3-02: Efficient recovery and dehydration of 1-butanol, acetone/butanol/ethanol, and isobutanol from dilute aqueous solutions using hybrid vapor stripping-vapor permeation

Butyl alcohols are receiving increased attention as renewable liquid biofuels with 1-butanol and isobutanol (aka 2-methyl-1-propanol) drawing most of the recent interest.  Fermentative production of butanol is limited to low concentrations, typically less than 2 wt% solvent, due to product inhibition. The result is high separation energy demand by conventional distillation approaches, despite favorable vapor-liquid equilibrium and partial miscibility with water. In previous work from our group, a process integrating steam stripping, vapor compression, and vapor permeation separation was evaluated for separating ethanol from water. Such a Membrane Assisted Vapor Stripping (MAVS) process is considered in this work for 1-butanol/water, acetone/butanol/ethanol/water (ABE/water), and isobutanol/water separation. Using process simulations, the earlier MAVS design was estimated to require 6.2 MJ-fuel/kg-butanol to produce 99.5 wt% 1-butanol from a 1 wt% 1-butanol feed, representing an energy savings of 63% relative to a benchmark distillation/decanter system and only a fraction of the 34.4 MJ/kg lower heating value of 1-butanol. Adding a fractional condensation step to the original MAVS design is predicted to reduce energy demand to only 4.8 MJ-fuel/kg-butanol and reduce membrane area by 65%. In the hybrid distillation/membrane MAVS systems, the stripping column provides high butanol recovery and low effluent concentration while the vapor compression and membrane steps enable the efficient recovery of latent and sensible heat from both the retentate and permeate streams from the membrane system. The operability and energy savings of a pilot MAVS system tested with simple ABE/water solutions and an ABE fermentation broth will be discussed.