Distillation columns

A distillation column is a tall vessel full of trays (or packing) where rising vapor meets falling liquid. At each tray they come close to equilibrium: the heavy component condenses out of the vapor and the light one boils out of the liquid. Up the column the vapor gets purer in the light component; down it, the liquid gets purer in the heavy one.

Heat goes in at the bottom through a reboiler, which boils some of the bottom liquid to make the rising vapor. At the top a condenser turns the vapor back to liquid. Part of that liquid is drawn off as the distillate (top product) and the rest is sent back down the column as reflux, which is what supplies the falling liquid. The feed enters partway up.

The reflux ratio, R = reflux ÷ distillate, is the main design choice. More reflux means more washing on each tray, so you need fewer trays, but more vapor to boil and condense, so more energy every year. At total reflux (nothing drawn off) you need the fewest stages, given by the Fenske equation: N_min = ln[(x_D/(1 − x_D)) × ((1 − x_B)/x_B)] ÷ ln α. For 95% top and 5% bottom with α = 2.5: ln(19 × 19) ÷ ln 2.5 = 6.4 stages.

At the other end is the minimum reflux ratio, below which no number of trays can do the job. Real columns are usually designed at about 1.2 to 1.5 times minimum reflux: a balance between the cost of a taller column and the cost of the energy. A graphical method (McCabe–Thiele) or simulation software then gives the actual number of trays.

Distillation is reliable and scales up well, but it's energy hungry: it accounts for a large share of the energy used by the chemical industry. That's why plants use heat exchangers to recover heat between columns, and look for other separations when α is small.