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Mass-Transfer Behaviors in Alcohol Solutions in an Internal-Loop Airlift Reactor of 5-m Height Zhonghuo Deng, Tiefeng Wang,* Nian Zhang, and Zhanwen Wang Beijing Key Laboratory of Green Reaction Engineering and Technology Department of Chemical Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: Gasliquid mass-transfer behaviors in aqueous alcohol solutions were studied in an internal-loop airlift reactor of 5-m height and 0.28-m i.d. in the superficial gas velocity (Ug) range from 2.0 to 10.0 cm/s. Air and aqueous n-butanol solutions were used as the gas and liquid phases, respectively. It was found that the volumetric mass-transfer coefficient decreased with increasing n-butanol concentration (CA) below a critical CA value of 0.5 wt % and increased with increasing CA above this concentration. To confirm this result, further experiments were also carried out in a bubble column of 0.12-m i.d. at Ug = 6 cm/s, and similar results were obtained. Further analysis showed that the value of kla/αg was independent of the superficial gas velocity and equal to 0.21 1/s in the airwater system; however, it decreased with increasing CA up to 0.25 wt % in n-butanol solutions, and further addition of n-butanol had no effect on kla/αg. A critical CA value of 0.5 wt % was also found for the liquid-side mass-transfer coefficient (kl). Below this concentration, kl decreased with increasing CA, whereas above this concentration, further addition of n-butanol had no effect on kl.
1. INTRODUCTION Airlift reactors are widely used in biochemical industrial processes, such as fermentation and wastewater treatment, because of their simple construction, good heat transfer, low shear rate, low power input, and easy scale-up.1,2 In many biochemical processes, alcohols are added as an external source of carbon.3,4 Aqueous alcohol solutions differ from water in terms of surface tension, and this causes significant differences in the hydrodynamics and mass transfer. Some previous studies have reported the influence of alcohols on the gas holdup,37 liquid velocity,3,4 bubble rise velocity,8 and bubble formation.9,10 Albijanic et al. summarized some of these works.3 However, research on the influence of alcohol addition on mass transfer is still very limited,3,4,7,11 and contradicting conclusions have been drawn in the existing studies. Posarac and Tekic,11 using a bubble column, and Albijanic et al.,3 using an internal airlift reactor, reported that the volumetric masstransfer coefficient (kla) increased with increasing alcohol concentration, while Al-Masry and Dukkan,7 using an external airlift reactor, and El Azher et al.,4 using a split rectangular airlift reactor, reported the opposite trend. The difference was possibly caused by the two opposing effects of alcohol addition:3,4 On one hand, increased gas holdup and decreased bubble diameter caused by the presence of alcohol tends to increase the interfacial area (a), whereas on the other hand, alcohol molecules in the solution tends to be absorbed and accumulate at the gasliquid interface to form a “rigid” bubble, which leads to a smaller liquid-side mass-transfer coefficient (kl). Such an analysis, however, is just a qualitative explanation, and more detailed data are needed to provide a better understanding of the mass-transfer mechanism in the presence of alcohol. Most studies in the literature have focused on the experimental determination of kla, which is a global parameter that depends on the reactor geometry, operating conditions, and physical properties. The common approach used to describe kla is to correlate it with the factors that affect it. The gasliquid mass-transfer rate in an airlift r 2011 American Chemical Society
reactor depends on the gas holdup, flow regime, bubble size distribution, bubble breakup and coalescence, interfacial area, and liquid-side mass-transfer coefficient.12 The separation of kl and a can allow the identification of whether kl or a controls the mass-transfer rate. In this work, we performed such a study on the influence of alcohol addition. The reactor size also has a significant influence on the hydrodynamics and mass-transfer rate.13 It is commonly accepted that the hydrodynamics becomes independent of the column size only when the column diameter (D), column height (H), and aspect ratio (H/D) are larger than certain threshold values.2 Wilkinson et al. suggested that H should be larger than 13 m.14 However, most works on airlift reactors in the literature have used reactors of about 2-m height,15,16 and only some works have used reactors of 4-m height.17,18 Therefore, an investigation using a larger airlift reactor will be valuable for a better understanding of the scale-up behavior. This work aimed to study the gasliquid mass-transfer behaviors in a 5-m-high internal-loop airlift reactor with water and aqueous solutions of n-butanol. The effects of surface tension and superficial gas velocity (Ug) on the gas holdup, bubble size, volumetric mass-transfer coefficient, interfacial area, and liquidside mass-transfer coefficient were investigated.
