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Konjac glucomannan blending membrane for application in pervaporation dehydration of caprolactam solution
Journal of Industrial and Engineering Chemistry 18 (2012) 934–940
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
New chitosan/Konjac glucomannan blending membrane for application in pervaporation dehydration of caprolactam solution Wenhai Lin *, Qin Li, Tianrong Zhu College of Chemistry and Molecule Science, Wuhan University, Wuhan 430072, P.R. China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 June 2011 Accepted 9 September 2011 Available online 3 February 2012
Chitosan/Konjac glucomannan blending membranes were investigated to propose an improved caprolactam pervaporation (PV) dehydration process. The composite membranes were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction and differential scanning calorimetry measurements to assess the intermolecular interactions of membranes. The effects of different temperature, CPL concentration in feed and KGM proportion in the membranes on the pervaporation performance were investigated. Data showed that KGM/CS blending membranes displayed good swelling and pervaporation performance, and the blending membrane M-1 had superior separation factor exceed 3000 at every temperature for 70 wt.% caprolactam. The results revealed that the separation performances of KGM/CS blending membranes were strongly related to their reaction degree, intrinsic structure and operating parameters. Crown Copyright ß 2012 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. All rights reserved.
Keywords: Chitosan Konjac glucomannan Membrane e-Caprolactam Pervaporation
1. Introduction Traditional separation techniques, such as evaporation, recrystallization and many other ways have high operating costs, high energy consumption and the secondary pollution at phase transferred and other separation problems [1–3]. Due to a requirement of high purity of raw material in the modern industry and a substantial increasing in the price of fuels, the development of energy conservative and efficient separation process has gain a great attention and become one of the hot topics in chemical industry field. Among the separation techniques, pervaporation (PV) is a novel membrane separation technique and has a potential application in chemical industry [4–7]. Unlike conventional separation techniques, pervaporation is a better alternative because of its ability to separate liquid mixtures without heating all of the mixtures to high temperature and this membrane technique can remove the minor component from the dilute solution [8]. PV can separate monomer from industrial mixture crude product with a very low energy cost [9], so it is an attractive process for producing high quality chemical materials from complicated mixture. Pervaporation membrane serves as a separating barrier that the component with a higher affinity transported across the membrane to accomplish a desired separation, thus the properties of the membranes determined
* Corresponding author. Tel.: +86 27 68752469; fax: +86 27 68776726. E-mail address: [email protected] (W. Lin).
the performance of the separation. Membrane material is the key of PV technique, and the development of high efficiency PV membrane is a major challenge [10]. Recently, membrane materials derived from natural renewable resources used as pervaporation membranes have aroused great interest for researchers because of the 3R (reduce, reuse and recycle) concept [11]. Chitosan derived through the deacetylation of chitin under alkaline conditions has become one of the most studied biodegradable pervaporation membranes due to its good film-forming characteristics, excellent hydrophilicity, and chemical resistance [12,13]. However, pure chitosan membranes have some drawbacks such as excessive swelling of the membrane in aqueous solutions [13]. Cross-linking and blending with other additives were usually used to improve their separation performance [14,15]. Konjac glucomannan (KGM) is a polysaccharide derived from the konjac tuber [16]. The deep development and exploits of KGM have been paid great attention in recent years. It can be prepared into various derivatives easily because of its good biocompatibility and biodegradable activity [17]. The studies on the applications of KGM and its derivatives have been extended greatly from gelling agent, film former and emulsifier to various fields, such as bio-technical and chemical industry etc. [18]. KGM has very good film-forming ability. Several kinds of films which blend KGM with chitosan, polyacylamide, sodium carboxymethylcellulose, polyvinylpyrrolidone, sodium aginate and cellulose were invented [19–21]. The results indicated that the intermolecular interactions between KGM and these substances were formed by hydrogen
1226-086X/$ – see front matter. Crown Copyright ß 2012 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. All rights reserved. doi:10.1016/j.jiec.2011.09.008
W. Lin et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 934–940
bond. The thermal stability and mechanic properties of the films were improved by blending KGM with these substances [20]. e-Caprolactam (CPL) is a monomer used extensively in the manufacture of high quality nylon-6 fibers. Traditional ecaprolactam (CPL) purification separation techniques, such as crystallization and three-effect evaporation, have many disadvantages, especially low energy coefficient [22]. Therefore, our group introduced PV technique into the CPL purification using PVA/CS blending membranes [23]. But PVA is a linear chain structure polymer with high crystallization and orientation, so limited the membrane performance. In the present study, KGM was used as a modifier to incorporate into a chitosan membrane. To optimize the separate ability, the solution was crosslinked before KGM being incorporated into the chitosan matrix. The performances of blending membranes with different proportion of KGM/CS for separating CPL–water mixtures have been explored. The morphological structure, thermal stability, and mechanical properties of the modified film were assessed using infrared (IR), scanning electron microscope (SEM), wide-angle X-ray diffraction (XRD), derivative thermogravimetric curve (DTG), differential scanning calorimetry (DSC) and swelling test. The effect of PV operating parameters such as temperature and feed concentration had been evaluated. The relationship between the structure and the separation property was discussed. 2. Experimental 2.1. Materials Industrially pure CPL was supplied by Baling Petrochemical Co. Ltd. (SINOPEC, China), and glutaraldehyde (GA, 25 wt.% in water) was purchased from Guoyao Chemicals Co., Ltd. (Si Chuan, China). Konjac glucomannan (KGM) was purchased from Qiangsen Konjac Corp., Wuhan, China. Chitosan ((hydrolyzed 91%, with average Mw = 500,000) was purchased from Guoyao Chemicals Co., Ltd. (Si Chuan, China)). Other chemicals were reagent grade and used without further purification. Porous ultrafiltration membrane of polyacrylonitrile (PAN) (Cut-off MW 5 104) was supplied by the Development Center of Water Treatment Technology (China). Deionized water was used in preparing the CPL feed solutions for the pervaporation experiments. 2.2. Preparation of crosslinked composite membranes KGM was dissolved in deionized water to prepare a concentration of 1 wt.% solution. 2.5 wt.% CS solutions were prepared by dissolving CS in 2 wt.% acetic acid solution at ambient temperature with stirring for 5 h. Series amount 50, 33 and 20 wt.% of the total solute of KGM solution and a certain amount of cross-linking agent (glutaraldehyde, GA) were added. The solution was continuously stirred at room temperature for 24 h, and the reaction was stopped by neutralization using sodium hydroxide solution. The resulting homogenous solution was used in the sequent process after degassing. The substrate membranes, hydrolyzed PAN microporous membranes, were prepared by immersing PAN ultrafiltration membrane in 5 wt.% NaOH aqueous solution at 50 8C for 1 h, washing thoroughly with deionized water until neutral and then immersing in 1N HCl aqueous solution for 20 min, and washing with water until neutral [23]. Then, a certain amount of prepared KGM/CS solution was cast on the porous PAN substrate membranes which were held on a glass plate. The composite membranes were allowed to evaporate slowly until dried at room temperature. Finally, the composite membranes were treated in an air-circulating oven at temperature 120 8C for 1 h
