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Mass transfer performance of a rotating packed bed equipped with blade packings in removing methanol and 1-butanol from gaseous streams
Chemical Engineering and Processing 53 (2012) 76–81
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Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Mass transfer performance of a rotating packed bed equipped with blade packings in removing methanol and 1-butanol from gaseous streams Chia-Chang Lin ∗ , Yu-Chiao Lin Department of Chemical and Materials Engineering, Chang Gung University, Taoyuan, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 26 March 2011 Accepted 14 December 2011 Available online 22 December 2011 Keywords: Rotating packed bed Absorption Mass transfer VOCs Blade packings
a b s t r a c t The removal of methanol and 1-butanol from gaseous streams by absorption with water was investigated in the RPB equipped with blade packings. The overall volumetric gas-phase mass transfer coefficient (KG a) for methanol and 1-butanol absorption was observed to increase with the rotational speed, the gas flow rate, and the liquid flow rate. Also, the local volumetric gas-phase mass transfer coefficient (kG a) was estimated, and then the portion of the total resistance to mass transfer in gas phase was determined. The result indicated that more than 90% of the total resistance to mass transfer in methanol and 1butanol absorption was found to be due to the gas phase. Comparison with the conventional packed tower demonstrated that mass transfer efficiency in the RPB equipped with blade packing was higher than that in the conventional packed tower. Consequently, the RPB equipped with blade packings would be an excellent absorber for the removal of alkanols from the exhausted gases. © 2012 Elsevier B.V. All rights reserved.
1. Introduction To remove VOCs from gaseous streams under various industrial processes, various methods, including thermal oxidation, adsorption, condensation, membrane separation, absorption, and biological treatment, have been developed. Absorption has been generally considered to be a fast, safe, and economically feasible method. To enhance the VOCs removal, the conventional VOCs absorption process used some gas–liquid contactors such as packed columns, spray columns, and bubble columns. Owing to that significant mass transfer limitation existed in the conventional gas–liquid contactors, a large space for installation and operation was needed, thus leading to high capital and operating costs. Recently, several novel types of gas–liquid contactors for VOCs absorption have been developed. In particular, a promising alternative of intensifying mass transfer is to create a significantly more rapid regeneration of the gas–liquid phase interface. This type of process can be achieved by contacting liquid and gas under a very high centrifugal acceleration using a rotating doughnut-shaped packing element. According to this concept, Ramshaw and Mallinson [1] invented a rotating packed bed (RPB) for enhancing gas–liquid mass transfer in distillation and absorption. This unique technology is referred to as “Higee” (an acronym for high gravity). Under RPB operation, thin films and tiny droplets generated owing to a rigorous centrifugal acceleration could provide an
∗ Corresponding author. Tel.: +886 3 2118800x5760; fax: +886 3 2118800x5702. E-mail address: [email protected] (C.-C. Lin). 0255-2701/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2011.12.007
enhancement in the gas–liquid mass transfer. Furthermore, the RPB could be operated higher gas and/or liquid flow rates owing to the low tendency of flooding relative to that in the conventional packed tower. Therefore, the gas–liquid mass transfer would frequently be enhanced by a factor of 10–100 and the dramatic reduction in the equipment size would be achieved, thereby reducing the capital and operating costs [2]. A variety of studies have appeared in which the attention was focused on the applications of the RPB in diverse processes such as distillation [3–5], VOCs absorption [6–12], CO2 absorption [13–21], O3 absorption [22–24], ozonation [25–32], reactive precipitation [33–44], and stripping [45,46]. To reduce the pressure drop of the conventional RPB equipped with random and structured packings used for treating VOCs from gaseous streams of huge flow rate, Lin and Jain [9] developed a novel RPB by adopting blades as packings, as shown in Fig. 1. Their results demonstrated that the RPB equipped with blade packings could provide some superior characteristics such as low gas pressure drop and high gas-phase mass transfer coefficient [9,10]. To examine the mass transfer performance of the RPB equipped with blade packings for VOCs removal, various VOCs absorption processes should be carried out in the RPB equipped with blade packings. Therefore, the aim of this work is to remove methanol and 1-butanol from air streams by absorption using water as the absorbent in the RPB equipped with blade packings. The interested operating parameters in this work are rotational speed, gas flow rate, and liquid flow rate. Also, comparison with conventional packed tower is investigated. Results in this work provide further insight into the feasibility of applying the RPB equipped with blade packings to the removal of VOCs from gaseous streams.
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Fig. 1. Schematic diagram of RPB equipped with blade packings.
2. Experimental Fig. 2 indicates the experimental procedure for VOCs removal using the RPB equipped with blade packings. Water stream was pumped into the inner border of the packed bed with the help of a liquid distributor. Water stream flowed outward due to centrifugal acceleration, and then left the packed bed at the outer border. During normal operation, an air stream first was introduced to two bubblers containing VOCs, and then mixed with another air stream for obtaining the VOCs concentration of 2000 ppmv in inlet air stream. Meanwhile, in order to maintain stable VOCs concentration, a buffer flask was added into the process. Next, VOCs-air stream entered at the outer border of the packed bed and flowed inward. Then, VOCs-air stream with a low VOCs concentration left from the top of the RPB. Owing to that VOCs-air stream contacted with water stream counter-currently in the RPB, VOCs in VOCs-air stream could be removed into the water stream. Finally, VOCs-rich aqueous solution was expelled from the bottom of the RPB. 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. The liquid distributor consisted of a tube, which had five holes of 0.1 cm diameter with 0.6 cm interval in a vertical direction. The RPB could be operated at the rotational speed of 400–1800 rpm, providing 7–149 times gravitational acceleration based on the arithmetic mean radius. The gas flow rate could be varied at the range of 5–60 L/min and the liquid flow rate could be varied at the range of 0.1–0.7 L/min.
