THE COOKING PROCESS Action of Aqueous Solutions of Pure Sodium Hydrosulfide on Aspen Woodl RALPH L. HOSSFELD2, ROSS AIKEN GORTNERa, AND FRANK H. KAUFERT Minnesota Agricultural Experiment Station, St. Paul, Minn.
due, while the organic potentialities of the residual liquor have been neglected. Therefore, a study of the liquor remaining after removal of the lignin was undertaken.
Alcohol-benzene extracted aspen sawdust has been cooked under varying conditions of time, temperature, and concentration with aqueous solutions of pure sodium hydrosulfide. The resulting cooks have been individually separated into seven fractions, including steam-volatile oil, residual wood, volatile acid, water-insoluble pitch, etherextractable nonvolatile oil, lignin, and a methanol-soluble sirup. In all cases, except when impure sodium hydrosulfide was used, the residual wood was completely unpulped. Most interesting is the etherextractable, nonvolatile oil, a sulfur-containing phenolic material occurring to the extent of about 29 per cent of the original wood in some cases.
COOKING CONDITIONS
All cooks were made in a solid, cast, stainless steel autoclave of 2-liter capacity, heated by direct flame; each cook consisted of 100 grams of air-dry, alcohol-benzene extracted aspen sawdust and 1 liter of cooking liquor. The sodium hydrosulfide was prepared by saturating an alcoholic solution of sodium ethoxide with hydrogen sulfide according to Rule (6). The resulting crystalline product was filtered, washed with ether, dried under vacuum, and stored in air-tight containers in the refrigerator. The pure material was white and practically odorless. The purity of the sodium hydrosulfide has a profound effect on the results. The presence of even the smallest amount of decomposition products, presumably sodium hydroxide and sulfide, cause the resulting cooks to resemble those made from sodium sulfide and the residual woods to be quite well pulped. The resulting cooks were individually separated into seven major fractions as follows: The entire contents of the autoclave were transferred to a steam distillation apparatus, and the volatile oils removed. T h e residual liquor from the steam distillation was filtered to remove the residual wood which was then washed and dried a t 105" C. The combined filtrate and washings were neutralized with sulfuric acid and again steam-distilled to remove the volatile acids which were recovered from the distillate as the sodium salts. After the residual liquor from the steam distillation had cooled, a water-insoluble pitch separated which mas
T
HE literature contains numerous reports on the role
of sulfur in the kraft pulping process. Many have dealt with the pulping action of sodium sulfide alone (2) or in combination with sodium hydroxide over a wide range of conditions. Others have described sovdium hydrosulfide as a cooking agent ( 1 , 3, 4). Almost without exception the prime motive of these investigations has been t o correlate the ratios and proportions of cooking agents with the I quantity and quality of pulp obtained. Some attention has been given to the ligneous material which can be isolated from the black liquor, and attempts have been made D ( I ) to carry out solvent fractionation. 3 > Numerous elementary analyses have indin I cated that combined sulfur is present. Apparently no systematic attempts have been made t o seek and study new compoi: nents of the sodium hydrosulfide black liquor $ a other than the generally recognized lignin, volatile acids, and sugars. In other words, the major effort has been centered about the pulp and alkali-soluble ligneous resi-
\
ALCOHOL EXTRACTION OF RESIDUAL WOOD 0 LOSS IN WT. OF RESIDUAL WOOD WT OF EXTRACTED MATERIAL A D I F F E R E N C E Cm MINUSO) 0 LOSS OF LIGNIN ON EXTRACTION
f
6
2
Previous papers in this aeries appeared in INDUSTRIAL AND ENQINEDRINQ C H E M I S T R Y in 1930, 1933, 1934, 1935, 1936, 1937, and 1938. * Present address, Marathon Chemical Comusnv, Rothschild. Wis. a Deceased. 1
6
CONC. COOKING LIQUOR-PER
8
CENT N d S H
Figure 1. Effect of Sodium Hydrosulfide Concentration on Alcohol Extractives in Residual Wood Cooked at 180° C. for 2 Hours 717
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Vol. 35, No. 6 YIELDS
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.RESIDUAL WOOD O T O T A L ETHER EXT.
