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Removal of styrene from petrochemical wastewater using pervaporation process
Desalination 284 (2012) 116–121
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Removal of styrene from petrochemical wastewater using pervaporation process Majid Aliabadi a, Abdolreza Aroujalian a, b,⁎, Ahmadreza Raisi a, b a b
Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Hafez Ave., P.O. Box 15875-4413, Tehran, Iran Food Process Engineering and Biotechnology Research Center, Amirkabir University of Technology (Tehran Polytechnic), Hafez Ave., P.O. Box 15875-4413, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 14 April 2011 Received in revised form 19 August 2011 Accepted 22 August 2011 Available online 9 September 2011 Keywords: Pervaporation Styrene PDMS Membrane separation Wastewater
a b s t r a c t In the present study, polydimethylsiloxane (PDMS) composite membranes were used to study the removal of styrene from dilute aqueous streams by pervaporation. The influences of styrene feed concentration, permeate pressure, operating temperature, feed flow rate and membrane thickness on the pervaporation performance were investigated. The results showed that with the increase in concentration of styrene in the feed solution, both the permeation flux and the styrene enrichment factor increase. Additionally, with a decrease in thickness of the membrane, the permeation flux was observed to enhance but the enrichment factor decreased. It was also indicated that increasing in the permeate pressure, reduces driving force for mass transfer and consequently the pervaporation performance dropped. Finally, the activation energy for pervaporation of both styrene and water calculated from Arrhenius plot indicated that the permeation of water through the membrane is higher temperature dependant than styrene. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Wastewater containing volatile organic compounds (VOCs) generated from various production lines in the chemical and petrochemical industries have recently caused serious environmental problems [1]. Styrene, as a VOC, is classified in EPA's Toxic Release Inventory as a carcinogen. Styrene is used predominantly in the production of polystyrene plastics and resins. It is also used as an intermediate in the synthesis of materials used for ion exchange resins and to produce copolymers as styrene–acrylonitrile (SAN), acrylonitrile–butadiene– styrene (ABS) and styrene-butadiene rubber (SBR). The most important physical/chemical properties of styrene are summarized in Table 1. Styrene enters the environment during the manufacture, use, and disposal of styrene-based products. The principal sources of styrene releases to water are industrial effluents. Styrene has been detected in effluents from the chemical, textile latex and coal gasification plants [2,3]. More stringent requirements for the removal of VOCs in recent years necessitate the development of innovative, cost-effective treatment alternatives. Traditional VOC control technologies such as adsorption [4] and biological treatment [5,6] have been studied for the removal of styrene from aqueous streams. However, these
⁎ Corresponding author at: Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Hafez Ave., P.O. Box 15875-4413, Tehran, Iran. Tel.: +98 21 64543163; fax: +98 21 66405847. E-mail address: [email protected] (A. Aroujalian). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.08.044
technologies do not always provide a complete and economic solution for some of these wastewater applications. According to the survey of AICHE's Center for Waste Reduction Technologies, membrane separation has become one of the emerging technologies for VOC control [7]. In recent years, membrane separation technology such as reverse osmosis [8], membrane air stripping process [9], membrane distillation [10], nanofiltration [11], pervaporation [12–14] and hybrid process [15,16] has been studied for the removal of VOCs from aqueous solutions. Among these technologies, pervaporation is a promising process for separation of VOC from water because of the compact design, recycling of recovered VOC and no emission problem [17]. Pervaporation is a membrane separation process in which a liquid stream containing two or more components is in contact with one side of a membrane while a vacuum or gas purge is applied to the other side. The components in the liquid stream sorb into the membrane, permeate through the membrane, and evaporate into the vapor phase. The condensed permeate liquid often separates into two phases due to limited solubility of VOCs in water. The organic phase can be treated for reuse, and the aqueous phase saturated with VOCs can be recycled to the feed stream for reprocessing. Pervaporative removal of various VOCs, such as benzene [18], toluene [19], ethyl benzene [20], xylene [21] and chlorinated solvents [22–24], from aqueous streams has been extensively investigated. However, to our knowledge, the removal of styrene from aqueous solutions has not been studied. In this study, the potential of the pervaporation process for treating styrene contaminated water, simulated petrochemical wastewater, was evaluated. According to the field data obtained
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Table 1 Physical and chemical properties of styrene and water [2]. Components
Molecular weight
Boiling point (°C)
Density at 20 °C (g cm− 3)
Vapor pressure at 20 °C (Pa)
Molar volume (cm3 mol− 1)
Solubility in water at 20 °C (mg l− 1)
Styrene Water
104.15 18.02
145.2 100
0.9059 0.998
656.4 2340
115.0 18.06
310 –
from a local plant, the concentration of styrene adjusted to the range of 20–300 mg/l, and the effect of operating conditions on the pervaporation performance was investigated.
2. Experimental 2.1. Materials PDMS/PVDF/PP composite membranes with functional layer of polydimethylsiloxane used in the experiments were kindly supplied by Helmholtz-Zentrum Geesthacht Zentrum für Material und Küstenforschung GmbH (Geesthacht, Germany). Membranes were cut into 15× 20 cm pieces and placed in a flat frame membrane module. Styrene (reagent grade, purity 99%), acetonitrile (HPLC grade, purity 99.9%) and water (HPLC grade, purity 99.9%) were purchased from Merck Co. Ltd. (Darmstadt, Germany) and deionized laboratory water was used for making aqueous mixtures.
