Negative Impact of High Stirring Speed in Laboratory-Scale Three-Phase HydrogenationsInci 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-transferlimitations in fast three-phase hydrogenations that are performed in a stirred tank. We provide experimental evidence for a 300-mL stirred reactor that, above a certain impeller speed, the efficiency of gas−liquid mass-transfer decreases, resulting in thedecreased reaction rate. The phenomenon is attributed to the high degree of gas recirculation with large cavities behind theblades. The recirculation may decrease hydrogen concentration in the remainder of the tank, thus decreasing the concentrationgradient that controls mass transfer. The model reaction in this work was 2-methyl-3-butyn-2-ol semihydrogenation with Lindlarcatalyst Pd−Pb/CaCO3. The test impellers were a Rushton turbine, a down-pumping pitched blade turbine, and up-pumpingA340 impellers. The kinetic experiments were combined with the measurement of volumetric gas−liquid mass-transfercoefficient, flow pattern analysis and impeller power demand calculations. Although the study does not include kinetic analysis, itprovides guidance to the three-phase reaction system analysis that the highest stirring speed may enhance mass-transferlimitations and should not be used without caution.

1. INTRODUCTION

Three-phase catalytic hydrogenations in semibatch reactors arewidely used in the synthesis fof ine chemicals. They represent aclassical example of a three-phase catalytic process, in whichhydrogen dissolves in the liquid phase and, along with theliquid reactant, diffuses toward the external solid catalystsurface, followed by the internal diffusion. If the intrinsicsurface reaction is relatively fast, such as an alkyne hydro-genation on a Pd catalyst, the process is most likely to be mass-transfer limited, and the higher the reaction temperatures andhydrogen 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 underMTL, since the retarded diffusion of a fresh reactant and atarget product to/from the catalyst surface promotes theproduct’s overhydrogenation. Both gas−liquid MTLs, andliquid−solid limitations by a hydrogenated reactant and/ordissolved hydrogen have been documented for such reac-tions.1−5

In eliminating MTL in gas−liquid−solid three-phase reactionsystems, two operational criteria should be considered: gasdistribution and solids (catalyst) suspension and distribution.At solids concentrations above 10 wt %, an interface calledcloud height can separate the solids-rich volumes and the clearliquid volumes.6 Typically, much lower catalyst quantities areused, such as 0.3 wt % as in the current study, thus, the reactionsystem is dominated by gas dispersion. Gas can be introducedinto the reactor with a sparger, or by using self-inducing shaftsand impellers.7 To achieve gas induction and good gasdistribution, dual impellers are recommended.8,9 In this study,we propose two dual-impeller configurations to eliminateMTLs and provide good mixing between the phases, and onetraditional single impeller configuration as a test model. Theimpeller configurations are (1) a Rushton turbine (RT) and a

down-pumping pitched blade turbine (PBT) where the RT isthe lower impeller, and (2) two up-pumping Lightnin A340impellers. In these configurations, the lower impeller isresponsible for solids suspension and gas distribution, and theupper impeller is responsible for the incorporation of the upperliquid layer in the main flow circulation, as well as furtherdistribution of gas and solids. The single RT is one of the mostwidely used stirrers in the chemical industry.10,11 Somecombinations of RT and PBT or multiple PBTs employ themost studied dual-impeller systems.9,12,13 The wide-bladehydrofoils, A340s, are relatively new, with few formal studiesin the open literature. The hydrofoil impellers work well for gasdispersion, and this configuration is very promising for thisapplication. The experiments were carried out in a 300-mLstirred vessel with a hollow gas-inducing shaft that is frequentlyused for laboratory kinetic studies.High stirring speed is generally considered as one of the a

priori requirements to eliminate external MTLs. In the currentwork, we show that there is an optimal stirring speed, and itsfurther increase is detrimental to a selected three-phase process.The tests were carried out for an industrially important three-phase reaction by evaluating reaction rates and selectivity withthe three impeller configurations and stirring rates, as well asmeasuring corresponding hydrogen volumetric mass-transfercoefficients (kLa) and analyzing the flow patterns in the reactor.The model reaction is a known fast hydrogenation of 2-methyl-3-butyn-2-ol (MBY) to 2-methyl-3-buten-2-ol (MBE) that is anintermediate in vitamin A, vitamin E, and perfume production(see Scheme S1 in the Supporting Information). The undesired

Received: May 3, 2014Revised: October 8, 2014Accepted: November 6, 2014

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overhydrogenation product is 2-methylbutan-2-ol (MBA).Commercial Lindlar catalyst was applied in a semibatch processwith ethanol as a solvent. Based on the catalytic testing resultsand impeller power consumption calculations, we also proposethe most optimal impeller configuration and stirring speedcombination among the three configurations studied. Althoughthe study does not include kinetic analysis, it provides guidanceto the three-phase reaction system analysis that the higheststirring speed may enhance MTLs and should be used withcaution. The applicability of the data to a larger scale requiresfurther investigation.The experimental details and impeller designs are provided in

the Supporting Information.

