regulates temperature in upper stage of column.
Distillation Control
Figure 6. Manipulating distillate flow regulates temperature in upper stage of column.

Distillation provides an example of such a requirement. Figure 6 illustrates the upper section of a column. The overhead vapor is totally condensed. Some of the condensate exits as distillate product; the remaining condensate returns to the column as reflux.

The control configuration consists of a temperature-to-flow cascade. Distillate flow manipulation regulates the temperature on the control stage in the upper section of the column. The temperature loop responds slowly, so providing a controller for distillate flow is advisable. Reflux manipulation regulates vessel level.

Reflux flow can't be zero; the minimum depends on the column internals. In the simple feedback configuration shown in Figure 6, you must convert this minimum flow to a corresponding valve opening and impose a minimum on the opening. The availability of a reflux flow measurement permits implementing a level-to-flow cascade. This enables imposing the minimum reflux flow via a lower limit on the reflux flow set point.

In developing control configurations for distillation, the first priority is to establish how to regulate product compositions. To maintain overhead composition at its desired value, the scheme shown in Figure 6 uses the temperature on a control stage in lieu of an overhead composition measurement. When you must control both overhead and bottoms compositions, interaction between these two loops is a given — anything that affects one composition impacts the other (a consequence of the material balances). Using a steady-state separation model, you can perform an interaction analysis to determine whether to control overhead composition using reflux flow or distillate flow. When the answer is distillate flow, the configuration in Figure 6 is appropriate.

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