Abstract:
A method for removal of pollutants from a pulping process byproduct liquor, the method comprising injecting an oxygen-containing gas into said liquor and condensing the water vapor from the stripping gas so as to produce a condensate comprising pollutants. Also a method of controlling the flow rate and/or temperature of a pulping process byproduct liquor, the process comprising injecting an oxygen-containing gas into the liquor in a reactor, processing the liquor in a flash tank so as to create a cooled liquor and combining at a least a portion of the cooled liquor with new liquor as the new liquor enters, or while the new liquor is in, the reactor.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention is directed toward the reduction of hazardous air pollutants and reduced sulphur compounds from a byproduct stream produced in a pulping process which contains water where a portion of the water is removed in a multiple effect evaporation system and a further portion of the water is removed in a direct contact evaporator (DCE).  
           [0002]    The Kraft pulping process produces cellulose fibers and black liquor. Black liquor is a water based mixture of organic wood derivatives and alkaline pulping chemicals, chiefly containing degraded lignin, organic acid salts, resin, sodium hydroxide, and sodium salts including carbonate, sulfide, sulfate, sulfite, and thiosulfate. Trace compounds in the black liquor include, methanol, benzene, methyl mercaptan and dimethyl disulfide. Weak black liquor contains 15 wt % dissolved and suspended solids of which about 80% are organic and the remainder are inorganic compounds.  
           [0003]    Weak black liquor is concentrated to about 45-50 wt % solids by multiple effect evaporation and further evaporated to about 65 wt % solids in a Direct Contact Evaporator (DCE), where the liquor is contacted with the flue gases from the recovery boiler. The concentrated liquor is combusted in a recovery boiler to raise steam and to recover sulfur and sodium for reuse in the pulping step. Oxidation of the sodium hydrosulfide in the black liquor is necessary prior to its introduction into the direct contact evaporator to minimize the emissions of hydrogen sulfide in the flue gas from the recovery boiler. Newer mills have replaced direct contact evaporators with indirectly-heated concentrators, which substantially reduces the total reduced sulfur emissions in the flue gas of the recovery boiler.  
           [0004]    Most oxidation processes use air as the oxidant. In these processes the liquor from the multiple effect evaporators is conveyed to an atmospheric reaction vessel where air is sparged through the liquor and a portion of the oxygen in the air reacts with the sulfide in the liquor to produce sodium thiosulphate. Unreacted oxygen, nitrogen, water vapor and volatile compounds, which have been transfer from the black liquor to the gas phase, are typically vented to the atmosphere. The volatile compounds include methanol and benzene, which are considered hazardous air pollutants (HAP), and methyl mercaptans and dimethyl disulfide, which are considered total reduced sulfur (TRS) pollutants. The total amount of HAP and TRS emissions in the vent of the air oxidizer is a function of their concentration in the black liquor entering the oxidation system, the amount of air used in the process and the amount of oxygen that reacts with the black liquor.  
           [0005]    After the oxidation step, the liquor is conveyed to the direct contact evaporator where the liquor contacts the hot flue gases from the recovery boiler. This process humidifies the flue gas and concentrates the black liquor. A portion of the remaining volatile compounds in the black liquor are transferred to the gas phase and discharged to the atmosphere in the flue gas from the DCE. Hydrogen sulphide can also be produced and transported to the gas phase via the reaction NaSH+H 2 O→H 2 S (g) +NaOH. The amount of H2S (g)  produced is a function of the concentration of NaSH in the black liquor. The total amount of HAP and TRS emissions in the flue gas is primarily a function of the concentration of these compounds in the liquor entering the DCE. The total amount of HAP and TRS emissions from the combination of air oxidizer and DCE are primarily a function of their concentration in the black liquor from the multiple effect evaporators.  
           [0006]    An alternative to the air oxidation process is described in U.S. Pat. Nos. 4,239,589, and 4,313,788, which discloses the oxidation of black liquor in which high recovery of the heat of reaction is accomplished by integration with multiple effect evaporator stages. The process uses a gas with a high concentration of oxygen, typically greater than 99%. The oxidized liquor from the evaporators is typically sent to storage and then passed to the DCE. The higher concentration of HAP and TRS in the black liquor entering the DCE as compared to black liquor from an air oxidation system results in a proportionate increase in the transfer of these compounds to the flue gas from the DCE. Therefore it is generally believed that the total emissions of HAP and TRS from the DCE will be approximately equal to the amount emitted from the combination of an air oxidation system and a DCE.  
           [0007]    It is a generally desirable to minimize the emission of HAP and TRS compounds into the atmosphere. As a result, Kraft pulp mills consider methods of reducing these emissions. Two identified methods to lower these emissions are the collection and incineration of the vent gas from the air oxidation system or conversion of the DCE recovery boiler to a low odor configuration. Unfortunately both options are expensive.  
           [0008]    Improved methods of reducing the total HAP and TRS emissions from the recovery area of a pulp mill are needed. The present disclosure and the claims, which follow, describe such an improved method.  
