Distillation feedforward control

Ratio and Multiplicative Feedforward Control. In many physical and chemical processes and portions thereof, it is important to maintain a desired ratio between certain input (independent) variables in order to control certain output (dependent) variables (1,3,6). For example, it is important to maintain the ratio of reactants in certain chemical reactors to control conversion and selectivity the ratio of energy input to material input in a distillation column to control separation the ratio of energy input to material flow in a process heater to control the outlet temperature the fuel—air ratio to ensure proper combustion in a furnace and the ratio of blending components in a blending process. Indeed, the value of maintaining the ratio of independent variables in order more easily to control an output variable occurs in virtually every class of unit operation. 

Figure 11.5a shows a typical implementation of feedforward controller. A distillation column provides the specific example. Steam flow to the reboiler is ratioed to the feed flow rate. The feedforward controller gain is set in the ratio device. The dynamic elements of the feedforward controller are provided by the lead-lag unit. 

For control purposes, somewhat simplified mathematical models usually are adequate. In distillation, for instance, the Underwood-Fenske-Gilliland model with constant relative volatilities and a simplified enthalpy balance may be preferred to a full-fledged tray-by-tray calculation every time there is a perturbation. In control situations, the demand for speed of response may not be realizable with an overly elaborate mathematical system. Moreover, in practice not all disturbances are measurable, and the process characteristics are not known exactly. Accordingly feedforward control is supplemented in most instances with feedback. In a well-designed system (Shinskey, 1984, p. 186) typically 90%... 

Controlling Quality of Two Products Where the two products have similar values, or where heating and cooling costs are comparable to product losses, the compositions of both products should be controlled. This introduces the possibility of strong interaction between the two composition loops, as they tend to have similar speeds of response. Interaction in most columns can be minimized by controlling distillate composition with reflux ratio and bottom composition with boil-up, or preferably boil-up/bottom flow ratio. These loops are insensitive to variations in feed rate, eliminating the need for feedforward control, and they also reject heat balance upsets quite effectively. 

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. 

The task of the lights column is to remove the light components from the recycled EDC, with chloroprene and tri-chloroethylene being the most important impurities. Therefore, a concentration-cascade scheme was implemented, with chloroprene concentration and reboiler duty as controlled and manipulated variables, respectively. The distillate to feed ratio was kept constant using feedforward control. This ratio can be used to adjust the level of tri-chloroethylene in the bottom product. The level in the condenser drum was controlled by the reflux. Note that fixing the reflux and controlling the level by distillate does not work, because the distillate rate is very small. 

Control of the heavies column is simpler. A fixed fraction of the feed is taken as bottom product, in a feedforward manner. The reboiler duty controls the level in the column sump. Note that this arrangement, which is required because of the small bottoms stream, cannot be implemented if a kettle reboiler is used. The column is operated at constant reflux, while the distillate rate controls the condenser level. 

If the distillate is fed to a downstream unit, the variability in flowrate will be a disturbance. So what can we do We can make use of feedforward control to anticipate the required changes in reflux and distillate flowrates (ratio distillate to feed with the ratio reset by the composition contoller). 

So what can we do in this case If column operation requires that we stick to this control structure, feedforward control will help to reduce the swings in distillate flowrate. However, better plantwide performance can be achieved if we can switch the control structure to one in which the vapor distillate is not used to control pressure. One possible alternative is shown in Fig. 6.27. Condenser cooling is used to control pressure, reflux flowrate controls reflux drum level (with P-only control), and the flowrate of the vapor distillate is ratioed to reflux flowrate. With this structure we allow the disturbances that the column energy... 

Consider the feedforward control of a distillation column. What kind of dynamic feedforward element will be needed lag-lead, lag only, lead only, gain only Give a rather qualitative explanation. 

V.23 Suppose that the transfer functions xd/F and xofx f for the distillation column of Problem V.22 are not precisely known. Consequently, the feedforward controllers designed in part (a) of Problem V.22 give imperfect control with a certain amount of residual difference between xD and the desired set point. Assume that td = 0.5 min, r = 1 min, and the following new expressions for the actual transfer functions ... 

Figure 2.2 Three different systems for the distillate composition control of a simple distillation column (a) feedback (b) feedforward (c) inferential.

Describe an on-line adaptive procedure for a typical feedforward control system (see Chapter 21). Do the same for the inferential control of a distillation column (see Example 22.S). 

An illustration of the use of chromatography in this industry is in the control of distillation towers. Distillation uses the difference in composition between a liquid and the vapor formed from that liquid as the basis for separation. The efficiency of the process is affected by temperature, pressure, feed composition, and feed flow-rate. Chromatography is used to monitor the composition of the feedstock and to apply feedforward control of the heat input (temperature) to the tower, or to monitor and control the composition of the product. In this latter case, the chromatograph output is simply compared with a set point, and the controller (using feedback) manipulates the temperature, pressure, or feed flow-rate by activating the appropriate final operator. Both types of distillation control are widely employed in petroleum refining. 

Obviously, all three of these control structures could be improved by using feedforward control. In CS1 and CS3, the reboiler heat input could be ratioed to the feed flow rate (with the ratio reset by the temperature controller), and in CS3, the condenser heat removal could be ratioed to the feed flow rate. Figure 8.16 illustrates that the improvement of the QR-to-F ratio provides for CS3 with the large distillate case. 

From the previous example, it was pointed out that a composition controller may need a proportional band as high as 1,000 percent. And because the period of the closed loop may be from 20 min to 2 hr, reset time of 10 min to 1 hr is commonly encountered. The integrated error caused by a load change was shown earlier to equal the product of the proportional band times reset time. Distillation is characterized by a large proportional-reset, product , compared to other processes. And because integrated error in product quality can be costly, distillation is a prime candidate for feedforward control. 

The basis for feedforward control of any mass transfer operation is the material balance. Earlier in this chapter the distillate to feed ratio was shown to be the principal factor affecting composition of either product stream. The feedforward control model is nothing more than an on-line solution to the material balance ... 

In general, feedforward control systems can be designed for multicomponent separations almost as easily as for binary separations. The first relevant question is how many product streams there are. If there are only two, distillate flow can be calculated as the sum of those components which pass overhead ... 

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 Fig. 11.25, product quality without feedforward control is seen to vary from 0.95 to 0.99, averaging 0.97, in order to respect the established minimum of 0.95. If feedforward were only to improve control by 4 1, the average could be reduced to 0.955, a reduction of 0.015. Since distillate flow and composition are inversely proportional, a reduction of 0.015 in average composition represents an increase in recovery by about the same amount ... 

Luyhen, W. L., and J. A. Gerster Feedforward Control of Distillation Columns, Ind. Eng. Chem., October, 1964. 

A particular column is fed a binary mixture containing 80 to 90 percent ght component. Distillate is to be controlled to a purity of 99.9 percent i rite the feedforward control equation assuming a constant V/F ratio. Repeat )r constant heat input. 

Notice the resemblance of Eq. (12.8) to the feedforward control equation for binary distillation. 

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