WO2022042882A1

WO2022042882A1 - A system and a method for reducing settle-out pressure using multiple collection vessel sections

Info

Publication number: WO2022042882A1
Application number: PCT/EP2021/025318
Authority: WO
Inventors: Lorenzo GALLINELLI; Michael BALFE; Marco Pelella; Leonardo Baldassarre
Original assignee: Nuovo Pignone Tecnologie - S.R.L.
Priority date: 2020-08-26
Filing date: 2021-08-23
Publication date: 2022-03-03
Also published as: EP4204749A1; EP4204748A1; WO2022042879A1

Abstract

The system for reducing settle-out pressure in a thermodynamic system (1) includes a chilling arrangement (51) which can be fluidly coupled with a compressor (31) adapted to circulate a working fluid in the thermodynamic system. The chilling arrangement (51) includes a heat exchanger (57) and a collection vessel (59) divided into a first vessel section (59.1) and a second vessel section (59.2) for collecting condensed working fluid at to different temperature levels, to separate heavier and lighter components of the working fluid.

Description

A system and a method for reducing settle-out pressure using multiple collection vessel sections

Description

TECHNICAL FIELD

[0001] The present disclosure concerns thermodynamic systems and methods. Specifically, disclosed herein are methods and systems for reducing the settle-out pressure (SOP) of a closed circuit in a thermodynamic system following shutdown of a pressure boosting apparatus, such as a compressor, to facilitate startup of the system.

BACKGROUND ART

[0002] In thermodynamic systems, where a working fluid is processed in a closed circuit and undergoes thermodynamic transformations comprising phase transitions between a liquid state and a gaseous state, shutdown of the compressor or other pressure boosting facility, causes pressure to equalize out in the closed circuit. The equalized pressure is called settle-out pressure. The settle-out pressure may depend, among others, upon the temperature of the circuit.

[0003] Settle-out pressure can dramatically increase and adversely affect the startup capability of the compressor driver. This is particularly the case where the thermodynamic system comprises a refrigeration circuit and is arranged in a hot environment. When the thermodynamic system is shut down and remains inoperative for a relatively long time at high ambient temperature, the thermodynamic system starts heating up. Possible liquid present in the closed compression loop begins to vaporize and pressurize the closed circuit, until the equalized pressure at ambient temperature or at the temperature of the metallic structure defining the closed circuit is achieved. This temperature may be as high as 50°C or higher, due to solar irradiation, for instance. The resulting settle-out pressure may be well above the design point and may be such that the compressor driver is incapable of starting up the compressor again.

[0004] In some compression systems, for instance those using mixed refrigerant (hereafter shortly also MR), such as propane-mixed refrigerant or APCI® LNG processes, compressor suction drums are usually dry, and no liquid accumulation is usually present therein. Consequently, the compressor settle-out pressure mainly depends on pressure equalization between the suction and discharge volume of the compressor section, occurring during the emergency shutdown transient. Also in this case, however, the SOP is critical, since the equalized pressure in the compressor may reach such values as to prevent compressor start-up. Indeed, the torque required to accelerate the compressor during compressor re-start can exceed the driver capability, preventing compressor start up. Hence, depressurization is required.

[0005] Generally speaking, similar issues may arise in thermodynamic systems comprising a pressurized circuit adapted to contain a working fluid and comprising at least one working fluid collection vessel, adapted to contain at least two phases of a working fluid, specifically a liquid phase and a gaseous phase (which may be or include a vapor phase) in a condition of thermodynamic equilibrium. Since the equilibrium pressure in a bi-phase system depends inter alia upon the temperature of the fluid, when the temperature increases, the equilibrium pressure in the system increases as well and may become higher than a threshold pressure. This may prejudice or adversely affect one or more functionalities of the system or prevent operation thereof altogether. If this situation occurs, venting the thermodynamic system is required or a dedicated compressor is needed to circulate the fluid in a condenser, to lower the pressure therein. Venting may cause loss of valuable products, cause environmental pollution, or entail other disadvantages.

[0006] WO2019/138049 discloses a novel thermodynamic system and a method for reducing settle-out pressure by collecting and condensing the working fluid of the thermodynamic system in a collection vessel, which is functionally coupled to a chilling arrangement.

[0007] Further improvements of this novel system and relevant method would be beneficial to enhance efficiency, for instance in cases where the working fluid includes a mixture of components, having different molecular weights.

SUMMARY

[0008] A system and a method for reducing settle-out pressure in a thermodynamic system are disclosed. The system includes a chilling arrangement adapted to be fluidly coupled with a processing section, e.g. including a compressor arrangement adapted to circulate a working fluid in the thermodynamic system. The chilling arrangement includes a heat exchanger and a collection vessel. The heat exchanger is adapted to chill working fluid flowing from the processing section through the chilling arrangement. The collection vessel includes at least a first vessel section and a second vessel section for collecting condensed working fluid at to different temperature levels. The heat exchanger includes a low-temperature section and a high-temperature section to chill the working fluid at two different temperature levels. Heavier and lighter components of the working fluid are thus collected in the at least two vessel sections.

[0009] According to embodiments disclosed herein, the thermodynamic system comprises a processing section, adapted to circulate a multicomponent working fluid therein, and a chilling arrangement. The chilling arrangement comprises in turn a collection vessel adapted to collect therein a liquid phase and a gaseous or vapor phase of the multicomponent working fluid in thermodynamic equilibrium. The collection vessel is adapted to be fluidly coupled to the processing section to remove working fluid therefrom and re-introduce working fluid therein during a settle-out pressure reduction step. The chilling arrangement further includes a heat exchanger functionally coupled to the collection vessel. The heat exchanger includes a hot side adapted to receive circulated working fluid, and a cold side adapted to receive chilling fluid in heat exchange relationship with the working fluid to remove heat therefrom.

[0010] According to embodiments disclosed herein, the heat exchanger includes at least a low-temperature section and a high-temperature section, each section including a hot side and a cold side. In in operation, i.e. during a settle-out pressure reduction process, chilling fluid circulates in the cold side of the low-temperature section and in the cold side of the high-temperature section and working fluid circulates in the hot side of the high-temperature section and in the hot side of the low-temperature section of the heat exchanger. As will become apparent from the detailed description of embodiments, in currently preferred embodiments the same chilling fluid can circulate in both sections of the heat exchanger, the cold sides whereof can be arranged in series. In other embodiments, the two cold sides of the heat exchanger sections may be arranged in parallel. In yet further embodiments, different chilling fluids can be used in the at least two heat exchanger sections, e.g. two chilling fluids at two different inlet temperatures.

[0011] In turn, the collection vessel includes at least a first vessel section, fluidly coupled to an outlet of the hot side of the high-temperature section of the heat exchanger and a second vessel section fluidly coupled to an outlet of the hot side of the low-temperature section of the heat exchanger. The first and second vessel sections can be arranged in series, i.e. in sequence, such that uncondensed working fluid collected in the first vessel section can flow towards the second vessel section, preferably passing through the low-temperature section of the heat exchanger. As will become apparent from the following detailed description of embodiments, however, other (currently less preferred) embodiments are possible, wherein the two vessel sections are not arranged in series with one another.

[0012] According to a further aspect, methods for reducing a settle-out pressure of a multicomponent fluid in a thermodynamic system are disclosed. In embodiments, the method comprises the following steps: feeding working fluid from the processing section through a hot side of a high- temperature section of the heat exchanger in heat exchange relationship with chilling fluid circulating in a cold side of the high-temperature section of the heat exchanger; collecting liquefied working fluid from the high-temperature section of the heat exchanger in a first vessel section of the collection vessel in a first condition of liquid-vapor equilibrium therein; feeding working fluid from the processing section through a hot side of a low- temperature section of the heat exchanger in heat exchange relationship with chilling fluid circulating in a cold side of the low-temperature section of the heat exchanger; collecting liquefied working fluid from the low-temperature section of the heat exchanger in a second vessel section of the collection vessel in a second condition of liquid-vapor equilibrium therein, at a temperature lower than in the first vessel section; and returning uncondensed working fluid from the collection vessel towards the processing section.

[0013] Further features and embodiments of the system and of the method according to the present disclosure are outlined below and set forth in the enclosed claims, which form an integral part of the present description. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] A more complete appreciation of the disclosed embodiments of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

Fig. l illustrates a schematic of a natural gas liquefaction plant including an arrangement for reducing settle-out pressure in case of compressor shutdown (emergency shutdown or during normal shutdown);

Fig.2 illustrates a schematic of the chilling arrangement in one embodiment; and Figs.3, 4, 5, 6, 7, 8, 9, 10 and 11 illustrate schematics of further embodiments.

DETAILED DESCRIPTION

[0015] According to one aspect, the present subject matter is directed to systems and methods for facilitating the startup of a thermodynamic system following tripping of a compressor or other pressure boosting arrangement, as a consequence of which the settle-out pressure (shortly also referred to as SOP) inside the thermodynamic system has increased. Specifically, in embodiments disclosed herein a thermodynamic system is provided, which includes a closed circuit adapted to circulate a working fluid, which undergoes cyclic thermodynamic transformations. In operation, a compressor boosts the pressure of the working fluid and circulates the working fluid in the closed circuit. The closed circuit comprises several sections, such as a low-pressure circuit section and a high-pressure circuit section. The pressure boosting arrangement sucks low-pressure working fluid from the low-pressure circuit section and delivers pressurized working fluid in the high-pressure circuit section.

[0016] To facilitate re-starting of the thermodynamic system, the working fluid is chilled and at least partly condensed and collected in a collection vessel. The pressure and amount of fluid present in the closed circuit, and specifically in the compressor section, are thus gradually reduced, until suitable conditions for restarting the thermodynamic system are achieved.

[0017] According to the present disclosure, a novel and useful chilling arrangement is disclosed, for improved chilling and condensing a multicomponent working fluid. More in detail, the chilling arrangement includes a heat exchanger, having at least one hot side where the working fluid circulates and at least one cold side, where a chilling fluid circulates in heat exchange relationship with the working fluid. Heat is thus removed from the working fluid by the chilling fluid. In some embodiments, as will be described in more detail hereafter, multi-stream heat exchangers can be used, which may have more than one hot side and/or more than one cold side, i.e. at least two hot streams and/or two cold streams. In other embodiments, the multi-stream heat exchanger may be replaced by a heat exchanger network, i.e. a combination of heat exchangers in series and/or parallel, performing the same cooling service.

[0018] To improve efficiency of the chilling process, the heat exchanger and the collection vessel are divided into at least two sections. A high-temperature section of the heat exchanger receives the entire working fluid flowrate. A portion of the working fluid exiting the high-temperature section of the heat exchanger partly collects in a condensed (liquid) phase in a first vessel section of the collection vessel. Working fluid still in gaseous, i.e. in vapor phase, flows from the first vessel section through the low- temperature section of the heat exchanger and is at least partly condensed and collected in a liquid phase in a second vessel section of the collection vessel. Condensed heavier components of the multicomponent working fluid (i.e. components having a higher molecular weight and a higher boiling temperature) mainly collect in the first vessel section. Condensed lighter components of the multicomponent working fluid (i.e. components having a lower molecular weight and a lower boiling temperature) mainly collect in the second vessel section.

