EP1864064A1

EP1864064A1 - Method for the liquefaction of a hydrocarbon-rich system

Info

Publication number: EP1864064A1
Application number: EP06724956A
Authority: EP
Inventors: Sander Kaart
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 2005-03-09
Filing date: 2006-03-07
Publication date: 2007-12-12
Also published as: AU2006222005A1; WO2006094969A1; AU2006222005B2; US20080173043A1; RU2007137274A; RU2386090C2

Abstract

The present invention relates to a method for the liquefaction of a hydrocarbon-rich stream, preferably a natural gas containing stream, by heat exchanging against a refrigerant (1a-d) . The liquid refrigerant (19) is evaporated using heat from the hydrocarbon-rich stream, thereby obtaining an evaporated refrigerant (3a-d) . The evaporated refrigerant (3a-d) is subsequently compressed (5), cooled (10) against ambient thereby fully condensing the compressed refrigerant. Next, the fully condensed compressed refrigerant (12) is further sub-cooled (14) by indirect heat exchange against an auxiliary refrigerant being cycled in an auxiliary refrigerant. Then the subcooled refrigerant (16) is expanded (18) thereby forming the liquid refrigerant (19).

Description

METHOD FOR THE LIQUEFACTION OF A HYDROCARBON-RICH STREAM

Field of the invention

The present invention relates to a method for the liquefaction of a hydrocarbon-rich stream, preferably a natural gas containing stream, wherein the hydrocarbon- rich stream to be liquefied is heat exchanged against a refrigerant thereby cooling the hydrocarbon-rich stream. Background of the invention

US patent 6,272,882 discloses a plant and a process of liquefying a gaseous methane-rich feed to obtain liquefied natural gas. The plant comprises a pre-cooling stage for pre-cooling the feed, followed by a natural gas liquids extraction stage, followed by further cooling of the gaseous feed in a mixed-refrigerant operated main cryogenic heat exchanger to obtain a pressurised liquid natural gas. The pressurised liquid natural gas is finally flashed to atmospheric pressure in a flashing stage .

The disclosed pre-cooling stage is based on a propane refrigerant cycle, wherein evaporated propane is compressed in a propane compressor. The propane is next condensed in an air cooler, where after the condensed propane at elevated pressure is passed to heat exchangers. In the heat exchangers, heat is to be transferred from the product stream into the propane refrigerant. Before entering into the heat exchangers, the condensed propane is allowed to expand to a high intermediate pressure over an expansion valve. A gaseous fraction of propane is formed by drawing heat from the product stream drawn from the heat exchangers and passed to an inlet in the propane compressor. The liquid fraction is passed to a consecutive heat exchanger. Before entering in the consecutive heat exchanger, the propane is allowed to expand to a low intermediate pressure over another expansion valve.

Further methods for the liquefaction of a hydrocarbon-rich stream have been disclosed in US 5,611,216, US 4,094,655, US 6,449,984 and US 2003/0177786. In spite of the known plants and processes, there is a continuing demand for improving the process efficiency.

There is also a continuing demand for reducing the refrigerant flow in any refrigerant cycle. The lower flow can for instance be used to lower capital expenditure required for safety measures, or to increase the production of refrigerated product stream by operating under normal refrigerant flow. Summary of the invention

It is an object of the present invention to meet the above demands.

It is a further object of the present invention to provide an alternative method for the liquefaction of a hydrocarbon-rich stream.

One or more of the above or other objects can be achieved according to the present invention by providing a method for the liquefaction of a hydrocarbon-rich stream, preferably a natural gas containing stream, wherein the hydrocarbon-rich stream to be liquefied is heat exchanged against a refrigerant, the method at least comprising the steps of:

(a) evaporating a liquid refrigerant using heat from the hydrocarbon-rich stream, thereby obtaining an evaporated refrigerant; (b) compressing the evaporated refrigerant, thereby obtaining a compressed refrigerant;

(c) cooling the compressed refrigerant against ambient thereby fully condensing the compressed refrigerant;

(d) expanding the fully condensed compressed refrigerant thereby forming said liquid refrigerant; wherein, before expanding in step (d) , the fully condensed compressed refrigerant is further sub-cooled by indirect heat exchange against an auxiliary refrigerant being cycled in an auxiliary refrigerant cycle comprising an auxiliary compressing step followed by drawing heat from the fully condensed compressed refrigerant for its further sub-cooling. Further sub-cooling of the already fully condensed compressed refrigerant has an advantage that less flash vapour will be formed in the expanding. Such flash vapour has to be circulated through the refrigeration cycle, while it hardly contributes to refrigerating the product stream. Specifically, power is lost in recompressing the flash vapour.

