- Email: [email protected]
Mass Transfer Enhancement of Gas Absorption by Adding the Dispersed Organic Phases
RESEARCH NOTES Chinese Journal of Chemical Engineering, 19(6) 1066ü1068 (2011)
Mass Transfer Enhancement of Gas Absorption by Adding the Dispersed Organic Phases* ZHANG Zhigang (ᄝ)ذ, XU Tianxing (༘ඟ), LI Wenxiu (हำ༎)**, JI Zhiling (ޠᄦঊ) and XU Guangrong (༘ڛఔ) College of Chemical Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China Abstract Mass transfer enhancement of gas absorption by adding a dispersed organic phase has been studied in this work. Various dispersed organic phases (heptanol, octanol, isoamyl alcohol, heptane, octane, and isooctane) were tested respectively in the experiment. According to the theoretical model and experimental data, the overall volumetric mass transfer coefficient and enhancement factor were obtained under different dispersed organic phase volume fraction and stirring speed. The experimental results indicate that gas-liquid mass transfer is enhanced at different level by adding a dispersed organic phase. The best performance of enhancement were achieved with the dispersed organic phase volumetric fraction of 5% and under an intermediate stirring speed of 670 r·min1. Among the organic phases tested in the experiment, alcohols show better performance, which gave 20% higher enhancement of overall volumetric mass transfer coefficient than adding alkanes. Keywords gas absorption, dispersed organic phase, enhancement factor, the interfacial area
1
INTRODUCTION
2
Gas-liquid-liquid systems arouse great interest in the past decades. The study centers mainly on reducing the mass transfer resistance and improving the mass transfer rate [13]. Researchers attempt to enhance gas-liquid mass transfer by different means. In recent years, adding mass transfer reinforcing agent into gas-liquid systems has become an important research topic [48]. For example, some organic phases, including perfluorocarbon, silicone oil and C12C16 alkanes, were once used as oxygen transfer accelerants for promoting oxygen mass transfer [9, 10]. Quijano et al. found that oxygen transfer rate enhanced by 65% in multi-tube air-lift loop reactor and 84% in continuous stirred tank reactor when adding 10% silicon oil [11]. As the diversity and complexity of three-phase systems, the mechanism of mass transfer enhancement can not been completely explained or fully quantified at present [12]. In this work, we studied the absorption in carbon dioxide-water system by adding dispersed organic phases. The gas absorption rate is characterized by the overall volumetric mass transfer coefficient (kLa). And enhancement factor (E) is defined to evaluate the absorption effect by adding different organic phases. By exploring the effect of dispersed organic phases, more insight on the mechanism may be obtained. Table 1
EXPERIMENTAL
The experimental setup consists of a gas premixing section, a thermostatic reactor tank and an analysis section, which is schematically shown in Fig. 1 and the main parameters of stirred tank are listed in Table 1. In the premixing section the feed gas is prepared by mixing CO2 and N2 gases to get a gas mixture of specified composition. The feed gas was introduced at the bottom of the stirred tank through a gas sparger.
