Cracking product distributions, role

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... 

NHs adsorption microcalorimetry has been used to characterize the acid sites of a H-USY zeolite and another USY sample in which the strong Lewis acid sites were poisoned with ammonia. Poisoning of the Lewis acid sites did not affect the rate of deactivation, the cracking activity, or the distribution of cracked products during 2-methylpentane cracking. Thus, strong Lewis acid sites do not seem to play any important role in cracking reactions [148]. 

Zeolite ZSM-5, as a member of the family of pentasil zeolites, has aroused tremendous interest after its first discovery by the research group of Mobile Company in the year 1972 [1]. With its adjustable framework A1 content (from 0 to about 8A1 per unit cell), two dimensional micropore channels (0.55 nm x 0.54 nm Fig. la), sinusoidal pore geometry along c axis (Fig.lb) and easy insertion of hetero-T atoms, this material plays an important role in many of crucial catalytic processes such as hydro-cracking, de-waxing, alkylation, etc., [2-5] as well as in separation of organic compounds with different sizes and shapes [6]. In the case when zeolite ZSM-5 was used as catalyst, most of reactions are diffusion-controlled [7]. This means that the product distribution largely depend on the nature and location of active sites in the crystalline framework of catalyst. Thus, the increase of the... 

The CFB catalytic cracking reactor plays an important role in the petroleum industry because of its better gas-solids contact and narrow residence time distribution, but its non-uniform radial flow structure and the extensive backmixing of gas and solids lead to a lower conversion rate and poorer selectivity to desired intermediate products [14]. 

Dealuminated Y zeolites which have been prepared by hydrothermal and chemical treatments show differences in catalytic performance when tested fresh however, these differences disappear after the zeolites have been steamed. The catalytic behavior of fresh and steamed zeolites is directly related to zeolite structural and chemical characteristics. Such characteristics determine the strength and density of acid sites for catalytic cracking. Dealuminated zeolites were characterized using X-ray diffraction, porosimetry, solid-state NMR and elemental analysis. Hexadecane cracking was used as a probe reaction to determine catalytic properties. Cracking activity was found to be proportional to total aluminum content in the zeolite. Product selectivity was dependent on unit cell size, presence of extraframework alumina and spatial distribution of active sites. The results from this study elucidate the role that zeolite structure plays in determining catalytic performance. 

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