This chapter deals with protein aggregation, which is a key issue in biopharmaceutical processes. Several experimental techniques to characterize the aggregate size and content are presented and fundamentals on the kinetic modelling of aggregation mechanisms are provided. The impact of operating conditions on the aggregation rate is reviewed and the steps critical for aggregate formation in biopharmaceutical processes are identified. Finally, methods aiming at reducing the aggregate content are proposed. These methods focus either on improving protein stability or on removing the formed aggregates. The former can be achieved by synthesizing aggregation-resistant proteins, tuning operating conditions, or designing processes with a shorter residence time (e.g. perfusion bioreactors or counter-current chromatography). The latter method is mainly achieved by filtration and chromatography. In particular, the simulated moving bed process is shown to be very advantageous for aggregate removal with size exclusion chromatography: it allows improving productivity, decreasing eluent consumption and increasing the outlet protein concentration as compared to single-column processes.

This chapter provides basic concepts about multi-column counter-current chromatography. The benefit of counter-current contact in separation processes is demonstrated considering cascades of equilibrium stages. Based on this, the true moving bed (TMB) process and the simulated moving bed (SMB) process are introduced. Then, the design space of the TMB and SMB processes leading to a complete separation is identified, starting with simplistic systems and progressively introducing more complex effects, namely non-linear adsorption isotherms and mass transfer limitations. Finally, two process design approaches for multi-column chromatographic processes are presented: an empirical one, which allows to obtain a first guess of the operating conditions from a single-column experiment, and a model-based one, which allows a more rigorous determination of the process variables.

This chapter focusses on the polishing steps encountered during protein purification by chromatography. The first part provides a detailed description of the mechanistic phenomena at stake, whose complexity may greatly vary from one case to the other depending on the number and type of impurities, the selectivity for the target protein as well as non-linear and competitive effects. Several multi-column counter-current processes allowing the implementation of the polishing steps in a continuous manner are presented in the second part of the chapter. Modifications of the classical SMB process introduced to better cope with the specific needs of the biopharmaceutical industry are discussed. These modifications primarily aim at recovering more than two fractions and at implementing modifier concentration gradients. Two process design approaches for multi-column processes are presented: an empirical one, which provides a first guess of the operating conditions from single-column experiments, and a model-based one, which allows a more rigorous determination of the process variables. The model-based approach is used to compare the performance of multi-column and single-column processes.