zeolite membranes

zeolite membranes

Chemical Engineering & Processing: Process Intensification 118 (2017) 47–53 Contents lists available at ScienceDirect Chemical Engineering & Process...

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Chemical Engineering & Processing: Process Intensification 118 (2017) 47–53

Contents lists available at ScienceDirect

Chemical Engineering & Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep

Pervaporation dehydration of binary and ternary mixtures of acetone, isopropanol and water using polyvinyl alcohol/zeolite membranes

MARK



Akbar Malekpour , Behnaz Mostajeran, Gholam Ali Koohmareh Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran

A R T I C L E I N F O

A B S T R A C T

Keywords: Pervaporation Dehydration Acetone Isopropanol Composite membrane

Mixed matrix membranes were prepared by uniformly dispersing of zeolite particles in a polyvinyl alcohol matrix. After membrane characterization, the ability of the prepared membranes was examined toward pervaporation dehydration of binary and ternary mixtures of acetone, isopropanol and water because of their industrial importance. The effect of different parameters on the membrane performance was investigated and the best conditions were obtained. The results showed that, NaA zeolite particles significantly improved the separation performance of the membranes. The selectivity was increased with an increase of zeolite loading up to 5% wt. The best results were obtained for dehydration of isopropyl alcohol where the separation factor was calculated as 1881. Based on the findings of the research, the incorporation of NaA zeolites into the polymer matrix can be very effective for dehydration of acetone and isopropanol. The results show that the prepared membrane can be a good candidate for removal of water in acetone production from isopropyl alcohol.

1. Introduction Acetone (ACE) is one of the most important solvents that is mainly used as a solvent and as a raw material for the production of methyl methacrylate and bisphenol A. This compound may be produced by several methods. One of the common methods is its production from isopropyl alcohol (IPA) by catalytic oxidation or dehydrogenation of isopropyl alcohol. In most cases, regardless of the process used for its preparation, manufactured acetone contains a substantial amount of water. Therefore, dehydration of acetone still remains a critical issue. Although acetone and water do not form an azeotrope, a strong reflux, a large column and high energy cost are required during the distillation process to obtain high purity acetone. On the other hand, dehydrogenation of IPA is a very important process for producing of ACE in Western Europe. A ternary system ACE/IPA/Water is formed in the reactor container during the dehydrogenation process of IPA [1]. The separation of IPA and water is difficult due to the formation of an azeotropic mixture between IPA and water (at the mole fraction of water: 0.3167 mol mol−1 and the boiling temperature of 353.55 K at atmospheric pressure) [2]. The membrane technology can be used in the acetone production industries through two methods: in the selective removal of water from the water/ACE or ACE/IPA/Water mixtures by pervaporation processes and in the membrane reactor manufactured using appropriate membranes for simultaneous removal of water along the acetone production.



Corresponding author. E-mail address: [email protected] (A. Malekpour).

http://dx.doi.org/10.1016/j.cep.2017.04.019 Received 1 December 2016; Received in revised form 3 April 2017; Accepted 27 April 2017 Available online 06 May 2017 0255-2701/ © 2017 Elsevier B.V. All rights reserved.

Pervaporation is a membrane separation technique which has a significant ability for removing water from liquid mixtures [3]. In the conventional distillation processes, the separation depends on the vapor–liquid equilibrium, but in the pervaporation, separation of the liquid mixtures can perform based on the difference in diffusivity of each liquid component in a membrane [4]. In this technique, the separation of mixtures is achieved by a partial vaporization of components through a suitable membrane. Due to the vaporization of permeating components, pervaporation is the most economical one when the concentration in the feed mixture is low for the favorable permeating components [5,6]. In the case of pervaporation, the difference in fugacity between feed and permeate side of the membrane, can be expressed as driving force for transferring the components through the membrane matrix [7]. One of the most important applications of pervaporation is separation of close boiling mixtures [8,9]. Because of requiring no additional chemicals and operating at moderate pressures and temperatures, pervaporation is reported as eco-friendly, economical and safe method [10–12]. Pervaporation is an economical separation technology and also an environmentally clean technology in which potential pollution sources for azeotropic distillation are not needed [13]. A hydrophilic membrane might be used to separate water from water/organic mixtures. Recent trends in membrane separation involve the development of composite membranes by incorporating zeolites as the reinforcing fillers. A mixed-matrix membrane might be useful for