2. EXPERIMENTAL SECTION 2.1. Experimental Apparatus. A schematic of the experimental apparatus is shown in Figure 1. The internal-loop airlift reactor used was made of Plexiglas. It comprised four main parts: Received: January 27, 2011 Accepted: August 29, 2011 Revised: August 16, 2011 Published: August 29, 2011 11537
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conductivity probe. The distance between the two tips was 1.25 mm. The measuring principle was based on the different conductivities of the gas and liquid, which gave different output voltage signals when the probe tip was in contact with different phases. The bubble chord length and rise velocity were obtained from the measured signals by the use of a previously published algorithm.20 In brief, the downstream signal with the dual-tip probe had a slight lag compared with the upstream signal because of the distance between the two tips. The lag time was determined by a correlation method, and then, using the distance between the tips, the bubble rise velocity was calculated. The bubble chord length was calculated by multiplying the bubble rise velocity and the duration of time that the probe tip was in the bubble. The bubble rise velocity was calculated as ub ¼
Figure 1. Schematic of the experimental apparatus. (Legend: 1, riser; 2, downcomer; 3, separator; 4, gas distributor; 5, electrical conductivity probe port; 6, oxygen probe port; 7, personal computer; 8, differential pressure transducer; 9, flow meter; and 10, compressor.)
surface tension
(wt%)
(mN/m)
0
72.1
0.05
70.8
0.25 0.5
66.4 55.9
1.0
47.3
0
where V1 and V2 are the signals from the two tips. Δt was determined as the value of τ when c(τ) reached a maximum. The bubble chord length was obtained using the equation lb ¼ ub ðt2 t1 Þ
annular riser, downcomer, gasliquid separator, and gas distributor. The total height of the reactor was 5 m. The riser was 0.28 m in i.d. and 4.1 m in height. The separator was 0.48 m in i.d. and 0.9 m in height. The draft tube was 0.19 m in o.d., 0.18 m in i.d., and 4.0 m in height. The gas distributor was an annular perforated plate with 196 holes of 1-mm diameter; thus, the gas was injected only into the annular riser. 2.2. Materials and Measuring Method. Air was used as the gas phase. Tap water and aqueous n-butanol solution of 0.051 wt % were used as the liquid phase. All experiments were carried out at 21 ( 1 °C. Surface tension was measured by a surface tension apparatus (QBZY-2, Shanghai Fangrui Instrument Company, Shanghai, China). The measured values are listed in Table 1. 2.2.1. Gas Holdup. The global gas holdups in the riser and downcomer were measured by the pressure drop method, as described in our former work.19 The global gas holdup in the reactor is αg ¼
αgr Ar þ αgd Ad Ar þ Ad
ð2Þ
where d0 is the distance between the two tips and Δt is the time difference of the signals from the two tips. Δt was determined by the correlation function of the two signals, which was Z ∞ V1 ðtÞ V2 ðt τÞ dt cðτÞ ¼ ð3Þ
Table 1. Surface Tensions of n-Butanol Solutions n-butanol concentration
d0 Δt
ð1Þ
where Ar and Ad are the cross-sectional areas of the annular riser and downcomer, respectively. 2.2.2. Bubble Diameter. The bubble characteristics were measured 2.0 m above the gas distributor with a dual-tip electrical
ð4Þ
where t1 and t2 are the beginning and end times, respectively, of a bubble signal. It is commonly accepted that the bubble size in an airwater system can be described by a log-normal distribution.21 The bubble size distribution can be obtained from the bubble chord length distribution by a distribution transform algorithm given by Wang et al.20 However, we found a bimodal bubble chord length distribution at high gas velocities (Ug > 6 cm/s) in the alcohol solutions. Camarasa et al. also reported a bimodal bubble size distribution with the addition of alcohol in a bubble column.22 In these cases, it is difficult to derive the bubble size distribution from the bubble chord length distribution. At low gas velocities (Ug < 6 cm/s), the bubble chord length distribution can be described by a log-normal distribution function, and the bubble size distribution can be obtained using the distribution transform algorithm. The bubble Sauter diameter calculated from the bubble size distribution was 1.21.3 times the average bubble chord length. Because the average bubble chord length varied only slightly in the experiments in this work, the bubble Sauter diameter was estimated with the following correlation dS ¼ 1:25lb, avg
ð5Þ
