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Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations
CJChE-00448; No of Pages 8 Chinese Journal of Chemical Engineering xxx (2016) xxx–xxx
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Chinese Journal of Chemical Engineering journal homepage: www.elsevier.com/locate/CJChE
Fluid Dynamics and Transport Phenomena
Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations☆ Jinjin Zhang, Zhengming Gao, Yating Cai, Ziqi Cai, Jie Yang ⁎, Yuyun Bao ⁎ State Key Laboratory of Chemical Resource Engineering, School of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e
i n f o
Article history: Received 11 February 2015 Received in revised form 26 June 2015 Accepted 28 June 2015 Available online xxxx Keywords: Stirred vessel Gassed power Mass transfer Triple-impeller combination Gas–liquid system
a b s t r a c t The gassed power demand and volumetric mass transfer coefficient (kLa) were investigated in a fully baffled, dished-base stirred vessel with a diameter of 0.30 m agitated by five triple-impeller combinations. Six types of impellers (six-half-elliptical-blade disk turbine (HEDT), four-wide-blade hydrofoil impeller (WH) pumping down (D) and pumping up (U), parabolic-blade disk turbine (PDT), and CBY narrow blade (N) and wide blade (W)) were used to form five combinations identified by PDT + 2CBYN, PDT + 2CBYW, PDT + 2WHD, HEDT + 2WHD and HEDT + 2WHU, respectively. The results show that the relative power demand of HEDT + 2WHU is higher than that of other four impeller combinations under all operating conditions. At low superficial gas velocity (uG), kLa differences among impeller combinations are not obvious. However, when uG is high, PDT + 2WHD shows the best mass transfer performance and HEDT + 2WHU shows the worst mass transfer performance under all operating conditions. At high uG and a given power input, the impeller combinations with high agitation speed and big projection cross-sectional area lead to relatively high values of kLa. Based on the experimental data, the regressed correlations of gassed power number with Froude number and gas flow number, and kLa with power consumption and superficial gas velocity are obtained for five different impeller combinations, which could be used as guidance for industrial design. © 2016 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.
1. Introduction Mechanically agitated gas–liquid reactors with single or multiple impellers are often used to intensify the contact between gas and liquid. Hydrogenation, chlorination, polymerization, sewage treatment, and aerobic fermentation are examples of such processes. Since the mass transfer of gas into liquid is often the rate-limiting step when low solubility gases are involved, the volumetric mass transfer coefficient, kLa, is considered a key parameter for operation, design, and scale-up of stirred reactors. Based on the flow patterns generated, an impeller can be characterized as an axial flow impeller such as marine propeller, Techmix 335 (TX) [1], Lightnin A315 (LTN) [2], four-wide-blade hydrofoil impeller (WH) [3], KHX [4] and CBY [5], and a radial flow impeller such as Rushton turbine (RT), pitched blade (PB), concave-bladed disk turbine (CD) [6], half elliptical-blade disk turbine (HEDT) [7], parabolic blade disk turbine (PDT) [5] and Narcissus (NS) [2]. Compared with the stirred reactors equipped with a single impeller, the dual-impeller [8–10] and triple-impeller [11–14] stirred reactors have such advantages as increased gas hold-up, superior gas distribution, long residence of gas bubbles, improved liquid circulation characteristics, low power
☆ Supported by the National Natural Science Foundation of China (21206002, 21376016). ⁎ Corresponding authors. E-mail addresses: [email protected] (J. Yang), [email protected] (Y. Bao).
consumption per impeller, and even distribution of shear stress and energy dissipation, thus, effective gas utilization. Over the past few decades, many publications presenting various hydraulic and mass transfer characteristics in multi-impeller stirred vessels can be found in the literature. Nocentini et al. [11] and Linek et al. [15] used multiple Rushton turbines on a common shaft. Arjunwadkar et al. [8] used dual impeller of RT and PB, and Suhaili et al. [16] used dual impeller of RT and CD. kLa for various impeller combinations was discussed by Moucha et al. [1] using 18 combinations of RT, PB and TX and their combinations, and Fujasová et al. [2] using 28 combinations of RT, PB, TX, LNT and NS and their combinations. Results show that radial flow impellers exhibit 20% to 50% higher oxygen transfer efficiency than axial flow impellers. However, radial flow impellers have the weakness of bad homogenization [2,12], and relatively high power consumption [17]. Compared with radial flow impeller combinations or axial flow impeller combinations, the mixed flow impeller combinations with both radial and axial flow impellers showed better homogenization performance and mass transfer performance at the same power consumption. However, few reports present the mass transfer characteristics for mixed flow impeller combinations. In the mixed flow impeller combinations, the radial flow impeller is often installed in the bottom to break and disperse the gas introduced from gas sparger and the axial flow impellers are installed above to circulate the gas–liquid flow. RT is the radial flow impeller usually used [1,2,12], but it has weakness of significant drop in gassed power draw. This disadvantage can be tackled by retrofitting of RT with streamlined impellers, such as impellers of CD, HEDT and PDT.
http://dx.doi.org/10.1016/j.cjche.2015.12.008 1004-9541/© 2016 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.
Please cite this article as: J. Zhang, et al., Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations, Chin. J. Chem. Eng. (2016), http://dx.doi.org/10.1016/j.cjche.2015.12.008
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However, research on the mass transfer characteristics for these modified impellers is quite rare [12], especially for impellers HEDT and PDT. Moreover, the superficial gas velocity (uG) for kLa determination reported in multi-impeller stirred reactors is usually small (less than 0.016 m·s− 1) and the research on kLa at higher uG (more than 0.016 m·s− 1) is open for further investigation. To date, data on kLa for the mixed flow impeller combinations are still not enough, especially for the new-type impeller combinations and at high gas rates. The aim of this work is to provide gassed power demand and mass transfer characteristics for five mixed flow impeller combinations with new-type axial flow impellers of WH (pumping down and pumping up) and CBY (narrow blade and wide blade) with radial flow impellers of HEDT and PDT at uG between 0.0078 and 0.039 m·s− 1. General correlations representing the dependence of gassed power and kLa on operating conditions are developed. kLa differences among these mixed flow impeller combinations are discussed and analyzed.
2. Experimental 2.1. Experimental setup All experiments were carried out in a dished-base cylindrical vessel equipped with triple impellers, as shown in Fig. 1. The diameter (T) of the tank was 0.30 m and the height of the tank (HT) was 0.75 m. The ungassed liquid level was maintained at a height H = 1.8 T with a deionized water volume of 0.036 m3. As a standard configuration, four 0.03 m-wide baffles were symmetrically mounted in the tank. The impellers used were six-half-elliptical-blade disk turbine (HEDT), parabolic-blade disk turbine (PDT), four-wide-blade hydrofoil impeller (WH), and CBY in Fig. 2. The diameter of all the impellers (D) was 0.4 T, the same as the clearance between the lowest impeller and the base of the tank, and the distance between two adjacent impellers was 0.48 T. WH can be used for up-pumping and down-pumping operating modes, identified as WHU and WHD, respectively. CBY can be classified as CBYN and CBYW by the impeller tip width of 0.1D and 0.14D, respectively. The shorthand notation used for defining the agitation combinations is straightforward: PDT + 2CBYN means parabolic-blade disk turbine at the bottom and two CBY impellers with an impeller tip width of 0.1D at mid-level and top-level. PDT + 2CBYW , PDT + 2WHD , HEDT + 2WH D and HEDT + 2WHU represent similar implications. Air was introduced from a sparger located at a height of 0.33 T above the tank bottom. The ring sparger was 0.8D in diameter with 27 symmetrical downward-directed holes of 0.002 m diameter.
Fig. 2. Impellers used in the experiment: (a) HEDT; (b) PDT; (c) WH; and (d) CBY.
