AU2007253406B2

AU2007253406B2 - Method and apparatus for treating a hydrocarbon stream

Info

Publication number: AU2007253406B2
Application number: AU2007253406A
Authority: AU
Inventors: Hillegonda Bakker; Joannes Ignatius Geijsel; Mark Antonius Kevenaar
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 2006-05-19
Filing date: 2007-05-16
Publication date: 2010-12-16
Anticipated expiration: 2027-05-16
Also published as: US20090301131A1; RU2436024C2; GB2450666B; GB0819580D0; RU2008150385A; AU2007253406A1; WO2007135062A3; GB2450666A; WO2007135062A2

Description

METHOD AND APPARATUS FOR TREATING A HYDROCARBON STREAM Background of the Invention 5 The present invention relates to a method and apparatus for treating a hydrocarbon stream such as natural gas, in particular in a process for the production of liquefied natural gas. 10 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 high pressures. 15 US 6,401,486 Bl relates to a method and apparatus for high recovery of hydrocarbon liquids from methane-rich natural gases in association with liquefied natural gas production. Figure 1 of US 6,401,486 shows a pre-treatment process in a liquefaction plant in which the lighter stream from the NGL recovery column can be re-compressed by 20 an expander/compressor. A problem of the arrangement in Figure 1 is that the expander/compressor requires a separate driver prior to the refrigeration system. A driver is an expensive item both in terms of capital and running costs. Object of the Invention 25 It is the object of the present invention to substantially overcome or at least ameliorate one or more of the disadvantages of the prior art, or to provide a useful alternative.

