METHOD FOR SEPARATING C5-C8 HYDROCARBONS AND ACID GASES FROM A FLUID STREAM

Abstract

A process for removing C.sub.5-C.sub.8-hydrocarbons and acid gases from a fluid stream is described, where a) the fluid stream is brought into contact with an absorption medium comprising at least one amine in an absorption zone to obtain a deacidified fluid stream and an acid-gases-laden absorption medium, b) the laden absorption medium is heated in a first heat exchanger and decompressed into a decompression zone to a pressure of from 5 to 10 bar to obtain a C.sub.5-C.sub.8-hydrocarbons-comprising gas phase and a hydrocarbon-depleted laden absorption medium, c) the hydrocarbon-depleted laden absorption medium is heated in an optional second heat exchanger and passed into a stripper in which at a pressure of 1 to 2.5 bar the acid gases are at least partially liberated by supplying heat to obtain a regenerated absorption medium and an acid-gas-comprising stream, and d) the regenerated absorption medium is recycled into the absorption zone.

Claims

1. A process for removing C.sub.5-C.sub.8-hydrocarbons and acid gases from a fluid stream, the process comprising: a) contacting the fluid stream with an absorption medium comprising at least one amine in an absorption zone to obtain a deacidified fluid stream and an acid-gases-laden absorption medium; b) heating the laden absorption medium in a first heat exchanger and decompressing the heated laden absorption medium into a decompression zone to a pressure of from 5 to 10 bar to obtain a C.sub.5-C.sub.8-hydrocarbons-comprising gas phase and a hydrocarbon-depleted laden absorption medium; c) passing the hydrocarbon-depleted laden absorption medium into a stripper in which at a pressure of 1 to 2.5 bar the acid gases are at least partially liberated by supplying heat to obtain a regenerated absorption medium and an acid-gas-comprising stream, the hydrocarbon-depleted laden absorption medium optionally being heated in a second heat exchanger before being introduced into the stripper; and d) recycling the regenerated absorption medium into the absorption zone, wherein: if the second heat exchanger is present, the heating medium used in the second heat exchanger is the regenerated absorption medium and the heating medium used in the first heat exchanger is the regenerated absorption medium after it has passed through the second heat exchanger, or, if the second heat exchanger is not present, the heating medium used in the first heat exchanger is the regenerated absorption medium; and the amount of heat transferred in the first heat exchanger is such that the C.sub.5-C.sub.8-hydrocarbons-comprising gas phase comprises 50% to 97% by volume of acid gases.

2. The process according to claim 1, where the second heat exchanger is present and where before the heating in the second heat exchanger the hydrocarbon-depleted laden absorption medium is decompressed into a desorption zone to a pressure of 1 to 2.5 bar and preheated by direct heat exchange with the acid-gas-comprising stream.

3. The process according to claim 1, wherein the fluid stream is brought into contact with the absorption medium in the absorption zone at a pressure of 50 to 80 bar.

4. The process according to claim 1, wherein the partial pressure of the acid gases in the supplied fluid stream is less than 1 bar.

5. The process according to claim 1, wherein the volume fraction of the acid gases in the supplied fluid stream is less than 3.3% by volume.

6. The process according to claim 1, wherein the temperature of the supplied fluid stream is less than 70 C.

7. The process according to claim 1, wherein the C.sub.5-C.sub.8-hydrocarbons-comprising gas phase comprises 70% to 95% by volume of acid gases.

8. The process according to claim 1, wherein the laden absorption medium heated in the first heat exchanger is decompressed into the decompression zone to a pressure of >6 to 10 bar.

9. The process according to claim 1, wherein the C.sub.5-C.sub.8-hydrocarbons are at least partially condensed out of the C.sub.5-C.sub.8-hydrocarbons-comprising gas phase and the uncondensed constituents are passed into the absorption zone.

10. The process according to claim 1, wherein the C.sub.5-C.sub.8-hydrocarbons comprise aromatic hydrocarbons selected from benzene, toluene, ethylbenzene and xylenes.

11. The process according to claim 1, wherein the supplied fluid stream is selected from fluid streams comprising 0.5% to 10% by volume of acid gases and 10 to 5000 ppmv of C.sub.5-C.sub.8-hydrocarbons.

