Open Loop Operation

The presence of multiple steady states in reactive distillation can influence the way a reactive distillation column should be operated and controlled. Transitions between parallel steady states were discussed in Chapter 8 (Section 8.4) and it was shown that the column conditions and product compositions can change without any change in the manipulated variables used for composition control. For example, some perturbations in either the feed rate or the feed composition can initiate subtle changes in the composition profile, which results in the column stabilising to a new steady state. While the feed rate is unlikely to change without warning, the feed composition might be reasonably expected to vary due to the effects of upstream processes and raw materials (e.g. crude oil) compositions. This immediately creates an incentive to avoid open-loop operation if multiple steady states are possible.

A steady state transition is also possible if both the feed rate and feed composition are constant. In this case, a perturbation in an external stream can initiate the sudden change. For example, the reboiler duty is often manipulated (indirectly) by regulating the steam flow to the reboiler. Since the duty that is obtained from this arrangement depends on both the steam flow and the steam pressure, a slight decrease in the supply pressure will result in a decrease in the reboiler duty even if the flow rate is not changed. Thus, an unobserved perturbation in the steam pressure could result in a transition from a desirable to an undesirable steady state even if the feed rate, feed composition, reflux rate and steam rate to the reboiler are constant. This possibility provides a further motivation for implementing closed-loop composition control of at least one product.

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