Rumpus, Josephine Mary; (1997) Dawatering and scale down of solid recovery in industrial centrifuges. Doctoral thesis (Ph.D.), University College London.

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Abstract

The scale up of centrifugal operations has been carried out in the past using a combination of [sigma] theory applied to laboratory bottle centrifugation and expert experience. The aim of the work presented in this thesis is to scale down industrial centrifuges so that they can be used with small amounts of material to obtain process information at an early stage in process development. Practical work was carried out using real process systems such as homogenised yeast flocculated with polyethylenimine (PEI) to selectively remove contaminating lipids, nucleic acids and cell debris from a solution of soluble protein. The separation capacity of a scroll decanter centrifuge is limited by solids conveyance, a turbulent settling zone and shear breakage. Rheological instrumentation was used to predict dewatering performance of biological sediments in a series of mass balanced pilot scale trials. Laboratory scale separations yield enough information to predict initial pilot scale trials using an operating line for the scroll decanter centrifuge where the viscoelastic properties of dewatered sediments are related to the turning force per revolution of the conveyor: log10G* MsendN(T-T0) Operation is then constrained only by economic and machine limitations. Scale down experiments revealed that shear of suspension in the feed zone creates permanent floe damage and also that the shearing action of the conveyor assists dewatering and contributes to softening of the sediment. The separation performance of a disc stack centrifuge with a bowl volume of 0.6 L was found to be the same as for a scaled down disc stack centrifuge with a bowl volume of 3 L used in previous studies. 25% scale down of separation area in the smaller disc stack centrifuge was achieved according to Z theory using a dilute suspension of polyvinylacetate. The position of the active discs was adjusted to compensate for variations in flow conditions across the stack using this robust system before applying the method to shear sensitive systems. Solids recovery of homogenised yeast flocculated with PEI was slightly higher in the scaled down stack due to the lower flow rates which reduce shear break up in the feed zone. Dewatering was over estimated in the scaled down stack due to longer sediment residence time.

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