water solutions

Journal of Membrane Science 369 (2011) 13–19

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Cobalt-doped silica membranes for pervaporation dehydration of ethanol/water solutions Jinhui Wang, Toshinori Tsuru ∗ Department of Chemical Engineering, Hiroshima University, 1-4-1 Kagayami-yama, Higashi-Hiroshima 739-8527, Japan

a r t i c l e

i n f o

Article history: Received 12 May 2010 Received in revised form 18 October 2010 Accepted 23 October 2010 Available online 3 November 2010 Keywords: Cobalt-doped silica membranes Pervaporation Ethanol Gas permeation Dehydration

a b s t r a c t Cobalt-doped silica (Co–SiO2 ) membranes were successfully fabricated by sol–gel processing. After measurement of single-gas permeation through the membranes, they were used in pervaporation (PV) experiments with ethanol feed concentrations ranging from 50 to 94 wt% and temperatures from 50 to75 ◦ C. The membrane prepared by firing at 550 ◦ C showed a He permeance of 2.6 × 10−6 mol/(m2 s Pa) with permeance ratios of 20 for He/N2 and 113,000 for He/SF6 , respectively. In addition, a permeate water flux of 1.1 kg/(m2 h) with a separation factor of 2530 was obtained at 75 ◦ C in 90 wt% ethanol. In PV dehydration experiments, the Co–SiO2 membranes showed a decrease in permeate flux and an increase in separation factor during the first several hours, then reached a steady state and subsequently remained constant. When the firing temperature of the top layer was increased from 350 to 550 ◦ C, both the singlegas permeance and the PV flux decreased, while the gas permeance ratio (He/N2 ) and the PV separation factor increased, suggesting an increase in the density of the network structure. In the PV experiments, water and ethanol showed approximately the same permeances as that of single-gas permeation, suggesting the permeation mechanism of water and ethanol in PV was dominated by molecular sieving. Moreover, cobalt-doped silica membranes showed good stability in an aqueous environment during the long-term PV experiments, which lasted 150 days. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Pervaporation (PV) is a promising alternative to conventional energy-intensive processes such as distillation and evaporation. PV is a “clean technology,” which is especially effective for the treatment of volatile organic compounds. Pervaporation can be used to break azeotropes, as its mechanism of separation is very different from that of distillation [1]. Polymeric, inorganic membranes and mixed matrix membranes have been applied to the dehydration of alcohols — mostly concentrations of ethanol/water and IPA/water systems. Zeolite membranes that are both highly selective and permeable have been reported for dehydration of ethanol/water solutions. For example, the zeolite NaA membranes developed by Kita et al. [2] had total fluxes of 2.15 and 1.10 kg/(m2 h) at 75 ◦ C with separation factors of 10,000 and 16,000 for ethanol aqueous solutions with 10 and 5 wt% of water, respectively. Due to extensive efforts to develop zeolite membranes with improved performance, zeolite membranes have been commercialized by several manufacturers [3–5]. The main drawbacks of zeolite membranes are that ceramic

∗ Corresponding author. Tel.: +81 824 24 7714; fax: +81 824 22 7191. E-mail address: [email protected] (T. Tsuru). 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.10.062

membranes are more expensive than polymeric membranes and their stability in acidic solutions is inadequate. Silica membranes comprise another class of inorganic membranes. Silica or silicabased membranes prepared by sol–gel processing also are highly selective to permeation of smaller molecules. An advantage of silica-based membranes is that they are relatively inexpensive [6] and quite stable in acidic solutions [7]. Therefore, silica membranes should be investigated as an alternative to zeolite membranes. Pervatech [8] has commercialized silica membranes with a water flux of 0.1–10 kg/(m2 h) and a separation factor greater than 350 in 96.4 wt% ethanol solutions at 70 ◦ C [9]. Generally, silica membranes are quite stable in dry conditions, but are unstable under high humidity or aqueous conditions. Under humid conditions, both permeability and selectivity gradually decrease at high temperatures [10] and at room temperature [11]. An attempt has been made to improve the stability of pure silica membranes by increasing the hydrophobicity of the organic–inorganic hybrid silica and by doping metal ions into the silica networks. The incorporation of methyl groups into microporous silica membranes, which was realized by use of mixed alkoxide precursors such as methyltriethoxysilane (MTES) and tetraethoxysilane (TEOS), enhanced the membrane lifetime for the dehydration of a butanol/water mixture at 95 ◦ C from a few weeks to more than 18 months with a water flux of about 4 kg/(m2 h) and a selectivity between 500 and 20,000 [12].