The first bottle of Na 2 SO 3 solution with a concentration of 500 mol·m− 3 was used to absorb the oxygen in the air so that no variation of Na2SO3 concentration in the second bottle would be resulted. The Na2SO3 solution with a concentration of 200 mol·m− 3 in the second bottle was fed into the tank by a peristaltic pump, and the ion concentration in the stirred tank was kept below 50 mol-ion·m− 3 to maintain the water as a coalescent system [18]. The concentration of dissolved oxygen was measured online by a DO probe (VISIFERM DO Arc 120, Hamilton, Switzerland) and recorded with time serial as data files by the computer. 2.2. Power consumption The power consumption of the impeller was measured by using a rotary torque sensor (AKC-205, China Academy of Aerospace Aerodynamics, China) installed between the impeller shaft and the motor bearings. The specific power consumption PTm (in W·kg−1), which is the sum of the potential energy of the sparged gas and the agitation power consumption calculated by measuring the torque of the stirring shaft and the agitation speed, is an important parameter to characterize the performance of an agitator. The total power consumption of the stirring system was calculated by PT ¼ Pg þ Pe
ð1Þ
P Tm ¼ P T =ρL V L
ð2Þ
where Pg is the gassed agitation power given by Pg = 2πNM and Pe is the potential energy of sparged gas calculated by Pe = ρL gHSQ g. 2.3. Measurement of kLa kLa was measured by the steady-state sulfite feeding method (SFM) [18], which is based on the measurement of dissolved oxygen concentrations under steady-state conditions of equilibrium between sulfite addition and oxygen dissolution. kLa is calculated by kL a ¼
Fig. 1. Experimental setup: (1) 1st bottle of Na2SO3 solution; (2) 2nd bottle of Na2SO3 solution; (3) peristaltic pump; (4) motor; (5) rotary torque sensor; (6) sparger; (7) DO probe; and (8) computer.
Q sCs 2V L C Ai ð1−C As =C Ai Þ
ð3Þ
where Q s is the feed rate of the sulfite solution, m3·s−1; Cs is the concentration of sulfite in the feed solution, mol·m−3; VL is the volume of the liquid in the aerated stirred tank, m3; CAi is the equilibrium
Please cite this article as: J. Zhang, et al., Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations, Chin. J. Chem. Eng. (2016), http://dx.doi.org/10.1016/j.cjche.2015.12.008
J. Zhang et al. / Chinese Journal of Chemical Engineering xxx (2016) xxx–xxx
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concentration of the oxygen at the gas–liquid interface, mol·m−3; and CAs is the oxygen concentration measured in water in steady state with the sulfite solution fed continuously, mol·m−3. The experimental results are reproducible twice or thrice with an average relative error less than 5%.
much lower than HEDT + 2WHD at the same operation conditions. This indicated that the bottom impeller can influence greatly the power consumption. Nevertheless, the similar RPD for PDT + 2WHD and HEDT + 2WHD shows that the influence of the bottom impeller on RPD is little.
3. Results and Discussion
3.1.2. Influence of blade width The effect of the blade width on RPD can be seen by the comparison of RPD for PDT + 2CBYN, PDT + 2CBYW and PDT + 2WHD, having the same bottom impeller and increasing width of blade for two upper impellers. Table 1 shows that both b and c increase with increasing blade width of upper impellers from CBYN, CBYW to WHD with the same bottom PDT impeller. The increasing values of b and c imply that RPD decreases more when more gas is introduced or stirred at higher agitation speed. It could be explained that the wider blade of upper impellers can supply bigger interaction area for both liquid and gas, introduce more gas into the liquid and keep bubbles staying for a longer time in liquid, thus the gas holdup increases, so RPD decreases more when FlG or Fr increases.
The superficial gas velocity in this work ranges from 0.0039 to 0.039 m·s−1 (equivalent to 0.463–4.63 vvm). We labeled the superficial gas velocity of 0.0039 to 0.0078 m·s−1 as low gas velocity, and 0.024 to 0.039 m·s−1 as high gas velocity based on the criterion for industrial operation. The impeller speeds for each impeller combination were above the complete gas dispersion speed (NCD) defined by Nienow et al. [19] during all of the measurements of power demand and volumetric mass transfer coefficient. The highest NCD at the highest superficial gas velocity of 0.039 m·s−1 for all impeller combinations used is 6.83 s−1, and the impeller speed used in the experiments was chosen from 8 to 13 s−1. 3.1. Relative power demand (RPD) The ratio of gassed to ungassed power (RPD, RPD = Pg/P0) for different impeller combinations at various agitation speeds is shown in Fig. 3. It can be seen that RPD decreases with increasing gas flow number (FlG) for all impeller combinations because as more gas is introduced into the stirred tank, the gas holdup increases and the average density of the gas–liquid mixture phase decreases. The decreasing average density causes the decrease of RPD with increasing FlG. Under all operating conditions, RPD of all impeller combinations is mostly above 0.65, higher than those of impeller combinations only using the radial impellers or axial impellers reported in the literature [20], meanwhile RPD of axial impellers 3WHD decreases dramatically to 0.4–0.5 when FlG is larger than 0.05 and RPD of radial impellers 3CD is smaller than 0.55 when Fl G is larger than 0.10. The higher RPD obtained in this research shows mixed flow impeller combinations having higher energy utilization efficiency and less loss of capacity for mass transfer. Power number (NP = P0/ρLN3D5) is one of the most basic parameters for stirred reactors and the gassed power number (NPG = Pg/ρLN3D5) is for the power number under aeration. Based on the experimental data of gassed power under different conditions, we use the following correlation −b
NPG ¼ mFlG Fr −c
ð4Þ
to regress NPG with the Froude number and gas flow number for different impeller combinations, and the results are shown in Table 1. Exponent b indicates the sensitivity of RPD on gas flow for different impeller combinations and is affected by the cavities behind impeller blades or/and the average density of the medium. The effects of gas flow and agitation speed on RPD for different impeller combinations will be discussed from the following three aspects. 3.1.1. Influence of bottom impeller In Fig. 3, RPD for PDT + 2WHD and HEDT + 2WHD presents the similar trend for different agitation speeds although with different bottom impellers. Both PDT and HEDT impellers are streamlined modification of RT with different curvatures. Vasconcelos et al. [6] studied the effect of blade shape on the performance of impellers and found that the blade streamlining can lead to a lower ungassed power number (NP). Therefore, it can be referred that NP for PDT is lower than HEDT. In this work, NPG for these two impeller combinations of PDT + 2WHD and HEDT + 2WHD was regressed in Table 1, while NP for PDT + 2WHD and HEDT + 2WHD without aeration was also obtained as 2.8 and 3.6, respectively. It can be seen that both NP and NPG for PDT + 2WHD are
3.1.3. Influence of up- and down-pumping operating modes The obviously different RPD trends for HEDT + 2WH U and HEDT + 2WHD show that the operating mode for the axial upper impellers can influence RPD greatly. RPD for HEDT + 2WHU are all above 0.75, much higher than those for other impeller combinations. That is because the flow field of liquid caused by HEDT + 2WHU , which was computed by Min et al. [21], has the same direction of the gas flow, leading to a rapid release of bubbles from the liquid, thus to a reduction in the retained gas and further to a high average medium density. The smaller exponent b for HEDT + 2WHU than HEDT + 2WHD in Table 1 indicates that the gas flow rate has less influence on RPD for HEDT + 2WHU than HEDT + 2WHD because of the shorter residence time of retained gas in liquid and lower gas holdup for HEDT + 2WHU. From the comparison of the different RPD trends for HEDT + 2WHU and HEDT + 2WH D, and the similar RPD for PDT + 2WHD and HEDT + 2WHD, it can be seen that two upper impellers play a more important role on the change of RPD than the bottom impeller. 3.2. Volumetric mass transfer coefficient (kLa) 3.2.1. Influence of impeller combinations at various superficial gas velocities kLa for different impeller combinations at various superficial gas velocities (uG) is shown in Fig. 4. It can be seen that for a given specific power consumption, kLa changes only about 10% for five different impeller combinations when u G is small (e.g. u G = 0.0039– 0.0078 m·s − 1 ). However, with u G increasing from 0.016 to 0.039 m·s− 1, the difference of kLa for five different impeller combinations increases gradually from 20% to as high as 80%. The impeller combination of PDT + 2WHD has the dominant mass transfer performance when uG ranges from 0.0078 to 0.039 m·s−1 in this work. It is also worth mentioning that the only up-pumping mode impeller combination HEDT + 2WHU presents much smaller mass transfer coefficient than other impeller combinations when uG is high (e.g. uG = 0.031–0.039 m·s−1). The regression results of kLa for different impeller combinations based on correlation (5) proposed by Cooper et al. [22] are shown in Table 2. β kL a ¼ AP α Tm uG
ð5Þ
Table 2 shows that the exponent β is bigger than α for all impeller combinations, indicating that it is more efficient to increase kLa by increasing the gas velocity than by increasing the power input, especially for the impeller combination of PDT + 2WHD, the exponent β is twice as big as α. Compared with other impeller combinations, PDT as the
Please cite this article as: J. Zhang, et al., Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations, Chin. J. Chem. Eng. (2016), http://dx.doi.org/10.1016/j.cjche.2015.12.008
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(a) N=8 s-1
(b) N=9 s-1
(c) N=10 s-1
(d) N=11 s-1
(e) N=12 s-1
(f) N=13 s-1
Fig. 3. RPD for different impeller combinations at different agitation speeds.
