Research Note pubs.acs.org/IECR
Negative Impact of High Stirring Speed in Laboratory-Scale ThreePhase Hydrogenations Inci Ayranci, Suzanne Kresta, Jing Shen, and Natalia Semagina* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2 V4 Canada S Supporting Information *
ABSTRACT: An increase in stirring speed is generally considered to be an a priori means of reducing external mass-transfer limitations in fast three-phase hydrogenations that are performed in a stirred tank. We provide experimental evidence for a 300mL stirred reactor that, above a certain impeller speed, the efficiency of gas−liquid mass-transfer decreases, resulting in the decreased reaction rate. The phenomenon is attributed to the high degree of gas recirculation with large cavities behind the blades. The recirculation may decrease hydrogen concentration in the remainder of the tank, thus decreasing the concentration gradient that controls mass transfer. The model reaction in this work was 2-methyl-3-butyn-2-ol semihydrogenation with Lindlar catalyst Pd−Pb/CaCO3. The test impellers were a Rushton turbine, a down-pumping pitched blade turbine, and up-pumping A340 impellers. The kinetic experiments were combined with the measurement of volumetric gas−liquid mass-transfer coefficient, flow pattern analysis and impeller power demand calculations. Although the study does not include kinetic analysis, it provides guidance to the three-phase reaction system analysis that the highest stirring speed may enhance mass-transfer limitations and should not be used without caution.
1. INTRODUCTION Three-phase catalytic hydrogenations in semibatch reactors are widely used in the synthesis fof ine chemicals. They represent a classical example of a three-phase catalytic process, in which hydrogen dissolves in the liquid phase and, along with the liquid reactant, diffuses toward the external solid catalyst surface, followed by the internal diffusion. If the intrinsic surface reaction is relatively fast, such as an alkyne hydrogenation on a Pd catalyst, the process is most likely to be masstransfer limited, and the higher the reaction temperatures and hydrogen pressures, leading to much faster reaction kinetics, the higher the negative impact of the mass-transfer limitations (MTL). Not only activity, but also selectivity decreases under MTL, since the retarded diffusion of a fresh reactant and a target product to/from the catalyst surface promotes the product’s overhydrogenation. Both gas−liquid MTLs, and liquid−solid limitations by a hydrogenated reactant and/or dissolved hydrogen have been documented for such reactions.1−5 In eliminating MTL in gas−liquid−solid three-phase reaction systems, two operational criteria should be considered: gas distribution and solids (catalyst) suspension and distribution. At solids concentrations above 10 wt %, an interface called cloud height can separate the solids-rich volumes and the clear liquid volumes.6 Typically, much lower catalyst quantities are used, such as 0.3 wt % as in the current study, thus, the reaction system is dominated by gas dispersion. Gas can be introduced into the reactor with a sparger, or by using self-inducing shafts and impellers.7 To achieve gas induction and good gas distribution, dual impellers are recommended.8,9 In this study, we propose two dual-impeller configurations to eliminate MTLs and provide good mixing between the phases, and one traditional single impeller configuration as a test model. The impeller configurations are (1) a Rushton turbine (RT) and a © XXXX American Chemical Society
down-pumping pitched blade turbine (PBT) where the RT is the lower impeller, and (2) two up-pumping Lightnin A340 impellers. In these configurations, the lower impeller is responsible for solids suspension and gas distribution, and the upper impeller is responsible for the incorporation of the upper liquid layer in the main flow circulation, as well as further distribution of gas and solids. The single RT is one of the most widely used stirrers in the chemical industry.10,11 Some combinations of RT and PBT or multiple PBTs employ the most studied dual-impeller systems.9,12,13 The wide-blade hydrofoils, A340s, are relatively new, with few formal studies in the open literature. The hydrofoil impellers work well for gas dispersion, and this configuration is very promising for this application. The experiments were carried out in a 300-mL stirred vessel with a hollow gas-inducing shaft that is frequently used for laboratory kinetic studies. High stirring speed is generally considered as one of the a priori requirements to eliminate external MTLs. In the current work, we show that there is an optimal stirring speed, and its further increase is detrimental to a selected three-phase process. The tests were carried out for an industrially important threephase reaction by evaluating reaction rates and selectivity with the three impeller configurations and stirring rates, as well as measuring corresponding hydrogen volumetric mass-transfer coefficients (kLa) and analyzing the flow patterns in the reactor. The model reaction is a known fast hydrogenation of 2-methyl3-butyn-2-ol (MBY) to 2-methyl-3-buten-2-ol (MBE) that is an intermediate in vitamin A, vitamin E, and perfume production (see Scheme S1 in the Supporting Information). The undesired Received: May 3, 2014 Revised: October 8, 2014 Accepted: November 6, 2014
A
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Research Note
overhydrogenation product is 2-methylbutan-2-ol (MBA). Commercial Lindlar catalyst was applied in a semibatch process with ethanol as a solvent. Based on the catalytic testing results and impeller power consumption calculations, we also propose the most optimal impeller configuration and stirring speed combination among the three configurations studied. Although the study does not include kinetic analysis, it provides guidance to the three-phase reaction system analysis that the highest stirring speed may enhance MTLs and should be used with caution. The applicability of the data to a larger scale requires further investigation. The experimental details and impeller designs are provided in the Supporting Information.
