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Hydrodynamics of a Bubble Column Reactor Staged with Sintered Metal Fiber Catalyst for Continuous Three-Phase Hydrogenation Martin Grasemann, Natalia Semagina, Albert Renken, and Lioubov Kiwi-Minsker* Ecole polytechnique fe´ de´ rale de Lausanne (EPFL), GGRC-ISIC, CH-1015 Lausanne, Switzerland
An innovative staged bubble column reactor (SBCR) with a structured catalyst based on sintered metal fibers (SMFs) coated with a thin ZnO layer has been designed for continuous three-phase hydrogenations. ZnO/ SMF thin plates (thickness ≈ 0.3 mm) were found to be suitable stages for the SBCR because of their rigid open structure with high porosity (>90%) and good adhesion of ZnO. The hydrodynamic characteristics of the SBCR, including pressure drop and residence time distribution, were investigated at varying superficial liquid (uL0 e 0.6 cm/s) and gas (uG0 e 10 cm/s) velocities for different gas/liquid systems. A semiempirical model was developed for describing the influence of the superficial fluid velocities as well as the gas and liquid physical properties on the pressure drop during SBCR operation and was found to be consistent with experiments. 1. Introduction Selective catalytic three-phase hydrogenations are widely used for the manufacture of fine chemicals and pharmaceuticals.1,2 The efficiency of these catalytic processes is strongly dependent not only on the catalysts’ intrinsic activity and selectivity, but also on the interaction of heat/mass transfer and the reaction kinetics, which are entirely controlled by the reactor and catalyst design. The proper choice of reactor is critical in terms of pressure drop, liquid hold-up, residence time distribution, and mass-transfer limitations. The hydrogen concentration in the liquid has been shown to be important for product selectivity.3 The novel approach for three-phase reactor design described herein is based on structured catalytic materials. Recently, we suggested fiber catalysts in the form of woven cloths for use in staged bubble column reactors (SBCR).4-6 As the diameter of a single fiber is usually in the micrometer range, internal masstransfer limitations can be effectively avoided, while the open macrostructure of the material leads to low pressure drop and also acts as a micromixer, enhancing bubble redistribution on each stage. Both glass fibers and activated carbon fiber cloths have shown promising results in this type of reactor,4-6 but their low rigidity complicates their use in SBCRs. An interesting alternative to woven fabrics are threedimensional sintered metal fibers (SMFs) in the form of porous sheets of ∼0.3 mm thickness consisting of fibers of ∼20 µm diameter. These SMF sheets exhibit high mechanical, thermal and chemical stabilities and can be easily shaped into any macrostructured catalytic bed. Their open structure favors intense phase mixing at low pressure drop.7-13 Furthermore, the high thermal conductivity of SMFs ensures isothermal conditions on the catalyst surface during highly exothermic reactions such as C-C triple-bond hydrogenations. The multilevel structure of an SMF-based catalyst is schematically presented in Figure 1. A catalyst based on SMFs with a thin Pd/ZnO coating showed excellent performance in the semibatch three-phase hydrogenation of an acetylenic alcohol in water.14 The general scheme of this model reaction is shown in Figure 2. To explore the potential of SMF-based catalysts for threephase reactions, we performed a study of the hydrodynamics * To whom correspondence should be addressed. Tel.:+41-21-693 3182. Fax:+41-21-693 3160. E-mail:
[email protected].
