Abstract:
A process and apparatus for continuously removing ions from solution in proportion to their prevalence in solution using an ion exchange media. The process comprises: (a) mixing fresh or regenerated ion exchange media and a feed solution containing diverse ions; (b) reacting the resulting slurry to produce a product slurry comprised of loaded ion exchange media and stripped product solution; (c) separating the loaded ion exchange media from the product slurry; (d) regenerating the loaded ion exchange media by counter current contact with a regenerant; and (e) conducting the process steps continuously and concurrently, whereby a continuous circuit is produced for dosing, loading, separating, and regenerating the ion exchange media, and whereby more concentrated ions are preferentially depleted in the product solution. An apparatus particularly adapted to practice the process and to treat sodic water is also provided.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to continuous ion exchange, and more specifically relates to the partial removal of diverse ions in proportion to their respective concentrations in solution.  
       BACKGROUND OF THE INVENTION  
       [0002]     Many surface and groundwater resources are classified as sodic or saline-sodic. Sodic water and saline-sodic water both contain high concentrations of monovalent sodium ions in solution relative to lower concentrations of divalent calcium and magnesium ions. Sodic water is defined as water having a sodium adsorption ratio (SAR) value greater than 15 where the SAR value is defined by the following equation:  
       SAR   =       [     Na   +     ]             [     CA     2   +       ]     +     [     Mg     2   +       ]       2             
 
         [0003]     Where the concentration terms have units of milliequivalents per liter. Sodic water is found in many arid and semi-arid areas of the world and is also a high volume waste of fossil fuel production. To render sodic water suitable for beneficial use in agriculture, the concentration of the predominant monovalent cations must be reduced without substantially reducing the concentration of the divalent cations in solution.  
         [0004]     As described in Perry&#39;s Chemical Engineers&#39; Handbook, 7 th  ed., chapter 16, page 14, and in Kirk-Othmer&#39;s Encyclopedia of Separation Technology, Vol. 2, pages 1074-1076, commercially available ion exchange media are selective and will remove divalent and multivalent cations in preference to monovalent cations. When ion exchange media are employed in conventional fixed or moving bed reactors, divalent cations will be removed to a greater extent than the monovalent cations. Divalent cations, even in low concentrations, will replace monovalent cations on the ion exchange media. Consequently, as shown by EMIT Water Discharge Technology, Sep. 17, 2003, commercially available produced water treatment schemes that use cation exchange media for sodium removal also quantitatively remove calcium and magnesium. Restoring divalent cations to the solution adds to process complexity and requires conditioning of treated water by chemical addition or mineral contacting plus blending of treated and untreated water streams.  
         [0005]     Selectivity of cation exchange media for calcium and magnesium over sodium and potassium has been the major impediment to simple, economical, single contact treatment of sodic water by ion exchange.  
       SUMMARY OF THE INVENTION  
       [0006]     In one embodiment of the present invention, a continuous selective ion exchange process of the present invention removes the above-described impediment and provides a simple and economical treatment of sodic water by ion exchange.  
         [0007]     In another embodiment, the invention may be characterized as a process for continuously removing ions from solution in proportion to their prevalence in solution using a continuous circuit for dosing, loading, separating, and regenerating ion exchange media, whereby sodic water can be rendered non-sodic in a single pass through a reaction volume.  
         [0008]     A continuous selective ion exchange process in accordance with an aspect of the present invention provides a simple method for controlled, continuous, removal of diverse ions in solution in proportion to their respective concentrations in solution. The process can be used to selectively remove monovalent cations in solution when using commercially available ion exchange media that is selective for divalent cations. Process equipment is simple, easily scaled, and suitable for modular assembly and application. These capabilities and characteristics render the continuous selective ion exchange process particularly suitable for treatment of sodic and saline-sodic waters such as those produced during fossil fuel exploration and development, and as found naturally in many arid regions of the world  
         [0009]     Accordingly, there are several objects and advantages of the present invention some of which are: 
        (a) to provide a selective ion exchange process that will allow preferential removal of monovalent cations from solutions containing both monovalent and divalent cations, when using commercially available ion exchange media that exhibits selectivity for divalent cations,     (b) to provide a simple continuous ion exchange process for treating sodic water, for beneficial use, using commercially available cation exchange media,     (c) to provide an ion exchange process for treating sodic water in a single pass through an ion exchange reactor,     (d) to provide an ion exchange process for removing ions from solution in proportion to their prevalence in solution despite inherent ion exchange media selectivity,     (e) to provide a method and apparatus for controlling the duration of contact between ion bearing solution and ion exchange media during continuous ion exchange,     (f) to provide a method and apparatus for continuously contacting ion exchange media and ion bearing solution at predetermined stoichiometric ratios,     (g) to provide a method and apparatus for continuously regenerating and dosing ion exchange media,     (h) to provide a method and apparatus for continuously controlling the degree of loading and regeneration of ion exchange media,     (i) to provide a method and apparatus to reduce consumption of ion exchange media due to breakage and attrition.        
 
