Numerical closure
Inspect residuals, tolerances, warnings and unresolved specifications. A status flag may summarize only part of the model.
ENGINEERING EXPLAINED / CONVERGED
Converged does not mean correct.
Close the recycle loop, change the condenser specification, and see the checks that belong between a green tick and a reported answer.
REFLUX FOR YOUR WORK
Engineering judgment,
with an extra pair of hands.
Join the Reflux trial waitlist. Tell us which simulator you use, and we’ll email you about availability.
Aspen Plus on Windows? Try Reflux Student →The guide below is free. No signup needed.
01 / WHAT THE TICK ACTUALLY CHECKS
The numbers agreed.
Now check the plant.
For a recycle solved by tearing a stream, the solver assumes a value, runs the downstream calculations, and compares the returned value with its guess. Convergence means the specified numerical tolerances were met. Other equations and specifications can have their own convergence checks.
A SCALAR TEACHING EXAMPLE
result = 20 + 0.6 × guess
Start from zero. Feed each result back as the next guess. The fixed point is 50, in arbitrary consistent flow units.
Not converged
Guess0.00
Returned result20.00
Absolute residual20.000Teaching tolerance: ≤0.10
This example uses simple fixed-point iteration and one scalar. Real recycle streams include composition, temperature, pressure and flow. Solver algorithms, tolerances and iteration limits vary; 30 evaluations is not a universal rule.
02 / A PHASE OUTLET CHANGES THE PROBLEM
Total or partial?
Give the gas somewhere to go.
In the professor’s anecdote, a column with a non-condensable in the overhead was specified with a total condenser. The reported result was −250° and a converged status. Changing the specification to a partial condenser allowed a vapour product.
THE REPORTED BAD RESULT
−250° · converged
A total-condenser specification asks for a liquid product. If the overhead includes material that will not condense at the intended conditions, that specification can demand an unrealistic state or fail.
The anecdote did not establish a temperature unit, mixture, pressure or corrected temperature. This toggle illustrates the outlet change; it does not calculate phase equilibrium.
CONDENSED LIQUID ↓ · UNCONDENSED VAPOUR ↑
A partial condenser permits both vapour and liquid products. It still needs a valid phase-equilibrium model, pressure, duty, reflux specification and feasible cooling utility. It is not a universal fix for convergence problems.
03 / BEFORE ANYONE TRUSTS THE RESULT
Five checks after
the solver stops.
Phases and temperatures
Check that the phase split, temperatures and pressures fit the chemistry, operating envelope and chosen property method.
Duty signs
Declare the convention. With heat into the system positive, a heater adds heat and a cooler rejects it. Investigate unexpected signs.
Flows and balances
Check mass and energy closure and unexpected negative physical flow rates. Signed reverse-flow conventions and iteration intermediates need context.
Utilities and equipment
Check approach temperatures, utility availability, pressure drop, size, materials and operability. A converged 200-stage result does not establish a practical column.
These are review prompts for an engineer or an agent supervising a simulator. Passing them is a starting point for design review, not a certification that the plant is safe or economical.
Why the example does not show a “correct” replacement temperature
Adding a vapour outlet changes the allowable phase split. The resulting temperature depends on the mixture, pressure, condenser duty and remaining specifications. Reporting a new temperature without those inputs would repeat the exact mistake this guide is about.
- Aspen Plus v10.2 User Guide, flowsheet sequencing and convergence: a primary reference for tear-stream iteration. This is an old edition; it does not establish current defaults.
- The 200-stage and −250° examples are a professor’s account supplied for the reel, anonymized here. They are illustrative anecdotes, not independently reproduced simulation cases.
Original generated equipment illustrations. The loop is a teaching diagram; the numerical example is deliberately separate from a full process simulation.