JPH0686920B2 - Storage method and storage container for volatile substances - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for storing volatile substances, and more particularly to a method for preserving volatile substances in a liquid state or a solid state at room temperature in which a loss due to vaporization is minimized. The present invention relates to a method for storing volatile substances that is kept to a minimum.

Furthermore, the present invention relates to a storage container suitable for storing liquid helium.

[Prior Art] Conventionally, volatile substances have been stored in a closed container, stored in a high-pressure cylinder, or cooled and stored.

Generally, relatively many methods of storing volatile substances that are solid or liquid at room temperature are known.

However, the method of storing a substance that is gaseous at room temperature in a liquid or solid state is limited.

For example, substances such as liquefied natural gas, liquid air, liquid nitrogen, liquid oxygen, liquid hydrogen, and liquid helium have a high saturated vapor pressure at room temperature, and are usually stored under pressure or cooled.

Therefore, in order to store these substances stably for a long period of time, the greatest effort is made to keep the low temperature and prevent the inflow of heat.

Dewar bottles and other heat-insulating containers are usually used to store these substances.

Among these substances, much effort has been made to preserve liquid He.

This is because liquid He has different physical properties from general liquefied gas.

That is, the liquid He is very low boiling point at one atmosphere, ⁴ He
Is about 4.2K and ³ He is about 3.2K. Therefore, in order to store the liquid He, it is necessary to use a container made of a metal with low thermal conductivity such as stainless steel or a heat insulating material such as glass. The so-called Deure bottle is used.

Normally, when using a Deuar bottle, the inside is made into a double structure, liquid He is put in the inner Deuar bottle, and heat radiation and heat conduction by accumulating liquid nitrogen etc. between the inner Deuar bottle and the outer Deuar bottle. The method of suppressing the inflow of heat due to is taken.

[Problems to be Solved by the Invention] However, in the conventional method as described above, when a substance that is a gas at room temperature and pressure is liquefied or solidified and stored, particularly during long-term storage, unnecessary vaporization may occur. Loss is inevitable.

Especially in the case of liquid helium, the loss due to vaporization is a problem to be overcome because it is expensive in itself.

Therefore, an object of the present invention is to provide a method for storing a volatile substance that can be stably stored for a long period of time by minimizing the evaporation of the volatile substance due to vaporization.

A further object of the present invention is to provide a storage container suitable for storing liquid helium among volatile substances.

[Means for Solving the Problems] The above object is achieved by the present invention described below.

That is, the present invention is a method for preserving a volatile substance, characterized in that the volatile substance is cooled and the saturated vapor pressure of the substance is kept below atmospheric pressure. A container for storing a certain liquid helium, characterized in that a means for suppressing a superfluid surface flow of the liquid helium is provided on an inner wall or an outer wall of the container, the container for liquid helium.

[Action] When a volatile substance, especially a liquefied gas, is stored at its boiling point, a gas of 1 atm or 760 Torr exists on the surface of the liquid, so that gas convection occurs and the inflow of heat due to this is inevitable. Therefore, vaporization of the liquefied gas proceeds.

For example, the boiling point of liquid nitrogen is 77K, and the saturated vapor pressure at that temperature is 1 atm or 760 Torr.

Therefore, if liquid nitrogen is stored at about 77K, its amount will decrease sharply due to vaporization.

The triple point of nitrogen is about 63 K, and when liquid nitrogen is cooled to this temperature, its saturated vapor pressure becomes about 94 Torr,
When the temperature is further lowered, the liquid nitrogen solidifies and its saturated vapor pressure can be made lower, so that the vaporization of the liquid nitrogen is suppressed.

Liquid oxygen has a saturated vapor pressure of 100 Torr or less at about 75 K, and when the temperature is further reduced to the triple point (about 54 K), the saturated vapor pressure can be reduced to 0.76 Torr.

Liquid oxygen in this state has extremely less evaporation due to vaporization as compared with the case of liquid nitrogen described above. And since it can be sufficiently decompressed, convection is reduced and it can be stably stored for a long time.

Liquid hydrogen has a saturated vapor pressure of 100 Torr or less at about 15 K, and when the temperature is further lowered to the triple point (about 13.8 K), the saturated vapor pressure drops to 52 Torr.

These liquefied gases solidify as the temperature is lowered, the heat of vaporization is relatively large, and the troublesome phenomena such as superfluidity do not occur. Therefore, the liquefied gas can be easily stored under reduced pressure.

Thus, by cooling the volatile substance and maintaining the saturated vapor pressure of the substance below atmospheric pressure, it is possible to minimize the loss due to unnecessary vaporization.

