1 A p p l i c a t i o n s in VS .NET

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the product must be manipulated to control composition, however, not pressure.
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Manipulation of Reflux
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Smooth delivery of reflux is extremely important in maintaining a steady composition profile within a column. Oscillations in flow will be propagated far down the column if they are long enough in period. This is the main reason why most columns are presently operated with a constant tlow of reflux. If dist illate flow is to be manipulated for control of composition, reflux must be a dependent variable in a,11 cases except that shown in Fig. 11.10. In operation without a flooded condenser, then, liquid level in the overhead accumulator must manipulate reflux. As mentioned in Chap. 3, control of the level of boiling and condensing liquids is complicated by transport and thermal problems. Furthermore, the need for heavy damping requires a wide proportional band, and subsequently, reset. It is therefore absolutely essential to close a loop around the control valve; otherwise hysteresis will promote an intolerable limit cycle. Either a valve positioner or a cascade flow loop will suffice for this purpose. The difficulty in controlling accumulator level through the manipulation of reflux poses another problem, however. It is actually the material balance on the top tray which determines what the composition profile will be. Changing the rate of flow of distillate being withdrawn from the accumulator has no effect on composition if the flow of reflux or vapor is not altered accordingly. In the long run, the level controller will bring this about, but the time lag of the accumulator intervenes. In this sense, the capacity of the accumulator significantly impedes composition control. If reflux flow were made to respond to the same control signal as distillate flow, the time lag of the accumulator could be eliminated. W i t h this arrangement, a decrease in reflux flow would occur simultaneously with an equal increase in distillate flow, and the accumulator level would remain stationary. But much more can be gained with such a configuratiom6 Reflux can be made to decrease more than distillate flow increases. This would cause accumulator level to rise, instead of falling as it did when reflux was left alone. The level controller will eventually return reflux to the correct steady-state value. But lead action has been introduced int o the material balance at t,he top tray, increasing the speed of the composition loop severalfold. In effect, the accumulator has been converted from a disadvantage to an advantage, from a lag to a lead. Figure 11.12 ~110~s how this is brought about.
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Distillation
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From composition controller
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Distillate
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FIG 11.12.
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This arrangement injects lead action into the composition loop.
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Reflux flow is programmed from distillate flow and the output of the level controller:
L=nz-KD
(11.13)
AS the curve in Fig. 11.12 indicates, reflux flow is given a lead-lag characteristic following distillat,e flow changes. Coefficient K sets bhe lead-lag ratio, while the lag time varies with the time constant of the accumulator and the setting of the level controller. If K is zero, as it would be in the absence of programming, reflux responds as a lag. Because Eq. (11.13) is a summation, the flow signals must be linearized. Otherwise, lead time will be different with each value of flow. In practice, coefficient K should be adjusted to minimize the period of the cornposition loop, although the actual setting is not especially critical. The fact that K has a real value is of primary concern.
Response of the Composition Loop
If a product analyzer is used for control, the response of the closed loop is considerably slower than if a temperature measurement were used. First, the analyzer would be located at one end of the tower, Tvhereas the temperature element is normally nearer the center, sensing changes in the material balance sooner. Second, an analyzer usually suffers from delay in the sampling system, and a chromntograph, in particular, exhibits delays in the separatJion of its sample. A vapor sample is recommended to minimize response time. j The dynamic response of composit ion to a change in dist illate flow exhibits considerable dead time, as is expected in a multicapacit y process. But the presence of an addit8ional feature is indicnt,ed by step-response test s. Figure 11.13 illustrates results which are typically encountered. The response is the sort which would be seen in a transmission line with
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