How long can liquid nitrogen be stored in a dewar flask? Liquefied gases are stored in dewars

How can liquid nitrogen be dangerous indoors?

When liquid nitrogen evaporates, gaseous nitrogen with a low temperature is formed, and its density is greater than that of air. Therefore, nitrogen after evaporation can accumulate initially at the lower level of the room and then gradually create an increased concentration throughout the room. This leads to a decrease in the concentration of oxygen in the air and when its value drops below 18%, a person in such a room is in serious danger - a disturbance in the breathing rhythm occurs, the pulse quickens, then a disturbance of consciousness, a decrease in sensitivity, the ability to move is lost, nausea and vomiting appear, consciousness turns off, and within a few minutes death occurs. The particular danger is that this happens painlessly and the person is not aware of his condition.

In rooms with natural ventilation, work with open cryogenic vessels is allowed if the volume of the room in m 3 exceeds the volume of liquid in Dewar vessels in liters by at least 7 times.

What to do if liquid nitrogen gets on or spills on your hands?

Brief contact of the skin with liquid nitrogen is not dangerous, since an air cushion with low thermal conductivity is formed on the skin, which protects the skin from direct contact with liquid nitrogen. Prolonged contact of liquid nitrogen or liquid nitrogen-cooled material with skin or eyes can cause serious injury. Handle liquid nitrogen with care! If liquid nitrogen is spilled, ventilate the room.

Questions about liquid nitrogen and Dewar flasks

How quickly does liquid nitrogen evaporate from a Dewar flask?

This depends on the type of Dewar and its volume, as well as the manufacturer. Dewar vessels from American and European manufacturers have best characteristics in terms of liquid nitrogen evaporation and are in the range from 0.10 to 0.20 l/day for vessels with a volume of 2 to 50 liters. Dewar flasks from most other manufacturers typically have evaporation rates in the range of 0.15 to 0.40 l/day.
Typically, data on the amount of evaporation is indicated in the passport or operating instructions for the Dewar vessel.
The average storage time of liquid nitrogen until it completely evaporates can range from several weeks to a year.

How do Ukrainian Dewar vessels differ from French ones?

First of all, volatility. For example, the SDS35Bio60 vessel manufactured in Ukraine holds nitrogen for 210 days, and its French analogue B2036 - 360 days. Another important difference is the more attractive appearance French Dewar flasks.

Do I need to close the Dewar flask to prevent nitrogen from evaporating?

It is prohibited to tightly close the neck of the vessel with any foreign plugs. Only standard plugs and caps should be used, which, among other things, prevent the formation of ice in the neck of the Dewar flask due to condensation of moisture from the atmosphere. If mechanical damage and/or a “snow coat” appears on the outer surface of the vessel (especially if it is completely frozen!), it is necessary to empty the vessel of liquid nitrogen, put the vessel on heating and contact us for consultation.

Why can't you put a stick of cotton wool into a Dewar flask?

Foreign objects in a liquid nitrogen vessel can create an ice block and cause the vessel to collapse.

Is it possible to smoke in a car while transporting a Dewar flask with liquid nitrogen?

Are any documents required to transport liquid nitrogen in a car?

According to the Rules of the European Agreement concerning the International Carriage of Dangerous Goods (ADR), liquid nitrogen in quantities up to 333 kg can be transported without complying with the restrictions established for dangerous goods. This rule is confirmed by Order of the Ministry of Transport of the Russian Federation dated 08.08.1995 No. 73.

Allowed for transportation in one transport unit without complying with the above Rules of Dewar vessels SK-16 filled with liquid nitrogen in an amount of up to 15 pcs.

Is it possible to order nitrogen delivery for tomorrow? On a specific day?

Nitrogen is delivered within two working days from the date of order.

Is the overflow device connected to the electrical network?

What is the operating principle of the overflow device? Is there a pump?

The action of the overflow device is based on increasing the pressure in the cryogenic vessel by introducing a “warm” mass into the liquid and using the gas-lift effect. The evaporated part of the liquid after sealing the neck of the vessel creates in it overpressure, which forces the liquid to flow through the siphon of the device into the container to be filled.

None additional devices no liquid transfer required.

Why is the overflow device not suitable for all Dewar flasks?

The part of the overflow device that is inserted into the Dewar vessel is a rigid structure and is only suitable for a vessel of a certain height.

The use of an overflow device for Dewar vessels with necks of different diameters is possible by using an additional seal.

Which Dewar vessel is best to order for refilling CryoFrost and why?

