This disclosure relates to a method for maintaining a subcooled state of a cryogenic fluid such as liquefied natural gas (LNG) in a storage vessel and to an improved system for dispensing the cryogenic fluid.

Liquefied natural gas is composed primarily of methane, which comprises about 85 to 98% of the LNG on a molar basis. Lesser components that may be present include ethane, propane, carbon dioxide, oxygen and nitrogen. For the purposes of illustration, the properties of pure methane will be used to characterize LNG.

Liquefied natural gas bulk storage vessels, especially those used in refuelling stations, are subject to both heat load, from natural heat-in-leak and refuelling operations, and from returned gas associated with the fuelling operation. This causes a significant heat load to the storage vessel, which typically results in gas venting. This venting is both a loss of valuable product, as well as a significant environmental issue because natural gas is a powerful greenhouse gas. Maintaining the contents of the bulk storage vessel in a subcooled state (temperature below the boiling point corresponding to the storage tank pressure) will prevent most or all of this venting. However, the amount of subcooling available depends on the temperature of the supplied liquid to the bulk storage vessel, and will be lost through warming after a period of time. Hence venting from LNG storage vessels is routine and a significant impediment to successful implementation of natural gas as a vehicle fuel. LNG vehicle fuel tanks typically have an optimum storage pressure of about 6-8 barg in order to deliver the fuel to the engine without the assistance of a pump or compressor. If the liquid supplied during refuelling is at a temperature above the saturation temperature corresponding to the optimum storage pressure then the fuel tank must typically vent during refuelling. It is therefore desirable for the temperature of the LNG supplied from the bulk storage tank to be at or somewhat below the saturation temperature

corresponding to the optimum on-board storage pressure. For example, at 6 barg the saturation temperature is about -131 °C (142°K). The bulk storage vessel head-pressure is sufficiently high to promote the flow of subcooled liquid into the vehicle fuel tank. However due to the subcooling, the liquid that reaches the vehicle tank is not of the corresponding condition, but significantly cooler. This allows the refuelling to occur with little or no venting, and the storage tank is filled at close to the optimum on-board storage pressure. Further, in the case of an on-board fuel tank that is initially at an elevated pressure relative to the optimum pressure, it is generally advantageous to first introduce subcooled LNG in order to collapse the existing gas in the fuel tank. WO2013/102794 provides a method for maintaining a subcooled state within a cryogenic fluid such as liquefied natural gas in a storage vessel comprising removing a portion of the cryogenic fluid, cooling the removed portion of cryogenic fluid and reintroducing the removed portion of cryogenic fluid back into the liquid region of the storage vessel. US6336331 provides a method for refrigerating the contents of a tank containing cryogenic liquid.

Accordingly, it is desirable to provide an alternative method for handling LNG, and/or tackle at least some of the problems associated with the prior art or, at least, to provide a commercially useful alternative thereto.

According to a first aspect, the present disclosure provides a vacuum insulated storage vessel for storing cryogenically cooled pressurised fluids,

the storage vessel encapsulating first and second fluid storage volumes, each of the first and second fluid storage volumes comprising an inlet for filling and one or more outlets for dispensing,

the vessel further comprising a first heat exchanger within the first fluid storage volume for cooling fluid held in the first fluid storage volume, the first heat exchanger being in fluid communication with an outlet of the second fluid storage volume.

The present invention will now be further described. In the following passages different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

Advantageously, the vessel described herein provides a low-cost apparatus for the dispensing of a cryogenically cooled fluid. In particular, the present disclosure provides a vacuum-insulated storage vessel for holding cryogenically cooled pressurised fluids. The use of vacuum-insulated storage vessels for holding cryogenic fluids is well known in the art. Cryogenic fluids suitable for the present invention include liquefied natural gas, liquid nitrogen, liquid oxygen, liquid air, and liquid argon and mixtures of these fluids. Other fluids and fluid mixtures, such as ethylene, while not typically classified as cryogenic are also suitable for the present invention. When these fluids or mixtures of fluids are stored in a vessel, it is natural for liquid and vapour fractions of the fluid to form and separate. Where mixtures of these fluids are contained as the sole contents of a storage vessel, then the molar ratio of the components will be different in the liquid and vapour phases according to equilibrium thermodynamics.

While the detailed description of the invention below discusses liquefied natural gas as the cryogenic fluid that is present in the storage vessel, and liquid nitrogen as the coolant, the invention would be applicable to other cryogenic fluids such as liquid nitrogen, liquid oxygen, liquid air, liquid argon, and ethylene and mixtures of these fluids.

