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Effect of aerosol loading on breakthrough characteristics of activated charcoal cartridges
Journal of Aerosol Science 55 (2013) 57–65
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Journal of Aerosol Science journal homepage: www.elsevier.com/locate/jaerosci
Effect of aerosol loading on breakthrough characteristics of activated charcoal cartridges Yu-Mei Kuo a, Chih-Wei Lin b, Sheng-Hsiu Huang b, Kuang-Nan Chang b, Chih-Chieh Chen b,n a
Department of Occupational Safety and Health, Chung-hwa University of Medical Technology, 89, Wen-Hwa 1st St., Jen-Te District, Tainan 712, Taiwan, ROC National Taiwan University, Institute of Occupational Medicine and Industrial Hygiene, College of Public Health, 17 Xu-Zhou Road, Room 718, Taipei 10055, Taiwan, ROC
b
a r t i c l e i n f o
abstract
Article history: Received 8 April 2012 Received in revised form 31 July 2012 Accepted 18 August 2012 Available online 25 August 2012
Charcoal cartridges are commonly employed to provide protection against a wide range of organic vapors at workplace owing to their excellent adsorption. In practical situations, not only organic vapors but also aerosol particles may exist in the workplace atmosphere. Focusing on aerosol particles, this study aimed to evaluate the effect of aerosol loading on both sorption capacity and breakthrough time of activated charcoal. Polydisperse corn oil aerosols with a mode of 0.2 mm and monodisperse acrylic powder of 0.15 and 10 mm were generated to load the test activated charcoal separately for conducting vapor sorption experiments. For liquid aerosol loading tests, the sorption capacities reduced by 14%, 38% and 46% and the 10% breakthrough times were shorten by 14%, 28% and 75% after loading 0.06, 0.14 and 0.25 g of corn oil aerosols per gram charcoal, respectively. For charcoal loaded with solid aerosols, the sorption capacity remained unchanged whereas the 10% breakthrough times were reduced by 26%, 27% and 35% after loading 0.019, 0.027 and 0.029 g of 0.15-mm acrylic powder per gram charcoal, and by 5%, 43% and 44% after loading 0.074, 0.13 and 0.18 g of 10-mm acrylic aerosols per gram charcoal, respectively. It was concluded that the breakthrough characteristics were affected by aerosol loading and the sorption capacity of activated charcoal can only be reduced by the loading of liquid aerosols. For the use of gas respirators with activated charcoal cartridges, it should be noted that the service life of the charcoal cartridges could be shortened due to aerosol loading in a work environment, from the perspective of respiratory protection. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Activated charcoal Breakthrough time Aerosol loading Sorption capacity
1. Introduction The sorption of organic vapors to activated charcoals occurs mainly through physical adsorption. The equilibrated amount, i.e., the sorption capacity, depends on the adsorption isotherm. Owing to the excellent adsorption capacity of activated carbon for organic vapors, charcoal cartridges are commonly used in gas respirators to protect workers’ health in the industrial environment. Charcoal has a porous structure with variable gaps of molecular dimensions, being the micropores (Innes et al., 1989). The micropores of charcoal, generally in slit-shape, are characterized by the slit width in the size range of 0.4 to 2 nm and represent the bulk of the adsorption capacity. The mesopores with size varying from 2 to
n
Corresponding author. Tel.: þ886 2 33668086; fax: þ 886 2 23938631. E-mail address: [email protected] (C.-C. Chen).
0021-8502/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jaerosci.2012.08.002
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50 nm are important for transport properties while the macropores denote those pores whose size exceed 50 nm. Micropores have higher surface area to pore volume ratios than mesopores and macropores. Micropores and available contact surfaces in the adsorbents play a dominant role in the adsorption and it is basically a process of volume filling. Then adsorption begins to shift gradually to multilayer adsorption in the meso- and macropores as the vapor pressure continues to increase. The adsorption behavior in mesopores depends not only on vapor-wall interactions but also on the attractive forces between vapor molecules, which may lead to capillary (pore) condensation (Stoeckli, 1990). Previous studies have concluded that two major barriers to gas molecule diffusion existed, namely pore entry and diffusion along the pore. The rate-limiting step in highly microporous carbons was pore entry (Rao, 1985). A variety of studies on the factors affecting adsorption capacity and/or breakthrough time of a charcoal cartridge have been carried out (Nelson & Correia, 1976; Yoon & Nelson, 1990, 1992; Yoon et al., 1996). Studies showed that the breakthrough characteristics of charcoal cartridges may be affected by the constituent(s) and the concentration(s) of the vapor(s) adsorbed (Nelson and Harder, 1974; Nelson and Correia, 1976; Tanaka et al., 1999; Dharmarajan et al., 2001), temperature (Nelson et al., 1976; James et al., 1984; Wood, 1985), moisture content in the gas flow in terms of relative humidity (Nelson et al., 1976; Yoon & Nelson, 1990; Tsai, 1994; Wood, 2004; Kaplan, 2006; Li, 2008; Ye, 2008; Cao, 2010; Bradley, 2011), gas flow rate (Nelson & Correia, 1976), and charcoal packing density (Trout, Breysse et al., 1986). A large number of previous studies have been on the performance of charcoal on vapor adsorption and influencing factors. However, little is known about the effect of aerosols deposited on the surface of activated charcoals. For practical situations, not only organic vapors but also aerosol particles may exist in the atmosphere at workplace. Subsequently, aerosol particles might penetrate and deposit on the charcoal inside the respirator cartridge and, therefore, induce extra air resistance. Moreover, the particles deposited on the surface of the granulated activated carbon may alter the breakthrough characteristics, thus affecting the service life of charcoal cartridges which is of great importance for respiratory protection in industrial applications. Therefore, this study aimed to evaluate the effect of aerosol loading on the breakthrough characteristics of activated charcoal. 2. Materials and methods 2.1. Test activated charcoal The activated charcoal taken from the same lot of commercially available respirator cartridges (3M 6001, St. Paul, MN, USA) was employed to conduct the experiments in this work. This organic vapor cartridge was approved by the National Institute of Occupational Safety and Health (NIOSH). The granular size of the charcoal was measured by US standard testing sieves to be in the size range of 12–20 mesh (equivalent to 0.84–1.68 mm). Under the STP condition, the N2-BET specific surface area was 1319 m2/g. The micropore volume and the micropore area were 0.48 cm3/g and 1047 m2/g, respectively. The granular charcoal was prepared by drying at 150 1C for at least 2 h, followed by purging with clean filtered air for another 2 h prior to test. After being preparation, the charcoal was packed in a cylindrical stainless steel sample holder, which is 7.16 cm in diameter and 2.56 cm in height. The homemade cartridge accommodated 35 g of granular charcoal with a calculated packed density of 0.34 g/cm3 for all the tests unless otherwise specified. The homemade charcoal cartridge was first tested for its sorption efficiency as a function of time to obtain the characteristic breakthrough curve of the cartridge before aerosol loading. Then the used cartridge was regenerated by following the same protocol of charcoal preparation as aforementioned. After regeneration, the charcoal cartridge was loaded with a given amount of aerosols in specified size range and was tested for the sorption efficiency again to obtain the breakthrough curve of the aerosol loaded cartridge. 2.2. Cyclohexane sorption test The operating conditions for the sorption efficiency tests followed mainly the Chinese National Standard certification method (CNS 6636 Z2023): 3000 ppm cyclohexane as the challenge agent (Furuse et al., 2001), 15 L/min gas flow rate and 50 75% relative humidity. Fig. 1 shows the schematic diagram of the experimental system set-up for charcoal sorption and aerosol loading tests. Cyclohexane vapor was generated by passing 15 L/min filtered air through the surface of liquid cyclohexane in a jar dipped in a water bath. The temperatures of the cyclohexane bath and water bath were set the same, 37 70.5 1C. The cyclohexane vapor concentration was controlled at 3000 ppm by refilling the escaped cyclohexane at an injection rate of 0.204 mL/min using a syringe pump. The relative humidity of the gas flow was monitored by a hygrometer and was maintained at 50 71% at 25 1C by passing the filtered air over a water reservoir. The gas mixture then passed through the test charcoal cartridge. The cyclohexane concentrations upstream and downstream of the cartridge were monitored by a flame ionization detector (FID, model 2005A, China Chromatography Co., Taiwan). The cyclohexane breakthrough curve as a function of time was characterized by calculating the ratio of downstream vapor concentration (C) in ppm to upstream vapor concentration (C0) in ppm. The sorption capacity of activated charcoal in mg-cyclohexane/g-charcoal was calculated by dividing the adsorbed amount of cyclohexane vapor by the amount of activated charcoal used in the cartridge. The adsorbed amount was estimated by the following equation. ! n X Q M:W: ðC i þ 1 þ C i ÞDt a W ads ¼ C 0 t ads 103 24:46 2 i¼1
Y.-M. Kuo et al. / Journal of Aerosol Science 55 (2013) 57–65
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Fig. 1. Schematic diagram of the experimental system set-up for charcoal adsorption and aerosol loading test.
