Optimization of structured adsorbents for gas separation processes

Conventional gas separation processes using packed beds of beads or granules suffer from high pressure drop and mass transfer resistance when higher throughputs are required, leading to lower productivity and recovery and higher power consumption. This restricts the adsorption processes using traditional adsorbents in the form of beads or granules to low throughputs and makes such processes less attractive compared to other processes such as distillation for large volume production and high productivity. Such problems could be reduced if structured adsorbents are developed and replace the traditional beds of beads or granules. Although much research has been devoted to the development of structured adsorbents over the last decade, there is still a need to increase the understanding of structured adsorbents. Therefore, this work aimed to increase the fundamental understanding of structured adsorbents using two different approaches; a general approach by which numerical models were developed to predict the performance of structured adsorbents with different geometries in pressure/vacuum swing adsorption (PSA/VSA) processes. The effects of parameters such as porosity, density and surface area on the performance of structured adsorbents with different geometries were evaluated. Comparisons based on mass and heat transfer, adsorbent loading and pressure drop characteristics of PSA systems for COz/N2 separation were carried out. The obtained results demonstrate the potential advantage of structured adsorbents in rapid cycle adsorption processes. The even flow distribution, very low mass and heat transfer resistances and low pressure drop in combination with considerable adsorption capacity in the best structured adsorbents indicate that these novel configurations are promising adsorbents for advanced PSA/VSA applications. In another part of the general approach, the optimization procedure for the structure of gas adsorbents at the pore-scale level was performed taking into account the effects of pore geometry, porosity and size on ultimate working capacity of adsorbents used in a PSA system. As a most remarkable finding of theoretical results by this optimization technique, the branched structure with a porosity of less than 50% represents the optimum structure with higher working capacity. Furthermore, at faster cycles the advantage of a branched structure is more obvious indicating its ability in reducing diffusion limitations more efficiently than other structures. The second approach was to evaluate the performance of zeolite coated monoliths prepared and tested experimentally by numerical modelling. The effects of wall porosity, channel width distribution and zeolite film thickness on the dynamic behavior of the adsorbents were examined. The model indicated that the film thickness could be increased up to about 10 Ilm to increase adsorption capacity without increasing the dispersion in the system further. In addition, it was shown that employment of monoliths with lower wall porosity would lead to better performance of the structured adsorbents.