[0019] The working fluid flowrate through the low-temperature section of the heat exchanger is substantially smaller than the flowrate through the high-temperature section of the heat exchanger, such that a smaller heat exchanger surface suffices to achieve low condensing temperatures in the second vessel section of the collection vessel.

[0020] In the following description of embodiments the thermodynamic system, in which the novel chilling arrangement is embodied, is a system for producing liquefied natural gas (LNG), shortly referred to also as “LNG system”. Those skilled in the art will, however, will appreciate that several features of the chilling arrangement and method disclosed herein and the relevant advantages thereof may be beneficially used also in different thermodynamic systems, where tripping of the system may cause increase of the settle-out pressure and may require reduction of the settle-out pressure achieved by a multicomponent fluid, prior to restart the system.

[0021] Moreover, in the present description reference will be made specifically to a processing section including a compressor, wherein reduction of the settle-out pressure is beneficial in order to facilitate compressor start-up after tripping. However, the arrangement disclosed herein may be used with advantage also in combination with other plants including a processing section, wherein reduction of a settle-out pressure may be required or beneficial for other purposes. For instance, the processing section may include a tank or reservoir, which undergoes pressure increase in case of tripping or for any other reason, and which may require pressure reduction without venting gas from the process section.

[0022] In the following description of embodiments, the chilling arrangement is adapted to remove working fluid from the compressor arrangement and thus reduce the pressure therein. In more general terms, the chilling arrangement can be adapted to remove working fluid from a generic processing section, which may form part of a more complex thermodynamic system, for instance including a closed circuit, wherein the settle-out pressure in the processing section needs to be lowered to re-start the thermodynamic system after tripping or normal shutdown.

[0023] Referring now to the drawings, Fig.1 illustrates an exemplary embodiment of an LNG system comprised of two combined refrigeration circuits for liquefying natural gas. Each refrigeration circuit is adapted to circulate a respective working fluid (refrigerant) therein. In the example of Fig.1 the refrigeration circuits include a pro- pane/mixed refrigerant system, including a first refrigerant circuit (propane circuit) using propane as working fluid, and a second refrigerant circuit (mixed refrigerant circuit) using mixed refrigerant (shortly MR) as a working fluid.

[0024] In the embodiment of Fig.1, the mixed refrigerant circuit comprises means for reducing the pressure of the MR in the circuit, e.g. following a period of inactivity of the MR compressor, which may lead to increased SOP. The mixed refrigerant is a multicomponent refrigerant containing fractions of fluids having different molecular weights, i.e. lower and higher molecular weights, condensing at different temperatures.

[0025] The thermodynamic system of Fig. 1 also comprises a storage unit or tank to store liquefied process fluid, i.e. liquefied natural gas. In embodiments disclosed herein the liquefied natural gas is used as a chilling fluid to reduce SOP in the MR circuit following tripping or shutdown.

[0026] While in Fig.1 the LNG system includes a combination of a propane refrigerant circuit and an MR refrigerant circuit, in some embodiments, LNG system may include more than two refrigerant circuits and/or different refrigerant fluids operating at different temperatures. The refrigerant of one said circuits can be used to reduce the pressure in another of said circuits.

[0027] For instance, in embodiments the LNG system may include a low-temperature nitrogen circuit with a nitrogen storage facility, where liquefied nitrogen is stored. Liquefied nitrogen can be used to reduce the pressure in a higher-temperature refrigeration circuit, for instance a mixed refrigerant circuit.

[0028] In embodiments, the LNG system may include a liquefied nitrogen storage tank, which is not used as a refrigerant in a refrigeration cycle, but rather just as a chilling fluid for SOP reduction purposes.

[0029] Those skilled in the art of gas liquefaction will understand that novel features of the method and system disclosed herein for reducing SOP at start-up can be embodied in other LNG systems, using different refrigeration circuits and refrigerant fluids, such as a Cascade® cycle, single mixed refrigerant (SMR) or dual mixed refrigerant (DMR) circuits, Linde® liquefaction systems, AP-X® liquefaction systems, and the like.

[0030] Additionally, features disclosed herein can be used in liquefaction facilities designed for the production of liquefied gases other than natural gas, such as ethane, propane, butane, pentane, propylene, ammonia, nitrogen, hydrogen and the like, as well as in chemical or petrochemical plants in general. In general, the liquefied gas can be stored in a storage unit, for example in a condition of vapor/liquid equilibrium. The liquefied gas can be used to chill a working fluid in a thermodynamic circuit, for example to reduce the pressure in a refrigerant circuit containing a refrigerant working fluid.

[0031] Turning now to Fig.1, the thermodynamic system is labeled 1 as a whole and comprises a first closed refrigerant circuit 3, wherein a first refrigerant working fluid is adapted to circulate and to undergo cyclic thermodynamic transformations, including compression, condensation, cooling and expansion. As mentioned above, by way of example in Fig. 1 the first closed refrigerant circuit 3 is a closed propane circuit.

[0032] The working fluid circulates in the closed refrigerant circuit 3 by means of a pressure boosting arrangement 5. In the schematic of Fig.1, the pressure boosting arrangement 5 comprises a compressor 7 having a suction side 7S and a delivery side 7D and a plurality of side streams. Each suction side can be provided with a suction drum (not shown) positioned upstream of the relevant compressor suction flange. In other embodiments, not shown, the pressure boosting arrangement 5 can include more than one compressor, in any configuration, for instance a plurality of compressors arranged in series and/or in parallel.

[0033] Downstream of the pressure boosting arrangement 5, with respect to the direction of the working fluid flow schematically represented by arrow FF, a heat removal and fluid condensing arrangement 9 is provided. The heat removal and fluid condensing arrangement 9 can include a heat exchanger, for instance a liquid/air or liquid/liquid heat exchanger. In other embodiments, the heat removal and fluid condensing arrangement 9 can include any other kind of heat removal arrangement.

[0034] A condensate collecting vessel or fluid collection vessel 11 is arranged downstream of the heat removal and fluid condensing arrangement 9. Propane in a bi-phase condition of liquid/gas (or vapor) equilibrium can be contained in the fluid collection vessel 11.

[0035] An expansion section 13 and a heat exchange arrangement 15 are further provided along the closed refrigeration circuit 3. A sub-cooler 12 can be arranged between the fluid collection vessel 11 and the expansion section 13.

[0036] The expansion section 13 may include one or more expanders, such as turboexpanders or hydraulic turbines (if the process fluid flowing there through is in the liquid state), or expansion valves, such as Joule-Thomson valves. The heat exchange arrangement 15 can include one or more evaporators, in which the condensed and expanded working fluid from the expansion section 13 is heated by heat exchange with a flow of a process fluid to be chilled, as will be described later on. [0037] In the schematic of Fig.1, the pressure boosting arrangement 5 further comprises a driver 19, which generates the mechanical power required to drive the compressor 7 in rotation. The driver 19 can be an electric motor. In other embodiments, as schematically shown in Fig. 1, the driver 19 can be a mechanical-power-generating turbomachine, such as a gas turbine engine, a turboexpander or a steam turbine. In yet further embodiments, the driver 19 can include a reciprocating, internal combustion engine. A combination of different drivers can also be used. In such case, drivers of the same kind, or else drivers of different nature can be combined with each other. For instance, a gas turbine engine can be used in combination with an electric motor or with a steam turbine.

[0038] The expansion section 13 and the heat exchange arrangement 15 are configured to expand propane at different, decreasing pressure levels, corresponding to decreasing temperatures of the propane. The propane is used to pre-cool a flow of natural gas flowing in a natural gas delivery line 21 and is further used to cool a flow of mixed refrigerant MR circulating in a second closed refrigeration circuit 4, described in greater detail below.

[0039] In the exemplary embodiment of Fig. 1, the expansion section 13 comprises a first set of expansion valves, or a set of expanders or of hydraulic turbines, shown at 13 A. The expansion section 13 further comprises a second set of expansion valves or expanders or hydraulic turbines shown at 13B. The expansion valves of each set 13A and 13B are arranged in series, to expand propane at gradually decreasing pressures and generate partial streams of propane at said decreasing pressures. The partial streams of expanded propane at the different pressure levels obtained by the expansion valves 13 A exchange heat in heat exchangers 15A of the heat exchange arrangement 15, at variable temperatures with a flow of natural gas flowing in the natural gas delivery line 21. The partial streams of expanded propane from the expansion valves 13B exchange heat at variable temperatures in heat exchangers 15B with the mixed refrigerant MR circulating in the second closed refrigeration circuit 4. The partial streams are then returned as side streams to the compressor 7.

[0040] In the exemplary thermodynamic system 1 the second closed refrigeration circuit 4 circulates a second refrigerant, e.g. a mixed refrigerant, in heat exchange relationship with the refrigerant (propane) processed in the first closed refrigerant circuit 3 and in heat exchange relationship with natural gas to be liquefied.

[0041] In embodiments, the second closed refrigeration circuit 4 includes a processing section 30, including a compressor arrangement 31, comprised of one or more compressors in series, driven by one or more drivers 33, e.g. electric motors, gas turbine engines, steam turbines or other mechanical power generating machines, or combinations thereof. In embodiments, the compressor arrangement 31 includes a low- pressure compressor section 31A and a high-pressure compressor section 3 IB (or a medium-pressure and high-pressure compressor section 3 IB) arranged in series. An intercooler 32 can be arranged between the low-pressure compressor section 31 A and the high-pressure compressor section 3 IB. Moreover, the compressor arrangement 31 may comprise at least one suction drum on the low-pressure compressor section 31A (not shown) upstream of compressor suction flange.

[0042] While in the embodiment schematically shown in Fig.l the compressor arrangement 31 and the compressor 7 are driven by different and separate drivers 33 and 19, respectively, in other embodiments (not shown) the propane compressor 7 and the mixed refrigerant compressor arrangement 31 can be arranged on the same shaft line and can be driven by the same driver or combination of drivers. In yet further embodiments (not shown), the high-pressure compressor section 3 IB and the propane compressor 7 can be on the same shaft line and driven by the same driver, while the low- pressure and/or medium-pressure mixed refrigerant compressor sections can be arranged on a separate shaft line driven into rotation by a different driver.

[0043] The compressed working fluid (mixed refrigerant, MR) of the closed refrigeration circuit 4 is cooled in a cooler 35 and further cooled and at least partly condensed in the heat exchangers 15B by heat exchange against expanded propane of the closed refrigerant circuit 3. Partly liquefied mixed refrigerant MR is delivered to a liquid-vapor separator 36 and the separate liquid and vapor streams from the separator 36 are expanded and circulated in a main cryogenic heat exchanger 37 (MCHE). Expanded mixed refrigerant further chills and liquefies the natural gas by heat exchange therewith in the main cryogenic heat exchanger 37. Vaporized mixed refrigerant MR is then delivered to compressor arrangement 31 to be compressed and circulated again in the above described loop. [0044] The closed refrigeration circuit 4 is thus divided into a high-pressure circuit section between the delivery side of the compressor arrangement 31 and the main cryogenic heat exchanger 37, and a low-pressure circuit section between the main cryogenic heat exchanger 37 and the suction side of the compressor arrangement 31.