where Wads is the adsorbed amount in mg; Q is the air flow rate in L/min; M.W. is the molecular weight; C0 is the challenge vapor concentration in ppm; tads is the adsorption period in min; Ci is the concentration of the ith breakthrough sample determined by the FID from the beginning of adsorption in ppm; and Dta is the sampling interval of the adsorption process in min which is 1 in the present study. To evaluate the sorption curve of activated charcoal quantitatively, the 10%, 50% and 90% breakthrough times were determined. The 10%, 50% and 90% breakthrough times were defined as the times when the downstream vapor concentration reached 10%, 50% and 90% of the challenged vapor concentration, respectively. 2.3. Aerosol penetration test Aerosol penetration test was conducted to gain the information regarding aerosol penetration through the charcoal cartridge as a function of particle size. This information was essential and provided the basis for our further study on aerosol loading effect. Fig. 2 shows the schematic diagram of experimental setup for aerosol penetration test. Corn oil was chosen as the challenge. A constant output atomizer (model 3076, TSI Inc., St. Paul, MN) and an ultrasonic atomizing nozzle (model 8700-120, Sonotek Inc., Highland, NY) were utilized to generate polydisperse submicrometer-sized and micrometer-sized aerosol particles, respectively. To avoid the particle charge effect, the aerosol output was passed through a Kr-85 radioactive source to neutralize the aerosol particles to the Boltzmann charge equilibrium. Two aerosol size spectrometers were used. They included a scanning mobility particle sizer (SMPS, model 3934, TSI Inc.) for particles smaller than 0.8 mm and an aerodynamic particle sizer (APS, model 3321A, TSI Inc.) for particles larger than 0.8 mm. These two instruments measured both aerosol concentrations and size distributions upstream and downstream of the charcoal cartridge. Although SMPS and APS size particles by different principles, data combination has been shown feasible after proper calibration and operation (Kuo et al., 2005). The aerosol penetration efficiency as a function of aerosol size was calculated as the ratio of the downstream aerosol concentration to upstream aerosol concentration in a given size range across the charcoal cartridge. 2.4. Aerosol loading test Corn oil and acrylic powder (MP series, Soken Chemical & Engineering Co., Tokyo, Japan) were employed to generate liquid and solid aerosols, separately. Acrylic powder has been tested for cyclohexane adsorption and no significant adsorption was found for a test period of 120 min at a flow rate of 15 L/min and 3000-ppm cyclohexane. Corn oil was atomized by a constant output atomizer (model 3076, TSI Inc., St. Paul, MN, USA) to generate polydisperse submicrometersized droplets with a mode of 0.2 mm and geometric standard deviation of 1.96. A dust feeder (RGB1000, Palas Gmbh, Karlsruhe, Germany) was utilized to generate monodisperse 0.15- or 10-mm acrylic powders, respectively. Under the designed flow rate (15 L/min in this study), the dominating filtration mechanisms of these two sizes of powders should be
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Fig. 2. Schematic diagram of experimental system set-up for aerosol penetration test.
distinctly different; that is, diffusion for 0.15-mm and inertial impaction for 10-mm. The loading amounts were 0.06, 0.14 and 0.25 g/g-charcoal for corn oil aerosols, 0.019, 0.027 and 0.029 g/g-charcoal for 0.15-mm acrylic powder aerosols and 0.074, 0.13 and 0.18 g/g-charcoal for 10-mm acrylic powder aerosols. The pressure drop across the charcoal cartridge was recorded by a pressure transducer (OMEGA, PX164-010D5V), which was calibrated against an inclined manometer. The amount of aerosol loaded in the test charcoal cartridge was determined by gravimetric analysis.
3. Results and discussion 3.1. The adsorption characteristics of activated charcoal cartridges Commercially available respirator cartridges were used in the present study. The variability of adsorption characteristics of five charcoal cartridges was shown in Fig. 3. The adsorption capacity of the cartridges was estimated to be 361 76 mg-cyclohexane/g-charcoal. The 10%, 50% and 90% breakthrough times were 60 714, 84 74 and 98 72 min, respectively. The pressure drop across the cartridges at 15 L/min gas flow rate was 3.670.8 mm H2O while the increase in pressure drop throughout the whole adsorption process was no more than 3%. The results demonstrated significant variations in adsorption capacity and breakthrough time among tested cartridges. Owing to the variations of performance of charcoal cartridges, the charcoals of the same lot were pooled and mixed for charcoal preparation. If possible, the same cartridge of charcoals was repeatedly used for adsorption tests after regeneration. The effect of regeneration on both adsorption capacity and breakthrough characteristics of activated charcoals after repeated regeneration for four times is shown in Fig. 4. The adsorption capacity of the cartridge was 355 72 mg-cyclohexane/g-charcoal. The 10%, 50% and 90% breakthrough times were 57 72, 8372 and 10172 min, respectively. The experimental results showed an excellent reproducibility of adsorption capacity and breakthrough curve of regenerated charcoals, indicating the effectiveness of the regeneration protocol employed in the present study.
3.2. Aerosol penetration through activated charcoal cartridges The aerosol penetration of the charcoal cartridge tested under 15 L/min gas flow rate is shown in Fig. 5. As expected, the aerosol penetration increased with increasing particle size for particles smaller than the most penetrating size, about 0.3 mm in this case. This was because aerosol deposition efficiency due to diffusion decreased as aerosol particles became larger. For micrometer-sized particles (or particles larger than the most penetrating size), the aerosol penetration decreased with increasing particle size due to stronger inertial impaction as well as interception.
Y.-M. Kuo et al. / Journal of Aerosol Science 55 (2013) 57–65
Fig. 3. Variation of sorption capacity and breakthrough time among different charcoal cartridges.
