Patent Grant
12135165
This application is a 371 of International Application No. PCT/FR2020/051319, filed Jul. 21, 2020, which claims priority to French Patent Application No. 1908807, filed Aug. 1, 2019, the entire contents of which are incorporated herein by reference.
The present invention relates to a method for liquefying a hydrocarbon stream, such as natural gas, said method using a mixed two-phase cooling stream that vaporizes against the hydrocarbon stream to be liquefied in a heat exchanger of the plate and fin type.
It is desirable for natural gas to be liquefied for a number of reasons. By way of an example, natural gas can be stored and transported over long distances more easily in the liquid state than in the gaseous state, since it occupies a smaller volume for a given mass and does not need to be stored at high pressure.
Several methods exist for liquefying a natural gas stream in order to obtain liquefied natural gas (LNG). Typically, a cooling stream, generally a mixture of several constituent elements, such as a mixture containing hydrocarbons, is compressed by a compressor, then introduced into an exchanger, where it is completely liquefied and sub-cooled to the coldest temperature of the method, typically that of the liquefied natural gas stream. At the coldest outlet of the exchanger, the cooling stream is expanded, forming a liquid phase and a gaseous phase. These two phases are remixed and reintroduced into the exchanger. The cooling stream introduced into the exchanger in the two-phase state is vaporized therein against the hydrocarbon stream that liquefies. Documents WO-A-2017/081374 and US 2007-A-0227185 describe such known methods.
The use of aluminum brazed plate and fin heat exchangers allows highly compact devices to be obtained that provide a large exchange surface area, which improves the energy performance capabilities of the liquefaction method described above.
These exchangers comprise a stack of plates that extend in two dimensions, lengthwise and widthwise, thus forming a stack of several series of passages, with some being intended for circulating a heat-transfer fluid, in this case the hydrocarbon stream to be liquefied, and others being intended for circulating a refrigerant, in this case the two-phase cooling stream to be vaporized.
Heat exchange structures, such as heat exchange waves, are generally disposed in the passages of the exchanger. These structures comprise fins that extend between the plates of the exchanger and allow the heat exchange surface area of the exchanger to be increased. Conventionally, these heat exchange structures have uniform properties and structures along the passages of the exchanger.
However, certain problems continue to arise with the known liquefaction methods, in particular due to the two-phase composition of the cooling stream reintroduced into the exchanger, and in particular when its vaporization occurs in an upward vertical flow.
Indeed, the two-phase cooling stream is introduced at the cold end of the exchanger, i.e. the end where a fluid is introduced with the lowest temperature of the temperatures of the exchanger, located at the lower end of the exchanger. The partial vaporization rate (or “flash”) is very low. As the cooling stream flows through the passages of the exchanger toward the upper end forming the hot end, the rate of partial vaporization, and therefore the amount of gas contained in the cooling stream, increases.
However, gas needs to be present in order to entrain the liquid phase of the cooling stream in order to compensate for the effect of gravity. As the amount of gas is lower at the cold end of the exchanger, entraining the liquid with the gas is more difficult. Therefore, the flow rate of the cooling stream is lower at the cold end and then increases toward the upper end of the exchanger, as the cooling stream is vaporized. This results in an inhomogeneous distribution of the cooling stream along the length of the exchanger.
In order to overcome the shortage of gas at the cold end, a known solution involves reducing the cross-section of the exchanger. The cross-section available for circulating the cooling stream is reduced, which allows the volume flow and the flow rate of the cooling stream to be increased at the cold end.
However, this solution results in a major disadvantage. Indeed, the cross-section of the exchanger is designed by considering the cold end, where the flow rate of the cooling stream is the lowest. However, this speed continues to increase along the flow path of the cooling stream, as the amount of gas increases, which leads to an excessively high level of pressure drops at the hot end, due to the reduced cross-section of the exchanger. This results in a degradation of the energy performance capabilities of the method.
The aim of the present invention is to overcome all or some of the aforementioned problems, in particular by proposing a method for liquefying a hydrocarbon stream against a two-phase cooling stream, said method using a heat exchanger ensuring more homogeneous distribution of said cooling stream in the length of the exchanger.
