Mixing process feedforward control

Draw three different feedforward control configurations for the mixing process of Example 4.11. 

From the earlier discussion and from Table 57.1 [11], we would expect that a combined feedforward-feedback control system would retain the superior performance of the feedforward controller and the insensitivity of the feedback controller to uncertainties and inaccuracies. Figure 57.4 shows the main components of a feedforward-feedback control system they include the feedforward controller incorporated with the process model, the feedback controller, and the dynamic compensator. The function of feedback element is to correct the action of the feedforward controller in the case of measurement and modeling inaccuracy. Courtois et al. [12] and Bruce and McFarlane [13] implemented the combined feedforward-feedback control system for mixed-flow grain dryers. 

Flow controllers set the rates of both streams, one being under flow-ratio control. In principle, either caustic soda or dilution water can be the master stream, with the other following it to maintain the ratio. Blending is controlled by a feedforward system, ultimately reset by the product concentration or density. Feedback from caustic concentration measurement (usually by density) could be used for final adjustment, but the concentration of the hypochlorite solution is the more important variable. The simple flow-ratio controller mentioned here can be replaced by a multi-stream version that allows use of other streams in addition to the principal 50% NaOH and dilution water. A cooler downstream of the mixing point removes the heat of dilution. The standard design is a titanium plate exchanger, which can also provide turbulence to complete the mixing process. Chlorine joins the diluted caustic in the reactor. Its rate of addition is controlled by an oxidation-reduction potential (ORP) instrument. The reaction mass recirculates from a collection tank around the system to reduce the increase of temperature across the reactor and to promote turbulence. The net production is removed from the tank, normally under level control. 

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