In a process for converting lingnocellulosic
biomass to
ethanol, the improvement of obtaining higher fermentable soluble
sugar yields by
drying acid impregnated
biomass particles, comprising: a) feeding moist
lignocellulosic biomass into an acid impregnator to render it acid-soaked and draining the acid-soaked
biomass to about 30% to 35% by weight solids; b)
dewatering the acid-soaked biomass by
drying or
centrifugation to prevent compaction of the biomass and arrive at about 40% to 60% by weight solids; c) subjecting the acid-impregnated biomass to a first-stage
hydrolysis reactor at a temperature of from 130° C. to 220° C. and discharging formed
hydrolysate into a flash tank at about 120° C. to 140° C. to hydrolyze most of the remaining soluble oligosaccharides to monomeric sugars, and flashing remaining
hydrolysate to a second flash tank at a lower temperature than the first flash tank-the second flash tank serving as a feed
surge tank for a counter-current extractor; d) washing the
hydrolysate, adjusting the pH of the
sugar extract to about 5, and recovering more than 95% of the soluble sugars in the first-stage hydrolysate
slurry by a counter-current extractor; e) subjecting remaining washed-first stage solids of pretreated biomass to a second-stage acid and
metal salt impregnator and
dewatering by
drying or
centrifugation to prevent compaction of biomass to arrive at 40% to 60% by weight solids; f) subjecting the acid and
metal salt-impregnated biomass to a second-stage
hydrolysis reactor at a temperature from 190° C. to 240° C. and discharging formed hydrolysate into a flash tank, at about 120° C. to 140° C. to hydrolyze most of the remaining soluble oligosaccharides to monomeric sugars and flashing remaining hydrolysate to a second flash tank at a lower temperature than the first flash tank, the second flash tank serving as a feed
surge tank for second-stage fementors; g) cooling pH-adjusted extract from the counter-current extractor, feeding the extract to a first-stage fermentor and
air sparging the first-stage fermentor at a rate sufficient to promote enough
yeast growth to compensate for loss through second-stage fermentors; h) pH adjusting second-stage hydrolysate
slurry to 4.5, cooling the
slurry and adding it into the top of the first fermentor of a two-fermentor
train in the second stage fermentors, pumping broth from the bottom of the first stage fermentors to the second stage fermentors while the
yeast is in the
growth phase for a period sufficient to consume over 95% of fermentable sugars; and i) recovering
ethanol.