I N D U S T R I A L A N D ENGINEERING CHEMISTRY
Februarv. 1924
141
T h e Gelatinization of Lignocellulose' TI-Action
of Dilute Sodium Hydroxide and Cuprammonium Solutions on the Pentosans By A. W.Schorger C . F. BURGESSLABORATORIES,
MADISON,
WIS.
s
Rinterstein7 found that; .NCE the discovery of The amount of pentosans remoued from aspen wood by treatment the residue remaining after the occurrence of penwith dilute sodium hydroxide solutions in the cold rapidly reaches boiling beech wood with 5 tosans in seed plants, a minimum, the extent of remoual of the pentosans depending on the per cent sulfuric acid for there have been two schools strength of the alk-aline solution. 3 hours still contained 10.16 of opinion regarding the No adsorption of pentosans took. place when aspen was ground in per cent xylan. He constate in which pentosans an alkaline solution of added pentosans. cluded t,hat the xylan is exist in the lignocelluloses When birch wood, giuen a preliminary extraction with concenpresent in two forms, the and the cellulosestherefrom. trated ammonia, was ireated with cuprammonium solution, the one being removed by dilute Most investigators have ratio of pentosan to orthoglucosan in the dissolued portion was acids and Schulze's reagent, held that a portion of the approximately the same as in the Cross and Beuan cellulobe. When the other resisting these pentosans are chemically treated with cuprammonium solution, the dissolued portion did not agents; a part of the xylan combined with orthoglucohaue the same composition as the residue with respect to pentosans is combined with the cellusan,2 while others, particand lignin. lose. I n marked contrast ularly in recent years, have No clean-cut proof exists that pentosans are chemically combined the mannan of the conifers considered them as held in with orthoglucosan in wood cellulose, but the weight of experimental can be readily and quantia state of adsorption or euidence is in fauor of chemical combination as against the adsorption tatively removed by hysolid solution. I n view hypothesis. drolysis with dilute acids.* of the recent extensive Heuser and Haugv found review and speculations on wood cellulose by Wise3wherein the pentosans are considered that the pentosan content of crude straw could not be as ad:;orbed, it is desirable to present briefly the literature reduced below 9.7 per cent xylan with boiling 6 per cent caustic soda. Commercial straw pulp obtained by alkaline from 1he opposite angle. Widicenus,* the chief exponent of the adsorption theory, digestion under pressure a t 140" to 150" C. still contained has not precluded the possibility of chemical reactions sqch 29 per cent xylan. More recently, Heuser andBoedekeP as esterification. His experimental work, upon which is have asserted that all plant celluloses, when properly purified, based the theory that the cell wall grows by adsorption by a are alike and have the empirical composition of cotton cellulose gel framework of colloids from the sap, is not partic- cellulose. Wood cellulose was practically freed from pentoularly convincing if we consider it from a cytological stand- sans by repeated extraction with hot 6 per cent caustic soda point After the formation of the cell by division a layer of or cold 17 per cent caustic soda, but with the simultaneous pectin is formed between it and its sister cell, this layer later destruction of a considerable part of the cellulose. The view becoming the middle lamella; layers of cellulose are then of these authors has been supported by Wise and Russell1' deposited, followed by lignification during the later stages. on the ground of having obtained comparable yields of The fundamental syntheses take place within the protoplasm cellobiose octacetate from cotton and spruce cellulose after which is in intimate contact with the eel1 wall, and there is the latter had been treated with 17.5 per cent caustic soda no reason to believe that any substances other than crystal- to remove pentosans. I n the writer's opinion the use of a loids can pass in quantity through the plasma membrane. boiling 6 per cent or a cold 17 per cent solution of caustic The cbhanges that the crystalloids undergo after leaving the soda is too drastic a treatment upon which to base these conclusions; certainly, none of the cellulose esters prepared plasma membrane are purely hypothetical. Schulze and Tollens6 found that all the pentoses were not in the laboratory remain unaffected by them. It is to be removed from spent maIt