GB2454344A

GB2454344A - Method and apparatus for controlling a refrigerant compressor, and a method for cooling a hydrocarbon stream.

Info

Publication number: GB2454344A
Application number: GB0819783A
Authority: GB
Inventors: Francois Chantant; Sander Kaart
Original assignee: SHELL INT RESEARCH; Shell Internationale Research Maatschappij BV
Current assignee: SHELL INT RESEARCH; Shell Internationale Research Maatschappij BV
Priority date: 2007-11-02
Filing date: 2008-10-28
Publication date: 2009-05-06
Also published as: US20090188277A1; GB0819783D0

Abstract

A refrigerant stream 40 is passed through at least one heat exchanger 32, 34 to provide a first at least partially evaporated stream 40a and a second at least partially evaporated stream 40b. A first compressor feed stream 20a is provided from the first partially evaporated stream 40a, and a second compressor feed stream 20b is provided from the second partially evaporated stream 40b. The first compressor feed stream 20a is passed through a first refrigerant compressor 12 and the second compressor feed stream 20b through a second compressor 14, to provide first and second compressed streams 50a, 50b, which are combined at a common pressure. The first refrigerant compressor 12 is controlled by heating the second at least partially evaporated stream 40b or the second compressor feed stream 20b, or vice versa. In use, the method and apparatus of control are used to cool a hydrocarbon stream, such as in a process which provides liquefied natural gas (LNG), and the method and apparatus limit the likelihood of compressor surge by maintaining an adequate volume flow rate.

Description

-2454344

METHOD AND APPARATUS FOR CONTROLLING A REFRIGERANT

COMPRESSOR, AND METHOD FOR COOLING A HYDROCARBON STREAM.

The present invention relates to a method and apparatus for controlling a refrigerant compressor.

Such a method and apparatus may be used in a method of cooling a hydrocarbon stream, such as a natural gas S stream. Accordingly, aspects of the invention relate to a method of cooling a hydrocarbon stream, and to a use of the apparatus in a process for cooling, and a use of the apparatus in a process for liquefying, a hydrocarbon feed stream.

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 pressure 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 refrigeration circuits with compressors, to progressively reduce its temperature until liquefaction is achieved.

Compressors for gaseous streams are used in many situations, systems and arrangements. Typically there is a vapour recycle or recirculation line around the compressor to avoid surge. 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 being lower than the pressure downstream of the compressor outlet. This can cause rapid pulsations in flow, which is generally termed surge' Surge is often accompanied 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 sometime, 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 recycle line to avoid surge is a simple and common operation without complications. Any change in temperature of the compressor flow is not significant.

Compressors used in refrigerant circuits have particular problems associated to them, especially when they are driven by fixed rotational speed drivers, such as a gas turbine. Refrigerant circuits are used in liquefaction systems, facilities and plants, such as for the production of a liquefied hydrocarbon stream such as liquefied natural gas (LNG). 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. Refrigerant compressors running at an effectively constant speed require relatively constant inflow of a gaseous stream to their suction side. Where the inflow of a gaseous stream 3..

falls for whatever reason below a certain minimum value, surge can occur.

US 4,464,720 discloses a surge control system which utilizes an algorithm to calculate a desired orifice differential pressure, and which compares 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 thus 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 address any of the problems described above.

The present invention provides a method of controlling a refrigerant compressor being one of at least a first refrigerant compressor and a second refrigerant compressor having a common post-compression communication in a refrigerant circuit circulating a refrigerant stream, the method at least comprising the steps of: (a) passing the refrigerant stream through at least one heat exchanger to provide a first at least partially evaporated stream and a second at least partially evaporated stream; (b) providing a first compressor feed stream from the first partially evaporated stream and a second compressor feed stream from the second partially evaporated stream; (c) passing the first compressor feed stream through the first refrigerant compressor and the second compressor feed stream through the second compressor to provide first and second compressed streams; (d) combining the first and second compressed streams at a common pressure; and (e) either heating the first at least partially evaporated stream or the first compressor feed stream to control the second compressor, or heating the second at least partially evaporated stream or the second compressor feed stream to control the first compressor.

