Posts Tagged global warming

SF6 Sulfur Hexafluoride

Posted by on Saturday, 19 March, 2011

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

Specific Gravity:

  • 5.11 @ 68 F

Specific Volume:

  • 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

Liquid phase

  • 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

Gaseous phase

  • 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 point

  • Critical pressure  : 37.59 bar
  • Critical temperature  : 45.5 °C

Triple point

  • 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.

Precautions:

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,

  1. Medical applications: electrical insulation in medical equipment (e.g. X-ray machines), or surgery,
  2. Electrical insulation in scientific equipment: (electron microscopes, particle accelerators such as Van der Graf generators),
  3. Acoustic insulation in double glazed windows
  4. Tracer gas for studying airflow in ventilation systems (for instance in mines) or in the high atmosphere.
  5. Tracer for leak detection in pressurised systems.
  6. To provide a special atmosphere for metallurgical processing (aluminium and magnesium) for military purpose.

 

References:

http://www.esrl.noaa.gov/gmd/hats/insitu/cats/conc/mlosf6.html

http://www.euractiv.com/en/sustainability/climate-meps-give-gas-bill-green-boost/article-145749

http://encyclopedia.airliquide.com/Encyclopedia.asp?GasID=34

http://www.asria.org/events/hongkong/05sept/index_html/lib/gail_kendall.pdf

http://toxnet.nlm.nih.gov/cgi-bin/sis/search/r?dbs+hsdb:@term+@rn+2551-62-4

Fact sheet:

http://www.c-f-c.com/specgas_products/sulfurhex_sf6.htm

http://www.asria.org/events/hongkong/05sept/index_html/lib/gail_kendall.pdf

http://encyclopedia.airliquide.com/Encyclopedia.asp?GasID=34

http://www.advancedspecialtygases.com/Dowloadable%20Files/SF6.pdf

http://www.advancedspecialtygases.com/Dowloadable%20Files/msds/SF6%20ASG.pdf

Pictures:

http://electrical-engineering-portal.com/wp-content/uploads/sf6-molecule-structure.jpg

http://upload.wikimedia.org/wikipedia/commons/thumb/8/89/Sulfur-hexafluoride-2D-dimensions.png/566px-Sulfur-hexafluoride-2D-dimensions.png

http://www.chem.tamu.edu/rgroup/soriaga/FYC/HTML_Presentation_folder8/img050.jpg

 

 

Related Terms in the Glossary:

Sulphur Hexafluoride

Greenhouse Gas

Global Warming

 

 

 

 

 


Carbon Dioxide Fertilisation

Posted by on Friday, 11 March, 2011

The issue of carbon dioxide fertilization applies most immediately and understandably to managing forests and woodlands to reduce fire risk. In addition, carbon dioxide fertilization may have an effect on plant competition that contributes to shifts in species distribution, including post-fire recovery. This factor complicates projections about how southwestern forest and woodlands will be affected by global warming. Experiments testing the effects of carbon dioxide fertilization indicate rising atmospheric levels will result in an increase in herbaceous production (Nowak et al. 2004). This increase will translate into more fine fuels that can carry fire in forests and woodlands. Meanwhile, the improved growth of trees exposed to carbon dioxide fertilization indicates that rising levels of this greenhouse gas may exacerbate the tendency toward increasingly dense forests. Plants in lower light levels (i.e., understory plants) survive better in conditions of elevated carbon dioxide. The reintroduction of a surface fire regime can help counteract this tendency toward increased density.

Forest protection and reforestation are widely acknowledged means for sequestering carbon from the atmosphere and storing it in plants, at least until a stand-replacing fire occurs. Not only does a stand-replacing fire release carbon dioxide into the atmosphere as it burns plants and wood, it arguably may cause a reduction in the disturbed stand’s ability to sequester carbon until a full tree canopy is reestablished. Carbon dioxide fertilization may improve seedling survival rates after a large-scale disturbance, but this has not been tested in the field.

Reducing the risk of large-scale crown fires by treatments such as thinning understory trees could be seen as a means of keeping carbon sequestered in forests. Forestry practices such as thinning treatments, intermediate, shelterwood and seed-tree harvest cuts, as opposed to clear-cuts, also leave many mature trees standing. Carbon dioxide continues to be taken up by the remaining trees, which can grow better with the reduction of competition for limited resources. Meanwhile, carbon is also sequestered in the harvested lumber for decades or more. When small-diameter wood is used as biomass for heat or energy production, it displaces the need for using fossil fuels for this purpose.

Land managers may want to incorporate some of the information on carbon dioxide fertilization effects, including the value of intact forests for carbon sequestration, into their educational materials about the need to treat stands to reduce fire risk. They may also be interested in the scientific literature that contains many reports of carbon dioxide fertilization experiments involving different wildland species.

Carbon Fertilization


online pharmacies
on line pharmacy