Ind. Eng. Chem. Res. 1988,27, 1377-1382
1377
The Stability Limits of the Catalytically Stabilized Thermal Combustion of Methane and Ethane Stephen L. Hung, Timothy A. Griffin, and Lisa D. Pfefferle* Department of Chemical Engineering, Yale University, New Haven, Connecticut 06520
The lean stability limits for the catalytically stabilized thermal combustion of various mixtures of methane/ethane over supported platinum catalyst were determined. The reference inlet velocity of the flow over the catalyst was maintained a t 97 cm/s, and the catalytic bed was well-insulated to achieve adiabatic conditions. When ambient air is used as the oxidizer, the results show that the minimum operating temperatures required for stable combustion were 1680 and 1510 K for pure methane and pure ethane, respectively. It was found that the minimum stable operating temperature for methane combustion could be reduced by nearly 80 K by adding ethane as approximately 20% of the total fuel flow. Comparisons with heterogeneous/homogeneous ignition characteristics of methane/ethane mixtures over a flat catalytic plate, also determined in this study, demonstrate that ethane extends the methane stability limits by decreasing the ignition delay time of the homogeneous gas-phase reactions. The feasibility of using a catalyst to improve the stability of a gas-phase combustor was first demonstrated by Pfefferle in 1970. Since that time, many studies on the catalytically stabilized thermal (CST) combustor have found that the CST combustor is not only more stable than conventional combustors, but soot and gaseous pollutant emissions are also reduced. Because the combustion in the CST burner can be stabilized at lower fuel concentrations, it can operate at much lower temperatures than a conventional combustor. The lower operating temperature, the presence of the catalyst, and the excess of available oxygen all act to reduce the amounts of NO, and other common combustion pollutants emitted (Pfefferle, 1975, 1978; Wampler et al., 1976; Folson et al., 1978; Chu and Kesselring, 1978; Prasad et al., 1980,1981a-c; Bruno et al., 1983a,b). These advantages make the CST combustor ideal for application in situations where the control of pollutants is important. The objective of this study was to determine the practicality of using a CST burner for the combustion of natural gas. The content of natural gas varies widely depending on the source. Although natural gas contains mostly methane and ethane, it may also contain N2,COz, 02,and H2S as well as higher hydrocarbons. The methane content of natural gas may vary from as little as 64.5% (Libyan) to as much as 98% (Soviet Union). In the United States, the methane content of natural gas varies from 70% to 95% and the ethane content from 3% to 18% with higher hydrocarbons, N2 and C02 making up most of the balance (Chigier, 1981). Because natural gas varies in methanelethane content, the stability limits of the CST combustor were studied as a function of the methane/ ethane fuel ratio. Air at ambient temperature, the most commonly available form of oxidizer, was used throughout this study. The results of this study may be applicable to finding a more efficient means of using natural gas through design of a CST combustor system that can run optimally over wide ranges of fuel compositions. Previous studies by other researchers have done much to characterize the operation of CST combustors. Many fuels have been used in these studies, but most of the published papers have reported the use of propane as the fuel (Prasad et al., 1981a-c; Bruno et al., 1983a,b;Wampler et al., 1976). Little has been documented on the CST combustion of methane short of diffusive fiber catalytic "combustors" described by Radcliffe et al. (1975) and Trimm and Lam (1980). These burners, however, are not 0888-5885/88/2627-1377$01.50/0
true gas-phase combustors and operate as catalytic oxidation reactors where the reaction rate is limited by the rate of mass transfer to the catalytic surface. Other researchers used methane as a fuel to study NO, formation by the addition of NH, (Folson et al., 1978) but did not report the effect of the fuel on combustion stability limits. In previous studies of CST combustion, the feed gases were preheated in order to lower the minimum equivalence ratio required for stable operation. Preheating has two advantages: A lower stable equivalence ratio allows lower burner operating temperatures; also, if the preheating temperature is high enough, the feed will autoignite in the catalytic bed. When propane in air is used at an equivalence ratio of 0.20 with inlet feed preheat of 600 K, the CST burner can be stabilized at an operating temperature as low as 1200 K (Prasad et al., 1981a-c). In contrast to previous studies, the current study did not employ preheating of the fuel or oxidizer. Although this significantly increases the operating temperature and shortens the catalyst life span, it demonstrates that preheating is not required to sustain CST combustion.
