Sedimentation is the movement of particles or macromolecules in an inertial field. Its applications in separation technology are extremely widespread. Extremes of applications range from the settling due to gravity of tons of solid waste and bacteria in wastewater treatment plants to the centrifugation of a few microliters of blood to determine packed blood cell volume (“hematocrit”) in the clinical laboratory. Accelerations range from 1 × g in flocculation tanks to 100,000 × g in ultracentrifuges for measuring the sedimentation rates of macromolecules. In bioprocessing, the most frequent applications of sedimentation include the clarification of broths and lysates, the collection of cells and inclusion bodies, and the separation of fluids having different densities. Unit operations in sedimentation include settling tanks and tubular centrifuges for batch processing, continuous centrifuges such as disk centrifuges, and less frequently used unit operations such as field-flow fractionators and inclined settlers. Bench scale centrifuges that accommodate small samples can be found in most research laboratories and are frequently applied to the processing of bench scale cell cultures and enzyme preparations. Certain high-speed ultracentrifuges are used as analytical tools for the estimation of molecular weights and diffusion coefficients. The chapter begins with a description of the basic principles of sedimentation, followed by methods of characterizing laboratory and larger-scale centrifuges. Two important production centrifuges, the tubular bowl centrifuge and the disk-stack centrifuge, are analyzed in detail to give the basis for scale-up. Ultracentrifuges, important for analytical and preparative work, are then analyzed. The effect of flocculation of particles on sedimentation is presented, and sedimentation of particles at low accelerations is discussed. The chapter concludes with a description of centrifugal elutriation.
Keywords: Boltzmann distribution rule, Carman-Kozeny equation, Einstein diffusion equation, Péclet number, Reynolds number, Stokes drag force, Stokes-Einstein equation, linear gradient mixer, sigma factor
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