Patent Description:
In ocean tankers carrying cargoes of liquid natural gas (LNG), as well as land based storage tanks, a portion of the liquid is lost through evaporation as a result of heat leak through the insulation surrounding the LNG storage receptacle. Moreover, heat leakage into LNG storage containers on both land and sea causes some of the liquid phase to vaporize thereby increasing the container pressure. Regulations prohibiting tanker disposal of hydrocarbon-containing streams by venting or flaring within the vicinity of metropolitan areas coupled with an increased desire to conserve energy costs have led to incorporation of reliquefiers into the design of new tankers for recovering LNG BOG.

One existing approach to BOG reliquification has been the use of a compression cycle, in which the BOG is compressed to an elevated pressure, cooled, and expanded before being returned to the storage vessel. The equipment required to compress the BOG is large, which is not ideal on tanker or other floating applications due to space contraints. In addition, the BOG is circulated through portions of the system at high pressure, which creates an elevated risk of leaks of flammable gas.

<CIT> describes an LNG BOG reliquefication process in which the predominantly methane BOG is compressed, then cooled sensibly by gaseous nitrogen in a closed loop nitrogen recycle refrigeration process, then condensed using boiling liquid nitrogen.

<CIT> describes an LNG boil-off gas reliquification process in which boil-off gas is condensed in an open loop methane refrigeration cycle where boil-off gas is warmed, compressed, cooled with ambient cooling then flashed to a low pressure to form liquid. In this case the BOG is warmed to ambient temperature before being compressed and cooled.

<CIT> describes a method for re-condensing a boil-off gas stream comprising natural gas from a storage tank according to the preamble of claim <NUM> and a boil-off gas re-condensation system according to the preamble of claim <NUM>.

There is a need for an improved BOG liquification system that is capable of reliquifying BOG without the need for compressing the BOG or the need to subcool the BOG.

Several aspects of the systems and methods are outlined below.

According to a first aspect of the present invention, there is provided a method for re-condensing a boil-off gas stream comprising natural gas from a storage tank, the method comprising:.

Said step (i) may further comprise combining the expanded refrigerant stream with the gaseous refrigerant stream and a portion of the cooled refrigerant stream before performing step (c).

Step (a) may further comprise at least partially condensing the boil-off gas stream in the first heat exchanger at a substantially constant temperature against the two phase refrigerant stream to form the at least partially condensed boil-off gas stream and the gaseous refrigerant stream.

The method may further comprise:
(j) maintaining the boil-off gas at a pressure that is no more than <NUM>% of a pressure of the storage tank during the performance of steps (a) and (b).

Step (a) may further comprise at least partially condensing the boil-off gas stream in a first vessel of the first heat exchanger against the two phase refrigerant stream flowing through a second vessel to form the at least partially condensed boil-off gas stream and the gaseous refrigerant stream, the first vessel being contained within the second vessel.

In a preferred embodiment, the two phase refrigerant stream comprises at least <NUM>% nitrogen.

The method may further comprise:
(k) using energy recovered from the performance of step (h) to drive at least a portion of the compression system or a generator.

Step (i) may comprise combining the expanded refrigerant stream with the gaseous refrigerant stream after a portion of the heating of step (c) has been performed on the gaseous refrigerant stream.

The method may further comprise:
(I) condensing a natural gas stream against the gaseous refrigerant stream in the second heat exchanger.

The method may further comprise:
(m) providing a blower that results in increased flow of the boil-off gas stream through the condensing heat exchanger.

Step (a) may comprise at least partially condensing the boil-off gas stream in the first heat exchanger located within a head space of the storage tank against the two phase refrigerant stream to form the at least partially condensed boil-off gas stream and the gaseous refrigerant stream, the two phase refrigerant stream comprising at least <NUM>% nitrogen and having the gas phase portion and the liquid phase portion in the first heat exchanger.

The method may further comprise:
(n) before performing step (b), phase separating the at least partially condensed boil-off gas stream into a vapor stream and a liquid stream and performing step (b) on only the liquid stream.

The method may further comprise:
(o) pumping liquid natural gas from the storage tank through a spray header located in a vapor space of the storage tank.

Step (q) may further comprise setting the first set point as the function of the pressure of the storage tank and a power consumption of the compression system.

The method may further comprise:
(r) maintaining a difference between a temperature of the gaseous refrigerant stream before performing step (c) and a temperature of cooled refrigerant stream within a second predetermined range by controlling a position of an expansion valve located in fluid flow communication with the cooled refrigerant stream downstream from the second heat exchanger and upstream from the first heat exchanger.

