The objective of harvest and clarification unit operations is the removal of cells and cell debris to enable product capture on a chromatographic column. The first step in the harvest of monoclonal antibodies from mammalian cell culture is cell removal, followed by filtration unit operations for additional clarification. Centrifugation and microfiltration have been the primary harvest techniques adopted industrially. Although depth filtration can be used as a primary harvest method, it is more common to see this unit operation follow the primary harvest step to provide additional clarification. Flocculants are sometimes added before harvest to improve harvest and clarification operations. Filtration through absolute pore size membranes is typically the final clarification step before capture chromatography. Expanded bed chromatography has been developed as an integrated unit operation that combines harvest with product capture, but to date, practical limitations have kept this technique from being adopted in commercial-scale operations.
Centrifugation uses the density difference between solids and the surrounding fluid. The centrifugal force accelerates the settling that would normally occur during sedimentation. Most industrial applications use disk stack centrifuges to remove cells and cell debris.2 Disk stack centrifuges offer continuous operation, making their throughput consistent with the desire to limit the time for harvest operations. Figure 1 shows the schematic for a disk stack centrifuge.
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The basic principles of centrifugation involve a balance between the buoyant force acting on solid particles and Stokes's law, which expresses the drag force. The ratio of flow rate to the effective settling area (termed Σ) is held constant during scale-up of centrifugal operations. For a disk stack centrifuge, Σ is expressed as:
in which n is the number of disks, r 0 and r i are the outer and inner radii of the disk, and θ is the angle of the disks from the vertical.
Operating conditions are often first screened at laboratory scale in a tubular bowl centrifuge and then translated to a disk stack centrifuge. For the two centrifuges: