Method for ethane recovery, using a refrigeration cycle with a mixture of at least two coolants, gases obtained by said method, and installation therefor

Abstract

The invention concerns a method and an installation for refrigerating gas mixtures, for cryogenic separation of a pressurised gas (1) constituents. The method comprises a refrigerating cycle wherein a fluid (13) is separated in a separator tank (B2) into a less volatile fraction (4), for producing refrigeration at a relatively high first temperature in an exchanger (E1), and into a more volatile second fraction (5) for producing refrigeration at a relatively low temperature in an exchanger (E2). The heated and expanded fractions (4, 5) are brought together then compressed in a compressor (K1). A fraction (26) derived from said compressor (K1) is cooled to supply the fraction (13).

Description

[0001] The present invention relates in general, and according to a first of its aspects, to the gas industry, and in particular to a method of recovering the ethane contained in a pressurized gas comprising methane and C2 and higher hydrocarbons, operating a multicomponent refrigeration cycle.

[0002] The term “multicomponent refrigeration cycle” should be understood to mean a refrigeration cycle using a refrigerant mixture composed of at least two refrigerants.

[0003] More precisely, the invention relates, according to its first aspect, to a process for recovering the ethane contained in a pressurized gas containing methane and C2 and higher hydrocarbons, operating a refrigerating cycle in which a relatively less-volatile first refrigerant is compressed, cooled and expanded in order thereafter to be used for cooling said pressurized gas to be separated or first separation products to a relatively high first temperature, and in which a relatively more-volatile second refrigerant is compressed, cooled and expanded in order thereafter to be used for cooling at least second separation products obtained from said pressurized gas to a relatively low second temperature.

[0004] Refrigeration processes of this type are well known to those skilled in the art and have been used for many years.

[0005] These refrigeration processes have drawbacks as regards operating costs, because of energy consumption associated with the low thermodynamic efficiency of these refrigeration cycles.

[0006] Such known processes also have drawbacks as regards operating costs generated by maintenance difficulties and by the frequency of intervention work, for example on compression plants, pumps or else on measurement and control equipment.

[0007] These drawbacks themselves entail excessive costs that extend the period of amortization of the financial investment in such plants because of production stoppages.

[0008] In this context, it is a first object of the present invention to provide a process, which moreover is in accordance with the generic definition given in the above preamble, which is essentially characterized in that the first and second refrigerants are used as a mixture when they are compressed and cooled, in that this mixture then undergoes a separation into a first fraction essentially containing the relatively less-volatile first refrigerant and into a second fraction essentially containing the relatively more-volatile second refrigerant, in that the first refrigerant is used in the form of the first fraction, for cooling to the relatively high first temperature, and in that the second refrigerant is used in the form of the second fraction, for cooling to the relatively low second temperature.

[0009] This process allows the operating costs of the plant, especially as regards energy consumption, to be limited by a higher thermodynamic efficiency, and also, as regards maintenance, by reducing the number of items of equipment in the plant by combining two refrigeration circuits into a single one. Thus, the maintenance operations are simplified, the time required to determine the causes of a failure of the plant is shortened and, consequently, any production stoppage will be shorter than when plants using a process according to the prior art are used.

[0010] According to a first aspect of the process of the invention, the first fraction may be cooled in a first exchanger, expanded to give a first expanded fraction, then warmed in the first exchanger, before being introduced into a low-pressure stage of a compressor.

[0011] According to the first aspect of the process of the invention, the second fraction may be cooled in the first exchanger and then in a second exchanger, expanded, then warmed in the second exchanger and mixed with the expanded first fraction.

[0012] According to the first aspect of the process of the invention, a third fraction may be tapped off the first fraction after it has been cooled in the first heat exchanger and the third fraction may be expanded and warmed in the first exchanger in order to deliver an expanded and warmed fourth fraction, which may be introduced into a medium-pressure stage of the compressor.

[0013] According to the first aspect of the process of the invention, a gaseous fifth fraction may be tapped off from the refrigerants undergoing compression in the compressor (K1) at a medium pressure, slightly above that of the expanded and warmed fourth fraction, and then cooled and expanded to the same pressure as said fourth fraction and then mixed with the latter.

