(331d) Techno-Economic Analysis of Hybrid Membrane/Distillation Column Systems for Ethanol-Water Separation

We have carried out a comprehensive analysis of the technical and economic challenges associated with the implementation of ethanol selective membrane systems for the separation of ethanol from water.

Membranes have the potential to displace current technologies such as distillation. Such a displacement is only attractive if energy and costs savings are possible. Our goal was to investigate the minimum membrane performance characteristics needed to accomplish such a savings. The case study chosen was the ethanol-water distillation in the dry-grind corn ethanol process.

The distillation column in the base case was modeled using the McCabe-Thiele method. The ethanol product purity and recovery of the base case provides the performance targets for the novel configurations suggested here.

We have modeled the membrane unit as a hollow fiber counter-current membrane system with nitrogen sweep gas. An initial analysis of a stand-alone membrane system shows that this is incapable of performing the required separation. 

Next, we considered a hybrid system composed of a membrane and a distillation column. Our analysis has shown that significant economic savings are possible when the membrane follows a small distillation column. This hybrid system has the capability to better capitalize on the VLE characteristics of the ethanol-water system than systems composed uniquely of distillation columns or membranes.

We have analyzed the effect of different values of membrane ethanol flux and selectivity on the performance of the hybrid system. It was found that changes in selectivity have a greater effect on the product purity than changes in flux and that changes in flux have a greater effect on the required membrane area (and therefore cost) than changes in selectivity. We have found that a minimum selectivity of 30 is required for the target product purity of 95 wt% ethanol.

Finally, we have formulated a net present value analysis of the resulting hybrid system. This has allowed us to generate a set of performance targets in terms of the membrane’s flux, selectivity and costs.