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Thermodynamic Analysis of Catalytic Cracking Reactions As the First Stage in the Development of Mathematical Description

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Thermodynamic Analysis of Catalytic Cracking Reactions As the First Stage in the Development of Mathematical Description Available online at www.sciencedirect.com ScienceDirect Procedia Chemistry 15 ( 2015 ) 342 – 349 16th International Scientific Conference “Chemistry and Chemical Engineering in XXI century” dedicated to Professor L.P. Kulyov, CCE 2015 Thermodynamic analysis of catalytic cracking reactions as the first stage in the development of mathematical description Galina Nazarova a*, Emiliya Ivanchina a, Elena Ivashkina a, Svetlana Kiseleva a, Valeria a Stebeneva a Tomsk Polytechnic University, Lenin Avenue 30, Tomsk, 634050, Russia Abstract In this work thermodynamic analysis of catalytic cracking reaction involving the high molecular weight hydrocarbons was carried out using quantum chemical method of calculation realized in Gaussian software. The method of calculation is DFT (Density Functional Theory), theoretical approximation is B3LYP model, 3-21G basis. The list of catalytic cracking reactions for calculation was prepared on the basis of the theoretical data about catalytic cracking, laboratory and experimental data from the industrial unit. The enthalpy and Gibbs energy values of the main catalytic cracking reactions are presented under the process conditions. The results of this work will be used to develop a kinetic model of catalytic cracking of petroleum feedstock. ©© 2015 2015 The The Authors. Authors. Published Published by byElsevier Elsevier B.V. B.V. This is an open access article under the CC BY-NC-ND license (Peerhttp://creativecommons.org/licenses/by-nc-nd/4.0/-review under responsibility of Tomsk Polytechnic). University. Peer-review under responsibility of Tomsk Polytechnic University Keywords: advanced petroleum refining, catalytic cracking, high molecular weight hydrocarbons, enthalpy and Gibbs energy , kinetic model. 1. Introduction Increased proportion of extracted heavy oil dictates a need to improve existing oil refining installations and orients the industrial sector on the resource efficient use of petroleum feedstock, improvement of the environment on a global scale, while the fuel requirement (gasoline and diesel fuel) is increasing every year. * Galina Nazarova. Tel.: +7-952-181-8788 E-mail address: [email protected] 1876-6196 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Tomsk Polytechnic University doi: 10.1016/j.proche.2015.10.054 Galina Nazarova et al. / Procedia Chemistry 15 ( 2015 ) 342 – 349 343 Catalytic cracking is important processes providing advanced petroleum refining. This process is aimed at manufacturing of light oil products such as gasoline from heavy fraction of petroleum feedstock. The proportion of catalytic cracking at the Russian refinery is not more than 10%, while the foreign indicators are 27-30%. In order to create competitive refining and petrochemical sector corresponding to the modern level of foreign refineries (the depth of oil refining is 85-95%), construction of facilities for oil refining (including catalytic cracking) is organized at the domestic enterprises for increasing the raw materials conversion level up to 82–85% by 2020 1. During operation of industrial oil installations, it is important to consider the multifold factors that equally influence composition, quantity, and quality of the main products 2,3,4. The problem of improving the energy and resource efficiency of catalytic cracking can be solved by the development of adequate mathematical model based on physical and chemical regularities of catalytic cracking reactions with a high predictive ability in the advanced petroleum refining. Approaches to formalization of the chemical reaction mechanism at the modelling of advanced petroleum process feedstock are known. The main ones are: the formation of pseudocomponent groups at the fractional composition5- 9 and combined models that take into account the interactions of pseudocomponents – product streams (gasoline, light and heavy gasoil, liquid and dry gas) and the conversion of the main group components of the feedstock (paraffins, naphthenes, aromatics) 10-14. One of the first stages in mathematical description development of the refining process is the thermodynamic analysis of the alleged reactions15,16. Determination of the thermodynamic regularities of catalytic cracking is possible to perform with the quantum chemical methods of calculation. At present, software for quantum chemical calculation implementation is submitted by numerous domestic and foreign computer products with different technical possibilities. The quantum calculations, in particular via density functional theory (DFT), are an essential tool in many