System and method for vaporizing liquefied natural gas for measurement

Abstract

1. A method for vaporizing liquefied natural gas (LNG) to determine its constituents, the method comprising: receiving LNG from a main pipeline into a pressurizer; pressurizing the LNG above its critical pressure using the pressurizer; directing a first portion of the pressurized LNG to a heater; heating the first portion of the pressurized LNG above its critical temperature using the heater; directing the pressurized and heated LNG to a vaporizer; and reducing the pressure of the heated LNG to a pressure below the critical pressure using the vaporizer to vaporize it.

Description

[Technical Field]

[0001] The present invention relates to a system and method for vaporizing liquefied natural gas (LNG) for sampling, and more particularly to a system and method for vaporizing a sample of an LNG cargo so that the composition of the sample and, therefore, the cargo, can be accurately determined. [Background technology]

[0002] LNG is typically sold on a British thermal unit (BTU) basis, so it is crucial for LNG suppliers and buyers to know exactly what the BTU value of their LNG cargo is, as well as the composition of the cargo.

[0003] To estimate the expected selling price of a particular LNG cargo, a supplier might identify the BTU value of the cargo when it is loaded onto a tanker. Complicating matters, the BTU value of an LNG cargo may fluctuate during transit (e.g., due to vaporization) and may not be the same value when the cargo arrives at the buyer. Therefore, a buyer of an LNG cargo may also want to identify the BTU value of the LNG cargo when it is unloaded from the tanker to avoid overpaying for the LNG cargo. Tanker operators may also be interested in BTU value fluctuations, especially since they often burn vaporized LNG during transit and are financially responsible for the amount burned.

[0004] Known methods for estimating the BTU value and constituents of LNG involve taking samples, often intermittently or continuously, from the cargo as it is unloaded from the tanker, and then vaporizing and analyzing at least a portion of the sample to identify the BTU value and constituents of the sample to estimate these properties of the LNG cargo. However, such methods are not sufficiently accurate.

[0005] One reason for the inaccuracy is due to the fact that LNG consists of a variety of different hydrocarbon compounds, each with different properties, such as different boiling points. Therefore, at certain pressures and temperatures, LNG is vulnerable to fractionation, where its components exist in different phases (i.e., liquid and gas). Therefore, inaccuracies can occur if, for example, part of an LNG sample being tested by gas chromatography vaporizes prior to the actual vaporization stage of the sampling system.

[0006] A common approach to avoid premature partial vaporization of an LNG sample is to use the shortest and smallest possible diameter lines and pipes and insulate them to help maintain a subcooled LNG sample before it reaches the vaporizer, but such an approach is not sufficiently reliable for maintaining a subcooled LNG sample, leaving the LNG sample vulnerable to partial vaporization prior to the vaporization stage.

[0007] Another common approach is to rapidly heat an LNG sample in a vaporizer to vaporize it. This process is intended to instantly heat an LNG sample that is in an all-liquid phase to an all-vapor phase. However, the process of heating an LNG sample is usually not instantaneous, and the sample passes through a mixture of liquid and vapor phases during this process. This approach results in the sample fractionating.

[0008] These approaches typically 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 cargo from which the LNG sample was taken.

[0009] Given that LNG cargoes can be worth tens of millions of dollars, even small uncertainties or inaccuracies in the measurement of LNG samples can have a significant impact on the price of an LNG cargo and therefore on the bottom line for LNG suppliers, purchasers, distributors and end users.

[0010] There is a need to address the above and / or at least provide a useful alternative. Summary of the Invention

[0011] According to a first aspect of the present invention, there is provided a method for vaporizing liquefied natural gas (LNG) to determine its constituents, the method comprising: receiving LNG from a main pipeline into a pressurizer; pressurizing the LNG above its critical pressure by the pressurizer; directing a first portion of the pressurized LNG to a heater; heating the first portion of the pressurized LNG above its critical temperature by the heater; directing the pressurized and heated LNG to a vaporizer; and reducing the pressure of the heated LNG by the vaporizer to a pressure below the critical pressure so as to vaporize the LNG.

[0012] In an embodiment of the invention, the vaporizer includes a regulator, and the pressurized and heated LNG is depressurized as it leaves the regulator.