la Summary of the Invention The present invention in a first aspect provides a method of treating a hydrocarbon stream such as natural gas from a feed stream comprising at least the steps of: 5 (a) passing the feed stream through a distillation column to provide a gaseous stream and a C2+ liquid stream; (b) compressing at least part of the gaseous stream through one or more feed compressors to provide a compressed stream; and (c) commonly driving at least one of the feed compressors with one or more io separate refrigerant compressors for one or more separate refrigerant circuits, by mechanically interconnecting said compressors. The present invention in a second aspect provides an apparatus for treating a hydrocarbon stream such as liquefied natural gas from a feed stream, the apparatus at is least comprising: a distillation column having an inlet for the feed stream and a first outlet for a gaseous stream and a second outlet for a C 2 ' liquid stream; one or more feed compressors for compressing at least a part of the gaseous stream; and 20 a common driver for driving one or more of the feed compressors with one or more separate refrigerant compressors for one or more separate refrigerant circuits. The present invention at least in a preferred embodiment reduces the capital and/or running costs of a liquefaction plant involving liquefying apparatus. 25 The present invention at least in a preferred embodiment provides an alternative method and apparatus for liquefying natural gas. The present invention at least in a preferred embodiment provides a method of WO 2007/135062 PCT/EP2007/054772 -2 treating a hydrocarbon stream such as natural gas from a feed stream comprising at least the steps of: (a) passing the feed stream through a distillation column to provide a gaseous stream and a C 2 + liquid stream; 5 (b) compressing at least part of the gaseous stream through one or more feed compressors to provide a compressed stream; and (c) commonly driving at least one of the feed compressors with one or more separate refrigerant compressors for one 10 or more separate refrigerant circuits, by mechanically interconnecting said compressors. Refrigerant circuit(s) use large compressors which can be driven by powerful drivers. Gas turbines are usually used to drive the compressors as the gas turbines 15 can be fuelled by feed gas, and in particular methane extracted from feed gas. It is also common to perform a number of pre treatment processes on a feed stream such as natural gas liquid extraction prior to liquefaction, including 20 separation or extraction of heavier hydrocarbon fractions. Many of such pre-treatment processes also require the use of compressors driven by drivers such as gas turbines. As each and every gas turbine or other driver in a 25 liquefaction plant is expensive, this contributes a considerable percentage to the overall capital and running costs of the cooling. It has been surprisingly found that by combining the driver of at least one of the feed compressors and at 30 least one of refrigerant compressors, the capital costs can be reduced. Furthermore it has been found that the resultant ratio of compressor power input for the 3 compressors can be brought closer to an optimum ratio, hence reducing capital and running costs. Refrigerant circuits are known in the art. They generally involve one or more 5 heat exchangers, one or more expansion devices or units, and one or more compressors. Whilst each refrigerant circuit can be separate from other refrigerant circuits, one or more parts of a refrigerant circuit can be connected or interconnected with another refrigerant circuit(s), or at least involve an interconnection of actions or combination of materials and/or flow with other circuit(s). 10 Each refrigerant circuit is separate from the pathway of the hydrocarbon stream and its treated and/or subsequent cooled and/or liquefied forms. Thus the or each refrigerant is not mixed with or directly derived from the hydrocarbon stream and its subsequent forms. That is, the fluid passing through the or each relevant feed compressor is is different to the fluid passing through the or each commonly driven refrigerant compressor. Preferably, the method further comprises the step of: cooling the compressed stream by passing the compressed stream against one or more refrigerants being in one or 20 more of the refrigerant circuits. In one preferred embodiment of the present invention, the method of treating a hydrocarbon stream such as natural gas from a feed stream comprises at least the steps of: (a) passing the feed stream through a distillation column to provide a 25 gaseous stream and a C 2 * liquid stream; (b) compressing at least part of the gaseous stream through one or more feed compressors to provide a compressed stream; and 4 (c) cooling the compressed stream by passing the compressed stream against one or more refrigerants being in one or more refrigerant circuits involving one or more refrigerant compressors; wherein at least one of the feed compressors in step (b) and at least one of the 5 refrigerant compressors in step (c) are mechanically interconnected and are arranged to be driven by a common driver. Although the method according to at least a preferred embodiment of the present invention is applicable to various hydrocarbon feed streams, it is particularly suitable for 10 natural gas streams to be cooled, and in particular liquefied. As the person skilled in the art readily understands how to cool and liquefy a hydrocarbon stream, this is not further discussed here in detail. Preferably, the cooling of the compressed stream involves at least two cooling is stages, each stage including at least one refrigerant circuit. More preferably, the cooling of the compressed stream involves one pre-cooling stage and one main cooling stage, and the or a feed compressor of step (b) is mechanically interconnected with a refrigerant compressor of the pre-cooling refrigerant circuit. 20 Pre-cooling of the feed stream can be carried out to reduce its temperature to below O'C, such as between -1O'C to -50'C. Preferably, the feed stream is cooled upstream of step (a) against at least part of the gaseous stream produced in step (a). 25 Main cooling of a pre-cooled stream can be to further reduce its temperature to below -100'C, such as between - 120'C to -170'C. 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 30 LNG may be depressurized by means of a Joule-Thomson valve or by means of a 5 cryogenic turbo-expander. Also, further intermediate processing steps between the gas/liquid separation in the distillation column and the cooling may be performed. The hydrocarbon stream may be any suitable feed stream to be treated, but is 5 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 10 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 is as some aromatic hydrocarbons. The natural gas stream may also contain non hydrocarbons such as H 2 0, N 2 , CO 2 , H 2 S and other sulphur compounds, and the like. If desired, the feed stream containing the natural gas may be pre-treated before passing it to the distillation unit. This pre-treatment may comprise removal of undesired 20 components such as CO 2 and H 2 S, or other steps such as upstream cooling, pre pressurizing or the like. These steps are well known to the person skilled in the art, they are not further discussed here. Where upstream cooling is included, another preferred embodiment of the 25 present invention is a method of treating a hydrocarbon stream such as natural gas from a feed stream further the step of: 6 cooling the feed stream upstream of step (a) by passing the feed stream against one or more refrigerants being in one or more of the refrigerant circuits. In this way, the present invention at least in a preferred embodiment provides a 5 method of treating comprising at least the steps of: (a) cooling the feed stream against one or more refrigerants being in one or more refrigerant circuits involving one or more refrigerant compressors; (b) passing the cooled feed stream through a distillation column to provide a gaseous stream and a C 2 ' liquid stream; 10 (c) compressing at least part of the gaseous stream through one or more feed compressors to provide a compressed stream; and (d) cooling the compressed stream; wherein at least one of the feed compressors in step (c) and at least one of the refrigerant compressors in step (a) are mechanically interconnected and are arranged to be driven by a common driver. 15 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 20 or more compounds or substances, including but not limited to sulfur, carbon dioxide, water, and C2+ hydrocarbons. The compressors are mechanically interconnected in the sense that there is physical linkage therebetween which relates motion therebetween. Preferably said 25 mechanically interconnected compressors are arranged on a common drive shaft of the driver, further reducing the capital infrastructure.

