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Dehydration of glycerin solution using pervaporation: HybSi and polydimethylsiloxane membranes
Journal of Membrane Science 450 (2014) 440–446
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Dehydration of glycerin solution using pervaporation: HybSi and polydimethylsiloxane membranes Shee-Keat Mah, Siang-Piao Chai n, Ta Yeong Wu Chemical Engineering Discipline, School of Engineering, Monash University, Jalan Lagoon Selatan, Bandar Sunway, 46150 Selangor Darul Ehsan, Malaysia
art ic l e i nf o
a b s t r a c t
Article history: Received 30 July 2013 Received in revised form 20 September 2013 Accepted 24 September 2013 Available online 29 September 2013
The dehydration of glycerin solution is an important step in glycerin purification process. In present work, pervaporative dehydration of glycerin solution was studied using HybSi and polydimethylsiloxane (PDMS) pervaporation membranes. The effects of physical property of membrane, operating parameters and glycerin concentrations on the membrane permeate flux and selectivity were examined. The present studies showed that hydrophilic HybSi membrane could provide higher permeate flux as compared to hydrophobic PDMS membrane under an identical operating condition. Remarkable difference in permeate flux was observed when the process was operating at room temperature. The influence of crossflow rate on the permeate flux was relatively low as compared to operating pressure and temperature. Operating temperature was found to possess the greatest influence on permeate flux and selectivity. The speculated molecule movement during pervaporation was discussed based on boundary layer theory and hydrogen bonding theory. The final permeate flux and permeance water content of 13.63 kg/m2 h and 98 wt%, respectively, were achieved by HybSi membrane in the dehydration process of 90 wt% glycerin solution into 99 wt% glycerin solution over a period of 90 min. & 2013 Elsevier B.V. All rights reserved.
Keywords: PDMS Ceramic Water extraction Boundary layer Hybrid membrane
1. Introduction Glycerin is one of the most versatile organic compounds that have been widely used in medical, food product, pharmaceutical, cosmetics, textile industries and preservative [1,2]. It is a colorless, odorless, non-toxic, viscous liquid with a sweet taste, and is derived from both natural and petrochemical feedstocks [3]. Besides, the production of biodiesel through transesterification of triglycerides produces glycerin as a major byproduct, accounting for up to 10–12% of the total biodiesel production capacity [4,5]. The raw glycerin byproduct can be further purified into high grade glycerin for commercial use. The traditional glycerin purification process usually involves glycerin washing, chemical and physical pretreatment, evaporation and distillation processes [4,6,7]. In general, evaporation and distillation involved in glycerin dehydration process usually demand for intensive energy consumption. In this regard, filtration process using pervaporation membrane appears to be an economical and environmental friendly alternative in replacement of these conventional glycerin dehydration processes. A variety of pervaporation separation processes has been demonstrated in the past decade, particularly in the application involving the separation of water from organic mixtures [8–13].
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0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.09.048
Several studies [14,15] have been reported on the dehydration of glycerin solution using polymeric pervaporation membranes at low operating temperature and constant pressure. To date, no study has been exploited on the dehydration of glycerin solution using both hydrophilic and hydrophobic membranes in crossflow configuration under different operating temperatures, pressures and crossflow rates. In the present work, the effects of physical property of membrane, operating crossflow rate, operating temperature, operating pressure as well as the concentration of glycerin solution, were investigated with respect to the membrane permeate flux and selectivity. The suitable membrane and experimental condition would be identified, and to be employed in the later stage for the investigation of the possibility to dehydrate glycerin solution up to 99 wt%. The experimental findings from this study are important in providing prior knowledge for the determination of suitability of employing pervaporation as an alternative to dehydrate glycerin solution. 2. Experimental 2.1. Chemical and membranes The analytical grade glycerin with a purity of 99 % was provided by Acros Organics. The model solutions containing 15, 30, 60 and 90 wt% of glycerin, respectively, were prepared using ultrapure water (0.055 mS/cm) produced from a TKA smart2pure standard
S.-K. Mah et al. / Journal of Membrane Science 450 (2014) 440–446
reached steady state. The vacuum pressure of permeate side in the pervaporation module was later adjusted to 10 mbar and kept constant thereafter. The data of weight reduction in feed tank as a function of time was collected by a data logger attached to the digital balance. After each experimental run, the total reduction of weight in the feed tank was derived from the results logged into the data recorder of digital balance. The weight of the sample collected in the sample collector was measured and compared to the total reduction of weight in the feed tank. Typically, 5% or less difference was achieved in all experimental runs. The reproducibility of all experimental runs for permeate flux was within 73%.
Chiller
Condenser Tubular pervaporation module Vacuum pump
Vacuum pressure gauge Thermometer Pressure gauge
441
2.3. Membrane performance evaluation and transport mechanism
Sample collector
Flow meter Feed Tank Data Logger
Heater
Digital balance Peristaltic pump Fig. 1. Experimental setup of pervaporation installation.
water dispenser (Thermo Electron LED, GmbH). Two tubular pervaporation membranes were selected for the separation of water/glycerin mixtures. They were hydrophilic HybSi (organic– inorganic hybrid silica membrane, contact angle 701) and hydrophobic PDMS (polymeric membrane, contact angle 1051) pervaporation membranes which were supplied by Energy research centre, Netherlands. The molecular structures of HybSi and PDMS membranes can be found in [16–18]. In brief, hydrophilic HybSi membrane is mainly used for the dehydration of water from organic mixtures while hydrophobic PDMS membrane is primarily used for the removal of organic compounds [16,19,20].
The performance of pervaporation membrane was evaluated by examining the permeate flux and permeance water content. The permeate flux was calculated using Eq. (1). The glycerin percentage of permeance was calculated using the refractive index method. The refractometer used was Lab Digital Refractometer Brix-300034 (Sper Scientific, USA) with reflective index resolution up to 0.0001RI (Accuracy, 70.0002RI). Standard calibration chart of water/glycerin mixtures were prepared from 0–80 wt% by plotting refractive index versus glycerin concentration. To measure any sample with glycerin concentration higher than 80 wt%, a known amount of ultrapure water was added to dilute the permeance. Permeate flux ¼
W A Δt
ð1Þ
where W is the weight of permeate (kg), A is the effective membrane surface area (m2) and Δt is the sampling time (h). The effects of physical property of membrane, glycerin concentrations and operating conditions such as crossflow rate (12– 36 L/h), pressure (0–2 bar) and temperature (25–160 1C) on the permeate flux and permeance water content were also studied. The transportation mechanism of water molecule was investigated based on the physicochemical properties of water, glycerin and membrane.
2.2. Pervaporation experiment 3. Results and discussion A crossflow tubular pervaporation experimental setup is shown in Fig. 1. This experimental setup consists of a closed feed tank, a peristaltic pump (masterflex, USA), a custom made high power heater, a tubular pervaporation module (Pervatech, Netherlands), a custom made condenser integrated with PolyScience chiller (PolyScience, USA) and a vacuum pump (Value, China). All the components in this pervaporation setup were wrapped by insulating material to prevent heat loss. A tubular pervaporation membrane (active area of 0.005 m2) was inserted inside the tubular pervaporation module and sealed on both ends with high temperature compatible Kalrez rubber O-rings. A 3 kg feed solution was filled into the feed tank and the peristaltic pump was used to circulate the feed to the membrane module and return back to the feed tank. The crossflow rate, feed temperature and pressure were piloted in a range of 12–36 L/h, 25–160 1C and 0 (atmospheric pressure) – 2 bar, respectively, depending on the requirement of the operating condition for each experiment. The pervaporation membrane was allowed to equilibrate with the feed solution at the designated glycerin concentration, crossflow rate, pressure and temperature for 1 h prior to the commencement of experiment. Next, the weight of the glycerin solution in the feed tank was measured using a digital balance. Once the weight reading from the digital balance attained a constant value at the designated temperature, pressure and crossflow rate, the system has now
3.1. Effects of physical property of membrane The effects of physical property of membrane on permeate flux and permeance water content were studied using 15 wt% glycerin solution as feed. The crossflow rate and pressure of the feed were set at 36 L/h and gauge pressure of 0 bar. The temperatures of the feed solution were fixed at 25, 45, 65, 85 and 98 1C to prevent boiling of the feed solution. The steady permeate fluxes of HybSi and PDMS membranes at different temperatures were summarized in Fig. 2. Besides, viscosity of 15 wt% glycerin solution in different temperatures was calculated using the model reported in [21] and stated in Fig. 2. By means of the refractive index method, it was assessed that all the permeate samples contained 100 wt% of water. The loss of glycerin in feed due to permeation was not observed. Fig. 2 clearly demonstrates that HybSi outperformed PDMS membrane in terms of permeate flux. At 25 1C, the permeate flux of HybSi membrane (1.56 kg/m2 h) was significantly higher than PDMS membrane (0.72 kg/m2 h), which could be explained by the hydrophilic nature of HybSi provided advantage of membrane surface water attraction as compared to hydrophobic PDMS [8,22]. Besides, high wettability of HybSi membrane also provided easier passage of water molecule through the membrane as compared to polymeric membranes [23].
