60/398, 880, filed July 25, 2002, the disclosure of which is hereby incorporated by reference in its entirety.

Background of the Invention Field of the Invention The present disclosure generally relates to the pulping of lignocellulosic materials, and more specifically, to the treatment of black liquor.

Description of the Related Art Among the steps in the production of paper is the manufacture of pulp, or cellulose fibers, from lignocellulosic materials, for example, wood, wheat straw, rice straw, bamboo, bagasse, hemp, flax, reeds, and agricultural wastes. In addition to the manufacture of paper, pulp is also used in such diverse applications as the manufacture of cement, the production of fibers for textiles, and the manufacture of film and sheet products.

The pulping process extracts the cellulose from the lignocellulosic materials. Wood is typically pulped using the kraft or sulfate process, in which wood chips are digested using pulping chemicals, usually sodium hydroxide and sodium sulfide, under steam pressure. The kraft process is effective in digesting wood, which is high in lignin. The <BR> <BR> extracted fibers are washed with water. The washings are referred to as"black liquor. " Kraft black liquor contains principally lignin, polysaccharides (e. g., hemicellulose), <BR> <BR> oligosaccharides, monosaccharides (e. g. , hexoses and pentoses), and sugar acids, as well as spent pulping chemicals. The black liquor is typically burned in a recovery boiler, which permits the recovery of the pulping chemicals. Because of its high lignin content, burning the black liquor from wood pulping generates a great deal of heat, which is used in the pulping process.

Non-wood materials are typically pulped by soda pulping, which is similar to kraft pulping, except no sulfur compounds are used. Because annual lignocellulosic materials typically contain less lignin than wood, the kraft process is not required for these materials.

Examples of such materials include rice straw, wheat straw, reed, bamboo, and bagasse.

The black liquor from a soda pulping mill typically contains a lower concentration of lignin. Another component of the black liquor is silica. The content of silica in the black

liquor in kilograms per ton of dry solids is provided in TABLE I for a variety of lignocellulosic raw materials.

TABLE I Raw Material Silica (Si02) (kg/ton of raw material) Hardwood (birch) 1 Bagasse 12 Bamboo 20-22 Reed 25-50 Wheat Straw 45-80 Rice Straw 110-160 Wheat straw and rice straw are widely used non-wood pulp sources, for example, in China. Because of the high silica content, the black liquor is unsuitable for burning in a recovery boiler. The silica fouls the heat transfer surfaces in the boiler, necessitating frequent shut downs for cleaning. Moreover, the lower lignin content means that burning the black liquor generates less heat. The black liquor is also unsuitable for discharge, requiring extensive treatment because of the high chemical oxygen demand (COD) and biological oxygen demand (BOD). The cost of treatment renders paper and pulping mills using wheat or rice straw economically nonviable.

Lignocellulosic materials are also pulped using mechanical, chemical-mechanical, and sulfite pulping processes. Each of these processes generates an aqueous waste stream.

Summary of the Invention Disclosed herein is an apparatus and method for treating black liquor, as well as for extracting useful products from the black liquor. One embodiment provides an apparatus and method for making black liquor non-hazardous. In addition to treating a hazardous waste stream, certain embodiments also provide useful products derived from the black liquor. These products may include lignin, polysaccharides, oligosaccharides, disaccharides, monosaccharides, and water.

According, an embodiment of the disclosed invention provides an apparatus comprising a mechanical separator and a first membrane filter for treating black liquor. The pH of the black liquor is adjusted to provide a supernatant and a precipitate comprising lignin ; the mechanical separator is configured to separate the precipitate from the supernatant; and the first membrane filter separates the supernatant into a first retentate,

which comprises substantially all of the lignin remaining in the supernatant, and a first permeate. In another embodiment, the apparatus further comprises a second membrane filter, wherein the second membrane filter separates the first permeate into a second retentate and a second permeate. Another embodiment further comprises a third membrane filter, wherein the third membrane filter separates the second permeate into a third retentate and a third permeate.

