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Carbon dioxide absorption in a cross-flow rotating packed bed
chemical engineering research and design 8 9 ( 2 0 1 1 ) 1722–1729
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Carbon dioxide absorption in a cross-flow rotating packed bed Chia-Chang Lin ∗ , Bor-Chi Chen Department of Chemical and Materials Engineering, Chang Gung University, 259 Wen-Hwa 1st Road, Kwei-Shan, Taoyuan 333, Taiwan, ROC
a b s t r a c t This work investigates the feasibility of applying the cross-flow rotating packed bed (RPB) to the removal of carbon dioxide (CO2 ) by absorption from gaseous streams. Monoethanolamine (MEA) aqueous solution was used as the model absorbent. Also, other absorbents such as the NaOH and 2-amino-2-methyl-1-propanol (AMP) aqueous solutions were compared with the MEA aqueous solution. The CO2 removal efficiency was observed as functions of rotor speed, gas flow rate, liquid flow rate, MEA concentration, and CO2 concentration. Experimental results indicated that the rotor speed positively affects the CO2 removal efficiency. Our results further demonstrated that the CO2 removal efficiency increased with the liquid flow rate and the MEA concentration; however, decreased with the gas flow rate and the CO2 concentration. Additionally, the CO2 removal efficiency for the MEA aqueous solution was superior to that for the NaOH and AMP aqueous solutions. Based on the performance comparison with the conventional packed bed and the countercurrent-flow RPB, the cross-flow RPB is an effective absorber for CO2 absorption process. © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Rotating packed bed; Cross flow; Absorption; Mass transfer; Carbon dioxide
1.
Introduction
Ramshaw and Mallinson (1981) invented the rotating packed bed (RPB) for enhancing the gas–liquid mass transfer in distillation and absorption processes. This novel technology was referred to as “Higee” (an acronym for high gravity). Under RPB operation, an enhancement by a factor of 10–100 in the gas–liquid mass transfer was achieved by thin films and/or tiny droplets generated due to a rigorous centrifugal acceleration (2000–10,000 m s−2 ). Therefore, the dramatic reduction in the size of the RPB would be obtained, thereby reducing the capital and operating costs (Ramshaw, 1983). The original RPB could be operated at the mode of gas–liquid countercurrentflow. The countercurrent-flow RPB has been proved to enhance gas–liquid mass transfer in diverse processes such as distillation (Kelleher and Fair, 1996; Lin et al., 2002; Wang et al., 2008), absorption (Munjal et al., 1989; Kumar and Rao, 1990; Chen and Liu, 2002; Lin et al., 2003, 2004, 2009a,b, 2010; Tan and Chen, 2006; Cheng and Tan, 2006; Lin and Jian, 2007; Jassim et al., 2007; Lin and Chien, 2008; Lin and Su, 2008; Cheng et al., 2010), stripping (Keyvani and Gardner, 1989; Singh et al., 1992;
∗
Liu et al., 1996; Y.S. Chen et al., 2005a,b; Lin and Liu, 2006, 2007), reactive precipitation (Chen et al., 2000, 2003; J.F. Chen et al., 2004; Wang et al., 2004) and ozone oxidation (Lin and Liu, 2003; Y.H. Chen et al., 2004, 2005; Shang et al., 2006; Chiu et al., 2007; Ku et al., 2008; Chang et al., 2009). To remove CO2 from a gas stream containing CO2 with a huge flow rate, Lin and Chen (2007) first adopted the crossflow RPB for the CO2 removal by chemical absorption with the NaOH aqueous solution, proposing that the cross-flow RPB had a great potential for the CO2 removal from the exhausted gases. Moreover, Lin and Chen (2008) investigated the mass transfer efficiency of the cross-flow RPB for removing CO2 from a gas stream containing 1 vol% CO2 by chemical absorption with the NaOH aqueous solution. They proposed that the mass transfer efficiency of the cross-flow RPB was comparable to that of the countercurrent-flow RPB. Furthermore, Lin et al. (2008) investigated the mass transfer efficiency of the crossflow RPB for removing CO2 from a gas stream containing 10 vol% CO2 by chemical absorption with the NaOH aqueous solution. Their experimental results confirmed that the mass transfer efficiency of the cross-flow RPB was higher than that
Corresponding author. Tel.: +886 3 211 8800x5760; fax: +886 3 211 8800x5702. E-mail address: [email protected] (C.-C. Lin). Received 13 May 2010; Received in revised form 18 November 2010; Accepted 25 November 2010 0263-8762/$ – see front matter © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2010.11.015
chemical engineering research and design 8 9 ( 2 0 1 1 ) 1722–1729
1723
Fig. 1 – Experimental setup for CO2 absorption. of the countercurrent-flow RPB. Based on these results, the cross-flow RPB could be an excellent alternative with high mass transfer efficiency to handle a gas stream containing huge flow rates and high CO2 concentrations. However, there are no existing literatures until now to examine how other absorbents affect the CO2 removal efficiency of the cross-flow RPB. Moreover, investigating the CO2 absorption performance of the cross-flow RPB with various absorbents is necessary to design the cross-flow RPB accurately and economically. Consequently, the objective of this work is to examine how operating parameters affect the CO2 absorption performance of the cross-flow RPB using the monoethanolamine (MEA) aqueous solution. The interested operating parameters in this work are rotor speed, gas flow rate, liquid flow rate, MEA concentration, and CO2 concentration. Additionally, other absorbents such as the NaOH and 2-amino-2-methyl-1-propanol (AMP) aqueous solutions were used to be compared with the MEA aqueous solution. The CO2 absorption performance comparison between the cross-flow RPB and other absorbers was also given. The results in this work could provide further insight into the feasibility of applying the cross-flow RPB to the removal of CO2 from gaseous streams.
2.
RPB, while the CO2 -rich MEA aqueous solution was expelled from the bottom of the RPB. Fig. 2 illustrates the photograph of the cross-flow RPB used in this work for the CO2 removal. The cross-flow RPB had an inner radius of 2.4 cm, an outer radius of 4.4 cm, and an axial height of 12 cm. The packing adopted in this work was stainless steel wire mesh that had a configuration of interconnected filaments with a mean diameter of 0.22 mm and an average mesh diameter of 3 mm. The packing was arranged within the cross-flow RPB with a specific surface area of 677 m2 /m3 and a voidage of 0.95. The liquid distributor consisted of a tube with a vertical set of holes that had twelve 0.1 cm diameter holes with 0.95 cm interval. Both sealings were adopted for prevent-
Experimental
Fig. 1 shows the experimental setup for CO2 absorption. During normal operation, the CO2 –N2 stream traveled axially from the bottom of the RPB owing to the pressure drop. At the same time, the prepared MEA aqueous solution was introduced from the tank into the inner edge of the packing through a liquid distributor. The MEA aqueous solution moved radially within the packing due to the centrifugal force and, then, exited the packing from the outer edge. Both the CO2 –N2 stream and the MEA aqueous solution were in contact with the cross-flow mode within the packing, in which CO2 in the CO2 –N2 stream reacted with MEA in the liquid stream. The exiting CO2 –N2 stream, containing low CO2 concentration, finally left the top of the packing and, then, was discharged from the top of the
Fig. 2 – Photograph of cross-flow RPB used in this work.
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ing gas from bypassing the packing and keeping the cross-flow operating mode. In general, the cross-flow RPB could be operated at the rotor speed of 540–1600 rpm, providing 11–97 times gravitational force based on the arithmetic mean radius. The CO2 concentration in inlet CO2 –N2 stream was varied at 1, 4, 7, and 10% (mole fraction) and the MEA concentration in inlet liquid stream was set at 0.2, 0.5, 0.7, and 1.0 mol/L. During operation, the gas flow rate (axial direction) was varied at the range of 10–70 L/min and the liquid flow rate (radial direction) was varied at the range of 0.2–0.5 L/min. The CO2 concentrations in inlet and outlet CO2 –N2 streams were measured by an infrared (IR) CO2 analyzer (Draeger, Ploytron Transmitter IR CO2 ). During CO2 absorption, the CO2 concentration in outlet CO2 –N2 streams were observed to drop rapidly and then reached a steady value within 10–15 min. The reproducibility tests under almost all of the operating conditions were carried out in this work. The CO2 concentration in outlet CO2 –N2 streams was observed to be reproduced with a deviation of less than 5%. The CO2 concentration in outlet liquid stream was determined by the standard method (Aroonwilas and Tontiwachwuthikul, 1997). Mass balance on inlet and outlet of both the gas and liquid streams indicated that the errors were within 10%. All experiments were conducted at an average temperature of 28 ◦ C with atmospheric pressure.