As less of the refrigerant needs to be cycled, associated equipment such as piping and/or flaring capacity may be downgraded. Alternatively, the refrigerant flow is maintained despite that less refrigerant is required, whereby the surplus refrigerating capacity is employed to increase the production of refrigerated stream.

According to a particularly preferred embodiment of the present invention, the fully condensed compressed refrigerant is sub-cooled to a temperature that is lower than ambient temperature. The further sub-cooling is preferably performed to a temperature that is less than 30 ⁰C above a bubble point temperature of the refrigerant after the subsequent expanding. Generally, the closer the temperature of further sub-cooling is to the bubble point of the refrigerant after the subsequent expanding, the less flash vapour will be generated. Thus, it is preferred that the further sub-cooling is preferably performed to a temperature that is less than 10 ⁰C - more preferably less than 4 ⁰C - above a bubble point temperature of the refrigerant after the subsequent expanding.

By performing the further sub-cooling to a temperature that is on a bubble point temperature of the refrigerant after the subsequent expanding, it can be avoided that any flash vapour is formed. Consequently, the full volume of refrigerant can be available for cooling the product stream.

However, it also costs more refrigerating power in the auxiliary cycle to lower the temperature. It has been found that there can be cross-over point where the additional auxiliary refrigerating power exceeds the main refrigerating power benefit. It has been found that the cross over point can be as close as 1.0 ⁰C, or even as close as 0.5 ⁰C, or even as close as 0.1 ⁰C above the bubble point of the main refrigerant after the subsequent expanding. For this reason, it is preferred that the further sub-cooling is performed to a temperature of not lower than 0.1 °C above the bubble point temperature of the refrigerant after the subsequent expanding. The auxiliary refrigerant is cycled in an auxiliary cycle comprising an auxiliary compressing step followed by drawing heat from the fully condensed compressed refrigerant. The auxiliary refrigerant cycle can be a dedicated auxiliary cycle, allowing to add-on to an existing process an additional sub-cooling process without having to modify the existing process in other places . The above and other features of the present invention will be illustrated below by way of example only and with reference to the accompanying non-limiting drawing. Brief description of the drawing

In the accompanying drawing, Figure 1 schematically shows an apparatus for carrying out one embodiment of the method of the invention;

Figure 2 schematically shows cooling and depressurisation trajectories in a schematic phase diagram;

Figure 3 schematically shows an apparatus for carrying out a comparative method;

Figure 4 schematically shows an apparatus for carrying out an alternative embodiment of the method of the invention;

Figure 5 schematically shows an apparatus for carrying out another alternative embodiment of the method of the invention;

Figure 6 schematically shows an apparatus for carrying out another alternative embodiment of the method of the invention; and

Figure 7 schematically shows an apparatus for carrying out still another alternative embodiment of the method of the 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. Detailed description of an embodiment

Figure 1 schematically shows an apparatus representing a process scheme for refrigerating a hydrocarbon-rich product stream. The apparatus comprises a heat exchanger arrangement 1 in the form of so-called kettles Ia to Id wherein a liquid refrigerant 19 is allowed to evaporate using heat from the product stream (not shown) . Here four kettles are depicted, each operating at a different pressure level, but the invention can also employ other types of heat exchangers or a different number of heat exchangers including a single heat exchanger.

In the shown four-stage heat exchanging, part of the liquid refrigerant in kettle Ia is evaporated, using heat from the product stream, whereby a liquid fraction of the liquid refrigerant is retained and separated from the evaporated part to be fed to the next kettle Ib, from where another part can be evaporated and so on.

Evaporated refrigerant is removed from the kettles Ia to Id via lines 3a to 3d, and fed to a compressor 5 wherein the evaporated refrigerant is subsequently compressed. The compressor 5 has consecutive pressure compression stages 5a to 5d, and the lines 3a to 3d fluidly connect with corresponding pressure level inlets 6a to 6d. Alternatively, a train of compressors with different pressure levels can be employed, or a single compressor. Another possible alternative for the present invention is a split compressor arrangement as published in US patent 6,637,238. The compressed refrigerant is expelled from compressor 5 via line 8 and contains a lot of heat, notably super heat in the vapour phase and evaporation heat. The compressed refrigerant is cooled against ambient in ambient cooler 10, here provided in the form of an air cooler, whereby the super heat and the evaporation heat is removed from the compressed refrigerant resulting in a fully condensed compressed refrigerant 12. Alternatively, a water cooler can be employed instead of or in combination with the air cooler 10. Depending on the ambient temperature and the composition of the refrigerant, the fully condensed compressed refrigerant 12 may be sub-cooled against the ambient when some additional heat is removed from the refrigerant by the ambient.

