The Dempster Plant

R. & J. Dempster carry out the refining in two stages, the first to split up the crude benzole into primary fractions, the second to convert the primary fractions into pure products.

The primary fractionating still, as shown in Fig. 170, is constructed on the same principle as the refining still, without the analyser. It is used for first distillations or for the production of crude or unseparated fractions, and is also admirably adapted for blowing over crude benzole, and will produce 90's benzole from washed products. The working is the same as described for the final still, bearing in mind that there is no analyser.

The final fractionating still (Fig. 171) produces pure products and should not be used for unwashed oils. In actual practice stills of this construction have produced regularly products testing to 100 per cent with a range of less than 1° C. They are very speedy in their action and are very easily controlled by skilled operators.

The still bottom a is charged with the required amount of oil, which is vaporised by steam at high pressure contained in a series of longitudinal tubes placed in the bottom of the still. The vapour passes up the fractionating column, which is made of sufficient height to condense and separate all the heavy and intermediate fractions. From the top of the fractionating column the vapour passes into the analyser, which contains a series of water pipes controlled by cocks, where the final separation takes place, leaving vapour which, on condensation, yields a pure product. This vapour passes down to the final cooler and, after condensing, the refined product passes out through an ordinary overflow column e which renders the flow of oil visible to the operator. The analyser can be fed with water, either warm from the cooler below or cold from the water main, and the depth of the water in the analyser can also be controlled. The still is fitted with front and back steam chests, steam control valves, oil control valves, and the usual vacuum pressure valves. Thermometers and pressure gauges are attached to the front of the still to guide the operator in working, and thermometers are attached to the analyser where efficient control is very necessary.

Fig. 170.

Dempster Primary Fractionating Still.

Fig. 171. Dempster Final Fractionating Still.

It is essential that in designing the fractionating column the maker should have in view the products to be obtained, the diameter and the pitch of the perforations in the various trays being regulated accordingly.

Blair, Campbell & M'Lean Plant. - Messrs. Blair, Campbell & M'Lean, Ltd., of Glasgow, manufacture a rectifying plant which gives good results (Fig. 172). Some of the special features are the facility with which the steam heating coils may be withdrawn from the still, when any repairs are necessary, without dismantling the whole still. The column is made with plates having bells and dip pipes of extra large area to prevent blocking. The dephlegmator, the rectifier, and the condenser are constructed in such a way that repairs and replacements of tubes, when necessary, can be carried out with the minimum of trouble. The whole still is of the most substantial design and is provided with all the necessary thermometers, gauges, etc., to enable the various operations to be carried out with the greatest ease and under perfect control of the operator.

Continuous Types

It is only within the last few years that plants of practical value for the continuous fractionation of crude benzole or naphthas have been introduced. They are economically far superior to the intermittent stills, but up to now pure products have not been obtained to any large extent by means of them. Blair, Campbell & M'Lean, Ltd., make a double and triple column continuous rectifying still and maintain that it will yield pure products, but it has not been possible to get details of the plant for publication. It is no doubt similar in many respects to those described later, and with the continual improvements and the amount of research being done on this subject, one may hope to see such apparatus generally installed in the near future.

Early investigators realised the importance of continuous stills, and a good many attempts have been made to produce them. E. Sorel (Eng. Pat. 243, 1900) used such a plant which was based on fractional condensation of the steam distillates of naphtha; for he observed that mixtures of steam and naphthalene boiled at 98° C ; steam and xylenes at 93° C.; steam and toluene at 80° C.; steam and benzene at 68.5° C. W. R. Bonsfield (Eng. Pat. 25699, 1901) endeavoured to work continuously by means of fractional condensation.

Fig. 172. - Blair, Campbell & M'Lean Plant.

A, rectifying still; Ax, manhole ; A2, inlet valve ; A3, safety valve; A4, air-cock; B, rectifying column; C, tubular dephlegmator ; D, tubular rectifier ; E, tubular condenser ; F, receiver ; G, water inlets ; H, water outlets ; I, thermometers ; J, observation glass ; J1 gauge glass ; K, drain-cock ; K1, run-off valve : L, steam inlet.

F. W. Ilges (Eng. Pat. 3302, 1909) developed the principle of using two stills continuously, each still being provided with two condensers, kept at different temperatures. Thus four fractions can be obtained. The modern plants described later are developments of this idea.

Reference may also be made to the continuous still of Barbet et Fils (Fr. Pat. 434677, 1910).

Sadler and Bellerby (Eng. Pat. 105395, 1917) were one of the first to make a continuous still of any practical importance, but their apparatus must be very delicate to control and liable to give inconsistent fractions.

Simon Carves Plant. Simon Carves, Ltd., Manchester, and Samuel Walker and Sons, Ltd., make very similar plants, so that a description of the former will suffice. This plant is shown diagrammatically in Fig. 173. It consists essentially of a column - still provided with reheating chambers as described later, but the products of distillation after leaving each reheating chamber are separately washed in fractionating columns and collected. Referring to the drawing - the crude benzole, which must be entirely free from water, is passed through a steam heater a, and then into the base of the fractionating column b. The benzole vapour coming off the top of the heater is delivered into the still column lower down than the point of entry of the liquid benzole. The benzole vapour therefore takes the place of the live steam jets in the usual type of still. In the base of the still, large heating chambers, A1 and A2, are fixed, the size of these depending of course largely upon the amount of benzole to be dealt with. These chambers are similar to those provided in the ordinary still as already described. At the head of the still is placed a large dephleg-mator c, with the usual vapour connections to the condenser. The temperatures in the still, dephlegmators, etc., are so regulated that only benzole vapour is allowed to pass forward to the condenser.

Fig. 173. - Simon Carves Plant.

A1; A2, A3, A4, steam heaters; B, B1, B2, fractionating trays ; C, C1 dephlegmators ; D, trays.

The less volatile products escape at the base of the still and overflow into the second still, which is provided for the recovery of toluol. The operation of this still is similar to that of the first, but the size is naturally smaller.

The residue, naphtha and creosote, from this still overflow into the third or naphtha still, which is suitably proportioned and provided with a steam inlet at the base.

The residual creosote, when overflowing from this still, runs back to the store tank.

The fractionating column is 3 feet 6 inches in diameter, and 12 feet high, and contains 14 trays, each tray being of the usual type and having 9 bubbling bells and the usual overflow pipes. There is also a mid-feather in each tray, and the liquor flows in alternately on each side of the mid-feather.

The dephlegmator casing is 3 feet in diameter and 9 feet high, and contains 9 cast iron gilled radiating pipes of 4 inches bore, secured to tube plates at top and bottom. The gilled pipes are, therefore, 9 feet long.

The toluol column is 3 feet in diameter and 12 feet high and takes 16 trays ; the dephlegmator is of corresponding size.

A plant of this description gives good results, surpassing those obtained in the usual blow-over still, and will convert 8000 gallons, in some cases even 12,000 gallons, of crude benzole per day into 90's benzole, 90's toluol and solvent naphtha.