The present invention relates to a method and apparatus for treating a mixed hydrocarbon stream. In another aspect, the present invention relates to a method of cooling an initial hydrocarbon stream. A common example of a mixed hydrocarbon stream is natural gas, which often consists of multiple components.

Natural gas is a useful fuel source, as well as being a source of various hydrocarbon compounds. It is often desirable to liquefy natural gas in a liquefied natural gas (LNG) plant at or near the source of 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 small volume and does not need to be stored at high pressure.

Usually, natural gas, comprising predominantly methane, enters an LNG plant at elevated pressures and is pre-treated to produce a purified feed stream suitable for liquefaction at cryogenic temperatures. The purified gas is processed through a plurality of cooling stages using heat exchangers to progressively reduce its temperature until liquefaction is achieved. The liquid natural gas is then further cooled and expanded to final atmospheric pressure suitable for storage and transportation.

In addition to methane, natural gas usually includes some heavier hydrocarbons and impurities, including but not limited to carbon dioxide, sulphur, hydrogen sulphide and other sulphur compounds, nitrogen, helium, water and other non-hydrocarbon acid gases, ethane, propane, butanes, C5+ hydrocarbons and aromatic hydrocarbons.

These and any other common or known heavier hydrocarbons and impurities either prevent or hinder the usual known methods of liquefying the methane, especially the most efficient methods of liquefying methane. Most if not all known or proposed methods of liquefying hydrocarbons, especially liquefying natural gas, are based on reducing as far as possible the levels of at least most of the heavier hydrocarbons and impurities prior to the liquefying process.

Hydrocarbons heavier than methane and usually ethane are typically condensed and recovered as natural gas liquids (NGLs) from a natural gas stream. The NGLs are usually fractionated to yield valuable hydrocarbon products, either as products steams per se or for use in liquefaction, for example as a component of a refrigerant .

Meanwhile, methane recovered from the NGL recovery is usually recompressed for use or reuse either in the liquefaction, such as a fuel gas, or being recombined with the main methane stream being liquefied, or it can be provided as a separate stream.

EP 1 031 803 A2 describes a method and apparatus for maximising the production rate of NGL in a gas processing plant. Natural gas passes through a turboexpander, a recompressor and a booster compressor, each having an antisurge valve and a cold recycle valve. If an operating point of either compressor achieves a set point of its CRIC controller or of the two UIC controllers, specific signals open the cold recycle valve first.

A problem with EP 1 031 803 A2 is that cold recycle of a fully compressed stream around the recompressor will also affect the pressure in the stream it joins from the separator, which will affect the pressure in the separator itself, changing the operation of the separator and thus its separation efficiency. In a first aspect, the present invention provides an apparatus for treating a mixed hydrocarbon feed stream, the apparatus at least comprising an NGL recovery system comprising :

(a) an expander to expand the mixed hydrocarbon feed stream into a mixed-phase hydrocarbon stream;

(b) a first gas/liquid separator to separate the mixed- phase hydrocarbon stream to provide at least a light overhead stream and a heavy bottom stream;

(c) a first compressor having a first inlet and a first outlet, to compress the light overhead stream and to provide a first compressed light stream;

(d) a second compressor having a second inlet and a second outlet, to compress the first compressed light stream to provide a further compressed light stream;

(e) a first compressor recycle line around the first compressor between the first outlet and the first inlet; and

(f) a second compressor recycle line provided with one or more in-line coolers around the second compressor between the second outlet and the second inlet.

This apparatus may be comprised in an apparatus, such as a plant or facility, for liquefying a hydrocarbon stream, such as natural gas. Such a liquefying apparatus may further comprise at least one or more cooling stages upstream and/or downstream of the NGL recovery system. In a particular embodiment, the liquefying apparatus may further comprise - A -

a first cooling stage of the one or more cooling stages, able to cool an initial hydrocarbon stream to provide a cooled and partly condensed initial hydrocarbon stream; a separator able to separate the cooled and partly condensed initial hydrocarbon stream to provide the mixed hydrocarbon feed stream and a methane enriched overhead stream; and a second cooling stage of the one or more cooling stages, able to cool the overhead stream to provide a combined liquefied hydrocarbon stream.