[0013] In an embodiment of the present invention, the regulator includes a pressure control valve.

[0014] In an embodiment of the invention, the method further comprises directing the vaporized LNG to a downstream measurement system, the downstream measurement system configured to measure the constituents of the vaporized LNG.

[0015] In an embodiment of the invention, the method further comprises directing the vaporized LNG to a downstream device to collect a representative sample of the LNG.

[0016] In an embodiment of the invention, the step of receiving the LNG into the pressurization device comprises receiving the LNG at a temperature and pressure below the critical temperature and critical pressure, respectively.

[0017] In an embodiment of the present invention, the step of receiving the LNG into the pressurization device includes receiving the LNG at a temperature of approximately -160°C and a pressure of approximately 1 to 4 Barg.

[0018] In an embodiment of the invention, the step of pressurizing the LNG comprises pressurizing the LNG to approximately 80 Barg such that the pressure of the LNG exceeds the critical pressure.

[0019] In an embodiment of the invention, the step of heating the first portion of LNG comprises heating the first portion of LNG to a temperature between approximately -10°C and approximately 45°C such that the temperature of the LNG exceeds the critical temperature.

[0020] In an embodiment of the invention, the step of depressurizing the first portion of LNG comprises reducing the pressure of the first portion of LNG to approximately 4 Barg so that the pressure of the first portion of LNG is below the critical pressure, thereby vaporizing the LNG.

[0021] In an embodiment of the invention, the method further comprises returning a second portion of the pressurized LNG from the pressurizer to the main pipeline.

[0022] In an embodiment of the invention, the method further comprises returning vaporized LNG from the pressurization device to the main pipeline.

[0023] In an embodiment of the invention, the method further comprises checking that the LNG has been pressurized above the critical pressure before directing the first portion of the LNG to the heater, and returning the LNG to the main pipeline if the LNG output from the pressurization device is not above the critical pressure.

[0024] In an embodiment of the invention, the method further comprises priming the pressurization device with LNG received from the main pipeline and returning the LNG to the main pipeline.

[0025] According to a second aspect of the present invention, there is provided a system for vaporizing LNG to measure its constituents, the system including: a pressurizer for receiving LNG to be measured from a main pipeline, the pressurizer being configured to pressurize the LNG above its critical pressure; a heater for heating a first portion of the pressurized LNG from the pressurizer above its critical temperature; and a vaporizer for reducing the pressure of the pressurized and heated LNG from the heater to a pressure below the critical pressure so as to vaporize it.

[0026] In an embodiment of the invention, the vaporizer includes a regulator configured such that the LNG is reduced in pressure to below the critical pressure when it leaves the regulator.

[0027] In an embodiment of the present invention, the regulator includes a pressure control valve.

[0028] In an embodiment of the invention, the system further comprises a downstream measurement system configured to measure the constituents of the vaporized LNG.

[0029] In an embodiment of the invention, the system further comprises a downstream device configured to collect a representative vapor sample of the LNG to be measured.

[0030] In an embodiment of the invention, the pressurization device is configured to receive the LNG at a temperature and pressure below the critical temperature and pressure, respectively.

[0031] In an embodiment of the present invention, the pressurization device is configured to receive the LNG at a temperature of approximately -160°C and a pressure of approximately 1 to 4 Barg.

[0032] In an embodiment of the invention, the pressurization device is configured to pressurize the LNG to approximately 80 Barg so that the pressure of the LNG exceeds the critical pressure.

[0033] In an embodiment of the invention, the heater is configured to heat the first portion of the pressurized LNG to a temperature between approximately -10°C and approximately 45°C such that the temperature of the first portion of LNG exceeds the critical temperature.

[0034] In an embodiment of the invention, the vaporizer is configured to reduce the pressure of the first portion of LNG output from the heater to approximately 4 Barg so that the pressure of the first portion of LNG is below the critical pressure, thereby vaporizing the LNG.

[0035] In an embodiment of the invention, the system is configured to return a second portion of the pressurized LNG from the pressurizer to the main pipeline.

[0036] In an embodiment of the invention, the system is configured to return vaporized LNG from the pressurizer to the main pipeline.