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J. Wang, T. Tsuru / Journal of Membrane Science 369 (2011) 13–19

Doping with metal is another option by which to improve the stability of pure silica membranes. Various types of metal ions were incorporated during sol preparation. Ni-doped silica membranes [13] fired in a steam atmosphere (partial pressure: 90 kPa) at 650 ◦ C had an asymptotic steady permeance of 1.6 × 10−5 for He and 4.6 × 10−6 m3 (STP)/(m2 s kPa) for H2 with a high selectivity of 1450 (He/N2 ) and of 400 (H2 /N2 ) even after being kept in steam (steam: 90 kPa) at 500 ◦ C for about 6 days. Co-doped silica membranes [14] fired at 600 ◦ C under a steam partial pressure of 90 kPa showed stable gaseous permeances and a H2 permeance of approximately 2.00–4.00 × 10−6 m3 (STP)/(m2 s kPa) with a selectivity of 250–730 (H2 /N2 ) even after 60 h of exposure to steam (steam pressure, 300 kPa) at 500 ◦ C. Battersby et al. [15] also reported cobalt-doped silica (Co–SiO2 ) membranes with a H2 permeance of 1 × 10−8 mol/(m2 s Pa) at 250 ◦ C and a single gas selectivity of 4500 for He/N2 . To date, doping of metals into silica membranes has been investigated for increased hydrothermal stability at high temperatures and for possible application to membrane reactors, such as steam reforming of methane [16]. Because doping with metals has been shown to consistently increase membrane performance, the applicability of metal doping to PV dehydration should be investigated. Identification of the permeation mechanism is essential for understanding membrane phenomena and for development of membranes with improved separation performance. In terms of the mechanism by which gaseous molecules permeate thorough porous silica membranes, different mechanisms dominate the transport through porous membranes, depending on the membrane pore size, the mean free path of gas molecules and the size of the permeating molecules: viscous flow, Knudsen flow, surface diffusion and activated diffusion. The solution–diffusion mechanism, which has been generally accepted for polymeric membranes, was extended to describe the transport mechanism of binary liquids through pervaporation membranes. The resulting adsorption–diffusion model has been used to describe the dehydration performance of microporous silica membranes. The adsorption–diffusion model assumes that water flux is proportional to its own driving force [17], covering both configuration diffusion and activated surface diffusion [18]. Direct comparison of the permeance of pure gas with PV through microporous membranes will offer additional information on the transport mechanism, including the pore size distributions of PV membranes and the contribution of the molecular sieving effect. However, to the best of our knowledge, the present study is the first to report this direct comparison. In the present study, Co–SiO2 membranes were fabricated and applied to PV dehydration of an ethanol/water system. Preparation conditions such as the firing temperature, the firing atmosphere and the coating sols were investigated to tune membrane performance. The concentration- and temperature-dependence of ethanol feed solutions were evaluated to investigate the transport mechanism. Moreover, gas permeation rates for He, N2 and SF6 were measured and compared with those of ethanol and water during PV.

Fig. 1. Schematic figure of single-gas permeation. (1) Gas cylinder; (2) pressure gauge; (3) heater; (4) membrane; and (5) soap flow meter.

stirred to facilitate hydrolysis and condensation at room temperature. After adding a specific amount of nitric acid and a large amount of water to control both the pH and concentration, the solution was boiled for about 12 h to obtain the Co–SiO2 colloidal sols. The average size of the colloidal sols was controlled by changing the TEOS concentration in the final sols from 0.5 to 2 wt%. Higher TEOS concentrations produced larger sols [19]. Porous ␣-alumina tubes (porosity: 50%, average pore size: 1 ␮m, outside diameter: 10 mm, length: 100 mm) were used as the supports for the Co–SiO2 membranes. Three layers were coated on the support. First, fine ␣-alumina particles (average particle diameter: 0.2–1.9 ␮m) were coated onto the outer surface of a porous support and fired at 550 ◦ C to produce a smooth surface and to remove large pinholes that might remain in the support. Then, silica–zirconia (Si/Zr = 1/1) colloidal sols were coated onto the substrate using hotcoating methods [20] — fired at 550 ◦ C in air by so called “flash firing” method. Finally, Co–SiO2 colloidal sols were coated and fired at 350–550 ◦ C in air. 2.2. Characteristic of Co–SiO2 powders Powder samples were obtained by a so-called “quick drying” method, in which the colloidal sols were dried in less than one second by dripping the colloidal sols onto a hot platinum plate at 180 ◦ C [7]. N2 -adsorption measurements of powder samples were performed at 77 K using BELSORP28SA equipment (BELL Co., Japan). XRD observations of the silica and Co–SiO2 powders were performed (RINT2000, RIGAKU Co., Japan). 2.3. Single-gas permeation experiments Single-gas permeation experiments were carried out at 200 ◦ C with the apparatus shown in Fig. 1. Prior to measurement, the membranes were pretreated under flowing He at 300 ◦ C to remove the adsorbed water and other impurities from the membrane pores. A high-purity gas (He, N2 , CO2 , or SF6 ) was fed into the system upstream of the cylindrical membrane, while the downstream pressure was kept constant at atmospheric pressure. The permeation rate was measured using a soap-film flow meter. After measurement of the permeation rate, the membranes were kept in a dry oven (0 ◦ C, <5 RH%) until use in subsequent experiments. 2.4. PV dehydration experiments

