Ind. Eng. Chem. Res. 1993,32, 2969-2974
2969
Simultaneous Measurement of Adsorption, Reaction, and Coke Using a Pulsed Microbalance Reactor Frank Hershkowitz' and Paul D. Madiara Exxon Research & Engineering Company, Route 22 East, Annandale, New Jersey 08801
A new research tool has been developed that allows in-situ measurement of the transient adsorption and coke deposition that occurs on zeolitic catalysts during short contact-time interactions with reactants. The tool is a microbalance pulse reactor (MPR), whose central feature is a new kind of microbalance that can accurately weigh a catalyst bed without regard to high velocities of vapor passing through. This TEOM microbalance measures mass by inertia instead of weight. In the MPR, zeolitic catalysts are exposed to short pulses of vapor-phase adsorbate carried through the catalyst bed on helium carrier gas. Substituting nonporous quartz for the catalyst allows quantitation of the vapor pulse shape. Adsorption responses t o pulses of n-decane, isopropylbenzene and triisopropylbenzene on La3+Yzeolite are reported. The MPR has application to understanding the mechanisms of catalytic cracking, which include a complex interaction of adsorption, shape-selective diffusion, and rapid deactivation by coke.
Introduction Catalytic cracking is the heart of a modern refinery, the chief unit operation for converting low-value,high-boiling feeds to high-value naphtha components of gasoline. Yet, despite this importance, many of the fundamental mechanisms of the catalytic reaction remain obscure. The reasons for this obscurity are well rooted in a regime of catalystlhydrocarbon contacting that is extraordinarily difficult to simulate in the laboratory, as well as an absence of analytical tools that can probe catalyst/hydrocarbon state during contacting. This paper describes a new research tool that addresses both of these difficulties and promises to illuminate many aspects of these mechanistic unknowns. In the process of cat cracking, a liquid gas-oil (-300540 "Cbp)at -300OCissprayedinto -750OCregenerated catalyst, and the resulting -500 "C mixture of catalyst and vaporized hydrocarbon travels up a riser for several seconds before being discharged into a reactorlstripper vessel that separates the catalyst from hydrocarbon. The catalyst is -75-pm spheres composed of 20-40% zeolite (-1-pm faujasite crystals) with clay, Si or A1 binder, and other additives. The ratio of catalyst to oil is sufficiently low (between 3 and 8) so that adequate oil is present to saturate all catalyst surface, if adsorption strength allows. In order toreact, a hydrocarbon molecule must penetrate the catalyst particle, become activated (e.g., cation, radical cation) by a surface or by active species near the surface, react, and desorb and exit the catalyst. The majority of the surface (and particularly the most active surface) is within the microporous zeolite crystal, for which access also requires a shape selective diffusion path. Over the 2-10 s that the feed is cracking, the nature of the catalyst is believed to change profoundly from a state of high activity to one of low activity, as coke builds up on the catalyst. The conversion and selectivity that are obtained in cat crackingresult directly from a combination and interaction of very fast and nonuniform catalyst/oil contacting, a complex catalyst surface, a strong role of competitive adsorption and shape-selective diffusion, a broad range of feed componenta, and a catalyst whose activity is rapidly changing. Thus, understanding about fundamental mechanisms of cat cracking must fit within this contacting framework in order to be relevant to cat cracking. Clearly, performance of fundamental research requires some 0888-5885/93/2632-2969$04.00/0
compromises in terms of experimental complexity. This paper presents a new approach that includes fewer compromises than was previously possible. The adsorption and diffusion of reactants in zeolitic catalysts are traditionally studied at lower temperature and with lower reactivity zeolites and then extrapolated to reaction conditions to understand their mechanistic roles. For example, Jlinchen and Stach (1985) measure n-decane adsorption on sodium-Y zeolite, which is less active than the rare-earth or hydrogen forms used in cracking catalysts, and a t temperatures from 290 to 570 K, which are well below the -773 K temperatures used for cracking. Diffusion parameters may be measured by many means, including sorption, NMR, and chromatographic methods (Ruthven, 1983)and frequency response methods (Yasudaand Yamamoto, 19851,but the avoidance of reaction drives practitioners of each to use low reactivity (smaller) hydrocarbons, low reactivity zeolites, and temperatures usually below 500 K. Some information of the role of size selectivity can also be obtained in reaction studies, a classic example being Nace's (1970) comparison of C16 species hexadecane and perhydropyrene. In order to study deactivation, one must break up the reaction into deactivation increments. This may be done with any reaction variable that contributes to severity. Commonly used approaches include a combination of lower temperature, less reactive hydrocarbon, lower pressure, and very short contact time. For example, Magnoux et al. (1987) studied deactivation using a low reactivity hydrocarbon (heptane) at moderate temperature (450 "C) and pressure (30 kPa). Under these conditions deactivation could be stretched out to take between 2 min and 6 h, time durations very easily accessible in the lab. However, such conditions exclude the observation of any role of competitive adsorption or of mass-transfer limitations. Our approach is to use a microbalance to weigh the bed of catalyst as it is being exposed to pulses of hydrocarbon vapor. A crucial, new aspect of this work is the use of an inertial microbalance that can determine bed mass without regard to high velocities of vapor passing through. Our reactor system is designed to capture more of the contacting complexity of cat cracking by using very short pulses of larger, more reactive hydrocarbons at near-commercial temperatures and near-commercial pressures. The microbalance aspect of the reactor permits on-line measurement of extent of hydrocarbon adsorption on the catalyst, 0 1993 American Chemical Society
2970 Ind. Eng. Chem. Res., Vol. 32, No. 12, 1993
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Figure 1. Flow diagram for the microbalance pulse reactor unit.
and of coke, which is expected to be closely associated with deactivation. The rapid and transient measurement of adsorption during reaction creates the opportunity, for the first time, to deconvolve cat cracking mechanisms into their reaction, adsorption, and mass-transfer phenomena during reaction under commercially relevant conditions. Reactor systems with the capability to weigh the catalyst bed have been built based on thermogravimetric analysis (TGA) instrumentation, for example by Lin et al. (1983) and by Dean and Dadyburjor (1988). There are two critical differences between our microbalance reactor and TGAbased designs. First, because our balance is based on an inertial design, it can enclose a packed bed of catalyst through which vapor may be forced at considerable rate. This supports short contact cracking as well as sharp hydrocarbon concentration gradients, which are useful in quantifying mass transfer. Second is our capability to measure transient mass gains and losses on a 0.11-s time scale, permitting the observation of the transient adsorption and desorption that occurs with each pulse.
Description of the Microbalance Pulse Reactor Reactor Flow and Control System. The block flow diagram for the microbalance pulse reactor (MPR) unit is shown in Figure 1. The gas manifold uses Brooks mass flow controllers to deliver metered amounts of air and helium to the unit. The carrier gas for reactions is helium that is introduced into the top of the preheat furnace. Air can also be directed to that location to regenerate the catalyst. Helium is also used as the reference gas in the thermal conductivity detector (TCD), and as one component of the purge gas to the microbalance. Hydrocarbon is introduced into the system by syringe, through a septum located at the top of the preheat furnace. After volatilization, the pulse shape is measured by the TCD, and the pulse travels through the microbalance to the product valve. This four-way valve either directs product to vent (with gas chromatography (GC) flow from the GC gas supply) or directs product directly into the GC's inlet splitter (with GC gas supply going to vent). The TCD reference cell outlet and the reactor vent are both directed to a back-pressure regulator for maintenance of reactor pressure. That back pressure is normally set equivalent to the GC's split pressure so that product-valve position changes do not affect reactor pressure. The GC is an HP-5890/5970 GCMS, with a 0.326-mm DB-1 capillary column and effluent splitting between mass spectroscopic (MS) and flame ionization (FID) detectors. Process variables, such as temperatures and flows, are controlled by a PC-based process control system. The system consists of Intellution's "FIX" software running
Figure 2. Schematic of the TEOM microbalance for suspended particulate measurement, as presented by Patashnick et al. (1980).
