Abstract:
Disclosed are processes and apparatuses for producing a crystalline product. The processes and apparatuses may extend the operational time of an evaporative crystallizer by providing an internal volume or large deposit inventory for fouling deposits to reside without impacting the unit operation.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to International Patent Application No. PCT/US2012/69761, filed Dec. 14, 2012, which claims priority to U.S. Provisional Application No. 61/578,986, filed Dec. 22, 2011, all of which are hereby incorporated herein by reference in their entireties. 
    
    
     FIELD 
     This invention relates to processes and apparatuses for forced circulation evaporative crystallization. In particular, this invention relates to processes and apparatuses for prolonging the operational time of an evaporative crystallizer by reducing build up due to fouling deposits. 
     BACKGROUND 
     Evaporative crystallizers are used to produce valuable crystalline products, such as tetrasodium ethylenediaminetetraacetic acid (“Na4EDTA”) and disodium EDTA. However, the operation of evaporative crystallizers is often limited in length of reliable operation due to the build-up of fouling deposits inside the evaporative crystallizer vessel. These deposits can interfere with the evaporative crystallizer equipment by partially or fully plugging pumps, transfer lines, and/or heat exchangers, thus requiring that the system frequently be shut down for cleaning 
     A typical design for a forced circulation evaporative crystallizer includes an outlet flow leaving the evaporative crystallizer at the bottom of the vessel and an inlet on the side of the vessel. Because fouling deposits accumulate at the bottom of the vessel, these deposits exit through the outlet and enter a circulation loop, thus partially or fully plugging the pumps, transfer lines, and/or heat exchangers in that loop. Thus, a need exists for a forced circulation evaporative crystallization system which allows for the accumulation of fouling deposits in order to avoid clogging of the circulation loop. 
     BRIEF SUMMARY 
     In one aspect, an illustrative embodiment provides an apparatus comprising an evaporative crystallizer, wherein the evaporative crystallizer includes a deposit accumulation volume located at the bottom of the evaporative crystallizer. The apparatus further comprises a first inlet for supplying a first flow to the evaporative crystallizer; and an outlet, wherein the outlet is located above the deposit accumulation volume and wherein the first inlet comprises a particle exit positioned above the outlet. 
     In another aspect, an illustrative embodiment provides a process comprises providing a feedstock of a solvent and a solute to a recirculation loop and heating the feedstock with a heat exchanger to provide a heated feedstock. The process further comprises supplying the heated feedstock to an evaporative crystallizer through a first inlet to produce a slurry, wherein the evaporative crystallizer includes a deposit accumulation volume; and returning the slurry to the recirculation loop through an outlet. 
     In another aspect, an illustrative embodiment provides a process comprises providing a feedstock of a solvent and a solute to a recirculation loop; heating the feedstock with a heat exchanger to provide a heated feedstock; and supplying the heated feedstock to an evaporative crystallizer through a first inlet to produce a slurry, wherein the evaporative crystallizer includes a deposit accumulation volume, and wherein fouling deposits accumulate in the deposit accumulation volume. The process further comprises returning the slurry to the recirculation loop through an outlet; extracting a portion of the slurry from the recirculation loop; supplying a first portion of the extracted slurry to the evaporative crystallizer through a second inlet, wherein the first portion of the extracted slurry sweeps crystalline product away from the deposit accumulation volume; and recovering crystalline product in a recovery system. 
     The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an apparatus for evaporative crystallization. 
         FIG. 2  is a top view of an apparatus for evaporative crystallization. 
         FIG. 3  is a schematic diagram of an apparatus for producing a crystalline product. 
         FIG. 4  is a graph showing the outlet flow over time for an apparatus for evaporative crystallization with a deposit accumulation volume. 
         FIG. 5  is a graph showing the outlet flow over time for an apparatus for evaporative crystallization without a deposit accumulation volume. 
         FIG. 6  is a graph showing the number of particles at various sizes for an apparatus for evaporative crystallization with a deposit accumulation volume and for an apparatus for evaporative crystallization without a deposit accumulation volume. 
         FIG. 7  is a graph showing analysis of run time by system product. 
     
