METHOD AND APPARATUS FOR LIQUEFYING A HYDROCARBON STREAM

The present invention relates to a method and apparatus for liquefying a hydrocarbon stream such as natural gas, in particular in a process for the production of liquefied natural gas . Several methods of liquefying a natural gas stream thereby obtaining liquefied natural gas (LNG) are known. It is desirable to liquefy a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form because it occupies a smaller volume and does not need to be stored at a high pressure .

EP 1 008 823 Bl relates to a process for liquefying a pressurized feed gas, wherein the feed stream and various refrigerant streams are passed through a heat exchanger. The mixed refrigerant exit stream is collected from the heat exchanger as vapour, compressed, cooled, and then flashed to provide a vapour refrigerant and liquid refrigerant which are either combined or separately re- introduced into the heat exchanger. A problem of all the arrangements shown in EP 1 008 823 Bl is that the heat exchanger is not cooling the feed stream with a high heat transfer coefficient characteristic of evaporative cooling along its length, if the refrigerant exit stream is wholly vapour. Thus, there is the problem that there is not maximization of the cooling capacity of the heat exchanger .

DE 199 37 623 Al relates to a process for liquefying a hydrocarbon-rich stream. It comprises carrying out

indirect heat exchanger with coolants, and each outflowing coolant mixture is present as a two-phase stream before compression. At each stage of heat exchange, the two phases of the evaporated outflowing coolant are separated, and then the liquid phase is combined with the re-compressed gaseous phase prior to recycling. For the second and third heat exchangers E2 and E3 in Figure 1, the recombined coolant is cooled through the preceding heat exchangers El and El + E2 respectively.

In DE 199 37 623 Al, such a system is not inefficient where the two phases are at the same temperature when combined, for example +40 ⁰ C for first stage coolant streams 13 and 14 of Figure 1. The recombined first coolant stream 10 is cooled sufficiently by heat exchanger El before use as a coolant stream through the same heat exchanger.

A problem with the arrangement shown in DE 199 37 623 Al comes in the recombining of the streams 24 and 25 after the separator D3, and the third stage coolant streams 35 and 36 after the separator D4. The liquid coolant streams 24 and 35, having been separated from the two-phase streams 23 and 34, will be at the same temperature as the two phase streams 23 and 34 respectively, for example about -40 ⁰ C and about

-100 ⁰ C. However, the separated gaseous streams 25 and 36, after recompression through compressors C3 and C4, will be relatively hot, and even with the water coolers E6 and E7, the temperatures of the streams after C3 and C4 will normally still be higher than for example +40 ⁰ C.

Recombining stream 24 (at -40 ⁰ C) and stream 25 (at about +40 ⁰ C) creates an intermediate temperature stream 20.

Such combining of streams 24 and 25 with a (mismatching) temperature that is significantly different is inefficient .

Moreover, the combined coolant stream 20 requires cooling to bring it closer to -40 ⁰ C or so desired for stream 21 prior to entry into the heat exchanger E2. This cooling is provided by heat exchanger El, which must therefore provide additional work (i.e. remove additional heat) for stream 20 as well as streams 10 and 1. This inefficient situation is even greater for the recombination of streams 35 and 36. Their temperature mismatch on mixing is greater than for streams 24 and 25, and stream 30 requires pre-cooling through both heat exchangers El and E2, requiring both heat exchangers to have an additional duty.

It is an object of the present invention to maximize heat transfer across a heat transfer area in a liquefying heat exchanger, particularly, but not exclusively, in a liquefaction plant involving liquefying apparatus. It is another object of the present invention to reduce energy inefficiencies caused by temperature mismatches when using and/or recombining separated coolant streams .

It is a further object to provide an alternative and more efficient method and apparatus for liquefying natural gas .