2.2. Pervaporation experiments A schematic diagram of the test unit is shown in Fig. 1. The system was operated with continuous recirculation of the aqueous phase to the feed tank. A flat frame membrane module (Osmonics Inc., Minnetonka, MN, USA) with effective area of 138 cm 2 was used in cross-flow mode. A peristaltic pump was used to recirculate the liquid feed from the feed tank through the membrane. The temperature of feed stream was controlled by a PID controller. During the experiments the membrane downstream pressure was controlled by a needle valve and a vacuum pump (Busch Inc., Switzerland). A pressure meter calibrated by a vacuum gauge was used to monitor the downstream pressure. The condenser system consisted of two traps refrigerated by liquid nitrogen (− 196 °C) that can be used alternately, allowing the permeate stream to be sampled continuously without interrupting operation of the unit. The feed flow rate was measured by a flow meter. Sampling was done carefully to avoid any loss. Samples were collected in a small 10 ml glass vial and capped immediately with a Teflon-lined cap to avoid styrene loss.
Fig. 1. Schematic diagram of the pervaporation system used in this study.
2.3. Analytical facilities The concentration of styrene in the aqueous solutions was measured by an Agilent series 1200 HPLC equipped with a reversephase Eclipse XDB C18 Agilent column (5 μm, 4.6 × 250 mm). The mobile phase was 75% acetonitrile and 25% water. Styrene concentration was determined using an injection volume of 10 μl at wavelength of 245 nm (UV detector) and a mobile phase flow rate of 1 ml/min. 2.4. Calculated quantities The fluxes of styrene and water were obtained from following equation [19]: Ji ¼
mi Am τ
ð1Þ
where Ji is the permeation flux of component i, gm −2 h −1; mi is the weight of component i in permeate, g; Am is the effective membrane area, m 2 and τ is the permeation time, h. The selectivity of the membrane may be quantified by two alternative dimensionless ratio, enrichment factor, β, and separation factor, α, defined as follows [25]: β¼
y1 x1
ð2Þ
α¼
y1 =y2 x1 =x2
ð3Þ
where x1 and x2 are the concentration of styrene and water in the feed solution as well as y1 and y2 being the concentration of styrene and water in the permeate. In the present study, the effect of feed flow rate, initial feed concentration, feed temperature, permeate pressure and functional layer thickness of membrane on the performance of pervaporation process for the removal of styrene from its aqueous solutions was investigated. Time duration of each experiment was 6 h and the collected permeate was weighed and analyzed every 2 h. All experimental conditions were repeated three times and the average values are reported.
Fig. 2. The effect of feed flow rate on the total and partial fluxes at styrene feed concentration of 150 ppm, temperature of 30 °C and permeate pressure of 1 mm Hg.
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3. Results and discussion 3.1. Effect of feed flow rate The effect of feed flow rate on the total flux and styrene partial flux at the styrene concentration of 150 ppm is shown in Fig. 2. The Reynolds number, Re, which can be calculated from Eq. (4), is well known dimensionless description of the hydrodynamic conditions in the feed flow: Re ¼
ρudh μ
ð4Þ
where v, ρ and μ are velocity, density and viscosity of feed and dh is hydraulic diameter of pervaporation cell. It can be seen from Fig. 2 that the styrene and total flux enhance with an increase in Reynolds number. For example, the styrene flux goes from 1.9 to 4.6 g −2 h −1 when the Reynolds number is raised from 430 to 2150 (which corresponded to feed flow rate from 1 to 5 l min −1) for the thick PDMS membrane. These results can be attributed to the effect of concentration polarization in the liquid boundary layer adjacent to the membrane surface. Concentration polarization tends to decrease the permeation rate of the more permeable component (styrene) and increase the permeation rate of the less permeable component (water), resulting in a lesser extent of separation [20]. However an increase in the feed flow rate could reduce the effect of concentration polarization, and thus the styrene flux should increase as observed. Additionally, water flux, which is controlled by the rate of diffusion through the membrane, should be independent of feed flow rate. As a result, the styrene/water enrichment factor increases as the feed flow rate increases as shown in Fig. 3. 3.2. Effect of feed concentration The effect of styrene feed concentration on the water and styrene partial fluxes and the enrichment factor was determined at the styrene concentration range of 20–300 ppm and different temperatures. The partial flux of styrene is plotted in Fig. 4 as a function of the styrene feed concentration for PDMS membrane with functional layer thickness of 18 μm. It is indicated that the partial flux of styrene enhances with an increase in the styrene feed concentration. This behavior can be attributed to the increase in styrene activity in the feed mixture due to enhancement in the styrene concentration. Besides, the results showed that the water flux decreases with the increase in styrene concentration in the feed solution, as shown in Fig. 5. For example, the water flux decreases from 91.2 to 79.5 g − 2 h − 1 when the feed concentration of styrene is increased from 20 to 300 ppm. This phenomenon can be explained by the water clustering effect.
Fig. 3. The effect of feed flow rate on the enrichment factor of styrene at styrene feed concentration of 150 ppm, temperature of 30 °C and permeate pressure of 1 mm Hg.
Fig. 4. The effect of styrene feed concentration on the styrene flux at different temperatures and permeate pressure of 1 mm Hg.
