It has already been suggested that a vital first stage of control system design is to accurately and appropriately determine the control objectives so that the process performance is optimised for some given economic constraints. In many cases, this step is neglected to the detriment of the controller's effectiveness. An important consideration in this regard is that it is normal for the economic constraints to shift periodically and that this can affect the control objectives both quantitatively and qualitatively. Indeed, a single set of control objectives may not be satisfactory.
The management of the control objectives in response to changing economic conditions is illustrated here for the 10 stage ETBE column described previously. The two most important process parameters for this column were considered to be the ETBE purity in the bottoms product and the isobutene conversion. In other columns, the ethanol concentration in the distillate (or another process parameter) could be highly important, and a different control system would be required. However, the current assumption of purity and conversion targets provides adequate scope for the exploration of an integrated control scheme designed to reflect the variation in economic values. Three different operating modes were considered in order to allow the process profitability to be maximised under a wide range of conditions.
The first mode of operation controls both the bottoms purity and isobutene conversion to pre-determined targets that correspond to downstream processing requirements or the results of a plant-wide optimisation. In the second mode, the bottoms purity is still controlled to a pre-determined target but the second control parameter is used to maximise the reactant conversion. In the third mode, both the purity and conversion are maximised using the two available degrees of freedom.
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