[0045] Liquefied natural gas from the main cryogenic heat exchanger 37 is collected and stored in a storage unit 39, wherefrom it can be delivered to one or more users or facilities, such as transportation facilities, e.g. to an LNG carrier.

[0046] In some circumstances the pressure inside the closed refrigeration circuit 4 can increase, for instance if the circulation of refrigerant working fluid is interrupted for whatever reason. The pressure between suction side and delivery side of the compressor arrangement 31 will equalize and achieve a settle-out pressure, which may increase in case the environment temperature increases. This may require action to be taken in order to reduce the pressure inside the compressor arrangement 31 and re-start working fluid circulation in the closed refrigeration circuit 4.

[0047] As will be described herein in connection with Fig.2, cooling of the working fluid (mixed refrigerant, MR) and thus reduction of the settle-out pressure in the closed refrigeration circuit 4 can be achieved by means of a chilling arrangement schematically shown at 51 in Fig.1, which can use liquefied natural gas (LNG) from the system as the chilling fluid. While in the schematic drawing liquefied natural gas is drawn from the storage unit 39, in some embodiments LNG can be drawn from the main cryogenic heat exchanger (MCHE) 37 in addition to or instead of using LNG from the storage unit 39.

[0048] In the schematic of Fig.1, an LNG conduit 53 and a cryogenic pump 54 fluidly connect the LNG storage unit 39 to the chilling arrangement 51. As will be described in connection with Fig.2, LNG vaporizes by heat exchange with the working fluid (mixed refrigerant) contained in the compressor arrangement 31 of processing section 30. Vaporized LNG is delivered through a boil-off gas (BOG) conduit 55 (BOG header) to a boil-off gas system (BOG system) schematically shown at 57 and recovered. By removing heat from the processing section 30, the SOP therein is reduced until a pressure value suitable for restarting the compressor arrangement 31 is achieved. [0049] Specifically, in some embodiments the compressor arrangement 31 can be isolated from the remaining of the refrigeration circuit 4 and the pressure therein can be reduced by condensing the mixed refrigerant contained therein through heat exchange against LNG, or another chilling fluid.

[0050] With continuing reference to Fig.1 , Fig.2 illustrates a schematic of an embodiment of the chilling arrangement 51 in more detail.

[0051] The chilling arrangement 51 includes a heat exchanger 57, functionally coupled to a working fluid collection vessel 59 adapted to collect components of the mixed refrigerant (hereafter referred to as “working fluid”) in a bi-phase condition, i.e. in a condition of thermodynamic equilibrium between a liquid phase and a gaseous (or vapor) phase. As will become apparent from the following description, the heat exchanger 57 includes at least a cold side, through which the chilling fluid circulates. The heat exchanger 57 further includes at least a hot side, through which the working fluid circulates. The heat exchanger 57 or at least one section thereof, can be a multistream heat exchanger. In such case the heat exchanger or the at least one section thereof will include a further fluid path, in addition to the hot side and the cold side, in heat exchange relationship among each other. In general, each heat exchanger section can be designed to include two hot and/or two cold streams. For example, in the schematic of Fig.2, one heat exchanger section 57.2 (to be described) includes two cold streams and one hot stream.

[0052] According to one aspect, the heat exchanger 57 includes a low-temperature section 57.1 and a high-temperature section 57.2 arranged in sequence in an upstreamdownstream direction, referred to the direction of flow of the chilling fluid, i.e. the chilling fluid flows through the cold side of the low-temperature section 57.1 first and subsequently through the cold side of the high-temperature section 57.2. Conversely, the working fluid flows through the hot side of the high-temperature section 57.2 first and subsequently through the hot side of the low-temperature section 57.1. As such, the working fluid and the chilling fluid are broadly flowing in a counter-flow mode. In each section of the heat exchanger 57, heat is removed from the working fluid by the chilling fluid.

[0053] In the exemplary embodiment illustrated in Fig.2, both the low-temperature section 57.1 and the high-temperature section 57.2 are designed as counter-now heat exchanger sections. However, in other embodiments, not shown, one or both sections or parts thereof can be designed to work in a co-current mode, i.e. with the working fluid and the chilling fluid flowing in the same direction across the heat exchanger.

[0054] As will be described in detail below, the working fluid flowrate through the two sections 57.1 and 57.2 of the heat exchanger 57 are different. More specifically, the flowrate through the high-temperature section 57.2 is higher than the flowrate through the low-temperature section 57.1, since part of the working fluid exiting the high-temperature section 57.2 is collected in a condensed state in a section of the collection vessel 59.

[0055] While in the embodiment of Fig.2 the heat exchanger 57 is divided into two sections, in other embodiments, the heat exchanger 57 can be divided into a higher number of sections.

[0056] Moreover, each heat exchanger section 57.1 and 57.2 can be designed as a single heat exchanger equipment or as a section of a main heat exchanger, or their combination.

[0057] In other embodiments, the heat exchanger may be also split in more sections each arranged in a network (series and/or parallel) to perform the cooling service

[0058] Moreover, several heat exchanger types can be used, for example plate and fin, shell and tube, kettle, harpin, coil wound, or their combination. Each heat exchanger section may have a different design, i.e. for instance one section may include a kettle configuration, and another section a shell and tube configuration.

[0059] The collection vessel 59 incudes a plurality of vessel sections. In embodiments, the number of vessel sections is identical to the number of heat exchanger sections. In Fig.2 the collection vessel 59 includes a first vessel section 59.1 and a second vessel section 59.2 arranged in series, the first vessel section 59.1 being arranged upstream of the second vessel section 59.2 with respect to the direction of flow of the working fluid.

[0060] The first vessel section 59.1 is fluidly coupled, through an inlet conduit 59.3, with the outlet of the hot side of the high-temperature section 57.2 of the heat exchanger 57. The first vessel section 59.1 is further fluidly coupled, through an outlet conduit 59.4, with the inlet of the hot side of the low-temperature section 57.1 of the heat exchanger 57. The inlet of conduit 59.4 is arranged at the top of the first vessel section 59.1, to collect uncondensed working fluid contained therein and to deliver the flow of uncondensed working fluid through the low-temperature section 57.1 of the heat exchanger 57. In use, condensed working fluid, mainly containing heavier components thereof, collects in the bottom of the first vessel section 59.1, while lighter, uncondensed components of the working fluid will flow through the low-temperature section 57.1 of the heat exchanger 57and will at least partly condense and collect in the second vessel section 59.2. To that end, the second vessel section 59.2 is fluidly coupled, through an inlet conduit 59.5, with the outlet of the hot side of the low-temperature section 57.1 of the heat exchanger 57.

[0061] The second vessel section 59.2 is further fluidly coupled to an outlet conduit 59.6, which is in turn fluidly coupled with the compressor arrangement 31 through the high-temperature section 57.2 of the heat exchanger 57, which for such purpose includes three streams. A conduit 59.7 branched-off from outlet conduit 59.6 is provided for bypassing the high-temperature section 57.2 of the heat exchanger 57, if needed.

[0062] Flowing the working fluid from the second vessel section 59.2 through the heat exchanger 57 in heat exchange relationship with warmer working fluid from the processing section before returning to the processing section 30, prevents working fluid to reach the compressor arrangement 31 at cryogenic temperatures.

[0063] In the illustrated embodiment, the high-temperature section 57.2 of the heat exchanger 57 is a multi-stream section, including a cold side through which chilling fluid flows, fed from conduit 62 or from bypass line 69 and exiting the cold side through a return conduit to be described. The high-temperature section 57.2 of the heat exchanger 57 further comprises a hot side through which the working fluid from the processing section 30 flows in heat exchange relationship with the chilling fluid circulating in the cold side of the high-temperature section 57.2 of the heat exchanger 57. Additionally, working fluid returning from the second vessel section 59.2 flows in heat exchange relationship with working fluid flowing through the hot side of the high- temperature section 57.2 of the heat exchanger 57. Thus, the working fluid arriving at the high-temperature section 57.2 of the heat exchanger 57 from the processing section 30 is chilled by heat exchange against the chilling fluid and against the cold working fluid returning from the second vessel section 59.2 towards the processing section 30.

[0064] In other embodiments, not shown, the low-temperature section 57.1 of the heat exchanger 57 can be a multi-stream heat exchanger section, where working fluid returning from the second vessel section 59.2 can be in heat exchange relationship with working fluid flowing from the first vessel section 59.1 to the second vessel section 59.2.

[0065] In operation, the collection vessel 59 operates as a fluid separator, wherein working fluid in a liquid state is collected and maintained in a condition of thermodynamic equilibrium with a gaseous or vapor phase. Uncondensed working fluid flows from the first vessel section 59.1 to the second vessel section 59.2, where it partly condenses due to heat removal through the chilling fluid in the low-temperature section 57.1 of the heat exchanger 57. The second vessel section 59.2 will thus contain liquid and gaseous working fluid in thermodynamic equilibrium at a temperature lower than in the first vessel section 59.1. In operation, uncondensed working fluid is vented through the conduit 59.6 towards the compressor arrangement 31 as explained below.

[0066] The chilling fluid is delivered to the heat exchanger 57 through conduit 53 and through a further conduit 61. A return conduit 65 returns LNG, which has vaporized in the heat exchanger 57, from the heat exchanger 57 to a liquid-vapor separator 67. Vaporized LNG flows from the separator 67 through boil-off conduit 55 towards the boil-off system 56 (Fig.l).

[0067] More specifically, the conduit 61 is fluidly coupled to the inlet of the cold side of the low-temperature section 57.1 of the heat exchanger 57. The outlet of the cold side of the low-temperature section 57.1 is in turn fluidly coupled through a conduit 62 with the inlet of the cold side of the high-temperature section 57.2 of the heat exchanger 57. The outlet of the cold side of the high-temperature section 57.2 is fluidly coupled through return conduit 65 with the liquid-gas separator 67.

[0068] As mentioned, the high-temperature section 57.2 of the heat exchanger 57 includes two cold streams. One cold stream receives working fluid from the second vessel section 59.2 through conduit 59.6. The other cold stream of the high-temperature section 57.2 of the heat exchanger 57 receives LNG from conduit 62 and/or from by-pass line 69. In other embodiments, the manner in which cold and hot streams are split may be different from the one shown in Fig. 2. For instance, a cold stream may be shared between sections 57.1 and 57.2 of the heat exchanger 57, or a stream can be extracted in a different position of the heat exchanger 57.

[0069] In other embodiments, not shown, the two sections 57.1 and 57.2 of the heat exchanger 57 can be embodied as two portions of a single heat exchanger, in which case an external coupling conduit 62 may be dispensed with.