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Table 1 Typical sample preparation and designation of KGM/CS composites membrane. Membranes
M-1 M-2 M-4 M-K M-C
Concentration (wt.%) Konjac glucomannan
Chitosan
Glutaraldehyde
49.75 33.17 19.9 99.5 0
49.75 66.33 79.6 0 99.5
0.5 0.5 0.5 0.5 0.5
for thermal cross-linking. The membranes in different KGM mass ratio (KGM:CS = 1:1, 1:2 and 1:4) were respectively designated as M-1, M-2, and M-4, as shown in Table 1. Pure crosslinked KGM and CS membrane were designated M-K, M-C, respectively. 2.3. Swelling experiments Precisely weighed dry membrane sheets were immersed in a closed bottle containing in-feed mixtures of CPL/water with CPL from 30 to 70 wt.% for a period at 40 8C. Only the active layer of the membrane was tested for sorption in this study. The sorption data only indicated the swelling behavior of the active membrane layer. The masses of dry membranes were first determined and then were equilibrated by soaking in different compositions of feed mixtures in a sealed vessel for 48 h. After swelling, the membranes were subsequently blotted between tissue papers to remove excess solvent and weighed as soon as possible. All the experiments were performed at least three times and the results were averaged. The degree of swelling (Ds) was calculated using the following equation: S ð%Þ
Ws Wd 100% Wd
(1)
where Wd and Ws are the weight of dry membrane and swollen membrane, respectively. 2.4. Characterization 2.4.1. Fourier transform infrared (FT-IR) spectroscopy The cross-linking reaction of KGM/CS with GA was confirmed by the Fourier transform infrared (FT-IR) spectroscopy. FT-IR spectra of various membranes were scanned using Nicolet AVATAR 360 FT-IR spectrometer. FTIR spectra were recorded within the range of 4000–500 cm1. 2.4.2. Scanning electron microscopy (SEM) Morphologies of the various composite membranes were examined by scanning electron microscopy (SEM). All specimens were coated with a conductive layer of sputtered gold. The morphologies of the KGM/CS blending membranes were observed with SEM (FEI Quanta 200, Holland). 2.4.3. X-ray diffraction (XRD) analysis X-ray diffraction (XRD) measurements were analyzed using a Shimadzu XRD-6000 (Japan) diffractometer equipped with graphite monochromatized Cu Ka radiation (l = 1.54060 A˚) at 40 kV and 30 mA with a scan rate of 48/min. Angle of diffraction varied from 58 to 458 to identify any changes in the crystal structure. 2.4.4. Thermal analyses Thermal gravimetric analysis was conducted with SETSYS 16 instrument (France) under a nitrogen atmosphere with a flow capacity of 50 ml/ min. The scan was carried out at a heating rate of
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3. Results and discussion 3.1. Membrane characterization
Fig. 1. FTIR spectra of pure CS, KGM and blending membranes with different KGM contents.
10 8C/min from 20 to 600 8C. The sample weight was about 5– 10 mg and analyzed using a-Al2O3 crucible. 2.4.5. Pervaporation experiments The tested membrane was allowed to equilibrate for about 1– 2 h at the corresponding temperature before performing the PV experiment with fixed compositions of the feed mixture. After establishment of a steady state, the permeate vapor was collected in a trap immersed in the liquid nitrogen jar on the downstream side at a fixed time of intervals. The feed mixtures were varied from 30 to 70 wt.% CPL. PV experiments were conducted in the range of 40–60 8C [22]. Based on PV data, separation performance of membranes was assessed in terms of flux (J) and separation factor (a). These were calculated respectively using the following equations:
a¼
J¼
yw =yCPL xw =xCPL
W At
(2)
(3)
where in Eq. (2) xw, yw are the mole fractions of water in the feed and the permeate, xCPL, yCPL are the mole fraction of CPL in the feed and the permeate; in Eq. (3), W (g), A (m2) and t (h) are the weight of permeates, effective membrane area and time, respectively.
3.1.1. FT-IR analysis FTIR spectra of original CS, KGM and CS/KGM blending membranes are illustrated in Fig. 1. From this spectrum of pure CS (Fig. 1M-C), it can be seen that the absorption band of –OH groups was at 3388 cm1. The band at 1650 cm1 predominantly arose from –NH2 groups asymmetric stretching [23]. Adsorption peak at 1063 cm1 was dominated by the C6–OH. The peak at 1728 cm1 was provided the evidence of characteristic absorption band of the C5 5O in the carbonyl of acetyl groups. The spectrum of Fig. 1M-K showed characteristic absorption peak of KGM. The absorption band of the 3382 cm1, 2886 cm1, 1724 cm1, 1636 cm1, 1061 cm1 were dominated by the –OH, C–H, C5 5O, C–O and C6–OH groups respectively [24]. While the absorption bands at 870 and 819 cm1 were characteristic of the vibrations of the mannose unit in KGM. Compared with the spectrum of M-C and M-K, the following changes had taken place in the blending films. The absorption band around 3440 cm1 shifted to a lower wave number with the increase of KGM, which was indicated the intermolecular hydrogen bonds between CS and KGM. The stretching of carbonyl at 1724 cm1 of KGM was disappeared; and the new stretching of bonds at 1638 cm1 was due to the absorption peak of amide carbonyl stretching. FTIR results confirmed that crosslinked reacting and new hydrogen bonds between CS and KGM molecules in the blending membranes have been occurred [24]. 3.1.2. SEM analysis Fig. 2 shows the cross-section morphology for KGM/CS blending membrane. The multilayer structure of membrane was observed very clearly: an active layer and a supported porous layer, which was similar to our previous work [22]. The thickness of the KGM/CS active layer can be estimated about 3–5 mm, whereas the thickness of the PAN porous support layer was approximately 50 mm in Fig. 2(A) and (B). It can be seen that the surface of active layer was smooth and compact without any cracks which indicates that the compatibility between CS and KGM was quite good. The porous layer was also arranged very regularly, which was benefit to separate components through the membranes and improve of the flux [23]. 3.1.3. XRD analysis The X-ray diffraction curves of KGM, CS, M-1, M-2 and M-3 films are shown in Fig. 3. Chitosan (Fig. 3M-C) exhibited a typical peak at 2u = 128 and 208 [25]. These two peaks were related to two types of
Fig. 2. The cross-section morphology of the KGM/CS blending membrane M-2.
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Fig. 5. DSC curves of the pure CS, KGM and blending membranes. Fig. 3. X-ray diffraction pattern of the pure CS, KGM and KGM/CS blending membranes.
crystals. The peak at 128 had notability effect on the properties of separation [26]. The pure KGM film was showed a non-crystalline state and only had a very broad peak around 2u = 208 [24]. When the blending membranes loading more konjac glucomannan, the diffraction peak corresponding to 128 of chitosan became wider and lower. The same regularity would be drawn in regard to diffraction peak around 2u = 208. From these patterns, it is clear that the crystallinity was decreased with increasing KGM loading in the blending membranes. This result implied that the CS/KGM blending membranes appeared more amorphous morphology after reaction. This trend was mainly attributed to the interaction between KGM and CS which strengthened with the increasing of konjac glucomanan loading. 3.1.4. Thermal analysis Thermal stability analysis of polymer material is helpful in the selection of materials which usually used in PV technique. Fig. 4 shows the detailed thermo-gravimetric data for the CS/KGM blending membranes. M-K, M-C, M-1 and M-2 had onset weight loss peaks at 73.3, 87.9, 75.8 and 81.6 8C, which could be assigned to the loss of adsorbed water from the membranes [27]. It was found that the onset temperatures of releasing of water molecules from the blending membranes were higher than KGM, but lower than CS. With the increase of the temperature, KGM and CS began the step of maximum weight loss at 280.7 and 248.9 8C while that of M-1, M-2 was shown at 223.9, 230.7 8C respectively. These degradation peaks may be due to the decomposition of KGM and
Fig. 4. DTG thermograms of the pure CS, KGM and KGM/CS blending membranes.
