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An Introduction to Pressure Swing Adsorption
The Popular Technology Used for Upgrade Biogas
Pressure swing adsorption is used for recovering high-purity gas products from crude process
gas streams, for example in hydrogen production, or as an alternative to hauled-in atmospheric gas products or
onsite cryogenic air separation processes.
Also known as PSA, this purification method separates the Carbon Dioxide, Nitrogen, Oxygen and Water from the raw
biogas stream by adsorbing gases at high pressure and desorbing them at low pressure as waste. The PSA system
commonly comprises four different adsorption columns working in sequence; Adsorption, depressurizing, desorption
and re-pressurizing.
The raw biogas is compressed and fed into the bottom of the adsorption column where it is purified. During this
time the remaining columns regenerate, such that there is always 1 absorber column actively cleaning gas. PSA does
not scrub hydrogen sulphide so this most be removed before it enters the compressor.
Pressure swing adsorption is a preferable technology if an adsorbent that allowing a largecoefficient of separation
for the CH4/N2 system is available.
Pressure swing adsorption is known to be one of the most economic processes to recover carbon dioxide in flue
streams from power plants, biogas plants, and incinerators. Carbon dioxide is hard to desorb or separate from
adsorbents such as zeolite and activated carbon; conventional processes, however, suffer from low productivity and
heavy electricity expenses for desorption by vacuum pumps.
Adsorbents play a major role in separation technologies due to their ability to selectively adsorb one or more
components from a mixture. One particular feature of adsorbents of great importance is their internal porous
structure.
PSA processes can be used to separate gases in a mixture because different gases tend to be attracted to different
solid surfaces more or less strongly. If a gas mixture such as air, for example, is passed under pressure through a
vessel containing an adsorbent bed that attracts nitrogen more strongly than it does oxygen, part or all of the
nitrogen will stay in the bed, and the gas coming out of the vessel will be enriched in oxygen.
The most commonly used system was developed from the Skarstrom cycle utilizing two or more packed adsorbent beds
and using four steps to comprise the cycle consisting of pressurization, adsorption, counter current blow-down, and
counter current purge.
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