Absorption of Hydrophobic Volatile Organic Compounds by a Rotating

Jun 28, 2012 - A rotating packed bed, which replaces the gravity force with a centrifugal force, can enhance the efficiency of mass transfer in severa...
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Absorption of Hydrophobic Volatile Organic Compounds by a Rotating Packed Bed Chia-Ying Chiang, Yi-Ying Liu, Yu-Shao Chen, and Hwai-Shen Liu* Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, ROC ABSTRACT: The high development of industries leads to significant amount of waste hydrophobic volatile organic compounds (VOCs), and these have caused serious environmental concerns. Because of the physical properties of the hydrophobic VOCs, a hydrophobic absorbent is needed in order to make the process more efficient if absorption is considered. However, most of these absorbents have high viscosities which leads to a low mass transfer coefficient. Thus, a cross-flow rotating packed bed (RPB) was evaluated for the feasibility of absorbing the hydrophobic VOCs, xylene, and toluene, by silicon oil, a model hydrophobic absorbent. The result shows that the absorption percentage could be up to 98% within a second contact of liquid and gas.

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correlated into the very first design equation (eq 1) that fitted very well in both their experimental result and others in the open literature.

INTRODUCTION Significant amounts of volatile organic compounds (VOCs) escaping from industrial processes cause serious environmental concerns, such as acid rain and carcinogenesis, and these need to be well-regulated. There are many possible unit operations to capture these VOCs. Among them, absorption is a wellaccepted and popular process. The hydrophilic VOCs may be absorbed satisfactorily by water-based absorbents, but not the hydrophobic ones. That is, conventional absorption processes with water-based absorbent seems inappropriate for hydrophobic VOCs because of their low solubility. On the other hand, while better solubility may be expected with oil-based absorbents for hydrophobic VOCs, poor mass transfer and hydrodynamic behavior, mainly due to the high viscosity, would hinder the applicability of these potential oil-based absorbents such as vegetal oil,1 solar oil,2 polyglycols,3−6 and silicon oil.2,7 Resulting from poor mass transfer, the bulky size of absorption columns is inevitable and thus leads to high capital and operating costs. To overcome this difficulty associated with oilbased absorbents, the concept of taking centrifugal force as a power to intensify the mass transfer efficiency was proposed, and such a facility was often named as a rotating packed bed (RPB) or Higee. A rotating packed bed, which replaces the gravity force with a centrifugal force, can enhance the efficiency of mass transfer in several applications, such as stripping,8−12 absorption,13−17 adsorption,18,19 and distillation.20,21 In 1997, Guo et al.22 experimentally reported the absorption performance of an NH3−water system. The mass transfer coefficient was found to be proportional to the gas flow rate. However, there was no obvious influence of rotational speed as centrifugal force went up to 15g. Furthermore, in 2006, Lin et al.23 performed the absorption process of isopropyl alcohol by water in a pilot-scale cross-flow RPB. Their result showed that the gas-side mass transfer coefficient (KGa) was mainly affected by the rotational speed and gas flow rate by the power of 0.52−1.11 and 0.48− 0.96, respectively. More complete results in a cross-flow RPB were reported in 2008 by Chen et al.16 Their experiments included the absorption of isopropyl alcohol, acetone, and ethyl acetate, into water and the corresponding KGa values were © 2012 American Chemical Society

K Ga DGat

2

= 0.0186ReG0.389ReL 0.534GrG 0.245Hy−0.185

(1)

It could be found that KGa increased with the increase of gas flow rate, liquid flow rate and rotational speed by the powers of 0.389, 0.534, and 0.490, respectively, but decreased with the increase of Henry’s constant by the power of 0.185. An RPB is expected to enhance mass transfer efficiency dramatically from previous experience and it is considered to be applicable to handle oil-based absorbents which can absorb hydrophobic VOCs efficiently. As a result, the idea of hydrophobic VOCs absorption by an oil-based absorbent, that is, toluene and xylene by silicone oil, in a novel mass transfer unit, cross-flow RPB, was evolved. Toluene and xylene are common hydrophobic VOCs in industry and possess low solubility in water (0.47 g/L for toluene and 0.11 g/L for xylene at 25 °C), but quite soluble in silicone oil. Although silicone oil may not be the best available absorbent for toluene and xylene, it could serve as a model compound to evaluate the feasibility.

