Patent Publication Number: US-2016237102-A1

Title: Processes and apparatus for lignin separation in biorefineries

Description:
PRIORITY DATA 
     This patent application is a divisional application of U.S. Pat. No. 9,322,072 issued on Apr. 26, 2016, which in turn claims priority to U.S. Provisional Patent App. No. 61/735,738, filed Dec. 11, 2012, each of which is hereby incorporated by reference herein. 
    
    
     FIELD 
     The present invention generally relates to fractionation processes for converting biomass into fermentable sugars, cellulose, and lignin. 
     BACKGROUND 
     Biomass refining (or biorefining) is becoming more prevalent in industry. Cellulose fibers and sugars, hemicellulose sugars, lignin, syngas, and derivatives of these intermediates are being used by many companies for chemical and fuel production. Indeed, we now are observing the commercialization of integrated biorefineries that are capable of processing incoming biomass much the same as petroleum refineries now process crude oil. Underutilized lignocellulosic biomass feedstocks have the potential to be much cheaper than petroleum, on a carbon basis, as well as much better from an environmental life-cycle standpoint. 
     Lignocellulosic biomass is the most abundant renewable material on the planet and has long been recognized as a potential feedstock for producing chemicals, fuels, and materials. Lignocellulosic biomass normally comprises primarily cellulose, hemicellulose, and lignin. Cellulose and hemicellulose are natural polymers of sugars, and lignin is an aromatic/aliphatic hydrocarbon polymer reinforcing the entire biomass network. Some forms of biomass (e.g., recycled materials) do not contain hemicellulose. 
     There are many reasons why it would be beneficial to process biomass in a way that effectively separates the major fractions (cellulose, hemicellulose, and lignin) from each other. Cellulose from biomass can be used in industrial cellulose applications directly, such as to make paper or other pulp-derived products. The cellulose can also be subjected to further processing to either modify the cellulose in some way or convert it into glucose. Hemicellulose sugars can be fermented to a variety of products, such as ethanol, or converted to other chemicals. Lignin from biomass has value as a solid fuel and also as an energy feedstock to produce liquid fuels, synthesis gas, or hydrogen; and as an intermediate to make a variety of polymeric compounds. Additionally, minor components such as proteins or rare sugars can be extracted and purified for specialty applications. 
     In light of this objective, a major shortcoming of previous process technologies is that one or two of the major components can be economically recovered in high yields, but not all three. Either the third component is sacrificially degraded in an effort to produce the other two components, or incomplete fractionation is accomplished. An important example is traditional biomass pulping (to produce paper and related goods). Cellulose is recovered in high yields, but lignin is primarily consumed by oxidation and hemicellulose sugars are mostly degraded. Approximately half of the starting biomass is essentially wasted in this manufacturing process. State-of-the-art biomass-pretreatment approaches typically can produce high yields of hemicellulose sugars but suffer from moderate cellulose and lignin yields. 
     There are several possible pathways to convert biomass into intermediates. One thermochemical pathway converts the feedstock into syngas (CO and H 2 ) through gasification or partial oxidation. Another thermochemical pathway converts biomass into liquid bio-oils through pyrolysis and separation. These are both high-temperature processes that intentionally destroy sugars in biomass. 
     Sugars (e.g., glucose and xylose) are desirable platform molecules because they can be fermented to a wide variety of fuels and chemicals, used to grow organisms or produce enzymes, converted catalytically to chemicals, or recovered and sold to the market. To recover sugars from biomass, the cellulose and/or the hemicellulose in the biomass must be hydrolyzed into sugars. This is a difficult task because lignin and hemicelluloses are bound to each other by covalent bonds, and the three components are arranged inside the fiber wall in a complex manner. This recalcitrance explains the natural resistance of woody biomass to decomposition, and explains the difficulty to convert biomass to sugars at high yields. 
     Fractionation of biomass into its principle components (cellulose, hemicellulose, and lignin) has several advantages. Fractionation of lignocellulosics leads to release of cellulosic fibers and opens the cell wall structure by dissolution of lignin and hemicellulose between the cellulose microfibrils. The fibers become more accessible for hydrolysis by enzymes. When the sugars in lignocellulosics are used as feedstock for fermentation, the process to open up the cell wall structure is often called “pretreatment.” Pretreatment can significantly impact the production cost of lignocellulosic ethanol. 
     One of the most challenging technical obstacles for cellulose has been its recalcitrance towards hydrolysis for glucose production. Because of the high quantity of enzymes typically required, the enzyme cost can be a tremendous burden on the overall cost to turn cellulose into glucose for fermentation. Cellulose can be made to be reactive by subjecting biomass to severe chemistry, but that would jeopardize not only its integrity for other potential uses but also the yields of hemicellulose and lignin. 
     Many types of pretreatment have been studied. A common chemical pretreatment process employs a dilute acid, usually sulfuric acid, to hydrolyze and extract hemicellulose sugars and some lignin. A common physical pretreatment process employs steam explosion to mechanically disrupt the cellulose fibers and promote some separation of hemicellulose and lignin. Combinations of chemical and physical pretreatments are possible, such as acid pretreatment coupled with mechanical refining. It is difficult to avoid degradation of sugars. In some cases, severe pretreatments (i.e., high temperature and/or low pH) intentionally dehydrate sugars to furfural, levulinic acid, and related chemicals. Also, in common acidic pretreatment approaches, lignin handling is very problematic because acid-condensed lignin precipitates and forms deposits on surfaces throughout the process. 
     One type of pretreatment that can overcome many of these disadvantages is called “organosolv” pretreatment. Organosolv refers to the presence of an organic solvent for lignin, which allows the lignin to remain soluble for better lignin handling. Traditionally, organosolv pretreatment or pulping has employed ethanol-water solutions to extract most of the lignin but leave much of the hemicellulose attached to the cellulose. For some market pulps, it is acceptable or desirable to have high hemicellulose content in the pulp. When high sugar yields are desired, however, there is a problem. Traditional ethanol/water pulping cannot give high yields of hemicellulose sugars because the timescale for sufficient hydrolysis of hemicellulose to monomers causes soluble-lignin polymerization and then precipitation back onto cellulose, which negatively impacts both pulp quality as well as cellulose enzymatic digestibility. 
     An acid catalyst can be introduced into organosolv pretreatment to attempt to hydrolyze hemicellulose into monomers while still obtaining the solvent benefit. Conventional organosolv wisdom dictates that high delignification can be achieved, but that a substantial fraction of hemicellulose must be left in the solids because any catalyst added to hydrolyze the hemicellulose will necessarily degrade the sugars (e.g., to furfural) during extraction of residual lignin. 
     Contrary to the conventional wisdom, it has been found that fractionation with a solution of ethanol (or another solvent for lignin), water, and sulfur dioxide (SO 2 ) can simultaneously achieve several important objectives. The fractionation can be achieved at modest temperatures (e.g., 120-160° C.). The SO 2  can be easily recovered and reused. This process is able to effectively fractionation many biomass species, including softwoods, hardwoods, agricultural residues, and waste biomass. The SO 2  hydrolyzes the hemicelluloses and reduces or eliminates troublesome lignin-based precipitates. The presence of ethanol leads to rapid impregnation of the biomass, so that neither a separate impregnation stage nor size reduction smaller than wood chips are needed, thereby avoiding electricity-consuming sizing operations. The dissolved hemicelluloses are neither dehydrated nor oxidized (Iakovlev, “SO 2 -ethanol-water fractionation of lignocellulosics,” Ph.D. Thesis, Aalto Univ., Espoo, Finland, 2011). Cellulose is fully retained in the solid phase and can subsequently be hydrolyzed to glucose. The mixture of hemicellulose monomer sugars and cellulose-derived glucose may be used for production of biofuels and chemicals. 
     Commercial sulfite pulping has been practiced since 1874. The focus of sulfite pulping is the preservation of cellulose. In an effort to do that, industrial variants of sulfite pulping take 6-10 hours to dissolve hemicelluloses and lignin, producing a low yield of fermentable sugars. Stronger acidic cooking conditions that hydrolyze the hemicellulose to produce a high yield of fermentable sugars also hydrolyze the cellulose, and therefore the cellulose is not preserved. 
