Process for digesting lignocellulosic material

Abstract

 A process for digesting lignocellulosic material. Lignocellulosic material is introduced into a digestive zone along with a liquid composed of an ester with an organic lignin solvent capable of dissolving lignin, and water. The lignocellulosic material is digested at a temperature of 0°C to 230°C for a period of time sufficient to solubilize the lignin in the ester and the solvent. The process of the invention reduces the amount of water in the digestive zone to inhibit the hydrolysis of the cellulose, accelerate the delignification and provide a more economical chemical recovery system.

Description

Background of the Invention

[0001] A number of pulping processes have been utilized in the past to separate cellulose fiber from wood and other lignocellulosic materials. The conven­tional pulping processes use inorganic chemicals, such as sodium, calcium, magnesium, ammonia, or sulfur as sulfides or sulfates, in water. Since lignin is insol­uble in water, the most successful processes in the past have modified the lignin to water soluble alkali lignins or lignosulfonates. The Kraft and sulfite processes have been widely used commercially and these processes are geared to the use of recycling systems for the recovery of the inorganic chemicals. However, there has been a notable lack of success in utilizing the organic residue from these processes. In general, the inorganic chemicals and the organic residues are concentrated as spent liquors. These spent liquors are burned to recover or destroy the inorganic chemicals. It is a major concern and financial burden on the pulp­ing industry to purify the air and water effluents from the concentration and burning operations required to recover the pulping chemicals.

[0002] To eliminate the problems associated with the use of inorganic chemicals in pulping, processes have been investigated utilizing lignin solvents, such as acetic acid, formic acid, acetone, dioxane, poly­glycols, alcohol, glycerol, phenol, and the like. The solvents are utilized to dissolve the lignin from the lignocellulosic material. However, these processes have proven to be unattractive for industrial applica­tion due to the high cost to make up and recover the solvents and the lower quality pulp produced.

[0003] Of the processes used in the past employing lignin solvents, processes utilizing organic acids and alcohols have proven to be of greatest interest, since they produce good quality pulp and these solvents can be produced from cellulose and/or hemicellulose por­tions of the lignocellulosic materials.

[0004] United States Patent No. 3,553,076 discloses a method of pulping using acetic acid. In accordance with the process of this patent, the presence of excess water is shown to be detrimental to producing good quality pulp due to the hydrolyzing of the cellulose and slowing down the rate of delignification. Conse­quently, concentrations of acetic acid of greater than 50% by weight are used. However, the use of high con­centrations of acetic acid makes the recovery system unacceptable from a cost standpoint. Further, wood chips normally contain about 50% by weight of water, so the amount of acetic acid required to achieve the high concentrations is excessive.

[0005] United States Patent No. 4,100,016 discloses a method of pulping utilizing ethanol as the lignin solvent. In this process a 50% by weight aqueous solu­tion of ethanol is utilized as the cooking liquor. Due to the high concentrations of alcohol required the recovery system is expensive, and due to high alcohol losses it is necessary to contend with government res­trictions to assure that the alcohol does not leave the mill.

[0006] Major research efforts have also been direct­ed to the production of synthetic fuels from biomass. The most successful efforts of this type consist of hydrolyzing the cellulosic components to fermentable sugars using sulfuric acid as a catalyst. The major problems with processes of this type are the low yields, destruction of the lignin and the hemicellulose components while hydrolyzing the cellulose, the high recovery costs to separate the products produced, and the excessive energy required.

Summary of the Invention

[0007] The invention is directed to an improved process for digesting lignocellulosic material, and more particularly to a pulping process which reduces the amount of water in the digestion to reduce the hydrolysis of the cellulose, as well as accelerating the delignification and providing a more economical chemical recovery system.

[0008] In accordance with the invention, a ligno­cellulosic material, such as wood chips is introduced into a digester with a cooking liquor composed of an ester, an organic lignin solvent and water.

