The effectiveness of a supervisory control system for this column is strongly dependent on two factors: the accuracy of the process model used for the optimisation; and the controller tunings. Plant-model mismatch can result in non-optimal changes so that there is no increase in profitability even after the process transient is complete. Regular feedback from the actual process, including data reconciliation, is required to ensure that the plant-model mismatch is minimised. Aggressive controller tunings are also required to minimise the effect of making set-point changes. Poorly tuned controllers result in large production losses before the new optimal conditions are attained. On the other hand, tightly tuned controllers allow more regular updating of set-points and, therefore, increase the economic benefit of the supervisory system.
The benefits of reactive distillation for the synthesis of fuel ethers such as ETBE have been clearly demonstrated but the dual process objectives, which are inherent in the combination of reaction and separation, require a careful rationalisation of operating conditions in order to maximise profitability. A satisfactory process optimisation can only be achieved by considering the net values and costs of both distillation products, the feed and utilities consumption, and the influence of process equipment constraints.
Although the optimum conditions for maximum profitability in a reactive ETBE column do not coincide with either maximum product purity or maximum reactant conversion, it is possible to implement a supervisory control system based on a relatively simple optimisation framework. It was shown, using dynamic simulations, that a supervisory control system could be used to increase the net profitability of a column by ensuring that the regulatory control system set-points were reset to their optimum values following process disturbances.
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