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KINETICS, CATALYSIS, AND REACTION ENGINEERING Kinetic Study of the Thermal Oxidation of p-Xylene to Terephthaldehyde Won Jae Lee, Won-Ho Lee,* Jong Hyun Chae, Dong Il Lee, Hyun-Kyung Yoon, In Kyu Park, and Ji Hyang Son Corporate Research and DeVelopment, LG Chem, Ltd., Research Park, 104-1 Moonji-Dong, Yuseong, Daejon 305-380, South Korea
Thermal oxidation of p-xylene to terephthaldehyde (TPAL) was investigated in a tubular reactor at 1 bar with various reaction variables, that is, temperature (803-863K), space time (10620-46980 L‚s/mol), and feed concentration of p-xylene (2500 and 7500 ppm). A kinetic modeling was performed on the basis of a power-law rate equation using a proposed simplified molecular reaction scheme. The reaction scheme consists of three reaction pathways: (1) p-xylene to p-tolualdehyde (PTAL), (2) p-xylene to TPAL, and (3) p-xylene to COx (CO and CO2). Model parameters were estimated from the experimental data through minimization of the objective function. The activation energy for the formation of TPAL from p-xylene was 49.2 kJ/mol. The order of reaction was approximated to zero with respect to partial pressure of p-xylene. The kinetic model yields an excellent fit of the experimental data. 1. Introduction The aromatic aldehydes are very important monomers in the chemical and pharmaceutical industries. Their applications include a variety of chemicals, specialty polymers, and drugs. Terephthaldehyde (TPAL), one of the most important aromatic aldehydes, has been produced via a liquid-phase Friedel-Crafts reaction. This process, however, is not environmentally benign because the toxic chlorinated compounds are produced. With the concern over the environment, the increasing demand for alternative oxidation processes has stimulated the development of the selective gas-phase oxidation using heterogeneous catalysts. The catalysts investigated so far for the oxidation of p-xylene to terephthaldehyde (TPAL) are Fe/ Mo/borosilicate molecular sieve,1 Fe/Mo/zeolites,2 WO3-MoO3/ Al2O3,3 W-Bi-Mo/Al2O3,4 W-Sb-Fe/Al2O3,5 and unsupported W-Sb.6 Recently, the W-Sb-based catalysts have received attention because of a high yield of TPAL.5,6 However, because a relatively high temperature is required to achieve a high yield of TPAL on these catalysts, a certain extent of p-xylene loss is inevitable due to thermal oxidation occurring in the zones without catalysts or in the void fraction of the catalyst bed itself.7 The present investigation aims to address the significance of thermal oxidation for the catalytic oxidation of p-xylene to TPAL and to provide a simple methodology to model the thermal oxidation of p-xylene to TPAL. Previous studies of the thermal oxidation of xylenes, which are important components of gasolines and jet fuels, have been mainly focused on the reactions at extremely high temperature. Emdee and co-workers8 studied the mechanism for the oxidation of m- and p-xylene in an atmospheric flow reactor at temperatures ranging from 1093 to 1199 K. Gaı¨l and Dagaut9 presented the detailed mechanism for the oxidation of p-xylene in a jetstirred reactor (JSR) at atmospheric pressure over the temper* To whom correspondence should be addressed. Tel.: +82-42866-2408. Fax: +82-42-863-7466. E-mail:
[email protected].
ature range 900-1300 K. More recently, Battin-Leclerc et al.10 modeled the oxidation of three xylene isomers in a shock tube at temperatures from 1300 to 1820 K and pressures from 6.7 to 9 bar and reported a similar oxidation reactivity among these isomers. The slow combustion of xylenes at less than 1000 K has also been investigated. Wright11 performed the gas-phase oxidation of three xylene isomers in a static reactor at subatmospheric pressures over the temperature range from 683 to 823 K. The overall activation energy and the order of reaction were reported, whereas the detailed reaction mechanism was not presented. Barnard and Hawtin12,13 examined the oxidation of p-xylene in a static system at temperatures between 733 and 785 K. They proposed the reaction mechanism in which formaldehyde plays an important role as the degenerate branching intermediate. To the best of our knowledge, any literature focusing on the kinetic investigation for the formation of TPAL, the desired product in the catalytic oxidation of p-xylene on the W-Sb catalysts, from the thermal oxidation of p-xylene between 803 and 863 K has not been reported. Kinetic studies for the thermal oxidation of p-xylene investigated so far by most of the researchers have the following limitations for the application to the present work: (1) Most of the kinetic studies performed at less than 900 K mainly dealt with the overall consumption of p-xylene, ignoring the formation of TPAL, the desired product. (2) The product distribution obtained for the combustion of p-xylene at temperatures greater than 1000 K is different from that in this work. Furthermore, the complete set of kinetic models derived from the detailed free radical mechanism, which obviously represents what occurs in the thermal oxidations, cannot be readily used to simulate and design a chemical reactor in that the solution of differential equations accounting for all of the reacting species involved in the radical reactions is extremely complicated and time-consuming. This is why the simplified molecular reaction schemes have been frequently implemented for the thermal cracking of hydrocarbons.14,15
10.1021/ie070294y CCC: $37.00 © 2007 American Chemical Society Published on Web 08/10/2007
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Figure 1. Experimental setup for the thermal oxidation of p-xylene to TPAL.