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45 PDMS
Permeate flux (kg/m2.h)
Permeate flux (kg/m2.h)
40 35 30
viscosity =0.4873 mPa.s
25 20 15 10 5
viscosity =1.4456 mPa.s
5
viscosity =0.4151 mPa.s
HybSi
viscosity =0.9241 mPa.s
viscosity =0.6494 mPa.s
12 L/h
4.5
24 L/h
4
36 L/h
viscosity = 0.4151 mPa.s
3.5 3 2.5 2 1.5 1 0.5
viscosity = 1.4456 mPa.s
0
0 25
45
65
85
viscosity = 0.3559 mPa.s
25
98
98
Fig. 2. The steady permeate flux of HybSi and PDMS membrane at different temperatures.
112
Temperature (°C)
Temperature (°C)
Fig. 3. The steady permeate flux of HybSi membrane in different crossflow rates.
70
3.2. Effects of crossflow rate Glycerin solution with concentration of 15 wt% was used as the model solution to study the effect of crossflow rate on permeate flux and permeance water content. In this regard, crossflow rate of the pervaporation experiment was piloted at 12, 24, and 36 L/h. At the temperature of 112 1C, the operating pressure of 1 bar was applied to prevent boiling of feed steam. When operated at 25 1C and 0 bar gauge pressure, the increase in crossflow rate from 12 to 36 L/h showed insignificantly escalating influence on the permeate flux. The slight increase of permeate flux with respect to the increasing crossflow rate might be due to the reduced effect of concentration polarization of glycerin molecule on the membrane surface [26]. In order to verify this deduction, crossflow rates of 12, 24 and 36 L/h were applied at three different operating temperatures, notably 25, 98 and 112 1C. The results shown in Fig. 3 ascertain that crossflow rate possessed little effect on the permeate flux despite being carried out at different operating
0 bar
60
Permeate flux (kg/m2.h)
Swelling effect is a major drawback which is generally encountered in hydrophilic pervaporation membrane. This effect could increase the pore size of the membrane which would subsequently increase the permeate flux as well. Consequently, the issue of reduction in selectivity of the filtration process arises in conjunction with the use of hydrophilic pervaporation membrane [8,22]. The higher permeate flux and total permeance water content attained with the application of HybSi membrane could be reasoned by the physical property of the membrane. Low swelling ability, high thermal stability and high chemical resistance on feed solution possessed by the HybSi membrane contributed to its outstanding performance [24,25]. However, as the operating temperature was elevated further apart from room temperature, the permeate flux recorded for both HybSi and PDMS shifted closer to each other with apparently smaller differences. One of the reasons to this phenomenon could be due to significant reduction of glycerin viscosity at elevated temperature. Lower viscosity of glycerin solution would promote the rate of permeation of water through the membrane. This is in associate of the decrease in hydrogen bonding between glycerin molecules and water molecules. By comparing the permeate flux and permeance water content of both HybSi and PDMS membranes, HybSi membrane was selected predominantly to separate water from glycerin solution due to its capability in providing higher permeate flux under same operating condition. In view of this, only HybSi membrane would be employed in the later stage in this study to examine the effects of operating parameters on the permeate flux and permeance water content.
0.5 bar 1 bar
50
1.5 bar
Slopes of flux increment
40 30 20 10 0 25
45
65
85
98
112
120
Temperature (°C) Fig. 4. The steady permeate flux of HybSi membrane in different pressures and temperatures.
temperatures. Another noticeable observation to be deduced from Fig. 3 would be the effect of operating temperature on permeate flux which was substantially higher than that of crossflow rate. The water content of all permeance sample was found to be 100 wt%. Considering that crossflow rate had very minor influence on permeate flux and no effect on permeance water content, low crossflow rate of 12 L/h was favored as less pumping and heating energy would be required. 3.3. Effects of different temperatures and pressures 3.3.1. Pervaporation of 15 wt% glycerin solution In this section, glycerin solution with concentration of 15 wt% was used as the feed solution to study the effect of operating temperature and pressure on permeate flux and permeance water content. The operating pressure was fixed at 0, 0.5, 1 and 1.5 bar gauge pressure while the operating temperature was controlled at 25, 45, 65, 85, 98, 112 and 120 1C. As a precaution to prevent boiling of feed solution during pervaporation process, the operating pressure was set at 1 and 1.5 bar for 112 1C while 1.5 bar for 120 1C. The crossflow rate of 12 L/h was selected in this section of investigation as elucidated in Section 3.2. In Fig. 4, it is clearly shown that the permeate flux increased exponentially with increasing temperature. In addition, all permeance samples were measured to be containing 100 wt% of water. Temperature is an important parameter in membrane filtration because it affects the properties of both feed solution and membrane. According to Figs. 2 and 3, the viscosity of 15 wt% glycerin solution decreased rapidly from 1.4456 to 0.3559 mPa s
S.-K. Mah et al. / Journal of Membrane Science 450 (2014) 440–446
3.3.2. Pervaporation of 30 and 60 wt% glycerin solutions In this section, 30 and 60 wt% glycerin solutions were used as feed solutions. To avoid boiling in the 30 and 60 wt% glycerin solutions during the pervaporation process, the operating pressure was adjusted to 1 and 1.5 bar at 112 1C while 1.5 bar at 120 1C. The permeate fluxes at different temperatures and pressures are shown in Figs. 5 and 6 accompanied with the corresponding viscosities at different temperatures. All permeate samples were
70
Permeate flux (kg/m2.h)
60
0 bar 0.5 bar 1 bar 1.5 bar
Viscosity = 0.5191 mPa.s
Viscosity = 0.4754 mPa.s
50 Viscosity = 0.6154 mPa.s
40 Viscosity = 0.7364 mPa.s
30 20 10
Viscosity = 2.5748 mPa.s
Viscosity = 1.5311 mPa.s
Viscosity = 1.0203 mPa.s
0 25
45
65
85
98
112
120
Temperature (°C) Fig. 5. The steady permeate flux of 30 wt% glycerin solution in different temperatures and pressures. 70 60
Permeate flux (kg/m2.h)
when the temperature was increased from 25 to 112 1C. When the operating temperature was increased to 120 1C, the viscosity of feed solution was further reduced to 0.3283 mPa s. The rapid decrease in the viscosity of the 15 wt% glycerin solution with the increase in temperature is strongly related to hydrogen bonding [3]. With an increase in the glycerin solution temperature, the number of hydrogen bonds between water–water and water– glycerin decreased due to rapid molecules movement [27,28]. The lifetime of hydrogen bond between water–water is significantly shorter than water–glycerin due to the fact that the water molecules re-orientational motions are faster than alcohol molecules [29,30]. The faster molecules movement and shorter lifetime of hydrogen bond between water–water molecules could promote formation of hydrogen bonding between hydrophilic HybSi and water molecules [16] and thus increasing the water permeation through the HybSi membrane. At the temperatures of 25, 45 and 65 1C, an increase in the operating pressure from 0 to 1.5 bar increased the permeate flux as shown in Fig. 4. The effects of operating pressure on the increasing permeate flux could be highlighted by “slopes of increment” as indicated in Fig. 4. This could be explained by the phenomenon in which higher operating pressure would provide a greater pressure gradient across the membrane, thus forcing the water molecules to diffuse across the membrane with greater ease. As both operating temperature and pressure increased in the experiment, it was observed that the higher operating pressure contributed to the increase in permeate flux with a greater extent at higher temperature. This was evident in Fig. 4 as the “slope of increment” was found to be increased from 25 to 98 1C. However, this was only observed in conditions with temperatures at 98 1C and below. At higher temperatures, the reduction of viscosity in the 15 wt% glycerin solution resulted in lower number and shorter lifetime of hydrogen bonding between water–water molecules in the feed solution. Higher operating pressure, on the other hand, provided a greater pressure gradient which would promote the diffusion of water molecules across the membrane. The combination of effects from both increased operating temperature and pressure thus offered superior performance of the membrane in terms of permeate flux. When the experiment was carried out at 65 1C, further increase of operating pressure from 0.5 to 1.5 bar did not show any significant effect on the permeate flux. On the other hand, when the operating temperature was further increased to 85 and 98 1C, the maximum permeate fluxes of 24.24 and 46.44 kg/m2 h, respectively, were achieved at 1 bar of pressure. The advancement of operating pressure from 1 to 1.5 bar at the aforementioned temperatures did not increase the permeate flux any further. On the contrary, the permeate flux dropped to a value close to the corresponding permeate flux when operated at 0.5 bar. The preferable operating pressure was found to be at 1 bar as to achieve the highest permeate flux without compromising on the selectivity. At the temperature of 112 1C, a decrease in permeate flux from 1 to 1.5 bar was also observed in Fig. 4. In this case, the results clearly stated that at operating temperature in a range of 65–112 1C, the preferred operating pressure should not exceed 1 bar.