2. Experimental 2.1. Preparation of Co–SiO2 sols and membranes Co–SiO2 colloidal sols (molar ratio Si/Co = 2/1) were prepared by hydrolysis and condensation of tetraethoxysilane (TEOS) and Co (NO3 )2 ·6H2 O in ethanol and water, as reported previously [14]. Briefly, a specific amount of TEOS (Kishida chemical company, Japan) was added to ethanol (Sigma–Aldrich, ≥99.5% v/v) with Co(NO3 )2 ·6H2 O (Sigma–Aldrich, Japan, ≥98%), and the solution was

PV experiments were carried out using a typical experimental apparatus equipped with a screw, heater and vacuum pump, as shown in Fig. 2. All experiments were performed between 40 and 75 ◦ C using aqueous ethanol solutions that consisted of 50–94 wt% ethanol. The feed solution was rapidly circulated to reduce the effects of concentration and temperature polarization. The permeate was kept under a vacuum to reduce the pressure to less than 1 kPa. The permeate was then collected during a predetermined time interval using a cold trap that was cooled using liquid nitro-

J. Wang, T. Tsuru / Journal of Membrane Science 369 (2011) 13–19

15

Intensitty [a.u.]

Co3O4

Mixture 550°C

Fig. 2. Schematic figure of the PV apparatus. (1) Membrane; (2) heater; (3) screw; (4) cold trap; and (5) vacuum pump.

gen. The silica membranes were dried at 180 ◦ C in an oven for at least 2 h before each experiment. The separation factor was defined by the following equation: Yw /Ye ˛= Xw /Xe

Jx px

350°C SiO2

20

30

40

(2)

where Px is the permeance of x (mol/(m2 s kPa)), Jx is the permeate flux (mol/(m2 s)), and px is the partial pressure difference (Pa). The activity coefficients of water and ethanol were determined using the Wilson model, and the pure component vapor pressures were calculated using the Antoine equation.

50

70

80

Fig. 3. XRD patterns of the SiO2 powder, the Co–SiO2 powders fired at 350, 450 or 550 ◦ C, and the simple mixture of SiO2 and cobalt powder fired at 550 ◦ C. Simple mixture: SiO2 powder were mixed with Co3 O4 powder.

kinetic diameter. The M-350 membrane showed an He permeance of 2.6 × 10−6 mol/(m2 s kPa) with a permeance ratio of 10 for He/N2 and of 23,500 for He/SF6 . Because the permeance of SF6 , which has a molecular size of 0.55 nm, was quite low for all three membranes compared with the permeance of He, it was concluded that there were no pinholes left in the membranes. By increasing the firing temperature from 350 to 550 ◦ C, the permeance of He decreased to 2.1 × 10−6 mol/(m2 s Pa) with an increase in the He/N2 permeance

He 10-5

3. Results and discussion

H2 CO 2 N2

SF6 M-350 M-450

3.1. Characteristics of Co–SiO2 powders and membranes

M-550 Co-SiO 2 2 Pa)] Permeanc e [mol/(m ·s·P

Fig. 3 shows the XRD patterns of pure silica powder, Co–SiO2 powders fired at 350, 450 or 550 ◦ C, and a simple mixture (SiO2 powder were mixed with Co3 O4 powder) of the silica and cobalt powders. The XRD patterns show that the peak intensity of the Co–SiO2 powders was lower and much broader than that of the simple mixture. Because XRD detects crystalline materials and the XRD peak intensity reflects the amount of the crystals, some of the Co impregnated in the silica matrix was undetectable by XRD. In addition, some of the Co formed crystalline Co3 O4 — approximately 20 nm in size, as determined by Sherrer’s equation — that existed outside the silica network [14]. There was no apparent difference in peak intensity between the three Co–SiO2 powders fired at different temperatures ranging from 350 to 550 ◦ C. This result suggests that the doped cobalt was well incorporated into the silica network even at the low temperature of 350 ◦ C and did not agglomerate even when fired at 550 ◦ C. It should be noted that typical composite oxides, such as SiO2 –ZrO2 , start to crystallize even in the amorphous phase at 450–500 ◦ C. This result confirmed that cobalt was well incorporated and stable in the silica matrix. Three Co–SiO2 membranes were prepared by firing the top layer at 350, 450 or 550 ◦ C. Fig. 4 shows the single-gas permeance of He, CO2 , N2 and SF6 , measured at 200 ◦ C, as a function of

60

2θ [°]

(1)

where Yw and Ye are the mole percentages of water and ethanol on the permeate side of the membrane, respectively. Xw and Xe are the mole percentages of water and ethanol on the feed side of the membrane. The permeances of water and ethanol were calculated using the following equation: Px =

450 C 450°C

10-7

10-9

200 ° C -11

10

2.0

4.0

6.0

Kinetic diameter [Å] Fig. 4. Single-gas permeance of the Co–SiO2 membranes M-350, M-450 and M-550, the top layer of which was fired at 350, 450 or 550 ◦ C, respectively. The open symbols [14] represent the Co–SiO2 membrane fired at 550 ◦ C in a steam atmosphere; permeance was measured at 500 ◦ C.

