Patent EA-000800-B1 (EA)
Title: Method for removal aromatic and/or higher-molecular hydrocarbons from a methane-based gas stream by condensation and stripping and associated apparatus therefor
Inventor: YAO JAME
IPC: F25J3/08
Filed: 1997-03-19
Abstract: 1. A process for removing and concentrating the higher molecular weight hydrocarbon species from a methane-based gas stream comprising the steps of: (a) condensing a minor portion of the methane-based gas stream thereby producing a two-phase stream; (b) feeding said two-phase stream into the upper section of a column; (c) removing from the upper section of said column a heavies-depleted gas stream; (d) removing from the lower section of said column a heavies-rich liquid stream; (e) contacting via indirect heat exchange the heavies-rich liquid stream with a methane-rich stripping gas stream thereby producing a warmed heavies rich stream and a cooled methane-rich stripping gas stream; (f) feeding said cooled methane-rich stripping gas stream to the lower section of the column; and (g) contacting the two-phase stream and the cooled methane-rich stripping gas stream in said column thereby producing the heavies-depleted gas stream and the heavies-rich liquid stream. 2. A process for removing benzene and other aromatics from a methane-based gas stream comprising the steps of: (a) condensing a minor portion of the methane-based gas stream thereby producing a two-phase stream; (b) feeding said two-phase stream into the upper section of a column; (c) removing from the upper section of said column a benzene/aromatic-depleted gas stream;  (d) removing from the lower section of said column a benzene/aromatic-rich liquid stream; (e) contacting via indirect heat exchange the benzene/aromatic-rich liquid stream with a niethane-rich stripping gas stream thereby producing a warmed benzene/aromatic-rich stream and a cooled methane-rich stripping gas stream; (f) feeding said cooled methane-rich stripping gas stream to the lower section of the column; and (g) contacting the two-phase stream and the cooled methane-rich stripping gas stream in said column thereby producing the benzene/aromatic-depleted gas stream and the benzene/aromatic-rich liquid stream. 3. A process according to claim 1 additionally comprising: (h) feeding the warmed benzene/aromatic-rich stream of step (e) to a demethanizer comprised of a fractionator column, a reboiler and a condenser thereby producing a benzene/aromatic-rich liquid stream and a methane-rich vapor stream. 4. A process according to claim 3 wherein a major portion of the cooling duty for the condenser is provided by the higher molecular hydrocarbons or the benzene/aromatic-rich liquid stream produced by step (d) or step (e). 5. A process according to claim 3 wherein a major portion of the cooling duty for the condenser is provided by flowing through an indirect heat exchange means in contact with the higher molecular hydrocarbons the or the benzene/aromatic-rich liquid stream of step (d) and the resulting treated the higher molecular hydrocarbons the or the benzene/aromatic-rich liquid stream becomes the higher molecular hydrocarbons the or the benzene/aromatic-bearing feed stream to step (e). 6. A process according to claim 3 wherein the cooling duty is provided by splitting the overhead vapor stream into a first vapor stream and a second vapor stream, cooling and partially condensing said first stream via indirect heat exchange with the higher molecular hydrocarbons the or the benzene/aromatic rich liquid stream of step (d) thereby producing a cooled, partially condensed first stream, combining said first stream and said second stream, feeding said combined stream to a gas-liquid separator from which is produced the reflux stream to the fractionating column and the methane rich vapor stream. 7. A process according to claim 6 wherein the flow rate of the reflux stream is controlled by calculating for the overhead vapor stream a two-phase stream temperature corresponding to the desired liquids content at equilibrium conditions, measuring the temperature of the two-phase stream, maintaining constant the flow rate of the first stream and the amount of cooling imparted to said stream, and adjusting the flow rate of said second stream responsive to the two-phase stream temperature such that the calculated two-phase stream temperature is approached. 8. A process according to claim 4 additionally comprising between steps (d) and (e) the additional step of: (i) flashing the higher molecular hydrocarbons or the benzene/aromatic-rich liquid stream to a lower pressure  thereby further decreasing the temperature of said stream. 9. A process according to claim 8 additionally comprising the step of: (j) condensing the higher molecular hydrocarbons or the benzene/aromatic depleted gas stream thereby producing a liquefied natural gas stream. 10. A process according to claim 9 wherein said condensing is comprised of flowing the higher molecular hydrocarbons or the benzene/aromatic depleted gas stream through an indirect heat exchange means cooled by said second refrigerant stream. 