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Modification of crosslinked chitosan membrane using NaY zeolite for pervaporation separation of water–isopropanol mixtures
Accepted Manuscript Title: Modification of Crosslinked Chitosan Membrane Using NaY Zeolite for Pervaporation Separation of Water–Isopropanol Mixtures Author: H.G. Premakshi K. Ramesh Mahadevappa Y. Kariduraganavar PII: DOI: Reference:
S0263-8762(14)00508-5 http://dx.doi.org/doi:10.1016/j.cherd.2014.11.014 CHERD 1745
To appear in: Received date: Revised date: Accepted date:
16-7-2014 16-11-2014 26-11-2014
Please cite this article as: Premakshi, H.G., Ramesh, K., Kariduraganavar, M.Y.,Modification of Crosslinked Chitosan Membrane Using NaY Zeolite for Pervaporation Separation of WaterndashIsopropanol Mixtures, Chemical Engineering Research and Design (2014), http://dx.doi.org/10.1016/j.cherd.2014.11.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Graphical abstract The SEM micrographs of NaY zeolite (40 mass%) incorporated crosslinked chitosan
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an
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membrane: (A) Surface view and (B) Cross-sectional view.
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Highlights Membrane with 40 mass% of NaY zeolite showed an excellent PV performance.
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Modified crosslinked chitosan membranes overcome the trade-off phenomenon.
Membranes have lower Epw compared to EpIPA, showing higher separation efficiency.
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Membranes exhibit positive Hs values, involving Henry’s mode of sorption.
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Modification of Crosslinked Chitosan Membrane Using NaY Zeolite for Pervaporation Separation of Water–Isopropanol Mixtures a
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Premakshi H. Ga., Ramesh K.b and Mahadevappa Y. Kariduraganavara,* Department of Chemistry, Karnatak University, Dharwad-580 003, India b
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Indian Institute of Science, Bangalore-560 012, India
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ABSTRACT
The blocked diisocyanate crosslinked chitosan membrane was modified by incorporating
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different mass% of NaY zeolite. The physico-chemical properties of resulting composite membranes were studied using Fourier transform infrared spectroscopy (FTIR), wide-
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angle X-ray diffraction (WAXD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The mechanical
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properties of the membranes were studied using universal testing machine (UTM). After
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measuring the equilibrium swelling, membranes were subjected to pervaporation for
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separation of water-isopropanol mixtures. Both flux and selectivity were increased with increasing NaY zeolite content in the membranes. The membrane containing 40 mass% of NaY zeolite exhibited the highest separation selectivity of 11,241 with a flux of 11.37 x 10-2 kg/m2h for 10 mass% of water in the feed. The total flux and flux of water are almost overlapping each other, suggesting that these membranes could be effectively used to break the azeotropic point of water-isopropanol mixture. From the temperature dependent diffusion and permeation values, the Arrhenius activation parameters were estimated. All the composite membranes exhibited lower activation energy compared to crosslinked membrane, indicating that the permeants require less energy during the process because of molecular sieving action attributed to the presence of sodalite and 3
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super cages in the framework of NaY zeolite. The Henry’s mode of sorption dominates the process, giving an endothermic contribution. Keywords: Chitosan; NaY zeolite; Pervaporation; Selectivity; Activation energy.
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*Corresponding author: Dr. M. Y. Kariduraganavar ([email protected])
Introduction
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1.
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Fax: +91-836-2771275; Phone: +91-836-2215286 (Extn. 23).
To remain competitive in the market, industries are investing a huge capita to improve
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the quality of the product and reduce the cost of production. Pervaporation (PV) is one such type of industries with a wide range of uses such as solvent dehydration, azeotropic
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separation and solvent recovery (Schafer and Crespo, 2005; Baker, 2004). It is a membrane based process, in which one side of the membrane is in contact with the feed
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mixture while the other side of the membrane is with a carrier gas or vacuum (Bruschke,
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2001; Winston Ho, 1992; Huang, 1991). This separation process offers many advantages
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such as high selectivity, low energy consumptions, moderate cost to performance ratio, compact and modular design (Baker, 2004). Polymeric membranes are widely used in PV process due to their flexibility in
design and low cost (Qiao et al., 2006). Membrane stability, high flux and selectivity, low production costs are always most important criteria for developing membranes for specific applications. Multi-layer composite membranes were successfully developed with a crosslinked poly(vinyl alcohol) or cellulose acetate thin film on the top of poly(acrylonitrile) or poly(sulfone) (Bruschke, 1983). Unfortunately, these membranes failed to perform in a harsh condition owing to their instability and high swelling nature. On the contrary, composite membranes exhibit better structural stability, excellent 4
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chemical resistance and withstand to higher operating temperature (Shah et al., 2000; Verkerk et al., 2001). Particularly, composite membranes made with zeolite demonstrated high flux and good selectivity owing to their unique molecular sieving action and high
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adsorption (Bowen et al., 2004). Recently, Shah et al. (2000), Kita et al. (1995) and Jafari, et al. (2013) developed zeolite based composite membranes and they all exhibited
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impressive flux and high separation factor, and have become superior to traditional
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polymeric membranes. But, the fabrication of pure zeolite membrane without any defects is extremely difficult and expensive as well. In order to overcome this problem, mixed
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matrix membranes were developed by the incorporation of inorganic adsorbent such as zeolite into a polymer matrix (Kittur et al., 2005; Varghese et al., 2009; Kulprathipanja et
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al., 1988; Hua et al., 2014; Shi et al., 2012). These endeavors produced promising results
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in the separation of water from isopropanol. Unfortunately, these membranes often fail to
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exhibit good physical and mechanical properties. Therefore, crosslinking method is the most convenient and effective approach to improve the physical and mechanical
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properties of the membrane (Lee et al., 1997; Jegal and Lee, 1999; Goissis et al., 1999; Wei et al., 1992; Schmidt and Baier, 2000; Rachipudi et al., 2013; Jin et al., 2004; Kuboe et al., 2004; Kulkarni et al., 2004; Ghazali et al., 1997; Devi et al., 2005; Choudhari et al., 2007). Various crosslinking agents including sulphuric acid (Lee et al., 1997), sulfosuccinic acid (Jegal and Lee, 1999), gluteraldehyde (Goissis et al., 1999), epoxy compound (Wei et al., 1992), dialdehyde starch (Schmidt and Baier, 2000), silane (Rachipudi et al., 2013), non-toxic nature agents (Jin et al., 2004; Kuboe et al., 2004), tetraethylorthosilicate (Kulkarni et al., 2004) and diisocyanates (Ghazali et al., 1997; Devi et al., 2005; Choudhari et al., 2007)
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have been employed as crosslinker for
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chitosan. Among these, diisocyanates are the important class of crosslinking reagents for chitosan, as isocyanates possess high reactivity towards amine as well as hydroxyl groups of chitosan. Choudhari et al. (2007) reported that diisocyanate crosslinked chitosan
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membrane had shown a good separation performance for the selective removal of water from isopropanol. The membrane containing 40 mass% of diisocyanate demonstrated the
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highest separation selectivity of 5,918 at 5 mass% of water in the feed. Unfortunately, the
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performance of the same membrane was very poor in yielding the flux. The best alternative for the improvement of this membrane is the incorporation of zeolite. Among
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the zeolites, NaY zeolite is the best choice, considering its unique properties such as high surface area (up to 1000 m2/g), high void volume (30% of the total volume of zeolite),
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uniform pore size distribution, presence of sodalite and super cages in the framework. In
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view of this, it is also being used widely in chemical and physical processes such as
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shape selective catalysis and separation media (Jia et al., 1991). The incorporation of such zeolite or porous material in the crosslinked membrane can improve the separation
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performance of pervaporation (Jia et al., 1991; te Hennepe et al., 1987; Kim et al., 2001). In addition, it is demonstrated that zeolite incorporated membranes have a high mechanical strength, good thermal and chemical stability, and thus the membranes incorporated with zeolites can be used over a wide range of operating conditions. For instance, Gao et al. (1996), and Chen et al. (2001) studied the pervaporation separation of hydrophilic zeolite-filled PVA and chitosan membranes for organic–water system. Similarly, our research group also reported hydrophobic zeolite-filled PVA membranes (Kittur et al., 2003), hydrophilic zeolite-filled sodium alginate and chitosan membranes (Kariduraganavar et al., 2004; Kittur et al., 2005) for the separation of water–isopropanol
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mixtures. In case of sodium alginate and chitosan membranes, both separation factor and flux were increased simultaneously, although it was uncommon in PV process. Understanding the pros and cons mentioned above, we have made an attempt to
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modify the blocked diisocyanate crosslinked chitosan membrane developed previously
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(Choudhari et al., 2007) by incorporating the NaY zeolite in different mass%. Before subjecting to PV study, the physicochemical changes in the resulting membranes were
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investigated using different techniques such as FTIR, WAXD, DSC, TGA and SEM. The mechanical properties of the membranes were also studied using UTM and correlated
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with the structure. The values of permeation flux, separation selectivity and diffusion
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coefficients were evaluated. From the temperature dependence of permeation flux and
Experimental section
2.1.
Materials
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2.
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diffusion coefficients, the Arrhenius activation parameters were estimated.
Chitosan ( Mw ~ 200,000; N-deacetylation degree 75-85%) and hexamethylene diisocyanate were purchased from Sigma-Aldrich Chemicals, USA. Isopropanol, acetic acid, sodium metabisulfite and acetone were procured from s. d. fine Chemicals Ltd., Mumbai,
India.
Hexamethylene
1,6-di(aminocarboxysulfonate)
was
synthesized
according to the procedure reported by Choudhari et al. (2007). NaY zeolite was supplied by Indian Petrochemicals Corp., Baroda, India. The characteristic properties of NaY zeolite are given in Table 1 (Venkalcom et al., 1997a; Venkalcom et al., 1997b). All the chemicals were of analytical grade and used without further purification. Deionized water was used throughout the study.
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2.2.
Membrane preparation
Chitosan (3 g) was dissolved in 100 ml of deionized water containing 2% of acetic acid and stirred for about 24 h at room temperature. The solution was filtered to remove the
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undissolved residue particles. It was then kept in an ultrasonic bath at a fixed frequency
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of 38 kHz (Grant XB6, UK) for 30 min to break the possible aggregated molecules. To this solution, 40 mass% of blocked diisocyanate as a crosslinker was added and stirred
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for 2 h at room temperature. To remove the air bubbles, the solution was left overnight. The resulting homogeneous solution was spread onto a clean glass plate. After being
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dried in air for about 48 h, the membrane was peeled-off and subsequently annealed at 60 o
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C for 24 h in an inert atmosphere and was designated as M.
To prepare composite chitosan membranes, a known amount of NaY zeolite was
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added into the above homogeneous solution. The amount of chitosan and crosslinker was
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kept constant for each membrane. To break the aggregated zeolite particles and to improve their dispersion in the polymer matrix, the mixed solution was stirred for about
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24 h and then subjected to sonication for about 30 min at a fixed frequency of 38 kHz. The resulting solution was poured onto a clean glass plate and the membrane was dried as mentioned above. The amount of zeolite particles with respect to chitosan was varied as 10, 20, 30, and 40 mass%, and the membranes thus obtained were designated as M-1, M2, M-3, and M-4, respectively. An attempt was also made to incorporate higher content of zeolite, but even for 45 mass% of zeolite, the membrane became brittle and lost the membrane property. Hence, the loading of zeolite content was restricted up to 40 mass%. The chemical structure and reaction route of the composite membranes are illustrated in Fig. 1. The thickness of these membranes was measured at different points using a
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Peacock dial thickness gauge (Model G, Ozaki Mfg. Co. Ltd., Japan) with an accuracy of ± 2 µm and the average thickness was considered for the calculation. Thickness of these
2.3.
