A Report on

NANOFILTRATION

Submitted by : Shohita Choudhry S.R. No. : 42/08

Roll no. :0804551048 Branch : 3rd B.Tech Chemical engg.

Index

What is nanofiltration ?

Nanofiltration is a relatively recent membrane filtration process used most often with low total dissolved solids water such as surface water and fresh groundwater, with the purpose of softening (polyvalent cation removal) and removal of disinfection by-product precursors such as natural organic matter and synthetic organic matter.

Modern crossflow filtration technology has principally evolved during the last thirty years, following the significant advancements in polymer chemistry over the same time. Today, a vast majority of crossflow filtration installations utilize polymeric membranes. Virtually all commercial nanofiltration membranes are polymeric.

Nanofiltration (NF) is a crossflow, pressure driven process that is characterized by amembrane pore size corresponding to molecular weight cutoff of approximately 200 –1000 dalton, and operating pressures of 150–500 psi (10 –34 bar). NF is primarily used to separate lowmolecular weight organics and multivalent salts from monovalent salts and water. Starting in the late 1970s, NF membrane processes gradually found their way into industrial applications, and serve as a viable alternative to more traditional separation processes like extraction, evaporation and distillation.

The first industrial systems using nanofiltration membranes were installedin 1978 using tubular membranes for desalination of dyes and brighteners.

Nanofiltration is mainly utilized for producing softened water for industry or potable water from a brackish source. This type of semi permeable membrane has high rejection of multi valent ions such as Calcium and moderate rejection of single valent ions such as Sodium. These propertiesallow this type of membrane to operate at greatly reduced operating pressures as compared to Reverse Osmosis.

Development of nanofiltration

In the early 1960s, the phase-inversion process for the manufacture of polymeric membranes was developing fast. Followed the development, filtration and filtration-related activity was of a burst. All of those led to the establishment of three membrane separation processes: reverse osmosis, ultrafiltration and microfiltration. The separation spectrum before these processes was the traditional cut point limit of standard filtration of around 0.01 mm.And after that, the separation spectrum become the very finest distinct solids, a few nanometers in size, and enabled the separation of large molecules from solution. Thought the actual size ranges vary somewhat from source to source, there is general agreement that microfiltration covers the range 10μm down to 0.1μm and ultrafiltration covered 0.1μm down to 0.005μm. Reverse osmosis was designed to retain the very small sodium chloride molecule. in reverse osmosis processes, nothing can pass but water.

The burst of filtration and filtration-related activity that followed the development of the phase-inversion process for the manufacture of polymeric membranes, in the early 1960s, led to the establishment of three membrane separation processes: reverse osmosis, ultrafiltration and, more recently, microfiltration. These processes took the separation spectrum from the traditional cut point limit of standard filtration of around 0.01mm to the very finest distinct solids, a few nanometers in size, and enabled the separation of large molecules from solution. The actual size ranges vary

somewhat from source to source, but there is general agreement that microfiltration covers the range 10μm down to 0.1μm, while ultrafiltration covered 0.1μm down to 0.005μm,in terms of discrete particles or Molecular Weight Cut-Off (MWCO) figures of 300,000 down to around 300 Daltons for dissolved materials. Reverse osmosis, of course, was designed to retain the very small sodium chloride molecule, which meant passing nothing else but water. During the 1970s and 1980s, membrane development was fairly rapid. At the end of the 1980s, “nanofiltration” was used to name the membrane development. Since that, nanofiltration is a fairly recent development in the range of membrane separation processes. Nanofiltration deals with not the distinct particles suspended in the liquid but also materials that are dissolved in a liquid. Reverse osmosis retains monovalent salts, while nanofiltration allows them to pass, and then retains divalent salts such as sodium sulphate.This is the key different between nanofiltration and reverse osmosis.

