PULPING OF LIGNOCELLULOSIC MATERIALS AND RECOVERY OF RESULTANT BY-PRODUCTS

Abstract: Filtrates from the bleaching and delignification of the pulp are recycled for reuse in pulping, separation, and recovery of lignin (66) and other by-products which results in significant energy savings and mitigation if not the elimination of pollution typically associated with bleaching. This invention also relates to products derived from the process and apparatus for carrying out the process. Lignins of various molecular weights and by-products of the pulping process are also recovered (66). The lignins are precipitated in high yields and at a high rate from a black liquor (40) produced by pulping wood at high temperature and pressures with an aqueous lower aliphatic alcohol solvent. As a by-product of this process a purified furfural product is recovered (71). This furfural may be recycled for use in the recovery of the low molecular weight lignin of this process.

Description: PULPING OF LIGNOCELLULOSIC MATERIALS AND RECOVERY OF RESULTANT BY-PRODUCTS CROSS-REFERENCE TO RELATED APPLICATION This is a continuation-in-part of application Serial No. 07/232,298, filed August 15, 1988, entitled "LIGNIN RECOVERY" which is a divisional application of Serial No. 06/940,460, filed December 11, 1986, now U.S. Patent 4,764,596 entitled "RECOVERY OF LIGNIN", which is a continuation-in-part of application Serial No. 06/795,069, filed November 5, 1985 entitled "PROCESS FOR LIGNIN RECOVERY" now abandoned. BACKGROUND OF THE INVENTION This invention generally relates to the pulping of lignocellulosic materials and the recovery of the resultant by-products resulting from the pulping process. More specifically, this invention relates to pulping of lignocellulosic materials in a water miscible organic solvent (e.g. lower aliphatic alcohol) and recovering and recycling the organic solvent and various by-products of the pulping process (e.g. cellulose, lignin, furfural hemicellulose and sugars). Pulping of the lignocellulosic material and the recovery of the resultant by-products may be carried out in either continuous or batch processes. Processes for treating wood with organic solvents, such as alcohols, to separate the wood's lignin, hemicellulose, sugar and cellulose fractions are now well known. See, for example, Kleinert et al U.S. Patent 1,856,567 and Kleinert U.S. Patent 3,585,104. Such solvent pulping processes have appeared to be attractive alternatives to conventional chemical pulping processes, such as kraft and sulfite pulping processes, which suffer from relatively high equipment costs and pollution problems. One solvent pulping process, disclosed in Diebold et al U.S. Patent 4,100,016, has appeared to be particularly attractive in providing highly efficient recovery of its alcohol solvent, separation of the cellulose and lignin fractions of wood, and recovery of 'cellulose pulp with no appreciable air or water pollution or solid waste products. This patented process has also provided hardwood pulps with yields. Kappa numbers, viscosities, fiber strengths and bleachability characteristics that are equal to or better than kraft and sulfite hardwood pulps. However, the recovery of lignin and other by¬ products from the alcohol/water black liquor, generated by the solvent pulping process of Diebold et al U.S. Patent 4,100,016, has been relatively inefficient and difficult to control. Lignin has been recovered from the black liquor in this patent by first stripping (preferably vacuum stripping) alcohol from the black liquor and then separating the lignin which precipitates from the stripper bottoms or tails (preferably by thickening and then centrifuging the settled solids from the stripper bottoms). However, a portion of the lignin has tended to precipitate as a sticky tar or gum on the internal surfaces of the stripper, thereby fouling the stripper and reducing its efficiency in recovery of alcohol from the black liquor. The lignin also has tended to precipitate from the stripper bottoms as a sticky amorphous mass which has been difficult to handle and has required substantial crushing to convert the lignin mass into a powder. As a result, more efficient ways have been sought for removing lignin and other by-products from the black liquor produced by a solvent pulping process such as is disclosed in Diebold et al U.S. Patent 4,100,016. One method has involved precipitating lignin from the alcohol/ water black liquor by diluting it with water. See Rydholm, "Pulping Processes', pp. 672-673, Interscience Publishers, New York (1971). However, this method has resulted in very slow settling rates of the lignin, and in some cases, a very stable colloidal suspension of the lignin has been formed which has been difficult to filter or centrifuge. There has been a continuing need, therefore, for a relatively simple way of recovering lignin and other by-products from an alcohol/water black liquor in high yields and at high rates in an easy to handle and useful form. Moreover, while solvent pulping processes produce hardwood pulps that are comparable in strength, brightness and cleanliness to kraft pulps produced from the same wood species, the resultant pulp from such pulping operations contains higher residual lignin in the pulp. Therefore pulps resulting from solvent pulping processes generally have a higher residual lignin content as measured by the pulp kappa number, and require, among other things, a large quantity of bleaching chemicals to produce satisfactorily bleached pulps. The increased use of large quantities of bleaching chemicals using conventional bleaching techniques created a need for devising new bleaching methods and/or systems for the disposal or recycling of these chemicals. Further, solvent pulping processes produce as z. by product, furfural, which can accumulate in the pulping solvent and interfere with delignification. There is therefore a distinct need for methods, apparatus and/or systems, which provide energy efficient, environmentally attractive and economically feasible means for pulping lignocellulosic materials and recovering the by-products of the pulping process. The Lora et al, U.S. Patent No. 4,764,596, addresses some of the foregoing problems. In Lora, lignin and other by-products of the pulping process are recovered from a "black liquor" produced when wood or other lignocellulosic material is contacted with a water miscible organic solvent (e.g. ethanol/water solvent) at elevated temperatures and pressures. The Lora process successfully allows for the recovery of many by-products, including most of the lignin liberated during pulping. It has also been discovered through improvements of the Lora et al patent as set forth herein that additional by¬ products, including lower molecular weight lignin and furfural, may be recovered while simultaneously increasing the overall energy efficiency of the process, reducing the consumption of solvent required throughout the system and significantly reducing or eliminating environmentally undesirable effluent. The invention described herein includes the foregoing improvements and additionally accelerates delignification of the black liquor and allows for the recovery of additional by-products and streams from the pulping process. This invention not only recovers furfural from the residual black liquor filtrates produced during lignin recovery but uses same to recover low molecular weight lignin by recycling same into the system. Also incorporated herein is a system which not only accommodates novel bleaching techniques but provides for the recycling of bleaching effluent filtrates, which accelerate delignification and mitigate against pollution. SUMMARY OF THE INVENTION This invention provides for the recovery of lignin and other by-products from pulping of fibrous material. In accordance with this invention, solvents and filtrates are recovered and recycled for reuse. This results in a significant solvent and energy savings. In accordance with this invention, filtrates from the bleaching and delignification of the pulp of this invention are recycled for reuse in the continuous pulping, separation and recovery of lignin and other by¬ products. The reuse of the bleaching filtrates results in accelerated delignification and greater operating efficiency. The net result is a significant energy savings and mitigation if not the elimination of pollution typically associated with bleaching. In accordance with this invention, lignin is recovered from a black liquor comprising a solution of lignin, hemicellulose, and a water miscible organic solvent by precipitating lignin solids by diluting the black liquor with water and acid under conditions to form a diluted residual black liquor including a diluted residual black liquor supernatant and precipitated lignin solids which are free from the formation of tarry lignin precipitates and recovering the lignin by separating the lignin solids from the diluted residual black liquor supernatant. In accordance with another aspect of this invention, a novel lignin is precipitated by the process of this invention. A preferred lignin is characterized by: a number average molecular weight of about 800 to 1500 g/mol; a polydispersity of less than about 4; and a methoxyl content approximately equal to that of native lignin. In accordance with another aspect of this invention, a novel low molecular weight lignin is obtained. A preferred lignin is characterized by: a number average molecular weight in the range of less than 500 g/mol, a glass transition in the range of from about 24° to 60°C and a syringhaldehyde to vanillin molar ratio of about 5.3:1, especially as it relates to hardwoods. It is expected that a more soluble lignin having a novel structure is produced. In accordance with another aspect of this invention, a purified furfural product is recovered. The furfural product contains from about 95 to 98% furfural, from about 0.1 to 0.5% ethanol and from about 0.1 to 2% water. Other aspects of this invention will be apparent from a reading of the remainder of this specification, including the drawings and claims. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a flow chart of a process for producing cellulose pulp from wood by treating the wood with an aqueous alcohol solvent, and for recovering lignin and other by-products from the alcohol/water black liquor. Figure 2 is a schematic sectional view of an example of an apparatus for precipitating lignin from the alcohol/water black liquor from the processes of Figures 1, 3 and 4. Figures 3 and 4 represent a flow chart of a continuous process for producing cellulose pulp from wood by treating the wood with an aqueous alcohol solvent and for recovering lignin, and other by-products from the alcohol/water black liquor produced in the pulping Drocess. Figure 5 is a schematic representation of crude furfural upgrading and purification by liquid/liquid cross-current extraction followed by pervaporation. Figure 6 is a schematic representation of crude furfural upgrading and purification by liquid/liquid counter-current extraction followed by pervaporation. Figure 7 is a flow chart for the recovery of low molecular weight lignin. Figure 8 is a schematic representation of a continuous co-current/counter-current extractor used in the processes of Figures 3 and 4. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The process shown in Figure 1 initially involves pulping a batch of wood chips or other fibrous material that are loaded from a hopper 1 into an extractor 2. The extractor 2 is operated in accordance with Diebold et al U.S. Patent 4,100,016 at an elevated temperature (e.g., about 180° to 210°C) and an elevated pressure (e.g., about 20 to 35 atmospheres) and with a solvent comprising: about 40 to 80% (by volume) of a water miscible lower aliphatic alcohol of 1 to 4 carbon atoms (e.g., methanol, ethanol, isopropanol or tert- butanol); 20 to 60% water; and if needed, a small amount of a strong water soluble acid, such as a mineral aid (e.g., hydrochloric, sulfuric, phosphoric or nitric acid) or an organic acid (e.g., oxalic acid, preferably acetic, formic or peroxy acids). Preferably, the wood chips in the extractor 2 are preheated with low pressure steam, and then, a twice- used 60% ethanol/40% water, primary solvent from a primary EET solvent accumulator 3 contacts the wood chips in the extractor 2. The primary solvent is rapidly recirculated through the extractor 2 and through a peak load (e.g., steam-heated) heat exchanger 4 to raise the temperature of the wood chips to about 190° to 200°C in a few minutes (preferably not more than about 5 minutes). Alternatively, in large scale plant operations, a time of from about 5 to 60 minutes is required. After this first pulping step is completed, the resulting extract or "black liquor" in the extractor 2 is displaced into a recovery feed accumulator 5 by a once-used 60% ethanol/40% water, secondary solvent (preferably heated to 190° to 200°C) from a secondary solvent accumulator 6. The black liquor which contains lignin, hemicelluloses, other saccharides and extractives (e.g., resins, organic acids, phenols and tannins) from the wood and the ethanol is recovered at a temperature of from about 180° to 210°C and under a pressure of from about 20 to 35 atmospheres in the recovery feed accumulator 5. At the end of the black liquor displacement, the secondary solvent in the extractor 2 is displaced into the primary solvent accumulator 3 by a fresh 60% ethanol/40% water solvent (preferably heated to 190° to 200°C) from a fresh solvent accumulator 7. The fresh solvent in the extractor 2 is then drained into the secondary solvent accumulator 6. Once the extractor 2 has been drained, it is vented, alcohol-rich vapors from the extractor are condensed in a water-cooled ("C.W.") condenser 8, and the resulting ethanol/water mixture from the condenser 8 is recycled to the fresh solvent accumulator 7. After venting the extractor 2, residual alcohol in the pulp in the extractor is then stripped with low pressure steam, and the resulting alcohol/water vapors are condensed and recovered as discussed below. After steam stripping, the pulp in the extractor 2 is sluiced with water, piped to a holding tank 9 and pumped through a pulp screen 10. The pulp can then be suitably subjected to a conventional pulp handling, bleaching and paper-making procedures. The extractor 2 can be loaded with another batch of wood chips from the hopper 1, and the wood chips can be contacted by the primary, secondary and fresh solvents from accumulators 3, 6 and 7 as described above. Alternatively, a continuous pulping process is shown in Figures 3 and 4. Initially, wood chips (50% moisture) or other fibrous material are pre-steamed in steaming bin 31. Saturated steam is injected into steaming bin 31 at atmospheric pressure. The chips are wetted and passed into metering screw 32 which is positioned at an angle. The excess water from the steam condensates in metering screw 32 and the wet chips are passed thru a first low pressure sluice 33, heated in line 46 by direct steam injection at a temperature of from about 270°F to about 330°F and at a pressure of from about 30 to about 100 psig. Line 46 is equipped with a steam barrier which helps prevent backup of alcohol-containing vapors into low pressure sluice 33. The steamed wood chips are passed thru a second low pressure sluice 34 and are mixed in line 45 with a solvent from line 36. The solvent is mixed with the chips in high pressure sluice 35 and partially impregnates the chips. The resultant slurry passes through line 37 and enters extractor 100 at inlet 38. As the cooking mixture enters extractor 100 at inlet 38, a liquid separator 101 regulates the flow of the mixture into extractor 100. Excess cooking mixture liquid overflows extractor 100 at outlet 39, is recycled thru line 57 and pumped back into high pressure sluice 35. The excess cooking liquid from high pressure sluice 35 is pumped thru line 58 and recycled back into surge tank 50. The cooking mixture in surge tank 50 is mixed internally thru line 51. Any overflow cooking mixture from surge tank 50 is pumped thru line 54 into line 37. The partially impregnated chips in extractor 100 are digested and extracted with solvent which is fed into extractor 100 at inlets 52 and 53. Preferably, the solvent is similar to that used in the process shown in Figure 1. The alcohol concentration in the solvent is regulated by mixing appropriate quantities of makeup alcohol, introduced at 49, with recovered alcohol from the alcohol and by-products recovery system introduced at 7, and with alcohol/water filtrate from counter-current washer 44. The solvent contained in line 36 is heated in pressure diffuser 47 by heat exchange with the pulp leaving extractor 100 at outlet 41. The type of extractor used is not critical, however it should be adaptable to the continuous pulping of the cooking mixture. Typical extractor dimensions depend on the required capacity of the extractor. For example, extractor 100 is operated in a continuous co- current/counter-current mode and at a pressure range of from about 200 to about 650 psig. Such an extractor is comprised of sequential reaction zones and means to add and remove solvent. The latter can be in the form of liquor extraction screens equipped with wipers or other cleaning devices that prevent screen plugging such as steam injectors. In one particular extractor configuration as shown in Figure 8, the cooking mixture which typically contains 5% chips in 95% solvent passes through extractor 100 and is exposed sequentially to six reaction zones. With this particular extractor configuration, further alcohol impregnation