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Effective interfacial area and liquid-side mass transfer coefficients in a rotating bed equipped with baffles
Separation and Purification Technology 144 (2015) 139–145
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Effective interfacial area and liquid-side mass transfer coefficients in a rotating bed equipped with baffles Ching-Yi Tsai, Yu-Shao Chen ⇑ Department of Chemical Engineering, Chung Yuan University, Chung-Li 320, Taiwan
a r t i c l e
i n f o
Article history: Received 23 June 2014 Received in revised form 6 February 2015 Accepted 7 February 2015 Available online 21 February 2015 Keywords: Rotating packed bed Baffle Mass transfer coefficient Effective interfacial area
a b s t r a c t This study examines the mass transfer characteristics of a rotating bed that is equipped with static baffles. The volumetric liquid-side mass transfer coefficient (kL a) and the effective gas–liquid interfacial area were determined using a deoxygenation system and a chemisorption of CO2 in NaOH solution, respectively; the local liquid-side mass transfer coefficient (kL ) was thus calculated. The effects of liquid flow rate, rotational speed, baffles and type of packing on these mass transfer parameters were investigated. Experimental results show that the bed with baffles had a higher interfacial area and lower kL than that without baffles. Adding baffles to the bed did not significantly affect the kL a values. Additionally, the packing with a larger specific interfacial area yielded higher values of kL a mainly because the effective interfacial area was higher. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction A rotating packed bed (RPB) is commonly introduced as a gas– liquid contactor with a high mass transfer efficiency as it replaces gravity with a centrifugal force. The liquid that flows through the high-speed rotor undergoes high acceleration, yielding thin liquid films, tiny liquid droplets and chaotic flow. Accordingly, the mass transfer and mixing efficiency can be substantially increased in an RPB. An RPB typically has a mass transfer coefficient that is one to two orders of magnitude higher than that of a conventional packed column. The considerable reduction in size and capital that are provided by the RPB have made it important in the field of process intensification. Applications of the RPB in absorption [1–4], stripping [5], distillation [6,7], transesterification [8] and the production of particles [9,10] have been widely explored. The characteristics of fluid flow within an RPB have been investigated by observation with a camera [11,12], electro-conductivity measurement [12] and CFD analysis [13]. One of the unique flow behaviors in an RPB is the synchronous rotation of fluids with the rotor, which is caused by the drag of the packing, and is supposed to be an important factor that limits the improvement of mass transfer efficiency. As the liquid enters the rotor, it is accelerated by the packing over a very short distance and then rotates synchronously with rotor. Strong mass transfer occurs in the zone close to the inner radius of the rotor because of the tangential velocity of the liquid relative to the packing is high. Outside this ⇑ Corresponding author. Tel.: +886 3 2654131; fax: +886 3 2654199. E-mail address: [email protected] (Y.-S. Chen). http://dx.doi.org/10.1016/j.seppur.2015.02.008 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
region, the relative velocity is very small, causing a severe mal-distribution of liquid in the packing [11,12]. Consequently, some of the packing becomes un-wetted and the gas–liquid interfacial area is reduced. On the other hand, synchronous rotation of the gas in the rotor was observed [14]. Some studies have revealed that the gas-side mass transfer coefficient (kG) is similar to that in a conventional packed column because of the lack of tangential velocity of the gas as it passes through the rotor [15,16]. The increase in the effective gas–liquid interfacial area is responsible for most of the increase in KGa that is caused by the centrifugal force in a gas-side resistance-controlled process. Recently, several novel designs that improve the hydraulic performance and mass transfer efficiency of RPBs were proposed. These designs have focused on limiting the synchronous rotation of fluids and enhancing the velocity of the fluids relative to the packing. Chandra et al. [17] developed an RPB with split packing and found that it exhibited enhanced tangential slip velocity. Reddy et al. [18] reported that KGa values in an RPB with split packing were two orders of magnitude higher than those in a conventional packed column. Wang et al. [19] presented a rotating zigzag bed for an ethanol–water distillation system and showed that the volume of the equipment could be made an order of magnitude smaller than that of a conventional packed column. Luo et al. [20] studied the mass transfer in an RPB with blades inside the rotor; their results revealed that the rotor with blades considerably increased the effective interfacial area and kL. Our earlier work developed a rotating blade bed with static baffles, whose pressure drop and overall mass transfer coefficient were examined using an isopropyl alcohol absorption process
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Nomenclature a C CO2 C CO2 C L;i C L;o C NaOH C Na2 CO3 DCO2 DCO2 ;w E H HCO2 Ha I k1 k2 1 k2 kG kL
gas–liquid interfacial area (1/m) concentration of carbon dioxide in liquid stream (mol/L) concentration of carbon dioxide at the gas–liquid interface (mol/L) concentration of solute in the inlet liquid stream (mol/L) concentration of solute in the outlet liquid stream (mol/L) concentration of sodium hydroxide (mol/L) concentration of sodium carbonate (mol/L) diffusion coefficient of carbon dioxide in liquid (m2/s) diffusion coefficient of carbon dioxide in water (m2/s) enhancement factor of the gas–liquid flux due to chemical reaction (–) Henry’s constant [(mol/L)/(mol/L)] Henry’s constant [(mol/L)/atm] Hatta number defined in Eq. (4) (–) ionic strength of solution (mol/L) reaction rate constant (L/s) reaction rate constant (L2/mol s) reaction rate constant in infinite dilution (L2/mol s) gas-side mass transfer coefficient (m/s) liquid-side mass transfer coefficient (m/s)
