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Pervaporation characteristics of trichlorinated organic compounds through Silicalite-1 zeolite membrane
Desalination 245 (2009) 754–762
Pervaporation characteristics of trichlorinated organic compounds through Silicalite-1 zeolite membrane Hyoseong Ahn, Dongjae Jeong, Heon-Kyu Jeong and Yongtaek Lee* Department of Chemical Engineering, Chungnam National University, 220 Gungdong, Daejeon, Korea Tel. +82 (42) 821-5686, Fax +82 (42) 822-8995; email: [email protected] Received 15 July 2008; revised 17 December 2008; accepted 09 January 2009
Abstract Silicalite-1 membrane synthesized on the inside of a porous stainless steel tube was used for pervaporation of trichloromethane, 1,1,2-trichloroethane and trichloroethylene from their aqueous solutions. Fluxes of trichloromethane, 1,1,2-trichloroethane and trichloroethylene were observed to be 3.0~39.8 g/m2/h, 0.3~4.6 g/m2/h and 0.1~2.0 g/m2/h while their separation factors were 16.4~29.6, 4.2~10.3 and 5.7~18.1, respectively depending on the mole fraction of a trichlorinated organic compound in the feed solution and the operating temperature. The silicalite-1 membrane showed the best separation ability for trichloromethane, since trichloromethane has the smallest kinetic diameter and the highest fugacity among the trichlorinated organic compounds. Keywords: Organic/water separation; Trichlorinated organic compounds; Pervaporation; Silicalite-1; Zeolite membrane
1. Introduction Separation of organic compounds from their aqueous solution is a required process in a point of view from not only prevention of organic pollutants emissions but also recycling of valuable organic materials. A hydrophobic membrane could be used to separate organic compounds from their aqueous solution. Even though hydrophobic polymer membranes could show a high selectivity, their application might be limited *Corresponding author.
by operating conditions such as concentration and temperature since their thermal, chemical and mechanical stabilities are not good enough for certain operating conditions. Because of well known unique pore structure, mechanical, chemical and biological stabilities, zeolite membranes have been widely studied on various applications: gas permeation, vapor permeation, pervaporation, etc.[1–19]. Pervaporation is an economic separation technology utilizing a membrane since it only needs electric powers to maintain the permeate side in vacuum. Also pervaporation is an environmentally clean technology in which potential air
Presented at the conference Engineering with Membranes 2008; Membrane Processes: Development, Monitoring and Modelling – From the Nano to the Macro Scale – (EWM 2008), May 25–28, 2008, Vale do Lobo, Algarve, Portugal. 0011-9164/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2009.02.048
H. Ahn et al. / Desalination 245 (2009) 754–762
pollution sources such as entrainers for azeotropic distillation are not needed [20,21]. A hydrophobic zeolite membrane might be useful for separation of organic materials with a pervaporation technique since it shows not only a molecular sieve effect but also a good physicochemical stability [22]. Chlorinated organic compounds are useful materials which have been used for various applications such as solvents, coating agents, extractants, cleaning agents and so on [23]. Chlorinated organic compounds are strongly regulated on a discharge so that the air and water pollutions can be prevented. Much research have been carried out for the adsorption of chlorinated organic compounds on zeolite powders. Adsorption of trichlorinated organic compounds on synthetic sorbents [24] have been reported. Also, the decomposition or the catalytic oxidation of chlorinated organic compounds were reported using ion-exchanged zeolites such as Co-Y, Cr-Y, Mn-Y [25], H-Y, NaY [26] and H-Y, H-ZSM-5 [27]. The silicalite-1 zeolite membrane classified as a MFI structure has been studied for separation of organic materials from their aqueous solution since it is known to be quite hydrophobic [22]. Noble et al. [22] studied the separation of methanol from its aqueous solution using a silicalite-1 zeolite membrane by pervaporation. The total flux increased as the feed concentration increased and the highest separation factor could be obtained at 16.5 wt% of the feed concentration of methanol. Their membrane showed more outstanding separation behavior on acetone/water pervaporation than on methanol/water. They obtained the highest separation factor of 255 at the acetone feed concentration of 0.8 wt% and the highest acetone flux of 950 g/m2/h at the acetone concentration of 43 wt%. Sano et al. [7] synthesized a silicalite-1 zeolite membrane on a sintered stainless steel tube as a porous support. Their membrane was used for pervaporation of acetic acid from its aqueous solution. It was shown that the constant total flux was obtained at the concentration of above 20 vol% of
755
