Figure 149

Material balance signal flow diagram—liquid sidestream drawoff mean that high- and low-level protection by means of overrides on reflux flow would be needed. This is discussed in Chapters 9 and 16.

It is sometimes argued that where reflux flow is much greater than top-product flow, one may control top composition more easily by adjusting top-product flow than by adjusting reflux flow. Actually a little algebra will show that there is not much difference, and that the difference is against the argument rather than in favor of it. If, for example, a change in feed rate or feed composition changes overhead composition, there will be a certain change in reflux flow and another change in top-product flow required to restore the top composition. These two required changes are the same in the steady state regardless of which variable is manipulated to control top composition. Composition control via distillate (top product) has the disadvantage that no change in composition takes place until the reflux flow changes. Since reflux is controlled by level, the dynamics of the level control loop appear in the composition control loop. This means, generally, that we cannot use averaging level control; we must design for tight level control. For this reason we normally prefer to control composition via reflux.

A similar line of reasoning may be followed at the base of the column, and leads to the conclusion that we would normally prefer to control base composition by manipulating boilup. Controlling base level by steam has another disadvantage if a thermosyphon reboiler is used; interchange of inventory between column base and reboiler sometimes leads to severe dynamic problems. This is discussed in Chapters 4, 15, and 16.

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