Hydrothermal-mechanical conversion of lignocellulosic biomass to ethanol or other fermentation products

Abstract

A low-cost process is provided to render lignocellulosic biomass accessible to cellulase enzymes, to produce fermentable sugars. Some variations provide a process to produce ethanol from lignocellulosic biomass (such as sugarcane bagasse or corn stover), comprising introducing a lignocellulosic biomass feedstock to a single-stage digestor; exposing the feedstock to a reaction solution comprising steam or liquid hot water within the digestor, to solubilize the hemicellulose in a liquid phase and to provide a cellulose-rich solid phase; refining the cellulose-rich solid phase, together with the liquid phase, in a mechanical refiner, thereby providing a mixture of refined cellulose-rich solids and the liquid phase; enzymatically hydrolyzing the mixture in a hydrolysis reactor with cellulase enzymes, to generate fermentable sugars; and fermenting the fermentable sugars to produce ethanol. Many alternative process configurations are described. The disclosed processes may be employed for other fermentation products.

What is claimed is: 
     
         1 . A process to produce ethanol from lignocellulosic biomass, said process comprising:
 (a) introducing a lignocellulosic biomass feedstock to a single-stage digestor, wherein said feedstock contains cellulose, hemicellulose, and lignin;   (b) exposing said feedstock to a reaction solution comprising steam or liquid hot water within said single-stage digestor, to solubilize at least a portion of said hemicellulose in a liquid phase and to provide a cellulose-rich solid phase;   (c) enzymatically hydrolyzing said cellulose-rich solid phase in a first hydrolysis reactor configured for cellulose liquefaction, thereby providing a first hydrolyzed cellulose mixture containing said liquid phase;   (d) refining said first hydrolyzed cellulose mixture in a mechanical refiner to reduce average particle size of said first hydrolyzed cellulose mixture, thereby providing a second hydrolyzed cellulose mixture;   (e) enzymatically hydrolyzing said second hydrolyzed cellulose mixture in a second hydrolysis reactor configured for cellulose saccharification, thereby generating fermentable sugars, wherein said second hydrolysis reactor includes a self-cleaning filter to remove cellulosic fiber strands;   (f) recycling at least a portion of said cellulosic fiber strands back to said first hydrolysis reactor and/or to said mechanical refiner; and   (g) fermenting at least some of said fermentable sugars in a fermentor to produce ethanol.   
     
     
         2 . The process of  claim 1 , wherein said lignocellulosic biomass feedstock is selected from the group consisting of hardwoods, softwoods, sugarcane bagasse, sugarcane straw, energy cane, corn stover, corn cobs, corn fiber, and combinations thereof 
     
     
         3 . The process of  claim 1 , wherein said lignocellulosic biomass feedstock is pretreated, prior to step (a), using one or more techniques selected from the group consisting of cleaning, washing, presteaming, drying, milling, particle size-classifying, and combinations thereof. 
     
     
         4 . The process of  claim 1 , wherein at least a portion of said reaction solution is introduced to said feedstock in a pre-impregnator prior to step (b). 
     
     
         5 . The process of  claim 1 , wherein step (b) includes a digestor residence time from about 2 minutes to about 4 hours. 
     
     
         6 . The process of  claim 1 , wherein step (b) includes a digestor temperature from about 150° C. to about 220° C. 
     
     
         7 . The process of  claim 1 , wherein step (b) is conducted at a digestor liquid-solid weight ratio from about 1 to about 4. 
     
     
         8 . The process of  claim 1 , wherein step (b) is conducted at a digestor pH from about 3 to about 5. 
     
     
         9 . The process of  claim 1 , wherein said reaction solution further comprises acetic acid. 
     
     
         10 . The process of  claim 1 , wherein vapor is separated from said liquid phase prior to step (c). 
     
     
         11 . The process of  claim 10 , wherein heat is recovered from at least some of said vapor. 
     
     
         12 . The process of  claim 10 , wherein at least some of said vapor is condensed or compressed and returned to said digestor. 
     
     
         13 . The process of  claim 1 , wherein said mechanical refiner is selected from the group consisting of a hot-blow refiner, a hot-stock refiner, a blow-line refiner, a disk refiner, a conical refiner, a cylindrical refiner, an in-line defibrator, an extruder, a homogenizer, and combinations thereof. 
     
     
         14 . The process of  claim 1 , wherein said second hydrolysis reactor is a multiple-stage hydrolysis reactor. 
     
     
         15 . The process of  claim 1 , wherein said self-cleaning filter is disposed between two stages of said multiple-stage hydrolysis reactor. 
     
     
         16 . The process of  claim 1 , wherein step (e) includes enzymatic hydrolysis of hemicellulose oligomers to generate fermentable monomer sugars. 
     
     
         17 . The process of  claim 16 , wherein at least a portion of said fermentable monomer sugars are also fermented to produce said ethanol. 
     
     
         18 . The process of  claim 1 , said process further comprising concentrating said ethanol by distillation. 
     
     
         19 . An ethanol product produced by a process according to  claim 1 . 
     
     
         20 . An ethanol product produced by a process according to  claim 18 .