Removal of SO2 from Industrial Effluents by a Novel Twin Fluid Air

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Ind. Eng. Chem. Res. 2008, 47, 7833–7840

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SEPARATIONS Removal of SO2 from Industrial Effluents by a Novel Twin Fluid Air-Assist Atomized Spray Scrubber B. Rajmohan, S. N. Reddy, and B. C. Meikap* Department of Chemical Engineering, Indian Institute of Technology (IIT), Kharagpur, Midnapur (W), West Bengal, Pin 721 302, India

Wet scrubbers are employed in process industries for scrubbing both gases and particulates. To achieve a high efficiency for scrubbers and meet pollution control standards, industries need a new type of scrubber. In this paper an attempt has been made to design a new spray scrubber by a twin fluid air-assisted atomizer for the removal of SO2. Experimental results for scrubbing of lean sulfur dioxide gas by a novel spray tower using a twin fluid air-assisted atomizer have been presented. The efficiency of the spray column was found to increase with increase in spray liquid flow rate (8.35 × 10-6-33.34 × 10-6 m3/s) and concentration of SO2 gas (400-1200 ppm). A maximum of almost 99.99% efficiency was observed for 1200 ppm at 3.354 × 10-3 m3/s gas flow rate and 33.34 × 10-6 m3/s spray liquid flow rate. Introduction Since the onset of the industrial revolution, there has been a steady change in the composition of the atmosphere, mainly due to the combustion of fossil fuels used for the generation of energy and transportation. Air pollution is a major environmental problem affecting the global environment in developing countries. The effects of air pollution on health are very complex as there are many different sources and their individual effects vary from person to person. It is not only the ambient air quality in the cities but also the indoor air quality in the rural and urban areas that are affected due to the problem of air pollution. In fact, in the developing world the highest air pollution exposures occur in the indoor environment. Air pollutants that are inhaled have a serious impact on human health, affecting the lungs and the respiratory system. These pollutants are also accumulated in soil, plants, and water, causing further increase in effects on human health. About 99% of the sulfur dioxide in air emitted from industrial sources is due to burning fossil fuels containing sulfur. In addition, during roasting and smelting of minerals containing sulfur, sulfur dioxide is emitted. Sulfur dioxide is also found in the exhaust of motor vehicles due to the ignition of fuel. The highest concentration levels of sulfur dioxide in air are found around petroleum refineries, chemical manufacturing industries, mineral ore processing plants, and thermal power stations. Sulfur dioxide emission can be reduced by burning coal with lower sulfur content. Emissions of sulfur dioxide may also be reduced by using less-polluting fuels, particularly gaseous fuel. All these are source correction techniquqes. However, if all source correction techniques fail to minimize the generation of SO2, then control techniques are used. In this direction wet scrubbers play an important role. With the passage of the 1971 Clean Air Act, American industry experienced a significantly increased need to reduce air pollution. The application of existing and new technologies resulted in the development of many air pollution control * To whom correspondence should be addressed. E-mail: bcmeikap@ che.iitkgp.ernet.in.

devices. In the recent past it has been found that conventional wet scrubbers are less effective to control both gaseous and dust particulate matter. A literature survey reveals that various wet scrubbers, viz., multistage bubble column,1 packed column,2 plate column, venturi,3 spray column,4 etc., offer choices among the scrubbing systems. Huckaby and Ray5 reported the absorption of SO2 into growing and evaporating droplets of water. Schmidt and Stichlmair6 reported experimental investigations on spray scrubbers of different sizes with cocurrent flow gas and liquid systems. The effects of different operating variables play an important role for removal of pollutants from gaseous mixtures. The literature survey also reveals that theoretical modeling and experimental studies for absorption of SO2 using

Figure 1. Schematic diagram of the experimental setup for SO2 scrubbing from flue gas.

