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Dehydration of diethylene glycol using a vacuum membrane distillation process
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Journal of the Taiwan Institute of Chemical Engineers 0 0 0 (2017) 1–5
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Dehydration of diethylene glycol using a vacuum membrane distillation process Tai-Hsiang Chen, Yao-Hui Huang∗ Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 19 August 2016 Revised 17 February 2017 Accepted 25 February 2017 Available online xxx Keywords: Membrane Vacuum membrane distillation VMD Diethylene glycol
a b s t r a c t The use of a porous PTFE membrane (pore size = 0.2 μm) in the dehydration of diethylene glycol (DEG) by a vacuum membrane distillation (VMD) method was studied. Experiments on VMD for the treatment of 85 wt. % aqueous DEG were carried out at temperatures from 70 to 90 °C, with stirring from 600 to 1700 rpm and vacuum degrees from 650 to 715 torr. The water contents in the feed and the permeate solution were detected by Karl Fisher titration. VMD for 100 min reduced the water content in the feed solution from 15 wt. % to 2 wt. % at 90 °C, 1500 rpm and 715 torr. The separation factor increased from 2500 to 9500 and the total flux declined from 18.5 to 1 kg/m2 h. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction In recent years, rapid growth in solar energy and semiconductor industries has increased the need for wafer slicing. The wire saw is the most frequently used tool for slicing wafers. When it is used, cutting oil is added for cooling or lubrication. In Taiwan, diethylene glycol (DEG) is typically used as the cutting oil. DEG with the formula (HOCH2 CH2 )2 O is a co-product with ethylene glycol and triethylene glycol. It is a colorless, sticky, and practically odorless liquid and miscible in water, alcohol, and ether. Therefore DEG can easily be washed away by water and make abrasive like SiC particles suspend in it. When a wafer is sliced, tons of waste cutting oil which consists of oils, water, SiC and Si particles is generated. The SiC and Si particles in waste cutting oil are generally separated into filter cake, and the DEG is mixed with water and other infinitesimal traces of impurities in the filtrates. Owing to the high water content, the filtrate cannot be immediately recycled as cutting oil. Distillation or vacuum distillation is the traditional way to dehydrate an organic solution. However, the cost-effectiveness of distilling DEG is rather low because the boiling point of DEG is high (245.3 °C). Membrane distillation (MD) is a good alternative to other presently used distillation processes because it uses less energy. MD involves membrane and distillation systems. Findley developed MD in 1967 [1,2]. Until 1983 MD was used in desalination [3]. More recently, MD has been used in several fields, such as
∗
Corresponding author. E-mail address: [email protected] (Y.-H. Huang).
precious metal recovery [4], bioethanol separation [5,6], waste water treatment [7,8] and the food industry [9,10]. MD is a low-cost, energy-saving alternative to conventional separation processes such as distillation and reverse osmosis (RO) [11]. The four types of MD systems are (1) direct contact membrane distillation (DCMD) [4,12–14], (2) air gap membrane distillation (AGMD) [13,15], (3) sweeping gas membrane distillation (SGMD) [5,7,16], and (4) [1,6,17–20] vacuum membrane distillation (VMD) systems. They have similar upstream sides of the membrane but different means of condensing vapor(s). The various advantages of MD include lower operating temperatures, lower operating pressure, fewer requirements of the mechanical properties of the membrane, weaker chemical interaction between the membrane and processing solutions, and smaller vapor spaces than those of conventional distillation processes, and 100% rejection of ions, macro-molecules, colloids, cells, and other non-volatiles [11]. In the MD process, a porous membrane is used. The upstream side of the membrane comprises hydrophobic materials, such as polytetrafluoroethylene (PTFE) [6], poly(vinylidenefluoride) (PVDF) [21], and polypropylene (PP) [20]. The feed solution comes into direct-contact with the hydrophobic upstream side of the membrane. The solution forms a vapor–liquid (V/L) interface over the pores because the material is hydrophobic. The volatile components of the feed solution evaporate at the V/L interface. These components then diffuse through the pores to the downstream side of membrane and are condensed using a cold stream (DCMD), a condensing plate (AGMD), or a condenser (SGMD/VMD). The driving force is the pressure drop that arises from either the vacuum degree on the downstream side or the hot/cold vapor pressure. [9,18,22,23]
http://dx.doi.org/10.1016/j.jtice.2017.02.028 1876-1070/© 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: T.-H. Chen, Y.-H. Huang, Dehydration of diethylene glycol using a vacuum membrane distillation process, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.02.028
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Fig. 1. The schematic VMD apparatus.
