Abstract:
Methods and apparatus for introducing a gas into the reaction zone of a reactor. Such methods and apparatus can more evenly distribute the gas throughout the reaction zone. Spargers for introducing a gas into the reaction zone of a reactor can be employed in systems and methods for carrying out the liquid-phase oxidation of an oxidizable compound, such as para-xylene.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    Various embodiments of the present invention generally relate to methods and apparatus for enhancing gas distribution in a reactor. More particularly, various embodiments of the present invention relate to spargers providing improved gas distribution in bubble column reactors. 
         [0003]    2. Description of the Related Art 
         [0004]    Liquid-phase oxidation reactions are employed in a variety of existing commercial processes. For example, liquid-phase oxidation is currently used for the oxidation of aldehydes to acids (e.g., propionaldehyde to propionic acid), the oxidation of cyclohexane to adipic acid, and the oxidation of alkyl aromatics to alcohols, acids, or diacids. A particularly significant commercial oxidation process in the latter category (oxidation of alkyl aromatics) is the liquid-phase catalytic partial oxidation of para-xylene to terephthalic acid. Terephthalic acid is an important compound with a variety of applications. The primary use of terephthalic acid is as a feedstock in the production of polyethylene terephthalate (“PET”). PET is a well-known plastic used in great quantities around the world to make products such as bottles, fibers, and packaging. 
         [0005]    In a typical liquid-phase oxidation process, including partial oxidation of para-xylene to terephthalic acid, a liquid-phase feed stream and a gas-phase oxidant stream are introduced into a reactor and form a multi-phase reaction medium in the reactor. The liquid-phase feed stream introduced into the reactor contains at least one oxidizable organic compound (e.g., para-xylene), while the gas-phase oxidant stream contains molecular oxygen. At least a portion of the molecular oxygen introduced into the reactor as a gas dissolves into the liquid phase of the reaction medium to provide oxygen availability for the liquid-phase reaction. If the liquid phase of the multi-phase reaction medium contains an insufficient concentration of molecular oxygen (i.e., if certain portions of the reaction medium are “oxygen-starved”), undesirable side-reactions can generate impurities and/or the intended reactions can be retarded in rate. If the liquid phase of the reaction medium contains too little of the oxidizable compound, the rate of reaction may be undesirably slow. Further, if the liquid phase of the reaction medium contains an excess concentration of the oxidizable compound, additional undesirable side-reactions can generate impurities. 
         [0006]    Conventional liquid-phase oxidation reactors are equipped with agitation means for mixing the multi-phase reaction medium contained therein. Agitation of the reaction medium is supplied in an effort to promote dissolution of molecular oxygen into the liquid phase of the reaction medium, maintain relatively uniform concentrations of dissolved oxygen in the liquid phase of the reaction medium, and maintain relatively uniform concentrations of the oxidizable organic compound in the liquid phase of the reaction medium. 
         [0007]    Agitation of the reaction medium undergoing liquid-phase oxidation is frequently provided by mechanical agitation means in vessels such as, for example, continuous stirred tank reactors (“CSTRs”). Although CSTRs can provide thorough mixing of the reaction medium, CSTRs have a number of drawbacks. For example, CSTRs have a relatively high capital cost due to their requirement for expensive motors, fluid-sealed bearings and drive shafts, and/or complex stirring mechanisms. Further, the rotating and/or oscillating mechanical components of conventional CSTRs require regular maintenance. The labor and shutdown time associated with such maintenance adds to the operating cost of CSTRs. However, even with regular maintenance, the mechanical agitation systems employed in CSTRs are prone to mechanical failure and may require replacement over relatively short periods of time. 
         [0008]    Bubble column reactors provide an attractive alternative to CSTRs and other mechanically agitated oxidation reactors. Bubble column reactors provide agitation of the reaction medium without requiring expensive and unreliable mechanical equipment. Bubble column reactors typically include an elongated upright reaction zone within which the reaction medium is contained. Agitation of the reaction medium in the reaction zone is provided primarily by the natural buoyancy of gas bubbles rising through the liquid phase of the reaction medium. This natural-buoyancy agitation provided in bubble column reactors reduces capital and maintenance costs relative to mechanically agitated reactors. Further, the substantial absence of moving mechanical parts associated with bubble column reactors provides an oxidation system that is less prone to mechanical failure than mechanically agitated reactors. 
