System and method for vaporizing liquefied natural gas for its measurement

Abstract

A method of vaporizing liquified natural gas (LNG) to measure its constituent components includes receiving LNG from a main line in a pressurization device, pressurizing the LNG beyond its critical pressure by the pressurization device, directing a first portion of the pressurized LNG to a heater, heating the first portion of the pressurized LNG beyond its critical temperature by the heater, directing the pressurized and heated LNG to a vaporization device, and depressurizing the heated LNG to a pressure below the critical pressure by the vaporization device to vaporize the LNG.

Description

TECHNICAL FIELD

[0001] The present invention relates to systems and methods for vaporising liquefied natural gas (LNG) for sampling. In particular, the present invention relates to systems and methods for vaporising a sample of an LNG shipment so that the composition of the sample, and hence the composition of the shipment, can be accurately determined. BACKGROUND

[0002] LNG is typically sold on the basis of its British thermal unit (BTU) value. It is therefore important for suppliers and purchasers of LNG to accurately know the BTU value of any LNG shipment, and its constituent components.

[0003] To estimate the expected sale price of a particular LNG shipment, a supplier can determine the BTU value of the shipment when it is loaded onto a tanker. The complicating factor is that the BTU value of an LNG shipment can change during transportation (for example, due to vaporisation), and so the BTU value can be different once the shipment reaches the buyer. The buyer of the LNG shipment can therefore also want to determine the BTU value of the LNG shipment when it is unloaded from the tanker, to avoid paying an excessive price for the LNG shipment. The operator of the tanker can also be interested in any change in the BTU value, particularly as they often burn the LNG that has vaporised during transportation and so have a financial responsibility for the amount that is burned.

[0004] Known methods for estimating the BTU value and constituent components of LNG include taking samples from the shipment (often intermittently or continuously) as it is being unloaded from the tanker. At least some of these samples are then vaporised and analysed to determine the BTU value and constituent components of the samples, and hence to infer the properties of the LNG shipment. However, such methods are not sufficiently accurate.

[0005] One reason for the inaccuracy is caused by the fact that LNG is made up of a range of different hydrocarbons, each of which has different properties, such as different boiling points. LNG is therefore susceptible to fractionation at particular pressures and temperatures, such that its components exist in different phases (that is, in liquid and gas phases). Inaccuracy can therefore occur if the part of the LNG sample that is being tested (for example, by gas chromatography) vaporises before the actual vaporisation stage of the sampling system.

[0006] Common methods that attempt to avoid premature partial evaporation of the LNG sample include using pipes and tubing that are as short as possible and as small in diameter as possible, and insulating them to help keep the LNG sample in a subcooled state before it reaches the vaporiser. However, such methods are not sufficient to reliably keep the LNG sample in a subcooled state, thereby making the LNG sample susceptible to partial evaporation before the vaporisation stage.

[0007] Another common method includes vaporising the LNG sample by rapidly heating the sample in a vaporiser. This process is intended to instantaneously heat the LNG sample, which is in a full liquid phase, to a full gas phase. However, this process of heating the LNG sample is typically not instantaneous, and therefore, when this process is performed, the sample transitions through a mixed liquid-gas phase. This method causes fractionation of the sample.

[0008] These methods often result in test results that do not accurately reflect the composition of the LNG sample, and therefore, do not accurately reflect the composition of the LNG transport cargo from which the LNG sample was taken.

[0009] Given that LNG transport cargos can be worth tens of millions of dollars, even small uncertainties and inaccuracies in the measurement of LNG samples can have a significant impact on the price of the LNG transport cargo, and therefore, the profits of LNG suppliers, purchasers, distributors, and end consumers.

[0010] There is a need to address the above problems, and / or at least provide a useful alternative. SUMMARY

[0011] According to a first aspect of the present invention, there is provided a method of vaporising liquified natural gas (LNG) to measure its constituent components, the method comprising:

[0012] receiving the LNG from a main line in a pressurising device;

[0013] pressurising the LNG above its critical pressure by the pressurising device;

[0014] directing a first portion of the pressurised LNG to a heater;

[0015] heating the first portion of the pressurised LNG above its critical temperature by the heater;

[0016] directing the pressurised and heated LNG to a vaporising device; and

[0017] vaporising the heated LNG by the vaporising device by depressurising the heated LNG to a pressure below the critical pressure.

