The growing demand on natural gas leads to the necessity to upgrade sub-quality gas. This sub-quality gas partly contains high amounts of nitrogen and/or carbon dioxide, sometimes helium is even a component of natural gas. To achieve pipeline and sales gas specification it is necessary to remove non desired components. Higher hydrocarbons such as ethane, propane, butane and pentane and he-lium can form valuable by-products. It is economically feasible to recover helium, if the amount in natural gas is higher than 0.1 mol%. Some of the recovery processes are well known from air separation: -...