Cracking (Chemistry)
Cracking in chemistry refers to the process of breaking down large hydrocarbon molecules into smaller, more useful ones. This is typically done through the application of heat and pressure, and it is an important step in the production of fuels and other petroleum-based products. The resulting smaller molecules can be used for various industrial and commercial purposes.
7 Key excerpts on "Cracking (Chemistry)"
eBook - ePub- James G. Speight(Author)
- 2019(Publication Date)
- Gulf Professional Publishing(Publisher)
...Therefore, gasoline that was not originally in the unrefined crude oil could be manufactured. Later processes, designed to raise the yield of gasoline from crude oil, decomposed higher molecular weight constituents into lower molecular weight products by processes known as cracking. And like typical gasoline, several processes produce the blending stocks for gasoline manufacture (Fig. 3.2). Thermal cracking, employing heat and high pressures, was introduced in 1913 but was accompanied in refineries after 1937 by catalytic cracking, the application of catalysts that facilitates chemical reactions producing more gasoline. Thermal cracking processes (Table 3.6) for converting the high-boiling fraction of crude oil to lower-boiling products still play an important role in the modern refinery through upgradation of heavy residue and improving the economics of the refinery through the production of lower-boiling distillates and other valuable products such as hydrocarbon gases and petroleum coke. The predominant reactions of thermal cracking are: (i) cracking of side chains aromatic nuclei, (ii) dehydrogenation of naphthene derivatives to form aromatic derivatives, (iii) condensation of aliphatic derivatives to form aromatic products, (iv) condensation of aromatic derivatives to form higher molecular weight aromatic derivatives, and (v) dimerization or oligomerization...
eBook - ePub- James G. Speight(Author)
- 2010(Publication Date)
- Gulf Professional Publishing(Publisher)
...This is achieved by using high pressures and temperatures without a catalyst, or lower temperatures and pressures in the presence of a catalyst. The source of the large hydrocarbon molecules is often the naphtha fraction or the gas oil fraction from the fractional distillation of petroleum. These fractions are obtained from the distillation process as liquids, but are re-vaporized before cracking. In the process, the high-molecular-weight hydrocarbons are decomposed in a random manner to produce mixtures of lower-molecular-weight hydrocarbons, some of which have carbon–carbon double bonds. In thermal cracking, high temperatures (typically in the range of 450–750°C) and pressures (up to about 70 atmospheres) are used to break the large hydrocarbons into smaller ones. Thermal cracking gives mixtures of products containing high proportions of hydrocarbons with double bonds – alkenes. Thermal cracking does not involve ionic intermediates but the carbon–carbon bonds are broken so that each carbon atom ends up with a single electron free radical. Two general types of reaction occur during cracking: 1. The decomposition of large molecules into small molecules (primary reactions): 2. Reactions by which some of the primary products interact to form higher-molecular-weight materials (secondary reactions): Thermal cracking is a free radical chain reaction; a free radical is an atom or group of atoms possessing an unpaired electron. Free radicals are very reactive, and it is their mode of reaction that actually determines the product distribution during thermal cracking. A free radical reacts with a hydrocarbon by abstracting a hydrogen atom to produce a stable end product and a new free radical. Free radical reactions are extremely complex, and it is hoped that these few reaction schemes illustrate potential reaction pathways...
