IT202000020479A1 - A SYSTEM AND METHOD FOR REDUCING SETTLE PRESSURE USING A BOOSTER COMPRESSOR - Google Patents

Description

A SYSTEM AND METHOD FOR REDUCING SETTLE PRESSURE USING A BOOSTER COMPRESSOR

Description

TECHNICAL FIELD

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

EARLIER ART

[0002] In thermodynamic systems, in which a working fluid is processed in a closed circuit and ? subjected to thermodynamic transformations comprising phase transitions between a liquid state and a gaseous state, the shutdown of the compressor or of another arrangement to increase the pressure causes an equalization of the pressure in the closed circuit. Is the pressure equalised? called settling pressure. The settling pressure can? depend, among other things, on the temperature of the circuit.

[0003] The settling pressure can? dramatically increase and negatively affect the ability? compressor actuator start. This ? particularly the case when the thermodynamic system includes a refrigeration circuit and ? placed in a warm environment. When the thermodynamic system is stopped and remains inoperative for a relatively long period of time at a high ambient temperature, the thermodynamic system begins to heat up. Any liquid present in the closed compression circuit begins to vaporize and pressurize the closed circuit, until the equalization pressure is reached at room temperature or at the temperature of the metal structure which defines the closed circuit. This temperature can reach 50?C and above due to, for example, solar radiation. The settling pressure can? be much higher than the point of the project and pu? be such that the actuator of the compressor is not ? able to start the compressor again.

[0004] In some compressor systems, for example those using a mixed refrigerant (hereinafter also briefly referred to as MR), such as propane-mixed refrigerant systems or APCI<? >for the liquefaction of natural gas, the suction drums of the compressor are usually dry, and in them usually there is no? a buildup of liquid. Consequently, the settling pressure of the compressor mainly depends on the equalization pressure between the suction volume and the discharge volume of the compressor section, which occurs during the emergency stop transient. Also in this case, however, the SOP ? criticism, because the equalization pressure in the compressor can? reach values such as to prevent the compressor from starting. In fact, the torque required to accelerate the compressor during compressor restart can exceed the capacity of the actuator, preventing the compressor from starting. Therefore ? depressurization is required.

[0005] In general terms, similar problems can arise in thermodynamic systems comprising a pressurized circuit suitable for containing a working fluid and comprising at least one working fluid collection tank, suitable for containing at least two phases of a working fluid, specifically a liquid phase and a gaseous phase (which may include, or consist of, a vapor phase) in a condition of thermodynamic equilibrium. because the equilibrium pressure in a two-phase system depends, among other things, on the temperature of the fluid, when the temperature increases, the equilibrium pressure in the system also increases and pu? exceed a threshold pressure. There? can? jeopardize or negatively influence one or more? of the features? system or prevent it from functioning altogether. If this situation occurs, ? necessary to vent the thermodynamic system, or ? a dedicated compressor is needed to circulate the fluid in a condenser, to lower the pressure in it. The vent can result in loss of valuable products, cause environmental pollution, or cause other inconvenience.

[0006] WO2019/138049 describes a new thermodynamic system and a new method for reducing the settling pressure by collecting and condensing the working fluid of the thermodynamic system in a collection tank, which is functionally coupled to a cooling system.

[0007] Further improvements of this new system and related method would be advantageous for improving its efficiency, for example in cases where the working fluid comprises a mixture of components having different molecular weights.

SUMMARY

[0008] According to the embodiments described herein, a thermodynamic system is provided, comprising a processing section, able to circulate a working fluid therein. The thermodynamic system further includes a cooling arrangement adapted to condense at least some working fluid from the processing section when the thermodynamic system is to be restarted following a lockout or shutdown, and the settling pressure in the processing section is to be be reduced. The cooling system comprises a collection tank able to collect therein a liquid phase and a gaseous phase of the working fluid in thermodynamic equilibrium.

[0009] The collection tank ? adapted to be placed in fluid connection with the processing section to remove working fluid therefrom and reintroduce working fluid thereto. The cooling arrangement further comprises a heat exchanger functionally coupled to the collecting tank for removing heat from the working fluid and at least partially condensing the working fluid by heat exchange with the cooling fluid circulating therein. The system further includes a connection adapted to fluid couple the heat exchanger and the processing section, to remove working fluid from the processing section and circulate working fluid through the heat exchanger to the reservoir. Also provided are a booster compressor, a compressor controller and a valve switching arrangement, wherein the valve switching arrangement is adapted to selectively fluid connect a suction side of the booster compressor with the processing section and a discharge side of the booster compressor with the heat exchanger to promote the flow of working fluid from the processing section to the collecting tank.

[0010] The auxiliary compressor ? controlled to promote or stimulate circulation of the working fluid from the processing section to the heat exchanger when such circulation cannot? be maintained thermodynamically by effect of temperature and pressure gradients across the processing section and cooling arrangement.

[0011] By providing an auxiliary compressor, is it possible? obtain an effective reduction of the settling pressure (SOP for short) even in cases where the working fluid consists or comprises components having a low molecular weight and a low evaporation temperature, for example when the working fluid comprises a large percentage of nitrogen.

According to a further aspect, a method of reducing the settling pressure in a thermodynamic system using a booster compressor as outlined above is disclosed herein.

[0013] Further features and embodiments of the system and method of the present description are defined in the claims and outlined in the following detailed description of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] A better appreciation of the embodiments of the invention and of the many advantages thereof can be be obtained from the detailed description which follows with reference to the accompanying drawings, in which:

Fig.1 illustrates a diagram of a natural gas liquefaction plant including an arrangement for reducing the settling pressure in the event of a compressor shutdown (emergency shutdown or during a normal shutdown); And

Fig.2 illustrates a diagram of the cooling arrangement in one embodiment.

DETAILED DESCRIPTION

[0015] A thermodynamic system is described here, which includes a processing station, which can be subject to an increase in the settling pressure in the event of blockage or shutdown of the thermodynamic system. In embodiments described herein, the processing section comprises a compressor arrangement, in turn comprising one or more compressors. compressors. To reduce the settling pressure before restarting the system, working fluid is removed from the processing section, cooled by heat exchange with a cooling fluid in a heat exchanger, at least partially condensed, and collected in a holding tank, where fluid of cooled and liquefied work? in thermodynamic equilibrium with one of its gaseous or vapor phases. The circulation of the working fluid from the processing section to the collection tank ? usually ensured thermodynamically, due to the effect of the temperature and pressure difference between the processing section and the collection tank. The latter contains working fluid which condenses at a low temperature. Condensation of the working fluid in the holding tank reduces the pressure in the tank and draws additional working fluid from the processing section. This spontaneous circulation of working fluid supported by thermodynamic factors reduces and tends to stop when a certain pressure is reached in the processing section.

[0016] This pressure can not be optimal for restarting the thermodynamic system. That is? a further reduction of the pressure could be desirable or advantageous for a pi? efficient restart of the thermodynamic system after blockage or shutdown.

[0017] To improve the operation of the cooling arrangement comprising the heat exchanger and the collecting tank, a booster compressor is provided. The booster compressor? adapted to promote circulation of the working fluid from the processing section through the heat exchanger and into the holding tank when the thermodynamically stimulated circulation becomes too low or stops altogether.

[0018] In the following description a thermodynamic system is illustrated, in which the cooling fluid ? liquefied natural gas and in which the working fluid ? a multi-component working fluid, for example so-called mixed coolant. To improve some aspects of the SOP reduction process in a thermodynamic system containing a multi-component working fluid, the holding tank ? divided into two or more sections and similarly the heat exchanger ? in turn designed to include a plurality? Sections: This allows working fluid components having different molecular weights to condense at different temperatures in sequentially arranged sections of the holding tank.