In this work, VMD is used to dehydrate aqueous DEG. The goal is to prove that MD technique reduces the water content of the DEG solution to less than 5 wt. % under optimal conditions for its dehydration. 2. Materials and methods 2.1. Material The porous PTFE membranes that were used in this work were purchased from King Membrane Energy Technology Inc. The mean pore size of each membrane was 0.2 μm. DEG was purchased from Alfa Aesar and its purity was 99%. 2.2. VMD experiments Fig. 1 schematically depicts the MD instruments. The membrane was cut into a circle with a diameter of 6.5 cm and put in a membrane cell. The effective area of the membrane was 19.6 cm2 (5 cm in diameter). The feed solution was 85 wt. % aqueous DEG (75 g). The permeate was trapped in a tube through an ice cooling system. Samples of the feed and permeate solutions were extracted at intervals. The feed samples were took every 10 min. and the permeate samples were took every 30 min. The water content in the feed solution samples was measured using a Karl Fischer titrator (787 KF Titrino, Metrohm). A total organic carbon analyzer (Sievers InnovOx Laboratory TOC Analyzer) was used to measure the concentrations of DEG in the permeate samples because those samples were dilute. The experimental variables, including temperature, stirring rate and vacuum degree, were controlled to observe their effects on the efficacy of VMD dehydration. To determine the performance of VMD system, the total flux (F, kg/m2 h) and separation factor (α ), defined by Eqs. (1) and (2), were utilized:
F = W/At
(1)
α = [Ywater /(1 − Ywater )]/[Xwater /(1 − Xwater )]
(2)
where W (kg) is the total amount of the permeate that was collected at experimental time t (h); A is the effective area of the membrane, and Xwater and Ywater are the weight fractions of the water in the feed and permeate solutions. Referring to pervaporation separation index (PSI) used to describe the overall performance in pervaporation, the efficiency of VMD dehydration (EVMD , kg/m2 h) can be described as Eq. (3).
EV MD = F × (α − 1 )
(3)
Fig. 2. Effect of temperature on water content in feed solution (stirring rate = 600 rpm, vacuum degree = 715 torr).
3. Results and discussion 3.1. Effect of temperature The stirring rate and the vacuum degree were 600 rpm and 715 torr, respectively, and the temperature was controlled from 70 °C to 90 °C. Figs. 2 and 3 present the obtained results. Initially, Xwater decreases rapidly to a level at 50 min that decreases as the temperature increases. Obviously, the VMD process has limited in the dehydration of DEG. This phenomenon resulted from the concentration polarization. The solution diffuses through the boundary close to the surface of the membrane by two mechanisms, advection and molecular diffusion [5]. Water more easily diffuses through the membrane than does DEG because it has a lower boiling point. Therefore, the water content on the membrane surface is lower than that in the feed solution (Xwater ). Xwater declines as water is extracted from the feed solution, and the water content on the membrane surface falls. When the water cannot diffuse to the surface of the membrane, Xwater becomes constant. Therefore, the boundary layer limits the final Xwater . Meanwhile, increasing the temperature can reduce the final Xwater . A higher temperature corresponds to faster molecular diffusion in the boundary layer. A rising temperature increases the pressure drop. According to the Clausius–Clapeyron relation, the vapor pressure of water increases
Please cite this article as: T.-H. Chen, Y.-H. Huang, Dehydration of diethylene glycol using a vacuum membrane distillation process, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.02.028
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3
Fig. 3. Effect of temperature on total flux and separation factor (stirring rate = 600 rpm, vacuum degree = 715 torr).
Table 1 Flux at different temperature and Xwater calculated from Eqs. (3), (4) and (6). Temperature ( °C)
70 75 90
Table 2 −Ea /R at different Xwater calculated by Arrhenius equation.