One or more of the above or other objects can be achieved by the present invention providing a method of liquefying a hydrocarbon stream such as natural gas from a feed stream comprising at least the steps of:

(a) passing the feed stream against a mixed refrigerant being cycled through a heat exchanger, to

provide an at least partly liquefied hydrocarbon stream having a temperature below -100 ⁰ C;

(b) outflowing the mixed refrigerant as a liquid and vapour outflow refrigerant stream from the heat exchanger;

(c) passing the liquid and vapour outflow refrigerant stream through a first separator to provide a vapour refrigerant stream and a liquid refrigerant stream; (d) recycling without substantial heat exchange the liquid refrigerant stream of step (c) into the heat exchanger of step (a) ;

(e) compressing the vapour refrigerant stream of step (c) to provide a compressed refrigerant stream; (f) cooling the compressed refrigerant stream to provide a cooled compressed stream having a temperature below 0 ⁰ C; and

(g) recycling the cooled compressed stream into the heat exchanger of step (a) . The outflowing mixed refrigerant is in the form of a combination of liquid and vapour: that is, the mixed refrigerant has not been fully vapourised as it outflows from the liquefying heat exchanger. Thus cooling is still being effected by evaporation of the mixed refrigerant along the full length or extent of the cooling passage of the mixed refrigerant through the heat exchanger to the outflow. This increases the heat transfer coefficient along the final extent of heat transfer area or volume for cooling, i.e. the heat transfer surface or surfaces available to the mixed refrigerant in the heat exchanger.

The cooling effected by the heat exchanger provides an at least partly, preferably wholly, liquefied

hydrocarbon stream below -100 ⁰ C, to provide, for example, liquefied natural gas. Such cooling is known in the art for the main cryogenic cooling of hydrocarbon streams such as natural gas. In the present invention, the liquid refrigerant stream separated from the liquid vapour outflow refrigerant stream is directly recycled back into the heat exchanger without requiring heat exchange by passage through another heat exchanger or cooler, although some minimal heat exchange may occur, usually to slightly reduce or help maintain the temperature of the liquid refrigerant stream. Any such heat exchange is minimal, and is not intended to significantly change the temperature of the liquid refrigerant stream. That is, any temperature change of the liquid refrigerant stream between its separation from the liquid and vapour outflow refrigerant stream, and its recycle, either directly or indirectly (for example in combination with another stream) , back into the heat exchanger, should be less than 40 ⁰ C, preferably less than 30 ⁰ C, or less than 20 ⁰ C or even less than 10 ⁰ C. Some minimal heat exchange may also still occur in the liquid refrigerant stream due to its position or arrangement, for example the length or positioning of the piping of the liquid refrigerant stream carrying it into the heat exchanger, and/or its proximity to the other streams or combination with other refrigerant streams . Such minimal heat exchange should again be less than 40 ⁰ C, preferably less than 30 ⁰ C, or less than 20 ⁰ C or even less than 10 ⁰ C (in case there is no heat exchange at all) .

By avoiding any substantial heat exchange at all or having only minimal heat exchange, an advantage of the

present invention is to make the method of liquefying a hydrocarbon stream more efficient, by not mixing or combining or recombining any mismatched 'cold' and 'hot', refrigerant streams together. Such recombining of mismatched streams with a significant different temperature requires extra cooling for use in liquefying hydrocarbon streams, as shown in DE 199 37 62 Al.

WO 2006/007278 A2 shows an arrangement of refrigerant streams that is typical for a plate fin heat exchanger arrangement. Figure 4 of the document shows using two heat exchangers, and the liquid streams from the separators 510A and 520B are collected and pumped to be combined with the compressed vapour streams from the same separators, (to form a recycled mixed stream 402 which is only completely condensed within the first heat exchanger 200 into a liquid stream 404) . However, this has the problem of combining low temperature liquid streams (from the separators) with the much higher temperature (after its compression) vapour stream, which destroys exergy, i.e. the total energy that can be converted to work with respect to a set of reference ambient conditions, due to the temperature mismatch, and so is less efficient, despite any vapour coolers provided. The recycled mixed stream 402 is still a mixed liquid and vapour stream. US 4,112,700 shows an arrangement involving a four heat exchanger pre-cooling of natural gas where the first multicomponent mixture exits the fourth pre-cooling heat exchanger with mixed liquid and vapour phases, but without direct recycling of the liquid phase into the same pre-cooling heat exchanger. Further, there is no consideration of the exergy benefit of minimising

temperature mis-matching of liquid refrigerant streams below 0 ⁰ C into a main cryogenic heat exchanger.