Fig. 3. Structure of blade packings.
Two VOCs, namely methanol and 1-butanol, were used in this work. The VOCs concentrations in inlet and outlet VOCs-air streams from the gas-collecting tubes were measured by a gas chromatography analyzer (China GC-FID Model 8700F) equipped with a FID and a capillary column (DB-624). Nitrogen was used as the carrier gas. The injector, column, and detector temperatures for methanol were set at 130, 90, and 200 ◦ C, respectively. The injector, column, and detector temperatures for 1-butanol were set at 130, 140, and 200 ◦ C, respectively. All experiments were conducted at 25 ± 1 ◦ C with atmospheric pressure. During normal operation, the steady state was observed after 10–15 min by monitoring the VOCs concentration in outlet VOCs-air streams. The reproducibility tests under almost all of the operating conditions were carried out in this work. The VOCs concentrations in outlet VOCs-air streams were observed to be reproduced with a deviation of less than 5%.
3. Results and discussion The overall volumetric gas-phase mass transfer coefficient (KG a) for VOCs absorption process in the RPB equipped with blade packings can be obtained with the concept of mass transfer and transfer unit. First, we consider the cylindrical control volume having an
Air Out Flowmeter
Gas-Collecting Tube
Buffer Flask Motor Water
Pump Sampler
Liquid Level Tank
Bubbler
Blade Packings Flowmeter
Water Bath Air In VOCs
Effluent Tank Fig. 2. Experimental setup for VOCs absorption process.
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8
inside radius, r, height zb , and thickness dr. The mass balance for a solute in this control volume for a dilute process is QG dCG = KG a(CG − CG∗ )2rzb dr
(1)
CG∗ H
−0
1-Butanol ; Liquid Flow Rate : 0.65 L/min
4
(2) 2
i.e., 1 (CG − CG,o ) A
(3)
where CG,o is the VOCs concentration in the outlet air stream, QL is the volumetric flow rate of liquid, and H is the Henry’s law constant of a solute defined as the ratio of the molar concentration in the gas phase to that in the liquid phase. The H values of methanol and 1-butanol at 25 ◦ C are 2.03 × 10−4 and 4.89 × 10−4 (mol/m3 )/(mol/m3 ) [47], respectively. Based on the Henry’s constants, the solubility of methanol in water is higher than that of 1-butanol in water. Besides, A is absorption factor defined as QL HQG
(4)
Then we substitute Eq. (3) into Eq. (1) and integrate the equation from R = Ri to R = Ro with the boundary conditions CG = CG,o and CG = CG,i , respectively. KG a =
QG (Ro2 − Ri2 )Zb
ln[(1 − 1/A)(CG,i /CG,o ) + 1/A] (1 − 1/A)
(5)
where CG,i is the VOCs concentration in the inlet air stream, Zb is the axial height of the RPB, and Ri and Ro are the inner and outer radius of the RPB, respectively. A higher KG a values represents the superior gas–liquid mass transfer. To evaluate mass transfer of the RPB equipped with blade packings for methanol and 1-butanol removal using absorption with water, the KG a values calculated by Eq. (5) were examined as functions of the main operating parameters, namely rotational speed (ω), gas flow rate (QG ), and liquid flow rate (QL ). Fig. 4 illustrates the effect of the rotational speed on the KG a values at a given gas flow rate of 55 L/min for methanol and 1-butanol absorption. It was expected that the KG a values increased with an increasing rotational speed at the range from 600 to 1800 rpm. This result implied that the mass transfer in methanol and 1-butanol absorption could be enhanced by an increasing rotational speed. Moreover, the KG a values for methanol absorption were higher than those for 1-butanol absorption. This behavior was attributed to the fact that the solubility of methanol in water is greater than that of 1-butanol in water [47]. It was concluded that the centrifugal force induced from the RPB equipped with blade packings did not affect the thermodynamic property of VOCs. Besides, the exponent (x) of KG a ∝ ωx was correlated. For methanol absorption, the x values were obtained as 0.31 and 0.30 at the liquid flow rates of 0.14 and 0.65 L/min, respectively. This result suggested that an increase in KG a by the rotational speed was independent of the liquid flow rate in methanol absorption. This behavior was also observed in 1-butanol absorption, indicating that the x values varied from 0.23 to 0.20 as the liquid flow rate was increased from 0.14 to 0.65 L/min. Fig. 5 shows the effect of the gas flow rate on the KG a values at a given rotational speed of 1800 rpm for methanol and 1-butanol absorption. It was realized that the resistance to mass transfer
0 400
800
1200
1600
2000
Rotational Speed (rpm) Fig. 4. Effect of rotational speed on KG a for VOCs absorption at QG = 55 L/min.