In a second aspect, the present invention also provides a method of treating a mixed hydrocarbon feed stream, the method at least comprising the steps of: (a) providing a mixed hydrocarbon feed stream;

(b) expanding the mixed hydrocarbon feed stream through an expander to provide a mixed-phase hydrocarbon stream;

(c) passing the mixed-phase hydrocarbon stream into a first gas/liquid separator to provide at least a light overhead stream and a heavy bottom stream;

(d) passing the light overhead stream through a first compressor having a first inlet and a first outlet, to provide a first compressed light stream; (e) passing the first compressed light stream through a second compressor having a second inlet and a second outlet, to provide a further compressed light stream; and (f) selectively recycling at least a fraction of the first compressed light stream through a first compressor recycle line, around the first compressor between the first outlet and the first inlet; and (g) selectively cooling and recycling at least a fraction of the further compressed light stream through a second compressor recycle around the second compressor between the second outlet and the second inlet, the second compressor recycle line provided with one or more in-line coolers. In a third aspect, the present invention also provides a method of cooling, preferably liquefying, an initial hydrocarbon stream, such as a natural gas stream, comprising at least the steps of:

(i) passing the initial hydrocarbon stream through a separator to provide a stabilized condensate stream and a mixed hydrocarbon feed stream;

(ii) treating the mixed hydrocarbon feed stream by a method according to the third aspect of the invention; and

(iii) cooling, preferably liquefying, at least a fraction of the one or more further compressed light streams to provide a cooled, preferably liquefied, hydrocarbon stream.

Embodiments and examples of the present invention will now be described by way of example only with reference to the accompanying non-limited drawings in which; Figure 1 is a diagrammatic scheme for a first apparatus and method for treating a mixed hydrocarbon stream according to one embodiment of the present invention;

Figure 2 is a diagrammatic scheme of a method of cooling an initial hydrocarbon stream including embodiments shown in Figure 1; and

Figure 3 is a diagrammatic scheme for a second apparatus and method for cooling an initial hydrocarbon stream including treating a mixed hydrocarbon stream according to a second embodiment of the present invention .

For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line.

The methods and apparatus disclosed herein may form part of or be used in a multi-column natural gas liquids (NGL) recovery system and arrangement. An apparatus for treating a mixed hydrocarbon feed stream wherein recompression of the light overhead stream from the first gas/liquid separator is carried out by one or more first compressors and one or more second compressors, each of the compressors having a separate recycle line therearound, is provided.

An advantage of this arrangement is a simplified apparatus and simplified and easier individual compressor control compared with that shown in EP 1 031 803 A2.

A second advantage of the apparatus disclosed herein is that the or each recycle line around the or each first compressor does not need to be cooled by a cooler, such as an expensive water and/or air cooler, as the or each first recycle line is dedicated to the or each first compressor. Thus, only the recycle stream around the second compressor requires cooling, significantly reducing the cooling duty required compared to the cooling duty required for the complete recompressed stream in EP 1 031 803 A2. This therefore significantly reduces the CAPEX and OPEX required for cooling only a recycle stream.

By ^λ recycle line' is meant a branch line from downstream of the or each outlet of the one or more first or second compressors which is connected upstream from the or each inlet of the one or more first or second compressors respectively.

For example, the one or more first compressor recycle lines may have one or more first recycle line outlets upstream of the one or more first inlets of the one or more first compressors and one or more first recycle line inlets downstream of the one or more first outlets of the one or more first compressors. As a further example, the one or more second compressor recycle lines may have one or more second recycle line outlets upstream of the one or more second inlets of the one or more second compressors and one or more second recycle line inlets downstream of the one or more second outlets of the one or more second compressors. More preferably the one or more second recycle line outlets lie downstream of the one or more first recycle line inlets.