Background There are several reasons why the CST combustor is more stable and more efficient than conventional combustors. As an igneslytic combustor (Pfefferle and Pfefferle, 1987), it is not only catalytically stabilized but is also thermally stabilized by the ceramic monolithic catalyst support. Without preheating, the feed gases are unable to autoignite in the presence of a cold catalyst, and an external source of energy must be introduced to help ignite the burner. This may be done electrically or by using a flame. After the reactor is ignited and the combustion is self-sustaining, the catalyst reaches a temperature close to the adiabatic flame temperature of the incoming fuellair mixture. The cold incoming gases contact the hot catalyst walls to ignite surface oxidation reactions. The wall reactions generate energy and maintain the wall near the adiabatic flame temperature, heating the gas adjacent to the walls sufficiently to ignite gas-phase combustion. The wall reactions may also influence the production of free radicals that help initiate the gas-phase reactions. Once the gas phase ignites, the homogeneous reactions quickly propagate to the center of the tube and consume the bulk of the combustible material in a flame zone that we have shown to be less than 100 pm thick in this study. Combustion byproducts, such as carbon monoxide, which are 0 1988 American Chemical Society
1378 Ind. Eng. Chem. Res., Vol. 27, No. 8, 1988
1 1
?atred aed I8 r ~ n gC n a v D e r
nq S c r e e n s and Sta less Steel Woo Mix
Li-1-P I Catolyst Coated honeycaTQed Mono1 ti-
!Is"
01
ng
AsneycJmbed 2l-.la I n s
Figure l. Schematic diagram of catalytically stabilized burner apparatus.
inherently slower to oxidize, react quickly in the presence of the catalyst immediately downstream of the main combustion zone. The ability of the CST combustor to combine both heterogeneous wall reactions and homogeneous gas reactions gives it the increased stability and efficiency as well as a high throughput. A more complete account of the operation and characteristics of CST combustion may be found in the literature (Pfefferle and Pfefferle, 1986, 1987; Trimm, 1983).
Experimental Setup A schematic diagram of the experimental test burner is shown in Figure 1. The apparatus consists of a mixing chamber and a reaction chamber through which the gases pass horizontally. The reaction chamber contains the catalytic bed and insulation. The fuel and air are introduced into the mixing chamber which is constructed from a 23 cm long stainless steel tube packed with copper beads. The mixing chamber is 5.1 cm in diameter and is fitted with baffles along the sides to prevent channeling. Immediately after the mixing section is a 5.7-cm-diameter stainless steel tube that is adjustable in length to accommodate varying lengths of catalyst beds (so that residence time over the catalytically active section can be varied). The length of this tube was varied from 7.6 to 12.7 cm according to the length of the catalytic bed used. The tube is packed with stainless steel wool and screens to help maintain a uniform flow profile. Immediately downstream of this tube and upstream of the catalytic bed is a piece of noncatalytic ceramic honeycombed monolith 2.5 cm in length. It is used to distribute the flow and to shield the catalyst from radiative energy losses. The catalyst-coated monolith that follows was varied from 2.5 to 7.6 cm in length for this study. A cross-sectional diameter of 5-7 cm was open to the flow in the catalytic bed. Immediately downstream of the catalytic section is another 10.7 cm of noncatalytic monolith to insulate the catalytic bed from heat losses and to provide further residence time for gas-phase reactions. The catalytic monolith and the insulating monolith are separated using a 0.16 cm thick ceramic felt gasket. The 5.7-cm-diameter adjustable tube and the monoliths are all insulated by ceramic wool blankets and are supported inside a 10.2-cm-diameterpipe, which is further insulated on the exterior. Thermocouple ports in the reactor chamber are located every 1.3 cm in the axial direction. Experiments were also carried out in a boundary layer reactor to allow probing of conditions near the catalyst surface in the CST combustor. In the experiments, fuel and air mixtures were flowed over an externally heated catalytic wall to study the effect of surface reactions on gas-phase ignition in a combustion boundary layer. The fuels used in this experiment were varying mixtures of methane and ethane. The methane has an assayed purity of 99.91% , with the major impurities being ethane and propane. The ethane has an assayed purity of 99.3970,
