Sparger Type Influence on the Volumetric Mass ... - ACS Publications

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Sparger Type Influence on the Volumetric Mass Transfer Coefficient in the Draft Tube Airlift Reactor with Diluted Alcohol Solutions Ivana M. Šijački,* Milenko S. Tokić, Predrag S. Kojić, Dragan Lj. Petrović, Miodrag N. Tekić, and Nataša Lj. Lukić Department of Chemical Engineering, Faculty of Technology, University of Novi Sad, 403063 Novi Sad, Serbia ABSTRACT: In this work, the influence of addition of normal aliphatic alcohols (from methanol to n-octanol) and the gas sparger type (single orifice, perforated plate, and sinter plate) on the volumetric mass transfer coefficient in a draft tube airlift reactor was investigated. The results showed that the addition of alcohols from methanol to n-hexanol led to an increase in the volumetric mass transfer coefficient, in comparison to water, while n-heptanol and n-octanol had the opposite effect. The influence of the gas sparger was dominant at low superficial gas velocities. At higher gas velocities, the liquid-phaseproperties and the sparger type, being opposite in effect on mass transfer, annulled one another’s influence. Also, simple correlation was proposed to predict the volumetric mass transfer coefficient. The analysis of the parameters in the proposed correlation showed that the liquid-phaseproperties, expressed through the surface tension gradient, and the gas sparger type, mainly through initial bubble size, had a marked influence on the mass transfer.

1. INTRODUCTION

tangential shear stress. The drag on the bubble is increased and the rise velocity is reduced.3−5 Only a few researches reported that addition of small quantities of aliphatic alcohols influenced mass transfer in bubble columns6,7 (BCs) and airlift reactors.5,8−14 Pošarac and Tekić6 conducted their research in a bubble column with aqueous alcohol solutions from methanol to n-butanol. It was reported6 that volumetric mass transfer coefficient (kLa) increased with addition of alcohols in comparison to the one obtained in water. This increase was more pronounced in alcohols with a higher number of C atoms. On the contrary, Koide et al.7 found that addition of n-hexanol, n-heptanol, and n-octanol in minute quantities led to a decrease in kLa in a rectangular bubble column. They7 concluded that the surface active substances adsorb at the gas−liquid interface and, thus, retard the surface flow leading to a reduction in the liquid-phase mass transfer coefficient (kL) values. Although a significant increase in specific gas−liquid interfacial area (a) was obtained, the resulting volumetric mass transfer coefficient (kLa) was lower than the one measured in water.7 In an external-loop airlift reactor (EL-ALR), the addition of short-chained alcohols (methanol to n-butanol) led to an increase in kLa,9,12 but the addition of n-octanol led to a decrease in kLa.5 The increase in alcohol concentration intensified this influence, whether it was a decrease or an increase in observed kLa.5,9,10,12 In a sequence from methanol to n-propanol, the effect is favored by an alcohol with a longer carbon chain length, but only up to a critical alcohol concentration.9,10 El Azher et al.15 reported that addition of alcohols led to a decrease in the volumetric mass transfer coefficient in a split-rectangular airlift reactor (SRALR), which is more obvious with higher concentrations and

Airlift reactors (ALRs) have been found as suitable for many different processes (biomass or metabolites production, wastewater treatment, gas−liquid or gas−liquid−solid chemical reactions, etc.). However, for specific industrial applications, there are still many issues to be solved concerning the behavior, design, and scale-up of these reactors. Many investigations have been conducted in order to define the influence of operating conditions, reactor geometry, gas sparger design, and specific liquid-phaseproperties on the hydrodynamics and mass transfer in ALRs. In order to simulate the complex real industrial systems, dilute nonviscous alcohol solutions have been used as the liquid phase in several studies. The addition of alcohols, being known as surfactants, inhibits the coalescence of bubbles and, therefore, has a strong influence on the hydrodynamics and mass transfer in these contactors. The alcohol molecule consists of a hydrophobic part (nonpolar carbon chain) and a hydrophilic part (polar −OH group). Such a chemical structure causes the accumulation of alcohol molecules on the gas−liquid interface with the carbon chain oriented toward the center of the bubble, leading to a formation of a monolayer around the gas bubble and, consequently, to a rigid bubble surface. When two bubbles approach, a thin liquid film forms between them, with a tendency to flow outward as the bubbles get closer. Normally, the gas−liquid interface on both sides of the film would stretch. However, the film has a lower alcohol concentration in comparison to the one on the bubble surface. This causes an increase in the surface tension of the film, resulting in surface tension gradient forces. The stretching of the bubble surface is retarded, the drainage of the liquid film between the bubbles is slower, and consequently, the coalescence of bubbles is hindered.1−3 When a bubble moves through the liquid, the adsorbed surfactants are pushed to the back of the bubble, which results in the surface tension gradient opposed to the © 2013 American Chemical Society

