Cracking (Chemistry)

9 Key excerpts on "Cracking (Chemistry)"

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  The Chemistry and Technology of Petroleum

  • James G. Speight(Author)
  • 2014(Publication Date)
  • CRC Press

    (Publisher)

  These processes may also be characterized by the physical state (liquid and/or vapor phase) in which the decomposition occurs. The state depends on the nature of the feedstock as well as conditions of pressure and temperature (Speight and Ozum, 2002). From the chemical viewpoint, the products of cracking are very different from those obtained directly from crude petroleum. When a twelve-carbon atom hydrocarbon typical of straight-run gas oil is cracked, there are several potential reactions that can lead to a variety of products, for example CH CH CH CH CH CH CH CH 3 2 10 3 3 2 8 3 2 2 ( ) ( ) Æ + H33527 CH CH CH CH CH CH CH CHCH 3 2 10 3 3 2 7 3 2 3 ( ) ( ) Æ + H33527 CH CH CH CH CH CH CH CHCH CH 3 2 10 3 2 6 3 2 2 ( ) ( ) 3 3 Æ + H33527 CH CH CH CH CH CH CH CH CH CH 3 2 10 3 3 2 5 3 2 2 2 3 ( ) ( ) ( ) Æ + H33527 CH CH CH CH CH CH CH CH CH CH 3 2 10 3 3 2 4 3 2 2 3 3 ( ) ( ) ( ) Æ + H33527 CH CH CH CH CH CH CH CH CH CH 3 2 10 3 3 2 3 3 2 2 4 3 ( ) ( ) ( ) Æ + H33527 CH CH CH CH CH CH CH CH CH CH 3 2 10 3 3 2 2 3 2 2 5 3 ( ) ( ) ( ) Æ + H33527 CH CH CH CH CH CH CH CH CH CH 3 2 10 3 3 2 3 2 2 6 3 ( ) ( ) Æ + H33527 CH CH CH CH CH CH CH CH CH 3 2 10 3 3 3 2 2 7 3 ( ) ( ) Æ + H33527 CH CH CH CH CH CH CH CH 3 2 10 3 4 2 2 8 3 ( ) ( ) Æ + H33527 The products are dependent on temperature and residence time and the simple reactions shown about do not take into account the potential for isomerization of the products or secondary and even tertiary reactions that can (and do) occur. 484 The Chemistry and Technology of Petroleum The hydrocarbons with the least thermal stability are the paraffins, and the olefins produced by the cracking of paraffins are also reactive. Cycloparaffins (naphthenes) are less easily cracked, their stability depending mainly on any side chains present, but ring splitting may occur, and dehydroge-nation can lead to the formation of unsaturated naphthenes and aromatics.

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  Chemistry of Fossil Fuels and Biofuels

  • Harold Schobert(Author)
  • 2013(Publication Date)
  • Cambridge University Press

    (Publisher)

  16 Thermal processing in refining 16.1 Thermal cracking In the progression through the oil window, thermally driven reactions break kerogen and larger hydrocarbon molecules into smaller ones. In a refinery, analogous processes take some of the heavier products and break them into smaller molecules. This shifts the molecular weight downward, increasing the amounts of relatively small molecules boiling in the gasoline range. To operate on a human rather than a geologic time scale requires running at much higher temperatures than are encountered in the oil window. Processes that rely entirely on heat for breaking down large petroleum molecules into smaller ones are called thermal cracking. With the steady increase in use of automobiles and trucks in the early decades of the past century, market demand for gasoline exceeded what could be supplied by straight- run gasoline, even augmented with natural gasoline. Cracking processes can increase the relative proportion of molecules in the C 5 –C 10 range, at the expense of larger molecules in products having a lower value than gasoline. Refinery cracking processes are of two types, thermal (that rely entirely on tempera- ture to drive the cracking reactions) or catalytic. Chapter 14 included a discussion of catalytic cracking reactions and processes. Thermal processes were developed starting around 1913. Numerous thermal cracking processes were developed in the early decades of the twentieth century. They helped meet the increasing demand for gasoline in the 1920s and 30s. The process developed by C.P. Dubbs [A] provides an example (see Figure 16.1). The basic purpose of thermal cracking, before about 1950, was to enhance the yield of light and middle distillates from the heavier components. Depending on feedstock and cracking conditions, thermal cracking could roughly double the output of gasoline relative to straight-run gasoline.

