The packing

The packing inside a fractionating column is very important and many articles in the scientific literature are devoted exclusively to this topic. Everyone has his own ideas on what constitutes the ideal packing and the writer is no exception. Unlike scientific texts, however, cost is a consideration here. 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 and should be of such a shape that they leave plenty of free space for vapour to rise up against a descending flow of liquid;

1/4 " x 3/8 compression fitting

1/4 " x 3/8 compression fitting


b) they should pack uniformly in order to avoid channeling, and c) they should have a large surface area and crevices where liquid can be trapped.

Scientific glass columns frequently use short, e.g. 6 mm lengths of 6 mm glass or ceramic tubing called Raschig rings. Ceramic saddles are another popular shape. Glass marbles might be used in large diameter columns but do not have sufficient surface area for a small diameter column such as ours. Also, unlike Raschig rings, they do not have any pockets where liquid can be trapped, so are rather inefficient.

The packing which we recommend has a very domestic origin but is cheap and highly effective. It consists of the scrubbers or scourers used for cleaning pots and pans and found in any supermarket. These are not the fine steel wool pads impregnated with soap but the much coarser scrubbers made from lathe turnings which usually come in a ball. They are available in copper, brass and stainless steel, and the ones to choose are the stainless steel. Several will be required for the column. Commercial packings using the same principle are available (at a price), and are very neat and uniform in surface distribution because the stainless steel filaments are woven into a blanket and the blanket is then rolled into a cylinder to exactly fit the inside of the column.

Packing the column is relatively simple if you have a 1V4" union joining the base of the column to the boiler because then there is no bottleneck and you have the full width of the column to work with. Pull out the balls of tangled filaments into a sausage-shape, dip them in soapy water to reduce friction, and carefully shove them up the column with a minimum of compaction. This type of packing only occupies about 4% of the column, leaving 96% open space, appreciably better than Raschig rings. It also has a much larger surface area, so you will find it very effective.

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