         [0019]     The foregoing objects and advantages are merely a representation of the full scope of the present invention. Further objects and advantages are to provide a sodic water treatment process and apparatus that can be easily and reliably scaled to any desired size, and that is simple and inexpensive to manufacture and operate, and is suitable for unattended operation in remote, harsh environments. Still further objects and advantages will become apparent from a consideration of the ensuing description and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     Various additional objects and advantages and a more complete understanding of the present invention are apparent and more readily appreciated by reference to the following Detailed Description and to the appended claims when taken in conjunction with the accompanying Drawings wherein:  
         [0021]      FIG. 1  is a material flow and major equipment arrangement diagram for a preferred embodiment of the continuous selective ion exchange process.  
         [0022]      FIG. 2  is a material flow and major equipment arrangement diagram for a simplified embodiment of the continuous selective ion exchange process. 
     
    
     DETAILED DESCRIPTION  
       [0023]     Kinetic studies with ion exchange media dispersed in ion bearing solutions have shown that the rate of removal of cations is proportional to the square root of the product of the cation concentration and the concentration of unused ion exchange media in the reaction volume. The form of the kinetic equation for removal of target ionic species is: 
 
 r   A   =k   A ( C   IX   C   A ) 0.5  
 
         [0024]     Where r A  is the removal rate of species “A”, k A  is the rate constant and C IX  and C A  are the respective concentrations of the unused ion exchange media and target ions in solution. Similar expressions can be written for each ionic species in solution, and the relative removal rate for any two species at a given ion exchange media concentration is:  
           r   A       r   B       =         k   A       k   B       ⁢       (       C   A       C   B       )     0.5           
 
         [0025]     Since the rate constants k A  and k B  depend largely on the reaction conditions and transport properties of the fluid, which are the same for both ionic species, the rate constants are approximately equal. Therefore, the initial relative rate of removal of two ionic species is approximated by the square root of the ratio of their concentrations in solution. For example, if sodium ions are present at nine times the concentration of calcium ions in solution, fresh ion exchange media will remove sodium ions at a rate approximately three times as fast as it will remove the calcium ions.  
         [0026]     The hereinabove discussed equations show that the rate of removal of a specific ionic species is a function of the stoichiometric ratio of the concentration of unused ion exchange media capacity and the concentration of the target ions in solution. The most rapid removal of a target ion will occur when fresh ion exchange media is well mixed with solution exhibiting a high concentration of the target ion. As exchange sites on the media are filled and the media approaches full loading, the rate of removal for all species declines and the relative selectivity of the media for specific ionic species controls its equilibrium loading.  
         [0027]     Consequently, preferential removal of the more concentrated species can be accomplished by reducing the contact time, increasing the media-to-ion stoichiometric ratio, and controlling the degree of mixing of fresh or partially loaded ion exchange media and the ion bearing solution. The present invention is designed to provide simple and easy control of media-solution contact time, media-solution stoichiometric ratio, and media-solution mixing as needed to take advantage of the aforementioned kinetic phenomena, and thereby allow preferential removal of monovalent ionic species using commercially available ion exchange media that exhibit selectivity for divalent ionic species.  
         [0028]     Methods used to acquire kinetic data for ion exchange reactions and to design reactors based on kinetic data are well known to practitioners having ordinary skill in the art.  
       DRAWINGS—REFERENCE NUMERALS  
       [0029]    
       
         
               
               
               
               
             
           
               
                   
               
               
                   
               
             
             
               
