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Single Event Microkinetics of (Cyclo)Alkane Cracking on Acid Catalysts

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Faculteit Ingenieurswetenschappen Chemische Proceskunde en Technische Chemie Laboratorium voor Petrochemische Techniek Directeur: Prof. Dr. Ir. Guy B. Marin Single event microkinetics of (cyclo)alkane cracking on acid catalysts Author: Pilar Díaz Ríos Promoters: Prof. Dr. Ir. G. B. Marin Prof. Dr. Lic. M.-F. Reyniers Coach: Ir. R. Van Borm Thesis work submitted to obtain the degree of chemical engineer 2006-2007 FACULTEIT INGENIEURSWETENSCHAPPEN Chemische Proceskunde en Technische Chemie Laboratorium voor Petrochemische Techniek Directeur: Prof. Dr. Ir. Guy B. Marin Opleidingscommissie Scheikunde Verklaring in verband met de toegankelijkheid van de scriptie Ondergetekende, Pilar Díaz Ríos afgestudeerd aan de UGent in het academiejaar 2006 - 2007 en auteur van de scriptie met als titel: “Single event microkinetics of (cyclo)alkane cracking on acid catalysts” verklaart hierbij: 1. dat hij/zij geopteerd heeft voor de hierna aangestipte mogelijkheid in verband met de consultatie van zijn/haar scriptie: de scriptie mag steeds ter beschikking gesteld worden van elke aanvrager de scriptie mag enkel ter beschikking gesteld worden met uitdrukkelijke, schriftelijke goedkeuring van de auteur de scriptie mag ter beschikking gesteld worden van een aanvrager na een wachttijd van jaar de scriptie mag nooit ter beschikking gesteld worden van een aanvrager 2. dat elke gebruiker te allen tijde gehouden is aan een correcte en volledige bronverwijzing Gent, 31 may 2007 ___________________________________________________________________________________________ Krijgslaan 281 S5, B-9000 Gent (Belgium) tel. +32 (0)9 264 45 16 • fax +32 (0)9 264 49 99 • GSM +32 (0)475 83 91 11 • e-mail: [email protected] http://www.tw12.ugent.be Single event microkinetics of (cyclo)alkane cracking on acid catalysts by Pilar Díaz Ríos Thesis work submitted to obtain the degree of chemical engineer Academic year: 2006-2007 Universiteit Gent Faculteit Ingenieurswetenschappen Promotors: Prof. Dr. Ir. G. B. Marin Prof. Dr. Lic. M.-F. Reyniers Coach: Ir. R. Van Borm Overview The study of pure hydrocarbons has been a continuous practice for obtaining a better insight in the chemistry of the FCC process. Single event microkinetic modelling is an advanced kinetic methodology in catalytic cracking. However, there are issues which are still to be addressed, such as coke formation and catalyst effects. The objective of this thesis is to obtain a better insight in the influence of the catalyst properties during catalytic cracking of (cyclo)alkanes in absence of coke formation. In the first chapter, the aim and justification of this work is explained and a general introduction about the fluid catalytic cracking process and the use of zeolites in this area is given. In the second chapter, the set up in which the experiments are performed, is described. Next, the mechanism of hydrocarbon cracking is explained and more specific information about the catalyst used is given. In the fourth and fifth chapter, experimental results of the catalytic cracking of iso-octane and methylcyclohexane over different catalysts are presented. The reactor effluent is analysed by means of gas chromatography. Based on these results feed conversion and product selectivities can be calculated, as discussed in chapter 4 and 5. The estimation of acid dependent parameters for catalytic cracking of iso-octane according to the SEMK is developed in chapter 6, and general conclusions of the whole work performed are presented in chapter 7. Keywords: catalytic cracking, single event microkinetic modelling, iso-octane, methylcyclohexane, acid sites, pore structure, Y zeolite, ZSM-5 zeolite, BIPOM zeolite Single event microkinetics of (cyclo)alkane cracking on acid catalysts Pilar Díaz Ríos Supervisor: Rhona Van Borm Abstract–The use of zeolites for hydrocarbon conversions is II. EXPERIMENTAL PROCEDURE based on their acid properties and their framework topology. This thesis aims at investigating the influence of these catalyst properties on the rates of the elementary reactions occurring A. Iso-octane cracking during catalytic cracking of alkanes and cycloalkanes. Cracking Catalytic cracking experiments have been performed in a experiments using iso-octane and methylcyclohexane as model recycle electrobalance reactor set up equipped with online gas components have been performed in a recycle electrobalance chromatographic effluent analysis. The reactant 2,2,4-triMe- reactor set up at a broad range of operating conditions. Based on pentane (iso-octane) is evaporated and diluted with nitrogen. the experimental data obtained for iso-octane cracking, acid Iso-octane cracking was performed over CBV 500 (Y zeolite), parameters can be estimated using the single event microkinetic modeling approach, for future optimizing the FCC process. CBV 3020 (ZSM-5 zeolite) and BIPOM3 (BIPOM material) and the activities and product selectivities obtained were Keywords–catalytic cracking, zeolite properties, Y zeolite, compared with previously available data. An overview of the ZSM-5 zeolite, BIPOM