10.1021/ie800712a CCC: $40.75  2008 American Chemical Society Published on Web 09/11/2008

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Figure 2. Twin fluid air-assisted atomizer with atomization.

water and dilute alkaline solution of sodium hydroxide give around 99% removal efficiency. Meikap et al.7 have used a horizontal cocurrent flow ejector system to scrub SO2 gas mixtures by fine droplets that absorbed SO2. Removal efficiencies on the order of 98.62% from a lean gas mixture at 840 ppm SO2 with water and 100% from a rich gas mixture at 1900 ppm SO2 with 0.005 N NaOH solution as scrubbing liquid were achieved. A very high percentage removal of SO2 was also reported from an air-SO2 mixture in a modified multistage bubble column without using any additives or pretreatment. A few modified systems such as the droplet column by Muller et al.8 and the spray column by Rajmohan et al.9,10 reported high scrubbing efficiencies for particulate matter. Absorption and Wet Scrubbing Equipment The aim of wet scrubbing equipment is to remove gaseous and particulate matter simultaneously from the exhaust stream. The absorption greatly depends on the solubility, mass transfer mechanism, and equilibrium concentration of the gas in solution. The rate of particulate matter collection at constant pressure drops is inversely proportional to the aerodynamic mean diameter of the particulate matter and scrubber droplets.

For gas removal the maximum equilibrium concentration in solution is described by Henry’s law: Cgas ) HCliquid

(1)

where H ) Henry’s constant, Cgas ) concentration in the gas stream, and Cliquid ) concentration in the liquid stream. Experimental Setup and Technique The schematic diagram of the experimental setup is shown in Figure 1. It consists of a vertical Perspex column 2.40 m long and 0.125 m in diameter, fitted with a frusto-conical top at the outlet. The maximum diameter of the frusto-conical outlet is 0.125 m, and the minimum diameter is 0.055 m. The height of the frusto-conical section is 0.128 m. Around 0.565 m from the bottom was fitted the hot air inlet duct coming through a blower of 2.2 kW capacity. SO2 is mixed with air, and they together flow in the upward direction of the column and droplets flow in the downward direction. Water is pumped from a water tank to the top of the column where it is atomized by highpressure air from a compressor. The water level in the tank is maintained constant by overflow or by adjusting water from

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Figure 3. Calibration curve of SO2 by UV spectrophotometric method.

Figure 5. Effect of liquid flow rate on percent removal efficiency of SO2 at constant gas flow rate and 800 ppm SO2.

isokinetics manner for SO2. A demister is fitted at the outlet of the frusto-conical part. The design of the setup ensures an optimum entrainment of water by the gas at high velocity and uniform dispersion and distribution of spray droplets throughout the column. The droplet diameter was predicted from the following equation as reported by Nukiyama-Tanasawa:11 d0 )

Figure 4. Effect of liquid flow rate on percent removal efficiency of SO2 at constant gas flow rate and 400 ppm SO2.

the main supply tank. A twin fluid, air blast atomizing nozzle (shown in Figure 2) is fitted in the frusto-conical section at the top of the column for disintegrating water into fine droplets. The dimensions of the atomizer are 60 mm long and 15 mm outside diameter with a 15° angle tapering ending at the mouth of the nozzle of 10 mm diameter. The mouth has an integrated opening with eight holes of 0.8 mm at the periphery for water jets and at the center a hole of 1 mm diameter for the air jet to atomize the liquid jet into fine droplets. Sample ports Si (i ) 1, 2) are connected by a probe placed in the column, and a representative sample was collected in an

[

]

( )

QL 16291.7 + 9005.8 Vr QG

1.5

(2)

where d0 ) Sauter mean droplet size (SMD; µm), Vr ) relative velocity, QG ) gas flow rate (Nm3/s), and QL ) liquid flow rate (m3/s). Experimental Procedure. Atmospheric air is allowed to pass through the spray column through a venturi mixer and an air rotameter. The air flow rate is set by a gate valve for which the SO2 flow rate is also set to give the desired concentrations (400, 800, and 1200 ppm) at the inlet of the spray column. The SO2 gas is mixed with the air stream through a venturi mixer for better mixing and uniform concentration of the gas in the air stream. The liquid and gas flow rates (AWRs ) air to water ratios) to the twin fluid air-assisted atomizer is set for scrubbing the lean SO2 gas in the spray column. The scrubbing water flow rate is varied (different AWR), and the scrubbing efficiency of the spray column for constant gas flow rate and concentration has been estimated using eq 3a. ηSO2% )

CSO2,inlet - CSO2,outlet CSO2,inlet

× 100

(3a)

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Figure 8. Effect of gas flow rate on percent removal efficiency of SO2 at constant liquid flow rate and 800 ppm. Figure 6. Effect of liquid flow rate on percent removal efficiency of SO2 at constant gas flow rate and 1200 ppm SO2.

Figure 7. Effect of gas flow rate on percent removal efficiency of SO2 at constant liquid flow rate and 400 ppm.

