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Binary VOCs absorption in a rotating packed bed with blade packings
Journal of Environmental Management 98 (2012) 175e182
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Binary VOCs absorption in a rotating packed bed with blade packings Ling-Jung Hsu, Chia-Chang Lin* Department of Chemical and Materials Engineering, Chang Gung University, Taoyuan, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 September 2010 Accepted 23 November 2011 Available online 31 January 2012
This investigation addressed the mass transfer of rotating packed beds with blade packings in removing methanol and 1-butanol from binary mixtures by absorption using water as the absorbent. The dependences of the overall volumetric gas-phase mass transfer coefficient (KGa) on the inlet methanol concentration, the inlet 1-butanol concentration, the rotational speed, the gas flow rate, and the liquid flow rate, were explored. The results demonstrated that the inlet methanol and 1-butanol concentrations had a negligible effect on the KGa values of methanol and 1-butanol. The KGa values of methanol and 1-butanol increased with the rotational speed, the gas flow rate, and the liquid flow rate. The dependence of KGa on the gas flow rate was higher than that on the liquid flow rate, revealing that the mass transfer in binary VOCs absorption may be controlled considerably by the mass transfer in gas phase. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Rotating packed bed Absorption Mass transfer VOCs Blade packings
1. Introduction Volatile organic compounds (VOCs), which are widely used as solvents and detergents in chemical industry, are harmful to the atmosphere when emitted in the exhausted gases. For example, they cause a serious pollution due to the formation of photochemical smog. Additionally, VOCs might harm human’s eyes, skin, kidney, liver, respiratory system, and other organs. Hence, governments made laws concerning concentrations and fluxes of VOCs. Various studies have proposed some common methods for the removal of VOCs, including thermal oxidation (Ruddy and Carroll, 1993; Khan and Ghoshal, 2000), absorption (Khan and Ghoshal, 2000; Lalanne et al., 2008; Heymes et al., 2006), condensation (Khan and Ghoshal, 2000; Dwivedi et al., 2004), membrane separation (Khan and Ghoshal, 2000; Ji et al., 1994), adsorption (Khan and Ghoshal, 2000; Dwivedi et al., 2004), and biological treatment (Khan and Ghoshal, 2000; Daubert et al., 2001). Among these methods, absorption has been proven to be an effective method for the removal of VOCs because it is fast, safe, and economically feasible. In the conventional absorption process, a traditional packed bed (TPB) is used to remove VOCs with an appropriate absorbent. However, due to its poor mass transfer between gas and liquid, an TPB has the bulky volume, thus leading to high capital and operating costs. Accordingly, an alternative to intensify the mass transfer between gas and liquid was proposed by
* Corresponding author. Tel.: þ886 3 2118800x5760; fax: þ886 3 2118800x5702. E-mail address: [email protected] (C.-C. Lin). 0301-4797/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvman.2011.11.011
Ramshaw and Mallinson (1981), who invented a rotating packed bed (RPB). The RPB, which differs from the TPB, offers a high gravity field to which liquid flow is subjected. Because rotating doughnutshaped packings induces the affiliation of centrifugal force, unusual hydrodynamics and mass transfer characteristics can be observed in the RPB. For example, Burns and Ramshaw (1996) proposed that centrifugal force can disperse liquid to thin films or tiny droplets with the help of packings. Thus, the RPB can increase the area of contact between gas and liquid and reduce the total resistance to mass transfer. Therefore, the RPB has smaller volume and lower capital and operating costs as compared with the TPB (Ramshaw, 1983). The RPB has been widely employed to distillation (Lin et al., 2002; Wang et al., 2008), VOCs absorption (Chen and Liu, 2002; Lin et al., 2004, 2009a; Lin and Jian, 2007; Lin and Chien, 2008; Chiang et al., 2009), CO2 absorption (Lin et al., 2003, 2008, 2010; Tan and Chen, 2006; Cheng and Tan, 2006), O3 absorption (Lin and Su, 2008; Lin et al., 2009b), ozone oxidation (Lin and Liu, 2003; Chen et al., 2005; Shang et al., 2006; Chiu et al., 2007; Chang et al., 2009), reactive precipitation (Chen et al., 2000; Shen et al., 2004; Hu et al., 2008; Zhao et al., 2009), and stripping (Lin and Liu, 2006; Li et al., 2009). Chen and Liu (2002) carried out an experimental study on the removal of single VOCs by absorption using an RPB with random packings. The investigated VOCs were isopropyl alcohol, acetone, and ethyl acetate. Their experimental results showed that centrifugal force intensified the mass transfer for the absorption of single VOCs in an RPB. Similarly, Lin et al. (2004) used an RPB with structured packings for removing single VOCs by absorption. They utilized isopropyl alcohol and ethyl acetate as model single VOCs. They found that the mass transfer efficiency of
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Fig. 1. Schematic diagram of RPB with blade packings.
an RPB with structured packings was equivalent to that of an RPB with random packings. To obtain high mass transfer efficiency, Lin and Jian (2007) first adopted the blade as packings in an RPB for the removal of isopropyl alcohol by absorption, as shown in Fig. 1. They proposed that an RPB with blade packings have many superior operating characteristics such as low pressure drop and high mass transfer efficiency. Moreover, our previously published studies presented more results concerning the removal of single VOCs by absorption using an RPB with blade packings (Lin and Chien, 2008; Lin et al., 2009a). Methanol, ethanol, acetone, methyl ethyl ketone, ethyl acetate, and methyl acetate were employed in these studies. According to these results, an RPB with blade packings can be an excellent alternative to remove singe VOCs from a gas stream. However, no previous studies have attempted to investigate the removal of VOCs from binary mixtures using an RPB with blade packings. Moreover, the effect of binary VOCs concentrations on the mass transfer in an RPB with blade packings was very essential for the industrial design. Therefore, the purpose of this investigation is to evaluate the overall volumetric gas-phase mass transfer coefficient (KGa) of an RPB with blade packings in binary VOCs absorption. Additionally, the effects of the VOCs concentrations, the rotational speed, the gas flow rate, and the liquid flow rate on KGa were examined. 2. Experimental Fig. 2 displays the experimental setup for binary VOCs absorption. Two sub-systems were necessary to generate a binary VOCs stream containing methanol and 1-butanol. An air stream of a low flow rate passed through two bubblers that contained methanol.
Fig. 3. Configuration of blade packings in RPB.
Simultaneously, another air stream of a low flow rate flowed through two bubblers that contained 1-butanol. Then, these air streams were combined with an air stream of a high flow rate to yield the desired concentrations of methanol and 1-butanol. Meanwhile, in order to maintain stable the concentrations of methanol and 1-butanol, two buffer flasks were added into the process. The combined air stream containing methanol and 1-butanol moved through the outer edge of the packed bed toward the inner edge of the packed bed. The clean air stream eventually exited from the top of the RPB with blade packings. Water traveled through a liquid distributor into the inner edge of the packed bed, and then exited at the outer edge of the packed bed due to centrifugal acceleration. Finally, water containing methanol and 1-butanol was expelled at the bottom of the RPB with blade packings. Fig. 3 presents the details of schematic design for twelve blades spacing 30 apart used as packings in the RPB. Each blade had an inner radius of 1.95 cm, an outer radius of 6.25 cm, and an axial height of 2.95 cm. The blade packings, which were made of the stainless steel wire mesh, had a specific surface area of 93 m2/m3 and a voidage of 0.99. During operation, the rotational speed of the RPB with blade packings can be varied from 600 to 1800 rpm, providing 17e149 times gravitational acceleration based on the arithmetic mean radius. The range of gas flow rate was 5e55 L/min and the range of liquid flow rate was 0.15e0.65 L/min.
Air Out Flowmeter
Gas-Collecting Tube Buffer Flask Bubbler Motor
water
Blade Packings
Pump Sampler
Liquid Level Tank Bubbler
Water Bath Air In Methanol
Water Bath Effluent Tank
Flowmeter
Buffer Flask
1-Butanol Air In
Fig. 2. Experimental setup for binary VOCs absorption.
