Patent Application
20230288029
This invention relates to cryogenic storage tank operation for storing liquid cryogens and preventing the loss of vaporized cryogen.
Cryogenic gases used commercially, including nitrogen, oxygen, hydrogen, and helium, are often transported and stored in their liquid state, at relatively cold temperatures (less than −175° C.). Because of the cold temperature and presence of both liquid and vapor phases, special handling is required. However, the greater density and lower pressure make it economically advantageous to handle these cryogens as liquids instead of gases. While the recovery methods included in this description may be applied to all cryogenic storage tanks, the near term application is for hydrogen storage tanks used in refueling vehicles. Hydrogen is being stored on vehicles in three ways at the present time: first as high pressure gas in bottles at pressure as high as about 700 MPa (10,500 psi), second as a hydride in bottles at a lower pressure, and third as a liquid at pressures near atmospheric pressure. The lower the pressure of a cryogenic liquid tank, the greater the density of the liquid and the lighter the weight of the tank; but the cost of cooling to a lower temperature increases the cost of cooling the cryogen. Hydrogen fueling stations that deliver gas at a high pressure use a pump that compresses the liquid to a high pressure then warm it to ambient temperature in a vaporizer and store it in high pressure gas cylinders. The liquid pumps are operated intermittently and warm up between uses, thus generating a lot of vapor as they are cooled down. Hydrogen fuel stations that refill liquid storage tanks transfer the liquid through vacuum jacketed transfer lines that vaporize the initial flow of liquid as the lines cool down. In both of these systems the vapor can be returned to the top of the storage tank which results in an increase in pressure; the increase depending on the amount of vapor relative to the size of the tank, the fraction of liquid in the tank, etc. These systems have to be designed and operated to keep within limits that avoid unnecessary venting of hydrogen. These cryogen storage tanks are typically operated with only the vapor of the cryogen in the space above the liquid—the vapor and liquid being in equilibrium. Specifically, the top layer of liquid next to the vapor has the temperature and pressure that is the saturation temperature and saturation pressure associated with the pressure of the vapor above the liquid. Cryogenic liquids become denser as they are cooled so the liquid becomes stratified, with the coldest liquid on the bottom. Similarly the warmest vapor is at the top of the tank. Sometimes, for an operating purpose, the pressure in the tank is temporarily raised by introducing additional gas (vapor) above the liquid. When this is done, some of the vapor condenses into the top layer of liquid, raising its temperature to the new, higher saturation temperature, but leaving the lower layers of liquid at approximately their original temperature and therefore, even more sub-cooled relative to the new, higher pressure.
During storage, a small amount of heat (heat leak) passes through the tank insulation and vaporizes some of the liquid cryogen, building pressure in the tank. Cryogen storage tanks that transport liquid cryogens are sealed during transport and the pressure is allowed to rise until the cryogen is delivered, or vented through a pressure relief valve. This is also typical of storage tanks that deliver the cryogen intermittently. Liquid is typically delivered from a storage tank by one of three methods. The first is to extend a line (tube) from the bottom of the tank through the top of the tank and forcing the liquid out by having the pressure above the liquid be greater than the delivered pressure. The second is to use a small pump in the bottom of the tank connected to a line (tube) from the bottom of the tank through the top of the tank and pumping the liquid out. The third is to have a line at the bottom of the tank that allows the liquid cryogen to drain out by the force of gravity or possibly be assisted by a pump. This invention applies to the second and third types of storage tanks in which vapor that results from cooling down objects outside the storage tank can be returned as gas to the top of the tank and if it is, it can then flow to a recovery cryostat external to the storage tank, be cooled down, condensed, and returned to the storage tank as liquid.
Existing systems that use a liquid hydrogen storage tank and deliver hydrogen gas at high pressure recover the boil off gas from cooling down the liquid pump by returning the boil off gas to the storage tank, taking gas from the storage tank and warming it to near ambient temperature, then processing it through a high pressure compressor. The volume of gas is minimized by operating the storage tank near the critical pressure, but even so the compressor is inefficient compared to the liquid pump because the volume of gas entering the compressor at 270 K and 0.6 MPa is over 100 times that of liquid entering the cold pump at the same pressure and mass flow rate. The compressibility of the liquid is much less than the gas which means that the clearance volume in the compressor can be much less; requiring the use of a diaphragm compressor rather than a less expensive piston type compressor. The cryogenic recovery cryostat of this invention can replace the gas compressor.