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Nomenclature α A AC E FT-IR GA GC IPA J ??0

M NaA PV PVA R SEM T TCD w Wd Ws Wt%

Separation factor Membrane surface (m2) Acetone Activation energy (kJ/mol) Fourier transform infrared Glutaraldehyde Gas chromatography Isopropyl alcohol Permeation flux (kg/m2.h) Pre-exponential factor of permeation

Molar (mol/L) Zeolite 4A Pervaporation Polyvinyl alcohol Gas constant Scanning electron microscopy Temperature (k) Thermal conductively detector Permeate mass (kg) Dry membrane weight Swollen membrane weight Weight percent

2.2. Synthesis and characterization of NaA zeolite particles

water pervaporation since it shows not only molecular sieve effects, but also good thermal, chemical and mechanical stabilities [14,15]. Polyvinyl alcohol (PVA) is a proper candidate as a bulk of membranes because of its excellent membrane-forming properties, convenient physical properties, low cost, high hydrophilicity, process ability and suitable chemical resistance in many pervaporation separation processes [16–18]. Nevertheless, especially when the membrane is operated under high fraction of water in feed mixtures, the –OH groups along PVA main-chains make the PVA-based membranes suffered from an excessive swelling [19]. Among the zeolites, NaA zeolite has appropriate molecular sieving action, high selective adsorption capacity and strong hydrophilic nature [20,21]. The molecular sieving effect is provided by the porous structure of the embedded-zeolite, with 0.4 nm pore diameter. The zeolite NaA allows water molecules with a kinetic diameter of 0.296 nm to diffuse easily through its apertures, but excludes larger organic molecules such as ACE and IPA [22] (kinetic diameters of ACE and IPA are 0.46 and 0.47 nm respectively). Incorporating the zeolite NaA with the PVA membranes will enhance the water permeation flux while reducing the membrane swelling and giving a higher degree of water selectivity. In addition, zeolites have a high mechanical strength, good thermal and chemical stability and the membranes incorporated with these fillers can be used over a wide range of operating conditions. The aim of this paper is pervaporation separation of the binary ACE/ Water, IPA/Water and ternary ACE/IPA/water solutions using high efficiency PVA/NaA composite membrane. The effects of some parameters such as zeolite contents and feed temperature on pervaporation performance were also investigated.

The hydrothermal synthesis of NaA zeolite particles was performed as follows: an aluminate solution was prepared by dissolving sodium hydroxide (4.04 g) and aluminum chloride (4.04 g) in distilled water (16 mL). A silica solution was prepared by dissolving 2.30 g sodium hydroxide and 1.63 g silica gel in 18 mL distilled water. Then aluminate solution was immediately added to the silica solution, and then it was stirred vigorously for 14 min. The prepared gel was hydrothermally treated for 3 h at 100 °C. The molar composition of the resulting gel was: 3.165 Na2O: Al2O3:1.926 SiO2: 128 H2O [23]. The crystal structure of the synthesized zeolite particles was examined with a thin-film X-ray diffraction using Cu kα radiation (XRD, Bruker, AXS Co., Germany, 30 kW). The XRD pattern of the synthesized NaA particles was compared to the standard NaA zeolite crystals. The surface morphology of the zeolite particles was observed using TESCAN Scanning Electron Microscopy (SEM). 2.3. Membrane preparation The PVA/NaA composite membrane was prepared by uniformly dispersing NaA particles into the polymer matrix. Because of hydrophilic properties of PVA and NaA zeolite particles, water is a favored permeating component. The integrated flat sheet membrane (4 cm in diameter) was placed in a test cell built of polyethylene and had an effective area of 50.24 cm2. In a typical synthesis, a polymeric solution was prepared with dissolving proper portions of dry PVA powder (solvent = 90 wt.% and polymer = 10 wt.%) in double distilled water at 100 °C to obtain a clear solution. This solution was mixed with different amounts of zeolite particles to reach final contents of 0, 2.5, 5, 7, 10, 12 wt fractions. The PVA-zeolite dispersion was stirred at 70 °C for 12 h to allow the cross linking reaction of the gel to proceed and propagation of particles become complete, followed by the addition of 0.5 mL GA cross linking agent [24,25]. After rigorous stirring for 2 h the prepared solution was sonicated for 15 min and immediately was cast on the plate. A thin film with about 60–70 μm thickness was formed after water evaporation at room temperature for 24 h.