         [0009]    When liquid-phase partial oxidation of para-xylene is carried out in a conventional oxidation reactor (CSTR or bubble column), the product withdrawn from the reactor is typically a slurry comprising crude terephthalic acid (“CTA”) and a mother liquor. CTA contains relatively high levels of impurities (e.g., 4-carboxybenzaldehyde, para-toluic acid, fluorenones, and other color bodies) that render it unsuitable as a feedstock for the production of PET. Thus, the CTA produced in conventional oxidation reactors is typically subjected to a purification process that converts the CTA into purified terephthalic acid (“PTA”) suitable for making PET. 
         [0010]    Although advances have been made in the art of liquid-phase oxidation reactions, improvements are still needed. 
       SUMMARY 
       [0011]    One embodiment of the present invention concerns a reactor defining a reaction zone therein. The reactor of this embodiment comprises a sparger disposed in the reaction zone for introducing fluid into the reaction zone. The sparger of this embodiment comprises at least three radially-extending fluid distribution conduits, where each fluid distribution conduit defines at least three fluid discharge openings, where the radial spacing of the fluid discharge openings associated with each of the fluid distribution conduits decreases outwardly, and where the sparger has a maximum diameter that is at least 90 percent of the diameter of the reaction zone at the elevation where the sparger is disposed. 
         [0012]    Another embodiment of the present invention concerns a reactor defining a reaction zone therein. The reactor of this embodiment comprises a sparger disposed in the reaction zone for introducing fluid into the reaction zone, where the sparger comprises one or more fluid distribution conduits defining in the range of from 20 to 300 fluid discharge openings, where when the sparger is theoretically partitioned into four annular regions of equal area, the cumulative discharge opening area of the fluid discharge openings located in one of the annular regions is within 25 percent of the cumulative discharge opening area of the fluid discharge openings located in at least one other of the annular regions, where the sparger has a total flow-through open area of at least 25 percent, where the fluid discharge openings have an average mean diameter in the range of from about 0.5 to about 2.0 mm, where greater than 50 percent of the fluid discharge openings are positioned to discharge the fluid in a normally downward direction, where the sparger has a maximum diameter in the range of from about 0.5 to about 6 meters, and where the sparger has a maximum diameter that is at least 90 percent of the diameter of the reaction zone at the elevation where the sparger is disposed. 
         [0013]    Yet another embodiment of the present invention concerns a system for at least partially oxidizing an oxidizable compound by contacting at least a portion of the oxidizable compound with a gas-phase oxidant. The system of this embodiment comprises a first oxidation reactor; a second oxidation reactor in downstream fluid-flow communication with the first oxidation reactor; a bubble column reactor in downstream fluid-flow communication with the second oxidation reactor and defining a reaction zone; and a sparger disposed within the reaction zone and configured to discharge at least a portion of the gas-phase oxidant into the reaction zone. In this embodiment, the sparger comprises one or more fluid distribution conduits defining a plurality of fluid discharge openings. Also, when the sparger is theoretically partitioned into four annular regions of equal area, the cumulative discharge opening area of the fluid discharge openings located in one of the annular regions is within 25 percent of the cumulative discharge opening area of the fluid discharge openings located in at least one other of the annular regions. Further, the sparger has a total flow-through open area of at least 25 percent and has a maximum diameter that is at least 90 percent of the diameter of the reaction zone at the elevation where the sparger is disposed. 
         [0014]    Still another embodiment of the present invention concerns a method for producing a dicarboxylic acid. The method of this embodiment comprises (a) contacting an oxidizable compound with a first gas-phase oxidant thereby forming a crude dicarboxylic acid slurry; (b) purifying at least a portion of the crude dicarboxylic acid slurry thereby forming a purified dicarboxylic acid slurry; and (c) contacting at least a portion of the purified dicarboxylic acid slurry with a second gas-phase oxidant in a reaction zone of a bubble column reactor, wherein at least a portion of the second gas-phase oxidant is introduced into the reaction zone via a sparger disposed in the reaction zone. The sparger of this embodiment comprises one or more fluid distribution conduits defining a plurality of fluid discharge openings, where when the sparger is theoretically partitioned into four annular regions of equal area, the cumulative discharge opening area of the fluid discharge openings located in one of the annular regions is within 25 percent of the cumulative discharge opening area of the fluid discharge openings located in at least one other of the annular regions, where the sparger has a total flow-through open area of at least 25 percent, where the sparger has a maximum diameter that is at least 90 percent of the diameter of the reaction zone at the elevation where the sparger is disposed. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0015]    Embodiments of the present invention are described herein with reference to the following drawing figures, wherein: 
