A system for capturing steam 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 based on U.S. Provisional Patent Application No. 63 / 318,555, filed Mar. 10, 2022, which is incorporated herein by reference.

[0002] (Technical Field) The present invention relates to the operation of cryogenic storage tanks for storing liquid cryogenic substances (cryogens) and preventing loss of the vaporized cryogenic substances.

Background Art

[0003] Cryogenic gases such as nitrogen, oxygen, hydrogen, and helium, which are used commercially, are often transported and stored in a liquid state at relatively low temperatures (less than - 175°C). Due to their low temperature and the presence of both liquid and gas phases, special handling is required. However, it is economically advantageous to handle these cryogenic substances as liquids rather than gases because of their high density and low pressure. The recovery methods included herein are applicable to all cryogenic storage tanks, but in the near future, application to hydrogen storage tanks used in fueling vehicles is expected. Currently, hydrogen is stored in vehicles in three ways. The first is to store it in cylinders as a high - pressure gas of about 700 MPa (10,500 psi), the second is to store it in cylinders as a lower - pressure hydride, and the third is to store it 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, the lighter the weight of the tank, but the cost for cooling to low temperature is high and the cooling cost of the cryogenic liquid increases.

[0004] In hydrogen fueling stations that supply gas at high pressure, pumps are used to compress the liquid to high pressure, then the liquid is heated to ambient temperature in a vaporizer before being stored in high-pressure gas cylinders. The liquid pump operates intermittently and warms up between uses, generating a large amount of steam as the liquid pump cools. Hydrogen fueling stations that replenish liquid storage tanks transport the liquid through a transfer line equipped with a vacuum jacket. The transfer line cools as it vaporizes the initial flow of liquid. In both of these systems, steam can be returned to the top of the storage tank, which increases the pressure. The degree of pressure increase depends on the amount of steam 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 unnecessarily release hydrogen.

[0005] These cryogen storage tanks typically operate solely on the cryogenic vapor in the space above the liquid, with the vapor and liquid in equilibrium. Specifically, the temperature and pressure of the uppermost layer of liquid adjacent to the vapor become the saturation temperature and saturation pressure of the vapor above the liquid. As the cryogenic liquid cools, its density increases, causing it to form layers, with the coldest liquid at the bottom. Similarly, the hottest vapor is at the top of the tank. In some cases, for operational purposes, additional gas (vapor) may be introduced above the liquid to temporarily increase the pressure in the tank. When additional gas is introduced, some of the vapor condenses to form the uppermost layer of liquid, changing the temperature of the uppermost layer of liquid to a higher saturation temperature, while the lower layers of liquid remain nearly at their original temperature, resulting in a more subcooled state under higher pressure.

[0006] During storage, a small amount of heat (heat leakage) passes through the tank's insulation, causing some of the liquid cryogenic material to vaporize, thus increasing the pressure inside the tank. Cryogenic material storage tanks transporting liquid cryogenic materials are sealed during transport and allow the pressure to rise until the cryogenic material is supplied or vented through a pressure relief valve. This is also true for storage tanks that supply cryogenic materials intermittently.

[0007] Liquids are typically supplied from storage tanks by one of three methods. The first method involves extending a line (tube) from the bottom to the top of the tank, pushing the liquid out by creating a pressure above the liquid that is higher than the supply pressure. The second method involves pumping the liquid using a small pump located at the bottom of the tank, connected to a line (tube) from the bottom to the top of the tank. The third method involves installing a line at the bottom of the tank, allowing the cryogenic liquid to be discharged by gravity, or with the assistance of a pump.

[0008] The present invention is applicable to second and third types of storage tanks, enabling the return of vapor generated by cooling an object outside the storage tank as gas to the top of the tank, and further enabling the subsequent flow of the gas into a recovery cryostat outside the storage tank for cooling and condensation, and then return to the storage tank as liquid.

[0009] In existing systems that use liquid hydrogen storage tanks to supply hydrogen gas at high pressure, the boil-off gas generated by cooling the liquid pump is returned to the storage tank, the gas is removed from the storage tank, heated to near ambient temperature, and processed by a high-pressure compressor to recover the boil-off gas. Although the gas volume is minimized by operating the storage tank near critical pressure, the volume of gas flowing into the compressor at 270K and 0.6MPa is more than 100 times that of the liquid flowing into the cooling pump at the same pressure and mass flow rate, making the compressor less efficient than the liquid pump. The gap volume within the compressor is very small, requiring the use of a diaphragm compressor rather than a less expensive piston compressor, and the compressibility of liquids is very low compared to gases. The cryogenic recovery cryostat of the present invention can replace the gas compressor.

