METHODS AND APPARATUSES FOR COOLING AND/OR LIQUEFYING A

HYDROCARBON STREAM

The present invention relates to a method and apparatus for cooling a hydrocarbon stream to provide a cooled hydrocarbon stream. In another aspect, the present invention also relates to a method of liquefying a hydrocarbon stream.

A common hydrocarbon stream to be cooled and/or liquefied comprises, or preferably essentially consists of, natural gas.

Several methods of cooling, usually 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.

As an example of liquefying natural gas, the natural gas, comprising predominantly methane, enters an LNG plant at elevated pressures and is pre-treated to produce a purified feed steam suitable for liquefying at cryogenic temperatures. The purified gas is processed through a plurality of cooling stages using heat exchangers involving one or more refrigerant circuits to progressively reduce its temperature until liquefaction is achieved.

In a refrigerant circuit, the refrigerant is evaporated in one or more stages to cool the hydrocarbon stream, and one or more refrigerant compressors are used to recompress the evaporated refrigerant in one or more stages.

Typically, one or more gas turbine drivers are used to provide the power for a cooling and/or liquefaction plant, especially with the usual availability from the hydrocarbon stream of fuel gas for the gas turbine (s) . It is known that the available power from a gas turbine driver decreases as the ambient temperature increases, because air density decreases with temperature. In contrast, it is also known that the overall power requirement of a cooling and/or liquefaction plant increases as the ambient temperature increases or the production rate (production volume per unit time) decreases, as then more compression is required to achieve the same level of refrigeration. These relationships are illustrated in a simplified manner in the accompanying Figure 1.

At a certain transition ambient temperature (labeled "T _r " in Figure 1) for a typical plant design, it is known that the power provided by the driver (s) can no longer match the overall plant power requirement, such that there will be a decrease in the production of cooled and/or liquefied product. Thus, when designing a cooling and/or liquefaction plant, there is factored into the design a temperature range in which the plant is expected to operate. This range is usually from a 'low ambient design temperature' ("T]_") to a 'hot ambient design temperature' ("Tj ₁ ") . The latter temperature is the highest temperature at which the plant is expected to

operate, and which is up to or slightly lower than the T _r temperature as shown in Figure 1.

However, there can be periods, even if brief, when the ambient temperature is abnormally at or above the hot ambient design temperature for the plant. Such conditions can occur with abnormal weather, especially heat spikes, in any location around the world, but particularly in hotter locations . A temperature at or above the hot ambient design temperature will hereinafter be referred to as 'high ambient temperatures' .

As shown in the accompanying Figure 1, once the high ambient temperature is above the T _r temperature for the plant design, the available driver power is less than the plant power requirement to achieve the same level of cooled product production. Thus, it has been hitherto accepted in the art that there will be periods of time where there is a decrease in cooled hydrocarbon production, often by around 50 wt%, until the high ambient temperature is again below the T _r temperature. This interrupts the desire to be able to maintain a certain level (usually a maximum level) of cooling and/or liquefaction, and hence a certain level (usually a maximum level) of cooled and/or liquefied hydrocarbon production . It is also desired to maintain constant volume flow through a refrigerant compressor and avoid surge at a high ambient temperature. To achieve this, typically one or more vapour recirculation valves are opened to provide a vapour recirculation stream and to put the refrigerant compressor into what is termed 'vapour recirculation mode'. However, with increasing vapour recirculation, there is also a decrease in the amount of compressed refrigerant then available to cool a hydrocarbon stream,

so that there is a further reduction in the cooled hydrocarbon stream production.

The higher the ambient temperature around the plant, the greater these effects, and the greater is the loss of cooled hydrocarbon production.

A compressor is said to be 'in deep surge' when the main flow through the compressor reverses its direction. Normally, this is related to a discharge pressure lower than the pressure downstream of the compressor outlet. This can cause rapid pulsations in flow, which is generally termed 'surge'. Surge is often symptomised by excessive vibration and noise. This flow reversal is accompanied with a very violent change in energy, which causes a reversal of the thrust force. The surge process can be cyclic in nature, and if allowed to continue for some time, irreparable damage can occur to the compressor .

Where a compressor is dealing with ambient temperature gases or other non-critical situations, recycling of discharge gas via a vapour recirculation line to avoid surge is a simple and common operation without complications. However, refrigerant compressors used in refrigerant circuits have particular problems associated to them due to the particular pressure and temperature requirements, especially when they are driven by fixed rotational speed drivers, such as a gas turbine commonly used in liquefaction plants such as for LNG.

US 4,464,720 discloses a surge control system which utilizes an algorithm to calculate a desired orifice differential pressure and compare the calculated result with an actual differential pressure. Pressure and temperature measurements are made on both the suction side and discharge side of a centrifugal compressor and

enter a control system so that the actual differential pressure is substantially equal to the desired differential pressure. A suction temperature of gas entering the centrifugal compressor is measured and used. However, the complex algorithm and values required for the calculations in US 4,464,720 do not relate to a refrigerant compressor, nor do they address the problem described above of high ambient temperature. Thus, US 4,464,720 is not sympathetic to the operation of refrigerant circuit (s) and compressor ( s ) at a high ambient temperature.

US 3,527,059 discloses a method of balancing the operation of a plurality of parallel-operating refrigerant compressors in which gaseous refrigerant from the discharge of the compressors is contacted with liquid refrigerant to cool and saturate the vapours, which are then recycled to the suction-side of the compressors. US 3,527,059 and particularly Figure 2 does not disclose the source of the liquid refrigerant used to cool and saturate the gaseous refrigerant. There is thus the problem of how to provide the liquid refrigerant, which cools and saturates the recycled gaseous refrigerant.

