Abstract: In a process in which a turbo-expander is utilized to drive a compressor, the pressure of the gas flowing to the turbo-expander is maintained at a desired level by manipulating the speed of the turbo-expander. If the turbo-expander should tend to overspeed, the discharge output of the compressor is recycled to the suction inlet of the compressor to thereby increase the loading on the compressor which will result in a reduced turbine speed. More gas can then be fed through the turbo-expander which will maintain the desired pressure. If the pressure of the gas flowing to the turbo-expander should continue to increase, a bypass valve is utilized to bypass the turbo-expander to thereby maintain a desired pressure of the gas flowing to the turbo-expander.