Water clustering is developed in the membrane arising from repulsive interaction between water and organic component absorbed [20]. It has actually been shown that the permeation of water through polymer membranes can be hindered by the formation of a water cluster [26,27].Water itself exists in the form of hydrogen-bonded clusters, depending on its circumstance. Thus, free water molecules may diffuse accompanied by clustered molecules. This implies that as the diffusion size of water increases, the diffusion coefficient decreases. The effect of feed concentration on the styrene enrichment factor is presented in Fig. 6. It can be seen from this figure that the enrichment factor increases as the styrene feed concentration goes to higher levels. For example, the enrichment factor increases from 493 to 812 when the feed concentration of styrene changes from 20 to 300 ppm at 30 °C. The enrichment factor is related to the ratio of styrene partial flux to water flux. The water flux declines with increase in the styrene feed concentration while the styrene flux rises. Therefore, the ratio of the two fluxes which is equal to the styrene enrichment factor increases when the feed concentration enhances. Similar results have been reported by other authors [28,29]. As an example, Lau et al. [29] reported that an increase in chloroform concentration resulted in the expected increase in chloroform flux, but a decrease in water flux, yields an increase in the separation factor from 1170 to 1800. 3.3. Effect of permeate pressure Permeate pressure is an important parameter that affects the performance of the pervaporation process. Since the driving force for permeation of components in pervaporation is the vapor pressure difference between the feed and permeate side, the partial fluxes
Fig. 5. The effect of styrene feed concentration on the water flux at temperature of 30 °C and permeate pressure of 1 mm Hg.
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Fig. 6. The effect of styrene feed concentration on the enrichment factor at different temperatures and permeate pressure of 1 mm Hg.
Fig. 8. The effect of permeate pressure on the enrichment factor of styrene at styrene feed concentration of 50 ppm and temperature of 30 °C.
are expected to increase with decreasing downstream pressure. Permeate pressure greatly affects the operating cost of the process because the cost of maintaining vacuum is substantial. It also determines the component concentration in the permeate stream and affects the membrane selectivity [30]. The effect of permeate pressure on the partial fluxes of water and styrene as well as enrichment factor was investigated over the permeate pressure range of 1–30 mm Hg at 30 °C. The partial fluxes of styrene and water are depicted in Fig. 7 as a function of permeate pressure for the PDMS membrane. This figure shows that partial fluxes of styrene and water decrease with an increase in the permeate pressure. For example, the styrene flux decreases from 1.56 to 0.15 g −2 h −1 when the permeate pressure is increased from 1 to 30 mm Hg. Also, it was found that the decrease in partial fluxes was more pronounced for the styrene flux in comparison to the water flux. These observations are a consequence of an increase in the vapor pressure of components in the permeate and hence, reducing in the vapor pressure differences across the membrane. Furthermore, the effect of permeate pressure on the enrichment factor of styrene at feed temperature of 30 °C and styrene concentration of 50 ppm was studied. The results are presented in Fig. 8. A significant decline in the separation efficiency with increasing permeate pressure was observed. This phenomenon can be related to more pronounced decrease of the styrene flux in comparison to the water flux and hence a less concentrated styrene permeate. Similar observations were found by other researchers [1,25,30], namely a clear decrease of the membrane selectivity caused by an increase in the downstream pressure of the pervaporative separation of organics/water mixtures. The effect of the permeate pressure on the enrichment factor is however dependent on the relative volatility between the selective and non-selective permeating compounds. It has been proven that
for binary mixtures, the permeate concentration depends on the permeate pressure as follows [19]:
Fig. 7. The effect of permeate pressure on the partial fluxes at styrene feed concentration of 50 ppm and temperature of 30 °C.
Fig. 9. The effect of feed temperature on the total and partial flux of styrene at styrene feed concentration of 50 ppm and permeate pressure of 1 mm Hg.
x1 ¼
p1 p2 1 p − 1 p2 −p1 pl p2 −p1
ð4Þ
where 1 and 2 represent the fast and the slow permeating component respectively, x1 is the mole fraction of component 1 in the permeate, p* stands for the saturated vapor pressure at operational temperature and pl is the downstream pressure. Derivation of Eq. (4) yields: dx1 p p 1 ¼ 1 2 2 : dp p1 −p2 p
ð5Þ
l
If the fast permeating compound is also the more volatile (p1∗ − the permeate concentration increases as permeate pressure is raised. In the opposite case, a steep decrease in the permeate concentration is observed. The decrease in the enrichment factor of styrene can be explained according to this fact. Styrene is less volatile than water, but shows higher permeability through PDMS membrane. Thus, the styrene enrichment factor decreases when the permeate pressure goes to higher levels, as demonstrated in Fig. 8.
p2∗ N 0),
3.4. Effect of feed temperature Feed temperature is a key factor in the pervaporation process and affects all of the constituent steps of solute transport from feed solution to permeate side, as well as the driving force for mass transfer. The effects of feed temperature on the total flux and partial fluxes of styrene and water as well as enrichment factor were investigated over the temperature range of 30–60 °C for a 50 ppm aqueous styrene solution. The results are shown in Figs. 9 and 10. As depicted, the
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M. Aliabadi et al. / Desalination 284 (2012) 116–121 Table 2 Activation energy values that were calculated from Arrhenius plots. Component
Styrene Water Total
Fig. 10. The effect of feed temperature on the enrichment factor of styrene at styrene feed concentration of 50 ppm and permeate pressure of 1 mm Hg.
