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HISTORY OF TECHNOLOGY, TECHNOLOGY, OBJECTS AROUND US
Cracking process. History of invention and production
Directory / The history of technology, technology, objects around us Cracking, cracking process - high-temperature processing of oil and its fractions in order to obtain, as a rule, products of lower molecular weight - motor fuel, lubricating oils, etc., as well as raw materials for the chemical and petrochemical industries. Cracking proceeds with the breaking of C-C bonds and the formation of free radicals or carbanions. Simultaneously with the rupture of C-C bonds, dehydrogenation, isomerization, polymerization and condensation of both intermediate and starting substances occur. As a result of the last two processes, so-called. cracked residue (fraction with a boiling point over 350 °C) and petroleum coke. The world's first industrial installation for continuous thermal oil cracking was created and patented by engineer V. G. Shukhov and his assistant S. P. Gavrilov in 1891 (patent of the Russian Empire No. 12926 dated November 27, 1891). An experimental setup has been made. The scientific and engineering solutions of V. G. Shukhov were repeated by W. Barton during the construction of the first industrial plant in the USA in 1915-1918. The first domestic industrial cracking installations were built by V. G. Shukhov in 1934 at the Soviet cracking plant in Baku.
Oil is an oily liquid with a characteristic pungent odor and a different color depending on the place of extraction. According to its chemical structure, it is an extremely complex mixture of various chemical compounds, primarily organic substances - hydrocarbons. Hydrocarbons are so called because they are chemical compounds of simple elements: carbon and hydrogen. In addition to them, the composition of oil includes sulfur, nitrogen, oxygen and many other impurities (including water and sand). Despite the fact that hydrocarbons include only two elements, their number is huge. This is explained by the fact that carbon and hydrogen can combine with each other in various combinations and proportions. Therefore, the properties of hydrocarbons are very different, and a large section of chemistry, the chemistry of organic substances, is studying them. Hydrocarbons can be liquid, gaseous or solid. Some are lighter than water and boil at lower temperatures, while others are heavier and boil at higher temperatures. Their specific gravity or density is very different (recall that specific gravity is a number showing how many times the volume of a substance is heavier or lighter than the same volume of water taken at 4 degrees). The most important property of oil and its products, on which the primary distillation of oil is based, is their ability to evaporate. The composition of oil includes such hydrocarbons that begin to evaporate even at ordinary temperatures. If oil is left to stand in an open vessel without heating for more or less a long time, then part of it will evaporate, and the rest will become denser and thicker. Due to the fact that oil contains various hydrocarbons with different boiling points, oil does not have a constant boiling point, such as water. If we begin to heat water in a vessel, we will notice the following phenomenon: a thermometer immersed in water will initially show a constant increase in temperature, but as soon as the temperature reaches 100 degrees, the increase will stop. And further, no matter how much we heat the vessel, the temperature will not rise until all the water has evaporated. This is due to the homogeneity of water, that is, the fact that water consists of identical molecules. We will observe a completely different picture when heated in a vessel of oil. In this case, no matter how much heat we supply, the temperature rise will not stop. Moreover, at the beginning of heating, the lightest hydrocarbons in terms of specific gravity will evaporate, from the mixture of which gasoline is obtained, then heavier hydrocarbons - forming kerosene, diesel fuel and lubricating oils. The primary distillation of oil was based on this principle.
Before the invention of cracking in large kerosene plants, distillation was carried out in large stills, into which superheated water vapor was constantly admitted in large quantities and at the same time oil was heated from the furnace under the boiler, burning coal or combustible gas. Passing through the oil, the steam carried along the lightest of the oil compounds with a low boiling point and a small specific gravity. This mixture of kerosene and gasoline with water was then sent to the refrigerator and settled. Since the products of distillation were much lighter than water, they were easily separated from it. Then the spill happened. First, the top layer with a specific gravity of up to 0,77 drained - gasoline, which was sent to a separate tank. Then kerosene was poured, that is, heavier hydrocarbons with a density of up to 0. The crude kerosene thus obtained burned poorly. It needed cleaning. To do this, it was first treated with a strong (86%) solution of sulfuric acid, and then with a solution of caustic soda. The result was refined kerosene - completely colorless, without a pungent odor and burning with an even flame, without burning and soot. The composition of oil also includes such heavy hydrocarbons, which, before reaching their boiling point, begin to decompose, and the more the oil is heated, the more intense the decomposition will occur. The essence of this phenomenon boils down to the fact that several smaller molecules with different boiling points and different specific gravity are formed from one large molecule of a heavy hydrocarbon. This decomposition began to be called cracking (from English to crack - to break, split). Thus, cracking should be understood as the decomposition under the influence of high temperature (and not only temperature, decomposition can occur, for example, from high pressure and for some other reasons) of complex and large hydrocarbon particles into simpler and smaller ones. The essential difference between the cracking process and primary distillation is that during cracking, a number of hydrocarbons undergo a chemical change, while during primary distillation, there is a simple separation of individual parts, or, as they say, fractions, of oil, depending on their boiling points.
