Abstract:
Desiccant tablets including solid potassium formate are used to dry gas, especially in natural gas transmission lines. Preferred tablets comprise 1-4% potassium formate (most preferably 2-3%), up to 1% of a surfactant as a lubricant, and the balance desiccant salts, preferably calcium chloride.

Description:
RELATED APPLICATION 
     This application is a continuation-in-part of my application of the same title filed Mar. 9, 2001, Ser. No. 09/803,072 now U.S. Pat. No. 6,368,384, which is based upon and incorporates the entire disclosure and claims of my Provisional Application Serial No. 60/217,805 filed Jul. 12, 2000. 
    
    
     TECHNICAL FIELD 
     Tablets containing potassium formate are useful for drying gases because of their deliquescent properties. 
     BACKGROUND OF THE INVENTION 
     In the past, desiccants have been commonly used to dry gases such as natural gas, particularly for its transmission through pipelines. Typically the pipelines carry natural gas over hundreds of miles, and it is necessary and desirable to remove whatever moisture is present in the gas and/or in the pipeline. To this end, the art has employed common desiccant salts such as calcium chloride, made into tablets which tend to absorb the moisture. 
     An early patent to, Hutchinson, U.S. Pat. No. 2,804,940, suggests passing a partially dehydrated natural gas upwards through a bed of solid particulate deliquescent material such as calcium, magnesium, or lithium chloride, or mixtures of them. Certain molar ratios of chlorides and bromides are suggested by Heath in U.S. Pat. No. 2,143,008 and by Thomas in U.S. Pat. No. 5,733,841; see also Thomas U.S. Pat. No. 5,766,309 for a good illustration of the placement of the solid deliquescent materials in a contemporary drying vessel; this patent (U.S. Pat. No. 5,766,309) is incorporated by reference herein in its entirety. 
     A solution of 40-70% potassium formate is suggested for use as a moisture absorbent in dehumidifiers and similar devices by Atkinson in U.S. Pat. No. 5,846,450. 
     SUMMARY OF THE INVENTION 
     Tablets are made of powdered or crystalline potassium formate and used to remove moisture from gases such as natural gas. The tablets may contain from 0.5% to 100% by weight potassium formate, the balance being alkali or alkaline earth metal formates (preferably sodium, calcium, cesium and/or magnesium formate), chlorides and/or bromides (preferably calcium chloride, potassium chloride, cesium chloride, lithium chloride, or mixtures thereof). The tablet is preferably made under compression and, for use in gas dryers in natural gas transmission lines, preferably weighs about 7 to 15 grams so it can conveniently form a bed in a more or less conventional gas dryer. A pillow shape is convenient, but no particular shape is required for this invention. Preferred tablets comprise 97.5% calcium chloride and 2.5% potassium formate. An additional 0.5% of a binder, which is preferably sodium lauryl sulfonate, may also be used. The preferred tablets need not use these exact proportions, however—they may comprise 0.5% to 100% potassium formate, an additional 0.01% by weight to 1% by weight of a surfactant used as a lubricant, and the balance one or more alkali metal or alkaline earth metal halides. 
     Tablets made as described above and containing 1-4% potassium formate, most preferably 2-3% potassium formate, are particularly advantageous not only for their gas drying abilities but also because recrystallization in the sump is minimal or nonexistent. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Comparisons have been made of the tablets of the present invention, containing potassium formate, to various tablets containing calcium chloride. For the comparisons, tablets were made of each of the materials indicated. Tablets of each description were placed in a screen basket which in turn was placed in a humidifier and permitted to remove moisture from the same air. The baskets retained the tablets but permitted liquid to drain into a pan. The pans were preweighed and weighed again periodically. Results are reported in terms of weight gain (liquid) in the pans, in grams, over time. Table I presents the data obtained. 
     In Table 1 and elsewhere herein, CaCl2 77% is calcium chloride containing 23% moisture. 10% KCOOH means 77%CaCl 2  having an additional (based on the calcium chloride) 10% potassium formate. “94%+10%KF” means, in flake form, 94% calcium chloride, 6% moisture and an additional 10% potassium formate based on the calcium chloride and moisture. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Weight increase over time (grams) 
               