bottom impeller has a better gas dispersion performance because of its lower power number and then higher agitation speed for a given power consumption. WHD as the upper impeller has a better effect to keep
Table 1 Regression results based on correlation (4) Impeller combination
m
b
c
R2
PDT + 2CBYN PDT + 2CBYW PDT + 2WHD HEDT + 2WHD HEDT + 2WHU
1.056 1.025 1.242 1.741 2.340
0.091 0.120 0.180 0.151 0.102
0.070 0.095 0.112 0.122 0.090
0.948 0.958 0.991 0.990 0.994
more bubbles in liquid because of its large blockage area and downpumping operation mode. Thus, the impeller combination PDT + 2WHD can lead to a higher gas holdup, lower RPD and better mass transfer performance than other impeller combinations, and this superiority becomes more evident with the increase of the gas velocity. Another impeller combination HEDT + 2WHU is also worth mentioned that the exponents α and β are relatively small, which can be ascribed to the up-pumping operation mode of the two top impellers. Min et al. [21] have simulated the flow field produced by HEDT + 2WHU used in the present experiments with the result shown in Fig. 5. The impellers discharge gas from the outermost edges of the impellers. The fluid at the level of the middle impeller is moving rapidly in a big Loop 2. This stream carries and
Please cite this article as: J. Zhang, et al., Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations, Chin. J. Chem. Eng. (2016), http://dx.doi.org/10.1016/j.cjche.2015.12.008
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/
(b) uG=0.0078 m·s-1
/
/
(a) uG=0.0039 m·s-1
/
/
(c) uG=0.016 m·s-1
/
/
(d) uG=0.024 m·s-1
/
/
(f) uG=0.039 m·s-1
(e) uG=0.031 m·s-1
Fig. 4. kLa for different impeller combinations and superficial gas velocities.
accelerates upwards bubbles leaving the blade, thus reduces the residence time of bubbles in liquid, leading to a relatively high RPD, low gas holdup and bad mass transfer performance. The
Table 2 Regression results based on correlation (5) Impeller combination
A
α
β
R2
PDT + 2CBYN PDT + 2CBYW PDT + 2WHD HEDT + 2WHD HEDT + 2WHU
2.435 1.668 4.918 2.104 0.931
0.623 0.657 0.394 0.536 0.516
0.769 0.685 0.876 0.730 0.540
0.984 0.993 0.990 0.977 0.975
detailed comparison and analysis of kLa for different impeller combinations will be discussed in the following sections. 3.2.2. Influence of bottom impeller Fig. 6 shows that when uG is in the range of 0.0039–0.0078 m·s−1, kLa for PDT + 2WHD and HEDT + 2WHD is almost the same, indicating that both PDT and HEDT can disperse gas well at low uG. But at higher uG when the frequency of bubble collision and coalescence increases, kLa for PDT + 2WHD is obviously higher than that for HEDT + 2WHD. This is because the agitation speed of PDT + 2WHD is about 1 s−1 higher than that of HEDT + 2WHD for a given power consumption. Such as the specific power consumption Pgm equaling to 1.16 W·kg−1 (corresponding to uG = 0.039 m·s− 1) in Fig. 7, the agitation speed is 10 s− 1 for
Please cite this article as: J. Zhang, et al., Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations, Chin. J. Chem. Eng. (2016), http://dx.doi.org/10.1016/j.cjche.2015.12.008
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Fig. 7. Comparison of Pgm-N for PDT + 2WHD and HEDT + 2WHD.
Fig. 5. Predicted flow field of liquid stirred by HEDT + 2WHU [21].
Fig. 8. Comparison of kLa operated by PDT + 2WHD and PDT + 2CBY.
main reason is that the projection cross-sectional area of WH D is much bigger than that of CBY. The larger contact area of WH D on the liquid flow supplies more fluid circulation in the whole vessel, increasing the refresh frequency of the gas–liquid interfacial area and thus increasing kLa eventually. Fig. 6. Comparison of kLa operated by PDT + 2WHD and HEDT + 2WHD.
PDT + 2WHD and 9 s−1 for HEDT + 2WHD. The stronger shear stress caused by the higher agitation speed can break the big bubbles into small ones more easily. Smaller bubbles can retain in liquid for longer time and increase the gas–liquid interfacial area, thus increase kLa eventually. 3.2.3. Influence of mid and top impellers Fig. 8 shows the comparison of kLa obtained by impeller combinations PDT + 2CBYN, PDT + 2CBYW and PDT + 2WHD, having different upper impellers with gradually increasing blade width. At low u G from 0.0039 to 0.0078 m·s− 1 , k L a is almost the same for all three impeller combinations; but at high superficial gas velocity, kLa for impeller combination PDT + 2WHD is 30% higher than that for PDT + 2CBY, including PDT + 2CBYN and PDT + 2CBYW . The
3.2.4. Influence of operating mode for same impeller Fig. 9 shows that when uG is in the range of 0.0039–0.024 m·s−1, kLa for HEDT + 2WHU and HEDT + 2WHD is almost same for a given power consumption. As uG increases, kLa for HEDT + 2WHD is about 15% higher than that for HEDT + 2WHU. With more gas dispersed, the bubble ascending velocity increases, the residence time of bubbles decreases, with a higher bubble escape velocity from the free surface. Fig. 5 [21] and Fig. 10 [23] show the flow field of liquid stirred by HEDT + 2WHU and HEDT + 2WHD, respectively. The liquid flow field produced by uppumping WHU will accelerate the process of bubbles escaping while the down-pumping WHD can decelerate the escaping process of bubbles. The opposite moving direction between gas and liquid flow produced by HEDT + 2WHD restrains bubble escape and increases the residence time of bubbles in liquid, then increases the gas holdup. Experimental results of Hao et al. [24] indicated that the gas holdup for HEDT + 2WHD is obviously higher than that for HEDT + 2WHU, and the exponent of uG in
Please cite this article as: J. Zhang, et al., Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations, Chin. J. Chem. Eng. (2016), http://dx.doi.org/10.1016/j.cjche.2015.12.008
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Fig. 9. Comparison of kLa operated by HEDT + 2WHU and HEDT + 2WHD.
Fig. 10. Predicted flow field of liquid stirred by HEDT + 2WHD [23].
