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CORRESPONDENCE Comments on “Removal of SO2 from Industrial Effluents by a Novel Twin Fluid Air-Assist Atomized Spray Scrubber” Amitava Bandyopadhyay* Department of Chemical Engineering, UniVersity of Calcutta, 92 APC Road, Kolkata 700 009, India Sir, Rajmohan et al. presented in their article1 a study on the removal of SO2 from industrial effluents by a novel twin fluid air-assist atomized spray scrubber. The title reflected that the investigation was on an industrial system, but a thorough reading of the article revealed that the experiments were carried out on a simulated system in the laboratory. Therefore, the title is not correct in respect to the research investigation reported. In the Introduction section, the background research work on SO2 scrubbing was appraised six times on different types of devices2-8 other than spray columns and twice with spray columns.9,10 Furthermore, particulate scrubbing has been described as background research in connection with the presented gas scrubbing thrice.6-8 This is not correct since the physics of particle collection and SO2 scrubbing are different. Interestingly, the study of a “modified multistage bubble column2” was referenced as background research work for “spray columns” in the article under discussion. This is also not correct, since the hydrodynamic regime of these two systems is entirely different. Legions of research investigations have been reported in the literature11-14 on spray scrubbing of SO2, but these were not referenced. In the Experimental Setup and Technique section, it was reported that the experimental column was a “vertical Perspex column” and “the bottom was fitted the hot air inlet duct coming through a blower of 2.2 kW capacity”. Clearly, this indicated that the scrubbing of hot gas was studied in the Perspex column. It is well-known that Perspex would soften and lose its form or shape if subjected to higher temperatures for any period of time. This issue of deformation of Perspex at higher temperatures cannot be ruled out unless the composition of the Perspex material, its glass transition temperature, and the operating temperature of scrubbing are all reported. Surprisingly, Figure 1 did not indicate the “hot air inlet duct coming through a blower”. Furthermore, droplet evaporation would also take place during hot gas scrubbing. Existence of an effective droplet phase in the column would not be conceivable in such hot conditions unless otherwise a heat balance is offered. In the Experimental Setup and Technique section, it was also reported that “sample was collected in an isokinetics manner for SO2”. This is not incorrect. Sampling efficiency in gaseous sampling is always 1, and hence, isokineticity is not a matter of concern in such a situation.15 On the other hand, sampling efficiency in particulate sampling is always 1 for all particles only in the case of isokinetic conditions, and as a result, it is carried out in isokinetic conditions. Hence, the statement made is incorrect. The use of the word * To whom correspondence should be addressed. E-mail: amitava.
[email protected].
“isokinetics” is also not correct. In this section, it was further reported that “The droplet diameter was predicted from the following equation as reported by Nukiyama-Tanasawa”. The correlation proposed by Nukiyama and Tanasawa is generally applied in the case of atomization in venturis. The application of the correlation in a twin fluid atomizer is not done owing to its different process of atomization than in the case of venturis. It is interesting to note here that the same atomizer was used for the atomization reported by Rajmohan et al.16,17 within the same range of parameters; for example, the atomizer dimension and the atomizing parameters. Under these circumstances, the relative velocity “(Vr)” below 100 m/s reported16 was also applicable to the presented article. However, the correlation proposed by Nukiyama and Tanasawa is valid for relative velocities (Vr) ranging between 100 m/s and sonic velocities. Hence, the applicability of the Nukiyama and Tanasawa correlation in the presented article is not correct. Comparison of Figures 4 and 7 revealed a very interesting facet of the article. The data retrieved from these figures are presented in Table 1. It can be seen from the table that the same experimental conditions yielded different percentages of removal of SO2 in the same spray column. This is not correct. Similarly, Figures 5, 6, 8, and 9 are not correct. Furthermore, the exponents in Figures 4-9 for gas and liquid flow rates either in the legend or in the x-axis are incoherent. In the subsection Effect of Liquid Flow Rate on the Percentage Removal of SO2 in the Result and Discussion section, the values of removal efficiency reported are not matching with the values in the respective figures. The values reported in the text are incorrect. This is presented in Table 2. It can be seen from the table that the values reported in the text were entirely different from those readable from the respective figures. In the subsection Effect of Gas Flow Rate on Droplet Size Distribution in the Result and Discussion section, it was reported that “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 values of gas flow rate in Figure 10 resemble the values of flow Table 1. Comparison of Figures 4 and 7a parameters SO2 removal efficiency for QG ) 3.584 × 10-3 m3/s QL ) 16.67 × 10-6 m3/s inlet SO2 concentration 400 ppm a
Figure 7 is a cross-plot of Figure 4.
10.1021/ie801789d CCC: $40.75 2009 American Chemical Society Published on Web 05/07/2009
figure 4
figure 7
82.5%
69%
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Table 2. Critical Examination of Removal Efficiency Data Reported in Subsection Effect of Liquid Flow Rate on the Percentage Removal of SO2 and in Figures 4-6 findings from the figure and comparison with values in the text
statement made in text by the authors
related figure no.
