1. Introduction
Separation process based on inorganic membrane, organic polymeric membrane, thin film composite membrane, ion exchange membrane is extremely popular due to easy processability cost effective and better tune ability[1-5]. Stability of inorganic is best among all due to high mechanical strength, thermal as well as chemical stability for which it can be used under any harsh conditions. Zeolite is one of the example of inorganic membrane widely used due to the excellent porosity, organized structure and easy tuning of hydrophobicity or hydrophilicity by controlling the Si/Al ratio. Pervaporation is liquid vapor separation process in which hydrophilicity of the membrane is one of the important properties to separate valuable solvent diluted with water.
Dehydration of organic solvent under wide range of pH requires membrane having stability in these conditions. Diluted acetic acid is by product of many chemical industries dealing with acetic acid, vinyl acetate and various other solvent producing industries. So removal of water and concentrating acetic acid through membrane separation process is one of the key issues. Through pervaporation process of separation of organic- organic mixture, temperature sensitive materials and azeotropic solvent are most efficient and cost effective techniques. One of the example is separation of methyl methacrylate (MMA) and methanol[6]. Composite membrane made up of silicates are used extensively for recovery of ethanol from water with separation factor of 24. 6[7]. Different types of zeolites membranes are used for the recovery of ethanol[8-11].
Dehydration of dimethoxymethane by membrane pervaporation process by zeolite membrane are successfully utilized[12]. Effeciency of esterification improved by rapid dehydration through zeolite membrane[13]. The classification of the review is presented schematically in Fig. 1.
2. Ethanol Dehydration
Yugawaralite zeolite membrane was synthesized through a novel and simple method[14]. M. Azarshab et al. made use of blast furnace slag (BFS), gross waste of steel industry, to fabricate the membranes through alkali activation. They first prepared a primary geopolymer gel and casted membranes at 25°C for 24 h. Then, these membranes after polymerization process form the zeolite layer. Also, they used four different water levels and alkali levels and found the optimum membrane was the one with 42 wt% water and 4 wt% alkali which showed permeation flux of 0.33 kg/m2⋅h, selectivity of 2579.48, and PSI value of 847.62. The thickness of the synthesized zeolite layer which was obtained by SEM analysis was about 5 μm. From these results, they concluded that the most effective parameters for synthesizing these membranes are water content and solution alkalinity.
A novel seeding method called interfacial polymerization through dip coating (IPDC) was devised by Y. Cao et al. to dehydrate ethanol aqueous solution[15] (Figs. 2 & 3).
Microporous α-Al2O3 support was coated with NaA crystals whose size is about 150 nm. Polyamide (PA) was produced by interfacial polymerization (IP) and made a continuous and dense NaA seed layer. Even though only 0.5 wt% seed content was put in a coating suspension, a very thin seed layer that has high quality and good adhesion was obtained by dip coating once more without drying after the first dip coating. Separation factor of above 10000 with 9.0 kg/m2⋅h flux was achived in PV process through zeolite membrane with 90 wt% ethanol aqueous solution at 75°C.
Another NaA zeolite membranes were developed by D. Liu et al. for pervaporation dehydration of ethanol[ 16]. The membranes were synthesized on top of 4 channel α-Al2O3 hollow fiber (4CF) surface by first seeding 4CF supports through hydrothermal crystallization. The as-synthesized membranes displayed a good water flux of 12.8 kg/m2⋅h and a separation factor of over 10000 when dehydrating 90 wt% ethanol/ water mixtures at 75°C. Since the membranes showed high reproducibility, as well, 4CF-supported NaA zeolite membranes have good potential in industrial applications.
Making a homogeneous mixture of inorganic fillers in polymer matrix is challenging for the matrix membranes (MMMs) to be used in PV[17]. To solve this issue G. Liu et al. devised an effective dispersion method of grafting n-octyl chains and then coating a thin polydimethylsiloxane (PDMS) layer on ZSM-5 surface. ZSM-5 filled PDMS mixed matrix/ceramic composite membranes that have homogeneously dispersed ZSM-5 zeolite in PDMS were fabricated by this method and evaluated. The interaction between PDMS and zeolite was calculated by molecular dynamics simulations, and it was turned out that their interaction between zeolite particles and PDMS matrix could highly be improved by modifying the surface of zeolite particles. SEM was carried out to confirm the uniform dispersions of modified particles in the matrix. The FT-IR analysis display lower hydroxyl group and contact angle measurement indicate increase of hydrophobicity by increasing zeolite loading.
V. Sebastian et al. utilized microwave heating in the preparation of capillary MFI-type zeolite membranes for PV application[18]. The MFI-type zeolite membranes were fabricated on both sides of the ceramic α -Al2O3 capillaries which have a high area-to-volume ratio of over 1000 m2/m3 which was tested for water/ ethanol (95/5 wt%) mixture. At a pervaporation temperature of 45°C, the double-sided membranes showed the best flux of 1.5 kg/m2⋅h and selectivity of 54. Plus, membrane quality improved when the synthesis is performed at 155°C instead of 160°C.