Another embodiment provides a method for treating black liquor comprising the steps of (1) adjusting the pH of the black liquor to provide a supernatant and a precipitate comprising lignin; (2) separating the precipitate from the supernatant using a mechanical separator; and (3) filtering the separated supernatant using a first membrane filter to produce a first retentate, which comprises substantially all of the lignin remaining in the supernatant, and a first permeate. In another embodiment, the method further comprises the step of (4) filtering the first permeate using a second membrane filter to produce a second retentate and a second permeate. Another embodiment further comprises the step of (5) filtering the second permeate using a third membrane filter to produce a third retentate and a third permeate.

In a preferred embodiment, the black liquor is a produced in the soda pulping of a non-wood lignocellulosic material, more preferably, wheat straw and rice straw. In one embodiment, the pH of the black liquor is adjusted to below about 3, preferably using sulfuric acid or hydrochloric acid. The pH of the black liquor preferably is adjusted using an in-line mixer. A flocculent is optionally added to the black liquor.

Preferably, the mechanical separator is a centrifuge, more preferably, a continuous decanter centrifuge. Preferably, the separation is performed at less than about 80 °C, more preferably, from about 40 °C to about 55 °C. In certain embodiments, the mechanical separator removes at least about 90% or about 98% of the precipitated lignin. In some embodiment, the precipitate comprises silica.

Preferably, the first membrane filter retains compounds with molecular weights of greater than about 2,500, more preferably, greater than from about 8,000 to about 10,000. In one embodiment, the first permeate comprises polysaccharides, oligosaccharides, disaccharides, and monosaccharides. In another embodiment, the retentate is recycled into a feed stream of the mechanical separator.

In certain embodiments, any one or all of the membrane filters is spiral wound and/or made from polyamide.

Preferably, the second membrane filter retains compounds with molecular weights of greater than from about 150 to about 300. In one embodiment, the second retentate comprises compounds selected from the group consisting of polysaccharides, oligosaccharides, and combinations thereof. In certain embodiments, the polysaccharides comprise xylan and/or the oligosaccharides comprise xylooligosaccharides. In some embodiments, the second permeate comprises a compound selected from the group consisting of xylooligosaccharides, xylose disaccharide, xylose, and arabinose.

Preferably, the third membrane filter is a reverse osmosis membrane filter. In one embodiment, the third retentate comprises compounds selected from the group consisting of oligosaccharides, disaccharides, monosaccharides, and combinations thereof. In certain embodiments, the disaccharides comprise xylose disaccharide and/or the monosaccharides comprise xylose, arabinose, or a combination thereof. In certain embodiments, the third permeate is dischargeable water and/or recyclable water. Preferably, the chemical oxygen demand of the third permeate is less than about 5000 ppm and/or the biological oxygen demand of the third permeate is less than about 5000 ppm. Preferably, the pH of the third permeate is from about 2.5 to about 6. In one embodiment, the pH of the third permeate is adjusted to from about 6 to about 8.

In some embodiments, the pulp or paper mill is the source of the black liquor.

Another embodiment provides an apparatus for manufacturing lignin comprising a mechanical separator and a first membrane filter. The apparatus manufactures lignin from black liquor; the pH of the black liquor is adjusted to provide a supernatant and a precipitate comprising lignin ; the mechanical separator is configured to separate the precipitate from the supernatant; and the first membrane filter produces from the supernatant a first retentate, which comprises substantially all of the lignin remaining in the supernatant, and a first permeate.

Another embodiment provides a method for manufacturing lignin from black liquor comprising (1) adjusting the pH of the black liquor to provide a supernatant and a precipitate comprising lignin; (2) separating the precipitate from the supernatant using a mechanical separator ; and (3) filtering the separated supernatant using a first membrane filter to produce a first retentate, which comprises substantially all of the lignin remaining

in the supernatant, and a first permeate. Another embodiment provides lignin manufactured by the disclosed method.

Also provided is an apparatus and a method for manufacturing yellow liquor, as well as yellow liquor manufactured by the disclosed method. Further provided is an apparatus and a method for manufacturing a high molecular weight sugar composition, as well as a high molecular weight sugar composition manufactured by the disclosed method.