A N O N VOLATILE OIL WATER-INSOL. PITCH O L l G N l N RESIDUE 9 N A SALT VDL. ACID
The residual wood was, in general, black in color, brash, and wholly unpulped. After 3 hours of beating in a ball mill, the material had the appearance of being little more than ground sawdust. It is interesting that these residual woods are unpulped, in spite of the fact that they contain only about 6-7 per cent of lignin which is the average content of a n unbleached, medium sulfite pulp. This observation might well indicate that the degree of lignin removal from a wood is not a true measure of the degree to which the material has been pulped. As illustrated in Table I and Figure 1, a part of the residual wood was soluble in alcohol, in agreement with the findings of Aronovsky (9) in the case of sodium sulfide cooks. The loss of lignin from the pulp on extraction was in close agreement with the amount of material extracted by alcohol. However, if the percentage of material extracted is calculated. and the difference taken between these Galues and those for the percentage loss in weight of the residual wood, in every case the weight
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4 6 CONC. COOKING LIQUOR- PER CENT N d S H
10
Figure 2. Effect of Sodium Hydrosulfide Concentration i n Cooking Liquor on Yields a t 180' C. and a Cooking Time of 2 Hours - .-. .
further fractionated into an ether-soluble and an ether-insoluble part, the latter being combined with the ligneous residue obtained in a following step. After removal of the water-insoluble pitch, the residual liquor was continuously extracted (6) with ether to give an ether-soluble nonvolatile oil. The ether-extracted liquor was then filtered to remove the ligneous residue. The remaining filtrate was neutralized and the whole evaporated t o dryness t o give a mass of crystals which were triturated with methanol in order t o separate the inorganic material from the water-soluble sirup. The resulting methanol solution of organic material was decolorized with Norite t o give.a light yellow sirup which would not crystallize. Both the steam-volatile and the ether-extracted oils contained organically combined sulfur. However, attempts t o separate these products into their constituent compounds have been unsuccessful because of their extremely labile nature.
iRESIDUAL WOOD
r/
0 TOTAL ETHER EXT. OIL
ANONVOLATILE
m
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0
9
4
6
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5ok
0 LIGNIN RESIDUE Q N A SALT VOL.ACID
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.RESIDUAL WOOD OTOTAL ETHER EXT.
60
6 WAT~R-INSOL. PITCH'
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A N O N VOLATILE OIL &WATER INSOL.PITCH OLlGNlN RESIDUE T N A SALT VOL. ACID
L TIME-
HOURS
Figure 4. Effect of Cooking Time on Yields at 4 Per Cent Sodium Hydrosulfide Concentration i n Liquor Cooked a t 180' C.
E
5 p - - - 7 ? - : 1
0 5
I
? O 140
170 TEMPERATURE-
I80 DEGREES CENTIGRADE
? 190
Figure 3. Effect of Temperature on Yields a t 4 Per Cent Sodium Hydrosulfide Concentration in Liquor Cooked for 2 Hours
of the extracted residue is about 2 per cent in excess of what it should have been on the basis of the material removed by extraction. This may be explained as adsorption or chemical combination of the alcohol by the pulp. Table I1 shows that the yield of the ether-extractable oil varies directly with the amount of lignin unaccounted for. I n other words, as the material isolated as lignin disappears from the lignin balance, the amount of ether-extractable oil increases. Table IIIA and Figure 2 indicate that sodium hydrosulfide concentrations greater than about 7 Der cent in the cook liquor do not griatly affect the yields*of the various
INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1943
719
Table 111, B and C, and Figures 3 and 4 indicate that the TABLEI. EFFECT OF SODIUM HYDROSULFIDE CONCENTRATION cooking time does not greatly affect the yield of products, IN COOKING LIQUOR ON ALCOHOL EXTRACTIVES AND LIQNIN with the exception of the water-insoluble pitch which inCONTENT OF RESIDUAL WOOD creases slightly with increming time, apparently a t the coho1 Extractivesa expense of the nonvolatile ether-extractable oil. An increase Difference (gain in Lignin in Residual Tooda Basis loss Basis in temperature causes a slight increase in the yield of etherwt. of Before After in wt. of wt. of extractable nonvolatile oil and water-insoluble pitch. Howextrac- Difference, residual extracCook residual extractive, % wood), % tion, % tion, % No. wood, % % ever, Figure 5, which shows the interrelation between tem17
18
19 20 21 0
-- 0.736 0.52
2.24C 2.31 2.27 2.56 2.24
1.51 1.79 2.47 5.68 12.35
+ 0.20 + 3.21 +10.10
6.31 6.56 8.93 13.78 27.65
4.88 4.92 6.86 9.25 15.75
1.43 1.64 2.07 4.53 11.90
Calculated on the basis of oven-dry unextracted residual wood.
a Negative loss indicated a n actual inirease in weight on extraction.
