Creating reaction mechanism without Arrhenius

[Ismael Wouters]

Dear all,

I am quite new to Cantera and Thermochemistry as a whole so forgive my naiveté on the topic. The queries that I have, which I will explain below, are more so about cantera itself and less so about my personal code.

I am currently modeling a Fischer-Tropsch reactor based off of a certain "real-life" project. I struggled to find an appropriate mechanism so decided to go ahead and try making one myself.

Unfortunately, I am missing Arrhenius coefficients from that "real-life" project. As such I'm quite unsure as to how I can approach it. My preference is to model the FT-synthesis using thermo equilibrium and not a kinetic approach. My questions are then:

1) Can I model thermodynamic equilibrium of the reactions using a YAML file that I made which contains species and reactions etc but NOT Arrhenius constants?. i.e. I Leave the portion where you fill in Arrhenius constants blank. 

2)  What would be the limitations of this approach should it work? 

3) Is a thermodynamic equilibrium of a catalytic processes even possible in cantera?

4) Is there perhaps an appropriate mechanism file out there that I haven't been able to find yet? It pertains to a reaction between H2 and CO from biomass derived synthesis gas over a Cobalt catalyst to produce HC chains (biofuels C5+). Perhaps I can modify the Pt_combust file? (Arrhenius constants issue would still present itself)

Thank you very much for your time! Any piece of advice is valuable to a novice like me please don't hesitate!

cheers,

Ismael

[Ray Speth]

Hi Ismael,

Equilibrium calculations are done using only the species thermodynamic data, not any reactions that may be defined. So if you define a phase with no reactions, you can still calculate the equilibrium state. The limitation to this approach is that if the real process is controlled by the kinetics, you won't get the correct solution, and the equilibrium calculation can't tell you whether the kinetic rates are the controlling parameter.

For the equilibrium calculations, since you don't need to define the reactions or their rates, there's no need to model the catalyst at all.

Regards,

Ray

[Ismael Wouters]

Hi Ray,

Thanks for the response!

I was able to generate an input file via the the thermobuild website. As you mentioned, I was able to perform equilibrium calculations using this mechanism. Sadly, I haven’t  been able to get the right composition of products when using minimisation of gibbs free energy to calculate equilibrium state. 

I'm getting mainly CH4 as a product but my goal is to produce C5+ alkenes. You mentioned that kinetics may be the controlling parameter here, this may indeed be the case which is unfortunate. Should you perhaps have any more suggestions I'm all ears!

Thank you in advance,

Ismael