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SEPARATIONS Process Intensification in a HIGEE with Split Packing K. Jagadeswara Reddy, Amit Gupta, and D. P. Rao* Department of Chemical Engineering, Indian Institute of Technology, Kanpur, Kanpur U.P. 208016, India
O. P. Rama Department of Chemical Engineering, Harcourt Butler Technological Institute, Kanpur U. P. 208002, India
Process intensification in rotating packed beds has attracted attention. The intensification is due to the large specific surface area of the packing and high liquid- and gas-side mass-transfer coefficients. Recent studies indicate that the slip velocity between the gas and the liquid in a rotating bed is in the same range as in conventional packed columns. Hence, the intensification is limited to a higher surface area of the packing if the resistance for mass transfer is on the gas side. To overcome this limitation, we proposed a rotating packed bed with split packing to promote the slip velocity as high as 30 m/s. To assess its efficacy, we have measured the mass-transfer coefficients on the gas side using the absorption of SO2 from air into aqueous NaOH solution and the one on the liquid side by the stripping of oxygen from water into nitrogen. The volumetric masstransfer coefficients on the gas side and the liquid side, respectively, were in the ranges 35-280× and 25250× compared to those of packed columns. Correlations for the local mass-transfer coefficients have been presented. The studies indicate a volume reduction of the units by 2 orders of magnitude is feasible. 1. Introduction The earth’s gravity dictates the allowable liquid and gas throughputs and the attainable mass-transfer rates in distillation and absorption columns. A rotating packed bed (also known as a HIGEE) provides a means for replacing the gravitational acceleration (g) by a centrifugal acceleration. A high centrifugal force permits the use of packing with a large surface area and enhances the mass-transfer coefficient on the liquid side. Recently, Dhiman et al.1 demonstrated that high transfer rates are possible in a rotating trickle-bed reactor and a volume reduction of ∼40× less compared to the conventional tricklebed reactor is feasible, if the rate-controlling step is the mass transfer through the liquid. It has been shown earlier2 that the centrifugal acceleration has little or no effect on the masstransfer coefficient on the gas side. The controlling resistance is on the gas side in several cases of distillation and absorption. Therefore, for these applications, the process intensification in rotating beds would be only due to the high surface area of the packing. This is a serious limitation of the rotating bed. Guo et al.3 reported visual studies on gas and liquid flow in the rotor by employing a video camera mounted on the rotor. Their studies indicate that the liquid breaks up into chunks as it enters the packing. Some sticks to the packing, and the rest goes through intense motion in a zone within 7-10 mm from the liquid entry. Thereafter, the liquid attaches itself to the packing. Beyond the entry zone, the liquid flows as films and flying drops. Further, they observe that a generation of large interfacial area in the entry region leads to a high mass-transfer rate. Taking the cue from their work, we propose to split the packing into annular rings of 7.5 mm thickness and with a gap * To whom correspondence is to be addressed. Tel.: 91 512 2597432. Fax: 91 512 2590104. E-mail:
[email protected].
of 4 mm between the rings. Further, if the adjacent rings are made to rotate in opposite direction, the tangential slip velocity could be as high as 30 m/s at the entry compared to 2-3 m/s in a rotor with a single packing element. There is a need to enhance the throughputs as well. Guo et al.4 report that gas velocities as high as 15 m/s could be used with the cross-flow of gas and liquid. Hence, we could enhance the throughputs if the cross-flow is ensured in the region of gas withdrawal. We propose to withdraw the gas in the axial direction through the packing ring and through the holes made on the side plate of the rotor rather than through the eye of the rotor, as is generally done. The objective of the present work is to assess the efficacy of the split packing in the enhancement of mass-transfer rates in rotating packed beds. 2. Experimental Setup Figure 1 shows a schematic diagram of the experimental setup. The outline of the rotor used in this study can be seen in the figure. In brief, the metal-foam packing was split into annular rings with gaps between the adjacent rings to avoid touching one another. A set of annular rings was fixed onto one of the side plates of the rotor, and the other set was fixed onto the other side plate. The gas was withdrawn through the second inner packing ring, rather than from the eye of the rotor, to enhance the allowable throughputs. Two types of metal-foam packing with specific surface areas of 1700 and 2500 m2/m3 (supplied by RECEMAT International B. V. Holland) were used. Tables 1 and 2 present the details of the rotor, the casing, and the packing rings. Further details of the rotor assembly are available elsewhere.5 To drive the side plates of the rotor in counter- or corotation, each one was connected to a three-phase AC motor. The rotational speed of the motors was varied using an inverter drive. The liquid was circulated through the unit
10.1021/ie051382q CCC: $33.50 © 2006 American Chemical Society Published on Web 05/02/2006
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Figure 1. Schematic diagram of experimental setup. Table 1. Details of Rotor and Casing disk material disk plate thickness outer radius axial width of packing gap between packing and disk cloth thickness porosity ring radial thickness axial width for liquid flow
Perspex 1.5 cm 15 cm 3.0 cm 0.2 cm 4 mm 0.9 7.5 mm 3.0 cm
Casing casing material casing geometry length (l) breadth (b) height
Perspex rectangular box 55 cm 28 cm 50 cm
Table 2. Dimensions of Packing Rings ring no.