on a Dell System 200 PC with an IBM "ARTIC" coprocessor board. The computer uses front-end components by OPTO-22 and Texas Instruments to interface with the unit. TEOM Microbalance. The tapered element oscillating microbalance was first reported by Patashnick et al. (1980)as a device that could be used to quantify amounts of dust suspended in a gas. A diagram of that analyzer is shown in Figure 2. Rupprecht and Patashnick Company ("R&P") custom manufactured our TEOM microbalance (TEOM is a registered trademark of R&P Co.) in 1988. At the time it represented the first such high-temperature, high-flow, fast time resolution microbalance in existence. R&P engineered the balance to specifications that are discussed below. Referring to Figure 2, these changes included replacing the filter with a catalyst-containing pocket, reversing the flow direction, building a custom tapered element that fit the specifications, and enclosing the tapered element assembly in a pressurizable housing. The design has evolved over the years that we have operated the balance, with the current design being comparable with a balance that R&P now manufactures as the "Model 1500 Pulse Mass Analyzer". The balance operates by using mechanical energy, which is controlled via a feedback circuit, to drive the tapered glass element to oscillateat ita natural frequency. This natural frequency is a function of the mechanical properties of the glass, as well as the inertia of the glass and catalyst bed. As this inertia changes, so does the natural frequency. Software and hardware provided by R&P performs the required frequency counting and mass calculation as often as once per 0.11 s, displaying the result on a PC screen and storing to disk. TEOM Design Criteria. Two different kinds of mass measurements are desired from the TEOM: (1) The measurement of transient phenomena such as adsorption and mass transfer and (2) measurement of steady-state changes in mass, such as due to coke buildup. For the former, mass varies rapidly between zero and the adsorption capacity of the catalyst, which is approximately 15% on zeolite or 2-5% on composite catalyst. The steadystate measurement (of coke buildup) permits longer data averaging times, but requires measurement of smaller masses, from 0 to 2 % on composite catalyst. These criteria, plus the desire to use a minimum amount of catalyst (to permit direct coupling of the reactor to a capillary GC column) formed the design specification that was provided the balance manufacturer. In collaboration with R&P, a design emerged that called for a catalyst bed sized for 50 mg of catalyst. Fifty milligrams was the smallest load for which the above criteria could be met. Such a minimum occurs only because there is an irreducible amount of glass
Ind. Eng. Chem. Res., Vol. 32, No. 12, 1993 2971 needed to form the tapered element and catalyst-bed holder, and minimum sensitivity is proportional to this, so-called “rest-mass”. On the basis of a 50-mg catalyst bed (or, alternatively a 10-mgbed of pure zeolite), the balance requirements are (1)very rapid (0.1 s) sensitivity to mass changes that are a significant fraction (- 10%) of the 1.5-mgadsorption capacity and (2) ultimate sensitivity to mass changes that are a small fraction (- 1% ) of the maximum commercial coke level, which is 1mg. For experiments that seek to relate coke formation to deactivation, a third performance requirement is that of a stable mass reading over the long period of time during which successive pulses of hydrocarbon may be injected. In addition, the desire to have sharp pulses of hydrocarbon in the catalyst bed leads to a desire for the microbalance (and flow system leading to it) to have a minimum of volume, and in particular, a minimum of dead volume that might disperse a pulse. A fundamental limitation for any microbalance reactor is the inability to distinguish between mass that is in the form of vapor between particles and mass that is hydrocarbon adsorbed on catalyst. Thus, the microbalance reactor concept is probably uniquely suited to zeolitic catalysts, which have strong adsorption strength and large capacity. An additional microbalance requirement emerges from the need to be able to distinguish mass of hydrocarbon in the vapor phase from massof hydrocarbon in the adsorbed phase. To distinguish vapor from adsorbed mass, the volume of the vapor space in the “weighed” portion of the balance must be limited to containing a much smaller amount of vapor-phase hydrocarbon than the expected adsorption level on the catalyst. These requirements were met by fabricating the microbalance and flow unit with small diameter tubing, and by fabricating the sample-holding region of the microbalance to have very little volume before and after the catalyst bed. For example, at 200 kPa and 300 “C, a maximum of 0.3 mg of decane could be in the -50 p L of balance vapor space, which is significantly less than the 1-10 mg that could be adsorbed by a 50-mg bed of zeolitic catalyst.
