Cryogens

8

Cryogens

Cryogenics, or low-temperature technology, is the science of producing and maintaining very low temperatures usually below 120 K, as distinct from traditional refrigeration which covers the temperature range 120 to 273.1 K. At or below 120 K, the permanent gases including argon, helium, hydrogen, methane, oxygen and nitrogen can be liquefied at ambient pressure as exemplified by Table 8.1. Any object may be cooled to low temperatures by placing it in thermal contact with a suitable liquefied gas held at constant pressure. Applications can be found in food processing, rocket propulsion, microbiology, electronics, medicine, metal working and general laboratory operations. Cryogenic technology has also been used to produce low-cost, high-purity gases through fractional condensation and distillation. Cryogens are used to enhance the speed of computers and in magnetic resonance imaging to cool high conductivity magnets for non-intrusive body diagnostics. Low-temperature infrared detectors are used in astronomical telescopes. Table 8.1 P r o p e r t i e s of common cryogens

Gas

Boiling point (~

Volume of gas produced on evaporation of 1 litre of liquid (litres)

Helium

- 269

757

Hydrogen

- 253

851

Neon

- 246

1438

Nitrogen

- 196

696

Fluorine

- 187

888

Argon

- 186

847

Oxygen

- 183

860

Methane

- 161

578

Krypton

- 151

700

Xenon

- 109

573

Chlorotrifluoromethane

- 81

Carbon

- 78.5

dioxide

553

Every gas has a critical temperature above which it cannot be liquefied by application of pressure alone (Chapter 4). As a result, gases used, e.g., as an inert medium to reduce oxygen content of atmospheres containing flammable gas or vapour (Chapter 6) are often shipped and stored as cryogenic liquid for convenience and economy. In the laboratory, a range of 'slush baths' may be used for speciality work. These are prepared by cooling organic liquids to their melting points by the addition of liquid nitrogen. Common examples are given in Table 8.2. Unless strict handling precautions are instituted, it is advisable to replace the more toxic and flammable solvents by safer alternatives.

LIQUID OXYGEN 259

Table 8.2 Working temperatures of cryogenic slush baths

Bath liquid

Temperature (~

Carbon tetrachloride Chiorobenzene Solid carbon dioxide in acetone or methylated spirits (1) Toluene Carbon disulphide Diethyl ether Petroleum ether

-

23 45 63 78 95 - 112 - 120 - 140

(1)Liquid nitrogen is omitted from this mixture and the solvent is used to improve the heat transfer characteristics of cardice.

Typical insulating materials include purged rockwool or perlite, rigid foam such as foam-glass or urethane, or vacuum. However, because perfect insulation is not possible heat leakage occurs and the liquefied gas eventually boils away. Uncontrolled release of a cryogen from storage or during handling must be carefully considered at the design stage. The main hazards with cryogens stem from: 9 The low temperature which, if the materials come into contact with the body, can cause severe tissue bums. Flesh may stick fast to cold uninsulated pipes or vessels and tear on attempting to withdraw it. The low temperatures may also cause failure of service materials due to embrittlement; metals can become sensitive to fracture by shock. 9 Asphyxiation (except with oxygen) if the cryogen evaporates in a confined space. 9 The very large vapour-to-liquid ratios (Table 8.1) so that a large cloud, with fog, results from loss of liquid. 9 Catastrophic failure of containers as cryogen evaporates to cause pressure build-up within the vessel beyond its safe working pressure (e.g. pressures <280 000 kPa or 40 600 psi can develop when liquid nitrogen is heated to ambient temperature in a confined space). 9 Flammability (e.g. hydrogen, acetylene, methane), toxicity (e.g. carbon dioxide, fluorine), or chemical reactivity (fluorine, oxygen). 9 Trace impurities in the feed streams can lead to combination of an oxidant with a flammable material (e.g. acetylene in liquid oxygen, solid oxygen in liquid hydrogen) and precautions must be taken to eliminate them. 9 Several materials react with pure oxygen so care in selection of materials in contact with oxygen including cleaning agents is crucial. Key precautions are given in Table 8.3. The cryogens encountered in greatest volume include oxygen, nitrogen, argon and carbon dioxide. Their physical properties are summarized in Table 8.4.

Liquid oxygen Liquid oxygen is pale blue, slightly heavier than water, magnetic, non-flammable and does not produce toxic or irritating vapours. On contact with reducing agents, liquid oxygen can cause explosions.