The phenomenon of oil decomposition was noticed long ago, but in ordinary oil distillation such decomposition was undesirable, therefore, superheated steam was used here, which contributed to the evaporation of oil without decomposition. The oil refining industry has gone through several stages in its development. At the beginning (from the 60s of the XNUMXth century until the beginning of the XNUMXth century), oil refining had a pronounced kerosene character, that is, kerosene was the main product of oil refining, which remained the main source of light for half a century. At Russian refineries, for example, the lighter fractions formed during distillation were treated as waste: they were burned in pits or dumped into reservoirs. However, the intensive development of road transport has placed other accents. If in the USA in 1913 there were 1 million 250 thousand cars, then in 1917 - about 5 million, in 1918 - 6 million, and in 25 - already 1922 million. Gasoline, which in the 12th century found very little use and was almost an unnecessary waste, gradually became the main goal of distillation. From 1900 to 1912, world gasoline consumption increased 115 times. Meanwhile, during the distillation of even oil rich in light fractions, gasoline accounted for only about 1/5 of the total output. Then the idea arose to subject the heavy fractions released after the primary distillation to cracking and thereby obtain lighter gasoline fractions from them. Soon it was found that not only heavy fractions (diesel oil or fuel oil), but also crude oil can serve as the feedstock for cracking. It also turned out that cracked gasoline is superior in quality to that obtained by conventional distillation, since it contains hydrocarbons that burn smoothly in engine cylinders without explosions (detonation). An engine running on such gasoline does not knock and lasts longer. In liquid cracking, the main points that determine the essence of the whole process are: the temperature and the time during which the product is under the influence of this temperature. Oil begins to decompose already at 200 degrees. Further, the higher the temperature, the more intense the decomposition. Similarly, the longer cracking lasts, the greater the yield of light fractions. However, at too high a temperature and a long cracking time, the process does not go at all as required - the molecules are not split into equal parts, but are crushed so that, on the one hand, too light fractions are obtained, and on the other, too heavy. Or, in general, there is a complete decomposition of hydrocarbons into hydrogen and carbon (coke), which, of course, is very undesirable. The optimal conditions for cracking, giving the highest yield of light gasoline fractions, were found at the beginning of the 1890th century by the English chemist Barton. Back in 1913, Barton was engaged in the distillation under pressure of Russian heavy oils (fuel oil) in England to obtain kerosene from them, and in 1916 he took out an American patent for the first ever method of obtaining gasoline from heavy petroleum fractions. For the first time, the cracking process according to the Barton method under industrial conditions was carried out in 1920, and by 800 more than XNUMX of its units were in production. The most favorable temperature for cracking is 425-475 degrees. However, if crude oil is simply heated to such a high temperature, most of it will evaporate. Cracking products in the vapor state was associated with some difficulties, so Barton's goal was to keep the oil from evaporating. But how to achieve such a state that when heated, the oil does not boil? This is possible if the entire process is carried out under high pressure. It is known that under high pressure any liquid boils at a temperature higher than under normal conditions, and this temperature is the higher, the greater the pressure.