    
    
     DETAILED DESCRIPTION 
     In one aspect, an apparatus for producing a crystalline product through evaporative crystallization is provided. The apparatus may be structured to reduce the build-up of fouling deposits and may prolong the operational time of an evaporative crystallizer between cleanings. 
       FIG. 1  illustrates an apparatus  100  for evaporative crystallization. The apparatus  100  may include a lower evaporative crystallizer section  101 , a first inlet  102 , an outlet  103 , a second inlet  104 , and a cone portion  105  allowing for the formation of a liquid-vapor interface. The first inlet  102  may enter the lower evaporative crystallizer section  101  at a position offset from the center or lowest point of the lower evaporative crystallizer section  101 , which may allow for complete drainage from the lower evaporative crystallizer section  101 . The first inlet  102  may comprise a particle exit positioned above the outlet  103 . The outlet  103  may be positioned above the lowest point of the lower evaporative crystallizer section  101 , thus forming a deposit accumulation volume  106 . Fouling deposits from the crystallization process may accumulate in the deposit accumulation volume  106 . For example, fouling deposits may form at the liquid-vapor interface and may fall to the deposit accumulation volume  106 . Collecting these deposits may prevent such deposits from clogging a recirculation line. The deposit accumulation volume  106  may have a volume of between about 1 and about 50 percent of the volume of the lower evaporative crystallizer section  101 , more preferably between 2 percent and 10 percent of the volume of the lower evaporative crystallizer section  101 . For example, the lower evaporative crystallizer section  101  may have a volume of about 11 cubic meters (about 3000 gallons) and the deposit accumulation volume  106  may have a volume of about 1.9 cubic meters (about 500 gallons). 
     The lower evaporative crystallizer section  101  may have a substantially vertical sidewall  107 . The second inlet  104  may be located at an angle of between about 45 degrees and about 90 degrees from the substantially vertical sidewall  107 .  FIG. 2  shows a top view of apparatus  100 . The second inlet  104  may enter tangentially to or perpendicular to the substantially vertical sidewall  107 , preferably in the lower quartile range of the vessel or more preferably from about 10 degrees to about 50 degrees from a tangent line  108 . The second inlet  104  may provide a secondary flow that may sweep crystalline product particles away from the deposit accumulation volume  106  without sweeping the large fouling deposits out of the deposit accumulation volume  106 . The secondary flow may also be used for providing solvent to clean the evaporative crystallizer at the end of a product run. The secondary flow may be between about 0.1 percent and about 20 percent of the flow through the first inlet  102 , more preferably between about 0.5 percent and about 5 percent of the flow through the first inlet  102 . For example, the flow through the first inlet  102  may be about 15 cubic meters per minute (about 4000 gallons per minute) and the secondary flow may be about 0.15 cubic meters per minutes (about 40 gallons per minute). 
       FIG. 3  illustrates an apparatus  200  for producing a crystalline product. A feedstock  201  is provided to a recirculation system  202 . The feedstock  201  may comprise a solvent and a solute. The solvent may be, for example, water. The solute may be, for example, tetrasodium EDTA or disodium EDTA. Other commonly known solvents and solutes may also be used. The recirculation system  202  may include a first inlet  203 , an outlet  204 , a heat exchanger  205 , and a recirculation pump  206 . Shell and tube, plate, finned, and other types of well-known heat exchangers may be used; such as, for example, the shell and tube type of heat exchanger with the process fluid residing within the tubes of the heat exchanger. The feedstock  201  may enter the recirculation system  202 , where the recirculation pump  206  may pump the feedstock  201 , plus recirculating fluid entering the circulation loop at crystallizer outlet  204 , to the heat exchanger  205 . The heat exchanger  205  may heat the recirculating fluid  201  above the solvent boiling point. Generally the recirculating fluid is heated to achieve a temperature rise of between 0.1° C. to 10° C. above the solvent boiling point, more preferably between 1° C. to 2° C. above the solvent boiling point at the vapor liquid interface. The heated feedstock  201  may then enter an evaporative crystallizer  207  through the first inlet  203 . The first inlet  203  may be offset from the center of the evaporative crystallizer  207  in order to allow for complete drainage from the evaporative crystallizer  207 . The feedstock  201  may form a slurry in the evaporative crystallizer  207  as a portion of the feedstock  201  plus recirculating fluid evaporates to form vapor, causing a portion of the solute content to precipitate out of solution in the form of solid particles. The slurry may exit the evaporative crystallizer  207  into the recirculation system  202  through the outlet  204 . 
     A portion of the slurry may be extracted from the recirculation system  202  as extracted slurry  208 . This extraction may occur before the feedstock  201 . Alternatively, this extraction may occur at another point of the recirculation system  202 , or, alternatively, a nozzle may be added to the crystallizer  207  in such a location as to allow the removal of a portion of the slurry contents. The non-extracted portion of the slurry may flow back to the recirculation pump  206 , the heat exchanger  205 , and return to the evaporative crystallizer  207 . The extracted slurry  208  may enter a first pump  209 . After the first pump  209 , the extracted slurry  208  may be divided into a first portion  210  and a second portion  211 . The first portion  210  may be supplied to the evaporative crystallizer  207  through a second inlet  212 . The first portion  210  may be introduced into the crystallizer at a direction sufficient to sweep crystalline product away from the deposit accumulation volume. The second portion  211  may be supplied to a recovery system  213  in order to recover a crystalline product. The second portion  211  may be about 10 percent of the flow of the first portion  210 . For example, the first portion  210  may have a flow rate of 0.15 cubic meters per minute (40 gallons per minute) and the second portion  210  may have a flow rate of 0.015 cubic meters per minute (4 gallons per minute). The recovery system  213  may comprise a cooling crystallizer  214 , a centrifuge  215 , a drier  216 , and a packaging apparatus  217 . The second portion  211  may be supplied to the cooling crystallizer  214  to produce cooled crystalline slurry  218 . The cooling crystallizer  214  may include a stirrer  219 . The cooling crystallizer  214  may cool the second portion  211  to decrease the solubility of the crystalline product in the solvent. The cooled crystalline slurry  218  may be supplied to a second pump  220 , then to the centrifuge  215 , and then to the drier  216  in order to produce a crystalline product  221 . The crystalline product  221  may then be sent to a packaging apparatus  217 . A portion of stream  218  can be returned to the cooling crystallizer  214  via stream  222 . 
     EXAMPLES 
     An evaporative crystallizer with an about 11 cubic meter operating volume (about 3000 gallons) that has a deposit inventory volume of approximately 0.28 cubic meters, or about 2.5 percent of the total working inventory is used. Steam is used to evaporate water from an approximate 40 percent solution of Na4EDTA to form Na4EDTA tetrahydrate crystals. The evaporative crystallizer includes a primary recycle with heating flowing at approximately 12.5 cubic meters per minute (about 3300 gallons per minute) and secondary tangential entry recycle that operates at approximately 0.28 cubic meters per minute (about 75 gallons per minute). The process is fed at a rate of approximately 2700 kg per hour (about 6000 pounds per hour) with an estimated 30 percent boil off rate. 
     The evaporative crystallizer operates continuously for nine days without plugging of the evaporative crystallizer primary or secondary recycle flows or the evaporator heat exchanger located in the primary flow recycle loop (as shown in  FIG. 4 ). This compares to 4-5 days operation for comparable systems using agitation for mixing, internal coils for heat transfer, no equivalent primary flow, and a secondary recycle flow of approximately 0.21 cubic meters per minute (about 55 gallons per minute) (as shown in  FIG. 5 ). The operational run time between required system washes for the system using the primary flow recycle loop is 338 hours. This compares to 150 hours of operational run time between required system washes for the system using agitation for mixing (as shown in  FIG. 7 ). Table 1 below shows the calculations used in  FIG. 7 . 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Calculations used for analysis of run time (as shown in FIG. 7) 
               