           [0016]      FIG. 1  is a side view of a reactor constructed in accordance with one embodiment of the present invention, particularly illustrating the introduction of slurry and gas streams into the reaction zone of the reactor, and the withdrawal of an off-gas and a treated slurry from the top and bottom of the reactor, respectively; 
           [0017]      FIG. 2  is a cross-sectional view of the reactor depicted in  FIG. 1  taken along line  2 - 2 , particularly illustrating a sparger having straight radially-extending fluid distribution conduits for introducing a fluid into the reaction zone of the reactor; 
           [0018]      FIG. 3  is a bottom view of an alternate sparger suitable for use in the reactor depicted in  FIG. 1 , particularly illustrating a sparger having curved radially-extending fluid distribution conduits for introducing a fluid into the reaction zone of the reactor; 
           [0019]      FIG. 4  is a bottom view of an alternate sparger suitable for use in the reactor depicted in  FIG. 1 , particularly illustrating a sparger having circular fluid distribution conduits for introducing a fluid into the reaction zone of the reactor; 
           [0020]      FIG. 5  is a bottom view of an alternate sparger suitable for use in the reactor depicted in  FIG. 1 , particularly illustrating a sparger having square fluid distribution conduits for introducing a fluid into the reaction zone of the reactor; 
           [0021]      FIG. 6  is a bottom view of an alternate sparger suitable for use in the reactor depicted in  FIG. 1 , particularly illustrating a sparger having octagonal fluid distribution conduits for introducing a fluid into the reaction zone of the reactor; 
           [0022]      FIG. 7  is a schematic view of a system for oxidizing an oxidizable compound, particularly illustrating a primary oxidation reactor, a primary oxidation side-draw reactor, a secondary oxidation reactor, and a secondary oxidation side-draw reactor having a sparger disposed therein. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Various embodiments of the invention concern a sparger for introducing a fluid into the reaction zone of a reactor, such as a bubble column reactor. Such a sparger can be employed in a system for the liquid-phase oxidation of an oxidizable compound, which can be carried out in the liquid phase of a multi-phase reaction medium contained in one or more agitated reactors. Suitable agitated reactors include, for example, bubble-agitated reactors (e.g., bubble column reactors), mechanically agitated reactors (e.g., continuous stirred tank reactors), and flow agitated reactors (e.g., jet reactors). 
         [0024]    Referring initially to  FIG. 1 , a sparger  10  is shown disposed in a bubble column reactor  12 . As used herein, the term “bubble column reactor” shall denote a reactor for facilitating chemical reactions in a multi-phase reaction medium, where agitation of the reaction medium is provided primarily by the upward movement of gas bubbles through the reaction medium. As used herein, the term “agitation” shall denote work dissipated into the reaction medium causing fluid flow and/or mixing. As used herein, the terms “majority,” “primarily,” and “predominately” shall mean more than 50 percent. As used herein, the term “mechanical agitation” shall denote agitation of the reaction medium caused by physical movement of a rigid or flexible element(s) against or within the reaction medium. For example, mechanical agitation can be provided by rotation, oscillation, and/or vibration of internal stirrers, paddles, vibrators, or acoustical diaphragms located in the reaction medium. As used herein, the term “flow agitation” shall denote agitation of the reaction medium caused by high velocity injection and/or recirculation of one or more fluids in the reaction medium. For example, flow agitation can be provided by nozzles, ejectors, and/or eductors. In various embodiments of the present invention, less than about 40, less than about 20, or less than 5 percent of the agitation of the reaction medium in the bubble column reactor is provided by mechanical and/or flow agitation. 
         [0025]    Referring still to  FIG. 1 , the bubble column reactor  12  is illustrated as comprising the sparger  10 , a vessel shell  14 , a gas inlet  16 , a slurry inlet  18 , a gas entry conduit  20 , and an off-gas outlet  22 . The bubble column reactor  12  can be configured for a counter-current reaction scheme, such that, in operation, a slurry can be introduced via the slurry inlet  18  at or near the normally upper-portion of the bubble column reactor  12  and can flow in a downward direction through a reaction zone  24  defined in the bubble column reactor  12 . A gas (e.g., a gas-phase oxidant) can be introduced into the bubble column reactor  12  via the inlet  16  and be dispersed into the reaction zone  24  via the sparger  10  located at or near the normally lower-portion of the bubble column reactor  12 . The gas can then travel in a substantially upward manner through the reaction zone  24 . Thereafter, a treated slurry can be withdrawn from the bottom of the bubble column reactor  12  via a slurry outlet  26 . In various embodiments, the flow behavior in the reaction zone  24  can be bubbly flow or substantially bubbly flow. Furthermore, in various embodiments, the flow behavior in the reaction zone  24  can be plug flow or substantially plug flow, where there is negligible convective mixing of mass with surrounding mass while flowing through the reaction zone  24 . In various embodiments, plug flow or near plug flow patterns can be achieved by increasing gas distribution in the reaction zone  24 , such that the same amount or substantially the same amount of gas phase oxidant is introduced into each area of the reaction zone  24 . In other words, plug flow or near plug flow behavior can be achieved by employing even or substantially even gas distribution across the entire or substantially the entire horizontal cross-section of the reaction zone  24 . 