[0009] The ester is produced from an aliphatic alcohol having 1 to 4 carbon atoms in the molecule and an aliphatic acid having 1 to 4 carbon atoms in the molecule, while the lignin solvent is preferably either an organic acid, containing 1 to 4 carbon atoms in the molecule, or an alcohol containing 1 to 4 carbon atoms in the molecule, or a mixture thereof.

[0010] The lignin solvent is miscible in both the ester and the water and the three components of the liquid composition are characterized by the ability to form a separate organic phase and aqueous phase under certain concentrations limits.

[0011] The cellulosic material is maintained in the digester at a temperature of 0°C to 230°C for a suffic­ient time to solubilize the lignin in the ester and solvent. The liquid composition is then discharged from the digester and flashed to atmospheric pressure causing cooling. Cooling of the liquid will reduce the solubility of the lignin in the organic components with the result that a portion of the solubilized lignin will precipitate and be suspended in the water phase. The liquid is then transferred to a separation vessel where the organic phase, containing dissolved lignin, and the aqueous phase containing suspended particles of liquid, are separated.

[0012] The organic phase containing dissolved lignin is returned to the digester for a succeeding digestive cycle, while the aqueous phase is further treated to remove residual organic materials and separate the lignin from the water.

[0013] Due to the use of the high proportion of organic components, the delignification operation is accelerated over conventional processes utilizing inorganic chemicals. The major portion of the organic materials i.e. the ester and acid solvent, are separ­ated in the organic phase from the water and are re­cycled back to the digester. This greatly simplifies the recovery system for the organic materials.

[0014] As the amount of water required in the diges­ter is reduced over conventional processes, hydrolysis of the cellulose is minimized and higher quality pulps are achieved.

[0015] The process of the invention also employs lower cooking temperatures for given pressures in the digestive zone and the time required to produce pulp having commercially acceptable properties is reduced.

[0016] As a further advantage, the process of the invention eliminates the use of sulfur compounds which cause series environmental pollution problems.

[0017] The bleaching process can be substantially simplified by introducing bleaching chemicals into the washing cycle for the pulp.

[0018] Other objects and advantages will appear in the course of the following description.

Description of the Drawings

[0019] The drawings illustrate the best mode presently contemplated of carrying out the invention.

[0020] In the drawings:

Fig. 1 is a phase diagram showing the relative solubilities of ethyl acetate, acetic acid and water in a ternary system;

Fig. 2 is a flow sheet illustrating a batch pulping system incorporating the invention; and

Fig. 3 is a flow sheet showing a continuous pulping system incorporating the invention.

Description of the Illustrated Embodiment

[0021] In the process of the invention, lignocellu­losic material, such as wood chips, is contacted in a digestive zone with a cooking liquor. Wood is composed essentially of cellulose, hemicellulose and lignin with small quantities of extractives and minerals, the rela­tive quantities of which vary depending on the wood species. An ideal pulping operation would be one that would modify the wood structure and/or separate the structure into its component parts and enable those parts to be reassembled as products that maximize the benefits derived from the individual physical and chem­ical properties of the component parts.

[0022] The digesting liquor is composed of an ester, an organic solvent for lignin and water. The ester is formed from an aliphatic acid and alcohol each contain­ing one to four carbon atoms in the molecule. Specific examples of esters that can be used are methyl acetate, ethyl acetate, propyl acetate and butyl acetate made from reacting the appropriate alcohol with acetic acid. Similar products can be made from formic, propionic and butyric acids. Lignin is soluble in the ester but to a lesser degree than in the organic solvent.

[0023] The lignin solvent can be an organic ali­phatic acid containing 1 to 4 carbon atoms, such as formic, acetic, propionic, or butyric acid, or an ali­ phatic alcohol containing 1 to 4 carbon atoms, such as methanol, ethanol, propanol and butanol, or mixtures of the acid and alcohol.

