The Adam Still

The Adam Patent Continuous Benzole Still is worthy of note and is shown in Fig. 174, which portrays a plant capable of distilling 10,000 gallons of crude benzole in 24 hours. It is employed in the primary separation of crude benzole into fractions, roughly representing the final products, and the elimination of the small proportion of creosote.

There are three stills, a1, a2 and a3, through which the crude benzole flows in succession, and from each a fraction is obtained corresponding to the benzole, toluol and solvent naphtha final products.

The advantages usually sought for in continuous working are uniformity of operation and heat economy, and these are obtained in this plant mainly by the use of control thermometers and the utilisation of a large proportion of the heat latent in the vapours.

The crude benzole is first circulated through a tubular preheater B, where it exchanges heat with the vapour of the benzole fraction from the first still a1; next, direct steam heat is applied in a special heater c, and this, broadly speaking, brings the temperature to the distilling point. The crude benzole now enters the first still, where it is heated by steam of 80 to 90 lbs. pressure. The still is a flat-bottomed one of D-shaped section, and the crude benzole follows a channel which repeatedly traverses the still longitudinally. The steam heating coil lies in the same channel, and the steam and benzole flow in reverse directions. The crude benzole on leaving the still should have parted with nearly all its benzole. That this is the case is indicated by its temperature which is registered by a thermometer in the outlet pipe. The crude benzole is similarly treated in a second, a2, and third still,

A3, of the same type, and the final outflow contains no light products.

Each still is fitted with a fractionating column of dimensions corresponding to the amount of vapour released and to the dephlegmation required. The first, d1, is the largest (18 feet high x 5 feet in diameter in 10 sections), the others being respectively, d2 (15 feet high x 3 feet in diameter in 8 sections) D3 (7 feet high x 2 feet in diameter in 7 sections). Each section contains an arrangement whereby the ascending vapour is compelled to pass through about 2 inches of refluxed liquid, in fine bubbles, from a perforated plate. This makes a very efficient column. It should be noted that in any section of the column the composition of the liquid and vapour remains the same, whereas in an intermittent type of still it continually alters. The liquid which .flows down the column is produced by a tubular analyser or reflux condenser, E1 and e2 of the ordinary type (this is omitted in the last still), and the vapour then passes to the condensing system,

1' 2 and G3.

As previously stated, the cold crude benzole plays a part in condensing the first or benzole fraction of the distillates. The rate of flow of crude benzole into the plant is seen on a meter h, of the Venturi type at the inlet to the preheating system, and each fraction at its outflow from the condenser passes over a V-notch calibrated to act as a meter. Further controls are provided by thermometers in the pipes carrying the vapours from the fractionating columns, and at various other points.

The products are worked up by the usual acid treatment, and are refractionated from stills of the usual type, specially designed for preparing pure products.

Treatment of Fractions

As the various plants required and in use for the fractionation of the naphthas have been very fully described, a survey of the method of splitting up the fractions will be of interest.

This, of course, cannot be a fixed method as it is dependent on so many factors, such as the quality of the crude material available and the kind of plant employed. However, roughly speaking, the distiller endeavours to make his fractions somewhat on the lines of those shown in Fig. 175, always bearing in mind that the fewer the fractions the less complicated the whole fractionation. Also the endeavour obviously is to get from the crude product to the finished one with as few intermediate fractions as possible. In the figure the distillations are shown as follows : - primary distillation ----------, secondary---------, tertiary

----------, final............. The figures below each fraction indicate approximately the boiling range.

1. Primary Distillation. - The crude naphtha or light oil, distilled in a fire still, produces crude benzole, crude solvent, crude heavy solvent and creosote.

2. Secondary Distillation. - The crude benzole, which should show a test of 60 to 65 per cent distilling off in a retort below 120° C, is the raw material for this distillation, and is obtained from the crude naphtha or from the washing of coke oven or coal gas with creosote. The crude is washed in the manner already described before it is run into the steam still. It can be worked in two different ways, that is, after the forerunnings containing large quantities of carbon di-sulphide have been sweated off, the charge can be run direct for pure benzene and pure toluene, or 90's benzole and 90's toluol.

In the former case the splitting of the distillate is very complicated as so many fractions have to be collected. They are, in order of collection as follows : - forerunnings, pure benzene, light crude toluol, pure toluene, heavy crude toluol, crude solvent, crude heavy solvent and residues. To obtain the pure products a very efficient plant has to be employed and, even so, the method is hardly practicable, for just before and just after the pure hydrocarbons come over, the still has to be run very slowly indeed (40 to 50 gallons per hour) and the intermediate fractions are necessarily large. It is better to sweat off some of the forerunnings and collect the fractions rapidly as 90's benzole, 90's toluol, crude solvent and crude heavy solvent. The last two fractions may either be blown over by means of raw steam or, if preferred, by distilling in vacuo, in which case the fractionation is more marked and better separation is obtained. The residue contains mostly creosote, but, to ensure that no naphtha is wasted, it may conveniently be put back into the crude naphtha for further working.

Fig. 175.

3. Tertiary Distillation. - The fractions should, as far as possible, be worked in rotation, so that the residue from each is the base on which a fresh charge of the next fraction is run. The 90's benzole produces a little forerunnings, pure benzene, light crude toluol and a little pure toluene.

When running from any fraction for pure products, the principle of working the still is always the same. The stage just before coming on to the pure, which can be ascertained by reference to the thermometer in the still-head pipe, is the one that has to be most carefully watched ; the still is then run very slowly, and the temperature of the water in the dephlegmator and analyser is kept very constant and well below the boiling point of the pure product. As soon as the pure commences to distil over, the rate may be very considerably increased, even up to 200 gallons per hour, and kept at that until there are indications of a change, when again the rate is decreased.

It is of very great importance to work a still at the speed which gives the best results, and this can only be ascertained by trial for each plant and for each fraction. Diligent research on this matter will enable the distiller to obtain the best possible yields of pures with the smallest quantities of intermediates.

After the pure benzene has been collected and the temperature has risen a few degrees, the charge of 90's toluol is added to the residue from the 90's benzole in the still; the distillation is then continued, light crude toluol, pure toluene and heavy crude toluol being collected.

The distillation is again stopped, the crude solvent added to this residue and the distillation recommenced, when the first fraction to come over consists of heavy crude toluol.

The other products from this distillation, namely the solvent naphtha and crude heavy solvent, are either obtained by blowing in raw steam or by distillation in vacuo. The former is the cheaper way, but in the opinion of the author the latter has advantages which more than compensate for the extra cost.

The last of the tertiary distillations, that of the crude heavy solvent, must be carried out in a fire still with a column having no dephleg-mator. In this case solvent naphtha and heavy naphtha are obtained, and the residue is creosote.

All the fractions before distillation are washed with sufficient amounts of acid and soda to meet the necessary requirements of purity.

4. The Quaternary Distillation. - This is the splitting of the light crude toluol into pure benzene and pure toluene, and of the residue, after addition of a new charge of heavy crude toluol, into pure toluene and solvent naphtha. In each of these cases small quantities of intermediates are formed which have to go back again to their respective fractions in the tertiary stage.

The quaternary distillation is conducted in the manner already described except that it may be generally carried out at a greater speed.

The figure shows the simplest form of separation of the various naphthas from the crude and one which would be nearly perfect from a distiller's point of view. Unfortunately,, in practice, a few intermediates between those mentioned are obtained, and have to be put back to the fractions to which they most nearly agree in boiling-point range.

The control of a refinery making all these products is not a simple matter, and should be in the hands of a capable chemist who can translate the results of tests obtained in the laboratory into terms of works production.