RECOVERY OF HYDROCARBONS FROM A GASEOUS STREAM

Abstract

A process (10) for the recovery of hydrocarbons from a Fischer-Tropsch tail gas includes providing a hydrocarbon rich Fischer-Tropsch tail gas (30) which includes hydrocarbons and carbon dioxide, compressing (14) the hydrocarbon rich Fischer-Tropsch tail gas (30) to provide a compressed hydrocarbon rich Fischer-Tropsch tail gas (36, 54), and contacting the compressed hydrocarbon rich Fischer-Tropsch tail gas (36, 54) with a lean oil (64, 54) to recover the hydrocarbons from the compressed hydrocarbon rich Fischer-Tropsch tail gas (36, 54) and to produce a hydrocarbon rich oil (66). Carbon dioxide is stripped (20) from the hydrocarbon rich oil (66) at a pressure which is below the pressure at which the hydrocarbon rich Fischer-Tropsch tail gas (36, 54) is contacted with the lean oil (64, 54), to provide a stripped hydrocarbon oil product (86).

Claims

1. A process for the recovery of hydrocarbons from a Fischer-Tropsch tail gas, the process including: providing a hydrocarbon rich Fischer-Tropsch tail gas which includes hydrocarbons and carbon dioxide; compressing the hydrocarbon rich Fischer-Tropsch tail gas to provide a compressed hydrocarbon rich Fischer-Tropsch tail gas, the difference in pressure between the compressed hydrocarbon rich Fischer-Tropsch tail gas and the hydrocarbon rich Fischer-Tropsch tail gas being in the range of 10 bar to 40 bar; contacting the compressed hydrocarbon rich Fischer-Tropsch tail gas with a lean oil to recover the hydrocarbons from the compressed hydrocarbon rich Fischer-Tropsch tail gas and to produce a hydrocarbon rich oil, the hydrocarbon rich oil also including carbon dioxide absorbed from the compressed hydrocarbon rich Fischer-Tropsch tail gas; and stripping carbon dioxide from the hydrocarbon rich oil at a pressure which is below the pressure at which the hydrocarbon rich Fischer-Tropsch tail gas is contacted with the lean oil, thereby to provide a stripped hydrocarbon oil product which includes hydrocarbons recovered from the compressed hydrocarbon rich Fischer-Tropsch tail gas.

2. The process according to claim 1, in which the compressed hydrocarbon rich Fischer-Tropsch tail gas is contacted with the lean oil in a sponge oil column.

3. The process according to claim 1, in which the carbon dioxide is stripped from the hydrocarbon rich oil in a stripping column or desorber, thereby to provide the stripped hydrocarbon oil product.

4. The process according to claim 1, which includes cooling the Fischer-Tropsch tail gas and separating the cooled Fischer-Tropsch tail gas into said hydrocarbon rich Fischer-Tropsch tail gas and a hydrocarbon condensate in a separator prior to compressing the hydrocarbon rich Fischer-Tropsch tail gas.

5. The process according to claim 1, which includes cooling the compressed hydrocarbon rich Fischer-Tropsch tail gas prior to contacting the compressed, thus cooled hydrocarbon rich Fischer-Tropsch tail gas with the lean oil.

6. The process according to claim 5, which includes separating a hydrocarbon condensate and a dirty water stream from the compressed, cooled hydrocarbon rich Fischer-Tropsch tail gas in a separator, prior to contacting the compressed, cooled hydrocarbon rich Fischer-Tropsch tail gas with the lean oil.

7. The process according to claim 1, in which the lean oil is, or includes, a Fischer-Tropsch hydrocarbon condensate.

8. The process according to claim 1, in which the stripped hydrocarbon oil product contains hydrocarbons predominantly in the range of from about C.sub.3 to about C.sub.30 hydrocarbons.

9. The process according to claim 2, which includes cooling an overhead gas stream from the sponge oil column, and using at least a portion of the cooled overhead gas stream to cool the compressed hydrocarbon rich Fischer-Tropsch tail gas.

10. The process according to claim 1, in which the carbon dioxide is stripped from the hydrocarbon rich oil at a pressure which is at least 10 bar or at least 15 bar or at least 20 bar or at least 25 bar below the pressure at which the hydrocarbon rich Fischer-Tropsch tail gas is contacted with the lean oil.

