The operating conditions (flow rate, RPM) for the disk stack centrifuge still require some empirical optimization even once the appropriate flow rate has been determined. This has been ascribed to a certain degree of cell disruption because of shear during centrifugation, which generates smaller particles that cannot be efficiently removed by the centrifuge under standard operating conditions.³ In general, long residence times (slow flow rates) will lead to a clearer centrate but at the expense of process throughput. The clarification efficiency of a centrifuge can be determined by a relative measurement of turbidity in the feed stream and the centrate. Not all aspects of a disk stack centrifuge can be captured effectively by a laboratory-scale tubular bowl centrifuge. In particular, shear stresses during entry and discharge from the bowl are difficult to scale. A scaled-down version of a disk stack centrifuge that requires less than 10 L of broth to operate has been developed to accelerate experimental development of operating conditions.⁴

Other operating parameters that require optimization for a continuous disk stack centrifuge operation include the discharge frequency, the discharge type, and the predischarge flush solution and volume. These parameters are determined empirically. The effectiveness of a centrifuge in removing particulates decreases as the bowl fills with the deposited solid sludge. However, too-frequent discharges will risk decreasing product yield and increase operating time. A practical compromise is to discharge the bowl after it fills up to 50–70% of the bowl volume. The discharge frequency is determined by the solids content of the cell culture broth. Experimentation with the ideal case of a continuous discharge in a nozzle centrifuge continues for the harvest of mammalian cell culture media but has not yet found favor in large-scale operation.⁵ In the case of discontinuous bowl discharge, the discharge type can be full or partial depending on whether the entire bowl contents are ejected during the discharge. Before discharge, a flush volume is used to push the contents of the bowl through the centrifuge so that yield losses during discharge are minimized. A buffer or water is often used for the flush. An important consideration is whether the osmotic difference between the flush fluid and the cell culture broth will cause lysis of cells releasing cell debris, host cell protein contaminants, DNA, and proteases, which can impact product quality negatively.

TANGENTIAL FLOW MICROFILTRATION

Mass transport limitations resulting from the formation of a concentration polarization layer of particles close to the membrane surface remain a significant limitation of MF harvest operations.⁸ Various alternative flow configurations have been proposed to mitigate the effects of concentration polarization. One alternative is to use rotating disk filters that augment the cross-flow velocity close to the membrane surface, thus sweeping away the concentration polarization layer.⁹ Further developments have used Taylor vortices.¹⁰ The use of Dean vortices created by flow patterns in the MF device offers the possibility of reducing concentration polarization without the need for moving parts in the MF system.¹¹ Yet another strategy has been the use of periodic backflushing to sweep the membrane surface clean.¹²

A comprehensive review of the fundamentals of cross-flow microfiltration and the phenomena involved during concentration polarization and fouling has been provided elsewhere.¹³ The flux decline during microfiltration has been described in terms of a series of physical phenomena.¹⁴