METHOD AND APPARATUS FOR CONTROLLING A REFRIGERANT COMPRESSOR, AND USE THEREOF IN A METHOD OF COOLING A

HYDROCARBON STREAM

The present invention relates to a method and apparatus for controlling a refrigerant compressor, and to their use in a method of cooling a hydrocarbon stream.

In other aspects, the invention relates to the use of said method and apparatus to avoid surge in such a refrigerant compressor.

The refrigerant compressor ( s ) may be used in one or more refrigerant circuits for cooling, optionally including liquefying, a hydrocarbon stream such as a natural gas stream. Thus, in another aspect, the invention relates to a method of cooling, optionally including liquefying, a hydrocarbon 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 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 refrigeration circuits

to progressively reduce its temperature until liquefaction is achieved.

Compressors for gaseous streams are used in many situations, systems and arrangements. Usually 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 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 sometime, irreparable damage can occur to the compressor.

Where the compressor is dealing with ambient temperature gases or other non-critical situations, recycling of discharge gas via the vapour recycle line to avoid surge is a simple and common operation without complications . Any change in temperature of the compressor flow is not critical.

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

Because the intake of a refrigerant compressor is cold compared to the temperature of the recycled or recirculated vapour, problems occur in the use of the normal method to operate the refrigerant compressor in recycle mode. In a refrigerant system, when the recycle valve is opened, a fast initial flow increase occurs, but then the refrigerant compressor flow quickly drops to a level below the initial value, only to then slowly rise to a higher new steady state value over time. However, the time required for this is much longer than the typical time-scale in which surge phenomena occur.

The accompanying Figure 4 explains this observed phenomenum: i.e. the relationship between refrigerant compressor flow and the pressure ratio across the refrigerant compressor being different for various inlet gas temperatures.

In Figure 4, the initial operating point of a refrigerant system is marked by the triangle α. Once the recycle valve is opened, the system responds with an initial flow rise. This flow rise is the consequence of a quick increase of the suction pressure that is caused by opening the recycle valve. However, opening the recycle valve also causes warmer recycled vapor to mix with the cold vapor that was initially flowing through the refrigerant compressor. This creates an increase in the temperature of the combined flow into the refrigerant compressor, which causes the refrigerant compressor to

operate at a lower volumetric flow at a given pressure ratio, so that the rise in suction temperature causes the flow to drop.

Once both suction and discharge pressure adapt to the new equilibrium, a higher flow rate is obtained at a lower pressure ratio across the refrigerant compressor, and finally the intent of the control action is met: increasing the refrigerant compressor flow by opening the recycle valve. However, the recycle valve control action has also had the undesired effect of temporarily reducing the volume flow through the refrigerant compressor.

The path of the flow over the opening of the recycle valve is shown in Figure 4 by line A, ending at circle β on a different (and higher at -16.5°C) temperature performance curve. The changes that have occurred over the path of line A can result in causing surge rather than preventing it .

This problem will occur both in mixed and pure component refrigeration systems. For pure component refrigeration systems in particular, the pressure ratio is partly set by the temperature of the liquid inventory of the heat exchanger (s) at the suction side, and of the accumulator at the discharge side of the refrigerant compressor. For such a system the compressor system pressure ratio is even slower to adapt to changes in valve settings or flow, and therefore the problem is particularly severe for pure component refrigerant systems .

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.

US 3,527,059 discloses a method of balancing the operation of a plurality of parallel-operating refrigerant compressors. The method comprises recycling a portion of the compressed gaseous refrigerant from the final stage of compression to both the first stage and second stage compression zone of each compressor in the system. The amount of recycle from the final stage of compression to the first stage compression zone is controlled by maintaining at least a predetermined minimum difference between the flows of refrigerant gases exteriorly entering the second stage compression zones and leaving such zone. The amount of compressed gaseous refrigerant recycled from the final stage of compression to the second stage compression zone is that quantity which will maintain at least a predetermined minimum refrigerant flow rate from the second stage compression zone. Compression efficiency is enhanced by cooling the recycled refrigerant, by passing it through spargers or distributors contained below the liquid level of a vessel containing liquid refrigerant. Since the hot recycled refrigerant vapours thus thoroughly contact the liquid refrigerant, the recycled refrigerant leaves the vessel as saturated vapour at its dew point temperature.