F (kg/m2 h) 15 wt. %
14 wt. %
13 wt. %
12 wt. %
11 wt. %
8.75 10.74 17.32
6.90 9.20 15.53
5.06 7.67 13.74
3.22 6.13 11.95
1.37 4.59 10.16
Xwater (wt. %)
15
14
13
12
11
Ea /R Adj. R2 Standard error
4202 0.996 181.2
4906 0.979 507.9
5933 0.941 1034
7613 0.869 2019
11,200 0.713 4577
with the temperature. Therefore, the final Xwater reduces from 10 wt. % to 5 wt. % as the temperature rises from 70 °C to 90 °C. Fig. 3 indicates that F increases with temperature. Moreover, α increases with temperature from 70 °C to 75 °C but decreases as the temperature increases further, perhaps because of evaporation of DEG. As the temperature increases, not only the water but also DEG evaporates more easily. However, the concentration of DEG in the permeate rises only from 0.21 wt. % to 0.33 wt. %. For EVMD , the highest value is 29,400 kg/m2 h at 80 °C A very clear linear relationship exists between F and Xwater . The linear relationships between F and Xwater at different temperatures obtained from the data in Fig. 3 are as follows,
F = 1.844 Xwater − 18.912 at 70◦ C
(4)
F = 1.538 Xwater − 12.329 at 75◦ C
(5)
F = 1.129 Xwater − 5.323 at 80◦ C
(6)
F = 1.790 Xwater − 9.531 at 90◦ C
(7)
All adjusted R-squares of the above equations, except Eq. (6), exceed 0.99. Xwater at different temperatures when the flux was zero was calculated by Eqs. (4), (5) and (7). Those results corresponded with the final Xwater were shown in Fig. 2. Furthermore, the flux can also be calculated by Eqs. (4), (5) and (7) when Xwater from 11 wt. % to 15 wt. % as well as list the results in Table 1. The relation between the flux and temperature can be described by Arrhenius equation (ln F = −Ea /RT + ln A). The calculation results list in Table 2. The increasing Ea accompanied with decreasing Xwater indicates the more influence of temperature as Xwater decreasing.
Fig. 4. Effect of stirring rate on water content in feed solution (temperature = 70 °C, vacuum degree = 715 torr).
3.2. Effect of stirring rate The effect of stirring on the performance of VMD was studied by varying the rate of stirring from 0 rpm (no stirring) to 1700 rpm at 70 °C and 715 torr. Figs. 4 and 5 present the results thus obtained. As mentioned above, advection and molecular diffusion are the two mechanisms by which water is transported through the boundary layer. Without stirring, advection does not occur, and the water is transferred across the boundary only by molecular diffusion. Without stirring, Xwater falls negligibly. The final Xwater reduces from 14.35% to 6.92% as the stirring rate increased from
Please cite this article as: T.-H. Chen, Y.-H. Huang, Dehydration of diethylene glycol using a vacuum membrane distillation process, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.02.028
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Fig. 5. Effect of stirring rate on total flux and separation factor (temperature = 70 °C, vacuum degree = 715 torr).
60 0 rpm to 150 0 rpm. However, when the stirring rate reaches 1700 rpm, the final Xwater is 7.78%, probably because the unstable vacuum degree which is caused by the rapid stirring rate. The rapid stirring rate makes most of the feed solution thrown on the wall of the tank. The membrane cannot be completely covered by the remainder of the feed solution. Consequently, both F and α increase with the stirring rate up to 1500 rpm, beyond which they decrease. The highest EVMD is about 66,0 0 0 kg/m2 h when the stirring rate was 1500 rpm. The final Xwater is about 7 wt. % under the optimal conditions of 1500 rpm, 70 °C and 715 torr. 3.3. Effect of vacuum degree
Fig. 6. Effect of vacuum degree on water (temperature = 90 °C, stirring rate = 600 rpm).
content
in
feed
solution
According to the results of the first two variables, temperature and stirring rate were set respectively at 90 °C and 1500 rpm. The vacuum degree was adjusted from 715 torr to 650 torr. Figs. 6 and 7 show the results thus obtained. The final Xwater increased with the vacuum degree from 650 torr to 715 torr. The vacuum drove the flow of water to the permeate solution. F decreased as the vacuum degree decreased. The value of α varied insignificantly as the vacuum degree was reduced from 715 torr to 700 torr, but declined
Fig. 7. Effect of vacuum degree on total flux and separation factor (temperature = 90 °C and stirring rate = 600 rpm).
