Ind. Eng. Chem. Res. 1993,32, 510-514
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RESEARCH NOTES Experimental Study of Reaction in Trickle-Bed Reactors with Liquid Maldistribution Richard L. McManus,+Gregory A. Funk,$Michael P. Harold, and Ka M. Ng' Department of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts 01003
Experiments were performed to study the effect of liquid maldistribution on trickle-bed reactor performance using a-methylstyrene hydrogenation as the model reaction. Different degrees of liquid flow uniformity were generated with various inlet distributors. The rate of reaction was measured at different liquid flow rates, and the temperatures within the reactor were monitored as a function of time. The experimental data confirmed qualitatively all the predictions of a previously published reactor model.
Introduction Liquid-phase maldistribution is an important factor in the design, scale-up,and operation of commercial tricklebed reactors,in which gas and liquid reactants react within the catalytic particles. For reasons such as an ineffective liquid inlet distributor or packing anisotropy,as in columns loaded with cylindrical catalytic extrudates, large regions of the bed might be bypassed by the liquid phase (Stanek et al., 1981; Ng and Chu, 1987). This can lead to two undesirable outcomes. With no fresh supply of the liquid reactants, there is essentially no reaction in these regions and the reactor is not fully utilized. In contrast, if a sufficient amount of the liquid reactants is vaporized, reactions can still occur in these nonwetted regions. Without the liquid phase as a heat carrier, however, this can result in hot-spot formation for highly exothermic reactions. Thus, a basic understanding of the impact of liquid maldistribution on reactor performanceis essential. To this end, a discrete model was developed for elucidatingthe effectof maldistribution during isothermal reaction in a trickle-bed reactor (Funk et al., 1990). It is based on a computer-generated bed packed with equalsized spherical catalyst pellets. The rectangular bed can have any height or width but has a thickness of only one sphere diameter. Therefore, although it is a threedimensional bed, it is basically a two-dimensional model. In a simulation, the configuration of the liquid inlet distributor is specified. Then, the flow on each and every catalyst pellet is determined, resulting in predictions of the degree of wetting on each individual pellet as well as the overall liquid flow pattern in the bed. CAT (computerassisted tomography) scans of trickle beds under experimental conditions similar to those in the simulations (Lutran et al., 1991)showed that the predicted liquid flow patterns are realistic. Coupled with models for intraparticle diffusionand reaction in a catalytic pellet completely or partially wetted on its external surface (Harold and Ng, 1987; Harold, 1988; Funk et al., 1988,1989;Funk et al., 19911,the sphere-pack model provides the overall rate of reaction for a given liquid flow pattern. The key + Current address: Procter and Gamble Co., Sharon Woods Technical Center, 11511 Reed Hartman Highway, Cincinnati, OH 45241. t Current address: U.O.P. Research Center, Des Plaines, IL 60011.
0888-588519312632-0510$04.00/0
advantage of the sphere-pack model over other conventional or approximate models is its ability for the quantification of irregular wetting features. This is important because the reaction rate of a pellet is often strongly dependent on its degree of wetting. The present spherepack model is limited to isothermal conditions and low gas-phase flow rates. However, these assumptions can be readily removed with a more detailed simulation. A number of interesting observations for the hydrogenation of a-methylstyrene over a Pd/Al203 catalyst have been made and elucidated with the sphere-pack model. Due to the low solubility of hydrogen in a-methylstyrene and the relatively fast transport rate of gas-phasehydrogen to the nonwetted surface of the catalyst, this system has been experimentallydemonstrated to exhibit effectiveness enhancement; i.e., the maximum reaction rate of an individual catalyst pellet occurs under partial wetting conditions (Herskowitzet al., 1979;Funk et al., 1991).