Abstract:
A method to wash and remove dissolved solids from biomass including: discharging a biomass slurry from a pretreatment vessel to a biomass slurry retention device; adding recovered wash liquid to dilute the biomass slurry in the retention device, wherein the recovered wash liquid is extracted from a drainer device upstream of the first retention device; discharging the diluted biomass slurry from the retention device to the drainer device; separating wash liquid with dissolved solids from the diluted biomass slurry in the drainer device and discharging a concentrated biomass slurry from the drainer device, and recovering the wash liquid from the diluted biomass slurry in the drainer device and transferring the recovered wash liquid to the retention device.

Description:
CROSS RELATED APPLICATION 
       [0001]    This application claims the benefit of application Ser. No. 61/333,481 filed May 11, 2010, which is incorporated in its entirety by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to pre-hydrolysis of cellulosic biomass to extract five carbon sugars (C5) and six carbon sugars (C6) for production of bio-fuels and chemicals. In particular, the invention relates to methods and devices for early extraction of hemi-cellulosic sugars dissolved from the biomass during pre-hydrolysis and washing the dissolved solids from the biomass discharged from a pre-hydrolysis vessel. 
         [0003]    Biomass includes lignocellulosic material such as wood, including wood chips and sawdust, and fibrous plants. Biomass feed stock is the biomass material conveyed to an inlet of a processing vessel, such as a pre-hydrolysis vessel. Biomass also includes agricultural residues (such as stalks, stover and hulls), straws and grasses or forest and sawmill residues (wood chips and shredded thinnings). Biomass typically excludes fossil fuels which have been transformed by geological processes into substances such as coal or petroleum. Biomass can be grown from numerous types of plants, including miscanthus, switch grass, hemp, corn, poplar, willow, sorghum, sugarcane, and varieties of tree species, ranging from eucalyptus to oil palm (palm oil). 
         [0004]    Pre-hydrolysis, which may be performed as auto-hydrolysis, refers to cooking of cellulosic biomass feed stocks at elevated temperatures of, for example, 110 degrees Celsius (° C.) to 160° C., for approximately 10 to 120 minutes in a solution that dissolves and hydrolysizes hemi-cellulose from the biomass. Auto-hydrolysis refers to using acetic acid released from the acetyl groups in biomass during auto-hydrolysis conditions. 
         [0005]    To enhance the pre-hydrolysis of hemi-cellulose mild acids, such as SO 2 -gas, oxygen and compressed air along with ammonia or other catalyzing agents may be added to the pre-hydrolysis reactor vessel. The pre-hydrolysis and auto-hydrolysis reactions dissolve and separate hemi-cellulose in the biomass to form dissolved C5 sugars, such as xylose and arabinose, and amorphous sugars. Pre-hydrolysis and auto-hydrolysis of soft-wood hemi-cellulose typically yields gluco-mannan, and of hard woods yields xylose and arabinose. 
         [0006]    A conventional pretreatment vessel is a pre-hydrolysis reactor vessel that receives cellulosic biomass feed stock, subjects the biomass feed stock to a hydrolysis reaction, and discharges a slurry of liquid and biomass to an optional bin or other intermediate storage silo or tank, or to a life bottom hopper or similar (with the optional provision to pre-steam) with a discharge device and feeder. The intermediate storage devices may operate under elevated pressures or temperatures of, for example, 110 degrees Celsius (° C.) to 160° C. 
         [0007]    From these intermediate storage devices or directly from the pretreatment vessel, prehydrolysed biomass may be fed to a draining device, such as an inclined drainer, such as a pressing device, which feeds the slurry of biomass and liquid to a sealing device. Conventionally, the draining device removes liquid from the biomass (“dewatering”) to increase the suspended solids (SS) level in the biomass, and does not change the dissolved solids (DS) level of the biomass. The sealing device may be a rotary valve, a modular screw device (MSD—such as the Impressafiner™ sold by the Andritz Group), another high compression, extruder like screw device, or a plug screw feeder that feeds the biomass under pressure to a subsequent reactor vessel, such as a fermentation vessel, a hydrolysis reactor or cooking vessel, e.g., digester. 