The present invention also provides a method of cooling a hydrocarbon stream such as natural gas, the method at least comprising the steps of: (a) providing a hydrocarbon feed stream; (b) cooling the hydrocarbon feed stream by heat exchange against a refrigerant stream to provide a cooled hydrocarbon stream and an at least first and second partly evaporated refrigerant streams; (c) compressing the first and second partly evaporated refrigerant streams using at least first and second refrigerant compressors in a method as herein defined.

The present invention also provides apparatus for controlling a refrigerant compressor being one of at least a first refrigerant compressor and a second refrigerant compressor having a common post-compression communication in a refrigerant circuit circulating a refrigerant stream, the apparatus at least comprising: at least one heat exchanger through which the refrigerant stream passes to provide a first at least partially evaporated stream and a second at least partially evaporated stream; a first compressor feed stream provided from the first partially evaporated stream; a second compressor feed stream provided from the second partially evaporated stream; a first refrigerant compressor through which passes the first compressor feed stream to provide a first compressed stream; a second compressor through which passes the second compressor feed stream to provide a second compressed stream; a combiner to combine the first and second compressed streams at a common pressure; and one or more heaters to either heat the first at least partially evaporated stream or the first compressor feed stream to control the second compressor, or to heat the second at least partially evaporated stream or the second compressor feed stream to control the first compressor.

The present invention also provides the use of such an apparatus in a process for cooling a hydrocarbon stream, wherein said cooling comprises passing the hydrocarbon stream through the at least one heat exchanger of the apparatus whereby heat is transferred from the hydrocarbon stream to the refrigerant stream.

The present invention also provides the use of such an apparatus in a process for liquefying a hydrocarbon stream, wherein said liquefying comprises cooling of the hydrocarbon stream, said cooling comprising passing the hydrocarbon stream through the at least one heat exchanger of the apparatus whereby heat is transferred from the hydrocarbon stream to the refrigerant stream.

Embodiments and examples 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 schematically shows a method of controlling a refrigerant compressor according to a first embodiment of the present invention; Figure 2 schematically shows amethod of controlling four refrigerant compressors according to another embodiment of the present invention; Figure 3 schematically shows an alternative arrangement to the method shown in Figure 2.

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.

The present invention is particularly suitable for, but not limited to, controlling two or more connected and multi-stage refrigerant compressors having a plurality of inlets accepting refrigerant at different pressure levels. The refrigerant compressor(s) may be used in one or more refrigerant circuits for cooling, optionally including liquefying, a hydrocarbon stream such as natural gas.

It has been surprisingly found that where two refrigerant compressors have a common post-compression communication, for example a common discharge or common accumulator, the heating of one compressor feed stream, or its main component being its evaporated stream from a heat exchanger, can control the other connected refrigerant compressor. This is because the heating of such a stream affects its subsequent compression, and thus the pressure of the other stream being compressed via their common post-compression communication. This is further discussed hereinafter.

The present invention may provide an improved method of controlling one or more refrigerant compressors, in particular two or more refrigerant compressors, used for compression of a multi-pressure level refrigerant.

The present invention is particularly suitable, but not limited to, where there are two or more refrigerant compressors in the same refrigerant circuit for different compressor feed streams, more particularly where the compressor feed streams are at different pressure levels.

When using multiple refrigerant compressors, either as separate compressors or one or more compressors having multiple pressure sections and having multiple inlets for different gas pressures, and optionally multiple recirculation lines, the method of the present invention allows all the refrigerant compressors to be controlled, especially to avoid surge.

The present invention is 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 is for controlling three or more, preferably four, refrigerant compressors, and two, three, four or five compressor feed streams based on two, three, four or five at least partly evaporated refrigerant streams respectively, wherein at least one of the evaporated refrigerant streams or at least one of the compressor feed streams is heated prior to passing through its respective refrigerant compressor.

