Reflux

Reflux is condensed vapor returned to a distillation column, where it flows downward and contacts rising vapor. This countercurrent contact improves separation by promoting repeated vaporization and condensation.

Distillation column: A vessel that separates a mixture by repeated vaporization and condensation. Reflux flows through the column, contacting vapor across its stages.
Fractional distillation: A separation method that repeatedly vaporizes and condenses components with different volatilities. Reflux is central to achieving repeated fractionation in a column.
Total reflux: A distillation condition in which all condensed overhead liquid returns to the column and no distillate is withdrawn. It maximizes internal reflux but produces no overhead product.
Minimum reflux: The limiting reflux condition at which a target separation requires an infinite number of ideal stages. Operating near this limit demands very tall columns for the desired separation.
Reflux ratio: The ratio of liquid returned to a distillation column to liquid product withdrawn. It quantifies how much condensed liquid returns to support separation.
Continuous distillation: A process that continuously feeds a mixture and withdraws separated products. Continuous columns return overhead condensate while products are withdrawn.
Minimum reflux ratio: The lowest reflux ratio at which a specified separation can be achieved, requiring infinitely many ideal stages. It marks the limiting low-reflux case against practical operating ratios.
Flooding (distillation): A column operating condition in which excessive vapor flow impedes downward liquid flow. Excessive reflux can raise liquid traffic and contribute to capacity limits.
Theoretical plate: An idealized stage at which vapor and liquid leaving are in equilibrium. Reflux helps each column stage approach vapor–liquid equilibrium.
Batch distillation: A distillation process in which a fixed charge is separated over time. Batch columns can vary reflux during a run to balance purity and production rate.

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