Cracking

Cracking or cracking is a chemical process by which the molecules of a compound are broken, thus producing simpler compounds.

Shúkhov, Baku, USSR, 1934

The original process, still in use, used heat and pressure and is called "thermal cracking" at a temperature of 850-810°C ("Shukhov cracking process"). Then a new method was devised: “catalytic cracking”, at a temperature of 450-500 °C, which uses a catalyst (a substance that determines other chemical changes without modifying itself). In the case of this type of cracking, the catalyst (such as Al₂O₃ or SiO₂) is a kind of clay that can occur in the form of lumps, pills, small grains or as a superfine powder and whose disintegrating action, added to that of heat and pressure, favors fractionation into lighter components and produces more and better compounds as a result.

A modern form of catalytic cracking of petroleum is the fluid process. This uses a fluid cat cracker, which is a machine, in some cases, up to two hundred feet high. Large amounts of steam, air, and pulverized catalyst circulate at high temperatures through miles of pipelines and reactors. At a certain height of the operation, the very fine grains of the catalyst are coated with carbon separated from the oil, and then stop acting through the action of a regenerator; however, the carbon is burned and consumed, and the catalyst is again able to function again and again.

In the fluid process, the catalyst is so fine that when it is agitated in a mixture with air or other gases, its volume increases and flows like a liquid, thus being able to be controlled by valves. This way of working with a solid substance as if it were a fluid has been an advance in refinery work.

Oil cracking makes it possible to obtain twice as much light fraction (naphtha) from a barrel of crude oil than that extracted by simple distillation. It is currently a fundamental procedure for the production of high octane gasoline.

History and patents

Among several variants of thermal cracking methods (known as "Shukhov cracking process", "Burton cracking process", "Burton-Humphreys cracking process&#34 and 'Dubbs cracking process') Vladimir Shukhov, a Russian engineer, invented and patented the first in 1891 (Russian Empire, patent no. 12926, November 7, 1891). One facility was used to a limited extent in Russia, but development was not followed up. In the first decade of the 20th century, American engineers William Merriam Burton and Robert E. Humphreys independently developed and patented a process similar to that of U.S. Patent 1,049,667, June 8, 1908. Among its advantages was the fact that both the condenser and the boiler were kept continuously under pressure.

In its earlier versions it was a batch process, rather than continuous, and many patents were to follow in the United States and Europe, though not all were practical. In 1924, a delegation from the American Sinclair Oil Corporation visited Shukhov. Sinclair Oil apparently wished to suggest that the Burton and Humphreys patent, in use by Standard Oil, was derived from Shukhov's patent for cracking petroleum, as described in the Russian patent. If that could be established, it could strengthen the hand of rival US companies who want to invalidate the Burton-Humphreys patent. In the event Shukhov satisfied the Americans that, in principle, Burton's method closely resembled his 1891 patents, though his own interest in the matter was mainly to establish that "the Russian oil industry could easily build a cracking apparatus according to any of the systems described without being accused by Americans of borrowing for free".

At the time, just a few years after the Russian revolution and brutal Russian civil war, the Soviet Union was desperate to develop industry and earn foreign exchange, so its oil industry eventually sourced much of its technology from oil companies. foreigners, mostly Americans. At about this time, fluid catalytic cracking was being explored and developed and soon replaced most purely thermal cracking processes in the fossil fuel processing industry. The replacement was not complete, as many types of cracks, including pure thermal cracks, are still in use, depending on the nature of the raw material and the products needed to meet market demands. Thermal cracking remains important, for example, in the production of naphtha, diesel oil and coke, and more sophisticated forms of thermal cracking have been developed for various purposes. These include visbreaking, steam cracking, and coke.

Cracking methodologies

Thermal cracking

Modern high pressure thermal cracking operates at absolute pressures of approximately 7000 kPa. A general disproportionation process can be observed, where "light" rich in hydrogen at the expense of heavier molecules that condense and run out of hydrogen. The actual reaction is known as homolytic fission and produces alkenes, which are the basis for the economically important production of polymers.

Thermal cracking is currently used to "upgrade" very heavy fractions or to produce light fractions or distillates, burner fuel and/or petroleum coke. Two extremes of thermal cracking in terms of product range are represented by the high temperature process called "steam cracking" or pyrolysis (ca. 750 °C to 900 °C or higher) which produces valuable ethylene and other feedstocks for the petrochemical industry. The mildest temperature retarded coke (approx. 500 °C) can produce, under the right conditions, needle coke, that is, a highly crystalline petroleum coke used in the production of electrodes for the steel and aluminum industries.
William Merriam Burton developed one of the first thermal cracking processes in 1912 which operated at 370–400 °C and an absolute pressure of 90 psi (620 kPa) and was known as the Burton process. Soon after, in 1921, CP Dubbs, an employee of the Universal Oil Products Company, developed a somewhat more advanced thermal cracking process that operated at 400–460 °C and was known as the Dubbs process. The Dubbs process was widely used by many refineries until the early 1940s, when catalytic cracking came into use.

