Separation of dichloromethane-nitrogen mixtures by adsorption: Experimental and molecular simulation studies

Experimental and grand canonical Monte Carlo simulation results for the separation of a CH₂Cl₂ (1.5 mol%)-N₂ binary gas mixture in molecular sieve materials are presented. AlPO₄-5 and MCM-41 molecular sieves have been used as the selective adsorbents because they consist of uniform arrays of uni-dimensional channels of micro and meso length scales, respectively. Adsorption isotherms were measured at 318 K and at pressures between 50 kPa and 130 kPa. Two MCM-41 materials have been used, one with a 33 Å pore diameter and the other with a 42 Å pore diameter. For AlPO₄-5 at 110kPa the total amount adsorbed from experiment was found to be independent of equilibration time at 0.0542, 0.0538 and 0.0547 mmol per g AlPO₄-5 for 2, 24 and 48 hours, respectively. However, the selectivity for CH₂Cl₂ was found to increase with time from 1.29, to 4.59, to 10.74. For MCM-41 at 110 kPa the selectivity for CH₂Cl₂ was found to be dependent on pore size. On increasing the pore size from 33 Å to 42 Å the selectivity for CH₂Cl₂ increased considerably. Grand canonical Monte Carlo simulations agreed qualitatively with the experimental results, showing a greater selectivity for CH₂Cl₂ than for N₂. The simulations indicate that MCM-41 has a lower selectivity for CH₂Cl₂ than AlPO₄-5, which contradicts the experimental results. Reasons for these discrepancies are presented and discussed.