Petroleum

by Albert Lidgett

The ammonia water got from the atmospheric condensers is pumped through a heater, in which it is raised in temperature by the waste water flowing from the still, and passes into the top of the still, which is circular in shape, about 30 feet high, and has a series of cast-iron shelves or trays fixed horizontally every 2 feet or thereabouts from the top to near the bottom. Steam is put into the bottom of the still at a pressure of 40 lb., and passes to the top through a series of conical arrangements on the shelves carrying with it the volatile ammonia, while the water, after traversing the whole area of each tray, passes out into a concrete tank containing a cast-iron worm, which is the heater already referred to, for the ammonia water on its way to the still. During its progress from the top to the bottom of the still, the water is diverted into a chamber containing milk of lime, setting free the fixed ammonia which cannot be got by steaming.

The steam and ammonia gas liberated in the still pass over into a large lead-lined tub or saturator, and bubbles through holes in a lead worm placed round the circumference at the bottom of the vessel. Sulphuric acid is at the same time run into the saturator, and, at a certain temperature, sulphate of ammonia is formed. The sulphate falls into a well, formed in the centre of the bottom of the vessel, in which are placed two steam ejectors, and these blow it out along with some liquor. This mixture is delivered into hutches having perforated bottoms, through which the ammonia liquor drains off, the solid sulphate being left in the hutch. This is now run by an overhead railway to the drying or storage stalls, and from these it is packed up and dispatched to the market. The exhaust steam and waste gases from the saturator are passed into the retorts, and utilized for the formation of ammonia from the shale, while the spent water is pumped to the spent shale bing, and thoroughly filtered before being allowed to escape from the works.

For dealing with the weak acid water recovered from the refinery, the Pumpherston plant consists of lead-lined tubs or crackers, into which a quantity of the acid water is run, and saturated with ammonia gas until it is near the salting point, when it gravitates into settling vessels in order to separate any tar carried over with the acid water. The clear liquid is then drawn into the saturator, where it is quickly converted into sulphate and blown out in the manner already described.

So up to date is the whole of the system governing the treatment of the shales and the resulting products, that the pumping of water from the mines, the haulage of the shale to the refineries, as well as driving of machinery in the works, is performed by electric power, the exhaust steam from the engines driving the generators, as in the case of the sulphate of ammonia exhaust, being sent to the retorts for use in the production of ammonia.

The process of refining the crude oil obtained from the shale into the various products is somewhat complicated and perplexing to those unassociated with the industry on account of the many distillations and treatments which have to be carried out before a good marketable article is produced. The following outline, however, will give a fair idea of the process adopted throughout Scotland.

The crude oil is delivered at the refinery into large tanks, which are placed at a sufficient height to feed the stills by gravitation. The crude oil is allowed to settle for twelve or more hours at a temperature sufficiently high to separate any water that may have passed the test at the retorts, and after this water has been run off, the oil is fed into the centre boiler of a battery of oil boilers. The lightest fraction of the oil—ultimately motor spirit and illuminating oils—is distilled off the feeding boiler and condensed in a coil of cast-iron pipes immersed in water in a tank, cold water being continuously run into the tank, while heated water is run off. The boilers on each side of the feed vessel receive their oil by a pipe connecting with the bottom of the latter, and they also distil over the lighter portion of oil with which they have been fed, the heavier portions passing on to a third boiler, where the process of distillation is repeated.

The oil now left is delivered into a cast-iron pot-still, in which it is ultimately distilled to dryness, the residue left in the still forming oil coke, which is valuable as a fuel on account of its high percentage of fixed carbon and low yield of ash. Steam is admitted to the still in large quantities at all distillations. The various stages of distillation are carried through in almost identically the same manner as that of crude oil, and, therefore, need not be described in detail.

The treatment or washing of the oil to remove the impurities that cannot be eliminated by distillation, consists in stirring the oil by compressed air for a given time in an iron vessel, with a fixed quantity of sulphuric acid, allowing it to settle, and running off the heavy mixture of tar and acid which separates. The acid-treated oil is then run into another similar vessel, treated with a solution of caustic soda, settles, and the soda tar which separates is run off. The acid tars are steamed and washed, the resulting acid water being sent to the sulphate of ammonia house for the manufacture of sulphate of ammonia, whilst the tar is mixed with that from the soda treatments and burned under the stills as liquid oil. As there is more than sufficient of this tar to distil all the oil at the various stages, the distillation is carried out without cost for fuel, excepting that necessary for steam-raising purposes.

A portion of the oil distilled at the second distillation, or green oil stage, is sent from the stills to the paraffin sheds to be cooled and the scale extracted, this eventually being made into paraffin wax. Stored in tanks until brought down to atmospheric temperature, the oil is pumped into the inner chamber of a cooler, which consists of a series of four vessels having inner and outer compartments. At the same time, anhydrous ammonia is forced into the outer compartment or jacket, and absorbs heat from the cooler, freezing the oil in the inner jacket into a pasty mixture of liquid oil and solid crystals of wax.

This mixture is then pumped into filter-presses, where a portion of the oil flows away through the cloth, while the wax is left behind in solid cakes, still containing a quantity of oil. These cakes are delivered by conveyors to the back of the hydraulic presses, where they are wrapped in cloth and placed on shelves between iron frames in the presses, most of the remaining oils being thus squeezed out. The material obtained from the hydraulic presses is known to the trade as paraffin scale, and as it is discoloured by the small quantity of oil which cannot be removed by pressing, a process of sweating by steaming in large brick compartments is adopted, in order to remove the oil. The scale, consequent upon the removal of the oil therefrom, becomes whiter and of higher melting point, and after further treatment is finally passed through filter paper and run into moulding trays. When cooled, this product is known as paraffin wax, of which there are many grades. One cannot enter into the technical arrangements involved, for obvious reasons, the chief one of which is that these cannot interest the reader; but sufficient has already been written in this chapter to suggest to the reader the perfection which has now been reached in the treatment of the shales of the Midlothians.