Another embodiment provides paper produced in a paper or pulp mill in which the black liquor is treated according to the disclosed method. Another embodiment provides water produced by the disclosed method.

Another embodiment provides an aqueous composition comprising xylan, xylooligosaccharides, xylose disaccharide, xylose, arabinose, and substantially no lignin.

Preferably, the weight of each sugar as a percentage of the total weight of the sugars is xylan from about 20% to about 35%; xylooligosaccharide from about 35% to about 50%; xylose disaccharide from 0% to about 10%; xylose from about 15% to about 30%; and arabinose from about 5% to about 20%.

Another embodiment provides an aqueous composition comprising xylan and xylooligosaccharides, and substantially no lignin, xylose disaccharide, xylose, or arabinose.

Preferably, the weight of each sugar as a percentage of the total weight of the sugars is xylan from about 25% to about 40%; xylooligosaccharide from about 35% to about 50%.

Brief Description of the Drawings FIG. 1 illustrates an embodiment of the disclosed apparatus for treating black liquor.

FIG. 2 is a flowchart illustrating an embodiment of the disclosed method for treating black liquor.

Detailed Description of the Preferred Embodiment The apparatuses, methods, and compositions disclosed herein are described in reference to black liquor produced in the soda pulping of wheat or rice straw and are particularly desirable for this purpose. Certain advantages, however, are also applicable to black liquor produced by any pulping method of any lignocellulosic material. Pulping methods include soda, kraft, sulfite, chemical-mechanical, and mechanical. Lignocellulosic materials include wood and non-wood materials. Wood includes hardwood and softwood.

Non-wood materials include materials purpose grown for fiber, including bamboo, hemp, flax, and kenaf. Other non-wood materials include agricultural wastes, for example,

bagasse, and cereal straw, including wheat, rice, maize, oat, rye, sorghum, barley, and millet straw.

As used herein, the term"polysaccharide"is used in its usual sense as well as with the particular meaning of a sugar polymer of greater than about 9 sugar units. Xylan is a xylose polysaccharide. The term"oligosaccharide"is used in its usual sense as well as with the particular meaning of a sugar polymer of from about 3 to about 8 sugar units.

Xylooligosaccharides are xylose oligosaccharides of from about 3 to about 8 xylose units.

Xylooligosaccharides are also referred to herein as"xylose n-mer,"where fz is the number of xylose units.

Disclosed herein is an apparatus and method for treating black liquor, as well as for extracting useful products from the black liquor. The apparatus and methods are described with reference to black liquor derived from soda pulping of wheat or rice straw. Those skilled in the art will realize that aspects of the disclosed apparatus and methods are also applicable to black liquor derived from the pulping of other lignocellulosic materials and/or different pulping methods. The products derived from the black liquor will depend on factors including the particular lignocellulosic material, as well as the pulping method used.

FIG. 1 illustrates an embodiment 100 of the disclosed apparatus for treating black liquor. The apparatus 100 is typically situated within or in close proximity of a paper or pulp mill. Lignocellulosic material is pulped in a paper or pulp mill 101. Black liquor, a by- product of the pulping, is the effluent from washing the pulp. A black liquor stream 102 from a paper of pulp mill 101 is stored a storage tank 104. The storage tank 104 is fluidly connected to a reaction tank 106. A pH adjuster 110 is provided for adjusting the pH of the black liquor by adding an acid or base. The reaction tank is fluidly connected to a mechanical separator 112, which divides the black liquor stream into a high solids stream 114 and a low solids stream 116. A first membrane filter 120 separates the low solids stream 116 into a first retentate 122 and a first permeate 124. The first retentate 122 is recycled into the feed stream before the pH adjuster 110 in the illustrated embodiment. A second membrane filter 130 separates the first permeate 124 into a second retentate 132 and a second permeate 134. A third membrane filter 140 separates the second permeate 132 into a third retentate 142 and a third permeate 144.

