NOVELTY - Production of pure methane comprises rectifying liquefied methane from a natural gas storage tank.    USE - The process can be used to produce methane with a purity of 99.9 mole%.    ADVANTAGE - Pure methane can be produced without expensive modification to existing natural gas liquefaction units.    DETAILED DESCRIPTION - Production of pure methane from natural gas that has been purified to remove water vapor, carbon dioxide and higher-boiling (C2+) hydrocarbons comprises:    (a) indirectly cooling the natural gas by evaporation of liquid nitrogen in a natural gas storage tank to liquefy the methane;    (b) withdrawing a gas comprising components with a lower boiling point than methane from the natural gas storage tank;    (c) rectifying the liquefied methane in a rectification column;    (d) withdrawing pure methane from the rectification column as a liquid bottoms stream; and    (e) recycling the overhead gas from the rectification column to the natural gas storage tank.    An INDEPENDENT CLAIM is also included for apparatus for carrying out the process, comprising a natural gas storage tank with a gas space equipped with a condenser, a methane storage tank equipped with a heater, and a rectification column, where the methane storage tank and rectification column are integrated into a separating column, the top of which is connected via a gas line to the gas space in the natural gas storage tank and via a liquid line to the liquid space of the natural gas storage tank to form a separatory circuit.    DESCRIPTION OF DRAWING(S) - The figure shows an apparatus for carrying out the process, comprising a natural gas storage tank with a gas space equipped with a condenser, a methane storage tank equipped with a heater, and a rectification column, where the methane storage tank and rectification column are integrated into a separating column, the top of which is connected via a gas line to the gas space in the natural gas storage tank and via a liquid line to the liquid space of the natural gas storage tank to form a separatory circuit.    Gas line (2)    Natural gas storage tank (3)    Condenser (4)    Liquid line (6)    Rectification column (7)    Methane storage tank (9)    Heater (10)