[21]. Experimental results demonstrated that adding the static baffles in the rotating blade bed reduced the pressure drop by 53% and improved KGa by 117%. These results indicated that installing static baffles inside the rotating blade bed effectively disturbed the rotation of the gas, reducing the centrifugal pressure drop and increasing the tangential slip velocity of the gas flow. Additionally, adding the baffles made the liquid flow behavior in the device very different from that in a conventional RPB because the liquid is repeatedly accelerated between the blades and the baffles. Consequently, the liquid-side mass transfer coefficients (kLa and kL) and the effective interfacial area of the proposed device were investigated in present study. Many works have presented experimental data and empirical equations for the volumetric liquid-side mass transfer coefficient in an RPB [22]. Oxygen–water stripping or absorption processes are typically conducted to obtain kLa because the gas-side mass transfer resistance is negligible in these processes. Moreover, several investigations have demonstrated the measurement of the effective gas–liquid interfacial area in an RPB using a CO2–NaOH absorption process. Munjal et al. [23] obtained the effective interfacial area of glass beads and commercial high-porosity packing in an RPB. Their experimental results showed that the interfacial area was proportional to the rotational speed to a power of 0.42 for glass beads and 0.28 for the high-porosity packing, respectively. Yang et al. [24] measured the effective interfacial area in an RPB with rotors of different radii. Higher interfacial area was observed with rotors with smaller outer radii, revealing a significant end effect in the RPB. Rajan et al. [25] measured the interfacial area in a split-packing RPB and found that counter-rotation of the packing yielded a higher interfacial area than co-rotation. Luo et al. [26] evaluated the interfacial area in RPBs that were packed with various types of wire mesh. They proposed that the effective interfacial area was proportional to the centrifugal acceleration raised to the power of 0.12. Guo et al. [27] investigated the effect of the shell zone on the interfacial area in an RPB; their results revealed that the interfacial area in the shell zone can reach 30% of that in the whole RPB. In our previous work, the rotating bed with baffles showed a high values of KGa. However, the fundamental characteristics of mass transfer in this device are not clear and a systematic
KGa kL a m n NCO2 PCO2 p q QG QL r CO2 ri ro S T z
overall volumetric gas-side mass transfer coefficient (1/s) volumetric liquid-side mass transfer coefficient (1/s) exponent in Eq. (15) (–) exponent in Eq. (15) (–) CO2 gas–liquid flux (mol/m2 s) partial pressure of CO2 in the gas phase (atm) exponent in Eq. (16) (–) exponent in Eq. (16) (–) gas flow rate (m3/s) liquid flow rate (m3/s) rate of reaction (mol/s) inner radius of the packed bed (m) outer radius of the packed bed (m) G stripping factor defined as S ¼ HQ Q L (–) temperature (K) axial height of the packing (m)
Greek letters e porosity of the packing (–) x rotational speed (rpm)
study on the individual mass transfer coefficients and the effective interfacial area is required. Therefore, this study further investigated the liquid-side mass transfer coefficients and the effective interfacial area to understand the mechanism of mass transfer of the proposed device. An oxygen-stripping process and a CO2 absorption process were utilized to determine kLa and the effective gas– liquid interfacial area, respectively, in a rotating bed with baffles. The kL values were also calculated. The effects of rotational speed, liquid flow rate, baffles and type of packing on these mass transfer parameters were examined. 2. Experimental Fig. 1 displays the structure of a rotating blade bed with baffles. The device herein comprised mainly of a rotating disk and a static disk. Three sets of blades, each has eight, 16 and 16 blades, were installed in the annular packing regions of the rotating disk, shown as Fig. 1(b). Each blade, which was covered by a layer of stainless steel mesh, had a radial width of 1.2 cm and an axial height of 1.8 cm. The inner and outer radii of the rotor were 1.8 and 7.8 cm, respectively, and the specific surface area and porosity were 163 1/m and 0.99, respectively. Between packing regions, two sets of baffles were fixed on the static disk, shown as Fig. 1(c). The baffle was made of a stainless steel sheet which was not perforated. It had a radial width and an axial height of 0.6 and 1.5 cm, respectively and was designed to be removable to enable the effect on mass transfer to be determined. The structure and number of the blades and baffles were discussed in our previous work [21]. The wire-mesh packing was also investigated by replacing the blades with stainless steel wire mesh in the annular packing regions. The specific surface area and porosity of the wire-mesh packing were 397 1/m and 0.98, respectively. The bed was operated from 600 to 1800 rpm, which provided a centrifugal force of 19 to 174-fold gravitational force. Consequently, the effect of gravity on the flow of fluids in the rotor can be neglected. The liquid entered the bed from a liquid distributor at its center; was sprayed toward the inner edge of the packing region, and was moved outward by the centrifugal force. Some of the liquid was captured by the static baffles and it would flow down along
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Gas inlet
Blade Rotational disk
Gas outlet
Seal
(b)
Liquid inlet
Baffle
Liquid outlet
(a)
(c)
Fig. 1. Schematic diagram of the device used in this study: (a) structure of the RPB with blade packings and baffles, (b) blades on the rotational disk and (c) baffles on the stationary disk. [21].
Rotameter Gas Out Nitrogen Blade Motor
packing. The liquid then splashed onto the static housing and exited at the bottom. The gas was introduced from the static housing, flowed inward through the packing and was expelled from the pipeline at the center of the bed. In the rotor, the synchronous rotation of gas is retarded by the baffles, which enhances the tangential velocity. 2.1. Measurement of kLa
Baffle Rotameter
Water
Water bath
DO Probe
Liquid Out
Fig. 2. Diagram of the experimental setup for stripping of oxygen.
the baffle surface and fall to the adjacent rotating packing region. Then the liquid can be evenly distributed into the packing due to the high relative velocity between the liquid droplet and rotating
The volumetric local liquid-side mass transfer coefficient was determined by a deoxygenation process because the solubility of oxygen in water was very low and the mass transfer resistance in the gas phase was negligible. Fig. 2 presents the experimental setup. Water controlled by a water bath at 30 °C and nitrogen at room temperature (27–28 °C) contacted in the RPB. Experimental results showed that the temperature difference between the inlet and outlet water streams was less than 1 °C. The dissolved oxygen (DO) concentrations in the inlet and outlet water streams were measured using a DO probe (DO-5509, Lutron electronic). The kLa values were calculated using the following equation.
ln QL kL a ¼ 2 2 pðro ri Þz
h C L;i 1i 1 1S C L;o þS 1 1S
ð1Þ
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Gas Out Rotameter CO2 analyzer CO2+air Blade Motor Baffle Rotameter
Liquid sampling point NaOH solution
Liquid Out
Fig. 3. Diagram of the experimental setup for absorption of CO2.
