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Technical question for the chemists among you

Community Veteran
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Technical question for the chemists among you

http://www.bbc.co.uk/news/uk-scotland-north-east-orkney-shetland-17505448
Quote
[Elgin platform gas leak: Exclusion zone in place
<snip>
Very flammable
Dr Simon Boxall, an oceanographer at Southampton University, told BBC Scotland that this was not a deepwater drilling rig and platform but it was unusual in that they were drilling down 5km (3.1 miles) into the sea bed.
He said: "It is a very deep well. The gas they are bringing up is what we call sour gas.
"That gas has a high proportion of hydrogen sulfide and carbon dioxide and that makes it very flammable and quite poisonous.

Now why should having a high proportion of CO2 make it more flammable and, other than making it extremely stinky and poisonous, why would hydrogen sulphide make it more flammable than normal gas.
http://www.offshore-technology.com/projects/elgin/
Quote
The sour condensate gas, which has an average carbon dioxide content of 3.5% and up to 40ppm of hydrogen sulphide, required purpose-designed equipment and an innovative development plan under which commercial gas would be produced on an offshore processing platform.
Just to add that at 40ppm hydrogen sulphide isn't particularly toxic and that assumes you inhale the undiluted gas which would kill you anyway due to lack of oxygen
8 REPLIES
itsme
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Re: Technical question for the chemists among you

hydrogen sulphide = flammable
carbon dioxide =  poisonous
Community Veteran
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Re: Technical question for the chemists among you

Carbon dioxide isn't poisonous - they actually add it to many commercial greenhouses to improve crop yield
Hydrogen sulphide has a lower explosion limit of 4% whereas natural gas has an LEL of 5.1% so there should be no way that a small percentage of hydrogen sulphide should have any significant effect
JayG
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Re: Technical question for the chemists among you

Carbon dioxide is regarded as a toxin rather than a poison, although the distinction is possibly a bit hair-splitting these days.
Hydrogen sulphide is flammable and also unstable when heated - don't know how it behaves when mixed in small concentrations with hydrocarbon gases but it's hard to imagine anyone wanting to be anywhere near it when it's escaping! Roll eyes
http://www.hpa.org.uk/Topics/ChemicalsAndPoisons/CompendiumOfChemicalHazards/HydrogenSulphide/
itsme
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Re: Technical question for the chemists among you

There are health affects with Carbon Dioxide at 3.5% concentration. So not poisonous but if you are putting out warnings it's easier to say it is than
Quote
Breathing increases to twice normal rate and becomes laboured. Weak
narcotic effect. Impaired hearing, headache, increased blood pressure
and pulse rate.
Community Veteran
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Re: Technical question for the chemists among you

The carbon dioxide in this respect is a bit of a red herring. All it does is define what kind of gas it is because of its proportion in the mixture. See below. The problem originates from the hydrogen sulphide (correct spelling because I'm British!!!) which as you quite rightly say, is inflammable and toxic.
Although the terms acid gas and sour gas are sometimes used interchangeably, strictly speaking, a sour gas is any gas that specifically contains hydrogen sulfide in significant amounts, whereas an acid gas is any gas that contains significant amounts of acidic gases such as carbon dioxide.
Community Veteran
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Re: Technical question for the chemists among you

I refer back to the original statement
"That gas has a high proportion of hydrogen sulfide and carbon dioxide and that makes it very flammable and quite poisonous.
So if it is normal methane - that isn't very flammable just mildly flammable. Why don't I believe that
With respect to CO2 toxicity and 3.5% that refers to the CO2 content of the air we breathe and the fact that the gas bubbling out has 3.5% CO2 is not relevant as breathing neat methane would kill you rather quicker. My point is that the presence of CO2 in the gas doesn't increase the toxicity to any serious extent at those concentrations and 40ppm of hydrogen sulphide, with subsequent dilution in the atmosphere isn't too  toxic either
Community Veteran
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Re: Technical question for the chemists among you

I think the original quote is misleading in that it seems to place the problem with the CO2, but the emphasis should be on the hydrogen sulphide, not the carbon dioxide. The latter is mentioned because it is referred to as a sour gas. The H2S is the culprit, not the CO2. Hence my reference to the red herring. You are quite right Jim in believing that the CO2 isn't a problem because it's not the part of the toxicity or inflammability problem. It just defines the gas as "sour" because it is the minor part of the gas mixture (ie less than H2S), nothing more.
Edit: And H2S is much heavier than either air or CO2 or methane. It tends to layer, so the toxicity and inflammability might be "pooled" in small areas on the sea surface.
alanf
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Re: Technical question for the chemists among you

Quote from: nozzer
The problem originates from the hydrogen sulphide (correct spelling because I'm British!!!)

The Royal Society of Chemistry is also British and now adheres to the international standard spelling sulfur, sulfate, sulfide etc.
http://www.rsc.org/periodic-table/element/16/sulfur
"Traditionally, the names of three elements have been spelled differently in US and British English. For articles about chemistry-related topics, Wikipedia follows the recommendations of the International Union of Pure and Applied Chemistry (IUPAC) as follows:[1][2]
Aluminium not      Aluminum
Sulfur      not      Sulphur
Caesium      not      Cesium
These international standard spellings should be used in all chemistry-related articles on English Wikipedia, even if they conflict with the other national spelling varieties used in the article.
This convention should also be applied to all compounds and derivative names of these chemicals: e.g. sulfate not sulphate; sulfuric not sulphuric; etc."
http://en.wikipedia.org/wiki/Wikipedia:Naming_conventions_%28chemistry%29
It will be interesting to see how many American publications stop referring to aluminum!