[0010] Cryostats, designed to keep objects such as magnetic resonance imaging (MRI) magnets at low temperatures, typically have a Gifford-McMahon (GM) cryogenic expander attached to the neck tube above the magnet. In the first stage, the GM cryogenic expander cools the radiation shield to a temperature of, for example, 50K, and in the second stage, it recondenses boiling helium at approximately 4K.

[0011] U.S. Patent No. 7,434,407 describes cooling a hydrogen storage tank using a Stirling pulse tube refrigerator, which in the first stage cools a cold shield and in the second stage prevents liquid hydrogen (H2) from boiling. Heat is transferred from the storage tank to the refrigerator by circulating helium through individually wound tubes around the cold shield and the internal tank, and by cooling the helium in the tubes in the first and second stages of the refrigerator. The disclosed application is use as a liquid hydrogen fuel container mounted in a vehicle. U.S. Patent No. 7,165,408 discloses a method of operating a liquid hydrogen storage tank designed for use in automobiles that minimizes the amount of gas discharged between tank replenishments. This patent describes changes in pressure and density over time. [Overview of the project] [Problems that the invention aims to solve]

[0012] This invention provides a cryogenic system and method for recovering gases that vaporize when a liquid cryogenic substance (cryogen) flows out of a cryogenic storage tank and cools an external mass. [Means for solving the problem]

[0013] The storage tank is a type of storage tank that can drain liquid from the tank by gravity or a pump and return the vaporized gas to the vapor space above the liquid in the storage tank. The system of the present invention comprises a recovery cryostat located outside the storage tank, which uses a cryocooler to condense the vapor received from the storage tank and return it to the storage tank as a liquid.

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

[0015] These and other advantages are realized, for example, by a cryogenic system that condenses vapors of 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 comprises 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 contain vapors from the liquid cryogenic material that boil when the external element and connection lines are cooled. The recovery cryostat is configured to receive vapors from the storage tank through a gas line and is connected to a cryocooler configured to condense the vapors contained from the storage tank into a liquid, and is configured to return the liquid to the storage tank through a liquid line. [Brief explanation of the drawing]

[0016] The drawings illustrate one or more embodiments based on the concept of the present invention and are not intended to limit the invention. In the drawings, similar elements are given the same reference numerals.

[0017] [Figure 1] Figure 1 shows a schematic diagram of an embodiment in which an external recovery cryostat, including a cryocooler, is added to a cryogenic material storage tank. Lines are shown that supply liquid to the pump, cool the pump and return steam to the storage tank, and connect the steam supply line and liquid return line to the recovery cryostat. [Figure 2] Figure 2 shows a schematic diagram of an embodiment in which an external recovery cryostat, including a cryocooler, is added to a cryogenic material storage tank. Lines are shown for supplying liquid to a liquid container, cooling and filling the liquid container and then returning steam to the storage tank, and connecting the steam supply line and liquid return line to the recovery cryostat. [Figure 3] Figure 3 shows a schematic diagram of an embodiment that includes a steam cooling heat shield inside a cryogenic storage tank and an external recovery cryostat including a cryocooler. Lines are shown supplying liquid to the pump, cooling the pump and returning steam to the storage tank, and connecting the steam supply line and liquid return line (through the heat shield) to the recovery cryostat. [Figure 4] Figure 4 shows a step-by-step diagram illustrating one embodiment of a method for recovering boiling steam in a cryogenic system. [Figure 5] Figure 5 shows a step in another embodiment of a method for recovering boiling steam in a cryogenic system. [Modes for carrying out the invention]

[0018] Some embodiments of the present invention will be described in more detail with reference to the accompanying drawings showing preferred embodiments of the present invention. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. These embodiments fully and completely disclose the present disclosure and are provided so that the scope of the present invention can be understood by those skilled in the art. In the drawings, the same or similar parts have the same reference numbers, and in principle, the descriptions thereof will not be repeated.

