RESIDENCE TIME DISTRIBUTION OF FOOD SUSPENSIONS CONTAINING LARGE PARTICLES WHEN FLOWING IN TUBULAR SYSTEMS

The behavior of concentrated aqueous suspensions containing cube-shaped particles (with edges of 1 cm) in tube flow has been investigated. Both the liquid and the solid phase residence time distributions were measured. In the case of the liquid phase, the pulse method was used by introducing as a tracer an aqueous solution containing citric acid and recording the pH variation of the liquid at the outlet section of the pipe. A step method was used for the solid phase in which potato cubes, previously colored by dipping them in a solution of ethyl alcohol and iodine, served as a tracer and the residence times of the particles were measured by video recording the particles through a transparent window positioned at the outlet of the pipe. The variables studied were flow rate and particle concentration. It has been found that the presence of the particles modifies the residence time distribution of the fluid. In particular, as the particle concentration increases, the mean residence time of the liquid phase increases and the flow regime departs from ideal plug flow. It has, also, proved that both flow rate and solid concentration affect the residence time distribution of the solid phase. With increasing flow rate, as well as with increasing solid content of the suspension, the residence time distribution of the solid phase approaches that of the liquid phase. Consequently, the particle mean residence time decreases and the flow regime of the solid phase approaches ideal plug flow.