by a 6-hour digestion with 4 per understood that it is not yet known how the pentosans cent sulfuric acid. The residue, after having been extracted are held, an ether linkage being generally assumed. On cookwith 5 per cent caustic soda, gave on extraction with cupram- ing sulfite cellulose with calcium hydroxide or barium hydroxmonium solution a cellulose that still contained pentoses. ide under pressure, Schwalbe and Becker12 found that the The impossibility of separating the xylan from the cellulose pentosan content could not be reduced below 5 per cent and led them to conclude that the two did not occur as a simple advanced the opinion that pentosans are combined with the mixture but as a compound. A similar conclusion was cellulose molecule. The hemicelluloses, such as xylan, are, in general, soluble reached by Schulze,6 who found that the xylan in beech wood was not entirely destroyed by a 3-hour boiling with in cuprammonium solution. According to Reiss13some hemi1.5 per cent sulfuric acid followed by a 14-day treatment celluloses are soluble, others not. Schulze14found that the with F. Schulze's reagent (nitric acid and potassium chlorate). hemicelluloses of various seeds were insoluble in cuprammonium solution; however, after boiling with dilute hydro1 Presented before the Division of Cellulose Chemistry a t the 65th chloric acid or treating with 10 per cent hydrochloric acid Meeting of the American Chemical Society, New Haven, Conn., April 2 t o 7,1923. 2 There is no suitable name for expressing a glucose anhydride grouping such as exists in cotton cellulose, and the term "orthoglucosan" is accordingly proposed; the name glucosan has long been used for an entirely different product. 8
T H I S JOURNAL, 15,
4
Wislicenus and Kleinstuck, 2. Chem. I n d . Kolloide, 6, 17,87 (1910). A n n . , 271, 55 (1892). L . physiol. Chem., 16, 435 (1892).
6 6
711 (1923).
' 2 . physiol. Chem., 17, 381 (1893). 1 Schorger, THIS JOURNAL, 9, 748 (1917). 9 2. angeu. Chem., 31, 168 (1918). 10 Ibid., 34, 461 (1921). 11 THISJOURNAL, 14, 288 (1922). 1 2 J . pvakl. Chem., 100, 37 (1919). 13 Landw. Jahvb., 18, 747 (1889). 1 4 2. physiol. Chem., 14, 245, 266 (1890); 16, 410 (1892);Bey., 22, 1194 (1889).
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
in the cold for 24 hours, they were readily soluble. This would indicate that hydrolysis was essential to solution. Fremy16 as long ago as 1859 pointed out that vegetable ivory (phytelephas) dissolves readily in cuprammonium solution while the fibers of wood do not, the difference being accordingly unexplainable as regards the relative densities of the materials. Hoffmeisterl6 found that by alternate treatment of lignocelluloses with acid and ammonia the cellulose became soluble in cuprammonium solution. I n an attempt to throw some light 6n the state in which . pentosans occur in lignocelluloses, wood was treated with dilute sodium hydroxide solutions and cuprammonium solutions; in the former only the pentosans are soluble, in the latter both pentosans and cellulose. In order to obtain as thorough an extraction as possible, use was made of the ball mill whereby gelatinization was attained as described in a previous paper." The insoluble residues remaining from treating aspen, three times in each case, with cold 1, 2, and 5 per cent sodium hydroxide contained 12.48, 9.23, and 6.71 per cent pentosans, respectively. The cell structure was completely destroyed by the method of grinding employed, leaving the particles in a swollen condition, thus offering with a minimum of chemical change a maximum surface for extraction. With each concentration of alkali the amount of pentosans removed rapidly reached a minimum and in no case was the removal of the pentosans approximately complete. If the alkali does not produce a chemical change in the wood, it is not apparent why a 5 per cent solution of caustic soda removes so much more xylan than a 1 per cent solution. It has long been recognized that xylan undergoes a change through extraction with caustic soda. Schmidt and Graumann18 have recently stated that the xylan in beech wood is not affected by chlorine dioxide, but when isolated in the usual way by means of caustic soda it is attacked. I n the case of dilute alkali, having a definite solvent and chemical reaction with the xylan, it is not apparent how adsorption could play the same part as where a neutral solvent is employed. The case is not analogous to colloidal ferric hydroxide, from which, as is well known, it is impossible