TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The device comprises at least one additional permeate separation stage. The permeate separation stage separates the first permeate stream in a third retentate stream and a third permeate stream, which is discarded or extracted or further processed as a product. The permeate pressure of the feed stream separation stage through permeate control device in the first permeate stream and/or through retentate control device in the third retentate stream is increased or decreased. The permeate control device and/or the retentate control device are controlled on the basis of measured values of at least one feed stream measuring devices in the first feed stream and/or in the raw gas stream and/or at least one retentate stream measuring devices in the second retentate. A gas separation membrane module with a mixed gas selectivity of carbon dioxide/methane of at least 30, preferably at least 45 are used in the feed stream separation step, preferably in all three membrane separation stages. The second permeate and/or the third retentate streams are supplied to the feed stream. The second permeate and/or the third retentate streams are led to a suction side of the compressor for resist processing. The second permeate stream and/or the third retentate stream are introduced between two compression stages in the compressor. The concentration of at least one separation step permeate gas of the feed stream is increased after recirculation of second permeate stream and the third retentate stream in the feed stream, preferably is increased by at least 2%, preferably 3 to 40%, compared to the concentration in the raw gas stream. An iterative trial or a process simulation is carried out prior to the start of the operation of the gas separation plant, where it is determined whether a change in the composition of the product stream or the feed- or the crude gas stream or a change in the flow of the feed- and crude gas is counteracted by means of increase or decrease of the pressure in the first permeate stream by means of the permeate control device and/or the retentate control device. The concentration of a heavy permeate component B1 of the second retentate stream determined by means of retentate stream measuring device falls below a predetermined target point, which increases or decreases the pressure of the first permeate stream by means of the permeate control device and/or the retentate control device until the concentration lies in target area. The membrane separation step comprises many gas separation membrane modules, which are connected parallely and/or serially. The gas separation membrane module comprises a hollow fiber membrane and/or a flat membrane. The multi-stage compressor is present in the device. The compressor is arranged in the device such that it generates a pressure drop in the feed stream separation step. The device comprises a control device, which adjusts the capacity of the compressor, preferably its rotational speed to change the second permeate stream and/or the third retentate stream and/or the crude gas stream, where the control device is controlled based on measured values of measuring devices into the respective streams. The flow-meters are used as measuring device in the feed stream. An online or offline measuring device for determining the composition of the gas mixture is used in the second retentate stream. A proportional-integral-derivative controller processes the data of the retentate stream measuring device and/or the feed stream measuring device and controls the permeate control device and/or the retentate control device. The materials for the separation-active layer of the membranes is amorphous or semi-crystalline plastics, preferably polyimide, preferably polyimide derivative of formula (I) and (II), polyamide, polysulfone, cellulose acetates or its derivatives, polyphenylene oxides, polysiloxanes, polymers of intrinsic microporosity, mixed matrix membranes, facilitated transport membranes, polyethylene oxides and/or polypropylene oxides. The concentration of the heavy permeating component B1 of the second retentate stream determined by means of retentate stream measuring device rises above a predetermined target point, which increases or decreases the pressure of the first permeate stream, by means of the permeate control device and/or the retentate control device until the concentration lies within the target value. The volume flow of the second retentate stream, preferably the composition of the second retentate stream, lies within limits, which drops the volume flow of the feed stream determined by means of the feed stream measuring device. The pressure of the first permeate stream increases or decreases by means of the permeate control device and/or the retentate control device until the composition, the pressure or volume flow of the second retentate stream lies within target value. The concentration of the heavy permeating component B1 increases the feed stream or the crude gas stream determined by means of the feed stream measuring device. The pressure of the first permeate stream is lowered by means of the permeate control device and/or the retentate control device until the composition or the pressure or the volume flow of the second retentate stream lies within the target value. The concentration of the heavy permeate component B1 reduces the feed stream or the crude gas stream determined by means of the feed stream measuring device. The pressure of the first permeate stream is lowered by means of the permeate control device and/or the retentate control device until the composition, the pressure or the volume flow of the second retentate stream, is within the target range. The pressure drop over the feed stream separation step is adjusted to 1-30 bar, preferably 3-10 bar. The pressure drop over the feed stream separation step and the retentate separation step is adjusted to 1-100 bar, preferably 10-70 bar. The partial pressure difference between the retentate- and the permeate side in the membrane separation stages is used as a driving force for the separation. The partial pressure is produced by the compressor, which is arranged on the feed side of the feed stream separation step and optionally by means of at least two vacuum pumps in the second and/or third permeate stream and/or through a permeate purge gas stream. The pressure of the permeate of the feed stream separation stage is the same or increased compared to the ambient pressure such that the partial pressure difference is present between retentate and permeate of the permeate separation stage and a driving force is provided, when the permeate of the permeate separation stage is applied to ambient pressure or vacuum. The capacity of the compressor, preferably its rotational speed, is adjusted to changes of the second permeate, the third retentate stream and/or the crude gas stream by means of a control device. The control device is controlled based on measured values of measuring devices in the streams. The method is designed such that varying amounts of recirculated gas from the second permeate stream and/or the third retentate stream is compensated preferably automatically with a control of the supplied amount of raw gas, preferably via crude gas control device without the changing speed of the compressor. The plant capacity is increased or decreased by changing the displacement volume of the compressor.x = 0-0.5;y = 0.5-1; andR = 1,3,5-trimethyl-benzene-2,4-diyl, 1-methyl-benzene-2,4-diyl, 2-methyl-benzene-1,3-diyl or 4,4'-methylenebis(4,1-phenylene. EXAMPLE - No suitable example given.