Fundamental Understanding of Removal of Water from a Moist Porous Medium in the Absence and Presence of Ultrasound Mechanism

Noori O'Connor, Zahra

Etd

Fundamental Understanding of Removal of Water from a Moist Porous Medium in the Absence and Presence of Ultrasound Mechanism

Public Deposited

In energy-hungry industry sectors such as papermaking, food processing, chemicals, or pharmaceuticals, the main challenge is to improve the energy efficiency of the process. For example, in the fabrication of paper, a slurry with cellulose fibers and other matter is drained, pressed, and dried. The latter step requires a considerable energy consumption. Therefore, improving the current drying technologies as well as developing novel and more efficient drying technologies is essential in the related industries. In the structure of wet paper, there are two different types of water: free water and associated/bound water. Free water can be easily removed. However, removing bound water consumes a large amount of energy during the process. Bound water is mainly defined as the water inside the nanopores of the product. The intermediate region, i.e., the transition from free water removal to bound water removal, is not studied in the literature. One of the main goals of this dissertation is to improve the energy efficiency of the current drying technologies and understand the dominant mechanisms in paper drying. Study of the bound water removal is out of the scope of this research. The focus is on the intermediate stage of drying, where the remaining free water is either present on the surfaces of the fibers in the form of a liquid film or the water is trapped inside the cellulose fibers. For this purpose, first the physics of removing a thin liquid film trapped between fibers in the paper drying process is explored. In this conjugate heat transfer problem, the film is assumed to be incompressible, viscous, and subject to evaporation, thermo-capillarity, and surface tension. By using a volume of fluid (VOF) model, the effect of above-mentioned parameters on drying behavior of the thin film is investigated. Second, a single cellulose fiber is considered, heated from its outer surface with the water removed from the two open ends of the fiber. The effect of different parameters on the removal of the trapped water is studied. These parameters encompass applied heat flux, water properties (including surface tension), fiber surface properties, geometry, and the dimensions of a given fiber in micro-sale. The governing transport equations, along with the corresponding boundary conditions are solved numerically using VOF method. Using the temperature measurements, the heat flux in the dynamic three-phase vapor/water/fiber contact line is studied. The effect of impurities and Marangoni flows on evaporation is discussed. The results of this research provide fundamental understanding of water removal at the intermediate stage of paper drying. To help with innovation and electrification of drying/dehydration processes, another main goal of this dissertation is to develop an innovative drying technology that improves the energy efficiency and product quality, significantly in high energy industry sectors such as papermaking. The current drying technique in the papermaking industry is contact drying, which depends on the conductive and/or convective heat transfer. In this study, for the first time, a systematic study is conducted using an innovative technology for paper drying by applying ultrasound mechanism, both direct-contact and non-contact (airborne). Specially, for airborne ultrasound, there are limited information available in the literature. The advantages of ultrasonic drying include greater energy efficiency, lower time and temperature of drying, improvement of the product quality, and it is considered a green and sustainable technology. Hence, fundamental understanding as well as applied analysis are required to help reduce the energy consumption and carbon footprint. For the direct-contact ultrasonic drying, the effects of initial moisture content, final thickness, and refining condition of the pulp are studied for two different types of pulps (hardwood and softwood) using 23 factorial design of experiments. The results of Analysis of Variance (ANOVA) show that in the range of the studied parameters in this research, thickness has the maximum effect on the ultrasonic drying time followed by the initial moisture content. In addition, using a linear regression model, two relationships for the total time of drying and the area under the drying curve as functions of the studied factors are provided. The results confirmed that ultrasonic drying is more efficient at higher moisture contents and higher thickness of the sample. These results are related to the structural characteristics of the samples such as porosity, pore distribution, and surface roughness. Therefore, microscopic images of the surfaces are studied, and the quality of the ultrasonically dried papers is measured using colorimeter analysis and tensile test measurements. For the airborne ultrasonic drying, three controlling factors are considered in the experiments including the initial moisture content, basis weight, and refining condition. The outcome of the experiments is compared to the results for direct-contact ultrasonic drying of paper. The results confirm that similar to direct-contact, for airborne ultrasonic drying, the basis weight/thickness of the sample is the most important factor in ultrasonic drying and it is followed by the effect of initial moisture content. Using linear regression model, two correlations for predicting the total time of airborne ultrasonic drying and the area under the drying curve are provided. Quality of the dried samples are evaluated, and the permeability measurements confirmed the effect of pore characteristics on ultrasonic drying. The analysis for energy consumption reveals that ultrasonic drying is more efficient at higher moisture contents. To fundamentally understand the physics behind ultrasonic drying, for the first-time ultrasonic atomization of a liquid layer and the mechanism of droplet formation is numerically studied in great details, both in direct-contact and airborne setups. The effect of different controlling parameters and thermophysical properties of the fluid (including frequency, liquid height, surface tension, viscosity, density, and gravity) on the onset amplitude for direct-contact ultrasonic atomization and mean droplet size are investigated. A correlation for predicting the mean droplet size as a function of non-dimensional numbers (including Reynolds number, Weber number, Froude number or the non-dimensional amplitude, and Strouhal number) is provided. Furthermore, the mechanism of ultrasonic atomization for a direct-contact setup is compared to an airborne setup. This dissertation provides a complete package about the potentials of ultrasonic drying, both direct-contact and non-contact mechanisms and gives insights on the methods of improving the process and reducing carbon footprint. In addition, it sheds light on the physics of ultrasonic atomization and optimizing the design of ultrasonic atomizers in terms of cost and efficiency for different applications.

Creator