2.2. Measurement of interfacial area The effective gas–liquid interfacial area was measured by the chemical method in a CO2–NaOH system [23–28]. Fig. 3 shows the experimental setup. A CO2–N2 mixed gas stream with a CO2 concentration of 10 vol.% was brought into contact with 1 M NaOH solution at 30 °C. The CO2 concentrations in the inlet and outlet gas streams were measured using an infrared CO2 analyzer (Polytron IR CO2, Drager). The pH value and the concentration of the carbonate ions of the outlet liquid stream were determined by potentiometric titration. The rate of reaction can be given by a second-order rate equation.
r CO2 ¼ k2 C CO2 C NaOH
ð2Þ
The reaction can be treated as a pseudo-first-order reaction when the following inequality is satisfied.
Ha <
1 C NaOH 1þ 2 C CO2
! ð3Þ
where Ha is the Hatta number, which is given by
Ha ¼
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DCO2 k2 C NaOH
The rate constant of the pseudo-first-order reaction is expressed as,
k1 ¼ k2 C NaOH
ð5Þ
The absorption rate of CO2 (N CO2 ) can be expressed as,
NCO2 ¼
EkL aC CO2
p
ðr2o
r 2i Þz
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ Ha2
ð6Þ
ð7Þ
If the reaction is fast (Ha > 3), then the enhancement factor can be expressed as,
E ¼ Ha
ð8Þ
The Ha values were from 8 to 9.5 in this study. Substituting Eqs. (4), (5) and (8) into Eq. (6), yields the absorption rate as,
NCO2 ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DCO2 k1 aC CO2 pðr 2o r 2i Þz
DCO2 ¼ 1 ð0:129C NaOH þ 0:261C Na2 CO3 Þ DCO2 ;w
ð10Þ
2119 DCO2 ;w ¼ 2:35 106 exp T
ð11Þ
The values of k2 can be expressed as follows [31].
log
k2 1 k2
¼ 0:1987I 0:2012I2
1
log k2 ¼ 11:895
2:382 T
ð12Þ
ð13Þ
C CO2 ¼ HCO2 PCO2
ð14Þ
The effects of ionic strength and temperature on Henry’s law constant, HCO2 , were taken from Danckwerts [28] and Versteeg et al. [30]. 3. Results and discussion
where E is the enhancement factor of the absorption rate enhanced by the chemical reaction and is calculated using the following equation [28].
E¼
In Eq. (9), the absorption rate can be calculated from the CO2 concentrations of the inlet and outlet gas streams. The values of DCO2 can be calculated as follows [29,30].
Furthermore, since the CO2 absorption process is controlled by the liquid film, C CO2 can be estimated by Henry’s law as follows.
ð4Þ
2
kL
Fig. 4. Dependence of kL a on rotation speed with blade packing.
ð9Þ
3.1. Volumetric liquid-side mass transfer coefficient The concentration of dissolved oxygen in the outlet liquid stream and the concentration of carbon dioxide in the outlet gas stream were monitored online by a DO probe and CO2 analyzer, respectively. All the experiments were performed for up to 10 min, when the system reached a steady state. The error bounds on the experimental data were estimated within ±10%. Figs. 4 and 5 display the kL a values in a rotating bed with and without the static baffles. Experimental results clearly revealed that kL a increased with rotational speed and liquid flow rate. A 35–73% increase of kL a was observed as the rotational speed increased from 600 to 1800 rpm. This is mainly because the thickness of the boundary layer and the size of the liquid droplets were reduced, which increases the kL and a as rotational speed and liquid flow rate
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Fig. 5. Dependence of kL a on rotation speed with wire-mesh packing.
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Fig. 6. Dependence of a on rotation speed with blade packing.
increased. The dependence of kL a on the rotational speed and liquid flow rate is expressed as follows.
kL a / xm Q nL
ð15Þ
Table 1 presents the exponents m and n that were obtained in this work and those of other RPBs in the literature [22,32]. The m value of the proposed device with blade packing was from 0.38 to 0.43 and that of the device with wire-mesh packing was from 0.29 to 0.32. The n values herein varied from 0.99 to 1.15. The m values were lower than the exponents in the literature, and the n values were slightly higher. Additionally, in Figs. 4 and 5, adding the baffles into the bed did not significantly affect the kL a values, suggesting that baffles in a bed are more suitable for use in a gas-side mass transfer resistance-controlled system than in a liquid-side-controlled system. The types of packing had a stronger effect on kL a: the kL a of a wire-mesh bed exceeded that of a blade bed, especially at higher liquid flow rates. 3.2. Effective interfacial area Figs. 6 and 7 present the effective gas–liquid interfacial area as measured by a CO2–NaOH absorption process. Experimental results showed that the effective interfacial area increased with the rotational speed, mainly because the thickness of the liquid films and the size of the liquid droplets were reduced in a centrifugal field, increasing the gas–liquid interfacial area. Moreover, a higher effective interfacial area was obtained when the static baffles were added into the beds. Adding the static baffles in a blade bed increased the interfacial area by 16–34%; adding them in a
Table 1 The m and n values (Eq. (15)) in literature and in this work. Literature
Packing type
m
n
Chen et al. [22]
Random packings Wire-mesh packing
0.6
0.77
Lin and Jian [32]
Blade packing
0.55
0.91
This work
Blade packing (with baffles) Blade packing (without baffles) Wire-mesh packing (with baffles) Wire-mesh packing (without baffles)
0.43 0.38 0.32 0.29
1.08 0.99 1.06 1.15
Fig. 7. Dependence of a on rotation speed with wire-mesh packing.
wire-mesh bed increased by 2–16%. In a rotating bed without baffles, vigorous collision of the liquid occurred only in the region close to the inner radius of the bed owing to the high relative velocity between the liquid and packing. After it leaves this region, the liquid moves synchronously with the packing and the tangential velocity of the liquid relative to the packing declines. Hence, the liquid cannot easily be distributed laterally in the bed, preventing parts of the packing from being wetted by the liquid and decreasing the interfacial area. In contrast, in the bed with static baffles, the baffles captured the liquid, which was redistributed into the rotating sections repeatedly as it passed through the bed. Vigorous impingements of liquid on the packing and baffles occurred, generating fine liquid droplets, which reduce the degree of maldistribution of the liquid and, therefore, increasing the interfacial area. According to Figs. 6 and 7, the wire-mesh packing provided a higher effective interfacial area than did the blades, probably because the wire-mesh packing had a higher surface area.