Referring to the drawings, Figure 1 shows an apparatus and method for treating a mixed hydrocarbon stream 10 such as an NGL recovery system 1, and Figure 2 shows a simplified and first general scheme of a liquefied natural gas plant 2 for a method for cooling an initial hydrocarbon stream 100, including the NGL recovery system 1 of Figure 1.

An initial hydrocarbon stream 100 may be any suitable hydrocarbon stream such as, but not limited to, a hydrocarbon-containing gas stream able to be cooled. One example is a natural gas stream obtained from a natural gas or petroleum reservoir. 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 such an initial hydrocarbon stream is comprised substantially of methane. Preferably such an initial feed stream comprises at least 50 mol% methane, more preferably at least 80 mol% methane.

The NGL recovery system 1 usually involves one or more gas/liquid separators 14 such as distillation columns and/or scrub columns to separate the mixed hydrocarbon feed stream 10 into at least a light stream and one or more heavy streams at relatively low pressure, for example in the range of 20 to 35 bar. As the mixed hydrocarbon feed stream 10 is usually provided from a high pressure initial hydrocarbon stream 100, for example in the range of 40 to 70 bar, it needs to be expanded prior to the separator, for instance using one or more expanders 12.

Any form of gas/liquid separator adapted to provide at least one overhead stream, usually a gaseous overhead stream, and usually an overhead stream enriched in one or more lighter hydrocarbons such as methane, and at least one bottom stream, usually a liquid stream, and usually enriched in one or more heavier hydrocarbons, is suitable. In certain circumstances, an overhead stream and/or a bottom stream may be a mixed phase stream.

An example of a suitable first gas/liquid separator 14 is a "demethanizer" designed to provide a methane- enriched overhead stream, and one or more C2+ streams in the form of liquid streams at or near the bottom enriched in C2+ hydrocarbons. However, depending on composition of the mixed hydrocarbon feed stream and the desired specification of the light overhead stream, the first gas/liquid separator 14 may be a de-ethanizer, a de- propanizer, or a de-butanizer or a scrub column, instead of a de-methanizer .

The term "mixed hydrocarbon feed stream" as used herein relates to a feed stream comprising methane (C]_) and at least 5 mol% of one or more hydrocarbons selected from the group comprising: ethane (C2), propane (C3) , butanes (C4), and C5+ hydrocarbons.

The terms "light" and "heavy" are defined relative to each other, and make reference to the overhead stream respectively the bottom stream from the one or more gas liquid separators 14. The composition of the "light" and "heavy" hydrocarbon streams depends on the composition of the feed gas as well as on the design and operation conditions of the gas liquid separators.

The term "heavy hydrocarbon stream" relates to a stream comprising a relatively higher content of heavier hydrocarbons than the light overhead stream. For instance, the heavy hydrocarbon stream could be a C2+ hydrocarbon stream, which predominantly comprises ethane (C2) and heavier hydrocarbons. The relative amount of ethane is higher than the relative amount of ethane in the feed stream, but a C2+ stream could still comprise some methane. Likewise, a C3+ hydrocarbon stream, a C4+ hydrocarbon stream or a C5+ hydrocarbon stream is relatively rich in propane and heavier, butanes and heavier, or, respectively, pentanes and heavier.

The light overhead stream may still comprise a minor (<10 mol%) amount of C2+ hydrocarbons (ethane and heavier) , but is preferably >80 mol%, more preferably >95 mol% methane.

Figure 2 shows an initial hydrocarbon stream 100 containing natural gas, which is cooled by a first cooling stage 104 to provide a cooled and partly condensed initial hydrocarbon stream 110. The first cooling stage 104 may comprise one or more heat exchangers either in parallel, series or both, in a manner known in the art. The provision of cooling to the first stage cooling 104 is known to the person skilled in the art .

The cooling of the initial hydrocarbon stream 100 may be part of a liquefaction process, such as a pre- cooling stage involving a propane refrigerant circuit (not shown), or a separate process.