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Performance of System and Components
TEOM Performance. The balance’s capability to meet these performance criteria are quantified in terms of baseline drift and in terms of the standard deviation of the mass data as a function of averaging time. Four baseline experiments are shown in Figure 3. It is typical of the microbalance that its quality of operation varies from day to day and catalyst to catalyst. Most typical is the baseline of 11/2/89, in which the maximum move over a 4-h period was 0.031 mg, or 3% of the maximum expected coke level. Under ideal conditions (2/2/90) much better stabilities can be achieved. The standard deviation of the TEOM mass data is a strong function of the duration over which mass values are averaged, as shown in Figure 4. At the fastest data rate of 0.11 s per point, standard deviation is nearly 0.1 mg. As averaging time increases over 1 s, standard deviation quickly drops beneath 0.01 mg. The balance has two modes of operation, one with a time base of 0.11 s, and the other with a time base of 1.67 s. This time is the interval over which frequency of oscillation is measured. The longer time base provides lower error in the 1-10-s region where data for both modes was measured. At long average times the error drops as low as 0.003 mg. A minimum in the curve is possible, as the standard deviation at very long average times becomes dominated
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an equilibrium sample different days with aquartz bed (9/25,9/26/89), of commercial FCC catalyst (11/02/89),and 20% Las+Y zeolite in quartz (2/2/90).Data of 9/26,11/2,and 2/2 are offset by 0.0002, 0.0004, and O.ooo6 g, respectively. 0.1
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by baseline drift. For example, the low values at 10 and 60 s and the 600-9 value were all from the same 11/02 baseline run. This performance is consistent with the criteria that were identified previously. The standard deviation of 0.08 mg at 0.11 s represents 5% of the nominal expected adsorption capacity (compared to desired sensitivity of 10%). The desired steady-state sensitivity of 0.01 mg is met with average times between 1 and 10 s. The longterm stability is variable, as discussed above. Even on good days (such as 11/2) it introduces a long-term uncertainty that can be much greater (0.03 mg/4 h) than the standard deviation of 60-s data (0.003 mg). Thus, the coke yield from a single pulse can usually be measured more accurately than can the cumulative relationship between deactivation and coke that is added in many pulses over a long time. Creation and Observation of Hydrocarbon Pulses. With helium as carrier gas, the gas-phase difference in density between the helium and the hydrocarbon produces a mass response in the microbalance due to the feed pulse alone. When a bed of inert fused-quartz particles is used, the mass response quantifies the shape and size of hydrocarbon vapor pulses passing through. Although the TCD shown in Figure 1 can be used to measure pulse shape, we prefer, when possible, to measure that shape by performing a series of “blank” injections of hydrocarbon
2972 Ind. Eng. Chem. Res., Vol. 32, No. 12, 1993 10-
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over fused quartz at the exact same conditions that are used for the adsorptive/reactive runs. Such a measurement has the advantage of including in the measured pulse shape any pulse dispersion that might occur in between the TCD and the TEOM, and of being a data type that is identical to that of the adsorption measurement. The "quartz-blank" feed pulse representation is prepared from a series of five or more injections, as shown in Figure 5. The need for many injections arises from the low signal, and hence low signal-to-noise ratio of the raw quartz-blank signal, as shown in Figure 5a. In order to combine the members of the series, an 11-point rolling average of the data (Figure 5b) is used to calculate a time intercept for the initial rise of the mass. The members of the series are synchronized on those time intercepts and then averaged together, resulting in a data set such as in Figure 5c.
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Measurements of Transient Adsorption
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The performance of the microbalance reactor system is demonstrated by mass responses obtained with a catalyst bed containing 8 mg (dry) of La3+Yzeolite. The zeolite, which has a Si/Al atomic ratio of 2.4 and asorption capacity of 14.6 g/100 g of octane at 100 "C, was made into larger particles by pilling at 17 MPa followed by crushing and sievingto a -170/+250 mesh size. The catalyst was diluted in 40 mg of similarly sized fused quartz during loading into the balance. Hydrocarbons were injected in amounts of 5 pL over freshly regenerated (2 h with air at 500 "C) catalyst. The carrier gas for the pulses was helium at a flow rate of 100 cm3 (STP)/min. The extraordinary capability of the zeolite to rapidly sorb hydrocarbon (and of the balance to follow this sorption) is demonstrated by the response to a decane injection at 300 "C, which is shown in Figure 6. The TEOM vapor density response to an injection at these conditions is also shown. At this density, hydrocarbon would enter the catalyst bed a t a peak rate of 1.9mg/s, which compares well to the 1.4 mg/s rate of mass increase in the bed. The zeolite crystals go from empty to near-saturation in about a half a second. The top of the mass response curve is roughly flat, with concentration of 0.8 mg, or 10% on zeolite. When the decane pulse ends, desorption of decane begins. Desorption occurs over about 10s, leavingaresidue of about 0.16% "coke" on zeolite. Conversion,as measured using the FID detector on the GCMS, was 4%.
