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Gaspermeation uses a dense polymeric membrane (polyimide) with different solubilities and diffusivities for the gas components contained in the biogas. As a result, the diffusion of the various gas components through the membrane differs significantly causing a quantitative separation of the gas species contained in the biogas. Membranes for biogas upgrading are made of materials that are permeable for carbon dioxide, water and ammonia. Hydrogen sulphide, oxygen and nitrogen permeate through the membrane to a certain extent and methane passes only to a very low extent. Typical membranes for biogas upgrading are made of polymeric materials like polysulfone, polyimide or polydimethylsiloxane. These materials show favourable selectivity for the methane/carbon dioxide separation combined with a reasonable robustness to trace components contained in typical raw biogases.

The driving force for the diffusion of a gas species through the membrane is the difference of the partial pressures of this species in the feed and in the permeate phase. A high transmembrane flow can be realised by applying a high pressure in the feed and a very low pressure in the permeate (almost atmospheric). The applied membrane material allows for a quantitative removal of most unwanted gas components. The separation procedure itself is marked by its simple, compact and steady character. Only nitrogen shows a behaviour similar to methane and can therefore not be removed from the biogas stream but remains in the product gas. Almost all modern biogas upgrading technologies show this behaviour regarding nitrogen today. Therefore, it is necessary to reduce the nitrogen content in the raw biogas itself by appropriate measures. The grade of upgrading quality is principally not limited by applying the gaspermeation technique. Sufficient product gas quality can be guaranteed by applying sufficient membrane area and suitable operational parameters.

Figure: Principles of biogas upgrading using gaspermeation

The major advantages of gaspermeation compared to other upgrading technologies are the stable operation and the compact plants together with the coinstantaneous drying and separation of trace components like ammonia and hydrogen sulphide just in one process step. A proper gas pre-treatment is indispensable because the combination of a wet and condensing gas with high amounts of ammonia or hydrogen sulphide would lead to a derogation of the membrane material. The stable and continuous operation of a gaspermeation upgrade unit makes this technique easily automatable and controllable. Furthermore, no expensive and energy-intense thermal regeneration or additional chemicals are involved in this process. Therefore, the whole upgrading plant ends up very simple and compact and can be realised in a very economical way regarding investment and operational cost. Additionally, it has been proved, that biogas upgrading using gaspermeation is very energy-efficient, which means that only a very low amount of energy is needed for the upgrading step (about 3% related to the higher heating value of the produced natural gas substitute).

To provide sufficient membrane surface area in compact plant dimensions these membranes are applied in form of hollow fibers with the high-pressure feed and retentate at the inner side and the low pressure permeate at the outside of the fiber. A high number of these hollow fibres are combined to a complete membrane module also holding the connectors for the feed, retentate and permeate piping. An exactly defined number of these modular components finally sum up to the total membrane area of the biogas upgrading plant.

Figure: left: SEM-micrograph of one hollow fibre membrane (magnification about 650); right: Depiction of a membrane module

After the compression to the applied operating pressure the raw biogas is cooled down for drying and removal of ammonia. After reheating with compressor waste heat the remaining hydrogen sulphide is removed by means of adsorption on iron or zinc oxide. Finally, the gas is piped to a single- or multi-staged gaspermeation unit. The numbers and interconnection of the applied membrane stages are not determined by the desired biomethane quality but by the requested methane recovery and specific compression energy demand. Modern upgrading plants with more complex design offer the possibility of very high methane recoveries and relatively low energy demand. Even multi-compressor arrangements have been realised and proved to be economically advantageous. The operation pressure and compressor speed are both controlled to provide the desired quality and quantity of the produced biomethane stream.

Figure: Flowsheet of a typical biogas upgrading unit applying the membrane technology gaspermeation

Gaspermeation is probably the youngest technology to be applied for biogas upgrading and thus it is relatively new. Nevertheless, a number of plants have already been realised showing the technological and economical feasibility of the process.

Figure: Biogas upgrading plant Kisslegg, Germany with a raw biogas capacity of 500m³/h (Source: AXIOM Angewandte Prozesstechnik)