A drawback of this known method is that it does not allow for independent control of the temperature of the

compressor feed stream because the temperature of the recycle stream is fixed at its dew point.

It is an object of the present invention to overcome the problems presented above. It is another object of the present invention to provide an improved method of controlling one or more refrigerant compressors at a normal operating temperature, in particular two or more refrigerant compressors, used for compression of a multi-pressure level refrigerant.

The present invention provides a method of controlling one or more refrigerant compressors for one or more gaseous streams at a normal operating temperature, at least one refrigerant compressor having a vapour recirculation line, the method at least comprising the steps of:

(a) providing a compressor feed stream from a combination of a vapour recirculation stream from the vapour recirculation line and an at least partly evaporated refrigerant stream;

(b) passing the compressor feed stream through a suction drum to provide a compressor gaseous stream;

(c) passing the compressor gaseous stream through the refrigerant compressor (s) ; (d) measuring the temperature Ti of the compressor gaseous stream at the inlet of at least one refrigerant compressor; and

(e) cooling of one or more of the group consisting of: the vapour recirculation stream, the at least partly evaporated refrigerant stream, the compressor feed stream and the compressor gaseous stream; the cooling controlled in response to the temperature T ₁ to seek to provide the

compressor gaseous stream at the normal operating temperature of at least one compressor.

The present invention also provides for a use of the above defined method for the cooling of a hydrocarbon stream, such as a natural gas stream. Accordingly, there is provided a method of cooling a hydrocarbon stream, such as a natural gas stream, the method at least comprising the steps of:

- providing a hydrocarbon feed stream; - cooling the hydrocarbon feed stream by heat exchange against a refrigerant stream to provide a cooled hydrocarbon stream and an at least partly evaporated refrigerant stream;

- providing a compressor feed stream from a combination of a vapour recirculation stream and the at least partly evaporated refrigerant stream;

- passing the compressor feed stream through a suction drum to provide a compressor gaseous stream;

- passing the compressor gaseous stream through one or more refrigerant compressors having a vapour recirculation line, to provide a compressed refrigerant stream;

- determining the temperature T ₁ of the compressor gaseous stream at the inlet of at least one refrigerant compressor; and

- cooling of one or more of the group consisting of: the vapour recirculation stream, the partly evaporated refrigerant stream, the compressor feed stream and the compressor gaseous stream, the cooling controlled in response to the temperature T]_, to seek to provide the compressor gaseous stream at the normal operating temperature .

The hydrocarbon stream may partly or wholly liquefy, for instance as a result of the cooling or subsequently to the cooling, so as to provide a liquefied hydrocarbon stream, such as LNG. The present invention also provides apparatus for controlling one or more refrigerant compressors for one or more gaseous streams at a normal operating temperature, the apparatus at least comprising: a suction drum to receive a compressor feed stream from a combination of a vapour recirculation stream and the at least partly evaporated refrigerant stream, and to provide a compressor gaseous stream; at least one refrigerant compressor having an inlet for the compressor gaseous stream and an outlet for providing a compressed refrigerant stream; one or more pathways for re-circulating part or all of the compressed refrigerant stream as a vapour recirculating stream through the refrigerant compressor; and a temperature controller to determine the temperature Ti of the compressor gaseous stream at the inlet of the refrigerant compressor (12), and to control either :

(i) one or more coolers to cool one or more of the group consisting of the vapour recirculation stream, the at least partly evaporated refrigerant stream, the compressor feed stream and the compressor gaseous stream, to seek to provide the compressor gaseous stream at the normal operating temperature; or (ii) one or more streams cooler than the vapour recirculating stream to be combined with one or more of the group consisting of : the vapour recirculation stream, the at least partly evaporated refrigerant stream, the

compressor feed stream and the compressor gaseous stream, to seek to provide the compressor gaseous stream at the normal operating temperature; or

(iii) one or more at least partially liquid streams to be combined with one or more of the group consisting of: the vapour recirculation stream, the at least partly evaporated refrigerant stream, the compressor feed stream and the compressor gaseous stream, to seek to provide the compressor gaseous stream at the normal operating temperature; or

(iv) a combination of two of more of (i) - (iii) ; in response to the temperature T _] _ .