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membranes was found to be 40 ± 2 µm. Fourier transform infrared spectroscopy
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The analyses of crosslinked chitosan and its NaY zeolite incorporated composite
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membranes were carried out using FTIR spectroscopy (Nicolet, Impact-410, USA). The polymer membranes were ground well with KBr under a hydraulic pressure of 400
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kg/cm2. The spectra were recorded in the frequency range of 400 to 4000 cm-1. In order to estimate the changes in the intensities of the characteristic peaks with respect to the
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amount of NaY zeolite in each scan, the amount of membrane sample and KBr were kept
Wide-angle X-ray diffraction
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2.4.
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constant.
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To know the level of dispersion of NaY zeolite in the crosslinked polymer matrix and to understand the extent of crystallinity, X-ray diffraction measurements were performed using a Brucker’s D-8 advanced wide-angle X-ray diffractometry. The X-ray source was Ni-filtered Cu Kα radiation (40 kV, 30 mA). The dried samples of uniform thickness (40 ± 2 μm) were mounted on a sample holder. The scanning angle (2θ) ranged from 5 to 50° with a scanning rate of 8°/min. 2.5.
Differential scanning calorimetry
The effect of NaY zeolite on the glass transition temperature (Tg) of the crosslinked chitosan membrane was studied using a differential scanning calorimetry (Perkin-Elmer
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DSC) at a temperature range from ambient to 400 oC under nitrogen atmosphere. In each measurement, 8-10 mg sample was used with heating rate of 10 oC/min. Thermogravimetric analysis
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2.6.
Thermal stability and decomposition temperature of the membrane samples were carried
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out using a thermogravimetric analysis (DSC Q 20, TA Instruments, Waters LLC, USA)
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at a heating rate of 10 °C/min under nitrogen atmosphere. The weight of the samples
2.7.
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taken for each record was 8-10 mg. Scanning electron microscopy
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The surface and cross-sectional views of the crosslinked chitosan and its NaY zeolite incorporated composite membranes were investigated using a scanning electron
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microscope (JEOL, JSM-400 Å, Tokyo, Japan). Before photographing, all the samples
Tensile properties
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2.8.
te
were vacuum dried and coated with a conductive layer (400 Å) of sputtered gold.
The mechanical properties such as tensile strength and percent elongation at break of the crosslinked chitosan and its NaY zeolite incorporated composite membranes were measured at 25 oC using universal testing machine (Hounsfield H10KS) with a speed of 50 mm/min. The gauge dimension of the test sample was 25 mm x 50 mm. For each sample, three specimens were tested and results were averaged. 2.8.
Swelling measurement
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The sorption properties of the crosslinked chitosan and its composite membranes were carried out at 30 °C. The dried membrane samples were first weighed and then they were immersed in different compositions of water-IPA mixtures in sealed vessels for 24 h. The
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swollen membrane samples were taken out, gently blotted with tissue paper and weighed
membranes was calculated using the following equation:
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100
(1)
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W Wd DS (%) s Wd
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as quickly as possible in a closed container. The percent degree of sorption (DS) of the
2.9.
Pervaporation experiments
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where Ws and Wd are the masses of the swollen and dry membranes, respectively.
The pervaporation cell was assembled from two cylindrical half-cells made of stainless
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steel fastened together by nuts and bolts. The membrane was supported on a perforated stainless steel plate placed at the junction of two cells. The feed temperature was
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maintained by circulating water through the cell jacket. The capacity of the feed cell was about 250 cm3 and the effective surface area of the membrane in contact with the feed mixture was 34.23 cm2. For all the measurements, the downstream pressure was maintained at 1.333224 x 103 Pa (10 Torr) using a two-stage vacuum pump (Toshniwal, Chennai, India). The water composition in the feed mixture was varied from 5 to 25 mass%. Before performing the PV experiments, the test membrane was allowed for swelling to attain equilibrium with a known volume of feed mixture for about 2 h in the feed compartment. The permeate vapor was collected in a glass trap suspended inside the liquid nitrogen jar. The experiments were carried out at 30, 40 and 50 °C. The flux was calculated by weighing the permeate and the compositions of water and isopropanol were 11
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estimated by measuring the refractive index of the permeate within an accuracy of ± 0.0001 units using an Abbe’s refractometer (Atago-3T, Japan). All the pervaporation runs were repeated thrice and the results were averaged. The results of permeation for water-
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IPA mixtures during the pervaporation were reproducible within an admissible range.
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From the PV data, separation performance of the membranes was estimated in
pervaporation run using Eq. (2): W A.t
(2)
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J
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terms of total flux (J) and it can be calculated from the weight of permeate collected after
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where W is the total weight of permeate (kg) collected in time t (h) and A is the effective membrane area (m2). The separation selectivity (αsep) was calculated using the following
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equation:
Pw / PIPA Fw / FIPA
(3)
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sep
where Pw and PIPA are the weight percent of water and isopropanol in the permeate, respectively. Fw and FIPA are the respective weight percent of water and isopropanol in the feed. 3. 3.1.
Results and discussion
Membrane characterization
3.1.1. FTIR studies The FTIR spectra of the crosslinked chitosan and its composite membranes are shown in Fig. 2. A broad characteristic band was observed at around 3450 cm-1 in all the
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membranes, which corresponds to O–H stretching vibrations of the hydroxyl groups. The small bands appeared at 1650 and 1570 cm−1 were respectively assigned to amide I and amide II bands of chitosan. The multiple bands appeared between 1000 and 1200 cm-1 in
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membranes were assigned to C–O stretching vibrations. All these bands are in good agreement with the data reported by our group (Choudhari et al., 2007). However, the
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intensity of these bands was marginally increased as the content of zeolite was increased
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in the crosslinked chitosan matrix. This suggests that the hydrophilicity of the membranes was increased owing to hydrophilic nature and molecular sieving action attributed to the
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presence of sodalite and super cages in the framework of NaY zeolite.
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3.1.2. WAXD studies
Fig. 3 shows the diffraction patterns of crosslinked chitosan and its zeolite incorporated
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composite membranes. The crosslinked chitosan membrane exhibited two sharp peaks at
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around 2θ = 14 and 17o, and a broad peak around 21o, which are respectively attributed to
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the presence of crystalline and amorphous region in the polymer. After the incorporation of NaY zeolite content in the crosslinked membrane, the intensity of these peaks diminishes in the diffraction patterns. However, as the content of zeolite was increased, these patterns were shifted gradually towards the zeolite structure, showing more reflection peaks of zeolite in the diffraction patterns. This clearly indicates that crystallinity of the crosslinked chitosan membrane was changed to amorphous domain. 3.1.3. DSC studies The differential scanning calorimetric study was carried out for crosslinked chitosan and its NaY zeolite incorporated composite membranes. The transition patterns of membranes
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were recorded in the temperature range of ambient to 400 oC and the resulting thermograms are presented in Fig. 4. A special care was taken while recording owing to their tendency to absorb moisture, which can strongly affect the transitions. From the
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patterns, it is noticed that the Tg of the crosslinked chitosan membrane was around 98 oC. Upon incorporating different amounts of NaY zeolite in the membranes, the Tg of the
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membranes did not change appreciably. This is expected, because the added zeolite only
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fills the free space available in the membrane matrix. However, the added zeolite significantly changed the melting temperature of the membranes, which was respectively
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ranged from 285 to 310 oC for the membranes M to M-4. The shift in the melting temperature from membrane M to M-4 is due to an electrostatic interaction that was
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occurred between zeolite and the crosslinked chitosan.
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3.1.4. TGA studies
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Thermal stability and degradation behavior of the crosslinked chitosan and NaY zeolite From the patterns, it is
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incorporated composite membranes are depicted in Fig. 5.
observed that irrespective of the zeolite loading, membranes underwent weight loss in two stages. The first stage of weight loss occurred from ambient to 150 oC, which corresponds to around 16 to 20 weight%. This loss attributes to the dehydration of membranes. This is expected due to increased hydrophilic character of the membrane by the addition of zeolite. The second stage of decomposition starts around 240 to 510 oC and this is accounted for 39 - 52%, attributing to a weight loss due to degradation of polymer network. When we looked into the individual patterns of all the membranes, it is observed that the weight loss was significantly decreased from membrane M to M-4. This is expected owing to the zeolite and its content in the membrane. Further, it is observed
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that as the content of zeolite was increased in the membranes, the decomposition temperature was marginally shifted from 240 to 280 oC. This is because of electrostatic interaction occurred between NaY zeolite and the chitosan. All these observations
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suggest that the added zeolite enhances the thermal stability of the membranes.
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3.1.5. SEM studies
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Fig. 6 (A and B) illustrates the surface morphology of the crosslinked and its composite membranes. From the surface micrographs (Fig. 6A), it is clear that all the membranes
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have a smooth, compact and homogeneous surface. This indicates that NaY zeolite was distributed evenly throughout the membrane matrix with no apparent clusterings.
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However, some of the zeolite particles are pulled out of the matrix, but their ends are embedded in the chitosan matrix. This suggested that there is an interfacial adhesion
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between chitosan and NaY zeolite. On the contrary, the cross-sectional views (Fig. 6B) of
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the membranes showed some cavities and are prominent at higher concentration of
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zeolite. This is very well supported from the WAXD patterns and FTIR stretchings. 3.1.6. Mechanical properties
Tensile strength and percent elongation of a membrane often tell its suitability for PV applications. The effect of zeolite on the tensile strength and percent elongation of the crosslinked membranes were studied and the data thus obtained are summarized in Table 2. It could be seen from the data that tensile strength of the membrane was increased marginally with increasing the zeolite content from membrane M to M-3. This is due to a uniform distribution of zeolite particles in the crosslinked membrane matrix. However, when the content of zeolite was increased to 40 mass% there was an abrupt change in the
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tensile strength (M-4). This may be because of dominant role of zeolite. On the contrary, the percent elongation at break was gradually decreased from membrane M to M-3. Again the effect of this was more pronounced for membrane M-4. Despite of the fact, the
Effects of feed composition and NaY zeolite content on membrane swelling
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3.2.
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resulting membranes are suitable for the PV separation.
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To study the effects of feed composition and NaY zeolite loading on membrane swelling, the percent degree of swelling of all membranes was plotted with respect to different
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mass% of water in the feed at 30 C as shown in Fig. 7. It is evident from the plot that the degree of swelling was increased almost linearly for all the membranes with increasing
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the mass% of water in the feed. This is attributed to a strong interaction occurring between water molecules and the membrane containing functional groups such as -OH
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and -NH2, and Na+ ions present in the zeolite cages. On the other hand, the degree of
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swelling was enhanced greatly with increasing the zeolite loading in the membranes (M-1
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to M-4). This is due to increased hydrophilic nature of the membrane owing to the presence of Na+ ions in the sodalite and supercages in the framework of NaY zeolite which tend to enhance the electrostatic force of attraction between water molecules and the membrane and thereby adsorption of water molecules increases resulting to a greater degree of swelling than that of crosslinked membrane. 3.3.
Effects of feed composition and the content of NaY zeolite on pervaporation
In order to study the effects of feed composition and the content of NaY zeolite on PV, the total permeation flux was plotted for all the membranes as a function of different mass% of water in the feed (Fig. 8). It is observed that the total permeation flux was
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increased for all the membranes with increasing water composition in the feed. This is attributed to an increased selective interaction between water molecules and the composite membranes. Similarly, the total permeation flux was increased with increasing
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the zeolite loading in the membranes. This trend remains same for all the membranes throughout the investigated feed compositions. This is expected owing to a combined
These are together responsible for increasing the
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(Kariduraganavar et al., 2004).