styrene flux as well as the water flux increase with an enhancement in the feed temperature, but the increase in water flux is more significant than the styrene flux. For example, the water flux goes from 90 to 378 g − 2 h − 1 when the temperature increases from 30 to 60 °C. It can be observed from Fig. 10 that the styrene enrichment factor decreases with an increase in the feed temperature. The enrichment factor is related to the ratio of styrene flux to water flux. The styrene and water flux increase with an enhancement in the feed temperature, but the increase in water flux is more significant than the styrene flux. Therefore, the ratio of the two fluxes which is equal to styrene enrichment factor increases when the temperate goes to higher level. During the pervaporation process, permeating components diffuse through free volumes of the membrane. Thermal motions of polymer chains in amorphous regions randomly produce free volumes. As temperature increases, frequency and amplitude of polymer jumping chains increase, resulting in the increase of free volume of the membrane. As a consequence, the diffusion rate of individual permeating components increases at higher temperatures leading to high permeation fluxes. Moreover, diffusivity and viscosity of styrene in the feed solutions and the permeability of this compound into the membrane are affected by the temperature variation in the feed. Another reason for the increase of permeation flux with increase in feed temperature is that by increasing temperature, the vapor pressure of each component increases which results in high permeation flux of all components. Thus, feed temperature affects the feed/membrane characteristics and the driving force of the process. 3.5. Activation energy of pervaporation Generally, variations of total and individual fluxes are related by the Arrhenius equation [31]:
Ji ¼ J0 exp
−Eai
,
! RT
C = 50 ppm
C = 300 ppm
Ea (kJ mol− 1)
R2
Ea (kJ mol− 1)
R2
4.398 40.231 39.891
0.993 0.990 0.990
5.051 41.042 38.402
0.997 0.986 0.998
where Ji (gm −2 h −1) represents the flux of component i, J0 (g −2 h −1) is pre-exponential factor, Eai (J mol −1) the apparent activation energy of pervaporation, R (J mol −1 K −1) the universal gas constant and T is absolute temperature (K). The apparent activation energy of permeation is determined from the slopes of the ln(Ji) versus 1/T plots. The logarithm of the partial flux of styrene and water versus the reciprocal temperature are plotted in Fig. 11. The linear relationship in this figure indicates that the behavior of the membrane followed the Arrhenius law. This is consistent with other studies have been reported in the literature [25,31]. From the slope of the lines, the activation energy of pervaporation for styrene and water was found to be 4.398 and 40.231 kJ mol −1, respectively. The activation energy values at different feed concentration are presented in Table 2. As can be seen from this table, all values of activation energy are positive, so an increase in the feed temperature results in higher flux. Furthermore, the activation energies vary with the styrene feed concentration and the activation energy of water is higher than that of styrene, indicating that the permeation of water through the membrane is higher temperature dependant than styrene. When activation energy is high, the flux will be more sensitive to temperature changes; therefore water is more sensitive to temperature variations. The difference in activation energy for each component may rise from several factors such as the molecular size, affinity between the permeant and membrane, and the interaction between the permeating molecules. 3.6. Effect of membrane thickness The effects of membrane thickness on the permeation flux and enrichment factor were studied for pervaporation of a 50 ppm aqueous styrene solution at 30 °C by two PDMS membranes having the functional layer thickness of 1 and 18 μm. The results are presented in Fig. 12. Both the total flux and partial flux of water decrease with increase in membrane thickness. This is due to the fact that at higher membrane thickness the feed components diffuse through a longer tortuous path to reach the downstream side and thus the resistance towards diffusion increases. The effect of membrane thickness on the enrichment factor is also indicated in Fig. 13. The styrene enrichment factor increases from 128
ð6Þ
Fig. 11. Arrhenius type relation on the total and styrene fluxes at styrene feed concentration of 50 ppm and permeate pressure of 1 mm Hg.
Fig. 12. The effect of membrane thickness on the water and styrene fluxes at styrene feed concentration of 50 ppm, temperature of 30 °C and permeate pressure of 1 mm Hg.
M. Aliabadi et al. / Desalination 284 (2012) 116–121
[3]
[4] [5]
[6]
[7] [8] Fig. 13. The effect of membrane thickness on the enrichment factor of styrene at styrene feed concentration of 50 ppm, temperature of 30 °C and permeate pressure of 1 mm Hg.
to 557 as the membrane thickness changes from 1 to 18 μm. The increase in selectivity of styrene with membrane thickness can be explained by the fact that the PDMS membrane being hydrophobic in nature retards permeation of water molecules more at higher membrane thickness while the styrene flux was not impacted significantly. Thus, the separation factors of styrene increases. Similar trends have been observed by other researchers [31,32]. 4. Conclusions The experimental study was conducted on the pervaporation process for the removal of styrene from aqueous solutions using flat sheet PDMS membranes. The membranes were found to be styrene selective, with enrichment factor as high as 812 and permeate flux as high as 402 g − 2 h − 1. The results of experiments showed that water clustering may be developed in the membrane arising from repulsive interaction between adsorbed styrene and water. As a consequence, the water flux decreased, as the styrene concentration in the feed solution increased. When the feed flow rate and temperature increased or permeate pressure decreased, the permeation flux increased but styrene enrichment factor showed different trends with variation in these operating parameters. In general, the effect of feed temperature and permeate pressure on pervaporation performance depends on the properties of the permeant compounds and the membranes. The activation energies of pervaporation determined from the Arrhenius plots showed that the permeation of water through the PDMS membrane is higher temperature dependant than styrene. Higher membrane thickness give as a result higher enrichment factor, though partial fluxes decrease considerably. Finally, it can be concluded that pervaporation process offers an interesting prospect for its application to the treatment of wastewater streams containing styrene because of the concentration of the contaminant in the permeate and the possibility for its reuse.
