ARTICLE pubs.acs.org/IECR
Absorption of Carbon Dioxide with Ionic Liquid in a Rotating Packed Bed Contactor: Mass Transfer Study Liang-Liang Zhang,† Jie-Xin Wang,*,† Yang Xiang,‡ Xiao-Fei Zeng,*,‡ and Jian-Feng Chen†,‡ †
Key Lab for Nanomaterials, Ministry of Education, and ‡Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing, 100029, People's Republic of China ABSTRACT: Using ionic liquids for CO2 capture is of great interest due to their unique characteristics. However, low gasliquid mass transfer rates in conventional gasliquid contactors due to the high viscosities of ionic liquids lead to the significant limitation for large-scale application of CO2 capture using ionic liquids. Therefore, there is an urgent demand to develop a novel gasliquid contactor for the intensification of the mass transfer efficiency in such a system. In this article, CO2 absorption with an ionic liquid in a rotating packed bed (RPB) gasliquid contactor is first reported. It was found that the RPB markedly enhanced the physical absorption of CO2 in the ionic liquid in a very short contact time, within seconds. Only one cycle gasliquid contact in the RPB could make the saturation ratio of CO2 in the ionic liquid reach 60%. The effects of various operation conditions on the liquid side volumetric mass transfer coefficient (kLR) were elucidated. Increasing the rotating speed from 1100 to 3100 rpm doubled the kLR. The increase of the liquid flow rate also benefited the enhancement of the kLR. The experimentally measured kLR in the RPB is at least 1 order of magnitude higher than that in a conventional packed tower. A model based on penetration theory was proposed to describe the mechanism of gasliquid mass transfer in the RPB. The predicted kLR was in good agreement with the experimental data with a deviation of less than 15%. The RPB shows great potential for the industrial application of CO2 capture using ionic liquids.
’ INTRODUCTION Carbon dioxide (CO2) is the most crucial greenhouse gas. The increase of anthropogenic carbon dioxide concentration in the atmosphere over the past century is known to be part of the cause of global warming. There is little doubt that petroleum, natural gas, and coal will continue to be the primary global fuel and chemical feedstock sources for the next few years. Thus carbon dioxide emissions from fossil fuel combustion are continuing to be a major contributor to global climate change.1,2 The UN Intergovernmental Panel on Climate Change (IPCC), Nobel Peace Prize laureate for 2007, has already established that CO2 capture and storage (CCS) “would be an option in the portfolio of actions for stabilization of greenhouse gas concentrations while allowing for the continued use of fossil fuels”.3 So far, a number of CO2 capture technologies are already being practiced on a laboratory scale or have been industrially demonstrated that require various processes involving physisorption/chemisorption,46 membrane separation7,8 or molecular sieves,9 carbamation,10 amine physical absorption,11 amine dry scrubbing,12 mineral carbonation,1315 etc. Mostly amine-based technologies, using for example monoethanolamine, diethanolamine, and methyldiethanolamine, are being used for CO2 capture through carbamate/carbonate formation. Although such classical capture technologies have been running in industry for a period of time, some exposed shortcomings have never been overcome. Recurrent drawbacks being attributed to all these systems, along with insufficient carbon dioxide capture capacity, are degradation of costly reagents and thermal stability, equipment corrosion, and high energy consumption during regeneration owing to the evaporation of the solvent.16 The regeneration step may increase the total operating costs of the capture plant up to 70%.3 All these r 2011 American Chemical Society
discrepancies have to be overcome and addressed on a wider scale for more efficiency but less cost. Therefore, a nonvolatile solvent that could facilitate CO2 capture without the loss of solvent into the gas stream has been sought. The recent concept of using ionic liquids for CO2 capture is gaining interest due to their unique characteristics, i.e., wide liquid ranges, thermal stabilities, negligible vapor pressures up to their thermal decomposition points, tunable physicochemical characters, and high CO2 solubilities.1726 However, ionic liquids commonly are high or superhigh viscosity liquids with poor fluidities. A significant limitation for large-scale application of a continuous CO2 capture process by ionic liquid is the