acetic acid in the feed solution and the permeate concentration of acetic acid was fairly higher than the feed. Nomura et al. [19] reported pervaporation characteristics of ethanol/water mixture by a silicalite-1 zeolite membrane synthesized on a sintered stainless steel tube. Concentrations of ethanol in the permeate were higher than those obtained by the vapor–liquid equilibrium at each feed concentration of ethanol. It was shown that the ethanol flux was increased with increase of a feed concentration of ethanol, and the highest separation factor was observed at 4.65 wt% of ethanol in the feed. In our previous work [28], it has been reported that the pervaporation of dichloromethane, 1,2dichloroethane and trans-1,2-dichloroethylene from their aqueous solution through the silicalite-1 membrane was reasonably successful. The flux of dichlorinated organic compounds was observed to be 0.2~71 g/m2/h while the separation factor was in the range from 12 to 332 depending on the concentration of a dichlorinated organic compound in the feed solution and the operation temperature. In this study, the silicalite-1 zeolite membrane was prepared by the secondary-growth method as reported in our previous work where the seed crystals were spread on the inside of a porous stainless steel support [28]. The secondarygrowth method may provide some advantages: preparation of a thin and uniform silicalite-1 layer, and a relatively short synthesis time. The synthesized silicalite-1 zeolite membrane was used to separate trichlorinated organic compounds (TCOs) from an aqueous feed solution. The effect of a temperature and a feed concentration are presented on TCOs/water separation.
2. Experimental 2.1. Membrane preparation and pervaporation The synthesis procedure of silicalite-1 zeolite membranes and the pervaporation apparatus were the same as in our previous work [28]. In Table 1, the physical properties of TCOs are represented.
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Table 1 Physical properties of TCOs Name
Trichloromethane
1,1,2-trichloroethane
Trichloroethylene
CHCl3 119.38 61.17
C2H3Cl3 133.41 113.45
C2HCl3 131.39 87.21
0.82
0.45
0.107
26.41 32.62 39.96 58.64
3.19 4.13 5.29 8.46
9.65 12.13 15.13 22.99
Chemical structure Molecular weight Boiling point(οC) Solubility (g/100g H2O, 20 οC) Vapor pressure (kPa)a 25 (οC) 30 (οC) 35 (οC) 45 (οC) a
Vapor pressure was calculated from the KDB correlation equation provided by Ref. [29].
2.2. Pervaporation characteristics Pervaporation characteristics might be presented in terms of a flux and a separation factor defined as follows: Flux =
P A⋅t
Separtion factor =
(1)
yTCOs / yH 2O xTCOs / xH 2O
(2)
where P denotes the weight of the permeate through the membrane, A represents the area of the membrane, and t means the time needed for collection of the permeate. y and x represent the mole fraction of a component in the permeate and the feed, respectively. 3. Results and discussion 3.1. Fluxes of TCOs and water TCOs and water fluxes through the silicalite1 membrane are plotted in Fig.1. Fluxes of TCOs increased with a mole fraction of TCOs and temperature. However, water fluxes decreased with
a mole fraction of TCOs and increased with temperature. This might be due to the increment of the fugacity of TCOs in the feed side shown in Table 2. A great transportation of TCOs might be resulted from an increase of the driving force of TCOs through the silicalite-1 zeolite, which is increased with fugacity. The change in flux of TCOs is almost proportional to the change in fugacity. It is expected that the flux of TCOs depends strongly on the competitive adsorption between TCOs and water. Since the silicalite-1 zeolite is quite hydrophobic, the increase of TCOs coverage on the zeolite would result in the decrease of water coverages as the concentration of TCOs increases. A decrease of water coverage would result in a decrease of the driving force for water transportation and accordingly the water flux decreases as shown in Fig.1 (b). The water flux is observed to be much higher than the TCOs flux at all the experimental conditions. Water can be adsorbed and transported since adsorption of water occurs more easily than adsorption of TCOs under the experimental situation where water is the dominant component: the mole fraction of water is over 0.999. Water is found to be preferably transported over TCOs as shown in Fig.1 (b). However, the water flux drastically decreased with an increase of mole frac-
H. Ahn et al. / Desalination 245 (2009) 754–762
g/m2/h at the feed concentration of 5 wt% (0.12 mole fraction) and 75οC. The flux of TCOs for our study was ranged between 3 g/m2/h and 40 g/m2/h below 0.001 mole fraction of TCOs in a feed solution. The flux of TCOs seems reasonable because the experiments were carried out under a very low mole fraction in the feed
tion of TCOs in the feed solution. This suggests that the fugacity of TCOs seriously affects the coverages of TCOs at a relatively high concentration of TCOs. The fugacity of TCOs is shown in Table 2. Kita et al. [30] reported that their tubular NaA zeolite module showed the water flux of 1,100
Trichlorinated organics flux (g/m2/h)
50
40
30
20
1,1,2-trichloroethane, 25°C 1,1,2-trichloroethane, 35°C 1,1,2-trichloroethane, 45°C trichloroethylene, 25°C trichloroethylene, 35°C trichloroethylene, 45°C trichloromethane, 25°C trichloromethane, 30°C trichloromethane, 35°C
10
0
1 × 10−5 1 × 10−4 1 × 10−3 Concentration of trichlorinated organic compounds (mole fraction) (a) 800 1,1,2-trichloroethane, 25°C 1,1,2-trichloroethane, 35°C 1,1,2-trichloroethane, 45°C trichloroethylene, 25°C trichloroethylene, 35°C trichloroethylene, 45°C trichloromethane, 25°C trichloromethane, 30°C trichloromethane, 35°C
600 water flux (g/m2/h)
757
400
200
0 1 × 10−5 1 × 10−4 1 × 10−3 Concentration of trichlorinated organic compounds (mole fraction) (b)
Fig.1. Fluxes through silicalite-1 membrane. (a) Fluxes of TCOs; (b) Fluxes of water.