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Table 2 The p and q values (Eq. (16)) in literature and in this work. Literature
Types of packing
e
p
q
Munjal et al. [23]
Glass beads High-porosity packing
0.43 0.95
0.42 0.28
0.26–0.30
Luo et al. [26]
Wire-mesh packing
0.96
0.24
0.77
This work
Blade packing (with baffles) Blade packing (without baffles) Wire-mesh packing (with baffles) Wire-mesh packing (without baffles)
0.99
0.22 0.20 0.33
0.34 0.36 0.23
0.33
0.26
0.98
The dependence of the interfacial area on the rotational speed and the liquid flow rate can be expressed as follows.
a / xp Q qL
ð16Þ
Table 2 presents the exponents p and q that were obtained in this work and those in the literature [23,26]. The p values, which varied over a wide range of 0.12–0.42, were governed primarily by the porosity of the packing: more porous packing had a smaller p value, as displayed in Fig. 8. The results confirmed the finding of Munjal et al. [23] that the centrifugal force has a weaker effect on more porous packing because less momentum is imparted to the liquid. The relation between p and packing porosity can be expressed empirically as,
p ¼ 0:453ð1 eÞ0:142
Fig. 8. Comparison of experimental values of p with results calculated using Eq. (17).
ð17Þ
As shown in Fig. 8, the exponent p can be reasonably predicted by Eq. (17). The q values obtained in this work were similar to those of Munjal et al. [23] but were smaller than those of Luo et al. [26] Though the data available in literature were included, more experimental data are required to further ensure the validity of Eq. (17). 3.3. Liquid-side mass transfer coefficient According to the empirical equation provided by Onda et al. [33], the variation of the effective interfacial area between two different solutions mainly depends on the physical properties such as density, viscosity and surface tension. Since these properties of a 1 M NaOH solution and water are similar, it is reasonably suggested that the interfacial area of these two aqueous phases do not change significantly [34,35]. Additionally, the liquid flow in the bed was suggested not influenced by the gas flow because the system was far from flooding. Therefore, the liquid-side mass transfer coefficient (kL ) can be calculated from the experimental results of deoxygenation and CO2 absorption using the following equation.
kL ¼
kL a a
ð18Þ
Figs. 9 and 10 plot the effect of rotational speed on kL of the two types of packing with and without static baffles. As expected, kL clearly increased with the liquid flow rate to the power of 0.63– 0.89. However, the dependence of kL on rotational speed differed between a blade bed and wire-mesh bed. In a blade bed, the value of kL increased with rotational speed to the power of 0.18–0.21, but in a wire-mesh bed, it was almost independent of rotational speed. Small liquid fragments in a high shear flow environment are commonly held to reduce mass transfer resistance. However, the velocity of the film flow on the packing surface probably decreased because of the increase in the interfacial area with rotational speed, leading to a reduction in kL . Consequently, kL was less
Fig. 9. Dependence of kL on rotation speed with blade packing.
enhanced by the centrifugal force in a wire-mesh bed than in a blade bed because more of the liquid traveled as a thin film over a wire-mesh bed. Figs. 9 and 10 also present the effect of static baffles on kL . The bed with static baffles had lower kL values. Adding static baffles increased the relative velocity and rate of collision of the liquid, increasing the mass transfer coefficient. However, the static baffles reduced the average radial velocity of the liquid that flowed through the bed, probably reducing kL . These results reveal that although adding static baffles in a rotating bed increased the effective interfacial area, doing so reduced kL , resulting in similar values of kL a in beds with and without baffles. This finding suggests that the use of static baffles in an RPB is more favorable for a gas-side mass transfer resistance-controlled process than for a liquid-sidecontrolled process.
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Fig. 10. Dependence of kL on rotation speed with wire-mesh packing.
4. Conclusions The volumetric liquid-side mass transfer coefficients and effective gas–liquid interfacial area were investigated by performing experiments that involved the deoxygenation and chemisorption of CO2 in aqueous NaOH in a rotating blade bed and a wire-mesh bed that were equipped with static baffles. The local liquid-side mass transfer coefficient was calculated by dividing kL a by the interfacial area. Experimental results demonstrated that kL a and the effective interfacial area increased with rotational speed and liquid flow rate. The centrifugal force had a smaller effect on interfacial area of more highly-porous packing because less momentum was imparted to the liquid. The value of kL increased with rotational speed in a blade bed but was almost independent of rotational speed in a wire-mesh bed. Adding static baffles in the rotating bed increased the interfacial area by 16–34% by the generation of vigorous flow between the rotor and the baffles. However, lower kL values were observed in the bed with baffles because the static baffles reduced the average radial velocity of the liquid. The effective interfacial area and kL a were lower in the blade bed than in the wire-mesh bed. These results suggest that packing with a higher specific surface area is more suitable for a liquid-side mass transfer resistance-controlled system, while the use of baffles is favorable in a gas-side-controlled system. Acknowledgment The support of the ROC Ministry of Science and Technology (NSC 102-2221-E-033-043; MOST 103-3113-E-007-002) is greatly appreciated. References [1] C.C. Lin, B.C. Chen, Y.S. Chen, S.K. Hsu, Feasibility of a cross-flow rotating packed bed in removing carbon dioxide from gaseous streams, Sep. Purif. Technol. 62 (2008) 507–512. [2] C.Y. Chiang, Y.Y. Liu, Y.S. Chen, H.S. Liu, Absorption of hydrophobic volatile organic compounds by a rotating packed bed, Ind. Eng. Chem. Res. 51 (2012) 9441–9445. [3] L.L. Zhang, J.X. Wang, Q. Sun, X.F. Zeng, J.F. Chen, Removal of nitric oxide in rotating packed bed by ferrous chelate solution, Chem. Eng. J. 181–182 (2012) 624–629.
