CA1131415A - Pulping of lignocellulose with aqueous methanol/ catalyst mixture - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Lignocellulosic plant material is chemically converted to separated fiber of superior pulp properties having almost native cellulose strength with low residual lignin, by cooking fragments of lignocellulose in an aqueous methanol solvent mixture containing a recoverable metal salt catalyst, or a combination of a metal salt and a minor quantity of strong acid such as HC1, in a pressure vessel at temperatures between 185°C and 205°C, and preferably between 190 C and 200°C, for durations from 15 minutes to over one hour.

A wood/liquor proportion between 1:4 and 1:20 or higher is maintained,with a preferred ratio 1:5 to 1:10, and the water/
methanol ratio is between 1:1 to 1:4, the preferred ratio being 3:7. The metal salt catalyst is a chloride or nitrate of one of the metals calcium, magnesium or barium, and is useful in concentr-ations between 0.005 molar to 1.0 molar. For cooking hardwood the preferred range is 0.05 to 0.1 molar, while for softwood the range preferred is 0.2 molar to 0.05 molar. Addition of HCl when cooking hard-to-delignify wood such as Spruce, in amount 0.0005 normal to 0.008 normal enhances rate and extent of delignification without significant damage to cellulose.

High pulp yields of the order of 54% to 56% of wood weight for Spruce with TAPPI (0.5) viscosity of 20 cps at Kappa number 35 are obtained, within 45 minutes, while Aspen can be cooked to a pulp yield of 62% with TAPPI (0.5) viscosity 22 cps at Kappa number 25 in less than 30 minutes.

Lignin is recovered by precipitation through liquor cooling and volatilizing methanol therefrom, as an undegraded low molecular weight powder soluble in usual lignin solvents; the part of the hemicelluloses converted to dissolved monomers and oligosaccharides is recoverable from the residue, and the catalyst may be obtained for re-use.

Description

This invention is in a new process for 'creating ligno-cellulose with a solvent mixture comprised of water, methyl alcohol, and a dissolved metal salt catalyst in a pressure vessel at a temperature in the range 180C to 2~C to produce a chemical pulp of fibrous material.

The process is effective to depolymerize and dissolve the greater part of the lignin and at least part of the hemicellulose materials of cell walls, and is particularly successful in producing pulps having high fiber strength in which cellulose retains nearly its natural, undegraded state. Such pulps are suitable for making high quality paper, and for dissolving purposes.

Heretofore, in processes wherein wood is subjected to hydrolysis by an acid catalyst in a solvent mixture made up of water and an alcohol having two or more carbon atoms in a straight chain at a temperature between 165C and 210C, the cellulose is attacked rapidly, so that before the lignin and hemicellulose constituents of cell walls are dissolved, extensive cellulose degr-adation has occurred. Even when a buffered strong acid or relat-ively weak dicarboxylic organic acid is used as catalyst to promote hydrolytic solvolysis, the fibers when liberated have a degree of polymerization reduced significantly below their natural condition, so that paper sheet made from such pulps lacks the high tear strength, burst strength, and breaking length properties desirable for industrial uses.

While known processes have advantages of requiring very short cooking time and yield a soluble lignin and dissolved sugars having considerable value, it has remained a desirable objective that liberation of cellulose fiber should preserve almost the native strength of cellulose fibrils when the lignin content of the pulp has attained acceptably low values. Another desirable objective is that the process should recover virtually all of the catalyst substance employed without losses. A further d~esirable 3~3~