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  Chemical Reaction Technology

  • Dmitry Yu. Murzin(Author)
  • 2015(Publication Date)
  • De Gruyter

    (Publisher)

  Chapter 6

  Cracking

  6.1 General

  Cracking is a process of breaking carbon-carbon bonds in complex organic molecules into simpler ones, which are more valuable. The cracking rate and selectivity depend on the presence of a catalyst and temperature. This chapter describes thermal cracking (visbreaking and steam cracking) and several catalytic processes such as fluid catalytic cracking (FCC) and hydrocracking.

  Thermal cracking is currently applied for transformation of either very heavy fractions at milder temperature (ca. 500°C) or to produce light fractions such as ethylene, propylene, and other feedstock for petrochemical industry by pyrolysis in the presence of steam (steam cracking) at higher temperatures (ca. 750°C to 900°C or higher). FCC produces a high yield of gasoline and liquefied petroleum gas (LPG), while hydrocracking is a major source of diesel, jet fuel, LPG, and naphtha.

  These processes (visbreaking, hydrocracking, FCC, and steam cracking) will be considered below following an increase in the process severity.

  6.2 Visbreaking

  The quantity of heavy residues is expected to increase in the future in view of the progressively increasing heavier nature of the crudes. The most promising technologies to process such feedstock involve the conversion of vacuum residue and extra heavy crude oil into light and middle distillate products.

  Visbreaking (viscosity reduction, viscosity breaking) is a relatively mild thermal non-catalytic cracking process mainly used to reduce vacuum tower bottom viscosities and pour points and to reduce the amount of cutting stock required for residue dilution to meet fuel oil specifications. Heavy fuel oil production can be reduced from 20% to 35% and cutter stock for dilution by 20% to 30% by visbreaking. This increases the yield of more valuable distillates directly converted from visbreaking or used as catalytic cracker feedstock.

  Visbreaker feeds comprise saturates, aromatics, resins, and asphaltenes. Saturates are found to have an average carbon number in the range C38–50 with relatively low heteroatom content. They consist of long alkyl chains with few or negligible naphthenic and aromatic rings. The aromatics have a carbon number in the range C41–53. Asphaltenes are the heaviest and the most polar class of components in the crude oil, containing larger amounts of heteroatoms than the rest of the components in the crude oil. They comprise molecules with 4–10 condensed aromatic rings,

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  Thermal and Catalytic Processing in Petroleum Refining Operations

  • James G. Speight(Author)
  • 2023(Publication Date)
  • CRC Press

    (Publisher)

  4 Thermal Cracking Processes

  DOI: 10.1201/9781003184904-4

  4.1 Introduction

  Distillation has remained a major refinery process and a process to which almost every crude oil that enters the refinery is subjected. However, not all crude oils yield the same distillation products. In fact, the composition and properties of the feedstock dictate the processes that may be required for refining and balancing product yield with demand is a necessary part of refinery operations (Parkash, 2003 ; Gary et al., 2007 ; Speight, 2014 ; Hsu and Robinson, 2017 ; Speight, 2017 ). Basic processes for this are still the so-called cracking processes in which relatively high-boiling constituents carbons are cracked, that is, thermally decomposed into lower-molecular-weight, smaller, lower-boiling molecules, although reforming alkylation, polymerization, and hydrogen-refining processes have wide applications in making premium-quality products (Parkash, 2003 ; Gary et al., 2007 ; Speight, 2014 ; Hsu and Robinson, 2017 ; Speight, 2017 ).

  After 1910 and the conclusion of World War I, the demand for automotive (and other) fuels began to outstrip the market requirements for kerosene and refiners, needing to stay abreast of the market pull, were pressed to develop new technologies to increase gasoline yields. There being finite amounts of straight-run distillate fuels in crude oil, refiners had, of necessity, the urgency to develop processes to produce additional amounts of these fuels. The conversion of coal and oil shale to liquid through the agency of cracking had been known for centuries, and the production of various spirits from crude oil through thermal methods had been known since at least the inception of Greek fire in earlier centuries.

  The discovery that higher-molecular-weight (higher-boiling) materials could be decomposed to lower-molecular-weight (lower-boiling) products was used to increase the production of kerosene and was called cracking distillation. In the process, a batch of crude oil was heated until most of the kerosene was distilled from it, and the overhead material became dark in color. At this point, the still fires were lowered, the rate of distillation decreased, and the feedstock was held in the hot zone, during which time some of the large hydrocarbons were decomposed and rearranged into lower-molecular-weight products. After a suitable time, the still fires were increased and distillation continued in the normal way. The overhead product, however, was low boiling (low viscosity) suitable for kerosene instead of the high-boiling (high-viscosity) oil that would otherwise have been produced. Thus, it was not surprising that such technologies were adapted for the fledgling crude oil industry.