in gas phase was reduced with an increasing gas flow rate, and thus higher KG a values were obtained at higher gas flow rates for methanol and 1-butanol absorption at the liquid flow rates of 0.14 and 0.65 L/min. In addition, the exponent (y) of KG aQG y was correlated. For methanol absorption, the y values were obtained as 0.79 and 0.81 at the liquid flow rates of 0.14 and 0.65 L/min, respectively. This result implied that an increase in KG a by the gas flow was independent of the liquid flow rate in methanol absorption. This feature was not found in 1-butanol absorption, indicating that the y values varied from 0.73 to 0.79 while the liquid flow rate was increased from 0.14 to 0.65 L/min. This result suggested that an increase in KG a by the gas flow rate was obvious at higher liquid flow rates in 1-butanol absorption. Fig. 6 presents the effect of the liquid flow rate on the KG a values at a given rotational speed of 1800 rpm for methanol and 1-butanol absorption. It was found in Fig. 6 that higher KG a values were obtained at higher liquid flow rates for methanol and 1-butanol absorption at the gas flow rates of 22 and 55 L/min. This characteristic was probably owing to that increasing the liquid flow rate
8
Methanol ; Liquid Flow Rate : 0.65 L/min Methanol ; Liquid Flow Rate : 0.14 L/min 1-Butanol ;Liquid Flow Rate : 0.65 L/min 1-Butanol ; Liquid Flow Rate : 0.14 L/min
6
KGa (1/s)
CG∗ =
A=
1-Butanol ; Liquid Flow Rate : 0.14 L/min
6
KGa (1/s)
where QG is the volumetric flow rate of gas and CG is the concentration of a solute in gas phase. CG∗ stands for the equilibrium concentration associated with the concentration of a solute in liquid phase. Also, the overall mass balance can be expressed as for the case of solute-free liquid input (CL,i = 0): QG (CG − CG,o ) = QL (CL − CL,i ) = QL
Methanol ; Liquid Flow Rate : 0.65 L/min Methanol ; Liquid Flow Rate : 0.14 L/min
4
2
0 0
15
30
45
60
Gas Flow Rate (L/min) Fig. 5. Effect of gas flow rate on KG a for VOCs absorption at ω = 1800 rpm.
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Table 1 Absorption of methanol and 1-butanol in water. ω (rpm)
QG (L/min)
QL (L/min)
VOCs
KG a (1/s)
kL a (1/s)
kG a (1/s)
% of total resistance in gas phase
600 1000 1400 1800 600 1000 1400 1800 600 1000 1400 1800 600 1000 1400 1800 600 1000 1400 1800 600 1000 1400 1800 600 1000 1400 1800 600 1000 1400 1800
22 22 22 22 22 22 22 22 55 55 55 55 55 55 55 55 22 22 22 22 22 22 22 22 55 55 55 55 55 55 55 55
0.14 0.14 0.14 0.14 0.65 0.65 0.65 0.65 0.14 0.14 0.14 0.14 0.65 0.65 0.65 0.65 0.14 0.14 0.14 0.14 0.65 0.65 0.65 0.65 0.14 0.14 0.14 0.14 0.65 0.65 0.65 0.65
Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol
1.62 2.06 2.55 2.60 2.35 2.78 3.11 3.39 2.96 3.77 3.80 4.26 4.50 4.89 5.23 6.50 0.83 0.91 1.01 1.12 1.00 1.38 1.46 1.58 1.31 1.48 1.57 1.70 2.11 2.22 2.30 2.72
0.0089 0.0117 0.0141 0.0161 0.0356 0.0471 0.0567 0.0651 0.0089 0.0117 0.0141 0.0161 0.0356 0.0471 0.0567 0.0651 0.0089 0.0117 0.0141 0.0161 0.0356 0.0471 0.0567 0.0651 0.0089 0.0117 0.0141 0.0161 0.0356 0.0471 0.0567 0.0651
1.68 2.13 2.65 2.69 2.38 2.81 3.15 3.43 3.17 4.03 4.02 4.50 4.62 4.99 5.33 6.63 0.87 0.95 1.05 1.16 1.02 1.40 1.48 1.60 1.41 1.58 1.66 1.79 2.18 2.28 2.34 2.78
96.3 96.4 96.3 96.7 98.7 98.8 98.9 98.9 93.3 93.5 94.5 94.6 97.4 97.9 98.1 98.0 95.4 96.2 96.5 96.6 98.6 98.6 98.7 98.8 92.8 93.8 94.5 94.8 97.1 97.7 98.0 98.0
would result in more liquid films spreading over the packing and more liquid droplets flying within the voidage of the bed, thus leading to an increase in gas–liquid interfacial area and a higher KG a. Besides, the exponent (z) of KG aQL z was correlated. For methanol absorption, the z values were obtained as 0.15 and 0.25 at the gas flow rates of 22 and 55 L/min, respectively. This result implied that an increase in KG a by the liquid flow rate was more evident at higher gas flow rates in methanol absorption. This characteristic was also found in 1-butanol absorption, indicating that the z values varied
8
Methanol ; Gas Flow Rate : 22 L/min 1-Butanol ; Gas Flow Rate : 55 L/min
KGa (1/s)
4
2
0 0.2
0.4
1 1/KG a − H/kL a
kL adp = 0.478Re0.906 Gr0.275 L L DL at
1-Butanol ; Gas Flow Rate : 22 L/min
0.0
kG a =
(5 )
where KG a could be obtained from the experimental data for VOCs absorption in this work and kL a could be calculated using the correlation for the RPB equipped with blade packings proposed by Lin and Jain [9], as follows
Methanol ; Gas Flow Rate : 55 L/min
6
from 0.23 to 0.31 while the gas flow rate was increased from 22 to 55 L/min. This result indicated that an increase in KG a by the liquid flow rate was more pronounced at higher gas flow rates in 1-butanol absorption. To evaluate the local volumetric gas-phase mass transfer coefficient (kG a), a relation based on two-film theory was presented, as follows
0.6
0.8
Liquid Flow Rate (L/min) Fig. 6. Effect of liquid flow rate on KG a for VOCs absorption at ω = 1800 rpm.
