Process and apparatus for treating green liquor

Abstract

A method of and apparatus for treating a green liquor stream produced in a kraft pulp process to separate green liquor from dregs present in the stream by utilizing a filtration vessel that has a pressure differential between the green liquor inlet and outlet to drive the dregs from the green liquor. In addition, negative pressure is utilized in the filtration vessel to thereby drive water vapor from and consequently cool the filtered green liquor.

1 . A method of treating a stream containing green liquor and solid dregs that is produced in a kraft pulp process, comprising
 (a) directing the stream to a filter unit for filtering dregs from the stream having an inlet for the stream, an outlet for filtered green liquor and an outlet for dregs, wherein the unit has at least one filtration element and a pressure differential between the filtered green liquor outlet and the inlet, with the filtered green liquor outlet being maintained at a lower pressure than the inlet, to thereby facilitate the removal of dregs from the green liquor by filtration; and   (b) maintaining that portion of the filter unit downstream from the filtration element at a negative pressure to thereby drive water vapor from and cool and concentrate the filtered green liquor.   
     
     
         2 . The method of  claim 1  comprising the further step of directing the filtered green liquor and water vapor to a separator vessel in which the vapor is separated from the green liquor. 
     
     
         3 . The method of  claim 2  wherein the separator vessel is maintained at a negative pressure. 
     
     
         4 . The method of  claim 2  further comprising utilizing the cooled green liquor in a recausticizing step to generate white liquor. 
     
     
         5 . The method of  claim 1  wherein the pressure differential between the green liquor inlet and green liquor outlet ranges from about 0.2 Bar to about 1.8 Bar. 
     
     
         6 . The method of  claim 5  wherein the pressure differential ranges from about 0.5 to about 1.5 bar. 
     
     
         7 . The method of  claim 1  wherein the negative pressure at the green liquor outlet will range from just below atmospheric to about negative 0.7 bar. 
     
     
         8 . The method of  claim 2  wherein the water vapor removed from the mixture is condensed back into water. 
     
     
         9 . The method of  claim 8  wherein at least a portion of said condensed water is combined with the separated green liquor downstream from the separator vessel and prior to the recausticizing step. 
     
     
         10 . The method of  claim 8  wherein none of said condensed water is combined with the separated green liquor. 
     
     
         11 . The method of  claim 8  wherein at least a portion of the condensed water is directed to the filter unit and is utilized as dregs cake wash to wash dregs separated from the green liquor and is thereafter discharged from the filter unit. 
     
     
         12 . A method of treating a green liquor stream produced in a kraft pulp process to separate green liquor from the stream for use in a recausticizing step to generate white liquor, comprising
 (a) directing the stream to a filter unit having a green liquor inlet and a green liquor outlet, wherein the unit has a pressure differential between the green liquor inlet and green liquor outlet to facilitate the removal of dregs from the green liquor by filtration, with the green liquor outlet being maintained at a negative pressure to thereby drive water vapor from and cool the filtered green liquor;   (b) directing the filtered green liquor and water vapor to a separator vessel in which the vapor is separated from the green liquor; and   (c) condensing the water vapor removed from the green liquor back into water.   
     
     
         13 . The method of  claim 12  wherein at least a portion of the condensed water is combined with the green liquor downstream from the separation step and prior to the recausticizing step. 
     
     
         14 . The method of  claim 12  wherein none of the condensed water is combined with the green liquor. 
     
     
         15 . The method of  claim 12  wherein at least a portion of the condensed water is directed to the filter unit and is utilized to wash dregs separated from the green liquor and is thereafter discharged from the filter unit. 
     
     
         16 . The method of  claim 12  wherein the pressure differential between the green liquor inlet and green liquor outlet is about one bar. 
     
     
         17 . The method of  claim 12  wherein the negative pressure at the green liquor outlet will range from just below atmospheric to about negative 0.7 bar 
     
     
         18 . The method of  claim 12  wherein the separator vessel is maintained at a negative pressure. 
     
     
         19 . Apparatus for use in a recausticizing process for separating green liquor from dregs comprising
 a filtration vessel containing a green liquor inlet and a green liquor outlet;   means to maintain a pressure differential between the green liquor inlet and the green liquor outlet to facilitate the removal of dregs from the green liquor by filtration;   means to maintain the green liquor outlet at a negative pressure to thereby drive water vapor from and cool the filtered green liquor;   means to remove the filtered dregs from the filtration vessel; and   means to remove the water vapor and the filtered green liquor from the filtration vessel.