NOVELTY - Methane (CH4) in liquefied natural gas (LNG) vapor is purified by pumping mixed gas from LNG storage tank and entering to pressure swing adsorption device to process separation of gas; introducing mixed gas to first adsorption tower to adsorb, where nth adsorption tower is vacuumizing, and rest adsorption towers are pressure equalizing; introducing concentrated purified gas with high purity CH4 to helium liquefier system; and introducing low-temperature mixed gas when first adsorption tower finishing adsorption.    USE - Method of purifying methane in liquefied natural gas vapor (claimed).    ADVANTAGE - The method realizes effective cyclic utilization of energy, guarantees safety production, and reduces air pollution. It has simple process, convenient operation, and good safety performance, and is suitable for large-scale production. The method can continuously obtain LNG vapor with high pure CH4, where purity can reach to 95-99.9%, and yield is more than 90%.    DETAILED DESCRIPTION - Methane (CH4) in liquefied natural gas (LNG) vapor is purified by pumping mixed gas from LNG storage tank and entering to pressure swing adsorption device to process separation of gas; introducing mixed gas to first adsorption tower to adsorb, where nth adsorption tower is vacuumizing, and rest adsorption towers are pressure equalizing; introducing concentrated purified gas with high purity CH4 to helium liquefier system, and transferring again to high purity LNG; and introducing low-temperature mixed gas when first adsorption tower finishing adsorption, where nth adsorption tower is stopped form vacuumizing. Desorption stage is processed at constant pressure or vacuum degree of 0.08 MPa, where adsorption period is 4-10 minutes, and adopting multiple pressure equalizing. Method is processed at low-temperature by pressure swing adsorption and temperature of - 150-(- 90) degrees C. Pressure swing adsorption device uses more than nth adsorption towers to adsorb, where n is not less than 4.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic block diagram illustrating method of purifying methane in liquefied natural gas vapor (Drawing includes non-English language text).