The present invention provides a method of cooling a hydrocarbon stream using one or more refrigerant circuits, at least one refrigerant circuit including one or more refrigerant compressors and a vapour recirculation line, the method comprising at least the steps of : (a) heat exchanging the hydrocarbon stream against one or more at least partly condensed refrigerant streams in the at least one refrigerant circuit to provide a cooled hydrocarbon stream and an at least partly evaporated refrigerant stream at a pressure Pl;

(b) providing one or more compressor feed streams at a pressure not higher than Pl for at least one of the refrigerant compressor ( s ) from at least a combination of:

(i) at least a fraction of the at least partly evaporated refrigerant stream,

(ii) an at least partially liquid stream which is at least partly vapourised to cool at least one of the one or more compressor feed streams, said at least partially liquid stream being provided from the at least one refrigerant circuit;

(c) operating at least one of the refrigerant compressor (s) thereby providing at least one or more compressed refrigerant streams;

(d) condensing at least a portion of the one or more compressed refrigerant streams to provide the one or more at least partly condensed refrigerant streams of step (a) .

The cooling of the hydrocarbon stream by the present invention is either fully or partly a method of liquefying a hydrocarbon stream so as to provide a liquefied hydrocarbon stream, such as for instance LNG. Thus, the present invention provides a method of liquefying a hydrocarbon stream to provide a liquefied hydrocarbon stream, comprising cooling of the hydrocarbon stream in accordance with a method as hereinbefore described.

The present invention also provides an apparatus for cooling a hydrocarbon stream, the apparatus at least comprising: one or more refrigerant circuits including one or more refrigerant compressors, to provide one or more compressed refrigerant streams, and including a condenser to condense at least a portion of the one or more

compressed refrigerant streams to provide one or more at least partly condensed refrigerant streams; a first heat exchanger in which the hydrocarbon stream and the one or more at least partly condensed refrigerant streams can exchange heat to provide a cooled hydrocarbon stream and one or more at least partly evaporated refrigerant streams; a combiner to provide a compressor feed stream for at least one of the refrigerant compressors, from at least:

(i) at least a fraction of one or more of at least partly evaporated refrigerant stream(s), (ii) an at least partially liquid stream able to be at least partly vapourised to cool the compressor feed stream ( s ) ; means to obtain said at least partially liquid stream from the at least one refrigerant circuit.

The present invention will now be further illustrated by way of example only, and with reference to embodiments, examples, and the accompanying non-limiting drawings in which:

Figure 1 shows changes in maximum available driver power and plant power requirement over changing ambient temperature; Figure 2 schematically shows a method of cooling a hydrocarbon stream according to one embodiment of the present invention;

Figure 3 shows changes in LNG production over changing ambient temperature for a ' non-controlled' compression and for controlled compression according to an embodiment of the present invention;

Figure 4 schematically shows a method of cooling a hydrocarbon stream according to a second embodiment of the present invention; and

Figure 5 schematically shows an alternative arrangement to the method shown in Figure 4.

For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line. Same reference numbers refer to similar components. There may be references herein to various types of

"valves" including flow-control valves, recirculation valves and expansion valves. Some valves required in any circuit or process may not be specifically or generally mentioned or referenced herein. The skilled man is aware of the type and arrangement of valve or valves required to affect processing of a line, stream, flow, circuit, etc .

The present invention is suitable to assist any refrigerant compressor ( s ) involved in cooling a hydrocarbon stream.

By using, as a constituent of a compressor feed stream, an at least partially liquid stream that then at least partly vapourises, the temperature of the compressor feed stream formed therewith can be lowered, so that the compressor feed stream is easier to compress whilst maintaining the same volume flow through the subsequent refrigerant compressor.

As a consequence, more cooling duty is available from the compressed refrigerant to cool (and optionally liquefy) a hydrocarbon stream such as natural gas than if the compressor feed stream would be made using a liquid that is at least partially vaporised to cool the compressor feed stream. Alternatively, there is a

reduction in the decrease of the cooling duty available from the refrigerant to cool (and optionally liquefy) the hydrocarbon stream where a reduction in available driver power has occurred. Suitably, the liquid is provided from the at least one refrigerant circuit, so that the total refrigerant inventory remains the same over time independently of the amount of liquid used in the combination that provides the one or more compressor feed streams. This saves on operation and control complexity over embodiments wherein the liquid is obtained from a separate source.

A further advantage of the present invention is that the inlet, or suction side, temperature of a compressor gaseous stream provided from such a compressor feed stream can be closer to or maintained more closely to the normal operating temperature of the compressor where there is a reduction of compressor driver power in any situation, particularly but not limited to where there is a high ambient temperature which results in the available driver power being lower than the plant power requirement, especially the refrigerant compressor ( s ) power requirement. By better maintaining the normal operating temperature of the, or each, compressor gaseous stream, there is less reduction in the compression, and/or amount of refrigerant compressed by the, or each, compressor .

A still further advantage of the method and apparatus disclosed herein is that an integrated system is provided in which the at least partially liquid stream which is combined with at least a fraction of the at least partly evaporated refrigerant stream is provided from the at least one refrigerant circuit. In this way, the method and apparatus disclosed herein dispenses with

the need for an external cooling system to provide the at least partially liquid stream.

Thus, in the present methods and apparatuses a reduction of available power from a driver for a refrigerant compressor in a refrigerant circuit can be accommodated, in particular if the compressor feed stream is made as hereinbefore described.