7 The distillation column may be any column or arrangement adapted to separate the feed gas into a gaseous stream, which will generally be methane-enriched, and a C2+ liquid stream, which will generally have >40 mol% of one or more hydrocarbons heavier than methane, such as ethane, propane, butanes, pentanes, C 6 +, etc, as well as usually a 5 proportion of methane. At least part of the heavier stream is commonly used to produce a natural gas liquid product or products, such as usually C 3 , C 4 , etc. The distillation column is preferably a natural gas liquid recovery column, generally for C 3 and C 4 hydrocarbons, optionally C 5 hydrocarbons, and which generally 1o operates at a low pressure. Following its separation in the distillation column, at least a part of the gaseous stream, usually all of the gaseous stream, is compressed. Said compression may be considered a 'recompression' where the feed gas is provided in a pressurized form. It is is desirable to compress or recompress the gaseous stream as it is easier to liquefy natural gas at high pressure. The compression of the gaseous stream can be carried out by one or more feed compressors such as natural gas booster compressors. In one preferred embodiment of the 20 present invention there are two feed compressors. In another preferred embodiment to the present invention, one, usually the first, feed compressor is mechanically interconnected to an expander adapted to expand the feed gas prior to its passage through the distillation column, so as to be partly, 25 substantially or wholly driven thereby. This provides a more efficient arrangement. Drivers for a compressor are known in the art. They may be a single generator, or a combination of generators. They include gas turbines, steam turbines and electric motors, and the generator or combination can be adapted to suit the apparatus, 30 arrangement or system, some of which may include generator assistance from one or 8 more other parts of the apparatus. In one arrangement, there is an auxiliary engine such as an electric motor to help start-up, and a gas turbine for the main running for the driver. In one preferred embodiment of the present invention, the cooling of the 5 compressed stream involves liquefying the compressed stream thereby obtaining a liquefied hydrocarbon stream such as liquefied natural gas. The present invention at least in a preferred embodiment also provides a method of treating a hydrocarbon stream such as natural gas from a feed stream comprising at 1o least the steps of: (a) liquefying the feed stream by cooling the feed stream against one or more refrigerants being in one or more refrigerants circuits involving one or more refrigerant compressors, to provide a liquefied feed stream; (b) passing the liquefied feed stream through an end separation vessel to is provide an end gaseous stream and an end liquid stream; and (c) compressing at least a part of the end gaseous stream through one or more end compressors to provide a compressed end gaseous stream; wherein at least one of the refrigerant compressors in step (a) at least one of the end compressors in step (c) are mechanically interconnected and are arranged to be driven 20 by a common driver. The feed stream may be the compressed stream that has been compressed through the one or more feed compressors as described hereinabove. 25 The end separation vessel may be an end flash vessel. The liquefying of the hydrocarbon stream in step (a) may involve at least a sub cooling stage, said sub-cooling stage having a sub-cooling refrigerant circuit, said circuit involving at least one sub-cooling refrigerant compressor, and wherein at least one said 9 sub-cooling refrigerant compressor may be mechanically interconnected with at least one end compressor. This method has similar benefits as hereinbefore described, in particular higher 5 process efficiency and lower capital and running costs. The present invention at least in a preferred embodiment includes a combination of any and all of the methods hereinbefore described, and mechanical interconnection of more than two compressors with a common driver. 10 The present invention at least in a preferred embodiment also provides apparatus for treating a hydrocarbon feed stream such as natural gas, the apparatus at least comprising: a distillation column having an inlet for the feed stream and a first outlet for a is gaseous stream and a second outlet for a C 2 + liquid stream; one or more feed compressors for compressing at least a part of the gaseous stream; and a common driver for driving one or more of the feed compressors with one or more separate refrigerant compressors for one or more separate refrigerant circuits. 20 Preferably, the apparatus includes a cooling system which may involve at least one pre-cooling stage and at least one main cooling stage, each stage including at least one refrigerant circuit and at least one compressor. 25 Also preferably, the cooling system involves or comprises a liquefying system, more preferably able to provide an LNG stream. Optionally, at least one compressor of both the pre-cooling and main cooling refrigerant circuits are also mechanically interconnected and are arranged to be driven by 30 a common driver.