with the major impurity being methane. The compressed air was dried and filtered prior to use. Ceramic cordierite (Celcor, 2Mg0.2Al2O3&3iO2; mp, 1720 K) monoliths supplied by Corning Glass Works were used as the catalyst support. The monoliths are cylindrical and have small parallel open square channels with the cross section in a honeycomb configuration. The average wall thickness of the monolith was 0.03 cm, making the wall thermally thin. This geometry has the advantage of combining a high surface area with a lower pressure drop than a packed bed. The monoliths were used for both the catalytic bed and the insulation before and after the bed. The monoliths have a cell density of 31 cells/cm2 and have an open cross-sectional area of 70%. Although other materials such as alumina can withstand higher temperatures, they have poor thermal shock characteristics and are not well-suited for repeated use a t combustion conditions. To increase the surface area of the cordierite support before impregnation with the catalyst, the monolith was washcoated by applying a thin layer of alumina. The alumina used was Versa1 150 supplied by Kaiser Alumina. A technique was developed for the washcoating so that the monoliths had a weight gain of 10% from the alumina after being calcined at 1400 K for 6 h. The weight gain was consistent to within 2% between the different catalysts used. After the alumina washcoating, the platinum was applied in the form of H2PtC1, solution. The monolith was then calcined again at 1400 K for 6 h to convert the H,PtCl, to platinum. The platinum loading was varied from 2.2 X lo-* to 8.1 X lo-* g/cm2 based on apparent macroscopic surface area to test the effect of platinum loading on the stability characteristics of the burner. But, as will be shown, the CST combustor was always operated under mass-transfer-controlled conditions, and the platinum loading in this regime does not influence the CST combustor stability limits. The temperature was monitored by using TungstenRhenium (3%-25%) thermocouples. The thermocouples were placed every 1.3 cm along the length of the reaction chamber into both the catalytic and insulating monoliths. Ports for the thermocouples were 0.32 cm in diameter and were drilled to the center of the monolith from the sides, perpendicular to the flow direction. Those channels in the monoliths through which the thermocouple ports passed were sealed using an alumina-based cement to prevent disturbances from the reacting gases. In this manner, the thermocouples were isolated from the flow and were able to measure the monolith wall temperature profile in the axial direction. Radial variations in the monolith temperature were measured by changing the vertical position of the thermocouples in their ports. The thermocouples were calibrated to a reference thermocouple and have a relative precision to each other of f 2 K. The data were recorded continously with the aid of a computer. The flow rate is characterized by the reference inlet velocity. This is the calculated average velocity of the unreacted gases just before it reaches the catalytic bed. The reference inlet velocity was maintained at 97 cm/s for all the runs in this experiment.
Results Velocity and fuel concentration profiles in the catalytic bed were shown to be radially uniform across the monolith. Pressure measurements were used to confirm that the velocity profile was uniform to within 2%. Flame ionization detector measurements of total hydrocarbons showed that the fuel concentration was uniform to within at least 1% for all radial positions. Radial uniformity was determined by taking gas samples from a cold burner at
Ind. Eng. Chem. Res., Vol. 27, No. 8, 1988 1379 1600
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OF CATALYTIC MONOLITH (cm) Figure 2. Minimum stable burner temperature as a function of catalyst bed length. Fuel, ethane; T (inlet) = 294 K; Urer= 97 cm/s; g/cm2. platinum loading = 2.7 X LENGTH
different radial positions within the catalytic bed. Adiabatic conditions in the catalytic bed and the downstream noncatalytic monolith were also demonstrated by a near constant measured flame temperature profile in the axial direction. The axial temperature measured in the monolith section varied by less then 3 K/cm until approximately 4 cm from the exhaust end of the burner, where heat losses were high. This heat loss does not affect the catalytic bed, as there was a 10.7 cm long insulating monolith downstream of the bed. The temperature of the monolith upstream of the catalytic bed remained relatively cool (1550 K) showed that the flame front was stabilized about 0.2 cm from the front face of the catalytic monolith and that the temperature reached its adiabatic value within 100 ps. These results demonstrate that the length of the catalyst bed did not influence the steady-state combustor stability limits for nonpreheated feeds for the range of catalytic monolith lengths tested. This finding is of practical importance because it implies that the catalyst bed can be