Butanol Production from Soybean Hull and Soy Molasses by Acetone-Butanol-Ethanol Fermentation

Abstract

Butanol is an important intermediate and solvent in the chemical industry, and a promising biofuel with a higher energy density and lower vapor pressure than ethanol. With the increased crude oil prices, the cost of producing butanol from petrochemical feedstocks has also increased dramatically. It is thus of high interest to produce butanol from low-cost renewable biomass such as soybean hull and soy molasses, two little-used byproducts from soybean biorefinery, by acetone-butanol-ethanol (ABE) fermentation. However, no solventogenic Clostridium strains can directly ferment cellulose and hemicellulose present in soybean hull, or the oligosaccharides present in soy molasses. In order to use these soy byproducts in ABE fermentation, acid pretreatment and enzymatic hydrolysis processes were developed to produce the hydrolysates containing fermentable sugars, which were then evaluated for butanol production by different Clostridium strains. It was found that cell growth and butanol production were severely inhibited by some lignin-degradation products present in soybean hull hydrolysate. However, after detoxification by activated carbon adsorption, soybean hull hydrolysate gave good butanol production. Soy molasses treated with α-galactosidase also gave good butanol production. Two processes for butanol production from soybean hull and soy molasses, respectively, are then designed for plants with an annual production of ~7.5 MM gal (23,000 ton) butanol. The projected butanol production cost is $4.06 /gal for soybean hull and $2.71/gal for soy molasses, compared to $4.41/gal for corn as the feedstock and ~$6.75/gal for the current market price for butanol. In conclusion, soybean hull and soy molasses can be used as feedstocks for the production of n-butanol in ABE fermentation, and the process should be economically feasible and attractive for industrial application.