10 Brief Description of the Drawings Embodiments of the present invention will now be described by way of example only, and with reference to the accompanying non-limiting drawings, in which: Figure 1 is a general scheme of a treatment process according to one 5 embodiment of the present invention; Figure 2 is a general scheme of a treatment process according to a second embodiment of the present invention; and Figure 3 is a general scheme of a treatment process according to a third embodiment of the present invention. 10 Detailed Description of the Preferred Embodiments 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. 15 Referring to the drawings, Figure 1 shows a treatment process for a hydrocarbon feed stream, such as 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. 20 Following any pre-treatment, the feed stream 10 could be expanded through a vapour expander 22 and then fed via inlet 52 into a distillation column 12 being a natural gas liquid recovery column, which preferably operates between O'C and -100'C, and at an above ambient pressure such as 10-30 bar. 25 The expander 22 can be in the form of a turbine. It reduces the pressure of the feed gas 10. In the distillation column 12, the expanded feed stream 10a is separated into a 30 gaseous stream 20, generally being a methane enriched stream and provided through a first outlet 54, and a C 2 * liquid stream 30, WO 2007/135062 PCT/EP2007/054772 - 11 generally being a heavier hydrocarbon rich stream and provided through a second outlet 56. The gaseous stream 20 from the first outlet 54 can then be re-compressed to make its subsequent cooling, 5 preferably liquefaction, easier. Preferably, prior to re compression, the gaseous stream 20 is heat-exchanged (not shown) against feed stream 10 to cool feed stream 10. The recompression may be to the same or similar pressure of the feed stream 10 before it entered the expander 22, or 10 may be different. In Figure 1, the recompression can be achieved in two stages. Firstly, the gaseous stream 20 is passed through a first feed compressor 14. This first compressor 14 is driven by the expander 22 via an interconnecting drive shaft 24, so recovering at least 15 some of the energy created by the expansion of the feed gas 10 passing through the expander 22. Thereafter, the part-compressed gaseous stream 20a is passed through a second feed compressor 16. Such second compressor 16 is usually larger than the first feed 20 compressor 14. Hitherto, the second feed compressor 16 has been driven by its own dedicated driver, generally being a gas or steam turbine, and usually with an electric start-up motor as well. As mentioned above, every separate driver 25 increases the capital and running costs of the plant. Indeed, thirty to forty percent of the capital cost in a liquefied natural gas (LNG) plant lies in the cooling plant proper. Thus minimizing the capital cost of a cooling operation is particularly advantageous. 30 An advantage of the present invention is to mechanically interconnect the second feed compressor 16 with a refrigerant compressor 18 of one of the refrigerant circuits described hereinafter. Preferably, WO 2007/135062 PCT/EP2007/054772 - 12 the two interconnected compressors 16, 18 are driven by a common driver 28. The driver 28 could be a gas turbine having an associated starter/helper motor. According to an embodiment of the present invention 5 involving two or more feed compressors, at least one of the feed compressors is mechanically interconnected with at least one of the refrigerant compressors. Where there are multiple feed compressors and the first is mechanically interconnected with an expander adapted to 10 expand the feed stream prior to separation, then it is the largest feed compressor which is preferably driven with the refrigerant compressor. Following recompression of the gaseous stream 20 through the first and second compressors 14, 16, the 15 compressed stream 40 is optionally first cooled by one or more ambient water and/or air coolers (not shown) known in the art, followed by cooling against one or more refrigerants as discussed further below. The cooling of the compressed stream 40 can be 20 carried out by a cooling system having or including any number of cooling stages. One common arrangement involves a pre-cooling or first cooling stage, and one or more, usually one, main cooling stages. Each cooling stage generally involves at least one refrigerant, which 25 refrigerant(s) are generally being circulated in a refrigerant circuit(s). Each cooling stage may also involve one or more steps, levels or sections, and there may also be a final sub-cooling stage. In the embodiment shown in Figure 1, the cooling is 30 or involves liquefying of the compressed stream 40 by a liquefying system which has one or more cooling and/or liquefying stages. This could involve a pre-cooling stage 26 and a main cooling stage 62. The pre-cooling stage 26 WO 2007/135062 PCT/EP2007/054772 - 13 involves a pre-cooling refrigerant circuit 32 which has at least one separate refrigerant compressor 18 for compressing its refrigerant, such as propane or a mixed refrigerant, to a higher pressure. A heat exchanger, 5 which could be one or more air coolers or other condensers, is provided downstream of the compressor 18 to cool the refrigerant with heat exchanged with a coolant, typically air and/or water. Whilst a single refrigerant compressor 18 is shown in 10 the pre-cooling refrigerant circuit 32 in Figure 1, it is envisaged that two or more separate or single casing compressors, which could optionally also be mechanically interconnected, may be used, optionally arranged on a common drive shaft. 