The fact that benzol solidifies when cooled to a low temperature supplies a method for its puri fication on a large scale. The solidified mass is submitted to pressure, by which the benzol is freed from its liquid impurities ; by redistillation alone, this mrould be almost iinpossible. Benzols which become solid at —1° (30° F.), are prepared for special purposes.
In commerce, it is rarely met with in such a state of purity ; but is sold as 90 per cent., 40 per cent., Sze., which means that when the sample is distilled, 90 per cent., 40 per cent., &e , passes over at or below 100° (212° F.) : 90 per cent. is the highest commercial rectification, and in preparing pure benzol it is better to work upon this basis. Pure benzol is obtained from the commercial article by repeating the process for the production of the latter, and expressing the congealed mass.
It is of great importance when purchasing benzol, toluol, naphtha, &c., first to have an under standing with the seller, as to the conditions of performing the distillation :— 1. The quantity to be placed in the retort for distilling.
2. The capacity of the retort.
3. Whether the bulb of the thermometer is to be immersed in the liquid or in the vapour.
4. The mte, whether in drops or in a stream, at which the distillate shall pass over.
5. The method of applying the beat.
6. When the distillation shall be considered complete.
Corrections for barometric pressure may sometimes be necessary, and, in all cases, the distillate should be measured at the ssrne temperature as the liquid, the loss due to imperfect refrigeration and condensation being considered as a part of the distillate. The residue, if measured off immediately the distillation is ended, will be mrarm ; if left in the retort to cool, the few drops collected in the receiver after the distillation is ended must be regarded as part of the residue. This may easily give rise to a difference of 2 or 3 per cent.
Manufacture and Rectification.—Several forms of apparatus for separating benzol from once-run naphtha have been already mentioned. The essential feature in all of them consists in receiving the vapours in a condenser kept at such a temperature that only those liquids can pass over which are vaporizable at that degree, those vapours which are condensed at this same degree being returned to.the still. By using a senes of condensers, the liquid may be fractionized to any extent at one operation. This portion of the factory is kept cool, and is placed as far as possible from boilers and furnaces, on account of the volatility of benzol and the inflammability of its vapour. The distillation
is effected by steam or heated water, a,s it is then easier to regulate the temperature ; direct heat colours a portion of the liquid. The vapours are collected in another boiler, placed on end, provided with a thermometer, and jacketed so that it can be kept at any required temperature by steam or water ; those vapours which are condensable at that temperature are here "arrested, whilst the others pass on to a properly cooled receiver, whence they are delivered into a vessel provided with two or more taps, commuoicating with stowage tanks. Wlaen the temperature of the intermediate condenser is so high as to keep the benzol in a state of vapour, the product will be richer. When it is desired to collect the distillate at different temperatures, the jacketed boiler is heated to, say, 82° (180° F.), so as to allow the benzol to pass over ; as the temperature is raised, the cock leading to the benzol tank is closed, and another cock is opened, so as to collect the second distillate ; thus any number of products may be drawn over from the same still of crude naphtha.
The crude benzol is washed by thorough agitation with strong sulphuric acid in well-closed, lead-lined vats ; on the large scale, the agitation is performed by st,eam power ; the acid is then drawn off, and the henzol is well washed with water. These operations are repeated according to the degree of purity required, or until the acid drawn off is nearly colourless ; the final washing is with lime water, or a weak alkaline solution. When viell washed, the benzol will suffer no change of colour on addition of sulphurio acid. The sulphuric acid may be used aft,erwards for the treatment of crude naphtha. The beuzol is then carefully distilled. To avoid the loss due to heating, the sulphuric acid is added slowly and at long intervals. The treatment viitla acid and alkali, and final washing with water, are best carried out in separate vessels ; by arranging them one above another, the liquids are easily decanted, and the sludge can be caught in receptacles placed beneath. Lime is cheaper than soda ; but it forms, with many of the tar acids, less soluble com pounds, wbich are thus less easily removed by washing. The alkali waste from this and similar operations is now frequently burnt for reconversion.