Should I use marbles Ive heard of Raschig rings but what are they and are they suitable for my equipment Ive heard that pot scrubbers make the best packing for small stills but that sound silly

This subject is a hotbed of opinions and, quite frankly, a lot of half-baked ideas. We need to go back to first principles to try and shed some light on this contentious subject.

Most people just don't understand how a plate in a large column operates. This is made worse by the fact that many of the equations used to describe the process use the term "theoretical plate". This leads to the notion that all you need to do is put flat plates in the right places inside a column, and the job will be done.

A plate in a large column is not just a flat plate. It's a trap to collect and hold reflux coming down the column, keeping it in intimate contact with the vapor rising up the column, so the equilibration processes can occur. Plates in big commercial columns have many different designs. One simple one is to stud each tray with numerous "bubble caps" that look like small metal mushrooms. Rising vapor enters the hollow stems of these "mushrooms" and then bubbles through the liquid because the outer lips of the caps are submerged in the liquid held in the tray.

The resulting foam of bubbles provides the large surface area needed for equilibrium exchange. If another way could be found to provide this large contact area between liquid and vapor then trays wouldn't be needed at all!

Bubble caps and foam are clearly out of the question for small columns, so the large liquid/vapor interface area is provided not by a foam of bubbles, but by the surface area of materials put in the column - the packing. The liquid reflux coats the large surface area of this material is held in intimate contact with the vapor. The greater the surface area, the better the interaction between liquid and vapor.

We can't use any material with a large surface area, because vapor and liquid have to flow freely through the column. Sand has a large surface area for a given volume, but the particles are so small that liquid would fill the tiny gaps between the grains and no vapor could pass through. Pebbles would permit vapor to rise as the gaps would be much larger, but the surface area would be very much less. We have to find the best compromise between the volume occupied by the packing material, the surface area available, and the space left for vapor and liquid to flow up and down. Let's examine a few frequently mentioned packing materials.

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