Method and system for cooling and separating a hydrocarbon stream

Abstract

The present invention relates to a method of cooling and separating a hydrocarbon stream: (a) passing an hydrocarbon feed stream (7) through a first cooling and separation stage to provide a methane enriched vapour overhead stream (110) and a methane depleted liquid stream (10); (b) passing the methane depleted liquid stream (10) to a fractionation column (200) to obtain a bottom condensate stream (210), a top stream enriched in C1-C2 (220) and a midstream enriched in C3-C4 (230), (c) cooling the upper part of the fractionation column (201) by a condenser (206), (d) obtaining a split stream (112) from the methane enriched vapour overhead stream (110) and obtaining a cooled split stream (112) by expansion-cooling the split stream (112), (e) providing cooling duty to the top of the fractionation column (201) using the cooled split stream (112).

Claims

1. A method of cooling and separating a hydrocarbon stream, the method comprising at least the steps of: (a) passing a hydrocarbon feed stream through a first cooling and separation stage to provide a methane enriched vapor overhead stream and a methane depleted liquid stream; (b) passing the methane depleted liquid stream to a fractionation column to separate the methane depleted liquid stream in a bottom condensate stream, a top stream enriched in C1-C2 and a midstream enriched in C3-C4, (c) cooling the upper part of the fractionation column by a condenser, (d) splitting the methane enriched vapor overhead stream in a main overhead stream and a split stream and obtaining a cooled split stream by expansion-cooling the split stream, (e) feeding a condenser feed stream to the condenser, the condenser feed stream comprising the cooled split stream, to provide cooling duty to the top of the fractionation column, (g) obtaining a condenser outlet stream from the condenser and combining the condenser outlet stream and the top stream enriched in C1-C2 providing a combined stream, and (h) feeding a first feed stream to a second cooling stage, the first feed stream comprising the combined stream, to obtain a further cooled liquefied hydrocarbon stream.

2. Method according to claim 1, wherein obtaining a cooled split stream is done by passing the split stream through an expander or valve to obtain the cooled split stream.

3. Method according to claim 1, wherein the method comprises (f) feeding a second feed stream to the second cooling stage, the second feed stream comprising the main overhead stream to obtain a cooled liquefied hydrocarbon stream.

4. Method according to claim 3, wherein splitting the methane enriched vapor overhead stream in a main overhead stream and a split stream is done upstream of the second cooling stage.

5. Method according to claim 1, the method comprising: (f) feeding a second feed stream to the second cooling stage, the second feed stream comprising the methane enriched vapor overhead stream; wherein splitting the methane enriched vapor overhead stream is done at an intermediate position in the second cooling stage.

6. Method according to claim 1, wherein the main overhead stream and the combined stream are cooled in parallel cooling paths in the second cooling stage.

7. Method according to claim 3 wherein the method comprises (i) combining the cooled liquefied hydrocarbon stream and the further cooled liquefied hydrocarbon stream downstream of the second cooling stage.

8. Method according to claim 1, wherein step (d) comprises controlling a mass flow of the split stream in response to one or more of the following parameters: a temperature indication (T) of the top stream enriched in C1-C2, a temperature indication of the cooled split stream, composition of the top stream enriched in C1-C2.

9. Method according to claim 3, wherein the feed stream to the second cooling stage further comprises the midstream enriched in C3-C4.

10. Method according to claim 1, wherein step (a) comprises (a1) passing the hydrocarbon feed stream through a pre-cooler obtaining a pre-cooled, partially condensed hydrocarbon feed stream, (a2) passing the pre-cooled, partially condensed hydrocarbon feed stream to a first separator to provide at least the methane depleted liquid stream.

11. Method according to claim 10, wherein step (a) further comprises (a3) obtaining a pre-cooled vapor hydrocarbon stream as top stream from the first separator and passing the pre-cooled vapor hydrocarbon stream to a further pre-cooler obtaining a further pre-cooled hydrocarbon feed stream, (a4) passing the further pre-cooled hydrocarbon feed stream to a second separator to provide at least the methane enriched vapor overhead stream.

