Abstract:
The present invention relates to a process for the removal of solvent from a polymer emulsion or suspension, by use of a decanter centrifuge ( 10 ). The decanter centrifuge ( 10 ) provides an efficient thickening method. The process is especially useful for emulsion or suspension polymer compositions having high levels of solvent (low polymer solids) that could benefit from a thickening or dewatering step prior to drying in a dryer ( 14 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a process for the removal of solvent from a polymer emulsion or suspension, by use of a decanter centrifuge. The decanter centrifuge provides an efficient thickening method. The process is especially useful for emulsion or aqueous suspension polymer compositions having high levels of solvent which may include water (low polymer solids) that could benefit from a thickening or dewatering step prior to drying. 
       BACKGROUND OF THE INVENTION 
       [0002]    A decanter centrifuge comprises a hollow drum cylindrical/conical cross-section rotatably supported by bearings and having a helical conveyor therein, rotatably supported by bearings relative to the drum. These centrifuges are used primarily where the mixture to be treated has a relatively large content of solids, or where the content of particles in a mixture varies greatly or varies greatly in size. They also find use in applications where relatively large volumes of liquid are treated. They provide a satisfactory separation despite large flows per unit time. Decanter centrifuges are used to separate solids from a liquid mixture in which the solids in the form of particles are suspended in a liquid having a density either greater or less than that of the particles. The major use of decanter centrifuges is in the treatment of sludge from sewage treatment plants (U.S. Pat. No. 6,123,656 and U.S. Pat. No. 6,537,191), though they have also found use in oil recovery systems (U.S. Pat. No. 6,214,236), and for defatting meats and other food products (U.S. Pat. No. 5,944,597). 
         [0003]    Decanter centrifuges have also been used to concentrate dehydrated emulsions of water soluble polymers in a water-in-oil polymer emulsion (U.S. Pat. No. 6,110,995). 
         [0004]    U.S. Pat. No. 2,718,452 describes a process for forming polytetrafluoroethylene organosols, in which the layer containing the organosol is decanted, and then centrifuged to remove occluded water. U.S. Pat. No. 3,536,683 described a process for isolating fluorine-containing polymers involving agglomerating the polymer by adding electrolyte, followed by separation of the agglomerates from the supernatant liquid in a bowl centrifuge, washing and re-centrifuging prior to drying. The washing helps remove electrolyte, which would be an undesired impurity in the final dried products, but the washing step adds complexity to the process, and adds additional water to the polymer, making drying more difficult. 
         [0005]    It is known to remove water or aqueous solution from polymer latex emulsion by various means. Some examples are thickener, filter, etc. In a current commercial practice fluoropolymer latex is processed through an agitated vessel(s) with or without latex washing, latex thickening and finally dried. Optionally, latex washing can be conducted in an agitated vessel or after the agitated vessel as a separate unit operation. The conventional thickening operation has its limitation on the dewater efficiency and overall yield, and the dewatered polymer latex stream still contains a large amount of water, requiring added processing time and energy for its removal. Furthermore, the carryover in the aqueous discharge can be high and reduces the overall process yield. Such fluoropolymer slurry typically floats to the top of the thickening device since its density is less than that of water leading to a low thickening efficiency. In some fluoropolymer operations, reverse osmosis has been mentioned for thickening, in which the operating difficulties, such as membrane fouling, is known. 
         [0006]    There is a need for a thickening or dewatering process that operates at higher throughput, higher efficiency, removes more solvent or water (produces a higher polymer solids) and generates higher yield than current processes. The need is especially great for emulsion or aqueous suspension polymers produced at a relatively low solids level, and therefore containing large amounts of solvent or water. 
