Method and System for Semi-Chemical Pulping

Abstract

Method and apparatus for minimizing adherence of lignin to an extraction screen in a semi-chemical pulping process. The method and apparatus generally relate to supplying white liquor and NaOH to as to increase the pH of cooking liquor at or near an extraction screen.

1 . A method for inhibiting fouling of a black liquor extraction screen in a cooking vessel for semi-chemical pulping, the method comprising the steps of:
 feeding lignocellulosic material and green liquor to the cooking vessel for semi-chemical pulping;   cooking the lignocellulosic material in the cooking vessel for semi-chemical pulping such that some or all lignin is separated from the lignocellulosic material, and wherein lignin is dissolved into a cooking liquor;   extracting via an extraction screen black liquor comprising dissolved lignin, wherein the black liquor is formed during the semi-chemical pulping process from the cooking vessel; and   adding white liquor or a base to the cooking vessel at or near the extraction screen, such that a pH at or near the extraction screen is elevated and inhibits lignin from adhering to the extraction screen.   
   
   
       2 . The method of  claim 1 , wherein the step of adding white liquor or a base comprises adding white liquor and a base to the cooking vessel. 
   
   
       3 . The method of  claim 1 , wherein the base comprises NaOH. 
   
   
       4 . The method of  claim 1 , wherein the step of feeding lignocellulosic material to the cooking vessel further comprises feeding at least one of NaOH, Na 2 CO 3 , Na 2 SO 3  or white liquor to the cooking vessel. 
   
   
       5 . The method of  claim 1 , wherein the step of adding white liquor or the base to the cooking vessel at or near the extraction screen occurs near the end of a cook when most lignin has been extracted from the lignocellulosic material. 
   
   
       6 . The method of  claim 1 , wherein the step of adding white liquor or the base to the cooking vessel at or near the extraction screen occurs using a central pipe running through the center of the cooking vessel. 
   
   
       7 . The method of  claim 1 , wherein the step of adding white liquor or the base to the cooking vessel at or near the extraction screen occurs using a circulation loop comprising a mixture of white liquor or NaOH and black liquor comprising dissolved lignin. 
   
   
       8 . The method of  claim 1 , wherein the step of adding white liquor or the base to the cooking vessel at or near the extraction screen creates a localized elevated pH at or near the extraction screen higher than the pH of the cooking liquor above the extraction screen. 
   
   
       9 . The method of  claim 1 , wherein a pH of a cooking liquor at an end of a cook is 10 or higher 
   
   
       10 . The method of  claim 1 , wherein a pH of a cooking liquor at an end of a cook is between 10 and 13. 
   
   
       11 . The method of  claim 1 , wherein a pH of a cooking liquor at an end of a cook is between 10 and 12. 
   
   
       12 . A cooking vessel for a process of semi-chemical pulping of lignocellulosic material that inhibits fouling of a black liquor extraction screen in the cooking vessel, the cooking vessel comprising:
 a first input conduit to the cooking vessel that carries green liquor;   a second input conduit to the cooking vessel that carries lignocellulosic material;   a cavity in which lignin is separated from the lignocellulosic material and dissolved into a cooking liquor;   an extraction screen connected to an extraction conduit that extracts black liquor comprising dissolved lignin, wherein the black liquor is formed during the semi-chemical pulping process from the cooking vessel; and   a third input conduit for delivering white liquor or a base to the cooking vessel at or near the extraction screen, such that a pH at or near the extraction screen is elevated and inhibits lignin from adhering to the extraction screen.   
   
   
       13 . The cooking vessel of  claim 12 , wherein the third input conduit delivers white liquor and a base. 
   
   
       14 . The cooking vessel of  claim 12 , wherein the base comprises NaOH. 
   
   
       15 . The cooking vessel of  claim 12 , wherein the first and second input conduits comprise a single input conduit. 
   
   
       16 . The cooking vessel of  claim 12  further comprising a fourth input conduit for feeding at least one of NaOH, Na 2 CO 3 , Na 2 SO 3  or white liquor to the cooking. 
   
   
       17 . The cooking vessel of  claim 12 , wherein the extraction screen is located near the end of a cook where most lignin has been extracted from the lignocellulosic material. 
   
   
       18 . The cooking vessel of  claim 12 , wherein the third input conduit comprises a central pipe running through the center of the cooking vessel. 
   
   
       19 . The cooking vessel of  claim 12 , wherein the third input conduit comprises a circulation loop for recirculating a portion of the black liquor with a mixture of white liquor or the base. 
   
   
       20 . The cooking vessel of  claim 12 , wherein the third input conduit is located at or near the extraction screen so as to create a localized elevated pH at or near the extraction screen higher than the pH of the cooking liquor above the extraction screen.