(pressure drop and back-mixing characteristics) in an SBCR staged with SMF layers. To characterize the back-mixing, the residence time distribution (RTD) was determined for various gas and liquid superficial velocities for the system water/air. The pressure drop was measured using SMF layers both with and without a ZnO coating. The influence on the pressure drop of liquid properties (viscosity and liquid/gas surface tension) and the distance between the layers was determined for various gas/liquid superficial velocities. The liquids used in this study cover a broad range of viscosities and surface tensions for both aqueous and organic reaction systems. 2. Experimental Section The SMF plates used for the hydrodynamics studies were supplied by Southwest Screens & Filters SA, Sprimont, Belgium, and were made of FeCrAl alloy (20% Cr, 4.75% Al, 0.27% Y, ∼1-2% other elements, balance Fe) in the form of uniform porous panels (0.29 mm thick, 71% porosity, 675 g/m2, ∼20 µm fiber diameter). The coating of the SMF surface with ZnO (4.5 wt %) was carried out according to a procedure described elsewhere,11 yielding a homogeneous surface layer of approximately 1.5 µm thickness. An identical coating was used for the hydrogenation experiments described in ref 14. All experiments were carried out for co-current gas and liquid up-flow using the setup shown in Figure 3. The bubble column reactor consisted of eight ring segments with 40 mm internal diameter and 15 mm height, resulting in a total height of the staged bubble column section of 120 mm. The segments and the conical inlet and outlet sections were made of acrylic glass, allowing visual observation of the column interior. Both the inlet and outlet sections were packed with glass beads (d ) 1 mm) to reduce the free volume and to provide a homogeneous radial distribution of the fluids at the column inlet. The SMF layers were fixed between adjacent reactor segments. To determine the influence of the distance between SMF layers on the pressure drop, one-half of the SMF sheets were removed, thus increasing the distance between layers to 30 mm. The air flow rates were regulated by mass flow controllers (Bronkhorst). The liquid flow rates were regulated using a valveless piston pump (Fluid Metering Inc.) set at a high frequency to limit flow pulsation. Inlet and outlet pressures were measured at the first and last reactor segments at a vertical distance of 120 mm.
10.1021/ie0700718 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/26/2007
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Figure 1. Three-level structure of the SMF-based Pd catalysts: Macrostructure of the layered fibrous bed for three-phase reactions, mesostructure of the porous SMF layer, and microstructure of the coated fibers (from left to right).
3. Results and Discussion
Figure 2. Reaction scheme of selective triple-bond hydrogenation. Table 1. Values for uG0 and uL0 Used in the Extended 22 FFD superficial gas velocity, uG0 (cm/s)
superficial liquid velocity, uL0 (cm/s)
FFD
2.6 7.56
0.143 0.367
center point
5.08
0.255
external points
0.83 2.6
0.143 0.04
2.1. Residence Time Distribution. For the residence time distribution (RTD) measurements, two ring segments, each holding a conductivity cell, were added at the top and bottom of the column. The cells were designed as two parallel copper grids at 4 mm vertical distance over the whole column cross section, as suggested by van Gelder and Westerterp.15 Both measuring cells were filled with glass beads (d ) 1 mm) to accelerate the cells’ response and to minimize their impact on the measured RTD in the column. The applicability of this design for staged bubble columns has been demonstrated by Ho¨ller et al.4 As a tracer, 0.4 mL of an aqueous solution of potassium chloride (40 g/L) was injected at the bottom of the packed inlet section. The cells’ response signals were recorded at 125 ms intervals. 2.2. Pressure Drop. The effects of various parameters on the pressure drop were determined using an experimental program based on a two-parameter full-factorial experimental design (22 FFD).16 The parameters investigated included the gas and liquid superficial velocities (uG0, uL0), the liquid type, and the distance between SMF layers (DL). As only the superficial fluid velocities could be varied continuously and independently, an FFD for these two parameters was repeated for the accessible values of DL (15 and 30 mm) and three different liquids (water, ethanol, and a mixture of 10 wt % 2-propanol in water). The simultaneous change of liquid viscosity and surface tension with the change of liquid type could not be avoided. Two external points and one center point were added to the basic 22 FFD to check for nonlinear contributions of the fluid velocities to the pressure drop. All measurement points were repeated three times; for the center point, five repetitions were done. Where possible, measurements were done in a random order. Table 1 lists the values for uG0 and uL0 used in the extended 22 FFD.