2. IMPELLER AND STIRRING SPEED EFFECTS ON THE PRODUCT YIELD, MASS TRANSFER, AND FLOW PATTERN Table 1 compares the maximum yield of the target product MBE and the time to achieve it for three impeller Table 1. Time to Maximum MBE Yield and Maximum MBE Yield at Different Stirring Speedsa 500 rpm impeller RT RT + PBT A340
900 rpm
1200 rpm
time, t (min)
MBE yield (%)
time, t (min)
MBE yield (%)
time, t (min)
MBE yield (%)
50 56
92.8 92.1
38 41
91.5 89.7
48 44
92.7 92.3
56
92.1
44
91.9
51
92.5
a
Errors for repeatability are ±0.5% for the yield and ±2 min for the time.
Figure 1. (Top) Concentration versus time and (bottom) selectivity− conversion profiles for the MBY hydrogenation with a Rushton turbine.
configurations at 500, 900, and 1200 rpm. Typical kinetic curves and MBE selectivity versus MBY conversion plots are shown for the Rushton turbine in Figure 1. The concentration corresponds to the weight percent of each of MBY, MBE, and MBA in their reaction mixture (solvent is excluded). The corresponding experimental kLa values are provided in Table 2. The lowest reaction rates (highest times to the maximum yield) are observed at 500 rpm for all impellers, which is consistent with the lowest kLa values. Typically, functionalized alkyne hydrogenations are characterized by a first-order equation, relative to the dissolved hydrogenation concentration,14 so the lower hydrogen concentration results in the lower intrinsic reaction rate. The lowest rates at 500 rpm are also responsible for higher selectivity to MBE: there is likely enough time for MBE to diffuse from the catalyst surface to the bulk before it gets overhydrogenated on the catalyst surface, as similarly discussed by Nijhuis et al. for 3-methyl-1-pentyn-3-ol hydrogenation.4 At 900 rpm, all impellers allowed the highest reaction rates and, hence, lowered the selectivity to MBE. The kLa values are higher than at 500 rpm, which can be explained by larger interfacial surface area with increased stirring speed. A surprising behavior was observed for the stirring speed of 1200 rpm. For all of the impellers, the kLa, reaction rate, and selectivity values were generally found between those values for the 500 rpm and 900 rpm speeds. Increasing the stirrer speed above 900 rpm was detrimental to the observed catalytic performance. This trend may be related to the gas−liquid mixing phenomena that occurs in turbulent systems.7 Alkyne hydro-
Table 2. Experimental kLa Values (s−1) for Hydrogen in Ethanol at 40 °C in the Pressure Range of 1.8−4 bar (Gauge Pressure) Stirring Speed impeller
500 rpm
900 rpm
1200 rpm
RT RT + PBT A340
0.056 ± 0.008 0.065 ± 0.008 0.067 ± 0.011
0.064 ± 0.012 0.068 ± 0.011 0.069 ± 0.018
0.058 ± 0.006 0.066 ± 0.013 0.069 ± 0.009
genations are known to be highly susceptible to the gas−liquid MTL, meaning that the gas flow pattern in the reactor is important. The high degree of gas recirculationgas returning back to the impeller, as opposed to that spargedthat occurs at high stirrer speeds (1200 rpm in this case) may reduce hydrogen concentration in the remainder of the tank, decreasing the concentration gradient that controls mass transfer,7 leading to the decrease in experimental kLa values and the decrease in reaction rate. To verify the hypothesis on the possible gas recirculation for the system under study, we performed an analysis of the flow patterns and operating regimes. The analysis is performed for the Rushton turbine, since all governing equations are known. The Supporting Information contains detailed calculations, while Figure 2 summarizes the results. The hollow shaft allows gas induction from the space above the liquid to the holes in the sparger, because of the pressure difference when the impeller is rotating. Zero pressure difference, meaning the gas is just drawn in, is characterized B
