GB892277A - Continuous digester - Google Patents

Abstract

892,277. Continuous digesters. BAUER BROS. CO. Nov. 29, 1960 [Nov. 30, 1959], No. 41055/60. Class 64(1) A continuous digesting apparatus for fragmentary fibrous material such as wood chips consists of a vertical digester vessel with an inlet and outlet at the top and bottom respectively, means for continuously supplying the wood chips to the inlet, means for supplying cooking liquor and steam to the upper part of the vessel to raise the temperature and pressure within, and means for supplying liquor under pressure into the lower region of the vessel to force the wood pulp through the outlet under pressure. A closed drainer device is in communication with the outlet and separates the liquid from the wood pulp. The system control includes a control over the quantity of pulp to be produced, the cooking temperature, the time of cook, the quantity of cooking liquor and its concentration. In the figures, the movement of solids and fluids through the system is indicated by solid lines. Broken lines denote electrical connections. A chip distributing conveyer 17 supplies the untreated wood chips to conveyer 16. A variable speed motor 55 drives a rotary screw 18 which feeds the wood chips to a rotary inlet valve 15, running at a predetermined speed, any overflow being discharged at 19 for return to the conveyer 17. The valve 15 registers with the inlet fitting 11 at the top of the cylindrical digester vessel 10. Steam and cooking liquor is supplied via inlets 14 and 13 respectively to the upper interior of the vessel 10 where the liquid level is kept sensibly constant at 22. The wood chips form a column in the vessel 10 which falls under gravity when the pulp is discharged through outlet 12. Nozzles 23 and 24 supplied with liquor, serve to dilute the pulp and sluice it through the outlet 12. Supplementary nozzles 88 and 89 are used when needed to speed up the sluicing. Conduit 26 connects the outlet 12 with a closed body 27 forming part of a drainer device 28. A rotary screw conveyer 31 conducts the pulp to a rotary outlet valve 35, whilst the liquor is drained off at an outlet 32 and fed back into the system via line 38. Any excess pulp and liquor mixture is fed back into the digester via line 52. The quantity of production depends on the volume of chips being fed to the system. This volume is selected by adjusting the speed of the motor 55, and may be effected manually or automatically in response to changes in the chip level in the digester vessel 10. The volume flow of chips to the digester is also related to the quantity flow of liquor to a supply tank 73, there being a connection 113 from a chemical liquor flow indicator 103 to a control indicator 57. White liquor from a make-up tank 92 of a known or measured concentration is pumped through a quantity control valve 99 to the tank 73. The liquid level in this tank is maintained by the addition of black liquor from a tank 71 through the operation of a transmitter 112 measuring the level and a valve 111. The cooking temperature within the digester is measured by a thermal means 75 which controls a valve 79 in a supplementary supply line 77 to the inlet 13. Heat exchangers 76 and 78 supplied with steam, raise the temperature of the incoming liquor to a required amount. A chip level sensing means 58 receives gamma rays from an emitter 59 and sends signals to the control indicator 57 and a valve 86 in the supplementary nozzles' line 87. Sensing the liquid level within the digester is a device 62 which ultimately controls valves 67 and 68 via an indicator 64.