METHOD AND APPARATUS FOR LIQUEFYING A HYDROCARBON STREAM

The present invention relates to a method and apparatus for liquefying a hydrocarbon stream, particularly but not exclusively 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 .

US 6,105,389A shows a method and device for liquefying natural gas using a compressed coolant mixture which is sub-cooled, expanded and vapourized, followed by a second cooling stage to liquefy the natural gas. In the first cooling stage, fractions of the first coolant mixture are expanded by expansion valves to cool the coolant mixture. However, the problem with expansion valves is that they are isenthalpic, so that the work which is available from the expansion of a fluid such as a coolant as it is let down in pressure in such a valve, is essentially lost.

It is an object of the present invention to improve the efficiency of a liquefying process and apparatus. 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, the method at least comprising the steps of:

(a) circulating a first refrigerant stream in a first refrigerant circuit;

(b) cooling the first refrigerant stream in one or more heat exchangers of a first cooling stage to provide a cooled first refrigerant stream;

(c) passing at least part of the cooled first refrigerant stream through one or more expanders to provide one or more expanded cooled first refrigerant streams ; (d) passing the or at least one of the expanded cooled first refrigerant streams and the feed stream through the first one or more heat exchangers to provide a cooled hydrocarbon stream;

(e) passing the cooled hydrocarbon stream through a second cooling stage against a second refrigerant stream to provide a liquefied hydrocarbon stream; wherein at least one expander is an expansion turbine whose work energy created in step (c) is used in the first refrigerant circuit. An advantage of the present invention is that by expanding at least a part of the first refrigerant stream through one or more expansion turbines to decrease the pressure of the first refrigerant prior to its use for cooling the feed stream in the heat exchanger ( s ), the expansion of the first refrigerant is near isentropic, which improves the efficiency of the first cooling. The near isentropic expansion of any gas to a lower pressure reduces the internal energy out of the gas during the expansion, and produces at least a part of this energy as useful work. This work energy can then be transferred and usefully employed elsewhere in the liquefying method, process and/or apparatus, such as to help drive another

unit or device or part thereof, such as a compressor, pump, or electric generator.

The hydrocarbon stream may be any suitable gas stream to be liquefied, 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 use. This pre-treatment may comprise reduction and/or removal of undesired components such as CO2 and H2S or other steps such as early 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 "feed stream" as used herein relates to any hydrocarbon-containing composition usually containing a large amount of methane. In addition to methane, natural gas contained various amounts of ethane, propane and heavier hydrocarbons. The composition varies depending upon the type and location of the gas. Hydrocarbons heavier than methane generally need to be removed from natural gas for several reasons, such as having different

freezing or liquefaction temperatures that may cause them to block parts of a methane liquefaction plant. C2-4 hydrocarbons can be used as a source of natural gas liquids . Thus, the term "feed stream" also includes a composition prior to any treatment, such treatment including cleaning, dehydration and/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, sulfur compounds, carbon dioxide, water, and C2+ hydrocarbons.

The term "expander" includes any unit, device or apparatus able to reduce the pressure in a stream. This includes valves as well as expansion turbines, and also includes one or two-phase expanders. Where at least one expander of the heat exchangers of the first cooling stage is an expansion turbine, one or more other expanders may be valves. Preferably, all of the heat exchangers of the first cooling stage are expansion turbines .

The first cooling stage of the present invention is intended to reduce the temperature of the cooled hydrocarbon stream to below 0 ⁰ C, usually in the range - 20 ⁰ C to -70 ⁰ C. Such a cooling stage is sometimes also termed a ^y pre-cooling' stage.

The second cooling stage is preferably separate from the first cooling stage. That is, the second cooling stage comprises one or more separate heat exchangers using a second refrigerant circulating in a second refrigerant circuit, although the refrigerant of the second refrigerant stream may also pass through at least

one heat exchanger of the first cooling stage, preferably all the heat exchangers of the first cooling stage.

Preferably, the first refrigerant circuit comprises one or more ambient coolers prior to the first heat exchanger.

In one embodiment of the present invention, the created work energy is used to increase the pressure of the first refrigerant stream prior to the first heat exchanger. One example of this is to use the work energy to drive one or more pumps circulating the first refrigerant stream.

The use of work energy from one or more expansion turbines in a first cooling stage (designed to reduce the temperature of a hydrocarbon stream to below 0 ⁰ C) , is particularly advantageous where the temperature of the associated refrigerant stream is not intended to be significantly low, such as -50 ^° C or -100 ^° C, prior to expansion. In this way, the increase in temperature in the refrigerant stream caused by its passage through one or more pumps, (where its increase in pressure causes an increase in temperature), can be more easily extracted by the use of one or more ambient coolers such as water and/or air coolers known in the art. The use of ambient coolers to reduce the temperature of a refrigerant stream intended to cool a hydrocarbon stream to a much lower temperature, such as below -100 ⁰ C, would be clearly insufficient .

