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Carbon dioxide emissions to the atmosphere lead to global warming and unpredictable climate change via the greenhouse effect. Carbon dioxide from industrial sources can be captured by the membrane separation technique to mitigate the greenhouse effect. In this research, a polymer solution was prepared by blending a cellulose triacetate (CTA) polymer with a tri-n-butyl phosphate (TBP) additive. The polymer solution was coated on an alumina (-Al2O3) tube, which acted as a support material, to prepare a composite membrane for the CO2 separation. The composite membrane that was prepared was characterized by Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermo-gravimetric analysis (TGA), and differential scanning calorimetry (DSC) analyses. The CO2 permeance and the selectivity for the prepared composite membrane were evaluated using a constant pressure-variable volume method. The influence of the concentration of CTA and TBP, polymer solution preparation time, number of sequential dip coating, and the feed gas pressure on the CO2, N-2, and CH4 gas separation performances was examined. The highest CO2 permeance of 129 GPU and the selectivity of 19.9 versus N-2 and 10.6 against CH4 were obtained for the pure gases, and a CO2 permeance of 116 GPU and selectivity of 17.1 against N-2 were obtained for a mixture of gases (15% CO2/85% N-2). (c) 2019 Society of Chemical Industry and John Wiley & Sons, Ltd.