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Time, seconds
Figure 7. Mass response to 5-fiL pulses of (a) isopropylbenzene or (b) triisopropylbenzene at 400 OC, 180 kPa, and carrier gas flow of 100 cms (STP)/m helium. Individual data points ( 0 )for bed of 8 mg of La3+Yzeolite in 40 mg of fused quartz; line is for 50 mg of fused quartz alone. All except isopropylbenzene/La3+Yrepresent averages of five pulses.
The MPR system's ability to quantify the effects of shape selectivity during reaction are demonstrated in the comparison of cracking of isopropylbenzene to 1,3,5triisopropylbenzene, as shown in Figure 7. In both cases, 8 mg (dry) of La3+Y is exposed to 5-pL pulses of hydrocarbon at 400 "C, 180 kPa,and 100 cm3 (STP)/min of helium carrier gas flow. In both cases mass was also measured over a bed of fused quartz alone (which measures mass in vapor phase). For the isopropylbenzene, the mass measured over zeolite was significantly higher than that measured over fused quartz, indicating adsorption to an extent of about 0.3 mg. For the triisopropylbenzene, there is no discernible difference between zeolite and fused quartz mass uptake, indicating that the zeolite is excluding the triisopropylbenzene from its internal pore structure. Consistent with this difference in adsorption behavior, the isopropylbenzene is converted to the extent of 38 % , compared to -0.1 % for the triisopropylbenzene.
Ind. Eng. Chem. Res., Vol. 32, No. 12,1993 2973 1.o
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Determination of Adsorption Parameters Measurements of adsorption during reaction, such as that shown in Figure 6, can be used to determine adsorption parameters at temperatures above those at which other methods cease to become usable. To accomplish this determination, we use a regression tool to find the best fit of a numerically-simulated curve to the measured curve. A schematic diagram for this process is shown in Figure 8. The numerical simulation uses the measured feed pulse, the measured conversion, unit operation parameters (e.g., flow rate), and adsorption/reaction model parameters to calculate a simulated transient adsorption curve. For the results discussed below, the adsorption/reaction model is based on Langmuir adsorption and first-order kinetics of decane reaction to products and coke. Plug flow in the catalyst bed is approximated as two perfectly stirred reactors in sequence. We have found that such a simulation/regression system works best when the signallnoise ratio of the data is improved in a preprocessing step that averages together a number of mass measurements in regions where the adsorption is changing little. To achieve this preprocessing, we calculate at each data point the local first and second time derviatives of mass (m’& m”) using the formulas of Savitzky and Golay (1964). Then, at each data point a value nt is calculated as: with limits ~1 IIiI “,I i,
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Finally, sets of consecutive data points are averaged such that the number of points averaged together is always less than or equal to the smallest value of nt within the set. The parameters a and b are chosen such that nt = 1in the regions of rapid uptake and desorption and nt 10 in the flat tail end. Typical values for a and b are 0.05 mg/s and .005 mg/s2,respectively. The impact of the preprocessing can be seen in the mass response data shown in Figure 9. The quality of the fit of this model to a transient adsorption curve is excellent, as shown in Figure 9 for a decane/LaY exposure identical to the one shown in Figure 6. Five parameters are regressed in this case: rate constants for the cracking and coking reactions, Langmuir saturation and Henry’s law parameters, and a time offset between vapor pulse and adsorption pulse. Fits of this quality are common, in this case with standard deviation of the adsorption data from the simulated response being 0.0192 mg. This particular regression results in Langmuir adsorption parameters of 10.3g/ 100g for saturation loading and 469 for the dimensionless Henry’s law constant (K of
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Figure 10. Comparison of the regressedHenry’s law value from this work).( tovalues approximatedfrom the work of Janchen and Stach (1985) and Stach et al. (1986) for adsorption of decane on NaY and US-Ex, dealuminated faujasite (-o-). (.e.)