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  Fossil Hydrocarbons

  Chemistry and Technology

  • Norbert Berkowitz(Author)
  • 1997(Publication Date)
  • Academic Press

    (Publisher)

  CHAPTER 9 Processing 1. THE CHEMICAL FOUNDATIONS Unlike preparation, which concerns itself mainly with removal of unwanted matter in fossil hydrocarbons by physical means [ 1], processing effects chemi- cal modifications designed to enhance the economic value of the feedstock or meet specialized market demands, and for the most part, these objectives are achieved by some form of thermal cracking or hydrogenation. Crackingmin essence, a controlled LT pyrolysismwill, if sufficiently mild, reduce the number of C atoms in a molecule without appreciably changing the H/C ratio of the feed or, if more severe, yield an H-enriched fraction by H-disproportionation and rejection of an equivalent amount of excess carbon to high-molecular- weight polynuclear aromatics and/or coke. The other method, hydrogenation, inserts H atoms into the host molecule, and thereby increases the H/C ratio without necessarily changing the molecular size unless the required hydrogena- tion depth--i.e., the extent of hydrogenationmdemands reaction conditions sufficiently severe to cause concurrent thermal cracking. Both approaches offer wide operational freedoms and are used in diverse formats- and if appropriately adapted to feedstock characteristics, both are technically capable of accepting any fossil hydrocarbon feedstock [2]. THERMAL CRACKING Although limited decomposition and molecular rearrangement can occur at low temperatures [3], significant thermal cracking can only proceed above ---300-350°C, where all hydrocarbons other than CH4 and C2H6 are less stable than their constituent elements.

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  The Chemistry of Hydrocarbon Fuels

  • Harold H. Schobert(Author)
  • 2013(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  Subsequent addition of hydrogen to the two fragments results in the formation of two smaller alkane molecules. The hydrocracking can be represented as the formation of methane and a new alkane shorter by one carbon atom, as for example in the hydrocracking of pentane: CH3CH2CH2CH2CH3 + H 2 -» CH3CH2CH2CH3 + CH4 245 The actual position along the chain at which hydrocracking occurs depends on the nature of the catalyst; nickel favors hydrocracking to methane as shown in the example above, whereas platinum favors hydrocracking near the middle of the chain. Hydrocracking generally requires high temperatures and metals which form very strong carbon-metal bonds. Both straight-chain and branched-chain compounds can undergo hydrocracking. Hydrocracking can also occur on the acidic sites of the oxide portion of the catalyst. Again, hydrocracking is like isomeration in that it is facilitated if the alkane is first dehydrogenated to an alkene on the metal surface. An important characteristic of hydrocracking is that the rate increases very rapidly as a function of the molecular weight of the reacting species. For example, hexadecane cracks three times as fast as dodecane. This strong dependence of rate on molecular weight means that the undesirable long-chain alkanes are cracked out of the feedstock while the more desirable shorter chain compounds undergo hydrocracking much more slowly and therefore tend to survive into the product stream. Considered overall, reforming is not a hydrogénation process, because dehydrogenation, dehydroisomerization, and dehydrocyclization all produce hydrogen, even though the hydrocracking and hydrode-sulfurization consume hydrogen. Thus in some cases the total reforming process could result in a net loss of hydrogen from the species undergoing reforming. In another sense, the reforming process can produce hydrogen, but removing it from some of the species being reformed.

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  The Petroleum Handbook

  • Shell(Author)
  • 1986(Publication Date)
  • Elsevier Science

    (Publisher)

  In the U S A , S t a n d a r d Oil of N e w Jersey operated a plant at Baton Rouge, La. *, producing gasoline from a Venezuelan k e r o s i n e / * Now owned by Exxon, USA. HYDROCRACKING 295 light gas oil fraction. Operating conditions in those units were c o m p a r a b l e : a p p r o x i m a t e reaction temperature 4 0 0 ° C a n d reaction pressures of 2 0 0 -3 0 0 bar. After the war, commercial hydrocracking was stopped because the process was too expensive. Hydrocracking research, however, continued intensively. By the e n d of the 1950s, the process h a d b e c o m e economic, for which a n u m b e r of reasons can be identified. T h e development of improved catalysts m a d e it possible to operate the process at considerably lower pressure, viz. 7 0 -1 5 0 bar. This in turn resulted in a reduction in e q u i p m e n t wall thickness, whereas, simultaneously, advances were m a d e in mechanical engineering, especially in the field of reactor design. These factors, together with the availability of relatively low-cost hydrogen from the b u d d i n g steam reforming process, b r o u g h t h y d r o -cracking b a c k on the refinery scene. T h e first units of the second generation were built in the U S A to meet the d e m a n d for conversion of surplus fuel oil (cycle oil from fluid catalytic cracking) in the gasoline-oriented refineries. Hydrocracking is n o w a well-established process, which is offered by m a n y licensors. Shell has developed three basic configurations, which are described below. Basis for the Choice of Conversion Route Refiners are continuously faced with trends towards increased conversion, better p r o d u c t qualities a n d m o r e rapidly changing p r o d u c t patterns. Various processes are available that c a n meet the requirements to a greater or less degree: coking, v i s b r e a k i n g / t h e r m a l cracking, catalytic cracking a n d hydrocracking.

