ARTICLE pubs.acs.org/IECR
Kinetics of Toluene Disproportionation: Modeling and Experiments Marcos W. N. Lob~ao,† Andre L. Alberton,‡ Sílvio A. B. V. Melo,† Marcelo Embiruc-u,† Jose L. F. Monteiro,§ and Jose Carlos Pinto*,§ †
Programa de Engenharia Industrial, Escola Politecnica, Universidade Federal da Bahia, Rua Aristides Novis, no 2, Federac-~ao, Salvador, 40210-630 BA, Brasil ‡ Pontifícia Universidade Catolica do Rio de Janeiro, Rua Marqu^es de S~ao Vicente 225-371 L, Rio de Janeiro, 22451-900 RJ, Brasil § Programa de Engenharia Química/COPPE, Universidade Federal do Rio de Janeiro, Cidade Universitaria, CP: 68502, Rio de Janeiro, 21941-972 RJ, Brasil ABSTRACT: This work presents modeling and experimental studies on the kinetics of toluene disproportionation in operation ranges that include real industrial operation conditions. The influence of reaction temperature, reactor pressure, feed composition, and residence time on conversion of reactants and product selectivity was investigated. Experiments were performed according to a sequential experimental design strategy, in order to provide maximum accuracy for model predictions. Statistical treatment of parameter estimates and model adequacy was performed with the help of maximum likelihood principles. Excellent agreement between model predictions and available experimental data was obtained in the full ranges of investigated experimental conditions.
derived from mechanisms that consider toluene adsorption3,6,19 or surface reaction3,20 as the RDS. In all these studies, the catalysts used to promote toluene disproportionation were based on Y-zeolites,3,15,20 ZSM-5 zeolites,4,5,10,12 and more commonly H-mordenite.68,10,11,13,14,1618 As briefly reviewed above, although the kinetics of toluene disproportionation has been investigated by several researches, distinct reaction mechanisms have been proposed and distinct kinetic models have been derived. Discrepancies can possibly be ascribed to the different analyzed catalyst types, operating conditions, conversion levels, and coking of catalysts; nevertheless, it is certain that agreement has yet to be reached regarding the kinetics of toluene disproportionation over heterogeneous catalysts. It is important to emphasize that many works published in the literature were performed at low pressures and not at the relatively high pressures practiced in real industrial environments.47,12,15,19,20 When the operation conditions resembled the actual industrial operation conditions, simplified models based on pseudohomogeneous approach were proposed, specially for the study of catalyst deactivation.810 However, the use of simplified kinetic models leads to a poorer fits, when compared to the LHHW and ER expressions. 19 Besides, as one can observe in Table 1, many studies were performed at low toluene conversions, reducing the reliability of the models for prediction of real industrial operation conditions. Based on the previous paragraphs, the use of LHHW models to fit experimental data obtained at operation conditions that resemble the actual industrial conditions seems appealing. For this reason, the main objective of the present work is to
1. INTRODUCTION Toluene disproportionation is an important chemical transformation for most refineries and chemical complexes, constituting the heart of economically relevant commercial processes used for direct production of aromatic products and indirect manufacture of polymer resins, synthetic fibers, and plasticizers.1 The toluene disproportionation reaction can be represented as 2T a B þ X
ðR1Þ
where T is toluene, B is benzene, and X is xylenes. Toluene disproportionation is performed commercially with the help of heterogeneous catalysts.1,2 One of the most important processes is the Tatoray process, where typical industrial conditions include temperatures of 350530 °C, pressures of 1050 bar, and H2/aromatic ratios between 5 and 12/1.1,2 The high hydrogen to aromatic ratio is necessary to avoid the catalyst deactivation.3 Reliable kinetic studies are very important for design, analysis, and control of the industrial process. Although most toluene disproportionation technologies are mature, kinetics of toluene disproportionation over heterogeneous catalysts is not completely understood even in ranges of experimental conditions normally employed by industrial processes. Table 1 presents a summarized review about the kinetic models proposed in the literature, including the experimental conditions and the conversion ranges investigated by the authors. Different kinetic models have been used so far to describe the catalytic disproportionation of toluene. Many authors proposed a pseudohomogeneous kinetics of first order4,5 or second order610 with respect to the toluene partial pressure in order to describe the reaction rates. Good fits have been reported for LangmuirHinshelwood-Hougen-Watson (LHHW) kinetic rate expressions, derived from mechanisms that consider surface reactions1118 as the rate determining step (RDS), and EleyRideal (ER) kinetic rate expressions, r 2011 American Chemical Society
Received: July 17, 2011 Accepted: November 7, 2011 Revised: October 11, 2011 Published: November 07, 2011 171
dx.doi.org/10.1021/ie2015526 | Ind. Eng. Chem. Res. 2012, 51, 171–183
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. Kinetic Studies of Toluene Disproportionation over Zeolites Catalyst (Adapted from Marques17) year
ref
catalyst
P (bar)
T (°C)
kinetic modelsa
XT (%)
1979
(3)
HY/AlF3/Cu
211.1
400500
e22
E-R reaction surface or toluene adsorption as rds
1981
(7)
modified mordenite
1
350450
