System for capturing vapors from cryogenic storage tanks

Abstract

A recovery cryostat, external to the cryogenic material storage tank, uses a cryocooler to condense the cryogenic material vapor received from the storage tank and return it to the storage tank as liquid. This process can be continuous or periodic, depending on the location of the recovery cryostat. If the recovery cryostat is located in a position where it can return liquid to the storage tank, the process can be continuous. If liquid cannot be returned to the storage tank, the liquid return line valve is closed while the cryocooler is condensing the vapor, the vapor supply line valve is closed, and then the liquid return line valve is opened to increase pressure in the recovery cryostat and drain the liquid. The storage tank is of a type that allows vapor boiled outside the storage tank to be returned to the vapor space above the liquid in the storage tank.

Description

[Technical field]

[0001] (CROSS REFERENCE TO RELATED APPLICATIONS) This application claims the benefit of priority to U.S. Provisional Patent Application No. 63 / 318,555, filed March 10, 2022, which is incorporated by reference herein.

[0002] (Technical field) This invention relates to the operation of cryogenic storage tanks for storing liquid cryogen and preventing loss of vaporized cryogen. [Background technology]

[0003] Commercially used cryogenic gases such as nitrogen, oxygen, hydrogen, and helium are often transported and stored in a liquid state at relatively low temperatures (below -175°C). Due to the low temperatures and the existence of both liquid and gas phases, special handling is required. However, due to the high density and low pressure, it is economically advantageous to handle these cryogenic substances as liquids rather than gases. The recovery methods contained herein are applicable to all cryogenic storage tanks, but are expected to be applied to hydrogen storage tanks used in refueling vehicles in the near future. Currently, hydrogen is stored on vehicles in three ways. First, hydrogen is stored in cylinders as a high-pressure gas at approximately 700 MPa (10,500 psi), second, hydrogen is stored in cylinders as a hydride at a lower pressure, and third, hydrogen is stored as a liquid at a pressure close to atmospheric pressure. The lower the pressure of the cryogenic liquid tank, the greater the density of the liquid and the lighter the weight of the tank, but the higher the cost of low temperature and the higher the cost of cooling the cryogenic liquid.

[0004] Hydrogen fuel stations that supply gas at high pressure use a pump that compresses a liquid to high pressure, then warms the liquid in a vaporizer to ambient temperature before storing it in a high-pressure gas cylinder. The liquid pump runs intermittently and warms up between uses, generating a lot of vapor as the liquid pump cools down. Hydrogen fuel stations that refill liquid storage tanks transfer the liquid through a transfer line that is vacuum jacketed. As the transfer line vaporizes the initial flow of liquid, the transfer line cools down. In both of these systems, the vapor can be returned to the top of the storage tank, which causes a pressure increase. The amount of pressure increase depends on the amount of vapor relative to the size of the tank, the fraction of liquid in the tank, etc. These systems must be designed and operated within limits that do not release hydrogen unnecessarily.

[0005] These cryogen storage tanks typically operate with only the vapor of the cryogen in the headspace above the liquid, and the vapor and liquid are in equilibrium. Specifically, the temperature and pressure of the top layer of liquid adjacent to the vapor are at their saturation temperature and pressure relative to the pressure of the vapor above the liquid. As the cryogenic liquid cools, it becomes denser and the liquid stratifies, so the coldest liquid is at the bottom. Similarly, the hottest vapor is at the top of the tank. Sometimes, for operational purposes, additional gas (vapor) is admitted above the liquid to temporarily increase the pressure in the tank. When the additional gas is admitted, some of the vapor condenses and becomes the top layer of liquid, changing the temperature of the top layer of liquid to the higher saturation temperature, while the bottom layer of liquid remains at approximately its original temperature, making it more subcooled for the higher pressure.

[0006] During storage, a small amount of heat (thermal leakage) can penetrate the tank's insulation, vaporizing some of the liquid cryogen, causing the pressure inside the tank to increase. Cryogenic storage tanks that transport liquid cryogen are sealed during transport and allow pressure to increase until cryogen is dispensed or the tank is vented through a pressure relief valve. This is also the case for storage tanks that have an intermittent supply of cryogen.

