Air flow required by a nitrogen generator 2

[Sergio.Martinez]

Hello friends , I work for a engineering company in Perú.
I want to get the air flow required by a nitrogen generator .
I have the flow of nitrogen .

Q = 71.52 L/s = 151.54 CFM
Pressure = 145 psig

The site conditions of the project are :

P atm = 56 kPa
T average = 7°C

But , the mining company is asking us if the actual nitrogen generator NitroSource Hifluxx is able to handle this flow.
And if the answer is positive , what would be the flow of compressed air.

As we can see , the relationship between air compressed and nitrogen with 95% of purity is 2.6 .
So in case I feed the nitrogen generator with air at 100 psig , the flow of air required would be 151.54 CFM *2.6 = 394 SCFM.
But the pressure we need is 175 psig in order to get 145 psig of pressure of the nitrogen.
I am tempted to use the correction factor by pressure of the table 4.2.2 , but I am not sure that the relation of 2.6 holds
in this case.
Please , if someone with experience in nitrogen generators could help me , I would be greatful.

 

[rotw]

Your N2 flow (requirement of the consumer) of 151.54 CFM (assume it is actual flow) is given at condition of 145 psig. Assume the temperature is ambient (you did not specify) = 7 deg C.
This same flow converted to Nm3/h (atm. pressure, 0 degC) is equivalent to 2739 Nm3/h.
This is by far exceeding the nominal capacity of the generator at any purity level.
Either something is wrong in the assumptions how it is stated (in this case please correct), or you would need several generator units in parallel. So I guess you need to sort this out first.

In regard to correction factors, application seems pretty simple: if your feed pressure deviate from specification, correct N2 delivery capacity based on table 4.6. If your ambient temperature at the site of consideration deviates from specification, the performance of the unit will be degraded, so you need to correct both N2 delivery flow AND air use flow by factors given in Table 4.7 and 4.8 respectively.

 

[LittleInch]

Sergio

First off you need to use some consistent units and explain this a bit more.

your demand of 151 cfm - Is that at the outlet pressure of 145 psi or concerted to a standard CFM

I'm guessing the SCFM as it seems to work with your figures, however 151 is more than the maximum 120 as listed in the data sheet?? Do you have two working together?

I'm assuming your 2.6 is a disvision from table 4.2.1 by 4.2

I can't see why table 2.2 isn't on top of table 4.2 and therefore your 151 can be met at your 145 psi output. Incidentally I don't know where your data 175 psi in to 141 psi out comes from??

NOTE: It's clear you're at an altitude of >4,000m if your local air pressure is only 56 kPa. The tables assume operation at sea level. How this affects the Nitrogen Plant I don't know, but it will impact the operation of the air compressor a LOT, but if you're used to those altitudes I guess you know that. So you will still need to check with the vendor for operation at your local altitude.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[rotw]

LittleInch,

Regarding your note, the air compressor is fed with air that is already pressurized. Would it still be affected?
Otherwise 100% agree with your comments.

 

[LittleInch]

rotw, thanks

I was just making the point that the OP also needs to consider the issue with the air compressor at those sorts of altitudes. If the supply is provided by others then there is no issue.

The big question is whether the 151 CFM is 151 SCFM....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Sergio.Martinez]

Hello friends , thank you very much for your responses.
Now , with respect to the Nitrogen generator. I finally think it will not cover the 151 CFM with 145 psig.
First , I would like to clarify the issue of the units.
In Table 4.2 the units are Nm3/h , but here "N" doesnt refer to normal conditions of 100 kPa and 20°C.
It is about Nitrogen. The normal pressure of the air is 100 psig and the inlet pressure of the air establish an normal outlet pressure of the Nitrogen of something between 70 - 74 psig. This is what I learned after reading a lot of technical information of the suppliers. And it makes sense
to me because in the membrane separator , there has to be a pressure difference in order to get the flow and finally the separation of the gases.
Besides , taking account that the molar fraction of the Nitrogen in the air is 0.78 , the final pressure is near to the value of the partial pressure of 78 psig.
Well , with this in mind I convert my 151.54 CFM with 145 psig to normal flow with 70 psig as I justified above.(I will keep the temperatures in 20°C ). So I get 151.54 cfm or 257.5 m3/h , and now

Normal flow = 257.5 m3/h * (145 psig)/70 = 533 Nm3/h .
This makes sense because with a greater density I get more mass in the same "Normal" volume . So this is the equivalent of my actual flow with 145 psig and high density in the units of the previous "normal" flow with less density.