 Thought nanofiltration is a liquid-phase separation removing dissolved solids with a relatively high transmembrane pressure carried out by means of membranes, the progress of much of the filtration business is being driven by demands for finer and finer cutpoints, in both liquid and gas filtration. And now the demands are being met by the use of correspondingly finer fibres to make the filter media. The fine filtration is taking the separation process that is effectively microfiltration to much lower cutpoints. The materials are also being referred to as membranes. It is hoped that the nanofiltration and filtration with nanofibres which are sufficiently different so as to avoid their confusion covered in the rest of this article. The very fine filtration that can be achieved with these nanoweb media is taking the separation process that is effectively microfiltration to much lower cutpoints. The materials are also being referred to as membranes, even though they are very different in format from the semipermeable plastic sheet still most commonly thought of when membranes are mentioned. Recent developments of membranes for NF have greatly extended their capabilities in very high or low pH environments, and in their application to non-aqueous liquids. The plastic media are highly cross-linked, to give long-term stability and a practical lifetime in more aggressive environments. NF membranes tend to have a slightly charged surface, with a negative charge at neutral pH. This surface charge plays an important role in the transportation mechanism and separation properties of the membrane.

During the 1970s and 1980s, membrane development was fairly rapid. At the end of the 1980s, “nanofiltration” was used to name the membrane development.  Since that, nanofiltration is a fairly recent development in the range of membrane separation processes. Nanofiltration deals with not the distinct particles suspended in the liquid but also materials that are dissolved in a liquid. Reverse osmosis retains monovalent salts, while nanofiltration allows them to pass, and then retains divalent salts such as sodium sulphate.This is the key different between nanofiltration and reverse osmosis.

Principle of nanofiltration

Nanofiltration (NF) is a cross-flow filtration technology which ranges somewhere between ultrafiltration (UF) and reverse osmosis (RO). The nominal pore size of the membrane is typically about 1 nanometre. Nanofilter membranes are typically rated by molecular weight cut-off (MWCO) rather than nominal pore size. The MWCO is typically less than 1000 atomic mass units (daltons). The transmembrane pressure (pressure drop across the membrane) required is lower (up to 3 MPa) than the one used for RO, reducing the operating cost significantly. However, NF membranes are still subject to scaling and fouling and often modifiers such as anti-scalants are required for use.The fundamental principle of Nanofiltration membrane technologyis the use of pressure to separate soluble ions from water througha semi permeable material. The membrane, unlike a dead endfilter, operates under a different hydraulic profile which is knownas cross flow filtration.

Most Nanofiltration membranes are composite materials that aresupported by a polymer substrate and manufactured in a spiralconfiguration as opposed to a flat sheet or tube geometry. Thepredominant model used today for industrial applications is thespiral configuration.

Transport Mechanism in Nanofiltration

Nanofiltrationmembranes are often categorized as “loose” reverse osmosis (RO) membranes. The differences between the two, however, are significant. The most notable difference is the ability of NF membranes to selectively reject divalent ions, while passing monovalent ions. It is a common belief that NF and RO membranes do not have distinct pores, as in ultrafiltration and microfiltration membranes. Although recent studies using Atomic Force Microscopy (AFM) suggest that pores of NFmembranes can be viewed, most membrane scientists choose to describe the pores as the distances between the polymer chains of the membrane building material.

The mechanism of transport and rejection of NF membrane is quite complexand is still a point of debate between scientists.Many models have been developed to identify the effect of different parameterson the transport mechanism and to predict the NF membrane performance. Solution diffusion theory describes the membrane as a porous film into which both water and solute (ion) dissolve.The solute moves in the membrane mainly under concentration gradient forces, while the water transport is dependent on the hydraulic pressure gradient. The transport of the solute through themembrane depends on hindered diffusion and convection. The transportation of a non-charged solute through an NF membrane is considered to be determined by a steric exclusion mechanism. Steric exclusion applies to NF membranes as well as ultrafiltration and microfiltration membranes. A separation between two different non-charged solutes is determined predominantly by the difference in their size and shape.

