A reflux column is a vertical tube, 60-120 cm tall and 5-10 cm across, packed with copper mesh, glass, stainless pads or ceramic so that rising vapour repeatedly condenses and re-boils, building ethanol up from about 10% to 92-95%. Collect only while a thermometer in the vapour at the top reads 78-80°C, set the heat so the vapour never floods the packing, and keep the column insulated so its own reflux, not heat loss, does the work.
Part of Ethanol Fuel Production
A reflux column enriches ethanol vapour in stages by letting part of it condense on packing and run back down to meet fresh vapour.
Reflux Is The Dial That Trades Speed For Strength
At any height in the column, rising vapour meets liquid running down. Where they meet, the more volatile ethanol moves into the vapour and the water into the liquid, so the vapour gets richer as it climbs. The reflux ratio R is the amount of condensate sent back divided by the amount taken as product. The boiler must make (R + 1) times the product as vapour, so higher R gives a purer product more slowly and at more heat per litre. The parent credits a well-made column with 92-95% ethanol in one pass. The corpus has no sourced plate count or optimum R, so this page gives the structure and the checks, not a number to build to.
What You Need
- Column tube: copper or stainless pipe, 60-120 cm tall, 5-10 cm across
- Packing: copper mesh, glass marbles, stainless steel scouring pads or ceramic pieces; not zinc, galvanised or aluminium
- Thermometer reading the vapour at the top of the column, not touching liquid or metal
- Condenser at the top (to make reflux and product) and a product tube, with a way to throttle product take-off
- Lagging (cloth, straw, clay-and-fibre) to insulate the column
- A controllable heat source
Method
Step 1: Heat gently to boiling, with the column packed and the take-off closed so all the condensate returns (total reflux). Wait until the top temperature steadies near 78°C.
Step 2: Open the take-off slowly and collect only while the thermometer holds 78-80°C. A higher reading means water is coming over; the parent says stop collecting above 85°C.
Step 3: Hold the heat steady. Increase product take-off and R falls and strength drops; reduce it and strength rises but the run lengthens.
Step 4: Watch for flooding. Surging, spitting or gurgling means liquid is being held up in the packing.
Worked Example: How Fast Can The Vapour Go Up The Tube?
Take a 2 kW heat input. Vapour that is mostly ethanol takes about 930 kJ/kg to produce, so 2,000 ÷ 930 = 2.15 g/s. That is 2.15 ÷ 46.07 = 0.0467 mol/s, and as an ideal gas at 78°C (351 K) one mole is 22.4 × 351 ÷ 273 = 28.8 litres, so the flow is 0.0467 × 28.8 = 1.35 L/s. In a 5 cm tube (area 1.96 × 10⁻³ m²) that is 1.35 × 10⁻³ ÷ 1.96 × 10⁻³ = about 0.69 m/s up the open tube, faster through the gaps in the packing. Vapour speed scales with heat and inversely with the tube’s cross-section: double the power and it doubles; double the diameter and it falls to a quarter. If a 5 cm column floods at some power, a 10 cm column should carry about four times it. (The flooding power itself is not sourced here; find it by raising heat slowly until surging begins and then backing off.)
A rough output check: at R = 3, 1 L of ethanol product (about 0.85 kg) needs 4 × 0.85 = 3.4 kg of vapour, 3.4 × 930 = 3.2 MJ, plus 2.7 MJ to warm 9.9 kg of wash, so about 5.9 MJ per litre. A 2 kW supply, 7.2 MJ an hour, then gives at most about 1.2 litres of ethanol an hour with no heat lost. Real stills are slower.
Common Mistakes
- Uninsulated column. Heat lost through the wall condenses vapour unevenly, and the top gets weak vapour; lag it.
- Too much heat. The packing floods and the product is weak.
- Packing that is too fine or too tightly crammed, which chokes the vapour path.
- Reading the thermometer in the liquid or against the wall. It then reads the pot, not the vapour.
- Sealing the top of the column so no vapour can leave. A column with a blocked take-off and a hot pot builds pressure.
Never close a still completely
The condenser and product line must always have an open route out; a column with a blocked outlet and a heater on will rupture. Ethanol vapour ignites at 3.3-19% in air and runs along the floor; use no open flame and keep every joint sealed (Sealing Joints), as set out in Distillation.
What’s Next
- Learn next: Achieving 90%+ Purity, for measuring and reaching the target
- Combine with: Condenser Coil, which sizes the cooling