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BACKGROUNDAlcohols, including ethanol and butanol, are receiving increased attention as renewable liquid biofuels. Alcohol concentrations may be low in a biological process due to product inhibition and, for non-starch feedstocks, limited substrate concentrations. The result is high separation energy demand by conventional distillation scenarios, despite favorable vapor-liquid equilibrium and, for butanol, partial miscibility with water. A hybrid vapor stripping-vapor permeation process, termed membrane assisted vapor stripping (MAVS), incorporating a fractional condensation step was found to be at least 65% more energy efficient than conventional distillation approaches. The effect of process design, component performance, and capacity changes on the energy usage and processing cost of MAVS systems for separating ethanol, 1-butanol, and acetone/butanol/ethanol (ABE) mixtures from water was studied.

RESULTSFor the recovery of 1-butanol from a 1 wt% aqueous solution, the 99.5 wt% 1-butanol product contained 7.0 times as much heating value energy as the MAVS process required to recover and dry it. The calculated cost to perform this separation was 0.126 US$ kg(-1)-product (0.102 US$ L-1) for a heating value cost of 3.69 US$ GJ(-1), far below the current values for crude oil and conventionally-produced ethanol. Energy (electricity, natural gas) was 23% of this cost. The largest capital cost item was the compressor on the overhead vapor stream from the stripping column. Capital costs for membranes/modules was the 6th highest cost category, representing only 4% of the capital cost. A 10-fold increase in membrane cost caused the cost of production to increase 38%.

CONCLUSIONHybrid MAVS processes are an energy- and cost-efficient means to recover alcohols from water. Despite recent fluctuations, fossil fuel costs are projected to increase. Thus processes utilizing mechanical energy to recapture and transfer thermal energy, including MAVS, should have a greater cost advantage in the future. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.