Hydrocarbons, “atomic cracking

The production of hot radicals and molecules by chemical reaction was recognized by early workers.4 Two important instances were the deuterium-methane exchange mechanism of Taylor and co-workers,5 and the atomic-cracking degradation reaction of hydrocarbons, suggested by Steacie and co-workers.2 Equation (I), in which one of the radicals, Ri and R2, may be a hydrogen atom, generalizes the possibilities in these two processes ... 

Higher hydrocarbons. Thermal cracking of higher hydrocarbons is believed to occur with Rice-Herzfeld-type mechanisms [45,46], Of course, with more carbon atoms in the molecule, more free radicals of different carbon numbers appear and produce a greater variety of products. As a still relatively simple example, the network of principal steps in cracking of n-butane is [47,48] ... 

In the heating and cracking phase, preheated hydrocarbons leaving the atomizer are intimately contacted with the steam-preheated oxygen mixture. The atomized hydrocarbon is heated and vaporized by back radiation from the flame front and the reactor walls. Some cracking to carbon, methane, and hydrocarbon radicals occurs during this brief phase. 

Cracking, or beta-scission, is a key feature of ionic cracking. Beta-scission is the splitting of the C-C bond two carbons away from the positive-charge carbon atom. Beta-scission is preferred becau.se the energy required to break this bond is lower than that needed to break the adjacent C-C bond, the alpha bond. In addition, short-chain hydrocarbons are less reactive than long-chain hydrocarbons. The rate of... 

Fortunately, we have a different set of carbon starting materials available to us, derived from nature but inaccessible to her hydrocarbons, which are more advanced than carbon dioxide. Most come from the oil left for us by prehistoric lifeforms. The petrochemical industry cracks it into smaller unsaturated blocks containing at most eight carbon atoms, from which almost 90% (by weight) of all useful synthetic organics are made (Scheme 12.2) [4],... 

Figure 7.7b shows the essential features of a refinery catalytic cracker. Large molar mass hydrocarbon molecules are made to crack into smaller hydrocarbon molecules in the presence of a solid catalyst. The liquid hydrocarbon feed is atomized as it enters the catalytic cracking reactor and is mixed with the catalyst particles being carried by a flow of steam or light hydrocarbon gas. The mixture is carried up the riser and the reaction is essentially complete at the top of the riser. However, the reaction is accompanied by the deposition of carbon (coke) on the surface of the catalyst. The catalyst is separated from the gaseous products at the top of the reactor. The gaseous products leave the reactor... 

Hydrogenolysis of 2-methylpentane, hexane, and methylcyclopentane has been also studied on tungsten carbide, WC, a highly absorptive catalyst, at 150-350 °C in a flow reactor [80], These reforming reactions were mainly cracking reactions leading to lower molar mass hydrocarbons. At the highest temperature (350 °C) all the carbon-carbon bonds were broken, and only methane was formed. At lower temperatures (150-200 °C) product molecules contained several carbon atoms. 

H2, hydrogen, is a colorless, odorless, tasteless, nonpolar, diamagnetic, diatomic gas with the lowest atomic weight and density of any known substance. It has low solubility in water and is very flammable. Hydrogen is prepared by reactions of metals with water, steam or various acids, electrolysis of water, the water gas reaction and thermal cracking of hydrocarbons. It combines with metals and nonmetals to form hydrides. 

They are not found to any extent in natural products, but are produced in the destructive distillation of complex natural substances, such as coal, and are formed in large amounts in petroleum refining, particularly in the cracking process. The first member of the series is ethylene, C2H4. The dienes contain two double bonds between pairs of carbon atoms in the molecule. They are related to the complex hydrocarbons in natural rubber and are important in the manufacture of synthetic rubber and plastics. The most important members of this series are butadiene, C4H5 and isoprene, CsHg. 

Olefins are hydrocarbon compounds with at least two carbon atoms and having a double bond. Their unstable nature and tendency to polymerize makes them one of the very important building blocks for the chemical and petrochemical industry (Gary and Handwerk, 1994). Although olefins are produced by fluid catalytic cracking in refineries, the main production source is through steam cracking of liquefied petroleum gas (LPG), naphtha or gas oils. 

In a cracking process the long, saturated, hydrocarbon chains in the crude oil are transformed into shorter ones, which, for lack of hydrogen atoms, contain double bonds, (-C=C ), which provide polymerisability. 

Fig. 1. Conversion of 1-methylnaphthalene (Xi M.Np) and product yields (Y on zeolite HY at 200 °C and 300 °C (Tr = transaUwlation product i. e., naphthalene and dlmethylnaphtha-lenes Cr = cracked products with less than 11 carbon atoms Qi = hydrocarbons with 11 carbon atoms).

Rings possessing three or fewer carbon atoms attached to the ring are quite resistant to cracking long hydrocarbon chains are usually broken and freed from the aromatic ring... 

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