         [0008]    Some liquor may be conventionally extracted from the pretreatment vessel to extract a portion of the dissolved hemi cellulosic material from biomass. The biomass discharged from the pretreatment vessel is conventionally transferred directly to a sealing device, e.g., a MSD, and then to subsequent processing, such as to enzymatic hydrolysis or fermentation stages or to a second stage pretreatment stage—for example a high pressure reactor—which may discharge the cooked biomass via steam-explosion or just be diluting and discharging as a slurry. 
         [0009]    The fermentation stages are intended to generate alcohols, e.g. ethanol, from cellulosic feed stocks, e.g., biomass, where high concentrations of a catalyst, e.g., acids, are used and which stages occur at high temperatures and pressures. Removing inhibitors to fermentation, like aldehydes (such as HMF, furfural, and formaldehyde), monomeric phenolics (such as vanillin and coniferylaldehyde), hemi-cellulosic compounds, acids (such as acetic acid, and formic acid) and other components and chemicals should increase the alcohol yield in the following fermentation. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0010]    Hemi cellulose is dissolved from biomass feed stock in a pre-hydrolysis reactor or auto-hydrolysis reactor. After the pre-hydrolysis or auto-hydrolysis reactor, the biomass is washed and dewatered (and optionally washed and dewatered repeatedly) to extract the dissolved hemi-cellulose (C5 sugars) and thereby separate the C5 sugars in the biomass from other sugars, e.g., C6 (glucose), in the biomass. The washing and dewatering of the biomass, and the associated extraction of the hemi cellulose, is performed before the biomass is transferred to a sealing device, e.g., MSD, and further process stages, such as fermentation. 
         [0011]    Extracting hemi-cellulose facilitates the subsequent conversion of other separated sugars to other products. For example, the C5 and C6 sugars separated from hardwoods, herbaceous biomass and agricultural residues may be converted to xylose and other food additives, biogas (through aerobic or anaerobic fermentation), methyl-furan (for use as a high octane oxygenate) or as aqueous sugars for conversion with micro-organisms to alcohols, e.g. ethanol. 
         [0012]    If dissolved hemi-cellulose produced in a pre-hydrolysis reactor is not extracted before the biomass is further processed in fermentation or other processes, the fermentation or other processes may convert the C5 sugars to components and chemicals that inhibit subsequent fermentation steps, such as the fermentation of the extracted C6 sugars. This inhibition of fermentation occurs in conventional processes that include pre-hydrolysis stages and pre-treatment processes in which the C5 sugars are not extracted before the fermentation stages. 
         [0013]    A method is disclosed herein to wash and remove dissolved solids from biomass comprising: discharging a biomass slurry from a pretreatment vessel to a biomass slurry retention device; adding recovered wash liquid to dilute the biomass slurry in the retention device, wherein the recovered wash liquid is extracted from a drainer device upstream of the first retention device; discharging the diluted biomass slurry from the retention device to the drainer device; separating wash liquid with dissolved solids from the diluted biomass slurry in the drainer device and discharging a concentrated biomass slurry from the drainer device, and recovering the wash liquid from the diluted biomass slurry in the drainer device and transferring the recovered wash liquid to the retention device. 
         [0014]    In the method, the pretreatment vessel may be a pre-hydrolysis reactor in which the biomass slurry undergoes hydrolysis, and the recovered wash liquid includes hemi cellulosic material dissolved from the biomass slurry. The method may include discharging a portion of the recovered wash liquid to a hydrolysate recovery device. In the method, the retention device may be a pressurized tank or a pressurized dilution conveyor and the drainer device is an inclined drainer including a screw or auger or an inclined pressing device, a lower inlet for the biomass, an upper outlet for the biomass and a lower liquid outlet. In the method, a portion of the recovered wash liquid is introduced to a lower portion of the pre-hydrolysis reactor vessel or other pretreatment vessel. 