[0037] In an embodiment of the invention, the system further includes a pressure control system configured to monitor the pressure of the LNG output from the pressurizer, and if the pressure of the output LNG is lower than the critical pressure, return the LNG output from the pressurizer to the main pipeline, and if the pressure of the output LNG is higher than the critical pressure, direct the first portion of the LNG output from the pressurizer to the heater and return a second portion of the LNG output from the pressurizer to the main pipeline.

[0038] In an embodiment of the invention, the system is configured such that the pressurization device is primed with LNG withdrawn from the main pipeline, which LNG is then returned to the main pipeline. [Brief explanation of the drawings]

[0039] In order that the present invention may be more readily understood, embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0040] [Figure 1] 1 is a piping and instrumentation diagram (P&ID) of a system embodying the present invention. [Figure 2] FIG. 2 is an enlarged view of the left side of the P&ID in FIG. 1. [Figure 3] FIG. 2 is an enlarged view of the right side of the P&ID in FIG. 1. [Figure 4] 1 is a P&ID of a system embodying the present invention. [Figure 5] FIG. 5 is an enlarged view of the left side of the P&ID in FIG. 4. [Figure 6] FIG. 5 is an enlarged view of the right side of the P&ID in FIG. 4. DETAILED DESCRIPTION OF THE INVENTION

[0041] Figures 1-6 are piping and instrumentation diagrams (P&IDs) illustrating one embodiment of the present system 200 and method. The P&IDs are prepared in accordance with standardized P&ID symbols and notations, as detailed in standards such as ISA S5.1, ISO 10628, and ISO 14617. The P&IDs of Figures 4-6 are modified versions of the P&IDs of Figures 1-3, respectively, to include symbols.

[0042] In the illustrated embodiment, LNG is drawn from a main pipeline 202 and channeled through vacuum insulated hoses 204 to a pressurization system 206. In the illustration, pressurization system 206 includes a vacuum insulated sump pump 208 and a pump, such as a cryogenic pump 210.

[0043] Advantageously, LNG is extracted from the main pipeline 202 at a sufficient rate and static pressure (approximately 1-4 Barg) so that the energy of the LNG is available to prime the cryogenic pump 210. Connected to the cryogenic pump 210 is a valve 212 that can be opened intermittently to vent vapor trapped in the cryogenic pump 210, thereby assisting the priming process. The LNG used to prime the pressurization device 206 is recycled back to the main pipeline 202. In an alternative embodiment of the invention, the LNG may be recycled back to the main pipeline 202 by an eductor (not shown).

[0044] Once the cryogenic pump 210 is primed, the LNG is immediately pressurized by the cryogenic pump 210. The cryogenic pump 210 is configured to pressurize the LNG above its critical pressure, preferably to a pressure of approximately 80 Barg. It is envisioned that the pressurization will occur at a relatively stable and controlled temperature. To this end, the temperature element 214 is configured to monitor the seal temperature, and if the seal temperature drops below ambient temperature, it indicates an LNG leak and provides a signal to shut down the system 200.

[0045] After the LNG is pressurized to above critical pressure by cryogenic pump 210, at least a first portion of the LNG is output through insulated pipeline 216 toward heater 218. A second, typically substantially larger, portion of the pressurized LNG is returned to the main pipeline via path 228 of system 200. Thus, when system 200 is operating, the pressurized LNG output from pressurizer 206 is continuously recirculated back to main pipeline 202.

[0046] Before the first portion of pressurized LNG enters heater 218, it is important to ensure that the pressurized LNG has not experienced a pressure loss and is still at the desired pressure of approximately 80 Barg, or at least above the critical pressure. To this end, a pressure control loop 220 is located downstream of cryogenic pump 210 and upstream of heater 218. Pressure control loop 220 includes a pressure indicating transmitter 222. Pressure indicating transmitter 222 monitors the pressure of the LNG output from cryogenic pump 210 and communicates with a pressure control valve 224. Pressure control valve 224 opens or closes based on the pressure reading from pressure indicating transmitter 222 to ensure that the LNG entering heater 218 is at or above the minimum desired pressure of 80 Barg. If the LNG pressure is too low, pressure control valve 224 closes, allowing the LNG pressure to increase to the desired pressure. If the LNG is above the desired pressure, pressure control valve 224 is configured to open, allowing LNG to flow into main pipeline 202 to maintain the pressure at the desired pressure and avoid over-pressurization in system 200. In this manner, pressure control loop 220 ensures that only LNG above the critical pressure enters heater 218. In this manner, there is little to no risk of fractionation of the pressurized LNG in heater 218 prior to heating therein.