objective is to avoid damage to polyglucan through attack by acids and by water-miscible volatile organic solvents such as ethanol and acetone, which have been discovered to rapidl~ degrade crystalline cellulose at temperatures above 150C particularly at the disordered regions.
The present invention overcomes the major deficiencies of known chemical pulping processes employing acid-catalysed aqueous solvent mixtures, and essentially consists in the method for converting lignocellulose such as softwood, hardwood, or agricult-ural residue to the form of separated fibers by cooking the ligno-cellulose with a solvent mixture at least four times the weight of material to be pulped containing water and methanol in the propor~
tion of 1 to 1 to 1 to 4, and containing from 0.005 to 1.0 moles of a metal salt or a mixture of metal salts selected from the chlorides and the nitrates of anv of the metals magnesium, calcium and barium, at 180C to 210C for a time generally between 30 minutes and about 90 minutes.
The method of the invention is particularly effective when the salt is magnesium or calcium chloride or nitrate at a molar concentration between 0.02 molar to 0.05 molar and the ratio, by weight, of water to methanol of the solvent mixture is about 3:7, and the cooking is carried out at a temperature between 190 and 200C
When Spruce is cooked by the novel process a pulp is obtained which retains appreciable amounts of hemicelluloses, yet has low residual lignin content and a degree of polymerization (DP) comparable to that of a pulp obtained bv the kraft process; the cooking time need be only from 30 to 45 minutes to yield a pulp with Kappa number 63, ~3~5 TAPPL viscosity (0.5) of 28.2 centipoises, or a degree of polymerization of 1550. Such pulp is far more easily bleached from its as-cooked brightness of 45 to ~0~ to the desired 80~ or higher brightness than is a kraft spruce pulp of comparable residual lignin content with a much lower starting brightness typically 35%.
According to the present invention, therefore, there is provided a method for chemically converting lignocellulose to the form of separated fibers which comprises cooking lignocellulose with an aqueous solvent mixture of at least four times the weight of the lignocellulose r the mixture comprising from ]. to 4 volumes of-methanol per volume of water and a metal salt catalyst dissolved therein in a concentration between 0.005 molar to 1 molar, the salt being selected from the group consisting of the chlorides and nitrates of magnesium, calcium and barium, the cooking temperature bein~ between about 180C
and 210C and the cooking time being not longer than two hours and sufficient to effect at least partial depolymeriza-tion and dissolution of the lignin, the hemicellulose and the other cell wall materials encasing the cellulose, and recovering the separated fibers from the liquor residue.

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The invention will be more particularly described in and by the following stlt~men-ls of preferred modes of carrying it into effect, together with tables and examples of the method.
Wood-To-Liquor Ratio The method preferably employs an amount of solvent mixture which cannot be less than the void voiume of wood fra~ments and must be sDfficient to provide solvent action for li~nin and carbohydrates during hydrolysis, and hence should be at least four times the wood weight or a greater weight where effective convective or forced circulation of liquor is essential. It has been found that the cooking action is fully effective if the solvent mixture is in the range between about 5 times the weight of wood and about 30 times the weight of the wood, and that cooking with even greater proportions, for example 50 to 100 times, delays the separation of fibers to the extent that such proportions are economically undesirable. Efficient cooking ;s realised with wood: liquor ratios from about 1:5 to about 1:10 as may be seen from Table I following.
T A B L E
PULP PROPERTIES, SPRUOE WOOD COOKED WITH AQUEOUS ~rH~NOL
__ AND CaC12 USING DIFFERENT WOOD/LIQUOl~ PROPORTIONS
CaC12 H20~ethanol Wood/ Temp. Time Yield Kappa TAPPI (0 5) Molarity Liquor C min Wt. % No. Visc. cp . __ . _ . _ _ ._ __ _._ - _ A . . . __ 0.053 : 7 1 : 10 205 60 45.4 19 9 0.053 : 7 1 : 100 205 60 54.0 65 22 0.103 : 7 1 : 10 200 30 59.0 61 19 0.103 : 7 1 : 100 200 30 No fiber separation .

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s Ratio of Water to Methanol Even though methanol is known to be a poor solvent for natural lignin or lignin polymer groups resulting from cleaving of natural lignin by hydrolytic action, when a solvent mixture is formulated with a water/methanol ratio between 1 : 1 and about 1 : 4 and the wood/liquor ratio is about 1 : 10, at the elevated temperatures of the process, near 200C, satisfactory dissolution of depolymerized lignin as well as of sugars formed by hydrolysis of hemicellulose is obtained.

The effectiveness of the process in providing high yields of separated fiber from Spruce which is difficult to delignify by aqueous organic solvent mixtures is enhanced by employing the higher proportions of methanol, with correspondingly higher TAPPI (0.5) viscosities but with correspondingly higher retained lignin, as shown by Table II.