Please cite this article as: T.-H. Chen, Y.-H. Huang, Dehydration of diethylene glycol using a vacuum membrane distillation process, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.02.028
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as it was reduced further to 650 torr. The highest EVMD is about 46,800 kg/m2 h when the vacuum degree was 715 torr. The lowest final Xwater was 2.2 wt. %. With stirring, all Ywater exceeded 99 wt. %. EVMD at 90 °C, 1500 rpm and 715 torr (46,800 kg/m2 h) was lower than that at 70 °C, 1500 rpm and 715 torr (66,0 0 0 kg/m2 h). However, the objective of this study was to reduce the water content of the DEG solution to less than 5 wt. %. The optimal conditions of this study were temperature = 90 °C, stirring rate = 1500 rpm, and vacuum degree = 715 torr. 4. Conclusion The boundary layer close to the surface of the membrane is the main mass transfer resistance of VMD process. Increasing the operating temperature and the stirring rate is a very efficient means of overcoming the mass transfer resistance of the boundary layer. The dehydration of DEG in this study was optimized at 90 °C, 1500 rpm, and 715 torr. Under these conditions, Xwater was reduced from 15 wt. % to 2.26 wt. %. At the same time F, α and EVMD between 0 to 10 min were 18.5 kg/m2 h, 2500 and 46,800 kg/m2 h. References [1] Abu-Zeid MAE-R, Zhang Y, Dong H, Zhang L, Chen H-L, Hou L. A comprehensive review of vacuum membrane distillation technique. Desalination 2015;356:1–14. [2] Findley ME. Vaporization through porous membranes. Ind Eng Chem Process Des Dev 1967;6:226–30. [3] Carlsson L. The new generation in sea-water deslination SU membrane distillation system. Desalination 1983;45:221–2. [4] Teng-Chien C, Gaw-Hao H, Chuh-Shun C, Yao-Hui H. Reducing industrial wastewater and recovery of gold by direct contact membrane distillation with electrolytic system. Sustainable Environ Res 2013;23:209–14. [5] Shirazi MMA, Kargari A, Tabatabaei M. Sweeping gas membrane distillation (SGMD) as an alternative for integration of bioethanol processing: study on a commercial membrane and operating parameters. Chem Eng Commun 2015;202:457–66. [6] Shi J-Y, Zhao Z-P, Zhu C-Y. Studies on simulation and experiments of ethanol-water mixture separation by VMD using a PTFE flat membrane module. Sep Purif Technol 2014;123:53–63.
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[7] Shirazi MMA, Kargari A, Tabatabaei M, Ismail AF, Matsuura T. Concentration of glycerol from dilute glycerol wastewater using sweeping gas membrane distillation. Chem Eng Process 2014;78:58–66. [8] Madhumala M, Madhavi D, Sankarshana T, Sridhar S. Recovery of hydrochloric acid and glycerol from aqueous solutions in chloralkali and chemical process industries by membrane distillation technique. J Taiwan Inst Chem Eng 2014;45:1249–59. [9] Gryta M, Tomaszewska M, Grzechulska J, Morawski AW. Membrane distillation of NaCl solution containing natural organic matter. J Membr Sci 2001;181:279–87. [10] Kotsanopoulos KV, Arvanitoyannis IS. Membrane processing technology in the food industry: food processing, wastewater treatment, and effects on physical, microbiological, organoleptic, and nutritional properties of foods. Crit Rev Food Sci Nutr 2015;55:1147–75. [11] Lawson KW, Lloyd DR. Membrane distillation. J Membr Sci 1997;124:1–25. [12] Liming S, Zidu M, Xiaohong L, Kosaraju PB, Irish JR, Sirkar KK. Pilot plant studies of novel membranes and devices for direct contact membrane distillation-based desalination. J Membr Sci 2008;323:257–70. [13] Summers EK, Lienhard JH. Experimental study of thermal performance in air gap membrane distillation systems, including the direct solar heating of membranes. Desalination 2013;330:100–11. [14] Lee J-G, Kim Y-D, Kim W-S, Francis L, Amy G, Ghaffour N. Performance modeling of direct contact membrane distillation (DCMD) seawater desalination process using a commercial composite membrane. J Membr Sci 2015;478:85–95. [15] Bahar R, Hawlader MNA, Ariff TF. Channeled coolant plate: a new method to enhance freshwater production from an air gap membrane distillation (AGMD) desalination unit. Desalination 2015;359:71–81. [16] Zhao S, Feron PHM, Xie Z, Zhang J, Hoang M. Condensation studies in membrane evaporation