Structured zeolite adsorbents for PSA applications

Sammanfattning: Zeolites are microporous crystalline aluminosilicates with unique properties, and are therefore used in adsorption processes, for membrane separation, and as catalysts. Thin zeolite films grown on honeycomb monolith supports represent a new type of material and may be used as structured catalysts and adsorbents, and improve the processes significantly. Pressure swing adsorption (PSA) and vacuum swing adsorption (VSA) are commonly used technologies for gas separation. Currently, adsorbents in form of zeolite beads or pellets are used in PSA and VSA processes. Although these adsorbents exhibit high adsorption capacity, the main drawback of these materials is the limited mass- and heat-transfer, which reduce the performance of the PSA and VSA processes. Novel structured adsorbents with reduced heat and mass transfer limitations may represent a competitive alternative to currently used adsorbents. In this work novel adsorbents in the form of thin zeolite films of NaX and ZSM-5 type were grown on ceramic cordierite monoliths and the CO2 and NOx adsorption properties were studied. NaX zeolite films with controllable thickness and free from sediments were grown with a multiple step synthesis procedure on the walls of porous ceramic cordierite monolith supports with different cell density. The structured adsorbents have lower adsorption capacity per gram adsorbent compared to beads. However, the pressure drop for the structured adsorbents is much lower than for beads. CO2 breakthrough fronts for the best structured NaX adsorbents were much sharper than for NaX beads. The sharp breakthrough fronts indicate that the flow distribution in the structured adsorbent is even and that the mass transfer resistance is very low due to the small film thickness and high effective diffusivity for CO2 in the film. At the same time, the adsorption capacity of the structured adsorbents is considerable. NOx adsorption for structured ZSM-5 adsorbents with varying Si/Al and Na/Al ratio was also systematically studied for the first time. An increased aluminium and sodium content in the films resulted in a higher amount of physisorbed NO2 at low temperature, due to increased number of weak adsorption sites. At higher temperature, the amount of NO2 adsorbed was nearly independent of aluminium and sodium concentration for all ZSM-5 film samples, which indicated that an increase in Al and Na concentration in the films increased only the number of weak adsorption sites. The VSA separation performance of the structured adsorbents was compared with that of adsorbent beads by numerical simulation. These results demonstrate the potential advantage of structured adsorbents, in particular in rapid cycle adsorption processes. The even flow distribution, very low mass transfer resistance and low pressure drop in combination with considerable adsorption capacity in the best structured adsorbents indicate that these novel materials are promising adsorbents for PSA and VSA applications.