[0019] FIG. 1 shows a preferred embodiment of a cryogenic system 100 configured to store cryogenic substances (cryogens) in a storage tank and capture the vapor from the storage tank. The cryogenic substance includes any one of helium, hydrogen, neon, oxygen, nitrogen, and argon. All the illustrated elements are insulated by existing means to minimize the heat taken in from the external environment into the cryogenic substance.

[0020] The cryogenic substance storage tank 10 contains a liquid cryogenic substance 15 and a vapor cryogenic substance 20. A cryocooler 40 is attached to the upper part of the recovery cryostat 45 and condenses the vapor 20 that flows through the gas lines 50, 51 and can be blocked by the gas line supply valve 32. The reference number 16 indicates the liquid accumulated in the recovery cryostat 45, and the reference number 21 indicates the vapor above the liquid 16. The liquid 16 returns to the storage tank 10 through the liquid lines 56, 55 and can be blocked by the liquid line return valve 33 on the liquid line 56.

[0021] The storage tank 10 is replenished through a line 36 from a supply trailer and a detachable coupling 35. The recovery cryostat 45 can be disposed 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 a device used with the present invention. Before turning on the pump 60 to deliver the liquid cryogenic substance, it is necessary to cool the pump 60 to the temperature of the liquid. To remove the sensible heat of the pump 60, valve 33 is closed and valves 30 and 31 are opened. It is illustrated that the liquid cryogenic substance 15 flows through pipe 55 and valve 30 by gravity and into the pump 60. When the temperature of the pump 60 is higher than the temperature of the liquid, the liquid vaporizes due to the sensible heat of the pump 60. The vapor passes through valve 31 and line 52 and branches. 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 pressure in the tank rises due to the warm gas accumulating at the top of the storage tank 10, and the liquid 15 becomes subcooled.

[0023] Cooling by evaporation continues until the pump 60 is cooled to the saturation temperature of the liquid. Thereafter, the pump 60 is operated to increase the pressure of the liquid and deliver it through the discharge line 61. When the liquid is discharged from the tank 10, the vapor 20 expands and the pressure and temperature decrease. When a large amount of liquid is discharged, the pressure at the inlet of the pump 60 decreases and the cryogenic substance begins to boil, so it is necessary to turn off the pump 60. When discharging the cryogenic substance batchwise from the large storage tank 10, turn off the pump 60 before the cryogenic substance begins to boil. When the pump 60 is turned off, the valve settings are returned to the positions before the pump 60 was turned on.

[0024] The cryocooler 40 is designed to provide slightly more cooling than required to match the long-term average heat loss, so usually it is not sufficient to condense the rate of gas generated by cooling the pump 60. The cryocooler 40 is connected or attached to the cryostat 45 and cools the vapor within the cryostat 45. The cryocooler 40 is either a GM, pulse tube, Stirling, or reverse Brayton type cryocooler.

[0025] The size of the cryocooler 40 is selected to be the size required to condense the gas vaporized by the heat leak and to maintain the storage tank 10 at a pressure lower than the pressure at which a safety release valve (not shown) releases some of the cryogenic material. The size of the recovery cryostat 45 is selected to allow for the storage of the condensed cryogenic material during the interval when the condensed cryogenic material returns to the storage tank 10.

[0026] If the recovery cryostat 45 is positioned so that the liquid can be discharged and returned to the storage tank, the valves 32 and 33 are not required, or can be kept in an open position at all times, allowing the condensation process to be carried out continuously.

[0027] If the liquid cannot be discharged and returned to the storage tank, valve 33 on the liquid return line 56 is closed while the cryocooler 40 condenses the steam. When the conditions for returning the liquid are met, valve 32 on the steam supply line 51 is closed. When the pressure in the recovery cryostat 45 rises sufficiently, the liquid is pushed out through valve 33 (check valve). Within the cryostat 45, the extent to which the pressure in the recovery cryostat 45 needs to exceed the required level is determined by the difference in liquid surface height and the pressure drop at the desired flow rate passing through valve 33 and lines 56, 55.

