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4.2. Carbenium Ion Chemistry of Catalytic Cracking

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4.2. Carbenium Ion Chemistry of Catalytic Cracking Faculteit Ingenieurswetenschappen Chemische Proceskunde en Technische Chemie Laboratorium voor Petrochemische Techniek Directeur: Prof. Dr. Ir. Guy B. Marin Single-event microkinetic modelling of the catalytic cracking of hydrocarbons over acid zeolite catalysts in the presence of coke formation Author: Carmen M. Alonso Romero 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, Carmen M. Alonso Romero afgestudeerd aan de UGent in het academiejaar 2006 - 2007en auteur van de scriptie met als titel: Single-event microkinetic modelling of the catalytic cracking of hydrocarbons over acid zeolite catalysts in the presence of coke formation 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, 20 august 2007 (Carmen M. Alonso Romero) ___________________________________________________________________________________________ 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 microkinetic modelling of the catalytic cracking of hydrocarbons over acid zeolite catalysts in the presence of coke formation by Carmen M. Alonso Romero Thesis work submitted to obtain the degree of chemical engineer Academic year: 2007 Universiteit Gent Faculteit Ingenieurswetenschappen Promotors: Prof. Dr. Ir. G. B. Marin Prof. Dr. Lic. M.-F. Reyniers Coach: Ir. R. Van Borm Overview Fluid catalytic cracking is one of the major processes in refining. A point of interest concerning catalytic cracking is the formation and deposition of coke on the zeolite catalyst. Numerous efforts have been made to add knowledge of the FCC process. In chapter 1 the Fluid catalytic cracking process and catalytic cracking catalyst will be discussed briefly. After that in chapter 2 coke formation and catalyst deactivation will be explained. Next, in chapter 3 both TEOM and recycle electrobalance reactor set up is described. Chapter 4, is devoted to explain SEMK (single-event microkinetic) modeling. The experimental results from the recycle electrobalance reactor of the catalytic cracking and coking of iso-octane and methylcyclohexane over different catalysts are presented in chapter 5. The experimental results obtained with TEOM reactor for the mixture (n-decane + methylcyclohexane + buthylcyclohexane/ 1-octene) over LZ-Y20 zeolite are in chapter 5 too. In chapter 6 the experimental data acquired with the TEOM reactor is used to estimate the kinetic parameters of the extended SEMK model to describe the cracking and coking process during the catalytic cracking of hydrocarbons. Finally, in chapter 7, the conclusions of the whole work performed are presented. Keywords: catalytic cracking, single-event microkinetic modelling, Y zeolites, coke formation. Single-event microkinetic modelling of the catalytic cracking of hydrocarbons over acid zeolite catalysts in the presence of coke formation Carmen M. Alonso Romero Supervisor: Rhona Van Borm Abstract-The first part of this work is devoted to acquire The types of elementary steps of carbenium/carbonium ion experimental data to estimate the kinetic parameters of the chemistry most important in coke formation are mainly extended SEMK model to describe the cracking and coking alkylation, cyclization, (de)protonation, and hydride transfer. process during the catalytic cracking of hydrocarbons. The Cyclization does not increase the size of the carbenium ion experiments are performed in TEOM reactor and allow to study but leads to ring formation, a prerequisite for coke formation. the influence of process conditions on the kinetics of the cracking Transformation into aromatic rings is possible by a succession and coking process. In the second part the influence of process conditions on the kinetics of the cracking and coking is analyzed of deprotonation and hydride transfer steps [1]. for different zeolites using a recycle electrobalance reactor. Keywords-catalytic cracking, coke formation, single-event microkinetic modelling, Y-zeolites B. Single-event approach The single-event concept is based on the fundamental framework of transition state theory. This concept factors the I. INTRODUCTION structural contribution associated with a single elementary In recent petrochemical refineries fluid catalytic cracking step out of the