Process for recovering helium from a gas stream

A process for recovering helium from a hydrocarbon gas feed stream, containing nitrogen, at least 0.1 mol % helium, and hydrocarbons, which comprises:

• A. countercurrently contacting said hydrocarbon gas feed stream with a lean physical solvent selected from the group consisting of:
• 1) paraffinic solvents having molecular weights ranging from 70 to 140 and UOP characterization factors ranging from 12.0 to 13.5, said factors being independent of the aromatic content of said paraffinic solvents,
• 2) naphthenic solvents having mclecular weights ranging from 70 to 130 and UOP characterization factors ranging from 10.5 to 12.0, said factors being independent of the aromatic content of said naphthenic solvents,
• 3) aromatic solvents selected from the group consisting of benzene, toluene, C.sub.8 -C.sub.10 aromatic compounds having methyl, ethyl or propyl aliphatic groups specifically constituting a subgroup of o-xylene, m-xylene, pxylene, hemimellitene, pseudocumene, mesitylene, cumene, o-ethyltoluene, m-ethyltoluene, p-ethyltoluene, n-propylbenzene, isopropylbenzene, indane, durene, isodurene, prehnitene, crude xylenes, toluene transalkylation reaction effluents, extracted C.sub.9 naphtha reformates, C.sub.9 heart cuts of said reformates which are enriched in C.sub.9 alkylbenzenes, C.sub.7 -C.sub.9 alkyl aromatics, and mixtures thereof, and
• 4) dialkyl ethers of polyalkylene glycol, N-methyl pyrollidone, dimethylformamide, propylene carbonate, sulfolene, and glycol triacetate,
• to produce an overhead stream which contains at least nitrogen and helium and a hydrocarbons-rich solvent bottom stream;
• B. flashing said solvent bottom stream at least twice to produce a methane-rich gas product and a lean solvent stream, the first overhead flashed gas stream being compressed and recycled to said contacting of said step A;
• C. cooling and recycling said lean solvent stream to said contacting of said step A;
• D. feeding said overhead stream of said step A to at least one membrane unit to produce a reject nitrogen stream and a permeate helium-rich stream which contains at least 50 mol % helium;
• and
• E. compressing said permeate stream to a pressure ranging from 200 to 3,000 psia.

A process for separating C.sub.1 + hydrocarbons and nitrogen from a nitrogen-rich gas stream containing at least 3 mol % of said nitrogen and at least 0.1 mol % of helium, comprising the following steps:

• A. contacting said nitrogen-rich gas stream with a lean oil selected from the group consisting of paraffinic, naphthenic, and aromatic hydrocarbons and mixtures thereof having molecular weights ranging between about 70 and 250, at temperatures no lower than -40.degree. F. and pressures ranging between 70 and 1,320 psia, to produce a nitrogen enriched stream, in which the ratio of said helium to said nitrogen is essentially unchanged, as an overhead product and a methane-rich oil stream as a bottom product;
• B. flashing said bottom methane-rich oil stream to recover a methane-rich overhead gas product and a lean oil-rich bottom stream;
• C. recycling said lean-oil rich bottom stream to said contacting of step A;
• D. feeding said nitrogen enriched stream produced in step A to a membrane unit comprising at least one thin, nonporous membrane exhibiting good selectivity for helium over nitrogen and methane and therein separating said nitrogen enriched stream into a low-pressure helium product as a permeate stream containing at least 50 mol % helium and a high-pressure nitrogen reject stream which is not recycled to said contacting of said step A;
• and
• E. compressing said permeate stream to pressures ranging from 200 to 3,000 psia.