The solution according to the invention then involves a method for liquefying a hydrocarbon stream, such as natural gas, using a heat exchanger comprising a plurality of plates parallel to each other and to a longitudinal direction that is substantially vertical, said exchanger having a length measured in the longitudinal direction, the plates being stacked with spacing so as to define between them at least one first series of passages for the flow of at least part of a two-phase cooling stream vaporizing by exchanging heat with at least the hydrocarbon stream, said method comprising the following steps:
As appropriate, the invention can comprise one or more of the following feature(s):
Preferably, the refrigerants flow upward in the longitudinal direction in the respective additional refrigerant passages of the heat exchanger.
More preferably, at least one additional refrigerant passage comprises at least one additional heat exchange structure comprising a plurality of additional series of fluid guiding walls, said series following each other in the longitudinal direction and having additional leading edges extending orthogonally to the longitudinal direction so as to fully or partly face the two-phase refrigerants, said additional heat exchange structure having a cross-sectional area of leading edges that decreases in the longitudinal direction.
The expression “natural gas” relates to any composition containing hydrocarbons, at least including methane. This comprises a “crude” composition (prior to any treatment or scrubbing) and also any composition which has been partially, substantially or completely treated for the reduction and/or removal of one or more compounds, including, but without being limited thereto, sulfur, carbon dioxide, water, mercury and certain heavy and aromatic hydrocarbons.
The present invention will now be better understood by virtue of the following description, which is provided solely by way of a non-limiting example and with reference to the accompanying figures, in which:
Preferably, the natural gas stream comprises, as a mole fraction, at least 60% methane, preferably at least 80%.
The natural gas 102 can be fractionated, i.e. part of the C2+ hydrocarbons containing at least two carbon atoms is separated from the natural gas using a device that is known to a person skilled in the art. The collected hydrocarbons C2+ are sent into fractionating columns comprising a de-ethanizer. The light fraction collected at the top of the de-ethanizer can be mixed with natural gas 102. The liquid fraction collected at the bottom of the de-ethanizer is sent to a de-propanizer.
The hydrocarbon stream 102 and the cooling stream 202 enter the exchanger E2, respectively via a third inlet 20 and a first inlet 21, in order to circulate therein in dedicated passages of the exchanger in directions parallel to the longitudinal direction z, which is substantially vertical during operation. These streams exit via a third outlet 22 and a first outlet 23.
Advantageously, the first inlet 21 for the cooling stream 202 and the third inlet 20 for the hydrocarbon stream are arranged so that the cooling stream 202, and optionally the hydrocarbon steam 102, flow co-currently in the downward direction, toward a second end 2b of the exchanger, which is located at a level below the level of a first end 1a of said exchanger. Preferably, the first end 2a corresponds to the hot end of the exchanger E2, i.e. the point of entry of the exchanger where a fluid is introduced at the highest temperature of the exchanger temperatures, in this case the third inlet 20.
Preferably, the hydrocarbon stream 102 is introduced into the heat exchanger E2 in the completely gaseous or partially liquefied state at a temperature ranging between −80 and −35° C.
According to another possibility, the hydrocarbon stream 102 is introduced into the exchanger E2 in the completely liquefied state at a temperature ranging between −130 and −100° C.
The cooling stream 201 exiting the exchanger E2 is expanded by an expansion component T3, such as a turbine, a valve or a combination of a turbine and a valve, so as to form a two-phase cooling stream 203 comprising a liquid phase and a gaseous phase. At least part of the two-phase cooling stream 203 originating from the expansion is reintroduced into the exchanger E2 via at least one second inlet 41 located in the region of the second end 2b and supplying a first series of passages 10 of the exchanger.
Preferably, the second end 2b corresponds to the cold end, which corresponds to a point of entry of the exchanger where a fluid is introduced at the lowest temperature of the exchanger temperatures, in this case the second inlet 41.
It should be noted that, within the scope of the invention, the reintroduction of said at least part of the two-phase cooling stream 203 can be achieved in several ways.