of the chips occurs at a constant temperature of from about 100° to 130°C in separation zone (a) for about 2 to about 20 minutes. In separation zone (a), a vapor head space is maintained with the level of the solvent in the cooking mixture higher than the level of the chips. Any excess solvent is removed thru outlet 39 and recycled as described above. The temperature of the cooking mixture chips is elevated as the cooking mixture passes into preheating zone (b) and is preheated to from about 150° to 180°C in about 50 minutes. The heating of the cooking mixture in preheating zone (b) is achieved by circulating the cooking solvent counter- currently thru a heat exchanger (typically of the tube and shell type) which is heated with steam. The heat exchanger temperature is maintained at a level sufficient to cause the cooking mixture in preheating zone (b) to heat to from about 150° to 180° C. The preheated cooking mixture is further heated in primary extraction zone (c) to from about 160° to 205° C and subjected to digestion and extraction for about 70 minutes. The cooking mixture is heated in primary extraction zone (c) by circulating the cooking solvent co-currently through a heat exchanger as described above. In zone (c) , a hot ethanol/water extract or "black liquor" is produced during the digestion and extraction process. The hot black liquor which contains lignin, hemicellulose, other saccharides and extractives (e.g.. resins, organic acids, phenols and tannins) is separated from the cooking mixture through line 40 and subsequently treated to recover the lignin and other by-products of the pulping process. The cooking mixture is further digested and extracted for about 60 minutes in secondary extraction zone (d) at a temperature of from about 150° to 180°C. The temperature is cooled in secondary extraction zone (d) by recirculating the cooking solvent in a heat exchanger as describe above. The heat exchanger temperature is maintained at a level sufficient to achieve the cooling of the cooking mixture to maintain a temperature of from about 150° to 180° C in secondary extraction zone (d). The cooking mixture is further digested and extracted for about 45 minutes in tertiary extraction zone (e) and the mixture is cooled to a temDerature of from about 130° to EET 160°C by recirculating the cooking solvent co-currently thru a heat exchanger as described above. The cooking mixture is further cooled to from about 70° to 100°C in cooling zone (f) for about 22 minutes and broken up into pulp with mixer 102. Cooling of the cooking mixture in cooling zone (f) is achieved by mixing the mixture with the solvent introduced at inlet 52 in a counter-current fashion and at inlet 53 in a co-current fashion. The solvent mixture consists of makeup alcohol, recycled alcohol from the alcohol and by-product recovery and alcohol/water filtrate from counter-current washer 44. The pulp is separated from the cooking mixture and exits extractor 100 through line 41. The pulp is washed in pressure diffuser 47 with recycled alcohol through line 7 and cooled to a temperature below 80°C while simultaneously additional lignin is removed and recycled through line 36 and the kappa number is reduced to a bleachable grade. Further defiberisation occurs as the pulp passes through pressure reduction valve 42. Pressure reduction valve 42 is preferably a blow valve. The pulp is further washed in multistage counter-current washer 44 by water introduction through line 43 and cooled to a temperature of from about 40° to 70°C. Counter- current washer 44 replaces conventional, less energy efficient, steam stripping methods and removes from about 50 to about 90% additional alcohol from the pulp. After water washing, the pulp is piped to tank 9 and pumped through a pulp screen 10. The pulp can then be suitably subjected to conventional pulp handling, bleaching and paper-making procedures. In one bleaching technique, the pulp now referred to as brownstock can be delignified by treating with oxygen coupled with a prior peroxy treatment using a peroxy compound such as peracetic acid or hydrogen Deroxide. Filtrates thus obtained under acidic conditions can be recycled as wash water for brown stock washing. These filtrates are introduced at inlet 43, mixed with water and become part of the solvent in line 36. Since the rate of delignification is directly proportional to the acidity of the solvent, it is believed that these acidic filtrates will accelerate the rate of delignification. It is also believed that the presence in the filtrates of organic acids of sodium lignate accelerate the rate of delignification. It is believed that such acid catalyzed delignification will result in lowering the operating temperature and pressure in extractor 100. Additionally, if several peroxy compound treatments are used sequentially, counter-current filtrates can be recycled. Alternatively, after pH adjustment, filtrates from alkaline oxygen delignification can also be used. For example, delignification of pulp with oxygen can be carried by first mixing a pulp slurry at from about 9 to 15% consistency by weight of pulp solids with a solution of sodium hydroxide (caustic) and further mixing at high shear with oxygen gas. The amount of caustic added can preferably be from about 2 to 8%, more preferably from about 3 to 6% based on % wt/wt of oven dry (o.d.) pulp. The temperature of the reaction mixture can preferably be between about 60°C and 110°C, more preferably between about 70°C and 90°C, and oxygen pressure in the bleaching vessel can preferably be maintained at from about 40 to 110 psig, more preferably at from about 80 to 100 psig for oxygen delignification and at from about 32 to 60 psig for delignification using oxidative extraction. The reaction time with oxygen can preferably be from about 6 to 60 minutes, more preferably from about 40 to 50 minutes. Additional chemical agents which may be added to help preserve strength properties include 0.5% to 1% magnesium sulfate, 0.5% diethylene triamine pentaacetic acid (DTPA), and up to 3% sodium silicate. For example, peroxy treatment of pulp can be achieved by mixing peracetic acid with the pulp at a pH cf about 2 to about 10 and in amounts of from about 0.5 to about 4% by weight of peracetic acid per weight of oven dried pulp. The pulp can be of any consistency, but is preferably between about 10 and 12% by weight of pulp solids. The reaction time can preferably be from about 20 minutes to about 3 hours at a temperature of from about 40°C to 90°C. Alternatively, peroxy treatment of pulp can also be achieved by mixing hydrogen peroxide with the pulp in amounts of from about 0.5% to about 4% hydrogen peroxide and at a pH of from about 2 and 11. The pulp can be of any consistency, but is preferably between about 10% and 12% by weight of pulp solids and the temperature of the reaction can be maintained at from about 40° and 90°C. Magnesium sulfate at from about 0.5% to 1.0% may be added for viscosity protection of the pulp, and DTPA may be added at from about 0.05 to 0.5% to prevent catalytic decomposition of the peroxide by metal ions such as manganese, copper, and iron. As shown in Figures 1, 3 and 4, the black liquor is flashed into a flash tank 11 to recover part of the ethanol. The flash tank 11 can be at atmospheric pressure for simplicity of operation or at reduced pressure to further cool the black liquor and enhance the alcohol recovery. The reduction in pressure in the flash tank 11 causes partial vaporization of the ethanol and leaves the residual black liquor in the flash tank with an ethanol content of about 30 to 45%, preferably abou'_ 35 to 40%. The residual black liquor is cooled during this step to a temperature of less than about 95°C, preferably down to about 80° to 92°C, but not below about 70°C to avoid premature precipitation of lignin in the flash tank 11. The black liquor can be heated by steam injection or indirect heating before flashing in flash tank 11 to vaporize more ethanol, therefore decreasing the ethanol concentration of the liquor to from about 25 to 34% and reducing the amount of dilution water needed for precipitation by from about 20 to 70%. Steam injection or indirect heating can also be introduced directly into flash tank 11 or in any of the flash tanks that can be used in sequential series with flash tank 11. The ethanol/water vapors obtained are condensed in condenser 8 and recycled, along with any makeup ethanol, water and/or acid, for use in treating subsequent batches of wood chips. Alternatively, in the continuous process as shown in Figures 3 and 4, the ethanol/water vapors from flash tank 11 (or flash tanks in sequential series with flash tank 11) can be recycled in reboiler 24, thus providing energy for distillation in solvent recovery tower 14. Solvent recovery can be further enhanced by interfacing