T A B L E I I
PULP PROPERTIES: SPRUCE WOOD COOKED 30 MIN. METHANOL/WATER
CONTAINING 0.16 Molar CONCENTRATION OF CaC12 COOKING LIQUOR COMPOSITION PULP YIELD KAPPA No. TAPPI (0.5) Water vol. Methanol vol. Wt. Percent lignin Visc. cp 1 1.5 51 44 14.0 1 2.33 53 55 20.5 1 4.0 58 63 18.0 .
From the foregoing it will be seen that at the constant salt concentration of 0.16 molar calcium chloride, a proportion of 30 volumes water to 70 volumes methanol yields a pulp containing the highest proportion of high polymer weight cellulose with an excellent yield of separated fiber and at an acceptably low lignin content.
The pulping effectiveness of the novel process and the superiority of the pulp obtained may be contrasted with prior observations on use of aqueo~s~methanol mixtures; see "The Cooking Process", S. I. Aronovsky and R. A. Gortner, Industrial and Engineering Chemistry 28,(1936) pp. 1270--76.

Metal Salt Selection and Concentration The process exhibits a high tolerance to large variation in the molar concentration of the metal salt used, at constant cooking conditions. A hardwood such as Aspen may be cooked to a fully separated fiber state using a concentration of calcium chloride as low as 0.005 molar, in aqueous methanol at 1:10 wood/
liquor ratio, in a time well below one hour, while at a concentr-ation as high as 1. molar a fully separated fiber state is reached in about 15 minutes. Hardwoods generally may be cooked with lower salt concentrations of any of the preferred catalysts than softwoods require; for example, Aspen cooks with calcium or magnesium chloride or nitrate less than 30 minutes yield fully cooked fragments at salt concentrations 0.05 molar to 0.10 molar.
Softwoods such as Spruce will generally require a higher salt concentration between 0.05 molar to 0.20 molar, and in certain instances improved fiber separa~ion may be gained by a concentration approaching 0.5 molar or higher~ and by addition of HCl.

The chlorides and nitrates of calcium, magnesium and barium have been found to be effective when used at the concentra-tions mentioned, for a wide range of woody plant materials. The preferred salts are the chlorides of magnesium and calcium, whichhave the advantages of lowest cost, and being tolerated well in fermentation processes to which the liquor residue may be fed to convert oligosaccharides dissolved in the liquor following recovery of the pulp and removal of lignin and methanol. Nevertheless the nitrates are effective catalysts, although their salts represent a higher cost. Table III shows representative data on pulp properties for one hardwood, Aspen, and one softwood, Spruce, when cooked with aqueous methanol at 200C for various metal salts, with a constant wood/liquor ratio of 1 : 10.

1~3L41 5 TABLE III

PULP PROPERTIES OF WOOD SPECIES COOKED WITH METHANO~-H2O AT 200C
USING SELECTED METAL SALTS AND CONCENTR~TIONS.

CATALYST COOKING PULP LIGNIN TAPPI
WOOD SALT MOLFS TIME, YIELD,KAPPA (0.5) VISC. D P
min % NUMBER cp ~lgC12 0.01 30 62 27 20 1320 Q CaC12 0.01 30 63 30 21 1360 o CaC12 0.10 25 61 16 18.7 1280 z;
~ MgC12 0.10 25 59 15 19 1300 BaC12 0.01 30 69 46 Poor fibre separation MgC12 0.05 30 59 51 17 1200 MgC12 0.10 30 54 29 18 1270 a g CaC12 0.05 30 66 60 20 1320 -CaCl 0.10 30 54 35 19 1300

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Mg(N~3)~0.10 45 57 55 23 1410 P~ -~ Ca(NO3~0.10 45 58 62 29 1570 All wood:liquor ratios 1:10.

In the foregoing the degree of polymerization (DP) values are derived from TAPPI Standard viscosity measurements and use of nomogram published by Rydholm, p. 1120. Excellent fibre separations were obtained by use of both chlorides and nitc~t~s of magnesium and calcium with low residual lignin and reten~io~ o~ significant amounts of hemicelluloses.
EFFECTS OF DURATION OF COOKING

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Da~/as ~re presented for a set of pulps -cooked t~ith liquor ~of water:me~an-olxatlo~ 3:7 and wood liquor ratio of 1:10 using 0.01 molar CaC12 as catalyst. Both spruce and aspen woods were cooked for extended durations. The residual DP o(f the pulps is observed to decrease with lengthened digestion times, however excellent fibre separations were observed and relatively good ~3~

