Methods of washing cellulose-rich solids from biomass fractionation to reduce lignin and ash content

Abstract

The present invention provides a process for fractionating lignocellulosic biomass, comprising: digesting a biomass feedstock in the presence of a solvent for lignin, an acid, and water, to produce cellulose-rich solids; separating and washing the cellulose-rich solids with a wash solvent; washing the cellulose-rich solids with water, to generate washed cellulose-rich solids and a wash liquor comprising fines, wherein the wash liquor is introduced to or in contact with a classifier to remove the fines; and separating the fines and recycling the remaining water. The classifier may include a screen with mesh size in the range of 10 to 500, such as 200. The washed cellulose-rich solids will typically have a lower Kappa number (lignin content) and ash content compared to cellulose-rich solids from a process without a classifier that removes fines.

What is claimed is: 
     
         1 . A process for fractionating lignocellulosic biomass, said process comprising:
 (a) digesting a lignocellulosic biomass feedstock under effective conditions in the presence of a solvent for lignin, an acid or acid precursor, and water, to produce cellulose-rich solids in a digestor liquor;   (b) separating said cellulose-rich solids from said digestor liquor and washing said cellulose-rich solids with a first wash liquid comprising a wash solvent for lignin, to generate first washed cellulose-rich solids;   (c) washing said first washed cellulose-rich solids with a second wash liquid comprising water, to generate second washed cellulose-rich solids and a wash liquor comprising fines, wherein said wash liquor is introduced to or in contact with a classifier to remove at least a portion of said fines in a liquid fines-containing stream;   (d) recovering said second washed cellulose-rich solids; and   (e) optionally separating said fines from said fines-containing stream and recycling water contained in said fines-containing stream back to step (c).   
     
     
         2 . The process of  claim 1 , wherein said lignocellulosic biomass feedstock is a hardwood. 
     
     
         3 . The process of  claim 1 , wherein said lignocellulosic biomass feedstock is an annual plant or agricultural residue. 
     
     
         4 . The process of  claim 1 , wherein said solvent for lignin is ethanol. 
     
     
         5 . The process of  claim 1 , wherein said solvent for lignin is the same as said wash solvent for lignin. 
     
     
         6 . The process of  claim 1 , wherein said acid or acid precursor is sulfur dioxide. 
     
     
         7 . The process of  claim 1 , wherein said classifier comprises a screen with mesh size in the range of 10 to 500. 
     
     
         8 . The process of  claim 7 , wherein said classifier comprises a screen with mesh size in the range of 150 to 250. 
     
     
         9 . The process of  claim 1 , wherein during step (b) and/or step (c), a disperser is utilized to liberate said fines from said second washed cellulose-rich solids. 
     
     
         10 . The process of  claim 1 , wherein during step (c), at least 95% of said fines contained in said wash liquor are removed into said liquid fines-containing stream. 
     
     
         11 . The process of  claim 1 , wherein during step (b) and/or step (c), one or more additives are introduced to remove minerals remaining in said first washed cellulose-rich solids and/or said second washed cellulose-rich solids. 
     
     
         12 . The process of  claim 1 , wherein said second washed cellulose-rich solids have a lower Kappa number compared to cellulose-rich solids from an otherwise-identical process without a classifier to remove at least a portion of said fines. 
     
     
         13 . The process of  claim 1 , wherein said second washed cellulose-rich solids have a lower ash content compared to cellulose-rich solids from an otherwise-identical process without a classifier to remove at least a portion of said fines. 
     
     
         14 . The process of  claim 1 , wherein said second washed cellulose-rich solids have a lower hemicellulose content compared to cellulose-rich solids from an otherwise-identical process without a classifier to remove at least a portion of said fines. 
     
     
         15 . The process of  claim 1 , wherein said second washed cellulose-rich solids contain about 75% or more cellulose, about 7 wt % or less lignin, about 5 wt % or less hemicellulose, and about 10 wt % or less ash. 
     
     
         16 . The process of  claim 15 , wherein said second washed cellulose-rich solids contain about 80% or more cellulose, about 3 wt % or less lignin, about 5 wt % or less hemicellulose, and about 8 wt % or less ash. 
     
     
         17 . The process  claim 1 , wherein steps (a)-(c) are conducted countercurrently. 
     
     
         18 . The process of  claim 1 , wherein said process is batch or semi-continuous, wherein step (b) and/or step (c) is conducted in simulated countercurrent fashion, and wherein multiple wash streams are generated. 
     
     
         19 . A method of separating fines from cellulose-rich solids, said method comprising:
 (a) obtaining a biomass digestor liquor comprising cellulose-rich solids;   (b) separating said cellulose-rich solids from said digestor liquor and washing said cellulose-rich solids with a first wash liquid, to generate first washed cellulose-rich solids;   (c) washing said first washed cellulose-rich solids with a second wash liquid, to generate second washed cellulose-rich solids and a wash liquor comprising fines, wherein said wash liquor is introduced to or in contact with a classifier to remove at least a portion of said fines in a liquid fines-containing stream;   (d) recovering said second washed cellulose-rich solids; and   (e) optionally separating said fines from said fines-containing stream and recycling water contained in said fines-containing stream back to step (c).   
     
     
         20 . A method of separating fines from cellulose-rich solids, said method comprising:
 (a) obtaining a biomass digestor liquor comprising cellulose-rich solids;   (b) separating said cellulose-rich solids from said digestor liquor and washing said cellulose-rich solids with a wash liquid, to generate washed cellulose-rich solids and a wash liquor comprising fines, wherein said wash liquor is introduced to or in contact with a classifier to remove at least a portion of said fines in a liquid fines-containing stream;   (c) recovering said second washed cellulose-rich solids; and   (d) optionally separating said fines from said fines-containing stream and recycling water contained in said fines-containing stream back to step (b).