Thus, the present invention is particularly advantageous where the work energy provided by each expansion turbine is used in a first cooling or pre- cooling stage, as no additional cooling is required in a first refrigerant circuit which already involves one or more ambient coolers prior to the heat exchanger (s) . In

this way, there is maximum usage of the work energy or exergy created, or at least minimum energy wastage, in the overall method of liquefaction.

In another embodiment of the present invention, the first cooling stage comprises two or three heat exchangers, and preferably, each heat exchanger has an associated expansion turbine through which at least a part of the first cooled refrigerant stream passes to provide an expanded cooled first refrigerant stream to its respective heat exchanger.

In a further aspect, the present invention provides apparatus for liquefying a hydrocarbon stream such as natural gas stream from a feed stream, the apparatus at least comprising: a first refrigerant circuit circulating a first refrigerant stream; a first cooling stage having one or more heat exchangers to receive the first refrigerant stream and to provide a cooled first refrigerant stream; one or more expanders to expand at least a part of the cooled first refrigerant stream to provide one or more expanded first refrigerant streams, at least one expander being an expansion turbine whose work energy created by the expansion is used in the first refrigerant circuit; the or each heat exchanger having a first inlet to pass the feed stream thereinto, and a second inlet to pass the or at least one of the expanded first refrigerant streams thereinto to cool the feed stream and so provide a cooled hydrocarbon stream; and a second cooling stage comprising one or more heat exchangers arranged to receive the cooled hydrocarbon

stream from the first cooling stage and to provide a liquefied hydrocarbon stream.

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 general scheme of a liquefying 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. Same reference numbers refer to similar components.

Figure 1 shows a general scheme for a liquefying a hydrocarbon stream such as natural gas . It shows an initial feed stream containing natural gas 10, which feed stream may be pre-treated to separate out at least some heavier hydrocarbons and impurities such as carbon dioxide, nitrogen, helium, water, sulfur and sulfur compounds, including but not limited to acid gases.

The feed stream 10 passes through a first cooling stage 100 using a first refrigerant being cycled in a first cooling refrigerant circuit 110, thereby obtaining a cooled hydrocarbon stream 40b. The first refrigerant may be any suitable component or, preferably, a mixture thereof, including one or more, preferably two or more, of nitrogen, methane, ethane, ethylene, propane, propylene, butane, pentane, etc.

The feed stream 10 passes into a first heat exchanger 14a through inlet 32a and passes therethrough as line 73a. Meanwhile, the first refrigerant passes into the first heat exchanger 14a through inlet 36a and passes through the first heat exchanger 14a as line 71a. In this way, the first refrigerant line 71a is also cooled in the first heat exchanger 14a.

Outflowing the first heat exchanger 14a through outlet 38a is a cooled first refrigerant stream 20a. By means of a suitable divider (not shown), such as split piping, there is created first and second fractions or parts 21a, 21b respectively of the cooled first refrigerant stream 20a. The first part 21a is directed towards a first expansion turbine 12a, which expands the first cooled first refrigerant fraction in a near isentropic manner to provide an expanded cooled first refrigerant stream 30a. The expanded stream 30a has an internal energy that is lower than the internal energy of stream 21a.

The difference in energy can be used as work, e.g. to drive a unit or device in the first refrigerant circuit 110. In the arrangement shown in Figure 1, the energy provided by the first expansion turbine 12a is used to drive a first pump 28a which is part of the first refrigeration circuit 110, and whose function is described hereinafter. The ratio of the first and second parts 21a, 21b of the cooled first refrigerant stream 20a in Figure 1 can be any suitable ratio known in the art. Where the first cooling stage 100 has two heat exchangers 14a, 14b, and uses a mixed refrigerant, an example ratio for two pressure levels is 35/65. An example of possible pressure levels in the heat exchangers 14a, 14b is 8 bar for a high pressure exchanger and 3 bar for a low pressure exchanger.

The expanded stream 30a is recycled into the first heat exchanger 14a at or near the top thereof, and as the expanded stream 30a passes downwardly through the heat exchanger 14a and vapourises, it provides cooling to the lines in the first heat exchanger 14a carrying the feed

stream 10 (line 73a) and the first refrigerant (line 71a), as is known in the art. This provides a cooled hydrocarbon stream 40a.

