Selective Removal of Water From Supercritical Natural Gas

Abstract

Abstract The demand for natural gas has pushed energy industries toward the discovery of remote offshore reservoirs. Consequently, new technologies have to be developed to efficiently produce and transport natural gas to consumption centers. Common design challenges in all gas processing methods for offshore applications are the compactness and reliability of process equipment. Water vapour is the most common impurity in natural gas mixtures. At very high gas pressures within the transportation systems hydrate can easily form even at relatively high temperatures. Gas dehydration or hydrate inhibition systems for offshore gas production/processing facilities should meet these requirements. It should also be noted that at certain pressure and composition conditions, the presence of heavy hydrocarbons (C2+) in natural gas increases pipeline flow capacity and improves compression efficiencies. Therefore, the development of a compact high pressure system capable of selectively removing water from high pressure natural gas streams without affecting the hydrocarbon content will be needed for especial applications and therefore it will be addressed in this paper. Most hydrate inhibition/water removal systems can only work below certain pressure conditions, are relatively large, and not selective towards water. Therefore, some hydrocarbon condensate is also removed during water dew pointing. The developed technique proposed in this study can be customized for the emerging marine transportation of gas in CNG form where the removal of heavier hydrocarbons might not be necessary and will be equally suitable for any other offshore/onshore natural gas production and processing including subsea production of oil and gas. This paper concentrates on the development of simulation techniques needed to accurately estimate dehydration efficiency to control hydrate in a supercritical flow using supersonic nozzles. A simulation model linked to a thermodynamic property generator is needed to predict the water removal efficiency under various flow conditions. The computer simulation results for water removal from a typical offshore natural gas stream under various conditions will be presented and compared with conventional techniques. Intensive water dew points down to about -50 to - 60 ºC can be achieved without any cryogenic cooling or use of solid adsorption techniques. Introduction Natural gas contains different amounts of water vapour, which is considered as the most common impurity in natural gas mixtures. This vapour causes operational problems such as hydrate formation, corrosion, high pressure drop, and consequently slugging flow and reduction in gas transmission efficiency. Water vapour also reduces the heating value of the gas and increases its specific value. One of the most serious problems in the gas industry is the possibility of the obstruction of gas flow due to formation of hydrates within the flow lines. A mass of hydrate is very porous and light in weight and looks like packed snow. The point at which the gas hydrate becomes a solid compound and a cause for trouble depends on temperature, pressure, gas composition, and water content [1]. The gas specific gravity, also a function of gas composition, affects the temperature at which the hydrate will form. At the very high pressure of gas within the transportation system hydrate can easily form even at relatively high temperatures (close to or above 20 ºC). One way to assure that hydrates do not form is to keep the amount of water vapour less than the amount required to fully saturate the gas. The amount of water present in the gas depends on the system pressure and temperature. When the gas contains water then the minimum temperature of the line should be kept above the hydrate point [3] or by reducing the water content, the water dew point of the gas mixture should be kept below the lowest temperature within the gas transmission/transport system. The use of hydrate inhibitors (e.g., methanol, glycols, or kinetic inhibitors) and line heating are commonly practiced to control hydrate formation in the gas transmission systems usually when the gas is transported from the wellhead to a processing facility. Liquid or solid desiccants may then be used to remove water vapour from gas streams and prepare the gas for transmission in long pipeline systems. Water removal may also be performed by reducing the gas temperature and therefore condensing the water out of the gas mixture and subsequently lowering the dew point of the gas (e.g., in LTX facilities) [6, 7].