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Ethylene Epoxidation on Ag-Cs/r-Al2O3 Catalyst: Experimental Results and Strategy for Kinetic Parameter Determination David Lafarga, Mohammed A. Al-Juaied, Christina M. Bondy, and Arvind Varma* Department of Chemical Engineering, University of Notre Dame, Notre Dame, Indiana 46556
The kinetics of ethylene epoxidation network were studied experimentally over a cesium-doped silver catalyst supported on R-Al2O3 pellets in a differential reactor. A variety of rate functions were considered and among these, expressions based on a dual-site Langmuir-Hinselwood mechanism: ri ) kiPEPnOi/(1 + KiE PE)2, fitted the data best with an average error of 13.0 and 10.7% for the epoxidation and the complete combustion reactions, respectively. Under the experimental conditions, both reactants influence both reaction rates; however, product influence is negligible because of their small partial pressures present in the reactor. High oxygen/ethylene ratios and lower temperature favored selectivity to the epoxidation reaction. The kinetic parameters obtained from the differential reactor experiments were refined to match the fixedbed reactor experiments. With this, the average error in predicting both ethylene conversion and selectivity to ethylene oxide was 4.3%. The apparent activation energies for the epoxidation and combustion reactions were 60.7 and 73.2 kJ/mol, respectively. 1. Introduction The epoxidation of ethylene to obtain ethylene oxide is an industrially important reaction, as the product is a valuable intermediate in the chemical industry. Some seven billion pounds of ethylene oxide are produced in the United States alone, making it one of the top chemicals.1 The reaction scheme is generally considered to be parallel, with two competing reactions involveds the epoxidation and the complete combustion:
Depending on the conditions, a third reaction involving the oxidation of ethylene oxide can also occur: +5/2O2
C2H4O 9 8 2CO2 + 2H2O (3)
(2)
but its rate is generally much smaller than those of reactions 1 and 2. We have recently investigated ethylene epoxidation over cesium-doped silver catalyst supported on R-Al2O3 pellets in a packed-bed membrane reactor, PBMR.2,3 It was demonstrated that significant improvements in ethylene oxide selectivity and yield can be achieved over a conventional fixed-bed reactor, FBR. In our approach, one of the reactants is distributed along the catalyst bed, segregating the components of the feed, which allows controlling local reactant concentrations. We investigated ethylene epoxidation in a PBMR both in the absence and presence of organohalide inhibitors (1,2dichloroethane, DCE), which are widely used in industry * To whom correspondence should be addressed. Telephone: 1-219-631-6491. Fax: 1-219-631-8366. E-mail:
[email protected] to improve catalyst performance.4 The performance of the membrane reactor (with either oxygen or ethylene permeating through the membrane, i.e., PBMR-O or PBMR-E, respectively) was compared with that of a conventional FBR. Several feed reactant concentrations, temperatures, residence times, and DCE concentrations were analyzed. Our previous experimental studies were conducted over the relevant range of temperatures (210-270 °C), but for a fixed overall flow (200 sccm) and a limited range of reactant concentrations (C2H4 and O2: 3-12%, balance N2). To optimize the operating conditions that maximize ethylene oxide production, including flow rates and reactant concentrations, it is necessary to develop a reactor model that is verified by experimental results. The model must include intrinsic reaction kinetics, membrane transport, as well as convective flow, inter- and intraparticle transport processes on the catalyst.5 Prior to optimizing reactor conditions and membrane permeation characteristics, the model should be validated by comparisons with experimental data from both fixed-bed and packed-bed membrane reactors. Accurate rate expressions are needed for both reactor models. Thus, it is essential to determine the intrinsic kinetics for the reaction network for the specific catalyst used, and this is the focus of the present work. 2. Review of Kinetic Models Available in the Literature Silver is the main ingredient in all ethylene epoxidation catalysts. Its uniqueness lies in the fact that oxygen can dissociatively adsorb on it, and the relatively weak bond strength of oxygen to silver permits formation of epoxide upon reaction with ethylene.6 The catalyst performance can be improved by support selection, use of alkaline promoters,4 chlorine containing compounds as feed additives,7,8 and optimal distribution of catalyst in pellets.9,10 There is no general agreement either about the reaction mechanism, or about the reaction kinetics. Various surveys that summarize different aspects of this