[0070] What matters, is that at least two sections can be identified in the heat exchanger 57, where two different flowrates of the working fluid are chilled in heat exchange relationship with the chilling fluid, such that heavier components of the working fluid condense at a higher temperature and collect in the first vessel section 59.1, while lighter components of the working fluid condense at a lower temperature and collect in the second vessel section 59.2.

[0071] In embodiments, under certain operating conditions, at least part of the chilling fluid may bypass the low-temperature section 57.1 of the heat exchanger 57 through the bypass line 69, and flow from conduit 61 directly through the high-tem- perature section 57.2 of the heat exchanger 57. A controlled valve 68 can selectively close and partially or entirely open the by-pass line 69.

[0072] In embodiments, a quenching conduit 63 branches-off from the conduit 53 and is fluidly coupled to the return conduit 65 upstream of the liquid-vapor separator 67. The quenching conduit 63 is mainly used to cool down the boil-off gas (BOG) flowing from the high-temperature section 57.2 of the heat exchanger 57 to a specific temperature before delivering it into the BOG header 55. In embodiments, the flow of liquefied natural gas in the quenching conduit 63 is regulated by a dedicated temperature controller 66 as described below. The temperature controller 66 can be positioned downstream of the liquid-vapor separator 67. Quenching is also used at start-up of the chilling arrangement 51, as will be explained in further detail below.

[0073] The chilling arrangement 51 is connected to the processing section 30 and specifically to the compressor arrangement 31 of the mixed refrigerant circuit 4, through a low-pressure inlet conduit 71 and through a high-pressure inlet conduit 72. The processing section 30, and specifically the compressor arrangement 31, can be isolated from the remaining part of the refrigeration circuit 4 by means of a suction side isolation valve 73 and a delivery side isolation valve 74. The low-pressure compressor section 31 A and the high-pressure compressor section 3 IB are fluidly coupled to one another. In embodiments, an intercooler 32 (Fig.1, not shown in Fig.2) can be arranged therebetween. In embodiments, an inter-stage isolation valve 70 can be provided between the two sections 31 A, 3 IB. The inter-stage isolation valve 70, if present, as well as isolation valves 73 and 74 are closed when the compressor arrangement 31 trips or is shut down, in order to isolate the low-pressure compressor section 31 A and the high-pressure compressor section 3 IB from one another and from the remaining circuit 4.

[0074] In the embodiment of Fig.2, the low-pressure inlet conduit 71 is coupled to the closed refrigeration circuit 4 between the suction-side isolation valve 73 and the suction side of the compressor arrangement 31. The high-pressure inlet conduit 72 is coupled to the closed refrigeration circuit 4 between the delivery side of the high-pressure compressor section 3 IB and the delivery-side isolation valve 74.

[0075] The low-pressure inlet conduit 71 can be fluidly coupled with the heat exchanger 57 through conduits 77 and 79. A check valve 81 is arranged along conduit 77 and a controlled valve 83 is arranged along working fluid feed conduit 79. In some embodiments, the controlled valve 83 is controlled by a pressure controller 85, which detects the pressure in conduit 79 downstream of the controlled valve 83. In other embodiment, the controlled valve 83 can be controlled by other controllers, for example a flowrate controller.

[0076] In embodiments, the high-pressure inlet conduit 72 can be fluidly coupled to conduit 79 via a further controlled valve 87, which can be controlled by the same pressure controller 85. In other embodiments, the controlled valve 87 can be controlled by other controllers, for example a flowrate controller. In embodiments, a check valve 89 is arranged along high-pressure inlet conduit 72, upstream of controlled valve 87.

[0077] In embodiments, a closing valve 75 is arranged along the low-pressure inlet conduit 71 and a closing valve 91 is arranged along the high-pressure inlet conduit 72.

[0078] The high-pressure inlet conduit 72 can be fluidly connected via a connection duct 93 to conduit 79. The connection duct 93 can be selectively opened or closed through a closing valve 95 arranged along connection duct 93.

[0079] In some embodiments, the chilling system 51 can further include an auxiliary compressor 101, which can be used for achieving sufficiently low pressures in the compressor arrangement 31 when the working fluid includes a relatively large amount of components having a low boiling temperature (lighter components). When the working fluid comprises a mixed refrigerant, the auxiliary compressor 101 may be beneficial when the mixed refrigerant comprises a high percentage of nitrogen, for instance.

[0080] The auxiliary compressor 101 has a suction side, which can be fluidly coupled with the high-pressure inlet conduit 72 through a closing valve 103. The auxiliary compressor 101 further has a delivery side fluidly coupled with conduit 79. A cooler 105 can be positioned between the delivery side of the auxiliary compressor 101 and the conduit 79. Furthermore, in the schematic of Fig.2, a check valve 102 and a discharge isolation valve 106 are positioned between the cooler 105 and the conduit 79.

[0081] In embodiments, the auxiliary compressor 101 can be driven by a driver 101.1, for instance an electric motor. Reference number 101.5 indicates a further driver, for instance a further electric motor driving a fan of the cooler 105. A recirculation line 107 can be provided in anti -parallel to the auxiliary compressor 101, between the outlet of the cooler 105 and the suction side of the auxiliary compressor 101. Along the recirculation line 107 a controlled valve 109 can be provided, which selectively opens and closes the recirculation line 107. The controlled valve 109 can be controlled by a compressor controller 111. Furthermore, a throttling valve 104 can be provided between the closing valve 103 and the tie-in of the recirculation line 107. The throttling valve 104 can be operated by compressor controller 111.

[0082] The auxiliary compressor 101 can be a positive displacement compressor. In some embodiments, the auxiliary compressor 101 can be a reciprocating compressor, which is particularly inexpensive and adapted to compress the small flowrate, which the auxiliary compressor 101 is required to process.

[0083] The main purpose of the auxiliary compressor 101 is to pump working fluid from the compressor arrangement 31 towards the heat exchanger 57 when spontaneous flow cannot be achieved thermodynamically thanks to the temperature difference and pressure difference between the interior of the compressor arrangement 31 and the interior of the collection vessel 59, as will be explained later on in further detail.

[0084] The operation of the chilling arrangement 51 described so far is as follows.

[0085] When the LNG system is running and working fluid (mixed refrigerant) is processed through the compressor arrangement 31 and caused to circulate through the refrigeration circuit 4, the valves 75 and 91 are closed and the chilling arrangement 51 is inoperative and isolated from the LNG system. A closing valve 113 can be provided along the LNG conduit 53, to prevent LNG from circulating through the heat exchanger 57.

[0086] If the compressor arrangement 31 trips for whatever reason, the isolation valves 73 and 74 will close and the pressure in the processing section 30 will equalize and achieve settle-out pressure (SOP). The SOP is usually too high to restart the compressor arrangement 31 with the installed driver, without taking some action to reduce the pressure of the fluid therein. The chilling arrangement 51 is thus activated to temporarily remove working fluid from the compressor arrangement 31 and condense working fluid in the heat exchanger and accumulate the condensed (liquid) working fluid in the collection vessel 59, thus reducing the pressure in the compressor arrangement 31, until a re-start pressure value is achieved therein.

[0087] The chilling arrangement 51 can be put into operation just prior to restarting the compressor arrangement 31, or any time after tripping or shutdown.

[0088] When the chilling arrangement 51 shall start, the valves 73, 70 and 74, as well as valves 83 and 87 are closed. Circulation of LNG through the heat exchanger 57 is prevented by valves 117 and 113, which are initially closed.

[0089] A preliminary cooling step is performed first, to bring the heat exchanger 57 and the liquid-vapor separator 67 at a desired starting pressure and temperature. This preliminary step may be required, since the chilling arrangement 51, whereof the liquid-vapor separator 67 and heat exchanger 57 form part, may remain inoperative for a long period of time and the pressure and temperature therein may be different from the values required to start depressurization of the compressor arrangement 31.

[0090] The preliminary cooling step can be performed as follows. [0091] The valve 113 on the LNG conduit 53 is opened and a controlled valve 119 on the quenching conduit 63 is gradually opened. LNG flows through conduit 63 and valve 119 and therefrom into the liquid-vapor separator 67 through the last portion of return conduit 65. The LNG is at cryogenic temperature and thus cools the liquid-vapor separator 67.

[0092] Once the liquid-vapor separator 67 has been cooled, the heat exchanger 57 must be cooled to the required temperature. LNG is caused to flow through the cold side of both the low-temperature section 57.1 and the high-temperature section 57.2 of the heat exchanger 57 by opening the controlled valve 117 and allowing a controlled flowrate of LNG to flow through the heat exchanger 57 and the return conduit 65 towards the liquid-vapor separator 67. The boil-off gas temperature, i.e. the temperature of the boil-off gas exiting the liquid-vapor separator 67, is controlled through the temperature controller 66, which acts upon controlled valve 119, such that an LNG quench flowrate is maintained in the conduit 63 at the required value to obtain the desired boil- off gas temperature in conduit 55.

[0093] While the liquid-vapor separator 67 and the heat exchanger 57 are gradually cooled down to a required cryogenic temperature with the above described procedure, the collection vessel 59 (both sections 59.1 and 59.2 thereof) cools down progressively during the subsequent depressurization step, during which the working fluid from the compressor arrangement 31 is continuously partially condensed at cryogenic temperature in the heat exchanger 57 and is collected in the liquid condensed state in the collection vessel 59.

[0094] At the beginning of the cooling down, the first working fluid condensed in the heat exchanger 57 and received in the collection vessel 59 will vaporize completely, since the collection vessel 59 is still at higher temperature. Vaporization of the condensed working fluid entering the collection vessel 59 after condensation in the heat exchanger 57 initiates cooling down of the collection vessel 59 towards cryogenic temperature. When the collection vessel 59 is sufficiently cold, working fluid in liquid state starts collecting therein at the pressure inside the collection vessel 59. As the depressurization of the compressor arrangement 31 proceeds, the collection vessel 59 continues to be cooled down, since more working fluid condensed at cryogenic temperature collects in the collection vessel 59. [0095] As an alternative to a progressive cooling down, the collection vessel 59 can be preliminary cooled down through a procedure disclosed in more detail below.

[0096] During the preliminary cooling step, the bypass line 69 can be opened if required, e.g. when an excessive LNG flow is routed to the low-temperature section 57.1 of the heat exchanger 57 and an excessive amount of boil-off gas (BOG) is generated due to LNG vaporization. Since the low-temperature section 57.1 of the heat exchanger 57 is smaller than the high-temperature section 57.2 of the heat exchanger 57, the excessive flowrate of boil-off gas could otherwise choke the low-temperature section 57.1.

