NOVELTY - Device for separating gases comprises feed stream separation stage (1) as a membrane separation stage, a retentate separation stage (2), at least one compressor (4), which is arranged on the feed side of the feed stream separation stage, at least one, preferably at least two vacuum pump, which are arranged such that they produce a partial pressure difference between the retentate and permeate side of the membrane, and a permeate control device. The feed stream separation stage divides a feed stream (5) made of at least two components into a first permeate stream.    USE - The device is useful for separating gases.    DETAILED DESCRIPTION - Device for separating gases comprises feed stream separation stage (1) as a membrane separation stage, a retentate separation stage (2), at least one compressor (4), which is arranged on the feed side of the feed stream separation stage, at least one, preferably at least two vacuum pump, which are arranged such that they produce a partial pressure difference between the retentate and permeate side of the membrane, and a permeate control device. The feed stream separation stage divides a feed stream (5) made of at least two components into a first permeate stream (6), which is discarded or removed or further processed as a second product stream, and a first retentate is separated. The retentate separation stage separates the first retentate stream into a second permeate stream and a second retentate stream, which is removed as a product or is further processed. The permeate control device increases or decreases the permeate pressure of the feed stream separation stage. At least one feed stream measuring device is controlled in the feed stream and/or in the raw gas stream and/or at least one retentate stream measuring device is controlled in the second retentate stream, respectively based on the measured values. INDEPENDENT CLAIMS are also included for:    (1) controlling a gas separation plant, comprising measuring the change in the composition of the second retentate stream with the retentate stream measuring device and/or measuring the change in the flow or composition of the feed stream and/or the crude gas stream with the feed stream measuring device, and increasing or decreasing the pressure in the first permeate stream by means of the permeate control device and/or the retentate control device, depending on the measured values of the retentate stream measuring device and/or the feed stream measuring device; and    (2) a biogas plant comprising the device.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic representation of the arrangement with three membrane separation steps.    Feed stream separation stage (1)    Retentate separation stage (2)    Compressor (4)    Feed stream (5)    First permeate stream (6)