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Obtaining Oil from Oil Shale

Adapted by Harry Knox from his book "The Scottish Shale Oil Industry & The Mineral Railway Lines"

Shale Retorts

The retort design had evolved over the life of the industry and it is a fact that the Scottish oil companies led the world in this retort technology. The first retorts to be used at Bathgate by James Young, were horizontal “D”-shaped common retorts, heated by a coal-fired oven. The cannel coal was fed into the retort by a rotating screw and the “residence” or “dwell” time (ie. the length of time the coal or shale remained in the retort under heat) was 16 to 24 hours. The oil yield from cannel coal exceeded 100 gallons per ton. With the expiration of Dr. Young’s Patent, so the search for more efficient types of retort began. The most common design of retort thereafter, was to be the vertical retort. Various engineers employed within the industry applied their thinking to improving retort design and retort performance.

The earliest type of vertical retort was designed by A.C. Kirk. This was elliptical in cross-section and widened progressively towards the foot of the retort thus providing a better and more uniform heating surface. This retort could process about 28 cwt of shale every 24 hours and was widely used across the industry, but was obsolete by 1883.

Norman Macfarlane Henderson, Manager of Oakbank Paraffin Oil Works, designed an improved form of retort which provided a better, and more efficient heating process for the shale, and offering a greater throughput of shale and yield of far higher quantities of related products. This proved to be a popular retort which was adopted by the Broxburn Oil Company Ltd and was also used at Champfleurie (Bridgend) Oil Works and Binnend Oil & Mineral Works.

By this time, retorts were being set up in banks (benches) numbering up to 88 retorts per bench and mass production was well and truly established.

Along came George Thomas Beilby and he introduced a completely new type of vertical retort which heated the shale in two stages, a cooler stage in the upper level, with significantly increased temperatures being employed in the lower part. This Beilby retort increased the yield of sulphate of ammonia, a valuable by-product of shale oil and in great demand as a fertiliser. However, the Beilby retort was otherwise to prove a commercial failure.

Undeterred, Beilby joined forces with William Young of the Clippens Oil Works, to improve on the original Beilby design and together they invented the Pentland retort. This embodied all the concepts of the original Beilby retort but in an improved form, and was capable of continuous operation and could process around 28 cwt of shale at one charge, the dwell time being reduced to around 18 hours. The Pentland retort soon was to become the industry’s preferred option.

In 1894, James Bryson of the Pumpherston Oil Company Ltd introduced the Pumpherston retort which retained all the desirable features of the Pentland retort but with added mechanical design and improvement. This Pumpherston retort had a cast iron upper part and a firebrick lower part and could process 4 to 5 tons of shale every 24 hours with dwell time reduced to a mere 4½ hours. The Pumpherston retort became the industry standard and was also known as the “Scottish” retort.

Norman Macfarlane Henderson then introduced a further modified and further improved version of the Pentland retort to give even greater production figures. This improved Pentland retort could cope with 3.2 tons of shale at each charge and dwell time was further reduced to around 4 hours. This retort was known as the Broxburn retort and soon became noted for its sheer reliability.

A further "Improved Broxburn” retort followed in 1941, which offered significant energy savings coupled with increased production. This was the retort which was adopted at the new (and last) Westwood Crude Oil Works at West Calder and was known, naturally as the Westwood retort. Neither of these improved retorts however, eclipsed the Pumpherston retort.

All these retorts however, worked on the same basic principles.

Crushing the Shale

Once the shale had been mined, it was taken, normally by rail, to the Company’s oil works where it was first put through a crusher. This was a series of steel rollers fitted with specially hardened metal teeth, which reduced the shale into fairly uniform 4 inch cubed pieces. This made the raw shale easier to load in to the retorts and ensured the uniformity of heating as the shale descended by gravity down the length of the retort.

