Method for cryogenic separation of carbon dioxide from hydrocarbons

In a method of treating hydrocarbon feed gas having relatively high carbon dioxide content to produce a liquid ethane-containing hydrocarbon product substantially free of carbon dioxide and a gaseous product comprised of methane and carbon dioxide wherein said treating includes:

• (a) expanding a precooled gaseous portion of said hydrocarbon feed gas through a turbine to produce a mixture of gaseous hydrocarbon, including methane, and liquid hydrocarbon, including liquid ethane, said mixture being at a temperature below -80.degree. F.;
• (b) passing said mixture to a demethanizer separation column at a temperature below -80.degree. F.;
• and
• (c) separating said mixture in said demethanizer such that essentially all the methane and gaseous carbon dioxide are recovered overhead as said gaseous product and essentially all said liquid hydrocarbon, including liquid ethane, is recovered at the bottom of said demethanizer as said liquid ethane-containing hydrocarbon product, while maintaining nongaseous carbon dioxide dissolved in said liquid ethane-containing hydrocarbon in said turbine and demethanizer, the specific improvement comprising;
• injecting from outside the demethanizer column into said ethane-containing liquid hydrocarbon at the bottom of said demethanizer separation column an effective amount of an inert sweep gas which does not liquefy under operating conditions such as to stabilize and desensitize the separation process occurring within said demethanizer with respect to temperature variations of said liquid hydrocarbon at the bottom of said column.