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reaction network are available in the literature.4,6,11-13 Most authors agree that the epoxidation and complete combustion are coupled with each other and that the ratio of r1/r2 depends on the interaction of the different species with the silver surface.12 Three different oxygen species (molecular, atomic, and subsurface) can be found on the catalyst surface. Their role has not been clearly assigned, and although initially molecular oxygen was considered to be responsible for epoxidation and atomic oxygen for combustion, recent works show that atomic species may be responsible for both reactions.6 The partial pressures of ethylene, oxygen, carbon dioxide, ethylene oxide, and water have been reported to influence the reaction rates. There is also no general consensus about the influence of individual reactant partial pressures, and very different values of the reaction orders for both oxygen and ethylene can be found in the literature.12 Both reaction rates go through a maximum respect to the ethylene partial pressure, and value of the latter where the maximum occurs depends on the oxygen partial pressure.14,15 The same effect is also reported for oxygen partial pressure.16 In addition, several studies have shown that all product species can inhibit both reactions,16-19 but the reported results are sometimes contradictory. Various models have been used to fit kinetic data, with both Langmuir-Hinshelwood and Eley-Rideal mechanisms, using different active species and limiting step assumptions, without any definitive conclusions at the present. To summarize the general trends, a brief survey is given next. Klugherz and Harriot14 proposed a model in which ethylene and oxygen compete for adsorption on a partially oxygenated silver surface, and a bimolecular reaction between adsorbed ethylene and either atomic (for r1) or molecular (for r2) oxygen is the rate-determining step. In their mechanism, Metcalf and Harriot16 considered molecular oxygen for both reactions, and developed rate expressions to account for inhibition by reaction products. Petrov et al.20 considered a single site Eley-Rideal mechanism and reported similar expressions for both epoxidation and complete combustion reactions. Ghazali et al.21 and Park and Gau15 found that the catalytic surface is partly covered by carbonaceous deposits under reaction conditions, and used dual-site Langmuir-Hinshelwood mechanism to represent the data. On the basis of the same mechanism, Borman and Westertep13 presented expressions accounting for the dependence of reaction rates on partial pressures of all components. The rate-determining step was assumed to be a bimolecular reaction between adsorbed ethylene and dissociatively adsorbed oxygen, with the competing reactions occurring on different catalytic sites. Stoukides and Pavlou17 assumed that epoxidation and complete combustion occur on the same site, and also accounted for the combustion of ethylene oxide. Al-Saleh et al.22 reported rate expressions where carbon dioxide was the only compound inhibiting both reactions. We may note from the above survey that very different rate expressions have been reported in the literature, where some are mechanistic while others empirical. Further, the operating conditions (temperature range, reactant feed concentrations, overall flows) and reactor types (internal and external recycle, CSTR, cooled tubular reactor, integral, differential, vacuum microreactor) also varied substantially between the