The installation had the following device. The pressurized boiler (1) was located above the furnace (1a) equipped with a fire tube (4). The boiler was made of good strong iron with a wall thickness of about 2 cm and was carefully riveted. A rising pipe (5) led to a water cooler (6), from where a pipeline (7) led to a collection tank (8). After the cracked product passed through the liquid counting apparatus (10), the pipe (9) located at the bottom of this tank branched into two side pipes (10a and 10b). Each side tube was supplied with a control valve; one of them led to pipe 11, and the other to pipe 12. At the beginning of cracking, the boiler (1) was filled with fuel oil. Due to the heat of the furnace (1a), the contents of the boiler slowly heated up to approximately 130 degrees. At the same time, the remains of the water contained in it evaporated from the fuel oil. Condensed in the refrigerator (6), the water then flowed into the reservoir (8), from which it descended through the pipe (21) into the ditch (22). At the same time, air and other gases escaped from the fuel oil. They also got through the cooler into the tank (8) and were discharged through the pipe (14a) to the pipeline (16). After the fuel oil got rid of water, air and gases dissolved in it, it was ready for cracking. The firebox was strengthened, and the temperature in the boiler slowly increased to 345 degrees. At the same time, the evaporation of light hydrocarbons began, which even in the refrigerator remained in a gaseous state. They fell into the tank (8), and then through the pipe 14a (the outlet cock of which was closed) into the pipeline (17), pipe (14) and back into the tank (8). Since these light gaseous fractions did not find an outlet, the pressure inside the plant began to rise. When it reached 5 atm, light hydrocarbons could no longer escape from the main boiler. These compressed gases maintained the same pressure in the boiler (1), cooler (6) and reservoir (8). Meanwhile, under the influence of high temperature, the process of splitting heavy hydrocarbons took place, which turned into lighter ones, that is, into gasoline. At a temperature of about 250 degrees, they evaporated, fell into the refrigerator and condensed here. From the refrigerator, gasoline flowed into the tank (8) and through pipe 9, and then 11 or 12 entered special sealed boilers. Here, under reduced pressure, light gaseous hydrocarbons dissolved in it evaporated from gasoline. These gases were gradually removed from the boilers, and the resulting crude gasoline was poured into special tanks. As the light fractions evaporated with increasing temperature, the contents in the boiler (1) became more and more resistant to heat. Work was interrupted as soon as more than half of its contents were converted into gasoline and passed through the refrigerator. (This amount was easy to calculate thanks to the liquid meter (10).) After that, the connection to the pipeline (17) was interrupted, and the valve of the pipeline (14a) connected to the compressor was opened, and the gas slowly escaped into the low pressure compressor (simultaneously, the pipeline was closed ( 9), interrupting the connection of the installation with the received gasoline). The firebox was extinguished, and when the contents of the boiler (1) cooled down, it was drained. Then the boiler was cleaned of coke deposits and prepared for the next start.
The cracking method developed by Barton marked the beginning of a new stage in the oil refining industry. Thanks to him, it was possible to increase the yield of such valuable oil products as gasoline and aromatic hydrocarbons by several times. Author: Ryzhov K.V.
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Comments on the article: Anatoly The distillation of oil in oil distillation stills of periodic action was started by the Nobel brothers in Baku, which were improved by Inchik, Tavrizov and Dmitry Mendeleev. Then, in 1885, VG Shukhov and F. A. Inchik developed a new method of continuous distillation in still batteries with a distillation column (this scheme is erroneously indicated as oil cracking). Nowhere and never has oil been subjected to cracking. In 1891, V. G. Shukhov, together with S. P. Gavrilov, patented a scheme for thermal cracking not of oil, but of oil products (patent No. 12926 dated November 27, 1891). This is the most ingenious invention of Shukhov, which was intercepted by the Americans, and which is still working for them. Heavy residues, light distillates and gas are subjected to cracking, which are combined in the REMOTE REACTION CHAMBER, where deep transformations of heavy raw materials occur due to cracking products of light distillates and gas pyrolysis. The gas and gasoline of this unique plant have a high yield of unsaturated hydrocarbons, including diene hydrocarbons, which are petrochemical raw materials. The cracked residue goes to vacuum distillation, diesel fuel - to hydrotreatment. The United States carefully hides the scheme of a simple and cheap process and in every possible way advertises catalytic cracking and hydrocracking, which are significantly superior to Shukhov cracking both in operating and especially in capital costs. And the primitive VISBREKING, artificially introduced into the scheme of our refineries, with high operating costs due to low mileage and high coke formation, looks completely humiliating.
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