V(g). Chemical Safety - Cryogens
A. Cryogenic Systems
Cryogenics is the science of very low temperatures. An accepted temperature used to distinguish between refrigeration and cryogenics is -73.3oC (-100oF). Low temperatures in the cryogenic range are generally obtained by the liquidification or solidification of gases. The most commonly used cryogens and their properties are listed in Appendix V(g)-A.
The primary hazard of cryogenic materials is their extreme coldness, which can result in frostbite and severe tissue damage. Accumulated vapors may also act as an asphyxiant. Liquid gases are extremely concentrated relative to room temperature gases and, consequently, their potential hazards are magnified. Liquefied inert gases, such as nitrogen, in contact with cold metal surfaces can cause condensation of oxygen from the room air resulting in an oxygen enriched atmosphere and, consequently, an increase fire hazard. The low temperatures involved also affect the properties of other materials; for example, rubber may become brittle and disintegrate and some metal alloys may become brittle.
Cryogens have very high liquid: vapor expansion ratios. For example, liquid nitrogen expands to 700 times its initial volume when it vaporizes. For liquid hydrogen, 22 cubic feet can expand to fill 16,000 cubic feet. This rapid expansion can cause a displacement of oxygen and consequently a life threatening asphyxiant atmosphere.
B. Storage And Handling Precautions
These are general precautions. The faculty or supervising staff member responsible for the cryogenic operation should establish more specific safety guidelines.
- Store and use cryogens only in containers and equipment recommended for cryogenic service.
- Avoid confined areas where vaporization occurs (e.g., do not put your head in the dry ice container).
- Ensure that all apparatus is properly vented to prevent accumulation of pressure and be cognizant of ice blocks that could block vent lines.
- Always wear chemical splash goggles and a face shield in cases where there is a likelihood of contact.
- Always wear long sleeves and/or a lab coat.
- Watches, rings, or other jewelry that could trap the material next to the skin should not be worn.
- If gloves are necessary to handle containers or cold metal parts, they should be impervious and loose enough to be thrown off in the case of contamination.
- Neither liquid nitrogen, liquid air, or any other cryogen with a normal boiling point < 183oC should be used to cool a flammable mixture in the presence of air as oxygen can condense from the air leading to an explosive mixture.
- Equipment must be kept clean to avoid contamination of organics with a cryogenic oxidant (e.g., liquid oxygen) or oxidants with cryogenic fuel (e.g., liquefied natural gas).
- When flammable gases are being used, eliminate potential ignition sources.
- Flammable and toxic gases should only be used in a fume hood.
- If liquid nitrogen has a blue tint, it has been contaminated with oxygen and should be replaced. The contaminated material is dangerous and potentially explosive.
- When spilled, liquid oxygen soaks into materials it comes into contact with, and the resulting mixture may be explosive.