Dewar vessels SK-16 or SK-25 depending on the flow of patients. The cryosurgical apparatus CryoFrost must be periodically refilled, so you need to choose vessels that have an overflow device. Pouring through the funnel will be inconvenient and difficult.

Liquefied gases are stored in Dewar flasks, which are glass or metal flasks with double walls (Fig. 1). Air is pumped out from the space between the walls, which leads to a decrease in their thermal conductivity. Since it is impossible to pump out all the air, the remaining molecules will transfer heat from environment to the contents of the Dewar flask. This residual thermal conductivity of the walls causes the liquefied gas in the vessel to continuously evaporate. When filling a Dewar flask with liquid nitrogen, the boiling point of which at normal atmospheric pressure is 77.3 K, it turned out that a mass M 1 of nitrogen evaporated per unit time. What mass of gas will evaporate from the same vessel per unit time if it is filled with liquid hydrogen, the boiling point of which is 20.4 K? The ambient temperature in both cases is 300 K.
Heat transfer occurs during such deviations from the state of thermodynamic equilibrium when different parts of the system have different temperatures. Under normal conditions, the mechanism of thermal conductivity of gas is as follows: molecules from a “hotter” region, as a result of chaotic movement, move in all directions and, colliding with molecules from “colder” regions, transfer part of their energy to them. Each molecule can transfer “excess” thermal energy over a distance on the order of the mean free path λ. Therefore, the total heat flow from a region with a higher temperature to a region with a lower temperature is proportional to the concentration of molecules n and their mean free path.
Each of the values ​​of n and λ depends on the pressure at which the gas is located. But their production does not depend on pressure.

nλσ≈1 (1)

The value σ=Πd 2 (d is the diameter of the molecule) does not depend on pressure. Therefore, the product nλ does not depend on pressure, although the concentration of molecules n is proportional to pressure.
Thus, under normal conditions, the thermal conductivity of a gas does not depend on Pressure, because all other quantities included in the expression for heat flow (temperature difference, wall area and distance between them) also do not depend on pressure.
So why do Dewar flasks pump out air from the space between the walls? The thing is that at very low gas pressure, when the free path of molecules is greater than the distance between the walls, the mechanism of thermal conductivity becomes different! gas molecules fly freely from one wall to another without colliding with each other, and transfer “excess” energy directly from wall to wall. Now thermal conductivity does not depend on the free path of molecules - it is only important that it exceeds the distance l between the double walls of the vessel. Since the heat flow, of course, in this case is proportional to the concentration of molecules, the lower the pressure of the air remaining between the walls, the lower its thermal conductivity will be.
In order to estimate the heat flow from the outer wall of the Dewar vessel to the cold inner wall, we will assume that each air molecule leaving the wall of the vessel has energy corresponding to the temperature of this wall. Colliding with another wall, the molecule transfers its entire energy to it. In other words, we believe that the interaction of molecules with the wall has the nature of an inelastic impact. If the impact of molecules on the wall were absolutely elastic, then the gas molecules would not transfer heat at all.
We will assume that the outer wall of the vessel has a temperature T0 equal to the ambient temperature. The liquefied gas in the Dewar vessel boils away little by little all the time, therefore, despite the continuous supply of heat, its temperature remains unchanged. The neck of the Dewar flask is kept open so that the evaporated gas can freely escape into the atmosphere - in otherwise the vessel will certainly explode due to the continuous increase in pressure. Thus, the temperature of the inner wall is equal to the boiling point T 1 of the liquefied gas at atmospheric pressure.
The energy flow transferred by air molecules from the hot wall to the cold one is proportional to the energy of the escaping molecule (i.e., the temperature of the hot wall T 0) and the number of molecules z leaving the hot wall per unit time. How many molecules leave the hot wall? Obviously, the same amount as flies to her from the cold wall. The number of such molecules is proportional to the concentration of molecules having a cold wall temperature T 1 and their average speed ‹v 1 ›:

z~n 1 ‹v 1 › (2)

Therefore, the energy flow from the hot wall to the cold one is proportional to the product T 0 z~T 0 n 1 ‹v 1 ›. Similarly, the energy flow transferred by molecules from a cold wall to a hot one is proportional to the product T 1 z~T 1 n 1 ‹v 1 ›. Consequently, the heat flow Q from the hot wall to the cold one, equal to the difference in the counter flows of energy, is proportional to the difference in temperature, concentration and average speed of molecules:

Q~(T 0 -T 1)n 1 ‹v 1 › (3)

