Membrane and Desalination Technologies - TCE Moodle Website

122 L. Song and K.Guan Tay
Permeate flux (m/s)
1.6×10 –5
1.4×10 –5
1.2×10 –5
1.0×10 –5
8.0×10 –6
6.0×10 –6
4.0×10 –6
2.0×10 –6
0
25
50
75
100
0.0
0 1 2 3 4 5 6
Distance from channel entrance (m)
Fig. 3.14. Evolvement of evenly distributed permeate flux profile of the membrane channel with time.
The reason for the decline in permeate flux along the membrane channel in Fig. 3.14 is the
build-up of salt concentration as water is lost as permeate when the feed water travels along
the channel. Figure 3.15 shows the variation of salt concentration along the membrane
channel at different times. At the start of the operation, salt concentration is seen to increase
along the membrane channel until the point about 4.5 m from the channel entrance, where it
remains unchanged for the rest of the channel. The increase in salt concentration increases the
osmotic pressure and reduces the net driving pressure. However, there is a limit for the salt
concentration in the membrane channel. When the osmotic pressure reaches the driving
pressure at some point in the membrane channel, salt concentration ceases to increase.
Crossflow velocity is found in Fig. 3.16 to decrease along the channel due to water loss.
Like the permeate flux and membrane resistance, the variations of both salt concentration and
crossflow velocity along the membrane channel became more moderate as fouling continues
to develop in the membrane system.
The changes in the patterns of the localized membrane resistance, permeate flux, salt
concentration, and crossflow velocity in Figs. 3.13–3.16 give strong evidence that membrane
fouling is occurring in the membrane channel. However, the initial stage of membrane
fouling does not affect the average permeate flux of a long membrane channel. The length
of the initial period of constant average flux is strongly affected by the driving pressure
employed in the membrane process. The average permeate flux of the same membrane
channel (using parameters given in Table 3.3) is simulated for two driving pressures of
1.21 10 8 and 1.38 10 8 and the results are shown in Fig. 3.17. It can be seen that the 15%