CS. The results indicated that the incorporation of copolymer altered the thermal decomposition, which decreased the thermostability of the natural polymer. So it is believed that CS/KGM membranes were mainly incorporated [28]. 3.1.5. DSC analysis The DSC curves of the CS, KGM and blending membranes (Fig. 5) were used to determine the compatibility of the blending by means of testing the endothermic peaks. In the CS curve, the main feature was an endothermic peak at 245.6 8C. The presence of this peak which evident in the DSC traces was essentially characterized the thermal behavior of chitosan. The presence of this peak was presumably resulted from the thermal decomposition of the glycosidic bonds of chitosan [24]. Similarly, KGM revealed the decomposition peak at 276.1 8C. The DSC curve of the CS/KGM blending membranes only exhibited one decomposition peak which shifted to a lower temperature than the peak of pure CS and KGM. In addition, with the increased CS in the blending membranes the decomposition temperature was nearly to the pure polymer curve. These results showed that the thermal stability of CS/KGM blending membranes decreased a little compared with the original CS and KGM. The interaction of two polysaccharides could disrupt their crystalline structures. The changes of number and location of the endothermic peaks in DSC curve indicated that konjac glucomannan and chitosan form a hydrogen-bonding interaction between the blending membranes. 3.2. Pervaporation characteristics 3.2.1. Swelling experiments Fig. 6 shows the degree of swelling which was obtained by soaking the CS/KGM blending membranes in the different CPL concentration solution at 40 8C. The transport of permeating molecules was controlled partially by sorption mechanism. Therefore, to study the effects of KGM on membrane swelling, the degree of swelling was plotted with different temperatures, ranging from 30 8C to 50 8C, in CPL 70 wt.% feed for 72 h, as shown in Fig. 6. It was noticed that the degree of swelling of the membranes decreased when the CPL loading in the solution increased. This was due to the fact that those membranes were hydrophilic and increased the strong interaction between water molecules and the membrane containing –OH groups and NH2 groups [29]. At same concentration solution, the degree of swelling reduced when the composite membranes with more KGM, due to the interaction with CS and KGM. The van der Waals force was weaked between KGM and CS, hydrogen bonding interaction, hydrophilic interaction and electrostatic interaction played important roles in the interfacial
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W. Lin et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 934–940 Table 2 Comparison of saturated vapor pressure of water and CPL at different temperatures.
Fig. 6. Degree of swelling of the pure CS, KGM and KGM/CS blending membranes at 40 8C.
formation. Between KGM and CS there was existing plenty of hydrogen bonding in the chains. In the three kinds of molecular force, the hydrogen bonding was the most important force than the others. More reaction between CS and KGM groups reduced the activity of molecular chains, the hydrophilic and hydrogen bonding interaction [22]. After blending, three-dimensional networks formed in the KGM and CS molecules by non-covalent bonding in the molecules. The hydrogen bonding included net-in and nettran forms in KGM/CS blending membranes [27]. This means that solvent molecules were difficult to absorb which would reduce the degree of swelling of membranes sharply. 3.2.2. Effect of operating temperature Operation temperature has a significant effect on the PV performance of the blending membrane [30]. So we typically chosen M-4 on the condition of 60 wt.% of CPL in feed to study the effects of temperature, and the resulting values are presented in Fig. 7. It displayed the effects of operation temperature on the separation factors and flux of total, water and CPL, respectively. It was observed that all of flux increased significantly with the temperature increasing. As shown in Fig. 7, it was attributed to the increase in operation temperature and the partial pressure of the water and CPL in the feed side (Table 2). As a consequence, the driving force for mass transfer across the blending membrane increased [23], resulting in the increase of the flux. In addition, high temperature accelerated the diffusion of the water and CPL
Temperature (8C)
Water saturated vapor pressure (Pa)
CPL saturated vapor pressure (Pa)
40 45 50 55 60
7362 9683 12,392 15,905 20,127
27 35 45 59 75
molecules and enlarged free volume in polymer matrix, which favored the permeation of the process, thus the flux increased. However, the flux increased with the expense of the separation factor. While the total flux increased following the temperature rising, the separation factor decreased synchronously (Fig. 7). This ‘‘trade-off’’ phenomenon may be traditionally explained by the increase in frequency and amplitude of polymer chain thermally induced expansion of the free volume in membranes, and CPL molecular passed easily [31]. Increase of smaller molecules diffusivities was the other source of this behavior. In addition, with the feed temperature increasing, the vapor pressure in the feed compartment increased (Table 2), but there was no increasing in vapor pressure at the permeate side. It resulted in an increasing of driving force with increasing temperature. Therefore, the permeation of diffusing molecules and the associated molecules through the membrane became easier, leading to an increasing of total permeation flux, while suppressing the separation factor. 3.2.3. Activation energy analysis Temperature dependent flux data have been fitted to the Arrhenius relationship to estimate the activation parameters. The Arrhenius relationship is expressed in Eq. (4) [22]. E p J ¼ J0 exp RT
(4)
J, Ep, J0, are permeation flux, activation energy for permeation, permeation rate constant, respectively. T is the absolute temperature and R is the molar gas constant. The activation energy (DEa) of water and CPL permeates through M-2 with 40 wt.% and 50 wt.% CPL concentrations were calculated on the basis of the Arrhenius formula as shown in Fig. 8, and their results are summarized in Table 3. It suggested that water flux has controlled over total flux and activation energies of total flux and water permeations were
Fig. 7. Effects of the operating temperature on the pervaporation performances in terms of water and CPL flux, total flux and separation factor through M-4 blending membranes at 60 wt.% CPL aqueous solution.
W. Lin et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 934–940
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Fig. 8. Temperature dependence of (a) ln[water and CPL flux]; (b) ln[total flux] for 40 wt.% and 50 wt.% CPL concentration for M-2.
Fig. 9. Effect of the feed concentration on the pervaporation performances of M-1 at 60 8C.
nearly in the same membrane at the same feed concentration. In addition, the Ew were obviously lower for the ECPL in this study. It implied that water consumed less amount of energy for permeation and had higher permeability than CPL. This fact proved that KGM/CS blending membranes were suitable to be used for PV separation CPL aqueous solution [23]. 3.2.4. Effect of feed composition Feed composition effect of flux and separation factor are displayed in Fig. 9. It showed the effects of feed concentration on the total permeation flux and separation factor vs. concentrations of 30–70 wt.% CPL in feed at different temperature with M-1 membranes. It can be seen that the flux of M-1 increased from 792 to 1327 g/(m2 h) for feeds containing 30–70 wt.%. It was easily noticeable to see the superiority of flux in lower CPL feed concentration and superiority separation factor in higher CPL feed concentration. These results indicated that flux and separation factor were strongly dependent on the feed composition. Those
behaviors were well agreed with our previous study, the KGM/CS blending membranes had the obvious advantage in separation factor [22,23]. The flux results increased due to the membranes swelling greatly at higher concentration of water in the feed, with the cost of separation factor. This phenomenon was explained by the amorphous regions of the KGM/CS crosslink structure was become more flexible and swollen more, both water and CPL molecules were more easily diffusion across the membranes. So the flux increased and the separation factor decreased. This was known as trade-off rule, which was generally observed in other pervaporation processes [15,23].
