Nonhydrocarbon gas separation device and nonhydrocarbon gas separation method

A non-hydrocarbon gas separation method for separating, from a natural gas containing a non-hydrocarbon gas, the non-hydrocarbon gas,

• the non-hydrocarbon gas separation method comprising the steps of:
• supplying the natural gas to a first separation module in which a separation membrane is housed, from a first supply line connected to the first separation module;
• allowing the non-hydrocarbon gas contained in the natural gas supplied from the first supply line to permeate the separation membrane in the first separation module to be separated, followed by discharging the non-hydrocarbon gas having been separated from the natural gas from a first discharge line connected to the first separation module;
• allowing the natural gas having been separated from the non-hydrocarbon gas to flow through the first separation module, followed by allowing the natural gas to outflow from a first outflow line connected to the first separation module;
• supplying the natural gas having outflowed from the first separation module to a second separation module in which a separation membrane is housed, and which is connected to the first separation module in series through a second supply line doubling as the first outflow line, from the second supply line;
• allowing the non-hydrocarbon gas contained in the natural gas supplied from the second supply line to permeate the separation membrane in the second separation module to be separated, followed by discharging the non-hydrocarbon gas having been separated from the natural gas from a second discharge line connected to the second separation module;
• allowing the natural gas having been separated from the non-hydrocarbon gas to flow through the second separation module, followed by allowing the natural gas to outflow from a second outflow line connected to the second separation module;
• increasing pressures of the non-hydrocarbon gas by compressors arranged in the discharge lines of the first separation module and the second separation module respectively, wherein the discharge line of the second separation module is connected to a suction side of the compressor arranged in the discharge line of the first separation module;
• detecting a pressure in the discharge line of the first separation module with a first pressure detection unit arranged at a suction side of the corresponding compressor arranged in the discharge line of the first separation module, and detecting a pressure in the discharge line of the second separation module with a second pressure detection unit arranged at a suction side of the corresponding compressor arranged in the discharge line of the second separation module;
• and
• controlling a first pressure control valve arranged between the first pressure detection unit and the corresponding compressor along the discharge line for the first separation module based on a value of pressure detected with the first pressure detection unit so that said value of pressure detected is a first preset target pressure, and controlling a second pressure control valve arranged between the second pressure detection unit and the corresponding compressor along the discharge line for the second separation module based on a value of pressure detected with the second pressure detection unit so that said value of pressure detected with the second pressure detection unit is a second preset target pressure, wherein said first pressure control unit and said second pressure control unit are controlled so that said first preset target pressure is maintained higher than said second preset target pressure;
• wherein a pressure of the first separation module on a first discharge line side is higher than a pressure of the second separation module on a second discharge line side under the pressure control implemented by said first pressure control valve and said second pressure control valve, and
• wherein the compressor arranged in the discharge line of the second separation module is configured to increase a pressure of the non-hydrocarbon gas to a pressure equal to or higher than a suction pressure of the compressor arranged in the discharge line of the first separation module.