solvent recovery tower 14 with additional solvent recovery towers and reboilers arranged in sequential series with solvent recovery tower 14. In accordance with this invention, lignin is then separated from the residual black liquor discharged from the flash tank 11. This step is carried out by diluting and preferably cooling the residual black liquor, as it leaves the flash tank 11, with water and acid to form a diluted residual black liquor with: a) an alcohol content of less than about 30% (by volume), preferably about 10 to 25%, particularly about 12 to 21%, with an alcohol content of about 8% being a practical minimum for subsequently recovering the alcohol economically; b) a temperature of less than about 75°C, preferably less than about 60°C, particularly about 35° to 55°C; and c) a pH of less than about 3, preferably less than about 2.5, particularly about 1.5 to 2.5. In this step, particular temperatures are not critical, although providing higher temperatures in the diluted residual black liquor will generally increase settling rates of the lignin but will yield a darker colored lignin and may decrease its yields. About 75°C is a maximum temperature to avoid the formation of tarry lignin precipitates, ambient temperatures (e.g., about 20°C) is a practical minimum, although lower temperatures (e.g., down to about 0°C) can be used if low settling rates can be tolerated. Temperatures below about 65°C, particularly below 60°C, provide a significantly lighter colored lignin precipitate. Alternatively, in large scale plant operations,- about 30°C is a maximum temperature in order to avoid the formation of tarry lignin precipitates. Also, particular pH's of the diluted residual black liquor are not critical in this step, but lower pH's increase the yield of precipitated lignin from the diluted residual black liquor and permit the use of higher temperatures in the diluted residual black liquor. However, lowering pH below about 1 provides little or no additional improvement in yield, and for this reason, a pH of about 1 is a practical minimum although lower pH's can be used. At a pH of less than about 3, lignin will precipitate from the diluted residual black liquor in high yield and at a high rate as fine solids. These lignin solids can then be separated from the remaining diluted residual black liquor supernatant in a conventional manner. Preferably, the lignin solids are separated by: allowing them to settle out as a paste of about 6 to 12% (by weight) solids in a conventional clarifier or settling tank 12; then concentrating this paste of lignin solids in a conventional centrifugal separator to form a wet cake of about 30 to 40% solids; and then drying this wet cake to form a uniform fine, free flowing powder. Alternatively, in large scale plant operations, the lignin solids are preferably separated by using large scale filters (e.g. belt filter and filter press, preferably drum filter) which allows easy washing of the lignin cake. In diluting the residual black liquor from the flash tank 11 with the water and acid to precipitate lignin, any conventional water soluble acid can be utilized which will provide the diluted residual black liquor with a pH of less than about 3.0, preferably less than about 2.50. For example, a strong mineral acid (e.g., hydrochloric, nitric, sulfuric or phosphoric acid) or a strong organic acid (e.g., oxalic acid, preferably acetic, formic or peroxy acids) can be used. Alternatively, as shown in Figures 3 and 4, filtrates from the bleaching step can also be used. They can be added to the water in line 43 and alternatively, they can be added to mixing tank 20. Additionally, oxygen delignification filtrates, after pH adjustment, are able to precipitate lignin at a pH above 3.0, preferably between approximately 3.5 and approximately 4.0. Preferably, the water and acid are mixed together before they are used to dilute the residual black liquor. In this regard, a particularly preferred mixture of acid and water is a residual black liquor supernatant that is derived from a previous batch of wood chips and that has been recycled and used to dilute the residual black liquor from the flash tank 11 after: a) the supernatant has been separated from the lignin solids from the previous batch of wood chips in the settling tank 12 and the centrifugal separator 13, or as shown in Figure 4, in large scale filter 63; and b) the alcohol content of the supernatant has been recovered in a conventional solvent condenser 15 as described below. The recycled residual black liquor supernatant or stripper bottoms, when used for diluting the residual black liquor from the flash tank 11, provides higher yields and faster settling of lignin solids precipitating in the settling tank 12 and centrifugal separator 13. In precipitating lignin from the residual black liquor from the flash tank 11, the method of diluting the residual black liquor with the water and acid also is not critical, so long as there is rapid and intimate mixing of the residual black liquor with the acid and water. For example, the residual black liquor can be suitably diluted by adding it to the acid and water in a conventional static dispersion mixer or a mixing tank, generally 20. The residual black liquor can also be diluted by adding it as a finely divided stream to a stream comprising a solution of the water and acid, for example, by means of a venturi-type device, generally 20, as shown schematically in Figure 2. The residual black liquor from flash tank 11 in Figures 1, 3 and 4 can be pumped through a small nozzle 21 located at about the center of a pipe 22 in the venturi-type device 20 in Figure 2, and the acid and water solution can flow in the pipe 22 towards the settling tank 12. As the residual black liquor is injected by the nozzle 21 into the acid and water solution in the pipe 22, the residual black liquor is rapidly diluted and cooled by the acid and water in the pipe 22. Lignin rapidly precipitates as fine solids from the resulting diluted residual black liquor in the pipe 22, which solids can be easily collected and concentrated in the settling tank 12 and centrifugal separator 13. Alternatively, as shown in Figure 4, the residual black liquor exits mixing tank 20 and enters a liquid/solid separation system consisting of large scale filter 63 (e.g. belt filter, filter press, preferably drum filter), and dryer 66. Filtrates of alcohol and dissolved solids, including hemicellulose, are extracted from filter 63 to be distilled in solvent recovery tower 14. Precipitated lignin cake is discharged from filter 63 and is dried to a powder-like form in dryer 66. In precipitating lignin in accordance with this invention, the yield and settling rates of the lignin are generally a function of: a) the wood species; b) the process conditions utilized in the extractor 2; c) the temperature, pH and solids content of (i) the residual black liquor from the flash tank 11 and (ii) the acid and water used to dilute it; and d) the ratio of residual black liquor to the acid and water used to dilute it. For example, the lignin from softwoods, such as spruce, is preferably precipitated at a temperature after dilution of about 40° to 60°C using an acid and water solution with a pH of about 1.5 to 2.5 and with a ratio of residual black liquor to the acid and water solution of about 0.5 to about 1. For hardwoods such as aspen, it is preferred to use an acid and water solution with a pH of about 1.2 to 2.2 and a temperature after dilution of less than about 50°C. In this regard, it is preferred to use a ratio of residual black liquor to the acid and water solution of: a) about 0.2 to 0.8 if the temperature after dilution is about 40°C; and b) about 0.6 to 1.0 if the temperature after dilution is less than about 40°C (e.g., down to ambient temperature). For hardwoods, such as sweetgum, maple and oak, it is preferred to use a temperature after dilution of about 40° to 60°C, an acid and water solution with a pH of about 1.5 to 2.5, and a ratio of residual black liquor to the acid and water solution of about 0.35 to 0.7. The clarified residual black liquor filtrate from the lignin solids separation step contain alcohol, furfural, wood sugars, acetic acid and low molecular weight lignin fragments that were not captured in the precipitation procedure. As shown in Figures 1, 3 and 4, the ethanol content is preferably recovered in the solvent recovery tower 14 and solvent condenser 15. The ethanol content of the supernatant can be stripped (e.g., down to about 200 ppm) in a conventional manner in the solvent recovery tower 14 at atmospheric pressure. Preferably, the tower 14 is heated by heating and recycling a portion of the bottoms stream from the tower 14 in a heat exchanger 24, using the low pressure steam used to strip residual ethanol from the pulp in the extractor 2. Alternatively, when