In this work, we present a simple description for the thermal oxidation of p-xylene to TPAL using the molecular reaction scheme rather than relying on the radical reaction scheme and then propose a kinetic model based upon a power-law kinetics, which can be eventually coupled with that for catalytic reaction to design an industrial-scale multitubular reactor for mass production of TPAL from catalytic oxidation of p-xylene. 2. Experimental Procedures Figure 1 shows a flow diagram of the experimental setup. Liquid p-xylene (Aldrich) was fed by a HPLC constant pressure pump (Lab Alliance). p-Xylene was vaporized and mixed with air in a preheater. The mass flow rate of air was controlled by a mass flow controller (Brooks). The gaseous mixture of p-xylene and air was heated to 673 K prior to injection to the top of reactor. No appreciable thermal degradation of p-xylene at this temperature was observed. The tubular reactor has a length of 1.445 m and an inner diameter of 0.0284 m. The reactor is made of chromium-nickel alloy (Inconel 600) that is designed for use at high temperature and under oxidizing conditions. In addition to the homogeneous thermal reactions, heterogeneous reactions catalyzed by carbonaceous material20 and/or reactor material may take place on the wall, so that these reactions should be diminished as much as possible for the rigorous kinetic modeling of homogeneous thermal reactions. In this paper, however, the wall effect was not discriminated because we focus on developing the kinetic model of thermal oxidation of p-xylene that can be easily used to design a catalytic industrial-scale multitubular reactor, where some extent of heterogeneous reactions on the reactor wall cannot be avoided. Therefore, the term “thermal reaction” includes both the homogeneous thermal reaction and a part of the heterogeneous reactions by wall effect. The reactor was heated by five heaters. To operate the reactor isothermally the reactor was immersed in a molten salt bath. Prior to conducting experiments, a proprietary mixture of solid salts was packed inside the bath. As the temperature increases, the solid salts turn to the molten state. For a long-run operation of reactor great attention should be paid to maintain the appropriate level of molten salts, which are continuously consumed due to vaporization, by adding the solid salts into the bath. The temperature of the molten salt was measured by
three thermocouples positioned in an axial direction. An axial temperature distribution inside the reactor was obtained using 24 thermocouples located at regular intervals in the thermowell, which has an outer diameter of 0.008 m and is placed inside the reactor. We observed no appreciable temperature gradient ((0.5 °C around target temperature) over 95% of the total reactor length. Most of the effluent from the bottom of the reactor flowed through the condenser unit to the vent system. A small portion of the effluent was directed to the sampling valve for the GC analysis. All of the process tubes before the GC were heated around 593 K to prevent the deposition of solid TPAL inside the tube. The on-line GC analysis was performed using an Agilent 6890 equipped with a TCD and a FID. The light gaseous components, that is, N2, O2, CO, and CO2, were separated by two packed columns (6 ft Porapak and 6 ft Molecular Sieve) and sent to the TCD. The hydrocarbon components, that is, benzene, toluene, p-xylene, TPAL, p-tolualdehyde (PTAL), benzaldehyde (BAL), p-hydroxylbenzaldehyde (PHBAL), and a trace of unidentified heavy hydrocarbons, were separated by a capillary column (30 m sol-gel Wax) and analyzed by the FID. Because the concentrations of benzene, toluene, BAL, and PHBAL were extremely low, these compounds were not taken into account for the kinetic modeling. On the basis of the GC analysis, the conversion of p-xylene (XPX), selectivity to product j (Sj) and yield of product j (Yj) are calculated as
XPX (%) )
Sj (%) )
F0PX - FPX F0PX Fj
F0PX
Yj (%) )
- FPX Fj
F0PX
× 100
× 100
× 100
where F0PX is the feed molar flow rate of p-xylene in mol/s, FPX is the effluent molar flow rate of p-xylene in mol/s, and Fj is the effluent molar flow rate of product j in mol/s.
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Figure 2. Comparison of conversion of p-xylene (A), yield of TPAL (B), yield of PTAL (C), and yield of COx (D) obtained by experimental (symbols) and calculation from kinetic model (solid lines) at Ptotal ) 1 bar, p-xylene feed concentration ) 2500 ppm, and T ) 803 K (b), 833 K (O), and 863 K (9).
Figure 3. Comparison of conversion of p-xylene (A), yield of TPAL (B), yield of PTAL (C), and yield of COx (D) obtained by experimental (symbols) and calculation from kinetic model (solid lines) at PTotal ) 1 bar, p-xylene feed concentration ) 7500 ppm, and T ) 803 K (b), 833 K (O), and 863 K (9).
Table 1. Reaction Conditions for the Kinetic Study of the Thermal Oxidation of p-Xylene total pressure, bar temperature, K O2 feed concn, mol % p-xylene feed concn, ppm V/F0PX, (L‚s)/mol of p-xylene
1.0 803, 833, 863 20.95, 20.84 2500, 7500 10620-46980
3. Results and Discussion 3.1. Experimental Results. The reaction conditions used for the kinetic study of the thermal oxidation of p-xylene are listed in Table 1. Because the lower and upper flammability limits of p-xylene at the standard temperature and pressure are 1.1 and 6.6 vol % in air,22 respectively, the feed concentration of p-xylene never exceeds 7500 ppm for safety. Space time, V/F0PX, is defined as the reactor volume divided by the feed molar flow rate of p-xylene in (L‚s)/mol of p-xylene. Experimental data were gathered by analyzing the exit sample several times at the same reaction conditions using the online GC. The data shown in the following figures are averages of those values. The standard deviation of each point is so small (