         [0015]    The method may further comprise: introducing the concentrated biomass slurry from the drainer to a second retention device; adding wash liquid recovered from a second drainer upstream of the second retention device to dilute the concentrated biomass slurry in the second retention device; discharging the diluted biomass slurry from the second retention device to the second drainer device; separating wash liquid with dissolved solids from the diluted biomass slurry in the second drainer device and discharging a concentrated biomass slurry from the second drainer device, and recovering the wash liquid from the diluted biomass slurry in the second drainer device and transferring the recovered wash liquid to the second retention device. 
         [0016]    The method may further comprise the introduction of a catalyst or a solvent before or between the various washing stages. The retention device and the inclined draining device may maintain the biomass at elevated temperatures and pressures to improve the diffusion of dissolved solids in the biomass, to enhance washing of the biomass and for energy efficiency by reducing the need to add heat or steam in a subsequent processing vessel. 
         [0017]    The method may further comprise: discharging the concentrated biomass slurry from one of the drainer device or the second drainer device to a high compression extruder; compressing the concentrated biomass slurry in the high compression extruder to further concentrate the biomass slurry, and recovering the wash liquid from the high compression extruder. In the method, the concentrated biomass slurry or the further concentrated biomass slurry may be transferred to a liquefaction reactor where the cellulose will be converted into C6 sugars (hydrolyzed through enzymes or organisms) or a fermentation unit in which C6 sugars in the biomass ferment. 
         [0018]    A system is disclosed for washing processed biomass and removing dissolved solids from the biomass comprising: a pre-hydrolysis reactor vessel having a lower outlet to discharge a biomass slurry, wherein the pre-hydrolysis reactor vessel is operated at conditions that promote hydrolysis of the biomass; a retention tank coupled to a conduit to receive the biomass slurry discharged through the lower outlet of the pre-hydrolysis reactor vessel, wherein the retention tank receives recovered wash liquid and adapted to dilute the biomass slurry in the tank with the recovered wash liquid and is adapted to discharge diluted biomass slurry; a drainer having a lower inlet coupled to a conduit adapted to transfer the discharged diluted biomass slurry from the retention tank to the inlet to the drainer, an upper solids outlet to discharge concentrated biomass slurry, and a lower liquids drain to discharge the recovered wash liquid, wherein the lower liquids drain is coupled to a conduit to transfer the recovered wash liquid to the retention tank. 
         [0019]    The system may further comprise a hydrolysate recovery tank coupled to the conduit for the recovered wash liquid. The retention device may be a pressurized tank and the drainer device is an inclined drainer including a screw or auger, between the lower inlet and the upper solids outlet. The pre-hydrolysis reactor vessel may include a nozzle on a lower portion of the reactor vessel and coupled to the conduit to receive the recovered wash liquid. The system may further comprise: a high compression extruder having an inlet coupled to a conduit for the discharged concentrated biomass slurry from the drainer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIGS. 1A and 1B  are a flow chart of an exemplary process for extracting hemi cellulosic components from biomass and reducing the dissolved solids content of biomass following pre-hydrolysis. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]      FIGS. 1A and 1B  are a flow chart of an exemplary process for extracting hemi cellulosic components from biomass and reducing the dissolved solids content of biomass following pre-hydrolysis. 
         [0022]    A biomass supply  10 , e.g., a chip bin, provides biomass feed stock to a pretreatment vessel  12 , such as a pre-hydrolysis or auto-hydrolysis reactor vessel. The biomass feedstock  10  may be conveyed to the pressurized vessel  12  by gravity or mechanically, e.g., via a screw conveyor or a conveyor belt. 
         [0023]    The biomass feed stock may be conveyed to the pretreatment vessel  12  as dry biomass material or with a partial liquid content. The feeding of the biomass feed stock may include an optional injection of pre-steam and a sealing device that pressurizes the biomass to a pressure suitable for feeding to the vessel  12 . The sealing device  11  may be a rotary valve, a MSD Impressafiner™ (which is a high compression, extruder like screw device) or a plug screw feeder. Alternatively, the biomass feed stock may be pumped directly to a downstream reactor vessel, e.g., a fermentation vessel in a manner similar to a turbo-feed system used in pulping processes. 
