that is another can of worms with a cryogenic system, no??????....these gases can't just be pumped down into a liquid, (and thus through a phase change!!) without some serious cooling considerations........
if you liquify propane, butane and cetane with the compressor, there is no need to remove the nitrogen
in this system, which is said to be more cost effective than a cryogenic one, most of the nitrogen is concentrated via membrane, then is used in the fuel mix......
Gas containing 8% to 15% nitrogen is compressed and passed across a first set of membrane modules. The permeate, which contains 4% nitrogen, is sent to the pipeline; the nitrogen-rich residue gas is passed to a second set of membrane modules. These modules produce a residue gas containing 50% nitrogen and a nitrogen-depleted permeate containing 10% to 20% nitrogen. The residue gas is used as fuel; the permeate is mixed with the incoming feed gas for further recovery.