This is illustrated in Figure 2, which schematically depicts a phase diagram for a typical refrigerant, where enthalpy H is set out on a horizontal axis and pressure P on a vertical axis. Line 20 represents a phase envelope, underneath which liquid and vapour phases of the refrigerant coexist and separate. Point W represents the compressed refrigerant 8 at high pressure Po and high enthalpy (or temperature) . In the air cooler 10 the compressed refrigerant is cooled to point Y, i.e. its enthalpy is lowered, at essentially constant pressure. The removal of super heat is indicated by line 22 and the removal of evaporation heat is indicated by line 24. X represents the refrigerant as it has just fully condensed at the given pressure level P₀. The optional sub-cooling against ambient to point Y is indicated via line 26. Referring back to Figure 1, the fully condensed compressed refrigerant 12 is further sub-cooled by indirect heat exchange against an auxiliary refrigerant 40, for instance in an auxiliary heat exchanger arrangement 14, resulting in a further sub-cooled fully condensed compressed refrigerant stream 16. Suitably, the auxiliary heat exchanger arrangement 14 can comprise one single heat exchanger or a set of two or more heat exchangers arranged in series, wherein the auxiliary refrigerant is allowed to evaporate at one or more pressure levels. During the further sub-cooling the pressure is essentially maintained at the compressed level. In the phase diagram of Figure 2, the resulting further sub- cooled fully condensed compressed refrigerant 16 is represented by point Z, and the further sub-cooling at constant pressure by line 28.

Finally, the sub-cooled fully condensed compressed refrigerant 16 is expanded in an expansion means, for instance over a Joule-Thompson valve 18, and the resulting refrigerant stream 19 is fed into the first kettle Ia where it is allowed to evaporate using heat extracted from the product stream. Preferably no expander is present between cooler 10 and the auxiliary heat exchanger 14; in other words, the pressure drop of the refrigerant between the cooling in cooler 10 and the sub- cooling against the auxiliary refrigerant in auxiliary heat exchanger 14 is less than 10 bar, preferably less than 5 bar, more preferably less than 2 bar, even more preferably less than 1 bar.

As the formation of flash vapour is now suppressed by the further sub-cooling of the fully condensed compressed refrigerant, it is now also possible to employ dynamic expansion instead of or preceding expansion in valve 18. Some of the heat released is picked up to be used elsewhere, and extra energy is drawn from the refrigerant so that the cooling requirement in heat exchanger arrangement 14 can be lower.

The invention at least also covers an alternative embodiment, wherein the first heat exchange stage Ia is performed in two or more kettles or heat exchangers arranged in parallel with each other. In such arrangement, the further subcooled fully condensed compressed refrigerant 16 can be split in two or more branches and expanded over two or more valves provided in the branches. Both the evaporated fractions as well as the liquid fractions drawn from the parallel heat exchangers are recombined, whereby the evaporated fraction is subjected to recompression and the liquid fraction fed to a consecutive serially arranged second cooling stage. An example of such parallel first stage is shown in US patent 6,389,844.

In a variation of the alternative embodiment, the further sub-cooled fully condensed compressed refrigerant 16 is expanded in one expansion means such as the Joule- Thompson valve 18 of the embodiment of Figure 1, and subsequently split over the two or more branches as discussed in the above-paragraph.

It is in the present invention preferred that the fully condensed compressed refrigerant stream is sub- cooled to such an extent that the expanded refrigerant stream in line 19 stays fully in the liquid region of the phase diagram. Any vapour that is formed over the expansion, so-called flash vapour, is lost for cooling purposes, but still has to go through the compression cycle via the first kettle Ia and line 3a. By reducing the amount of flash-evaporated refrigerant formed prematurely during expansion, compression power can thus be saved. In practice this means that the fully condensed compressed refrigerant 12 is preferably sub-cooled to a temperature whereby the subsequent expanding brings the temperature below the bubble point temperature of the refrigerant after the subsequent expanding, to avoid flashing during the expansion step altogether.