[0014] According to a second aspect of the process of the invention, the first and second refrigerants can be used as a mixture with a third refrigerant.

[0015] According to the second aspect of the process of the invention, the refrigerants may be methane, ethylene, and propane.

[0016] According to a third of its aspects, the invention relates to a methane-enriched gas and to an ethane-enriched product that are obtained by the present process as well as to a product enriched in C2 and higher hydrocarbons, obtained by the present process.

[0017] According to a fourth of its aspects, the invention relates to a plant for recovering the ethane contained in a pressurized gas containing methane and C2 and higher hydrocarbons, operating in particular a multicomponent refrigeration cycle, this plant using a refrigeration cycle and comprising means for compressing, cooling and expanding a relatively less-volatile first refrigerant, means for cooling, by means of the first refrigerant, said pressurized gas to be separated or first separation products to a relatively high first temperature, and means for compressing, cooling and expanding a relatively more-volatile second refrigerant, means for cooling, by means of the second refrigerant, at least second separation products obtained from said pressurized gas to a relatively low second temperature, characterized in that the first and second refrigerants are used as a mixture when they are compressed and cooled and in that this plant comprises means for this mixture to undergo a separation into a first fraction essentially containing the relatively less-volatile first refrigerant and into a second fraction essentially containing the relatively more-volatile second refrigerant, the first refrigerant being used in the form of the first fraction for cooling to the relatively high first temperature and the second refrigerant being used in the form of the second fraction for cooling to the relatively low second temperature.

[0018] The invention will be more clearly understood and further objects, features, details and advantages thereof will become more clearly apparent in the course of the description that follows, with reference to the appended schematic drawings given solely by way of example but implying no limitation, and in which:

[0019] FIG. 1 shows a functional block diagram of a plant according to one embodiment of the prior art;

[0020] FIG. 2 shows a functional block diagram of a plant according to a preferred embodiment of the invention.

[0021] The following symbols may in particular be read from both these figures: “FC” stands for “flow controller”, “GT” stands for “gas turbine”, “LC” stands for “liquid level controller”, “PC” stands for “pressure controller”, “SC”, stands for “speed controller” and “TC” stands for “temperature controller”.

[0022] For the sake of clarity and conciseness, the lines used in the plants shown in FIGS. 1 and 2 will be indicated by the same reference numbers as the gaseous fractions that flow therein.

[0023] Referring to FIG. 1, the plant shown is intended for treating a dry charge gas, in particular for isolating therefrom, on the one hand, a fraction composed mainly of methane essentially free of C2 and higher hydrocarbons and, on the other hand, a fraction composed mainly of ethane and other C2 and higher hydrocarbons essentially free of methane.

[0024] This plant has three independent circuits. A first circuit corresponds to the path followed by a gas to be purified, a second circuit corresponds to the cooling cycle of a refrigeration unit, the refrigerant of which is ethylene, and a third circuit corresponds to the cooling cycle of a refrigeration unit whose refrigerant is propane.

[0025] More precisely, in the first circuit, a charge gas 1, available at 15° C. and 18 bar, with a flow rate of 3903 kmol/h, is cooled in an exchanger E1 in order to deliver a cooled gas 302 at −17.52° C. and 17.8 bar. The latter is further cooled in a second exchanger E2, in order to deliver a partially condensed, cooled fluid 303 at −30.00° C. and 17.6 bar. The stream 1 is composed of 0.1% carbon dioxide, 24.3% methane, 74.4% ethane and 1.2% propane.

[0026] The fluid 303 is then introduced into a tank V1 where it undergoes separation of its liquid and gaseous constituents:

[0027] the gas phase, stream 304, available with a flow rate of 2219 kmol/h, is cooled to −60° C. and partially condensed in an exchanger E3 in order to deliver a fluid 305 at 17.4 bar. This fluid 305 feeds a distillation column T1 in its upper part;

[0028] the liquid phase, stream 306, available with a flow rate of 1684 kmol/h, is pumped by a pump P1, flows in a line that includes a controlled valve 321, the opening of which depends on a liquid level controller present in the tank V1, in order to deliver a stream 307 at −29.8° C. and 19.6 bar. The latter stream is then introduced into a middle part of the distillation column T1.