US 4,180,123 shows an arrangement where an admixed two phase flow exits a heat exchanger, but the liquid phase is only recycled back into the heat exchanger after the involvement of two coolers. Again, there is no consideration of the exergy benefit of minimising temperature mis-matching of liquid refrigerant streams below 0 ⁰ C into a main cryogenic heat exchanger. Another advantage of the present invention is that the first liquid refrigerant stream separated from the mixed refrigerant outflow is recycled back into the heat exchanger as at least part of the overall mixed refrigerant, reducing the power input required to effect the cooling of the heat exchanger, thus making the liquefaction process more efficient.

Although the method according to the present invention is applicable to various hydrocarbon feed streams, it is particularly suitable for natural gas streams to be liquefied.

Further the person skilled in the art will readily understand that after liquefaction, the liquefied natural gas may be further processed, if desired. As an example, the obtained LNG may be depressurized by means of a Joule-Thomson valve or by means of a cryogenic turbo- expander. Also, further intermediate processing steps between the gas/liquid separation in the gas/liquid separation vessel and the cooling may be performed.

The hydrocarbon stream may be any suitable gas stream to be treated, but is usually a natural gas stream obtained from natural gas or petroleum reservoirs. As an alternative the natural gas stream may also be obtained

from another source, also including a synthetic source such as a Fischer-Tropsch process.

Usually the natural gas stream is comprised substantially of methane. Preferably the feed stream comprises at least 60 mol% methane, more preferably at least 80 mol% methane.

Depending on the source, the natural gas may contain varying amounts of hydrocarbons heavier than methane such as ethane, propane, butanes and pentanes as well as some aromatic hydrocarbons. The natural gas stream may also contain non-hydrocarbons such as H2O, N2, CO2, H2S and other sulphur compounds, and the like.

If desired, the feed stream containing the natural gas may be pre-treated before feeding it to the main (cryogenic) heat exchanger. This pre-treatment may comprise reduction and/or removal of undesired components such as CO2 and H2S, or other steps such as pre-cooling, pre-pressurizing or the like. As these steps are well known to the person skilled in the art, they are not further discussed here.

The term "natural gas" as used herein relates to any hydrocarbon-containing composition which is at least substantially methane. This includes a composition prior to any treatment, such treatment including cleaning or scrubbing, as well as any composition having been partly, substantially or wholly treated for the reduction and/or removal of one or more compounds or substances, including but not limited to sulfur, carbon dioxide, water, and C2 ^"1" hydrocarbons . The separator may be any vessel, unit, column or arrangement adapted to separate the mixed refrigerant

into a vapour refrigerant stream and a liquid refrigerant stream. Such separators are known in the art and are not further discussed here.

The heat exchanger may be any column, tower, unit or other arrangement adapted to allow the passage of a number of streams therethrough, and to affect direct or indirect heat exchange between one or more lines of refrigerant, and one or more feed streams. Examples include a tube-in-shell heat exchanger or a spool-wound heat exchanger. Preferably, the heat exchanger is a cryogenic spool-wound heat exchanger .

The present invention also provides a method of treating a hydrocarbon stream such as natural gas from a feed stream comprising at least the steps of: (a) passing the feed stream against a mixed refrigerant being cycled through a heat exchanger, to provide a cooled hydrocarbon stream;

(b) outflowing the mixed refrigerant as a liquid and vapour outflow refrigerant stream from the heat exchanger;

(c) passing the liquid and vapour outflow refrigerant stream through a first separator to provide a vapour refrigerant stream and a liquid refrigerant stream; and (d) directly recycling the liquid refrigerant stream of step (c) into the heat exchanger.

The present invention includes a combination of any and all of the methods hereinbefore described.

