Cracking, thermal ratio

Activation in industrial plants is usually carried out by means of the feed stream of steam and hydrocarbons and sometimes with hydrogen addition at a high steam-to-carbon ratio (5—10) and at low pressure [389]. The hydrocarbons crack thermally and the resulting hydrogen or carbon acts as an initiator for the reduction process. As soon as metallic nickel is available, the steam reforming process will produce sufficient hydrogen for a quick reduction of the catalyst. 

Carbon black is produced by the partial combustion or the thermal decomposition of natural gas or petroleum distillates and residues. Petroleum products rich in aromatics such as tars produced from catalytic and thermal cracking units are more suitable feedstocks due to their high carbon/hydrogen ratios. These feeds produce blacks with a... 

This paper is concerned with the synthesis of Y zeolite with Si02/Al203 ratio of 4.5 from kaolin taken in Yen Bai-Vietnam and their catal3dic activity for the cracking of n-heptane. The synthesized sample (NaYl) showed the Y zeolite crystallinity of 53% and PI zeolite crystallinity of 32%, and exhibited good thermal stability up to 880 C. The activity and the stability of HYl turned out to be lower than those of standard sample (HYs), but the toluene selectivity was higher. The conversion of n-heptane to toluene might be due to the metal oxide impurities, which was present in the raw materials and this indicates the potential application of this zeolite for the conversion of n-parafRn to aromatics. 

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). 

Different procedures can be used in practice to activate the zeolite, and the choice of a particular method will depend on the catalytic characteristics desired. If the main objective is to prepare a very active cracking catalyst, then a considerable percentage of the sodium is exchanged by rare earth cations. On the other hand, if the main purpose is to obtain gasoline with a high RON, ultrastable Y zeolites (USY) with very low Na content are prepared. Then a small amount of rare earth cations is exchanged, but a controlled steam deactivation step has to be introduced in the activation procedure to obtain a controlled dealumination of the zeolite. This procedure achieves a high thermal and hydrothermal stability of the zeolite, provided that silicon is inserted in the vacancies left by extraction of A1 from the framework (1). The commercial catalysts so obtained have framework Si/Al ratios in the... 

For the sake of brevity, the yield data for all the individual components are not reported in Table II and subsequent tables. The yield of unreported components (usually Cs-i- olefins and naphthenes) can be calculated as 100 minus percentage yield of the reported components. Results shown in Table ll indicate that thermally treated ZSM-5 produced a high yield of Ce to Ce aromatics, Cs and C4 hydrocarbons. Steam treatment of ZSM-5 reduced cracking activity and increased the selectivity for Cs to C aliphatics at the expense of aromatics. The olefin to paraffin ratio in the product also increased upon steaming. 

H6]dimethylcyclobulane (4) was pyrolyzed at 338 401 °C under initial pressures of 10 and 30 Torr. The results arc inconclusive, showing that there is less than 4% racemization and no geometric isomerization in the recovered cyclobutane after its 11.7% cleavage to 2-methyl-propene.74 However, it appears that the ratio of the rate constants for the thermal cracking to geometric isomerization is a function of substituent size, as evidenced by the fact that dimethyl... 

COMBUSTOR OVER-ALL FUEL-AIR RATIO. In general, coke and smoke both increase with increasing fuel-air ratio, although some investigations have shown that smoke can attain a peak point beyond which it decreases. However, the location of this peak value was variable and dependent on other factors. These fuel-air ratio effects can be attributed to more fuel wash on surfaces, richer local fuel-air ratios, and increased thermal cracking of the fuel. Increased burning and erosion might lower coke and smoke formation, however. 

Figure 1.5 shows ways of designing tubular reactors to include heat transfer. If the amount of heat to be transferred is large, then the ratio of heat transfer surface to reactor volume will be large, and the reactor will look very much like a heat exchanger as in Fig. 1.5b. If the reaction has to be carried out at a high temperature and is strongly endothermic (for example, the production of ethylene by the thermal cracking of naphtha or ethane—see also Section 1.7.1, Example 1.4), the reactor will be directly fired by the combustion of oil or gas and will look like a pipe furnace (Fig. 1.5c).

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