(To followthe discussions in this Research Note, it is essential that the reader consults some of the figures in Funk et al. (1990). Those figures will be marked by SPM-spherepack model below.) Let us review some of the key predictions of SPM Figure 5, in which the overall reaction rate is plotted as a function of the liquid volumetric flow rate to the reactor, qr. Three liquid inlet distributors were used in the simulations-a one-tube, a three-tube, and a uniform inlet distributor. Three sets of data were obtained for each distributor type; each data set corresponds to a different computer-generated packing. Four key results were obtained. First, the phenomenon of effectiveness enhancement was conspicuous for the uniform and three-tube liquid inlet distributors, but was rather unnoticeable with the nonuniform one-tube inlet distributor. Second,the overall rate of reaction depended on packing geometry,which influencedthe wetting pattern. In other words, under otherwise identical conditions, a different reaction rate was obtained for each of the three data sets as the packing was changed. Third, at a constant but low liquid flow rate, the overall reaction rate declined as the liquid inlet distribution became less uniform. Fourth, at a constant but higher liquid flow rate, the overall reaction rate was essentially independent of the degree of uniformity of the liquid inlet distribution. A literature review revealed that there were no experimental data suitable for testing all these model predictions. On the contrary, extensive precautions were taken 0 1993 American Chemical Society
Gas Effluent
Hydrogen 1
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Reactor
Inert Alumina
7 Gas Effuent Line F Water Bath
Figure 1. Schematic of equipment used in the packed-bed experiment. Table I. Physical Properties of Catalyst pellet diameter = 0.32 cm surface area = 250 m2/g pellet density = 0.75 g/cm3
solid density = 3.1 g/cm3 void fraction = 0.76
in previous studies to avoid maldistribution. Therefore, the objective of this experimental study was to specifically generate trickle-bed reactor data with liquid maldistribution. The overall setup was very similar to the singlepellet reactor system used in Funk et al. (1991). For the same reasons given in the single-pellet study, hydrogenation of a-methylstyrene at 40 "C and l atm over a Pd/ A1203 catalyst was chosen to serve as the test reaction system.
Experimental Setup and Procedures The experimental setup can be broken down into three main components-the reactor, liquid flow loop, and gas delivery system (Figure 1). Two cylindrically-shaped, Pyrex reaction vessels were used: one had an inner diameter of 3.0 cm (to be referred to as reactor I) while the other an inner diameter of 5.1 cm (to be referred to as reactor 11). Both had a total length of 35 cm. Thus, we could study the effect of the reactor to catalyst particle diameter ratio on liquid maldistribution, and the rate of reaction. For reactor I (II), inert alumina spheres were placed at the bottom of the bed to a height of about 6 cm (8cm) in order to minimize any effects of the liquid leaving the sphere pack. These pellets rested upon a stainless steel screen that was supported by a concentric ring made of fritted glass. Pd-impregnated pellets with a total mass of 28.8 g (84.5 g) were positioned on top of the inert pellets to form the active portion of the bed. The height of this section was approximately 5 cm (6 cm). The y-alumina spheres used in both sections were identical (Norton Co.). Table I is a summary of the physical properties provided by the Norton Co. The impregnation procedure used to form the 1.25 wt 3'5 Pd/A1203 catalyst was the same as that used in the single-pellet study. Despite precautions taken, some pellets appeared slightly darker than others indicating that the intraparticle palladium distribution was not exactly the samefor different pellets. In addition, some of the individual pellets appeared to have variations throughout their interior. An annular water jacket provided a uniform wall temperature for the entire length of the bed. The reactor temperature was controlled by varying the voltage applied to a heating cord wrapped around the outside of the water jacket. Note that for clarity the heating cord as well as the water jacket is not included in Figure 1. Thermocouples were inserted into the gas and liquid entrance lines at points just before the reactants enter the reactor.