great resistance of mass transfer and low gasliquid mass transfer rate in conventional gasliquid contactors due to the high viscosity. Therefore, it is significant to develop a gasliquid contactor with high mass transfer efficiency for such a high viscosity system. To intensify the mass transfer process, huge volume and investment of equipment are needed to reduce the thin film and to provide a large mass transfer area. As compared to the above equipment, the rotating packed bed (RPB) as a novel and highly efficient contactor has been utilized to intensify the mass transfer2729 and successfully applied to absorption,5 distillation, polymer devolatilization,32 reactive crystallization,30,31,33,34 etc. The basic principle of RPB is to create a high-gravity environment via the action of centrifugal force such as the so-called “Higee”. The fluids going through the packing are spread out or Received: December 30, 2010 Accepted: April 14, 2011 Revised: April 1, 2011 Published: April 14, 2011 6957
dx.doi.org/10.1021/ie1025979 | Ind. Eng. Chem. Res. 2011, 50, 6957–6964
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. Temperature-Dependent Physical Properties of [bmim][PF6] T, K
μ, Pa 3 s
D, 1011 m2/s
σ, N/m
F, kg/m3
293
0.3682
4.575
0.049 57
1371.7
298 305
0.2609 0.1686
5.310 6.726
0.048 51 0.047 03
1367.4 1361.5
315
0.1021
0.044 90
1353.4
326
0.0710
12.64
0.042 57
1344.3
336
0.0591
14.95
0.040 44
1336.1
9.438
split into very fine droplets, threads, and thin films by the strong shear force, resulting in a significant intensification of micromixing and mass transfer between the fluid elements and gas.35,36 According to previous studies, the RPB has a higher gasliquid mass transfer efficiency which is evidenced by the fact that the volumetric mass transfer coefficient in an RPB is much higher than that in a conventional packed bed. Consequently, the size and the capital of the processing system would be extremely reduced. Handling of high viscosity liquid in an RPB was previously reported,32,37 such as polymerization of butyl rubber and polymer devolatilization. High-quality production was obtained in an RPB due to the significant intensification of micromixing. This demonstrates that an RPB could be used as an ideal contactor to intensify the mass transfer in high viscosity liquid. To the best of our knowledge, CO2 absorption with ionic liquid in the RPB contactor is reported for the first time in this article. The saturation ratio of CO2 in an ionic liquid was surveyed after a gasliquid contact within seconds in the RPB. The effects of the operating variables were investigated on the liquid side volumetric mass transfer coefficient (kLR). Dimensionless numbers were used to correlate kLR in the RPB contactor over wide ranges of operating conditions. It was found that kLR in the RPB was 1 order of magnitude higher than that in the packed tower under similar experimental conditions. Further, a model based on penetration theory was established to describe the mechanism of gasliquid mass transfer of ionic liquid system in the RPB and this model was validated by the experimental results.
’ EXPERIMENTAL SECTION Experimental Apparatus and Materials. The ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) was purchased from the Center for Green Chemistry and Catalysis of Chinese Academy of Science. In our experiments, it was first degassed and dried under vacuum for 24 h. The ionic liquid was collected after experiment and reconditioned (i.e., degassed and dried under vacuum) for further use. The ionic liquid was repeatedly recovered for CO2 absorption with no observed loss of efficiency. Its physical properties dependent on temperature are presented in Table 1. The viscosity was measured by a viscometer (Haake-rs-150). This ionic liquid was proved to be a Newtonian fluid by the data under different shear rates. The diffusivity of CO2 in the ionic liquid was computed from the experimental diffusion coefficient reported by Shiflett and Yokozeki.22 The surface tension of the solution was evaluated from the experimental results of Martino.38 The density value was from the data reported by Harris and Woolf.39 N2 with a purity of 99.99% and the mixture gas with a CO2 molar fraction of 10.0% and N2 molar fraction of 89.9% were purchased from Beijing Ruyuanruquan Technology Co., Ltd., China.
Figure 1. Scheme of experimental setup. (1) Gas cylinder. (2), (6) Gas flowmeter. (3), (7) IR gas analyzer. (4) Gas inlet. (5) Gas outlet. (8) Liquid storage. (9) Pump. (10) Liquid flowmeter. (11) Liquid inlet. (12) Liquid outlet.