     The dominant pulping process today is the Kraft process. Kraft pulping does not fractionate lignocellulosic material into its primary components. Instead, hemicellulose is degraded in a strong solution of sodium hydroxide with or without sodium sulfide. The cellulose pulp produced by the Kraft process is high quality, essentially at the expense of both hemicellulose and lignin. 
     Sulfite pulping produces spent cooking liquor termed sulfite liquor. Fermentation of sulfite liquor to hemicellulosic ethanol has been practiced primarily to reduce the environmental impact of the discharges from sulfite mills since 1909. However, ethanol yields do not exceed one-third of the original hemicellulose component. Ethanol yield is low due to the incomplete hydrolysis of the hemicelluloses to fermentable sugars and further compounded by sulfite pulping side products, such as furfural, methanol, acetic acid, and others fermentation inhibitors. 
     Solvent cooking chemicals have been attempted as an alternative to Kraft or sulfite pulping. The original solvent process is described in U.S. Pat. No. 1,856,567 by Kleinert et al. Groombridge et al. in U.S. Pat. No. 2,060,068 showed that an aqueous solvent with sulfur dioxide is a potent delignifying system to produce cellulose from lignocellulosic material. Three demonstration facilities for ethanol-water (Alcell), alkaline sulfite with anthraquinone and methanol (ASAM), and ethanol-water-sodium hydroxide (Organocell) were operated briefly in the 1990s. 
     In view of the state of the art, what is desired is to efficiently fractionate any lignocellulosic-based biomass (including, in particular, softwoods) into its primary components so that each can be used in potentially distinct processes. While not all commercial products require pure forms of cellulose, hemicellulose, or lignin, a platform biorefinery technology that enables processing flexibility in downstream optimization of product mix, is particularly desirable. An especially flexible fractionation technique would not only separate most of the hemicellulose and lignin from the cellulose, but also render the cellulose highly reactive to cellulase enzymes for the manufacture of fermentable glucose. 
     The AVAP® fractionation process developed by American Process, Inc. and its affiliates is able to economically accomplish these objectives. Improvements are still desired in the areas of lignin separation and recovery. 
     One of the biggest and well-known challenges in many biorefineries is dealing with lignin. Lignin is a major component of biomass. It is typically between 15-35 wt % (dry basis) of the biomass material. Lignin has good fuel value, similar to some types of coal. 
     Lignin is a natural polymer and is an essential part of wood and other forms of cellulosic biomass, including agricultural crop residues such as sugarcane bagasse. Lignin performs multiple functions that are essential to the life of the plant, including transport of nutrition and durability of the biomass. Lignin imparts rigidity to the cell walls and acts as a binder, creating a flexible composite cellulose-hemicellulose-lignin material that is outstandingly resistant to impact, compression, and bending. 
     After polysaccharides (polymers of sugar), lignin is the most abundant organic polymer in the plant world. Lignin is a very complex natural polymer with many random couplings, and therefore lignin has no exact chemical structure. The molecular structure of lignin consists primarily of carbon ring structures (benzene rings with methoxyl, hydroxyl, and propyl groups. 
     Various processes can be used to remove and isolate lignin from biomass. Each process, however, produces material of different composition and properties. Generally there are four important factors to take into account when working with lignin: 
     1. Source of the lignin. 
     2. Method used to remove lignin from the biomass. 
     3. Method(s) used to purify the lignin. 
     4. Nature of the chemical modification of the lignin after isolation. 
     These factors influence the properties of the lignin. Important properties of lignin formulations include molecular weight, chemical composition, and the type and distribution of chemical functional groups. 
     Separation and recovery of lignin is quite difficult. It is possible to break the lignin-cellulose-hemicellulose matrix and recover the lignin through a variety of treatments on the lignocellulosic material. However, known lignin recovery methods generally have one or more important commercial-scale limitations. Lignin purification from biomass is a classic chemical-engineering problem with complex chemistries and transport phenomena, criticality of reactor design and scale-up, serious analytical challenges, and many practical issues arising from lignin&#39;s propensity to stick to equipment and piping. 
     Lignin can be difficult to process in biorefineries because it has a tendency to deposit on solid surfaces and cause plugging. Although lignin handling has always been known to be a challenge, there remains a need in the art for ways to either avoid lignin precipitation or to deal with it after it occurs. Other difficulties are caused by downstream fermentation inhibition caused by lignin, as well as lignin fragments and derivatives (e.g., phenolics, acids, and other compounds). 
     Lignin separations challenges appear to be particularly troubling problem for acidic pretreatments of biomass or biomass-derived liquors. For example, in van Heiningen et al., “Which fractionation process can overcome the techno-economic hurdles of a lignocellulosic biorefinery,” Proceedings of the AIChE Annual Meeting, Minneapolis, Minn. (2011), it is cautioned that “an operating problem which has mostly been overlooked for acidic pretreatment is formation and precipitation of sticky lignin on reactor walls and piping.” The lack of R&amp;D attention to this problem is stated to be that it only “becomes apparent in continuous larger scale operation after one to two week operation.” 
     In view of the aforementioned needs in the art, improvements are clearly needed to avoid, or deal with, lignin precipitation during acidic hydrolysis of biomass and/or biomass hydrolysates (such as hemicellulose-containing liquid extracts). 
     SUMMARY 
     The present invention addresses the aforementioned needs in the art. 
     In some variations, the invention provides a process for fractionating lignocellulosic biomass, the process comprising: 
     (a) providing a feedstock comprising lignocellulosic biomass; 
     (b) in a digestor, fractionating the feedstock under effective fractionation conditions in the presence of a solvent for lignin, an acid or acid precursor (such as sulfur dioxide), and water, to produce a liquor containing hemicellulose, cellulose-rich solids, and lignin; 
     (c) substantially removing the cellulose-rich solids from the liquor; 
     (d) introducing a solid additive to the liquor (in the digestor or otherwise), wherein the solid additive combines with at least a portion of the lignin; 
     (e) separating a mixture comprising the lignin and the solid additive, each in free or combined form, from the liquor; 
     (f) optionally recovering the hemicellulose; and 
     (g) optionally hydrolyzing the cellulose-rich solids to glucose. 
     In some embodiments, the solid additive has a density of at least 1.5 g/cm 3 , such as at least 2.0 g/cm 3 . The solid additive may be introduced to the digestor as a slurry or dissolved in a solution. In some embodiments, the solid additive is present in the digestor at a concentration of at least 0.1 g/L, at least 1 g/L, or at least 10 g/L. 
     In some embodiments, the solid additive is selected from the group consisting of metal sulfates, metal sulfate hydrates, metal sulfate derivatives, ammonium sulfate, ammonium sulfate derivatives, native lignin, acid-condensed lignin, sulfonated lignin, lignin derivatives, and combinations thereof. In certain embodiments, the solid additive is selected from the group consisting of anhydrite, calcium sulfate hemihydrate, calcium sulfate dihydrate (gypsum), and combinations thereof. 
     The solid additive may comprise or consist essentially of gypsum. The solid additive may comprise or consist essentially of gypsum and lignin. The gypsum may be recycled gypsum that is generated following step (f). The lignin may be recycled lignin that is removed following step (f). 
     In some embodiments, the solid additive is selected from the group consisting of minerals, diatomaceous earth, silica, alumina, ash, zeolites, metal alums, ammonium alum, dust, cellulose, nanocellulose, sawdust, agricultural residue pith, biomass fines, and combinations thereof. 
     Without being limited by theories or hypotheses, at least a portion of the solid additive may combine, chemically or physically, with the lignin to form a lignin-additive complex that has a higher density than the density of the lignin. At least a portion of the solid additive may combine, chemically or physically, with the lignin to form a lignin-additive complex that has one or more of the following properties: (i) a higher settling rate than that of the lignin; (ii) a higher viscosity than that of the lignin; (iii) a higher ratio of density to viscosity compared to the lignin; and (iv) reduced tackiness compared to the lignin. 