                 10 
                 Fluidized Bed Reactor 
                 12 
                 Media Elutriation Line 
               
               
                 14 
                 Media Regenerator 
                 16 
                 Media Separator 
               
               
                 18 
                 Primary Rotary Valve 
                 20 
                 Secondary Rotary Valve 
               
               
                 22 
                 Feed Solution 
                 24 
                 Fresh Regenerant 
               
               
                 26 
                 Purge Solution 
                 28 
                 Product Solution 
               
               
                 30 
                 Spent Regenerant 
                 32 
                 Media Transport Line 
               
               
                 34 
                 Fresh Ion Exchange Media 
                 36 
                 Loaded Ion Exchange Media 
               
               
                 38 
                 Reactor Standpipe 
                 40 
                 Fluid Distributor 
               
               
                 42 
                 Reactant Slurry 
                 44 
                 Regenerator Standpipe 
               
               
                 46 
                 Regenerated Ion Exchange 
                 48 
                 Product Slurry 
               
               
                   
                 Media 
               
               
                   
               
             
          
         
       
     
         [0030]     Although present devices are functional, they are not sufficiently accurate or otherwise satisfactory. Accordingly, a system and method are needed to address the shortfalls of present technology and to provide other new and innovative features.  
         [0031]     Referring first to  FIG. 1 , a continuous selective ion exchange process is performed in an apparatus comprised of a fluidized bed reactor  10  equipped with a fluid distributor  40 , a media elutriation line  12 , a media separator  16  and a media regenerator  14 . A primary rotary valve  18  regulates flow rate of regenerated ion exchange media  46  particles from the media regenerator  14  to the fluidized bed reactor  10  through a reactor standpipe  38 . A secondary rotary valve  20  regulates flow rate of loaded ion exchange media  36  particles from the media separator to the media regenerator  14 . Feed solution  22 , fresh regenerant  24 , purge solution  26 , and fresh ion exchange media  34  are fed to the process at appropriate locations. Likewise, product solution  28 , and spent regenerant  30  are discharged from the process at appropriate locations.  
         [0032]     Referring next to  FIG. 2 , the simplified embodiment of a continuous selective ion exchange process uses a media transport line  32  and omits the separate ion exchange reactor  10  shown in  FIG. 1 .  
         [0033]     During operation of the continuous selective ion exchange process, ion exchange media are continuously circulated through the fluidized bed reactor  10 , media elutriation line  12 , media separator  16 , and media regenerator  14 . Target ions are removed from feed solution in the ion exchange reactor  10  and during transport through the elutriation line  12 . The reaction volume of a fluidized bed reactor can be increased or reduced by simple adjustment of the vertical position of the lower end of the media elutriation line  12 . Placing the lower end of the media elutriation line closer to the fluidized bed reactor&#39;s  10  fluid distributor  40  reduces the reaction volume and, therefore, reduces the contact time between the ion exchange resin and the feed solution  22 . If the desired ion exchange reactions are sufficiently fast, the fluidized bed reactor  10  shown in  FIG. 1  may be omitted and, as shown in  FIG. 2 , the ion exchange reaction will be accomplished in the media transport line  32 .  
         [0034]     In the embodiment shown in  FIG. 1 , feed solution is brought into contact with the fresh or regenerated ion exchange media  46  in the fluidized bed reactor to produce a reactant slurry  42 . Ion exchange reactions occur in the fluidized bed reactor  10  and the elutriation line  12  yielding a product slurry  48  that flows through the elutriation line  12  and into the media separator  16 .  
         [0035]     In the embodiment shown in  FIG. 2 , feed solution  22  is directly mixed with regenerated or fresh ion exchange media  46  to form the reactant slurry  42 . Ion exchange reactions occur in the media transport line  32  that discharges product slurry  48  into the media separator  16 .  
         [0036]     The media separator  16  recovers ion exchange media from the product slurry and discharges clarified product solution  28 , which is the primary process product. Thus, the feed solution is treated in one pass through the reaction volume. Media separation may be accomplished by any method that will separate the product slurry components into saturated settled media particles plus clarified product solution. Preferred methods of separating ion exchange media and product solution are gravity settling, straining, and cyclone separation because these methods of separation are simple, have no moving parts, and minimize mechanical breakage and attrition of the media.  