Moreover, the present methods and apparatuses help to reduce a decrease in a production rate of the cooled hydrocarbon stream, particularly where at least one of the refrigerant compressor ( s ) is driven by one or more drivers and the ambient temperature is at or above a hot ambient design temperature, by providing the one or more compressor feed streams as defined above. The normal operating temperature of a refrigerant compressor is the temperature of the compressor gaseous stream at the inlet or suction side of the compressor when no or minimal vapour recirculation occurs (i.e. any vapour recirculation valve is closed) . This is the case when the refrigerant compressor is operated outside the vapor recirculation mode.

Optionally, the method further comprises a step (e) of bringing the vapour recirculation line in a vapour recirculation mode, whereby the one or more compressor feed streams is made to comprise a vapour recirculation stream consisting of a vaporous portion of the at least one of the one or more compressed refrigerant streams. Accordingly, the apparatus may optionally comprise a vapour recirculation line, that is capable of combining a vapour recirculation stream consisting of a vaporous portion of the one or more compressed refrigerant streams into the one or more compressor feed streams. This allows for keeping the flow rate through the refrigerant

compressor sufficiently high to avoid surge. The liquid recirculation stream ensures that the vapor recirculation stream can be maintained over a longer period of time .

The refrigerant compressor ( s ) can be driven by one or any combination of drivers such as gas and steam turbines . Preferably, the driver power is provided by one or more gas turbines. The one or more turbines powering the refrigerant compressor ( s ) may be directly driving such compressor ( s ), or indirectly driving such compressor (s) (for example via electricity generation), or a combination of same.

The driver (s) provide the "available driver power", whose maximum over increasing ambient temperature is shown line 13 in Figure 1. It is a characteristic of drivers, in particular gas turbines, that the available driver power therefrom is dependent upon the ambient temperature therearound. As discussed hereinabove, this characteristic is factored into the design of a cooling and/or liquefaction plant to achieve desired cooling and/or liquefaction production within the expected operating ambient temperature range, most especially up to the hot ambient design temperature (Tj ₁ ) .

The available driver power of a plant will be different in different circumstances. For example, where a plant is in a normally cold ambient environment such as in northerly latitudes of the world such as in the arctic, then the available driver power may be based on a normal ambient temperature of less than 10 ⁰ C. Thus, a high ambient temperature for such a plant may only be slightly above that temperature, such as 15°C or 20 ⁰ C. Where the plant is in a hot location, such as near the equator, then the available driver power may be based on

a normal ambient temperature of 20-30 ⁰ C, such that a high ambient temperature is >35°C.

Thus, the hot ambient design temperature (Tj ₁ ) and a high ambient temperature for a cooling and/or liquefaction plant or process, will be different in different circumstances .

The hydrocarbon stream to be cooled may comprise natural gas or may essentially consist of a natural gas stream. Natural gas may be obtained from a natural gas or petroleum reservoir. Alternatively, it may comprise synthetic gas from a synthetic source such as for instance from a Fisher-Tropsch process.

Refrigerant circuits for providing a refrigerant stream to cool a hydrocarbon stream are known in the art. Such refrigerant circuits generally comprise: one or more refrigerant compressors, subsequent cooling of the compressed refrigerant stream by one or more heat exchangers, commonly ambient heat exchangers such as water and/or air coolers, a valve or other expander to reduce the pressure and therefore cool the refrigerant stream prior to heat exchange with the hydrocarbon stream, following which the refrigerant stream is at least partly evaporated, commonly >90% or 100% evaporated, prior to recompression. The at least partly evaporated refrigerant stream may originate from a heat exchanger wherein the refrigerant stream has received heat through heat exchange with another stream (e.g. the hydrocarbon stream to be cooled) in a refrigeration zone. The vapour recirculation line bypasses the refrigeration zone comprising the heat exchanger such that the vapour recirculation stream bypasses the refrigeration zone and/or the heat exchanger.

Commonly, the at least partially evaporated refrigerant stream is provided as a compressor feed stream to a pre-compressor suction drum to remove any liquid fraction, the overhead stream from which is a compressor gaseous stream ready for recompression.

In the case of a pure component refrigerant compressor, the normal operating temperature of the compressor gaseous stream is the dew point. A person skilled in the art understands that the vapour feed to the compressor may be slightly superheated (less than a few degrees centigrade) due to pressure drop between the refrigerant vaporizer and actual compressor inlet. In this case, the invention preferentially maintains any temperature rise from the dewpoint of the compressor gaseous stream during a vapour recycle or recirculation operation to less than 10 ⁰ C, more preferably less than 5°C.

In the case of a mixed refrigerant compressor, the normal operating temperature can be the dew point, but can also be well above the dew point. In this case, the invention preferentially maintains any temperature variation of the compressor gaseous stream to 10 ⁰ C above or below the normal operating temperature, more preferably less than 5°C above or below, during a vapour recycle or recirculation operation.

The present invention is suitable to assist any refrigerant compressor ( s ) involved in cooling a hydrocarbon stream without limitation to the nature of the refrigerant (s), the refrigerant circuit (s), or the manner of cooling.

For example, the present invention may be useful where there are two or more refrigerant compressors for different compressor feed streams, more particularly

where the compressor feed streams are at different pressure levels. When using multiple compressors, or compressor ( s ) having multiple pressure sections and having multiple inlets for different gas pressures, and usually multiple recirculation lines, the simple and effective reduction of feed stream temperature allows all the compressors to maximise refrigerant compression and/or avoid surge following any reduction in the design power of the compressor driver (s) . The present invention may be particularly useful where a refrigerant stream is evaporated at different pressure levels, but requiring each evaporated fraction to be recompressed to a unified pressure for re-use as a refrigerant . Thus, in one embodiment, the method comprises two or more, preferably two or four or five, refrigerant compressors, and two, three, four or five compressor gaseous streams.