the gas holdup correlation for HEDT + 2WHD is about twice of that for HEDT + 2WHU. The increasing gas holdup will enlarge kLa; therefore, kLa for HEDT + 2WHD is larger than that for HEDT + 2WHU. 4. Conclusions Five impeller combinations were used to study the effect of impeller combinations on gassed power and kLa at different superficial gas velocities in a baffled stirred reactor. RPD is a vital characteristic in gas–liquid stirred tank design and operation. RPD decreases with increasing gas flow number (FlG) for all impeller combinations. Under all uG and rotation speeds, RPD of HEDT + 2WHU is higher than that of all other combinations. That is mainly because the flow field of liquid produced by HEDT + 2WHU has the same direction of the gas flow, leading to a rapid release of bubbles from the liquid, thus to a reduction in the retained gas and so to a high average mixture density. At low superficial gas velocity, kLa for all impeller combinations under a given gassed power is almost the same. However, the impeller
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combination of PDT + 2WHD shows an obviously superior mass transfer performance at high superficial gas velocity. Several independent factors affecting the mass transfer performance were analyzed, including bottom impeller, combination of two upper impellers, and the operating mode of WH impeller. As the bottom impeller playing an important role in gas dispersion, PDT has a higher agitation speed leading to a higher shear rate than HEDT for a given power consumption. The higher shear rate can break the big bubbles into small ones more easily, thus increase the gas–liquid interfacial area, and be beneficial to the mass transfer performance. Blade width is another important parameter affecting kLa. WH has a larger project area than CBYN and CBYW, offering a better gas–liquid flow circulation and increasing contact time and the interfacial refreshment rate for gas and liquid. Considering the superiorities of both the bottom impeller and the blade width of two upper impellers, PDT + 2WHD is superior to other combinations, especially when the gas rate is high. In addition, the down pumping impeller produces downward axial flow to increase the bubble residence time, making PDT + 2WHD attractive. Finally, based on the experimental data, the regressed correlations of NPG with the Froude number and gas flow number and that of k L a with specific power consumption and superficial gas velocity for five different impeller combinations were obtained and analyzed, providing helpful guidance in industrial stirred tank design. Nomenclature CAi equilibrium concentration of oxygen at the gas–liquid interface, mol·m−3 CAs oxygen concentration with sulfite solution fed continuously, mol·m−3 Cs concentration of sulfite in the feed solution, mol·m−3 D impeller diameter, m FlG gas flow number, FlG = Qg/ND3 Fr Froude number; Fr = N2D/g g gravitational constant, m·s−2 H height of the liquid without gas input, m Hs depth of the sparger below the free surface, m HT height of the tank, m kLa volumetric mass transfer coefficient, m·s−1 M torque, N·m−1 N rotation speed, s−1 NCD just complete dispersion speed, s−1 NP power number, NP = P0/ρLN3D5 NPG gassed power number, NPG = Pg/ρLN3D5 Pe potential energy of sparged gas, W Pg gassed agitation power, W Pgm agitation power per mass, W·kg−1 PT total power consumption, W PTm mean total specific energy dissipation rate, W·kg−1 P0 ungassed agitation power, W Qg inlet gas flow rate, m3·s−1 Qs feed rate of the sulfite solution, m3·s−1 RPD ratio of gassed to ungassed power, RPD = Pg/P0, – T tank diameter, m uG superficial gas velocity, m·s−1 VL volume of liquid phase, m3 α,β exponent in correlation (5) ρL liquid density, kg·m−3 π constant References [1] T. Moucha, V. Linek, E. Prokopova, Gas hold-up, mixing time and gas–liquid volumetric mass transfer coefficient of various multiple-impeller combinations: Rushton turbine, pitched blade and Techmix impeller and their combinations, Chem. Eng. Sci. 58 (9) (2003) 1839–1846.
Please cite this article as: J. Zhang, et al., Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations, Chin. J. Chem. Eng. (2016), http://dx.doi.org/10.1016/j.cjche.2015.12.008
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[2] M. Fujasová, V. Linek, T. Moucha, Mass transfer correlations for multiple-impeller gas–liquid contactors. Analysis of the effect of axial dispersion in gas and liquid phase on “local” kLa values measured by the dynamic pressure method in individual sections of the vessel, Chem. Eng. Sci. 62 (6) (2007) 1650–1669. [3] L. Chen, Y.Y. Bao, Z.M. Gao, Void fraction distributions in cold-gassed and hotsparged three phase stirred tanks with multi-impeller, Chin. J. Chem. Eng. 17 (6) (2009) 887–895. [4] S.F. Yang, X.Y. Li, G. Deng, C. Yang, Z.S. Mao, Application of KHX impeller in a lowshear stirred bioreactor, Chin. J. Chem. Eng. 22 (10) (2014) 1072–1077. [5] Y.Y. Bao, J. Yang, B.J. Wang, Z.M. Gao, Influence of impeller diameter on local gas dispersion properties in a sparged multi-impeller stirred tank, Chin. J. Chem. Eng. 23 (4) (2015) 415–422. [6] J.M.T. Vasconcelos, S.C.P. Orvalho, A.M.A.F. Rodrigues, S.S. Alves, Effect of blade shape on the performance of six blade disk turbine impellers, Ind. Eng. Chem. Res. 39 (1) (2000) 203–213. [7] J. Zhao, Z.M. Gao, Y.Y. Bao, Effects of the blade shape on the trailing vortices in liquid flow generated by disc turbines, Chin. J. Chem. Eng. 19 (2) (2011) 232–242. [8] S.J. Arjunwadkar, K. Sarvanan, P.R. Kulkarni, A.B. Pandit, Gas–liquid mass transfer in dual impeller bioreactor, Biochem. Eng. J. 1 (2) (1998) 99–106. [9] H. Wu, V. Arcella, M. Malavasi, A study of gas–liquid mass transfer in reactors with two disk turbines, Chem. Eng. Sci. 53 (5) (1998) 1089–1095. [10] D. Garcia-Cortes, C. Xuereb, P. Taillandier, U. Jauregui-Haza, J. Bertrand, Effect of dual impeller-sparged geometry on the hydrodynamic and mass transfer in stirred vessels, Chem. Eng. Technol. 27 (9) (2004) 988–999. [11] M. Nocentini, D. Fajner, G. Pasquali, F. Magelli, Gas–liquid mass transfer and hold-up in vessels stirred with multiple Rushton turbines: water and water–glycerol solutions, Ind. Eng. Chem. Res. 32 (1) (1993) 19–26. [12] M.H. Xie, J.Y. Xia, Z. Zhou, G.Z. Zhou, J. Chu, Y.P. Zhuang, S.L. Zhang, Flow pattern, mixing, gas hold-up and mass transfer coefficient of triple-impeller configurations in stirred tank bioreactors, Ind. Eng. Chem. Res. 53 (14) (2014) 5941–5953.
[13] M.S. Puthli, V.K. Rathod, A.B. Pandit, Gas–liquid mass transfer studies with triple impeller system on a laboratory scale bioreactor, Biochem. Eng. J. 23 (2005) 25–30. [14] X.Y. Li, G.Z. Yu, C. Yang, Z.S. Mao, Experimental study on surface aerators stirred by triple impellers, Ind. Eng. Chem. Res. 48 (18) (2009) 8752–8756. [15] V. Linek, T. Moucha, J. Sinkule, Gas–liquid mass transfer in vessels stirred with multiple impellers — I. Gas–liquid mass transfer characteristics in individual stages, Chem. Eng. Sci. 51 (12) (1996) 3203–3212. [16] N. Suhaili, J.S. Tan, M. Mohamed, M. Halim, A.B. Ariff, Effects of dual impeller system of Rushton turbine, concave disk turbine and their combinations on the performance of kojic acid fermentation by Aspergillus flavus Link 44-1, Asia Pac. J. Chem. Eng. 10 (2015) 65–74. [17] J. Markopoulos, C. Christofi, I. Katsinaris, Mass transfer coefficients in mechanically agitated gas–liquid contactors, Chem. Eng. Technol. 30 (7) (2007) 829–834. [18] Y. Imai, H. Takei, M. Matsumura, A simple Na2SO3 feeding method for kLa measurement in large-scale fermentors, Biotechnol. Bioeng. 29 (8) (1987) 982–993. [19] A.W. Nienow, M. Konno, W. Bujalski, Studies on three-phase mixing: a review and recent results, Chem. Eng. Res. Des. 64 (1) (1986) 35–42. [20] J.G. Long, Y.Y. Bao, Z.M. Gao, Gas–liquid dispersion in a stirred tank with different impeller combinations, J. Beijing Univ. Chem. Technol. 32 (5) (2005) 1–5 (in Chinese). [21] J. Min, Y.Y. Bao, L. Chen, Z.M. Gao, J.M. Smith, Numerical simulation of gas dispersion in an aerated stirred reactor with multiple impellers, Ind. Eng. Chem. Res. 47 (18) (2008) 7112–7117. [22] C.M. Cooper, G.A. Fernstrom, S.A. Miller, Performance of agitated gas–liquid contactors, Ind. Eng. Chem. 36 (6) (1944) 504–509. [23] J. Zhao, X.N. Cheng, Z.M. Gao, Experimental study and numerical simulation of fluid flow in a liquid multiple impeller stirred tank, J. Beijing Univ. Chem. Technol. 38 (3) (2011) 22–27 (in Chinese). [24] Z.G. Hao, Y.Y. Bao, Z.M. Gao, Gas–liquid dispersion in a multi-impeller stirred tank, J. Chem. Eng. Chin. Univ. 18 (5) (2004) 547–552 (in Chinese).