additional recovery towers are used sequentially with solvent recovery tower 14, the tower 14 can be operated under vacuum or pressurized. The ethanol/ water vapors from the tower 14 are condensed in a conventional manner in the water-cooled condenser 15 (or by heat exchange with the stripper feed) and are then recycled together with the ethanol/water mixture which condenses from the low pressure steam in the heat exchanger 24. In accordance with this invention, the ethanol content of the supernatant from the settling tank 12 and centrifugal separator 13, or alternatively from large scale filter 63 as shown in Figure 4, can be suitably recovered in high yield in a simple manner, without lignin precipitating within the solvent recovery tower 14 and forming tarry or gummy deposits on the internal surfaces of the tower. The clarified residual black liquor filtrates from the lignin solids separation step typically contains from about 0.2 to 0.8% furfural, from about 10 to 15% alcohol, from about 0.5 to 10% dissolved solids, and water. As shown in Figures 1, 3 and 4, the black liquor is fed to solvent recovery tower 14 and a furfural side draw is removed which contains from about 12 to 30% furfural. The furfural side draw is taken at a plate above the feed plate at line 700. The furfural side draw is cooled by indirect heat exchange in heat exchanger 701 to a temperature of less than about 50° C and separates in decanter 71 into a crude furfural layer which contains from about 60 to 75% furfural, and an alcohol rich layer which is returned directly to solvent recovery tower 14. The crude furfural can be upgraded to from about 85 to 91% furfural using liquid/liquid extraction and can be further purified to from about 95 to 98% furfural using pervaporation. Other purification techniques include freeze concentration, dehydration and the use of a desiccant. The crude furfural layer which typically contains from about 60 to 75% furfural, from about 5 to 15% ethanol, from about 0.5 to 2% methanol and from about 7 to 15% water can be upgraded using liquid/liquid extraction. Cross-current liquid extraction can be used, and as shown in Figure 5, the crude furfural is mixed in mixer 71 using mechanical agitation with a solvent, preferably water. When the crude furfural and water separate in settler 72, a furfural raffinate and an alcohol rich water extract are obtained. Several extractions steps (N=extraction steps in Figure 5) can be used with more than one mixer and settler arranged in sequential series. However, upgrading of the crude layer can be satisfactorily achieved with preferably two or three sequential cross-current extractions. Water and crude furfural are mixed in a volume ratio of from about 1:1 to 3:2, for about 30 minutes, and at a temperature of from about 0° to 50° C. As shown in Figure 5, the alcohol extract which contains from about 0.5 to 6% ethanol, from about 0.2 to 1% methanol and from about 6 to 10% furfural is returned to solvent recovery tower 14. A furfural raffinate is obtained which contains from about 89 to 91% furfural, from about 0.1 to 0.2% ethanol, and from about 4.2 to 4.6% water. Alternatively, as shown in Figure 6, the crude furfural can be upgraded using counter-current extraction. The crude furfural is extracted with a solvent, preferably water in counter-current extractor 73. A temperature of from about 0° to 50°C is used, and the flow of crude furfural to water is about 3:2. An upgraded furfural raffinate is obtained which contains from about 85 to 90% furfural, from about 0.2 to 1% ethanol and from about 4 to 7% water. An alcohol extract typically containing from about 1 to 12% ethanol, from about 0.4 to 1.5% methanol and from about 6 to 10% furfural is returned to solvent recovery tower 14. The upgraded furfural raffinate can be further purified to remove the water (e.g. by dehydration or pervaporation) . Figures 5 and 6 are illustrative of purification by pervaporation. The pervaporation system 80 is comprised of a cell separated by a membrane 82 into two compartments 81 and 83. Membrane 82 is preferably a hydrophyllic membrane, for example, a polyvinyl alcohol membrane. The upgraded furfural is fed into compartment 81 and the water contained in the upgraded furfural is preferentially attracted by membrane 82. The water travels thru membrane 82 into compartment 83. A vacuum pressure of from about 0.8 to 8 psia (preferably from about 1 to 2 psia) is maintained in tank 83 to vaporize the water. A final furfural product is obtained which contains from about 95 to 98% furfural, from about 0.1 to 0.5% ethanol and from about 0.1 to 2% water and removed from compartment 81. Alternatively, the crude furfural can be upgraded using conventional distillation. An upgraded furfural distillate is obtained which contains from about 95 to 99% furfural, from about 0.2 to 1% ethanol and from about 0.2 to 1% water. Although a high purity furfural distillate is obtained, upgrading and purification of furfural using liquid/liquid extraction and pervaporation is the preferred method over a single distillation since this results in an energy savings of at least two-fold and the resultant furfural product contains less ethanol and water. The bottoms stream removed from the solvent recovery tower 14 contains wood sugars, low molecular weight lignins, acetic acid, ash and other minor components. A portion of the bottoms stream is preferably concentrated in a conventional manner, for example, in multiple effect evaporators 26. In this step, scaling or fouling cf the evaporation equipment is not a significant problem because there are no substantial amounts of high molecular weight lignin in the bottoms stream from the solvent recovery tower 14. The resulting syrup, containing hemicelluloses together with small amounts of other saccharides, extractives and very low molecular weight lignin (i.e., lignin with a molecular weight of less than about 400 g/ mol), can be burned to recover its fuel value, use as animal feed, or converted to other chemical products. Alternatively, the low molecular weight lignin can be recovered. The low molecular weight lignin corresponds to lignin fragments that were not captured by the precipitation process due to their low molecular weight and water solubility and several different fractions can be isolated. In general, low molecular weight lignin can be characterized by a low molecular weight fraction in the range of less than 500 g/ mol and a low glass transition temperature in the range of from about 24° to 60°C. Another characteristic, when hardwoods are pulped, is that the low molecular weight lignin is predominantly of the syringyl type, since by nitrobenzene oxidation, it yields a syringaldehyde to vanillin molar ratio of about 5.3:1. This low molecular weight lignin can be used as an extender in phenolic wood adhesive systems. It can also be used in applications requiring water solubility during processing (e.g. fiberglass binders) and as an intermediate for the production of syringaldehyde and other chemicals. Figure 7 illustrates the recovery of low molecular weight lignin. A portion of the bottom streams removed from solvent recovery tower 14 is concentrated by multiple effect evaporator 600 to a syrup containing from about 10 to 30% solids. Optionally, the pH of the syrup is raised to a pH of from about 2.0 to about 6.0 by alkaline addition before concentrating the bottom streams. The low molecular weight lignin fragments separate as a tarry organic phase 90 which contains from about 30 to 70% of the low molecular weight lignin that was present in the bottoms stream, and an aqueous fraction. This tarry organic phase 90 contains from about 60 to 90% low molecular weight lignin solids and at a temperature of from about 60 to 95°C, it has a viscosity of from about 400 to 3000 cps. This organic phase is separated in decanter 91 from the aqueous phase. The aqueous phase containing the remaining low molecular weight lignin is concentrated in evaporator 92 to from about 40 to 65% solids, and is extracted with an organic solvent (preferably 1:1 volume ratio) in counter-current liquid/ liquid extraction column 93. Organic solvents such as dieth l ether, cyclohexane, furan, and 3-hexanol can be used, however furfural is a particularly preferred solvent since it removes in excess of 70% of the low molecular weight lignin present in the aqueous phase in a single extraction. The furfural obtained by upgrading and purification as shown in Figures 5 and 6 can also be used to extract the low molecular weight lignin as described above. The raffinate from column 93 contains from about 5 to 20% low molecular weight lignin, from about 70 to 85% furfural and from about 5 to 15% water. The raffinate is vacuum distilled in column 94. The bottoms stream from column 94 contain low molecular weight lignin and the