         [0026]    Referring now to  FIG. 2 , a cross-section of the bubble column reactor  12  taken along line  2 - 2  is provided depicting the sparger  10  in greater detail. The sparger  10  comprises twelve straight or substantially straight radially-extending fluid discharge conduits  28 , each comprising eight fluid discharge openings  30 . Although the sparger  10  is depicted having twelve of the radially-extending fluid discharge conduits  28 , in various embodiments of the invention, the sparger  10  can have at least 3, at least 4, at least 6, at least 8, or at least 10 of the radially-extending fluid discharge conduits  28 . Additionally, in one or more embodiments, the sparger  10  can have in the range of from 3 to 20, in the range of from 6 to 18, or in the range of from 9 to 15 of the radially-extending fluid discharge conduits  28 . 
         [0027]    As depicted in  FIG. 2 , each of the radially-extending fluid distribution conduits  28  is coupled in fluid-flow communication with a vertical member  32  of the gas entry conduit  20  and extends radially therefrom. In one or more embodiments, each of the radially-extending fluid distribution conduits  28  can be equally or substantially equally spaced around the vertical member  32 . As used herein, the term “substantially equally spaced” shall mean that the spacing between each of the radially-extending fluid distribution conduits  28  varies by less than 5 percent. In various embodiments, each of radially-extending fluid distribution conduits  28  can be cylindrical or substantially cylindrical. Additionally, each of the radially-extending fluid distribution conduits  28  can have a length in the range of from about 0.25 to about 3 meters, or in the range of from 0.5 to 2.5 meters. Furthermore, each of the radially-extending fluid distribution conduits  28  can have an outer diameter in the range of from about 1 to about 10 cm, or in the range of from about 2 to about 5 cm. In various embodiments, each of the radially-extending fluid distribution conduits  28  can have an outer diameter of about 3 cm. 
         [0028]    As noted above, each of the radially-extending fluid distribution conduits  28  defines a plurality of the fluid discharge openings  30 . In various embodiments, each radially-extending fluid distribution conduit  28  can comprise at least 3, at least 4, at least 6, or at least 8 of the fluid discharge openings  30 . Additionally, each of the radially-extending fluid distribution conduits  28  can comprise in the range of from 3 to 20, in the range of from 5 to 17, or in the range of from 7 to 14 of the fluid discharge openings  30 . In various embodiments, each of the radially-extending fluid distribution conduits  28  can comprise 8 fluid discharge openings. In various embodiments, the sparger  10  can comprise a total of at least 20, at least 50, or at least 90 of the fluid discharge openings  30 . Furthermore, the sparger  10  can comprise a total number of fluid discharge openings  30  in the range of from 20 to 300, in the range of from 50 to 250, or in the range of from 80 to 220. 
         [0029]    In one or more embodiments, the radial spacing of the fluid discharge openings  30  associated with each of their respective radially-extending fluid distribution conduits  28  can decrease outwardly from the axial center of the sparger  10 . The radial spacing is considered to “decrease” in the arrangement where, if the distances between adjacent pairs of fluid discharge openings  30  (Y values) were plotted as a function of each distance&#39;s location relative to the center of the sparger (X values), the resulting linear trend line (i.e., the linear regression) would have a negative slope. What is meant by the distance&#39;s relative location is that the distance between the innermost adjacent pair of fluid discharge openings would be assigned an arbitrary X value of 1, the distance between the next outwardly spaced pair of fluid discharge openings would be assigned an arbitrary X value of 2, and so on. In various embodiments, the radial spacing can decrease between each subsequent outwardly-spaced pair of fluid discharge openings  30 . However, while it is permissible, it is not necessary for the radial spacing to decrease between each subsequent outwardly-spaced pair of fluid discharge openings  30 , so long as the plot described above has an overall negatively-sloped linear regression. By way of example, a fluid distribution conduit having the fluid discharge opening spacing described by the hypothetical data in Table 1 would have a slope of −5, even though the distances of space designations  2  and  6  are larger than their preceding space designations: 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Hypothetical Example of Decreasing Radial Spacing 
               
