Recycle: don't waste unreacted feed
Many reactions only convert a fraction of the feed per pass (ammonia synthesis: ~25%). Throwing away 75% of your nitrogen and hydrogen would be madness, so you separate the product and recycle the unreacted reactants back to the reactor.
This creates a loop. To solve a loop, the engine tears one stream: it guesses that stream's conditions, marches around the loop, compares the computed value to the guess, and iterates until they agree. LensaSim accelerates that with the Wegstein method and shows it live in the convergence monitor.
Recycle reclaims unreacted feed; the solver tears the loop and iterates to close it.