[0029] Produced at the top of the column T1 is a vapor 308 at −65.79° C. and 17.2 bar, available with a flow rate of 1358 kmol/h, which is cooled in an exchanger E4 in order to deliver a partially condensed fluid 309 at −90° C. and 17.0 bar. This fluid is then separated in a tank V2 into a gas fraction 310 in an amount of 971 kmol/h, composed of 0.1% carbon dioxide, 94.9% methane and 5.0% ethane, and into a liquid fraction 311 in an amount of 387 kmol/h, composed of 0.4% carbon dioxide, 47.6% methane and 52.0% ethane, which is pumped by a pump V2 along a line 312. This line 312 includes a controlled-opening valve 322, the opening of which depends on the flow rate in this same line.

[0030] The liquid fraction transported in the line 312 is then introduced into the last stage of the column T1.

[0031] The gas fraction 310 coming from the tank V2, at a temperature of −90.0° C., flows through a heat exchanger E6 in order to deliver a warmed fraction 313 at −35.0° C., this fraction 313 then flows through a heat exchanger E7 in order to deliver a warmed fraction 326, before passing into a controlled-opening valve 317, the opening of which depends on the pressure in the line 326. On leaving the valve 317, the product is collected in a delivery line 320 at 20.0° C. and leaves the plant.

[0032] In its lower part, the distillation column T1 has several trays that are connected in pairs via warming circuits, two of which have been shown. These are the circuits 315, 316 and 318, 319. Each of these warming circuits constitutes a lateral reboiler in the case of the circuit 315, 316 and a column bottom reboiler in the case of the circuit 318, 319.

[0033] The fluid flowing in the line 315, with a flow rate of 3000 kmol/h and at a temperature of −20.26° C., is warmed in the heat exchanger E1 by heat exchange with the charge gas 1 in order to deliver a warmed fluid 316 at −16.61° C. which is then introduced onto a tray below the tray where the withdrawal of the fluid 315 takes place. The temperature of the fluid flowing in this circuit 315, 316 is regulated by means of a controlled-opening valve 323 placed in a branch line of the circuit 315, 316 that does not pass through the exchanger E1. The opening of this valve 323 is controlled by a temperature controller connected to the line 302.

[0034] Similarly, the fluid flowing in the line 318 with a flow rate of 3341 kmol/h. and at a temperature of −16.15° C., which is located at a stage below the stage where the warmed fluid 316 is introduced, is warmed in a heat exchanger E5 by heat exchange with a refrigerant consisting of propane, in order to deliver a warmed fluid 319 at −14.87° C. This fluid is introduced onto a tray below the tray from which the fluid 318 is withdrawn. The temperature of the fluid flowing in this circuit 318, 319 is regulated by means of a controlled-opening valve 324, placed in a branch line for the refrigerant transported in lines 220, 221, which branch line does not pass through the exchanger E5. The opening of this valve 324 is controlled by a temperature controller connected to the line 319.

[0035] Finally, the residual liquid obtained at the bottom of a column T1, which is enriched in C2 and higher hydrocarbons, is withdrawn at a temperature of −14.87° C. and a pressure of 17.4 bar, in an amount of 2932 kmol/h via a line 314. The latter has a valve 325, the opening of which is controlled by a liquid level controller for the liquid at the bottom of column T1.