Preferably, the present invention provides a method involving two or more of the compressor feed streams having two or more different pressures, for example four compressor feed streams at four different pressures passing through two or four refrigerant compressors, being separate compressors, one or more compressors with multiple pressure sections, or a combination of same.

As stated above, the at least one refrigerant compressor may have a vapour recirculation line which may be operated in a vapour recirculation mode. Preferably, at least one of the compressor feed streams further comprises at least a fraction of the vapour recirculation stream in the vapour recirculation line.

The at least partially liquid stream could be added at any suitable location in a refrigerant circuit to be

part of the compressor feed stream. For example, it could be added before, after, or simultaneously with the combination of the at least partially evaporated refrigerant stream, and preferably the vapour recirculation stream. The liquid stream may or may not have a temperature cooler than one or both of the other streams, but its at least part vapourisation will cool one of the other streams or their combination.

The present invention is not limited by the position or location of the at least part vaporisation of the at least partially liquid stream. The at least part vaporisation, preferably full vaporisation, may occur within the at least partly evaporated refrigerant stream, or in one or more streams or lines in advance (upstream of) of the subsequent combination to provide the compressor feed stream, as long as a cooling effect of the vapourisation is able to be relayed, at least partly, to the compressor feed stream.

Preferably, the at least partially liquid stream is combined with the at least fraction of vapour recirculation stream prior to their combination with the at least partly evaporated refrigerant stream. More preferably, the at least partially liquid stream is fully vapourised following its combination with the at least a fraction of the vapour recirculation stream.

The at least partly evaporated refrigerant stream from the heat exchange in step (a) is provided at a pressure Pl. The stream pressure determined herein can be measured by any of the devices known in the art for this purpose, such as hydrostatic, aneroid, thermal conductivity or ionization gauges. The pressure of the one or more compressor feed streams in step (b) should

not be greater than the pressure Pl of the at least partly evapourated refrigerant stream.

By maintaining the pressure of the one or more compressor feed streams in step (b) equal to or lower than the pressure of the at least partly evapourated stream, an increase in pressure of the refrigerant streams between the heat exchanger and the suction inlet of the compressor is avoided. Thus, the addition of the at least partially liquid stream in step (b) (ii) does not lead to an increase in pressure during the at least partial vapourisation to cool at least one of the one or more compressor feed streams. This also ensures that the cooling of the at least one of the one or more compressor feed streams in step (b) (ii) occurs before the stream first enters the compressor, rather than the cooling of a partially compressed stream, in order to achieve the advantages disclosed herein.

The or each refrigerant compressor used in the present invention may be any suitable compressor, optionally having two or more compression stages or pressure sections. Use of the term 'refrigerant compressors or compressor ( s ) ' herein extends to a single refrigerant compressor having multiple pressure sections within the same shell or casing, and able to receive (usually through different inlets) two or more gaseous streams at different pressures. The hydrocarbon cooling or liquefying plant or facility may also involve one or more other refrigerant compressors not involved in the present invention. The or each recirculation line used herein may be any suitable line able to transfer a recirculation stream, which can be liquid, gaseous or mixed phase, from the discharge side of a compressor to the suction side.

Any recirculation stream bypasses a refrigeration zone, and in particular the heat exchangers comprised in a refrigeration zone . The or each recirculation line may be divided or fractionated in a manner known in the art, to supply a recirculated stream fraction to two or more refrigerant compressors.

The refrigerant stream may comprise a single component such as propane or nitrogen, or comprise a mixture of two or more selected from the group comprising: nitrogen, methane, ethane, ethylene propane, propylene, butanes, and pentanes.

In particular since the compressor feed stream(s) ar made from a combination involving at least an at least partially liquid stream, it is recommended to pass the compressor feed stream(s) through one or more suction drums to provide one or more compressor gaseous streams as overhead stream from the respective suction drums, which can be passed through the refrigerant compressors ( s ) . Herewith, any remaining liquid fraction, such as but not limited to any unevaporated or remaining liquid that has been added in step (b) (ii), is removed from the compressor feed stream(s) so that it is not fed to the refrigerant compressor.

Optionally, the vaporous portion of the at least one of the one or more compressed refrigerant streams for the vapor recirculation stream is obtained by dividing at least one of the compressed refrigerant streams into at least a first continuing stream and the vapour recirculation stream. The method may further comprise the step of:

(f) cooling the first continuing stream to provide an at least partially condensed first continuing stream, optionally followed by the step of:

(g) dividing the at least partially condensed first continuing stream into a second continuing stream and a liquid recirculation stream comprising at least some of the liquid phase that has condensed in step (f ) . This way, the one or more at least partly condensed refrigerant streams, against which the hydrocarbon stream to be cooled is heat exchanged, may be obtained from the second continuing stream. The liquid recirculation stream is suitably used as the source of the partially liquid stream of step (b) (ii), such that the at least partially liquid stream of step (b) (ii) may be obtained from the liquid recirculation stream of step (g) .

Thus, in one embodiment of the present invention, the liquid recirculation stream of step (g) provides the at least partially liquid stream.

In another embodiment of the present invention, a further at least partially liquid stream can be provided separately from the refrigerant circuit (s) of step (a), for example from one or more separate sources. Referring to the drawings, Figure 1 shows a typical maximum available driver power (line 13) and a typical power requirement for a cooling and/or liquefaction plant (line 16) against an increasing ambient temperature. As the ambient temperature increases, the available driver power decreases. On reaching a transition temperature T _r , the available driver power no longer matches the plant power requirement to maintain the desired production level of hydrocarbon cooling and/or liquefaction. Above the T _r temperature, cooled hydrocarbon production must decrease in line with the reducing available driver power. In practice, Tj ₁ may be chosen close to T _r in typical plant designs.

