In this section, comments or suggestions regarding required holdup will be primarily from the standpoint of getting good, or at least adequate, control of the column with which the holdups are associated. Small holdups favor good composition control. But when the holdups are part of a feed system for another process step, requirements may be much greater. This is discussed in more detail in Chapter 5.
Level control in condensate receivers or reflux drums is commonly achieved by manipulating either top product flow or reflux flow. Less commonly, overhead level control is accomplished by adjusting boilup or by adjusting condenser cooling water. For the first two cases, a relatively simple control system can be used.
For maximum flow smoothing, it uses the cascade PI level-control to flow-control scheme of Figure 3.26. For this example level is maintained by throttling distillate flow. Note that the PI level controller must be enhanced with high— and low—overrides (called "auto overrides") to keep level within the vessel. (With electronic analog or microprocessor controls, an alternate design with nonlinear gain and reset may be used—see reference 12.) Since, however, there are two outflows, one must also have overrides on reflux for the same reason. The quantitative design is discussed in Chapter 16. Note that the flow measurement must be linear (or linearized) for stability reasons. Cascade control is used to eliminate flow changes caused by control-valve upstream and downstream pressure variations.
For level control via reflux-flow manipulation, it is necessary to sacrifice flow smoothing in the interest of good composition control. If a PI controller
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