Investigation of the Kinetics of Ethylbenzene Pyrolysis Using a

A new technique of gathering kinetic data has been examined. This involved gathering raw data at the end of a plug-flow reactor undergoing temperature...
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Ind. Eng. Chem. Res. 2001, 40, 5878-5884

KINETICS, CATALYSIS, AND REACTION ENGINEERING Investigation of the Kinetics of Ethylbenzene Pyrolysis Using a Temperature-Scanning Reactor S. B. Domke,* R. F. Pogue, F. J. R. Van Neer, and C. M. Smith The Dow Chemical Company, Freeport, Texas 77541-3257

B. W. Wojciechowski Department of Chemical Engineering, Queen’s University, Kingston, Ontario, Canada

A new technique of gathering kinetic data has been examined. This involved gathering raw data at the end of a plug-flow reactor undergoing temperature ramping. The data acquired in this way was made to yield many isothermal sets of rate data suitable for fitting to candidate rate expressions. This technique was applied to the thermal decomposition of ethylbenzene. The resulting fit to a rate expression proposed previously in the literature was good. The parameters obtained compare well with literature values, and our fitted rate expressions yield calculated conversions in agreement with those reported in the literature. We believe that temperaturescanning methods used in experimental reactors yield correct rate data and are considerably faster for rate data acquisition than conventional isothermal reactors. Introduction As global competition increases in the marketplace, the need for obtaining data quickly becomes critical. The theory of temperature scanning (TS) allows for the rapid collection of kinetic data assuming that certain boundary conditions are obeyed.1-3 Valid kinetic data can be collected by ramping the reactor and feed temperature in several runs, each at a different space velocity. A set of such rampings constitutes a temperature-scanning reactor (TSR) experiment from which large numbers of rates can be extracted over the range of temperatures and conversions encountered. The method increases the speed at which kinetic data can be acquired by orders of magnitude and was used here to study the thermal decomposition of ethylbenzene (EB), an industrially relevant reaction in the production of styrene. Traditional kinetic methods used in the past involved singlepoint data collection after a reactor had reached a condition of steady state. These methods were time- and resource-consuming and consequently only examined a limited range of conditions. Reaction kinetics, therefore, were normally determined from curves consisting of no more than a few error-containing points, resulting in rate data of low precision and wide confidence limits. The TSR allows increased model accuracy and predictive ability by collecting a large number of data. A well-documented kinetic model of the thermal cracking of EB with steam dilution is useful for predicting the thermal conversion of EB under a variety of industrial catalytic reactor conditions. An accurate model would allow the separation of the thermal contribution to total conversion and to individual product * Corresponding author. Phone: (979) 238-7314. Fax: (979) 238-0028. E-mail: [email protected].

yields and consequently would permit the quantification of catalyst conversion and selectivity in the industrial reactor system. The massive amounts of kinetic data made available by TS is expected to allow us to formulate the necessary broadly applicable, and well documented, model of the thermal cracking reaction. The thermal cracking of EB was studied using a temperature-scanning plug-flow reactor (TS-PFR) at various temperatures, pressures, flow rates, and steamto-oil ratios. EB pyrolysis has been studied extensively but not over such a wide range of conditions as presented here.4-11 In addition, studies with steam dilution are limited.10,11 The simplified thermal cracking model developed on the basis of this data is to be combined with our current catalytic models to provide a complete description of the reactor performance. Experimental Methods Hardware. The TS-PFR system used is shown in Figure 1. Brooks and Porter mass flow controllers were used for all gas and liquid flows, respectively. The liquids were vaporized prior to entering the reactor. The reactor consisted of a 1-in.-o.d. 316 stainless steel tube. The reaction zone was 10 in. long and filled with inert ceramic balls, resulting in a reactor void fraction of 0.4. The furnace was a clam-shell design with two 1500-W heaters embedded in ceramic fiber insulation. A 1/8-in. thermowell was placed up through the bottom of the reactor and into the reaction zone. Four 1/16-in. thermocouples were equally spaced through the length of the reaction zone (see Figure 1) to monitor temperatures. This was the first TSR system to utilize a clamshell furnace rather than a convection oven so that higher temperatures could be studied. The furnace was designed to be isothermal, and tests were performed to