Figure 1 Schematic diagram of experimental setup 1ücarbon dioxide inlet; 2ünitrogen inlet; 3ügas premixing tank; 4ücontrol value; 5ügas sparger; 6üthermostatic bath; 7üreactor tank; 8üthermometer; 9üvent value; 10üstirrer; 11üpressure gauge; 12,13üdesiccant; 14ücarbon dioxide infrared analyzer
Equipment specifications
Height of reactor tank (H)/m
Height of stirrer (L)/m
Width of stirring paddle (W)/m
Liquid height (h)/m
Diameter of stirring paddle (D)/m
Diameter of reactor tank (T)/m
0.22
0.08
0.002
0.12
0.045
0.115
Received 2010-11-09, accepted 2011-07-25. * Supported by the National Natural Science Foundation of China (20776086). ** To whom correspondence should be addressed. E-mail: [email protected]
Chin. J. Chem. Eng., Vol. 19, No. 6, December 2011
The unabsorbed carbon dioxide and nitrogen leaved the tank from the free surface. The carbon dioxide volumetric fraction was analyzed by an infrared analyzer. The stirred tank was maintained at a constant experimental temperature of (303.15 ± 0.15) K with a thermostatic bath. In the experiment, water is taken as the continue phase, and heptanol, octanol, isoamyl alcohol, heptane, octane and isooctane are taken as dispersed organic phase, separately. The physical properties of the chemicals used are shown in Table 2. Table 2
Viscosity /mPa·s1
Interfacial tension /mN·m1
Solubility in water /g·(100 g)1
Density /kg·m3
heptanol
818.24
5.8
26.5
0.12
octanol
814.98
4.94
25.13
0.59
isoamyl alcohol
800.87
3.27
24.55
2.75
heptane
698.74
0.486
20.59
0.005
octane
676.27
0.376
19.86
0.002
isooctane
668.36
0.442
17.88
0.013
water
995.68
0.8007
71.20
ü
3
3.1.2 The enhancement factor The enhancement factor E is defined as the ratio of the overall volumetric mass transfer coefficient in the presence of the dispersed organic phase to that in the absence of the dispersed organic phase. For a gas-liquid mass transfer process, all the impact factors such as mass flux, interfacial area and interfacial concentration of carbon dioxide will eventually affect the mass transfer rate with the enhancement factor as an effective index. 3.2 Effect of the dispersed organic phase volumetric fraction
The physical properties of the chemicals
Reagent
1067
3.2.1 Effect of organic phase on kLa Figure 2 shows the influence of various dispersed organic phase volumetric fraction on the overall volumetric mass transfer coefficient kLa. The experiment was carried out at a constant temperature of 303.15 K and 101.325 kPa pressure and a stirring speed of 670 r·min1. When the dispersed organic phase volumetric fraction is in the range of 1%10%, kLa increases first and then decreases, and kLa reaches the maximum at the optimum volumetric fraction (about 5%).
RESULTS AND DISCUSSION
3.1
Characteristic parameters
3.1.1 The overall volumetric mass transfer coefficient According to shuttle mechanism [13, 14], we mainly study two parameters, the overall volumetric mass transfer coefficient kLa and the enhancement factor E. The overall volumetric mass transfer coefficient kLa is actually an average in the measurement period as calculated by kL a
dc 1 dt c* c
(1)
where c is the instantaneous concentration of carbon dioxide in the liquid phase (mol·L1), c* is the equilibrium concentration of carbon dioxide in the liquid phase (mol·L1) in the same period of measurement. Therefore, c and c* can be calculated based on the data acquisition: tª
[in u 0.5
[ out u 0.5 º
c
1 22.4VL
³ 0 « 60 1 [in 60 1 [out »dt
c*
1 22.4VL
³ 0 « 60 1 [in 0 1 [out » dt
¬
t*
¼
ª [in u 0.5
[ out u 0.5 º
(2)
(3) ¬ ¼ where VL is the overall volume of the liquid phase (L), t is an instantaneous time in the absorption period, t* is the time when absorption reaches the equilibrium (s), ȟin is the volume fraction of inlet carbon dioxide, ȟout is the volumetric fraction of outlet carbon dioxide. The average relative deviation is 2.9% according to the calculation results.