Ind. Eng. Chem. Res., Vol. 32, No. 3, 1993 571 Also, a thermocouple was placed in the constant-temperature bath to ensure that the desired liquid feed temperature was maintained. Three micro-thermocouples (Omega,subminiaturethermocouple probe; diameter 0.503 mm) were inserted into various locations of the active section of the catalyst bed. All of the experiments were performed using totalrecycle of the liquid phase, which consisted of a-methylstyrene (Kodak, 98% or higher purity) and the reaction product (cumene). In order to remove a polymerization inhibitor (4-tert-butylcatechol) and any moisture, the a-methylstyrene was soaked in a flask containing approximately20 g of y-alumina pellets prior to being used in the experiments (Germain et al., 1974). A steady flow of the liquid was delivered to the reactor I (11) from the 150-mL (300-mL) feed tank by a pump (Fh-idMetering Inc., Model QD). Except for a section of stainless steel tubing between this pump and the reactor, the rest of the tubing in the liquid flow loop was made of Viton. The section of stainless tubing was maintained at the same temperature as the reactor using a heating cord. After flowing through the bed, the liquid was collected in a shallow pool at the bottom of the reactor before being sent back to the feed tank with a second pump. The liquid pool prevented hydrogen from entering the liquid recycle line. The liquid feed tank was submerged in a constanttemperature water bath to keep the liquid at the reaction temperature. Hydrogen was bubbled through the liquid in order to presaturate the liquid. The agitation caused by the bubbling appeared to provide good mixing in the feed tank. In order to reduce the amount of liquid lost through vaporization, a condenser was installed at the gas outlet of the feed tank. Tap water served as the cooling liquid. For reactor I, in addition to the single-tube inlet shown in Figure 1,a triple-tube inlet configurationwas also used to provide an improved liquid inlet distribution. The three inlet tubes were arranged in such a way that they would occupy the vertices of an equilateral triangle. Prior to entering the reactor, the flow was split into three equal streams. Q u a l flow rates were achieved by adjusting the flow reeistance in each line with three precision valves (Nupro, SS-2MG). For reactor 11,in addition to the singletube inlet, a two-tube inlet configuration and a four-tube configuration were also used to provide an improved liquid inlet distribution. The four-tube inlet was a showerheadlike Pyrex fixture connected to the single inlet; the inlet tubes were arranged in such a way that they would occupy the corners of a square. For this four-tubeinlet distributor, the reactor was filled to a height of 3 cm with inert particlea, followed by the same 6 cm of catalytic pellets and then 5 cm of inert particles. This additional layer of inert pellets was expected to allow more liquid spreading by the time the liquid reached the active middle section, thereby enhancing the uniformity of the liquid pattern. A continuous gas flow of prepurified hydrogen (Aero All Gas,99.9% or higher purity) was delivered to the reactor using an electronic mass flow controller (Datametrics, Model 825). Heating cords wrapped around the stainless steel inlet line were used to preheat the gas to the reaction temperature. A typical run consisted of the following sequence of steps: First, the feed tank was charged with a-methylstyrene. Then, the reactor was preflooded by closing the reactor gas effluent line and pumping liquid from the feed tank to the reactor. The liquid inlet pump was shut off once all the pellets were submerged. After the pellets were
672 Ind. Eng. Chem. Res., Vol. 32, No. 3, 1993 7-
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soaked for 10 min, the recycle pump was turned on and the gas effluent line opened. Aa soon as the bed was drained, the liquid inlet pump was restarted and set to the desired volumetric flow rate. The recycle pump was adjusted to maintain a liquid pool at the bottom of the reactor for reasons discussed above. During this period of time, nitrogen was passed over the bed to prevent the catalyst from coming into contact with any undesirable agents in the surroundings. Next, hydrogen was bubbled through the feed tank at a flow rate of 1.67 mL/s. After the temperatures reached 40O C , nitrogen was turned off and hydrogen was delivered to the reactor at a flow rate of 3.33 mL/s (8.33 mL/s) for reactor I (11). From this point, the system was allowed between 2 and 3 h to reach steady state. The reaction rate was then determined by measuringthe steady buildup of cumene in the liquid phase. One milliliter samples of the liquid were withdrawn from the feed tank at 20-min intervals and analyzed using gas chromatography. The amount of time selected to reach steady state was shown to be sufficient since the plot of the buildup of cumene concentration versus time was linear (Funk, 1991). The method used to determine the reaction rate from the measured buildup of cumene concentration was identical to the one used in the single-pellet reactor study (Funk et al., 1991). Therefore, the details are not repeated here. The gas chromatograph(Hewlett Packard, Model 5890) was operated in the splitleasinjection mode with a capillary column (J & W,Durobond Wax)and a flame ionization detector. Peak areas were calibrated using the method of internal standardization (Grob, 1977). The internal standard was n-propylbenzene.
