Sources Of Benzene And Its Homologues

The chief source of benzene, toluene, and solvent naphtha is not coal tar but coke oven gas and coal gas, from which these substances are extracted by washing with various solvents such as tar or special creosotes. Some idea of the proportion of crude benzole obtained from the various sources can be obtained from the following interesting statistics :Production of Crude 65's Benzole from Coke Oven Plants

Figures compiled by the Ministry of Munitions for the crude benzole produced by gas companies during the latter part of the war period are as follows: -

No figures are obtainable for the crude benzole produced by the tar distilleries, that is to say, directly from the distillation of tar, but it is estimated that these distilleries make approximately 4,000,000 gallons per annum, and this quantity does not vary very much from year to year. It will be seen that the height of production was reached in the years 1917 and 1918, when about 44,000,000 and 43,000,000 gallons respectively were produced from all three sources. Since this time the production has rapidly dwindled down until in 1920 the estimated production from all sources was not much more than one half of this amount. The reduction in output is chiefly from the coke ovens and gas companies, as the extraction of the benzole at the present time is not a paying proposition.

M. Grebel1 gives the following interesting figures about the production of crude benzole in France :-

Production in 1913 from coke ovens was 2,600,000 gallons. With proper legislations and enterprise he foretells that the annual productions could be increased to1 H. Grebel, Gas Journal, 1920, 149, 364.

Fractionation of Naphthas

Plant Required

The plant required for fractionating the naphthas may be conveniently described in four separate parts as follows: 1. The Still, or heating medium, in which the liquid is converted into vapour.

2. The Column, or scrubbing tower, in which the vapour and condensed liquid are brought into intimate contact.

3. The Dephlegmator, analyser or constant temperature still-head, in which the vapour is partially condensed and passed back into the column.

4. The Condenser and Receiver, where the desired vapour is finally condensed and received.

1. The Still

It is not necessary to further describe the fire still which is used for the primary distillation of the naphthas, and again for the preparation of the refined heavy naphthas. The plant is simple and has been fully discussed earlier.

The steam still is simple in construction and can be made of either wrought or cast iron, and of any size from 2000 to 10,000 gallons capacity according to requirements. The shape of the still is usually a horizontal cylinder, although any convenient second-hand iron vessel will suit. This latter is often used, as the wear and tear on it is very small.

The coils, which must be of sufficient length to give the required heating capacity, are made of cast iron in sections, or preferably of wrought iron in one piece so that only two joints are required, one at the entrance and one at the exit. The cost of installing these wrought-iron coils is greater, but considerable economy is gained by saving heat, and also the chance of leaks is thereby reduced.

Copper coils should not be used, for although they are recommended by some makers, they do not work out economically. Their initial cost is high, and for some unknown reason they rapidly corrode and wear thin.

The slightest leakage of steam into the still is fatal at stages in 'the distillation when the very intensive fractionation is being carried out. In a large still several sets of coils are used so that more economical regulation of temperature can be effected. The outlet of the coil is connected with a steam trap so that constant pressure and temperature are maintained in the coil. The still is also supplied with a raw steam inlet at the bottom which enables the solvent naphtha to be brought off by steam, as the temperature required for this is too high for the steam boiler pressure found in most works. There is a charging pipe from the washer. The end of the still has a large manlid, generally the whole width of the still, which permits of easy cleaning and also allows the one-piece steam coil to be taken in and out. The column either rests directly on the still, the vapour passing into it through the large outlet hole, or is separated from it, the vapour being then conveyed through a wide connecting pipe.

2. The Column

This part of the plant is by far the most important of the whole apparatus, and too much attention cannot be given to it. Its function is to attain intimate contact between the vapour and condensed liquid.

It has been shown by Rosanoff and Bacon1 that the washing or scrubbing in a column is quite apart and distinct from the cooling attained in a dephlegmator or analyser and is far more important. So that although dephlegmation is an important adjunct to a fractionating apparatus it is essential that the column should be of sound construction before it is added.

The evolution of the present-day column, so far as coal tar distillation is concerned, is an interesting one and may be described in order of employment as (a) Packed Column, (6) Perforated Plate Column, (c) Cup and Seal Column.