The Fractionating Still is a compact device for increasing the amount of condensation and evaporation of reflux. A fractionating still has a vertical tube called a "fractionating column", filled with packing material, between the boiler and the final product condenser.
It's important to note that the "fractionating column" is just a part of the whole still, just as the boiler is a part. If this "column" were empty and had no packing material, then it wouldn't be a "fractionating" column, but would simply be a tube carrying vapor from one place to another, like the Swan's Neck in a whiskey still. The addition of packing promotes the process of "fractionating" we saw happening on the copper dome of a whiskey still. A "fractionating column" is used in both the "fractionating still" which we're about to describe, and the "compound still" we describe later.
The packing material in the fractionating column needs to have as much surface area as possible, and at the same time have as much open area as possible so vapor and liquid can flow freely through it. Many different kinds have been developed over the years. We discuss packing at length in Chapter 8.
The vapor rising through the column condenses on the packing, heating it up. As more vapor enters the column and condenses, the heat released re-evaporates the more volatile components of the reflux, increasing the strength of the vapor in the column and weakening the condensed reflux. The new, stronger vapor has a lower boiling point because of its higher volatile content, and will condense further up the column, starting the process over again. The depleted liquid has a higher boiling point, and will drip down the column until it finds a hotter region and evaporates again.
As this process repeats again and again, the most volatile components find their way to the top of the column, and the least volatile drip back into the boiler, and the temperature in the column is lowest at the top and highest at the bottom. The longer the column is, the more cycles of condensation and evaporation, and the better the separation of components.
In a 1 meter (just over a yard) fractionating column filled with high-efficiency packing, hundreds or even thousands of condensation and re-evaporation cycles take place. This is equivalent to re-distilling dozens of times with a pot still. The net result is a product with a very high percentage of volatiles indeed, even up to 90%. The key limitation of a fractionating column is that the vapor flow must be slow enough for the fractionating process to work efficiently. In a fractionating still, the relationship between fractionating column capacity and boiler energy input is critical. These factors are discussed in detail in Chapter 8.
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