Figure 2 Influence of dispersed organic phase volumetric fraction on kLa Ƶ heptanol;ƽoctanol;Ʒisoamyl alcohol;ͩheptane; Ż octane; Ź isooctane
3.2.2 Effect of the dispersed organic phase on E The enhancement factor first increases with the increasing of dispersed organic phase volumetric fraction at the beginning and then decreases gradually. The maximum value is also at f 5% (Fig. 3). At the low fraction of alcohols (f<1%), E jumps to about 1.2, suggesting very high efficiency of added alcohols. This phenomenon deserves careful scrutiny. The possible reason is that the presence of hydroxyl makes alcohols more easily dissolve in water. The alcohols-water interface is thinner than alkanes-water interface which leads to lower mass transfer resistance and advantages the enhancement of gas-liquid mass transfer. 3.3
Effect of the stirring speed on mass transfer
The overall volumetric mass transfer coefficient kLa is presented in Fig. 4 for different stirring speeds
1068
Chin. J. Chem. Eng., Vol. 19, No. 6, December 2011
1.468, 1.446, 1.222, 1.216, and 1.206 for heptanol, octanol, isoamyl alcohol, heptane, octane and isooctane respectively) are found at the optimum stirring speed (670 r·min1) and the suitable dispersed organic phase volumetric fraction (5%). Therefore, it is concluded that alcohols show the best performance among all of the dispersed organic phases tested in the experiment. The reason is maybe that dioxide carbon owns higher solubility in alcohols and the presence of hydroxyl affects the gas-liquid mass transfer process. Figure 3 Influence of dispersed organic phase volumetric fraction on E Ƶ heptanol;ƽoctanol;Ʒisoamyl alcohol;ͩheptane; Ż octane; Ź isooctane
REFERENCES 1 2 3 4
5 6
Figure 4 Influence of stirring speed on kLa Ƶ water;ƽheptanol;Ʒoctanol;ͩisoamyl alcohol; Ż heptane; Ź octane;ƹisooctane
with f 5%. kLa reaches the maximum when stirring speed was 670 r·min1. As it is shown in Figs. 4 and 5, the highest enhancement factor under almost all conditions (1.489,
7 8 9 10 11 12
13
Figure 5 Influence of stirring speed on E Ƶ heptanol;ƽoctanol;Ʒisoamyl alcohol;ͩheptane; Ż octane; Ź isooctane
14
Cents, A.H.G., Brilman, D.W.F., Versteeg, G.F., “Gas absorption in an agitated gas-liquid-liquid system”, Chem. Eng. Sci., 56, 10751083 (2001). Lekhal, A., Chaudhari, R.V., Wilhelm, A.M., Delmas, H., “Gas-liquid mass transfer in gas-liquid-liquid dispersions”, Chem. Eng. Sci., 52, 40694077 (1997). Van Ede, C.J., Van Houten, R., Beenackers, A.A.C.M., “Enhancement of gas to water mass transfer rates by a dispersed organic phase”, Chem. Eng. Sci., 50, 29112922 (1995). Dumont, E., Andres, Y., Le Cloirec, P., “Effect of organic solvents on oxygen mass transfer in multiphase systems: Application to bioreactors in environmental protection”, Biochem. Eng. J., 30, 245252 (2006). Dumont, E., Delmas, H., “Mass transfer enhancement of gas absorption in oil-in-water systems: A review”, Chem. Eng. Progr., 42, 419438 (2003). Chaumata, H., Billet-Duquennea, A.M., Augierb, F., Mathieub, C., Delmasa, H., “Mass transfer in bubble column for industrial conditionsüeffects of organic medium, gas and liquid flow rates and column design”, Chem. Eng. Sci., 60, 59305936 (2005). Dai, Y.J., Deng, T., Wang, J., Xu, K., “Enhancement of oxygen gas-liquid mass transfer with colloidal gas aphron dispersions”, Colloids Surfaces A Phys. Eng. Aspects, 240, 165171 (2004). Littlejohns, J.V., Daugulis, A.J., “Oxygen transfer in a gas–liquid system containing solids of varying oxygen affinity”, Chem. Eng. J., 129, 6774 (2007). Deziel, E., Comeau, Y., Villemur, R., “Two-liquid-phase bioreactors for enhanced biodegradation of hydrophobic/toxic compounds”, Biodegradation, 10, 219233 (1999). Munoz, R., Villaverde, S., Guieysse, B., Revah, S., “Two partitioning bioreactors for treatment of volatile organic compounds”, Biotechnol. Adv., 25, 410422 (2007). Guillermo, Q., Sergio, R., Mariano, G.R., Luis, B.F.C., Frederic, T., “Oxygen transfer in three-phase airlift and stirred tank reactors using silicone oil as transfer vector”, Proc. Biochem., 44, 619624 (2009). Dumont, E., Andres, Y., Le Cloirec, P., “Effect of organic solvents on oxygen mass transfer in multiphase systems: application to bioreactors in environmental protection”, Biochem. Eng. J., 30, 245252 (2006). Cesairio, M.T., Beverloo, W.A., Tramper, J., Beeftink, H.H., “Enhancement of gas-liquid mass transfer rate of apolar pollutants in the biological waste gas treatment by a dispersed organic solvent”, Enzyme Microb. Technol., 21, 578588 (1997). Brilman, D.W.F., Goldschmidt, M.J.V., Versteeg, G.F., van Swaaij, W.P.M., “Heterogeneous mass transfer models for gas absorption in multiphase systems”, Chem. Eng. Sci., 55 (12), 27932812 (2000).
