Abstract:
The present invention provides for a method for delignifying softwood or hardwood pulp in a fiber line of a pulp and papermaking system having a slurry at low consistency by adding to the pulp slurry oxygen and adding this pulp slurry reaction mixture to a reaction system comprising an efficient mixer and contact tank. The reaction system is preferably physically located after the knotters and before the brown stock washers to make more effective use the residual alkali and heat content in the incoming black liquor.

Description:
[0001]     This application claims priority from U.S. Provisional patent application Ser. No. 60/ 514,946 filed Oct. 28, 2003. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a method for delignifying softwood or hardwood pulp in a slurry at low consistency using a reaction device and oxygen.  
         [0003]     Oxygen delignification of cellulosic material is generally practiced at medium consistency (8 to 16%) a region where it has been determined makes it both relatively easy to operate and economical. In such operations partially delignified pulp is reacted with oxygen in the presence of alkali (pH&gt;11) using high shear mixers and pressurized upflow reactors with retention times of 20 to 60 minutes at pressure in excess of 90 psi and temperatures as high as 110° C. The high shear mixers facilitate the contacting of the pulp slurry with the oxygen containing gas in a very turbulent state lasting a few seconds prior to entering the pressurized vessels.  
         [0004]     Using such equipment in single and two-stage configurations have resulted in reported delignification of up to 45%. However, many of such systems perform below 40% kappa reduction due to less than optimal contacting of oxygen with the pulp. In addition, implementation of medium consistency oxygen delignification require high capital expenditures for high shear mixers, medium consistency pumps and large contact towers and have thus far excluded many mills from economically modifying their processes and taking advantage of this technology. Low consistency oxygen delignification has not been practiced commercially due to cost of chemicals and steam that would be required.  
         [0005]     Oxygen delignification systems have traditionally been located after the brownstock washers on a fiber line where the slurry is relatively cold and have been stripped of all caustic thus requiring the addition of the full complement of steam and caustic. As a result of the high capital cost, energy costs and anticipated higher cost of chemicals, low consistency, or below 8% cellulosic material, oxygen delignification has not been practiced commercially.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention provides a method for delignifying softwood or hard wood pulp in a slurry at low consistency by adding to the pulp slurry oxygen and adding this pulp slurry reaction mixture to a reaction system comprising an efficient mixer and contact tank. Preferably the pulp is removed from the brownstock processing section (between the blow tank and the bleach section of the papermaking system. The reaction system is physically located after the knotters and before the brown stock washers to make more effective use of the residual alkali and heat content in the incoming black liquor. The physical location of the reaction system can be at a convenient position along the papermaking system; only the pulp slurry that is removed is “fixed” in that it is removed from the point between the knotters and the brown stock washers.  
         [0007]     For purposes of the present invention, “low consistency” is defined as that pulp containing less than 8% by weight of cellulosic material.  
         [0008]     In practicing the methods of the present invention, the reaction system may also be physically located and the pulp slurry drawn from the section of the fiber line after the digester and before the brown stock washers, or in between any two stages of a multi-stage brownstock washing step.  
         [0009]     The hot pulp slurry is taken from the knotters with the black liquor still present. The black liquor results from cooking pulpwood in an aqueous solution in the sulfate or kraft papermaking process. Steam may be added to heat the slurry as well as caustic which can be used to adjust the pH. Oxygen is injected into the mixture which is then sent to the gas liquid mixer. The pulp slurry containing the oxygen is mixed at conditions of temperature and pressure whereby the oxygen begins to react with the lignin to effect delignification. This reaction mixture is then sent from the mixer to a retention tank which is also capable of being kept at pressure, where the delignification reaction will continue further. The reaction products are then sent into a blow tank or a gas-liquid separator where reaction gases and unused oxygen gas is disengaged or removed from the liquid product and scrubbed and either vented to the atmosphere or recycled.  