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Figure 1. TS-PFR system. Table 1. Experimental Design for the TS-PFR Thermal Cracking Study run

steam-to-oil ratio

pressure (psia)

1 2 3 4 5 6 7 8

1.2 1.0 2.0 3.0 5.0 0.5 1.2 1.2

14.7 14.7 14.7 14.7 14.7 14.7 10.0 5.0

run

steam-to-oil ratio

pressure (psia)

9 10 11 12 13 14 15

1.2 0.5 0.5 0.5 5.0 5.0 5.0

25.0 10.0 5.0 25.0 10.0 5.0 25.0

ensure reproducible temperature ramps prior to beginning the experiments. The axial temperature gradient through the catalyst bed was 4 °C. All of the functions of the equipment were logged and controlled using SE Reactors’ TSR software and a duTec interface. During a typical experiment, the reactor temperature was ramped at a constant rate. Throughout the temperature ramp, EB and product yields were determined by periodically sampling a portion of the reactor effluent using an online gas chromatograph (GC), instead of a mass spectrophotometer (MS) for which the TS technique was originally designed. The stream composition analysis was obtained using a modified Hewlett-Packard 5890 Series II GC equipped with multiple columns, a flame ionization detector (FID), and a thermal conductivity detector (TCD). The aromatics portion (i.e., benzene, toluene, EB, and styrene) of the effluent stream was analyzed using a DB-Wax, 10-m, 0.18-mm-i.d. narrowbore, 0.3-µm-filmthickness column and a FID detector. The lighter products (i.e., hydrogen, methane, carbon monoxide, carbon dioxide, ethane, and ethylene) were analyzed using dual columns and a TCD detector. The first column was a 25-m, 0.32-mm-i.d., Poraplot Q column, and the second was a 15-m, 0.32-mm-i.d., 25-µm-filmthickness, molecular sieve 5A column. Periodically, the GC was calibrated using known standards. Visual Basic macros were developed to communicate between the GC ChemStation and the TSR software to create a file in the correct format for access by the TSR data analysis software. Experimental Design. Steam-to-oil ratio, temperature, and pressure were the variables used in the experimental design shown in Table 1. Experiments were performed over a temperature range of 540-700 °C using a ramping rate of 1.0 °C/min. Each experiment consisted of 13 runs (i.e., temperature ramps) at various space velocities (i.e., space times or τ’s). Space time, τ,

Figure 2. Raw mole fraction data from the GC for EB, styrene, and benzene at τ ) 5.6 s from run 7 (S/O ) 1.2 (w/w), P ) 10.0 psia).

is a fundamental measure of the time of contact between reactants and the reaction zone used in kinetic rate expressions and is defined by

τ ) (volume of the reaction zone)/ (total volumetric flow rate of the feed vapors at standard temperature and pressure) (1) The space times of our runs ranged from 1.8 to 17.0 s, with the run at τ ) 5.6 s repeated twice to ensure reproducibility within the experiment. The space time range selected was limited by the high and low limits of the EB mass flow controller. The Peclet (Pe) and Bodenstein (Bo) numbers were used to determine that axial dispersion and wall effects were negligible, which ensured operation in a plug-flow regime. Data Processing and Handling. After the raw data was collected, it was processed through filtering and rate extraction steps using the TSR software. Because the TSR software provides rate data, parameter estimation for the rate expressions was performed using the Microsoft Excel Solver. Each of the 15 experiments performed ran for approximately 3 days. Data was collected every 4 min, resulting in over 500 raw readings (data points)/experiment. For each data point, the reactor pressure, input flow rate, output temperature, product composition, and various other readings were collected in a file for subsequent analysis by the TSR software. Figure 2 illustrates the reproducibility of the raw analytical data from the GC. Once a significant amount of EB conversion was achieved (>10%), the error for EB and styrene was 5% and that for benzene was 15%. There tended to be more scatter in the data for small concentration components such as benzene because of the vagaries of the GC analysis. This was unavoidable in realistic situations but could be handled by sophisticated filters because of the nature of the errors and because of the masses of data available. The raw data was smoothed using two-dimensional filters built into the TSR software. This process is akin to removing noise from communication signals and elsewhere where there is a large amount of data containing known or expected interdependencies. In this case the interdependencies are that we expect the underlying, “true”, curves to be smooth and the gap

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Figure 3. EB conversion versus oven temperature for various space times, τ, for run 14 (S/O ) 5.0, P ) 5.0 psia).