           [0009]    As much as the background of the invention described above and the details of the invention described below relate the Kraft pulping process, it is to be understood that there are other pulping processes, such as soda pulping that do not use sulfur in the process and therefore do not require the oxidation of a byproduct stream for odour control. However, these processes typically product a byproduct stream which is subject to concentration in a multiple effect evaporator system and a direct contact evaporation system and in which there are HAP. In this regard, the present invention can be employed to decrease the HAP in the byproduct stream fed to the DCE thereby reducing the HAP emissions from the DCE.  
         BRIEF SUMMARY OF THE INVENTION  
         [0010]    In one general aspect, the invention is an improvement in a pulping process that employs a multiple effect evaporator such that said evaporator produces a byproduct liquor, the improvement comprising:  
           [0011]    a) injecting an amount of oxygen-containing gas into a reactor, said gas comprising oxygen, said oxygen at a concentration at least 22% (v/v), such that said injecting results in greater than 40% of said oxygen reacting with the components of the liquor and such that said injecting results in both a further concentrated liquor and a stripping gas comprising both water vapor and methanol that was in the liquor prior to step (a);  
           [0012]    b) condensing said water vapor from said stripping gas so as to produce a condensate comprising said methanol; and  
           [0013]    c) separating said condensate from said further concentrated liquor.  
           [0014]    The invention is expressed in terms of methanol. However, the concentration of many other pollutants in the liquor will be decreased as that of methanol is decreased.  
           [0015]    The measurement of the percentage of oxygen that reacts can be calculated based on knowledge of the amount of oxygen that is inputted in step (a) and the amount of unreacted oxygen that is expelled as vent gas from the condensor that condenses the flash tank vapor. Oxygen content of a vent gas can be achieved by well known techniques. (For example, a Servomex oxygen analyzer which employs the paramagnetic property of oxygen.)  
           [0016]    In particular embodiments increasing concentrations of oxygen and/or the percentage of oxygen reacting with the liquor and/or the total amount of oxygen reacting with the liquor may be appropriate. As elaborated upon elsewhere herein, it can be commercially advantageous, to react as much of the oxygen-containing gas with the liquor as possible so as to minimize the amount of unreacted gas that has to be expelled from the system as vent gas—while preferably still effectively removing methanol from, or decreasing its concentration in, the liquor.  
           [0017]    Particular embodiments of the invention can be summarized as situations where the oxygen-containing gas comprises at least 70% (v/v) oxygen (at least a high percentage), where the oxygen-containing gas comprises at least 95% (v/v) oxygen (a very high percentage), where the percentage of oxygen reacting is at least 70%, where the percentage of oxygen reacting is at least 95%, and combinations thereof.  
           [0018]    In some embodiments, the oxygen-containing gas comprises an inert gas at a concentration of 1% to 40% v/v.  
           [0019]    In many preferred embodiments, the liquor contains about 40% -55% solids (w/w).  
           [0020]    In many preferred embodiments, subsequent to step (a) the liquor is processed in a flash tank so as to produce a further concentrated liquor and both water vapor and methanol and wherein said water vapor is condensed to produce a condensate comprising said methanol, and wherein said condensate is separated from said further concentrated liquor.  
           [0021]    In one preferred embodiment, the oxygen-containing gas in step (a) is the oxygen-richer gas, and at a time prior to step (a), there is a step comprising injecting an oxygen-containing gas into said concentrated liquor, said gas being the oxygen-poorer gas by virtue of the fact that it contains a lower percentage of oxygen than the oxygen-richer gas does. The oxygen-poorer gas can be air.  
           [0022]    In another preferred embodiment, the method further comprises combining, with new byproduct liquor that has not yet been processed through steps (a) and (b), at least a portion of the further concentrated liquor so as create a mixture of said new byproduct liquor and said portion of further concentrated liquor and, subsequent to said combining, processing at least a portion of said mixture through the steps (a), (b) and (c) described for a byproduct liquor.  
           [0023]    In another preferred embodiment, the majority of inert gases and unreacted oxygen in the oxygen-containing gas are separated from the by-product liquor at a point subsequent to reaction of the oxygen at a pressure of greater than 2 bar(g).  
           [0024]    In another general aspect, the invention is an improvement in a pulping process that employs a multiple effect evaporator such that said evaporator produces a byproduct liquor, the improvement comprising controlling the flow rate and/or temperature of the byproduct liquor, said improvement comprising:  
           [0025]    a) injecting an oxygen-containing gas into said liquor, said liquor being in a reactor designed for said purpose;  
           [0026]    b) subsequent to step (a), processing the liquor in a flash tank so as to create a cooled liquor; and  
           [0027]    c) combining at a least a portion of said cooled liquor with new liquor either previous to said new liquor entering, as said new liquor enters, or while said new liquor is in, said reactor;  
           [0028]    wherein step (a) is done at temperatures less than 175° C.  
           [0029]    In preferred embodiments, step (a) is done at a temperature less than 175° C. most preferably at a temperature in the range 120 to 135° C.  
           [0030]    In preferred embodiments, the pulping process is one in which the oxidation of the liquor takes place in a pressurized reactor operating at greater than 2.0 bar(g).  
