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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.
I N D U X T R I A L A N D ENGINEERING CHEMISTRY
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145
d
E $ 4
/
2
3
4
5
6
D
7
/
FIG.1
5
6
7
FIG.2
for any given-conditions. It will be noted that in the usual batch process a t the start the reacting substances are large while the product formed is small, whereas just the reverse is true at the end. We would, therefore, expect extremely rapid digestion of the wood at the start, and very slow cleaning up of the lignins a t the end. An examination of the curve of concentrations os. time in Fig. 1 shows the actual results. Fig. 2 gives the rate of caustic soda consumption as experimentally. determined by Sutermeister.6
PREVIOUS EXPERIMENTAL WORK An examination of these curves cannot but lead one to wonder if a process of countercurrent digestion, in which the caustic concentration would be highest a t the end where the lignin concentration is lowest, might not greatly increase the capacity of a digester. The fact that cellulose is attacked by caustic must be seriously considered, but by suitable time, Iemperature, and concentration control this might be kept a t a minimum. In a review of the literature we find that Ungerer,' an Austrian chemist, noted that certain lignin compounds, which were soluble a t low temperatuie during the early part of the cook, were again precipitated at the higher temperatures existing a t the end of the digestion. I n 1872 he secured a British patent, the chief object of which was to obtain a better bleaching pulp by removing the dissolved lignins by means of a countercurrent process. For that purpose he proposed to use nine digestem in series, the temperature of each being successively higher and the caustic being forced from one to the other until spent. There is nothing in the patent to indicate that he appreciated any other advantages that might result from the application of this principle. No record of the commercial application of his patent was found, which is not surprising when one considers the large British Patent 257 (1872).
4
3
number of factors which he proposed to vary and the probable unreceptive attitude of the industry to such radical changes a t that time. I n 1921 Clark and Cook* began experimental work to determine the effect of a countercurrent process on time of digestion and yield of pulp. For their purpose a small experimental digester shown in Fig. 3 was used. The digester consisted of a 5-foot section of &inch steel pipe, A , fitted with a removable flange cover and reducing coupling. The chips were charged through the cover, the caustic was added through the trap G a t the same rate the solution was drawn off through J . Recirculation was effected by the centrifugal pump D. The solution was heated by burners about the pipe E,steam being used only to heat up the digester. The charge was blown through the cock F to a tank, where it was washed, sampled, and weighed. The pressure was reduced through G and H. Clark and Cook first ran a series of batch cooks, of which Fig. 1 is an example; and then a series of countercurrent cooks. The net result of their work showed that digestion could be completed in one-half the time required by the batch process, but that the yield averaged 8 per cent lower, the extremes being 4.1 and 13.0 per cent. Examination of the pulp showed that even with such a decrease in time overcooking had occurred. It was evident that considerable experience would be SKETCH of fl P P R R R T U 5 necessary t o determine the exact end conditions in order to prevent a serious loss of cellulose. Their work was continued the next year by Wagner, Sippel, and Miskellylg who likewise found a time reduction of one-third to one-half over the batch process and a yield averaging 4.4 per cent lower. It is important to note that the a-cellulose in the pulp from the countercurrent digestion averaged 91.7 per cent as compared with 86.9 per cent
FIG.3
8
M. I. T. Undergraduate Thesis,
i921. @
M. I. T.Thesis. 1 9 U .
I N D U S T R I A L A N D ENGINEERING CHEMIXTRY
146
from the batch process. For special purposes where a high acellulose content is very desirable, such a process might be a distinct advantage, for not only would the troublesome p- and y-cellulose be removed, but the yield, based on a-cellulose, would be only 1.5 per cent lower than the batch process, 43.7 per cent as compared with 45.2 per cent. I n all cases the pulp from the countercurrent process was found to be much superior in color and bleaching quality, confirming the observations of Ungerer. /#O
6 ' 0
L 9,
>
.=60 &
%
$40
3
8
",
m
kl 0
/
2
3
FIG.4
As examination of the curves in Fig. 4 shows that the concentration of caustic soda at the end of the countercurrent cook was much higher than a t the beginning of the batch cook-in fact, accounting for a 2 per cent decrease in yield based on Sutermeister's6 investigations. The actual decrease in yield in the cooks represented by the curves was 2.4 per cent, the a-cellulose content of the countercurrent pulp was 5.9 per cent higher, and the Tinglelo bromine figure 0.40 as compared with 0.27 for the batch process. It would appear 10
THISJOURNAL, 14, 40 (1922).
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Vol. 16, No. 2
entirely possible to bring the yield up to standard by less drastic conditions at the end.
COMMERCIAL APPLICATIONS The commercial application of such a process can best be understood by a study of the plant layout shown in Fig. 5. The six digesters A to F are in ring series. Let us say that F is being filled with chips and that digestion is at progressive stages of completion in the others until A , which is being blown, is reached. The wood in B, being nearest complete digestion, would receive strong, fresh caustic, which passes through C, D, and E, until i t is drained a t E and passed through the recovery system shown. When B is blown, F becomes the last digester in the liquor cycle, C becomes the first, while A is filled with chips and the cycle repeated. The difficulties which beset the commercial adoption of such a process are apparent, especially to those familiar with the industry. It would be unwise t o attempt plant operation without going through an intermediate stage in which a number of small digesters, such as the one shown, are used to determine more detailed information in regard t o circulation and concentration of liquor, temperature, and time of digestion. Such experimentation would serve to train operators for the commercial unit and prevent costly mistakes through overcooking at large charges later on. The extra piping, storage tanks, and pumps required should not be excessive if well-balanced operation is maintained. One objection which might be raised is the loss of caustic due to blowing a digester just charged with fresh liquor. If the tank system of washing pulp were used, this loss would indeed be serious, but it is thought that systems employing continuous filters of the drum type or continuous centrifugals would make possible the recovery of this caustic with very little dilution. The most serious question is that of yield, for even a 1 per cent decrease would represent a large yearly loss. It is encouraging to note that in the second investigation the average loss was decreased 50 per cent, and had the final treatment not been so drastic the yield would probably have been much better. It might conceivably pay to maintain a margin of safety by slightly undercooking the wood, the screening being returned to the digester to prevent excessive loss. As the bleaching quality of the pulp is much improved anyway, the increase in bleach consumption due to undercooking would be no detriment. Experience has shown that wherever a batch process has been changed t o a continuous process, almost invariably costs are reduced, and better quality and uniformity of product result because of the standardization of operation required. The increased capacity of the digester room, one of the most expensive units of a mill, the higher a-cellulose content and better bleaching quality of the pulp, the more concentrated and uniform strength of black liquor to the evaporators, and the more complete utilization of soda are advan-. tages which invite the industrial application of this process.
Colloid Symposium Monograph The papers and discussions presented a t the First Annual Colloid Symposium, held at Madison, Wis., in June, 1923, have been published by the Department of Chemistry of the University of Wisconsin. The monograph comprises 420 pages, and is one of the most important books on the subject of colloids ever published in this country, The twenty-two papers cover a wide range of theoretical considerations and applications to medicine and the industries. Copies may be obtained a t cost of publication ($2.75 per copy, including postage) from the University Cooperative Co., Madison, Wis., to whom all remittances should be made. As the edition is limited, those contemplating purchase of copies should place orders promptly.
