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Adsorption–desorption characteristics of volatile organic compounds over various zeolites and their regeneration by microwave irradiation
Mesostructured Materials Recent Progress in Mesostructured D. Zhao, S. Qiu, Y. Tang and C. Yu (Editors) © 2007 Elsevier B.V. All rights reserved.
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Adsorption-desorption characteristics of volatile organic compounds over various zeolites and their regeneration by microwave irradiation K.-J. Kima, Y.-H. Kima, W.-J. Jeongb, N.-C. Parkc, S.-W. Jeongd and H.-G. Ahna* "Dept. ofChem. Eng., Sunchon National Univ., 315 Maegok-dong, Suncheon, Jeonnam, 540-742 Korea Opt. Eng. Res. Institute, Mokpo National Univ., Jeonnam, 453-729, Korea c Dept. ofChem. Eng., Chonnam National Univ., 300 Yongbong-dong, Buk-gu, Gwangju, 500-757 Korea Jeonnam Techno Park, 315 Maegok-dong, Suncheon, Jeonnam, 540-742 Korea.
Adsorption of volatile organic compounds (VOCs) over various zeolites was performed to determine the relationship between adsorption capacity and physical properties of zeolites. The desorption characteristics was investigated by means of conventional heating and microwave irradiation. FAU, Z-HY4.8, and MS-13X were better adsorption performance than the others. Desorption efficiency by microwave heating was better than that by conventional heating. 1. Introduction The volatile organic compounds (VOCs) exhaust that is caused by increase of the various organic solvent and the paints has been gradually increased. The VOCs are present in many types of waste gases, and the adsorption by adsorbents is often used to remove them [1, 2]. Desorption to reutilize the adsorbent saturated (or polluted) with VOCs can be carried out by microwave irradiation, so the adsorbent should be repeatedly utilized [3, 4]. The regeneration of the adsorbent using microwave due to dielectric heating is very effective because the microwave irradiates the VOC molecule directly. If the VOC is a nonpolar compound, the supply of the water is required for the microwave heating. Therefore, the microwave heating by dielectric heating can be performed in a relatively short period of time, which implies a lower
224
consumption of energy. And the main advantage of the microwave heating is that treatment utility for removing the VOCs is simpler and tail gas treatment capacity is smaller than that of direct heating method. Activated carbon cannot be used in this study as adsorbent because it is an electric conductor, so the electric current flows in electric field. The spark discharge occurs from the portion, and shows the light emission phenomenon. The zeolites were chosen for removing the VOCs, which were various pore structures and acidities. In this study, the physical properties of various zeolites and its adsorption capacity for VOCs were investigated. Desorption characteristics of the adsorbed VOCs was mainly investigated by means of microwave irradiation, and it was compared to that by the conventional heating. 2. Experimental Section Physical characteristics of various zeolites such as specific surface area, pore volume and pore size distribution was investigated with BET method (ASAP 2010, Micromeritics, USA). Adsorption and desorption of VOCs were performed using a flow system. Model gases were benzene, toluene, o-, m-, pxylenes, methanol, ethanol, i-propanol, and methylethylketone (MEK). All reagents for model gas were GR gr. (Junsei Chem., 99.0% ~ 99.5%). Concentration of VOCs was controlled with vaporizing individual VOCs in the saturator by He stream. He flow rate (mainly 40ml/min) was controlled using mass flow controller. Before adsorption experiment, the adsorbents (O.lg) were pretreated for lhr at 250°C. The adsorbents used as adsorbent were Molecular Sieve 13X [MS-13X], Mordenites [JRC-Z-HM10(2), Na-mordenite], Y-zeolites [JRC-Z-HY4.8, JRC-Z-HY5.6(2), and Faujasite (FAU)]. In the conventional heating, adsorbent column (o.d. 1/4", ss) of U-type was used. It was heated from 25 to 500°C by 5°C /min. In microwave experiment, adsorbent column (o.d. 3/8", quartz) of a U-type was used. All fittings for fixing an adsorbent column in oven were made of Teflon. Microwave source was generated from a microwave producer (2.45GHz). Concentration of VOCs was monitored with TCD of GC (GC-14B, Shimadzu, Japan). 3. Results and Discussion Physical characteristics by N2 adsorption was investigated for various zeolites. The BET surface area and pore volume by t-plot were shown Table 1, containing the amounts of toluene and MEK adsorbed only. The maximum surface area was observed on FAU. FAU and Z-HY4.8 showed the maximum total pore volume, and micropore was well formed in Z-HY4.8. The used zeolites are possessed of different acidities, Si/Al ratios, and average pore sizes, but the properties could not be related to the adsorption capacity. The adsorption capacity of all VOCs adsorbed depended on physical characteristics such as BET surface area and mesopore volume. The FAU with large surface
225
area and pore volume was maximum capacity, so it was considered to be a promising adsorbent for removing the VOCs. Table 1. Physical characteristics and VOCs adsorbed amount of various zeolites Zeolites
Amount of VOCs
Pore volume [m3/g]
BET surface
adsorbed [mmol/g]
area [m2/g] . Total pore
Micropore
Mesopore
Toluene
MEK
Z-HM10(2)
370.0
0.08
0.06
0.02
4.30
2.08
Na-Mor.