           [0031]    In other particular embodiments, the oxygen-containing gas comprises at least 70% (v/v) oxygen, more preferably, at least 95% (v/v) oxygen. Similarly, additional particular embodiments are where the liquor contains about 30%-55% solids (w/w).  
         BRIEF DESCRIPTION OF THE DRAWINGS  
         [0032]    [0032]FIG. 1 is a schematic flow diagram of a prior art method of black liquor oxidation, concentration and combustion, which utilizes direct contact evaporation prior to the recovery boiler, wherein the oxidation process uses air and the vent gases from the oxidation process are collected, cooled and incinerated.  
           [0033]    [0033]FIG. 2 is a schematic flow diagram of a prior art method of black liquor oxidation, concentration and combustion, which utilizes direct contact evaporation prior to the recovery boiler, wherein the oxidation process uses oxygen and the oxidation process is performed within the multiple effect evaporator system.  
           [0034]    [0034]FIG. 3 is a schematic flow diagram of the present invention, which is an improvement to the prior art method of reducing HAP and TRS emissions.  
           [0035]    [0035]FIG. 4 shows a relationship between MeOH and vent flow based on an EPA-Model BLO unit and conditions specified elsewhere herein.  
           [0036]    [0036]FIG. 5 shows a relationship between energy recovery and vent temperature based on an EPA-Model BLO unit and conditions specified elsewhere herein.  
           [0037]    [0037]FIG. 6 shows a relationship between MeOH and vent flow under conditions according to the present invention as specified elsewhere herein.  
           [0038]    [0038]FIG. 7 is a schematic flow diagram of the present invention regarding a method of controlling the flow rate and/or temperature of the black liquor.  
           [0039]    [0039]FIG. 8 is a schematic diagram of an aspect of the present invention combining the advantages of FIGS. 3 and 7.  
           [0040]    [0040]FIG. 9 is a schematic diagram of a variation of the invention illustrated in FIG. 3.  
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0041]    The invention is an improved method of reducing the emissions of HAP and TRS from the recovery area of a pulp mill. The improvement comprises contacting the black liquor, which has been processed through the multiple effect evaporators, with an oxygen-containing gas wherein the oxygen concentration of the gas is at least 22%. At least 40% of the oxygen is made to react with compounds in the black liquor and the heat of reaction is used to produce a stripping gas of sufficient temperature and volume to transfer a substantial portion of the HAP and TRS compounds from the black liquor to the gas phase. Optionally, a flash tank can be used in the process at a point subsequent to the reactor so as to produce the majority of the stripping gas. The amount of stripping gas and hence the amount of HAP and TRS transferred to the gas phase is controlled by varying the amount of oxygen fed to the system and/or the amount of oxygen reacted and/or the concentration of oxygen in the gas. The gas phase, which is primarily water vapour, is cooled whereupon the majority of the water vapor is condensed. A large portion of the HAP, primarily methanol, and a portion of the TRS compounds are transferred to the condensed water vapor (i.e., condensate). The remaining low volume gas stream is discharged to the atmosphere or conveyed to a combustion source for incineration. The volume of gas in the cooled vent stream is controlled by choosing an appropriate oxygen concentration in the oxygen-containing gas and designing the reactor to react with a specific percentage of the injected oxygen. The volume of gas in the vent stream can be controlled to ensure that the concentration of combustible gases is below the lower flammability limit thereby allowing for their discharge into a High Volume, Low Concentration, Non-Condensible (HVLC-NCG) gas system or can be controlled such that the concentration of oxygen in the vent stream is below the minimum oxygen concentration to support combustion, whereupon the vent can be discharged into a Low Volume, High Concentration, Non-Condensible (LVHC-NCG) gas system.  
         [0042]    The present invention allows for the controlled removal of HAP and TRS compounds from the black liquor while producing a specific volume of cooled vent gas. The process depends particularly on the knowledge that the concentration of HAP, primarily methanol, in a stripping gas is sensitive to the temperature of that gas.  
         [0043]    The improvement of the present invention over the prior art of air oxidation can be understood by an initial review of FIG. 1 Referring to FIG. 1, weak black liquor  1  from the wood pulping and washing steps is fed to multiple effect evaporation system  101 . Weak black liquor contains water, dissolved lignin and other wood constituents, sodium salts (particularly sodium sulfide and other unoxidized sulfur compounds), and sodium hydroxide. Trace compounds in the black liquor include HAP such as, methanol, and benzene, and TRS such as methyl mercaptan and dimethyl disulfide. The liquor is typically at 90° C. and 1.5 bar(g), and typically contains 15 wt % solids. The liquor is concentrated in multiple effect evaporator system  101  heated by steam  3  as is known in the art to yield partially concentrated black liquor  5  and residual steam/condensate  7 . A portion of the HAP and TRS compounds in the weak black liquor are transferred to the steam/condensate  7  and a portion remain in the partially concentrated black liquor  5 .  