3.
Results and discussion
explained by the fact that the centrifugal acceleration could induce thinner liquid films and/or tiny droplets; a thinner boundary layer for mass transfer would be induced, thus leading to a higher gas–liquid mass transfer according to penetration theory. Similar trends were also found in the CO2 removal by chemical absorption with the NaOH, MEA, and MEA/AMP aqueous solutions in the countercurrent-flow RPB (Lin et al., 2003) and the CO2 removal by chemical absorption with the NaOH aqueous solution in the cross-flow RPB (Lin and Chen, 2007). At a low gas flow rate of 10 L/min, E varied ωx with the x values of 0.10 and 0.01 for the liquid flow rate of 0.2 and 0.5 L/min, respectively. This observation implied that an increase of the E values by the rotor speed was more pronounced at a low liquid flow rate. At a low liquid flow rate, the cross-flow RPB could provide an improved liquid distribution for enhancing the contact of gas and liquid by increasing centrifugal acceleration. This characteristic was more obvious at a high gas flow rate of 70 L/min, as shown in Fig. 3, indicating that the x values varied from 0.32 to 0.14 as the liquid flow rate was increased from 0.2 to 0.5 L/min. Moreover, for a given liquid flow rate, the sensitivity of the E values to variations in the rotor speed at a high gas flow rate was higher than that at a low gas flow rate. Hence, the rotor speed significantly affected the E values at low liquid flow rates and high gas flow rates.
3.1.
Absorption behavior of MEA absorbent
3.1.2.
The removal efficiency of CO2 in the cross-flow RPB is defined as E=
Ci − Co × 100 Ci
(1)
where E is the removal efficiency of CO2 , and Ci and Co are the concentrations of CO2 in inlet and outlet CO2 –N2 streams, respectively. The E values were measured at various values of the operation variables, including rotor speed (ω), gas flow rate (QG ), liquid flow rate (QL ), MEA concentration (CMEA ), and CO2 concentration (CCO2 ) to evaluate the CO2 absorption performance of the cross-flow RPB using the MEA absorbent.
3.1.1.
Effect of rotor speed
Fig. 3 summarizes the E values as a function of the rotor speed from 540 to 1600 rpm at the CO2 concentration of 1% and the MEA concentration of 1.0 mol/L. As expected, increasing the rotor speed enhanced the E values. This result could be
Effect of gas flow rate
Fig. 4 presents the effect of varying the gas flow rate from 10 to 70 L/min on the E values at the CO2 concentration of 1% and the MEA concentration of 1.0 mol/L. The gas flow rate influenced the E values; that is, the E values decreased with the gas flow rate for a given liquid flow rate and rotor speed. Owing to that an increasing gas flow rate provided a larger amount of CO2 in the gas stream and a reduction in the contact time, the CO2 removal was limited at a high gas flow rate for a given MEA concentration. Similar trends were also found in the CO2 removal by chemical absorption with the NaOH, MEA, and MEA/AMP aqueous solutions in the countercurrentflow RPB (Lin et al., 2003) and the CO2 removal by chemical absorption with the NaOH aqueous solution in the cross-flow RPB (Lin and Chen, 2007). At a high liquid flow rate of 0.5 L/min, E was proportional −y to QG with the exponent y varying from 0.23 to 0.16 for the rotor speed from 540 to 1600 rpm. This feature implied that at a high liquid flow rate, the sensitivity of E to variations in the gas flow rate at a low rotor speed was slightly higher than that
100
100
75
75
QG= 10 (L/min); QL= 0.5 (L/min)
50
QG= 10 (L/min); QL= 0.2 (L/min) QG= 70 (L/min); QL= 0.5 (L/min)
CO2: 1 (%); MEA: 1.0 (mol/L)
E (%)
E (%)
CO2: 1 (%); MEA: 1.0 (mol/L)
QL= 0.5 (L/min); ω = 1600 (rpm)
50
QL= 0.5 (L/min); ω = 540 (rpm) QL= 0.2 (L/min); ω = 1600 (rpm)
QG= 70 (L/min); QL= 0.2 (L/min)
25
QL= 0.2 (L/min); ω = 540 (rpm)
25
0
0 200
600
1000
1400
1800
Rotor Speed (rpm)
Fig. 3 – Effect of rotor speed on E.
0
20
40
60
80
Gas Flow Rate (L/min)
Fig. 4 – Effect of gas flow rate on E.
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chemical engineering research and design 8 9 ( 2 0 1 1 ) 1722–1729
100
100
75
75
CO2: 1 (%); ω: 1600 (rpm)
QG= 10 (L/min); ω = 1600 (rpm)
50
E (%)
E (%)
CO 2: 1 (%); MEA: 1.0 (mol/L)
QG= 10 (L/min); ω = 540 (rpm) QG= 70 (L/min); ω = 1600 (rpm)
QG= 10 (L/min); QL= 0.5 (L/min)
50
QG= 10 (L/min); QL= 0.2 (L/min) QG= 70 (L/min); QL= 0.5 (L/min)
QG= 70 (L/min); ω = 540 (rpm)
QG= 70 (L/min); QL= 0.2 (L/min)
25
25
0
0 0.1
0.2
0.3
0.4
0.5
0.0
0.6
0.3
Liquid Flow Rate (L/min)
0.9
1.2
Fig. 6 – Effect of MEA concentration on E.
Fig. 5 – Effect of liquid flow rate on E.
at a high rotor speed. As the liquid flow rate was decreased to 0.2 L/min, this characteristic was also found, as shown in Fig. 4, indicating that the y values varied from 0.46 to 0.34 when the rotor speed was increased from 540 to 1600 rpm. Moreover, for a given rotor speed, the sensitivity of the E values to variations in the gas flow rate at a low liquid flow rate was higher than that at a high liquid flow rate. Hence, the gas flow rate provided a significant effect on the E values at low rotor speeds and low liquid flow rates.
3.1.3.
0.6
MEA Concentration (mol/L)
Effect of liquid flow rate
Fig. 5 indicates the effect of the liquid flow rate ranging from 0.2 to 0.5 L/min on the E values at the CO2 concentration of 1% and the MEA concentration of 1.0 mol/L. The liquid flow rate had an influence on the E values; that is, an increase in the liquid flow rate yielded an increase in the E values for a given gas flow rate and rotor speed. This behavior was attributed to the fact that more MEA used to absorb CO2 at a high liquid flow rate was favorable to the CO2 removal at a given MEA concentration. Similar trends were also found in the CO2 removal by chemical absorption with the NaOH, MEA, and MEA/AMP aqueous solutions in the countercurrent-flow RPB (Lin et al., 2003) and the CO2 removal by chemical absorption with the NaOH aqueous solution in the cross-flow RPB (Lin and Chen, 2007). At a low gas flow rate of 10 L/min, E correlated QLz with the exponent z varying from 0.19 to 0.08 for the rotor speed from 540 to 1600 rpm. This suggested that at a low gas flow rate, the sensitivity of E to the variation in the liquid flow at a low rotor speed was higher than that at a high rotor speed. As the gas flow rate was increased to 70 L/min, this phenomenon was more evident, as shown in Fig. 5, implying that the z values varied from 0.65 to 0.46 when the rotor speed was increased from 540 to 1600 rpm. Moreover, for a given rotor speed, the dependence of the E values on the liquid flow rate at a high gas flow rate was much higher than that at a low gas flow rate.
Hence, the liquid flow rate provided a considerable effect on the E values at low rotor speeds and high gas flow rates. The results in Fig. 5 can be fitted with the logarithmic equations indicated in Table 1. According to these equations, the liquid flow rates required to achieve a CO2 removal of 95% were obtained under various operating conditions, and the corresponding QG /QL ratio was represented by (QG /QL )95 . As listed in Table 1, the values of (QG /QL )95 increased with the gas flow rate for the same rotor sped at the MEA concentration of 1.0 mol/L. Moreover, the rotor speed provided an increase in (QG /QL )95 for the same gas flow rate at the MEA concentration of 1.0 mol/L. However, it was known that the energy consumption would be increased with an increasing rotor speed. As a result, the optimum between the rotor speed and the removal efficiency should be determined for industrial-scale applications.
