Patent ID: 7237407

Claim:
Process for simultaneous production of a liquefiable natural gas and a natural gas liquids (NGL) cut from a feed natural gas containing nitrogen, methane, C 2 to C 5 hydrocarbons and C 6 + heavy hydrocarbons; the method comprising the following steps: (a) pretreating said feed natural gas in order to obtain a pretreated natural gas; (b) cooling the pretreated natural gas resulting from step (a) to a temperature close to its dew point; (c) expanding the cooled pretreated natural gas resulting from step (b) and introducing the expanded natural gas into an NGL recovery unit comprising at least one main distillation column and therein producing, on the one hand, as column top product, a purified natural gas and, on the other hand, said NGL cut; and (d) forming the liquefiable natural gas from the purified natural gas resulting from step (c): wherein step (a) comprises the following substeps: (a1) cooling the feed natural gas to a temperature close to its dew point; (a2) introducing said cooled feed natural gas resulting from step (a1) into an auxiliary distillation column equipped with an overhead condenser that produces a reflux, operating the auxiliary distillation column at a pressure between 45 and 65 bar and producing, as top product, said pretreated natural gas, wherein the pretreated natural gas no longer contains practically any C 6 + hydrocarbons, the auxiliary distillation column furthermore producing a cut of essentially C 6 + heavy hydrocarbons; wherein step (c) comprises the following substeps: (c1) introducing the cooled pretreated natural gas resulting from step (b) into a separation drum in order to obtain a liquid stream and a gas stream; (c2) expanding the gas stream resulting from step (c1) in a turbine coupled to a first compressor; (c3) introducing the stream resulting from step (c2) into the main column at an intermediate level; and (c4) expanding the liquid stream resulting from step (c1) and introducing this expanded liquid stream into the main column at a lower level below the intermediate level; and the liquefiable natural gas furthermore includes a portion of the pretreated natural gas coming directly from step (a), and wherein said process further comprises a start-up phase in which the liquefiable natural gas consists mostly or completely of the pretreated natural gas coming directly from step (a), said liquefiable natural gas being relatively enriched with C 2 to C 5 hydrocarbons, and a subsequent production phase of adjusting the portion of pretreated natural gas coming directly from step (a) in the liquefiable natural gas and adjusting it according to the desired C 2 to C 5 hydrocarbon content in the liquefiable natural gas.