Feedforward control material balance

The example simulation THERMFF illustrates this method of using a dynamic process model to develop a feedforward control strategy. At the desired setpoint the process will be at steady-state. Therefore the steady-state form of the model is used to make the feedforward calculations. This example involves a continuous tank reactor with exothermic reaction and jacket cooling. It is assumed here that variations of inlet concentration and inlet temperature will disturb the reactor operation. As shown in the example description, the steady state material balance is used to calculate the required response of flowrate and the steady state energy balance is used to calculate the required variation in jacket temperature. This feedforward strategy results in perfect control of the simulated process, but limitations required on the jacket temperature lead to imperfections in the control. 

The feedforward system imposes an external material balance as well as a an internal material balance on the process. The internal balance is maintained by liquid level control on the discharge of each effect. Analysis of a level loop indicates that a narrow proportional band (less than 10%) can achieve stable control. However, because of the resonant nature of the level loop which causes the process to oscillate at its natural frequency, a much lower controller gain must be used (proportional bands 50-100%). A valve positioner is recommended to overcome the nonlinear nature of valve hysteresis. 

On many columns the decision is not clear cut. Here the approach should be to make a preliminary selection of one of the schemes, identify its limitations and attempt to enhance the scheme to deal with these. If this fails then switch to the alternative and enhance this one. For example we might have good reasons to select the material balance scheme but the column is subject to changes in feed rate. Installation of the feedforward scheme shown in Figure 12.60 will maintain a constant D/F ratio and so overcomes this limitation. While not quite as simple as drawn, a full description of feedforward control is presented later in this chapter. 

Figures 12.110and 12.111 show the same test with the material balance scheme in place. Again the distillate quality would be better controlled with no feedforward, rather than with just one of the ratios kept constant. While keeping either ratio constant marginally improves control of bottoms quality, keeping both constant will result in virtually no change.

As shown in Ref 33, the feedforward control equations were obtained by writing component material balances around the recycle addition point. For example, for monomer A this balance is given by Eq. (101). 

Temperature and composition are both properties of a flowing stream. Heat and material balances involve multiplication of these variables by flow, producing a characteristic nonlinear process model. Feedforward systems for control of these variables are similarly characterized by multiplication and division. The general form of process model for the applications is... 

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 ... 

The bottom half of the figure shows the material balance control scheme in which the top quality is controlled by the distillate draw-off However, the draw-off does not affect the top quality but rather the level. The level in turn affects the reflux flow, which subsequently affects the top quality. This means that there is a severe degree of mutual interaction between the control loops. It was found that the control structure of Fig. 34.8b would result in an oscillatory behavior of the quality control loop for feed flow changes of +10%. Only addition of a feedforward loop from distillate flow to reflux would stabilize the quality control loop. In that case, the distillate flow changes were subtracted from the reflux flow changes calculated by the level controller. After addition of feed-forward control, the response of the top and bottom qualities were similar to the responses of the energy balance control scheme for this situation with virtually no deviation from setpoint of the top quality and a bottom quality response similar to Fig. 34.7. 

Ls indicated in Chapter 1, it is most convenient, when starting the design of controls for a new or modernized plant, first to lay out all of the material-balance controls. These are mostly liquid level controls. It was also indicated that feedforward compensation could be used to supplement feedback composition controls to achieve more constant compositions. In the absence of feedback composition controls—usually because adequate composition measurements are lacking—feedforward compensation is almost mandatory. 

We will begin with combinations of level control and feedforward compensation for applications where material-balance control is in the direction opposite to flow. Then we will consider schemes in which material-balance control is in the direction of flow. Unfavorable schemes—those that are hard to design or to make work—will be pointed out their use should be avoided unless no suitable option is available. 

A useful interpretation of feedforward control is that it continually attempts to balance the material or energy that must be delivered to the process against the demands of the disturbance (Shinskey, 1996). For example, the level control system in Fig. 15.3 adjusts the feedwater flow so that it balances the steam demand. Thus, it is natural to base the feedforward control calculations on material and energy balances. For simplicity, we will first consider designs based on steady-state balances using physical variables rather than deviation variables. Design methods based on dynamic models are considered in Section 15.4. 

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