NL2005598C2

NL2005598C2 - Method and apparatus for cooling a hydrocarbon stream.

Info

Publication number: NL2005598C2
Application number: NL2005598A
Authority: NL
Inventors: Louis Zuurhout
Original assignee: Louis Zuurhout
Priority date: 2010-10-29
Filing date: 2010-10-29
Publication date: 2012-05-02
Also published as: WO2012057626A8; WO2012057626A3; WO2012057626A2

Description

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P30494NL00/AZE

METHOD AND APPARATUS FOR COOLING A HYDROCARBON STREAM

5 The present invention relates to a method and an apparatus for cooling a hydrocarbon stream. In particular it relates to a method wherein the hydrocarbon stream comprises natural gas and that the natural gas is cooled to liquefaction, yielding liquefied natural gas (LNG).

Methods for cooling hydrocarbons to form liquefied natural gas to obtain more convenient forms for transportation and storage are well known. Any LNG liquefaction 10 process requires one, two or three refrigerant cycles using various refrigerant gases to cool natural gas down to -162°C in a main cryogenic heat exchanger or cold-box where methane liquefies. Refrigerant gases that have been used include propane, propylene, ethane, ethylene, methane, nitrogen, carbon dioxide, or combinations of the preceding refrigerants (’’mixed refrigerant systems”). The size, number and types of the refrigerant cycles determine 15 the efficiency and cost of different liquefaction process. Each cycle takes warm, pre-treated feed gas and cools it till it condenses into LNG product. To produce the cold temperatures required to produce LNG, work must be put into the cycle through a refrigerant compressor and heat must be rejected from the cycle through (e.g. air or water) coolers. The amount of work (which depends on the size of refrigerant compressors, the efficiency of the compressor 20 and drivers and refrigerant flow rate) is a function of the liquefaction process, the feed gas conditions and the required delta in temperature. Commonly, before hydrocarbonaceous natural gas is cooled it is treated to remove compounds such as ammonia, hydrogen sulphide and carbon dioxide. Usual treatments include absorption in an amine-containing solution. Subsequently, the natural gas is cooled. During the cooling process other constituents of the 25 feed gas with higher boiling points are removed. These constituents may include remaining water, carbon monoxide, carbon dioxide, but also ethane, propane, butane and heavier hydrocarbons. Also nitrogen is removed as this inert gas lowers the heating value of LNG. Ultimately the product of the liquefaction process is predominantly liquefied methane.

Over the years liquefaction plants have increased in size. These larger plants with 30 more refrigerant cycles have become more complex and more costly to build. Amongst the economic considerations that are of importance in the design and operation of a liquefaction plant, the capital expense, the operating expense, and the availability are key. By “availability” is understood the time that the plant is producing hydrocarbons. By hydrocarbons are understood both LNG and heavier hydrocarbons, also known as 35 condensates. LNG plants may be onshore or offshore. Offshore plants, also called floating LNG plants, are being developed for exploitation of remote gas fields at sea.

The vast majority of LNG plants use single cycle heavy-duty gas turbines to drive one or more refrigerant compressors. Other plant designs use steam turbines. Turbine outages -2- cause a major portion of the production losses due to scheduled and unscheduled outages and output reduction due to fouling, degradation and temperature de-rating. Turbine driven compression trains currently cause a disproportional share of the total production losses.

Typical LNG liquefaction train availability is in the order of 95-98% due to scheduled 5 and unscheduled outages of the liquefaction compressors and associated drivers. The typical average LNG plant uptime is in the order of 92 to 96% and leaves room for improvement.

The availability is hampered by the turn-down ratio. This turn-down ratio is defined as the ratio of the minimum flow of the refrigerant, when the plant has to be shut down, to the maximum flow of the refrigerant. This ratio is influenced by limitations of the heat exchanger 10 design (e.g., pressure margin) and by the refrigerant compressor design. This causes a limitation to the amount of refrigerant that can go through the heat exchanger and thus the quantity of LNG the heat exchanger can produce. Given the limited pressure margin and safety requirements, the heat exchanger should not be operating at pressures significantly over its design conditions. The refrigerant inlet flow can be managed by controlling the flow 15 and discharge pressure from the refrigerant compressors. The turn-down ratio of the heat exchanger can thus be defined as a percentage of refrigerant inlet flow as a percentage of the design capacity. Atypical turn down ratio is 70%.

