Experimental study and modelling of the residence time distribution in a scraped surface heat exchanger during sorbet freezing

Abstract

Scraped surface heat exchangers (SSHEs) are widely used for crystallization applications in several food processes (i.e. crystallization of margarine, tempering of chocolate, freezing of ice cream and sorbet). The final quality of these food products is highly related to crystal size distribution and apparent viscosity, both of which are determined by the operating conditions of the process. During the freezing of sorbet, the increase in the ice volume fraction leads to an increase in the apparent viscosity of the product. This effect modifies the fluid flow behavior, the residence time distribution (RTD) and the temperature profile inside the equipment. This work aimed at studying the influence of the operating conditions on the RTD and the axial temperature profile of the product in a SSHE, so as to characterize the product flow behavior. RTD experiments were carried out in a continuous laboratory pilot-scale SSHE by means of a colorimetric method. Experiments showed that high product flow rates led to a narrowing of the RTD and thus to less axial dispersion, due to the enhancement of the radial mixing with the decrease in the apparent viscosity of the product. Spreading of the RTD was obtained for lower refrigerant fluid temperatures, due to a higher radial temperature gradient between the wall and the center of the exchanger, leading to a higher gradient of the apparent viscosity. This effect increased the difference in axial flow velocities and thus the axial dispersion. These results can be useful for the optimization and modelling of crystallization processes in SSHEs.