valve 13 a one way valve 15 in the gas return line 12 prevents back flow into return gas line 12 in alternative to returning the expanded gas from the turbo expander unit 10 to the gas feed line 8 on the intake side of the compressor 1 as illustrated in continuous lines in fig3 the expanded gas may be returned further upstream on the intake side of the compressor such as to the separator or to the bi phase well stream fed into the separator as illustrated in fig3 by dash dot lines extending the gas return line 12 to the upstream side of the separator the latter alternative may be advantageous e g in a case where liquid is precipitated from the expanded gas on the discharge side of the turbo expander unit 10 the expansion turbine 16 in the turbo expander unit 10 is drivingly connected to a pump wheel or rotor 17 in the liquid pump 2 in operation the pump 2 draws liquid from the separator 3 via liquid feed line 18 for injection into the compressed gas discharge line 9 via liquid injection line 19 which connects to the discharge line 9 at a liquid injection point re cycling of liquid back to the separator 3 can be accomplished via liquid return loop 20 and flow control valve 21 connecting the separator with the liquid injection line.