     Step (d) may be carried out prior to, simultaneously with, or following removal of the solvent for lignin from the liquor. Also, step (d) may be carried out prior to, simultaneously with, or following removal of the sulfur dioxide from the liquor. The solid additive is optionally recovered and recycled. 
     Some variations provide a process for fractionating lignocellulosic biomass, the process comprising: 
     (a) providing a feedstock comprising lignocellulosic biomass; 
     (b) in a digestor, fractionating the feedstock under effective fractionation conditions in the presence of a solvent for lignin, sulfur dioxide, and water, to produce a liquor containing hemicellulose, cellulose-rich solids, and lignin; 
     (c) substantially removing the cellulose-rich solids from the liquor; 
     (d) introducing gypsum to the digestor, wherein the gypsum combines with at least a portion of the lignin; 
     (e) separating a mixture comprising the lignin and the gypsum, each in free or combined form, from the liquor; 
     (f) neutralizing an acidic solution of the hemicellulose with lime, thereby generating produced gypsum; 
     (g) recovering the hemicellulose; and 
     (h) recycling at least a portion of the produced gypsum to step (d). 
     The additive precursor or precursors comprises lime and the additive comprises gypsum, in certain embodiments of the invention. 
     A process for fractionating lignocellulosic biomass, according to some embodiments, includes the steps of: 
     (a) providing a feedstock comprising lignocellulosic biomass; 
     (b) in a digestor, fractionating the feedstock under effective fractionation conditions in the presence of ethanol, sulfur dioxide, and water, to produce a liquor containing hemicellulose, cellulose-rich solids, and lignin; 
     (c) substantially removing the cellulose-rich solids from the liquor; 
     (d) introducing a solid additive to the digestor, wherein the solid additive combines with at least a portion of the lignin; 
     (e) separating a mixture comprising the lignin and the solid additive, each in free or combined form, from the liquor; and 
     (f) recovering the hemicellulose. 
     Some variations provide a process for fractionating lignocellulosic biomass, the process comprising: 
     (a) providing a feedstock comprising lignocellulosic biomass; 
     (b) in a digestor, fractionating the feedstock under effective fractionation conditions in the presence of a solvent for lignin, an acid or acid precursor, and water, to produce a liquor containing hemicellulose, cellulose-rich solids, and lignin; 
     (c) substantially removing the cellulose-rich solids from the liquor; 
     (d) in a hydrolysis reactor, hydrolyzing the hemicellulose contained in the liquor, in the presence of a hydrolysis catalyst, to produce fermentable hemicellulosic sugars; 
     (e) introducing a solid additive to the hydrolysis reactor, wherein the solid additive combines with at least a portion of the lignin; 
     (f) separating a mixture comprising the lignin and the solid additive, each in free or combined form, from the fermentable hemicellulosic sugars; and 
     (g) recovering the fermentable hemicellulosic sugars. 
     In some embodiments, step (d) is carried out prior to, simultaneously with, or following removal of the solvent for lignin from the liquor. Also, step (d) may be carried out prior to, simultaneously with, or following removal of the sulfur dioxide from the liquor. 
     When a solid additive is introduced to the hydrolysis reactor, the process may further include introducing a solid additive to the digestor. The solid additive introduced to the digestor may be the same as, or different than, the solid additive introduced to the hydrolysis reactor. Any of these solids additives may be recovered and recycled. 
     The above-described or other variations provide methods of improving lignin separation during lignocellulosic biorefining, the method comprising the steps of (i) catalyzing fractionation or hydrolysis with an acid to release sugars into an acidified solution containing lignin, wherein the solution also contains a solvent for lignin; (ii) neutralizing the acidified solution with a base to form a salt in a neutralized solution; (iii) in a separation unit, separating the salt and the lignin, each in free or combined form, from the neutralized solution; and then (iv) recycling a portion of the salt and optionally a portion of the lignin to step (i) to combine, physically or chemically, with the lignin, to improve lignin separation in the separation unit. 
     Known apparatus may be employed for these methods. The separation unit may be selected from the group consisting of a filter, a membrane, a decanter, a clarifier, a hydrocyclone, and a centrifuge. The salt may be selected from the group consisting of metal sulfates, metal sulfate hydrates, metal sulfate derivatives, ammonium sulfate, ammonium sulfate derivatives, and combinations thereof. For example, the salt may be selected from the group consisting of anhydrite, calcium sulfate hemihydrate, calcium sulfate dihydrate (gypsum), and combinations thereof. 
     Some embodiments provide a method of improving lignin separation during lignocellulosic biorefining, the method comprising the steps of (i) catalyzing fractionation or hydrolysis with a sulfur-containing acid and/or sulfur dioxide to release sugars and lignin into an acidified solution, wherein the solution also contains a solvent for lignin; (ii) neutralizing the acidified solution with lime to form gypsum in a neutralized solution; (iii) in a separation unit, separating the gypsum and the lignin, individually or in combination, from the neutralized solution; and then (iv) recycling a portion of the gypsum and optionally a portion of the lignin to step (i) to combine, physically or chemically, with the lignin released in step (i), to improve lignin separation in the separation unit in step (iii). 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURE 
         FIG. 1  depicts an exemplary process embodiment of the invention to fractionate biomass into cellulose, hemicellulose, and lignin with introduction and/or recycle of one or more solid additives at one or more locations to improve lignin separation. 
     
    
    
     DETAILED DESCRIPTION OF SOME EMBODIMENTS 
     This description will enable one skilled in the art to make and use the invention, and it describes several embodiments, adaptations, variations, alternatives, and uses of the invention. These and other embodiments, features, and advantages of the present invention will become more apparent to those skilled in the art when taken with reference to the following detailed description of the invention in conjunction with any accompanying drawings. 
     As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly indicates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this invention belongs. All composition numbers and ranges based on percentages are weight percentages, unless indicated otherwise. All ranges of numbers or conditions are meant to encompass any specific value contained within the range, rounded to any suitable decimal point. 
     Unless otherwise indicated, all numbers expressing parameters, reaction conditions, concentrations of components, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending at least upon a specific analytical technique. 
     The term “comprising,” which is synonymous with “including,” “containing,” or “characterized by” is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. “Comprising” is a term of art used in claim language which means that the named claim elements are essential, but other claim elements may be added and still form a construct within the scope of the claim. 
     As used herein, the phase “consisting of” excludes any element, step, or ingredient not specified in the claim. When the phrase “consists of” (or variations thereof) appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole. As used herein, the phase “consisting essentially of” limits the scope of a claim to the specified elements or method steps, plus those that do not materially affect the basis and novel characteristic(s) of the claimed subject matter. 
     With respect to the terms “comprising,” “consisting of,” and “consisting essentially of,” where one of these three terms is used herein, the presently disclosed and claimed subject matter may include the use of either of the other two terms. Thus in some embodiments not otherwise explicitly recited, any instance of“comprising” may be replaced by “consisting of” or, alternatively, by “consisting essentially of.” 
     The present invention, in some variations, is premised on the discovery that lignin separation may be improved, to a surprising extent, by introducing certain additives directly or indirectly into the digestor and/or hydrolysis reactor, as a precursor to later lignin separation by (for example) sedimentation, centrifugation, or filtration, or other separation operations to increase the lignin separation efficiency. 
     Certain exemplary embodiments of the invention will now be described. These embodiments are not intended to limit the scope of the invention as claimed. The order of steps may be varied, some steps may be omitted, and/or other steps may be added. Reference herein to first step, second step, etc. is for illustration purposes only. 
     Some embodiments can be understood with reference to  FIG. 1 . When a process sequence includes a hydrolysis reactor followed by a separation unit for removing lignin, such as depicted in  FIG. 1 , it is preferable in some embodiments to minimize precipitation of lignin in the digestor or hydrolysis reactor itself. The dotted lines in  FIG. 1  are optional streams. 