to remove all the alkali by washing with water. It is still less apparent how adsorption could play a large part in preventing the complete and rapid removal of xylan from wood by hydrolysis with acids, the xylan in this case being hydrolyzed to definitely crystalline products. Spoehrlg considers as cellulose the carbohydrate residue resisting so mild a hydrolysis as heating on a water bath for 3 hours with 1 per cent hydrochloric acid. The writer has also ground aspen in an alkaline solution of xylan and found that, no adsorption of pentosans took place. No great importance is attached to this fact per se, but it supports the contention that a portion of the pentosans is held in chemical combination; otherwise, all the pentosans would pass into solution, since under the experimental conditions employed there is every reason to believe that the sodium hydroxide comes into contact with all the pentosans. The latter condition may not be so readily attained with wood in its original irreversible condition. Birch wood, on account of its high xylan content, was treated with cuprammonium solution (1) to determine if the solution exerted a highly preferential solvent action on the xylan, which should be the case if the xylan is not in chemical combination; and (2) to determine if, in the pprtion of the wood dissolved, the xylan had a constant relationship to orthoglucosan. When the wood was treated directly with cuprammonium solution a certain preferential solvent action Compt. uend., 48, 277 (1859). Landw. Jahrb., 17, 261 (1888). 17 THISJOURNAL, 15, 812 (1923). 18 Ber., 54, 1867 (1921). 19 Cornegie Inst. Pub., 287,28 (1919). 16 16
Vol. 16, KO.2
was exerted on the pentosans that was to be expected owing to the presence of easily soluble pentosans; however, after two extractions the wood still contained 10 per cent pentosans. On the other hand, when the wood was first extracted with concentrated ammonia followed by cuprammonium solution, the dissolved portion showed a pentosan-orthoglucosan ratio fairly close to that existing in the Cross and Bevan cellulose. Whether this circumstance is fortuitous remains to be determined by further work, particularly on a number of species.
EXTRACTION WITH DILUTESODIUM HYDROXIDE The material extracted was the comminuted wood of the aspen (Populus tremuloides) representing trees about 15 years old. It contained 22.94 per cent pentosans and 23.44 per cent lignin, as determined with 72 per cent sulfuric acid. The 25-gram sample (23.3975 grams of dry, ash-free substance) was ground in a ball mill with alkali for three periods of 48, 24, and 18 hours each. During the first grinding 1000 cc. of alkali were used. The amount of solution was reduced in subsequent grindings in proportion to the loss in weight; e. g., if the residue weighed 20 grams only 800 cc. of alkali were used. The ground material was placed on a fine muslin filter, the first runnings being returned to the funnel. The filtrate was opalescent, though it showed no change in appearance when passed through a filter paper. The gelatinous mass was washed with cold water until the filtrate was neutral to litmus, allowed to stand until it formed a coherent mass, and transferred to a large evaporating dish, the last portions of the wood on the filter cloth being removed by the careful use of a spatula and wash bottle. During drying, conducted a t 40" C., the material was frequently crushed with a pestle so as to obtain a rather fine, granular mass before it had dried completely; if dried en masse the material is extremely hard arid difficult to manipulate. The residue wasthen weighed to determine the loss by grinding, and portions were taken for the determination of pentosans, moisture, and ash. All results are corrected for moisture and ash, as well as for the amounts removed for the various analyses, in order that the figures for the three grindings may be comparable. TABLEI-EXTRACTIONO F ASPENWITH DILUTE ALKALI Strength of NaOH -PENTOSANIN RESIDUESolution Extraction W O O D DISSOLVED Per cent of Percent No. Grams Per cent Per cent Total Pentosans 5 1 8.62 25.82 7.3218 31.29 2 0.7909 3.38 6.82 19.42 3 0.3928 1.68 6.71 18.62 36.44 27.82 11.58 2 1 6.5095 9.67 29.37 0.5891 2.52 2 3 0.4005 1.71 9.23 27.34 52.60 24.64 16.01 1 1 5.7645 42.50 3.57 13.58 0.8349 2 38.14 1.69 12.48 0.3962 3