More preferably, the method of the present invention comprises four compressor feed streams at four different pressures passing through four refrigerant compressors.

The cooling of the hydrocarbon stream may partly or wholly liquefy the hydrocarbon stream so as to provide a liquefied hydrocarbon stream, such as LNG. The cooling of the hydrocarbon stream to provide the cooled hydrocarbon stream may also be followed by further cooling of the cooled hydrocarbon stream to further liquefy and/or sub-cool the cooled hydrocarbon stream.

The or each refrigerant compressor involved in the present invention may be any suitable refrigerant compressor, optionally having two or more compression stages or pressure sections. Use of the term refrigerant compressors' herein extends to a single refrigerant compressor having multiple pressure sections in one casing, and able to receive two or more gaseous streams at different pressures. The hydrocarbon cooling or liquefying plant or facility may also involve one or more other refrigerant or other compressors not involved in the present invention.

The first and second compressors may have a vapour recirculation line. Preferably, the first and second compressor feed streams are a combination of at least a fraction of the vapour recirculation stream in the vapour recirculation line and the first and second at least partially evaporated refrigerant streams respectively.

The or each recirculation line useable in the present invention 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 refrigerant compressor to its suction side or a relevant suction drum. The or each recirculation line may be divided or separated in a manner known in the art, to supply a recirculated stream part or fraction to two or more refrigerant compressors.

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

The heating of the relevant stream or streams in step (e) can be provided by any method of heating. The heating may involve use of one or more dedicated heaters, optionally using a fuel gas, and/or by heat exchange against a stream capable of releasing heat to the relevant stream of streams, such as a warmer stream or streams, in one or more heat exchangers. In a cooling and/or liquefaction process or plant, there are typically a number of sources of a warmer stream or streams, which could be routed to provide, when required, the heating for step (e) The common post-compression communication of the first and second refrigerant compressors, which allows combining of the first and second compressed streams at a common pressure, can be any form of combining, either by a dedicated combiner such as a junction or T-piece, or by their common collection, for example in a cooler or accumulator.

In another embodiment, the method of the present invention is able to prevent surge in a refrigerant compressor being one of at least a first refrigerant compressor and a second refrigerant compressor having a common post-compression communication in a refrigerant circuit circulating a refrigerant stream. Preferably heating the first at least partially evaporated stream or the first compressor feed stream is to prevent surge in the second compressor, and heating the second at least partially evaporated stream or the second compressor feed stream is to prevent surge in the first compressor.

Referring to the drawings, Figure 1 shows a simplified and general scheme 1 of a method for -10 -controlling a refrigerant compressor for a refrigerant stream.

In Figure 1, there is also shown a method of cooling a hydrocarbon stream such as natural gas. A hydrocarbon feed stream 5 passes through a refrigeration zone 42 to provide a cooled hydrocarbon stream 6, for example, having a temperature below 0°C, for example between -10°C and -70°C; optionally partly liquefied. The refrigeration zone 42 may be part of a liquefaction plant (not shown), wherein the cooled hydrocarbon stream 6 is subsequently fully liquefied.

The hydrocarbon feed stream 5 is cooled by heat exchange against a refrigerant stream 40 circulating in a refrigerant circuit 2. The refrigerant stream 40 passes through at least two heat exchangers (32, 34) in the refrigeration zone 42.

In the scheme 1 shown in Figure 1, there is a first heat exchanger 32, from which a fraction of the refrigerant stream 40 is expanded and evaporated to provide a first at least partially evaporated stream 40a, and a second heat exchanger 34, usually at a lower pressure than that of the first heat exchanger 32, which receives the remaining fraction of the refrigerant stream from the first heat exchanger 32, expands and evaporates the remaining fraction, to provide a second at least partially evaporated stream 40b. Expansion may be performed by means of a valve upstream of the respective relevant heat exchanger, such as valve 66 upstream of the first heat exchanger 32 for the expansion in the first heat exchanger 32 and another valve (not shown) in the line between the first and second heat exchangers for the expansion in the second heat exchanger 34.