Cryostats that are designed to keep objects such as magnetic resonance imaging (MRI) magnets cold typically have a Gifford-McMahon (GM) type cryogenic expander mounted in a neck tube at the top of the magnet that cools a radiation shield at a first stage temperature e.g. 50 K and a second stage that re-condenses helium boil off at about 4 K. U.S. Pat. No. 7,434,407 describes the use of a Stirling type pulse tube refrigerator to cool a hydrogen storage tank, using the first stage to cool a cold shield and the second stage to keep liquid hydrogen (H2) from boiling off. Heat is transferred from the storage tank to the refrigerator by circulating helium through tubes wrapped independently around the cold shield and inner tank, and cooling the helium in the tubing on the first and second stages of the refrigerator. The application that is described is its use as a liquid hydrogen fuel container on a vehicle. U.S. Pat. No. 7,165,408 describes a method of operating a liquid hydrogen storage tank, designed for use in an automobile, which minimizes the amount of gas that vents between refilling the tank. The patent has interesting descriptions of the changes in pressure and density with time.
The disclosed invention provides cryogenic systems and methods for the recovery of gas that vaporizes when a liquid cryogen flows from a cryogenic storage tank as it cools down an external mass. The storage tank is a type that has liquid flow from the tank by force of gravity or by being pumped, and in which the vaporized gas can be returned to the vapor space above the liquid in the storage tank. The system of the disclosed invention comprises a recovery cryostat external to the storage tank, which uses a cryocooler to condense vapor received from the storage tank and to return it to the storage tank as a liquid. The process may be continuous or cyclical depending on the orientation of the recovery cryostat. If the recovery cryostat is located such that liquid can drain back to the storage tank, then the process can be continuous. If the liquid cannot be drained back, then a valve on the liquid return line is closed while the cryocooler condenses the vapor, a valve on the vapor supply line is then closed, the valve on the liquid return line is opened, and pressure in the recovery cryostat is increased to drive the liquid out.
These advantages and others are achieved, for example, by a cryogenic system for condensing vapor of cryogen from a cryogenic storage tank in an external recovery cryostat and returning the cryogen to the storage tank as a liquid. The cryogenic system includes a storage tank and a recovery cryostat connected to the storage tank. The storage tank is configured to store a liquid cryogen and to deliver the liquid cryogen to an external component. The storage tank is also configured to receive vapor from the liquid cryogen that boils off when said external component and connecting lines are cooled down. The recovery cryostat is configured to receive vapor from the storage tank through a gas line, is coupled to a cryocooler that is configured to condense the vapor received from the storage tank into liquid, and is configured to return the liquid to the storage tank through a liquid line.
The drawing figures depict one or more implementations in accord with the present concepts, by way of example only, not by way of limitations. In the figures, like reference numerals refer to the same or similar elements.
In this section, some embodiments of the invention will be described more fully with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will convey the scope of the invention to those skilled in the art. Parts that are the same or similar in the drawings have the same numbers and descriptions are usually not repeated.
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Pump 60 is used intermittently and warms up between uses. It is provided as an example of equipment that may be used with the present invention. Before pump 60 can be turned on to pump liquid cryogen, it must be cooled down to the liquid temperature. To remove the sensible heat of pump 60, valve 33 is closed and valves 30 and 31 are opened. Liquid cryogen 15 is shown flowing to pump 60 by gravity through pipes 55 and valve 30. As long as the pump is warmer than the liquid, the sensible heat of pump 60 vaporizes the liquid. The vapor flows through valve 31 and line 52, then splits. Most or all of the vapor returns to storage tank 10 through line 50. Some may flow through line 51 and valve 32 into recovery cryostat 45. The warm gas that collects at the top of storage tank 10 increases the pressure in the tank, causing liquid 15 to become subcooled.