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Res. 53 (2014) 4052–4058. [28] P.V. Danckwerts, Gas–Liquid Reactions, McGraw-Hill, New York, 1970. [29] G.F. Versteeg, W.V. Swaalj, Solubility and diffusivity of acid gases (CO2, N2O) in aqueous alkanolamine solutions, J. Chem. Eng. Data 33 (1988) 29–34. [30] R.D.V. Bhat, J.A.M. Kuipers, G.F. Versteeg, Mass transfer with complex chemical reactions in gas–liquid systems: two-step reversible reactions with unit stoichiometric and kinetic orders, Chem. Eng. J. 76 (2000) 127–152. [31] R. Pohorecki, W. Moniuk, Kinetics of reaction between carbon dioxide and hydroxyl ions in aqueous electrolyte solutions, Chem. Eng. Sci. 43 (1988) 1677–1684. [32] C.C. Lin, G.S. Jian, Characteristics of a rotating packed bed equipped with blade packings, Sep. Purif. Technol. 54 (2007) 51–60. [33] K. Onda, H. Takeuchi, Y. Okumoto, Mass transfer coefficient between gas and liquid phases in packed columns, J. Chem. Eng. Jpn. 1 (1968) 56–62. [34] G. Vazquez, E. Alvarez, R. Varela, A. Cancela, J.M. 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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Effective interfacial area and liquid-side mass transfer coefficients in a rotating bed equipped with baffles Ching-Yi Tsai, Yu-Shao Chen ⇑ Department of Chemical Engineering, Chung Yuan University, Chung-Li 320, Taiwan
a r t i c l e
i n f o
Article history: Received 23 June 2014 Received in revised form 6 February 2015 Accepted 7 February 2015 Available online 21 February 2015 Keywords: Rotating packed bed Baffle Mass transfer coefficient Effective interfacial area
a b s t r a c t This study examines the mass transfer characteristics of a rotating bed that is equipped with static baffles. The volumetric liquid-side mass transfer coefficient (kL a) and the effective gas–liquid interfacial area were determined using a deoxygenation system and a chemisorption of CO2 in NaOH solution, respectively; the local liquid-side mass transfer coefficient (kL ) was thus calculated. The effects of liquid flow rate, rotational speed, baffles and type of packing on these mass transfer parameters were investigated. Experimental results show that the bed with baffles had a higher interfacial area and lower kL than that without baffles. Adding baffles to the bed did not significantly affect the kL a values. Additionally, the packing with a larger specific interfacial area yielded higher values of kL a mainly because the effective interfacial area was higher. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction A rotating packed bed (RPB) is commonly introduced as a gas– liquid contactor with a high mass transfer efficiency as it replaces gravity with a centrifugal force. The liquid that flows through the high-speed rotor undergoes high acceleration, yielding thin liquid films, tiny liquid droplets and chaotic flow. Accordingly, the mass transfer and mixing efficiency can be substantially increased in an RPB. An RPB typically has a mass transfer coefficient that is one to two orders of magnitude higher than that of a conventional packed column. The considerable reduction in size and capital that are provided by the RPB have made it important in the field of process intensification. Applications of the RPB in absorption [1–4], stripping [5], distillation [6,7], transesterification [8] and the production of particles [9,10] have been widely explored. The characteristics of fluid flow within an RPB have been investigated by observation with a camera [11,12], electro-conductivity measurement [12] and CFD analysis [13]. One of the unique flow behaviors in an RPB is the synchronous rotation of fluids with the rotor, which is caused by the drag of the packing, and is supposed to be an important factor that limits the improvement of mass transfer efficiency. As the liquid enters the rotor, it is accelerated by the packing over a very short distance and then rotates synchronously with rotor. Strong mass transfer occurs in the zone close to the inner radius of the rotor because of the tangential velocity of the liquid relative to the packing is high. Outside this ⇑ Corresponding author. Tel.: +886 3 2654131; fax: +886 3 2654199. E-mail address: [email protected] (Y.-S. Chen). http://dx.doi.org/10.1016/j.seppur.2015.02.008 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
region, the relative velocity is very small, causing a severe mal-distribution of liquid in the packing [11,12]. Consequently, some of the packing becomes un-wetted and the gas–liquid interfacial area is reduced. On the other hand, synchronous rotation of the gas in the rotor was observed [14]. Some studies have revealed that the gas-side mass transfer coefficient (kG) is similar to that in a conventional packed column because of the lack of tangential velocity of the gas as it passes through the rotor [15,16]. The increase in the effective gas–liquid interfacial area is responsible for most of the increase in KGa that is caused by the centrifugal force in a gas-side resistance-controlled process. Recently, several novel designs that improve the hydraulic performance and mass transfer efficiency of RPBs were proposed. These designs have focused on limiting the synchronous rotation of fluids and enhancing the velocity of the fluids relative to the packing. Chandra et al. [17] developed an RPB with split packing and found that it exhibited enhanced tangential slip velocity. Reddy et al. [18] reported that KGa values in an RPB with split packing were two orders of magnitude higher than those in a conventional packed column. Wang et al. [19] presented a rotating zigzag bed for an ethanol–water distillation system and showed that the volume of the equipment could be made an order of magnitude smaller than that of a conventional packed column. Luo et al. [20] studied the mass transfer in an RPB with blades inside the rotor; their results revealed that the rotor with blades considerably increased the effective interfacial area and kL. Our earlier work developed a rotating blade bed with static baffles, whose pressure drop and overall mass transfer coefficient were examined using an isopropyl alcohol absorption process
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Nomenclature a C CO2 C CO2 C L;i C L;o C NaOH C Na2 CO3 DCO2 DCO2 ;w E H HCO2 Ha I k1 k2 1 k2 kG kL
gas–liquid interfacial area (1/m) concentration of carbon dioxide in liquid stream (mol/L) concentration of carbon dioxide at the gas–liquid interface (mol/L) concentration of solute in the inlet liquid stream (mol/L) concentration of solute in the outlet liquid stream (mol/L) concentration of sodium hydroxide (mol/L) concentration of sodium carbonate (mol/L) diffusion coefficient of carbon dioxide in liquid (m2/s) diffusion coefficient of carbon dioxide in water (m2/s) enhancement factor of the gas–liquid flux due to chemical reaction (–) Henry’s constant [(mol/L)/(mol/L)] Henry’s constant [(mol/L)/atm] Hatta number defined in Eq. (4) (–) ionic strength of solution (mol/L) reaction rate constant (L/s) reaction rate constant (L2/mol s) reaction rate constant in infinite dilution (L2/mol s) gas-side mass transfer coefficient (m/s) liquid-side mass transfer coefficient (m/s)
