Adsorption of N-butane gas on zeolites and fundamental adsorption unit design

Abstract

Gas chromatographic techniques were used to study the adsorption of n-butane on NaY and Offretite/Erionite zeolites. The adsorption equilibrium constants were measure at T = 100-200℃ for NaY zeolite and at T = 200-250℃ for Offretite/Erionite zeolite. Particle sizes used are 0.169, 0.212, and 0.279 mm. Under experimental conditions, the particle size and the flow rate have no effect on the adsorption equilibrium constant. The adsorption ability of Offretite/Erionite zeolite is better than NaY zeolite. The adsorption equilibrium constant increases with increasing column temperature. The heats of adsorption for NaY and Offretite/Erionite zeolites were found to be 9.01 and 9.98 kcal/mol. K respectively. Furthermore, the results also showed that the axial dispersion seemed to be independent of the flow rate and particle size. Therefore, it is concluded that molecular diffusion is the major factor for axial dispersion phenomena under measurements. The external tortuosity factors were calculated of 3.17 and 3.95 for Nay and Offretite/Erionite zeolites respectively. The main macropore diffusivity for both zeolites is due to the Knudsen diffusion. The approximate order of the crystal diffusivity for both zeolites were found to be 10-10 cm2 / s which characterize the crystal diffusion as the activated process. The overall rate of adsorption was studied in terms of the overall effective rate coefficient. At constant temperature, the overall effective rate coefficient increases with increasing particle size. The flow rate increases the turbulent mixing in axial dispersion term and this results to an increase of the overall effective rate coefficient. In addition, the overall rate coefficient also increases with temperature which is caused by the increase of the overall mass transfer of the adsorbent particle. At the same temperature, the overall mass transfer of Nay zeolte is higher than of Offretite/Erionite zeolite. In designing the fundamental adsorption unit for LPG separation, the experiments were carried out for propane and n-butane gases by NaY zeolite to find the condition which gave the satisfied resolution between two peaks. The satisfied condition was found to be T = 75℃ and flow rate = 24.5 cm3/min which gave the value of RAB> 1. The column length for LPG separation was approximated to be 17.28 cm, i.d. = 3.34 mm, and packed with NaY zeolite of particle size = 0.212 mm. The design data provide useful approximate guidance concerning the optimal choice of operating parameters for LPG separation.