2. IMPELLER AND STIRRING SPEED EFFECTS ON THEPRODUCT YIELD, MASS TRANSFER, AND FLOWPATTERN

Table 1 compares the maximum yield of the target productMBE and the time to achieve it for three impeller

configurations at 500, 900, and 1200 rpm. Typical kineticcurves and MBE selectivity versus MBY conversion plots areshown for the Rushton turbine in Figure 1. The concentrationcorresponds to the weight percent of each of MBY, MBE, andMBA in their reaction mixture (solvent is excluded). Thecorresponding 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 isconsistent with the lowest kLa values. Typically, functionalizedalkyne hydrogenations are characterized by a first-orderequation, relative to the dissolved hydrogenation concen-tration,14 so the lower hydrogen concentration results in thelower intrinsic reaction rate. The lowest rates at 500 rpm arealso responsible for higher selectivity to MBE: there is likelyenough time for MBE to diffuse from the catalyst surface to thebulk before it gets overhydrogenated on the catalyst surface, assimilarly discussed by Nijhuis et al. for 3-methyl-1-pentyn-3-olhydrogenation.4

At 900 rpm, all impellers allowed the highest reaction ratesand, hence, lowered the selectivity to MBE. The kLa values arehigher than at 500 rpm, which can be explained by largerinterfacial 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, andselectivity values were generally found between those values forthe 500 rpm and 900 rpm speeds. Increasing the stirrer speedabove 900 rpm was detrimental to the observed catalyticperformance.This trend may be related to the gas−liquid mixing

phenomena that occurs in turbulent systems.7 Alkyne hydro-

genations are known to be highly susceptible to the gas−liquidMTL, meaning that the gas flow pattern in the reactor isimportant. The high degree of gas recirculationgas returningback to the impeller, as opposed to that spargedthat occurs athigh stirrer speeds (1200 rpm in this case) may reducehydrogen concentration in the remainder of the tank,decreasing the concentration gradient that controls masstransfer,7 leading to the decrease in experimental kLa valuesand 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 flowpatterns and operating regimes. The analysis is performed forthe 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 pressuredifference when the impeller is rotating. Zero pressuredifference, meaning the gas is just drawn in, is characterized

Table 1. Time to Maximum MBE Yield and Maximum MBEYield at Different Stirring Speedsa

500 rpm 900 rpm 1200 rpm

impellertime, t(min)

MBEyield (%)

time, t(min)

MBEyield (%)

time, t(min)

MBEyield (%)

RT 50 92.8 38 91.5 48 92.7RT +PBT

56 92.1 41 89.7 44 92.3

A340 56 92.1 44 91.9 51 92.5aErrors for repeatability are ±0.5% for the yield and ±2 min for thetime.

Figure 1. (Top) Concentration versus time and (bottom) selectivity−conversion profiles for the MBY hydrogenation with a Rushtonturbine.

Table 2. Experimental kLa Values (s−1) for Hydrogen inEthanol at 40 °C in the Pressure Range of 1.8−4 bar (GaugePressure)

Stirring Speed

impeller 500 rpm 900 rpm 1200 rpm

RT 0.056 ± 0.008 0.064 ± 0.012 0.058 ± 0.006RT + PBT 0.065 ± 0.008 0.068 ± 0.011 0.066 ± 0.013A340 0.067 ± 0.011 0.069 ± 0.018 0.069 ± 0.009

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by the critical impeller speed (NC). At the impeller speedsbelow NC, the mass transfer occurs only on top of the liquid; atvalues above NC, the gas is drawn down to the blades, wherethe bubbles are created.15 The NC value determined for oursystem is 459 rpm, so our lowest applied 500 rpm speed issufficient 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), whichcorresponds to the homogeneous regime in the tank, with themonomodal distribution of gas bubbles (typically between 0.5mm and 4 mm).7 In this regime, the impeller controls the flowpattern and bubble size. The Froude number (Fr) and flow rate(Fl) number were evaluated to characterize the transitionsbetween different gas flow pattern regimes:7

=FrN D

g

2

(1)

=FlQ

NDg

3 (2)

where N is the rotational speed, D the impeller diameter, g theacceleration due to gravity, and Qg the gas flow rate. Thefollowing 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 thetank diameter.

• At Fl > 0.025(D/T)−0.5, gas accumulates behind theblades, forming cavities, which may obstruct the liquiddischarge from the impeller, causing poor mixing andmass transfer.

• At Fl < 13Fr2(D/T)5.0, gas recirculates back to theimpeller, causing a decrease in the mean gas phaseconcentration driving force for the gas−liquid masstransfer.

