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Catalytic Cracking of Methylcyclohexane Over Fcc Catalysts in a Mini-Fluidized Crec Riser Simulator

Mustafa Al-Sabawi and Hugo de Lasa. Chemical & Biochemical Engineering, University of Western Ontario, London, ON N6A5B8, Canada

The limited availability of high value light hydrocarbon feedstocks and the rise in crude prices have led to international recognition of the vast potential of Canada's oil sands. There are, however, technical challenges that come with Canadian bitumen production and refining, one of which is the significant presence of aromatics and cycloparaffins in bitumen-derived feedstocks. Aromatic fractions limit FCC bottoms conversion, decrease yield and quality of valuable products, increase levels of polyaromatic hydrocarbons prone to form coke on the catalyst, and ultimately compromise the unit performance. Cycloparaffin (naphthene) conversion, on the other hand, is a complex process involving competing reaction steps such as cracking, hydride transfer, ring opening and isomerization. In this regard, cycloparaffins are important in cracking chemistry as precursors for aromatics [1]. It is through the detailed understanding of cycloparaffin catalytic cracking that one can influence the composition of the resulting gasoline, minimizing the total aromatics fraction and making it more environmentally friendly.

There is scarce information in the open literature concerning the catalytic cracking of cycloparaffins over USHY zeolites under actual fluid catalytic cracking (FCC) conditions. Cycloparaffin studies that are currently found in the literature were either conducted using fixed-bed tubular reactors, MAT-reactors or autoclaves; units that do not provide adequate simulation of large-scale FCC units in terms of reactant partial pressure, reaction contact time, temperature and catalyst/hydrocarbon ratios and fluidization regime. Moreover, little work has been done with respect to the kinetic modeling of cycloparaffin catalytic cracking. The major concern with these models is that they fail to consider the important effects of hydrocarbon diffusion and adsorption in the catalyst pore network, but instead, describe the combined effect of diffusion, adsorption and reaction using “pseudo-parameters”. Thus, such models are inadequate for defining the critical role of diffusion and adsorption in FCC.

The objective of the present work is to determine the processability of methylcyclohexane (MCH) using USY catalysts with different crystallite sizes and to establish a kinetic model that accurately represents the catalytic conversion of MCH. To meet such objectives, catalytic cracking experiments using MCH on USY zeolite catalysts were carried out in the mini-fluidized CREC riser simulator [2], a novel unit that overcomes the technical problems of the standard micro-activity test (MAT). To secure the value of the catalytic cracking of MCH, modeling studies were developed under relevant FCC process conditions in terms of partial pressures of gas oil, temperatures (450-550oC), contact times (3-7 seconds, both for the hydrocarbons and the catalyst), and catalyst-gas oil mass ratios (5), and using well-fluidized catalysts. MCH overall conversions ranged between 4 to 10 wt% at low reaction temperatures and 10 to 16 wt% at high temperatures, with slightly higher conversions obtained using the larger zeolite crystallites. Moreover, it was found that MCH may undergo several reactions, including protolytic cracking, ring opening, isomerization, hydrogen transfer and transalkylation. A heterogeneous kinetic model for the conversion of MCH describing thermal effects as well as adsorption and reaction phenomena was established. Adsorption constants were determined to be in the range of 4 to 26 cm3/gcatalyst and the heat of adsorption was -40 kJ/mol. Kinetic parameters were also calculated, including thermal and catalytic intrinsic activation energies, which were in the range of 42-69 kJ/mol and 49-74 kJ/mol, respectively. It was also determined that MCH does not experience mass transport limitations within the USY zeolite network.

The present approach of this study represents a step forward in the study of the adsorption, diffusion and reaction of cycloparaffins in zeolitic catalysts, ahead of traditional studies at lower temperature and with lower-reactivity zeolites.