15 The refrigerant compressor 18 of the pre-cooling refrigerant circuit 32 is driven by a gas turbine 28 which could have an associated starter/helper motor. The second (and main) feed compressor 16 is mechanically interconnected to the separate refrigerant compressor 18 20 of the pre-cooling refrigerant circuit 32, preferably by being provided on a common drive shaft, whereby the gas turbine 28 also drives the second feed compressor 16, thus avoiding the need for two separate gas turbines or other drivers and associated capital and running costs. 25 The distribution of power from the gas turbine 28 between the pre-cooling refrigerant compressor 18 and the second feed compressor 16 can be freely chosen, such that the optimal power balance can be achieved. In particular, it is desired for the compressor power input from the gas 30 turbine 28 to be as close as possible to the optimal ratio for natural gas liquefaction. Large industrial gas turbines are known which are able to provide such power, and a changeable power input allows for variation in WO 2007/135062 PCT/EP2007/054772 - 14 loads of the compressors where non-steady state conditions are involved. Typically, the liquefying system also includes a main cooling stage 62 which has a separate refrigeration 5 circuit 64, and which generally also includes one or more separate refrigerant compressors 66. A non-limiting example of a typical main refrigerant is a mixture of compounds having different boiling points in order to obtain a well-distributed heat transfer. One mixture is 10 nitrogen, ethane and propane. Like the refrigerant of the pre-cooling circuit 32, the main refrigerant is separate from the feed stream 10, compressed stream 40 and its downstream pre-cooled form 50. The main cooling stage 62 could have two or more 15 compressors for compressing the main refrigerant. The main refrigerant could then be passed through a heat exchanger where it is cooled by heat exchange with a coolant, such as water. The main refrigerant could then be further cooled by heat exchange with the refrigerant 20 of the pre-cooling refrigerant circuit 32. In the or a heat exchanger of the main cooling stage 62, the expansion of the main refrigerant further cools and optionally liquefies the pre-cooled stream 50 from the pre-cooling process by heat exchange, to provide a 25 further cooled, preferably liquefied, product stream 60. It is possible that one or more of the main refrigerant compressors 66 could be mechanically interconnected with the pre-cooling refrigerant compressor 18 of the pre-cooling refrigerant circuit 32, 30 and/or the second feed compressor 16, so as to be driven by a common driver, possibly on a common drive shaft. Figure 2 shows a second arrangement for treatment of a hydrocarbon stream prior to any cooling and/or WO 2007/135062 PCT/EP2007/054772 - 15 liquefaction. Compared to the arrangement shown in Figure 1, the feed stream 10 is cooled prior to its expansion through the expander 22. The cooling can be carried out by an upstream or prior cooling stage 34 wherein the feed 5 stream 10 is cooled against an upstream refrigerant circuit 36. The upstream refrigerant circuit 36 involves a compressor 38 for compressing its separate refrigerant media to a higher pressure. Similar to Figure 1, in the arrangement in Figure 2, 10 the expanded feed gas 10a is fed via an inlet 52 into a distillation column 12, thereby providing a gaseous stream 20 via a first outlet 54, and providing a C2+ liquid stream 30 via a second outlet 56. The gaseous stream 20 can then be recompressed through a first feed 15 compressor 14 and second feed compressor 16. The compressed light stream 40 can then be cooled and/or liquefied as is known in the art, such as the liquefying system of Figure 1. In the arrangement shown in Figure 2, the second feed 20 compressor 16 is mechanically interconnected with the compressor 38 in the upstream refrigerant circuit 36 for the upstream cooling stage 34. Preferably, the two interconnected compressors 16, 38 are driven by a common driver 42. The driver 42 could be a gas turbine having an 25 associated starter/helper motor. The arrangement shown in Figure 2 has an advantage like that shown in Figure 1, that is avoiding the need for two separate gas turbines or other drivers for the second feed compressor 16 and an upstream refrigerant 30 compressor 38. As with the arrangement shown in Figure 1, the distribution of power from the gas turbine 42 between the compressors 16, 38 can be freely chosen, such that the optimal power balance can be achieved.