260

CRYOGENS

Table 8.3 General precautions with cryogenic materials Obtain authoritative advice from the supplier. Select storage/service materials and joints with care, allowing for the reduction in ductility at cryogenic temperatures. Provide special relief devices as appropriate. Materials of construction must be scrupulously clean, free of grease etc. Use only labelled, insulated containers designed for cryogens, i.e. capable of withstanding rapid changes and extreme differences in temperature, and fill them slowly to minimize thermal shock. Keep capped when not in use and check venting. Glass Dewar flasks for small-scale storage should be in metal containers, and any exposed glass taped to prevent glass fragments flying in the event of fracture/implosion. Large-scale storage containers are usually of metal and equipped with pressure-relief systems. In the event of faults developing (as indicated by high boil-off rates or external frost), cease using the equipment. Provide a high level of general ventilation taking note of density and volume of gas likely to develop: initially gases will slump, while those less dense than air (e.g. hydrogen, helium) will eventually rise. Do not dispose of liquid in a confined area. Prevent contamination of fuel by oxidant gases/liquids. With flammable gases, eliminate all ignition sources (refer to Chapter 6). Possibly provide additional high/low level ventilation; background gas detectors to alarm, e.g. at 40% of the LEL. With toxic gases, possibly provide additional local ventilation; monitors connected to alarms; appropriate air-fed respirators. (The flammable/toxic gas detectors may be linked to automatic shutdown instrumentation.) Limit access to storage areas to authorized staff knowledgeable in the hazards, position of valves and switches. Display emergency procedures. Wear face shields and impervious dry gloves, preferably insulated and of loose fit. Wear protective clothing which avoids the possibility of cryogenic liquid becoming trapped near the skin: avoid turnups and pockets and wear trousers over boots, not tucked in. Remove bracelets, rings, watches etc. to avoid potential traps of cryogen against skin. Prior to entry into large tanks containing inert medium, ensure that pipes to the tank from cryogen storage are blanked off or positively closed off: purge with air and check oxygen levels. If in doubt, provide air-fed respirators and follow the requirements for entry into confined spaces (Chapter 13). First aid measures include" Move casualties becoming dizzy or losing consciousness into fresh air and provide artificial respiration if breathing stops. Obtain medical attention (Chapter 13). In the event of 'frost-bite' do not rub the affected area but immerse rapidly in warm water and maintain general body warmth. Seek medical aid. Ensure that staff are trained in the hazards and precautions for both normal operation and emergencies.

Gaseous oxygen is colourless, odourless and tasteless. It does not burn but supports combustion of most elements. Thus upon vaporization liquid oxygen can produce an atmosphere which enhances fire risk; flammability limits of flammable gases and vapours are widened and fires burn with greater vigour. It may cause certain substances normally considered to be non-combustible, e.g. carbon steel, to inflame. In addition to the general precautions set out in Table 8.3, the following are also relevant to the prevention of fires and explosions" 9 Prohibit smoking or other means of ignition in the area. 9 Avoid contact with flammable materials (including solvents, paper, oil, grease, wood, clothing) and reducing agents. Thus oil or grease must not be used on oxygen equipment. 9 Purge oxygen equipment with oil-flee nitrogen or oil-flee air prior to repairs. 9 Post warning signs. 9 In the event of fire, evacuate the area and if possible shut off oxygen supply. Extinguish with

LIQUID CARBON DIOXIDE 261

Table 8.4 Physical properties of selected cryogenic liquids

Oxygen

Property of liquid Molecular weight Boiling point (at atmospheric pressure) Freezing point Critical temperature Density of liquid (at atmospheric pressure)

Nitrogen

Argon

32

28

40

~ ~ K

-183 -219 154.8

-196 -210 126.1

-186 -190 150.7

kg/m 3

1141

807

1394

Carbon dioxide 44 -78

1562

(solid) Density of vapour (at NBP) Density of dry gas at 15~ and at atmospheric pressure Latent heat of vaporization at NBP and atmospheric pressure Expansion ratio (liquid to gas at 15~ and atmospheric pressure) Volume per cent in dry air

kg/m 3

4.43

4.59

5.70

2.90

kg/m 3

1.34

1.17

1.67

1.86

kJ/kg

214

199

163

%

842 20.95

682 78.09

822 0.93

151 538 (1) 0.03

NBP Normal boiling point (1)From liquid CO2 at 21 bar-18~

water spray unless electrical equipment is involved, when carbon dioxide extinguishers should be used.

Liquid nitrogen and argon Liquid nitrogen is colourless and odourless, slightly lighter than water and non-magnetic. It does not produce toxic or irritating vapours. Liquid argon is also colourless and odourless but significantly heavier than water. Gaseous nitrogen is colourless, odourless and tasteless, slightly soluble in water and a poor conductor of heat. It does not burn or support combustion, nor readily react with other elements. It does, however, combine with some of the more active metals, e.g. calcium, sodium and magnesium, to form nitrides. Gaseous argon is also colourless, odourless and tasteless, very inert and does not support combustion. The main hazard from using these gases stems from their asphyxiant nature. In confined, unventilated spaces small leakages of liquid can generate sufficient volumes of gas to deplete the oxygen content to below life-supporting concentrations: personnel can become unconscious without warning symptoms (Chapter 5). Gas build-up can occur when a room is closed overnight. Also, because the boiling points of these cryogenic liquids are lower than that of oxygen, if exposed to air they can cause oxygen to condense preferentially, resulting in hazards similar to those of liquid oxygen.

