d e t y in the chemical laboratory
edited by MALCOLM M. RENFREW University of Idaho MOSCOW. Idaho 83843
The Safe Use of Cryogenic Fluids in the Research Laboratory Mary Jane Hofmam Central Research and Development Department, E. I. du Pont de
The use of cryogenic fluids, primarily characterized by having a boiling point of less than -73.3OC 1-1W0F). for ultra-lowtemperature technology i s common in chemical and hioloeical research. Nitroeen. .. . helium, and argon are the materials most communly used. Also, slush mixtures of dry ice (sdid carbon dioxide) with iropropanol or acetone, achieving an equilibrium point of - 7 8 T (-108.4 'F),are used as cold traps to condense volatile vapors from a system. Liquid oxygen should not he used in a research environment (see Flammability section). Using cryogenic fluids improperly may produce physical and personnel hazards. Hazards associated with cryogenics are not always obvious and special precautions are required for safe handling. T h e fallowing information regarding characteristics and hazards of crvoeenic fluids and safe oractices for t h e i r & is oresented to addiess the needs of both the academic and the industrial research laboratory. Physical Propertles The table contains the two principle physical properties of cryogenic fluids. These are important to recognize when selecting a material for cryogenic service. Hazards The hazards in using cryogenic fluids or cryogenic systems can be summarized in four major areas, some of whieh do not apply to every material.
Mary Jane Hdmann gradvated from Bowling seen State University. Bowling Green. OH, wnh a BS in chamisby and mathematics. She has w&ed for Du Pont lor over 15 years in research, information services, and safety. At present, she is a Safety Engineer and a Radiological Safely Officer fov me Experimental Station. Du Pont's basic research facility in Wilmington. DE. She is a member of American Chemical Society and Its Division of Chemical Heakh end Safety.
Nemours 8 Co.. Inc., Wilmington. DE 19898 Physlcal Propertles of Cryogenic Flulds
Liquefied
Bolting Poim
Gas
i"c)
Volume Expansion Ratio Liquid to Gas Phase
carbon dioxide methane oxygen argon nitrogen heiium-4 helium3
-78.5 -161.4 -183.0 -185.7 -195.8 -268.9 -269.9
553 to 1 578 to 1 860 to 1 847 to 1 696 to I 757 to I 757 to 1
Personnel The primary hazard to personnel is skin or eye contact with the cryogenic liquids, since these liquids are prone to splash in use due to the large volume expansion ratio when the liquid warms up. Accidental contact will cause a freezing injury similar to a hum. Eyes and skin should he protected whenever the possibility of contact with the liquid, cold pipes and cold equipment, or the cold gas exists. Safety goggles or a face shield should he worn if liquid ejection or splashing may occur or cold gas may issue forcefullv from eouioment. Clean. insulated gloves that can h i e h v removed and lone sleevrs are recommended for pmtertron of hands and arms For protectron oflega, long pants are recommended. Although msny of the cryogenic fluids are nontoxic in the gas phase, they are all capable of causing asphyxiation by displacing oxygeu-containing air. Since many of them are also odorless, colorless, and tasteless, there will he no advance warning of their presence. Prudent placement of equipment in adequately ventilated areas is therefore necessary whenever cryogenic liquids are stored or used.
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Flammability This is an obvious hazard when a material such as liquid hydrogen is used, but liquefied inert gases such as liquid nitrogen or liauid helium are caoable of condensineu orygen from the air, causing oxygen enrichment or entrapment in confined areas. I t is thus easy to create combinations of oxygen and combustible substances, such as fuels and other organic substances, which will ig'nite easily, burn intensely, and detonate in a ~
~
fire. Liquid oxygen itself is an extremely dangerous material because it is a vigorous oxidizer. High PIessure Gas Liquefied gases are generally stored at atmospheric pressure in an insulated container whieh keeps them near their boiling point, with some gas present. The large enoansion in volume that takes olace when the liquid bcromev a gas meana that pressure ran build up in an unvented or unrelieved rontainer and in transfer linra and piping. System design must take this expansion ratio into account. Materials The selection of materials to he used in cryogenic service is important because of the changes in physical properties of meterials at verv low temoeratures. Some materials become extremely trittle. losing their usefulness, while "them retain their ductility.Chcmical interactions between thecryugenic liquid and its container or equipment must also be evaluated. S t o r a g e a n d Transfer Cryogenic fluids are stored in vacuuminsulated containers designed to minimize hoil-off of the product. The most common container is a double-walled metal or glass container known as a Dewar flask. The evacuated space between the two walls usually contains. an insulating material. The Dewar should he kept covered with a cap that al-
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lows escape of built-up pressure and keeps air and moisture out. Glass Dewars not in a metal container should be shielded either by being placed in a metal can or hy taping the outside. This will guard against injuries should the Dewar imolode. I)ewar vessels should be examined r e p Iarly and maintained so that there is little pndurt loasand no possibility ofan ice plug forming in the neck with a resulting pressure build-up. Liquid-nitrogen Dewars present few problems, and, if the container and the cap appear in good condition, the Dewar is aceeotahle for use. Modern liauid-helium D w n r s are mow romplrr with two rel~ef valves. a pressure gauge, n l q u ~ dlmc access and a gas-phase valve, all protected by an aluminum chime ring. If any of these components appear broken or defective, the Dewar should be returned to the vendor. Transfer of cryogenic liquids from large Dewar vessels should be accomplished with soecid transfer tubes desiened for the oarticulsr application. Frequently, the vendor 01 the crycgtnrc materiala will have a liquid withdrawal device that pressure-transfers liquid nitrogen. Alternatively, gaseous nitrogen at 5 psi can be applied in conjunction with a transfer tube to remove the liquid nitrogen. Tipping or tilting to pour the liquid may damage large Dewars so this method of transfer is not acceptable. Do not use heat guns or similar equipment to warm transfer tubing quickly for disconnection. The fumes given off from the tubing are likely to cause the activation of any smoke detectors in the area. To protect the vacuum insulation system of Dewars, handle containers carefully. Large Dewars should be olaced on dollies which mow freely so that there is no pmpibiliry of personnel injury or damage to the supported Dewars.
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Dlsposal If it is necessary to dispose of large amounts of cryogenic fluids, they should be ieleased only in an open outdoor area that will provide the ventilation needed for rapid, safe evaporation. Protective leather or thermal barrier gloves and a face shield must be worn.
Suggested Addlllonal Readlng Ordin, Paul M. "Safety in the Handling and Usa of cryogenic Fluids", in "Liquid Cryrigem"; CRC: BoeaRaton, FL, 1983:Vol I, pp 1-57. Compressed Gas Association "Safe Handling and Uaa of cryogenic Luqida", in "Handbook of compreaud ca. sea'', 2nd d.; Van Noatrand Reinhold:New York. 1981; pp 205--227. National Research Cavneil '"PrudentPractices for Handling Hazardow Chemicals in Laboratmias": National Academy: Washington, 1981; pp 26.76.
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