To avoid confusion, it may be well to point out that the treatment of tars varies ac,cording to their qualities and sources; unless otherwise stated, the remarks in this article apply to English tars, ordinarily obtained from London and country gasworks. London tar weighs 11 to 12i lb. a. gall.; country tar, about 10i lb. The tars known 843 "Midlands" are tbe most sought after by distillers; having been distilled at a lower temperature than the London tars, they arc richer in bonze]. When the heating is very low, tbe benzol may be replaced by eupion. London tar is principally obtained from Newcastle cannel or bituminous coals. Scotch cannel yields 15i gall. of tar and 32 gall. liquor a ton ; caking coal yields about half as much. Tbe nature of the tar from the same coal will vary according to whether the retorts used are of elay OF of iron ; clay retorts are more strongly heated, which circumstance is said to diminish the benzol and increase the naphtha lene. This may explain why country tars are preferred to London tar.
The form and capacity of still used in the " fractionizing," or fractional distillation, of coal-tar vary, principally, according to the extent of the works. The smallest capacity considered economical is 500 gall.; in large factories, stills holding from 1200 to 2500 gall., and even 4000 to 7000 gall. arc used. A great advantage with the larger size is that it enables a manufacturer to dispense with much stowage space in the form of tanks, and consequently saves a great deal of labour ; on the other hand, an accident is of much more serious consequence, and the charge cannot 2 T be run over without involving nightwork. For snaall works, those stillii are preferred which admit of working off a charge in ten or twelve hours. The largest stills are convenient for dealing with special descriptions of tar, and in special modes of distilling. The most convenient, size is 2500 gall., which, on the English system, is run over in about thirty-six hours.
In arranging the plant for a tar distillery, the great object is to enable the tar to be worked up as rapidly as possible, and with the smallest amount of labour. In the winter months, increased supplies frequently involve the erection of a few extra stills. The varying consistence of the tar may necessitate modifications in the method of distilling, so that it is almost impossible to prescribe any general rules in erecting the plant for this business. A few extra stills should always be put up ready for use.
There is no doubt that eventually most large gas companies will utilize their own tar ; in such cases, it would be easy to determine what plaut should be erected, because, as a rule, the coal used is the same, and the conditions of heating are likely to be kept uniform. The manufacturer who purchases his tar from several gasworks has to put up with a variable product.
The English stills are constructed of On. boiler plates riveted together ; the portions whic.h are likely to be more strongly heated are made thicker. If the distillation is to be carried to coking, cast iron should be used, as it does not burn out so rapidly, and needs no joints. To prevent burning, the bottom of the still rests upon brickwork, except in the coking process. The form of still
chiefly used in England is that shown in Fig. 481. The "head" consists of a bent pipe a, communicating with a series of iron pipes b arranged in a tank as a condenser. At the top of the still is a manhole o, and an inlet d for the tar ; in same cases, the same inlet is used for injecting super heated steam, either for distilling purposes, or for driving out the pitch after the charge is finished.
Around the neck of the still is a channel for collecting the portions which condense in the head ; these are added to the distillates, as their falling back into the still may cause frothing, which is dan gerous. Near the bottom of the still is a pipe for removing the residue. Tbe stills are heated by separate furnaces, and are arranged under a light shed, so as to protect the vrorkmen from the weather. They are sometimes enclosed in brickwork, as shown, with the view of preventing loss of heat. The heating flues should not reach above the space occupied by the contents of the still at the end of the operatiou.
For a tar which is rich in light hydrocarbons, such as that yielded by Scotch cannel coal, and by the better kinds of lignite, another kind of still is required, as the application of fire-heat would be accompanied by danger. The method adopted with these tars is to distil them by steam-heat, in order to remove their more volatile constituents, until the residue in the retorts is reduced to a thick mass, known as " boiled tar." This is then withdrawn into tanks, or is pumped into other stills, in which the distillation is caaried on in the usual way. These tars, being richer in benzol and anthraeene oils, are much sought after by distillers, though the separation of water from them is more troublesome. The process, which is known as the " Scotch method," is conducted in the following manner :—The " green," or raw, tar is pumped from the settling tanks into stills—S, Fig. 482—having a capacity of 4000 to 5000 or more gallons, and made of i-in. boiler plate. A perforated steam-coil lies at the bottom of the still, or steam is admitted through branches p, with valves V, from an inlet pipe I. The stills should not be more than I filled, say to W, as plenty of room must be allowed. The distillation is carried on until the distillate acquires a sp. gr. of 0'930; the residue in the retort, known as "'boiled tar," is then run out, through a hole near the bottom, into tanks, or is pumped into other stills at once, for further distillation over a naked fire. This process commando itself for several reasons ; the most important are that the rapid recovery of the volatile constituents of the tar allows a large quantity to be worked up expeditiously, and that the naphtha obtained is more free from smell, and will keep better on exposure to the light. When the lighter portions are drawn over, the residue is stowed away for working up iu slack time. Much of the so-called Scotch " solvent naphtha," now met with in commerce, behaves under acids so differently from English naphthas that it is questionable how far it can be used as a substitute for them. A. great merit of the best Scotch naphthas the absence of naphthalene from the " solvent " fraction.