Abstract:
A pulp bleaching line has an initial stage using gaseous bleaching reagent followed by its subsequent stages without intervening washing or pumping steps. The pulp is transported from the initial stage through a mixer in which the pulp is heated and/or dosed with bleaching chemicals and through the subsequent stage by retained gas pressure developed in the initial stage. A portion of the retained gas may be separated and purged from the mixer through a pressure regulating device to optimize pressure for processes which follow the mixer/heater. This permits elimination of a washer and pump normally provided between the initial reactor and subsequent bleaching stage.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to wood pulp bleaching processes and more particularly to wood pulp bleaching processes employing gaseous bleaching reagents. 
     Wood pulp bleaching with gaseous reagents, such as oxygen and ozone, promise significant reduction of objectionable pulp mill effluent to streams and other bodies of water. Elimination of chlorine compounds from the bleaching sequence promises great economic and ecological benefits. However, incorporation of these bleaching reagents can impose significant capital costs on the pulp mill. 
     For example, the incorporation of ozone has been hindered in commercial applications to a large extent by high capital costs which are inherent to common prior art bleaching sequences which usually require that an ozone bleaching stage is followed by a pulp washing step. From the washer, the pulp is pumped at medium consistency to a mixer wherein alkaline chemicals, such as caustic soda together with any one of a number of reinforcing agents, eg. oxygen, hydrogen peroxide, sodium hypochlorite, or the like are added to the pulp. At the same time, the pulp is heated to increase its temperature above that at which it was discharged from the ozone reactor. The heated and alkalized pulp is then discharged from the mixer to the alkaline extraction stage. 
     Addition of the cost of the washer and pump to the cost of an existing bleaching operation in order to incorporate an ozone bleaching stage, when considered together with other difficulties and costs associated with ozone bleaching, often makes ozone bleaching economically undesirable. Any reduction of capital equipment requirements clearly would improve the acceptance of ozone bleaching and would increase its use. 
     The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, this is accomplished by a method for transporting pulp to subsequent bleaching stages from an initial gaseous bleaching stage for medium consistency pulp, including retaining gas pressure of the initial bleaching stage, discharging pulp from the initial bleaching stage under retained gas pressure, and allowing the retained gas pressure to transport the pulp to a mixer and onward through subsequent stages. 
     The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view illustrating a gaseous bleaching stage in a portion of a pulp processing line of the prior art; 
     FIG. 2 is a schematic view illustrating a portion of a pulp processing line as in FIG. 1 this time incorporating the present invention; 
     FIG. 3 is a view as in FIG. 2 illustrating another embodiment of the present invention; and 
     FIG. 4 presents another possible embodiment. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a portion of a pulp processing line incorporating (as an example) an ozone bleaching stage of the prior art. The pulp is washed in washer  10  and discharged through conduit  15  to thick stock pump  20 , from which it is pumped through conduit  25  to ozone mixer  30 . Ozone from the ozone supply, together with its carrier gas, is added to the pulp in the mixer, and the mixture is fed through conduit  35  to ozone/pulp contactor  40 . 
     After reaction with and finally separation from the pulp in contactor  40 , the remaining ozone and its carrier gas, together with gaseous reaction products, are removed through conduit  49  for reprocessing or other disposition. The pulp is discharged through conduit  45  into washer  42  and from there into medium consistency pump  50  through conduit  48 . It is then pumped through conduit  55  to mixer and heater  60 , where its temperature is increased and alkaline chemicals are added. 
     The resulting heated mixture is fed through conduit  65  to alkaline extraction reactor  70 . After a required reaction time, the pulp is discharged through conduit  75  to subsequent washing and other processing. It should be noted that discharge, from the contactor  40 , of residual ozone, together with its carrier gas and gaseous reaction products, through conduit  49  results in a decrease of pressure in the system downstream of the pulp ozone contactor reactor  40 . This necessitates addition of pump  246  to transport the pulp to the washer, and a pump  50  to transport the pulp beyond washer  42 . If there is sufficient hydrostatic head, there should be no need for pump  246  prior to washer  42  in the sequence. Pump  50 , provides the pressure necessary to transport the pulp, usually at medium consistency, to mixer  60  and on through the balance of the subsequent alkaline stage. 
     Referring to FIG. 2, an embodiment of the present invention can be seen, as applied to ozone bleaching followed by an alkaline stage, in which washer  10 , thick stock pump  20 , and ozone mixer  30  and their connecting conduits  15 ,  25 , and  35  are the same as in FIG.  1 . However, ozone/pulp contactor  140  is somewhat different in that it has no conduit  49  through which to vent gases. Rather, the ozone and carrier gas which enter through conduit  35  must exit with the pulp and gaseous reaction products only through conduit  47 . This limitation retains the gas pressure developed in the ozone bleaching stage and enables the retained gas pressure to transport the pulp from contactor  140 , through conduit  47 , into mixer  60  and onward to at least an immediately subsequent bleaching stage comprising the alkaline stage. 