[0036] In the second circuit, which corresponds to the cooling cycle of a refrigeration unit whose refrigerant is ethylene, a stream of liquid ethylene 100 with a flow rate 2570 kmol/h, a temperature of −30° C. and a pressure of 19.58 bar is withdrawn from a storage tank V5. This stream 100 is divided into:

[0037] (a) a first stream 117 with a flow rate of 1993 kmol/h, which is expanded to 6.79 bar and cooled to −63° C. by passing it through a valve 120 in order to deliver a stream 101 that is mixed with a stream 104 in order to give a stream 102 that feeds the exchanger E3 with refrigerating ethylene. The opening of the valve 120 is controlled by a liquid level controller in the exchanger E3;

[0038] (b) a second stream 114 with a flow rate of 577 kmol/h which is expanded to 18.58 bar and cooled to −80° C. in the exchanger E6 in order to give a warmed stream 115.

[0039] The 577 kmol/h of the stream 115 are divided into:

[0040] (a) 417 kmol/h constituting a first stream 116 which is expanded to 1.83 bar and cooled to −93° C. by passing through a valve 121 in order to deliver a stream 106 that feeds the exchanger E4 with refrigerating ethylene. The opening of the valve 121 is controlled by a liquid level controller contained in the exchanger E4. In addition, this liquid level controller is servocontrolled by another liquid level controller contained in the separating tank V2;

[0041] (b) 160 kmol/h constituting a second stream, that flows in a line 105 provided with a valve 122, the opening of which depends on the flow rate in the line 105, in order to give a stream 104 at −79.62° C. and 6.79 bar. This stream 104 is mixed with the stream 101 in order to give a stream 102, prior to it being introduced into the exchanger E3.

[0042] Vaporizing the ethylene contained in the exchanger E4 allows the stream 8 coming from the top of the column T1 to be cooled. The ethylene vapor thus obtained—stream 107 at −93° C. and 1.83 bar—is sent into the low-pressure stage of the compressor K1, passing via the suction tank V3.

[0043] Vaporizing the ethylene contained in the exchanger E3 allows the stream 4 coming from the tank V1 to be cooled. The ethylene vapor thus obtained—stream 103 at −62.83° C. and 6.79 bar—is sent into the medium-pressure stage of the compressor K1, passing via the suction tank V4.

[0044] The compressed ethylene obtained at the outlet of K1 delivers a fluid 112 at 17.75° C. and 20.6 bar with a flow rate of 2570 kmol/h, which is cooled and condensed by successively passing through the exchanger E8, to give a fraction 118 at −7° C. and 20.1 bar, and then the exchanger E9, to give a fraction 119 at −30° C. and 19.6 bar, before feeding the tank V5 with liquid ethylene.

[0045] In the third circuit, corresponding to the cooling cycle of a refrigeration unit whose refrigerant is propane, a pressurized stream 220 of liquid propane with a flow rate of 4340 kmol/h is withdrawn from a storage tank V6 at 42° C. and 18 bar. This stream 220 is cooled in the exchanger E5 by heat exchange with the liquid flowing in the lines 18, 19 in order to deliver a cooled fluid 221 at 33.64° C. and 17.5 bar. In parallel with the cooling circuit, passing through the exchanger E5, a line having a valve 24 allows the energy exchanges within E5 to be regulated.

[0046] The 4340 kmol/h of the cooled fluid 221 are then separated into two streams:

[0047] a first stream 200 of 4030 kmol/h, which is expanded by passing through a valve 226 in order to deliver a stream 201 at 3.46 bar and −10° C. The opening of the valve 226 is controlled by a liquid level controller contained in the exchanger E8. The stream 201 feeds the exchanger E8 with refrigerating propane;

[0048] a second stream 222 of 310 kmol/h, which is cooled in the exchanger E7 in order to give the stream 223 at −25° C.

[0049] The stream 223 is expanded by passing through a valve 229, the opening of which is controlled by the flow rate in the line, to give an expanded stream 224 at 1.48 bar.

[0050] The stream of propane 201 introduced into the exchanger E8 is partially vaporized in order to give a vapor phase 203 in an amount of 1387 kmol/h and a liquid phase 204 in an amount 2643 kmol/h. This stream 204 is divided into two streams:

[0051] 1700 kmol/h constituting a stream 205, which is expanded by passing through a valve 227, the opening of which depends on the liquid level contained in the exchanger E9, in order to deliver a stream 206 at 1.48 bar and −33° C. that feeds the exchanger E9 with refrigerating propane;

[0052] 943 kmol/h constituting a stream 208 which is expanded by passing through a valve 228, the opening of which depends on the liquid level contained in the exchanger E2, in order to deliver a stream 225 at 1.48 bar and −33° C., which feeds the exchanger E2 with refrigerating propane.