(6)
By substituting Eq. (6) and KG a obtained in this work into Eq. (5), the local volumetric gas-phase mass transfer coefficient (kG a) for the RPB equipped with blade packings can be obtained, as listed in Table 1. Also, the percentage of total resistance to mass transfer in gas phase was evaluated, as shown in Table 1. The result of Table 1 indicates that in the absorption of both methanol and 1-butanol the resistance in gas phase represented an appreciable portion of the total resistance to mass transfer. In methanol absorption, the resistance in gas phase constituted as much as 93.3–96.7% of the total resistance at a low liquid flow rate and as much as 97.4–98.9% of the total resistance at a high liquid flow rate. In 1-butanol absorption, the resistance in gas phase constituted as much as 92.8–96.6% of the total resistance at a low liquid flow rate and as much as 97.1–98.8% of the total resistance at a high liquid flow rate. Based on above discussion, in methanol and 1-butanol absorption the total resistance to mass transfer was significantly dominated by the resistance in gas phase. This
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Table 2 Comparison of conventional packed tower and rotating packed bed.
Operation conditions Pressure (atm) Temperature (K) Gas phase Gas loading (m3 /(m2 s)) Liquid phase Liquid loading (m3 /(m2 h)) VOCs Solute Concentration (ppmv) Packing Type Bed volume (cm3 ) Voidage KG a (1/s)
Acknowledgement
Conventional packed tower [48]
Rotating packed bed
The financial support of National Science Council of the Republic of China (NSC 97-2221-E-182-013-MY2) is greatly appreciated.
1 300
1 298
References
0.12
0.13
2.5–9.8
1.3–5.7
Methanol 10,000–25,000
Methanol 2000
1-in. Raschig rings 43,146 0.72 0.4–0.7
Blades 327 0.99 4.0–6.6
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finding was also observed in the RPB equipped with blade packings for isopropyl alcohol absorption [9] and the conventional packed tower for methanol absorption [48]. This result suggested that the resistance to mass transfer in gas phase would be predominant for alkanols absorption owing to that the solubility of alkanols in water was higher than that of other VOCs in water. To verify the mass transfer performance of the RPB equipped with blade packings, the KG a values obtained in this work were compared with the data of the conventional packed tower [48]. The conventional packed tower used by Houston and Walker [48] was 1.94 feet in height and 12 in. in diameter, and dealt with the methanol absorption using water. It was seen from Table 2 that the KG a values in the RPB equipped with blade packings were about 10 times higher than those in the conventional packed tower. This behavior was similar to the result in the RPB equipped with structured packings for ethanol absorption [12], presenting that the KG a values in the RPB equipped with structured packings was 3–9 times higher than those in the conventional packed tower. Therefore, the size of an absorber can be significantly reduced if the RPB instead of the conventional packed tower is used to remove VOCs from the gas stream. 4. Conclusions This work has examined the removal of methanol and 1-butanol from an air stream by absorption using water as the absorbent in the RPB equipped with blade packings under different operating conditions. The overall volumetric gas-phase mass transfer coefficients (KG a) were estimated as functions of the rotational speed, the gas flow rate, and the liquid flow rate. The KG a values for methanol and 1-butanol absorption increased with the rotational speed, the gas flow rate, and the liquid flow rate. The KG a values for methanol absorption were higher than those for 1-butanol absorption owing to that the solubility of methanol in water is higher than that of 1-butanol in water. Therefore, the thermodynamic property of VOCs was not affected by the centrifugal force in the RPB equipped with blade packings. Based on the percentage of total resistance to mass transfer in gas phase, the total resistance to mass transfer in methanol and 1-butanol absorption was observed to be substantially controlled by the resistance in gas phase. The comparison between the RPB equipped with blade packings and the conventional packed tower demonstrated that mass transfer efficiency of the RPB equipped with blade packing was higher than that of conventional packed tower. Consequently, the RPB equipped with blade packings is an excellent alternative with high mass transfer efficiency to remove alkanols from the exhausted gases.
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Contents lists available at SciVerse ScienceDirect
Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Mass transfer performance of a rotating packed bed equipped with blade packings in removing methanol and 1-butanol from gaseous streams Chia-Chang Lin ∗ , Yu-Chiao Lin Department of Chemical and Materials Engineering, Chang Gung University, Taoyuan, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 26 March 2011 Accepted 14 December 2011 Available online 22 December 2011 Keywords: Rotating packed bed Absorption Mass transfer VOCs Blade packings
a b s t r a c t The removal of methanol and 1-butanol from gaseous streams by absorption with water was investigated in the RPB equipped with blade packings. The overall volumetric gas-phase mass transfer coefficient (KG a) for methanol and 1-butanol absorption was observed to increase with the rotational speed, the gas flow rate, and the liquid flow rate. Also, the local volumetric gas-phase mass transfer coefficient (kG a) was estimated, and then the portion of the total resistance to mass transfer in gas phase was determined. The result indicated that more than 90% of the total resistance to mass transfer in methanol and 1butanol absorption was found to be due to the gas phase. Comparison with the conventional packed tower demonstrated that mass transfer efficiency in the RPB equipped with blade packing was higher than that in the conventional packed tower. Consequently, the RPB equipped with blade packings would be an excellent absorber for the removal of alkanols from the exhausted gases. © 2012 Elsevier B.V. All rights reserved.
