Requirement for Optimisation

The benefits of reactive distillation arise from the combination of reaction and separation within a single unit operation. As indicated previously, the overall capital cost of a reactive distillation process is likely to be considerably less than a comparable two-stage flowsheet and process advantages sometimes arise from the constant recycling of reactants to the reaction zone. However, these advantages are partially offset by the increased process complexity and potential difficulties with operation and control.

The main source of difficulty is that it is desirable to simultaneously produce both a high reactant conversion and a high product purity. That is, it must duplicate both functions of the two-stage process. However, the operating conditions that maximise reactant conversion do not coincide with the conditions that maximise the product purity so that the dual objectives are not entirely compatible. In an industrial environment, the best solution to this dilemma is to continuously optimise the process according to an objective function that is based on economic considerations and is dependent on both parameters (purity and conversion).

The optimisation should be undertaken at three levels: at the primary level, an effective regulatory control system is required to stabilise the process and reject the effects of disturbances; at the secondary level, the controller set-points should be regularly updated to reflect changing production targets; and at the tertiary level, the parameters of the optimisation should be updated to reflect changes in the external economic conditions. The primary level of optimisation is discussed in some detail in the following chapters while the tertiary level is outside the scope of this work. The framework for implementing the secondary level of process optimisation is discussed here.

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