The residues from the last grinding with 5,2, and 1 per cent caustic soda solutions contained 20.36, 21.07, and 20 48 per cent of lignin, respectively, a: uniformity unexpected and not easily explained. Contrary to the general opinion, the pentosans were very appreciably soluble in 1 per cent caustic soda. With 5 per cent and 2 per cent caustic soda the amount of xylan removed rapidly approached a minimum after the second extraction; if the curves of Fig. 1 were extended they would be expected to be asymptotic. ADSORPTION OF PENTOSANS To determine if pentosans are adsorbed by wood the following experiment was made: T o 50 grams of aspen 500 cc. of 2 per cent ammonium hydroxide solution were added, allowed to stand 2 weeks, filtered, washed, and the residue air-dried. The latter was treated with 500 cc. of 5 per cent sodium hydroxide solution, allowed to stand 48 hours with frequent shaking, then fltered and washed with 5 per cent sodium
February, 1924
INDUSTRIAL A N D ENGINEERING CHEMISTRY
hydroxide solution until the filtrate amounted to 600 cc. The pentosan content of the solution was then determined Twenty-five grams of aspen were then ground 48 hours with 500 cc. of 5 per cent sodium hydroxide solution and 500 cc. of the above filtrate containing 4.264 grams of pentosan, thereby increasing the pentosan content about 60 per cent. The gelatinized material was then washed and analyzed as usual. It contained 8.50 per cent pentosan in comparison with 8.62 per cent pentosan obtained in the absence of added pentosans. This shows that there was no adsorption of pentosans, though the gelatinous condition of the wood would presumably be favorable for it.
143
TABLE11-ACTION OF AMMONIA ON BIRCH ---PENTOSANA B Original wood Residue from NHiOH extraction Ammonia solution Cuprammonium soIution
%
%
27.86 21.51 4.98 1.81
25.86 21.21 4.86 1.39
In determining the pentosans in the cuprammonium solution, 200 cc. were used; most of the ammonia was removed by a vacuum a t room temperature The solution was then made up to 12 per cent hydrochloric acid, and after distilling down to 70 cc. the usual procedure for determining pentosans was followed. It will be noted thrtt the addition of cupric hydroxide to the ammonia soluticn resulted in an appreEXTRACTIOS WITH CUPRAMMONIUM SOLUTION ciable loss of pentosans or furfural-producing substances; The material used for treatment with cuprammonium was furthermore, the phloroglucide precipitate from the cupramyellow birch (Betula Zutea), a representative sample from a monjum solution was soluble to the extent of 25.3 per cent tree 01-er 60 years old. The wood contained 27.86 per cent in alcohol. It will also be seen that treatment with ammonia pentosans (or, by extraction of the phloroglucide with alcohol, has destroyed to a considerable extent the so-called methyl25.86 per cent pentosan and 2.21 per cent methylpentosan), pentosans. The sum of the corrected pentosan values for and 61.9 per cent cellulose by the Cross and Bevan method, the residue and the ammonia extract in B is in good agreement with that for the original wood, but in A there is a loss of 1.4 the latter in turn containing 28.3 per cent pentosans. per cent. In the writer's opinion figures for methylpentosans as now determined are largely fictitious. Previous extraction of the sample with concentrated ammonia in the cold removes easily soluble pentosans and reduces the amount of lignin in the cellulose subsequently removed by cuprammonium solution. The cellulose solution does not appear to undergo appreciable change on standing. In one experiment the birch was extracted with concentrated ammonia, well washed, and dried; 25 grams were then ground in 1000 cc. of cuprammonium solution prepared according to Gibsonz0for 24 hours. After allowing to settle, a portion of the solution A was filtered under pressure; for this purpose a 2.54-cm. (1-inch) threaded brass tube with a perforated brass cap was used, the latter containing a :Dorcelain filter plate and a layer of asbestos. The cellulose solution mixed with fibrous kieselguhr was poured into the tube and an air pressure of 40 pounds applied. In this way a perfectly clear filtrate was obtained. It was impossible to filter the cellulose solution by suction. The cellulose in the filtrate was precipitated I with hydrochloric acid and washed until the wash water gave no test for chlorides. The remainder of the cellulose solution B, after standing 12 days in a stoppered flask in a cool, dark 60 80 0 20 40 IO0 126 place, was filtered and precipitated in the same way. The HOURS GROUND analyses of the two precipitates are given below. FIG,1-PENTOSAN
CONTENT O F ASPENAFTER EXTRACTION WITH DILUTE SODIUM HYDROXIDE
