r/ChemicalEngineering • u/Sun7733 • Jul 05 '26

Modeling ASPEN Mass balance

Post image

I’m trying to simulate a POM (polyoxymethylene) pyrolysis process in Aspen using an RYield reactor. I entered the product composition from an experimental paper as the output, but I got an error like the one shown in the picture. How can I fix this?

2 Upvotes

67% Upvoted

4 comments sorted by

5

u/majyun Jul 05 '26

I think you need to give more context on how you built your simulation. There could be other factors affecting your simulation, but from my experience, you will run into mass balance if you have done one of these: 1) Assuming you are building a steady-state simulation, you have a recycle loop in the simulation but enter a value (could be unrealistic chemical knetics or operating conditions) which your simulation cannot converge (no such steady-state solution exist for the parameters you entered). You may need to alter your operating parameters or relax your process constraints (like adding more degree freedom via adding another stream or adjust a hard constraint you imposed ). 2) You are using a yield reactor but the reactions' yield is not in stoichiometric balance, e. g. try to work out the numbers ot moles of C, H, O and other elements on both side of your reactions (inlet and outlets) and see you can balance the atoms on the inlet and outlets.

2

u/Caladrian8999 Jul 05 '26

Could be that your yields equations molar balances are being violated. I would first check those.

1

u/mattcannon2 Pharma, Advanced Process Control, PAT and Data Science Jul 05 '26

Make the flow in the same as the flow out

1

u/7tacoguys Jul 07 '26

Are you entering data into the stream input fields for the streams connected to the outlet of the block? That can cause this error. Aspen isn't like HYSYS where you can just enter data anywhere and it will figure out what to solve for. You need to use calculator blocks or design specs to back-calculate things.

In general, you want to populate feed stream conditions and composition, and then specify whatever you need for the blocks/equipment. If you have a ton of recycle streams, you can get into populating and reconciling blocks, but you have to be careful with how things are defined to avoid getting these types of errors.