3.1.4.
Effect of MEA concentration
Fig. 6 displays the E values as functions of the MEA concentration from 0.2 to 1.0 mol/L at the rotor speed of 1600 rpm and the CO2 concentration of 1%. At a given gas flow rate and liquid flow rate, the E values increased with an increasing MEA concentration. This characteristic was caused by the fact that increasing MEA concentration could give higher amounts of hydroxide ions per unit volume for more CO2 absorption at a given gas flow rate and liquid flow rate. Similar trends were also found in the CO2 removal by chemical absorption with the NaOH, MEA, and MEA/AMP aqueous solutions in the countercurrent-flow RPB (Lin et al., 2003) and the CO2 removal by chemical absorption with the NaOH aqueous solution in the cross-flow RPB (Lin and Chen, 2007). At a low gas flow rate of 10 L/min, the E values were proportional to the MEA concentration raised to the w power. The w values varied from 0.04 to 0.15 as the liquid flow rate was decreased from 0.5 to 0.2 L/min. This result suggested that an enhancement of the E values by the MEA concentration was more pronounced at a low liquid flow rate. This finding was also observed at a high gas flow rate of 70 L/min, as shown in Fig. 6, showing that the w values increased from 0.21 to 0.30
Table 1 – (QG /QL )95 under various operating conditions. ω (rpm)
QG (L/min)
CMEA (mol/L)
Logarithmic equation
(QL )95 a (L/min)
(QG /QL )95
540 540 1600 1600
10 70 10 70
1.0 1.0 1.0 1.0
E = 16.8 ln(QL ) + 108 E = 29.3 ln(QL ) + 78 E = 7.5 ln(QL ) + 103 E = 27.0 ln(QL ) + 90
0.46 1.8 0.34 1.2
22 39 29 58
a
The liquid flow rate required to achieve E = 95.
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100
75
E (%)
MEA:1 (mol/L); ω: 1600 (rpm) QG= 10 (L/min); QL= 0.5 (L/min)
50
QG= 10 (L/min); QL= 0.2 (L/min) QG= 70 (L/min); QL= 0.5 (L/min) QG= 70 (L/min); QL= 0.2 (L/min)
25
0 0
3
6
9
12
CO2 Concentration (%)
Fig. 7 – Effect of CO2 concentration on E. with the decrease in the liquid flow rate from 0.5 to 0.2 L/min. Moreover, for a given liquid flow rate, the dependence of the E values on the MEA concentration at a high gas flow rate was higher than that at a low gas flow rate. Hence, the MEA concentration offered a larger effect on the E values at low liquid flow rates and high gas flow rates.
3.1.5.
Effect of CO2 concentration
Fig. 7 shows the effect of the CO2 concentration from 1 to 10% on the E values at the rotor speed of 1600 rpm and the MEA concentration of 1.0 mol/L. At a given gas flow rate and liquid flow rate, the E values decreased with an increasing CO2 concentration. This finding was explained by the fact that more CO2 must be removed at high CO2 concentrations for a given
MEA concentration. Similar trends were also found in the CO2 removal by chemical absorption with the NaOH, MEA, and MEA/AMP aqueous solutions in the countercurrent-flow RPB (Lin et al., 2003). At a low gas flow rate of 10 L/min, the E values were proportional to the CO2 concentration to the −0.01 power at the liquid flow rate of 0.5 L/min. This power would vary to −0.06 at the liquid flow rate of 0.2 L/min. This feature implied that a reduction of the E values by the CO2 concentration was more evident at a low liquid flow rate. This observation was more obvious at a high gas flow rate of 70 L/min, as shown in Fig. 7, revealing that this power varied from −0.17 to −0.46 with the decrease in the liquid flow rate from 0.5 to 0.2 L/min. Moreover, for a given liquid flow rate, the sensitivity of the E values to variations in the CO2 concentration at a high gas flow rate was much higher than that at a low gas flow rate. Hence, the CO2 concentration gave a substantial effect on the E values at low liquid flow rates and high gas flow rates.
3.2.
Comparison of absorbents
Absorbent type is considered another important factor affecting the removal efficiency of CO2 in the cross-flow RPB. A particular absorbent virtually yields a specific range of the E values. Table 2 shows the E values for the aqueous solutions containing NaOH, MEA, and AMP under various operating conditions at the absorbent concentration of 1 mol/L and the CO2 concentration of 1%. According to Table 2, it can be seen that the MEA aqueous solution exhibited the best removal efficiency of CO2 over the aqueous solutions containing NaOH and
Table 2 – E values for CO2 –NaOH, CO2 –MEA, and CO2 –AMP systems at absorbent concentration of 1 mol/L and CO2 concentration of 1%. ω (rpm)
QG (L/min)
QL (L/min)
E (NaOH) (%)
E (MEA) (%)
E (AMP) (%)
540 900 1260 1600 540 1600 540 1600 540 900 1260 1600 540 1600 540 1600 540 1600 540 1600 540 900 1260 1600 540 1600 540 1600 540 900 1260 1600
10 10 10 10 30 30 50 50 70 70 70 70 10 10 70 70 10 10 70 70 10 10 10 10 30 30 50 50 70 70 70 70
0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.3 0.3 0.3 0.3 0.4 0.4 0.4 0.4 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
81 87 88 90 45 63 36 50 32 37 41 43 83 92 34 52 92 96 45 59 96 97 98 98 75 85 63 75 55 63 65 68
82 90 91 92 53 68 40 53 33 40 44 47 85 92 40 54 94 96 50 66 97 97 98 98 81 88 71 79 61 68 69 71
57 66 69 69 34 43 28 35 26 28 30 32 63 77 27 36 78 84 35 43 82 85 87 88 55 65 43 51 38 40 43 45
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Table 3 – E values for CO2 –NaOH, CO2 –MEA, and CO2 –AMP systems at absorbent concentration of 1 mol/L and CO2 concentration of 10%. ω (rpm)
QG (L/min)
QL (L/min)
E (NaOH) (%)
E (MEA) (%)
E (AMP) (%)
540 900 1260 1600 540 1600 540 1600 540 900 1260 1600 540 1600 540 1600 540 1600 540 1600 540 900 1260 1600 540 1600 540 1600 540 900 1260 1600
10 10 10 10 30 30 50 50 70 70 70 70 10 10 70 70 10 10 70 70 10 10 10 10 30 30 50 50 70 70 70 70
0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.3 0.3 0.3 0.3 0.4 0.4 0.4 0.4 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
53 69 76 77 18 35 11 21 6.8 9.0 11 14 61 79 8.2 18 79 93 15 29 92 94 94 95 59 78 37 56 26 35 39 42
61 71 78 80 22 35 13 20 9.6 12 15 16 72 85 14 25 91 96 28 41 94 95 96 96 62 79 40 61 31 38 43 46
33 41 48 52 14 19 9.0 12 6.7 7.8 8.7 9.2 40 60 8.1 16 68 80 16 22 77 81 83 84 36 49 23 32 17 20 22 23
AMP. Additionally, the E values for the NaOH aqueous solution were higher than those for the AMP aqueous solution. For example, absorbing CO2 with the MEA aqueous solution was associated with the gave the E values varying from 61 to 71 as the rotor speed was increased from 540 to 1600 rpm at the gas flow rate of 70 L/min and the liquid flow rate of 0.5 L/min. However, at the same operating condition, the E values for the NaOH aqueous solution varied from 55 to 68 and the E values for the AMP aqueous solution varied from 38 to 45. Similar trends were also found at the CO2 concentration of 10%, as listed in Table 3. The same phenomenon was observed in the conventional packed bed (Aroonwilas et al., 1999). The difference between the E values for the NaOH aqueous solution and those for the AMP aqueous solution was primarily affected by the reaction rate of CO2 with the absorbent. The greater the reaction rate, the higher the E values would be expected. As pointed out by Aroonwilas et al. (1999), the reaction rate in the NaOH aqueous solution was much higher than that in the AMP aqueous solution, thus leading to a higher E value. However, the discrepancy between the E values for the MEA aqueous solution and those for the NaOH aqueous solution was not affected by the reaction rate of CO2 with the absorbent. As proposed by Aroonwilas et al. (1999), the reaction rate in the NaOH aqueous solution is higher than that in the MEA aqueous solution. Thus, a higher E value for the MEA aqueous solution was believed to be attributed to a higher gas–liquid interfacial area existed in the cross-flow RPB. The MEA aqueous solution has a lower surface tension in comparison with the NaOH aqueous solution (Aroonwilas et al., 1999).
