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  
       [0001]    This application is a continuation-in-part of my application of the same title filed Mar. 9, 2001, Ser. No. 09/803,072, 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  
         [0002]    Tablets containing potassium formate are useful for drying gases because of their deliquescent properties.  
         BACKGROUND OF THE INVENTION  
         [0003]    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.  
           [0004]    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.  
           [0005]    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  
         [0006]    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.  
           [0007]    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  
       [0008]    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.  
         [0009]    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                  
 
         [0010]    [0010]                                     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                    
         [0011]    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                  
 
         [0012]    [0012]                                                       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                        
         [0013]    [0013]                                                                           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%   3.9704   11.7115   13.8077   17.9961   19.2392   21.8174       KCOOH       +5.0%   3.8633   11.5287   14.0292   17.9577   19.0072   21.1038       KCOOH       +7.5%   4.8065   10.7662   13.1887   16.4462   17.7821   19.7666       KCOOH       +10.0%   4.7849   10.0518   12.7825   15.8435   17.2899   18.937       KCOOH                    
         [0014]    For 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 +   0.8145   2.1562   8.3738   9.5873   15.6393       10% NaCl       94% CaCl2 +   1.4649   2.6146   8.4068   9.303   14.9175       20% NaCl       100% KCOOH +   2.0998   3.8347   10.8299   12.3011   15.5572       20% NaCl       100% KCOOH +   1.3877   2.6183   9.3758   11.2575   14.1278       20% NaCl       94% CaCl 2  +   0.8164   1.5235   6.7693   7.7308   13.4968       20% CaBr 2                    
 
         [0015]    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.  
         [0016]    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.  
         [0017]    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.  
         [0018]    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   Sat.   Sat.   Sat.   Sat.       (lbs H 2 O/MMscfd)       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                  
 
         [0019]    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.