             
          
           
               
                 Product 
                 3 hours 
                 18 hours 
                 25 hours 
                 90 hours 
               
               
                   
               
             
          
           
               
                 CaCl2 77% 
                 0.7295 
                 4.609 
                 6.5831 
                 19.5703 
               
               
                 CaCl2 77% 
                 0.4096 
                 3.2685 
                 4.5123 
                 17.9454 
               
               
                 +10% KCOOH 
                 0.6026 
                 6.0434 
                 7.6411 
                 19.6084 
               
               
                 +10% KCOOH 
                 0.781 
                 5.8817 
                 7.7085 
                 22.1131 
               
               
                 94% + 10% KF 
                 0.002 
                 4.5483 
                 5.6003 
                 16.0585 
               
               
                 94% + 10% KF 
                 0.0931 
                 3.2718 
                 4.294 
                 16.8932 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Weight increase over time (grams) 
               
             
          
           
               
                 Product 
                 16 hours 
                 20 hours 
                 24 hours 
                 40 hours 
               
               
                   
               
             
          
           
               
                 CaCl2 77% 
                 8.9011 
                 9.6635 
                 10.0889 
                 12.4836 
               
               
                 94% + 2.5% KF 
                 6.9944 
                 8.4999 
                 9.5212 
                 14.1517 
               
               
                 94% + 5.0% KF 
                 8.245 
                 9.0611 
                 9.6312 
                 12.8922 
               
               
                 94% + 7.5% KF 
                 7.8536 
                 8.7734 
                 9.9184 
                 13.9462 
               
               
                 94% + 10% KF 
                 7.259 
                 8.1575 
                 9.1607 
                 13.2313 
               
               
                   
               
             
          
         
       
     
     In Table 2, “94%+10%KF” means 94% calcium chloride, 6% moisture and an additional 10% potassium formate based on the calcium chloride and moisture. “2.5%KF,” “5%KF,” and “7.5%KF” mean the indicated percentages of potassium formate in addition to the 94% calcium chloride. In Table 3, the term NH4F means ammonium formate. 
     
       
         
               
             
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Weight Increase Over Time (grams) 
               
             
          
           
               
                   
                 Hours 
               
             
          
           
               
                 Product 
                 16 hrs 
                 20 hrs 
                 24 hrs 
                 40 hrs 
               
               
                   
               
             
          
           
               
                 77% CaCl2 
                 6.547 
                 8.5854 
                 9.5619 
                 15.3974 
               
               
                 94% CaCl2 + 5% NH4F 
                 4.964 
                 6.3635 
                 7.2397 
                 12.9733 
               
               
                 94% CaCl2 + 7.5% NH4F 
                 8.2826 
                 10.3386 
                 11.3232 
                 15.2833 
               
               
                 94% CaCl2 + 10% NH4F 
                 6.4756 
                 8.0257 
                 9.0649 
                 13.1451 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Weight Increase Over Time 
               
             
          
           
               
                   
                 2 hours 
                 5 hours 
                 24 hours 
               
               
                   
                   
               
             
          
           
               
                 LiCl 
                 0.2913 
                 1.8077 
                 9.8172 
               
               
                 KCOOH 
                 0.5877 
                 2.2647 
                 9.993  
               
               
                 77% CaCl2 
                 0.0111 
                 0.4246 
                 4.9159 
               
               
                 Sm. Mesh CaCl2 
                 0.2023 
                 0.5881 
                 4.9159 
               
               
                 Prilled CaCl2 
                 0.0991 
                 0.7136 
                 4.6106 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Weight Increase Over Time 
               
             
          
           
               
                   
                 7 hrs 
                 22 hrs 
                 29 hrs 
                 46 hrs 
                 53 hrs 
                 70 hrs 
               
               
                   
                   
               
             
          
           
               
                 77% CaCl2 
                 2.7918 
                 10.2764 
                 12.082 
                 15.7432 
                 16.809 
                 19.0542 
               
               
                 +2.5% KCOOH 
                 3.9704 
                 11.7115 
                 13.8077 
                 17.9961 
                 19.2392 
                 21.8174 
               
