Koch Membrane Systems' Henia and Jorge Yacubowicz outline the uses and benefits of nanofiltration.
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 utilise polymeric membranes. Virtually all commercial nanofiltration membranes are polymeric.
Nanofiltration (NF) is a crossflow, pressure driven process that is characterised by a membrane 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 low molecular weight organics and multivalent salts from monovalent salts and water.
Starting in the late 1970s, NF membrane processes have gradually found their way into industrial applications, to serve as a viable alternative to more traditional separation processes like extraction, evaporation and distillation. The first industrial systems using NF membranes were installed in 1978 using tubular membranes for desalination of dyes and brighteners.
NF membranes are manufactured using two preparation techniques:
• Polymer phase inversion resulting in a homogeneous asymmetric membrane;
• Interfacial polarisation of a thin film composite layer on top of a substrate ultrafiltration membrane or other porous substrate.
Cellulose acetate and sulfonated polysulfone are two common materials used for making homogeneous asymmetric NF membranes. Thin film composite NF membranes use crosslinked polyamide polymers, reacted to carboxylic group or other charged “pendant.” Substrate materials commonly used for thin film composite membranes are polysulfone (PS), polyethersulfone (PES), polyvinyledene fluoride (PVDF), polyacrylonitrile (PAN), and Polyether ether Ketone (PEEK).
Recent developments of NF membranes that have exceptional stability in very low or high pH, very high temperature, or organic solvent media, required membrane manufacturers to seek new materials for membrane manufacturing. The materials used for these innovative membranes are highly crosslinked, to allow long term stability and practical membrane life in aggressive environments. Nanofiltation membranes have a slightly charged surface. Most NF membranes are negatively charged at neutral pH. This surface charge plays a major role in the transportation mechanism and separation properties of NF membranes.
Industrial applications of NF membranes are common in food and dairy, chemical process, pulp and paper, electronic and textile industries. The primary application of NF membranes continues to be in water treatment.
Most NF membranes are packed into spiral wound elements; however, tubular, hollow fiber and flat sheet/plate and frame modules are also available.
Transport mechanism in NF membranes
NF membranes are often categorised 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 NF membranes 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 complex and is still a point of debate between scientists. Many models have been developed to identify the effect of different parameters on the transport mechanism and to predict the NF membrane performance. The two major schools are Sourirajan's “sorption surface-capillary flow” approach, and the “solution-diffusion” theory.
Sorption surface-capillary flow describes the preferential sorption of water molecules in the membrane and the desorption of multivalent ions (by dielectric forces) causing exclusion of charged solutes, even smaller than the membrane pores, from movement into the membranes (Donnan exclusion). Effective charge density, pore radii and the ionic strength determine the rejection of monovalent ions, but generally speaking, for NF membranes the rejection of monovalent ions will be between 0 and 50%. 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 the membrane 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.
Parameters affecting the performance of NF membranes
When designing a NF process, one should consider several operating parameters. The most important operating parameters affecting the performance of NF membranes are similar to those for most crossflow filtration processes:
• Pressure: Pressure difference is the driving force responsible for a NF process. The effective driving pressure is the supplied hydraulic pressure less the osmotic pressure applied on the membrane by the solutes. NF 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 is not dependent significantly on the process temperature.
• Crossflow velocity: Increasing the crossflow velocity in an NF membrane process 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 NF membrane at too high a crossflow velocity may cause premature failure of membranes and modules.
• pH: pH affects performance of NF membranes in more than one way. The charged sites on the NF membrane surface (i.e. carboxylic group, 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 different chemistries to produce their thin film composite layer, the pH dependency of a membrane should be determined for each membrane type.
In addition to the effect of pH on the membrane itself, pH can be responsible for changes in the feed solution, causing changes in membrane performance. Two examples are change of solubility of ions at different pH regimes, causing different rejection rate; and change in the dissociation state of ions at different pH ranges.
Applications of NF membranes have grown significantly as knowledge of their operation has expanded. Well beyond its roots as “loose RO”, NF is now an integral step in many processes.