GB945958A - Continuous pulping process - Google Patents

Abstract

945,958. Digesters. ASSOCIATED PULP & PAPER MILLS Ltd. Dec. 29, 1960 [Feb. 8. l960], No. 44571/60. Heading B1X. [Also in Division D2] Pulp is made from wood or other cellulosebearing material by the alkaline processes (see Division D2) in a continuous digester in which (Fig. 1) wood chips are fed from hopper 11 via chip-meter 12 and low pressure feeder 12<SP>1</SP> to a steaming vessel 13 fitted with a screw conveyer 13<SP>1</SP> and then through a high pressure feeder 14 and pipe line 15 to point 16, at the upper end of digester 10. The chips are then fed downwardly into the digester via the rotating screw 19<SP>1</SP> within the cylindrical screen 19<SP>2</SP> To assist in the feeding of the chips, liquor is withdrawn from the digester at outlet 45 and via pipe line 17 to feeder 14. Cooking liquor is supplied through pipe line 30, pump 31, pump 26, and steam heater 27 and inlet 22 to the cooking zone of the digester, i.e. the part between the sets of screens 20 and 23 and may be circulated via liquor outlet 25 until the required temperature, e.g. 150-180‹ C., is reached. The end point of the cook is determined by testing a pulp sample taken at test cock 32. Black liquor is then introduced under pressure from pump 44 and passes via inlet 42 upwardly into the diffusion-displacement zone of the digester, i.e. the part between screen 23 and the base. Meanwhile, black liquoris withdrawn through nozzle 21. Cooking liquor is meanwhile introduced continuously via 30, 31, 26, 27, 29 and 22, withdrawn through 24 and recirculated through 27, thus maintaining the cooking temperature in the digester. Discharge of pulp is then commenced via outlet 33, black liquor being separated at concentrator 38 and recirculated via pipe line 39, valve 40 and inlet 37. Liquor balance in the digester is achieved by control of operations such that the rate of discharge of black liquor in gallons per minute at 21 is equal to or slightly greater than the sum of (U+S+M+W), where U is the rate of upflow of black liquor in the diffusiondisplacement zone of the digester, S is the rate of introduction of condensed steam with the wood, M the rate of introduction of moisture in the wood, and W the rate of introduction of cooking liquor. To compensate for variations in digester pressure, an inflow of black liquor is arranged through inlet 48 which is controlled by a pressure controller 49 connected to the digester at 50 and responsive to changes in digester pressure. In Fig. 2 water or aqueous acid solution may be introduced at a point 46 above the black liquor outlet 21, heated to the required temperature by an external steam heater (not shown), and passed upwardly through the digester countercurrent to the downward flow of wood chips and discharged at point 47 near the point of entry of the wood. In a further modification (not shown) the flow of liquor and wood may be concurrent during prehydrolysis. Specification 945,957 is referred to.