Received: Revised: Accepted: Published: 6812

November 21, 2012 March 7, 2013 April 22, 2013 April 22, 2013 dx.doi.org/10.1021/ie303211u | Ind. Eng. Chem. Res. 2013, 52, 6812−6821

Industrial & Engineering Chemistry Research

Article

Table 1. Review of Studies of Gas−Liquid Mass Transfer in Dilute Alcohol Solutions in Airlift Reactors reference

reactor type

geometry of reactor

distributor type and characteristics

liquid

investigated parameters εG, kLa

Pošarac12

EL-ALR

DR = DD = 0.106 m

single orifice, do = 4 mm

Petrović et al.11

DT-ALR

perforated plate, do = 1 mm

Al-Masry and Dukkan5 Freitas and Teixeira16 El Azher et al.15

EL-ALR

D = 0.1 m DR/D = 40, 50, 75% DR = DD = 0.225 m

0.5 and 1 wt % methanol and n-butanol, 0.5 wt % ethanol and n-propanol 0.5 wt % n-butanol


30−100 ppm n-octanol

εG, WLD, tC, kLa εG, ULC, kLa

D = 0.142 m DR/D = 44% 0.2 × 0.2 m


10 g/L ethanol

kLa

single orifice, do = 3.5 mm

εG, WLD, kLa

Albijanić et al.8

DT-ALR

Miyahara and Nagatani9 Miyahara et al.10

EL-ALR

D = 0.106 m DR/D = 51% DR = DD = 0.14 m

EL-ALR

DR = DD = 0.14 m

Moraveji et al.13

SC-ALR

D = 0.136 m AR/AD = 2.136

0.05 wt % methanol, 0.01−0.1 wt % n-propanol, 0.05 wt % n-butanol 1 wt % methanol, ethanol, n-propanol, isopropanol, nbutanol 0.01−1 wt % methanol, 0.01−1 wt % ethanol, 0.002−0.5 wt % n-propanol 0.01−2 wt % methanol, 0.01−1 wt % ethanol, 0.002−1.2 wt % n-propanol 0.25, 0.5, 0.75, and 1 wt % methanol, ethanol, n-propanol, and n-butanol

Moraveji et al.14

SC-ALR

this paper

DT-ALR

D = 0.136 m AR/AD = 2.136 D = 0.106 m DR/D = 51%

DT-ALR SR-ALR

single orifice, do = 4 mm porous plate, mean diameter do = 300 μm porous plate, mean diameters do = 90, 165, 230, 300, 400 μm sinter ball, no data about mean diameter sinter ball, no data about mean diameter single orifice, do = 4 mm perforated plate, do = 1 mm sinter plate, do = 100−160 μm

higher numbers of C atoms. However, Moraveji et al.13 gave a contrary conclusion for a split-cylinder airlift reactor (SC-ALR). In their13 research, the kLa in alcohol solutions from methanol to n-butanol increased in comparison to water. Still, the effect was favored by higher numbers of C atoms and higher alcohol concentration.13 Some research8,11,16 published so far deals with the influence of alcohol addition on mass transfer in a draft tube airlift reactor (DT-ALR). In such a reactor configuration, an increase in kLa was reported, probably due to the marked increase in specific interfacial area if alcohols were present, as the authors suggested.8,11,16 Albijanić et al.8 also implied that alcohols with a higher number of C atoms led to a higher increase in kLa. However, all previous research in DT-ALR concerning the influence of alcohol addition was done only up to n-butanol. Table 1 presents a review of studies on mass transfer conducted in ALRs with diluted alcohol solutions. In previous research, it has been shown that the design of the gas sparger affects hydrodynamics and mass transfer in BCs17−19 and ALRs20−22 only at low gas inputs. As the gas input increases, in the heterogeneous regime, the size of the bubbles in the dispersion is generally independent of the size at birth and is controlled by the equilibrium between the dynamic pressure force, which tends to break the bubble, and the surface tension force, which attempts to preserve the shape and size.20 Merchuk et al.22 applied seven different perforated and sinter spargers in a DT-ALR. Their22 extensive research showed that the hydrodynamics is greatly affected by the sparger design in uniform bubbly flow and transition flow. Spargers with smaller pore size created smaller bubbles which were easily dragged in the downcomer.22 Similar research was done by Cao et al.23 in an EL-ALR with four spargers of different geometry and pore size. On the other hand, other researchers20,24,25 reported that the distributor had no effect on hydrodynamics and, later, mass transfer. The discrepancies about the influence of the gas sparger on hydrodynamics and mass transfer could be ascribed to the different reactor scale, the reactor configuration, and the gas flow rate applied in the research.20 However, in non-