Figure 4. Equilibrium approach by TS-PFR for run 2 (S/O ) 1.0, P ) 14.7 psia) at τ ) 8.7 s (0.315 std L/min of total flow rate).

between the successive runs to show smooth behavior also. This gave a basis for filtering the large number of data points from each experiment to create a smooth kinetic surface in the [conversion (X), temperature (T), and space time (τ)] coordinates. Figure 3 shows such a surface from one of our runs. The smoothed (X, T, τ) surface allowed rates ()dX/dτ) to be calculated and yields a smooth set of [rate (r), conversion (X), and temperature (T)] data. During rate extraction, the TSR software applied the required algorithm, which indicates the direction on the (X, T, τ) surface along which slopes must be taken for valid rates to be calculated. The (r, X, T) triplets were the data used in fitting of rate expressions where, in general, r ) f(X,T).3 Results In our experiments EB conversions ranged from 0 to 70%. At low temperatures, at the beginning of a run, the EB conversion was far from equilibrium, but as the temperature was increased, the difference between the actual and equilibrium conversion became smaller. The approach to equilibrium is illustrated in Figure 4. For all experiments, EB conversion always increased with increasing temperature but decreased with increasing flow rate (decreasing τ). Figure 5 shows the EB mole fractions for run 3 at various τ’s. In all cases the major product from the thermal cracking (dehydrogenation) was styrene (Figure 6). It was observed that styrene selectivity went through a maximum with increasing temperature. The maximum varied with the experimental conditions but, in general, was around 627 °C. Selectivity to benzene, toluene, methane, and carbon monoxide increased with increasing temperature and pressure. At 700 °C, maximum selectivity to benzene ranged from 20% at 5 psia to 30% at 25 psia. The reaction was equally selective to toluene and methane (Figures 7 and 8) with selectivities ranging from 8% at 5 psia to 15% at 25 psia at 700 °C. At the high temperatures, carbon monoxide selectivity ranged from 7 to 15% as the pressure increased. Selectivity to ethylene also increased with temperature but remained relatively constant with pressure at around 20% at high temperatures. Throughout the range of conditions ethylene selectivity was approximately 4 times that of

Figure 5. Mole percent EB versus clock time at various τ’s for run 3 (S/O ) 2.0, P ) 14.7 psia). The top curve represents the results at the lowest τ (highest flow rate), and the bottom curve represents the data at the highest τ (lowest flow rate).

ethane. There was also little effect of pressure on the selectivity to ethane. Selectivity to carbon dioxide decreased with temperature to around 5% at 700 °C. Modeling EB conversions and product yields were calculated assuming 0% conversion and yield at the entrance of the reactor. Conversion was defined as the amount of EB converted divided by the amount of initial EB, and yield was defined as the amount of product formed divided by the amount of initial EB. Sets of isothermal rates for each of the individual components were extracted from the data using the TSR program. The temperature range for these calculations was 567-697 °C. The rate extractions resulted in files containing triplets of rate, conversion (or yield), and temperature.

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Figure 6. Mole percent styrene versus clock time at various τ’s for run 3 (S/O ) 2.0, P ) 14.7 psia). The top curve represents the results at the highest τ (lowest flow rate), and the bottom curve represents the data at the lowest τ (highest flow rate).

