The recovery of biological particles in high-speed continuous centrifuges with special reference to feed-zone break-up effects
In the first part of this thesis the means are described by which an industrial disc stack centrifuge may be scaled-down to process in a meaningful fashion small volumes of particle suspensions. The centrifuge separation characteristics so measured were suitable for direct scale-up predictions of centrifuge performance. Experiments with a dye tracer and a reduced number of discs indicated that the flow through the disc centrifuge is influenced by the position of the separating discs within the disc stack. This influence was also reflected in the separation performance of a particulate suspension. The results were shown to be in the range obtained using a full-scale industrial disc centrifuge. Experiments also showed a distinct trend that the flow rate and in particular the single passage throughput influenced the separation performance. Using a non-linear curve-fitting algorithm it was shown that the grade efficiency curve of a disc centrifuge may be approximated by a two-parameter model equation. In the second part of this thesis the means are described by which various feed zones designs may be examined in respect to particle breakup using small volumes of a diluted protein precipitate suspension. It has been shown that in the high shear fields which are present in the feed zones to centrifuges the precipitate particles are readily disrupted into smaller fragments, the fragment size depending on the strength of the shear field. Examination of three different feed zone types revealed that particle breakup increased in the following order: full-hermetic < hydro-hermetic < semi-hermetic It has also been demonstrated that the bowl speed and in particular the rotational velocity of the distributor ribs has a significant influence on the level of particle breakup in the hydro-hermetic feed zone, while the effects were less pronounced in the full-hermetic feed zone. Based on the results obtained during the above studies the major mechanism of particle breakup is thought to be due to a combination of turbulence and collision of the particle suspension with the rotating distributor ribs. It has been shown that the level of particle breakup is directly related to the circumferential velocity of the distributor ribs.