Comparison of Traditional and Structured Adsorbents for CO₂ Separation by Vacuum-Swing Adsorption

The development of structured adsorbents with attractive characteristics is an important step in the improvement of adsorption-based gas-separation processes. The improved features of structured adsorbents include lower energy consumption, higher throughput, and superior recovery and purity of product because of the even flow distribution, very low mass-transfer resistance, and low pressure drop in combination with a reasonable adsorption capacity. This study examines the vacuum-swing adsorption (VSA) CO₂ separation performance of structured adsorbents in the form of thin NaX films grown on the walls of ceramic cordierite monoliths, and the results are compared with NaX pellets. Adsorption equilibrium and dynamic properties are explored experimentally. The CO₂ breakthrough front for the NaX film grown on the 400 cells/in. ² (cpsi) monolith was close to ideal and indicated that axial dispersion was very small and that the mass-transfer resistance in the film was very low. The breakthrough front for the structured adsorbent with 400 cpsi was sharper than that for the structured adsorbent with 900 cpsi and only shifted to shorter breakthrough times because of the lower amount of zeolite and higher effective diffusivity of the former sample. In addition, the CO₂ breakthrough fronts for the 400 and 900 cpsi structured adsorbents were both sharper than the breakthrough front for NaX beads. This indicates that the flow distribution in the structured adsorbents is more even and that the mass-transfer resistance in the film is very low because of the small film thickness and high effective diffusivity for CO₂ in the NaX film. Experimental data were used to obtain overall mass-transfer linear-driving-force constants, which were subsequently used in a numerical simulation program to estimate the performance of the adsorbents for CO₂/N₂ separation in a VSA process. It was found that the recovery of structured adsorbents was superior to that of a packed bed because of the much shorter mass-transfer zone. The purity, on the other hand, was not as high as that obtained with a packed bed because of excessive voidage in the structured adsorbents. Increased cell density or improved zeolite loading of the structured adsorbents would improve the CO₂ purity without sacrificing recovery for the structured adsorbents, and this represents a path forward to improved VSA performance for CO₂ capture.