[0028] This process is usually performed when valves 30 and 31 are closed, but it may also be performed when one or both are open. When valve 30 is open, the pressure required to supply the liquid directly to an external element, such as a pump 60, is less than or equal to the pressure required to return the liquid to the storage tank 10. The pressure inside the cryostat 45 can be increased by turning off the cryocooler 40, turning on a heater (not shown) inside the cryostat 45, or by pressurizing the cryostat 45 with the same gas as the steam. If the storage tank 10 is of a type that has a pump for supplying the liquid, the pump is usually of an impeller type that can reverse the 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 the saturation temperature at the surface of the liquid, 2,000 kJ needs to be removed. To cool the pump, assuming that steam is discharged from the pump 60 while it is cooled to a temperature and saturation pressure equivalent to 28 K and 587 kPa, 4.0 kg of liquid hydrogen in 1,700 L of steam is required. This is less than 3% of the volume of the storage tank 10, and if the tank is 85% filled with liquid, the pressure rise inside the tank will be less than 100 kPa.

[0030] The cryocooler 40 on the recovery cryostat 45 needs to provide sufficient cooling to compensate for heat loss in the hydrogen storage tank (typically less than 40W for a tank of that size), condensation of boil-off gas (2,000kJ), and other losses in the line and recovery cryostat (estimated 25W). If the liquid pump 60 is operated every 6 hours, the load to remove 2,000kJ of heat is 93W, so the total load on the cryocooler 40 is approximately 160W at approximately 28K. If this example is applied to a hydrogen refueling station, the interval for cooling the pump will be shorter during the day and longer at night, so the average pressure in the storage tank will rise during the day and fall at night.

[0031] Figure 2 shows an embodiment of the cryogenic system 200, which has a liquid container instead of a pump. While system 100 maintains the pressure in the storage tank 10 near critical pressure, the cryogenic system 200 shown in Figure 2 maintains the pressure near atmospheric pressure. Examples of applications of the system for filling / replenishing the cryogenic container, illustrated as the liquid hydrogen container 11, are illustrated in U.S. Patents 7,434,407 and 7,165,408. Both of these patents describe liquid hydrogen containers that can be used in vehicles.

[0032] Lines 12 and 13 are vacuum-insulated and connected to detachable couplings 37 and 38 on container 11, respectively. Container 11 is filled by opening valves 30 and 31, allowing liquid to flow into container 11 through line 55, valve 30, line 12, and coupling 37. These lines, and any steam after cooling container 11 if possible, and steam 22 discharged as liquid 17 fills container 11, return to the top of storage tank 10 through coupling 38, line 13, valve 31, and lines 52 and 50. Once container 11 is filled, valves 30 and 31 are closed, and the steam that has returned to storage tank 10 and recovery cryostat 45 is condensed and returns to storage tank 10 as well as system 100. Hydrogen can flow out of container 11 as a liquid through valve 14, and can also flow in or out as a gas through valve 18.

[0033] Figure 3 shows an embodiment of cryogenic system 300, a variation of cryogenic system 100. In some cryogenic storage tanks, boil-off steam is circulated through line 57 to cool the cold shield to block heat leakage into the liquid, and then the steam is discharged. System 300 recovers the gas by allowing the gas that has flowed out of storage tank 10 to flow into recovery cryostat 45 through line 58 and valve 32. Cooling of external elements such as the liquid pump 60 and filling container 11 is performed as described in systems 100 and 200. The difference from these systems is that while valves 30 and 31 are closed, steam 20 flows out of storage tank 10 through lines 57 and 58 instead of line 50 and into recovery cryostat 45. Liquid 16 returns to storage tank 10 as described in system 100. Liquid hydrogen alternately flows into liquid hydrogen container 11 through valve 30 or returns as gas through valve 31, as described in system 200.

[0034] Figure 4 shows a process diagram of Method 400, which uses a recovery cryostat 45 to recover boiling steam in a cryogenic system. Method 400 can be used with any of the cryogenic systems 100, 200, and 300 described above. Method 400 can be used in situations where the recovery cryostat 45 is positioned so that the liquid can be discharged and returned to the storage tank, and the condensation process is performed continuously. For example, the storage tank 10 is configured to supply liquid cryogenic material to the external element by gravity, and the recovery cryostat 45 is configured to return the condensed liquid to the storage tank 10 by gravity through liquid lines 55, 56.

