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Cracking process


Cracking process
Oil is an oily liquid with a pungent smell and different depending on the place of production, color. According to their chemical structure, it is extremely complex mixture of different chemical compounds, especially organic substances - hydrocarbons. Hydrocarbons are so called because they are chemical compounds of simple elements: carbon and hydrogen. In addition to them in the oil include sulfur, nitrogen, oxygen, and many other impurities (including water and sand). Although hydrocarbons include only two elements, their number is enormous. This is because the carbon and hydrogen can be interconnected in various combinations and proportions. Therefore, hydrocarbon properties are very different, and their study is engaged in a large section of chemistry - the chemistry of organic compounds.

Hydrocarbons may be liquid, gaseous and solid. Some of them are lighter than water and boil at low temperatures, other heavier - and boil at higher temperatures. Very different in their specific gravity or density (recall that specific weight is the number that indicates how many times the amount of a substance heavier or lighter than the same volume of water taken at 4 degrees). The most important property of oil and its products, which is based on primary oil refining, is their ability to evaporate. The composition of such oil include hydrocarbons which vaporize begin even at ordinary temperature. If the oil is allowed to stand in an open container without heating more or less long time, part of it will evaporate and the rest will become denser and denser.

Because of the oil include various hydrocarbons with different boiling point, the oil has a constant boiling point, such as water. If we heat the water in the vessel, then observe the following phenomena: the thermometer dipped into the water, will initially show a constant increase in temperature, but as soon as the temperature reaches 100 degrees, raising stops. And then, however much we heated vessel, the temperature will not rise until all the water has evaporated. This is due to the homogeneity of water, ie, the fact that water is made up of the same molecules.

Quite a different picture we will see when heated in the oil vessel. In this case, no matter how much we no heat was applied, the temperature increase will not stop. And at the beginning of heat will evaporate the lightest on the specific weight of hydrocarbons from a mixture of gasoline that is obtained, then the heavier - forming kerosene, diesel oil and lubricating oil. This principle is the primary distillation of crude oil was established. Before the invention of cracking in large kerosene plant distillation was conducted in large stills, in which constantly let in large amounts of superheated steam and simultaneously heated oil from the furnace under the boiler by burning coal or fuel gas. Passing through the oil vapor entrains oil from the lightest compounds with low boiling point and low specific weight. This mixture of kerosene and gasoline with water and then heading to the refrigerator and defended. As distillates were much lighter than water, they are easily separated from it. Then came the sink. First dripped top layer with a specific density of up to 0, 77 - petrol, which is sent to a separate tank. Then they poured kerosene, i.e. heavier hydrocarbons with a density of 0 to 86. The thus obtained crude bad burning kerosene. I demand it clean. For this purpose, it is first treated with a strong (66%) sulfuric acid solution and then with sodium hydroxide solution. The result was a refined kerosene - completely colorless, who did not have a sharp odor and burning with a steady flame, and no burning soot.

The composition of oil includes also such heavy hydrocarbons, which, before reaching its boiling point, begin to decompose, and the more heat the oil, the decomposition will occur more intensively. The essence of this phenomenon is reduced to that of one large heavy hydrocarbon molecules formed by several small molecules with different boiling points and different specific weight. This expansion came to be called cracking (from English to crack - break apart, break up). Thus, it is necessary to understand cracking under the expansion under the influence of heat (and not only the temperature, the decomposition may occur, for example, by high pressure and for some other reasons) complex and large particles into smaller and smaller hydrocarbons. The essential difference cracking process from primary distillation is that cracking occurs in a number of chemical modification of hydrocarbons, whereas with primary distillation is easy separation of the individual pieces, or both said fractions of oil, depending on their boiling points.

The phenomenon of oil decomposition was noticed long ago, but at ordinary oil refining such an expansion was unwanted, so here and use superheated steam, which contributed to the evaporation of the oil without decomposition. Refining industry has evolved through several stages. At the beginning (with the 60-ies. In the XIX. And until the beginning of XX century.) Refining kerosene had an obvious nature, that is the main product of oil refining was kerosene, which remained for half a century the main source of light. On the Russian refineries, for example, formed during distillation lighter fractions considered as waste: they were burned in pits or dumped into waterways.

However, the rapid development of road transport has placed more emphasis. If there were 1 million 250 thousand in the United States in 1913 cars, in 1917 -. 5 million, in 1918 - 6, 25 million, and in 1922 - already 12 million gasoline, which in the XIX century, very few are used. and it was almost unnecessary garbage, gradually became the main purpose of distillation. From 1900 to 1912 the global consumption of gasoline has increased by 115 times. Meanwhile, during the distillation even rich light oil fractions in gasoline accounted for only about 1/5 of the total output volume. Then came the idea to expose heavy fractions isolated after primary distillation, cracking and receive from them thus lighter gasoline fractions. It was soon found that the feedstock for cracking can serve not only heavy fraction (diesel oil or fuel oil), and crude oil. It was also found that the cracked-gasoline is superior to the one that is obtained by ordinary distillation since incorporates those hydrocarbons which are burned smoothly into engine cylinders without explosions (detonation). The engine, running on petrol that does not knock, and lasts longer.