443
0 bar 0.5 bar 1 bar 1.5 bar
Viscosity = 1.2878 mPa.s
50
Viscosity = 1.1473 mPa.s
40 30 Viscosity = 2.0721 mPa.s
20 10
Viscosity = 12.8080 mPa.s
Viscosity = 5.9218 mPa.s
Viscosity = 1.6185 mPa.s
Viscosity = 3.2851 mPa.s
0 25
45
65
85
98
112
120
Temperature (°C) Fig. 6. The steady permeate flux of 60 wt% glycerin solution in different temperatures and pressures.
found to contain 100 wt% of water. HybSi membrane showed superior performance in terms of glycerin retention when compared to the results reported by Khairnar et al. [15] and Burshe et al. [14]. Similar trend of permeate flux with respect to operating temperature and pressure was observed in the 30 and 60 wt% glycerin solutions. Analogous to 15 wt% glycerin solution used in Section 3.3.1, higher operating temperature contributed to higher permeate flux as shown in Figs. 5 and 6. The factor that allows higher permeate flux at different concentrations of glycerin solution is strongly correlated to its viscosity. Besides that, operating temperature as high as 112 and 120 1C would promote the water molecule movement in the glycerin solution as a consequence of the reduction in the number of hydrogen bonds and thus greatly improved the permeate flux [27,28]. When comparing the permeate flux at the operating temperatures of 112 and 120 1C in Figs. 4 and 5, the values were close despite the fact that the concentration of glycerin was increased from 15 wt% to 30 wt%. However, when the glycerin concentration was increased to 60 wt%, the permeate flux at different temperatures and pressures was significantly low. The reason could be due to the considerably higher viscosity of 60 wt% glycerin solution than that of 15 and 30 wt% glycerin solutions. By carefully examining Figs. 4, 5 and 6, operating pressure at 1 bar at different temperatures was found to be favorable by providing better permeate flux without any trade-off of glycerin rejection. The ultimate finding in this section concluded that increasing the operating temperature could be an option to increase the processing rate of the glycerin filtration process using pervaporation
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membrane. According to Burshe et al. and Khairnar et al., the highest permeate fluxes obtained from pervaporation of 60 wt% glycerin solution at 70 1C were 1.21 and 0.85 kg/m2 h, respectively [14,15]. In the present work, the use of HybSi pervaporation membrane at atmospheric pressure and 65 1C was capable of achieving permeate flux as high as 1.80 kg/m2 h with total glycerin retention outperformed the pervaporation membranes as reported in [14,15].
3.3.3. Effects of operating temperature and pressure on water transport mechanism During the separation of glycerin and water mixtures, water molecules would be attracted to the active surface of hydrophilic HybSi membrane, followed by diffusion through the membrane and subsequently desorbed on the posterior side of the membrane. The water molecules permeated to the posterior side of the membrane would be evaporated under vacuum condition to form permeance [31]. The decrease in permeate flux when the operating pressure was increased from 1 to 1.5 bar at the operating temperatures above 65 1C could be explained by both fluid mechanics theory and hydrogen bonding theory [21,28,29,32,33]. The aforementioned phenomenon was observed and depicted in Sections 3.3.1 and 3.3.2. It was speculated that a series of event had contributed to this phenomenon, as described in the following: (i) operating temperature above 65 1C provided greater ease of water movement as
well as evacuation due to the negative correlation between operating temperature and viscosity of glycerin solution; (ii) the diffusion of water molecules which were previously attached to the periphery of glycerin molecules into the membrane would immediately produce a layer of polarized and concentrated glycerin molecules; and (iii) the crossflow mechanism was unable to provide sufficient shear force to “sweep” away the glycerin molecules that were superficially condensed as a thin layer on the membrane surface due to operating pressure as high as 1.5 bar which was pushing all the molecules against the membrane; as a result, (iv) the condensed layer of glycerin molecules that was being “pushed” towards the membrane active surface would cause blockage to a certain extent and substantially reduce the availability of membrane active surface area. The formation of this thin glycerin gel layer within the glycerin polarization layer was responsible for the reduction in the permeability of water in the glycerin dehydration process using pervaporation membrane [34]. Based on the fluid mechanics theory on boundary layer, an illustrated scenario of the distribution of glycerin molecules inside the tubular pervaporation membrane is shown in Fig. 7.
3.4. Dehydration of 90 wt% of glycerin solution With respect to the findings obtained in Sections 3.2 and 3.3, the preferable crossflow rate and operating pressure to achieve better pervaporation efficiency would be 12 L/h and 1 bar, respectively. In this section, the pervaporation membrane was utilized
Zero velocity layers
Pervaporation membrane
Glycerin gel layer
Concentration polarization layer
Pervaporation membrane
Water diffusion
Water diffusion
Shear stress
Flow velocity
Glycerin molecule Fig. 7. An illustration of tubular pervaporation process with concentration polarization and glycerin gel layers.
S.-K. Mah et al. / Journal of Membrane Science 450 (2014) 440–446
250
60
Table 1 The dehydration results of 90 wt% glycerin solution to 99 wt%.
Viscosity
50
200
40 150 30 100 20
Viscosity (mPa.s)
Run Glycerin number concentration (wt%)
Experimental time (min)
Permeance water content (wt%)
Final permeate flux (kg/m2 h)
1 2 3
89 89 90
98.82 98.73 98.64
13.69 13.63 13.58
99.01 99.02 99.01
50
10
25
45
65
85
98
112
120
130
140
150
160
0
Temperature (°C) Fig. 8. The steady permeate flux and viscosity of 90 wt% glycerin solution at different operating temperatures.
