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Dehydration with deliquescing dessicants

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Deliquescing desiccants are salts that adsorb water vapor; the water then condenses and dissolves the salt. The water drops down as brine and is removed from the vessel. In the past, the common deliquescing desiccant was calcium chloride (CaCl2).[1]

Deliquescing desiccants

Deliquescing desiccants have been used to dehydrate natural gas since the early 1920s.

Operational problems

Early desiccant systems were plagued with numerous operational problems, such as channeled gas flow and bridging, which caused washouts and velocity problems within the desiccant bed and ultimately resulted in inadequate dehydration of the gas stream. Almost all the operational problems associated with the early systems can be attributed to the poor quality of desiccant that was used.

Briquette form

Early desiccants were made in briquette form and were very soft, crumbly, and porous. The shape of the briquettes was irregular and, when partially consumed, became more irregular.

As gas flowed through the desiccant bed, it found the easiest path and bypassed the rest of the bed. Once started, this process accelerated desiccant consumption in the flow zone, and a channel would form through the desiccant bed. The drying process stopped because wet gas no longer contacted the desiccant.

Also because of its porous nature, gas could penetrate through the briquette, causing it to hydrate internally. This caused the briquette to expand and bloom, making the desiccant fuse together into a hard mass. The operator had to mechanically break up the bed, which was not easy.

Dry forming desiccant

Significant advancements have been made over the past 10 years to greatly improve deliquescing desiccant dehydration. New dry-forming technology produces a desiccant that is:

  • hard
  • nonporous
  • low permeability

Hydration can only occur on the outside of the desiccant, which helps to maintain its general shape as it is consumed. Flow efficiency remains relatively constant as the desiccant bed is consumed.

Fig. 1 shows a typical improved gas dehydration system using deliquescing desiccants. Gas containing water vapor enters the bottom of the vertical dehydration vessel and flows up through a series of full diameter diffusion baffles, which evenly distribute the gas stream across the diameter of the vessel. As the wet inlet gas stream flows up through the desiccant bed, the desiccant starts to hydrate, removing water vapor from the gas stream. This water accumulates on the desiccant surface and drips off the desiccant into the vessel sump as the hygroscopic brine on the desiccant surface continues to remove water vapor from the gas stream. This process, known as “deliquescing,” causes desiccant salts to dissolve into the fresh water accumulating on the desiccant. Desiccant is consumed at a rate based on the dilution factor of each desiccant formulation.

Vapor pressure

The ability of each desiccant to remove water vapor is based on the vapor pressure difference between the hydrate of the metal halide salt used in the desiccant and the partial pressure of water vapor in the inlet gas stream. Gas exiting the vessel top has been dried to a point consistent with the equilibrium point of each desiccant. There are several different hygroscopic grades of desiccant available. Selection of the correct desiccant is based on the inlet gas conditions and the required outlet moisture content. If more hygroscopic grades are needed, it is normally more economical to use several grades in series, flowing from lowest grade to highest in separate vessels, rather than using a high desiccant grade in one vessel. An exception is for very low flow rates such as instrument or fuel gas in which operating-cost savings might not offset additional equipment costs incurred by using multiple vessels.

Adding desiccants

As desiccant is consumed, new desiccant must be added periodically by isolating and depressurizing the vessel, removing the top service closure and pouring desiccant into the vessel. This interval is predictable, and if necessary, the vessel is simply oversized to provide a longer interval between service operations.

Hazards

One of the most hazardous operations is the opening of vessels that have been previously under pressure. Even with a safety interlock, a small amount of pressure could remain trapped in the vessel. Small “telltale” vents can become plugged, giving the operator a false sense that there is no internal pressure. When opening such a system, bolts should be loosened but not removed until the pressure seal is broken and a knife can be inserted into the pressure-containing cavity.

Brine

Water removed from the gas combines with salts in the desiccant to form brine water, which accumulates in the sump. This brine is removed (typically with automatic controllers) to brine storage, where it can normally be disposed of as common oilfield brine. There are no other products or emissions. Desiccant is not typically affected by high Btu gas; however, inlet gas should flow through standard filters or separators to remove liquids, as required in any dehydration process design.

Because deliquescing desiccant systems are closed, there are no volatile organic compound (VOC) or benzene, toluene, ethylbenzene, and xylene (BTEX) emissions. Because equipment is relatively simple compared to glycol systems, capital cost is normally less than that of triethylene glycol (TEG) systems. This is especially true if emission control systems are required with the TEG unit. Deliquescing desiccant systems are very simple to operate and require minimal maintenance. There are no moving parts other than a diaphragm-operated dump valve used to discharge brine water, and 100% turndown is possible with desiccant dehydration. However, the cost associated with manually refilling the desiccant can be substantial, as can be the maintenance cost associated with very corrosive brine.

Operating costs

The operating cost associated with a deliquescing desiccant system is directly proportional to the amount of water vapor removed from the inlet gas stream. Operating cost is lower for high-pressure and low-temperature gas streams in which small quantities of water vapor exist. Compared to conventional TEG dehydration, more gas can be sold with desiccant dehydration because there is no fuel or pump gas consumption.

References

  1. Redus, F.R. 1966. Field Operating Experience With Calcium Chloride Gas Dehydrators. World Oil (1 February): 63.

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