Air In
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The Henry’s constants of methanol and 1-butanol in water at 25 C are 2.03 104 and 4.89 104 (mol/m3)/(mol/m3) (Gupta et al., 2000), respectively, implying that the solubility of methanol is higher than that of 1-butanol. The concentrations of methanol and 1-butanol in the inlet and outlet air streams were measured by gas chromatography analyzer (China GCeFID Model 8700F) equipped with a FID and a capillary column (DB-624). The GCeFID used nitrogen as the carrier gas. The injector, oven, and detector temperatures were set to 130, 130, and 200 C, respectively. During normal operation, it took about 5e10 min to reach a steady state by monitoring the concentrations of methanol and 1-butanol in the outlet air stream. All binary VOCs absorption tests were performed at 25 1 C and 1 atm. 3. Results and discussion The overall volumetric gas-phase mass transfer coefficient (KGa) can be calculated using the following equation (Lin and Jian, 2007; Lin and Chien, 2008).
1 Ci 1 ln 1 þ QG A Co A KG a ¼ 1 p R2o R2i Zb 1 A
(1)
where Ci and Co are the concentrations of VOCs in the inlet and outlet air streams, respectively. QG is the volumetric flow rate of
177
gas, Zb is the axial height of the RPB, and Ri and Ro are the inner and outer radius of the RPB, respectively. The absorption factor A is defined as QL/HQG, where QL are the volumetric flow rate of liquid. H is the Henry’s constant of VOCs defined as the ratio of the molar concentration in the gas phase to that in the liquid phase [(mol/m3)/(mol/m3)]. To evaluate the mass transfer performance of the RPB with blade packings for the removal of VOCs from binary mixtures using water as the absorbent, the KGa values were calculated by Eq. (1) with the absorption factor and the corresponding concentrations in VOCs-air steams at various values of the operating parameters, including the concentrations of methanol and 1-butanol in the inlet air stream (Ci,m, Ci,b), the rotational speed (u), the gas flow rate (QG), and the liquid flow rate (QL). Table 1 presents the effect of the 1-butanol concentration in the inlet air stream on the KGa values of methanol and 1-butanol with a fixed methanol concentration of 2000 ppmv in the inlet air stream under various operating conditions. It can be seen from Table 1 that the KGa values of methanol were independent of the inlet 1-butanol concentration under the same operating conditions. For example, the KGa values of methanol varied from 2.97 to 2.89 1/s as the inlet 1-butanol concentration was increased from 250 to 2000 ppmv at a rotational speed of 1800 rpm, a gas flow rate of 20 L/min, and a liquid flow rate of 0.65 L/min. This behavior was attributed to the fact that the transfer rate of methanol into water was not hindered in the presence of 1-butanol due to the same inlet methanol concentration. Moreover, the KGa values of 1-butanol were independent of the inlet 1-butanol concentration under the
Table 1 Dependence of KGa on inlet 1-butanol concentration.
u (rpm)
QG (L/min)
QL (L/min)
Ci,m (ppmv)
Ci,b (ppmv)
KGa (methanol) (1/s)
KGa (1-butanol) (1/s)
600 600 600 600 1000 1000 1000 1000 1400 1400 1400 1400 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800
50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 5 5 5 5 20 20 20 20 35 35 35 35 50 50 50 50 50 50 50 50 50 50 50 50
0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.15 0.15 0.15 0.15 0.25 0.25 0.25 0.25 0.45 0.45 0.45 0.45
2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000
250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000
5.22 5.24 4.99 4.85 6.04 6.14 5.90 5.41 6.11 6.59 6.57 6.16 6.81 6.91 6.88 6.47 0.91 0.88 0.89 0.87 2.97 2.89 2.92 2.89 4.99 4.85 4.87 4.63 3.84 3.22 3.29 3.24 5.47 5.43 5.44 5.31 6.54 6.44 5.99 5.75
3.82 3.57 3.51 3.43 4.40 4.32 4.19 3.75 4.58 4.80 4.74 4.47 5.10 5.07 4.93 4.67 0.58 0.68 0.67 0.64 2.38 2.17 2.15 2.14 3.65 3.69 3.60 3.31 3.02 2.67 2.59 2.37 4.49 4.27 4.13 3.88 4.79 4.75 4.62 4.27
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same operating conditions. For example, the he KGa values of 1-butanol varied from 2.38 to 2.14 1/s while the inlet 1-butanol concentration was increased from 250 to 2000 ppmv at a rotational speed of 1800 rpm, a gas flow rate of 20 L/min, and a liquid flow rate of 0.65 L/min. This phenomenon revealed that the transfer rate of 1-butanol into water increased with the inlet 1-butanol concentration at the same inlet methanol concentration. Similarly, Table 2 indicates the effect of the methanol concentration in the inlet air stream on the KGa values of methanol and 1-butanol with a fixed 1-butanol concentration of 2000 ppmv in the inlet air stream under various operating conditions. As listed in Table 2, the KGa values of 1-butanol were independent of the inlet methanol concentration under the same operating conditions. For example, the KGa values of 1-butanol varied from 0.60 to 0.64 1/s as the inlet methanol concentration was increased from 250 to 2000 ppmv at a rotational speed of 1800 rpm, a gas flow rate of 5 L/min, and a liquid flow rate of 0.65 L/min. This feature was attributed to the fact that the transfer rate of 1-butanol into water was not retarded in the presence of methanol due to the same inlet 1-butanol concentration. Moreover, the KGa values of methanol were independent of the inlet methanol concentration under the same operating conditions. For example, the he KGa values of methanol varied from 0.75 to 0.87 1/s while the inlet methanol concentration was increased from 250 to 2000 ppmv at a rotational speed of 1800 rpm, a gas flow rate of 5 L/min, and a liquid flow rate of 0.65 L/min. This characteristic suggested that the transfer rate of methanol into water increased
with the inlet methanol concentration at the same inlet 1-butanol concentration. According to these results, the following discussions concerning the effects of the rotational speed, the gas flow rate, and the liquid flow rate were explored at an inlet methanol concentration of 2000 ppmv and an inlet 1-butanol concentration of 2000 ppmv. Fig. 4 illustrates the effect of the rotational speed varying from 600 to 1800 rpm on the KGa values. As shown in Fig. 4, there was a trend that the KGa values of methanol and 1-butanol were enhanced by the rotational speed under the same operating conditions. This behavior was also observed in single VOCs absorption in the RPB with blade packings (Lin and Jian, 2007; Lin and Chien, 2008). This result indicated that increasing the rotational speed could improve the mass transfer between gas and liquid because the centrifugal force was able to reduce the total resistance to mass transfer in VOCs absorption. Moreover, the KGa values of methanol were higher than those of 1-butanol under the same operating conditions. Higher solubility of methanol than one of 1-butanol in water could explain this phenomenon. According to Fig. 4, the KGa values of methanol and 1-butanol increased in proportion to the rotational speed raised to the x power as
KG afux
(2)
The x values were calculated by nonlinear regression using Origin Software (Version 8, OriginLab Corporation, Northampton, MA, USA), as listed in Table 3.
Table 2 Dependence of KGa on inlet methanol concentration.
u (rpm)
QG (L/min)
QL (L/min)
Ci,m (ppmv)
Ci,b (ppmv)
KGa (methanol) (1/s)
KGa (1-butanol) (1/s)
600 600 600 600 1000 1000 1000 1000 1400 1400 1400 1400 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800
50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 5 5 5 5 20 20 20 20 35 35 35 35 50 50 50 50 50 50 50 50 50 50 50 50
0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.15 0.15 0.15 0.15 0.25 0.25 0.25 0.25 0.45 0.45 0.45 0.45
250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000
2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000
4.21 4.38 4.87 4.85 4.23 5.58 5.64 5.41 6.50 6.16 6.08 6.16 5.96 6.13 6.28 6.47 0.75 0.79 0.83 0.87 2.67 2.79 2.77 2.89 4.66 4.70 4.98 4.63 2.79 2.97 3.12 3.24 4.52 5.04 4.98 5.31 5.99 5.71 5.93 5.75
3.45 3.35 3.37 3.43 3.36 3.69 3.83 3.75 4.75 4.53 4.37 4.47 4.26 4.52 4.42 4.67 0.60 0.60 0.61 0.64 1.95 2.01 1.98 2.14 3.39 3.25 3.48 3.31 2.21 2.32 2.31 2.37 3.53 3.99 4.03 3.88 4.25 4.03 4.30 4.27
L.-J. Hsu, C.-C. Lin / Journal of Environmental Management 98 (2012) 175e182
a
8
methanol 1-butanol
b
8
methanol 1-butanol 6
KGa (1/s)
KGa (1/s)
6
4
2
0 500
4
2
1000
1500
0 500
2000
8
methanol 1-butanol
d
1500
2000
8
6
KGa (1/s)
6
KGa (1/s)
1000
Rotational Speed (rpm)
Rotational Speed (rpm)
c
179
4
2
4
2
methanol 1-butanol 0 500
1000
1500
2000
0 500
Rotational Speed (rpm)
1000
1500
2000
Rotational Speed (rpm)
Fig. 4. Effect of rotational speed on KGa at Ci,m ¼ 2000 ppmv and Ci,b ¼ 2000 ppmv (a) QG ¼ 20 L/min and QL ¼ 0.15 L/min; (b) QG ¼ 20 L/min and QL ¼ 0.65 L/min; (c) QG ¼ 50 L/min and QL ¼ 0.15 L/min; (d) QG ¼ 50 L/min and QL ¼ 0.65 L/min.
When the gas flow rate was 20 L/min, the x values of methanol and 1-butanol at a liquid flow rate of 0.15 L/min were higher than those at a liquid flow rate of 0.65 L/min. This behavior was more evident at a gas flow rate of 50 L/min. This result implied that the rotational speed had a greater effect on the KGa values of methanol and 1-butanol at a low liquid flow rate. Furthermore, when the liquid flow rate was 0.65 L/min, the x values of methanol and 1butanol at a gas flow rate of 50 L/min were greater than those at a gas flow rate of 20 L/min. This feature was more obvious at a liquid flow rate of 0.15 L/min. This result revealed that the rotational speed had a higher effect on the KGa values of methanol and 1-butanol at a high gas flow rate. Hence, according to these observations, the rotational speed yielded a considerable effect on the mass transfer for binary VOCs absorption by increasing the gas flow rate and decreasing the liquid flow rate. Based on the obtained x values ranging from 0.15 to 0.67, KGa increased as a function of the centrifugal acceleration to the power of 0.08e0.34, which was close to 0.18 for single VOCs absorption in the RPB with random packings (Chen and Liu, 2002) and 0.10e0.29 for single VOCs absorption in the RPB with blade packings (Lin and Chien, 2008), but was lower than 0.27e0.43 for single VOCs absorption in the RPB with structured packings (Lin et al., 2004). Fig. 5 presents the relationship between the KGa values and the gas flow rate. Considering Fig. 5, the KGa values of methanol and 1-butanol increased with an increasing gas flow rate under the
same operating conditions. This behavior was mainly attributed to the fact that the resistance to mass transfer in gas phase depended on the thickness of the boundary layer. With an increase in the gas flow rate, the boundary layer tended to be thin at a high gas velocity, leading to a great benefit to the mass transfer. Therefore, an increase in gas flow rate would yield an increase in KGa. Similar characteristic was also found in single VOCs absorption in the RPB with blade packings (Lin and Jian, 2007; Lin and Chien, 2008). As shown in Fig. 5, the KGa values of methanol and 1-butanol were proportional to the gas flow rate raised to the y power as
KG afQGy
(3)
Table 3 Variation of x with operating conditions. QG (L/min)
QL (L/min)
VOCs
x
20 20 20 20 50 50 50 50
0.15 0.15 0.65 0.65 0.15 0.15 0.65 0.65
methanol 1-butanol methanol 1-butanol methanol 1-butanol methanol 1-butanol
0.29 0.35 0.15 0.22 0.67 0.53 0.27 0.30
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a
b
8
methanol 1-butanol
methanol 1-butanol 6
KGa (1/s)
KGa (1/s)
6
8
4
2
4
2
0
0 0
10
20
30
40
50
60
0
10
Gas Flow Rate (L/min)
c
d
8
methanol 1-butanol
30
40
50
60
8
6
KGa (1/s)
6
KGa (1/s)
20
Gas Flow Rate (L/min)
4
2
4
2
methanol 1-butanol 0
0 0
10
20
30
40
50
60
0
Gas Flow Rate (L/min)
10
20
30
40
50
60
Gas Flow Rate (L/min)
Fig. 5. Effect of gas flow rate on KGa at Ci,m ¼ 2000 ppmv and Ci,b ¼ 2000 ppmv (a) QL ¼ 0.15 L/min and u ¼ 600 rpm; (b) QL ¼ 0.15 L/min and u ¼ 1800 rpm; (c) QL ¼ 0.65 L/min and u ¼ 600 rpm; (d) QL ¼ 0.65 L/min and u ¼ 1800 rpm.
The y values were calculated by nonlinear regression using Origin Software (Version 8, OriginLab Corporation, Northampton, MA, USA), as listed in Table 4. With a liquid flow rate of 0.65 L/min, the y values of methanol and 1-butanol at a rotational speed of 1800 rpm were higher than those at a rotational speed of 600 rpm. This behavior was more evident at a liquid flow rate of 0.15 L/min. This result implied that the gas flow rate had a greater effect on the KGa values of methanol and 1-butanol at a high rotational speed. Furthermore, at a rotational speed of 1800 rpm, the y values of methanol and 1-butanol.at a liquid flow rate of 0.65 L/min were greater than those at a liquid flow rate of 0.15 L/min for This feature was also found at a rotational speed of 600 rpm. This result revealed that the gas flow rate had a higher effect on the KGa values of methanol and 1-butanol at a high liquid flow rate. Therefore, based on these observations, the Table 4 Variation of y with operating conditions. QL (L/min)
u (rpm)
VOCs
y
0.15 0.15 0.15 0.15 0.65 0.65 0.65 0.65
600 600 1800 1800 600 600 1800 1800
methanol 1-butanol methanol 1-butanol methanol 1-butanol methanol 1-butanol
0.43 0.59 0.63 0.64 0.76 0.79 0.88 0.87
gas flow rate yielded a marked effect on the mass transfer for binary VOCs absorption by increasing the rotational speed and the liquid flow rate. The dependence of KGa on the gas flow rate to the power of 0.43e0.88 was close to the reported range of 0.48e0.71 for single VOCs absorption in the RPB with blade packings (Lin and Chien, 2008) and the reported range of 0.62e0.77 for single VOCs absorption in the RPB with structured packings (Lin et al., 2004), but was higher than the exponent (0.32) for single VOCs absorption in the RPB with random packings (Chen and Liu, 2002). The effect of the liquid flow rate, varying from 0.15 to 0.65 L/min, on the KGa values was shown in Fig. 6. It was found in the figure that increasing the liquid flow rate increased the KGa values. This behavior was probably because increasing the liquid flow rate would provide more liquid films spreading over the packing and more liquid droplets moving within the void of the bed, thus leading to a large area of contact between gas and liquid. Moreover, considering Fig. 6, the KGa values of methanol and 1-butanol were proportional to the liquid flow rate raised to the z power as
KG afQLz
(4)
The z values were calculated by nonlinear regression using Origin Software (Version 8, OriginLab Corporation, Northampton, MA, USA), as listed in Table 5. With a gas flow rate of 20 L/min, the z values of methanol and 1-butanol at a rotational speed of 600 rpm were higher than those at a rotational speed of 1800 rpm. This behavior was more evident
L.-J. Hsu, C.-C. Lin / Journal of Environmental Management 98 (2012) 175e182
a
b
8
methanol 1-butanol
8
methanol 1-butanol 6
KGa (1/s)
KGa (1/s)
6
4
2
4
2
0
0 0.0
0.2
0.4
0.6
0.8
0.0
0.2
Liquid Flow Rate (L/min)
c
0.4
0.6
0.8
Liquid Flow Rate (L/min)
d
8
methanol 1-butanol
8
6
KGa (1/s)
6
KGa (1/s)
181
4
2
4
2
methanol 1-butanol 0
0 0.0
0.2
0.4
0.6
0.8
0.0
0.2
Liquid Flow Rate (L/min)
0.4
0.6
0.8
Liquid Flow Rate (L/min)
Fig. 6. Effect of liquid flow rate on KGa at Ci,m ¼ 2000 ppmv and Ci,b ¼ 2000 ppmv (a) QG ¼ 20 L/min and u ¼ 600 rpm; (b) QG ¼ 20 L/min and u ¼ 1800 rpm; (c) QG ¼ 50 L/min and u ¼ 600 rpm; (d) QG ¼ 50 L/min and u ¼ 1800 rpm.