Figure 2 shows a simplified and general scheme 1 for a method of cooling a hydrocarbon stream, such as part of a method of liquefying a hydrocarbon stream for liquefied natural gas production, according to one embodiment of the present invention. The scheme 1 can be part of a cooling and/or liquefaction plant .

In Figure 2, a hydrocarbon feed stream 5 passes through a first heat exchanger 21, which may comprise one or more heat exchangers in series parallel or both, to provide a cooled hydrocarbon stream 6, for example, having a temperature below 0 ⁰ C, for example between -10 ⁰ C and -70 ⁰ C; optionally at least partly liquefied.

The hydrocarbon feed stream 5 is cooled by heat exchange with an at least partly condensed, preferably fully condensed, refrigerant stream 2Oe in a refrigerant circuit 2. The cooling of the hydrocarbon feed stream 5 by the at least partly condensed refrigerant stream 2Oe provides an at least partly, usually mostly, and preferably fully, evaporated refrigerant stream 8 at a pressure Pl. The at least partly evaporated refrigerant stream 8 now requires to be recompressed for reuse. As such it is partly the source of a compressor feed stream 10a which has a pressure not greater than that of the partly evaporated refrigerant stream 8 i.e. the pressure Pl. Compressor feed stream 10a passes through a suction drum 11 to provide a compressor gaseous stream 10 as an overhead stream. The suction drum 11 may also provide a minor liquid bottom stream 10b.

The compressor gaseous stream 10 passes through the inlet 14 of a refrigerant compressor 12 driven by a refrigerant compressor driver 15. In the refrigerant compressor 12, the compressor gaseous stream 10 is

compressed to provide a compressed refrigerant stream 20 through outlet 16.

Figure 2 shows a point labelled "T\-_" at which it is desired to maintain as close as possible the normal operating temperature of the compressor gaseous stream 10 going into the inlet 14. The actual temperature of the compressor gaseous stream 10 at the inlet 14 is not critical to the present invention, such that measurement of the actual temperature is not necessary, only that it is maintained at or as close as possible to the normal operating temperature of the refrigerant compressor 12, for example within j^lO°C, preferably j^5°C.

The arrangement shown in Figure 2 allows for the adjustment of the compressor feed stream 10a to alter, preferably to minimize change in, the temperature of the compressor gaseous stream 10 and inlet temperature T\-_ so that it is as close to that desired, or as close as can be maintained where the reduction in the power output of the driver 15 is such that there is inevitably at least some refrigerant compression loss, resulting in at least some hydrocarbon cooling loss.

The arrangement comprises both a vapour recirculation line carrying vapour recirculation streams 30, 30a and 30b, and a liquid recirculation line carrying liquid recirculation streams 40 and 32.

Optionally, there is provided a second heat exchanger 26, usually in the form of an ambient heat exchanger such as one or more water and/or air coolers, after the outlet 16 to cool the compressed refrigerant stream 20 and provide a cooler compressed refrigerant stream 20a.

The compressed refrigerant stream 20 (or cooler compressed stream 20a) is divided by a divider 18, e.g.

in the form of a stream-splitter, into a first continuing refrigerant stream 20b and a vapour recirculation stream 30 passing through a vapour recirculation line 30. The divider 18 may be any arrangement able to divide a stream into two or more fractions or parts, such as a manifold or dedicated unit, or more simply, a T-piece.

In general, the division of the compressed refrigerant stream 20 or cooler compressed refrigerant stream 20a into the vapour recirculation stream 30 and the first continuing refrigerant stream 20b can yield the vapour recirculation stream 30 to contain any mass percentage of between 0% to 100% of the compressed refrigerant stream 20. That is, there may be occasions during the operation of the refrigerant compressor 12 where no recirculation (i.e. the vapour recirculation stream 30 is 0% of the compressed refrigerant stream 20) is required to maintain minimum flow through the inlet 14. There may be alternative occasions where 100% of the compressed refrigerant stream 20 is re-circulated as the vapour recirculation stream 30, for example during start up of the refrigerant compressor 12.

For the purposes of the embodiment shown in Figure 2, the vapour recirculation stream 30 and any other recirculation stream is in operation, and the refrigerant compressor 12 is therefore in vapour recirculation mode. At least a fraction, preferably all, of the vapour recirculation stream 30 is combined with at least a fraction, preferably all, of the at least partly evaporated refrigerant stream 8 coming from the first heat exchanger 21. By way of example only, the vapour recirculation stream 30 is a fraction such as 10 vol% of the compressed refrigerant stream 20.

The vapour recirculation stream 30 passes through an expander such as a recirculation valve 22 known in the art to provide an expanded vapour recirculation stream 30a, which can be combined with the at least partly evaporated refrigerant stream 8 by a combiner 24 to provide the compressor feed stream 10a. It is preferred that the expanded vapour recirculation stream 30a is directly injected into the at least partly evaporated refrigerant stream 8. The continuing refrigerant stream 20b passes through one or more coolers 17 provide an at least partially, preferably fully, condensed first continuing stream 20c, prior to expansion through a valve 7 and recirculation through the first heat exchanger 21. The condensed first continuing stream 20c can be divided into a second continuing stream 2Od and a liquid recirculation stream 40, the latter comprising at least a partially, preferably fully, liquid phase derived from the condensed portion of the condensed continuing stream 20c. This can be done using an at least partially divider 19a known in the art, which may be similar to the divider

18 described hereinbefore.