The experimental setup for CO2 absorption is schematically shown in Figure 1. The RPB contactor mainly consisted of a packed rotator, a fixed casing, a liquid inlet, and a gas inlet. The inner and outer diameters of the rotator were 20 and 60 mm, respectively, and the axial length of the rotator was 20 mm. The bed was packed with stainless wire mesh, whose porosity and surface area were 0.90 and 850 m2/m3, respectively. The packing consisted of 11 layers. The rotator was installed inside the fixed casing and rotated at an adjustable speed. The inner diameter of the gas pipeline was 12.0 mm. The inner diameter of the liquid pipeline between the liquid storage and the liquid inlet was 0.5 mm. The inner diameter of the liquid pipeline linked with the liquid outlet was 0.9 mm. All the pipelines and equipment were swept by nitrogen prior to use in order to remove moisture and air. For a comparison, similar absorption experiments were also performed in a conventional packed tower. The packed tower had a 25 mm inner diameter and a 200 mm packing height. The packing was a 3 3 mm mesh ring (theta shape), which was made of stainless wire and had a surface area of 2000 m2/m3. Process of Absorption Experiments. The CO2 absorption experiment was performed as followed. Before the gas and liquid entered the RPB, the packing was preheated to the same temperature by the water in the casing. The mixture gas stream from the gas cylinder as the continuous phase flowed inward from the outer edge of the RPB by a pressure-driving force, while the ionic liquid as the dispersed phase was sprayed into the packing through three holes (the diameter was 0.5 mm) of the liquid distributor in the center of RPB. The ionic liquid moved outward and left from the outer edge of the RPB through a centrifugal force. Gas and liquid streams had a countercurrent contact in the RPB. Consequently, CO2 in the gas stream was dissolved in the liquid stream before the gas and liquid streams left the RPB from the gas outlet and the liquid outlet, respectively. The CO2 concentrations in the inlet and outlet gas streams were measured by two infrared gas analyzers (GXH-3010F, Huayun Analytical Instrument Institution, China, CO2 molar fraction ranging from 0 to 10%). The gastightness was carefully checked to ensure no leakage of the gas before each experiment. In our study, the pressure of inlet gas was maintained at 0.10 MPa. The pressure drop from the inlet to the outlet was always lower than 2 kPa, which could be ignored. In order to maintain a higher gravity level, the rotating speed varied from 1130 to 3164 rpm, providing a centrifugal acceleration from 297 to 2327 m s2 based on the quadratic mean radii of the RPB. All 6958
dx.doi.org/10.1021/ie1025979 |Ind. Eng. Chem. Res. 2011, 50, 6957–6964
Industrial & Engineering Chemistry Research
ARTICLE
The CO2 removal efficiency was assessed by the overall volumetric mass transfer coefficient in this study. For the physical absorption of CO2 in ionic liquid, it contained the liquid side and gas side mass transfer coefficients. KL R ¼
1 1 1 þ kG RH kL R
ð1Þ
In view of the high Henry coefficient and no accurate values of kGR available now, this absorption process in the RPB was based on the assumption that the gas-phase resistance was negligible. The major mass transfer resistance lay in the liquid film. It was a liquid film controlled process. KL R ¼ kL R
ð2Þ
Figure 2. Effect of absorbing cycle on accumulative saturation and molar ratio of CO2 in ionic liquid.
On the basis of the above assumption, mass balances could be employed for CO2 and N2.
the data were obtained until a steady-state operation was maintained for 2 min.
Gin Yin Gout Yout LF ¼ þ ðXout Xin Þ M Vm Vm
ð3Þ
GN2 ¼ Gin ð1 Yin Þ ¼ Gout ð1 Yout Þ
ð4Þ
’ RESULTS AND DISCUSSION Absorption of CO2 in Ionic Liquid. It is hard for the saturation ratio of CO2 in an ionic liquid to reach 100% after one gasliquid mass transfer process, so the collected ionic liquid from the liquid outlet was pumped into the liquid inlet for the next gasliquid contact process. One gasliquid mass transfer process in the RPB was called a cycle. Figure 2 exhibits the evolution of saturation ratio and molar ratio of CO2 in the ionic liquid with increasing cycle number under different gas flow rates. It could be clearly seen that the saturation ratio reached 60% (CO2 molar ratio 0.13%) just through one cycle. It took about 2.1 s to complete one cycle of gasliquid contact by our calculation, which was from the known thickness of the packing and the average liquid flow rate in the packing of RPB obtained from eqs 13 and 14. With the increase of cycle number, the increase of absorption gradually became small. This was mainly ascribed to the fact that the mass transfer driving force decreases with the enhanced CO2 concentration in the liquid phase. After five or six cycles (about 1012 s), the accumulative absorption reached up to 96% saturation ratio (CO2 molar ratio 0.21%). Liquid Volumetric Mass Transfer Coefficient in RPB. Before the mass balance equations were derived, the following assumptions were employed in dealing with the absorption process: 1. Steady-state condition prevailed. 2. One-dimensional diffusion was assumed: The concentration of every component varied only in the radial direction of the packing because liquid and gas had little circumferential motion. Liquid motion in the RPB hardly had back mixing, so plug-flow condition was applicable to both gas and liquid phases. 3. The pressure drop in the RPB was ignored and the nitrogen was not absorbed by the ionic liquid. Owing to the extremely low solubility of nitrogen in the ionic liquid, neglecting the loss of nitrogen can simplify the calculation of absorption of CO2 in the ionic liquid. 4. Dilute solution was assumed: Henry’s law was appropriate for this absorption process. By fitting the solubility data in the literature,18,22,23 this assumption was well verified at low temperature and pressure.