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limit is also exceeded, which is dramatic for 1200 rpm. This may explain the reduced kLa values, as well as the lower reaction rate at this highest rotation speed. Although recirculation is also likely for 900 rpm, the recirculation limit is an order of magnitude lower than that observed for the 1200 rpm stirring speed. The negative effect of recirculation on kLa and the reaction rate at 900 rpm seem to be outweighed by improved mass transfer, as compared to 500 rpm, because of higher gas velocities. These observations refer to the Rushton turbine, but they are likely to qualitatively explain the similar observed phenomena (the highest rate is observed at 900 rpm; see Table 1) for the RT+PBT configuration. The A340 impeller is specifically designed to eliminate cavity formation; that is why it provides the highest kLa values among all impellers for all stirring speeds. The A340 impeller allows higher kLa values and, hence, lower selectivities than the Rushton turbine, which is consistent with the general observed trend. For the A340 impeller, however, the reaction rate dependence on kLa is inverted: equally high kLa values for all three stirring speeds (∼0.069 s−1) provide the lowest rates among all tested impellers at a fixed stirring rate (see Table 1). This is likely due to a different flow pattern in the reactor with the A340 impeller, which is out of the scope of the current work and cannot be referenced, because of rather few reported studies.
Figure 2. Flow map for a single Ruston turbine (D/T = 0.55). Regimes: (1) below minimum dispersion speed, (2) vortex cavities, no recirculation, (3) vortex cavities with recirculation, (4) flooded, (5) loaded with large cavities, and (6) large cavities with recirculation.
by the critical impeller speed (NC). At the impeller speeds below NC, the mass transfer occurs only on top of the liquid; at values above NC, the gas is drawn down to the blades, where the bubbles are created.15 The NC value determined for our system is 459 rpm, so our lowest applied 500 rpm speed is sufficient for gas induction. The superficial gas velocities were found between 0.02 and 0.04 (the lower value is for a stirring speed of 500 rpm), which corresponds to the homogeneous regime in the tank, with the monomodal distribution of gas bubbles (typically between 0.5 mm and 4 mm).7 In this regime, the impeller controls the flow pattern and bubble size. The Froude number (Fr) and flow rate (Fl) number were evaluated to characterize the transitions between different gas flow pattern regimes:7 Fr =
Fl =
N 2D g
3. IMPELLER SELECTION As seen from Table 1, the reaction rates vary within a small range for all impellers and stirring speeds, and in the synthesis of fine chemicals, because of the high target product cost and high E-factors (waste/product ratio), selectivity is more important than the reaction times, if they are within an acceptable range. Since the MBE yields are lowest for a stirring speed of 900 rpm for all stirrers, this stirring speed should not be selected for any of the impellers. In terms of the MBE yield, RT@500 rpm, RT@1200 rpm and A340@1200 rpm outperform other combinations. Impeller power consumption is an additional factor to select the proper impeller−speed combination. The power consumption calculations are presented in the Supporting Information. As seen from Figure 3, the power demand for the A340 impeller is lower than for the Rushton turbine, when compared
(1)
Qg
