Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/260

242 PARAFFIN Refinery. The first operation in oil refining consists in submit ting the crude oil to distillation in large pot-shaped stills capable of holding 1200 or 1400 gallons. The distillation is continued till only a pure vesicular coke remains in the still, and the vapours (condensed by the ordinary worm-pipe arrangement) constitute once-run oil," which from its bright green colour is also known as green oil. The once-run oil is the material from which, by a repeated series of washings with sulphuric acid and caustic soda and fractional distillations, the graduated series of purified pro ducts is finally obtained. Washing. Once-run oil contains a scries of basic and acid com ponents. To separate these the oil is first repeatedly treated with sulphuric acid of different degrees of strength, which is thoroughly intermixed and brought in contact with the oil by mechanical means in an agitating tank or washer. The acid first used is a weak tarry acid which has been already used in a subsequent stage of the manufacture. This produces a copious tarry deposit, which is removed ; the process is repeated with a similar result ; and there after the oil is further treated with tvo successive washings of strong vitriol. After settling and removal of the precipitated tars, a similar series of washings with caustic soda solutions of increasing strength, and corresponding precipitation and removal of tars which combine with the alkali, are carried out. During both the acid and the soda treatments the oil is maintained at a tempera ture of about 1 00 F. by the circulation of steam through the tanks in coiled pipes. The sulphuric acid tars are to some extent used as fuel in the fractionating stills. Fractional Distillation. The purified once-run oil is a very mixed substance, giving off vapours within a wide range of temperatures, which condense into products of varied specific gravity. By the series of fractional distillations to which it is submitted a series of products are ultimately obtained comparatively homogeneous in constitution, which distil within relatively narrow limits of tem perature. The ordinary method of fractionating once-run oil consists in running it into large cylindrical boiler stills heated by furnaces in which the acid tar already spoken of is consumed. The stills have led into them steam-pipes, through which steam is injected into the oil in process of distillation as required. When the heat is first raised, superheated steam is injected to aid in carry ing off the lighter vapours, which are condensed as naphtha or spirit." As the distillation proceeds, and the gravity of the con densed product increases, it is run into separate receivers, and thus a series of fractionated intermediate products is produced, the first portion up to 0750 specific gravity being naphtha, while from 750 to 850 is the burning oil portion, and the subsequent portion separated is heavy oil containing paraffin. The portion remaining in the still is removed to the residue stills, in which it is distilled till the still contains only coke. The oil driven off from the residue stills is called "heavy oil and paraffin," and passes to the paraffin house for treatment there. Improved Fractionating Stills. Many attempts have been made to adapt the fractionating still to a system of continuous working by keeping the contents at a constant level as the distillation pro ceeds. For a long period continuous distillation was only imper fectly applicable, and yielded unsatisfactory results. The lighter fractions alone were driven off, and as the distillation progressed the density of the contents of the still gradually increased, making the difference between the oil added to the still and that within it increasingly great. In the end the contents of the still had to be removed and completely distilled as one charge in a separate still. In 1883 Mr Norman M. Henderson, the patentee of the Henderson retort, patented a continuous process of distillation which com pletely obviates all difficulties, and largely reduces the time, labour, and cost of fractionation as compared with the ordinary intermittent method. According to Henderson s system, purified once-run oil is fractionated continuously in a connected series of three cylindrical stills. Each still is fitted with inlet and outlet pipes, the months of which opening upwards are placed at opposite extremities of the still. The outlet pipe of No. 1 passes as inlet into No. 2, and similarly outlet of No. 2 is connected as inlet with No. 3, while the outlet of No. 3 passes into one or more common residue stills. The inlet or feed pipe of No. 1 traverses the long horizontal con densing pipes of the whole three, and thus the once-run oil, while absorbing heat before entering No. 1 still, also aids the condensation of the vapours. In working there is a constant feeding of heated once-run oil into No. 1 still, a like steady flow from No. 1 to No. 2, from No. 2 to No. 3, and from No. 3 to a residue still. The oil of course increases in density as it passes onwards ; but the specific gravity in each still is practically constant, and, as the heat applied is increased in proportion to the gravity, the oil vaporized in each separate still is of uniform quality and specific gravity. In No. 3 still, where, in consequence of the high gravity and temperature, there is a tendency to deposit cai bonaceous matter, circulating plates or dishes hinged to each side of the still, and concentric with the bottom shell, are placed. The circulation of the oil from the bottom up the sides in the space between the shell and the circulating plates is directed