segmented into different sections for different functions. Platinum, which can ignite surface reactions at lower temperatures, is needed only very close to the inlet; other catalysts (for example, transition-metal oxides) can be used downstream for economic reasons, for temperature stability, and/or for the reduction of specific pollutants (i.e., Ni catalyst for the reduction of NO,). In fact, our results indicate that, in our test burner, platinum is needed only on the front 0.2 cm of the monolith to sustain stable combustion. It should also be mentioned that the near adiabatic condition of the catalytic bed contributes to the shortening of the primary flame front by minimizing heat losses that lower the wall temperature. If the heat losses were significant, then the temperature at the front may be lowered enough to broaden the flame front and the length of the catalyst required to maintain combustion stability would increase. For the remainder of this study, only 2.5 cm long catalytic monoliths were used. A platinum loading based on apparent macroscopic surface area of 2.7 X gm/cm2 was used in the methane/ethane mixture experiments. The consistency of the results between the catalyst beds was checked by coating three separate monoliths to this loading and finding the minimum stable equivalence ratio of each using ethane. The results are consistent to within 3.5%. The sensitivity of the combustor to the amount of catalyst used was also checked by varying the catalyst loading on three other 5.4 X lo", monoliths. These had loadings of 2.2 X and 8.1 X gm/cm2 on monoliths that were 2.5 cm in length. These results are presented in Figure 3 and show that the burner stability had little dependence on the catalyst loading in the regime of loading used. This is because at temperatures above 850 K, the rate of the heterogeneous fuel oxidation reactions is limited by the rate of mass transfer to the surface and is not a function of the amount of catalyst loading. The lean stability limits of the CST combustor as a function of methane to ethane mixture ratio were also investigated. The change in the minimum operating temperature necessary to maintain burner stability as a function of methane fuel fraction is plotted in Figure 4. This experiment was repeated several times using different
1380 Ind. Eng. Chem. Res., Vol. 27, No. 8, 1988 I6O0
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w U
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I 2.0
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PLATINUM LOADING
( i x10’
8.0
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g/cm2)’
Figure 3. Minimum stable burner temperature as a function of platinum loading. Fuel, ethane; T (inlet) = 294 K; Uref= 97 cm/s; (*) platinum loading is given per apparent macroscopic surface area. I
I
I
limits
0 Flat plate gas phase combustion limits at
0
e e
e
* 0
1 0
0.0
0.2
0.4
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0.8
1.o
METHANE MOLE FRACTION IN FUEL MIXTURE
Figure 4. Minimum stable burner temperature as a function of methane mole fraction in a methane/ethane fuel mixture. ( 0 )CST Flat plate reactor data. T (inlet) = 294 K; Uref= burner data. (0) 97 cm/s; platinum loading = 2.7 X g/cm2.
catalyst sections, and the stability curve was always reproduced within the accuracy of the thermocouples. Note that since no external means of heating was used, the adiabatic flame temperature is directly determined by the equivalence ratio. The results of three separate runs using different catalyst beds show that pure ethane could be stabilized in the CST combustor at a temperature of 1510 K. The minimum temperature required to stabilize pure methane was found to be 1670 K in two separate runs. The fact that a higher operating temperature (equivalence ratio) of methane than ethane is required to stabilize the burner was expected. Methane has a lower burning velocity and has longer ignition delay times than ethane (and other higher order hydrocarbons). This is due to the relatively stronger C-H bonds in methane compared with the C-C or C-H bonds found in the other hydrocarbons. In addition, the methyl radicals produced in methane decomposition are much less reactive than ethyl radicals, and do readily decompose to form additional H atoms. The results show that small amounts of ethane can cause a drastic difference in the stability of mostly methane fuel mixtures. The minimum CST combustor temperature needed to maintain combustion stability was decreased by nearly 80 K by the addition of approximately 20% ethane