               
                   
               
             
             
               
                 Means and Std Deviations 
               
             
          
           
               
                   
                   
                   
                   
                 Std Err 
                   
                   
               
               
                 Level 
                 Number 
                 Mean 
                 Std Dev 
                 Mean 
                 Lower 95% 
                 Upper 95% 
               
               
                   
               
               
                 New 
                 5 
                 338.000 
                 55.5608 
                 24.848 
                 269.01 
                 406.99 
               
               
                 Old 
                 15 
                 150.133 
                 65.0482 
                 16.795 
                 114.11 
                 186.16 
               
               
                   
               
             
          
           
               
                 Means Comparisons 
               
               
                 Comparisons for each pair using Student&#39;s t 
               
             
          
           
               
                   
                 t 
                 Alpha 
               
               
                   
                   
               
               
                   
                 2.10092 
                 0.05 
               
               
                   
                   
               
             
          
           
               
                   
                 Abs(Dif)-LSD 
                 New 
                 Old 
               
               
                   
                   
               
               
                   
                 New 
                 −83.79 
                 119.45 
               
               
                   
                 Old 
                 119.45 
                 −48.38 
               
               
                   
                   
               
               
                   
                 Positive values show pairs of means that are significantly different. 
               
             
          
         
       
     
     Impact on particle size distribution is also improved by decreasing the amount of small particles being generated. Particles sizes that are too small may create a particle dust, whereas particle sizes that are too large will not easily dissolve. A comparison of the number of particles at various sizes for a forced circulation system with a deposit accumulation volume (inventory) and for an agitated evaporative crystallizer utilizing internal heating coils is shown in  FIG. 6 . 
     While the invention has been described above according to its preferred embodiments, it can be modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using the general principles disclosed herein. Further, the application is intended to cover such departures from the present disclosure as come within the known or customary practice in the art to which this invention pertains and which fall within the limits of the following claims.