Mass Transfer Enhancement of Gas Absorption by Adding the Dispersed Organic Phases* ZHANG Zhigang (ᄝ)ذ, XU Tianxing (༘ඟ), LI Wenxiu (हำ༎)**, JI Zhiling (ޠᄦঊ) and XU Guangrong (༘ڛఔ) College of Chemical Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China Abstract Mass transfer enhancement of gas absorption by adding a dispersed organic phase has been studied in this work. Various dispersed organic phases (heptanol, octanol, isoamyl alcohol, heptane, octane, and isooctane) were tested respectively in the experiment. According to the theoretical model and experimental data, the overall volumetric mass transfer coefficient and enhancement factor were obtained under different dispersed organic phase volume fraction and stirring speed. The experimental results indicate that gas-liquid mass transfer is enhanced at different level by adding a dispersed organic phase. The best performance of enhancement were achieved with the dispersed organic phase volumetric fraction of 5% and under an intermediate stirring speed of 670 r·min1. Among the organic phases tested in the experiment, alcohols show better performance, which gave 20% higher enhancement of overall volumetric mass transfer coefficient than adding alkanes. Keywords gas absorption, dispersed organic phase, enhancement factor, the interfacial area
1
INTRODUCTION
2
Gas-liquid-liquid systems arouse great interest in the past decades. The study centers mainly on reducing the mass transfer resistance and improving the mass transfer rate [13]. Researchers attempt to enhance gas-liquid mass transfer by different means. In recent years, adding mass transfer reinforcing agent into gas-liquid systems has become an important research topic [48]. For example, some organic phases, including perfluorocarbon, silicone oil and C12C16 alkanes, were once used as oxygen transfer accelerants for promoting oxygen mass transfer [9, 10]. Quijano et al. found that oxygen transfer rate enhanced by 65% in multi-tube air-lift loop reactor and 84% in continuous stirred tank reactor when adding 10% silicon oil [11]. As the diversity and complexity of three-phase systems, the mechanism of mass transfer enhancement can not been completely explained or fully quantified at present [12]. In this work, we studied the absorption in carbon dioxide-water system by adding dispersed organic phases. The gas absorption rate is characterized by the overall volumetric mass transfer coefficient (kLa). And enhancement factor (E) is defined to evaluate the absorption effect by adding different organic phases. By exploring the effect of dispersed organic phases, more insight on the mechanism may be obtained. Table 1
EXPERIMENTAL
The experimental setup consists of a gas premixing section, a thermostatic reactor tank and an analysis section, which is schematically shown in Fig. 1 and the main parameters of stirred tank are listed in Table 1. In the premixing section the feed gas is prepared by mixing CO2 and N2 gases to get a gas mixture of specified composition. The feed gas was introduced at the bottom of the stirred tank through a gas sparger.
Figure 1 Schematic diagram of experimental setup 1ücarbon dioxide inlet; 2ünitrogen inlet; 3ügas premixing tank; 4ücontrol value; 5ügas sparger; 6üthermostatic bath; 7üreactor tank; 8üthermometer; 9üvent value; 10üstirrer; 11üpressure gauge; 12,13üdesiccant; 14ücarbon dioxide infrared analyzer
Equipment specifications
Height of reactor tank (H)/m
Height of stirrer (L)/m
Width of stirring paddle (W)/m
Liquid height (h)/m
Diameter of stirring paddle (D)/m
Diameter of reactor tank (T)/m
0.22
0.08
0.002
0.12
0.045
0.115
Received 2010-11-09, accepted 2011-07-25. * Supported by the National Natural Science Foundation of China (20776086). ** To whom correspondence should be addressed. E-mail: [email protected]
Chin. J. Chem. Eng., Vol. 19, No. 6, December 2011