Steam cracks

Steam cracking is a petrochemical process in which saturated hydrocarbons are broken down into smaller, often unsaturated hydrocarbons. It is the main industrial method for producing the lightest alkenes (or commonly olefins), including ethene (or ethylene) and propene (or propylene). Steam cracking units are facilities in which a feedstock such as naphtha, liquefied petroleum gas (LPG), ethane, propane, or butane is thermally cracked using steam in a bank of pyrolysis furnaces to produce lighter hydrocarbons..

In steam cracking, a liquid or gaseous hydrocarbon feed such as naphtha, LPG or ethane is diluted with steam and heated very little in a furnace without the presence of oxygen. Typically, the reaction temperature is very high, around 850 °C, but the reaction is only allowed very briefly. In modern cracking furnaces, the residence time is reduced to milliseconds to improve performance, resulting in gas velocities up to the speed of sound. After reaching cracking temperature, the gas is quenched to stop the reaction in a transfer line heat exchanger or within a quench header using quench oil.

The products produced in the reaction depend on the composition of the feed, the ratio of hydrocarbons to steam, and on the cracking temperature and residence time of the furnace. Light hydrocarbon feeds such as ethane, LPG, or light naphtha provide product streams rich in lighter alkenes, including ethylene, propylene, and butadiene. Heavier hydrocarbon feeds (full range and heavy naphthas, as well as other refinery products) give some of these, but also give products rich in aromatic hydrocarbons and hydrocarbons suitable for inclusion in gasoline or fuel oil.

Higher cracking temperature (also known as gravity) favors the production of ethene and benzene, while lower gravity produces larger amounts of propene, C4 hydrocarbons, and liquid products. The process also results in the slow deposition of coke, a form of carbon, on the reactor walls. This degrades the efficiency of the reactor, so the reaction conditions are designed to minimize this. However, a steam cracker can generally only run for a few months at a time between coker cooks. Decokes require the furnace to be isolated from the process and then a stream of steam or a steam/air mixture is passed through the furnace coils. This converts the hard solid carbon layer into carbon monoxide and carbon dioxide. Once this reaction is complete, the furnace can be returned to service.

Process Details

The areas of an ethylene plant are:

1. Steam-rich furnaces;
2. recovery of primary and secondary heat with rapid cooling;
3. a dilution steam recycling system between ovens and the rapid cooling system;
4. primary compression of craqueado gas (3 compression stages);
5. elimination of hydrogen sulfide and carbon dioxide (acid gas emissions);
6. Secondary compression (1-3 stages);
7. cracked gas drying;
8. cryogenic treatment;
9. The whole cold current of cracked gas goes to the detaching tower. The aerial current from the demetanizer tower consists of all the hydrogen and methane that was in the cracked gas stream. The cryogenic treatment (−157 °C) that treats this air current separates the hydrogen from methane. The recovery of methane is critical for the economic operation of an ethylene plant.
10. The bottom current from the detaching tower goes to the desethanizing tower. The air current from the desethanizing tower consists of all the C2 that were in the cracked gas stream. The current contains acetylene, which is explosive above 200 kPa (29 psi). If partial acetylene pressure is expected to exceed these values, currents are partially hydrogenated. The ethylene product is taken from the top of the tower and the ethank that comes from the bottom of the divisor is recycled to the furnaces to be cracked again;
11. The bottom current from the desetanizer tower goes to the depropanizer tower. The air current from the depropanizer tower consists of all the C3 that were in the cracked gas stream. Before feeding the C3 to the divisor C3, the current is hydrogenic to convert the mixture of methylacetylene and propadiene (aleno). This sequence is sent to the divisor C3. The divisor is propylene product and the lower current is propane that is sent back to the furnaces for its craqueo or used as fuel.
12. The lower current of the depropanizer tower feeds to the debutanizer tower. The overall flow from the debutanizer is all the C4 that were in the cracked gas current. The lower current of the debutanizer (light Pyrolysis Gasoline) consists of everything in the cracked gas current that is C5 or heavier.

Since ethylene production is energy intensive, a lot of effort has gone into recovering heat from the gas leaving the kilns. Most of the energy recovered from cracked gas is used to produce high pressure (1200 psig) steam. This steam, in turn, is used to drive the turbines to compress cracked gas, the propylene refrigeration compressor, and the ethylene refrigeration compressor. An ethylene plant, once in operation, does not need to import steam to drive its steam turbines. A typical world scale ethylene plant (about 1.5 billion pounds of ethylene per year) uses a 45,000 horsepower (34,000 kW) cracked gas compressor, a 30,000 hp (22,000 kW) propylene compressor and a 15,000 hp (11,000 kW) ethylene compressor.