According to a second aspect of the present invention, there is provided a boil-off gas re-condensation system comprising:.

The first heat exchanger may be adapted to at least partially condense the boil-off gas stream at a substantially constant temperature.

The system may be adapted to maintain the boil-off gas at a pressure that is no more than <NUM>% of a pressure of the storage tank from the point at which the boil-off gas is withdrawn from the storage tank as the boil-off gas stream to the point at which the boil-off gas is returned to the storage tank as the at least partially condensed boil-off gas stream.

The first heat exchanger may comprise an inner vessel in fluid flow communication with the boil-off gas stream and an outer vessel in fluid flow communication with the two phase refrigerant stream, the inner vessel being contained within the outer vessel.

The at least one controller may be further adapted to maintain a difference between a temperature of the gaseous refrigerant stream and a temperature of cooled refrigerant stream within a second predetermined range by controlling a position of an expansion valve located in fluid flow communication with the cooled refrigerant stream downstream from the second heat exchanger and upstream from the first heat exchanger.

<FIG> show embodiments according to the current invention.

The ensuing detailed description provides preferred exemplary embodiments only, and is not intended to limit the scope, applicability, or configuration thereof. Rather, the ensuing detailed description of the preferred exemplary embodiments will provide those skilled in the art with an enabling description for implementing the preferred exemplary embodiments.

Reference numerals that are introduced in the specification in association with a drawing figure may be repeated in one or more subsequent figures without additional description in the specification in order to provide context for other features.

The application includes a plurality of exemplary BOG recondensation systems. Features that are present in more than one BOG recondensation system are represented by reference numerals that differ by a factor of <NUM>. For example, the storage tank <NUM> of the BOG recondensation system of <FIG> corresponds to the storage tank <NUM> of <FIG> and the storage tank <NUM> of <FIG>. Unless a feature is specifically described as being different from other BOG recondensation systems in which it is shown in the drawings, that feature can be assumed to have the same structure and function as the corresponding feature in the BOG recondensation system in which it is described. Moreover, if that feature does not have a different structure or function in a subsequently-described BOG recondensation system, it may not be specifically referred to in the specification.

The term "fluid flow communication," as used in the specification and claims, refers to the nature of connectivity between two or more components that enables liquids, vapors, and/or two-phase mixtures to be transported between the components in a controlled fashion (i.e., without leakage) either directly or indirectly. Coupling two or more components such that they are in fluid flow communication with each other can involve any suitable method known in the art, such as with the use of welds, flanged conduits, gaskets, and bolts. Two or more components may also be coupled together via other components of the system that may separate them, for example, valves, gates, or other devices that may selectively restrict or direct fluid flow.

The term "conduit," as used in the specification and claims, refers to one or more structures through which fluids can be transported between two or more components of a system. For example, conduits can include pipes, ducts, passageways, and combinations thereof that transport liquids, vapors, and/or gases.

The term "natural gas", as used in the specification and claims, means a hydrocarbon gas mixture consisting primarily of methane.

The terms "hydrocarbon", "hydrocarbon gas", or "hydrocarbon fluid", as used in the specification and claims, mean a gas/fluid comprising at least one hydrocarbon and for which such hydrocarbon(s) comprise at least <NUM>%, and more preferably at least <NUM>% of the overall composition of the gas/fluid.

In the claims, letters are used to identify claimed steps (e.g. (a), (b), and (c)). These letters are used to aid in referring to the method steps and are not intended to indicate the order in which claimed steps are performed, unless and only to the extent that such order is specifically recited in the claims.

Directional terms may be used in the specification and claims (e.g., upper, lower, left, right, etc.). These directional terms are merely intended to assist in describing exemplary embodiments, and are not intended to limit the scope thereof. As used herein, the term "upstream" is intended to mean in a direction that is opposite the direction of flow of a fluid in a conduit from a point of reference. Similarly, the term "downstream" is intended to mean in a direction that is the same as the direction of flow of a fluid in a conduit from a point of reference.