TAPPI (o.5) viscosi-ty values and DP values are retained at low residual lignin Kappa number values, Eollowing 35 minutes digestion of the softwood and after about 20 minutes digestion of the hardwood, as shown in Table lV.
T A B L E I V

PULP PROPERTIES FOR WOOD COOKS IN H20-MET~ANOL 3:7, 200 C

WOOD/LIQUOR RATIO 1:10, SALT 0.10 Molar CaC12 Duration of Pulp yield, KAPPA TAPPI (0.5) p SpeCles cook, Min. Weight % Number visc., cp D

72 103 (no fiber separation) 62 63 28.2 1550 SPRUCE 35 56 46 18.2 1275 52 42 15.3 1160 99 (no fiber separation) 73 61 25.0 1450 ASPEN 20 63 22 20.5 1340 61 16 18.7 1280 Effect on Fiber Separation of Temperature, and Cooking Range Softwoods when cooked by the process of the invention exhibit a doùbling of the reaction rate, as expressed by the duration of cooking required to attain a specified residual lignin content, generally below KAPPA number 45, for each 10 C
rise in temperature within the effective range of temperature bounded by an upper scorching temperature limit of about 210C
and a lower delignification threshold at about 180C, at which the digestion time interval does not exceed about 90 minutes.

As may be understood by reference to Table I and the following Table V, cellulose damage increases for a hardwood such as Aspen as temperature increases above 18G, which is a lower temperature limit at which delignification and fiber separ-ation is assured within 40 minutes. An upper digestion tempera-ture of 210set by onset of scorching need never be reached, as the degradation of cellulose for practical cooking durations ~IL3~

of about 25 minutes above 190C will dictate usual].y not exceeding the latter temperature.
Table V below reports analyses of Spruce and Aspen pulps cooked with 0.1 molar CaC12 at different temperatures and for varying durations, in aqueous methanol of 3:7 volume ratio with wood/liquor ratio 1 : 10.

T A B L E V

PULP PROPERTIES FOR WOOD COOKS IN H20-METHANOL 3:7 WOOD/LIQUOR RATIO 1 : 10, SALT 0.10 Molar CaCl Species Duration of OTemp. Pulp Yield, KAPPA TAPPI (0,5) Cook, Min. C Weight % Number Visc. cp DP

ASPEN 40 190 57 9.4 21 1360 200 53 6.0 12.5 1050 SPRUCE 60 190 59 53 26.4 1470 190 56 40 23.2 1410 200 59 6~ 19.0 1300 Because each lignocellulosic material represents a different composition and character of its lignin, hemicellulose, and other cell wall constituents, the practice of the invention will necessarily require some experimentation with a given material to obtain the optimum pu.lp properties. Some guida~ce may be obtained by reference to Table VI which records pulp analyses of cooking conditions for seven different species which yielded high quality pulps typical of good results, althou~h it is to be understood that these conditions are illustrative of good practice rather than optimal conditions. The Table includes handsheet strength properties of interest after the pulps were beated to 300 Csf, sheet testing being carried out according to prescribed TAPPI standards. The pulps were treated only by a hot aqueous methanol wash to remove precipitated lignin, and further washed and suspended .i.n water hefore beating and handsheet formation.

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Pulping With ~queous Methanol and ~etal Salt, ~cidified b~ HC1.

It is known that for rapid bulk delignification in acid-catalysed aqueous organic solvent mixtures large initial proton concentrations are required. ~ further improvement of the present invention for rapid wood delignification and pulping is obtained by mild acidification of the solvent mixture containing the metal salt catalyst earlier described. The addition of hydrochloric acid in concentrations of 0.0005 to 0.006 Normal to methanol-wa-ter solutions containing 0.05 M CaC12 was found to improve the rate of delignification significantly without significantly affecting the cellulose, by the time that fiber-separation of the wood was evident.

The acid enhances the initial bulk delignification and hydrolysis of hemicelluloses by providing high initial proton concentrations at a time when the cellulose is still well protected from the hydrogen ions by the encrusted lignin since in its native structure cellulose is less aceessible to protons when swelling by solvents is restricted by the lignin-hemicellulose matrix.

The functions and effects of the metal salt catalysts are essentially unchanged from their roles as described hereinbefore, the metal salt catalyst serving both as a proton-generating agent as well as providing protection to the cellulose especially at the later stages of the cooking against degradation by hydrolytic solvolysis.

A study of the effects of metal salt catalyst alone, and when combined with very minor concentrations of hydrochloric acid, was made by a series of six cooking operations using a laboratory scale pressure vessel measuring about 20 cm by 8 cm diameter, of stainless steel. Air-dry wood chips containing about 8% moisture were made up into 10.0 gram batches, the wood being Spruce, not 30 previously subjected to any treatment nor elevated temperatures~
The cooking liquor was prepared using 3 volumes of distilled water and 7 volumes of methanol, 100 grams of the solution being loaded ~- - L 0 into the vessel at room temperature, the initial batch ~nd all subsequent batches having added thereto an amount of calcium chloride (CaC12) as a solution in the water component used as cooking liquor, to provide a concentration in the li~uor 0.05 Molar.
The first batch was cooked by heating the sealed vessel in a pre-heated oil bath held at 200C. It was noted that at the end of 7 minutes, the vessel temperature had reached 200C. The cooking time, counted from immersion time in the bath, was extended to 40 minutes. The vessel was then chilled by removing from the bath and running cold water over it, for one minute, when the vessel was opened. The liquor was decanted, and the pulp removed. The removed pulp was washed with a hot aqueous methanol containing no additive, and drained. The washed pulp was mechanically disintegra-ted to make a dispersion in water about 4% by weight.
The cooled residue with precipitated lignin was dumped into an equal volume of cold water, whereupon about 70% of the dissolved lignin removed fro~ the wood was available as a precipitate. The liquor was filtered and evaporated to further precipitate lignin and yield a syrup containing the calcium chloride salt. The washed pulp was analyzed by atomic absorption techniques, and residual cation was observed to be insignificant as compared to the cation originally present in the woodO

Five further batches were cooked each including concentra-tions of HCl in addition to CaC12, for varying cooking times. Data for the solvent mixtures and pulp analysis are shown in Table VII, showing high DP values and TAPPI viscosities at low Kappa numbers.

T A B L E V I I
Conc. Conc. Solution Cooking Pulp Kappa TAPPI ( ' 5)DP
HClCaCl2 pH Time Yield Number Visc. cp NM before after mln Wt. % _ _ -- 0.05 5.8 3.6 40 54 44 20 1320 0.001 0.05 2.8 4.1 40 56 58 30 1600 0.002 0.05 2.6 4.0 40 54 48 21 1360 0.002 0.05 2.6 3.9 35 58 57 24 1440 0.004 0.05 2.5 3.7 40 53 37 23 1420 0.004 0.05 2.5 3.9 ~5 52 30 19 1300 Material: SPRUCE WOOD chips 11 -THE COOKING PROCES_ The process must be carried out in a pressure vessel capable of withstanding internal pressure of the order of 30 atmospheres corresponding to the vapor pressure of the lowest-boiling component, methanol, at a maximum tempera-ture at least 205C, whether stationary batch type of cooking or continuous cooking is practiced.
The material of which the vessel is made requires to be non-corrodible under process condi-tions, in which the liquor pH is observed to range from an initial value of about pH 4.25 after the solvent mixture has impregnated the charge of wood chips, to a slightly smaller pH at the end of the cooking.interval, which increase in acidity is ascribable to the generation of small amounts of organic acids by conversion of lignocellulosic components at the elevated temperature conditions, and by hydrol-ytic action.
The weakly acidic character of the liquor may be understood to arise when wood is impregnated by the solvent mixture by cation exchange phenomena, which, per se, are well kno~n:~rom the literature; see for example, H. Masters, "Reactions of Cellulose with Sodium Chloride and other Neutral Salt Solutions"
p. 2032, J. Am. Soc. 121, 1922 wherein reference is made -to observed acidic effects.with solutions of barium chloride and calcium chloride, ordinarily neutral to litmus paper in aqueous solution.
The formation of a metal complex by the cation formed upon dissoc-ation of the dissolved metal salt and subsequent exchange with carboxylic groups present, as depicted by the following relation, releases two protons for each bivalent metal ion which participates in the ion exchange:
LIGNOCELLULOSE _ COOH + M =======LIGNOcELLuLOsEcooH-M
-~ 2(H3O) The hemicellulose and lignin constituents which carry a multiplicity -.12 -of funetional groups may be presumed to be the chief sources of proton formation by such complexing. Table VIII presents data recording pH measurements made with several effective eatalyst salts according to the invention at several wood/liquor proportions, illustrating aeidifieation of the solvent mixture upon steeping several speeies with water, with aqueous salt solutions, and with cooking liquor.

The eooking proeess, reeovery of the pulp fibers and of dissolved products, and recovery of methanol and of metal salt are equally successful and feasible when a minor amount of the strong acid catalyst is ineluded in the cooking liquor. The use of the auxiliary eatalyst, sueh as HCl, is particularly advanta-geous when cooking certain Gymnosperms ~ notably Spruce wood.
No modification of the cooking parameters or of the liquor need be made when using the very low proportions represented by 0.0005 N
to 0.008 N HCl, i.e. not over 0,30 gm HC1 per dm of solvent mixture.

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The insensitivity of any of the pre~erred metal salts herein disclosed to temperatures of 200C or hiyher allows the process to be carried out with heat exchange apparatus arranyed to heat the vessel or the solvent mixture which is circulated, throughout the duration of the cooking interval. The cooking vessel may be of any form suitable for single stage or dual or multi-stage processes whether by batching or in a continuous manner, as desired for the recovery of specific byproducts.

For recovery of oligosaccharides formed by hydrolysis of hemicelluloses with least heat losses by degradation, means must be provided to drain and exchange part of the cooking liquor throughout the cooking interval. Lignin of low molecular weight state is not significantly affected by the process conditions since the cook durations are short and acid concentration is too low to effect recondensation. Due to the successive hydrolyses and a degree of alcoholysis which may temporarily alter the solubility of lignin macromolecules, suitably depolymerized molecular aggregates as recovered from residual liquors after 30 minute cooks have measured molecular weights in the range from about 320 to 12,000 with an average weight about 3600.

Delignification may be arranged to take place in digesters, vessels, or extraction towers commonly used in pulping of wood. In any case it will be desirable to remove from the digester such amounts of liquor and at predetermined rates as will allow removal of methanol, lignin and oligosacch-arides by steam stripping, flash evaporation, filtration/
centrifuging, thickening, crystallization and replenishment of water, methanol, and metal salt.

The removal of pulp from the pressure vessel in a batch operation may be arranged to initially drain the hot liquor from the pulp and thereby lowering the vessel pressure, ~3~

and washing the pulp with a hot wash liquid which is ac~ueous methanol, followed by rinsing with cold water. The hot liquor and the wash liquid may be steam-stripped to recover methanol, and to precipitate lignin.

The nature of the hydrolysis reactions during the cooking operation may be deduced from an analysis of the spectrum of oligomeric and monomeric sugars present in the residual liquor. Such analysis reveals that the liquor is directly suitable, or after a mild secondary hydrolysis, for fermentation and recovery of alcohol. The origin oE the major part of any glucose in solution can be traced to hemicellulose, indicating that degradation of cellulose structure is minor, as shown in Table ~X. The data presented shows, as a progression, the gross analysis of Aspen and Spruce wood constituents; the distribution of constituents folowing the cooking; and a comparison of carbohydrate analyses Eor the whole wood and the pulp and the cooking liquor.
T A B L E IX
_ C 0 ~5 P O S I T I O N A N A L Y S I S
Species Holocellulose lignin extract-Weight % Wt % ables ASPEN WOOD 77.4 19.7 2.9 SPRUCE WOOD 72.3 _26.5 1.2 P O S T - C O O K I N G C O M P O S I T I O N
CookingPulp Yield ~PPI Dissolved Sugars AspEN PULP T2i5me, Min 6~ 2.1 V sc;
LIQUOR 16.3 9.1 SPRUCE PULP 45 52 2.9 19 LIQUOR 23.0 8.9 C A R B O H Y D R A T E A N A L Y S I S
Species Glucan Xylan Galactan Arabinan Mannan Uronic Unaccounted Anhydr.
ASPEN WOOD 57.9 13.2 0.6 0.2 3.4 1.0 ASPEN PULP 53.1 2.8 0.1 trace 2.2 0.06) 9 0 LIQUOR 0.4 7.2 0.5 trace 0.8 0.2 ) SPRUCE WOOD 49.9 5.7 1.8 1.1 11.9 0.8 SPRUCE PULP 43.1 2.0 - ~ 2.5 trac~ 14.7 LIQUOR 1.7 1.4 1.0 0.6 4.7 0.1 _ __. ___ -~3~5 CONSIDERATIONS OF PROTON SOUl~.CES