11. A process according to claim 1 or 2 wherein step (a) is comprised of splitting the methane-based gas stream into a first stream and a second stream, cooling said first stream thereby producing a partially condensed first stream, and combining said partially condensed first stream with the second stream thereby producing said two-phase stream. 12. A process according to claim 11 wherein the amount of liquids in said two phase stream is controlled by determining for the methane-based gas stream a two-phase stream temperature corresponding to the desired liquids content at equilibrium conditions, measuring the temperature of the two-phase stream, maintaining constant the flow rate of the first stream and the amount of cooling imparted to said stream, and adjusting the flow rate of said second stream responsive to the two-phase stream temperature such that the two-phase stream temperature approximates the calculated two-phase stream temperature. 13. A process according to any one of the preceding claims additionally comprising the step of (h) sequentially cooling the methane-based gas stream prior to step (a) by flowing said stream through at least one indirect heat exchange means in contact with a first refrigerant stream thereby producing a cooled methane-based gas stream and flowing the cooled methane-based gas stream through at least one indirect heat exchange means in contact with a second refrigerant stream wherein the boiling point of the second refrigerant stream is less than the boiling point of the first refrigerating tream thereby producing the feed stream to step (a). 14. A process according to any one of the preceding claims wherein said first refrigerant stream is comprised in major portion of propane and said second refrigerant stream is comprised in major portion of ethane, ethylene or a mixture thereof. 15. A process according to claim 4 further comprising: i) withdrawing a side stream from the methane-based gas stream at a location downstream of one of the indirect heat exchange means and employing said side stream as the methane-rich stripping gas in step (e). 16. A process according to any one of the preceding claims wherein (i) said cooling by at least one indirect heat exchange means in contact with a first refrigerant stream is comprised of flowing said gas stream to be cooled through two or more indirect heat exchange means in a sequential manner (ii) and wherein the first refrigerant to each such indirect heat exchange means has been flashed to a progressively lower temperature and pressure in a sequentially consistent manner (iii) and wherein said cooling by at least one indirect heat exchange means in contact with a second refrigerant stream is comprised of flowing said gas stream to be cooled through two or more indirect heat exchange means in a sequential manner and  (iv) wherein the second refrigerant to each indirect heat exchange means has been flashed to a progressively lower temperature and pressure in a sequentially consistent manner. 17. A process according to claim 16 wherein three indirect heat exchange means are employed for cooling by the first refrigerant stream and two or three indirect heat exchange means are employed for cooling by the second refrigerant stream. 18. A process according to any one of the preceding claims wherein the absolute pressure of the methane-based feed gas is 500 to 900 psia (35,15-63,27 kg/cm<2>), preferably about about 575 to about 650 psia (40,42-45,70 kg/cm<2>). 19. A process according to claim 4 additionally comprising the steps of: (k) flashing in one or more steps the liquefied product of step (j) to approximately atmospheric pressure thereby producing an LNG product stream and one or more methane vapor streams; (l) compressing a majority of the vapor streams of step (k) to a pressure of 500 to 900 psia (35,15-63,27 kg/cm<2>); (m) cooling said compressed vapor stream of step (1); and (n) combining the resulting cooled stream with the methane-based gas stream fed to step (a) or the resulting product from one of the indirect heat exchange means of step (h). 20. A process according to claim 19 wherein the methane-rich vapor stream of step (h) is combined with one of the vapor streams of step (k) prior to step (I) and/or in which the absolute pressure of the methane-based feed gas and the gas stream from step (l) is about 575 to about 650 psia (40,42-45,70 kg/cm<2>). 21. A process according to claim 1 wherein the column provides two to fifteen theoretical stages of gas-liquid contacting, preferably three to ten theoretical stages of gas-liquid contacting. 22. A apparatus comprising: (a) a condenser; (b) a column; (c) a heat exchanger providing for indirect heat exchange between two fluids; (d) a conduit between said condenser and the upper section of the column for flow of a two-phase stream to the column; (e) a second conduit connected to the upper section of the column for the removal of a vapor stream from the column; (f) a conduit between said column and heat exchanger for flow of a cooled gas stream from the heat exchanger; (g) a conduit between said column a