Natural Gas Sweetening - Glycol Dehydration

Production Facility

Produced natural gas contains water vapor. The gas is usually saturated under reservoir conditions of temperature and pressure. As it flows up the tubing, some of this water vapor may condense as "free water." The remainder will remain as water vapor. The process of removing water vapor from a gas stream is called "gas dehydration. The amount of water vapor contained in a gas stream can be expressed in terms of concentration, mg/m3 in metric units (in the U.S., the normal units are pounds of water vapor per million standard cubic feet of gas, lbs/MMSCF), or in terms of the "dew point" of the gas. The dew point is the temperature at which water will condense from the gas stream as it is cooled. The higher the concentration of water vapor in a given gas stream the higher its dew point. Thus, gas dehydration is also called "dew point depression," the lowering of the dew point of a gas by lowering the concentration of water vapor in the gas. Gas is dehydrated to prevent hydrate formation, to prevent corrosion, or to meet a sales gas contract. Hydrates are loosely-linked, crystal-like chemical compounds of hydrocarbon and water resembling dirty ice. If hydrates form, they can accumulate in valves and fittings, blocking or restricting gas flow. In order for hydrates to form, water must be present in liquid form, and the gas must be cooled to below its hydrate formation temperature, which is a function of gas composition and pressure. Thus, a gas stream which is dehydrated so that its dew point is lower than the temperature to which it will be cooled will have no water condensing from the gas, and hydrates will not form.

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Water condensing in a gas line which is cooled below its dew point can cause corrosion, especially if the gas contains carbon dioxide (CO2) or hydrogen sulfide (H2S). Also, water in a gas line reduces the line capacity, increases the pressure drop in the line, and can produce undesirable or damaging slugging in the pipeline. For these reasons most gas sales contracts specify a maximum amount of allowable water vapor in the gas. For the Southern U.S. the limit is normally 112 mg/m3 (7 lbs/MMSCF), for the Northern U.S. 64 mg/m3 (4 lbs/MMSCF), and for Canada 32 to 64 mg/m3 (2 to 4 lbs/MMSCF). These limits are low enough to prevent water from dropping out in the line at normal transmission pressures and the lowest anticipated gas line temperature. These values correspond to dew points of approximately 0°C for 112 mg/m3 (32°F for 7 lbs/MMSCF), -7°C for 64 mg/m3 (20°F for 4 lbs/MMSCF) and -18°C for 32 mg/m3 (0°F for 2 lbs/MMSCF) in a 6900 kPa (1,000 psi) gas line.

Gas can be dehydrated by cooling and separating the condensed liquids, by using specially designed low temperature separation processes, by using solid desiccants, or by using liquid desiccants. Cooling the gas stream and removing the free water with a separator is the simplest method of dehydration. However, this method is limited by the hydrate formation temperature unless some other hydrate preventative method has been taken.

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