where lb,avg is the average bubble chord length. Similar correlations have been used by previous researchers with constants between 1 and 1.5.2325 It should be pointed out that bubble measurements using a two-tip probe are reliable in bubbly flow, whereas at high superficial gas velocities, bubble measurements become more complex and less reliable because of bubble deformation and random velocity directions. Equation 5 is based on the results at low superficial gas velocity (Ug < 6 cm/s) and was extended for use at high superficial gas velocities considering that the average bubble chord length did not change significantly 11538
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Figure 3. Effects of Ug and n-butanol concentration on kla.
where klal is the volumetric mass-transfer coefficient per unit volume of liquid, Csensor is the oxygen concentration measured by the sensor, ksensor is the time constant of the sensor, and Cl* is the oxygen value when the probe was placed in an air-saturated liquid for a sufficiently long time. The relationship between klal and kla can be expressed as kl al ¼
Figure 2. Effects of Ug and n-butanol concentration on gas holdup: (a) riser, (b) (αgr αgd)/αgr, (c) overall.
in this work. Further improvement in these measurements will provide more reliable results. Xue et al. developed a four-point optical probe technique with a data processing algorithm.26 With the greater amount of information obtained by the four-point probe, they obtained reliable results over a wider range of operating conditions based on Ishii’s definition of interfacial area.27,28 2.2.3. Mass Transfer. The volumetric mass-transfer coefficient, kla, was determined by the oxygen desorption technique. A fluorescence oxygen probe (LDO HQ10, Hach Company, Loveland, CO) placed 2.0 m above the gas distributor was used to measure the change in oxygen concentration with time. The operating principle of the fluorescence oxygen probe is based on the correlation of the oxygen concentration with the time of light emission of the luminescent material. The probe is coated with a luminescent material. When blue light from a light-emitting diode (LED) is transmitted to the sensor surface, the luminescent material is excited and emits red light. The more oxygen that is present, the shorter the time it takes for the red light to be emitted. This time was measured and correlated with the oxygen concentration. The continuous stirred-tank reactor (CSTR) model was used to determine kla. Assuming that the liquid was perfectly mixed and that oxygen accumulation in the gas phase was negligible, the mass-transfer rate measured by the sensor is given by Csensor 1 ¼ ðksensor ekl al t kl al eksensor t Þ ksensor kl al Cl
ð6Þ
kl a ð1 αg Þ
ð7Þ
The liquid-side mass-transfer coefficient was determined as kla/a, and the interfacial area a was calculated by 6αg/dS. Others details for the calibration of the time constant of the sensor and the determination of the volumetric mass-transfer coefficient, interfacial area, and liquid-side mass-transfer coefficient were given in our previous work.19
3. RESULTS AND DISCUSSION 3.1. Gas Holdup. Figure 2ac shows the influence of Ug and n-butanol addition on the gas holdup in the riser, the gas holdup difference ratio, and the overall gas holdup. The gas holdup increased with increasing Ug and n-butanol concentration (CA), in agreement with the results reported in the literature.35,7 The increase of the gas holdup with increasing CA slowed in the range of high CA values. For example, at Ug = 8 cm/s, the overall reactor gas holdup increased from 14.0% to 18.9% when CA was increased from 0 to 0.05 wt %, but it increased from 18.9% to only 22.1% when CA was increased from 0.05% to 0.25 wt %. The influence of n-butanol addition on bubble coalescence was studied. Alcohols are coalescence-hindering materials.22,29,30 An alcohol has a hydrophobic part (carbon chain) and a hydrophilic part (hydroxyl group). When dissolved in water, alcohol molecules are absorbed and accumulate at the gasliquid interface, with the hydrophobic part facing away from the bulk of the solution to form a “protective” monolayer. This monolayer hinders bubble coalescence and leads to smaller bubble sizes and lower bubble slip velocities. When a bubble rises through the liquid, the alcohol molecules at the gasliquid interface are pushed to the back of the bubble, and a surface tension gradient is formed. This surface tension gradient opposes the tangential shear stress and leads to an increase in the drag force and a decrease in the bubble slip velocity.3,5 In addition, the decrease in the bubble slip velocity enhances the entrainment of bubbles into 11539
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Figure 4. Effects of Ug and n-butanol concentration on kla in different reactors.