And , only now I am in conditions to compare with the table 4.2 . And I have to apply the factor 2.1 , because the pressure of the inlet air is not 100 psig , it is 175 psig.

So , with main unit and 5 sub-units I have : 204 Nm3/h * 2.1 = 422 Nm3/h.

As we can see my requirement is 533 Nm3/h and the maximum capacity of the generator is 422 Nm3/h. So this generator can not handle this flow , with the actual number of sub-units , but again , with some calculations each sub-unit gives 68 Nm3/h. In theory I will need 2 aditional
sub-units to get my actual consume of Nitrogen.

Now I have to face another "little" problem . My boss (the leader of discipline) and the project leader are not convinced with these calculations and still think this generator can manage the flow.

 

[rotw]

"Now I have to face another "little" problem . My boss (the leader of discipline) and the project leader are not convinced with these calculations and still think this generator can manage the flow."

I am not convinced either (though I think the generator is under-sized to your flow requirement).
To me, for [Nm3/h] to be anything else than normal m3/h - it needs to be stated by manufacturer or you would need to document this.
Till then [Nm3/h] is normal m3/hr in my world.

 

[LittleInch]

Sergio,

I don't think you are correct.

The manufacturers website makes this a little clearer.

In the table you post the N mean nominal and the s in the CFM table means standard.

Now so long as your demand is also in scfm then I don't think you have an issue. The pressure correction factor at 175 psio is 2.1, i.e. for 150scfm at your required pressure of 145 psi you only need a unit capable of about 75 scfm at the normal air inlet pressure of 7 bar which is what the table above is based on.

But you will need around 2.6 times as much air as the volume of nitrogen you need according to the tables above. So about 450-500 scfm compressed air at 12 bar. The air compressor is where you need to be careful about inlet air conditions at your altitude, but let someone else worry about that and just ask for 500 scfm at 12 bar.




Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[rotw]

Just to second LittleInch and summarize for OP some steps to follow maybe it is of help:

A/ Check if your consumer demand of N2= 151.54 is in <actual> or <standard> CFM?

​

A-1/ If this is actual, which I really doubt now based on management feedback, no way this can be a fit -> Substantiate/ Inform Management.

​

A-2/ If standard is confirmed (151.54 SCFM), check your air feed pressure capability (I suspect it is 100 psig) at your site/facility.
This will determine the number of units required. I think you need to have at least ~110 psig of feed air pressure (considering ~0.9 capacity correction factor for ambient) to fit with the option of Main+5 sub units (I suspect the more costly for a 95% purity).
B/ When you have identified your sizing / number of units, proceed with check of air flow requirement.

​

B-1/ Apply correction factors and check altitude impact as already pointed out.

Sergio,
Just a small note: In the table posted just above, you could see a footnote for your reference stating the Nitrogen capacity (m3/h) reference:
m3 reference standard 20°C, 1013 millibar(a), 0% relative water vapour pressure.

 

[KEVINZY]

Hi
At first, I believed the flow of Nitrogen should be 151.54 cfm or scfm, but not actual flow. The unit is the same as nitrogen generator.
You'd better to confirm the flowrate in cfm or scfm, the difference is 20℃ or 0℃. That 151.54 cfm =0.931*151.54 scfm
And the pressure should be 145psig, consider the pressure drop of menbrane, the inlet air pressure should be 175 or 190 psig. Choose 175 psig.
According to 4.2.2 the pressure correction factor should be 2.1
Then we choose main unit and 3 sub-units the flowrate will be 80*2.1=168 cfm
And you should consider the site temperature is only 7℃.
The temperature correction factor is 0.9 in 10℃. So the flowrate is 168*0.9=151.2scfm. That's OK

 

[ARenko]

Assuming you need 151 SCFM at 145 psig, and that you need 175 psig at the membrane inlet to get 145 psig out (is this from the manufacturer?)...

Your pressure correction is 2.1, so nominal unit requirement is 151/ 2.1 = 72 SCFM. The main plus three sub units will handle this (delivering 2.1*80 = 168 SCFM). But you also need to accomodate for temperature. And it's not site temperature - you need to correct for temperature at the inlet of the membranes. Do you know this temp? I expect it is hotter than ambient. Say it's only 3C hotter at 10C... the correction factor is .9, so your 168 SCFM is 151 SCFM.

If you have a main plus 5 sub units like you suggested (it's not clear if that's what you actually have though), then you will have no problem. You can isolate a couple of sub units and do fine provided someone can deliver 393 SCFM at 175 psig. Actually they can probably deliver a bit less air - in my experience the recovery (N2/ Feed air in %, i.e the 2.6 factor you mention) goes up with higher pressures in the membrane.