(2) ND3 where N is the rotational speed, D the impeller diameter, g the acceleration due to gravity, and Qg the gas flow rate. The following regimes can be distinguished:7 • At Fr < 0.04, there is no impeller influence. • At Fl > 30Fr(D/T)3.5, impeller flooding occurs (gas flow swamps the impeller, resulting in poor mixing); T is the tank diameter. • At Fl > 0.025(D/T)−0.5, gas accumulates behind the blades, forming cavities, which may obstruct the liquid discharge from the impeller, causing poor mixing and mass transfer. • At Fl < 13Fr2(D/T)5.0, gas recirculates back to the impeller, causing a decrease in the mean gas phase concentration driving force for the gas−liquid mass transfer. These regimes are plotted on a flow map for our impeller-totank diameter ratio (D/T) of 0.55 (see Figure 2), along with the loci for 500, 900, and 1200 rpm stirring speeds. As seen from Figure 2, the impeller is not flooded, but the formation of large cavities behind the blades of the Rushton turbine is likely, because the cavity formation limit (Fl = 0.034) is exceeded. For the two larger stirring speeds, the recirculation
Figure 3. Energy consumption in MBY hydrogenation to achieve maximum yield of MBE for three impeller configurations: RT only, RT +PBT, and two A340s. C
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at the same impeller speeds, and the vibration level is significantly lower for the A340 impeller, since the up-pumping A340 impeller provides flow in the same direction of the bubble rise, eliminating opposing flows that can cause vibrations. However, the highest MBE yield at an acceptable reaction time is found for the Rushton turbine operating at 500 rpm, and this combination may be recommended for the studied tank− reaction system to achieve maximum product yield at the lowest power consumption. However, the final decision should also take into consideration the impeller operational expenses versus product cost. At 500 rpm, the energy consumption of RT is 87% higher than of the two A340 impellers but the Rushton turbine allows 0.7% higher MBE yield; the optimal choice depends on the product and further purification cost. It should also be noted that the solid catalyst concentration in this reaction system is significantly low, implying that a cloud height does not form. At higher catalyst concentrations, the flow field and mixing times can vary significantly. Scale-up studies will be required in extrapolating these results to larger scales.
AUTHOR INFORMATION
Corresponding Author
*Tel.: +1-(780)-492-2293. Fax: +1-(780)-492-2881. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank NSERC−Discovery Grant and Lightnin for funding this research, and they thank Lightnin for providing the impellers.
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REFERENCES
(1) Nijhuis, T. A.; van Koten, G.; Moulijn, J. A. Optimized palladium catalyst systems for the selective liquid-phase hydrogenation of functionalized alkynes. Appl. Catal., A 2003, 238, 259−271. (2) Meyberg, M.; Roessler, F. In situ measurement of steady-state hydrogen concentrations during a hydrogenation reaction in a gasinducing stirred slurry reactor. Ind. Eng. Chem. Res. 2005, 44, 9705− 9711. (3) Bruehwiler, A.; Semagina, N.; Grasemann, M.; Renken, A.; KiwiMinsker, L.; Saaler, A.; Lehmann, H.; Bonrath, W.; Roessler, F. Threephase catalytic hydrogenation of a functionalized alkyne: Mass transfer and kinetic studies with in situ hydrogen monitoring. Ind. Eng. Chem. Res. 2008, 47, 6862−6869. (4) Nijhuis, T. A.; van Koten, G.; Kapteijn, F.; Moulijn, J. A. Separation of kinetics and mass-transport effects for a fast reaction: The selective hydrogenation of functionalized alkynes. Catal. Today 2003, 79−80, 315−321. (5) Creeze, E.; Hoffer, B. W.; Berger, R. J.; Makkee, M.; Kaptejin, F.; Moulijn, J. A. Three-phase hydrogenation of D-glucose over a carbon supported ruthenium catalystMass transfer and kinetics. Appl. Catal., A 2003, 251, 1−17. (6) Bittorf, K. J.; Kresta, S. M. Prediction of cloud height for solids suspensions in stirred tanks. Trans. Inst. Chem. Eng. 2003, 81 (Part A), 568−577. (7) Middleton, J. C.; Smith, J. M. Gas-liquid mixing in turbulent systems. In Handbook of Industrial Mixing: Science and Practice: Paul, E. L., Atiemo-Obeng, V. A., Kresta, S. M., Eds.; John Wiley & Sons: New York, 2003; pp 585−638. (8) Dohi, N.; Matsuda, Y.; Itano, N.; Shimizu, K.; Minekawa, K.; Kawase, Y. Mixing characteristics in slurry stirred tank reactors with multiple impellers. Chem. Eng. Commun. 