Overview of Membrane Separations with insight on ceramic membranes
Magan Khakharia1, Sachin Jadhav2, Arvind Sikarwar2, Bhaskar Thorat2
– Magan Khakharia, Microfilt India Pvt Ltd; Sachin Jadhav, Arvind Sikarwar, Bhaskar Thorat, ICT-Mumbai
Article provides a quick review of the developments in membrane separations, particularly on membrane materials with a brief on the latest introduction, ceramic membranes.
Membrane separations have progressed very fast, overtaking many of the conventional separations like distillation, evaporation etc. The advantages of membrane separations are many and it is also finding increasing applications in newer areas. With advances in materials development, new membrane materials like ceramic membranes are increasing the applications bandwidth to a wide range of process parameters apart from longer life of the membrane.
are organic or inorganic porous materials that allow the separation of different substances (soluble or not) present in a liquid or gas. The separation is performed by applying the pressure gradient between the feed side and the permeate side of the membrane. It results in the passage of particles or molecules smaller than the membrane pore size.
Year Evolution 1748: Discovery of semi permeable membrane by Abbott Nollec
1900: Production of artificial membrane
1950: Sea water desalination program in the USA, Uranium enrichment program in the France by the CEA with the help of mineral membrane 1960: Discovery of cellulose acetate asymmetric membrane by Loeb and by Sourirajan
1970: Many industrial units for sea water desalination 1971: Beginning of UF application for the extraction of milk protein
1980: Industrial production of the first Inorganic membrane in the world CARBOSEP applied to UF for the food and beverages and bio application
Interest of membrane separation and domains
The retention of molecules are realized without any phase change as against the other ubiquitous unit operations such as evaporation, crystallization and liquidliquid extraction. In majority of the cases, the interested
solute is elegantly separated from the source without any distortion or di-orientation of molecules as it may happen in other separation techniques such as chromatography.
Membrane processes can be roughly classified as: micro-filtration (MF), ultra-filtration (UF), nano-filtration (NF), reverse osmosis (RO), pervaporation (PV) and electro-dialysis (ED). Out of these,the mirco-, ultra-, nano-filtration, and reverse osmosis work on size exclusion principle. On the other hand, pervaporation is based on membrane affinity towards the components and electrodialysis makes use of charge based separation. A detailed Osmonics Chart is presented in Figure 1 to understand the differentiation between the above mentioned filtration processes.
Hydrodynamics of membrane process
Membrane processes can be largely classified as Dead End Filtration and Cross Flow Filtration. The dead end filtration is a batch process where pressure is applied on one side of the membrane to draw filtrate on the other side. Here, the filtration layer forms over the membrane over the period of time, thus increasing the pressure drop across. This type of filtration system is popular in laboratory scale filtration studies. Most industrial membrane processes uses cross flow mechanism and hence it is also known as cross flow filtration or tangential flow filtration. These filtration equipment can be then further scaled up and utilized at commercial scale. Figure 2(a) and 2(b)depicts the operational features of cross flow and dead end filtration, respectively.
Membrane materials classification
A wide range of synthetic membranes are available for a variety of separation assignments. They can be produced from organic materials such as polymers and liquids, as well as inorganic materials. The most of commercially utilized synthetic membranes in separation industry are made of polymeric materials. Polymeric membranes lead the membrane separation industry market because they are very competitive in perfor-
mance and economics. They can be classified based on their surface chemistry, bulk structure, morphology, and production method. Figure 3 shows the classification of the membranes. The most common polymers that are employed commercially are cellulose acetate, nitrocellulose, and cellulose esters (CA, CN, and CE), polysulfone (PS), polyether sulfone(PES), polyacrilonitrile (PAN), polyamide, polyimide, polyethylene and polypropylene (PE and PP), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinylchloride (PVC). Carbon, ceramic and SS membranes are the major types of inorganic membranes.
Ceramic membranes are majorly produced from zirconium, Titanium or Alumina. The pores are obtained by the superimposition of several layers of porous media of decreasing granulometry. The layers are bound together by sintering. The values of thickness of the layers are specified below:
1. 1st porous media – 1-5 nm
2. 2nd porous media – 5-100 nm
3. 3rd porous media – 0.1-1.5 μm
4. 4th porous media – 5-10 μm
Filtration layer types
Synthetic membranes can also be categorized based on their morphology, namely, symmetric membranes and asymmetric membranes. As the names suggest the symmetric membranes consists of homogeneous symmetric layer of pores throughout the membrane materials as shown in Figure 4. On the other hand, the asymmetric membranes are composed of a filtration layer (or active layer) supported by another layer called as support layer (Figure 5). Both of these layers are essentially made up of different materials. The support layer is usually porous in nature and offers low mass transfer resistance.