332.3
0.16
0.14
0.02
2.46
2.53
Z-HY5.6(2)
650.0
0.22
0.19
0.03
5.79
8.31
Z-HY4.8
663.0
0.28
0.24
0.04
10.38
10.18
MS-13X
581.7
0.21
0.19
0.02
13.20
10.53
FAU
691.0
0.29
0.18
0.11
17.10
10.76
Desorption characteristics of VOCs by the microwave irradiation on the saturated adsorbents was investigated for their regeneration. Also desorption efficiency of microwave heating was examined, compared to conventional heating by electric furnace. Temperature rising curve in conventional heating was linear, and its rising rate was very slow. But temperature rising rate by microwave heating was very fast. Desorption curve of toluene and MEK by microwave irradiation on FAU and MS-13X was investigated, and only desorption curves on MS-13X were shown in Fig. 1. As a whole, desorption by microwave heating was reached to completion faster than that by conventional heating. In microwave heating, desorption concentration of MEK was higher than that of toluene. It is thought because microwave is irradiated to only toluene or MEK in a moment. The desorbed amount from MS-13X was more than that from FAU because of the difference of their dielectric constant. Desorption rate of both MS-13X and FAU were almost same. After initial adsorption, first desorption rate and second desorption rate by heating with electric furnace of 500°C were nearly same. By the way, desorption rate from the saturated adsorbents by microwave irradiation for 5min was low, but desorption rate by microwave irradiation for lOmin was above 95%. The desorbed amounts of toluene and MEK by microwave heating were dependent on irradiation time. Also, desorption performance from MS-13X was more efficient than that from FAU. Besides, MS-13X was pretreated in electric furnace of 250°C for lhr. When the MS-13X saturated with VOCs was irradiated for above lOmin, the desorption rate was higher than 100%. The MS-13X has the undesorbed water and impurities even though it was pretreated at 250°C. They can be removed by microwave irradiation of 10 min. This was therefore considered to be a reason for
226
desorprion rate of over 100%. In other words, undesorbed water and impurities might be desorbed with the adsorbed VOCs. These facts suggest that the microwave heating of the adsorbent for regeneration was very efficient. As a result, the microwave heating was known to be an effective means for desorption of VOCs adsorbed on zeolites. Toluene CH-3O0°C CH-500°C'"
CH-etxfc^1 MW-5min MW-10min
! !
1 1 1 1 1i
ft jj
/
1,
' —
r Time on stream [min]
Time on stream [min]
Fig. 1. Desorption curves of toluene and MEK on MS-13X by conventional heating and microwave heating.
4. Conclusion Adsorption capacity and desorption characteristics of VOC over various zeolites and their regeneration by microwave irradiation were investigated. Among the various zeolites used, FAU and MS-13X showed the greatest adsorption capacity, and took the maximum BET surface area, total pore volume, micropore and mesopore. The microwave heating was very effective for desorption of toluene and MEK on FAU and MS-13X. Desorption rate by microwave heating of MS-13X was very excellent. Microwave heating of toluene and MEK depended on irradiation time. It was known that the microwave heating was very effective for regenerating the polluted zeolites. 5. Acknowledgement This subject is supported by Ministry of Environment as "The Eco-technopia 21 project". 6. References [1] [2] [3] [4]
C. L. Chuang and P. C. Chiang, Chemophere, 53 (2003) 17 . K.-J. Kim, et al., Catal. Today, 111 (2006) 223. P. S. Schmidt and J. R. Fair, Waste Management, 14 (1994) 3. C. O. Ania, J. A. Menendez, J. B. Parra and J.J . Pis, Carbon, 42 (2004) 1383.