Optionally, one or more accumulators 19 are provided downstream of cooler (s) 17, in which case the liquid recirculation stream 40 can be drawn from the accumulator

19 (not shown), e.g. as a bottom stream. Alternatively, the optional accumulator 19 may be upstream of the divider 19a, as shown in Figure 2.

The second continuing stream 2Od passes through an optional valve 7, to provide an expanded at least partially condensed refrigerant stream 2Oe ready for recirculation through the first heat exchanger 21.

The liquid recirculation stream 40 passes through a controlling valve 40a to provide an expanded liquid recirculation stream 32. This stream is at least partially in liquid phase, and it is usually cooler than the liquid recirculation stream 40. It can be combined with the expanded vapour recirculation stream 30a via combiner 32a to provide a combined recirculation stream 30b. Preferably the expanded liquid recirculation stream 32 is directly injected into the expanded vapour recirculation stream 30a. The effect of the combination with the expanded vapour recirculation stream 30a is to at least partly vaporise the at least partially liquid stream 32, and thus influence, usually reduce, the combined temperature thereof . The temperature of the combined recirculation stream 30b can be such as to affect the combination of this stream with the evaporated refrigerant stream 8 and so provide a compressor gaseous stream 10 having a lower temperature.

Thus, in accordance with this embodiment, the means to obtain the at least partially liquid stream from the at least one refrigerant circuit comprises the cooler 17 wherein the liquid is formed and the divider 19a in the refrigerant circuit 2 downstream of the cooler 17, with the optional accumulator 19 upstream thereof. A further at least partially liquid stream can be provided from a separate line 34b (such as from one or more alternative sources of a liquid stream, usually a liquid refrigerant stream from another store or circuit), to be passed through another controlling valve 34c and thus be provided as a cooler at least partially liquid stream 34 to be combined with the evaporated refrigerant stream 8 by a combiner 34a.

An advantage of the present invention is to achieve a lower inlet temperature to the refrigerant compressor 12. In this way, over reliance on the vapour recirculation line 30 can be reduced. This is particularly important where the ambient temperature around the refrigerant compressor 12 and/or driver 15 is above the hot ambient design temperature of the plant using the scheme 1 of Figure 2.

In this way, the scheme of Figure 2 also shows a method of accommodating a reduction of available power from the driver 15 and a method of reducing the decrease in the production of the cooled hydrocarbon stream 6, where the ambient temperature around the plant using the scheme is above its hot ambient design temperature and/or its transition temperature.

Figure 3 shows examples of loss of LNG production in a two-stage natural gas liquefaction plant as a consequence of an increasing high ambient temperature. In a theoretical case where there is no loss or reduction in the available driver power to the plant, then line A in Figure 3 is followed.

When, on the other hand, a realistic loss or reduction in the available driver power is assumed, due to a high ambient temperature period during which the one or more refrigerant compressors in the pre-cooling stage of a conventional plant design have vapour recirculation, then Line C could be followed. In the present calculation this may lead to as much as 50% loss in LNG production at a temperature of T+6°C above the transition temperature (T _r ) for the plant.

The present invention is similar to the arrangement in Line C, but now includes use of an at least partially liquid stream, preferably using a secondary circulation

line, allowing an at least partially liquid injection into the vapour recirculation line of the refrigerant compressor also. The secondary circulation line may be referred to as a "liquid recirculation line". Using such a controlled compression, the loss of LNG production follows Line B, resulting in a reduction in the loss of LNG production which is clearly less than that of Line C, for example 10% less at T+6°C. This is a significant reduction in the loss of LNG production where the ambient temperature has increased by only a few degrees. Such a situation may not occur over a long period (for example only during an abnormal 'hot weather' period), but on an industrial scale, the difference in production levels is significant . Figure 2 further shows that the inlet temperature T]_ of the compressor gaseous stream 10 can be relayed to one or more expanders such as valves which influence the flow and/or cooling of relevant streams in the general scheme 2 shown in Figure 2. For example, the inlet temperature T]_ could be relayed via one or more of the dashed lines 9 to one or both of the control valves 40a and/or 34c, whose operation influences the flow of the expanded stream 32 or 34. The present invention therefore also provides rapid control and feedback from the inlet temperature T]_ to one or more of the control valves.

By maintaining the normal operating temperature T _] _ of the compressor gaseous stream 10 at a 'constant' level, or as close to such a level as is possible, for example within j^lO°C, the effect is that there is the required suction flow rate of compressor gaseous stream 10 into the inlet 14 to also avoid surge of the refrigerant compressor 12.

With maintenance of the inlet temperature T]_ of the compressor gaseous stream 10 as required, the refrigerant compressor 12 will also be more efficient by the reduction of an unnecessary high vapour recycle rate. For example, US 4,464,720 shows a centrifugal compressor having a single re-circulation line connected between the suction and discharged sides of the compressor. However, opening and closing of the blow-off valve shown in US 4,464,720 will vary the pressure and therefore the temperature of the gas being re-circulated back into the centrifugal compressor.

In Figure 2, the refrigerant compressor 12 may be a single refrigerant compressor for compressing a single refrigerant stream, or it may be one of a number of refrigerant compressors involved in compressing one or more refrigerant streams, and/or it may be a compressor having two or more inlets for the compression of one or more refrigerant streams at different pressures, optionally in a single casing. Figure 4 shows a second refrigerant circuit 3 comprising a refrigeration zone 41. The refrigeration zone 41 may comprise two or more, such as four, separate heat exchangers (133, 134, 135, 136), or it may comprise a single heat exchanger involving outlets of refrigerant at different pressure levels (43, 44, 45, 46) . Such arrangements are well known in the art, and examples are shown in WO 01/44734 A2 and WO 2005/057110 Al.

