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Ind. Eng. Chem. Res. 2005, 44, 30-40
Monolithic Catalysts with High Thermal Conductivity for Improved Operation and Economics in the Production of Phthalic Anhydride Thorsten Boger* and Monica Menegola Corning GmbH, Abraham-Lincoln-Strasse 30, D-65189 Wiesbaden, Germany
Recent research on extruded monoliths made from materials with high thermal conductivity has shown that such catalyst supports can provide for excellent radial heat-transfer properties. In this paper the use of monolithic catalyst supports in the oxidation of o-xylene to phthalic anhydride is discussed. It is found that for the key catalyst design variables, e.g., amount of active material per reactor volume and thickness of coating, the values for conventional catalysts could be exceeded. Evaluation of the reactor performance is done by means of a detailed mathematical model and literature kinetics, which were adapted to match the performance of industrial reactors and catalysts. The enhanced heat-transfer properties result in better control of the hot spot and allow for operation at more severe conditions, e.g., higher feed concentrations. At equal feed conditions, a higher coolant temperature or catalyst loading is required to balance the lower reaction rates as a result of the lower reactor temperature. The simulation results suggest that higher yields of phthalic anhydride can be achieved at equal or lower hot-spot temperatures. If desired, a significant increase in the reactor capacity is expected by operating at higher feed concentrations. An economic evaluation of the results showed that annual cost reductions on the order of $1 million or more are expected for a 45 kt/year unit. Introduction Until recently, the use of monolithic catalysts for highly exothermic reactions was assumed to not be possible because of poor radial heat-transfer characteristics.1 This assumption, however, was based on properties characteristic of conventional ceramic honeycombs with low thermal conductivity. Recently, Corning Inc. developed a process to prepare extruded metal monoliths with high thermal conductivity,2 made, for example, from copper (see Figure 1). Following some theoretical studies,3 heat transfer and reactive CO oxidation experiments4,5 have shown that, when packaged appropriately into reactor tubes, these monoliths provide excellent radial heat-transfer properties. No radial temperature gradients and overall heat-transfer coefficients on the order of 400-500 W/m2‚K were measured at superficial gas flow rates as low as 0.1 kg/m2‚s. For comparison, typical values for randomly packed catalyst beds operated at 1-2 kg/m2‚s are on the order of 150200 W/m2‚K. Groppi and Tronconi6 discuss the concept of using monolithic catalysts with high thermal conductivity in two selective oxidations, ethylene oxide and formaldehyde, by means of modeling results. Experimental data obtained in an industrial pilot reactor for the oxychlorination of ethylene to ethylene dichloride with the active catalyst supported on metal monoliths showed that the hot spot is significantly reduced compared to the conventional ring-type pellet catalyst.7 In this paper, we investigate the potential of such highly conductive monolithic catalyst structures in the process of producing phthalic anhydride by selective oxidation of o-xylene. Phthalic anhydride is a key intermediate in the production of plasticizers, e.g., for poly(vinyl chloride) as well as for polyesters and resins. The annual production is in excess of 3000 kt, and * To whom correspondence should be addressed. Tel.: +49 611 7366 168. Fax: +49 611 7366 112. E-mail: BogerT@ corning.com.
Figure 1. Photograph of a copper-based monolithic catalyst support with high thermal conductivity.
typical unit capacities range from 15 kt/year for older units to about 75 kt/year in modern plants, although some larger plants were built as well. Today, phthalic anhydride is primarily produced by gas-phase oxidation of o-xylene. Because the reaction is highly exothermic, multitubular reactors are used to control the reactor temperature and the undesired oxidation to CO2. Some typical characteristics and operating conditions of commercial reactors are summarized in Table 1. Figure 2 shows a simplified flowsheet of a phthalic anhydride plant with o-xylene as the feedstock. Actually, monolithic catalyst structures are already used in several phthalic anhydride plants although not yet in the main reactor (shaded areas in Figure 2). The off-gas contains some organic compounds, which need to be removed before it can be emitted to the environment. One technology for phthalic anhydride off-gas purification is catalytic combustion with monolithic catalyst supports.8,9 Eberle et al.10,11 describe the use of monolithic catalysts in postreactors, which are integrated into the heat-exchanger train between the main reactor and the
10.1021/ie040088f CCC: $30.25 © 2005 American Chemical Society Published on Web 12/04/2004
Ind. Eng. Chem. Res., Vol. 44, No. 1, 2005 31
Figure 2. Simplified flowsheet of an o-xylene-based phthalic anhydride process. Table 1. Typical Reactor and Operating Characteristics for the Oxidation of o-Xylene to Phthalic Anhydride parameter no. of tubes per reactor tube inner diameter, mm tube length, m length of the catalyst bed, m heat-exchange medium/coolant coolant circuits standard new units flow rate per tube (air), Nm3/h o-xylene feed concentration, g/Nm3 old, low-productive catalyst state-of-the-art catalyst pressure inlet temperature, °C reactor catalyst bed salt bath inlet temperature, °C salt bath flow
typical value or range Reactor 10 000-20 000 25 3-3.5 2.8 molten salt 1 2
Design of a Monolithic Catalyst for the Selective Oxidation of o-Xylene Commercial catalysts for the selective oxidation of o-xylene to phthalic anhydride are egg-shell-type and are mainly based on ring-shaped supports made from an inert, dense ceramic material. Typical dimensions for the rings are ca. 8 mm outer diameter and 5 mm length. The geometric surface area, void fraction, and packing density are about 600 m2/m3, 55%, and 1100 kg/m3, respectively (all based on the reactor volume). The catalytic active phase is made primarily out of V2O5/ TiO2, with the titania being in the anatase form and the vanadia content being in the range of 2-15 wt %, based on the coating.12 Modern catalysts also contain promoters for improved selectivity.12 The thickness of
15 500 25 2.8 NaNO2/KNO3 1
Process 3-4.5
switch condenser to improve the product quality and extend the catalyst life in the main reactor. Two successful commercial installations in India are reported.10 The objective of the present paper is to explore the upstream extension of monolithic catalyst structure use into the main conversion reactor. For this purpose, the general design of a suitable monolithic catalyst as well as the operation will be discussed by means of detailed numerical simulation. We will also discuss the impact on the process economics for a typical commercial unit.