WO 2007/135062 PCT/EP2007/054772 - 16 Liquefied natural gas 60 from a liquefying system can be passed into 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 5 liquefied natural gas product can be transferred to a storage vessel or other storage or transportation apparatus. Figures 1 and 2 exemplify two possible arrangements of the present invention for commonly driving at least 10 one of the feed compressors with one or more separate refrigerant compressors for one or more separate refrigerant circuits. Figure 3 shows a third scheme for treating a hydrocarbon feed gas stream 100, particularly the end 15 flash processing of an LNG stream. One source of the hydrocarbon feed gas stream 100 is the compressed stream 40 shown in Figures 1 and 2. Liquefaction of a hydrocarbon stream 100 by a liquefying system is known by the skilled person in the 20 art. Liquefaction can be carried out in a number of stages, and for the sake of simplicity only, Figure 3 shows a final sub-cooling stage 102 only. The sub-cooled stream 110 can pass through an expander 104 to provide stream 110a, and a water and/or air cooler 106 to provide 25 stream 110b, which passes via inlet 122 into an end separation vessel 108, in this instance being an end flash vessel known in the art. In general, an end flash system such as that shown in Figure 3 can be used at the downstream end of the sub 30 cooling stage 102 to optimize liquefied natural gas (LNG) production. It usually includes an end compressor driven by a separate electric drive motor. The power needed to WO 2007/135062 PCT/EP2007/054772 - 17 drive the end compressor is a usually smaller than the required compressor power for the sub-cooling stage. The end separation vessel 108 has a first outlet 124 to provide a liquid stream 120 such as LNG, which, via a 5 pump 128, can create a final LNG stream 130 for storage and/or transportation. From a second outlet 126 of the end separation vessel 108 there is provided an end gaseous stream 140, which could be combined with, for example, a boil-off gas 10 stream 150 known in the art. The combined stream 160 can be compressed by an end compressor 114 to provide a useable stream 170, such as for fuel gas. The sub-cooling stage 102, involves a refrigerant circuit 132 having a first refrigerant compressor 116. 15 Optionally, the sub-cooling refrigerant circuit 132 also includes an air or water cooler 117. In the arrangement shown in Figure 3, the first refrigerant compressor 116 is mechanically interconnected with the end compressor 114. Preferably, the two 20 interconnected compressors 114, 116 are driven by a common driver 134. The driver 134 could be a gas turbine having an associated starter/helper motor. The arrangement shown in Figure 3 has an advantage like that shown in Figures 1 and 2, that is avoiding the 25 need for two separate gas turbines or other drivers for the end compressor 114 and first refrigerant compressor 116. As with the arrangement shown in Figures 1 and 2, the distribution of power from the driver 134 between the compressors 114, 116 can be freely 30 chosen, such that the optimal power balance can be achieved. The liquefaction in Figure 3 may involve other cooling stages such as pre-cooling and main cooling WO 2007/135062 PCT/EP2007/054772 - 18 stages. Any or each such cooling stage of the liquefaction may involve one or more refrigerant circuits, and involve one or more refrigerant compressors. It is possible that one or more of the other 5 refrigerant circuit compressors used in the liquefaction, from any stage thereof, could be mechanically interconnected with the end compressor 114, so as to be driven by a common driver, possibly on a common drive shaft. 10 The following Table provides example temperature, pressure, flowrate and phase data for an embodiment of the present invention exemplified in Figure 1. Temperature Pressure Flowrate Phase Stream 0 C Bar kmol/s 15 10 -28 57 17 vapor 10a -69 22 17 mixed 30 -58 22 3 liquid 20 -76 22 18 vapor 20a 48 29 18 vapor 40 88 73 18 vapor 50 -27 71 18 vapor 20 60 -150 66 18 liquid 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.