2. Materials and methods 2.1. Materials PVA (98%, molecular weight = 72000, Applichem) was used as a dense membrane material. Glutaraldehyde (GA, 25 wt%, Merck) was used as cross-linking agent. HCl (Merck) as a 0.1 M solution was used for hydrolyzing the polymer during the cross linking process. ACE and IPA were supplied with purity > 0.998 kg kg−1 and used without any further purification. The following chemicals were utilized to synthesize the NaA zeolite particles: sodium hydroxide (NaOH, 1.0 mol/L) as a Na source, aluminum chloride with a content of 98 wt% as an Al source, silica gel as a Si source. Commercially available NaA zeolite (code: 233668, molecular sieve, powder, < 5 μm) and isobutanol (GC grade 99.5%) were purchased from Merck. For the calibration of GC (in the analytical step for ternary mixtures) the chemicals (ACE, IPA and isobutanol) were obtained from Merck with a guaranteed purity of ≥0.999. Double distilled water was used throughout the research work.

2.4. Characterization The surface morphology of the cast composite membrane was observed by scanning electron microscopy (SEM). The samples were prepared by fracturing in liquid N2 and coated with a conductive layer of sputtered gold. Moreover, the FT-IR spectra of the composite membrane was obtained from Jasco, 6300 FT-IR Spectrometer, operating in the range of 4000–750 cm−1. The swelling tests were performed for the membranes containing different amount of zeolite (0, 2.5, 5, 7, 10, and 12) after they were dried in vacuum at room temperature for 12 h. The dried membranes were weighted (wd) and immersed in the ternary mixture for 20 days. 48

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the zeolite surface (Al–OH, Si–OH) and PVA active surface groups, compatibility of the zeolite surface with PVA was improved. Fig. 3 shows the FT-IR spectra of PVA/NaA composite membrane. The spectrum revealed the absorption bands at 600 and 900 cm−1 which are correspondent to Al–O and Si–O bonds of the AlO4 and SiO4 tetrahedral in zeolite framework. The peaks around 1717 cm−1, 2850 cm−1 and 3200 cm−1 are attributed to C–O, C–H and O–H stretching vibrations respectively. By comparing FT-IR spectra of the PVA with the composite, it was confirmed that the functional groups of polymer, remained intact with no chemical interaction with zeolite particles. The prepared PVA-based polymeric membranes showed pronounced swelling in aquatic media, which affects permeate fluxes and selectivities. As shown in Fig. 4 the swelling degree is maximum for neat PVA membrane. The zeolite incorporation into the polymer matrix was causing the decrease of the swelling. Swelling assuagement by adding NaA particles is due to a lower water adsorption capability of the zeolite particles [27], as compared with PVA because zeolites are more resistant to swelling in comparison with PVA. In fact, the presence of zeolite particles between the hydrophilic polymer chains causes reduction of loosening of the polymer chains in the composite membrane. By increasing the amount of zeolites the capacity of the membrane for water adsorption was increased. The latter is due to adsorption of water by zeolites which they keep the water molecules in their channel and cavities. Except water, ACE and IPA can also be adsorbed by the PVA matrix, while their adsorption in NaA zeolite is limited because of the narrow pore size of NaA zeolites.

The membranes were then taken out and the excess solvent were carefully wiped off. After that, the swollen membranes were weighted again (ws). The swelling degree of membrane was calculated using the following equation:

swelling degree (%) =

(ws − wd) × 100 wd

(1)

The interaction between water molecules and the –OH pendant groups of PVA causes the membrane to prefer water molecules [26]. 2.5. Pervaporation experiments A laboratory-scaled setup was designed and assembled for the pervaporation experiments. In Fig. 1 the schematic view of the setup is shown. All main parts were manufactured from stainless steel or polyethylene. In this regard, a pump was used to circulate the feed solution from the feed tank (capacity = 2 L) to the membrane module. A vacuum pump continuously evacuated the system down to 2.6 mbar and generated the driving force for the membrane separation by lowering the partial pressure on the permeate side. A glassy cold trap, immersed in liquid nitrogen was used to trap permeate. The feed temperature was adjusted by a water bath and measured using a digital thermometer. Before each run, the membranes were allowed to equilibrate with the feed solution for about 1 h. The permeation flux of the membrane was calculated by weighting the collected permeate using a digital microbalance. Samples of the feed mixtures were collected from the feed tank. All the experiments were repeated three times and average results were presented. The membrane performance in pervaporation experiments was studied by calculating total permeation flux (J) and separation factor (α) using the following equations:

J=

w A. t

3.2. Pervaporation tests Different experiments were conducted with the binary ACE/Water, IPA/Water and ternary ACE/IPA/Water mixtures for evaluation of the prepared membranes. Also the effects of various variables such as feed temperature and zeolite percentage in the polymer matrix were investigated. The selected mixtures are very important in acetone and isopropanol manufacturing plants. The main separation task in this work was to overcome the azeotrope point between IPA and water. Thus, the water mass fraction of the feed was selected near to the azeotropic region. In binary mixture, the azeotropic point of IPA and water is in 0.14 kg/kg. The feed temperature was varied between 30–60° C. For the binary ACE/Water mixture, the feed temperature was varied between 30–50° C to prevent a loss of feed mixtures due to the vaporization. The water mass fraction in the feed was set to 0.1 kg/kg. To identify the influence of a third component on the separation characteristic, the experiments have been performed with the ternary mixtures. The feed composition was selected as 30:30:40%wt for ACE: IPA: Water. The effect of zeolite amounts on the membrane performance was investigated by changing the zeolite content between 0 and 12 wt.%.

(2)

Where J is total permeation flux in kg/m2 h, w is the permeate mass in kg, A is the membrane surface area in m2 and t is the permeation time in h. Separation factor (α) is defined as:

α=

yp / yf xp / xf

(3)

Where xf and yf are weight fractions of water and organic compound (ACE or IPA) in feed and xp and yp are weight fractions of water and organic compound in permeate streams. 2.6. Head space-GC analysis A GC chromatograph (GC, Philips, PU 9100) with OV1 packed column, equipped with a thermal conductivity detector (TCD) was used for analysis of the samples. 0.5 mL of each sample was put into isolated vials. The vials were placed in a water bath (50° C) for 15 min. After equilibrium conditions were achieved, 500 μL of the vapor phase from the sample headspace was injected into the GC column. The mass fractions of the organic compounds (ACE and IPA) were obtained directly from the evaluation of the chromatogram, based on singlecomponent calibration curve using isobutanol as internal standard. Water mass fraction of each sample was calculated by subtraction of the sum of the ACE and IPA from 100%.

3.2.1. Effect of feed temperature The influence of feed temperature on the separation characteristics

3. Results and discussion 3.1. Characterization The SEM images of the prepared zeolite and its composite form with PVA are shown in Fig. 2a and b, respectively. The images demonstrate that NaA zeolite particles are uniformly distributed on the membrane surface and it can also be clearly seen that no aggregation of the zeolite particles is observed. Due to the formation of hydrogen bonds between

Fig. 1. Simplified flow sheet of the laboratory-scale setup for pervaporation experiments.

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Fig. 2. SEM micrograph of (a) NaA zeolite particles and (b) PVA/NaA composite.

For binary ACE/Water mixture, an increase in the operating temperature caused the increase in permeate flux (Fig. 6). While it had no specific influence on water percentage in the permeate side. These findings showed that the changes in separation factor by changing temperature were not significant. The comparison between the amounts of activation energy of water in ACE/Water and IPA/Water mixtures did not show any significant differences. While, the activation energy for ACE is slightly higher than that for IPA. The separation studies on the ternary ACE/IPA/Water mixture were shown that permeation of both organic compounds (ACE and IPA) had a substantial increase with increasing feed temperature, especially at higher water mass fractions of feed (Fig. 7). This subject can justify the decreasing trend of separation factor at higher feed temperatures. The apparent activation energy values for IPA, ACE and water, for binary and ternary solutions were calculated from the slopes of Arrhenius plots by the least square method and the related data are presented in Table 1. The results showed that the water activation energy in this system is almost the same as its values that calculated in binary systems. On the other hand, in the binary systems the activation energies for water permeation is lower than the ones for organic species. The comparison between binary and ternary data from the point of view of activation energies, denotes that for all three species, these energies are significantly lower than the ones determined for the binary systems.

Fig. 3. FT-IR spectra of PVA/NaA composite membrane.

3.2.2. Effect of zeolite loading All of the data related to permeation and separation factor of PVA/ NaA composite membrane at different percentages of zeolite loading (0, 2.5, 5, 7, 10 and 12 wt%) are shown in Fig. 8. The results were obtained for 90 wt.% IPA aqueous solution at room temperature. Compared to the neat PVA membrane, total permeation flux and separation factor were improved with increasing zeolite loading up to 5 wt.%. This is due to this fact that the hydrophilic nature of NaA zeolite can increase the PVA matrix hydrophilicity. In fact, zeolite particles with molecular sieving properties have a significant effect on the transport of water molecules through the membrane. The presence of these particles, could facilitate the water transport, despite of a reduction in free

Fig. 4. the effect of zeolite loading percentage on the swelling of PVA/NaA composite membrane.

of PVA/NaA composite membrane, in terms of total permeate flux and water separation factor is shown in Fig. 5. An increase in the feed temperature was enhanced strongly the total permeate flux, whereas separation selectivity was decreased. It seems that at lower temperature only water molecules can transport through the membrane matrix. While at higher temperatures, because of plasticizing effect and loosening of the polymeric chains, different molecules can permeate through the membrane. It was also observed that the diffusion of water molecules through the membrane is enhanced at higher temperatures, due the more flexibility of membrane matrix [28], therefore, the permeation fluxes were increased. Temperature increasing showed an increasing influence on IPA diffusion through the membrane. Accordingly, the membrane water selectivity was decreased at higher feed temperatures. The temperature dependency of experimental permeate flux data is often described using an Arrhenius-type relation (Eq. (3)).

⎛ −E ⎞ J = J0 exp ⎜ ⎟ ⎝ RT ⎠

(4)

which J is permeation flux, J0 is pre-exponential factor of permeation, R is the gas constant, E is activation energy and T is temperature in Kelvin. The graphical representations of the Arrhenius-type relation were constructed and a linear behavior was observed.

Fig. 5. Effect of feed temperature on separation factor and total permeation flux of 90 wt. % IPA/water solution with 5 wt.% composite membrane.

