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Cracking (chemistry)
Cracking (chemistry)
Cracking (chemistry)
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In petrochemistry, petroleum geology and organic chemistry, cracking is the process whereby complex organic molecules such as kerogens or long-chain hydrocarbons are broken down into simpler molecules such as light hydrocarbons, by the breaking of carbon–carbon bonds in the precursors. The rate of cracking and the end products are strongly dependent on the temperature and presence of catalysts. Cracking is the breakdown of large hydrocarbons into smaller, more useful alkanes and alkenes. Simply put, hydrocarbon cracking is the process of breaking long-chain hydrocarbons into short ones. This process requires high temperatures.[1]

More loosely, outside the field of petroleum chemistry, the term "cracking" is used to describe any type of splitting of molecules under the influence of heat, catalysts and solvents, such as in processes of destructive distillation or pyrolysis.

Fluid catalytic cracking produces a high yield of petrol and LPG, while hydrocracking is a major source of jet fuel, diesel fuel, naphtha, and again yields LPG.

History and patents

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Refinery using the Shukhov cracking process, Baku, Soviet Union, 1934

Among several variants of thermal cracking methods (variously known as the "Shukhov cracking process", "Burton cracking process", "Burton–Humphreys cracking process", and "Dubbs cracking process") Vladimir Shukhov, a Russian engineer, invented and patented the first in 1891 (Russian Empire, patent no. 12926, November 7, 1891).[2] One installation was used to a limited extent in Russia, but development was not followed up. In the first decade of the 20th century the American engineers William Merriam Burton and Robert E. Humphreys independently developed and patented a similar process as U.S. patent 1,049,667 on June 8, 1908. Among its advantages was that both the condenser and the boiler were continuously kept under pressure.[3]

In its earlier versions it was a batch process, rather than continuous, and many patents were to follow in the US and Europe, though not all were practical.[2] In 1924, a delegation from the American Sinclair Oil Corporation visited Shukhov. Sinclair Oil apparently wished to suggest that the patent of Burton and Humphreys, in use by Standard Oil, was derived from Shukhov's patent for oil cracking, as described in the Russian patent. If that could be established, it could strengthen the hand of rival American companies wishing 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 primarily to establish that "the Russian oil industry could easily build a cracking apparatus according to any of the described systems without being accused by the Americans of borrowing for free".[4]

At that time, just a few years after the Russian Revolution and Russian Civil War, the Soviet Union was desperate to develop industry and earn foreign exchange. The Soviet oil industry eventually did obtain much of their technology from foreign companies, largely American ones.[4] At about that time, fluid catalytic cracking was being explored and developed and soon replaced most of the purely thermal cracking processes in the fossil fuel processing industry. The replacement was not complete; many types of cracking, including pure thermal cracking, still are in use, depending on the nature of the feedstock and the products required to satisfy market demands. Thermal cracking remains important, for example, in producing naphtha, gas oil, and coke; more sophisticated forms of thermal cracking have since been developed for various purposes. These include visbreaking, steam cracking, and coking.[5]

Cracking methodologies

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Thermal cracking

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Modern high-pressure thermal cracking operates at absolute pressures of about 7,000 kPa. An overall process of disproportionation can be observed, where "light", hydrogen-rich products are formed at the expense of heavier molecules which condense and are depleted of hydrogen. The actual reaction is known as homolytic fission and produces alkenes, which are the basis for the economically important production of polymers.[6]

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 the thermal cracking in terms of the 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, and the milder-temperature delayed coking (ca. 500 °C) which can produce, under the right conditions, valuable needle coke, a highly crystalline petroleum coke used in the production of electrodes for the steel and aluminium industries.[citation needed]

William Merriam Burton developed one of the earliest thermal cracking processes in 1912 which operated at 700–750 °F (370–400 °C) and an absolute pressure of 90 psi (620 kPa) and was known as the Burton process. Shortly thereafter, in 1921, C.P. Dubbs, an employee of the Universal Oil Products Company, developed a somewhat more advanced thermal cracking process which operated at 750–860 °F (400–460 °C) and was known as the Dubbs process.[7] The Dubbs process was used extensively by many refineries until the early 1940s when catalytic cracking came into use.[1]

Steam cracking

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Main article: Steam cracking

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

In steam cracking, a gaseous or liquid hydrocarbon feed like naphtha, LPG or ethane is diluted with steam and briefly heated in a furnace without the presence of oxygen. Typically, the reaction temperature is very high, at around 850 °C, but the reaction is only allowed to take place very briefly. In modern cracking furnaces, the residence time is reduced to milliseconds to improve yield, resulting in gas velocities up to the speed of sound. After the cracking temperature has been reached, the gas is quickly quenched to stop the reaction in a transfer line heat exchanger or inside a quenching header using quench oil.[citation needed][8]