along with 90 wt% glycerin solution at the aforementioned operating conditions. Seeing that the boiling point of 90 wt% glycerin solution at atmospheric pressure is 138 1C [35], the permissible operating temperature would now range from 25 to 160 1C as the maximum operating pressure was increased to 2 bar. This is in view of the potential of maximizing the dehydration efficiency of the pervaporation process by manipulating the operating temperature and pressure. As to prevent boiling of feed solution, the operating temperatures were only allowed to increase to 150 and 160 1C when the operating pressures were fixed at 1.5 and 2 bar, respectively. The steady permeate fluxes of the pervaporation process using 90 wt% glycerin solutions evaluated at different operating temperatures are shown in Fig. 8, accompanied with its respective viscosities. At 25 1C, the viscosity of 90 wt% glycerin solution was 208.71 mPa s. The viscosity reading decreased dramatically to 59.40 mPa s when the operating temperature was at 45 1C. However, the permeate flux was rather insensitive to the reduction in viscosity of the glycerin solution, particularly in the range of 45– 120 1C. On the other hand, when the operating temperature was increased from 120 to 130 1C where the viscosities remained almost unchanged, a sharp increase in permeate flux was observed. The viscosities of glycerin solution were measured to be reduced from 4.27 to 3.49 mPa s. This indicated that the permeate flux was basically insensitive to the operating temperature up to 120 1C. Significant effect of operating temperature on permeate flux could only be discovered when the experiment was conducted at 120 1C. In the range of 130–140 1C, the permeate flux was found to have a gradual increasing trend. It is important to take note that all the experimental runs conducted at 140 1C and below were subjected to operating pressure at 1 bar. The result suggested that when the temperature was at 130 1C, the water molecules in the feed solution would obtain sufficiently large energy to move relatively freely in the 90 wt% glycerin solution. This would subsequently increase the rate of diffusion of water molecules through the pervaporation membrane. The quality of permeance from all experimental runs described in Fig. 8 was carefully examined using the refractive index method. The results showed that the permeance obtained from 90 wt% glycerin pervaporation at operating temperature ranged from 25 to 150 1C was found to contain 100 wt% of water. However, the permeance collected at the temperature of 160 1C contained 99.91 wt% of water. This shows that glycerin diffused through the membrane at this particular temperature and pressure. As the operating temperature was increased to 150 and 160 1C, the recorded permeate fluxes were 20.04 and 57.60 kg/m2 h, respectively. The result also suggests that the operating temperature of 160 1C could be considered as the most efficient operating
Weight of gycerin soution in feed tank (g)
Permeate flux (kg/m2.h)
Permeate flux
0
445
1850 Concentration of glycerin solution in feed tank with respect to experimental time
90 wt%
1800 1750 1700 1650 1600 1550
99 wt%
1500 1450 1400 1350 0
10
20
30
40
50
60
70
80
90
100
110
120
Time (minute) Fig. 9. The dehydration process of 90 wt% glycerin solution.
temperature for the dehydration of glycerin solution. This is in associated with its ability to obtain permeate flux of 188% higher at the stipulated conditions as compared to the permeate flux achieved using operating temperature of 150 1C. 3.5. Dehydration of glycerin solution to 99 wt% In this section, the pervaporative dehydration process was used to concentrate 90 wt% glycerin solution to 99 wt%. According to the findings described in Sections 3.2, 3.3 and 3.4, the operating condition of dehydration process was controlled at 12 L/h, 2 bar and 160 1C. The entire system was filled with 3 kg of 90 wt% glycerin solution. Upon reaching steady state, the weight recorded for the glycerin solution contained in the feed tank was 1.8 kg, while the remaining could be found in pipelines and other auxiliary equipment. Three separated experimental runs were conducted as shown in Table 1. Based on the experimental data, 3 kg of 90 wt% glycerin solution required around 90 min of continuous dehydration process to concentrate the feed solution up to 99 wt% with the permeance water content well above 98 wt%. The trend of weight reduction in the feed solution with respect to time was demonstrated in Fig. 9. A gradual weight reduction of the feed solution was observed when the concentration of the glycerin in the feed solution progressively increased from 90 to 99 wt% as a result of the permeation of water through the membrane. The weight reduction in the feed solution would eventually level out as the process continued beyond 90 min.
4. Conclusions The performance of hydrophilic HybSi and hydrophobic PDMS membranes on pervaporation of glycerin solution was investigated. The hydrophilic HybSi membrane was found to be capable of providing higher water flux as compared to the hydrophobic PDMS membrane when subjected to identical operating conditions. The water permeate flux was mostly insensitive to crossflow rate at different operating temperatures. The viscosity of feed solution was found to be influencing the permeate flux extensively. In general,
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increasing the operating pressure would gradually increase the permeate flux at different operating temperatures. However, when the operating pressure was increased to 1.5 bar coupled with operating temperature higher than 65 1C, a decrease in permeate flux was observed. The collective understanding of fundamental knowledge on glycerin and membrane physicochemical properties as well as boundary layer theory is crucial in providing a comprehensive insight on the molecule movement during the pervaporation process. During the dehydration of 90 wt% glycerin solution, permeate flux as high as 57.60 kg/m2 was achieved at 160 1C and 2 bar with 99.91 wt% of water content in the permeance. In the process of concentrating 90 wt% glycerin solution to 99 wt%, a 3 kg of feed solution required approximately 90 min to dehydrate the glycerin solution and eventually produced final permeate flux and permeance water content of around 13.63 kg/m2 h and 98 wt%, respectively. Lastly, the HybSi pervaporation membrane was proven to be capable of dehydrating glycerin solution up to 99 wt% at 160 1C and 2 bar without much compromising on the retention of glycerin.
Acknowledgments The authors thank the Monash University (Sunway Campus) for the financial support in this work (Major Grant EM-2-10). References [1] A. Behr, J. Irawadi, J. Leschiski, F. Findner, Improved utilization of renewable resource: new important derivative of glycerol, Green Chem. 10 (2008) 13–30. [2] A. Singhabhandhu, T. Tezuka, A perspective in incorporation of glycerin purification process in biodiesel plants using waste cooking oil as feedstock, Energy 35 (2010) 2493–2504. [3] M. Pagliaro, M. Rossi, The Future of Glycerol: New Uses of a Versatile Raw Material, Royal Society of Chemistry, 2008 (chapter 1). [4] Y. Zhang, M.A. Dube, D.D. McLean, M. Kates, Biodiese production from waste cooking oil: 1. Process design and technological assessment, Bioresour. Technol. 89 (2003) 1–16. [5] H.W. Alex, D. Posarac, N. Ellis, Assessment of four biodiesel production processes using HYSYS. Plant, Bioresour. Technol. 99 (2008) 6587–6601. [6] N. Ziels, L.B. Company, Hammond, Indiana, Recovery and purification of glycerol, J. Am. Oil Chem. Soc. 33 (1956) 556–565. [7] G. D'souza, The importance of glycerol in the fatty acid industry, J. Am. Oil Chem. Soc. 56 (1979) 812A–818A. [8] 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. [9] H.I. Shaban, Pervaporation separation of water from organic mixtures, Sep. Purif. Technol. 2 (1997) 119–126. [10] X. Qiao, T. Chung, W.F. Gou, T. Matsuura, M.M. Teoh, Dehydration of isopropanol and its comparison with dehydration of butanol isomers from thermodynamic and molecular aspects, J. Membr. Sci. 252 (2005) 37–49. [11] C. Hu, B. Li, H.W. Ruili Guo, Z. Jiang, Pervaporation performance of chitosan– poly(acrylic acid) polyelectrolyte complex membranes for dehydration of ethylene glycol aqueous solution, Sep. Purif. Technol. 55 (2007) 327–334.
[12] S. Xia, X. Dong, Y. Zhu, W. Wei, F. Xiangli, W. Jin, Dehydration of ethyl acetate– water mixtures using PVA/ceramic composite pervaporation membrane, Sep. Purif. Technol. 77 (2011) 53–59. [13] Q.W. Yeang, S.H.S. Zein, A.B. Sulong, S.H. Tan, Comparison of the pervaporation performance of various types of carbon nanotube-based nanocomposites in the dehydration of acetone, Sep. Purif. Technol. 107 (2013) 252–263. [14] M.C. Burshe, S.B. Sawant, V.G. Pangarkar, Dehydration of glycerin–water mixtures by pervaporation, J. Am. Oil Chem. Soc. 76 (1999) 209–214. [15] D.B. Khairnar, V.G. Pangarkar, Dehydration of glycerin/water mixtures by pervaporation using homo and copolymer membranes, J. Am. Oil Chem. Soc. 81 (2004) 505–510. [16] H.M. van Veen, M.D.A. Rietkerk, D.P. Shanahan, M.M.A. van Tuel, R. Kreiter, H.L. Castricum, J.E. ten Elshof, J.F. Vente, Pushing membrane stability boundaries with HybSi pervaporation membranes, J. Membr. Sci. 380 (2011) 124–131. [17] Technology, HybSi, ECN. Available from: 〈http://www.hybsi.com/technology/〉. [18] Y. Luo, S. Tan, H. Wang, F. Wu, X. Liu, L. Li, Z. Zhang, PPMS composite membranes for the concentration of organics from aqueous solutions by pervaporation, Chem. Eng. J. 137 (2008) 496–502. [19] R. Kreiter, M.D.A. Rietkerk, H.L. Castricum, H.M. van Veen, J.E. ten Elshof, J. F. Vente, Stable hybrid silica nanosieve membranes for the dehydration of lower alcohols, ChemSusChem 2 (2009) 158–160. [20] PDMS-Based Organophilic Pervaporation Membranes, Pervatech. Available from: 〈http://www.pervaporation-membranes.com/PDMS-based-organophilic-perva poration-membranes.html〉. [21] N.S. Cheng, Formula for the viscosity of glycerol–water mixture, Ind. Eng. Chem. Res. 47 (2008) 3285–3288. [22] S.I. Semenova, H. Ohya, K. Soontarapa, Hydrophilic membranes for pervaporation: an analytical review, Desalination 110 (1997) 251–286. [23] 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. [24] H.L. Castricum, R. Kreiter, H.M. van Veen, D.H.A. Blank, J.F. Vente, J.E. ten Elshof, High-performance hybrid pervaporation membranes with superior hydrothermal and acid stability, J. Membr. Sci. 324 (2008) 111–118. [25] I. Agirre, M.B. Güemez, H.M. van Veen, A. Motelica, J.F. Vente, P.L. Arias, Acetalization reaction of ethanol with butyraldehyde coupled with pervaporation, semi-batch pervaporation studies and resistance of HybSi membranes to catalyst impacts, J. Membr. Sci. 371 (2011) 179–188. [26] K.J. Lee, R.M. Wu, Simulation of resistance of cross-flow microfiltration and force analysis on membrane surface, Desalination 233 (2008) 239–246. [27] J. Zielkiewicz, Structural properties of water: comparison of the SPC, SPCE, TIP4P, and TIP5P models of water, J. Chem. Phys. 123 (2005) 104501. [28] J.C. Crittenden, R.R. Trussell, D.W. Hand, K.J. Howe, G. Tchobanoglous, Water Treatment – Principles and Design, 2nd edition, John Wiley & Sons, Hoboken, New Jersey, USA. 2005. [29] C. Chen, W.Z. Li, Y.C. Song, J. Yang, Hydrogen bonding analysis of glycerol aqueous solutions: a molecular dynamics simulation study, J. Mol. Liq. 146 (2009) 23–28. [30] E. Guàrdia, J. Martí, J.A. Padró, L. Saiz, A.V. Komolkin, Dynamics in hydrogen bonded liquids: water and alcohols, J. Mol. Liq. 96–97 (2002) 3–17. [31] F.U. Baig, Advance Membrane Technology and Applications, Chapter 17 Pervaporation, John Wiley & Sons, Hoboken, New Jersey, USA. 2008. [32] P. Kundu, I.D.D. Cohen, Fluid Mechanics, Fifth edition, Elsevier, Boston, USA. 2012. [33] C. Jim, Q. Li, M. Elimelech, In situ monitoring techniques for concentration polarization and fouling phenomena in membrane filtration, Adv. Colloid Interface 107 (2004) 83–108. [34] J.C. Kim, F. Garzotto, D.N. Cruz, C.Y. Goh, F. Nalesso, J.H. Kim, E. Kang, H.C. Kim, C. Ronco, Enhancement of solute removal in a hollow-fiber hemodialyzer by mechanical vibration, Blood Purif. 31 (2011) 227–234. [35] Physical Properties of Glycerine and its Solutions, Glycerine Producers' Association, 1963.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Dehydration of glycerin solution using pervaporation: HybSi and polydimethylsiloxane membranes Shee-Keat Mah, Siang-Piao Chai n, Ta Yeong Wu Chemical Engineering Discipline, School of Engineering, Monash University, Jalan Lagoon Selatan, Bandar Sunway, 46150 Selangor Darul Ehsan, Malaysia
art ic l e i nf o
a b s t r a c t
Article history: Received 30 July 2013 Received in revised form 20 September 2013 Accepted 24 September 2013 Available online 29 September 2013
The dehydration of glycerin solution is an important step in glycerin purification process. In present work, pervaporative dehydration of glycerin solution was studied using HybSi and polydimethylsiloxane (PDMS) pervaporation membranes. The effects of physical property of membrane, operating parameters and glycerin concentrations on the membrane permeate flux and selectivity were examined. The present studies showed that hydrophilic HybSi membrane could provide higher permeate flux as compared to hydrophobic PDMS membrane under an identical operating condition. Remarkable difference in permeate flux was observed when the process was operating at room temperature. The influence of crossflow rate on the permeate flux was relatively low as compared to operating pressure and temperature. Operating temperature was found to possess the greatest influence on permeate flux and selectivity. The speculated molecule movement during pervaporation was discussed based on boundary layer theory and hydrogen bonding theory. The final permeate flux and permeance water content of 13.63 kg/m2 h and 98 wt%, respectively, were achieved by HybSi membrane in the dehydration process of 90 wt% glycerin solution into 99 wt% glycerin solution over a period of 90 min. & 2013 Elsevier B.V. All rights reserved.
Keywords: PDMS Ceramic Water extraction Boundary layer Hybrid membrane
1. Introduction Glycerin is one of the most versatile organic compounds that have been widely used in medical, food product, pharmaceutical, cosmetics, textile industries and preservative [1,2]. It is a colorless, odorless, non-toxic, viscous liquid with a sweet taste, and is derived from both natural and petrochemical feedstocks [3]. Besides, the production of biodiesel through transesterification of triglycerides produces glycerin as a major byproduct, accounting for up to 10–12% of the total biodiesel production capacity [4,5]. The raw glycerin byproduct can be further purified into high grade glycerin for commercial use. The traditional glycerin purification process usually involves glycerin washing, chemical and physical pretreatment, evaporation and distillation processes [4,6,7]. In general, evaporation and distillation involved in glycerin dehydration process usually demand for intensive energy consumption. In this regard, filtration process using pervaporation membrane appears to be an economical and environmental friendly alternative in replacement of these conventional glycerin dehydration processes. A variety of pervaporation separation processes has been demonstrated in the past decade, particularly in the application involving the separation of water from organic mixtures [8–13].
n
Corresponding author. Tel.: þ 60 355146234; fax: þ 60 355146207. E-mail address: [email protected] (S.-P. Chai).
0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.09.048
Several studies [14,15] have been reported on the dehydration of glycerin solution using polymeric pervaporation membranes at low operating temperature and constant pressure. To date, no study has been exploited on the dehydration of glycerin solution using both hydrophilic and hydrophobic membranes in crossflow configuration under different operating temperatures, pressures and crossflow rates. In the present work, the effects of physical property of membrane, operating crossflow rate, operating temperature, operating pressure as well as the concentration of glycerin solution, were investigated with respect to the membrane permeate flux and selectivity. The suitable membrane and experimental condition would be identified, and to be employed in the later stage for the investigation of the possibility to dehydrate glycerin solution up to 99 wt%. The experimental findings from this study are important in providing prior knowledge for the determination of suitability of employing pervaporation as an alternative to dehydrate glycerin solution. 2. Experimental 2.1. Chemical and membranes The analytical grade glycerin with a purity of 99 % was provided by Acros Organics. The model solutions containing 15, 30, 60 and 90 wt% of glycerin, respectively, were prepared using ultrapure water (0.055 mS/cm) produced from a TKA smart2pure standard
S.-K. Mah et al. / Journal of Membrane Science 450 (2014) 440–446
reached steady state. The vacuum pressure of permeate side in the pervaporation module was later adjusted to 10 mbar and kept constant thereafter. The data of weight reduction in feed tank as a function of time was collected by a data logger attached to the digital balance. After each experimental run, the total reduction of weight in the feed tank was derived from the results logged into the data recorder of digital balance. The weight of the sample collected in the sample collector was measured and compared to the total reduction of weight in the feed tank. Typically, 5% or less difference was achieved in all experimental runs. The reproducibility of all experimental runs for permeate flux was within 73%.