at a gas flow rate of 50 L/min. This result implied that the liquid flow rate had a greater effect on the KGa values of methanol and 1butanol at a low rotational speed. Furthermore, at a rotational speed of 1800 rpm, the z values of methanol and 1-butanol.at a gas flow rate of 50 L/min was higher than those at a gas flow rate of 20 L/min. This feature was more obvious at a rotational speed of 600 rpm. This result revealed that the liquid flow rate had a higher effect on the KGa values of methanol and 1-butanol at a high gas flow rate. Accordingly, owing to these observations, the liquid flow rate yielded a substantial effect on the mass transfer for binary VOCs absorption by decreasing the rotational speed and increasing the gas flow rate. Based on the obtained z values, KGa was proportional to the liquid flow rate raised to the power of 0.29e0.54, which was close to Table 5 Variation of z with operating conditions. QG (L/min)
u (rpm)
VOCs
z
20 20 20 20 50 50 50 50
600 600 1800 1800 600 600 1800 1800
methanol 1-butanol methanol 1-butanol methanol 1-butanol methanol 1-butanol
0.37 0.38 0.29 0.32 0.54 0.46 0.38 0.37
0.24e0.67 for single VOCs absorption in the RPB with blade packings (Lin and Chien, 2008), 0.34e0.49 for single VOCs absorption in the RPB with structured packings (Lin et al., 2004), and 0.33 for single VOCs absorption in the RPB with random packings (Chen and Liu, 2002). According to Tables 4 and 5, the dependence of KGa on the gas flow rate for binary VOCs absorption was higher than that on the liquid flow rate, implying that the total resistance to mass transfer in binary VOCs absorption may be controlled considerably by the resistance in gas phase (Strigle, 1987).
4. Conclusions This investigation studied the mass transfer of an RPB with blade packings in binary VOCs absorption. The overall volumetric gasphase mass transfer coefficients (KGa) were estimated as functions of the inlet methanol concentration, the inlet 1-butanol concentration, the rotational speed, the gas flow rate, and the liquid flow rate. The KGa values of methanol and 1-butanol were independent of the inlet methanol concentration and the inlet 1-butanol concentration. However, the KGa values of methanol and 1-butanol increased with the rotational speed, the liquid flow rate, and the gas flow rate. The rotational speed could provide a stronger effect on the KGa values of methanol and 1-butanol at higher gas flow rates and lower liquid flow rates. The gas flow rate could provide a stronger effect on the KGa values of methanol and 1-butanol at higher rotational speed and
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higher liquid flow rates. The liquid flow rate could provide a stronger effect on the KGa values of methanol and 1-butanol at lower rotational speeds and higher gas flow rates. According to the dependence of KGa on the gas and liquid flow rates, the mass transfer in binary VOCs absorption was observed to be dominated by the mass transfer in gas phase. Acknowledgment The financial support of National Science Council of the Republic of China (NSC 97-2221-E-182-013-MY2) is greatly appreciated. References Burns, J.R., Ramshaw, C., 1996. Process intensification: visual study of liquid maldistribution in rotating packed beds. Chem. Eng. Sci. 51, 1347e1352. Chang, C.C., Chiu, C.Y., Chang, C.Y., Chang, C.F., Chen, Y.H., Ji, D.R., Yu, Y.H., Chiang, P.C., 2009. Combined photolysis and catalytic ozonation of dimethyl phthalate in a high-gravity rotating packed bed. J. Hazard. Mater. 161, 287e293. Chen, Y.S., Liu, H.S., 2002. Absorption of VOCs in a rotating packed bed. Ind. Eng. Chem. Res. 41, 1583e1588. Chen, J.F., Wang, Y.H., Guo, F., Wang, X.M., Zheng, C., 2000. Synthesis of nanoparticles with novel technology: high-gravity reactive precipitation. Ind. Eng. Chem. Res. 39, 948e954. Chen, Y.H., Chang, C.Y., Su, W.L., Chen, C.C., Chiu, C.Y., Yu, Y.H., Chiang, P.C., Chang, C.F., Shie, J.L., Chiou, C.S., Chiang, S.I.M., 2005. Ozonation of CI Reactive Black 5 using rotating packed bed and stirred tank reactor. J. Chem. Technol. Biotechnol. 80, 68e75. Cheng, H.H., Tan, C.S., 2006. Reduction of CO2 concentration in a zinc/air battery by absorption in a rotating packed bed. J. Power Sources 162, 1431e1436. Chiang, C.Y., Chen, Y.S., Liang, M.S., Lin, F.L., Tai, C.Y., Liu, H.S., 2009. Absorption of ethanol into water and glycerol/water solution in a rotating packed bed. J. Taiw. Inst. Chem. Eng. 40, 418e423. Chiu, C.Y., Chen, Y.H., Huang, Y.H., 2007. Removal of naphthalene in Brij 30-containing solution by ozonation using rotating packed bed. J. Hazard. Mater. 147, 732e737. Daubert, I., Lafforgue, C., Maranges, C., Fonade, C., 2001. Feasibility study of a compact process for biological treatment of highly soluble VOCs polluted gaseous effluent. Biotechnol. Prog. 17, 1084e1092. Dwivedi, P., Gaur, V., Sharma, A., Verma, N., 2004. Comparative study of removal of volatile organic compounds by cryogenic condensation and adsorption by activated carbon fiber. Sep. Purif. Technol. 39, 23e37. Gupta, A.K., Teja, A.S., Chai, X.S., Zhu, J.Y., 2000. Henry’s constants of n-alkanols (methanol through n-hexanol) in water at temperatures between 40 C and 90 C. Fluid Phase Equilib. 170, 183e192. Heymes, F., Demoustier, P.M., Charbit, F., Fanlo, J.L., Moulin, P., 2006. A new efficient absorption liquid to treat exhaust air loaded with toluene. Chem. Eng. J.115, 225e231. Hu, T.T., Chiou, H., Chan, H.K., Chen, J.F., Yun, J., 2008. Preparation of inhalable salbutamol sulphate using reactive high gravity controlled precipitation. J. Pharm. Sci. 97, 944e949. Ji, W., Sikdar, S.K., Hwang, S.T., 1994. Modeling of multicomponent pervaporation for removal of volatile organic compounds from water. J. Membr. Sci. 93, 1e19.