1. Introduction To remove VOCs from gaseous streams under various industrial processes, various methods, including thermal oxidation, adsorption, condensation, membrane separation, absorption, and biological treatment, have been developed. Absorption has been generally considered to be a fast, safe, and economically feasible method. To enhance the VOCs removal, the conventional VOCs absorption process used some gas–liquid contactors such as packed columns, spray columns, and bubble columns. Owing to that significant mass transfer limitation existed in the conventional gas–liquid contactors, a large space for installation and operation was needed, thus leading to high capital and operating costs. Recently, several novel types of gas–liquid contactors for VOCs absorption have been developed. In particular, a promising alternative of intensifying mass transfer is to create a significantly more rapid regeneration of the gas–liquid phase interface. This type of process can be achieved by contacting liquid and gas under a very high centrifugal acceleration using a rotating doughnut-shaped packing element. According to this concept, Ramshaw and Mallinson [1] invented a rotating packed bed (RPB) for enhancing gas–liquid mass transfer in distillation and absorption. This unique technology is referred to as “Higee” (an acronym for high gravity). Under RPB operation, thin films and tiny droplets generated owing to a rigorous centrifugal acceleration could provide an
∗ Corresponding author. Tel.: +886 3 2118800x5760; fax: +886 3 2118800x5702. E-mail address: [email protected] (C.-C. Lin). 0255-2701/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2011.12.007
enhancement in the gas–liquid mass transfer. Furthermore, the RPB could be operated higher gas and/or liquid flow rates owing to the low tendency of flooding relative to that in the conventional packed tower. Therefore, the gas–liquid mass transfer would frequently be enhanced by a factor of 10–100 and the dramatic reduction in the equipment size would be achieved, thereby reducing the capital and operating costs [2]. A variety of studies have appeared in which the attention was focused on the applications of the RPB in diverse processes such as distillation [3–5], VOCs absorption [6–12], CO2 absorption [13–21], O3 absorption [22–24], ozonation [25–32], reactive precipitation [33–44], and stripping [45,46]. To reduce the pressure drop of the conventional RPB equipped with random and structured packings used for treating VOCs from gaseous streams of huge flow rate, Lin and Jain [9] developed a novel RPB by adopting blades as packings, as shown in Fig. 1. Their results demonstrated that the RPB equipped with blade packings could provide some superior characteristics such as low gas pressure drop and high gas-phase mass transfer coefficient [9,10]. To examine the mass transfer performance of the RPB equipped with blade packings for VOCs removal, various VOCs absorption processes should be carried out in the RPB equipped with blade packings. Therefore, the aim of this work is to remove methanol and 1-butanol from air streams by absorption using water as the absorbent in the RPB equipped with blade packings. The interested operating parameters in this work are rotational speed, gas flow rate, and liquid flow rate. Also, comparison with conventional packed tower is investigated. Results in this work provide further insight into the feasibility of applying the RPB equipped with blade packings to the removal of VOCs from gaseous streams.
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77
Fig. 1. Schematic diagram of RPB equipped with blade packings.
2. Experimental Fig. 2 indicates the experimental procedure for VOCs removal using the RPB equipped with blade packings. Water stream was pumped into the inner border of the packed bed with the help of a liquid distributor. Water stream flowed outward due to centrifugal acceleration, and then left the packed bed at the outer border. During normal operation, an air stream first was introduced to two bubblers containing VOCs, and then mixed with another air stream for obtaining the VOCs concentration of 2000 ppmv in inlet air stream. Meanwhile, in order to maintain stable VOCs concentration, a buffer flask was added into the process. Next, VOCs-air stream entered at the outer border of the packed bed and flowed inward. Then, VOCs-air stream with a low VOCs concentration left from the top of the RPB. Owing to that VOCs-air stream contacted with water stream counter-currently in the RPB, VOCs in VOCs-air stream could be removed into the water stream. Finally, VOCs-rich aqueous solution was expelled from the bottom of the RPB. 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. The liquid distributor consisted of a tube, which had five holes of 0.1 cm diameter with 0.6 cm interval in a vertical direction. The RPB could be operated at the rotational speed of 400–1800 rpm, providing 7–149 times gravitational acceleration based on the arithmetic mean radius. The gas flow rate could be varied at the range of 5–60 L/min and the liquid flow rate could be varied at the range of 0.1–0.7 L/min.
Fig. 3. Structure of blade packings.
Two VOCs, namely methanol and 1-butanol, were used in this work. The VOCs concentrations in inlet and outlet VOCs-air streams from the gas-collecting tubes were measured by a gas chromatography analyzer (China GC-FID Model 8700F) equipped with a FID and a capillary column (DB-624). Nitrogen was used as the carrier gas. The injector, column, and detector temperatures for methanol were set at 130, 90, and 200 ◦ C, respectively. The injector, column, and detector temperatures for 1-butanol were set at 130, 140, and 200 ◦ C, respectively. All experiments were conducted at 25 ± 1 ◦ C with atmospheric pressure. During normal operation, the steady state was observed after 10–15 min by monitoring the VOCs concentration in outlet VOCs-air streams. The reproducibility tests under almost all of the operating conditions were carried out in this work. The VOCs concentrations in outlet VOCs-air streams were observed to be reproduced with a deviation of less than 5%.
