Self-Sourced Superheated Steam Generation In Combination With Latent Heat Reclamation For Food Dehydration:System Design And Experiments

There are many disadvantages in the currently available super-heated steam dehydration systems.The design of these systems is complicated,requires separate superheated steam generation,difficult to reclaim the exhaust heat for more efficient energy use,and can cause deterioration of product quality during falling rate drying.The objective of this study was to design a new dehydration system that utilizes the moisture released from the dehydrating materials for superheated steam generation in combination with latent heat reclamation(3SG-LHR)for more energy-efficient food dehydration.This is a two-state dehydration processing consisting of a super-heated steam dehydration chamber that utilized the moisture evaporated from the dehydrating material for the generation of superheated steam in the first stage and a latent heat recovery drying unit for continued dehydration in the second stage.In the first stage,the product was heated,allowing the water to evaporate into the chamber.As the dehydration process progressed,the original air in the chamber was gradually replaced by moisture vapor,which was eventually converted into superheated steam.The superheated steam worked as the drying medium to remove the remaining water in the dehydrating material to produce a semi-moist product,which were transferred to the second stage for continued drying.The exhaust heat from the first chamber transferred into a compact heat exchanger of the second stage for latent heat recovery,in which he latent heat of exhausted gas was released and reused as the heat source in the heat recovery drying unit to dry the semi-dried product produced in the first stage.In this study,kelp was investigated for dehydration in this system.A numerical model was developed to accurately predict and analyze moisture content and temperature distribution of kelp during dehydration.By comparing energy consumption from the hot air drying and the 3SG-LHR dehydration process,energy saving from the 3SG-LHR process was calculated.The following summarizes the main accomplishments and conclusions of this study:1.The system design for the self-sourced superheated steam generation in combination with latent heat reclamation(3SG-LHR)for food dehydration.An energy-efficient 3SG-LHR system was designed with key components,such as heater,blower fan,deflector,and heat exchanger,integrated.With 3SG-LHR,no separate superheated steam generation system was needed.Energy recovery was accomplished by reusing the exhaust heat from the first stage in the second stage.2.The design and analysis of compact heat exchanger and its effect on dehydration.Numerical analysis was used to study the temperature distribution,velocity field,and the volumetric distribution of condensed water in the compact heat exchanger tubes using Fluent~?.To improve the utilization of recovered latent heat by the compact heat exchanger,the effects of different drying modes on heat transfer,mass transfer,and energy efficiency during kelp dehydration were evaluated.3.Thermal properties of kelp.The effect of temperature,moisture content on the thermal properties(thermal conductivity,specific heat and thermal diffusivity)of kelp was studied.Empirical equations of these thermal properties were expressed as a function of moisture content and temperature,which were used for investigation of mass and heat transfer during kelp dehydration.4.Dehydration characteristics and kinetics of kelp.The process of kelp dehydration was studied in the 3SG-LHR system.The results indicated that the increase of temperature reduced dehydration time.The relationship between effective moisture diffusivity of kelp and temperature was found.The energy required for kelp dehydration in this system was 26.189kJ/mol.Eight thin-layer mathematical models were used to study the dehydration curves of kelp.The Midilli and Kucuk model was chosen for describing the dehydration kinetics of kelp in the superheated steam drying chamber.5.Theoretical analysis of the dehydration process in the superheated steam dehydration chamber.In this chamber,the dehydration process could be separated into two stages.One-dimensional heat and mass transfer models were developed for each stage.The mass and heat transfer equations and boundary conditions were solved numerically by using the finite difference method,and validated with experimental data.Results demonstrated a good agreement between the model and experimental data,suggesting that the mathematical models could be used to estimate moisture and heat transfer during dehydration.6.Comparison of conventional hot air drying(CHAD)and 3SG-LHR dehydration.Both processes were used to dry kelp.The energy efficiency and product quality(color,rehydration ratio,and shrinkage)were evaluated.The dehydration with the 3SG-LHR process achieved better color retention,higher rehydration ratio,and smaller shrinkage than the CHAD process,practically achieving a 44.29%reduction in energy consumption,in comparison to a theoretical energy saving of 55.79%.