the downcomer, which results in a reduction of the difference between the gas holdups in the riser and downcomer and a decrease in the liquid circulation velocity.3,4 In this work, the average gas holdup difference decreased from 15% to 7.5% when CA was increased from 0 to 0.25 wt %. Both a decreased bubble rise velocity and a decreased liquid velocity lead to an increase in bubble residence time and, consequently, to an increase in gas holdup. 3.2. Mass-Transfer Rate. 3.2.1. Volumetric Mass-Transfer Coefficient. The effects of Ug and CA on kla are shown in Figure 3. It can be seen that kla increased with increasing Ug. In addition, kla showed different trends with CA in different ranges, with a critical CA value of 0.5 wt % where the trend changed. Below this critical concentration, kla decreased with increasing CA, whereas above this concentration, the trend reversed. Such a change in trends has not previously been reported in the literature,3,4,7 which might due to the much narrower range of alcohol concentrations investigated in those works. To confirm this behavior, we carried out similar experiments in a bubble column of 0.12-m i.d. at Ug = 6 cm/s, and the results are shown in Figure 4. Similar trends were observed for both the airlift reactor and bubble column, including the same critical concentration of 0.5 wt %. To gain a better understanding of this phenomenon, the value of kla/αg is analyzed in this section, and a separation of kl and a is performed in the next section. The gasliquid mass-transfer rate is closely related to the hydrodynamics; that is, the trend of kla is similar to that of the gas holdup. Letzel and Stankiewicz reported that the kla/αg ratio in the nitrogenwater system was almost constant and equaled 0.5 1/s at different system pressures.31,32 Jordan and Schumpe also reported that kla/αg in the nitrogendecalin system was almost independent of Ug and gas density, at a value of 0.45 1/s.33 Vandu and Krishna reported that kla/αg in the airwater system was 0.48 1/s and was practically independent of the column diameter and Ug.13 A rough estimation can be made to gain a better understanding of the value of kla/αg. Considering eq 7, kla/αg can be rewritten as kla/αg = 6kl/dS. A typical value of kl in the airwater system is about 0.0004 m/s; thus, a bubble Sauter diameter of 46 mm results in values of 0.60.4 for kla/αg. These results are interesting and important because they indicate that a simple rule can be used to estimate kla from the gas holdup, which is much easier to measure than kla. However, this simple correlation needs more validation with other liquids and reactor
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Figure 5. Variation of kla/αg with Ug and n-butanol concentration.
Figure 6. Effects of Ug and n-butanol concentration on bubble Sauter diameter.