3.16 – 5.62 9.59
37.56 0.56 1.11 44.05 51.31 0.80 1.59 1.11 2.21 0.999 0.99995 0.9999
3.88 11.53 26.57 4.54 13.52 31.16 5.29 15.74 44.05 0.9999 0.9997 0.9993 3.16 4.24 5.62 –
0.001 0.00005 0.0001
0.0001 0.0003 0.0007 3.30 4.72 6.55 0.9997
0.0003
1,1,2-trichloroethane
3.16 – 5.62 9.59
5.46 0.12 0.37 0.73 7.80 0.17 0.51 1.03 10.83 0.23 0.70 1.40 0.9995 0.99999 0.99997 0.99994
0.0005 0.00001 0.00003 0.00006
Trichloroethylene
1.22 1.71 2.34 0.9999
0.0001
a
Activity coefficient was obtained from UNIFAC calculation. Fugacity was calculated by the equation, f i = xiγ i Pi sat , xi, mole fraction, yi, activity coefficient, Pisat, saturated vapor pressure [kPa].
Mole fraction of organic compounds Fugacity 25(οC) of organics 30(οC) 35(οC) (kPa)a 45(οC) Mole fraction of water Fugacity 25(οC) of 30(οC) water 35(οC) a (kPa) 45(οC)
Trichloromethane
Table 2 Fugacity of the components in the feed solution
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H. Ahn et al. / Desalination 245 (2009) 754–762
solution and at a low temperature compared to Kita’s experimental condition. 3.2. Separation factor The separation factor of TCOs was plotted as a function of the feed mole fraction of TCOs through the silicaite-1 membrane as shown in Fig.2. The separation factor increased as both the mole fraction of TCOs and the temperature increased. The separation factor was not proportional to a change of the fugacity of TCOs in the feed solution. The increase of the separation factor could be explained by the definition of the separation factor equation. As shown in the definition of the separation factor, the increasing rate of the numerator ( yTCOs / yH 2O ) according to the increment of TCOs fluxes in Fig.1 is much greater than the increasing rate of the denominator ( xTCOs / xH 2O ) from the increment of the feed mole fraction.
3.3. Comparison of separation behavior of TCOs Organic fluxes at the same mole fraction of 0.0001 in the feed solution are shown in Fig. 3 to compare the pervaporation behavior among TCOs as a function of the temperature. Trichloromethane showed the highest flux while 1,1,2-trichloroethane showed the lowest flux. This may be due to the fugacity difference of each compound. It can be said that the coverage of trichloromethane is the highest because it has the highest fugacity among TCOs. Further the molecule of trichloromethane can diffuse quite easily since its kinetic diameter of 5.389Å [31] is small enough to pass the pore of silicalite-1 zeolite which is known to be 5.1Å ~ 5.6Å. Though 1,1,2-trichloroethane and trichloroethylene might have similar kinetic diameters, 1,1,2-trichloroethane fluxes are lower than trichloroethylene since the fugacity of trichloroethylene is much higher than that of 1,1,2-trichloroethane as shown in Table 2.
