Excerpt from THE COMPLEAT DISTILLER page 64
For associated images see the full version under “Recipes & Information”
If you’re on the quest for the purest product that can be made by distillation, the compound still is your tool. The price of the distinctly higher purity is a slightly more complex operation and longer operating runs.
The compound still is simply a fractionating still with a condenser on the top and a means of returning controlled amounts of condensed liquid back to the column as induced reflux. The fractionating still achieves reasonable separation through the naturally occurring reflux in its packing, and the longer its packed column the better it works. The compound column accomplishes significantly better separation to occur by recycling the reflux in the upper portion of its column over and over again, until the components of the liquid/vapor mix in the column settle into dynamic equilibrium.
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.
After two hours have elapsed at 100% reflux, the system is in equilibrium, and the top section filled with the foreshots. You need to collect this product very slowly, (1-2 drops per second), so you will not disturb the equilibrium. You must proceed cautiously at this stage, because there is only a small quantity of the foreshots and it would be easy to disturb the small zone they inhabit in the top of the column. You will begin collecting them at a temperature as low as 60ºC (140º F), and as they are collected, the temperature will slowly rise. Discard them until the vapor temperature stops rising, and then begin collecting the heads, which may be added to the next batch for ethanol recovery. By this time, the temperature should be stable at about 78º C (173º F). You can expect 100 – 150 ml of heads from an average batch. Your nose should be the best judge of when the heads have all been collected, but if you wish to be absolutely sure (or if you have a cold!), collect 250 – 300 ml (about a cup).THE COMPLEAT DISTILLER 65
From this point on, an almost pure azeotropic mix of ethanol and water will be coming through and may be collected as the main product. The temperature should remain absolutely stable throughout the collection of the main product, and you can increase the rate of collection to around 4 drops per second. We give guidelines in terms of drops per second, but the size of a drop is affected by many things – its composition, its temperature, and the size of the tube it is dripping from. The real measure is the production rate, remembering that we want to collect the main body of ethanol at 90% reflux. 750
Watts will produce about 50 ml of 95% ethanol per minute, so you want to be collecting between 4 and 5 ml per minute to establish a reflux ratio of 90%.
The best way to measure this is to establish a slow flow, begin collecting it in a small graduated cylinder, and measure the time it takes to collect 10 ml. Keep adjusting the valve until it takes two to two and a half minutes. When you are satisfied with the collection rate, count the number of drops you get in 10 seconds. Do this three or four times, and take the average.
In the future, you can reset the still to the same 90% reflux ratio by adjusting the valve until you have the correct number of drops falling in 10 seconds. This adjustment may be made quite rapidly. In our experience, the correct rate of collection is found at about 35 drops in 10 seconds – not far off the 4 drops we recommended above! For the best accuracy, though, you should make this measurement yourself and calibrate your system. Calculating on the basis that the 20 liters put in the boiler contains 8 liters of ethanol, it will take around 14 hours to collect all of it. You could get slightly better purity if you reduced the draw-off rate to 2 drops a second, but the run would take twice as long.
To achieve the highest purity, you will take the slow road, and you probably won’t want to watch the still for hours, not to mention the extra attention required near the end, checking for the onset of tails. Fortunately, all the toxic compounds and heads have been removed, so it’s perfectly safe to stop the run at any time and resume it later. You will have to equilibrate the column again, but since the most volatile component is now ethanol, it will only take an hour at 100% reflux. Once equilibrium is achieved, you can start collecting product immediately.
Since you know how much ethanol was in the boiler, and you know the rate of collection, you can calculate when the end of the run should be. About an hour before this time, you should start collecting in small bottles and checking them for taste and smell before adding them to the main container. Your sense of smell is much more sensitive than the thermometer.
When you detect tails in the product, switch to a tails container. You can raise the collection rate now, and continue until the temperature has risen a few degrees, indicating that all the ethanol has been collected. You can add the collected tails to the boiler of the next batch along with the heads.
The product from a compound still is often much purer than commercially produced vodka, and most people are very satisfied with it as it comes from the still. If you want the ultimate in purity, process the product with activated carbon to remove the last hint of any congeners. We especially recommend carbon treatment if you are planning on making perfumes, essences, or delicately flavored liqueurs. It shouldn’t be necessary if you want to make strongly flavored beverages or mixed drinks. The final choice, of course, is up to you, and you should have some fun experimenting with these options. Let your own taste and preference be your guide.