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influence of ionic species (Na+) present in the zeolite cages and porous structure
permeation flux with the zeolite loading.
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The overall selectivity of a membrane in PV process is generally described on the
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basis of interaction between membrane and the permeating molecules, molecular size of the permeating species and pore diameter of the membrane. Fig. 9 shows the effect of
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water composition on the selectivity of all membranes. It is observed that the selectivity
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was decreased drastically for all the membranes with increasing the water composition in the feed. This effect is more predominant particularly for the membranes having higher
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loading of zeolite. This is because, at higher composition of water, the membrane swells greatly due to an establishment of strong interaction between the membrane and water molecules, and thereby suppressing the interaction between NaY zeolite and chitosan. This in turn may be the reason for the development of very minute narrow openings at the interface between the NaY zeolite and chitosan moiety (Chen et al., 2001). This allows the IPA molecules to pass through the membrane along with the selective water molecules. As a result, the selectivity underwent a drastic decrease at higher concentration of water in the feed. However, the selectivity was greatly enhanced for all the membranes with increasing the zeolite loading. It is more pronounced at lower water
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composition (5 and 10 mass%). This is mainly due to a greater interaction between NaY zeolite and crosslinked chitosan matrix owing to the presence of Na+ ions in the framework of sodalite and supercages. However, at higher water composition in the feed,
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the interaction between water molecules and membrane matrix becomes predominant and suppresses the interaction between NaY zeolite and chitosan, which becomes responsible
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for an abrupt decrease in selectivity. This can be clearly observed from Fig. 10, wherein
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both the flux and selectivity were plotted as a function of NaY zeolite content in the membrane at 10 mass% of water in the feed.
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Generally, with increasing the packing density in the polymer matrix, permeation
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flux decreases and selectivity increases (Kittur et al., 2003; Kurkuri et al., 2002). However, in the present study both permeation flux and selectivity were increased
d
simultaneously with increasing the NaY zeolite content in the membrane. Although, this
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is in contrast to a trade-off phenomenon existing between flux and selectivity in PV process, a significant enhancement of hydrophilicity, selective adsorption and
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establishment of molecular sieving action overcome the situation by the incorporation of porous NaY zeolite in the polymer matrix. This can be further explained in such a way that when we use water selective NaY zeolite incorporated composite membranes, the transport of water molecules through the membrane occurs in a straight path through the NaY zeolite pores with subsequent adsorption at the feed side followed by desorption at the permeate side, which in turn are responsible for higher water flux. If sufficient water is available inside the membrane, then the NaY zeolite pores will be largely occupied by water molecules, preventing the IPA molecules entering into the NaY zeolite pores. Thus, on their way through the membrane the IPA molecules have to move around the
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NaY zeolite cages. The higher water concentration inside the membrane close to the permeate side of the membrane and the fact that water can travel along the straight path whereas IPA has to follow a more tortuous path, in which the membrane can act together
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in explaining the way in which membrane performance is enhanced, both in terms of flux and selectivity when NaY zeolite are incorporated into the membrane matrix (te Hennepe
cr
et al., 1987).
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In PV process, the efficiency of the membrane is generally assessed on the basis of permeation of individual components. Therefore, the extent of permeation of
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individual components was determined by plotting the total flux and fluxes of water and
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IPA as a function of zeolite content in the membrane for 10 mass% of water in the feed (Fig. 11). From the plot, it is clear that the total flux and flux of water are overlapping
d
each other, whereas the flux of IPA is negligibly small for all the membranes, indicating
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that the membranes developed in the present study are highly selective towards water. Further, it is concluded that membrane containing 40 mass% of NaY zeolite
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demonstrated an excellent flux of 11.37 x 10-2 kg/m2h with a selectivity of 11,241 for 10 mass% of water in the feed at 30 oC among the membranes developed in the present study. 3.4.
Diffusion coefficient
In PV process, mass transport of binary liquid molecules through a polymer matrix is generally described by the solution-diffusion mechanism with three consecutive steps: sorption, diffusion and desorption (Lee et al., 1989). The penetrants undergo sorption at the feed side of a membrane and subsequently they diffuse through the membrane. Finally, desorption of penetrant takes place in the form of vapor at the downstream side 19
Page 19 of 51
of the membrane. Thus, the solubility and diffusivity of penetrants in membrane play an important role in the membranes’ selectivity and permeation flux. In PV process, because of the establishment of fast equilibrium distribution between the bulk feed and upstream
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surface of a membrane, the diffusion step controls the transport of penetrants (Kittur et al., 2003; Hwang and Kammermeyer, 1975). It is therefore important to estimate the
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diffusion coefficient (Di) of penetrating molecules to understand the mechanism of
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transport.
(Yamasaki et al., 1994):
dC i dx
(4)
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J i Di
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From Fick’s law of diffusion, the diffusion coefficient can be expressed as
where J is the permeation flux per unit area (kg/m2s), D is the diffusion coefficient
d
(m2/s), C is the concentration of permeant in the membrane (kg/m3), subscript i stands for
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water or isopropanol, and x is the diffusion length (m). For simplicity, it is assumed that
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the concentration profile along the diffusion length is linear. Thus, diffusion coefficient Di can be calculated with the following modified equation (Kusumocahyo and Sudoh, 1999):
Di
J i Ci
(5)
where is the membrane thickness. The calculated values of Di at 30 oC are presented in Table 3. It is observed that the diffusion coefficient of water as increased from M to M-4, while decreasing the diffusion coefficient of IPA. This trend remains same for all investigated water composition. However, the magnitude of the diffusion coefficient for water is quite high in comparison with that of IPA. This further supports that the 20
Page 20 of 51
membranes developed in the present study by the incorporation of zeolite have remarkable separation ability for the separation of water from IPA. As discussed in PV study, this was attributed to increased hydrophilicity, selective adsorption and
ip t
establishment of molecular sieving action owing to the presence of sodalite and
Effect of temperature on PV performance
us
3.5.
cr
supercages in the framework of NaY zeolite.
Temperature is an important operating parameter in pervaporation process as it greatly
an
induces the sorption and diffusion, and hence it has shown significant affect on the performance of the membranes. The effect of operating temperature on the pervaporation
M
performance for water-isopropanol mixture was studied for all the membranes at 10 mass% of water in the feed and the resulting values are presented in Table 4. It is
d
observed that the permeation rate was increased from 30 to 50 oC for all the membranes,
te
while suppressing the separation selectivity. Generally, this happens because of two
Ac ce p
reasons. Firstly, as the temperature increases the viscosity of the permeating molecules decreases due to decreased cohesive forces between the permeants. Secondly, an increase of thermal energy intensifies the motion of polymer chain segments creating more freevolume in the polymer matrix. However in the present study, the later reason is ruled out since the experiments were performed well below the glass transition temperature of the crosslinked chitosan. Therefore, the viscosity of permeating molecules played a major role in allowing the associated molecules along with the selective permeants. This results to an increase in total permeation flux while decreasing the selectivity. This effect prompted us to estimate the activation energies for permeation and diffusion using the Arrhenius type equation (Huang and Yeom, 1991):
21
Page 21 of 51
Ex X X o exp RT
(6)
where X represents permeation (J), or diffusion (D). Xo is a constant representing pre-
ip t
exponential factor of Jo or Do. Ex represents the activation energy for permeation or diffusion depending upon the transport process under consideration, and RT is the usual
cr
energy term. As the feed temperature increases, the vapor pressure in the feed
us
compartment also increases, but the vapor pressure at the permeate side is not affected. This leads to an increase of driving force with increasing the temperature.
an
Arrhenius plots of log J and log D versus temperature are shown in Fig. 12 and
M
13, respectively. In both the cases, a linear behavior was observed, suggesting that permeability and diffusivity follow an Arrhenius trend. From the least-squares fits of
d
these linear plots, the activation energies for total permeation (Ep) and total diffusion (ED)
te
were estimated. Similarly, we have also estimated the activation energies for permeation of water (Epw) and isopropanol (EpIPA), but the plots are not given to avoid the crowdness.
Ac ce p
The values thus obtained are presented in Table 5. From Table 5, it is observed that activation energy values of both Ep and ED were
decreased with increasing the NaY zeolite content in the membrane (M-1 to M-4). This is expected due to increased hydrophilicity and molecular sieving action attributed to the presence of sodalite and supercages in the framework of NaY zeolite, and obviously reduces the energy required for transport of selective permeants. It can also be seen that the apparent activation energy values obtained for water permeation (Epw) are much lower than those of isopropanol (EpIPA), and this difference was increased around 5 to 12 times as the content of zeolite was increased, suggesting that membranes developed here have
22
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higher separation efficiency towards water. The activation energy values of water permeation (Epw) and total permeation (Ep) are almost close to each other, signifying that coupled-transport of both water and isopropanol is minimal due to higher selective nature
ip t
of the membranes. Further, the estimated Ep and ED values ranged between 9.18 and 18.15 kJ/mol and 8.91 and 17.81 kJ/mol, respectively. Using these values, we have
us
H s E p E D .
cr
calculated the heat of sorption as:
(7)
an
The resulting Hs values are included in Table 5. It is noticed that the Hs values obtained in the present study are positive for all the membranes, suggesting that Henry’s
Conclusions
d
4.
M
mode of sorption is predominant, giving an endothermic contribution.
te
Using a solution casting technique, crosslinked chitosan membrane was modified by
Ac ce p
incorporating different mass% of NaY zeolite and these were successfully employed for the separation of water-IPA mixtures. An increase of NaY zeolite content in the membrane results to a simultaneous increase of both permeation flux and selectivity. This was explained on the basis of significant enhancement of hydrophilicity, selective adsorption and establishment of molecular sieving action. Both total flux and flux of water are almost overlapping with each other, signifying that the modified membranes are highly selective towards water. The highest separation selectivity was found to be 11,241 with a flux of 11.37 x 10-2 kg/m2h for the membrane having 40 mass% NaY zeolite loading at 30 °C for 10 mass% of water in the feed. With regard to temperature effect, the permeation rate was found to increase while suppressing the selectivity. The
23
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modified membranes exhibited lower activation energy values compared to crosslinked membrane, indicating that the permeants require less energy during the process owing to molecular sieving action attributed to the presence of sodalite and supercages in the
ip t
framework of NaY zeolite. For all the membranes, Henry`s mode of sorption dominates
cr
the process, giving an endothermic contribution.
us
Acknowledgements
Authors are thankful to the UGC, New Delhi for providing financial assistance (Grant
an
No. 37-245/3009, SR). Authors also thank the Department of Physics, Indian Institute of
Ac ce p
te
d
M
Science, Bangalore, for extending wide-angle X-ray diffraction facility.
24
Page 24 of 51
Nomenclature molecular weight
A
effective membrane area (m2)
DS
degree of swelling (%)
Do
pre-exponential factor for diffusion
ED
activation energy for diffusion (kJ/mol)
EDw
activation energy for diffusion of water (kJ/mol)
Ep
activation energy for permeation (kJ/mol)
Epw
activation energy for permeation of water (kJ/mol)
EDIPA
activation energy for diffusion of IPA (kJ/mol)
Ex
activation energy for permeation or diffusion (kJ/mol)
∆Hs
heat of sorption (kJ/mol)
IPA
Isopropanol
J
total flux (kg/m2h)
Jo
pre-exponential factor for permeation
P and F
mass percent of permeate and feed
R
gas constant
T W
cr
us
an
M
d
te
Ac ce p
t
ip t
Mw
Ws and Wd
permeation time (h) temperature (K) mass of permeate (kg) mass of the swollen and dry membranes
Greek letters δ
membrane thickness (40 m)
sep
separation factor
25
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References Baker, R.W., 2004. Membrane technology and applications, second ed. Wiley, Inc. Menlo Park, California.
ip t
Bowen, T.C., Noble, R.D., Falconer, J.L., 2004. Fundamentals and applications of pervaporation through zeolite membranes. J. Membr. Sci. 245, 1-33.
us
the pervaporation method. German Pat. DE 3220570.
cr
Bruschke, H., 1983. Multilayered membrane and its use in separating liquid mixtures by
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Choudhari, S.K., Kittur, A.A., Kulkarni, S.S., Kariduraganavar, M.Y., 2007.