Table 3 Activation energy data of M-2 on pervaporation at different concentration CPL aqueous solution. CPL concentration (wt.%)
30 40 50 60 70
DEa activation energy (kJ/mol) Et
Ew
ECPL
21.17 19.71 28.34 29.56 31.52
21.14 19.68 28.31 29.53 31.49
79.45 77.99 91.11 89.10 88.40
Fig. 10. Effects of different loading of KGM in blending membranes on pervaporation performance for different temperatures in 70 wt.% CPL concentration in feed solution.
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W. Lin et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 934–940
4. Conclusions
Fig. 11. Effects of different loading of KGM in blending membranes on pervaporation performance for different CPL concentration at 50 8C.
3.2.5. Effect of KGM loading Figs. 10 and 11 show the effects of KGM loading in blending membranes on permeate flux and separation factor for 70 wt.% CPL in feed at different temperatures and 50 8C at different CPL concentration. The KGM content had significant effects on both permeation flux and separation factor. When the KGM loading increased, the permeation flux increased which was consistent with the changing trend of crystallinity. Another important factor to affect the separation flux was the swelling behavior of the membranes in the feed mixture. The free volume and the swelling behavior of the membranes increased with the addition of KGM. Whereas the KGM:CS ratio increased from 1:4 to 1:2, the separation factor had nearly no change, and then dropped a few when the ratio reached 1:1. This separation factor variable trend was due to the membranes’ internal structure. Introduction of KGM could significantly enhanced the hydrophilicity of the membranes as it had strong hydrophilic groups and disturbed CS regularity structure. But when the exceeding swell of membranes and the increase of free volume reached a certain level, the permeation of CPL could be accelerated and the separation factor could be dropped, which was confirmed by the XRD measurement [32]. Compared with PVA and PVA/CS membranes [22,23], it could be found that KGM/CS blending membranes had the near flux and higher separation factor under the same condition. The membranes with more KGM reacting with CS, due to the fewer active groups of unit mass and denser network structure, had lower total flux but more excellent separation factor. When KGM and CS were blended, their molecule structures were disturbed. The active groups in polysaccharides formed three-dimensional networks. Due to that KGM had high molecular weight, characterized gel behavior, and formed stable inter chains with CS [27]. So the membranes of M-1 had the best flux and M-4 had the best separation factor. These results agree well with the test of the degree of swelling. It showed that the flux and separation factor could reach a balance state by the modification loading of KGM.
KGM/CS blending membranes were prepared and pervaporation dehydration experiments were conducted. The introducing of KGM affected the pervaporation performance of the composite membranes. FTIR spectra and XRD curves showed the changes of structure, and SEM displayed the morphology of the composite membranes. The results of TGA and DSC indicated that the incorporation of KGM altered the thermal decomposition of CS matrix, which decreased the thermo-stability. With increasing the KGM proportion, the permeation flux increased with a drop of separation factor. With increasing of both temperature and feed water concentration, the total flux increased and the separation factor decreased. Swelling results followed those trends. Meanwhile, it was successful for the application of KGM/CS blending membranes in CPL/water system on pervaporation. The data showed the improved separation factor for CPL/water compared with the former PVA and PVA-CS membranes. The experimental results also indicated that the KGM/CS blending membranes had superior dehydration performances for caprolactam solution. References [1] W.W. Ding, Y.T. Wu, X.Y. Tang, L. Yuan, Z.Y. Xiao, J. Chem. Technol. Biotechnol. 86 (2011) 82. [2] B.K. Hur, L.P. Hao, X.J. Cao, J. Ind. Eng. Chem. 14 (2008) 639. [3] I.S. Moon, M. Matheswaran, T.O. Kwon, J.W. Kim, J. Ind. Eng. Chem. 13 (2007) 965. [4] P. Garg, R.P. Singh, V. Choudhary, Sep. Purif. Technol. 76 (2011) 407. [5] I.C. Kim, Y.H. Ka, J.Y. Park, K.H. Lee, J. Ind. Eng. Chem. 10 (2004) 115. [6] Y.I. Park, C.K. Yeom, S.H. Lee, B.S. Kim, J.M. Lee, H.J. Joo, J. Ind. Eng. Chem. 13 (2007) 272. [7] S. Araki, Y. Kiyohara, S. Imasaka, S. Tanaka, Y. Miyake, Desalination 266 (2011) 46. [8] S. Khoonsap, S. Amnuaypanich, J. Membr. Sci. 367 (2011) 182. [9] J.W. Rhim, D.S. Kim, D.H. Kim, B.S. Lee, G.Y. Moon, H.K. Lee, N.S. Yong, J. Ind. Eng. Chem. 15 (2009) 393. [10] S. Maji, S. Banerjee, J. Membr. Sci. 360 (2010) 380. [11] Y.T. Ong, A.L. Ahmad, S.H.S. Zein, S.H. Tan, Braz. J. Chem. Eng. 27 (2010) 227. [12] S.Y. Cho, T.Y. Kim, S.J. Kim, J.H. Yang, J. Ind. Eng. Chem. 10 (2004) 201. [13] C.H. Kim, K.S. Choi, J. Ind. Eng. Chem. 8 (2002) 71. [14] M.B. Patil, T.M. Aminabhavi, Sep. Purif. Technol. 62 (2008) 128. [15] M. Nawawi, M. Ghazali, R.Y.M. Huang, J. Membr. Sci. 124 (1997) 53. [16] M.A.K. Williams, T.J. Foster, D.R. Martin, I.T. Norton, M. Yoshimura, K. Nishinari, Biomacromolecules 1 (2000) 440. [17] C.L. Xu, S. Willfor, B. Holmbom, Bioresources 3 (2008) 713. [18] D.T. Tian, H.Q. Xie, Polym. Bull. 61 (2008) 277. [19] K.S. Mikkonen, M.P. Yadav, P. Cooke, S. Willfor, K.B. Hicks, M. Tenkanen, Bioresources 3 (2008) 178. [20] J. Lu, J.H. Zhang, C. Xiao, J. Appl. Polym. Sci. 106 (2007) 1972. [21] H.Q. Yu, A. Huang, C.B. Xiao, J. Appl. Polym. Sci. 100 (2006) 1561. [22] L. Zhang, P. Yu, Y.B. Luo, J. Membr. Sci. 306 (2007) 93. [23] Q. Li, P. Yu, T.R. Zhu, L. Zhang, Q. Li, Y.B. Luo, Desalination 16 (2010) 304. [24] X. Ye, J.R. Kennedy, B. Li, B.J. Xie, Carbohydr. Polym. 64 (2006) 532. [25] R.J. Samuels, J. Polym. Sci.: Polym. Phys. 19 (1981) 1081. [26] B. Focher, M. Dave, E.Marsano Tamagno, Macromolecules 28 (1995) 3531. [27] G. Zhou, Y.B. Li, L. Zhang, H. Li, M.B. Wang, L. Cheng, Y.Y. Wang, H.N. Wang, P.J. Shi, J. Mater. Sci. 42 (2007) 2591. [28] G. Zhou, Y.B. Li, L. Zhang, Y. Zuo, A. John, Jansen, J. Biomed. Mater. Res. A 83 (2007) 931. [29] G. Dhanuja, B. Smitha, S. Sridhar, Sep. Purif. Technol. 44 (2005) 130. [30] L.K. Pandey, C. Saxena, V. Dubey, J. Membr. Sci. 227 (2003) 173. [31] S.S. Kulkarni, S.M. Tambe, A.A. Kittur, M.Y. Kariduraganavar, J. Appl. Polym. Sci. 99 (2006) 1380. [32] T.R. Zhu, Y.B. Luo, Y.W. Lin, Q. Li, P. Yu, M. Zeng, Sep. Purif. Technol. 74 (2010) 242.