Delignification of lignocellulose b~ the aqueous methanol-metal salt system of this invention is believed to proceed by combined alcoholysis and proton catalysed hydrolysis that reduce the size of native lignin molecules by bond breaking within the macro-molecules and between the lignin-hemicellulose complexes. A limited reaction of methanol in association with water of the solvent mixture may play a role in temporary and permanent modification of lignin functional groups at high temperature to increase the rate of lignin dissolution. The simultaneous removal of carbohydrates and cleavage of the lignin-carbohydrate bond facilitate a widened pathway for passage of fragmented lignin from the cell wall. As both lignin and t~e less stable carbohydrates are removed by a similar mechanism and require substantially lower activation energy in hydrolysis than the cellulose, this limited proton source and the possible protective effect of the exchanged and adsorbed cations is believed to be responsible for minimization of the degradation of cellulose as observed in Table IX.
Cations of magnesium, calcium and barium appear to have the ability of restricted catalysis of delignification while restraining cellulose degradation. The sources of protons produced in the system comprised of impregnated lignocellulosics and the catalysed aqueous solvent mixture are considered to result from phenomena of hydrolysis and alcoholysis of the solvent-cation complexes, cation exchange on uronic acids, carbonyl, ester and ether functions on both carbohydrates and lignin and finally through increased dissociation and complexing of weak organic acids (acetic, formic etc),formed during the course of cooking, due to the x alteration of their activity coefficients by adding the alakli earth metal salts to the solution as predicted by the Debye-Hùckel theory. A combination of these factors is held responsible for the pH drop when the catalyst-containing aqueous methanol solution is added to lignocellulosics as demonst~ated in Table VIII. The proton activity is further enhanced by the relatively high temperature and pressure. It was found most advantageous~to work at the higher temperatures (180 to 200C) as in this case the reaction times are sufficiently short to favourably influence the kinetics of delignification while cellulose degradation remains suppressed. There were no reprecipitation or secondary condensation tendencies of either lignin or the hemicelluloses observed under these conditions.

Claims (26)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for chemically converting lignocellulose to the form of separated fibers, which comprises, - cooking lignocellulose with an aqueous solvent mixture of at least four times the weight of the lignocellulose, - the said mixture comprising from 1 to 4 volumes of methanol per volume of water and a metal salt catalyst dissolved therein in a concentration between 0.005 molar to 1 molar, - the salt being selected from the group consisting of the chlorides and nitrates of magnesium, calcium,and barium, - the cooking temperature being between about 180°C and 210°C
and the cooking time being not longer than two hours and sufficient to effect at least partial depolymerization and dissolution of the lignin, the hemicellulose and the other cell wall materials encasing the cellulose, and recovering the separated fibers from the liquor residue.

2. The method of Claim 1 wherein the cooking is carried out at a temperature between about 190°C to 200 C.

3. The method of Claim 1 further comprising the step of volatilizing methanol from said liquor residue and condensing the volatile material to recover methanol.

4. The method of Claim 3 wherein the remaining liquor residue is cooled to precipitate lignin and the precipitated lignin is separated from the liquor residue.

5. The method of Claim 4 wherein the precipitate is washed with hot aqueous methanol and dried to powder state.

6. The method of Claim 4 wherein the precipitate is washed with aqueous acetone and dried to powder state.

7. The method of Claim 4 wherein the liquor residue following removal of lignin is diluted and supplied as an ingredient to the wort in a fermentation process to convert sugars in said residue to alcohol, and the alcohol is recovered by distillation.

8. The method of Claim 7 wherein the spent wort is at least partially dehydrated to a salt-rich residue and crystalline salt is recovered.