What is the concentration n 1 of “cold” air molecules in the space between the walls? If we denote by n 0 the concentration of “hot” molecules, i.e. those that have left the outer wall, then the sum n 1 + n 0 is equal to the total air concentration n between the walls:

n=n 1 +n 0 (4)

As already noted, the same number of molecules fly to the hot wall per unit time as to the cold one. Therefore

n 1 ‹v 1 ›=n 0 ‹v 0 › (5)

Since the average speed is proportional to the root of the thermodynamic temperature, then from equality (5) we have

Substituting n 0 into relation (4), we find

Now expression (3) for the heat flow can be rewritten as

Due to this heat flow, a mass of liquefied gas M 1 evaporates per unit time, equal to the ratio Q to the specific heat of vaporization Λ:

Exactly the same expression will be valid in the case when the Dewar vessel is filled with another liquefied gas, whose boiling point is T 2 and the specific heat of vaporization is Λ 2. All proportionality coefficients omitted from qbop-mule (9) do not depend on what kind of gas is in the vessel. Therefore, for the ratio of the masses of different gases evaporating per unit time from the same Dewar flask, we obtain

Substituting here the values ​​of the specific heat of vaporization of hydrogen Λ 2 = 4.5 * 10 5 J/kg, nitrogen Λ 1 = 2.0 * 10 6 J/kg and their boiling temperatures T 2 = 20.4 K, T 1 = 77, 3 K, we find M 2 /M 1 ≈0.34.
It turned out that by mass, hydrogen boils away from the Dewar flask more slowly than nitrogen, although the boiling point of hydrogen is lower. However, with boiling rate and volume, everything is different. The density of liquid hydrogen is approximately 0.07 g/cm 3, nitrogen 0.8 g/cm 3, therefore for the ratio of the volumes of evaporated hydrogen V 2 and nitrogen V 1 we obtain V 2 /V 1 = 3.89, i.e. hydrogen boils away approximately 4 times faster than nitrogen.
From formula (9) it is clear that the mass of the evaporating gas is proportional to the concentration n of the air remaining between the walls of the Dewar vessel. Therefore, the less this air, the better the thermal insulation. Typically, Dewar flasks are evacuated to a high vacuum (10 -3 -10 -5 mm Hg). This corresponds to the concentration of the remaining air n=p/kT 0 ~10 11 -10 13 cm -3. At such concentrations, the mean free path will be, as can be seen from relation (1), a value of the order of λ≈1/(nΠd 2)~10-10 3 cm. The distance between the double walls l usually equal to several millimeters. Therefore, at such a pressure of the remaining air, the average free path significantly exceeds the distance between the walls and the mechanism of thermal conductivity is exactly the same as that considered in the problem.
When the air pressure between the walls is about 10 -2 mm Hg. Art. the free path becomes comparable to the distance between the walls. Therefore, pumping to such or greater pressure makes no sense at all, since under such conditions the thermal conductivity of air does not depend on pressure.
The surfaces of the vessel walls that form the vacuum space are usually coated with a thin layer of silver to reduce radiant heat transfer between the walls. Therefore, in this problem we did not take into account the radiant component of the heat flow.
Dewar flasks are also used to store substances at temperatures higher than the ambient temperature. Thermoses, common in everyday life, are glass Dewar flasks enclosed in a metal or plastic shell to protect them from damage.

Safety instructions

1. When operating, service personnel must have clothing that completely covers the surface of the body. It is necessary to use gloves GOST 12.4.010, goggles GOST 12.4.013 or a plexiglass shield. Avoid touching bare body surfaces to metal parts cooled with liquid nitrogen.
2. The room where work with liquid nitrogen is carried out must be equipped with supply and exhaust ventilation. When filling vessels, it is not allowed to overflow liquid nitrogen through the neck onto the floor.
3. The neck of the vessel should be closed only with a standard insert that has channels for releasing nitrogen vapor from the vessel.
4. To avoid increased nitrogen evaporation, it is not recommended to place vessels near heating devices or in direct sunlight.
5. If frost or a “snow coat” appears on the surface of the vessel lid, the layer of which increases as nitrogen evaporates, which is a sign of loss of vacuum in the insulating cavity of the vessel, it is necessary to immediately transfer the biomaterial reserves to a working vessel, then drain the liquid nitrogen and place the defective vessel hectares of heating for two days in a room with limited access to people.
These measures are necessary to prevent possible destruction of the vessel due to the release of gases when the adsorbent is heated.