[0018] In embodiments of the application, the vaporization apparatus includes a regulator, and the pressurized and heated LNG is depressurized upon exiting the regulator.

[0019] In embodiments of the application, the regulator includes a pressure control valve.

[0020] In embodiments of the application, the method further includes directing the vaporized LNG to a downstream measurement system configured to measure constituent components of the vaporized LNG.

[0021] In embodiments of the application, the method further includes directing the vaporized LNG to a downstream apparatus to collect a representative sample of the LNG.

[0022] In embodiments of the application, the step of receiving the LNG in the pressurization apparatus includes receiving the LNG at a temperature and pressure that are respectively below the critical temperature and critical pressure.

[0023] In embodiments of the application, the step of receiving the LNG in the pressurization apparatus includes receiving the LNG at a temperature of about -160°C and a pressure of about 1 to 4 Barg.

[0024] In embodiments of the application, the step of pressurizing the LNG includes pressurizing the LNG to about 80 Barg, such that the pressure of the LNG exceeds its critical pressure.

[0025] In embodiments of the application, the step of heating the first portion of LNG includes heating the first portion of LNG to a temperature of between about -10°C and about 45°C, such that the temperature of the LNG exceeds its critical temperature.

[0026] In embodiments of the application, the step of depressurizing the first portion of LNG includes reducing the pressure to about 4 Barg, such that the pressure of the first portion of LNG is below the critical pressure, thereby vaporizing the LNG.

[0027] In embodiments of the application, the method further includes sending the second portion of the pressurized LNG from the pressurization apparatus back to the main line.

[0028] In embodiments of the application, the method further includes sending vaporized LNG from the pressurization apparatus back to the main line.

[0029] In embodiments of the application, the method further includes:

[0030] prior to directing the first portion of LNG to the heater, checking whether the LNG has been pressurized above the critical pressure; and

[0031] if the LNG output from the pressurization apparatus is not above the critical pressure, sending the LNG back to the main line.

[0032] In embodiments of the application, the method further comprises charging the pressurizing device with LNG received from the main line, and returning the LNG to the main line.

[0033] According to a second aspect of the application, there is provided a system for vaporizing LNG for measuring its constituent components, the system comprising:

[0034] a pressurizing device for receiving LNG to be measured from a main line, the pressurizing device configured to pressurize the LNG above its critical pressure;

[0035] a heater for heating a first portion of the pressurized LNG from the pressurizing device above its critical temperature; and

[0036] a vaporizing device for depressurizing the pressurized and heated LNG from the heater to a pressure below the critical pressure, to vaporize the LNG.

[0037] In embodiments of the application, the vaporizing device comprises a regulator configured to depressurize the LNG to a pressure below the critical pressure as it exits the regulator.

[0038] In embodiments of the application, the regulator comprises a pressure control valve.

[0039] In embodiments of the application, the system further comprises a downstream measurement system configured to measure constituent components of the vaporized LNG.

[0040] In embodiments of the application, the system further comprises a downstream device configured to collect a representative vaporized sample of the LNG to be measured.

[0041] In embodiments of the application, the pressurizing device is configured to receive the LNG at a temperature and pressure below the critical temperature and pressure, respectively.

[0042] In embodiments of the application, the pressurizing device is configured to receive the LNG at a temperature of about -160°C and a pressure of about 1 to 4 Barg.

[0043] In embodiments of the application, the pressurizing device is configured to pressurize the LNG to about 80 Barg, such that the pressure of the LNG exceeds its critical pressure.

[0044] In embodiments of the application, the heater is configured to heat the first portion of the pressurized LNG to a temperature of between about -10°C and about 45°C, such that the temperature of the LNG of the first portion exceeds its critical temperature.

[0045] In an embodiment of the invention, the evaporation apparatus is configured to reduce the pressure of the first portion of LNG output from the heater to about 4 Barg, thereby reducing the pressure of the first portion of LNG below its critical pressure for evaporation.

[0046] In an embodiment of the invention, the system is configured to return a second portion of the pressurized LNG from the pressurization unit back to the main pipeline.

[0047] In an embodiment of the invention, the system is configured to return evaporated LNG from the pressurization unit to the main pipeline.