This is schematically shown in Figure 2, whereby refrigerant stream 16 is preferably flash-expanded from point Z via line 30 to point Zl where the pressure P_a represents the pressure level of operation of the first kettle Ia.

Assuming that cooling of the product stream in the first kettle Ia involves evaporation of the liquid refrigerant, it is preferred that the fully condensed compressed refrigerant in line 12 has been sub-cooled to a temperature between the temperature in the first kettle Ia and 9 ⁰C higher than that temperature, preferably between the temperature in the first kettle Ia and 5 ⁰C higher than that temperature, more preferably the temperature in the first kettle Ia and 3 °C higher than that temperature.

The present invention also covers alternative routes from X to Zl, such as for instance sub-cooling against ambient from point X to Y, and then further sub-cooling while simultaneously letting off pressure; or such as for instance sub-cooling against ambient from point X to Y, letting off pressure to an intermediate value that is higher than P_a, then further sub-cooling, and then letting off more pressure and so on until point Zl is reached.

The auxiliary refrigerant 40 can be at least partially evaporated after having picked up enthalpy from the refrigerant stream 12 in heat exchanger arrangement 14. In the embodiment of Figure 1, the auxiliary refrigerant is cycled in an auxiliary cycle 55, whereby the auxiliary refrigerant stream is recompressed in an auxiliary compressor 45. The auxiliary compressor 45 optionally comprises two or more compression stages 45a and 45b. A fully compressed auxiliary refrigerant stream 42 is then cooled against ambient in heat exchanger 44. The resulting fully compressed cooled auxiliary refrigerant stream 46 is then optionally separated in a liquid fraction 52 and a vapour fraction 50 in separator 48, whereby the vapour fraction 50 is fed back to the auxiliary compressor 45 at an intermediate pressure inlet point. More optionally the fully compressed cooled auxiliary refrigerant stream 46 is partly flashed off by letting down the pressure upstream of the separator 48. For this reason a Joule- Thompson valve 54 can be provided, optionally preceded by a dynamic expansion device if possible. Such so-called "economizer" line-up reduces power consumption as part of the flash vapour is circulated at a higher pressure level than if the full pressure drop were made in one expansion step in valve 56.

Just before drawing heat from the condensed compressed refrigerant 12, the pressure in the auxiliary refrigerant stream 52 can be let down, for which a Joule- Thompson valve 56 can be provided, optionally preceded by a dynamic expansion device if possible. By letting the pressure down to a predetermined selected value, the bubble point temperature can be chosen in accordance with the desired temperature to be reached in the fully condensed compressed refrigerant 16.

Instead of a dedicated auxiliary cycle for cycling the auxiliary refrigerant as shown in Figure 1, the auxiliary refrigerant 40 can be derived from a cold slip stream from somewhere else in the process. For instance, in a when the product stream is a natural gas stream and the ultimate goal of refrigeration is liquefying the natural gas, then the auxiliary refrigerant can for instance be a slip stream of so-called end-flash gas. An advantage of such alternative is that no additional refrigeration cycle has to be built and operated and that the additional heat integration within the entire process can increase the power-efficiency of the entire process. In a pre-cooling stage of a liquefaction process for natural gas, a single component refrigerant, typically propane (i.e. comprising at least 90 mol% propane, preferably substantially 100%), is often employed as the refrigerant. In particular when the refrigerant is propane, a suitable choice for the auxiliary refrigerant is butane (i.e. comprising at least 90 mol% butane, preferably substantially 100%). Butane is suitable because it has a slightly higher boiling temperature than propane refrigerant when determined under equal pressure condition. This enables a suitable selection of heat exchange conditions in heat exchanger arrangement 14 whereby the auxiliary refrigerant 40 can evaporate by picking up heat from the fully condensed compressed refrigerant 12.

Another reason making butane suitable as a choice for the auxiliary refrigerant 40 is that it has a higher heat of evaporation than the fully condensed compressed refrigerant 12. Therefore sub-cooling of a certain flow rate of the fully condensed compressed refrigerant 12 can be achieved using a smaller flow rate of the auxiliary refrigerant 40. The auxiliary compression power is further lowered by the fact that the required compression ratio is smaller provided that the temperature to which the compressed auxiliary refrigerant 42 is cooled against ambient in heat exchanger 44 is the same as the temperature of the fully condensed compressed refrigerant 12.

