FIG 11.23. Feedforward control without dynamic compensation produces an S-shaped step response. 0 in .NET

Make QR Code ISO/IEC18004 in .NET FIG 11.23. Feedforward control without dynamic compensation produces an S-shaped step response. 0

FIG 11.23. Feedforward control without dynamic compensation produces an S-shaped step response. 0
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FIG 11.24. The desired response can be modeled reasonably well with two lags and a lead. 0
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In fact, the compensation shown in Fig. 11.24 can then be achieved with a lag in the forward loop. This is especially desirable when feedforward control is applied to a train of closely coupled towers. When bottoms flow is the manipulated variable, as in the alternate arrangement mentioned earlier, the dynamic response is different.2 Without dynamic compensation, an increase in feed rate will cause an immediate increase in bottoms flow. This, in turn, will start bottoms level falling, thereby reducing heat input through the action of the bottoms-level controller. If bott80ms flow is but a small fraction of the feed, the effect will be reduced. But, dynamic compensation which will prevent a change in heat input is desirable. This would ordinarily take the form of a multiple lag. Whenever heat input is manipulated through a forward loop, as in Figs. 11.14 and 11.15, dynamic compensation is considerably different. If a step increase in feed rate is converted instantaneously into a proportional increase in boilup, a momentous imbalance in the vapor-liquid distribution is forced up the column, Eventually the flow of reflux will increase in order to restore balance, and when equilibrium returns, the material balance will be unaffected. The sudden increase in boilup will, however, carry bottoms product upward, lowering the level in the reboiler, until reflux flow increases commensurately. Consequently, an analyzer or temperature measurement anywhere within the column will indicate a transient overcorrection. Fortunately the remedy is simple. The dynamic element in the heatinput loop, g,,(t), should be a lag, adjusted to favor bottoms-composition regulation; the dynamic element in the distillate loop, go(L), must also be a lag, adjusted for distillate-composition control after the other is set. This arrangement is favored even when reflux is manipulated from accumulator level; the steam loop act)s as a formidable accelerating agent.
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Feedforward control loops always contain more instruments than feedback loops. So there must be some justification for their use. Feedforward loops can be designed to maintain constant product quality
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and/or to operate the process at least cost. Economic justification is different in each case. For control, justification must be based on the advantages to be gained over feedback. If there is no measurement available for feedback, there is also no contention. Optimizing programs fall into this category, because they have no measurement of profit or loss from which to feed back. If feed rate and composition are invariant, there seems to be no purpose for a forward loop. Although feed rate to a column may be on flow control, this does not mean that it is invariant-it means that the stream is only subject to intentional disturbances. Supply of feed stock must come from somewhere, and its source cannot, have infinite capacity. The smaller the supply capacity, the more often feed rate will have to be adjusted. And whether feed rate is subject to random variations or intentional set-point adjustments, it can change rapidly-far more rapidly than a feedback loop on product quality can respond. A forward loop on feed rate is very simple, just requiring a lead-lag unit and a divider between the feed-flow transmitter and the distillateflow controller, as in Fig. 11.15. The second forward loop from feed composition to distillat,e flow is much more costly and less valuable. Feed composition cannot- change instantaneously without supply sources being switched. If feed stock comes from a single source, whether a tank or another processing unit, its composition can only change at a limited rate. It is entirely possible, in many columns, that the time response of the feed source is slower than that of the product-quality feedback loop. In these cases, feedback control is quite effective in coping with variations in feed composibion. TO add a forward loop on feed composition requires an analyzer, a multiplier, and possibly a summing device. Compare Figs. 11.14 and 11.17 with 11.15. Strictly speaking, this forward loop should have its own dynamic compensation, apart from t hat chosen for the feed-rate loop. 3 But because feed-composition changes are riormally so slow, they contain but small dynamic components, so t he use of a separate compensator is hardly just#ifiable. In the absence of a feedback loop, a forward loop from feed composition is import,ant,. But if feedback is available, this forward loop is generally not warranted. The worth of fcedforward control stems from four principal contributions : 1. Consistent product quality 2. Increased recovery 3. Reduced cone.rmption of utilities 4. Reduced t,ankage requirements
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