In the present invention, the saturated vapor pressure of volatile substances is 300 Torr or less,
It is effective that the pressure is preferably 100 Torr or less, more preferably 50 Torr or less.

On the other hand, in the case of liquid helium, it does not solidify to absolute zero unless it is under pressure, it becomes a superfluid state at 2.17 K or less, and it shows a property that is significantly different from other liquefied gas, etc. It is necessary to further devise the method.

Generally, in order to stably store liquid helium, 1)
Suppressing the inflow of heat due to convection of helium vapor,
2) It is necessary to suppress the superfluid surface flow, 3) suppress the intrusion of heat due to heat conduction, and 4) suppress the inflow of radiant heat.

Regarding the above 3) and 4), various methods have been conventionally used, and the double dewar mentioned above is one of them.

Regarding 1), it is a method that has not been performed conventionally when storing helium. This is because depressurization of liquid helium was usually avoided because helium vaporization proceeded.

Certainly the above reason is correct when the temperature of liquid helium is stored at 4.2K. However, if the temperature of liquid helium is further lowered to 2.5K or lower, the action will be completely different.

In such a low temperature state, the saturated vapor pressure of liquid helium is about 100 Torr or less, and in this temperature range, heat inflow due to convection of helium vapor is suppressed.

Especially when the liquid helium temperature is about 1 to 1.5 K, the saturated vapor pressure is about 0.1 to 1 Torr, and the vapor pressure on the helium liquid surface cannot rise any more, so the convection of helium can be almost ignored. The amount of evaporation also becomes extremely small.

When the saturated vapor pressure of helium is set to 0.1 Torr or less, the above effect becomes particularly remarkable.

However, ⁴ He becomes superfluid below 2.17K. This causes liquid helium to rise up the walls of the container and out of the container. At this time, since the surface area of the liquid helium is remarkably increased, the evaporation of the liquid helium is promoted and the loss amount of helium is increased.

In addition, it was impossible to perform reduced pressure cooling on superfluid liquid helium. This is because it was necessary to cool to below 1K due to the expansion of the liquid surface due to superfluidity, and it was impossible to cool to this temperature.

Therefore, in the present invention, in order to suppress the superfluid flow of liquid helium, a means for suppressing the superfluid surface flow is provided on the inner wall or the outer wall of the container storing the liquid helium.

Examples of the means include treating the inner wall or the outer wall of the container with a substance for suppressing the superfluid surface flow, or providing irregularities for suppressing the superfluid surface flow.

Examples of substances for treating the container to suppress the superfluid surface flow include fluorine-containing substances such as fluororesins.

For example, a Teflon container or a container whose surface is coated with a fluororesin is extremely effective. Further, it is also possible to use those obtained by surface-treating the surface of organic polymer, graphite, glass or the like with CF ₄ plasma or the like.

Further, a fluorine-containing surfactant or an organic acid, for example, wax such as pentadecafluorocaprylic acid or paraffin may be applied to the surface of the container.

Polyethylene and other organic polymers are layered on the surface, and a polyethylene container is also effective in suppressing surface flow.

The inner wall of the container may be burred or bulged with such a material, and as shown in FIG. 1, the surface flow suppressing material is applied in a strip shape on the inner wall 1-2 of the container above the helium liquid level 1-3. Is also good.

Although such materials are known as water-repellent materials, it has been hitherto unknown that superfluid surface flow is suppressed by these materials.

On the other hand, as an example of providing unevenness for suppressing the superfluid surface flow, as shown in FIG. 2 (a), a streak-like shape having a finite angle with respect to the flow direction of the superfluid surface flow is formed on the inner wall or the outer wall of the container. An example is provided in which the protrusion or the groove 2-2 is provided.

FIG. 2 (b) shows a stripe-shaped protrusion or groove 2-5, 2-6,
An example is shown in which a plurality of 2-7 and 2-8 are provided on the inner wall or the outer wall of the container 2-4.

FIG. 3 (a) is a diagram showing the cross-sectional shape of the protrusion shown in FIG. In the figure, R indicates the radius of curvature of the tip of the protrusion 3-3, and in order to suppress the superfluid surface flow, R is 5 times or less, preferably 3 times the thickness of the surface flow 3-2. Should be: Since the surface flow thickness 3-2 is usually quite thin, R is in the range of 1 μm or less, preferably 1000 Å or less. Moreover, if the height of the protrusion is 1000 Å or more, it is particularly effective for suppressing the surface flow.

