Feed Composition Feedforward

Feedforward on feed composition can be a valuable enhancement but may not be practical. Firstly it requires an on-stream analyser on feed. Few plant owners would install this as standard and there may not be sufficient economic justification to add it later. Secondly it may not be possible to acquire an analyser that responds quickly enough. If the change in feed composition affects tray temperatures and/or inferentials before being reported by the analyser then the feedback controller(s) will take corrective action. A delayed measurement of feed composition would then be less valuable than having no measurement. 

However, if the feed is produced by an upstream imit and routed directly to the column (i.e. not via storage), it may be possible to develop an inferential based on the operating conditions in that unit. 

If we have either version of the material balance scheme then feedforward to distillate flow or bottoms flow is straightforward. Since these are linearly rated to LKf. Differentiating Equation (12.25) 

The situation is more complex if the energy balance scheme is installed. The sign of the gradient reverses for both MVs - reboil and reflux. And the reversal does not occur at the same feed composition. Care needs to be taken therefore that, for the range of possible feed compositions, it is reasonable to assume a linear relationship. 

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Another difficulty may be in the determination of the feedforward gain (K) that should be used. Unlike feed rate feedforward, feed composition feedforward requires a bias not a ratio algorithm and so K is not 1 (see Chapter 6 for explanation). Figure 12.114 shows how each of the possible MVs should be adjusted as feed composition changes. The shape of these lines was explained earlier in the chapter (see Figure 12.68). K is the gradient of the line. 

The disturbance must be detected. If we cannot measure it, we cannot use feedforward control. This is one reason why feedforward control for throughput changes is commonly used, whereas feedforward control for feed composition disturbances is only occasionally used. The former requires a flow-measurement device, which is usually available. The tatter requires composition analyzer, which may or may not be available. 

Whenever the feed composition is unpredictable, one must directly control the compositions of both products. The main benefit of dual composition control is minimized energy consumption. The main limitation is caused by the interactions between the two composition loops. On the left of Figure 2.93 an example of a feedforward dual composition control system is shown. In this configuration, the distillate flow is manipulated to control the distillate composition by maintaining the relationship ... 

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. 

In terms of composition control the performance of this version of the scheme is unlikely to be distinguishable from that shown in Figure 12.64. The additional complexity therefore might not be justified. But while it is still not correct to keep the modified reflux ratio constant as feed composition changes. Figure 12.70 shows a more linear relationship. This would be helpful if feedforward on feed composition was being contemplated. 

Feed enthalpy feedforward is, rather fortunately, not often of great benefit. Changes in enthalpy are usually small compared to reboiler duty. If feedforward is justified then measuring enthalpy may present a problem. Provided the feed is below its bubble point or above its dew point then, provided its composition and pressure are reasonably constant, it is sufficient to use temperature as the DV. However if the feed is partially vaporised a measurement of wetness is not possible. Since change in wemess represents a large change in enthalpy it is probably not realistic to make any assumption about its value. 

To illustrate this approach the polymerization process previously described in Figure 12.33 is considered. The presence of the recycle stream introduces disturbances in the reactor feed which perturb the polymer properties. The objective of the feedforward control is to compensate for these disturbances by manipulating the fresh feeds in order to maintain constant feed composition and flow to the reactor. Feedforward control of the recycle allows the designer to separate the control of the reactor from the rest of the process. 

Feedforward controller, previously discussed, which maintains the total monomer feed rate and monomer A feed composition at the appropriate set points for the specific polymer grade being produced. The feedforward controller can be easily extended to n monomers by specifying the total monomer reactor feed rate and the feed composition for n — 1 monomers. 

The A s represent departures from average operating conditions. The constants Kfi Kfi may be cdculated approximately by the column designer. Luyben has shown that it is necessary to be quite careful in designing feedforward compensation for feed composition changes, particularly when the column is not making a sharp separation. 

It should be noted, too, that feedforward from feed composition may not be needed if the feed comes from a process step with discharge composition control. Feedforward compensation for other process variables, such as bottom product or distillate demand flow, is discussed in Chapter 6. 