0.25, 0.5, 0.75 and 1 wt % n-butanol 3.2 wt % methanol, 0.46 wt % ethanol, 0.032 wt % npropanol, 0.032 wt % isopropanol, 0.011 wt % n-butanol, 0.0057 wt % n-pentanol, 0.0051 wt % n-hexanol, 0.002 wt % n-heptanol, and 0.002 wt % n-octanol

εG, WLD, tC, kLa εG, ULC, dVS, kLa, kL, a kLa εG, WLC, tmix, kLa, kL, a kLa, dB kLa

coalescing systems, the impact of the gas distributor is emphasized. Camarasa et al.1 conducted experiments in the BC with three different distributors: single orifice, multiple orifice, and porous plate. With the porous plate as the gas sparger and in n-butanol and n-pentanol solutions, a narrow distribution of small bubbles is observed, and they are almost the same size within the column as those formed at the distributor.1 The research of Zahradniḱ et al.26 showed that, even in the case of two perforated plate distributors with the same free plate area, but different hole diameters, the alcohol addition (from ethanol to n-octanol) to viscous saccharose solutions led to a significant difference in the bubble bed behavior in the BC. The favorable effect of alcohols increased with the increasing length of their carbon chain.26 The synergistic effect of sparger design and coalescence inhibitors (aqueous solutions of inorganic salts) on the hydrodynamics of an EL-ALR was described in the research of Snape et al.27 The reason for suppression of the distributor’s effect was ascribed to the influence of superposed liquid flow on the two-phase flow pattern and bubble rise velocity in the riser.27 Bovonsombut et al.28 concluded that kLa could become 2 or 3 times higher in a DT-ALR with the porous plate in comparison to the perforated ring sparger if inorganic salts were added to the liquid−phase. On the other hand, organic compounds brought a decrease in kLa for both distributors used, as the coefficient kL decreased to a greater extent in comparison to a slight enhancement of the specific area a. Most of the previous research mentioned above deals with the influence of the gas distributor on the hydrodynamics. Although it is known that mass transfer directly depends upon the changes in hydrodynamics, some additional research about the influence of distributor type on mass transfer should be done. In this experimental work, three of the most commonly used gas spargers were employed. Therefore, the results obtained through this research could be comparable with the previous investigations and the quantitative analysis of the distributor’s effect could be reported. 6813

dx.doi.org/10.1021/ie303211u | Ind. Eng. Chem. Res. 2013, 52, 6812−6821


Article

Figure 1. Experimental setup.

the main hydrodynamic parameters in a DT-ALR with diluted alcohol solution and three gas sparger types: single orifice, perforated, and sinter plate. The validity of these correlations30 on kLa prediction will be examined through this experimental work. None of the studies published so far have gone into details concerning the possible synergistic influence of alcohol addition and the gas sparging effectiveness on the mass transfer of the DT-ALR. The aim of this paper is to investigate the influence of both alcohol addition and the type of gas sparger on kLa, and also, to quantify their influence through analysis of parameters in the proposed correlation.

Based on investigations in different systems, correlations for prediction of the volumetric mass transfer coefficient in ALRs have been proposed in various forms. Akita and Yoshida29 developed the correlation for kLa prediction in bubble columns. This correlation included dimensionless numbers where, among others, liquid−phase diffusivity, liquid kinematic viscosity, surface tension, and density were incorporated as factors that conceivably affect kLa. Pošarac and Tekić6 used the same correlation29 and showed that it is applicable with a very satisfying error on diluted alcohol solutions in bubble columns. However, it is more than obvious that the only property of diluted alcohol solutions that differs considerably from water is the surface tension. Koide et al.7 developed a correlation for bubble columns and diluted alcohol solutions (n-hexanol, nheptanol, and n-octanol) of different concentrations. This correlation included “surface pressure”, a variable dependent on surface tension and alcohol concentration. In a sequence, normal aliphatic alcohols could also be presented through a number of C atoms (CN) in a molecule. Such a correlation, which included the CN, and also the alcohol concentration, was proposed by Miyahara et al.10 For kLa prediction in DT-ALR, Albijanić et al.8 proposed a simpler correlation which included a surface tension gradient as a variable independent of alcohol concentration. Also, by choosing this variable they8 were able to explain the phenomena on the bubble−liquid interface if alcohols from methanol to n-butanol were added. All of these correlations6−8 were developed for diluted alcohol solutions on the basis of experiments with one type of gas sparger. In our previous work,30 simple correlations were developed to predict