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Desalination journal homepage: www.elsevier.com/locate/desal
Removal of styrene from petrochemical wastewater using pervaporation process Majid Aliabadi a, Abdolreza Aroujalian a, b,⁎, Ahmadreza Raisi a, b a b
Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Hafez Ave., P.O. Box 15875-4413, Tehran, Iran Food Process Engineering and Biotechnology Research Center, Amirkabir University of Technology (Tehran Polytechnic), Hafez Ave., P.O. Box 15875-4413, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 14 April 2011 Received in revised form 19 August 2011 Accepted 22 August 2011 Available online 9 September 2011 Keywords: Pervaporation Styrene PDMS Membrane separation Wastewater
a b s t r a c t In the present study, polydimethylsiloxane (PDMS) composite membranes were used to study the removal of styrene from dilute aqueous streams by pervaporation. The influences of styrene feed concentration, permeate pressure, operating temperature, feed flow rate and membrane thickness on the pervaporation performance were investigated. The results showed that with the increase in concentration of styrene in the feed solution, both the permeation flux and the styrene enrichment factor increase. Additionally, with a decrease in thickness of the membrane, the permeation flux was observed to enhance but the enrichment factor decreased. It was also indicated that increasing in the permeate pressure, reduces driving force for mass transfer and consequently the pervaporation performance dropped. Finally, the activation energy for pervaporation of both styrene and water calculated from Arrhenius plot indicated that the permeation of water through the membrane is higher temperature dependant than styrene. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Wastewater containing volatile organic compounds (VOCs) generated from various production lines in the chemical and petrochemical industries have recently caused serious environmental problems [1]. Styrene, as a VOC, is classified in EPA's Toxic Release Inventory as a carcinogen. Styrene is used predominantly in the production of polystyrene plastics and resins. It is also used as an intermediate in the synthesis of materials used for ion exchange resins and to produce copolymers as styrene–acrylonitrile (SAN), acrylonitrile–butadiene– styrene (ABS) and styrene-butadiene rubber (SBR). The most important physical/chemical properties of styrene are summarized in Table 1. Styrene enters the environment during the manufacture, use, and disposal of styrene-based products. The principal sources of styrene releases to water are industrial effluents. Styrene has been detected in effluents from the chemical, textile latex and coal gasification plants [2,3]. More stringent requirements for the removal of VOCs in recent years necessitate the development of innovative, cost-effective treatment alternatives. Traditional VOC control technologies such as adsorption [4] and biological treatment [5,6] have been studied for the removal of styrene from aqueous streams. However, these
⁎ Corresponding author at: Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Hafez Ave., P.O. Box 15875-4413, Tehran, Iran. Tel.: +98 21 64543163; fax: +98 21 66405847. E-mail address: [email protected] (A. Aroujalian). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.08.044
technologies do not always provide a complete and economic solution for some of these wastewater applications. According to the survey of AICHE's Center for Waste Reduction Technologies, membrane separation has become one of the emerging technologies for VOC control [7]. In recent years, membrane separation technology such as reverse osmosis [8], membrane air stripping process [9], membrane distillation [10], nanofiltration [11], pervaporation [12–14] and hybrid process [15,16] has been studied for the removal of VOCs from aqueous solutions. Among these technologies, pervaporation is a promising process for separation of VOC from water because of the compact design, recycling of recovered VOC and no emission problem [17]. Pervaporation is a membrane separation process in which a liquid stream containing two or more components is in contact with one side of a membrane while a vacuum or gas purge is applied to the other side. The components in the liquid stream sorb into the membrane, permeate through the membrane, and evaporate into the vapor phase. The condensed permeate liquid often separates into two phases due to limited solubility of VOCs in water. The organic phase can be treated for reuse, and the aqueous phase saturated with VOCs can be recycled to the feed stream for reprocessing. Pervaporative removal of various VOCs, such as benzene [18], toluene [19], ethyl benzene [20], xylene [21] and chlorinated solvents [22–24], from aqueous streams has been extensively investigated. However, to our knowledge, the removal of styrene from aqueous solutions has not been studied. In this study, the potential of the pervaporation process for treating styrene contaminated water, simulated petrochemical wastewater, was evaluated. According to the field data obtained
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Table 1 Physical and chemical properties of styrene and water [2]. Components
Molecular weight
Boiling point (°C)
Density at 20 °C (g cm− 3)
Vapor pressure at 20 °C (Pa)
Molar volume (cm3 mol− 1)
Solubility in water at 20 °C (mg l− 1)
Styrene Water
104.15 18.02
145.2 100
0.9059 0.998
656.4 2340
115.0 18.06
310 –
from a local plant, the concentration of styrene adjusted to the range of 20–300 mg/l, and the effect of operating conditions on the pervaporation performance was investigated.
2. Experimental 2.1. Materials PDMS/PVDF/PP composite membranes with functional layer of polydimethylsiloxane used in the experiments were kindly supplied by Helmholtz-Zentrum Geesthacht Zentrum für Material und Küstenforschung GmbH (Geesthacht, Germany). Membranes were cut into 15× 20 cm pieces and placed in a flat frame membrane module. Styrene (reagent grade, purity 99%), acetonitrile (HPLC grade, purity 99.9%) and water (HPLC grade, purity 99.9%) were purchased from Merck Co. Ltd. (Darmstadt, Germany) and deionized laboratory water was used for making aqueous mixtures.