The representative samples are collected at the inlet and outlet sample ports (S1 and S2) of the spray column and analyzed

Figure 9. Effect of gas flow rate on percent removal efficiency of SO2 at constant liquid flow rate and 1200 ppm.

for SO2 by the “Tetrachloro Mercurate Method” (The West & Gaeke Method) (IS: 5182, Part-VI).12

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Figure 10. Effect of gas flow rate on droplet size determined by Nukiyama-Tanasawa equation. Droplet size was obtained based on the gas-liquid flow rates used in the scrubbing process.

Figure 12. Effect of gas flow rate on NTUog of spray column scrubbing at different concentrations and constant liquid flow rate of 25.0 × 10-6 m3/s.

Determination of SO2 in Flue Gases through the West and Gaeke Method. The West and Gaeke Method is applicable for the determination of sulfur dioxide (SO2) in ambient air in the concentration range of about 25-1000 µg/m3. The working sample of flue gas has the concentration range in the abovementioned range, so this method is being employed to measure the concentration. SO2 in the gas sample is absorbed in 0.1 M sodium tetrachloromercurate solution. Nonvolatile dichlorosulfitomercurate ion is formed in this process. An acid-bleached pararosaniline solution, sulfanilic acid, and formaldehyde were added to the mixture to form a complex ion, which produces red-purple pararosaniline methyl sulfonic acid. The absorbance was noted for a wavelength of 560 nm by a UV-vis spectrophotometer. This method is very sensitive and is not subject to interference from other acidic or basic gases or solids such as SO3, H2SO4, NH3, or lime dust. Even the presence of NO2 and ozone do not interfere when their concentrations are less than that of SO2 in the ambient air. Interference by NOx is eliminated by sulfamic acid and ozone. NH3, sulfides, and aldehydes do not interfere. After sample collection the solutions are relatively stable. Samples are stored at 5 °C for 30 days without any loss of SO2. The rate of decay is independent of the concentration of SO2. The SO2 concentrations were measured by using eq 3. SO2 (µg/m3) ) µg of SO2/mL × vol of absorbing reagent × 1000 Figure 11. Effect of gas flow rate on NTUog of spray column scrubbing at different concentrations and constant liquid flow rate of 33.34 × 10-6 m3/s.

vol of air sampled (L)

(3)

(SO2in ppm ) SO2 in µg/m3 × 3.82 × 10-4). A calibration

7838 Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 Table 1. Calculated Values of NTUog, HTUog, and Overall Mass Transfer Coefficient QG, 10-3 m3/s QL, 10-6 m3/s G (mol/s) Kg (cm/s) Kl (cm/s) H (mol L-1 atm-1) 3.584 3.584 3.584 4.584 4.584 4.584 5.584 5.584 5.584 3.584 3.584 3.584 4.584 4.584 4.584 5.584 5.584 5.584 3.584 3.584 3.584 4.584 4.584 4.584 5.584 5.584 5.584

16.67 25.0 33.34 16.67 25.0 33.34 16.67 25.0 33.34 16.67 25.0 33.34 16.67 25.0 33.34 16.67 25.0 33.34 16.67 25.0 33.34 16.67 25.0 33.34 16.67 25.0 33.34

0.16 0.16 0.16 0.2046 0.2046 0.2046 0.2492 0.2492 0.2492 0.16 0.16 0.16 0.2046 0.2046 0.2046 0.2492 0.2492 0.2492 0.16 0.16 0.16 0.2046 0.2046 0.2046 0.2492 0.2492 0.2492

0.62 0.619 0.617 0.704 0.701 0.698 0.777 0.773 0.77 0.62 0.619 0.617 0.704 0.701 0.698 0.777 0.773 0.77 0.62 0.619 0.617 0.704 0.701 0.698 0.777 0.773 0.77

0.7 2.28 3.78 5.28 6.78 8.28 9.78 11.28 15 0.7 2.28 3.78 5.28 6.78 8.28 9.78 11.28 15 0.7 2.28 3.78 5.28 6.78 8.28 9.78 11.28 15

curve has been constructed as shown in Figure 3 to find out the SO2 concentrations at the inlet and at the outlet. Results and Discussion SO2 scrubbing in the spray column with tap water was conducted to study the effect of gas and liquid flow rates on the SO2 removal efficiency by using a twin fluid air-assisted nozzle.

Figure 13. Effect of gas flow rate on NTUog of the spray column scrubbing at different concentrations and constant liquid flow rate of 16.67 × 10-6 m3/s.

