8 replies to this topic

[sanderson51]

Does anyone have experience with nitrogen on a flare stack instead of natural gas?  This would be to save cost and reduce greenhouse gas emissions for stand by operation. The obvious downside is that you are 100% reliant on the pilots to light the flame in a release.

 

Also what is experience with realistic natural gas purge rates to manage burn back in the tip?  I see velocity requirements of 0.006 to 0.012 meters/second for a velocity seal.  The calculated purge rate on this basis seems incredibly low.

 

 

[breizh]

Hi,
Let you talk to specialist: John ZinK. CO.I don't feel comfortable with this idea (safety point of view) and I'm puzzled with your comments about cost.

Good luck

Breizh

[shvet1]

Does anyone have experience with nitrogen on a flare stack instead of natural gas?  

I have. This is an expensive option.

 

 

Also what is experience with realistic natural gas purge rates to manage burn back in the tip?  I see velocity requirements of 0.006 to 0.012 meters/second for a velocity seal.  The calculated purge rate on this basis seems incredibly low.

Velocity depends on (1) purge gas, (2) type, and (3) design of seal so you should consult seal Manufacturers as velocity is a competitive benefit and proprietary information.

Edited by shvet1, 10 May 2023 - 12:26 AM.

[sanderson51]

Hi,
Let you talk to specialist: John ZinK. CO.I don't feel comfortable with this idea (safety point of view) and I'm puzzled with your comments about cost.

Good luck

Breizh

I have talked to John Zink and will talk to Callidus and ZEECO.  John Zink says that nitrogen purge is becoming more common on flare systems that have flare gas recovery compressors.  JZ was confident that the pilots would light the flame when there is an actual release.  Nevertheless, I am interested in real world experience. 

 

Assume that a 20" flare with a velocity seal requires 0.006 m/s of purge gas velocity.  0.006 m/s is 0.02 ft/sec.  The cross sectional area of a 20" flare is 2.2 ft2.  Therefore the required purge rate is 0.044 ft3/sec or 2.6 cfm.  Cryogenic nitrogen purity is not required to purge a flare.  Membrane separation can produce 95% purity.  The power consumption to produce 10 cfm of 95% purity nitrogen is about 4.5 kW (6 HP) or in other words 4.5 kWh/hr.  Assume a power cost of 0.1 $/kWh.  The power cost is 4.5 * 24 * 0.1 = $10.8 per day or $3900/yr.

 

I have heard a required velocity of 0.2 ft/sec for a natural gas purge to prevent burn back in the tip.  0.2 ft/sec * 2.2 ft2 = 0.44 ft/sec or 26 cfm.  That works out to 37,000 cfd.  If natural gas costs $10/mcf that is $370 per day or $135,000 per year.

 

What am I missing?

[breizh]

Hi,

It seems that you have support from reputable companies. You are considering OPEX but what about CAPEX for your N2 membrane process, I expect a air compressor, dyer, spare equipment ?

Good luck

Breizh 

[sanderson51]

Hi,

It seems that you have support from reputable companies. You are considering OPEX but what about CAPEX for your N2 membrane process, I expect a air compressor, dyer, spare equipment ?

Good luck

Breizh 

It a refinery so there is a good sized instrument air system which produces dry air and probably high enough pressure already to supply a membrane unit.  Even starting from scratch a delta OPEX of $100,000 will justify $500,000 of CAPEX.

[Bobby Strain]

You should get the purge for both nitrogen and natural gas from each vendor. Don't make stuff up. And, get the prices right! $10 gas and $.10 electricity don't seem consistent.

 

Bobby

[sanderson51]

You should get the purge for both nitrogen and natural gas from each vendor. Don't make stuff up. And, get the prices right! $10 gas and $.10 electricity don't seem consistent.

 

Bobby

 

 

I posted here to get real world experience.  The vendors are incentivized to understate purge rates to prevent burn back.  Been there, done that.

 

The heat rate to produce power from a combine cycle gas turbine is about 6000 btu/kwh.  So the energy cost is $0.06/kWh with $10/mmbtu gas.  If natural gas is $10/mmbtu and electrical is $0.25/kWh it makes no difference.  In the client's location natural gas prices were as high as $35/mmbtu last year.

[breizh]

Hi,

Consider this brochure from John Zink. You may find pointers.

Breizh