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

A higher cracking temperature (also referred to as severity) favors the production of ethylene and benzene, whereas lower severity produces higher amounts of propylene, C4-hydrocarbons and liquid products. The process also results in the slow deposition of coke, a form of carbon, on the reactor walls. Since coke degrades the efficiency of the reactor, great care is taken to design reaction conditions to minimize its formation. Nonetheless, a steam cracking furnace can usually only run for a few months between de-cokings. "Decokes" require the furnace to be isolated from the process and then a flow of steam or a steam/air mixture is passed through the furnace coils. This decoking is essentially combustion of the carbons, converting the hard solid carbon layer to carbon monoxide and carbon dioxide.

Fluid catalytic cracking

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Main article: Fluid catalytic cracking
Schematic flow diagram of a fluid catalytic cracker

The catalytic cracking process involves the presence of solid acid catalysts, usually silica-alumina and zeolites. The catalysts promote the formation of carbocations, which undergo processes of rearrangement and scission of C-C bonds. Relative to thermal cracking, cat cracking proceeds at milder temperatures, which saves energy. Furthermore, by operating at lower temperatures, the yield of undesirable alkenes is diminished. Alkenes cause instability of hydrocarbon fuels.[9]

Fluid catalytic cracking is a commonly used process, and a modern oil refinery will typically include a cat cracker, particularly at refineries in the US, due to the high demand for gasoline.[10][11][12] The process was first used around 1942 and employs a powdered catalyst. During WWII, the Allied Forces had plentiful supplies of the materials in contrast to the Axis Forces, which suffered severe shortages of gasoline and artificial rubber. Initial process implementations were based on low activity alumina catalyst and a reactor where the catalyst particles were suspended in a rising flow of feed hydrocarbons in a fluidized bed.[citation needed]

In newer designs, cracking takes place using a very active zeolite-based catalyst in a short-contact time vertical or upward-sloped pipe called the "riser". Pre-heated feed is sprayed into the base of the riser via feed nozzles where it contacts extremely hot fluidized catalyst at 1,230 to 1,400 °F (666 to 760 °C). The hot catalyst vaporizes the feed and catalyzes the cracking reactions that break down the high-molecular weight oil into lighter components including LPG, gasoline, and diesel. The catalyst-hydrocarbon mixture flows upward through the riser for a few seconds, and then the mixture is separated via cyclones. The 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.[citation needed]

During the trip up the riser, the cracking catalyst is "spent" by reactions which deposit coke on the catalyst and greatly reduce activity and selectivity. The "spent" catalyst (equilibrium catalyst) is disengaged from the cracked hydrocarbon vapors and sent to a stripper where it contacts steam to remove hydrocarbons remaining in the catalyst pores. The "spent" catalyst 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" catalyst then flows to the base of the riser, repeating the cycle.[citation needed]

The gasoline produced in the FCC unit has an elevated octane rating but is less chemically stable compared to other gasoline components due to its olefinic profile. Olefins in gasoline are responsible for the formation of polymeric deposits in storage tanks, fuel ducts and injectors. The FCC LPG is an important source of C3–C4 olefins and isobutane that are essential feeds for the alkylation process and the production of polymers such as polypropylene.[citation needed]

Typical yields of a UOP Fluid Catalytic Cracker (volume, feed basis, ~23 API feedstock and 74% conversion)[13]

Feedstock % End Product %
Mixed Gasoil with a Feed sg of 0.916 100 Offgas (methane, ethane, CO) 2.32
C3 -C4 vol% 27.2
Gasoline vol% 57.2
Light Cycle Oil (diesel) vol% 17.1
Clarified (slurry) oil vol% 8.9
Coke wt% 5 estimated
Total 100 Total 117.2

Hydrocracking

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Hydrocracking is a catalytic cracking process assisted by the presence of added hydrogen gas. Unlike a hydrotreater, hydrocracking uses hydrogen to break C–C bonds (hydrotreatment is conducted prior to hydrocracking to protect the catalysts in a hydrocracking process). In 2010, 265 million tons of petroleum was processed with this technology. The main feedstock is vacuum gas oil, a heavy fraction of petroleum.[14][15]

The products of this process are saturated hydrocarbons; depending on the reaction conditions (temperature, pressure, catalyst activity) these products range from ethane and LPG to heavier hydrocarbons consisting mostly 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.[14]