12. A system for cooling and separating a hydrocarbon stream comprising: a first cooling and separation stage for receiving a hydrocarbon feed stream and generate a methane enriched vapor overhead stream and a methane depleted liquid stream; a fractionation column comprising an inlet arranged to receive the methane depleted liquid stream, a bottom outlet for discharging a bottom condensate stream, a top outlet for discharging a top stream enriched in C1-C2, and a mid-outlet for discharging a midstream enriched in C3-C4, an expansion-cooling device arranged to receive the split stream from the methane enriched vapor overhead stream and generate a cooled split stream by expansion-cooling the split stream, a condenser being positioned in the upper part of the fractionation column to provide cooling duty to the fractionation column, the condenser comprising an inlet for receiving a condenser feed stream, wherein the condenser feed stream comprises the cooled split stream, and an outlet for providing a condenser outlet stream from the condenser; the system being arranged to combine the condenser outlet stream and the top stream enriched in C1-C2 to provide a combined stream, and to feed a first feed stream to a second cooling stage, the first feed stream comprising the combined stream, to obtain a further cooled liquefied hydrocarbon stream.

13. A liquefied natural gas plant or facility including a system according to claim 12.

Description

(1) Embodiments and examples of the present invention will now be described by way of example only and with reference to the accompanying non-limiting drawings in which:

(2) FIG. 1 schematically shows a first embodiment;

(3) FIG. 2 schematically shows a second embodiment; and

(4) FIG. 3 schematically shows a third embodiment.

(5) For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line.

(6) It is suggested to perform NGL extraction using a fractionation column providing a bottom condensate stream, a top stream enriched in C1-C2 and a midstream enriched in C3-C4. This saves plot space, weight and equipment count/CAPEX. The condensate stream is enriched in C5+.

(7) Instead of a series of fractionation columns, each having associated auxiliary equipment including re-boilers, condensers, accumulators, pumps and associated piping, it is proposed to use a single fractionation column, preferably having an internal condenser and internal reboiler, thereby obtaining a significant reduction on hardware elements. This allows for a more compact design with a reduced equipment count and reduced piping and thus with a reduced weight. A reduced weight can result in reduced required foundation and structural steel when the fractionation section is integrated inside a module. It also makes transportation of moveable modules easier and is advantageous for floating applications.

(8) By combining multiple fractionation columns into a single fractionation column having three outlet streams as described above, the temperature of the top of the single fractionation column is typically below ambient temperature and therefore using ambient air or cooling water to provide sufficient condenser duty to the column is not practicable, if not impossible.

(9) By using a split stream of the methane enriched vapour overhead stream 110 to provide cooling duty instead of an ambient (water/air) stream or a refrigerant stream, the equipment count (less auxiliary equipment, less piping) can be further reduced.

(10) A method of and a system for cooling and separating a hydrocarbon stream is provided as for instance shown in FIG. 1.

(11) A hydrocarbon feed stream is passed through a first cooling and separation stage, thereby providing a methane enriched vapour overhead stream 110 and a methane depleted liquid stream 10. This corresponds with (a) as mentioned above.

(12) According to an embodiment (a), being the first cooling and separation stage as defined above comprises

(13) (a1) passing the hydrocarbon feed stream 7 through a pre-cooler 14 obtaining a pre-cooled, partially condensed hydrocarbon feed stream 8,

(14) (a2) passing the pre-cooled, partially condensed hydrocarbon feed stream 8 to a first separator 16 to separate the methane depleted liquid stream 10 from the pre-cooled partially condensed hydrocarbon feed stream 8.

(15) This is schematically shown in FIG. 1, showing a first cooling and separation stage comprising the pre-cooler 14 and the first separator 16. The first separator 16 may be a knock-out drum. The pre-cooler 14 is arranged to receive a hydrocarbon feed stream 7, cool the hydrocarbon feed stream 7 and pass it to the first separator 16. The pre-cooler 14 is arranged to receive a refrigerant stream 100 and discharge a warmed refrigerant stream 100a. The refrigerant cycle, typically comprising a compressor, condenser and expansion valve, is not shown.

(16) The pre-cooler 14 is schematically shown as a single heat exchanger 14, but may comprise a plurality of parallel and/or serial heat exchangers. The pre-cooler 14 may cool the hydrocarbon feed stream using a first refrigerant, which may be a single component refrigerant, such as propane, or a mixed refrigerant, for instance comprising a selection of the following components: propane, ethane/ethylene and butane.

(17) The separator 16 has an overhead outlet from which, in use, a methane enriched vapour overhead stream 110 is obtained and a bottom outlet from which in use a methane depleted liquid stream 10 is obtained.

(18) According to an embodiment, shown in FIG. 2, (a) (being the first cooling and separation stage), further comprises

(19) (a3) obtaining a pre-cooled vapour hydrocarbon stream 9 as top stream from the first separator 16 and passing the pre-cooled vapour hydrocarbon stream 9 to a further pre-cooler 14 obtaining a further pre-cooled hydrocarbon feed stream 9,

(20) (a4) passing the further pre-cooled hydrocarbon feed stream 9 to a second separator 17 to separate the methane enriched vapour overhead stream 110 from the further pre-cooled hydrocarbon feed stream 9.

(21) Both the pre-cooler 14 and the further pre-cooler 14 may be cooled by the same refrigerant. Again, the pre-cooler 14 and further pre-cooler are schematically shown as single heat exchangers, but may each comprise a plurality of parallel and/or serial heat exchangers.

(22) According to this embodiment, the first separator is a scrub column 16, comprising a plurality of trays and may be provided with an (internal) reboiler (not shown).

(23) The second separator 17, which may be a knock-out vessel, may further produce a bottom stream 18 which is passed to the first separator 16, e.g. using a pump 19, and is provided as reflux to the first separator 16. The bottom stream 18 may be introduced in the first separator 16 at a higher level than the pre-cooled, partially condensed hydrocarbon feed stream 8 is introduced.

(24) Both in the embodiment depicted in FIGS. 1 and 2, the methane depleted liquid stream 10 is passed to a fractionation column 200 to separate the methane depleted liquid stream 10 in a bottom condensate stream 210, a top stream 220 enriched in C1-C2 and a midstream 230 enriched in C3-C4.

(25) The top stream 220 is enriched in C1-C2, i.e. enriched in methane, ethane, the mid-stream 230 is enriched in C3-C4, i.e. enriched in propane and butane, and the bottom condensate stream 210 is enriched in components heavier than butane, such as pentane.

(26) The term enriched is used to indicate that the mol % of the indicated component(s) has/have been increased with respect to the mol % of the same component(s) from which the enriched stream was obtained, in this case the methane depleted liquid stream 10.

(27) The flow rate of the methane depleted liquid stream 10 can be controlled by a valve 11.

(28) Fractionation column 201 comprises any suitable mass transfer equipment such as packing or a plurality of internal trays, positioned at different levels in the fractionation column 201. The fractionation column 201 may be provided with a reboiler 207 positioned below the rectifying section, e.g. below the lowest tray. The re-boiler may be an external reboiler, but preferably is an internal reboiler 207 to have a more compact design.

(29) The fractionation column 201 is schematically depicted comprising an internal condenser 206 and internal reboiler 207. It will be understood that the fractionation column 201 may alternatively comprise an external condenser and/or external reboiler.

(30) Furthermore, for all embodiments described, the fractionation column 201 may comprise additional hardware for further optimization of the fraction column 201, i.e. the fractionation column 201 may be a divided wall column or equipped with a side-stripper on stream 230, etc.

(31) The upper part of the fractionation column 201 comprises an internal condenser 206 to provide sufficient cooling duty to the fractionation column 201. The internal condenser 206 is positioned above the rectifying section, e.g. above the upper tray.

(32) The methane enriched vapour overhead stream 110 is split in a split stream 112 and a main overhead stream 111. In the figures this is done at junction 130, which may be a splitting device, such as a T-piece with two independent outlet valves.

