Processes and systems for handling knots in a chemical pulping process

Abstract

Processes and systems are provided for cooking wood chips containing knots in a digester to produce a chemical pulp. Specifically, a low pressure slurry of comminuted cellulosic fibrous material is formed in a vessel operating at essentially atmospheric pressure by mixing therein a liquid and the comminuted cellulosic fibrous material. The low pressure slurry is thereafter supplied to a low pressure inlet of a high pressure transfer device while liquid is removed from the slurry through a low pressure outlet thereof. A high pressure liquid is supplied to the transfer device (e.g., via a slurry pump) so as to form a high pressure slurry of the comminuted cellulosic material which is then discharged from a high pressure outlet of the transfer device. Knots are removed via a knot separator from a slurry of uncooked knot-containing cellulosic material at essentially atmospheric pressure. The removed knots are thereafter transferred to the vessel so that the knots become part of the low pressure slurry supplied to the low pressure inlet of the transfer device.

What is claimed is:  
     
         1 . A process for cooking wood chips containing knots in a digester to produce a chemical pulp, comprising the steps of: 
 (a) forming a low pressure slurry of comminuted cellulosic fibrous material in a vessel operating at essentially atmospheric pressure by mixing therein a liquid and the comminuted cellulosic fibrous material;    (b) supplying the slurry to a low pressure inlet of a high pressure transfer device while removing liquid from the slurry through a low pressure outlet thereof;    (c) supplying a high pressure liquid to the transfer device and forming a high pressure slurry of the comminuted cellulosic material;    (d) discharging the high pressure slurry from a high pressure outlet of the transfer device,    (e) removing knots from a slurry of uncooked knot-containing cellulosic material at essentially atmospheric pressure, and thereafter transferring the removed knots to the vessel so that the knots become part of the low pressure slurry supplied to the low pressure inlet of the transfer device.    
     
     
         2 . The process of  claim 1 , comprising using a knot drainer to remove the knots from the slurry of uncooked comminuted cellulosic material.  
     
     
         3 . The process of  claim 1 , wherein the vessel includes a chip chute, and a metering device which discharges a metered amount of the comminuted cellulosic material to the chip chute, and wherein step (e) includes the step of introducing the removed knots to the low pressure slurry at the chip chute.  
     
     
         4 . The process of  claim 1 , comprising recirculating at least a portion of the liquid removed from the slurry at the low pressure outlet of the transfer device to the vessel.  
     
     
         5 . The process of  claim 4 , wherein the vessel includes a chip chute, and a metering device which discharges a metered amount of the comminuted cellulosic material to the chip chute, and wherein step (e) includes the step of introducing the removed knots to the low pressure slurry at the chip chute.  
     
     
         6 . A system for producing a chemical pulp from a slurry of knot-containing cellulosic material, comprising: 
 (a) a digester for pressurized cooking the knot-containing cellulosic material;    (b) a vessel operating at essentially atmospheric pressure for forming a low pressure slurry of comminuted cellulosic fibrous material;    (b) a high pressure transfer device which includes a low pressure inlet, a low pressure outlet, a high pressure inlet and a high pressure outlet, wherein the low pressure inlet receives the low pressure slurry from the vessel and the low pressure outlet allows liquid to be removed from the slurry;    (c) a slurry pump for supplying a high pressure liquid to the transfer device through the high pressure inlet and thereby form high pressure slurry of the comminuted cellulosic material which is discharged through the high pressure outlet and supplied to the digester;    (e) a knot separator for removing knots from a slurry of uncooked knot-containing cellulosic material at essentially atmospheric pressure, and thereafter transferring the removed knots to the vessel so that the knots become part of the low pressure slurry supplied to the low pressure inlet of the transfer device.    
     
     
         7 . The system of  claim 6 , wherein said knot separator includes a knot drainer to remove the knots from the slurry of uncooked comminuted cellulosic material.  
     
     
         8 . The system of  claim 6 , wherein the vessel includes a chip chute, and a metering device which discharges a metered amount of the comminuted cellulosic material to the chip chute, and wherein step (e) includes the step of introducing the removed knots to the low pressure slurry at the chip chute.  
     
     
         9 . The system of  claim 6 , comprising a recirculation conduit for recirculating at least a portion of the liquid removed from the slurry at the low pressure outlet of the transfer device to the vessel.  
     
     
         10 . The process of  claim 9 , wherein the vessel includes a chip chute, and a metering device which discharges a metered amount of the comminuted cellulosic material to the chip chute, and wherein the knots removed by said knot separator are introduced to the low pressure slurry at the chip chute.