Scaling up

For small scale distilling, the primary factors that can be easily altered are boiler power and column diameter. Column length will depend on the efficiency of separation, but if the packing is not altered too much then it doesn't make much difference, and 100-120cm (3 to 4 feet) is fine. Change the type of packing, and you're in a whole new ball game.

As column diameter increases, the capacity of the column and the area you are trying to distribute reflux over both increase dramatically (by the square of the diameter). We know of many people running columns in the 75-150mm (3-6 inch) range, and many of the people using 100mm (4 inch) and larger have had to resort to trays or other liquid spreading devices to get adequate performance. Scrubbers can be used to pack a 75mm column, but it takes a lot of them, and they have to be pulled out to a pancake shape with great care. A random filling like Raschig rings, or similar commercial packing material, is easier to use with large columns (please don't use marbles - see Chapter 8)

A second issue with increased column diameter is that the quality of separation can suffer, because turbulence within the column has more room to express itself and cause mixing between the theoretical plates. This is one of the reasons that large industrial columns use physical plates to hold liquid in place and force vapor into contact with it under strictly controlled conditions.

As boiler power is increased, more vapor and liquid condensate are produced (45 liters of vapor every minute for each kilowatt of power). If column diameter is increased proportionately, the main problem will be assuring the even spread of the condensate across the width of the column. If column diameter is not increased, two phenomena can appear when the power becomes too high. These are channeling and choking.

Channeling occurs when the downward flow of reflux becomes large enough that it doesn't spread evenly over the packing material, but begins to flow downward as a rapid stream. This is because the layer of liquid spread over the packing material becomes too thick for wetting forces to hold it to the packing, and it begins flowing rapidly down the column. Once channeling has started, it will continue even though the boiler power or reflux rate is turned down dramatically, because a preferred path through the packing has been established. Channeling dramatically reduces the efficiency of the column and the purity of the product. About the only way to detect channeling is through the temperature of the vapor at the top of the column and the quality of the product. If you are using scrubbers, the flow level that will trigger channeling changes every time you re-pack the column, so the best advice is to operate well below the maximum possible power.

Choking is caused when the volume of liquid working its way down the column fills enough of the column area that vapor cannot flow freely upward. Choking is hard to miss, because the column begins to literally rumble and shake, and large puffs of vapor can shoot out of the condenser. Choking not only ruins the operation of the still, it can be dangerous! Channeling does not always occur before choking - one is a surface area issue, and the other is an open area issue.

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