elementary rate coefficient. The number of (FCC) is one of the principal processes applied to convert distinct configurations taken by a reactant and its transition crude oil into lighter more valuable transportation fuels. A state is related to changes in their symmetry numbers to point of interest concerning catalytic cracking is the formation account for all possible occurrences of identical single events and deposition of coke on the zeolite catalyst. The deposited making up the elementary step [2]. The elementary rate coke deactivates the catalyst leading to a decrease in coefficient k can be written as a function of the single event ~ hydrocarbon conversion and energetic efficiency of the FCC rate coefficient k : process. ~ To optimally operate an industrial FCC unit, understanding = eknk and quantifying the influence of feedstock composition, Where n is the number of single events, which is calculated process conditions and catalyst properties on the kinetics of e the catalytic cracking in the presence of coke formation is as the ratio between the global symmetry number of the indispensable. The kinetic modelling of the elementary reactant σ ,rgl and that of the transition stateσ gl,≠ . reactions occurring during the catalytic cracking of hydrocarbons is based on the single-event microkinetic III. EXPERIMENTAL RESULTS (SEMK) modelling approach, which has been developed at the “Laboratorium voor Petrochemische Techniek”. This work involves the use of a TEOM reactor, working at A. TEOM REACTOR different conditions of pressure, space time and temperatures With the experimental data of the catalytic cracking of a and using a mixture of n-decane+methylcyclohexane mixture n-decane + methylcyclohexane + butylcyclohexane / +butylcyclohexane/1-octene as feed over LZ-Y20. The 1-octene over LZ-Y20 zeolite in the TEOM reactor, the effect experimental data obtained allow to determine the influence of the operating conditions on conversion, product of process conditions on coke formation. distribution and coke formation is assessed. A quantitative In the second part of this work is based on experimental data analysis of the experimental data is very useful for the obtained with the recycle electrobalance reactor. The coke construction of the kinetic model. formation has been studied for two feeds, methylcyclohexane In figure 1 it is observed that n-decane conversion and iso-octane, different zeolites and different process dramatically drops during the first 20 minutes of reaction. conditions. This way, the influence of coke formation can be This occurs for all the components of the mixture and at all studied on different zeolites. reaction conditions applied. This is due to the deactivating effect of coke formation. It is generally known that the initial II. SINGLE-EVENT MICROKINETIC MODEL catalyst cracking activity is mainly associated to very strong Brønsted acid sites. The presence of strong acid sites accelerates the protolysis reactions, whose activation energy A. Coke formation in terms of elementary steps is typically high. Moreover, due to a higher carbenium ion Coke is defined as a carbenium ion with a size and structure lifetime adsorbed on strong acid sites than on weaker acid preventing its desorption, thus permanently covering the sites, propagation of the catalytic cracking reactions via active site(s) it was formed upon. hydride transfer is favored (Quintana-Solorzano, 2007). active sites Conversion vs time (nC10) Catalyst Si/Al bulk Si/Al frame structure (mol NH3/kg) 100 W/F = 5 kgs/mol LZY20 H-USY 2,6 30 0,99 FAU 80 W/F = 10 kgs/mol CBV 720 H-USY 15 16 0,60 FAU 60 W/F = 15 kgs/mol W/F = 20 kgs/mol CBV 760 H-USY 30 100 0,23 FAU 40 W/F = 25 kgs/mol CBV 500 NH4-USY 2,6 3,9 1,50 FAU Conversion % Conversion 20 Table 1 Physical properties of the zeolites investigated. 0 0 2000 4000 6000 8000 10000 Time (s) of coke on the zeolites decreases in the following order: CBV Figure 1 n-Decane conversion as a function of time for different space 500>CBV 720>LZY20>CBV 760. The Si/Al framework ratio times, at 753 K and 400 103 Pa. decreases in the same order. The high deposition of coke in CBV 500 might be due to a higher number of active sites (1.5 Conversion increases with increasing space time and mmol NH3/g). The same trend is observed for both feeds but temperature. Higher space
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