Further, the angle θ of the protrusion 3-3 shown in FIG. 3 (a) is preferably 30 ° or more.

FIG. 3B is a diagram showing a cross-sectional structure of the surface flow suppressing groove.

In FIG. 3 (b), it is the protrusion 3-7 that substantially suppresses the surface flow 3-6. The depth H of the groove 3-5 is 1000Å or more, the width of the groove 3-5 is 1000Å or more, the radius of curvature R ₁ of the protrusion 3-7,
R ₂ preferably complies with the conditions shown in FIG. 3 (a).

That is, in FIG. 3 (b), R ₁ and R ₂ should be 5 times or less, preferably 3 times or less the thickness of the surface flow 3-6. Since the surface flow 3-6 is usually quite thin, R ₁ and R ₂ are preferably 1 μm or less, and more preferably 1000 Å or less. The angles θ ₁ and θ ₂ of the protrusion 3-7 are preferably 30 ° or more.

As shown in FIG. 2 (b), a plurality of the irregularities for suppressing the surface flow described above are more effective when formed on the inner wall or the outer wall of the container.

In other words, it is not always easy to process the protrusion,
It is difficult to form protrusions that have no defects. As a result, liquid helium gets over the protrusions from the defective portions of the protrusions, and the superfluid surface flow cannot be sufficiently suppressed. Further, even if foreign matter such as dust adheres to the protrusions, the suppression of the surface flow is hindered.

Such a problem can be solved by providing a plurality of protrusions.

Further, when the amount of liquid helium in the container decreases and the liquid level of liquid helium decreases, there is a space between the liquid surface and the protrusion, and the wall surface of this space is covered with superfluid helium. Then, the surface area of the liquid helium increases, so that the evaporation amount increases and the consumption amount of the liquid helium increases.

However, by providing multiple protrusions from the portion near the inlet of the inner wall of the container to the portion near the bottom of the container, the surface flow is always suppressed by the protrusions near the liquid surface of helium.
It is possible to prevent an increase in surface area.

As a method of forming the unevenness, for example, a cutting technique used when forming a diffraction grating or a combination of photolithography and etching may be used.

In the manufacture of ICs, undercutting at the time of etching becomes a problem, but in the present invention, the undercut can be utilized rather to form a small R-shaped projection when forming the projection.

The method for storing volatile substances of the present invention is not limited to simply storing a small amount of liquefied gas in a laboratory. For example, the effect is particularly large when used for cooling with liquid helium used for large-scale superconducting coils used for nuclear fusion, electric power storage, and the like. Because in such a large cooling system, if bubbles are formed due to vaporization of helium, they will be accumulated in a specific place of the cooling system to hinder the cooling of that portion, or damage due to an increase in pressure will occur. If superfluid helium can be used for such a large-scale device, its high thermal conductivity can be used for efficient cooling. Also, if superfluid helium is used for analytical equipment, use a pump. Instead, liquid helium can be sent to any location to help cool the sample. Furthermore, when it is necessary to create an inert surface in the vacuum reactor, if liquid helium is cooled until the vapor pressure becomes, for example, 10 ^-4 Torr or less, the liquid will be stored in a vacuum container of about 10 ^-4 Torr. It is possible to create an inert surface covered with helium. If such a method is used, it can be used for the inner surface treatment of a hydrogen atom gas storage container. When a large amount of liquid helium is transported, the loss of liquid helium during transportation can be extremely reduced by utilizing the method of the present invention for a liquid helium tank of a tank truck or a tanker.

Unnecessary loss of liquid helium can be minimized even in a transportation system using superconductivity such as a linear motor car using magnetic levitation by a superconducting coil.

[Examples] Hereinafter, the present invention will be described in detail based on Examples.

Example 1 As shown in FIG. 4, a double dewar for liquid helium.
Styrofoam block 4-2 was placed in 4-3 and 4-4, and a Teflon graduated cylinder (3000cc) 4-1 was placed on it.

After pouring liquid nitrogen into the space 4-5 between the two double dewers 4-3 and 4-4 to cool the inner dew 4-3, and further thoroughly squeezing the inner dew 4-3 with liquid nitrogen, Liquid helium was filled into the inner Dewar 4-3 and cooled.

After the whole is sufficiently cooled, liquid helium is poured into the Teflon graduated cylinder in the inner dewar 4-3 and sealed with a lid, then the exhaust port 4-8 is connected to the rotary pump to decompress the inside and The inside was about 1 Torr.