types. The variation of kla/αg with Ug in this work is shown in Figure 5. In the airwater system, the value of kla/αg was almost independent of Ug and was 0.211/s. This value is lower than that reported in the literature, which is probably due to the different type and geometry of reactor used. The reactor in this work was a 5-m-high internal airlift reactor. The limited oxygen in the small bubbles recirculated in the reactor can be rapidly depleted in the riser, and thus, these bubbles in the downcomer contribute significantly to the gas holdup but insignificantly to the gas liquid mass transfer. The volumetric mass-transfer coefficient determined by the CSTR model is based on the volume of the whole reactor, and therefore, kla and the value of kla/αg in this work are much smaller than those in the bubble column reported in the literature. With the addition of n-butanol, kla/αg was also approximately constant at Ug > 4 cm/s but had a smaller value than that in water, namely, 0.14 1/s in the 0.05 wt % n-butanol solution and 0.11 1/s in the other three solutions. Thus, simple correlations of kla = 0.21αg for water and kla = 0.11αg for the n-butanol solution with CA values larger than 0.25 wt % can be used to estimate the mass-transfer rate from the gas holdup. The predicted and experimental data for the n-butanol solutions are compared in Figure 5, and good agreement was obtained. The parameter kla/αg can be interpreted as the volumetric mass-transfer 11540
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Figure 7. Effects of Ug and n-butanol concentration on interfacial area.
Figure 8. Effects of Ug and n-butanol concentration on liquid-side mass-transfer coefficient.
coefficient per unit volume of bubbles.13 The constant value of kla/αg in n-butanol solutions with CA g 0.25 wt % was about one-half that in water, which indicates that the mass-transfer rate per unit volume of bubbles in these solutions was only one-half that in water. The detailed mechanisms for the decrease of mass transfer upon addition of n-butanol are discussed in the next section. 3.2.2. Interfacial Area and Liquid-Side Mass-Transfer Coefficient. The influences of Ug and the n-butanol concentration on the bubble Sauter diameter are shown in Figure 6. In the airwater system, the Sauter diameter increased with increasing Ug. In the n-butanol solutions, the bubble Sauter diameter was independent of Ug, with a constant value of 5.6 mm for 0.05, 0.25, and 0.5 wt % n-butanol solutions and 5.1 mm for 1 wt % n-butanol solution. A constant bubble diameter in alcohol solutions was also observed by Loubiere and Hebrard.10 Figure 7 shows that the interfacial area a increased with increasing Ug and CA. This is a result of an increase in gas holdup with an unchanged bubble Sauter diameter. The variation of kl with Ug and CA is shown in Figure 8. In the airwater system, kl had a constant value of 2.2 104 m/s in the heterogeneous regime. Upon addition of n-butanol, kl decreased with increasing Ug, especially in the low concentration
Figure 9. Effects of n-butanol concentration on mass-transfer characteristics.
range. A critical CA value of 0.5 wt % was also found for kl. Below this concentration, kl decreased with increasing CA, whereas above this concentration, further addition of n-butanol had no effect on kl. The reason for the decrease of kl with addition of n-butanol is as follows: First, the protective monolayer at the gasliquid interface formed by absorbed n-butanol molecules enhances bubble rigidity and hinders bubble coalescence, which reduces oxygen renewal and oxygen diffusivity. This aspect of the mechanism was already proposed in works on the effect of surfactants.3437 Second, the driving force for oxygen mass transfer is reduced in the presence of n-butanol because many small bubbles 11541
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Industrial & Engineering Chemistry Research are formed.3,4 The small bubbles recirculate in the reactor and contribute significantly to gas holdup but insignificantly to mass transfer. This is because the limited oxygen in these small bubbles can be rapidly depleted but no more oxygen is obtained from other bubbles by bubble coalescence. This is also the reason for the slight decrease of kl with increasing Ug. The increase of Ug has two effects: One is to increase turbulence and surface renewal, which tends to increase kl, and the other is to increase small inert bubbles, which tends to decrease kl. In this work, these two effects were of comparable magnitudes, so kl decreased only slightly with increasing Ug. In summary, as shown in Figure 9, the different trends of kla with CA in different ranges can be comprehended as follows: Below the critical CA value of 0.5 wt %, with increasing CA, the decrease of kl overcame the increase of interfacial area and led to a decrease in kla. In contrast, above this critical CA value, with increasing CA, kl remained constant, and the increase of the interfacial area led to an increase in kla. A comparison of this work with those in the literature is needed to understand the above conclusions about the influence of alcohol on the mass-transfer rate. However, this is difficult because there have been only a few investigations on this subject. For instance, the Ug values used in the studies of El Azher et al.4 and Albijanic et al.3 were only up to 4 cm/s, and the n-butanol concentration ranges were narrow and cannot be compared with those used in this work. More importantly, there is a lack of studies including a separation of kl and a in the presence of an alcohol. A detailed study of mass-transfer rates when an alcohol is present is still needed, and the following aspects should be paid special attention: First, the relationship between the gas holdup and mass-transfer rate needs to be further studied. The ratio between the values of kla/αg in water and n-butanol solutions with CA g 0.25 wt % was found here to be approximately 2.0. In addition, it should be noticed that this ratio is close to the ratio of kl in water and n-butanol solutions with CA g 0.25 wt %. If this ratio can be verified to be almost constant for all alcohol solutions, then it can be used to estimate kla in alcohol solutions from the abundant data available for pure water. Second, a more in-depth investigation of the critical concentration of alcohol at which the trend of kl with CA changes is needed. This critical value is significant because it determines the transition of the factors that control the mass-transfer rate. Given that the measurement uncertainty of kl was up to 20% in this work, the critical concentration of n-butanol was not very precisely determined, and a more detailed investigation is needed. Furthermore, the influence of the type of alcohol used on the critical concentration should also be studied.