The fully condensed compressed refrigerant 12 is best sub-cooled to a temperature whereby the subsequent expanding in valve 18 brings the temperature below the bubble point temperature of the liquid refrigerant 19 after the subsequent expanding and before subsequent expanding in valve 2a. Comparative example Figure 3 represents a comparative apparatus for carrying out a comparative process. The difference with the embodiment of Figure 1 is that the auxiliary heat exchanger arrangement 14 and the auxiliary refrigerant cycle is not present. Thus, the further sub-cooling step of the invention is not employed. This can result in that the fully condensed compressed refrigerant 12 (corresponding to point Y in Figure 2) is partly wasted to flash vapour during expansion in valve 18, as is schematically represented in Figure 2 wherein line 32 crosses the phase envelope 20 on its way to point Yl. When the system is in point Yl, the refrigerant phase- separates into a liquid fraction in point Zl and vapour fraction in point U whereby the total available enthalpy Hy is divided over the liquid fraction in which an enthalpy of R^ will ^be vested and a the vapour fraction in which an enthalpy of H_\j will be vested.

The recompression in compressor 5 of evaporated refrigerant discharged from kettle Ia via line 3a, in each of the embodiments of Figure 1 and Figure 3, is schematically depicted in Figure 2 by line 34 indicating that compression heat is added to the evaporated refrigerant 3a in point U and the pressure is increased. After recompression, the recompressed refrigerant 8 is back in the starting point W and the cycle is completed.

Both in the comparative embodiment and the embodiment of Figure 1, part of the liquid refrigerant 19 is evaporated in a first stage in the first kettle Ia, using heat from the product stream, after expanding the liquid refrigerant over valve 18. A retained liquid fraction of the refrigerant is drawn from the first kettle Ia and let down to a lower pressure level over valve 2a (or equivalent means, optionally in combination with a dynamic expander) before it is fed to the second kettle Ib where cooling of the product stream can proceed in a second stage. In this way even more consecutive cooling stages, each time at a lower pressure level, can be executed using the same liquid refrigerant to each time enable vapourisation at a lower temperature.

The power balance in the process of Figure 1 has been calculated for both the main refrigerant cycle and the auxiliary refrigerant cycle, assuming that a total of 148.7 MW of heat is drawn from the product stream in kettles Ia to Id. Butane was selected as the auxiliary refrigerant cycling through compressor 45, and propane as the main refrigerant cycling through compressor 5. The result of this calculation is shown in columns 2 and 3 in Tables I and II given below, for relevant lines in the process of Figure 1 as set out in column 1 of Tables I and II.

As comparison, the power balance in the propane cycle of the process of Figure 3 has been calculated, assuming that the same kettle duty in kettles Ia to Id of 148.7 MW as was assumed in the calculation of the process of Figure 1. The result is shown in columns 4 and 5 of Tables I and II. Of Tables I and II, Table I shows temperature (in columns 2 and 4) and pressure data (columns 3 and 5), and Table II power data.

Column 6 in each of the Tables I and II shows the difference between the processes of Figure 1 and 3.

Table I Temperature and pressure data

1 (line) 2 3 4 5 6

Inv • (Fig • D Comp. (Fig. 3) Diff .

3a 12. 2 ⁰C 682 kPa 12.2 ° C 682 kPa 0

3b -4. 5 °C 413 kPa -4.5 ° C 413 kPa 0

3c -21 .4 ⁰C 232 kPa -21.4 ⁰C 232 kPa 0

3d -39 .8 ⁰C 111 kPa -39.8 ⁰C 111 kPa 0

8 2073 kPa 2073 kPa 0

12 42 ⁰C 42 ⁰C 0

16 (12) 15 ⁰C (42 ⁰C ) -27 ⁰C

40 13 ⁰C 164 kPa — —

46 42 °C 402 kPa — —

50 257 kPa — —

Table II Power balance

In these calculations, an isentropic efficiency of 80% has been assumed for all compressor stages 5a to 5d and 45a and 45b. Condensation of the propane in line 8 is assumed to occur at a temperature of 57 ⁰C. In the process of Figure 1, a power of 36.2 MW is transferred in heat exchanger arrangement 14 from the main refrigerant to the auxiliary refrigerant.