[0097] More specifically, during this preliminary cooling step, the closing valve 75 is open and the controlled valve 83 can slightly open to allow a small working fluid flowrate from the low-pressure compressor section 31A of the compressor arrangement 31 towards the high-temperature section 57.2 of the heat exchanger 57 and through the hot side thereof. Bi-phase working fluid exiting the high-temperature section 57.2 enters the first vessel section 59.1. Liquid working fluid collects in the bottom of the first vessel section 59.1, where it can vaporize, while the first vessel section 59.1 is still at high (non-cryogenic) temperature. Vaporization cools down the first vessel section 59.1. The working fluid in the vapor phase from the first vessel section 59.1 enters the hot side of the low-temperature section 57.1 of the heat exchanger 57 through outlet conduit 59.4 and flows in heat exchange relationship with the incoming LNG and finally enters the second vessel section 59.2. The working fluid partly condenses in the low-temperature section 57.1 of the heat exchanger 57 at a temperature lower than the condensing temperature in first vessel section 59.1. Bi-phase working fluid exiting the low-temperature section 57.1 of the heat exchanger 57 is collected in the second vessel section 59.2, where it can at least partly vaporize and thus cool down the second vessel section 59.2. The second vessel section 59.2 is therefore brought at a temperature lower than the first vessel section 59.1.

[0098] The preliminary cooling step is conducted until the boil-off gas (boil-off conduit 55) achieves a pre-set temperature, for instance between around 0°C and around 15°C. This temperature range is merely by way of example and shall not be construed as a limitation. In other embodiments, the pre-set temperature may be for instance lower than 0°C, e.g. even substantially lower, such as between around -20°C and around -60°C. In general, the pre-set temperature may be selected based on heat exchanger design considerations. Once this temperature is achieved, the working fluid flowrate through conduit 79 can be gradually increased by gradually opening the controlled valve 83. At the same time, the LNG flowrate through the heat exchanger 57 increases gradually by opening the controlled valve 117. Based on the quench flow delivered though conduit 63, the LNG flowrate can be controlled to target a desirable boil-off flow to keep it constant during the cooling down transient. The temperature of the vaporized LNG in boil-off conduit 55 is instead maintained at a desired temperature through the temperature controller 66. In other embodiments, a flowrate controller can be provided on the conduit 55. Irrespective of the kind of chosen control procedure, a flowrate set point can be generated, which is applied to a flowrate controller 116 provided on LNG conduit 53 and adapted to act upon the controlled valve 117.

[0099] In other embodiments, a preliminary cooling down of the collection vessel 59 can be performed. By way of example, a dedicated small working fluid condenser (e.g. a shell and tube, a plate and fin or kettle type condenser) can be used to condense a small amount of working fluid coming from the compressor arrangement 31 using a cryogenic source, such as LNG for example. Liquid working fluid thus obtained can be fed to collection vessel sections 59.1 and 59.2 cooling them down and pressurizing them.

[0100] In other embodiments, the reciprocating compressor 101 can be activated for circulating the working fluid trapped inside the collection vessel 59 and relevant piping to the heat exchanger 57 though the conduits 122, 77, 93 and 79 or different ones not shown. In other embodiments a combination of both solutions can be used.

[0101] As during the preliminary cooling step, also in the following compressor depressurization step working fluid from the low-pressure compressor section 31 A of the compressor arrangement 31 is partly condensed in the heat exchanger 57 and progressively collects in the collection vessel 59.

[0102] More specifically, working fluid in the vapor (gaseous) state flows through conduits 71, 77, 79 and controlled valve 83 and through the hot side of the high-tem- perature section 57.2 of the heat exchanger 57. Here the working fluid in the vapor phase is cooled by heat exchange against LNG flowing through the cold side of the high-temperature section 57.2 of the heat exchanger 57. The working fluid exiting the high-temperature section 57.2 of the heat exchanger 57 through conduit 59.3 is in a biphase state, i.e. contains a liquid phase and a vapor (gaseous) phase. When entering the first vessel section 59.1 part of the liquid working fluid can vaporize, thus causing further temperature reduction. Liquid working fluid collects at the bottom of the first vessel section 59.1, while working fluid in the vapor phase flows through conduit 59.4, hot side of the low-temperature section 57.1 of the heat exchanger 57 and conduit 59.5 into the second vessel section 59.2 of the collection vessel 59. Flow of the working fluid through the low-temperature section 57.1 is caused by the pressure difference between first vessel section 59.1 and the second vessel sections 59.2, due to the different temperatures therein.

[0103] Since a large part of the i ncoming working fluid condenses in the first vessel section 59. 1 and only working fluid in the vapor state flows through the low-temperature section 57.1 of the heat exchanger 57, the working fluid flowrate through the low- temperature section 57.1 of the heat exchanger 57 can be smaller than the high-tem- perature section 57.2 thereof.

[0104] As different temperatures are achieved by the working fluid in the two vessel sections 59.1, 59.2, the condensing working fluid in the two sections has a different composition. Heavier fractions (i.e. gaseous components having a higher molecular weight and a higher liquefaction temperature) of the working fluid mainly condense in the first vessel section 59.1. Lighter fractions (i.e. gaseous components having a lower molecular weight and a lower liquefaction temperature) mainly condense in the second vessel section 59.2.

[0105] During this step, the LNG flowrate through the heat exchanger 57 and the working fluid flowrate towards the heat exchanger 57 can be controlled by a temperature controller 86 arranged along LNG return conduit 65 and by the pressure controller 85. Temperature controller 86 and pressure controller 85 can be activated in a cascade arrangement, in that the set point of the pressure controller 85 is determined by the temperature controller 86.

[0106] More specifically, the temperature controller 86 is aimed at maintaining the temperature of the vaporized LNG in the boil-off conduit 55 at a pre-set value by modifying the set-point of the pressure controller 85 between a minimum value and a maximum value. For instance, control is performed such as to cause a continuous increase of the working fluid flowrate through valve 83, which is controlled by pressure controller 85 until a maximum flowrate value is achieved, which depends upon the availability of LNG. In fact, the working fluid flowrate delivered through valve 83 to the heat exchanger 57 will be the one sufficient to maintain the temperature of the vaporized LNG in return conduit 65 at a desired value, and the pressure inside the vessel section 59.1 at a given value.

[0107] For instance, if the temperature in the LNG return conduit 65 drops below the temperature set-point of temperature controller 86, which means that an excessive amount of LNG is available, the temperature controller 86 can modify the pressure setpoint of pressure controller 85, which in turns causes further opening of the controlled valve 83 and consequently increases the flow of working fluid from the compressor arrangement 31 to be condensed, in turn causing an increase of the pressure in the collection vessel 59.

[0108] In other embodiments, a different logic can be used to adjust the working fluid and LNG flowrates for the cooling down service. For example, the temperature controller 86 could be directly connected to the valve 83 in this first part of the cooling down step, or a flow controller could be used instead of pressure controller 85.

[0109] In addition to temperature controller 86 and pressure controller 85, during this step also a further pressure controller 121 can be activated, which is arranged along a return conduit 122, connecting outlet conduit 59.6 and bypass conduit 59.7 to inlet conduit 71, via a controlled valve 123. The pressure controller 121 controls the controlled valve 123 such that pressure inside the second vessel section 59.2 is maintained at a pressure set-point, which is higher than the pressure inside the low-pressure compressor section 31A of the compressor arrangement 31. When the pressure inside the second vessel section 59.2 increases above the pressure set-point of pressure controller 121, the latter opens valve 123 and vents uncondensed working fluid accumulated in the second vessel section 59.2 into the low-pressure compressor section 31A of the compressor arrangement 31 through conduits 59.6, 122 and 71.

[0110] The pressure set-point of the pressure controller 121 can be calculated based upon the pressure inside the low-pressure compressor section 31 A, detected by a pressure sensor 125. The pressure set-point of pressure controller 121 can be calculated by adding a pressure difference (e.g. approximately 0.1-1 bar) to the pressure inside the low-pressure compressor section 31A of the compressor arrangement 31, detected by pressure sensor 125.

[OHl] Since the working fluid is a mixed refrigerant, containing a mixture of different components, such as methane, propane, ethane and nitrogen, the working fluid flowing back towards the low-pressure compressor section 31 A of the compressor arrangement 31 will have a higher content of those components of the mixed refrigerant having a lower vaporization temperature, i.e. a lower molecular weight, thus mainly nitrogen and methane. Heavier components condense and collect in the first vessel section 59.1 and second vessel section 59.2 of the collection vessel 59.

[0112] Usually, during the above described step of depressurizing the low-pressure compressor section 31A of the compressor arrangement 31, the controlled valve 123 remains closed, since the pressure in the collection vessel 59 is always below the pressure in the low-pressure compressor section 31 A, since the collection vessel 59 is fed by the low-pressure compressor section 31A during depressurization thereof.

[0113] In the exemplary embodiment of Fig.2 the same conduit 71 is used to flow working fluid from the low-pressure compressor section 31A towards the collection vessel 59, and also to return uncondensed working fluid from the collection vessel 59 towards the low-pressure compressor section 31 A. In other embodiments, not shown, separate conduits can be provided. In some embodiments, for example, the conduit 79 can be fluidly coupled to the low-pressure compressor section 31 A downstream of the relevant compressor, i.e. at the outlet of the compressor or downstream thereof.

[0114] As depressurization of the low-pressure compressor section 31A proceeds, the pressure inside the low-pressure compressor section 31A and the pressure inside the collection vessel 59 tend to become equal. This causes the pressure controller 85 to further open controlled valve 83, until the latter becomes fully open, to achieve the pressure set-point calculated by the temperature controller 86. Once valve 83 is completely open, control will shift from valve 83 to valve 87, i.e. the pressure controller 85 will start gradually opening controlled valve 87, such that working fluid from the high-pressure compressor section 3 IB will start flowing through conduit 72 towards the heat exchanger 57 and towards the collection vessel 59. When the pressure controller 85 starts opening controlled valve 87, the valve 83 can be closed. Before opening valve 87, valve 91 has been opened to allow working fluid into inlet conduit 72.

[0115] Once controlled valve 87 starts opening, the pressure inside the high-pressure compressor section 3 IB will start dropping, as working fluid contained therein flows through valve 91, high-pressure inlet conduit 72, valve 87 and conduit 79 into the heat exchanger 57. Since the controlled valve 87 is smaller than the controlled valve 83, once valve 87 is completely open, further pressure drop inside the high-pressure compressor section 3 IB can be obtained by shifting the control of pressure controller 85 from controlled valve 87 back to controlled valve 83 and diverting the flow from the high-pressure inlet conduit 72 towards conduit 79. This is achieved by opening valve 95 on the connection duct 93. Gradual opening of the controlled valve 83 under the control of pressure controller 85 will cause the pressure in the high-pressure compressor section 3 IB to further drop.

[0116] During depressurization of the high-pressure compressor section 3 IB, the working fluid contained therein is delivered to the high-temperature section 57.2 of the heat exchanger 57, cooled by heat exchange against LNG and against cold working fluid returned from the low-temperature section 57.1. The working fluid from the high- pressure compressor section 3 IB thus partially condenses in the first vessel section 59.1. Uncondensed working fluid further flows from the first vessel section 59.1 through the hot side of the low-temperature section 57.1 of the heat exchanger 57 and partly condenses in the second vessel section 59.2 at a temperature lower than the temperature in the first vessel section 59.1.

[0117] As mentioned, also during this phase working fluid components having a higher boiling temperature condense mainly in the first vessel section 59.1, while components having a lower boiling temperature mainly condense in the second vessel section 59.2.