and sweeping gas membrane distillation. J Membr Sci 2014;462:9–16. [17] Bandini S, Saavedra A, Sarti GC. Vacuum membrane distillation: experiments and modeling. AICHE J 1997;43:398–408. [18] Chel-Ken C, Sarbatly R. Vacuum membrane distillation processes for aqueous solution treatment: a review. Chem Eng Process: Process Intensif 2013;74:27–54. [19] Zhou T, Yao Y, Xiang R, Wu Y. Formation and characterization of polytetrafluoroethylene nanofiber membranes for vacuum membrane distillation. J Membr Sci 2014;453:402–8. [20] Tang N, Peng Y, Jia Z, Zhang L, Xiang J, Yuan L, et al. Vacuum membrane distillation simulation of desalination using polypropylene hydrophobic microporous membrane. J Appl Polym Sci 2015;132(11). [21] Yang X, Wang R, Shi L, Fane AG, Debowski M. Performance improvement of PVDF hollow fiber-based membrane distillation process. J Membr Sci 2011;369:437–47. [22] Allan FM, Qatanani N, Barghouthi I, Takatka KM. Dusty gas model of flow through naturally occurring porous media. Appl Math Comput 2004;148:809–21. [23] El-Bourawi MS, Ding Z, Ma R, Khayet M. A framework for better understanding membrane distillation separation process. J Membr Sci 2006;285:4–29.
Please cite this article as: T.-H. Chen, Y.-H. Huang, Dehydration of diethylene glycol using a vacuum membrane distillation process, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.02.028
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Journal of the Taiwan Institute of Chemical Engineers 0 0 0 (2017) 1–5
Contents lists available at ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Dehydration of diethylene glycol using a vacuum membrane distillation process Tai-Hsiang Chen, Yao-Hui Huang∗ Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 19 August 2016 Revised 17 February 2017 Accepted 25 February 2017 Available online xxx Keywords: Membrane Vacuum membrane distillation VMD Diethylene glycol
a b s t r a c t The use of a porous PTFE membrane (pore size = 0.2 μm) in the dehydration of diethylene glycol (DEG) by a vacuum membrane distillation (VMD) method was studied. Experiments on VMD for the treatment of 85 wt. % aqueous DEG were carried out at temperatures from 70 to 90 °C, with stirring from 600 to 1700 rpm and vacuum degrees from 650 to 715 torr. The water contents in the feed and the permeate solution were detected by Karl Fisher titration. VMD for 100 min reduced the water content in the feed solution from 15 wt. % to 2 wt. % at 90 °C, 1500 rpm and 715 torr. The separation factor increased from 2500 to 9500 and the total flux declined from 18.5 to 1 kg/m2 h. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction In recent years, rapid growth in solar energy and semiconductor industries has increased the need for wafer slicing. The wire saw is the most frequently used tool for slicing wafers. When it is used, cutting oil is added for cooling or lubrication. In Taiwan, diethylene glycol (DEG) is typically used as the cutting oil. DEG with the formula (HOCH2 CH2 )2 O is a co-product with ethylene glycol and triethylene glycol. It is a colorless, sticky, and practically odorless liquid and miscible in water, alcohol, and ether. Therefore DEG can easily be washed away by water and make abrasive like SiC particles suspend in it. When a wafer is sliced, tons of waste cutting oil which consists of oils, water, SiC and Si particles is generated. The SiC and Si particles in waste cutting oil are generally separated into filter cake, and the DEG is mixed with water and other infinitesimal traces of impurities in the filtrates. Owing to the high water content, the filtrate cannot be immediately recycled as cutting oil. Distillation or vacuum distillation is the traditional way to dehydrate an organic solution. However, the cost-effectiveness of distilling DEG is rather low because the boiling point of DEG is high (245.3 °C). Membrane distillation (MD) is a good alternative to other presently used distillation processes because it uses less energy. MD involves membrane and distillation systems. Findley developed MD in 1967 [1,2]. Until 1983 MD was used in desalination [3]. More recently, MD has been used in several fields, such as
∗
Corresponding author. E-mail address: [email protected] (Y.-H. Huang).
