Nanofiltration System

Nanofiltration like other membrane separation processes uses membranes to separate different fluids or ions. Organic semipermeable membrane, forms a separation layer where the filtration process takes place. Once the pressure difference between the feed (retentate) and the filtrate (permeate) is created, the separation process starts. Membrane, because of its selectivity, retain one or many components of a dissolved mixture whereas water and substances having a molecular weight < 200 D are able to permeate the semipermeable separation layer. The most important aspect of nanofiltration membrane is that, monovalent ions pass the membrane whereas divalent and multivalent ions are rejected.

Nanofiltration uses cellulosic acetate and aromatic polyamide type Membranes which can reject salts from 95% for divalent salts to 40% for monovalent salts. The membrane used by Nanofiltration have pores that are typically much larger than the membrane pores that are used in reverse osmosis. The rejection rate of divalent and multivalent cations especially calcium and magnesium is higher in nanofiltration due to this, it is used extensively in industrial water softening applications as well as pre-treatment for reverse osmosis. Nanofiltration can operate at lower pressure and is capable of passing some of the inorganic salts hence they can be used where high organic removal and moderate inorganic removals are desired.

Nanofiltration Versus Reverse Osmosis
The membrane pore structure of nanofiltration is larger than reverse osmosis membrane. That is why it is also sometimes referred to as "loose" RO. More salt is allowed to pass in nanofiltration as compared to reverse osmosis. NF can operate at higher recoveries as compared to reverse osmosis, which enables the conservation of water usage because of lower concentrate stream flow rate.

The other main difference between reverse osmosis and the NF lies in the removal of monovalent ions such as chlorides. The monovalent ion level removed by the RO is 98-99% level at 200 psi. But in case of NF membrane's removal of monovalent ions ranges between 50% to 90%. The removal of monovalent ion by NF depends on the material and manufacture of the membrane due to this, there is a variety of Nanofiltration membranes available. Nanofiltration uses less fine membrane hence the feed pressure required in the system is lower as compared to RO systems. The fouling rate in NF system as compared to RO system is also lower.


Application of Nanofiltration

Properties of Nanofiltration
Following are the properties of nanofiltration:

• The nanofiltration membranes pore size is equivalent to a molecular weight cut off value of approximately 300-500 g/mole. Hence the components possessing this molecular weight can be easily separated from the components possessing higher molecular weight.
  
  
• The surface of the NF membranes is slightly charged. Charge interaction plays major role because the dimensions of the pores are less than one order of magnitude larger than the size of ions. This property also helps in separating ions of different valencies.

Transfer of Mass in Nanofiltration
Mass transfer can be graphically represented as follows:
Where:
P = External pressure
(J) = Solvent flux
(R) = Rejection
solvent flux is given by:


P = Effective transmembrane pressure [N/m2]
n = The permeate viscosity [Pa.s]
Rtot = The total resistance towards solvent flow [m-1].

For solute behavior characterization, the rejection is given by:

Cb = Concentration level l
= Final concentration of solute in the permeate

External Resource
Properties and applications of Nanofiltration Membrane: Prof.dr.ir. J.T.F. Keurentjes and prof.ir. J.A. Wesselingh