Chiller
Condenser Tubular pervaporation module Vacuum pump
Vacuum pressure gauge Thermometer Pressure gauge
441
2.3. Membrane performance evaluation and transport mechanism
Sample collector
Flow meter Feed Tank Data Logger
Heater
Digital balance Peristaltic pump Fig. 1. Experimental setup of pervaporation installation.
water dispenser (Thermo Electron LED, GmbH). Two tubular pervaporation membranes were selected for the separation of water/glycerin mixtures. They were hydrophilic HybSi (organic– inorganic hybrid silica membrane, contact angle 701) and hydrophobic PDMS (polymeric membrane, contact angle 1051) pervaporation membranes which were supplied by Energy research centre, Netherlands. The molecular structures of HybSi and PDMS membranes can be found in [16–18]. In brief, hydrophilic HybSi membrane is mainly used for the dehydration of water from organic mixtures while hydrophobic PDMS membrane is primarily used for the removal of organic compounds [16,19,20].
The performance of pervaporation membrane was evaluated by examining the permeate flux and permeance water content. The permeate flux was calculated using Eq. (1). The glycerin percentage of permeance was calculated using the refractive index method. The refractometer used was Lab Digital Refractometer Brix-300034 (Sper Scientific, USA) with reflective index resolution up to 0.0001RI (Accuracy, 70.0002RI). Standard calibration chart of water/glycerin mixtures were prepared from 0–80 wt% by plotting refractive index versus glycerin concentration. To measure any sample with glycerin concentration higher than 80 wt%, a known amount of ultrapure water was added to dilute the permeance. Permeate flux ¼
W A Δt
ð1Þ
where W is the weight of permeate (kg), A is the effective membrane surface area (m2) and Δt is the sampling time (h). The effects of physical property of membrane, glycerin concentrations and operating conditions such as crossflow rate (12– 36 L/h), pressure (0–2 bar) and temperature (25–160 1C) on the permeate flux and permeance water content were also studied. The transportation mechanism of water molecule was investigated based on the physicochemical properties of water, glycerin and membrane.
2.2. Pervaporation experiment 3. Results and discussion A crossflow tubular pervaporation experimental setup is shown in Fig. 1. This experimental setup consists of a closed feed tank, a peristaltic pump (masterflex, USA), a custom made high power heater, a tubular pervaporation module (Pervatech, Netherlands), a custom made condenser integrated with PolyScience chiller (PolyScience, USA) and a vacuum pump (Value, China). All the components in this pervaporation setup were wrapped by insulating material to prevent heat loss. A tubular pervaporation membrane (active area of 0.005 m2) was inserted inside the tubular pervaporation module and sealed on both ends with high temperature compatible Kalrez rubber O-rings. A 3 kg feed solution was filled into the feed tank and the peristaltic pump was used to circulate the feed to the membrane module and return back to the feed tank. The crossflow rate, feed temperature and pressure were piloted in a range of 12–36 L/h, 25–160 1C and 0 (atmospheric pressure) – 2 bar, respectively, depending on the requirement of the operating condition for each experiment. The pervaporation membrane was allowed to equilibrate with the feed solution at the designated glycerin concentration, crossflow rate, pressure and temperature for 1 h prior to the commencement of experiment. Next, the weight of the glycerin solution in the feed tank was measured using a digital balance. Once the weight reading from the digital balance attained a constant value at the designated temperature, pressure and crossflow rate, the system has now
3.1. Effects of physical property of membrane The effects of physical property of membrane on permeate flux and permeance water content were studied using 15 wt% glycerin solution as feed. The crossflow rate and pressure of the feed were set at 36 L/h and gauge pressure of 0 bar. The temperatures of the feed solution were fixed at 25, 45, 65, 85 and 98 1C to prevent boiling of the feed solution. The steady permeate fluxes of HybSi and PDMS membranes at different temperatures were summarized in Fig. 2. Besides, viscosity of 15 wt% glycerin solution in different temperatures was calculated using the model reported in [21] and stated in Fig. 2. By means of the refractive index method, it was assessed that all the permeate samples contained 100 wt% of water. The loss of glycerin in feed due to permeation was not observed. Fig. 2 clearly demonstrates that HybSi outperformed PDMS membrane in terms of permeate flux. At 25 1C, the permeate flux of HybSi membrane (1.56 kg/m2 h) was significantly higher than PDMS membrane (0.72 kg/m2 h), which could be explained by the hydrophilic nature of HybSi provided advantage of membrane surface water attraction as compared to hydrophobic PDMS [8,22]. Besides, high wettability of HybSi membrane also provided easier passage of water molecule through the membrane as compared to polymeric membranes [23].
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45 PDMS
Permeate flux (kg/m2.h)
Permeate flux (kg/m2.h)
40 35 30
viscosity =0.4873 mPa.s
25 20 15 10 5
viscosity =1.4456 mPa.s
5
viscosity =0.4151 mPa.s
HybSi
viscosity =0.9241 mPa.s
viscosity =0.6494 mPa.s
12 L/h
4.5
24 L/h
4
36 L/h
viscosity = 0.4151 mPa.s
3.5 3 2.5 2 1.5 1 0.5
viscosity = 1.4456 mPa.s
0
0 25
45
65
85
viscosity = 0.3559 mPa.s
25
98
98
Fig. 2. The steady permeate flux of HybSi and PDMS membrane at different temperatures.
112
Temperature (°C)
Temperature (°C)
Fig. 3. The steady permeate flux of HybSi membrane in different crossflow rates.
70
3.2. Effects of crossflow rate Glycerin solution with concentration of 15 wt% was used as the model solution to study the effect of crossflow rate on permeate flux and permeance water content. In this regard, crossflow rate of the pervaporation experiment was piloted at 12, 24, and 36 L/h. At the temperature of 112 1C, the operating pressure of 1 bar was applied to prevent boiling of feed steam. When operated at 25 1C and 0 bar gauge pressure, the increase in crossflow rate from 12 to 36 L/h showed insignificantly escalating influence on the permeate flux. The slight increase of permeate flux with respect to the increasing crossflow rate might be due to the reduced effect of concentration polarization of glycerin molecule on the membrane surface [26]. In order to verify this deduction, crossflow rates of 12, 24 and 36 L/h were applied at three different operating temperatures, notably 25, 98 and 112 1C. The results shown in Fig. 3 ascertain that crossflow rate possessed little effect on the permeate flux despite being carried out at different operating
0 bar
60
Permeate flux (kg/m2.h)
Swelling effect is a major drawback which is generally encountered in hydrophilic pervaporation membrane. This effect could increase the pore size of the membrane which would subsequently increase the permeate flux as well. Consequently, the issue of reduction in selectivity of the filtration process arises in conjunction with the use of hydrophilic pervaporation membrane [8,22]. The higher permeate flux and total permeance water content attained with the application of HybSi membrane could be reasoned by the physical property of the membrane. Low swelling ability, high thermal stability and high chemical resistance on feed solution possessed by the HybSi membrane contributed to its outstanding performance [24,25]. However, as the operating temperature was elevated further apart from room temperature, the permeate flux recorded for both HybSi and PDMS shifted closer to each other with apparently smaller differences. One of the reasons to this phenomenon could be due to significant reduction of glycerin viscosity at elevated temperature. Lower viscosity of glycerin solution would promote the rate of permeation of water through the membrane. This is in associate of the decrease in hydrogen bonding between glycerin molecules and water molecules. By comparing the permeate flux and permeance water content of both HybSi and PDMS membranes, HybSi membrane was selected predominantly to separate water from glycerin solution due to its capability in providing higher permeate flux under same operating condition. In view of this, only HybSi membrane would be employed in the later stage in this study to examine the effects of operating parameters on the permeate flux and permeance water content.