The refrigeration zone 41 can be for withdrawing heat from a stream, for example one or more hydrocarbon streams (not shown) such as a natural gas stream to be liquefied. Examples of methods for liquefying nature gas are mentioned in US 6 389 844 and US 6,370,910 Bl which are hereby incorporated by reference. In these patent

documents, a plant is described for liquefying natural gas wherein the plant comprises a pre-cooling heat exchanger having an inlet for natural gas and an outlet for cooled natural gas, and a pre-cool refrigerant circuit for removing heat from the natural gas in the pre-cooling heat exchanger.

The refrigeration zone 41 may be equivalent to or part of the heat exchanger 21 shown in Figure 2. For example, where cooling, preferably liquefying, of a hydrocarbon stream 5 involves two or more stages, such as a first stage to lower the temperature of the hydrocarbon stream 5 below 0 ⁰ C, and a second stage to further lower the hydrocarbon stream to a temperature below -90° or -100 ⁰ C, the refrigeration zone 41 could act as the cooling for the first stage.

As the apparatus and operation of a refrigeration zone 41 is well known, it is only shown schematically here for the sake of clarity. The refrigeration zone has an inlet 42 intended for normally fully condensed refrigerant 60 at a refrigeration pressure. More than one inlet may be present.

In the arrangement shown in Figure 4, the refrigeration zone 41 has first, second, third and fourth outlets 43, 44, 45, 46 respectively for the part of the refrigerant 60 that has evaporated at different pressure levels, within decreasing pressure from the first outlet 43 to the fourth outlet 46. For example, the first outlet 43 is intended for gaseous refrigerant originating from the refrigerant 60 and released at a high-high pressure as a first evaporated stream 70, the second outlet 44 for gaseous refrigerant released at a high pressure as a second evaporated stream 80, the third outlet 45 for gaseous refrigerant released at a medium pressure as a

third evaporated stream 90, and fourth outlet 46 for gaseous refrigerant released at a low pressure as a fourth evaporated stream 100. The refrigeration zone 41 may have further outlets. Each evaporated stream 70, 80, 90, 100 is passed into a corresponding suction drum, which may each be provided in the form of a gas/liquid separator such as knockout drums 48a, 48b, 48c and 48d, from which there are respective overhead gaseous streams 70a, 80a, 90a, 100a, which are fed to one or more refrigerant compressors, here shown in the form of first to fourth compressors 58, 56, 54, 52.

The fourth knock-out drum gaseous stream 100a passes into the first compressor 58 to provide a fourth compressed stream 100b, which is combined with the second knock-out gaseous stream 80a to enter a second compressor 56 to provide a first combined compressed stream 120. The first and second compressors 58, 56 may be separate compressors, or may be combined in one casing, having two inlets and two sections to accommodate the different pressure levels of the second and fourth knock-out gaseous streams 80a, 100a.

Similarly, the third knock-out gaseous stream 90a passes into a third compressor 54, and its compressed stream 90b is combined with the first knock-out gaseous stream 70a to pass into a fourth compressor 52 and provide a second combined compressed stream 110. As above, the third and fourth compressors 54, 52 may be separate compressors, or may be combined in one casing having two inlets and different sections to accommodate the different pressures of the first and third knock-out gaseous streams 70a, 90a.

The arrangement of the refrigeration zone 41, the outlets and gaseous streams therefrom, and the compressors 52-58 are known in the art, and are shown and described for example in WO 01/44734 A2. The or each driver or drivers, which may be gas turbines, steam turbines, electric motors using electric power originating from gas turbines, or any suitable other driver or combinations thereof, for the four compressors 52-58 are not shown in Figure 4. The first and second combined compressed streams

110, 120 are themselves combined to form an overall compressed stream 130, which is cooled by a first cooler 62 such as an ambient water and/or air cooler known in the art. The first cooler 62 may comprise one or more coolers in parallel, series or both, and provides a cooled compressed stream 137.

In the same manner as described above for the arrangements shown in Figure 2, the cooled compressed stream 137 can be divided between a first continuing stream 160 and a vapour recirculation stream 150 by use of a stream splitter 72. The vapour recirculation stream 150 can be divided into four separate fraction streams 150a, 150b, 150c, 15Od to pass through separate respective control valves 122a to 122d, and combine with the evaporated refrigerant streams 70, 80, 90 and 100 respectively .

The first continuing steam 160 is further cooled and mostly or fully condensed, for example by a second cooler 64 being one or more coolers such as water and/or coolers, which provides a cooled first continuing steam 170. The cooling provided by the second cooler 64, preferably fully condenses the cooled first continuing stream 170. The cooled first continuing stream 170 passes

into an accumulator 66, which accumulator may be a separate unit, or a simple divider of the cooled first continuing stream 170.

The accumulator 66 provides a second continuing stream 190 which comprises, or preferably essentially consists of, a liquid phase, and which can be further cooled by a third cooler 68, being one or more coolers such as water and/or air coolers, to provide a reconstituted or reformed generally liquid refrigerant stream ready for passage through a valve 77 for return and re-use in the refrigeration zone 41.

The accumulator 66 also provides a convenient source for a liquid recirculation stream 180, which is an at least partially liquid stream, and which is usually cooler than the vapour recirculation stream 150.

