Figure 317

Column pressure control with flooded condenser is often easier to maintain, which is especially important if fouling and corrosion are problems. If an overhead condenser is used, a more expensive column-supporting structure is required, particularly if the overhead surge drum is also located at the top of the column. The support problem can be minimized, however, by building the condenser into the top of the column. But one also needs a higher head cooling-water pump, and it is harder to remove the condenser tube bundle for maintenance. If, however, one uses the arrangement of direct return reflux and overflow distillate, then the reflux does not come from the overhead surge drum and this vessel can be located at ground level. Since this tank with its contents is often far heavier than the condenser, the condenser can be located overhead with only a modest increase in structural requirements over a ground-located condenser. Overall a properly designed gravity-flow reflux system is significandy cheaper than a pumped-back reflux system; it is also probably safer since there is no pump to fail. For all gravity-return reflux systems, one must be careful to design the vapor piping and condenser to have a low pressure drop compared with the difference in head between the point of reflux return to the column and the condensate receiver liquid level.

Reflux Flow or Flow-Ratio Control

When reflux is flow or flow-ratio controlled, piping and instrumentation can be very simple. Perhaps the most common arrangement is that of Figure

3.18. Here reflux drum level is controlled by throttling distillate flow. A disadvantage, unless the drum has a large cross section, is that variations in level will cause momentary changes in both reflux and distillate flows. The reflux flow or flow-ratio controller will usually be fast enough that this will not be a problem for reflux flow. The level controller, on the other hand, may have to be cascaded to distillate flow control.

To avoid this problem, one may design a distillate overflow system that provides constant head for reflux. Consider, for example, the scheme of Figure

3.19, which features a vapor—liquid disengagement space built into the lower section of a vertical-tube, coolant-in-shell condenser. For maximum effectiveness the liquid pool in the vapor—liquid disengagement space should have a large cross-sectional area, and the overflow weir should permit a wide range of overflows with only a small change in head. Then, with head across the reflux line fixed, flow will vary only when the valve position is changed. For this application a valve with linear trim will have a linear installed characteristic if line drop is negligible; that is, a plot of reflux flow versus valve stem position will be a straight line. If the individual valve is shop calibrated, then valve stem position can be accurately related to reflux flow.

Use of this technique leads to the arrangement of Figure 3.20. Since the surge tank needs a level controller, there is no savings in instrumentation, but the equipment that needs to be installed at a high elevation is minimized.

Another method of controlling gravity return reflux is shown in Figure 3.21. Here a reflux flow measurement is coupled through a controller to a distillate valve. When this valve is pinched, it backs liquid up into the receiver, causing more reflux to overflow. An elegant way of doing this is to cause reflux to overflow through a Sutro weir, as shown in Figure 3.22. The Sutro weir has the advantage of being a linear weir.6

Distillate Flow or Flow-Ratio Control

For those columns with gravity return reflux, a severe oscillation in overhead vapor flow to the condenser is sometimes encountered. This is commonly called "reflux cycle" and has a typical period of several minutes. It has been observed primarily in columns where reflux flow is the difference between rate of condensation and distillate flow rate; that is, where distillate is on automatic flow control or column-composition control. Reversing the controls—that is, employing automatic reflux flow or flow-ratio control and allowing distillate to be the difference flow—provides a positive cure.

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