serial conversion reaction failing to converge

[thubelihle]

Hi. The following conversion reactor is failing to converge, but the conversion adds upto 1

[jasper]

You have in the feed, 100 kmol/h methane and 21 kmol/h Oxygen.

The reactions are serial, the first reaction

Methane + 0.5 * Oxygen => Carbon Monoxide + 2 Hydrogen

is set to convert 0.4 * 100 kmol/h Methane, so 40 kmol/h methane. In doing so, it also consumes 20 kmol/h Oxygen. Remains, 60 kmol/h methane and 1 kmol/h Oxygen.

The next reaction

Methane + Oxygen => Carbon Dioxide + 2 Hydrogen

is asked to convert 60 percent of the remaining methane (serial box is checked), so 0.6 * 60 kmol/h = 36 kmol/h. In doing so, this reaction requires 36 kmol/h Oxygen, but you have only 1 kmol/h Oxygen left. Hence, the error is

error: calculate failed for unit FixedConversionReactor_5: Conversion of 0.6 for compound Methane for reaction 2 caused negative concentrations of compound Oxygen

[ioncube]

thubelihle, Attached is the flowsheet where you can change controller options & get a dynamic setting of your feed flow

[thubelihle]

thank you so much jasper and ioncube, learnt a lot. ioncube, can you explain to me how the dynamic control system works, looks supercool to me. does it measure the ratio of methane to oxygen or you simple put the manually the feed flowrate of air? it looks like a feedback control system, thanks

[ioncube]

1] Measure Element ...detects O2 mole flow (it can be changed) in Product stream
2] An information stream transfers this to Controller as input
3] Controller is given a set point to adjust above to say zero
4] Output from controller (another information stream) sends this information to a control element (flow constriction in feed) to meet set point

[thubelihle]

Thank you so much, l appreciate this

[jasper]

This particular case is hard to control with a controller, as the solution is at the edge of the feasible region.

A simpler, non iterative solution exists.

For each kmol/s methane, you already know you need 0.56 kmol/s oxygen. You can measure the methane flow rate in the methane feed, and calculate directly the required oxygen flow using an information calculator. And put this number into the flow constraint.