US3585104A - Organosolv pulping and recovery process - Google Patents

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• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C3/00—Pulping cellulose-containing materials
  • D21C3/20—Pulping cellulose-containing materials with organic solvents or in solvent environment

Definitions

• the spent cooking liquor is recovered from the digester and is conducted at approximately the pulping conditions of temperature and pressure into a multistage flash evaporator from which the organic solvent is recovered continuously and can be recycled for make-up of the pulping liquor.
• the multistage flash evaporation also permits recovery of lignin and water soluble components containing sugars, oligosaccharides and organic acids.
• This invention relates to an organosolv pulping and recovery process and to an apparatus for carrying out said process.
• this invention relates to a process and an apparatus for subjecting to pulping subdivisions of various fibrous plant materials, such as wood, bamboo, bagasse, cereal straws, etc. at an elevated temperature, using aqueous mixtures of water-miscible, volatile organic solvents as pulping agents and recovering not only the cellulosic pulp and the volatile organic components of the pulping agent, but also the noncellulosic components of the fibrous plant materials solubilized during the pulping.
• aqueous solutions of inorganic chemicals such as the various sulfites, sodium hydroxide, mixtures of sodium hydroxide and sodium sulfide, are used as delignifying agents.
• inorganic chemicals such as the various sulfites, sodium hydroxide, mixtures of sodium hydroxide and sodium sulfide
• pulping agents For the so called hydrotropic pulping, sulfonates of various aromatic compounds, for instance, xylene sulfonate, have been suggested as pulping agents, exhibiting only negligible volatility.
• penetration of the pulping chemicals into the subdivided plant materials prior to heating to maximum reaction temperature is a prime factor.
• the process of the present invention because of the volatility of the pulping agents, results in rapid and uniform penetration without the necessity of preimpregn-ation when the subdivided fibrous plant material is directly contacted with the liquor at maximum pulping temperature and pressure.
• the process of the present invention has the advantage that a separate stage of liquor impregnation of the fibrous plant material prior to pulping can be eliminated.
• pulping is carried out at relatively mild conditions of liquor pH below and above the neutral point and the pulping liquor containing dissolved non-cellulosi components of the fibrous plant material is separated from the pulp dur- 3,585,104 Patented June 15, 1971 ing or soon after the pulping reaction without cooling ice this liquor i.e. at approximately the pulping conditions.
• pulping of a fibrous plant material is carried out continuously with flow of the pulping liquor countercurrently to the flow of the subdivided plant material, for example wood chips, and the organic components of the spent pulping agent are recovered by multistage flash evaporation.
• aqueous mixtures of the lower aliphatic alcohols such as methanol, ethanol, propanol
• aqueous mixtures of the lower aliphatic ketones such as acetone
• Preferential pulping agents are aqueous mixtures of ethanol in the range between about 20% and ethanol by weight.
• the elevated temperature used during the pulping reactions should be between about C. and about 200 C., preferably between and 195 C. with the reaction time at a chosen temperature varying from one or more hours at the lowest temperatures to a few minutes at the highest temperatures.
• pulping is carried out in the aqueous mixture containing the vola tile organic solvent with immediately reacting the subdivided fibrous plant material with the pulping agent at maximum pulping temperature and the corresponding pressure without requiring immediate previous impregnation of the plant material with the liquor.
• a brief pre-steaming of the subdivided fibrous plant material as commonly practised in conventional pulping, for instance, ten minutes steaming at 1.05-2.00 kg./cm. steam pressure, is advisable not only in order to remove air from the plant material, but also to increase the cellwall permeability and thus to promote leaching out of the non-cellulosic portions from the pulps solubilized during pulping.
• the spent pulping liquor is subjected to multistage flash evaporation, resulting in the recovery of aqueous condensates of the volatile organic components of the pulping agent for re-use.
• the vapors formed are used as heating media in successive bodies before being withdrawn as condensates.
• This is achieved by introducing the spent pulping liquor into the first body of a multistage flash evaporator having a pressure which, although being the highest in the evaporator system, is appreciably lower than the digester pressure.
• partial evaporation is caused by the heat content of the liquor with formation of vapor richer in said volatile organic component of the pulping agent than the remaining liquor portions with concurrent cooling of these liquor portions prior to entering the next body of lower pressure.
• this multistage flash evapo ration is operated by means of the pressure differentials between successive bodies of the evaporator while the remaining liquor portions flow from body to body and the vapors formed in each body are used for heating the next body while undergoing condensation with continuously withdrawing and recovering the enriched condensates for further use.
• enriched aqueous ethanol fractions are recovered from ethanol-containing spent pulping liquors by multi stage flash evaporation.