1999, 171, 211−229. (9) Kasundra, R. B.; Kulkarni, A. V.; Joshi, J. B. Hydrodynamic and mass transfer characteristics of single and multiple impeller hollow selfinducing reactors. Ind. Eng. Chem. Res. 2008, 7, 2829−2841. (10) Guillard, F.; Tragardh, C. Mixing in industrial Rushton turbineagitated reactors under aerated conditions. Chem. Eng. Process. 2003, 42, 373−386. (11) Zlokarnik, M. Stirring. In Ullmann’s Encyclopedia of Industrial Chemistry, Vol. 34; Wiley−VCH: Weinheim, Germany, 2012; pp 434− 469. (12) Mishra, V. P.; Joshi, J. B. Flow generated by a disc turbine. IV: Multiple impellers. Chem. Eng. Res. Des. 1994, 72 (5), 657−668. (13) Patwardhan, A. W.; Joshi, J. B. Hydrodynamics of a stirred vessel equipped with a gas inducing impeller. Ind. Eng. Chem. Res. 1997, 36, 3904−3914. (14) Semagina, N.; Grasemann, M.; Xanthopoulos, N.; Renken, A.; Kiwi-Minsker, L. Structured catalyst of Pd/ZnO on sintered metal fibers for 2-methyl-3-butyn-2-ol selective hydrogenation. J. Catal. 2007, 251, 213−222. (15) Zieverink, M. M. P.; Kreutzer, M. T.; Kapteijn, F.; Moulijn, J. A. Gas-liquid mass transfer in benchscale stirred tanksFluid properties and critical impeller speed for gas induction. Ind. Eng. Chem. Res. 2006, 45, 4574−4581.
4. CONCLUSIONS Experimental study of a fast alkyne semihydrogenation in a 300-mL bench-scale stirred tank with RT, RT+PBT, and two A340 impellers operating at stirring speeds of 500, 900, and 1200 rpm was combined with the measurement of volumetric gas−liquid mass-transfer coefficients (kLa) and flow pattern analysis. The following conclusions were obtained: • The lowest kLa value for the RT@500 rpm is responsible for the relatively lower reaction rate but the largest product selectivity, because there is likely enough time for MBE to diffuse from the catalyst surface to the bulk before it becomes overhydrogenated. • The decrease in kLa values and reaction rates observed at 1200 rpm, as compared to that observed at 900 rpm, is attributed to the high degree of gas recirculation in the system with large cavities behind the blades (based on Rushton turbine (RT) analysis), which decreases the concentration gradient for efficient mass transfer. • The RT@500 rpm, RT@1200 rpm, and A340@1200 rpm combinations were found to be the best, in terms of MBE yield for the studies in the bench-scale reactor, with the lowest power consumption by the RT@500 rpm, which is a recommended combination. • The study shows that the increase of the stirring speed may be detrimental to the desired process outcome in a stirred tank, and it must be evaluated on a case-to-case basis.
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Research Note
ASSOCIATED CONTENT
S Supporting Information *
Section S1 is the experimental section: it describes the materials, impeller designs, catalytic tests, the measurement of kLa, and hydrogen absorption experiments. Section S2 gives a flow regime analysis. Section S3 gives the impeller power consumption calculations. Figure S1 shows the RT, RT+PB, and A340s impeller designs with dimensions. Scheme S1 shows the model three-phase reaction. Table S1 shows the flow pattern analysis for the Rushton turbine (D/T = 0.55). This material is available free of charge via the Internet at http:// pubs.acs.org. D
dx.doi.org/10.1021/ie5017927 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