As used in the specification and claims, the terms "high-high", "high", "medium", "low", and "low-low" are intended to express relative values for a property of the elements with which these terms are used. For example, a high-high pressure stream is intended to indicate a stream having a higher pressure than the corresponding high pressure stream or medium pressure stream or low pressure stream described or claimed in this application. Similarly, a high pressure stream is intended to indicate a stream having a higher pressure than the corresponding medium pressure stream or low pressure stream described in the specification or claims, but lower than the corresponding high-high pressure stream described or claimed in this application. Similarly, a medium pressure stream is intended to indicate a stream having a higher pressure than the corresponding low pressure stream described in the specification or claims, but lower than the corresponding high pressure stream described or claimed in this application.

Unless otherwise stated herein, any and all percentages identified in the specification, drawings and claims should be understood to be on a weight percentage basis. Unless otherwise stated herein, any and all pressures identified in the specification, drawings and claims should be understood to mean gauge pressure.

As used in the specification and claims, the term "compression system" is defined as one or more compression stages. For example, a compression system may comprise multiple compression stages within a single compressor. In an alternative example, a compression system may comprise multiple compressors.

Unless otherwise stated herein, introducing a stream at a location is intended to mean introducing substantially all of the stream at the location. All streams discussed in the specification and shown in the drawings (typically represented by a line with an arrow showing the overall direction of fluid flow during normal operation) should be understood to be contained within a corresponding conduit. Each conduit should be understood to have at least one inlet and at least one outlet. Further, each piece of equipment should be understood to have at least one inlet and at least one outlet.

<FIG> shows a boil-off gas (BOG) re-condensing system <NUM> in which LNG is contained with in a storage tank <NUM>. Boil-off gas exits the storage tank <NUM> as a BOG stream <NUM>, which flows through a condensing heat exchanger <NUM> and is at least partly condensed, forming partially condensed BOG stream <NUM>, which is returned to the storage tank <NUM> by gravity, either to the top of the tank if partially condensed or near the bottom if fully condensed.

In this arrangement, the condensing heat exchanger <NUM> is a plate fin heat exchanger <NUM> located within in a vessel <NUM> containing boiling liquid nitrogen (LIN). In this arrangement, the condensing heat exchanger <NUM> is located above the storage tank <NUM>. Alternatively, the condensing heat exchanger <NUM> could be located inside the storage tank <NUM>, for example, on the surface of a heat exchanging coil containing boiling LIN.

A gaseous nitrogen (GAN) stream <NUM> is withdrawn from the condensing heat exchanger <NUM> and combined with an expanded GAN stream <NUM> to form a combined GAN stream <NUM>. The combined GAN stream <NUM> is warmed to near ambient temperature in a heat exchanger <NUM> against a high pressure GAN stream <NUM> (described herein), forming a warmed GAN stream <NUM>. Alternatively, the expanded GAN stream <NUM> could be combined with the GAN stream <NUM> after GAN stream <NUM> has been partly warmed in the heat exchanger <NUM>. This is depicted by the broken line representing the alternate expanded GAN stream 108A.

The warmed GAN stream <NUM> is then compressed in a compressor <NUM> to form a compressed GAN stream <NUM>. The compressed GAN stream <NUM> is then is cooled to near ambient temperature against cooling water or ambient air (not shown) in a heat exchanger <NUM> to form a high pressure GAN stream <NUM>. Compressor <NUM> could optionally include multiple stages of compression with cooling water or air intercoolers (not shown).

The high pressure GAN stream <NUM> is cooled in the heat exchanger <NUM> against the combined GAN stream <NUM> to an intermediate temperature to form a high pressure cooled GAN stream <NUM>. A portion <NUM> of the high pressure cooled GAN stream <NUM> is then expanded isentropically in an expander <NUM>. Work produced by the expander <NUM> may be recovered as electrical energy in a generator, or the expander <NUM> could be mechanically coupled to the compressor <NUM> to provide part of the compression energy required to press the warmed GAN stream <NUM>.

The remaining portion <NUM> of the high pressure cooled GAN stream <NUM> is then further cooled in heat exchanger <NUM> exiting as a cooled GAN stream <NUM>, which has a temperature slightly warmer than the GAN stream <NUM>. The cooled GAN stream <NUM> is flashed across a JT valve <NUM>, forming two phase nitrogen stream <NUM>, which is fed to the shell side of the condensing heat exchanger <NUM>.