[0047] The LNG is conditioned as it enters the heater 218 by an adjustable restricted orifice and capillary. Heat is added to the pressurized LNG in the heater 218 to heat the LNG sample above its critical temperature. In this manner, the LNG output from the heater 218 is pressurized above its critical pressure and heated above its critical temperature without entering a mixed liquid-gas phase. The heated and pressurized LNG is then ready to be rapidly vaporized by reducing its pressure below its critical pressure, as described below.

[0048] The heated and pressurized LNG is output from heater 218 to vaporizer 226, where it may be depressurized to a pressure below the critical pressure so that the LNG rapidly vaporizes. The term "vaporizer" should be understood to refer to any device or devices in system 200 configured to effect the depressurization of the LNG output from heater 218. In the illustrated embodiment, the vaporizer includes a regulator, shown in the form of pressure control valve 226 of system 200. As the LNG exits pressure control valve 226, the pressure of the LNG is reduced to approximately 4 Barg, below the critical pressure, thereby vaporizing the LNG. Advantageously, this low pressure is suitable for downstream systems and devices used to measure the constituents of the vaporized LNG because, having been pressurized above the critical pressure and heated above the critical temperature prior to vaporization, the LNG is not vulnerable to fractionation during the vaporization process. In this manner, the resulting vapor gas constitutes a sample whose composition accurately reflects the composition of the LNG cargo. In this way, downstream measurement and analysis of vapor gas samples provides accurate compositional information of the LNG cargo.

[0049] 1, it is generally preferred to locate the pressurizer 206 close to the main pipeline 202 to minimize pressure losses that may occur when transferring the LNG to the cryogenic pump 210. Pressure losses can result in inadvertent boiling of the LNG before it reaches the pressurizer 206. Additional measures, such as shortening the length and / or diameter of the interconnecting piping, incorporating an impact probe, etc., can help reduce pressure losses.

[0050] Advantageously, when LNG is pressurized above its critical pressure, the pressurized LNG is less susceptible to boiling or fractionation. Thus, downstream components in system 200, such as heater 218 and vaporizer 226, may be located relatively more distally from LNG main pipeline 202 and / or in more customized locations, as needed. Thus, system 200 may be better adapted to different locations and / or vessel configurations and layouts.

[0051] Many modifications to the above-described embodiment will be apparent to those skilled in the art without departing from the scope of the present invention. For example, the pressure and temperature may be varied as needed, provided that pressurizer 206 pressurizes LNG above its critical pressure and heater 218 heats the pressurized LNG above its critical temperature. Similarly, the specific configurations and arrangements shown in the drawings are merely exemplary and may be varied without departing from the scope of the present invention to achieve vaporization of LNG with little to no possibility of inadvertent fractionation during vaporization.

[0052] Throughout this specification and the claims that follow, unless the context requires otherwise, the word "comprise" and variations such as "comprises" and "comprising" will be understood to mean the inclusion of a stated integer or step or group of integers or steps and not to the exclusion of other integers or steps or groups of integers or steps.

[0053] Reference herein to any prior publication (or information derived therefrom) or known matter is not, and should not be taken as, an acknowledgment or admission or in any way suggestion that the prior publication (or information derived therefrom) or known matter forms part of the general general knowledge in the field of endeavor to which this application pertains.

[0054] <Parts list> The P&ID incorporates item labels, tag numbers and codes to reference the equipment, devices and other features of embodiments of the present invention, as summarized in the table below.

[0055] [Table 1]

Claims

1. 1. A method for vaporizing liquefied natural gas (LNG) for measuring its constituents, comprising: receiving LNG from a main pipeline at a temperature of approximately -160°C and a pressure of approximately 1 to 4 Barg into a pressurization system comprising a pump; pressurizing the LNG above its critical pressure with the pressurizing device; directing a first portion of the pressurized LNG to a heater; heating the first portion of pressurized LNG above its critical temperature with the heater; directing the pressurized and heated LNG to a vaporizer without entering a liquid-gas mixture; and reducing the pressure of the heated LNG to below the critical pressure by the vaporizer so as to vaporize the heated LNG.