Previous to the use of cuprarnmonium solution it was of interest to determine the effect of concentrated ammonia alone. Yellow birch passing a 60-mesh sieve lost 10 9 per cent of its weight when treated with concentrated ammonia in the cold; the insoluble residue contained 25.8 per cent pentosan. Since the original wood contained 27.86 per cent pentosans, 17.49 per cent of the total pentosans were removed by the ammonia. When 25 grams of birch were ground with 1liter of concentrated ammonia for 24 hours, 14.49 per cent of the wood dissolved. The insoluble residue was filtered off and washed thoroughly. A pentosan determination was made on this residue and on a portion of the undiluted ammoniacal filtrate. A portion of the latter was also treated with an excess of crystalline copper hydroxide and allowed to stand with frequent shaking for 1 week to determine the effect on the pentosans. The solution was then filtered by pressure and the pentosans determined. The results are given below: A represents the pentosans calculated from the total phloroglucide precipitate, and B, after extracting the latter with alcohol.
TABLE 111 Cellulose A
B
Lignin Per cent
5.61 5.70
Pentosan Per c'nt
27.39 26.20
Pentosan on Lignin-Free Basis Per cent
28.70 27.99
The difference in pentosan content may be considered as within experimental error. Birch wood, first extracted with concentrated ammonia, gave with cuprammonium solution in various experiments celluloses which on a lignin-free baciis contained from 25 to 29 per cent of pentosans. I n view of the various factors involved in the use of cuprammoniiim solution, this may be considered in fair agreement with the pentosan content (28.3 per cent) of the Cross and Bevan cellulose. The writer has made numerous extractions with cuprammonium solution and has failed to obbain therefrom a cellulose constant in composition as regards the lignin and pentosan content. This is contrary to the statement of Cross and Bevan21 that the soluble portion of the lignocelluloses has the same composition as the residue. I n one experiment 40 grams of birch that had not been given a preliminary treat20 21
J . Chem. SOC.(London), 117,492 (1920). "Cellulose," 1916, p. 114.
INDUSTRIAL A N D ENGINEERlNG CHEMISTRY
144
ment of any kind beyond dry grinding were ground for 72 hours with 2 liters of cuprammonium solution. This product was centrifuged and subjected to the following treatmenks: A , solution obtained by centrifuging; B, cellulose precipitated from a portion of the centrifuged solution, washed with hot water; C, portion of centrifuged solution filtered through the brass tube; D, solution obtained by treating the sludge from the centrifuge with 1 liter of cuprammonium solution and let stand with frequent shaking for 5 days and centrifuging; E, sludge from treatment D. The centrifuged solutions were allowed to stand 24 hours and again centrifuged; they were still cloudy. The cellulose was precipitated as usual and washed with cold water unless otherwise specified. The analyses of the various celluloses are given in Table 1V. It is apparent that there is no uniformity in the precipitates in this table, and that a preferential solvent action was exercised on the pentosans. The lower lignin and pentosan content of B is readily explained by washing with hot water in which the pentosans, once dissolved, are appreciably soluble;
Vol. 16, No. 2
after washing with cold water the first washing with hot water was colored a decided yellow. There is no reason to expect that the pentosan in the cuprammonium solution is in chemical combination with orthoglucosan, though the optimum conditions for adsorption exist; as a matter of fact, the pentosan can be readily removed with dilute alkali and to an appreciable extent by prolonged treatment with hot water. The kigh pentosan contents of D and E indicate the great resistance of the pentosans to solution. The chlorine-sodium sulfite reaction for lignin was applied to the lignins isolated from A and D by means of 72 per cent sulfuric acid, the characteristic color reaction being obtained in both cases. TABLE IV-CELLULOSESE&TRACTED FROM BIRCHBY CUPRAMMONIUM SOLUTION
Precipitate A
B
C D E
Lignin Per cent
13.58 9.77 13.97 12.84 34.11
Pentosan Per cent
38.82 36.91 33.07 17.68 10.01
Pentosan on Lignin-Free Basis Per cent
44.92 40.91 38.44 20.28 15.32
Countercurrent Digestion of Wood1l2 By R. T. Haslam and W. P. Ryan MASSACHUSETTS INSTITUTE OP TECHNOLOGY, CAMBRIDGE, MASS
the lignin content becomes less.
