(583c) Comparison of Adsorptive Separation Performance of a Hollow Fiber Bed and a Packed Bed: A Modeling Study Using Zeolite 13X and a Mixture of Propylene and Propane

Authors: 
Sen, T., Georgia Institute of Technology
Kawajiri, Y., Georgia Institute of Technology
Realff, M. J., Georgia Institute of Technology
Since its discovery by Carl Wilhelm Scheele in 1773, adsorption has become one of the commonly used industrial techniques for fluid (gas/liquid) separation, mostly fueled by the invention of tunable synthetic zeolites in the 1940s and the development of cyclic schemes which allowed for product recovery and adsorbent regeneration. 1 Traditionally, adsorbent crystals are formed into pellets (with or without a binder) and then packed into beds. However, pressure drops are usually incurred due to the random and tortuous nature of the packing void space which influences the cost of pressurizing the feed gas and the shape of the adsorption front. Heat transfer is inefficient for operating a TSA cycle and there is scope for improvement of fluid to adsorbent mass transfer coefficient. 2-4

To overcome these difficulties, several structured configurations including, monoliths, laminates and foams, have been developed. Detailed reviews and comparisons of these configurations are available in literature 2,3,5,6. More recently, a novel hollow fiber-based solid sorbent system has been proposed and experimentally validated4,7-9 which provides an additional degree of control over the heat transfer efficiency as described here. The hollow fiber configuration relies on solid sorbents embedded in a porous polymeric hollow fiber matrix. Several identical fibers are assembled inside a module which resembles a shell and tube heat exchanger. The bore of the hollow fiber has an impermeable lumen layer which allows the flow of a cooling/heating medium to mitigate the temperature of operation of the process despite heat of adsorption/desorption. 4

Compared to the traditional packed bed, the configuration of the hollow fiber bed offers several advantages: (1) the structured packing has a significantly lower pressure drop (2) the diffusion length for the fluid to the interior of the hollow fibers is significantly lower than the spherical packed bed (3) the bore allows for a heating/cooling medium to efficiently transfer heat to/from the adsorbent phase. However, in terms of adsorbent density per unit bed volume, the hollow fiber bed has a clear disadvantage.

From the previous discussion it is clear that while the use of a hollow fiber adsorbent bed has several advantages, it also has at least one significant drawback. It has been hypothesized that the advantages will outweigh the disadvantages of using a hollow fiber bed, but testing and understanding the drivers of the relative performance is needed. The aim of this computational study is therefore, to perform a detailed comparison of the properties and separation performance of the novel hollow fiber bed configuration with that of the traditionally used packed bed. To enable unbiased comparison, an equivalent model for the packed bed and the hollow fiber bed was developed. A preliminary parametric comparison of properties such as heat and mass transfer co-efficients, adsorbent density, and pressure drop was performed on the basis of these models. Optimized five step single bed cycles for the separation of propylene and propane on zeolite 13X were also studied. For the same recovery and purity, the hollow fiber bed was found to have a productivity that was five times higher than the packed bed. The hollow fiber bed also showed higher productivity, when parameters such as the desorption pressure and the ratio of the purge to feed velocity were varied in both beds.

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