Distillation dynamic compensation

Feedforward control system that provides constant separation by manipulating the distillate flow (top). At the bottom, a variety of dynamic compensators are shown, which can be used to match the "dynamic personality" of the process. 

A schematic of the stripping section of a distillation column with a ratio controller for the bottom products composition is shown in Figure 15.53. This application is similar to ratio control except that dynamic compensation is added to the measured column feed rate. If the steam flow to the reboiler were increased immediately for an increase in column feed rate, the corrective action would initially be an overcorrection. This results because, when a feed rate change occurs, it takes some time for the bottom product composition to respond. The purpose of the dynamic compensation (DC) element is to allow for the correct timing for the compensation for feed rate changes. 

As is typical of feedforward control loops, dynamic compensation is necessary to ensure that the effect of a distillate-rate change be manifest at the same time as the feed-rate change which promoted it. Because feed enters the tower at a location considerably removed from where distillate is withdrawn, their dynamic effects upon composition diff er by a corresponding amount. The response of a tower due to a change in feed rate appears as the sum of an incident and a reflected wave, just as is the case with distillate rate, but the incident path is longer and the reflected path is shorter. Figure 11.21 illustrates the difference in the length of the paths. 

In practice, since feed flow rate changes constitute by far the major disturbances to most distillation columns, there has been a tendency to concentrate on compensating for them. Further, it has been found that major improvement results from using only static feedforward the incremental improvement obtained with the various G( ) terms is comparatively small. Usually, therefore, one finds in practice a static gain term with a first-order lag or simple lead—lag dynamic compensation. It is likely, however, that more exact dynamic functions will be beneficial in at least some cases. 

The initial screen uses both DSC and DTA dynamic heating method tests to compensate for some of the problems inherent in each test. The DSC test is fast, simple, sensitive, and quantitative. It requires only a small amount of sample. The small sample size, however, can be a problem with some samples, such as distillation residues, because of a lack of sample homogeneity. Also, the only inexpensive sample containers for DSC are aluminum or stainless steel. The containers can sometimes cause problems because of chemical reactions between the sample and pan. The DTA test addresses both of these problems. It uses a 2-5 g sample and the containers are glass. Its disadvantages are that it lacks the sensitivity of the DSC and that it is not quantitative. 

Control structures CSl and CS3 drive both product compositions back close to the desired values at the new steady state. There are significant dynamic departures, particularly in the bottoms composition when using CS3. Consider the effect of the large increase in feed flow rate at time equal to 7 h. There is a dynamic 10-fold increase in the impurity of propane in the bottoms at about 7.5 h. This occurs because the increase in feed flow rate brings more light material into the column that affects bottoms composition quickly before the corresponding drop in Stage 8 temperature can increase reboiler heat input to compensate for the increase in feed flow rate. Remember that the feed is liquid, so it affects the bottoms much more quickly and drastically than the distillate. In addition, there... 

The improvement in control by the use of pressure compensation has been quantitatively demonstrated. The implementation of this type of structure in Aspen Dynamics has been outlined. A simple procedure for deriving the relationships between temperature, pressure, and composition has been illustrated. Pressure compensation should be considered in distillation columns where pressure changes at the control tray are significant. 

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