In the equipment section we have illustrated and discussed three different types of stillhead, two in copper and one in glass. For simplicity, the following discussion will be restricted to the model shown in Figures 3 and 5, the one we call the "Mexican cactus".
In contrast to the situation during beer-stripping, in the case of fractional distillation the small draw-off valve in the horizontal part of the still-head is completely closed initially so that all the vapour condensed at the top will run back down the column to the boiler. Under these conditions the column is said to be operating under "total reflux".
While the boiler is heating up, keep an eye on the operation until the thermometer in the stillhead suddenly rises and you know that the hot vapours from the boiler have heated the column and its contents and have risen into the condenser where they are being cooled and converted back to liquid. It is prudent, for the reason discussed in the next paragraph, not to walk away from the still and let this event take place in your absence.
The boil-up rate must not be greater than the column can handle. A packed column provides only a limited path for liquid to flow down against a rising stream of vapour so, if the boil-up rate is excessive, the column will choke with liquid and become ineffective. This is unlikely to be a problem with the 11/4-inch diameter column and the type of packing described in the equipment section, especially if the heat input is reduced to 750 watts by changing the immersion heater in the boiler as recommended. With a glass column choking is easily detected because liquid can be seen bubbling away in the packing, but with a metal column this is not possible. So listen. Choking or flooding may be detectable by a slight rumbling noise. The other method of detection is to look at the thermometer. Liquid rising from the boiler is much hotter than the vapour so, instead of registering 70+°C. the thermometer may register 90+°C. If this happens, switch off and try again. It is somewhat like a smoking chimney — once the chimney is warmed up the smoke stops billowing into the room.
The next several hours are spent equilibrating the column. This is the period during which the various components of the mixture sort themselves out with the more volatile components moving to the top of the column and the least volatile moving to the bottom. To understand why this takes time consider the following homely analogy. A long corridor is packed with people of different heights waiting to get through a door at the end in order to enter the store. The store manager announces that before he lets anyone in he wants everyone to sort themselves out by height, the short people at the front and the tall people at the rear, with a uniform height gradient between. There is a lot of shuffling about and it takes quite a while for a perfectly even gradient from shortest to tallest to be established. The same is true of a mixture of liquids of different B.P. in a packed column.
The progress of equilibration can be followed by watching the temperature of the vapour at the top of the column. Ethyl alcohol has a boiling point between 78 and 79 deg. C., the exact figure depending on the atmospheric pressure (see Appendix V), while the heads such as acetone and methanol have a lower B.P. The thermometer will register this and, although a temperature of 78°C. might be registered at first it will slowly fall a few degrees as the acetone and methanol find their way to the head of the queue.
Periodically crack open the valve in the stillhead a fraction to bleed off these heads into a spoon, leaving room for the ethanol to rise a bit higher in the column. Referring back to the analogy of people of different height shuffling about, if you let some of the shorter people through the door, even if the sorting out isn't quite complete, you will make it easier for the remainder to get organized. A suitable withdrawal rate would be 2 or 3 drops per second.
These heads not only have a strong smell (test them with a spoon) but also a terrible taste so you can congratulate yourself that you're getting rid of them and not drinking them. They are highly inflammable and make an excellent fondue fuel or starter fluid for the barbecue. As the heads are bled off the temperature will slowly rise to 78+ deg. C. indicating that most of the heads have now been drawn off and ethyl alcohol is beginning to appear.
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