             
          
           
               
                   
                 Space Designation 
                 Distance Between Adjacent Openings 
               
               
                   
                   
               
               
                   
                 1 
                 35 cm 
               
               
                   
                 2 
                 40 cm 
               
               
                   
                 3 
                 25 cm 
               
               
                   
                 4 
                 20 cm 
               
               
                   
                 5 
                 15 cm 
               
               
                   
                 6 
                 20 cm 
               
               
                   
                 7 
                  5 cm 
               
               
                   
                   
               
             
          
         
       
     
         [0030]    In one or more embodiments, each of the radially-extending fluid distribution conduits  28  can comprise an innermost fluid discharge opening  34 , an outermost fluid discharge opening  36 , and one or more intermediate fluid discharge openings  38 . As can be seen in  FIG. 2 , the distance between the innermost fluid discharge opening  34  and its adjacent intermediate fluid discharge opening  38   a  can be greater than the distance between the outermost fluid discharge opening  36  and its adjacent intermediate fluid discharge opening  38   f . In various embodiments, the distance between the innermost fluid discharge opening  34  and its adjacent fluid discharge opening  38   a  can be at least 1, at least 5, or at least 10 percent greater than the distance between the outermost fluid discharge opening  36  and its adjacent intermediate fluid discharge opening  38   f . Additionally, in various embodiments, the distance between the innermost fluid discharge opening  34  and its adjacent intermediate fluid discharge opening  38   a  can be greater than the distance between two adjacent intermediate fluid discharge openings  38  (e.g., fluid discharge openings  38   a  and  38   b ). In one or more embodiments, the distance between the innermost fluid discharge opening  34  and its adjacent intermediate fluid discharge opening  38   a  can be at least 1, at least 5, or at least 10 percent greater than the distance between two adjacent intermediate fluid discharge openings  38  (e.g., fluid discharge openings  38   a  and  38   b ). Furthermore, in various embodiments, the distance between the innermost fluid discharge opening  34  and its adjacent intermediate fluid discharge opening  38   a  can be greater than each of the distances between adjacent fluid discharge openings  38 . Also, the distance between the innermost fluid discharge opening  34  and its adjacent intermediate fluid discharge opening  38   a  can be at least 1, at least 5, or at least 10 percent greater than each of the distances between adjacent fluid discharge openings  38 . Moreover, in various embodiments, the distance between adjacent intermediate fluid discharge openings  38  can decrease with outward radial placement from vertical member  32 . In one or more embodiments, the distance between adjacent intermediate fluid discharge openings  38  can decrease by at least 1, at least 5, or at least 10 percent among each subsequent outwardly placed adjacent pairs of intermediate fluid discharge openings  38 . For example, the distance between intermediate fluid discharge openings  38   b  and  38   c  can be at least 1, at least 5, or at least 10 percent less than the distance between intermediate fluid discharge openings  38   a  and  38   b . In all embodiments described herein relating to the spacing of the fluid discharge openings  30 , the distance between fluid discharge openings  30  shall be determined from the center of one fluid discharge opening to the center of its adjacent fluid discharge opening. 
         [0031]    In one or more embodiments, fluid discharge openings  30  can be spaced equiannularly or substantially equiannularly on each of fluid distribution conduits  28 . As used herein, the term “equiannular” when used to describe fluid discharge opening spacing shall denote spacing such that the annular areas of theoretical concentric or substantially concentric rings defined by the centers of fluid discharge openings  30  are equal. As used herein, the term “substantially” when modifying the term “equiannular” shall mean that the annular areas of the theoretical concentric rings vary by less than 1 percent between any two annular areas. 
         [0032]    In one or more embodiments, when the sparger  10  is theoretically partitioned into four annular regions of equal area, the cumulative discharge opening area of all the fluid discharge openings  30  located in a first selected annular region can be within 25, within 10, within 5, or within 1 percent of the cumulative discharge opening area(s) of the fluid discharge openings  30  located in at least one, at least two, or all three of the remaining annular regions. In other words, at least two, at least three, or all four of the annular regions can have cumulative discharge opening areas of the fluid discharge openings  30  that are within 25, within 10, within 5, or within 1 percent of each other. Additionally, when the sparger  10  is theoretically partitioned into four annular regions of equal area, the cumulative discharge opening area of all the fluid discharge openings  30  located in the outermost annular region can be within 25, within 10, within 5, or within 1 percent of the cumulative discharge opening area of all the fluid discharge openings  30  located in the innermost annular region. Furthermore, when the sparger  10  is theoretically partitioned into four annular regions of equal area, the cumulative discharge opening area of all the fluid discharge openings  30  located in the outermost annular region can be within 25, within 10, within 5, or within 1 percent of the cumulative discharge opening area of all the fluid discharge openings  30  located in the inner-intermediate annular region. Also, when the sparger  10  is theoretically partitioned into four annular regions of equal area, the cumulative discharge opening area of all the fluid discharge openings  30  located in the outermost annular region can be within 25, within 10, within 5, or within 1 percent of the cumulative discharge opening area of all the fluid discharge openings  30  located in the outer-intermediate annular region. Moreover, when the sparger  10  is theoretically partitioned into four annular regions of equal area, the cumulative discharge opening area of all the fluid discharge openings  30  located in the innermost annular region can be within 25, within 10, within 5, or within 1 percent of the cumulative discharge opening area of all the fluid discharge openings  30  located in the inner-intermediate annular region. Additionally, when the sparger  10  is theoretically partitioned into four annular regions of equal area, the cumulative discharge opening area of all the fluid discharge openings  30  located in the innermost annular region can be within 25, within 10, within 5, or within 1 percent of the cumulative discharge opening area of all the fluid discharge openings  30  located in the outer-intermediate annular region. Also, when the sparger  10  is theoretically partitioned into four annular regions of equal area, the cumulative discharge opening area of all the fluid discharge openings  30  located in the inner-intermediate annular region can be within 25, within 10, within 5, or within 1 percent of the cumulative discharge opening area of all the fluid discharge openings  30  located in the outer-intermediate annular region. It should be understood that, if the boundary of a theoretical annular region bisects a fluid discharge opening  30 , then each portion of the bisected fluid discharge opening  30  will count only toward the cumulative discharge opening area of the respective annular region in which that portion lies. 
