Alan W. Peters

Influence of zeolite particle size on selectivity during fluid catalytic cracking

Abstract NaY zeolites of varying average particle size were synthesized and incorporated into fluid cracking catalysts. Scanning Electron Micrographs showed that these zeolites exhibited agglomeration and intergrowths. Hence, external surface area measured by N2 adsorption t plot was used to calculate effective particle size. Effective particle size of these zeolite samples ranged from 0.06 to 0.65 μm. The t plot micropore volume was determined to be the appropriate method of characterization of crystallinity of these zeolites. Cracking of West Texas Heavy Gas oil is limited by zeolitic diffusion at 500°C for a zeolite with 0.65 μm effective particle size. Hence, the catalyst containing smaller particle zeolite exhibited improved activity and selectivity to intermediate cracked products like gasoline and light cycle oil. Selectivity differences can be explained by considering the effect of diffusion resistance on the rate constants for cracking of gas oil and gasoline.

Environmental Fluid Catalytic Cracking Technology

Abstract The fluid catalytic cracking (FCC) process converts heavy oil into voluable fuel products and petrochemical feedstocks. Environmental regulations are a key driving force for reducing FCC process air-pollutant emissions and for changing the composition of fuel products. Environmental considerations are affecting the design and operation of the FCC and are providing opportunities for the development of in-process additives. The present article reviews developments in these environmental technologies.

Vanadium poisoning of cracking catalysts: mechanism of poisoning and design of vanadium tolerant catalyst system

The mechanism of vanadium poisoning of cracking catalysts is described. Experimental results identify the poison precursor as volatile vanadic acid, H3VO4 which is formed under FCC regenerator conditions by the reaction V2O5(s) + 3H2O(v) 2H3VO4(v). The concentration of H3VO4 in a typical regenerator (730 °C, 20% H2O, 2 atm total pressure) is 1–10 ppm. Since H3VO4 is a strong acid analogous to H3PO4, it can destroy the zeolite by hydrolysis of the zeolite SiO2Al2O3 framework. A basic solid with reasonable pore structure should be an effective scavenger. Basic alkaline earth oxides such as MgO or CaO are shown to be effective for vanadium scavenging. Microactivity testing shows excellent activity retention when 20% MgO is blended with cracking catalyst at vanadium loadings of 0.67% and 1.34% V by weight on catalyst. However, the SOx, in the regenerator flue gas can form a sulfate that competes with the formation of the vanadate. The degree of competition will be thermodynamically controlled. Since the formation of the vanadate from the oxide expands the lattice, pore structure effects exist similar to those observed for the reaction of calcium oxide with sulfur oxides.

Preparation and Properties of Fluid Cracking Catalysts for Residual Oil Conversion

Abstract Catalytic cracking of petroleum to produce gasoline began in about 1912. The early pioneering work was carried out by Eugene Houdry [1]. Modern fluid catalytic cracking (FCC) was conceived at Exxon and commercially developed in about 1940 [2] using amorphous catalysts. Fluid catalysts are small spherical particles ranging from 40 to 150 um in diameter with acid sites capable of cracking large petroleum molecules to products boiling in the gasoline range. One advantage of the FCC process is the absence of the diffusion limitations present in conventional gas oil cracking due to the small size of the catalyst particle. Since 1964 virtually all catalysts contain faujasite, a stable, large pore, Y-type zeolite dispersed in a silica/alumina matrix [3]. The catalytic aspects of contemporary FCC processes have been reviewed by Venuto and Habib [4], Gates, Katzer, and Schuit [5], Magee and Blazek [6], and Magee [7]. A more recent update of refinery trends has been made available by Blazek [8].

Effect of exchange cations and silica to alumina ratio of faujasite on coke selectivity during fluid catalytic cracking

Hydrothermally treated fluid-cracking catalysts containing zeolites of varying chemical composition were used in cracking experiments with commercial gas oils and a model hydrocarbon compound. An observed linear relationship between coke yields for a given catalyst and a second-order kinetic conversion parameter was used to assess coke selectivity as a function of zeolite composition as measured by the unit cell constant. Coke selectivity was sensitive to the unit cell constant in the 24.57- to 24.33-A range. Both parallel and consecutive coking reactions were suppressed in the presence of the smaller unit cell zeolite catalyst. Below 24.33 A, coke selectivity was less sensitive to the unit cell constant. Exchange cations like sodium and mixed rare earths did not independently influence coke selectivity at a given unit cell constant. Reduction in coke selectivity for the zeolites with smaller unit cell constants can be attributed to the lower density of acid sites in those zeolites.

