Characteristics Of A Good Still-Head

In choosing or constructing a still-head, the first point to be considered is its efficiency in separating the components of a mixture. It frequently happens, however, that the quantity of liquid available is small and, in any case, when a series of fractional distillations has to be carried out, some of the fractions eventually become very small: in such a case, of two still-heads of equal efficiency, that one is the more useful which allows of the distillation of the smaller quantity of liquid. Now there is necessarily, at any moment, a certain amount of condensed liquid in every efficient still-head and, obviously, the smallest quantity of a substance that can be usefully distilled must be considerably greater than that of the liquid and vapour in the still-head. It is therefore of great importance to construct the still-head in such a manner that - consistently with efficiency - the quantity of condensed liquid in it at any moment shall be as small as possible. It is also important that, after the removal of the source of heat, this liquid shall return as completely as possible to the still.

Among other points to be considered are (a) ease of construction, (b) freedom from liability to fracture, (c) convenience in handling.

Comparison of Still-heads. Mixture distilled. - In comparing the efficiency and usefulness of different still-heads it is necessary always to distil a mixture of the same composition. Equal weights of pure benzene and toluene may conveniently be employed.

Rate Of Distillation

It is necessary also that the rate of distillation should always be the same, and it is best to collect the distillate at the rate of one drop per second. A good plan is to have a seconds pendulum - a weight attached to a string 39.1 inches or, say, 1 metre long, serves the purpose very well - swinging behind the receiver, so that the drops may be easily timed.

That the rate of distillation does greatly influence the separation was pointed out by Brown,1 and is well shown by Table 48, in which are recorded the results of three distillations, each of 50 grams of the benzene-toluene mixture, at the rate of 30, 60 and 120 drops per minute. An improved still-head was used, and the distillate was collected in eleven fractions, the twelfth consisting of the residue (calculated by difference) after the temperature had reached the boiling point of pure toluene. For convenience, the results are stated as percentages.

Table 48

* Temperature barely reached 110.6° ; residue not quite pure toluene.

It will be seen that the separation is greatly improved by diminishing the rate of distillation.

Explanation Of Tables

In Tables 49 to 53, which show the relative efficiency and usefulness of different still-heads, the following data are given.

1. The vertical height from the bottom of the still-head to the side delivery tube.

2. The final temperature for each fraction.

3. The percentage weight of distillate.

4. The weight of vapour and liquid in the still-head during the distillation when pure or nearly pure toluene was coming over. This was arrived at by continuing the distillation as nearly as possible to the last drop and weighing the liquid, when cold, in the flask (still) and - if necessary and possible - in the still-head. From this amount was subtracted the constant weight of liquid and vapour in the flask itself, which was estimated to be 0.85 gram.

1 Brown, " The Comparative Value of Different Methods of Fractional Distillation," Trans. Chem. Soc, 1880, 37, 49.

For the very long still-heads, which could not conveniently be weighed, the estimated loss by evaporation +0.85 gram was subtracted from the difference between the original weight of the mixture and the sum of the weights of the fractions. The actual loss by evaporation, when measured, varied from 0.00 to 065 gram, and was greater for the large than the small still-heads.

The Plain Vertical Still-head, - Although the plain vertical still-head is less efficient than any other, yet a certain amount of fractionation does take place in it, and it may be well to consider the effect of altering (a) its length, (b) its internal diameter. The influence of such alterations is shown in Table 49.