The effect of adsorbent heterogeneity on pure-gas and multicomponent adsorption equilibrium, especially at low pressures, and accurate prediction adsorption equilibrium

• Main Author: Yufeng, H.
• Published: University of Edinburgh 2005
• Subjects: 530.0724
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.664187

A Tian-Calvet type microcalorimeter has been constructed and used to measure the adsorption isotherms and the isosteric heat of adsorption of pure methane, ethane and CO₂, and the binary mixtures of the above-mentioned three adsorptives in three pure-silica MCM-41 samples with different pore diameters at room temperature and at pressures up to 1 bar. Ideal Adsorption Solution Theory (IAST) was applied to predict the binary adsorption isotherms and isosteric heats for individual components in the mixture. It is found that all the three binary mixtures behave ideally in MCM-41 at room temperature and at low pressures. The adsorption isotherms of binary ethane/CO₂ mixtures in the three MCM-41 samples was measured using a high-pressure volumetric apparatus at 264.6 K and at pressures up to 30 bar. IAST was used to study the adsorption system, and it gives quite accurate predictions of multicomponent adsorption equilibrium at low pressures and shows some deviations at moderate and high pressures, presumably due to the chemical dissimilarity of the two adsorptives. Grand canonical Monte Carlo simulations have been carried out to study the same adsorption system. The simulations were carried out using three different models for MCM-41 with different degrees of surface heterogeneity. The model that has an amorphous structure, generated by an energy-minimization procedure, gives the best predictions for ethane adsorption, especially at low pressures, suggesting that this model incorporates a good representation of the heterogeneity of the real MCM-41 material. Excellent predictions of the adsorption of pure CO₂ and binary mixtures of ethane and CO₂ in MCM-41 are obtained with the model, further confirming the realism of this model Long-ranged electrostatic interactions are included for the simulation of CO₂, these interactions, which play an important role, are treated by a simple one-dimension summation method, which gives an accurate calculation of the potential.