Storage tank 104 is sized and configured to store the black liquor 102 from the pulping operation. The size and configuration of the storage tanlc 104 will depend on factors

including the capacity of the pulping operation, and whether the pulping is a continuous or batch process. The storage tank 104 is constructed from a material that will withstand the black liquor, which is both hot (up to about 90 °C) and caustic. Examples of suitable materials include stainless steel, and polymer-lined tanks. Several storage tanks 104 may be used instead of a single tank for reasons such as facilitating maintenance, adjusting plant capacity usage, reducing construction cost, and the like.

The black liquor is transferred into the reaction tank 106 by any means known in the art, for example, by gravity or using a pump. The reaction tank 106 is sized and configured according to the capacity of the pulping operation and is constructed from a material that will withstand the corrosive characteristics of the black liquor, which is described in greater detail below. The reaction tank optionally comprises a mixer. In a preferred embodiment, the mixer is a low speed mixer. As is discussed below, lignin precipitates from the black liquor in the reaction tank 106. Overly vigorous mixing makes separating the precipitated lignin from the supernatant more difficult.

The pH of the black liquor is adjusted using a pH adjuster 110 of any type known in the art. In the illustrated embodiment, the pH adjustment occurs while the black liquor is transferred to the reaction tank 106, for example, using an in-line mixer. In another embodiment, the pH is adjusted in the reaction tank 106, for example, by dispensing acid directly into the reaction tank. In one embodiment, the pH is adjusted to a predetermined value under automated control without monitoring or intervention by an operator. The black liquor in the storage tank 104 is basic. In the reaction tank 106, the pH of the black liquor is acidic, thereby precipitating the lignin. Typically, some of the lignin will remain dissolved in the black liquor, however. Lowering the pH will also tend to precipitate dissolved silicate as silica. Both the lignin and the silica become less soluble at lower pH. In certain embodiments, the pH of the black liquor is adjusted to less than about 6, less than about 5, less than about 4, or less than about 3. In a preferred embodiment, the pH is adjusted to about 3. In one embodiment, the black liquor is acidified using a mineral acid, for example, sulfuric acid or hydrochloric acid. In a preferred embodiment, the acid is sulfuric acid. The acidification reaction increases the temperature of the black liquor, which may be greater than 90 °C.

In another embodiment, a flocculent is added to the black liquor mixture in the reaction tank 106. The flocculent is selected to assist the precipitation and aggregation of

the precipitated lignin, generating larger flocs. Larger floes are more easily separated by the mechanical separator 112 used in certain embodiments, described in greater detail below.

Suitable flocculents include inorganic and polymeric flocculents, for example, anionic and cationic polymer flocculents.

The acidified black liquor is transferred to the mechanical separator 112 by any means known in the art, for example, using a pump or by gravity. In a preferred embodiment, the pump is a low-shear pump, for example, a screw pump. The reduced mixing provided by a low shear pump provides a mixture from which the precipitated lignin is more easily separated, as is discussed in greater detail below. A pump can also provide a higher input pressure to the mechanical separator 112.

The mechanical separator 112 separates the precipitated solids from the supernatant.

The mechanical separator 112 is any means known for separating a liquid from precipitated solid, for example, a centrifuge, a filter, a membrane filter, or decanter. The mechanical separator 112 separates the acidified black liquor into a high-solids stream 114, which has a higher concentration of precipitates, and a low-solids stream 116, which has a lower concentration of precipitates.

In one embodiment, the mechanical separator 112 is a centrifuge or a plurality of centrifuges. The centrifugation may be a batch or continuous process. In a preferred embodiment, the centrifugation is continuous. A continuous process permits a more efficient use of the pulping operation, as well as for the other components of the black liquor treament system. Examples of suitable centrifuges include continuous decanter centrifuges, disk-stack, and high-speed centrifuges. Commercially available examples of suitable centrifuges include the CHNX series from Alfa Laval (Lund, Sweden) and the CQ series from CentriQuip (Derbyshire, Lac).