         [0033]    In various embodiments, the fluid discharge openings  30  can be circular or substantially circular. Additionally, the fluid discharge openings  30  can have an average mean diameter in the range of from about 0.5 to about 2.0 mm, in the range of from about 0.6 to about 1.8 mm, in the range of from about 0.7 to about 1.6 mm, or in the range of from 0.8 to 1.4 mm. Furthermore, in various embodiments, the fluid discharge openings  30  can all be substantially the same size, having a variation in the mean diameter of less than 0.5 mm, less than 0.3 mm, less than 0.1 mm, or less than 0.05 mm between any two of the fluid discharge openings  30 . 
         [0034]    In various embodiments, at least a portion of the fluid discharge openings  30  can be positioned so as to be able to discharge a fluid in a normally downward direction. As used herein, the term “downward” shall denote any direction extending below the normally underneath side of the sparger  10  within 15° of vertical. In one or more embodiments, at least 50, at least 75, at least 90, or at least 95 percent of the fluid discharge openings  30  are positioned so as to be able to discharge a fluid in a normally downward direction. Furthermore, all or substantially all of the fluid discharge openings  30  can be configured to discharge a fluid in a normally downward direction. 
         [0035]    In one or more embodiments, the sparger  10  can have a total flow-through open area of at least 25 percent, at least 50 percent, or at least 75 percent. As used herein, the term “flow-through open area” shall denote the total horizontal area occupied by a sparger defined by a theoretical perimeter of its outermost points minus the percentage of area occupied by the fluid distribution conduits of the sparger. For instance, with respect to the sparger  10 , the total horizontal space occupied by the sparger  10  would be defined by the outermost ends of the fluid distribution conduits  28 , while the total flow-through open area would be the sum of the wedge-shaped open areas  40  between fluid distribution conduits  28 . The wedge-shaped open areas  40  are measured on a theoretical horizontal plane passing through the sparger  10  at an elevation where the fluid distribution conduits  28  have their maximum horizontal diameters. In various embodiments, the sparger  10  can have a total flow-through open area in the range of from about 25 to about 99 percent, in the range of from about 50 to about 95 percent, or in the range of from 75 to 90 percent. 
         [0036]    The sparger  10  can have any dimensions suited for use in a bubble column reactor. In one or more embodiments, the sparger  10  can have a maximum diameter of at least 0.5 meters, at least 0.75 meters, or at least 1 meter. Furthermore, the sparger  10  can have a maximum diameter in the range of from about 0.5 to about 6 meters, in the range of from about 0.75 to about 5 meters, or in the range of from 1 to 4 meters. Also, when the sparger  10  is disposed in the reaction zone of a bubble column reactor, such as the reaction zone  24  of the bubble column reactor  12  depicted in  FIG. 1 , the sparger  10  can have a maximum diameter that is at least at least 90 percent, at least 95 percent, at least 96 percent, or at least 97 percent of the diameter of the reaction zone at the elevation of the reaction zone  24  where the sparger  10  is disposed. The elevation of the sparger  10  relative to the reaction zone shall be determined using to the centroid of the sparger  10 . The centroid of the sparger  10  shall be determined on the basis of the sparger alone and shall not be calculated including other members, such as the vertical member  32 . 
         [0037]    Referring again to  FIG. 1 , as noted above, the bubble column reactor  12  can be configured to facilitate counter-current contact between a slurry (e.g., a purified terephthalic acid (“PTA”) slurry) and a gas-phase stream (e.g., a gas-phase oxidant). Accordingly, in various embodiments, the slurry inlet  18  of the bubble column reactor  12  can be located so as to introduce a slurry within the normally-uppermost 50 percent, the normally-uppermost 30 percent, the normally-uppermost 20 percent, or the normally-uppermost 10 percent region of the reaction zone  24  of the bubble column reactor  12 . Additionally, in various embodiments, the sparger  10  can be disposed within the normally-lowermost 30 percent, the normally-lowermost 20 percent, or the normally-lowermost 10 percent region of the reaction zone  24  of the bubble column reactor  12 . 
         [0038]    In various embodiments, the sparger  10  can be configured to introduce a gas, such as a gas-phase oxidant (e.g., air or a combination of air and steam), into the reaction zone  24 . In various embodiments, the gas flow rate to the sparger  10  can be at least 25, at least 50, at least 75, at least 100, or at least 150 kg/hour. Furthermore, the gas flow rate to the sparger  10  can be in the range of from about 25 to about 700 kg/hour, in the range of from about 50 to about 600 kg/hour, or in the range of from 75 to 500 kg/hour. Additionally, a gas can be introduced into the reaction zone  24  at such a rate as to produce a superficial gas velocity (“U g ”) in the reaction zone  24  in the range of from about 0.01 to about 0.9 cm/s, in the range of from about 0.05 to about 0.4 cm/s, or in the range of from 0.1 to 0.2 cm/s. Superficial gas velocity, as known in the art, is simply the ratio of volumetric gas flow rate to the average cross-sectional area of the reaction zone  24 . In various embodiments, the superficial gas velocity in the reaction zone  24  can be about 0.16 cm/s. Moreover, the gas holdup in the reaction zone  24  can be in the range of from about 0.5 to about 3 percent, or in the range of from 1 to 2 percent. As known in the art, “gas holdup” is simply the volume fraction of a multi-phase reaction medium that is in the gaseous state. Also, in various embodiments, the pressure drop associated with introducing the gas-phase stream into the reaction zone  24  can be at least 1, at least 2, or at least 2.5 pounds per square inch (“psi”). Furthermore, the pressure drop associated with introducing the gas-phase stream into the reaction zone  24  can be in the range of from about 1 to about 10 psi, in the range of from about 2 to about 7.5 psi, or in the range of from 2.5 to 5 psi. Pressure drop is determined according to the following formula: 
         [0000]      Δ P= 0.36(ρ)( U   0   2 )
 
