During the enzymatic hydrolysis of the potassium salt of Penicillin-G (PenGK) into Phenylacetic acid (PAA) and potassium salt of 6-Aminopenicillanic acid (6-APA), the pH of the reaction mixture falls on account of accumulation of PAA. This lowers the stability and activity of the enzyme used, viz., Penicillin-G acylase (PGA). A new approach of extracting the PAA by a long-chain tertiary amine, which is in the dispersed phase, as a liquid ion exchanger (LIX), is presented. A mathematical model has been developed for this slurry phase reactor with PenGK in the continuous aqueous phase, the amine alongwith a diluent in the dispersed organic phase and the immobilized PGA enzyme as the solid catalyst. Effects of various parameters affecting the conversion of PenG have been discussed. The model has been solved for batch and semi-batch modes. It has been shown that the semi-batch mode yields a higher productivity. This approach can also be advantageously used for other intermediates like 7-ADCA for cephalosporins.