         [0010]     The invention may be implemented as a retrofit in existing mills as well as new ones at relatively low capital cost. The placement of the system takes advantage of the residual caustic and heat that will be present in the slurry as it leaves the knotters. Furthermore a low consistency oxygen delignification reaction will be easier to control compared to operations at medium and high consistencies of pulp. Accordingly low consistency oxygen delignification before the brown stock washers of a fiber line is achieved.  
         [0011]     The reactor assembly consists of a controlled cavitation device or other efficient mixing device, coupled with a residence tank.  
         [0012]     The heat and alkali requirements for the delignification reaction will be obtained primarily from the residual alkali and heat content in the incoming black liquor and an oxygen containing gas.  
         [0013]     The amount of oxygen present can be an excess so as to react with some of the organic acids in the black liquor to reduce the heat value of the black liquor, and to react with sulfur compounds in the black liquor to reduce or eliminate the potential for total reduced sulfur (TRS) emissions.  
         [0014]     A further advantage of the present invention is that un-oxidized white liquor is employed as the source of needed alkali. Additionally the excess oxygen is employed to oxidize some of the sulfur compounds in the white liquor. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The FIGURE is a schematic representation of an oxygen delignification process according to the practice of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     The present invention provides for the low consistency oxygen delignification of pulp and comprises the following steps:  
         [0017]     Taking pulp slurry at consistencies between 2 to 8% from the fiber line, at an appropriate location;  
         [0018]     The pulp slurry drawn from the fiber line may be removed between the knotters and the brown stock washers, or before the brown stock washers and after the digester, or in between any two stages of a multi-stage brownstock washing step.  
         [0019]     When necessary, adding a small amount of caustic to bring the slurry pH to at least 11 to 12;  
         [0020]     Adding steam (when needed) to bring slurry temperature to between 85° C. to 120° C.;  
         [0021]     Introducing the slurry and an oxygen containing gas into a high efficiency mixer for intense mixing under pressure of between 20 and 160 psig for up to 2 minutes;  
         [0022]     Sending the slurry and oxygen mixture to a holding tank and maintaining it under pressure of 20 to 160 psig for 5 to 30 minutes;  
         [0023]     Sending the reaction mixture to a disengagement tank where the reaction gases are separated from the slurry and sent for treatment and disposal; and  
         [0024]     Sending the slurry to the next stage in the bleach line of the papermaking process, which could be the screens or brown stock washers.  
         [0025]     In the embodiment shown in  FIG. 1 , pulp slurry from the knotters is contacted with oxygen in the presence of caustic, temperature and allowed a short residence time in a retention tank for the oxygen to react with and delignify the pulp. The oxygen also reacts with the organic solid in the black liquor resulting in a reduction of the heat value of the black liquor going to the recovery boiler. Additional reactions take place between the oxygen and sulfur compounds in the liquor, thus reducing or eliminating potential sources of total reduced sulfur (TRS) emissions by the pulp mill.  
         [0026]     In the process, pulp slurry from the knotters with the black liquor still present and at a consistency of between 2 to 8% is transferred by a pump  30  through pipelines  10  and  40  to a steam mixer  50 . Caustic, if needed may be added to the pulp slurry through side pipe  20  to maintain the pH of the pulp slurry between 11 and 12. At the steam mixer, steam is added to the pulp slurry through pipe line  130  to bring the temperature of the slurry to between 90 and 110° C. before it flows into the efficient mixing device which could be a cavitation type reactor  70  through line  60 . Oxygen containing gas is introduced through a side inlet  140  and enters the efficient mixing device with the pulp slurry stream.  
         [0027]     The pulp slurry and the oxygen containing gas are brought into intimate contact by the shearing and cavitation and mechanical forces generated as the rotor rotates at a high speed in the mixing device  70 . The contacting is performed under suitable conditions of temperature and pressure whereby the oxygen begins to react with the lignin under caustic conditions to effect delignification. The reaction mixture proceeds through pipe line  80  into a retention tank  90  where under conditions of temperature, pressure and time the delignification reaction is continued further.  
