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
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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
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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.