These regimes are plotted on a flow map for our impeller-to-tank diameter ratio (D/T) of 0.55 (see Figure 2), along withthe 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 Rushtonturbine is likely, because the cavity formation limit (Fl = 0.034)is exceeded. For the two larger stirring speeds, the recirculation

limit is also exceeded, which is dramatic for 1200 rpm. Thismay explain the reduced kLa values, as well as the lowerreaction rate at this highest rotation speed. Althoughrecirculation is also likely for 900 rpm, the recirculation limitis an order of magnitude lower than that observed for the 1200rpm stirring speed. The negative effect of recirculation on kLaand the reaction rate at 900 rpm seem to be outweighed byimproved mass transfer, as compared to 500 rpm, because ofhigher 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 theRT+PBT configuration. The A340 impeller is specificallydesigned to eliminate cavity formation; that is why it providesthe highest kLa values among all impellers for all stirring speeds.The A340 impeller allows higher kLa values and, hence, lowerselectivities than the Rushton turbine, which is consistent withthe general observed trend. For the A340 impeller, however,the reaction rate dependence on kLa is inverted: equally highkLa values for all three stirring speeds (∼0.069 s−1) provide thelowest rates among all tested impellers at a fixed stirring rate(see Table 1). This is likely due to a different flow pattern inthe reactor with the A340 impeller, which is out of the scope ofthe current work and cannot be referenced, because of ratherfew reported studies.

3. IMPELLER SELECTION

As seen from Table 1, the reaction rates vary within a smallrange for all impellers and stirring speeds, and in the synthesisof fine chemicals, because of the high target product cost andhigh E-factors (waste/product ratio), selectivity is moreimportant than the reaction times, if they are within anacceptable range. Since the MBE yields are lowest for a stirringspeed of 900 rpm for all stirrers, this stirring speed should notbe selected for any of the impellers. In terms of the MBE yield,RT@500 rpm, RT@1200 rpm and A340@1200 rpm outper-form other combinations. Impeller power consumption is anadditional factor to select the proper impeller−speedcombination. The power consumption calculations arepresented 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

Figure 2. Flow map for a single Ruston turbine (D/T = 0.55).Regimes: (1) below minimum dispersion speed, (2) vortex cavities, norecirculation, (3) vortex cavities with recirculation, (4) flooded, (5)loaded with large cavities, and (6) large cavities with recirculation.

Figure 3. Energy consumption in MBY hydrogenation to achievemaximum yield of MBE for three impeller configurations: RT only, RT+PBT, and two A340s.

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at the same impeller speeds, and the vibration level issignificantly lower for the A340 impeller, since the up-pumpingA340 impeller provides flow in the same direction of the bubblerise, eliminating opposing flows that can cause vibrations.However, the highest MBE yield at an acceptable reaction timeis found for the Rushton turbine operating at 500 rpm, and thiscombination may be recommended for the studied tank−reaction system to achieve maximum product yield at thelowest power consumption. However, the final decision shouldalso take into consideration the impeller operational expensesversus product cost. At 500 rpm, the energy consumption ofRT is 87% higher than of the two A340 impellers but theRushton turbine allows 0.7% higher MBE yield; the optimalchoice depends on the product and further purification cost. Itshould also be noted that the solid catalyst concentration in thisreaction system is significantly low, implying that a cloud heightdoes not form. At higher catalyst concentrations, the flow fieldand mixing times can vary significantly. Scale-up studies will berequired in extrapolating these results to larger scales.

4. CONCLUSIONS

Experimental study of a fast alkyne semihydrogenation in a300-mL bench-scale stirred tank with RT, RT+PBT, and twoA340 impellers operating at stirring speeds of 500, 900, and1200 rpm was combined with the measurement of volumetricgas−liquid mass-transfer coefficients (kLa) and flow patternanalysis. 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 productselectivity, because there is likely enough time for MBE todiffuse from the catalyst surface to the bulk before it becomesoverhydrogenated.• The decrease in kLa values and reaction rates observed at

1200 rpm, as compared to that observed at 900 rpm, isattributed to the high degree of gas recirculation in the systemwith large cavities behind the blades (based on Rushton turbine(RT) analysis), which decreases the concentration gradient forefficient mass transfer.• The RT@500 rpm, RT@1200 rpm, and A340@1200 rpm

combinations were found to be the best, in terms of MBE yieldfor the studies in the bench-scale reactor, with the lowest powerconsumption by the RT@500 rpm, which is a recommendedcombination.• 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.

■ ASSOCIATED CONTENT

*S Supporting InformationSection S1 is the experimental section: it describes thematerials, impeller designs, catalytic tests, the measurement ofkLa, and hydrogen absorption experiments. Section S2 gives aflow regime analysis. Section S3 gives the impeller powerconsumption calculations. Figure S1 shows the RT, RT+PB,and A340s impeller designs with dimensions. Scheme S1 showsthe model three-phase reaction. Table S1 shows the flowpattern analysis for the Rushton turbine (D/T = 0.55). Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Tel.: +1-(780)-492-2293. Fax: +1-(780)-492-2881. E-mail:semagina@ualberta.ca.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors would like to thank NSERC−Discovery Grant andLightnin for funding this research, and they thank Lightnin forproviding the impellers.

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