         [0007]    Surprisingly it has been found that the use of a decanter centrifuge in the processing of aqueous polymer emulsions and suspensions, such as a fluoropolymer latex, offers a high throughput and improves the thickening efficiency while providing a consistent solid content in the dewatered polymer latex. Optionally, a horizontal decanter centrifuge is used. The decanter centrifuge can be used to process any polymer suspension, solution or mixture having significantly large particles (which can be masses consisting of more than one polymer particle, such as agglomerates or coagulants), where there is a density different (either more or less) between the polymer particles and the continuous aqueous phase. In the case of fluropolymers, it was found that the polymer agglomerates were less dense than the aqueous phase. Polymer agglomerates are typically derived from the combination or aggregation of colloidal polymer particles dispersed or suspended in a liquid into clusters or matrixes. 
         [0008]    The decanter result for the fluoropolymers is unexpected since conventional operation of a decanter centrifuge typically involves a solid component that is denser than the liquid component on the theory that the denser phase travels faster than the less dense phase in a centrifugal field. In the process of the invention the polymer-agglomerates are less dense than the continuous (solvent) phase. Depending on the operating parameters of the decanter centrifuge, the polymer agglomerates can be discharged as the higher solids floater. The solid content of the solid discharge can be either increased or decreased by adjusting operating parameters of the decanter centrifuge. 
       SUMMARY OF THE INVENTION  
       [0009]    The problems described above are solved by a method for processing a polymer emulsion or suspension comprising the step of thickening said emulsion or suspension using a decanter centrifuge. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0010]      FIG. 1 : shows a flow diagram for one method of processing a polymer latex or suspension using a horizontal decanter centrifuge. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]    The term “polymer agglomerates” as used herein means clusters, matrixes or aggregates that typically derived from the combination or aggregation of colloidal polymer particles that had been dispersed or suspended in a liquid. 
         [0012]    By “thickening” of a polymer emulsion or suspension containing a continuous phase and polymer agglomerates, as used herein, means to increase the weight percentage of polymer agglomerates by removing some of the continuous phase. 
         [0013]    The invention relates to the use of a decanter centrifuge in the thickening operation of a polymer emulsion or suspension composition. The decanter centrifuge is especially useful in the processing of polymer compositions having large amounts of water or other solvents (low polymer solids levels). By increasing the level of solids in the thickening operation, throughput through a dryer can be increased, and energy consumption decreased. 
         [0014]    The process of the invention is especially useful for polymer compositions in the form of an aqueous emulsion or suspension of polymer particles. In a preferred embodiment, the polymer is a fluoropolymer. Fluoropolymers for which the present process is useful include, but are not limited to, homopolymers, such as polyvinylidene fluoride, or copolymers, such as, but not limited to, vinylidene fluoride/hexafluoropropylene copolymer, vinylidene fluoride/chlorotetrafluoroethylene copolymer, vinylidene fluoride/tetrafluoroethylene copolymer, ethylene/tetrafluoroethylene copolymer, fluorinated ethylene propylene (FEP) and mixtures thereof. The solvent or continuous phase includes, but is not limited to, water and organic solvents. 
         [0015]    Preferably the fluoropolymer is a polyvinylidene fluoride polymer. “PVDF” or PVDF resin or PVDF polymer refers not only to homopolymers of PVDF but also to copolymers prepared from at least about 75% by weight of vinylidene fluoride (VDF) monomer. Comonomers may include other fluorinated monomers such as: hexafluoropropylene (HFP), chlorotetrafluoroethylene (CTFE), tetrafluoroethylene (TFE), and vinyl fluoride. 