The storage vessel encapsulates first and second fluid storage volumes. That is, the storage vessel has an outer wall encompassing and enclosing the first and second fluid storage volumes. The first and second fluid storage volumes may be separate high pressure containers. Preferably the storage vessel has a partition defining the first and second fluid storage volumes; that is, there is a single high pressure container within the vacuum insulated vessel with an internal partition. Preferably the first and second fluid storage volumes are encapsulated within a common evacuated atmosphere.

The first fluid storage volume is for holding a cryogenically cooled pressurised fluid for dispensing. For example, the first fluid storage volume may contain liquid natural gas which can be dispensed into automobiles as a fuel. The second fluid storage volume is for holding a cryogenically cooled pressurised fluid for use as a coolant. Typically a cryogenic fluid such as liquid nitrogen is used as a coolant. However other cryogenic fluids such as liquid air, oxygen, and argon and mixtures of these fluids can be employed or a heat transfer fluid cooled by other means may be employed. Preferably the second fluid storage vessel comprises liquid nitrogen (also known as LIN or LN2).

The provision of the first and second fluid storage volumes in the single vacuum- insulated vessel provides a number of benefits. Specifically, the fluids share the structure and insulation of the vessel. By being a single vessel, the apparatus is a transportable system and has the additional benefits of significantly reduced civils-costs and complexity, alongside plug-and-play installation and commissioning. Commissioning and decommissioning of static systems typically requires time-consuming pressure testing, purging, venting and filling processes so as ensure a flammable atmosphere does not exist and that the system is leak-free. The transportable system described herein eliminates purging, pressure testing and venting processes associated with the vessel, having the ability to be moved to site with product stored within. Advantageously, the dual-product vessel can be 'plug and play' incorporating simple electronic connections for ease of installation. Advantageously the vessel described herein comprises on-board dual storage for of LNG and liquid nitrogen in volumes suitable for product conditioning. The vessel also comprises a heat exchanger, such as a conditioning coil, for the stored fluid which can be used to sub-cool LNG prior to filling.

Advantageously the vessel may further comprise:

• A heat exchanger, such as a condensing coil for storage top gas, allowing

improved pressure control.

· A saturation vaporiser system to allow for post-bulk-fill warming of the liquid.

• Inter-jacket pipework and valves to minimize pipe runs and heat in-leak.

• A valve arrangement to allow for the use of 'waste' LIN from the sub-cooling coil to further cool the top-gas (pressure control), further reducing LIN consumption. Preferably the vessel is a tank container. A tank container is a vessel typically made of stainless steel surrounded by an insulation and protective layer of usually polyurethane and aluminium. The outer jacket of the vessel may be of Stainless steel or carbon steel construction. The vessel is provided in the middle of a steel frame. The frame may be made according to ISO standards which is 12.192 meters long, 2.438 meters wide and 2.591 meters or 2.896 meters high. The contents of the tank range from 27,000 to 4,000 litres. There are both smaller and larger tank containers, which usually have a size different from the ISO standard sizes. An ISO tank container is shown in figure 1.

Each of the first and second fluid storage volumes comprise an inlet for filling and one or more outlets for dispensing. Therefore the vessel can be refilled with fluids. The dispensing outlet for the first volume permits the use of the LNG as a fuel. The one or more outlets for the second volume permit the use of LIN as a coolant in the first and optionally the second and/or further heat exchangers disclosed herein. The vessel comprises a first heat exchanger within the first fluid storage volume for cooling fluid held in the first fluid storage volume. The first heat exchanger is in fluid communication with an outlet of the second fluid storage volume. Heat exchangers are well known in the art. The provision of the first heat exchanger permits the use of the coolant in the second fluid storage volume to be used to subcool the fluid in the first fluid storage volume.

The amount of cryogenic fluid supplied to the first heat exchanger is adjusted to maintain the desired degree of subcooling of the cryogenic fluid present in the storage vessel. This cooling can also be provided by other cryogenic fluids, or a heat transfer fluid cooled by other means. The cryogenic fluid is vented from the heat exchanger after performing its heat exchange duties. The nitrogen vented from the external heat exchanger may be further employed for other purposes (cooling or otherwise). Preferably the first heat exchanger is provided in a lower portion of the first fluid storage volume. That is the cooled portion of cryogenic fluid in the first fluid storage volume is near the bottom of the storage vessel. This will help establish a uniform bottom subcooled layer in the storage vessel. Conditioning of suitably subcooled LNG through immersed coil arrangement with liquid nitrogen minimises and/or eliminates the venting of natural gas due to heat in leak and pressure-equalisation whilst refuelling on-board tanks by the maintenance of subcooled LNG in the storage container. The first heat exchanger will, in use, be submerged in the liquid LNG.

