It is extremely important to give the column enough time to equilibrate. When this state is achieved, the composition of this mixture remains the same in each zone of the column, even though molecules are entering and leaving that point constantly. In particular, the liquid/vapor mix at the very top of the column will eventually consist of one, and only one compound. However, this will only be true when all the vapor reaching the top is returned to the column as liquid reflux. As soon as some of the condensed vapor is collected as product, then other components will rise a bit higher in the column. This has been covered in previous chapters, but it is worth repeating here, because understanding this is vital to the correct operation of a compound still.
Once the boiler is at temperature, and the power controller is turned down to around 750 watts, make sure that all the condensed liquid is returned to the column, and that the condenser on top is condensing all the vapor. The column is then left alone for two hours, while it reaches equilibrium and all of the most volatile component has had a chance to make its way to the top.
Many people worry that if the reflux is cooled too much, it will upset the equilibrium in the column, and many ingenious designs have been proposed to minimize this "super-cooling". Fortunately, these fears are groundless. What happens when the reflux is cooled below its condensation temperature (which is very difficult to avoid if you want to condense all the vapor) is that a "buffer zone" is established in the very top layer of packing. Here, the cool reflux is quickly warmed up to its boiling point, and both equilibrium and the overall reflux ratio of the column are maintained. See Chapter 8 for a full discussion of how this works.
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