[0024] The digesting liquor has the following com­position in weight percent:
Ester 1% - 98%
Acid or Alcohol 1% - 90%
Water 1% - 80%

[0025] A preferred compositional range for the liquor utilizing an acid as the solvent is as follows:
Ester 5 - 95%
Acid 5 - 80%
Water 10 - 60%

[0026] The preferred compositional range for the liquor containing an alcohol as the solvent is as follows:
Ester 5% -95%
Alcohol 10% - 45%
Water 10% - 60%

[0027] Wood chips contain approximately 50% by weight of water and the specific composition of the cooking liquor falling within the above ranges is determined by the desired spent liquor composition which takes into account the liquid to wood ratio and the water content of the wood. In general, the liquor is used in a weight ratio of about 1:1 to 20:1 with respect to the wood chips.

[0028] The pulping systems of Figs. 2 and 3 will be described as using ethyl acetate as the ester, acetic acid as the lignin solvent, and water, in the digesting liquor, but it is contemplated that other esters and acids and/or alcohols can be utilized.

[0029] As shown in the phase diagram of Fig. 1, acetic acid is soluble in both ethyl acetate and water, while ethyl acetate has only limited solubility in water. For concentrations of acetic acid under about 22% by weight, the liquid composition will separate into two phases, an organic phase and a second aqueous phase. During digesting, it is preferred to maintain the concentration of acetic acid under 40% by weight, so that when the liquor is discharged from the digester it can be made to separate into two phases as will hereinafter be described.

[0030] Fig. 2 is a flow sheet showing a batch pulp­ing system incorporating the invention. The wood chips or other biomass is fed to a digester 1 through line 2, while the digesting liquor contained in tank 3 is pump­ed through lines 4 and 5 to the lower end of digester 1.

[0031] The digesting process is carried out at a temperature of 0°C to 230°C and preferably at a temper­ature of about 100°C to 200°C. If an elevated tempera­ture is utilized, the liquor being introduced to diges­ter 1 can be heated by steam in a heat exchanger 7.

[0032] The digestion is carried out for a time suf­ficient to substantially delignify the lignocellulosic material, usually ranging from about 1 minute to 16 hours. The temperature and time are dependent upon the quality desired, the yield of pulp and the quantity and quality of the by-products to be produced.

[0033] During the digesting, gaseous products dis­charged from the upper end of digester 1 through line 8, are passed through a heat exchanger 9 to cool the gases and are then delivered to a chemical recovery unit 10.

[0034] In chemical recovery unit 10, the gaseous products of the digestion, such as methanol, methyl acetate, furfural, and the like, are separated from the ethyl acetate, acetic acid and water vapors and dis­charged through line 11. The recovered ester, acid and water are recyled from chemical recovery unit 10 through line 12 and returned to digester 1.

[0035] During the digestion process, a portion of the liquor is discharged from the digester through line 13 and recyled back to line 4 and returned to the digester through heat exchanger 7 to maintain the digestion temperature.

[0036] After the digestion is completed, the liquor is discharged from the lower end of digester 1 through line 14 and delivered to a flash tank 15 which is at approximately atmospheric pressure. As the digester operates at a higher pressure, the flashing to atmos­pheric pressure will reduce the temperature of the liquor. As an example, if the digester is operating at a temperature of 180°C and a pressure of150 psi, flashing to atmospheric pressure will reduce the tem­perature to about 100°C. The vapors resulting from the flashing are conducted through line 16 to the chemical recovery unit 10, while the liquid is discharged through line 17, and passes through screens 18 to re­move fibrous material. The liquid then flows through line 19 to a primary phase separation tank 20.

[0037] In the separation tank 20, the liquor will be separated into two phases, an organic phase consisting primarily of the ester and acid and an aqueous phase consisting primarily of water. As previously noted, if the concentration of the acetic acid in the digester is below 22%, the liquid will separate into the two phases. If the concentration of acetic acid in the digester 1 is maintained above 22% so that a two-phase separation will not occur, additional quantities of water and/or ethyl acetate can be added to the liquid after flashing to reduce the acetic acid concentration to a value below 22% so that the phase separation occurs in tank 20. If additional water is supplied, it can come from the bottom of the raffinate solvent re­covery column 27 through line 33 or evaporator conden­sate through line 50.