The sub-cooling arrangement of the conditioning coil provides a much simpler arrangement for zero-loss LNG vehicle refuelling compared to prior art pressure-decant or more complex pump designs. Immersed coils eliminate heat-in-leak as a result of gas locks and from moving product from the vessel into external heat exchangers as per the prior art. Additional control elements, as necessary, such as control valves, or temperature or pressure sensing devices may also be used to control the degree and rate of subcooling.

Advantageously, the provision of the first heat exchanger within the first fluid storage volume minimises the introduction of heat into the system.

Preferably there is further provided a second heat exchanger which is in fluid

communication with an outlet of the second fluid storage volume and is provided in an upper portion of the first fluid storage volume. The heat exchanger thus placed in the vessel ullage-space can be used to condense vapours so as to prevent venting of gaseous product, and to facilitate a method of pressure-control. The condensing coil re- liquefies storage tank top gas and reduces the pressure and thus prevents venting to atmosphere, eliminating losses of the product stored within the first fluid storage volume and preventing environmental harm. The used LIN from the first heat exchanger may be passed to the second heat exchanger for further use before being lost.

The heat exchanger technology from WO2013102794 appears to have difficulty controlling boil off effectively in low utilisation installations due to the lack of convective heat transfer within the fluid. Cooling fluid at the base of the vessel was not always effective at controlling pressure in the ullage space (top) of the vessel. Venting caused by process heat-in-leak and low utilisation results in the loss of valuable product, and also the rejection of a powerful greenhouse gas. The loss has therefore got both financial and environmental implications. The provision of a second heat exchanger at the top of the first fluid storage volume serves to mitigate this problem.

According to a further aspect there is provided a system for dispensing cryogenically cooled pressurised fluid, the system comprising:

the vacuum insulated storage vessel of any of the preceding claims;

a third fluid storage volume comprising a further heat exchanger;

a dispensing duct in fluid communication with an outlet of the first fluid storage volume for connection to a remote storage vessel to be filled;

a coolant duct providing fluid communication between an outlet of the second fluid storage volume and the further heat exchanger; and

a venting duct in fluid communication with the third fluid storage volume for connection to a remote storage vessel to be filled,

the system further comprising a pressure sensor in communication with the first duct for detecting excess pressure in the remote storage vessel,

whereby the system is configured to dispense fluid from the second fluid storage volume to the further heat exchanger when an excess pressure is detected.

The third fluid storage volume may be within the vacuum insulated storage vessel to benefit from the structural convenience and insulation.

The system permits the dispensing of LNG, such as to fill a high pressure vessel in an automobile. The filling may be by Pressure Decant (PD). The principle of PD is well known in LNG dispensing systems and can be used to establish close control of vehicle on-board storage pressures so as to create a pressure differential, and initiate/sustain flow. Recently, there has been a movement towards pumped LNG dispensers due to their relative simplicity and ability to overcome vent recovery requirements due to lower pressure storage volume operation. Pumped systems require three phase electricity, complicated cool-down procedures, power electronics and longevity is unknown. A decision has therefore been made to develop a pressure-decant LNG dispenser without such requirements. Pressure-decant systems utilise the pressure within the first fluid storage volume to overcome the vehicle tank pressure during refuel. This pressure can advantageously be easily controlled with the second heat exchanger disclosed herein. Vent recovery is a challenge for pressure decant systems due to the high pressures required within the first fluid storage volume (vapour from the vehicle tank cannot simply be taken back into the first fluid storage volume headspace as per pumped system utilising low vessel storage pressures). Vapour recovery may be conducted by the cooling of the third storage volume with pressure-controlling coolant coil. This reduces the pressure in the remote storage vessel and permits pressure-decant filling to occur. In a less preferred embodiment, a large coil within the first fluid storage volume may be used to collapse the first fluid storage volume pressure and create the pressure differential to allow vapours to flow from the remote storage vessel into the primary storage vessel. The vessel will then require pressure raising prior to filling.

It is expected that very few vehicles require vent recovery prior to filling. The vent recovery system works on the principle of condensation of on-board vapours in a condenser sump (third fluid storage volume). A pressure transducer shall be used to determine that a vehicle of high tank pressure has been connected. Upon initiation of the fill, the condenser shall be cooled by flowing LN2 through. A temperature element can regulate LN2 flow, and ensure heat exchanger efficiency. Upon reduction of the pressure of the on-board tank (indicated by the pressure sensor) to below vent recovery set-point, the fill shall commence.

