Nitrogen Onsite

[cougarfan]

We are looking at getting a nitrogen onsite system for 99.5 % nitrogen at 14,000 to 15,000 scfh , 90 psig requirement - hwoever, I need to know there are many systems out there - how does one pick which one is best - it will be required 24 hrs/7 days - with the max flow being just under this value - how to I pick onsite, PSA or Membrane????

HELP......

 

[pmover]

dideyjohn,
It might be beneficial to post this request within the process engineering forum as well. You will likely get good responses from process engineers whom have experience with this subject matter. I am familiar with the membrane type, but certainly not at the capacities specified. Good Luck. - pmover

 

[MortenA]

Why dont you ty to contact a number of vendors (pref. some that make both)?

Best regards

Morten

 

[cougarfan]

I have done so and know trying to decifer what they say and I thought it might be a good idea to get an unbiased response from those who may have more experience than I do.....

 

[TD2K]

I have only some general comments on membrane systems, most of which you already know.

Purity from membranes is dependent on the rate you take the N2 product, the higher than rate, the more O2 remains in the N2 stream. 99.5% is going to be at the lower production rates for a membrane unit thus you'll need multiple units compared to a user that can accept 95% N2.

Membrane units also use a lot of air. The ones I looked at required 3 to 4 times the amount of feed air compared to N2 product. Most of the compressed air simply leaks across the membrane and is vented as an O2 rich stream. I would suspect this increases their operating costs for a rate such as yours.

Good filtration and preferably oil free compressors are recommended as lube oil will contaminate the membranes over time and they can not be cleaned. You can provide coalescing filters and activated carbons as protection but I'd prefer to eliminate the problem with oil free compressors.

PSA units are frequently used now I understand for mid-range capacities of high purity O2, not too sure where they fit with 99.5%. Air separation plants are for high volume users.

You might want to take a look at Chemical Engineering magazine, they had an article on this a couple of years back if I remember correctly. Check out their website. You could also look at Hydrocarbon processing magazine or plant engineering.

At the end of the day, it's going to come down to upfront cost, operating cost and reliability. All of these units can be highly reliable so I don't think that makes a difference. A refinery near me is installing an O2 separation plant to provide O2 to the plant for air enrichment, that plant is designed to operate unattended with remote monitoring (it's a leased unit)