• the upper layer is an aqueous relatively light colored solution containing the water-soluble products derived from the fibrous plant material by the pulping reactions, such as monomer sugars, oligosaccharides, organic acids etc.
• the lower layer is a dark-brown, quasi molten flowing mass of greater specific gravity, mainly consisting of plasticized lignin substances. Based upon this finding and the appreciable differences of the specific gravity, the aqueous layer can be easily separated by known means from the layer of the highly plasticized, quasi molten lignin substances.
• lignin substances are highly plasticized and exhibit pronounced chemical reactivity. They can be used as raw materials for chemical conversion such as hydrogenation, oxidation, sulfonation, and other chemical reactions of the lignin, and also for condensation reactions, for instance, in the production of plastic materials.
• the monomer sugars contained in the aqueous layer or formed by hydrolysis from the oligosaccharides also present in the aqueous solution can be used in fermentation reactions, for instance to produce ethanol, or in protein production by microbiological processes.
• Acetic acid present in the aqueous layer, especially in hardwood pulping can be recovered by known methods such as solvent extraction with appropriate water nonmiscible solvents, such as ethyl ether, isopropyl ether, butyl acetate, amyl acetate and others.
• the process of the present invention allows the separate recovery for further use of these two groups of substances derived from the fibrous plant materials, namely, the water-soluble products on the one hand, and the bulk of the plasticized lignin substances on the other.
• the new process is preferably designed for continuous operation, it can also be carried out in a series of batch digesters with liquor flow from one pressure vessel to the next one in a quasi continuous countercurrent flow operation. In such case, for emptying and refilling at the end of the digestion period, each digester must be taken out of operation, while continuing the liquor flow in the remaining digesters.
• the multistage flash evaporator is nevertheless one of continuous operation.
• the new process is practically odorless, and by recovering the organic components of the fibrous plant materials solubilized during the pulping reactions, it eliminates also stream pollution.
• the pulping and recovery process of the present invention eliminates the problem of air pollution and also greatly reduces the problem of water and stream pollution.
• the new process can be used for the pulping of various fibrous plant materials, especially of various wood species.
• it can be used for the pulping of mixtures of very different hardwoods. Since the volatile organic pulping agents penetrate easily and uniformly into all kinds of chipped wood, the new process can also be used for the pulping of hardwoods of high density.
• the new process can be used in the whole range of pulps produced from high-yield pulps of relatively high lignin content to low-lignin bleachable pulps.
• the present inventional also comprises an apparatus for carrying out the process described, comprising a vertical elongated cylindrical digester, means for conducting subdivided fibrous plant material into said digester, means for conducting to said digester a pulping liquor comprising an aqueous volatile organic component, means for contacting said liquor and said subdivided fibrous plant material in said digester countercurrently, a pulping zone and a cooling zone in said digester, means for withdrawing from the cooling zone said pulping liquor and heating said pulping liquor to maximum pulping temperature and re-introducing the said heated liquor into the pulping zone of the digester, means for recovering digested wood chips, a multi-stage flash evaporator, means for withdrawing the spent cooking liquor from the pulping zone and for feeding said liquor at the operating temperature and pressure of said digester into said multi-stage flash evaporator, means for recovering from said multi-stage evaporator a condensate enriched in said aqueous volatile organic component and a residual liquid low in said aqueous volatile
• the present invention comprises an apparatus for carrying out the process described, said apparatus comprising a vertical elongated cylindrical digester having means for continuously introducing thereinto subdivided fibrous plant material against the digester pressure at the top of the digester, means for continuously withdrawing the pulp produced in a partly Washed and cooled state at the bottom of the digester, this digester having strainer rings for continuously introducing and withdrawing pulping liquor to provide liquor upflow in two temperature stages With circulation and heating means for the liquor between these two stages, the uppermost strainer ring being used to withdraw spent pulping liquor for recovery and also to provide a constant liquor level in the digester with a small vapor phase above connected to release means, the outlet of the digester for the spent pulping liquor being connected to a multistage flash evaporator, means for Withdrawing the condensates from said evaporator, the last body of said evaporator discharging the remaining liquor into a separation tank having withdrawing means for separately recovering the water insoluble highly plasticized lignin portions of relatively high
• EXAMPLE Commercial aspen chips of about inch length are continuously introduced into chip bin 1 from which they enter low-pressure steaming vessel 3 through low-pressure rotary valve 2. While being transported through vessel 3 by screw conveyor 4, the chips are treated by a countercurrent flow of low-pressure steam (1.2 kg./cm. supplied to vessel 3 through pipe 5 and withdrawn through exhaust pipe 6 connected to rotary valve 2 and fractionating condenser column 23.
• the spent steam contains small amounts of ethanol leaking from the digester through high-pressure rotary valve 7, and after being withdrawn through exhaust pipe 6, it is subjsected to fractionated condensation first in condenser column 23 and then in condenser column 21 connected by pipe 20.