[0007] Liquid is usually supplied from a storage tank in one of three ways. The first is by running a line (tube) from the bottom of the tank to the top of the tank and forcing the liquid out by making the pressure above the liquid greater than the supply pressure. The second is by pumping the liquid out using a small pump at the bottom of the tank with a line (tube) connecting the bottom of the tank to the top of the tank. The third is by running a line at the bottom of the tank and allowing the liquid cryogen to drain by gravity or with the assistance of a pump.

[0008] The present invention applies to the second and third types of storage tanks and allows vapor generated by cooling an object outside the storage tank to be returned to the top of the tank as a gas, and then the gas can be passed to a recovery cryostat outside the storage tank to be cooled and condensed and returned to the storage tank as a liquid.

[0009] In existing systems that use liquid hydrogen storage tanks and supply hydrogen gas at high pressure, boil-off gas is recovered by cooling the liquid pump and returning the generated boil-off gas to the storage tank, and then removing the gas from the storage tank, heating it to near ambient temperature, and processing it with a high-pressure compressor. Although the volume of gas is minimized by operating the storage tank near the critical pressure, the compressor is inefficient compared to a liquid pump because the volume of gas entering the compressor at 270K and 0.6MPa is more than 100 times that of liquid entering the cooling pump at the same pressure and mass flow rate. The clearance volume in the compressor is very small, necessitating the use of a diaphragm compressor instead of a cheaper piston compressor, and the compressibility of liquid is very low compared to gas. The cryogenic recovery cryostat of the present invention can replace a gas compressor.

[0010] Cryostats designed to keep objects such as magnetic resonance imaging (MRI) magnets cold usually have a Gifford-McMahon (GM) cryogenic expander attached to the neck tube at the top of the magnet. The GM cryogenic expander cools the radiation shield at a temperature of, say, 50 K in a first stage, and recondenses the boiled helium at about 4 K in a second stage.

[0011] US Patent No. 7,434,407 describes cooling a hydrogen storage tank using a Stirling type pulse tube refrigerator, which cools the cold shield in a first stage and prevents liquid hydrogen (H2) from boiling in a second stage. Heat is transferred from the storage tank to the refrigerator by circulating helium through tubes individually wrapped around the cold shield and inner tank, cooling the helium in the tubes in the first and second stages of the refrigerator. The disclosed application is as a liquid hydrogen fuel container on board a vehicle. US Patent No. 7,165,408 describes a method of operating a liquid hydrogen storage tank designed for use in a vehicle, which minimizes the amount of gas discharged during refilling of the tank. The patent describes the changes in pressure and density over time. Summary of the Invention [Problem to be solved by the invention]

[0012] The present invention provides a cryogenic system and method for recovering gas that vaporizes as liquid cryogen flows out of a cryogenic storage tank to cool an external mass. [Means for solving the problem]

[0013] The storage tank is a type of storage tank that allows liquid to flow out of the tank by gravity or a pump, returning vaporized gas to a vapor space above the liquid in the storage tank. The system includes a recovery cryostat external to the storage tank that uses a cryocooler to condense vapor received from the storage tank and return it to the storage tank as a liquid.

[0014] This process can be performed continuously or periodically depending on the position (orientation) of the recovery cryostat. If the recovery cryostat is positioned so that liquid can be returned to the storage tank, the process can be performed continuously. If liquid cannot be returned to the storage tank, the valve on the liquid return line is closed while the cryocooler condenses the vapor, then the valve on the vapor supply line is closed and the valve on the liquid return line is opened to increase pressure in the recovery cryostat and drain the liquid.

[0015] These and other advantages are realized, for example, by a cryogenic system that condenses vapor of a cryogenic material from a cryogenic storage tank in an external recovery cryostat and returns the cryogenic material as a liquid to the storage tank. The cryogenic system includes a storage tank and a recovery cryostat connected to the storage tank. The storage tank is configured to store liquid cryogenic material and to supply the liquid cryogenic material to an external element. The storage tank is also configured to receive vapor from the liquid cryogenic material that boils when the external element and connecting lines are cooled. The recovery cryostat is configured to receive vapor from the storage tank through a gas line and is connected to a cryocooler configured to condense the vapor received from the storage tank into a liquid and return the liquid to the storage tank through a liquid line. [Brief description of the drawings]

[0016] The drawings illustrate one or more embodiments consistent with the concepts of the invention and are not intended to limit the invention, in which like elements are designated with like reference numerals.

