by jasper » 18 May 2013, 19:19
I do not think that for this one I will find a ready made solution. As you can see from the Gibbs reactor, the conversion of the reaction is about 100%*(1-1.2e-8), or 99.9999988%. The Gibbs reactor converges because it is looking for a minimum and terminates where it can no longer get the value smaller, so chances are there is a pretty big relative error on your calculated equilibrium constant. The Equilibrium and CSTR reactors however want to converge on a relative tolerance of the given equilibrium constant, by variation of reaction extent. It has an error tolerance of 1e-4 by default, so this means you need to converge the reaction extent to a relative precision of about 1e-12 or so. This is not feasible. The errors in the underlying thermo routines are several orders of magnitude larger to begin with.
Are you really interested in the exact remaining concentrations of oxygen and methane? As a good approximation, it all burns. Why do you not model this as a reaction with a fixed conversion of 100%?