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.