Hydrocarbon Compression

The first step in evaluating and/or designing a gas dehydration system is to determine the water content of the gas. The water content of a gas is dependent upon gas composition, temperature, and pressure. For sweet natural gases containing over 70% methane and small amounts of “heavy ends,” the McKetta-Wehe pressure-temperature correlation, as shown in Figure 8-1, may be used. As an example, assume it is desired to determine the water content for a natural gas with a molecular weight of 26 that is in equilibrium with a 3% brine at a pressure of 3,000 psia and a temperature of 150°R From Figure 8-1 at a temperature of 150°F and pressure of 3,000 psia there is 104 Ib of water per MMscf of wet gas. The correction for salinity is 0.93 and for molecular weight is 0.98. Therefore, the total water content is 104 x 0.93 x 0.98 = 94.8 Ib/MMscf.

A correction for acid gas should be made when the gas stream contains more than 5% CO2 and/or H2S. Figures 8-2 and 8-3 may be used to determine the water content of a gas containing less than 40% total concentration of acid gas. As an example, assume the example gas from the previous paragraph contains 15% H2S. The water content of the hydrocarbon gas is 94.8 Ib/MMscf. From Figure 8-3, the water content of H2S is 400 Ib/MMscf. The effective water content of the stream is equal to (0.85X94.8)+ (0.15)(400) or 141 Ib/MMscf.

Gas dehydration is the process of removing water vapor from a gas stream to lower the temperature at which water will condense from the stream. This temperature is called the “dew point” of the gas. Most gas sales contracts specify a maximum value for the amount of water vapor allowable in the gas. Typical values are 7 Ib/MMscf in the Southern U.S., 4 Ib/MMscf in the Northern U.S. and 2 to 4 Ib/MMscf in Canada. These values correspond to dew points of approximately 32°F for 7 lb/ MMscf, 20°F for 4 lb MMscf, and 0°F for 2 Ib/MMscf in a 1,000 psi gas line.

Dehydration to dew points below the temperature to which the gas will be subjected will prevent hydrate formation and corrosion from condensed water. The latter consideration is especially important in gas streams containing CO2 or H2S where the acid gas components will form an acid with the condensed water.

The capacity of a gas stream for holding water vapor is reduced as the stream is compressed or cooled. Thus, water can be removed from the gas stream by compressing or cooling the stream. However, the gas stream is still saturated with water so that further reduction in temperature or increase in pressure can result in water condensation.

This section discusses the design of liquid glycol and solid bed dehydration systems that are the most common methods of dehydration used for natural gas. In producing operations gas is most often dehydrated by contact with triethylene glycol. Solid bed adsorption units are used where very low dew points are required, such as on the inlet stream to a cryogenic gas plant where water contents of less than 0.05 Ib/MMscf may be necessary.