Laboratory Scale Distillation

observed. No forced reflux is used in this method of rectification; all the vapour reaching the head is condensed as efflux. In order to minimize entrainment of droplets, some form of dephlegmator can be interposed between the pot and the head. This technique can be conducted at pressures ranging from atmospheric to the lowest pressure available; pressures as low as 0.01 torr are not unusual. Very small quantities of product can be distilled using a device known as a 'kugelrohr (ball-tube) apparatus'; it is constructed of two or more spherical globes connected linearly by glass tubing to each other and to a vacuum source. The globe most remote from the vacuum source is charged with the distilland and vacuum is applied; the unit is rotated by hand in a horizontal position while the globe containing the distilland is heated by flame or oil bath. The vapour condenses in the globe nearer the vacuum source.

It must be stated that the development of preparative gas chromatography, a powerful and convenient method, has in most cases obviated the necessity of distillation of quantities less than 25 g. Separations hardly realizable with the most sophisticated and cumbersome techniques can be accomplished in minutes using this method; the fractions taken are usually of a high degree of purity as isolated.

The remaining method, fractional distillation, is normally used to purify quantities of crude product larger than approximately 100 g, although apparatus such as the Podbielniak column, constructed of a metal helix fitted snugly into a small-diameter glass column, have been utilized successfully to fractionate amounts less than a tenth as large. The seperation efficiency of such a column can be very high but throughput is very low. As stated, preparative gas chromatography is currently the method of choice for small quantities. For rectification of larger quantities a variety of apparatus is available, and the choice of equipment rests upon the predicted difficulty of the separation to be accomplished. As mentioned, on the bench one may be able to choose a synthetic scheme which results is an easily purified crude product; when this is not feasible recourse is had to a more efficient system capable of separating components whose boiling points lie close together.

As a rule of thumb, if the components of a mixture differ in boiling point by at least 30°, satisfactory partition can be achieved by use of Vigreux column, which is constructed of a glass tube 16-25 mm in diameter 20 cm to 1 m in length having tiers of indentations spaced 1.5-2 cm apart. The fractionating efficiency of the Vigreux column is quite good considering the ease and economy of construction of the unit.

Due to its relative openness the throughput of such a column is quite high, and as a result this type of column has become the workhorse in many preparative laboratories; its efficiency may be further en-chanced by operating it under forced reflux. Throughput of 0.1-1.5 Lh-1 are common when using this column. An additional advantage to use of this column is the ease of repair; most repairs can be made in the laboratory.

In cases where the boiling point differentials between component is less than 30°C, use of a more efficient column to effect partition will usually be found advantageous. These units are of three basic types; the first in which contact between liquid and vapour in the column is forced by mechanical construction of the column, the second in which the structured packing is used to present a large surface area to promote vapour-liquid interaction in the column (such as a bubble-cap column), and the third, a column filled with glass or metal objects of various geometries such as metal saddles, glass beads, and glass helices. The last type is often referred to as 'dumped packing'.

Units of the first type, whereby the vapour and liquid are forced into contact by the construction of the column are fairly efficient and can be designed for a reasonably high throughput. They are, however, very expensive and quite fragile when constructed of glass; thus, while they have found major application in industrial processes, where the construction is only of metal, in the laboratory they have not found use to the extent to which packed columns are used. Repairs to glass columns of this type are difficult and expensive.

The second type, structured column packings, are usually made up of a fairly closely woven metal mesh, which is then crimped and rolled into cylindrical sections which are pushed into a glass or metal column; the packed length of such a column typically will be 0.6-3.0m for laboratory use, with diameters of 25-100 mm. The packing itself can be fabricated from many different metals, for instance stainless steels, monel, or tantalum depending on the chemical nature of the product to be purified. The advantages of this type of column are relatively high throughput, low pressure drop and moderate to high fractionation efficiency. They can be designed to operate at pressures as low as 0.1 mmHg and even greater than atmospheric pressure. In practice, however, the high vapour velocities and low vapour densities encountered at pressures less than 1 mmHg result in significant degradation of column efficiency, especially if a moderate throughput is required. As to size, a column of this type whose internal diameter is about 25 mm would be suitable for use with a distillation pot of 5 L capacity, while for use on a 50 L pot a column of 75-100 mm internal diameter would suffice. In addition, this type of column is quite easily constructed, easily repaired if broken, and can be used to predict the performance of an industrial unit of similar construction. In the laboratory, throughput of 0.1-2.0 Lh-1, depending on pressure, can be expected.

The last group of columns, the so-called 'dump packed' type, are as a group the most efficient at fractionation and can be the most tedious to use. They can be filled with a variety of packings, from Raschig rings (short sections of glass tubing) to columns packed with single turn glass helices dropped individually into the column. The latter is one of the most efficient fractionating columns ever devised, but as will be seen, is more suitable for distillation of smaller batches. In practice, columns of this design intended for laboratory use are 0.6-2.5 m in length, with internal diameters of 15-60 mm. Packing fabricated of perforated metal should be purchased after consultation with the manufacturer; a nominal size of 0.4-1.0 cm is usually chosen for bench use. Of the various packing components, a column filled with glass helices of 4.5-8.0 mm diameter has proven to have the greatest efficiency per unit column length but this advantage is offset to some extent by the low throughput and high pressure drops attendant with use of these units. Raschig rings are cut to a length approximating their external diameter from glass tubing 6-12 mm in size. Columns filled with dumped packing have one advantage other than efficiency in that they can easily be emptied and refilled with some other type of packing should such be desirable; conversely, structured packing, once installed can be difficult if not impossible to remove from the column without out ruining it. The disadvantages of dumped packings are a tendency to form channels through which vapour can pass without contacting liquid, low throughput, and, in most cases, high pressure drop, in addition to being prone to flooding. As a result these columns are seldom used at pressures below 10 mmHg; fractionation efficiency and throughput suffer markedly at lower pressures. In spite of these disadvantages columns of this construction have found use in many laboratories because of their ability to successfully perform separations not possible with columns of other design. Throughputs of 0.025-0.5 Lh_1 can be expected from these columns.

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