If, further, Xin = 0: Xout
GN2 MIL Yin Yout ¼ LFVm 1 Yin 1 Yout
ð5Þ
Then the equilibrium concentrations of CO2 in the ionic liquid inlet and outlet could be calculated by Henry’s law, respectively: Xe, in ¼
PYin H
ð6Þ
PYout ð7Þ H By building a differential mass balance equation for dissolved CO2 in the bulk liquid over an elementary volume and assuming that kLR was constant, kLR could be regarded as the overall volumetric mass transfer coefficient of the physical absorption process based on the liquid film controlled process assumption. Xe, out ¼
LF dX ¼ kL RCm ðXe XÞhð2πÞr dr MIL
ð8Þ
By integrating eq 8, we could derive the equation including kLR in the packing region as Z Z R LF Xout dX ¼ kL RCm ð2πÞh r dr ð9Þ MIL Xin Xe X Ri Since Henry’s law was appropriate for this absorption process, the equilibrium line was a straight line. Integration could be used instead of the logarithmic mean method. The experimental kLR in the RPB could be calculated as kL R ¼
LF NTU MIL Cm πhðR 2 Ri 2 Þ
Xe, out Xout Xe, in Xin NTU ¼ ðXe, out Xout Þ ðXe, in Xin Þ ðXout Xin Þ ln
ð10Þ
ð11Þ
where NTU is the number of transfer units. 6959
dx.doi.org/10.1021/ie1025979 |Ind. Eng. Chem. Res. 2011, 50, 6957–6964
Industrial & Engineering Chemistry Research
Figure 3. Effect of rotating speed on liquid side volumetric mass transfer coefficient in RPB.
Figure 4. Effect of liquid flow rate on liquid side volumetric mass transfer coefficient in RPB.
To evaluate the CO2 absorption performance in the RPB, the kLR values were measured at different operating variables including the rotating speed, gas flow rate, liquid flow rate, and temperature. Reproducibility tests at almost all of the operating conditions were performed in the study. The data on the CO2 outlet concentration and the calculated kLR were observed to be repeatable with a deviation of less than 5%. Figure 3 shows the effect of rotating speed on the measured kLR. It could be seen that kLR obviously increased with an increase in the rotating speed ranging from 1130 to 2150 rpm. This was mainly attributed to the reduced liquid side mass transfer resistances from the smaller liquid droplet, the thinner liquid film, and diffusion depth. However, when the rotating speed was further increased, only a small effect on kLR was observed. A possible reason was that the reduction extent in mass transfer resistance at higher rotating speed was compensated by the decreased gasliquid contact time unfavorable to absorption. Increasing the rotating speed from 1100 to 3100 rpm doubled kLR. Furthermore, it could be seen that the increase of the liquid flow rate contributed more to the enhanced kLR than the gas flow rate by comparing three curves, well proving the liquid film controlled absorption process.
ARTICLE
Figure 5. Effect of gas flow rate on liquid side volumetric mass transfer coefficient in RPB.
Figure 4 gives the effect of liquid flow rate on the measured kLR. As a whole, kLR rose with the increase of the liquid flow rate. However, kLR rapidly increased with the increasing liquid flow rate from 30 to 60 mL/min. When the liquid flow rate exceeded 60 mL/min, kLR slightly increased. Obviously, an increase in the liquid flow rate would be beneficial to the formation of thinning liquid boundary layer and the corresponding decrease of mass transfer resistance, and would simultaneously result in the shortening of the contact time. The former benefited kLR, while the latter had a inverse effect. Moreover, a higher liquid flow rate would provide more absorbent to absorb CO2. The above results indicated that a decrease in the liquid side mass transfer resistance and an increase in the amount of absorbent offered greater effects on the absorption over a reduction in the contact time. Figure 5 plots the effect of gas flow rate on the measured kLR. Clearly, the gas flow rate had only a very small effect on kLR at two fixed liquid flow rates, mainly explained by the increase of gas holdup and turbulence of the system. This is in good agreement with the above results that the major mass transfer resistance lay in the liquid film. Figure 6 presents the effect of temperature on the measured kLR. With increasing temperature from 293 to 336 K, kLR had an obvious increase. This was because the increase of temperature lowered the viscosity of the ionic liquid, thereby leading to the faster flow, as well as the subsequent better gasliquid mixing and mass transfer in the packing. On the other hand, the diffusion coefficient was proportional to temperature and inversely proportional to viscosity. High temperature made the diffusion of CO2 in ionic liquid easier, thereby causing the increase of kLR. A correlation of kLR for CO2 absorption in ionic liquid was achieved in eq 12 from the experimental results shown in Figures 36. kL Rdp ¼ 8:183Sc0:5 Gr 0:37 We0:335 DRt
ð12Þ
In eq 12, dp is the spherical equivalent diameter of the packing, Sc is the Schmidt number, Gr is the Grashof number, and We is the Weber number. Because of the small liquid mass flux and high viscosity, the Reynolds number was very low (lower than 0.032) and had only a small change. Therefore, the correlation did not include the Reynolds number. According to eq 12, it was found 6960
dx.doi.org/10.1021/ie1025979 |Ind. Eng. Chem. Res. 2011, 50, 6957–6964
Industrial & Engineering Chemistry Research
ARTICLE
Figure 6. Effect of temperature on liquid side volumetric mass transfer coefficient in RPB.