Fig. 4. Effect of regeneration on the sorption capacity and breakthrough time of the same cartridge of activated charcoal.
Fig. 5. Aerosol penetration through charcoal cartridge as a function of particle size, operated under 15 L/min gas flow rate.
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3.3. Effect of liquid aerosol loading on adsorption characteristics of activated charcoal The effect of liquid aerosol loading on adsorption capacity and breakthrough characteristics of activated charcoal cartridge is shown in Fig. 6. The experimental data show reduction in adsorption capacities by 14%, 38% and 46% for loadings of 0.06, 0.14 and 0.25 g-corn oil/g-charcoal, respectively. Moreover, the decrease in adsorption capacity was exponentially proportional to increase in loading mass. Corn oil aerosols once deposited tended to form films covering the charcoal surface, thus reducing the active site for vapor adsorption. In addition, such reduction might also decrease the volume of porosity as well as cause restrictions and blockages, which impeded access to other volumes of pores. All these explained why the adsorption capability of charcoals was worsened after corn oil aerosol loading. The 10%, 50% and 90% breakthrough times reduced by 14%, 13% and 11% for aerosol loading of 0.06 g-aerosol/gcharcoal. These three breakthrough times were shortened by 28%, 23% and 20% for aerosol loading of 0.14 g-aerosol/gcharcoal loading, and 75%, 36% and 27% for aerosol loading of 0.25 g-aerosol/g-charcoal loading, respectively. It was concluded that when challenged with liquid aerosols, the breakthrough curve shifted to the lower end as the loading mass increased. This also implies a reduction of the service life of charcoal cartridge. 3.4. Effect of solid aerosol loading on adsorption characteristics of activated charcoal The effect of solid aerosol loading on breakthrough characteristics of activated charcoals was examined using 0.15- and 10-mm acrylic powders, as shown in Figs. 7 and 8, respectively. Fig. 7 illustrates the variations in adsorption capacity and breakthrough time for loadings of 0.019, 0.027 and 0.029 g/g-charcoal of 0.15-mm acrylic powder aerosols. As can be seen, no significant deviations in adsorption capacity were observed after aerosol loading. However, the slope of the breakthrough curves became less sharp. The 10% breakthrough times reduced by 26%, 27% and 35% for loadings of 0.019, 0.027 and 0.029 g/g-charcoal of 0.15-mm acrylic aerosols, respectively. The increases of 15%, 25% and 29% on the 90% breakthrough time were found after loading 0.019, 0.027 and 0.029 g/g-charcoal of 0.15-mm acrylic aerosols. It was concluded that the loading of solid aerosols reduced the 10% breakthrough time but increased the 90% breakthrough time, with the 50% breakthrough time remaining unchanged. These results imply that solid aerosol loading might shorten the service life of charcoal cartridge.
Fig. 6. Breakthrough curves of charcoal cartridges before and after loading (A) 0.06 g, (B) 0.14 g and (C) 0.25 g of corn oil aerosols per gram of activated charcoal.
Fig. 7. Breakthrough curves of charcoal cartridges before and after loading (A) 0.019 g, (B) 0.027 g and (C) 0.029 g of 0.15-mm acrylic powder aerosols per gram of activated charcoal.
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Fig. 8. Breakthrough curves of charcoal cartridges before and after loading (A) 0.074 g, (B) 0.13 g and (C) 0.18 g of 10-mm acrylic powder aerosols per gram of activated charcoal.
Fig. 9. The SEM image of 10-mm acrylic powder aerosols deposited on the activated charcoal granule (3000X).
The breakthrough curves of charcoal cartridges before and after loading for 0.074, 0.13 and 0.18 g/g-charcoal of 10-mm acrylic powder aerosols are shown in Fig. 8. Similar to that for 0.15-mm acrylic powder aerosols, no significant deviations in adsorption capacity were observed after aerosol loading. The 10% breakthrough times reduced by 5%, 43% and 44% after loading 0.074, 0.13 and 0.18 g/g-charcoal of acrylic aerosols, respectively. The increases of 1%, 2% and 9% on the 50% breakthrough time and 0.002%, 29% and 37% on the 90% breakthrough time were found after loading 0.074, 0.13 and 0.18 g/g-charcoal of acrylic aerosols, respectively. Judging from the abovementioned data, the loading of solid aerosols dramatically reduced the 10% breakthrough time but significantly increased the 90% breakthrough times. Acrylic powder has been tested for cyclohexane adsorption and no significant adsorption was found for a test period of 120 min at a flow rate of 15 L/min and 3000-ppm cyclohexane. Moreover, experimental results showed that no difference in adsorption capacity was found when the charcoals were loaded with solid aerosols (acrylic powders). However, the slope of the breakthrough curve became less sharp with shorter 10% breakthrough time and longer 90% breakthrough time after loading with solid aerosols. The deposited solid aerosols might block portions of pore entrance and act as a barrier to slow down gas flow to reach the activated sites in the deeper region. This led to an early breakthrough of challenged organic vapor, i.e., shorter 10% breakthrough time. However, the total activated sites for the adsorption of the organic vapor were apparently not affected by the deposition of solid aerosols, except more time was needed to reach the equilibrium state of vapor adsorption, i.e., prolonged 90% breakthrough time. Fig. 9 illustrates the SEM image of the highly porous structure of charcoals and the 10-mm acrylic powder aerosols deposited on the charcoal surface. Since the adsorption capacity remained the same, the deposited solid particles probably acted only as an obstacle to slow down the adsorption rate when going toward equilibrium. This is to say, the deposition of solid aerosols (acrylic powders) on charcoal would not affect the adsorption capacity regardless of particle size, although small 0.15-mm particles appeared to be more poisonous than the big 10-mm particles when the comparison is mass-based. In real situations, aerosols might vary in shape and chemical composition, but the morphology of the solid aerosols is not expected to affect the sorption capacity as long as the challenge aerosols are inert to organic vapor. For smaller particles
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Fig. 10. Effect of granular size on the breakthrough curves of charcoal cartridges.