[0019] Those skilled in the art will understand, however, that at least some of the advantages obtained with the booster compressor described here can also be obtained in a different thermodynamic system, for example where ? used a different cooling fluid and/or where ? used a one-component working fluid. Furthermore, the use of an auxiliary compressor can? also be advantageous in systems and installations where the collection tank is not? divided into multiple sections and in which the entire working fluid flows through a single heat exchanger.

[0020] For example, an auxiliary compressor can? also be used in a thermodynamic system as described in WO2019138049.

[0021] In the following description of embodiments, the thermodynamic system, in which ? incorporated a cooling arrangement, ? a system for producing liquefied natural gas (LNG), also briefly referred to as the "LNG system". Those skilled in the art, however, will appreciate that various features of the cooling arrangement and method disclosed herein and the respective advantages thereof can be used to benefit even in different thermodynamic systems, where shutdown of the system may occur. cause an increase in the settling pressure and pu? require a reduction in the settling pressure achieved by a multi-component fluid before restarting the system.

[0022] Furthermore, in the present description it will be referring specifically to a processing section that includes a compressor, in which the reduction of the settling pressure? advantageous in order to facilitate the start of the compressor following a shutdown. However, the arrangement described here can be used to advantage also in combination with other systems which include a processing section, in which a reduction in the settling pressure can? be in demand, or be advantageous, for other reasons. For example, the processing section can? include a container or tank, which undergoes an increase in pressure in the event of a stop or for any other reason, and which can? request a pressure drop without venting gas from the processing section.

[0023] In the following description of embodiments, the cooling arrangement is designed to remove working fluid from the arrangement of the compressor and reduce cos? the pressure in it. In terms more general, the cooling arrangement can? be able to remove working fluid from a generic processing section, which can? form part of a thermodynamic system pi? complex for example comprising a closed circuit, in which the settling pressure in the processing section can? require to be lowered to restart the thermodynamic system after a sudden stop or a normal stop.

Referring now to the drawings, Fig.1 illustrates an exemplary embodiment of an LNG system comprising two combined refrigeration circuits for liquefying natural gas. Each refrigeration circuit ? adapted to circulate a respective working fluid (refrigerants) therein. In the example of Fig.1, the refrigeration circuits comprise a mixed propane/refrigerant system, comprising a first refrigerant circuit (propane circuit) using propane as the working fluid, and a second refrigerant circuit (mixed refrigerant circuit) using a mixed refrigerant (MR for short) as working fluid.

[0025] In the embodiment of Fig.1 the mixed refrigerant circuit comprises means for reducing the pressure of the MR in the circuit, for example following a period of inactivity? of the compressor of MR, which can? lead to an increase in SOP. Mixed refrigerant? a multi-component refrigerant containing fractions of fluids having different molecular weights, cio? lower and higher molecular weights, which condense at different temperatures.

[0026] The thermodynamic system of Fig.1 also comprises a unit? storage or storage tank, to store liquefied process fluid, the cio? liquefied natural gas. In embodiments described herein, the liquefied natural gas is used as a coolant to reduce the SOP in the MR circuit following a shutdown or lockout.

[0027] While in Fig.1 the LNG system comprises a combination of a propane refrigerant circuit and an MR refrigerant circuit, in some embodiments, the LNG system can understand more of two refrigerant circuits and/or different refrigerant fluids operating at different temperatures. The coolant of one of these circuits can? be used to reduce the pressure in another of said circuits.

[0028] For example, in embodiments, the LNG system can include a low temperature nitrogen loop with a nitrogen storage arrangement, in which liquid nitrogen is stored. Liquid nitrogen can be used to reduce the pressure in a refrigeration circuit at a temperature pi? high, for example a mixed refrigerant circuit.

[0029] In embodiments, the LNG system can include a container for storage of liquid nitrogen, which is not ? used as a refrigerant in a refrigeration cycle, bens? rather only as a coolant for SOP reduction purposes.

Those skilled in the gas liquefaction art will appreciate that novel features of the method and system described herein, to reduce startup SOP, can be incorporated into other LNG systems using different refrigeration circuits and different refrigerant fluids, such as a Cascade™ cycle, Single Mixed Refrigerant (SMR) or Dual Mixed Refrigerant (DMR) loops, Linde™ liquefaction systems, AP-X™ liquefaction systems, and the like.

Furthermore, features disclosed herein may be used in liquefaction plants 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 and petrochemical plants in general. In general terms, the liquefied gas can? be stored in a unit? storage, for example in a vapor/liquid equilibrium condition. The liquefied gas can be used to cool a working fluid of a thermodynamic circuit, for example to reduce the pressure in a refrigerant circuit containing a refrigerant working fluid.

[0032] Returning now to Fig.1, is the thermodynamic system ? marked 1 as a whole and comprises a first closed coolant circuit 3, in which a first coolant working fluid ? able 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? a closed loop of propane.

[0033] The working fluid circulates in the closed refrigerant circuit 3 by means of a pressure boosting arrangement 5. In the diagram 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 lateral flows. Each suction section pu? include a suction drum (not shown) positioned upstream of the respective compressor suction flange. In other embodiments, not shown, the pressure boosting arrangement 5 may understand more of a single compressor in any configuration, for example a plurality? of compressors arranged in series and/or in parallel.

[0034] Downstream of the pressure increase arrangement 5, with respect to the working fluid flow direction schematically represented by the arrow FF, ? A heat removal and fluid condensation arrangement 9 is provided. The heat removal and fluid condensation arrangement 9 can? include a heat exchanger, such as a liquid/air heat exchanger or a liquid/liquid heat exchanger. In other embodiments, the heat removal and fluid condensation arrangement 9 may include any type of heat removal arrangement.

[0035] A condensate collection tank, or fluid collection tank 11, ? disposed downstream of the fluid condensation and heat removal arrangement 9. Propane in a two-phase condition of liquid/gas (or vapor) in equilibrium can? be contained in the fluid collection tank 11.

[0036] An expansion section 13 and a heat exchange arrangement 15 are also provided along the closed refrigeration circuit 3. A sub-cooler 12 can? be arranged between the fluid collection tank 11 and the expansion section 13.

[0037] The expansion section 13 can? include one or more expanders, such as turboexpanders or water turbines (if the working fluid flowing through them is in a 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, coming from the expansion section 13, is heated by heat exchange with a flow of process fluid to be cooled, as will be? described more forward.

[0038] In the diagram of Fig.1, the pressure boosting arrangement 5 further comprises an actuator 19, which generates the mechanical power required to rotate the compressor 7. The actuator 19 can? be an electric motor. In other embodiments, as schematized in Fig.1, the actuator 19 can? be a turbomachinery generating mechanical power, such as a gas turbine engine, turboexpander or steam turbine. In some further embodiments, the actuator 19 may include a reciprocating internal combustion engine. Can? A combination of different actuators can also be used. In this case, actuators of the same type or actuators of a different nature can be combined with each other. For example, a gas turbine engine can be used in conjunction with an electric motor or a steam turbine.

The expansion section 13 and the heat exchange arrangement 15 are configured to expand propane at different and decreasing pressure levels corresponding to decreasing propane temperatures. The propane ? used to precool a stream of natural gas, which flows in a natural gas supply line 21, and ? It is also used to cool a flow of mixed refrigerant MR which circulates in a second closed refrigeration circuit 4, described in greater detail below. forward.

[0040] In the exemplary embodiment of Fig.1, the expansion section 13 comprises a first series of expansion valves, or a series of expanders or hydraulic turbines, indicated with 13A. 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 series 13A and 13B are arranged in series, to expand propane at gradually decreasing pressures and generate partial flows of propane at those decreasing pressures. The partial streams of expanded propane at different pressure levels obtained by the expansion valves 13A exchange heat in heat exchangers 15A of the heat exchange arrangement 15, at varying pressures with a natural gas stream flowing in the natural gas supply line 21 The partial streams of expanded propane from expansion valves 13B exchange heat at varying temperatures in heat exchangers 15B with mixed refrigerant MR circulating in the second closed refrigeration loop 4. The partial streams are then returned as side streams to the compressor 7.