Figure 8. Mole percent toluene versus clock time at various τ’s for run 3 (S/O ) 2.0, P ) 14.7 psia). The top curve represents the results at the highest τ (lowest flow rate), and the bottom curve represents the data at the lowest τ (highest flow rate). Table 2. Rate Parameters Determined from Experimental Data component

ln(A)

Ea/R (K)

n

R2

EB styrene benzene toluene methane ethylene ethane

31.49 ( 0.05 27.35 ( 0.06 33.28 ( 0.08 31.13 ( 0.04 31.13 ( 0.10 34.10 ( 0.03 29.70 ( 0.03

-31356 ( 26 -27440 ( 46 -34655 ( 72 -33219 ( 30 -33219 ( 91 -35818 ( 25 -32667 ( 20

1.00 ( 0.14 0.70 ( 0.09 1.00 ( 0.15 1.00 ( 0.12 1.00 ( 0.09 1.00 ( 0.03 1.00 ( 0.11

0.90 0.89 0.88 0.82 0.87 0.79 0.90

irreversible reaction with an Arrhenius form of the rate constant, ki.

ri,calc ) kiPEBn ) Ai exp(Ea,i/RT)PEBn

Figure 7. Mole percent methane versus clock time at various τ’s for run 3 (S/O ) 2.0, P ) 14.7 psia). The top curve represents the results at the highest τ (lowest flow rate), and the bottom curve represents the data at the lowest τ (highest flow rate).

The TSR rates were calculated using the definition of τ at standard conditions (0 °C and 1 atm) and had units of (change in fractional conversion or yield)/(change in space time). These TSR rates were converted to rates at actual operating conditions with units of (change in partial pressure)/(change in time). Volume expansion due to reaction was taken into account with the unit conversions. The unit conversions followed the form of eq 2 where T and P are the actual reactor operating

ri ) ri,TSR

(1 + x) (273T K)(1 atm P )

(2)

conditions, x is the conversion,  is the change in moles when the reaction is completed divided by the total moles fed. Several model forms were tested for the rate expressions. The best fit was with the rate expression for an

(3)

The preexponential factor (Ai), activation energy (Ea,i), and order (n) were determined by minimizing the sum of the residuals. Rate parity plots for various components are shown in Figure 9. Table 2 lists the parameters determined for the various components. The rate parameters were then used to predict compositional data from the TSR. Figures 10 and 11 show examples of the TSR experimental data versus predictions for EB and toluene at all temperatures, pressures, and steam-to-oil ratios used in this study. The EB parity plot appears to have breaks in the data due to the different initial EB mole fractions used in the experiments. Deviations are greatest at low initial EB concentrations and high EB conversions. The simplicity of the model, its irreversibility, and unaccounted secondary reactions of the products probably account for the deviations at high conversions.4,5 For EB conversions of less than 30%, the model equations predict the experimental partial pressures of the major components to within 10%. In Table 3, the rate parameters determined for styrene, benzene, and toluene are compared with parameters found in the literature. The literature values listed in the table were determined by different methods. Three of the authors used carrier gases in conventional flow systems (Clark and Price,4 toluene; Bruins-

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Figure 9. Rate parity plots of predicted rate versus experimental rate for EB, styrene, benzene, and toluene. Table 3. Rate Parameter Comparison with the Literature Data styrene

benzene

toluene

author

Ea/R (K)

ln(A)

n

Ea/R (K)

ln(A)

n

Ea/R (K)

ln(A)

n

this work Clark and Price Brooks et al. Bruinsma et al. Hausmann and King

-27440 -32246 -43240 -26582 -34524

27.35 29.2 41.9 26.4 29.0

0.7 1.0 0.6 1.0 0.5

-34655 -26000 -38658

33.28 20.9 38.2

1.0 1.0 1.0

-33219 -35300 -35097

31.13 33.8 33.0

1.0 1.0 1.0

ma et al.,6 argon; Hausmann and King,5 nitrogen). Brooks et al.7 used a conventional static reactor for their experiments. Temperatures studied by these authors varied from 500 to 815 °C. The styrene parameter values in this work are within the range of the literature values, although some difference in these values may be expected because of the different reaction orders used. The benzene and toluene values also compare well with the literature

values. As another confirmation of our model, the experimental data from Clark and Price was predicted using our model and parameters, as shown in Figure 12. The Clark and Price experiments were conducted at temperatures of 637-816 °C and reaction times of 9-18 min. The majority of the data points are within 15% of their predicted values. We note that we might have introduced some error into the Clark and Price experimental data because conversion was not explicitly

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Figure 13. Comparison of the literature values (Crowne et al.8) with the predicted values of conversion from the rate parameters determined with the TSR. Figure 10. EB mole fraction parity plot for all pressures, temperatures, τ’s, and steam-to-oil ratios.

