OPTIMIZATION OF REVERSE-FLOW, 2-TEMPERATURE, DILUTE-ACID PRETREATMENTTO ENHANCE BIOMASS CONVERSION TO ETHANOL

A reverse-flow, two-temperature dilute-acid prehydrolysis process of c ommercial yellow poplar sawdust using two percolation reactors was des igned to simulate countercurrent flow of the biomass solids and prehyd rolysis liquor, and to exploit the xylan biphasic kinetics. Lower temp eratures (150-174 degrees C) are initially applied to hydrolyze the ea sily hydrolyzable xylan, and higher temperatures (180-204 degrees C) a re applied to hydrolyze the remaining xylan. Two reactors were used to optimize each temperature range, using Varying concentrations of sulf uric acid from 0.073-0.73 wt% and reaction times. Yields of soluble xy lose, as high as 97% of theoretical, expressed as monomeric and oligom eric xylose, have been achieved with only 2.9% of the xylan being degr aded to furfural, at concentrations of total potential sugar between 2 .4 and 3.7 wt% before flashing. Depending on the combined severity of the acid concentration, residence time of the solids and liquor, and t emperature of prehydrolysis, 81-100% of the hemicellulose, 3-32% of th e glucans, and up to 46% of the Klason lignin could be solubilized. Th e lignocellulosic substrate produced from the pretreatment is readily converted to ethanol at a yield of approx 91% of theoretical, with eth anol concentrations of up to 4.0 wt% in 55 h via a simultaneous saccha rification and fermentation (SSF) process. In terms of xylose recovery and ethanol production level and rate, the present results are far su perior to those previously reported using a single-temperature, dilute -acid pretreatment.