Please cite this article as: J. Zhang, et al., Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations, Chin. J. Chem. Eng. (2016), http://dx.doi.org/10.1016/j.cjche.2015.12.008
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Chinese Journal of Chemical Engineering journal homepage: www.elsevier.com/locate/CJChE
Fluid Dynamics and Transport Phenomena
Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations☆ Jinjin Zhang, Zhengming Gao, Yating Cai, Ziqi Cai, Jie Yang ⁎, Yuyun Bao ⁎ State Key Laboratory of Chemical Resource Engineering, School of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e
i n f o
Article history: Received 11 February 2015 Received in revised form 26 June 2015 Accepted 28 June 2015 Available online xxxx Keywords: Stirred vessel Gassed power Mass transfer Triple-impeller combination Gas–liquid system
a b s t r a c t The gassed power demand and volumetric mass transfer coefficient (kLa) were investigated in a fully baffled, dished-base stirred vessel with a diameter of 0.30 m agitated by five triple-impeller combinations. Six types of impellers (six-half-elliptical-blade disk turbine (HEDT), four-wide-blade hydrofoil impeller (WH) pumping down (D) and pumping up (U), parabolic-blade disk turbine (PDT), and CBY narrow blade (N) and wide blade (W)) were used to form five combinations identified by PDT + 2CBYN, PDT + 2CBYW, PDT + 2WHD, HEDT + 2WHD and HEDT + 2WHU, respectively. The results show that the relative power demand of HEDT + 2WHU is higher than that of other four impeller combinations under all operating conditions. At low superficial gas velocity (uG), kLa differences among impeller combinations are not obvious. However, when uG is high, PDT + 2WHD shows the best mass transfer performance and HEDT + 2WHU shows the worst mass transfer performance under all operating conditions. At high uG and a given power input, the impeller combinations with high agitation speed and big projection cross-sectional area lead to relatively high values of kLa. Based on the experimental data, the regressed correlations of gassed power number with Froude number and gas flow number, and kLa with power consumption and superficial gas velocity are obtained for five different impeller combinations, which could be used as guidance for industrial design. © 2016 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.
1. Introduction Mechanically agitated gas–liquid reactors with single or multiple impellers are often used to intensify the contact between gas and liquid. Hydrogenation, chlorination, polymerization, sewage treatment, and aerobic fermentation are examples of such processes. Since the mass transfer of gas into liquid is often the rate-limiting step when low solubility gases are involved, the volumetric mass transfer coefficient, kLa, is considered a key parameter for operation, design, and scale-up of stirred reactors. Based on the flow patterns generated, an impeller can be characterized as an axial flow impeller such as marine propeller, Techmix 335 (TX) [1], Lightnin A315 (LTN) [2], four-wide-blade hydrofoil impeller (WH) [3], KHX [4] and CBY [5], and a radial flow impeller such as Rushton turbine (RT), pitched blade (PB), concave-bladed disk turbine (CD) [6], half elliptical-blade disk turbine (HEDT) [7], parabolic blade disk turbine (PDT) [5] and Narcissus (NS) [2]. Compared with the stirred reactors equipped with a single impeller, the dual-impeller [8–10] and triple-impeller [11–14] stirred reactors have such advantages as increased gas hold-up, superior gas distribution, long residence of gas bubbles, improved liquid circulation characteristics, low power
☆ Supported by the National Natural Science Foundation of China (21206002, 21376016). ⁎ Corresponding authors. E-mail addresses: [email protected] (J. Yang), [email protected] (Y. Bao).
consumption per impeller, and even distribution of shear stress and energy dissipation, thus, effective gas utilization. Over the past few decades, many publications presenting various hydraulic and mass transfer characteristics in multi-impeller stirred vessels can be found in the literature. Nocentini et al. [11] and Linek et al. [15] used multiple Rushton turbines on a common shaft. Arjunwadkar et al. [8] used dual impeller of RT and PB, and Suhaili et al. [16] used dual impeller of RT and CD. kLa for various impeller combinations was discussed by Moucha et al. [1] using 18 combinations of RT, PB and TX and their combinations, and Fujasová et al. [2] using 28 combinations of RT, PB, TX, LNT and NS and their combinations. Results show that radial flow impellers exhibit 20% to 50% higher oxygen transfer efficiency than axial flow impellers. However, radial flow impellers have the weakness of bad homogenization [2,12], and relatively high power consumption [17]. Compared with radial flow impeller combinations or axial flow impeller combinations, the mixed flow impeller combinations with both radial and axial flow impellers showed better homogenization performance and mass transfer performance at the same power consumption. However, few reports present the mass transfer characteristics for mixed flow impeller combinations. In the mixed flow impeller combinations, the radial flow impeller is often installed in the bottom to break and disperse the gas introduced from gas sparger and the axial flow impellers are installed above to circulate the gas–liquid flow. RT is the radial flow impeller usually used [1,2,12], but it has weakness of significant drop in gassed power draw. This disadvantage can be tackled by retrofitting of RT with streamlined impellers, such as impellers of CD, HEDT and PDT.
http://dx.doi.org/10.1016/j.cjche.2015.12.008 1004-9541/© 2016 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.
Please cite this article as: J. Zhang, et al., Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations, Chin. J. Chem. Eng. (2016), http://dx.doi.org/10.1016/j.cjche.2015.12.008
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However, research on the mass transfer characteristics for these modified impellers is quite rare [12], especially for impellers HEDT and PDT. Moreover, the superficial gas velocity (uG) for kLa determination reported in multi-impeller stirred reactors is usually small (less than 0.016 m·s− 1) and the research on kLa at higher uG (more than 0.016 m·s− 1) is open for further investigation. To date, data on kLa for the mixed flow impeller combinations are still not enough, especially for the new-type impeller combinations and at high gas rates. The aim of this work is to provide gassed power demand and mass transfer characteristics for five mixed flow impeller combinations with new-type axial flow impellers of WH (pumping down and pumping up) and CBY (narrow blade and wide blade) with radial flow impellers of HEDT and PDT at uG between 0.0078 and 0.039 m·s− 1. General correlations representing the dependence of gassed power and kLa on operating conditions are developed. kLa differences among these mixed flow impeller combinations are discussed and analyzed.
2. Experimental 2.1. Experimental setup All experiments were carried out in a dished-base cylindrical vessel equipped with triple impellers, as shown in Fig. 1. The diameter (T) of the tank was 0.30 m and the height of the tank (HT) was 0.75 m. The ungassed liquid level was maintained at a height H = 1.8 T with a deionized water volume of 0.036 m3. As a standard configuration, four 0.03 m-wide baffles were symmetrically mounted in the tank. The impellers used were six-half-elliptical-blade disk turbine (HEDT), parabolic-blade disk turbine (PDT), four-wide-blade hydrofoil impeller (WH), and CBY in Fig. 2. The diameter of all the impellers (D) was 0.4 T, the same as the clearance between the lowest impeller and the base of the tank, and the distance between two adjacent impellers was 0.48 T. WH can be used for up-pumping and down-pumping operating modes, identified as WHU and WHD, respectively. CBY can be classified as CBYN and CBYW by the impeller tip width of 0.1D and 0.14D, respectively. The shorthand notation used for defining the agitation combinations is straightforward: PDT + 2CBYN means parabolic-blade disk turbine at the bottom and two CBY impellers with an impeller tip width of 0.1D at mid-level and top-level. PDT + 2CBYW , PDT + 2WHD , HEDT + 2WH D and HEDT + 2WHU represent similar implications. Air was introduced from a sparger located at a height of 0.33 T above the tank bottom. The ring sparger was 0.8D in diameter with 27 symmetrical downward-directed holes of 0.002 m diameter.
Fig. 2. Impellers used in the experiment: (a) HEDT; (b) PDT; (c) WH; and (d) CBY.
