Predicting influence of membrane properties on filtration
performance
This section evaluates the role of membrane properties (i.e., membrane
pore size and porosity) in determining the filtration performance of an
EMR. As discussed in the section on theoretical simulation, the membrane
pore size determines the convective and diffusive hindrance factors
(i.e., Kc in eq. (33) andKd in eq. (34)) and thus affects the sieving
efficiency of solutes. Figure 5 shows the permeate flux and
permeate concentration of an EMR as a function of membrane pore size. It
is evident that the tighter membrane (smaller pore size) suffers from
more severe concentration polarization leading to lower permeate flux37 (Figure 5a ). The flux difference becomes
smaller in the later stage, which is attributed to the reduction of
degree of concentration polarization as more products passed through the
membrane. Since the tighter membrane rejects more dextran, it has a low
permeate concentration compared to the membrane with larger pore size at
the same operation time in the initial stage (Figure 5b ).
Although the membrane with large pore size has a higher filtration
efficiency, the membrane with small pore size benefits from prolonged
retention time which enables sufficient hydrolysis of substrate and in
turn produces more uniform and smaller oligodextran molecules10. However, further increase in operation time leads
to decrease of permeate concentration (Figure 5b ) because
substrate concentration decreases with increase of the operation time
under the water feeding mode.
The membrane porosity is also of particular significance because, on one
hand, it affects the relationship between the convection and diffusion
transportation of solutes (i.e, Pem in eq. (29))
and, on the other hand, it determines the membrane intrinsic resistance
(i.e, Rm in eq. (3)) and retention time that
relate to separation efficiency. Figure 6 presents the
variations of permeate flux and permeate concentration at different
membrane porosities. The membranes with different porosities present a
similar filtration performance in the initial filtration process.
However, as more oligodextran products are collected in the permeate,
the concentration polarization decreases and the role of membrane
resistance in the total filtration resistance is enhanced. A higher
membrane porosity causes a decrease of the membrane resistance (i.e.
smaller Rm in eq. (3)). As a result, the membrane
with higher porosity has larger permeate flux in the later filtration
process, which leads to decrease of the permeate concentration
(Figure 6 ). This result is explained by the increase in
oligodextran rejection due to the dilution effect at higher flux38. The developed model elaborates the inherent
relationship between membrane properties and reaction-separation
efficiency and shows that selection of a membrane with large porosity
and uniform pore size distribution in an EMR is beneficial for the
improvement of filtration efficiency and quality of products.
Figure 5. Simulation investigation of the effect of the pore
size of membrane on (a) permeate flux and (b) permeate concentration
with membrane porosity of 0.8. Substrate concentration: 50 g/L, enzyme
concentration: 0.05 g/L, operating pressure: 3 bar, agitation speed:
1000 rpm, and feeding mode: water feeding.
Figure 6. Simulation investigation of the effect of membrane
porosity on (a) permeate flux and (b) permeate concentration with
membrane pore size of 10 nm. Substrate concentration: 50 g/L, enzyme
concentration: 0.05 g/L, operating pressure: 3 bar, agitation speed:
1000 rpm, and feeding mode: water feeding.