(C) Cup And Seal Column

The difficulties experienced in practice in the use of the perforated plate column induced makers to devise a different form, and Heckmann (Ger. Pat. 39557) may be said to be the first to develop the cup and seal column, which is so efficient that it has, with slight alterations, come into general use.

Heckmann Column

Fig. 167 shows a sectional diagram of the column in which a is a fixed plate with several (according to the diameter of the column) ascending vapour pipes b. The vapour is prevented from being carried direct into the next section by means of the inverted hood, dome or cup c. The cups, which have serrated edges, are partially immersed in the condensed liquid, the depth of this liquid on the plate being regulated by the height of reflux pipe d.

The vapour when it strikes the dome c is forced down and through the liquid on the tray, and so an actual scrubbing action between liquid and vapour is attained. The return pipes D are placed on opposite sides of the column in each tray and dip into the liquid in the tray immediately below. Thus the returning liquid flows across each tray, and the ascending vapour can only rise through the pipes b.

Egrot's plant was devised on similar lines to that of Heckmann, and was used very successfully in France in the 'nineties of last century, and was then considered a great improvement on Savalle's perforated plate column. Many patents have been brought out since, making slight variations on the cup and seal column. For instance, Metallwerke Nehien A.-G. (Ger. Pat. 294781, 1914) introduced baffles on each plate of the column to ensure the ascending vapour taking a more zigzag course through the liquid on the plate. The improvements effected are not, however, very noteworthy.

The practical advantages of the cup and seal column are that the column is not so sensitive to irregularity of working caused by variation in atmospheric conditions and by change of steam pressure. It can also be worked at almost any speed with efficiency, although, of course, as in all cases of distillation, the slower the speed within certain limits the better the fractionation. The inverted cup plays an important part in the column, as there is no doubt that the vapour, as it bubbles through the liquid on a tray, mechanically carries with it a certain amount of that liquid to the next tray above. The result is that there is a continual ascension of liquid of too high a boiling point, being mechanically carried upwards with the vapour. The inverted cup lessens this action, as it allows a less violent ebullition on the trays.

3. Dephlegmation

Dephlegmation is the word usually employed at the present time to describe the action of an apparatus above the column which condenses further portions of the vapour and returns them to the column.

Dephlegmators, analysers, reflux condensers or constant temperature still-heads, as they are variously called, may all be said to perform the same function of dephlegmation. No plant can be thoroughly efficient without some form of dephlegmator, although makers have often paid too much attention to the dephlegmator and too little to the column itself. Unless the construction of the column is correct no amount of complicated dephlegmating apparatus will produce good results, but the careful combination of the two is essential.

The usual form of dephlegmator is shown in Fig. 168, in which it will be seen that the vapour passes through the nest of tubes and is partially condensed by water surrounding them.

It is found practical, in some cases, to allow the vapours to go outside the tubes and the water inside as the tubes can be more easily cleaned of lime deposit.

E. Bonnell (Fr. Pat. 395040,1909) employs hexagonal tubes immersed in a water bath, by which he obtains greater cooling surface without unduly increasing the size of the apparatus.

C. Still (Eng. Pat. 3269, 1911) uses a horizontal dephlegmator so arranged that the vapours pass in at the top and traverse a zigzag course downwards while the water flows in the reverse direction.

Fig. 168. - Dephlegmator or Analyser.

Other patentees of note of constant temperature still-heads and dephlegmators are - Rosanoff (Fr. Pat. 443054, 1912), Chenard (Fr. Pat. 443499, 1912 and 1914), Allen (Fr. Pat. 481134, 1916) and Aylsworth (U.S. Pat. 1250760), who regulates the temperature of his still-head by means of a thermostat.

The usual form of analyser, as shown in Fig. 168, is slightly different in principle from the usual dephlegmator in that separate condensates are obtained. Each condensate is then returned separately to a different part of the column. If the water is allowed to flow in at a and out at B, the heaviest portion of the vapour will be condensed in C, and the lightest in f. The condensate in c is, therefore, returned to a section about half-way down the column, and in d, e, f, respectively to successively higher sections in the column.

Many makers, and particularly some of the earlier ones such as Coupier, Vedle and Egrot, paid great attention to their analysers and maintained that they were the chief factor in obtaining their good results.

The author has experimented with a plant with an ordinary perpendicular tube dephlegmator and horizontal analyser, as described (p. 404), attached to a cup and seal column. The results showed that the analyser did practically no work at all and accounted for no extra separation whatever. The reason may have been that the plant was working so efficiently that before the vapours reached the analyser the separation was so complete that any further change was negligible ; at all events, this horizontal type of analyser can be of little practical value. The principle of all these forms of dephlegmator is exactly the same. Water is usually the cooling medium, although in come cases the raw material for the still charge is used in order to obtain a heat interchange. The inflow of cooling liquid is regulated so that the constant difference of temperature between cooling liquid and vapour is about 20° to 25° C. If this difference is maintained, a sufficient quantity of vapour is condensed and refluxed back into the column, thus ensuring a good descending flow of liquid through the column and a consequent intimate contact between vapour and liquid.