i.d. (mm)
o.d. (mm)
1 2 3 4 5 6 7 8 9
81 106 131 156 181 206 231 256 281
96 121 146 171 196 221 246 271 296
using a centrifugal pump. A cylindrical tube used for liquid distribution was of 2.5 cm outer diameter, 1.4 cm inner diameter, and 33 cm length. It was provided with 36 holes of 2 mm diameter in 4 or 6 rows spread over 2.5 cm at the closed end of the tube to introduce the liquid as jets on to the periphery of the inner ring. The tube was held stationary. A 25-hp blower was used to supply the air. 2.1. Gas-Side Mass Transfer. Vidwans and Sharma,6 Scheffe and Weiland,7 Weiland and Ahlgren,8 and Sandilya et al.9 have used the absorption of SO2 from air into an aqueous solution of NaOH to obtain the gas-side mass-transfer coefficient. The liquid-side resistance is negligible for this system.8 We have chosen this system for determination of the gas-side masstransfer coefficient. The concentration of SO2 in the inlet air was maintained below 4000 ppm, the maximum limit of the SO2 analyzer. The initial concentration of NaOH solution was 1.5 N. It was made up whenever it fell below 20% of the original
strength. The solution was renewed after every two runs. The liquid-free gas samples drawn from (i) the gas inlet, (ii) the gas outlet, and (iii) the casing (see Figure 1) were analyzed using the SO2 analyzer supplied by TSI, U.S.A. (model CA-6050; range ) 0-4000 ppm, accuracy ) (5 ppm in the range of 0-200 ppm, (5% of reading between 200 and 1000 ppm, and (10% of reading between 1000 and 4000 ppm). The preliminary runs revealed that the SO2 concentration in the gas outlet was below 1 ppm, outside the range of the SO2 analyzer. Similar observations were made by Weiland and Ahlgren.8 Therefore, we removed the outer packing rings and carried out the mass-transfer studies with only five inner rings. A significant fraction of mass transfer took place to the liquid held at the bottom of casing due to the agitation induced by the rotating gas. To minimize it, we have provided the inclined baffles in the casing (see Figure 1). We carried out the runs at different gas flow rates and rotor speeds. 2.2. Liquid-Side Mass Transfer. The stripping of oxygen from tap water into pure nitrogen for obtaining the liquid-side mass-transfer coefficients has been widely used (Akita and Yoshida,10 Deckwer et al.,11 and Chen et al.12,13). The setup shown in Figure 1 was employed with minor changes to determine the liquid-side mass-transfer coefficient. The flow rate of nitrogen was monitored by a mass flow meter (Cole Parmer, model 32707-16). The flow rate was maintained high enough so that the oxygen concentration in the gas was negligible in the casing. Fresh water was continuously fed to the unit. The oxygen content in the water at the inlet and the outlet was measured using a dissolved oxygen meter supplied by Thermo Electron Corporation (Orion model 850Aplus, range ) 0 to 20 ppm, and accuracy ) 1%). It took ∼45-60 min to attain steady state. The concentration of oxygen fell below the range of the oxygen analyzer even with five rings. Therefore, the runs were carried out using only four rings. A significant amount of mass transfer took place in the casing. To account for this, we have also carried out runs with the innermost ring. 3. Results and Discussion From the visual observations, it appeared that liquid flowed as jets from the distributor onto the inner periphery of the innermost packing ring. The liquid left the outer periphery of
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Table 3. Range of Parameters Employed in Present Work air-SO2-caustic system
parameters gas velocity (m/s) liquid velocity (m/s) rotational speed (RPM) SO2 concentration (ppm) temperature (°C) pressure (atm) nitrogen flow rate (mL/min) NaOH concentration (N)
0-4 0.0015, 0.0045 500-1300 2500-4000 25-28 1 atm
oxygen-water system 0.001-0.028 500-1300 25-28 1 atm 30
1.2-1.5
this ring as a spray of fine droplets and entered the counterrotating next ring. The width of the liquid spray, from the outermost ring onto the casing wall, was the same as the width of the rotor. The spread of individual drops hitting the casing wall could be observed at low liquid flow rates. From these observations, it appeared that the droplet size was 100 g. Much higher throughputs could be achieved using the suggested scheme of gas withdrawal in this work. This area has not received much attention. An open demonstration of process intensification in distillation and absorption on an industrial scale is not available, though claims to this effect have been made by ICI27 and, recently, by Dow Chemicals.16 4. Conclusions We have presented the individual volumetric mass-transfer coefficients for a rotating bed with split foam-metal packing. These are much higher than those reported with a single packing element and are ∼2 orders of magnitude higher. However, the advantages of split packing with counterrotation need to be established. The present study indicates a volume reduction of distillation and absorption units by 2 orders of magnitude is feasible if throughputs are enhanced. A demonstration of the volume reduction on an industrial scale is warranted. Acknowledgment We applaud RECEMAT International B. V. Holland for supplying the tailor-made foam-metal rings free of cost.
