Purification and recovery of biofuels using inorganic membranes

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Interest in renewable fuels has prompted a significant amount of research and investment in the production of ethanol from biomass. Purifying the ethanol product from fermentation broth represents a significant process hurdle as a result of the presence of azeotropes, significantly impeding the economic production of renewable, fermentation based biofuels. Membrane separation, and in particular reverse osmosis, represents a low-energy alternative to conventional distillation processes for separating and purifying chemical systems. Unfortunately, the widespread application of conventional reverse osmosis membrane technology for chemical fractionation has not materialized because of two main roadblocks: 1) High concentrations of organic chemicals generally degrade conventional polymeric reverse osmosis membranes; and 2) Extreme osmotic back pressures develop within the membrane requiring excessive if not unattainable trans-membrane pressures. This work proposes the use of inorganic alumina membranes in conjunction with countercurrent reverse osmosis for the selective removal of water from alcoholic fermentation products. Alumina membranes synthesized by anodic aluminum oxidation, followed by atomic layer deposition, provide uniform, nanometre sized pores necessary for reverse osmosis separations in addition to providing excellent stability against high concentrations of organic chemicals. The ability of atomic layer deposition to tune the pore size in 0.1 nm increments means that any two molecules with a size difference of > 0.1 nm can theoretically be separated using a membrane. In order to overcome the osmotic pressure limitations that develop during reverse osmosis, a countercurrent flow of an extractant on the permeate side of the membrane will be employed. This countercurrent flow can consist of either the recirculation of a portion of the high concentration retentate (e.g. reflux) or it may be an independent extractant (a process referred to as perstraction). Once perfected, this membrane process could be adapted to other alcohol-water separations, fruit juice concentration, desalination, as well as a multitude of other non-aqueous chemical systems.