[0053] The streams 225 and 224 are merged, prior to their being introduced into the exchanger E2, to give a stream 209.

[0054] Vaporizing the propane in the exchanger E2 allows the stream 2 to be cooled and partially condensed. The propane vapor thus obtained—stream 210 at −33° C. and 1.48 bar—is mixed with a gas stream 207 coming from the exchanger E9 in order to give a stream 211 which is sent firstly to a suction tank V7 and then sent to the low-pressure stage of a compressor K2.

[0055] Vaporizing the propane in the exchanger E9 allows the stream 118 to be cooled and partially condensed. The propane vapor thus obtained—stream 207 at −33° C. and 1.48 bar—is mixed with the gas stream 210 coming from the exchanger E9 in order to give the stream 211, which is sent firstly to the suction tank V7 and then sent to the low-pressure stage of the compressor K2.

[0056] Vaporizing the propane in the exchanger E8 allows the stream 112 to be cooled and partially condensed. The propane vapor thus obtained—stream 203 at −10° C. and 3.46 bar—is firstly sent to the suction-tank V8 and then sent to the medium-pressure stage of the compressor K2.

[0057] The compressor K2 delivers a stream 217 of hot compressed propane gas at 78.02° C. and 18.6 bar, with a flow rate of 4340 kmol/h. This stream 217 is cooled in a first exchanger E10, to deliver a cooled stream 218 at 52.36° C. and 18.3 bar, and then in a second exchanger E11, to deliver a liquid stream 219 at 42° C. and 18.0 bar. The latter liquid is then stored in the tank V6.

[0058] Referring now to FIG. 2, the plant shown is intended for treating a dry charge gas, in particular for isolating therefrom, on the one hand, a fraction composed mainly of methane essentially free of C2 and higher hydrocarbons and, on the other hand, a fraction composed mainly of ethane and other C2 and higher hydrocarbons essentially free of methane.

[0059] This plant has two independent circuits. A first circuit corresponds to the path followed by a gas to be purified and a second circuit corresponds to the cooling cycle of a refrigeration unit, the refrigerant of which is a mixture of at least three different products that may in particular be propane, ethylene and methane.

[0060] More precisely, in the first circuit, a charge gas 1, available at 15° C. and 18 bar, with a flow rate of 3903 kmol/h is cooled to −60° C. and 17.7 bar in an exchanger E1, which here is a plate exchanger, in order to deliver a cooled gas 303. The latter feeds a distillation column T1 in its upper part. The stream 1 is composed of 0.1% carbon dioxide, 24.3% methane, 74.4% ethane and 1.2% propane.

[0061] In the same way as in the process described in FIG. 1, a vapor 308 is produced at the top of column T1, at −66.21° C. and 17.0 bar and a flow rate of 1342 kmol/h, which is cooled in an exchanger E2 in order to deliver a partially condensed fluid 309. The streams 308 and 309 are composed of 0.16% carbon dioxide, 81.8% methane and 18.0% ethane. The stream 309 is then separated in a tank V2 into a gas fraction 310 and into a liquid fraction 311. This liquid fraction 311 is transported under gravity in a line that includes a controlled-opening valve 322, the opening of which depends on the liquid level in the tank V1.

[0062] The liquid fraction 311 is then introduced into the final stage of the column T1.

[0063] The gas fraction 310 coming from the tank V2 is composed of 0.1% carbon dioxide, 94.9% methane and 5.0% ethane. This fraction enters a heat exchanger E2 at −90° C. in order to deliver a warmed fraction 326 at −70° C., then passes in succession through the exchanger E1 and through a controlled valve 317, the opening of which depends on the pressure in the line 326. On leaving the valve 317, the product is collected in a delivery line 320 at 39° C. and leaves the plant.

