Efficient, Scalable Clarification of Diverse Bioprocess Streams Using a Novel Pilot-Scale Tubular Bowl Centrifuge

NOVEMBER 2005 C entrifugation is ubiquitous in the biotechnology industry, primarily in cell harvest and cell lysate clarification. The technology provides advantages over microfiltration, particularly for large macromolecules such as vaccines that are either virus-like particles or loosely structured polysaccharides or nucleic acids. When large pore-size cutoffs are needed, membranes are susceptible to filter fouling by cell debris. Scale-up can become problematic due to uncertain modeling of the fouled condition, as witnessed by the development of complex operational algorithms such as Cwall (1) and optimized flow patterns such as backpulsing (2) and co-current flow (3). Clarification by centrifugation, on the other hand, circumvents the problem altogether and eliminates the cost of batch filter replacement. Several centrifuge designs are used to clarify biological streams, with scroll decanters, disc-stack centrifuges, and tubular bowl centrifuges being the most common. The choice for a given application depends on the nature and concentration of solid particles to be sedimented. For example, because of low sedimentation forces, the use of scroll decanters is limited to streams containing large particles such as mycelia or flocculated cells (4). Disc-stack centrifuges can generate much higher centrifugal forces, process large volumes of material, and handle large quantities of solids, discharging sediment as a wet slurry (5). That can be either ejected continuously (nozzle discharge) or intermittently (solids ejecting discharge). Disc-stack centrifuges fit a wide range of uses in bioprocessing and have long been used to harvest microbial cells and inclusion bodies (4) or to clarify cell lysates (12). A relatively new application for discstack centrifuges is in mammalian cell harvest, for which low expression rates of therapeutic proteins in cell culture (<2 g/L) require large volumes of cell cultures to be produced at manufacturing scale (>15,000-L bioreactors). The high throughput of disc-stack machines makes them a good choice at manufacturing scale (5– 11). But mammalian cells are shear sensitive (8), so the high-shear entrance region of such machines was redesigned with a hydrohermetic seal to eliminate a problematic gas–liquid interface. Even so, careful operation is critical to preventing high fluid stress. Centrifuges must generally be operated at less than full speed to achieve optimum centrate clarity in mammalian cell harvest (10, 11). Biotech companies must quickly convert new leads into purified therapeutics for clinical trials and, ultimately, for the market. A key element in that process is the dual role of pilot facilities: to make GMPcompliant material for clinical trials and demonstrate process scale-up B I O P R O C E S S TECHNICAL