Patent Application
20210284917
THIS INVENTION relates to the recovery of hydrocarbons from a gaseous stream. In particular, the invention relates to a process for the recovery of hydrocarbons from a Fischer-Tropsch tail gas.
Commercial Fischer-Tropsch synthesis processes principally produce a hydrocarbon product comprising hydrocarbons over a wide range of molecular weights, e.g. from C1 to heavier, waxy hydrocarbon products which may be liquid or solid at ambient temperature and pressure, and an aqueous stream, commonly referred to as a reaction water stream. The reaction water stream is typically condensed from a gaseous overhead product from a Fischer-Tropsch synthesis reactor by cooling the gaseous overhead product to a temperature in the range of from about 30° C. to about 80° C. by air and/or water cooling. Cooling the gaseous overhead product from the Fischer-Tropsch synthesis reactor also condenses valuable condensable hydrocarbons from the gaseous overhead stream. The condensed hydrocarbons and condensed reaction water are typically separated in a three-phase gas-liquid separator from gaseous components remaining after the cooling step, thereby to produce a Fischer-Tropsch tail gas, a Fischer-Tropsch reaction water stream and Fischer-Tropsch hydrocarbon condensate. The Fischer-Tropsch tail gas is further typically processed to recover at least some additional hydrocarbon products and water that remain in the Fischer-Tropsch tail gas at the temperature and pressure conditions of the three-phase gas-liquid separator. The additional hydrocarbon products are typically in the range of C3 to C10 hydrocarbons and are thus valuable.
Various approaches have been used commercially to recover the additional hydrocarbon products from the Fischer-Tropsch tail gas exiting the three-phase gas-liquid separator. These approaches include cryogenic processes and hydrocarbon wash processes.
Cryogenic processes typically make use of very low temperatures to separate the hydrocarbons in the Fischer-Tropsch tail gas from other gaseous components in a cryogenic separation step. Certain organic acids and oxygenated species present in Fischer-Tropsch tail gas are harmful to downstream processing operations and must be removed. This is typically accomplished by employing a water wash. Water is problematic in cryogenic processes (which operate well below the freezing point of water) and so the Fischer-Tropsch tail gas must be dried downstream of the washing step, prior to the cryogenic separation step. Cryogenic processes are equipment and capital intensive. They are also high energy consumers and expensive to operate due to the large refrigeration duties required to achieve the low temperatures required for the cryogenic separation of the hydrocarbon products from other gaseous components in the Fischer-Tropsch tail gas.
Hydrocarbon wash processes make use of a sponge oil or lean oil as an absorption medium. The sponge oil is contacted with Fischer-Tropsch tail gas so that hydrocarbons present in the Fischer-Tropsch tail gas are absorbed into the sponge oil to form a hydrocarbon rich sponge oil. Some of the other gaseous components in the Fischer-Tropsch tail gas are also absorbed into the sponge oil, in particular carbon dioxide. This is undesirable as carbon dioxide is an unwanted species in the downstream processing of the hydrocarbon rich sponge oil. It is therefore necessary to remove absorbed carbon dioxide from the hydrocarbon rich sponge oil. This is typically done by stripping any dissolved carbon dioxide from the hydrocarbon rich sponge oil. Some of the valuable hydrocarbons in the hydrocarbon rich sponge oil may be undesirably lost in the carbon dioxide stripping process.
Conventional hydrocarbon wash processes thus have the advantage that they avoid the low temperatures required to separate valuable hydrocarbon components from Fischer-Tropsch tail gas in cryogenic processes. They are also less complex than cryogenic processes and therefore less capital intensive to construct and less expensive to operate. However, they are not as effective in separating the valuable hydrocarbons from the Fischer-Tropsch tail gas. Consequently, conventional hydrocarbon wash processes do not achieve as high hydrocarbon recoveries as cryogenic processes.
A hydrocarbon wash process for the recovery of hydrocarbons from a Fischer-Tropsch tail gas that at least ameliorates the shortcomings of the hydrocarbon wash processes of the prior art would thus be desirable and beneficial.
In this specification, the term “Fischer-Tropsch tail gas” is intended to mean a gaseous stream derived from the gaseous overhead product of a Fischer-Tropsch synthesis reactor.
Also, in this specification, the term “lean oil” is intended to mean an oil used to absorb hydrocarbons in the range of from about C3 hydrocarbons to about C10 hydrocarbons from a gaseous stream.
According to the invention, there is provided 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;
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.
The hydrocarbon rich Fischer-Tropsch tail gas may be provided at a pressure P1, the compressed hydrocarbon rich Fischer-Tropsch tail gas may be at a pressure P2 which is greater than the pressure P1, and the compressed hydrocarbon rich Fischer-Tropsch tail gas may be contacted with the lean oil at a pressure which is greater than the pressure P1.