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 various embodiments of the present invention; Figure 2 schematically shows a method of controlling a number of refrigerant compressors according to another embodiment of the present invention;

Figure 3 schematically shows an alternative arrangement to the method shown in Figure 2; and

Figure 4 shows changes in volumetric flow over pressure ratio for a prior art refrigerant compressor and for a refrigerant compressor controlled according to one embodiment of the present invention. For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line. Same reference numbers refer to similar components.

There are 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 presently disclosed methods and apparatuses allow for a controlled cooling to a variable degree of one or more of the group consisting of: a vapour recirculation stream, a partly evaporated refrigerant stream, a compressor feed stream and a compressor gaseous stream, being controlled in response to the temperature T _] _ of the compressor gaseous stream at the inlet of the refrigerant compressor, to seek to provide the compressor gaseous stream at the normal operating temperature. This allows for an improved temperature control of the compressor inlet stream that is independent of the recirculation flow rate.

In the present disclosure, a compressor gaseous stream, generated from a compressor feed stream, is passed through a refrigerant compressor having a vapour recirculation line for a vapour recirculation stream. Present embodiments apply cooling of one or more of the vapour recirculation stream, a refrigerant stream, the compressor feed stream, and the compressor gaseous stream to keep the compressor gaseous stream at a normal operating temperature associated with the refrigerant compressor.

By maintaining the inlet or suction side temperature of the compressor gaseous stream at or close to its normal operating temperature, the vapour recirculation

stream is able to maintain the flow rate through the refrigerant compressor within its operable range, and thus surge is prevented.

The normal operating temperature of a refrigerant compressor is the temperature of the refrigerant compressor gaseous stream at the inlet or suction side of the refrigerant compressor when no or minimal vapour recycle occurs (i.e. any vapour recycle valve is closed) . 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 refrigerant compressor may be slightly superheated (less than a few degrees celcius) 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 compressor feed stream is provided from a combination of the vapour recirculation stream from the vapour recirculation line and an at least partly evaporated refrigerant stream. 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. a 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.

The present invention is suitable for, but not limited to, controlling two or more multi-stage refrigerant compressors having a plurality of inlets accepting refrigerant at different pressure levels.

Accordingly, the present invention is particularly suitable, but not limited to, when there are two or more refrigerant compressors for different compressor gaseous streams, more particularly where the gaseous streams are at different pressure levels. When using multiple refrigerant compressors, or refrigerant compressor (s) having multiple pressure sections and having multiple inlets for different gas pressures, and usually multiple recirculation lines, the simple and effective maintenance of suction side temperature ( s) allows all the refrigerant compressors 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 disclosed method comprises two or more, preferably two or four, 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 gaseous streams

having two or more different pressures, for example four compressor gaseous streams at four different pressures passing through two or four refrigerant compressors, being separate refrigerant compressors, one or more refrigerant compressors with multiple pressure sections in one casing, or a combination of these.

Maintenance of the suction side temperature of a refrigerant compressor can be achieved during vapour recycle in a number of ways. For example, the temperature of the recirculation stream in the vapour recirculation line can be altered, usually cooled, so as to adjust the temperature of the suction side gaseous stream, which comprises the recirculation stream.

Preferably, the suction side temperature can be maintained by adding one or more additional streams to one or more of the group selected from: one or more at least partially evaporated refrigerant streams, the vapour recirculation stream, the compressor feed stream, the compressor suction drum, and the compressor gaseous stream. Such one or more additional streams either have a cooler temperature than that to which they are added, and/or they are wholly or substantially liquid, so that the gaseous stream suction side temperature can be changed as required. These cooler and/or cooling streams may be directly injected to change the temperature of the gaseous stream suction side temperature as required. The source of one or more of the additional streams may be part of the refrigeration cycle, circuit or system comprising the refrigerant compressor. The or each refrigerant compressor used 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, or not in recycle mode at the same time as those of the present invention.

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 the suction side. 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, or comprise a mixture of two or more selected from the group comprising: nitrogen, methane, ethane, propane, butanes, pentanes .

