NOVELTY - Device comprises at least two membrane units, a compressor (3) placed upstream from the first membrane unit. The membrane units exhibit a gas inlet, a retentate outlet and a permeate outlet. The retentate outlet of a first membrane unit (1) is connected to the gas inlet of a second membrane unit (2), the permeate outlet of the second membrane unit (2) is connected on the intake side with the compressor (3) or the gas supply leading into the condenser, and the compressor is conductively connected with the gas inlet of the first membrane unit.    USE - The device is useful for controlling the concentration of product gas in an offgas during separating a gas mixture into product gas and offgas by gas permeation, preferably for the separating the gas mixture comprising methane and carbon dioxide into methane as product gas and carbon dioxide as offgas (all claimed).    ADVANTAGE - The device provides improved product gas yield and is cost efficient.    DETAILED DESCRIPTION - Device comprises at least two membrane units, a compressor (3) placed upstream from the first membrane unit. The membrane units exhibit a gas inlet, a retentate outlet and a permeate outlet. The retentate outlet of a first membrane unit (1) is connected to the gas inlet of a second membrane unit (2), the permeate outlet of the second membrane unit is connected on the intake side with the compressor or the gas supply leading into the condenser, and the compressor is conductively connected with the gas inlet of the first membrane unit. The product gas is obtained via the retentate outlet, and offgas is obtained via the permeate outlet. The permeate outlet of the first membrane unit is connected to the line between the permeate outlet of the second membrane unit and the compressor or the gas supply leading into the condenser are connected. An INDEPENDENT CLAIM is also included for controlling the concentration of product gas in an offgas during separating a gas mixture into product gas and offgas by gas permeation, comprising using the retentate of a first membrane unit as the feed gas of a second membrane unit, mixing the permeate of the second membrane unit with the pressurized feed gas of the first membrane unit, removing the retentate of the second membrane unit as the product gas, and the permeate of the first membrane unit as the offgas. The permeate of the second membrane unit is mixed with a second permeate of the first membrane unit, and the mixture is mixed with the pressurized feed gas of the first membrane unit.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic block diagram of the device (Drawing includes non-English language text).    First membrane unit (1)    Second membrane unit (2)    Compressor (3)    Additional compressor (4)    Control valve (5)