Minimizing Natural Gas Dehydration Costs With Proper Selection of Dry Bed Desiccants and New Dryer Technology
Authors: Vavro Matthew E. 
Published: December 2007

The key to effectively drying natural gas is planning and proper design. With increasingly stringent environmental concerns, dry bed desiccants are gaining rapid acceptance over traditional methods such as triethylene glycol. By properly selecting the correct desiccant, and operating the system in a way that is conducive to effective dehydration, costs can be kept to a minimum. Engineering support, coupled with adequate product choices, has dramatically reduced dehydration costs. Proper design of the entire system is probably the single largest cost saver. Historically, gas dehydration equipment has been non-integrated, with parts being purchased separately and assembled in the field, without considering how the plumbing, vessels, and desiccants work together. Our new approach offers an integrated system, where plumbing, gas flow, dryer design, and desiccant performance are all considered. Additionally, new technology has enabled us to dry gas at nearly two linear feet per second, compared with traditional velocities of 0.5 – 0.75 ft/s, while at the same time greatly increasing the amount of water removed per pound of desiccant (referred to as "dilution rate"). The benefit to the operator is that vessels can be smaller, thereby reducing equipment costs. Consequently, gas dehydration units can contain multiple vessels on the same skid, allowing either multi-stage drying, which greatly reduces costs by removing as much water as possible with lower-cost desiccants, or increased flow capacity by flowing gas in parallel through several vessels.

Introduction

Natural gas contains varying amounts of water vapor, which must normally be removed before combustion. This vapor also causes operational problems such as hydrate formation, free liquid precipitation, and corrosion during production, regulation, metering, and transportation.

Historically, gas has been dehydrated by many different methods. Four of the most common are (1) Glycols, (2) Regenerative adsorption systems, (3) Membrane filters, and (4) Deliquescents, commonly referred to as dry bed desiccants.

Glycol systems have been in operation throughout the petroleum industry since 1949, and have provided fairly reliable dehydration. However, they are labor intensive, the associated technology has not advanced significantly, operating costs can vary dramatically, and there are substantial environmental problems due to fugitive emissions, soil contamination, and fluid disposal problems. Emissions from glycol dehydrators will be subject to regulations that stem from Title III of the 1990 Clean Air Act Amendments, and oil and gas production facility standards must be promulgated by November 15, 1997.

Regenerative adsorption systems have also been used for years, but represent substantial capital expenditures. While normal operating costs are relatively low, these systems are sensitive to fouling. Slugs of water or liquid hydrocarbon can decrease the adsorptive capacity of the unit, and can result in loss of the desiccant, decreasing bed life to less than one year.

Membrane filters are used to remove CO2 from gas streams, and can also be used for water vapor removal. However, they are normally expensive, are easily fouled by contaminants, and require high pressure to operate efficiently.

Deliquescents, primarily various blends of alkali earth metal halide salts, have been used for gas dehydration for years, but only under certain conditions. Their use has been limited by pressure, rate, required outlet moisture content, economics, and primarily by poor design and application.

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