Linear Reagent Demand Control

It is critical that filters be judiciously used because they add delay to the response to unmeasured load upsets and provide an illusion of better control by providing an attenuated version of the real world. They should only be used to help screen out transients that are so fast they will be smoothed out by a downstream volume and any response by a controller would potentially do more harm than good. It is also important to realize that there will be an offset in the actual pH of the effluent compared to the average pH signal because of measurement errors and noise and any nonsymmetry of the titration curve about the set point. The effect can be estimated by translation of the oscillations on the titration curve. Normally the offeet is relatively small (less than 1 pH) and can be reduced by a simple bias of the upstream set point [Ref. 1.2). The use of process variable signal characterization to imbed the titration curve and create a linear reagent demand controller described in Chapter 8 for the inline mixer can inherently eliminate the offset. 

The operator may prefer to use a dummy pH controller even though the linear reagent demand controller is doing the work behind the scene. 

If Coriolis and magnetic flowmeters are installed to measure the reagent and influent flows, cascade control, flow feedforward, and online identification of a titration curve should be utilized to facilitate start-up and recovery from the flat portions of the titration curve and improve over system performance and knowledge. If the titration curve is relatively fixed or can be calculated or identified online, linear reagent demand control should be used to suppress oscillations and noise and restore... 

Why should a linear reagent demand controller be used for adaptive control ... 

For extremely variable titration curves, the titration curve can be identified online for linear reagent demand pH control if the pH extremes... 

Qnite obvionsly, the control of the molar mass of the product of these reactions is very important. Very-high-molar-mass material may be too difficult to process, whereas low-molar-mass polymer may not exhibit the properties desired in the end product, and one mnst be able to stop the reaction at the required value of p. Consequently, the reactions are particularly demanding with respect to the purity of the reagents, and acenrate control of the amount of each species in the mixture is cardinal. It is symptomatic of these critical requirements that only the following four types of reaction usefully produce linear polymers with M > 25,000 g mol ... 

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