         [0044]    The partially concentrated black liquor  5 , typically at 90° C. and 1.0 bar(g) containing typically 48 wt % solids flows to air black liquor oxidation system  103  in which air  9  oxidizes the sodium hydrosulfide to sodium thiosulphate via the reaction: 
         2NaSH+2O 2 →Na 2 S 2 O 3 +H 2 O 
         [0045]    and a portion of the organic material in the black liquor.  
         [0046]    The amount of air sparged into the liquor is controlled so as to convert at least 95% of the sodium hydrosulfide to sodium thiosulphate. The percent of oxygen in the air 9 that specifically reacts with black liqor is typically no greater than 40%. To achieve a NaSH concentration of less than 2 g/l in the black liquor a significant excess of air  9  is required and the percent of oxygen in the air that reacts with the black liquor is generally no greater than 30%.  
         [0047]    Unreacted oxygen, the nitrogen in the air, water vapour, and HAP and TRS compounds are vented  11  from the reactor to a heat transfer device  109 , herein called a condenser, where the gas  11  is cooled and a portion of the water vapor and HAP and TRS are condensed and discharged from the condensor in stream  32 . The cooled vent gas  28  flows to a combustion device  111  where the HAP and TRS compounds are incinerated and the products of combustion  29  are discharged to the atmosphere. The concentration of volatile compounds, including methanol, benzene, acetaldehyde, acetone, methyl mercaptans, and dimethyl disulfide, in the vent gas  11  is a function of thermodynamic equilibrium. The total amount of these compounds in the vent gas  11  is dependent upon their concentration in the feed black liquor  5 , the amount of air fed  9  to the oxidizer system, the temperature of the feed black liquors and the amount of oxygen reacted.  
         [0048]    Oxidized black liquor  13 , now containing typically less than 2 g/l of sodium sulfide, and a lower concentration of HAP and TRS compounds, passes to the direct contact evaporation system  105  in which the black liquor is further concentrated by direct contact with hot flue gas  15  from recovery boiler  107 . Fully concentrated black liquor  17  and final flue gas  19  flow from the evaporator system  105 . Flue gas  19  contains, water vapor, combustion products, hydrogen sulfide and HAP and TRS compounds. The concentration of HAP and TRS compounds in the flue gas  19  is a function of thermodynamic equilibrium. The total amount of these compounds in the flue gas  19  is dependent primarily upon their concentration in the feed black liquor  13 . The black liquor at this point typically contains 65 wt % solids at 115° C. Fully concentrated black liquor  17  is combined with sodium sulfate (salt cake) makeup  20  and passes into recovery boiler  107  in which the organic materials are combusted with air to generate heat withdrawn as steam  21  for use elsewhere in the mill. The inorganic sulfur, largely sodium thiosulfate is reduced to sodium sulfide in the boiler and smelt  23 , containing molten sodium sulfide and sodium carbonate, is withdrawn for preparing green liquor. Flue gas streams  27  and  19  pass to a cleanup system for particulate removal typically an electrostatic precipitator. The oxidation of sodium sulfide in black liquor oxidation system  103  is required in order to reduce the amount of hydrogen sulfide formed in direct contact evaporator  105  and carried therefrom to the atmosphere in final flue gas  19 .  
         [0049]    Data on the concentration and the total emissions of HAP and TRS from an air oxidizer  103  and a DCE  105  have been collected. In the EPA document entitled ‘Chemical Recovery Combustion Sources at Kraft and Soda Mills’ data is presented on the emissions of Hazardous Air Pollutants from air oxidation systems and direct contact recovery boilers. Emissions from Air-sparging BLO systems were 2.03×10 −4  kilograms per kilogram of black liquor solids (Kg/Kg) of which, 1.73×10 −4  Kg/Kg was methanol (i.e., 85%). HAP emissions in the flue gas from direct contact recovery boilers were 4.32×10 −4  Kg/Kg of which, 1.70×10 −4  Kg/Kg was methanol (i.e., 39%). TRS emissions from Air-sparging BLOX systems are estimated to be 2.93×10 −5  Kg/Kg, based on data in the document. TRS emissions, of which the majority is H 2 S (g) , from the DCE boiler are estimated to be 1.21×10 −4  Kg/Kg.  
         [0050]    Further data was compiled by the National Council of the Paper Industry for Air and Stream Improvements Inc. in Technical Bulletin No. 680 entitled: ‘Volatile Organic Emissions from Pulp and Paper Mill Sources, Part VI-Kraft recovery furnaces and black liquor oxidation systems’. In this document, the emissions from 2 air oxidation systems and 2 direct contact evaporator recovery boilers were presented.  
         [0051]    The HAP emissions from the air oxidation system at a mill designated Mill D were 1.85×10 −4  Kg/Kg of which, 1.65×10 −4  Kg/Kg was methanol (i.e., 89%). The HAP emissions from the recovery boiler of Mill D were 1.00×10 −4  Kg/Kg of which 0.65×10 −4  Kg/Kg was methanol (i.e., 65%).  
         [0052]    The HAP emissions from the air oxidation system at a mill designated Mill H were 1.40×10 −4  Kg/Kg of which, 1.05×10 −4  Kg/Kg was methanol (i.e., 75%). The HAP emissions from the recovery boiler of Mill H were 1.40×10 −4  Kg/Kg of which 1.0×10 −4  Kg/Kg was methanol (i.e., 71%).  
