Non-Uniform Flow in Packed Bed Adsorbers

Adsorption is a commonly applied industrial separation process employing the use of packed beds. The packed bed is made using adsorbent particles that preferentially remove one or more components from a gas or liquid mixture passed over it. For many systems, the bed loading is not in continuous equilibrium with the concentration of the adsorbate(s) in the gas/liquid and a mass transfer zone develops along the length of the bed. The length of this mass transfer zone is a major concern when designing any adsorption system. If the predicted mass transfer zone is shorter than that realised, then breakthrough of the adsorbate occurs and the result is contamination of the product. Alternatively, if the bed is over designed then unnecessary capital is spent on adsorbent and the operating cost increases due to the compressor/pump having to drive material through a larger bed. For pressure swing systems, compressor costs also increase for the bed-pressurizing step. Many factors determine the length of the mass transfer zone and have been accounted for in numerous one-dimensional plug flow models. However, shorter mass transfer zones are consistently predicted numerically compared to experimental measurements. In general, this has led to the models being corrected by using an axial dispersion term. Empirical correlations are available for determining the amount of axial dispersion in a packed bed, taking into account molecular diffusion and turbulent mixing. However, in general this tends to be a " catch-all " quantity which is used to fit experimental data with mathematical models. Unfortunately, the value obtained for the axial dispersion term determined in this way tends not to scale well with adsorber size and therefore expensive pilot-scale adsorbers are needed before industrial-scale systems can be built. The purpose of the study carried out was to investigate flow inhomogeneity in the adsorption system and determine whether or not an appreciable effect on performance is observed. It is felt that flow mal-distribution is a contributory factor to axial dispersion in adsorption systems and could give an insight into problems with scale up. A two-dimensional modelling approach is used to qualitatively investigate beds where radial voidage varies. The model takes into account both radial flow and bulk adsorption effects, which have not been accommodated previously in the literature. It is found that mal-distribution factors can have a marked effect on adsorber performance and the length of the mass transfer zone.