Typically, the process includes stripping carbon dioxide from the hydrocarbon rich oil in a stripping column or desorber thereby to provide the stripped hydrocarbon oil product.
The hydrocarbon rich oil may contain hydrocarbons predominantly in the range of from about C3 to about C30 hydrocarbons.
In this specification, where a stream is described as containing hydrocarbons “predominantly in the range”, it is intended to mean that the hydrocarbons in the range described constitute at least 60 wt %, preferably at least 70 wt %, more preferably at least 80 wt % of the hydrocarbons in the stream.
In addition to hydrocarbons and carbon dioxide, the gaseous overhead product of a Fischer-Tropsch synthesis reactor, and hence the hydrocarbon rich Fischer-Tropsch tail gas, may include carbon monoxide, hydrogen, methane and water. The hydrocarbons in the gaseous overhead product of a Fischer-Tropsch synthesis reactor may be predominantly in the range of from about C2 to about C30 hydrocarbons.
The hydrocarbons in the hydrocarbon rich Fischer-Tropsch tail gas may be predominantly in the range of from about C2 to about C10 hydrocarbons, typically in the range of from about C3 to about C10 hydrocarbons.
Hydrocarbon rich Fischer-Tropsch tail gas is obtained or derived from the gaseous overhead product of a Fischer-Tropsch synthesis reactor, after at least one unit operation has been conducted on the gaseous overhead product of a Fischer-Tropsch synthesis reactor. Thus, typically, hydrocarbon rich Fischer-Tropsch tail gas is obtained or derived from the gaseous overhead product of a Fischer-Tropsch synthesis reactor by cooling the gaseous overhead product at least once (e.g. to about 70° C.) and by removing reaction water and Fischer-Tropsch condensate at least once from the gaseous overhead product, thereby providing a Fischer-Tropsch tail gas which may be used as a hydrocarbon rich Fischer-Tropsch tail gas.
If desired, hydrocarbon rich Fischer-Tropsch tail gas may however be provided by cooling the Fischer-Tropsch tail gas again at least once, and again removing water, i.e. so-called dirty water, and condensate.
The process of the invention may thus include cooling the gaseous overhead product of a Fischer-Tropsch synthesis reactor and separating the cooled gaseous overhead product into a Fischer-Tropsch tail gas and Fischer-Tropsch condensate. Typically, also reaction water is separated from the cooled gaseous overhead product. The Fischer-Tropsch tail gas thus obtained may be the hydrocarbon rich Fischer-Tropsch tail gas which is compressed.
Preferably, the process of the invention includes further cooling the Fischer-Tropsch tail gas and separating the cooled Fischer-Tropsch tail gas into the hydrocarbon rich Fischer-Tropsch tail gas and a first hydrocarbon condensate in a first separator prior to compressing the hydrocarbon rich Fischer-Tropsch tail gas. A first dirty water stream may also be withdrawn from the first separator.
The hydrocarbon rich Fischer-Tropsch tail gas may be provided at a pressure in the range of from about 10 bar(g) to about 40 bar(g), preferably in the range of from about 15 bar(g) to about 35 bar(g), e.g. about 20 bar(g).
The difference in pressure between the compressed hydrocarbon rich Fischer-Tropsch tail gas and the hydrocarbon rich Fischer-Tropsch tail gas may be in the range of from about 1 bar(g) to about 50 bar(g), preferably in the range of from about 10 bar(g) to about 40 bar(g), e.g. about 30 bar(g).
The hydrocarbon rich Fischer-Tropsch tail gas may thus be compressed to a pressure in the range of from about 40 bar(g) to about 60 bar(g), preferably in the range of from about 45 bar(g) to about 55 bar(g), e.g. about 50 bar(g).
As will be appreciated, compressing the hydrocarbon rich Fischer-Tropsch tail gas will increase the temperature of the compressed hydrocarbon rich Fischer-Tropsch tail gas, e.g. to a temperature in excess of 100° C.
Following compression, the Fischer-Tropsch tail gas may be cooled before separating the cooled Fischer-Tropsch tail gas into the hydrocarbon rich Fischer-Tropsch tail gas and the first hydrocarbon condensate in the first separator. The Fischer-Tropsch tail gas may be cooled to a temperature in the range of from about 15° C. to 60° C., more preferably 30° C. to 50° C., e.g. about 45° C. Typically, the Fischer-Tropsch tail gas is further cooled in a heat exchanger by indirect heat exchange against cooling water.
The hydrocarbon rich Fischer-Tropsch tail gas may have a carbon dioxide content in the range of from about 1 vol % to about 50 vol %, typically in the range of from about 20 vol % to about 40 vol %, e.g. about 32 vol %.
The first hydrocarbon condensate may contain hydrocarbons predominantly in the range of from about C4 to about C12 hydrocarbons.
The process may include cooling the compressed hydrocarbon rich Fischer-Tropsch tail gas prior to contacting the compressed hydrocarbon rich Fischer-Tropsch tail gas with the lean oil. The compressed hydrocarbon rich Fischer-Tropsch tail gas may be cooled to a temperature of less than about 40° C., typically in the range of from about 0° C. to about 10° C., e.g. about 5° C.
The compressed hydrocarbon rich Fischer-Tropsch tail gas may be cooled by indirect heat exchange against cooling water or chilled cooling water, by indirect heat exchange in a feed/product heat exchanger against a process stream, by indirect heat exchange against a refrigerant, e.g. propane or propylene, or by any combination thereof.
The process may further include separating a second hydrocarbon condensate and a second dirty water stream from the compressed, cooled hydrocarbon rich Fischer-Tropsch tail gas in a second separator, prior to contacting the compressed, cooled hydrocarbon rich Fischer-Tropsch tail gas with the lean oil.
The second dirty water stream may be combined with the first dirty water stream.
The compressed hydrocarbon rich Fischer-Tropsch tail gas, downstream of the second separator, may contain hydrocarbons predominantly in the range of from about C3 hydrocarbons to about C5 hydrocarbons.
The second hydrocarbon condensate stream may contain hydrocarbons predominantly in the range of from about C3 hydrocarbons to about C9 hydrocarbons.
The compressed hydrocarbon rich Fischer-Tropsch tail gas may be contacted with the lean oil in a sponge oil column, i.e. in an absorber.
The sponge oil column may be a packed column or a trayed column. Preferably, the sponge oil column is a trayed column.
The sponge oil column may be operated at an operating pressure in excess of about 40 bar(g), typically in the range of from about 40 bar(g) to about 60 bar(g), preferably in the range of from about 45 bar(g) to about 55 bar(g), e.g. about 46 bar(g).
The sponge oil column may be operated at an operating temperature of less than about 20° C., typically in the range of from about 0° C. to about 10° C., e.g. about 8° C., measured at the top thereof.
An overhead gas stream may be recovered from the sponge oil column. The overhead gas stream may be divided into a fuel gas and a recycle gas.
The pressure of the fuel gas may be reduced by passing the fuel gas through a turbo-expander.
The recycle gas may be recycled to a synthesis gas generation unit, such as a reformer, a gasifier or a partial oxidation (POX) unit. Preferably, the recycle gas is recycled to an autothermal reformer.
The synthesis gas generation unit may generate a synthesis gas for use in the Fischer-Tropsch synthesis reactor which provides the hydrocarbon rich Fischer-Tropsch tail gas.
If desired, the fuel gas and the recycle gas may however be provided by cooling the overhead gas steam from the sponge oil column and removing condensate.
Additional hydrocarbons may be recovered from the overhead gas steam from the sponge oil column by further cooling the overhead gas steam from the sponge oil column and removing condensate.
The process may thus include cooling the overhead gas stream from the sponge oil column by indirect heat exchange with a refrigerant, e.g. propane or propylene. The overhead gas stream may be cooled to a temperature of less than about below 40° C., typically in the range of from about 0° C. to about 10° C., e.g. about 5° C. The cooled overhead gas stream may be separated into a chilled overhead gas stream and a chilled reflux stream.
The chilled overhead gas stream may be the process stream used to cool the compressed hydrocarbon rich Fischer-Tropsch tail gas, e.g. by indirect heat exchange in a feed/effluent heat exchanger as hereinbefore described.
The chilled overhead gas stream may be divided into the fuel gas and the recycle gas. Typically, the chilled overhead gas stream is divided only after having been used as a coolant to cool the compressed hydrocarbon rich Fischer-Tropsch tail gas.
The process may thus include 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.
The process may include increasing the pressure of the lean oil before contacting the compressed hydrocarbon rich Fischer-Tropsch tail gas with the lean oil. The pressure of the lean oil may be increased, e.g. by pumping. The pressure of the lean oil may be increased to a pressure in excess of about 40 bar(g), typically in the range of from about 40 bar(g) to about 60 bar(g), preferably in the range of from about 45 bar(g) to about 55 bar(g), e.g. about 50 bar(g).
The process may include cooling the lean oil prior to contacting the compressed hydrocarbon rich Fischer-Tropsch tail gas with the lean oil. The lean oil may be cooled to a temperature of less than about 40° C., typically in the range of from about 0° C. to about 10° C., e.g. about 5° C.
The lean oil may be cooled by indirect heat exchange against cooling water or chilled cooling water, or by indirect heat exchange against a refrigerant, e.g. propane or propylene, or by a combination thereof.
The lean oil may contain hydrocarbons predominantly in the range of from about C5 to about C30 hydrocarbons.
The lean oil may be, or may include, a Fischer-Tropsch hydrocarbon condensate.