         [0053]    The above data is summarized in Table I.  
                                                   TABLE I                           Values are in Kg/Kg black liquor solids (BLS)                EPA   Mill D   Mill H                        Air Oxidizer Vent                   HAP   2.03 × 10 −4     1.85 × 10 −4     1.40 × 10 −4         MEOH   1.73 × 10 −4     1.65 × 10 −4     1.05 × 10 −4         TRS   2.93 × 10 −5         DCE Flue Gas       HAP   4.32 × 10 −4     1.00 × 10 −4     1.40 × 10 −4         MEOH   1.703 × 10 −4      0.65 × 10 −4     1.00 × 10 −4         TRS   1.21 × 10 −4                    
 
         [0054]    Data on vent gas volumes are presented in the NCASI report. Models of air sparged black liquor oxidation systems are presented in the EPA report that specific the flow of black liquor solids, the vent gas flow rate, the moisture content of the vent gas and the methanol emissions in kg/day.  
         [0055]    The data is summarized in Table II.  
                                                         TABLE II                                   Black Liquor   Vent gas Flow       Methanol           Firing Rate   Dry Standard   Moisture Content   Emissions           Kg/day   m3/sec   %   Kg/d                                    EPA   1.2   7.3   35   212       Mill D   0.76   4.19   54   124       Mill H   1.175   2.84   44.6   101.6                  
 
         [0056]    A thermodynamic analysis based on the operating conditions as specified in the EPA model was conducted to estimate the concentration of methanol in the feed black liquor  5  to the air oxidizer and the amount of oxygen in the air  9  that reacts with the black liquor. The estimated MeOH concentration in the black liquor was 150 mg/l and approximately 19.2% of the total oxygen in the air (i.e., 1186 kg/hr) was estimated to react with compounds in the black liquor.  
         [0057]    A thermodynamic analysis was conducted to estimate the total amount of methanol that would be transferred to the flue gas of the DCE if the air oxidation system was eliminated. It was found that the total emissions increased by an amount which was almost equal to the amount of methanol in the vent of the air oxidizer.  
         [0058]    Maximizing the amount of HAP and TRS in the vent gas from an air oxidizer, which is subsequently collected and incinerated, would be advantageous, as it would lower the HAP and TRS emissions from the DCE. One potential means of increasing the total amount of HAP and TRS in the vent is through an increasing in the amount of air  9  fed to the air oxidation system. However, there are distinct limits to this approach. It is known that the amount of oxygen reacting with the black liquor  5  fed to an air oxidation system is primarily dependent upon the size of the atmospheric reaction vessel and the amount of material in the black liquor that can be oxidized at the operating temperature of the air oxidizer. Therefore, an increase in airflow to the oxidizer will not cause an proportionate increase in the amount of oxygen reacted. As a result, the temperature of the vent gas and the liquor will decrease due to the cooling effects of humidification. Equilibrium calculations show that the concentration of HAP in the vent gas, primarily methanol, declines with a decrease in vent temperature. This is illustrated in FIG. 4 where data from the EPA model was used wherein the black liquor solids in the feed liquor  5  was 50 mt/hr, the concentration of black liquor solids in the feed black liquor  5  was 48 wt %, the moisture content of the vent gas  11  was 35 wt % and the vent gas volume  11  at 54° C. was 12.7m 3 /sec. Based upon this data to was estimated the concentration of MeOH in the feed liquor  5  was 150 mg/l, and the air flow  9  was 8.16 m 3 /sec. The air flow  9  was varied and the new equilibrium methanol concentration and flow rate of the vent gas  11  were calculated. The total amount of oxygen in the air  9  reacting with the black liquor was constant at 1186 kg/hr. The x-axis in FIG. 4 represents the ratio of vent gas flow to black liquor solids flow. The y-axis represents both the equilibrium methanol concentration in ppm, (dashed line), and the total emission of methanol in kg/hr, (solid line), in the vent gas  11 . It can be seen that the concentration of methanol declines as the vent gas flow is increased. This decline diminishes the positive impact of a higher vent gas volume on the transfer of HAP and TRS to the gas phase. Under these particular conditions, an increase in vent flow from 0.75 m3/kg BLS to 1.4 m3/kg BLS, an 87% increase, causes an increase in total methanol in the vent stream from 8.65 kg/hr to 9.07 kg/hr, a 4.9% increase. Typically, the potential reduction in HAP and TRS emissions from the DCE resulting from an increase in the air oxidizer vent gas flow does not justify the cost to collect, cool and incinerate the additional vent gas.  
         [0059]    An additional drawback of the prior art is poor energy recovery. The amount of energy recovered from the vent gas  11  is dependent upon the temperature of the gas  28  exiting the condensor. This is illustrated in FIG. 5 which is based on the EPA model. The x-axis is the temperature of the gas exiting the condenser. The y-axis represents the amount of energy transferred from the feed vent gas  11  to the cooling medium  30 , which is normally water. At most Pulp mills there is an excess amount of warm water  31  and in many cases, water at a temperature of less than 60° C. has no value.  
