Residence time distribution measurements and simulation of the flow pattern in a scraped surface heat exchanger during crystallisation of ice cream Original Research Article

Scraped surface heat exchangers are used in the food industry to process highly viscous fluids, and ice-cream in particular, but most of the time, the influence of operating conditions on product quality is poorly understood. The objective of the study is to develop simple tools to help industrials understand, and then optimise their fabrication process. Residence Time Distribution (RTD) has been characterised in an industrial pilot system during real ice cream production, after the method had been validated in an experimental set-up with a simple mixture of water and sucrose. It has been shown that in its dimensionless form, RTD depend slightly on flow rate and scraper rotational speed. A simple model of flow pattern applicable to SSHE during crystallisation was developed to reproduce the observed RTD. It distinguishes two zones: the volume of fluid near the cooling wall where ice is generated and which is swept by the blades and the volume of fluid near to the rotor. Therefore, the model considers two parallel plug flow reactors with axial dispersion, and which exchange fluid by radial mixing. After adjustment of the model parameters, a good agreement was obtained with experimental results. The flow rate is lower in the zone near the cooling wall; this can be due to a higher ice concentration leading to higher viscosity. This approach can contribute to better understand, optimise and control SSHE used for ice cream production.