The present invention further provides apparatus for liquefying a hydrocarbon stream such as natural gas from a feed stream, the apparatus at least comprising:

a heat exchanger for liquefying the hydrocarbon stream against a mixed refrigerant stream, the heat exchanger having a feed inlet for the feed stream, a feed outlet for its at least partly liquefied stream, one or more mixed refrigerant inlets, and a mixed refrigerant outlet for a vapour and liquid refrigerant outflow stream; a first separator for separating the liquid and vapour outflow refrigerant stream into a vapour and a liquid, the first separator having a first outlet to provide a vapour refrigerant stream and a second outlet to provide liquid refrigerant stream; a refrigerant inlet in the heat exchanger to inflow the liquid refrigerant stream into the heat exchanger; a compressor for compressing the vapour refrigerant stream to provide a compressed refrigerant stream; one or more coolers for cooling the compressed refrigerant stream to provide a cooled compressed stream having a temperature below 0 ⁰ C; and a pathway for recycling the cooled compressed stream into the heat exchanger.

An embodiment of the present invention will now be described by way of example only, and with reference to the accompanying non-limiting drawing, in which :- Figure 1 is a diagrammatic scheme of a treatment process according to one embodiment of the present invention .

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. The same reference numbers refer to similar components.

Referring to the drawing, Figure 1 shows a liquefying process for a hydrocarbon feed stream 10. The feed stream 10 may be a pre-treated natural gas stream, wherein one or more substances or compounds, such as sulfur, sulfur compounds, carbon dioxide, and moisture or water, are reduced, preferably wholly or substantially removed, as is known in the art.

Optionally, the feed stream 10 may have undergone one or more pre-cooling stages such as are known in the art. One or more of such pre-cooling stage (s) may involve one or more refrigeration circuits. As an example, a natural gas feed stream is commonly processed from an initial temperature of 30-50 ⁰ C, such as 40 ⁰ C. Following one or more pre-cooling stages, the temperature of the natural gas feed stream can be reduced to -30 to -70 ⁰ C, such as -40 ⁰ C to -50 ⁰ C.

In Figure 1, the heat exchanger 12 is preferably a spool-wound cryogenic heat exchanger adapted to have three lines wholly or partially therethrough. Cryogenic heat exchangers are known in the art, and can have various arrangements of their feed stream (s) and refrigerant stream(s) . Further, such heat exchangers can also have one or more lines running therethrough for the passage of other fluids, such as refrigerant streams for other cooling stages or parts of a treatment, preferably liquefaction plants. Any such other lines or streams are not shown in Figure 1 for simplicity.

The feed stream 10 enters the heat exchanger 12 via a feed inlet 52, passes through the heat exchanger via line 150, and outflows through a feed outlet 54 to provide an at least partly liquefied hydrocarbon stream 20. This liquefied stream 20 is preferably wholly

liquefied, and may be further processed as discussed hereinafter. Where the liquefied stream 20 is liquefied natural gas, an example temperature can be approximately -150 ⁰ C. The liquefying of the feed stream 10 is provided by a refrigerant circuit 160. The refrigerant circuit 160 circulates a mixed refrigerant, preferably being a mixture of gases, more preferably being selected from the group comprising nitrogen, methane, ethane, ethylene, propane, propylene, butane, pentane, etc. The composition of the mixed refrigerant can vary according to the conditions and parameters desired for the heat exchanger 12, as is known in the art.

Many arrangements are known for the inlet, outlet and flow of refrigerant through a heat exchanger to affect cooling of a feed stream. One or more lines of refrigerant through the heat exchanger may also be being cooled themselves by the heat exchanger, rather than affecting cooling on another line or stream. In the arrangement shown in Figure 1, there is a vapour inflow refrigerant stream 30 which passes through first inlet 66 and along line 130 through the heat exchanger 12, before outflowing through first refrigerant outlet 68. In its passage through line 130, the vapour inflow refrigerant stream 30 is cooled and/or liquefied such that the first outflow refrigerant stream 45 is a liquid stream. The first outflow refrigerant stream 45 passes through one or more pressure reducing devices such as a throttling valve 14, so as to provide a first reduced pressure refrigerant stream 50 being both vapour and liquid. This first reduced pressure refrigerant stream 50 re-enters the heat exchanger 12 via inlet 72

and can be passed downwardly through the heat exchanger 12 via a first distribution manifold 34 in a manner known in the art. As the refrigerant stream passes downwardly through the heat exchanger 12 from the first distribution manifold 34, the liquid refrigerant changes in part from liquid to vapour, and so effects cooling of the feed stream line 150 and the vapour refrigerant stream line 130 at a high heat transfer coefficient.