50
Separation factor
40
30
1,1,2-trichloroethane, 25°C 1,1,2-trichloroethane, 35°C 1,1,2-trichloroethane, 45°C trichloroethylene, 25°C trichloroethylene, 35°C trichloroethylene, 45°C trichloromethane, 25°C trichloromethane, 30°C trichloromethane, 35°C
20
10
0 1 × 10−5
759
1 × 10−4
1 × 10−3
Concentration of trichlorinated organics compounds (mole fraction)
Fig. 2. Separation factors of TCOs through silicalite-1 membrane.
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trichloromethane 1,1,2-trichloroethane trichloroethylene
Organic flux (g/m2/hr)
6
4
2
0 20
25
30
35
40
45
50
Temperature (°C)
Fig. 3. Fluxes of TCOs at the same feed mole fraction of 0.0001.
trichloroethane shows the lowest. The separation factor difference among TCOs might be understood by the difference of the permeation flux according to Fig.3. The separation factor
The separation factors of TCOs at the same mole fraction of 0.0001 in the feed mixture are plotted in Fig. 4. The separation factor of trichloromethane shows the highest and 1,1,230 1,1,2-trichloromethane 1,1,2-trichloroethane trichloroethylene
25
Separation factor
20
15
10
5
0 20
25
30
35
40
Temperature (°C)
Fig. 4. Separation factors of TCOs at the same feed mole fraction of 0.0001
45
50
H. Ahn et al. / Desalination 245 (2009) 754–762
increased as the temperature increased which leads to the increment of the fugacity. At the same operation condition, the separation factor of trichloromethane was observed to be 2~3 times larger than that of 1,1,2-trichloroethane and the separation factor of trichloroethylene was 2 times higher than that of 1,1,2trichloroethane as seen in Fig. 4. 4. Conclusions Synthesized silicalite-1 membranes showed reasonable fluxes and separation factors for the binary aqueous solution of trichloromethane/water, 1,1,2-trichloroethane/water and trichloroethylene/water. The fluxes and the separation factors through silicalite-1 zeolite membranes were strongly affected by both the feed concentration and the operating temperature which results in the fugacity changes of the components. The separation factors of trichloromethane/water mixture and trichloroethylene/water were found to be 3 times and 2 times higher than that obtained from 1,1,2trichloethane/water mixture, respectively. Silicalite-1 membrane might be one of the potential pervaporation membranes for separation of trichlorinated organic compounds from their aqueous solutions. Acknowledgement This work was supported by Korea Science and Engineering Foundation Grant (R05-2003000-10119-0).
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[4] H. Kita, K. Horii, Y. Ohtoshi, K. Tanaka and K. Okamoto, J. Mater. Sci. Lett., 14 (1995) 206–208. [5] M. Kondo, M. Komori, H. Kita and K. Okamoto, J. Membrane Sci., 133 (1997) 133–141. [6] D. Shah, K. Kissick, A. Ghorpade, R. Hannah and D. Bhattacharyya, J. Membrane Sci., 179 (2000) 185–205. [7] T. Sano, H. Yanagishita, Y. Kiyozumi, F. Mizukami and K. Haraya, J. Membrane Sci., 95 (1994) 221– 228. [8] S. Lu, C. Chiu and H. Huang, J. Membrane Sci., 176 (2000) 159–167. [9] H. Takaba, R. Koshita, K. Mizukami, Y. Oumi, N. Ito, M. Kubo, A. Fahmi and A. Miyamoto, J. Membrane Sci., 134 (1997) 127–139. [10] W.J.W. Bakker, F. Kapteijn, J. Poppe and J. Moulijn, J. Membrane Sci., 117 (1997) 57–78. [11] C. Bai, M. Jia, J.L. Falconer and R.D. Noble, J. Membrane Sci., 105 (1995) 79–87. [12] J. Dong, K. Wegner and Y.S. Lin, J. Membrane Sci., 148 (1998) 233–241. [13] M. Yang, B.D. Crittenden, S.P. Perera, H. Moueddeb and J.A. Dalmon, J. Membrane Sci., 156 (1999) 1–9. [14] M. Nomura, T. Yamaguchi and S.I. Nakao, J. Membrane Sci., 187 (2001) 203–212. [15] M. Noack, P. Kölsch, R. Schäfer, P. Toussaint, I. Sieber and J. Caro, Micropor. Mesopor. Mater., 49 (2001) 25–37. [16] M. Arruebo, J. Coronas, M. Meméndez and J. Santamaria, Sep. Puri. Technol., 25 (2001) 275– 286. [17] M. Noack, P. Kölsch, P.J. Caro, M. Schneider, P. Toussaint and I. Sieber, Micropor. Mesopor. Mater., 35–36 (2000) 253–265. [18] W. Zhu, F. Kapteijn and J.A. Moulijn, Sep. Puri. Technol., 32 (2003) 223–230. [19] M. Nomura, T. Yamaguchi and S. Nakao, J. Membrane Sci., 144 (1998) 161–171. [20] C.C. Pereira, A.C. Habert, R. Nobrega and C.P. Borges, J. Membrane Sci., 138 (1998) 227–235. [21] D. Hofmann, L. Fritz and D. Paul, J. Membrane Sci., 144 (1998) 145–159. [22] Q. Liu, R.D. Noble, J.L. Falconer and H.H. Funke, J. Membrane Sci., 117 (1996) 163–174. [23] R.E. Kirk and D.F. Othmer, Encyclopedia of chemical technology, 4th ed., John Wiley & Sons, New York, 1991. [24] G. Rexwinkel, J.T.A. M. Berkhout and A.B.M. Heesink, Chem. Eng. Sci., 58 (2003) 1301–1314. [25] S. Chatterjee, H.L. Greene and Y.J. Park, J. Catal., 138 (1992) 179–194.
