5 replies to this topic

[dideyjohn]

I work at a batch chemical facility and use onsite nitorgen for our products to blanket and purge with. The onsite system is rated at 99.5 % purity level and we were looking at going to a lower purity level say 98 % which would push the amount of flow out of the system almost 2x as much which would save us money. However, we do not want to get into the position of doing this without all safety taken into consideration and want to know if there is a table or calculation method where upon one can check the values of MOC for eahc product in question.

[Diederik Zwart]

You can estimate the Limiting Oxygen Concentration (LOC) i.e. the oxygen concentration below which no combustion is possible. The calculation involves the LEL of the fuel and the stoichiometric ratio fuel/oxygen for complete combustion:

y fuel + x O2 -> a CO2 + b H2O
LEL = Z% v/v fuel in air

LOC = Z*x/y [% v/v O2 in Nitrogen]

This gives a slightly more conservative value than experimentally determined LOC's, so you'll be on the safe side. Normally if you stay below 10% O2, you'll be OK for most chemicals, except hydrogen (LOC = 4%).

I hope this helps,
Diederik Zwart

[Art Montemayor]

I believe what Diederik has supplied is exactly the tool(s) you need to resolve your need for a safe, cost-effective reactor purging system.

What I can add to your tool chest is a website that may be of additional help or information in the future:

http://www4.us.airli...sp?0&OID=-14860

This your access to Air Liquide's data library on compressed gas applications - some of which are Nitrogen purging systems for process equipment. I'm of the opinion that the "optimum" (least total cost) Nitrogen purity is not necessarily the customary 99.5% (vol.) purity normally supplied.

I hope this helps.

Art Montemayor

[fares]

I believe the limit maximum is 5% Oxygen in hydrocarbon and if you put Hydrocarbon in Air(20.9% Oxygen) should not also exceed 5% as Lower Explosion limit and in related to Hydrogen as amaximum may be 2.5% Only, The Nitrogen Purity on any Hydro cabon should not be less than 96% at any condition ,the issue is very dangoures and it may cause explosion very easy

[JWHohman]

Another resource you might try is NFPA 69. It lists limiting oxidant concentrations for various flammable gases. It also advises safety factors that should be applied to the LOC depending on the system design, instrumentation, etc.

[Tony Bird]

In response to Fares comments.

The "5% as Lower Explosive Limit (LEL)" that Fares refers should be 5% of the LEL which is generally used as the "safe" level when conducting gas tests either before or during hot work or when detecting flammable gases. I have used this 5% of LEL on remotely operated compressor houses, breather vents on water bath heaters (to detect tube rupture) and detecting leakage on propane refrigerant systems. I am sure that this is what Fares wanted to say but felt I needed to explain further.

For explanation, here a few of the LEL's for hydrocarbon gases in air C1 = 4.4%, C2 = 2.5%, C3 = 1.7% (in volume %). The percentages continue to decrease with longer chain lengths. The data is from the Australian Standard which will have its origin in an American or British standard