Controlling the dew point of natural gas
Meeting water and hydrocarbon dew point specifications for natural gas without acid gas components
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Natural gas transported through pipelines to users will generally need to meet both water and hydrocarbon dew point specifications. Several proven processing options are well known for meeting these specifications, but the cost effectiveness and suitability of these available options depend on a particular application including the required dew point targets. To evaluate these options for a particular project, the design team typically needs to perform process simulation with proper thermodynamics packages, size the items of equipment required, and estimate the capital and operating costs for each of the evaluated options. However, there are application areas where one of these options generally becomes favourable or the most cost effective. This article highlights these application areas and summarises options for controlling both water and hydrocarbon dew points of natural gas.
Gas dehydration is necessary as hydrate or ice may form in non- dehydrated gas at low temperatures; this will be encountered in cold climates or when gas reduces in pressure through the delivery system to the users. Condensation of water in the natural gas stream also leads to corrosion problems, especially from acid gas components such as CO2 and H2S. In addition to corrosion, the acid gas content affects the equilibrium water saturation of natural gas and the design and selection of the required dehydration process. However, this article excludes topics on acid gas removal and focuses on controlling both the water dew point (WDP) and hydrocarbon dew point (HDP) of natural gas with no or insignificant contents of acid gas.
To avoid condensation of heavy components when the gas pressure reduces in the delivery system, the HDP of natural gas also needs to be reduced as necessary. Condensation of heavy components increases pipeline pressure drop and necessitates higher frequencies of pigging operation to maintain the targeted deliver capacity.
Controlling or reducing both the water and hydrocarbon dew points can be accomplished via a number of processing options. References 1 and 2 are examples of numerous publications covering WDP and HDP control units. Most of the HDP control options involve lowering gas temperatures to condense heavy components, and the feed gas is typically dehydrated first to prevent gas from freezing and equipment from plugging. This article discusses typical dehydration schemes first, followed by comparative evaluation of the HDP options where minimisation of both capital and operating costs typically depends on the feed gas composition, available feed supply pressure, and the targeted delivery pressure, in addition to the gas specification required by the user.
Natural gas from reservoirs typically contains water, and treated natural gas from an acid gas removal unit is typically water-saturated. Water-saturated gas is commonly dehydrated through adsorption or an absorption process. Adsorption utilises solid desiccants such as silica gels, alumina, and zeolite molecular sieves. These desiccants vary in adsorption isotherms and dehydration performance characteristics. Among these sorbents, the molecular sieve is the only option which can dehydrate the feed gas to less than 0.1 ppmv of water or with a WDP as low as -150°F (-100°C) or less. A molecular sieve is commonly used upstream of cryogenic gas processing such as liquefied natural gas (LNG) plants and demethaniser units for ethane recoveries. In addition to water, other polar compounds such as acid gases like CO2 and H2S are also adsorbed in certain types of molecular sieves.
Water content specifications for pipeline-transported natural gas typically range from 1-7 lb/MMSCF (20 ppmv to 140 ppmv), and glycol absorption is commonly utilised for gas dehydration with this specification target range. A glycol dehydration unit (see Figure 1) consists of a contactor absorbing water from the feed stream, and a glycol regeneration system stripping off absorbed water in the rich glycol and generating lean glycol. Glycol absorption via a contactor to meet the 1-7 lb/MMSCF target range requires less capital and fewer operating costs relative to those of the molecular sieve option. When the WDP target is very low and requires molecular sieve or another solid sorbent, a combination of glycol absorption and molecular sieve adsorption possibly improves the economics of the dehydration system.
Commonly used glycols are ethylene glycol (EG) and triethylene glycol (TEG). A TEG dehydration unit with a conventional regeneration scheme can reach about 99 wt% lean TEG. The weight per cent of the regenerated TEG increases at lower regenerator pressures and higher reboiler temperatures. At temperatures higher than 404°F (207°C) however, TEG starts to degrade. The glycol regeneration system can become more costly when the regenerator off-gas stream contains compounds for which emission is regulated. About 99.96 wt% TEG can be attained with stripping gas in the regenerator. Licensed regeneration schemes can reach 99.99 wt% TEG and higher.
Regenerating TEG to a higher weight per cent enables further reduction of the water content (or concentration) of the dehydrated gas and the associated WDP. Water absorption in the TEG contactor is somewhat less sensitive to pressures up to 1500 psig, and the required minimum (or equilibrium) weight per cent of lean TEG entering the contactor can be estimated from published correlations.3 For a given lean TEG wt% and flow rate to the contactor, the water content in lb/MMSCF of the dehydrated gas from the contactor reduces to a lower level when the contactor operates at a lower temperature. Most contactors, however, operate at 60-110°F (15-43°C) as excessively low operating temperatures sharply increase the viscosity of TEG and condensation of heavy components in the feed gas.
Moreover, the dehydrated gas WDP will also depend on the number of equilibrium stages in the contactor. Generally, 99.96 wt% lean TEG can dehydrate gas down to -40°F (-40°C) WDP at 80°F (27°C) contactor temperature. However, for a given water content or concentration of the dehydrated gas, the WDP increases proportionally with pressure. To meet a given WDP specification target, higher operating pressures require lower water content in the dehydrated gas, and lower water contents require higher concentrations of lean TEG.
The water content of a gas in equilibrium with a hydrate can be lower than the water content of the same gas in equilibrium with supercooled or meta-stable water. Mainly depending upon the gas composition and water saturation levels of natural gas, hydrate formation temperatures for a specific gas composition and water content may be different from the WDP temperatures in equilibrium with meta-stable water. This difference is especially common for gases at very low WDP temperature regions where thermodynamically unstable (meta-stable) water is formed, and the targeted water content may need to be reduced to avoid hydrate formation.
Other common dehydration schemes include injection with a hydrate inhibitor (see Figure 2). Methanol or ethylene glycol (EG) are the most commonly used inhibitors and are typically applied in sufficient quantities upstream of process equipment cooling the gas temperature. The injected inhibitor absorbs water in the gas to prevent freezing or hydrate formation, and a regeneration still recovers the inhibitor at a targeted concentration from the water-rich inhibitor. Generally, EG can be considered when downstream processing temperatures are not excessively low (above -40°F), and methanol is adequate for much lower downstream processing temperatures (as low as below -140°F) and relatively more susceptible to vapour losses at high operating temperatures. Applicable regulations should also be reviewed before finalising the selection of an inhibitor.
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