         [0016]    Typically, following polymerization, the polymer is agitated, thickened and dried. The step of washing is optional, which can remove impurities and improve product quality. The drying step is used when dry powder or pellets are desired, though the concentrated polymer could be sold in a wet state, and further processed into final product. Coagulation or agglomeration is optionally and preferably used to increase the effective particle size for easier separation from the continuous phase. Typically polymer aggregates for separation in the decanter centrifuge have an average diameter of more than 1 micron, preferably more than 2 microns, and most preferably over 5 microns. A preferred range for the average diameter is from 2 to 20 microns. 
         [0017]    One embodiment of the invention is illustrated in  FIG. 1 . The polymerization reaction is carried out in a polymerization reactor vessel  2 , either batchwise or continuous. Monomer, initiator, additives, etc. are fed into vessel  2  through line  1 . After the reaction is complete, the reactor effluent is filtered through a screener  4  and then sent to an agitated vessel  6  and an optional wash or additive addition vessel  8 . Additives can be optionally added to the polymer composition during the processing of the polymer. The typical polymer solids content in line  9  is 1-30 weight %, and preferably 2 to 20 weight percent. The effluent is then sent to a decanter centrifuge  10  for thickening. Preferably, the decanter centrifuge  10  is a horizontal type centrifuge. In this invention, the residence time is defined as the volumetric flow rate of the feed divided by the volume of the continuous phase pool of the decanter centrifuge. Depending on rotation speed, throughput, particle size and shape, density difference, solution viscosity, polymer concentration, etc., the polymer content in the centrifuge effluent line  11  can vary from over 20 weight % to over 45 weight %. It has been found that an advantage to the process of the invention is that the polymer content in the effluent of line  11  is extremely consistent for the same set of process conditions of the decanter centrifuge (G force, feed rate, scroll speed, etc.). The consistency is generally within +/−0.5 weight percent, while conventional processes will generally vary by up to +/−2.0 weight percent in solids. Line  11  can then be fed to a dryer  14 . Useful dryers include, but are not limited to, belt dryers, rotary dryers, fluid bed dryers, spray dryers, etc. Optionally, a slurry feed pump  12  can be added in Line  11  to facilitate the feed pump to the dryer of choice. 
         [0018]    The decanter centrifuge is typically run at a pool volume of 10% to 95% of the maximum pool volume, and preferably between 40% and 90%. In one preferred embodiment, the decanter centrifuge contains a floater disc in the discharge side of the continuous phase. A typical beach angle for the decanter centrifuge is between 5 to 25 degrees and preferably between 8 and 15 degrees. A typical bowl length to bowl diameter ratio is from 1:1 to 4:1, and preferably 2:1 to 4:1. 
         [0019]    In another process, the concentrated solids product from the decanter can be sold without further drying, and be processed into final product. 
         [0020]    The decanter centrifuge is typically run at from 100 to 5000 times gravity, and preferably at from 200 to 3500. It was found that the level of thickening increased as the centrifugal force was increased. Residence time is generally between 5 seconds and 10 minutes, and preferably from 10 seconds to 5 minutes. The optimal centrifugal force and residence time is dependent on many factors, including but not limited to, polymer composition, feed rate, density, solids level, particle size and shape, solution viscosity, density difference between the polymer and aqueous phase, diameter, pool depth, channel configuration, beach angle, length/diameter ration of the subject decanter centrifuge, etc. 
       EXAMPLES 
       [0021]    The following examples are set forth to demonstrate the invention but are not to be construed as narrowing the breadth thereof. PVDF is used in the examples. 
         [0022]    The stream from the wash/additive addition vessel as used herein has the following typical content and properties: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Average polymeric particle size: 
                 5-10 microns 
               