[0041] In the exemplary thermodynamic system 1, the second closed refrigeration circuit 4 circulates a second refrigerant, for example a mixed refrigerant, in heat exchange ratio with the refrigerant (propane) processed in the first closed refrigerant circuit 3 and in ratio of heat exchange with natural gas to be liquefied.

[0042] In embodiments, the second closed refrigeration circuit 4 comprises a processing section 30, comprising a compressor arrangement 31, comprising one or more compressors in series, operated by one or more? actuators 33, such as electric motors, gas turbine engines, steam turbines or other mechanical power generating machines, or combinations thereof. In embodiments, the compressor arrangement 31 comprises a low pressure compressor section 31A and a high pressure compressor section 31B (or a medium pressure and high pressure compressor section 31B) arranged in series. A 32 intercooler can? be disposed between the low pressure compressor section 31A and the high pressure compressor section 31B. Also, the compressor arrangement 31 can including at least one suction drum on the low pressure compressor section 31A (not shown) upstream of the compressor suction flange.

[0043] While in the embodiment schematically shown in Fig.1 the compressor arrangement 31 and the compressor 7 are driven by different and separate actuators 33 and 19, respectively, in other embodiments (not shown) the propane compressor 7 and the mixed refrigerant compressor arrangement 31 may be arranged on the same shaft line and may be actuated by the same actuator or combination of actuators. In still further embodiments (not shown), the high pressure compressor section 31B and the propane compressor 7 may be on the same shaft line and driven by the same actuator, while the low pressure and/or high pressure sections medium pressure mixed refrigerant compressor may be located on a separate shaft line rotated by a different drive.

[0044] The compressed working fluid (mixed refrigerant, MR) of the closed refrigeration circuit 4 ? cooled in cooler 35 and further cooled and partially condensed in heat exchangers 15B by heat exchange to expanded propane of closed refrigeration circuit 3. Partially liquefied mixed refrigerant MR is fed to liquid-vapour separator 36 and the liquid and vapor separate streams from separator 36 are expanded and circulated in a main cryogenic heat exchanger (MCHE) 37. Expanded mixed refrigerant further cools and liquefies? the natural gas by heat exchange therewith in the main cryogenic heat exchanger 37. A vaporized mixed refrigerant MR is then fed to the compressor arrangement 31 to be compressed and circulated again in the loop described above.

[0045] Is the refrigeration circuit closed 4 ? then divided into a high pressure circuit section between the discharge 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.

[0046] Liquefied natural gas from main cryogenic heat exchanger 37 ? collected and stored in a? unit? of storage 39, from which it is fed to one or more? utilities or facilities, such as transportation facilities, such as an LNG tanker.

[0047] In some circumstances the pressure inside the closed refrigeration circuit 4 can increase, for example if the circulation of the refrigerant working fluid ? discontinued for any reason. Will the pressure between the suction side and the discharge side of the compressor arrangement 31 equalize? and reach a settling pressure, which can? increase if the ambient temperature increases. This can request to take actions in order to reduce the pressure inside the compressor arrangement 31 and restart the circulation of working fluid in the closed refrigeration circuit 4.

[0048] How will it be? described in connection with Fig.2, the cooling of the working fluid (mixed refrigerant, MR) and thus the reduction of the settling pressure in the closed refrigeration circuit 4 can be achieved by means of a cooling arrangement schematically indicated at 51 in Fig. .1, which can? use liquefied natural gas (LNG) from the system as the coolant. While in the schematic drawing liquefied natural gas is taken from the unit? of storage 39, in some embodiments LNG can? be taken from the main cryogenic heat exchanger (MCHE) 37 in addition to, or instead of?, using LNG from the unit? storage 39.

[0049] In the diagram of Fig.1, an LNG pipeline 53 and a cryogenic pump 54 place the unit in fluid connection? of LNG storage 39 and the cooling arrangement 51. How will it come? described in relation to Fig.2, LNG vaporizes by heat exchange with the working fluid (mixed refrigerant) contained in the compressor arrangement 31 of the processing section 30. Vaporized LNG is fed through a boil-off gas (BOG) pipe 55 (BOG manifold) 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 to a pressure value capable of restarting the compressor arrangement 31.

[0050] Specifically, in some embodiments the compressor arrangement 31 may be isolated from the rest of the refrigeration circuit 4 and the pressure inside can? be reduced by condensing the mixed refrigerant contained in it by heat exchange with LNG, or another cooling fluid.

[0051] Continuing to refer to Fig.1, Fig.2 illustrates in greater detail a diagram of an embodiment of the cooling arrangement 51.

The cooling arrangement 51 comprises a heat exchanger 57, functionally coupled to a working fluid collection tank 59 adapted to collect components of the mixed coolant (hereinafter referred to as the "working fluid") in a two-phase condition , that is? in a condition of thermodynamic equilibrium between a liquid phase and a gaseous (or vapour) phase. How will it turn out? clear from the following description, the heat exchanger 57 comprises at least one cold side, through which the cooling fluid circulates. The heat exchanger 57 further comprises at least one hot side, through which the working fluid circulates. The heat exchanger 57, or at least a section thereof, can be a multi-flow heat exchanger. In this case, the heat exchanger or at least a section of it will include? a further fluid path, in addition to the hot side and the cold side, in heat exchange relationship between them. In general terms, each section of the heat exchanger can? be designed to include two hot and/or cold streams. For example, in the diagram of Fig.2, a heat exchanger section 57.2 (to be described) comprises two cold streams and one hot stream.

[0053] According to one aspect, the heat exchanger 57 comprises a low temperature section 57.1 and a high temperature section 57.2 arranged in sequence in an upstream-downstream direction, with reference to the flow direction of the cooling fluid, i.e. of the coolant flowing first through the cold side of the low-temperature section 57.1 and then through the cold side of the high-temperature section 57.2. Conversely, the working fluid flows first through the hot side of the high temperature section 57.2 and then through the hot side of the low temperature section 57.2. In this way the working fluid and the coolant flow generically in mode? against the tide. In each section of the heat exchanger 57, heat is removed from the working fluid by the cooling fluid.

In the exemplary embodiment shown in Fig.2, both the low-temperature sections 57.1 and the high-temperature sections 57.2 are designed as counterflow heat exchanger sections. However, in other embodiments, not shown, one or both sections or portions thereof may be designed to work in synchronous mode. equi-current, that is? with the working fluid and the cooling fluid flowing in the same direction through the heat exchanger.

[0055] How will it be? described in more detail further on, the flow rate of working fluid through the two sections 57.1 and 57.2 of the heat exchanger 57 are different. Pi? specifically, the flow rate through the high temperature section 57.2 ? greater than the flow rate through the low temperature section 57.1, since? part of the working fluid leaving the high-temperature section 57.2 is collected in a condensed state in a section of the collecting tank 59.

[0056] 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 larger number of sections.

[0057] Furthermore, each heat exchanger section 57.1 and 57.2 can be designed as a single heat exchanger or as a section of a main heat exchanger, or combinations thereof.

[0058] In other embodiments, the heat exchanger can also be divided into more? sections, each arranged in a network (in series and/or parallel) to provide the cooling service.

[0059] Furthermore, different types of heat exchangers can be used, for example plate and fin, shell and tube, kettle, harpin, coil, or combinations thereof. Each section of heat exchanger can? have a different configuration, cio? for example, a section pu? understand a configuration kettle and another section pu? include a shell and tube configuration.

[0060] The collection tank 59 comprises a plurality of tank sections. In embodiments, the number of tank sections ? equal to the number of heat exchanger sections. In Fig.2 the collection tank 59 comprises a first tank section 59.1 and a second tank section 59.2 arranged in series, the first tank section 59.1 being arranged upstream of the second tank section 59.2 with respect to the fluid flow direction of work.

