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Pressure swing adsorption


Gas separation using adsorption is based on different adsorption behaviour of various gas components on a solid surface under elevated pressure. Usually, different types of activated carbon or molecular sieves (zeolites) are used as the adsorbing material. These materials selectively adsorb carbon dioxide from the raw biogas, thus enriching the methane content of the gas. After the adsorption at high pressure the loaded adsorbent material is regenerated by a stepwise decrease in pressure and flushing with raw biogas or biomethane. During this step offgas is leaving the adsorber. Afterwards, the pressure is increased again with raw biogas or biomethane and the adsorber is ready for the next sequence of loading.


Figure: Flowsheet of a typical biogas upgrading unit applying pressure swing adsorption

Industrial scale upgrading plants implement four, six or nine adsorber vessels in parallel at different positions within this sequence in order to provide a continuous operation. During the decompression phase of the regeneration the composition of the offgas is changing as the also adsorbed methane is released earlier (at higher pressures) and the bulk of carbon dioxide is preferentially desorbed at lower pressures. Thus, the offgas of the first steps of decompression is typically piped back to the raw biogas inlet in order to reduce the methane slip. Offgas from later steps of regeneration could be led to a second stage of adsorption, to the offgas treatment unit or could be vented to the atmosphere. As water and hydrogen sulphide contents in the gas irreversibly harm the adsorbent material these components have to be removed before the adsorption column.


Figure: Biogas upgrading plant Mühlacker, Germany with a raw biogas capacity of 1000m³/h (Source: Schmack CARBOTECH)