This would result in a higher gas–liquid interfacial area for the MEA aqueous solution, thus leading to a higher E value.
3.3. Comparison with conventional packed bed and countercurrent-flow RPB To compare the CO2 absorption performance of the cross-flow RPB with other absorbers such as the conventional packed bed and the countercurrent-flow RPB, the mass transfer coefficient is considered a simple but representative design parameter. For CO2 absorption process, the experimental overall volumetric gas-phase mass transfer coefficients (KG a) of the cross-flow RPB can be evaluated by the following equation (Lin and Liu, 2007): KG a =
QG (Ro 2 − Ri 2 )Zb
ln
C i
Co
(2)
where QG is the volumetric flow rate of gas, Z is the axial length of the cross-flow RPB, Ri and Ro are the inner and outer radii of the cross-flow RPB, respectively. Ci and Co are the concentrations of CO2 in the inlet and outlet CO2 –N2 streams, respectively. The estimated KG a values in the cross-flow RPB were compared to the reported KG a values in the conventional packed bed when the NaOH aqueous solution was used as the absorbent (Aroonwilas and Tontiwachwuthikul, 1997). The conventional packed bed had a height of 110 cm and an internal diameter of 1.9 cm. As shown in Table 4, the average KG a value (1.42) in the cross-flow RPB was about 0.75 of that (1.96)
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Table 4 – Comparison of cross-flow RPB and conventional packed bed. Conventional packed bed (Aroonwilas and Tontiwachwuthikul, 1997) Operation conditions Pressure (atm) Temperature (K) Gas phase CO2 concentration (mol%) Gas flow rate (L/min) Gas loading (m3 /(m2 h)) Liquid phase NaOH concentration (mol/L) Liquid flow rate (mL/min) Liquid loading (m3 /(m2 h)) KG a (s−1 ) KG a (based on liquid loading) (s−1 )
Cross-flow RPB
1 297
1 301
1 4.9–10.4 1044–2196
1 10–70 140–983
0.2–1.0 23–67 4.9–14.2 1.38–2.41 0.70–1.15
0.2–1.0 200–500 0.5–1.2 0.30–2.54 0.30–2.54
Table 5 – Comparison of cross-flow RPB and countercurrent-flow packed bed. Countercurrent-flow RPB (Lin et al., 2003) Operation conditions Pressure (atm) Temperature (K) Gas phase CO2 concentration (mol%) Gas flow rate (L/min) Gas loading (m3 /(m2 h)) Liquid phase MEA concentration (mol/L) flow rate (mL/min) Liquid loading (m3 /(m2 h)) KG a (s−1 )
1 300
1 301
10 13.1 111
10 10 140
1.0 42 0.4 0.21
in the conventional packed bed. As pointed out by Strigle (1987), KG a in the conventional packed bed varied with the liquid loading to the 0.3 power. When the liquid loadings in the cross-flow RPB were used to evaluate KG a in the conventional packed bed, Table 4 indicates that the average KG a value (1.42 s−1 ) in the cross-flow RPB was higher than that (0.93 s−1 ) in the conventional packed bed. Additionally, the higher the gas loading, the higher the KG a values would be expected. Thus, the cross-flow RPB performed better than the conventional packed bed. When the MEA aqueous solution was used as the absorbent, Table 5 shows that the KG a value in the cross-flow RPB was 2.5 times higher than that in the countercurrent-flow RPB, which had an inner radius of 3.8 cm, an outer radius of 8.0 cm, and an axial height of 2.0 cm. Based on the comparison in KG a, the cross-flow RPB shows its applicability in removing CO2 from gaseous streams.
4.
Cross-flow RPB
Conclusions
This work has examined the absorption performance of the cross-flow RPB using CO2 absorption by the MEA aqueous solution from gaseous streams. The results were considered in relation with the removal efficiency, E values. The E values obviously increased with the rotor speed. Also, the E values appeared to increase with an increasing liquid flow rate and an increasing MEA concentration, but decrease with an increasing gas flow rate and an increasing CO2 concentration. Moreover, the (QG /QL )95 values increased with the rotor speed and the gas flow rate. Based on the comparison between the MEA aqueous solution and other absorbents such as the NaOH and AMP aqueous solutions, the MEA aqueous
1.0 200 0.5 0.51
solution yielded the superior CO2 removal efficiency. Additionally, according to the performance comparison with the conventional packed bed and the countercurrent-flow RPB, the cross-flow RPB is believed to be an alternative gas–liquid contactor for the removal of CO2 from gaseous streams.
Acknowledgement The financial support of National Science Council of the Republic of China (NSC 97-2221-E-182-013-MY2) is greatly appreciated.
References Aroonwilas, A., Veawab, A., Tontiwachwuthikul, P., 1999. Behavior of the mass-transfer coefficient of structured packing in CO2 absorbers with chemical reactions. Ind. Eng. Chem. Res. 38, 2044. Aroonwilas, A., Tontiwachwuthikul, P., 1997. High-efficiency structured packing for CO2 separation using 2-amino-2-methyl-1-propanol (AMP). Sep. Purif. Technol. 12, 67. 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, 287. Chen, J.F., Li, Y.L., Wang, Y.H., Yun, J., Cao, D.P., 2004. Preparation and characterization of zinc sulfide nanoparticles under high-gravity environment. Mater. Res. Bull. 39, 185. Chen, J.F., Shao, L., Zhang, C.G., Chen, J.M., Chu, G.W., 2003. Preparation of TiO2 nanoparticles by a rotating packed bed reactor. J. Mater. Sci. Lett. 22, 437. Chen, J.F., Wang, Y.H., Guo, F., Wang, X.M., Zheng, C., 2000. Synthesis of nanoparticles with novel technology:
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high-gravity reactive precipitation. Ind. Eng. Chem. Res. 39, 948. Chen, Y.H., Chang, C.Y., Su, W.L., Chen, C.C., Chiu, C.Y., Yu, Y.H., Chiang, P.C., Chiang, S.I.M., 2004. Modeling ozone contacting process in a rotating packed bed. Ind. Eng. Chem. Res. 43, 228. 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, 68. Chen, Y.S., Lin, C.C., Liu, H.S., 2005a. Mass transfer in a rotating packed bed with Viscous Newtonian and Non-Newtonian Fluids. Ind. Eng. Chem. Res. 44, 1043. Chen, Y.S., Lin, C.C., Liu, H.S., 2005b. Mass transfer in a rotating packed bed with various radii of the bed. Ind. Eng. Chem. Res. 44, 7868. Chen, Y.S., Liu, H.S., 2002. Absorption of VOCs in a rotating packed bed. Ind. Eng. Chem. Res. 41, 1583. 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, 1431. Cheng, H.H., Shen, J.F., Tan, C.S., 2010. CO2 capture from hot stove gas in steel making process. Int. J. Greenh. Gas Control 4, 525. 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, 732. Jassim, M.S., Rochelle, S., Eimer, D., Ramshaw, C., 2007. Carbon dioxide absorption and desorption in aqueous monoethanolamine solutions in a rotating packed bed. Ind. Eng. Chem. Res. 46, 2823. Kelleher, T., Fair, J.R., 1996. Distillation studies in a high-gravity contactor. Ind. Eng. Chem. Res. 35, 4646. Keyvani, M., Gardner, N.C., 1989. Operating characteristics of rotating beds. Chem. Eng. Prog. 85, 48. Ku, Y., Ji, Y.S., Chen, H.W., 2008. Ozonation of o-cresol in aqueous solutions using a rotating packed-bed reactor. Water Environ. Res. 80, 41. Kumar, M.P., Rao, D.P., 1990. Studies on a high-gravity gas–liquid contactor. Ind. Eng. Chem. Res. 29, 917. Lin, C.C., Chao, C.Y., Liu, M.Y., Lee, Y.L., 2009a. Feasibility of ozone absorption by H2 O2 solution in rotating packed beds. J. Hazard. Mater. 167, 1014. Lin, C.C., Chao, C.Y., Liu, M.Y., 2010. Removal of ozone from air by absorption in a rotating packed bed. J. Ind. Eng. Chem. 16, 140. Lin, C.C., Chen, B.C., 2007. Carbon dioxide absorption into NaOH solution in a cross-flow rotating packed bed. J. Ind. Eng. Chem. 13, 1083. Lin, C.C., Chen, B.C., 2008. Characteristics of cross-flow rotating packed beds. J. Ind. Eng. Chem. 14, 322. 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. 62, 507. 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, 138.