               
                 +5.0% KCOOH 
                 3.8633 
                 11.5287 
                 14.0292 
                 17.9577 
                 19.0072 
                 21.1031 
               
               
                 +7.5% KCOOH 
                 4.8065 
                 10.7662 
                 13.1887 
                 16.4462 
                 17.7821 
                 19.7666 
               
               
                 +10.0% KCOOH 
                 4.7849 
                 10.0518 
                 12.7825 
                 15.8435 
                 17.2899 
                 18.937 
               
               
                   
               
             
          
         
       
     
     For or Table 5, the indicated percentages of potassium formate were added to the standard 77% calcium chloride, which contained 23% by weight moisture at the beginning of the test. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 Weight Increase Over Time 
               
             
          
           
               
                   
                 4 hrs 
                 8 hrs 
                 24 hrs 
                 28 hrs 
                 48 hrs 
               
               
                   
                   
               
             
          
           
               
                 94% CaCl2 + 10% NaCl 
                 0.8145 
                 2.1562 
                 8.3738 
                 9.5873 
                 15.6393 
               
               
                 94% CaCl2 + 20% NaCl 
                 1.4649 
                 2.6146 
                 8.4068 
                 9.303 
                 14.9175 
               
               
                 100% KCOOH + 20% NaCl 
                 2.0998 
                 3.8347 
                 10.8299 
                 12.3011 
                 15.5572 
               
               
                 100% KCOOH + 20% NaCl 
                 1.3877 
                 2.6183 
                 9.3758 
                 11.2575 
                 14.1278 
               
               
                 94% CaCl 2  + 20% CaBr 2   
                 0.8164 
                 1.5235 
                 6.7693 
                 7.7308 
                 13.4968 
               
               
                   
               
             
          
         
       
     
     For Table 6, the indicated additions of sodium chloride and calcium bromide were made to 94% calcium chloride in the case of the first, second and fifth tests, and to a 100% aqueous solution of potassium formate in the third and fourth tests. 
     In each case where potassium formate was present, the formation of the drained solution was faster than with compositions not including potassium formate. Potassium formate clearly absorbs more moisture, more readily, than the other salts tested, and clearly enhances these abilities for any of the solution combinations. 
     Thus it will be seen that my invention comprises a deliquescent gas drying tablet comprising 0.5% to 100% solid potassium formate and 0% to 99.5% alkali, alkaline earth metal or ammonium formates, chlorides, bromides or mixtures thereof. The composition is preferably comprises about 5% to about 99.5% by weight potassium formate and most preferably about 5% to about 15% potassium formate with the balance being desiccant salts such as alkali (including cesium), alkaline earth metal or ammonium formates, chlorides, bromides or mixtures thereof, with calcium chloride being preferred. 
     Following in Table 7 is a comparison of a tablet of the present invention, comprising 77% calcium chloride, 2.5% potassium formate, and water (Tablet Z), to a tablet containing no potassium formate—that is, comprising 77% calcium chloride and water (Tablet A). All four tests were conducted at the same natural gas line site. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 7 
               
               
                   
               
             
             
               
                 Meter pressure 
                 611    
                 612    
                 621    
                 631    
               
               
                 Gas flow rate (MMscfd) 
                  1.19 
                  1.183 
                  1.071 
                  0.97 
               
               
                 Inlet temperature (° F.) 
                 68   
                 80   
                 90   
                 100    
               
               
                 Inlet gas moisture 
               
               
                 (lbs H 2 O/MMscfd) 
                 Sat. 
                 Sat. 
                 Sat. 
                 Sat. 
               
               
                 Outlet gas moisture 
               
               
                 (lbs H 2 O/MMscfd) 
               
               
                 Tablet A 
                 5.0 
                 5.8 
                 6.4 
                 11.0  
               
               
                 Tablet Z 
                 2.9 
                 4.9 
                 5.7 
                 6.1 
               
               
                   
               
             
          
         
       
     
     Tablet Z, containing 2.5% potassium formate, was not only clearly superior in drying the gas, but also showed no signs of recrystallizing in the sump, in contrast to liquid generated by Tablet A.