         [0024]    The pre-hydrolysis vessel  12  may be a horizontal, vertical or inclined reactor. A horizontal or inclined reactor may include an internal auger or screw to move the biomass slurry through the reactor. A vertical reactor may be similar in structure to a continuous digester vessel conventionally used for producing pulp from wood chips. 
         [0025]    The pretreatment reactor vessel  12  may be a sealed vessel operating at an elevated temperature of, for example, above 100 degrees Celsius (100° C.) and under pressure above atmospheric pressure. The reaction conditions inside the pretreatment reactor vessel  12 , e.g., a pre-hydrolysis reactor, may include a temperature in a range of 110 degrees Celsius (° C.) to 160° C. and a pressure in a range of 1.5 gauge bar to 6 gauge bar. The retention period of biomass in the vessel  12  may be in a range of 10 minutes (min) to 120 min. The retention time is the period from when the biomass enters the pretreatment reactor vessel  12  to when the biomass is discharged from the vessel. These reaction conditions and retention times are exemplary. The reaction conditions in an operational embodiment of the pre-treatment reactor vessel will depend on the biomass material being processed and conditions specific to that process. 
         [0026]    Mild acids, SO 2 -gas, oxygen, compressed air, ammonia or other catalyzing agents  14  may be optionally added to the pretreatment reactor vessel to promote the hydrolysis reaction of the biomass in the vessel. Alternatively, the biomass may undergo an auto-hydrolysis using, for example, the acetic acids released from the acetyl groups in the biomass under the auto-hydrolysis conditions. 
         [0027]    Combinations of steam, ammonia and other heating medium(s) may be used to provide heat energy to the pressurized reactor vessel  12  or to the biomass feed stock  10  prior to entering the vessel  12 . The addition of heat may be unnecessary if the biomass feedstock  10 , liquor  16  and pressure conditions in the pretreatment vessel  12  are sufficient to elevate the temperature in the vessel to promote hydrolysis. 
         [0028]    Various liquors  16 , e.g. chemicals, water and other liquids, may be added to the biomass in the treatment vessel. For example, acids, e.g., sulfur-dioxide SO 2 , may be added to promote a hydrolysis reaction in the vessel. The liquid, chemicals and biomass form a biomass slurry that is discharged from the pretreatment reactor vessel at the vessel bottom outlet  14 . 
         [0029]    The flow rates of solids and liquor through the pretreatment reactor vessel depend on the individual conditions of the process for treating the biomass and can be determined by a person of ordinary skill in the art of treating biomass. The proportion of suspended solids (SS) in the pretreatment vessel may be 20% solids and 80% liquids. The liquid in the slurry may, for example, contain twenty five percent (25%) dissolved solids (DS). The dissolved solids in the slurry include the hemi cellulosic material extracted from the fibers in the biomass during the hydrolysis process occurring in the pretreatment vessel. 
         [0030]    The process shown in  FIGS. 1A and 1B  are envisioned to provide a continuous flow of biomass through the pretreatment vessel and the subsequent tanks and drainers. Alternatively, the process disclosed herein may be embodied as a batch process in which biomass sequentially and in separate steps fills, is treated and thereafter is discharged from the pretreatment reactor vessel. In a batch process, the biomass discharged from the pretreatment vessel cyclically and periodically flows to the tanks and drainers downstream of the pretreatment vessel. 
         [0031]    Recovered wash liquid  18  is introduced at or near the bottom of the pretreatment reactor vessel  12 . The recovered wash liquid may have a lower dissolved solids content, e.g. about 18%, than the biomass slurry in the upper portion of the pretreatment vessel. The addition of recovered wash liquid reduces the portions of solids in the biomass slurry at the bottom outlet  20  of the vessel  12  to, for example, 12% suspended solids and 20% dissolved solids. The recovered wash liquid  18  may also cool the biomass slurry to suppress the hydrolysis reaction in the biomass as the biomass is discharged from the vessel. 
