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

Andrés García, Roque Rubén (2020). Low temperature drying process intensification by application of power ultrasound. Design and development of ultrasonic drying equipment and systems. Thesis (Doctoral), E.T.S.I. Industriales (UPM). https://doi.org/10.20868/UPM.thesis.65959.

Description

Title: Low temperature drying process intensification by application of power ultrasound. Design and development of ultrasonic drying equipment and systems
Author/s:
  • Andrés García, Roque Rubén
Contributor/s:
  • Riera Franco de Sarabia, Enrique Fernando
  • Recuero López, Manuel
Item Type: Thesis (Doctoral)
Date: 2020
Subjects:
Faculty: E.T.S.I. Industriales (UPM)
Department: Ingeniería Mecánica
Creative Commons Licenses: Recognition - No derivative works - Non commercial

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Abstract

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 (CHAD) 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 the limitations of CHAD by increasing the drying rate or accelerating the process even if it takes place at lower temperatures. Generally, power ultrasonic energy is used to produce permanent changes in the treated medium. Its use is based on the adequate exploitation of a series of mechanisms activated by the high-intensity ultrasonic waves, such as radiation pressure, acoustic streaming, agitation, instability at the interfaces, and structural diffusion. 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 (AFD). Nevertheless, food dehydration processes assisted by power ultrasounds need high energy ultrasonic field around the samples are placed. Airborne power ultrasonic transducers (APUT) are the tools capable of generating the high-intensity ultrasonic field. 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. The attenuation of acoustic waves when propagating through a gas media has a quadratic relationship with frequency, so, ultrasonic waves attenuate much faster than acoustic waves with frequencies in the audible range. On the other hand, it is necessary to have a good impedance matching between the ultrasonic transducer and the propagating medium. 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 main goal of the development of airborne power ultrasonic transducers with extensive radiators is 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 AFD 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. The document has been divided into six chapters, as follows: - CHAPTER 1 provides a description of fundamentals of power ultrasonics, explaining the general principles of ultrasounds, and with a brief state-of-the-art of industrial applications assisted by power ultrasound, more specifically food dehydration processes. - CHAPTER 2 focuses on the description of airborne power ultrasonic systems. The nonlinear behaviors of these systems are also presented here, as well as the transducers we will work with. - CHAPTER 3 deals with the numerical simulation processes of the two systems and the estimation of the generated ultrasonic field for each case. - CHAPTER 4 describes all the experiments carried out for the full knowledge of each transducer, including the low power characterization, stability tests, nonlinear dynamics characterization, modal analysis, and acoustic measurements in the near and far field. Some examples of other transducers are also included in this chapter. - CHAPTER 5 is dedicated to the whole development of the transducers that is the full characterization applying the techniques presented in the previous chapter. This chapter also includes a preliminary design of an adaptive mechanical amplifier to improve the performance of the transducer. - CHAPTER 6 shows the influence of the application of the APUT with stepped-grooved circular plate in the enhancement of atmospheric freeze-drying (AFD) experiments over apple slices at a laboratory scale. - CHAPTER 7 summarizes the principal aspects of this work and the gains achieved after the whole development of the two systems. This work is a first attempt to scale up AFD process by considering a drying chamber with a new airborne power ultrasonic transducer with stepped-grooved circular radiator, capable of accommodating a larger amount of food samples. The APU application with this new system during AFD kinetics of apple slices resulted in an increased drying rate and a reduced drying time. As a conclusion, the ultrasonically assisted AFD can be an interesting alternative to an expensive and high-demand energy process such as vacuum freeze-drying. What is now required is to increase efforts directed toward scaling up APU application taking into consideration issues such as drying chamber configuration which will improve the ultrasonic efficiency as well as procedures for the mass production of ultrasonic transducers. So, future research lines would be the industrial scaling of this technology, using ultrasonic systems with circular radiators and ultrasonic systems with flat rectangular radiators and a set of reflectors.

Funding Projects

TypeCodeAcronymLeaderTitle
Government of SpainBES-2013-064748UnspecifiedUnspecifiedContrato predoctoral para la formación de doctores
Government of SpainEEBB-I-15-10006UnspecifiedUnspecifiedMovilidad Predoctoral para la realización de estancias breves en Centros de I+D españoles y extranjeros
Government of SpainEEBB-C-16-00694UnspecifiedUnspecifiedMovilidad Predoctoral para la realización de estancias breves en Centros de I+D españoles y extranjeros
Government of SpainEEBB-C-17-12673UnspecifiedUnspecifiedMovilidad Predoctoral para la realización de estancias breves en Centros de I+D españoles y extranjeros

More information

Item ID: 65959
DC Identifier: https://oa.upm.es/65959/
OAI Identifier: oai:oa.upm.es:65959
DOI: 10.20868/UPM.thesis.65959
Deposited by: Archivo Digital UPM 2
Deposited on: 27 Jan 2021 09:33
Last Modified: 26 Jul 2021 22:30
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