Meanwhile, the vapourized first refrigerant can be collected and outflowed via outlet 42a from the first heat exchanger 14a as a first vapour exit stream 90a at or near its bottom end, and passed to the first compressor 22 for recompression and circulation in the first refrigerant circuit 110. Both the second fraction or part 21b of the first cooled first refrigerant stream 20a, and the cooled hydrocarbon stream 40a, then enter the second heat exchanger 14b at or near its bottom end via inlets 36b and 32b respectively, and pass upwardly therethrough. The first refrigerant stream passes through outlet 38b at or near the top of the second heat exchanger 14b as a further cooled first refrigerant stream 20b, and it is expanded through a second expansion turbine 12b to provide a second expanded first refrigerant stream 30b, which can then be passed back into the second heat exchanger 14b via inlet 44b for downward passage therethrough. As the second expanded stream 30b passes downwardly though the second heat exchanger 14b, it provides cooling to the lines of hydrocarbon stream (line 73b) and first refrigerant (line 71b) passing up through the second heat exchanger 14b in a manner known in the art. The second expanded refrigerant stream 30b is vapourized during its passage downwardly through the second heat exchanger 14b, and can be collected through outlet 42b as a second vapour exit stream 90b which passes to the first compressor 22 for recompression and circulation in the first refrigeration circuit 110.

The second expanded stream 30b is expanded by the second expansion turbine 12b in a near isentropic manner, and the difference in energy between the pre-expanded stream 20b and the post-expanded stream 30b can also be used as work in the first refrigeration circuit 110.

As is shown in Figure 1, the first refrigerant collected from the first and second heat exchangers 14a, 14b is compressed by compressor 22 to provide a compressed stream 95, and cooled by a water and/or air cooler 26 to provide a re-condensed stream 70 for reentry into the heat exchangers 14a, 14b. To assist circulation of the re-condensed first refrigerant stream 70, it is passed consecutively through the first and second pumps 28b, 28a to provide a pumped stream 70a prior to a final ambient cooler 29, which may comprise one or more ambient coolers such as water and/or air coolers, to provide a cooled pumped stream 70b ready for re-use in the heat exchangers 14a, 14b..

Pumps are common in refrigerant circuits, but still require to be driven, i.e. energy input. Moreover, they also increase the temperature of a stream passing therethrough as it is pressurised. In the present invention, it is particularly advantageous to drive or work the pumps 28a, 28b by using the work energy created by the close or neighbouring expansion turbines 12a, 12b. The work energy of the expansion turbines 12a, 12b can be transferred to the associated pumps 28a, 28b through any mechanical linkage that interconnects them, such as a common shaft. In a hot or warmer climate for a liquefying plant, the temperature of the re-condensed stream 70 is typically 40 ⁰ C to 60 ⁰ C, for example 50 ⁰ C. Each pump in a first refrigerant circuit can raise the temperature

of a re-condensed stream by a few degrees Celsius, so that for the arrangement shown in Figure 1 based on a hot climate, a typical temperature for the pumped stream 70a can be for example 53 ⁰ C to 56 ⁰ C. One or more ambient coolers such as the ambient cooler 29 shown in Figure 1 can then reduce the temperature of the pumped stream 70a by 10 to 20 ⁰ C, so that a typical temperature for the cooled pumped stream 70b is 40 ⁰ C. In a cold or colder climate, the temperatures of these streams may be 10 to 20 ⁰ C lower, but the increased temperature effect of the pumps is the same.

The drive for one or both of the pumps 28a, 28b may be partly, substantially or wholly provided by the work energy from the expansion turbines 12a, 12b. In this way, the external energy requirement of the first refrigerant circuit 110 is reduced, thereby making it more efficient.

Where they may be further serial heat exchangers in the first cooling stage 100, the further cooled first refrigerant stream 20b could have been split to provide a further line of first refrigerant to pass therethrough, and so forth for any further heat exchangers, until the last heat exchanger recycles all the first refrigerant as shown in Figure 1 for stream 20b.

The hydrocarbon stream exits the second heat exchanger 14b through outlet 34b as a cooled hydrocarbon stream 40b.

Each expansion turbine 12a, 12b allows the expansion of the first refrigerant to be near isentropic, which improves the efficiency of the first refrigeration circuit 110. That is, the internal energy of streams 30a, 30b is lowered, which benefits the efficiency of the process. Moreover, a lower internal energy, e.g. lower entropy, of the recycled first refrigerant stream equates

to a cooler refrigerant temperature, such that the recycled refrigerant streams provide better cooling to the heat exchangers, and/or less work for the refrigerant compressor ( s ) to recompress the refrigerant. Preferably, the first cooling stage 100 cools the feed stream 10 to below 0 ⁰ C, such as between -20 ⁰ C and - 70 ⁰ C, preferably either between -20 ⁰ C and -35°C, or between -40 ⁰ C and -70 ⁰ C, generally depending on the type of first cooling stage process. In one embodiment of the present invention, each heat exchanger of a multi-stage first cooling stage 100 involves a different first refrigerant pressure. The expanded refrigerant from each pressure stage could be compressed in one or more compressors, for example, using different compressors for different refrigerant entry pressures .