     In some variations, the invention provides a process for fractionating lignocellulosic biomass, the process comprising: 
     (a) providing a feedstock comprising lignocellulosic biomass; 
     (b) in a digestor, fractionating the feedstock under effective fractionation conditions in the presence of a solvent for lignin, sulfur dioxide, and water, to produce a liquor containing hemicellulose, cellulose-rich solids, and lignin; 
     (c) substantially removing the cellulose-rich solids from the liquor; 
     (d) introducing a solid additive to the digestor, wherein the solid additive combines with at least a portion of the lignin; 
     (e) separating a mixture comprising the lignin and the solid additive, each in free or combined form, from the liquor; and 
     (f) recovering the hemicellulose. 
     In some variations, the invention provides a process for fractionating lignocellulosic biomass, the process comprising: 
     (a) providing a feedstock comprising lignocellulosic biomass; 
     (b) in a digestor, fractionating said feedstock under effective fractionation conditions in the presence of a solvent for lignin, an acid or acid precursor, and water, to produce a liquor containing hemicellulose, cellulose-rich solids, and lignin; 
     (c) substantially removing said cellulose-rich solids from said liquor; 
     (d) introducing a solid additive to said liquor, wherein said solid additive combines with at least a portion of said lignin; and 
     (e) separating a mixture comprising said lignin and said solid additive, each in free or combined form, from said liquor. 
     The acid or acid precursor may be a sulfur-containing acid or compound, such as (but not limited to) sulfur dioxide, sulfurous acid, sulfur trioxide, sulfuric acid, lignosulfonic acid, or combinations thereof. Other acids or acid precursors may be employed. An acid precursor will generate some amount of acid in situ, under the liquor conditions of the digestor. 
     In some embodiments, the solid additive has a density of at least 1.5 g/cm 3 , such as at least 2.0 g/cm 3 . The solid additive may be introduced to the digestor as a slurry or dissolved in a solution. In some embodiments, the solid additive is present in the digestor at a concentration of at least 0.1 g/L, at least 1 g/L, or at least 10 g/L. 
     In some embodiments, the solid additive is selected from the group consisting of metal sulfates, metal sulfate hydrates, metal sulfate derivatives, ammonium sulfate, ammonium sulfate derivatives, native lignin, acid-condensed lignin, sulfonated lignin, lignin derivatives, and combinations thereof. In certain embodiments, the solid additive is selected from the group consisting of anhydrite, calcium sulfate hemihydrate, calcium sulfate dihydrate (gypsum), and combinations thereof. 
     The solid additive may comprise or consist essentially of gypsum. The solid additive may comprise or consist essentially of gypsum and lignin. The gypsum may be recycled gypsum that is generated following step (f). The lignin may be recycled lignin that is removed following step (f). 
     In some embodiments, the solid additive is selected from the group consisting of minerals, diatomaceous earth, silica, alumina, ash, zeolites, metal alums, ammonium alum, dust, cellulose, nanocellulose, sawdust, agricultural residue pith, biomass fines, and combinations thereof. 
     Without being limited by theories or hypotheses, at least a portion of the solid additive may combine, chemically or physically, with the lignin to form a lignin-additive complex that has a higher density than the density of the lignin. At least a portion of the solid additive may combine, chemically or physically, with the lignin to form a lignin-additive complex that has one or more of the following properties: (i) a higher settling rate than that of the lignin; (ii) a higher viscosity than that of the lignin; (iii) a higher ratio of density to viscosity compared to the lignin; and (iv) reduced tackiness compared to the lignin. 
     Step (d) may be carried out prior to, simultaneously with, or following removal of the solvent for lignin from the liquor. Also, step (d) may be carried out prior to, simultaneously with, or following removal of the sulfur dioxide from the liquor. The solid additive is optionally recovered and recycled. 
     Some variations provide a process for fractionating lignocellulosic biomass, the process comprising: 
     (a) providing a feedstock comprising lignocellulosic biomass; 
     (b) in a digestor, fractionating the feedstock under effective fractionation conditions in the presence of a solvent for lignin, sulfur dioxide, and water, to produce a liquor containing hemicellulose, cellulose-rich solids, and lignin; 
     (c) substantially removing the cellulose-rich solids from the liquor; 
     (d) introducing gypsum to the digestor, wherein the gypsum combines with at least a portion of the lignin; 
     (e) separating a mixture comprising the lignin and the gypsum, each in free or combined form, from the liquor; 
     (f) neutralizing an acidic solution of the hemicellulose with lime, thereby generating produced gypsum; 
     (g) recovering the hemicellulose; and 
     (h) recycling at least a portion of the produced gypsum to step (d). 
     The additive precursor or precursors comprises lime and the additive comprises gypsum, in certain embodiments of the invention. 
     A process for fractionating lignocellulosic biomass, according to some embodiments, includes the steps of: 
     (a) providing a feedstock comprising lignocellulosic biomass; 
     (b) in a digestor, fractionating the feedstock under effective fractionation conditions in the presence of ethanol, sulfur dioxide, and water, to produce a liquor containing hemicellulose, cellulose-rich solids, and lignin; 
     (c) substantially removing the cellulose-rich solids from the liquor; 
     (d) introducing a solid additive to the digestor, wherein the solid additive combines with at least a portion of the lignin; 
     (e) separating a mixture comprising the lignin and the solid additive, each in free or combined form, from the liquor; and 
     (f) recovering the hemicellulose. 
     Some variations provide a process for fractionating lignocellulosic biomass, the process comprising: 
     (a) providing a feedstock comprising lignocellulosic biomass; 
     (b) in a digestor, fractionating the feedstock under effective fractionation conditions in the presence of a solvent for lignin, sulfur dioxide (or other acid or acid precursor), and water, to produce a liquor containing hemicellulose, cellulose-rich solids, and lignin; 
     (c) substantially removing the cellulose-rich solids from the liquor; 
     (d) in a hydrolysis reactor, hydrolyzing the hemicellulose contained in the liquor, in the presence of a hydrolysis catalyst, to produce fermentable hemicellulosic sugars; 
     (e) introducing a solid additive to the hydrolysis reactor, wherein the solid additive combines with at least a portion of the lignin; 
     (f) separating a mixture comprising the lignin and the solid additive, each in free or combined form, from the fermentable hemicellulosic sugars; and 
     (g) recovering the fermentable hemicellulosic sugars. 
     The acid or acid precursor may be a sulfur-containing acid or compound, such as (but not limited to) sulfur dioxide, sulfurous acid, sulfur trioxide, sulfuric acid, lignosulfonic acid, or combinations thereof. Other acids or acid precursors may be employed. An acid precursor will generate some amount of acid in situ, under the conditions of the hydrolysis reactor. 
     In some embodiments, step (d) is carried out prior to, simultaneously with, or following removal of the solvent for lignin from the liquor. Also, step (d) may be carried out prior to, simultaneously with, or following removal of the sulfur dioxide from the liquor. 
     When a solid additive is introduced to the hydrolysis reactor, the process may further include introducing a solid additive to the digestor. The solid additive introduced to the digestor may be the same as, or different than, the solid additive introduced to the hydrolysis reactor. Any of these solids additives may be recovered and recycled. 
     The above-described or other variations provide methods of improving lignin separation during lignocellulosic biorefining, the method comprising the steps of (i) catalyzing fractionation or hydrolysis with an acid to release sugars into an acidified solution containing lignin, wherein the solution also contains a solvent for lignin; (ii) neutralizing the acidified solution with a base to form a salt in a neutralized solution; (iii) in a separation unit, separating the salt and the lignin, each in free or combined form, from the neutralized solution; and then (iv) recycling a portion of the salt and optionally a portion of the lignin to step (i) to combine, physically or chemically, with the lignin, to improve lignin separation in the separation unit. 
     Known apparatus may be employed for these methods. The separation unit may be selected from the group consisting of a filter, a membrane, a decanter, a clarifier, a hydrocyclone, and a centrifuge. The salt may be selected from the group consisting of metal sulfates, metal sulfate hydrates, metal sulfate derivatives, ammonium sulfate, ammonium sulfate derivatives, and combinations thereof. For example, the salt may be selected from the group consisting of anhydrite, calcium sulfate hemihydrate, calcium sulfate dihydrate (gypsum), and combinations thereof. 