12. The process according to claim 1, wherein the supplied fluid stream is natural gas.

13. The process according to claim 1, wherein is less than 1 and is defined as: $=\frac{{\stackrel{.}{m}}_a.Math.c_{\mathrm{pa}}}{{\stackrel{.}{m}}_f.Math.c_{\mathrm{pf}}}$ where: {dot over (m)}.sub.a is the mass flow rate of the absorption medium passed into the absorption zone, {dot over (m)}.sub.f is the mass flow rate of the supplied fluid stream, c.sub.pa is the specific heat capacity of the absorption medium passed into the absorption zone, and c.sub.pf is the specific heat capacity of the supplied fluid stream.

14. The process according to claim 1, wherein the amine is a sterically hindered amine or a tertiary amine.

15. The process according to claim 1, wherein the absorption medium further comprises a sterically unhindered primary or secondary amine.

Description

[0109] The invention is more particularly elucidated by the appended drawings and the examples which follow.

[0110] FIG. 1 is a schematic diagram of a plant comprising only the second heat exchanger for performing a noninventive process.

[0111] FIG. 2 is a schematic diagram of a plant comprising the first and the second heat exchanger for performing the inventive process.

[0112] FIG. 3 is a schematic diagram of a plant comprising only one heat exchanger for performing a noninventive process where before warming in the second heat exchanger the hydrocarbon-depleted laden absorption medium is decompressed in a desorption zone to a pressure of 1 to 2.5 bar and preheated via direct heat exchange with the acid-gas-comprising stream. The heat exchanger of the process shown in FIG. 3 corresponds to the optional second heat exchanger of the process according to the invention.

[0113] FIG. 4 is a schematic diagram of a plant comprising the first and the second heat exchanger for performing the inventive process where before heating in the second heat exchanger the hydrocarbon-depleted laden absorption medium is decompressed in a desorption zone to a pressure of 1 to 2.5 bar and preheated via direct heat exchange with the acid-gas-comprising stream.

[0114] In accordance with FIG. 1 a fluid stream 1 is passed into the lower part of an absorption column 2. The absorption column 2 comprises an absorption zone 3 which comprises two sectors arranged on top of one another and comprising packings. In the absorption zone 3 the fluid stream is brought into contact, in countercurrent, with an absorption medium which is introduced into the absorption column 2 via the conduit 4 above the absorption zone. The deacidified fluid stream is withdrawn via conduit 17. Fresh water is supplied via conduit 18.

[0115] The acid-gases-laden absorption medium is withdrawn at the bottom of the absorption column 2 and decompressed into the decompression zone of the flash tank 5 to a pressure of 5 to 10 bar via a throttle valve (not shown). The decompression results in desorption of coabsorbed constituents of the fluid stream and of a portion of the acid gases which are withdrawn via stream 16. The absorption medium decompressed to a pressure of 5 to 10 bar is passed via the heat exchanger 19 and conduit 6 into the stripper 7. The stripper 7 comprises a regeneration zone 8 which comprises two sectors arranged on top of one another and comprising packings. In the lower part of the stripper 7 the decompressed absorption medium is heated via the evaporator 9 and partially evaporated. The temperature increase liberates the absorbed acid gases. The acid-gases-comprising stream is discharged via the conduit 10 at the top of the stripper 7 and sent to the cooler 11. An acid gas condensate is obtained at the cooler 11, collected in the phase separation vessel 12 and recycled into the stripper. The acid gases are withdrawn as stream 13. The regenerated absorption medium 14 is recycled back into the absorption column 2 via the heat exchanger 19, cooler 15, a pump (not shown) and conduit 4.

[0116] FIG. 2 shows an inventive embodiment. The reference numerals in FIG. 2 have the same meanings as in FIG. 1. In contrast to FIG. 1, the acid-gases-laden absorption medium withdrawn at the bottom of the absorption column 2 is passed through a heat exchanger 20 and then decompressed into the decompression zone of the flash tank 5 to a pressure of 5 to 10 bar via the throttle valve (not shown). The regenerated absorption medium 14 is recycled back into the absorption column 2 via the heat exchanger 19, heat exchanger 20, cooler 15, a pump (not shown) and conduit 4.