Table 2. Comparison of Liquid Side Volumetric Mass Transfer Coefficient Values between Packed Tower and RPB packed tower
RPB
temperature, K
293
293
pressure, atm
1
1
packing volume, cm3 packing surface area, m2/m3
98.17 2000
50.26 850
CO2 concentration, mol %
10
10
gas flow rate, L/min
0.71.2
0.61
liquid flow rate, mL/min
36.573.0
29.2102.2
gasliquid ratio
1133
634
kLR, 102 s1
0.0630.19
0.953.9
that kLR was proportional to the liquid viscosity to the power of 0.24. As compared to the results reported by Delaloye et al. for a packed tower,40 liquid viscosity had less influence on mass transfer in the RPB than in a packed tower. The kLR was proportional to the centrifugal acceleration to the power of 0.37, indicating that the centrifugal force was still revealed as effective in enhancing mass transfer in highly viscous media. Comparison of Liquid Side Volumetric Mass Transfer Coefficient between the RPB and Packed Tower. Table 2 gives a comparison of the liquid side volumetric mass transfer coefficient in the RPB and a packed tower at similar operating conditions. kLR was measured to be (0.953.9) 102 s1 for the RPB and (0.631.9) 103 s1 for packed tower. The former was at least 1 order of magnitude higher than the latter. This completely indicated that the RPB still possessed excellent gasliquid mass transfer performance for high viscosity media. Thus it could be envisioned that the RPB would exhibit a great potential for various industrial applications with ionic liquids, especially for CO2 capture using ionic liquids.
’ MODEL DEVELOPMENT Until now extensive modeling works on an RPB were reported in the literature.8,24 For example, Guo et al.41 developed a model describing three types of mass transfer process in a cross-flow RPB. Chen et al.42 proposed a model for the ozonation process in an RPB to achieve some empirical correlations. The liquid flow
in an RPB had been observed to be pore flow, film flow, and droplet flow based on visual observations reported by Burns and Ramshaw.43 The diameter of the droplet and the thickness of the film had been determined.4345 The viewpoint that the intensification was achieved by a larger gasliquid effective interfacial area being produced under a centrifugal field was widely accepted. The complex movement of liquid in the packing was researched, but the renewal effect of the packing on the liquid was neglected. Hence, a model reflecting the absorption process in an RPB more directly and predicting the mass transfer coefficient exactly should be developed. Before establishing a model for the absorption process, the following assumptions were employed: 1. According to the study of Basic and Dudukovic43,46,47 and Guo,44 it was assumed that the liquid had the same residence time in the packing and the liquid flow pattern in the radial direction was plug flow. 2. The majority liquid phase inside the RPB under high gravity existed as dispersed droplets by the strong shear force and impingement of the packing based on previous visual observations of liquid flow in an RPB.48 The total surface areas of the droplets were regarded as the gas liquid effective interfacial ones. 3. The rotor in the RPB consisted of 11 layers of packing. Liquid droplets were renewed every time it passes through one layer of packing. The correlation obtained by Burns45 was applied to calculate the liquid holdup: !0:5 ω2 r u 0:6 ν 0:22 εL ¼ 0:039 ð13Þ g0 u0 ν0 The relationship between the liquid holdup and the average radial liquid flow rate in RPB could be described as u¼
Lf εL
ð14Þ
By calculating eqs 13 and 14, the liquid holdup and the liquid flow rate were obtained. The rotating packing led to unceasing collision between the liquid and packing which was thought to cause rapid renewal of the liquid film surface. The liquid droplet was renewed when it passed through the packing layer, and the renewal frequency was determined by S¼u
Ns R Ri
ð15Þ
Based on penetration theory, the liquid side mass transfer coefficient kL could be expressed as pffiffiffiffiffiffiffiffiffiffiffiffi kL ¼ DCO2 S ð16Þ Liquid in the RPB existed as liquid droplets, on which the mass transfer took place. The diameter of a droplet could be calculated as 0:5 σ ð17Þ d ¼ 0:7284 2 ω RF The total surface areas of the droplets were regarded as the gasliquid effective interfacial one. It could be calculated as R ¼ εL 6961
6 d
ð18Þ
dx.doi.org/10.1021/ie1025979 |Ind. Eng. Chem. Res. 2011, 50, 6957–6964
Industrial & Engineering Chemistry Research
ARTICLE
’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ86-10-64447274 (J.-X.W.); þ86-10-64446466 (X.-F.Z.). Fax: þ86-10-64423474 (J.-X.W.); þ86-10-64434784 (X.-F.Z.). E-mail:
[email protected] (J.-X.W.);
[email protected]. edu.cn (X.-F.Z.).
’ ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Nos. 20821004 and 20990221), the National “973” Program of China (No. 2009CB219903), and the National “863” Program of China (No. 2009AA033301).
Figure 7. Comparison of predicted and experimental liquid side volumetric mass transfer coefficient values.
