Bleaching of medium consistency pulp with ozone without high shear mixing

Abstract

Methods are disclosed for bleaching medium consistency cellulose pulp including generating a stream of ozone-containing gas having an ozone concentration of at least 10% by weight and radially injecting the stream of ozone-containing gas into a stream of cellulose pulp flowing through a reactor so as to provide a stream of bleached cellulose pulp. A low to medium intensity mixer operating at no more than 800 rpm may be used, downstream from the injection site.

1 . A method for bleaching medium consistency cellulose pulp comprising providing a stream of ozone-containing gas and radially injecting the stream of ozone-containing gas in a single ozone addition step into a stream of cellulose pulp flowing through a reactor so as to provide a stream of bleached cellulose pulp, wherein said reactor operates in plug flow mode at a pressure of at least 3 bar and at a temperature of 50° C. or less and said reactor does not include a high shear mixer.  
     
     
         2 . The method of  claim 1  wherein said ozone-containing gas has an ozone concentration of at least 10 percent by weight.  
     
     
         3 . The method of  claim 1  wherein said ozone-containing gas has an ozone concentration of at least 15 percent by weight.  
     
     
         4 . The method of  claim 1  wherein said ozone-containing gas has an ozone concentration of at least 20 percent by weight.  
     
     
         5 . The method of  claim 1  including radially injecting said stream of ozone-containing gas into said stream of cellulose pulp by means of a plurality of porous metal injectors.  
     
     
         6 . The method of  claim 1  wherein the mean residence time of said pulp in said reactor is between about 10 and about 150 s.  
     
     
         7 . The method of  claim 6  wherein the mean residence time of said pulp in said reactor is between about 15 and about 40 s.  
     
     
         8 . The method of  claim 1  wherein the stream of ozone-containing gas is generated from a mixture of pressurized oxygen and at least one other gas or liquid.  
     
     
         9 . A reactor for bleaching medium consistency pulp wherein said reactor comprises a vessel, means for radially injecting an ozone-containing gas into a stream of cellulose pulp flowing through said vessel, and said reactor further comprising a mixer of the rotatable helix screw type having a first pitch in the center and a second pitch at the periphery, the second pitch being lower than the first pitch, downstream from said injecting means.  
     
     
         10 . The reactor of  claim 9  wherein the first pitch is in the range of 20 to 45 degrees and the second pitch is in the range of 5 to 30 degrees.  
     
     
         11 . The reactor of  claim 9  including baffles on the vessel inner wall.  
     
     
         12 . The reactor of  claim 11  wherein said baffles are of the helix type, having a negative pitch in respect to the mixer.  
     
     
         13 . The reactor of  claim 12  wherein said baffles have a pitch in the range of 5 to 45 degrees.  
     
     
         14 . The reactor of  claim 11  wherein said baffles comprise flat disc baffles dividing the reactor volume.  
     
     
         15 . A method for bleaching medium consistency cellulose pulp comprising providing a stream of ozone-containing gas and radially injecting the stream of ozone-containing gas in a single ozone addition step into a stream of cellulose pulp flowing through a reactor so as to provide a stream of bleached cellulose pulp, wherein said reactor operates in plug flow mode at a pressure of at least 3 bar and at a temperature of 50° C. or less and said reactor includes a mixer operating at not more than 800 rpm downstream from the injection site.  
     
     
         16 . The method of  claim 15  wherein said ozone-containing gas has an ozone concentration of at least 10 percent by weight.  
     
     
         17 . The method of  claim 15  wherein said ozone-containing gas has an ozone concentration of at least 15 percent by weight.  
     
     
         18 . The method of  claim 15  wherein said ozone-containing gas has an ozone concentration of at least 20 percent by weight.  
     
     
         19 . The method of  claim 15  wherein said mixer includes a static mixer.  
     
     
         20 . The method of  claim 15  wherein said mixer includes a dynamic mixer.  
     
     
         21 . The method of  claim 15  wherein said mixer includes a rotatable helical screw mixer.  
     
     
         22 . The method of  claim 15 , including radially injecting said stream of ozone-containing gas into said stream of cellulose pulp by means of a plurality of porous metal injectors.  
     
     
         23 . The method of  claim 15  wherein the mean residence time of said pulp in said reactor is between about 10 and about 150 s.  
     
     
         24 . The method of  claim 23  wherein the mean residence time of said pulp in said reactor is between about 15 and about 40 s.  
     
     
         25 . The method of  claim 15  wherein the stream of ozone-containing gas is generated from a mixture of pressurized oxygen and at least one other gas or liquid.