Optionally, the present invention further comprises one or more of the following further steps:

(f) dividing the compressed refrigerant stream into at least a first continuing stream and a vapour recirculation stream;

(g) cooling the first continuing stream to provide an at least partially condensed first continuing stream;

(h) dividing the at least partially condensed first continuing stream into a second continuing stream and a second recirculation stream;

(i) allowing at least part of the second continuing stream to evaporate to form the at least partly evaporated refrigerant stream of step (a) ; and

(j) using the second recirculation stream as one or more of the cold streams.

The second recirculation stream may suitably be added to the vapour recirculation stream., Referring to the drawings, Figure 1 shows a simplified and general scheme 2 of various methods for 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 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 -1O ⁰ C and -70 ⁰ C; optionally partly liquefied.

The hydrocarbon feed stream 5 is cooled by heat exchange with a refrigerant stream 2Oe, which provides an at least partly, usually mostly, and preferably fully, evaporated, refrigerant stream 8. The mostly evaporated refrigerant stream 8 now requires to be recompressed for reuse. As such it is partly or wholly the source of a compressor feed stream 10a, which passes through a suction drum 11 to essentially remove any liquid that may be present in the compressor feed stream 10a and thereby 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. In the refrigerant compressor 12, the gaseous stream is compressed to provide a compressed refrigerant stream 20 through outlet 16.

Optionally, there is provided a first heat exchanger 26, usually 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 or stream-splitter into a 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. The vapour recirculation stream 30 is preferably essentially fully in vapour phase. The continuing refrigerant stream 20b passes through one or more coolers 17 and one or more accumula ^¬ tors 19 prior to expansion through an expansion valve 7 and recirculation through the refrigeration zone 21. In general, the division of the compressed refrigerant stream 20 can provide any percentage between 0% to 100% of the vapour recirculation stream 30. 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%) 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 present invention, the recirculation stream 30 and any other recirculation stream is in operation, and is therefore combined with the at least partly evaporated refrigerant stream 8

coming from the 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 first refrigerant 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. Figure 1 shows a temperature controller Tl which measures the temperature Ti of the compressor gaseous stream 10 at the inlet 14 of refrigerant compressor 12. The temperature controller Tl can be any device known in the art for such a purpose. The actual temperature of the compressor gaseous stream 10 at the inlet 14 is not critical to the present invention, only that it is maintained at or close to the normal operating temperature of the refrigerant compressor 12, for example within ^ ₁ IO ⁰ C. Thus, in one embodiment the temperature controller Tl can determine the difference in temperature between the temperature T ₁ of the compressor gaseous stream 10 at the inlet 14 of the refrigerant compressor 12 and the normal operating temperature of the refrigerant compressor 12. In a further embodiment, the normal operating temperature of refrigerant compressor 12 can be input to temperature controller Tl as a set point, or a set point range, and the temperature controller Tl will seek to maintain the measured inlet temperature Ti within this range.

Figure 1 shows the temperature controller Tl on the compressor gaseous stream line 10. However, this can be

positioned on any line from which the temperature T ₁ at the compressor inlet can be determined.

Figure 1 shows a number of possible arrangements to allow for the adjustment of the compressor gaseous stream 10 to maintain its inlet temperature T _] _ as desired.

In one arrangement, the first heat exchanger 26 can be used to adjust the temperature of the cooler compressed refrigerant stream 20a, which will therefore affect the temperature of the divided vapour re- circulation stream 30. This adjustment in temperature can be used all along the recirculation line 30, and affect the temperature of the combination of the expanded vapour recirculation stream 30a and the at least partly evaporated refrigerant stream 8, so as to maintain the inlet temperature T _] _ of the compressor gaseous steam 10 as desired.

In a second arrangement, there is provided a second heat exchanger 28 in the path of the compressor feed stream 10a. This second heat exchanger 28 can adjust the temperature of the compressor gaseous stream 10 to be maintained at or close to normal operating temperature.