In another embodiment, the mechanical separator 112 is filter or series of filters.

Any filtration means for separating precipitates known in the art may be used. In one embodiment, the filtration means is a wafer dish media filter, commercially available from Filtrex (Hayward, CA). In another embodiment, the filters may be a set of progressive bag filters. In a preferred embodiment, the filtration means is a combination of an upstream wafer dish media filter and a downstream set of bag filters. The filtration means may be cleaned by backwashing with air, water or another fluid. By automating the backwashing, for example, at a predetermined pressure differential, time, or volume of black liquor, the

filters are made self-cleaning. In another embodiment, the mechanical separator comprises a combination of a centrifuge and a filter, for example, a continuous centrifuge in series with bag filters.

At lower temperatures, a greater proportion of the lignin is precipitated. In one embodiment, the acidified black liquor is cooled to less than 30 °C for the separation.

Cooling the black liquor requires time and energy, however. In one embodiment, separating the lignin at from about 40 °C to about 55 °C provides a good recovery of lignin, particularly at a pH of less than about 3. In another embodiment, the separation is performed at less than about 80 °C. In other embodiments, the separation is performed at less than about 40 °C, less than about 50 °C, less than about 60 °C, less than about 70 °C, less than about 80 °C, or less than about 90 °C.

Separating the lignin from the acidified black liquor prior to the downstream membrane filtration minimizes fouling of the membrane filters by the lignin. Consequently, maximizing the recovery of the precipitated lignin makes the later membrane filtration steps more efficient. In one embodiment, the high solids stream 114 contains at least about 80% of the precipitated lignin. In a preferred embodiment, the high solids stream 114 contains at least about 98% of the precipitated lignin. In other embodiments, the high solids stream 114 contains at least about 60%, at least about 70%, at least about 80%, or at least about 90% of the precipitated lignin.

The high solids stream 114 contains the separated lignin and silica as well as any other precipitated materials. The high solids stream 114 may range from a slurry containing a significant amount of the supernatant to a dry cake containing no or almost no supernatant. The lignin in the high solids stream 114 has a high caloric content.

Consequently, the high solids stream 114 may be burned on site to generate steam.

Alternatively, the high solids stream 114 may be dried or otherwise further processed into lignin-containing products.

The low solids stream 116 contains largely supernatant and may also contain some precipitated lignin and silica, as well as dissolved materials, including dissolved lignin, dissolved silica, polysaccharides, oligosaccharides, monosaccharides, organic acids, and salts.

The apparatus desirably uses membrane filters. The membrane filters may be of any configuration known in the art, for example, plate-and-frame, spiral wound, tubular, hollow

fibers, or combinations thereof. The membrane filters may be fabricated using any material knows in the art, for example, an organic polymer or an inorganic material. Examples of suitable organic polymers include cellulose acetate, polyamide, polyvinylidene fluoride, polysulfone, polycarbonate, polypropylene, polyethylene, and PTFE. Examples of inorganic materials include ceramic and metal oxide aggregate.

Different membrane materials are compatible with different pH ranges of the process stream, as is known in the art.

As is known in the art, higher temperatures generally provide higher flow rates. The maximum temperature at which a filter can operate depends on the membrane material. For example, cellulose acetate filters have a maximum temperature of about 40 °C ; polyamide filters, about 80 °C ; polysulfone, about 85 °C. Other filter materials may be used at higher temperatures. Changing the temperature of the feed requires energy and extra equipment, and introduces complications in the process. In one embodiment, the apparatus is configured for use without temperature adjustment steps.

Higher pressures will also tend to provide higher flow rates. Pressures of up to about 100 psi, about 200 psi, and about 300 psi are typical, but certain configurations and materials may be used at up to about 400 psi, about 500 psi, about 600 psi, about 700 psi, about 800 psi, about 900 psi, and about 1000 psi. Operating at higher pressures requires more robust equipment, which is typically more expensive and presents additional safety and maintenance issues.

The design recoveries of the membrane filters range from about 40% to about 90%.