2.2. Pervaporation experiments A schematic diagram of the test unit is shown in Fig. 1. The system was operated with continuous recirculation of the aqueous phase to the feed tank. A flat frame membrane module (Osmonics Inc., Minnetonka, MN, USA) with effective area of 138 cm 2 was used in cross-flow mode. A peristaltic pump was used to recirculate the liquid feed from the feed tank through the membrane. The temperature of feed stream was controlled by a PID controller. During the experiments the membrane downstream pressure was controlled by a needle valve and a vacuum pump (Busch Inc., Switzerland). A pressure meter calibrated by a vacuum gauge was used to monitor the downstream pressure. The condenser system consisted of two traps refrigerated by liquid nitrogen (− 196 °C) that can be used alternately, allowing the permeate stream to be sampled continuously without interrupting operation of the unit. The feed flow rate was measured by a flow meter. Sampling was done carefully to avoid any loss. Samples were collected in a small 10 ml glass vial and capped immediately with a Teflon-lined cap to avoid styrene loss.
Fig. 1. Schematic diagram of the pervaporation system used in this study.
2.3. Analytical facilities The concentration of styrene in the aqueous solutions was measured by an Agilent series 1200 HPLC equipped with a reversephase Eclipse XDB C18 Agilent column (5 μm, 4.6 × 250 mm). The mobile phase was 75% acetonitrile and 25% water. Styrene concentration was determined using an injection volume of 10 μl at wavelength of 245 nm (UV detector) and a mobile phase flow rate of 1 ml/min. 2.4. Calculated quantities The fluxes of styrene and water were obtained from following equation [19]: Ji ¼
mi Am τ
ð1Þ
where Ji is the permeation flux of component i, gm −2 h −1; mi is the weight of component i in permeate, g; Am is the effective membrane area, m 2 and τ is the permeation time, h. The selectivity of the membrane may be quantified by two alternative dimensionless ratio, enrichment factor, β, and separation factor, α, defined as follows [25]: β¼
y1 x1
ð2Þ
α¼
y1 =y2 x1 =x2
ð3Þ
where x1 and x2 are the concentration of styrene and water in the feed solution as well as y1 and y2 being the concentration of styrene and water in the permeate. In the present study, the effect of feed flow rate, initial feed concentration, feed temperature, permeate pressure and functional layer thickness of membrane on the performance of pervaporation process for the removal of styrene from its aqueous solutions was investigated. Time duration of each experiment was 6 h and the collected permeate was weighed and analyzed every 2 h. All experimental conditions were repeated three times and the average values are reported.
Fig. 2. The effect of feed flow rate on the total and partial fluxes at styrene feed concentration of 150 ppm, temperature of 30 °C and permeate pressure of 1 mm Hg.
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3. Results and discussion 3.1. Effect of feed flow rate The effect of feed flow rate on the total flux and styrene partial flux at the styrene concentration of 150 ppm is shown in Fig. 2. The Reynolds number, Re, which can be calculated from Eq. (4), is well known dimensionless description of the hydrodynamic conditions in the feed flow: Re ¼
ρudh μ
ð4Þ
where v, ρ and μ are velocity, density and viscosity of feed and dh is hydraulic diameter of pervaporation cell. It can be seen from Fig. 2 that the styrene and total flux enhance with an increase in Reynolds number. For example, the styrene flux goes from 1.9 to 4.6 g −2 h −1 when the Reynolds number is raised from 430 to 2150 (which corresponded to feed flow rate from 1 to 5 l min −1) for the thick PDMS membrane. These results can be attributed to the effect of concentration polarization in the liquid boundary layer adjacent to the membrane surface. Concentration polarization tends to decrease the permeation rate of the more permeable component (styrene) and increase the permeation rate of the less permeable component (water), resulting in a lesser extent of separation [20]. However an increase in the feed flow rate could reduce the effect of concentration polarization, and thus the styrene flux should increase as observed. Additionally, water flux, which is controlled by the rate of diffusion through the membrane, should be independent of feed flow rate. As a result, the styrene/water enrichment factor increases as the feed flow rate increases as shown in Fig. 3. 3.2. Effect of feed concentration The effect of styrene feed concentration on the water and styrene partial fluxes and the enrichment factor was determined at the styrene concentration range of 20–300 ppm and different temperatures. The partial flux of styrene is plotted in Fig. 4 as a function of the styrene feed concentration for PDMS membrane with functional layer thickness of 18 μm. It is indicated that the partial flux of styrene enhances with an increase in the styrene feed concentration. This behavior can be attributed to the increase in styrene activity in the feed mixture due to enhancement in the styrene concentration. Besides, the results showed that the water flux decreases with the increase in styrene concentration in the feed solution, as shown in Fig. 5. For example, the water flux decreases from 91.2 to 79.5 g − 2 h − 1 when the feed concentration of styrene is increased from 20 to 300 ppm. This phenomenon can be explained by the water clustering effect.
Fig. 3. The effect of feed flow rate on the enrichment factor of styrene at styrene feed concentration of 150 ppm, temperature of 30 °C and permeate pressure of 1 mm Hg.
Fig. 4. The effect of styrene feed concentration on the styrene flux at different temperatures and permeate pressure of 1 mm Hg.