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Lin, C.C., Ho, T.J., Liu, W.T., 2002. Distillation in a rotating packed bed. J. Chem. Eng. Jpn. 35, 1298. Lin, C.C., Jian, G.S., 2007. Characteristics of a rotating packed bed equipped with blade packings. Sep. Purif. Technol. 54, 51. Lin, C.C., Lin, Y.C., Chien, K.S., 2009b. VOCs absorption in rotating packed beds equipped with blade packings. J. Ind. Eng. Chem. 15, 813. Lin, C.C., Liu, W.T., 2003. Ozone oxidation in a rotating packed bed. J. Chem. Technol. Biotechnol. 78, 138. 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, 455. Lin, C.C., Liu, W.T., 2007. Mass transfer characteristics of a high-voidage rotating packed bed. J. Ind. Eng. Chem. 13, 71. 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, 2381. Lin, C.C., Su, Y.R., 2008. Performance of rotating packed beds in removing ozone from gaseous streams. Sep. Purif. Technol. 61, 311. 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, 1471. Liu, H.S., Lin, C.C., Wu, S.C., Hsu, H.W., 1996. Characteristics of a rotating packed bed. Ind. Eng. Chem. Res. 35, 3590. Munjal, S., Dudukovic, M.P., Ramachandran, P.A., 1989. Mass transfer in rotating packed beds. I. Development of gas–liquid and liquid–solid mass-transfer coefficients. Chem. Eng. Sci. 44, 2245. Ramshaw, C., Mallinson, R.H., 1981, Mass transfer process. U.S. Patent 4,283,255. Ramshaw, C., 1983. Higee distillation – an example of process intensification. Chem. Eng. 389, 13. 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, 2299. Singh, S.P., Wilson, J.H., Counce, R.M., Villiers-Fisher, J.F., Jennings, H.L., Lucero, A.J., Reed, G.D., Ashworth, R.A., Elliott, M.G., 1992. Removal of volatile organic compounds from groundwater using a rotary air stripper. Ind. Eng. Chem. Res. 31, 574. 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, 174. Wang, G.Q., Xu, O.G., Xu, Z.C., Ji, J.B., 2008. New HIGEE-rotating zigzag bed and its mass transfer performance. Ind. Eng. Chem. Res. 47, 8840. Wang, M., Zou, H.K., Shao, L., Chen, J.F., 2004. Controlling factors and mechanism of preparing needlelike CaCO3 under high-gravity environment. Powder Technol. 142, 166.
Contents lists available at ScienceDirect
Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd
Carbon dioxide absorption in a cross-flow rotating packed bed Chia-Chang Lin ∗ , Bor-Chi Chen Department of Chemical and Materials Engineering, Chang Gung University, 259 Wen-Hwa 1st Road, Kwei-Shan, Taoyuan 333, Taiwan, ROC
a b s t r a c t This work investigates the feasibility of applying the cross-flow rotating packed bed (RPB) to the removal of carbon dioxide (CO2 ) by absorption from gaseous streams. Monoethanolamine (MEA) aqueous solution was used as the model absorbent. Also, other absorbents such as the NaOH and 2-amino-2-methyl-1-propanol (AMP) aqueous solutions were compared with the MEA aqueous solution. The CO2 removal efficiency was observed as functions of rotor speed, gas flow rate, liquid flow rate, MEA concentration, and CO2 concentration. Experimental results indicated that the rotor speed positively affects the CO2 removal efficiency. Our results further demonstrated that the CO2 removal efficiency increased with the liquid flow rate and the MEA concentration; however, decreased with the gas flow rate and the CO2 concentration. Additionally, the CO2 removal efficiency for the MEA aqueous solution was superior to that for the NaOH and AMP aqueous solutions. Based on the performance comparison with the conventional packed bed and the countercurrent-flow RPB, the cross-flow RPB is an effective absorber for CO2 absorption process. © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. Keywords: Rotating packed bed; Cross flow; Absorption; Mass transfer; Carbon dioxide
1.
Introduction
Ramshaw and Mallinson (1981) invented the rotating packed bed (RPB) for enhancing the gas–liquid mass transfer in distillation and absorption processes. This novel technology was referred to as “Higee” (an acronym for high gravity). Under RPB operation, an enhancement by a factor of 10–100 in the gas–liquid mass transfer was achieved by thin films and/or tiny droplets generated due to a rigorous centrifugal acceleration (2000–10,000 m s−2 ). Therefore, the dramatic reduction in the size of the RPB would be obtained, thereby reducing the capital and operating costs (Ramshaw, 1983). The original RPB could be operated at the mode of gas–liquid countercurrentflow. The countercurrent-flow RPB has been proved to enhance gas–liquid mass transfer in diverse processes such as distillation (Kelleher and Fair, 1996; Lin et al., 2002; Wang et al., 2008), absorption (Munjal et al., 1989; Kumar and Rao, 1990; Chen and Liu, 2002; Lin et al., 2003, 2004, 2009a,b, 2010; Tan and Chen, 2006; Cheng and Tan, 2006; Lin and Jian, 2007; Jassim et al., 2007; Lin and Chien, 2008; Lin and Su, 2008; Cheng et al., 2010), stripping (Keyvani and Gardner, 1989; Singh et al., 1992;
∗
Liu et al., 1996; Y.S. Chen et al., 2005a,b; Lin and Liu, 2006, 2007), reactive precipitation (Chen et al., 2000, 2003; J.F. Chen et al., 2004; Wang et al., 2004) and ozone oxidation (Lin and Liu, 2003; Y.H. Chen et al., 2004, 2005; Shang et al., 2006; Chiu et al., 2007; Ku et al., 2008; Chang et al., 2009). To remove CO2 from a gas stream containing CO2 with a huge flow rate, Lin and Chen (2007) first adopted the crossflow RPB for the CO2 removal by chemical absorption with the NaOH aqueous solution, proposing that the cross-flow RPB had a great potential for the CO2 removal from the exhausted gases. Moreover, Lin and Chen (2008) investigated the mass transfer efficiency of the cross-flow RPB for removing CO2 from a gas stream containing 1 vol% CO2 by chemical absorption with the NaOH aqueous solution. They proposed that the mass transfer efficiency of the cross-flow RPB was comparable to that of the countercurrent-flow RPB. Furthermore, Lin et al. (2008) investigated the mass transfer efficiency of the crossflow RPB for removing CO2 from a gas stream containing 10 vol% CO2 by chemical absorption with the NaOH aqueous solution. Their experimental results confirmed that the mass transfer efficiency of the cross-flow RPB was higher than that
Corresponding author. Tel.: +886 3 211 8800x5760; fax: +886 3 211 8800x5702. E-mail address: [email protected] (C.-C. Lin). Received 13 May 2010; Received in revised form 18 November 2010; Accepted 25 November 2010 0263-8762/$ – see front matter © 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2010.11.015
chemical engineering research and design 8 9 ( 2 0 1 1 ) 1722–1729
1723
Fig. 1 – Experimental setup for CO2 absorption. of the countercurrent-flow RPB. Based on these results, the cross-flow RPB could be an excellent alternative with high mass transfer efficiency to handle a gas stream containing huge flow rates and high CO2 concentrations. However, there are no existing literatures until now to examine how other absorbents affect the CO2 removal efficiency of the cross-flow RPB. Moreover, investigating the CO2 absorption performance of the cross-flow RPB with various absorbents is necessary to design the cross-flow RPB accurately and economically. Consequently, the objective of this work is to examine how operating parameters affect the CO2 absorption performance of the cross-flow RPB using the monoethanolamine (MEA) aqueous solution. The interested operating parameters in this work are rotor speed, gas flow rate, liquid flow rate, MEA concentration, and CO2 concentration. Additionally, other absorbents such as the NaOH and 2-amino-2-methyl-1-propanol (AMP) aqueous solutions were used to be compared with the MEA aqueous solution. The CO2 absorption performance comparison between the cross-flow RPB and other absorbers was also given. The results in this work could provide further insight into the feasibility of applying the cross-flow RPB to the removal of CO2 from gaseous streams.
