Distillation column filling

A good column filling should be 5-8 mm in size, have a large surface area and be smooth (glazed, polished or glass) in order to impart a fast and even runback. Glass spheres (marbles) and the like are generally too large to give a good fractionation in the column. One can say that a column filling should be one tenth of the diameter of the column, but this is not absolutely correct. For example, with

5 mm glass spheres in a 50 mm wide column the vapour resistance will be too high. If the spheres are the right size, the surface area will be too small. The column filling is a once-only cost, so it must be correct from the start. Raschigrings of porcelain (should be glazed) are the best for 50 to 75 mm wide columns. They look like small pieces of cup tube and have an enormous surface area (both inside and out). The surface area is 930 against less than 300 for the corresponding spheres without holes through, and they are the best that is available.

Raschigrings are used widely as boiling stones (for distributing heat in boiling vessel) both in the chemical industry and for boiling wort, for example, where it is desired to keep an even and exact boiling rate. Raschigrings are available from specialist suppliers. 1 litre of filling is sufficient for a normal column (600 mm high and

50 mm in diameter). As a stopgap one can use the small glass cubes resulting from breaking safety glass (a toughened vehicle windscreen with platic laminations between layers), brass, or stainless steel scrubs pads or lath fillings or 6 mm stainless steel nuts (expensive), etc.

If one wishes to compare the results of using Rachi rings and most other fillings the difference is as night is to day. Both the column and column filling must be cleaned thoroughly before each distillation, and must be cleaned after each distillation. A good cleaner is a winemaking cleaning agent used for cleaning of demijohns and bottles.

After the apparatus has been used the column should always be rinsed out with (preferably hot) water. One Hushes water through the distillation channel in the opposite direction through the entire column. This flushes out most of the impurities that have stuck in the column and column filling. Then it is a simple matter to remove the column filling for a thorough cleaning.

For a modern stainless steel still, both gas and electricity are suitable sources of heat, but electricity is safer to work with then a open flame. If the still has straight run-through cooling in the column the hotplate need not be infinitely variable. An ordinary boiling plate is suitable, but the best is a standard hotplate. These have a higher capacity. If the cooker has a cooker hood above it,

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