11. The process according to claim 2, in which the hydrocarbon condensate, or at least a portion of the hydrocarbon condensate, is fed to the sponge oil column to form part of the lean oil.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0073] The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawing which shows one embodiment of a process to recover hydrocarbons from a Fischer-Tropsch tail gas in accordance with the invention.

[0074] In the drawing, reference numeral 10 generally indicates a process for the recovery of hydrocarbons from a Fischer-Tropsch tail gas in accordance with the invention.

[0075] The process 10 broadly includes a first separator 12, a tail gas compressor 14, a second separator 16, a sponge oil column 18 and a carbon-dioxide stripper 20.

[0076] A Fischer-Tropsch tail gas 22 and a Fischer-Tropsch hydrocarbon condensate 24 are withdrawn from a three-phase gas-liquid separator (not shown) located downstream of a Fischer-Tropsch synthesis reactor and also downstream of an air cooler (also not shown). The air cooler and the three-phase gas-liquid separator are respectively used to cool the gaseous overhead product of a Fischer-Tropsch reactor and to separate the gaseous overhead product into the Fischer-Tropsch tail gas 22, reaction water and the Fischer-Tropsch hydrocarbon condensate 24. A reaction water stream (not shown) is thus also withdrawn from the three-phase gas-liquid separator.

[0077] The Fischer-Tropsch tail gas 22 is typically withdrawn from the three-phase gas-liquid separator at a temperature of 70° C. and at a pressure of 20 bar(g). The Fischer-Tropsch tail gas 22 is cooled to 45° C. by indirect heat exchange against cooling water in a heat exchanger 26, providing cooled Fischer-Tropsch tail gas. The cooled Fischer-Tropsch tail gas 28 is then fed into the first separator 12, from which a first hydrocarbon rich tail gas 30, a first hydrocarbon condensate 32 and a first dirty water stream 34 are withdrawn. The first separator 12 is typically a three-phase separator that allows for gaseous components to be withdrawn from a top section thereof (as the first hydrocarbon rich tail gas 30) and allows the first hydrocarbon condensate and the first dirty water stream to be separated under gravity and to be withdrawn as the streams 32 and 34 respectively.

[0078] The first hydrocarbon rich tail gas 30 is routed to the inlet of the tail gas compressor 14 where the pressure is increased from 19 bar(g) to 50 bar(g), providing a compressed first hydrocarbon rich tail gas 36. The temperature of the compressed first hydrocarbon rich tail gas 36 rises to 131° C. as a result of the compression.

[0079] The compressed first hydrocarbon rich tail gas 36 is then cooled in a series of heat exchangers. In the embodiment shown, the compressed first hydrocarbon rich tail gas 36 is cooled in a sequence of four heat exchangers: first by indirect heat exchange against a recycle gas 38 in a heat exchanger 40, secondly by indirect heat exchange against cooling water in a heat exchanger 42, thirdly by indirect heat exchange against the recycle gas 38 in a heat exchanger 44 and finally by indirect heat exchange against a propylene refrigerant in a heat exchanger 46.

[0080] The temperature of the compressed first hydrocarbon rich tail gas 36 is reduced from 131° C. as it exits the tail gas compressor 14 to 85° C. after the heat exchanger 40, to 45° C. after the heat exchanger 42, to 24° C. after the heat exchanger 44 and to 5° C. after the heat exchanger 46, providing a cooled, compressed first hydrocarbon rich tail gas 48. It will be appreciated that the heat exchanger arrangement 40, 42, 44 and 46 could be configured differently to achieve the same end temperature (5° C.) of the cooled, compressed first hydrocarbon rich tail gas 48, e.g. based on the availability of the recycle gas 38, cooling water or propylene refrigerant as heat exchange media.

[0081] The cooled, compressed first hydrocarbon rich tail gas 48 is then routed to the second separator 16 where components condensed as a result of the cooling in the heat exchangers 40, 42, 44 and 46 are separated from any remaining gaseous components. A second dirty water stream 50 is removed from the bottom of the second separator 16 and mixed with the first dirty water stream 34 from the first separator 12. A combined dirty water stream 35 is sent for further processing (not shown).