Ac ce p
Development of novel blocked diisocyanate crosslinked chitosan membranes for pervaporation separation of water-isopropanol mixture. J. Membr. Sci. 302, 197206.
Devi, D.A., Smitha, B., Sridhar, S., Aminabhavi, T.M., 2005. Pervaporation separation of isopropanol/water mixtures through crosslinked chitosan membranes. J. Membr. Sci. 262, 91-99. Ghazali, M., Nawawi, M., Huang, R.Y.M., 1997. Pervaporation dehydration of isopropanol with chitosan membranes. J. Membr. Sci. 124, 53-62. Gao, Z., Yue, Y., Li, W., 1996. Application of zeolite-filled pervaporation membranes. Zeolite 16, 70-74. 26
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Goissis, G., Marcantonio, Jr., E., Marcantonio, R.A.C., Lia, R.C.C., Cancian, D.C.J., Carvalho, W.M.D., 1999. Biocompatibility studies of anionic collagen membranes with different degree of glutaraldehyde cross-linking. Biomaterials 20, 27-34.
ip t
Hua, D., Ong, Y.K., Wang, Y., Yang, T., Chung, T.S., 2014. ZIF-90/P84 mixed matrix membranes for pervaporation dehydration of isopropanol. J. Membr. Sci. 453, 155-
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Huang, R.Y.M., 1991. Pervaporation membrane separation processes, elsevier, Amsterdam.
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Huang, R.Y.M., Yeom, C.K., 1991. Pervaporation separation of aqueous mixtures using crosslinked poly vinyl alcohol membranes. III. Permeation of acetic acid-water
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mixtures. J. Membr. Sci. 58, 33-47.
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Hwang, S.T., Kammermeyer, K., 1975. Membrane in separations, Wiley-Interscience,
Jafari, M., Bayat, A., Mohammadi, T., Kazemimoghadam, M., 2013. Dehydration of
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ethylene glycol by pervaporation using gamma alumina/NaA zeolite composite membranes. Chem. Eng. Res. Des. 91, 2412-2419.
Jegal, J., Lee, K.H., 1999. Chitosan membranes crosslinked with sulfosuccinic acid for the pervaporation separation of wáter/alcohol mixtures. J. Appl. Polym. Sci. 71, 671-675.
Jia, M., Peinemann, K.V., Behling, R.D., 1991. Molecular sieving effects of the zeolitefilled silicone rubber membrane in gas permeation. J. Membr. Sci. 57, 289-292. Jin, J., Song, M., Hourston, D.J., 2004. Novel chitosan-based films cross-linked by genipin with improved physical properties. Biomacromolecules 5,162-168.
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Kariduraganavar, M.Y., Kittur, A.A., Kulakarni, S.S., 2004. Development of novel pervaporation membranes for the separation of water-isopropanol mixtures using sodium alginate and NaY zeolite. J. Membr. Sci. 238, 165-175.
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Kim, K.J., Park, S.H., So, W.W., Moon, S.J., 2001. Pervaporation separation of aqueous organic mixtures through sulfated zirconia-poly(vinyl alcohol) membrane. J. Appl.
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Polym. Sci. 79, 1450-1455.
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Kita, H., Horii, K., Ohtoshi, Y., Tanka, K., Okamoto, K.-I., 1995. Synthesis of a zeolite NaA membrane for pervaporation of water–organic liquid mixtures, J. Mater. Sci.
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Kittur, A.A., Kariduraganavar, M.Y., Toti, U.S., Ramesh, K., Aminabhavi, T.M., 2003.
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Pervaporation separation of water-isopropanol mixtures using ZSM-5 zeolite
d
incorporated poly(vinyl alcohol) membranes. J. Appl. Polym. Sci. 90, 2441-2448.
te
Kittur, A.A., Kulkarni, S.S., Aralaguppi, M.I., Kariduraganavar, M.Y., 2005. Preparation and characterization of novel pervaporation membranes for the separation of water–
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isopropanol mixtures using chitosan and NaY zeolite. J. Membr. Sci. 247, 75–86.
Kuboe, Y., Tonegawa, H., Ohkawa, K., Yamamoto, H., 2004. Quinone cross-linked polysaccharide hybrid fiber. Biomacromolecules 5, 348-357.
Kulkarni, S.S., Kittur, A.A., Aralaguppi, M.I., Kariduraganavar, M.Y., 2004. Synthesis and characterization of hybrid membranes using poly(vinyl alcohol) and tetraethylorthosilicate for the pervaporation separation of wáter-isopropanol mixture. J. Appl. Polym. Sci. 94, 1304-1315. Kulprathipanja, S., Neuzil, R.W., Li, N.N., 1988. Separation of fluids by means of mixed matrix membranes, 4740219th ed. US.
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Kurkuri, M.D., Toti, U.S., Aminabhavi, T.M., 2002. Synthesis and characterization of blend membranes of sodium alginate and poly(vinyl alcohol) for the pervaporation separation of water-isopropanol mixtures. J. Appl. Polym. Sci. 86, 3642-3651.
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Kusumocahyo, S.P., Sudoh, M., 1999. Dehydration of acetic acid by pervaporation with charged membranes. J. Membr. Sci. 161, 77-83.
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Lee, Y.M., Bourgeois, D., Belfort, G., 1989. Sorption, diffusion and pervaporation of
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organics in polymer membranes. J. Membr. Sci. 44, 161-181.
Lee, Y.M., Nam, S.Y., Woo, D.J., 1997. Pervaporation of ionically surface crosslinked
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chitosan composite membranes for water/alcohol mixtures. J. Membr. Sci. 133, 103-110.
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Qiao, X., Chung, T.S., Rajagopalan, R., 2006. Zeolite filled p84 co-polyimide
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membranes for dehydration of isopropanol through pervaporation process, Chem.
Rachipudi, P.S., Kittur, A.A., Sajjan, A.M., Kariduraganavar, M.Y., 2013. Synthesis and
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characterization of hybrid membranes using chitosan and 2-(3,4-epoxycyclohexyl) ethyltrimethylsilane. J. Membr. Sci. 441, 83-92.
Schafer, T., Crespo, J.G., 2005. Vapour permeation and pervaporation, in: Afonso, C.A.M., Crespo, J.G. (Eds.), In green separation processes. Wiley-VCH, Weinheim, pp. 271-289.
Schmidt, C.E., Baier, J.M., 2000. Acellular vascular tissues: natural biomaterials for tissue repair and tissue engineering. Biomaterials 21, 2215-2231. Shah, D., Kissick, K., Ghorpade, A., Hannah, R., Bhattacharyya, D., 2000. Pervaporation of alcohol water and dimethylformamide–water mixtures using hydrophilic zeolite
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NaA membranes: mechanisms and experimental results. J. Membr. Sci. 179, 185-205. Shi, G.M., Yang, T., Chung, T.S., 2012. Polybenzimidazole (PBI)/zeolitic imidazolate
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frameworks (ZIF-8) mixed matrix membranes for pervaporation dehydration of alcohols. J. Membr. Sci. 415-416, 577-586.
cr
te Hennepe, H.J.C., Bargeman, D., Mulder, M.H.V., Smolders, C.A., 1987. Zeolite-filled
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silicone rubber membranes. I. Membrane preparation and pervaporation results. J. Membr. Sci. 35, 39-55.
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Venkalcom, I.F.J., Beukelaer, S.D., Uytterhoeven, J.B., 1997a. Sorption and pervaporation of aroma compounds using zeolite-filled PDMS membranes. J. Phys.
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Chem. B 101, 5186-5190.
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Venkalcom, I.F.J., Dotremont, C., Morobe, M., Uytterhoeven, J.B., Vandecasteele, C.,
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1997b. Zeolite-filled PDMS membranes I. Sorption halogenated hydrocarbons, J. Phys. Chem. B 101, 2154-2159.
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Varghese, J.G., Kittur, A.A., Kariduraganavar, M.Y., 2009. Dehydration of THF-water mixtures using zeolite-incorporated polymeric membranes. J. Appl. Polym. Sci. 111, 2408-2418.
Verkerk, A., Male, P.V., Vorstman, M., Keurentjes, J., 2001. Properties of high flux ceramic pervaporation membranes for dehydration of alcohol/water mixtures. Sep. and Purif. Technol. 22-23, 689-695. Wei, Y.C., Hudson, S.M., Mayer, J.M., Kaplan, D.L., 1992. The crosslinking of chitosan fibers. J. Polym. Sci. Pol. Chem. 30, 2187-2193. Winston Ho, W.S., 1992. Membrane Handbook Chapman & Hall, New York.
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Yamasaki, A., Iwatsubo, T., Masuoka, T., Misoguchi, K., 1994. Pervaporation of ethanol/water through a poly(vinyl alcohol)/cyclodextrin (PVA/CD) membranes. J.
Ac ce p
te
d
M
an
us
cr
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Membr. Sci. 89, 111-117.
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FIGURE CAPTIONS Fig. 1 - Modification of crosslinked chitosan membrane with NaY zeolite.
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Fig. 2 - FTIR spectra of crosslinked chitosan and its composite membranes: (M) 0 mass%; (M-1) 10 mass%; (M-2) 20 mass%; (M-3) 30 mass%; (M-4) 40 mass% of
cr
NaY zeolite.
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Fig. 3 - Wide-angle X-ray diffraction patterns of crosslinked chitosan and its composite membranes: (M) 0 mass%; (M-1) 10 mass%; (M-2) 20 mass%; (M-3) 30
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mass%; (M-4) 40 mass% of NaY zeolite.
Fig. 4 - DSC thermograms of crosslinked chitosan and its composite membranes:
M
(M) 0 mass%; (M-1) 10 mass%; (M-2) 20 mass%; (M-3) 30 mass%; (M-4) 40
d
mass% of NaY zeolite.
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Fig. 5 - Thermogravimetric analysis of crosslinked chitosan and its composite membranes: (M) 0 mass%; (M-1) 10 mass%; (M-2) 20 mass%; (M-3) 30 mass%;
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(M-4) 40 mass% of NaY zeolite.
Fig. 6 - SEM micrographs of crosslinked chitosan and its composite membranes: (A) Surface views and (B) Cross-sectional views. Fig. 7 - Variation of degree of swelling of crosslinked chitosan and its composite membranes with different mass% of water in the feed. Fig. 8 - Variation of total pervaporation flux of crosslinked chitosan and its composite membranes with different mass% of water in the feed. Fig. 9 - Variation of separation selectivity of crosslinked chitosan and its composite membranes with different mass% of water in the feed. 32
Page 32 of 51
Fig. 10 - Variation of total flux and selectivity of membranes with different mass% of NaY zeolite at 10 mass% of water in the feed. Fig. 11 - Variation of total flux, and fluxes of water and isopropanol with different
ip t
mass% of NaY zeolite in the membranes at 10 mass% of water in the feed.
cr
Fig. 12 - Variation of log J of crosslinked chitosan and its composite membranes
us
with temperature at 10 mass% of water in the feed.
Fig. 13 - Variation of log D of crosslinked chitosan and its composite membranes
Ac ce p
te
d
M
an
with temperature at 10 mass% of water in the feed.
33
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Table 1 - Physicochemical properties of hydrophilic NaY zeolite. Na+
SiO2/Al2O3
2.6
Density
1.27 g/ml
Pore size
0.5-2.0 m
Pore volume
0.47 ml/g
Topology
FAU (Faujasite)
Nature
Hydrophilic
Ac ce p
te
d
M
an
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cr
ip t
Counter ion
34
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Table 2 - Mechanical properties of crosslinked chitosan and its NaY zeolite incorporated composite membranes.
15.32 15.90 16.19 17.16 26.11
8.21 7.90 7.60 7.32 6.25
ip t
Elongation at break (%)
Ac ce p
te
d
M
an
us
M M-1 M-2 M-3 M-4
Tensile Strength (MPa)
cr
Membrane
35
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Table 3 - Diffusion coefficients of water and isopropanol at different mass% of water in the feed for different membranes. Dw x 108 (cm2/s)
Mass
DIPA x 1010 (cm2/s)
M
M-1
M-2
M-3
M-4
M
M-1
5
9.21
13.43
17.11
20.70
21.43
0.62
0.39
10
10.30
14.80
18.01
20.93
22.61
1.54
15
10.62
16.30
18.11
21.21
23.72
20
11.52
16.91
19.61
21.31
23.80
25
11.73
17.32
19.82
22.10
24.02
M-2
M-3
M-4
0.37
0.36
0.28
0.97
0.92
0.68
cr
water
ip t
% of
us
1.07 3.39
2.02
1.66
1.54
8.69
7.46
6.43
5.15
3.83
25.72
25.71
23.61
23.50
23.20
Ac ce p
te
d
M
an
3.79
36
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Table 4 - Pervaporation flux and separation selectivity for different membranes at different temperatures for 10 mass% of water in the feed. J x102 (kg/m2h)
Temp.
sep
ip t
°C M-1
M-2
M-3
M-4
M
M-1
M-2
M-3
M-4
30
5.05
7.21
8.32
10.29
11.37
2186
4491
6420
7491
11241
40
7.37
8.05
8.44
10.90
11.71
572
711
50
7.89
8.50
9.21
11.63
12.61
441
741
774
809
us
cr
M
634
683
607
Ac ce p
te
d
M
an
520
37
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Table 5 - Arrhenius activation parameters for permeation and diffusion, and heat of sorption.
M-2
M-3
Ep
18.15
16.68
14.11
12.98
ED
17.81
16.35
13.80
12.68
Epw
17.49
16.06
13.53
EpIPA
83.14
94.47
100.41
Hs
0.34
0.33
9.18
us
8.91 9.74
105.92
118.82
an
12.46
0.30
0.27
Ac ce p
te
d
M
0.31
M-4
ip t
M-1
cr
M
38
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Ac ce p
te
d
M
an
us
cr
ip t
Figure 1
Page 39 of 51
Ac ce p
te
d
M
an
us
cr
ip t
Figure 2
Page 40 of 51
Ac ce p
te
d
M
an
us
cr
ip t
Figure 3
Page 41 of 51
Ac ce p
te
d
M
an
us
cr
ip t
Figure 4
Page 42 of 51
Ac
ce
pt
ed
M
an
us
cr
i
Figure 5
Page 43 of 51
Ac ce p
te
d
M
an
us
cr
ip t
Figure 6
Page 44 of 51
Ac
ce
pt
ed
M
an
us
cr
i
Figure 7
Page 45 of 51
Ac
ce
pt
ed
M
an
us
cr
i
Figure 8
Page 46 of 51
Ac
ce
pt
ed
M
an
us
cr
i
Figure 9
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Ac
ce
pt
ed
M
an
us
cr
i
Figure 10
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Ac
ce
pt
ed
M
an
us
cr
i
Figure 11
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Ac
ce
pt
ed
M
an
us
cr
i
Figure 12
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Ac
ce
pt
ed
M
an
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cr
i
Figure 13
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S0263-8762(14)00508-5 http://dx.doi.org/doi:10.1016/j.cherd.2014.11.014 CHERD 1745
To appear in: Received date: Revised date: Accepted date:
16-7-2014 16-11-2014 26-11-2014
Please cite this article as: Premakshi, H.G., Ramesh, K., Kariduraganavar, M.Y.,Modification of Crosslinked Chitosan Membrane Using NaY Zeolite for Pervaporation Separation of WaterndashIsopropanol Mixtures, Chemical Engineering Research and Design (2014), http://dx.doi.org/10.1016/j.cherd.2014.11.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Graphical abstract The SEM micrographs of NaY zeolite (40 mass%) incorporated crosslinked chitosan
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te
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M
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us
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membrane: (A) Surface view and (B) Cross-sectional view.
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Highlights Membrane with 40 mass% of NaY zeolite showed an excellent PV performance.
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Modified crosslinked chitosan membranes overcome the trade-off phenomenon.
Membranes have lower Epw compared to EpIPA, showing higher separation efficiency.
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Membranes exhibit positive Hs values, involving Henry’s mode of sorption.
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Modification of Crosslinked Chitosan Membrane Using NaY Zeolite for Pervaporation Separation of Water–Isopropanol Mixtures a
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Premakshi H. Ga., Ramesh K.b and Mahadevappa Y. Kariduraganavara,* Department of Chemistry, Karnatak University, Dharwad-580 003, India b
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Indian Institute of Science, Bangalore-560 012, India
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ABSTRACT
The blocked diisocyanate crosslinked chitosan membrane was modified by incorporating
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different mass% of NaY zeolite. The physico-chemical properties of resulting composite membranes were studied using Fourier transform infrared spectroscopy (FTIR), wide-
M
angle X-ray diffraction (WAXD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). The mechanical
d
properties of the membranes were studied using universal testing machine (UTM). After
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measuring the equilibrium swelling, membranes were subjected to pervaporation for
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separation of water-isopropanol mixtures. Both flux and selectivity were increased with increasing NaY zeolite content in the membranes. The membrane containing 40 mass% of NaY zeolite exhibited the highest separation selectivity of 11,241 with a flux of 11.37 x 10-2 kg/m2h for 10 mass% of water in the feed. The total flux and flux of water are almost overlapping each other, suggesting that these membranes could be effectively used to break the azeotropic point of water-isopropanol mixture. From the temperature dependent diffusion and permeation values, the Arrhenius activation parameters were estimated. All the composite membranes exhibited lower activation energy compared to crosslinked membrane, indicating that the permeants require less energy during the process because of molecular sieving action attributed to the presence of sodalite and 3
Page 3 of 51
super cages in the framework of NaY zeolite. The Henry’s mode of sorption dominates the process, giving an endothermic contribution. Keywords: Chitosan; NaY zeolite; Pervaporation; Selectivity; Activation energy.
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*Corresponding author: Dr. M. Y. Kariduraganavar ([email protected])
Introduction
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1.
cr
Fax: +91-836-2771275; Phone: +91-836-2215286 (Extn. 23).
To remain competitive in the market, industries are investing a huge capita to improve
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the quality of the product and reduce the cost of production. Pervaporation (PV) is one such type of industries with a wide range of uses such as solvent dehydration, azeotropic
M
separation and solvent recovery (Schafer and Crespo, 2005; Baker, 2004). It is a membrane based process, in which one side of the membrane is in contact with the feed
d
mixture while the other side of the membrane is with a carrier gas or vacuum (Bruschke,
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2001; Winston Ho, 1992; Huang, 1991). This separation process offers many advantages
Ac ce p
such as high selectivity, low energy consumptions, moderate cost to performance ratio, compact and modular design (Baker, 2004). Polymeric membranes are widely used in PV process due to their flexibility in
design and low cost (Qiao et al., 2006). Membrane stability, high flux and selectivity, low production costs are always most important criteria for developing membranes for specific applications. Multi-layer composite membranes were successfully developed with a crosslinked poly(vinyl alcohol) or cellulose acetate thin film on the top of poly(acrylonitrile) or poly(sulfone) (Bruschke, 1983). Unfortunately, these membranes failed to perform in a harsh condition owing to their instability and high swelling nature. On the contrary, composite membranes exhibit better structural stability, excellent 4
Page 4 of 51
chemical resistance and withstand to higher operating temperature (Shah et al., 2000; Verkerk et al., 2001). Particularly, composite membranes made with zeolite demonstrated high flux and good selectivity owing to their unique molecular sieving action and high
ip t
adsorption (Bowen et al., 2004). Recently, Shah et al. (2000), Kita et al. (1995) and Jafari, et al. (2013) developed zeolite based composite membranes and they all exhibited
cr
impressive flux and high separation factor, and have become superior to traditional
us
polymeric membranes. But, the fabrication of pure zeolite membrane without any defects is extremely difficult and expensive as well. In order to overcome this problem, mixed
an
matrix membranes were developed by the incorporation of inorganic adsorbent such as zeolite into a polymer matrix (Kittur et al., 2005; Varghese et al., 2009; Kulprathipanja et
M
al., 1988; Hua et al., 2014; Shi et al., 2012). These endeavors produced promising results
d
in the separation of water from isopropanol. Unfortunately, these membranes often fail to
te
exhibit good physical and mechanical properties. Therefore, crosslinking method is the most convenient and effective approach to improve the physical and mechanical
Ac ce p
properties of the membrane (Lee et al., 1997; Jegal and Lee, 1999; Goissis et al., 1999; Wei et al., 1992; Schmidt and Baier, 2000; Rachipudi et al., 2013; Jin et al., 2004; Kuboe et al., 2004; Kulkarni et al., 2004; Ghazali et al., 1997; Devi et al., 2005; Choudhari et al., 2007). Various crosslinking agents including sulphuric acid (Lee et al., 1997), sulfosuccinic acid (Jegal and Lee, 1999), gluteraldehyde (Goissis et al., 1999), epoxy compound (Wei et al., 1992), dialdehyde starch (Schmidt and Baier, 2000), silane (Rachipudi et al., 2013), non-toxic nature agents (Jin et al., 2004; Kuboe et al., 2004), tetraethylorthosilicate (Kulkarni et al., 2004) and diisocyanates (Ghazali et al., 1997; Devi et al., 2005; Choudhari et al., 2007)
5
have been employed as crosslinker for
Page 5 of 51
chitosan. Among these, diisocyanates are the important class of crosslinking reagents for chitosan, as isocyanates possess high reactivity towards amine as well as hydroxyl groups of chitosan. Choudhari et al. (2007) reported that diisocyanate crosslinked chitosan
ip t
membrane had shown a good separation performance for the selective removal of water from isopropanol. The membrane containing 40 mass% of diisocyanate demonstrated the
cr
highest separation selectivity of 5,918 at 5 mass% of water in the feed. Unfortunately, the
us
performance of the same membrane was very poor in yielding the flux. The best alternative for the improvement of this membrane is the incorporation of zeolite. Among
an
the zeolites, NaY zeolite is the best choice, considering its unique properties such as high surface area (up to 1000 m2/g), high void volume (30% of the total volume of zeolite),
M
uniform pore size distribution, presence of sodalite and super cages in the framework. In
d
view of this, it is also being used widely in chemical and physical processes such as
te
shape selective catalysis and separation media (Jia et al., 1991). The incorporation of such zeolite or porous material in the crosslinked membrane can improve the separation
Ac ce p
performance of pervaporation (Jia et al., 1991; te Hennepe et al., 1987; Kim et al., 2001). In addition, it is demonstrated that zeolite incorporated membranes have a high mechanical strength, good thermal and chemical stability, and thus the membranes incorporated with zeolites can be used over a wide range of operating conditions. For instance, Gao et al. (1996), and Chen et al. (2001) studied the pervaporation separation of hydrophilic zeolite-filled PVA and chitosan membranes for organic–water system. Similarly, our research group also reported hydrophobic zeolite-filled PVA membranes (Kittur et al., 2003), hydrophilic zeolite-filled sodium alginate and chitosan membranes (Kariduraganavar et al., 2004; Kittur et al., 2005) for the separation of water–isopropanol
6
Page 6 of 51
mixtures. In case of sodium alginate and chitosan membranes, both separation factor and flux were increased simultaneously, although it was uncommon in PV process. Understanding the pros and cons mentioned above, we have made an attempt to
ip t
modify the blocked diisocyanate crosslinked chitosan membrane developed previously
cr
(Choudhari et al., 2007) by incorporating the NaY zeolite in different mass%. Before subjecting to PV study, the physicochemical changes in the resulting membranes were
us
investigated using different techniques such as FTIR, WAXD, DSC, TGA and SEM. The mechanical properties of the membranes were also studied using UTM and correlated
an
with the structure. The values of permeation flux, separation selectivity and diffusion
M
coefficients were evaluated. From the temperature dependence of permeation flux and
Experimental section
2.1.
