Study On Preparation And Performance Of Modified NaAlg Composite Membranes For CO₂Separation

The fixed carrier membrane is promising for CO₂separation and the carriergenerally includes the pyridyl group, the amine group, and the carboxyl group, etc.The membrane with the carboxyl group as the fixed carrier is antioxidative, which isgreatly potential for CO₂separation from flue gas. In this work, attempts were madeto develop new fixed carrier membranes with the carboxyl group as the fixed carrer.Sodium alginate （NaAlg）, a kind of cheap, easily available and natural polymer, wasused as membrane material, and modified in a physical or chemical method toimprove CO₂permeance of the membrane.NaAlg was modified by low molecular weight sodium tartrate （ST） containingcarboxyl groups to prepare NaAlg-ST coating solutions. A series of NaAlg-ST/PScomposite membranes were prepared by casting the NaAlg-ST solutions on thepolusulfone （PS） ultrafiltration membranes. The results of ATR-FTIR spectra showedthat cross-linking framework formed between NaAlg and ST due to the intermolecularhydrogen bond and indicated that the concentration of the carrier （-COO-） increasedwith increasing ST-to-NaAlg mass ratio （mST/mNaAlg） in the coating solution. TheCO₂（15vol%）/N₂（85vol%） separation performance of NaAlg-ST/PS compositemembranes was investigated. The results showed that cross-linking NaAlg with STcan effectively improve the permselectivity of the membrane. Meanwhile, thereexisted an optimal mST/mNaAlgin NaAlg-ST coating solution which could maximizeCO₂permeance of the membrane. The endurability of the NaAlg-ST/PS compositemembrane was investigated and it was confirmed that the NaAlg-ST/PS membranewith the carboxyl group as the fixed carrier was antioxidative and SO₂-resistant.According to the experimental results of the NaAlg-ST/PS membrane with maximumCO₂permeance in this work, the performance of the two-stage membrane process wasroughly evaluated. The economic evaluation results indicated that the two-stageNaAlg-ST/PS membrane process used for CO₂/N₂separation was competitive againstthe traditional chemical absorption method.To improve gas permeance, the hydroxyl group in NaAlg was silylated with theTMS group to develop silylated sodium alginate （SNaAlg）. SNaAlg with different TMS content was developed by changing the mole ratio of reactants. The SNaAlg/PScomposite membrane was developed with SNaAlg as the separation layer andpolysulfone （PS） as the support. The chemical characterization of SNaAlg wasconducted on Fourier transform infrared （FTIR） spectrometer, nuclear magneticresonance （NMR） spectrometer and inductively coupled plasma mass spectrometry（ICP-MS） and TMS content was calculated based on the measured value of Si. TheCO₂（15vol%）/N2（85vol%） separation performance of SNaAlg/PS compositemembranes was investigated. The results showed that the incorporation of TMSgroups increased gas permeance. However, plasticization emerged with increasingTMS content, which is greatly harmful for the selectivity. According to theexperimental results of the SNaAlg/PS membrane with maximum CO₂permeance inthis work, the performance of the two-stage membrane process was roughly evaluated.The economic evaluation results indicated that the two-stage SNaAlg/PS membraneprocess used for CO₂/N2separation was competitive against the traditional chemicalabsorption method.