Crude Oil Production

The crushed shale was taken by hutch up a sloping tramway to the top of the retort bench and loaded into a steel hopper at the top of each retort, and each hopper was charged around every 6 hours. The shale then passed, by gravity, into the upper part of the retort, a cast iron vertical tube around 11 feet in length and about 2 feet in diameter. The retorts were arranged in sets of four within a common firebrick heating chamber. When in the retort, the shale was then subjected to continuous heat from an external source with the temperature in the upper portion being around 270ºF and increasing to around 480ºF at the bottom of the cast iron section. The shale, still moving down by gravity, then passed into the lower firebrick portion of the retort, this being around 18 feet in length and increasing in diameter to about 3 feet. Here, the heat applied became more intense, ranging from 950ºF through to 1800ºF. In this lower stage, water and air were injected into the retort with about 75 gallons of water being added to each ton of shale. This water provided the hydrogen necessary to create ammonia from the nitrogen released by the shale. This action also had the result of creating a superheated steam (a gas) which both increased and stabilised the temperature of the shale throughout the diameter of the retort to protect the ammonia and oil vapours from further “cracking” or decomposition, thus ensuring an oil of the highest quality. These oil vapours were swept up from the lower part of the retort into a large diameter outlet pipe passing through the neck of the retort and onwards into the atmospheric condensers.

The column of shale inside the retort was supported on a firebrick cone and the dwell time was about 4 hours in the most modern retorts. The spent shale, at the completion of the retorting process, passed down into a cast iron chamber where it was swept into a discharge hopper by a rotating steel arm. From the discharge hopper, the spent shale was loaded into hutches to be taken by a continuous rope haulage system to the waste tip eventually forming the great red bings that are, even today, landmarks on the West Lothian skyline.

The external heating in the firebrick heating chambers was achieved, in the most modern of the retorts, by burning the otherwise incondensable gases given off by the shale under heating, to supplement the coal gas produced “in house” at the oil works. The burning gases were swept upwards in a spiralling flow through the heating chambers providing a continuous heating source at constant temperatures, and finally escaping into the atmosphere through the chimneys on top of the retorts.

The crude oil and ammoniacal liquid, having passed through the retort condensers were then passed to a separator where the crude oil, being lighter, rose to the surface and was drawn off via a pipe leading to a receiver.

The ammoniacal liquid being heavier, settled to the bottom of the separator and was drawn off in to another separate receiver. Any uncondensed gases from the retort condensers were cooled and scrubbed by water to recover any ammonia which might still be present, plus any naphtha which had escaped condensation.

Recovery of Naphtha

The gases, after being scrubbing by water in a scrubbing tower, were sprayed with shale gas oil which then absorbed any naphtha present. This oil / naphtha mixture was then passed into a naphtha still and, having had steam passed through the mixture, any naphtha present was condensed back into unrefined naphtha. The gases were also cooled and re-circulated back into the scrubbing process which was a continuous process.

Around 3 gallons of naphtha per ton of shale were recovered by this process.

Recovery of Ammonia

The ammonia present in the gases was scrubbed by a similar process to the naphtha but using water in the scrubbing tower instead of gas oil as the absorbing liquid. The ammonia recovered by this process, plus the ammonia obtained from the retort condensers was mixed. The ammonia, now expelled as a vapour, was absorbed in a mixture of sulphuric acid and water, and the solution, when evaporated left sulphate of ammonia in a crystalline form.

During WWI, the Ministry of Munitions required large quantities of concentrated ammoniacal liquor containing at least 25% of ammonia for the manufacture of nitrate of ammonia, a key constituent of AMATOL, a high explosive. One crude oil works in the Lothians, in all probability Addiewell, commenced production of this high grade liquor, producing 20 tons daily (7,300 tons annually) with an ammonia content of 27.3%, to help the war effort.

Refining Crude Oil

The objectives of refining the crude oil were to obtain the various types of refined oil and wax of a quality consistent with market demands, at the lowest possible production costs and the general principles can be listed as follows:

The crude oil remaining, at this first refining stage, and still in a semi-solid state (at 87º) was then heated in a still and, since the specific gravity of oils are related to the boiling point, separation or "cracking" of the oil was effected by condensing all the oil vapours and leading the condensed liquid oils into separate receivers, the vapours of those with the lower boiling points distilling over the first stages and those with the higher boiling points distilling over the latter stages.

