Electrohydrodynamics for dehydration of soft, heat-sensitive biological materials
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Date
2023
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Doctoral Thesis
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Abstract
Background and motivation:
Reducing post-harvest losses is one of the major measures considered by food system experts to increase global food security. Fruits and vegetables have the highest share of post-harvest losses, with 37 to 50% loss. Preserving fruits and vegetables as dried materials is one of the oldest and most established techniques for increasing off-season availability. Nevertheless, drying is an energy-intensive process due to the high latent heat of evaporation required to evaporate water. In addition, the existing drying methods are energy-intensive and costly, hence adding a substantial cost for high-volume, low-value products such as fruits and vegetables. Therefore, researchers and industry are continuously looking for more sustainable alternative solutions. Electrohydrodynamic (EHD) drying is a promising yet not commercialized technology for drying different biomaterials, including fruits and vegetables. This drying technique has gained the attention of researchers due to its promising features, such as low energy consumption, operation at ambient temperature, and good preservation of the nutritional content and sensory appeal of dried fruits and vegetables. Despite all these promising characteristics, researchers and industry have not yet been able to implement EHD drying as an industrial-scale unit operation. Currently, we lack comprehensive knowledge about the physics behind the EHD drying process, its energy consumption and exergy situation, its impact on product quality, its scalability, and the associated costs and economics. These challenges hinder the initiation of the next steps that could finally lead to industrial implementation.
Scope and approach:
This thesis addresses these challenges by exploring the underlying physics of EHD drying, introducing an optimized and scalable EHD drying configuration, and benchmarking the performance of EHD drying against standard conventional drying methods. To this end, the three main tools of a scientific approach, namely theoretical development, modeling and simulation, and experimental verification, are employed. First, the main dehydration mechanisms in EHD drying are identified, modeled by theoretical formulation, and analyzed. Using these theoretical models, we quantified the relative contribution of each dehydration mechanism to the overall mass transfer and ranked them based on their contribution. Then, a physics-based model which couples EHD-generated airflow directly to the convective heat and mass transfer from the food was developed and solved using the finite element method. This validated model enabled us to design a scalable configuration in silico. Different arrangements of electrodes were tested in silico to improve the energy efficiency and drying kinetics of this scalable configuration. After selecting the configuration based on the simulation results, a lab-scale setup was developed to experimentally verify the scalability of this novel configuration. Moreover, the sustainability of the scalable EHD drying was evaluated by quantifying the Key Performance Indicators (KPIs). The performance indicators were selected based on the current concerns and interests of the food industry, namely, drying kinetics, energy consumption and environmental impact, product quality, and final product cost. By comparing the KPIs between EHD drying and conventional drying methods, this thesis provides a clear overview for industry, farmers, and other stakeholders about the advantages and disadvantages of using EHD drying as an alternative to their current technologies, such as hot-air, solar, and freeze-drying. Pre-treatment methods were also combined with EHD drying to improve the drying rate of the EHD drying process to satisfy the high throughput demand of the food processing industry.
Key findings and conclusions:
Theoretical modeling of the different EHD-driven dehydration mechanisms shows that convective dehydration by ionic wind is the dominant dehydration mechanism, with a contribution of about 93% to the overall water flux for a capillary-porous material. The contribution of all the other water transport mechanisms, such as transmembrane flow and electro-osmosis, is only 7%. These findings prove that EHD drying is a convective-based drying and convection should be the focus of any design and process optimization in the next steps. Our experiments and simulations showed that using a mesh collector instead of a plate collector and smart activation of wires in the mesh results in more energy-efficient and uniform dehydration than the conventional EHD drying configurations employed in previous studies. Using these insights, we introduced a new configuration for EHD drying. This novel design is independent of fruit loading density showing better scalability in terms of production capacity compared to the conventional EHD drying designs. In addition, it significantly improves the drying rate by more than 65% and energy consumption by more than 60% compared to the conventional configurations. We also observed a 50% decrease in drying time compared to control drying (i.e., natural convection) by operating EHD at only 1 W. Finally, this technology was benchmarked against other conventional drying methods by KPI evaluation. The results show that compared to the currently used drying methods for small to medium-scale drying, EHD was found to be a more exergy and energy-efficient, cost-effective, and sustainable alternative that can provide higher-quality dried products. Using pre-treatment methods is one of the options to increase the drying rate in EHD drying. Therefore, pulsed electric fields (PEF), ultrasound, and blanching pre-treatment methods were studied to explore their impacts on the drying rate. Results show that only PEF pre-treatment could significantly (by 39%, p<0.05) decrease the drying time. However, it resulted in a 26% higher browning index than the untreated EHD-dried apples, which is not appealing to consumers. Other quality attributes, such as antioxidant activity, total phenolics, and rehydration ratio, were not significantly affected by the applied pre-treatment methods. The obtained insights into the EHD drying process, and the significant improvements of the EHD dryer with our optimized and up-scalable design, together with the holistic evaluation of its performance, could provide the push needed to finally implement EHD drying as an industrial unit on a full scale.
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Examiner : Defraeye, Thijs
Examiner : Six, Johan
Examiner : Mellmann, Jochen
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ETH Zurich
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Subject
Food processing; Industrial dryers; Sustainability; Ionic wind; Techno-economic analysis; Physics-based simulation; Multiphysics modeling; Dehydration processes; Pretreatment
Organisational unit
03982 - Six, Johan / Six, Johan