TOPIC:

How can return loops, such as an internal recycling flow, be modelled? Thanks.

Well, I do not have so much of experience with material and STAN.
But I will try to answer your question.

For solid waste management, my observation is that recycled materials are considered as an output from the recycling process, along with some residues (going to the landfill).

Hope it helps.

Thanks for your reply, but I am afraid it doesn't solve my problem. I want to model a return loop, ie an output from a process that goes back as an input to the same process. In my case it concerns reuse, but it could also be a case of e.g. internal heat recovery in an industrial process. So my question is if it is possible to model such return loops directly or if I have to go via a dummy process. Hope my question is clear!

Ok.
So I think, now I understand your problem. Thanks for the clarification.

I have tried to work with your problem and made two rough sketches (please see attached).
While Fig. 1 shows only the system without its sub-systems, Fig. 2 provides the details of sub-systems and recycling flow.

Hope it helps.

Hi everyone!
It is possible to model internal loops. But be aware that these flows can not be computed by a mass balance (see attached example) because the balance equation would be F1+F2+F5=F3+F4+F5, what is the same as F1+F2=F3+F4. I.e. the value of the internal loop has to be given in order to be displayed.
All the best,
Oliver