         [0032]    A discharge scraper or a discharge screw  19  in a lower region of the interior of the pretreatment reactor vessel assists in discharging the slurry of biomass from bottom discharge outlet  20  of the pretreatment reactor vessel  12 . 
         [0033]    The biomass slurry discharged from the pretreatment reactor vessel is washed to remove chemicals and dissolved solids and to extract hydrolysate, e.g., hemi-cellulosic (C5 sugars). From the bottom outlet  20  of the pretreatment reactor vessel  12 , the biomass slurry flows under the force of gravity to an upper inlet of a first retention tank  22 . The retention tanks, e.g.,  22 , may be pressurized to a pressure the same as or similar to the pressure in the pretreatment reactor vessel  12  to maintain pressure on the biomass material and retain heat energy in the biomass material. The temperature in the retention tanks  22 ,  34 ,  42 ,  48  may be above 100° C. The biomass slurry flowing to the tank  22  may have a suspended solids content of 12% and a dissolved solids content of 20%. 
         [0034]    The first retention tank  22  receives recovered wash liquid  18  from biomass drainers  26 ,  36 ,  44  and  50  that are downstream of the tank  22 . The drainers are devices that extract liquid from the biomass, and may include screws, augers, presses and other devices that pull liquid out of the biomass. 
         [0035]    The wash liquid from the drainers is recovered by being used in the tanks upstream in the process of where the wash liquid is extracted from the biomass. The recovered wash liquid has a lower or equal dissolved solids (DS) level as compared to the dissolved solids level of the biomass in the first retention tank. For example, the recovered wash liquid  18  added to the first retention tank  22  may have a DS level of 18% as compared to the DS level of 20% of the biomass entering the upstream retention tank(s). Fresh wash liquid  24 , such as water, may be added to each of the retention tanks  22 ,  34 ,  42  and  48 . 
         [0036]    The combination of fresh wash liquid and recovered wash liquid dilutes the slurry of biomass in the retention tanks, and promotes the migration of hemi cellulose and other solids from the fibers in the biomass to the liquid in the biomass slurry. The biomass may be retained for a period of, for example, 3 to 15 minutes, in the first retention tank  22 . 
         [0037]    The biomass settles at the bottom of the first retention tank  22  and is discharged from the bottom to a first drainer  26 , such as MSD screw press. The dissolved solids content of the biomass slurry discharged from the first retention tank is lower than the biomass slurry entering the tank. For example, the biomass slurry at the discharge of the tank  22  may have a DS level of 18% as compared to a DS level of 20% at the inlet to the tank. The suspended solids (SS) level of the biomass slurry also drops in the tank due to the addition of fresh and wash liquids to the tank. For example, the SS level of the biomass slurry at the discharge of the tank  22  may be 8% as compared to a SS level of 12% at the inlet to the tank. 
         [0038]    The drainer  26  dewaters the biomass material to reduce its suspended solids (SS) level. For example, the drainer may reduce the SS level to 20% from 8% of the biomass slurry. If the drainer does not add wash liquid, the drainer does not change the dissolved solids (DS) level of the biomass. 
         [0039]    If one or both of fresh and recovered wash liquid are added to the drainer, the DS level of the biomass may be reduced by the drainer. Wash liquid may be added to the drainer by nozzles mounted on the housing of the drainer to inject the wash liquid into the biomass move upwardly through the drainer. If the biomass in the drainer has sufficient retention time to allow solids, e.g., hemi cellulose, in the biomass to migrate to dissolved solids in the liquid, one or more of the retention tanks may be unnecessary. 