The cool down vaporization continues until the pump is cooled to the saturation temperature of the liquid. Then the pump can be operated, pumping liquid to higher pressure through discharge line 61. Removing liquid from tank 10 causes vapor 20 to expand, dropping in pressure and temperature. If a lot of liquid is removed, the pressure at the inlet to the pump 60 will drop to the point where the cryogen will start to boil and the pump has to be turned off. For batch removal of cryogen from a large storage tank, pump 60 is turned off before this happens. After pump 60 is turned off the valve settings are returned to the positions they were in before the pump was turned on.
Cryocooler 40 is designed to provide slightly more refrigeration than required to match the average heat losses over an extended period of time and is usually insufficient to condense the gas at the rate it is generated by cooling down the pump. Cryocooler 40 is coupled or attached to cryostat 45 to cool down the vapor 21 in the cryostat 45. Cryocooler 40 may be one of a GM, pulse tube, Stirling, or reverse Brayton type of cryocooler. The size of cryocooler 40 is selected to condense gas that vaporizes due to heat leak, and that required to keep storage tank 10 below a pressure that would cause a safety vent valve (not shown) to vent some of the cryogen. The size of recovery cryostat 45 is selected to store condensed cryogen between time intervals when it is returned to storage tank 10.
If recovery cryostat 45 is located such that liquid can drain back to the storage tank, then valves 32 and 33 may not be needed or may be always at open positions and the condensing process may be continuous. If the liquid cannot be drained back, then valve 33 on liquid return line 56 is closed while cryocooler 40 condenses the vapor. When conditions are reached for returning liquid, valve 32 on the vapor supply line 51 is closed. Pressure in recovery cryostat 45 is then increased sufficiently to force the liquid out through valve 33, which may be a check valve. How much the pressure in cryostat 45 has to exceed in recovery cryostat 45 depends on the difference in elevation between the liquid surfaces and the pressure drop at the desired flow rate through valve 33 and lines 56 and 55. This process typically takes place while valves 30 and 31 are closed, but may take place while one or both are open. If valve 30 is open, the pressure needed to supply liquid direct to the external component (for example, pump 60) will be less than that needed to return liquid to storage tank 10. The pressure in cryostat 45 may be increased by turning off the cryocooler 40, turning on a heater (not shown) in cryostat 45, or pressurizing cryostat 45 with the same gas as the vapor. If storage tank 10 is the type that has a pump to deliver liquid, the pump is usually an impeller type that would allow liquid to flow through it in reverse when it is not running.
An example is given for a storage tank 10 that can hold 80,000 L of hydrogen, and a liquid pump 60, that requires the removal of 2,000 kJ to cool from 160 K to 28 K, the saturation temperature at the surface of the liquid. Cooling the pump requires 4.0 kg of liquid hydrogen which has a vapor volume of 1,700 L, assuming the vapor leaves pump 60, as it cools down, at its temperature and at the saturation pressure corresponding to 28 K, 587 kPa. This is less than 3% of the volume of storage tank 10 and results in a pressure increase in the tank of less than 100 kPa if the tank is 85% full of liquid. Cryocooler 40 on recovery cryostat 45 has to provide enough cooling to match the heat loss in a hydrogen storage tank, typically less than 40 W for this size tank, condensing the boil-off gas, 2,000 kJ, and other losses in the lines and recovery cryostat estimated at 25 W. If liquid pump 60 is operated every 6 hours, the load is 93 W to remove 2,000 kJ of heat, thus the total load on the cryocooler 40 is about 160 W at about 28 K. If this example is applied to a hydrogen refueling station, the time interval between cooling down the pump might be shorter during the day and longer at night so the average pressure in the storage tank might increase during the day and decrease at night.
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The terms and descriptions used herein are set forth by way of illustration only and are not meant as limitations. Those skilled in the art will recognize that many variations are possible within the spirit and scope of the invention and the embodiments described herein.
This application claims priority to U.S. Provisional Patent Application No. 63/318,555, filed Mar. 10, 2022, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 63318555 | Mar 2022 | US |