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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
New chitosan/Konjac glucomannan blending membrane for application in pervaporation dehydration of caprolactam solution Wenhai Lin *, Qin Li, Tianrong Zhu College of Chemistry and Molecule Science, Wuhan University, Wuhan 430072, P.R. China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 June 2011 Accepted 9 September 2011 Available online 3 February 2012
Chitosan/Konjac glucomannan blending membranes were investigated to propose an improved caprolactam pervaporation (PV) dehydration process. The composite membranes were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction and differential scanning calorimetry measurements to assess the intermolecular interactions of membranes. The effects of different temperature, CPL concentration in feed and KGM proportion in the membranes on the pervaporation performance were investigated. Data showed that KGM/CS blending membranes displayed good swelling and pervaporation performance, and the blending membrane M-1 had superior separation factor exceed 3000 at every temperature for 70 wt.% caprolactam. The results revealed that the separation performances of KGM/CS blending membranes were strongly related to their reaction degree, intrinsic structure and operating parameters. Crown Copyright ß 2012 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. All rights reserved.
Keywords: Chitosan Konjac glucomannan Membrane e-Caprolactam Pervaporation
1. Introduction Traditional separation techniques, such as evaporation, recrystallization and many other ways have high operating costs, high energy consumption and the secondary pollution at phase transferred and other separation problems [1–3]. Due to a requirement of high purity of raw material in the modern industry and a substantial increasing in the price of fuels, the development of energy conservative and efficient separation process has gain a great attention and become one of the hot topics in chemical industry field. Among the separation techniques, pervaporation (PV) is a novel membrane separation technique and has a potential application in chemical industry [4–7]. Unlike conventional separation techniques, pervaporation is a better alternative because of its ability to separate liquid mixtures without heating all of the mixtures to high temperature and this membrane technique can remove the minor component from the dilute solution [8]. PV can separate monomer from industrial mixture crude product with a very low energy cost [9], so it is an attractive process for producing high quality chemical materials from complicated mixture. Pervaporation membrane serves as a separating barrier that the component with a higher affinity transported across the membrane to accomplish a desired separation, thus the properties of the membranes determined
* Corresponding author. Tel.: +86 27 68752469; fax: +86 27 68776726. E-mail address: [email protected] (W. Lin).
the performance of the separation. Membrane material is the key of PV technique, and the development of high efficiency PV membrane is a major challenge [10]. Recently, membrane materials derived from natural renewable resources used as pervaporation membranes have aroused great interest for researchers because of the 3R (reduce, reuse and recycle) concept [11]. Chitosan derived through the deacetylation of chitin under alkaline conditions has become one of the most studied biodegradable pervaporation membranes due to its good film-forming characteristics, excellent hydrophilicity, and chemical resistance [12,13]. However, pure chitosan membranes have some drawbacks such as excessive swelling of the membrane in aqueous solutions [13]. Cross-linking and blending with other additives were usually used to improve their separation performance [14,15]. Konjac glucomannan (KGM) is a polysaccharide derived from the konjac tuber [16]. The deep development and exploits of KGM have been paid great attention in recent years. It can be prepared into various derivatives easily because of its good biocompatibility and biodegradable activity [17]. The studies on the applications of KGM and its derivatives have been extended greatly from gelling agent, film former and emulsifier to various fields, such as bio-technical and chemical industry etc. [18]. KGM has very good film-forming ability. Several kinds of films which blend KGM with chitosan, polyacylamide, sodium carboxymethylcellulose, polyvinylpyrrolidone, sodium aginate and cellulose were invented [19–21]. The results indicated that the intermolecular interactions between KGM and these substances were formed by hydrogen
1226-086X/$ – see front matter. Crown Copyright ß 2012 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. All rights reserved. doi:10.1016/j.jiec.2011.09.008
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bond. The thermal stability and mechanic properties of the films were improved by blending KGM with these substances [20]. e-Caprolactam (CPL) is a monomer used extensively in the manufacture of high quality nylon-6 fibers. Traditional ecaprolactam (CPL) purification separation techniques, such as crystallization and three-effect evaporation, have many disadvantages, especially low energy coefficient [22]. Therefore, our group introduced PV technique into the CPL purification using PVA/CS blending membranes [23]. But PVA is a linear chain structure polymer with high crystallization and orientation, so limited the membrane performance. In the present study, KGM was used as a modifier to incorporate into a chitosan membrane. To optimize the separate ability, the solution was crosslinked before KGM being incorporated into the chitosan matrix. The performances of blending membranes with different proportion of KGM/CS for separating CPL–water mixtures have been explored. The morphological structure, thermal stability, and mechanical properties of the modified film were assessed using infrared (IR), scanning electron microscope (SEM), wide-angle X-ray diffraction (XRD), derivative thermogravimetric curve (DTG), differential scanning calorimetry (DSC) and swelling test. The effect of PV operating parameters such as temperature and feed concentration had been evaluated. The relationship between the structure and the separation property was discussed. 2. Experimental 2.1. Materials Industrially pure CPL was supplied by Baling Petrochemical Co. Ltd. (SINOPEC, China), and glutaraldehyde (GA, 25 wt.% in water) was purchased from Guoyao Chemicals Co., Ltd. (Si Chuan, China). Konjac glucomannan (KGM) was purchased from Qiangsen Konjac Corp., Wuhan, China. Chitosan ((hydrolyzed 91%, with average Mw = 500,000) was purchased from Guoyao Chemicals Co., Ltd. (Si Chuan, China)). Other chemicals were reagent grade and used without further purification. Porous ultrafiltration membrane of polyacrylonitrile (PAN) (Cut-off MW 5 104) was supplied by the Development Center of Water Treatment Technology (China). Deionized water was used in preparing the CPL feed solutions for the pervaporation experiments. 2.2. Preparation of crosslinked composite membranes KGM was dissolved in deionized water to prepare a concentration of 1 wt.% solution. 2.5 wt.% CS solutions were prepared by dissolving CS in 2 wt.% acetic acid solution at ambient temperature with stirring for 5 h. Series amount 50, 33 and 20 wt.% of the total solute of KGM solution and a certain amount of cross-linking agent (glutaraldehyde, GA) were added. The solution was continuously stirred at room temperature for 24 h, and the reaction was stopped by neutralization using sodium hydroxide solution. The resulting homogenous solution was used in the sequent process after degassing. The substrate membranes, hydrolyzed PAN microporous membranes, were prepared by immersing PAN ultrafiltration membrane in 5 wt.% NaOH aqueous solution at 50 8C for 1 h, washing thoroughly with deionized water until neutral and then immersing in 1N HCl aqueous solution for 20 min, and washing with water until neutral [23]. Then, a certain amount of prepared KGM/CS solution was cast on the porous PAN substrate membranes which were held on a glass plate. The composite membranes were allowed to evaporate slowly until dried at room temperature. Finally, the composite membranes were treated in an air-circulating oven at temperature 120 8C for 1 h
