Feed Enthalpy Feedforward

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. 

If we can successfully measure enthalpy we next need to determine what action to take when it changes. We could simply maintain the heat balance by adjusting the reboiler duty to compensate for the change. For this we need to be able to measure reboiler duty in consistent units. Further since energy is entering the column at a different point, the liquid and vapour traffic in part of the column will change. So maintaining the heat balance is not sufficient to maintain product compositions. 

Alternatively, we could attempt to obtain the feedforward gains (AO empirically by plant testing, providing that we can introduce a disturbance into feed enthalpy. We may be able to determine K from analysis of historical data but if these were collected while tray temperature (or some other composition) control was in service then it will only be possible to model steady state behaviour. Similarly we could identify K from steady state simulation. Dynamic compensation would then have to be tuned by trial and error. 

So far we have considered schemes that control either the distillate composition or the bottoms composition but not both simultaneously. On most columns the two controllers would interact to the point of instability. There are a number of techniques available which help alleviate this problem. 

We have seen that the Ryskamp scheme largely breaks the interaction in one direction so that corrections made to the bottoms composition have little impact on the distillate. Although the converse is not true an adjustment to the reflux ratio will affect the bottoms composition, but when its controller takes corrective action it will not light the distillate composition controller. 

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In addition to the recovery of the latent heat of vapor streams, in many cases it is practical to recover part of the sensible heat in the column bottom product and steam condensate by exchange with column feed. Such schemes have been used in the chemical and petroleum industries for years. Since feed flow is typically set by level controllers or flow-ratio controllers, its flow rate will not be constant. The feed enthalpy or temperature, therefore, is apt to be variable. This may make column-composition control difficult unless one employs either feedforward compensation or a trim heater with control for constant temperature or enthalpy. (See Chapters 5 and 11.)... 

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. 

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