Simulation of Membrane-photobioreactor for Carbon Dioxide Removal by Microalgal Photosynthesis

Elevated CO₂ level in the closed space or room has much been concerned. Control of CO₂ within certain range is one of the most important contents in the life support system. In this paper, a 10 liter membrane-photobioreactor operated with dead end mode was prepared to remove CO₂ from air by using the photosynthetic alga, Chlorella vulgaris. The results showed that gas exchange efficiency was improved greatly when the membrane module was adopted. Compared to that in the ordinary photobioreactor, not only the retention time of the smaller and more uniform gas bubble is increased from 2 s to more than 20 s, but also the dissolved oxygen (DO) dropped by a factor of 30, resulting in that the CO₂ fixation rate was enhanced 2 from 80 to 260 mg l^-1 h^-1. When the operating condition was controlled: cell density of 2.0×10⁷ cells ml^-1, inlet gas rate of 3 l min^-1, and light intensity of 165 μE m^-2 s^--1 at 25∼ 30 °C, 1.0 % CO₂ in the aeration gas could be reduced to 0.3% in the discharged gas; and for room air (0.04% CO₂), the CO₂ concentration could be decreased to the boundary value.

Taking account of conditions including the velocity of flow, the degree of mixing, the gas-liquid mass transfer and the rate of photosynthesis, the corresponding model for prediction of DO and pH in airlift column photobioreactor used for culturing microalgae was developed. Experimental data were also used to verify the model which could potentially be applied to rational design of the photobioreactor and removal of CO₂.