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Table 1 Typical sample preparation and designation of KGM/CS composites membrane. Membranes
M-1 M-2 M-4 M-K M-C
Concentration (wt.%) Konjac glucomannan
Chitosan
Glutaraldehyde
49.75 33.17 19.9 99.5 0
49.75 66.33 79.6 0 99.5
0.5 0.5 0.5 0.5 0.5
for thermal cross-linking. The membranes in different KGM mass ratio (KGM:CS = 1:1, 1:2 and 1:4) were respectively designated as M-1, M-2, and M-4, as shown in Table 1. Pure crosslinked KGM and CS membrane were designated M-K, M-C, respectively. 2.3. Swelling experiments Precisely weighed dry membrane sheets were immersed in a closed bottle containing in-feed mixtures of CPL/water with CPL from 30 to 70 wt.% for a period at 40 8C. Only the active layer of the membrane was tested for sorption in this study. The sorption data only indicated the swelling behavior of the active membrane layer. The masses of dry membranes were first determined and then were equilibrated by soaking in different compositions of feed mixtures in a sealed vessel for 48 h. After swelling, the membranes were subsequently blotted between tissue papers to remove excess solvent and weighed as soon as possible. All the experiments were performed at least three times and the results were averaged. The degree of swelling (Ds) was calculated using the following equation: S ð%Þ
Ws Wd 100% Wd
(1)
where Wd and Ws are the weight of dry membrane and swollen membrane, respectively. 2.4. Characterization 2.4.1. Fourier transform infrared (FT-IR) spectroscopy The cross-linking reaction of KGM/CS with GA was confirmed by the Fourier transform infrared (FT-IR) spectroscopy. FT-IR spectra of various membranes were scanned using Nicolet AVATAR 360 FT-IR spectrometer. FTIR spectra were recorded within the range of 4000–500 cm1. 2.4.2. Scanning electron microscopy (SEM) Morphologies of the various composite membranes were examined by scanning electron microscopy (SEM). All specimens were coated with a conductive layer of sputtered gold. The morphologies of the KGM/CS blending membranes were observed with SEM (FEI Quanta 200, Holland). 2.4.3. X-ray diffraction (XRD) analysis X-ray diffraction (XRD) measurements were analyzed using a Shimadzu XRD-6000 (Japan) diffractometer equipped with graphite monochromatized Cu Ka radiation (l = 1.54060 A˚) at 40 kV and 30 mA with a scan rate of 48/min. Angle of diffraction varied from 58 to 458 to identify any changes in the crystal structure. 2.4.4. Thermal analyses Thermal gravimetric analysis was conducted with SETSYS 16 instrument (France) under a nitrogen atmosphere with a flow capacity of 50 ml/ min. The scan was carried out at a heating rate of
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3. Results and discussion 3.1. Membrane characterization
Fig. 1. FTIR spectra of pure CS, KGM and blending membranes with different KGM contents.
10 8C/min from 20 to 600 8C. The sample weight was about 5– 10 mg and analyzed using a-Al2O3 crucible. 2.4.5. Pervaporation experiments The tested membrane was allowed to equilibrate for about 1– 2 h at the corresponding temperature before performing the PV experiment with fixed compositions of the feed mixture. After establishment of a steady state, the permeate vapor was collected in a trap immersed in the liquid nitrogen jar on the downstream side at a fixed time of intervals. The feed mixtures were varied from 30 to 70 wt.% CPL. PV experiments were conducted in the range of 40–60 8C [22]. Based on PV data, separation performance of membranes was assessed in terms of flux (J) and separation factor (a). These were calculated respectively using the following equations:
a¼
J¼
yw =yCPL xw =xCPL
W At
(2)
(3)
where in Eq. (2) xw, yw are the mole fractions of water in the feed and the permeate, xCPL, yCPL are the mole fraction of CPL in the feed and the permeate; in Eq. (3), W (g), A (m2) and t (h) are the weight of permeates, effective membrane area and time, respectively.
3.1.1. FT-IR analysis FTIR spectra of original CS, KGM and CS/KGM blending membranes are illustrated in Fig. 1. From this spectrum of pure CS (Fig. 1M-C), it can be seen that the absorption band of –OH groups was at 3388 cm1. The band at 1650 cm1 predominantly arose from –NH2 groups asymmetric stretching [23]. Adsorption peak at 1063 cm1 was dominated by the C6–OH. The peak at 1728 cm1 was provided the evidence of characteristic absorption band of the C5 5O in the carbonyl of acetyl groups. The spectrum of Fig. 1M-K showed characteristic absorption peak of KGM. The absorption band of the 3382 cm1, 2886 cm1, 1724 cm1, 1636 cm1, 1061 cm1 were dominated by the –OH, C–H, C5 5O, C–O and C6–OH groups respectively [24]. While the absorption bands at 870 and 819 cm1 were characteristic of the vibrations of the mannose unit in KGM. Compared with the spectrum of M-C and M-K, the following changes had taken place in the blending films. The absorption band around 3440 cm1 shifted to a lower wave number with the increase of KGM, which was indicated the intermolecular hydrogen bonds between CS and KGM. The stretching of carbonyl at 1724 cm1 of KGM was disappeared; and the new stretching of bonds at 1638 cm1 was due to the absorption peak of amide carbonyl stretching. FTIR results confirmed that crosslinked reacting and new hydrogen bonds between CS and KGM molecules in the blending membranes have been occurred [24]. 3.1.2. SEM analysis Fig. 2 shows the cross-section morphology for KGM/CS blending membrane. The multilayer structure of membrane was observed very clearly: an active layer and a supported porous layer, which was similar to our previous work [22]. The thickness of the KGM/CS active layer can be estimated about 3–5 mm, whereas the thickness of the PAN porous support layer was approximately 50 mm in Fig. 2(A) and (B). It can be seen that the surface of active layer was smooth and compact without any cracks which indicates that the compatibility between CS and KGM was quite good. The porous layer was also arranged very regularly, which was benefit to separate components through the membranes and improve of the flux [23]. 3.1.3. XRD analysis The X-ray diffraction curves of KGM, CS, M-1, M-2 and M-3 films are shown in Fig. 3. Chitosan (Fig. 3M-C) exhibited a typical peak at 2u = 128 and 208 [25]. These two peaks were related to two types of
Fig. 2. The cross-section morphology of the KGM/CS blending membrane M-2.
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Fig. 5. DSC curves of the pure CS, KGM and blending membranes. Fig. 3. X-ray diffraction pattern of the pure CS, KGM and KGM/CS blending membranes.
crystals. The peak at 128 had notability effect on the properties of separation [26]. The pure KGM film was showed a non-crystalline state and only had a very broad peak around 2u = 208 [24]. When the blending membranes loading more konjac glucomannan, the diffraction peak corresponding to 128 of chitosan became wider and lower. The same regularity would be drawn in regard to diffraction peak around 2u = 208. From these patterns, it is clear that the crystallinity was decreased with increasing KGM loading in the blending membranes. This result implied that the CS/KGM blending membranes appeared more amorphous morphology after reaction. This trend was mainly attributed to the interaction between KGM and CS which strengthened with the increasing of konjac glucomanan loading. 3.1.4. Thermal analysis Thermal stability analysis of polymer material is helpful in the selection of materials which usually used in PV technique. Fig. 4 shows the detailed thermo-gravimetric data for the CS/KGM blending membranes. M-K, M-C, M-1 and M-2 had onset weight loss peaks at 73.3, 87.9, 75.8 and 81.6 8C, which could be assigned to the loss of adsorbed water from the membranes [27]. It was found that the onset temperatures of releasing of water molecules from the blending membranes were higher than KGM, but lower than CS. With the increase of the temperature, KGM and CS began the step of maximum weight loss at 280.7 and 248.9 8C while that of M-1, M-2 was shown at 223.9, 230.7 8C respectively. These degradation peaks may be due to the decomposition of KGM and
Fig. 4. DTG thermograms of the pure CS, KGM and KGM/CS blending membranes.