Preparing the product for use

1. Preparing the vessel for work consists of conducting an external inspection of the vessel, stabilizing evaporation, loading canisters with a biological product and refueling.
2. During an external inspection, the absence of dents and cracks on the casing, the integrity of the insert and contamination of the internal cavity of the vessel are checked.
If there are dents on the casing, the vessel must be filled with liquid nitrogen and checked that the casing is not frozen. Contamination of the internal cavity of the vessel is eliminated with warm water and cleaning solutions. After washing, the vessel must be thoroughly dried.
3. is carried out through the filling hose of the transport tank or funnel. Refilling is carried out in small portions of liquid nitrogen, avoiding blocking the cross-section of the neck with a stream of nitrogen and overflowing it through the neck. The end of the funnel should be lower than the base of the neck.
4. After filling is completed, keep the Dewar vessel for 5-6 hours to stabilize the thermal regime and stop the rapid boiling of the liquid.
5. Load canisters with a bioproduct by slowly lowering them into liquid nitrogen. Place the handles of the canisters in the slots at the upper end of the neck of the Dewar vessel, placing the canisters evenly around the circumference.
6. After loading the canisters with bioproducts, top up with liquid nitrogen to the bottom of the neck, install the insert in the neck and close with the lid.

Operating procedure

1. Place the Dewar vessel and the canisters at the place of use. Periodically check the liquid nitrogen level in. Check the level with a special probe or a clean thin-walled metal tube. A sudden release of nitrogen vapor from the tube means that the tube has touched the liquid surface.
2. It is recommended to produce 25-15% of the volume of liquid nitrogen after the residue in the Dewar vessel. The frequency of refills depends on the number of canister loads. Therefore, it is recommended to open the Dewar flask and remove the canisters as little as possible during operation.
3. The Dewar vessel should be emptied of liquid nitrogen by turning it over or squeezing it out with nitrogen gas at a pressure of up to 0.03 MPa (0.3 kg/cm) using a special device.

Liquid nitrogen is a cryogenic liquid that is odorless and colorless. The temperature indicator of the cryofluid is 196 ºС.

Liquid nitrogen is safe only if certain requirements are met when working with this cryogenic liquid and the container in which it is placed:

1. Storage and movement of the vessel (even empty) is carried out only in a vertical position.
2. When working and transporting the vessel, avoid sharp shocks and impacts on the container.
3. Tightly closing the neck of a vessel with foreign objects and stoppers is prohibited! It is allowed to use only standard lids that prevent the formation of an ice crust on the neck and on the outer surface of the vessel.
4. It is advisable to fill the vessel using a special device such as a flexible metal hose or special. transfusion devices. The storage period of liquid nitrogen is from 54 to 213 days.
5. If you are filling a warm vessel, then you need to pour the product slowly and following all the rules, avoiding strong evaporation and splashing of the liquid.
6. When transferring liquid nitrogen, it is important not to allow drops of liquid to fall on the outer surface of the vessel. If liquid nitrogen does spill, it is necessary to check after 24 hours to ensure that there is no icing on the neck before using the vessel again and before installing cryogenic product transfer equipment.
7. It is not recommended to place the vessel near heating devices or in direct sunlight.
8. The room in which Dewar flasks with liquid nitrogen are located must be regularly ventilated, because nitrogen constantly evaporates and displaces oxygen, which can lead to oxygen deficiency and asphyxia.
9. Do not fill a container that has lost vacuum with nitrogen. Vacuum loss can be detected by observing whether frost forms on the outside of the vessel.
10. If there is damage on the outer surface of the vessel or you find frosting, then it is important to clean the container from the cryoproduct. It is forbidden to carry out repairs yourself.
11. It is prohibited to place foreign objects (cotton wool, cryomassage sticks, etc.) into the vessel. A foreign object inside a vessel is one of the reasons for its damage.

When using liquid nitrogen it is prohibited:

1. Store cryogenic liquid in containers not intended for cryogenic liquids.

For small volumes of liquid nitrogen, specialists use special 1-liter thermoses and Dewar flasks with a volume of 6 to 50 liters.

If nitrogen is used constantly and in large volumes, then cryogenic tanks, containers, and reservoirs are needed for this.

2. Allow prolonged contact of liquid nitrogen with the skin

If the cryoproduct ends up on a skin area, it disappears and does no harm.

However, if skin areas come into contact with liquid nitrogen for a long time, a person can get severe burns.

At the moment of contact with nitrogen, an insulating protective shell of steam is formed on the skin for a split second. Consequently, during performances and when organizing experiments with the product, experts do not allow the skin to come into contact with liquid nitrogen for more than a second.

