Research On Removing Low Concentration CO₂ By Membrane Reactor Filled With Nanocomposite Hydrogel Immobilized Enzyme

Control of CO₂ 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.Aiming at above-mentioned objective,in this work,carbonic anhydrase(CA) which is the most efficient catalyst for CO₂ hydration and dehydration was immobilized into the poly (acrylic acid-co-acrylamide)/hydrotalcite(PAA-AAm/HT) nanocomposite hydrogels with porous network structure.A hollow fiber membrane reactor filled with immobilized CA enzyme by nanocomposite hydrogel was prepared and used for low concentration CO₂ from mixed gas streams.Mainly research of the thesis includes the following four aspects:1.Preparation of PAA-AAm/HT nanocomposite hydrogel with appropriate crosslinking density,uniform particle size,porous network structure and high water absorbency and salt-resistant performance.Firstly,highly crystalline Mg/Al hydrotalcite(Mg/Al-HT) was synthesized by urea method.Secondly,HT was organo-modified by sodium methyl allyl sulfonate(SMAS) and possible representation for the structure of organo-modified Mg/Al-HT was modeled by Gauss View software. Finally,PAA-AAm/HT nanocomposite hydrogel was prepared via in-situ polymerization.Results showed that when the content of HT was 3 wt%the highest absorbency for deionized water and 0.9 wt%NaCl_(aq) were 1100 g/g and 145 g/g, respectively.Porous network structures were observed by CryoSEM and free water in the porous structures was also proved.TEM result showed that HT was dramatically exfoliated into nanoscale and homogeneously dispersed in the polymer matrix and PAA-AAm/HT nanoacomposite hydrogel belonged to exfoliated type nanocomposite.2.Immobilization of CA via PAA-AAm/HT nanocomposite hydrogel.On the one hand,PAA-AAm/HT nanocomposite hydrogel was directly used to immobilize enzyme by embedding method.Result showed that the immobilized CA could retain 90.9%of the specific activity of the free CA.Because of the formation of a microenvironment almost all composed of free water in the porous network structure, which provided the nicer survival condition for the enzyme.Moreover,thermal stability of immobilized CA was also improved greatly and the activity of the immobilized CA could retain 65%of its original activity after 60min at 50℃.On the other hand,in order to enhance the immobilization capacity of hydrogel, PAA-AAm/HT nanocomposite hydrogels were activated by N-hydroxysuccinimide (NHS) in the presence of N,N'-dicyclohexylcarbodiimide(DCC) and activated hydrogels were used to immobilize CA by embedding and covalent coupling.Results showed that the immobilize capacity of activated hydrogels was 4.56mg/g.in comparison with the 2.68mg/g for embedding method,immobilize capacity of activated hydrogels was enhanced greatly.However,compared with un-activated hydrogels,activated hydrogels could retain lower enzyme activities,76.8%of the specific activity of the free CA.It was because that multi-point covalent linkage would partially destroy the active sites of CA.In addition,dispersion of CA in the nanocomposite hydrogel was also obtained by fluorescence microscopy and the microstructure change of hydrogels before and after enzyme immobilization was observed by CryoSEM.3.Design of hollow fiber membrane reactor filled with immobilized enzyme by nanocomposite hydrogel and study of low concentration CO₂ removal.The membrane reactor was constructed by two independent sets of intimately commingled hydrophobic microporous PVDF hollow fibers containing immobilized CA enzyme by PAA-AAm/HT nanocomposite hydrogels.Results showed that the membrane reactor performed very well.CO₂/N₂ selectivity was 820,CO₂/O₂ selectivity was 330,and CO₂ permeance was 1.65×10^-8 mol/(m².s.Pa) at the best operating conditions:0.1%CO₂, 20mM Tris-HC1(pH 8.0),temperature 20℃,V_s=300ml/min,V_F=100ml/min,1g/l CA,1g nanocomposite hydrogel.Moreover,after CA enzyme was immobilized by hydrogels,thermal and running stability were improved greatly.Prolonged runs lasting 30 hours showed that permeation performance of the membrane reactor were quite stable.4.A model of CO₂ facilitated transport across the membrane reactor was established by analysing the mass transfer mechanism and typical concentration profiles for CO₂ across the membrane reactor filled with immobilized CA were obtained.Results showed that concentration profiles was accorded with transfer mechanism of CA catalyzed CO₂ and experimental data was in good agreement with the simulation results and colud reflected CO₂ facilitated transport across the membrane reactor.This could provide a theoretical basis for further optimization and scale-up design of the membrane reactor.In summary,high-performance membrane reactor filled with nanocomposite hydrogel immobilized CA enzyme will provide theoretical basis and design foundation for the eventual realization of CO₂ removal technology with high efficient,safe and reliable from the closed spaces,such as spaceship and submarine.