2.
RPB, while the CO2 -rich MEA aqueous solution was expelled from the bottom of the RPB. Fig. 2 illustrates the photograph of the cross-flow RPB used in this work for the CO2 removal. The cross-flow RPB had an inner radius of 2.4 cm, an outer radius of 4.4 cm, and an axial height of 12 cm. The packing adopted in this work was stainless steel wire mesh that had a configuration of interconnected filaments with a mean diameter of 0.22 mm and an average mesh diameter of 3 mm. The packing was arranged within the cross-flow RPB with a specific surface area of 677 m2 /m3 and a voidage of 0.95. The liquid distributor consisted of a tube with a vertical set of holes that had twelve 0.1 cm diameter holes with 0.95 cm interval. Both sealings were adopted for prevent-
Experimental
Fig. 1 shows the experimental setup for CO2 absorption. During normal operation, the CO2 –N2 stream traveled axially from the bottom of the RPB owing to the pressure drop. At the same time, the prepared MEA aqueous solution was introduced from the tank into the inner edge of the packing through a liquid distributor. The MEA aqueous solution moved radially within the packing due to the centrifugal force and, then, exited the packing from the outer edge. Both the CO2 –N2 stream and the MEA aqueous solution were in contact with the cross-flow mode within the packing, in which CO2 in the CO2 –N2 stream reacted with MEA in the liquid stream. The exiting CO2 –N2 stream, containing low CO2 concentration, finally left the top of the packing and, then, was discharged from the top of the
Fig. 2 – Photograph of cross-flow RPB used in this work.
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ing gas from bypassing the packing and keeping the cross-flow operating mode. In general, the cross-flow RPB could be operated at the rotor speed of 540–1600 rpm, providing 11–97 times gravitational force based on the arithmetic mean radius. The CO2 concentration in inlet CO2 –N2 stream was varied at 1, 4, 7, and 10% (mole fraction) and the MEA concentration in inlet liquid stream was set at 0.2, 0.5, 0.7, and 1.0 mol/L. During operation, the gas flow rate (axial direction) was varied at the range of 10–70 L/min and the liquid flow rate (radial direction) was varied at the range of 0.2–0.5 L/min. The CO2 concentrations in inlet and outlet CO2 –N2 streams were measured by an infrared (IR) CO2 analyzer (Draeger, Ploytron Transmitter IR CO2 ). During CO2 absorption, the CO2 concentration in outlet CO2 –N2 streams were observed to drop rapidly and then reached a steady value within 10–15 min. The reproducibility tests under almost all of the operating conditions were carried out in this work. The CO2 concentration in outlet CO2 –N2 streams was observed to be reproduced with a deviation of less than 5%. The CO2 concentration in outlet liquid stream was determined by the standard method (Aroonwilas and Tontiwachwuthikul, 1997). Mass balance on inlet and outlet of both the gas and liquid streams indicated that the errors were within 10%. All experiments were conducted at an average temperature of 28 ◦ C with atmospheric pressure.
3.
Results and discussion
explained by the fact that the centrifugal acceleration could induce thinner liquid films and/or tiny droplets; a thinner boundary layer for mass transfer would be induced, thus leading to a higher gas–liquid mass transfer according to penetration theory. Similar trends were also found in the CO2 removal by chemical absorption with the NaOH, MEA, and MEA/AMP aqueous solutions in the countercurrent-flow RPB (Lin et al., 2003) and the CO2 removal by chemical absorption with the NaOH aqueous solution in the cross-flow RPB (Lin and Chen, 2007). At a low gas flow rate of 10 L/min, E varied ωx with the x values of 0.10 and 0.01 for the liquid flow rate of 0.2 and 0.5 L/min, respectively. This observation implied that an increase of the E values by the rotor speed was more pronounced at a low liquid flow rate. At a low liquid flow rate, the cross-flow RPB could provide an improved liquid distribution for enhancing the contact of gas and liquid by increasing centrifugal acceleration. This characteristic was more obvious at a high gas flow rate of 70 L/min, as shown in Fig. 3, indicating that the x values varied from 0.32 to 0.14 as the liquid flow rate was increased from 0.2 to 0.5 L/min. Moreover, for a given liquid flow rate, the sensitivity of the E values to variations in the rotor speed at a high gas flow rate was higher than that at a low gas flow rate. Hence, the rotor speed significantly affected the E values at low liquid flow rates and high gas flow rates.
3.1.
Absorption behavior of MEA absorbent
3.1.2.
The removal efficiency of CO2 in the cross-flow RPB is defined as E=
Ci − Co × 100 Ci
(1)
where E is the removal efficiency of CO2 , and Ci and Co are the concentrations of CO2 in inlet and outlet CO2 –N2 streams, respectively. The E values were measured at various values of the operation variables, including rotor speed (ω), gas flow rate (QG ), liquid flow rate (QL ), MEA concentration (CMEA ), and CO2 concentration (CCO2 ) to evaluate the CO2 absorption performance of the cross-flow RPB using the MEA absorbent.
3.1.1.
Effect of rotor speed
Fig. 3 summarizes the E values as a function of the rotor speed from 540 to 1600 rpm at the CO2 concentration of 1% and the MEA concentration of 1.0 mol/L. As expected, increasing the rotor speed enhanced the E values. This result could be
Effect of gas flow rate
Fig. 4 presents the effect of varying the gas flow rate from 10 to 70 L/min on the E values at the CO2 concentration of 1% and the MEA concentration of 1.0 mol/L. The gas flow rate influenced the E values; that is, the E values decreased with the gas flow rate for a given liquid flow rate and rotor speed. Owing to that an increasing gas flow rate provided a larger amount of CO2 in the gas stream and a reduction in the contact time, the CO2 removal was limited at a high gas flow rate for a given MEA concentration. Similar trends were also found in the CO2 removal by chemical absorption with the NaOH, MEA, and MEA/AMP aqueous solutions in the countercurrentflow RPB (Lin et al., 2003) and the CO2 removal by chemical absorption with the NaOH aqueous solution in the cross-flow RPB (Lin and Chen, 2007). At a high liquid flow rate of 0.5 L/min, E was proportional −y to QG with the exponent y varying from 0.23 to 0.16 for the rotor speed from 540 to 1600 rpm. This feature implied that at a high liquid flow rate, the sensitivity of E to variations in the gas flow rate at a low rotor speed was slightly higher than that
100
100
75
75
QG= 10 (L/min); QL= 0.5 (L/min)
50
QG= 10 (L/min); QL= 0.2 (L/min) QG= 70 (L/min); QL= 0.5 (L/min)
CO2: 1 (%); MEA: 1.0 (mol/L)
E (%)
E (%)
CO2: 1 (%); MEA: 1.0 (mol/L)
QL= 0.5 (L/min); ω = 1600 (rpm)
50
QL= 0.5 (L/min); ω = 540 (rpm) QL= 0.2 (L/min); ω = 1600 (rpm)
QG= 70 (L/min); QL= 0.2 (L/min)
25
QL= 0.2 (L/min); ω = 540 (rpm)
25
0
0 200
600
1000
1400
1800
Rotor Speed (rpm)
Fig. 3 – Effect of rotor speed on E.
0
20
40
60
80
Gas Flow Rate (L/min)
Fig. 4 – Effect of gas flow rate on E.