The two phases of the two-phase stream 203 can be separated beforehand in a separator component 27 before being recombined outside the exchanger and reintroduced into the exchanger E2 in the liquid/gas mixture state via the same inlet 41, as shown in
According to an alternative embodiment (not shown), the liquid and gaseous phases can be separately introduced into the exchanger via separate inlets, then mixed together within the exchanger, by means of a mixing device, as described in FR-A-2563620 or WO-A-2018172644, for example. These devices are typically machined parts comprising a particular arrangement of separate channels for a liquid phase and a gaseous phase and of orifices placing these passages in fluid communication in order to dispense a liquid/gas mixture. The two-phase stream 203 is thus entirely or almost entirely reintroduced.
According to another alternative embodiment (not shown), only the liquid phase separated from the two-phase stream 203 is reintroduced via the second inlet 41. This liquid phase forms said part of the two-phase cooling stream 203. The gaseous phase is preferably diverted from the exchanger E2, i.e. it is not introduced therein. It should be noted that the two-phase fluid optionally can be directly reintroduced in the liquid/gas mixture state following expansion.
Preferably, said at least part of the two-phase cooling stream 203 is introduced into the heat exchanger E2 at a first temperature T1 ranging between −120 and −160° C. and exits the heat exchanger E2 at a second temperature T2 higher than the first temperature T1, preferably with T2 ranging between −35 and −130° C.
Said at least part of the two-phase cooling stream 203 flows through the passages 10 in an upward direction and is vaporized by counter-currently refrigerating the natural gas 102 and the cooling stream 202.
The vaporized cooling stream exits the exchanger E2 via a second outlet 42 in order to be compressed by a compressor and then cooled in the indirect heat exchanger by exchanging heat with an external cooling fluid, for example, water or air (in 26 in
In the method described in
Preferably, the cooling stream 202 contains hydrocarbons with a number of carbon atoms of at most 5, preferably at most three, more preferably at most two.
Preferably, the cooling stream 202 is formed, for example, by a mixture of hydrocarbons and nitrogen, such as a mixture of methane, ethane and nitrogen, but can also contain propane, butane, pentane and/or ethylene.
The mole fraction proportions (%) of the components of the cooling stream can be:
Nitrogen: 0% to 10%;
Methane: 30% to 70%;
Ethane: 30% to 70%;
Propane: 0% to 10%;
The natural gas exits the exchanger E2 in the at least partially liquefied state 220 at a temperature that preferably is at least 10° C. higher than the bubble point of the liquefied natural gas produced at atmospheric pressure (the bubble point refers to the temperature at which the first vapor bubbles form in a liquid natural gas at a given pressure) and at a pressure identical to the natural gas inlet pressure, to the nearest pressure drops. For example, the natural gas exits the exchanger E2 at a temperature ranging between −105° C. and −145° C. and at a pressure ranging between 4 MPa and 7 MPa. Under these temperature and pressure conditions, the natural gas does not remain entirely liquid after expansion to atmospheric pressure.
The hydrocarbon stream 102 circulates in a second series of passages (not shown) that are fully or partly arranged alternating and/or adjacent to all or part of the passages 10 of the first series. The flow of fluids in the passages generally occurs parallel to the longitudinal direction z, which is vertical during the operation of the exchanger.
The sealing of the passages 10 along the edges of the plates is generally provided by lateral and longitudinal sealing strips 4 attached to the plates. The lateral sealing strips 4 do not completely seal the passages 10, but leave inlet 41 and outlet 42 openings. The inlets and outlets 41, 42 of the overlaid passages 10 are joined by manifolds 71, 82 used for introducing and discharging the cooling stream 203.
Conventionally, the passages 10 comprise one or more heat exchange structures S disposed between the plates 2. The purpose of these structures is to increase the heat exchange surface area of the exchanger. Specifically, the heat exchange structures are in contact with the fluids circulating in the passages and transfer heat flows by conduction as far as the adjacent plates.
The heat exchange structures also act as spacers between the plates 2, in particular during the assembly of the exchanger by brazing, and to avoid any deformation of the plates when pressurized fluids are being used. They also provide guidance for the flows of fluid in the passages of the exchanger.