[0117] The reference numerals in FIG. 3 have the same meanings as in FIG. 1. The absorption medium decompressed to a pressure of 5 to 10 bar is passed via conduit 23 into the low-pressure flash tank 24. Conduit 23 connects between the desorption zone disposed below it in the low-pressure flash tank and a rectifying zone 25 disposed above it in the low-pressure flash tank. The acid-gases-comprising stream is discharged via the conduit 10 at the top of the stripper 7 and sent to the low-pressure flash tank 24. Conduit 10 connects to the low-pressure flash tank 24 in the region between the bottom and the desorption zone. In the desorption zone the acid-gases-comprising stream supplied via conduit 10 is brought into contact, in countercurrent, with the absorption medium supplied via conduit 23. The acid-gases-comprising stream subsequently passes through the rectifying zone 25 and is discharged via conduit 26 at the top of the low-pressure flash tank 24 and sent to the cooler 27. An acid gas condensate is obtained at the cooler 27, collected in the phase separation vessel 28 and recycled into the low-pressure flash tank 24 in the region above the rectifying zone 25. The acid gases are withdrawn as stream 13.

[0118] The absorption medium decompressed to 1 to 2.5 bar is withdrawn, via a conduit, from the bottom of the low-pressure flash tank and passed into the stripper 7 with the aid of the optional pump 29 via the heat exchanger 19 and conduit 30. Below the regeneration zone 8 the absorption medium is collected on collecting tray 31, heated and partially evaporated via the evaporator 9 and passed into the bottom region below the collecting tray 31. The regenerated absorption medium 14 is recycled back into the absorption column 2 via the heat exchanger 19, cooler 15, pump 22 and conduit 4. The scrubbing zone 21 is disposed above the absorption zone 3 in the absorption column 2. Feeding of fresh water 18 is effected above the scrubbing zone 21.

[0119] FIG. 4 shows an inventive embodiment. The reference numerals in FIG. 4 have the same meanings as in FIGS. 1, 2 and 3. In contrast to FIG. 3, the acid-gases-laden absorption medium withdrawn at the bottom of the absorption column 2 is passed through a heat exchanger 20 and then decompressed into the flash tank 5 to a pressure of 5 to 10 bar via the throttle valve (not shown). The regenerated absorption medium 14 is recycled back into the absorption column 2 via the heat exchanger 19, heat exchanger 20, cooler 15, pump 22 and conduit 4.

EXAMPLES

General Information

[0120] The composition, flow rate, temperature and pressure of the fluid stream 1 for the examples which follow were:

2.0000% by volume CO.sub.2
0.0004% by volume H.sub.2S
1.0000% by volume N.sub.2
92.9796% by volume CH.sub.4
2.0000% by volume C.sub.2H.sub.6
1.0000% by volume C.sub.3H.sub.8
1.0000% by volume C.sub.6H.sub.14
0.0050% by volume benzene
0.0050% by volume toluene
0.0050% by volume ethylbenzene
0.0050% by volume o-xylene
Flow rate (dry): 500 000 m.sup.3(STP)/h
Flow rate (water): 74 m.sup.3(STP)/h
Flow rate (overall): 500 074 m.sup.3(STP)/h
Flow rate (overall): 401 605 kg/h

Temperature: 30.0 C.

Pressure: 66.0 bar

[0121] All pressures reported in the present document are absolute pressures.

[0122] In the present document m.sup.3(STP)/h is the volume flow rate reported in standard cubic meters per hour. A standard cubic meter refers to a temperature of 273.15 K and a pressure of 1.01325 bar. All values reported in the unit % by volume also refer to these conditions.

[0123] The temperature of the deacidified (to a residual content of not more than 0.005% by volume of CO.sub.2 in each case) fluid stream 17 was 56 C. in each case. The pressure of the deacidified fluid stream 17 was 65.9 bar in each case.

