Process with stock but without volume flow?

Hi there,

I am currently struggling with adding the following process to my MFA: I have a change in concentration of a compound in solution while the concentration doesn't change. Thus, the volume flow of dStock is = 0 but the mass concentration in another layer is != 0. I tried to do this with a stock, but if the volume flow is 0, then there is no change in the stock in another layer. I then tried to get this done by using a subsystem but I ran into the same problem. My next attempt was to split it into two processes, with one of them resembling the vessel and the second one resembling the increase/decrease in a compound (no chemical reaction involved). However, it doesn't work as I thought. Any hint on how to solve this issue? I appreciate any hint!

Kind regards,
Anil Tellbüscher

...and one remark: The smiley was supposed to look friendly