         [0060]    Therefore, generally there is limited value in the energy recovered from an air oxidizer vent gas.  
         [0061]    An additional drawback of this prior art is the large volume of cooled vent gas  28  and the high HAP and TRS concentration in the cooled vent gas  28 . Equilibrium calculations were conducted based on the EPA model with the assumption that the cooled vent gas  28  is cooled to 54° C. At these conditions, the cooled vent gas  28  flow rate was 12.7 m3/sec and approximately 82% of the methanol entering the condenser in stream  11 , remained in the cooled vent gas stream. Vent gas stream  28  is then passed directly to combustion device  111  or discharged into the High Volume Low Concentration, Non-condensible gas system (HVLC-NCG). The combustion device is typically an existing unit at the mill such as a power &amp;steam boiler, the recovery boiler, or an independent incinerator. The high volume of the cooled vent gas stream  28  requires large diameter ducting to transfer the gas to the combustion device, and the high moisture content and low oxygen content of the stream can cause operational problem in the combustion device or in the case where the combustion device is a dedicated incinerator, there is a substantial capital investment requirement.  
       EXAMPLE 1  
       [0062]    The present invention comprises improvements to the prior art methods of FIG. 1. One embodiment of the invention is illustrated in FIG. 3. Partially concentrated black liquor  5 , which has been processed through the multiple effect evaporators  101 , is passed to a reactor  113  where the liquor is pressurized and an oxygen containing gas  33  having a concentration of 95% is injected into the stream. 99% of the oxygen is made to react with the sulfide and organic material in the black liquor causing an increase in the sensible heat of the black liquor. The oxidized liquor  49  is then depressurized in flashtank  117  where the sensible heat is converted to latent heat in the form of water vapor. A portion of the HAP and TRS compounds in the liquor are transferred to the gas phase due to thermodynamic equilibrium. The vent gas  45  is conveyed to a condensor  109  where the gas is cooled and the majority of the water vapour is condensed and removed in condensate stream  32 . Additionally, the majority of the methanol and some of the HAP and TRS are condensed and discharged in the condensate stream  32 . The vent gas exiting the condensor  28  is discharged to atmosphere or conveyed to an incineration device  111  or discharged into either the HVLC-NCG system or the low volume, high concentration non-condensible gas system (LVHC-NCG). The desired total amount of HAP and TRS transferred to the vent gas is controlled through the reaction of a sufficient amount of oxygen  33  with the black liquor  5 .  
         [0063]    Oxidized black liquor  13 , now containing typically less than 2 g/l of sodium sulfide, and a controlled concentration of HAP and TRS compounds, passes to the direct contact evaporation system  105  in which the black liquor is further concentrated by direct contact with hot flue gas  15  from recovery boiler  107 . Fully concentrated black liquor  17  and final flue gas  19  flow from the evaporator system  105 . Flue gas  19  contains, water vapor, combustion products, hydrogen sulfide and HAP and TRS compounds. The concentration of HAP and TRS compounds in the flue gas  19  is a function of thermodynamic equilibrium. The total amount of these compounds in the flue gas  19  is dependent primarily upon their concentration in the feed black liquor  13 , which concentration is controlled through the adding of a specific amount of oxygen  33 .  
         [0064]    The fully concentrated black liquor  17  proceeds in the same manner as described in the prior art.  
         [0065]    An improvement of the present invention over the prior art of air oxidation comprises the ability to control the transfer of HAP and TRS into the vent gas  45  by controlling the amount of oxygen  33  reacting with the black liquor S. This is illustrated in FIG. 6 where data from the EPA model was used. In this graph, the x-axis represent the ratio of vent gas flow  45  of the invention and vent gas flow  11  of the prior art to black liquor solids flow. The y-axis represents the total transfer of methanol in kg/hr from the black liquor to the air oxidation system vent stream  11  and the present invention vent stream  45 . In the present invention, 9 kg/hr of methanol can be transferred to the vent gas  45  at a total vent gas ratio of 0.4 m3/kg BLS. In contrast, the vent flow  11  from the air oxidation system must be 1.2 m3/kg BLS to transfer the same 9 Kg/hr. Also, in the present invention, the reaction of a sufficient amount of oxygen  33  to produce a vent stream volume of 0.6 m3/kg BLS will cause a transfer of 10 kg/hr of methanol to the vent stream  45  whereas it is clear from the data that an air oxidation system cannot achieve this amount of transfer.  
         [0066]    An additional improvement to the prior art of air oxidation is a significantly lower cooled vent gas flow  28  as compared to the vent flow  28  FIG. 1 from an air oxidation system. Equilibrium calculations were conducted based on the EPA model parameters with the assumption that the cooled vent gas  28  is cooled to 54° C. At these conditions, the present invention cooled vent gas  28  flow rate was less than 0.035 m3/sec and approximately 1% of the methanol entering the condenser in stream  45 , remained in the cooled vent gas stream  28  whereas the cooled vent gas  28  flow rate from the air oxidation system was 12.7 m 3 /sec and approximately 82% of the methanol entering the condenser in stream  45 , remained in the cooled vent gas stream  28 . This very low flow gas stream from the present invention, which has been depleted in oxygen can be discharged safely into the existing low volume, high concentration, non-condensible gas system LVHC. The gases of the LVHC system are typically combusted in the lime kiln at a Kraft mill. Therefore no additional incineration device is required.  