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EXPERIMENTS The main structure of a cross-flow rotating packed bed (RPB) has the radii of stationary housing, inner and outer radii, and the height of the bed are 19.95, 1.3, 5.45, and 10.4 cm, respectively. Liquid flows outward from a liquid distributor which contains two sets of holes, and each set has 10 holes in line with 1 cm distance between holes. The diameter of the holes is 1 mm. As liquid sprays onto the rotating packing, it flies out quickly due to the centrifugal force and sprays on the stationary housing. On the other hand, the gas is introduced from the bottom of the RPB and contacts with the liquid which is in the form of tiny droplets and extremely thin film. After Received: Revised: Accepted: Published: 9441

September 20, 2011 June 8, 2012 June 28, 2012 June 28, 2012 dx.doi.org/10.1021/ie2021637 | Ind. Eng. Chem. Res. 2012, 51, 9441−9445

Industrial & Engineering Chemistry Research

Research Note

Figure 1. The experimental setup.

hydrophobic compounds (toluene or xylene) being absorbed into liquid from gas stream in a cross-flow pattern, the gas exists from the center piping line. The experimental setup is shown in Figure 1. Silicon oil, the absorbent, with the liquid flow rates of 165, 240, 348, and 500 mL/min was pumped into the RPB which was controlled at 28 °C. The diameter of the ports for gas inlet and outlet are 1.7 and 1.9 cm, respectively, while the diameter of ports for liquid inlet and outlet are 0.8 and 2 cm, respectively. Gas with the total flow rates of 10.4, 15.1, 20.3, and 25.8 L/min was separated into two streams; one passed through the hydrophobic VOC container to strip the VOC, such as xylene or toluene, out by the bubbling method24 and the other stream was used to dilute the VOC to a certain concentration quantified by a gas chromatography (China Chromatography, GC9800 series) equipped with a FID and a dimethyl silicone column. Nitrogen was used as the carrier gas. The injector, column, and detector temperatures were set at 160, 160, and 200 °C, respectively. Furthermore, the rotor speed was controlled at 500 to 1700 rpm corresponding to 9−108g force based on the arithmetic mean radius. The concentration of hydrophobic VOCs in silicon oil were measured by a UV spectrophotometer (Spekol 1300) and the viscosity of silicon oil was obtained by viscometer (Brookfield, model DV-II+). Henry’s constants were determined by the bubbling method.22

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RESULTS AND DISCUSSION Absorption Percentage. For an intuitive index of the efficiency of the absorption process, the absorption percentage is introduced. The definition is shown as following: absorption percentage (%) =

CG,i − CG,o CG,i

100 (2)

The dependence of absorption percentage on the rotational speed at various liquid flow rates was shown in Figure 2. For either xylene or toluene as VOC sources, the absorption percentage increased as the rotational speed increased, and the maximum absorption percentage can reach as high as 98% within a second contact of gas and liquid at 1700 rpm. This high absorption efficiency can be explained by a high centrifugal

Figure 2. Dependence of absorption percentage on the rotor speed at various gas flow rates: (a) xylene, (b) toluene.

force resulting in the thinner liquid film on the packing and the smaller droplets flying in the packing. Furthermore, an obvious 9442

dx.doi.org/10.1021/ie2021637 | Ind. Eng. Chem. Res. 2012, 51, 9441−9445

Industrial & Engineering Chemistry Research

Research Note

increase of the absorption percentage was found as the liquid flow rate increased. It reinforced the fact that an increase of liquid flow rate in a cross-flow rotating packed bed was important in several previous findings.16,22,23 This is mainly due to a better liquid distribution in the packing, especially the axial direction, leading to a better mass transfer efficiency. To understand whether the variation of gas inlet concentrations would affect the absorption in a cross-flow RPB or not, the dependence of absorption percentage on various gas inlet concentrations at different rotational speeds is shown in Figure 3. It was shown that there was no apparent

Figure 3. Dependence of stripping percentage on the rotor speed at various gas inlet concentrations.

difference of absorption percentage based on the various gas inlet concentrations, 300−1200 ppmv, when the rotational speeds were higher than 1300 rpm. At high rotational speed, higher mass transfer was expected which means more xylene was absorbed. Within this concentration (ppmv level) range, the ratio for solute distributed in gas and liquid (Henry’s constant) is relatively a constant, thus even though the total amount of absorbed xylene was increased, the absorption percentage remained constant. However, as the rotational speed decreased, an absorption percentage decrease was noted, especially at higher gas inlet concentrations. It might be because liquid tended to flow at the bottom of the packing at lower rotational speed. This led to an unevenly distributed liquid pattern and unused bed in the upper region. Therefore, the mass transfer area between gas and liquid phase was limited, and the absorption percentage decreased. Mass Transfer Coefficient. Due to the variation of concentrations in both radial and axial directions, the mass transfer coefficient could not be obtained directly as in a counter-current flow RPB; instead a numerical method is needed. By guessing a KGa value, the corresponding gas and liquid outlet concentrations could be estimated. Then, with the comparison of these concentration profiles with the one measured by experiments, the gas film mass transfer coefficient can be obtained by a trial-and-error method. The derivation of mass transfer coefficients in a cross-flow RPB was demonstrated in detail by Chen et al. in 2008.16 The dependence of KGa on the rotational speed at various liquid flow rates is shown in Figure 4 for rotational speeds of 500, 800, 1300, and 1700 rpm. As expected, the KGa values increased with increasing rotational speed which implied that