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[26] B. Ramachandran, H.L. Greene and S. Chatterjee, Appl. Catal. B: Environ., 8 (1996) 157–182. [27] J.R. González-Velasco, R. López-Fonseca, A. Aranzabal, J.J. Gutiérrez-Ortiz and P. Steltenpohl, Appl. Catal. B: Environ., 24 (2000) 233–242. [28] H. Ahn, and Y. Lee, J. Membrane Sci., 279 (2006) 459–465.
[29] http://www.cheric.org/research/kdb/hcprop/cmpsrch.php. [30] Y. Morigami, M. Kondo, J. Abe, H. Kita, and K. Okamoto, Sep. Puri. Technol., 25 (2001) 251–260. [31] R.C. Reid, J. M. Prausnitz and B.E. Poling, Properties of Gases and Liquids, 4th ed., McGraw-Hill, New York, 1987.
Pervaporation characteristics of trichlorinated organic compounds through Silicalite-1 zeolite membrane Hyoseong Ahn, Dongjae Jeong, Heon-Kyu Jeong and Yongtaek Lee* Department of Chemical Engineering, Chungnam National University, 220 Gungdong, Daejeon, Korea Tel. +82 (42) 821-5686, Fax +82 (42) 822-8995; email: [email protected] Received 15 July 2008; revised 17 December 2008; accepted 09 January 2009
Abstract Silicalite-1 membrane synthesized on the inside of a porous stainless steel tube was used for pervaporation of trichloromethane, 1,1,2-trichloroethane and trichloroethylene from their aqueous solutions. Fluxes of trichloromethane, 1,1,2-trichloroethane and trichloroethylene were observed to be 3.0~39.8 g/m2/h, 0.3~4.6 g/m2/h and 0.1~2.0 g/m2/h while their separation factors were 16.4~29.6, 4.2~10.3 and 5.7~18.1, respectively depending on the mole fraction of a trichlorinated organic compound in the feed solution and the operating temperature. The silicalite-1 membrane showed the best separation ability for trichloromethane, since trichloromethane has the smallest kinetic diameter and the highest fugacity among the trichlorinated organic compounds. Keywords: Organic/water separation; Trichlorinated organic compounds; Pervaporation; Silicalite-1; Zeolite membrane
1. Introduction Separation of organic compounds from their aqueous solution is a required process in a point of view from not only prevention of organic pollutants emissions but also recycling of valuable organic materials. A hydrophobic membrane could be used to separate organic compounds from their aqueous solution. Even though hydrophobic polymer membranes could show a high selectivity, their application might be limited *Corresponding author.