The gasliquid mass transfer coefficient could be obtained: kL R ¼
pffiffiffiffiffiffiffiffiffiffiffiffi DCO2 SR
ð19Þ
A comparison analysis was also performed between the experimental data and the predicted data, and the results are given in Figure 7. It could be found that this model offered a relatively precise prediction of kLR, with a deviation of 15% compared to the experimental values. The model indicated that the high kLR in the RPB was mainly achieved by two factors. One was the frequent renewal on the packing surface, which could significantly eliminate the surface effect and promote the gasliquid mass transfer. The other was the high gasliquid interfacial area of 2200 m2/m3 in the RPB, thereby leading to high kLR.
’ CONCLUSIONS In this article, an RPB was first used as a gasliquid contactor for the intensification of CO2 absorption with an ionic liquid. The saturation ratio reached 60% with only one cycle of gas liquid contact in the RPB. Increasing the rotating speed and temperature could dramatically enhance the gasliquid mass transfer of this viscous system. The liquid side volumetric mass transfer coefficient in the RPB obviously increased with the increase of the liquid flow rate, while it slightly increased with the increase of the gas flow rate. A correlation of the kLR for CO2 absorption in the ionic liquid was obtained, and there was less dependence of kLR on liquid viscosity in the RPB than in a packed tower. The liquid side volumetric mass transfer coefficient in the RPB was measured to be (0.953.9) 102 s1, which was an order of magnitude higher than that in a conventional packed tower. A model based on penetration theory was established to describe the mechanism of gasliquid mass transfer in the RPB, indicating that high kLR inside the RPB was mainly achieved by the large gasliquid interfacial area and its frequent renewal on the packing surface. It showed good agreement with the experimental data with a maximum relative error of less than 15%. In summary, the RPB exhibited great potential for various industrial applications with ionic liquids, especially for CO2 capture using ionic liquids.
’ NOTATION X = molar fraction of CO2 in liquid phase Y = molar fraction of CO2 in gas phase Z = saturation ratio of CO2 in liquid phase H = Henry’s law constant Vm = average gas molar volume, L/mol MIL = molar mass of ionic liquid, g/mol Cm = average molar concentration of liquid phase, mol/m3 kL = liquid side mass transfer coefficient, m/s kLR = liquid side volumetric mass transfer coefficient, s1 kGR = gas side volumetric mass transfer coefficient, s1 KLR = overall volumetric mass transfer coefficient, s1 L = liquid flow rate, mL/min Lf = liquid flux, m3/m2 s Ns = number of layers of wire-meshed packing N = rotating speed of RPB, rpm P = total pressure of the gas phase, Pa S = renewal frequency, s1 G = total gas flow rate, L/min GN2 = gas flow rate of nitrogen, L/min u = liquid velocity, m/s u0 = characteristic liquid velocity (=0.01 m/s), m/s g0 = characteristic centrifugal acceleration (=100 m/s2), m/s2 r = radius of packed rotator, m R = radius of outer packed rotator, m Ri = radius of inner packed rotator, m h = axial height of packing, m T = temperature, K d = diameter of liquid droplet, m dp = spherical equivalent diameter of packing (=6(1 ε)/Rt), m ac = centrifugal acceleration, m/s2 Greek Symbols
εL = liquid holdup, m3/m3 R = gasliquid interfacial area, m2/m3 Rt = surface area of packing, m2/m3 ν = kinematic viscosity, m2/s F = density of ionic liquid, kg/m3 ν0 = kinematic viscosity (=106 m2/s), m2/s σ = surface tension of ionic liquid, N/m Subscripts
cal = predicted data from the model exp = experimental data e = equilibrium out = outlet region of RPB in = inlet region of RPB 6962