The total amount of propane that is cycled through line 12 in the process of Figure 1 is 456 kg/s, while in the process of Figure 2 a propane flow of 589.3 kg/s was required to maintain the same heat transfer rate (chiller duty) of 148.7 MW in the kettles Ia to Id. For achieving this reduction of 133.3 kg/s of cycling propane, a flow rate of only 104.2 kg/s of auxiliary refrigerant in the form of butane through line 40 was needed, and only a total flow rate of 116.5 kg/s of butane needed to be cycled in line 46.

Hence, the propane cycle can be provided with smaller piping, or with the same piping a smaller pressure loss will be experienced. Also, safety flaring capacity could likely be reduced, as the largest refrigerant circuit (in this case the propane main refrigerant circuit) needs to contain less refrigerant.

It can be seen from Table I that the major difference between the process of Figure 1 and the process of Figure 3 is that the propane stream in line 16 of

Figure 1 is sub-cooled by 27 °C more than the propane stream in line 12 of Figure 2, bringing the temperature of the propane stream in line 16 upstream of the valve 18 to only 2.8 ⁰C above the temperature in the first kettle Ia.

As a result less vapour is thought to form in valve 18, so that less 133.3 kg/s less propane needs to be recycled and 9 MW less power needs be put into the highest pressure stage 5a of compressor 5. Only part (4.9 MW) of the power saved is used to drive the auxiliary compressor 45, so that 4.1 MW of total work is saved by the process of Figure 1 over the process of Figure 3. Accordingly, the rejected heat is 4.1 MW lower in the process of Figure 1 compared to the one of Figure 3.

Other embodiments

It is remarked that by letting off pressure in the valves 2a to 2c going from the first to the second pressure levels of Figures 1 and 3, some flash vapour can be formed similar to letting off the pressure in valve 18. This is schematically shown in Figure 2, where letting off pressure from point Zl results in phase separation from point Z2. In a further preferred embodiment, before expanding in the consecutive sub- stage, the liquid fraction is further sub-cooled (for instance to point V over trajectory 38 in Figure 2) by indirect heat exchange against a second auxiliary refrigerant. Upon subsequent letting off the pressure (from P_a to P)₃ to point Vl in Figure 2) flash vapour formation can also be reduced or avoided m the further pressure reductions. Alternative trajectories from Zl to Vl can be employed instead as explained above for trajectories to Zl.

Various embodiments incorporating this general principle are shown in Figures 4 to 6.

Starting with Figure 4, this embodiment is based on the embodiment of Figure 1 modified by the provision of a second auxiliary refrigerant cycle 155. Similar to the auxiliary refrigerant cycle 55, the second auxiliary refrigerant cycle 155 can comprise a second auxiliary compressor 145, an optional second separator 148, a second ambient heat exchanger 144. The second auxiliary compressor 145 optionally comprises two or more compression stages 145a and 145b.

In operation, the second auxiliary refrigerant stream is recompressed m the second auxiliary compressor 145. A fully compressed second auxiliary refrigerant stream 142 is then cooled against ambient in heat exchanger 144. The resulting fully compressed cooled second auxiliary refrigerant stream 146 is then optionally separated in a second liquid fraction 152 and a second vapour fraction 150 in second separator 148, whereby the second vapour fraction 150 is fed back to the second auxiliary compressor 145 at an intermediate pressure inlet point. More optionally the fully compressed cooled second auxiliary refrigerant stream 146 is partly flashed off by letting down the pressure upstream of the second separator 148. For this reason a Joule-Thompson valve 154 can be provided, optionally in combination with a dynamic expansion device. The second liquid fraction 152 is led to second auxiliary heat exchanger arrangement 114 where it draws heat from the liquid refrigerant leaving the first kettle Ia by indirect heat exchange. After being discharged from the second auxiliary heat exchanger arrangement 114, the second auxiliary refrigerant is recompressed in second auxiliary compressor 145.

Just before drawing heat from the liquid refrigerant leaving the first kettle Ia, the pressure in the second auxiliary refrigerant stream 152 can be let down, for which a Joule-Thompson valve 156 can be provided, optionally in combination with a dynamic expansion device .

