Modeling of vacuum membrane distillation (VMD) as an approach to separate butanol from aqueous solutions

Butanol is a promising biofuel due to its good fuel properties. One of the barriers for butanol to conquer the market is its product inhibition at 13 g/l during fermentation step, which results in a highly uneconomical energy demand during down streaming process. To elude this inhibition an on-line product separation is desirable, which can be implemented by means of VMD. This upstream membrane step, which concentrates butanol to a feasible level for state of the art distillation, leads to a hybrid process with an overall optimized process efficiency.

The aim of this work was to model a VMD process with the help of Aspen PlusTM(Aspen Tech). Since Aspen Plus does not contain membrane modules in its model library, a model was developed in Aspen Custom ModelerTM (Aspen Tech) and implemented in Aspen Plus.

To validate the simulation experimental data was collected during trials with different modules from the company MicrodynNadir GmbH. Two capillary modules with the area of 0,1 m² and 0,75 m² as well as two tubular modules with an area of 0,036 m² and 0,2 m² were used.

Experimental and simulation data of these four modules are compared in terms of selectivity and transmembrane flux. Parameters influencing the efficiency were investigated like alcohol feed concentration, feed flowrate, feed temperature as well as the set vacuum. Several sensitivity analyses were carried out to find the optimal process conditions.

The results show that different modules (e.g. more capillaries or tubes) result in different membrane performance. With these results possible validation for industrial application and upscale designs can be generated.