Model Study On CO₂ Removal By Membrane-Photobioreactor

The key point of study on CO2 removal by microalgae cultured in a MPBR (MPBR) is to improve CO2 removal capability. Photobioreactor (PBR) combined with hollow fiber membrane contactor has improved mass transfer effect and supplied a larger interfacial area between gas and liquid phase, so as to increase the ability of CO2 removal by microalgae. In this paper, the efficiency of CO2 removal both in MPBR and ordinary PBR has been discussed and compared by considering the concentration conditions of species in the tube side, shell side and membrane phase of the membrane contactor, and PBR conditions including the velocity of flow, the degree of mixing, the gas-liquid mass transfer and the rate of photosynthesis. A model of air-lift PBR was developed, and the model combining with the equations describing the concentrations of CO2 and O2 in the tube side, shell side and membrane phase of the membrane contactor has established the overall model study about MPBR. Time course of pH, dissolved oxygen (DO) in algae liquid and CO2, O2 concentrations in the outlet gas under different CO2 concentration in the aeration gas were thoroughly discussed and compared both in MPBR and ordinary PBR. Variation of pH, DO in algae liquid during a week has been also predicted and compared both in MPBR and ordinary PBR under the same experimental conditions.The simulations closely agreed with experimental data, and the results showed that pH, DO both in MPBR and ordinary PBR appeared periodic vibration with light and dark time alteration. CO2 concentration increased and O2 concentration decreased under the irradiation. In ordinary PBR, when CO2 concentration in the aeration gas was 0.23%, the ability of CO2 removal by Chlorella Vulgaris was higher and CO2 concentration in the outlet gas went down to 0.025%;while CO2 concentration in the aeration gas increased to 0.5%, CO2 concentration in the outlet gas decreased to 0.35%, and DO build-up was severe, so the efficiency of CO2 fixation by Chlorella Vulgaris significantly reduced. In MPBR, DO level in algae liquid was much lower than that of ordinary PBR;and when CO2 concentration in the inlet gas was 0.5%,CO2 concentration in the outlet gas went down to 0.02%, which was lower than standard CO2 concentration in the atmosphere, so satisfied the need for human respiration in closed space.In general, the ability of CO2 removal by MPBR was much higher than that of ordinary PBR. The DO concentration was reduced for the improvement of mass transfer effect and a larger interfacial area between gas and liquid phase in algae liquid in which CO2 was supplied by the membrane contactor. Therefore, in MPBR, photosynthesis of Chlorella Vulgaris has been improved and the efficiency of CO2 removal by Chlorella Vulgaris was increased. The model established could potentially be applied to rational design of the MPBR, high-density culture of microalgae and efficient removal of CO2.