         [0067]    An additional improvement of the present invention is the recover of the energy in the vent gas  45  at a temperature that is sufficiently high to be of value to the mill. In the presented embodiment, the flashtank  117  is operated at 1.0 bar(g). At this pressure approximately 98% of the energy in the vent gas is recovered at a temperature of approximately 90° C., thereby providing energy at a temperature that is of value to the mill. Alternatively, should a mill desire to recover the energy at a higher temperature, the flashtank can be made to operate at a higher pressure. This option is unavailable in an air oxidation system as the cost to operate the system at pressure would be prohibitively high.  
         [0068]    The improvement of the present invention over the prior art as described in U.S. Pat. Nos. 4,239,589 and 4,313,788 can be understood by an initial review of FIG. 2 Referring to FIG. 2, weak black liquor  1  from the wood pulping and washing steps is fed to multiple effect evaporation system  101 . Weak black liquor contains water, dissolved lignin and other wood constituents, sodium salts (particularly sodium sulfide and other unoxidized sulfur compounds), sodium hydroxide. Trace compounds in the black liquor include HAP such as, methanol, and benzene, and TRS such as methyl mercaptan and dimethyl disulfide. The liquor is typically at 90° C. and 1.0 bar(g), and typically contains 15 wt % solids. The liquor is concentrated in multiple effect evaporator system  101  heated by steam  3  as is known in the art to yield partially concentrated black liquor  13  and residual steam/condensate  7 . In the prior art, a liquor stream  47  is withdrawn from an intermediate location in the multiple effect evaporators  101 . The withdrawn liquor  47 , typically at 110° C. and 2.0 bar(g) containing 35 wt % solids flows to a reactor  113  where a gas stream  51  containing typically 99% oxygen, oxidizes the sodium hydrosulfide to sodium thiosulphate via the reaction: 
         2NaSH+2O 2 →Na 2 S 2 O 3 +H 2 O. 
         [0069]    and a portion of the organic material in the black liquor.  
         [0070]    The prior art specifically directs that the amount of oxygen made to react with the black liquor should be sufficient to convert a desired amount of NaSH to Na 2 S 2 O 3 . The oxidized liquor  49 , which is now at an elevated temperature, is returned to the multiple effect evaporators  101  where the sensible heat in the liquor is converted to latent heat in the effect receiving the heated black liquor. The liquor proceeds through the remaining effects of the multiple effect evaporators  101  and is discharged as partially concentrated and oxidized black liquor  13 , typically at 100° C. and 1.0 bar(g) containing 48 wt % solids flows  
         [0071]    A portion of the HAP and TRS compounds in the weak black liquor  1  are transferred to the steam/condensate  7  and a portion remain in the partially concentrated black liquor.  
         [0072]    Oxidized black liquor  13 , now containing typically less than 2 g/l of sodium sulfide, passes to the direct contact evaporation system  105  in which the black liquor is further concentrated by direct contact with hot flue gas  15  from recovery boiler  107 . Fully concentrated black liquor  17  and final flue gas  19  flows from the evaporator system  105 . Flue gas  19  contains, water vapor, combustion products, hydrogen sulfide and HAP and TRS compounds. The concentration of HAP and TRS compounds in the flue gas  19  is a function of thermodynamic equilibrium. The total amount of these compounds in the flue gas  19  is dependent primarily upon their concentration in the feed black liquor  13  which concentration is dependent upon the operating conditions of the multiple effect evaporators.  
         [0073]    The fully concentrated black liquor  17  proceeds in the same manner as described in FIG. 1.  
         [0074]    In the practice of this prior art, the concentration of HAP and TRS compounds in the liquor entering the direct contact evaporator system  105  is equal to the concentration in the liquor exiting the multiple effect evaporation system  101 . As a result, the amount of HAP and TRS emissions from the DCE  105  is greater than the prior air of air oxidation or the present invention.  
         [0075]    The improvement of the present invention over this prior art comprises the ability to substantially lower the HAP and TRS emissions from the DCE  105  by conducting the oxidation after the multiple effect evaporation system and by controlling the amount of oxygen reacting with the black liquor so as to control the transfer of HAP and TRS into vent gas  45 , FIG. 3.  
