The purpose of the still-head is to divide the vapour emerging from the column into two streams. This it does by first condensing the vapour to liquid in a heat-exchanger and then, as the liquid runs back down towards the column, diverting a portion of it to the outside world via a small valve. This valve has only a small volume of liquid to handle so for fine control choose a needle valve. Solder a short length of 5/16 inch tubing to the still-head as shown in the diagram and attach the valve with a compression ring fitting. This will avoid the necessity of having to heat the valve itself during soldering.
To make a strong joint, and to ensure a clear path for liquid flow, the following procedure is recommended: Before soldering, drill a 5/16 inch hole in the 11/4 inch elbow where it will overlap the inner tube. Then slip it over the 11/4 inch tubing and solder in place. Position the 5/16 inch tubing through the hole in the elbow and butting up against the inner tube. Solder in place. Drill right through the short length of 5/16 inch tubing, penetrating the inner tube.
When the valve is closed, all the liquid returns to the column and back down into the boiler. If the valve is wide open all the condensed liquid will run out through it and none return to the boiler. In practice, the valve is adjusted to a setting at which about 10% of the liquid is drawn off into a receiving bottle while 90% returns to the boiler. A valuable refinement is to have a tongue protruding about 3/4 inch into the column from the horizontal portion of the still-head so that the returning liquid cascades down the centre of the column. Without the tongue the liquid is liable to channel down the wall of the column and thereby fail to baste the packing uniformly. The tongue is shown in Figure 4 (but see alternative still head in Figure 7).
The condenser for cooling the vapour and returning it to the column is made from about 10 feet of 3/16 inch copper tubing.
A thermometer in the still-head measures the temperature of the vapour at the top of the column and is an excellent indicator of just when reflux has started. It also lets you know when the "heads" are coming over, when it is pure ethyl alcohol, and when the "tails" are starting to appear.
Packing: The packing inside a fractionating column is very important, and many articles in the scientific literature have been devoted to the subject. What is needed are pieces of glass, ceramic or metal which are inert to the liquid being refluxed and which have the following characteristics:
a) they should not pack tightly, but should be of such a shape that they leave plenty of free space for vapour to rise up against a descending flow of liquid; and b) they should have a large surface area and crevices where liquid can be trapped.
Scientific glass columns frequently use short (6 mm) lengths of 6 mm glass tubing called Raschig rings. If you decide to use a glass column the glassblower you employ will be able to supply you with them. For a metal column such as ours, an excellent and cheap packing is provided by ordinary scouring pads such as used in the kitchen for cleaning pots and pans. They are available in copper, brass, and stainless steel. The stainless steel ones are ideal but are not always stocked so if you have difficulty locating a supplier just use copper or brass. If they are held together by a rubber band, remove it and stretch out the balls of metal turnings into cylindrical shapes. Gently push the packing up the column, doing your best to avoid compaction. For a 3 foot column you will need about 8 scouring pads.
Another possibility for an effective column packing would be the spiral turnings from a lathe. See if you can find a local machine-shop which works in stainless steel and have them put some turnings aside for you. Since they normally go to the scrap-bin you can probably get them for nothing.
Stainless steel: The design will be just the same as in copper but you will find that the steel fabricator who makes it for you will probably use butt welding rather than fittings to join the pieces together. This adds to the labour costs so that the cost of the column, still-head and condenser will likely prove to be two or three times greater than the same equipment in copper where you have done most of the work yourself.
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