[0017] [Figure 1] 1 shows a schematic diagram of an embodiment in which an external recovery cryostat containing a cryocooler is added to a cryogenic storage tank. Shown are lines supplying liquid to the pump, lines that allow the pump to cool before returning vapor to the storage tank, and lines connecting the vapor supply and liquid return lines to the recovery cryostat. [Diagram 2] 2 shows a schematic diagram of an embodiment in which an external recovery cryostat containing a cryocooler is added to a cryogenic storage tank. Shown are lines supplying liquid to the liquid container, lines that cool and fill the liquid container and then return vapor to the storage tank, and lines connecting the vapor supply and liquid return lines to the recovery cryostat. [Diagram 3] 3 shows a schematic diagram of an embodiment with a vapor-cooled heat shield within a cryogenic storage tank and an external recovery cryostat containing a cryocooler. Shown are the lines supplying liquid to the pump, allowing the pump to cool before returning vapor to the storage tank, and lines connecting the vapor supply line and liquid return line (through the heat shield) to the recovery cryostat. [Figure 4] FIG. 4 illustrates the steps of one embodiment of a method for recovering boiled vapor in a cryogenic system. [Diagram 5] FIG. 5 illustrates the steps of another embodiment of a method for recovering boiled vapor in a cryogenic system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Some embodiments of the present invention will be described in more detail with reference to the attached drawings in which preferred embodiments of the present invention are shown. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. These embodiments are provided to fully and completely disclose the contents of this disclosure and to allow those skilled in the art to understand the scope of the present invention. The same or similar parts in the drawings have the same reference numerals, and as a rule, the description thereof will not be repeated.

[0019] 1 shows a preferred embodiment of a cryogenic system 100 configured to store a cryogen in a storage tank and capture vapor from the storage tank. The cryogen may include any of the following: helium, hydrogen, neon, oxygen, nitrogen, and argon. All of the illustrated elements are insulated by conventional means to minimize heat capture by the cryogen from the external environment.

[0020] Cryogenic storage tank 10 contains liquid cryogen 15 and vapor cryogen 20. A cryocooler 40 is attached to the top of recovery cryostat 45 and condenses vapor 20, which flows through gas lines 50, 51 but can be shut off by gas line supply valve 32. Reference numeral 16 denotes liquid that accumulates in recovery cryostat 45 and reference numeral 21 denotes vapor above liquid 16. Liquid 16 returns to storage tank 10 through liquid lines 56, 55 but can be shut off by liquid line return valve 33 on liquid line 56.

[0021] The storage tank 10 is refilled through a line 36 from a supply trailer and a removable coupling 35. A recovery cryostat 45 can be positioned above the storage tank 10, in which case the condensed liquid 16 returns to the storage tank by gravity.

[0022] The pump 60 is used intermittently and warms up between uses. The pump 60 is provided as an example of an apparatus for use with the present invention. Before the pump 60 is turned on to pump liquid cryogen, it must cool down to the temperature of the liquid. To remove the sensible heat of the pump 60, valve 33 is closed and valves 30, 31 are opened. The liquid cryogen 15 is shown flowing by gravity through pipe 55 and valve 30 into the pump 60. If the temperature of the pump 60 is higher than that of the liquid, the sensible heat of the pump 60 will vaporize the liquid. The vapor passes through valve 31 and line 52 and branches off. Most of the vapor returns to the storage tank 10 through line 50. A portion of the vapor flows through line 51 and valve 32 into the recovery cryostat 45. The warm gas that collects at the top of the storage tank 10 increases the pressure in the tank, causing the liquid 15 to become subcooled.

[0023] Evaporative cooling continues until the pump 60 has cooled to the saturation temperature of the liquid. The pump 60 is then turned on to pump the liquid at higher pressure through the discharge line 61. As liquid is discharged from the tank 10, the vapor 20 expands, reducing the pressure and temperature. If a large amount of liquid is discharged, the pressure at the inlet to the pump 60 drops and the cryogen begins to boil, necessitating the turning off of the pump 60. If a large storage tank 10 is being discharged in batches, the pump 60 is turned off before the cryogen begins to boil. Once the pump 60 is turned off, the valve settings are returned to the positions they were in before the pump 60 was turned on.

