The present invention relates to a process and apparatus for purifying a molecule of interest from a heterogeneous clarified fluid mixture. The apparatus of the invention generally comprises a continuous perfusion fermentation system, a continuous particle removal system integrated with the perfusion fermentation system； and a continuous purification system integrated with the particle removal system, which is maintained under sterile conditions. The process comprises filtering a heterogeneous clarified fluid mixture by continuous ultrafiltration at a specific flow rate below the transition point of the molecule of interest in the pressure-dependent region of the flux versus TMP curve, wherein the specific flow rate is maintained substantially constant throughout the continuous ultrafiltration.