(33) The split stream 112 is typically smaller than the main overhead stream 111. The flow rate of the split stream 112 is preferably controlled. This may be done in any suitable manner, including a controllable splitting device.

(34) The split stream 112 may for instance have flow of less than 25 mass % of the main overhead stream 111, or less than 15 mass % of the main overhead stream 111, or less than 10 mass % of the main overhead stream 111, for instance 9 mass % of the main overhead stream 111. The split stream 112 is passed through an expander 113 or a valve to obtain a cooled split stream 112 by expansion-cooling the split stream 112.

(35) The term expansion-cooling is used in this text to refer to an isotropic process, wherein the split stream 112 passes through an (turbo-) expander 113, work is extracted from the split stream 112, and the pressure and temperature of the split stream 112 are lowered, and to an isenthalpic process, wherein the split stream 112 passes through a (throttling) valve (e.g. Joule-Thompson valve), and the pressure and temperature of the split stream 112 are lowered.

(36) Thus, the split stream 112 may be passed over a (throttling) valve or an (turbo) expander 113 to reduce the temperature of the split stream to a temperature sufficiently low to provide cooling duty to the top of the fractionation column.

(37) A valve is less efficient in terms of recovering cold, but is typically more reliable as it doesn't comprise moving/rotating parts.

(38) According to an embodiment obtaining a cooled split stream 112 is done by passing the split stream 112 through an expander 113 or valve to obtain the cooled split stream 112.

(39) Using an expander 113, although being a more complex and expensive piece of hardware than a valve, is preferred as it creates a colder flow than a valve and therefore allows for a minimal flow rate of the split stream 112.

(40) The cooled split stream 112 may typically be cooled to below 60 C., for instance to below 80 C.

(41) According to an example, the split stream 112 is let down in pressure from 52 bara to 8 bara over a turbo-expander 113 and is thereby cooled from 28 C. (stream 112) to 80 C. (stream 112). If instead a JT valve would be used, the same pressure reduction would result in a cooling from 28 C. (stream 112) to 60 C. (stream 112).

(42) Typically, the temperature of the cooled split stream 112 is in the range 60 C. to 120 C. and has a pressure in the range of 8-10 bara. The internal condenser 206 is fed with a condenser feed stream 204. According to the embodiments described, the condenser feed stream 204 comprises the cooled split stream 112. Thereby, cooling duty is provided to the internal condenser 206 and thus to the top of the fractionation column 201 in an efficient manner.

(43) According to an embodiment the method further comprises

(44) (f) feeding a feed stream 231 to a second cooling stage, the feed stream 231 comprising the main overhead stream 111, to obtain a cooled liquefied hydrocarbon stream 225.

(45) FIG. 1 schematically shows the second cooling stage, schematically being represented by a single heat exchanger 115, although it will be understood that any type and number of parallel and/or serial heat exchangers may be used. The heat exchanger 115 of the second cooling stage is arranged to receive a second refrigerant stream 114 and discharge a warmed second refrigerant stream 114a. The refrigerant cycle of the second refrigerant, typically comprising a compressor, condenser and expansion valve or the like, is not shown. The second refrigeration cycle may include cooling the second refrigerant by the first refrigerant.

(46) The second cooling stage may comprise a main cryogenic heat exchanger through which the second refrigerant is cycled, wherein the second refrigerant is split in a light mixed refrigerant and a heavy mixed refrigerant.

(47) The first and second cooling stage may be refrigerated by first and second refrigeration cycles with first and second refrigerants (100/114) respectively, wherein the first refrigerant is heavier than the second refrigerant. The first and second refrigerants may be single component refrigerants and/or mixed refrigerants. The presence of additional cooling stages is of course not excluded.

(48) According to an embodiment, splitting the methane enriched vapour overhead stream 110 in a main overhead stream 111 and a split stream 112 is done upstream of the second cooling stage.