US 3,527,059 discloses a method for controlling parallel-operating refrigeration compressors. The objective is to make each compressor stage to operate efficiently at or 20 near its normal capacity. Each stage of such compressor is designed to operate at 80% or more of its normal capacity. One of the objectives of this operation is to control the temperature of compressed refrigerant gases.

In US 4,057,972 a process for the liquefaction of natural gas has been described wherein a multi-component refrigerant system is used. The multi-component refrigerant 25 system is a self-contained cycle. In addition the process employs an independent refrigeration system which is used to pre-cool the hydrocarbon feed gas to the stage that all potential solids have been removed. This process differs from the known so-called cascade refrigeration cycle, and has allegedly overcome the disadvantage that a cascade requires many compressors which is considered undesirable.

30 Neither US 3,527,059 nor US 4,057,972 addresses the problem of the availability of the LNG plant. It has now been found that when three or more compressors are used in parallel the availability of an LNG plant can be increased significantly, as the outage of one compressor would still enable a refrigerant flow and pressure that would be above the turndown ratio.

35 Accordingly, the present invention provides a method for cooling a hydrocarbon stream by passing the hydrocarbon stream through at least one heat exchanger, which method comprises the steps of: (a) compressing a refrigerant in a compressor unit to yield compressed refrigerant; -3- (b) expanding the compressed refrigerant, to yield cold refrigerant; (c) passing the hydrocarbon stream against the cold refrigerant in the heat exchanger, to yield a cooled hydrocarbon stream and at least partly evaporated refrigerant; (d) recovering the cold hydrocarbon stream; and 5 (e) recycling at least part of the at least partly evaporated refrigerant to the compressor unit, wherein the compressor unit comprises at least three compressors that operate in parallel.

The present invention allows that when the LNG plant operates at normal capacity the compressors also operate at normal capacity. When one compressor has to be taken out for scheduled or unscheduled maintenance, repair, cleaning or a similar reason, the other, at 10 least two, compressors will have sufficient capacity to provide an amount of refrigerant at sufficient pressure, temperature and flow rate to enable the LNG plant to operate above the turn down ratio.

The present invention improves LNG production capacity, availability, production efficiency and thermal efficiency of the compression trains in LNG liquefaction plants by 15 installation of surplus compression capacity to compensate for equipment fouling, deterioration, derating and compression outages by installing 3 or more compressors which can handle equal process conditions, such as gas composition, gas inlet temperature, pressure ratio, suction pressure and discharge pressure, and preferably identical capacity in terms of volumetric, mass or standard gas flow, that operate in parallel for one or more 20 cryogenic heat exchangers. LNG plant designers can optimize additional cost of capital equipment by combining two or more distinct refrigerant compression duties (e.g., propane and mixed refrigerants) in this parallel compression train configuration.

The present invention can be applied in conjunction with heat exchangers in, in particular, LNG plants. As indicated above, these LNG plants may be onshore or offshore.

25 Offshore plants, also called floating LNG plants, are being developed for exploitation of remote gas fields at sea. Especially for these expensive plants the improved availability obtainable with the present invention constitutes a significant advantage.

The capacity of an LNG plant is influenced by ambient air temperature. The power of a turbine is inversely proportional to ambient air temperature. If the heat sink temperature also 30 varies, e.g., because the temperature of cooling water or cooling air goes up, the capacity of the LNG plant is further influenced. Typically, the equipment in an LNG plant is designed such that it can handle temperature variations, by applying standard design margins. The equipment in an LNG plant is typically slightly overdesigned for standard operating conditions.

The slight overdesign has as additional benefit in the present invention that the 35 compressor would allow the compressors to operate at about 80% of their maximum capacity, thereby benefitting of advantages that were described in US 3527059. When one compressor is taken out for whatever reason, the other at least two compressors can be adjusted such -4- that they operate at or near maximum capacity, thereby compensating at least partly for the taking out of the one compressor. In this way the availability of the LNG plant is improved.