dx.doi.org/10.1021/ie1025979 |Ind. Eng. Chem. Res. 2011, 50, 6957–6964
Industrial & Engineering Chemistry Research Dimensionless Groups
Gr = Grashof number = dp3acF2/μ2 Sc = Schmidt number = μ/FD We = Weber number = Lf2/FRtσ
’ REFERENCES (1) Haszeldine, R. S. Carbon capture and storage: how green can black be?. Science 2009, 325, 1647–1651. (2) Keith, D. W. Why capture CO2 from the atmosphere. Science 2009, 325, 1654–1655. (3) Metz, B.; Davidson, O.; Coninck, H. D.; Loos, M.; Meyer, L. Special Report on Carbon Dioxide Capture and Storage; Intergovernmental Panel on Climate Change (IPCC): New York, NY, USA, 2005. (4) Mandal, B. P.; Bandyopadhyay, S. S. Simultaneous absorption of carbon dioxide and hydrogen sulfide into aqueous blends of 2-amino2-methyl-1-propanol and diethanolamine. Chem. Eng. Sci. 2005, 60, 6438–6451. (5) Yi, F.; Zou, H. K.; Chu, G. W.; Shao, L.; Chen, J. F. Modeling and experimental studies on absorption of CO2 by Benfield solution in rotating packed bed. Chem. Eng. J. 2009, 145, 377–384. (6) Boucif, N.; Favre, E.; Roizard, D. CO2 capture in HFMM contactor with typical amine solutions: a numerical analysis. Chem. Eng. Sci. 2008, 63, 5375–5385. (7) Ebner, A. D.; Ritter, J. A. State-of-the-art adsorption and membrane separation processes for carbon dioxide production from carbon dioxide emitting industries. Sci. Technol. 2009, 44, 1273–1421. gueda, V. I.; Sanz, A. (8) Delgado, J. A.; Uguina, M. A.; Sotelo, J. L.; A Simulation of CO2 absorption into aqueous DEA using a hollow fiber membrane contactor: evaluation of contactor performance. Chem. Eng. J. 2009, 152, 396–405. (9) Zelenak, V.; Badanicova, M.; Halamova, D.; Cejka, J.; Zukal, A.; Murafa, N.; Goerigk, G. Amine-modified ordered mesoporous silica: effect of pore size on carbon dioxide capture. Chem. Eng. J. 2008, 144, 336–342. (10) Gabrielsen, J.; Michelsen, M. L.; Stenby, E. H.; Kontogeorgis, J. M. A model for estimating CO2 solubility in aqueous alkanolamines. Ind. Eng. Chem. Res. 2005, 44, 3348–3354. (11) Chaffee, A. L.; Knowles, G. P.; Liang, Z. CO2 capture by adsorption: materials and process development. Int. J. Greenhouse Gas Control 2007, 1, 11–18. (12) Serna-Guerrero, R.; Da’na, E.; Sayari, A. New insights into the interactions of CO2 with amine-functionalized silica. Ind. Eng. Chem. Res. 2008, 47, 9406–9412. (13) Druckenmiller, M. L.; Maroto-Valer, M. M. Carbon sequestration using brine of adjusted pH to form mineral carbonates. Fuel Process. Technol. 2005, 86, 1599–1614. (14) Liu, N.; Bond, G. M.; Abel, A. Biomimetic sequestration of CO2 in carbonate form: role of produced waters and other brines. Fuel Process. Technol. 2005, 86, 1615–1625. (15) Zeman, F. Experimental Results for Capturing CO2 from the Atmosphere. AIChE J. 2008, 54, 1396–1399. (16) Dawodu, O. F.; Meisen, A. Degradation of alkanolamine blends by carbon dioxide. Can. J. Chem. Eng. 1996, 74, 960–966. (17) Zhang, S. J.; Chen, Y. H.; Li, F. W.; Lu, X. M.; Dai, W. B.; Mori, R. Fixation and conversion of CO2 using ionic liquids. Catal. Today 2006, 115, 61–69. (18) Zhang, X. C.; Liu, Z. P.; Wang, W. C. Screening of ionic liquids to capture CO2 by COSMO-RS and experiments. AIChE J. 2008, 54, 2717–2728. (19) Freemantle, M. New frontiers for ionic liquids. Chem. Eng. News 2007, 85, 23–26. (20) Muldoon, M. J.; Aki, S.; Anderson, J. L.; Dixon, J. K.; Brennecke, J. F. Improving carbon dioxide solubility in ionic liquids. J. Phys. Chem. B 2007, 111, 9001–9009. (21) Kim, Y. S.; Choi, W. Y.; Jang, J. H.; Yoo, K. P.; Lee, C. S. Solubility measurement and prediction of carbon dioxide in ionic liquids. Fluid Phase Equilib. 2005, 228, 439–445.
