Building simulations
Recycles, convergence and messages
A flowsheet without a recycle solves in one pass, block by block. A recycle has to be guessed and corrected until the guess and the result agree — and LensaSim shows you that happening.
How a recycle is solved
- The solver finds every loop and tears it at a stream: it assumes that stream, runs round the loop, and compares what comes back with what it assumed.
- It repeats, accelerating with Wegstein's method, until the two agree to one part in a million.
- Analysis → Convergence shows the torn streams and the residual at every iteration — a line falling steadily is a loop converging; one that stalls or wanders is not.
When a loop will not converge
- Give the material somewhere to leave. A recycle with no purge accumulates whatever enters and never leaves — an inert, a by-product — and cannot reach a steady state. Add a splitter and purge a little.
- Choose the tear. Click a stream in the loop and Mark as preferred tear stream. A stream with a steady composition, such as a recycle just after a separator, usually works best. A loop closed only through an energy stream must be torn at a material stream.
- Check the blocks in the loop alone. A column or reactor that fails on its own will fail inside a loop — the messages name the block.
- Make it easier first. Solve with a smaller recycle or a looser specification, then tighten it.
Reading the messages
- Errors stop the solve and name the block and the reason: a specification it cannot meet, a missing input, an unbalanced reaction.
- Warnings mean the result is there but carries a caveat — a property method outside its range, a two-phase feed to a single-phase block, a step count that is too coarse. Read them before you trust the number.
- The AI assistant in Analysis can read your flowsheet and its messages and explain them.
Something not covered here? Email us — a person replies.