Base level control via bottom product

Base-Level Control via Bottom-Product Withdrawal... 

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

Material balance control in direction of flow, reflux drum level control via distillate, base level control via bottom product... 

If the distillate is the demand stream, then base level control via bottom product and overhead level control via boilup will not control the column properly. This scheme is analogous to the preceding sdieme. 

COLUMN-BASE LEVEL CONTROL VIA BOTTOM-PRODUCT... 

Bottom-Product Demand Base-Level Control Via Feed... 

Bottom product demand, overhead level control via top product, base level via feed... 

In a recent project we had considerable difficulty with base level control via feed accompanied by steam/feed ratio control. In effect the level controller adjusts both feed and steam. Since bottom produrt was the demand flow, this was corrected by converting the steam/feed ratio control to steam/bottom-product ratio control. 

Step 7. Methane is purged from the gas recycle loop to prevent it from accumulating, and its composition can be controlled with purge flow. Diphenyl is removed in the bottoms stream from the recycle column, where steam flow controls base level. Here we control composition (or temperature with the bottoms flow. The inventory of benzene is accounted for via temperature and overhead receiver level control in the product column. Toluene inventory is accounted for via level control in the recycle column overhead receiver. Purge flow and gas-loop pressure control account for hydrogen inventory. 

Level control of column base via bottom product throttling, cascade arrangement... 

As indicated earlier, reflux drum level control via boilup is difficult unless a lot of holdup is available. Overhead composition may be controlled by trimming the reflux/distillate ratio base composition may be controlled by trimming the bottom-product/distillate ratio. Note that if either distillate or bottom product (or side product) is a demand flow, either reflux drum or base level control must manipulate feed rate. 

Reflux via reflux drum level control bottom product via base level control... 

Once the large internal flow rates have been set via appropriate control laws, the index of the DAE system (7.21) is well defined, and a state-space realization (ODE representation) of the slow subsystem can be derived. This representation of the slow dynamics of the column can be used for the derivation of a model-based nonlinear controller to govern the input-output behavior of the column, namely to address the control of the product purity and of the overall material balance. To this end, the small distillate and bottoms flow rates as well as the setpoints of the level controllers are available as manipulated inputs. 

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