down to nanometer range or close to the size of molecular clusters, they might act differently but it is not within the scope of the present study. 3.5. Pressure drops induced by aerosol loading No significant difference in pressure drop across the cartridge, before and after corn oil aerosol loading was observed. The deviation of pressure drop throughout the whole adsorption process was no more than 3% for each experiment. The resultant pressure drops induced by loading 0.019, 0.027 and 0.029 g/g-charcoal of 0.15-mm acrylic aerosols were 19071, 261 72 and 34272 mm H2O, respectively. The loading of acrylic aerosols led to an overwhelming increase in pressure drop across the charcoal cartridge as compared with that without aerosol loading (3.670.1 mm H2O).The pressure drops for loaded with 0.074, 0.13 and 0.18 g/g-charcoal of 10-mm acrylic powder aerosols were 4.170.2, 22.9 70.5 and 25.4 70.3 mm H2O, respectively. In comparison with the resultant pressure drop caused by the deposition of 0.15-mm acrylic powder aerosols, the pressure drop caused by 10-mm acrylic powder aerosols of equivalent mass was much lower, mainly because of less total surface area of big 10-mm acrylic powders. In the present study, both liquid and solid aerosols were used as loading aerosols. The deposition behavior of liquid aerosols was expected to be quite different from that of solid ones. Once deposited, liquid aerosols tended to form liquid films covering the surface of charcoals and hardly induced significant change in pressure drop across the charcoal cartridge, regardless of the aerosol size. On the other hand, solid particles deposition may overwhelmingly increase the pressure drop, especially for the small ones. 3.6. Influence of charcoal granular size on adsorption characteristics of activated charcoal The granular size of activated charcoals used in the respirator cartridge was not uniform. The mass median diameter of charcoal granules was 1.06 mm with the geometric standard deviation of 1.31. Those charcoal granules passing No. 20 mesh of the US standard sieve was defined as ‘‘fine’’ charcoals with granular size smaller than 0.84 mm, while those that failed to pass No. 12 mesh was classified as ‘‘coarse’’ charcoals with granular size larger than 1.41 mm, according to the US standard sieve designation. Fig. 10 compares the breakthrough characteristics of charcoal granules in these two size ranges. As can be seen, the adsorption capacity of fine charcoals was approximately 10% more than that of coarse charcoals. According to N2-BET measurements, the specific surface areas for charcoal granules smaller than 0.84 mm (or fine charcoals) and larger than 1.41 mm (or coarse charcoals) were 1285 and 1189 m2/g, respectively. The results demonstrated that the measured adsorption capacity for charcoals of different granular sizes were in good agreement with the corresponding specific surface areas. Between charcoal granules of the two size ranges mentioned above, no significant difference was observed for the 50% and 90% breakthrough times. However, the 10% breakthrough time of coarse charcoals was about 30% shorter than that of fine ones. That means the fine activated charcoals are more cost-effective, from the perspective of respiratory protection. 4. Conclusions and recommendations The variation of breakthrough curves of activated charcoal cartridges was high and might impede the study aim of investigating the effect of aerosol loading on adsorption characteristics of charcoal cartridges. Regeneration of the test charcoal effectively reduced the variability and provided a better basis for the present study.
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Liquid aerosol loading reduced the adsorption capacity of activated charcoals and the breakthrough curve was shifted to the lower end, indicating a shorter breakthrough time. On the other hand, solid aerosol loading did not change the adsorption capacity of activated charcoals, but the slope of the breakthrough curve became less sharp, i.e., shorter 10% breakthrough time and longer 90% breakthrough time. The 50% breakthrough time was almost unaffected. In short, the breakthrough time changed (decreased or increased) with increasing loaded aerosol mass. Liquid particles were more poisonous than solid ones because liquid particles might form films covering more activated site of charcoals. Liquid aerosol loading deteriorated the charcoal adsorption performance and shifted the entire breakthrough curve toward the lower end. The reduction in breakthrough time increased with increasing loaded mass. The pre-filter installed upstream of the charcoal unit would help protect against aerosol contamination, especially for liquid aerosols.
Acknowledgements This study was financially supported by Taiwan National Science Council through grant no. NSC 94-2211-E-273-003, NSC 95-2211-E-273-009 and NSC 96-2211-E-273-001. References Bradley, R.H. (2011). Surface studies of novel hydrophobic active carbons. Applied Surface Science, 257, 2912–2919. Cao, L. (2010). Effect of relative humidity on adsorption capacity of organic vapors on activated carbon and prediction by the modified D-R equations. 2010 International Conference on Digital Manufacturing and Automation (ICDMA). CNS 6636 Z 2023, /http://www.cnsonline.com.tw/previewJPG.jsp?general_no ¼0663600&language ¼C&pagecount ¼ 9&page¼ 5, accessed 2/10/2012S. Dharmarajan, V., Lingg, R.D., & Myer, H.E. (2001). Evaluation of organic-vapor respirator cartridge efficiency for hexamethylene diisocyanate vapor in the presence of organic solvents. Applied Occupational and Environmental Hygiene, 16, 397–404. Furuse, M., Kanno, S., Takano, T., & Matsumura, Y. (2001). Cyclohexane as an alternative vapor of carbon tetrachloride for the assessment of gas removing capacities of gas mask. Industrial Health, 39, 1–7. Innes, R.W., Fryer, J.R., & Stoeckli, H.F. (1989). On the correlation between micropore distribution obtained from molecular probes and from high resolution electron microscopy. Carbon, 27, 71–76. James, R., Dukes-Dobos, F., & Smith, R. (1984). Effects of respirators under heat/work conditions. American Industrial Hygienist Association Journal, 45, 399–404. Kaplan, D. (2006). Effects of high relative humidity on the dynamic adsorption of dimethyl methylphosphonate (DMMP) on activated carbon. Carbon, 44, 3247–3254. Kuo, Y.M., Huang, S.H., Shih, T.S., Chen, C.C., & Weng, Y.M., et al. (2005). Development of a size-selective inlet-simulating ICRP lung deposition fraction. Aerosol Science and Technology, 39, 437–443. Li, J. (2008). Effect of relative humidity on adsorption of formaldehyde on modified activated carbons. Chinese Journal of Chemical Engineering, 16, 871–875 . Nelson, G.O., & Correia, A.N. (1976). Respirator cartridge efficiency studies: VIII. summary and conclusions. American Industrial Hygienist Association Journal, 37, 514–525. Nelson, G.O., Correia, A.N., & Harder, C.A. (1976). Respirator cartridge efficiency studies: VII. Effect of relative humidity and temperature. American Industrial Hygienist Association Journal, 37, 280–288. Nelson, G.O., & Harder, C.A. (1974). Respirator cartridge efficiency studies. V. Effect of solvent vapor. American Industrial Hygienist Association Journal, 35, 391–410. Rao, M.B. (1985). Mathematical modeling of diffusive potentials within carbon molecular sieves. Extended Abstracts and Program—Biennial Conference on Carbon, 114–115. Stoeckli, H.F. (1990). Microporous carbons and their characterization: the present state of the art. Carbon, 28, 1–6. Tanaka, S., Nakano, Y., Tsunemori, K., Shimada, M., & Seki, Y. (1999). A study on the relative breakthrough time (RBT) of a respirator cartridge for forty-six kinds of organic solvent vapors. Applied Occupational and Environmental Hygiene, 14, 691–695. Trout, D., Breysse, P.N., Hall, T., Corn, M., & Risby, T. (1986). Determination of organic vapor respirator cartridge variability in terms of degree of activation of the carbon and cartridge packing density. American Industrial Hygienist Association Journal, 47, 491–496. Tsai, W.T. (1994). Surface chemistry of activated carbons and its relevance for effects of relative humidity on adsorption of chlorinated organic vapors. Chemosphere, 29, 2507 -2507. Wood, G.O. (1985). Effects of air temperatures and humidities on efficiencies and lifetimes of air-purifying chemical respirator cartridges tested against methyl iodide. American Industrial Hygienist Association Journal, 46, 251–256. Wood, G.O. (2004). Estimating service lives of organic vapor cartridges II: A single vapor at all humidities. Journal of Occupational and Environmental Hygiene, 472–492. Ye, P. (2008). Study of adsorption kinetics of perfluoroisobutene on fixed bed activated carbon. Journal of Chemical and Engineering Data, 53, 1262–1265. Yoon, Y.H., & Nelson, J.H. (1990). Effects of humidity and contaminant concentration on respirator cartridge breakthrough. American Industrial Hygienist Association Journal, 51, 202–209. Yoon, Y.H., & Nelson, J.H. (1992). Breakthrough time and adsorption capacity of 3 respirator cartridges. American Industrial Hygienist Association Journal, 53, 303–316. Yoon, Y.H., Nelson, J.H., & Lara, J. (1996). Respirator cartridge service-life: exposure to mixtures. American Industrial Hygienist Association Journal, 57, 809–819.
