Predicting influence of operating conditions on filtration
performance
We systematically investigated the influence of agitation speed,
operating pressure and feeding mode on the filtration performance of an
EMR. As expected, the permeate flux increases at high agitation speed
(Figure 7a ). This positive phenomenon is attributed to a
thinner concentration polarization layer at higher agitation (a strong
shear force on the membrane surface) 36.
There were no changes of the
permeate flux when agitation speed is less than 1000 rpm because the
shear force generated by lower agitation is not enough to mitigate
concentration polarization (Figure S1 ). The increased
concentration polarization caused higher solute flux (Figure
7b ). A similar trend is observed with the permeate flux with increasing
operating pressure, which is attributed to a greater driving force at
higher operating pressure (Figure 8a ). The intensified driving
force could accelerate the penetration of oligodextran products through
the membrane and thus result in higher solute flux (Figure 8b ).
However, increasing the agitation speed and operating pressure decreases
the permeate concentration due to a dilution effect at higher permeate
flux under the water feeding mode (Figure 7c and 8c ).
The substrate feeding mode is an alternative for enhancing the
concentration of oligodextran in the permeate. Unlike the water feeding
mode where pure water is continuously fed into the system, in the
substrate feeding mode the dextran at constant concentration is
continuously fed into the reaction system to maintain a constant feed
volume. The simulations on the influence of feeding mode are summarized
in Figure 9 . It is no doubt that the membrane suffers from more
severe fouling under the substrate feeding mode 10,
which thus leads to an obvious decrease of permeate flux (Figure
9a ). The decreased permeate flux under the substrate feeding mode
reduced the oligodextran passing through the membrane, as is evidenced
by the lower solute flux (Figure 9b ). The solute flux increases
monotonically with prolonged operation duration under the substrate
feeding mode, while the solute flux gradually decreases in the later
stages under the water feeding mode owing to reduced substrate
concentration in the EMR (Figure 9b ). This phenomenon is more
prominent in the variation of permeate concentration with operation time
(Figure 9c ). The increased
permeate concentration is beneficial in terms of decrease in energy
consumption (based on eq. (35)). This analysis was confirmed by the
finding that the substrate feeding mode exhibits a lower specific energy
consumption (2.4 kWh/kg) compared to the water feeding mode (3.5 kWh/kg)
(Figure 9d ). However, the oligodextran yield obtained under the
substrate feeding mode (28.2 %) is obviously lower than that obtained
under the water feeding mode (89.6 %) (Figure 9d ), which is
attributed to the higher substrate concentration in the feed of the
former mode so that the enzyme is not enough to achieve a high
conversation rate 39. The lower permeate flux under
the substrate feeding mode causes a smaller permeate volume, and even
with a higher permeate concentration, the mass of product is still
smaller. Therefore optimization of system operating conditions is of
great significance for the stable and efficient operation of an EMR
system.
Figure 7. Simulation investigation of the effect of agitation
speed on (a) permeate flux, (b) solute flux, and (c) permeate
concentration with membrane pore size of 10 nm and membrane porosity of
0.8. Substrate concentration: 50 g/L, enzyme concentration: 0.05 g/L,
operating pressure: 1 bar, and feeding mode: water feeding.
Figure 8. Simulation investigation of the effect of operating
pressure on (a) permeate flux, (b) solute flux, and (c) permeate
concentration with membrane pore size of 10 nm and membrane porosity of
0.8. Substrate concentration: 50 g/L, enzyme concentration: 0.05 g/L,
agitation speed: 1000 rpm, and feeding mode: water feeding.
Figure 9. Simulation investigation of the effect of feeding
mode on (a) permeate flux, (b) solute flux, (c) permeate concentration,
and (d) yield and specific energy consumption with membrane pore size of
10 nm and membrane porosity of 0.8. Substrate concentration: 50 g/L,
enzyme concentration: 0.05 g/L, agitation speed: 1000 rpm, and operating
pressure: 3 bar.