In this arrangment, the refrigeration duty for condensation of the BOG stream <NUM> is provided by nitrogen. In other arrangements, alternate refrigerants could be used, such as argon for example. It is preferable that the refrigerant comprise less than <NUM> mol% hydrocarbons. This improves safety by using a non-flammable refrigerant in portions of the system <NUM> that are operated under an elevated pressure. It is also preferable that the refrigerant have a purity of at least <NUM> mol% and, more preferably, at least <NUM>%. For example, if the refrigerant is nitrogen, then it comprises preferably at least <NUM> mol% nitrogen. The preferred purity of the refrigerant enables the boiling of the refrigerant in the condensing heat exchanger <NUM> and compression of the refrigerant in the compression system <NUM> to be performed more efficiently.

In this arrangement, the condensation of the BOG stream <NUM> is performed at a substantially constant temperature. In this context, "substantially constant temperature" means that the temperature difference between the BOG stream <NUM> as it enters the condensing heat exchanger <NUM> and the partially condensed BOG stream <NUM> as it exits the condensing heat exchanger is preferably less than <NUM> degrees Celsius.

The heat exchanger <NUM> may also be used to condense a warm natural gas stream <NUM> to form a condensed natural gas stream <NUM>. In addition, a supplemental LIN refrigeration stream <NUM> could optionally be directed to the cold end of the condensing heat exchanger <NUM>.

<FIG> shows another BOG re-condensing system <NUM>, which the condensing heat exchanger is located within the head space of the storage tank <NUM>. In this arrangement, the two phase nitrogen stream <NUM> is circulated through a heat exchanging coil <NUM> located in the head space of the storage tank <NUM>. BOG in the head space (represented by dashed line <NUM>) comes in contact with the outer surface of the heat exchanging coil <NUM>, becomes at least partially condensed (represented by dashed line <NUM>), a flows downwardly away from the heat exchanging coil <NUM>.

<FIG> shows another the BOG re-condensing system <NUM>, in which a blower <NUM> is used to overcome the frictional resistance of the piping and the condensing heat exchanger <NUM>. The blower <NUM> conveys a BOG stream <NUM> to the condensing heat exchanger <NUM>, where it is at least partly condensed. In this arrangement, some sensible cooling of the BOG occurs in the condensing heat exchanger <NUM>, but all of the cooling of the BOG stream <NUM> is still provided by boiling liquid nitrogen, in contrast with the prior art.

It is important to note that, even in the arrangement shown in <FIG>, the BOG remains substantially at the pressure of the storage tank <NUM> througout the re-liquification process. In this context, the term "substantially" means that the pressure of the BOG is only elevated to the extent required to overcome friction losses incurred as it circulates through the condensing heat exchanger <NUM> and the conduits that contain the BOG stream <NUM> and the partially condensed BOG stream <NUM>. Stated another way, the BOG is preferably maintained at a pressure that is no more than <NUM>%, more preferably no more than <NUM>%, and most preferably no more than <NUM>%, of the pressure of the storage tank <NUM>. For example, it is common for the pressure of a bulk LNG storage tank to be maintained at slightly above atmospheric pressure of <NUM> PSIA (<NUM> kPa). Based on a tank pressure of <NUM> PSIA (<NUM> kPa), it is preferable that the re-condensation process be performed on the BOG at a pressure that does not exceed <NUM> PSIA (<NUM> kPa) at any time during the process (i.e., from point at which the BOG stream <NUM> is withdrawn from the storage tank <NUM> to the point at which the partially condensed BOG stream <NUM> reenters the storage tank <NUM>). Among other advantages, this enables the portion of the system <NUM> through which flammable fluid circulates to operate at low pressure, which reduces the risk of a flammable leak.

<FIG> shows another BOG re-condensing system <NUM>, which is useful when the BOG stream <NUM> contains a substantial nitrogen fraction (e.g., more than <NUM> mol% nitrogen). When the BOG stream <NUM> contains a substantial nitrogen fraction, it is more efficient to provide the required cooling duty by only partly condensing it. The partially condensed BOG stream <NUM> is separated into a liquid stream <NUM> and vapor stream <NUM> in phase separator <NUM>. The liquid stream <NUM> is returned to the storage tank <NUM> and vapor stream <NUM> (which is nitrogen rich) may be burned or used as fuel.

For storage tanks <NUM> in which the LNG contains a substantial nitrogen fraction, the arrangement shown in <FIG> is useful because it prevents uncondensed nitrogen from accumulating in the vapor space of the storage tank <NUM>. If nitrogen accumulates in the vapor space, the temperature of the BOG stream <NUM> decreases. This decreased temperature increases the power required for condensation of the BOG stream <NUM> and may decrease the capacity of the BOG re-condensing system <NUM>. For condensation of BOG on an LNG transport ship, increased nitrogen levels in the BOG stream <NUM> may also negatively impact the ship engines that use BOG as fuel.