[0061] The first tank section 59.1 ? in fluid coupling, through an inlet conduit 59.3, with the hot side outlet of the high temperature section 57.2 of the heat exchanger 57. The first tank section 59.1 ? furthermore in fluid coupling, through an outlet conduit 59.4, with the inlet on the hot side of the low temperature section 57.1 of the heat exchanger 57. The inlet conduit 59.4 is also in fluid coupling. arranged in the upper part of the first tank section 59.1, for collecting non-condensed working fluid contained therein and for feeding the flow of non-condensed working fluid through the low-temperature section 57.1 of the heat exchanger 57. In use, working fluid condensed, containing mainly the components pi? heavy of it, is collected in the bottom of the first section of tank 59.1, while components pi? Non-condensed light of the working fluid will flow through the low temperature section 57.1 of the heat exchanger 57 and be at least partially condensed and collected in the second tank section 59.2. For this purpose, the second tank section 59.2 ? in fluid coupling, through an inlet duct 59.5, with the hot side outlet of the low temperature section 57.1 of the heat exchanger 57.

[0062] The second tank section 59.2 ? furthermore, in fluid coupling with an outlet duct 59.6, which ? in turn in fluid coupling with the compressor arrangement 31 via the high temperature section 57.2 of the heat exchanger 57, which comprises three flows for this purpose. A 59.7 branch conduit ? provided by the outlet duct 59.6 to bypass the high temperature section 57.3 of the heat exchanger 57, if required.

[0063] By flowing the working fluid from the second tank section 59.2 through the heat exchanger 57 in heat exchange relationship with the higher working fluid? heat coming from the processing section, before returning it to the processing section 30, prevents the working fluid from reaching the compressor arrangement 31 at cryogenic temperatures.

[0064] In the illustrated embodiment, the high temperature section 57.2 of the heat exchanger 57 is a multi-flow section comprising a cold side, through which coolant supplied by conduit 62 or bypass line 69 flows and exiting the cold side via 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 cooling fluid circulating in the cold side of the high temperature section 57.2 of the heat exchanger 57. Also, working fluid returning from the second tank section 59.2 flows in heat exchange ratio with the working fluid flowing through the hot side of the high temperature section 57.2 of the heat exchanger 57. Thus, the working fluid which arrives at the high temperature section 57.2 of the heat exchanger 57 coming from the processing section 30 is cooled by heat exchange with the cooling fluid and with the cold working fluid which returns from the second tank section 59.2 towards the processing section 30.

[0065] In other embodiments, not shown, the low temperature section 57.1 of the heat exchanger 57 can be a multi-flow heat exchanger section, in which working fluid returning from the second tank section 59.2 can be in heat exchange relationship with the working fluid flowing from the first tank section 59.1 to the second tank section 59.2.

[0066] In operation, the collection tank 59 functions as a fluid separator, in which the working fluid in the liquid phase is collected and maintained in a condition of thermodynamic equilibrium with the gaseous or vapor phase. Uncondensed working fluid flows from the first tank section 59.1 to the second tank section 59.2, in which it is partially condensed as a result of heat removal by the cooling medium in the low temperature section 57.1 of the heat exchanger 57. The second section of tank 59.2 will contain? therefore liquid and gaseous working fluid in thermodynamic equilibrium at a lower temperature than that of the first tank section 59.1. In operation, uncondensed working fluid is vented through conduit 59.6 to compressor arrangement 31 as illustrated below.

[0067] The cooling fluid is fed to the heat exchanger 57 through a conduit 53 and through a further conduit 61. A return conduit 65 carries LNG, which? been vaporized in the heat exchanger 57, from the heat exchanger 57 to a liquid-vapour separator 67. Vaporized LNG flows from the separator 67 through a boil-off conduit 55 to the boil-off system 56 (FIG. 1).

[0068] More specifically, the conduit 61 ? in fluid coupling with the cold side inlet of the low temperature section 57.1 of the heat exchanger 57. The cold side outlet of the low temperature section 57.1 ? in turn in fluid coupling through a conduit 62 with the cold side inlet of the high temperature section 57.2 of the heat exchanger 57. The cold side outlet of the high temperature section 57.2 is in fluid coupling via a return line 65 with the liquid-gas separator 67.

[0069] As mentioned, the high temperature section 57.2 of the heat exchanger 57 comprises two cold streams. A cold stream receives working fluid from the second tank section 59.2 through the conduit 59.6. The other cold stream of the high temperature section 57.2 of the heat exchanger 57 receives LNG from the line 62 and/or the bypass line 69. In other embodiments, how the hot and cold streams are split may be different. be different from the one shown in Fig.2. For example, a cold stream can be shared between sections 57.1 and 57.2 of heat exchanger 57, or a stream can? be extracted to a different position of the heat exchanger 57.

In other embodiments, not shown, the two sections 57.1 and 57.2 of the heat exchanger 57 may be configured as two portions of a single heat exchanger, in which case an external coupling duct 62 may be avoided.

[0071] There? what does it detect? that at least two sections can be identified in a heat exchanger 57, in which two different working fluid flow rates are cooled in heat exchange relationship with the cooling fluid, so what components? heavy components of the working fluid condense at a higher temperature and collect in the first section of the tank 59.1, while components more? lighter than the working fluid condense at a temperature pi? low and are collected in the second tank section 59.2.

[0072] In other embodiments, under some operating conditions at least a portion of the cooling fluid may bypass the low temperature section 57.1 of the heat exchanger 57 through the bypass line 69, and flow from the conduit 61 directly through the high temperature section 57.2 of the heat exchanger 57. A control valve 68 can? selectively close and partially or fully open the bypass line 69.

[0073] In embodiments, a quenching conduit 63 ? derived from the conduit 53 and ? in fluid coupling with the return line 65 upstream of the liquid-vapour separator 67. The quenching line 63 is mainly used to cool the boil-off gas (BOG) flowing from the high temperature section 57.2 of the heat exchanger 57 to a specific temperature before sending it to the BOG header 55. In embodiments, the liquefied natural gas flow in the quenching duct 63 ? regulated by a dedicated temperature controller 66, as described below. The temperature controller 66 can? be positioned downstream of the liquid-vapour separator 67. Quenching is also used at the start of the cooling arrangement 51, as will be? explained in greater detail pi? forward.

[0074] The cooling arrangement 51 ? 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 arrangement of compressor 31, pu? be isolated from the rest of the refrigeration circuit 4 by means of a suction-side isolation valve 73 and a discharge-side isolation valve 74. The low-pressure compressor section 31A and the high-pressure compressor section 31B are in fluid coupling with each other. In embodiments, an intercooler 32 ( Fig. 1 , not shown in Fig. 2 ) can be placed between them. In embodiments, an interphase isolation valve 70 may be provided between the two sections 31A, 31B. The interphase isolation valve 70, if present, as well as? the isolation valves 73 and 74 are closed when the compressor arrangement 31 stalls or is shut down, in order to isolate the low pressure compressor section 31A and the high pressure section 31B from each other and from the remaining circuit 4.

[0075] 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 line 72 is coupled to the closed refrigeration circuit 4 between the suction side isolation valve 73 and the suction side of the compressor arrangement 31. coupled to the closed refrigeration circuit 4 between the discharge side of the high pressure compressor section 31B and the discharge side isolation valve 74.

[0076] The low pressure inlet duct 71 can? be in fluid coupling with heat exchanger 57 via conduits 77 and 79. A check valve 81 ? disposed along the conduit 77 and a controlled valve 83 ? disposed along the working fluid supply line 79. In some embodiments, the controlled valve 83 is controlled by a pressure controller 85, which senses the pressure in the conduit 79 downstream of the controlled valve 83. In other embodiments, the controlled valve 83 may be controlled by a pressure controller 85. be controlled by another controller, such as a flow controller.

