Maximum Oxygen concentration

[cougarfan]

I work at a batch chemical facility and have a onsite nitrogen generation system onsite. I would like to lower the purity level down to 98 % to increase my flow rate out of the system and reduce backup liquid usage. How do I determine whether or not this can be done? Is there any easy way? Any website that has information on this?

 

[25362]

Is the nitrogen used as a blanket to avoid air (oxygen) entrance to prevent flammability or reactivity ? If yes, do you keep control of the MOC on a regular basis ? What is the MOC level you used until now ? Are there any particles or droplets that could enhance the chemical's reactivity ?

 

[TD2K]

I'm assuming you have a membrane system. The purity of these is typically set by the product rate, the more product you use, the higher the oxygen content in the product nitrogen and vice versa.

Talk to your vendor to get the performance curves on your unit and the restrictions. There is typically a maximum inlet pressure and a max dP for the unit if I recall correctly to avoid damage.

25362's points are also valid wrt reviewing the higher O2 limits' effect on your process.

 

[cougarfan]

25632 - we use the nitorgen to blanket the chemicals in the tanks and the purge vessels etc. We ra e using 99.5 % purity and looking at 98 % purity. The MOC for our centirfiuges is looked at carefully and alarams at 5 % but no one wants to put their foot down and say 1% o2 is okay or 2 % O2 is oaky in the gas stream.....

TD2K - Y3es it is a membrane and I spoke with the vendor of the system and yes, he says it will handle the increased flow

So, both of you, where do I get MOC values for all chemicals? Any websites out there?

 

[TrevorP]

Didleyjohn,
Two useful articles on MOC's published in Chemical Engineering magazine, "Use nitrogen to boost plant safety and product quality", June 1994, Laut and Johnstone; "Predict safe oxygen in combustible gases", December 1989, Subramaniam and Cangelosi. The first has some literature values, and both have estimation methods for predicting MOC for various compounds.
NFPA guidelines suggest running at a maximum of 80% of MOC for continuously monitored processes, and 40% max. if not continuously monitored.

 

[pmureiko]

A process safety reference book may have MOC data. Also try NFPA 68 and 69.

My opinion is a MOC of 2% should be safe for just about all compounds if you are protecting for flammability. I recall hydrogen is the most reactive gas at either 6 or 8% oxygen. Less reactive chemicals will have a higher MOC and further from the 2% from your blanketing gas.

 

[MarkkraM]

What are you're chemicals? Do they react at all with O2?
Are you concerned about forming trace amounts of substances from reaction with oxygen?

 

[cougarfan]

MarkkraM,
My 2 main concerns with having a lower purity are:
1. safety - is there any chemicals that have such a low MOC that the chances of explision or flammibility are greatly increased.

2. Degradation of the product - if exposed to small amounts of nitrogen, will this degrade the product used in our process.