Materials
Ac ce p
te
2.
d
diffusion coefficients, the Arrhenius activation parameters were estimated.
Chitosan ( Mw ~ 200,000; N-deacetylation degree 75-85%) and hexamethylene diisocyanate were purchased from Sigma-Aldrich Chemicals, USA. Isopropanol, acetic acid, sodium metabisulfite and acetone were procured from s. d. fine Chemicals Ltd., Mumbai,
India.
Hexamethylene
1,6-di(aminocarboxysulfonate)
was
synthesized
according to the procedure reported by Choudhari et al. (2007). NaY zeolite was supplied by Indian Petrochemicals Corp., Baroda, India. The characteristic properties of NaY zeolite are given in Table 1 (Venkalcom et al., 1997a; Venkalcom et al., 1997b). All the chemicals were of analytical grade and used without further purification. Deionized water was used throughout the study.
7
Page 7 of 51
2.2.
Membrane preparation
Chitosan (3 g) was dissolved in 100 ml of deionized water containing 2% of acetic acid and stirred for about 24 h at room temperature. The solution was filtered to remove the
ip t
undissolved residue particles. It was then kept in an ultrasonic bath at a fixed frequency
cr
of 38 kHz (Grant XB6, UK) for 30 min to break the possible aggregated molecules. To this solution, 40 mass% of blocked diisocyanate as a crosslinker was added and stirred
us
for 2 h at room temperature. To remove the air bubbles, the solution was left overnight. The resulting homogeneous solution was spread onto a clean glass plate. After being
an
dried in air for about 48 h, the membrane was peeled-off and subsequently annealed at 60 o
M
C for 24 h in an inert atmosphere and was designated as M.
To prepare composite chitosan membranes, a known amount of NaY zeolite was
d
added into the above homogeneous solution. The amount of chitosan and crosslinker was
te
kept constant for each membrane. To break the aggregated zeolite particles and to improve their dispersion in the polymer matrix, the mixed solution was stirred for about
Ac ce p
24 h and then subjected to sonication for about 30 min at a fixed frequency of 38 kHz. The resulting solution was poured onto a clean glass plate and the membrane was dried as mentioned above. The amount of zeolite particles with respect to chitosan was varied as 10, 20, 30, and 40 mass%, and the membranes thus obtained were designated as M-1, M2, M-3, and M-4, respectively. An attempt was also made to incorporate higher content of zeolite, but even for 45 mass% of zeolite, the membrane became brittle and lost the membrane property. Hence, the loading of zeolite content was restricted up to 40 mass%. The chemical structure and reaction route of the composite membranes are illustrated in Fig. 1. The thickness of these membranes was measured at different points using a
8
Page 8 of 51
Peacock dial thickness gauge (Model G, Ozaki Mfg. Co. Ltd., Japan) with an accuracy of ± 2 µm and the average thickness was considered for the calculation. Thickness of these
2.3.
ip t
membranes was found to be 40 ± 2 µm. Fourier transform infrared spectroscopy
cr
The analyses of crosslinked chitosan and its NaY zeolite incorporated composite
us
membranes were carried out using FTIR spectroscopy (Nicolet, Impact-410, USA). The polymer membranes were ground well with KBr under a hydraulic pressure of 400
an
kg/cm2. The spectra were recorded in the frequency range of 400 to 4000 cm-1. In order to estimate the changes in the intensities of the characteristic peaks with respect to the
M
amount of NaY zeolite in each scan, the amount of membrane sample and KBr were kept
Wide-angle X-ray diffraction
te
2.4.
d
constant.
Ac ce p
To know the level of dispersion of NaY zeolite in the crosslinked polymer matrix and to understand the extent of crystallinity, X-ray diffraction measurements were performed using a Brucker’s D-8 advanced wide-angle X-ray diffractometry. The X-ray source was Ni-filtered Cu Kα radiation (40 kV, 30 mA). The dried samples of uniform thickness (40 ± 2 μm) were mounted on a sample holder. The scanning angle (2θ) ranged from 5 to 50° with a scanning rate of 8°/min. 2.5.
Differential scanning calorimetry
The effect of NaY zeolite on the glass transition temperature (Tg) of the crosslinked chitosan membrane was studied using a differential scanning calorimetry (Perkin-Elmer
9
Page 9 of 51
DSC) at a temperature range from ambient to 400 oC under nitrogen atmosphere. In each measurement, 8-10 mg sample was used with heating rate of 10 oC/min. Thermogravimetric analysis
ip t
2.6.
Thermal stability and decomposition temperature of the membrane samples were carried
cr
out using a thermogravimetric analysis (DSC Q 20, TA Instruments, Waters LLC, USA)
us
at a heating rate of 10 °C/min under nitrogen atmosphere. The weight of the samples
2.7.
an
taken for each record was 8-10 mg. Scanning electron microscopy
M
The surface and cross-sectional views of the crosslinked chitosan and its NaY zeolite incorporated composite membranes were investigated using a scanning electron
d
microscope (JEOL, JSM-400 Å, Tokyo, Japan). Before photographing, all the samples
Tensile properties
Ac ce p
2.8.
te
were vacuum dried and coated with a conductive layer (400 Å) of sputtered gold.
The mechanical properties such as tensile strength and percent elongation at break of the crosslinked chitosan and its NaY zeolite incorporated composite membranes were measured at 25 oC using universal testing machine (Hounsfield H10KS) with a speed of 50 mm/min. The gauge dimension of the test sample was 25 mm x 50 mm. For each sample, three specimens were tested and results were averaged. 2.8.
Swelling measurement
10
Page 10 of 51
The sorption properties of the crosslinked chitosan and its composite membranes were carried out at 30 °C. The dried membrane samples were first weighed and then they were immersed in different compositions of water-IPA mixtures in sealed vessels for 24 h. The
ip t
swollen membrane samples were taken out, gently blotted with tissue paper and weighed
membranes was calculated using the following equation:
us
100
(1)
an
W Wd DS (%) s Wd
cr
as quickly as possible in a closed container. The percent degree of sorption (DS) of the
2.9.
Pervaporation experiments
M
where Ws and Wd are the masses of the swollen and dry membranes, respectively.
The pervaporation cell was assembled from two cylindrical half-cells made of stainless
te
d
steel fastened together by nuts and bolts. The membrane was supported on a perforated stainless steel plate placed at the junction of two cells. The feed temperature was
Ac ce p
maintained by circulating water through the cell jacket. The capacity of the feed cell was about 250 cm3 and the effective surface area of the membrane in contact with the feed mixture was 34.23 cm2. For all the measurements, the downstream pressure was maintained at 1.333224 x 103 Pa (10 Torr) using a two-stage vacuum pump (Toshniwal, Chennai, India). The water composition in the feed mixture was varied from 5 to 25 mass%. Before performing the PV experiments, the test membrane was allowed for swelling to attain equilibrium with a known volume of feed mixture for about 2 h in the feed compartment. The permeate vapor was collected in a glass trap suspended inside the liquid nitrogen jar. The experiments were carried out at 30, 40 and 50 °C. The flux was calculated by weighing the permeate and the compositions of water and isopropanol were 11
Page 11 of 51
estimated by measuring the refractive index of the permeate within an accuracy of ± 0.0001 units using an Abbe’s refractometer (Atago-3T, Japan). All the pervaporation runs were repeated thrice and the results were averaged. The results of permeation for water-
ip t
IPA mixtures during the pervaporation were reproducible within an admissible range.
cr
From the PV data, separation performance of the membranes was estimated in
pervaporation run using Eq. (2): W A.t
(2)
an
J
us
terms of total flux (J) and it can be calculated from the weight of permeate collected after
M
where W is the total weight of permeate (kg) collected in time t (h) and A is the effective membrane area (m2). The separation selectivity (αsep) was calculated using the following
d
equation:
Pw / PIPA Fw / FIPA
(3)
Ac ce p
te
sep
where Pw and PIPA are the weight percent of water and isopropanol in the permeate, respectively. Fw and FIPA are the respective weight percent of water and isopropanol in the feed. 3. 3.1.
Results and discussion
Membrane characterization
3.1.1. FTIR studies The FTIR spectra of the crosslinked chitosan and its composite membranes are shown in Fig. 2. A broad characteristic band was observed at around 3450 cm-1 in all the
12
Page 12 of 51
membranes, which corresponds to O–H stretching vibrations of the hydroxyl groups. The small bands appeared at 1650 and 1570 cm−1 were respectively assigned to amide I and amide II bands of chitosan. The multiple bands appeared between 1000 and 1200 cm-1 in
ip t
membranes were assigned to C–O stretching vibrations. All these bands are in good agreement with the data reported by our group (Choudhari et al., 2007). However, the
cr
intensity of these bands was marginally increased as the content of zeolite was increased
us
in the crosslinked chitosan matrix. This suggests that the hydrophilicity of the membranes was increased owing to hydrophilic nature and molecular sieving action attributed to the
an
presence of sodalite and super cages in the framework of NaY zeolite.
