A process for treating methane-containing gas including CO.sub.2 and at least one compound of volatile organic compounds (VOC) wherein the methane-containing gas mixture is compressed, cooled and supplied to an absorption apparatus absorbing liquid VOC reversibly and at least a portion of the VOC is absorbed from the methane-containing gas mixture yielding a methane-containing and VOC- and CO.sub.2-reduced gas mixture and VOC- and CO.sub.2-loaded absorption means. The VOC- and CO.sub.2-loaded absorption means is transported from the absorption apparatus to a desorption apparatus. The methane-containing and VOC-reduced gas mixture is supplied from the absorption apparatus to a separation apparatus for CO.sub.2 removal. A regeneration gas stream includes at least a portion of the CO.sub.2-enriched gas stream which regenerates the VOC- and CO.sub.2-loaded absorption means. The exhaust gas stream discharges from the desorption apparatus and the at least partially regenerated absorption means moves from the desorption apparatus into the absorption apparatus.

Provided are an acidic gas removing agent with smaller variation in amounts of carbon dioxide absorption even in the coexistence of water in removing carbon dioxide in a gas, an acidic gas removing method, an absorption device provided with the acidic gas removing agent, and an acidic gas cleaning device. Used is an acidic gas removing agent containing one or more selected from the group consisting of a hydroxy group-containing compound and a derivative of the hydroxy group-containing compound each having a number-average molecular weight of less than 500, containing a constituent unit having 3 or more carbon atoms and one or more oxygen atoms, and having a solubility parameter (LogS) value of 2.25 or less.

A process for treating methane-containing gas including CO.sub.2 and at least one compound of volatile organic compounds (VOC) wherein the methane-containing gas mixture is compressed, cooled and supplied to an absorption apparatus absorbing liquid VOC reversibly and at least a portion of the VOC is absorbed from the methane-containing gas mixture yielding a methane-containing and VOC- and CO.sub.2-reduced gas mixture and VOC- and CO.sub.2-loaded absorption means. The VOC- and CO.sub.2-loaded absorption means is transported from the absorption apparatus to a desorption apparatus. The methane-containing and VOC-reduced gas mixture is supplied from the absorption apparatus to a separation apparatus for CO.sub.2 removal. A regeneration gas stream includes at least a portion of the CO.sub.2-enriched gas stream which regenerates the VOC- and CO.sub.2-loaded absorption means. The exhaust gas stream discharges from the desorption apparatus and the at least partially regenerated absorption means moves from the desorption apparatus into the absorption apparatus.

An acidic gas absorbing solution of the present invention is an acidic gas absorbing solution that reversibly absorbs and desorbs carbon dioxide, the acidic gas absorbing solution containing: an oxygen-containing polymer and an amine compound, wherein the oxygen-containing polymer has at least one group selected from the group consisting of an oxyalkylene group, a carbonate group, and an ester group, has a number-average molecular weight of 250 to 20000, has a boiling point of 200 C. or more, and has a solubility parameter of 15.0 (MPa).sup.1/2 or more and less than 19.5 (MPa).sup.1/2, and the amine compound has a solubility parameter of 13.0 to 25.5 (MPa).sup.1/2.