Numerical simulation of lethality and f-value of a non-Newtonian fluid aseptic sterilization under the effect of fluid velocity and tube diameter in a double tubular exchanger

Numerical simulation of lethality and f-value of a non-Newtonian fluid aseptic sterilization under the effect of fluid velocity and tube diameter in a double tubular exchanger
Azadeh Ranjbar Nedamani
Received: Accepted:
Assistant Professor, Department of Biosystem Engineering, Faculty of Agricultural Engineering- Sari Agricultural Sciences and Natural Resources University- Sari- Mazandaran- Iran.

Abstract:
Introduction: Unit operation of heat processing is common in food process industries. Most unit operations such as canning, pasteurization, and sterilization are based on the heat process. In the traditional food heating process, the food is filled in a package and then the heat process starts, but in some new technologies such as the aseptic process, the food heat processing is before the packaging and after heating, food will be packaged in a sterilized condition. The aseptic process needs a high temperature for sterility. But in comparison with traditional heating sterilization can destroy the microorganisms at high rates and also can more deserve the texture, flavor, color, and nutritional elements of food. The foods which are processed under aseptic conditions show higher shelf-life, and nutritional characteristics and their packaging in different types and sizes is possible. These factors make the aseptic process more attractive for use in food sterilization. Today the numerical simulation is used for investigating the temperature and lethality to insurance how the food fluids reach the determined temperature during the aseptic heating process. Since the viscosity and velocity of the fluid and the design of an aseptic heat exchanger are effective on the temperature profile of fluids, in this manuscript the numerical simulation was used for studying the lethality and temperature profile inside a broken heating non-Newtonian fluid.
Materials and Methods: In this study, a 3.5% starch dispersion was used for simulation. Since the starch dispersion is a broken heating fluid, three temperature limitations were assumed for viscosity changes during sterilization. The first limit was 78-89℃, which relates to the starch pre-gelatinization limit and shows the starch dispersion viscosity is increasing to make a starch paste, and the second at 89.5-92.5℃ which relates to starch gelatinization and shows the starch dispersion viscosity is reaching the highest level, and the third at 92.5-121℃ which is for post-gelatinization of starch and shows the decrease in starch dispersion viscosity. This decrease relates to granules rupturing basically due to temperature increasing to the 121℃. For the simulation in this study, the two factors of inlet velocity of the fluid (0.5 and 1m/s) and inner tube diameter (inner tubes 4 and 8 cm) were considered. The lethality and F-value were calculated simultaneously with simulation in COMSOL software. The COMSOL 5.3a was used for simulation. The general alpha method and backward Euler time steps were used for solving the laminar flow and mass transfer equations. The geometry consists of a double tube heat exchanger of 2m in length. The outer tube diameter was 18cm. The steam with 121℃ is assumed as the heating medium for sterilization. 3.5% starch dispersion with an inlet temperature of 40℃ was used. The mesh quality in the simulation was 0.8. the tube material was stainless steel. The vapor is injected between two tubes. The lethality and F-value for Clostridium botulinum spores were calculated numerically. The simulated graphs and temperature were extracted from COMSOL after validation of the simulated data. The heat resistance of the inner tube and cooling phase was ignored. It was assumed that the fluid is hemogenic, and the velocity near the wall is zero.
Results and discussion: The changes in diameter and fluid inlet velocity had a significant effect on lethality and F-value. Reducing the inlet velocity leads to increasing the lethality. When the diameter of the inner tube was 4cm, the lethality in lower velocity shows a 3-fold increase. While the lethality at the inner diameter of 8cm was 6.5-fold which is considerable. This shows despite fluid flow, changes in tube diameter have a high effect on microorganisms’ lethality. When the inlet velocity was low, the temperature changes during the tube length were low but finally, at the end of the exchanger, the changes in temperature were high. This shows when the fluid velocity reduces, the needed holding time for reaching a determined temperature will be achieved. Temperature also is a function of the starch dispersion viscosity. The viscosity of starch at the first parts of the tube increases due to gelatinization. But after, from the center to the end of the tube the viscosity decreases. These changes in viscosity are a result of gelatinization and pos-gelatinization of starch. The heat leads to starch gelatinized in the first part of the tube and the temperature changes were high due to inconsistency of viscosity in all layers of fluid. When the diameter of the inner tube and the inlet velocity of the fluid increased, the increase of viscosity happened in total flow. Finally, the simulated data show that the changes in fluid viscosity in different places in a heat exchanger led to changes in temperature distribution and finally changes in the sterilization process. This is important when the fluid viscosity change with temperature or other unit operations combined with temperature.
Conclusion: The results show both factors of inner tube diameter and inlet fluid velocity have a significant effect on the temperature profile of fluid and the lethality and F-value of aseptic sterilization. Especially the flow behavior is a function of the tube diameter. This is important when the starch is a broken heating fluid and its viscosity changes during the heating process. These viscosity changes in the starch fluid are also dependent on temperature profile and show very exact calculations for lethality. The changes in inlet velocity of fluid, temperature, pumping conditions, and tube diameter are the most important factors that affect the viscosity of food fluids. This study shows that simulation of fluid behavior during heat processing is important before designing and manufacturing the food machinery. This can be helpful for reducing the challenges during unit operations, and errors, and can assure the sterility of the final food product.
Keywords: Numerical simulation, Aseptic, Double tube heat exchanger, Lethality