[0024] Cryocooler 40 is designed to provide slightly more cooling than is required to match the average heat loss over a long period of time, and therefore is typically insufficient to condense a percentage of the gas produced by cooling pump 60. Cryocooler 40 is coupled or attached to cryostat 45 and cools the vapor within cryostat 45. Cryocooler 40 may be either a GM, pulse tube, Stirling, or inverse Brayton type cryocooler.

[0025] Cryocooler 40 is sized as needed to condense any gas vaporized by thermal leaks and maintain storage tank 10 at a pressure lower than that which would cause a portion of the cryogen to be vented by a safety vent valve (not shown). Recovery cryostat 45 is sized to store the condensed cryogen between intervals during which it is returned to storage tank 10.

[0026] If the recovery cryostat 45 is located in a position where it can drain the liquid back into the storage tank, then valves 32, 33 may not be required or may be in a permanently open position and the condensation process may occur continuously.

[0027] If the liquid cannot be drained back to the storage tank, valve 33 on liquid return line 56 is closed while cryocooler 40 condenses the vapor. When the conditions for returning liquid are met, valve 32 on vapor supply line 51 is closed. When the pressure in recovery cryostat 45 rises sufficiently, liquid is forced through valve 33 (a check valve). How much excess pressure is required in cryostat 45 is determined by the difference in liquid surface height and the pressure drop at the desired flow rate through valve 33 and lines 56, 55.

[0028] This step is usually performed with valves 30, 31 closed, but may be performed with one or both open. When valve 30 is open, the pressure required to directly supply liquid to an external element, such as pump 60, is less than the pressure required to return the liquid to storage tank 10. The pressure in cryostat 45 can be increased by turning off cryocooler 40, by turning on a heater (not shown) in cryostat 45, or by pressurizing cryostat 45 with the same gas as the vapor. If storage tank 10 is of the type having a pump to supply liquid, the pump is usually an impeller type capable of reverse flow of liquid when not in operation.

[0029] An example is shown, consisting of a storage tank 10 capable of holding 80,000 L of hydrogen, and a liquid pump 60. To cool the liquid pump 60 from 160 K to 28 K, which is the saturation temperature at the surface of the liquid, 2,000 kJ must be removed. To cool the pump, 4.0 kg of liquid hydrogen with a vapor volume of 1,700 L is required, assuming that the vapor is discharged from the pump 60 while the pump 60 is cooled to a temperature and saturation pressure corresponding to 28 K and 587 kPa. This is less than 3% of the volume of the storage tank 10, and the pressure rise in the tank will be less than 100 kPa if the tank is 85% filled with liquid.

[0030] The cryocooler 40 on the recovery cryostat 45 must provide enough cooling to account for heat losses in the hydrogen storage tank (typically less than 40 W for a tank of that size), condensation of the boil-off gas (2,000 kJ), and other losses in the lines and the recovery cryostat (estimated 25 W). If the liquid pump 60 is run every six hours, the load to remove 2,000 kJ of heat is 93 W, so the total load on the cryocooler 40 is about 160 W at about 28 K. Applying this example to a hydrogen refueling station, the pump cooling intervals are shorter during the day and longer at night, so the average pressure in the storage tank increases during the day and decreases at night.

[0031] Figure 2 shows an embodiment of a cryogenic system 200 with a liquid container instead of a pump. While system 100 maintains the pressure in storage tank 10 near the critical pressure, cryogenic system 200 shown in Figure 2 maintains the pressure near atmospheric pressure. Application of the system for filling / replenishing a cryogenic container, shown as liquid hydrogen container 11, is exemplified in U.S. Patent Nos. 7,434,407 and 7,165,408, both of which describe liquid hydrogen containers that can be used in vehicles.