[0077] In embodiments, the high pressure inlet conduit 72 may be in fluid coupling with the conduit 79 through a further controlled valve 87, which can? be controlled by the pressure controller 85 itself. In other embodiments, the controlled valve 87 may be controlled by the pressure controller 85. be controlled by other controllers, such as a flow controller. In embodiments, a non-return valve 89 is arranged along the high pressure inlet pipe 72, upstream of the controlled valve 87.

[0078] In embodiments, a shutoff valve 75 is disposed along the low pressure inlet pipe 71 and a closing valve 91 ? arranged along the high pressure inlet pipe 72.

[0079] The high pressure inlet duct 72 can be in fluid coupling through a connecting conduit 93 with the conduit 79. The connecting conduit 93 can? be selectively opened or closed through a closing valve 95 arranged along the connecting duct 93.

[0080] In some embodiments, the cooling system 51 can furthermore include an auxiliary compressor 101, which can? be used to achieve sufficiently low pressures in the compressor arrangement 31 when the working fluid comprises an amount relatively large number of components having a low boiling point (light components). When is the working fluid ? a mixed refrigerant, the auxiliary compressor 101 pu? be advantageous for example when the mixed refrigerant comprises a high percentage of nitrogen.

[0081] The auxiliary compressor 101 has a suction side, which can? be in fluid coupling with the high pressure inlet conduit 72 via a shut-off valve 103. The booster compressor 101 further includes a discharge side in fluid coupling with the conduit 79. A cooler 105 may be in fluid coupling with the conduit 72. be positioned between the discharge side of the booster compressor 101 and the duct 79. Furthermore, in the diagram of Fig.2 between the cooler 105 and the duct 79 a non-return valve 102 and a discharge isolation valve 106 are arranged.

[0082] In embodiments, the booster compressor 101 can be operated by an actuator 101.1, for example an electric motor. The reference number 101.5 designates a further drive, for example a further electric motor which drives a fan of the cooler 105. A recirculation line 107 can? be provided in anti-parallel with respect 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 can? a controlled valve 109 may be provided, which selectively opens and closes the recirculation line 107. The controlled valve 109 can? be controlled by a compressor controller 111. Also, a throttle valve 104 can? be provided between shut-off valve 103 and recirculation line coupling 107. Throttle valve 104 can? be operated by the compressor controller 111.

[0083] The auxiliary compressor 101 can? be a volumetric compressor. In some embodiments, booster compressor 101 may be a reciprocating compressor, that ? particularly inexpensive and ? adapted to compress the low flow rate that ? requested to process booster booster 101.

[0084] The main purpose of the auxiliary compressor 101 ? that of pumping working fluid from the compressor arrangement 31 to the heat exchanger 57 when a spontaneous flow cannot? more be achieved thermodynamically as a result of the temperature difference and pressure difference between the interior of the compressor arrangement 31 and the interior of the holding tank 59, as will be achieved? clarified more forward in further detail.

[0085] Does the cooling arrangement 51 described up to now work? the following.

[0086] When the LNG system ? in operation and the working fluid (mixed refrigerant) is processed through the compressor arrangement 31 and circulated through the refrigerating circuit 4, the valves 75 and 91 are closed and the cooling arrangement 51 is closed. non-operational and isolated from the LNG system. Along the pipeline 53 of LNG pu? a shut-off valve 113 should be provided to prevent LNG from circulating through the heat exchanger 57.

[0087] If the compressor arrangement 31 fails for any reason, the isolation valves 73 and 74 will close and the pressure in the processing section 30 will equalize? and reach the settling pressure (SOP). The SOP? usually too high to restart the compressor arrangement 31 with the actuator installed without activating measures to reduce the fluid pressure therein. The cooling arrangement 51 is then activated to temporarily remove working fluid from the compressor arrangement 31 and condense the working fluid in the heat exchanger and accumulate the condensed working fluid (liquid) in the holding tank 59, thereby reducing the pressure in the compressor arrangement 31 until a restart pressure value is reached therein.

[0088] The cooling arrangement 51 can be put into operation just before restarting the compressor arrangement 31, or at any time after the shutdown or lockout.

[0089] When the cooling arrangement 51 is to be started, the valves 73, 70 and 74 as well as valves 83 and 87 are closed. The circulation of LNG through the heat exchanger 57 is prevented by the valves 117 and 113, which are initially closed.

[0090] A preliminary cooling phase is initially performed, to bring the heat exchanger 57 and the liquid-vapour separator 67 to a desired starting pressure and temperature. This preliminary phase can be required, since? the cooling arrangement 51, of which the liquid-vapour separator 67 and the heat exchanger 57 are part, can remain inoperative for a long period of time and the pressure and temperature therein may be different from the values required to initiate depressurization of the compressor arrangement 31.

[0091] The preliminary cooling phase takes place as follows.

[0092] The valve 113 on the LNG line 53 is opened and the controlled valve 119 on the quenching line 63 is opened gradually. LNG flows through conduit 63 and valve 119 and from these into liquid-vapour separator 67 through the last portion of return conduit 65. LNG? at cryogenic temperature and thus cools the liquid-vapour separator 67.

[0093] Once the liquid-vapour separator 67 ? been cooled, the heat exchanger 57 must be cooled to the required temperature. LNG is flowed through the cold side of both the low temperature 57.1 and high temperature 57.2 sections of the heat exchanger 57 by opening the controlled valve 117 and allowing a controlled flow rate of LNG to flow through the heat exchanger 57 and through the return duct 65 towards the liquid-vapour separator 67. The temperature of the boil-off gas, that is? the temperature of the boil-off gas exiting the liquid-vapour separator 67, ? controlled through the temperature controller 66, which acts on the controlled valve 119, so? that the quench LNG flow rate is maintained in conduit 63 at the value required to obtain the desired boil-off gas temperature in conduit 55.

[0094] While the liquid-vapour separator 67 and the heat exchanger 57 are gradually cooled to the required cryogenic temperature with the procedure described above, the holding tank 59 (both sections 59.1 and 59.2 thereof) cool progressively during the subsequent depressurization phase, during which the working fluid from the compressor arrangement 31 is continuously partially condensed at a cryogenic temperature in the heat exchanger 57 and is collected in a condensed liquid layer in the collecting tank 59.

[0095] At the start of cooling, the first working fluid condensed in the heat exchanger 57 and received in the collection tank 59 will vaporize? completely, because the collection tank 59 ? still at high temperature. The vaporization of the condensed working fluid entering the collecting tank 59 after condensing in the heat exchanger 57 initiates the cooling of the collecting tank 59 towards the cryogenic temperature. When the collection tank 59 ? sufficiently cooled, liquid-phase working fluid begins to collect therein at the temperature within the holding tank 59. As depressurization of the compressor arrangement 31 proceeds, the holding tank 59 continues to be cooled, since it is cooled. more working fluid condensed at cryogenic temperature collects in the collection tank 59.

[0096] As an alternative to progressive cooling, the collection tank 59 can be cooled preliminarily through a procedure described in greater detail below.

[0097] During the preliminary cooling phase, the bypass line 69 can be opened, if required, for example if an excessive flow of LNG is diverted to the low temperature section 57.1 of the heat exchanger 57 and ? generated a quantity? excessive boil-off gas (BOG) due to vaporization of LNG. because the low temperature section 57.1 of the heat exchanger 57 ? more of the high temperature section 57.2 of the heat exchanger 57, the excess flow rate of boil-off gas could otherwise block the low pressure section 57.1.

