Process operating optimization
For a fuller appraisal of the impact of operating conditions on
productivity, we performed simulations to evaluate the dependency of the
yield and energy consumption on agitation speed and operating pressure.
Under the water feeding mode, the production yield increases gradually
and then reached a plateau with increase of agitation speed and a fixed
operating pressure (Figure 10a ). This increase can be ascribed
to the enhanced transportation of solute across the membrane, which is
the result of less concentration polarization at higher agitation speed36. Higher permeate flux would increase the product
retention due to a dilution effect 38 that would lead
to the existence of a plateau of production yield. At the same time,
specific energy consumption increases monotonically owing to the greater
power consumption at higher agitation speed (based on eq. (37)). Since
the agitation speed is fixed, increasing operating pressure enhances the
yield monotonically (Figure 10a ) and thus decreases specific
energy consumption (Figure 10b ), which is attributed to the
monotonic increase of solute flux with operating pressure
(Figure 8b ). Although the higher operating pressure together
with lower agitation speed would lead to serious membrane fouling, it is
beneficial in enhancing the production efficiency of the EMR system
within an acceptable range of degree of fouling with the water feeding
mode.
Additional simulations on the effects of agitation speed and operating
pressure under the substrate feeding mode are summarized inFigure 11 . The variation trends of yield and energy consumption
with agitation speed obtained under the substrate feeding mode are
consistent with those observed under the water feeding mode. However,
when the agitation speed is fixed, the yield decreases gradually with
increasing operating pressure under the substrate feeding mode, which is
different from what is observed under the water feeding mode
(Figure 11a ). The reason is that the membrane surface suffers
from more serious fouling which results in a lower flux under the
substrate feeding mode as demonstrated in Figure 9a . In
addition, the solute flux first increases then decreases with increase
in operating pressure from 1 bar to 5 bar under the substrate feeding
mode (Figure S2 ). This observation correlates well with the
variation of the specific energy consumption as shown in Figure
11b . The specific energy consumption decreases first then increases to
result in an optimal specific energy consumption at each given agitation
speed (Figure 11b ). For instance, the optimal specific energy
consumption of 13.2 kWh/kg is obtained at an operating pressure of 2 bar
because the agitation is fixed at 1800 rpm. In conclusion, selection of
a higher agitation speed and an optimal operating pressure can promote
production of oligodextran and simultaneously decrease energy
consumption under the substrate feeding mode.
From a practical perspective, the substrate feeding mode is a better
alternative in large scale applications because of easy continuous
operation and low energy consumption, but severe membrane fouling
hinders its industrial implementation 6. The extremely
low permeate flux resulting from membrane fouling decreases the
filtration efficiency and thus leads to increased operating costs. In
addition, the substrate is easily overly hydrolyzed because the products
cannot be removed in a timely manner due to low permeate flux.
Consequently, development of membrane fouling control strategy in EMR
systems deserves further attention for its potential to be fully
realized.
Figure 10. Simulation investigation of the effect of agitation
speed and operating pressure on (a) production yield and (b) 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, operation duration: 240 min, and feeding mode: water feeding.
Figure 11. Simulation investigation of the effect of agitation
speed and operating pressure on (a) production yield and (b) 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, operation duration: 240 min, and feeding mode: substrate
feeding.