Low temperature drying process intensification by application of power ultrasound. Design and development of ultrasonic drying equipment and systems

• Autores: Roque Rubén Andrés García
• Directores de la Tesis: Enrique Fernando Riera Franco de Sarabia (dir. tes.), Manuel Recuero López (codir. tes.)
• Lectura: En la Universidad Politécnica de Madrid ( España ) en 2021
• Idioma: español
• Tribunal Calificador de la Tesis: Jesús Félez Mindán (presid.), Guillermo De Arcas Castro (secret.), Tomás Enrique Gómez Álvarez-Arenas (voc.), Andrew Mathieson (voc.), José Javier Benedito Fort (voc.)
• Programa de doctorado: Programa de Doctorado en Ingeniería Mecánica por la Universidad Politécnica de Madrid
• Materias:
  • Física
    • Acústica
      • Ultrasonidos
  • Ciencias tecnológicas
    • Tecnología electrónica
      • Transductores electroacústicos
• Enlaces
  • Tesis en acceso abierto en: Archivo Digital UPM
• Resumen

  • The application of power ultrasonics in several industrial processes has been proved to be beneficial in terms of acceleration of the process with lower energy consumption and an optimal result. Among the industrial processes that can be enhanced using power ultrasound, we can find defoaming, debubbling, particle agglomeration, supercritical CO2 extraction, or food dehydration, among others. Focusing on food dehydration, this process consists of transferring the moisture attached to the solid matrix of the food to the external gas media, like air. This mass transfer process is influenced by two parameters: the internal and the external resistance. Internal resistance results from the characteristics of the solid matrix and temperature while external resistance mainly depends on the boundary layer thickness. Currently, convective hot-air drying is one of the most common and widely used drying techniques used to extend the shelf life of food products. However, this is a highly time and energy-consuming thermal process, which also negatively affects the final quality properties of the dehydrated product such as color, texture, flavor, rehydration capacity, the content of vitamins, or other nutrients. Airborne power ultrasound (APU) application in drying systems may overcome some of these limitations by increasing the drying rate or accelerating the process even if it takes place at lower temperatures. Freeze drying (o lyophilization) is a food dehydration operation that involves freezing the products, minimizing the environmental pressure (keeping vacuum) and removing the wet content by sublimation. On the other hand, freeze drying assisted by power ultrasound needs a gas media for the ultrasonic waves to propagate, so, the environmental pressure is in this case atmospheric pressure. Hence, this process is known as lyophilization at atmospheric pressure, or atmospheric freeze drying. Food dehydration processes assisted by power ultrasounds need high energy ultrasonic field around the samples are placed. The generation of high-intensity ultrasonic field carries a series of issues that the transducer needs to solve. These issues are related to the difficulties of ultrasonic propagation through gas media and to the required high-amplitude vibrations of the transducer. Airborne power ultrasonic transducers with extensive radiator are the tools capable of generating the high-intensity ultrasonic field. An extensive radiator with a certain surface design, and vibrating with high displacements is capable to provide high acoustic energy at the samples, although it carries other consequences to solve like the appearance of undesired nonlinear effects that may affect the performance of the transducer. The development of airborne power ultrasonic transducers with extensive radiators has to guarantee that the system is capable to operate under a high-power regime, generating the required ultrasonic field and without suffering from undesired nonlinear effects. In this case, a novel ultrasonic technology has been specifically developed to improve atmospheric freeze drying processes. The development of this technology has followed the next steps: - Numerical design.- Using FEM methods, the design of each component has to tune the transducer to vibrate at the desired frequency and with the desired mode. On the other hand, the possibility of modal interaction with other near modes must be discarded in this step. - Experimental characterization.- After the transducer has been built, it must be tested to confirm that the system is capable to operate under a high-power regime without showing critical nonlinearities. - Operation.- The last step consists of determining the efficiency of the transducer in the specific process, like food dehydration. This work deals with these steps of the development of two airborne power ultrasonic transducers with extensive radiator, obtaining a good characterization of both cases, and an enhancement of freeze drying of food samples, for the transducer that has been use for this process.