[0038] As the lignin is less soluble in the organic components at a lower temperature after flashing than at the higher digesting temperature, a portion of the lignin will separate or precipitate from the organic phase and be suspended in the aqueous phase in tank 20. The organic phase, consisting primarily of ethyl acetate and acetic acid with dissolved lignin, is then returned through line 21 to line 4 and recycled to digester 1. The aqueous phase, consisting primarily of water containing suspended particles of lignin and also containing a limited amount of the ester and acid with lignin dissolved in the organic components, is then discharged from separator 20 through line 22 and is cooled to 20°C at cooler 22A and fed to centrifuge 34 where the suspended lignin is removed. The aqueous liquid is then introduced into the upper end of an extraction column 23 through line 35 or recycles back to primary phase separator 20. The extraction column, which is operating at a temperature of about 20°C, acts to separate the water from the organic components. By a conventional liquid extraction process, the organic components, consisting of the ester and acid, are dis­charged from the upper end of column 23 and introduced into the central portion of an extract solvent recovery column 25 through line 24, while water is discharged from the lower end of column 23 through line 26 and is fed to a raffinate solvent recovery column 27.

[0039] In column 25, the ester and acid are separat­ed by distillation and the acid containing dissolved lignin is discharged from the lower end of column 25 through line 28 and is returned to the liquor storage system 3. The ester is discharged from the upper end of column 25 through line 29 and is conducted through line 30 back to liquor storage system 3. In addition, a portion of the ester can be recycled from line 30 through line 31 to the lower end of extraction column 23 to aid in separating the organic components from water.

[0040] In column 27, the residual ester is separated from the aqueous phase and the ester is discharged from column 27 through line 32 which is connected to line 30, while the water containing dissolved sugars is discharged from the lower end of column 27 through line 33 to an evaporator 36 where a portion of the water is evaporated and the residue is delivered to a burning unit 37.

[0041] The lignin recovered from the centrifuge 34 is conducted through line 38 to the burner unit 37. By burning the lignin and the residue from the evapora­tion, steam and electrical power can be generated for industrial use.

[0042] After the digestion has been completed, the pulp in digester 1 is washed with the digester liquor. During the washing cycle, the liquor from storage system 3 is fed through lines 4 and 6 into the central portion of the digester and the liquor is drained from digester 1 through line 39 to a holding tank 40. The liquor in tank 40 is then returned to line 4 through line 41. The washing aids in removing residual lignin and sugars from the pulp.

[0043] Following the wash with the digester liquor, the pulp in digester 1 is subjected to a hot water wash. In this regard, hot water in line 42 is fed to pulp recovery system 43 where it is used on pulp wash­ers, then it is discharged from recovery system 43 through line 44 to line 4. The hot water, after enter­ing the digester through line 6, is discharged through line 14 to flash tank 15. From flash tank 15 the wash­ing water flows through line 17 and through screens 18 where it is transferred through lines 45 and 46 to secondary phase separator tank 47. In separator tank 47 the residual organic materials, i.e. the ester and acid will separate as an organic phase from the water. The organic phase is delivered from the upper end of tank 47 through line 48 to line 19 where it passes through the primary phase separator 20, while the aqueous phase is discharged through line 49 and is returned to line 4 for recycling of the wash water. Condensate from the evaporation process can be deliver­ed through line 50 to line 49 to add to the wash water.

[0044] After the washing has been completed, the pulp is flushed from digester 1 through line 51 to the pulp recovery system 43 where it is discharged through line 52 to other processing operations.

[0045] As the pulping operation is maintained at slightly acidic conditions, the pulp can be bleached with hydrogen peroxide or other bleaching chemicals during the digesting operation. More specifically, the bleaching chemicals can be introduced into the hot water during the water washing cycle so that the pulp can be bleached in the digester to thereby eliminate the expensive bleaching equipment which is normally required in conventional pulping processes.

[0046] Fig. 3 illustrates the invention as utilized in a continuous pulping process. Wood chips or other biomass are introduced through line 53 to a feed system 54 and the wood chips are continuously fed from feed system 54 through line 55 to the upper end of digester 56. Digester 56 will normally operate at a temperature in the range of 100°C to 200°C.

