Pretreatment method of cellulosic biomass via flowability control and reactive extrusion process

Abstract

In a pretreatment process for lignocellulosic biomass, lignin is separated from cellulose by a reactive extrusion process with flowability control of the biomass suspension. Flowability control via incorporation of flowability control agents renders biomass suspensions processable using equipment in which relatively high shearing stresses and pressures can be applied. In one embodiment, particulate cellulosic biomass is added to a fully-intermeshing twin-screw extruder with pH adjusters, water and flowability control agents. Upon the incorporation of the flowability control agent, the biomass suspension flows without phase separation and thus can be subjected to high mechanical shearing stresses and chemical reaction in the extruder. The screw configuration creates multiple mixing zones within the confines of the extruder. The flowability control agent may be a water-soluble polymer that allows a hydrogel to be formed or may be a recycled portion of the black liquor from the reactive extrusion process.

1 . A method of pretreatment of lignocellulosic biomass, comprising the steps of:
 suspending particles of a lignocellulosic biomass containing cellulose and lignin in a liquid thereby forming a suspension;   adding a flowability control agent to the suspension; and   mechanically shearing the suspension, whereby the size of the biomass particles is reduced and at least some of the lignin is separated into a liquid phase of recoverable black liquor and the cellulose remains in the cellulosic biomass.   
     
     
         2 . The method of  claim 1 , comprising the further step of adjusting the pH of the suspension so as to achieve a pH that promotes separation of the lignin from the cellulose; 
     
     
         3 . The method of  claim 1 , wherein the flowability control agent includes a water-soluble polymer that forms a hydrogel. 
     
     
         4 . The method of  claim 3 , wherein the flowability control agent is selected from the group of polymers consisting of carboxy methyl cellulose, chitosan, polyacrylic acid, a polyacrylate, polyethylene glycol, polyethylene oxide, an alginate and polyvinyl alcohol. 
     
     
         5 . The method of  claim 1 , where the flowability control agent is a polymer that imparts elasticity and viscosity to the biomass suspension such as to inhibit separation of said cellulosic biomass from said suspension under pressure and shear. 
     
     
         6 . The method of  claim 1 , wherein the flowability control agent includes an aqueous solution containing lignin. 
     
     
         7 . The method of  claim 1 , wherein the flowability control agent includes a portion of the black liquor in a recycle stream. 
     
     
         8 . The method of  claim 7 , further including the steps of separating the portion of the black liquor from the suspension, adjusting the pH of the portion of the black liquor to a pH that promotes separation of the lignin from the cellulose, and adjusting the water content of the portion of the black liquor to provide the recycle stream with an elasticity and a viscosity that inhibit separation of the cellulosic biomass from the suspension under pressure and shear upon mixing the portion of the black liquor into the suspension. 
     
     
         9 . The method of  claim 8 , wherein the recycle stream consists essentially of the portion of the black liquor. 
     
     
         10 . The method of  claim 8 , wherein the recycle stream includes NaOH at a molarity of about 4 M NaOH. 
     
     
         11 . The method of  claim 1 , wherein said steps are performed as a continuous process within a fully-intermeshing twin-screw extruder. 
     
     
         12 . The method of  claim 11 , wherein the extruder has intermeshing screws within a barrel, said screws being configured to create at least a first zone and a second zone within the barrel, the first and second zones being created such that the suspension passes first through the first zone then through the second zone. 
     
     
         13 . The method of  claim 12 , wherein the screws are further configured to promote the formation of a suspension seal between the first and second zones 
     
     
         14 . The method of  claim 12 , wherein the mechanical shearing step includes the steps of controlling the temperature of the first zone at a first temperature below the boiling temperature of water and controlling the temperature of the second zone at a temperature above the boiling temperature of water. 
     
     
         15 . The method of  claim 14 , wherein the first temperature is about 70° C. and the second temperature is about 120° C. 
     
     
         16 . The method of  claim 1 , including the further steps of collecting the suspension after the mechanical shearing step and separating the black liquor from the cellulosic biomass. 
     
     
         17 . The method of  claim 16 , including the further step of returning a portion of the separated black liquor to the suspension. 
     
     
         18 . The method of  claim 1 , wherein said method is performed as a continuous process. 
     
     
         19 . The method of  claim 16 , wherein the cellulosic biomass is suitable for fermentation to ethanol and/or biofuels.