4. CONCLUSIONS Gasliquid mass-transfer characteristics in a 5-m-high internal-loop airlift reactor were studied. The effects of the superficial gas velocity and n-butanol concentration on the global gas holdup, bubble size, volumetric mass-transfer coefficient, interfacial area, and liquid-side mass-transfer coefficient were investigated. The main conclusions from the results obtained in this work are as follows: (1) Gas holdup increased with increasing n-butanol concentration (CA), as a result of less bubble coalescence and an increase in the bubble drag force caused by the change in the surface tension.
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(2) The volumetric mass-transfer coefficient (kla) increased with increasing Ug. Below a critical CA value of 0.5 wt %, kla decreased with increasing CA, whereas the opposite trend was found above this concentration. The value of kla/αg was found to be 0.21 1/s in an airwater system, and it decreased with increasing CA up to 0.25 wt % in n-butanol solution. Further addition of n-butanol had no effect on kla/αg. (3) The interfacial area increased with increasing Ug and CA. The liquid-side mass-transfer coefficient (kl) decreased with increasing Ug and CA up to 0.5 wt %. Further addition of n-butanol had no effect on kl. (4) Detailed studies of mass-transfer rates when alcohol is present in a system are still needed. The relationships between the gas holdup and mass-transfer rate and the critical concentration of alcohol should be paid special attention.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: 86-10-62794132. Fax: 86-10-62772051. E-mail: wangtf@ tsinghua.edu.cn.
’ ACKNOWLEDGMENT The authors gratefully acknowledge the financial support of the Foundation for the Author of National Excellent Doctoral Dissertation of PR China (No. 200757) and the National 973 Project of China (No. 2007CB714302). The authors thank Professor D. Z. Wang for his helpful discussions and English revisions. ’ NOTATION a = gasliquid interfacial area per unit dispersion volume, m1 al = gasliquid interfacial area per unit liquid volume, m1 Ad = cross-sectional area of the downcomer, m2 Ar = cross-sectional area of the annular riser, m2 CA = n-butanol concentration, wt % Cl* = saturation oxygen concentration in the liquid, kg/m3 Csensor = liquid-phase oxygen concentration given by the sensor, kg/m3 d0 = distance between the two probe tips, mm dS = bubble Sauter diameter, mm h = height, m ksensor = time constant of the sensor, s1 kl = liquid-side mass-transfer coefficient, m/s kla = volumetric mass-transfer coefficient based on the dispersion volume, s1 klal = volumetric mass-transfer coefficient based on the liquid volume, s1 lb = bubble chord length, mm lb,avg = average bubble chord length, mm t = time, s ub = bubble rise velocity, m/s Ug = superficial gas velocity, cm/s Ugr = superficial gas velocity in the riser, cm/s Greek Symbols
αg = gas holdup, arbitrary units αgd = gas holdup in the downcomer, arbitrary units 11542
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d = downcomer g = gas phase l = liquid phase r = riser
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