[0047] The digesting liquor, as described in connec­tion with the batch process of Fig. 2, can be a com­ bination of ethyl acetate, acetic acid and water. The liquor is contained in liquor storage system 57 and is continuously fed through line 58 to the central portion of digester 56. During the digesting a portion of the liquor is continuously recirculated through external line 59 and can be heated by passing through heat exchanger 60 in heat transfer relation to steam or other heating medium. During the digesting, the diges­ting liquor is continuously withdrawn from the digester through line 61 and flows to a flash tank 62 where the liquor is flashed to approximately atmospheric pres­sure. The reduction of pressure will volatilize the volatile components and cause a substantial reduction in the temperature of the liquor. The volatilized components resulting from the flashing are discharged from tank 62 through line 63 to the volatile chemical recovery system 64. In recovery system 64 the highly volatile components such as methanol, methyl acetate, furfural and the like, are separated from the vaporized liquor and are discharged through line 65. The resid­ual liquified liquor is then discharged from system 64 through line 66 and is returned to the wood chip feed system 54. The liquor being returned through line 66 can be preheated by passing through a heat exchanger 67 in heat exchange relation with the volatilized products flowing through line 63.

[0048] The spent liquor is discharged from flash tank 62 through line 68 and passes through screens 69 to remove the fibrous material and then flows through line 70 to the phase separator tank 71 which operates in a manner similar to separator tank 20 of the first embodiment. The components of the digesting liquor are adjusted in concentration either in the digester or in tank 62, screens 69 or tank 71 so that a phase separa­tion occurs in tank 71. As previously described, liquor separates into an organic phase, which contains primarily the ester and acid with dissolved lignin, and an aqueous phase which contains precipitated lignin along with a minor amount of the organic components which, in turn, contain dissolved lignin. The organic phase is discharged from the separator tank 71 through line 72 and is continuously fed to the digester 56 through line 58, while the aqueous phase is discharged from the phase separator 71 through line 73 and cooler 100 to centrifuge 80 where the precipitated lignin is removed from the water. The water is discharged from centrifuge through line 81 and is pumped to the extrac­tion column 74, while the lignin discharged from cent­rifuge 83 is fed to a burning unit 84, similar to burn­ing unit 37 of the previous embodiment.

[0049] Extraction column 74 operates in a manner similar to column 23 and serves to separate the aqueous phase from residual organic phase. The residual organic phase, consisting of the ester and acid with dissolved lignin, is discharged from the upper end of extraction column 74 through line 75 and a portion of the organic components are fed to the central portion of extract solvent recovery column 77, while the remaining portion of the organic components pass through line 78 and is returned to the liquid storage system 57.

[0050] In column 77, the acetate and acetic acid are separated by distillation and the acetic acid is dis­charged from the lower end of column 77 through line 86 and is returned through line 78 to the liquor storage system 57, while the ester is discharged from the upper end of column 77 through line 87 and is recycled to extraction column 74 through line 88 or is returned to storage system 57 through lines 88 and 78.

[0051] In column 82, residual ester is separated from water and the ester is discharged from the upper end of column 82 through line 90 which is connected to line 88, while the water containing dissolved sugars is discharged from the lower end of column 82 through line 91. A portion of the water is recycled back to centri­fuge 80 while the remaining portion is fed to evapora­tor 92 for evaporation. The condensate from the evaporation process can be returned through line 93 to a pulp recovery system 94 and used for pulp washing, or to wash fibers on screens 69 through line 89.

[0052] After the digestion is completed and the spent liquor is removed from the digester 56, the pulp can be washed with hot water to remove residual spent liquor and lignin from the pulp. In this regard, the water is introduced into recovery system 94 through line 96 and the water flows through lines 95 and 97 to the lower end of the digester 56 and is circulated from the digester through line 61 to the flash tank 62 and phase separator 71, as previously described in the first embodiment.