2. EXPERIMENTAL SETUP The experiments were conducted at 20 ± 1 °C and atmospheric pressure in a glass DT-ALR, with geometrical details presented in Figure 1. The air, sparged into the draft tube, was used as the gas phase. The gas flow rates were controlled and measured by a rotameter. Three of the commonly used sparger types were used: a single orifice (4 mm i.d.), perforated plate (7 holes of 1 mm i.d., triangular pitch), and sinter plate (100−160 μm pores, average pore size 115 μm, porosity 8%). Tap water and dilute alcohol solutions from methanol to n-octanol were used as the liquid−phase. The concentrations of alcohols used in this experimental work and the main physical properties of the liquid-phaseat 20 °C are summarized in Table 2. The concentration of each alcohol was chosen on the basis of the research of Keitel.31 He reported that minimum and upper limiting concentrations exist, in which 6814



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concentration measurements. Still, most of the kLa values were obtained with an error under 10%. The size of bubbles during the experiments was estimated visually, taking into account the magnifying properties of the column glass wall. Several spherical particles of different, previously determined, diameters were inserted in the column and used as an etalon for approximate bubble sizing. The bubble size was the data important for the purpose of regime identification and evidence of coalescence.

Table 2. Surface Tension and Surface Tension Gradient of Used Liquid Phase at 20 °C

liquid

concn, CA (wt %)

surface tension, σ (N/m) × 103

tap water methanol ethanol n-propanol 2-propanol n-butanol n-pentanol n-hexanol n-heptanol n-octanol

0 3.2 0.46 0.036 0.036 0.011 0.0057 0.0051 0.002 0.002

72.4 66.1 70.4 71.9 71.7 71.8 71.7 69.9 67.8 65.0

surface tension gradient, −dσ/dCA [(N m2)/mol] × 103

R2 a

0.011 0.027 0.060 0.058 0.155 1.082 1.985 4.141 15.205

0.96 0.99 0.99 0.99 0.99 0.86 0.99 0.99 0.95

3. RESULTS AND DISCUSSION Hydrodynamic Regimes in the Downcomer. According to the findings of many authors,8,11,20,33−37 three different regimes were observed in the downcomer. They could be described using the following terms:35 no gas entrainment (regime I), gas entrainment but without gas recirculation (regime II), and recirculation of one part of the gas fraction (regime III). The regime characteristics and transition points depend upon reactor geometry and experimental conditions.20,35 Through visual observation and the obtained values of the downcomer gas holdup, the induced downcomer liquid velocity, and the difference between the riser and the downcomer gas holdup, which is proportional to the driving force for the liquid circulation, the existence of three different hydrodynamic regimes in the downcomer was confirmed in our previous work.30 Detailed descriptions of these regimes, including the superficial gas velocities that correspond to the transition points, can also be found in this paper.30 The phenomena concerning the hydrodynamicsquantity and size of bubbles, difference between the riser and the downcomer gas holdup, and liquid velocityinfluenced the volumetric mass transfer coefficient through both the kL and a. However, the transition between the regimes is not so sharp (Figure 2), on the basis of the evolution of the volumetric mass transfer coefficient with the superficial gas velocity. Individual effects of sparging effectiveness and alcohol addition on kL and a in a certain manner annul each other, resulting in almost negligible changes, if the mass transfer is expressed through the volumetric mass transfer coefficient. But, still, previously30 determined superficial gas velocities which correspond to the transition points between the first and second regime, and also the second and third regime, presented in Figure 3, could be identified, if the slopes of the kLa versus UG curves were calculated. Volumetric Mass Transfer Coefficient. Figure 2 illustrates the changes in the volumetric mass transfer coefficient in all liquid−phases and with all gas distributor types used. As can be seen from this figure, kLa increases with an increase in gas input. This increase is the most pronounced for very low superficial gas velocities (first regime), especially over the sinter plate as the gas distributor. Also, the increase is more pronounced in alcohol solutions with a lower number of C atoms. As UG reaches 0.01 m/s, the changes in volumetric mass transfer coefficient become less steep, especially when the CN and the distributor effectiveness increases. For the second and the third regime, the slopes of kLa versus UG curves could be determined by linear fitting of the experimental data. On the basis of the numeric values of these slopes, it is confirmed that, although in systems with sinter plate the highest kLa values were measured at very low superficial gas velocities, these systems tend much faster to a constant value of kLa, because the determined slopes are the lowest in the second regime (0.433− 0.680) and third regime (0.250−0.763). On the other hand, in

a Coefficient of determination for the estimation of the surface tension gradient.