by operating conditions such as concentration and temperature since their thermal, chemical and mechanical stabilities are not good enough for certain operating conditions. Because of well known unique pore structure, mechanical, chemical and biological stabilities, zeolite membranes have been widely studied on various applications: gas permeation, vapor permeation, pervaporation, etc.[1–19]. Pervaporation is an economic separation technology utilizing a membrane since it only needs electric powers to maintain the permeate side in vacuum. Also pervaporation is an environmentally clean technology in which potential air
Presented at the conference Engineering with Membranes 2008; Membrane Processes: Development, Monitoring and Modelling – From the Nano to the Macro Scale – (EWM 2008), May 25–28, 2008, Vale do Lobo, Algarve, Portugal. 0011-9164/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2009.02.048
H. Ahn et al. / Desalination 245 (2009) 754–762
pollution sources such as entrainers for azeotropic distillation are not needed [20,21]. A hydrophobic zeolite membrane might be useful for separation of organic materials with a pervaporation technique since it shows not only a molecular sieve effect but also a good physicochemical stability [22]. Chlorinated organic compounds are useful materials which have been used for various applications such as solvents, coating agents, extractants, cleaning agents and so on [23]. Chlorinated organic compounds are strongly regulated on a discharge so that the air and water pollutions can be prevented. Much research have been carried out for the adsorption of chlorinated organic compounds on zeolite powders. Adsorption of trichlorinated organic compounds on synthetic sorbents [24] have been reported. Also, the decomposition or the catalytic oxidation of chlorinated organic compounds were reported using ion-exchanged zeolites such as Co-Y, Cr-Y, Mn-Y [25], H-Y, NaY [26] and H-Y, H-ZSM-5 [27]. The silicalite-1 zeolite membrane classified as a MFI structure has been studied for separation of organic materials from their aqueous solution since it is known to be quite hydrophobic [22]. Noble et al. [22] studied the separation of methanol from its aqueous solution using a silicalite-1 zeolite membrane by pervaporation. The total flux increased as the feed concentration increased and the highest separation factor could be obtained at 16.5 wt% of the feed concentration of methanol. Their membrane showed more outstanding separation behavior on acetone/water pervaporation than on methanol/water. They obtained the highest separation factor of 255 at the acetone feed concentration of 0.8 wt% and the highest acetone flux of 950 g/m2/h at the acetone concentration of 43 wt%. Sano et al. [7] synthesized a silicalite-1 zeolite membrane on a sintered stainless steel tube as a porous support. Their membrane was used for pervaporation of acetic acid from its aqueous solution. It was shown that the constant total flux was obtained at the concentration of above 20 vol% of
755
acetic acid in the feed solution and the permeate concentration of acetic acid was fairly higher than the feed. Nomura et al. [19] reported pervaporation characteristics of ethanol/water mixture by a silicalite-1 zeolite membrane synthesized on a sintered stainless steel tube. Concentrations of ethanol in the permeate were higher than those obtained by the vapor–liquid equilibrium at each feed concentration of ethanol. It was shown that the ethanol flux was increased with increase of a feed concentration of ethanol, and the highest separation factor was observed at 4.65 wt% of ethanol in the feed. In our previous work [28], it has been reported that the pervaporation of dichloromethane, 1,2dichloroethane and trans-1,2-dichloroethylene from their aqueous solution through the silicalite-1 membrane was reasonably successful. The flux of dichlorinated organic compounds was observed to be 0.2~71 g/m2/h while the separation factor was in the range from 12 to 332 depending on the concentration of a dichlorinated organic compound in the feed solution and the operation temperature. In this study, the silicalite-1 zeolite membrane was prepared by the secondary-growth method as reported in our previous work where the seed crystals were spread on the inside of a porous stainless steel support [28]. The secondarygrowth method may provide some advantages: preparation of a thin and uniform silicalite-1 layer, and a relatively short synthesis time. The synthesized silicalite-1 zeolite membrane was used to separate trichlorinated organic compounds (TCOs) from an aqueous feed solution. The effect of a temperature and a feed concentration are presented on TCOs/water separation.
2. Experimental 2.1. Membrane preparation and pervaporation The synthesis procedure of silicalite-1 zeolite membranes and the pervaporation apparatus were the same as in our previous work [28]. In Table 1, the physical properties of TCOs are represented.
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H. Ahn et al. / Desalination 245 (2009) 754–762
Table 1 Physical properties of TCOs Name
Trichloromethane
1,1,2-trichloroethane
Trichloroethylene
CHCl3 119.38 61.17
C2H3Cl3 133.41 113.45
C2HCl3 131.39 87.21
0.82
0.45
0.107
26.41 32.62 39.96 58.64
3.19 4.13 5.29 8.46
9.65 12.13 15.13 22.99
Chemical structure Molecular weight Boiling point(οC) Solubility (g/100g H2O, 20 οC) Vapor pressure (kPa)a 25 (οC) 30 (οC) 35 (οC) 45 (οC) a
Vapor pressure was calculated from the KDB correlation equation provided by Ref. [29].