         [0040]    The drainer  26  may include an internal screw or auger to move biomass material upwardly through an internal passage in the drainer. The screw or auger moves the biomass material upwardly while allowing liquids to drain to the bottom outlet  28 . Further, the screen or auger may compress the biomass and thereby squeeze liquids with dissolved solids from the biomass. As the biomass moves up through the drainer, liquid and dissolved solids in the biomass remain in the bottom of the drainer and are extracted through the outlet  28 . A screen between the internal passage in the drainer and the outlet prevents biomass solid materials, e.g., fibers, from flowing through the drain outlet. 
         [0041]    The extracted wash liquid with dissolved solids, e.g., DS level of 18%, from the first drainer  26  flows through the outlet to a conduit  30  that directs a portion of the wash liquid to a hydrolysate tank  32  from which the dissolve hemi cellulosic (C5 sugars) material may be extracted and used in further processes. Another portion of the extracted wash liquid from the drainer is used as recovered wash liquid and directed to the pretreatment vessel  12  and to the first retention tank  22 , both of which are upstream of the first drainer  26 . The recovered wash liquid flows upstream, e.g., cross-current, to the flow direction of the biomass slurry through the series of retention tanks and drainers. 
         [0042]    The sequence of diluting the biomass material with wash liquid to reduce the dissolved solids (DS) level and thereafter dewatering the biomass material to reduce the suspended solids (SS) level may be repeated until the DS and SS levels of the biomass material is at desired levels. 
         [0043]    A second retention tank  34  receives the dewatered biomass slurry from the first drainer  26  and dilutes the biomass with fresh wash liquid  24  and wash liquid recovered from drainers downstream of the second retention tank. The structure, operation and purpose of the second retention tank is substantially the same as the first retention tank, with the exception that the second retention tank receives the biomass slurry from a drainer and at a higher SS level and a lower DS level than the biomass slurry received by the first tank  22  from the pretreatment vessel  12 . For example, the biomass slurry flowing to an upper inlet of the second retention tank  34  may have a SS level of 20% and a DS level of 18%, as compared to the biomass slurry with an SS level of 12% and a DS level of 20% entering the first retention tank. 
         [0044]    The DS level in the biomass slurry is reduced by sequentially (1) diluting the biomass slurry to reduce the DS level and (2) dewatering the biomass slurry by extracting wash liquid with dissolved solids. These two steps reduce the DS content of the biomass slurry, extract dissolved hemi cellulosic compounds and maintain the SS content to acceptable levels. The sequence of diluting and dewatering the biomass may be repeated with successive retention tanks and drainers until the DS level of the biomass has been reduced to a desired level, e.g., to below 10% and preferably below 8% and even below 7%. Alternatively, the sequence of diluting and dewatering the biomass slurry may be repeated until sufficient hemi cellulosic material is removed from the biomass. 
         [0045]    The second retention tank  34  receives the dewatered biomass material from the first drain  26 . Fresh wash liquid  24  and recycled wash liquid from a second drainer  36  and subsequent drainers are added to the second retention tank  34  to dilute the biomass slurry in the tank. The biomass slurry fed to an upper inlet to the second retention tank may have a DS level of 18% and a SS level of 20%. The biomass slurry may be discharged from the second retention tank with a DS level of 14% and a SS of 5%. The second drainer  36  dewaters the biomass slurry such that at the drainer discharge the slurry as a DS level of 14% and a SS level of 20%. The wash liquid outlet to the second drain  36  discharges liquid with dissolved solids, e.g., hemi cellulosic material, to a conduit  38  that feeds the wash liquid to one or more of the hydrolysate tank  32 , the upstream retention tanks  22 ,  34 , and the pretreatment reactor vessel  12 . 
         [0046]    The biomass discharged from an upper outlet of the second drainer  36  flows through conduit  40  to a third retention tank  42 . The biomass slurry may enter the third retention tank  42  with a DS level of 14% and a SS level of 20%. Fresh wash liquid  24  and recovered wash liquid  46 ,  54  flows into the third retention tank  42  to dilute the biomass slurry. The retention time of the biomass slurry in the third retention tank  42 , as well as all of the retention tanks  22 ,  34 ,  48 , is sufficient to allow hemi cellulosic material and other solids in the fibers of the biomass to migrate into the liquid as dissolved solids. At the lower discharge of the third retention tank  42 , the biomass slurry may have a DS level of 11% and a SS level of 5%. From the lower discharge the biomass slurry flows through conduit  43  to the third drainer  44 . 