precious metal recovery [4], bioethanol separation [5,6], waste water treatment [7,8] and the food industry [9,10]. MD is a low-cost, energy-saving alternative to conventional separation processes such as distillation and reverse osmosis (RO) [11]. The four types of MD systems are (1) direct contact membrane distillation (DCMD) [4,12–14], (2) air gap membrane distillation (AGMD) [13,15], (3) sweeping gas membrane distillation (SGMD) [5,7,16], and (4) [1,6,17–20] vacuum membrane distillation (VMD) systems. They have similar upstream sides of the membrane but different means of condensing vapor(s). The various advantages of MD include lower operating temperatures, lower operating pressure, fewer requirements of the mechanical properties of the membrane, weaker chemical interaction between the membrane and processing solutions, and smaller vapor spaces than those of conventional distillation processes, and 100% rejection of ions, macro-molecules, colloids, cells, and other non-volatiles [11]. In the MD process, a porous membrane is used. The upstream side of the membrane comprises hydrophobic materials, such as polytetrafluoroethylene (PTFE) [6], poly(vinylidenefluoride) (PVDF) [21], and polypropylene (PP) [20]. The feed solution comes into direct-contact with the hydrophobic upstream side of the membrane. The solution forms a vapor–liquid (V/L) interface over the pores because the material is hydrophobic. The volatile components of the feed solution evaporate at the V/L interface. These components then diffuse through the pores to the downstream side of membrane and are condensed using a cold stream (DCMD), a condensing plate (AGMD), or a condenser (SGMD/VMD). The driving force is the pressure drop that arises from either the vacuum degree on the downstream side or the hot/cold vapor pressure. [9,18,22,23]
http://dx.doi.org/10.1016/j.jtice.2017.02.028 1876-1070/© 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: T.-H. Chen, Y.-H. Huang, Dehydration of diethylene glycol using a vacuum membrane distillation process, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.02.028
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Fig. 1. The schematic VMD apparatus.
In this work, VMD is used to dehydrate aqueous DEG. The goal is to prove that MD technique reduces the water content of the DEG solution to less than 5 wt. % under optimal conditions for its dehydration. 2. Materials and methods 2.1. Material The porous PTFE membranes that were used in this work were purchased from King Membrane Energy Technology Inc. The mean pore size of each membrane was 0.2 μm. DEG was purchased from Alfa Aesar and its purity was 99%. 2.2. VMD experiments Fig. 1 schematically depicts the MD instruments. The membrane was cut into a circle with a diameter of 6.5 cm and put in a membrane cell. The effective area of the membrane was 19.6 cm2 (5 cm in diameter). The feed solution was 85 wt. % aqueous DEG (75 g). The permeate was trapped in a tube through an ice cooling system. Samples of the feed and permeate solutions were extracted at intervals. The feed samples were took every 10 min. and the permeate samples were took every 30 min. The water content in the feed solution samples was measured using a Karl Fischer titrator (787 KF Titrino, Metrohm). A total organic carbon analyzer (Sievers InnovOx Laboratory TOC Analyzer) was used to measure the concentrations of DEG in the permeate samples because those samples were dilute. The experimental variables, including temperature, stirring rate and vacuum degree, were controlled to observe their effects on the efficacy of VMD dehydration. To determine the performance of VMD system, the total flux (F, kg/m2 h) and separation factor (α ), defined by Eqs. (1) and (2), were utilized:
F = W/At
(1)
α = [Ywater /(1 − Ywater )]/[Xwater /(1 − Xwater )]
(2)
where W (kg) is the total amount of the permeate that was collected at experimental time t (h); A is the effective area of the membrane, and Xwater and Ywater are the weight fractions of the water in the feed and permeate solutions. Referring to pervaporation separation index (PSI) used to describe the overall performance in pervaporation, the efficiency of VMD dehydration (EVMD , kg/m2 h) can be described as Eq. (3).
EV MD = F × (α − 1 )
(3)
Fig. 2. Effect of temperature on water content in feed solution (stirring rate = 600 rpm, vacuum degree = 715 torr).