3. Results and discussion The overall volumetric gas-phase mass transfer coefficient (KG a) for VOCs absorption process in the RPB equipped with blade packings can be obtained with the concept of mass transfer and transfer unit. First, we consider the cylindrical control volume having an
Air Out Flowmeter
Gas-Collecting Tube
Buffer Flask Motor Water
Pump Sampler
Liquid Level Tank
Bubbler
Blade Packings Flowmeter
Water Bath Air In VOCs
Effluent Tank Fig. 2. Experimental setup for VOCs absorption process.
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8
inside radius, r, height zb , and thickness dr. The mass balance for a solute in this control volume for a dilute process is QG dCG = KG a(CG − CG∗ )2rzb dr
(1)
CG∗ H
−0
1-Butanol ; Liquid Flow Rate : 0.65 L/min
4
(2) 2
i.e., 1 (CG − CG,o ) A
(3)
where CG,o is the VOCs concentration in the outlet air stream, QL is the volumetric flow rate of liquid, and H is the Henry’s law constant of a solute defined as the ratio of the molar concentration in the gas phase to that in the liquid phase. The H values of methanol and 1-butanol at 25 ◦ C are 2.03 × 10−4 and 4.89 × 10−4 (mol/m3 )/(mol/m3 ) [47], respectively. Based on the Henry’s constants, the solubility of methanol in water is higher than that of 1-butanol in water. Besides, A is absorption factor defined as QL HQG
(4)
Then we substitute Eq. (3) into Eq. (1) and integrate the equation from R = Ri to R = Ro with the boundary conditions CG = CG,o and CG = CG,i , respectively. KG a =
QG (Ro2 − Ri2 )Zb
ln[(1 − 1/A)(CG,i /CG,o ) + 1/A] (1 − 1/A)
(5)
where CG,i is the VOCs concentration in the inlet air stream, Zb is the axial height of the RPB, and Ri and Ro are the inner and outer radius of the RPB, respectively. A higher KG a values represents the superior gas–liquid mass transfer. To evaluate mass transfer of the RPB equipped with blade packings for methanol and 1-butanol removal using absorption with water, the KG a values calculated by Eq. (5) were examined as functions of the main operating parameters, namely rotational speed (ω), gas flow rate (QG ), and liquid flow rate (QL ). Fig. 4 illustrates the effect of the rotational speed on the KG a values at a given gas flow rate of 55 L/min for methanol and 1-butanol absorption. It was expected that the KG a values increased with an increasing rotational speed at the range from 600 to 1800 rpm. This result implied that the mass transfer in methanol and 1-butanol absorption could be enhanced by an increasing rotational speed. Moreover, the KG a values for methanol absorption were higher than those for 1-butanol absorption. This behavior was attributed to the fact that the solubility of methanol in water is greater than that of 1-butanol in water [47]. It was concluded that the centrifugal force induced from the RPB equipped with blade packings did not affect the thermodynamic property of VOCs. Besides, the exponent (x) of KG a ∝ ωx was correlated. For methanol absorption, the x values were obtained as 0.31 and 0.30 at the liquid flow rates of 0.14 and 0.65 L/min, respectively. This result suggested that an increase in KG a by the rotational speed was independent of the liquid flow rate in methanol absorption. This behavior was also observed in 1-butanol absorption, indicating that the x values varied from 0.23 to 0.20 as the liquid flow rate was increased from 0.14 to 0.65 L/min. Fig. 5 shows the effect of the gas flow rate on the KG a values at a given rotational speed of 1800 rpm for methanol and 1-butanol absorption. It was realized that the resistance to mass transfer
0 400
800
1200
1600
2000
Rotational Speed (rpm) Fig. 4. Effect of rotational speed on KG a for VOCs absorption at QG = 55 L/min.
in gas phase was reduced with an increasing gas flow rate, and thus higher KG a values were obtained at higher gas flow rates for methanol and 1-butanol absorption at the liquid flow rates of 0.14 and 0.65 L/min. In addition, the exponent (y) of KG aQG y was correlated. For methanol absorption, the y values were obtained as 0.79 and 0.81 at the liquid flow rates of 0.14 and 0.65 L/min, respectively. This result implied that an increase in KG a by the gas flow was independent of the liquid flow rate in methanol absorption. This feature was not found in 1-butanol absorption, indicating that the y values varied from 0.73 to 0.79 while the liquid flow rate was increased from 0.14 to 0.65 L/min. This result suggested that an increase in KG a by the gas flow rate was obvious at higher liquid flow rates in 1-butanol absorption. Fig. 6 presents the effect of the liquid flow rate on the KG a values at a given rotational speed of 1800 rpm for methanol and 1-butanol absorption. It was found in Fig. 6 that higher KG a values were obtained at higher liquid flow rates for methanol and 1-butanol absorption at the gas flow rates of 22 and 55 L/min. This characteristic was probably owing to that increasing the liquid flow rate
8
Methanol ; Liquid Flow Rate : 0.65 L/min Methanol ; Liquid Flow Rate : 0.14 L/min 1-Butanol ;Liquid Flow Rate : 0.65 L/min 1-Butanol ; Liquid Flow Rate : 0.14 L/min
6
KGa (1/s)
CG∗ =
A=
1-Butanol ; Liquid Flow Rate : 0.14 L/min
6
KGa (1/s)
where QG is the volumetric flow rate of gas and CG is the concentration of a solute in gas phase. CG∗ stands for the equilibrium concentration associated with the concentration of a solute in liquid phase. Also, the overall mass balance can be expressed as for the case of solute-free liquid input (CL,i = 0): QG (CG − CG,o ) = QL (CL − CL,i ) = QL
Methanol ; Liquid Flow Rate : 0.65 L/min Methanol ; Liquid Flow Rate : 0.14 L/min
4
2
0 0
15
30
45
60
Gas Flow Rate (L/min) Fig. 5. Effect of gas flow rate on KG a for VOCs absorption at ω = 1800 rpm.