[0124] In all examples the absorption medium was an aqueous solution of 33.5% by weight of methyldiethanolamine and 6.5% by weight of piperazine in water. 247 t/h of absorption medium at a temperature of 40.0 C. were introduced into the absorption column 2 above the absorption zone via conduit 4. The temperature of the fresh water introduced via conduit 18 was likewise 40.0 C. The temperature of the acid-gases-laden absorption medium withdrawn at the bottom of the absorption column 2 was 35.4 C. in each case.

[0125] The examples are based on calculations performed using a simulation model. The phase equilibria were described using a model by Pitzer (K. S. Pitzer, Activity Coefficients in Electrolyte Solutions 2nd ed., CRC Press, 1991, Chapter 3, Ion Interaction Approach: Theory). The simulation of the absorption operations is described using a mass-transfer-based approach; details in this regard are described in Asprion (Asprion, N.: Nonequilibrium Rate-Based Simulation of Reactive Systems: Simulation Model, Heat Transfer, and Influence of Film Discretization, Ind. Eng. Chem. Res. (2006) 45(6), 2054-2069).

[0126] The absorption column 2 had a diameter of 3200 mm and comprised an absorption zone 3 comprising two random packings (INTALOX Metal Tower Packing IMTP 50, Koch-Glitsch, Wichita USA), each having a packing height of 7 meters. The stripper 7 had a diameter of 2300 mm and comprised a regeneration zone 8 comprising two random packings (INTALOX Metal Tower Packing IMTP 50, Koch-Glitsch, Wichita USA), each having a packing height of 5 meters. The flash tanks 5/24 each had a diameter of 2200 mm and comprised a decompression zone/a desorption zone, each comprising a random packing (INTALOX Metal Tower Packing IMTP 50, Koch-Glitsch, Wichita USA) having a packing height of 5 meters.

Comparative Example 1

[0127] A process was simulated in a plant according to FIG. 1. The acid-gases-laden absorption medium was decompressed into the flash tank 5 to a pressure of 6.2 bar. The temperature of both the gas phase discharged via stream 16 and the absorption medium discharged from the flash tank in a downward direction was 34.6 C. The stripper 7 was operated at a pressure of 1.7 bar. 1.87 t/h of fresh water were supplied via conduit 18.

[0128] The energy requirements for the coolers 11 and 15 and for the evaporator 9 and the amount of heat transferred via heat exchanger 19 are reported in table 1. The condensation temperature (cooler 11) was in the customary range and was identical in this example and the remaining examples and comparative examples. The temperature of the stream 13 was consequently also identical in all examples and comparative examples.

Example 2

[0129] A process was simulated in a plant according to FIG. 2. The acid-gases-laden absorption medium was decompressed into the flash tank 5 to a pressure of 6.2 bar. The temperature of both the gas phase discharged via stream 16 and the absorption medium discharged from the flash tank in a downward direction was 95.8 C. The stripper was operated at a pressure of 1.7 bar. 1.95 t/h of fresh water were supplied via conduit 18.

[0130] The energy requirements for the coolers 11 and 15 and for the evaporator 9 and the amount of heat transferred via heat exchangers 19 and 20 are reported in table 1.

Comparative Example 3

[0131] A process was simulated in a plant according to FIG. 3. The acid-gases-laden absorption medium was decompressed into the flash tank 5 to a pressure of 6.2 bar. The temperature of both the gas phase discharged via stream 16 and the absorption medium discharged from the flash tank in a downward direction was 34.6 C. The low-pressure flash tank was operated at a pressure of 1.7 bar and the stripper at a pressure of 1.8 bar. 1.86 t/h of fresh water were supplied via conduit 18.

[0132] The energy requirements for the coolers 11 and 15 and for the evaporator 9 and the amount of heat transferred via heat exchanger 19 are reported in table 1.

Example 4

[0133] A process was simulated in a plant according to FIG. 4. The acid-gases-laden absorption medium was decompressed into the flash tank 5 to a pressure of 6.2 bar. The temperature of both the gas phase discharged via stream 16 and the absorption medium discharged from the flash tank in a downward direction was 95.8 C. The low-pressure flash tank was operated at a pressure of 1.7 bar and the stripper at a pressure of 1.8 bar. 1.94 t/h of fresh water were supplied via conduit 18.