to the predominately methane mixture. This temperature difference translates into a large difference in the ignition delay time, which is determined by a high activation energy mechanism. The addition of a small amount of methane to a predominately ethane mixture had a relatively small effect. The minimum combustor temperature varied by only 40 K for the addition of up to 50% methane in ethane. This observation may be explained as follows: Small amounts of ethane in the fuel mixture help initiate the combustion reactions by forming a radical pool and releasing energy, accelerating the decomposition of the more stable methane. When there is a shortage of ethane available to form this radical pool, the methane mixture becomes significantly harder to burn. This behavior has been documented by other researchers (Crossley et al., 1972; Pfefferle and Churchhill, 1983; Westbrook and Pitz, 1983) in the numerical simulation of noncatalytic ignition properties of mixtures of methane and ethane. To check the assumption that the change in combustion stability is due primarily to the difference in the ignition delay times of homogeneous gas-phase reactions, we carried out experiments of boundary layer ignition over a heated flat catalytic plate. Mixtures of fuel and air were passed over a platinum-coated flat plate to form a laminar boundary layer. The catalytic flat plate was heated electrically and the temperature was controlled at a constant value between 1100 and 1600 K. Unlike the case of the experimental CST burner, this wall temperature was independent of the fuel/air ratio. OH radical concentrations were measured at different positions along the plate (in the flow direction) and perpendicular to the plate. Hydroxyl radical was chosen because it is a good indicator of combustion intensity, reaching a maximum concentration immediately downstream of a premixed flamefront. Laser-induced fluorescence (LIF) of OH, a standard technique in combustion research (Crosley and Smith, 1983), was used for these measurements. The details of the experimental setup and measurement technique are discussed elsewhere (Pfefferle et al., 1987). These measurements were carried out for various plate temperatures and methane/ethane fuel fractions to study the dependence of gas-phase ignition as a function of these parameters. An understanding of the ignition process in CST combustors can be enhanced by determining the influence of a hot, catalytic wall on temperature and species distributions in the boundary layer. Ignition delay times for initiation of gas-phase combustion can be approximated by following the OH concentration development as a function of position from the wall a t a fixed distance in the flow direction along the wall. The boundary layer formed by flowing fuel/air mixtures over a hot catalytic plate is, therefore, an ideal system in which to study the processes governing CST combustor operation. Furthermore, because the fluid dynamics of laminar boundary layers over flat plates have been characterized, we can model the effects of the wall on gas-phase ignition. The surface temperature of the platinum plate required for homogeneous gas-phase ignition at a distance of 2.5 cm from the leading edge of the flat plate (along the flow direction) along with the data for minimum stable operating temperature in the CST combustor are shown in Figure 4 for mixtures of methanelethane in air. Performing the experiments at a constant flow velocity (100 cm/s) and observing the ignition development at a fixed axial position along the flow is equivalent to setting a constant gas-phase ignition delay time. The similar curvature and nearly identical location of inflection points on the ignition temperature curve as compared with the data
Ind. Eng. Chem. Res., Vol. 27, No. 8, 1988 1381
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100
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Figure 5. OH concentration profile in a combustion boundary layer at 2.5 cm from leading edge of platinum-coated plate. Fuel: (A) 20% ethane, 80% methane; (B)10% ethane, 90% methane. (0) Plate temperature = 1570 K; (m) plate temperature = 1520 K.
for the CST stability limits suggest that the controlling parameter in maintaining a stable CST combustor is the induction time required to initiate homogeneous gas-phase reactions. The difference in the magnitude of the temperatures in the curves shown in Figure 4 is expected because of the different heat- and mass-transfer characteristics of the CST burner and the boundary layer combustor. The assumption that the induction time for gasphase ignition controls the minimum stable equivalence ratio is also supported by data from measurements of OH radical concentration profiles (Figure 5) over a heated catalytic plate before and after gas-phase combustion was initiated for different fuel compositions but the same overall equivalence ratio. Mixtures of ethane/methane in air were used with an total equivalence ratio of 0.5. Case A is for methane containing 20% ethane and case B is for methane containing 10% ethane as the fuel. For case A, ignition occurred at a plate