(49) According to an embodiment the method further comprises

(50) (g) obtaining a condenser outlet stream 205 from the condenser 206 and combining the condenser outlet stream 205 and the top stream enriched in C1-C2 220 providing a combined stream 222,

(51) (h) feeding a further feed stream 223 to a second cooling stage, the further feed stream 223 comprising the combined stream 222, to obtain a further cooled liquefied hydrocarbon stream 224.

(52) The pressure of the split stream 112 is preferably reduced when expansion-cooling the split stream to a pressure substantially equal to the pressure of the top stream 220 of the fractionation column 200. The term substantially is used here to indicate that the two streams can be combined without the need of further pressure reduction or pressure increasing devices like valves or pumps.

(53) Combining the condenser outlet stream 205 and the top stream enriched in C1-C2 220 providing a combined stream 222 can be done with any suitable combiner 221, such as a T-piece.

(54) According to an embodiment the main overhead stream 111 and the combined stream 222 are cooled in parallel cooling paths in the second cooling stage.

(55) The main initial overhead stream and the combined stream are cooled in parallel in the second cooling stage, preferably in the same one or more heat exchangers, usually referred to as the main cryogenic heat exchanger(s).

(56) The main initial overhead stream 111 and the combined stream 222 are passed through the second cooling stage at a different pressure. The main overhead stream 111 is passed through the second cooling stage at a first pressure and the combined stream 222 is passed through the second cooling stage at a second pressure, the first pressure being greater than the second pressure. As the initial overhead stream 111 is typically at a higher pressure than the combined stream 222 and cooling can be done most effectively at higher pressures, the streams are not combined prior to the second cooling stage, but are passed through the same heat exchanger(s) 115 of the second cooling stage via different cooling paths, e.g. cooling tubes, running in parallel.

(57) According to an alternative embodiment, the main overhead stream 111 and the combined stream 222 are combined upstream of the second cooling stage. Before combining, the combined stream 222 may be compressed and/or the main overhead stream 111 may be reduced in pressure to match the pressure of the main overhead stream 111 and combined stream 222 prior to combining. Compressing the combined stream 222 may be done with a compressor which can be partially driven by the expander 113. The compressor and the expander 113 may for instance have combined axis.

(58) According to an embodiment the method comprises

(59) (h) combining the cooled liquefied hydrocarbon stream 225 and the further cooled liquefied hydrocarbon stream 224 downstream of the second cooling stage.

(60) Combining may comprise equalizing the pressures of both the cooled liquefied hydrocarbon stream 225 and the further cooled liquefied hydrocarbon stream 224 by passing both streams through a respective valve or expander 301. The cooled liquefied hydrocarbon stream 225 is preferably passed through an expander 301 and the further cooled liquefied hydrocarbon stream 224 is preferably passed through a Joule-Thompson valve 301.

(61) Typically, the cooled liquefied hydrocarbon stream 225 and the further cooled liquefied hydrocarbon stream 224 are let down to 3 bar above the bubble point of the streams (e.g. 4-5 bara).

(62) The combined stream 303 is then fed to a LNG storage tank 302 or via an end-flash vessel to a LNG storage tank (not shown).

(63) According to an embodiment (d) comprises controlling a mass flow of the split stream 112 in response to one or more of the following parameters, but not limited to: a temperature indication (T) of the top stream enriched in C1-C2, a temperature indication of the cooled split stream 112, composition of the top stream enriched in C1-C2.

(64) The mass flow of the split stream 112 may be controlled in any suitable manner, such as by controlling the settings of two outlet valves (not shown) on the T-piece 130 or one valve (not shown) downstream junction 130 in conduit 112.

(65) The temperature indication may be a measured temperature of the top stream enriched in C1-C2 obtained by a direct temperature measurement or a temperature indication obtained from a temperature measurement at a top tray of the fractionation column 200 or a tray at the top part 201 of the fractionation column 200.

(66) A temperature controller 131 is provided which controls the mass flow of the split stream 112 based on a received temperature indication (T). According to the example shown in FIGS. 1 and 2, the temperature indication (T) is obtained from the top stream enriched in C1-C2.

(67) The midstream 230 enriched in C3 and C4 may be passed to a storage tank (not shown) to be sold separately or to be used as refrigerant make-up.