223 223
Adsorption-desorption characteristics of volatile organic compounds over various zeolites and their regeneration by microwave irradiation K.-J. Kima, Y.-H. Kima, W.-J. Jeongb, N.-C. Parkc, S.-W. Jeongd and H.-G. Ahna* "Dept. ofChem. Eng., Sunchon National Univ., 315 Maegok-dong, Suncheon, Jeonnam, 540-742 Korea Opt. Eng. Res. Institute, Mokpo National Univ., Jeonnam, 453-729, Korea c Dept. ofChem. Eng., Chonnam National Univ., 300 Yongbong-dong, Buk-gu, Gwangju, 500-757 Korea Jeonnam Techno Park, 315 Maegok-dong, Suncheon, Jeonnam, 540-742 Korea.
Adsorption of volatile organic compounds (VOCs) over various zeolites was performed to determine the relationship between adsorption capacity and physical properties of zeolites. The desorption characteristics was investigated by means of conventional heating and microwave irradiation. FAU, Z-HY4.8, and MS-13X were better adsorption performance than the others. Desorption efficiency by microwave heating was better than that by conventional heating. 1. Introduction The volatile organic compounds (VOCs) exhaust that is caused by increase of the various organic solvent and the paints has been gradually increased. The VOCs are present in many types of waste gases, and the adsorption by adsorbents is often used to remove them [1, 2]. Desorption to reutilize the adsorbent saturated (or polluted) with VOCs can be carried out by microwave irradiation, so the adsorbent should be repeatedly utilized [3, 4]. The regeneration of the adsorbent using microwave due to dielectric heating is very effective because the microwave irradiates the VOC molecule directly. If the VOC is a nonpolar compound, the supply of the water is required for the microwave heating. Therefore, the microwave heating by dielectric heating can be performed in a relatively short period of time, which implies a lower
224
consumption of energy. And the main advantage of the microwave heating is that treatment utility for removing the VOCs is simpler and tail gas treatment capacity is smaller than that of direct heating method. Activated carbon cannot be used in this study as adsorbent because it is an electric conductor, so the electric current flows in electric field. The spark discharge occurs from the portion, and shows the light emission phenomenon. The zeolites were chosen for removing the VOCs, which were various pore structures and acidities. In this study, the physical properties of various zeolites and its adsorption capacity for VOCs were investigated. Desorption characteristics of the adsorbed VOCs was mainly investigated by means of microwave irradiation, and it was compared to that by the conventional heating. 2. Experimental Section Physical characteristics of various zeolites such as specific surface area, pore volume and pore size distribution was investigated with BET method (ASAP 2010, Micromeritics, USA). Adsorption and desorption of VOCs were performed using a flow system. Model gases were benzene, toluene, o-, m-, pxylenes, methanol, ethanol, i-propanol, and methylethylketone (MEK). All reagents for model gas were GR gr. (Junsei Chem., 99.0% ~ 99.5%). Concentration of VOCs was controlled with vaporizing individual VOCs in the saturator by He stream. He flow rate (mainly 40ml/min) was controlled using mass flow controller. Before adsorption experiment, the adsorbents (O.lg) were pretreated for lhr at 250°C. The adsorbents used as adsorbent were Molecular Sieve 13X [MS-13X], Mordenites [JRC-Z-HM10(2), Na-mordenite], Y-zeolites [JRC-Z-HY4.8, JRC-Z-HY5.6(2), and Faujasite (FAU)]. In the conventional heating, adsorbent column (o.d. 1/4", ss) of U-type was used. It was heated from 25 to 500°C by 5°C /min. In microwave experiment, adsorbent column (o.d. 3/8", quartz) of a U-type was used. All fittings for fixing an adsorbent column in oven were made of Teflon. Microwave source was generated from a microwave producer (2.45GHz). Concentration of VOCs was monitored with TCD of GC (GC-14B, Shimadzu, Japan). 3. Results and Discussion Physical characteristics by N2 adsorption was investigated for various zeolites. The BET surface area and pore volume by t-plot were shown Table 1, containing the amounts of toluene and MEK adsorbed only. The maximum surface area was observed on FAU. FAU and Z-HY4.8 showed the maximum total pore volume, and micropore was well formed in Z-HY4.8. The used zeolites are possessed of different acidities, Si/Al ratios, and average pore sizes, but the properties could not be related to the adsorption capacity. The adsorption capacity of all VOCs adsorbed depended on physical characteristics such as BET surface area and mesopore volume. The FAU with large surface
225
area and pore volume was maximum capacity, so it was considered to be a promising adsorbent for removing the VOCs. Table 1. Physical characteristics and VOCs adsorbed amount of various zeolites Zeolites
Amount of VOCs
Pore volume [m3/g]
BET surface
adsorbed [mmol/g]
area [m2/g] . Total pore
Micropore
Mesopore
Toluene
MEK
Z-HM10(2)
370.0
0.08
0.06
0.02
4.30
2.08
Na-Mor.
