The pot still is also a very simple device, consisting of a boiler, a condenser, and a connecting tube. Many successful pot stills have been made from everyday kitchen pots and pans. Pressure cookers are a common choice because they are sturdy, and can also provide steam for processing botanicals by steam distillation.
A common pressure cooker is more than ample for producing steam for botanicals, but rather small for producing spirits from a batch of fermented brew. Amateur distillers frequently use boilers in the range of 4 to 50 liters (1 to 12 US gallons). You should choose your size by carefully considering how much you intend to ferment, and how often you plan to run your still. It is easy and common to build something much larger than you really need or want.
Once the boiler is built, you need to connect a short tube to the top to carry the vapor to the condenser. This is usually easy with a pressure cooker, because most of them have a nozzle on top for the pressure regulator. A short length of polyethylene tubing can connect this nozzle to a similar diameter metal tube (keeping plastic parts to a minimum). This metal tube is then connected in a similar way to a simple condenser like a Liebig. If you have a modified water heater, then the tube can be screwed directly into the outlet fitting. If you have custom-built a boiler, you will have to choose how to fit the tube to the pot, and many methods are detailed throughout this book.
The pot still is an attractive option for beginners because of its simplicity, and the speed of obtaining results. Much of the equipment you make or acquire to build a pot still will be useful later on, when making more advanced fractionating or compound stills. The boilers are exactly the same, but you need to add external heater control to use the more sophisticated designs.
Warning! An unmodified pressure cooker is really NOT suitable for fractionating or compound stills, because of the tiny nozzle. If you want to use one, you should replace the nozzle with a larger fitting, which usually ruins it as a pressure cooker!
Making a boiler for a fractionating still is exactly the same as for a pot still. However, the size and control of the heating element becomes very important. A fractionating column works because vapor repeatedly condenses and re-evaporates on the packing, constantly increasing the percentage of volatile compounds. The faster hot vapor flows into the bottom of a reflux column, the more difficult it is for reflux to condense, and the column loses efficiency. In the extreme case, you end up with a column full of hot, fast moving vapor and no reflux - a pot still.
There is considerable leeway in the operation of a fractionating column, but remember that the more slowly the vapor rises up the column, the more efficient the fractionating process will be. We have found that it works best to limit the heater element to no more than 1 kW for a 50mm (2 inch) column.
A fractionating still is only a little more difficult to make than a pot still. In fact, the only difference is the use of a packed column instead of an open tube between the boiler and the condenser. In countries where amateur distilling is legal, many manufacturers offer "reflux" stills. Should you choose to buy one, first make absolutely sure that it really is a fractionating still. Many of them do not have adequate surface area in the column to enable efficient fractionation, either because the column is too short, or because the packing is inadequate, or both. They are really just pot stills, and the advertising is a case of mutton dressed as lamb.
In Chapter 3, we showed that reflux is the key to effective separation of the components of a mixture by distillation. The composition of the vapor gradually changes as it rises up a reflux column, becoming richer and richer in the most volatile components until, at the top, the vapor is very rich in the most volatile components. You can increase this separation by lengthening the column, but you will quickly reach the point of diminishing returns, because the increase in concentration grows smaller with each cycle of condensation and evaporation. The vapor and dew lines get closer and closer together toward the bottom of the curves.
A very good amateur column can be made from 1 meter (39 inches) of 50mm (2 inch) tubing. Increasing the length of the column will increase its fractionating efficiency, but increasing the diameter runs the risk of the liquid channeling as it works its way downward. Experiments have shown that the greater the diameter of the column, the more likely it will be that the reflux will establish "preferred paths" and run down quickly instead of slowly dripping from level to level. If that happens, the purpose of the column is defeated and the still is once again just a pot still.
Large industrial columns use physical spreader trays to hold and distribute the liquid in the column, and it seems reasonable that a similar technique would work on a smaller scale. However, we've never found any reason to explore that because a 5 cm (2 inch) column is simple to pack and provides more than enough output. Please feel free to do a bit of research!
The Fractionating Still
The column should be loosely packed with packing material, leaving a gap of around 6 inches at the top and 2 inches at the bottom. These gaps allow you to insert a thermometer and other items you might want later. As we have said before, packing with marbles is a waste of time and effort. Chapter 8 tells why in detail. Commercial packing material, like Raschig rings will do very well, but with a packing made of small pieces you need a way to retain them in the column. Wire screening pushed into the end of the tube is a simple answer. Just make sure that the wire is not so fine that it impedes vapor flow. We prefer to use metal scrubber material for all the packing. It provides the best surface/volume ratio and holds itself in place in the tubing. It also gives you the choice of using copper as both packing material and chemical catalyst (again, see Chapter 8 for details).
You can use whatever means of connection best suits you at either end of the column. If you use a commercial water heater as a boiler then a simple threaded adapter will probably be best to connect the bottom of the column to the boiler. Don't worry about the small diameter of the coupling. For the rate at which you'll be operating, a 13 mm (1/2 inch) opening is quite capable of delivering all the vapor produced.
A tube leads from the top of the column to the condenser. The condenser can be as simple or as complicated as you like, but an ordinary straight Liebig condenser will do just fine. We suggest building one about 60 cm (2 feet) long, because the extra surface area will allow good cooling with low water flow. A tee junction should be on top of the column, so that a thermometer can be inserted. A simple cork will do a fine job of holding the thermometer in place, or you may wish to use a metal compression fitting as described in Chapter 7.
Finally, resist the temptation to put "cooling tubes" through the middle of the column. Those that advocate them believe that this increases the amount of reflux and so increases the efficiency of the "reflux" column. In Chapter 8 we will show that all these cooling tubes do is interrupt the thermal gradient in the column and consequently reduce the overall efficiency. The only place that induced reflux may be used profitably is at the top of the column, and this is the key feature of a compound still.
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