The availability of an LNG plant is further improved if the capacity of the compressors is increased even beyond the maximum capacity of the heat exchanger. The maximum 5 capacity of the heat exchanger can be expressed as the maximum flow of refrigerant that the heat exchanger can handle. The maximum flow of refrigerant already takes into account the design variations discussed above to cope with temperature variations. Additional refrigerant compression capacity is effectively an increment in refrigerant compressor flow to the heat exchanger. Therefore, the heat exchanger having a maximum inlet flow of refrigerant, the 10 compressor unit has suitably a capacity in excess of this maximum inlet flow. In this way the skilled person is enabled to operate at or near the standard design capacity of the heat exchanger even when one compressor has been taken out. Good results are obtainable when the compressor unit has a capacity that is from 105 to 150% of the maximum inlet flow. If the excess capacity is below 105%, there is still a chance that temperature variations may 15 become troublesome to operate above the turn down ratio. At an excess capacity of above 150% the capital expense will become very high. Moreover, there is a chance that when all compressors are in operation, the compressors have to operate at a capacity of below 80% of their design capacity, which may lead to efficiency losses. In order to optimise the efficiency, the availability and the capital expenses, the number of compressors in the compressor unit 20 suitably ranges from three to five. When there are more than one refrigerant compressor units, the excess capacity can be customised for each refrigerant compressor unit.

The operation of an LNG plant is well known. The compressors are driven by one or more drivers. These drivers include gas turbines, steam turbines or electrical motors. Accordingly, the compressors are coupled with at least one driver, selected from a gas 25 turbine, steam turbine and electrical motor. One suitable gas turbine is the aero-derivative gas turbine. Aero-derivative gas turbines have been derived from jet engines as used in airplanes. Suitably these are steam-injected gas turbines in which steam is injected into the combustor and gas path to increase the power output and the efficiency. Aero-derivative gas turbines are usually smaller than heavy duty gas turbines that are commonly used in LNG 30 plants. However, they are more energy efficient, have good mechanical drive capabilities, have good efficiencies and can be exchanged quickly reducing maintenance outages. Aero-derivative gas turbines are typically not used due to its relative small size compared to heavy duty gas turbines. However, in the present invention these aero-derivative gas turbines can be excellently used, since the use of a plurality of compressors makes it possible to employ 35 smaller turbines, such as aero-derivative gas turbines.

It is possible to apply one driver that is coupled to more than one refrigerant compressor. This may have a beneficial effect on the capital expenses. However, the flexibility of the operation is enhanced if each compressor in the compressor unit is coupled -5- with a separate driver. The skilled person has the option to optimise the capital costs on the one hand, which would benefit fewer turbines, and the flexibility and availability on the other hand, which would promote more drivers, such as one driver per compressor. It would also be possible to use one driver for more than one compressor, whereby the compressors would 5 provide refrigerant to different heat exchangers. In this way the advantages of flexibility and reduced number of drivers would be combined.

The capacity of the compressors in the compressor unit is suitably selected to provide an excess capacity of refrigerant with regard to the maximum inlet flow of the heat exchanger. The capacities of the compressors in each compressor unit may vary. However, in order to 10 facilitate the operation when one compressor has to be taken out, the capacity of each compressor in the compressor unit has substantially the same capacity. By substantially the same capacity is understood that the compressors have capacities that differ 5% or less from each other.

As indicated above, the refrigerants may be selected from a variety of compounds, 15 known in the art. Suitable refrigerants include nitrogen, methane, ethane, ethylene, propane, propylene, one or more butane isomers, one or more pentane isomers and mixtures thereof. The refrigerants may be used in a number of refrigerants streams. Hence, it is possible to employ one refrigerant stream, but also more streams are possible, e.g., two to four refrigerant streams. The refrigerant compounds used in the various refrigerant streams may 20 be the same or different. The use of different refrigerant compounds is preferred since that allows easy application of different temperature regimes when more than one heat exchanger is used.

The skilled person will understand that when the refrigerant is compressed in the compressor unit, the temperature of the compressed refrigerant will go up. Suitably, the 25 compressed refrigerant is cooled before being expanded. The cooling may be accomplished in any way known in the art. Typically air or water coolers are employed. However, it is also possible to use other streams that are available in the LNG process for cooling purposes. By using other process streams of the LNG process the thermal efficiency of the process may be optimised.