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Fig. 6. Effect of feed temperature on separation factor and total permeation flux of 80 wt. % ACE/Water solution with 5 wt.% composite membrane. Fig. 9. Effect of zeolite loading on separation factor and total permeation flux in binary ACE/water solution at room temperature.

decreased as the zeolite content became over 5 wt.%. This reduction can be due to the poor interfacial connection between zeolite and polymer matrix and agglomeration of NaA particles. In dehydration process of ACE, the effect of zeolite loading was investigated by calculating total permeate flux and separation factor at different zeolite contents at room temperature and feed composition of about 80 wt.%. Fig. 9 shows the influence of this parameter on the flux and separation factor. The results showed that the total permeation flux was increased with increasing the zeolite content. Similarly, for separation factor up to 5 wt.% the same trend was observed. But when more than 5% was used, this percent due to the reduction in adhesion between polymer and zeolite particles, was decreased [3]. The effect of zeolite loading on the total flux and separation factor of the PVA/NaA membrane when a ternary mixture of ACE/IPA/water was used is shown in Fig. 10. A considerable increase in the flux was observed when the NaA content was increased from 0 wt.% to 12 wt.%. The water selectivity showed an enhancement trend with increasing the amount of zeolite up to 5 wt.%. When the zeolite loading was increased to more than 5 wt.%, the organic contents in permeate were suddenly increased, implying that there was a leakage of organic species through defects of the membrane [32]. These defects were the outcome of agglomeration of NaA particles in high percentages, which provides an easy passage for all permeates. Finally, for showing the validation of the method, a comparison study with previously reported works was performed and the resulted data are shown in Table 2. Based on the finding, remarkable advantages in terms of amounts of fluxes and separation factor are obvious. In addition, the presented membranes can be prepared rapidly, costeffective and simply.

Fig. 7. Effect of feed temperature on water separation factor and total permeation flux of ternary ACE/IPA/Water solution with 5 wt.% composite membrane. Table 1 Calculated activation energies of the components for passing through the PVA/NaA composite membrane. The values for E (kJ/mol) were determined using total permeate fluxes. component

Binary IPA/water

Binary ACE/water

Ternary ACE/IPA/water

ACE IPA water

— 90.49 ± 13.60 20.17 ± 3.67

101.4 ± 2.62 — 25.69 ± 4.69

50.89 ± 6.50 42.39 ± 3.65 23.30 ± 0.67

4. Conclusions The PVA/NaA composite membranes were successfully prepared using PVA as polymer matrix and NaA zeolite particles as fillers. The Fig. 8. Effect of zeolite loading on separation factor and total permeation flux in binary IPA/water solution at room temperature.

volume [29,30]. The use of hydrophilic fillers, causes a decrease of the activation energy for permeation of water molecules [31], therefore, water flux and total permeation flux through the membrane are increased in comparison with the neat PVA membrane. Fig. 8 shows that the incorporation of NaA particles has a significant influence on the separation factor. Separation factor was increased by increasing the zeolite content and it was reached to its highest value at 5%. The calculated separation factor in this content was obtained 1881. It seems that the increasing of adhesion between polymer chain and zeolite particles and the reduction in free volumes in the membrane matrix is responsible for these observations. However, separation factor was

Fig. 10. Effect of zeolite loading on water separation factor and total permeation flux in ternary ACE/IPA/water solution at room temperature.

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Table 2 A comparison study about dehydration of solvents with different membranes. Membrane type

Target

T(°C)

Total Flux (kg/m2h)

PVA PVA PVA PVA PVA/NaA PVA/zeolite-g-PHEMA PVA/PANI PVA/NaX PVA/NaA PVA/NaAlg Zsm-5/PDMS PDMS/ceramic DMS/PI PVA/NaA

Acetone Ethylacetate Acetic acid Isopropanol Butanol acetone Isopropanol Isopropanol Ethylen glycol Isopropanol ethanol ethanol ethanol Isopropanol/Acetone

30 50 40 60 25 30 30 30 70 30 50 40 50 30

0.329 0.022 0.1 0.08 1.5 1.31 0.069 0.216 2.4 0.039 0.200 1.6 0.032 0.9

Separation factor

Total Selectivity

Water selectivity

79.9 5000 200 1492 15 2357.7 564.2 133 1520 11241 23.5 8.9 6.6 1881

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