Figure 5 relates to an embodiment wherein the first- mentioned auxiliary refrigerant cycle has been modified in that the optional separator 48 of Figure 1 is provided in the form of a kettle 58 or a heat exchanger. Line 12 passes through that kettle as its warm side. In operation, the fully condensed compressed refrigerant 12 is further sub-cooled by indirect heat exchange against the auxiliary refrigerant in at least two stages including the kettle 48 and the heat exchanger arrangement 14 at two pressure levels. The auxilary refrigerant circuit of the embodiment of Figure 5 can also be advantageously applied in an embodiment such as shown Figure 1 which is not provided with a second auxiliary refrigerant circuit. However, in another advantageous embodiment shown in

Figure 6, the apparatus of Figure 5 is modified in that line 146 is also passed though kettle 48. Herewith it is achieved that the fully compressed cooled second auxiliary refrigerant stream 146 is sub-cooled or further sub-cooled by indirect heat exchange against the first- mentioned auxiliary refrigerant before expanding it in expansion device 154. In this embodiment, the sub-cooling or further sub-cooling is applied on the second auxilary refrigerant in order to avoid unnecessary circulation of vapour through the second compressor stage 145a thereby saving a little more compression power in the second auxilary refrigerant cycle 155.

In the embodiments of Figure 4 to 6, the second auxiliary refrigerant should preferably be selected to have a lower bubble point temperature than that of the auxiliary refrigerant, but higher than that of the main liquid refrigerant, when determined under equal-pressure condition. In case of propane as a main refrigerant and butane as the auxiliary refrigerant, iso-butane is a suitable choice for the second auxiliary refrigerant.

Likewise, a third and fourth auxiliary refrigerant cycles can be employed between second and third, respectively third and fourth main refrigerant pressure stages. The possible power reduction is expected to be less with each stage, as the compression power put into each compression stage of 5a to 5d is lower with each consecutive stage. But, as the main refrigerant flow through lines 3c and 3d will be reduced by virtue of the invention, more of the product stream can be refrigerated before the maximum suction flow of the compressor 5 is reached. This is of particular importance in a colder ambient, as then the refrigerant pressure can be lower while at the same time the flow has to be higher in order to achieve the required volumetric flow. Provided that the maximum suction flow of the compressor is not exceeded, the lower refrigerant flow through lines 3c and 3d would help to maximise the amount of refrigerated product stream that can be produced.

The calculation that led to Tables I and II above has also been performed on the embodiments of Figure 4 to 6, whereby propane (C3) was chosen as the main refrigerant, normal butane (nC4) as the auxiliary refrigerant in circuit 55, and iso-butane (iC4) as the second auxiliary refrigerant in circuit 155.

The results on the energy balance and the refrigerant flow rates in the various circuits is shown in Table III. In order to compare the effect of the described modifications of the embodiment of Figure 1, the results for Figures 1 and 3 are repeated in Table III.

Table I I I

*) in kettle 58 instead of heat exchanger arrangement 14 **) in heat exchanger arrangement 14

In order to achieve this situation, in the processes of Figures 5 and 6 15.2 MW was transferred from line 12 to the auxiliary refrigerant via kettle 58 to bring the temperature of the refrigerant in line 12 upstream of heat exchanger arrangement 14 down to 30 ⁰C. In the process of Figure 6, an additional power of 1.6 MW was transferred from line 152 to the auxiliary refrigerant via kettle 58. The temperature of the liquid refrigerant just before expanding in valve 2a in the processes of Figures 4 to 6 has been lowered to -4.5 ⁰C in second auxiliary heat exchanger arrangement 114.

Still alternative embodiment of the method of the invention is illustrated in Figure 7. This embodiment employs a similar amount of hard ware as the embodiment described above with reference to Figure 1, but allows for further sub-cooling of the liquid refrigerant at two pressure levels in two consecutive stages. In this embodiment the compressor 5 is arranged in two compression sections 5a and 5b. Instead of separator 48 of Figure 1, a kettle 58 is employed wherein after letting off pressure in valve 54 the auxiliary refrigerant is both separated into vapour 50 and liquid 52 fractions and is allowed to further evaporate using heat drawn from the fully condensed compressed refrigerant 12. The fully condensed compressed refrigerant 12 is thereby further sub-cooled using kettle 58 for the function of heat exchanger arrangement 14 of Figure 1.

The resulting further sub-cooled fully condensed compressed refrigerant 16 is expanded over valve 18 and fed to the first kettle Ia where it is allowed to evaporate against heat drawn from the product stream. The residual liquid fraction is drawn from the kettle Ia and before expanding in valve 2, the liquid fraction is again further sub-cooled by indirect heat exchange in heat exchanger arrangement 14 against a second auxiliary refrigerant in the form of the liquid fraction 52 drawn from kettle 58. The pressure level in of the second auxiliary refrigerant in heat exchanger arrangement 14 can be lowered relative to the pressure level in kettle 58 to a desired pressure level by means of for instance Joule Thompson valve 56. The further sub-cooling before expanding the second time in valve 2 can reduce or avoid flash vapour formation in a similar way as before in kettle 58.

