Membranes - Principles of Chemical Separations with Environmental Applications - page 251

x

Permeate

3

m

40

hr

3 3

40 m r m

=

2

20 hr

Figure 9.9 Schematic flow diagram, Example 9.2.

Figure 9.10 RO membrane.

9.9.4 Nanofiltration

Nanofiltration spans the gap in particle size between reverse osmosis (hyperfiltration)
and ultrafiltration. It can separate high molecular weight compounds (100-1000) from
solvents, and can also separate monovalent from multivalent ions. The driving force is a
pressure difference of about 0.3-3 MPa (even greater than ultrafiltration). The nanofiltra-
tion process can reject selected (typically polyvalent) salts and may be used for selective
removal of hardness ions in a process known as membrane softening [10].

9.9.5 Reverse osmosis

Osmosis refers to the flow of a pure solvent, usually water, across a solvent-permeable
membrane. The flow is driven from the solvent phase towards a high salt concentration
phase. The result is a dilution of the high salt concentration solution. Osmosis is not useful
as a separation process since the solvent is moving in the wrong direction, resulting in
dilution as opposed to separation. The solvent can be transferred in the opposite direction
if a pressure is applied to the high salt concentration side of the membrane such that
the pressure driving force is greater than the osmotic pressure gradient. Referring to
Figure 9.10, the water flux is given by:

J w = K ( P − ) , (9.16)

where K = permeability coefficient

P 2
= 1 − 2
and

P = P 1 −

= CRT ;