[0082] A second hydrocarbon rich tail gas 52 is withdrawn from the second separator 16, as well as a second hydrocarbon rich condensate 54.

[0083] The Fischer-Tropsch hydrocarbon condensate 24 from the three-phase separator (not shown) is received at a temperature of 70° C. and a pressure of 20 bar(g) and is increased in pressure to 50 bar(g) via a pump 56, providing a pressurised Fischer-Tropsch hydrocarbon condensate 58. The pressurised Fischer-Tropsch hydrocarbon condensate 58 is then cooled in a series of heat exchangers 60, 62 to produce a pressurised, cooled Fischer-Tropsch hydrocarbon condensate 64. The heat exchanger 60 is a cooling water heat exchanger and the heat exchanger 62 is a propylene refrigerant heat exchanger. The temperature of the pressurised, cooled Fischer-Tropsch hydrocarbon condensate 64 after the heat exchanger 60 is 45° C. and after the heat exchanger 62 it is 5° C. Again, it will be appreciated that the heat exchanger arrangement 60, 62 could be configured differently depending on the availability of cooling water and propylene refrigerant as heat exchange media to achieve the same end temperature (5° C.) of the pressurised, cooled Fischer-Tropsch hydrocarbon condensate 64.

[0084] The pressurised, cooled Fischer-Tropsch hydrocarbon condensate 64 is then fed into the sponge oil column 18 where it is contacted in a counter-current arrangement with the second hydrocarbon rich tail gas 52. The sponge oil column 18 is a trayed column. The second hydrocarbon rich condensate 54 from the second separator 16 is also fed to the sponge oil column 18 and contacted with the second hydrocarbon rich tail gas 52. The sponge oil column is operated at a pressure of 46 bar(g) and a temperature of 8° C. in a bottom section and 5° C. in a top section thereof.

[0085] The pressurised, cooled Fischer-Tropsch hydrocarbon condensate 64 is fed as a lean oil to the sponge oil column 18 at a top section thereof. The second hydrocarbon rich condensate 54 is fed as additional lean oil near the top section thereof. The second hydrocarbon rich tail gas 52 is fed to a bottom section of the sponge oil column 18. C.sub.3 to C.sub.7 hydrocarbons in the second hydrocarbon rich tail gas 52 are preferentially absorbed into the lean oil and are withdrawn from the sponge oil column 18 as a hydrocarbon rich condensate oil 66.

[0086] To improve and enhance the recovery of C.sub.3 to C.sub.7 hydrocarbons from the second hydrocarbon rich tail gas 52, the sponge oil column 18 is equipped with a chilled reflux system. The chilled reflux system includes a propylene-cooled reflux condenser 68 and a separator drum 70. An overhead gas stream 72 from the top section of the sponge oil column 18 is cooled in the propylene-cooled reflux condenser 68, whereafter condensed components are separated from any remaining gaseous components in the separator drum 70. The condensed components, at a temperature of 5° C., are returned to the sponge oil column 18 as a chilled reflux stream 74. The remaining gaseous components, also at a temperature of 5° C., are withdrawn from the separator drum 70 as the chilled overhead gas stream 38.

[0087] The chilled overhead gas stream 38 is used as a heat exchange medium in the heat exchangers 44, 40 as hereinbefore described. Thereafter, a portion 76 of the chilled overhead gas stream 38 is withdrawn and reduced in pressure from 45 bar(g) to 25 bar(g) through a turbo-expander 78. A fuel gas 79 is withdrawn from the turbo-expander 78 and routed to a fuel gas system (not shown). A remaining portion of the chilled overhead recycle gas stream 38 is recycled at pressure as a recycle gas 39 to a synthesis gas generator, e.g. an auto-thermal reformer (not shown).

[0088] The hydrocarbon rich condensate 66 from the sponge oil column 18 is routed to a top section of the carbon dioxide stripper 20. The first hydrocarbon condensate 32 from the first separator 12 is also fed to the carbon dioxide stripper 20. In the carbon dioxide stripper 20, carbon dioxide is stripped from the hydrocarbon rich condensate 66 and from the first hydrocarbon condensate 32. Optionally, a portion 25 of the first hydrocarbon condensate 32 from the first separator 12 may be combined with the Fischer-Tropsch hydrocarbon condensate 24 prior to the pump 56, before being fed to the sponge oil column 18 as part of the lean oil.