The first bottle of Na 2 SO 3 solution with a concentration of 500 mol·m− 3 was used to absorb the oxygen in the air so that no variation of Na2SO3 concentration in the second bottle would be resulted. The Na2SO3 solution with a concentration of 200 mol·m− 3 in the second bottle was fed into the tank by a peristaltic pump, and the ion concentration in the stirred tank was kept below 50 mol-ion·m− 3 to maintain the water as a coalescent system [18]. The concentration of dissolved oxygen was measured online by a DO probe (VISIFERM DO Arc 120, Hamilton, Switzerland) and recorded with time serial as data files by the computer. 2.2. Power consumption The power consumption of the impeller was measured by using a rotary torque sensor (AKC-205, China Academy of Aerospace Aerodynamics, China) installed between the impeller shaft and the motor bearings. The specific power consumption PTm (in W·kg−1), which is the sum of the potential energy of the sparged gas and the agitation power consumption calculated by measuring the torque of the stirring shaft and the agitation speed, is an important parameter to characterize the performance of an agitator. The total power consumption of the stirring system was calculated by PT ¼ Pg þ Pe
ð1Þ
P Tm ¼ P T =ρL V L
ð2Þ
where Pg is the gassed agitation power given by Pg = 2πNM and Pe is the potential energy of sparged gas calculated by Pe = ρL gHSQ g. 2.3. Measurement of kLa kLa was measured by the steady-state sulfite feeding method (SFM) [18], which is based on the measurement of dissolved oxygen concentrations under steady-state conditions of equilibrium between sulfite addition and oxygen dissolution. kLa is calculated by kL a ¼
Fig. 1. Experimental setup: (1) 1st bottle of Na2SO3 solution; (2) 2nd bottle of Na2SO3 solution; (3) peristaltic pump; (4) motor; (5) rotary torque sensor; (6) sparger; (7) DO probe; and (8) computer.
Q sCs 2V L C Ai ð1−C As =C Ai Þ
ð3Þ
where Q s is the feed rate of the sulfite solution, m3·s−1; Cs is the concentration of sulfite in the feed solution, mol·m−3; VL is the volume of the liquid in the aerated stirred tank, m3; CAi is the equilibrium
Please cite this article as: J. Zhang, et al., Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations, Chin. J. Chem. Eng. (2016), http://dx.doi.org/10.1016/j.cjche.2015.12.008
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concentration of the oxygen at the gas–liquid interface, mol·m−3; and CAs is the oxygen concentration measured in water in steady state with the sulfite solution fed continuously, mol·m−3. The experimental results are reproducible twice or thrice with an average relative error less than 5%.
much lower than HEDT + 2WHD at the same operation conditions. This indicated that the bottom impeller can influence greatly the power consumption. Nevertheless, the similar RPD for PDT + 2WHD and HEDT + 2WHD shows that the influence of the bottom impeller on RPD is little.
3. Results and Discussion
3.1.2. Influence of blade width The effect of the blade width on RPD can be seen by the comparison of RPD for PDT + 2CBYN, PDT + 2CBYW and PDT + 2WHD, having the same bottom impeller and increasing width of blade for two upper impellers. Table 1 shows that both b and c increase with increasing blade width of upper impellers from CBYN, CBYW to WHD with the same bottom PDT impeller. The increasing values of b and c imply that RPD decreases more when more gas is introduced or stirred at higher agitation speed. It could be explained that the wider blade of upper impellers can supply bigger interaction area for both liquid and gas, introduce more gas into the liquid and keep bubbles staying for a longer time in liquid, thus the gas holdup increases, so RPD decreases more when FlG or Fr increases.
The superficial gas velocity in this work ranges from 0.0039 to 0.039 m·s−1 (equivalent to 0.463–4.63 vvm). We labeled the superficial gas velocity of 0.0039 to 0.0078 m·s−1 as low gas velocity, and 0.024 to 0.039 m·s−1 as high gas velocity based on the criterion for industrial operation. The impeller speeds for each impeller combination were above the complete gas dispersion speed (NCD) defined by Nienow et al. [19] during all of the measurements of power demand and volumetric mass transfer coefficient. The highest NCD at the highest superficial gas velocity of 0.039 m·s−1 for all impeller combinations used is 6.83 s−1, and the impeller speed used in the experiments was chosen from 8 to 13 s−1. 3.1. Relative power demand (RPD) The ratio of gassed to ungassed power (RPD, RPD = Pg/P0) for different impeller combinations at various agitation speeds is shown in Fig. 3. It can be seen that RPD decreases with increasing gas flow number (FlG) for all impeller combinations because as more gas is introduced into the stirred tank, the gas holdup increases and the average density of the gas–liquid mixture phase decreases. The decreasing average density causes the decrease of RPD with increasing FlG. Under all operating conditions, RPD of all impeller combinations is mostly above 0.65, higher than those of impeller combinations only using the radial impellers or axial impellers reported in the literature [20], meanwhile RPD of axial impellers 3WHD decreases dramatically to 0.4–0.5 when FlG is larger than 0.05 and RPD of radial impellers 3CD is smaller than 0.55 when Fl G is larger than 0.10. The higher RPD obtained in this research shows mixed flow impeller combinations having higher energy utilization efficiency and less loss of capacity for mass transfer. Power number (NP = P0/ρLN3D5) is one of the most basic parameters for stirred reactors and the gassed power number (NPG = Pg/ρLN3D5) is for the power number under aeration. Based on the experimental data of gassed power under different conditions, we use the following correlation −b
NPG ¼ mFlG Fr −c
ð4Þ
to regress NPG with the Froude number and gas flow number for different impeller combinations, and the results are shown in Table 1. Exponent b indicates the sensitivity of RPD on gas flow for different impeller combinations and is affected by the cavities behind impeller blades or/and the average density of the medium. The effects of gas flow and agitation speed on RPD for different impeller combinations will be discussed from the following three aspects. 3.1.1. Influence of bottom impeller In Fig. 3, RPD for PDT + 2WHD and HEDT + 2WHD presents the similar trend for different agitation speeds although with different bottom impellers. Both PDT and HEDT impellers are streamlined modification of RT with different curvatures. Vasconcelos et al. [6] studied the effect of blade shape on the performance of impellers and found that the blade streamlining can lead to a lower ungassed power number (NP). Therefore, it can be referred that NP for PDT is lower than HEDT. In this work, NPG for these two impeller combinations of PDT + 2WHD and HEDT + 2WHD was regressed in Table 1, while NP for PDT + 2WHD and HEDT + 2WHD without aeration was also obtained as 2.8 and 3.6, respectively. It can be seen that both NP and NPG for PDT + 2WHD are
3.1.3. Influence of up- and down-pumping operating modes The obviously different RPD trends for HEDT + 2WH U and HEDT + 2WHD show that the operating mode for the axial upper impellers can influence RPD greatly. RPD for HEDT + 2WHU are all above 0.75, much higher than those for other impeller combinations. That is because the flow field of liquid caused by HEDT + 2WHU , which was computed by Min et al. [21], has the same direction of the gas flow, leading to a rapid release of bubbles from the liquid, thus to a reduction in the retained gas and further to a high average medium density. The smaller exponent b for HEDT + 2WHU than HEDT + 2WHD in Table 1 indicates that the gas flow rate has less influence on RPD for HEDT + 2WHU than HEDT + 2WHD because of the shorter residence time of retained gas in liquid and lower gas holdup for HEDT + 2WHU. From the comparison of the different RPD trends for HEDT + 2WHU and HEDT + 2WH D, and the similar RPD for PDT + 2WHD and HEDT + 2WHD, it can be seen that two upper impellers play a more important role on the change of RPD than the bottom impeller. 3.2. Volumetric mass transfer coefficient (kLa) 3.2.1. Influence of impeller combinations at various superficial gas velocities kLa for different impeller combinations at various superficial gas velocities (uG) is shown in Fig. 4. It can be seen that for a given specific power consumption, kLa changes only about 10% for five different impeller combinations when u G is small (e.g. u G = 0.0039– 0.0078 m·s − 1 ). However, with u G increasing from 0.016 to 0.039 m·s− 1, the difference of kLa for five different impeller combinations increases gradually from 20% to as high as 80%. The impeller combination of PDT + 2WHD has the dominant mass transfer performance when uG ranges from 0.0078 to 0.039 m·s−1 in this work. It is also worth mentioning that the only up-pumping mode impeller combination HEDT + 2WHU presents much smaller mass transfer coefficient than other impeller combinations when uG is high (e.g. uG = 0.031–0.039 m·s−1). The regression results of kLa for different impeller combinations based on correlation (5) proposed by Cooper et al. [22] are shown in Table 2. β kL a ¼ AP α Tm uG
ð5Þ
Table 2 shows that the exponent β is bigger than α for all impeller combinations, indicating that it is more efficient to increase kLa by increasing the gas velocity than by increasing the power input, especially for the impeller combination of PDT + 2WHD, the exponent β is twice as big as α. Compared with other impeller combinations, PDT as the
Please cite this article as: J. Zhang, et al., Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations, Chin. J. Chem. Eng. (2016), http://dx.doi.org/10.1016/j.cjche.2015.12.008
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(a) N=8 s-1
(b) N=9 s-1
(c) N=10 s-1
(d) N=11 s-1
(e) N=12 s-1
(f) N=13 s-1
Fig. 3. RPD for different impeller combinations at different agitation speeds.
