(77d) Microfiber Adsorbents for Improving Packed Bed Adsorbers
AIChE Spring Meeting and Global Congress on Process Safety
Tuesday, April 24, 2007 - 3:15pm to 3:40pm
Industrial gas or liquid phase adsorptive separations are usually performed as column processes, where the adsorbent takes the form of pellets packed in a column. To limit pressure drop, these particles have to be fairly large (1-5mm) and especially in the liquid phase, these then exhibit a strong mass transfer limitation by diffusion in the macropores or even large crystals in the particles. As a result, the breakthrough fronts are often very broad, even with a favourable equilibrium. The strong reduction of the particles to some 10 micron or the use at smaller scale operations of monolithic porous columns reduces the MTZ, but this can only be performed for short column heights like in high pressure chromatography in fine chemical or chiral separations. For large scale industrial processes (environment and petrochemicals) this is a much too expensive option. The only possibility is then to reduce flow rates and results in low volumetric productivities, large adsorbent inventory and column size. Especially when using a highly selective adsorbent to remove impurities from a large stream, this is economically not feasible. Usually, the adsorbent is only very partially loaded when breakthrough already occurs. The use of monoliths, supported small particles or thin layers on fibers with very open structures, where diffusion distances are very short, dramatically improve mass transfer and overall performance. On a volume basis, these materials however have a limited capacity and would require very large columns for bulk separations. Their cost is also prohibitive for large scale application. Recently, they have been proposed for use at the exit of a conventional packed bed that takes most of the load, but where a short bed of fibers or other finely structured material is used at the exit to catch the traces during early breakthrough. A limited capacity, but coupled to very fast mass transfer then allows to increase the breakthrough time by a factor 2 tot 3, and hence increase the bed loading by the same factor. These fibers or monolithic structures are ideal for the removal of traces of impurities from large streams, given their fast mass transfer and low pressure drops. In these applications, a large capacity is not needed and a thin layer on a carrier is sufficient(10-20% loading with 90% porosity materials). Such modified processes not only result in more compact equipment and lower investment, but also in reduced energy consumption in regeneration of the beds. Layers of adsorbents such as zeolites of 100nm can be produced on various supports showing virtually no mass transfer limitation. This paper presents a modeling study, identifying the range of parameters and applications where such a combined bed system might be of use and economically feasible. Preliminary experiments of adsorptive purification with metal fibers coated with zeolites will also be presented.