9. The method of Claim 1 or Claim 2 wherein the ligno-cellulose is a hardwood and the salt concentration is in the range 0.05 molar to 0.01 molar.

10. The method of Claim 1 or Claim 2 wherein the ligno-cellulose is a softwood and the metal salt concentration is in the range 0.2 molar to 0.05 molar.

11. The method of Claim 1 or Claim 2 wherein the metal salt is calcium chloride.

12. The method of Claim 1 or Claim 2 wherein the metal salt is calcium nitrate.

13. The method of Claim 1 or Claim 2 wherein the metal salt is magnesium chloride.

14. The method of Claim 1 or Claim 2 wherein the metal salt is magnesium nitrate.

15. The method of Claim 1 or Claim 2 wherein the metal salt is barium chloride.

16. The method of Claim 1 or Claim 2 wherein the metal salt is barium nitrate.

17. A method for chemically converting lignocellulosic materials from plants of the order of Angiosperms to the form of separated fibers which comprises, - cooking fragmented material in an aqueous solvent mixture in amount between 5 and 10 times the weight of the material, - the said mixture comprising about 3 parts of water to 7 parts of methanol by weight and containing a metal salt selected from the group consisting of the chlorides and the nitrates of calcium and magnesium in a concentration between about 0.05 molar to 0.01 molar, - the cooking temperature being maintained between about 190 C
and 200°C for at least 15 minutes and sufficient to effect at least partial depolymerization and dissolution of lignin and hemicelluloses and other cell wall materials encasing the cellulose fibers, and - recovering the separated fibers from liquor residue.

18. A method for chemically converting lignocellulosic materials from plants of the order of Gymnosperms to the form of separated fibers which comprises, - cooking fragmented material in an aqueous solvent mixture in amount between 5 and 10 times the weight of the said material, - the said mixture comprising about 3 parts of water to 7 parts of methanol by weight and containing a metal salt selected from the group consisting of the chlorides and the nitrates of calcium and magnesium in a concentration between about 0.2 molar to 0.05 molar, - the cooking temperature being maintained between 195°C and 205°C for at least 20 minutes and sufficient to effect at least partial depolymerization and dissolution of lignin and hemicelluloses and other cell wall materials encasing the cellulose fibers, and - recovering the separated fibers from liquor residue.

19. The method of Claim 17 wherein the aqueous solvent mixture comprises hydrochloric acid in a concentration between about 0.0005 normal and 0.008 normal with respect to the mixture.

20. The method of Claim 17 or Claim 18 or Claim 19 wherein methanol is recovered from said residue by volatilization and condensation and lignin is recovered as a precipitate.

21. The method of Claim 17 or Claim 18 or Claim 19 wherein methanol is recovered from said residue by volatilization and condensation and lignin is recovered as a precipitate, and the residual liquor is evaporated to separate sugars.

22. The method of Claim 17 or Claim 18 or Claim 19 wherein the recovered pulp of separated fibers is washed with hot aqueous solvent selected from the group consisting of methanol and acetone and the washed pulp is rinsed with water.

23. The method of Claim 17 or Claim 18 or Claim 19 wherein the liquor residue is cooled to precipitate lignin and the lignin precipitate is separated from the liquor residue and dried to powder state.

24. The method of Claim 17 or Claim 18 or Claim 19 wherein the liquor residue is cooled to precipitate lignin and the lignin precipitate is separated from the liquor, and the liquor residue is then diluted and supplied as an ingredient to the wort in a fermentation process for conversion of dissolved sugars to alcohol and protein.

25. A method for chemically converting lignocellulose to the form of separated fibers which comprises cooking fragmented lignocellulose in a pressure vessel at a temperature between 185°C
and 210°C with an impregnating liquor in amount between 5 and 10 times the weight of lignocellulose, the liquor comprising water and methanol in weight ratio between 1:1 and 1:4 and including as catalyst for promoting at least partial depolymerization of lignin and hemicelluloses a dissolved metal salt selected from the group consisting of chlorides and nitrates of magnesium, calcium and barium in amount from 0.005 molar to 1.0 molar, until the fiber is separated.

26. The method of Claim 25 wherein the liquor comprises hydrochloric acid in a concentration between about 0.0005 Normal and 0.008 Normal with respect to the liquor.