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Table 1 Absorption of methanol and 1-butanol in water. ω (rpm)
QG (L/min)
QL (L/min)
VOCs
KG a (1/s)
kL a (1/s)
kG a (1/s)
% of total resistance in gas phase
600 1000 1400 1800 600 1000 1400 1800 600 1000 1400 1800 600 1000 1400 1800 600 1000 1400 1800 600 1000 1400 1800 600 1000 1400 1800 600 1000 1400 1800
22 22 22 22 22 22 22 22 55 55 55 55 55 55 55 55 22 22 22 22 22 22 22 22 55 55 55 55 55 55 55 55
0.14 0.14 0.14 0.14 0.65 0.65 0.65 0.65 0.14 0.14 0.14 0.14 0.65 0.65 0.65 0.65 0.14 0.14 0.14 0.14 0.65 0.65 0.65 0.65 0.14 0.14 0.14 0.14 0.65 0.65 0.65 0.65
Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol Methanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol 1-Butanol
1.62 2.06 2.55 2.60 2.35 2.78 3.11 3.39 2.96 3.77 3.80 4.26 4.50 4.89 5.23 6.50 0.83 0.91 1.01 1.12 1.00 1.38 1.46 1.58 1.31 1.48 1.57 1.70 2.11 2.22 2.30 2.72
0.0089 0.0117 0.0141 0.0161 0.0356 0.0471 0.0567 0.0651 0.0089 0.0117 0.0141 0.0161 0.0356 0.0471 0.0567 0.0651 0.0089 0.0117 0.0141 0.0161 0.0356 0.0471 0.0567 0.0651 0.0089 0.0117 0.0141 0.0161 0.0356 0.0471 0.0567 0.0651
1.68 2.13 2.65 2.69 2.38 2.81 3.15 3.43 3.17 4.03 4.02 4.50 4.62 4.99 5.33 6.63 0.87 0.95 1.05 1.16 1.02 1.40 1.48 1.60 1.41 1.58 1.66 1.79 2.18 2.28 2.34 2.78
96.3 96.4 96.3 96.7 98.7 98.8 98.9 98.9 93.3 93.5 94.5 94.6 97.4 97.9 98.1 98.0 95.4 96.2 96.5 96.6 98.6 98.6 98.7 98.8 92.8 93.8 94.5 94.8 97.1 97.7 98.0 98.0
would result in more liquid films spreading over the packing and more liquid droplets flying within the voidage of the bed, thus leading to an increase in gas–liquid interfacial area and a higher KG a. Besides, the exponent (z) of KG aQL z was correlated. For methanol absorption, the z values were obtained as 0.15 and 0.25 at the gas flow rates of 22 and 55 L/min, respectively. This result implied that an increase in KG a by the liquid flow rate was more evident at higher gas flow rates in methanol absorption. This characteristic was also found in 1-butanol absorption, indicating that the z values varied
8
Methanol ; Gas Flow Rate : 22 L/min 1-Butanol ; Gas Flow Rate : 55 L/min
KGa (1/s)
4
2
0 0.2
0.4
1 1/KG a − H/kL a
kL adp = 0.478Re0.906 Gr0.275 L L DL at
1-Butanol ; Gas Flow Rate : 22 L/min
0.0
kG a =
(5 )
where KG a could be obtained from the experimental data for VOCs absorption in this work and kL a could be calculated using the correlation for the RPB equipped with blade packings proposed by Lin and Jain [9], as follows
Methanol ; Gas Flow Rate : 55 L/min
6
from 0.23 to 0.31 while the gas flow rate was increased from 22 to 55 L/min. This result indicated that an increase in KG a by the liquid flow rate was more pronounced at higher gas flow rates in 1-butanol absorption. To evaluate the local volumetric gas-phase mass transfer coefficient (kG a), a relation based on two-film theory was presented, as follows
0.6
0.8
Liquid Flow Rate (L/min) Fig. 6. Effect of liquid flow rate on KG a for VOCs absorption at ω = 1800 rpm.
(6)
By substituting Eq. (6) and KG a obtained in this work into Eq. (5), the local volumetric gas-phase mass transfer coefficient (kG a) for the RPB equipped with blade packings can be obtained, as listed in Table 1. Also, the percentage of total resistance to mass transfer in gas phase was evaluated, as shown in Table 1. The result of Table 1 indicates that in the absorption of both methanol and 1-butanol the resistance in gas phase represented an appreciable portion of the total resistance to mass transfer. In methanol absorption, the resistance in gas phase constituted as much as 93.3–96.7% of the total resistance at a low liquid flow rate and as much as 97.4–98.9% of the total resistance at a high liquid flow rate. In 1-butanol absorption, the resistance in gas phase constituted as much as 92.8–96.6% of the total resistance at a low liquid flow rate and as much as 97.1–98.8% of the total resistance at a high liquid flow rate. Based on above discussion, in methanol and 1-butanol absorption the total resistance to mass transfer was significantly dominated by the resistance in gas phase. This
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Table 2 Comparison of conventional packed tower and rotating packed bed.
