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CRYOGENIC TECHNOLOGY

1. Differentiate between a cryogen and an ordinary liquid

Ordinary liquids are liquids at standard  room temperature and pressure .Cryogenic liquids (also known as cryogens) are gases  at low temperatures and high pressure, they are generally  in their liquid state. These liquids are extremely cold and have boiling points less than -150°C (- 238°F).

2. Explain two precautions  to be taken  when cryogenic liquids  are in a Dewar flask.

• Keep dewar flasks covered with a loose fitting cap. This method prevents air or moisture from entering the container yet allows pressure to escape.
• Use only the stopper or plug supplied with the container. Glass Dewar flasks are available but never use them to store combustible or oxidizing cryogenic liquids.
• Store cryogenic cylinders in cool dry environments with adequate ventilation

3. Explain  the removal of gas plug  from a Dewar vessel

If any air is allowed to enter a Dewar vessel containing a cryogenic fluid,  the air would solidify inside and as a result, these may cause a blockage of the neck tube (causes gas- plug ) and prevents the escape of boiled off gases. The gas –plug formed can be removed by directing a steam of room temperature helium gas down through the gas –plug so as to dislodge it.

4. Describe  methods of stocking  cryogenics in the laboratory.

Store all cryogenic liquid containers upright in well-ventilated areas. Handle them carefully, and avoid dropping, rolling or tipping them on their sides. Take the same precautions for storing liquid cryogens as are needed for storing these materials in their gas form.

5. List any four research field where cryogenic technology may be applied.

• In maintaining an intermediate heat sink  between  the room  temperature  and other parts that  are supposed to be maintained at low temperatures
• In providing pre-cooling in liquefies  and refrigerants
• In pre- cooling apparatus that will later  be used  at lower temperatures
• In cooling adsorbents used for  purifying gases  e.g. cold traps
• In storing and preservation of semen for artificial insemination
• In medicine especially in cryosurgery and cold embedding techniques
• In manufacture ,packaging and storage of pharmaceutical products.

6. Explain the potential hazards of cryogenics

There are three groups of health hazards associated with cyrogenic liquids: extreme cold, asphyxiation, and toxicity.

Extreme Cold Hazard

Cryogenic liquids and their associated cold vapours and gases can produce effects on the skin similar to a thermal burn. Brief exposures that would not affect skin on the face or hands can damage delicate tissues such as the eyes. Prolonged exposure of the skin or contact with cold surfaces can cause frostbite. The skin appears waxy yellow. There is no initial pain, but there is intense pain when frozen tissue thaws.

Unprotected skin can stick to metal that is cooled by cryogenic liquids. The skin can then tear when pulled away. Even non-metallic materials are dangerous to touch at low temperatures. Prolonged breathing of extremely cold air may damage the lungs.

Asphyxiation Hazard

When cryogenic liquids form a gas, the gas is very cold and usually heavier than air. This cold, heavy gas does not disperse very well and can accumulate near the floor. Even if the gas is non-toxic, it displaces air. When there is not enough air or oxygen, asphyxiation and death can occur. Oxygen deficiency is a serious hazard in enclosed or confined spaces.

Small amounts of liquid can evaporate into very large volumes of gas. For example, one litre of liquid nitrogen vapourizes to 695 litres of nitrogen gas when warmed to room temperature (21°C).

Toxic Hazards

Each gas can cause specific health effects. For example, liquid carbon monoxide can release large quantities of carbon monoxide gas, which can cause death almost immediately. Refer to the material safety data sheet for information about the toxic hazards of a particular cryogen.

7. State four properties of cryogenic substances

8. State two reasons why hydrogen is not widely used as cryogen

Hydrogen is not easily available and cost of production is high Unlike other gases, hydrogen is not readily available in the atmosphere. It requires processes like electrolysis of water for its production. This is a very costly process and time consuming.

9. Explain precautions to be taken when cryogenic liquids are in the dewar flask

• Keep dewar flasks covered with a loose fitting cap. This method prevents air or moisture from entering the container yet allows pressure to escape. Use only the stopper or plug supplied with the container
• display hazard-warning signs to alert people to the presence of ;
• cryogenic liquids;
• be restricted to the relevant personnel only;
• be no-smoking and no naked flame areas;
• be well ventilated, including make-up air;
• have an atmospheric oxygen monitor to detect for and warn about oxygen enrichment or deficiency;
• have a safe means of escape;

10. State five uses of nitrogen gas in the laboratory

• As a coolant
• For storage of semen
• As an inert gas sometimes used in GLC
• used to control a highly sensitive atmosphere and  Ideal atmospheric conditions  for maintaining a safe, clean and controlled environment where equipment and subsequent procedures depend on it.

