Thermal cracking features

Dynacracking A petroleum cracking process which combines the best features of the "catalytic cracking and Thermal cracking processes. It converts heavy oil feedstocks to fuel gas, gasoline, and fuel oil. No catalyst is used. Developed in the 1950s by Hydrocarbon Research, but not commercialized. 

One of the significant features of hydrocarbon free radicals is their resistance to isomerization, for example, migration of an alkyl group and, as a result, thermal cracking does not produce any degree of branching in the products other than that already present in the feedstock. 

Figure 7.13 presents a simplified flowsheet, which concentrates the essential features the balanced VCM technology, as conceptually developed in the previous sections, but this time with the three plants and recycles in place chlorination of ethylene (Rl), thermal cracking of EDC (R2) and oxyclorinahon of ethylene (R3). As mentioned in Section 7.3, from plantwide control three impurities are of particular interest (I]) chloroprene (nbp 332.5 K), (12) trichloroethylene (nbp 359.9K), and (13) tetrachloromethane (nbp 349.8). I, and 12 are bad , since the first can polymerize and plug the equipment, while the second favors the coke formation by EDC pyrolysis. On the contrary, I3 has a catalytic effect on the VCM formation, in some patents being introduced deliberately. 

Thermal cracking of organic substances is an important reaction in the petroleum industry and has been extensively studied for over seventy years. At least for simple alkanes, the decay is first order in good approximation and therefore was long believed to occur in a single, unimolecular step [21]. However, in the 1930s, Rice and coworkers [22-24] established the presence of free radicals under the conditions of the reaction by means of the Paneth mirror technique [25,26], This observation led Rice and Herzfeld to propose a chain mechanism [22,27,28], Extensive later studies proved the essential features of their mechanism to be correct not only for hydrocarbons, but also for many other types of organic substances. 

A lluidized-bed pyrolysis reactor is the most suitable for thermal cracking of MWP to obtain liquid/waxy product. Key features of the lluidized-bed pyrolysis include [3, 20] ... 

A common feature of these catalysts is their acidic nature (i.e., they all act as solid phase acids in the hot gas oil vapor stream). Synthetic silica/alumina catalyst composites, for example, have an acidity of 0.25 mEq/g distributed over an active surface area of some 500m /g). This acidity is the key feature that distinguishes catalytic cracking from straight thermal cracking. 

In a review article, Benson [29] distinguishes two broad categories of chain reactions that have somewhat different kinetic features pyrolytic chains, containing a unimoiecular step, and metathetical chains involving two reactants and only bimolecular steps. We consider the interesting and practical case of thermal cracking, or pyrolysis, to illustrate the principles. 

Fig. 8 presents a simplified flowsheet, which concentrates the essential features of an industrial process, but does not reproduce a particular technology. The chlorination of ethylene to DCE takes place in a gas-liquid type reactor (Rl). After purification, the thermal cracking of DCE takes place in a tubular reactor (R2). The by-product HCl is recovered as DCE by oxyclorination of ethylene in a fluid bed type (R3). The flowsheet may consider the processing of an external DCE stream. Thus, the amounts of ethylene and chlorine are balanced to consume the entire HCl produced in the cracking section. 

As newer and more powerful engines have been developed, higher octane gasoline has been requested. Moreover, aircraft engines needed fuel with octane ratings of nearly 100. Such features could not be achieved with the traditional thermal cracking process. Hence, the... 

V/ith considerable sliding rates ( >10 m/sec) and friction couple pressure ( > 1 MPa) chromium steels eind all kinds of surfacing are liable to thermal cracking effected by temperature gradients. Under these conditions hard alloys BK3, BK4, BK6 and BK8 give satisfactory results. The specific feature of these materials is that under conditions of liquid contact both elements of the couple are manufactured of hard... 

If one examines the evolution of new zeolite structures over the past decade the most interesting discoveries have been made with high silica compositions. Many of these phases can be prepared in essentially all silica forms. Purists would prefer to classify such molecular sieves as organosilicates or porosils (1), in part because the physical properties differ from more classical low Si/Al ratio zeolites. In particular, the high silica zeolites tend to be more thermally stable and chemically robust. Additionally, the higher the Si/Al ratio the more hydrophobic the zeolite. These features are desirable for catalysts that may be used in catalytic processes such as cracking (3). 

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