The major products from hydrocracking are jet fuel and diesel, but low sulphur naphtha fractions and LPG are also produced.[16] All these products have a very low content of sulfur and other contaminants with a goal of reducing the gasoil and naphtha range material to 10 PPM sulfur or lower.[7] It is very common in Europe and Asia because those regions have high demand for diesel and kerosene. In the US, 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 into lighter products under a wide range of very high pressures (1,000–2,000 psi) and fairly high temperatures (750–1,500 °F; 399–816 °C), in the presence of hydrogen and special catalysts.[14]

Indicative Isocracking (UOP VGO Hydrocracking) Yields[17]

Feedstock: Russian VGO 18.5 API, 2.28% Sulfur by wt, 0.28% Nitrogen by wt, Wax 6.5% by wt.

Feedstock Distillation Curve

Feedstock Quality Cut % Temp C
Starting Temperature 435
10 460
30 485
50 505
70 525
90 550
End Point 600

Products from a UOP Hydrocracker

Product wt % vol %
C5-180C 4.8 5.9
180-290C 15.4 17.4
290-370C 16.4 18.1
370-425C 13.7 15.0
425-475C 19.3 21.0
475C+ 27.4 29.6
Total 97.0 107.0

Hydrocracking is (mostly) a licensed technology due to its complexity. Typically the licensor is also the catalyst provider. Also, unit internals can often be patented by the process licensors and are designed to support specific functions of the catalyst load. Currently, the major process licensors for hydrocracking are:

  • UOP
  • Axens
  • Chevron Lummus Global
  • Topsoe
  • Shell Criterion
  • Elessent (formerly DuPont)
  • ExxonMobil (iso-dewaxing for lubricant hydrocracking)

Fundamentals

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Outside of the industrial sector, cracking of C−C and C−H bonds are rare chemical reactions. In principle, ethane can undergo homolysis:

CH3CH3 → 2 CH3⋅

Because C−C bond energy is so high (377 kJ/mol),[18] this reaction is not observed under laboratory conditions. More common examples of cracking reactions involve retro-Diels–Alder reactions. Illustrative is the thermal cracking of dicyclopentadiene to produce cyclopentadiene.

See also

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Cracking (chemistry)

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In chemistry, particularly in the petroleum and petrochemical industries, cracking is the process of breaking down large, complex molecules—such as those found in crude oil fractions like heavy gas oils or residuals, or lighter naphthas—into smaller, lighter molecules that are more valuable and easier to refine, primarily through the cleavage of carbon-carbon bonds using , pressure, and often catalysts or . This conversion is essential in oil refineries and petrochemical plants to increase the yield of high-demand products like , diesel, and olefins from lower-value heavy or gaseous feedstocks. The two primary types of cracking are thermal cracking and catalytic cracking. Thermal cracking relies on high temperatures (typically 455–590°C) and pressures (up to 17.6 kg/cm²) to generate free radical intermediates that decompose heavy hydrocarbons, with subtypes including visbreaking for reduction, coking for producing and lighter liquids, and for generating olefins such as and in applications. In contrast, catalytic cracking uses acidic catalysts, such as zeolites or silica-alumina, to promote ionic mechanisms involving intermediates, enabling more selective formation of branched alkanes and alkenes at moderate temperatures (470–525°C in reactors). (FCC), a common variant, circulates powdered catalyst with the feedstock in a riser reactor, followed by regeneration to burn off deposited coke, yielding high-octane components. Hydrocracking, another catalytic method, incorporates to saturate olefins and reduce content, producing cleaner distillates. These processes have revolutionized and production since the early , allowing facilities to maximize light product output—FCC alone accounts for a significant portion of U.S. production—and adapt to varying crude oil qualities, though they also generate byproducts like coke and emissions that require environmental controls. Cracking's efficiency stems from its ability to significantly increase the yield of usable fuels and chemicals from heavy or lower-value feedstocks, making it a cornerstone of the global .

Fundamentals

Definition and Purpose

In petroleum chemistry, cracking is the process of breaking down large molecules into smaller, more useful fragments through the cleavage of carbon-carbon bonds. This or catalytic transforms complex, high-molecular-weight hydrocarbons into simpler alkanes and alkenes that serve as essential building blocks for fuels and chemicals. The primary purpose of cracking in petroleum refining is to convert heavy, less valuable fractions of crude oil—such as those remaining after initial —into lighter, higher-demand products including , diesel, and alkenes. By enabling the production of these versatile compounds, cracking addresses the mismatch between crude oil composition and market needs, where lighter fractions are preferred for transportation fuels and feedstocks. Economically, cracking holds significant importance in the refining industry by maximizing yields from each barrel of crude oil, thereby reducing from heavy residues and boosting the overall value of outputs. This enhances profitability by converting low-value heavy oils into marketable products that command higher prices, supporting the global demand for fuels and chemicals while optimizing resource utilization. Feedstocks commonly include distillates like and gas oils derived from crude oil, yielding products such as shorter-chain alkanes for fuels, alkenes for , and aromatics for solvents and materials.

