Study On CO₂ Removal From Closed Air By Microalgae (Enzyme) Integrated Membrane Bioreactor

Control of CO₂ and O₂ levels within certain range is one of the most important tasks in life support systems, such as spaceship and submarine. Technology used for CO₂ removal from the closed space requires high efficient, safe and reliable CO₂ extraction systems characterized by small volume, low mass, low rate of energy consumption, minimal use of consumables, and little or no crew time for operation and maintenance. In this work, based on the advantage of hollow fiber membrane in high specific mass transfer area and low volume, two kinds of effective membrane transport carriers like Chlorella vulgris and carbonic anhydrase (CA) were selected to prepare the corresponding hollow fiber membrane-photobioreactor and hollow fiber contained enzyme bioreactor for the study on CO₂ removal and transformation from air.Chlorella vulgaris, with the features including the efficient photosynthesis superior to C4 plants, the fast proliferation rates, the wide variety of tolerance to extreme environments, and the high performance for recduction of atmospheric CO₂ was first used for the construction of a 3 L simple photobioreactor integrated with a hollow fiber membrane module. Factors like carbon source, pH, temperature, light intensity and gas aerated conditions were investigated. Under the optimum operating conditions including inlet gas flowing rate of 600 ml min^-1, inlet gas of 1% (v/v) CO₂, light intensity of 3500 1x, pH 8.5-9.5 at 25³0°C, and hollow fiber membrane module with pore size of 0.33 μm, the CO₂ removal ratio was 50% and the maximum CO₂ removal rate was 118 mg l^-1 h^-1.In the 10 L airlift photobioreactor, high density culture of C. vulgaris was obtained in fed-batch mode, with the maximum density increase from the original 2.3xlO7 cells/ml to 2.675×10⁸ cells/ml. And Effects of CO₂ and O₂ concentrations in the inlet gas on the photosynthesis of C. vulgris were investigated, with the results showing that CO₂ content in the normal air couldn't meet the needs of microalgal growth, while with pH oscillating at 8.5, the 1% CO₂- rich gas could support the densityhigher above 108 cells/ml. Although O2 evolution rate could be enhanced by reducing the O2 concentration in the inlet gas, its effect on CO2 removal and O2 evolution was not significant. Furthermore, the photosynthetic capability of C. vulgaris was closely related with the microalgal growth, which decreased sharply once it entered into the stationary stage. Consequently, improvement of CO2 removal rate couldn't be relied solely upon the high-density culture of C. vulgaris.In the 10 L membrane-photobioreactor, a hollow fiber membrane module was adopted to supply the needs of C. vulgaris for carbon source. Compared with the ordinary photobioreactor, the gas exchange efficiency was improved greatly when the membrane module was operated for aeration in the dead-end mode, i.e. not only the retention time of the smaller and more uniform gas bubbles increased from 2 s to more than 20 s, but also the dissolved oxygen (DO) released timely, resulting in the enhancement of the CO2 fixation rate from 80 to 260 mg I"1 h'1. When the operating conditions were controlled at cell density of 2.0xl07 cells ml"1, inlet gas flow rate of 3 1 min"1, and light intensity of 10000 lx at 25 -30 °C, the 1.0% CO2 in the aeration gas could be reduced to 0.3% in the discharged gas, and using normal room air (0.04% CO2) as feed, the CO2 concentration in the discharged gas could be decreased to the boundary value of 0.01%.A model of air-lift photobioreactor for CO2 removal was established by combination of conditions including the velocity of flow, the degree of mixing, the gas-liquid mass transfer and the rate of photosynthesis. Two corresponding simplified methods, such as time discretization and lumped parameters were put forward. Using the method of lumped parameters, a model for prediction of CO2,02 concentration in the outlet gas and simulation of time course of DO, pH in the column air-lift photobioreactor under different CO2 concentration in the aeration gas was thoroughly discussed. Experimental data were used to verify the model, which could be potentially applied to rational design of photobioreactor, high-density culture of microalgae, and simulation and prediction of CO2 removal from air.Carbonic Anhydrase, another kind of carrier for membrane facilicated transport of CO2, was also chosen to develop the hollow fiber contained enzyme membranebioreactor. A thin film of CA solutions was held in the interstitial space between two independent sets of intimately commingled hydrophobic microporous hollow fibers, with one set of fibers carrying the feed gas while the other set carrying a sweep gas or the permeated gas stream. The CO2 concentration could be reduced from 1.0% in the breathe air to lower than that in the normal air by 5 cascades when the appropriate operating conditions were controlled: CA concentration of 10 mg/L, CA activity of 2200 U mg"1, Tris-HCl buffer of 0.02 M (pH=8.0), liquid membrane thickness of 1 mm, the feed gas flow rate of 100 ml min"1, and the sweep Argon gas flow rate of 200 ml min"1 at 25°C. Furthermore, the aforementioned bioreactor showed sustainable CO2 removal capacity, i.e. the CO2 removal rate could be maintained after the regeneration of CA by sweep of inert gas Ar.Finally, a series of poly(acrylic acid-co-acrylamide) superabsorbents were synthesized for the immobilization culture of microalgae and CA hydrogel immobilization. The results showed that CA molecule could enter into the network of one of the biocompatible hydrogels, and the activity of CA trapped into which could be maintained without any reduction for 6 monthes. Due to overcoming the shortage of liquid membrane while retaining the water-rich ambience by trapping the water in the network of the hydrogel, the hollow fiber contained hydrogel-enzyme membrane bioreactor showed CO2 removal capability superior to the aforementioned hollow fiber contained liquid enzyme membrane bioreactor, reducing CO2 from 0.52% in the inlet feed gas to that lower than 0.090%. Consequently, as a more safe and reliable CO2 removal system, the hollow fiber contained hydrogel-enzyme membrane bioreactor could be potentially applied in closed space.