and assisted by jets of steam from a pipe laid along the bottom of the still. In this way the oil is kept in steady circulation up the sides and down the centre, and any deposit of coke which may take place forms on the inner surface of the circu lating plates, from which there is provision for its easy removal when required. The manufacturer has now his material split up into three pro ducts naphtha, burning oil, and heavy oil with paraffin. By renewed treatments with acid and alkali and fractional distilla tions, these products are further purified and differentiated. We cannot go into technical details, and in regard to the principles upon which the processes are founded reference may be made to what has been said above in connexion with corresponding laboratory methods. As a final result the following products (or a similar series of other products) are produced and sent out into the market : 1. Gasoline : a mixture of paraffins, so volatile that a current of air by being passed through it at ordinary temperatures is converted into combustible (non- explosive) gas. 2. Naphtha: a mixture of hydrocarbons which in volatility and otherwise arc equivalent to the crude benzol of the coal-gas industry. 3. Burning oil : a mixture of oils sufficiently volatile and light to be suitable for combustion in domestic lamps with wicks, and yet practically free of dangerously volatile inflammable components. 4. Heavy oil, corresponding to a range of very high boiling points; too heavy or viscid to be raised by the wick of a lamp, but well adapted for lubricating purposes. This part contains the solid paraffin which the manufacturer takes care to extract as completely as possible before the oil is sold as "lubricating oil." The several kinds of crude paraffin extracted are classed as "hard scab:" or " soft scale," according to their fusing points and consequent degrees of hard ness at ordinary temperatures. Separation of Hard Scale. The heavy oil forming the last of the three portions into which once-run oil is fractionated, at ordi nary atmospheric temperatures, becomes thick and pasty by the abundant formation of crystals of solid paraffin. This mixture of oil and paraffin is separated by draining through canvas bags, or, as is now the almost universal practice, by passing the magma into a filter press. This apparatus contains a series of thirty or forty perforated plates about 2 feet square, the faces of which are covered with filtering canvas. They are screwed up together in an oblong horizontal frame, so that a space or chamber about an inch wide is left between each pair of plates. Into these chambers the pasty mixture is forced under high pressure, the material pass ing into and filling each chamber through an orifice in the centre of the plates till the whole of the chambers are filled. The pressure being kept up, the fluid oil exudes through the canvas and perforations in the plates, leaving solid paraffin, which continues to accumulate till the chambers are filled with it in a comparatively dry condition. The soft cake from the filter press is further squeezed in canvas in an hydraulic press giving off more fluid oil, and the cake from this pressure consists of commercial hard scale or crude paraffin. Soft Scale. The heavy oils separated in the second and third fractionation of burning oils, and the oil from which the above hard scale is separated, hold dissolved in them paraffin of low melting point, which can only be crystallized out by bringing the oil to a very low temperature. For this purpose the oils are reduced to from 18 to 20 F. by artificial refrigeration. The method now employed consists in sufficiently cooling a continuous current of brine or of chloride of calcium solution by passing it through an ether refrigerating machine. This cold current of brine circulates through the interior of a large cylinder or drum, which revolves slowly, dipping into a trough containing the oil to be cooled. The cold surface of the drum in contact with the oil takes on a deposit of solid paraffin crystallized out of the mixture. It is removed by scrapers and made to fall into a separate receptacle, whence it goes to the filter press and the hydraulic press in the same way as the hard scale. Lubricating Oil. The oil from which hard and soft paraffin .ire separated as above stated exhibits a blue fluorescence, and is hence called blue oil. It receives an acid and soda series of washings, after which it is submitted to fractionation. The first portion given off, up to about 850 specific gravity, is transferred to the burning-oil series, with which it is mixed for further treatment. The remainder is received as various grades of lubricating oil, with specific gravity ranging from 860 to 890. These heavy oils are again refrigerated, yielding a further crop of soft scale, after which they get a final acid and alkali treatment, and are finished for use by having steam blown through them for a prolonged period, the effect of which is to reduce greatly their objectionable smell. Finally they are kept in warm settling tanks at a temperature of not less than 90 F. for eight or ten days, when they are ready for the market. Occasion has already been taken to name the advantages which this kind of mineral oil offers as a, lubricating agent. Let us now add that it cannot quite take the place of fatty lubricants, lack ing the degree and kind of viscosity which fits these for certain purposes. A mixture of fatty and mineral oil in proper proportions is often found to work better than either component would by itself. As mineral oil is far cheaper than all the fatty oils, it is largely used as adulterant of these. Such adulteration can