3.1. Residence Time Distribution. Figure 4 shows typical response curves as normalized inlet and outlet signals, C(t), for a tracer injection. The residence time distribution (RTD) can be characterized by the variance around the mean residence time. The tanks-in-series model was used to describe the RTD of the liquid phase in the studied SBCR.17 The age distribution, E(t), for N perfectly mixed tanks in series can be calculated as a function of the mean residence time, htB, and the number of tanks, N, as
E(t) )
tN-1 NN e-tN/thB N (N - 1)! ht B
(1)
The number of tanks N and the mean residence time in the bubble column were first estimated using the mean residence time and variances calculated from the response curves at the inlet and outlet17 (eqs 2 and 3)
N)
(thout - ht in)2 σout2 - σin2
ht B ) ht out - ht in
(2) (3)
A second method to estimate the mean residence time, ht, and the number of tanks in series, N, can be obtained with the convolution integral (eq 4)17 by fitting the calculated output signal, exp Ccalc out (t), to the measured output response signal, Cout (t) ti
Ccalc out (ti) )
∑0 Cexp in (tj) E(ti - tj) ∆t
(4)
For both methods, the calculated concentration curves are in fairly good agreement with the measured data. Figure 4 shows an example set of normalized inlet and outlet concentration responses together with the respective results for the tanks-inseries model. Figure 5 shows the estimated number of tanks obtained for different superficial gas velocity uG0. As can be seen, the backmixing characteristics of the SBCR are not influenced by the superficial gas velocity uG0 once the gas provides sufficient mixing in the single stages. It is interesting to note that, although the SBCR comprises only nine compartments between the top and bottom conductivity cells (see Figure 3), the model gives a higher value of N ≈ 10.5. Probably, the conductivity cells packed with glass beads influence the measured RTD by acting as compartments with lower mixing efficiencies.
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Figure 3. Scheme of the staged bubble column. Figure 5. Model parameter N for eight SMF layers (uL0 ) 0.29 cm/s).
on the empirical model reported for fibrous woven cloths studied in a similar setup18 and has the general form
∆pdyn ) ∆pf ) A ReBL ReCG G
Figure 4. Normalized concentration response for eight SMF stages ( htB ) 14.9 s, uG0 ) 9.9 cm/s, uL0 ) 0.29 cm/s): experimental data (O,b) and model prediction.
3.2. Pressure Drop. In the present study, only the dynamic pressure drop, ∆pdyn, was of interest. It was estimated from the mean residence time, htB, in the SBCR and the measured total pressure drop, ∆ptot, using the equation
∆pdyn ) ∆ptot - FLghL ) ∆ptot - FLg
QLht B AR
(5)
The parameters influencing the pressure drop through the SBCR with coated SMF layers were determined by an analysis of variance (ANOVA)16 based on a polynomial model of the experimental pressure drop data. The controlling parameters were the gas superficial velocity and the liquid type. The dynamic pressure drop was found to decrease slightly with increasing liquid superficial velocity. However, for all tested liquids, the chosen range of uL0 caused a maximum decrease in ∆pdyn of 2%. Therefore, it was discarded for the graphical representation of the results. Figure 6 shows the dynamic pressure drop per SMF stage as a function of the gas Reynolds numbers for three of the liquids investigated. The distance between the SMF layers showed no significant influence on the pressure drop in the investigated parameter ranges. This indicates that the energy dissipation in the gas/ liquid bulk was negligible compared to the energy dissipation needed to pass through the catalytic SMF stages. A semiempirical model predicting the frictional pressure drop over the SMF sheets was developed, including the influence of the fluid properties and the superficial gas and liquid velocities. It is based
(6)
where A, B, and C are model parameters and G is a factor characterizing the cloth’s thread and mesh geometry. In this case, the dynamic pressure drop is assumed to be purely frictional. For the SMF stages, a number of additional assumptions had to be made: (1) Because of the small pore diameter, capillary forces during the passage of the gas through the SMF material cannot be neglected. Therefore, a term describing the capillary entrance pressure, ∆pcap, was added to the dynamic pressure drop
∆pdyn ) ∆pcap + ∆pf
(7)
(2) ∆pcap can be approximated by the Young-Laplace equation,19 assuming a circular pore geometry with a minimum G-L curvature diameter equal to the equivalent maximum pore diameter dP (contact angle of 180°). The relevant surface tension in this case is defined by the G-L interface only. Thus, the Young-Laplace equation can be written as19
∆pcap )
4γG-L dP
(8)
(3) Figure 6 shows that the liquid type influences not only the level of ∆pdyn (defined by ∆pcap) but also the slope of ∆pdyn as a function of ReG. To account for this factor, ∆pcap was introduced into the basic equation for ∆pf (eq 6). This seems reasonable, as the capillary entrance pressure strongly influences the gas distribution over the SMF surface area and, consequently, the local gas velocity and the frictional pressure drop. (4) Although the influence of the liquid superficial velocity on the pressure drop was shown to be weak, ReL was included in the model. The semiempirical model resulting from these assumptions can be presented in the form
∆pdyn )
4γG-L 4γG-L B C +A Re Re dP,max dP,max L G
(9)
The model was fitted to the experimental data acquired for the systems water/air and ethanol/air using a nonlinear least-squares
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Figure 8. Dynamic pressure drops per SMF stage (reactor with eight SMF layers) for coated and uncoated SMFs (system water/air). Figure 6. Dynamic pressure drop per SMF stage (reactor with eight SMF layers) for different liquids.