Cooling of the initial hydrocarbon stream 104 may involve reducing the temperature of the initial hydrocarbon stream 104 to below -0 ⁰ C, for example, in the range -10 ⁰ C to -70°C.

The cooled initial hydrocarbon stream 110 is passed into a separator such as a condensate stabilisation column 108, usually operating at an above ambient pressure in a manner known in the art. The condensate stabilisation column 108 provides overhead a mixed hydrocarbon feed stream 10, preferably having a temperature below -0 ⁰ C, and a bottom stabilized condensate steam 120. The overhead mixed hydrocarbon feed stream 10 is an enriched-methane stream compared to the cooled initial hydrocarbon stream 110.

The mixed hydrocarbon feed stream 10 comprises methane and one or more of C2, C3, C4 and C5+ hydrocarbons. Typically, the proportion of methane in the mixed hydrocarbon feed stream 10 is 30-50 mol%, with significant fractions of ethane and propane, such as 5-10 mol% each.

In NGL recovery, it is desired to recover methane in a mixed hydrocarbon stream (for example, for use as a fuel or to be liquefied in the LNG plant 2 and provided as additional LNG) , and to provide at least a C2+ stream, optionally one or more of a C2 stream, a C3 stream, a C4 stream, and a C5+ stream (not shown) . In Figures 1 and 2, at least a fraction, usually all, of the mixed hydrocarbon feed stream 10 passes into the NGL recovery system 1. The mixed hydrocarbon feed stream 10 passes through one or more expanders 12 to provide a reduced pressure and mixed-phase (liquid and vapour) hydrocarbon stream 20, and then enters the first gas/liquid separator 14 at a suitable height. The first gas/liquid separator 14 is adapted to separate the liquid and vapour phases, so as to provide a light overhead stream 30 and a heavy bottom stream 50. The first gas/liquid separator 14 may include a reboiler and a first reboiler vapour return stream (not shown) in a manner known in the art.

The nature of the streams provided by the first gas/liquid separator 14 can be varied according to the size and type of separator, and its operating conditions and parameters, in a manner known in the art. For the arrangement shown in Figures 1 and 2, it is desired for the light overhead stream 30 to be methane-enriched, preferably to be >90 mol% methane.

The heavy bottom stream 50 can be >90 or >95 mol% ethane and heavier hydrocarbons, and can be subsequently fractionated or otherwise used in a manner known in the art for an NGL stream. The light overhead stream 30 can now be recompressed by one or more first compressors 16 and one or more second compressors 22. For this purpose, Figures 1 and 2 show one or more first compressors 16, having a first inlet 17 and first outlet 18, and one or more second compressors 22 having a second inlet 23 and a second outlet 24.

The one or more second compressors 22 are provided downstream of the one or more first compressors 16 such that a second inlet of the one or more second compressors 22 can receive at least part of the first compressed light stream from a first outlet of the one or more first compressors 16. Preferably, there is no cooler present in the line between the first outlet of the one or more first compressors 16 and the one or more second compressors 22, such that the one or more second compressors receive an uncooled first compressed light stream. Compression of a methane-rich gaseous stream is known in the art, and the first and second compressors 16, 22 may comprise any known apparatus, device or unit in one or more sections, steps or stages able to increase the pressure on the light stream. Types and forms of suitable compressors and recompressors are well known in the art .