Nanofiltration membranes and their properties

When designing a nanofiltration process, one should consider several operating parameters. The most important operating parameters affecting the performance of nanofiltration membranes are similar to those for most crossflow filtration processes:

• Pressure. Pressure difference is the driving force responsible for a nanofiltration process. The effective driving pressure is the supplied hydraulic pressure less the osmotic pressure applied on the membrane by the solutes. Nanofiltration provides good separation at net pressures of 150 psi (10 bar) or higher.

• Temperature. Increasing the process temperature increases the NF membrane flux due to viscosity reduction. The rejection of NF membranes isnot dependent significantly on the process temperature.

• Crossflow Velocity. Increasing the crossflow velocity in an NF membraneprocess increases the average flux due to efficient removal of fouling layer from the membrane surface. However, the mechanical strength of the membrane, and construction of the element and system hardware will determine the maximum crossflow velocity that can be applied. Running a nanofiltration membrane at too high a crossflow velocity may cause premature failure of membrane and modules.

• pH. pH affects performance of nanofiltration membranes in more thanone way. The charged sites on the NFmembrane surface (i.e. carboxylicgroup, sulfonic group) are negatively charged at neutral pH or higher, but lose their charge at acidic pH. It is well known that most NF and RO membranes have lower rejection at low pH, or after acid rinse. It should be noted, however, that since different membrane manufacturers use differentchemistries to produce their thin film composite layer, the pH dependency of a membrane should be determined for eachmembrane type. In addition to the effect of pH on the membrane itself, pH can be responsiblefor changes in the feed solution, causing changes in membrane performance.Two examples are change of solubility of ions at different pHregimes, causing different rejection rate; and change in the dissociationstate of ions at different pH ranges.

• Salinity. The effective pore radius of a charged pore will increase as the ionic strength of the surrounding liquid increases. Therefore, the rejection of monovalent ions will decrease as their concentration in the feed solution increases. The rejection of divalent ions will be affected to a lower extent.

The global market for nanofiltration membranes increased from $89.1 million in 2006 to an estimated $97.5 million by the end of 2007. It should reach $310.5 million by 2012, a compound annual growth rate (CAGR) of 26.1%.

The water treatment sector is projected to account for 72.7% of total revenues in 2007, worth an estimated $70.9 million in 2007 and expected to reach $238.2 million by 2012, a CAGR of 27.4%.

Continued growth in regulations aimed at protecting the environment will positively affect the future expansion of the nanofiltration membranes market.

 

Applications of nanofiltration

In much of the developing world, clean drinking water is hard to come by, and nanotechnology provides one solution. While nanofiltration is used for

the removal of contaminants from a water source, it is also commonly used for desalination. As seen in a recent study in South Africa, tests were run using polymeric nanofiltration in conjunction with a reverse osmosis process to treat brackish groundwater. These tests produced potable water, but as the researchers expected, the reverse osmosis removed a large majority of solutes. This left the water void of any essential nutrients (calcium, magnesium ions, etc.), placing the nutrient levels below that of the required World Health Organization standards. This process was probably a little too much for the production of potable water, as researchers had to go back and add nutrients to bring solute levels to the standard levels for drinking water consumptionProviding nanofiltration methods to developing countries, to increase their supply of clean water, is a very inexpensive method compared to conventional treatment systems. However, there remain issues as to how these developing countries will be able to incorporate this new technology into their economy without creating a dependency on foreign assistance.

Nanotechnology is being used to develop solutions to three very different problems in water quality.

One challenge is the removal of industrial water pollution, such as a cleaning solvent called TCE, from ground water. Nanoparticles can be used to convert the contaminating chemical through a chemical reaction to make it harmless. Studies have shown that this method can be used successfully to reach contaminates dispersed in underground ponds and at much lower cost than methods which require pumping the water out of the ground for treatment.

Another challenge is the removal of salt or metals from water. A deionization method using electrodes composed of nano-sized fibers shows promise for reducing the cost and energy requirements of turning salt water into drinking water.

The third problem concerns the fact that standard filters do not work on virus cells. A filter only a few nanometers in diameter is currently being developed that should be capable of removing virus cells from water.