[0118] Uncondensed, light components of the working fluid can be vented in the low-pressure compressor section 31 A through valve 123 when the pressure inside the second vessel section 59.2 raises above the set-point of pressure controller 121. The check valve 81 on line 77 prevents backflow m conduit 79. Before venting the uncondensed components of the working fluid to the low-pressure compressor section 31A through valve 123, working fluid vapor from the second vessel section 59.2 is heated by flowing in the relevant cold stream of the high-temperature section 57.2 of the heat exchanger 57.

[0119] When the pressure difference between the high-pressure compressor section 3 IB and the low-pressure compressor section 31A drops to a pre-set value, the interstage isolation valve 70 is opened such that the low-pressure compressor section 31A and the high-pressure compressor section 3 IB of the compressor arrangement 31 are placed in fluid communication and the pressure therein can achieve a balanced value.

[0120] Opening of the inter-stage isolation valve70 can occur also at a later stage, when the reciprocating compressor 101 is put into operation, as described later on.

[0121] As depressurization of the compressor arrangement 31 proceeds further, the pressure inside the compressor arrangement 31 will tend to become equal to the pressure in the collection vessel 59. This will cause the pressure controller 85 to fully open the controlled valve 83 and the pressure controller 121 to close the controlled valve 123. The working fluid flowrate will diminish and will finally stop when the pressure in the collection vessel 59 will reach the liquid-vapor equilibrium pressure at the temperature and composition of the working fluid in the collection vessel 59.

[0122] Depending upon the composition of the mixed refrigerant, this final pressure may be sufficiently low to re-start the compressor arrangement 31.

[0123] Conversely, in other situations, the final pressure achieved at the end of the above described procedure may be higher than the one required to restart the compressor arrangement 31. Specifically, when a mixed refrigerant with a large percentage of nitrogen is used, the final pressure in the chilling system 51 may be above the threshold required to restart the compressor arrangement 31. If a lower pressure in the compressor arrangement 31 is desired, the auxiliary compressor 101 is activated.

[0124] The auxiliary compressor 101 can start operating when the pressure in the high-pressure compressor section 3 IB has achieved a pre-set threshold. The auxiliary compressor 101 can be loaded when the working fluid flowrate measured by a flow controller 127 positioned on conduit 79 drops below a pre-set threshold. Alternatively, loading of the auxiliary compressor 101 can be controlled as a function of the pressure detected by the pressure controller 85, or by the pressure controller 121. To cause working fluid to flow from the high-pressure compressor section 3 IB through the high- temperature section 57.2 of the heat exchanger 57 and to the collection vessel 59 by means of the auxiliary compressor 101, the valve 95 is closed, and the valves 103 and 104 are opened. In some embodiments, the flow controller 127 on conduit 79 can be activated and used to control the auxiliary compressor 101. Working fluid will be processed by the auxiliary compressor 101 and flow from the high-pressure compressor section 3 IB, through the high-pressure inlet conduit 72, the auxiliary compressor 101, the cooler 105 and the conduit 79, in the high-temperature section 57.2 of the heat exchanger 57.

[0125] In some embodiments, a ratio controller 128 can be used to calculate the LNG flowrate required as a function of the working fluid flow processed through the auxiliary compressor 101. In other embodiments, the temperature controller 86 can reduce the LNG flowrate acting as an override controller, in case the minimum temperature on conduit 65 is reached. In general, an LNG flowrate set-point can be provided to the flowrate controller 116 arranged along the LNG conduit 53. The flowrate controller 116 acts upon valve 117 to maintain the required LNG flowrate through the chilling arrangement 51.

[0126] The set-point value of the pressure controller 123 may be reduced from the value achieved before starting operation of the auxiliary compressor 101 to a lower value, for instance from about 6-8 barA to about 3-6 barA. By keeping the auxiliary compressor 101 in operation, the pressure at the suction side of the auxiliary compressor 101 will drop as a consequence of working fluid being transferred from the compressor arrangement 31 into the collection vessel 59 and accumulated therein in a condensed state. Uncondensed components of the working fluid, i.e. components having a lower vaporization temperature, will be vented through controlled valve 123 in the compressor arrangement 31. In so doing, by means of the auxiliary compressor 101 gas with heavy components trapped in the volume inside the compressor arrangement 31 will be forced to flow from the low-pressure compressor section 31A and high- pressure compressor section 3 IB (pushed by the incoming lighter gas from valve 123) to the heat exchanger 57 for partial condensation. The condensed working fluid is accumulated in the collection vessel 59.

[0127] In addition to maintain the required pressure in the second vessel section 59.2, venting through valve 123 to the compressor arrangement 31 has also the advantage of lowering the molecular weight of the mixture trapped in the compressor arrangement 31. This in turn reduces the power absorbed by the compressor arrangement 31 during start up.

[0128] When the pressure value required for re-start of the compressor arrangement 31 is achieved, for instance 1-2.5 barA, which corresponds to the maximum value of the compression ratio of the auxiliary compressor 101, the isolation valves 75 and 91 as well as the inter-stage isolation valve 70 can be closed.

[0129] In other embodiments, the inter-stage isolation valve 70 can be kept open for startup purposes.

[0130] The controlled valve 109 will be fully opened and the auxiliary compressor 101 will be stopped. Also controlled valves 123, 83 and 87 will be closed.

[0131] The compressor arrangement 31 now contains a mixed refrigerant with a higher content of components having a low vaporization temperature (specifically nitrogen and methane) and at a low pressure. The compressor arrangement 31 can now be re-started and progressively accelerated to reach the minimum operating speed. When this latter has been achieved, working fluid contained (mainly in the liquid state) in the collection vessel 59 will be gradually re-introduced in the compressor arrangement 31. This can be done for instance by removing liquefied working fluid through a return conduit 131, which is fluidly coupled to the first vessel section 59.1 through an isolation valve 133 and to the second vessel section 59.2 through an isolation valve 135.

[0132] In some embodiments, the return conduit 131 can lead to a quench system installed on an anti-surge line of the low-pressure compressor section 31 A, or of the high-pressure compressor section 3 IB of the compressor arrangement 31 (not shown).

[0133] A quench valve 137 on the return conduit 131 can be controlled by a temperature controller 139, which is usually located on the outlet of suction drums of the low- pressure compressor section 31A and/or high-pressure compressor section 3 IB. Liquefied working fluid is thus delivered to the compressor arrangement 31 via quench valve 137 by sequentially opening the isolation valves 135 and 133. The quench valve 137 is controlled by the temperature controller 139, the set-point whereof can be gradually lowered.

[0134] Liquified working fluid from the collection vessel 59 is injected by the quenching system in the anti-surge line, where the liquified working fluid vaporizes by contacting the working fluid recycled through the anti-surge line of the compressor arrangement 31.

[0135] In embodiments, to keep the pressure in the collection vessel 59 at a substantially constant value, the auxiliary compressor 101 can be started and the pressure controller 121 and the flow controller 127 can be activated to operate with a fixed setpoint. In other embodiments, an additional after-cooler (not shown in figure) can be provided on conduit 93 to avoid recycling cryogenic fluid to the auxiliary compressor 101.

[0136] In other embodiments, a conduit 200 connecting the low-pressure compressor section 31 A of the compressor arrangement 31 to the suction side of the auxiliary compressor 101 can be put in service by opening a relevant isolation valve. The conduit 200 can be used in case an extra flow is required to maintain the pressure inside the first vessel section 59.1. The compressor controller 111 will decide the amount of flow to be routed to the auxiliary compressor 101 through conduit 200. During the operation with conduit 200 open, the valve 75 will be open to discharge working fluid towards the low-pressure compressor section 31 A, while valves 95, 83, 87 are closed. While in Fig.2 the conduit 200 connects the low-pressure compressor section 31A of the compressor arrangement 31 to the suction side of the auxiliary compressor 101, in other embodiments the conduit 200 can be omitted and the high-pressure inlet conduit 72 used for the same purpose.

[0137] In other embodiments, not shown, the liquified working fluid from the collection vessel 59 can be vaporized before being reintroduced in the compressor arrangement 31 by means of a vaporizer (not shown), e.g. an air evaporator.

[0138] Once the liquefied working fluid collected in the collection vessel 59 has been transferred back to the compressor arrangement 31, the isolation valves 133 and 135 can be closed and the auxiliary compressor 101 can be shut down. If needed, the remaining pressure in the collection vessel 59 can be reduced by opening the controlled valve 123 and the isolation valve 75.

[0139] If the auxiliary compressor 101 is not provided, the working fluid collected in collection vessel 59 can be transferred back gradually to the compressor arrangement 31 by means of an external pressurizing system, possibly provided with an evaporator.

[0140] While in the schematic circuit of Fig.2 the auxiliary compressor 101 is shown as an ad hoc device, added to the LNG system, in some embodiments a compressor already present in the LNG system can be used to operate as auxiliary compressor for the purpose described above.

[0141] For instance, a compressor section of an existing propane recovery compressor available for depressurizing the propane cycle can be used for that purpose. The propane recovery compressor is usually a reciprocating compressor adapted to perform the above described function of auxiliary compressor 101. Thus, the auxiliary compressor as understood herein may also be an existing compressor provided for in the plant for other uses.

[0142] While in Fig.2 the whole chilling duty is provided by liquefied natural gas, in other embodiments, two separate chilling fluids can be used in combination. Moreover, in Fig.2 the heat exchanger sections 57.1 and 57.2 are arranged in sequence. In other embodiments, a different arrangement of the heat exchanger sections can be foreseen. Figs. 3, 4, 5, 6, 7, 8, 9, 10 and 11 refer to several alternative embodiments involving alternative manners of providing the required chilling duty and/or different arrangements of the heat exchanger sections. The schematics of Figs 3, 4, 5, 6, 7, 8, 9, 10 and 11 illustrate only the heat exchanger 57 and relevant sections thereof, and the collection vessel 59 and relevant sections thereof. The remaining parts of the circuitry can be the same or similar as the one already shown in Fig.2 and described above.

[0143] With continuing reference to Figs. 1 and 2, the schematic of Fig.3 illustrates an embodiment, wherein two different chilling duties and two different chilling fluids are used to chill the multicomponent working fluid during a settle-out pressure reduction phase. The same reference numbers designate parts and components which are the same or functionally correspond to those already described above in connection with Fig.2.

[0144] Working fluid (mixed refrigerant MR) is delivered by working fluid feed conduit 79 to the hot side of the high-temperature section 57.2 of the heat exchanger 57. Chilled working fluid exits the high-temperature section 57.2 of the heat exchanger 57 and is delivered to the first vessel section 59.1 through the inlet duct 59.3, such that liquefied working fluid at a first temperature collects in the first vessel section 59.1 in a condition of thermodynamic equilibrium with vapor or gaseous phase thereof at a first temperature, as in the embodiment of Fig.2.