zeolite, single event microkinetic modelling. catalyst properties is given in table I. The catalyst pellet size varied from 0,5 to 0,71 mm for CBV 500 and BIPOM3. For CBV 3020 a larger pellet size (0,71-1 mm) was also used to study the absence of transport limitations. Temperature I. INTRODUCTION (475°C) and iso-octane partial pressure (7 kPa) were constant Fluid catalytic cracking is the most important process in a in all the experiments, but a broad range of space times was modern refinery. The growth in the use of USY zeolites in investigated. this process has improved the understanding of the nature and location of the catalytically actives sites in them. Elementary Table I. Properties of zeolites tested in iso-octane cracking. reactions such as isomerization, protonation or β-scission Active sites occur via carbenium ion intermediates. In the industrial FCC Catalyst Si/Al bulk Si/Al frame Structure (mol NH3/kg) process, an acid catalyst containing Y zeolite as main active CBV 500 2,6 3,9 1,50 FAU (NH4-USY) component is used [1]. CBV 3020 15 18,4 ? MFI The influence of the zeolite properties is studied by the (ZSM-5) Al-BIPOM1 50 47 ? MFI cracking of iso-octane and methylcyclohexane in a recycle (BIPOM) Al-BIPOM3 50 ? ? MFI electrobalance reactor over different zeolites. In the case of (BIPOM) the cracking of iso-octane, one Y zeolite is used to study the correct use of the set up, a type of ZSM-5 zeolite to study the absence of diffusion limitations and BIPOM samples to study B. Methylcyclohexane cracking the behavior of these new materials in catalytic cracking. For Experiments using methylcyclohexane as model component methylcyclohexane cracking, a set of Y zeolites differing in of cyclics were performed at similar operating conditions as acid characteristics have been tested at various operation with iso-octane (475°C, 7 kPa and a broad range of space conditions to study the influence of the catalyst properties on times) over four Y zeolites whose properties are shown in activity and selectivity. table II. The objective in this case is to compare activity and A single event kinetic model describing catalytic cracking product selectivities obtained on the catalysts. of hydrocarbons in absence of coke formation [2] has already been developed at the LPT but the kinetic parameters Table II. Properties of zeolites tested in methylcyclohexane cracking. determined still depend on the type of the catalyst used. A general application of this model in process optimization and Catalyst Si/Al bulk Si/Al frame Active sites structure (mol NH3/kg) catalyst development is consequently limited. Therefore, the LZY20 2,6 30 0,99 FAU kinetic model has been modified to be able to use it with other (H-USY) CBV 720 15 16 0,60 FAU catalysts. Currently, the model is extended to account for the (H-USY) influence of acid properties on alkane cracking. Acid CBV 760 30 100 0,23 FAU (H-USY) parameters were estimated based on available iso-octane CBV 500 2,6 3,9 1,50 FAU cracking data. (NH4-USY) C. Experimental results of iso-octane cracking III. SINGLE EVENT MICROKINETIC MODEL It has been demonstrated based on the experiments performed that there is a good agreement between previously A. Iso-octane cracking model available and currently obtained data over CBV 500 during The rate coefficients in the current single event microkinetic the cracking of iso-octane. This confirms the proper operation model [3] depend on the acid properties and pore structure of and functioning of the set up. the acid zeolite catalyst. Therefore it is important to derive a When comparing activity and product selectivities obtained kinetic model that can accurately describe the influence of the over CBV 3020 no influence of the pellet size on the catalyst properties on the rates of the elementary reaction experimental data was observed. This indicates that transport steps. The elementary steps considered in the cracking of limitations inside the ZSM-5 pellets were absent under the acyclics are (de)protonation, hydride transfer, protolytic conditions investigated. scission, β-scission and PCP-isomerization. Kinetic Two newly developed zeolitic materials have been tested in parameters for these elementary reactions had been iso-octane cracking. At similar operating conditions, both determined previously for the cracking of iso-octane over the BIPOM1 and BIPOM3 showed a relatively low activity reference catalyst LZ-Y20 by means of regression of compared to the other zeolites tested. To compare the product experimental data [4]. distribution obtained, propylene selectivity over all catalysts The influence of acidity has been implemented in the tested in iso-octane cracking is represented in Figure 1. As kinetic model by introducing a limited number of catalyst can be seen, CBV 500 and CBV 3020 follow the same trend descriptors: the number of active sites C , the influence on the as Y and ZSM-5 zeolites [4], respectively. It has been t protonation enthalpy ∆(∆Hpr), and the influence on the observed that BIPOM1 resembles Y zeolite behavior, while activation energies of the elementary reaction families BIPOM3 shows an intermediate behavior between Y and considered ∆Ea,reac.
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