A third possible arrangement shown in Figure 1 is the addition of a first cold, preferably liquid- containing, stream 32 to be combined by a combiner 32a with the expanded vapour recirculation stream 30a. Direct injection of the first cold, preferably liquid- containing, stream 32 into the expanded vapour recirculation stream 30a is preferred. The cold stream 32 vaporises on contact with the warmer vapour recirculation stream 30a to provide a first combined recirculation stream 30b having a temperature lower than the vapour recirculation stream 30a. By varying the proportion of the cold stream 32 combined with the warmer

vapour recirculation stream 30a, the temperature of the latter stream can be varied. Thus the temperature of the first combined re-circulation stream 30b can be such as to affect the combination of this stream with the at least partly evaporated refrigerant stream 8 to provide a compressor gaseous stream 10 having an inlet temperature as desired.

In a fourth possible arrangement shown in Figure 1, there is provided a second cold stream 34 to be combined by a combiner 34a, such as by direct injection of the second cold stream 34, with the compressor feed stream 10a. The temperature and/or phase of the second cold stream 34 can also be such as to affect the temperature of the compressor gaseous stream 10 having an inlet temperature at or close to its normal operating point.

The cooling of a stream by combining it with a cold stream or a cooling stream, such as in the third and fourth arrangements as described above may be referred to as direct heat exchange. Passing the stream and the cold stream or cooling stream through a heat exchanger, such as in the first and second arrangements described above may be referred to as indirect heat exchanging.

Two or more of the arrangements shown in Figure 1 may be used to affect or control the temperature of the compressor gaseous stream 10 prior to the inlet 14.

A person skilled in the art will be aware of the nature and provision of the first and second heat exchangers 26, 28, which may comprise one or more heat exchangers in parallel, series or both, as well as the nature and provision of first and second cold streams 32, 34.

For example, the second heat exchanger 28 can be supplied with a cooling stream 28a, which first passes

through a control/expansion valve 28b. The expansion of such streams and their use in heat exchangers is known in the art .

The first and second cold streams 32, 34 may be provided from any suitable source being separate or integral to the refrigerant circuit of the general scheme 2 shown in Figure 1. For example, after the continuing refrigerant stream 20b passes through the one or more coolers 17 and the one or more accumulators 19, its stream 20c can be divided by a divider 19a or stream splitter into a second continuing stream 2Od (which passes through the valve 7 to become the refrigerant stream 2Oe) and a second recirculation stream 40, which is preferably an at least partially liquid recirculation stream 40, in a second recirculation line 40. The liquid recirculation stream 40 preferably comprises a liquid phase, either in the form of a mixed liquid and vapour phase or an essentially fully liquid phase.

The source of the second cold stream 34 could be any suitable supply of an at least partially, preferably wholly, liquid stream 34b.

Preferably, the first and second sources 40, 34b of the first and second cold streams 32, 34, pass through respective first and second flow-control valves 40a, 34c. Optionally, one or more of the sources of the cold streams 32, 34 may also be cooled in a separate heat exchanger ( s ) .

Figure 1 further shows that the inlet temperature T]_ of the compressor gaseous stream 10 can be relayed, for instance by the temperature controller Tl, to one or more expanders such as valves, which influence the flow and/or cooling of one or more streams in the general scheme 2 shown in Figure 1. For example, the inlet temperature υ\-_

could be relayed via one or more of the (dashed) lines 9 to one or more of the valves 28b, 34c and 40a, whose operation controls the flow of the expanded stream thereafter, which therefore feeds through into the degree of cooling provided to one or more of the streams passing through one or more of the heat exchangers, or to one of the streams to be combined therewith.

The present invention can therefore provide rapid control and feedback from the inlet temperature T _] _ to one or more of the valves, ensuring maintenance of the temperature of the compressor gaseous stream 10 at or close to a normal operating temperature. In addition, a signal indicating the valve position of the recirculation valve 22 or a signal origination from a controller (not shown) that controls the recirculation valve 22, can be relayed to any of the other temperature controlling valves in the arrangement shown in Figure 1 to increase the response time of the temperature control. This provides feed forward or ratio control. Thus, the cooling in response to the temperature T]_ may suitably comprise controlling the operation of one or more valves. These one or more valves may control the flow and/or expansion of the cold stream or cooling stream used for the cooling of one or more of the steams defined in step (e) - suitably by direct or indirect heat exchange therewith - or otherwise influence the cooling of one or more of the streams defined in step (e) .