[0048] In embodiments of the present invention, the system further includes a pressure control system configured to:

[0049] Monitor the pressure of the LNG output from the pressurization unit;

[0050] If the pressure of the output LNG is lower than the critical pressure, the LNG output from the pressurization unit will be returned to the main pipeline; and

[0051] If the pressure of the output LNG is higher than the critical pressure, the first portion of the LNG output from the pressurization device will be directed to the heater, and the second portion of the LNG output from the pressurization device will be sent back to the main pipeline.

[0052] In an embodiment of the invention, the system is configured to fill the pressurization device with LNG drawn from the main pipeline and then return the LNG to the main pipeline. Attached Figure Description

[0053] To facilitate a better understanding of the present invention, embodiments will now be described by way of example only with reference to the accompanying drawings:

[0054] Figure 1 This displays a piping and instrumentation diagram (P&ID) of the system implementing the present invention.

[0055] Figure 2 yes Figure 1 A close-up view of the left side of the P&ID;

[0056] Figure 3 yes Figure 1 A close-up view of the right side of the P&ID;

[0057] Figure 4 P&ID is the system for implementing this invention;

[0058] Figure 5 yes Figure 4 A close-up view of the left side of the P&ID; and

[0059] Figure 6 is Figure 4 a close-up view of the right side of the P&ID of DETAILED DESCRIPTION

[0060] Figures 1 to 6 Piping and instrumentation diagrams (P&IDs) are shown that illustrate embodiments of the present system 200 and method. The P&IDs are made in accordance with standardized P&ID symbols and tags as detailed in standards such as ISA S5.1, ISO 10628, and ISO 14617. Figures 4 to 6 The P&IDs of Figures 1 to 3 are modified versions of the P&IDs of

[0061] In the illustrated embodiment, LNG is drawn from the main line 202 and directed through a vacuum insulated hose 204 to a pressurization device 206. In the figures, the pressurization device 206 includes a vacuum insulated tank pump 208 and a pump such as a cryogenic pump 210.

[0062] Advantageously, the LNG is drawn from the main line 202 at a sufficient velocity and static pressure (about 1 to 4 Barg) so that the energy of the LNG can be used to charge the cryogenic pump 210. Connected to the cryogenic pump 210 is a valve 212 that can be intermittently opened to vent any vapor trapped in the cryogenic pump 210 to assist the charging process. The LNG used to charge the pressurization device 206 is recirculated back to the main line 202. In alternative embodiments of the present invention, the LNG can be recirculated back to the main line 202 through an eductor (not shown).

[0063] Once the cryogenic pump 210 is charged, the LNG is pressurized by the cryogenic pump 210. The cryogenic pump 210 is configured to pressurize the LNG beyond its critical pressure, preferably to a pressure of about 80 Barg. Conceivably, the pressurization occurs at a relatively stable and controlled temperature. To this end, a temperature element 214 is configured to monitor the seal temperature, wherein if the seal temperature drops below ambient temperature, it indicates that there is a leak of LNG and provides a signal to shut down the system 200.

[0064] After the LNG is pressurized by the cryogenic pump 210 to a pressure above the critical pressure, at least a first portion of the LNG is output through an insulated line 216 to a heater 218. A second portion (typically a larger portion) of the pressurized LNG is sent back to the main line through a path 228 of the system 200. Thus, when the system 200 is operating, the pressurized LNG output from the pressurization device 206 is continuously recirculated back to the main line 202.

[0065] Before the first portion of pressurized LNG enters the heater 218, it is important to ensure that the pressurized LNG has not suffered any pressure loss and is still at the desired pressure of about 80 Barg, or at least above the critical pressure. To this end, a pressure control loop 220 is provided downstream of the cryogenic pump 210 and upstream of the heater 218. The control loop 220 includes a pressure indicating transmitter 222 which monitors the pressure of the LNG output from the cryogenic pump 210 and communicates with a pressure control valve 224. The pressure control valve 224 opens and closes based on the pressure reading of the pressure indicating transmitter 222 to ensure that the LNG entering the heater 218 is at or above the minimum desired pressure of 80 Barg. If the pressure of the LNG is too low, the pressure control valve 224 closes to allow the pressure of the LNG to increase to the desired pressure. If the LNG is above the desired pressure, the pressure control valve 224 is configured to open to allow the LNG to flow to the main line 202 to maintain the pressure at the desired pressure and avoid over-pressurization in the system 200. Thus, the pressure control loop 220 ensures that only LNG above its critical pressure enters the heater 218. In this way, any pressurized LNG in the heater 218 is at little risk of fractionating before it is heated in the heater.