30 The skilled person may employ any expansion method that he would like to use.

Typically some expansion takes place in the heat exchanger. In this way liquid refrigerant evaporates and absorbs heat from the hydrocarbon stream. However, it is advantageous to employ an expansion means that allows adiabatic expansion of the compressed refrigerant so that the temperature of the refrigerant is lowered before it enters the heat exchanger.

35 Suitable expansion means include pressure reduction valves, such as a Joule-Thomson valve, and expansion engines such as turbines or an expansion vessel. Therefore the compressed refrigerant is advantageously expanded via an expansion means, wherein a Joule-Thomson valve is preferred.

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It is possible to employ the method of the present invention for cooling any hydrocarbon stream. For that purpose one may pass the hydrocarbon stream suitably through one to four heat exchangers that operate in series and/or in parallel. With heat exchangers that operate in parallel the size of the equipment used, such as heat exchangers and/or 5 compressors may be designed in an optimal way. When a parallel operation of heat exchangers is desired, the number of parallel heat exchangers typically varies between two and four. With heat exchangers in series, the product is successively cooled so that a sufficiently cooled hydrocarbon is produced that may be used for various objectives. When the hydrocarbon streams contains heavier products, such as hydrocarbons with 2 or more 10 carbon atoms, such as ethane, ethylene, propane, butanes and pentanes, these heavier hydrocarbons may be easily recovered, in particular separately from each other. However, it is preferred to use the method in the preparation of liquefied natural gas. Therefore, the cold hydrocarbons stream preferably contains methane and the stream is cooled to liquefaction, yielding liquefied natural gas. The liquefaction may be obtained in one to four heat 15 exchangers that operate in series. When more than one heat exchanger is used, it is not required that all that exchangers operate in accordance with the method of the present invention. It is possible to run a number of heat exchangers in accordance with known operations and techniques. However, the method according to the present invention is preferably applied in all heat exchangers when more than one heat exchanger is used. The 20 methane-containing hydrocarbon stream is advantageously pre-treated to remove noxious contaminants from the hydrocarbon stream. Noxious components that are suitably removed before the hydrocarbons stream is subjected to liquefaction include carbon monoxide, carbon dioxide, hydrogen sulphide, thiols, ammonia, but also hydrocarbons with two or more carbon atoms per molecule.

25 The present invention is particularly useful for producing liquefied natural gas (LNG).

In such cases the hydrocarbon stream suitably comprises at least 60%vol methane. More preferably, the methane content in the hydrocarbon stream is at least 70 %vol, even more preferably at least 80%vol. The remaining portion of the hydrocarbon stream may contain nitrogen, and the other compounds mentioned above, viz. carbon monoxide, carbon dioxide, 30 hydrogen sulphide, thiols, ammonia, but also hydrocarbons with two or more carbon atoms per molecule. It is understood that some natural gas sources may contain very high concentrations of carbon dioxide and/or ammonia and/or hydrogen sulphide. Such concentrations may amount to 60%vol or higher. The skilled person will realise that such streams will be treated first in order to remove these components, e.g., by amine treatment.

35 The product thus obtained, containing at least 60 %vol methane, may be subjected to the process according to the present invention.

The conditions for producing LNG are well known. Such conditions may be applied in the process according to the present invention. Suitable conditions for the compressor units -7- include a compression of the refrigerants to a pressure ranging from 10 to 50 bar, preferably 25 to 45 bar. In the expansion stage the compressed refrigerant may be expanded to a pressure ranging from 8 to 0.1 bar, suitably to a pressure ranging from 0.5 to 5 bar. The skilled person may select the optimal conditions based on i.a. the number of heat exchangers 5 and the desired levels of cooling for the hydrocarbon stream. In a typical case the hydrocarbon stream is cooled in stages wherein the temperature may be brought down in two to four stages from about ambient temperature, e.g., 20 °C, to -165 °C. At a temperature of 70 to 0 °C the hydrocarbon stream is suitably treated to remove contaminants. In a further stage the stream may be cooled to a temperature of - 30 to -80 °C. In one or more further heat 10 exchangers, the temperature is then reduced to below the liquefaction temperature for methane, yielding LNG.