eBook - ePubOil
A Beginner's Guide
- Vaclav Smil(Author)
- 2017(Publication Date)
- Oneworld Publications(Publisher)
...The industry needed a process that would break C-C bonds to produce lighter compounds and catalytic cracking provided the solution. CATALYTIC CRACKING The first breakthrough in producing lighter products from heavier feedstocks came in 1913 when William M. Burton patented thermal cracking of crude oil. Burton’s process relied on the combination of heat and high pressure to break heavier hydrocarbons into lighter fractions. A year later Almer M. McAfee patented the first catalytic cracking process that became commercially available by 1923: crude oil was heated in the presence of aluminum chloride, a compound able to break long-chained hydrocarbon molecules into shorter, more volatile compounds, and gasoline yield was as much as 15% higher compared to thermal cracking. But because the relatively expensive catalyst could not be recovered and reused, thermal cracking (less effective but simpler and cheaper) remained dominant until 1936, when Sun Oil’s Pennsylvania refinery in Marcus Hook installed the first catalytic cracking unit designed by Eugène Houdry to produce high-octane gasoline. Houdry’s fixed-bed process allowed for the recovery of the catalyst but it required a temporary shutdown of the refining operation while the aluminosilicate catalytic compound was regenerated. Soon afterwards Warren K. Lewis and Edwin R. Gilliland replaced Houdry’s fixed catalyst with a more efficient moving-bed arrangement whereby the catalyst circulated between the reaction and the regeneration vessels. This process boosted gasoline yields by 15% and by 1942 90% of all aviation fuel produced for the US war effort was made using this system. An even higher yield was achieved with the invention of powdered catalyst suspended in the air stream (and behaving like a fluid) by four Standard Oil chemists in 1940. Fluid catalytic cracking (FCC) takes place in a reactor under high temperature (540°C) in less than four seconds...
eBook - ePub- Jacob A. Moulijn, Michiel Makkee, Annelies E. van Diepen(Authors)
- 2013(Publication Date)
- Wiley(Publisher)
...Factors contributing to its growing use are the increasing demand for transportation fuels, especially diesel, and the decline in the heavy fuel oil market. The increasing need for the production of clean fuels has also had a significant impact. As the name implies, hydrocracking involves the cracking of an oil fraction into lighter products in the presence of hydrogen. This distinguishes the process from the FCC process (Section 3.4.2), which does not have hydrogen in the feed, and from the hydrotreating process (Section 3.4.5.1), in which virtually no C–C bond breaking takes place. Hydrocracking is a very versatile and flexible process that can be aimed at the production of naphtha or at the production of middle distillates, namely jet and diesel fuel. Although at first sight it might be expected that hydrocracking competes with fluid catalytic cracking, this is certainly not the case; the processes are complementary. The fluid catalytic cracker takes the more easily cracked alkane-rich atmospheric and vacuum gas oils as feedstocks, while the hydrocracker mainly uses more aromatic feeds, such as FCC cycle oils and distillates from thermal cracking processes, although it also takes heavy atmospheric and vacuum gas oils and deasphalted oil. QUESTION: Will gasoline-range products from hydrocracking contain more or less sulfur than those from FCC? 3.4.5.2.1 Reactions and Thermodynamics Hydrocracking can be viewed as a combination of hydrogenation and catalytic cracking. The former reaction is exothermic while the latter reaction is endothermic...
eBook - ePubPetrochemistry
Petrochemical Processing, Hydrocarbon Technology and Green Engineering
- Martin Bajus(Author)
- 2020(Publication Date)
- Wiley(Publisher)
...The product distribution depends on the feedstock and on processing conditions. These conditions are determined by thermodynamic and kinetic factors (Sections 1.2.4 – 1.2.5). 7.1.2 Thermodynamics In general, lower alkenes, especially ethene, propene, and butadiene, are the desired products of SC. From a thermodynamic view, the reaction temperature should be high for sufficient conversion. The forward reaction is also favored by a partial pressure of the alkanes, because for every molecule converted, two molecules are formed. A process under vacuum would be desirable in this respect. In practice, it is more convenient to apply dilution with steam, which has essentially the same effect. 7.1.3 Mechanism The reactions involved in thermal cracking of hydrocarbons are quite complex and involve many radical steps (Section 1.2.8.2). The thermal cracking reaction proceeds via a free‐radical mechanism. Two types of reactions are involved: (i) primary cracking, where the initial formation of alkane and olefin takes place; and (ii) secondary cracking, where light products rich in olefins are formed. The total cracking reactions can be grouped as follows: Initiation reaction Propagation reaction Addition reaction Isomerization reaction Termination reaction Molecular cyclization reaction 7.1.4 Kinetics First‐order kinetics implies the rate of reaction of alkanes. The reactivity of alkanes increases with chain length...