• the chips, steam-treated in vessel 3, are -fed continuously through high-pressure rotary valve 7 into vertical, cylindrical digester 8.
• Digester '8 is equipped with a series of strainer rings 9, 10, 11, 12, namely, strainer rings 9 and 10 for introducing liquor and strainer rings 11 and 12, for withdrawing liquor from the digester.
• Freshly recovered aqueous ethanol of about 45.8% ethanol content having a pH of 4 and a temperature of C. and additional re-cycled liquor of similar composition and temperature, recovered from the partially washed pulp in the screw press 13 is forced into digester 8 by means of the rotary pump 28 via pipes 67 and 29 and strainer ring 9.
• a major portion of this liquor mixture flows in the digester upwards countercurrently to the pulp while partly washing and cooling it, and another portion of the liquor mixture flows concurrently with the cooled pulp through conduit 30 into the screw press 13, where the pressed pulp is discharged from the pressurized system of the digester.
• the liquor portions of relatively low temperature which, after being introduced into the digester through strainer ring 9, flow upwards countercurrently to the pulp are withdrawn through strainer ring 11.
• This liquor is re-cycled from strainer ring 11 to strainer ring 10 by rotary pump 15 via pipe 14, heater 16 and pipe 17. In this manner, the liquor is heated up to maximum pulping temperature C.) before being re-introduced into the digester. While the liquor again flows upwards in the digester between strainer rings 10 and 12 countercurrently to the plant material, pulping takes place at maximum temperature 180 C.) in liquid phase with solubilization of major portions of the noncellulosic components. Time of exposure of the plant material to the liquor at maximum temperature is about 30 minutes.
• strainer ring 12 Through strainer ring 12, the spent pulping liquor containing about 10% by weight of solubilized non-cellulosic portions of the Wood is continuously withdrawn from the digester via pipe 31 directly into body 32, the first body of the five-stage flash evaporator 66 consisting of bodies 32, 33, 34, 35, 36 and fractionating column 37 which operates at atmospheric pressure.
• strainer ring 12 a constant level of the pulping liquor is maintained in digester 8 and a small vapor phase is formed above the liquor level in the top section of the digester.
• these chips are briefly exposed to a vapor phase considerably richer in ethanol content than the corresponding hquor.
• the chips are not only rapidly heated to pulping temperature but concurrently are also impregnated with aqueous ethanol according to the ethanolwater vapour-phase liquid-phase equilibrium at pulping temperature.
• the chips when subsequently submerged mto the liquor are at cooking conditions, so that pulping starts immediately.
• the vapor phase in the top section of the digester is connected via pipe 18 and release valve 19 for continuously releasing sufi'icient amounts of the vapor mixture in order to remove permanent gases such as air and carbon dioxide accumulating in the vapor phase during pulping. In this way also, water portions are removed from the digesting system.
• Valve 19 is connected via pipe 20 to the fractionating condenser column 21 for recovery of aqueous ethanol from said withdrawn vapor.
• the fractionating condenser column 21 also, the ethanol-water vapors partially enriched in ethanol in condenser column 23 are further subjected to fractionated condensation.
• the total of the aqueous ethanol recovered in fractionating condenser column 21 flows via pipe 65 by means of the rotary pump 22 into the liquor make-up tank 27 for re-use.
• Non-condensable gases air and carbon dioxide
• the water is withdrawn from fractionating condenser column 21 through pipe 62 together with some condensed volatiles derived from the Wood in some cases, such as terpenes.
• the partly washed and cooled pulp together with portions of the liquor are fed by the digester pressure through conduit into the screw press 13.
• Liquor squeezed out from the pulp in this screw press is re-cycled via pipe 67 by rotary pump 28 and via pipe 29 into strainer ring 9 and enters the digester together with fresh liquor from liquor make-up tank 27 also supplied by rotary pump 28 via pipe 29.
• the pulp is treated in washer 24 in a countercurrent flow of weak aqueous sodium hydroxide (3-6 g. NaOH per liter) solution at 90-100" C. entering through pipe 25 and being withdrawn through pipe 26 for recovering the ethanol portions by fractionated distillation.
• the pulp is recovered for further treatment by the screw press 60 and the liquor thus removed is re-cycled into washer 24 via pipes 68 and 25.
• the spent pulping liquor withdrawn from the digester through strainer ring 12 at digester temperature (180 C.) and pressure (17.4 kg.-cm. contains per 100 kg., about 41.6 kg. ethanol, 48.4 kg. Water and 10 kg. noncellulosic solids solubilized from the wood during pulping.