[0098] More Specifically, during this preliminary cooling phase, the shutoff valve 75 is opened and the controlled valve 83 can be opened. open slightly to allow a small flow of working fluid from the low pressure compressor section 31A of the compressor arrangement 31 to the high temperature section 57.2 of the heat exchanger 57 and through the hot side thereof. Two-phase working fluid leaving the high temperature section 57.2 enters the first tank section 59.1. Liquid working fluid collects at the bottom of the first tank section 59.1, where it can vaporize, while the first section of the tank 59.1 ? still at elevated temperature (not cryogenic). Vaporization cools the first section of tank 59.1. The vapor phase working fluid from the first tank section 59.1 enters the hot side of the low temperature section 57.1 of the heat exchanger 57 through the outlet pipe 59.4 and flows in heat exchange relationship with the incoming LNG and finally enters the second tank section 59.2. The working fluid partially condenses in the low temperature section 57.1 of the heat exchanger 57 at a lower temperature than the condensing temperature of the first tank section 59.1. Two-phase working fluid leaving the low temperature section 57.1 of the heat exchanger 57 is collected in the second tank section 59.2, where it can partially vaporize and then cool the second tank section 59.2. The second tank section 59.2 ? therefore brought to a lower temperature than the first tank section 59.1.

[0099] The preliminary cooling step is carried out until the boil-off gas (boil-off line 55) reaches a pre-set temperature, for example between about 0°C and about 15°C. This temperature range? merely illustrative and should not be construed as a limitation. In other embodiments, the preset temperature can be for example lower than 0°C, for example even substantially lower, such as between about -50°C and about -60°C. In general, the preset temperature can? be selected based on heat exchanger design considerations. Once this temperature ? been reached, the flow rate of working fluid through the conduit 79 can? can be gradually increased by gradually opening the controlled valve 83. At the same time, the LNG flow rate through the heat exchanger 57 gradually increases by opening the controlled valve 117. Based on the quench flow fed through the conduit 63, the LNG flow rate can be increased gradually. be controlled towards a desired boil-off flow to keep this constant during the cool down transient. The temperature of the vaporized LNG in the boil-off conduit 55 is, conversely, maintained at a desired temperature by the temperature controller 66. In other embodiments, conduit 55 can be provided with a flow controller. Regardless of the type of control procedure chosen, pu? a flow set-point can be generated, which is applied to a flow controller 116 provided on the LNG duct 53 and capable of acting on the controlled valve 117.

[0100] In other embodiments, it can a preliminary cooling of the collection tank 59 can be performed. By way of example, a small dedicated working fluid condenser (for example, shell and tube, plate and fin, or kettle) can? be used to condense a small amount? of working fluid from the compressor arrangement 31 using a cryogenic source, for example LNG. Liquid working fluid cos? obtained pu? be fed to sections 59.1 and 59.2 of the collection tank by cooling and pressurizing them.

[0101] In other embodiments, the reciprocating compressor 101 can be activated to circulate working fluid trapped within holding tank 59 and related conduits to heat exchanger 57 through conduits 122, 77, 93 and 79 or others not shown. In other embodiments can? be used a combination of both solutions.

[0102] As in the pre-cooling phase, also in the subsequent depressurization phase of the working fluid compressor from the low-pressure compressor section 31A of the compressor arrangement 31 ? partially condensed in the heat exchanger 57 and gradually collects in the collection tank 59.

[0103] More Specifically, the vapor (gaseous) phase working fluid flows through the conduits 71, 77, 79 and the controlled valve 83 and through the hot side of the high temperature section 57.2 of the heat exchanger 57. Here the vapor phase working fluid is cooled by heat exchange with LNG flowing through the cold side of the high temperature section 57.2 of the heat exchanger 57. The working fluid leaving the high temperature section 57.2 of the heat exchanger 57 through the conduit 59.3? in a two-phase state, that is? contains a liquid phase and a vapor (gaseous) phase. When entering the first section of the tank 59.1 some of the working fluid can vaporize doing so? further reduce the temperature. Liquid working fluid collects at the bottom of the first tank section 59.1, while vapor phase working fluid flows through conduit 59.4, the hot side of low temperature section 57.1 of heat exchanger 57 and conduit 59.5 in the second tank section 59.1. tank 59.2 of the collecting tank 59. The flow of the working fluid through the low-temperature section 57.1 ? caused by the pressure difference between the first tank section 59.1 and the second tank section 59.2 due to the different temperatures therein.

[0104] Since? a large part of the incoming working fluid condenses in the first tank section 59.1 and through the low-temperature section 57.1 of the heat exchanger 57 only the working fluid in the vapor state flows, the working fluid flow rate through the low-temperature section 57.1 of the heat exchanger 57 pu? be lower than that in the high temperature section 57.2 of it.

[0105] Since? different temperatures are reached by the working fluid in the two tank sections 59.1, 59.2, the working fluid which condenses in the two sections has a different composition. fractions pi? heavy (i.e. gaseous components having a higher molecular weight and higher liquefaction temperature) of the working fluid condense mainly in the first tank section 59.1. fractions pi? lighter (i.e. gaseous components having a lower molecular weight and a lower liquefaction temperature) condense mainly in the second tank section 59.2.

[0106] During this stage, the flow rate of LNG through the heat exchanger 57 and the flow rate of the working fluid to the heat exchanger 57 can be controlled by a temperature controller 86 located along the LNG return line 65 and by the pressure controller 85. The temperature controller 86 and the pressure controller 85 can be activated in a cascade arrangement, in the sense that the set-point of the pressure controller 85 is determined by the temperature controller 86.

[0107] More specifically, the temperature controller 86 has the purpose of maintaining the temperature of the vaporized LNG in the boil-off duct 55 at a pre-set value by modifying the set-point of the pressure controller 85 between a minimum and a maximum value. For example, the control ? performed in such a way as to cause a continuous increase in the working fluid flow rate through the valve 83, which is controlled by the pressure controller 85 until reaching a maximum flow rate, which depends on the availability? of LNG. In fact, the working fluid flow rate fed through the valve 83 to the heat exchanger 57 will be? that sufficient to maintain the temperature of the vaporized LNG in the return pipe 65 at a desired value, and the pressure inside the tank section 59.1 at a given value.

[0108] For example, if the temperature in the return pipe of LNG 65 falls below the set-point temperature of the temperature controller 86, which means that ? an excess of LNG is available, the 86 temperature controller can? change the set point of the pressure controller 85, which in turn causes a 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 in the pressure in the collecting tank 59.

[0109] In other embodiments, it can A different logic may be used to regulate the LNG working fluid flow rates for the cooling service. For example, in this first part of the cooling phase, the temperature controller 86 could be connected directly to the valve 83, or a flow controller could be used instead of the valve 83. the pressure controller 85.

[0110] In addition to the temperature controller 86 and the pressure controller 85, during this phase it can also be activated a further pressure controller 121, which ? arranged along a return pipe 122, which connects the outlet pipe 59.6 and the by-pass pipe 59.7 to the inlet pipe 71, through a controlled valve 123. The pressure controller 121 controls the controlled valve 123, so that the pressure inside the second tank section 59.2 is maintained at a pressure set-point, which ? higher than the pressure inside the low pressure compressor section 31A of the compressor arrangement 31. When the pressure inside the second tank section 59.2 rises above the pressure set point of the pressure controller 121, this last opens the valve 123 and vents uncondensed working fluid accumulated in the second tank section 59.2 to the low pressure compressor section 31A of the compressor arrangement 31 through the conduits 59.6, 122 and 71.

[0111] The pressure set-point of the pressure controller 121 can? be calculated on the basis of the pressure inside the low pressure section of the compressor 31A, detected by a pressure sensor 125. The pressure set-point of the pressure controller 121 can? be calculated by adding a pressure difference (e.g. about 0.1-1 bar) to the pressure within the low-pressure compressor section 31A of the compressor arrangement 31, sensed by the pressure sensor 125.

