Ind. Eng. Chem. Res. 2002, 41, 1583-1588
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Absorption of VOCs in a Rotating Packed Bed Yu-Shao Chen and Hwai-Shen Liu* Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, R.O.C.
A Higee absorption process was developed to remove VOCs (volatile organic compounds) from air into an aqueous phase under a centrifugal field. The experimental results showed that the overall volumetric gas-phase mass-transfer coefficient (KGa) increased as a function of the gas Grashof number (GrG) to the power of 0.18. In analyzing kG and a individually, it was found that the enhancement of mass transfer by the centrifugal force can mainly be attributed to an increase in the effective gas-liquid interfacial area. The values of kG lie in a range similar to that for conventional packed beds. Introduction Recently, various studies have been performed on process intensification in high-gravity systems (i.e., Higee), such as absorption, stripping, adsorption, distillation, and extraction.1-10 The Higee system, first developed by Ranshaw and Mallinson,1 basically is a rotating packed bed in which gravity is replaced by a centrifugal force. Under a centrifugal field, thinner liquid films and smaller droplets could be obtained. Also, the system can be operated at higher gas/liquid ratios because of the decreased tendency for flooding. The most important feature of this design is that it frequently increases the mass-transfer coefficient by up to an order of magnitude and, thus, dramatically reduces the equipment size. In 1981, Ramshaw and Mallinson1 reported the very first data in a Higee system for the absorption of oxygen into water. The results showed that the overall volumetric liquid-phase mass-transfer coefficients (KLa) increased significantly with increasing rotational speed. Later, Tung and Mah11 derived a mass-transfer model for rotational packed beds based on the penetration theory and obtained a correlation for kL
kL )
()
DL 2 × 31/3 at ScL1/2ReL1/3 dp π a
1/3
GrL1/6
Sandilya et al.12 investigated a SO2-NaOH solution system and found that the kG values, estimated on the basis of the total surface area of the packing, were much lower than those of the conventional packed beds. However, they also suggested that the kG values of the rotating packed beds should be in a range similar to that for conventional packed beds in view of the negligible tangential slip velocity of the gas. They attributed the lower values of kG to a severe liquid maldistribution in the rotating packed bed. In other words, they thought that the gas-liquid interfacial area of the rotating packed bed was overpredicted by using the total surface area of the packing. However, by visual observation, Gao et al.13 found that unconventional gas-liquid interfacial area is possible in rotating packed beds. Thus, an alternative method is proposed in this study to estimate the effective gas-liquid interfacial area, as well as to obtain useful design data for the absorption of VOCs (volatile organic compounds). In particular, a deoxygenation experiment was performed to obtain the effective interfacial area, and the interfacial area data were then used to analyze VOC absorption results. By assuming similar effective interfacial areas for the same bed, the gas-phase mass-transfer coefficient, kG, and interfacial area, a, can be obtained and evaluated.
(1)
They found that this correlation could fit Ramshaw and Mallinson’s data1 well. In 1989, Munjal et al.,2,3 in a study of the chemical absorption of CO2 in NaOH solution, showed that both the gas-liquid interfacial area, a, and the liquid-phase mass-transfer coefficient, kL, increased with rotational speed. In gas-phase mass transfer, Ramshaw and Mallinson1 first reported NH3 absorption in a Higee system and found that KGa for NH3 absorption was 4-9 times higher in such systems than in conventional packed beds. Liu et al.7 also showed that KGa increased with increasing rotational speed for ethanol stripping. On the other hand, Guo et al.9 constructed a cross-flow rotating packed bed. They performed experiments involving the absorption of NH3 into water and the absorption of SO2 into ammonium sulfite solution. Their results showed that KGa was not influenced by rotational speed when the centrifugal acceleration was above 15g. In 2001, * Corresponding author. Fax: +886-2-2362-3040. E-mail:
[email protected].
Experiment The main structure of a rotating packed bed is shown in Figure 1. The liquid enters the packed bed from a liquid distributor, sprays from the inner bed, and moves outward as a result of the centrifugal force. The liquid distributor used in this study had four vertical sets of holes spaced 90° apart; each set had four 1-mmdiameter holes. The gas is introduced from the outer edge of the packing and flows inward. Thus, the gas and liquid come into contact countercurrently in the rotating packed bed. The packings used in this study were 2-mmdiameter acrylic beads, whose total specific surface area, at, was 1200 m2/m3, and whose porosity, , was 0.4. Stainless steel wire meshes were used as the packing support. The axial height of the bed was 2 cm. The inner and outer radii of the bed were 2 and 4 cm, respectively. The bed could be operated in the range 150-2100 rpm, providing 0.75-150g of force, as determined on the basis of the arithmetic mean radius. Figure 2 shows a diagram of the experimental setup for VOC absorption. Fresh water at a temperature of 25 °C was pumped into the rotating packed bed. An air
10.1021/ie010752h CCC: $22.00 © 2002 American Chemical Society Published on Web 02/23/2002
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Ind. Eng. Chem. Res., Vol. 41, No. 6, 2002 Table 1. Transport and Thermodynamic Properties of VOCs H (atm m3/mol)
VOC isopropyl alcohol acetone ethyl acetate
10-5
DL (cm2/s) 10-5
1.104 × 4.276 × 10-5 1.347 × 10-4
0.87 × 1.16 × 10-5 1.00 × 10-5
DG (cm2/s) 0.099 0.103 0.087
packed dimethylsilicone column. Nitrogen was used as the carrier gas. The injector, column, and detector temperatures were set at 140, 150, and 200 °C, respectively. In the deoxygenation experiments, fresh water was introduced into the rotating packed bed and contacted countercurrently with a nitrogen stream. The concentrations of dissolved oxygen (DO) in the inlet and outlet water were measured by a DO probe (Ingold type 170). Results and Discussion
Figure 1. Main structure of a rotating packed bed.
stream was introduced into a bubbler containing aqueous VOC, and the mixture was then diluted by another air stream to the desired VOC concentration. A buffer flask (in fact, an empty bottle) was used to maintain a stable gas concentration. Then, the water and gas streams were contacted countercurrently in the rotating packed bed. The temperatures of the outlet gas and liquid streams were measured to be 25 ( 1°C. Three VOCs, namely, isopropyl alcohol, acetone, and ethyl acetate, were used in this study. The transport and thermodynamic properties of these VOCs are listed in Table 1. The concentrations of the inlet and outlet gas streams from the gas-collecting tubes and the outlet liquid stream were measured by a gas chromatograph equipped with a FID and a 3-ft × 1/8-in. stainless-steel-
Figure 2. Diagram of the experimental setup for VOC absorption.
During the absorption of the VOCs, the outlet gas concentrations of VOCs were found to drop rapidly during the first minute and then decrease slowly to reach a steady value within 15 min. Increasing the rotor speed increased the removal of the VOCs while decreasing the time required to reach steady state (data not shown). This finding implies that the enhancement of mass transfer under a high centrifugal field is feasible. Material balances on the inlet and outlet of both the gas and liquid phases indicated that the errors were within 10%. A design equation, similar to the one by Liu et al.,7 can be obtained using mass balances and the transfer unit concept
KGa )
Gm
[( )(
ln 1 -
zptπ(r22 - r12)
) ]
1 Y1 - HyX2 1 + A Y2 - HyX2 A 1 -1 A
(2)
where A is an absorption factor defined by
A)
Lm HyGm
(3)
With eq 2, the overall volumetric gas-phase masstransfer coefficient can be calculated using the absorp-
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Figure 3. Dependence of KGa on rotor speed at various liquid flow rates for the absorption of (a) isopropyl alcohol, (b) acetone, and (c) ethyl acetate.
Figure 4. Dependence of KGa on rotor speed at various gas flow rates for the absorption of (a) isopropyl alcohol, (b) acetone, and (c) ethyl acetate.
tion factor and the corresponding stream concentrations. Thus, Figure 3 shows the dependence of KGa on the rotor speed for various liquid flow rates. It is clear that the mass-transfer coefficients increase with increasing rotational speed. Figure 4 shows an even steeper increase of KGa with increasing rotational speed for various gas flow rates. These characteristics are similar to those found in conventional packed beds, except that
the mass-transfer coefficient increases with increasing rotor speed in Higee systems. In other words, the centrifugal force reduces the mass-transfer resistance. A correlation was developed to predict KGa for the rotating packed bed. The following equation was obtained from a regression of Higee experimental data, including those for VOC absorption in this study as well
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Figure 5. Comparison of experimental values of KGa with results calculated using eq 8.