Contents lists available at SciVerse ScienceDirect
Journal of Aerosol Science journal homepage: www.elsevier.com/locate/jaerosci
Effect of aerosol loading on breakthrough characteristics of activated charcoal cartridges Yu-Mei Kuo a, Chih-Wei Lin b, Sheng-Hsiu Huang b, Kuang-Nan Chang b, Chih-Chieh Chen b,n a
Department of Occupational Safety and Health, Chung-hwa University of Medical Technology, 89, Wen-Hwa 1st St., Jen-Te District, Tainan 712, Taiwan, ROC National Taiwan University, Institute of Occupational Medicine and Industrial Hygiene, College of Public Health, 17 Xu-Zhou Road, Room 718, Taipei 10055, Taiwan, ROC
b
a r t i c l e i n f o
abstract
Article history: Received 8 April 2012 Received in revised form 31 July 2012 Accepted 18 August 2012 Available online 25 August 2012
Charcoal cartridges are commonly employed to provide protection against a wide range of organic vapors at workplace owing to their excellent adsorption. In practical situations, not only organic vapors but also aerosol particles may exist in the workplace atmosphere. Focusing on aerosol particles, this study aimed to evaluate the effect of aerosol loading on both sorption capacity and breakthrough time of activated charcoal. Polydisperse corn oil aerosols with a mode of 0.2 mm and monodisperse acrylic powder of 0.15 and 10 mm were generated to load the test activated charcoal separately for conducting vapor sorption experiments. For liquid aerosol loading tests, the sorption capacities reduced by 14%, 38% and 46% and the 10% breakthrough times were shorten by 14%, 28% and 75% after loading 0.06, 0.14 and 0.25 g of corn oil aerosols per gram charcoal, respectively. For charcoal loaded with solid aerosols, the sorption capacity remained unchanged whereas the 10% breakthrough times were reduced by 26%, 27% and 35% after loading 0.019, 0.027 and 0.029 g of 0.15-mm acrylic powder per gram charcoal, and by 5%, 43% and 44% after loading 0.074, 0.13 and 0.18 g of 10-mm acrylic aerosols per gram charcoal, respectively. It was concluded that the breakthrough characteristics were affected by aerosol loading and the sorption capacity of activated charcoal can only be reduced by the loading of liquid aerosols. For the use of gas respirators with activated charcoal cartridges, it should be noted that the service life of the charcoal cartridges could be shortened due to aerosol loading in a work environment, from the perspective of respiratory protection. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Activated charcoal Breakthrough time Aerosol loading Sorption capacity
1. Introduction The sorption of organic vapors to activated charcoals occurs mainly through physical adsorption. The equilibrated amount, i.e., the sorption capacity, depends on the adsorption isotherm. Owing to the excellent adsorption capacity of activated carbon for organic vapors, charcoal cartridges are commonly used in gas respirators to protect workers’ health in the industrial environment. Charcoal has a porous structure with variable gaps of molecular dimensions, being the micropores (Innes et al., 1989). The micropores of charcoal, generally in slit-shape, are characterized by the slit width in the size range of 0.4 to 2 nm and represent the bulk of the adsorption capacity. The mesopores with size varying from 2 to
n
Corresponding author. Tel.: þ886 2 33668086; fax: þ 886 2 23938631. E-mail address: [email protected] (C.-C. Chen).
0021-8502/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jaerosci.2012.08.002
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50 nm are important for transport properties while the macropores denote those pores whose size exceed 50 nm. Micropores have higher surface area to pore volume ratios than mesopores and macropores. Micropores and available contact surfaces in the adsorbents play a dominant role in the adsorption and it is basically a process of volume filling. Then adsorption begins to shift gradually to multilayer adsorption in the meso- and macropores as the vapor pressure continues to increase. The adsorption behavior in mesopores depends not only on vapor-wall interactions but also on the attractive forces between vapor molecules, which may lead to capillary (pore) condensation (Stoeckli, 1990). Previous studies have concluded that two major barriers to gas molecule diffusion existed, namely pore entry and diffusion along the pore. The rate-limiting step in highly microporous carbons was pore entry (Rao, 1985). A variety of studies on the factors affecting adsorption capacity and/or breakthrough time of a charcoal cartridge have been carried out (Nelson & Correia, 1976; Yoon & Nelson, 1990, 1992; Yoon et al., 1996). Studies showed that the breakthrough characteristics of charcoal cartridges may be affected by the constituent(s) and the concentration(s) of the vapor(s) adsorbed (Nelson and Harder, 1974; Nelson and Correia, 1976; Tanaka et al., 1999; Dharmarajan et al., 2001), temperature (Nelson et al., 1976; James et al., 1984; Wood, 1985), moisture content in the gas flow in terms of relative humidity (Nelson et al., 1976; Yoon & Nelson, 1990; Tsai, 1994; Wood, 2004; Kaplan, 2006; Li, 2008; Ye, 2008; Cao, 2010; Bradley, 2011), gas flow rate (Nelson & Correia, 1976), and charcoal packing density (Trout, Breysse et al., 1986). A large number of previous studies have been on the performance of charcoal on vapor adsorption and influencing factors. However, little is known about the effect of aerosols deposited on the surface of activated charcoals. For practical situations, not only organic vapors but also aerosol particles may exist in the atmosphere at workplace. Subsequently, aerosol particles might penetrate and deposit on the charcoal inside the respirator cartridge and, therefore, induce extra air resistance. Moreover, the particles deposited on the surface of the granulated activated carbon may alter the breakthrough characteristics, thus affecting the service life of charcoal cartridges which is of great importance for respiratory protection in industrial applications. Therefore, this study aimed to evaluate the effect of aerosol loading on the breakthrough characteristics of activated charcoal. 2. Materials and methods 2.1. Test activated charcoal The activated charcoal taken from the same lot of commercially available respirator cartridges (3M 6001, St. Paul, MN, USA) was employed to conduct the experiments in this work. This organic vapor cartridge was approved by the National Institute of Occupational Safety and Health (NIOSH). The granular size of the charcoal was measured by US standard testing sieves to be in the size range of 12–20 mesh (equivalent to 0.84–1.68 mm). Under the STP condition, the N2-BET specific surface area was 1319 m2/g. The micropore volume and the micropore area were 0.48 cm3/g and 1047 m2/g, respectively. The granular charcoal was prepared by drying at 150 1C for at least 2 h, followed by purging with clean filtered air for another 2 h prior to test. After being preparation, the charcoal was packed in a cylindrical stainless steel sample holder, which is 7.16 cm in diameter and 2.56 cm in height. The homemade cartridge accommodated 35 g of granular charcoal with a calculated packed density of 0.34 g/cm3 for all the tests unless otherwise specified. The homemade charcoal cartridge was first tested for its sorption efficiency as a function of time to obtain the characteristic breakthrough curve of the cartridge before aerosol loading. Then the used cartridge was regenerated by following the same protocol of charcoal preparation as aforementioned. After regeneration, the charcoal cartridge was loaded with a given amount of aerosols in specified size range and was tested for the sorption efficiency again to obtain the breakthrough curve of the aerosol loaded cartridge. 2.2. Cyclohexane sorption test The operating conditions for the sorption efficiency tests followed mainly the Chinese National Standard certification method (CNS 6636 Z2023): 3000 ppm cyclohexane as the challenge agent (Furuse et al., 2001), 15 L/min gas flow rate and 50 75% relative humidity. Fig. 1 shows the schematic diagram of the experimental system set-up for charcoal sorption and aerosol loading tests. Cyclohexane vapor was generated by passing 15 L/min filtered air through the surface of liquid cyclohexane in a jar dipped in a water bath. The temperatures of the cyclohexane bath and water bath were set the same, 37 70.5 1C. The cyclohexane vapor concentration was controlled at 3000 ppm by refilling the escaped cyclohexane at an injection rate of 0.204 mL/min using a syringe pump. The relative humidity of the gas flow was monitored by a hygrometer and was maintained at 50 71% at 25 1C by passing the filtered air over a water reservoir. The gas mixture then passed through the test charcoal cartridge. The cyclohexane concentrations upstream and downstream of the cartridge were monitored by a flame ionization detector (FID, model 2005A, China Chromatography Co., Taiwan). The cyclohexane breakthrough curve as a function of time was characterized by calculating the ratio of downstream vapor concentration (C) in ppm to upstream vapor concentration (C0) in ppm. The sorption capacity of activated charcoal in mg-cyclohexane/g-charcoal was calculated by dividing the adsorbed amount of cyclohexane vapor by the amount of activated charcoal used in the cartridge. The adsorbed amount was estimated by the following equation. ! n X Q M:W: ðC i þ 1 þ C i ÞDt a W ads ¼ C 0 t ads 103 24:46 2 i¼1
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Fig. 1. Schematic diagram of the experimental system set-up for charcoal adsorption and aerosol loading test.
