Direct and Indirect MB Control

Indirect MB control. Here, the composition (or temperature) controller does not directly regulate a material balance (i.e., a product) stream. Instead, it regulates reflux, boilup, or condensation. TTie product streams are controlled by level or pressure. Adjustments to the material balance are thus performed indirectly, i.e., by working through the pressure or levels. This becomes apparent when the stepwise action of the scheme is considered. The stepwise action of the indirect scheme in Fig. 16.4a is described in Sec. 16.3. 

Two indirect MB control schemes are common (Fig. 16.4a,6). For columns producing two liquid products, both products are on level control. In the Fig. 16.4a scheme, the composition (or temperature) controller regulates boilup, with reflux being the free stream. In the Fig. 16.46 scheme, these controls are interchanged. 

When the top product is vapor, it is on pressure control, with the accumulator level controlling the condensation rate. The Fig. 16.4c 

Direct MB control. Here the composition (or temperature) controller directly regulates a material balance (i.e., product) stream. The other product is regulated by a level or by pressure. The stepwise action of the Fig. 16.4d scheme is as follows. Suppose the concentration of lights rises in the column feed. This will be sensed by a drop in column temperature, and the temperature controller will increase distillate flow. Accumulator level will fall, and the level controller will reduce reflux. This will lower the bottom level, and the level controller will reduce bottom flow. 

One direct MB control scheme (Fig. 16.4d) is common. This scheme is similar to the Fig. 16.46 scheme, but with the reflux and product controls interchanged. The Fig. 16.4d scheme is suitable for two liquid products. An analogous scheme for a vapor top product requires that the composition controller regulate top product flow, a function better performed by the pressure controller. For this reason, a scheme analogous to the Fig. 16.4d scheme is seldom used with a vapor top product. 

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