         [0000]    where ΔP is the pressure drop, β is the gas density of the incoming gas-phase stream, and U 0  is the velocity of the gas-phase stream determined at the fluid discharge openings  30 . U 0  is determined according to the following formula: 
         [0000]        U   0 =[flow rate of gas-phase stream]/[ N (π/4)( d   0   2 )]
 
         [0000]    where N is the total number of the fluid discharge openings  30  and d o  is the average diameter of the fluid discharge openings  30 . 
         [0039]    In various embodiments, the operating pressure of the reaction zone  24 , measured at the off-gas outlet  22 , can be in the range of from about 0.4 to about 8 MPa, in the range of from about 0.5 to about 4 MPa, or in the range of from 1 to 2 MPa. Additionally, the operating temperature of the reaction zone  24 , measured at the slurry outlet  26 , can be in the range of from about 150 to about 280° C., in the range of from about 160 to about 240° C., or in the range of from 170 to 210° C. 
         [0040]    Referring now to  FIG. 3 , an alternate sparger  110  is depicted having a plurality of curved, radially-extending fluid distribution conduits  128 . Each of the fluid distribution conduits  128  can comprise a plurality of fluid discharge openings  130 , which include innermost fluid discharge openings  134 , intermediate fluid discharge openings  138 , and outermost fluid discharge openings  136 . Additionally, the sparger  110  comprises a gas entry conduit  120 . In various embodiments, the sparger  110  can be employed in a bubble column reactor (such as bubble column reactor  12 , described above with reference to  FIG. 1 ) to introduce a gas (e.g., a gas-phase oxidant) into the reaction zone of the reactor. The sparger  110 , the fluid distribution conduits  128 , and the fluid discharge openings  130  can each have the same or substantially the same dimensions and can operate in the same or substantially the same manner as the sparger  10 , the fluid distribution conduits  28 , and the fluid discharge openings  30  described above with reference to  FIGS. 1 and 2 . 
         [0041]    Referring now to  FIG. 4 , an alternate sparger  210  is depicted having a plurality of circular fluid distribution conduits  228 . The circular fluid distribution conduits  228  can be positioned in a concentric or substantially concentric manner. Additionally, in various embodiments, the circular fluid distribution conduits  228  can be spaced equiannularly or substantially equiannularly. As can be seen in  FIG. 4 , the fluid distribution conduits  228  present a plurality of fluid discharge openings  230 . The innermost fluid discharge openings  234  can be located on the innermost fluid distribution conduit  240 , the intermediate fluid discharge openings  238  can be located on their respective intermediate fluid distribution conduits  242 , and the outermost fluid discharge openings  236  can be located on the outermost fluid distribution conduit  244 . The number, spacing, and dimensions of the fluid discharge openings  230  can be the same or substantially the same as the fluid discharge openings  30  described above with respect to  FIG. 2 . Additionally, the sparger  210  can operate in the same or substantially the same manner as the sparger  10  described above with respect to  FIGS. 1 and 2 . 
         [0042]    Referring now to  FIG. 5 , an alternate sparger  310  is depicted having a plurality of square fluid distribution conduits  328 . The square fluid distribution conduits  328  can be positioned in a concentric or substantially concentric manner. As can be seen in  FIG. 5 , the fluid distribution conduits  328  present a plurality of fluid discharge openings  330 . In various embodiments, the square fluid distribution conduits  328  can be spaced such that the fluid discharge openings  330  are equiannularly or substantially equiannularly spaced. In one or more embodiments, the innermost fluid discharge openings  334  can be located on the innermost fluid distribution conduit  340 , the intermediate fluid discharge openings  338  can be located on their respective intermediate fluid distribution conduits  342 , and the outermost fluid discharge openings  336  can be located on the outermost fluid distribution conduit  344 . The number, spacing, and dimensions of the fluid discharge openings  330  can be the same or substantially the same as the fluid discharge openings  30  described above with respect to  FIG. 2 . Additionally, the sparger  310  can operate in the same or substantially the same manner as the sparger  10  described above with respect to  FIGS. 1 and 2 . 
         [0043]    Referring now to  FIG. 6 , an alternate sparger  410  is depicted having a plurality of octagonal fluid distribution conduits  428 . The octagonal fluid distribution conduits  428  can be positioned in a concentric or substantially concentric manner. As can be seen in  FIG. 6 , the fluid distribution conduits  428  present a plurality of fluid discharge openings  430 . In various embodiments, the octagonal fluid distribution conduits  428  can be spaced such that the fluid discharge openings  430  are equiannularly or substantially equiannularly spaced. In one or more embodiments, the innermost fluid discharge openings  434  can be located on the innermost fluid distribution conduit  440 , the intermediate fluid discharge openings  438  can be located on their respective intermediate fluid distribution conduits  442 , and the outermost fluid discharge openings  436  can be located on the outermost fluid distribution conduit  444 . The number, spacing, and dimensions of the fluid discharge openings  430  can be the same or substantially the same as the fluid discharge openings  30  described above with respect to  FIG. 2 . Additionally, the sparger  410  can operate in the same or substantially the same manner as the sparger  10  described above with respect to  FIGS. 1 and 2 . 
         [0044]    Referring now to  FIG. 7 , a sparger  510  can be employed in a bubble column reactor  512  in a system  514  for at least partially oxidizing an oxidizable compound (e.g., para-xylene) to form a dicarboxylic acid (e.g., terephthalic acid). The system  514  is depicted as comprising an initial oxidation reactor  516 , an initial oxidation side-draw reactor  518 , a secondary oxidation reactor  520 , and the bubble column reactor  512 , which can be a side-draw reactor. The sparger  510  can have the same or substantially the same dimensions and operate in the same or substantially the same manner as described above with reference to any of the spargers  10 ,  110 ,  210 ,  310 , or  410  described above with reference to  FIGS. 2 ,  3 ,  4 ,  5 , and  6 , respectively. Additionally, the bubble column reactor  512  can have the same or substantially the same dimensions and operate in the same or substantially the same manner as the bubble column reactor  12  described above with reference to  FIG. 1 . 