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100
100
75
75
CO2: 1 (%); ω: 1600 (rpm)
QG= 10 (L/min); ω = 1600 (rpm)
50
E (%)
E (%)
CO 2: 1 (%); MEA: 1.0 (mol/L)
QG= 10 (L/min); ω = 540 (rpm) QG= 70 (L/min); ω = 1600 (rpm)
QG= 10 (L/min); QL= 0.5 (L/min)
50
QG= 10 (L/min); QL= 0.2 (L/min) QG= 70 (L/min); QL= 0.5 (L/min)
QG= 70 (L/min); ω = 540 (rpm)
QG= 70 (L/min); QL= 0.2 (L/min)
25
25
0
0 0.1
0.2
0.3
0.4
0.5
0.0
0.6
0.3
Liquid Flow Rate (L/min)
0.9
1.2
Fig. 6 – Effect of MEA concentration on E.
Fig. 5 – Effect of liquid flow rate on E.
at a high rotor speed. As the liquid flow rate was decreased to 0.2 L/min, this characteristic was also found, as shown in Fig. 4, indicating that the y values varied from 0.46 to 0.34 when the rotor speed was increased from 540 to 1600 rpm. Moreover, for a given rotor speed, the sensitivity of the E values to variations in the gas flow rate at a low liquid flow rate was higher than that at a high liquid flow rate. Hence, the gas flow rate provided a significant effect on the E values at low rotor speeds and low liquid flow rates.
3.1.3.
0.6
MEA Concentration (mol/L)
Effect of liquid flow rate
Fig. 5 indicates the effect of the liquid flow rate ranging from 0.2 to 0.5 L/min on the E values at the CO2 concentration of 1% and the MEA concentration of 1.0 mol/L. The liquid flow rate had an influence on the E values; that is, an increase in the liquid flow rate yielded an increase in the E values for a given gas flow rate and rotor speed. This behavior was attributed to the fact that more MEA used to absorb CO2 at a high liquid flow rate was favorable to the CO2 removal at a given MEA concentration. Similar trends were also found in the CO2 removal by chemical absorption with the NaOH, MEA, and MEA/AMP aqueous solutions in the countercurrent-flow RPB (Lin et al., 2003) and the CO2 removal by chemical absorption with the NaOH aqueous solution in the cross-flow RPB (Lin and Chen, 2007). At a low gas flow rate of 10 L/min, E correlated QLz with the exponent z varying from 0.19 to 0.08 for the rotor speed from 540 to 1600 rpm. This suggested that at a low gas flow rate, the sensitivity of E to the variation in the liquid flow at a low rotor speed was higher than that at a high rotor speed. As the gas flow rate was increased to 70 L/min, this phenomenon was more evident, as shown in Fig. 5, implying that the z values varied from 0.65 to 0.46 when the rotor speed was increased from 540 to 1600 rpm. Moreover, for a given rotor speed, the dependence of the E values on the liquid flow rate at a high gas flow rate was much higher than that at a low gas flow rate.
Hence, the liquid flow rate provided a considerable effect on the E values at low rotor speeds and high gas flow rates. The results in Fig. 5 can be fitted with the logarithmic equations indicated in Table 1. According to these equations, the liquid flow rates required to achieve a CO2 removal of 95% were obtained under various operating conditions, and the corresponding QG /QL ratio was represented by (QG /QL )95 . As listed in Table 1, the values of (QG /QL )95 increased with the gas flow rate for the same rotor sped at the MEA concentration of 1.0 mol/L. Moreover, the rotor speed provided an increase in (QG /QL )95 for the same gas flow rate at the MEA concentration of 1.0 mol/L. However, it was known that the energy consumption would be increased with an increasing rotor speed. As a result, the optimum between the rotor speed and the removal efficiency should be determined for industrial-scale applications.
3.1.4.
Effect of MEA concentration
Fig. 6 displays the E values as functions of the MEA concentration from 0.2 to 1.0 mol/L at the rotor speed of 1600 rpm and the CO2 concentration of 1%. At a given gas flow rate and liquid flow rate, the E values increased with an increasing MEA concentration. This characteristic was caused by the fact that increasing MEA concentration could give higher amounts of hydroxide ions per unit volume for more CO2 absorption at a given gas flow rate and liquid flow rate. Similar trends were also found in the CO2 removal by chemical absorption with the NaOH, MEA, and MEA/AMP aqueous solutions in the countercurrent-flow RPB (Lin et al., 2003) and the CO2 removal by chemical absorption with the NaOH aqueous solution in the cross-flow RPB (Lin and Chen, 2007). At a low gas flow rate of 10 L/min, the E values were proportional to the MEA concentration raised to the w power. The w values varied from 0.04 to 0.15 as the liquid flow rate was decreased from 0.5 to 0.2 L/min. This result suggested that an enhancement of the E values by the MEA concentration was more pronounced at a low liquid flow rate. This finding was also observed at a high gas flow rate of 70 L/min, as shown in Fig. 6, showing that the w values increased from 0.21 to 0.30
Table 1 – (QG /QL )95 under various operating conditions. ω (rpm)
QG (L/min)
CMEA (mol/L)
Logarithmic equation
(QL )95 a (L/min)
(QG /QL )95
540 540 1600 1600
10 70 10 70
1.0 1.0 1.0 1.0
E = 16.8 ln(QL ) + 108 E = 29.3 ln(QL ) + 78 E = 7.5 ln(QL ) + 103 E = 27.0 ln(QL ) + 90
0.46 1.8 0.34 1.2
22 39 29 58
a
The liquid flow rate required to achieve E = 95.
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100
75
E (%)
MEA:1 (mol/L); ω: 1600 (rpm) QG= 10 (L/min); QL= 0.5 (L/min)
50
QG= 10 (L/min); QL= 0.2 (L/min) QG= 70 (L/min); QL= 0.5 (L/min) QG= 70 (L/min); QL= 0.2 (L/min)
25
0 0
3
6
9
12
CO2 Concentration (%)
Fig. 7 – Effect of CO2 concentration on E. with the decrease in the liquid flow rate from 0.5 to 0.2 L/min. Moreover, for a given liquid flow rate, the dependence of the E values on the MEA concentration at a high gas flow rate was higher than that at a low gas flow rate. Hence, the MEA concentration offered a larger effect on the E values at low liquid flow rates and high gas flow rates.
3.1.5.
Effect of CO2 concentration
Fig. 7 shows the effect of the CO2 concentration from 1 to 10% on the E values at the rotor speed of 1600 rpm and the MEA concentration of 1.0 mol/L. At a given gas flow rate and liquid flow rate, the E values decreased with an increasing CO2 concentration. This finding was explained by the fact that more CO2 must be removed at high CO2 concentrations for a given
MEA concentration. Similar trends were also found in the CO2 removal by chemical absorption with the NaOH, MEA, and MEA/AMP aqueous solutions in the countercurrent-flow RPB (Lin et al., 2003). At a low gas flow rate of 10 L/min, the E values were proportional to the CO2 concentration to the −0.01 power at the liquid flow rate of 0.5 L/min. This power would vary to −0.06 at the liquid flow rate of 0.2 L/min. This feature implied that a reduction of the E values by the CO2 concentration was more evident at a low liquid flow rate. This observation was more obvious at a high gas flow rate of 70 L/min, as shown in Fig. 7, revealing that this power varied from −0.17 to −0.46 with the decrease in the liquid flow rate from 0.5 to 0.2 L/min. Moreover, for a given liquid flow rate, the sensitivity of the E values to variations in the CO2 concentration at a high gas flow rate was much higher than that at a low gas flow rate. Hence, the CO2 concentration gave a substantial effect on the E values at low liquid flow rates and high gas flow rates.
3.2.