When fluid cracking highlights determining the nature of the whole process are: temperature and the time during which the product is under the influence of temperature. Oil begins to decompose already at 200 degrees. Further, the higher the temperature, the more is the decomposition. Likewise - cracking the longer, the greater the yield of light fractions. However, at too high temperature and duration of the cracking process is not as required - not molecules are split into equal parts, and split up so that on the one hand are too light fractions, and on the other - too heavy. Or even comes complete decomposition of the hydrocarbons into hydrogen and carbon (coke), which, of course, very undesirable.

Optimal conditions for cracking, giving the highest yield of light gasoline fractions, were found in the early XX century English chemist Barton. Even in 1890, Barton worked in England, under the pressure of Russian distillation of heavy oil (fuel oil) to produce kerosene from them, and in 1913 he took a US patent for the first in the history of a process for producing gasoline from heavy petroleum fractions. For the first time the cracking process by the method of Barton under industrial conditions was carried out in 1916, and by 1920 was in production for more than 800 of its units.

The most favorable temperature for cracking - 425-475 degrees. However, if a crude oil is heated to such a high temperature, a large part of it evaporates. Cracked products in the vapor phase was associated with some difficulties, so the aim was not to Barton oil evaporate. But how to achieve such a state that upon heating the oil does not boil? This is possible if carried out under high pressure the whole process. It is known that any high pressure liquid boils at a higher than normal, the temperature and the higher the temperature, the greater the pressure.

Installation was the next device. Working under the pressure of the boiler (1) is located above the combustion chamber (1a) provided with the smoke tubes (4). The boiler was made of a durable good iron with a wall thickness of about 2 cm and was thoroughly riveted. Climbing up the pipe (5) led to the water cooler (6), where the pipeline (7) went to the collection tank (8). After the cracked product passes through a counting device for liquids (10), located at the bottom of this tank a pipe (9) is branched into two lateral tubes (10a and 10b). Each side tube supplied with a control valve; one of them led to the pipe 11 and pipe 12 to the other.

At the beginning of the cracking kettle (1) filled with fuel oil. Due to the heat of the furnace (1a) of the boiler contents slowly heated to about 130 degrees. At the same time the remains of the oil evaporate water contained therein. Condensing in the refrigerator (6), then water flowed into the tank (8), which through the pipe (21) down into the gutter (22). At the same time of the oil out air and other gases. They also fall into the tank through the cooler (8) and the tube (14a), set aside in the conduit (16).

After oil rid of water dissolved therein and air gas, it was ready for cracking. Amplified furnace and the boiler temperature was increased slowly to 345 degrees. Thus began evaporation of light hydrocarbons that remain even in the refrigerator in the gaseous state. They fall into the tank (8) and then through the pipe 14a (which was shut off valve) into the conduit (17), the tube (14) and back to the reservoir (8). Since these light gaseous fraction found no outlet, the pressure inside the unit starts to rise. When it has reached 5 atm, light hydrocarbons could not escape from the main boiler. These compressed gases maintained the same pressure in the boiler (1), a refrigerator (6) and the reservoir (8). Meanwhile, under the influence of high temperatures, the process of splitting of heavy hydrocarbons, which are converted into lighter, ie gasoline. At a temperature of about 250 degrees, they evaporate, they fall into the fridge and condensed here. gas flowed from the cooler to the reservoir (8) and the pipe 9, and 11 or 12 and then fed into a special compressed boilers. Here, under a reduced pressure to evaporate from gasoline dissolved therein light gaseous hydrocarbons. These gases are gradually removed from the boiler, and the resulting crude gasoline blended into special tanks.

As the evaporation of light fractions contents with increasing temperature in the boiler (1) it became increasingly resistant towards heat. The work was interrupted as soon as more than half of its contents turned into gasoline and passed through a condenser. (This amount is easily calculated thanks to the counter fluid (10).) Thereafter, connection pipe (17) is interrupted, and the pipeline valve (14a) connected to the compressor, opened, and the gas slowly volatilized in the low pressure compressor (simultaneously closed conduit ( 9), interrupting the connection setup with the production of gasoline). Firebox quenched, and when the contents of the boiler (1) to cool, it was poured. Then the boiler was purified by coke deposit and prepared for the next run.

cracking method developed by Burton, it marked the beginning of a new stage in the refining industry. Thanks to him, was able to raise several times the output of oil products such as gasoline and aromatic hydrocarbons.

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