Algebraic difference between weight o[ extractive and loss in weight.of extracted residual wood shows an actual gain in weight of extracted material over what it should have been on the basis of extractives removed. c
TABLE11. RELATION BETWEEN LIGNINUNACCOUNTED FOR AND TOTAL ETHER EXTRACTIVES AT DIFFERENT CONCENTRATIONS OF SODIUM HYDROSULFIDE IN COOKINQ LIQUOR Cook No. NaSH concn., % Lignin, grams In original wood Recovered Unaccounted for (B) Total ether ext. (A), grams Ratio, A’/B
17 1
18 19 20 0 7 4 2
19.3 19.3 19.5 6.4 8 . 3 8.0 12.9 11.0 11.5 27.1 28.0 23.2 2.1 2.5 2.0
21 0
19.5 19.5 11.8 17.0 7.7 2.6 17.5 6.5 2.3 2.4
products. The maximum yield of ether-extractable oil was obtained when the sodium hydrosulfide concentration was about 10 per cent; this fact brings to mind the much discussed “optimum sulfidity” of kraft liquors, and would indicate a possible relation between this value and such an optimum. The most commonly reported values for optimum sulfidity seem to lie in the range of about 30 per cent which, according to Hagglund @), corresponds to a concentration of about 16 per cent sodium sulfide in the cookipg liquor.
Figure 5.
Three-Dimensional Relation between Figures 2 and 3
TABLE 111. EFFECTOF SODIUMHYDROSULFIDE CONCENTRATION, TEMPERATURE, AND TIXEON YIELDSOF PRODUCTS FROM ALCOHOL-BENZENE EXTRACTED WOOD Cock No.
NaSH Concn.
%
17 18 19 20 21
lo 7 4
22 23 24 19
Temp., C. 160 170 180 190
19 25 26
Time. Hr. 2 4 6
2 0
Volatile Oil
Grams
0.9ga 1 . 1 1.17 1.15 1 . 1 1.15 0 . 0 0.0 0.9
1.1
B. 0.6 0.9 1.1 1.3 C.
0
%
Nonvolatile Oil
Grams
%
WaterInsol. Pitch Cfroms
%
Total Ether Ext.
Drams
%
Residual Wood
Grams
%
A . Cooking Temperature 180° C., Time 2 Hours 22.12 23.4 5.0 5 . 3 27.1 28.7 50.0 52.8 22.7 24.1 5.3 5.6 28.0 29.6 51.0 53.8 18.0 18.9 5.2 5.4 55.0 57.5 23.2 24.2 15.0 16.7 2 . 5 2.6 62.5 65.5 17.5 18.3 6.1 6.4 0.0 0.0 6.1 6.4 63.0 66.0
Lignin Residue
Na. Salts of Volatile Acids
3.2 3 . 4 4.9 5 . 2 3.1 3.2 3.2 3.4 0.0 0 . 0
3 . 0 3.18 3 . 1 3.3 3.2 3.4 2.0 2.1 2.4 2 . 5
5 . 5 5.8 5.7 5.9 3.1 3.2 3.4 3.5
2.6 3.5 3.2 3.7
3.1 3.2 3 . 1 3.2 2.8 2.9
3 . 2 3.4 3.0 3.1 3.1 3.3
Grams %
Grams %
Sodium Hydrosulfide Concentration 4 Per Cent. Cooking Time 2 Hours 0.63 0.94 1.15 1.36
14.0 15.2 18.0 19.8
14.7 15.9 18.9 20.7
1.S 1.4
4.1 5.2 4.6
4.3 6.4 4.8
15.3 15.8 19.3 20.2 23.2 24.2 24.4 25.6
64.0 67.0 59.0 61.8 55.0 57.5 53.0 55.5
2.7 3.6 3.4 3.9
Sodium Hydrosulfide Concentration 4 Per Cent, Temperature 180° C.