For the sake of convenience, it is normal for heat exchange structures of the same type to be arranged along an exchanger passage. For example, when these structures are formed by waves, these have corrugations of the same type, in particular the same corrugation span and therefore the same density of fins, the same thickness, etc.
However, the inventors of the present invention have demonstrated that, with such a configuration, disparities in pressure drops and flow rates appear as the cooling stream flowed along the passages 10, in particular due to the progressive vaporization of said cooling stream.
In order to overcome these problems, the invention proposes arranging, in at least one passage 10 of the first series, a heat exchange structure allowing the pressure drops to be balanced along the length of said passage.
More specifically, at least one passage 10 comprises a heat exchange structure S, the cross-sectional area of leading edges of which decreases in the longitudinal direction z, i.e. toward the first end of the exchanger.
With reference to
According to the invention, the cross-sectional area is measured orthogonally to the longitudinal direction z and per meter of exchanger length. Determining the cross-sectional area A per unit of exchanger length allows a progressive variation of said cross-section to be qualified along the passage 10 and/or allows any differences in length to be overcome when different portions S1, S2, . . . of structures (see below) are considered. For example, in
The arrangement of an exchange structure, for which the cross-sectional area of leading edges decreases toward the first end 2a, allows any disparities in pressure drops experienced by the cooling stream 203 along the passage 10 to be compensated.
Thus, at the second end 2b, where the amount of gas present in the stream is still relatively low, a larger leading edge area per unit length allows the pressure drops and the flow rate to be increased, promoting the ascent of the stream 203. As it flows in the longitudinal z direction, reducing the leading edge area allows the pressure drops experienced by the cooling stream 203 to be reduced.
The exchanger according to the invention allows the pressure drops to be adjusted over the length of the passage and allows a reasonable level of pressure drops to be maintained at the first end 2a. The energy performance capabilities of the industrial installation integrating the exchanger according to the invention are improved.
This also allows high enough fluid flow rates to be provided over the entire length of the passage, in particular at the second end 2b where entraining the liquid phase is critical. This results in a more uniform distribution of the two-phase cooling stream and an improvement in the performance capabilities of the exchanger. The exchanger thus can be designed with reduced safety margins compared to the margins that should be provided in the absence of structures according to the invention.
In addition, the exchanger can operate in steps called reduced steps, i.e. a lower flow rate, whether in a transient operating mode or in a steady state mode.
According to an embodiment that is schematically shown in
According to another embodiment that is schematically shown in
More specifically, said portions each have a constant leading edge surface area, with a reduction of said surface area being obtained by a variation from one portion to another.
Preferably, said portions S1, S2, . . . form separate physical entities, formed from separate strips. For example, the portions S1, S2 are separate wave mats. Advantageously, the structural portions are brazed together in the passage 10. One and/or other of said portions S1, S2 also can be formed by a plurality of separate sub-portions, preferably in the form of wave mats, brazed together in said passage 10.
Preferably, a portion S2 arranged upstream of another portion S1 in the longitudinal direction z, preferably consecutively, has a cross-sectional area A2 of leading edges that is increased by a multiplying factor of at least 1.3, preferably ranging between 1.5 and 5 relative to the cross-sectional area A1 of leading edges of the other portion S1.
Such a multiplying coefficient allows effective balancing of the pressure drops experienced by the two-phase cooling stream, in particular when said stream flows through the other portion S1 with a liquid/gas volume ratio that is lower, by at least 2%, preferably lower by 2 to 20%, than the liquid/gas volume ratio of the stream flowing in the portion S2, which is closer to the second end 2b. In this case, this involves average liquid/gas volume ratios over the lengths of each considered portion.
It should be noted that, preferably, the two-phase cooling stream introduced into the heat exchanger E2 has a liquid/gas volume ratio ranging between 10 and 100%, preferably between 10 and 60%, with said ratio being defined as the ratio between the volume flow rate of the liquid phase and the volume flow rate of the gaseous phase of the two-phase cooling stream.