condensate from condenser 940 containing primarily from about 85 to 95% furfural and from about 5 to 15% water is recycled to column 93. The extract from column 93 containing from about 6 to 10% furfural, from about 4 to 5% sugars and from about 50 to 80% water is stripped from furfural in column 95. The condensate from column 95 in condenser 950 is decanted in decanter 96 and the heavy layer containing from about 85 to 95% furfural and from about 5 to 15% water is recycled to column 93. The stripper bottoms from column 95 contain sugars, mostly xyloses, which can be further used for furfural production by acid catalyzed dehydration. A second portion of the bottoms stream removed from the tower 14 is preferably used as the acid and water solution for diluting the residual black liquor from the flash tank 11 in order to precipitate lignin therefrom. In this regard, the second portion of the bottoms stream from the tower 14 is preferably cooled to a temperature of less than about 50°C, preferably about 25° to 40°C (about 0 β C being a practical minimum), and its pH is adjusted, if necessary, to about 1.0 to 3.0 by adding a strong water soluble acid to it. Then the cooled and acidified second portion of the bottoms stream (hereinbefore called the "recycled residual black liquor supernatant") is intimately and rapidly mixed (e.g., in the venturi-type device 20 of Figure 2) with the residual black liquor to dilute and cool the residual black liquor and precipitate lignin. The very pure lignin, which precipitates as fine solids from the diluted residual black liquor in the settling tank 12, can be subsequently removed from the centrifugal separator 13, or alternatively from large scale filter 63 as shown in Figure 4 , water-washed and dried in a conventional manner (e.g., by spin flash drying) to form a fine (e.g., -80 mesh) uniform, free flowing, water insoluble powder. This lignin can be characterized as having: a relatively low number average molecular weight of about 800 to 1500 g/mol, preferably about 900 to 1300 g/mol and a glass transit i on temperature which is preferably about 100° to 170°C, particularly about 130° to 150°C, although a glass transition temperature of preferably about 80° to 170°C, particularly about 90° to 150°C is also observed; a narrow molecular weight distribution, i.e., a polydispersity of less than about 4, preferably no more than about 3, particularly only about 1.5 to 2.7; and a methoxyl content approximately equal to the methoxyl content of native lignin (i.e., about 20% for hardwoods and about 14% for softwoods). This lignin also has a softening temperature which is preferably about 120° to 150°C, particularly about 130° to 150°C. These characteristics show, inter alia, the purity and low degree of chemical modification of the lignin of this invention. This lignin can be used for example, as a phenol formaldehyde resin extender in the manufacture of particle board and plywood. This lignin can also be used in the manufacture of molding compounds, urethane and epoxy resins, antioxidants, controlled-release agents and flow control agents. This invention and many of its attendant advantages will be understood from the foregoing description, and it will be apparent that various modifications and changes can be made without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the processes hereinbefore described being merely preferred embodiments. For example, the process for precipitating the lignin of this invention can alternatively be carried out by separately adding an acid and water to a solution of lignin dissolved in a water miscible organic solvent to form a diluted aqueous solution with a pH of less than about 3, an organic solvent content of less than about 30% and a temperature of less than about 75°C, from which diluted solution the lignin will precipitate as uniform fine solids. In this regard, the acid can be separately added to the residual black liquor from the flash tank 11 in Figure 1 by adding the acid to the primary solvent from the primary solvent accumulator 3 before the primary solvent is used in the extractor 2 for pulping wood chips to produce the black liquor (which becomes, after removal of ethanol in the flash tank 11, the residual black liquor). Also, the process of this invention can be carried out with a water miscible organic solvent other than a lower aliphatic alcohol (preferably ethanol), such as acetone, glycol or glycerol, or with a mixture cf such solvents. Also, these processes can be carried out using any fibrous plant material, such a bamboo, bagasse, kenaf, cereal straws, and not just wood.

Claims: We claim: 1. A process for pulping fibrous plant material in a water miscible organic solvent and for recovering said water miscible organic solvent, lignin and other by-products resulting from the pulping of said fibrous plant material comprising the steps of: wetting said fibrous plant material; heating said wetted fibrous plant material; impregnating said heated fibrous plant material with a water miscible organic solvent to form an impregnated fibrous plant material mixture; extracting said impregnated fibrous plant material mixture with said water miscible organic solvent to produce a cooking mixture, said cooking mixture comprising cellulose pulp and black liquor, said black liquor comprising said lignin and said by-products; and recovering said lignin from said black liquor. 2. The process of Claim 1 which further comprises the step of recovering said water miscible organic solvent and said by-products from said black liquor. 3. The process of Claim 2 wherein said extraction step comprises the steps of: impregnating said cooking mixture with said water miscible organic solvent in a separation zone; heating said cooking mixture produced in said separation zone in a preheating zone; heating said cooking mixture produced in said preheating zone in a primary extraction zone; digesting said cooking mixture during said heating in said primary extraction zone to produce said black liquor from said cooking mixture; simultaneously extracting said cooking mixture in said primary extraction zone during said digestion step; digesting said cooking mixture produced in said primary extraction zone in a secondary extraction zone; digesting said cooking mixture from said secondary extraction zone in a tertiary extraction zone; cooling said cooking mixture in said tertiary extraction zone; and cooling said cooking mixture from said tertiary extraction zone in a cooling zone. 4. The process of Claim 3 which comprises the step of separating said black liquor from said cooking mixture. 5. The process of Claim 4 wherein said water miscible organic solvent comprises makeup alcohol, recovered alcohol, alcohol/water filtrates and bleaching filtrates. 6. The process of Claim 5 which further comprises the step of separating said cellulose pulp from said cooking mixture. 7. The process of Claim 6 which further comprises the steps of: reducing said cellulose pulp kappa number and forming a brownstock pulp; washing said brownstock pulp with a washing medium to produce alcohol/water filtrates; bleaching said brownstock pulp to produce bleaching filtrates; combining said alcohol/water filtrates and said bleaching filtrates; and recycling said combined alcohol/water filtrates and bleaching filtrates. 8. The process of Claim 7 wherein said washing medium comprises said recycled alcohol/water and bleaching filtrates. 9. The process of Claim 8 which comprises the steps of: removing residue of said water miscible organic solvent during said washing step, said water miscible organic solvent comprising residue of said lignin; and recycling said water miscible organic solvent for use in said impregnation and extraction steps of Claim 1. 10. The process of Claim 3 which comprises the steps of: circulating said water miscible organic solvent counter-currently and heating said cooking mixture in said preheating zone; circulating said water miscible organic solvent co-currently and heating said cooking mixture in said primary extraction zone; circulating said water miscible organic solvent co-currently and cooling said cooking mixture in said secondary extraction zone; circulating said water miscible organic solvent co-currently and cooling said cooking mixture in said tertiary extraction zone; simultaneously mixing said cooking mixture with said water miscible organic solvent introduced countercurrently at a first inlet and with said water miscible organic solvent introduced countercurrently at a second inlet and cooling said cooking mixture in said cooling zone. 11. The process of Claim 10 which further comprises the steps of: flashing said black liquor and forming a residual black liquor; recovering alcohol from said flashing step; recycling said recovered alcohol; and combining said recycled recovered alcohol with said miscible organic solvent. 