The auxiliary refrigerant in the embodiment of Figure 7 is best selected by considering the bubble-point requirement in the second stage 14 using similar considerations as explained above. The bubble-point requirement in kettle 58 can then be achieved by selecting suitable pressure drops over valves 54 and 56.

For a propane refrigerant cycle, a suitable auxiliary refrigerant is iso-butane.

Energy balance calculations have been performed for the process of Figure 7 in the same way as for the other embodiments, again assuming the same chiller duty. The results have also been included in Table III. Like in the processes of Figures 4 to 6, the temperature of the liquid refrigerant just before expanding in valve 2a in has been lowered to -4.5 ⁰C in auxiliary heat exchanger arrangement 14.

It turns out that the overall power saved is 6.6%, as was the case for the embodiment of Figure 1. Adding a third stage to the iC4 cycle could enhance the savings. However, it is notable that this embodiment results in the highest propane flow reduction of all the embodiments .

The above-described embodiments can be used to cool any type of product stream, but can suitably be employed in a pre-cooling stage in the production of liquefied natural gas (LNG) wherein the product stream comprises a natural gas.

Instead of using the method of the invention to reduce the power consumption, as illustrated above, it is also possible to increase the duty (more heat exchanged) without needing to cycle a higher amount of refrigerant in the highest pressure stage 5a of the compressor 5 than would have been the case in the comparative embodiment.

In the above description, the compressors are driven by a suitable motor, such as for instance a gas turbine or an electrically driven motor or a combination thereof.

Claims

C L A I M S

1. Method for the liquefaction of a hydrocarbon-rich stream, preferably a natural gas containing stream, wherein the hydrocarbon-rich stream to be liquefied is heat exchanged against a refrigerant, the method at least comprising the steps of:

(a) evaporating a liquid refrigerant using heat from the hydrocarbon-rich stream, thereby obtaining an evaporated refrigerant;

(b) compressing the evaporated refrigerant, thereby obtaining a compressed refrigerant;

(d) expanding the fully condensed compressed refrigerant thereby forming said liquid refrigerant; wherein, before expanding in step (d) , the fully condensed compressed refrigerant is further sub-cooled by indirect heat exchange against an auxiliary refrigerant being cycled in an auxiliary refrigerant cycle comprising an auxiliary compressing step followed by drawing heat from the fully condensed compressed refrigerant for its further sub-cooling.

2. Method of claim 1, wherein the further sub-cooling is performed to a temperature that is lower than ambient temperature.

3. Method of claim 1 or 2, wherein the auxiliary refrigerant is selected to have a higher bubble point temperature than the liquid refrigerant when determined under equal-pressure condition.

4. Method according to one or more of the preceding claims, wherein the auxiliary refrigerant is selected to have a higher heat of evaporation than the liquid refrigerant .

5. Method according to one or more of the preceding claims, wherein the pressure drop of the refrigerant between the cooling in step (d) and the sub-cooling against the auxiliary refrigerant is less than 10 bar, preferably less than 5 bar, more preferably less than 2 bar.

6. Method according to one or more of the preceding claims, wherein the refrigerant comprises > 90 mol% propane and the auxiliary refrigerant comprises > 90 mol% butane .

7. Method according to one or more of the preceding claims, wherein expanding the fully condensed compressed refrigerant in step (d) is performed in at least consecutive first and second sub-stages, wherein the further sub-cooling of the fully condensed compressed refrigerant is performed to a temperature on or above a bubble point temperature of the refrigerant after the subsequent expanding and before expanding in the second sub-stage .

8. Method of claim 7, wherein part of the liquid refrigerant is evaporated in the first sub-stage, using heat from the hydrocarbon-rich stream, after expanding in the first sub-stage and before expanding in the second sub-stage, wherein a liquid fraction of the liquid refrigerant is retained and separated from the evaporated part and further expanded in the second sub-stage, wherein the liquid fraction is further sub-cooled by indirect heat exchange against a second auxiliary refrigerant before the further expanding in the second sub-stage .

9. Liquefied hydrocarbon-rich stream, in particular liquefied natural gas, obtained by heat exchanging against a refrigerant using the method according to one or more of the preceding claims .