The unabsorbed carbon dioxide and nitrogen leaved the tank from the free surface. The carbon dioxide volumetric fraction was analyzed by an infrared analyzer. The stirred tank was maintained at a constant experimental temperature of (303.15 ± 0.15) K with a thermostatic bath. In the experiment, water is taken as the continue phase, and heptanol, octanol, isoamyl alcohol, heptane, octane and isooctane are taken as dispersed organic phase, separately. The physical properties of the chemicals used are shown in Table 2. Table 2
Viscosity /mPa·s1
Interfacial tension /mN·m1
Solubility in water /g·(100 g)1
Density /kg·m3
heptanol
818.24
5.8
26.5
0.12
octanol
814.98
4.94
25.13
0.59
isoamyl alcohol
800.87
3.27
24.55
2.75
heptane
698.74
0.486
20.59
0.005
octane
676.27
0.376
19.86
0.002
isooctane
668.36
0.442
17.88
0.013
water
995.68
0.8007
71.20
ü
3
3.1.2 The enhancement factor The enhancement factor E is defined as the ratio of the overall volumetric mass transfer coefficient in the presence of the dispersed organic phase to that in the absence of the dispersed organic phase. For a gas-liquid mass transfer process, all the impact factors such as mass flux, interfacial area and interfacial concentration of carbon dioxide will eventually affect the mass transfer rate with the enhancement factor as an effective index. 3.2 Effect of the dispersed organic phase volumetric fraction
The physical properties of the chemicals
Reagent
1067
3.2.1 Effect of organic phase on kLa Figure 2 shows the influence of various dispersed organic phase volumetric fraction on the overall volumetric mass transfer coefficient kLa. The experiment was carried out at a constant temperature of 303.15 K and 101.325 kPa pressure and a stirring speed of 670 r·min1. When the dispersed organic phase volumetric fraction is in the range of 1%10%, kLa increases first and then decreases, and kLa reaches the maximum at the optimum volumetric fraction (about 5%).
RESULTS AND DISCUSSION
3.1
Characteristic parameters
3.1.1 The overall volumetric mass transfer coefficient According to shuttle mechanism [13, 14], we mainly study two parameters, the overall volumetric mass transfer coefficient kLa and the enhancement factor E. The overall volumetric mass transfer coefficient kLa is actually an average in the measurement period as calculated by kL a
dc 1 dt c* c
(1)
where c is the instantaneous concentration of carbon dioxide in the liquid phase (mol·L1), c* is the equilibrium concentration of carbon dioxide in the liquid phase (mol·L1) in the same period of measurement. Therefore, c and c* can be calculated based on the data acquisition: tª
[in u 0.5
[ out u 0.5 º
c
1 22.4VL
³ 0 « 60 1 [in 60 1 [out »dt
c*
1 22.4VL
³ 0 « 60 1 [in 0 1 [out » dt
¬
t*
¼
ª [in u 0.5
[ out u 0.5 º
(2)
(3) ¬ ¼ where VL is the overall volume of the liquid phase (L), t is an instantaneous time in the absorption period, t* is the time when absorption reaches the equilibrium (s), ȟin is the volume fraction of inlet carbon dioxide, ȟout is the volumetric fraction of outlet carbon dioxide. The average relative deviation is 2.9% according to the calculation results.
Figure 2 Influence of dispersed organic phase volumetric fraction on kLa Ƶ heptanol;ƽoctanol;Ʒisoamyl alcohol;ͩheptane; Ż octane; Ź isooctane
3.2.2 Effect of the dispersed organic phase on E The enhancement factor first increases with the increasing of dispersed organic phase volumetric fraction at the beginning and then decreases gradually. The maximum value is also at f 5% (Fig. 3). At the low fraction of alcohols (f<1%), E jumps to about 1.2, suggesting very high efficiency of added alcohols. This phenomenon deserves careful scrutiny. The possible reason is that the presence of hydroxyl makes alcohols more easily dissolve in water. The alcohols-water interface is thinner than alkanes-water interface which leads to lower mass transfer resistance and advantages the enhancement of gas-liquid mass transfer. 3.3