     In mixer  60 , the pulp temperature is increased by heating, and alkaline chemicals needed for the alkaline stage are added to the pulp. The resulting mixture is discharged from mixer  60 , still under the retained gas pressure, through conduit  65  into alkaline reactor  70 . Upon substantial completion of the alkaline reaction, the treated pulp, still under the retained gas pressure, is discharged through conduit  75  to subsequent processing. 
     The embodiment of FIG. 3 is, in all respects except one, identical to that of FIG.  2 . In this embodiment, mixer  80  is incorporated in place of the mixer ( 60 ) shown in FIG.  2 . Mixer  80  has a gas discharge conduit  87  through which a portion of the retained gas pressure may be released through pressure regulating device  90 . This purges a sufficient quantity of the retained gas to leave only sufficient pressure in the mixer for transport of the pulp to or to and through alkaline reactor  70 . FIG. 4 illustrates yet another embodiment of the invention which provides relatively fine pressure tuning capability for the bleaching system. 
     By incorporating gas separator  200 , the quantity of gas purged can be more accurately controlled. The prior art system of FIG. 1 has gas separation in the ozone/pulp contactor  40  as a consequence of its operating characteristics, and substantially all of the residual gas is removed. The embodiment of FIG. 3 takes advantage of the gas separation which tends to occur naturally in a gas/liquid system. This allows a portion of the gas pressure to be purged, as already described, but such purging is limited so that a desirable quantity of gas carries forward with the pulp to the subsequent bleaching stage. 
     In most cases, even though FIGS. 1-4 show either upflow or downflow through the vessels, flow may be in either direction according to conduit arrangements which are determined by desired operating conditions. Clearly, downflow of the pulp is aided by gravity, while upflow requires a driving pressure to overcome gravity. Accordingly, the pressure regulation of the present invention provides a degree of versatility which is not normally available without the use of pumps. 
     It is also clear that, due to the thermal balance in the system, it may be desirable to have an additional mixer to heat the pulp (usually using steam). 
     In the scheme of FIG. 4, a pressure reduction device  190  is interposed between ozone/pulp contactor  140  and gas separator  200 . The pulp from the gas separator flows into mixer  160  in which the temperature is increased by heating and alkaline chemicals are blended with the pulp as required by the alkaline stage. 
     The blended and heated pulp is discharged through conduit  165  to alkaline vessel  70 . After the reaction is finished, the pulp is discharged to subsequent processing through conduit  75 . 
     Gas from gas separator  200  is routed through conduit  205  to pressure regulator  90  and exhausted for reprocessing or other appropriate disposal through conduit  100 . The gas in gas separator  200  acts as a pneumatic spring whose stiffness is determined by the backpressure imposed by pressure regulator  90 . This maintains a relatively constant driving force for the pulp through mixer  160  and alkaline vessel  70 . 
     Of course, depending on the configurations of the vessels of the system, either upflow or downflow of the pulp may be desirable for given operating conditions. This will dictate the degree of pressure regulation required and will determine whether the embodiment of FIG. 2, with no pressure regulation, FIG. 3, with limited pressure regulation, or FIG. 4 with full pressure regulation, is the preferred embodiment, recognizing that each embodiment results in specific quantities of gas carried forward to subsequent bleaching stages. 
     Liebergott, et.al (1992 Non-Chlorine Bleaching Conference) showed that there may be a beneficial effect in delignification efficiency by eliminating the washing step between an ozone bleaching stage and an alkaline extraction stage. Of course, elimination of the washing step will require additional alkali to be used due to carry forward of acid from the first stage, but this is partially offset by savings in capital equipment costs. The result of this invention is to eliminate additional equipment to further reduce the capital cost of the project, and provide for oxygen gas to be carried forward into the subsequent stage or stages which has a further beneficial effect in delignification efficiency. This process is applicable to all systems employing bleaching agents in which the subsequent stage is enhanced by the presence of oxygen gas or is at least not affected in a negative manner. This is very desirable since the cost of purchasing and maintaining pumps which transport pulp at medium consistency is quite high and represents a financial burden on mill operations. 
     According to this invention, the second stage may be any alkaline stage whose performance is enhanced by exposing the pulp and reactants to oxygen gas, i.e., sodium hydroxide (E) alone or with hydrogen peroxide (P), or sodium hypolchlorite (H). In this example the performance of the stage is enhanced, or reinforced by oxygen. The conventional designations of these enhanced stages then are E O , E OP , or E OH  . It is understood that additional bleaching reagents which operate in an acid environment in the subsequent stage of bleaching may become commercially viable in the future, so this invention is not limited to that in which the first stage is acid and the second stage is alkaline. Therefore, according to this invention, the two or more stages of bleaching in which the motive force for transporting pulp through the subsequent stages is the gas pressure of the first stage, may be any combination of acid or alkaline stages. 
     By the methods of the invention described herein, it is possible to eliminate a pulp washer and a pulp transfer pump from a pulp processing line, thereby substantially reducing the cost of the pulp processing system.