The invention also provides an apparatus for cooling a hydrocarbon stream comprising - a heat exchanger provided with a first side for refrigerant having a first side inlet and a first 15 side outlet, and a second side for the hydrocarbon stream to be cooled, which second side is provided with a second side inlet and a second side outlet; - a compressor unit for compressing the refrigerant comprising at least three compressors, the first side outlet of the heat exchanger being connected with an inlet of each compressor of the compressor unit, and the first site inlet being connected to an outlet of each of the 20 compressors in the compressor unit; and - an expansion means that is provided in fluid communication with the outlets of each of the compressors and the first site inlet of the heat exchanger.

In order to arrive at the above-described advantages, the heat exchanger has a maximum inlet flow for the first site inlet, and the compressor unit has preferably a capacity in 25 excess of that maximum inlet flow. The expansion means that is contained in the apparatus according to the present invention is preferably a Joule-Thomson valve as described above. The invention will be further illustrated by means of the figures.

Figure 1 shows an embodiment of the present invention wherein LNG is produced in two refrigeration cycles. A refrigeration cycle is shown as a closed loop in the liquefaction 30 process wherein a refrigerant is compressed and expanded and withdraws heat in the heat exchanger and disposes of heat in a cooler system.

Figure 2 shows another embodiment wherein LNG is produced in two parallel series of heat exchangers, and wherein drivers are coupled with multiple compressors, each compressor being tied to its closed refrigerant cycle.

35 It is evident that the figures represent simplified schemes only. Auxiliary equipment has not been shown as will be clear to the skilled person.

The figures will be further explained, whilst referring to the production of LNG. It is emphasised that the invention may also be used for other purposes, such as the cooling of -8- hydrocarbon streams to obtain liquid hydrocarbons with 2 or more carbon atoms per molecule. In Figure 1 two heat exchangers 1 and 2 are shown. Natural gas, from which contaminants such as ammonia, hydrogen sulphide and carbon dioxide have been removed, is fed via a line 3 into heat exchanger 1. Into heat exchanger 1 are also fed a first refrigerant, 5 such as propane, mixed refrigerant or methane gas, via a line 30 and a second refrigerant, e.g., mixed refrigerant or nitrogen, via a line 10. The second refrigerant in line 10 is precooled in heat exchanger 1, together with the natural gas in line 3. The first refrigerant is passed through the heat exchanger 1 and leaves the heat exchanger via a line 31. Via an expansion means 46 where the first refrigerant expands and cools off, and a line 47 the first refrigerant is 10 passed to the heat exchanger 1. The first refrigerant is subsequently further evaporated in heat exchanger 1 by absorbing heat from the streams in lines 3 and 10. The heat that is absorbed by the adiabatic evaporation cools the natural gas in line 3 and the second refrigerant in line 10. The line 3 forms the second side and the environment in heat exchanger 1 around the line 3 forms the first side of the heat exchanger. Although line 3 has been shown 15 here as a single conduit, it is clear to the skilled person that the heat exchange surfaces may be made very large, e.g. via a bundle of conduits, to facilitate the heat exchange process. The first refrigerant is withdrawn from heat exchanger 1 via a line 32. From line 32 the refrigerant is split into three and is fed via lines 33, 34 and 35 to compressors 36, 38 and 40, respectively. Although in the figure three compressors are shown, it is clear that more 20 compressors, e.g., four or five, may also be used. These compressors are operating in parallel and have a combined capacity above the maximum capacity of the heat exchanger as expressed in the maximum refrigerant flow that can be fed via line 30. The compressors have been coupled with drivers, in this case gas turbines, 37, 39 and 41, respectively.

Compressed first refrigerant leaves the respective compressors via lines 42, 43 and 25 44, and is combined into line 30. The compressed refrigerant may have an elevated temperature. Therefore, it is suitably cooled in an air or water cooler, indicated by 45 before it is recycled into the heat exchanger 1.

