Organophilic Pervaporation of Butanol from an Aqueous Solution with POMS

Butanol is seen as a promising biofuel due to its good fuel properties. In the biotechnological production of butanol by using Aceton-Butanol-Ethanol (ABE) Fermentation, the product inhibition of microorganisms at 13 g/L is a major problem. Due to the low butanol concentration present in the broth, state of the art distillation results in a highly uneconomically energy demand. To counteract this inhibition an on-line butanol separation is desirable. A two-step separation process with downstream distillation leads to a low overall energy demand and an optimized efficiency. An attractive option to put this into practice is the product separation with organophilic pervaporation.

This study focuses on the separation performance of a poly(octhylmethyl siloxane) [POMS] membrane during pervaporative stripping of butanol from an aqueous solution. The influencing factors on the pervaporative separation like feed temperature, alcohol concentration, Reynolds number in the module as well as the set vacuum were investigated in order to find optimum process conditions. Feed temperature varied from 25 to 55 °C, initial feed concentration from 0.5 to 1.5 w% and volume flow from 100 - 200 L/h. The applied vacuum was set to the lowest possible value and varied from 10 - 4 mbar depending on the flux influenced by process parameters. On account of numerous investigated parameters a model was generated with design of experiment, which should summarize all interesting process conditions. The experimental results show that POMS membrane can selectively separate butanol from an aqueous solution. The highest selectivity was obtained at temperature of 55 °C and low feed concentration of 0.5 w%. The applied vacuum at this process conditions stayed at about 10 mbar. Selectivities of about 28 are reached.

A prior work analysed POMS membranes during pervaporative stripping of ethanol from aqueous solutions. Experiments with similar process conditions were investigated to compare the data obtained in this study with others given in the literature. The trend of increasing selectivity at low alcohol concentrations and high temperatures is concordant during all experiments for ethanol as well as for butanol. This conformity assures the aim of this study showing pervaporation as a great possibility to raise the efficiency of the overall process and to deal with the low butanol concentration obtained during ABEFermentation.