range the alcohol addition has a marked influence on the global hydrodynamics.31 Increasing the alcohol concentration above the upper limiting concentration value only enhances the liquid-phasefrothing and bubble coalescence1,32 and has no remarkable effect on the volumetric mass transfer coefficient.9 The chosen concentration could not be equal for all alcohols because of the limited solubility of alcohols with a higher number of C atoms and also the values of concentration ranges,31 which are quite different for different alcohols. However, the influence of alcohols was later described through the surface tension gradient, rather than concentration, because it explains a real physical occurrence at the bubble−liquid interface. Surface tensions of liquid−phases were obtained by tensiometer (Torsion Balance, United Kingdom, Model OS) with ±0.0001 N/m accuracy. The surface tension gradient (−dσ/dCA) was estimated from the slope of the experimental σ versus CA curve.30 The overall gas holdup was determined by the volume expansion technique with an error 0.45 m/s), the measured kLa in systems with sinter plate as the gas distributor was even lower than the one obtained over the single orifice. This was because the smallest bubbles and greatest number of bubbles are created over the sinter plate. Bubbles of smaller diameter were held longer in the dispersion and more easily dragged into the downcomer and, later on, recirculated. These small bubbles were usually depleted of oxygen and act as a sink when they recirculate and mix with the fresh bubbles in the riser.5 However, the differences in kLa values over all three distributors were mainly about 5% and did not exceed 10%, except in the first regime where they reached about 50%. Influence of Alcohols and Gas Distributor Type on Mass Transfer. The addition of alcohols led instantly to an obvious creation of smaller bubbles and, therefore, an increase in specific interfacial area (a). This conclusion was made by visual observation only. However, several researchers9,13,14 measured bubble diameter and specific interfacial area and came to the same result. On the other hand, addition of alcohol influences the liquid-phasemass transfer coefficient in an opposite manner. Alcohols, being known as surfactants, adsorb on the bubble surface and retard surface flow by the surface tension gradient.7 Hence, the values of kL in diluted alcohol solutions from methanol to n-octanol were reduced.7,13,38 Only Miyahara et al.9 found that kL increased in diluted solutions from methanol to n-propanol. The resulting volumetric mass transfer coefficient (kLa) depended upon the ratio of the specific interfacial area increase and the mass transfer coefficient decrease. In a sequence from methanol to n-hexanol, the resulting volumetric mass transfer coefficient increased in comparison to water. Figure 4 depicts the effect of alcohol type on kLa in systems with a single orifice as the gas distributor. Similar findings could be reported for perforated and sinter plates. However, the conclusion that the kLa increased with an

Figure 2. Influence of gas sparger type and alcohol addition on the volumetric mass transfer coefficient. Legend: squares, single orifice; circles, perforated plate; triangles, sinter plate; solid symbols, regime I (presence of small bubbles in the downcomer); half-solid symbols, regime II (stagnant swarm of bubbles in the downcomer); open symbols, regime III (circulation of one part of the bubbles through the column).

Figure 3. Superficial gas velocities corresponding to regime transition points.

systems with perforated plate as the gas distributor, the highest values of slopes were observed: 0.541−0.910 in second regime 6816