Cooled natural gas leaves heat exchanger 1 via a line 4 and is passed into the second heat exchanger 2. The cooled second refrigerant leaves the heat exchanger 1 via a line 11 30 and is also passed into the heat exchanger 2. The refrigerant leaves the heat exchanger 2 and is passed via a line 12 and an expansion means 13 and a line 14 back into the heat exchanger 2. In the expansion means the compressed refrigerant is at least partly adiabatically evaporated so that cold is created. The cold created by the evaporation is used to cool the natural gas in line 4 further to at least partial liquefaction. The at least partially 35 liquefied natural gas leaves the heat exchanger 2 via a line 5. It may be further expanded in an expansion means 6 and the expanded product is withdrawn via line 7. The products may be split into a fuel product, withdrawn via a line 8, and LNG, withdrawn via a line 9. The fuel product may comprise methane and nitrogen and/or heavier hydrocarbons.

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The second refrigerant that has been at least partly evaporated is withdrawn from the heat exchanger 2 via a line 15. Thereafter it is split into three. Via lines 16, 20 and 19 the refrigerant is passed to compressors 17, 21 and 23, respectively. The compressors are driven via gas turbines 18, 22 and 24, respectively. Although in the figure three compressors, lines 5 and turbines are shown, it is clear that more compressors, lines and turbines, e.g., four or five, may also be used.

Compressed second refrigerant leaves the respective compressors via lines 25, 26 and 27, and is combined into line 10. The compressed refrigerant may have an elevated temperature. Therefore, it is cooled in an air or water cooler, indicated by 28, before it is 10 recycled into the heat exchanger 1.

Figure 2 shows an alternative embodiment where each refrigeration cycle is conducted in two parallel heat exchangers. Natural gas is fed via line 103, split into two, and via lines 103a and 103b fed into heat exchanger 101a and 101b, respectively. Into heat exchangers 101a and 101b are also fed a first refrigerant via a line 130a and 130b, and a 15 second refrigerant via a line 110a and 110b. The second refrigerant in lines 110a and 110b is precooled in heat exchangers 101a and 101b, together with the natural gas in lines 103a and 103b. The first refrigerant is passed through the heat exchangers 101a and 101b and leaves the heat exchangers via lines 131a and 131b. It is passed through expansion means 161a and 161b, and is recycled to the heat exchangers 101a and 101b through lines 160a and 20 160b, respectively. The refrigerant is subsequently evaporated into heat exchanger 101a and 101b. The cold that is created by the adiabatic evaporation cools the natural gas in lines 103a and 103b and the second refrigerant in lines 110a and 110b. The lines 103a and 103b form the second side and the environment in heat exchangers 101a and 101b around the lines 103a and 103b form the first side in the heat exchangers. The first refrigerant is withdrawn 25 from heat exchangers 101a and 101b via lines 132a and 132b, and combined into line 132.

From the line 132 the refrigerant is split into three and is fed via lines 133, 134 and 135 to compressors 136, 137 and 138, respectively. Although in the figure three compressors, lines and turbines are shown, it is clear that more compressors, lines and turbines, e.g., four or five, may also be used.

30 These compressors are operating in parallel and have a combined capacity above the maximum capacity of the heat exchanger as expressed in the maximum refrigerant flow that can be fed via lines 130a and 130b.

Compressed first refrigerant leaves the respective compressors via lines 142, 143 and 144, and is combined into line 130. The compressed refrigerant may have an elevated 35 temperature. Therefore, it is suitably cooled in an air or water cooler, indicated by 145 before it is recycled into the heat exchanger 1. The cooler may be positioned before the line 130 is split. Alternatively, two smaller coolers may be positioned such that the refrigerants in lines 130a and 130b are cooled separately.