Water clustering is developed in the membrane arising from repulsive interaction between water and organic component absorbed [20]. It has actually been shown that the permeation of water through polymer membranes can be hindered by the formation of a water cluster [26,27].Water itself exists in the form of hydrogen-bonded clusters, depending on its circumstance. Thus, free water molecules may diffuse accompanied by clustered molecules. This implies that as the diffusion size of water increases, the diffusion coefficient decreases. The effect of feed concentration on the styrene enrichment factor is presented in Fig. 6. It can be seen from this figure that the enrichment factor increases as the styrene feed concentration goes to higher levels. For example, the enrichment factor increases from 493 to 812 when the feed concentration of styrene changes from 20 to 300 ppm at 30 °C. The enrichment factor is related to the ratio of styrene partial flux to water flux. The water flux declines with increase in the styrene feed concentration while the styrene flux rises. Therefore, the ratio of the two fluxes which is equal to the styrene enrichment factor increases when the feed concentration enhances. Similar results have been reported by other authors [28,29]. As an example, Lau et al. [29] reported that an increase in chloroform concentration resulted in the expected increase in chloroform flux, but a decrease in water flux, yields an increase in the separation factor from 1170 to 1800. 3.3. Effect of permeate pressure Permeate pressure is an important parameter that affects the performance of the pervaporation process. Since the driving force for permeation of components in pervaporation is the vapor pressure difference between the feed and permeate side, the partial fluxes
Fig. 5. The effect of styrene feed concentration on the water flux at temperature of 30 °C and permeate pressure of 1 mm Hg.
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Fig. 6. The effect of styrene feed concentration on the enrichment factor at different temperatures and permeate pressure of 1 mm Hg.
Fig. 8. The effect of permeate pressure on the enrichment factor of styrene at styrene feed concentration of 50 ppm and temperature of 30 °C.
are expected to increase with decreasing downstream pressure. Permeate pressure greatly affects the operating cost of the process because the cost of maintaining vacuum is substantial. It also determines the component concentration in the permeate stream and affects the membrane selectivity [30]. The effect of permeate pressure on the partial fluxes of water and styrene as well as enrichment factor was investigated over the permeate pressure range of 1–30 mm Hg at 30 °C. The partial fluxes of styrene and water are depicted in Fig. 7 as a function of permeate pressure for the PDMS membrane. This figure shows that partial fluxes of styrene and water decrease with an increase in the permeate pressure. For example, the styrene flux decreases from 1.56 to 0.15 g −2 h −1 when the permeate pressure is increased from 1 to 30 mm Hg. Also, it was found that the decrease in partial fluxes was more pronounced for the styrene flux in comparison to the water flux. These observations are a consequence of an increase in the vapor pressure of components in the permeate and hence, reducing in the vapor pressure differences across the membrane. Furthermore, the effect of permeate pressure on the enrichment factor of styrene at feed temperature of 30 °C and styrene concentration of 50 ppm was studied. The results are presented in Fig. 8. A significant decline in the separation efficiency with increasing permeate pressure was observed. This phenomenon can be related to more pronounced decrease of the styrene flux in comparison to the water flux and hence a less concentrated styrene permeate. Similar observations were found by other researchers [1,25,30], namely a clear decrease of the membrane selectivity caused by an increase in the downstream pressure of the pervaporative separation of organics/water mixtures. The effect of the permeate pressure on the enrichment factor is however dependent on the relative volatility between the selective and non-selective permeating compounds. It has been proven that
for binary mixtures, the permeate concentration depends on the permeate pressure as follows [19]:
Fig. 7. The effect of permeate pressure on the partial fluxes at styrene feed concentration of 50 ppm and temperature of 30 °C.
Fig. 9. The effect of feed temperature on the total and partial flux of styrene at styrene feed concentration of 50 ppm and permeate pressure of 1 mm Hg.
x1 ¼
p1 p2 1 p − 1 p2 −p1 pl p2 −p1
ð4Þ
where 1 and 2 represent the fast and the slow permeating component respectively, x1 is the mole fraction of component 1 in the permeate, p* stands for the saturated vapor pressure at operational temperature and pl is the downstream pressure. Derivation of Eq. (4) yields: dx1 p p 1 ¼ 1 2 2 : dp p1 −p2 p
ð5Þ
l
If the fast permeating compound is also the more volatile (p1∗ − the permeate concentration increases as permeate pressure is raised. In the opposite case, a steep decrease in the permeate concentration is observed. The decrease in the enrichment factor of styrene can be explained according to this fact. Styrene is less volatile than water, but shows higher permeability through PDMS membrane. Thus, the styrene enrichment factor decreases when the permeate pressure goes to higher levels, as demonstrated in Fig. 8.
p2∗ N 0),
3.4. Effect of feed temperature Feed temperature is a key factor in the pervaporation process and affects all of the constituent steps of solute transport from feed solution to permeate side, as well as the driving force for mass transfer. The effects of feed temperature on the total flux and partial fluxes of styrene and water as well as enrichment factor were investigated over the temperature range of 30–60 °C for a 50 ppm aqueous styrene solution. The results are shown in Figs. 9 and 10. As depicted, the
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M. Aliabadi et al. / Desalination 284 (2012) 116–121 Table 2 Activation energy values that were calculated from Arrhenius plots. Component
Styrene Water Total
Fig. 10. The effect of feed temperature on the enrichment factor of styrene at styrene feed concentration of 50 ppm and permeate pressure of 1 mm Hg.