Advantageously, each series of fluid guiding walls 121, 122, 123, 221, 222, 223 forms a corrugation comprising a plurality of fins 123, 223 following one another in the lateral direction y, with wave peaks 121, 221 and wave troughs 122, 222 alternately connecting said fins 123, 223. The fins preferably follow one another at regular intervals. Said corrugations have pitches p1, p2 defined as the distances between two successive fins of the same corrugation measured in the lateral direction y. To express the pitches p1 and p2 of the corrugations, the relations p1=25.4/n1 and p2=25.4/n2 can be used, where n1 and n2 respectively represent the number of fins 123, 223 per inch, with 1 inch being equal to 25.4 millimeters, of the corrugations, measured in the lateral direction y.
Preferably, the first and second fluid guiding walls extend parallel to the longitudinal direction z. They may also be arranged parallel or orthogonally to the plates 2.
Preferably, said corrugations have increasing pitches in the longitudinal direction z. In other words, the corrugations have a decreasing fin density in the longitudinal direction z.
Alternatively or additionally, said corrugations have wall thicknesses that decrease in the longitudinal direction z.
Increasing the thickness or reducing the pitch of the fins allows the cross-sectional area of leading edges perceived by the two-phase cooling stream to be increased toward the second end of the exchanger, which tends to increase any pressure drops, and therefore the flow rate in this zone.
For example, if a structure S is considered that comprises a first and a second portion S1, S2, with the first portion S1 being arranged downstream of the second portion S2, i.e. further from the second end 2b, the walls of the second portion 221, 222, 223 have a second thickness e2 that is greater than the first thickness e1 of the walls 121, 122, 123 of the first portion S1. The term “downstream” is used by considering the direction of flow of the two-phase stream 203 in the portions S1, S2, . . . .
The first portion S1 can have a first corrugation pitch p1 that is greater than the second corrugation pitch p2 of the second portion S2.
As corrugations for the series of walls of the heat exchange structures S, the various types of waves can be used that are normally implemented in exchangers of the brazed plate and fin type. The waves can be selected from among the known types of wave, such as straight waves, serrated (partially offset) waves or herringbone waves. These waves may be perforated or not perforated.
As can be seen in
In the case of a partially offset wave, the cross-sectional area A2 of the leading edges of the portion S2 corresponds to the sum of the cross-sectional areas A2i, A2i+1, A2i+2, measured orthogonally to the longitudinal direction z and expressed per meter of exchanger length, of the leading edges 224i, 224i+1, 224i+2 of each series of guiding walls.
The above description is applicable to the portion S1 shown in
Within the scope of the invention, the variation of the cross-sectional area of leading edges of the exchange structure S in the longitudinal direction z can be obtained by varying at least one characteristic dimension, such as the thickness, the wave pitch, the serration length, etc., within the structure.
In particular, this variation can occur between portions of structures of the same type. For example, the heat exchange structure S can comprise a plurality of partially offset wave portions, with the serration length increasing toward the first end.
Preferably, a serration length L1 for the other portion S1 will be selected that is greater than the serration length L2 of the portion S2 by a factor ranging between 1.7 and 7, with this being the case for a portion S2 arranged upstream of another portion S1, preferably adjacent thereto. Preferably, the serration lengths can range between 1 and 15 mm, preferably between 3 and 13 mm.
Preferably, the offset distances range between 1 and 20 mm, preferably between 3 and 15 mm.
Preferably, the characteristic dimensions of the waves other than the serration lengths, such as offset distances, thickness, wave pitch, etc., are identical within the exchange structure S.
It is also possible to implement a variation of the type of wave within the structure S in order to balance the pressure drops experienced by the refrigerants on these two portions. For example, arranging one or more portions S2, S3 with a partial offset at the second end and arranging one or more portions S1, S2 as a straight wave at the first end. A straight wave actually introduces fewer leading edges into the passage and thus less pressure drops.
With reference to
where Ly is the width of the passage 10 in which the two-phase cooling stream flows; and
For example, for a partially offset wave S2, called “⅛″ serrated” (1″=1 inch=25.4 mm), L2=25.4/8=3.18 mm. For a partially offset wave S1, called “⅕″ serrated” (1″=1 inch=25.4 mm), L1=25.4/5=5.08 mm.
