Mass and Momentum Transfer in Isothermal Sorption Vessels

Adsorption separation processes are finding increasing use in the process industries for splitting fluid mixtures into constituent components. In order to design full-size equipment, experimental studies are first carried out at laboratory- and pilot-scale. However, there are difficulties in scaling-up the information obtained from these sets of apparatus to allow accurate prediction of operational performance. In particular, industrial equipment is found to under-perform compared to equivalent systems of smaller size. Two reasons have been proposed in the literature for the performance degradation; thermal effects and flow maldistribution. Thermal effects arise because the adsorption/desorption processes are non-isothermal and this has already been extensively studyed and reported upon. However, information regarding flow maldistribution is either absent or contradictory. Flow maldistribution is caused by two mechanisms, voidage variation over the radius of the packed bed and poor distribution at the inlet and outlet. The variation in voidage in a packed bed is mainly due to the „wall effect‟, whereby the enclosing walls are found to increase the structural order of the packing material. This opens up paths through which material passes easily and therefore reduces the effectiveness of the packed bed. The literature contains many sources of information as to when the wall effect should be deemed a problem, however this is not always found to correspond to experimental results. A study is therefore required to ascertain the effect of voidage variation over a wide scale range. In order to cover such a large variety of scales, a simulation approach is applied. In order to incorporate sufficient information about the system, a 2-D semi-homogeneous model is used. Comparisons can then be made to the 1-D axially dispersed plug flow model in order to ascertain whether this simple approach is justifiable. A second study has also been carried out in regard to flow distribution at the inlet of packed beds. Good flow distribution at the inlet can be a problem in short length, large diameter packed beds. Unfortunately, due to economic objectives, these are the types used in industrial processes. It is therefore important to ascertain the effect of plenum chamber geometry and packed bed characteristics on material distribution at the inlet. This will allow comparison of the transport results obtained from laboratory-scale equipment to those from full-scale.