Influence of operating conditions on residence time distributions in a scraped surface heat exchanger during aerated sorbet production

The residence time distribution (RTD) inside a scraped surface heat exchanger (SSHE) during simultaneous crystallization-foaming process for continuous production of an aerated sorbet was studied. The effect of mix flowrate, air flowrate and refrigerant temperature on the unfrozen liquid/ice crystals phase RTD was investigated using a dye tracer method. The experimental results revealed that both mix and air flowrates increase leads to a lessening of the minimum and of the mean residence time inside the SSHE. Flow diagnosis showed the presence of a nearly stagnant air pocket which volume increase with the air flowrate increase, resulting in an augmentation of the liquid phase velocity. More axial dispersion was observed at higher air flowrates and at lower refrigerant temperature due to greater radial temperature and axial velocity gradients. These conclusions were confirmed by the parameters of the fitted flow models (axial dispersion, tank-in-series and gamma distribution models) even if only the gamma distribution model succeeded to well describe the flow patterns observed. The findings of this pioneering work in the field of gas-liquid flow in SSHEs might be useful for several applications involving multiphasic flow in SSHEs.