[0064] In its lower part, the distillation column T1 has several trays that are connected in pairs via warming circuits, two of which are shown. These are the circuits 315, 316 and 318, 319. Each of these warming circuits constitutes a lateral boiler in the case of the circuit 315, 316 and a column bottom reboiler in the case of the circuit 318, 319.

[0065] The fluid flowing in the line 315 with a flow rate of 1000 kmol/h and a temperature of −40.7° C. is warmed in the heat exchanger E1 in order to deliver a warmed fluid 316 at −19.14° C. This is then introduced onto a tray below the tray from which the fluid 315 is withdrawn. The temperature of the fluid flowing in this circuit 315, 316 is regulated by means of a controlled-opening valve 323 placed in a branch line of the circuit 15, 16, which branch line does not pass through the exchanger E1. The opening of this valve 323 is controlled by a temperature controller connected to the line 316 downstream of the point where the fluids flowing in the line 316 mixes with the fluid flowing in the branch line with the valve 323.

[0066] Similarly, the fluid flowing in the line 318 with a flow rate of 3790 kmol/h and a temperature of −17.36° C. is warmed in the heat exchanger E1 in order to deliver a warmed fluid 319 at −14.94° C. This is then introduced onto a tray below the tray from which the fluid 318 is withdrawn. The temperature of the fluid flowing in this circuit 318, 319 is regulated by means of a controlled-opening valve 324 placed in a branch line of the circuit 315, 316, which branch line does not pass through the exchanger E1. The opening of this valve 324 is controlled by a temperature controller connected to the line 316 downstream of the point where the fluid flowing in the line 319 mixes with the fluid flowing in the branch line with the valve 324.

[0067] Finally, the residual liquid obtained at the bottom of the column T1, which is enriched with C2 and higher hydrocarbons, is withdrawn via a line 314 that has a valve 325 whose opening is controlled by a liquid level controller for the liquid in the bottom of the column T1. This liquid, available at −14.94° C. and 17.4 bar, is composed of 0.1% carbon dioxide, 1% methane, 97.4% ethane and 1.5% propane.

[0068] In the second circuit, corresponding to the cooling cycle of a refrigeration unit whose refrigerant is a mixture of at least three products, a refrigerating mixture 13 composed of 5% methane 12, 25% ethylene 3, and 70% propane 2, at a temperature of 42° C. and at a pressure of 27.79 bar, and the flow rate of which is 3970 kmol/h, is separated in a tank V2 into a first fraction 4 essentially containing the less-volatile first refrigerant 2 and into a second fraction 5 essentially containing the more-volatile second refrigerant 3 and the more-volatile third refrigerant 12.

[0069] The stream 5, constituting the vapor phase of the separating tank V2, which is composed of 9.8% methane, 36.3% ethylene and 53.9% propane and the flow rate of which is 1469 kmol/h, is cooled and condensed in the exchanger E1 in order to give a stream 14 available at −60° C.

[0070] The stream 14 is then cooled in an exchanger E2 in order to give a stream 15 available at −90° C. and 27.1 bar. This stream 15 is expanded in a valve 16 to give a stream 17 at a pressure of 2.3 bar and a temperature of −96° C. The opening of the valve 16 is regulated by a temperature controller in the line 310.

[0071] The stream 17 is warmed in the exchanger E2 and partially vaporizes, so as to meet the refrigeration requirements of the exchanger E2, in order to deliver the stream 18 at a temperature of −67.9° C. and a pressure of 2.2 bar at the outlet of the exchanger.

[0072] The stream 4, constituting the liquid phase from the separating tank V2, is composed of 2.2% methane, 18.3% ethylene and 79.5% propane, the flow rate of which is 2501 kmol/h, is cooled in the exchanger E1 in order to give a stream 19 available at −60° C.

