The vapours coming from the still are condensed in coolers of various shapes, one of which is shown in figs. 2 and 3. The cooling-pipes are best made of cast-iron, say 4 in. wide inside and laid so as to have a continuous fall towards the bottom. A steam-pipe (b) is provided for heating the cooling water, which is necessary during the later part of the operation to prevent the stopping up of the pipes by the solidification of the distillates. A cock (a) allows steam to be injected into the condensing worm in order to clear any obstruction.
| Fig. 2.—Condensing Worm (Plan) |
The cooling-pipe is at its lower end connected with receivers for the various distillates in such a manner that by the turning of a cock the flow of the distillates into the receivers can be changed at will. In a suitable place provision is made for watching the colour, the specific gravity, and the general appearance of the distillates. At the end of the train of apparatus, and behind the vacuum pump or ejector, when one is provided, there is sometimes a purifier for the gases which remain after condensation; or these gases are carried back into the fire, in which case a water-trap must be interposed to prevent explosions.
Distillation of the Tar.—The number of fractions taken during the distillation varies from four to six. Sometimes a first fraction is taken as “first runnings,” up to a temperature of 105° C. in the still, and a second fraction as “light oil,” up to 210° C., but more usually these two are not separated in the first distillation, and the first or “light oil” fraction then embraces everything which comes over until the drops no longer float on, but show the same specific gravity as water. The specific gravity of this fraction varies from 0.91 to 0.94. The next fraction is the “middle oil” or “carbolic oil,” of specific gravity 1.01, boiling up to 240° C.; it contains most of the carbolic acid and naphthalene. The next fraction is the “heavy oil” or “creosote oil,” of specific gravity 1.04. Where the nature of the coals distilled for gas is such that the tar contains too little anthracene to be economically recovered, the creosote-oil fraction is carried right to the end; but otherwise, that is in most cases, a last fraction is made at about the temperature 270° C., above which the “anthracene oil” or “green oil” is obtained up to the finish of the distillation.
| Fig. 3.—Condensing Worm (side elevation). |
During the light-oil period the firing must be performed very cautiously, especially where the water has not been well removed, to prevent bumping and boiling over. It has been observed that, apart from the water, those tars incline most to boiling over which contain an unusual quantity of “fixed carbon.” During this period cold water must be kept running through the cooler. The distillate at once separates into water (gas-liquor) and light oil, floating at the top. Towards the end of this fraction the distillation seems to cease, in spite of increasing the fires, and a rattling noise is heard in the still. This is caused by the combined water splitting off from the bases and phenols and causing slight explosions in the tar.
As soon as the specific gravity approaches 1.0, the supply of cold water to the cooler is at least partly cut off, so that the temperature of the water rises up to 40° C. This is necessary because otherwise some naphthalene would crystallize out and plug up the pipes. If a little steam is injected into the still during this period no stoppage of the pipes need be feared in any case, but this must be done cautiously.
When the carbolic oil has passed over and the temperature in the still has risen to about 240° C., the distillate can be run freely by always keeping the temperature in the cooler at least up to 40° C. The “creosote oil” which now comes over often separates a good deal of solid naphthalene on cooling.
The last fraction is made, either when the thermometer indicates 270° C., or when “green grease” appears in the distillate, or simply by judging from the quantity of the distillate. What comes over now is the “anthracene oil.” The firing may cease towards the end as the steam (with the vacuum) will finish the work by itself. The water in the cooler should now approach the boiling-point.
The point of finishing the distillation is different in various places and for various objects. It depends upon the fact whether soft or hard pitch is wanted. The latter must be made where it has to be sold at a distance, as soft pitch cannot be easily carried during the warmer season in railway trucks and not at all in ships, where it would run into a single lump. Hard pitch is also always made where as much anthracene as possible is to be obtained. For hard pitch the distillation is carried on as far as practicable without causing the residue in the still to “coke.” The end cannot be judged by the thermometer, but by the appearance and quantity of the distillate and its specific gravity. If carried too far, not merely is coke formed, but the pitch is porous and almost useless, and the anthracene oil is contaminated with high-boiling hydrocarbons which may render it almost worthless as well. Hard pitch proper should soften at 100° C., or little above.