Khan, F.I., Ghoshal, A.K., 2000. Removal of volatile organic compounds from polluted air. J. Loss. Prev. Process. Ind. 13, 527e545. Lalanne, F., Malhautier, L., Roux, J.C., Fanlo, J.L., 2008. Absorption of a mixture of volatile organic compounds (VOCs) in aqueous solutions of soluble cutting oil. Bioresour. Technol. 99, 1699e1707. Li, W.Y., Wu, W., Zou, H.K., Chu, G.G., Shao, L., Chen, J.F., 2009. Process intensification of VOC removal from high viscous media by rotating packed bed. Chin. J. Chem. Eng. 17, 389e393. Lin, C.C., Chien, K.S., 2008. Mass transfer performance of rotating packed beds equipped with blade packings in VOCs absorption into water. Sep. Purif. Technol. 63, 138e144. Lin, C.C., Jian, G.S., 2007. Characteristics of a rotating packed bed equipped with blade packings. Sep. Purif. Technol. 54, 51e60. Lin, C.C., Liu, W.T., 2003. Ozone oxidation in a rotating packed bed. J. Chem. Technol. Biotechnol. 78, 138e141. Lin, C.C., Liu, W.T., 2006. Removal of an undesired component from a valuable product using a rotating packed bed. J. Ind. Eng. Chem. 12, 455e459. Lin, C.C., Su, Y.R., 2008. Performance of rotating packed beds in removing ozone from gaseous streams. Sep. Purif. Technol. 61, 311e316. Lin, C.C., Ho, T.J., Liu, W.T., 2002. Distillation in a rotating packed bed. J. Chem. Eng. Jpn. 35, 1298e1304. Lin, C.C., Liu, W.T., Tan, C.S., 2003. Removal of carbon dioxide by absorption in a rotating packed bed. Ind. Eng. Chem. Res. 42, 2381e2386. Lin, C.C., Wei, T.Y., Liu, W.T., Shen, K.P., 2004. Removal of VOCs from gaseous streams in a high-voidage rotating packed bed. J. Chem. Eng. Jpn. 37, 1471e1477. Lin, C.C., Chen, B.C., Chen, Y.S., Hsu, S.K., 2008. Feasibility of a cross-flow rotating packed bed in removing carbon dioxide from gaseous streams. Sep. Purif. Technol. l62, 507e512. Lin, C.C., Lin, Y.C., Chien, K.S., 2009a. VOCs absorption in rotating packed beds equipped with blade packings. J. Ind. Eng. Chem. 15, 813e818. Lin, C.C., Chao, C.Y., Liu, M.Y., Lee, Y.L., 2009b. Feasibility of ozone absorption by H2O2 solution in rotating packed beds. J. Hazard. Mater. 167, 1014e1020. Lin, C.C., Lin, Y.H., Tan, C.S., 2010. Evaluation of alkanolamine solutions for carbon dioxide removal in cross-flow rotating packed beds. J. Hazard. Mater. 175, 344e351. Ramshaw, C., Mallinson, R.H., 1981. Mass Transfer Process U.S. Patent 4, 283, 255. Ramshaw, C., 1983. Higee distillationdan example of process intensification. Chem. Eng. 389, 13e14. Ruddy, E.N., Carroll, L.A., 1993. Select the best VOC control strategy. Chem. Eng. Prog. 89, 28e35. Shang, N.C., Chen, Y.H., Yang, Y.P., Chang, C.H., Yu, Y.H., 2006. Ozonation of dyes and textile wastewater in a rotating packed bed. J. Environ. Sci. Health Part A-Toxic/ Hazard. Subst. Environ. Eng. 41, 2299e2310. Shen, Z.G., Chen, J.F., Yun, J., 2004. Preparation and characterizations of uniform nanosized BaTiO3 crystallites by the high-gravity reactive precipitation method. J. Cryst. Growth 267, 325e335. Strigle Jr., R.F., 1987. Random Packing and Packed Towers: Design and Applications, first ed. Gulf, Houston. Tan, C.S., Chen, J.E., 2006. Absorption of carbon dioxide with piperazine and its mixtures in a rotating packed bed. Sep. Purif. Technol. 49, 174e180. Wang, G.Q., Xu, Z.C., Yu, Y.L., Ji, J.B., 2008. Performance of a rotating zigzag beddA new HIGEE. Chem. Eng. Process. 47, 2131e2139. Zhao, H., Wang, J.X., Zhang, H.X., Shen, Z.G., Yun, J., Chen, J.F., 2009. Facile preparation of danazol nanoparticles by high-gravity anti-solvent precipitation (HGAP) method. Chin. J. Chem. Eng. 17, 318e323.
Contents lists available at SciVerse ScienceDirect
Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Binary VOCs absorption in a rotating packed bed with blade packings Ling-Jung Hsu, Chia-Chang Lin* Department of Chemical and Materials Engineering, Chang Gung University, Taoyuan, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 September 2010 Accepted 23 November 2011 Available online 31 January 2012
This investigation addressed the mass transfer of rotating packed beds with blade packings in removing methanol and 1-butanol from binary mixtures by absorption using water as the absorbent. The dependences of the overall volumetric gas-phase mass transfer coefficient (KGa) on the inlet methanol concentration, the inlet 1-butanol concentration, the rotational speed, the gas flow rate, and the liquid flow rate, were explored. The results demonstrated that the inlet methanol and 1-butanol concentrations had a negligible effect on the KGa values of methanol and 1-butanol. The KGa values of methanol and 1-butanol increased with the rotational speed, the gas flow rate, and the liquid flow rate. The dependence of KGa on the gas flow rate was higher than that on the liquid flow rate, revealing that the mass transfer in binary VOCs absorption may be controlled considerably by the mass transfer in gas phase. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Rotating packed bed Absorption Mass transfer VOCs Blade packings
1. Introduction Volatile organic compounds (VOCs), which are widely used as solvents and detergents in chemical industry, are harmful to the atmosphere when emitted in the exhausted gases. For example, they cause a serious pollution due to the formation of photochemical smog. Additionally, VOCs might harm human’s eyes, skin, kidney, liver, respiratory system, and other organs. Hence, governments made laws concerning concentrations and fluxes of VOCs. Various studies have proposed some common methods for the removal of VOCs, including thermal oxidation (Ruddy and Carroll, 1993; Khan and Ghoshal, 2000), absorption (Khan and Ghoshal, 2000; Lalanne et al., 2008; Heymes et al., 2006), condensation (Khan and Ghoshal, 2000; Dwivedi et al., 2004), membrane separation (Khan and Ghoshal, 2000; Ji et al., 1994), adsorption (Khan and Ghoshal, 2000; Dwivedi et al., 2004), and biological treatment (Khan and Ghoshal, 2000; Daubert et al., 2001). Among these methods, absorption has been proven to be an effective method for the removal of VOCs because it is fast, safe, and economically feasible. In the conventional absorption process, a traditional packed bed (TPB) is used to remove VOCs with an appropriate absorbent. However, due to its poor mass transfer between gas and liquid, an TPB has the bulky volume, thus leading to high capital and operating costs. Accordingly, an alternative to intensify the mass transfer between gas and liquid was proposed by
* Corresponding author. Tel.: þ886 3 2118800x5760; fax: þ886 3 2118800x5702. E-mail address: [email protected] (C.-C. Lin). 0301-4797/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvman.2011.11.011
Ramshaw and Mallinson (1981), who invented a rotating packed bed (RPB). The RPB, which differs from the TPB, offers a high gravity field to which liquid flow is subjected. Because rotating doughnutshaped packings induces the affiliation of centrifugal force, unusual hydrodynamics and mass transfer characteristics can be observed in the RPB. For example, Burns and Ramshaw (1996) proposed that centrifugal force can disperse liquid to thin films or tiny droplets with the help of packings. Thus, the RPB can increase the area of contact between gas and liquid and reduce the total resistance to mass transfer. Therefore, the RPB has smaller volume and lower capital and operating costs as compared with the TPB (Ramshaw, 1983). The RPB has been widely employed to distillation (Lin et al., 2002; Wang et al., 2008), VOCs absorption (Chen and Liu, 2002; Lin et al., 2004, 2009a; Lin and Jian, 2007; Lin and Chien, 2008; Chiang et al., 2009), CO2 absorption (Lin et al., 2003, 2008, 2010; Tan and Chen, 2006; Cheng and Tan, 2006), O3 absorption (Lin and Su, 2008; Lin et al., 2009b), ozone oxidation (Lin and Liu, 2003; Chen et al., 2005; Shang et al., 2006; Chiu et al., 2007; Chang et al., 2009), reactive precipitation (Chen et al., 2000; Shen et al., 2004; Hu et al., 2008; Zhao et al., 2009), and stripping (Lin and Liu, 2006; Li et al., 2009). Chen and Liu (2002) carried out an experimental study on the removal of single VOCs by absorption using an RPB with random packings. The investigated VOCs were isopropyl alcohol, acetone, and ethyl acetate. Their experimental results showed that centrifugal force intensified the mass transfer for the absorption of single VOCs in an RPB. Similarly, Lin et al. (2004) used an RPB with structured packings for removing single VOCs by absorption. They utilized isopropyl alcohol and ethyl acetate as model single VOCs. They found that the mass transfer efficiency of
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Fig. 1. Schematic diagram of RPB with blade packings.
