Secrets of Wiped Film Distillation - Part 1

Secrets of Wiped Film Distillation - Part 1

Molecule size - Wiped film stills can distill larger molecules than commonly believed

Sometimes people put limits on distillation. I remember being told that certain molecules are just too large to be volatile. Certain solvents should be avoided because you can't dry them, or remove them.

For example - dimethylformamide has a boiling point of 153°C. It was considered hard to remove and best avoided. It was considered impossible to eliminate down to acceptable levels of residual solvent.

In many cases, this is true of reactor systems and even rotovap systems. These larger solvents can hang around for several reasons. Limited vacuum and distance to condenser cause major problems in these systems. Many reactor systems only pull 50-100 Torr. In addition, when the amount of solvent lessens, it becomes harder and harder to fill the headspace and get the vapors up to the condenser to be collected as distillate.

However, wiped film stills defeat these problems by their design. Wiped film stills are capable of high vacuum (0.020 Torr or better). In addition, the material is in a thin film, so there is no barrier to evaporation.

By comparison, in a reactor or rotovap, the volatile is vaporized on the heated surface on the bottom of the liquid. Then the gas travels through the entirety of the liquid to get to the liquid/gas surface and vaporize. The volatile can redissolve into the liquid and fail to evaporate.

Another advantage of wiped films stills is they can be configured with an internal condenser, giving a "short path" from the wall to the condenser. This condenser acts like a magnet, quickly removing the vapors from the gas phase and preventing reabsorbing or failure to travel to the condenser.

Because of these design advantages, wiped film stills can remove even relatively large molecules from your feed material down to low ppms. In fact, molecules like DMF are considered tiny and easy to distill in wiped film stills.

One example is silicone polymer fluids. The residual cyclic starting materials (D4, D5, D6) have boiling points from 175 - 245°C. Despite this they can be removed down to less than 50 ppm by using high vacuum and moderate heat in a single pass (if the feed is degassed). Even though these cyclics typically start at only 1-5%, that trace amount can be effectively removed.

This is not possible in reactors or rotovaps. I would have never even attempted this type of distillation in other equipment. I would have tried to find alternative ways to purify the product or said it wasn't possible.

And yet, this power of wiped film stills is not well understood. That is why this is a secret of wiped film distillation: Too many talented chemists don't know what is possible with the equipment. Basically, if the atmospheric boiling point is under 600°C, it may be possible to distill it with a wiped film still.