In certain preferred embodiments, the design recovery is about 70%, about 75%, about 80%, or about 85%. Actual recoveries depend on the nature and quality of the feedstock.

Furthermore, with use, the membranes will tend to foul and actual recoveries will decrease.

Actual recoveries as low as about 10% are acceptable in some embodiments. The filters are typically flushed when the recovery decreases by about from 30% to 40% of the design specification.

The fluxes of the membrane filters will also depend on the nature and quality of the feedstock. The design flux preferably ranges from about 4 gal/day/ft2 to about 15 gal/day/ft2, more preferably, from about 8 gal/day/ft2 to about 10 gal/day/ft2. As with the recovery, the flux will decrease with use. Actual fluxes as low as from about 2 gal/day/ft 2 to about 6 gal/day/ft2 are usable in some embodiments.

The particular configuration and material is selected according to factors including the size discrimination desired in the filtration, the temperature of the process stream, the pH of the process stream, the pressure of the process stream, and the desired flow rate.

Each membrane filter stage may include a plurality of filter units in parallel, in series, or in a combination of parallel and series configurations to achieve the desired results. The filters may be configured for multistage recirculation (feed and bleed) or in a Christmas tree design. In one embodiment, the multiple filter units are configured for continuous operation, permitting filter maintenance, for example, cleaning, flushing, or replacing, without shutting down the apparatus.

The illustrated embodiment 100 desirably uses three membrane filter stages.

Typically, each successive filter stage retains smaller compounds than the previous stage, which reduces fouling. Those skilled in the art will understand that other embodiments may use fewer than three or greater than three membrane filter stages. The number of membrane filter stages used for a black liquor treatment process will depend on factors including the characteristics of the black liquor, the characteristics of the treatment steps and devices upstream of the membrane filters, the characteristics of the membrane filters, and the products isolated in each filter stream. In a preferred embodiment, the filters are selected to provide a useful product stream at each stage, as will be described in greater detail below.

Membrane filters are often defined according to the sizes of the retained compounds. Microfiltration (MF) retains particles larger than from about 0. 02 um to about 0. 1 um. Ultrafiltration (IJF) retains compounds larger than about 1,000 to about 20,000 MW. Nanofiltration (NF) retains compounds larger than about 150 to about 300 MW.

Reverse osmosis (RO) retains salts.

The first membrane filter 120 separates the low solids stream 116 into a first retentate 122 and a first permeate 124. The first retentate 122 includes all precipitates that were not removed by the mechanical separator, as well as certain dissolved compounds. In certain embodiments, the first membrane filter 120 retains dissolved compounds with molecular weights of greater than about 20,000, greater than about 10,000, greater than about 8, 000, greater than about 2,500, or greater than about 1,000. Most of the dissolved lignin is retained at a rejection of greater than about 10,000 molecular weight units.

In a preferred embodiment, the first membrane filter 120 is an ultrafiltration filter that retains compounds with molecular weights of greater than about 8,000 to 10,000,

which includes substantially all of the dissolved lignin. In this embodiment, the first permeate 124 is referred to as"yellow liquor. "Yellow liquor is an aqueous solution comprising dissolved sugars. The weight of each sugar as a percentage of the total weight of the sugars is provided in TABLE II.

TABLE I1 : Sugar wt% of total sugars Xylan 20-35% Xylooligo 3-8mer 35-50% Xylose disaccharide 0-10% Xylose 15-30% Arabinose 5-20% In one embodiment, the first membrane filter 120 is a spiral wound filter made from polyamide. The filtration is preferably performed at from about 100 psi to about 300 psi at from about 40 °C to about 80 °C.

For black liquor derived from the soda pulping of wheat or rice straw, yellow liquor comprises a range of sugars : xylan with a molecular weight of from about 1,200 to about 10,000 ; xylooligosaccharide with a molecular weight of from about 450 to about 1,200 (xylose 3-8mer); xylose disaccharide ; xylose; and arabinose. Yellow liquor contains substantially no lignin. Yellow liquor also contains salts, for example, of the counterion of the acid used to acidify the black liquor, and some silicate. The precise composition will vary with the composition of the lignocellulosic material.

