The development of a temperature gradient across the membranes enhances the flux of dual membrane by a factor of 25 with only a 3-5 °C temperature difference (Cath et al. 2005). However, this method cannot be applied to the microalgae broth culture

because the increased temperature would kill the microalgae. Operation at high or low flow rate also does not cause emulsion compared to conventional devices, which are subject to flooding at high flow rate and unloading at low flow rate, whereas the overall mass transfer is controlled by the liquid film resistance even at elevated pres­sures (Dindore et al. 2004). The flow in the liquid phase can be controlled at a steady condition, thus preventing the mass transfer reduction. The performance of the membrane contactor still can be improved when the concentration of the absor­bent and the liquid-gas pressure difference are designed properly, as shown in Yan et al. (2007). Increasing the number of capillaries and, at the same time, decreasing the gas flow rate enhanced the CO2 removal efficiency (Bottino et al. 2008). Treatment of the membrane, such as drawing in the axial direction and soaking in concentrated acid, improved the CO2 recovery rate from the absorbent (Takahashi et al. 2011).

The modification of membrane surface was also capable of enhancing the absorption flux. Bakeri et al. (2012) showed that the modification of the poly — etherimide hollow-fibre membrane showed 72% higher absorption flux than commercial polypropylene membrane contactors. The removal and mass trans­fer of CO2 from flue gas also can be increased by letting the flue gas flow inside the hollow-fibre membrane rather than inside the lumen membrane (Yanchao et al. 2012).