Claims

1. A method of treating a hydrocarbon stream such as natural gas from a feed stream comprising at least the steps of: 5 (a) passing the feed stream through a distillation column to provide a gaseous stream and a C 2 + liquid stream; (b) compressing at least part of the gaseous stream through one or more feed compressors to provide a compressed stream; and (c) commonly driving at least one of the feed compressors with one or more io separate refrigerant compressors for one or more separate refrigerant circuits, by mechanically interconnecting said compressors.

2. The method as claimed in claim 1, wherein the distillation column is a natural gas liquid recovery column. 15

3. The method as claimed in claim I or claim 2, wherein the feed stream is expanded prior to passing through the distillation column.

4. The method as claimed in any one of claims 1 to 3, wherein the compressing of 20 step (b) involves first and second feed compressors.

5. The method as claimed in any one of the preceding claims, the method further comprising the step of: cooling the compressed stream by passing the compressed stream against one or 25 more refrigerants being in one or more of the refrigerant circuits.

6. The method as claimed in claim 5, wherein the second feed compressor is mechanically interconnected with a refrigerant compressor of one or more of the refrigerant circuits cooling the compressed stream. 30

7. The method as claimed in claim 5 or claim 6, wherein the cooling of the compressed stream involves at least two cooling stages, each stage including at least one refrigerant circuit. 20

8. The method as claimed in claim 7, wherein the cooling of the compressed stream involves one pre-cooling stage and one main cooling stage, and the or a feed compressor of step (b) is mechanically interconnected with a refrigerant compressor of the pre cooling refrigerant circuit. 5

9. The method as claimed in any one of claims 5 to 8, wherein the cooling of the compressed stream involves liquefying the compressed stream thereby obtaining a liquefied hydrocarbon stream such as liquefied natural gas. io

10. The method as claimed in any one of the preceding claims, the method further comprising the step of: cooling the feed stream upstream of step (a) by passing the feed stream against one or more refrigerants being in one or more of the refrigerant circuits. 15

11. The method as claimed in claim 10, wherein the or a feed compressor of step (b) is mechanically interconnected with a refrigerant compressor of the refrigerant circuit cooling the feed stream.

12. The method as claimed in any one of the preceding claims further comprising the 20 step of: cooling the feed stream upstream of step (a) against at least part of the gaseous stream.

13. An apparatus for treating a hydrocarbon stream such as liquefied natural gas 25 from a feed stream, the apparatus at least comprising: a distillation column having an inlet for the feed stream and a first outlet for a gaseous stream and a second outlet for a C 2 + liquid stream; one or more feed compressors for compressing at least a part of the gaseous stream; and 30 a common driver for driving one or more of the feed compressors with one or more separate refrigerant compressors for one or more separate refrigerant circuits.

14. The apparatus as claimed in claim 13, wherein the commonly driven compressors are mechanically interconnected and arranged on a common drive shaft of 35 the driver. 21

15. The apparatus as claimed in claim 13 or claim 14, wherein the apparatus includes an expander to expand the feed stream upstream of the distillation column.

16. The apparatus as claimed in claim 15, wherein the expander is mechanically 5 interconnected to one or more of the feed compressors and is partly, substantially or wholly driven thereby.

17. The apparatus as claimed in any one of claims 13 to 16, wherein the apparatus further includes a cooling system for cooling the gaseous stream, which cooling system 10 involves at least one pre-cooling stage and at least one main cooling stage, each stage including at least one refrigerant circuit and at least one refrigerant compressor.

18. The apparatus as claimed in claim 15, wherein one feed compressor of step (b) is mechanically interconnected with a refrigerant compressor of a pre-cooling refrigerant is circuit.

19. The apparatus as claimed in claim 15 or claim 16, wherein at least one refrigerant compressor of both the pre-cooling and main cooling refrigerant circuits are mechanically interconnected and are arranged to be driven by said common driver. 20

20. The apparatus as claimed in any one of claims 13 to 19, wherein the cooling system includes a liquefying system to obtain a liquefied hydrocarbon stream such as liquefied natural gas. 25

21. A method of treating a hydrocarbon stream such as natural gas from a feed stream, substantially as hereinbefore described with reference to the accompanying drawings.

22. An apparatus for treating a hydrocarbon stream such as liquefied natural gas 30 from a feed stream substantially as hereinbefore described with reference to the accompanying drawings. Dated 17 May, 2010 Shell Internationale Research Maatschappij B.V. Patent Attorneys for the Applicant 35 SPRUSON & FERGUSON