-11 -Each of the evaporated refrigerant streams 40a, 40b are typically wholly evaporated to achieve maximum cooling in the heat exchangers 32, 34, although partial-evaporation is also a possibility covered by the invention. Usually, the first evaporated stream 40a is at a higher pressure than the second evaporated stream 40b although the present invention is not limited thereto.

Typically, each of the first and second evaporated streams 40a, 40b are passed through first and second suction drums 16, 18 respectively, to provide first and second compressor feed streams 20a, 20b respectively. The first and second compressor feed streams 20a, 20b then pass through first and second refrigerant compressors 12, 14 respectively to provide first and second compressed streams 50a, 50b respectively. The first and second compressed streams 50a, 50b can be combined by a combiner 52 to provide a combined stream 60, which can be first cooled by a first ambient heat exchanger such as one or more water and/or air coolers 54, to provide a cooler compressed stream 60a.

When desired or necessary, the cooler compressed stream 60a can be divided by a divider 56, for instance in the form of a stream-splitter, into a continuing refrigerant stream 60b and a vapour recirculation stream 30 passing through a vapour recirculation line 30. The divider 56 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. The continuing refrigerant stream 60b passes through one or more further coolers 58, one or more accumulators 62, one or more yet further coolers 64, prior to expansion through an expansion valve 66 to provide the refrigerant -12 -stream 40 and recirculation through the refrigeration zone 42.

The vapour recirculation stream 30 may then be divided by any suitable divider 31 into two part-recirculation streams 30a, 30b, expanded through suitable control valves 68a, 68b in a manner known in the art, and subsequently combined with the first and second at least partially evaporated streams 40a, 40b respectively to provide first and second combined feed streams lOa, lOb respectively.

It is known that vapour recirculation provides the ability to increase flow through a compressor, more particularly to maintain minimum flow where there is a reduction in the flow of evaporated refrigerant which normally forms the major component of the compressor feed stream.

Every compressor requires a balance between its inlet pressure and outlet or discharge pressure. For a refrigerant compressor in a refrigerant circuit, the inlet and outlet pressures are usually significantly determined by two vapour pressures; firstly from the heat exchangers using the cooling duty of the refrigerant stream, and secondly from the post-compression refrigerant accumulator.

However, there can be occasions or periods where the pressure balance is upset, such that the discharge pressure from a refrigerant compressor can be insufficient compared to the inlet pressure, leading to surge. One example of this is an insufficiency in the provision of an evaporated stream from a heat exchanger.

With an insufficient flow of evaporated stream to provide the compressor feed stream, the compressor can go towards surge.

-13 -A benefit of the present invention is recognising that where there are two or more refrigerant compressors having a common post-compression communication, such as a common discharge, then the discharge pressure from each refrigerant compressor can affect the other refrigerant compressor. That is, using the post-compression common pressure to allow the action of one refrigerant compressor to control an associated or connected refrigerant compressor.

Referring to the Scheme 1 shown in Figure 1, there is the possibility of the second refrigerant compressor 14 heading towards surge where, for example, the pressure of the second compressor feed stream 20b is decreasing, such that the relationship of its pressure with the pressure of the second compressed stream 50b is unbalanced.

The method of the present invention involves heating of the first at least partially evaporated stream 40a and/or the first combined feed stream ba and/or the first compressor feed stream 20a, to control the second compressor 14. For example, Figure 1 shows one or more heaters such as a heat exchanger 22 in the path of the first at least partially evaporated stream 40a, which allows heating, preferably rapid heating, of the first at least partially evaporated stream 40a upstream of the first suction drum 16.