<FIG> shows another a BOG re-condensing system <NUM>, which is also useful when the BOG stream <NUM> contains nitrogen. In this case, the partially condensed gas stream <NUM> is only partly condensed and returned to the top of the storage tank <NUM> in its vapor space <NUM>. In order to prevent nitrogen from accumulating in the vapor space <NUM>, a pump <NUM> is used to feed LNG to a spray header <NUM>, which keeps the liquid and vapor phases in equilibrium and prevents the accumulation or enrichment of nitrogen in the vapor space <NUM>. For LNG carrier ships, the pump <NUM> and spray header <NUM> are often needed for cool-down of the storage tank <NUM> prior to initial filling of the tank. Accordingly, the same pump <NUM> and spray header <NUM> may be used for both purposes.

An exemplary embodiment of a BOG re-condensing system <NUM> in accordance with the present invention is shown in <FIG>. In this embodiment, a valve controller <NUM> is used to indirectly control pressure in the storage tank <NUM> by modulating the capacity of the condensing heat exchanger <NUM>. The pressure controller <NUM> controls the pressure in the storage tank <NUM> by adjusting the setpoint SP1 of the valve controller <NUM> based on an output OP1 of a pressure controller <NUM>, which in turn controls the pressure of boiling LIN in the condensing heat exchanger <NUM> by manipulating valve <NUM>. As used herein, the terms "closing" and "opening" are indended to mean changing the position of a valve in one direction or another - not necessarily to change the valve position to a fully open or fully closed position.

When the boil-off rate is at the design capacity of the BOG re-condensing system <NUM>, the pressure of the storage tank <NUM> (measured by PV2) is at the setpoint SP2 and valve <NUM> is fully or nearly fully open. If the boil-off rate decreases below the design capacity, the pressure in the storage tank <NUM> will begin to fall and the pressure controller <NUM> will respond by increasing the setpoint SP1 to the valve controller <NUM>, which will respond by partly closing valve <NUM>, thereby increasing the pressure of the boiling LIN and in turn increasing the LIN temperature which decreases the driving force for heat transfer and the cooling duty so that the tank pressure is maintained at the setpoint. The pressures downstream of <NUM> and upstream of the JT valve <NUM> drop because the valve is closing and the mass flowrate of nitrogen is decreasing, while the volumetric flowrate remains roughly the same, allowing compressor <NUM> to continue to operate at or near peak efficiency. The liquid level in the condensing heat exchanger <NUM> increases because the inventory of gaseous nitrogen in the system decreases due to the reduced pressures on both the suction and discharge circuits connected to <NUM>, and in heat exchanger <NUM>. This method of turndown reduces the mass flowrate and power consumption of the compressor <NUM> by reducing system gaseous inventory without loss of nitrogen refrigerant.

Conversely, if the boil-off rate increases, the pressure controller <NUM> will respond by increasing the setpoint to the valve controller <NUM>, which will respond by opening valve <NUM>, thereby increasing the pressure of the boiling LIN and decreasing the temperature of the LIN which increases the driving force for heat transfer and the cooling duty so that the storage tank <NUM> pressure is maintained at the setpoint SP2. The liquid level in <NUM> then decreases, bringing additional nitrogen inventory into circulation and raising the pressures in the system downstream of valve <NUM> and upstream of the JT valve <NUM>.

As mentioned previously, the output OP2 of the pressure controller <NUM> is normally used as the setpoint SP1 of the valve controller <NUM>. At boil-off rates above the design point, the cooling duty may be such that the power needed approaches the maximum power available from the motor <NUM> used to drive the compressor <NUM>. To prevent motor overload, a power controller <NUM> is provided. The power controller <NUM> compares the power consumption of the motor PV3 to the user supplied setpoint SP3 (the maximum allowed power). If the boil-off rate is high and the power consumption PV3 approaches the setpoint SP3, the output OP3 from power controller <NUM> increases. This output OP3 is compared to the output OP2 from the pressure controller <NUM> in a selector block <NUM>, which passes the larger value as a setpoint SP1 to the valve controller <NUM>. If the output OP3 from the power controller <NUM> is greater than the output OP2 from the pressure controller <NUM>, the power controller output OP3 will override the pressure controller output OP2 to prevent overload of the motor <NUM>. In that case, the pressure in the storage tank <NUM> will exceed the setpoint SP2 and may activate pressure relief valves (not shown) and send excess BOG to flare or vent.

