Biogas is an eco-friendly energy source that is becoming in-creasingly important in today‘s energy supply. It can be used to generate power or heat or as a fuel, and provides a high energy yield per square meter of land. But an extensive up-grading and purification process is required before biogas is fed into the natural gas grid. New and highly selective poly-mer membranes from Evonik convert raw biogas simply and efficiently into highly pure biomethane. This increases yield and conserves valuable resources.

Product Story No. 44, January 11, 2012

Evonik Industries AG Rellinghauser Straße 1–1145128 EssenGermanyContactCorporate Press Alexandra Boy phone +49 201 177-3167 fax +49 201 177-3030 alexandra.boy@evonik.comTrade Press Thomas Langephone +49 2365 49-9227 fax +49 2365 49-809227 thomas.lange@evonik.com

Biogas from spaghetti?Membranes from Evonik upgrade biogas, generated from renewable raw materials, simply and efficiently.

Highly selective membranes Polyimides are high performance polymers with high pressure and temper- ature resistance. “For SEPURAN we rely on a specially optimized form of Evonik‘s proven and tested polyimide family. The membranes have consis-tently high selectivity and are particularly suitable for separation of CO2 and methane,” says Dr. Goetz Baumgarten of the Fibres and Membranes growth line of Evonik‘s High Performance Polymers Business Line.

How does the membrane work? Gas molecules are of different sizes and have different solubilities in polymers. The biogas to be cleaned is introdu-ced under high pressure at one end of the membrane. “The CO2 mole-cules are smaller than the methane molecules and also more soluble in po-lymers. As a result, they pass through the micropores of the membrane much faster and are separated from the methane,“ explains Baumgarten. CO2, water vapor, and traces of ammonia and hydrogen sulfide are drawn off at the low-pressure side (from the lateral opening in Figure 2), while the methane collects at the other end of the membrane, the high-pressure

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It looks like a bunch of spaghetti strands or a paintbrush, but is in fact a bundle of highly selective polyimide hollow fibers in Evonik’s new SEPURAN membrane module.

Green is in, with bicycles preferred to cars, organic produce to fast food, and energy class A+++ to A: In our modern society, decisions are increas- ingly being influenced by ecological considerations. Industry and business are also reacting to the trend towards sustainability and offering more and more “eco” products. And green energy is following the same track. Ac-cording to the Renewables Global Status Report (GSR) 2011, renewable energies today account for about 16 percent of global energy consump-tion; by the year 2050, this figure could rise to more than 50 percent, as predicted in a scenario of the World Climate Council in its Special Report on Renewable Energy Sources and Climate Change Mitigation (SRREN). With the major energy producers focusing mainly on wind, water and sun, biogas as an alternative energy source appears to have been somewhat overshadowed—quite unjustifiably, because it is a highly efficient energy source and an important component of decentralized supply structures.

Efficient biogas upgrading Biogas is produced by fermentation of biomass, an organic substance con-sisting of, for example, plants, liquid manure, or effluent sludge. But in ad-dition to the methane energy source, raw biogas also contains carbon dio-xide (CO2)and other trace gases. Because CO2 is not combustible, it lowers the calorific value of the gas and must therefore be separated out. The common separation methods such as pressurized water scrubbing, pressure swing adsorption, and amine scrubbing have considerable disad-vantages: They need comparatively large amounts of energy as well as auxiliary materials and chemicals. Wastes and wastewater are generated that must be treated and disposed of. Further, the biogas after upgrading is usually at low pressure. Before it is fed into a medium-pressure grid, it needs to be compressed to 15-20 bar by, for example, an additional com-pressor. Conventional upgrading plants are therefore usually cost effective only for raw biogas quantities significantly in excess of 500 standard cubic meters per hour (Nm³/h). This usually makes them unsuitable for decen-tralized energy supply with a large number of relatively small plants. Evonik Industries has developed a technology for cost- and energy-effici-ent separation of CO2. What appears at first sight to be a bunch of spa-ghetti strands or a paint brush, is in fact a bundle of highly selective membranes made up of multiple cylindrical polyimide hollow fibres. These are used in the new hollow fibre membrane modules of SEPURAN Green. side. The methane-rich gas is directly drawn off at the high-pressure side

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and needs no further compression for feeding into the grid. This saves the costs of an additional compressor. Further, the biomethane produced in this way can be used even by small plants and therefore allows decentra-lized supply of energy. In a test plant in Neukirchen an der Vöckla (Austria), Evonik‘s experts have been putting the production modules through their paces since early 2011. “Our experience so far is that the new SEPURAN Green membrane modules are a robust and simple tool for gas purification. The methane of the raw gas can be cleaned to a purity higher than 99 percent. In use, the modules are distinguished by higher plant availability, reduced energy requirements, and lower maintenance costs than in alternative processes,” says Baumgarten. These are compelling arguments in view of the globally increasing interest in green energy.

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Biogas is produced by microbial fermenta-tion of sustainable raw materials like corn, or from effluent sludge or liquid manure. Following separation of various minor com-ponents like water vapor, hydrogen sulfide, and particularly CO2, the biogas can then be used in the gas grid, in combined heat and power plants, and at filling stations.

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