In one embodiment disclosed herein, one or more of the expanders 12 prior to the first gas/liquid separator 14 are mechanically-linked to one or more of the first compressors 16. Such mechanical-linking may occur by any known linkage, one example of which is shared or common driveshaft 21. The mechanical linking of an expander and a compressor, in order to use some of the work energy provided from the expander by the expansion of a gas therethrough, to partly or fully drive a mechanically linked compressor, is known in the art. Figures 2 and 3 herewith show such a driveshaft 21, whilst the same is shown in Figure 1 in broken form for schematic layout purposes only. In this way, operation and performance of the first compressor 16 is related to operation and performance of the expander 12 as discussed further hereinafter. Each of the first compressor (s) 16 is able to compress at least a fraction of the light overhead stream 30 to provide a first compressed light stream 40 in a manner known in the art. Between the first outlet 18 and first inlet 17 of each first compressor 16, there is a first compressor recycle line 42 which is able to take at least a fraction of the first compressed light stream 40 from a first compressor recycle stream inlet 41 and recycle it back into the path of the light overhead stream 30 via first compressor recycle stream outlet 45. The division of the first compressed light stream 40 between a first compressed continuing stream 52 and a first recycle stream 42 may be carried out by any suitable divider or stream splitter known in the art. The division of the first compressed light stream may be anywhere between 0- 100% for each of the continuing stream 52 and first recycle stream 42 as discussed further hereinafter.

The first compressor recycle line 42 is a dedicated line around the first compressor 16 and preferably only includes one or more control valves 44 required to change the pressure of the first compressor recycle stream 42 to approximate or equate its pressure to the intended pressure of the C2 overhead stream 30 for the suction side of the first compressor 16. In particular, it is noted that there is no cooler or coolers on the first compressor recycle line 42 (adapted to change the temperature of the first compressor recycle stream 42, generally downwardly, whilst the pressure of the first compressor recycle stream 42 is wholly or substantially unchanged) . Thus, the CAPEX and OPEX of needing one or more coolers is avoided, whilst the first compressor recycle line 42 still provides anti-surge control around the first compressor 16.

In this way, the first compressor recycle stream 42 is uncooled, and/or the first compressor recycle line 42 is an uncooled recycle line.

The first compressed continuing stream 52, being some or all of the first compressed light stream 40, may then pass through an optional one or more throttle control valves 26, and then pass as a second compressor feed stream 54 into the one or more second compressors 22, each second compressor 22, to provide one or more further compressed light streams 60 in a manner known in the art. The or each second compressor 22 may be the same or similar to a ^λ boost' compressor, generally having a dedicated driver or drive mechanism separate from the one or more first compressors 16.

Around the or each second compressor 22 is a second compressor recycle line 32, such that the one or more further compressed light streams 60 can be divided by a divider or stream splitter known in the art, anywhere between 0-100%, between a final compressed stream 70 and a second compressor recycle stream 32. The second compressor recycle stream 32 has a second compressor recycle stream inlet 33. The second compressor recycle stream 32 includes one or more coolers 34, preferably one or more water and/or air coolers, known in the art and adapted to reduce the temperature of the second compressor recycle stream 32. The one or more air coolers 34 are followed by one or more control valves 36 to provide a final recycle stream 38 for re-injection into the main light stream in advance of the second inlet 23 of the second compressor 22 at second compressor recycle stream outlet 39. The second compressor recycle line 32 provides antisurge control around the second compressor 22 in a manner known in the art. The second compressor recycle line 32 is a dedicated line around the second compressor 22. In particular, it is noted that the one or more coolers 34 are only required to cool the percentage of the further compressed light stream 60 which is passed into the second compressor recycle line 32, which percentage is commonly zero or minimal, thus minimising the OPEX of the one or more coolers 34.

Figures 1 and 2 show a simplified arrangement of the recompression of a light stream using a first compressor 16 which has a dedicated first compressor recycle line 42 that does not require dedicated or external cooling, and a second compressor 22 with a dedicated second compressor recycle line 32. Thus, the first and second compressor recycle lines 32, 42 are independent, and can be independently controlled.

Figures 1 and 2 also show a first bypass line 80 around the expander 12 having a control valve 82. In this way, at least a fraction, optionally all, of a mixed hydrocarbon feed stream 10 not requiring to be passed through the expander 12 can pass through the first bypass line 80 to provide the mixed-phase hydrocarbon stream 20. This arrangement may occur during start up of the NGL recovery system 1, and/or during tripping of one or more of the expanders 12 as further discussed hereinafter.