Our use of feedforward compensation for distillation columns curroitiy is based almost entirely on feed-rate changes. Thus we have reflux-to-feed, steam-to-feed, and sometimes other ratio-control systems. But there are at least three other variables that can be important feed composition, feed enthalpy or q, and column pressure. 

Column gains for feedforward compensation. For example, if feed composition changes, what changes in boilup and reflux do we need to hold terminal compositions constant Steady-state accuracy is required. 

For any of the control systems discussed—Figure 19.2, 19.3, or 19.4— assume that top composition and bottom composition are held constant. We wish to find the changes in Lk, D, B, and V, required to hold compositions constant in the face of a feed composition change, Azp. This information could be used to design feedforward compensators to minimize transioit changes in terminal compositions. The variables chosen for feedforward compensation will depend on which feedback control scheme is used—Figure 19.2,19.3, or 19.4. 

Since holding interior tray temperatures constant does not hold terminal compositions constant in the face of feed-composition changes, it has occurred to some authors that one should vary interior tray temperatures as feed composition changes. Luyben, for example, has su ested a feedforward scheme for changing temperature controller set points. 

Df(z) can be determined from equation (21.13) once the nature of the load and the process model are known. Let us calculate the feedforward algorithm for a feed-composition disturbance from the infexmation in Table 21.1 for a step-load change. 

The nonlinear simulation was used to illustrate the closed-loop response of the controlled variable X2 following a 30 percent increase in feed composition. The results are shown in Figure 21.4b with the feedback-only dual and PID algorithms. Control is immensely improved with the feedforward action. The slight deviation in X2 with feedforward control is due to inaccuracies in the linear model and the long sampling time relative to the process dead time. The... 

Consider the blending system of Section 15.3, but now assume that a pneumatic control valve and an I/P transducer are used. A feedforward-feedback control system is to be designed to reduce the effect of disturbances in feed composition xi on the controlled variable, product composition X. Inlet flow rate 1V2 can be manipulated. Using the information given below, design the following control systems and compare the closed-loop responses for a +0.2 step change in xi. 

Ratio control is a simple form of feedforward control that is commonly employed in controlling reactor feed compositions and in blending operations. It is also used to control the fuel-to-air ratio in heaters and boilers and to control the reflux ratio in distillation columns. The flow rate of one stream is used to provide the set point for another stream so that that ratio of the two streams is kept constant even if the flow of the first stream varies. Alternatively, the actual ratio between two flows can be used as the input to a controller. 

We want to use a feedforward controller to make adjustments in steam flow to the reboiler, whenever the feed rate to the column changes, so that bottoms composition is held constant. The feedforward-controller design equation [Eq. (11.28)] gives... 

Figure 11.5h shows a combined feedforward-feedback system where the feedback signal is added to the feedforward signal in a summing device. Figure 11.Sc shows another combined system where the feedback signal is used to change the feedforward controller gain in the ratio device. Figure 11.6 shows a combined feedforward-feedback control system for a distiltetion column where feed-rate disturbances are detected and both steam flow and reflux flow arc changed to hold both overhead and bottoms compositions constant. Two feedforward controllers are required.

One of the most remarkable features of feedforward networks is the possibility to approximate, with an arbitrarily prescribed precision, even extremely complicated and extremely general dependencies [36-43]. In catalysis we are primarily interested in dependencies of catalyst performance, expressed as products yields, catalyst activity, conversion of feed molecules and products selectivity, on composition of the catalysts, their physical properties, and on reaction conditions. It is for the approximation of such dependencies that artificial neural networks have been used in catalysis so far [15-22]. 

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. 

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). 

It is clear that our first reaction is to use a composition analyzer to measure the concentration of pentane in the distillate and then using feedback control to manipulate the reflux ratio, so that we can keep the distillate 95% in pentane. This control scheme is shown in Figure 2.2a. An alternative control system is to use a composition analyzer to monitor the concentration of pentane in the feed. Then in a feedforward arrangement we can change the reflux ratio to achieve our objective. This control... 

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