styrene flux as well as the water flux increase with an enhancement in the feed temperature, but the increase in water flux is more significant than the styrene flux. For example, the water flux goes from 90 to 378 g − 2 h − 1 when the temperature increases from 30 to 60 °C. It can be observed from Fig. 10 that the styrene enrichment factor decreases with an increase in the feed temperature. The enrichment factor is related to the ratio of styrene flux to water flux. The styrene and water flux increase with an enhancement in the feed temperature, but the increase in water flux is more significant than the styrene flux. Therefore, the ratio of the two fluxes which is equal to styrene enrichment factor increases when the temperate goes to higher level. During the pervaporation process, permeating components diffuse through free volumes of the membrane. Thermal motions of polymer chains in amorphous regions randomly produce free volumes. As temperature increases, frequency and amplitude of polymer jumping chains increase, resulting in the increase of free volume of the membrane. As a consequence, the diffusion rate of individual permeating components increases at higher temperatures leading to high permeation fluxes. Moreover, diffusivity and viscosity of styrene in the feed solutions and the permeability of this compound into the membrane are affected by the temperature variation in the feed. Another reason for the increase of permeation flux with increase in feed temperature is that by increasing temperature, the vapor pressure of each component increases which results in high permeation flux of all components. Thus, feed temperature affects the feed/membrane characteristics and the driving force of the process. 3.5. Activation energy of pervaporation Generally, variations of total and individual fluxes are related by the Arrhenius equation [31]:
Ji ¼ J0 exp
−Eai
,
! RT
C = 50 ppm
C = 300 ppm
Ea (kJ mol− 1)
R2
Ea (kJ mol− 1)
R2
4.398 40.231 39.891
0.993 0.990 0.990
5.051 41.042 38.402
0.997 0.986 0.998
where Ji (gm −2 h −1) represents the flux of component i, J0 (g −2 h −1) is pre-exponential factor, Eai (J mol −1) the apparent activation energy of pervaporation, R (J mol −1 K −1) the universal gas constant and T is absolute temperature (K). The apparent activation energy of permeation is determined from the slopes of the ln(Ji) versus 1/T plots. The logarithm of the partial flux of styrene and water versus the reciprocal temperature are plotted in Fig. 11. The linear relationship in this figure indicates that the behavior of the membrane followed the Arrhenius law. This is consistent with other studies have been reported in the literature [25,31]. From the slope of the lines, the activation energy of pervaporation for styrene and water was found to be 4.398 and 40.231 kJ mol −1, respectively. The activation energy values at different feed concentration are presented in Table 2. As can be seen from this table, all values of activation energy are positive, so an increase in the feed temperature results in higher flux. Furthermore, the activation energies vary with the styrene feed concentration and the activation energy of water is higher than that of styrene, indicating that the permeation of water through the membrane is higher temperature dependant than styrene. When activation energy is high, the flux will be more sensitive to temperature changes; therefore water is more sensitive to temperature variations. The difference in activation energy for each component may rise from several factors such as the molecular size, affinity between the permeant and membrane, and the interaction between the permeating molecules. 3.6. Effect of membrane thickness The effects of membrane thickness on the permeation flux and enrichment factor were studied for pervaporation of a 50 ppm aqueous styrene solution at 30 °C by two PDMS membranes having the functional layer thickness of 1 and 18 μm. The results are presented in Fig. 12. Both the total flux and partial flux of water decrease with increase in membrane thickness. This is due to the fact that at higher membrane thickness the feed components diffuse through a longer tortuous path to reach the downstream side and thus the resistance towards diffusion increases. The effect of membrane thickness on the enrichment factor is also indicated in Fig. 13. The styrene enrichment factor increases from 128
ð6Þ
Fig. 11. Arrhenius type relation on the total and styrene fluxes at styrene feed concentration of 50 ppm and permeate pressure of 1 mm Hg.
Fig. 12. The effect of membrane thickness on the water and styrene fluxes at styrene feed concentration of 50 ppm, temperature of 30 °C and permeate pressure of 1 mm Hg.
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[3]
[4] [5]
[6]
[7] [8] Fig. 13. The effect of membrane thickness on the enrichment factor of styrene at styrene feed concentration of 50 ppm, temperature of 30 °C and permeate pressure of 1 mm Hg.
to 557 as the membrane thickness changes from 1 to 18 μm. The increase in selectivity of styrene with membrane thickness can be explained by the fact that the PDMS membrane being hydrophobic in nature retards permeation of water molecules more at higher membrane thickness while the styrene flux was not impacted significantly. Thus, the separation factors of styrene increases. Similar trends have been observed by other researchers [31,32]. 4. Conclusions The experimental study was conducted on the pervaporation process for the removal of styrene from aqueous solutions using flat sheet PDMS membranes. The membranes were found to be styrene selective, with enrichment factor as high as 812 and permeate flux as high as 402 g − 2 h − 1. The results of experiments showed that water clustering may be developed in the membrane arising from repulsive interaction between adsorbed styrene and water. As a consequence, the water flux decreased, as the styrene concentration in the feed solution increased. When the feed flow rate and temperature increased or permeate pressure decreased, the permeation flux increased but styrene enrichment factor showed different trends with variation in these operating parameters. In general, the effect of feed temperature and permeate pressure on pervaporation performance depends on the properties of the permeant compounds and the membranes. The activation energies of pervaporation determined from the Arrhenius plots showed that the permeation of water through the PDMS membrane is higher temperature dependant than styrene. Higher membrane thickness give as a result higher enrichment factor, though partial fluxes decrease considerably. Finally, it can be concluded that pervaporation process offers an interesting prospect for its application to the treatment of wastewater streams containing styrene because of the concentration of the contaminant in the permeate and the possibility for its reuse.
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