Similarly, Figures 1 and 2 also show a second bypass line 90 with a one-way valve 92 around the first compressor 16 so as to take at least a fraction, optionally all, of the light overhead stream 30 around the or each first compressor 16 to provide the second compressor feed stream 54 for the or each second compressor 22. The second bypass line 90 may be used during start-up of the NGL recovery system 1, especially where there is no driving power for first compressor 16, (which can be mechanically linked to and therefore driven by the expander 12) . The second bypass line 90 may also be useful where one or more of the first compressors 16 ^λ trips' as further discussed hereinafter.

As shown in Figure 2, the final compressed stream 70 may be wholly or partly used as fuel gas 72, or passed to a gas network e.g. to provide domestic gas, or subsequently cooled, preferably liquefied, to provide a cooled hydrocarbon stream such as LNG. The cooling and preferred liquefaction may be carried out after passage along line 71. The cooling and preferred liquefaction occurs in the second cooling stage 112, typically comprising one or more heat exchangers, to provide a liquefied hydrocarbon stream 130, as shown in Figure 2. Suitable liquefaction processes for such second cooling stages are known to the person skilled in the art and will not be further described here.

Figure 3 shows a simplified and second general scheme of a liquefied natural gas plant 2 for a method for cooling an initial hydrocarbon stream 100 as described for Figure 2, further including NGL recovery scheme 3 based on having a first expander and first compressor string A, and a second expander and first compressor string B.

In Figure 3, a mixed hydrocarbon feed stream 10 is divided by a stream splitter 11 into at least two, preferably two or three, part-feed streams 10a and 10b, which pass into respective expanders 12a and 12b which are mechanically linked by respective common driveshafts 21a and 21b to respective first compressors 16a and 16b. The division of the mixed hydrocarbon feed stream into the part-feed streams 10a and 10b may be any ratio or percentage, but will generally be equal during normal and conventional operation of the second NGL recovery stream 3 wherein the expanders 12a and 12b have the same capacity. Variations in the size, type, capacity, number and their balance of the expanders 12, and in consequence in the size, capacity, type, number and balance of the first compressors 16, are known to the skilled man in the art with knowledge of NGL recovery processes, operations and parameters .

Each expander 12a, 12b provides a mixed-phase hydrocarbon stream 20a, 20b respectively, which can be combined by a suitable combiner 15 such as a T-piece, to provide a single mixed-phase hydrocarbon stream 20 to pass into the first gas/liquid separator 14 as hereinabove described. Optionally, one or more of the mixed-phase hydrocarbon streams 20a and 20b may pass directly into the first gas/liquid separator 14 without combination with the or all of the other mixed-phase hydrocarbon streams.

The first gas/liquid separator 14 provides a light overhead stream 30, and a heavy bottom stream 50 as hereinbefore described. The light overhead stream 30 can then be divided by a stream splitter 31 in a manner known in the art, to provide at least two, preferably two or three, part-light streams 30a, 30b which pass respectively into the two first compressors 16a, 16b through their first inlets 17a, 17b to provide two respective first compressed light streams 40a, 40b at first outlets 17b, 18b. 0-100% of the first compressed light streams 40a, 40b may pass into two respective first compressor recycle lines 42a, 42b through first compressor recycle inlets 41a, 41b for recycle through respective control valves 44a, 44b and return to the suction sides of the two first compressors 16a, 16b via first compressor recycle outlets 45a, 45b as described hereinabove.

That fraction of each of the first compressed light streams 40a and 40b not passing into the first compressor recycle lines 42a, 42b provide first compressed continuing streams 52a, 52b, which can pass through respective throttle control valves 26a, 26b before being combined by a combiner 53 to provide a second compressor feed stream 54 which passes to a second compressor 22 through an inlet 23, and out through an outlet 24 as a further compressed light stream 60. As described above, a fraction between 0-100% of the further compressed light stream 60 can provide a second compressor recycle stream 32 via a second compressor recycle inlet 33 and second compressor recycle outlet 39, whilst a final compressed stream 70 can be used as described above, for example as one or more other fuel stream, export stream, or for cooling, preferably liquefying, to provide a liquefied hydrocarbon stream such as LNG.