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Cooled natural gas leaves heat exchangers 101a and 101b via lines 104a and 104b and is passed into second heat exchangers 102a and 102b. The cooled second refrigerant leaves the heat exchangers 101a and 101b via line 111a and 111b and is also passed into the heat exchangers 102a and 102b. The refrigerant streams leave the heat exchanger 102a 5 and 102b and are passed via lines 112a and 112b to expansion means 113a and 113b. From the expansion means 113a and 113b the refrigerant is returned to the heat exchangers 102a and 102b via lines 114a and 114b. In the expansion means 113a and 113b the compressed refrigerant is at least partly adiabatically expanded and optionally at least partly evaporated so that cold is created. The cold created by the evaporation is used to cool the natural gas in 10 lines 104a and 104b further to at least partial liquefaction. The at least partially liquefied natural gas leaves the heat exchangers 102a and 102b via lines 105a and 105b. The at least partly liquefied natural gas is combined into a line 105. It may be further expanded in an expansion means 106 and the expanded product is withdrawn via line 107. The products may be split into a fuel product, as indicated above, that is withdrawn via a line 108, and LNG, 15 withdrawn via a line 109.

The second refrigerant that has been at least partly evaporated is withdrawn from the heat exchangers 102a and 102b via lines 115a and 115b. The two streams are combined into a line 115. Thereafter it is split into three. Via lines 116, 120 and 119 the refrigerant is passed to compressors 117, 121 and 123, respectively. The compressors are driven via gas turbines 20 150, 151 and 152, respectively. Although in the figure three compressors, lines and turbines are shown, it is clear that more compressors, lines and turbines, e.g., four or five, may also be used.

Compressed second refrigerant leaves the respective compressors via lines 125, 126 and 127, and is combined into line 110. The compressed refrigerant may have an elevated 25 temperature. Therefore, it is suitably cooled, e.g., in an air or water cooler, indicated by 128, before it is recycled into the heat exchanger 101a and 101b.

The compressors 136,117,137,121,138 and 123 are coupled with three drivers. Compressors 136 and 117 are coupled with a gas turbine 150; compressors 137 and 121 are coupled with a gas turbine 151 and compressors 138 and 123 are coupled with a gas turbine 30 152. In this way the capital expense for a plurality of gas turbine drivers may be avoided.

However, when one of the compressors is to be taken out, there are two remaining compressor pairs that can provide sufficient capacity to keep the LNG plant on line.

Although in the Figures 1 and 2 two refrigerant lines are shown, it is clear that another number, e.g., one, three or four, refrigerant lines may also be used. The Figure 2 also shows 35 two parallel heat exchanger streams; however it is evident that another number, e.g., one or three, parallel heat exchangers may also be used.

The invention is described in the following clauses.

-11 -CLAUSES

1. Method for cooling a hydrocarbon stream by passing the hydrocarbon stream through at least one heat exchanger, which method comprises the steps of: (a) compressing a refrigerant in a compressor unit to yield compressed refrigerant; 5 (b) expanding the compressed refrigerant, to yield cold refrigerant; (c) passing the hydrocarbon stream against the cold refrigerant in the heat exchanger, to yield a cooled hydrocarbon stream and at least partly evaporated refrigerant; (d) recovering the cold hydrocarbon stream; and (e) recycling at least part of the at least partly evaporated refrigerant to the compressor unit, 10 wherein the compressor unit comprises at least three compressors that operate in parallel.

2. Method according to clause 1, wherein the heat exchanger has a maximum inlet flow of refrigerant and the compressor unit has a capacity in excess of that maximum inlet flow.

3. Method according to clause 2, wherein the compressor unit has a capacity that is 15 from 105 to 150% of the maximum inlet flow.

4. Method according to any one of clauses 1 to 3, wherein the compressor unit comprises from 3 to 5 compressors.

5. Method according to any one of clauses 1 to 4, wherein the compressors are coupled with at least one driver, selected from a gas turbine, steam turbine and electrical 20 motor.

6. Method according to clause 5, wherein each compressor in the compressor unit is coupled with a separate driver.

7. Method according to any one of clauses 1 to 6, wherein each compressor has substantially the same capacity.

25 8. Method according to any one of clauses 1 to 7, wherein the refrigerant is selected from the group consisting of nitrogen, methane, ethane, ethylene, propane, propylene, one or more butane isomers, one or more pentane isomers and mixtures thereof.

9. Method according to any one of clauses 1 to 8, wherein the compressed refrigerant is cooled before being expanded.

30 10. Method according to any one of clauses 1 to 9, wherein the compressed refrigerant is expanded via an expansion means, such as a Joule-Thomson valve.