By maintaining the temperature T _] _ of the compressor gaseous stream 10 at a 'constant' level, for example within +10 ⁰ C of the normal operating temperature, the effect is that the vapour recirculation stream is able to maintain the required suction flow rate of compressor

gaseous stream 10 into the inlet 14 to avoid surge of the refrigerant compressor 12.

With maintenance of the inlet temperature T _] _ of the compressor gaseous stream 10 as required, compressor surge by temporary flow reduction is avoided as shown by line A in Figure 4. With a minimum flow rate in the vapour recirculation line 30, the refrigerant compressor 12 will also be more efficient by the reduction of an unnecessary high recycle rate. For example, US 4,464,720 shows a centrifugal compressor having a re-circulation line connected between the suction and discharged sides of the compressor. However, there is not shown any control of the temperature of the recirculation line or stream therein, such that 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 as described above in relation to line A of Figure 4.

Without any control over the temperature of the incoming gas stream during pressure variations caused by changes in the recirculation line (for example its opening or closing), surge is possible in a refrigerant compressor. Also, in case of recirculation at a reduced driver power output, the compressor is forced into a less efficient operating regime. By maintaining the inlet temperature of the compressor gaseous stream 10 at the normal operating point, for example less than 10 ⁰ C above the dew point for pure component refrigerants and within 10 ⁰ C of the normal operating point for mixed refrigerants, the present invention avoids such variation and thus maintains the refrigerant compressor 12 in optimal performance during variation of the flow rate of the compressor gaseous stream 10.

Figure 4 shows, in line B, how the path of a refrigerant compressor response can be significantly improved by the present invention fixing the inlet temperature of the refrigerant compressor. In particular, the refrigerant compressor does not change its performance curve. Moreover, the temperature of the liquid inventory in the system no longer has to change. As shown in the Figure 4, the system moves from its initial steady state (triangle α) to the new steady state (asterisk γ) along one compressor performance curve only. Thus, as the pressure ratio decreases, the flow through the compressor continuously increases, avoiding the path of line A discussed hereinabove.

In Figure 1, 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 refrigerant compressor having two or more inlets for the compression of one or more refrigerant streams at different pressures, optionally in a single casing.

Figure 2 shows another refrigeration circuit 3 comprising a refrigeration zone 41. The refrigeration zone 41 may comprise two or more, such as four, separate heat exchangers, or it may comprise a single heat exchanger involving outlets of refrigerant at different pressure levels. 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 5 such as a natural gas stream to 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 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 1. 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 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 the refrigerant stream 60 at a refrigeration pressure. More than one inlet may be present .

In the arrangement shown in Figure 2, the refrigeration zone 41 has first, second, third and fourth outlets 43, 44, 45, 46 respectively for the parts of the refrigerant that has evaporated at different pressure levels, with decreasing pressure from the first outlet 43 to the fourth outlet 46. For example, the first outlet 43 is intended for gaseous refrigerant 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 .

The fourth knock-out drum gaseous stream 100a passes into a first refrigerant compressor 58 to provide a compressed stream 100b, which is combined with the second knock-out gaseous stream 80a to enter a second refrigerant compressor 56 to provide a first combined compressed stream 120. The first and second refrigerant compressors 58, 56 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 knock-out gaseous streams 80a, 100a.

Similarly, the third knock-out gaseous stream 90a passes into a third refrigerant compressor 54, and its compressed stream 90b is combined with the first knockout gaseous stream 70a to pass into a fourth refrigerant compressor 52 and provide a second combined compressed stream 110. As above, the third and fourth refrigerant compressors 54, 52 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 knock-out gaseous streams 70a, 90a.

Each refrigerant compressor (12, 52, 54, 56, 58) may have a vapour recirculation stream (30) or a vapour recirculation stream fraction (150a-d) . Likewise, each of the one or more refrigerant compressors may have a vapour recirculation line and an at least partially liquid recirculation line around the or each refrigerant compressor . The arrangement of the refrigeration zone 41, the outlets and gaseous streams therefrom, and the refrigerant compressors 52 - 58, are known in the art, and are shown and described for example in WO 01/44734 A2. 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 140.