Also, liquid nitrogen must not come into contact with clothing or jewelry located on parts of a person’s body. In this case, the person receives an instant burn.

3. Drink cryoproduct

This can cause internal burns and death.

In addition, when liquid nitrogen turns into a gaseous state, it increases 700 times, and if it enters the human body, nitrogen can lead to rupture of internal organs.

Working with an overflow device for Dewar Vessels "Dioxide"

Working with a PU type overflow device

1. Before installing the overflow device on the Dewar flask, the proper clamp level and correct height installation of the rubber seal. The rubber seal must be set to the required height, and the worker must tighten the clamps so that the overflow device does not move.

2.

• Heat exchangers are immersed in liquid nitrogen;
• The neck of the vessel is sealed using a rubber seal.
• Pressure arises due to the evaporation of the cryogenic liquid;
• The pressure in the tank increases, and liquid nitrogen is supplied through the overflow unit

It is important to monitor the nitrogen level in the container. If there is little cryo-liquid and the heat exchanger is not in nitrogen, then the overflow equipment will not work.

If there is enough nitrogen in the Dewar vessel, but the overflow process has stopped, it is necessary to remove the overflow device from the Dewar vessel and allow the heat exchanger to warm up to room temperature.

1. Heating of the heat exchanger should occur naturally. The heat exchanger must not be located near open heat sources.
2. Frame
3. Overflow tube
4. Rubber compressor
5. shield
6. Lever
7. Tip - adapter
8. Heat exchanger

Filter

1. Working with overflow device type YDB

• Before installing a YDB type overflow device on a Dewar Vessel, you must:
• install the overflow metal hose onto the overflow device and seal the threaded connection of the metal hose with fum tape to avoid loss of cryoproduct;
• check the liquid dispensing valve and the gas discharge valve: the cryo-liquid dispensing valve must be closed and the gas discharge valve must be open;

2. set the sealing fitting to the desired height and secure it by tightening the top nut;

• When installing an overflow device on a Dewar vessel:
• you immerse the overflow device body into the Dewar Vessel up to the sealing fitting;

3. tighten the lower nut of the fitting to seal the neck;

• To transfer liquid nitrogen, you must:
• close the gas release valve and open the liquid outlet valve;

start pumping the pump.

4. After the body of the overflow device and the metal hose have cooled down (usually this takes up to 1 minute), the supply of liquid nitrogen begins.

5. To maintain the liquid nitrogen overflow process, it is necessary to periodically pump up the pump.

6. To stop the overflow: open the gas release valve and, after the liquid stops overflowing, close the liquid outlet valve.

If the intervals between overflows are short (about 20 minutes), you can leave the overflow device on the Dewar vessel in a prepared state (after opening the gas release valve). It is not recommended to leave YDB installed on the Dewar for a long time due to increased product loss. Important!

1. When working with the overflow device, monitor the pressure level. To determine the pressure level, a pressure gauge is installed on the overflow device. Do not allow pressure to rise above 0.06 MPa! Excessive excess pressure can lead to the destruction of the vessel or even its explosion!
2. water pump
3. Heat exchanger
4. Pressure gauge
5. Gas relief valve
6. Liquid outlet valve
7. Frame
8. Sealing fitting

Overflow metal hose

Working with the cryo kit

A nitrogen thermos should be used to make honey. and cosmetic procedures are the most convenient. From Dewar flasks, liquid nitrogen is poured into a thermos using an overflow device.

Also, using a thermos is an excellent solution for performances and experiments.

Prohibited! To avoid explosions, close the lid of the thermos tightly even when there is liquid nitrogen in it.

2. Cryomassage wand

Designed for massage with liquid nitrogen. The end of the stick has notches that allow you to tightly secure cotton swabs. Before performing the procedure, the specialist dips the tip of the applicator with a cotton swab into nitrogen, and then runs the stick along the massage lines of the face.

3. Cryosurgical instrument Cryostik

Necessary for cryodestruction - removal of moles, warts, papillomas.

The cryosurgical instrument has 3 different types of tips. The specialist uses the tip whose diameter is optimal for removing the tumor.

To carry out the cryodestruction process, the tips are dipped in liquid nitrogen and then quickly applied to the unwanted tumor. Intracellular and intercellular fluids of pathological tissues are frozen, the cells die, and the neoplasm is destroyed.

After the procedure, healthy tissues gradually appear in place of the destroyed pathological tissues.