1.24 1.24 1.24 1.24 1.24 1.24 1.24 1.24 1.24 1.24 1.24 1.24 1.24 1.24 1.24 1.24 1.24 1.24 1.24 1.24 1.24 1.24 1.24 1.24 1.24 1.24 1.24

KG (cm/s) 0.416 320 8 0.238 909 2 0.170 014 9 0.135 64 0.110 260 4 0.092 880 8 0.081 223 4 0.071 381 2 0.054 921 5 0.416 320 8 0.238 909 2 0.170 014 9 0.135 64 0.110 260 4 0.092 880 8 0.081 223 4 0.071 381 2 0.054 921 5 0.416 320 8 0.238 909 2 0.170 014 9 0.135 64 0.110 260 4 0.092 880 8 0.081 223 4 0.071 381 2 0.054 921 5

A (cm2/m3) volume (cm3) 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000 7 500 000

0.024 0.024 0.024 0.024 0.024 0.024 0.024 0.024 0.024 0.024 0.024 0.024 0.024 0.024 0.024 0.024 0.024 0.024 0.024 0.024 0.024 0.024 0.024 0.024 0.024 0.024 0.024

NTUog

HTUog

4.683 608 4 2.687 728 5 1.912 667 9 1.193 064 2 0.969 829 3 0.816 962 4 0.586 483 9 0.515 417 0.396 568 1 4.683 608 4 2.687 728 5 1.912 667 9 1.193 064 2 0.969 829 3 0.816 962 4 0.586 483 9 0.515 417 0.396 568 1 4.683 608 4 2.687 728 5 1.912 667 9 1.193 064 2 0.969 829 3 0.816 962 4 0.586 483 9 0.515 417 0.396 568 1

0.418 480 8 0.729 240 3 1.024 746 6 1.642 828 6 2.020 974 1 2.399 131 3 3.341 950 3 3.802 746 4.942 404 0.418 480 8 0.729 240 3 1.024 746 6 1.642 828 6 2.020 974 1 2.399 131 3 3.341 950 3 3.802 746 4.942 404 0.418 480 8 0.729 240 3 1.024 746 6 1.642 828 6 2.020 974 1 2.399 131 3 3.341 950 3 3.802 746 4.942 404

Effect of Liquid Flow Rate on the Percentage Removal of SO2. The effect of liquid flow rate on the percentage removal efficiency at three different gas flow rates and at constant inlet SO2 concentration is shown in Figure 4. It can be seen from Figure 4 that the removal efficiency increases as the liquid flow rate increases. This may be because of an increase in the droplet-gas interfacial contact area as the number of droplets increases with respect to flow rate. Thus the percentage removal efficiency increases with an increase in liquid flow rate. This

Figure 14. Effect of gas to liquid flow rate ratio on NTUog of the spray column.

Ind. Eng. Chem. Res., Vol. 47, No. 20, 2008 7839

of SO2 gas at 800 and 1200 ppm. Results indicate a trend similar to that discussed earlier. Effect of Gas Flow Rate on Droplet Size Distribution. Figure 10 represents the droplet size distribution with the variation of gas flow rate. It is very interesting to note that as the air flow rate through the atomizer was increased the droplet size gradually decreases, which is very much desired to get high separation efficiency. The average droplet diameter in the present case was found to be in the range 100-220 µm. Number of Transfer Units (NTU) Calculation The mass transfer of SO2 gas into liquid droplets in a spray scrubber is expressed in terms of the number of transfer units (NTUog): NTUog ) ln(SO2 inlet/SO2 outlet) ) (KGAPV)/G

(4)

where SO2 inlet ) inlet concentration of sulfur dioxide gas (ppmv), SO2 outlet ) outlet concentration of sulfur dioxide gas (ppmv), KG ) overall mass transfer coefficient (mol/cm2 · s · atm), A ) interfacial mass transfer area per unit volume (cm2/m3), P ) absolute scrubber pressure (atm), V ) scrubber volume (m3), and G ) molar gas flow rate (mol/s). The overall mass transfer coefficient can be calculated from the individual mass transfer film coefficients in the gas and liquid sides as shown in eq 5: KG ) Figure 15. Comparison of experimental and empirical efficiencies of the removal efficiency of SO2.