J. Wang, T. Tsuru / Journal of Membrane Science 369 (2011) 13–19

10 4

10

4

1.5

10

Flux [kg/(m 2·h)]

Water 10 -1

10 2

Ethanol

10 -2

10

Flux [kg/(m 2 ·h)]

10 3

1

Sepa a ration factor, α [-]

Separation factor

M-550, 70°C, 94wt% 10

-3

0

500

1000

1500

Separation factor

10

Water 2

0.5

10

0 350

1

3

1.0

Ethanol 450

Top layer firing temperature [ ° C]

Separation factor, α [-]

16

10 550

PV Time [min]

ratio from 10 to 23. This result suggests that the network structures permeated by He and N2 (which have molecular sizes similar to water) became denser as a result of firing at higher temperatures. It should be noted that the permeance ratio of He/N2 through the Co–SiO2 membrane fired at 550 ◦ C in a steam atmosphere reached 800, as shown by the open symbols in Fig. 4 [14]. This result suggests that, compared with air, the steam atmosphere resulted in a much denser network structure. Therefore, it was concluded that the pore size or the structure of Co–SiO2 membranes could be tuned by changing either the firing temperature or the atmosphere of the top layer. The larger pore size of the membranes fired at 350–550 ◦ C in a dry air atmosphere may be favorable for achievement of a high permeate flux during the PV experiments because the molecular size of water is 0.3 nm. 3.2. PV performance of Co–SiO2 membranes Fig. 5 shows the time course of permeate flux and separation factor for an ethanol feed concentration of 94 wt% at 70 ◦ C using the Co–SiO2 membrane M-550. The membrane showed decreased permeate fluxes of both water and ethanol and a gradual increase in the separation factor for several initial hours. After about 600 min, the membrane reached steady fluxes for both ethanol and water. Yang et al. [21] concluded that the decrease in water flux with pervaporation time is probably due to the appearance of –OH groups, which make the pore surface much more hydrophilic but also reduce the effective pore size of SiO2 –ZrO2 (50 mol% ZrO2 ) membranes. Similar to SiO2 –ZrO2 membranes, after contact with water, silica [22] and Co–SiO2 membranes undergo hydrolysis of siloxane bonds, followed by the generation of silanol groups to which water and/or ethanol might be tightly adsorbed. Consequently, the effective pore size is reduced, thereby decreasing the permeation rate and increasing the separation factor due to the smaller molecular size of water. The steady-state PV performance of the Co–SiO2 membranes fired at 350, 450 and 550 ◦ C (i.e., M-350, M-450 and M-550) are summarized in Fig. 6. When the top layer firing temperature was increased from 350 to 550 ◦ C, the water flux deceased from 1.2 to 0.75 kg/(m2 h) and ethanol flux decreased from 0.3 to 0.01 kg/(m2 h), while the separation factor increased from 65 to 1670. As shown in Fig. 4, as the top layer firing temperature was increased, the He permeance decreased and the permeance ratio of He/N2 increased. This result suggests that the density of the network structure was increased at higher firing temperatures. Gas permeation properties show good concordance with both the PV flux and separation factor. Because the Co–SiO2 powders fired at 350–550 ◦ C had similar XRD patterns, we concluded that cobalt was homogenously incorporated into the silica matrix, irrespective of

Fig. 6. PV flux and separation factor of the Co–SiO2 membranes M-350, M-450, and M-550, as a function of the top-layer firing temperature. The PV experiments were carried out at 70 ◦ C using an ethanol feed concentration of 94 wt%.

firing temperature. Moreover, the Co–SiO2 sintered uniformly to form a denser structure at higher temperatures. Asaeda et al. [23] also concluded that a membrane firing a lower temperature (e.g., 400 ◦ C) would create much more porous structures in silica–zirconia membranes for higher water fluxes and higher separation performance. In conclusion, the membrane pore size could be tuned by changing the firing temperature because the silica structure was strongly affected by the firing temperature. Fig. 7 shows the PV separation factors of approximately 20 Co–SiO2 membranes plotted as a function of permeate total fluxes for a 94 wt% ethanol feed concentration at 70 ◦ C. Only steady-state data are plotted. Separation factors were varied from 60 to 4000, while total permeate fluxes ranged from 0.2 to 1.2 kg/(m2 h). Most membranes fell near the “trade-off” line. The Co–SiO2 membranes shown in Fig. 7 were prepared by firing at controlled temperatures using different coating silica sols. The effect of firing temperature on PV performance was discussed above. The colloidal sol size, measured by dynamic light-scattering, can be controlled over the range of several nanometers to several tens of nanometers by varying the TEOS, water, and acid concentrations [19]. Since membrane pores are formed as voids among the packed colloidal particles (i.e., interparticle pore) in the colloidal sol route, the size of the sol are important to determine pore size. In this study, the diameters of the Co–SiO2 sols ranged from 50 nm to 200 nm, and the weight percent ranged from 0.5 to 2 wt%. Generally speaking, use of larger sols in the top-layer coat results in higher permeate fluxes and lower separation factors. In conclusion, membrane separation performance can be tuned by changing both the firing temperature and the size of the coating sols. Fig. 8 shows the time course of the fluxes and separation factor over 150 days. The duration of the PV stability of the Co–SiO2 mem10 4 94 wt%, 70° C Separation factor, α [-]

Fig. 5. Time course of pervaporation performance using a fresh Co–SiO2 membrane M-550 (ethanol feed concentration of 94 wt%, 70 ◦ C).

10 3

10 2

10

1 0.1

1.0 2 Total flux [kg/(m . h)]

10.0

Fig. 7. PV separation factor as a function of the total permeate flux of several Co–SiO2 membranes at 70 ◦ C with a feed side ethanol concentration of 94 wt%. The dashed line is a visual guide.

J. Wang, T. Tsuru / Journal of Membrane Science 369 (2011) 13–19

103

1 Water

102

10-1 10-2 10-3

10

Ethanol Continuous

air

0

110 75 PV time [day]

2

94wt% EtOH

1

150

Fig. 8. Long-term PV performance of Co–SiO2 membrane M-550 (top layer fired at 550 ◦ C, dry air). After about 2 days of continuous PV, the membrane was kept in air and then was immersed in 94 wt% ethanol for a total of 150 days. The initial PV performances (fresh membrane) are shown at PV time of 0.

branes was evaluated as follows. First, continuous PV experiments were carried out for approximately 30 h. Then, the membrane was removed from the apparatus and kept in a dry oven for approximately 100 days. Then, the membrane was reused for PV. For an additional 50 days, the membrane was kept immersed in 94 wt% ethanol with periodic exchange of the used feed solution with fresh feed solution. The ethanol feed solution concentration was always kept at 94 wt% and all experiments were performed at 70 ◦ C. During the initial 2 days, both the water and ethanol fluxes tended to decrease, as explained in Fig. 5. Subsequently, no apparent decrease in the membrane separation performance was observed, even after the membrane was kept in air or 94 wt% ethanol for 150 days. The final stable water flux was approximately 70% of its initial value. It was previously reported that the water flux (feed composition of 5% water in n-butanol, 95 ◦ C) for a pure SiO2 membrane decreased only to approximately 6% of initial flux within a week [12]. Therefore, it was concluded that the Co–SiO2 membranes used in the present study showed good stability in an aqueous environment. 3.3. Effect of temperature on the PV performance of Co–SiO2 membranes Fig. 9(a) and (b) shows the separation properties of the M-550 membrane as a function of operation temperature. The experiments were performed as follows. The temperature was decreased

Water flux

ux [kg/(m2·h)] Flu

Separation factor

(a)

103

10-1 102 10-2 2 1

3.4. Effect of feed concentration on the PV performance of Co–SiO2 membranes Fig. 10(a) and (b) shows the PV flux, permeance and separation factor as a function of the ethanol feed concentration (50–94 wt%) at 70 ◦ C. The water flux decreased from 6.0 to 0.7 kg/(m2 h) and the ethanol flux was decreased much less, from 0.01 to 0.004 kg/(m2 h), when the ethanol feed concentration was increased from 50 to 94 wt%. With an increase in the water concentration on the feed side, it is reasonable that more water permeated through the membrane because of the increased driving force. However, it is quite interesting that the decrease in the ethanol permeate flux was observed irrespective of the increased ethanol concentration. This result was attributed to the interactions of water and ethanol with the silica pores. SiO2 –ZrO2 (ZrO2 : 50 mol%) membranes [21] showed an increase in ethanol flux at higher ethanol concentrations, and then a decreased flux as the concentration was increased further, showing a maximum at an ethanol feed concentration of approximately 60 mol%. At low feed concentrations, for example, at an ethanol feed concentration of 50 wt%, only a small amount of ethanol was adsorbed to the inner surface of the silica pores, and both water and ethanol could permeate through the pores,

104

Ethanol flux 10

10-6

Water

10-8

Ethanol

3

10-33 2.8

1 3.0 1000/T [K-1]

3.2

(b)

94 wt%, M-550

Separa ation factor, α [-]