different investigators. Taking into account the lack of agreement between the reported kinetics and the different experimental conditions, it was concluded that laboratory investigation of reaction kinetics was required. To ensure the applicability of the intrinsic kinetics for the subsequent reactor modeling and optimization effort, the same experimental conditions as used in the PBMR studies2,3 were employed. 3. Experimental Section 3.1. Catalyst Preparation and Characterization. The catalyst preparation was based on the procedure developed by Bhasin et al.23 The R-Al2O3 pellets (Norton SA 5102, cylindrical, 3 mm o.d.) were pretreated by acid leaching with hydrochloric acid and calcined at 1100 °C for 24 h. The dried pellets were used as support for impregnation with a solution containing silver oxide, cesium hydroxide, lactic acid, and hydrogen peroxide. A calcination treatment with N2 (500 °C for 5 h) was then performed to decompose the lactic acid. Finally, to stabilize the activity, the catalyst was treated alternatively under oxygen and hydrogen flows (3 h each, 100 sccm) in two oxidation-reduction cycles at 350 °C. The final catalyst contained 13.54 wt % Ag and 0.005 wt % Cs (based on the dried weight of the support and reduced catalyst). The support was densely covered with 0.3-0.5 µm silver crystallites. The silver surface area was 925 cm2 Ag/g-cat, while the BET surface area was 0.97 m2/g-cat. Additional details about catalyst characterization are available elsewhere.2 3.2. Differential Fixed-Bed Reactor Experiments. 3.2.1. Apparatus. A schematic diagram of the experimental setup is shown in Figure 1. It consists of three parts: feed mixing, reactor, and analysis zones. The reactor consisted of a 304 stainless steel tube (i.d.: 0.75 cm), packed with Ag-Cs/R-Al2O3 catalyst and inert R-Al2O3 powder forming a fixed-bed configuration. The volume of catalyst bed was 6.6 cm3, packed uniformly over ∼15 cm length zone. The reactor body was secured and placed vertically in a three-zone furnace. Two chromel-alumel (type K) thermocouples were placed inside the reactor, one at each end, with the tips located in the catalyst bed. The isothermal condition was monitored and assured in this manner. In all experiments, the temperature difference between the two thermocouple readings was less than 2 °C. The reactant gases were mixed and preheated before being introduced into the reactor bottom, and exited together from the top. The reactant gases used in the experiments were ethylene/argon mixture (Linde, 50%/50% custom-made), oxygen (Linde, zero grade) and argon (Linde, high purity). Prior to mixing, the hydrocarbon stream passed through a column containing drierite (Alltech Associates) to remove traces of water. The oxygen and argon streams, on the other hand, passed through columns containing drierite and molecular sieve 5A (Alltech Associates) in order to remove excess moisture and hydrocarbon contaminants. To ensure that no particles escaped from the gas purifiers, 7 µm filters (Swagelok Co.) were utilized. The volumetric flow rates of the individual gases were metered using Unit (models: UFC-1100A) mass flow controllers. The feed pressure was monitored and controlled using a pressure transducer and controller (Brooks 5866, Brooks Instruments). A Hewlett-Packard 5890 II gas chromatograph (TCD detector, Hayesep D column; 1/8 in. o.d., 6.1 m long) and
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Figure 1. Schematic diagram of the experimental apparatus.
Hewlett-Packard 3396 II integrator were used to analyze the stream compositions. The carrier gas was helium, and the analysis was carried out at 150 °C. The species concentrations in the feed and outlet streams were measured. The response factors for the different gases were determined using a calibration mixture (Matheson), with composition: ethylene (4.82%), ethylene oxide (1.04%), carbon dioxide (1.97%), and balance nitrogen. Carbon balances typically closed within (2%. Finally, a computer interface and data acquisition program was used. The hardware consisted of a multifunction board and an analog output board installed in a MacIntosh Centris 650 computer. LabVIEW 3.01 software was used for the various controls. The computer permitted continuous monitoring of the transient outputs from the thermocouples and the pressure transducer/controller. The computer also provided automated control of the mass flow rates, pressure, pneumatic GC injection valve and power supply to the preheater and each of the three zones within the reactor furnace. 3.2.2. Experimental Procedure. For the kinetic experiments, the catalyst pellets were crushed to obtain particles in the range 0.425-0.85 mm. The catalyst
particles were diluted with the same size particles of pretreated R-alumina. To fill the catalyst bed, 0.4 g AgCs/R-Al2O3 catalyst particles were mixed uniformly with 6.8 g R-Al2O3 diluent particles, resulting in a bed length of ∼15 cm. For the overall flows used, this packing resulted in a pressure drop through the bed that was always negligible (