[0112] Since? the working fluid? a mixed refrigerant, containing a mixture of different components, such as methane, propane, ethane and nitrogen, the working fluid returning to the low pressure compressor section 31A of the compressor arrangement 31 will have a more high content of those components of the mixed refrigerant that have a vaporization temperature pi? low, what? a lower molecular weight, and therefore mainly nitrogen and methane. Components more heavy particles condense and collect in the first tank section 59.1 and in the second tank section 59.2 of the collecting tank 59.

[0113] Usually, during the above-described depressurization phase of the low-pressure compressor section 31A of the compressor arrangement 31, the controlled valve 123 remains closed, since the the pressure in the collection tank 59 ? always lower than the pressure in the low-pressure compressor section 31A, since? the collection tank 59 ? supplied by the low pressure compressor section 31A during depressurization thereof.

[0114] In the embodiment of Fig.2 the same duct 71 is used to flow working fluid from the low pressure compressor section 31A to the collecting tank 59, and also to return uncondensed working fluid from the collecting tank 59 to the low pressure compressor section 31A. In other embodiments, not shown, separate ducts may be provided. In some embodiments, for example, conduit 79 can be in fluid coupling with the low pressure compressor section 31A downstream of the respective compressor, that is? at the compressor outlet or downstream of it.

[0115] As the depressurization of the low pressure compressor section 31A proceeds, the pressure inside the low pressure section of the compressor 31A and the pressure inside the collecting tank 59 tend to become equal. This causes the pressure controller 85 to further open the controlled valve 83, until it is completely open, to reach the pressure set-point calculated by the temperature controller 86. Once the valve 83 is ? completely open, the control will be? transferred from the valve 83 to the valve 87, that is? the 85 pressure controller will start? to gradually open the controlled valve 87, so? what working fluid from the high pressure section of the 31B compressor will start? to flow through the conduit 72 to the heat exchanger 57 and to the collection tank 59. When the pressure controller 83 begins to open the controlled valve 87, the valve 83 can? be closed. Before opening the valve 87, the valve 91 ? been opened to allow the working fluid to enter inlet conduit 72.

[0116] Once the controlled valve 87 begins to open, the pressure inside the high pressure compressor section 31B will begin to open. to fall, since? working fluid contained in it flow? through the valve 91, the high pressure inlet line 72, the valve 87 and the line 79 into the heat exchanger 57. the controlled valve 87 ? more small of the controlled valve 83, once the valve 87 ? fully open, further pressure drop across the high pressure compressor section 31B can This can be achieved by transferring control of the pressure controller 85 from the controlled valve 87 back to the controlled valve 83 and diverting the flow from the high pressure inlet conduit 72 to the conduit 79. This? ? obtained by opening the valve 95 on the connecting pipe 93. The gradual opening of the controlled valve 83 under the control of the pressure controller 85 will cause? a further pressure drop in the high pressure compressor section 31B.

[0117] During the pressurization of the high pressure compressor section 31B, the working fluid contained therein is fed to the high temperature section 57.2 of the heat exchanger 57, cooled by heat exchange with LNG and with the cold working fluid which returns from the low temperature section 57.1. Will the working fluid from the high pressure compressor section 31B, therefore, condense? partially in the first tank section 59.1. Uncondensed working fluid further flows from the first tank section 59.1 through the hot side of the low temperature section 57.1 of the heat exchanger 57 and partially condenses in the second tank section 59.2 at a lower temperature than the temperature of the first tank section 59.1 .

[0118] As mentioned, also during this step working components having a higher boiling temperature condense mainly in the first tank section 59.1, while components having a lower boiling temperature condense mainly in the second tank section 59.2.

[0119] Non-condensed light working fluid components can be vented into the low-pressure compressor section 31A via the valve 123 when the pressure within the second tank section 59.2 rises above the pressure controller set-point 121. Check valve 81 on line 77 prevents a backflow into line 79. Before venting uncondensed working fluid components to low pressure compressor section 31A via valve 123, working fluid vapor from the second tank section 59.2 is heated by making it flow into the respective cold flow of the high temperature section 57.2 of the heat exchanger 57.

[0120] When the pressure difference between the high-pressure compressor section 31B and the low-pressure compressor section 31A drops to a preset value, the interphase isolation valve 70 is opened, thus that the low-pressure compressor section 31A and the high-pressure compressor section 31B of the compressor arrangement 31 are placed in fluid communication and the pressure therein can? reach an equilibrium value.

[0121] The opening of the interphase isolation valve 70 can? also take place at a later time, when ? was the reciprocating compressor 101 put into operation, as described more? forward.

[0122] As the depressurization of the compressor array 31 proceeds, does the pressure within the compressor array 31 tend? to become equal to the pressure inside the collection tank 59. This far? yes, the pressure controller 85 will open? fully the controlled valve 83 and that the pressure controller 121 will close? the controlled valve 123. Will the flow of the working fluid decrease? and finally cease? when the pressure in the collection tank 59 will reach? the liquid-vapor equilibrium pressure at the temperature and composition of the working fluid in the collecting tank 59.

[0123] Depending on the composition of the mixed refrigerant, this final pressure could be low enough to restart the compressor arrangement 31.

[0124] Conversely, in some situations the final pressure reached at the end of the above described procedure can be greater than that required to restart the compressor arrangement 31. Specifically when a mixed refrigerant with a high percentage of nitrogen is used, the final pressure of the cooling system 51 can be greater than the threshold required to restart the compressor setting 31. If in the compressor setting 31 ? do you want a pressure pi? low, the auxiliary compressor 101 is activated.

[0125] The auxiliary compressor 101 can? start operating when the pressure in the high pressure compressor section 31B has reached a preset threshold. The auxiliary compressor 101 can? be loaded when the flow rate of the working fluid measured by the flow controller 127 positioned on the conduit 79 falls below a pre-set threshold. Alternatively, the load of booster compressor 101 can be controlled as a function of the pressure sensed by the pressure controller 85, or by the pressure controller 121. To flow working fluid from the high pressure compressor section 31B through the high temperature section 57.2 of the heat exchanger 57 and to the collection 59 by means of the booster compressor 101, the valve 95 is closed and the valves 103 and 104 are opened. In some embodiments, flow controller 127 on conduit 79 may be activated and used to control the auxiliary compressor 101. Working fluid will come? processed by the auxiliary compressor 101 and flow? from the high pressure compressor section 31B, through the high pressure inlet pipe 72, the booster compressor 101, the cooler 105 and the pipe 79, into the high temperature section 57.2 of the heat exchanger 57.

[0126] In some embodiments, a ratio controller 128 can be used to calculate the required LNG flow rate as a function of the working fluid flow processed through the booster compressor 101. In other embodiments, the temperature controller 86 may reduce the LNG flow acting as a higher level controller, in case the minimum temperature is reached on duct 65. In general, an LNG flow set-point can be supplied to the flow controller 116 disposed along the LNG conduit 53. The flow controller 116 acts on the valve 117 to maintain the required flow of LNG through the cooling arrangement 51.

[0127] The set-point value of the pressure controller 123 can? be reduced from the value reached before the start of operation of the auxiliary compressor 101 to a lower value, for example from about 6-8 barA to about 3-6 barA. By keeping the booster 101 running, the pressure on the suction side of the booster 101 will drop? as a result of the working fluid being transferred from the compressor arrangement 31 to the collecting tank 59 and stored therein in a condensed state. Non-condensed components of the working fluid, ie? components that have a pi? low vaporizing temperature, will be vented through the controlled valve 123 in the compressor arrangement 31.

What? doing, by means of the auxiliary compressor 101 gas with components pi? heavy trapped in volume within the arrangement of compressor 31 sar? forced to flow from the low pressure compressor section 31A and the high pressure compressor section 31B (pushed by the lighter gas entering from the valve 123) to the heat exchanger 57 for partial condensation. The condensed working fluid is accumulated in the collection tank 59.

