In this case, as shown in Figure 6.2, we have steam/bottom-product and distillate/bottom-product ratio controls.
In calculating reflux drum level controller settings, whether proportional only or PI, one should take care to account for reflux subcooling (see Chapter 16, Section 4). Overhead composition may be controlled by trimming the distillate/bottom-product ratio with perhaps a feedforward compensator connected into the overhead level control loop. Base composition may be controlled by trimming the steam/bottom-product ratio control.
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