Another feature of the control system is to maintain a constant temperature difference between the temperatures of the combined GAN stream <NUM> entering the cold end of the heat exchanger <NUM> (measured at PV6) and the cooled GAN stream <NUM> exiting the cold end of the heat exchanger <NUM> (measured at PV7). This temperature difference PV4 is measured by FY and fed by signal PV4 to a temperature difference controller <NUM>. The temperature difference controller <NUM> maintains the temperature difference PV4 at an operator supplied setpoint SP4 by manipulating the setpoint SP5 of a flow controller <NUM>. The flow controller <NUM>, in turn, controls the position of the JT valve, which controls the flow rate of nitrogen thorugh the JT valve <NUM>. If the temperature difference PV4 at the cold end of the heat exchanger <NUM> begins to exceed the setpoint SP4, the temperature difference controller <NUM> will decrease the setpoint SP5 to the flow controller <NUM>. The flow controller <NUM> will, in turn, begin to close the JT valve <NUM>, reducing the flow of the cooled GAN stream <NUM> which will reduce the temperature difference PV4.

In this exemplary embodiment, the expander <NUM> is equipped with flow control nozzles <NUM> that can be adjusted manually to change the flowrate and the outlet-inlet pressure difference across the expander <NUM> and the compressor <NUM> to improve efficiency.

Table <NUM> shows stream data for an example of a process conducted in accordance with the system of <FIG>, but without the warm natural gas stream <NUM>, alternate expanded GAN stream 108A, or the supplemental LIN refrigeration stream <NUM>. In this example, the total compression work of the compressor <NUM> is <NUM>,<NUM> hp and the work produced by the expander <NUM> is <NUM> hp for a net work requirement of <NUM>,<NUM> hp. The cooling duty of the condensing heat exchanger <NUM> is <NUM> kw in this example.

Claim 1:
A method for re-condensing a boil-off gas stream (<NUM>, <NUM>) comprising natural gas from a storage tank (<NUM>, <NUM>), the method comprising:
(a) at least partially condensing the boil-off gas stream (<NUM>, <NUM>) in a first heat exchanger (<NUM>, <NUM>) against a two phase refrigerant stream (<NUM>) to form an at least partially condensed boil-off gas stream (<NUM>, <NUM>) and a gaseous refrigerant stream (<NUM>, <NUM>), the two phase refrigerant stream comprising no more than <NUM> mol% hydrocarbons and at least <NUM> mol% of at least one selected from the group of nitrogen and argon, the two-phase refrigerant stream having a gas phase portion and a liquid phase portion in the first heat exchanger;
(b) returning the at least partially condensed boil-off gas stream (<NUM>, <NUM>) to the storage tank (<NUM>, <NUM>);
(c) heating the gaseous refrigerant stream (<NUM>, <NUM>) in a second heat exchanger (<NUM>, <NUM>) against a high pressure refrigerant stream (<NUM>) to form a warmed refrigerant stream (<NUM>, <NUM>);
(d) compressing the warmed refrigerant stream (<NUM>, <NUM>) in a compression system (<NUM>, <NUM>) to form a compressed refrigerant stream (<NUM>);
(e) cooling the compressed refrigerant stream (<NUM>) in a third heat exchanger (<NUM>) to form the high pressure refrigerant stream (<NUM>);
(f) cooling the high pressure refrigerant stream (<NUM>) against the gaseous refrigerant stream (<NUM>, <NUM>) in the second heat exchanger (<NUM>, <NUM>) to form a high pressure cooled refrigerant stream (<NUM>, <NUM>);
(g) separating the high pressure cooled refrigerant stream (<NUM>, <NUM>) into a first portion (<NUM>, <NUM>) and a second portion (<NUM>);
(h) expanding the second portion (<NUM>) of the high pressure cooled refrigerant stream to form an expanded refrigerant stream (<NUM>, <NUM>); characterised in that the method comprises:
(p) controlling a position of a first valve (<NUM>) as a function of a pressure of the gaseous refrigerant stream (PV1) and a first set point (SP1), the first valve being positioned downstream from the first heat exchanger (<NUM>) and upstream from the second heat exchanger (<NUM>) and in fluid flow communication with the gaseous refrigerant stream (<NUM>); and
(q) setting the first set point (SP1) as a function of a pressure of the storage tank (PV2)..