11. Method according to any one of clauses 1 to 10, wherein the hydrocarbon stream is passed through two to four heat exchangers that operate in series and/or in parallel.

12. Method according to any one of clauses 1 to 11, wherein the cold hydrocarbon 35 stream contains methane and is cooled to liquefaction, yielding liquefied natural gas.

13. Apparatus for cooling a hydrocarbon stream comprising - a heat exchanger provided with a first side for refrigerant having a first side inlet and a first - 12- side outlet, and a second side for the hydrocarbon stream to be cooled, which second side is provided with a second side inlet and a second side outlet; - a compressor unit for compressing the refrigerant comprising at least three compressors, the first side outlet of the heat exchanger being connected with an inlet of each of the 5 compressors of the compressor unit, and the first site inlet being connected to an outlet of each of the compressors in the compressor unit; and - an expansion means that is provided in fluid communication with the outlets of each of the compressors and the first site inlet of the heat exchanger.

14. Apparatus according to clause 13, wherein the heat exchanger has a maximum 10 inlet flow for the first site inlet and the compressor unit has a capacity in excess of that maximum inlet flow.

15. Apparatus according to clause 13 or clause 14, wherein the expansion means is a Joule-Thomson valve.

Claims

A method for cooling a hydrocarbon stream by passing the hydrocarbon stream through at least one heat exchanger, the method comprising the steps of: (a) compressing a refrigerant in a compressor unit to obtain a compressed refrigerant; (b) expanding the compressed refrigerant to obtain cold refrigerant; (C) directing the hydrocarbon stream against the cold coolant into the heat exchanger, which heat exchanger has a maximum coolant inlet stream, so that a cooled hydrocarbon stream and at least partially evaporated coolant are obtained; (d) recirculating at least a portion of the at least partially evaporated refrigerant to the compressor unit, the compressor unit including at least three compressors operating in parallel, and the compressor unit having a capacity greater than the maximum inlet flow of the heat exchanger .

2. Method according to claim 1, wherein the compressor unit has a capacity that is from 105% to 150% of the maximum inlet flow.

The method of claim 1 or 2, wherein the compressor unit comprises 3 to 5 compressors.

4. A method according to any one of claims 1 to 3, wherein the compressors are coupled to at least one drive device selected from a gas turbine, a steam turbine and an electric motor.

The method of claim 4, wherein each compressor in the compressor unit is coupled to a separate drive device.

The method of any one of claims 1 to 5, wherein each compressor has substantially the same capacity.

The method of any one of claims 1 to 6, wherein the coolant is selected from the group consisting of nitrogen, methane, ethane, ethylene, propane, propylene, one or more isomers of butane, one or more isomers of pentane, and mixtures thereof.

The method of any one of claims 1 to 7, wherein the compressed refrigerant is cooled before being evaporated.

The method of any one of claims 1 to 8, wherein the compressed refrigerant is expanded via an expansion member, such as a Joule-Thomson valve.

The method of any one of claims 1 to 9, wherein the hydrocarbon stream is passed through two to four heat exchangers operating in series. - 14-

The method of any one of claims 1 to 10, wherein the cold hydrocarbon stream contains methane and is cooled until it becomes liquid, resulting in liquefied natural gas.

12. Device for cooling a hydrocarbon stream comprising: 5. a heat exchanger provided with a cold side for coolant, which is provided with an inlet for the cold side and an outlet for the cold side, and a warm side for the hydrocarbon stream which must be cooled, which hot side is provided with an inlet for the hot side and an outlet for the hot side, and which heat exchanger has a maximum inlet flow for the cold side; 10. a compressor unit for compressing the coolant, comprising at least three compressors, the cold side outlet of the heat exchanger being connected to an inlet of each of the compressors of the compressor unit, and the cold side inlet of the heat exchanger is connected to an outlet of each of the compressors in the compressor unit, which compressor unit has a capacity that is greater than the maximum inlet current of the heat exchanger; and - an expansion member which is arranged in fluid-suitable connection with the outlets of each of the compressors and with inlet of the cold side of the heat exchanger.

The device of claim 12, wherein the expansion member is a Joule-Thomson valve. 25