an RPB with structured packings was equivalent to that of an RPB with random packings. To obtain high mass transfer efficiency, Lin and Jian (2007) first adopted the blade as packings in an RPB for the removal of isopropyl alcohol by absorption, as shown in Fig. 1. They proposed that an RPB with blade packings have many superior operating characteristics such as low pressure drop and high mass transfer efficiency. Moreover, our previously published studies presented more results concerning the removal of single VOCs by absorption using an RPB with blade packings (Lin and Chien, 2008; Lin et al., 2009a). Methanol, ethanol, acetone, methyl ethyl ketone, ethyl acetate, and methyl acetate were employed in these studies. According to these results, an RPB with blade packings can be an excellent alternative to remove singe VOCs from a gas stream. However, no previous studies have attempted to investigate the removal of VOCs from binary mixtures using an RPB with blade packings. Moreover, the effect of binary VOCs concentrations on the mass transfer in an RPB with blade packings was very essential for the industrial design. Therefore, the purpose of this investigation is to evaluate the overall volumetric gas-phase mass transfer coefficient (KGa) of an RPB with blade packings in binary VOCs absorption. Additionally, the effects of the VOCs concentrations, the rotational speed, the gas flow rate, and the liquid flow rate on KGa were examined. 2. Experimental Fig. 2 displays the experimental setup for binary VOCs absorption. Two sub-systems were necessary to generate a binary VOCs stream containing methanol and 1-butanol. An air stream of a low flow rate passed through two bubblers that contained methanol.
Fig. 3. Configuration of blade packings in RPB.
Simultaneously, another air stream of a low flow rate flowed through two bubblers that contained 1-butanol. Then, these air streams were combined with an air stream of a high flow rate to yield the desired concentrations of methanol and 1-butanol. Meanwhile, in order to maintain stable the concentrations of methanol and 1-butanol, two buffer flasks were added into the process. The combined air stream containing methanol and 1-butanol moved through the outer edge of the packed bed toward the inner edge of the packed bed. The clean air stream eventually exited from the top of the RPB with blade packings. Water traveled through a liquid distributor into the inner edge of the packed bed, and then exited at the outer edge of the packed bed due to centrifugal acceleration. Finally, water containing methanol and 1-butanol was expelled at the bottom of the RPB with blade packings. Fig. 3 presents the details of schematic design for twelve blades spacing 30 apart used as packings in the RPB. Each blade had an inner radius of 1.95 cm, an outer radius of 6.25 cm, and an axial height of 2.95 cm. The blade packings, which were made of the stainless steel wire mesh, had a specific surface area of 93 m2/m3 and a voidage of 0.99. During operation, the rotational speed of the RPB with blade packings can be varied from 600 to 1800 rpm, providing 17e149 times gravitational acceleration based on the arithmetic mean radius. The range of gas flow rate was 5e55 L/min and the range of liquid flow rate was 0.15e0.65 L/min.
Air Out Flowmeter
Gas-Collecting Tube Buffer Flask Bubbler Motor
water
Blade Packings
Pump Sampler
Liquid Level Tank Bubbler
Water Bath Air In Methanol
Water Bath Effluent Tank
Flowmeter
Buffer Flask
1-Butanol Air In
Fig. 2. Experimental setup for binary VOCs absorption.
Air In
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The Henry’s constants of methanol and 1-butanol in water at 25 C are 2.03 104 and 4.89 104 (mol/m3)/(mol/m3) (Gupta et al., 2000), respectively, implying that the solubility of methanol is higher than that of 1-butanol. The concentrations of methanol and 1-butanol in the inlet and outlet air streams were measured by gas chromatography analyzer (China GCeFID Model 8700F) equipped with a FID and a capillary column (DB-624). The GCeFID used nitrogen as the carrier gas. The injector, oven, and detector temperatures were set to 130, 130, and 200 C, respectively. During normal operation, it took about 5e10 min to reach a steady state by monitoring the concentrations of methanol and 1-butanol in the outlet air stream. All binary VOCs absorption tests were performed at 25 1 C and 1 atm. 3. Results and discussion The overall volumetric gas-phase mass transfer coefficient (KGa) can be calculated using the following equation (Lin and Jian, 2007; Lin and Chien, 2008).
1 Ci 1 ln 1 þ QG A Co A KG a ¼ 1 p R2o R2i Zb 1 A
(1)
where Ci and Co are the concentrations of VOCs in the inlet and outlet air streams, respectively. QG is the volumetric flow rate of
177
gas, Zb is the axial height of the RPB, and Ri and Ro are the inner and outer radius of the RPB, respectively. The absorption factor A is defined as QL/HQG, where QL are the volumetric flow rate of liquid. H is the Henry’s constant of VOCs defined as the ratio of the molar concentration in the gas phase to that in the liquid phase [(mol/m3)/(mol/m3)]. To evaluate the mass transfer performance of the RPB with blade packings for the removal of VOCs from binary mixtures using water as the absorbent, the KGa values were calculated by Eq. (1) with the absorption factor and the corresponding concentrations in VOCs-air steams at various values of the operating parameters, including the concentrations of methanol and 1-butanol in the inlet air stream (Ci,m, Ci,b), the rotational speed (u), the gas flow rate (QG), and the liquid flow rate (QL). Table 1 presents the effect of the 1-butanol concentration in the inlet air stream on the KGa values of methanol and 1-butanol with a fixed methanol concentration of 2000 ppmv in the inlet air stream under various operating conditions. It can be seen from Table 1 that the KGa values of methanol were independent of the inlet 1-butanol concentration under the same operating conditions. For example, the KGa values of methanol varied from 2.97 to 2.89 1/s as the inlet 1-butanol concentration was increased from 250 to 2000 ppmv at a rotational speed of 1800 rpm, a gas flow rate of 20 L/min, and a liquid flow rate of 0.65 L/min. This behavior was attributed to the fact that the transfer rate of methanol into water was not hindered in the presence of 1-butanol due to the same inlet methanol concentration. Moreover, the KGa values of 1-butanol were independent of the inlet 1-butanol concentration under the
Table 1 Dependence of KGa on inlet 1-butanol concentration.
