Diffusion and catalytic cracking of 1,3,5 tri-iso-propyl-benzene in FCC catalysts

Abstract

The present study describes catalytic cracking experiments developed in a novel CREC Riser Simulator using 1,3,5-Tri-iso-propyl-benzene and two FCC catalysts with different crystal sizes (0.4-μm and 0.9-μm diameter). The experiments are modeled using an unsteady state model for both gas and catalyst phases. It is found that a quasi-steady state approximation can be used for the catalyst and changes in the gas phase can be accounted, under the allowed model simplifications, with a relatively simple unsteady state equation. The model is completed using two catalytic decay models, with one of them involving a decay function based on "reactant converted". Experimental and modeling observations point towards an overall cracking reaction rate controlled by diffusion at 350°C -450°C with this rate shifting to one being controlled by the intrinsic cracking reaction at 500°C -550°C.(*) Author to whom correspondence may be addressed 2 1-IntroductionThe oil refining industry has experienced in recent years, a dramatic increase in the price and availability of crude oil. In this respect, new catalyst technology is needed to process heavier feedstocks and to produce more environmentally friendly products.The currently available 60-μm commercial FCC catalysts are manufactured with 1-2-μm Y-zeolite crystals dispersed in an amorphous silicaalumina matrix . The majority of the active sites are located within the zeolite pore structure. In order for the reaction to proceed, molecules have to diffuse through the large matrix pores into the zeolite crystals.As a result, only certain hydrocarbon species with a kinetic diameter smaller than 10.2 Å can penetrate the zeolite pores (Leiby, 1992). Hence the relationship between catalytic cracking reaction and diffusional processes can determine the extent of cracking and greatly influence the product selectivity. Such phenomena are complex and theoretical models to describe such systems are needed (Bidabehere and Sedran, 2000).Diffusion in catalysts belongs in many cases to the Knudsen regime.However, diffusion in zeolites falls more in a so-called "configurational regime", since the size of the hydrocarbon molecule is nearly the size of the zeolite inner channels (Karger and Ruthven, 1992; Gates et al,1979). As a result, diffusion of the hydrocarbon molecules is governed by an almost continuous interaction between the zeolite crystal and the diffusing molecules (Karger and Ruthven,1992;Gates et al,1979). In addition, there may be compounded effects 3 with larger hydrocarbon molecules hindering the diffusion of smaller molecules.As a result, lighter product molecules may be trapped or delayed in a zeolite pore network already filled with sorbed heavier molecules (Gates et al, 1979). The 1,3,5-TIPB is a relatively bulky molecule with a critical molecular diameter of about 9.5 Å (Bhatai, 1991). Due to its large size, it has been used to simulate the diffusional constrains expected in the catalytic cracking of gas oil (Aguiar et al, 1995). For instance, the cracking of 1,3,5-TIPB ...

The effect of Y-zeolite acidity on m-xylene transformation reactions

“…For example, the low olefinity obtained from MAT reactor, due to its higher reaction time (> 75 s) as compared to the riser (< 15 s), non uniform coke deposition (150 mm long catalyst bed), and temperature/concentration gradient, which are eliminated by the well-mixed characteristics and intense fluidization of the riser simulator. The riser simulator is fast becoming a valuable experimental tool for reaction evaluation involving model compounds [24,25] and also for testing and developing new fluidized catalytic cracking in vacuum gas oil cracking [26,27]. The riser simulator consists of two outer shells, the lower section and the upper section, which allow one to load or to unload the catalyst easily, as illustrated in Fig as well as intense gas mixing inside the reactor.…”

Impact of the Desilication Treatment of Y Zeolite on the Catalytic Cracking of Bulky Hydrocarbon Molecules

“…Resids have higher contents of contaminant metals, polynuclear aromatics, heteroatoms and complex macromolecular groups than standard hydrocarbon feedstocks [3], which are typically vacuum gas oils (VGO). This represents an important challenge for FCC technology, and improvements in the transport properties of the commercial catalysts would be attractive in order to improve the selectivity to desired products and decrease undesired secondary reactions [4][5][6][7][8].…”

“…The first one consists in synthesizing zeolites with extra-large pores [9] which, however, would show a different topology and low hydrothermal stability. The second one is based on the use of smaller Y zeolite crystal size, so as to increase the external specific surface area and decrease diffusion paths in the crystals [4,5,10]. A third possibility is the induction of intracrystalline mesopores, that is, cracks, cavities [11] and cylindrical channels [12] in the crystals in the mesopore size range, that are interconnected in a wormhole-like manner to each other [8].…”

Revisiting Oxidative Dehydrogenation of Ethane by W Doping into MoVMn Mixed Oxides at Low Temperature

“…Regarding catalyst deactivation, as deactivation functions can be expressed in terms of the catalyst time-on-stream ( = exp(− )), deactivation can also be related to the progress of the reaction [45], where is a catalyst deactivation constant. The deactivation function based on time-on-stream was initially suggested by Voorhies [46].…”