In this paper by applying the direct continuous-Time system identification methods an accurate and reliable control oriented model of the conductivity in the polishing phase of the purification process of the nimotuzumab, a monoclonal antibody, is derived. A brief introduction to the theoretical aspects of the direct continuous-Time system identification methods is presented. The complete direct system identification procedure, from the design of the experiment to model validation is developed. For this purpose, the CONTSID toolbox is used. The model validation results show that the model obtained reproduces with high accuracy the observed data, and therefore it can be used in the design of control systems and/or for prediction purposes.