Method and arrangement for producing liquefied methane gas (LMG) from various gas sources

A method of continuously producing a liquefied methane gas (LMG) from a pressurized mixed methane gas feed stream, the mixed methane gas feed stream containing methane and a variable concentration of nitrogen, the method including the simultaneous steps of:

• (A) passing the mixed methane gas feed stream through a first heat exchanger and then through a second heat exchanger to condense at least a portion of the mixed methane gas feed stream, the first heat exchanger using a first cryogenic refrigerant and the second heat exchanger using a second cryogenic refrigerant;
• (B) sending the mixed methane gas feed stream coming out of the second heat exchanger through a mid-level inlet of a fractional distillation column to separate the mixed methane gas feed stream into a methane-rich liquid fraction and a nitrogen-rich gas fraction;
• (C) withdrawing the methane-rich liquid fraction accumulating at a bottom of the fractional distillation column through a bottom outlet, the methane-rich liquid fraction constituting the LMG;
• (D) passing the LMG withdrawn from the bottom outlet in step (C) through a third heat exchanger to further cool the LMG;
• (E) withdrawing the nitrogen-rich gas fraction at a top of the fractional distillation column through a top outlet;
• (F) passing the nitrogen-rich gas fraction through a fourth heat exchanger and then through a fifth heat exchanger, the fourth heat exchanger using the first cryogenic refrigerant and the fifth heat exchanger using the second cryogenic refrigerant, at least a portion of the nitrogen-rich gas fraction undergoing a phase change to a liquid phase inside the fifth heat exchanger;
• (G) introducing the nitrogen-rich gas fraction coming out of the fifth heat exchanger into a nitrogen phase separator vessel where the liquid phase is separated from a gas phase;
• (H) withdrawing the liquid phase accumulating at a bottom of the nitrogen phase separator vessel and introducing the withdrawn liquid phase by gravity into the fractional distillation column as a reflux stream through an overhead inlet located above the mid-level inlet and below the top outlet;
• (I) withdrawing the gas phase from a top of the nitrogen phase separator vessel and passing the withdrawn gas phase directly into an expansion valve;
• (J) using the expanded gas coming out of the expansion valve as the first cryogenic refrigerant, the first cryogenic refrigerant circulating in an open-loop first refrigerant circuit originating at an outlet of the expansion valve and then passing through, in succession, the fourth heat exchanger and the first heat exchanger;
• (K) venting the first cryogenic refrigerant, coming from the first heat exchanger, out of the first refrigerant circuit;
• and
• (L) circulating the second cryogenic refrigerant in a closed-loop second refrigerant circuit, the second refrigerant circuit extending from an independent cryogenic refrigeration system to the fifth heat exchanger, from the fifth heat exchanger to the third heat exchanger, from the third heat exchanger to the second heat exchanger, and then from the second heat exchanger back to the independent cryogenic refrigeration system.

An arrangement for continuously producing a liquefied methane gas (LMG) from a pressurized mixed methane gas feed stream, the mixed methane gas feed stream containing methane and a variable concentration of nitrogen, the arrangement including:

• a fractional distillation column having a top outlet, a bottom outlet, a mid-level inlet and an overhead inlet located above the mid-level inlet and below the top outlet;
• a mixed methane gas feed stream circuit for a mixed methane gas feed stream, the mixed methane gas feed stream circuit extending, in succession, between an inlet of the mixed methane gas feed stream circuit, a first heat exchanger, a second heat exchanger, and the mid-level inlet of the fractional distillation column;
• a liquid methane gas (LMG) circuit, the LMG circuit extending between the bottom outlet of the fractional distillation column, a third heat exchanger, and an outlet of the LMG circuit;
• a nitrogen phase separator vessel having a mid-level inlet, a top outlet and a bottom outlet, the bottom outlet of the nitrogen phase separator vessel being in fluid communication with and positioned vertically above the overhead inlet of the fractional distillation column;
• an expansion valve in direct fluid communication with the top outlet of the nitrogen phase separator vessel;
• an opened-loop first refrigerant circuit for a first cryogenic refrigerant, the first refrigerant circuit extending, in succession, between an outlet of the expansion valve, a fourth heat exchanger, the first heat exchanger and a venting outlet of the first refrigerant circuit;
• a closed-loop second refrigerant circuit for a second cryogenic refrigerant, the second refrigerant circuit being in fluid communication with an inlet and an outlet of an independent cryogenic refrigeration system, the second refrigerant circuit extending, in succession, between the outlet of the independent cryogenic refrigeration system, a fifth heat exchanger, the third heat exchanger, the second heat exchanger and the inlet of the independent cryogenic refrigeration system;
• and
• a nitrogen-rich gas fraction circuit extending, in succession, between the top outlet of the fractional distillation column, the fourth heat exchanger, the fifth heat exchanger and the mid-level inlet of the nitrogen phase separator vessel.