NOVELTY - Pressure swing adsorption, separation and purification of carbon dioxide in biogas involves removing free water from biogas by a gas water separator (1); compressing and pressurizing dewatered biogas; when the biogas enters a buffer tank (3) then absorbing carbon dioxide and small amount of impurity in biogas by compound absorbent; exhausting and recycling the absorption bed and evacuating the other part; and (e) pumping out the carbon dioxide from the vacuum pump (5), regenerating the absorbent, and raising the pressure at the absorption pressure for next absorption and circulation.    USE - For pressure swing adsorption, separation and purification of carbon dioxide in biogas (claimed).    ADVANTAGE - The method is suitable for scale continuous production, is simple, continuous and convenient to operate; and saves energy consumption. The concentration of methane in the biogas to over 98%, enriches and purifies carbon dioxide to over 98.5% by decompression desorption, and regenerates the absorbent.    DETAILED DESCRIPTION - Pressure swing adsorption, separation and purification of carbon dioxide in biogas, involves (a) removing free water from biogas by a gas water separator (1); (b) compressing and pressurizing the dewatered biogas at 0.5-1.5 MPa; (c) when the biogas in step (b) enters a buffer tank (3), enabling the biogas to enter from the bottom of an abruption tower to the bed of the absorption tower (4), then absorbing the carbon dioxide and a small amount of impurity in the biogas by the compound absorbent, then exhausting the methane from an outlet of the absorption tower; when the front edge of the impurity approaches the outlet of the absorbent, closing a raw material gas inlet of the absorption tower and an outlet gas valve; then stopping absorbing; opening stroke control valves KV-3A and KV-4A; descending for 2-3 times stably at the same speed and pressure, then absorbing the carbon dioxide from the compound absorbent; (d) exhausting and recycling the absorption bed and evacuating the other part; and (e) pumping out the carbon dioxide from the vacuum pump (5), regenerating the absorbent till the purity is more than 98.5%; then raising the pressure at the absorption pressure for next absorption and circulation.    DESCRIPTION OF DRAWING(S) - The figure shows a diagram of method for pressure swing adsorption, separation and purification of carbon dioxide in biogas.    Gas water separator (1)    Buffer tank (3)    Absorption tower (4)    Vacuum pump (5)    Blower (7)