Alkanes catalytic cracking

Butanes are naturally occurring alkane hydrocarbons that are produced primarily in association with natural gas processing and certain refinery operations such as catalytic cracking and catalytic reforming. The term butanes includes the two stmctural isomers, / -butane [106-97-8] CH2CH2CH2CH2, and isobutane [79-28-9], (CH2)2CHCH2 (2-methylpropane). 

The principal sources of feedstocks in the United States are the decant oils from petroleum refining operations. These are clarified heavy distillates from the catalytic cracking of gas oils. About 95% of U.S. feedstock use is decant oil. Another source of feedstock is ethylene process tars obtained as the heavy byproducts from the production of ethylene by steam cracking of alkanes, naphthas, and gas oils. There is a wide use of these feedstocks in European production. European and Asian operations also use significant quantities of coal tars, creosote oils, and anthracene oils, the distillates from the high temperature coking of coal. European feedstock sources are 50% decant oils and 50% ethylene tars and creosote oils. 

It has been shown, however, that such catalysts may contain protons, either by design or because of the difficulty in removing all traces of moisture, and these protons have been shown to be superacidic with Hammett acidities up to —18. These protons will also play some role in the catalytic activity of these ionic liquids in practical situations. Ionic liquids in which superacidic protons have deliberately been generated by addition of small amounts of water, HCl or H2SO4 have been used to catalytically crack polyethene under relatively mild conditions. The main products are mixed C3-C5 alkanes, which would be a useful feedstock from waste polyethene recycling. In contrast to other cracking procedures no aromatics or alkenes are produced, although small amounts of polycyclic compounds are obtained. 

Catalytic cracking When a mixture of alkanes from the gas oil fraction (C12 and higher) is heated at very high temperature (-500 °C) in the presence of a variety of catalysts, the molecules break apart and rearrange to smaller, more highly branched alkanes containing 5-10 carbon atoms. 

At low temperature (375 and 400 °C), the product distribution obtained with the catalysts is very different from the one obtained under thermal cracking. With the catalytic cracking (ZSM-5), the obtained products are mainly n-alkanes, isomerised alkanes and alkenes with a carbon number between 1 to 6 whereas with the thermal cracking the whole range of n-alkanes with 1 to 9 carbon atoms and the 1 -alkenes with 2 to 10 carbon atoms are observed. This difference of product distribution can easily be explained by the cracking mechanisms. In one hand, the active intermediate is a carbocation and in the other hand it is a radical. 

As the proportions of the various fractions produced do not match consumer demand, catalytic cracking is used to break down larger hydrocarbon molecules in the heavy fractions into smaller molecules, such as those used as gasoline. In industry, the vapour of the alkane being cracked is passed over a heated catalyst of aluminium oxide in the absence of air. 

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

The modem gasolines are produced by blending products from cmde oil distillation, that is, fluid catalytic cracking, hydrocraking, reforming, coking, polymerization, isomerization, and alkylation.Two clear examples of the possible use of solid-acid catalysts in refining processes are the isomerization of lineal alkanes and the alkylation of isobutene with butanes. In both these cases, and due to the octane... 

In practice, short-chain alkanes and alkenes are normally used as feedstock for shape-selective catalytic formation of isooctanes at relatively low temperatures. Until the 1980s, lead alkyls (Section 18.1) were added to most automotive fuels to help suppress engine knock, but they have been phased out in North America because of the chronic toxicity of lead and lead compounds. The most commonly used nonlead antiknock additive is now methyl tert-butyl ether [MTBE CH30C(CH3)3], which is made by the reaction of methanol with 2-methylpropene, (CHs C—CH2 (see Section 7.4). The latter is obtained by catalytic cracking of petroleum fractions to give 1-butene, which is then shape-selectively isomerized on zeolitic catalysts. 

A more detailed interpretation of the chemistry of catalytic cracking was based on studies with pure hydrocarbons.121-123 A simplified summary put forward by Heinemann and coworkers123 (Fig. 2.1) shows how Cg open-chain and cyclic alkanes are transformed to benzene by the action of both the hydrogenating (metal) and acidic (halogenated alumina) functions of the catalyst. 

Alkenes are very reactive under the conditions of catalytic cracking and easily undergo secondary transformations even at low conversions of the initial reactant. Therefore the products have always been found to contain less alkenes than arenes or alkanes. 

Very few alkenes are found in nature. Most of the alkenes used by the petrochemical industry are obtained by breaking up larger, less useful alkane molecules obtained from the fractional distillation of crude oil. This is usually done by a process called catalytic cracking. In this process the alkane molecules to be cracked (split up) are passed over a mixture of aluminium and chromium oxides heated to about 500 °C. 

Catalytic cracking The decomposition of higher alkanes into alkenes and alkanes of lower relative molecular mass. The process involves passing the larger alkane molecules over a catalyst of aluminium and chromium oxides, heated to 500°C. 

Hence, primary products may be obtained which is not the case in catalytic cracking over monofunctional catalysts where formation of carbenium ions occurs by hydride abstraction from the n-alkane rather than via n-alkenes. 

This picture explains the great differences in product distributions of ideal hydrocracking and catalytic cracking. Furthermore, it is in agreement with the observation that carbon number distributions are similar in ideal hydrocracking of n-alkanes and in catalytic cracking of n-alkenes with the same carbon number (19). 

Unsteady-state reactor operation is traditionally considered to be related to the performance of catalytic processes which are characterized by quick loss in catalyst activity. For such processes as, for example, catalytic cracking (Section B.3.10) or dehydrogenation of alkanes (Section B.4.3), a sequence of reaction and regeneration stages is unavoidable and should be included into the design. 

After distillation, catalytic cracking converts some of the less valuable fractions to more valuable products. Catalytic cracking involves heating alkanes in the presence of materials that catalyze the cleavage of large molecules into smaller ones. Cracking... 

Catalytic cracking in the presence of hydrogen to give mixtures of alkanes, (p. 100)... 

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