temperature of 1520 K (filled squares). The plate temperature for case A was then increased to 1570 K (open squares), which is the plate temperature at which ignition occurred for case B. Note that, while ignition takes place at approximately 50 K lower for the fuel mixture containing 20% ethane, subsequent to gas-phase ignition the OH concentration profiles for the two mixtures are not only similar in shape but also in the magnitude of the concentrations. The presence of gasphase combustion in these plots is evidenced by the peak in OH concentration away from the catalytic surface. The tests showed that the amount of ethane in the fuel affects gas-phase ignition temperature, but once ignition has occurred, the different mixtures of fuels behave similarly. Hence, the effect of using different fuels on the stability of the CST combustor appears to be dominated by the characteristics of gas-phase kinetics of the fuel mixtures. We have shown in related work, however, that gas-phase combustion kinetics can be modified by a catalytically active surface and that the magnitude of this effect is influenced by fuel structure (Pfefferle et al., 1987). This coupling of heterogeneous and homogeneous reaction mechanisms will be investigated in our future work. Summary The steady-state stability limits of the CST combustor as a function of varying concentrations of methane and
ethane have been observed and explained. The results show that the minimum operating temperature of methane may be decreased by as much as 80 K by the addition of a moderate fraction of ethane. Although the addition of methane to a predominately ethane fuel mixture will increase the minimum CST burner operating temperature, this effect is relatively small. Comparison with the data from flat plate experiments indicates that the gas-phase ignition temperature (for a given flow time) is the mechanism controlling stability limits in the CST burner. OH profiles taken before and after gas-phase ignition show that ethane addition accelerates ignition but does not influence the extent to which combustion propagates into the boundary layer. A two-dimensional model including multistep gas-phase kinetics is currently being developed to further investigate the controlling mechanisms of the CST combustor. It was also found that only a small length (less than 2.5 cm) of catalytic bed is required to maintain combustor stability. The required length is most likely even shorter due to the steepness of the flame front. The results indicate that, for a segmented catalytic bed employing different catalysts for different functions, only a small length of catalyst is needed for maintaining the steady-state stability of the CST combustor. This is an encouraging result for the future design of a reduced emissions CST burner for natural gas utilization. Acknowledgment The authors thank Dr. W. C. Pfefferle for advice on catalyst preparation; Dr. David Crosley, Dr. G. P. Smith, and Mark Dyer (SRI International) for collaboration in the OH radical concentration measurements; and Corning Glass Works. The authors are grateful to the Gas Research Institute (Contract 5086-260-1249) for support of this study. Registry No. Pt, 7440-06-4; methane, 74-82-8; ethane, 74-84-0.
Literature Cited Bruno, C.; Walsh, P. M.; Santavicca, D. A.; Bracco, F. V. Int. J . Heat Mass Transfer 1983a, 26, 1109-1120. Bruno, C.; Walsh, P. M.; Santavicca, D. A.; Sinha, N.; Yaw, Y.; Bracco, F. V. Combust. Sci. Technol. 1983b, 31, 43-74. Chigier, N. Energy, Combustion, and Enuironment; McGraw-Hill: New York, 1981; p 81. Chu, E. K.; Kesselring, J. P. “Fuel NO, Control by Catalytic Combustion”. Proceedings of the 3rd Workshop on Catalytic Combustion, 1978; EPA Report 600/7-79-038, pp 291-329. Crosley, D. R.; Smith, G. P. Opt. Eng. 1983,22, 545-553. Crossley, R. W.; Dorko, E. A.; Scheller, K.; Burcal, A. Combust. Flame 1972, 19, 373-378. Folson, B. A.; Courtney, C.; Heap, M. P. “Environmental Aspects of Low BTU Gas-Fired Catalytic Combustion”. Proceedings of the 3rd Workshop on Catalytic Combustion, 1978; EPA Report 600/7-79-038, pp 345-383. Nowak, E. J. Chem. Eng. Sci. 1969,24, 421-423. Pfefferle, W. C. U S . Patent 3 928 961, Dec 30, 1975; original filing 1971. Pfefferle, W. C. “Catalytically Supported Thermal Combustion”. Belgian Patent 814 752, Nov 8, 1978. Pfefferle, L. D.; Churchill, S. W. “A Computer Study of the Constant Pressure Auto-Ignition of Mixtures of Ethane, Methane, Carbon Monoxide and Hydrogen in Air”. ASME 83-WA/HT-67, 1983. Pfefferle, W. C.; Pfefferle, L. D. Prog. Energy Combust. Sci. 1986, 12. Pfefferle, L. D.; Pfefferle, W. C. Cat. Reu.-Sci. Eng. 1987, 29(2,3), 219-267. Pfefferle, L. D.; Griffin, T. A.; Crosley, D. R.; Dyer, M. J., submitted for publication in Combust. Flame 1987. Prasad, R.; Kennedy, L. A.; Ruckenstein, E. Combust. Sci. Technol. 1980, 22, 271-280.