- Dan Bahadur Pal, Pardeep Singh, Dan Bahadur Pal, Pardeep Singh(Authors)
- 2022(Publication Date)
- CRC Press(Publisher)
...Catalytic cracking in economic process associates connecting a reactant which can be a gas oil fraction with a catalyst under suited position of heat (temperature), tension (pressure) and residence time. Through this process a considerable part (more than 50%) of the reactant is transformed into gasoline or lower-boiling-point products normally during a single-pass application. While carrying out the cracking process carbonaceous substance is usually accumulated on the catalyst, considerably weakening its action and replacement such deposition is necessary. The carbonaceous content accumulates arises from the thermal decaying of polar species of high molecular weight in the feed supplied. The elimination of the retentate from the catalysts is normally done by heating in the company of air as far as activity of catalyst is restored. Weight hourly space velocity (WHSV) is defined as the weight of feed flowing per unit weight of the catalyst per hour. 7.2.4 Biofuels Biofuels are sustainable power source (energy source) derived from wastes or biological substances, which can play beneficial act cutting down carbon dioxide discharge [ 21 ]. They are one of the biggest sources of sustainable power in today world. In the transport sector, biofuels are blended with diesel and gasoline. Biodiesels are alkyl esters long chain fatty acids. Such esters are received by vegetable oil through transesterification with ethanol or methanol. Biofuels which are obtained by inedible oil have good capacity of being alternate fuel. Biodiesels can be enticing alternate fuel as they are eco-friendly and can be obtained by treatment of non-edible or edible oils. A wide collection of plants which produce inedible oil are treated by chemical process for production of biodiesel. Few inedible oils are silk cotton tree, jatropha, mahua oil or madhuca indica, microalgae, rubber seed, karanja, neem etc. which are easily accessible in advance nations and are cheaper comparatively than edible oils...
- Robert J. Farrauto, Lucas Dorazio, C. H. Bartholomew(Authors)
- 2020(Publication Date)
- Wiley-AIChE(Publisher)
...The fluidized process allows for excellent heat transfer. The active life of the catalyst is only on the order of a second because of excessive coke deposition. The deactivated catalyst particles are separated from the product in a cyclone and transported into a separate reactor where they are regenerated with a limited amount of air. The regenerated catalyst is mixed with the incoming feed that is preheated by the heat of combustion of the coke. Coking, by olefin polymerization reactions, has activation energy of 40 kJ/mol, while the desired cracking reaction has activation energy two times higher (80 kJ/ mol). Thus, coking is kinetically favored and must be factored into the overall process. This is accomplished by utilizing the heat of combustion of the coke to preheat the feed. The gaseous products are distilled according to their boiling ranges as shown in Figure 10.7. 10.4.2 Hydrocracking This is usually used for heavy feeds such as vacuum oils and oil sands that are highly deficient in H 2 that must be reduced in size and increased in hydrogen content. Multiringed polyaromatic hydrocarbons are to be cracked to monoaromatic or substituted cyclohexane molecules with an increase in hydrogen content to be used for transportation or heating. Two different types of catalysts are used: (1) (Co, Mo)/Al 2 O 3 and (2) Pt and/or Pd ion exchanged onto zeolites (faujasite, mordenite, or ZSM-5). The presence of high-pressure hydrogen significantly reduces coke formation and thus extends the time between regenerations. 10.5 NAPHTHA REFORMING In the gasoline-fueled internal combustion engine, maximum power occurs when the fuel–air mixture combusts at maximum compression. This occurs when the piston reaches the top of the cylinder. By design, combustion occurs by action of the spark plug, which ignites the mixture at maximum compression. However, the fuel and air mixture temperature increases as it is adiabatically compressed...
