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Liquefied Natural Gas
Published in Arthur J. Kidnay, William R. Parrish, Daniel G. McCartney, Fundamentals of Natural Gas Processing, 2019
Arthur J. Kidnay, William R. Parrish, Daniel G. McCartney
Depending on the specific situation, not all plants will have all the processes shown, some plants may have additional processes. Note that nitrogen rejection is not included. Nitrogen is the most volatile LNG component. If present in high concentrations, it is removed by flashing the LNG. This gas typically becomes fuel gas for the plant. If in sufficiently high concentrations a nitrogen rejection unit (see Chapter 13) is used. As noted in Chapter 14, the off-gas can become a feedstock for helium recovery if the helium content is greater than 0.3 vol%. The large volumes of off-gas from baseload plants make helium recovery economically feasible even at lower concentrations. LNG facilities in Qatar and Australia recover helium from the liquefaction flash gas stream.
Potential for helium recovery and purification in Australia through membrane gas separation
Published in Australian Journal of Multi-Disciplinary Engineering, 2020
Conventionally, helium is extracted from the off-gas of the nitrogen rejection unit (NRU) in a natural gas process. The primary purpose of this unit is to remove nitrogen from the natural gas and has the secondary effect of concentrating helium. Hence, helium recovery is primarily a process of separation from nitrogen (Rufford et al. 2014; Scholes and Ghosh 2016). Traditional helium recovery and purification is undertaken through liquefaction, where the NRU off-gas is cryogenically cooled to below the boiling temperature of nitrogen (−195°C or 77 K). Helium remains a gas and is separated from the liquid nitrogen. Further purification is achieved through pressure swing adsorption (PSA), which is able to purify the helium to >99.9% by removing trace amounts of oxygen and hydrogen. However, for low grade reservoirs where the helium concentration is very low, liquefaction separation is very energy intensive because of the amount of nitrogen that must be condensed. Hence, alternative technologies are required that can undertake helium recovery and purification. A potential technology is membrane gas separation, which is based on a semi-permeable membrane, generally a polymeric film, which enables selective gases to pass through while other gases and vapours experience the membrane as a barrier (Ho and Sirkar 1992). The main driving force for separation is the partial pressure difference across the membrane, and hence the technology lends itself to the high pressure gases associated with natural gas processing. The solution-diffusion mechanism that is used to explain gas permeation through non-porous membranes is not viable for helium, because the gas’s chemically inert state means that no chemical interactions with the polymeric membrane occurs; and hence mass transport is through diffusion alone (Scholes and Ghosh 2017). Membrane gas separation is currently commercialised in the natural gas industry for acidic gas removal and hydrocarbon vapour recovery (Baker 2002), and there is potential for membrane gas separation to also recovery and purify helium. Indeed, helium separation was one of the first applications proposed for membranes when the technology was first developed in the 1960s (Stern et al. 1965). However, to date membranes have been limited to small scale helium recycling processes in niche industries, such as recovering helium from air mixtures used in deep sea diving.