Pour it back.
Boil it again.
The company is named after a ratio. Here is the ratio, what sits on each side of it, and what every drop you pour back costs at the bottom of the column.
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R = L / D, and what L and D are.
At the top of a distillation column the vapour leaves the last tray and goes into a condenser, where it becomes a liquid and collects in a reflux drum. From that drum the liquid splits two ways. One part leaves as distillate product, called D. The other part is poured back down onto the top tray, and that part is called L, the reflux.
The reflux ratio is just those two numbers divided: R = L / D. It is how much you pour back for every drop you keep. Nothing more complicated than that, and the whole design of the column hangs off it.
The reason reflux exists at all is that the liquid running back down the column is what makes separation happen. Each tray needs liquid flowing down across it to contact the vapour coming up. Take the liquid away and the trays have nothing to work with.
Both ends of the dial are useless.
Turn the ratio all the way up and all the way down and you get the two limits every column lives between.
Total reflux. Pour everything back and take no product at all. D goes to zero, so R goes to infinity. The operating line lies on the 45 degree diagonal, the driving force is as large as it can be, and the column separates with the fewest stages that are physically possible. It also makes nothing, which is why you only run a column this way at startup or while you are testing it.
Minimum reflux. Pour back as little as you can and the operating line pivots down until it touches the equilibrium curve. At the point where they touch there is no driving force left, so the steps get smaller and smaller and never get past it. That point is called a pinch, and the column would need infinite stages to do the separation. It is a lower bound, not a design.
Both diagrams use the same column: constant relative volatility of 2.5, saturated liquid feed at 50 mol%, distillate at 95 mol% and bottoms at 5 mol%. For this column the pinch sits at x = 0.50, y = 0.714, which gives Rmin = 1.10.
Move the dial yourself.
This steps the same column off for whatever reflux ratio you pick, counting theoretical stages the way you would by hand. Watch what happens to the stage count as you come down towards 1.0, and what happens to the boil-up as you go up.
Stages are stepped between the equilibrium curve and the operating lines, which is the McCabe-Thiele construction. It assumes constant molar overflow and a binary mixture with a constant relative volatility, so treat the count as the textbook answer rather than a tray count you would buy steel against. A real column adds tray efficiency, pressure drop and a safety margin on top.
What the pouring back actually costs.
Every drop of reflux that runs down the column arrives at the bottom and has to be boiled again. The vapour the reboiler has to raise is the reflux plus the product:
L = R D, and V = L + D, so V = D (R + 1).
That is the sentence the whole trade-off lives in. At R = 1.32 and 100 kmol/h of distillate, the reboiler is boiling 232 kmol/h, so more than half of what you boil never leaves as product. Double the reflux ratio to 2.64 and the boil-up goes to 364 kmol/h for exactly the same product.
So the ratio is a dial with capital on one side and energy on the other. More reflux means fewer stages, which means a shorter column and less steel. It also means more boil-up, which means a bigger reboiler and a bigger fuel bill for as long as the plant runs. Add the two costs together and the sum has a minimum, which is where the rule of thumb comes from.
Capital falls steeply as you come up off the minimum, because stages are disappearing fast. Energy climbs in a straight line, because boil-up is linear in R. The total bottoms out a little way up from the pinch, and that is the number the textbooks quote.
The 1.2 is a starting point, not a law.
Design texts put the economic optimum somewhere around 1.1 to 1.5 times the minimum reflux, and 1.2 is the value quoted most often. It is where the total cost curve is flat enough that being slightly wrong does not cost much, which is exactly why it is a good place to start and a bad place to stop.
The optimum moves with the price of energy, with how expensive the trays are, with how hard the separation is, and with whether the column is already built. Near an azeotrope the equilibrium curve comes down to the diagonal and Rmin climbs, which changes the arithmetic completely. Run the cost numbers for your own case rather than taking 1.2 off a page.
The relationships used here, R = L / D, the total and minimum reflux limits, the 1.1 to 1.5 design band and V = D (R + 1), are standard distillation results. Price, distillation lecture notes sets them out in one place.
Four in ten.
Distillation accounts for about 40% of the processing energy used in refining and continuous chemical processes. That figure is an energy share, not a share of costs, and it covers processing energy in those sectors rather than all industrial energy everywhere. US Department of Energy, distillation column modeling tools.
Most of that energy is reflux. Liquid boiled at the bottom, sent up the column, condensed at the top, and poured straight back down to be boiled again. The ratio decides how many times round that loop each drop goes before it leaves as product, which is why one number on one block is worth understanding properly.
And the name.
It is a play on words for reflux ratio. That is the honest answer. It is also a joke about acid reflux and what the current state of process software does to the people who have to use it, which is the other honest answer.
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