Aspen PLUS simulation of semi-batch reaction: RBatch reactor 1

[Chemluv2]

Dear,
I am trying to simulate a batch reaction between two chemicals, using the RBatch model in Aspen PLUS.
One of the chemicals is in liquid form at the specified reaction temperature, the other one is in vapour form at the same temperature. Although I "told" Aspen that reaction occurs both in liquid and vapour phase, when looking at the simulation results, it seems to me that RBatch assumes reaction only occurs at the interface between vapour and liquid phase. The simulation results show that there is no perfect mixing of the liquid and vapour phases. In reality, we have an external loop on the reactor, with a heat exchanger (to remove reaction heat), and a jet mixer inside the reactor to ensure good mixing.

Maybe in my Aspen batch simulation, I need to build the external loop as well ?
How to "tell" Aspen that the recirculating fluid is perfectly mixed with the reactor contents ?

Thanks for your feedback.

 

[moltenmetal]

I have no experience simulating batches with Aspen, but no simulator is going to be able predict reaction kinetics, especially when you don't know whether your reaction is limited by kinetics or by mass transfer.

It would be a rare reaction indeed which occurred in BOTH the liquid and gas phases as you're assuming.

Suppose reagent A is in the liquid and B is in the gas.

If the reaction is fast (i.e. the reaction between an acid gas and an alkaline scrubbing solution), A and B will react only at the interface and the rate will be limited by mass transfer, and hence will be entirely dependent on (k sub l * a), i.e. the reaction rate will be strongly determined by mixing and other factors affecting mass transfer. In this case, the simulator *might* have adequate correlations to help you simulate the mass transfer conditions (k sub l * a) generated by your jet mixer.

If A reacts slowly with B, you will essentially be saturating the liquid with B with minimal effort. The slow reaction will occur between A and a pseudo steady state concentration of dissolved B in the bulk of the liquid phase. In that case, additional mixing (improving mass transfer) won't appreciably increase the rate. If you know the reaction order with respect to both reagents and the reaction rate constant(s) and/or equilibrium constant(s) as well as the relationship between rate/equilibrium constants and temperature, you will be able to simulate the batch without caring about the jet mixer too much.

If the rate is in the middle of the range, both mass transfer and kinetics will be important, and you'll need to know a lot to make any meaningful predictions.

So: what exactly are you expecting from your simulator?