Molecular Falling Film Still

Dean Stark
Uncommon Stills

A glass molecular falling film still is an extremely simple apparatus that is useful for cleaning up a distillation run, performing a separation at a precise temperature, or (what we use it for) recovering hearts from the heads fraction.

The body of the still contains a heated core with a special spiraled edge along the length of the tube. This tube is heated using a solvent at its boiling point, and the solvent is completely isolated from the liquid to be distilled. For example, if you want to separate at 68-69’C, an ideal solvent is n-hexane, whose boiling point is 68.7’C. We operate this unit at atmospheric pressure, not under vacuum.

An outer shell surrounds the heated central spiral tube. The central tube is hot, the outer shell is cool. The model shown at left is produced by Aldrich.

The solvent is heated in a lower boiler flask and brought to its boiling point and held there. Eventually, the central spiral tube reaches this temperate and maintains this temperature. The solvent vapor is captured by a reflux condenser and returned to the boiler. We use a closed loop Tektronics cooler running at about 4’C to condense our solvents without issue.

Hot solvent reflux return to the boiler. The solvent is completely isolated from the wash.

At the top of the still, the wash is added dropwise and SLOWLY onto the central spiral tube. The special shape of the spiral’s edge creates a thin film out of each drop, which descends slowly down the spiral by gravity. Since the thin is very thin (“molecular”) with a wide surface area, it is brought up to the temperature of the central tube very quickly. There is not enough film applied to appreciably cool the spiral tube, so the apparatus is essentially a constant temperature still.

Wash driprate onto the heated spiral core.

As the hot thin film descends, those components in the wash with a boiling point less than or equal to the solvent boiling point go into the vapor phase, reach the outer shell of the still, and are immediately cooled and condensed. The condensate falls by gravity and reaches one receiver flask.

As the room temperature wash (purple) falls by gravity onto the heated central spiraled tube, low boiling point compounds vaporize and reach the outer shell and condense (red). Higher boiling point compounds remain in the liquid phase and travel down the spiral (blue).

The components in the wash with a boiling point greater than the solvent boiling point remain in the liquid phase and ride down the spiral by gravity, reaching another receiver flask.

The falling film wetting the spiral (watch at the center).

It is important to know that there are many potential azeotropic mixtures in a wash, not just the famous ethanol-water one at 78.1’C. For example, if we select n-hexane at 68’C as our boiling solvent, we can potentially separate out the methanol azeotropes:

Common azeotropic mixtures that can exist in a distiller’s wash. These boiling points should be considered when selecting a solvent.

As long as heat is applied to the solvent boiler, the condenser is cold, and as long as the wash is introduced dropwise to the top of the still, the unit operates continuously! We load ours up with 1L of heads at a time, and let it drip all morning or afternoon to recover some good ethanol while we are doing something else.

The best way to control your drop rate at the top of the still is with a constant addition funnel. These funnels have a special bottom valve with an equalizing arm and are pretty much “set it and forget it” once you have a good application rate.

Adding a long squirrel-cage type blower vertically near the still body provides a simple way to cool the outer shell and improve your separation.

Horizontal computer cooling fan mounted vertically to remove heat from the outer shell of the still. This method works very well.

The gold standard, in our opinion, is a jacketed shell, available from Aldrich, and we are always looking for one!

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