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cause flooding. Therefore fairly tight pressure control is required. This leads to large and rapid changes in the vapor distillate flowrate when it is used to control pressure. If the distillate is fed to a downstream unit, these large flowrate changes represent severe disturbances.

One of the main advantages of the CS1 structure is that the constant reflux flowrate establishes steady liquid flowrates down through the trays of the column. Changing vapor rates can be achieved fairly rapidly (in 20-30 s). Changing liquid rates takes much longer because of the hydraulic lags introduced by weirs and baffles. The rule of thumb is about 3-6 seconds per tray. So in a 30-tray column, it will take 2-3 minutes for a change in reflux flow to work its way down to the base of the column.

Control Structure CS2 Figure 8.5b presents an alternative control structure in which the condenser heat removal is fixed instead of fixing the reflux flowrate. Since pressure is controlled by manipulating the flowrate of vapor distillate, this structure has the same problem of distillate flowrate variability to a downstream unit.

This structure does not keep reflux flowrate constant, so internal liquid rates can fluctuate, which can lead to poor hydraulic performance.

Some distillation columns with partial condensers are constructed with the condenser installed at the top of the column inside the shell. There is usually no reflux drum. Vapor flows upward through the tubes of the condenser. The condensate liquid flows downward and drops into a liquid distributer above the top tray. These "dephlegmator" systems are frequently used when very toxic or dangerous chemicals are involved because they eliminate some pumps, extra vessels, and fittings and thus reduce potential leak problems.

In this type of system there are only two manipulated variables: vapor distillate leaving the top of the condenser and condenser heat removal. The absence of a reflux drum means that there is no surge capacity to attenuate disturbances. As a result, these systems have very poor dynamic disturbance behavior and should be avoided if possible.

A viable alternative is to place a large total trapout tray below the condenser that can serve as an internal reflux drum. Liquid reflux can be taken from this trapout tray and fed to the top tray through a control valve. This modified system requires additional column height, which means higher capital investment. But its dynamic controllability is much better.

Control Structure CS3 Figure 8.5c shows the third alternative control structure studied. Now condenser heat removal is used to control pressure, and reflux is used to control level.

Since the distillate flowrate cannot be held constant, the control scheme must permit it to change. This is achieved in this control structure by ratioing the distillate flowrate to the reflux flowrate.

Of course, maintaining a constant reflux ratio may or may not be the best structure to handle feed composition changes when single-end control is used in a distillation column. To address this issue, the curves shown in Figure 8.6 are generated by varying the feed composition (in terms of the light- and heavy-key components) while maintaining the purities at both ends of the column. The required changes in the reflux and reflux ratio are plotted in terms of ratios to their design values at the design feed composition (40 and 4 mol% propane, respectively, for the two cases).

Case 1: Large Distillate Flow; RR = 2.6

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