Polymeric materials and ceramic are the most widely used functional membrane material to prepare symmetric membranes for industrial scale applications. The asymmetric membranes are usually prepared form symmetric polymeric or ceramic membrane materials or both. In an asymmetric membrane, usually the support layer provides desired mechanical strength and the thin skin layer constituting either polymeric or ceramic materials caters towards the desired separation characteristics.
1.3.1 Polymeric membranes
Polymeric membranes are thin films of 10-100 µm thickness. Different types of polymers such as polysulphone (PSU), cellulose acetate (CA), polyamide (PA), polyethersulphone (PES), Polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polytetrafloroethylene (PTFE), polyetherimide (PEI), polypropylene (PP) are used widely to fabricate the polymeric membranes.
Advantages of polymeric membranes:
Wider ranges of pore sizes varying from MF to RO are available.
Comparatively low cost than ceramic membrane
Easy to fabricate
Ease to scale up
Disadvantages of polymeric membrane:
Low solvent resistance
Lower applicable range of pH and hence, low corrosion resistance
Applicable to low temperature ranges
Lower life span (12-18 months)
1.3.2 Ceramic membranes
Ceramic membranes are type of artificial membranes made from various inorganic materials such alumina, zirconia, silica, titania, kaolin etc. Ceramic membranes possess superior chemical, thermal and mechanical stability. The thickness of ceramic membrane is in the range of 2-5 mm and some times higher depending on the specific applications. Asymmetric ceramic membranes constitute thin film (10-100 µm) of ceramic coating over a thick porous symmetric support.
Advantages of ceramic membranes:
Very high corrosion resistance
Applicability to wider pH ranges (0.5-14)
Applicability of wider temperature ranges (350-500 oC). As a result they are used in industrial scale separations without any feed pre-conditioning steps.
Less fouling tendency
Inertness to common chemicals and solvents
Higher mechanical strength
Disadvantages of ceramic membranes:
Most ceramic membranes are available in pore diameters within the MF and UF range (0.010 – 10 µm)
Comparatively higher cost
Brittle in nature
1.3.3 Types of ceramic membranes
In general, ceramic membranes are broadly classified into two categories, i.e. dense and porous membranes, based on the absence or presence of the pores in the membrane structure.
However, the actual classification is based on the transport mechanism of the species through
the membrane. If the separation mechanism is mainly controlled by sieving the species (molecules, ions etc.,) then it is called as porous membranes. If the membrane follows general solution diffusion type mechanism of the transport of species, then the membranes are referred as dense membranes. The separation mechanism is attributed to the affinity (selective adsorption and diffusion) of molecules with membrane materials and it plays an important and dominant role when the pore size is relatively small.
Dense membranes
Two major types of dense ceramic membranes, metal and solid electrolyte, have been studied and developed extensively for gas separation. Dense metal membranes made of palladium and its alloys have been suggested for hydrogen separation and purification applications (Hsieh 1996). Other metals membranes fabricated using tantalum; vanadium and niobium possess better selectivity for hydrogen. Solid electrolyte membranes are made of mixed conducting oxides (doped zirconia and thoria) that are selective to certain ionic species.
Porous membranes
Porous ceramic membranes are prepared with several layers of porous materials. These layers have gradations of pore diameters and each layer is applied and stabilized so as to acts as the support for the next finer layer. In general, microfiltration, ultrafiltration and nanofiltration membranes are porous in nature and employed in various industrial liquid phase applications to concentrate or purify dilute (aqueous) solutions (Noble and Stern 1995). The microstructure of the membranes (shape, size, porosity, tortuosity etc.,) will strongly depend on the method of fabrication (Burggraaf and Cot 1996).
1.3.4 Applications of ceramic membranes in various industries
In earlier, the ceramic membranes are used in wastewater treatment applications. Later, its effective and potential usages cover the entire industries where media are filtered.
Ceramic membranes in chemical industries:
Catalysts separation.
Product separation and cleaning.
Recycling and cleaning of organic solvents.
Desalination of products.
Recovery of pigments and dyes.
Concentration of metal hydroxide solutions.
Concentration of polymer suspensions.
Ceramic membranes in metal industries:
Oil-in-water emulsions treatment
Heavy metals recovery.
Treating of wastewater from grinding processes.
Recycling and disposal of degreasing and rinsing wastewater.
Treatment of wastewater from glass and glass fiber production.
Ceramic membrane in food and beverage industries:
Purification of drinking water.
Dewatering of products.
Desalination of whey.
Clarification/Concentration of fruit juices and beers.
Sterilization of milk and whey.
Ceramic membrane in biochemical industries :
Separation, concentration and dewatering of biomass and algae.
Concentration fractionation, isolation and sterilization for antibiotics, enzymes, proteins, amino acids and vitamins.
Separation of yeast.
Disposal of fat emulsions.
Desalination.
Ceramic membrane in environment and recycling:
Chemical oxygen demand (COD) / Biological oxygen demand (BOD) removal.
Removal of heavy metals and radioactive substances.
Oil/water separation.
Retention of microorganism.
Recovery of pharmaceuticals and pesticides.
Purification of the drain of sewage plants.
Recycling of water from swimming pools.
1.3.5 Polymer vs. Ceramic membranes
Considering the advantages and disadvantages of both membranes; it can be observed that polymeric membranes are much useful for laboratory scale use. For laboratory scale use, applicability of the membrane technology towards any particular separation is the main aim but not their life span and cost. However, for industrial scale applications, cost and life span are the most relevant matters along with the separation efficiency. For most of the applications, the life span is evaluated as 12 – 18 months (extendable to 36 months by adopting cleaning schemes)
and 10 years for polymeric and ceramic membranes, respectively (Mulder 1991). Therefore, though ceramic membranes involve higher initial costs, their ability to prove higher flux and applicability to wide range of temperature, chemical processing conditions could favor them to be the choice in contrary to the polymeric membranes. Though the separation characteristics of ceramic membrane processes are similar to polymeric membranes, they are not yet widely applied in industrial scale applications owing to its higher cost. Under these circumstances, the development and usage of comparatively low cost membranes with longer life span is anticipated to drive the economic competitiveness of ceramic membranes in the industry.