Methods and systems for the use of recycled filtrate in bleaching pulp

Abstract

A system for the bleaching of lignocellulosic pulp including: a first oxygen reactor and a second oxygen reactor connected in series to the first oxygen reactor, wherein each reactor bleaches the pulp; an upstream pulp washer that washes pulp and discharges washed pulp to a first pulp conduit having an outlet coupled to the first oxygen reactor; an oxygen supply conduit connected to the first pulp conduit and supplying oxygen to the washed pulp flowing through the first pulp conduit; a downstream pulp washer that washes bleached pulp discharged from the second oxygen reactor; a filtrate conduit receiving filtrate discharged from the downstream pulp washer; a second pulp conduit receiving bleaches pulp discharged from the first oxygen reactor and conveying the bleached pulp to the second oxygen reactor, wherein the filtrate conduit is connected to the second pulp conduit such that at least a portion of the filtrate in the filtrate conduit flows into the second pulp conduit, and a second oxygen supply conduit connected to the second pulp conduit and introducing oxygen to the pulp flowing through the conduit.

1 . A system for bleaching of lignocellulosic pulp comprising:
 a first oxygen reactor and a second oxygen reactor each bleaching the pulp;   a downstream pulp washer that receives and washes bleached pulp discharged from the second oxygen reactor;   a filtrate conduit receiving filtrate discharged from the downstream pulp washer, and   a pulp conduit receiving the pulp discharged from the first oxygen reactor and conveying the discharged pulp to the second oxygen reactor, wherein the filtrate conduit is connected to the pulp conduit such that at least a portion of the filtrate in the conduit flows into the pulp conduit.   
     
     
         2 . The system of  claim 1  wherein the pulp washer is a drum displacement (DD) washer. 
     
     
         3 . The system of  claim 1  further comprising an oxygen supply conduit connected to the pulp conduit and introducing oxygen to the pulp flowing through the conduit. 
     
     
         4 . The system of  claim 1  further comprising a mixer coupled to the pulp conduit such that pulp flowing through the pulp conduit flows through the mixer and wherein the filtrate conduit is coupled to the mixer such that the filtrate flows into the mixer and enters the pulp conduit. 
     
     
         5 . The system of  claim 1  further comprising an upstream pulp washer discharging washed pulp to a second pulp conduit connected to the first oxygen reactor. 
     
     
         6 . The system of  claim 5  further comprising a second oxygen supply conduit connected to the second pulp conduit to introduce oxygen to the washed pulp in the second pulp conduit. 
     
     
         7 . The system of  claim 5  wherein the filtrate conduit includes a branch of the conduit extending to a fluid inlet to the upstream pulp washer, and a portion of the filtrate in the filtrate conduit flows through the branch and into the fluid inlet of the upstream pulp washer. 
     
     
         8 . The system of  claim 1  further comprising a heat exchanger coupled to the filtrate conduit and transferring heat to the filtrate flowing through the filtrate conduit. 
     
     
         9 . The system of  claim 1  further comprising a pump coupled to the filtrate conduit and pumping the filtrate in the filtrate conduit towards the pulp conduit. 
     
     
         10 . A system for the bleaching of lignocellulosic pulp comprising:
 a first oxygen reactor and a second oxygen reactor connected in series to the first oxygen reactor, wherein each reactor bleaches the pulp;   an upstream pulp washer that washes pulp and discharges washed pulp to a first pulp conduit having an outlet coupled to the first oxygen reactor;   an oxygen supply conduit connected to the first pulp conduit and supplying oxygen to the washed pulp flowing through the first pulp conduit;   a downstream pulp washer that washes bleached pulp discharged from the second oxygen reactor;   a filtrate conduit receiving filtrate discharged from the downstream pulp washer;   a second pulp conduit receiving bleaches pulp discharged from the first oxygen reactor and conveying the bleached pulp to the second oxygen reactor, wherein the filtrate conduit is connected to the second pulp conduit such that at least a portion of the filtrate in the filtrate conduit flows into the second pulp conduit, and   a second oxygen supply conduit connected to the second pulp conduit and introducing oxygen to the pulp flowing through the conduit.   
     
     
         11 . The system of  claim 10  wherein the upstream pulp washer and the downstream pulp washer are each a drum displacement (DD) washer. 
     
     
         12 . The system of  claim 10  further comprising a mixer coupled to the second pulp conduit such that pulp flowing through the second pulp conduit flows through the mixer and wherein an outlet to the filtrate conduit is coupled to the mixer such that the filtrate flows into the mixer and thereby enters the second pulp conduit. 
     
     
         13 . The system of  claim 10  wherein the filtrate conduit includes a branch of the conduit extending to a fluid inlet to the upstream pulp washer and a portion of the filtrate in the filtrate conduit flows through the branch and into the fluid inlet of the upstream pulp washer. 
     
     
         14 . The system of  claim 10  further comprising a heat exchanger coupled to the filtrate conduit and transferring heat to the filtrate flowing through the filtrate conduit. 
     
     
         15 . A method for bleaching of cellulosic pulp comprising:
 introducing a pulp to a first oxygen reactor;   bleaching the pulp in the first oxygen reactor and discharging from the first oxygen reactor at least partially bleached pulp;   mixing the at least partially bleached pulp with a recycled filtrate to generate a pulp-filtrate mixture;   introducing the pulp-filtrate mixture to a second oxygen reactor for bleaching the pulp-filtrate mixture and discharging a further bleached pulp;   washing the further bleached pulp in a washer, and   removing filtrate from the washer to generate the recycled filtrate.   
     
     
         16 . The method of  claim 15  including adding oxygen to the partially bleached pulp or pulp-filtrate mixture after the partially bleached pulp is discharged from the first oxygen reactor and before the pulp-filtrate mixture is introduced to the second oxygen reactor. 
     
     
         17 . The method of  claim 15  including washing the pulp and adding oxygen to the pulp before introducing the pulp to the first oxygen reactor. 
     
     
         18 . The method of  claim 15  including heating the recycled filtrate before the mixing of the recycled filtrate with the at least partially bleach pulp. 
     
     
         19 . The method of  claim 15  including pressurizing the recycled filtrate before the mixing of the recycled filtrate with the at least partially bleach pulp. 
     
     
         20 . The method of  claim 15  including introducing a portion of the removed filtrate to a second washer and discharging from the second washer as washed pulp the pulp to be introduced to the first oxygen reactor.