base case
4
2000 W/m2‚K. The heattransfer resistance due to conduction through the tube wall was considered by using the typical tube wall thickness of ∼2 mm and the thermal conductivity of stainless steel at 400 °C. The resulting value for λtube/ ttube is approximately 7500 W/m2‚K. The heat transfer from the tube to the coolant is considered by the correlation given by Jones16 for the shell-side heattransfer coefficient in shell and tube heat exchangers. Under typical operating conditions with molten salt as the coolant, values on the order of hext ) 2000 W/m2‚K are obtained. Finally, the contact resistance between the monolithic skin and the inner tube wall is considered by a constant value hgap. The values were derived from the experimental data reported by Tronconi et al.4 for a lump of the resistances of the tube (ttube/λtube), the monolith itself (dtube/Kmλr,mono), and the contact between the monolith and the tube (1/hgap). Among the four resistances, the contact resistance between the monolith and the tube, hgap, is the rate-limiting step, with values in the range of 500-600 W/m2‚K. The resulting overall heat-transfer coefficient heff is about 350 W/m2‚K and would be 450 W/m2‚K if the resistance on the shell side (salt bath) could be neglected. The approach chosen, eq 7, is identical with the common one-dimensional approach used for packed beds, when the effective heat-transfer coefficient, heff, is calculated from the effective radial conductivity (equivalent to the calculation for the monolith) and the wall heat-transfer coefficient (equivalent to hgap).17 The resistance of the tube and the external heat transfer to the coolant are usually neglected. For comparative purposes as well as to validate the model, we simulated the performance of industrial reactors filled with pellet catalysts. In this case, we used the same one-dimensional model with heat- and masstransfer parameters for pellet catalysts.13,14,17 In addition, in the enthalpy balance of the gas, a term was added that considers heat exchange with the coolant. It is recognized that pellet-filled tubular reactors are preferably simulated by using at least a two-dimensional model, but we believe that for the purposes of our study the model used here is sufficient.
Table 3. Physical Properties of the Catalyst and the Molten Salt (for the Highly Conductive Monolithic Support, the Values for the Porous Copper Were Used)
material density, kg/m3 heat capacity, kJ/kg‚K thermal conductivity, W/m‚K
(7)
(2)
n)1
∂ωSj
(6)
molten salt
pellet catalyst
active phase (V2O5/TiO2)
highly conductive monolithic support
1900 1.5 0.75
2500 1.0 1.5
1150 0.8 1.5
6200 0.386 130
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Ind. Eng. Chem. Res., Vol. 44, No. 1, 2005 Table 4. Kinetic Parameters Ki,0 and Ea from Calderbank18 and Ki,0 as Used in This Work Ki,0, kmol/kg‚s‚bar
Figure 4. Simplified kinetic network for the oxidation of o-xylene to phthalic anhydride. The solid lines represent the steps considered in this work.
Kinetics In Figure 4, a simplified network of the oxidation of o-xylene to phthalic anhydride is shown. There is a general consensus that phthalic anhydride is formed through the intermediate o-tolualdehyde, which is then further oxidized to phthalide and then to phthalic anhydride (steps 1, 2, and 5, respectively). In addition, some groups suggest the presence of a direct oxidation of o-xylene to phthalic anhydride (step 4)18 route as well as a direct step from o-tolualdehyde to phthalic anhydride (step 7),19 or both.20 With respect to the total oxidation to the undesired carbon oxides CO and CO2, most studies only consider the combustion of o-xylene (step 3)13,18,20 because the total oxidation of phthalic anhydride (step 6) was found to be negligible under practical conditions (