[0145] Uncondensed working fluid i s delivered through conduit 59.4 and through the hot side of the low-temperature section 57.1 of the heat exchanger 57. The outlet of the hot side of the low-temperature section 57.1 is fluidly connected to the second vessel section 59.2 through inlet conduit 59.5. Condensed working fluid collects in the second vessel section 59.2 in thermodynamic equilibrium with a gaseous or vapor phase of the working fluid at a temperature lower than the temperature of the first vessel section 59.2. Uncondensed working fluid is returned through outlet conduit 59.6 towards the processing section 30. As in the previously described embodiment, high molecular weight components of the working fluid condense mainly in the first vessel section 59.2 and low molecular weight components condense mainly in the second vessel section 59.2. Lighter components return through conduit 59.6 to the compressor arrangement 31 of the processing section 30.

[0146] In the embodiment of Fig.3, uncondensed working fluid returning to the processing section 30 is partially re-heated in the high-temperature section 57.2 of the heat exchanger 57 in heat exchange relationship with incoming working fluid, such as to prevent working fluid at cryogenic temperature to enter the processing section 30.

[0147] In the embodiment of Fig.3, two different chilling duties are provided for chilling the working fluid sequentially in the high-temperature section 57.2 and in the low-temperature section 57.1 of heat exchanger 57. For instance, liquefied natural gas LNG is used in the high-temperature section 57.2 and a second chilling fluid, preferably at a lower temperature than the liquefied natural gas, is used in the low-temperature section 57.1. In embodiments the low-temperature chilling fluid can be liquefied nitrogen (N2).

[0148] A similar embodiment is illustrated in Fig.4. The embodiment of Fig. 4 differs from the embodiment of Fig.3 in that both heat exchanger sections are multi-stream sections, and the uncondensed working fluid exiting the second vessel section 59.2 flows through the low-temperature section 57.1 and the high-temperature section 57.2 of the heat exchanger 57 before returning to the processing section 30. In both heat exchanger sections 57.1, 57.2 the uncondensed working fluid is partially heated by heat exchange against incoming working fluid.

[0149] With continuing reference to Figs. 1, 2, 3 and 4, a further embodiment of the collection vessel 59 and of the heat exchanger 57 is illustrated in Fig.5. The cold sides of the low-temperature section 57.1 and of the high-temperature section 57.2 of the heat exchanger 57 are configured as described in connection with Figs 3 and 4: two different chilling fluids are used to provide the required chilling duties.

[0150] While in the previously described embodiments the working fluid flows in sequence through the first vessel section 59.1 and the second vessel section 59.2, that are fluidly coupled to one another in an in-series arrangement, in the embodiment of Fig. 5 the working fluid flow delivered by feed conduit 79 is split into a first partial flow and a second partial flow.

[0151] The first partial flow is delivered through a first working fluid delivery conduit 79.1 through the hot side of the high-temperature section 57.2 of the heat exchanger 57 and collected in the first section 59.2 of the collection vessel 59. A second partial flow is delivered through a second working fluid delivery conduit 79.2 directly through the hot side of the low-temperature section 57.1 of the heat exchanger 57 and therefrom into the second vessel section 59.2.

[0152] In the embodiment of Fig.5, uncondensed working fluid from the first vessel section 59.1 is delivered through the hot side of the low-temperature section 57.1 of the heat exchanger 57 and further chilled and collected in the second section 59.2 of the collection vessel 59.

[0153] As in the previous embodiments, uncondensed working fluid returned towards the processing section 30 is heated in heat exchange relationship with incoming working fluid in one or the other or both sections 57.1 and 57.2 of heat exchanger 57. In the embodiment shown in Fig.5 uncondensed working fluid is returned through the high-temperature section 57.2 of the heat exchanger 57.

[0154] With continuing reference to Figs. 1, 2, 3, 4 and 5, a further embodiment of the collection vessel 59 and heat exchanger 57 is shown in Fig.6. The same reference numbers are used to designate the same or equivalent parts as previously described.

[0155] While in Fig.2 the chilling fluid flows sequentially through the cold side of the low-temperature section 57.1 and through the cold side of the high-temperature section 57.2 of the heat exchanger 57, which are arranged in series, in the embodiment of Fig.6 the two cold sides of the sections 57.1 and 57.2 of the heat exchanger 57 are arranged in parallel. The remaining circuitry is the same as in Fig.2 and will not be described again.

[0156] With continuing reference to Figs 1, 2, 3, 4, 5 and 6, a further embodiment is shown in Fig.7. The same reference numbers are used to designate the same or equivalent parts, already described in connection with the previous embodiments. In Fig.7 the cold sides of the low-temperature section 57.1 and high-temperature section 57.2 of the heat exchanger 57 are arranged in parallel as shown in Fig.6. The flow of the incoming working fluid from the working fluid delivery conduit 79 is split into two partial flows delivered through conduits 79.1 and 79.2 to the high-temperature section 57.2 and the low-temperature section 57.1 of the heat exchanger 57 as in the exemplary embodiment of Fig.5. The second vessel section 59.2 receives therefore two flows of chilled working fluid, a first one directly from conduit 79 and the other from the first vessel section 59.1.

[0157] While in the previously described embodiments uncondensed working fluid from the first vessel section 59.1 is fed to the low-temperature section 57.1 of the heat exchanger 57 and therefrom to the second vessel section 59.1, in other embodiments a different configuration may be provided.

[0158] With continuing reference to Figs. 1, 2, 3, 4, 5, 6 and 7, in Fig.8 an embodiment is illustrated, wherein uncondensed working fluid from the first vessel section 59.1 is returned to the processing section 30 directly. The incoming working fluid from conduit 79 is split into two partial flows as already described in connection with the embodiment of Fig.5.

[0159] In the embodiment of Fig.8 two chilling duties are provided for chilling working fluid in the low-temperature section 57.1 and in the high-temperature section 57.2 of the heat exchanger 57, in a manner similar to the embodiment of Figs. 3, 4 and 5.

[0160] With continuing reference to Figs. 1, 2, 3, 4, 5, 6, 7 and 8, a yet further embodiment is illustrated in Fig.9. The embodiment of Fig.9 differs from the embodiment of Fig.8 in that a single chilling duty is provided, and the same chilling fluid, specifically liquefied natural gas, flows sequentially through the cold sides of the low-temperature section 57.1 and high-temperature section 57.2 of the heat exchanger 57, which are arranged in series.

[0161] With continuing reference to Figs. 1, 2, 3, 4, 5, 6, 7, 8 and 9, further embodiments of the heat exchanger 57 and collection vessel 59 are shown in Figs 10 and 11. The two embodiments of Figs. 10 and 11 differ from one another as far as the cold sides of the sections 57.1 and 57.2 of the heat exchanger 57 are concerned. In Fig.10 the cold side of the low-temperature section 57.1 and the cold side of the high-temper- ature section 57.2 are arranged in series and a single chilling duty is provided, using liquefied natural gas as a chilling medium flowing sequentially through the cold side of both sections 57.1 and 57.2 of the heat exchanger 57.

[0162] In Fig.11 two chilling duties are provided for the two sections 57.1 and 57.2 of the heat exchanger 57, in a manner similar to Figs 3, 4, 5, 8.

[0163] In both embodiments of Figs. 10 and 11 the incoming working fluid fed through working fluid feed conduit 79 is split into a first partial flow and a second partial flow, which are delivered through conduits 79.1 and 79.2, in a manner similar to Figs. 5, 7, 8 and 9.

[0164] In both Figs. 10 and 11 the heat exchanger 57 includes further sections 57.3 and 57.4 arranged along the flow path of the first partial flow delivered through conduit 79.1 and along the flow path of the second partial flow delivered through conduit 79.2. The two sections 57.3 and 57.4 of the heat exchanger 57 are used to pre-cool or partially condense working fluid delivered from the processing section 30 through conduits 79.1 and 79.2 by heat exchange against uncondensed working fluid returning towards the processing section 30 from the second vessel section 59.2 through outlet conduit 59.6. The flowrate of uncondensed working fluid at cryogenic temperature exiting the second vessel section 59.2 is split into two flows, which are delivered through respective cold sides of the sections 57.3 and 57.4 of the heat exchanger 57, wherein the uncondensed working fluid returned towards the processing section 30 is heated by heat exchange against working fluid coming from the processing section 30 through working fluid feed conduit 79 and flowing through the hot sides of the sections 57.3 and 57.4 of the heat exchanger 57.

[0165] In the embodiments of Figs. 10 and 11 the pre-chilled working fluid exiting sections 57.3 and 57.4 of the heat exchanger 57 is delivered to the first vessel section

59.1. This latter is arranged in series with the second vessel section 59.2 in a way similar to what has been described with reference to Figs. 2, 3, 4, 6.

[0166] In the embodiments of Figs. 10 and 11 the partial flow delivered by conduit 79.2 bypasses the high-temperature section 57.2 of the heat exchanger 57 and is delivered directly from the section 57.4 of the heat exchanger 57 to the first vessel section

59.1, while the partial flow delivered by conduit 79.1 flows through the hot side of the high-temperature section 57.2 of the heat exchanger 57 before being delivered to the first vessel section 59.1 through inlet conduit 59.3.

[0167] While the invention has been described in terms of various specific embodiments, it will be apparent to those of ordinary skill in the art that many modifications, changes, and omissions are possible without departing form the spirt and scope of the appended claims. In addition, unless specified otherwise herein, the order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments.

Claims

A system and a method for reducing settle-out pressure using multiple collection vessel sections CLAIMS

1. A thermodynamic system (1) comprising: a) a processing section (30), adapted to circulate a multicomponent working fluid therein; b) a chilling arrangement (51) comprising:

- a collection vessel (59) adapted to collect therein a liquid phase and a gaseous or vapor phase of the multicomponent working fluid in thermodynamic equilibrium; wherein the collection vessel is adapted to be fluidly coupled to the processing section (30) to remove working fluid therefrom and re-introduce working fluid therein; and

- a heat exchanger (57) functionally coupled to the collection vessel (59); wherein the heat exchanger includes a hot side adapted to receive working fluid, and a cold side adapted to receive chilling fluid in heat exchange relationship with the working fluid to remove heat therefrom; wherein: the heat exchanger (57) includes at least a low-temperature section (57.1) and a high-temperature section (57.2), each section including a hot side and a cold side; wherein in operation chilling fluid circulates in the cold side of the low-temperature section (57.1) and in the cold side of the high-temperature section (57.2) and working fluid circulates in the hot side of the high-temperature section (57.2) and in the hot side of the low-temperature section (57.1) of the heat exchanger (57); and the collection vessel (59) includes: at least a first vessel section (59.1), fluidly coupled to an outlet of the hot side of the high-temperature section (57.2) of the heat exchanger (57), and a second vessel section (59.2) fluidly coupled to an outlet of the hot side of the low-temperature section (57.1) of the heat exchanger (57).

2. The thermodynamic system (1) of claim 1, wherein the cold side of the low-temperature section (57.1) and the cold side of the high-temperature section (57.2) of the heat exchanger (57) are arranged in series, and wherein an outlet of the cold side of the low-temperature section (57.1) is fluidly coupled to an inlet of the cold side of the high-temperature section (57.2), such that at least a portion of chilling fluid

38 flows sequentially through the low-temperature section (57.1) and the high-tempera- ture section (57.2) of the heat exchanger (57).