bottom impeller has a better gas dispersion performance because of its lower power number and then higher agitation speed for a given power consumption. WHD as the upper impeller has a better effect to keep
Table 1 Regression results based on correlation (4) Impeller combination
m
b
c
R2
PDT + 2CBYN PDT + 2CBYW PDT + 2WHD HEDT + 2WHD HEDT + 2WHU
1.056 1.025 1.242 1.741 2.340
0.091 0.120 0.180 0.151 0.102
0.070 0.095 0.112 0.122 0.090
0.948 0.958 0.991 0.990 0.994
more bubbles in liquid because of its large blockage area and downpumping operation mode. Thus, the impeller combination PDT + 2WHD can lead to a higher gas holdup, lower RPD and better mass transfer performance than other impeller combinations, and this superiority becomes more evident with the increase of the gas velocity. Another impeller combination HEDT + 2WHU is also worth mentioned that the exponents α and β are relatively small, which can be ascribed to the up-pumping operation mode of the two top impellers. Min et al. [21] have simulated the flow field produced by HEDT + 2WHU used in the present experiments with the result shown in Fig. 5. The impellers discharge gas from the outermost edges of the impellers. The fluid at the level of the middle impeller is moving rapidly in a big Loop 2. This stream carries and
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/
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/
/
(b) uG=0.0078 m·s-1
/
/
(a) uG=0.0039 m·s-1
/
/
(c) uG=0.016 m·s-1
/
/
(d) uG=0.024 m·s-1
/
/
(f) uG=0.039 m·s-1
(e) uG=0.031 m·s-1
Fig. 4. kLa for different impeller combinations and superficial gas velocities.
accelerates upwards bubbles leaving the blade, thus reduces the residence time of bubbles in liquid, leading to a relatively high RPD, low gas holdup and bad mass transfer performance. The
Table 2 Regression results based on correlation (5) Impeller combination
A
α
β
R2
PDT + 2CBYN PDT + 2CBYW PDT + 2WHD HEDT + 2WHD HEDT + 2WHU
2.435 1.668 4.918 2.104 0.931
0.623 0.657 0.394 0.536 0.516
0.769 0.685 0.876 0.730 0.540
0.984 0.993 0.990 0.977 0.975
detailed comparison and analysis of kLa for different impeller combinations will be discussed in the following sections. 3.2.2. Influence of bottom impeller Fig. 6 shows that when uG is in the range of 0.0039–0.0078 m·s−1, kLa for PDT + 2WHD and HEDT + 2WHD is almost the same, indicating that both PDT and HEDT can disperse gas well at low uG. But at higher uG when the frequency of bubble collision and coalescence increases, kLa for PDT + 2WHD is obviously higher than that for HEDT + 2WHD. This is because the agitation speed of PDT + 2WHD is about 1 s−1 higher than that of HEDT + 2WHD for a given power consumption. Such as the specific power consumption Pgm equaling to 1.16 W·kg−1 (corresponding to uG = 0.039 m·s− 1) in Fig. 7, the agitation speed is 10 s− 1 for
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Fig. 7. Comparison of Pgm-N for PDT + 2WHD and HEDT + 2WHD.
Fig. 5. Predicted flow field of liquid stirred by HEDT + 2WHU [21].
Fig. 8. Comparison of kLa operated by PDT + 2WHD and PDT + 2CBY.
main reason is that the projection cross-sectional area of WH D is much bigger than that of CBY. The larger contact area of WH D on the liquid flow supplies more fluid circulation in the whole vessel, increasing the refresh frequency of the gas–liquid interfacial area and thus increasing kLa eventually. Fig. 6. Comparison of kLa operated by PDT + 2WHD and HEDT + 2WHD.
PDT + 2WHD and 9 s−1 for HEDT + 2WHD. The stronger shear stress caused by the higher agitation speed can break the big bubbles into small ones more easily. Smaller bubbles can retain in liquid for longer time and increase the gas–liquid interfacial area, thus increase kLa eventually. 3.2.3. Influence of mid and top impellers Fig. 8 shows the comparison of kLa obtained by impeller combinations PDT + 2CBYN, PDT + 2CBYW and PDT + 2WHD, having different upper impellers with gradually increasing blade width. At low u G from 0.0039 to 0.0078 m·s− 1 , k L a is almost the same for all three impeller combinations; but at high superficial gas velocity, kLa for impeller combination PDT + 2WHD is 30% higher than that for PDT + 2CBY, including PDT + 2CBYN and PDT + 2CBYW . The
3.2.4. Influence of operating mode for same impeller Fig. 9 shows that when uG is in the range of 0.0039–0.024 m·s−1, kLa for HEDT + 2WHU and HEDT + 2WHD is almost same for a given power consumption. As uG increases, kLa for HEDT + 2WHD is about 15% higher than that for HEDT + 2WHU. With more gas dispersed, the bubble ascending velocity increases, the residence time of bubbles decreases, with a higher bubble escape velocity from the free surface. Fig. 5 [21] and Fig. 10 [23] show the flow field of liquid stirred by HEDT + 2WHU and HEDT + 2WHD, respectively. The liquid flow field produced by uppumping WHU will accelerate the process of bubbles escaping while the down-pumping WHD can decelerate the escaping process of bubbles. The opposite moving direction between gas and liquid flow produced by HEDT + 2WHD restrains bubble escape and increases the residence time of bubbles in liquid, then increases the gas holdup. Experimental results of Hao et al. [24] indicated that the gas holdup for HEDT + 2WHD is obviously higher than that for HEDT + 2WHU, and the exponent of uG in
Please cite this article as: J. Zhang, et al., Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations, Chin. J. Chem. Eng. (2016), http://dx.doi.org/10.1016/j.cjche.2015.12.008
J. Zhang et al. / Chinese Journal of Chemical Engineering xxx (2016) xxx–xxx
Fig. 9. Comparison of kLa operated by HEDT + 2WHU and HEDT + 2WHD.
Fig. 10. Predicted flow field of liquid stirred by HEDT + 2WHD [23].