Chemical Principles and Reactions

Cracking of hydrocarbons involves the cleavage of carbon-carbon (C-C) bonds to produce smaller molecules, primarily through free radical or pathways depending on the process conditions. In cracking, the reaction proceeds via a free radical chain mechanism, which dominates at high temperatures without catalysts. The free radical mechanism begins with the step, where energy induces homolysis of a C-C bond in a molecule, generating two alkyl radicals: R-R→2R∙\text{R-R} \rightarrow 2\text{R}^\bullet This step requires overcoming the bond dissociation energy, typically around 350 kJ/mol for C-C bonds in alkanes, which necessitates elevated temperatures to achieve significant rates. In the propagation phase, radicals sustain the chain through hydrogen abstraction and β-scission. Hydrogen abstraction involves a radical (R•) removing a from another (R'H), forming a stable (RH) and a new radical (R'•): R∙+R’H→RH+R’∙\text{R}^\bullet + \text{R'H} \rightarrow \text{RH} + \text{R'}^\bullet β-scission then cleaves the new radical at a β-position relative to the , yielding an olefin and a smaller alkyl radical, such as: \ceCH3CH2CH2∙−>CH3∙+CH2=CH2\ce{CH3CH2CH2^\bullet -> CH3^\bullet + CH2=CH2} These steps propagate the cracking, favoring the formation of alkenes and lighter alkanes. The chain terminates when two radicals combine to form a stable molecule, such as or recombination: R∙+R’∙→R-R’\text{R}^\bullet + \text{R'}^\bullet \rightarrow \text{R-R'} This reduces the radical concentration and halts the reaction. In contrast, catalytic cracking employs a (carbenium ion) mechanism facilitated by acid sites on catalysts like zeolites. Carbocations form via of olefins or from alkanes at Brønsted or Lewis acid sites: R-CH=CH2+H+→R-CH-CH3+\text{R-CH=CH}_2 + \text{H}^+ \rightarrow \text{R-CH-CH}_3^+ These positively charged intermediates are highly reactive. Rearrangements stabilize the through isomerization (hydride or alkyl shifts) or cyclization (intramolecular addition leading to rings or aromatics), relocating the positive charge to more stable tertiary positions. For example, a primary may shift to a secondary or tertiary one via 1,2-hydride migration. Cyclization can form cyclic intermediates, promoting in heavier feeds. Cracking concludes with β-scission, where the breaks a C-C bond β to the charged carbon, producing a lighter olefin and a new : R3C+−CH2−CH2−R’→R3C=CH2++CH2−R’\text{R}_3\text{C}^+-\text{CH}_2-\text{CH}_2-\text{R'} \rightarrow \text{R}_3\text{C=CH}_2 + ^+\text{CH}_2-\text{R'} This step is selective for shorter-chain products and occurs more readily than in free radical paths due to lower barriers. Thermodynamically, cracking is an with positive change (ΔH > 0), as bond breaking consumes , requiring continuous input to drive the reaction forward. The reaction is favored at high temperatures because the increase in the number of product molecules leads to a positive change (ΔS > 0), enhancing spontaneity via the relation ΔG = ΔH - TΔS. Kinetically, cracking faces high activation energy barriers, particularly for C-C bond cleavage in processes (often >300 kJ/mol), limiting rates without assistance. Catalysts lower these barriers by stabilizing transition states—e.g., acid sites reduce Ea for formation to ~100-200 kJ/mol—enabling reactions at milder conditions and improving selectivity.