• This liquor enters via pipe 31 into the first body 32 of the five-stage fiash evaporator 66, consisting of the bodies 32, 33, 34, 35, 36 and fractionation column 37, and operating at falling pressures and temperatures. While Residual liquor (1:51.)
• Pipes 42, 43, 44 and 45 connect the vapor phase of bodies 32, 33, 34 and 35 to the condenser-heater elements of the subsequent bodies 33, 34, 35 and 36, respectively.
• Pipe 46 connects the vapor phase of body 36 to the fractionating condenser column 37.
• Pipes 47, 48, 49 and 50 carry the residual liquor from the first body 32 to the subsequent bodies 33, 34, 35 and 36.
• Pipes 51, 52, 53, 54, 55, 56 and 57 feed the condensates of aqueous ethanol recovered from the bodies 33, 34, 35, 36 and from the fractionating column 37 and condenser 41 via pipe 57 into the liquor make-up tank 27 by means of the rotary pump 58.
• Pipes 51, 52, 53 and 54 are equipped with cooling devices for the condensates (not shown in the drawing) to reduce the temperature of the corresponding condensates to a temperature level not substantially higher than about 100 C.
• Valves 61, 62 and 63 indicate the water outlets from the fractionating condenser columns 37, 21 and 23 respectively.
• Liquor make-up tank 27 is equipped with known means (not shown in the drawing) for cooling or heating the recovered mixture of the pulping agent, and also with other means (not shown) for adjusting the resulting mixture to the desired level of concentration and temperature for re-use in pulping and introduction into digester by means of rotary pump 28 via pipe 29 and strainer ring 9.
• Table II shows the approximate amounts and composition of the residual liquor leaving the bodies 32, 33, 34, 35 and 36 of the five-stage flash evaporator 66, and those of the ethanol-water vapors formed in these bodies, and also the corresponding vapor pressures, assuming that all the vapors have been condensed.
• the residual liquor flows from body 32 through bodies 33, 34, 35 into body 36 partial evaporation of aqueous ethanol of relatively high ethanol content takes place in each body partly by the differences of the heat contents of the liquor in the various bodies, and partly by the condensation heats of the vapors from each preceding body used as heating medium in the condenser-heater elements resulting in condensation of the vapors.
• the residual liquor is withdrawn into the separation tank 38 where the water-insoluble, molten phase of highly plasticized lignin of relatively high specific gravity separates from the aqueous layer of lower specific gravity and accumulates in the lower part of tank 38.
• the vapors formed in body 36 undergo fractionated condensation in the fractionating condenser column 37 which operates at atmospheric pressure.
• the residual liquor (27.8 kg.) withdrawn from body 36 contains about 10 kg. non-cellulosic solids solubilized from the wood during pulping.
• this liquor separates easily into a non-aqueous, dark-brown, quasi molten phase of highly plasticized lignin (about 4.5- 5 kg. calculated on kg. of the initial spent pulping liquor) of about 1.5 specific gravity and an aqueous phase (22.823.3 kg.) of about 1.1-1.2 specific gravity containing about 5-5 .5 kg. monomer sugars, oligosaccharides and also acetic acid derived from the Wood.
• aqueous fraction withdrawn from the separation tank 38 into the fractionating column 40 residual ethanol and methanol split off from the wood during pulping can be recovered in vessel 41 and withdrawn therefrom through pipe 55 for further use.
• the aqueous fraction stripped of ethanol and methanol in this way is withdrawn from column 40 through valve 59.
• This fraction contains up to about 2% acetic acid calculated on the initial wood which can be recovered by extraction with isopropyl ether or butyl acetate.
• the aqueous solution of the wood sugars and oligosaccharides can be used as a nutrient for microorganisms in fermentation processes or for biological production of proteins. After removal of the water, the solids of the residue can be used as a fodder for stall-feeding cattle, pigs, etc.
• the highly plasticized lignin phase after recovering the small amounts of ethanol and methanol retained, preferably by steam stripping, can be used directly in the production of plastic materials or as raw material in the organic industry for chemical conversion such as hydrogenation, oxidation, sulfonation, etc.
• the first body of the flash evaporator can be equipped with heating elements for partly steam heating the liquor withdrawn from the digester. In this case, an increased amount of heat is available in the first body for liquor evaporation than without this additional heating. This first body can then be of reduced size, and also the number of bodies can be reduced from five to four.
• a continuous method for digesting subdivided fibrous plant material in a digester at an elevated digesting pressure and at an elevated digesting temperature without preimpregnation of pulping agent which consists essentially of (a) feeding said subdivided fibrous plant material to an inlet of said digester and moving said fibrous plant material through the digester to a fibrous plant material outlet remote from said inlet;

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• 1968
  • 1968-07-29 US US748320A patent/US3585104A/en not_active Expired - Lifetime

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