figure also reveals that the removal efficiency of 400 ppm SO2 is significantly high as the efficiency increases from 65% to 81% for liquid flow rates ranging from 16.67 × 10-6 m3/s to 33.34 × 10-6 m3/s for a gas flow rate of 3.584 × 10-3 m3/s. The results for the SO2 scrubbing for gas flow rates 4.584 × 10-3 m3/s and 5.584 × 10-3 m3/s showed a decrease in the efficiency as shown in Figure 4. Figure 5 represents the effect of liquid flow rate on SO2 removal efficiency for scrubbing of SO2 gas by the spray system at 800 ppm. The rate of increase in the efficiency seems to be gradual and almost linear. Due to the increase in the concentration, the efficiency range also seems to be higher than the previous one. The efficiency ranges from around 85% to 95.75% for the liquid flow rate of 33.34 × 10-6 m3/s and gas flow rate of 3.584 × 10-3 m3/s. Figure 6 represents the effect of liquid flow rate on SO2 removal efficiency for scrubbing of SO2 gas at 1200 ppm. It has been found that the efficiency observed was still higher for scrubbing of SO2 than that at 800 ppm. An efficiency of almost 99.98% of SO2 was observed for the liquid flow rate of 33.34 × 10-6 m3/s and gas flow rate of 3.584 × 10-3 m3/s at 1200 ppm. As the gas flow rate is increased, the efficiency also decreases. Effect of Gas Flow Rate on the Percentage Removal of SO2. The percentage removal efficiency of SO2 gas at 400 ppm under different gas flow rates in the spray scrubber has been plotted against gas flow rates and is shown in Figure 7. It has been observed that the SO2 removal efficiencies decrease with increase in the gas flow rate. This is because of as the gas flow rate increases the time of contact between the gas and the liquid droplets becomes shorter, and thus the absorption of the gas phase SO2 into the liquid phase water droplet decreases, leading to the decrease in the removal efficiency of SO2. Figures 8 and 9 present the effect of gas flow rate on the scrubbing efficiencies

1 1 H + Kg EKl

(5)

where Kg ) local gas phase mass transfer coefficient (mol/ cm · s2 · atm), Kl ) local liquid phase mass transfer coefficient (0.07-1.5 cm/s), E ) enhancement factor to account for diffusion of SO2 through the liquid as bisulfite and sulfite (E ) 1.1), and H ) Henry’s constant for SO2 in the scrubber liquid (H2O) (H ) 1.2 mol L-1 atm-1). The KG values are presented in Table 1. KG ) 0.4163 cm/s, Kg ) 0.4163 × 10-5 mol/cm2 · s · atm, actual interfacial area for absorption ) 75 × 104 cm2 /(QG, m3), QG ) 3.585 × 10-3 m3/s, pressure ) 1 atm, G ) molar gas flow rate (mol/s) ) 0.156 25, NTUog ) (KGAPV)/G ) 4.683, and HTUog ) Z/NTUog ) 1.96 (m)/4.683 ) 0.418. Figures 11, 12, and 13 show the effect of gas flow rate on NTUog of the scrubber at various inlet SO2 concentrations in the range 400-1200 ppm under different constant liquid flow rates. It has been found that as the inlet concentration of SO2 increases the NTUog value considerably increases. It is very interesting to note that at lower concentrations of SO2 the change in NTUog is negligible, whereas at higher concentration it changes very sharply. Figure 14 represents the effect of gas to liquid flow rate on the NTUog of the spray scrubber. It has been found that as the ratio of liquid to gas flow rate increases the NTUog sharply increases almost linearly. Development of Correlation for Removal Efficiency A dimensional analysis has been made to develop the empirical model for the prediction of SO2 removal efficiency. The Buckingham π theorem was used to develop the correlation, and the coefficients of the dimensional groups were estimated by multiple regression analysis. The parameters chosen for the dimensional analysis are as follows: (a) physical parameters: (i) inlet concentration of SO2 (CSO2(i)), (ii) gas viscosity (µG), (iii) liquid viscosity (µL), (iv) liquid surface tension (σL), (v) gas density (FG), (vi) liquid

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density (FL), (vii) diffusivity of SO2 in air (DSO2A), (viii) diffusivity of SO2 into liquid droplet (DSO2L) (b) geometric parameters: (ix) Sauter mean diameter of the droplet (d0), (x) spray column diameter (DC), (xi) height of the spray column (HC) (c) flow parameters: (xii) superficial liquid velocity (VL), (xiii) superficial gas velocity (VG). Since the velocity of the liquid droplet is not known, the velocity of the liquid to the atomizer has been used in this analysis.