12. The process of Claim 11 which further comprises the steps of: separating said lignin from said residual black liquor; and producing residual black liquor filtrate from said separation step, said residual black liquor filtrate comprising furfural, a lignin with a low molecular weight and said by-products. 13. The process of Claim 12 wherein said furfural in said black liquor filtrate is recovered by the steps of: producing a furfural side draw from said residual black liquor filtrate; cooling said furfural side draw and producing a crude furfural layer and an alcohol rich layer; separating said crude furfural layer from said alcohol rich layer; extracting said crude furfural layer with a solvent and producing a furfural raffinate and an alcohol extract; and purifying said furfural raffinate and producing a furfural product. 14. The process of Claim 13 which comprises the steps of: * recovering said alcohol from said alcohol rich layer; recycling said recovered alcohol; and combining said recycled recovered alcohol with said water miscible organic solvent. 15. The furfural product of Claim 14 comprising from about 95 to 99% furfural. 16. The process of Claim 15 which further comprises the steps of: producing residual black liquor bottoms stream from said residual black liquor filtrate, said residual black liquor bottoms stream comprising said low molecular weight lignin and said by products; separating a first portion of said bottoms stream comprising said low molecular weight lignin; and separating a second portion of said bottoms stream comprising said by-products. 17. The process of Claim 16 wherein said low molecular weight lignin is recovered by the steps of: concentrating said first portion of said residual black liquor bottoms stream, said residual black liquor bottoms stream comprising said low molecular weight lignin and forming a tarry organic phase and an aqueous phase from said concentration step; separating said tarry organic phase from said aqueous phase; concentrating said agueous phase; extracting said concentrated aqueous phase with an organic solvent, said organic solvent comprising an element of the group consisting of diethyl ether, cyclohexane, furan, 3-hexanol and furfural; producing an aqueous extract and a raffinate; distilling said raffinate to produce bottoms stream, said bottoms stream comprising said low molecular weight lignin and a condensate comprising said organic solvent; collecting said bottoms stream and mixing with said tarry organic phase; and separating said low molecular weight lignin comprised in said tarry organic phase and said bottoms stream. 18. The process of Claim 17 which further comprises the steps of: separating said aqueous extract from said organic solvent; recovering said organic solvent from said separation step; and combining said organic solvent from said stripping step with said organic solvent from said distillation step. 19. The process of Claim 18 which comprises the step of recycling said recovered organic solvent for use in said extraction step of Claim 16. 20. The process of claim 19 wherein said organic solvent comprises furfural recovered from said residual black liquor filtrate. 21. The low molecular weight lignin of Claim 20 having a number average molecular weight less than 500 grams/mol. 22. The low molecular weight lignin of Claim 21 having a glass transition temperature. 23. The low molecular weight lignin of Claim 22 wherein said glass transition temperature is in the range of from about 24° to 60°C. 24. The process of Claim 15 which comprises the step of recovering said by products from said second portion of said bottoms streams. 25. A process for the recovery of furfural from black liquor produced in the pulping of fibrous plant material with a water miscible organic solvent comprising the steps of: diluting said black liquor and forming a residual black liquor; producing residual black liquor filtrates from said residual black liquor, said residual black liquor filtrates comprising furfural; removing a furfural side draw from said residual black liquor filtrate; cooling said furfural side draw forming a crude furfural layer and an alcohol rich layer; separating said crude furfural layer from said alcohol rich layer, by decanting said alcohol rich layer; extracting said crude furfural layer with a solvent and producing a furfural raffinate and an alcohol extract; and purifying said furfural raffinate and producing a furfural product. 26. The process of Claim 25 which comprises the steps of: recovering said alcohol from said alcohol rich layer; recycling said recovered alcohol; and combining said recycled recovered alcohol with said water miscible organic solvent. 27. The process as in Claim 26 wherein said extraction step is cross-current solvent extraction. 28. The process as in Claim 27 wherein said crude furfural layer and said solvent are mixed in a volume ratio of from about 1:1 to 3:2. 29. The process as in Claim 26 wherein said extraction step is counter-current solvent extraction. 30. The process as in claim 29 wherein said crude furfural layer and said solvent are mixed in a flow ratio of about 3:2. 31. The process as in Claim 26 wherein said furfural raffinate is purified by pervaporation. 32. The process of Claim 31 which comprises the steps of: introducing said furfural raffinate into a first compartment, said first compartment separated from a second compartment by a membrane; separating into said second compartment said water comprised in said furfural raffinate; vaporizing said water in said furfural raffinate; and producing said furfural product from said compartment. 33. The furfural product of Claim 32 comprising from about 95 to 98% furfural. 34. The process as in Claim 26 wherein said furfural raffinate is purified by distillation. 35. The furfural product of Claim 34 comprising from about 95 to 99% furfural. 36. A process for the recovery of low molecular weight lignin from black liquor produced in the pulping of fibrous plant material with a water ..ύscible organic solvent comprising the steps of: producing residual black liquor filtrates from said residual black liquor; producing residual black liquor bottoms stream from said residual black liquor filtrate, said residual black liquor bottoms stream comprising said low molecular weight lignin; concentrating a portion of said residual black liquor bottoms stream comprising said low molecular weight lignin and producing a tarry organic phase and an aqueous phase; separating said tarry organic phase from said aqueous phase by decanting said tarry organic phase; concentrating said agueous phase; extracting said concentrated aqueous phase with an organic solvent comprising a member selected from the group consisting of diethyl ether, cyclohexane, furan, 3- hexanol and furfural; producing an aqueous extract and a raffinate; distilling said raffinate to produce bottoms stream comprising said low molecular weight lignin and a condensate comprising said organic solvent; collecting said bottoms stream; mixing said bottoms streams with said tarry organic phase; and separating said low molecular weight lignin comprised in said tarry organic phase and in said bottoms stream. 37. The process of Claim 36 which further comprises the steps of: separating said aqueous extract from said organic solvent; recovering said organic solvent from said separation step; and combining said organic solvent from said separation step with said organic solvent from said distillation step. 38. The process of Claim 37 which comprises the step of recycling said recovered organic solvent for use in said extraction step. 39. The low molecular weight lignin of Claim 38 having a number average molecular weight less than 500 grams/mol. 40. The low molecular weight lignin of Claim 39 having a glass transition temperature. 41. The low molecular weight lignin of Claim 40 wherein said glass transition temperature is in the range of from about 24° to 60°C. 42. A low molecular weight lignin having a number average molecular weight less than 500 grams/mol. 43. The low molecular weight lignin of Claim 42 having a glass transition temperature. 44. The low molecular weight lignin of Claim 43 wherein said glass transition temperature is in the range of from about 24° to 60°C. 45. A furfural product comprising from about 95 to 99% furfural. 46. In an apparatus for pulping wood or other fibrous Dlant material with a water miscible orσanic solvent to produce a black liquor as a by-product and for subsequently recovering the solvent from the black liquor; the improved apparatus which produces and recovers a precipitated lignin component comprising: means (a) for diluting the black liquor with an aqueous acid to precipitate lignin from the resulting diluted black liquor and leave a lignin-depleted supernatant; and means (b) for thereafter separating the precipitated lignin from the lignin-depleted supernatant. 