Results and Discussion This experimental study consisted of measuring the reaction rate at different liquid flow rates for two different inlet configurations for reactor I and three for reactor 11. For both reactors, the one-tube inlet simulate the behavior of a reactor with severe liquid maldistribution. Multiple inlet configurations were used to obtain improved liquid distribution. All of the experiments were performed at a feed temperature and average total pressure of 40 O C and 1atm, respectively. Temperature excursions within the bed were measured and are discwed below. Figure 2 presents the results for both inlet confiiations for reactor I. The filled symbols represent the one-tube inlet data, while the unfilled symbols represent the data obtained with the three-tube inlet. For each inlet configuration, the top of the reactor was opened once to
reposition the inlet tube(@. This was expected to affect the flow path of the liquid and thus the overall reaction rate. This was true even for the one-tube inlet because the tube could not be fabricated to be perfectly vertical and centered in the reactor. To show the effect of this factor on the rate of reaction, two different symbolssquares and circles-are used to signify a change in the liquid flow pattern. Because of equipment limitations, the lowest liquid volumetric flow rate, qr, was approximately 0.5 mL/s. The four data sets were extended with dotted lines to the origin, which represents the limiting case of no reaction because of an absence of the liquid reactant a-methylstyrene. The three-tube data conspicuously exhibit the phenomenon of effectiveness enhancement. The overall reaction rate was external mass-transfer controlled, and hydrogen was the limiting reactant. The hydrogen masstransfer rate was higher on the nonwetted part of a partially wetted pellet because the liquid film constituted an additional resistance. There was a maximum overall rate at an intermediate wetting efficiency due to a net increase in the supply rate of dissolved hydrogen. These data are in qualitative agreementwith the simulation results (SPM Figure 5). The reason can be seen in SPM Figures 7 and 8, which show the flow patterns at different liquid volumetric flow rates for a three-tube and a one-tube liquid inlet distributor, respectively. More liquid filaments are formed with the three-tube inlet, resulting in more partially wetted pellets. A larger number of partially wetted pellets produce a more pronounced effect of effectiveness enhancement. All the experimental reaction rates in Figure 2 are lower thanthe simulation data (SPM Figure 5). The discrepancy was expected because the literature kinetics parameters (Herskowitz et al., 1979) used in the simulations are not likely to be the same as those corresponding to our own catalyst. Also, the range of liquid volumetric flowrate was from 0 to 1m L / s in the simulations but from 0 to 3 mL/s in Figure 2. This is caused by the fact that the spherepack model is basically a two-dimensional model and has a different cross-sectional area than the experimental reactors. Thus, quantitative differences are expected but the qualitative trends should be the same for the simulations and experiments. The simulation data also provide the observation that the overall rate of reaction depends on packing geometry, which influences the wetting pattern. This was demonstrated by the two data seta represented by circles and squares. For example, the filled circles are consistently higher than the filled squares. As mentioned,rather than emptying and repacking the reactor, a new liquid pattern was obtained simply by opening the reactor top and repositioning the liquid inlet distributors. Over the entire range of liquid flow rates studied, the one-tube inlet generally gave significantly lower overall rates. The same general behavior was predicted by the sphere-pack model (SPM Figure 5). At the low end of the range of liquid flow ratea studied more catalyst pellets are partially wetted with the three-tube inlet than the onetube inlet (see, for example, SPM Figures 7c and &).Since nonwetted pellets are not supplied with the essentially nonvolatile liquid reactant, they do not participate in the reaction. Therefore,the overall reaction rate for the entire bed is lower with the one-tube inlet. At higher liquid flow rates, the simulations predict that the overall reaction rate is independent of the uniformity of liquid inlet distribution. A comparison of SPM Figures 7f and 8f reveals the underlying cause. More particles are
Ind. Eng. Chem. Res., Vol. 32,No. 3,1993 573
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Side View
Figure 4. Side and top views of the reactor showinn " the relative positions of the three micro-thermocouples. 0
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Figure 3. Dependence of reaction rate on liquid flow rate for the one-tube (crosses), two-tube (circles), and four-tube (squares) inlet configurations in reactor 11.
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wetted in the three-tube case (SPMFigure 74 but,oppite to the flow pattern at lower liquid flow rates, there are more partially wetted catalyst particles with the one-tube inlet (SPM Figure 84. Because of effectiveness enhancement, the one-tube inlet has on the average a higher rate of reaction per pellet, which compensatesthe contributions of more wetted pellets to the overall reaction rate in the three-tube inlet case. This is opposite to the experimental data which indicate that the overall reaction rate for the three-tube inlet is higher even at high liquid flow rates (Figure 2). The discrepancy is caused by the difference in the tube-to-sphere diameter ratio, which is equal to 15.7in the simulationsand 9.4 in the reactor I experiments. This likely implies that there might not be as many partially wetted pellets in the experiments as encountered in the simulation (SPM Figure 84. In fact, during the course of the experiments, liquid was observed to flow down the insidewall of reador I for liquid flow rates greater than approximately 1.67 mL/s. To prove this point, a reactor with a larger reactor-toparticle diameter ratio was used. Figure 3 presents the results for all three inlet configurationsfor reactor 11.The crosses, circles, and squares indicate the one-tube, twotube, and four-tube inlet data, respectively. In corroboration with the data for reactor I, the effectiveness enhancement phenomenon was evident for the two-tube and four-tube inlet configurations. Also,the reaction rate was higher with improved liquid distribution. In agreement with the simulation data, the overall reaction rate was independent of the uniformity of liquid inlet distribution at higher liquid flow rates. The reason is that, unlike reactor I, the tube-to-sphere diameter ratio for reactor I1 was 15.9,which is close to 15.7 used in the simulations. This indicates that there were more partially wetted catalytic particles in reactor 11, which had a larger diameter than reactor I. It should be pointed out that the reaction rate was somewhat higher for reactor I1 than reactor I over the entire range of liquid flow rates. For example, the average reaction rate for the one-tube inlet distributor is approximately 2.5 X 10-7mol/(g.s) for reactor I but is about 3.5 X lo-' mol/(g.s) for reactor 11. This is due to the fact that a new batch of freshly-prepared catalysts was used for reactor 11. Fortunately, this small difference in catalyst activity doesnot change the qualitative conclusionsderived from these data. Observations of Temperature Excursions. In addition to the reaction rate data, temperature profiles at three locations within the active section of each bed were recorded. Figure 4 is a sketch showing approximately the
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relative positions of the three micro-thermocouples for reactor I1 with the four-tube inlet distributor. Figure 5 presents the temperature versus time data for the same bed at a liquid volumetric flow rate of 1.33 mL/s. Before allowingthe reaction to take place, the reactor was allowed sufficient time to reach a temperature of 40 OC. However, as shown in Figure 5, the temperature profile showed a substantial spike shortly after the hydrogen flow was initiated (indicated by time zero). The micro-thermocouple located around the middle point within the active section of each bed, T, (squares), detected the largest temperature deviation. The final temperature value of approximately 50 "C was significantly higher than the desired temperature of 40 "C. The thermocouple close to the bottom, T b (circles),had a much smaller temperature spike, while the thermocouple close to the top, Tt (diamonds), had the smallest deviation from the desired temperature. The heat of reaction of the hydrogenation of u-methylstyrene, equal to -26 kcal/mol, accounts for these observations. A plausible explanation for the presence of temperature spikes is as follows. With preflooding, the internal void space of all particles was initially liquid filled. In other words, a sufficient amount of liquid reactant was present to react within each particle, regardless of the flow pattern. Hence, all particles contributed to the generation of heat due to the exothermic reaction,resulting in a substantial increase in temperature. Eventually the intraparticle liquid reactant for pellets in nonwetted regions of the reactor was depleted, and onlythose particles receiving a fresh supply of liquid reactant contributed to the reaction. Therefore, the generation of heat was significantlylower and a decreasein temperature resulted. Recently, Watson and Harold (1992) have carried out experiments with a single pellet reactor. The measured pellet temperature and weight as a function of time data confirmed the above explanation. Additionally, the different degrees of temperature deviation for the three thermocouples can be understood
574 Ind. Eng. Chem. Res., Vol. 32, No. 3, 1993
by considering two competing processes. As the gas and liquid streams flow cocurrently downward through the bed, they carry with them in the form of sensible heat the heat generated by reaction. Some of this energy is lost through the reactor walls to the water jacket. The top thermocouple, Tt, had the lowest temperature because the extent of reaction was still small at the top of the active section and it was located closest to the wall. The maximum temperature deviation occurred with the middle thermocouple, T,. It was located at the center of the active section. The bottom thermocouple, T b , showed a smaller deviation than the top one, indicating that the time for cooling by the external water jacket was sufficient at the lowest position. One can rule out the possibility that the thermocouple with the maximum temperature deviation was caused by overheated catalyst pellets in contact with the thermocouple junction. Data similar to Figure 5 were obtained for all other experimental runs.