The earliest stills were cast-iron horizontal stills but were prone to fracturing and were soon to be replaced by vertical steel stills. These stills were far from perfect and continued to give problems which led to the industry adoption of pot or coking stills. The crude oil on being processed through the pot stills produced high quantities of high quality oil products including paraffin. Steam was used in this process to facilitate the distillation of the various fractions and also to control the temperature to avoid any further decomposition of the products. A by-product of this process was a heavy, impure tar which was itself then burned to provide the necessary heat in this distillation process. Nothing was wasted!

During this first distillation process, the oil was washed using sulphuric acid and the product of this stage was known as “green oil” because of its colour.

Various types of still were used including the Henderson continuous boiler still which reduced the distillation process to a single stage. First used at Broxburn Oil Works this type of still was initially used at Grangemouth Refinery to refine the middle east crude oil, until replaced by the Scottish Oils pipe still.

These stills were at the heart of the cracking process and this process is, even today, an essential part of the process at the modern refineries. The very first version of a pipe still was designed and patented by James Young (Junior) who devised the process whereby the crude oil was heated under pressure (20 psi) in an oval-bottom pressure still. This process increased significantly, the yield and quality of paraffin and burning oils at that time, and, as the demand for motor spirit grew so this cracking process was used to increase the yield of petrol. Young’s early process was to be overtaken by the modern thermal pipe still crackers, and introduced at two shale oil refineries, and today, modern hydrocrackers (cat crackers) distill heavy oil products under pressure using hydrogen at 2000 psi in the presence of a catalyst, to produce gasoline and LDF products. The cat cracker was introduced at Grangemouth Refinery in 1970.

Refining Crude Oil: Second Distillation

The “green oil” obtained from the first distillation was again run through a distillation process very similar to the first stage, and again the oil was heated, but no steam was employed in this stage in order that the more crystalline form of solid paraffin could be extracted. The fractions obtained at this point in the process were:

The crude burning was then stirred and treated with sulphuric acid to remove nitrogen compounds and to polymerise the lower olefins likely to affect the stability of the final products, before being washed in a caustic soda solution. After settling, the oil was re-distilled in fine old boilers (boiler stills). At this stage with the residue being run off continuously, the fractions obtained were:

At this point, the lamp, power and lighthouse oils were drawn into storage tanks ready to be marketed. This residuum was blown into a separate single boiler still and re-distilled to produce:

Meanwhile, the heavy oils and paraffin were cooled to a very low temperature in refrigerated vessels charged with liquid ammonia, and when the required temperature had been achieved, the cold oil was forced, under pressure, into a series of filter presses. These presses were then further compressed, forcing all the remaining liquid oil out to be collected in another container. The filter press cakes of paraffin wax did, however, still contain traces of liquid oil and these cakes were then further pressed in linen or cotton cloth until only solid cakes of paraffin wax remained. This paraffin wax was removed for further refining and removal of coloration and impurities.

The oil remaining after this pressing process was termed “blue oil”, again because of colour. This blue oil was then treated with a solution of acid and caustic soda, and, when finally settled, was collected and re-distilled in the blue oil stills, to produce the following distillates:

The residuum was again drawn off and prepared for the markets as oil residuum, being in great demand for the manufacture of greases and wire-rope lubricant.

The gas-oil, cleaning and lubricating oils were further treated and when settled, run off and prepared for the markets.

Crude Solid Paraffin Wax

This crude paraffin wax was subjected to a sweating process in a large, purpose-built sweat house each of which were equipped with close-fitting iron doors and internally fitted with steam coils along the walls. These houses also contained a series of water-cooled horizontal pans. The raw melted wax was run into these pans from a charging tank above the building and when the wax had been cooled by the water, and solidified once more, the sweating process began. When the wax was once more in a liquid state due to the steam heating, the wax was clarified and impurities removed. The wax was cooled once more, then re-liquidised by sweating once more, yielding a fine, soft wax and a hard paraffin wax of good clear colour, with a melting point of between 119ºF and 122ºF. This quality wax was sweated once again to remove any residual colouring before being prepared for the market. In the early days of the industry, this wax was retained for the production of candles, each oil works of the time having an associated “candle house”. The candles produced from this wax were of exceptionally high quality, burning with an extremely bright light. Addiewell Chemical Works was noted for both the quality and intricacy of design and decoration of its candles.