“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” “The efficiency ranges from around 85% to 95.75% for the liquid flow rate of 33.34 × 10-6 m3/s and gas flow” “An efficiency of almost 99.98% of SO2 was observed for the liquid flow rate of 33.34 × 10-6 m3/s and”
Figure 4
It showed a maximum value in the range of about 82-95% instead of 65-81% SO2 removal efficiency.
Figure 5
It showed a maximum value in the range of about 69-82% instead of 85%-95.75% SO2 removal efficiency. It showed a maximum value of about 97.5% instead of 99.98% SO2 removal efficiency.
Literature Cited
Figure 6
rate of the simulated waste gas stream that was introduced at the bottom of the spray scrubber. Therefore, the atomizing airflow rates were identical to the flow rates of the waste gas stream. This indicates that the atomizing airflow rate was 92.4925-668.2635 times higher than the liquid flow rate. An atomization at air to water ratio of “668.2635” is not possible. The legend of Figure 10 reveals “Inlet solid loading S × 103 ) 0.00 kg/m3”. This legend is not necessary since only scrubbing of SO2 gas with water was conducted. Interestingly, an identical figure can be seen in the investigation reported by Rajmohan et al.18 [See Figure 9]. In the subsection Number of Transfer Units (NTU) Calculation in the Result and Discussion section, it was necessary to write the expression for calculating values of mass transfer parameters since mass transfer studies were neither reported in the article nor were they referenced. In Figure 15, the x-axis is much too compressed leading to a deviation that is less than what it would be. Two dimensionless numbers like the Weber number and Euler number were mentioned in eq 7. The “Column Diameter” was included in the Weber number. The physical significance of the Weber number would have been justified in this case if “a characteristic length of the flowing stream” was selected as per the definition instead of the column diameter. Hence, the Weber number as defined is not correct. On the other hand, the Euler number as defined
[
of the pressure force to the inertia force. However, the parameters used in the Euler number in this case suggest that this dimensionless group could be appropriately designated as “(modified) Dispersion number”. In the Conclusions section it was reported that “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)”. The efficiency of 98.98% is not available in the text of the article.
]
diffusivity of SO2 in air or in water superficial liquid velocity × spray column diameter is also not correct since the Euler number is defined as the ratio
(1) Rajmohan, B.; Reddy, S. N.; Meikap, B. C. Removal of SO2 from Industrial Effluents by a Novel Twin Fluid Air-Assist Atomized Spray Scrubber. Ind. Eng. Chem. Res. 2008, 47 (20), 7833. (2) Meikap, B. C.; Biswas, M. N. Fly-ash Removal Efficiency in a Modified Multistage Bubble Column Reactor. Sep. Purif. Technol. 2004, 36, 177. (3) Zenz, F. A. Designing Gas Absorption Towers. Chem. Eng. Process. 1972, 79, 120. (4) Wen, C. Y.; Fan, L. T. Models for Flow Systems and Chemical Reactors; Dekker: New York, 1975. (5) 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. (6) Muller, N.; Benadda, B.; Otterbein, M. Mass Transfer in a Droplet Column in Presence of Solid Particles. Chem. Eng. Process. 2001, 40, 167. (7) Raj Mohan, B.; Biswas, S.; Mohanty, C. R.; Meikap, B. C. Control of Air Pollutants by a Gas-Liquid Contacting Towers. Int. J. Chem. Sci. 2007, 59, 665. (8) Rajmohan, 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. (9) Mehta, K. C.; Sharma, M. M. Mass Transfer in Spray Column. Br. Chem. Eng. 1970, 15, 1440. (10) Schmidt, B.; Stichlmair, A. Two Phase Flow and Mass Transfer in Scrubbers. J. Chem. Eng. Technol. 1991, 14, 162. (11) Kohl, A. L.; Reisenfeld, F. C. Gas Purification; McGraw Hill Book Company, Inc.: New York, 1985. (12) Brogren, C.; Hans, T. K. Modeling the absorption of SO2 in a spray scrubber using the penetration theory. Chem. Eng. Sci. 1997, 52, 3085. (13) Chen, W. H. Unsteady absorption of sulfur dioxide by an atmospheric water droplet with internal circulation. Atmos. EnViron. 2001, 35, 2375. (14) Chen, W. H. Dynamics of sulfur dioxide absorption in a raindrop falling at terminal velocity. Atmos. EnViron. 2001, 35, 4777. (15) Zhang, Y. Indoor Air Quality Engineering: CRC Press: New York, 2005; p 213. (16) Rajmohan, B.; Jain, R. K.; Meikap, B. C. Comprehensive analysis for prediction of dust removal efficiency using twin-fluid atomization in a spray scrubber. Sep. Purif. Technol. 2008, 63, 269. (17) Rajmohan, B.; Meikap, B. C. Performance characteristics of the particulate removal in a novel spray-cum-bubble column scrubber. Chem. Eng. Res. Des., available online May 11,http://dx.doi.org/10.1016/ j.cherd.2008.05.011. (18) Rajmohan, B.; Biswas, S.; Meikap, B. C. Performance characteristics of the particulates scrubbing in a counter-current spray-column. Sep. Purif. Technol. 2008, 61, 96.
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