13. The Crowne et al. data was collected at temperatures between 633 and 737 °C, a constant contact time of 0.4 s, and a pressure of 0.02 atm. The R2 value for both predictions was 0.96. Conclusions

Figure 11. Toluene mole fraction parity plot for all pressures, temperatures, τ’s, and steam-to-oil ratios.

The TSR technique has been successfully extended to a more traditional reactor/furnace setup with GC analysis instead of the convection oven and MS analysis used previously.12,13 Using this very different system, useful kinetic information was obtained for a complex reaction over a wide range of conditions. The 15 experiments required approximately 45 days to collect over 7500 raw data points. TSR methods made the number of (r, X, T) triplets available for data fitting to candidate rate expressions quite large. We believe that it would be impossible to collect some 170 raw data points/day, over such a wide range of conditions, using conventional methods of data collection. This disparity will always be present because of the time needed for a conventional reactor to attain steady state after each change of reaction conditions. The data collected from the TS method was used to calculate kinetic parameters that compared well with parameters determined from conventional methods. The kinetic model was capable of predicting experimental results from the literature and the current study. Literature Cited

Figure 12. Comparison of the literature values (Clark and Price4) with the predicted values of conversion from the rate parameters determined with the TSR.

reported in the original paper and was calculated by us from the reported results. The model developed also predicts data from Crowne et al.,8 as shown in Figure

(1) Wojciechowski, B. W. The Temperature Scanning Reactor. I: Reactor Types and Modes of Operation. Catal. Today 1997, 36, 167. (2) Rice, N. M.; Wojciechowski, B. W. The Temperature Scanning Reactor. II: Theory of Operation. Catal. Today 1997, 36, 191. (3) Asprey, S. P.; Rice, N. M.; Wojciechowski, B. W. The Temperature Scanning Reactor. III: Experimental Procedures and Data Processing. Catal. Today 1997, 36, 209. (4) Clark, W. D.; Price, S. P. Free-radical and Molecular Processes in the Pyrolysis of Ethylbenezene. Can. J. Chem. 1970, 48, 1059. (5) Hausmann, E. D.; King, C. J. Pyrolysis of Ethylbenzene with and without Oxygen Initiation. Ind. Eng. Chem. Fundam. 1966, 5 (3), 295. (6) Bruinsma, O. S. L.; Geertsma, R. S.; Bank, P.; Moulijn, J. A. Gas-Phase Pyrolysis of Coal-related Aromatic Compounds in a Coiled Tube Flow Reactor. Fuel 1988, 67, 327. (7) Brooks, C. T.; Peacock, S. J.; Reuben, B. G. Pyrolysis of Ethylbenzene. J. Chem. Soc., Faraday Trans. 1 1982, 78, 3187.

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(8) Crowne, C. W. P.; Grigulis, V. J.; Throssell, J. J. Pyrolysis of Ethylbenzene by the Toluene Carrier Method. Trans. Faraday Soc. 1969, 65, 1051. (9) Szwarc, M. The C-C Bond Energy of Ethylbenzene. J. Chem. Phys. 1949, 17 (5), 431. (10) Chen, Q.; Froment, G. F. Thermal Cracking of Substituted Aromatic Hydrocarbons. II. Kinetic Study of the Thermal Cracking of n-Propylbenzene and Ethylbenzene. J. Anal. Appl. Pyrolysis 1991, 21, 51. (11) Webb, G. A.; Corson, B. B. Pyrolytic Dehydrogenation of Ethylbenezene to Styrene. Ind. Eng. Chem. 1947, 39 (9), 1153.

(12) Asprey, S. P.; Wojciechowski, B. W.; Peppley, B. A. Kinetic Studies using Temperature-scanning: the Steam-reforming of Methanol. Appl. Catal., A 1999, 179, 51. (13) Wojciechowski, B. W.; Asprey, S. P. Kinetic Studies using Temperature-scanning: the Oxidation of Carbon Monoxide. Appl. Catal., A 2000, 190, 1.

Received for review May 31, 2001 Revised manuscript received September 11, 2001 Accepted September 21, 2001 IE010483V