Cracking Methodologies

Thermal Cracking

Thermal cracking represents the original method of cracking, employing elevated temperatures and pressures in the absence of catalysts to break down large molecules into smaller, more valuable fractions. In this process, heavy feedstocks such as gas oils or crude residues are heated to temperatures typically ranging from 455 to 590°C under pressures of 2 to 18 , depending on the specific variant such as visbreaking or , promoting free radical reactions that cleave carbon-carbon bonds. The reaction proceeds through homolytic bond cleavage, hydrogen abstraction, and beta-scission steps, resulting in a of shorter-chain alkanes, alkenes, and some aromatics, though with limited control over product distribution due to the non-selective nature of radical propagation. Key variants of thermal cracking include visbreaking and delayed coking, each tailored to specific severity levels for residue upgrading. Visbreaking is a mild form conducted at 455–480°C and 3.5–17.6 , where topped crude or vacuum residuum is briefly exposed to heat in a furnace before , primarily to reduce for easier handling as or feedstock for further processing. In contrast, delayed coking applies more severe conditions at 480–590°C and low pressures of about 1.8–2.1 , using a semi-batch with coke drums to convert heavy residuals into lighter distillates while depositing solid coke as a byproduct. These variants exemplify the spectrum of thermal severity, from partial to near-complete conversion of heavy fractions. The primary advantages of thermal cracking lie in its simplicity and lack of need for expensive catalysts, enabling straightforward implementation in early refineries with basic equipment like furnaces and fractionators. However, it suffers from drawbacks such as low selectivity toward desired products like high-octane , excessive energy consumption due to the endothermic reactions, and significant coke formation that fouls equipment and requires frequent maintenance. Typical products include gasoline-range hydrocarbons (C5–C12), but with notable yields of olefins and aromatics that reflect the radical mechanism's tendency toward unsaturated compounds. A simplified representation of the cracking reaction is: C16H34→C8H18+C8H16\text{C}_{16}\text{H}_{34} \rightarrow \text{C}_{8}\text{H}_{18} + \text{C}_{8}\text{H}_{16} This equation illustrates the of a long-chain into a shorter and , a hallmark of the free radical pathway.

Steam Cracking

Steam cracking is a thermal used primarily to produce light olefins from feedstocks such as , , , and gas oils. In this method, the feedstock is mixed with in a typically ranging from 0.25–0.4 for gaseous feeds like to 0.5–1.0 for liquid feeds like , and the mixture is heated in tubular furnaces to temperatures between 750–900°C under low pressure of approximately 1.7–2.5 bar. This operates without catalysts, relying on free radical mechanisms initiated at high temperatures to break carbon-carbon bonds in the hydrocarbons. The role of steam is multifaceted: it acts as a to lower the of hydrocarbons, thereby reducing the likelihood of coke formation through reactions and promoting higher selectivity toward olefins over aromatics and heavier byproducts. By minimizing on furnace tubes, steam extends equipment life and maintains process efficiency, while also contributing to the endothermic cracking reactions. Optimal steam ratios, such as 0.3–0.4 for cracking, balance olefin yields with , as excessive steam increases downstream separation costs without proportionally improving product selectivity. The primary products of steam cracking are , , and , with yields reaching up to 80% from feeds under optimized conditions. For , typical selectivity is 60–75 wt%, accompanied by (around 5–10 wt%) and (2–5 wt%), while side products include (about 5–10 vol%) and (10–15 vol%). Heavier feedstocks like yield lower (25–35 wt%) but more (15–20 wt%) and C4+ fractions, highlighting the process's versatility for olefin production tailored to feedstock availability. Furnace design is critical for controlling reaction kinetics, featuring coiled radiant tubes within a firebox where the preheated mixture undergoes cracking for a short residence time of 0.1–0.5 seconds to favor primary radical reactions over secondary decomposition. These coils, often made of high-alloy materials to resist carburization and creep, allow precise heat transfer from burners, with the convective section preheating the feed and steam to 500–680°C before the radiant zone. The process is highly endothermic, requiring significant fuel input (typically natural gas) to sustain temperatures, and reactions are halted by rapid quenching in transfer line exchangers, cooling the effluent to 400–500°C within 0.02–0.1 seconds to preserve olefin yields and prevent over-cracking. This quenching also recovers heat to generate high-pressure steam for compression and other plant operations, mitigating some of the energy intensity.