     Some embodiments provide a method of improving lignin separation during lignocellulosic biorefining, the method comprising the steps of (i) catalyzing fractionation or hydrolysis with a sulfur-containing acid and/or sulfur dioxide to release sugars and lignin into an acidified solution, wherein the solution also contains a solvent for lignin; (ii) neutralizing the acidified solution with lime to form gypsum in a neutralized solution; (iii) in a separation unit, separating the gypsum and the lignin, individually or in combination, from the neutralized solution; and then (iv) recycling a portion of the gypsum and optionally a portion of the lignin to step (i) to combine, physically or chemically, with the lignin released in step (i), to improve lignin separation in the separation unit in step (iii). 
     In some embodiments, the solid additive has a density that is higher than the density of lignin, that is, higher than about 1-1.5 g/cm 3 . For example the solid additive may have a density of about 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5 g/cm 3  or higher. 
     The solid additive may be introduced to the hydrolysis reactor as a dry powder, a wet powder, a slurry, partially or completely dissolved in a solution, as an aqueous liquid, in ionic form (i.e. a salt that is dissociated as anions and cations), etc. The term “solid additive” is meant to refer to the phase of the additive in isolation from the hydrolysis reactor or solution. Within the hydrolysis reactor itself, the additive is by no means limited to being a distinct solid phase. 
     In some embodiments, the solid additive contains sulfur, such as in the form of a sulfate salt. When the solid additive contains sulfur, it is possible for that sulfur to be reactive with the lignin so that at least a portion of the sulfur reacts with lignin to generate sulfonated lignin. Sulfonated lignin may be advantageous for downstream separation efficiency, in some embodiments. 
     Many solid additives are possible in the present invention. The solid additive may be selected from the group consisting of metal sulfates, metal sulfate hydrates, metal sulfate derivatives, ammonium sulfate, ammonium sulfate derivatives, native lignin, acid-condensed lignin, sulfonated lignin, lignin derivatives, and combinations thereof. In some embodiments, the solid additive is selected from the group consisting of anhydrite (anhydrous calcium sulfate), calcium sulfate hemihydrate, calcium sulfate dihydrate (gypsum), and combinations thereof. 
     In certain embodiments, the solid additive comprises gypsum. The solid additive may consist essentially of gypsum. In certain embodiments, especially those employing internal process recycling, the solid additive comprises gypsum and lignin. The solid additive may consist essentially of gypsum and lignin. When the solid additive is gypsum (or includes gypsum), that gypsum may include, or consist entirely of, recycled gypsum that is generated following the step(s) of separating the gypsum and lignin and from the fermentable hemicellulosic sugars. When the solid additive includes lignin, that lignin may include, or consist entirely of, recycled lignin that is generated following the step(s) of separating the gypsum and lignin from the fermentable hemicellulosic sugars. 
     When a base other than lime is used to neutralize the sugar solution from acid hydrolysis, then the solid additive may preferably be something other than gypsum. For example, when ammonia or an ammonium alkali is used as the neutralizing base, and ammonium sulfate is produced, when a preferred solid additive is ammonium sulfate. 
     In some embodiments, the solid additive is not introduced directly but rather generated in situ, such as by introducing a base to react a portion of the catalyst with the base to form the additive. For example lime could be introduced, wherein the lime reacts with some acid to form gypsum as the solid additive. It would also be possible to add two or components that react with each other (not with the catalyst) in situ to make the solid additive. 
     Other solid additives are possible in the present invention as well. For example, in some embodiments the solid additive is selected from the group consisting of minerals, diatomaceous earth, silica, alumina, ash, zeolites, metal alums, ammonium alum, dust, cellulose, nanocellulose, sawdust, agricultural residue pith, biomass fines, and combinations thereof. 
     A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition and highly ordered atomic structure. Any of the more than 4,000 known minerals, according to the International Mineralogical Association, may be utilized as the solid additive in this invention. Zeolites include any known natural or synthetic zeolites, which are microporous, aluminosilicate minerals. Examples include, but are not limited to, talc, dolomite, olivine, and perlite. 
     Alums include any alums of general formula AM(SO 4 ) 2 .nH 2 O, where A is an alkali metal or ammonium, M is a trivalent metal, and n is greater than 1, such as from 6 to 20, e.g. 12. Exemplary alums include aluminum potassium sulfate (“potash alum” or simply “alum”), KAl(SO 4 ) 2 .12H 2 O; soda alum, NaAl(SO 4 ) 2 .12H 2 O; ammonium alum, NH 4 Al(SO 4 ) 2 .12H 2 O; and chrome alum, KCr(SO 4 ) 2 .12H 2 O. 
     Other solid additives include cellulose of various origins and particle size, including nanocellulose, or cellulosic materials such as sawdust, agricultural residue pith (such as corn stover pith), or other biomass particles or particles derived from biomass. 
     In addition to the solid additive, in some embodiments another material is introduced, such as a buffer, an emulsifier, a mixing agent, or flocculating agent. For example, polymer flocculating agents may be introduced. Polymers can flocculate colloidal suspensions generally through the mechanisms of charge neutralization, formation of patches of opposite charge and subsequent attraction, and bridging. Flocculation depends on the size of the polymer molecule both in solution and after adsorption, charge density, polymer concentration, presence of other electrolytes, and the mode of addition. In some embodiments, the selected solid additive performs one or more of these functions to some extent. For instance, alum may act as a buffering agent as well as a flocculating agent. 
     In some embodiments, at least a portion of the solid additive combines, chemically or physically, with the lignin to form a lignin-additive complex that has one or more of the following properties, compared to the lignin: a higher density; a higher viscosity; a higher density/viscosity ratio; a higher settling rate; and/or reduced tackiness. The solid additive may be classified as a lignin detackifier, in some embodiments. Other rheological modification may be accomplished, to alter the physical or mechanical properties of the complex, compared to lignin alone. 
     The solid additive may be present in the hydrolysis reactor at a concentration of at least 0.1 g/L, at least 1 g/L, or at least 10 g/L, such as about 0.2, 0.5, 0.8, 1.0, 1.5, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10, 11, 12, 15, 20 g/L or higher. 
     The hydrolysis catalyst may be added and then the solid additive added to the reactor. Or, the solid additive may be added and then the hydrolysis catalyst added to the reactor. Or, these components may be simultaneously introduced. 
     The process may further comprise recovering and recycling at least a portion of the hydrolysis catalyst, at least a portion of the solid additive, or both. 
     The lignin-additive complex may be separated from solution using a variety of separation devices. The separation unit may be selected from filters, membranes, decanters, clarifiers, centrifuges, decanting centrifuges, cyclones, hydrocyclones, precipitators, electrostatic precipitators, evaporators, flash vessels, distillation columns, and so on. The lignin-additive complex may be recovered in solid form, in slurry form, or as a dilute solution in liquid. 
     The lignin and additive may be recovered in combination, or they may be recovered separately, in one or multiple stages or units. When the lignin and additive are recovered together, a portion may be recycled back to the hydrolysis reactor (at least some should be purged at steady state to avoid lignin build-up). When the lignin and additive are recovered separately, it is possible to recycle all of the additive and some of the lignin, all of the additive and none of the lignin, some of each of the additive and lignin, etc. 
     The remainder of the process, in some variations, will now be described without limiting the principles of the invention. 
     This disclosure describes processes and apparatus to efficiently fractionate any lignocellulosic-based biomass into its primary major components (cellulose, lignin, and if present, hemicellulose) so that each can be used in potentially distinct processes. An advantage of the process is that it produces cellulose-rich solids while concurrently producing a liquid phase containing a high yield of both hemicellulose sugars and lignin, and low quantities of lignin and hemicellulose degradation products. The flexible fractionation technique enables multiple uses for the products. The cellulose is highly reactive to cellulase enzymes for the manufacture of glucose. Other uses for celluloses can be adjusted based on market conditions. 