11. What are the flammability hazards of cryrogenic liquids?

Several types of situations exist that may result in a flammability hazard including: fire, oxygen-enriched air, liquid oxygen, and explosion due to rapid expansion.

1. Fire Hazard

Flammable gases such as hydrogen, methane, liquefied natural gas and carbon monoxide can burn or explode. Hydrogen is particularly hazardous. It forms flammable mixtures with air over a wide range of concentration (4 percent to 75 percent by volume). It is also very easily ignited.

1. Oxygen-Enriched Air

Liquid hydrogen and liquid helium are both so cold that they can liquefy the air they contact. For example, liquid air can condense on a surface cooled by liquid hydrogen or helium. Nitrogen evaporates more rapidly than oxygen from the liquid air. This action leaves behind a liquid air mixture which, when evaporated, gives a high concentration of oxygen. This oxygen-enriched air now presents all of the same hazards as oxygen.

1. Liquid Oxygen Hazard

Liquid oxygen contains 4,000 times more oxygen by volume than normal air. Materials that are usually considered non-combustible, (such as carbon and stainless steels, cast iron, aluminum, zinc and teflon (PTFE),) may burn in the presence of liquid oxygen. Many organic materials can react explosively, especially if a flammable mixture is produced. Clothing splashed or soaked with liquid oxygen can remain highly flammable for hours.

1. Explosion Due to Rapid Expansion

Without adequate venting or pressure-relief devices on the containers, enormous pressures can build up. The pressure can cause an explosion called a “boiling liquid expanding vapour explosion” (BLEVE). Unusual or accidental conditions such as an external fire, or a break in the vacuum which provides thermal insulation, may cause a very rapid pressure rise. The pressure relief valve may not be able to handle this increased pressure. Therefore, the containers must also have another backup device such as a frangible (bursting) disc.

12. Explain the problem of boil- off  in storage of cryogens

A cryogenic fluid is typically kept at low temperatures in a storage vessel. The storage has a major challenge due to the inherent heat input from the environment. The storage has a major challenge due to the inherent heat input from the environment. The vapours created due to the ambient heat input (while maintaining constant pressure in the storage vessel) are called “boil-off”.

13. Using a sketch diagram, describe the main features of a typical  cryogenic storage vessel

14. Explain the removal of a gas-plug from a dewar flask

• Using a piece of tubing positioned just above the blockage, direct a flow of room temperature (or heated) helium gas onto the blockage until an opening is formed to relieve the helium space.
• Once an opening is formed, the rest of the blockage may be carefully chipped away if it is accessible, or melted away with

If the blockage has just formed, and or it is determined that very little pressure build-up is present in the helium space, the following alternate procedure may be used.

• Obtain a suitable length of copper or aluminum rod and affix a stop at a point such that the rod can not accidentally drop into the reservoir when the blockage is pierced.
• Warm the rod somewhat, if convenient.
• Lower the rod into the neck tube to contact the blockage, and allow it to remain there until its heat capacity has been completely transferred. Repeat until an opening is formed.
• Carefully chip away the remaining ice.

State eight methods of handling and storage of cryogens

• Handle cryogenic liquids carefully. …
• Never allow unprotected parts of your body to touch any objects cooled by liquid cryogens. …
• Use tongs or proper gloves to handle objects that are in contact with cryogenic liquids.
• Appropriate personal protective equipment should be worn when handling cryogenic liquids.
• Due to high expansion ratios and potential for asphyxiation cryogenic materials must only be used/stored only in well-ventilated areas. Cryogenic materials and dry ice must  be stored in cold rooms.
• Never dispose of cryogens  eg Nitrogenby pouring it on the floor or into a sink. It could displace enough oxygen and cause suffocation and cause damage to materials it contacts. (Nitrogen is colorless and odorless – the cloud that forms when you pour LN2 is condensed water vapor from the air, not nitrogen gas.)
• Do not allow any cryogens to touch any part of your body or become trapped in clothing near the skin.
• Do not touch any item that has been immersed in cryogensuntil it has warmed to room temperature.
• Metals to be used for equipment in cryogensmust possess satisfactory physical properties at the low operating temperatures.
• Introduction of a substance which is at normal room temperature into a cryogenic liquid or gas is always somewhat hazardous. There is a violent evolution of gas, and there is likely to be considerable splashing of liquid.