[0032] Lines 12, 13 are vacuum insulated and connect to removable couplings 37, 38, respectively, on container 11. Container 11 is filled by opening valves 30, 31 and allowing liquid to flow into container 11 through line 55, valve 30, line 12, and coupling 37. These lines, and possibly the vapor after cooling container 11, and vapor 22 expelled by liquid 17 filling container 11, return to the top of storage tank 10 through coupling 38, line 13, valve 31, and lines 52, 50. Once container 11 is filled, valves 30, 31 are closed and the vapor returning to storage tank 10 and recovery cryostat 45 is condensed and returned to storage tank 10 as in system 100. Hydrogen can exit container 11 as a liquid through valve 14, and can enter or exit as a gas through valve 18.

[0033] FIG. 3 shows an embodiment of a cryogenic system 300, which is a variation of the cryogenic system 100. Some cryogenic storage tanks drain the vapor after boil-off vapor in line 57, which cools the cold shield to block heat leakage to the liquid. System 300 recovers the gas by forcing it out of the storage tank 10 through line 58, valve 32 and into the recovery cryostat 45. Cooling of external elements such as the liquid pump 60 and the filling container 11 is performed as described for systems 100 and 200. The difference with these systems is that while valves 30, 31 are closed, the vapor 20 leaves the storage tank 10 through lines 57, 58 instead of line 50 and flows into the recovery cryostat 45. The liquid 16 returns to the storage tank 10 as described for system 100. The liquid hydrogen alternately flows through valve 30 into the liquid hydrogen container 11 or returns as a gas through valve 31 as described for system 200.

[0034] 4 shows a flow diagram of a method 400 of using a capture cryostat 45 to capture boiled vapor in a cryogenic system. Method 400 can be used with any of the cryogenic systems 100, 200, 300 described above. Method 400 can be used in situations where the condensation process is performed continuously, with the capture cryostat 45 located in a position where liquid can be discharged and returned to a storage tank. For example, the storage tank 10 is configured to supply liquid cryogen to an external element by gravity, and the capture cryostat 45 is configured to return condensed liquid by gravity through liquid lines 55, 56 to the storage tank 10.

[0035] The method 400 includes the steps of passing vapor from the storage tank 10 through gas lines 50, 51 and collecting it via a collection cryostat 45 (S401), condensing the vapor 21 into liquid 16 using a cryocooler 40 (S402), and returning the liquid 16 to the storage tank 10 through liquid lines 55, 56 (S403).

[0036] 5 shows a flow diagram of a method 500 of using the recovery cryostat 45 to recover boiled vapor in a cryogenic system. The method 500 can be used with any of the cryogenic systems 100, 200, 300 described above. The method 500 can be used in situations where the storage tank 10 is configured to supply liquid cryogenic material to an external element. The method 500 includes the steps of closing the liquid line return valve 33 (S501), opening the gas line supply valve 32 while keeping the liquid line return valve 33 closed (S502), condensing the vapor 21 in the recovery cryostat 45 (S503), closing the gas line supply valve 32 (S504), turning off the cryocooler 40, turning on a heater (not shown) in the recovery cryostat 45, and pressurizing the recovery cryostat 45 with the same gas as the vapor. , or any one or more of the above steps (S505), and when the pressure in the recovery cryostat 45 is sufficient to return the liquid 16 to the storage tank 10 or an external element, the steps of opening the liquid line return valve 33 (S506), closing the liquid line return valve 33 (S507), stopping the pressurization of the recovery cryostat 45 by reversing the pressurization step of S505 (S508), and opening the gas supply line valve 32 (S509).

[0037] The terms and descriptions used in this specification are used for the purpose of describing the embodiments of the present invention, and are not intended to limit the scope of the present invention. It is apparent to those skilled in the art that various modifications can be made within the spirit and scope of the present invention and the embodiments described herein.

Claims

1. 1. A cryogenic system for condensing a vapor of a cryogenic material from a cryogenic storage tank in an external recovery cryostat and returning the cryogenic material as a liquid to the storage tank, comprising: a storage tank configured to store a liquid cryogenic material and supply the liquid cryogenic material to an external element, the storage tank configured to contain vapor generated by boiling of the liquid cryogenic material when the external element and connecting lines are cooled; a recovery cryostat connected to the storage tank, the recovery cryostat connected to a cryocooler configured to receive vapor from the storage tank through a gas line and to condense the vapor received from the storage tank into a liquid, and configured to return the liquid to the storage tank through a liquid line; A cryogenic system comprising:

2. The cryogenic system of claim 1 , wherein the cryogen comprises any of helium, hydrogen, neon, oxygen, nitrogen, and argon.