Concluding Remarks The objective of this experimental project was to generate data suitable for testing some of the qualitative predictions of the isothermal trickle-bed reactor model (Funk et al., 1990). The experimental study involved measuring the overallreaction rate in a packed-bed reactor for different inlet configurations over a range of liquid flow rates. Because of the exothermicity of the model reaction system, the experiments were not exactly isothermal. The deviation from the set temperature did not seem to have changed the qualitative trends. In fact, the experimental data confirmed all the qualitative predictions of the sphere-pack model. Acknowledgment This research was supported in part by the National Science Foundation (Grant No. CBT-8700554) and by a Mobay Corp. research fellowship awarded to R.L.M.
Literature Cited Funk, G. A. Effect of Wetting on Catalytic Gas-Liquid Reactions. Ph.D. Dissertation, University of Maseachusetts, Amherst, 1990. Funk, G. A.; Harold, M. P.; Ng, K. M. Effectiveness of A Partially Wetted Catalyst for Bimolecular Reaction Kinetias. AZChE J. 1988, 34, 1361.
Funk, G. A.; Harold, M. P.; Ng, K. M. Reactant Adsorption Effects on Partially Wetted Catalyst Performance. Chem. Eng. Sci. 1989, 44, 2509.
Funk, G. A.; Harold, M. P.; Ng, K. M. A Novel Model for Reaction in Trickle Beds with Flow Maldietribution. Znd. Eng. Chem.Res. 1990, 29, 738.
Funk, G. A.; Harold, M. P.; Ng, K. M. Experimental Study of Reaction in a Partially Wetted Catalytic Pellet. AZChE J. 1991,37,202. Germain, A. H.; Lefebvre,A. G.; L’Homme, G. A. Experimental Study of a Catalytic Trickle Bed Reactor. ACS Symp. Ser. 1974,133, 164.
Grob, R. L., Ed. Modern Practice of Gas Chromatography; Wiley: New York, 1977. Harold, M. P. Partially Wetted Catalyst Performance in the Consecutive-Parallel Network. AZChE J. 1988,34,980. Harold, M. P.; Ng, K. M. Effectiveness Enhancement and Reactant Depletion in a Partially Wetted Catalyst. AZChE J. 1987, 33, 1448.
Herskowitz, M.; Carbonell, R. G.; Smith, J. M. Effectiveness Factor and Mase Transfer in Trickle-Bed Reactors. AZChE J. 1979,25, 272.
Lutran, P. G.; Ng, K. M.; Delikat, E. P. Liquid Distribution in Trickle Beds. An Experimental Study Using Computer-Assisted Tomography. Znd. Eng. Chem. Res. 1991,30, 1270. Ng, K. M.; Chu, C. F. Trickle-Bed Reactors. Chem. Eng. Prog. 1987, 38 (ll),55.
Stanek, V.; Hanika, J.; Hlavacek, V.; Tmka, 0. The Effect of Liquid Flow Distribution on the Behavior of a Trickle Bed Reactor. Chem. Eng. Sci. 1981, 36, 1045. Watson, P. C.; Harold, M. P. Dynamic Effects of Phase Transition with Exothermic Reaction in a Single Catalytic Pellet. AZChE J. 1992, in press . Received for reuiew July 16, 1992 Revised manuscript received December 1, 1992 Accepted December 28, 1992