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solutions also found that kLa values were lower than those in water. Although it was obvious that in these solutions the bubbles were the smallest in size, and therefore the specific interfacial area (a) was the highest, the decrease in kL overcame this phenomenon, leading to lower resulting kLa. The explanation for the above mentioned phenomena could be found in the adsorption behavior of alcohols. Although Wang et al.40 conducted the research on the adsorption behavior of alcohols on the solid surface, a similar occurrence at the gas−liquid interface is expected. They40 reported that methanol, ethanol, n-butanol, n-hexanol, and n-octanol formed homogeneous films while adsorbed on solid surfaces. Methanol and ethanol formed an upright standing bilayer structure with alkyl chains oriented toward the alcohol solution and the −OH group toward the solid surface. Alcohols with a higher number of C atoms formed a monolayer. n-Butanol created a wormlike pattern instead of a flat surface, while the alkyl chains alternate between vertical and tilted orientation. Similar to n-butanol, the n-hexanol molecules produced layers with tilted orientation. However, the hexanol films were smoother due to more compact arranging of molecules because of enhanced intermolecular interactions. n-Octanol showed different adsorption behavior; the molecules adsorbed with a 40° bevel of a hydrocarbon chain with respect to the surface. With the increase in the carbon chain length, the forces between the molecules themselves, and also between the molecules and the surface, were increased. As a result, the stability of a molecular layer was enhanced.40 In our case, expecting that similar phenomena occur on the bubble surface, this led to an increased resistance to oxygen diffusion through the gas−liquid interface. Although with an increase in CN, the corresponding surface tension gradient increased, thus increasing the stability of bubbles due to coalescence and leading to formation of smaller bubbles, the resistance to the mass transfer also increased, leading to a reduction in kL. Therefore, the opposing effects of alcohols on a and kL led to similar values of measured kLa in a sequence from methanol to n-butanol. Later on, the intensity of decrease in kL led to more pronounced changes and even to a decrease of the volumetric mass transfer coefficient in comparison to water, though much smaller bubbles were formed and the coalescence was almost completely suppressed. The ratio of the volumetric mass transfer coefficient and the overall gas holdup, for several liquid−phases (0.46 wt % ethanol, 0.011 wt % n-butanol, and 0.0051 wt % n-hexanol) and all gas distributors used, is presented in Figure 6. The data for the overall gas holdup was reported in our previous paper.30 Because the ratio kLa/εG is directly proportional to the ratio kL/ dVS, it can be concluded (Figure 6) that at low superficial gas velocities the Sauter mean diameter of bubbles (dVS) increases more intensively in comparison to the liquid-phasemass transfer coefficient (kL). When the third regime occurs, values of the kLa/εG tend to a constant value of about 0.2 ± 0.03 s−1 for all liquid−phases and all distributors used. As the number of C atoms increases, the system reached this asymptotic value faster. The ratio of kL/dVS, therefore, followed this trend and also tended to settle at a constant value at higher superficial velocities. Miyahara and Nagatani9 analyzed separately the influence of alcohol addition on Sauter mean bubble diameter (dVS) and mass transfer coefficient (kL) in the EL-ALR. They9 showed that dVS increased with UG increase, while the kL increased up to a superficial gas velocity of about 0.03 m/s and then became almost constant. This conclusion was in agreement with our findings. On the other hand, Moraveji et

Figure 4. Effect of alcohol type from methanol to n-hexanol on volumetric mass transfer coefficient.

increased number of C atoms in an alcohol molecule, as obtained by Albijanić et al.,8 could not be clearly withdrawn in this case. The differences could be ascribed to the different alcohol concentrations used in the above mentioned8 and this work. It has been reported6,13,14,39 that kLa increases with an increase in alcohol concentration. In all mentioned investigations,6,13,14,39 the concentrations of diluted alcohol from ethanol to n-butanol solutions were higher than those applied in this experimental work. The data of kLa in n-hexanol solutions of concentrations similar to ours could been found only in paper of Koide et al.7 They7 reported that, in n-hexanol solutions under the concentration of 100 ppm, higher kLa values, in comparison to water, were obtained. In our experiments, the kLa values in n-hexanol solutions were quite close to the one measured in water, but still higher. On the other hand, in n-heptanol and n-octanol solutions, measured kLa values were almost equal to or lower than those obtained in water, as can be seen in Figure 5. Figure 5 represents the system with a single orifice as gas distributor, but the same relative relations are obtained with the other two distributors, as in this case (Figure 2) the distributor has almost no influence on kLa. Koide et al.7 for both n-heptanol and noctanol solutions and Al-Masry and Dukkan5 for n-octanol

Figure 5. Effect of n-heptanol and n-octanol addition on volumetric mass transfer coefficient. 6817



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As presented in Table 1, several researchers8,11,16 have conducted experiments in DT-ALR with different alcohol solutions and different gas spargers. Figure 7 presents an

Figure 7. Comparison of experimental results for the kLa to results obtained by others in DT-ALR (Table 1).