332.3
0.16
0.14
0.02
2.46
2.53
Z-HY5.6(2)
650.0
0.22
0.19
0.03
5.79
8.31
Z-HY4.8
663.0
0.28
0.24
0.04
10.38
10.18
MS-13X
581.7
0.21
0.19
0.02
13.20
10.53
FAU
691.0
0.29
0.18
0.11
17.10
10.76
Desorption characteristics of VOCs by the microwave irradiation on the saturated adsorbents was investigated for their regeneration. Also desorption efficiency of microwave heating was examined, compared to conventional heating by electric furnace. Temperature rising curve in conventional heating was linear, and its rising rate was very slow. But temperature rising rate by microwave heating was very fast. Desorption curve of toluene and MEK by microwave irradiation on FAU and MS-13X was investigated, and only desorption curves on MS-13X were shown in Fig. 1. As a whole, desorption by microwave heating was reached to completion faster than that by conventional heating. In microwave heating, desorption concentration of MEK was higher than that of toluene. It is thought because microwave is irradiated to only toluene or MEK in a moment. The desorbed amount from MS-13X was more than that from FAU because of the difference of their dielectric constant. Desorption rate of both MS-13X and FAU were almost same. After initial adsorption, first desorption rate and second desorption rate by heating with electric furnace of 500°C were nearly same. By the way, desorption rate from the saturated adsorbents by microwave irradiation for 5min was low, but desorption rate by microwave irradiation for lOmin was above 95%. The desorbed amounts of toluene and MEK by microwave heating were dependent on irradiation time. Also, desorption performance from MS-13X was more efficient than that from FAU. Besides, MS-13X was pretreated in electric furnace of 250°C for lhr. When the MS-13X saturated with VOCs was irradiated for above lOmin, the desorption rate was higher than 100%. The MS-13X has the undesorbed water and impurities even though it was pretreated at 250°C. They can be removed by microwave irradiation of 10 min. This was therefore considered to be a reason for
226
desorprion rate of over 100%. In other words, undesorbed water and impurities might be desorbed with the adsorbed VOCs. These facts suggest that the microwave heating of the adsorbent for regeneration was very efficient. As a result, the microwave heating was known to be an effective means for desorption of VOCs adsorbed on zeolites. Toluene CH-3O0°C CH-500°C'"
CH-etxfc^1 MW-5min MW-10min
! !
1 1 1 1 1i
ft jj
/
1,
' —
r Time on stream [min]
Time on stream [min]
Fig. 1. Desorption curves of toluene and MEK on MS-13X by conventional heating and microwave heating.
4. Conclusion Adsorption capacity and desorption characteristics of VOC over various zeolites and their regeneration by microwave irradiation were investigated. Among the various zeolites used, FAU and MS-13X showed the greatest adsorption capacity, and took the maximum BET surface area, total pore volume, micropore and mesopore. The microwave heating was very effective for desorption of toluene and MEK on FAU and MS-13X. Desorption rate by microwave heating of MS-13X was very excellent. Microwave heating of toluene and MEK depended on irradiation time. It was known that the microwave heating was very effective for regenerating the polluted zeolites. 5. Acknowledgement This subject is supported by Ministry of Environment as "The Eco-technopia 21 project". 6. References [1] [2] [3] [4]
C. L. Chuang and P. C. Chiang, Chemophere, 53 (2003) 17 . K.-J. Kim, et al., Catal. Today, 111 (2006) 223. P. S. Schmidt and J. R. Fair, Waste Management, 14 (1994) 3. C. O. Ania, J. A. Menendez, J. B. Parra and J.J . Pis, Carbon, 42 (2004) 1383.