[0053] The pulp is then flushed from digester 56 through line 98 to pulp recovery system 94 and then can be conveyed through line 98 for further processing in a conventional manner. The refined, bleached pulp pro­duct leaves in line 99.

[0054] The following examples illustrate the process of the invention:

EXAMPLE I

[0055] Ethyl acetate, acetic acid and water in the proportions of 33.3%/33.3%/33.3%, respectively by weight, were added to wood chips containing about 83% by weight O.D. wood. The liquid to wood ratio ws varied from 4:1 to 10:1 as shown in Table I. The system was heated to 170°C for 2 hours. After heating, the ethyl acetate/acetic acid, water solution contain­ing the dissolved lignin, other extractives and dis­solved sugars and polysaccharides were drained from the wood fiber and the wood fiber washed first with fresh liquor in the same proportions as the cooking liquor and then with acetone. The yield of pulp, Kappa number and pulping conditions are shown in Table I.

EXAMPLE II

[0056] Additional experiments were carried out as described in Example I but at different cooking times as shown in Table II. A high yield pulp was obtained with a short cooking time and a post-refining stage as shown in the following table.

EXAMPLE III

[0057] Additional tests were carried out as described in Example I but with different ratios of the three primary components of the cooking liquor, namely, ethyl acetate/acetic acid/water, and at different times and temperatures as shown in Table III.

EXAMPLE IV

[0058] Additional tests were carried out as described in Example I but using spruce wood, as shown in Table IV.

[0059] In the pulping system of the invention, the two-phase liquid - liquid separation, in or outside of the digestive zone is controlled by the concentration of the individual components, i.e. the ester, lignin solvent and water. As the system substantially reduces the amount of water required in the digester, hydroly­sis of the cellulose is inhibited and delignification is accelerated. The invention also provides a more economical chemical recovery system.

[0060] The invention reduces the time required to produce pulp having acceptable chemical and physical properties and permits the use of lower cooking temper­atures for given pressures in the digestive zone.

[0061] The system also results in the production of higher yield pulps and higher strength pulps.

[0062] As a further advantage, the pulping system eliminates the use of sulfur compounds which cause serious environmental pollution problems.

[0063] The invention also facilitates bleaching of the pulp, for the bleaching chemicals can be added to the water wash so that the bleaching can be carried out without the need for the expensive and complicated auxili­ary bleaching equipment.

[0064] While the above description has illustrated the process of the invention used for pulp production, it is contemplated that the process can also be employ­ed to produce low molecular weight compound such as sugars and other chemicals for conversion to synthetic fuels from biomass.

Claims

1. A process for modifying the structure of a lignocellulosic material, which comprises digesting the lignocellulosic material, at 0 to 230 C, in the presence of a single-phase digesting liquor comprising an organic ester, an organic solvent capable of solubilising lignin, and water.

2. A process according to claim 1, wherein the organic ester is derived from a C_1-4 alkanoic acid and a C_1-4 alkanol, and the solvent is a C_1-4 alkanoic acid, a C_1-4 alkanol or a mixture thereof.

3. A process according to claim 1, wherein the ester is ethyl acetate and the solvent is acetic acid.

4. A process according to any preceding claim, wherein the liquor comprises from 1 to 98% by weight of the ester and from 1 to 90% by weight of the solvent, the balance being water.

5. A process according to any preceding claim, wherein the lignocellulosic material comprises wood chips.

6. A process according to any preceding claim, wherein from 1 to 20 parts by weight of the liquor are used per part by weight of the lignocellulosic material.

7. A process according to any preceding claim, which comprises the additional step of washing residual digested cellulosic material.

8. A process according to claim 7, wherein the washing liquid includes a bleaching chemical.

9. A process according to any preceding claim, wherein the relative concentrations of the components are maintained or adjusted such that, after digestion, the liquor separates into substantially aqueous and substantially non-aqueous phases.

10. A process according to claim 9, which comprises the additional steps of precipitating a portion of solubilised lignin from the non-aqueous phase, suspending the precipitated lignin in the aqueous phase, and separating the suspended lignin from the aqueous phase.

Drawing