M
3.1.2. WAXD studies
Fig. 3 shows the diffraction patterns of crosslinked chitosan and its zeolite incorporated
d
composite membranes. The crosslinked chitosan membrane exhibited two sharp peaks at
te
around 2θ = 14 and 17o, and a broad peak around 21o, which are respectively attributed to
Ac ce p
the presence of crystalline and amorphous region in the polymer. After the incorporation of NaY zeolite content in the crosslinked membrane, the intensity of these peaks diminishes in the diffraction patterns. However, as the content of zeolite was increased, these patterns were shifted gradually towards the zeolite structure, showing more reflection peaks of zeolite in the diffraction patterns. This clearly indicates that crystallinity of the crosslinked chitosan membrane was changed to amorphous domain. 3.1.3. DSC studies The differential scanning calorimetric study was carried out for crosslinked chitosan and its NaY zeolite incorporated composite membranes. The transition patterns of membranes
13
Page 13 of 51
were recorded in the temperature range of ambient to 400 oC and the resulting thermograms are presented in Fig. 4. A special care was taken while recording owing to their tendency to absorb moisture, which can strongly affect the transitions. From the
ip t
patterns, it is noticed that the Tg of the crosslinked chitosan membrane was around 98 oC. Upon incorporating different amounts of NaY zeolite in the membranes, the Tg of the
cr
membranes did not change appreciably. This is expected, because the added zeolite only
us
fills the free space available in the membrane matrix. However, the added zeolite significantly changed the melting temperature of the membranes, which was respectively
an
ranged from 285 to 310 oC for the membranes M to M-4. The shift in the melting temperature from membrane M to M-4 is due to an electrostatic interaction that was
M
occurred between zeolite and the crosslinked chitosan.
d
3.1.4. TGA studies
te
Thermal stability and degradation behavior of the crosslinked chitosan and NaY zeolite From the patterns, it is
Ac ce p
incorporated composite membranes are depicted in Fig. 5.
observed that irrespective of the zeolite loading, membranes underwent weight loss in two stages. The first stage of weight loss occurred from ambient to 150 oC, which corresponds to around 16 to 20 weight%. This loss attributes to the dehydration of membranes. This is expected due to increased hydrophilic character of the membrane by the addition of zeolite. The second stage of decomposition starts around 240 to 510 oC and this is accounted for 39 - 52%, attributing to a weight loss due to degradation of polymer network. When we looked into the individual patterns of all the membranes, it is observed that the weight loss was significantly decreased from membrane M to M-4. This is expected owing to the zeolite and its content in the membrane. Further, it is observed
14
Page 14 of 51
that as the content of zeolite was increased in the membranes, the decomposition temperature was marginally shifted from 240 to 280 oC. This is because of electrostatic interaction occurred between NaY zeolite and the chitosan. All these observations
ip t
suggest that the added zeolite enhances the thermal stability of the membranes.
cr
3.1.5. SEM studies
us
Fig. 6 (A and B) illustrates the surface morphology of the crosslinked and its composite membranes. From the surface micrographs (Fig. 6A), it is clear that all the membranes
an
have a smooth, compact and homogeneous surface. This indicates that NaY zeolite was distributed evenly throughout the membrane matrix with no apparent clusterings.
M
However, some of the zeolite particles are pulled out of the matrix, but their ends are embedded in the chitosan matrix. This suggested that there is an interfacial adhesion
d
between chitosan and NaY zeolite. On the contrary, the cross-sectional views (Fig. 6B) of
te
the membranes showed some cavities and are prominent at higher concentration of
Ac ce p
zeolite. This is very well supported from the WAXD patterns and FTIR stretchings. 3.1.6. Mechanical properties
Tensile strength and percent elongation of a membrane often tell its suitability for PV applications. The effect of zeolite on the tensile strength and percent elongation of the crosslinked membranes were studied and the data thus obtained are summarized in Table 2. It could be seen from the data that tensile strength of the membrane was increased marginally with increasing the zeolite content from membrane M to M-3. This is due to a uniform distribution of zeolite particles in the crosslinked membrane matrix. However, when the content of zeolite was increased to 40 mass% there was an abrupt change in the
15
Page 15 of 51
tensile strength (M-4). This may be because of dominant role of zeolite. On the contrary, the percent elongation at break was gradually decreased from membrane M to M-3. Again the effect of this was more pronounced for membrane M-4. Despite of the fact, the
Effects of feed composition and NaY zeolite content on membrane swelling
cr
3.2.
ip t
resulting membranes are suitable for the PV separation.
us
To study the effects of feed composition and NaY zeolite loading on membrane swelling, the percent degree of swelling of all membranes was plotted with respect to different
an
mass% of water in the feed at 30 C as shown in Fig. 7. It is evident from the plot that the degree of swelling was increased almost linearly for all the membranes with increasing
M
the mass% of water in the feed. This is attributed to a strong interaction occurring between water molecules and the membrane containing functional groups such as -OH
d
and -NH2, and Na+ ions present in the zeolite cages. On the other hand, the degree of
te
swelling was enhanced greatly with increasing the zeolite loading in the membranes (M-1
Ac ce p
to M-4). This is due to increased hydrophilic nature of the membrane owing to the presence of Na+ ions in the sodalite and supercages in the framework of NaY zeolite which tend to enhance the electrostatic force of attraction between water molecules and the membrane and thereby adsorption of water molecules increases resulting to a greater degree of swelling than that of crosslinked membrane. 3.3.
Effects of feed composition and the content of NaY zeolite on pervaporation
In order to study the effects of feed composition and the content of NaY zeolite on PV, the total permeation flux was plotted for all the membranes as a function of different mass% of water in the feed (Fig. 8). It is observed that the total permeation flux was
16
Page 16 of 51
increased for all the membranes with increasing water composition in the feed. This is attributed to an increased selective interaction between water molecules and the composite membranes. Similarly, the total permeation flux was increased with increasing
ip t
the zeolite loading in the membranes. This trend remains same for all the membranes throughout the investigated feed compositions. This is expected owing to a combined
These are together responsible for increasing the
us
(Kariduraganavar et al., 2004).
cr
influence of ionic species (Na+) present in the zeolite cages and porous structure
permeation flux with the zeolite loading.
an
The overall selectivity of a membrane in PV process is generally described on the
M
basis of interaction between membrane and the permeating molecules, molecular size of the permeating species and pore diameter of the membrane. Fig. 9 shows the effect of
d
water composition on the selectivity of all membranes. It is observed that the selectivity
te
was decreased drastically for all the membranes with increasing the water composition in the feed. This effect is more predominant particularly for the membranes having higher
Ac ce p
loading of zeolite. This is because, at higher composition of water, the membrane swells greatly due to an establishment of strong interaction between the membrane and water molecules, and thereby suppressing the interaction between NaY zeolite and chitosan. This in turn may be the reason for the development of very minute narrow openings at the interface between the NaY zeolite and chitosan moiety (Chen et al., 2001). This allows the IPA molecules to pass through the membrane along with the selective water molecules. As a result, the selectivity underwent a drastic decrease at higher concentration of water in the feed. However, the selectivity was greatly enhanced for all the membranes with increasing the zeolite loading. It is more pronounced at lower water
17
Page 17 of 51
composition (5 and 10 mass%). This is mainly due to a greater interaction between NaY zeolite and crosslinked chitosan matrix owing to the presence of Na+ ions in the framework of sodalite and supercages. However, at higher water composition in the feed,
ip t
the interaction between water molecules and membrane matrix becomes predominant and suppresses the interaction between NaY zeolite and chitosan, which becomes responsible
cr
for an abrupt decrease in selectivity. This can be clearly observed from Fig. 10, wherein
us
both the flux and selectivity were plotted as a function of NaY zeolite content in the membrane at 10 mass% of water in the feed.
an
Generally, with increasing the packing density in the polymer matrix, permeation
M
flux decreases and selectivity increases (Kittur et al., 2003; Kurkuri et al., 2002). However, in the present study both permeation flux and selectivity were increased
d
simultaneously with increasing the NaY zeolite content in the membrane. Although, this
te
is in contrast to a trade-off phenomenon existing between flux and selectivity in PV process, a significant enhancement of hydrophilicity, selective adsorption and
Ac ce p
establishment of molecular sieving action overcome the situation by the incorporation of porous NaY zeolite in the polymer matrix. This can be further explained in such a way that when we use water selective NaY zeolite incorporated composite membranes, the transport of water molecules through the membrane occurs in a straight path through the NaY zeolite pores with subsequent adsorption at the feed side followed by desorption at the permeate side, which in turn are responsible for higher water flux. If sufficient water is available inside the membrane, then the NaY zeolite pores will be largely occupied by water molecules, preventing the IPA molecules entering into the NaY zeolite pores. Thus, on their way through the membrane the IPA molecules have to move around the
18
Page 18 of 51
NaY zeolite cages. The higher water concentration inside the membrane close to the permeate side of the membrane and the fact that water can travel along the straight path whereas IPA has to follow a more tortuous path, in which the membrane can act together
ip t
in explaining the way in which membrane performance is enhanced, both in terms of flux and selectivity when NaY zeolite are incorporated into the membrane matrix (te Hennepe
cr
et al., 1987).
us
In PV process, the efficiency of the membrane is generally assessed on the basis of permeation of individual components. Therefore, the extent of permeation of
an
individual components was determined by plotting the total flux and fluxes of water and
M
IPA as a function of zeolite content in the membrane for 10 mass% of water in the feed (Fig. 11). From the plot, it is clear that the total flux and flux of water are overlapping
d
each other, whereas the flux of IPA is negligibly small for all the membranes, indicating
te
that the membranes developed in the present study are highly selective towards water. Further, it is concluded that membrane containing 40 mass% of NaY zeolite
Ac ce p
demonstrated an excellent flux of 11.37 x 10-2 kg/m2h with a selectivity of 11,241 for 10 mass% of water in the feed at 30 oC among the membranes developed in the present study. 3.4.
Diffusion coefficient
In PV process, mass transport of binary liquid molecules through a polymer matrix is generally described by the solution-diffusion mechanism with three consecutive steps: sorption, diffusion and desorption (Lee et al., 1989). The penetrants undergo sorption at the feed side of a membrane and subsequently they diffuse through the membrane. Finally, desorption of penetrant takes place in the form of vapor at the downstream side 19
Page 19 of 51
of the membrane. Thus, the solubility and diffusivity of penetrants in membrane play an important role in the membranes’ selectivity and permeation flux. In PV process, because of the establishment of fast equilibrium distribution between the bulk feed and upstream
ip t
surface of a membrane, the diffusion step controls the transport of penetrants (Kittur et al., 2003; Hwang and Kammermeyer, 1975). It is therefore important to estimate the
cr
diffusion coefficient (Di) of penetrating molecules to understand the mechanism of
us
transport.
(Yamasaki et al., 1994):
dC i dx
(4)
M
J i Di
an
From Fick’s law of diffusion, the diffusion coefficient can be expressed as
where J is the permeation flux per unit area (kg/m2s), D is the diffusion coefficient
d
(m2/s), C is the concentration of permeant in the membrane (kg/m3), subscript i stands for
te
water or isopropanol, and x is the diffusion length (m). For simplicity, it is assumed that
Ac ce p
the concentration profile along the diffusion length is linear. Thus, diffusion coefficient Di can be calculated with the following modified equation (Kusumocahyo and Sudoh, 1999):
Di
J i Ci
(5)
where is the membrane thickness. The calculated values of Di at 30 oC are presented in Table 3. It is observed that the diffusion coefficient of water as increased from M to M-4, while decreasing the diffusion coefficient of IPA. This trend remains same for all investigated water composition. However, the magnitude of the diffusion coefficient for water is quite high in comparison with that of IPA. This further supports that the 20
Page 20 of 51
membranes developed in the present study by the incorporation of zeolite have remarkable separation ability for the separation of water from IPA. As discussed in PV study, this was attributed to increased hydrophilicity, selective adsorption and
ip t
establishment of molecular sieving action owing to the presence of sodalite and
Effect of temperature on PV performance
us
3.5.
cr
supercages in the framework of NaY zeolite.