overview of the parameters’ ranges in the mentioned research. As can be seen in Figure 7, the results presented in this paper include the range published by Petrović et al.11 and Albijanić et al.,8 while our range is much wider than the one from experiments of Freitas and Teixeira.16 This relation is expected, as we conducted the experiments not only with higher air flows and a more effective distributor (sinter plate) but also with alcohols of higher CN. The data obtained in our experiments is processed through the correlation p ⎛ ⎛ d σ ⎞⎞ 3 p p p kLa = p1 UG2⎜⎜1 + ⎜ − ⎟⎟⎟ φ 4do 5 ⎝ dCA ⎠⎠ ⎝

(2)

where kLa could be predicted as the overall (for the whole range of superficial gas velocities) or in terms of the observed hydrodynamic regimes. The parameters p1−p5 are determined by applying the regression analysis on the experimental data. Because the correlations are highly nonlinear, LAB Fit software41 is chosen as suitable. The correlations are derived for the following ranges of the independent variables: 0.0055 m/s < UG < 0.07 m/s, 0.011 mN m2/mol < −dσ/dCA < 15.205 mN m2/mol, 0.0024 < φ < 0.0323, and 0.115 < do < 4 mm. To emphasize the influence of each independent variable, especially the influence of the surface tension gradient −dσ/dCA and the gas dispersion effectiveness (represented through φ and do), the parameters are also presented in terms of flow regimes observed30 in the DT-ALR. Table 3 contains the values of the estimated parameters in the proposed correlations, generally presented by eq 2. Also, standard quantifiersthe coefficient of determination (R2) and the errors of parameters, expressed as a percentage of a value are presented in Table 3. The significance of each parameter is expressed through its t value. Generally, eq 2 can be used for prediction of the volumetric mass transfer coefficient in a very satisfactory manner. This can beconfirmed through the values of both the determination coefficient (R2 = 0.87−0.94) and the relative average error (δ) for a fairly high number of available data points (n = 107−397). Observed in a whole range of superficial gas velocities, the

Figure 6. Evolution of the ratio of volumetric mass transfer coefficient and the overall gas holdup with superficial gas velocity in ethanol, nbutanol, and n-hexanol solutions.

al.13 showed that in a SC-ALR, kL reached a constant value at much lower superficial gas velocities: under 0.01 m/s.

4. CORRELATIONS As has already been proven,8,34 the correlations with the surface tension gradient as the variable are more than adequate to represent the phenomena concerning the hydrodynamic and mass transfer behavior in the DT-ALRs with diluted alcohol solutions as the liquid−phase. Investigation presented in a previous paper30 has shown that the distributor type also has strong influence on the primary dispersion and, therefore, on the global hydrodynamics of a DT-ALR. This influence can be characterized by modifying previously developed correlations,8,34 such as introducing the ratio of the sparger’s free area, the draft tube cross-sectional area (φ), and the orifice (or pore) diameter (do), as variables.30 6818



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Table 3. Values of Correlation Parameters for the Volumetric Mass Transfer Coefficient y (eq 2)

regime

statistic evaluation of parameter

−1

kLa, s

t, % kLa, s−1

I t, % II t, % III t, %

p1 ± error(p1) 0.21 17.2 0.77 8.1 0.30 14.8 0.14 16.2

± 5.2% ± 23.1% ± 11.2% ± 7.6%

p2 ± error(p2) 0.65 48.5 0.79 32.8 0.70 43.9 0.53 27.8

± 1.8% ± 5.7% ± 3.8% ± 4.5%

p3 ± error(p3)

p4 ± error(p4)

p5 ± error(p5)

−0.13 17.5 −0.24 25.7 −0.15 19.5 −0.15 31.2

−0.04 5.2 0.09 9.3 −0.02 2.2 −0.05 9.9

−0.07 11.5 −0.18 24.1 −0.11 19.6 −0.06 14.9

± 4.9% ± 7.2% ± 8.6% ± 4.0%

± 16.9% ± 19.5% ± 50.0% ± 13.1%

R2

n

δ, %

± 7.6%

0.94

397

12.7

± 7.6%

0.92

107

9.5

± 8.3%

0.88

167

10.0

± 8.5%

0.87

205

5.2

correlation overpredicted values to a greater extent (up to 30%) at very low superficial gas velocities (Figure 8). But, for a sinter

Figure 8. Comparison between experimental and calculated values of the volumetric mass transfer coefficient for the whole range of superficial gas velocities.

