Demand

Drum Level Control

to the restriction that column overrides, particularly high base level, can restrict feed flow.

Reflux Drum Level Control Via Distillate; Base Level Control Via Bottom Product (Figure 6.6)

This is one of the most commonly encountered column-control schemes. Both level control systems may be calculated by the method of Chapter 16.

Reflux/feed and steam/feed ratio controls should be provided. Top composition may be controlled by trimming the reflux/feed ratio; bottom composition by trimming the steam/feed ratio.

Reflux Drum Level Control Via Reflux; Base Level Control Via Bottom Product (Figure 6.7)

This is also a commonly encountered control scheme. Both level controls may be calculated by the method of Chapter 16; care should be taken to include the subcooled reflux effect on overhead level control.

Distillate/feed and steam/feed ratio controls should be provided. Top composition may be controlled by trimming the distillate/feed ratio and with perhaps additional feedforward to the reflux drum level controller. Base composition may be controlled by trimming the steam/feed ratio.

In many cases this scheme will require a buffer tank in the top-product line to the next piece of process equipment, unless the top product is going directly to storage.

Reflux Drum Level Control Via Distillate; Base Level Control Via Boilup (Figure 6.8)

Overhead level control may be calculated simply by the method of Chapter 16, Section 3, but base level control by boilup is very difficult. It is normally used only when the average bottom-product flow is very small. The characteristic time constant rH should be at least 15 minutes and other design factors should be as indicated in Chapter 16, Section 7. In most cases base level control by boilup requires a dynamic analysis, and perhaps supplementary plant tests. If steam flow is measured with an orifice, a square root extractor should be used.

Reflux/feed and bottom-product/feed ratio controls should be provided. Top composition may be controlled by trimming the reflux/feed ratio while base composition may be controlled by adjusting the bottom-product/feed ratio.

Reflux Drum Level Control Via Distillate; Base Level Control Via Boilup; Reflux Ratioed to Top Product

It is sometimes recommended that reflux be ratioed to distillate to diminish interactions between top and base feedback composition controls.3 If, however, reflux drum level is controlled by manipulating top product, and base level is controlled by manipulating boilup (see Figure 6.9), there will be a dynamics

Reflux Drum

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