Liquid helium 4-6 boiled violently during depressurization, but stabilized when the temperature dropped to 2.17K. When the pressure was reduced to 1 Torr, the temperature became approximately 1.3K. When the state of decrease of liquid helium was examined while keeping the pressure constant in this state, it was about 6 CC / hour. Even after 15 hours, 1.3K of superfluid helium was reduced by only about 100cc.

In addition, when a lead container was used instead of the Teflon container,
The reduction rate of liquid helium was about 100 cc / hour, which was much faster than that using the Teflon container.

Example 2 No. 4, except that liquid nitrogen was directly poured into the inner inner member without using the Styrofoam block and the graduated cylinder.
The inside was decompressed with a pump using the same double dual as in the figure. The temperature at this time was 55 K, and the liquid nitrogen was solidified.

When the storage was continued in this state, the weight of solid nitrogen in Dewar 4-3 was reduced by only 20% even after one month.

On the other hand, when it was stored at a temperature of about 77 K without decompression, it became empty after 3 days.

Example 3 Storage of liquid helium was carried out in the same manner as in Example 1 except that a graduated cylinder made of Teflon was not used, but instead paraffin was applied to the inner wall of the upper part of the Deurer and the pressure was set to 0.1 Torr. I did.

As a result, the reduction rate of liquid helium was about 10 cc / hour. On the other hand, when it was performed without applying paraffin, the reduction rate of liquid helium was about 150 cc / hour.

Example 4 In Example 1, a cylindrical glass graduated cylinder was used instead of the Teflon graduated cylinder.

In the circumferential direction of the inner wall surface of the graduated cylinder, streak-shaped projections were formed at intervals of 5 mm by etching.

Next, the whole was cooled with liquid helium, and liquid helium was poured into the graduated cylinder to cool under reduced pressure. When cooled to about 1.3K, the pressure dropped below 1 Torr. When liquid helium was stored in this state, the rate of decrease was 15
It was cc / hour.

As a comparison, the reduction rate was 80 cc / hour or more when a graduated cylinder without protrusions was used.

Example 5 A Teflon container 4-1 shown in FIG. 4 was charged with a superconducting coil made of Nb-Ti and having a diameter of 20 cm so that the current supply wire could come out of the container.

The double-dewar gap 4-5 was filled with liquid nitrogen, liquid helium was poured into a Teflon container containing a coil, and the temperature was lowered to 1.2 K by cooling under reduced pressure.

The saturated vapor pressure at this time was less than 1 Torr.

Next, when a permanent current was applied to the coil, no heat generation from the coil was observed.

At this time, the reduction rate of liquid helium in the Teflon container was 20 cc / hour. On the other hand, when not cooling under reduced pressure,
The rate of decrease was over 200cc / hour.

[Effect] As described above, according to the present invention, it is possible to minimize the loss due to vaporization of the volatile substance.

Further, when the volatile substance is superfluid helium, the surface flow can be suppressed by the present invention.
The reduction rate of helium was reduced to several tens.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 (a),
(B) is a diagram showing another embodiment of the present invention, FIG. 3 (a),
2B is a diagram showing the sectional structure of FIG. 2A, and FIG. 4 is a diagram showing another embodiment of the present invention. 1-1, 2-1, 2-4, 3-1, 3-4, 4-1: Container 1-2, 2-2, 2-5, 2-6, 2-7, 2-8: Surface Flow suppression means 1-3, 2-3, 2-9: Liquid helium liquid surface 3-2, 3-6: Surface flow 3-3, 3-7: Projection, 3-5: Groove 4-2: Fire Styrol block 4-3, 4-4: Dewar 4-5: Liquid nitrogen, 4-6: Liquid helium 4-7: Lid for vacuum seal, 4-8: Exhaust port

Claims (5)

[Claims] 1. A method for storing a volatile substance, which comprises cooling the volatile substance and maintaining a saturated vapor pressure of the substance at a normal pressure or lower. 2. The storage method according to claim 1, wherein the saturated vapor pressure of the substance is maintained at 300 Torr or less. 3. The storage method according to claim 1, wherein the saturated vapor pressure of the substance is maintained at 100 Torr or less. 4. The method according to claim 1, wherein the saturated vapor pressure of the substance is maintained at 50 Torr or less. 5. Cooling liquid helium, which is a volatile substance,
And a storage container used to store the liquid helium by keeping the saturated vapor pressure of the liquid helium below atmospheric pressure, and a means for suppressing the superfluid surface flow of liquid helium on the inner wall or the outer wall of the container. A storage container for liquid helium characterized by being provided.