(68) According to an embodiment the feed stream 231 to the second cooling stage further comprises the midstream enriched in C3-C4 230.

(69) The method according to this embodiment thus comprises combining the midstream enriched in C3-C4 and the main overhead stream 111 obtaining a combined stream, wherein the feed stream to the second cooling stage comprises the combined stream. Combining may be done by any suitable combiner 150, such as a T-piece.

(70) FIG. 3 schematically depicts an alternative embodiment, in which splitting the methane enriched vapour overhead stream 110 in a main overhead stream 111 and a split stream 112 is not done upstream of the second cooling stage, as depicted in FIGS. 1 and 2, but at an intermediate position in the second cooling stage, in particular at an intermediate position of the heat exchanger 115.

(71) The heat exchanger 115 may be a cryogenic main heat exchanger through which light and heavy mixed refrigerant are cycled through parallel (set of) tubes carrying the light and heavy mixed refrigerant respectively. The heat exchanger 115 also comprises a plurality of tubes carrying the stream(s) to be cooled by the light and heavy mixed refrigerants.

(72) The intermediate position may be chosen at a position where the tubes exit the heat exchanger to allow the heavy mixed refrigerant to expand and be reintroduced to the shell side of the heat exchanger 115 to provide cooling. The tubes carrying the light mixed refrigerant re-enter the heat exchanger and exit the heat exchanger at a downstream position (downstream with respect to the intermediate position) to be expanded and reintroduced to the shell side of the heat exchanger 115 to provide cooling.

(73) According to such an embodiment splitting the methane enriched vapour overhead stream 110 in a main overhead stream 111 and a split stream 112 is done at an intermediate position in the second cooling stage.

(74) The second cooling stage, in particular the heat exchanger(s) 115 comprised by the second cooling stage, comprises a second cooling stage inlet 233 for receiving the methane enriched vapour overhead stream 110 and a second cooling stage outlet 232 for discharging the cooled liquefied hydrocarbon stream 225, the intermediate position being at a position in between the second cooling stage inlet and outlet 233, 232.

(75) In FIG. 3, the second cooling stage is shown as a single heat exchanger in which a single refrigerant cooling path 114, 114 is shown.

(76) The split stream 112 is passed to a valve or expander 113 to obtain a cooled split stream 112 and fed as the condenser feed stream 204 to the internal condenser 206.

EXAMPLE

(77) Next, the functioning of the embodiment as shown in FIG. 2 will be described by way of example.

(78) A hydrocarbon feed stream 7 is passed through the first cooling and separation stage to produce a methane depleted liquid stream 10 of 6.1 kg/s, at a temperature of 24.6 C. and a pressure of 8.5 bara, comprising 30 mol % methane, 13 mol % ethane, 16 mol % propane, 24 mol % butane and 17 mol % C5+. The mass flow of methane depleted liquid stream 10 is 6.1 kg/s.

(79) A split stream 112 of 5.2 kg/s is obtained at a temperature of 27.6 C. and a pressure of 51.3 bara, comprising 1 mol % N2, 88 mol % methane, 8 mol % ethane and 3 mol % propane. After having passed expander 113, a cooled split stream 112 is obtained having a temperature of 87.4 C and a pressure of 8.3 bara.

(80) From fractionation column 200, the following streams are obtained: bottom condensate stream 210 of 1.9 kg/s, having a temperature of 124.1 C., a pressure 8.7 bara and comprising approximately 100 mol % C5+; a top stream enriched in C1-C2 220 of 3.2 kg/s, having a temperature of 11.9 C. and a pressure of 8 bar, comprising 42 mol % methane, 18 mol % ethane, 21 mol % propane and 19 mol % butane; midstream enriched in C3-C4 230 of 1 kg/s, having a temperature of 37.3 C. at a pressure of 8.4 bara comprising 4 mol % methane and ethane, 12 mol % propane, 81 mol % butane and 3 mol % of C5+.

(81) The person skilled in the art will understand that the present invention can be carried out in many various ways without departing from the scope of the appended claims.