u (rpm)
QG (L/min)
QL (L/min)
Ci,m (ppmv)
Ci,b (ppmv)
KGa (methanol) (1/s)
KGa (1-butanol) (1/s)
600 600 600 600 1000 1000 1000 1000 1400 1400 1400 1400 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800
50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 5 5 5 5 20 20 20 20 35 35 35 35 50 50 50 50 50 50 50 50 50 50 50 50
0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.15 0.15 0.15 0.15 0.25 0.25 0.25 0.25 0.45 0.45 0.45 0.45
2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000
250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000
5.22 5.24 4.99 4.85 6.04 6.14 5.90 5.41 6.11 6.59 6.57 6.16 6.81 6.91 6.88 6.47 0.91 0.88 0.89 0.87 2.97 2.89 2.92 2.89 4.99 4.85 4.87 4.63 3.84 3.22 3.29 3.24 5.47 5.43 5.44 5.31 6.54 6.44 5.99 5.75
3.82 3.57 3.51 3.43 4.40 4.32 4.19 3.75 4.58 4.80 4.74 4.47 5.10 5.07 4.93 4.67 0.58 0.68 0.67 0.64 2.38 2.17 2.15 2.14 3.65 3.69 3.60 3.31 3.02 2.67 2.59 2.37 4.49 4.27 4.13 3.88 4.79 4.75 4.62 4.27
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same operating conditions. For example, the he KGa values of 1-butanol varied from 2.38 to 2.14 1/s while the inlet 1-butanol concentration was increased from 250 to 2000 ppmv at a rotational speed of 1800 rpm, a gas flow rate of 20 L/min, and a liquid flow rate of 0.65 L/min. This phenomenon revealed that the transfer rate of 1-butanol into water increased with the inlet 1-butanol concentration at the same inlet methanol concentration. Similarly, Table 2 indicates the effect of the methanol concentration in the inlet air stream on the KGa values of methanol and 1-butanol with a fixed 1-butanol concentration of 2000 ppmv in the inlet air stream under various operating conditions. As listed in Table 2, the KGa values of 1-butanol were independent of the inlet methanol concentration under the same operating conditions. For example, the KGa values of 1-butanol varied from 0.60 to 0.64 1/s as the inlet methanol concentration was increased from 250 to 2000 ppmv at a rotational speed of 1800 rpm, a gas flow rate of 5 L/min, and a liquid flow rate of 0.65 L/min. This feature was attributed to the fact that the transfer rate of 1-butanol into water was not retarded in the presence of methanol due to the same inlet 1-butanol concentration. Moreover, the KGa values of methanol were independent of the inlet methanol concentration under the same operating conditions. For example, the he KGa values of methanol varied from 0.75 to 0.87 1/s while the inlet methanol concentration was increased from 250 to 2000 ppmv at a rotational speed of 1800 rpm, a gas flow rate of 5 L/min, and a liquid flow rate of 0.65 L/min. This characteristic suggested that the transfer rate of methanol into water increased
with the inlet methanol concentration at the same inlet 1-butanol concentration. According to these results, the following discussions concerning the effects of the rotational speed, the gas flow rate, and the liquid flow rate were explored at an inlet methanol concentration of 2000 ppmv and an inlet 1-butanol concentration of 2000 ppmv. Fig. 4 illustrates the effect of the rotational speed varying from 600 to 1800 rpm on the KGa values. As shown in Fig. 4, there was a trend that the KGa values of methanol and 1-butanol were enhanced by the rotational speed under the same operating conditions. This behavior was also observed in single VOCs absorption in the RPB with blade packings (Lin and Jian, 2007; Lin and Chien, 2008). This result indicated that increasing the rotational speed could improve the mass transfer between gas and liquid because the centrifugal force was able to reduce the total resistance to mass transfer in VOCs absorption. Moreover, the KGa values of methanol were higher than those of 1-butanol under the same operating conditions. Higher solubility of methanol than one of 1-butanol in water could explain this phenomenon. According to Fig. 4, the KGa values of methanol and 1-butanol increased in proportion to the rotational speed raised to the x power as
KG afux
(2)
The x values were calculated by nonlinear regression using Origin Software (Version 8, OriginLab Corporation, Northampton, MA, USA), as listed in Table 3.
Table 2 Dependence of KGa on inlet methanol concentration.
u (rpm)
QG (L/min)
QL (L/min)
Ci,m (ppmv)
Ci,b (ppmv)
KGa (methanol) (1/s)
KGa (1-butanol) (1/s)
600 600 600 600 1000 1000 1000 1000 1400 1400 1400 1400 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1800
50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 50 5 5 5 5 20 20 20 20 35 35 35 35 50 50 50 50 50 50 50 50 50 50 50 50
0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.65 0.15 0.15 0.15 0.15 0.25 0.25 0.25 0.25 0.45 0.45 0.45 0.45
250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000 250 500 1000 2000
2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000 2000
4.21 4.38 4.87 4.85 4.23 5.58 5.64 5.41 6.50 6.16 6.08 6.16 5.96 6.13 6.28 6.47 0.75 0.79 0.83 0.87 2.67 2.79 2.77 2.89 4.66 4.70 4.98 4.63 2.79 2.97 3.12 3.24 4.52 5.04 4.98 5.31 5.99 5.71 5.93 5.75
3.45 3.35 3.37 3.43 3.36 3.69 3.83 3.75 4.75 4.53 4.37 4.47 4.26 4.52 4.42 4.67 0.60 0.60 0.61 0.64 1.95 2.01 1.98 2.14 3.39 3.25 3.48 3.31 2.21 2.32 2.31 2.37 3.53 3.99 4.03 3.88 4.25 4.03 4.30 4.27
L.-J. Hsu, C.-C. Lin / Journal of Environmental Management 98 (2012) 175e182
a
8
methanol 1-butanol
b
8
methanol 1-butanol 6
KGa (1/s)
KGa (1/s)
6
4
2
0 500
4
2
1000
1500
0 500
2000
8
methanol 1-butanol
d
1500
2000
8
6
KGa (1/s)
6
KGa (1/s)
1000
Rotational Speed (rpm)
Rotational Speed (rpm)
c
179
4
2
4
2
methanol 1-butanol 0 500
1000
1500
2000
0 500
Rotational Speed (rpm)
1000
1500
2000
Rotational Speed (rpm)
Fig. 4. Effect of rotational speed on KGa at Ci,m ¼ 2000 ppmv and Ci,b ¼ 2000 ppmv (a) QG ¼ 20 L/min and QL ¼ 0.15 L/min; (b) QG ¼ 20 L/min and QL ¼ 0.65 L/min; (c) QG ¼ 50 L/min and QL ¼ 0.15 L/min; (d) QG ¼ 50 L/min and QL ¼ 0.65 L/min.
When the gas flow rate was 20 L/min, the x values of methanol and 1-butanol at a liquid flow rate of 0.15 L/min were higher than those at a liquid flow rate of 0.65 L/min. This behavior was more evident at a gas flow rate of 50 L/min. This result implied that the rotational speed had a greater effect on the KGa values of methanol and 1-butanol at a low liquid flow rate. Furthermore, when the liquid flow rate was 0.65 L/min, the x values of methanol and 1butanol at a gas flow rate of 50 L/min were greater than those at a gas flow rate of 20 L/min. This feature was more obvious at a liquid flow rate of 0.15 L/min. This result revealed that the rotational speed had a higher effect on the KGa values of methanol and 1-butanol at a high gas flow rate. Hence, according to these observations, the rotational speed yielded a considerable effect on the mass transfer for binary VOCs absorption by increasing the gas flow rate and decreasing the liquid flow rate. Based on the obtained x values ranging from 0.15 to 0.67, KGa increased as a function of the centrifugal acceleration to the power of 0.08e0.34, which was close to 0.18 for single VOCs absorption in the RPB with random packings (Chen and Liu, 2002) and 0.10e0.29 for single VOCs absorption in the RPB with blade packings (Lin and Chien, 2008), but was lower than 0.27e0.43 for single VOCs absorption in the RPB with structured packings (Lin et al., 2004). Fig. 5 presents the relationship between the KGa values and the gas flow rate. Considering Fig. 5, the KGa values of methanol and 1-butanol increased with an increasing gas flow rate under the
same operating conditions. This behavior was mainly attributed to the fact that the resistance to mass transfer in gas phase depended on the thickness of the boundary layer. With an increase in the gas flow rate, the boundary layer tended to be thin at a high gas velocity, leading to a great benefit to the mass transfer. Therefore, an increase in gas flow rate would yield an increase in KGa. Similar characteristic was also found in single VOCs absorption in the RPB with blade packings (Lin and Jian, 2007; Lin and Chien, 2008). As shown in Fig. 5, the KGa values of methanol and 1-butanol were proportional to the gas flow rate raised to the y power as
KG afQGy
(3)
Table 3 Variation of x with operating conditions. QG (L/min)
QL (L/min)
VOCs
x
20 20 20 20 50 50 50 50
0.15 0.15 0.65 0.65 0.15 0.15 0.65 0.65
methanol 1-butanol methanol 1-butanol methanol 1-butanol methanol 1-butanol
0.29 0.35 0.15 0.22 0.67 0.53 0.27 0.30
180
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a
b
8
methanol 1-butanol
methanol 1-butanol 6
KGa (1/s)
KGa (1/s)
6
8
4
2
4
2
0
0 0
10
20
30
40
50
60
0
10
Gas Flow Rate (L/min)
c
d
8
methanol 1-butanol
30
40
50
60
8
6
KGa (1/s)
6
KGa (1/s)
20
Gas Flow Rate (L/min)
4
2
4
2
methanol 1-butanol 0
0 0
10
20
30
40
50
60
0
Gas Flow Rate (L/min)
10
20
30
40
50
60
Gas Flow Rate (L/min)
Fig. 5. Effect of gas flow rate on KGa at Ci,m ¼ 2000 ppmv and Ci,b ¼ 2000 ppmv (a) QL ¼ 0.15 L/min and u ¼ 600 rpm; (b) QL ¼ 0.15 L/min and u ¼ 1800 rpm; (c) QL ¼ 0.65 L/min and u ¼ 600 rpm; (d) QL ¼ 0.65 L/min and u ¼ 1800 rpm.
