A countercurrent adsorptive reactor, based on the segregation tendency of dense sorbent particles through a liquid fluidized bed of light biocatalyst particles, is described for the acidifying production of the key pharmaceutical intermediate 6-Aminopenicillanic acid from Penicillin G. Mass transfer limited release of hydroxide from an ion exchange resin with simultaneous separation of the product by ion exchange, enabled axial control of the pH and product inhibition. The complexity of the countercurrent Trickle Flow Reactor requires a mathematical model, incorporating the effects of hydrodynamics, heterogeneous biocatalysis and multicomponent ion exchange. Such a model is presented and the parameters in the Three Phase -liquid. biocatalyst, sorbent- Model were determined from independent experiments. Model predictions gave an adequate description of experimental reactor performance. Using this model, the sensitivity of key parameters in the model, such as sorbent to liquid feed ratio and ion exchange selectivity and reactor optimization are readily studied.