Method for producing ethanol using cellulosic biomass as raw material
In the present disclosure, after a volatile organic acid is added as a catalyst to a slurry and the hydrolytic saccharification reaction of hemicellulose is performed, the volatile organic acid (including one added to the slurry and one produced as a by-product) contained in a C5 saccharified solution can be easily recovered by adding a small amount of sulfuric acid to the C5 saccharified solution and subjecting the C5 saccharified solution to air stripping treatment. The efficiency of hydrolytic saccharification of hemicellulose into C5 sugars using a volatile organic acid can also be continuously improved by adding the recovered volatile organic acid as a catalyst to another slurry of cellulosic biomass.
1. A method for producing ethanol using cellulosic biomass as a raw material, comprising:
a first hydrolytic saccharification step of adding a volatile organic acid to a slurry of cellulosic biomass and hydrothermally treating the slurry in a supercritical or subcritical state to saccharify/decompose hemicellulose contained in the cellulosic biomass into C5 sugars;
a first solid-liquid separation step of subjecting the slurry after the first hydrolytic saccharification step to solid-liquid separation;
a recovery step of adding sulfuric acid to a C5 saccharified solution obtained in the first solid-liquid separation step, and then subjecting the C5 saccharified solution to air stripping treatment or distillation treatment to recover the volatile organic acid contained in the C5 saccharified solution;
a second hydrolytic saccharification step of slurrying a dewatered cake obtained in the first solid-liquid separation step by adding water and hydrothermally treating the slurry in a supercritical or subcritical state to saccharify/decompose cellulose contained in the cellulosic biomass into C6 sugars;
a second solid-liquid separation step of subjecting the slurry after the second hydrolytic saccharification step to solid-liquid separation;
a fermentation step of subjecting the C5 saccharified solution after the recovery step and a C6 saccharified solution obtained in the second solid-liquid separation step to alcoholic fermentation; and
a distillation step of distilling a fermented liquid obtained in the fermentation step to concentrate ethanol,
wherein the volatile organic acid recovered in the recovery step is reused as all or part of the volatile organic acid to be added to the slurry in the first hydrolytic saccharification step.
2. The ethanol production method according to claim 1 , wherein in the first hydrolytic saccharification step, the volatile organic acid is added to the slurry in a concentration of 0.1% by mass or higher but 10% by mass or lower.
3. The ethanol production method according to claim 1 , further comprising a third solid-liquid separation step of adding lime to the C5 saccharified solution after the recovery step, and separating the resulting gypsum by solid-liquid separation to remove the sulfuric acid and a fermentation inhibitor contained in the C5 saccharified solution.
4. The ethanol production method according to claim 3 , wherein the third solid-liquid separation step is a solid-liquid separation step in which the gypsum is separated by solid-liquid separation using a thickener or a settling tank.
5. The ethanol production method according to claim 4 , wherein the C5 saccharified solution from which the gypsum has been removed in the third solid-liquid separation step is concentrated by a reverse osmosis membrane device.