Effective Purification of Biogas by a Condensing‐Liquid Membrane

Abstract

In a time of limited reserves of fossil fuels, the problem of biogas utilization becomes a fundamental question. For example, the city of Prague in central Europe with 1.25 million inhabitants requires 35 million liters of diesel per year for its 1180 buses. Diesel is expensive and significantly increases air pollution, mainly with aromatic hydrocarbons (including naphthalenes and alkyl benzenes) and carbon black. Biogas occurs as a result of 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). Therefore, biogas has the potential of becoming an alternative 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 problematic element. If we think about biogas as a fuel, the best alternative seems to be the purification of biogas produced in sewage plants, because it generally has the highest methane content and is easily accessible. Various residual compounds (water vapor, hydrogen sulfide, siloxanes, mercaptanes) present in biogas have already been described with complex analysis. Many different methods have been attempted 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 and cellulose acetate have already been described. Polyimide membranes are very popular and polyether block amide membranes have also been tested. Most of these membranes are effective for CH4/CO2 separation, but the majority of them cannot be used for biogas purification because they are destroyed by aggressive gases. Nevertheless, they have already been applied for inert gases. A very promising method of gas separation is represented by ionic-liquid membranes. Their main advantages are high fluxes through membranes and a very good selectivity. Many different ionic liquids have been used to separate methane from carbon dioxide and their effectiveness has been proved. However, ionic liquids appear to be too expensive for biogas treatment on an industrial scale, their hygroscopic properties lead to water degradation of the ionic liquid, and certain biogas compounds may cause their fast degradation. Moreover, their low chemical reactivity cannot preclude accumulation of unfavorable substances in membranes. That is why membrane processes for biogas treatment do exist but do not employ liquid membranes. We have proposed a newmethod of membrane separation called the “condensing-liquid membrane” (CLM). 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 permselective membrane; furthermore, condensed water passing through the membrane ensures selectivity of the whole separation. The CLM is in fact a liquid (water in this case) condensing on a porous hydrophilic membrane as a result of the temperature difference of the membrane and water-saturated biogas feed. The main difference between the CLM and an immobilized liquid membrane lies in the fact that the condensing membrane is being regenerated during its continuous operation. The feed mixture of gases (raw biogas from a sewage plant, see Table 1) is saturated by water vapor. The porous membrane (the optimal pore size must be found) has to be cool enough to make the liquid condense in its pores. In our case study, the feed biogas was thermostated at 27 8C and the porous membrane at 14 8C. Various operational conditions were followed and their effect on the separation of methane from unwanted gases was monitored.