The heat exchanger 12 also has a liquid inflow refrigerant stream 40 which passes into the heat exchanger via inlet 64, and along line 140 in the heat exchanger 12. It outflows the heat exchanger 12 through outlet 74, at an intermediate level between the top and bottom, to provide a second outflow refrigerant stream 60, which passes through an expander 16 to reduce its pressure and form a second reduced pressure refrigerant stream 70, which stream, being both liquid and vapour, passes via inlet 76 back into the heat exchanger 12, and passes down through the heat exchanger 12 via a second distribution manifold 36.

Preferably, the pressures of the first and second reduced pressure refrigerant streams 50 and 70 are essentially the same; that is, any variation in pressure is not significant or does not have an impact on the operation of the heat exchanger 12.

Hitherto, a liquid phase-changing refrigerant stream for recycle in a prior art heat exchanger is allowed to completely change phase to a vapour, and so be collected as a vapour outlet stream. This has created two main areas or zones within the prior art heat exchanger. There is a first zone in which the liquid in the liquid and vapour refrigerant stream(s) is allowed to change phase

to a vapour, and thus effect the cooling of a feed stream, and any refrigerant streams also being cooled. In such a zone, the heat exchanger can be defined as being in a 'wet mode' of operation. As the change of phase finishes, there is a second zone, being the area or volume within the heat exchanger (generally below the first zone) where the refrigerant is wholly vapour. As there is no longer any phase change occurring in the second zone, i.e. it is a single vapour phase, there is a heat transfer area in the heat exchanger which is only providing cooling with a significantly lower heat transfer coefficient. Some cooling may still be effected in the change of temperature of the vapour, but the heat transfer coefficient from a vapour is significantly less than the heat transfer coefficient of a liquid changing into a vapour. Thus, the second zone of a prior art heat exchanger is significantly less efficient in providing any cooling to a feed stream, etc. In Figure 1 of the present invention, the downflowing refrigerant stream in the heat exchanger 12 is outflowed from the heat exchanger 12 via outlet 62 as a liquid and vapour outflow refrigerant stream 80. As this outflow stream 80 still includes liquid refrigerant, there is liquid phase change occurring throughout the heat exchanger 12 from the first and second distribution manifolds 24, 26 to the outlet 62. Thus, there is no or minimal heat transfer area or volume within the heat exchanger 12 wherein the refrigerant is vapour only. That is, there is no area or volume or minimal area or volume where the downflowing refrigerant stream is not providing the most efficient heat transfer (with a high heat

transfer coefficient), and so the most efficient cooling effect on the feed stream line 150 (and other lines) . Thus, the heat transfer area or volume within the heat exchanger 12 for heat exchange with the feed stream 10 is maximized. This provides an increase in the effective heat transfer area, possibly of over 10%, for the same physical heat exchanger size and shape.

The heat exchanger 12 generally operates at a low pressure such as 1 to 10 bar. The temperature at the bottom of the heat exchanger 12 can be in the range -30 ⁰ C to -50 ⁰ C, so that the outflow refrigerant stream 80 is at this same temperature.

The vapour and liquid outflow refrigerant stream 80 is sent to a first liquid/vapour separator 18 via inlet 78, which also operates a low pressure such as 1 to 10 bar. The first separator 18 separates the outflow refrigerant stream 80 in a manner known in the art to provide via a first outlet 82 a vapour refrigerant stream 90, and via a second outlet 84, a liquid refrigerant stream 110, generally having a temperature in the same range of -30 ⁰ C to -50 ⁰ C.

The vapour refrigerant stream 90 can be compressed via any method known in the art involving one or more compressors and one or more coolers. Figure 1 shows as an example a compressor 22 to provide a compressed stream 95, having, by way of example only, a temperature of approximately 75 ⁰ C. This compressed stream 95 is then cooled by a water and/or air cooler 24 and a further cooler 25 to provide a cooled compressed refrigerant stream 100 preferably having a temperature in the range - 30 ⁰ C to -50 ⁰ C.