In the illustrated embodiment, the first retentate 122, which contains some precipitated lignin as well as dissolved lignin, is recycled into feed stream of the pH adjuster 110. Consequently, in the illustrated embodiment, substantially all of the lignin is recovered in high solids stream 114 produced by the mechanical separator 112.

The second membrane filter 130 separates the first permeate 124 into a second retentate 132 and a second permeate 134. In certain embodiments, the second membrane filter 130 is selected to retain compounds with molecular weights of greater than about 150, greater than about 300, or greater than about 500. In certain embodiments, the second membrane filter 130 is selected to retain compounds with molecular weights of greater than from about 150 to about 300, which retains hemicellulose, oligosaccharides, disaccharides, monosaccharides, and salts. In the case of wheat and rice straw, the hemicellulose is mostly

xylan, the oligosaccharides are mostly xylooligosaccharides, the disaccharides are mostly xylose disaccharide, and the monosaccharides are mainly xylose and arabinose. In a preferred embodiment, the second membrane filter 130 is a nanofiltration filter that retains the sugars xylan with a molecular weight of from about 1,200 to about 10,000, and xylooligosaccharide with a molecular weight of from about 450 to about 1, 200 (xylose 3- 8mer). In one embodiment, the second retentate 132 also contains some disaccharides, monosaccharides, or both. As a weight percentage of the total weight of the sugars in the retentate, the xylan is from about 25% to about 40%, and the xylooligosaccharide from about 35% to about 50%. The xylan and xylooligosaccharides may be isolated from the second retentate using methods Imown in the art. Xylooligosaccharides are used in the manufacture of pharmaceuticals and foodstuffs. Xylan is useful as a dough conditioner.

In one embodiment, the second membrane filter 130 is a spiral wound filter made from polyamide. The filtration is preferably performed at from about 200 psi to about 600 psi at from about 40 °C to about 80 °C.

The third membrane filter 140 separates the second permeate 134 into a third retentate 142 and a third permeate 144. The third membrane filter 140 is preferably a reverse osmosis filter. In one embodiment, the third membrane filter 140 is a spiral wound filter made from polyamide. The filtration is preferably performed at from about 300 psi to about 1000 psi at from about 40 °C to about 80 °C.

The third retentate 142 comprises monosaccharides and some disaccharides. hi one embodiment, the third retentate 142 also contains some oligosaccharides. In the case of wheat and rice straw, the monosaccharides are mostly xylose and arabinose, and the disaccharides are mostly xylose disaccharide. The sugars may be further, or used as a mixture, for example, as a fermentation feedstoclc. In a one embodiment, the third permeate 144 comprises water with a pH of from about 2.5 to about 6. The water is preferably recyclable into the pulping or papermaking operation. In one embodiment, the pH of the water is adjusted to from about 6 to about 8 using methods well known in the art, for example, lime.

The third permeate 144 preferably has low chemical oxygen demand (COD) and biological oxygen demand (BOD). In certain embodiments, the COD is less than about 5000 ppm, about 1000 ppm, or about 300 ppm. In certain embodiments, the BOD is less than about 5000 ppm, about 500 ppm, or about 150 ppm. Discharge requirements will vary

with locally applicable laws and regulations. In one embodiment, the third permeate 144 is dischargeable without further treatment. In another embodiment, the third permeate 144 is dischargeable with minimal treatment.

A flowchart illustrating an embodiment 200 of the disclosed method for treating black liquor is provided in FIG. 2 in reference to the apparatus illustrated in FIG. 1. Those skilled in the art will understand that the disclosed method may be practiced using an apparatus of another type or configuration. In step 210, black liquor 102 is transferred into a storage tank 104 from a pulping operation. In step 220, the black liquor is transferred into a reaction tank 106. hi step 230, the pH of black liquor is adjusted to precipitate dissolved lignin. In one embodiment, the black liquor is acidified. In a preferred embodiment, the acidified black liquor is held in the reaction tank 106 for from about 10 minutes to about 30 minutes, which appears to allow the precipitated lignin to coagulate. In certain embodiments, the reaction tank 106 is stirred during the holding period. In one embodiment, the stirring is performed using a low speed mixer, for example, less than about 200 rpm, or less than about 50 rpm.

