Radical steam cracking

Steam Cracking. Steam cracking is a nonselective process that produces many products from a variety of feedstocks by free-radical reactions. An excellent treatise on the fundamentals of manufacturing ethylene has been given (44). Eeedstocks range from ethane on the light end to heavy vacuum gas oil on the heavy end. All produce the same product slate but in different amounts depending on the feedstock. 

Propene and 1-butene, respectively, are produced in this free radical reaction. Higher hydrocarbons found in steam cracking products are probably formed through similar reactions. 

Only about 3wt% of ethane is observed in the steam cracking products, indicating that the formation of ethylene is the preferential fate of ethyl radicals. Note that most of ethane is formed via ethyl radical H-abstraction reactions, while less than 10% is due to the recombination reaction of methyl radicals. Similarly, propane formation is mostly due to the H-abstraction reactions of propyl radicals and only marginally to the recombination of methyl and ethyl radicals. 

In the temperature range of interest for steam cracking, isomerization and decomposition reactions compete and decomposition prevails only at temperatures higher than 1,100 K, while the 1-5 isomerization reaction dominates at temperatures lower than 900 K. Moreover, alkyl radicals can undergo competitive dehydrogenation reactions. 

As previously discussed, alkyl radicals decomposition reactions constitute an important fate and reaction path of alkyl radicals. Due to the very short lifetimes of alkyl radicals, Rice and Herzfeld (1933, 1934) suggested a complete decomposition mechanism where all the radicals larger than methyl were considered instantaneously decomposed into alkenes and H and CH3 radicals. In this mechanism, all the intermediate alkyl radicals decompose to directly form alkenes and smaller alkyl radicals. This would mean that the final ethylene production from a steam cracking process would be significantly overestimated when compared with the experimental measurements. For instance, the net and final result of the successive decomposition mechanism of 1-decyl radical would be 5 moles of ethylene and one H radical. 

The present studies have been concerned with the overall effect of surfaces on reactions occuring during steam cracking. The formation of gaseous and solid products has been related to the nature of the reactor surface. Steam cracking is, however, a high temperature pyrolysis reaction in which free radical intermediates play an important role. No attempt has been made to relate the experimental results to the nature and amount of free radicals present in the system. 

The results show clearly the importance of the chemical nature of the actual surface in determining the product spectrum observed during steam cracking. The surface has an important bearing on both the solid and gaseous products of the reaction. It would be very interesting to establish the exact mechanism by which this occurs, but the free radical intermediates involved are not amenable to study using the present system. 

Not all petrochemical processes are catalytic—the steam cracking of hydrocarbons to lower olefins is a thermal process at 700 to 800°C or more. However, excluding free-radical polymerization processes, this is a rare example, though severe conditions may still be required in some catalysed processes on thermodynamic grounds or to achieve acceptable rates (several mol h per litre of reaction volume). As we shall see in this and the following chapter, the major impact of catalysis is to provide a remarkably wide range of products from a small number of building blocks. 

Steam cracking Naphtha b.p. 30-190°C steam - 1 1 Ethane None 750-900°C Free radical chain, C-C cleavage. Ethene, Propene Butenes, Butadiene... 

Fulmer fields. In W. Europe and Japan, however, ethane is not available in sufficient quantity and naphtha has to be used. In the steam cracking of naphtha alkyl radicals are first formed by homolysis of C—C bonds and these then decompose to alkenes and smaller radicals. 

Ra = initial radical, or (b) disproportionation, exemplified for the reaction of two butyl radicals relevant in steam cracking. 

Steam cracking is an endothermic, radical, high-temperature reaction that is carried out in a tubular reactor, the cracking furnace. 

Steam cracking takes place without a catalyst at temperatures up to 900 °C. The process is complex, although it undoubtedly involves radical reactions. The high-temperature reaction conditions cause spontaneous homolytic breaking of C-C and C-H bonds, with resultant formation of smaller fragments. We might imagine, for instance, that a molecule of butane splits into two ethyl... 

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. 

In conclusion, we believe that cracking of gas-oil is taking place on zeolites via carbonium, carb mium ions and radicals. In the case of steam dealujninated samples, when more than 5 Al per unit cell (Si/Al<30) are present in the framework of the zeolite, the ionic mechanism is much more important than 1 he radical one. When the framev ork aluminum decreases and the number of defects increases, the radical mechanism becomes operative and eventually dominant when practically no aluminum is present in l he zeolite framework and superacid Brdnsted sites (at 3610 cm ) are not present. 

In heavily steam-dealuminated zeolites, most of the activity should come from the EFAL that is concentrated on the external siirface (3,4). We believe that Levyis acidity, which can stabilize radicals, can play an important role in the radical cracking observed with strongly steam-dealuminated HY zeolites. Furthermore,... 

These results show that, together with the typical carbocation cracking, a radical cracking mechanism is also taking place, especially on highly steam dealuminated zeolites. This radical mechanism, which will become proportionally more important when the framework aluminium content of the zeolite will decrease, could explain the strong increase in ethylene formation and decrease in branched/unbranched C4 ratio when decreasing the unit cell size of the zeolite. 

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