as those for ethanol stripping reported earlier by Liu et al.7
KGaHy0.27RT 2
DGat
) 0.077ReG0.323ReL0.328GrG0.18
(4)
As shown in Figure 5, the experimental results lie within (30% of the values estimated by eq 4. The enhancement in mass transfer by the centrifugal force can be clearly observed. We then proposed to determine the gas-phase mass-transfer coefficient, kG, and the effective gas-liquid interfacial area, a, individually and investigate how they are influenced by centrifugal force. The effective gas-liquid interfacial area in the rotating packed bed, a, could be evaluated using
a)
kLa kL
(5)
where the values of kLa could be obtained from the deoxygenation experiments assuming that the gasphase mass-transfer resistance could be neglected and kL could be calculated from eq 1. The results are listed in Table 2. It was found that the gas-liquid interfacial area increased as a function of the rotor speed to the power of 0.40, which is within the range estimated by Munjal et al.3 In addition, it was noted that the gasliquid interfacial area increased as a function of the liquid flow rate to the power of 0.66. To determine the values of kG, a relation based on two-film theory14 was proposed, as follows
kG )
1 1 H a KGa kLa
(
)
(6)
where KGa can be obtained from the experimental results for the absorption of VOCs, kL can be obtained from eq 1, and a can be obtained from Table 2. Figure 6 shows the dependence of kG on rotor speed. It can be seen that kG is not significantly influenced by the
Figure 6. Dependence of kG on rotor speed at various liquid and gas flow rates for the absorption of (a) isopropyl alcohol, (b) acetone, and (c) ethyl acetate.
centrifugal force. Moreover, Onda et al.15 proposed a correlation for conventional packed beds as follows
kG ) 2ReG0.7ScG1/3(atdp)-2 atDG
(7)
The values of kG obtained from eq 7 range from 0.32 to 0.59 g mol/(atm m2 s). This indicates that the gas-phase mass-transfer coefficients in Higee systems are in a
Ind. Eng. Chem. Res., Vol. 41, No. 6, 2002 1587 Table 2. Gas-Liquid Interfacial Area under Different Operating Conditions Obtained from Deoxygenation Experiments ω (rpm)
L (mL/min)
a (1/m)
600 900 1200 1500 600 900 1200 1500 600 900 1200 1500 600 900 1200 1500
258 258 258 258 435 435 435 435 612 612 612 612 822 822 822 822
372 444 480 504 540 696 732 780 636 840 888 972 756 948 1020 1140
range similar to that for conventional packed beds. This result seems to confirm the expectations of Sandilya et al.12 It also suggests that the centrifugal force enhances mass transfer perhaps primarily by increasing the gasliquid interfacial area. The gas-phase mass-transfer coefficients are expected to be quite similar to those in conventional packed beds. Conclusion According to the results of our experiments, it is clear that centrifugal force intensifies mass transfer during VOC absorption in a rotating packed bed. The outlet VOC concentration was reduced significantly with the aid of the centrifugal force within a short contact time. In addition, a correlation for KGa in the rotating packed bed was proposed, showing that KGa depends on the gas Grashof number (GrG) to the power of 0.18. Also, by analyzing the gas-phase mass-transfer coefficient (kL) and the effective gas-liquid interfacial area (a) individually, it was found that the effective interfacial area increases with the rotor speed to the power of 0.40, whereas the gas-phase mass-transfer coefficient is a weak function of centrifugal force. The values of kG lie in a range similar to that for conventional packed beds. Thus, the enhancement of mass transfer by the centrifugal force can perhaps mainly be attributed to an increase in the effective gas-liquid interfacial area. Acknowledgment The support from ITRI (Industrial Technology Research Institute) is greatly appreciated. Nomenclature A ) absorption factor ) Lm/HyGm a ) effective gas-liquid interfacial area per unit volume of the packed bed (m2/m3) at ) total particle surface area per unit volume of the packed bed (m2/m3) ac ) centrifugal acceleration (m/s2) DG ) diffusivity of the gas (m2/s) DL ) diffusivity of the liquid (m2/s) dp ) diameter of the packing (m) G ) gas volumetric flow rate (L/min) Gm ) gas molar flow rate (mol/s)
