Cracking of olefins

Petroleum-derived benzene is commercially produced by reforming and separation, thermal or catalytic dealkylation of toluene, and disproportionation. Benzene is also obtained from pyrolysis gasoline formed ia the steam cracking of olefins (35). 

Light hydrocarbons (Ci to C4) and aromatics (mainly Ce to Ce) were produced by ZSM-5 due to the the conversion of olefins and paraffins. Thus,these results provide evidence for cracking of olefins, paraffins and cyclization of olefins by ZSM-5 at 500 C. The steam deactivated ZSM-5 catalyst exhibited reduced olefin conversion and negligible paraffin conversion activity. 

We use here a simplified kinetic model of cracking reactions in order to illustrate the role of secondary cracking steps on product distribution. More detailed and rigorous models are available but the additional rigor is not essential to describe the concepts that we illustrate here. We assume that sites within transport-limited pellets (all kinetic-transport parameters as in Figs. 16, 17, and 19) catalyze the cracking of olefins with a probability given by... 

The previous results underline the importance of shape selectivity effects even for the transformation of small olefins such as butene. The results are in agreement with the early, related work by Haag et al. 59). who investigated cracking of olefins and paraffins catalyzed by the zeolite HZSM-5 and distinguished between restricted transition state shape selectivity and mass transport shape selectivity, ft is clear that the effects discussed here are best described in terms of restricted transition state shape selectivity. 

Tihe cracking of olefinic hydrocarbons has not been as well studied as the cracking of paraffins. V. V. Voevodsky theorized (25) that the allyl radicals, in addition to decomposition and addition to double bonds, can enter into disproportion with the starting olefin. The reaction results in the formation of diene and alkyl radicals of the type shown in Reaction 1 (Voevodsky Reaction)... 

As can be seen, there are two possible ways in which the chain-initiating carbonium ion is formed. If the first way, involving the formation of the catalyst-carbon linkage by loss of hydride ion from a paraffin is very rapid, then we should expect that paraffins should crack as readily as olefins, since the same intermediate structure is formed. Yet we do know that the catalytic cracking of olefins occurs at considerably lower temperatures than that of paraffins. Thus, it appears that the explanation given by Thomas (4) involving the formation of a small amount of olefin to serve as the proton acceptor is the more likely one. 

The cracking of olefins has been discussed to a certain extent in the part dealing with the cracking of paraffins. It was pointed out that, in general, olefins undergo cracking much more readily than paraffins. 

Greensfelder, B.S. and Voge H.H., "Catalytic Cracking of Pure Hydrocarbons. Cracking of Olefins", Ind. Eng. Chem. 37(10), 983-988 (1945b). 

Buchanan JS, Santiesteban JG, Haag WO. Mechanistic considerations in acid-catalyzed cracking of olefins. J Catal 1996 158 279-87. 

Olefins are produced primarily by thermal cracking of a hydrocarbon feedstock which takes place at low residence time in the presence of steam in the tubes of a furnace. In the United States, natural gas Hquids derived from natural gas processing, primarily ethane [74-84-0] and propane [74-98-6] have been the dominant feedstock for olefins plants, accounting for about 50 to 70% of ethylene production. Most of the remainder has been based on cracking naphtha or gas oil hydrocarbon streams which are derived from cmde oil. Naphtha is a hydrocarbon fraction boiling between 40 and 170°C, whereas the gas oil fraction bods between about 310 and 490°C. These feedstocks, which have been used primarily by producers with refinery affiliations, account for most of the remainder of olefins production. In addition a substantial amount of propylene and a small amount of ethylene ate recovered from waste gases produced in petroleum refineries. 

Certain CFCs are used as raw materials to manufacture key fluorinated olefins to support polymer apphcations. Thermolysis of HCFC-22 affords tetrafluoroethylene and hexafluoropropylene [116-15 ] under separate processing conditions. Dechlorination of CFC-113 forms chlorotrifluoroethylene [79-38-9]. Vinyhdene fluoride [75-38-7] is produced by the thermal cracking of HCFC-142b. 

Aromatic. Aromatic feedstreams (C-8, C-9, C-10) derived from the steam cracking of petroleum distillates are composed of styrene, iadene, vinyltoluenes (eg, meta- and i ra-methylstyrene), and their respective alkylated analogues. A typical aromatic feedstream might contain 50% reactive olefins with the remainder being alkylated benzenes and higher aromatics. 

Wax Cracking. One or more wax-cracked a-olefin plants were operated from 1962 to 1985 Chevron had two such plants at Richmond, California, and Shell had three in Europe. The wax-cracked olefins were of limited commercial value because they contained internal olefins, branched olefins, diolefins, aromatics, and paraffins. These were satisfactory for feed to alkyl benzene plants and for certain markets, but unsatisfactory for polyethylene comonomers and several other markets. Typical distributions were C 33% C q, 7% 25% and 35%. Since both odd and... 

Thermal polymerization is not as effective as catalytic polymerization but has the advantage that it can be used to polymerize saturated materials that caimot be induced to react by catalysts. The process consists of the vapor-phase cracking of, for example, propane and butane, followed by prolonged periods at high temperature (510—595°C) for the reactions to proceed to near completion. Olefins can also be conveniendy polymerized by means of an acid catalyst. Thus, the treated olefin-rich feed stream is contacted with a catalyst, such as sulfuric acid, copper pyrophosphate, or phosphoric acid, at 150—220°C and 1035—8275 kPa (150—1200 psi), depending on feedstock and product requirement. 

Polyisobutylene has the chemical properties of a saturated hydrocarbon. The unsaturated end groups undergo reactions typical of a hindered olefin and are used, particularly in the case of low mol wt materials, as a route to modification eg, the introduction of amine groups to produce dispersants for lubricating oils. The in-chain unsaturation in butyl mbber is attacked by atmospheric ozone, and unless protected can lead to cracking of strained vulcanizates. Oxidative degradation, which leads to chain cleavage, is slow, and the polymers are protected by antioxidants (75). 

Fired reactors contain tubes or coils in which an endothermic reaction within a stream of reac tants occurs. Examples include steam/ hydrocarbon reformers, catalvst-filled tubes in a combustion chamber pyrolyzers, coils in which alkanes (from ethane to gas oil) are cracked to olefins in both types of reac tor the temperature is maintained up to 1172 K (1650°F). 

Separation of olefins produced by cracking operations and subsequent conversion. This is the major route to aliphatic petrochemicals. 

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