By heating the first at least partially evaporated stream 40a, there will be the same volume flow in this stream, but the mass flow in the subsequent first compressor feed stream 20a is reduced. With less mass flow in the subsequent first compressed stream SOa, the same amount of cooling duty provided by the first and further coolers 54, 58 increases the amount or degree of cooling of the flow of refrigerant therethrough, such -14 -that the temperature of refrigerant in the accumulator 62 is lower than normal, such that there is less vapour pressure created therefrom. With less vapour pressure from the accumulator 62, this feeds back to reducing the pressure on the discharge side of the second compressor 14, such that it is less likely to go into surge.

In the same way, heating of the second at least partially evaporated stream 40b or the second combined feed stream lOb or the second compressor feed stream 20b provides control of the first compressor 12. In scheme 1, there is shown by way of example a heater such as a similar heat exchanger 24 in the path of the second at least partially evaporated stream 40b for this purpose.

A person skilled in the art will be aware of the nature and provision of the first and second heat exchangers 22, 24, which may comprise one or more heat exchangers in parallel, series or both.

Figure 2 shows a second scheme 101 involving another refrigeration circuit 102 comprising a second refrigeration zone 142. The second refrigeration zone 142 may comprise two or more, such as four, separate heat exchangers, or it may comprise a single heat exchanger involving the outlet of refrigerant at different pressure levels. Such arrangements are well known in the art, and examples are shown in WO 01/44734 A2 and The second refrigeration zone 142 can be for withdrawing heat from a stream, for example one or more hydrocarbon streams 100 such as a natural gas stream to * 30 be liquefied. Examples of methods for liquefying natural 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 -15 -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 second refrigeration zone 142 may be equivalent to or an extension of the first refrigeration zone 42 shown in Figure 1. For example, where cooling, preferably liquefying, of a hydrocarbon stream 100 involves two or more stages, such as a first stage to lower the temperature of the hydrocarbon stream 100 below 0°C, and a second stage to further lower the hydrocarbon stream to a temperature below _900 or -100°C, the second refrigeration zone 142 could act as the cooling for the first stage.

In the arrangement shown in Figure 2, the second refrigeration zone 142 has first, second, third and fourth heat exchangers 132, 134, 136, 138, and first, second, third and fourth outlets 143, 144, 145, 146, for a refrigerant stream 140 evaporated at different pressure levels, with decreasing pressure from the first outlet 143 to the fourth outlet 146. For example, the first outlet 143 is intended for gaseous refrigerant released at a high-high pressure as a first evaporated stream 140a, the second outlet 144 for gaseous refrigerant released at a high pressure as a second evaporated stream 140b, the third outlet 145 for gaseous refrigerant released at a medium pressure as a third evaporated stream 140c, and the fourth outlet 146 for gaseous refrigerant released at a low pressure as a fourth evaporated stream 140d. The second refrigeration zone 142 may have further outlets.

-16 -Each evaporated stream 140a-d is passed into a corresponding suction drum or gas/liquid separator such as knockout drums].48a-d, from which there are respective overhead gaseous compressor feed streams 120a-d.

S The fourth compressor feed stream 120d passes into a first refrigerant compressor ll2d to provide a compressed stream 150d, which is combined with the second compressor feed stream 120b to enter a second refrigerant compressor 112b to provide a first final compressed stream 160. The first and second refrigerant compressors ll2d, 112b may be separate refrigerant compressors, or may be in one casing, having two inlets and one or two sections to accommodate the different pressure levels of the second and fourth compressor feed streams 120b, 120d.

Similarly, the third compressor feed stream 120c passes into a third refrigerant compressor 112c, and its compressed stream 150c is combined with the first compressor, feed stream 120a to pass into a fourth refrigerant compressor 112a and provide a second final gaseous stream 170. As above, the third and fourth refrigerant compressors 112a, 112b may be separate refrigerant compressors, or may be in one casing having two inlets and different sections to accommodate the different pressures of the first and third streams 120a, 120c.

