The Compleat Distiller

THE COMPLEAT DISTILLER 55
A well-designed column can consistently produce a very pure product at reflux ratios around 90%.
Even taking only 10% of the condensate causes the reservoir zone to shrink slightly, but not enough to
effect the purity at the very top, where the product is being condensed and collected.
Equilibrium requires a delicate balance between the rate of vapor production, the rate of condensation,
and the capacity of the column for liquid and vapor flow. Like with the fractionating still, you can
apply all the power you want to the boiler until it begins to boil, but then the power has to be adjusted
to a gentle simmer, to provide an even flow of vapor.
As an example, 750 watts of heat will drive vapor up a 50 mm (2 inch) diameter column at a speed of
28 cm (11 inches) per second. For a column only 1meter long (around 1 yard), that's quite rapid.
Without any interference, the vapor will pass all the way through the column in only 5 seconds! In a
compound still, all the vapor has to be condensed and re-evaporated many times on the packing within
the column as well as being recycled time an again at the top, making the column a very busy place.
Control of the heat entering the boiler, and thus of the vapor entering a compound still is essential.
The compound still head
Unfortunately, you can’t make a compound still by just setting a reflux condenser on top of a
fractionating column. It's a little more complicated than that. The purpose of the reflux condenser is to
condense the vapor reaching it so it can be returned to the column. The job of the still head is to
control the percentage of the distillate that is collected rather than returned as reflux.
Once again, just about any design of boiler will work well, as long as it is properly controlled. Your
choice of condenser for the top of the column is affected by the fact that the vapor and the distillate will
be going in opposite directions. A Reflux Condenser is probably the simplest choice, though a Firebox
type will work well also, as long as the cooling tubes are large enough to avoid choking with liquid. A
simple Liebig condenser will also do the job, but is quite a bit longer, making the complete still very
tall.
Compound still management
Choosing the type of condenser is simple compared to the question of how to control reflux and
withdraw the product. There are three fundamental methods of accomplishing this:
• Liquid management. Divide and distribute the liquid condensate directly.
• Cooling management. Divide the vapor into two flows by managing the amount of cooling in the
reflux condenser.
• Vapor management. Divide the vapor into two flows mechanically.
Liquid management
The still head does two things: taps into the reflux stream and controls how much is withdrawn.
Neither the pot nor the fractionating still has this problem, because all the vapor arriving at the top is
removed. A compound still is able to produce excellent separation because only a small amount of the
vapor is collected. Large commercial operations solve this problem by collecting all reflux from the
condenser in a tank, and running two separately controlled lines from it, a small percentage to the
collection tank and the remainder back to the column.
A header tank is impractical for a small still, so the reflux dripping from the condenser is sampled
directly and product withdrawn through a valve that can control very small volumes of liquid. Getting
this process set up is touchy. You must have a good understanding of the rate at which reflux is being
produced, and a precision (needle-type) valve for controlling the withdrawal rate.