Method and an apparatus for obtaining energy by expanding a gas at a wellhead

A method for reducing the pressure of a fluid comprising a natural gas, water, and solid residues, said fluid being available at an extraction pressure at a wellhead of a natural gas field and for recovering energy directly from said natural gas at said wellhead, comprising the steps of:

• prearranging a vertical elongated container having an inlet nozzle for said fluid for connection with said wellhead and an outlet nozzle for a treated natural gas, said outlet nozzle above said inlet nozzle in said vertical elongated container;
• prearranging a cyclone centrifugal separation stage arranged in said vertical elongated container at said inlet nozzle, said cyclone centrifugal separation stage configured for separating said water and solid residues from said natural gas of said fluid,
• prearranging an inertial separation stage arranged in said vertical elongated container at said outlet nozzle for separating liquid droplets and mist from the gas,
• prearranging a gas-conditioning means arranged downstream of said outlet nozzle and configured to prevent a formation of ice in said natural gas during a subsequent expansion in a rotor,
• prearranging an energy recovery expansion device arranged downstream of said gas-conditioning means for expanding said natural gas exiting from said wellhead until a use pressure is reached, said energy recovery expansion device having said rotor rotatably arranged about an own axis and a stator, and a gas passageway defined between said rotor and said stator;
• connecting an electric power generator to said rotor of said energy recovery expansion device, so that a rotation of said rotor causes an electric power generation by said generator;
• conveying a stream of said fluid exiting from said wellhead in said inlet nozzle for separating said water and solid residues from said natural gas of said fluid in said cyclone centrifugal separation stage, and then conveying said natural gas in said vertical elongated container up to said inertial separation stage for separating liquid droplets and mist from the natural gas and up to said outlet nozzle;
• conditioning said natural gas exiting from said outlet nozzle in said gas-conditioning means for preventing a formation of ice in said natural gas during said subsequent expansion,
• conveying said natural gas in said passageway of said energy recovery expansion device in such a way to cause a rotation of said rotor and to cause said expansion, the pressure of said gas decreasing from said extraction pressure to said use pressure;
• extracting said electric power that is generated by said generator as a consequence of said rotation of said rotor.