q = Kc, where q and c have equivalent units of grams or moles per cubic centimeter, and zeolite concentration, q, is based on crystal volume). Similar values cannot be directly measured at these temperatures by use of conventional means because cracking and coking would be extensive over the time scale of the conventional measurement. However, we can compare these values to extrapolated values. The saturation loading is within 10% of the value of 11.9 that we would extrapolate (using the methods of Dubinin (1960)) from the zeolite’s low temperature adsorption capacity. The Henry’s law constant can be compared to the results reported by Jlinchen and Stach (19851, and Stach et al. (1986), who measured adsorption isotherms of decane on a variety of faujasitic zeolites at temperatures of 0-200 “C. We have calculated approximate Henry’s law constants from those works by taking the ratio of reported vapor- and adsorbed-phase concentrations at the lowest measured loading. Our regressed, transient-adsorption value of the Henry’s law constant is extremely close to the value that we would extrapolate from Stach et al.’s (1986) values for decane on a highly dealuminated Y zeolite (USEx), as shown in Figure 10. Such a close fit is fortuitous, as none of the Jlinchen and Stach zeolites exactly match our zeolite. The closeness of the fit confirms that what we measure is comparable in magnitude to the expected Henry’s law constant.
2974 Ind. Eng. Chem. Res., Vol. 32, No. 12, 1993
Conclusions Characterization of the functioning of zeolitic catalysts, particularly for the reaction conditions of catalytic cracking, has been encumbered by the unavailability of methods to measure key perfomance features such as adsorption on the catalyst and the coke produced by reaction under realistic reaction conditions of short contact times, high hydrocarbon loading, and high reaction temperatures. The present apparatus and method fill this need by providing a means to measure, in-situ, this adsorption and coke. Use of an inertial microbalance in a pulse reactor permits the measurement of amounts adsorbed on catalyst during exposure to short-duration, high-concentration pulses of hydrocarbon, permitting quantitation of the adsorption isotherms, as well as quantitation of kinetics that may be based on adsorbed concentrations. This apparatusmakes possible the in-situ measurement of coke from such catalytic reactions, opening the way to a better understanding of the kinetics and mechanisms of coke formation, as well as the effects of coke on subsequent reaction.
Acknowledgment The authors wish to acknowledge our appreciation of R&P Co.’s contribution in adapting their technology to our application. We are particularly grateful for the help of Dave Hassel and Harvey Patashnick, who personally undertook the challenge and made our balance a reality.
Chromatographic Analysis for Catalytic Cracking. Znd. Eng. Chem. Res. 1988,27(lo),1754-1759. (2) Dubinin, M. M. The potential theory of adsorption of gases and vapors for adsorbenta with energetically nonuniform surfaces. Chem. Rev. 1960,60,235-241. (3) Jhchen, J.; Stach, H. Dependence of the adsorption equilibrium of n-decane on the Si/Al-ratio of faujasite zeolites. Zeolites 1985,5,57-59. (4) Lin, C.; Park, S. W.; Hatcher, W. J., Jr. Zeolite Catalyst Deactivation by Coking. Znd. Eng. Chem. Process Des. Dev. 1983,22 (4),609-614. (5) Magnoux, P.;Cartraud, P.; Mignard, S.; Guisnet, M. Coking, Aging, and Regeneration of Zeolites-11. Deactivation of HY Zeolite during n-Heptane Cracking. J. Catal. 1987,106,235-241. (6) Nace, D. M. Catalytic Cracking over Crystalline Aluminosilicates. Znd. Eng. Chem. Prod. Res. Deu. 1970,9 (2),203-209. (7) Patashnick, H.; Rupprecht, G.; Wang, J. C. F. A New RealTime Monitoring Instrument for Suspended Particulate Mass Concentration: The TEOM. Prepr.-Am. Chem. Soc., Diu. Pet. Chern. 1980,25,188-193. (8) Ruthven, D. M. Diffusion in A, X, and Y Zeolites. ACSSymp. Ser. 1983,218,345. (9) Savitzky, A.; Golay, M. J. E. Smoothing and Differentiation of Data by Simplified Least Squares Proceedures. Anal. Chem. 1964, 36 (8),1627-1639. (10)Stach, H.; Lobe, U.; Thamm, H.; Schirmer, W. Adsorption equilibria of hydrocarbons on highly dealuminated zeolites. Zeolites 1986,6,74-90. (11) Yasuda, Y;Yamamoto,A. Zeolitic Diffusivities of Hydrocarbons by the Frequency Response Method. J.Catal. 1986,93,176 181. Received for review April 14, 1993 Revised manuscript received August 9, 1993 Accepted August 24, 1993*
Literature Cited (1) Dean, J. W.; Dadyburjor, D. B. An Ambient-Pressure Pulse Microreactor with Continuous Thermogravimetric and On-Line
Abstract published in Advance ACS Abstracts, October 15, 1993. @