In step 240, a mechanical separator 112 separates the acidified black liquor into a high solids stream 114 and a low solids stream 116, as discussed above. hi step 250, a first membrane filter 120 is used to separate the low solids stream 116 into a first retentate 122 and a first permeate 124, which are described in greater detail above.

In optional step 260, the first retentate 122, which contains some precipitated lignin as well as dissolved lignin, is recycled to the storage taillc 104. Consequently, substantially all of the lignin is recovered in high solids stream 114 produced by the mechanical separator 112. In other embodiments, the first retentate is recycled to another part of the apparatus, for example the reaction tank 106 or mechanical separator 112.

In step 270, a second membrane filter 130 separates the first permeate 124 into a second retentate 132 and a second permeate 134, which are described in greater detail above.

In step 280, a third membrane filter 140 separates the second permeate 134 into a third retentate 142 and a third permeate 144, which are described in greater detail above.

EXAMPLE A pilot plant was constructed according to the embodiment illustrated in FIG. 1 and used to treat black liquor 102 derived from soda pulping wheat straw, which was held in a storage tank 104. As the black liquor was pumped into a reaction tank 106, the pH was adjusted to about 3 with sulfuric acid using an in-line mixer 110. The acidified black liquor was held in the reaction tank 106 for about 30 minutes, then gravity transferred in to an Alfa Laval CHNX continuous decanter centrifuge 112. The high solids stream 114 from the centrifuge contained about 98% of the precipitated lignin. The pilot system used three membrane filters: ultrafiltration (IJF) 120, nanofiltration (NF) 130, and reverse osmosis (RO) 140. About 94% of the lignin remaining in the low solids stream 116 was retained in the ultrafiltration retentate 122, which was recycled into the storage tank 104. The three membrane filter stages, ultrafiltration 120, nanofiltration 130, and reverse osmosis 140 were constructed from filter units commercially available from GE Osmonics (Minnetonka, MN). TABLE III provides details of the configurations and operating parameters of the filters units at the time of a sampling of the feed and product streams provided in TABLE IV. The lignin and sugar compositions, and COD and BOD of selected feed and product streams are provided in TABLE IV in g/L (Note that the analytical method does not determine the concentration of xylooligosaccharide 6-8mers).

TABLE III 120 (UF) 130 (NF) 140 (RO) Model GM8040CJL DK4040CJL SE2540CJL Array Configuration 1 : 1 2: 1 1: 1: 1 Elements per housing 3 4 1 Temperature, °C 50 50 53 Feed Pressure, psig 210 385 640 Retentate Pressure, psig 170 375 600 Permeate Flow, gpm 2.51 2.6 0.55 Retentate Flow, gpm 6.22 2 0.5 Recirculation Flow, gpm 63. 8 0 2.8 Recovery, % 28. 7 56.5 52.4 Flux, gal/day/ft2 2.2 4.. 59

TABLE IV Black Liquor Yellow Liquor NF Retentate RO Retentate RO Permeate 102 124 132 142 144 Lignin 18. 26 0.03 Xylan 28. 68 5.63 5. 91 4. 19 (0. 41) Xylose 12.15 7.77 9.02 7.53 3.26 3-5mer Xylose 0.94 0. 75 2mer Xylose 10.24 5.81 6.09 5.72 1.51 Arabinose 4.63 1.6 1.96 2.13 0.14 COD 86,877 23,758 26,595 22,694 5,106 BOD 34,400 12,690 15,960 12,680 3,350 The embodiments illustrated and described above are provided as examples of certain preferred embodiments of the present invention. Various changes and modifications can be made to the embodiments presented herein by those skilled in the art without departure from the spirit and scope of this invention, the scope of which is limited only by the claims appended hereto.