Effect of the stirring speed on mass transfer
The overall volumetric mass transfer coefficient kLa is presented in Fig. 4 for different stirring speeds
1068
Chin. J. Chem. Eng., Vol. 19, No. 6, December 2011
1.468, 1.446, 1.222, 1.216, and 1.206 for heptanol, octanol, isoamyl alcohol, heptane, octane and isooctane respectively) are found at the optimum stirring speed (670 r·min1) and the suitable dispersed organic phase volumetric fraction (5%). Therefore, it is concluded that alcohols show the best performance among all of the dispersed organic phases tested in the experiment. The reason is maybe that dioxide carbon owns higher solubility in alcohols and the presence of hydroxyl affects the gas-liquid mass transfer process. Figure 3 Influence of dispersed organic phase volumetric fraction on E Ƶ heptanol;ƽoctanol;Ʒisoamyl alcohol;ͩheptane; Ż octane; Ź isooctane
REFERENCES 1 2 3 4
5 6
Figure 4 Influence of stirring speed on kLa Ƶ water;ƽheptanol;Ʒoctanol;ͩisoamyl alcohol; Ż heptane; Ź octane;ƹisooctane
with f 5%. kLa reaches the maximum when stirring speed was 670 r·min1. As it is shown in Figs. 4 and 5, the highest enhancement factor under almost all conditions (1.489,
7 8 9 10 11 12
13
Figure 5 Influence of stirring speed on E Ƶ heptanol;ƽoctanol;Ʒisoamyl alcohol;ͩheptane; Ż octane; Ź isooctane
14
Cents, A.H.G., Brilman, D.W.F., Versteeg, G.F., “Gas absorption in an agitated gas-liquid-liquid system”, Chem. Eng. Sci., 56, 10751083 (2001). Lekhal, A., Chaudhari, R.V., Wilhelm, A.M., Delmas, H., “Gas-liquid mass transfer in gas-liquid-liquid dispersions”, Chem. Eng. Sci., 52, 40694077 (1997). Van Ede, C.J., Van Houten, R., Beenackers, A.A.C.M., “Enhancement of gas to water mass transfer rates by a dispersed organic phase”, Chem. Eng. Sci., 50, 29112922 (1995). Dumont, E., Andres, Y., Le Cloirec, P., “Effect of organic solvents on oxygen mass transfer in multiphase systems: Application to bioreactors in environmental protection”, Biochem. Eng. J., 30, 245252 (2006). Dumont, E., Delmas, H., “Mass transfer enhancement of gas absorption in oil-in-water systems: A review”, Chem. Eng. Progr., 42, 419438 (2003). Chaumata, H., Billet-Duquennea, A.M., Augierb, F., Mathieub, C., Delmasa, H., “Mass transfer in bubble column for industrial conditionsüeffects of organic medium, gas and liquid flow rates and column design”, Chem. Eng. Sci., 60, 59305936 (2005). Dai, Y.J., Deng, T., Wang, J., Xu, K., “Enhancement of oxygen gas-liquid mass transfer with colloidal gas aphron dispersions”, Colloids Surfaces A Phys. Eng. Aspects, 240, 165171 (2004). Littlejohns, J.V., Daugulis, A.J., “Oxygen transfer in a gas–liquid system containing solids of varying oxygen affinity”, Chem. Eng. J., 129, 6774 (2007). Deziel, E., Comeau, Y., Villemur, R., “Two-liquid-phase bioreactors for enhanced biodegradation of hydrophobic/toxic compounds”, Biodegradation, 10, 219233 (1999). Munoz, R., Villaverde, S., Guieysse, B., Revah, S., “Two partitioning bioreactors for treatment of volatile organic compounds”, Biotechnol. Adv., 25, 410422 (2007). Guillermo, Q., Sergio, R., Mariano, G.R., Luis, B.F.C., Frederic, T., “Oxygen transfer in three-phase airlift and stirred tank reactors using silicone oil as transfer vector”, Proc. Biochem., 44, 619624 (2009). Dumont, E., Andres, Y., Le Cloirec, P., “Effect of organic solvents on oxygen mass transfer in multiphase systems: application to bioreactors in environmental protection”, Biochem. Eng. J., 30, 245252 (2006). Cesairio, M.T., Beverloo, W.A., Tramper, J., Beeftink, H.H., “Enhancement of gas-liquid mass transfer rate of apolar pollutants in the biological waste gas treatment by a dispersed organic solvent”, Enzyme Microb. Technol., 21, 578588 (1997). Brilman, D.W.F., Goldschmidt, M.J.V., Versteeg, G.F., van Swaaij, W.P.M., “Heterogeneous mass transfer models for gas absorption in multiphase systems”, Chem. Eng. Sci., 55 (12), 27932812 (2000).