[21]. Experimental results demonstrated that adding the static baffles in the rotating blade bed reduced the pressure drop by 53% and improved KGa by 117%. These results indicated that installing static baffles inside the rotating blade bed effectively disturbed the rotation of the gas, reducing the centrifugal pressure drop and increasing the tangential slip velocity of the gas flow. Additionally, adding the baffles made the liquid flow behavior in the device very different from that in a conventional RPB because the liquid is repeatedly accelerated between the blades and the baffles. Consequently, the liquid-side mass transfer coefficients (kLa and kL) and the effective interfacial area of the proposed device were investigated in present study. Many works have presented experimental data and empirical equations for the volumetric liquid-side mass transfer coefficient in an RPB [22]. Oxygen–water stripping or absorption processes are typically conducted to obtain kLa because the gas-side mass transfer resistance is negligible in these processes. Moreover, several investigations have demonstrated the measurement of the effective gas–liquid interfacial area in an RPB using a CO2–NaOH absorption process. Munjal et al. [23] obtained the effective interfacial area of glass beads and commercial high-porosity packing in an RPB. Their experimental results showed that the interfacial area was proportional to the rotational speed to a power of 0.42 for glass beads and 0.28 for the high-porosity packing, respectively. Yang et al. [24] measured the effective interfacial area in an RPB with rotors of different radii. Higher interfacial area was observed with rotors with smaller outer radii, revealing a significant end effect in the RPB. Rajan et al. [25] measured the interfacial area in a split-packing RPB and found that counter-rotation of the packing yielded a higher interfacial area than co-rotation. Luo et al. [26] evaluated the interfacial area in RPBs that were packed with various types of wire mesh. They proposed that the effective interfacial area was proportional to the centrifugal acceleration raised to the power of 0.12. Guo et al. [27] investigated the effect of the shell zone on the interfacial area in an RPB; their results revealed that the interfacial area in the shell zone can reach 30% of that in the whole RPB. In our previous work, the rotating bed with baffles showed a high values of KGa. However, the fundamental characteristics of mass transfer in this device are not clear and a systematic
KGa kL a m n NCO2 PCO2 p q QG QL r CO2 ri ro S T z
overall volumetric gas-side mass transfer coefficient (1/s) volumetric liquid-side mass transfer coefficient (1/s) exponent in Eq. (15) (–) exponent in Eq. (15) (–) CO2 gas–liquid flux (mol/m2 s) partial pressure of CO2 in the gas phase (atm) exponent in Eq. (16) (–) exponent in Eq. (16) (–) gas flow rate (m3/s) liquid flow rate (m3/s) rate of reaction (mol/s) inner radius of the packed bed (m) outer radius of the packed bed (m) G stripping factor defined as S ¼ HQ Q L (–) temperature (K) axial height of the packing (m)
Greek letters e porosity of the packing (–) x rotational speed (rpm)
study on the individual mass transfer coefficients and the effective interfacial area is required. Therefore, this study further investigated the liquid-side mass transfer coefficients and the effective interfacial area to understand the mechanism of mass transfer of the proposed device. An oxygen-stripping process and a CO2 absorption process were utilized to determine kLa and the effective gas– liquid interfacial area, respectively, in a rotating bed with baffles. The kL values were also calculated. The effects of rotational speed, liquid flow rate, baffles and type of packing on these mass transfer parameters were examined. 2. Experimental Fig. 1 displays the structure of a rotating blade bed with baffles. The device herein comprised mainly of a rotating disk and a static disk. Three sets of blades, each has eight, 16 and 16 blades, were installed in the annular packing regions of the rotating disk, shown as Fig. 1(b). Each blade, which was covered by a layer of stainless steel mesh, had a radial width of 1.2 cm and an axial height of 1.8 cm. The inner and outer radii of the rotor were 1.8 and 7.8 cm, respectively, and the specific surface area and porosity were 163 1/m and 0.99, respectively. Between packing regions, two sets of baffles were fixed on the static disk, shown as Fig. 1(c). The baffle was made of a stainless steel sheet which was not perforated. It had a radial width and an axial height of 0.6 and 1.5 cm, respectively and was designed to be removable to enable the effect on mass transfer to be determined. The structure and number of the blades and baffles were discussed in our previous work [21]. The wire-mesh packing was also investigated by replacing the blades with stainless steel wire mesh in the annular packing regions. The specific surface area and porosity of the wire-mesh packing were 397 1/m and 0.98, respectively. The bed was operated from 600 to 1800 rpm, which provided a centrifugal force of 19 to 174-fold gravitational force. Consequently, the effect of gravity on the flow of fluids in the rotor can be neglected. The liquid entered the bed from a liquid distributor at its center; was sprayed toward the inner edge of the packing region, and was moved outward by the centrifugal force. Some of the liquid was captured by the static baffles and it would flow down along
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141
Gas inlet
Blade Rotational disk
Gas outlet
Seal
(b)
Liquid inlet
Baffle
Liquid outlet
(a)
(c)
Fig. 1. Schematic diagram of the device used in this study: (a) structure of the RPB with blade packings and baffles, (b) blades on the rotational disk and (c) baffles on the stationary disk. [21].
Rotameter Gas Out Nitrogen Blade Motor
packing. The liquid then splashed onto the static housing and exited at the bottom. The gas was introduced from the static housing, flowed inward through the packing and was expelled from the pipeline at the center of the bed. In the rotor, the synchronous rotation of gas is retarded by the baffles, which enhances the tangential velocity. 2.1. Measurement of kLa
Baffle Rotameter
Water
Water bath
DO Probe
Liquid Out
Fig. 2. Diagram of the experimental setup for stripping of oxygen.
the baffle surface and fall to the adjacent rotating packing region. Then the liquid can be evenly distributed into the packing due to the high relative velocity between the liquid droplet and rotating
The volumetric local liquid-side mass transfer coefficient was determined by a deoxygenation process because the solubility of oxygen in water was very low and the mass transfer resistance in the gas phase was negligible. Fig. 2 presents the experimental setup. Water controlled by a water bath at 30 °C and nitrogen at room temperature (27–28 °C) contacted in the RPB. Experimental results showed that the temperature difference between the inlet and outlet water streams was less than 1 °C. The dissolved oxygen (DO) concentrations in the inlet and outlet water streams were measured using a DO probe (DO-5509, Lutron electronic). The kLa values were calculated using the following equation.
ln QL kL a ¼ 2 2 pðro ri Þz
h C L;i 1i 1 1S C L;o þS 1 1S
ð1Þ
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Gas Out Rotameter CO2 analyzer CO2+air Blade Motor Baffle Rotameter
Liquid sampling point NaOH solution
Liquid Out
Fig. 3. Diagram of the experimental setup for absorption of CO2.