Estimation of hydrogen transfer rates over zeolite catalysts

Abstract A kinetic model has been developed to describe the product distribution of the reaction of cyclohexene on acidic zeolite catalysts. The model was used to estimate the rate constants for hydrogen transfer, isomerization, oligomerization, cracking and coking for various zeolite catalysts. Over Y-zeolites, the selectivity toward hydrogen transfer increases by a factor of 1.7 as the number of Al atoms per unit cell increases from 4.6 to 33.4. The selectivity toward hydrogen transfer can be correlated with the fraction of paired Al sites in the zeolite as estimated by a statistical calculation. Over zeolites of comparable silica to alumina ratio, the ratio of isomerization selectivity to hydrogen transfer selectivity was the highest for H-ZSM-5, followed by H-beta and REUSY.

Penta-co-ordinated aluminium in zeolites and aluminosilicates

Solid state 27Al n.m.r. spectroscopy of various aluminosilicates (zeolites, clays, amorphous SiO2–Al2O3) shows that upon thermal or hydrothermal treatment a line appears at ∼30 p.p.m.; the position of this line suggests that penta-co-ordinated Al is present in all these aluminosilicates.

Chapter 6 Instrumental Methods of FCC Catalyst Characterization

Publisher Summary The Fluid Catalytic Cracking (FCC) catalyst is a coarse powder consisting of particles in the 40 to 100 micron size range with an average size of about 65 microns. It contains 20% to 50% of an active zeolite, usually faujasite, with a surface area of 900 meters2/gram. Instrumental methods are used to characterize the FCC catalyst particle, both fresh and after deactivation. The catalyst properties—such as zeolite content, the unit cell size of the zeolite, and the content and location of the active components—determine the activity and selectivity of the catalyst. Catalyst stability is measured by comparing the fresh properties of the catalyst with the properties of the operating or equilibrium catalyst. In most advanced technology catalysts, the zeolite is designed to hydrothermally dealuminate in a controlled and stable way to the intended unit cell size and surface area. The hydrothermal environment of the unit is, in a sense, part of the catalyst preparation. Also, during the use of contaminant, metals contained in the oil at the part-per-million level may deposit on the catalyst and cause changes in activity and selectivity. The control of these contaminants is one of the most important issues in catalytic cracking. It is important for the catalyst user, the manufacturer, and the researcher to be able to follow the chemical and physical changes that take place during the operation of the catalyst in the FCC Unit (FCCU). The operation of the FCCU is also described in the chapter.

Solid-state NMR studies of silanol groups in mildly and highly dealuminated faujasites

Abstract Experimental evidence for two different aluminum-bonded silanol species, SI(OSi)(OAl)2OH and Si(OSi)2(OAl)OH, in a mildly dealuminated faujasite is presented. A sample of a low unit cell size ( Si Al =4.1 ) templated Y zeolite was prepared for this study to minimize interferences from framework structures highly-coordinated to aluminum, such as Si(OSi)(OAl)3 and Si(OAl)4. Bonding of the silicon to OH groups was observed by 1H/29Si cross-polarization/magic-angle-spinning NMR experiments, while bonding of silicon to aluminum was observed by 29Si/27Al dipolar-dephasing-difference NMR experiments. The newly identified sites are of interest because these aluminum-bonded silanol sites may dehydrate to form strong Bronsted or Lewis acid sites and thus change the activity of the zeolite in FCC catalysts. Strong Lewis acid sites have been implicated as a source of enhanced cracking activity for mildly dealuminated samples of both ZSM-5 and faujasite in previous studies. As an extension of these studies, the structure of a highly dealuminated faujasite by solid-state 29Si NMR methods was also examined. Our purpose is to resolve conflicting chemical shift assignments in the literature. Based on the results of a variable-contact-time 1H/29Si cross-polarization experiment, new signals with chemical shifts of −102 and −104 ppm are assigned to silicon atoms that are directly connected through bridging oxygen atoms to silicon atoms in silanol groups.

The Structural Elements of Faujasite and their Impact on Cracking Selectivity

The location of alumina in the dealuminated faujasite framework is described. The selectivities of dealuminated and rare earth exchanged faujasite, including selectivities for octane, coke, hydrogen transfer, and isomerization, are shown to depend on the relative amounts of paired and isolated sites present. Pentacoordinated and octahedral nonframework alumina do not contribute to selectivities in cracking. Selectivity and activity is controlled by the framework aluminum sites.