Analysis of conveying and mixing in food processing using numerical simulation.

摘要

Melt conveying and mixing are important unit operations in food processing. It is often needed to transfer liquid or semi-solid materials. Mixing two or more ingredients to achieve value-added food products are becoming more popular in the food industry. Better understanding of these processes is required in order to achieve more efficient and better performing equipment designs.; Numerical simulation was used to investigate conveying and mixing processes. FEM (finite element method) and FDM (finite difference method) based models were developed to calculate flow field and temperature field developed during extrusion process inside single- and twin-screw, open end, deep channel extruders. Both Newtonian and non-Newtonian rheological models were simulated. System variables (SME, specific mechanical energy; MRT, mean resident time; TE, exit temperature) were defined and calculated from the flow and temperature fields. Also, a multi-mode viscoelastic model (Phan-Thien-Tanner) was used to calculate the complex flow and temperature fields in the intermeshing zone of a twin-screw cheese stretcher-cooker. The methods and results established can be used to optimize extrusion process, more specifically, the Mozzarella cheese stretching-cooking process.; Stretching and heating step in Mozzarella cheesemaking during which cheese curd was conveyed through a twin-screw stretcher-cooker was also investigated experimentally. Relationships between cheese properties (composition, function and microstructure) and system variables (SME, TE) were established. They can be used to achieve better control and design of cheese stretcher-cooker.; In addition, FEM models were developed to calculate flow fields created during mixing process in various geometrical mixer designs. Based on the flow field parameters, statistical procedures and criteria were developed to evaluate mixing performance of different mixers. Simple (no moving boundaries) and complex (moving boundaries) geometrical designs were investigated along with Newtonian and non-Newtonian material properties. The methods and models established can be used to optimize mixing process and mixing device designs.