N-2/CH4 separation behavior at elevated temperature on P84 hollow fiber carbon membrane

Abstract

Recently, natural gas consumption has grown over the years and this large demand requires efficient technology to purify it. Nitrogen (N2) is one of the main impurities in natural gas and could reduce its heat value. Therefore, it is necessary to separate nitrogen from natural gas. Since the diameter kinetic between CH4 and N2 is similar, it is rather hard to separate this gas mixture. This research aims to study the N2/CH4 separation at elevated temperature through a hollow fiber carbon membrane (HFCM) derived from P84 co-polyimide. A comprehensive investigation of P84 HFCM fundamental transport from 273 K to 373 K provided a better understanding of gas permeability as a function of permeation temperature. Moreover, the influence of carbonization heating rate towards the microstructure of HFCM and the N2/CH4 separation behavior was carefully investigated. The XRD analysis confirmed that the carbon membrane formation referred to the amorphous structures at 2θ of 22° and aromatic graphite at 42°, which refers to the (0 0 2) and (1 0 0), respectively. In addition, the SEM demonstrated the HFCM's dense structure with finger-like pores. The highest selectivity of N2/CH4 occurred at the 3 °C/min heating rate with 373 K permeation temperature (9.09) by N2 permeability of 186.92 Barrer because of the contribution of activation energy. Higher energy activation for N2 (13.91 kJ mol−1) makes a higher permeability improvement than CH4 (8.37 kJ mol−1). Thermodynamic studies confirm the selective adsorption effect of gas on HFCM. The influence of heating rate on N2/CH4 gas separation performance was studied at 1 to 3 °C/min. Adsorption and activated surface diffusion contributed to gas diffusion at 298 K and 373 K permeation temperatures) and 5 °C/min exhibited good separation performance above the Robeson upper bound. Overall, the study gives another alternative in separating the N2/CH4 mixture through the optimization of the operating process.