where Wads is the adsorbed amount in mg; Q is the air flow rate in L/min; M.W. is the molecular weight; C0 is the challenge vapor concentration in ppm; tads is the adsorption period in min; Ci is the concentration of the ith breakthrough sample determined by the FID from the beginning of adsorption in ppm; and Dta is the sampling interval of the adsorption process in min which is 1 in the present study. To evaluate the sorption curve of activated charcoal quantitatively, the 10%, 50% and 90% breakthrough times were determined. The 10%, 50% and 90% breakthrough times were defined as the times when the downstream vapor concentration reached 10%, 50% and 90% of the challenged vapor concentration, respectively. 2.3. Aerosol penetration test Aerosol penetration test was conducted to gain the information regarding aerosol penetration through the charcoal cartridge as a function of particle size. This information was essential and provided the basis for our further study on aerosol loading effect. Fig. 2 shows the schematic diagram of experimental setup for aerosol penetration test. Corn oil was chosen as the challenge. A constant output atomizer (model 3076, TSI Inc., St. Paul, MN) and an ultrasonic atomizing nozzle (model 8700-120, Sonotek Inc., Highland, NY) were utilized to generate polydisperse submicrometer-sized and micrometer-sized aerosol particles, respectively. To avoid the particle charge effect, the aerosol output was passed through a Kr-85 radioactive source to neutralize the aerosol particles to the Boltzmann charge equilibrium. Two aerosol size spectrometers were used. They included a scanning mobility particle sizer (SMPS, model 3934, TSI Inc.) for particles smaller than 0.8 mm and an aerodynamic particle sizer (APS, model 3321A, TSI Inc.) for particles larger than 0.8 mm. These two instruments measured both aerosol concentrations and size distributions upstream and downstream of the charcoal cartridge. Although SMPS and APS size particles by different principles, data combination has been shown feasible after proper calibration and operation (Kuo et al., 2005). The aerosol penetration efficiency as a function of aerosol size was calculated as the ratio of the downstream aerosol concentration to upstream aerosol concentration in a given size range across the charcoal cartridge. 2.4. Aerosol loading test Corn oil and acrylic powder (MP series, Soken Chemical & Engineering Co., Tokyo, Japan) were employed to generate liquid and solid aerosols, separately. Acrylic powder has been tested for cyclohexane adsorption and no significant adsorption was found for a test period of 120 min at a flow rate of 15 L/min and 3000-ppm cyclohexane. Corn oil was atomized by a constant output atomizer (model 3076, TSI Inc., St. Paul, MN, USA) to generate polydisperse submicrometersized droplets with a mode of 0.2 mm and geometric standard deviation of 1.96. A dust feeder (RGB1000, Palas Gmbh, Karlsruhe, Germany) was utilized to generate monodisperse 0.15- or 10-mm acrylic powders, respectively. Under the designed flow rate (15 L/min in this study), the dominating filtration mechanisms of these two sizes of powders should be
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Fig. 2. Schematic diagram of experimental system set-up for aerosol penetration test.
distinctly different; that is, diffusion for 0.15-mm and inertial impaction for 10-mm. The loading amounts were 0.06, 0.14 and 0.25 g/g-charcoal for corn oil aerosols, 0.019, 0.027 and 0.029 g/g-charcoal for 0.15-mm acrylic powder aerosols and 0.074, 0.13 and 0.18 g/g-charcoal for 10-mm acrylic powder aerosols. The pressure drop across the charcoal cartridge was recorded by a pressure transducer (OMEGA, PX164-010D5V), which was calibrated against an inclined manometer. The amount of aerosol loaded in the test charcoal cartridge was determined by gravimetric analysis.
3. Results and discussion 3.1. The adsorption characteristics of activated charcoal cartridges Commercially available respirator cartridges were used in the present study. The variability of adsorption characteristics of five charcoal cartridges was shown in Fig. 3. The adsorption capacity of the cartridges was estimated to be 361 76 mg-cyclohexane/g-charcoal. The 10%, 50% and 90% breakthrough times were 60 714, 84 74 and 98 72 min, respectively. The pressure drop across the cartridges at 15 L/min gas flow rate was 3.670.8 mm H2O while the increase in pressure drop throughout the whole adsorption process was no more than 3%. The results demonstrated significant variations in adsorption capacity and breakthrough time among tested cartridges. Owing to the variations of performance of charcoal cartridges, the charcoals of the same lot were pooled and mixed for charcoal preparation. If possible, the same cartridge of charcoals was repeatedly used for adsorption tests after regeneration. The effect of regeneration on both adsorption capacity and breakthrough characteristics of activated charcoals after repeated regeneration for four times is shown in Fig. 4. The adsorption capacity of the cartridge was 355 72 mg-cyclohexane/g-charcoal. The 10%, 50% and 90% breakthrough times were 57 72, 8372 and 10172 min, respectively. The experimental results showed an excellent reproducibility of adsorption capacity and breakthrough curve of regenerated charcoals, indicating the effectiveness of the regeneration protocol employed in the present study.
3.2. Aerosol penetration through activated charcoal cartridges The aerosol penetration of the charcoal cartridge tested under 15 L/min gas flow rate is shown in Fig. 5. As expected, the aerosol penetration increased with increasing particle size for particles smaller than the most penetrating size, about 0.3 mm in this case. This was because aerosol deposition efficiency due to diffusion decreased as aerosol particles became larger. For micrometer-sized particles (or particles larger than the most penetrating size), the aerosol penetration decreased with increasing particle size due to stronger inertial impaction as well as interception.
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Fig. 3. Variation of sorption capacity and breakthrough time among different charcoal cartridges.
Fig. 4. Effect of regeneration on the sorption capacity and breakthrough time of the same cartridge of activated charcoal.