the gas holdup correlation for HEDT + 2WHD is about twice of that for HEDT + 2WHU. The increasing gas holdup will enlarge kLa; therefore, kLa for HEDT + 2WHD is larger than that for HEDT + 2WHU. 4. Conclusions Five impeller combinations were used to study the effect of impeller combinations on gassed power and kLa at different superficial gas velocities in a baffled stirred reactor. RPD is a vital characteristic in gas–liquid stirred tank design and operation. RPD decreases with increasing gas flow number (FlG) for all impeller combinations. Under all uG and rotation speeds, RPD of HEDT + 2WHU is higher than that of all other combinations. That is mainly because the flow field of liquid produced by HEDT + 2WHU has the same direction of the gas flow, leading to a rapid release of bubbles from the liquid, thus to a reduction in the retained gas and so to a high average mixture density. At low superficial gas velocity, kLa for all impeller combinations under a given gassed power is almost the same. However, the impeller
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combination of PDT + 2WHD shows an obviously superior mass transfer performance at high superficial gas velocity. Several independent factors affecting the mass transfer performance were analyzed, including bottom impeller, combination of two upper impellers, and the operating mode of WH impeller. As the bottom impeller playing an important role in gas dispersion, PDT has a higher agitation speed leading to a higher shear rate than HEDT for a given power consumption. The higher shear rate can break the big bubbles into small ones more easily, thus increase the gas–liquid interfacial area, and be beneficial to the mass transfer performance. Blade width is another important parameter affecting kLa. WH has a larger project area than CBYN and CBYW, offering a better gas–liquid flow circulation and increasing contact time and the interfacial refreshment rate for gas and liquid. Considering the superiorities of both the bottom impeller and the blade width of two upper impellers, PDT + 2WHD is superior to other combinations, especially when the gas rate is high. In addition, the down pumping impeller produces downward axial flow to increase the bubble residence time, making PDT + 2WHD attractive. Finally, based on the experimental data, the regressed correlations of NPG with the Froude number and gas flow number and that of k L a with specific power consumption and superficial gas velocity for five different impeller combinations were obtained and analyzed, providing helpful guidance in industrial stirred tank design. Nomenclature CAi equilibrium concentration of oxygen at the gas–liquid interface, mol·m−3 CAs oxygen concentration with sulfite solution fed continuously, mol·m−3 Cs concentration of sulfite in the feed solution, mol·m−3 D impeller diameter, m FlG gas flow number, FlG = Qg/ND3 Fr Froude number; Fr = N2D/g g gravitational constant, m·s−2 H height of the liquid without gas input, m Hs depth of the sparger below the free surface, m HT height of the tank, m kLa volumetric mass transfer coefficient, m·s−1 M torque, N·m−1 N rotation speed, s−1 NCD just complete dispersion speed, s−1 NP power number, NP = P0/ρLN3D5 NPG gassed power number, NPG = Pg/ρLN3D5 Pe potential energy of sparged gas, W Pg gassed agitation power, W Pgm agitation power per mass, W·kg−1 PT total power consumption, W PTm mean total specific energy dissipation rate, W·kg−1 P0 ungassed agitation power, W Qg inlet gas flow rate, m3·s−1 Qs feed rate of the sulfite solution, m3·s−1 RPD ratio of gassed to ungassed power, RPD = Pg/P0, – T tank diameter, m uG superficial gas velocity, m·s−1 VL volume of liquid phase, m3 α,β exponent in correlation (5) ρL liquid density, kg·m−3 π constant References [1] T. Moucha, V. Linek, E. Prokopova, Gas hold-up, mixing time and gas–liquid volumetric mass transfer coefficient of various multiple-impeller combinations: Rushton turbine, pitched blade and Techmix impeller and their combinations, Chem. Eng. Sci. 58 (9) (2003) 1839–1846.
Please cite this article as: J. Zhang, et al., Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations, Chin. J. Chem. Eng. (2016), http://dx.doi.org/10.1016/j.cjche.2015.12.008
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[2] M. Fujasová, V. Linek, T. Moucha, Mass transfer correlations for multiple-impeller gas–liquid contactors. Analysis of the effect of axial dispersion in gas and liquid phase on “local” kLa values measured by the dynamic pressure method in individual sections of the vessel, Chem. Eng. Sci. 62 (6) (2007) 1650–1669. [3] L. Chen, Y.Y. Bao, Z.M. Gao, Void fraction distributions in cold-gassed and hotsparged three phase stirred tanks with multi-impeller, Chin. J. Chem. Eng. 17 (6) (2009) 887–895. [4] S.F. Yang, X.Y. Li, G. Deng, C. Yang, Z.S. Mao, Application of KHX impeller in a lowshear stirred bioreactor, Chin. J. Chem. Eng. 22 (10) (2014) 1072–1077. [5] Y.Y. Bao, J. Yang, B.J. Wang, Z.M. Gao, Influence of impeller diameter on local gas dispersion properties in a sparged multi-impeller stirred tank, Chin. J. Chem. Eng. 23 (4) (2015) 415–422. [6] J.M.T. Vasconcelos, S.C.P. Orvalho, A.M.A.F. Rodrigues, S.S. Alves, Effect of blade shape on the performance of six blade disk turbine impellers, Ind. Eng. Chem. Res. 39 (1) (2000) 203–213. [7] J. Zhao, Z.M. Gao, Y.Y. Bao, Effects of the blade shape on the trailing vortices in liquid flow generated by disc turbines, Chin. J. Chem. Eng. 19 (2) (2011) 232–242. [8] S.J. Arjunwadkar, K. Sarvanan, P.R. Kulkarni, A.B. Pandit, Gas–liquid mass transfer in dual impeller bioreactor, Biochem. Eng. J. 1 (2) (1998) 99–106. [9] H. Wu, V. Arcella, M. Malavasi, A study of gas–liquid mass transfer in reactors with two disk turbines, Chem. Eng. Sci. 53 (5) (1998) 1089–1095. [10] D. Garcia-Cortes, C. Xuereb, P. Taillandier, U. Jauregui-Haza, J. Bertrand, Effect of dual impeller-sparged geometry on the hydrodynamic and mass transfer in stirred vessels, Chem. Eng. Technol. 27 (9) (2004) 988–999. [11] M. Nocentini, D. Fajner, G. Pasquali, F. Magelli, Gas–liquid mass transfer and hold-up in vessels stirred with multiple Rushton turbines: water and water–glycerol solutions, Ind. Eng. Chem. Res. 32 (1) (1993) 19–26. [12] M.H. Xie, J.Y. Xia, Z. Zhou, G.Z. Zhou, J. Chu, Y.P. Zhuang, S.L. Zhang, Flow pattern, mixing, gas hold-up and mass transfer coefficient of triple-impeller configurations in stirred tank bioreactors, Ind. Eng. Chem. Res. 53 (14) (2014) 5941–5953.
[13] M.S. Puthli, V.K. Rathod, A.B. Pandit, Gas–liquid mass transfer studies with triple impeller system on a laboratory scale bioreactor, Biochem. Eng. J. 23 (2005) 25–30. [14] X.Y. Li, G.Z. Yu, C. Yang, Z.S. Mao, Experimental study on surface aerators stirred by triple impellers, Ind. Eng. Chem. Res. 48 (18) (2009) 8752–8756. [15] V. Linek, T. Moucha, J. Sinkule, Gas–liquid mass transfer in vessels stirred with multiple impellers — I. Gas–liquid mass transfer characteristics in individual stages, Chem. Eng. Sci. 51 (12) (1996) 3203–3212. [16] N. Suhaili, J.S. Tan, M. Mohamed, M. Halim, A.B. Ariff, Effects of dual impeller system of Rushton turbine, concave disk turbine and their combinations on the performance of kojic acid fermentation by Aspergillus flavus Link 44-1, Asia Pac. J. Chem. Eng. 10 (2015) 65–74. [17] J. Markopoulos, C. Christofi, I. Katsinaris, Mass transfer coefficients in mechanically agitated gas–liquid contactors, Chem. Eng. Technol. 30 (7) (2007) 829–834. [18] Y. Imai, H. Takei, M. Matsumura, A simple Na2SO3 feeding method for kLa measurement in large-scale fermentors, Biotechnol. Bioeng. 29 (8) (1987) 982–993. [19] A.W. Nienow, M. Konno, W. Bujalski, Studies on three-phase mixing: a review and recent results, Chem. Eng. Res. Des. 64 (1) (1986) 35–42. [20] J.G. Long, Y.Y. Bao, Z.M. Gao, Gas–liquid dispersion in a stirred tank with different impeller combinations, J. Beijing Univ. Chem. Technol. 32 (5) (2005) 1–5 (in Chinese). [21] J. Min, Y.Y. Bao, L. Chen, Z.M. Gao, J.M. Smith, Numerical simulation of gas dispersion in an aerated stirred reactor with multiple impellers, Ind. Eng. Chem. Res. 47 (18) (2008) 7112–7117. [22] C.M. Cooper, G.A. Fernstrom, S.A. Miller, Performance of agitated gas–liquid contactors, Ind. Eng. Chem. 36 (6) (1944) 504–509. [23] J. Zhao, X.N. Cheng, Z.M. Gao, Experimental study and numerical simulation of fluid flow in a liquid multiple impeller stirred tank, J. Beijing Univ. Chem. Technol. 38 (3) (2011) 22–27 (in Chinese). [24] Z.G. Hao, Y.Y. Bao, Z.M. Gao, Gas–liquid dispersion in a multi-impeller stirred tank, J. Chem. Eng. Chin. Univ. 18 (5) (2004) 547–552 (in Chinese).
Please cite this article as: J. Zhang, et al., Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations, Chin. J. Chem. Eng. (2016), http://dx.doi.org/10.1016/j.cjche.2015.12.008