Figure 4. Dependence of KGa on the rotor speed at various gas flow rates: (a) xylene, (b) toluene.

centrifugal force could effectively reduce the mass transfer resistance for VOCs absorption process in a cross-flow RPB. Figure 5 shows the dependence of KGa on gas flow rate at various liquid flow rates for both absorbents, xylene and toluene. It could be found that as gas and liquid flow rates increased, the mass transfer coefficient increased. However, the influence of gas flow rate was less than that of the liquid flow rate. This might be because the highly viscous absorbent seriously increased the liquid-phase mass transfer resistance and thus decreased the mass transfer coefficient. Only by increasing the mass transfer coefficient significantly, here by increasing liquid flow rate, with the help of centrifugal force, the KGa would apparently improve. Furthermore, as liquid flow rate was higher than 348 mL/min, the gas film mass transfer coefficient increase was observed. This might be due to the welldistributed thin liquid film and the increase of tiny liquid droplets flying in the voidage of the packing which led to a more mass transfer area.

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CONCLUSION The capability of dealing with high viscous absorbent to capture the hydrophobic VOCs from industries by a cross-flow RPB was demonstrated, while it is not a favorable unit operation in a 9443

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at = total particle surface area per unit volume of the packed bed (m2/m3) ac = centrifugal acceleration (m/s2) CGi = gas inlet concentration (mol/L) CGo = gas outlet concentration (mol/L) DG = diffusion coefficient in gas (m2/s) dp = Spherical equivalent diameter of the packing =((6(1ε))/(atψ)) (m) G = gas mass flux (kg/m2 s) g = gravitational force (m/s2) Hy = Henry’s constant [(mol/mol)/(mol/mol)] KGa = overall volumetric gas-film mass-transfer coefficient (1/s) L = liquid mass flux (kg/m2 s) Greek Letters

ε = porosity of the packing (−) μG = viscosity of the gas μL = viscosity of the liquid ρG = density of the gas (kg/m3) ψ = sphericity of packing (−) Dimensionless Quantities

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REFERENCES

(1) Pierucci, S.; Del Rosso, R.; Bombardi, D.; Concu, A.; Lugli, G. An Innovative Sustainable Process for VOCs Recovery from Spray Paint Booths. Energy J. 2005, 30, 1377. (2) Volodin, N. I.; Puzyreva, V. M.; Soroko, V. E. Absorption Treatment of Gasesto Remove Impurities of Organic Solvents. Russ. J. Appl. Chem. 1997, 70, 1745. (3) Porter, K. E.; Sitthiosoth, S.; Jenkins, J. D. Designing a Solvent for Gas Absorption. Trans IChemE. 1991, 69, 229. (4) Stockley, N. J. VOC Abatement by Absorption. Eur. Coat. J. 1994, 751. (5) Cotte, F.; Fanlo, J. L.; Le Cloirec, P.; Escobar, P. Absorption of Odorous Molecules in Aqueous Solutions of Polyethylene Glycols. Environ. Technol. 1995, 16, 127. (6) Wang, X.; Daniels, R.; Baker, R. W. Recovery of VOCs from High-Volume, Low-VOC-Concentration Air Streams. AIChE J. 2001, 47, 1094. (7) Majumdar, S.; Bhaumik, D.; Sirkar, K. K.; Simes, G A Pilot-Scale Demonstration of a Membrane Based Absorption Stripping Process for Removaland Recovery of Volatile Organic Compounds. Environ. Prog. 2001, 20, 27. (8) Singh, S. P.; Wilson, J. H.; Counce, R. M.; Villiersfisher, J. F.; Jennings, H. L.; Lucero, A. J.; Reed, G. D.; Ashworth, R. A.; Elliott, M. G. Removal of Volatile Organic-Compounds from Groundwater Using a Rotary Air Stripper. Ind. Eng. Chem. Res. 1992, 31, 574. (9) 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. (10) Chen, Y. S.; Lin, C. C.; Liu, H. S. Mass Transfer in a Rotating Packed Bed with Viscous Newtonian and Non-Newtonian Fluids. Ind. Eng. Chem. Res. 2005, 44, 1043. (11) Chen, Y. S.; Lin, C. C.; Liu, H. S. Mass Transfer in a Rotating Packed Bed with Viscous Radii of the Bed. Ind. Eng. Chem. Res. 2005, 44, 7868. (12) Chen, Y. S.; Lin, F. Y.; Lin, C. C.; Tai, C. Y.; Liu, H. S. Packing Characteristics for Mass Transfer in a Rotating Packed Bed. Ind. Eng. Chem. Res. 2006, 45, 6846. (13) Ramshaw, C.; Mallinson, R. H. Mass Transfer Apparatus and Process. U.S. Patent 4,400,275, 1981. (14) Chen, Y. S.; Liu, H. S. Absorption of VOCs in a Rotating Packed Bed. Ind. Eng. Chem. Res. 2002, 41, 1583.