     The biomass feedstock may be selected from hardwoods, softwoods, forest residues, industrial wastes, pulp and paper wastes, consumer wastes, or combinations thereof. Some embodiments utilize agricultural residues, which include lignocellulosic biomass associated with food crops, annual grasses, energy crops, or other annually renewable feedstocks. Exemplary agricultural residues include, but are not limited to, corn stover, corn fiber, wheat straw, sugarcane bagasse, sugarcane straw, rice straw, oat straw, barley straw, miscanthus, energy cane straw/residue, or combinations thereof. 
     As used herein, “lignocellulosic biomass” means any material containing cellulose and lignin. Lignocellulosic biomass may also contain hemicellulose. Mixtures of one or more types of biomass can be used. In some embodiments, the biomass feedstock comprises both a lignocellulosic component (such as one described above) in addition to a sucrose-containing component (e.g., sugarcane or energy cane) and/or a starch component (e.g., corn, wheat, rice, etc.). 
     Various moisture levels may be associated with the starting biomass. The biomass feedstock need not be, but may be, relatively dry. In general, the biomass is in the form of a particulate or chip, but particle size is not critical in this invention. 
     Reaction conditions and operation sequences may vary widely. Some embodiments employ conditions described in U.S. Pat. No. 8,030,039, issued Oct. 4, 2011; U.S. Pat. No. 8,038,842, issued Oct. 11, 2011; U.S. Pat. No. 8,268,125, issued Sep. 18, 2012; and U.S. patent application Ser. Nos. 13/004,431; 12/234,286; 13/585,710; 12/250,734; 12/397,284; 12/304,046; 13/500,916; U.S. Ser. Nos. 13/626,220; 12/854,869; and 61/732,047, including the prosecution histories thereof. Each of these commonly owned patent applications is hereby incorporated by reference herein in its entirety. In some embodiments, the process is a variation of the AVAP® process technology which is commonly owned with the assignee of this patent application. 
     In some embodiments, a first process step is “cooking” (equivalently, “digesting”) which fractionates the three lignocellulosic material components (cellulose, hemicellulose, and lignin) to allow easy downstream removal. Specifically, hemicelluloses are dissolved and over 50% are completely hydrolyzed; cellulose is separated but remains resistant to hydrolysis; and part of the lignin is sulfonated into water-soluble lignosulfonates. 
     The lignocellulosic material is processed in a solution (cooking liquor) of aliphatic alcohol, water, and sulfur dioxide. The cooking liquor preferably contains at least 10 wt %, such as at least 20 wt %, 30 wt %, 40 wt %, or 50 wt % of a solvent for lignin. For example, the cooking liquor may contain about 30-70 wt % solvent, such as about 50 wt % solvent. The solvent for lignin may be an aliphatic alcohol, such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutanol, 1-pentanol, 1-hexanol, or cyclohexanol. The solvent for lignin may be an aromatic alcohol, such as phenol or cresol. Other lignin solvents are possible, such as (but not limited to) glycerol, methyl ethyl ketone, or diethyl ether. Combinations of more than one solvent may be employed. 
     Preferably, enough solvent is included in the extractant mixture to dissolve the lignin present in the starting material. The solvent for lignin may be completely miscible, partially miscible, or immiscible with water, so that there may be more than one liquid phase. Potential process advantages arise when the solvent is miscible with water, and also when the solvent is immiscible with water. When the solvent is water-miscible, a single liquid phase forms, so mass transfer of lignin and hemicellulose extraction is enhanced, and the downstream process must only deal with one liquid stream. When the solvent is immiscible in water, the extractant mixture readily separates to form liquid phases, so a distinct separation step can be avoided or simplified. This can be advantageous if one liquid phase contains most of the lignin and the other contains most of the hemicellulose sugars, as this facilitates recovering the lignin from the hemicellulose sugars. 
     The cooking liquor preferably contains sulfur dioxide and/or sulfurous acid (H 2 SO 3 ). The cooking liquor preferably contains SO 2 , in dissolved or reacted form, in a concentration of at least 3 wt %, preferably at least 6 wt %, more preferably at least 8 wt %, such as about 9 wt %, 10 wt %, 11 wt %, 12 wt %, 13 wt %, 14 wt %, 15 wt %, 20 wt %, 25 wt %, 30 wt % or higher. The cooking liquor may also contain one or more species, separately from SO 2 , to adjust the pH. The pH of the cooking liquor is typically about 4 or less. 
     Sulfur dioxide is a preferred acid catalyst, because it can be recovered easily from solution after hydrolysis. The majority of the SO 2  from the hydrolysate may be stripped and recycled back to the reactor. Recovery and recycling translates to less lime required compared to neutralization of comparable sulfuric acid, less solids to dispose of, and less separation equipment. The increased efficiency owing to the inherent properties of sulfur dioxide mean that less total acid or other catalysts may be required. This has cost advantages, since sulfuric acid can be expensive. Additionally, and quite significantly, less acid usage also will translate into lower costs for a base (e.g., lime) to increase the pH following hydrolysis, for downstream operations. Furthermore, less acid and less base will also mean substantially less generation of waste salts (e.g., gypsum) that may otherwise require disposal. 
     In some embodiments, an additive may be included in amounts of about 0.1 wt % to 10 wt % or more to increase cellulose viscosity. Exemplary additives include ammonia, ammonia hydroxide, urea, anthraquinone, magnesium oxide, magnesium hydroxide, sodium hydroxide, and their derivatives. 
     The cooking is performed in one or more stages using batch or continuous digestors. Solid and liquid may flow cocurrently or countercurrently, or in any other flow pattern that achieves the desired fractionation. The cooking reactor may be internally agitated, if desired. 
     Depending on the lignocellulosic material to be processed, the cooking conditions are varied, with temperatures from about 65° C. to 175° C., for example 75° C., 85° C., 95° C., 105° C., 115° C., 125° C., 130° C., 135° C., 140° C., 145° C., 150° C., 155° C., 165° C. or 170° C., and corresponding pressures from about 1 atmosphere to about 15 atmospheres in the liquid or vapor phase. The cooking time of one or more stages may be selected from about 15 minutes to about 720 minutes, such as about 30, 45, 60, 90, 120, 140, 160, 180, 250, 300, 360, 450, 550, 600, or 700 minutes. Generally, there is an inverse relationship between the temperature used during the digestion step and the time needed to obtain good fractionation of the biomass into its constituent parts. 
     The cooking liquor to lignocellulosic material ratio may be selected from about 1 to about 10, such as about 2, 3, 4, 5, or 6. In some embodiments, biomass is digested in a pressurized vessel with low liquor volume (low ratio of cooking liquor to lignocellulosic material), so that the cooking space is filled with ethanol and sulfur dioxide vapor in equilibrium with moisture. The cooked biomass is washed in alcohol-rich solution to recover lignin and dissolved hemicelluloses, while the remaining pulp is further processed. In some embodiments, the process of fractionating lignocellulosic material comprises vapor-phase cooking of lignocellulosic material with aliphatic alcohol (or other solvent for lignin), water, and sulfur dioxide. See, for example, U.S. Pat. Nos. 8,038,842 and 8,268,125 which are incorporated by reference herein. 
     A portion or all of the sulfur dioxide may be present as sulfurous acid in the extract liquor. In certain embodiments, sulfur dioxide is generated in situ by introducing sulfurous acid, sulfite ions, bisulfite ions, combinations thereof, or a salt of any of the foregoing. Excess sulfur dioxide, following hydrolysis, may be recovered and reused. In some embodiments, sulfur dioxide is saturated in water (or aqueous solution, optionally with an alcohol) at a first temperature, and the hydrolysis is then carried out at a second, generally higher, temperature. In some embodiments, sulfur dioxide is sub-saturated. In some embodiments, sulfur dioxide is super-saturated. In some embodiments, sulfur dioxide concentration is selected to achieve a certain degree of lignin sulfonation, such as 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10% sulfur content. SO 2  reacts chemically with lignin to form stable lignosulfonic acids which may be present both in the solid and liquid phases. 