In some cases, vapour recovery may be required prior to fill in order to reduce on-board storage pressures to such an extent where PD fill process may take place. Vapour recovery can be completed by condensing vapours in a condensing vessel (third fluid storage volume), by utilizing liquefied nitrogen from the second fluid storage volume as a total-loss coolant. This condensing vessel may be located in the LNG receiver, as part of the Dispenser (Static Installations) or the ISO Skid (transportable installations). Vapour condensed in the vessel/exchanger is returned to the LNG storage vessel using an automated valve arrangement and natural pressure-raise from heat in-leak. Preferably the dispensing duct includes a mass flow meter, the system further comprising a thermosiphon duct for cooling the mass flow meter extending from an outlet of the first fluid storage volume to an inlet of the first fluid storage volume, wherein said inlet is above said outlet. The arrangement of the inlet and outlet encourages a thermosiphon of cooled LNG which reduces the temperature of the MFM. This can keep the temperature of the MFM within desired limits to maintain accuracy.

The proposed pressure-decant system has an ability to cool the mass flow meter (MFM) quickly prior to filling. MFMs require cooling and quality (minimal gas-phase) liquid flow prior to the vehicle fill valve opening in order to guarantee accuracy and reliability. The PD system also has the ability to minimise liquid boil-off during fills, and prevent venting of vapour to atmosphere.

According to a further aspect there is provided a system for dispensing cryogenically cooled pressurised fluid, the system comprising:

the vacuum insulated storage vessel of any of the preceding claims;

a dispensing duct in fluid communication with an outlet of the first fluid storage volume for connection to a remote storage vessel to be filled;

wherein the dispensing duct includes a mass flow meter, the system further comprising a thermosiphon duct for cooling the mass flow meter, the thermosiphon duct extending from an outlet of the first fluid storage volume to an inlet of the first fluid storage volume, wherein said inlet is above said outlet.

The LNG Dispenser system described herein is fundamentally based upon the "LIN Assist" system disclosed in WO2013/102794. That is, the system includes means to sub-cool liquid in the bottom of a cryogenic vessel. However, previous designs of PD dispenser have suffered with high Liquefied Nitrogen consumption due to procedural vaporisation of LNG due to poor process design. Sub-cooled liquid is required as on-board storage vessels are only fitted with top-fill capability and liquid warms during the fill. Vehicles require specific product conditions; too cold and the vehicle is unable to operate until the LNG warms, LNG too warm results in reduced range, poor filling, and the potential of venting from the on-board vessel. LIN Consumption is related to the rate of LNG consumption at the particular installation, and has ranged from 30-150% LIN:LNG by mass. The causes of this High LIN consumption were found to be:

• Procedural heat in-leak through use of external heat exchangers working upon the principle of head-pressure;

· The requirement to cool-down the mass-flow meter using thermosiphon action prior to fill;

• 'Normal' heat in-leak from the vessel and pipework;

• Poor temperature control at the base of the vessel, and inaccuracy in

temperature measurement;

· Gas-locking of pipework and inefficient thermosiphon action;

• Procedural cooling and vaporisation of LNG within the condenser (positioned within the 'normal' thermosiphon flow path). The present apparatus serves to address at least some of these disadvantages, In particular, it permits 'No loss' dispensing of conditioned Liquefied Natural Gas (LNG) to vehicles including vapour recovery (VR), and pressure control (PC) through a Pressure Decant (PD) Method for no vent operation and increased hold times of road/sea going LNG storage/transportation vessels.

The invention will now be described in relation to the following non-limiting figures, in which: Figure 1 shows an ISO pressurised gas container suitable for use as the vessel described herein.

Figure 2 shows a schematic of the vessel described herein. Figures 3a and 3b show alternative schematics of systems for dispensing cryogenically cooled pressurised fluid.

As shown in Figure 2 there is provided a storage vessel 1 for holding cryogenically cooled pressurised fluids. The vessel 1 is made of steel and is insulated, having an insulated outer wall 5.

The vessel 1 contains a first volume 8 divided from a second volume 9 by a partitioning wall 10. The second volume 9 contains compressed nitrogen which can form a gaseous coolant layer 1 1 and liquid coolant layer 12. The first volume 8 contains the fuel source for dispensing such as LNG. These can form a gaseous fuel layer 16 and a liquid fuel layer 17. The first volume 8 has an inlet 19 for refilling. The second volume 9 has an inlet 18 for refilling.