         [0028]     In order for the reaction to proceed as mentioned above, an oxygen containing gas must be used. Preferably oxygen with a minimum purity of 93% is employed. The preferred total reaction pressure should be not less than 80 psig and more preferably above 90 psig. The preferred minimum reaction temperature is 90° C. but could be as high as 110° C.  
         [0029]     The residence time of the reaction mixture in the retention tank is not less than 10 minutes but preferably up to 30 minutes. The reaction mixture is withdrawn through outlet  100  into a blow tank or gas-liquid separator  110  where reaction gases and unused oxygen gas disengages from the liquid product and is scrubbed and vented. The pulp slurry is withdrawn through line  120  and sent to the washing stage of the bleach line of the pulp and papermaking process.  
         [0030]     By practicing the methods of the present invention, bleaching can be done with much less capital, specifically by not having to build, own, operate and maintain separate pressure towers for traditional oxygen delignification. Un-oxidized white liquor can be used as the source of caustic as the oxidation of the white liquor also takes place in the mixer. Also, the exothermic heat of reaction of oxidation of white liquor raises the temperature of the pulp resulting in steam savings for oxygen delignification and most of the oxygen delignification reactions can be done at the existing pulp temperature.  
         [0031]     Oxygen delignification before the brown stock washers lowers the BTU value of the black liquor allowing for increased capacity of steam limited recovery boilers. Additionally, less oxidized white liquor will be needed in this sequence than traditional two stage oxygen delignification thus taking the load off the lime kiln and other white liquor systems.  
         [0032]     The black liquor produced will have a lower viscosity because of the reaction of the oxygen with the Na 2 S and a portion of the dissolved lignin allowing for a higher percentage solids firing in the recovery boiler. Methanol and other alcohols present in the black liquor before the brown stock washers will react to form a reducing environment in the pulp. With the availability of hydrogen limited, the side reactions that hurt pulp strength will also be limited. Indeed pulp strength may increase.  
         [0033]     The following is an example of a practice of the invention in accordance with the embodiment illustrated in  FIG. 1 . In this example a side stream of softwood slurry at a consistency of 2.5% and containing residual black liquor was withdrawn with a Gould&#39;s pump  30  at a flow rate of 50 gallons per minute (gpm) through a side pipe  10  on the stock line going from the knotters to the first brown stock (BS) washers. The pH of the slurry was above 11.5 and so no additional caustic was added. Slurry temperature was 87° C. The slurry was heated to 90° C. adding low-pressure steam through pipe  130  and mixed into the slurry with the steam mixer  50 . Pure oxygen was added to the slurry through a side pipe  140  at a rate of 60 pounds per hour before it entered a high efficiency gas-liquid mixer, in this case a controlled cavitation reactor  70 . The slurry and gas were mixed for a few seconds at about 90 pounds per square inch (psi) in the reactor  70 , the stock was transferred into a retention tank  90  and kept for 10 minutes and was finally returned to the bleach line just before the first BS washer. The pulp Kappa # is the amount of lignin in the pulp sample. Samples were taken from the exit slurry stream for analysis. The following results were obtained:  
                                                         TABLE 1                                       Inlet   Outlet   % Reduction                                        Pulp Kappa #   25.8   16.5   37           Na 2 S, mg/kg   21000   &lt;200   almost           solids       (non detect)   100           Dissolved Lignin,   32.1   29.1   9.3           As % solids                      
 
         [0034]     Comparatively, for conventional single stage oxygen delignification systems operating under similar conditions of temperature and pressure on a 10% consistency pulp, residence times of 40 minutes and higher will be required to achieve the 37% delignification.  
         [0035]     While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of this invention will be obvious to those skilled in the art. The appended claims and this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention.  
         [0036]     Having thus described the invention,