1

from 70 to 40 ◦ C, and then increased to 75 ◦ C. Finally, the temperature was decreased to 40 ◦ C. The data points shown in Fig. 9 are average values (at least three data points were taken) for each temperature. Both the permeate water and ethanol fluxes increased with increasing feed temperature, while the separation factor remained constant or increased slightly. After the permeation temperatures were varied between 75 and 40 ◦ C three times, the water flux was reproducible, while the ethanol flux fluctuated a little above or below the original value probably due to physical and/or chemical adsorption. Fig. 9(b) shows the permeances of water and ethanol as a function of feed solution temperature. Both the water and ethanol flux were approximately constant and/or slightly decreased with increasing temperature. Bettens et al. [18] reported that for pure water or ethanol permeation in PV, both the water and ethanol fluxes increased with temperature. In the present study, the increase in flux with temperature was ascribed to an increase in the feed vapor pressure, which was the driving force for permeation. The permeances were approximately constant, which demonstrates that the permeability of the membranes did not change with temperature. Again, the water permeance was constant after three cycles of temperature change.

Permeance [mol/(m2·s·Pa)]

2·h)] Flux [kg/(m [

Separation factor

Separattion factor, α [-]]

104

10

17

10 10-10 2.8

2 1

3

3.0 1000/T [K-1]

3.2

Fig. 9. Temperature-dependence of the Co–SiO2 membrane M-550. (a) Flux and separation factor, and (b) water and ethanol permeances calculated from the same flux data (the order of temperature change was as follows: (1st, 70 → 40 ◦ C) ⇒ (2nd, 40 → 75 ◦ C) ⇒ (3rd, 75 → 40 ◦ C)).

J. Wang, T. Tsuru / Journal of Membrane Science 369 (2011) 13–19

104

10 (a)

103

1 Separation factor 10-1

102

10-2

Permean nce [mol/(m2·s·P Pa)]

Flux x [kg/(m2·h)]

Water

Sepa aration factor, α [-]

18

10

Ethanol

(b) 10-6 Water

10-88

Ethanol

M- 550, 75°C 10-33 40

1 60

80

10 10-10 40

100

60

80

100

Feed EtOH concentration [wt%]

Feed EtOH concentration [wt%]

Fig. 10. Feed-side concentration-dependence of Co–SiO2 membranes M-550. (a) Flux of water and ethanol and (b) permeance of water and ethanol.

resulting in a lower separation factor. At high ethanol feed concentrations, for example, 94 wt%, water molecule would be displaced by ethanol as the concentration of water decreased [24]. So more ethanol molecules were adsorbed and the effective pore size for both water and ethanol were reduced. Therefore, both water and ethanol permeance were decreased. Because ethanol is larger than water, ethanol permeance decreased much more, resulting in a higher separation factor.

H2O 10

He

-5

CO2

N2 EtOH

M-350 M-450

3.5. Comparison between PV performance and single gas permeation performance

M-550 Permea ance [mol/(m 2 · s ·Pa)]

To better understand the mechanism of water and ethanol transport during PV, the permeances of single gases were compared with those in PV. The permeance of water and ethanol for the M-350, M-450 and M-550 membranes during PV dehydration experiments (70 ◦ C, 94 wt%) were plotted together with the single gas permeances, as shown in Fig. 11. The PV permeance were calculated from flux that had already been reported in Fig. 6, and the single gas permeances was the same as those in Fig. 4. The kinetic diameters generally agree with the molecular size for inorganic gases and the values reported for the gases used in the present study were as follows: He (0.260 nm), N2 (0.364 nm) and SF6 (0.55 nm) [25]. The kinetic diameters of polar molecules were obtained using the Stockmayer potential, which corrects for the effects of polar interactions. The kinetic diameter of H2 O was reported to be 0.265 [25], 0.2995 [26] or 0.317 nm [27]. Based on the permeation experiments with SiO2 membranes at high temperatures, the kinetic diameter of 0.3 nm, which was proposed by van Leeuwen based on vapor–liquid equilibrium data, is the most probable value [28]. The reported kinetic diameter of ethanol is 0.43 nm [26].

SF6

10

-7

-9

10

Open: PV,70 ° C Closed: Gas,200 ° C -11 10 2.0

4.0

6.0

Kinetic diameter [Å] Fig. 11. Permeance of single gases as a function of kinetic diameter for the three Co–SiO2 membranes (open symbols represent the water and ethanol permeances of the same membranes during the PV dehydration experiments).

Table 1 Separation performance of several membranes in ethanol/water systems. Membrane

Zeolite-A Zeolite-T ECN-silica Perv-silica M-350 M-450 M-550

T [◦ C]

70 70 70 70 70 70 70

Feed EtOH conc. [wt%]

89.9 89.9 89.7 89.0 94.3 94.2 94.1

PV performance

Ref.