[0134] The energy requirements for the coolers 11 and 15 and for the evaporator 9 and the amount of heat transferred via heat exchangers 19 and 20 are reported in table 1.

TABLE-US-00001 TABLE 1 Energy consumption for cooling, evaporation, heat exchange (unit: MW) cooler evaporator/ heat comparative exchanger ex. 1 example 2 comparative ex. 3 example 4 9 14.7 14.7 14.6 11.4 11/27* 6.0 5.7 0.1 2.4 15* 0.1** 0.1** 5.8 0.1** 19 20.1 3.8 14.9 4.5 20 16.1 16.1 *The energy requirements necessitated by cooling are reported with a negative prefix. In the simulation upon which comparative example 1 is based heat was supplied in 15 (and the prefix is therefore positive). **This cooler/heat exchanger would be omitted upon practical implementation of the process.

[0135] Properties of the streams 16 and 13 for examples 1 to 4 are reported in table 2.

[0136] As is evident from table 2, in the inventive examples the volume fraction of the C.sub.5-C.sub.8-hydrocarbons (C.sub.6H.sub.14, benzene, toluene, ethylbenzene, o-xylene) in the acid gases withdrawn via stream 13 is lower than in the comparative examples. It is thus possible to remove a greater proportion of C.sub.5-C.sub.8-hydrocarbons in accordance with the invention.

[0137] The C.sub.5-C.sub.8-hydrocarbons in stream 16 are moreover generated at a pressure of 6.2 bar so that no additional compression outlay is required to send them to the fuel gas system.

[0138] In example 4 the output of the evaporator 9 (i.e. the output required for the regeneration) was only 11.4 MW and thus 3.2 to 3.3 MW lower than in the examples 1 to 3. The energy requirements necessitated by cooling were also lower in this example than in the examples 1 to 3. The combination of the inventive procedure with the preheating of the hydrocarbon-depleted laden absorption medium by direct heat exchange with the acid-gas-comprising stream accordingly provides a synergistic effect in terms of energy requirements.

TABLE-US-00002 TABLE 2 comparative ex. 1 example 2 comparative ex. 3 example 4 stream 16 13 16 13 16 13 16 13 CO.sub.2 % by volume 2.5664 99.7065 75.1282 99.9048 2.5667 99.7066 75.1342 99.9048 H.sub.2S % by volume 0.0011 0.0200 0.0069 0.0209 0.0011 0.0200 0.0070 0.0209 N.sub.2 % by volume 0.5571 0.0007 0.1359 0.0002 0.5571 0.0007 0.1358 0.0002 CH.sub.4 % by volume 93.9195 0.2315 23.8084 0.0539 93.9191 0.2314 23.8026 0.0539 C.sub.2H.sub.6 % by volume 1.6625 0.0051 0.4304 0.0011 1.6625 0.0051 0.4303 0.0011 C.sub.3H.sub.8 % by volume 0.7047 0.0022 0.1832 0.0005 0.7047 0.0022 0.1831 0.0005 C.sub.6H.sub.14 % by volume 0.5404 0.0027 0.1493 0.0004 0.5404 0.0027 0.1493 0.0004 benzene % by volume 0.0156 0.0079 0.0251 0.0063 0.0156 0.0079 0.0251 0.0063 toluene % by volume 0.0145 0.0076 0.0386 0.0044 0.0145 0.0076 0.0386 0.0044 ethylbenzene % by volume 0.0092 0.0069 0.0435 0.0031 0.0092 0.0069 0.0435 0.0031 o-xylene % by volume 0.0090 0.0089 0.0504 0.0044 0.0090 0.0089 0.0504 0.0044 flow rate (dry) m.sup.3(STP)/h 231 9999 989 9241 231 9999 989 9241 water m.sup.3(STP)/h 2 508 143 469 2 492 143 455 flow rate (overall) m.sup.3(STP)/h 233 10507 1132 9711 233 10491 1132 9696 flow rate (overall) kg/h 184 20019 1766 18520 184 20006 1766 18508 temperature C. 34.6 95.8 34.6 95.8 pressure bar 6.2 1.6 6.2 1.6 6.2 1.6 6.2 1.6