The y values were calculated by nonlinear regression using Origin Software (Version 8, OriginLab Corporation, Northampton, MA, USA), as listed in Table 4. With a liquid flow rate of 0.65 L/min, the y values of methanol and 1-butanol at a rotational speed of 1800 rpm were higher than those at a rotational speed of 600 rpm. This behavior was more evident at a liquid flow rate of 0.15 L/min. This result implied that the gas flow rate had a greater effect on the KGa values of methanol and 1-butanol at a high rotational speed. Furthermore, at a rotational speed of 1800 rpm, the y values of methanol and 1-butanol.at a liquid flow rate of 0.65 L/min were greater than those at a liquid flow rate of 0.15 L/min for This feature was also found at a rotational speed of 600 rpm. This result revealed that the gas flow rate had a higher effect on the KGa values of methanol and 1-butanol at a high liquid flow rate. Therefore, based on these observations, the Table 4 Variation of y with operating conditions. QL (L/min)
u (rpm)
VOCs
y
0.15 0.15 0.15 0.15 0.65 0.65 0.65 0.65
600 600 1800 1800 600 600 1800 1800
methanol 1-butanol methanol 1-butanol methanol 1-butanol methanol 1-butanol
0.43 0.59 0.63 0.64 0.76 0.79 0.88 0.87
gas flow rate yielded a marked effect on the mass transfer for binary VOCs absorption by increasing the rotational speed and the liquid flow rate. The dependence of KGa on the gas flow rate to the power of 0.43e0.88 was close to the reported range of 0.48e0.71 for single VOCs absorption in the RPB with blade packings (Lin and Chien, 2008) and the reported range of 0.62e0.77 for single VOCs absorption in the RPB with structured packings (Lin et al., 2004), but was higher than the exponent (0.32) for single VOCs absorption in the RPB with random packings (Chen and Liu, 2002). The effect of the liquid flow rate, varying from 0.15 to 0.65 L/min, on the KGa values was shown in Fig. 6. It was found in the figure that increasing the liquid flow rate increased the KGa values. This behavior was probably because increasing the liquid flow rate would provide more liquid films spreading over the packing and more liquid droplets moving within the void of the bed, thus leading to a large area of contact between gas and liquid. Moreover, considering Fig. 6, the KGa values of methanol and 1-butanol were proportional to the liquid flow rate raised to the z power as
KG afQLz
(4)
The z values were calculated by nonlinear regression using Origin Software (Version 8, OriginLab Corporation, Northampton, MA, USA), as listed in Table 5. With a gas flow rate of 20 L/min, the z values of methanol and 1-butanol at a rotational speed of 600 rpm were higher than those at a rotational speed of 1800 rpm. This behavior was more evident
L.-J. Hsu, C.-C. Lin / Journal of Environmental Management 98 (2012) 175e182
a
b
8
methanol 1-butanol
8
methanol 1-butanol 6
KGa (1/s)
KGa (1/s)
6
4
2
4
2
0
0 0.0
0.2
0.4
0.6
0.8
0.0
0.2
Liquid Flow Rate (L/min)
c
0.4
0.6
0.8
Liquid Flow Rate (L/min)
d
8
methanol 1-butanol
8
6
KGa (1/s)
6
KGa (1/s)
181
4
2
4
2
methanol 1-butanol 0
0 0.0
0.2
0.4
0.6
0.8
0.0
0.2
Liquid Flow Rate (L/min)
0.4
0.6
0.8
Liquid Flow Rate (L/min)
Fig. 6. Effect of liquid flow rate on KGa at Ci,m ¼ 2000 ppmv and Ci,b ¼ 2000 ppmv (a) QG ¼ 20 L/min and u ¼ 600 rpm; (b) QG ¼ 20 L/min and u ¼ 1800 rpm; (c) QG ¼ 50 L/min and u ¼ 600 rpm; (d) QG ¼ 50 L/min and u ¼ 1800 rpm.
at a gas flow rate of 50 L/min. This result implied that the liquid flow rate had a greater effect on the KGa values of methanol and 1butanol at a low rotational speed. Furthermore, at a rotational speed of 1800 rpm, the z values of methanol and 1-butanol.at a gas flow rate of 50 L/min was higher than those at a gas flow rate of 20 L/min. This feature was more obvious at a rotational speed of 600 rpm. This result revealed that the liquid flow rate had a higher effect on the KGa values of methanol and 1-butanol at a high gas flow rate. Accordingly, owing to these observations, the liquid flow rate yielded a substantial effect on the mass transfer for binary VOCs absorption by decreasing the rotational speed and increasing the gas flow rate. Based on the obtained z values, KGa was proportional to the liquid flow rate raised to the power of 0.29e0.54, which was close to Table 5 Variation of z with operating conditions. QG (L/min)
u (rpm)
VOCs
z
20 20 20 20 50 50 50 50
600 600 1800 1800 600 600 1800 1800
methanol 1-butanol methanol 1-butanol methanol 1-butanol methanol 1-butanol
0.37 0.38 0.29 0.32 0.54 0.46 0.38 0.37
0.24e0.67 for single VOCs absorption in the RPB with blade packings (Lin and Chien, 2008), 0.34e0.49 for single VOCs absorption in the RPB with structured packings (Lin et al., 2004), and 0.33 for single VOCs absorption in the RPB with random packings (Chen and Liu, 2002). According to Tables 4 and 5, the dependence of KGa on the gas flow rate for binary VOCs absorption was higher than that on the liquid flow rate, implying that the total resistance to mass transfer in binary VOCs absorption may be controlled considerably by the resistance in gas phase (Strigle, 1987).
4. Conclusions This investigation studied the mass transfer of an RPB with blade packings in binary VOCs absorption. The overall volumetric gasphase mass transfer coefficients (KGa) were estimated as functions of the inlet methanol concentration, the inlet 1-butanol concentration, the rotational speed, the gas flow rate, and the liquid flow rate. The KGa values of methanol and 1-butanol were independent of the inlet methanol concentration and the inlet 1-butanol concentration. However, the KGa values of methanol and 1-butanol increased with the rotational speed, the liquid flow rate, and the gas flow rate. The rotational speed could provide a stronger effect on the KGa values of methanol and 1-butanol at higher gas flow rates and lower liquid flow rates. The gas flow rate could provide a stronger effect on the KGa values of methanol and 1-butanol at higher rotational speed and
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