[0066] The LNG is regulated as it passes through the adjustable orifice and capillary tube into the heater 218. Heat is applied to the pressurized LNG within the heater 218 to heat the LNG sample above the critical temperature. Thus, the LNG output from the heater 218 has been pressurized above the critical pressure and heated above the critical temperature, and the LNG will not enter the mixed liquid gas phase. Thus, the heated and pressurized LNG can be rapidly vaporized by reducing its pressure below the critical pressure, as described below.

[0067] The heated and pressurized LNG is output from the heater 218 to a vaporization device 226 through which the LNG can be depressurized to a pressure below the critical pressure to cause the LNG to rapidly vaporize. The term "vaporization device" should be understood to mean any one or more devices of the system 200 that are configured to effect the depressurization of the LNG output from the heater 218. In the illustrated embodiment, the vaporization device comprises a regulator, shown in the form of a pressure control valve 226 of the system 200. When the LNG exits the pressure control valve 226, the pressure of the LNG is reduced to below the critical pressure, to about 4 Barg, to vaporize the LNG. Advantageously, this lower pressure is suitable for use in downstream systems and devices that measure the composition of the vaporized LNG. Since the LNG is pressurized above its critical pressure and heated above its critical temperature before vaporization, the LNG is less susceptible to fractionation during the vaporization process. Thus, the composition of the sample of vaporized gas produced accurately reflects the composition of the LNG shipment. Accordingly, downstream measurement and analysis of the vaporized gas sample provides accurate composition information of the LNG shipment.

[0068] Reference is made to Figure 1 Preferably, the pressurization device 206 is located proximate to the main line 202 to minimize pressure losses that can occur in transferring the LNG to the cryogenic pump 210; pressure losses can cause the LNG to boil off unexpectedly before reaching the pressurization device 206. Additional measures, such as reducing the length and / or diameter of the interconnecting piping and / or including a surge probe, can help reduce pressure losses.

[0069] Advantageously, once the LNG is pressurized above its critical pressure, the pressurized LNG is less likely to boil off or fractionate; thus, downstream components in the system 200, such as the heater 218 and the vaporization device 226, can be located farther away from the LNG main line 202 and / or in a more customized arrangement, as desired. Thus, the present system 200 can be better adapted to different site and / or ship configurations and layouts.

[0070] Those skilled in the art will appreciate many modifications to the above-described embodiments without departing from the scope of the present invention. For example, the pressures and temperatures can be varied as desired, so long as the pressurization device 206 pressurizes the LNG above its critical pressure and the heater 218 heats the pressurized LNG above its critical temperature. Similarly, the specific components and their arrangement in the figures are merely exemplary and can be varied without departing from the scope of the present invention to achieve vaporization of the LNG with little likelihood of unintended fractionation during vaporization.

[0071] In this specification and the following claims, the word "comprise" and its variants are not intended to exclude other integers or steps than those mentioned in the claims. In the description and the following claims, the term "comprise" and its variants are not intended to exclude other integers or steps than those mentioned in the claims. In the description and the following claims, the term "comprising" and its variants are not intended to exclude other integers or steps than those mentioned in the claims.

[0072] The mention of any prior publication (or information derived from it), or anything of known prior art, in this specification is not, and should not be taken as, an admission that the publication (or information derived from it), or known prior art, is part of the common general knowledge in the field relevant to the present invention.

[0073] Parts list

[0074] The P&IDs include project designations, tag numbers, and drawing designations to represent the instruments, devices, and other features of the embodiments of the present invention, as shown in the following table:

[0075]

[0076]

Claims

1. A method for evaporating liquefied natural gas (LNG) to measure its composition, the method avoiding fractionation and comprising: LNG from the main pipeline (202) is received in a pressurization unit, and the LNG is at a temperature of -160°C and a pressure of 1 to 4 Barg; The LNG is pressurized beyond its critical pressure using this pressurization device; The first portion of the pressurized LNG is directed to the heater (218); The first portion of pressurized LNG is heated above its critical temperature by the heater (218), and the LNG does not enter the mixed liquid-gas phase; The pressurized and heated LNG is directed to the evaporation unit (226); The heated LNG is depressurized to below the critical pressure using the evaporation device (226) to evaporate the LNG; and The evaporated LNG is directed to a downstream measurement system configured to measure the composition of the evaporated LNG.

