Convection Limited Sorption Kinetics in Packed Beds

If the rate capacity of sorbent particles to sorb exceeds the bulk convection rate of sorbate to those particles, then the latter mechanism is rate controlling. In a continuous flow packed bed reactor, this occurs at some point in the sorption uptake zone. Here the concentration level of sorbate decreases continuously forming a sorbate concentration profile. Convection rate decreases proportionately, finally to the point that it is rate limiting. This critical transition point from particle kinetics to convection limited kinetics occurs at the inflection point of the concentration profile. Beyond the inflection point the profile shape is determined by the probability of contacts between the flowing sorbate molecules and the particles comprising the packed bed, which can be expressed as a decay function. The decay function was verified experimentally and an expression for sorption uptake rate was developed for the convection limited case. Removal of trace organics by activated carbon beds in water treatment is an example.