[0073] The stream 19 is then separated into two streams:

[0074] a stream 8, the flow rate of which is 1000 kmol/h, is expanded to 8.1 bar by passing through a valve 20, to give a stream 21. The latter is vaporized and warmed in the exchanger E1 to give a stream 9 at a temperature of 38.5° C. and a pressure of 7.8 bar;

[0075] a stream 22, the flow rate of which is 1501 kmol/h, is expanded to 2.2 bar in a valve 23 and then mixed with the stream 18 to give the stream 6. The latter, available at a temperature of −64.93° C. and a pressure of 2.2 bar, composed of 6.0% methane, 27.2% ethylene and 66.8% propane, is vaporized and warmed in the exchanger E1 in order to deliver a stream 7 available at 38.5° C. and 1.9 bar.

[0076] The stream 7 is sent to the low-pressure stage of a compressor K1, passing via a suction tank V3. A stream 11, coming from the compressor K1 with a flow rate of 2970 kmol/h corresponding to all of the stream 7 entering the low-pressure stage of the compressor, is introduced into a water exchanger E11 at a pressure of 8.0 bar and a temperature of 113.75° C. in order to produce a cooled stream 25 at 42.0° C. and 7.7 bar.

[0077] The stream 9 flows through a suction tank V4 and then mixes with the stream 25, to deliver a stream 10 with a flow rate of 3970 kmol/h, at 41.01° C. and 7.7 bar. The latter stream 10 is introduced into a medium-pressure stage of the compressor K1.

[0078] A stream 26 coming from a high-pressure stage of the compressor K1 with a flow rate of 3970 kmol/h, at 111.66° C. and 28.39 bar, is cooled in a water exchanger E10 in order to give a stream 27 at 54.36° C. Finally, this stream 27 is cooled to 42.0° C. in a water exchanger E12, to give the stream 13.

[0079] The performance characteristics of the two processes are now given by the means of comparative tables. 1 Comparison of the powers of the compressors (in kW): (The powers are based on 82% polytropic efficiencies) Conventional Process according process to the invention (FIG. 1) (FIG. 2) Ethylene 2124 compressor Propane 7406 compressor Refrigerant 8708 mixture compressor Total 9530 8708

[0080] The process according to the invention allows a power saving of 9.4%. 2 Comparison of the refrigerating water exchangers Conventional process Process of the invention Heat Exchange Heat Exchange Water exchanged MTD area exchanged MTD area exchanger (kW) (° C.) (m2) (kW) (° C.) (m2) E10 3239 22.1 293 5989 35.2 341 E11 16426 8.88 3700 4451 24.4 365 E12 8211 6.88 2388 Total 19665 3993 18651 3094

[0081] The area of the refrigerating water exchangers for the process according to the invention 29% is less than that of the conventional process. The water consumption is 5.4% less in the case of the process of the invention. 3 Comparison of the cryogenic exchangers: Conventional process Process of the invention Heat Exchange Heat Exchange exchanged MTD area exchanged MTD area Exchanger (kW) (° C.) (m2) (kW) (° C.) (m2) E1 2000 11.2 357 30100 7.27 8286 E2 5658 6.74 1679 2367 3.49 1356 E3 5514 15.7 702 E4 1397 11.4 245 E5 1258 53.2 47 E6 606 8.59 141 E7 570 12.4 595 E8 929 11.0 169 E9 7500 3.75 4000 Total 25432 7935 32467 9642

[0082] The process according to the invention uses a 21% higher total exchange area than the known process. However, the cost of these exchangers is lower. 4 Comparison of the number of equipment items: Conventional Process of the process invention Cryogenic 9 2 exchangers Water exchangers 2 3 Tanks 8 4 Column 1 1 Compressors 2 1 Pumps 2 0 Total 24 11

[0083] The process of the invention comprises only 11 equipment items instead of 24 in the case of the known process. 5 Comparison of the number of control systems: Conventional Process of the process invention Flow rate control 3 1 Level control 7 2 Temperature 2 4 control Pressure control 1 1 Total 13 8

[0084] The new process possesses 8 control systems instead of 13 in the case of the conventional process.

