## Reaction Rate Expression - Conversion Problem

Discusses use of COCO, the process simulation and modelling software suite from AmsterCHEM, downloadable from www.cocosimulator.org

Moderator: jasper

### Reaction Rate Expression - Conversion Problem

Hello,
I have assigned a reaction expression for the oxidation of sulfur dioxide with a catalyst (Heterogeneous Reaction). The reactor converts the sulfur dioxide to sulfur trioxide but with the 3rd reactor the conversion reaches more than 100% which is impossible.
I just want to make sure that the reaction expression is written correctly in the simulator as there are partial pressures in it.
Normally the first reactor would convert 60%, 2nd around 84% and the 3rd reactor would reach to 93%.

I am attaching the fsd file and the reaction expression.

Attachments
Untitled.png (15.96 KiB) Viewed 532 times
SulfuricAcid_Flowsheet.fsd
Chani

Posts: 6
Joined: 16 November 2020, 15:49

### Re: Reaction Rate Expression - Conversion Problem

The SO2 stream in F1/SDC is 608.37134 ton/day. The SO3 stream in F2/HSTII is 670.2688 ton/day.

Now in molar flows:

The SO2 stream in F1/SDC is 9496.336 kmol/day. The SO3 stream in F2/HSTII is 8371.7461 kmol/day. So the total conversion is 8371.7461/9496.336=88.2 %.

This is not more than 100%, but also not what you expect, I gather. But your expressions are listed without dimensions; I see you have implemented the rate as

((1e+03/3600*exp(12.160-5473/T))*P/101325*(X(Oxygen)*X("Sulfur Dioxide"))*(1-P/101325*(X("Sulfur Trioxide")/(exp(11300/T-10.68))/X("Sulfur Dioxide")/X(Oxygen)^0.5))/22.414*(1+P/101325*(exp(-9.953+8619/T)*X("Sulfur Dioxide")+exp(-71.745+52596/T)*X("Sulfur Trioxide")))^2)

I take it from there P in your expression is in atms - should you not have (*P/101325)^2 then? I am also missing a sqrt(P/101325) a bit later?

jasper

Posts: 967
Joined: 24 October 2012, 15:33
Location: Spain

### Re: Reaction Rate Expression - Conversion Problem

Hello,

Sorry but please could you explain why the partial pressure expression should be: (P/101325)^2?
If the pressure given is in Pascals you just need to divide it into 101325 to transform it to atmosphere, right?

Thank you x
Chani

Posts: 6
Joined: 16 November 2020, 15:49

### Re: Reaction Rate Expression - Conversion Problem

Yes, but it appears to me that P(Oxygen)*P("Sulfur Dioxide") = (X(Oxygen)*P*X("Sulfur Dioxide")*P) = X(Oxygen)*X("Sulfur Dioxide")*P^2, no?

jasper

Posts: 967
Joined: 24 October 2012, 15:33
Location: Spain

### Re: Reaction Rate Expression - Conversion Problem

Yes, correct!

Sorry for the misunderstanding.
Chani

Posts: 6
Joined: 16 November 2020, 15:49