In the same manner as described above for the arrangements shown in Figure 1, the cooled compressed stream 140 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 may be divided into four separate recirculation stream fractions 150a, 150b, 150c, 15Od to pass through separate respective control valves 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 liquid stream 190 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 and return and use as stream 60 in the refrigeration zone 41.

The accumulator 66 also provides a convenient source of a second generally liquid recirculation stream 180, which is cooler than the first recirculation stream 150. Thus, the second recirculation stream 180 can act a source of a cold stream 32 shown in Figure 1. Similar to the first recirculation stream 150, the second 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 2, to pass through respective flow-control valves and be combined, respectively, with the four vapour recirculation stream fractions 150a, 150b, 150c and 15Od prior to the knockout drums 48a-d. The cooler fraction streams 180a-d (being liquid in the arrangement shown in Figure 2, but possibly being two phase or even just gaseous in other arrangements), vapourise on contact and combination with the warmer vapour recirculation fractions 150a-d, for example by direct injection into 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 second 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 second recirculation stream 180 provides a recirculation stream having a temperature cooler than the first recirculation stream 150, such that their combination 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 refrigerant compressors 52-58. Thus, maintenance of the inlet temperatures of at least the first and third refrigerant compressors 58, 54, labelled in Figure 2 as inlet temperatures T2, T3 respectively, can be maintained preferably within j^5°C of their normal operating temperature. Operation of the temperature and flow of the vapour and second 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 stream 70- 100, can provide the finesse for optimal operation of the refrigerant compressors, 52-58 and their inlet temperatures T2 and T3.

If desired or necessary, the inlet temperatures prior to the fourth and second refrigerant compressors

52, 56 can be similarly maintained at or close to their normal operating temperature, including through control of the streams 70a, 80a, and their combination with the compressed streams 90b and 100b. Figure 3 shows a second refrigeration circuit 4 similar to the first refrigeration circuit 3 shown in Figure 2. However, in the second refrigeration circuit 4, the second evaporated refrigerant stream 80a is now combined with the second compressed stream 90b prior to passage of the combined stream into the fourth refrigerant compressor 52. Meanwhile, the first gaseous evaporated stream 70a is combined with the first compressed stream 100b for entry into the third refrigerant compressor 56. 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 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 second recirculation stream 180 is provided, and subsequently divided into four fractions 180a-d for addition to the respective vapour recirculation stream fractions 150a, 150b, 150c and 15Od as described hereinbefore . Again, operation of the temperature and flow of the vapour and second 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 refrigerant compressors 52-58 in the arrangement shown in Figure 3 to maintain their inlet temperatures T2 and T3 within at or close to their normal operating temperature.

In the arrangements shown in Figures 2 and 3, 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 refrigerant compressors, in such a way as to maintain the inlet temperature of at least one refrigerant compressor at or close to its normal operating temperature. Table 1 shows a comparison of various flows in a refrigeration circuit, firstly as per a prior art circuit based on the arrangement shown in Figure 2 herewith but with only vapour recirculation, and secondly, a working example of the arrangement of a refrigeration circuit of the present invention shown in Figure 2 using a second recirculation stream 180.

Table 1

Prior Art Recycle With stream 180 Recycle

Stream Flow P T Flow P T [kg/s] [bar] [° C] [kg/s] [bar] [° C]

130 693 21 .5 714 23.6

70a 5 .5 20 .9 5.4 7 .8

80a 3 .5 15 .3 3.0 -8 .0

90a 1 .4 -5 .1 1.2

33 .8

100a 1 .1 10 .3 0.73

46 .5

180a 20.2

180b 15.4

180c 13.3

18Od 11.3

150a 144 93.1

150b 86 56.6

150c 53 26.5

15Od 63 15.4

Table 1 shows the significant reductions in the temperatures of the suction drum overhead streams 70a, 80a, 90a, 100a, (which are then the full or further compressor gaseous streams for the refrigerant compressors 52-58), when these streams involve combination with a fraction 180a-d of the cooler second recirculation stream 180. The reduced temperatures maintain the combined streams 70a, 80a, 90a, 100a, (and so also the combination of streams 90b + 70a and 100b + 80a), at or close to the normal operating temperatures of the refrigerant compressors 52-58.

The methods and apparatuses as herein described may be used to avoid surge in one or more refrigerant compressors .

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.