Fluid Dynamic Analysis of the Solid–liquid Separation Process by Centrifugation

In this paper a theoretical analysis and preliminary experimental trials of the solid – liquid separation process by centrifugation has been carried out. This has been done to evaluate as better as possible the modifications should be made on the centrifuge to improve its performance. A new dimensionless number, the dragging number d, has been defined as the ratio between the mean settling time of the smallest particles and the mean dragging time, which is the time necessary to remove all the settled solids. It is shown that, in the case of the olive oil extraction process by centrifugation, if d increases too much, then it is possible that the transition zone between the two main directions of the fluid motion reaches the oil layer. Therefore, it is important to avoid this interaction, otherwise a mixing between water and oil will occur. In this case it is necessary to correctly evaluate the optimal value of the dragging number in order to leave a thick water layer over the sediment. Finally, it is pointed out that about the 75 % of the solids present in the olive paste, used in preliminary experimental trials, settle almost immediately, very near the feeding section. This means that suitable solutions are required to reduce the problem above, as to let the differential velocity to vary in function of the characteristics of the olive paste and/or to use barriers correctly located near the feeding section.