CS. The results indicated that the incorporation of copolymer altered the thermal decomposition, which decreased the thermostability of the natural polymer. So it is believed that CS/KGM membranes were mainly incorporated [28]. 3.1.5. DSC analysis The DSC curves of the CS, KGM and blending membranes (Fig. 5) were used to determine the compatibility of the blending by means of testing the endothermic peaks. In the CS curve, the main feature was an endothermic peak at 245.6 8C. The presence of this peak which evident in the DSC traces was essentially characterized the thermal behavior of chitosan. The presence of this peak was presumably resulted from the thermal decomposition of the glycosidic bonds of chitosan [24]. Similarly, KGM revealed the decomposition peak at 276.1 8C. The DSC curve of the CS/KGM blending membranes only exhibited one decomposition peak which shifted to a lower temperature than the peak of pure CS and KGM. In addition, with the increased CS in the blending membranes the decomposition temperature was nearly to the pure polymer curve. These results showed that the thermal stability of CS/KGM blending membranes decreased a little compared with the original CS and KGM. The interaction of two polysaccharides could disrupt their crystalline structures. The changes of number and location of the endothermic peaks in DSC curve indicated that konjac glucomannan and chitosan form a hydrogen-bonding interaction between the blending membranes. 3.2. Pervaporation characteristics 3.2.1. Swelling experiments Fig. 6 shows the degree of swelling which was obtained by soaking the CS/KGM blending membranes in the different CPL concentration solution at 40 8C. The transport of permeating molecules was controlled partially by sorption mechanism. Therefore, to study the effects of KGM on membrane swelling, the degree of swelling was plotted with different temperatures, ranging from 30 8C to 50 8C, in CPL 70 wt.% feed for 72 h, as shown in Fig. 6. It was noticed that the degree of swelling of the membranes decreased when the CPL loading in the solution increased. This was due to the fact that those membranes were hydrophilic and increased the strong interaction between water molecules and the membrane containing –OH groups and NH2 groups [29]. At same concentration solution, the degree of swelling reduced when the composite membranes with more KGM, due to the interaction with CS and KGM. The van der Waals force was weaked between KGM and CS, hydrogen bonding interaction, hydrophilic interaction and electrostatic interaction played important roles in the interfacial
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W. Lin et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 934–940 Table 2 Comparison of saturated vapor pressure of water and CPL at different temperatures.
Fig. 6. Degree of swelling of the pure CS, KGM and KGM/CS blending membranes at 40 8C.
formation. Between KGM and CS there was existing plenty of hydrogen bonding in the chains. In the three kinds of molecular force, the hydrogen bonding was the most important force than the others. More reaction between CS and KGM groups reduced the activity of molecular chains, the hydrophilic and hydrogen bonding interaction [22]. After blending, three-dimensional networks formed in the KGM and CS molecules by non-covalent bonding in the molecules. The hydrogen bonding included net-in and nettran forms in KGM/CS blending membranes [27]. This means that solvent molecules were difficult to absorb which would reduce the degree of swelling of membranes sharply. 3.2.2. Effect of operating temperature Operation temperature has a significant effect on the PV performance of the blending membrane [30]. So we typically chosen M-4 on the condition of 60 wt.% of CPL in feed to study the effects of temperature, and the resulting values are presented in Fig. 7. It displayed the effects of operation temperature on the separation factors and flux of total, water and CPL, respectively. It was observed that all of flux increased significantly with the temperature increasing. As shown in Fig. 7, it was attributed to the increase in operation temperature and the partial pressure of the water and CPL in the feed side (Table 2). As a consequence, the driving force for mass transfer across the blending membrane increased [23], resulting in the increase of the flux. In addition, high temperature accelerated the diffusion of the water and CPL
Temperature (8C)
Water saturated vapor pressure (Pa)
CPL saturated vapor pressure (Pa)
40 45 50 55 60
7362 9683 12,392 15,905 20,127
27 35 45 59 75
molecules and enlarged free volume in polymer matrix, which favored the permeation of the process, thus the flux increased. However, the flux increased with the expense of the separation factor. While the total flux increased following the temperature rising, the separation factor decreased synchronously (Fig. 7). This ‘‘trade-off’’ phenomenon may be traditionally explained by the increase in frequency and amplitude of polymer chain thermally induced expansion of the free volume in membranes, and CPL molecular passed easily [31]. Increase of smaller molecules diffusivities was the other source of this behavior. In addition, with the feed temperature increasing, the vapor pressure in the feed compartment increased (Table 2), but there was no increasing in vapor pressure at the permeate side. It resulted in an increasing of driving force with increasing temperature. Therefore, the permeation of diffusing molecules and the associated molecules through the membrane became easier, leading to an increasing of total permeation flux, while suppressing the separation factor. 3.2.3. Activation energy analysis Temperature dependent flux data have been fitted to the Arrhenius relationship to estimate the activation parameters. The Arrhenius relationship is expressed in Eq. (4) [22]. E p J ¼ J0 exp RT
(4)
J, Ep, J0, are permeation flux, activation energy for permeation, permeation rate constant, respectively. T is the absolute temperature and R is the molar gas constant. The activation energy (DEa) of water and CPL permeates through M-2 with 40 wt.% and 50 wt.% CPL concentrations were calculated on the basis of the Arrhenius formula as shown in Fig. 8, and their results are summarized in Table 3. It suggested that water flux has controlled over total flux and activation energies of total flux and water permeations were
Fig. 7. Effects of the operating temperature on the pervaporation performances in terms of water and CPL flux, total flux and separation factor through M-4 blending membranes at 60 wt.% CPL aqueous solution.
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Fig. 8. Temperature dependence of (a) ln[water and CPL flux]; (b) ln[total flux] for 40 wt.% and 50 wt.% CPL concentration for M-2.
Fig. 9. Effect of the feed concentration on the pervaporation performances of M-1 at 60 8C.
nearly in the same membrane at the same feed concentration. In addition, the Ew were obviously lower for the ECPL in this study. It implied that water consumed less amount of energy for permeation and had higher permeability than CPL. This fact proved that KGM/CS blending membranes were suitable to be used for PV separation CPL aqueous solution [23]. 3.2.4. Effect of feed composition Feed composition effect of flux and separation factor are displayed in Fig. 9. It showed the effects of feed concentration on the total permeation flux and separation factor vs. concentrations of 30–70 wt.% CPL in feed at different temperature with M-1 membranes. It can be seen that the flux of M-1 increased from 792 to 1327 g/(m2 h) for feeds containing 30–70 wt.%. It was easily noticeable to see the superiority of flux in lower CPL feed concentration and superiority separation factor in higher CPL feed concentration. These results indicated that flux and separation factor were strongly dependent on the feed composition. Those
behaviors were well agreed with our previous study, the KGM/CS blending membranes had the obvious advantage in separation factor [22,23]. The flux results increased due to the membranes swelling greatly at higher concentration of water in the feed, with the cost of separation factor. This phenomenon was explained by the amorphous regions of the KGM/CS crosslink structure was become more flexible and swollen more, both water and CPL molecules were more easily diffusion across the membranes. So the flux increased and the separation factor decreased. This was known as trade-off rule, which was generally observed in other pervaporation processes [15,23].
