Joel Padin

New sorbents for olefin/paraffin separations by adsorption via π-complexation: synthesis and effects of substrates

New adsorbents for olefin/paraffin separations are synthesized by effective dispersion of Ag+ cations on high-surface-area substrates. Two techniques for synthesis are compared: thermal monolayer dispersion and incipient wetness impregnation. The incipient wetness impregnation technique yields the best sorbents that show the highest selectivities, olefin capacities, reversibility and fast rates. AgNO3 is dispersed on three substrates (γ-Al2O3, SiO2 and MCM-41 mesoporous molecular sieve) via incipient wetness impregnation for ethane/ethylene and propane/propylene separations. The surface chemistry of the substrate plays an important role in olefin adsorption. The silica surface (on both silica gel and MCM-41) provides a better substrate due to its lack of Lewis acid sites (unlike γ-Al2O3), and consequently the Ag atoms in these sorbents are more capable of forming π-complexation bonds with olefins. ESCA results indicate a transfer of electron density from Ag+ to the Lewis acid sites of alumina, hence weakens the π-complexation bonds with olefins.

Molecular sieve sorbents for kinetic separation of propane/propylene

Abstract The separation of propane–propylene mixtures is one of great industrial importance. It is also one of the most difficult and costly to achieve. In this work, the feasibility of using sorbents based on kinetic (due to differences in diffusion rates) or steric (due to size exclusion) effect for gas-phase separation of propane–propylene was determined. Equilibrium isotherms and uptake curves were measured on NaA (4A), NaLiA, and AlPO4-14 zeolites. All of the sorbents were capable of selectively adsorbing propylene over propane. PSA simulations were used to compare these sorbents against π-complexation sorbents such as AgNO3/SiO2. The best sorbents were AlPO4-14 and AgNO3/SiO2. In both cases, over 99% propylene product purities could be obtained at reasonably high recoveries and throughputs.

Kinetic separation of methane/carbon dioxide by molecular sieve carbons

The bulk separation of CH4/CO2 mixture by pressure swing adsorption (PSA) on two different types of carbon molecular sieves (CMS) is analyzed. The two CMS are Bergbau–Forschung (BF) CMS and Takeda 3A CMS. Two different PSA cycles are considered, differing by whether a feed step is used. For a 50/50 feed mixture, it is shown that the separation is feasible with both sorbents to produce a methane product at over 90% purity and at reasonably high product recoveries and sorbent productivities. It is shown in this work that temperature is an important factor to consider for kinetics-based separations, and that a higher temperature is favorable for CH4/CO2 separation on molecular sieve where diffusion is slow. At 25°C, BF CMS yields better separation because the diffusion of both CO2 and CH4 are slow in the Takeda 3A CMS. Increasing the temperature decreases the equilibrium adsorption amounts, but increases the diffusivities. The separation results with Takeda 3A are substantially improved when the temperature is increased to 70°C, and the separation is better than that of BF CMS at 25°C. Therefore, the rate of diffusion (rather than equilibrium) is the dominating factor in kinetics-based separations. The effects of the sorbent working capacity (defined as the difference in the amounts adsorbed of the fast diffusing component between the ends of the adsorption step and the desorption step) on the separation are illustrated. For the kinetics-based separation, a high-pressure feed step is desirable while a low-pressure purge step is not desirable. The reasons are seen clearly from the bed profiles.