Temperature is an important operating parameter in pervaporation process as it greatly
an
induces the sorption and diffusion, and hence it has shown significant affect on the performance of the membranes. The effect of operating temperature on the pervaporation
M
performance for water-isopropanol mixture was studied for all the membranes at 10 mass% of water in the feed and the resulting values are presented in Table 4. It is
d
observed that the permeation rate was increased from 30 to 50 oC for all the membranes,
te
while suppressing the separation selectivity. Generally, this happens because of two
Ac ce p
reasons. Firstly, as the temperature increases the viscosity of the permeating molecules decreases due to decreased cohesive forces between the permeants. Secondly, an increase of thermal energy intensifies the motion of polymer chain segments creating more freevolume in the polymer matrix. However in the present study, the later reason is ruled out since the experiments were performed well below the glass transition temperature of the crosslinked chitosan. Therefore, the viscosity of permeating molecules played a major role in allowing the associated molecules along with the selective permeants. This results to an increase in total permeation flux while decreasing the selectivity. This effect prompted us to estimate the activation energies for permeation and diffusion using the Arrhenius type equation (Huang and Yeom, 1991):
21
Page 21 of 51
Ex X X o exp RT
(6)
where X represents permeation (J), or diffusion (D). Xo is a constant representing pre-
ip t
exponential factor of Jo or Do. Ex represents the activation energy for permeation or diffusion depending upon the transport process under consideration, and RT is the usual
cr
energy term. As the feed temperature increases, the vapor pressure in the feed
us
compartment also increases, but the vapor pressure at the permeate side is not affected. This leads to an increase of driving force with increasing the temperature.
an
Arrhenius plots of log J and log D versus temperature are shown in Fig. 12 and
M
13, respectively. In both the cases, a linear behavior was observed, suggesting that permeability and diffusivity follow an Arrhenius trend. From the least-squares fits of
d
these linear plots, the activation energies for total permeation (Ep) and total diffusion (ED)
te
were estimated. Similarly, we have also estimated the activation energies for permeation of water (Epw) and isopropanol (EpIPA), but the plots are not given to avoid the crowdness.
Ac ce p
The values thus obtained are presented in Table 5. From Table 5, it is observed that activation energy values of both Ep and ED were
decreased with increasing the NaY zeolite content in the membrane (M-1 to M-4). This is expected due to increased hydrophilicity and molecular sieving action attributed to the presence of sodalite and supercages in the framework of NaY zeolite, and obviously reduces the energy required for transport of selective permeants. It can also be seen that the apparent activation energy values obtained for water permeation (Epw) are much lower than those of isopropanol (EpIPA), and this difference was increased around 5 to 12 times as the content of zeolite was increased, suggesting that membranes developed here have
22
Page 22 of 51
higher separation efficiency towards water. The activation energy values of water permeation (Epw) and total permeation (Ep) are almost close to each other, signifying that coupled-transport of both water and isopropanol is minimal due to higher selective nature
ip t
of the membranes. Further, the estimated Ep and ED values ranged between 9.18 and 18.15 kJ/mol and 8.91 and 17.81 kJ/mol, respectively. Using these values, we have
us
H s E p E D .
cr
calculated the heat of sorption as:
(7)
an
The resulting Hs values are included in Table 5. It is noticed that the Hs values obtained in the present study are positive for all the membranes, suggesting that Henry’s
Conclusions
d
4.
M
mode of sorption is predominant, giving an endothermic contribution.
te
Using a solution casting technique, crosslinked chitosan membrane was modified by
Ac ce p
incorporating different mass% of NaY zeolite and these were successfully employed for the separation of water-IPA mixtures. An increase of NaY zeolite content in the membrane results to a simultaneous increase of both permeation flux and selectivity. This was explained on the basis of significant enhancement of hydrophilicity, selective adsorption and establishment of molecular sieving action. Both total flux and flux of water are almost overlapping with each other, signifying that the modified membranes are highly selective towards water. The highest separation selectivity was found to be 11,241 with a flux of 11.37 x 10-2 kg/m2h for the membrane having 40 mass% NaY zeolite loading at 30 °C for 10 mass% of water in the feed. With regard to temperature effect, the permeation rate was found to increase while suppressing the selectivity. The
23
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modified membranes exhibited lower activation energy values compared to crosslinked membrane, indicating that the permeants require less energy during the process owing to molecular sieving action attributed to the presence of sodalite and supercages in the
ip t
framework of NaY zeolite. For all the membranes, Henry`s mode of sorption dominates
cr
the process, giving an endothermic contribution.
us
Acknowledgements
Authors are thankful to the UGC, New Delhi for providing financial assistance (Grant
an
No. 37-245/3009, SR). Authors also thank the Department of Physics, Indian Institute of
Ac ce p
te
d
M
Science, Bangalore, for extending wide-angle X-ray diffraction facility.
24
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Nomenclature molecular weight
A
effective membrane area (m2)
DS
degree of swelling (%)
Do
pre-exponential factor for diffusion
ED
activation energy for diffusion (kJ/mol)
EDw
activation energy for diffusion of water (kJ/mol)
Ep
activation energy for permeation (kJ/mol)
Epw
activation energy for permeation of water (kJ/mol)
EDIPA
activation energy for diffusion of IPA (kJ/mol)
Ex
activation energy for permeation or diffusion (kJ/mol)
∆Hs
heat of sorption (kJ/mol)
IPA
Isopropanol
J
total flux (kg/m2h)
Jo
pre-exponential factor for permeation
P and F
mass percent of permeate and feed
R
gas constant
T W
cr
us
an
M
d
te
Ac ce p
t
ip t
Mw
Ws and Wd
permeation time (h) temperature (K) mass of permeate (kg) mass of the swollen and dry membranes
Greek letters δ
membrane thickness (40 m)
sep
separation factor
25
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ip t
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ip t
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ip t
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Verkerk, A., Male, P.V., Vorstman, M., Keurentjes, J., 2001. Properties of high flux ceramic pervaporation membranes for dehydration of alcohol/water mixtures. Sep. and Purif. Technol. 22-23, 689-695. Wei, Y.C., Hudson, S.M., Mayer, J.M., Kaplan, D.L., 1992. The crosslinking of chitosan fibers. J. Polym. Sci. Pol. Chem. 30, 2187-2193. Winston Ho, W.S., 1992. Membrane Handbook Chapman & Hall, New York.
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Yamasaki, A., Iwatsubo, T., Masuoka, T., Misoguchi, K., 1994. Pervaporation of ethanol/water through a poly(vinyl alcohol)/cyclodextrin (PVA/CD) membranes. J.
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an
us
cr
ip t
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FIGURE CAPTIONS Fig. 1 - Modification of crosslinked chitosan membrane with NaY zeolite.
ip t
Fig. 2 - FTIR spectra of crosslinked chitosan and its composite membranes: (M) 0 mass%; (M-1) 10 mass%; (M-2) 20 mass%; (M-3) 30 mass%; (M-4) 40 mass% of
cr
NaY zeolite.
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Fig. 3 - Wide-angle X-ray diffraction patterns of crosslinked chitosan and its composite membranes: (M) 0 mass%; (M-1) 10 mass%; (M-2) 20 mass%; (M-3) 30
an
mass%; (M-4) 40 mass% of NaY zeolite.
Fig. 4 - DSC thermograms of crosslinked chitosan and its composite membranes:
M
(M) 0 mass%; (M-1) 10 mass%; (M-2) 20 mass%; (M-3) 30 mass%; (M-4) 40
d
mass% of NaY zeolite.
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Fig. 5 - Thermogravimetric analysis of crosslinked chitosan and its composite membranes: (M) 0 mass%; (M-1) 10 mass%; (M-2) 20 mass%; (M-3) 30 mass%;
Ac ce p
(M-4) 40 mass% of NaY zeolite.
Fig. 6 - SEM micrographs of crosslinked chitosan and its composite membranes: (A) Surface views and (B) Cross-sectional views. Fig. 7 - Variation of degree of swelling of crosslinked chitosan and its composite membranes with different mass% of water in the feed. Fig. 8 - Variation of total pervaporation flux of crosslinked chitosan and its composite membranes with different mass% of water in the feed. Fig. 9 - Variation of separation selectivity of crosslinked chitosan and its composite membranes with different mass% of water in the feed. 32
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Fig. 10 - Variation of total flux and selectivity of membranes with different mass% of NaY zeolite at 10 mass% of water in the feed. Fig. 11 - Variation of total flux, and fluxes of water and isopropanol with different
ip t
mass% of NaY zeolite in the membranes at 10 mass% of water in the feed.
cr
Fig. 12 - Variation of log J of crosslinked chitosan and its composite membranes
us
with temperature at 10 mass% of water in the feed.
Fig. 13 - Variation of log D of crosslinked chitosan and its composite membranes
Ac ce p
te
d
M
an
with temperature at 10 mass% of water in the feed.
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Table 1 - Physicochemical properties of hydrophilic NaY zeolite. Na+
SiO2/Al2O3
2.6
Density
1.27 g/ml
Pore size
0.5-2.0 m
Pore volume
0.47 ml/g
Topology
FAU (Faujasite)
Nature
Hydrophilic
Ac ce p
te
d
M
an
us
cr
ip t
Counter ion
34
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Table 2 - Mechanical properties of crosslinked chitosan and its NaY zeolite incorporated composite membranes.
15.32 15.90 16.19 17.16 26.11
8.21 7.90 7.60 7.32 6.25
ip t
Elongation at break (%)
Ac ce p
te
d
M
an
us
M M-1 M-2 M-3 M-4
Tensile Strength (MPa)
cr
Membrane
35
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Table 3 - Diffusion coefficients of water and isopropanol at different mass% of water in the feed for different membranes. Dw x 108 (cm2/s)
Mass
DIPA x 1010 (cm2/s)
M
M-1
M-2
M-3
M-4
M
M-1
5
9.21
13.43
17.11
20.70
21.43
0.62
0.39
10
10.30
14.80
18.01
20.93
22.61
1.54
15
10.62
16.30
18.11
21.21
23.72
20
11.52
16.91
19.61
21.31
23.80
25
11.73
17.32
19.82
22.10
24.02
M-2
M-3
M-4
0.37
0.36
0.28
0.97
0.92
0.68
cr
water
ip t
% of
us
1.07 3.39
2.02
1.66
1.54
8.69
7.46
6.43
5.15
3.83
25.72
25.71
23.61
23.50
23.20
Ac ce p
te
d
M
an
3.79
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Table 4 - Pervaporation flux and separation selectivity for different membranes at different temperatures for 10 mass% of water in the feed. J x102 (kg/m2h)
Temp.
sep
ip t
°C M-1
M-2
M-3
M-4
M
M-1
M-2
M-3
M-4
30
5.05
7.21
8.32
10.29
11.37
2186
4491
6420
7491
11241
40
7.37
8.05
8.44
10.90
11.71
572
711
50
7.89
8.50
9.21
11.63
12.61
441
741
774
809
us
cr
M
634
683
607
Ac ce p
te
d
M
an
520
37
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Table 5 - Arrhenius activation parameters for permeation and diffusion, and heat of sorption.
M-2
M-3
Ep
18.15
16.68
14.11
12.98
ED
17.81
16.35
13.80
12.68
Epw
17.49
16.06
13.53
EpIPA
83.14
94.47
100.41
Hs
0.34
0.33
9.18
us
8.91 9.74
105.92
118.82
an
12.46
0.30
0.27
Ac ce p
te
d
M
0.31
M-4
ip t
M-1
cr
M
38
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Ac ce p
te
d
M
an
us
cr
ip t
Figure 1
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Ac ce p
te
d
M
an
us
cr
ip t
Figure 2
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Ac ce p
te
d
M
an
us
cr
ip t
Figure 3
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Ac ce p
te
d
M
an
us
cr
ip t
Figure 4
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Ac
ce
pt
ed
M
an
us
cr
i
Figure 5
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Ac ce p
te
d
M
an
us
cr
ip t
Figure 6
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Ac
ce
pt
ed
M
an
us
cr
i
Figure 7
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Ac
ce
pt
ed
M
an
us
cr
i
Figure 8
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Ac
ce
pt
ed
M
an
us
cr
i
Figure 9
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Ac
ce
pt
ed
M
an
us
cr
i
Figure 10
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Ac
ce
pt
ed
M
an
us
cr
i
Figure 11
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Ac
ce
pt
ed
M
an
us
cr
i
Figure 12
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Ac
ce
pt
ed
M
an
us
cr
i
Figure 13
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