         [0045]    In operation, a liquid-phase feed stream comprising an oxidizable compound (e.g., para-xylene) and a solvent (e.g., acetic acid and/or water) can be introduced into the initial oxidation reactor  516  for liquid-phase oxidation. A gas-phase oxidant (e.g., air) can also be introduced into the initial oxidation reactor  516  via a sparger  522 . In one or more embodiments, the initial oxidation reactor  516  can be a bubble column reactor, so that agitation of the resulting reaction medium in the reaction zone  524  of the initial oxidation reactor  516  is primarily provided by bubbles of the incoming gas-phase oxidant. Oxidation of the oxidizable compound can be a precipitation reaction producing a three-phase reaction medium. Following initial oxidation, the resulting off-gas can be discharged via a line  526 , and the resulting crude dicarboxylic acid slurry (e.g., a crude terephthalic acid (“CTA”) slurry) can be withdrawn via a side-draw conduit  528 . 
         [0046]    The slurry in the side-draw conduit  528  can be introduced into the initial oxidation side-draw reactor  518  where it can undergo further oxidation via contact with additional gas-phase oxidant (e.g., air or a combination of air and steam). The off-gas resulting from further oxidation in the initial oxidation side-draw reactor  518  can be withdrawn via a line  530 , while the resulting slurry can be withdrawn via a line  532 . 
         [0047]    The slurry from the line  532  can be introduced into the secondary oxidation reactor  520 . Additionally, additional gas-phase oxidant (e.g., air) can be mixed with the slurry from the line  532  prior to introduction into the secondary oxidation reactor  520 . Alternatively, additional gas-phase oxidant (e.g., air) can be introduced into the secondary oxidation reactor  520  separately. Additional solvent (e.g., acetic acid and/or water) can be introduced into the secondary oxidation reactor  520  via a sparger  534 . In one or more embodiments, the secondary oxidation reactor  520  can be a continuous stirred tank reactor (“CSTR”), so that agitation of the resulting reaction medium in the reaction zone  536  of the secondary oxidation reactor  520  is primarily provided by mechanical means. In alternate embodiments, the secondary oxidation reactor  520  can be a bubble column reactor. Following secondary oxidation, the resulting off-gas can be discharged via a line  538 , and the resulting purified dicarboxylic acid slurry (e.g., a purified terephthalic acid (“PTA”) slurry) can be withdrawn via a side-draw conduit  540 . 
         [0048]    The slurry in the side-draw conduit  540  can be introduced into the bubble column reactor  512  where it can undergo further oxidation via contact with additional gas-phase oxidant (e.g., air). As noted above, the additional gas-phase oxidant can be introduced into the reaction zone  542  of the bubble column reactor  512  via the sparger  510 , which can have the same configuration as any of the above-described spargers of  FIGS. 2-6 . The off-gas resulting from the additional oxidation in the bubble column reactor  512  can be withdrawn via a line  544 , while the resulting slurry (e.g., a terephthalic acid slurry) can be withdrawn via a line  546 . 
       DEFINITIONS 
       [0049]    It should be understood that the following is not intended to be an exclusive list of defined terms. Other definitions may be provided in the foregoing description, such as, for example, when accompanying the use of a defined term in context. 
         [0050]    As used herein, the terms “a,” “an,” and “the” mean one or more. 
         [0051]    As used herein, the term “and/or,” when used in a list of two or more items, means that any one of the listed items can be employed by itself or any combination of two or more of the listed items can be employed. For example, if a composition is described as containing components A, B, and/or C, the composition can contain A alone; B alone; C alone; A and B in combination; A and C in combination, B and C in combination; or A, B, and C in combination. 
         [0052]    As used herein, the terms “comprising,” “comprises,” and “comprise” are open-ended transition terms used to transition from a subject recited before the term to one or more elements recited after the term, where the element or elements listed after the transition term are not necessarily the only elements that make up the subject. 
         [0053]    As used herein, the terms “having,” “has,” and “have” have the same open-ended meaning as “comprising,” “comprises,” and “comprise” provided above. 
         [0054]    As used herein, the terms “including,” “includes,” and “include” have the same open-ended meaning as “comprising,” “comprises,” and “comprise” provided above. 
       Numerical Ranges 
       [0055]    The present description uses numerical ranges to quantify certain parameters relating to the invention. It should be understood that when numerical ranges are provided, such ranges are to be construed as providing literal support for claim limitations that only recite the lower value of the range as well as claim limitations that only recite the upper value of the range. For example, a disclosed numerical range of 10 to 100 provides literal support for a claim reciting “greater than 10” (with no upper bounds) and a claim reciting “less than 100” (with no lower bounds). 
         [0056]    The present description uses specific numerical values to quantify certain parameters relating to the invention, where the specific numerical values are not expressly part of a numerical range. It should be understood that each specific numerical value provided herein is to be construed as providing literal support for a broad, intermediate, and narrow range. The broad range associated with each specific numerical value is the numerical value plus and minus 60 percent of the numerical value, rounded to two significant digits. The intermediate range associated with each specific numerical value is the numerical value plus and minus 30 percent of the numerical value, rounded to two significant digits. The narrow range associated with each specific numerical value is the numerical value plus and minus 15 percent of the numerical value, rounded to two significant digits. For example, if the specification describes a specific temperature of 62° F., such a description provides literal support for a broad numerical range of 25° F. to 99° F. (62° F.+/−37° F.), an intermediate numerical range of 43° F. to 81° F. (62° F.+/−19° F.), and a narrow numerical range of 53° F. to 71° F. (62° F.+/−9° F.). These broad, intermediate, and narrow numerical ranges should be applied not only to the specific values, but should also be applied to differences between these specific values. Thus, if the specification describes a first pressure of 110 psia and a second pressure of 48 psia (a difference of 62 psi), the broad, intermediate, and narrow ranges for the pressure difference between these two streams would be 25 to 99 psi, 43 to 81 psi, and 53 to 71 psi, respectively. 
       Claims not Limited to Disclosed Embodiments 
       [0057]    The preferred forms of the invention described above are to be used as illustration only, and should not be used in a limiting sense to interpret the scope of the present invention. Modifications to the exemplary embodiments, set forth above, could be readily made by those skilled in the art without departing from the spirit of the present invention. 
         [0058]    The inventors hereby state their intent to rely on the Doctrine of Equivalents to determine and assess the reasonably fair scope of the present invention as it pertains to any apparatus not materially departing from but outside the literal scope of the invention as set forth in the following claims.