2.2. Measurement of interfacial area The effective gas–liquid interfacial area was measured by the chemical method in a CO2–NaOH system [23–28]. Fig. 3 shows the experimental setup. A CO2–N2 mixed gas stream with a CO2 concentration of 10 vol.% was brought into contact with 1 M NaOH solution at 30 °C. The CO2 concentrations in the inlet and outlet gas streams were measured using an infrared CO2 analyzer (Polytron IR CO2, Drager). The pH value and the concentration of the carbonate ions of the outlet liquid stream were determined by potentiometric titration. The rate of reaction can be given by a second-order rate equation.
r CO2 ¼ k2 C CO2 C NaOH
ð2Þ
The reaction can be treated as a pseudo-first-order reaction when the following inequality is satisfied.
Ha <
1 C NaOH 1þ 2 C CO2
! ð3Þ
where Ha is the Hatta number, which is given by
Ha ¼
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DCO2 k2 C NaOH
The rate constant of the pseudo-first-order reaction is expressed as,
k1 ¼ k2 C NaOH
ð5Þ
The absorption rate of CO2 (N CO2 ) can be expressed as,
NCO2 ¼
EkL aC CO2
p
ðr2o
r 2i Þz
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ Ha2
ð6Þ
ð7Þ
If the reaction is fast (Ha > 3), then the enhancement factor can be expressed as,
E ¼ Ha
ð8Þ
The Ha values were from 8 to 9.5 in this study. Substituting Eqs. (4), (5) and (8) into Eq. (6), yields the absorption rate as,
NCO2 ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DCO2 k1 aC CO2 pðr 2o r 2i Þz
DCO2 ¼ 1 ð0:129C NaOH þ 0:261C Na2 CO3 Þ DCO2 ;w
ð10Þ
2119 DCO2 ;w ¼ 2:35 106 exp T
ð11Þ
The values of k2 can be expressed as follows [31].
log
k2 1 k2
¼ 0:1987I 0:2012I2
1
log k2 ¼ 11:895
2:382 T
ð12Þ
ð13Þ
C CO2 ¼ HCO2 PCO2
ð14Þ
The effects of ionic strength and temperature on Henry’s law constant, HCO2 , were taken from Danckwerts [28] and Versteeg et al. [30]. 3. Results and discussion
where E is the enhancement factor of the absorption rate enhanced by the chemical reaction and is calculated using the following equation [28].
E¼
In Eq. (9), the absorption rate can be calculated from the CO2 concentrations of the inlet and outlet gas streams. The values of DCO2 can be calculated as follows [29,30].
Furthermore, since the CO2 absorption process is controlled by the liquid film, C CO2 can be estimated by Henry’s law as follows.
ð4Þ
2
kL
Fig. 4. Dependence of kL a on rotation speed with blade packing.
ð9Þ
3.1. Volumetric liquid-side mass transfer coefficient The concentration of dissolved oxygen in the outlet liquid stream and the concentration of carbon dioxide in the outlet gas stream were monitored online by a DO probe and CO2 analyzer, respectively. All the experiments were performed for up to 10 min, when the system reached a steady state. The error bounds on the experimental data were estimated within ±10%. Figs. 4 and 5 display the kL a values in a rotating bed with and without the static baffles. Experimental results clearly revealed that kL a increased with rotational speed and liquid flow rate. A 35–73% increase of kL a was observed as the rotational speed increased from 600 to 1800 rpm. This is mainly because the thickness of the boundary layer and the size of the liquid droplets were reduced, which increases the kL and a as rotational speed and liquid flow rate
C.-Y. Tsai, Y.-S. Chen / Separation and Purification Technology 144 (2015) 139–145
Fig. 5. Dependence of kL a on rotation speed with wire-mesh packing.
143
Fig. 6. Dependence of a on rotation speed with blade packing.
increased. The dependence of kL a on the rotational speed and liquid flow rate is expressed as follows.
kL a / xm Q nL
ð15Þ
Table 1 presents the exponents m and n that were obtained in this work and those of other RPBs in the literature [22,32]. The m value of the proposed device with blade packing was from 0.38 to 0.43 and that of the device with wire-mesh packing was from 0.29 to 0.32. The n values herein varied from 0.99 to 1.15. The m values were lower than the exponents in the literature, and the n values were slightly higher. Additionally, in Figs. 4 and 5, adding the baffles into the bed did not significantly affect the kL a values, suggesting that baffles in a bed are more suitable for use in a gas-side mass transfer resistance-controlled system than in a liquid-side-controlled system. The types of packing had a stronger effect on kL a: the kL a of a wire-mesh bed exceeded that of a blade bed, especially at higher liquid flow rates. 3.2. Effective interfacial area Figs. 6 and 7 present the effective gas–liquid interfacial area as measured by a CO2–NaOH absorption process. Experimental results showed that the effective interfacial area increased with the rotational speed, mainly because the thickness of the liquid films and the size of the liquid droplets were reduced in a centrifugal field, increasing the gas–liquid interfacial area. Moreover, a higher effective interfacial area was obtained when the static baffles were added into the beds. Adding the static baffles in a blade bed increased the interfacial area by 16–34%; adding them in a
Table 1 The m and n values (Eq. (15)) in literature and in this work. Literature
Packing type
m
n
Chen et al. [22]
Random packings Wire-mesh packing
0.6
0.77
Lin and Jian [32]
Blade packing
0.55
0.91
This work
Blade packing (with baffles) Blade packing (without baffles) Wire-mesh packing (with baffles) Wire-mesh packing (without baffles)
0.43 0.38 0.32 0.29
1.08 0.99 1.06 1.15
Fig. 7. Dependence of a on rotation speed with wire-mesh packing.
wire-mesh bed increased by 2–16%. In a rotating bed without baffles, vigorous collision of the liquid occurred only in the region close to the inner radius of the bed owing to the high relative velocity between the liquid and packing. After it leaves this region, the liquid moves synchronously with the packing and the tangential velocity of the liquid relative to the packing declines. Hence, the liquid cannot easily be distributed laterally in the bed, preventing parts of the packing from being wetted by the liquid and decreasing the interfacial area. In contrast, in the bed with static baffles, the baffles captured the liquid, which was redistributed into the rotating sections repeatedly as it passed through the bed. Vigorous impingements of liquid on the packing and baffles occurred, generating fine liquid droplets, which reduce the degree of maldistribution of the liquid and, therefore, increasing the interfacial area. According to Figs. 6 and 7, the wire-mesh packing provided a higher effective interfacial area than did the blades, probably because the wire-mesh packing had a higher surface area.