               
                   
                 Composition: 
               
               
                   
                 PVDF 
                  2-8% 
               
               
                   
                 Water 
                 89-97% 
               
               
                   
                 VF 2   
                 trace 
               
               
                   
                 Surfactant 
                 trace 
               
               
                   
                 Density: 
                 0.9-0.999 g/cc 
               
               
                   
                   
               
             
          
         
       
     
       Examples 1-2 (Comparative) 
       [0023]    In a commercial thickening operation, the PVDF stream is thickened in a settling vessel or thickener prior to spray drying. The typical performance of the thickener is summarized below: 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Polymer content in 
                 Polymer content in 
               
               
                   
                 Exp 
                 dewatered effluent, 
                 Aqueous effluent, 
               
               
                   
                 No. 
                 wt % 
                 ppm 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 PVDF 
                 1 
                 20-23% 
                 500-700 
               
               
                   
                 PVDF 
                 2 
                 16-19% 
                 500-700 
               
               
                   
                   
               
             
          
         
       
     
       Examples 3-13 
       [0024]    A PVDF sample from the wash/additive addition vessel was processed with a 6″ horizontal decanter centrifuge with the following equipment configuration:
       Bowl diameter: 152 mm   Bowl length: 356 mm   Beach angle: 10 degree
 
under various centrifugal forces and operating conditions. The polymer content in the feed is 3.3-4.1% by weight. The results are summarized in TABLE 2:
       
 
         [0000]                                                    TABLE 2                                       Polymer   Polymer                                   content   content                   Feed           in   in                   Rate   Scroll   Pond   PVDF   Aqueous   Solid           Exp   Centrifugal   in   Speed   Depth   Effluent   Effluent   Recovery       Sample   No.   Force in G   GPM   In rpm   In mm   wt %   vol %   in %                   PVDF   2                   16-19%                   (Base           Case)           3   1367xG   2.1   30.5   1.3-1.8   38.8   0.04   99.9           4   1367xG   1.7   30.5   1.3-1.8   38.1   0.07   99.9           5   3076xG   1.7   30.5   1.3-1.8   42.1   0.03   99.9           6   3076xG   0.8   30.5   1.3-1.8   42.5   0.05   99.9           7   3076xG   0.7   21.1   1.3-1.8   44.4   0.02   99.9           8   3076xG   1.6   21.1   1.3-1.8   44.1   0.03   99.9           9   1367xG   2.4   30.5   1.6-2.0   41.5   0.07   99.8           10    1367xG   2.1   30.5   1.6-2.0   36.8   0.13   99.5           11    1367xG   1.3   42.1   1.4-1.9       0.04           12    1367xG   1.9   42.1   1.4-1.9   35.8   0.08   99.7           13    1367xG   2.9   42.1   1.4-1.9   35.8   0.15   99.5                    
Compared to Example 2, the thickening efficiency with PVDF with horizontal decanter centrifuge was much higher when the centrifugal force was over 1367×G.
 
       Examples 14-16 
       [0028]    A PVDF sample from the wash/additive addition vessel was processed with a 6″ horizontal decanter centrifuge under various centrifugal forces and operating conditions. The decanter is equipped with plough tiles and a floater disc. The polymer content in the feed is approximately 4.4% by weight. The results are summarized in Table 3 below: 
         [0000]                                        TABLE 3                                   Polymer               Feed Rate   Scroll   Pond   content in       Exp   Centrifugal   in   Speed   Depth   PVDF       No.   Force in G   GPM   In rpm   In mm   Effluent wt %                   1 (Base                   16-19%       Case)       14   1489xG   1.19   28.2   1.3-1.8   29.6       15   1457xG   1.35   30.5   1.3-1.8   32.7       16   1266xG   1.13   23.7   1.3-1.8   36.7                    
Compared to Example 2, the thickening efficiency with PVDF with horizontal decanter centrifuge was much higher.
 
       Examples 17 
       [0029]    A PVDF sample from the wash/additive addition vessel was processed with a 6″ horizontal decanter centrifuge under various centrifugal forces and operating conditions. The decanter is equipped with plough tiles and a floater disc. The polymer content in the feed is approximately 4.4% by weight. The results are summarized below: 
         [0000]                                        TABLE 4                       Feed                           Rate   Scroll   Pond   Polymer content in           Centrifugal   in   Speed   Depth   PVDF Effluent       Exp No.   Force in G   GPM   In rpm   In mm   wt %                   1 (Base                   16-19%       Case)       17   219xG   1.494   3.2   1.3-1.8   27%                    
Compared to Example 2, the thickening efficiency with PVDF with horizontal decanter centrifuge was much higher even at a G force of 219.