3. Results and discussion 3.1. Effect of temperature The stirring rate and the vacuum degree were 600 rpm and 715 torr, respectively, and the temperature was controlled from 70 °C to 90 °C. Figs. 2 and 3 present the obtained results. Initially, Xwater decreases rapidly to a level at 50 min that decreases as the temperature increases. Obviously, the VMD process has limited in the dehydration of DEG. This phenomenon resulted from the concentration polarization. The solution diffuses through the boundary close to the surface of the membrane by two mechanisms, advection and molecular diffusion [5]. Water more easily diffuses through the membrane than does DEG because it has a lower boiling point. Therefore, the water content on the membrane surface is lower than that in the feed solution (Xwater ). Xwater declines as water is extracted from the feed solution, and the water content on the membrane surface falls. When the water cannot diffuse to the surface of the membrane, Xwater becomes constant. Therefore, the boundary layer limits the final Xwater . Meanwhile, increasing the temperature can reduce the final Xwater . A higher temperature corresponds to faster molecular diffusion in the boundary layer. A rising temperature increases the pressure drop. According to the Clausius–Clapeyron relation, the vapor pressure of water increases
Please cite this article as: T.-H. Chen, Y.-H. Huang, Dehydration of diethylene glycol using a vacuum membrane distillation process, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.02.028
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3
Fig. 3. Effect of temperature on total flux and separation factor (stirring rate = 600 rpm, vacuum degree = 715 torr).
Table 1 Flux at different temperature and Xwater calculated from Eqs. (3), (4) and (6). Temperature ( °C)
70 75 90
Table 2 −Ea /R at different Xwater calculated by Arrhenius equation.
F (kg/m2 h) 15 wt. %
14 wt. %
13 wt. %
12 wt. %
11 wt. %
8.75 10.74 17.32
6.90 9.20 15.53
5.06 7.67 13.74
3.22 6.13 11.95
1.37 4.59 10.16
Xwater (wt. %)
15
14
13
12
11
Ea /R Adj. R2 Standard error
4202 0.996 181.2
4906 0.979 507.9
5933 0.941 1034
7613 0.869 2019
11,200 0.713 4577
with the temperature. Therefore, the final Xwater reduces from 10 wt. % to 5 wt. % as the temperature rises from 70 °C to 90 °C. Fig. 3 indicates that F increases with temperature. Moreover, α increases with temperature from 70 °C to 75 °C but decreases as the temperature increases further, perhaps because of evaporation of DEG. As the temperature increases, not only the water but also DEG evaporates more easily. However, the concentration of DEG in the permeate rises only from 0.21 wt. % to 0.33 wt. %. For EVMD , the highest value is 29,400 kg/m2 h at 80 °C A very clear linear relationship exists between F and Xwater . The linear relationships between F and Xwater at different temperatures obtained from the data in Fig. 3 are as follows,
F = 1.844 Xwater − 18.912 at 70◦ C
(4)
F = 1.538 Xwater − 12.329 at 75◦ C
(5)
F = 1.129 Xwater − 5.323 at 80◦ C
(6)
F = 1.790 Xwater − 9.531 at 90◦ C
(7)
All adjusted R-squares of the above equations, except Eq. (6), exceed 0.99. Xwater at different temperatures when the flux was zero was calculated by Eqs. (4), (5) and (7). Those results corresponded with the final Xwater were shown in Fig. 2. Furthermore, the flux can also be calculated by Eqs. (4), (5) and (7) when Xwater from 11 wt. % to 15 wt. % as well as list the results in Table 1. The relation between the flux and temperature can be described by Arrhenius equation (ln F = −Ea /RT + ln A). The calculation results list in Table 2. The increasing Ea accompanied with decreasing Xwater indicates the more influence of temperature as Xwater decreasing.
Fig. 4. Effect of stirring rate on water content in feed solution (temperature = 70 °C, vacuum degree = 715 torr).
3.2. Effect of stirring rate The effect of stirring on the performance of VMD was studied by varying the rate of stirring from 0 rpm (no stirring) to 1700 rpm at 70 °C and 715 torr. Figs. 4 and 5 present the results thus obtained. As mentioned above, advection and molecular diffusion are the two mechanisms by which water is transported through the boundary layer. Without stirring, advection does not occur, and the water is transferred across the boundary only by molecular diffusion. Without stirring, Xwater falls negligibly. The final Xwater reduces from 14.35% to 6.92% as the stirring rate increased from
Please cite this article as: T.-H. Chen, Y.-H. Huang, Dehydration of diethylene glycol using a vacuum membrane distillation process, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.02.028
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Fig. 5. Effect of stirring rate on total flux and separation factor (temperature = 70 °C, vacuum degree = 715 torr).