In another embodiment of the present invention, the feed stream does not pass through all the heat exchangers of the first cooling stage, but may only pass through one or more of said heat exchangers. For example, in the scheme shown in Figure 1, the feed stream 10 could pass through the second heat exchanger 14b only to achieve the required cooling of the first cooling stage 100.

The final cooled hydrocarbon stream 40b from the second heat exchanger 14b is then sent to a second cooling stage 200 using a second refrigerant, preferably a mixed refrigerant as hereinbefore described, circulating in a second refrigerant circuit 80.

There can be various arrangements for the cooled hydrocarbon stream 40b and the second refrigerant circuit 80 in and through the second cooling stage 200. Such arrangements are known in the art. These can involve one or more cooling stages, optionally at different pressure

levels, and optionally within one vessel such as a cryogenic heat exchanger.

The second cooling stage 200 may reduce the temperature of the cooled hydrocarbon stream 40b to provide a liquefied hydrocarbon stream 60 at a temperature of about or lower than -130 ⁰ C.

In the simplified form shown in Figure 1, the second refrigerant circuit 80 passes the vapourised second refrigerant exit stream 50a through two second compressors 52 driven by a driver 54, and a water and/or air cooler 56. After the cooler 56, the condensing second refrigerant can pass through the heat exchangers 14a, 14b of the first cooling stage 100 as line 75a, 75b, to provide a cooler, liquefied second refrigerant stream 50 for use in the second cooling stage 200.

Thus, the second refrigerant is at least partly cooled by the first cooling stage 100. This arrangement simplifies the method of liquefying a hydrocarbon feed stream such as natural gas by combining a portion of the cooling duty of the first cooling stage of the feed stream with an equivalent portion of the cooling of the second refrigerant using the same equipment, rather than the second refrigeration circuit 200 requiring other heat exchangers to sufficiently cool the second refrigerant for use in the second cooling stage 200. This therefore reduces the level of apparatus and equipment needed, and reduces the capital and running costs for the process shown in Figure 1.

Additional cooling of the feed stream, cooled and/or liquefied hydrocarbon stream and/or the refrigerant ( s ) could be provided by one or more other refrigerants or refrigerant cycles in addition to cooling by the first and second cooling stages, optionally being connected

with another part of the method and/or apparatus for liquefying a hydrocarbon stream as described herein.

For example, the liquefied stream 60 could then undergo a third cooling stage (not shown), preferably sub-cooling. Sub-cooling can be provided by passing the liquefied stream through one or more stages using one or more sub-cooling heat exchangers. The or each heat exchanger of the sub-cooling would preferably be supplied with cooling by a mixed (third) refrigerant. Additional cooling of the liquefied stream and/or the sub-cooling refrigerant could be provided by one or more other refrigerants or refrigerant circuits, optionally being connected with another part of the method and/or apparatus for liquefying a hydrocarbon stream as described herein.

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 .

Table I gives an overview of the power requirements in three cases. Table 1

The specific power of a liquefaction process is defined as the refrigerant compressor power (in KW) required by the process, divided by the quantity (in tpd) of liquefied hydrocarbon stream (for example LNG) provided.

Case 1 relates to the power required to drive the compressor of a refrigeration circuit for a first or pre- cooling stage of a liquid natural gas liquefaction system involving two heat exchangers, and using expansion valves as described in US 6,105,389A.

Case 2 is the power required to drive the compressor 22 in the liquefying process shown in Figure 1 as described above, but without any pumps 28a, 28b.

Case 3 is the power required to drive the compressor 22 in Figure 1 wherein the first refrigerant circuit 110 includes the pumps 28a, 28b as shown.

As can be seen, the power requirement for Case 2 is lower than that for Case 1, such that the use of expansion turbines has reduced the power required to recompress the first refrigerant in the first refrigerant circuit 110. Less compression pressure is required as the expanded first refrigerant (after passing through the expansion turbines) has a lower energy, and is therefore cooler, than by use of expansion valves. Case 3 introduces the pumps 28a, 28b driven by the work output of the expansion turbines 12a, 12b. It shows that the power requirement of compressor 22 is even lower than that in Case 2, as further efficiency is being created in the first refrigerant circuit 110 by the use of the pumps 28a, 28b, thus reducing the general propulsion power created by the compressor 22 in order to maintain circulation of the first refrigerant in the first refrigerant circuit 110.

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 .