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  Upgrading Oilsands Bitumen and Heavy Oil

  • Murray R. Gray(Author)
  • 2015(Publication Date)
  • Pica Pica Press

    (Publisher)

  3 4.2 Mechanisms of Cracking Hydrocarbons Primary upgrading processes must operate at temperatures above 420 ° C to break C–C bonds at useful rates, and the contribution of any catalyst to this process will be limited. The thermal breakage of bonds in liquid- and vapour-phase hydrocarbons occurs via highly activated intermediates, each of which is a free radical with an unpaired electron. The energies required to dissociate these chemical bonds and form these free radicals (Table 4.1) are so high that the active species are never present in high concentrations. Nonetheless, these intermedi-ates participate in chain-reaction processes to yield significant conversions of the feed mixtures. 4.2.1 Reaction steps and kinetics of thermal cracking The elementary reaction steps and detailed kinetics of these chain-reaction processes are best illustrated by the example of cracking n-alkanes in a solvent. For simplicity, the following single-step mechanism is used to illustrate the elementary reactions in thermal cracking of n-alkanes in the vapour phase: Initiation: P k 1  →  2R · (4.4) Propagation: • Hydrogen abstraction: R · + P k 2  →  RH + P · (4.5) • b -Scission: P · k 3  →  R · + O (4.6) Termination: Radical + Radical →  Products (4.7) where P and P · represent the parent alkane and the corresponding parent radi-cals, respectively; R · and RH are lower alkyl radicals and their corresponding 154 Upgrading Oilsands Bitumen and Heavy Oil alkanes; and O represents olefins. Different termination reactions are possible as follows: R · + R · k 4  →  Products (4.8) R · + P · →  Products (4.9) P · + P · →  Products (4.10) Detailed kinetics will be derived below using the termination Reaction (4.8) as an example.

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  The Desulfurization of Heavy Oils and Residua

  • James G. Speight(Author)
  • 1999(Publication Date)
  • CRC Press

    (Publisher)

  The original incen-tive to develop cracking processes arose from the need to increase gasoline sup-plies (Buonara, 1998). Since cracking could virtually double the volume of gaso-line from a barrel of crude oil, the purpose of cracking was wholly justified. The process employs a variety of reactors with bed types varying from fixed beds to moving beds to fluidized beds. The fixed-bed process was the first to be used commercially and uses a static Table 7-8 Refinery Processes that Employ Catalysts Process Materials charged Products recovered Temperature of reaction Type of reaction Cracking Gas oil, fuel oil, heavy feed-stocks Gasoline, gas, and fuel oil 875-975°F 470-525°C Dissociation or splitting of mole-cules Hydrogenation Gasoline to heavy feedstocks Low-boiling products 400-850°F 205-455°C Mild hydrogenation; cracking; re-moval of sulfur, nitrogen, oxy-gen, and metallic compounds Reforming Gasolines, naphthas High-octane gasolines, aro-matics 850-1000°F 455-535°C Dehydrogenation, dehydroisom-erization, isomerization, hydro-cracking, dehydrocyclization Isomerization Butane C4H i 0 Isobutane C4H10 Rearrangement Alkylation Butylene and isobutane, C4H8 and C4H io Alkylate, C8H18 32-50°F 0-10°C Combination Polymerization Butylene, C4H8 Octene, C8H i 6 300-350°F 150-175°C Combination Desulfurization During Refining 275 276 Chapter Seven bed of catalyst in several reactors, which allows a continuous flow of feedstock to be maintained. Thus the cycle of operations consists of (1) flow of feedstock through the catalyst bed, (2) discontinuance of feedstock flow and removal of coke from the catalyst by burning, and (3) insertion of the reactor on-stream. The moving-bed process (Figure 7-14) uses a reaction vessel in which cracking takes place and a kiln in which the spent catalyst is regenerated, catalyst move-ment between the vessels is provided by various means.

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