Table 3 Activation energy data of M-2 on pervaporation at different concentration CPL aqueous solution. CPL concentration (wt.%)
30 40 50 60 70
DEa activation energy (kJ/mol) Et
Ew
ECPL
21.17 19.71 28.34 29.56 31.52
21.14 19.68 28.31 29.53 31.49
79.45 77.99 91.11 89.10 88.40
Fig. 10. Effects of different loading of KGM in blending membranes on pervaporation performance for different temperatures in 70 wt.% CPL concentration in feed solution.
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4. Conclusions
Fig. 11. Effects of different loading of KGM in blending membranes on pervaporation performance for different CPL concentration at 50 8C.
3.2.5. Effect of KGM loading Figs. 10 and 11 show the effects of KGM loading in blending membranes on permeate flux and separation factor for 70 wt.% CPL in feed at different temperatures and 50 8C at different CPL concentration. The KGM content had significant effects on both permeation flux and separation factor. When the KGM loading increased, the permeation flux increased which was consistent with the changing trend of crystallinity. Another important factor to affect the separation flux was the swelling behavior of the membranes in the feed mixture. The free volume and the swelling behavior of the membranes increased with the addition of KGM. Whereas the KGM:CS ratio increased from 1:4 to 1:2, the separation factor had nearly no change, and then dropped a few when the ratio reached 1:1. This separation factor variable trend was due to the membranes’ internal structure. Introduction of KGM could significantly enhanced the hydrophilicity of the membranes as it had strong hydrophilic groups and disturbed CS regularity structure. But when the exceeding swell of membranes and the increase of free volume reached a certain level, the permeation of CPL could be accelerated and the separation factor could be dropped, which was confirmed by the XRD measurement [32]. Compared with PVA and PVA/CS membranes [22,23], it could be found that KGM/CS blending membranes had the near flux and higher separation factor under the same condition. The membranes with more KGM reacting with CS, due to the fewer active groups of unit mass and denser network structure, had lower total flux but more excellent separation factor. When KGM and CS were blended, their molecule structures were disturbed. The active groups in polysaccharides formed three-dimensional networks. Due to that KGM had high molecular weight, characterized gel behavior, and formed stable inter chains with CS [27]. So the membranes of M-1 had the best flux and M-4 had the best separation factor. These results agree well with the test of the degree of swelling. It showed that the flux and separation factor could reach a balance state by the modification loading of KGM.
KGM/CS blending membranes were prepared and pervaporation dehydration experiments were conducted. The introducing of KGM affected the pervaporation performance of the composite membranes. FTIR spectra and XRD curves showed the changes of structure, and SEM displayed the morphology of the composite membranes. The results of TGA and DSC indicated that the incorporation of KGM altered the thermal decomposition of CS matrix, which decreased the thermo-stability. With increasing the KGM proportion, the permeation flux increased with a drop of separation factor. With increasing of both temperature and feed water concentration, the total flux increased and the separation factor decreased. Swelling results followed those trends. Meanwhile, it was successful for the application of KGM/CS blending membranes in CPL/water system on pervaporation. The data showed the improved separation factor for CPL/water compared with the former PVA and PVA-CS membranes. The experimental results also indicated that the KGM/CS blending membranes had superior dehydration performances for caprolactam solution. References [1] W.W. Ding, Y.T. Wu, X.Y. Tang, L. Yuan, Z.Y. Xiao, J. Chem. Technol. Biotechnol. 86 (2011) 82. [2] B.K. Hur, L.P. Hao, X.J. Cao, J. Ind. Eng. Chem. 14 (2008) 639. [3] I.S. Moon, M. Matheswaran, T.O. Kwon, J.W. Kim, J. Ind. Eng. Chem. 13 (2007) 965. [4] P. Garg, R.P. Singh, V. Choudhary, Sep. Purif. Technol. 76 (2011) 407. [5] I.C. Kim, Y.H. Ka, J.Y. Park, K.H. Lee, J. Ind. Eng. Chem. 10 (2004) 115. [6] Y.I. Park, C.K. Yeom, S.H. Lee, B.S. Kim, J.M. Lee, H.J. Joo, J. Ind. Eng. Chem. 13 (2007) 272. [7] S. Araki, Y. Kiyohara, S. Imasaka, S. Tanaka, Y. Miyake, Desalination 266 (2011) 46. [8] S. Khoonsap, S. Amnuaypanich, J. Membr. Sci. 367 (2011) 182. [9] J.W. Rhim, D.S. Kim, D.H. Kim, B.S. Lee, G.Y. Moon, H.K. Lee, N.S. Yong, J. Ind. Eng. Chem. 15 (2009) 393. [10] S. Maji, S. Banerjee, J. Membr. Sci. 360 (2010) 380. [11] Y.T. Ong, A.L. Ahmad, S.H.S. Zein, S.H. Tan, Braz. J. Chem. Eng. 27 (2010) 227. [12] S.Y. Cho, T.Y. Kim, S.J. Kim, J.H. Yang, J. Ind. Eng. Chem. 10 (2004) 201. [13] C.H. Kim, K.S. Choi, J. Ind. Eng. Chem. 8 (2002) 71. [14] M.B. Patil, T.M. Aminabhavi, Sep. Purif. Technol. 62 (2008) 128. [15] M. Nawawi, M. Ghazali, R.Y.M. Huang, J. Membr. Sci. 124 (1997) 53. [16] M.A.K. Williams, T.J. Foster, D.R. Martin, I.T. Norton, M. Yoshimura, K. Nishinari, Biomacromolecules 1 (2000) 440. [17] C.L. Xu, S. Willfor, B. Holmbom, Bioresources 3 (2008) 713. [18] D.T. Tian, H.Q. Xie, Polym. Bull. 61 (2008) 277. [19] K.S. Mikkonen, M.P. Yadav, P. Cooke, S. Willfor, K.B. Hicks, M. Tenkanen, Bioresources 3 (2008) 178. [20] J. Lu, J.H. Zhang, C. Xiao, J. Appl. Polym. Sci. 106 (2007) 1972. [21] H.Q. Yu, A. Huang, C.B. Xiao, J. Appl. Polym. Sci. 100 (2006) 1561. [22] L. Zhang, P. Yu, Y.B. Luo, J. Membr. Sci. 306 (2007) 93. [23] Q. Li, P. Yu, T.R. Zhu, L. Zhang, Q. Li, Y.B. Luo, Desalination 16 (2010) 304. [24] X. Ye, J.R. Kennedy, B. Li, B.J. Xie, Carbohydr. Polym. 64 (2006) 532. [25] R.J. Samuels, J. Polym. Sci.: Polym. Phys. 19 (1981) 1081. [26] B. Focher, M. Dave, E.Marsano Tamagno, Macromolecules 28 (1995) 3531. [27] G. Zhou, Y.B. Li, L. Zhang, H. Li, M.B. Wang, L. Cheng, Y.Y. Wang, H.N. Wang, P.J. Shi, J. Mater. Sci. 42 (2007) 2591. [28] G. Zhou, Y.B. Li, L. Zhang, Y. Zuo, A. John, Jansen, J. Biomed. Mater. Res. A 83 (2007) 931. [29] G. Dhanuja, B. Smitha, S. Sridhar, Sep. Purif. Technol. 44 (2005) 130. [30] L.K. Pandey, C. Saxena, V. Dubey, J. Membr. Sci. 227 (2003) 173. [31] S.S. Kulkarni, S.M. Tambe, A.A. Kittur, M.Y. Kariduraganavar, J. Appl. Polym. Sci. 99 (2006) 1380. [32] T.R. Zhu, Y.B. Luo, Y.W. Lin, Q. Li, P. Yu, M. Zeng, Sep. Purif. Technol. 74 (2010) 242.