Operation conditions Pressure (atm) Temperature (K) Gas phase Gas loading (m3 /(m2 s)) Liquid phase Liquid loading (m3 /(m2 h)) VOCs Solute Concentration (ppmv) Packing Type Bed volume (cm3 ) Voidage KG a (1/s)
Acknowledgement
Conventional packed tower [48]
Rotating packed bed
The financial support of National Science Council of the Republic of China (NSC 97-2221-E-182-013-MY2) is greatly appreciated.
1 300
1 298
References
0.12
0.13
2.5–9.8
1.3–5.7
Methanol 10,000–25,000
Methanol 2000
1-in. Raschig rings 43,146 0.72 0.4–0.7
Blades 327 0.99 4.0–6.6
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finding was also observed in the RPB equipped with blade packings for isopropyl alcohol absorption [9] and the conventional packed tower for methanol absorption [48]. This result suggested that the resistance to mass transfer in gas phase would be predominant for alkanols absorption owing to that the solubility of alkanols in water was higher than that of other VOCs in water. To verify the mass transfer performance of the RPB equipped with blade packings, the KG a values obtained in this work were compared with the data of the conventional packed tower [48]. The conventional packed tower used by Houston and Walker [48] was 1.94 feet in height and 12 in. in diameter, and dealt with the methanol absorption using water. It was seen from Table 2 that the KG a values in the RPB equipped with blade packings were about 10 times higher than those in the conventional packed tower. This behavior was similar to the result in the RPB equipped with structured packings for ethanol absorption [12], presenting that the KG a values in the RPB equipped with structured packings was 3–9 times higher than those in the conventional packed tower. Therefore, the size of an absorber can be significantly reduced if the RPB instead of the conventional packed tower is used to remove VOCs from the gas stream. 4. Conclusions This work has examined the removal of methanol and 1-butanol from an air stream by absorption using water as the absorbent in the RPB equipped with blade packings under different operating conditions. The overall volumetric gas-phase mass transfer coefficients (KG a) were estimated as functions of the rotational speed, the gas flow rate, and the liquid flow rate. The KG a values for methanol and 1-butanol absorption increased with the rotational speed, the gas flow rate, and the liquid flow rate. The KG a values for methanol absorption were higher than those for 1-butanol absorption owing to that the solubility of methanol in water is higher than that of 1-butanol in water. Therefore, the thermodynamic property of VOCs was not affected by the centrifugal force in the RPB equipped with blade packings. Based on the percentage of total resistance to mass transfer in gas phase, the total resistance to mass transfer in methanol and 1-butanol absorption was observed to be substantially controlled by the resistance in gas phase. The comparison between the RPB equipped with blade packings and the conventional packed tower demonstrated that mass transfer efficiency of the RPB equipped with blade packing was higher than that of conventional packed tower. Consequently, the RPB equipped with blade packings is an excellent alternative with high mass transfer efficiency to remove alkanols from the exhausted gases.
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[40] D.G. Wang, F. Guo, J.F. Chen, L. Shao, H. Liu, Z.T. Zhang, A two-step way to synthesize nano inner-modified aluminum trihydroxide, Colloid Surf. A-Physicochem. Eng. Aspects 293 (2007) 201–209. [41] H. Chiou, L. Li, T.T. Hu, H.K. Chan, J.F. Chen, J. Yun, Production of salbutamol sulfate for inhalation by high-gravity controlled antisolvent precipitation, Int. J. Pharm. 331 (2007) 93–98. [42] R.H. Zhao, C.P. Li, F. Guo, J.F. Chen, Scale-up preparation of organized mesoporous alumina in a rotating packed bed, Ind. Eng. Chem. Res. 46 (2007) 3317–3320. [43] T.T. Hu, H. Chiou, H.K. Chan, J.F. Chen, J. Yun, Preparation of inhalable salbutamol sulphate using reactive high gravity controlled precipitation, J. Pharm. Sci. 97 (2008) 944–949. [44] H. Zhao, J.X. Wang, H.X. Zhang, Z.G. Shen, J. Yun, J.F. Chen, Facile preparation of danazol nanoparticles by high-gravity anti-solvent precipitation (HGAP) method, Chin. J. Chem. Eng. 17 (2009) 318–323. [45] C.C. Lin, W.T. Liu, Removal of an undesired component from a valuable product using a rotating packed bed, J. Ind. Eng. Chem. 12 (2006) 455–459. [46] W.Y. Li, W. Wu, H.K. Zou, G.G. Chu, L. Shao, J.F. Chen, Process intensification of VOC removal from high viscous media by rotating packed bed, Chin. J. Chem. Eng. 17 (2009) 389–393. [47] A.K. Gupta, A.S. Teja, X.S. Chai, J.Y. Zhu, Henry’s constants of n-alkanols (methanol through n-hexanol) in water at temperatures between 40 ◦ C and 90 ◦ C, Fluid Phase Equilibr. 170 (2000) 183–192. [48] R.W. Houston, C.A. Walker, Absorption in packed towers – effect of molecular diffusivity on gas film coefficient, Ind. Eng. Chem. 42 (1950) 1105–1112.