[0085] The invention is therefore beneficial in limiting the energy consumption when producing purified gases. This objective is achieved while allowing a high level of selectivity in separating methane from the other constituents when operating the process.

[0086] Thus, the results obtained by the invention afford major advantages, these consisting in a substantial simplification and a substantial saving in both the construction and the technology of the equipment and in the methods of operating them, as well as in the quality of the products obtained by these methods.

Claims

1. A process for recovering the ethane contained in a pressurized gas (1) containing methane and C2 and higher hydrocarbons, operating a refrigerating cycle in which a relatively less-volatile first refrigerant (2) is compressed, cooled and expanded in order thereafter to be used for cooling said pressurized gas (1) to be separated or first separation products to a relatively high first temperature, and in which a relatively more-volatile second refrigerant (3) is compressed, cooled and expanded in order thereafter to be used for cooling at least second separation products obtained from said pressurized gas (1) to a relatively low second temperature *1, *2 characterized in that the first and second refrigerants (2, 3) are used as a mixture when they are compressed and cooled, in that this mixture then undergoes a separation into a first fraction (4) essentially containing the relatively less-volatile first refrigerant (2) and into a second fraction (5) essentially containing the relatively more-volatile second refrigerant (3), in that the first refrigerant is used in the form of the first fraction, for cooling to the relatively high first temperature, and in that the second refrigerant is used in the form of the second fraction, for cooling to the relatively low second temperature.

2. The process as claimed in claim 1, characterized in that the first fraction (4) is cooled in a first exchanger (E1), expanded to give a first expanded fraction (6), then warmed in said first exchanger, before being introduced into a low-pressure stage (7) of a compressor (K1).

3. The process as claimed in claim 2, characterized in that the second fraction (5) is cooled in the first exchanger (E1) and then in a second exchanger (E2), expanded, then warmed in said second exchanger and mixed with the expanded first fraction (6).

4. The process as claimed in claim 2 or claim 3, characterized in that a third fraction (8) is tapped off the first fraction (4) after it has been cooled in the first heat exchanger (E1) and in that said third fraction (8) is expanded and warmed in said first exchanger (E1) in order to deliver an expanded and warmed fourth fraction (9), which is introduced into a medium-pressure stage (10) of the compressor (K1).

5. The process as claimed in claim 4, characterized in that a gaseous fifth fraction (11) is tapped off from the refrigerants undergoing compression in the compressor (K1) at a medium pressure, slightly above that of the expanded and warmed fourth fraction (9), and is cooled and expanded to the same pressure as said fourth fraction (9) and then mixed with the latter.

6. The process as claimed in any one of the preceding claims, characterized in that said first and second refrigerants (2, 3) are used as a mixture with a third refrigerant (12).

7. The process as claimed in claim 6, characterized in that the refrigerants are methane, ethylene and propane.

8. A methane-enriched gas obtained by the process as claimed in one of the preceding claims.

9. An ethane-enriched product obtained by the process as claimed in one of claims 1 to 7.

10. A product enriched with C2 and higher hydrocarbons, obtained by the process as claimed in one of claims 1 to 7.

11. A plant for recovering the ethane contained in a pressurized gas (1) containing methane and C2 and higher hydrocarbons, this plant comprising means for compressing, cooling and expanding a relatively less-volatile first refrigerant (2), means for cooling, by means of the first refrigerant, said pressurized gas (1) to be separated or first separation products to a relatively high first temperature, and means for compressing, cooling and expanding a relatively more-volatile second refrigerant (3), means for cooling, by means of the second refrigerant (3), at least second separation products obtained from said pressurized gas (1) to a relatively low second temperature, characterized in that the first and second refrigerants (2, 3) are used as a mixture when they are compressed and cooled and in that this plant comprises means for this mixture to undergo a separation into a first fraction (4) essentially containing the relatively less-volatile first refrigerant (2) and into a second fraction (5) essentially containing the relatively more-volatile second refrigerant (3), the first refrigerant being used in the form of the first fraction for cooling to the relatively high first temperature and the second refrigerant being used in the form of the second fraction for cooling to the relatively low second temperature.