plate, the correlation underestimated values in a range 0.012 < UG < 0.02 m/s when a swarm of tiny bubbles was observed in the downcomer. If the correlation was applied in a range of UG for separately defined flow regimes, the deviations (δ) of about 80% of experimental data from their estimates were less than 15% (Figure 9). This was even more pronounced at higher gas throughputs, typical of reactor operation. Table 3 has included the results of modeling of the volumetric mass transfer coefficient for the entire range of gas throughputs, regardless of the individual flow regimes, and also in terms of three observed flow regimes. In comparison to the contribution of the superficial gas velocity, the influence of the liquid-phase-related (−dσ/dCA) and the sparger-related (φ and do) variables was lower, based on the significance of the corresponding parameters (p3, p4, and p5). However, their significance is far from being negligible. The analysis shows that suppression of coalescence with alcohol addition had a more pronounced effect than the sparger effectiveness and initial bubble size. This conclusion was more obvious in terms of observed flow regimes, based on the significance of the parameter p3. The aeration effectiveness, expressed through variable φ, was less influential in comparison to the bubble size at birth, related to the orifice size (do). In the second regime, when an almost static bubble swarm was present in the downcomer, the aeration factor had almost no influence. Also, on the basis of the significance of the parameter p5, it can be concluded that the influence of initial bubble size lessens as the superficial gas velocity increases, when coalescence occurs.

Figure 9. Comparison between experimental and calculated values of the volumetric mass transfer coefficient in terms of observed flow regimes.

5. CONCLUSIONS In this article, the influence of both the gas sparger type (single orifice, perforated plate, and sinter plate) and alcohol addition 6819



Article

C*L = equilibrium dissolved oxygen concentration (mol/m3) CN = number of C atoms in alcohol molecule chain dB = bubble diameter (mm) do = diameter of orifice (mm) dVS = Sauter mean bubble diameter (mm) D = diameter of column (m) DR = inner diameter of riser (m) kL = liquid-phasemass transfer coefficient (m/s) kLa = volumetric mass transfer coefficient (s−1) t = time (s) UG = superficial gas velocity, column based (m/s) ULC = superficial liquid circulation velocity (m/s) WLC = liquid circulation velocity, (m/s) WLD = downcomer interstitial liquid velocity, (m/s)

(from methanol to n-octanol) on volumetric mass transfer coefficient in a DT-ALR is studied. • The appearance of three different hydrodynamic regimes in the downcomer is confirmed. The changes in the volumetric mass transfer coefficient in terms of observed regimes (transition points) are less pronounced in comparison to changes in hydrodynamic parameters: downcomer gas holdup and induced liquid velocity. • The type of gas distributor has marked influence on the primary gas dispersion and, therefore, on gas−liquid mass transfer. The highest values of volumetric mass transfer coefficient are obtained with the sinter plate, the most efficient distributor, until the recirculation of one part of the gas bubbles started. As the third regime is reached in systems with the sinter plate, much lower kLa values are measured, especially if alcohols are present. The sinter plate creates very small bubbles, thus leading to a major increase in specific interfacial area (a). On the other hand, these small bubbles, which easily recirculate, are usually depleted from oxygen and act as a sink. • The addition of small amounts of normal aliphatic alcohols changes the gas−liquid mass transfer of a DTALR while decreasing the surface tension. This leads to an increase in the volumetric mass transfer coefficient in solutions from methanol to n-hexanol, in all investigated systems, in comparison to water. On the other hand, the addition of n-heptanol or n-octanol brought a decrease in kLa. Although alcohols act as surfactants and hinder bubble coalescence, thus leading to increase in specific interfacial area (a), they adsorb on the gas−liquid interface and retard the surface flow, causing a decrease in mass transfer coefficient kL. The resulting volumetric mass transfer coefficient depends upon the interrelation of kL and a. • The correlations to fit the experimental data are proposed. They include the superficial gas velocity as well as variables related to the liquid-phaseproperties and the gas distributor type. A very satisfactory agreement between the experimental and the calculated data is achieved by applying the developed correlations. Future research on this matter should include the parameters concerning the influence of reactor geometry on the gas−liquid mass transfer.

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Abbreviations

ALR = airlift reactor BC = bubble column DT-ALR = draft tube airlift reactor EL-ALR = external-loop airlift reactor SC-ALR = split-cylinder airlift reactor SR-ALR = split-rectangular airlift reactor Greek Letters

δ = average relative error (%) εG = gas holdup σ = surface tension (N/m) −dσ/dCA = surface tension gradient (N m2/mol)

Subscripts

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Project No. 172025).

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C = circulation D = downcomer G = gas phase L = liquid−phase mix = mixing R = riser

NOMENCLATURE

Notation

a = specific interfacial area (m−1) CA = concentration of alcohol (wt %) CL = instantaneous dissolved oxygen concentration (mol/ m3) 6820



Article

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