Ethanol production with dilute acid hydrolysis using partially dried lignocellulosics

In a process for converting lignocellulosic biomass to ethanol, the improvement of obtaining higher fermentable soluble sugar yields by drying acid impregnated biomass particles, comprising:

• a) feeding a moist lignocellulosic biomass feedstock into a dilute strong acid impregnator to effect pH values below about 3 for a sufficient residence time to render it acid-soaked and draining the acid-soaked biomass to about 30% to about 35% by weight solids;
• b) dewatering said acid-soaked biomass by drying or centrifugation in a manner so as to prevent densifying the biomass particles and obtain a solids content of about 40% to 60% wet weight basis;
• c) subjecting said acid-impregnated biomass to a first-stage hydrolysis reactor at a temperature sufficient to commence hydrolysis and discharging the formed hydrolysate into a first flash tank at a temperature sufficient to hydrolyze most of the soluble oligosaccharides to monomeric sugars and flashing remaining hydrolysate to a second flash tank at a lower temperature than the first flash tank—said second flash tank serving as a feed surge tank for a counter-current extractor;
• d) washing the hydrolysate, recovering more than about 95% of the soluble sugars in the first-stage hydrolysate slurry by a counter-current extractor, and adjusting the pH of the extract to about 5;
• e) subjecting remaining washed first stage pretreated solids to a second-stage acid and metal salt impregnator and dewatering by drying or centrifugation in a manner so as to prevent compaction of the biomass particles and obtain a solids content of about 40% to 60% wet weight basis;
• f) subjecting said acid and metal salt-impregnated biomass to a second-stage hydrolysis reactor at a temperature of from about 190° C. to about 240° C. and discharging the formed hydrolysate into a flash tank at about 120° C. to about 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—said second flash tank serving as a feed surge tank for second-stage fermentors;
• g) cooling the pH-adjusted extract from said 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 about 4.5, cooling the slurry and adding it into the top of the first fermentor of the second-stage fermentation train, pumping broth from the bottom of the first fermentor to the top of a second fermentor for a period sufficient for the carried over yeast to consume over about 95% of fermentable sugars;
• and
• i) recovering ethanol.