The arrangement of the refrigeration zone 142, the outlets and gaseous streams therefrom, and the refrigerant compressors ll2a-d are known in the art, and are shown and described for example in WO 01/44734 A2.

The first and second final compressed streams 160, are themselves combined to form an overall combined compressed stream 180, which is cooled by a first cooler 162 such as an ambient water and/or air cooler known in -17 -the art. The first cooler 162 may comprise one or more coolers in parallel, series or both, and provides a cooled compressed stream 190.

In the same manner as described above for the arrangement shown in Figure 1, the cooled compressed stream 190 can be divided when desired or necessary between a continuing stream 200 and a vapour recirculation stream 130 by use of a divider 156, e.g. in the form of a stream splitter. The vapour recirculation stream 130 may be divided into four separate fraction streams 130a-d to pass through separate respective control valves 152a-d and be combined with the evaporated refrigerant streams 140a-d respectively to provide combined feed streams llOa-d respectively.

The first continuing steam 200 is further cooled and mostly or fully condensed, for example by a second cooler 172 being one or more coolers such as water and/or air coolers, which provides a cooled continuing steam 210.

The cooling provided by the second cooler 172 preferably fully condenses the cooled continuing stream 210. The cooled continuing stream 210 passes into an accumulator 166.

The accumulator 166 provides a liquid stream 220 which can be further cooled by a third cooler 168, 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 177 and return and use as stream 140 in the second refrigeration zone 142.

In the same manner as described in Figure 1, heating of the first evaporated stream 140a, for example by using one or more heaters such as a heat exchanger 116a in its path, affects the volume flow of the subsequent -18 -compressor feed stream 120a, and hence the properties of the second combined compressed stream 170, the overall compressed stream 180, and therefore the pressure of the first final compressed stream 160, and hence the second compressor 112b. In the same way, heating of the second evaporated stream 140b, such as via a second heat exchanger 116b, will allow control of the fourth compressor ll2a. Similarly, heating of the third evaporated stream 140c by a third heat exchanger ll6c, will affect the first compressor ll2d, and heating of the fourth evaporated stream 140d by a fourth heat exchanger 116d, will affect the third compressor 112c.

Indeed, depending on the arrangement and relationship of the streams and compressors in the second scheme 101 shown in Figure 2, the heating of the first evaporated stream 140a (to affect the properties of the second combined compressed stream 170, the overall compressed stream 180, the pressure of the first final compressed stream 160, and hence the second compressor 112b), can also affect the first compressor ll2d, as it is in series with the second compressor 112b. Typically, the heating of the first evaporated stream 140a may have a greater effect on the second compressor ll2b than the first compressor 112d, but it may nevertheless assist a degree of control over the first compressor 112d.

Similarly, again depending on the arrangement and relationship of the streams and compressors in the second scheme 101 shown in Figure 2, the heating of the second evaporated stream 140b may allow control of the fourth compressor 112a and the third compressor ll2c. Similarly, heating of the third evaporated stream 140c by a third heat exchanger 116c may affect both the first compressor 112d and the second compressor ll2b, and heating of the -19 -fourth evaporated stream 140d by a fourth heat exchanger 116d may affect the third compressor 112c and the fourth compressor 112a.

Figure 3 shows a third scheme 301 using a third refrigeration circuit 302 similar to the second scheme 101 and second refrigeration circuit 102 shown in Figure 2. However, in the second refrigeration circuit 302, the second compressor feed stream l2Ob is now combined with the compressed stream 150c from the third compressor 112c prior to passage of the combined stream into the fourth refrigerant compressor 112a. Meanwhile, the first compressor feed stream 120a is combined with the compressed stream 150d from the first compressor 112d for entry into the second refrigerant compressor 112b.

This arrangement of evaporated refrigerant streams and refrigerant compressors from a refrigeration zone is shown in WO 2005/057110 Al.