G′ ) gas mass flux [kg/(m2 s)] H ) Henry’s constant (atm m3/mol) Hy ) dimensionless Henry’s constant KGa) overall volumetric gas-phase mass-transfer coefficient [g mol/(atm m3 s)] KLa ) overall volumetric liquid-phase mass-transfer coefficient (1/s) kG ) gas-phase mass-transfer coefficient [g mol/(atm m2 s)] kL ) liquid-phase mass-transfer coefficient (m/s) L ) liquid volumetric flow rate (mL/min) Lm ) liquid molar flow rate (mol/s) L′ ) liquid mass flux (kg/m2-s) pt ) total pressure of the system (atm) r1 ) inner radius of the packed bed (m) r2 ) outer radius of the packed bed (m) X2 ) outlet liquid-phase mole fraction of VOCs (mol/mol) Y1 ) outlet gas-phase mole fraction of VOCs (mol/mol) Y2 ) inlet gas-phase mole fraction of VOCs (mol/mol) z ) height of packed bed (m) Greek Letters ) porosity of the packing µG ) viscosity of the gas [kg/(m s)] µL ) viscosity of the liquid [kg/(m s)] FG ) density of the gas (kg/m3) FL ) density of the liquid (kg/m3) ω ) rotor speed (rpm) Dimensionless Quantities GrG ) gas Grashof number ) dp2‚ac‚FG2/µG2 ReL ) liquid Reynolds number ) L′/atµL ReG ) gas Reynolds number ) G′/atµG ScG ) gas Schmidt number )µG/FGDG ScL ) liquid Schmidt number ) µL/FLDL
Literature Cited (1) Ramshaw, C.; Mallison, R. H. Mass Transfer Process. U.S. Patent 4,383,255, 1981. (2) Mujal, S.; Dudukovic, M. P.; Ramachandran, P. A. Mass Transfer in Rotating Packed Beds: I. Development of Gas-Liquid and Liquid-Solid Mass-Transfer Coefficients. Chem. Eng. Sci. 1989, 44, 2245. (3) Mujal, S.; Dudukovic, M. P.; Ramachandran, P. A. Mass Transfer in Rotating Packed Beds: II. Experimental Results and Comparison with Theory and Gravity Flow. Chem. Eng. Sci. 1989, 44, 2257. (4) Keyvani, M.; Gardner, N. C. Operating Characteristics of Rotating Beds. Chem. Eng. Prog. 1989, 85, 48. (5) Singh, S. P.; Wilson, J. H.; Counce, R. H.; Villiers-Fisher, J. F.; Jennings, H. L.; Lucero, A. J.; Reed, G. D.; Ashworth, R. A.; Elliot, M. G. Removal of Volatile Organic Compounds from Groundwater Using a Rotary Air Stripper. Ind. Eng. Chem. Res. 1992, 31, 574. (6) Kumar, M. P.; Rao, D. P. Studies on a High-Gravity GasLiquid Contactor. Ind. Eng. Chem. Res. 1990, 29, 917. (7) Liu, H. S.; Lin, C. C.; Wu, S. C.; Hsu, H. W. Characteristics of a Rotating Packed Bed. Ind. Eng. Chem. Res. 1996, 35, 3590. (8) Kelleher, T.; Fair, J. R. Distillation Studies in a HighGravity Contactor. Ind. Eng. Chem. Res. 1996, 35, 4646. (9) Guo, F.; Zheng, C.; Guo, K.; Feng, Y.; Gardner, N. C. Hydrodynamics and Mass Transfer in Cross-Flow Rotating Packed Beds. Chem. Eng. Sci. 1997, 21/22, 3853. (10) Lin, C. C.; Liu, H. S. Adsorption in a Centrifugal Field: Basic Dye Adsorption by Activated Carbon. Ind. Eng. Chem. Res. 2000, 39, 161. (11) Tung, T. H.; Mah, R. S. H. Modeling Liquid Mass Transfer in Higee Separation Process. Chem. Eng. Commun. 1985, 39, 147. (12) Sandilya, P.; Rao, D. P.; Sharma, A.; Biswas, G. Gas-Phase Mass Transfer in a Centrifugal Contactor. Ind. Eng. Chem. Res. 2001, 40, 384. (13) Guo, K.; Guo, F.; Feng, Y.; Chen, J.; Zheng, C.; Gardner, N. C. Synchronous Visual and RTD Study on Liquid Flow in a Rotating Packed-Bed Contactor. Chem. Eng. Sci. 2000, 55, 1699.
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(14) Whitman, W. G. The Two-Film Theory of Absorption. Chem. Metall. Eng. 1923, 29, 147. (15) Onda, K.; Takeuchi, H.; Okumoto, Y. Mass Transfer Coefficient between Gas and Liquid Phases in Packed Columns. J. Chem. Eng. Jpn. 1968, 1, 56.
Received for review September 10, 2001 Revised manuscript received December 26, 2001 Accepted January 7, 2002 IE010752H