Comparison of absorbents
Absorbent type is considered another important factor affecting the removal efficiency of CO2 in the cross-flow RPB. A particular absorbent virtually yields a specific range of the E values. Table 2 shows the E values for the aqueous solutions containing NaOH, MEA, and AMP under various operating conditions at the absorbent concentration of 1 mol/L and the CO2 concentration of 1%. According to Table 2, it can be seen that the MEA aqueous solution exhibited the best removal efficiency of CO2 over the aqueous solutions containing NaOH and
Table 2 – E values for CO2 –NaOH, CO2 –MEA, and CO2 –AMP systems at absorbent concentration of 1 mol/L and CO2 concentration of 1%. ω (rpm)
QG (L/min)
QL (L/min)
E (NaOH) (%)
E (MEA) (%)
E (AMP) (%)
540 900 1260 1600 540 1600 540 1600 540 900 1260 1600 540 1600 540 1600 540 1600 540 1600 540 900 1260 1600 540 1600 540 1600 540 900 1260 1600
10 10 10 10 30 30 50 50 70 70 70 70 10 10 70 70 10 10 70 70 10 10 10 10 30 30 50 50 70 70 70 70
0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.3 0.3 0.3 0.3 0.4 0.4 0.4 0.4 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
81 87 88 90 45 63 36 50 32 37 41 43 83 92 34 52 92 96 45 59 96 97 98 98 75 85 63 75 55 63 65 68
82 90 91 92 53 68 40 53 33 40 44 47 85 92 40 54 94 96 50 66 97 97 98 98 81 88 71 79 61 68 69 71
57 66 69 69 34 43 28 35 26 28 30 32 63 77 27 36 78 84 35 43 82 85 87 88 55 65 43 51 38 40 43 45
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Table 3 – E values for CO2 –NaOH, CO2 –MEA, and CO2 –AMP systems at absorbent concentration of 1 mol/L and CO2 concentration of 10%. ω (rpm)
QG (L/min)
QL (L/min)
E (NaOH) (%)
E (MEA) (%)
E (AMP) (%)
540 900 1260 1600 540 1600 540 1600 540 900 1260 1600 540 1600 540 1600 540 1600 540 1600 540 900 1260 1600 540 1600 540 1600 540 900 1260 1600
10 10 10 10 30 30 50 50 70 70 70 70 10 10 70 70 10 10 70 70 10 10 10 10 30 30 50 50 70 70 70 70
0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.3 0.3 0.3 0.3 0.4 0.4 0.4 0.4 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
53 69 76 77 18 35 11 21 6.8 9.0 11 14 61 79 8.2 18 79 93 15 29 92 94 94 95 59 78 37 56 26 35 39 42
61 71 78 80 22 35 13 20 9.6 12 15 16 72 85 14 25 91 96 28 41 94 95 96 96 62 79 40 61 31 38 43 46
33 41 48 52 14 19 9.0 12 6.7 7.8 8.7 9.2 40 60 8.1 16 68 80 16 22 77 81 83 84 36 49 23 32 17 20 22 23
AMP. Additionally, the E values for the NaOH aqueous solution were higher than those for the AMP aqueous solution. For example, absorbing CO2 with the MEA aqueous solution was associated with the gave the E values varying from 61 to 71 as the rotor speed was increased from 540 to 1600 rpm at the gas flow rate of 70 L/min and the liquid flow rate of 0.5 L/min. However, at the same operating condition, the E values for the NaOH aqueous solution varied from 55 to 68 and the E values for the AMP aqueous solution varied from 38 to 45. Similar trends were also found at the CO2 concentration of 10%, as listed in Table 3. The same phenomenon was observed in the conventional packed bed (Aroonwilas et al., 1999). The difference between the E values for the NaOH aqueous solution and those for the AMP aqueous solution was primarily affected by the reaction rate of CO2 with the absorbent. The greater the reaction rate, the higher the E values would be expected. As pointed out by Aroonwilas et al. (1999), the reaction rate in the NaOH aqueous solution was much higher than that in the AMP aqueous solution, thus leading to a higher E value. However, the discrepancy between the E values for the MEA aqueous solution and those for the NaOH aqueous solution was not affected by the reaction rate of CO2 with the absorbent. As proposed by Aroonwilas et al. (1999), the reaction rate in the NaOH aqueous solution is higher than that in the MEA aqueous solution. Thus, a higher E value for the MEA aqueous solution was believed to be attributed to a higher gas–liquid interfacial area existed in the cross-flow RPB. The MEA aqueous solution has a lower surface tension in comparison with the NaOH aqueous solution (Aroonwilas et al., 1999).
This would result in a higher gas–liquid interfacial area for the MEA aqueous solution, thus leading to a higher E value.
3.3. Comparison with conventional packed bed and countercurrent-flow RPB To compare the CO2 absorption performance of the cross-flow RPB with other absorbers such as the conventional packed bed and the countercurrent-flow RPB, the mass transfer coefficient is considered a simple but representative design parameter. For CO2 absorption process, the experimental overall volumetric gas-phase mass transfer coefficients (KG a) of the cross-flow RPB can be evaluated by the following equation (Lin and Liu, 2007): KG a =
QG (Ro 2 − Ri 2 )Zb
ln
C i
Co
(2)
where QG is the volumetric flow rate of gas, Z is the axial length of the cross-flow RPB, Ri and Ro are the inner and outer radii of the cross-flow RPB, respectively. Ci and Co are the concentrations of CO2 in the inlet and outlet CO2 –N2 streams, respectively. The estimated KG a values in the cross-flow RPB were compared to the reported KG a values in the conventional packed bed when the NaOH aqueous solution was used as the absorbent (Aroonwilas and Tontiwachwuthikul, 1997). The conventional packed bed had a height of 110 cm and an internal diameter of 1.9 cm. As shown in Table 4, the average KG a value (1.42) in the cross-flow RPB was about 0.75 of that (1.96)
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Table 4 – Comparison of cross-flow RPB and conventional packed bed. Conventional packed bed (Aroonwilas and Tontiwachwuthikul, 1997) Operation conditions Pressure (atm) Temperature (K) Gas phase CO2 concentration (mol%) Gas flow rate (L/min) Gas loading (m3 /(m2 h)) Liquid phase NaOH concentration (mol/L) Liquid flow rate (mL/min) Liquid loading (m3 /(m2 h)) KG a (s−1 ) KG a (based on liquid loading) (s−1 )
Cross-flow RPB
1 297
1 301
1 4.9–10.4 1044–2196
1 10–70 140–983
0.2–1.0 23–67 4.9–14.2 1.38–2.41 0.70–1.15
0.2–1.0 200–500 0.5–1.2 0.30–2.54 0.30–2.54
Table 5 – Comparison of cross-flow RPB and countercurrent-flow packed bed. Countercurrent-flow RPB (Lin et al., 2003) Operation conditions Pressure (atm) Temperature (K) Gas phase CO2 concentration (mol%) Gas flow rate (L/min) Gas loading (m3 /(m2 h)) Liquid phase MEA concentration (mol/L) flow rate (mL/min) Liquid loading (m3 /(m2 h)) KG a (s−1 )
1 300
1 301
10 13.1 111
10 10 140
1.0 42 0.4 0.21
in the conventional packed bed. As pointed out by Strigle (1987), KG a in the conventional packed bed varied with the liquid loading to the 0.3 power. When the liquid loadings in the cross-flow RPB were used to evaluate KG a in the conventional packed bed, Table 4 indicates that the average KG a value (1.42 s−1 ) in the cross-flow RPB was higher than that (0.93 s−1 ) in the conventional packed bed. Additionally, the higher the gas loading, the higher the KG a values would be expected. Thus, the cross-flow RPB performed better than the conventional packed bed. When the MEA aqueous solution was used as the absorbent, Table 5 shows that the KG a value in the cross-flow RPB was 2.5 times higher than that in the countercurrent-flow RPB, which had an inner radius of 3.8 cm, an outer radius of 8.0 cm, and an axial height of 2.0 cm. Based on the comparison in KG a, the cross-flow RPB shows its applicability in removing CO2 from gaseous streams.
4.
Cross-flow RPB
Conclusions
This work has examined the absorption performance of the cross-flow RPB using CO2 absorption by the MEA aqueous solution from gaseous streams. The results were considered in relation with the removal efficiency, E values. The E values obviously increased with the rotor speed. Also, the E values appeared to increase with an increasing liquid flow rate and an increasing MEA concentration, but decrease with an increasing gas flow rate and an increasing CO2 concentration. Moreover, the (QG /QL )95 values increased with the rotor speed and the gas flow rate. Based on the comparison between the MEA aqueous solution and other absorbents such as the NaOH and AMP aqueous solutions, the MEA aqueous
1.0 200 0.5 0.51
solution yielded the superior CO2 removal efficiency. Additionally, according to the performance comparison with the conventional packed bed and the countercurrent-flow RPB, the cross-flow RPB is believed to be an alternative gas–liquid contactor for the removal of CO2 from gaseous streams.
Acknowledgement The financial support of National Science Council of the Republic of China (NSC 97-2221-E-182-013-MY2) is greatly appreciated.
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