Making and Inert Gas from Hydrogen or Natural Gas 2

[Nigel]

Is it possible to mix hydrogen or natural gas with another gas to make them inert? If so what would this gas be? Please be specific.

Thanks in advance, Nigel.

Nigel Waterhouse
n_a_waterhouse@hotmail.com

A licensed aircraft mechanic and graduate engineer. Attended university in England and graduated in 1996. Currenty,living in British Columbia,Canada, working as a design engineer responsible for aircraft mods and STC's.

 

[MortenA]

If you dillute the gas to below LOL with any inert gas (N2 CO2 whatever) then it will not ignite in conteact with oxygen/spark). But as long as they are kept apart form oxygen they themself are pretty inert.

Best Regards

Morten

 

[jonralph]

Yup. Combine it with oxygen (burn it) and you get a mixture N2, CO2 & H2O. Propably a bit of CO too plus a few others. (deliberatley vague!)

Inert gas generation units are now common in the oilfield for underbalanced drilling. Try

http://www.northlandenergy.com/control_press_drill_gassupply.htm

for a basic explanation, basically they burn a fuel gas, scrub it, and pump it down an oil well to lighten the hydrostatic head of the column of drilling fluid. Could also be used for purging, inerting. Tankers have been using a similar system for years to inert the cargo holds.

 

[Nigel]

MortenA (and anyone else who has some useful info)

If I had a tank of CNG, LPG, or hydrogen, what proportion of intert gas would I need to add in order to prevent combustion? In the event of a leak would they stay mixed long enough to prevent combustion? I am not a chemical engineer, so please forgive my apparent ignorance in this area.

If anyone knows where I could get more info on this matter please let me know.

Nigel Waterhouse
n_a_waterhouse@hotmail.com

A licensed aircraft mechanic and graduate engineer. Attended university in England and graduated in 1996. Currenty,living in British Columbia,Canada, working as a design engineer responsible for aircraft mods and STC's.

 

[MortenA]

Nigel

All gasses that can ignite has something called a Lower Explosive Limit (shorted _LEL_ the b..... editor turned it into a laughing face) and also a Upper Explosive Limit. You can find this in many physical databook (i dont have any here). If you have a mixture use weighted mol. average.

If you have a gas with no oxygene this gas could not ignite if you added a spark. If you start to add atmosperic air then at a certain dillution a sparc would cause an explosion. Continue to adding air and at another lower dillution the spark would no longer cause an explosion.

So if you dillute with N2 until below lower explosive limit then your gas mixture cannot ignite. But this is prob. below 20% of your HC/H mixture.

I dont know of any "magic component" that could be added in trace amounths to prevent ignition.

Best Regards

Morten

 

[jonralph]

The normal method of preventing ignition is just keeping the Oxygen out. Usually a tank is purged with Nitorgen or similar before use. As long as the tank is filled in a "closed system" then no air can get in, causing an explosive atmosphere. Do you have a gas barbecue? Ever taken the cylinder to get refilled, same principle, only propane in the cylinder so the Upper Explosive limit has been reached (I wondered what the heck the smiley face was)

Think of any combustion engine, think of the fuel mixture

Lower Explosive limit (LEL) = Lean mixture, not enough fuel/too much air
Upper Explosive Limit (UEL) = Rich mixture, too much fuel/ not enough air

 

[owg]

There is a Minimum Oxygen Content (MOC) for each flammable gas, which applies to the common inerting agents, Nitrogen and CO2. The NFPA suggests controlling about 2% below this minimum level for safe operation. For the natural gas inerted by N2 the MOC is about 11 mol%. Deduct 2% for a suggested oxygen content of 9 mol% or less. I forget the number for hydrogen but I think it is lower. Look up MOC in the NFPA or a good text book.

Use a reliable vendor for the nitrogen, Union Carbide, Praxair, Liquid Air, Air Products, etc. Some of these may have merged or changed names. Inert gas generators can make corrosive gas, not recommended.

Visit

http://www.curryhydrocarbons.ca

for more.

 

[TangoCleveland]

Check out Bureau of Mines Bulletin 627, "Flammability characteristics of Combustible Gases and Vapors." It has charts for adding CO2 or N2 to a flammable gas to make an inert mixture.

 

[Flareman]

Following up on OhioLarry's post, BOM-503 actually covers this a little better than BOM-627 but both are OK. The problem is that it's difficult to get either of these publications on the open market and you probably need to go to your nearest engineering library and ask if they have a copy.
The posted limits of flammability (lean limit) for gases actually represents the amount of quench inert (nitrogen) in the air at the measured condition. When you replace nitrogen with carbon dioxide (for example) you need to rebalance the numbers to keep the heat absorbtive power of the CO2 the same as it was for the Nitrogen. Sounds long winded when I explain it this way!! It's actually a matter of not exceeding a miminum theoretical adiabatic flame temperature with whatever constituents you have.
Getting back to your actual problem, 1 vol of Hydrogen is, theoretically, just flammable in 16.5 vols of N2 or 10.3 vols of CO2.
1 vol of CH4 is just flammable in 4.2 vols of N2 or 3.3 vols of CO2. You could also use steam (roughly 4/3 as much as N2 by vol). Apply your own FOS to these numbers and don't be afraid to be over-conservative (ie. safe). The NFPA recommend using a value which relates to 25% of the LEL concentration. You could try the NFPA forum for more info.
I can send you a relevant page from BOM-503 and an interpretive LEL/UEL chart of my own, if you drop me an email at flareman_xs@netzero.net.


David