are thought to have a major influence on model accuracy. The error bands in Figure 6 represent the model prediction range caused by a (5% variation in liquid surface tension. A comparison between the pressure drop of SMF stages with and without metal oxide coating for the system water/air showed a considerably increased dynamical pressure drop for the coated SMF stages (Figure 8). This can be expected, as the metal oxide coating decreases the SMF pore diameter, thus shifting ∆pcap toward higher values. However, it cannot be excluded that the pressure drop characteristics are also influenced by the changed wettability of the coated SM fibers. Conclusions
Figure 7. Parity plot for measured and predicted SMF dynamic pressure drops. Table 2. Fluid Properties at 20 °C20
water ethanol 2-propanol (10%) in water air
G-L surface tension, γ (mN/m2)
viscosity, µ (mPa s)
72.0 22.0 40.4 -
1.002 1.203 1.629 1.82 × 10-2
regression (Mathlab). A, B, C, and dP,max were chosen as model parameters. The latter represents the maximum equivalent pore diameter of the SMF material averaged over all layers. Table 2 lists the fluid properties used in the model. The best fit was obtained for A ) 3.36, B ) 0.11, C ) -0.04, and dP,max ) 0.5 mm. The surprisingly high value for dP,max suggests the heterogeneous microstructure of the SMF material, also visible in Figure 1. The decrease of the pressure drop with increasing liquid superficial velocity predicted by the ANOVA is confirmed by the fitted value for C. The fitted parameters were applied to the system water/2propanol/air to test the general validity of the model. Both the fitted and the predicted pressure drop curves for the three liquid/ gas systems are shown in Figure 6. Figure 7 shows the resulting parity plot for all gas/liquid systems studied. It can be seen that the semiempirical model describes the pressure drop through the SBCR with the SMF catalyst reasonably well. The observed deviations of the predicted behavior can have various causes. Impurities and the resulting changes in liquid surface tension
Sintered metal fiber (SMF) filters were used for the preparation of structured catalyst in staged bubble column reactors (SBCRs) for three-phase reactions. The SBCR hydrodynamics (pressure drop, residence time distribution) was studied for various superficial gas and liquid velocities. The frictional pressure drop over the SMF layers was found to depend mainly on the gas velocity, uG0, and the gas-liquid surface tension. The influence of the liquid superficial velocity was determined to be relatively small. A semiempirical model to predict the pressure drop over the SMF layers within the SBCR was developed. The model data obtained for various liquids were consistent with experimental results. A narrow residence time distribution (RTD) was obtained in the SBCR and described by a tanks-in-series model. The low pressure drop and narrow RTD suggest that SMFbased structured catalysts are suitable for SBCRs applied in continuous three-phase hydrogenations. Symbols Used A, B, C ) model parameters C ) normalized concentration DL ) distance between SMF layers (mm) AR ) reactor cross-sectional area (m2) dP,max ) maximum equivalent pore diameter (model parameter) (m) E(t) ) age distribution (s-1) g ) gravitational constant (m s-2) h ) height (m) N )number of perfectly mixed tanks ∆p ) pressure drop (Pa) Q ) volumetric flow rate (m3 s-1) t ) time (s) ht ) mean residence time (s) u0 ) superficial velocity (cm s-1)
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Greek Symbols m-1)
γ ) surface tension (mN F ) density (kg m-3) σ2 ) variance of the residence time distribution (s2) Subscripts B ) bubble column dyn ) dynamic f ) frictional G ) gas in ) reactor inlet L ) liquid out ) reactor outlet R ) reactor tot ) total Superscripts calc ) calculated exp ) experimental Acknowledgment Financial support from the Swiss National Science Foundation and the Swiss Commission for Technology and Innovation (CTI) is gratefully acknowledged. Literature Cited (1) Chen, B.; Dingerdissen, U.; Krauter, J. G. E.; Rotgerink, H. G. J. L.; Mobus, K.; Ostgard, D. J.; Panster, P.; Riermeier, T. H.; Seebald, S.; Tacke, T.; Trauthwein, H. New developments in hydrogenation catalysis particularly in synthesis of fine and intermediate chemicals. Appl. Catal. A 2005, 280, 17. (2) Molnar, A.; Sarkany, A.; Varga, M. Hydrogenation of carbon-carbon multiple bonds: Chemo-, regio- and stereo-selectivity. J. Mol. Catal. A: Chem. 2001, 173, 185. (3) Ro¨ssler, F.; Meyberg, M. Catalytic hydrogenation in the liquid phase: Influence of the dissolved hydrogen concentration on the selectivity of alkyne to alkene hydrogenations. In Book of Abstracts 7th International Symposium on Catalysis Applied to Fine Chemicals (CAFC-7); DECHEMA: Frankfurt, Germany, 2005; p 76. (4) Ho¨ller, V.; Radevik, K.; Kiwi-Minsker, L.; Renken, A. Bubble columns staged with structured fibrous catalytic layers: Residence time distribution and mass transfer. Ind. Eng. Chem. Res. 2001, 40, 1575.
