The compound still allows you to increase the number of cycles as much as you want without lengthening the column. Placing a condenser on the top of the fractionating column and refluxing all the condensate creates a closed loop, with the reflux condensing and re-evaporating continuously in the upper section of the column. The last little bit of separation, which is the hardest to achieve, takes place in this re-circulation zone. The distillation process can now produce the most volatile component of a mixture, pure and without contamination!
Construction and operation of a compound still require a clear understanding of the processes occurring in the fractionating column. At a reflux ratio of 100%, the flow of vaporized material going upwards is matched by an equal flow of reflux flowing downwards. As the most volatile component builds up at the top of the column, the less volatile ones migrate downward to areas of higher temperature, and the least volatile ones constantly drip back into the boiler. Molecules of the most volatile component are constantly moving into and out of the reservoir zone at the top, but less volatile components cannot make it to the top. This is the state of equilibrium, or dynamic balance.
When you begin to withdraw product from the top of the column, it is replaced by material from lower in the column. If the product is removed slowly enough, the equilibrium can maintain itself, and the size of the reservoir zone remains the same as long as more of the product is entering the column from the boiler. If you remove product too rapidly, the impure material will move up the column faster than the column can purify it, and the reservoir zone will shrink and become contaminated.
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.
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.
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