Desiccant tablets for gas drying
1. Method of drying gas comprising contacting gas with a bed of tablets comprising 1-4% potassium formate by weight thereby forming an aqueous solution comprising water from said gas and potassium formate from said tablet, and separating said solution from said bed of tablets.
2. Method of claim 1 wherein at least some of said solution is permitted to remain in contact with said bed of tablets and to contact said gas, whereby said solution continues to remove moisture from said gas until said tablets are substantially dissolved.
3. Method of claim 1 wherein said tablets comprise 2% to 3% potassium formate.
4. Method of claim 1 wherein said gas is natural gas.
5. Method of claim 1 wherein said tablets include desiccant salts.
6. Method of claim 5 wherein said desiccant salts comprise predominantly calcium chloride.
7. Method of claim 1 wherein said separating is accomplished by draining.
8. Method of claim 1 followed by replacing said bed of tablets with new tablets comprising solid potassium formate.
9. Method of claim 8 followed by another iteration of the method of claim 1.
10. Method of claim 5 wherein said desiccant salts include lithium chloride.
11. Method of claim 5 wherein said desiccant salts include cesium chloride.
12. Method of claim 1 wherein contacting of said gas with said bed of tablets is accomplished by flowing said gas upwardly through said bed of tablets.
13. A deliquescent gas drying tablet comprising 1% to 4% solid potassium formate and 0% to 99% alkali, alkaline earth metal or ammonium formates, chlorides, bromides or mixtures thereof.
14. A deliquescent tablet of claim 13 wherein the potassium formate comprises about 2% to about 3% by weight.
15. A deliquescent tablet of claim 14 comprising about 0.5% to about 97% calcium chloride.
16. A desiccant tablet comprising about 1% to about 4% by weight potassium formate, about 0.1 to 1% surfactant, and the balance at least one desiccant salt.
17. A desiccant tablet of claim 16 made by compression, wherein said surfactant is sodium lauryl sulfonate.
18. A desiccant tablet of claim 16 wherein said at least one desiccant salt comprises calcium chloride.
19. A desiccant tablet of claim 16 wherein said potassium formate comprises 2-3% by weight of said tablet.
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.sub.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.
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.
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