In a similar arrangement to that shown in Figure 2, the two final compressed steams 360, 370 are further combined as an overall combined compressed stream 180, -first cooled, and a. vapour recirculation stream 130 is optionally provided and divided into four fraction streams for combining with each of the evaporated refrigerant streams 140a-d. The first continuing stream 200 is further cooled by a second cooler 164, and passed into an accumulator 166 as described hereinbefore.

In the same manner as described in Figure 2, heating of the first evaporated stream 140a, for example by using a heat exchanger ll6a in its path, affects the volume flow of the subsequent compressor feed stream 120a, and hence the properties of the first final compressed stream 360, the overall combined compressed stream 180, and therefore the pressure of the second combined compressed -20 -stream 370, and hence the fourth compressor 112a (and possibly the third compressor 112c). In the same way, heating of the second evaporated stream 140b, such as via a second heat exchanger 116b, will allow control of the S second compressor 112b (and possibly the first compressor 112d). Similarly, heating of the third evaporated stream 140c by a third heat exchanger 116c, will affect the first compressor 112d (and possibly the second compressor 112b), and heating of the fourth evaporated stream 140d by a fourth heat exchanger llGd, will effect the third compressor 112c (and possibly the fourth compressor 112a) Table 1 shows percentage changes in pressures and volume flows for a working example of the present invention based on the arrangement for refrigerant compressors for a refrigerant circuit shown in Figure 2.

-21 -

Table 1

Heated Refrigerant Pressure (bar) Volume Flow Stream Compressor change % change 140a 112d 1.35 -0.50 112c 4.00 -4.34 112b 1.51 3.30 112a 7.09 -1.73 140b 112d 4.84 -5.99 112c 1.76 2.42 112b 6.42 -10.67 112a -0. 02 0.93 l4Oc 112d 0.32 0.28 112c 1.31 -1.69 ll2b 0.14 0.06 112a 0.23 -0.33 140d 112d 2.60 -3.56 112c 0.33 0.29 112b 0.67 -0.94 112a 0.09 -0.06 Heating of the first evaporated stream 140a increases the pressure of the subsequent compressor feed stream 120a into the fourth refrigerant compressor 112a (by 7.09) with a small reduction in its volume flow S (-1.73w) . The effect of this increase in pressure on the compressor feed stream 120a into the fourth refrigerant compressor 112a is to increase the volume flow through the second refrigerant compressor ll2b (by +3.30), such that the second compressor 112b moves away from surge.

Meanwhile, heating the second evaporated stream 140b increases the pressure of the second compressor feed stream (120b) on the second refrigerant compressor 112b -22 - (by +6.42), which at least increases the volume flow through the third refrigerant compressor ll2c.

Heating the third evaporated stream 140c increases the pressure of the third compressor feed stream 120c (by +1.31) , which has the effect of increasing the volume flow through both the first refrigerant compressor 112d (by + 0.28) and the second refrigerant compressor 112b (by 0.06), to move them both away from surge.

Lastly, heating of the fourth evaporated stream 140d increases the pressure of the fourth compressor feed stream 120d and the pressure of the first refrigerant compressor 112d, and increases the volume flow (by 0.29%) through the third refrigerant compressor 112c so that the third refrigerant compressor moves away from surge.

There are references hereinabove to various types of "valves" including flow-control valves, recirculation valves and expansion valves. Some valves reqi.iired 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..

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.

Claims

-23 - CLAIMS

1. A method of controlling a refrigerant compressor being dne of at least a first refrigerant compressor and a second refrigerant compressor having a common post-compression communication in a refrigerant circuit circulating a refrigerant stream, the method at least comprising the steps of: (a) passing the refrigerant stream through at least one heat exchanger to provide a first at least partially evaporated stream and a second at least partially evaporated stream; (b) providing a first compressor feed stream from the first partially evaporated stream and a second compressor feed stream from the second partially evaporated stream; (c) passing the first compressor feed stream through the first refrigerant compressor and the second compressor feed stream through the second compressor to provide first and second compressed streams; (d) combining the first and second compressed streams at a common pressure; and (e) either heating the first at least partially evaporated stream or the first compressor feed stream to control the second compressor, or heating the second at least partially evaporated stream or the second compressor feed stream to control the first compressor.

