An optimal recycle policy for multicomponent batch distillation

Abstract A physically and thermodynamically sensible approach is proposed for processing off-cut recycles in multicomponent batch distillation. For the proposed strategy an optimal control formulation is developed which allows calculating simultaneously the amounts and compositions of the intermediate cuts to be recycled, and the reflux ratio profiles to yield a minimum batch cycle time. A dynamic column model is used for the purpose. The optimal policies are generated by decomposing the overall problem into a sequence of pseudo-binary optimal control problems, each solved by a method previously applied by the authors to binary batch distillation. Application of the method to typical example problems demonstrates the idea and shows that large improvements in column performance could be achieved. The proposed optimal recycling strategy gives an overall batch time saving of more than 45% and productivity improvements of up to 67% in some cases. It is also shown that a measure q of “the degree of difficulty of separation” correlates well with the expected performance improvements and can be used in deciding whether a recycle should be used.