The combination of the first expander 12a, the mechanically linked first compressor 16a, and their associated lines, provide the first string A, whilst the combination of the second expander 12b, the mechanically linked first compressor 16b, and its associated lines, provide the second string B.

In this way, the user of the second NGL recovery scheme 3 is able to have greater options and flexibility concerning the flow of the mixed hydrocarbon feed stream 10 through the second NGL recovery scheme 3, in particular operations and flows through the expanders 12 and first compressors 16. As well as providing operational advantages during normal and/or conventional running of an NGL recovery scheme, this arrangement further provides two further advantages. Firstly, should any string of a multi-string NGL recovery scheme not be able to run normally, either by accident or design, the continuance of the NGL recovery is possible through one or more of the other strings. In particular, where a string should ^λ trip' , then the or each other string is able to continue operation of the

NGL recovery, even if the volume and/or mass of the mixed hydrocarbon feed stream continues at the same level, or continues at a significant level.

The ^λ tripping' of a expander-compressor string can occur for a number of reasons, and/or in a number of situations. Common examples include ^λ overspeed' , for instance where the driver produces more power than that required by the compressor and ^λ vibration' when the compressor is operating beyond the flow envelope and the flow angle with respect to the vane angle is incorrect.

A second particular advantage of the second NGL recovery scheme 3 shown in Figure 3 is during start-up of the recovery scheme. By providing two or more strings, each string can be separately started at a different time, and optionally with different starting parameters than each other strings. Thus, the user has greater options and control over the start-up of all the strings prior to full and normal operation of the overall second NGL recovery scheme 3. As an example, at the start-up of an NGL recovery scheme, the mixed hydrocarbon feed stream 10 is usually passed through a first bypass stream 80 to bypass the first expanders 12a, 12b to provide the mixed-phase hydrocarbon stream 20 because the pressure in the mixed hydrocarbon stream 10 may already be at a low level, such that expansion in first expanders 12a, 12b is unnecessary, or would result in too low a pressure in mixed-phase hydrocarbon stream 20. Bypassing the first expanders 12a, 12b provides a higher pressure in light overhead stream 30 than would otherwise occur.

Similarly, the light overhead stream 30 can pass through the second bypass line 90, and one-way valve 92 to bypass the first compressors 16a, 16b, especially where these are not provided with power or otherwise driven by the first expanders 12a and 12b which are being similarly by-passed.

It is a particular advantage of the method and apparatus disclosed herein that through pressure and flow control of each bypass stream and each part-stream, as the flow and/or pressure of the mixed-phase hydrocarbon stream 10 increases during start-up, one or more strings of a multi-string NGL recovery scheme can be separately started and brought up to normal operation as a controlled procedure. Thus, the two throttle control valves 26a, 26b in the paths of the first compressor continuing streams 52a, 52b, allow control of the introduction of light overhead streams 30a, 30b into the first compressors 16a, 16b in calculation with reduction of the flow of the second bypass stream 90. The two throttle valves 26a, 26b can control the pressure at the discharge of each of the first compressors 16a, 16b, especially near stonewall of each first compressor 16a, 16b, which most usually can occur during start-up and following any tripping of a string.

In this way, the pressure of the light stream in the second bypass line 90 does not hinder the start-up of each of the first compressors 16a, 16b, either together or independently. This arrangement seeks to ensure maximum forward flow through the or each first compressor, (and hence no overheating) , without operating in the stonewall region.

It is a further advantage of a multi-string NGL recovery scheme that one or more of the first compressors 16a, 16b can be isolated from the or each other first compressors, so as to reduce interaction between the first compressors 16a, 16b.

A person skilled in the art will readily understand that the present invention may be modified in many ways without departing from the scope of the appended claims.