Water flux [kg/(m2 h)]

Total flux [kg/(m2 h)]

Water permeance [mol/(m2 s Pa)]

Separation factor [−]

1.119 0.902 2.012 1.905 1.200 0.788 0.753

1.120 0.910 2.330 2.000 1.500 0.820 0.760

1.2E−06 9.6E−07 2.1E−06 1.9E−06 1.9E−06 1.2E−06 1.1E−06

18,000 1000 60 160 65 346 1675

[29] [29] [29] [29] This work This work This work

J. Wang, T. Tsuru / Journal of Membrane Science 369 (2011) 13–19

As shown in Fig. 11, both the water and ethanol permeances were similar to those of the single gases. H2 O (molecular size: 0.30 nm) had a permeance between that of He (0.260) and CO2 (0.32), while that of ethanol (0.43) was between N2 (0.364) and SF6 (0.55). Moreover, the order of permeance during PV of Co-doped silica membranes fired at different temperatures was identical to the permeances of the gases: 350 ◦ C > 450 ◦ C > 550 ◦ C. Although the permeances of water and ethanol were measured under different conditions from those used for gas measurements, we concluded that the mechanism of water transport through the Co–SiO2 membrane pores during PV was dominated by the molecular sieve mechanism, and water permeated as a gaseous molecule during PV. The PV performance of several inorganic membranes in ethanol/water system are summarized in Table 1, which includes data from an excellent paper [29] that extensively summarizes PV dehydration performance in many kinds of solvent systems. Because the PV experimental conditions were not always the same, permeances were calculated for comparison. A- and T-type zeolite membranes showed both high flux and the highest separation factors. In general, as an alternative technology, silica membranes have lower separation factors and reduced permeance. Among SiO2 membranes, the Co–SiO2 membranes described in the present study had relatively low fluxes and high separation factors with good stability in an aqueous environment. Further improvements in the preparation method are required to obtain higher flux, which is very important for practical applications. 4. Conclusions In the present study, Co–SiO2 membranes were successfully prepared and their performance was investigated using single-gas permeation and PV dehydration of ethanol/water mixtures. (1) When the top layer firing temperature was increased from 350 to 550 ◦ C, the single gas permeance of He decreased from 2.6 to 2.1 × 10−6 mol/(m2 s Pa), while the permeance ratios of He/N2 and He/SF6 increased from 10 to 23 and from 23,500 to 113,000, respectively. In PV dehydration of 94 wt% aqueous ethanol solutions, the total permeate flux decreased from 1.2 to 0.75 kg/(m2 h) and the separation factor increased from 65 to 1670. Both the gas permeation and PV results showed that increasing the top layer firing temperature resulted in smaller pore size and denser network structures. (2) Both the water and the ethanol fluxes decreased when the feed side temperature was decreased from 75 to 50 ◦ C, while the permeances of both permeate water and ethanol increased slightly. The separation factor was constant or increased slightly with increasing temperature. (3) When the ethanol feed concentration was reduced from 94 wt% to 50 wt%, the permeate water flux increased from 0.6 to about 6 kg/(m2 h), while the ethanol flux increased only slightly from 0.004 to 0.009 kg/(m2 h). Consequently, the separation factor decreased from 2700 to about 760. (4) The dependence of permeance during PV on the molecular size of water and ethanol showed a similar trend to the single-gas permeation curves of He, N2 and SF6 .This result suggested that the mechanism of water and ethanol permeation during PV was dominated by molecular sieving. (5) The Co–SiO2 membranes prepared in the present study exhibited good stability in water. After being immersed in 94 wt% ethanol and used in PV experiments, the water flux of the membrane remained to be 0.6 kg/(m2 h), and the separation factor was constant at approximately 3300. Co–SiO2 membranes

