Cracking, thermal selective

Catalytic dewaxiag (32) is a hydrocrackiag process operated at elevated temperatures (280—400°C) and pressures, 2,070—10,350 kPa (300—1500 psi). However, the conditions for a specific dewaxiag operatioa depead oa the aature of the feedstock and the product pour poiat required. The catalyst employed for the process is a mordenite-type catalyst that has the correct pore stmcture to be selective for normal paraffin cracking. Platinum on the catalyst serves to hydrogenate the reactive iatermediates so that further paraffin degradation is limited to the initial thermal reactions. 

A second method for synthesis of aHyl chloride is thermal dehydrochlorination, ie, cracking, of 1,2-dichloropropane, but this method is generally less satisfactory because of low aHyl chloride selectivity (50—60%) and operating temperatures of 500—600°C (4,7—10). The by-products of cracking are 1-chloropropene and 2-chloropropene, which have no significant commercial use. 

Cracking temperatures are somewhat less than those observed with thermal pyrolysis. Most of these catalysts affect the initiation of pyrolysis reactions and increase the overall reaction rate of feed decomposition (85). AppHcabiUty of this process to ethane cracking is questionable since equiUbrium of ethane to ethylene and hydrogen is not altered by a catalyst, and hence selectivity to olefins at lower catalyst temperatures may be inferior to that of conventional thermal cracking. SuitabiUty of this process for heavy feeds like condensates and gas oils has yet to be demonstrated. 

Selection of feedstock for thermal cracking to ethylene by linear programming... 

It is important to separate catalyst and vapors as soon as they enter the reactor. Otherwise, the extended contact time of the vapors with the catalyst in the reactor housing will allow for non-selective catalytic recracking of some of the desirable products. The extended residence time also promotes thermal cracking of the desirable products. 

Post-riser hydrocarbon residence time leads to thermal cracking and non-selective catalytic reactions. These reactions lead to degradation of valuable products, producing dry-gas and coke at the expense of... 

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. 

The effect of conversion on the structure of an asphaltene molecule has been reported to depend on the operating conditions and on the presence or not of a catalyst. The effect of thermal processing reaction of a vacuum residue resulted in the selective cracking of the aliphatic or naphthenic side chains of the molecule, leaving the highly condensed aromatic core structure almost intact (see Fig. 16) [116]. 

The development of composite micro/mesoporous materials opens new perspectives for the improvement of zeolytic catalysts. These materials combine the advantages of both zeolites and mesoporous molecular sieves, in particular, strong acidity, high thermal and hydrothermal stability and improved diffusivity of bulky molecules due to reduction of the intracrystalline diffusion path length, resulting from creation of secondary mesoporous structure. It can be expected that the creation of secondary mesoporous structure in zeolitic crystals, on the one hand, will result in the improvement of the effectiveness factor in hydroisomerization process and, on the other hand, will lead to the decrease of the residence time of products and minimization of secondary reactions, such as cracking. This will result in an increase of both the conversion and the selectivity to isomerization products. 

Prior to 1938, gasoline was obtained from thermal-cracking plants then the Houdry fixed-bed catalytic cracking process led to the development of a fluidized-bed process by Standard Oil for the catalytic production of motor fuels (4-8). Acid-treated clays of the montmorilIonite type were the first fluid-cracking catalysts widely employed by the industry. However, the ever greater demand for aviation fuels during the 1939-1945 period prompted the search for more active and selective catalysts. Research on novel catalyst... 

The effect of steam treatment of ZSM-5 on its cracking activity and selectivity was measured with experiments using n-hexadecane feed. With the thermally treated ZSM-5 catalyst, concentration of the unconverted n-hexadecane in the product was not measurable while 507e of the feed was unconverted with the steam treated ZSM-5 catalyst (Table II). The lower limit of conversion with the thermally treated catalyst corresponding to detection limit of n-hexadecane is 99.99%. This lower limit suggests at least an order of magnitude reduction in apparent first order rate constant of the ZSM-5 catalyst upon steam treatment. The small reduction in crystallinity upon steaming cannot fully explain the dramatic activity loss. Loss of active sites due to dealumination of ZSM-5 can be postulated to explain the reduction in activity. 

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. 

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