Fluid Catalytic Cracking

Fluid catalytic cracking (FCC) is a key refining process that converts heavy feedstocks, primarily vacuum gas oil (VGO) boiling in the range of 340–540°C, into valuable lighter products such as and olefins. In this process, the preheated VGO feedstock is injected into a riser reactor where it contacts a hot, fluidized catalyst, typically Y-type (also known as ultrastable Y zeolite or USY), which provides the acidic sites necessary for cracking. The reaction occurs at temperatures of 500–550°C and pressures of 1–2 , with short contact times of a few seconds to minimize overcracking. The catalyst consists of fine particles sized 50–100 μm, often around 60–75 μm, which behave like a when aerated with or hydrocarbon vapors, enabling efficient mixing and in the riser. This allows for a continuous operation where the catalyst circulates between the riser reactor and a separate regenerator vessel. In the regenerator, operating at approximately 700°C, deposited coke is burned off with air to restore the catalyst's activity, producing heat that is transferred back to the riser to sustain the endothermic cracking reactions. The catalyst-to-oil ratio is typically maintained at 5–10:1 to optimize conversion and product selectivity. The cracking mechanism proceeds via intermediates formed on the Brønsted acid sites of the catalyst, where of feedstock molecules leads to β-scission and skeletal rearrangements. This results in the production of branched alkanes, which enhance ratings, and olefins as primary products, alongside minor aromatics and coke. Unlike cracking, the catalytic nature of FCC promotes selective bond breaking at lower temperatures, favoring the formation of high-value branched and unsaturated hydrocarbons. Typical yields from FCC of VGO include 50–60% (primarily C5–C11 hydrocarbons with research octane number 90–94), 20% light cycle oil (LCO), and 10–20% light olefins such as . These yields can vary with operating conditions, such as higher catalyst-to-oil ratios increasing conversion but also coke formation. Compared to thermal cracking methods, FCC offers higher selectivity for and lower operating temperatures, improving energy efficiency; however, it suffers from catalyst deactivation due to coke deposition and metal contaminants (e.g., and from the feedstock), necessitating the continuous regeneration cycle.

Hydrocracking

Hydrocracking is a catalytic that converts heavy feedstocks, such as vacuum gas oil (VGO), into lighter, cleaner products by combining cracking with under high pressure. The employs bifunctional catalysts, typically nickel-molybdenum (Ni-Mo) supported on alumina-silica or zeolite-alumina composites, which facilitate both acid-catalyzed cracking and metal-catalyzed . Operating conditions generally include pressures of 50-150 (approximately 5-15 MPa) and temperatures ranging from 350-450°C, allowing the breakdown of large molecules while saturating olefins and removing impurities. The process typically proceeds in multiple stages within fixed-bed or ebullating-bed reactors. Initial hydrotreating removes , , and metal contaminants from the VGO feed using and a hydrotreating catalyst, protecting downstream cracking catalysts from . This is followed by the hydrocracking stage, where acid sites on the catalyst promote carbon-carbon bond cleavage to form smaller fragments, while metal sites enable to stabilize these fragments and prevent coke formation. Ebullating-bed configurations are particularly suited for heavier feeds, as they allow continuous catalyst addition and withdrawal to handle metal contaminants. A simplified representation of the hydrocracking reaction is: C20H42+H2→2C10H22\text{C}_{20}\text{H}_{42} + \text{H}_2 \rightarrow 2 \text{C}_{10}\text{H}_{22} This equation illustrates the cleavage and saturation of a long-chain alkane into shorter paraffins. Hydrocracking achieves high conversion rates exceeding 90%, enabling the production of low-sulfur diesel and jet fuel with minimal aromatic content, which enhances fuel stability and reduces emissions. Typical product yields include 40-60% middle distillates (diesel and kerosene) and about 20% naphtha from VGO feed, depending on process severity and catalyst selectivity. Hydrogen consumption ranges from 200-500 scf per barrel of feed, primarily for saturation and heteroatom removal. These attributes make hydrocracking essential for upgrading heavy residues into high-quality transportation fuels.

History and Development

Early Innovations

The initial observations of thermal decomposition in hydrocarbons emerged in the , driven by fundamental chemical research. In the , French chemist conducted systematic experiments on the effects of high temperatures—often red heat—on organic compounds, including hydrocarbons derived from fats and alcohols. These studies revealed that heating hydrocarbons led to their breakdown into simpler gases and liquids, such as and , laying the groundwork for understanding as a means to alter molecular structures. Berthelot's work, published in memoirs to the , emphasized the thermal instability of carbon-hydrogen chains under elevated conditions, though it remained largely academic at the time. The transition to practical innovations occurred in the early amid growing demand for lighter fuels like and . In 1891, Russian Vladimir Shukhov, along with Sergei Gavrilov, secured the world's first for a continuous cracking process ( Patent No. 12926, November 27). This method involved heating or heavy oils under in a tubular reactor to break long-chain hydrocarbons into shorter ones, enabling efficient, ongoing production without batch interruptions. Shukhov's design, implemented at the oil fields, marked the first industrial application of cracking, prioritizing steady output for fuel refining. World War I intensified the push for cracking technologies due to the urgent need for high-octane aviation gasoline to power engines. In 1912, American chemist William Merriam Burton, working at of , developed the Burton —a pressurized thermal cracking method operating at 700–750°F (370–400°C) and 70–90 psi. Patented in 1913 (U.S. Patent No. 1,049,667), it converted heavy residuum into gasoline yields up to twice those of straight , directly addressing wartime shortages by boosting light fuel production at the . This innovation proved pivotal, as Allied forces relied on such es for superior aviation fuels that enhanced performance. Early cracking faced significant hurdles that limited its efficiency and scalability. Yields of were initially modest, often below 40% from heavy feeds, with substantial byproducts including gases, , and coke that reduced overall selectivity and required frequent reactor cleaning. High operating temperatures promoted coke deposition, fouling equipment and shortening operational cycles, while corrosive byproducts like accelerated material degradation in reactors and . These issues demanded robust alloys and improvements to mitigate and risks. By the , overcoming these obstacles enabled widespread adoption in U.S. refineries, transforming supply. cracking units proliferated, elevating the yield from crude from roughly 15% via simple to 45% or more through cracking, with cracked comprising an increasing share of total output—reaching about 50% by 1930. This shift supported the booming , as refiners like scaled operations to meet surging demand, marking the onset of cracking as a cornerstone of modern petroleum processing.