The Refined Products

Spirits and Naphtha

Various spirits were produced over the years to meet changing market requirements, such as motor spirit (later to be petrol), extracting spirit, solvent naphtha (used in rubber-making and waterproofing), burning naphtha, white spirit (used in the manufacture of paint) and solvents used in the manufacture of linoleum. A much purified distillate of naphtha was used to produce oil gas, a product invented in 1851 by Julius Pintsch of Germany. When pressurised, this gas was to be much used for lighting of railway carriages, buoys and beacons. The use on the railway was of short duration after a number of serious accidents where, following derailments, escaping pressurised oil gas ignited and caused multiple fatalities as fire swept through the wreckage.

Lamp and Power Oil

Lamp oil was in great demand, particularly by the Railway Companies, as a continuously burning oil for use in signal lamps and train lamps. The Scottish Railway companies contracted to purchase lamp oil from the various oil companies, each taking about 25,000 gallons every two months at, of course, a favourable cost. The lighthouse oil was, with its high flash point, a safe oil to use and to store in quantity and as the name suggests, it was in great demand as a source of illumination in lighthouses.

Gas / Fuel Oil and Cleaning Oil

Gas oil was just another grade of fuel oil. The fuel oils (furnace fuel oils) were widely used in steam boilers as an alternative to coal, particularly in sea-going vessels. The fuel oil fuelled the ships of the Royal Navy during WWI.

Cleaning oil was a highly refined oil much in demand by railway companies for cleaning locomotives. It was also used in the manufacture of axle grease.

Batching Oil

This particular grade of oil was specially prepared for the spinning industry. It was used, in the main, in Dundee for jute spinning and in Lancashire in the cotton spinning trade. Mixed with water and sprayed over the fibres during the spinning process, it kept them soft and flexible, thus preventing breakage.

Lubricating Oil

As the name implies, this was an oil used to lubricate all sorts of moving machinery and was also so used in the manufacture of specialised greases.

Residuum Oil

This thick oil residue was a constituent part of heavy lubricating greases.

Motor Spirit

Later known as petrol and used by the internal combustion engine fitted to motor cars, and based on naphtha. Distilled at a higher temperature under pressure irt was treated with sulphur compounds using lead (the plumbite process) to create leaded petrol.

Diesel Oil

By 1938, there was an increasing demand for diesel oil and the Scottish Oils Ltd chemists found, by experimentation that by vapourising shale crude oil in a pipe still and separating the fractions by use of a bubble cap fractioning column, some 50% could be converted to diesel oil. This was achieved by firstly taking the naphtha, the hydrocarbon with the lowest boiling point, from the top of the column, mixing it with scrubbed naphtha and refining to produce motor spirit. The next fraction with an intermediate boiling point was taken from the upper part of the column, treated with sulphuric acid to remove the basic nitrogen compounds and re-active olefins, then washed with a caustic soda solution to remove acid tars and finally re-distilling it to produce diesel oil. The process produced little or no waste and all the impure materials were recycled back into the refining process.

Production of Sulphuric Acid

The sulphuric acid used in the refining process was made in house, firstly at Bathgate Chemical Works by Young’s and production continued there until closure in 1956. Sulphuric acid was also later produced at Broxburn Oil Works.

The process used iron pyrites ore (iron sulphide) imported from Spain. The ore was burned in air, in a furnace to produce sulphur dioxide gas. Passing the gas into lead chambers, the sulphur dioxide then reacted with nitrogen obtained from a spray containing a mix of water and sodium nitrate, producing dilute sulphuric acid. Using a process to recover and re-use the nitrogen oxides, this dilute acid was concentrated to an 80% concentration. It was possible to further concentrate this acid in heated silica basins if required and when highly concentrated, the acid, now known as oleum or disulphuric acid, could be safely transported between place of production, the other oil refineries and other consumers.

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