1 . 1 1.15 1 . 2 1.25 1 . 1 1.15
18.0 18.7 15.0
18.9 19.6 15.7
5.2 4.0 6.9
5.4 4.2 7.2
23.2 24.2 21.7 22.7 21.9 22.9
55.0 57.6 53.0 55.5 52.0 54.5
All percentages are calculated on the basis of oven-dry alcohol-benzene extracted wood.
Calculating this value in terms of hydrosulfide gives a value of 11.5 per cent, which is in fair agreement. It is apparent, therefore, that the appearance of the ether-extractable oil is in some way directly related to the action of sulfide in the kraft liquor, especially since a similar material can also be isolated from commercial black liquors.
perature and concentration of cook liquor, would lead us to predict that, if the temperature-concentration surface were defined in the higher ranges, a yield of slightly less than 30 per cent of ether-extractable material would be the maximum obtainable under any conditions. It will be interesting to see whether such a prediction is confirmed in fact, and
LIGNIN IN RESIDUAL WCK)D 0 ALCOHOL- E X T R A C T E D @ UNEXTRACTED
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Let us accept the not uiireasonable proposition that lignin exists in wood, not in the form in which it is commonly described, but as a labile complex of monomeric lignin units. It is further reasonable to propose that the increased efficiency of the kraft liquor over nonsulfide liquors is due to the presence of sodium hydrosulfide resulting from the hydrolysis of sodium sulfide, and that this hydrosulfide breaks down the primary lignin complex to give sulfurcontaining oils which are immediately soluble in the alkaline liquor. These oils are then polymerized by the action of the excess of alkali a t the elevated temperature of the cook to give the lignin as it appears in the black liquor. CONCLUSIONS
1. The nature of sodium hydrosulfide used in making up the cooking liquor has a profound effect on the cooking process. 2. Aspen wood is not pulped by cooking with solutions of pure sodium hydrosulfide under the conditions described. 3. A part of the residual wood from such cooks is soluble in alcohol, the quantity of alcohol-soluble material decreasing with increasing concentration of sodium hydrosulfide in the cooking liquor. 4. The lignin value of the alcohol-extracted residual wood decreases in direct relation n-ith the amount of extractives removed (Figure 6). 5 . Steam-volatile sulfur-containing oil is present in the black liquors resulting from cooks made mith aqueous solutions of pure sodium hydrosulfide under the conditions described (Figure 7). 6. Nonvolatile ether-extractable oil is present in the acidified black liquor from cooks made with aqueous solutions of pure sodium hydrosulfide under the conditions
sulfide concentration of about 10 per cent in the cooking liquor. 8. An increase in cooking time, at 180" C. and 4 per cent sodium hydrosulfide in the cooking liquor, does not appreciably affect the yields of various fractions, with the exception of water-insoluble pitch which increases with time, apparently a t the expense of the nonvolatile oil. 9. An increase in cooking temperature from 160" to 190" C. decreases the yield of residual wood and lignin residue, but increases the yield of ether-extractable oil and volatile acid. 10. A mechanism may be proposed for the action of sulfide in the kraft process: Sodium hydrosulfide is the ac-
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VOLATILE
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OL
OVARIATION WITH CONC COOK LIQUOR OVARIATION W I T H T E M P
0
TEMPERATURE- DEGREES CENTIGRADE
170
2
'90
4 m N c . COOKING Q LIUOR-
Figure 7.
6
PER CENT N& S H
190 8
10
Yields of Volatile Oils
by polymerization of these units through the further action of the excess alkali to giye an alkali-soluble amorphous thiolignin. LITERATURE CITED
(1) dhlm, C. E., P a p e r Trade J.,113, S o . 1 3 , 175 (1941). (2) Aronorsky, S. I., and Gortner. R . A . , ISD. EXG.CHEM.,26, 61 (1934). (3) Hagglund, E., arid Hedlund, R., Papier-Fabr., 50, 49-53, 61-5 (1332). (4) Hanson, F. S., Tech. Assoc. Papers 23, 163 (1341). ( 6 ) Hossfeld, R., ISD. ESG.CHEM.,hs.4~. ED., 14, 118 (1842). (6) Rule, A,, J . Chem. Soc., 39, 558 (1911).
PAFER2034, Minnesota Agricultural Experiment Station. Ph.D. thesis of Ralph L. Hossfeld.
Condensed from
