Pressure-still tar

Cracking. Materials such as cracking-still gas, pressure distillate, cracked gas oil, and pressure-still tar are products of cracking. By cracking we refer to the decomposition of heavy or high-boiling oils by exposure to extreme temperature. At temperatures exceeding about 680 F materials such as gas oil, fuel oil, and tars are decomposed into (1) gas, (2)... 

Several descriptions have been pubUshed of the continuous tar stills used in the CIS (9—11). These appear to be of the single-pass, atmospheric-pressure type, but are noteworthy in three respects the stills do not employ heat exchange and they incorporate a column having a bubble-cap fractionating section and a baffled enrichment section instead of the simple baffled-pitch flash chamber used in other designs. Both this column and the fractionation column, from which light oil and water overhead distillates, carboHc and naphthalene oil side streams, and a wash oil-base product are taken, are equipped with reboilers. 

Distillation System. The cmde condensate consists of the desired product, some low boiling constituents, and a smaller quantity of high boiling tar. Distillation separates the low boiling components, which are invariably incinerated, followed by the product fraction. Tar accumulates in the stiU ketdes, from which it is periodically removed, again to incineration. Stills work at atmospheric pressure and are vented to the incinerator. 

The various designs differ in the extent to which heat exchange is used, in the plan of the pipe-still furnace, in the distillation pressure, and in whether recycle of pitch or base tar is involved. 

Yield from Coal Tar.—The yield of coal tar in the original distillation of the coal is about 2 to 5 per cent, but it depends upon many physical or mechanical factors such as temperature and pressure of the distillation, the form of the still, the length of time and the temperature to which the volatile products are heated, etc. Approximate yields from the redistillations of the coal tar may be stated as follows ... 

The tar is distilled at a constant pressure in tube-stills where the distilled product passes into fractionating columns. These are divided into a series of chambers, or vats, where the rising vapors successively deposit their components in order of increasing volatility. 

Tar acid is an industrial product that contains a variety of aromatic compounds. Consider the distillation (still pressure 250 mm Hg) of such a system with the following mole percents (phenol, 35 o-cresol, 30 m-cresol, 30 xylenols, 15 heavy components or residue, 5). Volatilities relative to the residue (130°C to 170°C) for phenol, o-cresol, w-cresol, and xylenols, and residue are 14.37, 11.49, 8.16, 4.83, and 1.00. The fractionation is to... 

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