The Figure 6 shows the surface properties of a sample membrane. The multichannel membranes are the most used ceramic membranes. The channel diameter can range from 1.5 mm to 6 mm hydraulic diameter. The available diameter of tube can be from 10mm to 45mm. The number of channels in one multichannel membrane can vary from 7 to 93.The area for membrane can vary from 0.1 m2 to 1 m2. Figures 7a and b should provide a better understanding of the macro structures.
Table 1 also shows the comparison between the organic and ceramic membranes. The several advantages offered by ceramic membranes are high working temperatures, large working pH range, better solvent oxidation properties and longer life span.
Consider Figure 8 to understand the fundamentals of membrane filtration.
Let, P =Retentate pressure carton inlet; P = Retentate 1 2 pressure carton outlet; P =Permeate back pressure
The axial pressure drop is given by,
ΔP= P – P
The average pressure over membrane is given by, P = Average pressure = (P + P )/2
m 1 2
The pressure that is needed to push the filtrate through the membrane is called Trans membrane pressure (TMP). The TMP is defined as the pressure gradient of the membrane, or the average feed pressure minus the permeate pressure. The Transmembrane pressure is therefore given by,
TMP = P – P
As the filtration progresses, permeate is recovered from the filtrate side and the retentate is recycled to the feed tank. This results in the decreased feed volume over the entire filtration process unless the fresh feed is introduced to the feed tank. Here, the volume correction factor is applied to remove the inconsistencies given as follows: Initial feed volume Volume concentration factor =
Final retentate volume
The velocity along the membrane is a very critical parameter. It is recommended to keep it high so as to maintain the membrane surface clean. Typical velocities can vary from 2 m/s to 76 m/s, depending on the fouling tendency of the membrane
The membrane system can have different configurations such as:
- Sample batch system
- Batch system with retentate recycling
- Batch system with retentate recycling and feeding - Feed batch system
- Multi stage system for continuous operation.
Figure 8. Schematic of cross flow filtration process with possible configurations Global manufacturing and market share
Worldwide contribution of ceramic membranes is less than 15%. The rest is contributed by organic/poly-
meric membranes. Major ceramic membranes manufacturers are:
- PALL (Germany)
- Frana (Norway)
- FAME industries (France)
- Atech (Germany)
- JiuwuHitech (China)
- CTI (China)
The following numbers give the breakup of manufacturing cost.
- Labour – 7%
- Raw material – 75%
- Other manufacturing cost – 18%
Market share by geography
- North America – 22%
- China – 15%
- Europe – 28%
- Japan – 10%
- South East Asia – 10%
- India < 5%
- Rest of world < 10%
Market share by application:
- Biotechnology and Pharmaceuticals – 27% - Chemical Industry – 28%
- Food and Beverage – 17%
- Waste and Waste Water Treatment – 24%
- Others – 4%
Applications and processes:
- Biotechnology and Pharmaceuticals
- Extraction clarification of enzymes
- Production of lactic bacteria
- Extraction/clarification of Antibiotics - Extraction/clarification of amino acids - Pyrogen removal from water
- Concentration purification of whole cell vaccine Food and beverage:
- Glucose syrup clarification
- Whey concentration
- Milk concentration and bacteria removal
- Milk protein standardization
- Wine/juice clarification
- Plant extract/tea/coffee clarification Environment:
- Treatment of coating effluents
- Degreasing bath treatment
- Printing effluent treatment
- Dye penetration test effluent treatment
- Iron removal from water
- Strong corrosive acid/alkali separation at high temperature or pressure
- Salt chemicals, petrochemicals, fine chemicals, chloro-alkali industry.
Future prospects of ceramic membranes
The ceramic membrane market is projected to reach a market size of 5.1 billion USD by 2020. It has recorded a significant annual growth of approximately 11% on an average over last few years.The Asia–Pacific region will be the biggest market for ceramic membranes due to emerging economies like India, Japan and China (major focus on China). Waste water treatment is estimated to be the largest market for ceramic membranes. This is due to rising issue of water scarcity and the growing demand for clean water. In India, various leading institutes like ICT, CGCSMRI Kolkata, IIT’s, and CSMRI Bhavnagar are already working to develop indigenous ceramic membranes.
Figure 1. Size exclusion chart for various membrane filtration processes
Figure 2. Schematic of cross flow filtration and dead end filtration
Figure 3. Classification of membrane materials
Figure 7(a): Ceramic membrane geometry; (b): Ceramic membranes in housings