Pipelines are used to transport natural gas over thousands of miles, with compressors (pumping stations) at regular intervals along the pipeline to compensate for the pressure drop due to flow. Because of the long distances involved and the good heat transfer, the gas remains at an ambient temperature of 20°C. At each pumping station the gas in compressed with a singlestage compressor to 4 MPa, and then isobarically cooled back to 20°C. Because of safety and equipment constraints, the gas temperature during compression should not exceed 120°C. Assuming natural gas is methane that is described by the Peng-Robinson equation of state, how low a pressure is allowed before the gas should be recompressed?