47. An apparatus for pulping fibrous plant material and for recovering pulp and by-products of the pulping process comprising: means (a) for pulping fibrous plant material with a water miscible organic solvent to produce pulp and a black liquor; means (b) for subsequently separating the pulp from the black liquor; means (c) for thereafter removing the pulp from the black liquor; means (d) for thereafter diluting the black liquor with an aqueous acid to precipitate lignin from the resulting diluted black liquor and leave a lignin depleted supernatant; and means (e) for thereafter separating the liquor from the lignin-depleted supernatant. 48. An apparatus for pulping fibrous plant material and for recovering pulp and by-products of the pulping process comprising: an extractor for pulping fibrous plant material with a water miscible organic solvent to produce pulp and a black liquor and for separating and removing the pulp from the black liquor; a flash tank for separating and removing or the solvent from the black liquor leaving a residual black liquor; a condenser for condensing and recycling the flashed solvent; a mixer for rapidly and intimately mixing a mixture comprising the residual black liquor, water and acid such that lignin precipitates from the residual black liquor; a settling tank for separating the lignin from the mixture of residual black liquor, water and acid to form a lignin with residual black liquor, water and acid and a clarified residual black lignin supernatant; a centrifugal separator for further separating the lignin from the remaining residual black liquor, water and acid to form a powder-like lignin and additional clarified residual black liquor supernatant; a solvent recovery tower in which the solvent content of the supernatant introduced to the tower from the settling tank and centrifugal separator is stripped from the supernatant and recycled for use in pulping additional fibrous plant and in which a bottoms stream is formed; and an evaporator for concentrating the bottoms stream from which hemicellulose, saccharides, extractives and low molecular weight lignin may be recovered or burned as fuel. 49. An apparatus for pulping fibrous plant material in a water miscible organic solvent and for recovering said water miscible organic solvent, lignin and other by-products of the pulping process comprising: means (a) for pulping said fibrous plant material in said water miscible organic solvent to produce a cooking mixture, said cooking mixture comprising cellulose pulp and black liquor, said black liquor comprising said lignin and said by-products; means (b) for separating said cellulose pulp and said black liquor produced by pulping means (a); means (c) for separating brownstock pulp from said cellulose pulp separated by means (b); means (d) for recovering said brownstock pulp separated by means (c); means (e) for separating said lignin from said black liquor separated by means (b); means (f) for recovering said lignin from said black liquor separated by means (b); means (g) for separating said water miscible organic solvent, and said by-products from said black liquor separated by means (b); and means (h) for recovering said water miscible organic solvent, and said by-products separated by means (g). 50. The apparatus of Claim 49 which further comprises means (i) for recycling said water miscible organic solvent recovered by means (h) for further use in said apparatus. 51. The apparatus of Claim 50 which further comprises means (j) for separating a crude furfural from said black liquor separated by means (b). 52. The apparatus of Claim 51 which further comprises means (k) for separating a furfural product from said crude furfural separated by means (j). 53. The apparatus of Claim 52 which further comprises means (1) for recovering said furfural product separated by means (k). 54. The apparatus of Claim 53 which further comprises means (m) for separating a tarry organic phase from said black liquor separated by means (b). 55. The apparatus of Claim 54 which further comprises means (n) for separating a low molecular weight lignin from said tarry organic phase separated by means (m). 56. The apparatus of Claim 55 which further comprises means (o) for recovering said low molecular weight lignin separated by means (n). 57. The apparatus of Claim 56 which further comprises means (p) for recycling said furfural product, recovered by means (1), for further use in said apparatus. 58. The apparatus of Claim 57 wherein means (d) for recovering said brownstock further comprises means (q) for bleaching said brownstock. 59. The apparatus of Claim 58 wherein means (q) for bleaching said brownstock further comprises means (r) for separating bleaching filtrates from said bleached brownstock. 60. The apparatus of Claim 59 which further comprises means (s) for recovering said bleaching filtrates separated by means (r). 61. The apparatus of Claim 60 which further comprises means (t) for recycling said bleaching filtrates recovered by means (s) for further use in said apparatus. 62. An apparatus for pulping fibrous plant material in a water miscible organic solvent and for recovering said water miscible organic solvent, lignin and other by-products resulting from the pulping of said fibrous plant material comprising: an extractor for pulping said fibrous plant material with said water miscible organic solvent to produce a cooking mixture, said cooking mixture comprising cellulose pulp and black liquor and to separate said black liquor and said cellulose pulp from said cooking mixture; a separator for separating alcohol from said black liquor such that a residual black liquor is formed; a condenser for recovering said water miscible organic solvent; a mixer to mix said residual black liquor with water and acid such that lignin separates from said residual black liquor; and a separator for separating said lignin from said residual black liquor and to produce residual black liquor filtrate, said residual black liquor filtrate comprising furfural, a lignin with a low molecular weight and said by-products. 63. The apparatus of Claim 62 which further comprises: a solvent recovery tower to recover alcohol from said residual black liquor filtrate to recycle said recovered alcohol for use with said water miscible organic such that residual black liquor solvent bottoms stream and a side draw comprising furfural are formed; and an evaporator to concentrate said residual black liquor bottoms stream such that a lignin with a low molecular weight and said by-products are recovered from said black liquor bottoms stream. 64. The apparatus of Claim 63 which further comprises; a washer to wash said cellulose pulp with said recovered alcohol and to reduce said cellulose pulp kappa number to form a brownstock pulp such that alcohol/water filtrates are recovered from said washing; a washer to wash said brownstock pulp with water; and a bleaching system which bleaches said brownstock pulp such that bleaching filtrates are produced, recycled and combined with said alcohol/water filtrates. 65. The apparatus of Claim 64 which further comprises: a heat exchanger which cools said furfural side draw to produce a crude furfural layer and an alcohol rich layer; a decanter which separates said crude furfural layer from said alcohol rich layer; an extractor which extracts said crude furfural layer and to produce a furfural raffinate and an alcohol extract; and a purification system which purifies said furfural raffinate and to produce a furfural product. 66. The apparatus of Claim 65 which further comprises a condenser which condenses said alcohol from said alcohol rich layer. 67. The apparatus of Claim 66 which further comprises: an evaporator which concentrates a first portion of said residual black liquor bottoms stream comprising said low molecular weight lignin to form a tarry organic phase and an aqueous phase; a decanter which separates said tarry organic phase from said aqueous phase; an extraction column which separates said low molecular weight lignin in said aqueous phase to produce an aqueous extract and a raffinate; a solvent stripper which separates said aqueous extract from said organic solvent to recover said organic solvent; and a distillation column which distills said raffinate and to produce bottoms stream comprising said low molecular weight lignin.

Inventors: CRONLUND MALCOLM, GOPAL GOYAL C, KATZEN RAPHAEL, LEBLANC RON, LORA JAIRO H, WINNER STEPHEN R, WU CHIH FAE, CRONLUND, MALCOLM, GOPAL, GOYAL, C., KATZEN, RAPHAEL, LEBLANC, RON, LORA, JAIRO, H., WINNER, STEPHEN, R., WU, CHIH, FAE
Applicants: REPAP TECHNOLOGIES INC, REPAP TECHNOLOGIES INC.
Application year: 1992
Main classification: C07G1
IPC codes: C07D307, C07G1, C08H5, D21C3, D21C9, D21C11