2. The method as described in claim 1, wherein, The evaporation unit (226) includes a regulator, and the pressurized and heated LNG is depressurized when it leaves the regulator.

3. The method as described in claim 1 or 2, wherein, The step of pressurizing the LNG includes pressurizing the LNG to 80 Barg so that the pressure of the LNG exceeds its critical pressure.

4. The method as described in claim 1 or 2, wherein, The step of heating the first portion of LNG includes heating the first portion of LNG to a temperature between -10°C and 45°C, so that the temperature of the LNG exceeds its critical temperature.

5. The method as described in claim 1 or 2, wherein, The step of depressurizing the first portion of LNG includes reducing the pressure to 4 Barg so that the pressure of the first portion of LNG is below the critical pressure, thereby evaporating the LNG.

6. The method of claim 1 or 2, further comprising returning a second portion of the pressurized LNG from the pressurization unit to the main pipeline (202).

7. The method of claim 1 or 2, further comprising returning the evaporated LNG from the pressurization unit to the main pipeline (202).

8. The method of claim 1 or 2, further comprising: Before directing the first portion of the LNG to the heater (218), check whether the LNG has been pressurized beyond the critical pressure; as well as If the LNG output from the pressurization unit is not higher than the critical pressure, the LNG is returned to the main pipeline (202).

9. The method of claim 1 or 2, further comprising filling the pressurization device with LNG received from the main pipeline (202) and returning the LNG to the main pipeline (202).

10. A system (200) for evaporating LNG to measure its composition, the system (200) avoiding fractionation and comprising: A pressurizing device is used to receive LNG to be measured from the main pipeline (202), the LNG being at a temperature of -160°C and a pressure of 1 to 4 Barg, the pressurizing device being configured to pressurize the LNG beyond its critical pressure; A heater (218) is used to heat a first portion of the pressurized LNG from the pressurization device above its critical temperature, and the LNG does not enter the mixed liquid-gas phase; Evaporation unit (226) for depressurizing the pressurized and heated LNG from the heater (218) to a pressure below the critical pressure, in order to evaporate the LNG; and A downstream measurement system configured to measure the composition of the evaporated LNG.

11. The system (200) as claimed in claim 10, wherein, The evaporation unit (226) includes a regulator configured to depressurize the LNG to a pressure below the critical pressure when it leaves the regulator.

12. The system (200) as claimed in claim 10 or 11, wherein, The pressurization device is configured to pressurize the LNG to 80 Barg, thereby exceeding the LNG's critical pressure.

13. The system (200) as claimed in claim 10 or 11, wherein, The heater (218) is configured to heat the first portion of the pressurized LNG to a temperature between -10°C and 45°C, thereby causing the temperature of the first portion of the LNG to exceed its critical temperature.

14. The system (200) as claimed in claim 10 or 11, wherein, The evaporation unit (226) is configured to reduce the pressure of the first portion of LNG output from the heater (218) to 4 Barg, thereby reducing the pressure of the first portion of LNG below its critical pressure for evaporation.

15. The system (200) as claimed in claim 10 or 11, wherein, The system (200) is configured to return a second portion of the pressurized LNG from the pressurization unit back to the main pipeline (202).

16. The system (200) as claimed in claim 10 or 11, wherein, The system (200) is configured to return the evaporated LNG from the pressurization unit to the main pipeline (202).

17. The system (200) of claim 10 or 11 further includes a pressure control system configured to: Monitor the pressure of the LNG output from the pressurization unit; If the pressure of the output LNG is lower than the critical pressure, the LNG output from the pressurization unit will be returned to the main pipeline (202); and If the pressure of the output LNG is higher than the critical pressure, the first portion of the LNG output from the pressurization device will be directed to the heater (218), and the second portion of the LNG output from the pressurization device will be sent back to the main pipeline (202).

18. The system (200) as claimed in claim 10 or 11, configured to fill the pressurization device with LNG drawn from the main line (202) and then return the LNG to the main line (202).

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