Figure 5. Dependence of KGa on gas flow rate at various rotor speeds: (a) xylene, (b) toluene.

conventional absorption process due to its low mass transfer efficiency. In this report, up to 98% of xylene and toluene were shown to be efficiently absorbed by the hydrophobic viscous silicon oil absorbent. Silicon oil was only a model absorbent to demonstrate the feasibility and capability of absorption of the hydrophobic VOCs in this study because of its well-defined physical properties. There are other good candidates in terms of cost and solubility. Better results could be expected by choosing the absorbents properly depending on the VOCs.

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GrG = gas Grashof number = dp3acρG2/μG2 ReG = gas Reynolds number = G/atμG ReL = liquid Reynolds number = L/atμL

AUTHOR INFORMATION

Corresponding Author

*Tel.: +886-2-3366-3050. Fax: +886-2-2362-3040. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The support from National Science Council is greatly appreciated. NOMENCLATURE a = effective gas−liquid interfacial area per unit volume (m2/ m3) 9444

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(15) Lin, C. C.; Wei, T. Y.; Liu, W. T.; Shen, K. P. Removal of VOCs from Gaseous Streams in a High-Voidage Rotating Packed Bed. J. Chem. Eng. Jpn. 2004, 37, 1471. (16) Chen, Y. S.; Hsu, Y. C.; Lin, C. C.; Tai, C. Y. D.; Liu, H. S. Volatile Organic Compounds Absorption in a Cross-Flow Rotating Packed Bed. Environ. Sci. Technol. 2008, 42, 2631. (17) Chiang, C. Y.; Chen, Y. S.; Liang, M. S.; Lin, F. Y.; Tai, C. Y. D.; Liu, H. S. Absorption of Ethanol into Water and Glycerol/Water Solution in a Rotating Packed Bed. J. Taiwan Inst. Chem. Eng. 2009, 40, 418. (18) Lin, C. C.; Chen, Y. S.; Liu, H. S. Adsorption of Dodecane from Water in a Rotating Packed Bed. J. Chin. Inst. Chem. Eng. 2004, 35, 531. (19) Lin, C. C.; Liu, H. S. Adsorption in a Centrifugal Field: Basic Dye Adsorption by Activated Carbon. Ind. Eng. Chem. Res. 2000, 39, 161. (20) Kelleher, T; Fair, J. R. Distillation Studies in a High-Gravity Contactor. Ind. Eng. Chem. Res. 1996, 35, 4646. (21) Lin, C. C.; Ho, T. J.; Liu, W. T. Distillation in a Rotating Packed Bed. J. Chem. Eng. Jpn. 2002, 35, 1298. (22) Guo, F.; Zheng, C.; Guo, K.; Gardner, N. C. Hydrodynamics and Mass Transfer in Crossflow Rotating Packed Bed. Chem. Eng. Sci. 1997, 52, 3853. (23) Heymes, F.; Demoustier, P. M.; Charbit, F.; Fanlo, J. L.; Moulin, P. A. New Efficient Absorption Liquid to Treat Exhaust Air Loaded with Toluene. Chem. Eng. J. 2006, 115, 225. (24) Lin, C. C.; Wei, T. Y.; Hsu, S. K.; Liu, W. T. Performance of a Pilot-Scale Cross-Flow Rotating Packed Bed in Removing VOCs from Waste Gas Streams. Sep. Purif. Technol. 2006, 52, 274.

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