       EXAMPLE 2  
       [0076]    This example shows how a combination of oxygen based and air based black liquor oxidation can be used to maximize the stripping of HAP&#39;s and TRS from the black liquor The removal of methanol from the liquor is a function of the temperature of the vent gas. To achieve a low Na2S concentration a mill normally has 2 oxidizer, a primary and a secondary. The primary oxidizer converts about 90% of the Na2S and the percent of oxygen fed to the system, which reacts with the liquor, is high (i.e., 35%). This means the energy release per volume of vent gas is high (again on a relative basis). Therefore the temperature of the primary oxidizer vent will be reasonable high and thus the methanol stripped from the liquor (as Kg MeOH/Kg vent flow) will be high. In contrast, the amount of feed oxygen reacting with the liquor in the ‘secondary’ oxidizer is quite low. This means that the temperature of the vent stream is generally lower than the feed temperature of the black liquor (i.e., the energy needed to humidity the vent is greater than the energy released in the oxidation reaction). Therefore, the amount of methanol (HAP&#39;s) stripped from the liquor into the vent of the secondary oxidizer is low (relatively).  
         [0077]    This issue is further addressed in this Example and is illustrated in FIG. 9. One uses a certain amount of air (as the primary oxidant) in a primary air oxidizer  127 , and then replaces the second oxidizer with an oxygen-based reactor  113 , which is connected to the air oxidizer by line  50 . In this way, the total amount of vent gas  45  for an equal transfer of HAP&#39;s from the liquor would be less than an oxidation system employing secodary oxidizer. However, the cost is a factor. The primary reason to go to the oxygen process is to minimize the cooled vent stream from the condenser. If one opts for the above, the vent flow will be a function of the amount of ‘air’ oxidation employed. Since the cost of collection and incineration is high, due consideration must be made to the capital and operating costs.  
       EXAMPLE 3  
       [0078]    The logic is the same as in Example 2. However, one enriches the air stream sent to the primary air oxidizer. The intent is to maximize the HAP removal while minimizing the vent steam.  
       EXAMPLE 4  
       [0079]    One may not want to use 100% oxygen, but rather prefers to operate at an oxygen concentration such that the nitrogen (argon or other inert gas) that passes through the system and finally into the vent from the condenser in an amount sufficient to ensure that the concentration of combustible NCG (primarily methyl mercaptans and dimethyl disulfide) is well below the lower flammability limit. One operates at the oxygen concentration that dilutes the remaining NCG&#39;s to levels below the LFL. If this is not done, one has to add air or an inert gas to the condenser to control the concentration of NCG to below the LFL.  
       EXAMPLE 5  
       [0080]    The intent of the black liquor oxidation process is to convert NaSH to Na2S203. A competing reaction is the oxidation of organic material in the black liquor. Both reactions are exothermic, and cause a rise in liquor temperature. As the temperature increases, the amount of oxygen reacting with the organic material increases. Also, the solubility of calcium carbonate decreases with temperature. A process has been developed wherein a portion of the black liquor is recycled and cooled (by flashing) to controls the temperature of the oxidation reaction.  
         [0081]    When black liquor is oxidized 2 reactions can occur: 
         [0082]    2NaSH+202-→Na2S203+H2O  (1) 
         Oxygen+Organic-→acids 
         [0083]    It is well known that the amount of oxygen reaction with the organic material in the black liquor increases with an increase in reactor temperature. That is to say that the selectivity to NaSH decreases with increased temperature. Additionally, the solubility of calcium carbonate in black liquor decreases with an increase in temperature. Both of these temperature effects have caused problems in oxygen based black liquor oxidation systems.  
         [0084]    Also, high temperature reduces oxygen solubility and lowers the oxygen partial pressure in a closed system, which slows the rate of reaction.  
         [0085]    The temperature rise in an oxygen based black liquor oxidation system is depending upon the amount of oxygen reacted. The amount of oxygen reacted is a function of the NaSH concentration in the liquor. This concentration is dependent upon the amount of water present in the black liquor. From an energy recovery, and environmental perspective, it is best to conduct the oxidation at a solid concentration of between 40 and 60% solids. However, the NaSH concentration at these concentrations is high, at approximately 35 g/1. Therefore the temperature rise through the reactor is highest at these solids. The higher temperature lowers the selectivity which in turn increases the amount of oxygen needed for NaSH conversion, which increases the temperature.  
         [0086]    To address the foregoing, an oxidation system can be designed which allows for temperature control. The system is illustrated in FIG. 7, for a system using an indirect contact evaporator  106  and in FIG. 8 for a system using a direct contact evaporator  105 . A portion of the liquor is recycled using line  123  in FIG. 7 and line  121  in FIG. 8, which is otherwise the same as FIG. 3. In FIG. 8, the liquor is cooled by flashing in the flash tank  117  and then reintroduced into the reactor  113 .  
         [0087]    The advantages of the system include:  
         [0088]    1) Control of the reaction selectivity;  
         [0089]    2) A means of ensuring that the solubility limit of calcium carbonate in the black liquor i greater than the actual concentration of the calcium carbonate in the black liquor;  
         [0090]    3) Constant reactor operating conditions; and  
         [0091]    4) Oxidation of NaSH which is somewhat independent of the flow rate of black liquor to/from the system.  
         [0092]    Although illustrated and described herein with with reference to certain specific embodiments, the present invention is not intended to be limited to the detailed embodiments shown. One skilled in the art can understand the invention and make various modifications thereto without departing from the basic spirit thereof, and without departing from the scope of the claims which follow.