Advantageously, the method for liquefying a hydrocarbon stream according to the invention can implement one or more additional refrigeration cycles carried out upstream of the main refrigeration cycle described above, so as to pre-cool the hydrocarbon stream.
In this embodiment, the supply stream 110 arrives, for example, at a pressure ranging between 4 MPa and 7 MPa and at a temperature ranging between 30° C. and 60° C. With the supply stream 110 comprising a mixture of hydrocarbons, such as natural gas, the cooling stream 202 and the additional cooling stream 30 enter the exchanger E1 in order to circulate therein in parallel directions and co-currently in the downward direction.
A cooled, or even at least partially liquefied, hydrocarbon stream 102 exits the pre-cooling exchanger E1. Preferably, the hydrocarbon stream 102 exits in the gaseous or partially liquefied state, for example, at a temperature ranging between −35° C. and −70° C. Preferably, the cooling stream 202 exits the exchanger E1 completely condensed, for example, at a temperature ranging between −35° C. and −70° C. The stream 102 is then introduced into the exchanger E2.
As can be seen in
The additional cooling stream can be formed by a mixture of hydrocarbons, such as a mixture of ethane and propane, but also can contain methane, butane and/or pentane. The mole fraction proportions (%) of the components of the first cooling mixture can be:
Ethane: 30% to 70%;
Propane: 30% to 70%;
Butane: 0% to 20%;
In the method described in
In the additional exchanger E1, which is also of the plate and fin type, at least two partial streams originating from the additional cooling stream are withdrawn from the exchanger at two separate outlet points and then expanded to different pressure levels, thus forming the at least one first and one second separate refrigerant fluid F1 and F2 reintroduced into the exchangers via separate inlets 31, 32 selectively supplying additional refrigerant passages in order to be vaporized therein with the supply stream, the cooling stream and part of the additional cooling stream.
In the embodiment according to
The three vaporized refrigerants F1, F2, F3 are sent to different stages of the compressor K1, compressed, then condensed in the condenser C1 by exchanging heat with an external cooling fluid, for example, water or air. The first cooling stream originating from the condenser C1 is sent into the additional exchanger E1 via the pipe 30. The pressure of the first cooling stream at the outlet of the compressor K1 can range between 2 MPa and 6 MPa. The temperature of the additional cooling stream at the outlet of the condenser C1 can range between 10° C. and 45° C.
Preferably, the refrigerants F1, F2, F3 flow from one end 1b of the additional exchanger E1 to another end 1a in the longitudinal direction z, in the upward direction. The end 1b corresponds to the cold end of the additional exchanger E1 where the refrigerant F1 is introduced at the lowest temperature of the temperatures of the additional exchanger E1.
Advantageously, the exchanger E1 comprises at least one of the passages for the flow of refrigerants of the pre-cooling cycle, in which at least one additional heat exchange structure is arranged with a cross-sectional area of leading edges that decreases in the direction z. Said additional heat exchange structure can comprise one or more of the previously described features.
The arrangement of exchange structures with different cross-sectional areas of leading edges allows the pressure drops experienced by the refrigerants along the refrigerant passages to be balanced, while maintaining a reasonable level of pressure drops at the other end 1a of the exchanger E1. The energy performance capabilities of the industrial installation integrating the exchanger E1 are further improved.
Of course, the invention is not limited to the particular examples described and illustrated in the present application. Other alternative forms or embodiments within the competence of a person skilled in the art may also be contemplated without departing from the scope of the invention. For example, other configurations for injecting and extracting fluids into and from the exchanger, other directions of flow of the fluids, other types of fluids, other types of heat exchange structures, etc. clearly can be contemplated, depending on the constraints stipulated by the method to be implemented.
| Number | Date | Country | Kind |
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| 1908807 | Aug 2019 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2020/051319 | 7/21/2020 | WO |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2021/019153 | 2/4/2021 | WO | A |
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| Number | Date | Country | |
|---|---|---|---|
| 20220275997 A1 | Sep 2022 | US |