ARTICLE
(22) Shiflett, M. B.; Yokozeki, A. Solubilities and diffusivities of carbon dioxide in ionic liquids: [bmim][PF6] and [bmim][BF4]. Ind. Eng. Chem. Res. 2005, 44, 4453–4464. (23) Kumelan, J.; Kamps, A. P. S.; Tuma, D.; Maurer, G. Solubility of CO2 in the ionic liquids [bmim][CH3SO4] and [bmim][PF6]. J. Chem. Eng. Data 2006, 51, 1802–1807. (24) Cadena, C.; Anthony, J. L.; Shah, J. K.; Morrow, T. I.; Brennecke, J. F.; Maginn, E. J. Why is CO2 so soluble in imidazolium-based ionic liquids?. J. Am. Chem. Soc. 2004, 126, 5300–5308. (25) Hou, Y.; Baltus, R. E. Experimental measurement of the solubility and diffusivity of CO2 in room-temperature ionic liquids using a transient thin-liquid-film method. Ind. Eng. Chem. Res. 2007, 46, 8166–8175. (26) Betas, E. D.; Mayton, R. D.; Ntai, I.; Davis, J. H. CO2 capture by a task-specific ionic liquid. J. Am. Chem. Soc. 2002, 124, 926–927. (27) Ramshaw, C. Mass transfer process. European Patent 0002568, 1979. (28) Ramshaw, C.; Mallinson, R. Mass transfer process. U.S. Patent 4,263,255, 1981. (29) Ramshaw, C. The opportunities for exploiting centrifugal fields. Heat Recovery Syst. CHP 1993, 13, 493–513. (30) Chen, J. F.; Shao, L.; Guo, F.; Wang, X. M. Synthesis of nanofibers of aluminum hydroxide in novel rotating packed bed reactor. Chem. Eng. Sci. 2003, 58, 569–575. (31) Wang, M.; Zou, H. K.; Shao, L.; Chen, J. F. Controlling factors and mechanism of preparing needlelike CaCO3 under high-gravity environment. Powder Technol. 2004, 142, 166–174. (32) Chen, J. F.; Gao, H.; Zou, H. K.; Chu, G. W.; Zhang, L.; Shao, L.; Xiang, Y.; Wu, Y. X. Cationic polymerization in rotating packed bed reactor: experimental and modeling. AIChE J. 2010, 56, 1053–1062. (33) Chen, J. F.; Shao, L. Mass production of nanoparticles by high gravity reactive precipitation technology with low cost. China Particuol. 2003, 1, 64–69. (34) Chen, J. F.; Wang, Y. H.; Guo, F.; Wang, X. M.; Zheng, C. Synthesis of nanoparticles with novel technology: high-gravity reactive precipitation. Ind. Eng. Chem. Res. 2000, 39, 948–954. (35) Yang, H. J.; Chu, G. W.; Xiang, Y.; Chen, J. F. Characterization of micromixing efficiency in rotating packed beds by chemical methods. Chem. Eng. J. 2006, 121, 147–152. (36) Chen, J. F.; Zheng, C.; Chen, G. T. Interaction of macro- and micromixing on particle size distribution in reactive precipitation. Chem. Eng. Sci. 1996, 51, 1957–1966. (37) Yang, S.; Lin, C. C.; Tseng, I. M.; Liu, W. T.; Yu, H. T. Method for removing volatile components from a high viscosity liquid by using rotation pack bed. U.S. Patent 6,884,401, 2005. (38) Martino, W.; De la Mora, J. F.; Yoshida, Y.; Saito, G.; Wilkes, J. Surface tension measurements of highly conducting ionic liquids. Green Chem. 2006, 8, 390–397. (39) Harris, K. R.; Woolf, L. A. Temperature and pressure dependence of the viscosity of the ionic liquid 1-butyl-3-methylimidazoliam hexafluorophosphate. J. Chem. Eng. Data 2005, 50, 1777–1782. (40) Delaloye, M. M.; Stockar, U. V.; Lu, X. P. The influence of viscosity on the liquid-phase mass transfer resistance in packed columns. Chem. Eng. J. 1991, 47, 51–61. (41) Guo, F.; Zheng, C.; Guo, K.; Feng, Y. D.; Gardner, N. C. Hydrodynamics and mass transfer in cross-flow rotating packed bed. Chem. Eng. Sci. 1997, 52, 3853–3859. (42) 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. Modeling ozone contacting process in a rotating packed bed. Ind. Eng. Chem. Res. 2004, 43, 228–236. (43) Basic, A.; Dudukovic, M. P. Liquid holdup in rotating packed bed: examination of the film flow assumption. AIChE J. 1995, 41, 301–316. (44) Guo, K.; Guo, F.; Feng, Y.; Chen, J.; Zheng, C.; Gardner, N. C. Synchronous visual and RTD study on liquid flow in rotating packedbed contactor. Chem. Eng. Sci. 2000, 55, 1699–1706. (45) Burns, J. R.; Ramshaw, C. Process intensification: visual study of liquid maldistribution in rotating packed beds. Chem. Eng. Sci. 1996, 51, 1347–1352. 6963
dx.doi.org/10.1021/ie1025979 |Ind. Eng. Chem. Res. 2011, 50, 6957–6964
Industrial & Engineering Chemistry Research
ARTICLE
(46) Munjal, S.; Dudukovic, M. P.; Ramachandran, P. Mass transfer in rotating packed beds: I. Development of gas-liquid and liquid-solid mass-transfer coefficients. Chem. Eng. Sci. 1989, 44, 2245–2256. (47) Munjal, S.; Dudukovic, M. P.; Ramachandran, P. Mass transfer in rotating packed beds: II. Development of gas-liquid and liquid-solid mass-transfer coefficients. Chem. Eng. Sci. 1989, 44, 2257–2268. (48) Chen, J. F. High Gravity Technology and Application; Chemical Industry Press of China: Beijing, 2003.
6964
dx.doi.org/10.1021/ie1025979 |Ind. Eng. Chem. Res. 2011, 50, 6957–6964