[0128] In addition to maintaining the required pressure in the second tank section 59.2, venting through the valve 123 to the compressor arrangement 31 also has the advantage of lowering the molecular weight of the mixture trapped in the compressor arrangement 31. This in turn reduces the power drawn by the compressor arrangement 31 during startup.

[0129] When the pressure value required for the restart of the compressor arrangement 31 is reached, for example 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 interphase isolation valve 70 can be closed.

[0130] In other embodiments, the interface isolation valve 70 may be kept open for restart.

[0131] The controlled valve 109 will be completely open and the auxiliary compressor 101 will come? arrested. The controlled valves 123, 83 and 87 will also be closed.

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 restarted and accelerated gradually to reach the speed? minimum operational. When the latter ? been reached, the working fluid contained (mainly in the liquid state) in the collection tank 59 will come? gradually reintroduced into the compressor arrangement 31. There? can? be effected, for example, by removing liquefied working fluid through a return conduit 131 which ? in fluid coupling with the first tank section 59.1 via an isolation valve 133 and with the second tank section 59.2 via an isolation valve 135.

[0133] In some embodiments, the return conduit 131 can to a quench system installed on an anti-surge line of the low pressure compressor section 31A, or on the high pressure compressor section 31B of the compressor arrangement 31 (not shown).

[0134] A quench valve 137 on the return line 131 can? be controlled by a temperature controller 139, which ? usually arranged at the outlet of the suction drums of the low pressure compressor section 31A and/or the high pressure compressor section 31B. Liquefied working fluid is cos? supplied to the compressor arrangement 31 through the quench valve 137 sequentially opening the isolation valves 135 and 133. The quench valve 137 is controlled by the temperature controller 139, whose set-point can? be gradually lowered.

[0135] The liquefied working fluid from the holding tank 59 is injected by the quench system into the anti-surge line, where the liquefied working fluid vaporizes upon contact with the recycled working fluid through the anti-surge line of the compressor arrangement 31.

[0136] In embodiments, to maintain the pressure of the collection tank 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 set-point. In other embodiments, an aftercooler (not shown) can be arranged on the conduit 93 to avoid recirculating cryogenic fluid towards the auxiliary compressor 101.

[0137] In other embodiments, a conduit 200, which connects the low-pressure compressor section 31A of the compressor arrangement 31 to the suction side of the booster compressor 101, may be put into service by opening a respective isolation valve. The duct 200 can? be used where additional flow is required to maintain pressure within the first tank section 59.1. Will the compressor controller 111 decide? the quantity? of flow to be made to flow to the auxiliary compressor 101 through the conduit 200. During operation with the conduit 200 open, the valve 75 will be? open to discharge working fluid to the low pressure compressor section 31A, while the 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 booster compressor 101, in other embodiments the conduit 200 may be omitted and the high pressure inlet pipe 72 can? used for the same purpose.

[0138] In other embodiments, not shown, the liquefied working fluid from the holding tank 59 can be vaporized before being reintroduced into the compressor arrangement 31 by means of a vaporizer (not shown), such as an air evaporator.

[0139] Once the liquefied working fluid collected in the collection tank 59 ? been transferred back into the compressor arrangement 31, the isolation valves 133 and 135 can be closed and the booster compressor 101 can? be turned off. If necessary, the residual pressure in the collecting tank 59 can be reduced by opening controlled valve 123 and isolation valve 75.

[0140] If the auxiliary compressor 101 is not ? expected, working fluid collected in the collection tank 59 pu? be transferred back gradually to the compressor arrangement 31 by means of an external pressurization system, optionally provided with an evaporator.

[0141] While in the schematic circuit of Fig.2 the auxiliary compressor 101 ? 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 function as a booster for the purpose described above.

[0142] For example, a compressor section of an existing propane scavenge compressor to depressurize the propane cycle can be used for this purpose. The propane recovery compressor ? usually a reciprocating compressor able to perform the above described function of the auxiliary compressor 101. Therefore, the auxiliary compressor in the sense understood here can? be an existing compressor provided in the plant for other purposes.

[0143] While the invention? been described in terms of various specific embodiments, sar? it is clear to those skilled in the art that many changes, modifications and omissions are possible, without departing from the scope and spirit of the claims. Also, unless otherwise specified, the order or sequence of any process or method steps may be varied or reconfigured according to alternative embodiments.

Claims (8)

A SYSTEM AND METHOD FOR REDUCING SETTLE PRESSURE USING A BOOSTER COMPRESSOR CLAIMS 1. A thermodynamic system (1) comprising: a processing section (30), able to circulate a working fluid therein; a cooling arrangement (51) comprising: - a collection tank (59) capable of collecting therein a liquid phase and a gaseous or vapor phase of the working fluid in thermodynamic equilibrium; in which the collection tank ? adapted to be in fluid coupling with the processing section (30) to remove working fluid therefrom and reintroduce working fluid thereto; and - a heat exchanger (57) functionally coupled to the collection tank (59); wherein the heat exchanger (57) comprises a hot side adapted to receive the working fluid, and a cold side adapted to receive a cooling fluid in heat exchange relationship with the working fluid to remove heat therefrom; a connection adapted to fluid couple the heat exchanger (57) and the processing section (30), to remove working fluid from the processing section (30) and circulate working fluid through the heat exchanger (57) towards the collection tank (59); a booster compressor (101), a compressor controller (111) and a valve switching arrangement (103, 104, 106), adapted to selectively place a suction side of the booster compressor (101) in fluid communication with the section processing section (30) and a discharge side of the booster compressor (101) with the heat exchanger (57), to promote the flow of working fluid from the processing section (30) to the collecting tank (59). The thermodynamic system (1) of claim 1, wherein the compressor controller (111) is adapted to activate the booster compressor (101) to circulate working fluid from the processing section (30) to the heat exchanger (57) when a thermodynamically generated flow caused by the temperature and pressure difference? insufficient. The thermodynamic system (1) of claim 1 or 2, wherein the activation of the booster compressor (101) is controlled a controller (127; 85; 123) able to detect at least one parameter of the working fluid in the collection tank (59). 4. The thermodynamic system (1) of claim 3, wherein said parameter ? selected from the group consisting of: a working fluid flow from the processing section (30) to the collection tank (59); a pressure in the collecting tank (59), or in a conduit in fluid coupling therewith; a combination of them. 5. The thermodynamic system (1) of one or more? of the preceding claims, wherein the auxiliary compressor (101) is a volumetric compressor, in particular a reciprocating compressor. 6. The thermodynamic system (1) of one or more? of the preceding claims, wherein the processing section (30) comprises a compressor arrangement (31) having a low pressure compressor section (31A) and a high pressure compressor section (31B), and wherein the connection between the processing section (30) and the heat exchanger (57) comprises a first coupling (77, 79) between the low pressure compressor section (31A) and the heat exchanger (57), and a second coupling (32 , 79) between the high pressure compressor section (31B) and the heat exchanger (57). The thermodynamic system (1) of claim 6, wherein the booster compressor (101) comprises a suction side adapted to be in fluid coupling with the second coupling (72, 79) and a discharge side adapted to be in fluid coupling with the first coupling (77, 79). 8. A method for reducing a settling pressure in a thermodynamic system comprising: a processing section (30) adapted to circulate a working fluid therein; a cooling arrangement (51) comprising a collection tank (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 tank (59), wherein the heat exchanger (57) comprises a hot side adapted to circulate working fluid, and a cold side adapted to circulate a cooling fluid in heat exchange ratio with the working fluid to remove heat from it; the method including the following steps: supplying working fluid from the processing section (30) through the heat exchanger (57) in heat exchange relationship with the cooling fluid; collecting liquefied working fluid from the heat exchanger (57) in the collection tank (59); flowing non-condensed working fluid from the collection tank (59) to the processing section (30); And promoting the circulation of the working fluid from the processing section (30) through the heat exchanger (57) and to the collecting tank (59) with a booster compressor (101).