Inpetrochemistry,petroleum geology andorganic chemistry,cracking is the process whereby complexorganicmolecules such askerogens or long-chainhydrocarbons are broken down into simpler molecules such as light hydrocarbons, by the breaking ofcarbon–carbon bonds in the precursors. Therate of cracking and the end products are strongly dependent on thetemperature and presence ofcatalysts. Cracking is the breakdown of large hydrocarbons into smaller, more usefulalkanes andalkenes. 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 ofdestructive distillation orpyrolysis.
Fluid catalytic cracking produces a high yield ofpetrol andLPG, while hydrocracking is a major source ofjet fuel,diesel fuel,naphtha, and again yields LPG.
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 engineersWilliam 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 AmericanSinclair 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 theRussian Revolution andRussian 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 producingnaphtha,gas oil, andcoke; more sophisticated forms of thermal cracking have since been developed for various purposes. These includevisbreaking,steam cracking, andcoking.[5]
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 ashomolytic fission and producesalkenes, which are the basis for the economically important production ofpolymers.[6]
Thermal cracking is currently used to "upgrade" very heavy fractions or to produce light fractions or distillates, burner fuel and/orpetroleum coke. Two extremes of the thermal cracking in terms of the product range are represented by the high-temperature process called "steam cracking" orpyrolysis (ca. 750 °C to 900 °C or higher) which produces valuableethylene and other feedstocks for thepetrochemical industry, and the milder-temperaturedelayed coking (ca. 500 °C) which can produce, under the right conditions, valuableneedle coke, a highly crystalline petroleum coke used in the production ofelectrodes for thesteel andaluminium 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 theBurton process. Shortly thereafter, in 1921,C.P. Dubbs, an employee of theUniversal Oil Products Company, developed a somewhat more advanced thermal cracking process which operated at 750–860 °F (400–460 °C) and was known as theDubbs process.[7] The Dubbs process was used extensively by manyrefineries until the early 1940s when catalytic cracking came into use.[1]
Steam cracking is apetrochemical process in which saturatedhydrocarbons are broken down into smaller, often unsaturated, hydrocarbons. It is the principal industrial method for producing the lighteralkenes (or commonlyolefins), includingethene (orethylene) andpropene (orpropylene). Steam cracker units are facilities in which a feedstock such as naphtha, liquefied petroleum gas (LPG),ethane,propane orbutane 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 likenaphtha,LPG orethane 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 thespeed of sound. After the cracking temperature has been reached, the gas is quickly quenched to stop the reaction in a transfer lineheat 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 asethane, LPGs or lightnaphtha give product streams rich in the lighter alkenes, including ethylene, propylene, andbutadiene. Heavier hydrocarbon (full range and heavy naphthas as well as other refinery products) feeds give some of these, but also give products rich inaromatic hydrocarbons and hydrocarbons suitable for inclusion ingasoline orfuel oil. Typical product streams includepyrolysis gasoline (pygas) andBTX.
A higher crackingtemperature (also referred to as severity) favors the production ofethylene andbenzene, whereas lower severity produces higher amounts ofpropylene, C4-hydrocarbons and liquid products. The process also results in the slow deposition ofcoke, a form ofcarbon, 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.
Schematic flow diagram of a fluid catalytic cracker
The catalytic cracking process involves the presence ofsolid acid catalysts, usuallysilica-alumina andzeolites. The catalysts promote the formation ofcarbocations, 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 acat cracker, particularly at refineries in the US, due to the high demand forgasoline.[10][11][12] The process was first used around 1942 and employs a powderedcatalyst. 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 activityalumina catalyst and a reactor where the catalyst particles were suspended in a rising flow of feed hydrocarbons in afluidized bed.[citation needed]
In newer designs, cracking takes place using a very activezeolite-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 viacyclones. The catalyst-free hydrocarbons are routed to a mainfractionator 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 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 plusoxygen) is used to burn off the coke to restore catalyst activity and also provide the necessary heat for the next reaction cycle, cracking being anendothermic 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 elevatedoctane rating but is less chemically stable compared to other gasoline components due to itsolefinic profile. Olefins in gasoline are responsible for the formation ofpolymeric deposits in storagetanks, fuel ducts andinjectors. The FCC LPG is an important source ofC3–C4 olefins andisobutane that are essential feeds for thealkylation process and the production of polymers such aspolypropylene.[citation needed]
Typical yields of a UOP Fluid Catalytic Cracker (volume, feed basis, ~23 API feedstock and 74% conversion)[13]
Hydrocracking is a catalytic cracking process assisted by the presence of addedhydrogen gas. Unlike ahydrotreater, 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 aresaturated hydrocarbons; depending on the reaction conditions (temperature, pressure, catalyst activity) these products range fromethane, LPG to heavier hydrocarbons consisting mostly ofisoparaffins. Hydrocracking is normally facilitated by a bifunctional catalyst that is capable of rearranging and breakinghydrocarbon chains as well as adding hydrogen toaromatics andolefins to producenaphthenes andalkanes.[14]
The major products from hydrocracking arejet fuel anddiesel, but low sulphurnaphtha fractions and LPG are also produced.[16] All these products have a very low content ofsulfur and othercontaminants 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 andkerosene. In the US, fluid catalytic cracking is more common because the demand forgasoline 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]
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:
Outside of the industrial sector, cracking of C−C and C−H bonds are rarechemical reactions. In principle, ethane can undergohomolysis:
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 ofdicyclopentadiene to producecyclopentadiene.
^Kraus, Richard S."Petroleum Refining Process" in 78. Oil and Natural Gas, Kraus, Richard S., Editor, Encyclopedia of Occupational Health and Safety, Jeanne Mager Stellman, Editor-in-Chief. International Labor Organization, Geneva. 2011.Archived 2013-07-24 at theWayback Machine.
Colorado School of Mines - faculty member John Jechura is one of the best teachers of refining technology in the world, he puts his course materials online free of charge including a good presentation onhydrotreating/hydrocracking.