In the soda process for the manufacture of wood pulp, the digester is charged at the start with caustic soda and wood chips. Thus strong caustic is brought in contact with wood when the concentration of lignin is large, leaving weak caustic to digest the small amounts of lignin remaining at the end of the digestion.. This combination gives extremely rapid digestion of wood at the beginning and a very slow cleaning up of lignins at the end. For rapidity of digestion the law of mass saction indicates the advantage of using dilute caustic soda at the start when the lignin concentration is high, gradually increasing the strength of causfic as
This may be accomplished by causing the strong caustic fo flow in Q countercurrent direction to the wood chips that are being digested. Experimental work indicates that such a process has the advanfages of decreasing the time of digestion by one-third to one-half. increasing the a-cellulose content of the pulp, and of producing, in general, a better bleaching pulp. The disadvantage in the runs here reported consists in a reduction of yields amounting to 4.4 per cent. . A method is suggested of carrying out a countercurrent system of digestion using the customary vertical tank digesters.
HE present soda process for the production of wood pulp is based on the patent granted Charles Watt and Hugh Burgess3 in 1854, for, with the exception of the elimination of an intermediate chlorination practiced in the early days, few radical changes have been made. The digester, however, has been changed frequently, the tendency being always toward the large, vertical, stationary, direct heating type. The more important factors, such as charging mixture, concentration of caustic, pressure, temperature, and time of digestion, have been carefully studied, especially by Surface14who recommends the following:
(a) increasing caustic in charging mixture 2 per cent; ( b ) increasing the concentration 12 grams per liter; (c) increasing the pressure 5 pounds; (d) increasing the time 1.2 hours. It is evident that the type of digester will have an important effect on operation, for on it will depend the character and amount of circulation of the cooking liquor. However, each mill using data in the literature as a guide can work out the operating conditions best suited to it. As the chemistry of the process has become better known, it has been possible to eliminate much of the trial and error method of earlier days. It is generally believed that the action of caustic on the wood is one of alkaline hydrolysis, in which the lignocellulose is gradually broken down with the formation of acid products which combine with and neutralize the caustic.6 If this is true, we would expect that for a definite weight of wood a definite amount of caustic would always be neutralized, and on a certain wood caustic neutralized a t the end of digestion has been found t o be 16.7 pounds out of 25 pounds charge per 100 pounds of bone dry wood. From this it would B = C, in appear that we can regard the reaction as A which A represents the acid products resulting from hydrolysis, B the caustic, and C the sodium organic salts formed. By the mass law, A p = K
T
............ (2) Concentration oi liquor.. .....
(1) Charging mixture (3) Pressure..
..................
(4)Temperature.. . . . . . . . . . . . . . (5) Time of digestion..
..........
25 pounds NaOH per 100 pounds bone dry wood 70 grams NaOH per liter 100 pounds per square inch gage 338O F. 7 hours
Investigations by Sutermeister5 show that a decrease of 1 per cent in the yield results from each of the following: Presented before the Division of Industrial and Engineering Chemistry at the 64th Meeting of the American Chemical Society, Pittsburgh, Pa., September 4 to 8 , 1922. Received October 20, 1923. Contribution No. 31 from Department of Chemical Engineering, Massachusetts Institute of Technology. 8 U. S. Patent 11,343 (1854). "Effects of Varying Certain Cooking Conditions in Producing Soda Pulp from Aspen " U.S. De& dgr., Bull. 80. 6 "Chemistry of Pulp and Paper Making," p. 109.
'
+
DeCew, J . SOC.Chem. Ind., 26, 561 (1907);Klason, "Verhandelinger des Vereins des Papier und Zellstoffchemicher," 1909, p. 84; and others.
