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Cost-effective gas treatment by membranes in Liquid Bio Methane (LBM) and LNG process schemes
Article by Michiel van Aken, Technical Director at Osomo Projects – iLNG BV.
In the gas processing industry, it is generally considered impossible to apply simple and cost effective membrane gas separation, directly upstream of liquefaction of natural gas or biogas to Liquefied Natural Gas (LNG) or Liquid Bio Methane (LBM). The fundamental issue is that membrane gas separation is only efficient for bulk removal of contaminants. Whereas only traces of contaminants like CO2 and water are allowed in the treated gas prior to liquefaction.
In the large-scale LNG industry, amine gas treating is the standard technology to remove CO2, followed by gas dehydration by molsieves to remove moisture from the water-saturated gas. Many LNG peakshavers apply the adsorption process by molsieves for CO2 and water removal from pipeline gas. This gas treating technology requires a significant regeneration gas flow. The resulting ‘tail gas’ with high CO2 concentration has to be recompressed and returned to the gas grid. Consequently, sufficient downstream gas demand in the pipeline is essential for gas mixing and staying within gas specifications. These ‘state of the art’ gas treating technologies are efficient but relatively complex and require heat at different temperatures. Amine based systems usually also have significant operational challenges.
At the same time, upgrading of raw biogas (containing > 40% CO2) by membranes is a cost effective, simple and industry-standard technology to allow gas grid injection with about 2 mol% CO2 remaining in upgraded biogas. The same membranes are also extremely selective with respect to water removal from methane. A unique feature is that membrane technology is easily scalable in contrast with amine and molsieve systems. However, standard membranes cannot reach the low concentration of impurities in the treated gas as achievable with amine and molsieves. Therefore, gas treatment by membranes is not being applied in the LNG industry.
Osomo Projects BV have developed and built an integrated unit of biogas treating and liquefaction to LBM. The iLNG process scheme is still based on standard membranes, yet with a huge improvement of membrane performance by the patented ‘Flash-2-Sweep’ configuration. This is combined with a CO2 tolerant liquefaction and end-flash system, based on 3-phase separation in one vessel; ‘Cryo-3-Flash’. The concept allows processing a wide range of gas composition, has a low equipment count and is safe and simple to operate. This allows the production of LBM locally and efficiently from raw biogas at an extremely small, yet economical scale.
In addition to biogas, fossil gas can be treated as well by membrane gas separation. A single stage membrane, swept with end-flash gas, is directly followed by liquefaction and a 3-phase end flash system. Consequently, the process line-up becomes simple. No heat demand, no batch processes, no absorbents, no chemicals, no wet gas streams, nor liquid waste streams are involved.
New membrane materials with ultra-thin selective layers are being developed by several companies, who claim increasingly higher selectivity (ratio of permeance) of gas contaminants towards methane. Therefore, membrane gas separation will become more “pressure ratio limited”, in other words; a higher selectivity gives a progressively smaller improvement of separation performance. This limitation can be solved by the ‘Flash-2-Sweep” concept.
Although counter-intuitive, membranes have the potential to become a cost-effective and scalable gas treatment technology for CO2 and water removal in the LNG industry.