2.2. Pervaporation characteristics Pervaporation characteristics might be presented in terms of a flux and a separation factor defined as follows: Flux =
P A⋅t
Separtion factor =
(1)
yTCOs / yH 2O xTCOs / xH 2O
(2)
where P denotes the weight of the permeate through the membrane, A represents the area of the membrane, and t means the time needed for collection of the permeate. y and x represent the mole fraction of a component in the permeate and the feed, respectively. 3. Results and discussion 3.1. Fluxes of TCOs and water TCOs and water fluxes through the silicalite1 membrane are plotted in Fig.1. Fluxes of TCOs increased with a mole fraction of TCOs and temperature. However, water fluxes decreased with
a mole fraction of TCOs and increased with temperature. This might be due to the increment of the fugacity of TCOs in the feed side shown in Table 2. A great transportation of TCOs might be resulted from an increase of the driving force of TCOs through the silicalite-1 zeolite, which is increased with fugacity. The change in flux of TCOs is almost proportional to the change in fugacity. It is expected that the flux of TCOs depends strongly on the competitive adsorption between TCOs and water. Since the silicalite-1 zeolite is quite hydrophobic, the increase of TCOs coverage on the zeolite would result in the decrease of water coverages as the concentration of TCOs increases. A decrease of water coverage would result in a decrease of the driving force for water transportation and accordingly the water flux decreases as shown in Fig.1 (b). The water flux is observed to be much higher than the TCOs flux at all the experimental conditions. Water can be adsorbed and transported since adsorption of water occurs more easily than adsorption of TCOs under the experimental situation where water is the dominant component: the mole fraction of water is over 0.999. Water is found to be preferably transported over TCOs as shown in Fig.1 (b). However, the water flux drastically decreased with an increase of mole frac-
H. Ahn et al. / Desalination 245 (2009) 754–762
g/m2/h at the feed concentration of 5 wt% (0.12 mole fraction) and 75οC. The flux of TCOs for our study was ranged between 3 g/m2/h and 40 g/m2/h below 0.001 mole fraction of TCOs in a feed solution. The flux of TCOs seems reasonable because the experiments were carried out under a very low mole fraction in the feed
tion of TCOs in the feed solution. This suggests that the fugacity of TCOs seriously affects the coverages of TCOs at a relatively high concentration of TCOs. The fugacity of TCOs is shown in Table 2. Kita et al. [30] reported that their tubular NaA zeolite module showed the water flux of 1,100
Trichlorinated organics flux (g/m2/h)
50
40
30
20
1,1,2-trichloroethane, 25°C 1,1,2-trichloroethane, 35°C 1,1,2-trichloroethane, 45°C trichloroethylene, 25°C trichloroethylene, 35°C trichloroethylene, 45°C trichloromethane, 25°C trichloromethane, 30°C trichloromethane, 35°C
10
0
1 × 10−5 1 × 10−4 1 × 10−3 Concentration of trichlorinated organic compounds (mole fraction) (a) 800 1,1,2-trichloroethane, 25°C 1,1,2-trichloroethane, 35°C 1,1,2-trichloroethane, 45°C trichloroethylene, 25°C trichloroethylene, 35°C trichloroethylene, 45°C trichloromethane, 25°C trichloromethane, 30°C trichloromethane, 35°C
600 water flux (g/m2/h)
757
400
200
0 1 × 10−5 1 × 10−4 1 × 10−3 Concentration of trichlorinated organic compounds (mole fraction) (b)
Fig.1. Fluxes through silicalite-1 membrane. (a) Fluxes of TCOs; (b) Fluxes of water.
3.16 – 5.62 9.59
37.56 0.56 1.11 44.05 51.31 0.80 1.59 1.11 2.21 0.999 0.99995 0.9999
3.88 11.53 26.57 4.54 13.52 31.16 5.29 15.74 44.05 0.9999 0.9997 0.9993 3.16 4.24 5.62 –
0.001 0.00005 0.0001
0.0001 0.0003 0.0007 3.30 4.72 6.55 0.9997
0.0003
1,1,2-trichloroethane
3.16 – 5.62 9.59
5.46 0.12 0.37 0.73 7.80 0.17 0.51 1.03 10.83 0.23 0.70 1.40 0.9995 0.99999 0.99997 0.99994
0.0005 0.00001 0.00003 0.00006
Trichloroethylene
1.22 1.71 2.34 0.9999
0.0001
a
Activity coefficient was obtained from UNIFAC calculation. Fugacity was calculated by the equation, f i = xiγ i Pi sat , xi, mole fraction, yi, activity coefficient, Pisat, saturated vapor pressure [kPa].