2. The method of claim 1 , wherein the vaporizer includes a regulator, and the pressurized and heated LNG is depressurized upon exiting the regulator.

3. The method of claim 1 or 2, further comprising directing the vaporized LNG to a downstream measurement system, the downstream measurement system configured to measure the constituents of the vaporized LNG.

4. 4. The method of any one of claims 1 to 3, wherein the step of pressurizing the LNG comprises pressurizing the LNG to approximately 80 Barg such that the pressure of the LNG exceeds the critical pressure.

5. 5. The method of claim 1, wherein the step of heating the first portion of LNG comprises heating the first portion of LNG to a temperature between about -10°C and about 45°C such that the temperature of the LNG exceeds the critical temperature.

6. 6. The method of any one of claims 1 to 5, wherein depressurizing the first portion of LNG comprises reducing the pressure of the first portion of LNG to approximately 4 Barg so that the pressure of the first portion of LNG is below the critical pressure, thereby vaporizing the LNG.

7. The method of any one of claims 1 to 6, further comprising returning a second portion of the pressurized LNG from the pressurizer to the main pipeline.

8. The method of any one of claims 1 to 7, further comprising returning vaporized LNG from the pressurization device to the main pipeline.

9. checking that the LNG has been pressurized above the critical pressure before directing the first portion of the LNG to the heater; 9. The method of claim 1, further comprising: returning the LNG output from the pressurizer to the main pipeline if the pressure of the LNG is not above the critical pressure.

10. 10. The method of any one of claims 1 to 9, further comprising priming the pressurization device with LNG received from the main pipeline and returning the LNG to the main pipeline.

11. 1. A system for vaporizing LNG to measure its constituents, comprising: a pressurization system comprising a pump for receiving metered LNG from a main pipeline at a temperature of approximately -160°C and a pressure of approximately 1 to 4 Barg, the pressurization system being configured to pressurize the LNG above its critical pressure; a heater for heating a first portion of the pressurized LNG from the pressurization device above its critical temperature; a vaporizer for reducing the pressure below the critical pressure so as to vaporize the pressurized and heated LNG from the heater without entering a liquid-gas mixture phase.

12. The system of claim 11 , wherein the vaporizer includes a regulator configured to reduce the LNG pressure below the critical pressure as the LNG exits the regulator.

13. The system of any one of claims 11 to 12, further comprising a downstream measurement system configured to measure the constituents of the vaporized LNG.

14. A system according to any one of claims 11 to 13, wherein the pressurising device is configured to pressurise the LNG to approximately 80 Barg so that the pressure of the LNG exceeds the critical pressure.

15. 15. The system of any one of claims 11 to 14, wherein the heater is configured to heat the first portion of the pressurized LNG to a temperature between approximately -10°C and approximately 45°C such that the temperature of the first portion of the LNG exceeds the critical temperature.

16. 16. The system of claim 11, wherein the vaporizer is configured to reduce the pressure of the first portion of LNG output from the heater to approximately 4 Barg so that the pressure of the first portion of LNG is below the critical pressure, thereby vaporizing the LNG.

17. The system of any one of claims 11 to 16, wherein the system is configured to return a second portion of the pressurized LNG from the pressurizer to the main pipeline.

18. The system of any one of claims 11 to 17, wherein the system is configured to return vaporized LNG from the pressurizer to the main pipeline.

19. monitoring the pressure of the LNG output from the pressurizing device; If the pressure of the output LNG is lower than the critical pressure, the LNG output from the pressurizing device is returned to the main pipeline; 19. The system of any one of claims 11 to 18, further comprising a pressure control system configured to direct the first portion of the LNG output from the pressurizer to the heater and return a second portion of the LNG output from the pressurizer to the main pipeline if the pressure of the output LNG is higher than the critical pressure.

20. 20. A system as claimed in any one of claims 11 to 19, configured to prime the pressurisation device with LNG withdrawn from the main pipeline, which LNG is then returned to the main pipeline.

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