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[dre0324]

Hello, I am doing practice problems for a test and came up with this question. I got my test tomorrow so any help would be appreciated!

A mixture containing 80% C3H8 and 20% C4H10 is being used as a fuel for a gas welding machine. In order to generate high temperatures, the fuel is burned with an oxygen-enriched air consisting of 50% O2 and 50% N2. The flue gas contains 1.0% CO and 5.0% O2.

Calculate complete composition of the gas produced and the percentage excess oxygen fed to the welding machine.

I tried doing an atomic balance by choosing 100kmol of flue gas as a basis, but I get 3 unknowns and dont know how to get rid of them. I then did the same choosing a 100kmol basis of the fuel mixture and still go the same problem.

How can i go about solving this?

[kkala]

I believe attached Gascomb.xls will help (subject to comments) resulting in 11.9% excess (enriched) air and flue gases with N2=42.3%, CO2=21.8%, H2O=29.9% by volume. It is not easy to describe the procedure, which could be followed in the worksheet, but few hints may be useful.
1. Basis considered: 100 kmol of fuel gas.
2. Two options considered: Case A = all CO produced from C3H8 burning, Case B = all CO produced from C4H10 burning. Actual case would be between the two. No real difference was found between these two options.
3. Original arbitrary values were accepted for excess (enriched) air and the percentage of the compound burnt to CO. The latter is C3H8 (Case A) or C4H10 (CaseB).
4. Based on above, flue gas composition was calculated (shown in the spreadsheet).
5. By hand changing above excess air and percentage of compound burnt to CO, values were found to satisfy the 5% O2 and 1% CO in flue gas (trial and error).
6. Solver was tried for this, but did not gave results, saying "not enough memory" etc. I am using Office 98, probably solver can work in another computer to avoid hand work.
7. There may be simpler solution, but I understand matter is urgent.
8. Pay attention to the assumptions (e.g. fuel gas composition mol/mol, flue gas O2=5%, CO=1% mol/mol on wet basis), they may need correction if not real. These are pointed out on the spreadsheet.

Attached Files

•  Gascomb.xls   20KB   159 downloads

Edited by kkala, 26 January 2010 - 05:14 PM.