3. 10. The cryogenic system of claim 1, wherein the recovery cryostat is positioned above the storage tank, and the condensed liquid returns to the storage tank by gravity.

4. a supply valve on the gas line connecting the storage tank to the recovery cryostat; a return valve on the liquid line connecting the recovery cryostat to the storage tank; The cryogenic system of claim 1 further comprising:

5. The cryogenic system of claim 1 , wherein the external element is one of a liquid pump and a liquid container.

6. 10. The cryogenic system of claim 1, wherein the cryocooler is one of a GM cryogenic expander, a pulse tube, a Stirling type pulse tube, and a reverse Brayton type cryocooler.

7. 1. A method for recovering boiled vapor in a cryogenic system, comprising: The cryogenic system comprises: a storage tank configured to supply liquid cryogen to an external element by gravity, the storage tank configured to contain vapor resulting from boiling of the liquid cryogen when the external element and connecting lines are cooled; a recovery cryostat connected to a cryocooler configured to receive vapor from the storage tank through a gas line and condense the vapor received from the storage tank into a liquid, and configured to return the liquid to the storage tank by gravity through a liquid line; Equipped with The method comprises: receiving the vapor from the storage tank through the gas line to the recovery cryostat; condensing the vapor into the liquid using the cryocooler; Returning the liquid through the liquid line to the storage tank. A method comprising the steps of:

8. 8. The method of claim 7, wherein the cryogenic material is one of helium, hydrogen, neon, oxygen, nitrogen, and argon.

9. The method of claim 7 , wherein the external element is one of a liquid pump and a liquid container.

10. 8. The method of claim 7, wherein the recovery cryostat is positioned above the storage tank, and the condensed liquid returns to the storage tank by gravity.

11. a supply valve on the gas line connecting the storage tank to the recovery cryostat; a return valve on the liquid line connecting the recovery cryostat to the storage tank; The method of claim 7 further comprising:

12. 8. The method of claim 7, wherein the cryocooler is one of a GM cryogenic expander, a pulse tube, a Stirling type pulse tube, and an inverse Brayton type cryocooler.

13. 1. A method for recovering boiled vapor in a cryogenic system, comprising: The cryogenic system comprises: a storage tank configured to supply liquid cryogenic material to an external element, the storage tank configured to contain vapor resulting from boiling of the liquid cryogenic material when the external element and connecting lines are cooled; a recovery cryostat configured to receive vapor from the storage tank through a gas line having a gas line supply valve and to be connected to a cryocooler configured to condense the vapor received from the storage tank into a liquid, the recovery cryostat configured to return the liquid to the storage tank or to the external element through a liquid line having a liquid line return valve; Equipped with The method comprises: closing the liquid line return valve; keeping the liquid line return valve closed and opening the gas line supply valve; condensing the vapor in the recovery cryostat; Close the gas line supply valve; pressurizing the recovery cryostat by one or more of turning off the cryocooler, turning on a heater in the recovery cryostat, and pressurizing the recovery cryostat with the same gas as the vapor; opening the liquid line return valve based on pressure within the recovery cryostat to return the liquid to the storage tank or to the external element; closing the liquid line return valve; depressurizing the recovery cryostat; and Open the gas supply line valve, A method comprising the steps of:

14. 14. The method of claim 13, wherein the cryogenic material is one of helium, hydrogen, neon, oxygen, nitrogen, and argon.

15. The method of claim 13 , wherein the external element is one of a liquid pump and a liquid container.

16. 14. The method of claim 13, wherein the recovery cryostat is positioned above the storage tank, and the condensed liquid returns to the storage tank by gravity.

17. 14. The method of claim 13, wherein the cryocooler is one of a GM cryogenic expander, a pulse tube, a Stirling type pulse tube, and an inverse Brayton type cryocooler.

18. 14. The method of claim 13, wherein the step of opening the liquid line return valve comprises opening the liquid line return valve when pressure in the recovery cryostat is sufficient to push the liquid through the liquid line.

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