Question

I need help with this.

Answer 1

Can you post any calculations you've done? I can give you hand with this a bit later on when I have some time.

Answer 2

Okay @matt101, so I attempted it this way.

Taking 100 lb-moles as basis of the leaving gas from the burner;
SO2= 7.8 lb-mole
SO3= 0.8 lb-mole
From the equations of the chemical reactions occuring in the burner:
S+O2---->SO2
S+1/2O2----->SO3

So the total amount of sulphur burnt must be 7.8+0.8=8.6 lb-mole.

Using the equation PV=nRT, V comes to be 79311.23. Here P= 29.2, n=100, R=21.85, T=1060.

Weight of sulfur burnt= 32 x 8.6= 275.2 lb-mole.

Finally if 275.2 lb of sulphur yields 79311.23 ft^3 gas, then 1 lb of sulphur will yield 79311.23/275.2= 288.2 ft^3 of gases.

Answer 3

That's SO2+1/2O2---->SO3

Answer 4

That's the number I'm getting!

Sorry, was just having some trouble following the units. I'm used to working in metric :P

Answer 5

I used a slightly different method though because I didn't quite follow your lb-mole strategy. I'm posting it here as an FYI, but in the end I get the same answer as you!

1. You want to calculate the amount of gas per pound of sulfur. I chose to first find out how many moles of sulfur there are in a pound. It turns out there are 14.146 mol in a pound of sulfur.

2. This means there are also 14.146 mol of sulfur-containing gases. We can say this because in our case, all sulfur-containing gases only contain ONE atom of sulfur, so if every atom of solid sulfur reacts, an equal number of sulfur-containing gas molecules will be produced. (NOTE: If there were to be more than 1 sulfur atom in one or both gases, we would need to consider both the number of sulfur atoms in each gas as well as the relative amounts of each listed in your chart at this step).

3. I used PV=nRT to solve for the combined volume of SO2 and SO3. I had to convert to metric because I'm not sure what temperature unit you're using - what is it? I used P=98.88 kPa, n=14.146, R=8.314 L kPa/mol K, T=588.706 K and found that V=700.219 L. I converted this to ft^3 since that's what you're using and got 24.728 ft^3.

4. This value is ONLY taking into account the sulfur-containing gases, which based on your chart represent only 8.6% of the total volume. I divided 24.728 ft^3 by 0.086 to get 287.5 ft^3, which is the total volume of all gases in the mixture, including O2 and N2.

This number is close enough to yours when considering rounding errors, so although I don't quite understand what you did, what you did worked! Thanks though for giving my brain a workout with this question!

Have you figured out the other two parts?

Answer 6

I'm using rankine (R) as the unit for temperature.
Your way of solving this is much simpler, but I've been instructed to follow the method taught by my teacher. I've always used the metric system as well and this sudden transition is just too hard to get used to.
For the (b) part;
Since total sulfur is 8.6 lb-mole, the total amount of O2 required for complete oxidation to SO3 would be (8.6*1.5)=12.9lb-mole.
The total oxygen present is = 12.2 (free oxygen)+ 7.8(in SO2)+ 0.8*1.5(in SO3)=21.2 lb-moles.
So the excess is 21.2-12.9=8.3
%age excess is=8.3/12.9=64.3%.
Could you check if this answer is correct by your method?

Answer 7

That's really bizarre...I've only ever heard of the rankine in passing and have never used it in calculations. Too bad you need to deal with weird units!

For this question, I would actually follow the exact same process as what you did to get the same answer. I find it easier to use something like "relative units" than "lb-moles", which I still don't entirely understand, because we're really talking about ratios of things either in terms of relative volumes (i.e. %'s from the table) or stoichiometric coefficients (i.e. from the reactions). So just replace every "lb-moles" with "relative units" in your solution and you'll see how I solved it!

Answer 8

Thank you so much @matt101. I don't understand the lb-moles very well myself but I've to work with these units anyway. But now I can use your method of relative units to cross-check answers. Thanks once again! :)