Sulfur Hexafluoride SF6
Sulfur hexafluoride [SF6] is an inorganic colourless, odourless, liquefied, non-toxic and non-flammable greenhouse gas. It is shipped as a liquid under its own vapour pressure. It is generally transported as a liquefied compressed gas.
It has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom.
It is a hypervalent molecule – a molecule that contains one or more typical elements (group 1, 2, 13-18) formally bearing more than eight electrons in their valence shells.
Typical for a non-polar gas, it is poorly soluble in water but soluble in non-polar organic solvents. Sf6 is 5 times denser than air. It has a density of 6.12 g/L at sea level conditions, which is considerably higher than the density of air.
Method of Preparation:
The only industrial process currently in use is the synthesis of sulphur hexafluoride by allowing fluorine obtained by electrolysis to react with sulphur according to the exothermic reaction:
S + 3F2 → SF6 [+ 262 kcal]
During this reaction, a certain number of other fluorides of sulphur are formed, such as SF4, SF2, S2F2, S2F10, as well as impurities due to the presence of moisture, air and the carbon anodes used for the fluorine electrolysis. These by products are removed by various purification processes.
There is virtually no reaction chemistry for SF6. We can prepare it from the elements through “exposure of S8 to F2”.
There are also other methods to prepare- Sulfur fluorides are cogenerated, but these are removed by heating the mixture to disproportionate any S2F10 [which is highly toxic, unlike SF6 are non poisonous and odourless] and then scrubbing the product with NaOH to destroy remaining SF4
“[S2F10 - SF4=SF6]“.
Properties of SF6:
Molecular Weight of SF6
- Molecular weight : 146.05 g/mol
- 5 times denser than air
- 5.11 @ 68 F
- 2.5 cu.ft./lb @ 70 F
Solid phase- Latent heat of Fusion
- Latent heat of fusion (1,013 bar, at triple point) : 39.75 kJ/kg
- Latent heat of vaporization (1.013 bar at boiling point) : 162.2 kJ/kg
- Vapour pressure (at 21 °C or 70 °F) : 21.5 bar
- Liquid density (at triple point) : 1880 kg/m3
- Boiling point (Sublimation) : -63.9 °C
- Gas density (1.013 bar and 15 °C (59 °F)) : 6.27 kg/m3
- Heat capacity at constant pressure (Cp) (1.013 bar and 21 °C (70 °F)) : 0.097 kJ/(mol.K)
- Compressibility Factor (Z) (1.013 bar and 15 °C (59 °F)) : 0.9884
- Specific gravity (air = 1) (1.013 bar and 21 °C (70 °F)) : 5.114
- Viscosity (1.013 bar and 0 °C (32 °F)) : 0.000142 Poise
- Thermal conductivity (1.013 bar and 0 °C (32 °F)) : 12.058 mW/(m.K)
- Specific volume (1.013 bar and 21 °C (70 °F)) : 0.156 m3/kg
- Critical pressure : 37.59 bar
- Critical temperature : 45.5 °C
- Triple point temperature : -49.4 °C
- Triple point pressure : 2.26 bar
Solubility in Water
- Solubility in water (20 °C and 1 bar) : 0.007 vol/vol
Green House Gas Concerns of SF6:
SF6 is the most potent greenhouse gas with a global warming potential of 22,800 times that of CO2. SF6 is an anthropogenically produced compound, mainly used as a gaseous dielectric in gas insulated switchgear power installations. Given the low amounts of SF6 released compared to carbon dioxide, its overall contribution to global warming is estimated to be less than 0.2 percent. Sulfur hexafluoride is also extremely long-lived – they remain in the atmosphere for longer period than any other compound. SF6 acting as green house gas can have a heavy impact on the Global climate, and its concentration in the earth atmosphere is rapidly increasing. It is inert in the troposphere and stratosphere and has an estimated atmospheric lifetime of 800–3200 years. Sf6’s anthropogenic sources are di- electric mediums.
Health and Physiological effects:
During its working cycle, SF6 decomposes under electrical stress, forming toxic by-products that are a health threat for working personnel in the event of exposure. The danger with sulfur hexafluoride is that the degeneration products can be toxic, causing nausea and vomiting, pulmonary symptoms, and transient atelectasis. It may be contaminated with other fluorides of sulfur, such as sulfur pentafluoride and disulfur decafluoride, which are extremely toxic and are respiratory irritants. The gas can be inhaled in a small, safe amount and cause the breather’s voice to sound very deep. This is due to the gas’s large molar mass. It is possible to safely breathe sulfur hexafluoride – heavy gas as long as they include a 20% mixture of oxygen. Repeated high exposures can cause deposits of fluorides in the bones (fluorosis) that may cause pain, disability and mottling of the teeth. Repeated exposure can cause nausea, vomiting, loss of appetite, diarrhoea or constipation. Nosebleeds and sinus problems can also occur.
1. Oxy-fluoride levels or other by-product concentrations in the operating gas matrix should be traced to predetermine the overall gas toxicity
2. Contaminants should be systematically considered during maintenance, chamber evacuation and system opening process;
3. Small SF6 quantities leaking into air or stagnated pollutant concentrations in the operating field should be analyzed and compared to the threshold limit values and permissible exposure levels.
Uses of SF6
The unique properties of SF6 have led to its adoption for a number of industrial and scientific applications including,
- Medical applications: electrical insulation in medical equipment (e.g. X-ray machines), or surgery,
- Electrical insulation in scientific equipment: (electron microscopes, particle accelerators such as Van der Graf generators),
- Acoustic insulation in double glazed windows
- Tracer gas for studying airflow in ventilation systems (for instance in mines) or in the high atmosphere.
- Tracer for leak detection in pressurised systems.
- To provide a special atmosphere for metallurgical processing (aluminium and magnesium) for military purpose.
Related Terms in the Glossary: