Generalised model for heteroazeotropic batch distillation with variable decanter hold-up

• A general model of batch heteroazeotropic distillation is proposed.
• Both liquid phases can be refluxed or withdrawn as distillate.
• The hold-up of both liquid phases in the decanter is variable.
• Still path directions are determined for all the 16 possible operational policies.
• The still path directions are validated by rigorous dynamic simulation.

A general model of batch heteroazeotropic distillation is proposed. Both liquid phases present in the decanter can be refluxed or withdrawn as distillate, their hold-up can be increased, decreased or kept constant, as well. By assuming maximal separation, that is, that the composition of the condensate always equals to that of the heteroazeotrope, the still path equation was derived. The still path directions are determined for all the 16 possible operational policies. It is possible to steer the still path in a desired direction by changing the operational parameters, which allows the recovery of a pure component in the still. The still path directions are validated by rigorous simulations for three policies not published yet using the mixture water – formaldehyde – propyl formate. From the 16 operational policies, 11 can be considered as useful in practice. To demonstrate the advantage of using a non-traditional policy, the separation of the mixture aniline – ethylene glycol – water was investigated, as well. By using a non-traditional operational policy with hold-up reduction in the decanter a higher purity of ethylene glycol was obtained in the still.