branches of chemical research 17. Quantum chemical methods are used for describing the alkane adsorption in nanoporous crystalline and ordered acid catalysts with different pore geometry, enthalpy of alkane adsorption and its relation with the activation energy for the monomolecular cracking of n-paraffins18. Also such methods are used to establish the mechanism of substances formation on the catalyst surface and the stages of transition states formation19,20. The aim of the work is establishment of thermodynamic regularities of catalytic cracking of petroleum feedstock using quantum chemical methods of calculation for development of catalytic cracking kinetic model. Nomenclature ∆Н enthalpy change during a chemical reaction, kJ/mol ΔG Gibbs energy change during a chemical reaction, kJ/mol T temperature, K P pressure, MPa 2. Introduction Heavy fraction of petroleum feedstock such as vacuum distillate or residues mixture from the secondary manufacturing processes with the boiling point about 350-560 °C is used as catalytic cracking feedstock. The technological process is implemented in the riser reactor. The main characterized parameters of the reactor operation are the consumption of feedstock (160-365 m3/h), the pressure in the reactor sediment chamber (0.8-1.5) kg/cm2), the temperature of the feedstock entering the reactor (240-350 °C), the temperature of the products at the outlet from the ballistic separator (495-535°C). The products from the catalytic cracking unit are rich in gas and gasoline fractions, light gasoil (fraction of 195 - 310 °C), heavy gas oil (fraction of 310 - 420 °C). Chemical conversions of hydrocarbon feedstock are carried on carbonium ion mechanism through chemisorption of hydrocarbon molecules to the microsphere zeolite catalyst surface 21. This list includes: x cracking of high molecular weight n-paraffins and isoparafins C13–C40; x cracking of medium weight n-paraffins and isoparaffins C5–C12 ; 344 Galina Nazarova et al. / Procedia Chemistry 15 ( 2015 ) 342 – 349 x isomerization of medium weight paraffins С5-С12; x cracking of olefins; x hydrogen transfer; x dealkylation of naphthenes; x dealkylation of aromatic hydrocarbons; x dehydrogenation of naphthenes; x condensation of aromatic compounds; x coke formation; x cyclization of olefins to naphthenes. The list of the reactions was composed using the theoretical data about catalytic cracking, experimental data from the industrial plant and laboratory and experimental data from the industrial unit. 3. Approaches and Methods Thermodynamic analysis of the catalytic cracking reaction is one of the first stages in mathematical model developing which allows establishing the possibility of catalytic cracking reactions. Calculations to determine thermodynamic parameters of individual hydrocarbons involved in the catalytic cracking reactions of petroleum feedstock were performed using the quantum chemical methods. The values of Gibbs energy (ΔG) reaction characterize the probability of reaction, and the value is necessary for formalization of the hydrocarbon chemical conversion scheme, which will be the foundation to the catalytic cracking kinetic model. But there are some difficulties: since the catalytic cracking is an advanced refining process of petroleum feedstock, the thermodynamic parameters of the reactions involving the high molecular weight hydrocarbons are virtually absent in the directory. It is also important to take into account the real operating conditions of the industrial unit for calculation of the reaction thermodynamic parameters, as well as the interaction effects of hydrocarbon molecules with reaction mixture molecules. The software which implements the quantum chemical methods of calculation of the molecule electronic structure Gaussian and provides adequacy and sufficient accuracy of calculations for chemical reaction simulation of refinery processes was used to solve these difficulties. Comparison of quantum chemical calculation methods: ab initio methods (DFT, B3LYP) and semi-empirical (PM3) was conducted in this work. PM3 is a semi-empirical method, which uses parameters obtained from the experimental data to simplify the calculations. Thus the Schrodinger equation is solved with a certain approximation using the appropriate parameters of the systems studied. Different methods are characterized by different sets of parameters. The ab initio (non-empirical) method of DFT implemented in Gaussian software was used for the evaluation of each individual hydrocarbon formation thermodynamic properties. Theoretical approximation was B3LYP model (Becke’s DFT (B3) with theoretical approach using Lee, Yang and Parr electron correlation (LYP), 3-21G basis. The model substances participated in the chemical transformations were built in GaussView software. Also the choice of this theory (DFT) can be explained
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