3. The thermodynamic system (1) of claim 1, wherein the cold side of the low-temperature section (57.1) and the cold side of the high-temperature section (57.2) of the heat exchanger (57) are coupled in parallel and are adapted to receive a first flow and a second flow of a single chilling fluid.

4. The thermodynamic system (1) of claim 1, wherein the cold side of the low-temperature section (57.1) and the cold side of the high-temperature section (57.2) of the heat exchanger (57) are adapted to receive respectively a first chilling fluid and a second chilling fluid, different from the first chilling fluid.

5. The thermodynamic system (1) of one or more of the preceding claims, wherein the first vessel section (59.1) is fluidly coupled to an inlet of the hot side of the low-temperature section (57.1) of the heat exchanger (57), such that the first vessel section (59.1) and the second vessel section (59.2) are coupled in series.

6. The thermodynamic system (1) of one or more of claims 1 to 4, wherein the first vessel section (59.1) is fluidly coupled through a return conduit to the processing section (30).

7. The thermodynamic system (1) of one or more of the preceding claims, comprising a working fluid feed conduit (79) adapted to feed working fluid from the processing section (30) to the collection vessel (59).

8. The thermodynamic system (1) of claim 7, wherein the working fluid feed conduit (79) is fluidly coupled to the first vessel section (59.1) through a first working fluid delivery conduit (79.1) and to the second vessel section (59.2) through a second working fluid delivery conduit (79.2).

9. The thermodynamic system (1) of claim 8, wherein the first working fluid delivery conduit (79.1) extends through the hot side of the high-temperature section (57.2) and the second working fluid delivery duct (79.2) extends through the hot side of the low-temperature section (57.1) of the heat exchanger (57).

10. The thermodynamic system (1) of one or more of the preceding

39 claims, wherein the second vessel section (59.2) is fluidly coupled to the processing section (30) through a controlled valve (123) adapted to return working fluid from the collection vessel (59) towards the processing section (30).

11. The thermodynamic system (1) of claim 10, wherein the second vessel section (59.2) is fluidly coupled to the processing section (30) through at least one of the low-temperature section (57.1) and high-temperature section (57.2) of the heat exchanger (57), in heat exchange relationship with working fluid from at least one of the processing section (30) and the first vessel section (59.1).

12. The thermodynamic system (1) of one or more of the preceding claims, wherein the processing section (30) comprises a compressor arrangement (31).

13. The thermodynamic system (1) of one or more of the preceding claims, wherein the processing section (30) comprises an inlet provided with an inlet isolation valve (73) and an outlet provided with an outlet isolation valve (74); and wherein a compressor arrangement (31) is arranged between the inlet isolation valve and the outlet isolation valve.

14. The thermodynamic system (1) of claim 13, wherein the compressor arrangement (31) comprises a low-pressure compressor section (31 A) and a high-pressure compressor section (3 IB), and wherein preferably an isolation valve (70) is arranged between the low-pressure compressor section (31 A) and the high-pressure compressor section (3 IB).

15. The thermodynamic system (1) of claim 13 or 14, wherein the chilling arrangement (51) is connected to the processing section (30) through a low- pressure inlet conduit (71) fluidly coupled to the low-pressure compressor section (31 A) and through a high-pressure inlet conduit (72) fluidly coupled to the high-pressure compressor section (3 IB).

16. The thermodynamic system (1) of claim 15, wherein a first controlled valve (83) is provided along a working fluid feed conduit (79) adapted to feed working fluid from the processing section (30) towards the heat exchanger (57); wherein the first controlled valve (83) is adapted to be fluidly connected with the low- pressure inlet conduit (71) and with the high-pressure inlet conduit (72), selectively.

40

17. The thermodynamic system of claim 15 or 16, wherein a second controlled valve (87) is provided along the high-pressure inlet conduit (72).

18. The thermodynamic system (1) of claim 17, wherein the first controlled valve (83) and the second controlled valve (87) are adapted to be selectively opened and closed under the control of a controller (85).

19. The thermodynamic system (1) of one or more of the preceding claims, further comprising a closed circuit (4), adapted to circulate the working fluid therein, said closed circuit including the processing section (30).

20. The thermodynamic system of claim 19, wherein said closed circuit (4) comprises a high-pressure circuit section and a low-pressure circuit section; and wherein the processing section (30) is arranged between the low-pressure circuit section and the high-pressure circuit section.

21. The thermodynamic system (1) of claim 19 or 20, wherein the closed circuit (4) further comprises a heat exchange arrangement (37) adapted to circulate the working fluid in heat exchange relationship with a process fluid (NG; LNG) and remove heat therefrom, and a liquefied process fluid storage unit (39), adapted to collect liquefied process fluid (LNG) therein; wherein the heat exchanger (57) is adapted to be fluidly coupled to at least one of the heat exchange arrangement (37) and the liquefied process fluid storage unit (39) and circulate process fluid in the cold side of the heat exchanger (57).

22. The thermodynamic system (1) of one or more of the preceding claims, further comprising a return conduit (131) fluidly coupling the first vessel section (59.1) and the second vessel section (59.2) to the processing section (30), adapted to return working fluid collected in the collection vessel (59) towards the processing section (30).

23. The thermodynamic system (1) of claim 22, wherein the processing section (30) comprises a compressor (31) with an anti-surge line, and wherein the return conduit (131) is adapted to feed working fluid towards the anti-surge line through a quench valve (137).

24. A method for reducing a settle-out pressure of a multicomponent working fluid in a thermodynamic system (1) comprising: a processing section (30) adapted to circulate a multicomponent working fluid therein; a chilling arrangement (51) comprising a collection vessel (59) adapted to collect therein a liquid phase and a gaseous or vapor phase of the working fluid in thermodynamic equilibrium, and a heat exchanger (57) functionally coupled to the collection vessel; wherein the heat exchanger (57) includes a hot side adapted to circulate the working fluid, and a cold side adapted to circulate chilling fluid in heat exchange relationship with the working fluid to remove heat therefrom; the method comprising the following steps: feeding working fluid from the processing section (30) through a hot side of a high-temperature section (57.2) of the heat exchanger (57) in heat exchange relationship with chilling fluid circulating in a cold side of the high-temperature section (57.2) of the heat exchanger (57); collecting liquefied working fluid from the high-temperature section (57.2) of the heat exchanger (57) in a first vessel section (59.1) of the collection vessel (59) in a first condition of liquid-vapor equilibrium therein; feeding working fluid from the processing section (30) through a hot side of a low-temperature section (57.2) of the heat exchanger (57) in heat exchange relationship with chilling fluid circulating in a cold side of the low-temperature section (57.2) of the heat exchanger (57); collecting liquefied working fluid from the low-temperature section (57.2) of the heat exchanger (57) in a second vessel section (59.1) of the collection vessel (59) in a second condition of liquid-vapor equilibrium therein, at a temperature lower than in the first vessel section (59.1); and returning uncondensed working fluid from the collection vessel (59) towards the processing section (30).

25. The method of claim 24, further comprising the following steps: flowing uncondensed working fluid from the first vessel section (59.1) through the low-temperature section (57.1) of the heat exchanger (57) in heat exchange relationship with the chilling fluid; and collecting liquefied working fluid from the low-temperature section (57.1) of the heat exchanger (57) in the second vessel section (59.2) of the collection vessel (59).

26. The method of claim 24, further comprising the following steps: splitting a working fluid flow from the processing section (30) in a first partial flow and a second partial flow; feeding the first partial flow of working fluid through the hot side of the high- temperature section (57.2) of the heat exchanger (57); feeding the second partial flow of working fluid through the hot side of the low-temperature section (57.1) of the heat exchanger (57); and returning uncondensed working fluid from the first vessel section (59.1) and from the second vessel section (59.2) to the processing section (30).

27. The method of one or more of claims 24 to 26, further comprising the step of circulating chilling fluid in series through the cold side of the low-temperature section (57.1) and through the cold side of the high-temperature section (57.2) of the heat exchanger (57).

28. The method of one or more of claims 24 to 26, further comprising the step of circulating chilling fluid in parallel through the cold side of the low-temperature section (57.1) and through the cold side of the high-temperature section (57.2) of the head exchanger (57).

29. The method of one or more of claims 24 to 26, further comprising the steps of: circulating a first chilling fluid through the cold side of the low-temperature section (57.1) of the heat exchanger (57); circulating a second chilling fluid through the cold side of the high-temperature section (57.2) of the heat exchanger (57).

30. The method of one or more of claims 24 to 29, wherein the thermodynamic system (1) comprises a closed refrigeration circuit (4) adapted to circulate the working fluid therein, and including: a high-pressure circuit section; a low-pressure circuit section, the processing section (30) including a compressor arrangement (31) and being arranged between the high-pressure circuit section and the low-pressure circuit section; a heat exchange arrangement (37); and wherein the heat exchange arrangement (37) is adapted to liquefy a process fluid by heat exchange with the working fluid.

31. The method of claim 30, wherein the liquefied process fluid is used

43 as chilling fluid in the heat exchanger (57) of the chilling arrangement (51).

32. The method of claim 30 or 31, wherein the process fluid is natural gas.

33. A natural gas liquefaction system (1) comprising:

• a refrigerant circuit (4) adapted to circulate a multicomponent refrigerant fluid therein and comprising: a compressor arrangement (31) and a heat exchange arrangement (37), in heat exchange relationship with a natural gas delivery line (21);

• a liquefied natural gas storage unit (39) adapted to collect and store liquefied natural gas (LNG) therein;

• a chilling arrangement (51) comprising:

- a collection vessel (59) adapted to collect therein a liquid phase and a gaseous or vapor phase of the refrigerant fluid in thermodynamic equilibrium; wherein the collection vessel (59) is adapted to be fluidly coupled to the compressor arrangement (31) to remove refrigerant fluid therefrom and reintroduce refrigerant fluid therein; and

- a heat exchanger (57) functionally coupled to the collection vessel (59); wherein the heat exchanger (57) includes a hot side adapted to circulate the refrigerant fluid, and a cold side adapted to circulate chilling fluid in heat exchange relationship with the refrigerant fluid to remove heat therefrom; wherein: the heat exchanger (57) includes: at least a low-temperature section (57.1) comprising a hot side and a cold side; and a high-temperature section (57.2) including a hot side and a cold side; the collection vessel (59) includes: at least a first vessel section (59.1), fluidly coupled to an outlet of the hot side of the high-temperature section (57.2) of the heat exchanger (57) and a second vessel section (59.2) fluidly coupled to an outlet of the hot side of the low-temperature section (57.1) of the heat exchanger (57). during a settle-out pressure reduction phase, chilling fluid circulates in the cold side of the low-temperature section (57.1) and in the cold side of the high-temperature section (57.2) and refrigerant fluid circulates in the hot side of the high-temperature

44 section (57.1) and in the hot side of the low-temperature section (57.1) of the heat exchanger (57).

45