The lean oil may be a mixture of an externally sourced hydrocarbon condensate and Fischer-Tropsch hydrocarbon condensate obtained from the Fischer-Tropsch tail gas. The externally sourced hydrocarbon condensate may be an oil stream from a product work-up unit treating a Fischer-Tropsch synthetic crude.
The second hydrocarbon rich condensate, or at least a portion of the second hydrocarbon rich condensate, may be fed to the sponge oil column to form part of the lean oil. Alternatively, or in addition, the first hydrocarbon rich condensate, or at least a portion of the first hydrocarbon rich condensate, may be fed to the sponge oil column to form part of the lean oil.
As set out hereinbefore, carbon dioxide may be stripped or removed from the hydrocarbon rich oil in a stripping column or desorber, i.e. a carbon dioxide stripper, providing stripped hydrocarbon rich oil. The carbon dioxide stripper may be a packed column or a trayed column. Preferably, the carbon dioxide stripper is a trayed column.
The process may include a pressure-reducing step between the sponge oil column and the stripper.
The carbon dioxide stripper may be operated at a pressure below 40 bar(g), preferably below 35 bar(g), more preferably in the range of from about 10 bar(g) to about 30 bar(g), e.g. about 20 bar(g).
The carbon dioxide may be 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.
A carbon dioxide-rich overhead stream may be withdrawn from the carbon dioxide stripper. The carbon dioxide-rich overhead stream may contain about 25 vol % to about 99 vol % carbon dioxide, preferably about 50 vol % to about 90 vol % carbon dioxide, e.g. about 77 vol % carbon dioxide.
The carbon dioxide-rich overhead stream may contain hydrocarbons predominantly in the range of from about C1 to about C3 hydrocarbons. The carbon dioxide-rich overhead stream may also contain hydrogen and carbon monoxide, as well as water and hydrocarbons heavier than C3 hydrocarbons, e.g. C4 and C5 hydrocarbons.
The carbon dioxide-rich overhead stream may be withdrawn from the carbon dioxide stripper at a temperature in the range of from about 15° C. to about 35° C., preferably in the range of from about 20° C. to about 30° C., e.g. about 25° C.
The carbon dioxide-rich overhead stream may be divided into an off-gas stream and a carbon dioxide-rich recycle stream. As set out hereinbefore, the carbon dioxide-rich recycle stream may contain hydrocarbons predominantly in the range of from about C1 to about C5 hydrocarbons. These hydrocarbons are valuable and it is desirable to recover them. The carbon-dioxide rich recycle stream may thus be mixed with the hydrocarbon rich Fischer-Tropsch tail gas prior to the compression of the hydrocarbon rich Fischer-Tropsch tail gas.
Instead or in addition, the carbon dioxide-rich overhead stream, or at least a portion of the carbon dioxide-rich overhead stream, may be recycled to the synthesis gas generation unit.
The off-gas stream may be purged, e.g. to a flare system or to atmosphere at a safe location.
Stripped hydrocarbon rich oil may be withdrawn from the carbon dioxide stripper. The stripped hydrocarbon rich oil may be at a temperature in the range of from about 150° C. to about 300° C., preferably in the range of from about 200° C. to about 265° C., e.g. about 212° C.
A first portion of the stripped hydrocarbon rich oil may be heated in a reboiler and returned the carbon dioxide stripper. A second portion of the stripped hydrocarbon rich oil may be withdrawn as the stripped hydrocarbon oil product.
The stripped hydrocarbon oil product may contain hydrocarbons predominantly in the range of from about C3 to about C30 hydrocarbons.
The process may have a C3 hydrocarbon recovery in excess of 10%, preferably in excess of 20%, more preferably in excess of 30%, e.g. about 37%. The C3 hydrocarbon recovery may be measured as a ratio of a mass/time unit of C3 hydrocarbons in the stripped hydrocarbon oil product to a mass/time unit of C3 hydrocarbons in the gaseous overhead product of the Fischer-Tropsch synthesis reactor, expressed as a percentage.
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.
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.
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.
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.
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.
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.
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.
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.
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).
A second hydrocarbon rich tail gas 52 is withdrawn from the second separator 16, as well as a second hydrocarbon rich condensate 54.
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.
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.
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. C3 to C7 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.
To improve and enhance the recovery of C3 to C7 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.
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).
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.
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. C2 to C6 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.
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.
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.
The process 10 as exemplified has the advantage that hydrocarbon recovery, specifically C3 hydrocarbon recovery, is maximised by ensuring a maximum amount of C3 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 C3 hydrocarbons to the off-gas 82 withdrawn from the carbon dioxide stripper 20.
The process 10, as exemplified, enables hydrocarbons (C3, C4 and C5 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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016/07944 | Nov 2016 | ZA | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2017/056341 | 10/13/2017 | WO | 00 |