(5) Ho¨ller, V.; Yuranov, I.; Kiwi-Minsker, L.; Renken, A. Structured multiphase reactors based on fibrous catalysts: Nitrite hydrogenation as a case study. Catal. Today 2001, 69, 175. (6) Kiwi-Minsker, L.; Joannet, E.; Renken, A. Loop reactor staged with structured fibrous catalytic layers for liquid-phase hydrogenations. Chem. Eng. Sci. 2004, 59, 4919. (7) De Greef, J.; Desmet, G.; Baron, G. V. Micro-fiber elements as perfusive catalysts or in catalytic mixers. Flow, mixing and mass transfer. Catal. Today 2005, 105, 331. (8) Tribolet, P.; Kiwi-Minsker, L. Palladium on carbon nanofibres grown on metallic filters as novel structured catalyst. Catal. Today 2005, 105, 337. (9) Yuranov, I.; Renken, A.; Kiwi-Minsker, L. Zeolite/sintered metal fibers composites as effective structured catalysts. Appl. Catal. A 2005, 281, 55. (10) Cahela, D. R.; Tatarchuk, B. J. Permeability of sintered microfibrous composites for heterogeneous catalysis and other chemical processing opportunities. Catal. Today 2001, 69, 33. (11) Yuranov, I.; Kiwi-Minsker, L.; Renken, A. Structured combustion catalysts based on sintered metal fibre filters. Appl. Catal. B 2003, 43, 217. (12) Cerri, I.; Pavese, M.; Saracco, G.; Specchia, V. Premixed metal burners based on a Pd catalyst. Catal. Today 2003, 83, 19. (13) Meffert, M. W. Ph.D. Thesis. Preparation and characterization of sintered metal microfiber-based composite materials for heterogeneous catalyst applications. Auburn University, Auburn, AL, 1998. (14) Semagina, N.; Grasemann, M.; Xanthopoulos, N.; Kiwi-Minsker, L.; Renken, A. Structured catalyst of Pd/ZnO on sintered metal fibers for 2-methyl-3-butyn-2-ol selective hydrogenation. J. Catal. 2007, manuscript accepted. (15) van Gelder, K. B.; Westerterp, K. R. Residence time distribution and hold-up in a cocurrent upflow packed bed reactor at elevated pressure. Chem. Eng. Technol. 1990, 13, 27. (16) Box, G. E. P.; Hunter, J. S.; Hunter, W. G. Statistics for Experimenters; John Wiley & Sons, Inc.: New York, 2005. (17) Levenspiel, O. Chemical Reaction Engineering; John Wiley & Sons: New York, 1999. (18) Ho¨ller, V.; Wegricht, D.; Kiwi-Minsker, L.; Renken, A. Fibrous structured catalytic beds for three-phase reaction engineering: Hydrodynamics study in staged bubble columns. Catal. Today 2000, 60, 51. (19) Gregg, S. J.; Sing, R. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982. (20) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics, 82nd ed.; CRC Press: Boca Raton, FL, 2001.
ReceiVed for reView January 12, 2007 ReVised manuscript receiVed May 2, 2007 Accepted May 3, 2007 IE0700718