For pervaporation application, W. Yuan et al. devised good performance NaA zeolite-polyimide composite membranes[19]. Zeolite membrane ws prepared on porous seeded α-Al2O3 ceramic hollow fiber though linking with 3-aminopropyltriethoxysilane. The synthesis solution with ratio of 50Na2O:(TMA)2O:4.5SiO2:Al2O3:950H2O was pre-crystallized at 60°C for 2 hours followed by microwave heating. 10 μm in thick composite membrane was prepared though dip coating method. XRD results displayed that there is only NaA zeolite on the support. FT-IR confirmed that PI has been successfully fabricated on the surface of NaA zeolite membranes. SEM showed that the particles are twinborn on the surface. After pervaporation experiments on dehydration of 90 wt% aqueous ethanol solution, it was turned out that the selectivity of composite membranes was up to 13000.
3. Bio-oil Dehydration
Bio-oil derived from biomass is mixture of various type of components in which water is one of the main component that reduce the heating value. Dehydration of bio-oil is a complex issue due to its sensitivity to heat as well as complex nature of the compounds present. So PV process by use of zeolite membrane is one of the alternative way to get rid of water.
Since previous bio-oil dehydration method using HZSM-5 zeolite has limits on producing heavier biofuels in the distillate ranges, J. Kittikarnchanaporn et al. tested HY and HBeta zeolites, hoping they could get larger hydrocarbons[20]. The zeolites were even doped with group 5A oxides at 5 wt% of loading, aiming to increase distillate-range products. The researchers carried out experiments at 500°C and discovered that the distillate production depends on the channel opening size of zeolites regardless of the type of loaded oxides, while the yield also depends on other parameters like strength of acid and the pore channel. Plus, they confirmed that Group 5A oxides doped on zeolites indeed improve the oil yield. It would be better if catalyst support should have higher strength of Bronsted acid.
The dehydration of bio-oil is important since it can be applied as a renewable transportation fuel, so G. Li et al. suggested a T-type zeolite membrane[21]. XRD showed that the T-type zeolite layer grew successfully on the tubular α-Al2O3 support. SEM analysis showed α-Al2O3 was coated by rod-shaped T-type zeolite crystals with a thickness of approximately 5 μm. The crystallinity of T-type zeolite crystals is stable even after use for 2 weeks. A rapid reduction in the permeation flux caused by the easily fouled membrane in bio-oil due to a complex composition and found that calcination at 220°C can solve this problem. At the same time tuning the pore size and pore density mitigate the membrane fouling.
To make bio-oil pervaporation dehydration more efficient, G. Li et al. prepared ZSM-5 zeolite membranes[ 22] (Figs. 4 & 5).
They investigated the permeation performance and found that water mainly flow through ZSM-5 membranes through intercrystalline pores and fouling occur by blocking of these pore. Tuning the transport path is key and it was achieved by reducing membrane fabrication time. Since the robust membranes have abundant intercrystalline pores, permeation flux in bio-oil dehydration was enhanced.
A. V. Klinov et al. investigated PV dehydration of ethanol and isopropanol by using HybSi membranes[ 23]. The isopropanol dehydration showed better flux as well as separation factor than the ethanol dehydration. A mathematical model of the PV process was devised based on the “solution-diffusion” concept. The model consists 3 parameters: two permeability coefficients for pure components and an “active pores fraction”, not only help establish optimum the operating conditions for the pervaporation process, but also critically reduce the number of PV experiments needed for fabricating a PV pilot plant. The results calculated from the proposed model were compared with the results of PV dehydration of glycerin through HybSi membranes from other research and PV dehydration of ethanol through zeolite membranes. The calculated results met reasonably good with experimental data.
A two-step acid/alkali-catalytic method was devised by E. Lv et al. to make biodiesel from Firmiana platanifolia L.f. seed oil acquired from solvent extraction by absolute ethanol[24]. To reduce the content of free fatty acids (FFAs) below 1 wt%, the esterification of seed oil with ethanol was conducted with the cation exchange resin. A NaA zeolite membrane was utilized for dehydration. Alkali-catalysis transesterification was carried out in the presence of alkali. The combination of esterification with PV could move the reaction towards equilibrium. The FFAs conversion rates reached over 99.0% when it is operated for 7 hours at 78°C with ethanol to oil molar ratio of 15:1 and catalyst dosage of 40.0 wt%.
5. Conclusions
Zeolite membrane is one of the best alternative to polymeric membrane due to its chemical, mechanical, thermal stability as well as simplicity in designing uniform porous structure by solvothermal, sol-gel process etc. Hydrophilicity of the zeolite membrane can be easily tuned by varying the Al/Si ratio. Growth and crystallization of zeolite layer trough placement of a seed on the porous support is controlled by the temperature and duration of hydrothermal process. The pore diameter of the zeolite membrane usually about 0.41 nm is most suitable for dehydration by PV of ethanol, methanol and acetone. This review mainly discusses about dehydration of ethanol, bio-oil as well as few others by membrane PV.