Fig. 5. Aerosol penetration through charcoal cartridge as a function of particle size, operated under 15 L/min gas flow rate.
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3.3. Effect of liquid aerosol loading on adsorption characteristics of activated charcoal The effect of liquid aerosol loading on adsorption capacity and breakthrough characteristics of activated charcoal cartridge is shown in Fig. 6. The experimental data show reduction in adsorption capacities by 14%, 38% and 46% for loadings of 0.06, 0.14 and 0.25 g-corn oil/g-charcoal, respectively. Moreover, the decrease in adsorption capacity was exponentially proportional to increase in loading mass. Corn oil aerosols once deposited tended to form films covering the charcoal surface, thus reducing the active site for vapor adsorption. In addition, such reduction might also decrease the volume of porosity as well as cause restrictions and blockages, which impeded access to other volumes of pores. All these explained why the adsorption capability of charcoals was worsened after corn oil aerosol loading. The 10%, 50% and 90% breakthrough times reduced by 14%, 13% and 11% for aerosol loading of 0.06 g-aerosol/gcharcoal. These three breakthrough times were shortened by 28%, 23% and 20% for aerosol loading of 0.14 g-aerosol/gcharcoal loading, and 75%, 36% and 27% for aerosol loading of 0.25 g-aerosol/g-charcoal loading, respectively. It was concluded that when challenged with liquid aerosols, the breakthrough curve shifted to the lower end as the loading mass increased. This also implies a reduction of the service life of charcoal cartridge. 3.4. Effect of solid aerosol loading on adsorption characteristics of activated charcoal The effect of solid aerosol loading on breakthrough characteristics of activated charcoals was examined using 0.15- and 10-mm acrylic powders, as shown in Figs. 7 and 8, respectively. Fig. 7 illustrates the variations in adsorption capacity and breakthrough time for loadings of 0.019, 0.027 and 0.029 g/g-charcoal of 0.15-mm acrylic powder aerosols. As can be seen, no significant deviations in adsorption capacity were observed after aerosol loading. However, the slope of the breakthrough curves became less sharp. The 10% breakthrough times reduced by 26%, 27% and 35% for loadings of 0.019, 0.027 and 0.029 g/g-charcoal of 0.15-mm acrylic aerosols, respectively. The increases of 15%, 25% and 29% on the 90% breakthrough time were found after loading 0.019, 0.027 and 0.029 g/g-charcoal of 0.15-mm acrylic aerosols. It was concluded that the loading of solid aerosols reduced the 10% breakthrough time but increased the 90% breakthrough time, with the 50% breakthrough time remaining unchanged. These results imply that solid aerosol loading might shorten the service life of charcoal cartridge.
Fig. 6. Breakthrough curves of charcoal cartridges before and after loading (A) 0.06 g, (B) 0.14 g and (C) 0.25 g of corn oil aerosols per gram of activated charcoal.
Fig. 7. Breakthrough curves of charcoal cartridges before and after loading (A) 0.019 g, (B) 0.027 g and (C) 0.029 g of 0.15-mm acrylic powder aerosols per gram of activated charcoal.
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Fig. 8. Breakthrough curves of charcoal cartridges before and after loading (A) 0.074 g, (B) 0.13 g and (C) 0.18 g of 10-mm acrylic powder aerosols per gram of activated charcoal.
Fig. 9. The SEM image of 10-mm acrylic powder aerosols deposited on the activated charcoal granule (3000X).
The breakthrough curves of charcoal cartridges before and after loading for 0.074, 0.13 and 0.18 g/g-charcoal of 10-mm acrylic powder aerosols are shown in Fig. 8. Similar to that for 0.15-mm acrylic powder aerosols, no significant deviations in adsorption capacity were observed after aerosol loading. The 10% breakthrough times reduced by 5%, 43% and 44% after loading 0.074, 0.13 and 0.18 g/g-charcoal of acrylic aerosols, respectively. The increases of 1%, 2% and 9% on the 50% breakthrough time and 0.002%, 29% and 37% on the 90% breakthrough time were found after loading 0.074, 0.13 and 0.18 g/g-charcoal of acrylic aerosols, respectively. Judging from the abovementioned data, the loading of solid aerosols dramatically reduced the 10% breakthrough time but significantly increased the 90% breakthrough times. Acrylic powder has been tested for cyclohexane adsorption and no significant adsorption was found for a test period of 120 min at a flow rate of 15 L/min and 3000-ppm cyclohexane. Moreover, experimental results showed that no difference in adsorption capacity was found when the charcoals were loaded with solid aerosols (acrylic powders). However, the slope of the breakthrough curve became less sharp with shorter 10% breakthrough time and longer 90% breakthrough time after loading with solid aerosols. The deposited solid aerosols might block portions of pore entrance and act as a barrier to slow down gas flow to reach the activated sites in the deeper region. This led to an early breakthrough of challenged organic vapor, i.e., shorter 10% breakthrough time. However, the total activated sites for the adsorption of the organic vapor were apparently not affected by the deposition of solid aerosols, except more time was needed to reach the equilibrium state of vapor adsorption, i.e., prolonged 90% breakthrough time. Fig. 9 illustrates the SEM image of the highly porous structure of charcoals and the 10-mm acrylic powder aerosols deposited on the charcoal surface. Since the adsorption capacity remained the same, the deposited solid particles probably acted only as an obstacle to slow down the adsorption rate when going toward equilibrium. This is to say, the deposition of solid aerosols (acrylic powders) on charcoal would not affect the adsorption capacity regardless of particle size, although small 0.15-mm particles appeared to be more poisonous than the big 10-mm particles when the comparison is mass-based. In real situations, aerosols might vary in shape and chemical composition, but the morphology of the solid aerosols is not expected to affect the sorption capacity as long as the challenge aerosols are inert to organic vapor. For smaller particles
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Fig. 10. Effect of granular size on the breakthrough curves of charcoal cartridges.