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show promise for use in PV dehydration applications due their higher flux and better stability. References [1] P.D. Chapman, T. Oliveira, A.G. Livingston, K. Li, Membranes for the dehydration of solvents by pervaporation, J. Membr. Sci. 318 (2008) 5–37. [2] H. Kita, K. Horii, K. Tanaka, K.-I. Okamoto, Y. Ohtoshi, Synthesis of a zeolite NaA membrane for pervaporation of water/organic liquid mixtures, J. Mater. Sci. Lett. 14 (1995) 206–208. [3] Y. Morigami, M. Kondo, J. Abe, H. Kita, K. Okamoto, The first large-scale pervaporation plant using tubular-type module with zeolite NaA membrane, Sep. Purif. Technol. 25 (2001) 251–260. [4] K. Sato, K. Sugimoto, T. Nakane, Preparation of higher flux NaA zeolite membrane on asymmetric porous support and permeation behavior at higher temperatures up to 145 ◦ C in vapor permeation, J. Membr. Sci. 307 (2008) 181–195. [5] http://www.hitachi-chem.co.jp/english/. [6] T.A. Peters, J. Fontalvo, M.A.G. Vorstman, N.E. Benes, R.A. van Dam, Z.A.E.P. Vroon, E.L.J. van Soest-Vercammen, J.T.F. Keurentjes, Hollow fibre microporous silica membranes for gas separation and pervaporation: synthesis, performance and stability, J. Membr. Sci. 248 (2005) 73–80. [7] M. Asaeda, M. Ishida, T. Waki, Pervaporation of aqueous organic acid solutions by porous ceramic membranes, J. Chem. Eng. Jpn. 38 (2005) 336–343. [8] http://www.pervaporation-membranes.com/index.html. [9] H.M. van Veen, Y.C. van Delft, C.W.R. Engelen, P.P.A.C. Pex, Dewatering of organics by pervaporation with silica membranes, Sep. Purif. Technol. 22–23 (2001) 361–366. [10] B.-K. Sea, E. Soewito, M. Watanabe, K. Kusakabe, S. Morooka, S.S. Kim, Hydrogen recovery from a H2 –H2 O–HBr mixture utilizing silica-based membranes at elevated temperatures. 1. Preparation of H2 O- and H2 -selective membranes, Ind. Eng. Chem. Res. 37 (1998) 2502–2508. [11] M. Asaeda, S. Yamasaki, Separation of inorganic/organic gas mixtures by porous silica membranes, Sep. Purif. Technol. 25 (2001) 151–159. [12] J. Campaniello, C.W.R. Engelen, W.G. Haije, P.P.A.C. Pex, J.F. Vente, Long-term pervaporation performance of microporous methylated silica membranes, Chem. Commun. 7 (2004) 834–835. [13] M. Kanezashi, M. Asaeda, Hydrogen permeation characteristics and stability of Ni-doped silica membranes in steam at high temperature, J. Membr. Sci. 271 (2006) 86–93. [14] R. Igi, T. Yoshioka, Y.H. Ikuhara, Y. Iwamoto, T. Tsuru, Characterization of Codoped silica for improved hydrothermal stability and application to hydrogen separation membranes at high temperatures, J. Am. Ceram. Soc. 91 (2008) 2975–2981. [15] S. Battersby, T. Tasaki, S. Smart, B. Ladewig, S. Liu, M.C. Duke, V. Rudolph, J.C. Diniz da Costa, Performance of cobalt silica membranes in gas mixture separation, J. Membr. Sci. 329 (2009) 91–98. [16] S. Battersby, S. Smart, B. Ladewig, S. Liu, M.C. Duke, V. Rudolph, J.C. Diniz da Costa, Hydrothermal stability of cobalt silica membranes in a water gas shift membrane reactor, Sep. Purif. Technol. 66 (2009) 299–305. [17] A.W. Verkerk, P. van Male, M.A.G. Vorstman, J.T.F. Keurentjes, Description of dehydration performance of amorphous silica pervaporation membranes, J. Membr. Sci. 193 (2001) 227–238. [18] B. Bettens, S. Dekeyzer, J. Bart Van der Bruggen, C. Degre‘ve, Vandecasteele, Transport of pure components in pervaporation through a microporous silica membrane, J. Phys. Chem. B 109 (2005) 5216–5222. [19] T. Tsuru, Nano/subnano-tuning of porous ceramic membranes for molecular separation, J. Sol–Gel Sci. Technol. 46 (2008) 349–361. [20] M. Asaeda, J. Yang, Y. Sakou, Porous silica/zirconia (50%) membranes for pervaporation of isopropyl alcohol (IPA)/water mixtures, J. Chem. Eng. Jpn. 35 (2002) 365–371. [21] J. Yang, T. Yoshioka, T. Tsuru, M. Asaeda, Pervaporation characteristics of aqueous–organic solutions with microporous SiO2 –ZrO2 membranes: experimental study on separation mechanism, J. Membr. Sci. 284 (2006) 205–213. [22] Y. Ma, J. Wang, T. Tsuru, Pervaporation of water/ethanol mixtures through microporous silica membranes, Sep. Purif. Technol. 66 (2009) 479–485. [23] M. Asaeda, Y. Sakou, J. Yang, K. Shimasaki, Stability and performance of porous silica–zirconia composite membranes for pervaporation of aqueous organic solutions, J. Membr. Sci. 209 (2002) 163–175. [24] R.K. Iler, The Chemistry of Silica, John Wiley & Sons, 1979, p. 657 (a WileyInterscience publication). [25] D.W. Breck, Zeolite Molecular Sieves, John Wiley & Sons, New York, 1974. [26] M.E. van Leeuwen, Derivation of Stockmayer potential parameters for polar fluids, Fluid Phase Equilib. 99 (1994) 1–18. [27] H.J.C. Berendsen, J.R. Grigera, T.P. Straatsma, The missing term in effective pair potentialst, J. Phys. Chem. 91 (1987) 6269–6271. [28] T. Tsuru, R. Igi, M. Kanezashi, T. Yoshioka, S. Fujisaki, Y. Iwamoto, Permeation properties of hydrogen and water vapor through porous silica membranes at high temperatures, AIChE, in press, doi:10.1002/aic.12298. [29] S. Sommer, T. Melin, Performance evaluation of microporous inorganic membranes in the dehydration of industrial solvents, Chem. Eng. Process. 44 (2005) 1138–1156.