Still With Convex Bottom

A still which has been tried, and is in use in a large works to-day, is similar in all respects to the one just described except that it has a convex instead of a concave bottom. The heating surface is obviously the same, and it is more easily and effectively cleaned out. Its great drawback is that the draw-off pipe for the pitch must be connected in the centre of the bottom, and the junction is therefore subjected to the greatest heat during the distillation. The engineering problem how to overcome the consequent strains set up must be a difficult one.

Horizontal Still

The form of still just described is, as already mentioned, the usual one, but there are a few works that prefer a horizontal still, which, in most cases, has a similar dome-shaped bottom. There seems no distinct advantage in the horizontal still, but rather a disadvantage, as the fuel consumption is higher, in many cases even twice as high.

Tubular Heating

Many patents have been brought out for heating the tar in a still with tubes inserted through which the flue gases pass. In principle they are exactly similar to the ordinary water-tube steam boiler. This method of heating has given fairly good results, but it has been applied more successfully to continuous stills, details of which appear later.

Vapour Pipe

The vapour pipe is best made of cast iron and should be large enough to allow free flow of vapour. There is a tendency for manufacturers to make this pipe too small so that there is a danger of the vapour being throttled.

Condenser

The best metal for the coil is cast iron, as wrought iron corrodes too easily. Lead is sometimes used, but it is quite unnecessary to incur the high initial expense entailed by the employment of this metal.

Sight Box

The outlet of the coil from the condenser goes into a sight box or seal which allows all the gases to pass away by a gas pipe. This is very important, as the gases evolved just when the still begins to run, and that liberated when the anthracene oil is coming off, are very poisonous, and it is advisable to pass them through the gas pipe to an iron oxide purifier. The chief poisonous ingredient, hydrogen sulphide, is acted on by the oxide, and the purified gas is usually carried round and fed into the fire of the still or into some convenient fire.

It is customary in a works where there are several stills in a row to pass the gas from each still into a common gas pipe leading to the purifier. Where this is the practice great care should be taken, when cleaning out any individual still, to disconnect the gas pipe of that still; for if at the same time other stills are working, the gas can easily work back into the one under repair and so cause danger to the men inside. Several fatal accidents have occurred in this manner.

Receivers

The condensed vapour from the sight box is run into receivers. There are many simple devices by which the various fractions are made to run into their respective store tanks.

Pitch Cooler

The running-off pipe for the pitch is connected directly to the pitch cooler, which is placed as near to the still as possible. It must be remembered that when the distillation is complete the residual pitch has a temperature approximating to 300° C. The pitch is allowed to remain in the still for some hours until its temperature is considerably lowered, so that, when "tapped out," too much strain on the still shall not be caused by sudden cooling and contraction. The further strain on the metal of the still which would be caused by immediately charging it again with cold tar should be avoided. Even at the lower temperature it is not safe to expose the pitch freely to air, as it would be liable to catch fire.

For this reason an intermediate vessel or pitch cooler is used. It is either an iron boiler or sometimes a " pitch-house " made of iron work. The pitch is cooled in this vessel until it is only fluid enough to run into the cooling bays, where it remains until quite cold and sufficiently workable to send out.

Heat Interchanges Or Economisers

Most plants have some form of heat interchangers or economisers. They work on the principle of utilising the heat in the vapours leaving the still and that in the pitch to warm up the raw tar for the next charge.

These arrangements have been very thoroughly worked out and enough heat may be recovered to distil off all the water together with some naphthas from the raw tar while in the economiser, so that the actual still charge is hot enough to come on to the run soon after charging, and is free from water.

Lay-Out Of Still

Fig. 153 shows the lay-out of such a still; and it will be noticed that in this case there are two economisers : the first is fired in the same way as the still, in order to ensure the removal of all the water; the second is simply a condenser for the vapours from No. 1 with the coil surrounded with raw tar.

Frothing

A good many arrangements have been devised and patented with the object of preventing the tar from frothing and boiling over when distillation commences. The underlying idea is to cool the top portion of the still or the vapours in the top of the still. They are none of them very successful and, indeed, are little used, continuous dehydration giving better results, as will be shown later.

Continuous Methods of Distillation

Numerous attempts have been made to distil tar continuously, and although to-day the great difficulties that have to be faced are overcome, many distillers prefer the old intermittent methods. Continuous distillation is expensive, and can only pay where the still can be kept going continually, or when very few stoppages are required. This, then, is the great difficulty, as the tar available in most districts is limited, and very often the quantity differs greatly at different seasons of the year.

Some of the earlier experiments are very ingenious, and may be usefully mentioned.

Fig. 153. - Lay-out of Still.

A, pitch cooler ; B, tar still; C, No. 1 economiser ; D, No. 2 economiser; E, heavy oil condenser; F, naphtha condenser ; G, light oil condenser ; H1, H2, H3, outlets to receivers.