[0035] Method 400 comprises the steps of passing steam from the storage tank 10 through gas lines 50 and 51 and recovering it via a recovery cryostat 45 (S401), condensing the steam 21 into liquid 16 using a cryocooler 40 (S402), and returning the liquid 16 to the storage tank 10 through liquid lines 55 and 56 (S403).

[0036] Figure 5 shows a process diagram of Method 500, which uses a recovery cryostat 45 to recover boiling steam in a cryogenic system. Method 500 can be used with any of the cryogenic systems 100, 200, and 300 described above. Method 500 can be used in situations where the storage tank 10 is configured to supply liquid cryogenic material to an external element. Method 500 involves closing the liquid line return valve 33 (S501), opening the gas line supply valve 32 with the liquid line return valve 33 closed (S502), condensing the steam 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 steam. The process includes pressurizing the recovery cryostat 45 by one or more of the following (S505), opening the liquid line return valve 33 (S506) if the pressure inside the recovery cryostat 45 is sufficient to return the liquid 16 to the storage tank 10 or an external element, closing the liquid line return valve 33 (S507), stopping the pressurization of the recovery cryostat 45 by reversing the pressurization process of S505 (S508), and opening the gas supply line valve 32 (S509).

[0037] The terms and descriptions used herein are for illustrative purposes only and do not limit the scope of the invention. Those skilled in the art will see that various modifications falling within the spirit and scope of the invention and the embodiments described herein can be implemented.

Claims

1. A cryogenic system that condenses the vapor of cryogenic material from a cryogenic storage tank in an external recovery cryostat and returns the cryogenic material to the storage tank as a liquid, A storage tank configured to store a liquid cryogenic substance and to supply the liquid cryogenic substance to an external element, wherein the storage tank is configured to contain the vapor generated when the liquid cryogenic substance boils when the external element and connecting lines are cooled, in the upper part of the storage tank. A recovery cryostat connected to the storage tank, which is connected to a cryocooler configured to receive steam from the storage tank through a supply gas line and to condense the steam received from the storage tank into a liquid, and to return the liquid to the storage tank through a return liquid line connected to the bottom of the storage tank, A cryogenic system equipped with the following features.

2. The cryogenic system according to claim 1, wherein the cryogenic material comprises helium, hydrogen, neon, oxygen, nitrogen, and argon.

3. The cryogenic system according to claim 1 or 2, wherein the recovery cryostat is positioned above the storage tank and the condensed liquid is returned to the storage tank by gravity.

4. The storage tank is connected to the recovery cryostat by a supply valve on the supply gas line, The recovery cryostat is connected to the storage tank by a return valve on the return liquid line, The cryogenic system according to claim 1 or 2, further comprising:

5. The cryogenic system according to claim 1 or 2, wherein the external element is either a liquid pump or a liquid container.

6. The cryogenic system according to claim 1 or 2, wherein the cryocooler is one of a GM cryogenic expander, a pulse tube, a Stirling-type pulse tube, and an inverted Brayton-type cryocooler.

7. A method for recovering the vapor of a cryogenic substance that has boiled in a cryogenic system, This cryogenic system is A storage tank configured to supply the cryogenic substance as a liquid cryogenic substance to an external element, wherein the storage tank is configured to contain the steam generated when the liquid cryogenic substance boils when the external element and connecting lines are cooled, in the upper part of the storage tank. A recovery cryostat configured to receive vapor from a storage tank through a supply 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 connected to the bottom of the storage tank and configured to return the liquid to the storage tank or the external element through a return liquid line having a liquid line return valve, Equipped with, The method is, Close the liquid line return valve, With the liquid line return valve closed, open the gas line supply valve. The vapor in the recovery cryostat is condensed, Close the gas line supply valve, The recovery cryostat is pressurized by one or more of the following: turning off the cryocooler, turning on the heater in the recovery cryostat, and pressurizing the recovery cryostat with the same gas as the steam. Based on the pressure in the recovery cryostat to return the liquid to the storage tank or the external element, the liquid line return valve is opened. Close the liquid line return valve, The pressurization of the recovery cryostat is stopped, and, Open the gas supply line valve. A method comprising a process.

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

9. The method according to claim 7, wherein the external element is either a liquid pump or a liquid container.

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

11. The method according to claim 7, wherein the cryocooler is one of a GM cryogenic expander, a pulse tube, a Stirling-type pulse tube, and an inverted Brayton-type cryocooler.

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