         [0047]    The biomass slurry discharged from the third retention tank is dewatered in the third drainer  44 , which may be an horizontal, inclined or vertical drainer with screw or auger conveyors (as may be each of the drainers). The drainers  26 ,  36 ,  44 ,  50  may also be embodied as a pressing device that compresses the discharged biomass slurry to extract water (or other liquid) from the slurry. 
         [0048]    The third drainer  44  has an upper outlet that discharges biomass slurry having, for example, a DS level of 11% and a SS level of 20%. The third drainer has a lower liquid outlet that discharges extracted wash liquid to a conduit  46 . The discharged wash liquid may have a DS level of 11%, which is the same DS level as the biomass slurry moving through the drainers. Each of the drainers  26 ,  36   44  and  50 , generally discharge wash liquid having a DS level the same as the DS level of the biomass slurry passing through the drainer. The wash liquid discharged from the third drainer flows through conduit  46  to the hydrolysate tank  32  and the upstream retention tanks  34 ,  42  and pretreatment reactor vessel  12 . 
         [0049]    A fourth retention tank  48  receives biomass slurry discharged to conduit  45  from the third inclined drainer  44 . Fresh wash liquid  24  and recovered wash liquid from a discharge conduit  51  from a fourth drainer  50  is added to the fourth retention tank  48  to dilute the biomass in that tank. The biomass slurry entering the fourth retention tank  48  may have a DS level of 11% and a SS level of 20%. The biomass slurry discharged from the fourth retention tank may have a DS level of 7.4% and a SS level of 5%. 
         [0050]    The fourth drainer  50  receives and dewaters the biomass discharged to conduit  49  from the fourth retention tank  48 . The biomass discharged from an upper inlet to the fourth drainer may have a DS level of 7.4% and a SS level of 20%. The drainers may each elevate the SS level to a uniform level, such as to a SS level of 20%. The wash liquid extracted from the drainers has a DS level the same as the DS level of the biomass slurry flowing through the drainer. 
         [0051]    The biomass slurry discharged from the last drainer, e.g., the fourth drainer  50 , may flow via conduit  53  to a modular screw device (MSD)  52  or other high compression, extruder like screw device. An example of a MSD is the Impressafiner™ sold by the Andritz Group. The MSD  52  increases the SS level of the biomass slurry from, for example, 20% to 40%. The WASH liquid extracted from the biomass slurry by the MSD may flow through conduit  54  as recovered wash liquid that flows to the retention tanks and pretreatment reactor vessel  12 . The recovered wash liquid form the MSD  52  may also flow to the hydrolysate tank  32 . 
         [0052]    The biomass slurry discharged from the MSD has a high SS level, e.g., 40%, a low DS level, e.g., 7.4%, and has had extracted much of its dissolved hemi cellulosic material. The biomass slurry from the MSD may flow to a second reaction vessel  56  in which further processes are performed on the biomass such as a fermentation in a fermentation vessel, or enzymatic hydrolysis in a hydrolysis reactor or further cooking in a cooking vessel, e.g., a digester vessel. 
         [0053]    Four washing stages are shown in  FIGS. 1A and 1B  by way of example. The number of washing stages may be less than or greater than four, in other embodiments of the invention. The retention tanks,  22 ,  34 ,  42  and  48  may be pressurized tanks or other retention devices such as dilution conveyor, which is a pressurized vessel with an internal screw auger having cut-flights that evenly re-dilute the slurry inside the vessel of the conveyor. Further, a catalyst or a solvent may be added to the biomass slurry before or between the various washing stages, such as by introduction to the conduit between the screen of a drainer and a retention tank. 
         [0054]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.