The effective upgrading of raw biogas to methane by selective membranes

The aim of this work was to develop innovative membranes able to separate carbon dioxide and eventually other undesirable compounds from raw biogas. Such membranes should be stable in an aggressive environment and resistant to humidity present in biogas. Therefore, three completely different types of membranes were investigated in this work, namely a supported ionic liquid membrane, a water condensing membrane [1] and a water-swollen thin film composite membrane [2]. This work deals mainly with a model mixture, but raw biogas taken from a sewage plant was used to complete the results of the work. Biogas is produced by anaerobic digestion of organic waste, and consists mainly of methane, carbon dioxide, and a small amount of corrosive gases (water vapor, hydrogen sulfide, ammonia, and mercaptanes). The methane present gives biogas the potential to become an alternative source to classical fuels. Unfortunately, the composition of biogas, typically 50-70 vol% methane and 30-50 vol% carbon dioxide, depends on its origin and on the season. Consequently, it is most commonly used in ancillary combined heat and power plants connected to biogas sources, such as farms or sewage plants, where a change in the composition of biogas is not a problem. For use as a fuel, the best source of biogas is that produced in sewage plants, because it has generally the highest methane content and it is easily accessible. Many different methods have been studied to purify biogas to engine-fuel quality. Water scrubbing, polyethylene glycol scrubbing, or molecular sieves are used to remove carbon dioxide. Pressure-swing absorption is also very common. Hydrogen sulfide, which is problematic because of its corrosive effect, is captured on impregnated active coal or by absorption. Membrane separation represents the latest approach to biogas purification. Polymeric membranes made of silicone rubber [3] and cellulose acetate have already been described [4]. Polyimide membranes [5,6] are very popular and polyether block amide membranes have also been tested [7]. Most of these membranes are effective for CH4/CO2 separation, but the majority cannot be used for biogas purification because they are damaged by aggressive gases. Nevertheless, they have already been applied for inert gases [8]. A promising novel class of gas separation membranes is represented by ionic liquid membranes. Their main advantages are high fluxes through the membranes and a very good selectivity [9]. Many different ionic liquids have been used to separate methane from carbon dioxide [10-11] and their effectiveness has been proved. However, ionic liquids appear to be too expensive for biogas treatment on an industrial scale. Recently we have proposed a new method of membrane separation, using a so-called "condensing-liquid membrane" [1]. This type of membrane has a significant advantage over the usual liquid membrane. Unwanted and toxic gases are removed from its continuously refreshed surface with condensed water to avoid contamination of the perm-selective membrane; furthermore, condensed water passing through the membrane ensures selectivity of the whole separation. The method is in fact based on a liquid (water in this case), condensing on a hydrophilic membrane as a result of the temperature difference of the membrane and the water-saturated biogas feed. The feed gas mixture is saturated by water vapor. The membrane has to be cool enough to make the liquid condense on the surface. Various operational conditions were followed and their effect on the separation of methane from unwanted gases was monitored. Another type of membrane based on a similar principle is the swollen hydrophilic thin film composite membrane. The condensing water on the membrane or on the swollen hydrophilic thin film