0.5 bar 1 bar
50
1.5 bar
Slopes of flux increment
40 30 20 10 0 25
45
65
85
98
112
120
Temperature (°C) Fig. 4. The steady permeate flux of HybSi membrane in different pressures and temperatures.
temperatures. Another noticeable observation to be deduced from Fig. 3 would be the effect of operating temperature on permeate flux which was substantially higher than that of crossflow rate. The water content of all permeance sample was found to be 100 wt%. Considering that crossflow rate had very minor influence on permeate flux and no effect on permeance water content, low crossflow rate of 12 L/h was favored as less pumping and heating energy would be required. 3.3. Effects of different temperatures and pressures 3.3.1. Pervaporation of 15 wt% glycerin solution In this section, glycerin solution with concentration of 15 wt% was used as the feed solution to study the effect of operating temperature and pressure on permeate flux and permeance water content. The operating pressure was fixed at 0, 0.5, 1 and 1.5 bar gauge pressure while the operating temperature was controlled at 25, 45, 65, 85, 98, 112 and 120 1C. As a precaution to prevent boiling of feed solution during pervaporation process, the operating pressure was set at 1 and 1.5 bar for 112 1C while 1.5 bar for 120 1C. The crossflow rate of 12 L/h was selected in this section of investigation as elucidated in Section 3.2. In Fig. 4, it is clearly shown that the permeate flux increased exponentially with increasing temperature. In addition, all permeance samples were measured to be containing 100 wt% of water. Temperature is an important parameter in membrane filtration because it affects the properties of both feed solution and membrane. According to Figs. 2 and 3, the viscosity of 15 wt% glycerin solution decreased rapidly from 1.4456 to 0.3559 mPa s
S.-K. Mah et al. / Journal of Membrane Science 450 (2014) 440–446
3.3.2. Pervaporation of 30 and 60 wt% glycerin solutions In this section, 30 and 60 wt% glycerin solutions were used as feed solutions. To avoid boiling in the 30 and 60 wt% glycerin solutions during the pervaporation process, the operating pressure was adjusted to 1 and 1.5 bar at 112 1C while 1.5 bar at 120 1C. The permeate fluxes at different temperatures and pressures are shown in Figs. 5 and 6 accompanied with the corresponding viscosities at different temperatures. All permeate samples were
70
Permeate flux (kg/m2.h)
60
0 bar 0.5 bar 1 bar 1.5 bar
Viscosity = 0.5191 mPa.s
Viscosity = 0.4754 mPa.s
50 Viscosity = 0.6154 mPa.s
40 Viscosity = 0.7364 mPa.s
30 20 10
Viscosity = 2.5748 mPa.s
Viscosity = 1.5311 mPa.s
Viscosity = 1.0203 mPa.s
0 25
45
65
85
98
112
120
Temperature (°C) Fig. 5. The steady permeate flux of 30 wt% glycerin solution in different temperatures and pressures. 70 60
Permeate flux (kg/m2.h)
when the temperature was increased from 25 to 112 1C. When the operating temperature was increased to 120 1C, the viscosity of feed solution was further reduced to 0.3283 mPa s. The rapid decrease in the viscosity of the 15 wt% glycerin solution with the increase in temperature is strongly related to hydrogen bonding [3]. With an increase in the glycerin solution temperature, the number of hydrogen bonds between water–water and water– glycerin decreased due to rapid molecules movement [27,28]. The lifetime of hydrogen bond between water–water is significantly shorter than water–glycerin due to the fact that the water molecules re-orientational motions are faster than alcohol molecules [29,30]. The faster molecules movement and shorter lifetime of hydrogen bond between water–water molecules could promote formation of hydrogen bonding between hydrophilic HybSi and water molecules [16] and thus increasing the water permeation through the HybSi membrane. At the temperatures of 25, 45 and 65 1C, an increase in the operating pressure from 0 to 1.5 bar increased the permeate flux as shown in Fig. 4. The effects of operating pressure on the increasing permeate flux could be highlighted by “slopes of increment” as indicated in Fig. 4. This could be explained by the phenomenon in which higher operating pressure would provide a greater pressure gradient across the membrane, thus forcing the water molecules to diffuse across the membrane with greater ease. As both operating temperature and pressure increased in the experiment, it was observed that the higher operating pressure contributed to the increase in permeate flux with a greater extent at higher temperature. This was evident in Fig. 4 as the “slope of increment” was found to be increased from 25 to 98 1C. However, this was only observed in conditions with temperatures at 98 1C and below. At higher temperatures, the reduction of viscosity in the 15 wt% glycerin solution resulted in lower number and shorter lifetime of hydrogen bonding between water–water molecules in the feed solution. Higher operating pressure, on the other hand, provided a greater pressure gradient which would promote the diffusion of water molecules across the membrane. The combination of effects from both increased operating temperature and pressure thus offered superior performance of the membrane in terms of permeate flux. When the experiment was carried out at 65 1C, further increase of operating pressure from 0.5 to 1.5 bar did not show any significant effect on the permeate flux. On the other hand, when the operating temperature was further increased to 85 and 98 1C, the maximum permeate fluxes of 24.24 and 46.44 kg/m2 h, respectively, were achieved at 1 bar of pressure. The advancement of operating pressure from 1 to 1.5 bar at the aforementioned temperatures did not increase the permeate flux any further. On the contrary, the permeate flux dropped to a value close to the corresponding permeate flux when operated at 0.5 bar. The preferable operating pressure was found to be at 1 bar as to achieve the highest permeate flux without compromising on the selectivity. At the temperature of 112 1C, a decrease in permeate flux from 1 to 1.5 bar was also observed in Fig. 4. In this case, the results clearly stated that at operating temperature in a range of 65–112 1C, the preferred operating pressure should not exceed 1 bar.
443
0 bar 0.5 bar 1 bar 1.5 bar
Viscosity = 1.2878 mPa.s
50
Viscosity = 1.1473 mPa.s
40 30 Viscosity = 2.0721 mPa.s
20 10
Viscosity = 12.8080 mPa.s
Viscosity = 5.9218 mPa.s
Viscosity = 1.6185 mPa.s
Viscosity = 3.2851 mPa.s
0 25
45
65
85
98
112
120
Temperature (°C) Fig. 6. The steady permeate flux of 60 wt% glycerin solution in different temperatures and pressures.
found to contain 100 wt% of water. HybSi membrane showed superior performance in terms of glycerin retention when compared to the results reported by Khairnar et al. [15] and Burshe et al. [14]. Similar trend of permeate flux with respect to operating temperature and pressure was observed in the 30 and 60 wt% glycerin solutions. Analogous to 15 wt% glycerin solution used in Section 3.3.1, higher operating temperature contributed to higher permeate flux as shown in Figs. 5 and 6. The factor that allows higher permeate flux at different concentrations of glycerin solution is strongly correlated to its viscosity. Besides that, operating temperature as high as 112 and 120 1C would promote the water molecule movement in the glycerin solution as a consequence of the reduction in the number of hydrogen bonds and thus greatly improved the permeate flux [27,28]. When comparing the permeate flux at the operating temperatures of 112 and 120 1C in Figs. 4 and 5, the values were close despite the fact that the concentration of glycerin was increased from 15 wt% to 30 wt%. However, when the glycerin concentration was increased to 60 wt%, the permeate flux at different temperatures and pressures was significantly low. The reason could be due to the considerably higher viscosity of 60 wt% glycerin solution than that of 15 and 30 wt% glycerin solutions. By carefully examining Figs. 4, 5 and 6, operating pressure at 1 bar at different temperatures was found to be favorable by providing better permeate flux without any trade-off of glycerin rejection. The ultimate finding in this section concluded that increasing the operating temperature could be an option to increase the processing rate of the glycerin filtration process using pervaporation
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membrane. According to Burshe et al. and Khairnar et al., the highest permeate fluxes obtained from pervaporation of 60 wt% glycerin solution at 70 1C were 1.21 and 0.85 kg/m2 h, respectively [14,15]. In the present work, the use of HybSi pervaporation membrane at atmospheric pressure and 65 1C was capable of achieving permeate flux as high as 1.80 kg/m2 h with total glycerin retention outperformed the pervaporation membranes as reported in [14,15].