60 0 rpm to 150 0 rpm. However, when the stirring rate reaches 1700 rpm, the final Xwater is 7.78%, probably because the unstable vacuum degree which is caused by the rapid stirring rate. The rapid stirring rate makes most of the feed solution thrown on the wall of the tank. The membrane cannot be completely covered by the remainder of the feed solution. Consequently, both F and α increase with the stirring rate up to 1500 rpm, beyond which they decrease. The highest EVMD is about 66,0 0 0 kg/m2 h when the stirring rate was 1500 rpm. The final Xwater is about 7 wt. % under the optimal conditions of 1500 rpm, 70 °C and 715 torr. 3.3. Effect of vacuum degree
Fig. 6. Effect of vacuum degree on water (temperature = 90 °C, stirring rate = 600 rpm).
content
in
feed
solution
According to the results of the first two variables, temperature and stirring rate were set respectively at 90 °C and 1500 rpm. The vacuum degree was adjusted from 715 torr to 650 torr. Figs. 6 and 7 show the results thus obtained. The final Xwater increased with the vacuum degree from 650 torr to 715 torr. The vacuum drove the flow of water to the permeate solution. F decreased as the vacuum degree decreased. The value of α varied insignificantly as the vacuum degree was reduced from 715 torr to 700 torr, but declined
Fig. 7. Effect of vacuum degree on total flux and separation factor (temperature = 90 °C and stirring rate = 600 rpm).
Please cite this article as: T.-H. Chen, Y.-H. Huang, Dehydration of diethylene glycol using a vacuum membrane distillation process, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.02.028
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as it was reduced further to 650 torr. The highest EVMD is about 46,800 kg/m2 h when the vacuum degree was 715 torr. The lowest final Xwater was 2.2 wt. %. With stirring, all Ywater exceeded 99 wt. %. EVMD at 90 °C, 1500 rpm and 715 torr (46,800 kg/m2 h) was lower than that at 70 °C, 1500 rpm and 715 torr (66,0 0 0 kg/m2 h). However, the objective of this study was to reduce the water content of the DEG solution to less than 5 wt. %. The optimal conditions of this study were temperature = 90 °C, stirring rate = 1500 rpm, and vacuum degree = 715 torr. 4. Conclusion The boundary layer close to the surface of the membrane is the main mass transfer resistance of VMD process. Increasing the operating temperature and the stirring rate is a very efficient means of overcoming the mass transfer resistance of the boundary layer. The dehydration of DEG in this study was optimized at 90 °C, 1500 rpm, and 715 torr. Under these conditions, Xwater was reduced from 15 wt. % to 2.26 wt. %. At the same time F, α and EVMD between 0 to 10 min were 18.5 kg/m2 h, 2500 and 46,800 kg/m2 h. References [1] Abu-Zeid MAE-R, Zhang Y, Dong H, Zhang L, Chen H-L, Hou L. A comprehensive review of vacuum membrane distillation technique. Desalination 2015;356:1–14. [2] Findley ME. Vaporization through porous membranes. Ind Eng Chem Process Des Dev 1967;6:226–30. [3] Carlsson L. The new generation in sea-water deslination SU membrane distillation system. Desalination 1983;45:221–2. [4] Teng-Chien C, Gaw-Hao H, Chuh-Shun C, Yao-Hui H. Reducing industrial wastewater and recovery of gold by direct contact membrane distillation with electrolytic system. Sustainable Environ Res 2013;23:209–14. [5] Shirazi MMA, Kargari A, Tabatabaei M. Sweeping gas membrane distillation (SGMD) as an alternative for integration of bioethanol processing: study on a commercial membrane and operating parameters. Chem Eng Commun 2015;202:457–66. [6] Shi J-Y, Zhao Z-P, Zhu C-Y. Studies on simulation and experiments of ethanol-water mixture separation by VMD using a PTFE flat membrane module. Sep Purif Technol 2014;123:53–63.
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Please cite this article as: T.-H. Chen, Y.-H. Huang, Dehydration of diethylene glycol using a vacuum membrane distillation process, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.02.028