Introduction and Literature Review
1.3 Ceramic membranes
Ceramic membranes are made of various inorganic materials such as α-alumina, γ-alumina, zirconia, silica, titania, kaolin, etc. In comparison with polymeric membranes, ceramic membranes possess superior chemical, thermal and mechanical stability. It can also be used in harsh environment in chemical process industry. The thickness of ceramic membrane is in the range of 2-5 mm and it may be higher depending on the specific applications.
Asymmetric membranes constitute thin film in the range of 10-100 µm ceramic coating over a thick symmetric support.
Some advantages of ceramic membranes (Mulder 1991):
a) High corrosion resistance; in very few chemicals, such as hydrofluoric acid and phosphoric acid, ceramic membranes don’t have high corrosion resistance. One of the most useful features of the ceramic membranes is that their ability to tolerate strong doses of chlorine (up to 2000 mg/L in certain cases).
b) Applicable to wide range of pH (0.5-14).
c) Applicable to broad temperatures (350-500 °C). As a result, it can be used in industrial scale separations without any feed pre-conditioning steps.
d) Longer life span (5-10 years). In practical, there are many ceramic systems in industry, which are operating after 10-14 years of installation.
e) Less fouling tendency.
f) Inertness to common chemicals and solvents.
g) Higher mechanical strength.
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Few drawbacks of ceramic membranes:
a) Ceramic membranes are comparatively higher cost. For instance, in 1996 cost indices of polymeric spiral wound modules and ceramic systems were 50-100 $/m2 and 500- 3000 $/m2, respectively. Along with these cost including controls, pumps and fittings were estimated to be 225-350 $/m2 and 2200-3600 $/m2 for polymeric and ceramic membranes (Mulder 1991). This indicates that a tenfold higher cost of ceramic membranes when compared to polymeric membranes. In due course of time, though the price of ceramic membranes may have reduced, these costs have not been very competitive with polymeric membranes. Because of that, it is expected that the cost of ceramic membranes will be still higher to that extent of 2-3 orders of magnitude.
b) Ceramic membranes are brittle in nature. It may be broken or damaged if it drops.
1.3.1 General methods used for the preparation of ceramic membrane
Inorganic membranes have permselectivity’s that are five to ten times higher than the conventional polymeric materials. Furthermore, they are more stable in aggressive feeds. The majority of ceramic glass, carbon and zeolitic membranes cost between one and three orders of magnitude more per unit of membrane area when compared to polymeric membranes.
Moreover, they are complicated to fabricate into large and defect-free areas. The polymeric materials have an advantage that they can be processed into hollow fibers, which offer high separation output due to the inherently high surface area to volume ratio. Inorganic membranes classified into two major categories based on its structure: porous inorganic membrane and dense (non-porous) inorganic membranes. A few fabrication techniques of inorganic membranes are given in Fig. 1.3. Typically, the porous inorganic membranes have the structures of asymmetric and symmetric framework. The porous inorganic membrane with pores more than 0.3 nm usually work as a sieve for large molecules and particles. The
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commercially utilized porous inorganic membranes are glass, metal, alumina, zirconia, zeolite and carbon. Other inorganic materials such as cordierite, silicon carbide, silicon nitride, titania, mullite, tin oxide and mica are used to produce inorganic membranes. These membranes vary significantly in pore size, support material and configuration. On the other hand, dense membranes made of palladium and its alloys, silver, nickel and stabilized zirconia are used for the fabrication of inorganic membranes.
Fig. 1.3 Fabrication techniques of inorganic membranes
Mesoporous materials can be synthesized by different methods of preparation. In all these methods, common ingredients are (1) Silica source, (2) Templating agent i.e. structure directing agent, (3) Solvent and (4) Mineralising agent (NaOH, NH3, HCl).
1.3.2 Solvent evaporation techniques
The solvent evaporation techniques involve formation of a liquid film containing the solvent, surfactant and silica precursor followed by evaporation of the solvent. It is divided into two methods.
1.3.2.1 Dip coating
The dip coating technique is widely used in the preparation of composite membrane. In this Fabrication techniques
Anodic oxidation Phase separation
and leaching
Co-pressing Co-sintering or
Dip coating
Chemical vapor deposition
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solution is allowed to drain to a particular thickness. The thickness of the film is mainly determined by the rate of evaporation of the solvent and the viscosity of the solution. Fig. 1.4 displays the dip coating process.
Fig. 1.4 (a) Dip coating, (b) After coating
1.3.2.2 Spin coating
Spin coating is a technique that utilizes centrifugal forces produced by a spinning substrate to spread a coating solution over a surface. This type of coating technique is faster and efficient.
It can be controlled by a few parameters to yield a well defined coating coverage. The liquid solution is placed in the center of the sample and the sample spins at a given speed and time, where the centrifugal force will cause the liquid to spread evenly across the sample. Four stages (see Fig. 1.5) of spin-coating are: deposition of the surfactant/inorganic solution, spin- up, spin-off and evaporation. Initially, an excess of liquid is deposited on the surface of the substrate during the first stage. In the spin-up stage, liquid flows radially outwards by
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centrifugal force. In the spin-off stage, excess of liquid flows to the outwards and leaves in a form of droplets. In the final stage, evaporation takes place leading to the formation of uniform thin films.
Fig. 1.5 Spin coating steps
1.3.3 Growth from solution
The basic principle for the synthesis of ordered mesoporous films by growth from solution is to bring the synthesis solution (including solvent, surfactant and inorganic precursor) in contact with a second phase, e.g. solid (ceramic), gas (air) or another liquid (oil). The two- phase system is kept under specific conditions and the ordered film is formed at the interface.
Hydrothermal deposition technique comes under this process.
1.3.4 Sol-gel method
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products with high surface area, controllable and facile amalgamation of other compounds such as promoters or stabilizers, alteration of fine pore structure and direct casting of the selective layer over the membrane support (Agrafiotis and Tsetsekou 2002). Further, it generates high purity products with narrow pore size distribution and needs low sintering temperature. Being the less energy, it is widely used due to its facile method and does not require sophisticated instrumentation. A sol is a stable dispersion of colloidal very fine particles or polymer in a liquid. The particles may be amorphous or crystalline. A gel consists of three dimensional continuous networks, which enclosed a liquid phase where the network is built from agglomeration of colloidal particles. For the fabrication of composite membrane, two sol-gel routes are used. The first one corresponds to colloidal route that explores colloidal chemistry in aqueous media and the other corresponds to the polymeric route, which searches the chemistry of metal organic precursors with organic solvents. Fig. 1.6 demonstrates the sol-gel method for both colloidal and polymeric routes. In the sol-gel process, the first stage refers to the preparation of a sol using molecular precursors either with metal salts or metal organics. The condensation reactions occurred at the sol stage for both cases with the formation of colloids or clusters that coalesce finally to form gel. For the fabrication of active layer, the coating must be carried out with the sol whose rheological behavior is adaptable for the porous membrane support or substrate. Drying and sintering steps subsequently followed, which determine the chemical characteristics of the membrane.
In the drying process of coated sol, gelation takes place and cross linking of the gel particles are formed to produce desired product due to the thermal treatment. Usually, thin and crack free membrane layers are obtained with this technique.
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Fig. 1.6 Pictorial representation of inorganic membrane preparation by sol-gel method