2. Method as claimed in claim 1, wherein the first and second compressors have a vapour recirculation line, and wherein the first and second compressor -24 -feed streams are a combination of at least a fraction of the vapour recirculation stream in the vapour recirculation line and the first and second at least partially evaporated refrigerant streams respectively.

3. Method as claimed in claim 1 or claim 2, wherein the refrigerant in the refrigerant circuit is propane.

4. Method as claimed in one or more of the preceding claims, wherein the first and second evaporated streams have different pressures.

5. Method as claimed in one or more of the preceding claims for controlling three or more, preferably four, refrigerant compressors, and two, three, four or five compressor feed streams based on two, three, four or five at least partly evaporated refrigerant streams respectively, wherein at least one of the evaporated refrigerant streams or at least one of the compressor feed streams is heated prior to passing through its respective refrigerant compressor.

6. Method as claimed in claim 5, comprising four compressor feed streams at four different pressures passing through four refrigerant compressors.

7. A method as claimed in one or more of the preceding claims, wherein heating the first at least partially evaporated stream or the first compressor feed stream is to prevent surge in the second compressor, and heating the second at least partially evaporated stream or the second compressor feed stream is to prevent surge in the first compressor.

8. A method of cooling a hydrocarbon stream, the method at least comprising the steps of: (a) providing a hydrocarbon feed stream; -25 -(b) cooling the hydrocarbon feed stream comprising heat exchanging the hydrocarbon feed stream against a refrigerant stream to provide a cooled hydrocarbon stream and at least first and second partly evaporated refrigerant streams; (c) compressing the first and second partly evaporated refrigerant streams using at least first and second refrigerant compressors in a method as defined in one or more of Claims 1 to 7.

9. Method as claimed in claim 8, wherein the cooling comprises liquefying the hydrocarbon feed stream to provide a liquefied hydrocarbon stream.

10. Method as claimed in claim 9, wherein the liquefied hydrocarbon stream is an LNG stream.

11. Method as claimed in one or more of claims 8 to 10, wherein the hydrocarbon feed stream comprises natural gas.

12. Method as claimed in one or more of claims 8 to 11, wherein the hydrocarbon feed stream consists essentially of natural gas.

13. Apparatus for controlling a refrigerant compressor being one of at least a first refrigerant compressor and a second refrigerant compressor having a common post-compression communication in a refrigerant circuit circulating a refrigerant stream, the apparatus at least comprising: at least one heat exchanger through which the refrigerant stream passes to provide a first at least partially evaporated stream and a second at least partially evaporated stream; a first compressor feed stream provided from the first partially evaporated stream; -26 -a second compressor feed stream provided from the second partially evaporated stream; a first refrigerant compressor through which passes the first compressor feed stream to provide a first compressed stream; a second compressor through which passes the second compressor feed stream to provide a second compressed stream; a combiner to combine the first and second compressed streams at a common pressure; and one or more heaters to either heat the first at least partially evaporated stream or the first compressor feed stream to control the second compressor, or to heat the second at least partially evaporated stream or the second compressor feed stream to control the first compressor.

14. Use of the apparatus of claim 13 in a process for cooling a hydrocarbon stream, wherein said cooling comprises passing the hydrocarbon stream through the at least one heat exchanger of the apparatus whereby heat is transferred from the hydrocarbon stream to the refrigerant stream.

15. Use of the apparatus of claim 13 in a process for liquefying a hydrocarbon stream, wherein said liquefying comprises cooling of the hydrocarbon stream, said cooling comprising passing the hydrocarbon stream through the at least one heat exchanger of the apparatus whereby heat is transferred from the hydrocarbon stream to the refrigerant stream.