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Prasad, R.; Tsai, H. L.; Kennedy, L. A.; Ruckenstein, E. Combust. Sci. Technol. 1981a, 25,71-84. Prasad, R.; Tsai, H. L.; Kennedy, L. A.; Ruckenstein, E. Combust. Sci. Technol. 1981b, 26,51-64. Prasad. R.: Kennedv. L. A.: Ruckenstein. E. Combust. Sei. Technol. issic, 27,45-& ' Fkdcliffe, S. W.; Hickqan, R. G. J. Inst. Fuel 1975, 208,208-214. Trimm, D.L.Appl. Catal. 1983, 7,249-282. Trimm, D. L.;Lam, C . W . Chem. Eng. Sei. 1980, 35, 1405-1413.
Wampler, F. B.; Clark, D. W.; Galhes, F. A. Combust. Sei. Technol. 1976,14, 25-31. Westbrook, C. K.; Pitz, W. J. Combust. Sei. Technol. 1983, 33, 315-319.
Received for review August 17, 1987 Revised manuscript received February 23, 1988 Accepted March 11, 1988
Phenylacetonitrile Alkylation with Different Phase-Transfer Catalysts in Continuous Flow and Batch Reactors Vittorio Ragaini,* Giovanni Colombo, and Paolo Barzaghi Dipartimento di Chimica Fisica ed Elettrochimica, Uniuersitd di Milano, via Golgi 19, 20133 Milano, Italy
Emo Chiellini and Salvatore D'Antone Dipartimento di Chimica e Chimica Industriale, Universitd di Pisa, via Risorgimento 35, 56100 Pisa, Italy
T h e monoalkylation of phenylacetonitrile (PAN), in phase-transfer (PT) conditions, with butyl bromide in aqueous organic medium as catalyzed by both quaternary ammonium salts bound t o an insoluble polymer and soluble low molar mass analogues has been studied by using different experimental apparatus. Batch slurry r e a d o n or fiied bed reactors under continuous flow conditions and different stirring modes have been adopted within the scope of a basic investigation of the performance of different phase-transfer catalytic systems. A classification is provided of the different catalytic systems and apparatus in terms of specifie activity, and an optimization of experimental conditions useful for further kinetic investigations has been performed. The ethylation of phenylacetonitrile (PAN) with tetrabutylammonium bromide (TBAB) as a soluble catalyst in phase-transfer (PT).conditions in an aqueous organic system, has been studied (Komeili-Zadeh et al., 1978;Sol a r ~et al., 1980;Balakrishnan and Ford, 1981,1983)with the aim of analyzing the reaction kinetics. In particular, parameters such as stirring rate and base (NaOH) and Brconcentrations were analyzed. For the utilization of insoluble PT catalysts in fixed bed reactors (Ragaini and Saed, 1980,Ragaini et al., 1986),a fairly detailed study has been undertaken of the monoalkylation reaction of PAN (Scheme I), as catalyzed both by insoluble polymer-supported quaternary ammonium groups or glymes and soluble low molar mass analogues. The ultimate goal of the study was the evaluation of the specific activities of the different catalysts when used in either slurry (SR) or fixed bed (FB) reactors under different stirring modes such as ultrasound mixer (UM), turbine stirrer (TS), and in-line turbine mixer with recirculating pump (TP).
Scheme I PT Catalyst
+ C&Br
Toluene
NoOH/H20
+ (NaBrl
aq
Stirring
Scheme I1
Experimental Section Catalysts. In Table I the chemical and physical characteristics are collected for the catalysts whose structures are illustrated in Scheme 11. The low molar mass quaternary ammonium derivatives either in the salt or in the hydroxide forms as well as the insoluble polymeric catalysts IRA 904 and Duolite A101, A161,and A171 were from commercial sources. The other polymeric catalysts containing quaternary ammonium groups or glymes as active groups were prepared according to a general procedure as represented in Scheme 111. Styrene/divinylbenzene polymer matrices with different degrees of cross-linking were prepared by suspension polymerization of the two comonomers in the presence of n-octane as porogenic agent. The successive chloro-
* To whom correspondence should be addressed.
methylation of the basic resins followed by quaternization with tertiary amines or etherificationwith the monomethyl ether of tetraethylene glycol or tetraethylene glycol led to 0 1988 American Chemical Society