Fluid catalytic cracking

The catalytic cracking process involves the presence of solid acid catalysts, usually silica-alumina and zeolites. Catalysts promote the formation of carbocations, which undergo rearrangement processes and CC bond cleavage. Relative to thermal cracking, jack cracking occurs at milder temperatures, which saves energy. Also, when operating at lower temperatures, the yield of alkenes decreases. Alkenes cause instability of hydrocarbon fuels.

Fluid catalytic cracking is a commonly used process, and a modern oil refinery will typically include a cat cracker, particularly in refineries in the United States, due to the high demand for gasoline. The process was first used around 1942 and uses a powdered catalyst. During World War II, the Allied Forces had abundant supplies of the materials in contrast to the Axis Forces, which suffered severe shortages of gasoline and artificial rubber. Initial implementations of the process were based on a low activity alumina catalyst and a reactor where the catalyst particles were suspended in an upward flow of hydrocarbon feed in a fluidized bed.

In newer designs, cracking is accomplished using a highly active zeolite-based catalyst in a short-contact-time or upward-sloping vertical tube called a "riser". The preheated feed is sprayed into the base of the riser through feed nozzles where it comes into contact with the extremely hot 666 to 760°C fluidized catalyst. The hot catalyst vaporizes the feed and catalyzes cracking reactions that break down the high molecular weight oil into lighter components, including LPG, gasoline and diesel. The catalyst-hydrocarbon mixture flows up through the riser for a few seconds, and then the mixture is separated by means of cyclones. Catalyst-free hydrocarbons are routed to a main fractionator for separation into fuel gas, LPG, gasoline, naphtha, light-cycle oils used in diesel and jet fuel, and heavy fuel oil.

During the elevator ride, the cracking catalyst is "used up" by reactions that deposit coke on the catalyst and greatly reduce activity and selectivity. The "spent" it is decoupled from the broken hydrocarbon vapors and sent to a separator where it comes into contact with the vapor to remove hydrocarbons remaining in the catalyst pores. The "spent" it then flows into a fluidized bed regenerator where air (or in some cases air plus oxygen) is used to burn off the coke to restore catalyst activity and also provide the necessary heat for the next reaction cycle, cracking being an endothermic reaction.. The "regenerated" it then flows to the base of the riser, repeating the cycle.

Gasoline produced at the FCC unit has a high octane number, but is less chemically stable compared to other gasoline components due to its olefin profile. Olefins in gasoline are responsible for the formation of polymeric deposits in storage tanks, fuel lines, and injectors. FCC LPG is an important source of C₃ -C₄ olefins and isobutane which are essential feeds for the alkylation process and the production of polymers such as polypropylene.

Hydrocracking

Hydrocracking is a catalytic cracking process assisted by the presence of added hydrogen gas. Unlike a hydrotreater, hydrocracking uses hydrogen to break CC bonds (hydrotreating is done prior to hydrocracking to protect catalysts in a hydrocracking process). In 2010, 265 × 10⁶ tons of oil were processed with this technology. The main raw material is vacuum diesel, a heavy fraction of petroleum.

The products of this process are saturated hydrocarbons; depending on the reaction conditions (temperature, pressure, catalyst activity) these products vary from ethane, LPG to heavier hydrocarbons consisting mainly of isoparaffins. Hydrocracking is normally facilitated by a bifunctional catalyst that is capable of rearranging and breaking hydrocarbon chains, as well as adding hydrogen to aromatics and olefins to produce naphthenes and alkanes.

The main products of hydrocracking are jet fuel and diesel, but low-sulfur naphtha fractions and LPG are also produced. All these products have a very low content of sulfur and other contaminants. It is very common in Europe and Asia because those regions have high demand for diesel and kerosene. In the United States, fluid catalytic cracking is more common because the demand for gasoline is higher.

The hydrocracking process depends on the nature of the feedstock and the relative rates of the two competing reactions, hydrogenation and cracking. Heavy aromatic feedstock is converted to lighter products under a wide range of very high pressures (1,000-2,000 psi) and fairly high temperatures (400-800°C), in the presence of hydrogen and special catalysts.

The main functions of hydrogen are, therefore:

1. prevent the formation of polycyclic aromatic compounds if the raw material has a high paraffin content,
2. reduction of tar training,
3. reduction of impurities,
4. prevent coke accumulation in the catalyst,
5. convert sulfur and nitrogen compounds present in raw material into hydrogen and ammonia sulphide, and
6. achieving high-level swamp fuel.

Basics

Outside of the industrial sector, CC and CH bond cracking is a rare chemical reaction. In principle, ethane can undergo homolysis:

CH₃ CH₃ → 2 CH₃

Because the DC binding energy is so high (377 kJ/mol), this reaction is not observed under laboratory conditions. The most common examples of cracking reactions involve retro-Diels-Alder reactions. Illustrative is the thermal cracking of dicyclopentadiene to cyclopentadiene.