down to nanometer range or close to the size of molecular clusters, they might act differently but it is not within the scope of the present study. 3.5. Pressure drops induced by aerosol loading No significant difference in pressure drop across the cartridge, before and after corn oil aerosol loading was observed. The deviation of pressure drop throughout the whole adsorption process was no more than 3% for each experiment. The resultant pressure drops induced by loading 0.019, 0.027 and 0.029 g/g-charcoal of 0.15-mm acrylic aerosols were 19071, 261 72 and 34272 mm H2O, respectively. The loading of acrylic aerosols led to an overwhelming increase in pressure drop across the charcoal cartridge as compared with that without aerosol loading (3.670.1 mm H2O).The pressure drops for loaded with 0.074, 0.13 and 0.18 g/g-charcoal of 10-mm acrylic powder aerosols were 4.170.2, 22.9 70.5 and 25.4 70.3 mm H2O, respectively. In comparison with the resultant pressure drop caused by the deposition of 0.15-mm acrylic powder aerosols, the pressure drop caused by 10-mm acrylic powder aerosols of equivalent mass was much lower, mainly because of less total surface area of big 10-mm acrylic powders. In the present study, both liquid and solid aerosols were used as loading aerosols. The deposition behavior of liquid aerosols was expected to be quite different from that of solid ones. Once deposited, liquid aerosols tended to form liquid films covering the surface of charcoals and hardly induced significant change in pressure drop across the charcoal cartridge, regardless of the aerosol size. On the other hand, solid particles deposition may overwhelmingly increase the pressure drop, especially for the small ones. 3.6. Influence of charcoal granular size on adsorption characteristics of activated charcoal The granular size of activated charcoals used in the respirator cartridge was not uniform. The mass median diameter of charcoal granules was 1.06 mm with the geometric standard deviation of 1.31. Those charcoal granules passing No. 20 mesh of the US standard sieve was defined as ‘‘fine’’ charcoals with granular size smaller than 0.84 mm, while those that failed to pass No. 12 mesh was classified as ‘‘coarse’’ charcoals with granular size larger than 1.41 mm, according to the US standard sieve designation. Fig. 10 compares the breakthrough characteristics of charcoal granules in these two size ranges. As can be seen, the adsorption capacity of fine charcoals was approximately 10% more than that of coarse charcoals. According to N2-BET measurements, the specific surface areas for charcoal granules smaller than 0.84 mm (or fine charcoals) and larger than 1.41 mm (or coarse charcoals) were 1285 and 1189 m2/g, respectively. The results demonstrated that the measured adsorption capacity for charcoals of different granular sizes were in good agreement with the corresponding specific surface areas. Between charcoal granules of the two size ranges mentioned above, no significant difference was observed for the 50% and 90% breakthrough times. However, the 10% breakthrough time of coarse charcoals was about 30% shorter than that of fine ones. That means the fine activated charcoals are more cost-effective, from the perspective of respiratory protection. 4. Conclusions and recommendations The variation of breakthrough curves of activated charcoal cartridges was high and might impede the study aim of investigating the effect of aerosol loading on adsorption characteristics of charcoal cartridges. Regeneration of the test charcoal effectively reduced the variability and provided a better basis for the present study.
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Liquid aerosol loading reduced the adsorption capacity of activated charcoals and the breakthrough curve was shifted to the lower end, indicating a shorter breakthrough time. On the other hand, solid aerosol loading did not change the adsorption capacity of activated charcoals, but the slope of the breakthrough curve became less sharp, i.e., shorter 10% breakthrough time and longer 90% breakthrough time. The 50% breakthrough time was almost unaffected. In short, the breakthrough time changed (decreased or increased) with increasing loaded aerosol mass. Liquid particles were more poisonous than solid ones because liquid particles might form films covering more activated site of charcoals. Liquid aerosol loading deteriorated the charcoal adsorption performance and shifted the entire breakthrough curve toward the lower end. The reduction in breakthrough time increased with increasing loaded mass. The pre-filter installed upstream of the charcoal unit would help protect against aerosol contamination, especially for liquid aerosols.
Acknowledgements This study was financially supported by Taiwan National Science Council through grant no. NSC 94-2211-E-273-003, NSC 95-2211-E-273-009 and NSC 96-2211-E-273-001. References Bradley, R.H. (2011). Surface studies of novel hydrophobic active carbons. Applied Surface Science, 257, 2912–2919. Cao, L. (2010). Effect of relative humidity on adsorption capacity of organic vapors on activated carbon and prediction by the modified D-R equations. 2010 International Conference on Digital Manufacturing and Automation (ICDMA). CNS 6636 Z 2023, /http://www.cnsonline.com.tw/previewJPG.jsp?general_no ¼0663600&language ¼C&pagecount ¼ 9&page¼ 5, accessed 2/10/2012S. Dharmarajan, V., Lingg, R.D., & Myer, H.E. (2001). Evaluation of organic-vapor respirator cartridge efficiency for hexamethylene diisocyanate vapor in the presence of organic solvents. Applied Occupational and Environmental Hygiene, 16, 397–404. Furuse, M., Kanno, S., Takano, T., & Matsumura, Y. (2001). Cyclohexane as an alternative vapor of carbon tetrachloride for the assessment of gas removing capacities of gas mask. Industrial Health, 39, 1–7. Innes, R.W., Fryer, J.R., & Stoeckli, H.F. (1989). On the correlation between micropore distribution obtained from molecular probes and from high resolution electron microscopy. Carbon, 27, 71–76. James, R., Dukes-Dobos, F., & Smith, R. (1984). Effects of respirators under heat/work conditions. American Industrial Hygienist Association Journal, 45, 399–404. Kaplan, D. (2006). Effects of high relative humidity on the dynamic adsorption of dimethyl methylphosphonate (DMMP) on activated carbon. Carbon, 44, 3247–3254. Kuo, Y.M., Huang, S.H., Shih, T.S., Chen, C.C., & Weng, Y.M., et al. (2005). Development of a size-selective inlet-simulating ICRP lung deposition fraction. Aerosol Science and Technology, 39, 437–443. Li, J. (2008). Effect of relative humidity on adsorption of formaldehyde on modified activated carbons. Chinese Journal of Chemical Engineering, 16, 871–875 . Nelson, G.O., & Correia, A.N. (1976). Respirator cartridge efficiency studies: VIII. summary and conclusions. American Industrial Hygienist Association Journal, 37, 514–525. Nelson, G.O., Correia, A.N., & Harder, C.A. (1976). Respirator cartridge efficiency studies: VII. Effect of relative humidity and temperature. American Industrial Hygienist Association Journal, 37, 280–288. Nelson, G.O., & Harder, C.A. (1974). Respirator cartridge efficiency studies. V. Effect of solvent vapor. American Industrial Hygienist Association Journal, 35, 391–410. Rao, M.B. (1985). Mathematical modeling of diffusive potentials within carbon molecular sieves. Extended Abstracts and Program—Biennial Conference on Carbon, 114–115. Stoeckli, H.F. (1990). Microporous carbons and their characterization: the present state of the art. Carbon, 28, 1–6. Tanaka, S., Nakano, Y., Tsunemori, K., Shimada, M., & Seki, Y. (1999). A study on the relative breakthrough time (RBT) of a respirator cartridge for forty-six kinds of organic solvent vapors. Applied Occupational and Environmental Hygiene, 14, 691–695. Trout, D., Breysse, P.N., Hall, T., Corn, M., & Risby, T. (1986). Determination of organic vapor respirator cartridge variability in terms of degree of activation of the carbon and cartridge packing density. American Industrial Hygienist Association Journal, 47, 491–496. Tsai, W.T. (1994). Surface chemistry of activated carbons and its relevance for effects of relative humidity on adsorption of chlorinated organic vapors. Chemosphere, 29, 2507 -2507. Wood, G.O. (1985). Effects of air temperatures and humidities on efficiencies and lifetimes of air-purifying chemical respirator cartridges tested against methyl iodide. American Industrial Hygienist Association Journal, 46, 251–256. Wood, G.O. (2004). Estimating service lives of organic vapor cartridges II: A single vapor at all humidities. Journal of Occupational and Environmental Hygiene, 472–492. Ye, P. (2008). Study of adsorption kinetics of perfluoroisobutene on fixed bed activated carbon. Journal of Chemical and Engineering Data, 53, 1262–1265. Yoon, Y.H., & Nelson, J.H. (1990). Effects of humidity and contaminant concentration on respirator cartridge breakthrough. American Industrial Hygienist Association Journal, 51, 202–209. Yoon, Y.H., & Nelson, J.H. (1992). Breakthrough time and adsorption capacity of 3 respirator cartridges. American Industrial Hygienist Association Journal, 53, 303–316. Yoon, Y.H., Nelson, J.H., & Lara, J. (1996). Respirator cartridge service-life: exposure to mixtures. American Industrial Hygienist Association Journal, 57, 809–819.