5 replies to this topic

[Doctor_Ed]

I'm trying to calculate the temperature change in a gas-phase reaction. Thermodynamics is not my strong point (I'm an organic chemist), so this is a challenge for me. Anyway, here are the steps I'm following.

1. Calculate the enthalpy at the starting temperature Ti of each component in the feed by adding H-H(298) of the compound and subtracting H-H(298) of the elements (with correct coefficients).
2. Sum these enthalpies. This is H(f).
3. Calculate or estimate a product composition.
4. Start an iteration by guessing a product temp Tf.
5. Calculate the enthalpy at this temperature Tf of each component in the product by adding H-H(298) and subtracting H-H(298) of the elements.
6. Sum the product enthalpies. This is the trial H(p).
7. Compare H(f) and H(p). Most likely they will differ.
8. By trial and error (or better, by clever algorithm) calculate an improved product temperature.
9. Loop back to 5 until H(f) and H(p) are nearly the same. The temp that gives equal H(f) and H(p) is the final temp.

I've tried this with N2/H2/O2 to make N2/H2O, and also with N2/H2S/SO2 to make sulfur in N2. But my calculated product temperatures are always too high. What am I doing wrong?

The water and sulfur examples are attached.

Ed

Attached Files

•  Calc_temps.xls   24KB   46 downloads

[alokjaiswal]

Dear,

I have tried to solve your N2/H2/O2/H2O problem in excel. And my calculation gives the temperature of 500 deg C. I have used some standard data available with me for individual component. I am also attaching ur excel file with a new sheet name "Explanation-1". Please check if in order.

Alok Jaiswal

Attached Files

•  Calc_temps_rev.xls   45KB   46 downloads

[Doctor_Ed]

Thanks for your help. Yes, your values give the right answer. So, it's necessary to use the right enthalpies.

I've prepared a new spreadsheet including equations for calculating enthalpies. Now I get very good results for H2O, CO2, and NH3. But H2S + SO2 doesn't work. Sulfur is very tricky because of all the different states. I don't know the right way to calculate the enthalpies of sulfur compounds.

If you know how to do the sulfur, I would appreciate it if you could explain it.

Ed

Attached Files

•  Example 2.xls   73.5KB   33 downloads

[alokjaiswal]

Dear,

Can you specifically tell me which type of reaction between H2S and SO2 is? Where can I find its mention? Please elaborate so that I can work on it more.

Alok

[Doctor_Ed]

Dear,

Can you specifically tell me which type of reaction between H2S and SO2 is? Where can I find its mention? Please elaborate so that I can work on it more.

Alok

The reaction of H2S + SO2 takes place in the gas phase. At high temperatures it is endothermic. The products are H2O and 3/2 S2. I can calculate the final temperature without problem using known enthalpies for all the components.

At low temperature, H2S + SO2 react to make H2O and 3/8 S8, and the reaction is exothermic. I'm unable to calculate the final temperature for this case. There must be some problem with the calculated enthalpies of S8 (and S7 and S6).

I calculate the enthalpies of the sulfur species using the Shomate equation and Shomate parameters. See the following links.
http://webbook.nist....sk=1#Thermo-Gas
http://webbook.nist....21459041&Mask=1
http://webbook.nist....sk=1#Thermo-Gas

But I get incorrect heat data. I suspect there is a problem with the reference state, but I don't know exactly what is wrong.

Ed

[Doctor_Ed]

Okay, the problem is that the enthalpies of formation of S8 vapor etc at 298K are shown to be zero in the NIST tables. This is not helpful since the heat of formation with reference to S8 solid is considerable (around 101 kJ/mole). Furthermore, unless you are a paying customer, NIST won't reveal the heats of formation for the different sulfur species.

But JANAF will. http://kinetics.nist.gov/janaf/
And several other sources, like Rau et al., 1973.

Taking the heats of formation into consideration, the temperature calculations work correctly.

Ed

Dear,

Can you specifically tell me which type of reaction between H2S and SO2 is? Where can I find its mention? Please elaborate so that I can work on it more.

Alok

The reaction of H2S + SO2 takes place in the gas phase. At high temperatures it is endothermic. The products are H2O and 3/2 S2. I can calculate the final temperature without problem using known enthalpies for all the components.

At low temperature, H2S + SO2 react to make H2O and 3/8 S8, and the reaction is exothermic. I'm unable to calculate the final temperature for this case. There must be some problem with the calculated enthalpies of S8 (and S7 and S6).

I calculate the enthalpies of the sulfur species using the Shomate equation and Shomate parameters. See the following links.
http://webbook.nist....sk=1#Thermo-Gas
http://webbook.nist....21459041&Mask=1
http://webbook.nist....sk=1#Thermo-Gas

But I get incorrect heat data. I suspect there is a problem with the reference state, but I don't know exactly what is wrong.

Ed