The liquid recirculation stream 180 can act as a 'second recirculation' stream. Similar to the vapour recirculation stream 150, the liquid recirculation stream 180 can be divided into a number of fraction streams, such as the four fraction streams 180a, 180b, 180c, 18Od shown in Figure 4, to pass through respective valves 140a to d, and be combined, respectively, with the four vapour recirculation stream fractions 150a, 150b, 150c and 15Od prior to the knockout drums 48a-d. The usually cooler fraction streams 180a-d

(preferably being liquid in the arrangement shown in Figure 4, but possibly being two phase in other arrangements), vapourise on contact and combination with the warmer vapour recirculation fractions 150a-d, thereby reducing the temperatures of the combined streams prior to the relevant refrigerant compressors 52-58.

The flow and temperature of the liquid recirculation stream 180 and the four fraction streams 180a, 180b,

180c, 18Od can be controlled by regulating the flow from the accumulator 66 and/or regulating the valves through which the four fraction streams 180a, 180b, 180c, 18Od, pass prior to combination with vapour recirculation stream fractions 150a, 150b, 150c and 15Od. These are simple actions to regulate, so as to influence the temperature of the various streams once combined.

In this way, the liquid recirculation stream 180 provides a recirculation stream having a temperature cooler than, the vapour recirculation stream 150, such that their individual temperatures can be used to adjust the temperature of the combined refrigerant streams 70a, 80a, 90a and 100a prior to the entry of the combined streams into the compressors 52-58. Thus, maintenance of the inlet temperatures of at least the first and third compressors 58, 54, labelled in Figure 4 as inlet temperatures T2, T3 respectively, can be maintained as close as, preferably within +10 ⁰ C, more preferably within +5 ⁰ C of, their normal operating temperatures. Operation of the temperature and flow of the vapour and liquid recirculation streams 150 and 180, as well as the valves 122a-d and 140a-d for each of the fractions of the streams prior to their combination with the evaporated refrigerant stream 70-100, can provide the finesse for optimal operation of the compressors, 52-58 and their inlet temperatures T2 and T3.

If desired or necessary, the inlet temperatures prior to the fourth and second compressors 52, 56 could also be maintained at or as close to their normal operating temperatures.

Figure 5 shows a third refrigerant circuit 4 similar to the refrigerant circuit 3 shown in Figure 4. However, in the third refrigeration circuit 4, the second

evaporated refrigerant stream 80a is now combined with the compressed stream 90b prior to passage of the combined stream 90c into the fourth compressor 52. Meanwhile, the first gaseous evaporated stream 70a is combined with the first compressed stream 100b for entry into the third compressor 56. This arrangement of evaporated refrigerant streams and compressors from a refrigeration zone is shown in WO 2005/057110 Al.

In a similar arrangement to that shown in Figure 4, the two combined compressed steams 110a, 120a are further combined as an overall compressed stream 130, first cooled, and a vapour recirculation stream 150 is provided and divided into four fraction streams for combining with each of the evaporated refrigerant streams 70-100. The first continuing stream 160 is further cooled by a second cooler 64, and passed into an accumulator 66, where a liquid recirculation stream 180 in the form of an at least partially liquid stream, is provided and subsequently divided into four fractions 180a-d for addition to the respective vapour recirculation stream fractions 150a-d as described hereinbefore.

Again, operation of the temperature and flow of the vapour and liquid recirculation streams 150 and 180, as well as the valves for each of the fractions of the streams prior to their combination with the evaporated refrigerant streams 70-100, can provide the finesse for optimal operation of the compressors 52-58 in the arrangement shown in Figure 4 to maintain their inlet temperatures T2 and T3 within at or close to their normal operating temperature.

In the arrangements shown in Figures 4 and 5, there can be further recirculation streams provided, and/or different division of each recirculation stream, so as to

optimize adjustment of the temperature of the gaseous refrigerant streams entering each refrigerant compressor, or each section of the compressors, in such a way as to maintain the inlet temperature of at least one compressor at or close to its normal operating temperature, and maintain as high as possible cooled hydrocarbon production during high ambient temperature periods .

Table 1 shows a comparison of various modeled flows in two refrigeration circuits. The first circuit is based on the arrangement shown in Figure 4 herewith but with only vapour recirculation. The second circuit is a working example of the arrangement of a refrigeration circuit of the present invention shown in Figure 4 using an at least partially liquid stream as a liquid recirculation stream 180.

Table 1

Table 1 shows two clear advances by the present invention. Firstly, an increase in flow of stream 130 by the present invention (714 kg/s compared with 693 kg/s), i.e. total mass of refrigerant compressed by the compressors 52-58. Thus, there is more compressed refrigerant available in general.

Secondly, the decrease in the amount of compressed refrigerant required for vapour recirculation, and, in the present invention arrangement, as a source for an at least partially liquid stream (i.e. the 'liquid recirculation' stream 180) . The total flow of streams 150a-d for the prior art recycle is 346 kg/s, whereas the total for all the streams 150a-d and 180a-d for the present invention arrangement is only 251.6 kg/s. This

alone is a 27% reduction in the amount of compressed refrigerant that is needed for compressor recirculation.

The combination of the above two benefits further demonstrates the surprising advantages of the present invention. Subtracting the recirculation flows from the compressed stream flows 130 provides the overall amount of compressed refrigerant that is subsequently available for cooling a hydrocarbon stream in the refrigeration zone 41. In the first circuit, this overall amount of compressed refrigerant available for cooling is 347 kg/s (693 less 346) . The amount of compressed refrigerant that is available for cooling a hydrocarbon stream in the refrigeration zone 41 with the present invention arrangement is 462.2 kg/s (714 less 251.6) . This is a 33% increase; a significant increase based on the industrial scale of a liquefying plant.

Persons 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.