Liquid carbon dioxide Liquid carbon dioxide is usually stored under 20 bar pressure at-18~ Compression and cooling of the gas between the temperature limits at the 'triple point' and the 'critical point' will cause it

262 CRYOCENS to liquefy. The triple point is the pressure temperature combination at which carbon dioxide can exist simultaneously as gas, liquid and solid. Above the critical temperature point of 31 ~ it is impossible to liquefy the gas by increasing the pressure above the critical pressure of 73 bar. Reduction in the temperature and pressure of liquid below the triple point causes the liquid to disappear, leaving only gas and solid. (Solid carbon dioxide is also available for cryogenic work and a t - 7 8 ~ the solid sublimes at atmospheric pressure.) Liquid carbon dioxide produces a colourless, dense, non-flammable vapour with a slightly pungent odour and characteristic acid 'taste'. Physical properties are given in Table 8.5 (see also page 277). Figure 8.1 demonstrates the effect of temperature on vapour pressure.

Table 8.5 Physical properties of carbon dioxide Molecular weight Vapour pressure at 21~ Specific volume at 21~ 1 atm Sublimation point at 1 atm Triple point at 5.11 atm Density, gas at 0~ 1 atm Specific gravity, gas at 0~ 1 bar (air= 1) Critical temperature Critical pressure Critical density Latent heat of vaporization at triple point at 0~ Specific heat, gas at 25~ 1 atm Cp Cv ratio Cp/Cv Thermal conductivity at 0~ at 100~ Viscosity, gas at 21~ 1 atm Entropy, gas at 25~ 1 atm Heat of formation, gas at 25~ Solubility in water at 25~ 1 atm

44.01 57.23 bar 547 ml/g -

78.5oc

_

56.6oC

1.977 g/I 1.521 31~ 73.9 bar 0.468 g/ml 83.2 cal/g 56.2 cal/g 0.205 cal/g ~ 0.1565 cal/g ~

1.310 3.5 x 10-5 cal/s cm 2 ~ 5.5 x 10 -s cal/s cm 2 ~ 0.0148 cP 1.160 cal/g ~ - 2137.1 cal/g 0.759 vol/vol water

Inhalation of carbon dioxide causes the breathing rate to increase (Table 8.6): 10% C O 2 in air can only be endured for a few minutes; at 25% death can result after a few hours exposure. The 8 hr TWA hygiene standard (see Chapter 5) for carbon dioxide is 0.5%; at higher levels life may be threatened by extended exposure. The following considerations therefore supplement those listed in Table 8.3: 9 Ensure that operator exposure is below the hygiene standard. (Note: For environmental monitoring, because of its toxicity, a COe analyser must be used as distinct from simply relying on checks of oxygen levels.) 9 When arranging ventilation, remember that the density of carbon dioxide gas is greater than that of air. 9 Ensure that pipework and control systems are adequate to cope with the pressures associated with storage and conveyance of carbon dioxide, which are higher than those encountered with most other cryogenic liquids.

LIQUEFIED NATURAL GAS 1000 900 800 700 600

-

pressure 1071.6 p.s.i.a. at 31~

500 400 300 -

200

100 908070-

--

Freezing point

60o_ 09 {3,. v

50-

i,-

40-

(/)

30i.. 0 El.

20-

10. 9876543-

2 -

I

I

I

~

-150

1.

-125

I -100

-75

-50

-25

0

~

-101

-87

-73

-60

-46

-32

-18

,

I

I

I

25

50

75

100

125

-4

10

25

37

52

Temperature Figure 8.1

Carbon dioxide vapour pressure versus temperature

Liquefied natural gas Liquefied natural gas is predominantly methane. The cryogenic properties of methane are: Boiling point Critical temperature Critical pressure Liquid-to-gas ratio by volume

-162~ -82~ 45.7 atm 1 to 637

263

264 CRYOGENS Table 8.6 Effect of carbon dioxide exposure on breathing rates CO 2

in air (vol. %)

O.1-1 2 3 5

Increased lung ventilation slight, unnoticeable 50% increase 100% increase 300% increase; breathing becomes laborious

The impurities in LNG result in slightly different properties, and there are significant variations depending upon its source of supply. Natural gas is considered non-toxic but can produce an oxygen deficient atmosphere (p. 153). It is odourless (therefore an odorant is added for distribution by pipeline). Its physical properties are similar to those of methane, i.e.: Ignition temperature Flammable limits Vapour density

537~ 5 % to 15% 0.55

However, the safety considerations with LNG must account for: 9 The tendency, for economic reasons, to store it in very large insulated containers. 9 The requirement for special materials of construction to cater for storage at-162~ and for design of plant to cope with thermal differences. 9 The prevention of leaks, since liquid may generate large quantities of flammable gas. 9 The addition of odorants after vaporization, i.e. the liquid is odour-free. 9 The gas generated by vaporization is cold and therefore denser than air, i.e. it tends to slump. LPG and methane are discussed further in Chapter 9.