TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The first permeate stream is not subjected to recompression. At least in the feed stream separating unit, preferably in all the three membrane separation units, gas separation membrane modules having mixed gas selectivity for carbon dioxide and methane of at least 30, preferably at least 45 are used. At least one of the membrane separation units, comprises more than one gas separation membrane modules which are parallely and/or serially connected. The gas separation membrane modules comprise hollow fiber membranes and/or flat membranes. The device is designed such that the gas volume recycled in the second permeate stream and the third retentate stream, amounts to less than 60 vol.% of the volume of crude gas stream. The device is designed such that it increases the concentration of at least one permeate gas of the feed stream separating unit, in the feed stream, by at least 2%, preferably 3-40% compared to the concentration in the crude gas stream, after recycling the second permeate stream and the third retentate stream. The second permeate stream and the third retentate stream are conducted on suction side of the compressor. The device comprises a multi-step compressor, where the second permeate stream and/or the third retentate stream are introduced into the compressor between two compression steps. The compressor is arranged in the device such that it generates a pressure difference in the feed stream separating unit. The device comprises a control device which adjusts the power of the compressor, preferably rotational speed of the compressor with changes in the second permeate stream, the third retentate stream and/or the crude gas stream. The device is configured such that it automatically balances varying amounts of recycled gas from the second permeate stream and/or the third retentate stream, with a control of the supplied amount of crude gas through a crude gas control device, without changing the rotational speed of the compressor. The first retentate stream and/or the first permeate stream make use of flow meters as measuring device. The second retentate stream and/or the third permeate stream make use of an online- or offline measuring device for determining the composition of the respective gas mixture. The method comprises discharging at least 95%, preferably at least 99.5% of the retentate component of the feed stream separating unit, introduced into the device with the crude gas stream, through the second retentate stream, and discharging not greater than 5%, preferably not greater than 0.5% of the retentate component of the feed stream separating unit, introduced into the device with the crude gas stream, through the third permeate stream. Preferred Method: The determination of the concentration is carried out online and/or offline. In the method (v) when the volume flow of the first retentate stream increases, then the pressure of the second permeate stream is reduced using the permeate control device, until a property of the second retentate stream is correlated with the volume flow of the first retentate stream using a calibration curve, preferably until the composition of the second retentate stream is within limits, and (vi) when the volume flow rate of the first retentate stream decreases, then the pressure of the second permeate stream is increased using the permeate control device, until a property of the second retentate stream is correlated with the volume flow of the first retentate stream, using a calibration curve, preferably until the composition of the second retentate stream is within limits, and/or (vii) when the volume flow of the first permeate stream increases, then the pressure of the third retentate stream is increased using the retentate control device, until a property of the third permeate stream is correlated with the volume flow of the first permeate stream using a calibration curve, preferably until the composition of the third permeate stream is within limits, and (viii) when the volume flow of the first permeate stream decreases, then the pressure of the third retentate stream is reduced using the retentate control device, until a property of the third permeate stream is correlated with the volume flow of the first permeate stream, using a calibration curve, preferably until the composition of the third permeate stream is within limits. The controlling is carried out according to a calibration curve which comprises a correlation between flow and pressure for maintaining a concentration in another gas stream. The pressure drop through the feed stream separating unit is adjusted to 1-30 bars, preferably 3-10 bars. The pressure drop through the feed stream separating unit and the retentate separating unit is adjusted to 1-100 bars, preferably 10-70 bars. The driving force for the separation task, comprises a partial pressure difference between the retentate- and the permeate side in the respective membrane separation units, where the partial pressure difference is generated by a compressor which is arranged on the feed side of the feed stream separating unit, and optionally by at least one, preferably one or two vacuum pumps in the second- and/or the third permeate stream and/or third permeate stream and/or by a permeate side purge gas stream. The pressure of the permeate of the feed stream separating unit is same or increased compared to ambient pressure such that a partial pressure difference between the retentate- and permeate of the permeate separating unit, is present, and thus a driving force is provided when the permeate of the permeate separating unit is applied with ambient pressure or reduced pressure. The system output of the plant is increased or decreased by a change in the displacement volume of the compressor, where a caused change of the concentration of the severe permeating component in the second retentate stream is counteracted by steps (i) and (ii), a caused change of the concentration of the severe permeating component in the third permeate stream is counteracted by steps (iii) and (iv), a caused change of the flow of first retentate stream is counteracted by the steps (v) and (vi) and/or a caused change of the flow of the first permeate stream is counteracted by the steps (vii) and (viii). The method is carried out as a part of an operation of a biogas plant, where depending on filling level of the biogas plant, which is determined by pressure of the fermenter, or filling level of an intermediate storage, the rotational speed of the compressor, and thus the delivery volume of the compressor is controlled for changing or constantly maintaining the filling level in the fermenter and/or the intermediate storage.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The gas mixture comprises a mixture of carbon dioxide and methane, a mixture of hydrogen and methane or a mixture of carbon monoxide and hydrogen, crude biogas or crude natural gas.    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The membranes of the device, comprise materials for separation-active layer, comprising amorphous- or partially crystalline plastic, preferably polyimide, polyamide, polysulfone, cellulose acetate or its derivatives, polyphenylene oxide, polysiloxane, polymers of intrinsic microporosity, mixed matrix membranes, facilitated transport membranes, polyethylene oxide and/ polypropylene oxide. The material for the separation-active layer of the membrane, comprises polyimide polymers of formula (I) or (II), preferably polyimide with CAS no. 9046-51-9 and/or polyimide with CAS no. 134119-41-8.x = 0-0.5;y = greater than or equal to 1 or greater than or equal to 0.5; andR = 1,3,5-trimethylbenzene-2,4-diyl, 1-methylbenzene-2,4-diyl, 2-methylbenzene-1,3-diyl or benzylbenzene-4,4'-diyl. EXAMPLE - No suitable example given.