General Remarks On The Construction Of Bubbling Still-Heads

The results of the experiments which have been made serve to indicate the requirements which should be fulfilled in order that a still-head of this kind may give the best possible results.

1. Number of Sections. - It should be possible to greatly increase the number of sections without seriously adding to the difficulty of construction or to the fragility of the apparatus. This requirement is best fulfilled by the "Brown ' and the "Young and Thomas " still-heads.

2. Size of Constrictions, etc. - As the amount of condensed liquid flowing back at any level is greatest at the bottom of a still-head and least at the top, it follows that, in order to retard the flow sufficiently for a pool to be formed, more complete obstruction is necessary at the top of the tube than at the bottom. In the Le Bel-Henninger still-head, however, the constrictions are frequently made widest, and the platinum cones largest, at the top of the tube. There is thus a tendency, on the one hand, for the liquid to flow past the upper cones without forming a pool, and, on the other hand, for the quantity of liquid in the pools in the lower bulbs to be unnecessarily large. In the Young and Thomas apparatus it is advisable to make the upper constrictions somewhat deeper than the lower ones.

3. Width of Reflux Tubes and Depth of Traps. - It has been pointed out that, consistently with efficiency, the amount of condensed liquid in the still-head during distillation should be as small as possible. The reflux tubes should, therefore, not be made wider - in that part which is filled with liquid during the distillation - than is necessary freely to carry back the condensed liquid ; also the U-shaped parts, acting as traps, should be no deeper than is required to prevent the ascending vapour from forcing its way through them. Additional width or depth simply means waste liquid in the still-head, but if the number of sections is very large, the lower traps should be made rather wider than the upper ones. On the other hand, the upper part of the reflux tubes may be advantageously made fairly wide so as to facilitate the entrance of the condensed liquid, and also to prevent bubbles or columns of vapour from being caught and carried down with the liquid through the traps. Such columns of vapour, when formed, are liable to drive out the liquid, and the ascending vapour may then pass more easily through the traps than through the pools. formed by the obstructions.

The poor results obtained with the Le Bel-Henninger still-head, when only 25 grams of the mixture was distilled, were probably largely due to the excessive width and depth of the reflux tubes. The weight of liquid and vapour in the still-head was more than twice as great as in those of Glinsky or Young and Thomas.

4. Flow of Liquid through the Reflux Tubes. - It is of the utmost importance that there should be a rapid flow of condensed liquid through the reflux tubes, especially if they are outside the main tube and are not heated by the ascending vapour. To take an extreme case, suppose that there were no back flow at all through the reflux tubes and that the traps simply became filled with the first portions of condensed liquid. This most volatile liquid would thus remain lodged in the traps until the end of the distillation, and would then form part of the residue. In the distillation, for instance, of a mixture of benzene and toluene, the last fraction might consist of pure toluene, while the residue at the end of the fractionation would be very rich in benzene.

Of the three bubbling still-heads compared, the Young and Thomas is the best in this respect; in the Glinsky apparatus, on the other hand, the flow of liquid was exceedingly slow, and it is for this reason that, when only 25 grams of the mixture was distilled, the residue was so much richer in benzene than the last portions of distillate. The Glinsky apparatus is, indeed, quite unsuited for the distillation of very small quantities of liquid. The same fault is to be observed with the Le Bel-Henninger still-head, but it is not nearly so marked.

In fractionating tubes of the Glinsky or Le Bel-Henninger type, the upper end of the reflux tube should be wide and the junction with the bulb should be low down, in order that the quantity of liquid in the pool may not become unnecessarily large. This is a point that is frequently overlooked.

5. Arrangement of the Reflux Tubes. - To get the best results, there should be a reflux tube connecting each section with the one below it, so that the change in composition may be regular from bottom to top of the still-head. That is the case with the still-heads of Le Bel-Henninger, Brown, and Young and Thomas, but in the Glinsky apparatus there is only one reflux tube connecting the top bulb practically with the still. Thus the condensed liquid, which is returned to the still through the reflux tube, is richer in the more volatile component of the mixture than the liquid in the lower pools.

6. Return of Liquid from Still-head to Still after the Distillation is completed. - When the residual liquid is valuable, it is of importance that it should return as completely as possible from the still-head to the still. The weights of liquid actually left in the still-heads after cooling were as follows : - Glinsky, 0.2 gram ; Le Bel-Henninger, 1.4 grams ; Young and Thomas, 0.55 gram. In this respect, and this only, the Glinsky still-head gave the best results, but the amounts of liquid left in the "Rod and Disc," the "Pear," and the "Evaporator" tubes were far smaller.

Of the various still-heads that have been described, it may be concluded that, when only moderate efficiency is required, the "Rod

L and Disc " or "Pear" is to be most strongly recommended, but for great efficiency the "evaporator " still-heads give the best results.

Comparison of Improved with Plain Vertical Still-head.

- The relative efficiency of the different still-heads is well shown by the following comparison of the number of fractional distillations with a plain vertical tube of 30 cm. height, which give the same result as a single distillation with the improved apparatus.

In general, the improvement in the separation of toluene is better than in that of benzene.

Comparisons of the relative efficiency of various still-heads have been made by Rittman and Dean, J. Ind. Eng. Chem., 1915, 7, 754 ;' Bureau of Mines, Washington, 1916, Bull. 125, " Petroleum Technology," 34; Friedrichs, Zeit. f. angew. Chem., 1919, 32, 341, and others.

Fractionating columns are in many respects analogous to the scrubbing towers employed for the absorption of soluble gases. The theory of scrubbing towers has been considered mathematically by Donnan and Masson,1 and the conclusions arrived at as regards the conditions required for high absorption efficiency are applicable with respect to efficiency of separation in distillation.

The efficiency in each case depends on1. High interfacial area between gas (vapour) and liquid.

2. High relative motion of gas (vapour) and liquid within limits.

3. High degree of turbulent motion in one or both phases.

1 Donnan and Masson, "Theory of Gas Scrubbing Towers with Internal Packing," J. Soc. Chem. Ind., 1920, 39, 237 T. *

Other laboratory still-heads of various types have been described by-R. Rempel, Chem. Zeit., 1886, 10, 371. P. Monnet, Monit. Scient., 1887, (IV.), 1, 335. G. E. Claudon and E. C. Morin. Bull. Soc. Chim., 1888, 48, 804. M. Ekenberg, Chem. Zeit., 1892, 16, 958. E. Varenne, Bull. Soc. Chim., 1894, 11, 289. M. Otto, ibid., 1894, 11, 197. C. W. Volney, J. Amer. Chem. Soc, 1894, 16, 160. A. Tixier, Bull. Soc. Chim., 1897, 17, 392. H. Vigreux, ibid., 1904, 31, 1116. A. Golodetz, Chem. Ind., 1912, 35, 102, and 141.

4. Sufficient rate of flooding to secure the maximum drip effect. [As regards distillation this requirement is only applicable in certain cases.]

To put the matter shortly, what is required for both absorption and fractionation is the most perfect possible contact between liquid and gas (vapour).