Nomenclature a ) interfacial area (m2/m3) ap ) specific surface area of the packing (m2/m3) b ) breadth of casing (m) CLi ) concentration of solute in the inlet liquid stream (mol/L) CLo ) concentration of solute in the outlet liquid stream (mol/ L) CLc ) concentration of solute in the liquid stream entering the casing (mol/L) CGi ) concentration of solute in the inlet gas stream (mol/L) CGo ) concentration of solute in the outlet gas stream (mol/L) CGc ) concentration of solute in the gas stream entering the casing (mol/L) DG ) diffusion coefficient of gas phase (m2/s) DL ) diffusion coefficient of liquid phase (m2/s) dp ) effective diameter of packing, 6(1 - )/ap G ) gas velocity (m/s) kL ) liquid-side mass-transfer coefficient (s-1) kG ) gas-side mass-transfer coefficient (s-1) kLa|c ) average liquid-side volumetric mass-transfer coefficient in casing (s-1) kGa|c ) average gas-side volumetric mass-transfer coefficient in casing (s-1) kLa ) average liquid-side volumetric mass-transfer coefficient of packing (s-1) kGa ) average gas-side volumetric mass-transfer coefficient of packing (s-1) kLla ) local liquid-side volumetric mass-transfer coefficient of packing (s-1) kGla ) local gas-side volumetric mass-transfer coefficient of packing (s-1) L ) liquid velocity (m/s) l ) length of the casing (m) QL ) volumetric liquid flow rate (m3/s) QG ) volumetric gas flow rate (m3/s) ri ) inner radius (m) ro ) outer radius (m) VS ) volume of spray between casing and outer packing ring (m3) z ) axial width of the rotor (m) Greek Symbols ) porosity of the packing FG ) gas density (m2/m3) FL ) liquid density (m2/m3) µG ) viscosity of gas (kg/m‚s) µL ) viscosity of liquid (kg/m‚s) ω ) rotational speed (rpm) Dimensionless Groups ReG ) FGQG/ap(2πrz)µG GrG ) FG2dp3rω2/µG2 ScG ) µG/FGDG ScL ) µL/FLDL
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ReL ) FLQL/ap(2πrz)µL GrL ) FL2dp3rω2/µL2 Literature Cited (1) Dhiman, S. K.; Verma, V.; Rao, D. P.; Rao, M. S. Process Intensification in a Trickle-Bed Reactor: Experimental Studies. AIChE J. 2005, 51, 3186. (2) Rao, D. P.; Bhowal, A.; Goswami, P. S. Process intensification in Rotating Packed Beds (HIGEE): An Appraisal. Ind. Eng. Chem. Res. 2004, 43, 1150. (3) Guo, K.; Guo, F.; Feng, Y.; Chen. J.; Zheng, C.; Gardener, N. C. Synchronous Visual and RTD study in Liquid Flow in Rotating Packed Bed Contactor. Chem. Eng. Sci. 2000, 55, 1699. (4) Guo, F.; Zheng, C.; Guo, K.; Feng, Y.; Gardner, N. C. Hydrodynamics and Mass Transfer in Cross Flow Rotating Packed Bed. Chem. Eng. Sci. 1997, 52, 3853. (5) Chandra, A.; Goswami, P. S.; Rao, D. P. Characteristics of flow in a rotating packed bed (HIGEE) with Split Packing. Ind. Eng. Chem. Res. 2005, 44, 4051. (6) Vidwans, A. D.; Sharma, M. M. Gas-side mass transfer coefficient in packed column. Chem. Eng. Sci. 1967, 22, 673. (7) Scheffe, R. D.; Weiland, R. H. Mass Transfer Characterisitics of Valve Trays. Ind. Eng. Chem. Res. 1987, 26, 229. (8) Weiland, R. H.; Ahlgren, K. R. Mass Transfer Characteristics of some Structured Packings. Ind. Eng. Chem. Res. 