Optimization of the recycle structure of multiple stages molecular distillation

Abstract

In cases when the separation achieved by one molecular distillation stage is not sharp enough, it is resorted to multiple stage schemes. This brings the issue of how to recycle the intermediate streams to optimize the process. The present paper proposes to use the source-sink assignment methodology to decide the recycle structure. The counter current scheme is also taken into account because it is usually regarded as the design of choice to recycle streams. The approach was applied to optimize the recycle structure of a two stages process for the separation of free fatty acids from soybean oil deodorizer distillate, to get a concentrate of tocopherols in the residual stream, within specification of acidity. The counter current scheme was also implemented. For this case study the optimal recycle structure gets 10% more product with a profit 0.6% larger than the counter current scheme, even if it has larger operative and investment costs. So, in this particular case, the reward for using a rigorous methodology was not that significant. However, the approach proposed here is systematic and necessarily leads to a structure with a performance greater than or equal to any other, because the superstructure explored includes all of them.