([ ] [ ] [

ηSO2 ) f

DSO2A

a

VLDC

[(

DSO2W VLDC

b

σL VL2DCFL

] [ ] [ ] [ ]) c

CSO2 FL

d

VG VL

e

d0 DC

f

(6)

ηSO2 ) 0.18 exp - [EuSO2A]-0.372[EuSO2L]0.335[WeL]-0.122

[ ] [ ] [ ] )] CSO2 FL

-0.539

VG VL

0.425

d0 DC

-0.042

(7)

where ηSO2 (%) ) percentage removal efficiency of the SO2, EuA ) Euler number of gas (air), EuL ) Euler number of liquid, and WeL ) Weber number of liquid. The empirical equation obtained is given in eq 6, and the empirical model along with coefficients is given in eq 7. The regression coefficient was 0.99 and the root-mean-square error (rmse) was 0.0394. Figure 15 gives the comparison of both experimental and empirically predicted values of the spray scrubbing efficiencies of SO2. Conclusions A novel spray scrubber with a twin fluid, air blast atomizer has been designed and fabricated. The removal efficiency is found to be a function of inlet SO2 concentration, the air flow rate, and the liquid flow rate. The outcome of the results are summarized as follows: (1) The experimental investigation shows that a very high percentage removal of SO2 from 62.54% to 98.98% can be achieved from a lean air-SO2 mixture (400-1200 ppm SO2) without using any additives or pretreatment. (2) The increase in the efficiency with respect to concentration is mainly due to the increase in the concentration gradient

between the liquid and gas phase SO2 concentrations, thus increasing the driving force for more absorption of SO2 into the liquid droplets. (3) The results also indicate that an increase in liquid flow rate increases the SO2 removal efficiency, whereas an increase in the gas flow rate decreases the removal efficiency. (4) A correlation has been proposed to predict the SO2 removal efficiency under various operating conditions. The predicted removal efficiency agrees well with the experimentally observed values. Literature Cited (1) Meikap, B. C.; Biswas, M. N. Fly-ash Removal Efficiency in a Modified Multistage Bubble Column Reactor. Sep. Purif. Technol. 2004, 36, 177. (2) Zenz, F. A. Designing Gas Absorption Towers. Chem. Eng. Process. 1972, 79, 120. (3) Wen, C. Y.; Fan, L. T. Models for Flow Systems and Chemical Reactors; Dekker: New York, 1975. (4) Mehta, K. C.; Sharma, M. M. Mass Transfer in Spray Column. Br. Chem. Eng. 1970, 15, 1440. (5) Huckaby, J. L.; Ray, A. K. Absorption of Sulfur dioxide by Growing and Evaporating Water Droplets. Chem. Eng. Sci. 1989, 44, 2797. (6) Schmidt, B.; Stichlmair, A. Two Phase Flow and Mass Transfer in Scrubbers. J. Chem. Eng. Technol. 1991, 14, 162. (7) Meikap, B. C.; Satyanarayan, S.; Nag, A.; Biswas, M. N. Scrubbing of Sulfur dioxide from Waste Gas Stream by Horizontal Co-current Flow Ejector System. Indian J. EnViron. Prot. 1999, 19, 523. (8) Muller, N.; Benadda, B.; Otterbein, M. Mass Transfer in a Droplet Column in Presence of Solid Particles. Chem. Eng. Process. 2001, 40, 167. (9) Raj Mohan, B.; Biswas, S.; Mohanty, C. R.; Meikap, B. C. Control of Air Pollutants bya Gas-Liquid Contacting Towers. Int. J. Chem. Sci. 2007, 59, 665. (10) Raj Mohan, B.; Biswas, S.; Mohanty, C. R.; Meikap, B. C. Simultaneous Control of Gaseous and Dust Air Pollutants by a Gas-Liquid Contacting Towers. Process Plant Eng. 2007, 35, 28. (11) Nukiyama, S.; Tanasawa, Y. Experiment on Atomization of Liquid by means of Air Stream. Trans. Soc. Mech. Eng. Jpn. 1938, 4, 86. (12) Indian Standards, Method for Measurement of Air Pollution, Sulfur dioxide, Part VI; IS: 5182; 1969; p 4.

ReceiVed for reView February 26, 2008 ReVised manuscript receiVed July 13, 2008 Accepted July 25, 2008 IE800712A