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Table 2 The p and q values (Eq. (16)) in literature and in this work. Literature
Types of packing
e
p
q
Munjal et al. [23]
Glass beads High-porosity packing
0.43 0.95
0.42 0.28
0.26–0.30
Luo et al. [26]
Wire-mesh packing
0.96
0.24
0.77
This work
Blade packing (with baffles) Blade packing (without baffles) Wire-mesh packing (with baffles) Wire-mesh packing (without baffles)
0.99
0.22 0.20 0.33
0.34 0.36 0.23
0.33
0.26
0.98
The dependence of the interfacial area on the rotational speed and the liquid flow rate can be expressed as follows.
a / xp Q qL
ð16Þ
Table 2 presents the exponents p and q that were obtained in this work and those in the literature [23,26]. The p values, which varied over a wide range of 0.12–0.42, were governed primarily by the porosity of the packing: more porous packing had a smaller p value, as displayed in Fig. 8. The results confirmed the finding of Munjal et al. [23] that the centrifugal force has a weaker effect on more porous packing because less momentum is imparted to the liquid. The relation between p and packing porosity can be expressed empirically as,
p ¼ 0:453ð1 eÞ0:142
Fig. 8. Comparison of experimental values of p with results calculated using Eq. (17).
ð17Þ
As shown in Fig. 8, the exponent p can be reasonably predicted by Eq. (17). The q values obtained in this work were similar to those of Munjal et al. [23] but were smaller than those of Luo et al. [26] Though the data available in literature were included, more experimental data are required to further ensure the validity of Eq. (17). 3.3. Liquid-side mass transfer coefficient According to the empirical equation provided by Onda et al. [33], the variation of the effective interfacial area between two different solutions mainly depends on the physical properties such as density, viscosity and surface tension. Since these properties of a 1 M NaOH solution and water are similar, it is reasonably suggested that the interfacial area of these two aqueous phases do not change significantly [34,35]. Additionally, the liquid flow in the bed was suggested not influenced by the gas flow because the system was far from flooding. Therefore, the liquid-side mass transfer coefficient (kL ) can be calculated from the experimental results of deoxygenation and CO2 absorption using the following equation.
kL ¼
kL a a
ð18Þ
Figs. 9 and 10 plot the effect of rotational speed on kL of the two types of packing with and without static baffles. As expected, kL clearly increased with the liquid flow rate to the power of 0.63– 0.89. However, the dependence of kL on rotational speed differed between a blade bed and wire-mesh bed. In a blade bed, the value of kL increased with rotational speed to the power of 0.18–0.21, but in a wire-mesh bed, it was almost independent of rotational speed. Small liquid fragments in a high shear flow environment are commonly held to reduce mass transfer resistance. However, the velocity of the film flow on the packing surface probably decreased because of the increase in the interfacial area with rotational speed, leading to a reduction in kL . Consequently, kL was less
Fig. 9. Dependence of kL on rotation speed with blade packing.
enhanced by the centrifugal force in a wire-mesh bed than in a blade bed because more of the liquid traveled as a thin film over a wire-mesh bed. Figs. 9 and 10 also present the effect of static baffles on kL . The bed with static baffles had lower kL values. Adding static baffles increased the relative velocity and rate of collision of the liquid, increasing the mass transfer coefficient. However, the static baffles reduced the average radial velocity of the liquid that flowed through the bed, probably reducing kL . These results reveal that although adding static baffles in a rotating bed increased the effective interfacial area, doing so reduced kL , resulting in similar values of kL a in beds with and without baffles. This finding suggests that the use of static baffles in an RPB is more favorable for a gas-side mass transfer resistance-controlled process than for a liquid-sidecontrolled process.
C.-Y. Tsai, Y.-S. Chen / Separation and Purification Technology 144 (2015) 139–145
Fig. 10. Dependence of kL on rotation speed with wire-mesh packing.
4. Conclusions The volumetric liquid-side mass transfer coefficients and effective gas–liquid interfacial area were investigated by performing experiments that involved the deoxygenation and chemisorption of CO2 in aqueous NaOH in a rotating blade bed and a wire-mesh bed that were equipped with static baffles. The local liquid-side mass transfer coefficient was calculated by dividing kL a by the interfacial area. Experimental results demonstrated that kL a and the effective interfacial area increased with rotational speed and liquid flow rate. The centrifugal force had a smaller effect on interfacial area of more highly-porous packing because less momentum was imparted to the liquid. The value of kL increased with rotational speed in a blade bed but was almost independent of rotational speed in a wire-mesh bed. Adding static baffles in the rotating bed increased the interfacial area by 16–34% by the generation of vigorous flow between the rotor and the baffles. However, lower kL values were observed in the bed with baffles because the static baffles reduced the average radial velocity of the liquid. The effective interfacial area and kL a were lower in the blade bed than in the wire-mesh bed. These results suggest that packing with a higher specific surface area is more suitable for a liquid-side mass transfer resistance-controlled system, while the use of baffles is favorable in a gas-side-controlled system. Acknowledgment The support of the ROC Ministry of Science and Technology (NSC 102-2221-E-033-043; MOST 103-3113-E-007-002) is greatly appreciated. References [1] C.C. Lin, B.C. Chen, Y.S. Chen, S.K. Hsu, Feasibility of a cross-flow rotating packed bed in removing carbon dioxide from gaseous streams, Sep. Purif. Technol. 62 (2008) 507–512. [2] C.Y. Chiang, Y.Y. Liu, Y.S. Chen, H.S. Liu, Absorption of hydrophobic volatile organic compounds by a rotating packed bed, Ind. Eng. Chem. Res. 51 (2012) 9441–9445. [3] L.L. Zhang, J.X. Wang, Q. Sun, X.F. Zeng, J.F. Chen, Removal of nitric oxide in rotating packed bed by ferrous chelate solution, Chem. Eng. J. 181–182 (2012) 624–629.
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