To wet basis...

[AndreChE]

Hello

I am doing energy balances to cracking furnaces.

I have the %vol O2 at dry basis and I want to convert it to wet basis.

What is the best way to do it?

Thanks
AndreChE

 

[MortenA]

Calculate the composition of the wet gas - including water?

Best regards

Morten

 

[25362]

Of course humidity can be measured by a variety of instruments.

Otherwise, if you wish to estimate it, assuming perfect and complete combustion with excess (xs) air, knowing the % CO₂ on a dry basis you obtain y/x (i.e., the H/C atomic ratio) of your fuel in C_xH_y.

Since the volumes of CO₂ and H₂O aren't affected by xs air, and the volume of water from combustion is equal to (y/2x) times the vol of CO₂, one can estimate the volume of combustion water from that ratio.

For example, assume one is burning a hydrocarbon fuel with 20% xs air (about 3.5% oxygen in the flue gases) and measures 11% CO₂ on a dry basis, it would result in y/x=2.9 (a typical refinery gas). And y/2x=1.45, meaning water volume would be 1.45*11=15.95. Added to 100% dry gas results in 115.95 volumes of wet gas, and any % expressed on a dry basis should be divided by 1.1595 to get the % on a wet basis.

BTW, the xs air on a perfect and complete combustion of a hydrocarbon can be determined just by knowing the % O₂ with little appreciable error for any hydrocarbon fuel, as follows:

1% oxygen ~ 5% xs air; 2%, ~10% xs air; 3.5%, ~20% xs air; 5%, ~30% xs air, etc.

Don't forget that any fuel oil atomizing steam would have to be accounted for. I sincerely hope to be understood.

 

[hacksaw]

do it on a mol basis

 

[25362]

To convert the xs air -where air components are given in the ratio N₂/O₂=3.76- and %CO₂ content (dry basis), into y/x of your hydrocarbon fuel the equation is:

p = 100[÷][4.76n+(1.19n-0.25)(y/x)]​

where

n, is the xs air coefficient, for 10% xs air, 1.1; for 20% xs air, 1.2, and so forth.
p = %CO₂, dry basis.

The above formula is derived from the stoichiometric one giving all (wet) products of combustion of C_xH_y with xs air:

xCO₂+(y/2)H₂O+3.76n(x+y/4)N₂+(n-1)(x+y/4)O₂​

 

[mbeychok]

See thread798-103509

Milton Beychok
mbeychok@xxx.net (replace xxx with cox)
(Visit me at www.air-dispersion.com)

 

[25362]

AndreChE,

Summarizing, when the composition of the hydrocarbon fuel C_xH_y is not known, the (approximate) procedure is as follows:


1. From %O₂ (dry basis), A, find the excess
air coefficient n.
2. Measure %CO₂ (dry basis) = p.
3. From n and p determine y/x.
4. Find %H₂O, H = p(y/2x).
5. Find %O₂ (wet basis), B=A/(100+H).

All the % are volume or mol %.

I invite comments from experts in combustion engineering.