Mole fraction of organic compounds Fugacity 25(οC) of organics 30(οC) 35(οC) (kPa)a 45(οC) Mole fraction of water Fugacity 25(οC) of 30(οC) water 35(οC) a (kPa) 45(οC)
Trichloromethane
Table 2 Fugacity of the components in the feed solution
758 H. Ahn et al. / Desalination 245 (2009) 754–762
H. Ahn et al. / Desalination 245 (2009) 754–762
solution and at a low temperature compared to Kita’s experimental condition. 3.2. Separation factor The separation factor of TCOs was plotted as a function of the feed mole fraction of TCOs through the silicaite-1 membrane as shown in Fig.2. The separation factor increased as both the mole fraction of TCOs and the temperature increased. The separation factor was not proportional to a change of the fugacity of TCOs in the feed solution. The increase of the separation factor could be explained by the definition of the separation factor equation. As shown in the definition of the separation factor, the increasing rate of the numerator ( yTCOs / yH 2O ) according to the increment of TCOs fluxes in Fig.1 is much greater than the increasing rate of the denominator ( xTCOs / xH 2O ) from the increment of the feed mole fraction.
3.3. Comparison of separation behavior of TCOs Organic fluxes at the same mole fraction of 0.0001 in the feed solution are shown in Fig. 3 to compare the pervaporation behavior among TCOs as a function of the temperature. Trichloromethane showed the highest flux while 1,1,2-trichloroethane showed the lowest flux. This may be due to the fugacity difference of each compound. It can be said that the coverage of trichloromethane is the highest because it has the highest fugacity among TCOs. Further the molecule of trichloromethane can diffuse quite easily since its kinetic diameter of 5.389Å [31] is small enough to pass the pore of silicalite-1 zeolite which is known to be 5.1Å ~ 5.6Å. Though 1,1,2-trichloroethane and trichloroethylene might have similar kinetic diameters, 1,1,2-trichloroethane fluxes are lower than trichloroethylene since the fugacity of trichloroethylene is much higher than that of 1,1,2-trichloroethane as shown in Table 2.
50
Separation factor
40
30
1,1,2-trichloroethane, 25°C 1,1,2-trichloroethane, 35°C 1,1,2-trichloroethane, 45°C trichloroethylene, 25°C trichloroethylene, 35°C trichloroethylene, 45°C trichloromethane, 25°C trichloromethane, 30°C trichloromethane, 35°C
20
10
0 1 × 10−5
759
1 × 10−4
1 × 10−3
Concentration of trichlorinated organics compounds (mole fraction)
Fig. 2. Separation factors of TCOs through silicalite-1 membrane.
760
H. Ahn et al. / Desalination 245 (2009) 754–762
trichloromethane 1,1,2-trichloroethane trichloroethylene
Organic flux (g/m2/hr)
6
4
2
0 20
25
30
35
40
45
50
Temperature (°C)
Fig. 3. Fluxes of TCOs at the same feed mole fraction of 0.0001.
trichloroethane shows the lowest. The separation factor difference among TCOs might be understood by the difference of the permeation flux according to Fig.3. The separation factor
The separation factors of TCOs at the same mole fraction of 0.0001 in the feed mixture are plotted in Fig. 4. The separation factor of trichloromethane shows the highest and 1,1,230 1,1,2-trichloromethane 1,1,2-trichloroethane trichloroethylene
25
Separation factor
20
15
10
5
0 20
25
30
35
40
Temperature (°C)
Fig. 4. Separation factors of TCOs at the same feed mole fraction of 0.0001
45
50
H. Ahn et al. / Desalination 245 (2009) 754–762
increased as the temperature increased which leads to the increment of the fugacity. At the same operation condition, the separation factor of trichloromethane was observed to be 2~3 times larger than that of 1,1,2-trichloroethane and the separation factor of trichloroethylene was 2 times higher than that of 1,1,2trichloroethane as seen in Fig. 4. 4. Conclusions Synthesized silicalite-1 membranes showed reasonable fluxes and separation factors for the binary aqueous solution of trichloromethane/water, 1,1,2-trichloroethane/water and trichloroethylene/water. The fluxes and the separation factors through silicalite-1 zeolite membranes were strongly affected by both the feed concentration and the operating temperature which results in the fugacity changes of the components. The separation factors of trichloromethane/water mixture and trichloroethylene/water were found to be 3 times and 2 times higher than that obtained from 1,1,2trichloethane/water mixture, respectively. Silicalite-1 membrane might be one of the potential pervaporation membranes for separation of trichlorinated organic compounds from their aqueous solutions. Acknowledgement This work was supported by Korea Science and Engineering Foundation Grant (R05-2003000-10119-0).
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