3.3.3. Effects of operating temperature and pressure on water transport mechanism During the separation of glycerin and water mixtures, water molecules would be attracted to the active surface of hydrophilic HybSi membrane, followed by diffusion through the membrane and subsequently desorbed on the posterior side of the membrane. The water molecules permeated to the posterior side of the membrane would be evaporated under vacuum condition to form permeance [31]. The decrease in permeate flux when the operating pressure was increased from 1 to 1.5 bar at the operating temperatures above 65 1C could be explained by both fluid mechanics theory and hydrogen bonding theory [21,28,29,32,33]. The aforementioned phenomenon was observed and depicted in Sections 3.3.1 and 3.3.2. It was speculated that a series of event had contributed to this phenomenon, as described in the following: (i) operating temperature above 65 1C provided greater ease of water movement as
well as evacuation due to the negative correlation between operating temperature and viscosity of glycerin solution; (ii) the diffusion of water molecules which were previously attached to the periphery of glycerin molecules into the membrane would immediately produce a layer of polarized and concentrated glycerin molecules; and (iii) the crossflow mechanism was unable to provide sufficient shear force to “sweep” away the glycerin molecules that were superficially condensed as a thin layer on the membrane surface due to operating pressure as high as 1.5 bar which was pushing all the molecules against the membrane; as a result, (iv) the condensed layer of glycerin molecules that was being “pushed” towards the membrane active surface would cause blockage to a certain extent and substantially reduce the availability of membrane active surface area. The formation of this thin glycerin gel layer within the glycerin polarization layer was responsible for the reduction in the permeability of water in the glycerin dehydration process using pervaporation membrane [34]. Based on the fluid mechanics theory on boundary layer, an illustrated scenario of the distribution of glycerin molecules inside the tubular pervaporation membrane is shown in Fig. 7.
3.4. Dehydration of 90 wt% of glycerin solution With respect to the findings obtained in Sections 3.2 and 3.3, the preferable crossflow rate and operating pressure to achieve better pervaporation efficiency would be 12 L/h and 1 bar, respectively. In this section, the pervaporation membrane was utilized
Zero velocity layers
Pervaporation membrane
Glycerin gel layer
Concentration polarization layer
Pervaporation membrane
Water diffusion
Water diffusion
Shear stress
Flow velocity
Glycerin molecule Fig. 7. An illustration of tubular pervaporation process with concentration polarization and glycerin gel layers.
S.-K. Mah et al. / Journal of Membrane Science 450 (2014) 440–446
250
60
Table 1 The dehydration results of 90 wt% glycerin solution to 99 wt%.
Viscosity
50
200
40 150 30 100 20
Viscosity (mPa.s)
Run Glycerin number concentration (wt%)
Experimental time (min)
Permeance water content (wt%)
Final permeate flux (kg/m2 h)
1 2 3
89 89 90
98.82 98.73 98.64
13.69 13.63 13.58
99.01 99.02 99.01
50
10
25
45
65
85
98
112
120
130
140
150
160
0
Temperature (°C) Fig. 8. The steady permeate flux and viscosity of 90 wt% glycerin solution at different operating temperatures.
along with 90 wt% glycerin solution at the aforementioned operating conditions. Seeing that the boiling point of 90 wt% glycerin solution at atmospheric pressure is 138 1C [35], the permissible operating temperature would now range from 25 to 160 1C as the maximum operating pressure was increased to 2 bar. This is in view of the potential of maximizing the dehydration efficiency of the pervaporation process by manipulating the operating temperature and pressure. As to prevent boiling of feed solution, the operating temperatures were only allowed to increase to 150 and 160 1C when the operating pressures were fixed at 1.5 and 2 bar, respectively. The steady permeate fluxes of the pervaporation process using 90 wt% glycerin solutions evaluated at different operating temperatures are shown in Fig. 8, accompanied with its respective viscosities. At 25 1C, the viscosity of 90 wt% glycerin solution was 208.71 mPa s. The viscosity reading decreased dramatically to 59.40 mPa s when the operating temperature was at 45 1C. However, the permeate flux was rather insensitive to the reduction in viscosity of the glycerin solution, particularly in the range of 45– 120 1C. On the other hand, when the operating temperature was increased from 120 to 130 1C where the viscosities remained almost unchanged, a sharp increase in permeate flux was observed. The viscosities of glycerin solution were measured to be reduced from 4.27 to 3.49 mPa s. This indicated that the permeate flux was basically insensitive to the operating temperature up to 120 1C. Significant effect of operating temperature on permeate flux could only be discovered when the experiment was conducted at 120 1C. In the range of 130–140 1C, the permeate flux was found to have a gradual increasing trend. It is important to take note that all the experimental runs conducted at 140 1C and below were subjected to operating pressure at 1 bar. The result suggested that when the temperature was at 130 1C, the water molecules in the feed solution would obtain sufficiently large energy to move relatively freely in the 90 wt% glycerin solution. This would subsequently increase the rate of diffusion of water molecules through the pervaporation membrane. The quality of permeance from all experimental runs described in Fig. 8 was carefully examined using the refractive index method. The results showed that the permeance obtained from 90 wt% glycerin pervaporation at operating temperature ranged from 25 to 150 1C was found to contain 100 wt% of water. However, the permeance collected at the temperature of 160 1C contained 99.91 wt% of water. This shows that glycerin diffused through the membrane at this particular temperature and pressure. As the operating temperature was increased to 150 and 160 1C, the recorded permeate fluxes were 20.04 and 57.60 kg/m2 h, respectively. The result also suggests that the operating temperature of 160 1C could be considered as the most efficient operating
Weight of gycerin soution in feed tank (g)
Permeate flux (kg/m2.h)
Permeate flux
0
445
1850 Concentration of glycerin solution in feed tank with respect to experimental time
90 wt%
1800 1750 1700 1650 1600 1550
99 wt%
1500 1450 1400 1350 0
10
20
30
40
50
60
70
80
90
100
110
120
Time (minute) Fig. 9. The dehydration process of 90 wt% glycerin solution.
temperature for the dehydration of glycerin solution. This is in associated with its ability to obtain permeate flux of 188% higher at the stipulated conditions as compared to the permeate flux achieved using operating temperature of 150 1C. 3.5. Dehydration of glycerin solution to 99 wt% In this section, the pervaporative dehydration process was used to concentrate 90 wt% glycerin solution to 99 wt%. According to the findings described in Sections 3.2, 3.3 and 3.4, the operating condition of dehydration process was controlled at 12 L/h, 2 bar and 160 1C. The entire system was filled with 3 kg of 90 wt% glycerin solution. Upon reaching steady state, the weight recorded for the glycerin solution contained in the feed tank was 1.8 kg, while the remaining could be found in pipelines and other auxiliary equipment. Three separated experimental runs were conducted as shown in Table 1. Based on the experimental data, 3 kg of 90 wt% glycerin solution required around 90 min of continuous dehydration process to concentrate the feed solution up to 99 wt% with the permeance water content well above 98 wt%. The trend of weight reduction in the feed solution with respect to time was demonstrated in Fig. 9. A gradual weight reduction of the feed solution was observed when the concentration of the glycerin in the feed solution progressively increased from 90 to 99 wt% as a result of the permeation of water through the membrane. The weight reduction in the feed solution would eventually level out as the process continued beyond 90 min.
4. Conclusions The performance of hydrophilic HybSi and hydrophobic PDMS membranes on pervaporation of glycerin solution was investigated. The hydrophilic HybSi membrane was found to be capable of providing higher water flux as compared to the hydrophobic PDMS membrane when subjected to identical operating conditions. The water permeate flux was mostly insensitive to crossflow rate at different operating temperatures. The viscosity of feed solution was found to be influencing the permeate flux extensively. In general,
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increasing the operating pressure would gradually increase the permeate flux at different operating temperatures. However, when the operating pressure was increased to 1.5 bar coupled with operating temperature higher than 65 1C, a decrease in permeate flux was observed. The collective understanding of fundamental knowledge on glycerin and membrane physicochemical properties as well as boundary layer theory is crucial in providing a comprehensive insight on the molecule movement during the pervaporation process. During the dehydration of 90 wt% glycerin solution, permeate flux as high as 57.60 kg/m2 was achieved at 160 1C and 2 bar with 99.91 wt% of water content in the permeance. In the process of concentrating 90 wt% glycerin solution to 99 wt%, a 3 kg of feed solution required approximately 90 min to dehydrate the glycerin solution and eventually produced final permeate flux and permeance water content of around 13.63 kg/m2 h and 98 wt%, respectively. Lastly, the HybSi pervaporation membrane was proven to be capable of dehydrating glycerin solution up to 99 wt% at 160 1C and 2 bar without much compromising on the retention of glycerin.
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