Key Patents and Industrial Advancements

The development of thermal cracking processes in the marked a pivotal shift toward continuous operations in , with the Holmes-Manley process, patented by the Company in 1922, introducing an innovative method for heating heavy oils under pressure to achieve higher yields compared to batch methods. This process, refined by R.C. Holmes and F.T. Manley, utilized a continuous flow system that minimized downtime and improved efficiency, achieving yields up to 40% from crude oil fractions. Similarly, the Dubbs process, patented by Universal Oil Products Company in 1919 and commercialized in 1922, incorporated recirculation of heavier residues to enhance conversion rates, yielding with superior antiknock properties and enabling scalable industrial adoption. The transition to catalytic cracking began with the Houdry process in 1936, patented by Eugene J. Houdry and licensed to Socony-Vacuum Oil Company, which employed fixed-bed reactors with activated clay catalysts to selectively break carbon-carbon bonds at lower temperatures than methods, producing high-octane with yields exceeding 50%. This innovation, detailed in Houdry's U.S. Patent No. 2,078,945 (1937), revolutionized by reducing coke formation and allowing semi-continuous operation, with the first full-scale commercial unit at , for Sun Oil Company, demonstrating a 20-30% increase in gasoline production efficiency over thermal cracking. Fluid catalytic cracking (FCC) advanced this further through a 1942 patent by Standard Oil of New Jersey (now ExxonMobil), developed by Donald L. Campbell, Homer Z. Martin, Eger V. Murphree, and Charles W. Tyson, which introduced fluidized-bed technology for truly continuous processing. Covered in U.S. Patent No. 2,451,804 (1948, based on 1942 filings), the process suspended catalyst particles in an upward gas flow, enabling rapid and catalyst regeneration, with the first commercial unit at , boosting gasoline output by over 40% while minimizing operational interruptions. This fluidized-bed design became the cornerstone of modern FCC, processing millions of barrels daily and supporting wartime demands. Hydrocracking emerged in the as a hydrogen-integrated variant, with Chevron Research Company (formerly of ) patenting the Unicracking process in 1959, commercialized in 1962, which combined cracking with to produce cleaner, high-quality fuels from heavy residues. Detailed in patents from the early 1960s, Unicracking used bifunctional catalysts ( and metal sites) under high pressure to saturate olefins and reduce content, achieving near-100% conversion to diesel and with levels below 10 ppm, far superior to earlier methods. This process addressed the limitations of catalytic cracking by producing low-aromatic, high-cetane products, influencing the shift away from leaded additives phased out globally by the due to environmental regulations. In the early 2020s, advancements focus on sustainable integration, such as modified FCC catalysts incorporating zeolites with metal promoters to improve and reduce emissions in diverse feedstocks, including biofuels. Chevron's Unicracking iterations enable co- of renewable feeds with , supporting the production of lower-carbon fuels. These developments underscore the from high-emission processes to eco-efficient systems, though legacy aspects like lead-dependent boosting remain obsolete.

