Homogeneous Reactor Problems

[nrgeng]

I am reporting some behaviors of which you may not be aware. The simulations are of a homogeneous reaction in a CSTR Unit. I emailed the files to you.

Problem 1 {CSTR (Forum) err=LS V1.0.fsd}
Changing mole fraction, x(CO2), from 0.35 to 0.36 yields the following message:
error: calculate failed for unit CSTR: Solution failed: line search did not converge

Problem 2 {CSTR (Forum) err=k V1.0.fsd}
Clicking on solve provides a different value of conversion, X(CO2), each time. Which conversion is correct, the first or the last?

While changing the reaction rate constant, k, different values of conversion, X(CO2), were obtained based on the previous result.

Are these just characteristics of the operation of COFE?

[jasper]

Problem 1:

Your reaction is

CO2 + 3 H2 -> H2O + CH3OH

Your rate depends only on the concentration of CO2, so the reaction still takes place when the H2 concentration drops below zero.

This goes ok as long as you have sufficient H2 to react away all CO2, but as soon as you run out of H2, no solution exists that results a non-negative H2 concentration. Your forward rate should take into account the depletion of H2.

Note that at X[H2] = 0.65 mol/mol you are nearly running out of H2.

[jasper]

Problem 2:

It looks like the reaction is converged, in terms of flow rate vs reaction rate, due to the extremely low flow rate. Did you intend 40 mmol/h total flow rate? (1.4e-7 kg/s)?

What you are looking at is a truncation of the tolerance (which is normalized at flow rate) to prevent a division by zero.