         [0037]     Loaded ion exchange media  36  are transferred from the media separator  16  into the regenerator  14  by means of gravity transport through the secondary rotary valve  20  and via the regenerator standpipe  44 . The media transfer rate through the secondary rotary valve  20  is proportional to the secondary rotary valve  20  rotation speed.  
         [0038]     In the regenerator  14 , the ion exchange media are continuously regenerated by counter current contact with fresh regenerant  24 . Fresh regenerant  24  is introduced near the bottom of the regenerator  14  and flows upward counter to the descending ion exchange media. The regenerator  14  is designed so that the upward superficial velocity of the regenerant  24  is less than the superficial fluidizing velocity of the loaded ion exchange media. Spent regenerant  30  is withdrawn from the fluid filled headspace above the bottom end of the regenerator standpipe  44  and in the upper portion of the regenerator  14 . Optionally, a purge solution  26  may be introduced just below the secondary rotary valve  20  to minimize contamination of the product solution  28  by spent regenerant  30  that might otherwise be contained in the pocket flow and leakage through the secondary rotary valve  20 .  
         [0039]     Regenerated ion exchange media  46  are transferred from the regenerator  14  into the fluidized bed reactor  10  by means of gravity transport through the primary rotary valve  18  and via the reactor standpipe  38 . The ion exchange media transfer rate through the primary rotary valve  18  is proportional to the primary rotary valve rotation speed.  
         [0040]     By the process hereinabove discussed ion exchange media are continuously cycled through the fluidized bed reactor  10 , media elutriation line  12 , media separator  16 , media regenerator  14 , and back to the fluidized bed reactor  10 .  
         [0041]     The inventory of ion exchange media in the process circuit is initially charged or replenished through the fresh ion exchange media  34  line into the reactor standpipe and between the primary rotary valve  18  and the fluidized bed reactor  10 .  
         [0042]     The primary and secondary rotary valves  18  and  20  are preferably designed or operated such that the rotation speed of the secondary rotary valve  20  exceeds the rotation speed of the primary rotary valve  18  by a predetermined value. With this mode of operation, the primary rotary valve speed is used to easily regulate the overall ion exchange media circulation rate and, thereby, adjust the media-to-solution stoichiometric ratio as needed to remove target exchangeable ions in the feed solution.  
         [0043]     In the simplified embodiment ( FIG. 2 ) of the continuous selective ion exchange process, ion exchange media discharged from the primary rotary valve  18 , or introduced via the fresh ion exchange media  34  line, are directly entrained by the feed solution  22 . Desired ion exchange reactions occur during transport of the resulting slurry  42  in the media transport line  32 . The media transport line  32  may be provided in alternate configurations, (e.g., loops, coils, spirals, etc.) as needed to accomplish slurry transport, to control mixing of media and solution, and to provide optimum contact time for ion exchange. No separate ion exchange reactor is used. In all other respects, operation of the simplified embodiment of the instant process is the same as hereinabove discussed for the preferred embodiment.  
         [0044]     Thus, the reader will see that a continuous selective ion exchange process in accordance with one or more aspects of the present invention provides a simple method for controlled, continuous, removal of diverse ions in solution in proportion to their respective concentrations in solution. The process can be used to selectively remove monovalent cations in solution when using commercially available ion exchange media that is selective for divalent cations. This process equipment is simple, easily scaled, and suitable for modular assembly and application. These capabilities and characteristics render the continuous selective ion exchange process particularly suitable for treatment of sodic and saline-sodic waters such as those produced during fossil fuel exploration and development, and as found naturally in many arid regions of the world, although application to other industries is also contemplated.  
         [0045]     The foregoing description should not be construed as limiting the scope of the invention, but rather as an exemplification of preferred embodiments thereof. Other variations are possible. For example, orientation of major equipment items in other than a vertical configuration is not required if the rotary valves  18 ,  20  are replaced by appropriate slurry pumps. A variety of methods, such as centrifugation, cyclone separation, filtration, straining, and settling may be used to accomplish the media separation step. Depending on scale, different regenerator configurations and internals may be used to ensure efficient counter current regeneration of media with regenerant solution. A stirred tank or other type of ion exchange reactor may be substituted for the fluidized bed ion exchange reactor. The media transport tube  32  may be furnished in many (banked tubes, loops, coils, spirals, etc.) alternative configurations and lengths. The process may be applied to accomplish either cation or anion removal, or for chemical adjustment of solution ionic composition, ionic strength, or pH. More than one process arrangement may be employed in sequence to achieve concurrent continuous selective exchange of both cations and anions.  
         [0046]     Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.