     The concentration of sulfur dioxide, additives, and aliphatic alcohol (or other solvent) in the solution and the time of cook may be varied to control the yield of cellulose and hemicellulose in the pulp. The concentration of sulfur dioxide and the time of cook may be varied to control the yield of lignin versus lignosulfonates in the hydrolysate. In some embodiments, the concentration of sulfur dioxide, temperature, and the time of cook may be varied to control the yield of fermentable sugars. 
     Once the desired amount of fractionation of both hemicellulose and lignin from the solid phase is achieved, the liquid and solid phases are separated. Conditions for the separation may be selected to minimize the reprecipitation of the extracted lignin on the solid phase. This is favored by conducting separation or washing at a temperature of at least the glass-transition temperature of lignin (about 120° C.). 
     The physical separation can be accomplished either by transferring the entire mixture to a device that can carry out the separation and washing, or by removing only one of the phases from the reactor while keeping the other phase in place. The solid phase can be physically retained by appropriately sized screens through which liquid can pass. The solid is retained on the screens and can be kept there for successive solid-wash cycles. Alternately, the liquid may be retained and solid phase forced out of the reaction zone, with centrifugal or other forces that can effectively transfer the solids out of the slurry. In a continuous system, countercurrent flow of solids and liquid can accomplish the physical separation. 
     The recovered solids normally will contain a quantity of lignin and sugars, some of which can be removed easily by washing. The washing-liquid composition can be the same as or different than the liquor composition used during fractionation. Multiple washes may be performed to increase effectiveness. Preferably, one or more washes are performed with a composition including a solvent for lignin, to remove additional lignin from the solids, followed by one or more washes with water to displace residual solvent and sugars from the solids. Recycle streams, such as from solvent-recovery operations, may be used to wash the solids. 
     After separation and washing as described, a solid phase and at least one liquid phase are obtained. The solid phase contains substantially undigested cellulose. A single liquid phase is usually obtained when the solvent and the water are miscible in the relative proportions that are present. In that case, the liquid phase contains, in dissolved form, most of the lignin originally in the starting lignocellulosic material, as well as soluble monomeric and oligomeric sugars formed in the hydrolysis of any hemicellulose that may have been present. Multiple liquid phases tend to form when the solvent and water are wholly or partially immiscible. The lignin tends to be contained in the liquid phase that contains most of the solvent. Hemicellulose hydrolysis products tend to be present in the liquid phase that contains most of the water. 
     In some embodiments, hydrolysate from the cooking step is subjected to pressure reduction. Pressure reduction may be done at the end of a cook in a batch digestor, or in an external flash tank after extraction from a continuous digestor, for example. The flash vapor from the pressure reduction may be collected into a cooking liquor make-up vessel. The flash vapor contains substantially all the unreacted sulfur dioxide which may be directly dissolved into new cooking liquor. The cellulose is then removed to be washed and further treated as desired. 
     A process washing step recovers the hydrolysate from the cellulose. The washed cellulose is pulp that may be used for various purposes (e.g., paper or nanocellulose production). The weak hydrolysate from the washer continues to the final reaction step; in a continuous digestor this weak hydrolysate may be combined with the extracted hydrolysate from the external flash tank. In some embodiments, washing and/or separation of hydrolysate and cellulose-rich solids is conducted at a temperature of at least about 100° C., 110° C., or 120° C. The washed cellulose may also be used for glucose production via cellulose hydrolysis with enzymes or acids. 
     In another reaction step, the hydrolysate may be further treated in one or multiple steps to hydrolyze the oligomers into monomers. This step may be conducted before, during, or after the removal of solvent and sulfur dioxide. The solution may or may not contain residual solvent (e.g. alcohol). In some embodiments, sulfur dioxide is added or allowed to pass through to this step, to assist hydrolysis. In these or other embodiments, an acid such as sulfurous acid or sulfuric acid is introduced to assist with hydrolysis. In some embodiments, the hydrolysate is autohydrolyzed by heating under pressure. In some embodiments, no additional acid is introduced, but lignosulfonic acids produced during the initial cooking are effective to catalyze hydrolysis of hemicellulose oligomers to monomers. In various embodiments, this step utilizes sulfur dioxide, sulfurous acid, sulfuric acid at a concentration of about 0.01 wt % to 30 wt %, such as about 0.05 wt %, 0.1 wt %, 0.2 wt %, 0.5 wt %, 1 wt %, 2 wt %, 5 wt %, 10 wt %, or 20 wt %. This step may be carried out at a temperature from about 100° C. to 220° C., such as about 110° C., 120° C., 130° C., 140° C., 150° C., 160° C., 170° C., 180° C., 190° C., 200° C., or 210° C. Heating may be direct or indirect to reach the selected temperature. 
     The reaction step produces fermentable sugars which can then be concentrated by evaporation to a fermentation feedstock. Concentration by evaporation may be accomplished before, during, or after the treatment to hydrolyze oligomers. The final reaction step may optionally be followed by steam stripping of the resulting hydrolysate to remove and recover sulfur dioxide and alcohol, and for removal of potential fermentation-inhibiting side products. The evaporation process may be under vacuum or pressure, from about −0.1 atmospheres to about 10 atmospheres, such as about 0.1 atm, 0.3 atm, 0.5 atm, 1.0 atm, 1.5 atm, 2 atm, 4 atm, 6 atm, or 8 atm. 
     Recovering and recycling the sulfur dioxide may utilize separations such as, but not limited to, vapor-liquid disengagement (e.g. flashing), steam stripping, extraction, or combinations or multiple stages thereof. Various recycle ratios may be practiced, such as about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.95, or more. In some embodiments, about 90-99% of initially charged SO 2  is readily recovered by distillation from the liquid phase, with the remaining 1-10% (e.g., about 3-5%) of the SO 2  primarily bound to dissolved lignin in the form of lignosulfonates. 
     In a preferred embodiment, the evaporation step utilizes an integrated alcohol stripper and evaporator. Evaporated vapor streams may be segregated so as to have different concentrations of organic compounds in different streams. Evaporator condensate streams may be segregated so as to have different concentrations of organic compounds in different streams. Alcohol may be recovered from the evaporation process by condensing the exhaust vapor and returning to the cooking liquor make-up vessel in the cooking step. Clean condensate from the evaporation process may be used in the washing step. 
     In some embodiments, an integrated alcohol stripper and evaporator system is employed, wherein aliphatic alcohol is removed by vapor stripping, the resulting stripper product stream is concentrated by evaporating water from the stream, and evaporated vapor is compressed using vapor compression and is reused to provide thermal energy. 
     The hydrolysate from the evaporation and final reaction step contains mainly fermentable sugars but may also contain lignin depending on the location of lignin separation in the overall process configuration. The hydrolysate may be concentrated to a concentration of about 5 wt % to about 60 wt % solids, such as about 10 wt %, 15 wt %, 20 wt %, 25 wt %, 30 wt %, 35 wt %, 40 wt %, 45 wt %, 50 wt % or 55 wt % solids. The hydrolysate contains fermentable sugars. 
     Fermentable sugars are defined as hydrolysis products of cellulose, galactoglucomannan, glucomannan, arabinoglucuronoxylans, arabinogalactan, and glucuronoxylans into their respective short-chained oligomers and monomer products, i.e., glucose, mannose, galactose, xylose, and arabinose. The fermentable sugars may be recovered in purified form, as a sugar slurry or dry sugar solids, for example. Any known technique may be employed to recover a slurry of sugars or to dry the solution to produce dry sugar solids. 
     In some embodiments, the fermentable sugars are fermented to produce biochemicals or biofuels such as (but by no means limited to) ethanol, isopropanol, acetone, 1-butanol, isobutanol, lactic acid, succinic acid, or any other fermentation products. Some amount of the fermentation product may be a microorganism or enzymes, which may be recovered if desired. 