[0089] The carbon dioxide stripper 20 is equipped with a reboiler 80 that is supplied with saturated high pressure steam at 68 bar(g). The reboiler 80 heats the contents of the carbon dioxide stripper 20 thereby to strip any carbon dioxide from the hydrocarbon rich condensate 66 and from the first hydrocarbon condensate 32. A carbon dioxide-rich overhead stream 81 is removed from the top of the carbon dioxide stripper 20. Part of the carbon dioxide-rich overhead stream 81 is recycled as a carbon dioxide-rich recycle gas 84 and mixed with the first hydrocarbon rich tail gas 30 withdrawn from the first separator 12 upstream of the tail gas compressor 14 in order to recover at least some of any lighter hydrocarbons (e.g. C.sub.2 to C.sub.6 hydrocarbons) that may also report to the carbon dioxide-rich overhead stream 81 withdrawn from the carbon dioxide stripper 20. Part of the carbon dioxide-rich overhead stream 81 may be discarded as off-gas 82 and may be burned in a flare system or vented to atmosphere at a safe location as appropriate (not shown). Optionally, part of the carbon dioxide-rich overhead stream 81 may be recycled to a synthesis gas generation unit to recover at least some of the carbon dioxide in the carbon dioxide-rich overhead stream 81 (not shown). The off-gas 82 is withdrawn from the carbon dioxide stripper 20 at a temperature of 25° C.

[0090] A stripped hydrocarbon condensate 85 is withdrawn from the bottom of the carbon dioxide stripper 20 at a temperature of 212° C. Part of the stripped hydrocarbon condensate 85 is routed as a reboiler feed 87 to the reboiler 80 where is it heated and returned to the carbon dioxide stripper 20. Another part of the stripped hydrocarbon condensate 85 is withdrawn from the process 10 as stripped hydrocarbon condensate product 86.

[0091] Typically, the carbon dioxide stripper 20 is operated at a pressure of 20 bar(g), which is substantially lower than the operating pressure of the sponge oil column 18 which operates at 46 bar(g). An advantage of lowering the operating pressure of the carbon dioxide stripper 20 is that carbon dioxide can easily be stripped from the hydrocarbon rich condensate 66 at the lower pressure. A further advantage of lowering the operating pressure of the carbon dioxide stripper 20 is that the reboiler 80 can be operated at a lower temperature, thus mitigating the risk of thermally cracking the hydrocarbon rich condensate 66 fed to the carbon dioxide stripper 20.

[0092] The process 10 as exemplified has the advantage that hydrocarbon recovery, specifically C.sub.3 hydrocarbon recovery, is maximised by ensuring a maximum amount of C.sub.3 hydrocarbons are absorbed into the sponge oil in the sponge oil column 18 at elevated pressure (typically a pressure of more than 40 bar(g)) and low temperature (in the range typically of from about 5° C. to about 8° C. as hereinbefore described). Furthermore, the use of a separate carbon dioxide stripper 20 operated at a lower pressure than the sponge oil column 18, allows carbon dioxide to be stripped from the hydrocarbon rich condensate 66 at a lower reboiler temperature which mitigates thermal cracking of the hydrocarbon rich condensate 66 and consequent loss of C.sub.3 hydrocarbons to the off-gas 82 withdrawn from the carbon dioxide stripper 20.

[0093] The process 10, as exemplified, enables hydrocarbons (C.sub.3, C.sub.4 and C.sub.5 hydrocarbons) to be recovered from a Fischer-Tropsch tail gas at similar or even higher recoveries than for conventional low pressure cryogenic schemes, without (i) having to use a cryogenic section and associated upstream drying and acid wash units and (ii) without having to use multiple refrigerant cooling systems (e.g. ethylene and propylene/propane) with multiple compression stages.

[0094] The process of the invention thus is beneficial over the processes of the prior art at least in that it is a simple process with a reduced equipment count and therefore reduces the capital expenditure required for the process; in that it reduces equipment size due to the elevated pressure of the process (the aforesaid benefits are believed to result in a capital expenditure saving of the order of about 30% over conventional cryogenic systems); in that it mitigates the risk of thermal cracking of hydrocarbons; and in that it has an improved separation efficiency for hydrocarbons.