A process for separating methane from a natural gas mixture employs pressure swing adsorption in one or more vessels. Each vessel has an adsorbent material having a kinetic selectivity for contaminants over methane greater than 5. Contaminants within the natural gas mixture become gases kinetically adsorbed within the adsorbent material. The vessel is placed under pressure to cause contaminants to be adsorbed in the surfaces and micro-pores of the adsorbent material. The process includes releasing a product stream comprised at least 95% by volume methane from a first gas outlet in the vessel, and desorbing the contaminant gases from the adsorbent material by reducing the pressure within the vessel. The desorbing step is done without applying heat to the vessel, thereby delivering a waste gas stream comprised at least 95% by volume of the contaminant gases. An improved fractionation vessel having both major and minor flow channels is also provided.