Method for separating carbon dioxide from methane

Claim 1

A method for the separation of carbon dioxide from methane in a dried gas mixture wherein the carbon dioxide is from about 30 mol percent to about 90 mol percent of the gas mixture which comprises: (a) cooling the gas mixture to form a vapor-liquid binary system by dividing the mixture into three streams for partial cooling and recombining the streams for further cooling by heat exchange with the product streams of a fractionation step wherein a first stream is partially cooled by indirect heat exchange with a liquid side stream from the fractionation step; a second stream is partially cooled by indirect heat exchange with the overhead product stream from the fractionation step; and a third stream is partially cooled by indirect heat exchange with the bottoms product stream of the fractionation step prior to recombining the streams for further cooling; (b) fractionating the binary system to remove an overhead stream comprising predominately methane in the gaseous phase and a bottoms streams consisting essentially of carbon dioxide in the liquid phase; (c) warming the overhead and bottoms streams from the fractionating step through indirect heat exchange with inlet gas to cool the inlet gas (step a); provided that the entire method is performed, without formation of solids, at a pressure below the critical pressure and a temperature above the triple point for the carbon dioxide-methane mixture.

Claim 2

A method for the separation of carbon dioxide from methane in a dried gas mixture wherein the carbon dioxide is from about 30 mol percent to about 90 mol percent of the gas mixture which comprises: (a) cooling the gas mixture to form a vapor-liquid binary system by dividing the mixture into three streams for partial cooling and recombining the streams for further cooling by heat exchange with the product streams of a fractionation step wherein a first stream is partially cooled by indirect heat exchange with a liquid side stream from the fractionation step; a second stream is partially cooled by indirect heat exchange with the overhead product stream from the fractionation step; and a third stream is partially cooled by indirect heat exchange with the bottoms product stream of the fractionation step prior to recombining the streams for further cooling; (b) fractionating the binary system to remove an overhead stream comprising predominately methane in the gaseous phase and a bottoms streams consisting essentially of carbon dioxide in the liquid phase; (c) warming the overhead and bottoms streams from the fractionating step through indirect heat exchange with inlet gas to cool the inlet gas (step a); provided that the entire method is performed, without formation of solids, at a pressure below the critical pressure and a temperature above the triple point for the carbon dioxide-methane mixture.

Claim 3

A method for the separation of carbon dioxide from methane in a gas mixture, wherein the carbon dioxide is present in amounts of from about 30 mol percent to about 90 mol percent in the gas mixture, which comprises: (a) drying the gas mixture; (b) separating the gas mixture into three streams for precooling; (c) cooling the three streams by: (i) indirect heat exchange of a first precooling stream with a side stream of liquid from a fractionation step (f); (ii) indirect heat exchange of a second precooling stream with a bottoms product stream from the fractionation step (f); and (iii) indirect heat exchange of a third precooling stream with an overhead product stream from the fractionation step (f); (d) combining the precooled streams; (e) cooling the combined streams by indirect heat exchange with the product streams, such cooling to form a vapor-liquid binary system; (f) fractionating the binary system in a fractionation vessel, supplying heat to said vessel by indirect heat exchange with the first precooling stream of the gas mixture; (g) recovering as an overhead product a gas stream being predominantly methane, a portion of which overhead stream is condensed and returned to the fractionation vessel as a reflux stream; (h) recovering as a bottoms product a liquid stream consisting essentially of carbon dioxide and a small amount of methane; and (i) heating the product streams by indirect heat exchange with the gas mixture to take advantage of the energy present to reduce the temperature of the gas mixture; provided, that the above steps are performed without formation of solids and such that the gas mixture and vapor-liquid binary system remains at a pressure below the critical pressure and at a temperature above the triple point temperature of the gas mixture.

Claim 4

A method for the separation of carbon dioxide from methane in a gas mixture, wherein the carbon dioxide is present in amounts of from about 30 mol percent to about 90 mol percent in the gas mixture, which comprises: (a) drying the gas mixture; (b) separating the gas mixture into three streams for precooling; (c) cooling the three streams by: (i) indirect heat exchange of a first precooling stream with a side stream of liquid from a fractionation step (f); (ii) indirect heat exchange of a second precooling stream with a bottoms product stream from the fractionation step (f); and (iii) indirect heat exchange of a third precooling stream with an overhead product stream from the fractionation step (f); (d) combining the precooled streams; (e) cooling the combined streams by indirect heat exchange with the product streams, such cooling to form a vapor-liquid binary system; (f) fractionating the binary system in a fractionation vessel, supplying heat to said vessel by indirect heat exchange with the first precooling stream of the gas mixture; (g) recovering as an overhead product a gas stream being predominantly methane, a portion of which overhead stream is condensed and returned to the fractionation vessel as a reflux stream; (h) recovering as a bottoms product a liquid stream consisting essentially of carbon dioxide and a small amount of methane; and (i) heating the product streams by indirect heat exchange with the gas mixture to take advantage of the energy present to reduce the temperature of the gas mixture; provided, that the above steps are performed without formation of solids and such that the gas mixture and vapor-liquid binary system remains at a pressure below the critical pressure and at a temperature above the triple point temperature of the gas mixture.