First and second heat exchangers 20, 25 are provided in the first volume 8, spaced apart from one another. The heat exchangers 20, 25 may be conditioning coils. The first heat exchanger 20 may be submerged the liquid fuel layer 17. This is connected to the second volume 12 by duct 30. The second heat exchanger 25 may be located above the liquid fuel layer 17 in the gaseous fuel layer 16. This is connected to the second volume 12 by duct 40.

The first heat exchanger 20 is located more closely to the outlet 55 than is the second heat exchanger 25. The first heat exchanger 20 may be connected to the second heat exchanger 25 by duct 50 to reuse the coolant. Duct 50 thereby provides a flow path from the second volume 12 via the duct 30, the first heat exchanger 20, the second heat exchanger 25 and outlet 45 through the first volume. The use of this flow-path, rather than simply from the second volume 12 via the duct 30, the first heat exchanger 20, to the outlet 35, is controlled depending on the cooling required for the gas in the ullage to control the pressure. Waste coolant is lost through outlets 35 and 45 as appropriate.

Subcooled fuel, such as LNG, may be dispensed through outlet 55 and coolant, such as liquid nitrogen, can be dispensed through outlet 60 for use as described below.

Figure 3a shows a system 100 for dispensing cryogenically cooled pressurised fluid. The system 100 comprises the vacuum insulated storage vessel 1 discussed herein. The system 100 is for providing a remote storage vessel 105, such as a car fuel tank, with a fluid held in the first storage volume 8.

The system 100 comprises a third fluid storage volume 1 10 comprising a further heat exchanger 1 15. The further heat exchanger 1 15 is connected to a coolant duct 120 connected to an outlet of the second fluid storage volume 9. Spent coolant from the further heat exchanger 1 15 is vented from outlet 125.

The system 100 comprises a dispensing duct 130 for connecting the first fluid storage volume 8 to the remote storage vessel 105. The dispensing duct 130 comprises a mass flow meter 135 for measured dispensing of the LNG. The mass flow meter 135 is in thermal contact with a thermosiphon duct 140 from the first fluid storage volume for cooling the mass flow meter 135. The thermosiphon duct 140 extends from an outlet 141 of the first fluid storage volume 8 to an inlet 142 of the first fluid storage volume 8. The inlet 142 is above said outlet 141 .

A venting duct 145 is provided for connecting the remote storage vessel 105 to the third fluid storage volume 1 10. The third fluid storage volume 1 10 may be connected to the first fluid storage volume by a return duct 150. However, this is optional as shown in Figure 3b.

In the system shown in Figure 3b, there is only a single supply 141 , 140 to the mass flow meter 135 which communicates with both the dispensing duct 130 and the inlet 142. The dispensing duct 145 is in communication with a pressure sensor (not shown) for detecting excess pressure in the remote storage vessel 105. If excess pressure is detected then dispensing is not initiated. Instead, coolant 12 from the second fluid storage volume 9 is passed to the further heat exchanger 1 15 via the coolant duct 120. This cools the third fluid storage volume 1 10. While the third fluid storage volume 1 10 is in fluid communication with the remote storage volume 105, gas in said volume 105 will drop in pressure as LNG is condensed into the third storage volume 1 10 by the cooled heat exchanger 1 15. This condensed LNG can be returned to the first fluid storage volume 9 via the return duct 150. This will optionally pass through a mass flow meter for accurate metering.

Once the pressure sensor (not shown) indicates that there is a sufficiently low pressure in the remote storage vessel 105, pressure dispensing of LNG into the remote storage vessel 105 can commence. The pressure dispensing can be controlled using the second heat exchanger 25 to regulate pressure in the first storage volume 8. Before dispensing commences, the mass flow meter 135 is cooled by the thermosiphon duct 140 passing cooled LNG past the mass flow meter 135, preferably returning via duct 142. Preferably, thermosiphon duct 140 passes through the mass flow meter 135. As discussed herein, the particular advantages associated with the novel system are reduced cost and complexity, coupled with reduced LIN consumption. This is at least in part due to the possibility of eliminating the condenser from the fill process-pipework, improved process heat in-leak and in-tank (rather than remote) heat exchangers. Indeed, LIN:LNG consumption of 5% is achievable.

Although preferred embodiments of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the scope of the invention or of the appended claims. In particular, other cryogenic fluids can be employed in addition to LIN and LNG. For example, liquid air may be used in place of LIN for the cooling. Vent control of alternative cryogenic fluids, such as ethylene, argon or liquid air, may also be accomplished.