     When the fermentation will employ bacteria, such as Clostridia bacteria, it is preferable to further process and condition the hydrolysate to raise pH and remove residual SO 2  and other fermentation inhibitors. The residual SO 2  (i.e., following removal of most of it by stripping) may be catalytically oxidized to convert residual sulfite ions to sulfate ions by oxidation. This oxidation may be accomplished by adding an oxidation catalyst, such as FeSO 4 .7H 2 O, that oxidizes sulfite ions to sulfate ions, which is a well-known practice for fermentation to acetone/butanol/ethanol (ABE). Preferably, the residual SO 2  is reduced to less than about 100 ppm, 50 ppm, 25 ppm, 10 ppm, 5 ppm, or 1 ppm. 
     In some embodiments, the process further comprises recovering the lignin as a co-product. The sulfonated lignin may also be recovered as a co-product. In certain embodiments, the process further comprises combusting or gasifying the sulfonated lignin, recovering sulfur contained in the sulfonated lignin in a gas stream comprising reclaimed sulfur dioxide, and then recycling the reclaimed sulfur dioxide for reuse. 
     The process lignin separation step is for the separation of lignin from the hydrolysate and can be located before or after the final reaction step and evaporation. If located after, then lignin will precipitate from the hydrolysate since alcohol has been removed in the evaporation step. The remaining water-soluble lignosulfonates may be precipitated by converting the hydrolysate to an alkaline condition (pH higher than 7) using, for example, an alkaline earth oxide, preferably calcium oxide (lime). The combined lignin and lignosulfonate precipitate may be filtered. The lignin and lignosulfonate filter cake may be dried as a co-product or burned or gasified for energy production. The hydrolysate from filtering may be recovered and sold as a concentrated sugar solution product or further processed in a subsequent fermentation or other reaction step. 
     Native (non-sulfonated) lignin is hydrophobic, while lignosulfonates are hydrophilic. Hydrophilic lignosulfonates may have less propensity to clump, agglomerate, and stick to surfaces. Even lignosulfonates that do undergo some condensation and increase of molecular weight, will still have an HSO 3  group that will contribute some solubility (hydrophilic). 
     In some embodiments, the soluble lignin precipitates from the hydrolysate after solvent has been removed in the evaporation step. In some embodiments, reactive lignosulfonates are selectively precipitated from hydrolysate using excess lime (or other base, such as ammonia) in the presence of aliphatic alcohol. In some embodiments, hydrated lime is used to precipitate lignosulfonates. In some embodiments, part of the lignin is precipitated in reactive form and the remaining lignin is sulfonated in water-soluble form. 
     The process fermentation and distillation steps are intended for the production of fermentation products, such as alcohols or organic acids. After removal of cooking chemicals and lignin, and further treatment (oligomer hydrolysis), the hydrolysate contains mainly fermentable sugars in water solution from which any fermentation inhibitors have been preferably removed or neutralized. The hydrolysate is fermented to produce dilute alcohol or organic acids, from 1 wt % to 20 wt % concentration. The dilute product is distilled or otherwise purified as is known in the art. 
     When alcohol is produced, such as ethanol, some of it may be used for cooking liquor makeup in the process cooking step. Also, in some embodiments, a distillation column stream, such as the bottoms, with or without evaporator condensate, may be reused to wash cellulose. In some embodiments, lime may be used to dehydrate product alcohol. Side products may be removed and recovered from the hydrolysate. These side products may be isolated by processing the vent from the final reaction step and/or the condensate from the evaporation step. Side products include furfural, hydroxymethyl furfural (HMF), methanol, acetic acid, and lignin-derived compounds, for example. 
     The cellulose-rich material is highly reactive in the presence of industrial cellulase enzymes that efficiently break the cellulose down to glucose monomers. It has been found experimentally that the cellulose-rich material, which generally speaking is highly delignified, rapidly hydrolyzes to glucose with relatively low quantities of enzymes. For example, the cellulose-rich solids may be converted to glucose with at least 80% yield within 24 hours at 50° C. and 2 wt % solids, in the presence of a cellulase enzyme mixture in an amount of no more than 15 filter paper units (FPU) per g of the solids. In some embodiments, this same conversion requires no more than 5 FPU per g of the solids. 
     The glucose may be fermented to an alcohol, an organic acid, or another fermentation product. The glucose may be used as a sweetener or isomerized to enrich its fructose content. The glucose may be used to produce baker&#39;s yeast. The glucose may be catalytically or thermally converted to various organic acids and other materials. 
     In some embodiments, the cellulose-rich material is further processed into one more cellulose products. Cellulose products include market pulp, dissolving pulp (also known as α-cellulose), fluff pulp, purified cellulose, paper, paper products, and so on. Further processing may include bleaching, if desired. Further processing may include modification of fiber length or particle size, such as when producing nanocellulose or nanofibrillated or microfibrillated cellulose. It is believed that the cellulose produced by this process is highly amenable to derivatization chemistry for cellulose derivatives and cellulose-based materials such as polymers. 
     When hemicellulose is present in the starting biomass, all or a portion of the liquid phase contains hemicellulose sugars and soluble oligomers. It is preferred to remove most of the lignin from the liquid, as described above, to produce a fermentation broth which will contain water, possibly some of the solvent for lignin, hemicellulose sugars, and various minor components from the digestion process. This fermentation broth can be used directly, combined with one or more other fermentation streams, or further treated. Further treatment can include sugar concentration by evaporation; addition of glucose or other sugars (optionally as obtained from cellulose saccharification); addition of various nutrients such as salts, vitamins, or trace elements; pH adjustment; and removal of fermentation inhibitors such as acetic acid and phenolic compounds. The choice of conditioning steps should be specific to the target product(s) and microorganism(s) employed. 
     In some embodiments, hemicellulose sugars are not fermented but rather are recovered and purified, stored, sold, or converted to a specialty product. Xylose, for example, can be converted into xylitol. 
     A lignin product can be readily obtained from a liquid phase using one or more of several methods. One simple technique is to evaporate off all liquid, resulting in a solid lignin-rich residue. This technique would be especially advantageous if the solvent for lignin is water-immiscible. Another method is to cause the lignin to precipitate out of solution. Some of the ways to precipitate the lignin include (1) removing the solvent for lignin from the liquid phase, but not the water, such as by selectively evaporating the solvent from the liquid phase until the lignin is no longer soluble; (2) diluting the liquid phase with water until the lignin is no longer soluble; and (3) adjusting the temperature and/or pH of the liquid phase. Methods such as centrifugation can then be utilized to capture the lignin. Yet another technique for removing the lignin is continuous liquid-liquid extraction to selectively remove the lignin from the liquid phase, followed by removal of the extraction solvent to recover relatively pure lignin. 
     Lignin produced in accordance with the invention can be used as a fuel. As a solid fuel, lignin is similar in energy content to coal. Lignin can act as an oxygenated component in liquid fuels, to enhance octane while meeting standards as a renewable fuel. The lignin produced herein can also be used as polymeric material, and as a chemical precursor for producing lignin derivatives. The sulfonated lignin may be sold as a lignosulfonate product, or burned for fuel value. 
     The present invention also provides systems configured for carrying out the disclosed processes, and compositions produced therefrom. Any stream generated by the disclosed processes may be partially or completed recovered, purified or further treated, and/or marketed or sold. 
     In this detailed description, reference has been made to multiple embodiments of the invention and non-limiting examples relating to how the invention can be understood and practiced. Other embodiments that do not provide all of the features and advantages set forth herein may be utilized, without departing from the spirit and scope of the present invention. This invention incorporates routine experimentation and optimization of the methods and systems described herein. Such modifications and variations are considered to be within the scope of the invention defined by the claims. 
     All publications, patents, and patent applications cited in this specification are herein incorporated by reference in their entirety as if each publication, patent, or patent application were specifically and individually put forth herein. 
     Where methods and steps described above indicate certain events occurring in certain order, those of ordinary skill in the art will recognize that the ordering of certain steps may be modified and that such modifications are in accordance with the variations of the invention. Additionally, certain of the steps may be performed concurrently in a parallel process when possible, as well as performed sequentially. 
     Therefore, to the extent there are variations of the invention, which are within the spirit of the disclosure or equivalent to the inventions found in the appended claims, it is the intent that this patent will cover those variations as well. The present invention shall only be limited by what is claimed.