Applications and Impacts

Industrial Uses

In petroleum refining, (FCC) units play a central role by converting heavy fractions into valuable lighter products, particularly . These units account for more than 40% of global production, enabling refineries to maximize output from intermediate distillates like gas oils. Hydrocracking, another key cracking process, is widely employed in complex refineries to upgrade heavy oils into high-quality middle distillates, including , which constitutes a significant portion of the diesel yield from light cycle oil (LCO) feedstocks. This versatility allows refineries to process a broader range of crude oils, enhancing overall efficiency and product slate flexibility. In the petrochemical sector, steam cracking serves as the primary method for ethylene production, with global capacity reaching approximately 225 million metric tons per annum as of 2025. Ethylene derived from this process is the foundational building block for plastics manufacturing, supporting the production of polyethylene, ethylene oxide, and other derivatives essential to packaging, construction, and consumer goods industries. As of 2025, European steam cracker closures, totaling around 4 million metric tons per annum of ethylene capacity, highlight challenges in high-cost regions amid global shifts toward sustainable feedstocks. The integration of cracking technologies within refinery configurations is quantified by the Nelson Complexity Index (NCI), where higher NCI values—driven by cracking units like FCC and hydrocrackers—correlate with greater capability to handle lower-quality, heavier crude oils, thereby increasing the yield of high-value lighter products and elevating refinery valuations relative to simpler operations. Economically, FCC units typically operate at capacities of 50,000 to 100,000 barrels per day, processing substantial volumes to achieve scale efficiencies in and olefin output. Steam crackers, meanwhile, are designed for world-scale production, with average capacities exceeding 1.5 million metric tons of per year, underscoring their role in meeting surging demand for petrochemical feedstocks. Emerging trends include bio-cracking applications, where is adapted to process renewable and waste-based feedstocks, such as biomass-derived oils, to produce sustainable fuels and chemicals, addressing the need for lower-carbon alternatives in and petrochemical operations.

Environmental and Safety Considerations

Cracking processes in refining generate significant air emissions, particularly during the regeneration phase of (FCC) units, where oxides () and oxides () are released from the of coke deposited on catalysts. SOx emissions typically range from 10 to 200 mg/Nm³ in existing FCC units with controls, while NOx levels are between 30 and 250 mg/Nm³, depending on feed content and conditions. Volatile organic compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs) are emitted as fugitive pollutants from off-gases and handling operations, contributing to atmospheric reactivity and potential carcinogenic risks. Additionally, (FCC) accounts for 20–35% of total refinery CO2 emissions, with FCC alone producing 105,000 to 276,000 metric tons of CO2 per million tons of crude oil processed, stemming from energy-intensive heating and regeneration. Environmental impacts extend beyond air quality to resource consumption and waste generation. Steam cracking requires substantial water for quenching and cooling, with overall refinery water use ranging from 0.07 to 0.66 m³ per metric ton of crude oil, exacerbating scarcity in water-stressed regions. Hydrocracking and associated hydrotreating produce wastewater laden with hydrocarbons, hydrogen sulfide (H2S), ammonia (NH3), and phenols, at volumes of 0.1 to 1.5 m³ per metric ton of crude, necessitating treatment to prevent aquatic toxicity. Petroleum coke (petcoke), a byproduct from related coking processes integrated with cracking, poses disposal challenges due to its high carbon content and potential for heavy metal leaching, often requiring enclosed storage and dampening to minimize dust emissions and groundwater contamination. Regulatory frameworks have evolved to curb these impacts, with the U.S. Environmental Protection Agency (EPA) enforcing standards under the Clean Air Act since 1970, mandating maximum achievable control technology (MACT) for hazardous air pollutants from FCC units, including wet gas scrubbers to reduce by up to 90% and particulate matter. In the , the Industrial Emissions Directive (2010/75/EU) incorporates Best Available Techniques (BAT) reference documents for refineries and large-volume organic chemicals, setting associated emission levels (BAT-AELs) for ethylene crackers such as below 100 mg/Nm³ for new units and below 300 mg/Nm³, promoting low-emission designs like and sulfur recovery units achieving 99% efficiency. Safety hazards in cracking operations arise from extreme process conditions, including temperatures exceeding 500°C and pressures up to 40 bar in hydrocracking, heightening risks of from leaks or . Catalyst handling in FCC units generates respirable dust, posing inhalation risks and requiring and enclosed systems. Notable incidents include the 2015 ExxonMobil Torrance in an FCC , triggered by a release and ignition, injuring workers and releasing pollutants, and the 2000 rupture and in a catalytic cracking at a European , underscoring vulnerabilities in integrity. Mitigation efforts focus on integrating (CCS) into FCC regenerators to sequester up to 90% of CO2 emissions, alongside emerging from cracking byproducts for green fuel applications, such as blending with low-carbon to reduce lifecycle GHG by 40-95%. These strategies, including methane-to-methanol conversion in cracking, align with 2020s goals by enhancing and enabling net-zero transitions in .

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