1993, 32, 1411. (9) Sandilya, P.; Rao, D. P.; Sharma, A. Gas-Phase Mass Transfer in a Centrifugal Contactor. Ind. Eng. Chem. Res. 2001, 40, 384. (10) Akita, K.; Yoshida, F. Gas Holdup and Volumetric Mass Transfer Coefficient in Bubble Columns. Ind. Eng. Chem. Proc. Des. DeV. 1973, 12, 76. (11) Deckwer, N. D.; Buckhart, R.; Zoll, G. Mixing and Mass Transfer in Tall Bubble Columns.Chem. Eng. Sci. 1974, 29, 2177. (12) 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. (13) Chen, Y. S.; Lin, C. C.; Liu, H. S. Mass Transfer in a Rotating Packed Bed with Various Radii of the Bed. Ind. Eng. Chem. Res. 2005, 44, 7868. (14) Zheng, C.; Guo, K.; Feng, Y.; Yang, C.; Gardener, N. C. M. Pressure Drop of Centripetal Gas Flow Through Rotating Beds. Ind. Eng. Chem. Res. 2000, 39, 829.
(15) Jagadeswara Reddy, K. Hydrodynamic and mass transfer studies on a novel rotating packed bed. M.Tech. Thesis, Department of Chemical Engineering, Harcourt Butler Technological Institute, Kanpur, India, 2005. (16) Trent, D.; Tirtowidjojo, D. Commercial Operation of a Rotating Packed Bed (RPB) and Other Application of RPB Technology. In Better Processes for Better ProductssProcess Intensification for the Chemical Industry, Proceedings of the 4th International Conference, Bruges, Belgium, 2001; BHR Group Limited: Cranfield, Bedfordshire, U.K., 2001; p 11. (17) Liu, H. S.; Lin, C. C.; Wu, S. C.; Hsu, H. W. Characteristics of a Rotating Packed Bed. Ind. Eng. Chem. Res. 1996, 35, 3590-3596. (18) Kelleher, T.; Fair, J. R. Distillation Studies on a High-Gravity GasLiquid Contactor. Ind. Eng. Chem. Res. 1996, 35, 4646. (19) Kumar, M. P.; Rao, D. P. Studies on a High-Gravity Gas-Liquid Contactor. Ind. Eng. Chem. Res. 1990, 29, 917. (20) Tung, H. H.; Mah, R. S. H. Modeling of Liquid Mass Transfer in Higee Separation Process. Chem. Eng. Commun. 1985, 39, 147, 1985. (21) Munjal, S.; Dudukovic, M. P.; Ramachandran, P. Mass Transfer in Rotating Packed BedssI. Development of Gas-Liquid and LiquidSolid Mass Transfer Correlations. Chem. Eng. Sci. 1989, 44 (10), 2245. (22) Singh, S. P.; Wilson, J. H.; Counce, R. M.; Villiers-Fisher, J. F.; Jennings, H. L.; Lucero, A. J.; Reed, G. D.; Ashworth, R. A.; Elliot, M. G. Removal of Volatile Organic compounds from Groundwater using a Rotary of Air stripper. Ind. Eng. Chem. Res. 1992, 31, 574. (23) Kevyani, M.; Gardener, N. C. Operating Characteristics of Rotating Beds. Chem. Eng. Prog. 1989, 85, 48. (24) Onda, K.; Takeuchi, H.; Okumoto, Y. Mass Transfer Coefficients between Gas and Liquid Phases in Packed Columns. J. Chem. Eng. Jpn. 1968, 1, 56. (25) Bravo, J. L.; Rocha, J. A.; Fair, J. R. Mass Transfer in Guaze Packings. Hydrocarbon Process. 1985, 64, 91. (26) Sherwood, T. K.; Shipley, G. H.; Holloway, F. A. L. Flooding Velocities in Packed Columns. Ind. Eng. Chem. 1938, 30, 768. (27) Ramshaw, C. Higee DistillationsAn example of Process Intensification. Chem. Eng. 1983, Feb, 389.
ReceiVed for reView December 12, 2005 ReVised manuscript receiVed February 23, 2006 Accepted March 29, 2006 IE051382Q