Optionally, the further cooler 25 also provides at least some of the cooling in any pre-cooling of the feed stream 10 prior to the heat exchanger 12, as hereinbefore described. The cooled compressed refrigerant stream 100 can pass directly back into the heat exchanger 12. Preferably, it passes via inlet 86 into a second separator 26, to provide via a first outlet 88 the vapour inflow refrigerant stream 30, and via a second outlet 92 the liquid refrigerant inflow stream 40. The second separator operates at a relatively higher pressure such as in the range 30 to 60 bar, preferably 40-55 bar.

For its route back into the heat exchanger 12, the liquid refrigerant stream 110 could be combined with the cooled compressed refrigerant stream 100 prior to the second separator 26, or the liquid refrigerant stream 110 could pass directly into the second separator 26, or the liquid refrigerant stream 110 could re-enter the heat exchanger 12 separately to any other refrigerant stream. The liquid refrigerant stream 110 is recycled to the heat exchanger 12 without substantial heat exchange of the liquid refrigerant stream 110 between the outlet 84 of the first liquid/vapour separator 18 and the inlet of the heat exchanger 12. Preferably no heat exchange of the liquid refrigerant stream 110 at all takes place between the outlet 84 of the first liquid/vapour separator 18 and the inlet of the heat exchanger 12.

Preferably, the liquid refrigerant inflow stream 40 and the liquid refrigerant stream 110 (via pump 94) re- enter the heat exchanger 12 as a combined liquid refrigerant stream 120 through inlet 64, by passing through a combiner 28, such as a junction or union.

Preferably, the temperature difference between the liquid refrigerant stream 110 and the cooled compressed refrigerant stream 100, (and so also the liquid inflow refrigerant stream 40, which will be at the same or substantially the same temperature as the cooled compressed refrigerant stream 100), is less than 10 ⁰ C, preferably less than 5°C or even less than 3°C. This close matching of their temperatures minimises any exergy loss required to balance their temperatures prior to their re-entry into the heat exchanger 12.

As an example, the temperature of the liquid and vapour outflow refrigerant stream 80 could be in the range -40 ⁰ C to -50 ⁰ C, such that the liquid refrigerant stream 110 from the first separator 18 is at this approximate temperature or possibly slightly below. Where the temperature range of the liquid inflow refrigerant stream 40 from the second separator 26 is also in the range -40 ⁰ C to -50 ⁰ C, their combination by the combiner 28 to produce the combined liquid refrigerant stream 120 is close such as in the range -45°C to -50 ⁰ C.

This close matching of temperatures also applies to the introduction of the liquid refrigerant stream 110 elsewhere as mentioned above. Liquefied natural gas 20 from a liquefying system can be passed into for further cooling, for example a sub- cooling stage, and/or a final separator wherein vapour can be removed for use as fuel in the plant, for example for the gas turbines running the various compressors, and a liquefied natural gas product can be transferred to a storage vessel or other storage or transportation apparatus .

The final separator could be an end flash system, which can be used at the downstream end of the sub- cooling stage to optimize liquefied natural gas (LNG) production. It usually includes an end compressor driven by a separate electric drive motor. The power needed to drive the end compressor is a usually smaller than the required compressor power for the sub-cooling stage.

An example of the advantage of the present invention is illustrated by the data in Table 1 below. In the first column, Table 1 shows the data for the operation of a main heat exchanger (MCHE) in a prior art reference liquefaction process, with a throughput of around 17.5 kmol/s. The exchanger consists of a warm tube bundle and a cold tube bundle, with effective surface areas (UA) respectively of about 60,000 and 13,000 kW/K. The liquid content of the low pressure refrigerant leaving the shell side of the exchanger is 0%, i.e. the exchanger is operated in a 'dry mode'.

By changing the outflowing refrigerant composition according to the present invention such that the liquid content of the refrigerant leaving the heat exchanger becomes 0.5 mol%, the MCHE exchanger is now operating in a fully 'wet mode'. As a result of the wet mode operation, the heat transfer coefficient U in the lower part of the exchanger is the same across the full height of the heat exchanger. As shown by the second "wet" column in Table 1, this increases the effective surface area (UA) of the warm bundle of the heat exchanger by about 10%, and this provides an increase of production of about 1.6%, for identical refrigerant compression power and physical heat exchanger area. This is a significant increase on an industrial scale.

Table 1. Comparative data

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