Patent GB-669852-A (GB)
Title: Manufacture of pulp from wood and other cellulosic fibrous materials
IPC: D21C7/14
Filed: 1949-05-05
Abstract: <PICT:0669852/IV (a)/1> Cellulose pulp is obtained from wood or other cellulosic fibrous material by filling a digester with chips of the material; pre-heating an alkaline cooking liquor under pressure to at least a predetermined cooking temperature which is above 145 DEG  C., this liquor having a predetermined effective Na2O concentration not substantially in excess of 20 grams per litre; charging the liquor into the chip-filled digester under pressure and continuously circulating a quantity of liquor, in excess of that required to fill the digester, through the digester, then through a reservoir of similar liquor separate from the digester, and back to the digester; continuously replenishing the Na2O concentration and heat content of the liquor so circulated to bring them to the determined values; and continuing the circulation of the liquor until the reaction of the liquor on the chips is completed, and then blowing the digester. Preferably the active Na2O concentration should not be allowed to fall below 4 grams per litre and for best results it should be between 10 and 20 grams per litre when it enters the digester.  If desired a plurality of digesters may be used with a common reservoir or accumulator and the operation may be modified by filling the digesters successively with chips, placing the digesters in operation sequentially by bathing the chips in each digester as it is placed in operation in a continuously flowing stream of cooking liquor, the stream for each digester being caused to circulate through the digester to the common accumulator and back to the digester, the quantity of liquor so circulated being in excess of that necessary to completely fill the chip-containing digesters which are in operation at any given time.  The digesters are blown sequentially after completion of the cooking operations.  Black liquor is separated from the pulp and a portion of the black liquor is added to the circulating cooking liquor.  White liquor is also added to maintain the Na2O concentration at the required value.  As shown, digesters 1-5 each communicate with accumulator 6 by pipe 7.  The return flow from the accumulator passes to the digesters by pipe 8 and offtakes 1a-1e, each of which includes a pump 9 and a heater 10.  Fresh white liquor is supplied through pipe 11 and black liquor enters through pipe 11a. The supply of white and black liquor may be adjusted automatically.  As shown, the controller 12 measures the conductivity of cooking liquor from pipe 8 and controls it at a fixed value by actuating valve 12b; controller 13 measures the level of liquid in accumulator 6 by differential pressure and controls it by actuating valve 13b.  When starting the operation, the cooking liquor may be heated to the desired temperature by circulating it through pipe 8, heater 10, and pipe 15.  The cooking temperature may be as high as 175 DEG  C.  The cooking liquor may contain sodium sulphide as well as caustic soda and the effective Na2O concentration is calculated on the total concentration of caustic soda and sodium sulphide.