Željko Grbavčić

Analysis of Moisture Transfer During the Drying of Clay Tiles with Particular Reference to an Estimation of the Time-Dependent Effective Diffusivity

The description of the drying process was reduced to the establishment of a series of theoretical and empirical drying models. The complex processes of simultaneous moisture and heat transfer, which are often nonstationary, and the distinct nature and properties of the material to be dried further complicate the description of the drying process. Three theories—diffusion theory, capillary flow theory, and evaporation–condensation theory—have won general recognition for the explanation of moisture transfer in porous media. The mechanisms of moisture movement during drying in the constant and especially in the falling drying period are rather complex and, hitherto, there have been no generally accepted explanations that could identify the exact transition between possible drying mechanisms, such as liquid movement due to capillary forces, liquid diffusion due to concentration gradients, liquid and vapor flow due to differences in total pressure, vapor diffusion due to difference in vapor concentration, vapor diffusion due to partial vapor pressure gradients, Knudsen diffusion, thermodiffusion, and the evaporation–condensation mechanism. The goal of this study was to find a way to better understand the different drying mechanisms, to identify the exact transition between them, and to estimate the time-dependent effective diffusivity. The results presented in this article confirmed that the effective diffusivity represents an overall mass transport property of moisture that includes all possible moisture transport mechanisms that are simultaneously controlling the moisture migration process in a material during drying. The experimental investigations were performed on clay tiles in a laboratory recirculation dryer, for which the drying parameters (humidity, temperature, and velocity) could be programmed, controlled, and monitored during drying.

Mass transfer between fluid and immersed surfaces in liquid fluidized beds of coarse spherical inert particles

Mass transfer coefficients between fluid and immersed surfaces in liquid fluidized beds of spherical inert particles have been studied experimentally using fluidization columns 40 mm and 70 mm in diameter. Mass transfer data were obtained in three experimental systems: i) transfer of methylene blue dye from very dilute aqueous methylene blue solution as the fluidizing fluid to plane solid surfaces using the adsorption method [Koncar-Djurdjevic, Nature, 172 (1953) 858]; ii) transfer of benzoic acid from a large spherical particle to flowing water using the dissolution method, and iii) transfer of benzoic acid from a plane solid surface to flowing water also using the dissolution method. In all runs mass transfer rates were determined in the presence of inert fluidized particles 1.20, 1.94 and 2.98 mm in diameter. Measurements covered a particle range having Reynolds number from 15 to 400, and two Schmidt numbers (20 °C), 1361 and 1932. Experimental data are correlated by the widely used formula jD=0.261 Ga0.324Rep−0.97 (jD = mass transfer number; Ga = Galileo number; Rep = Reynolds number). The data show that an analogy exists between the mass transfer factor and fluid–particle interphase drag coefficient, since both quantities vary with voidage in a similar way. The normalized mass transfer factor in liquid–phase fluidized beds of active or inert particles and the dimensionless drag coefficient are shown to be the same. Experimental data agree reasonably well with the proposed model.

Mass transfer in liquid spout—fluid beds of ion exchange resin☆

Abstract Experiments were carried out to determine the liquid side mass transfer coefficient in spout-fluid beds of ion exchange resin. The ratio (Sh S—F ) overall /Sh fluidized is correlated with the ratio of the flow rate through the nozzle to the total flow rate for different experimental conditions. A method for the calculation of the initial conversion in a spout—fluid bed is given and a model of a spout—fluid bed ion exchanger involving liquid flow pattern is proposed. Particle motion in the annulus was measured. The model is based on the assumption of film diffusion control. Experimental results obtained in the study of the conversion of ion exchange resin “Amberlite IR-120” in Na + form and diluted hydrochloric acid in a spout—fluid bed are in agreement with theoretical predictions.

Mass transfer and fluid flow visualization in packed and fluidized beds by the adsorption method

Mass transfer coefficient (jD) between fluid and column wall in liquid packed and fluidized beds of spherical inert particle has been studied experimentally using adsorption method. Experiments were conducted in column 40 mm in diameter for packed and fluidized beds. In all runs mass transfer rates were determined in presence of spherical glass particles 2.06 mm in diameter. This paper introduced adsorption method as very suitable method for studies of mass transfer and for fluid flow visualization. The adsorption method is based on the dynamic adsorption of an organic dye onto a surface covered with a thin layer of a porous adsorbent. Local and average mass transfer coefficients were determinated from the surface color intensity of the foils of silica gel. Correlation jD = f(Re) was derived using mass transfer coefficients data.

Removal of volatile organic compounds from activated carbon by thermal desorption and catalytic combustion

The thermal desorption of saturated activated carbon discharged from an industrial adsorber and catalytic oxidation of desorbed products over a Pt/Al2O3 catalyst were investigated. The activated carbon is almost completely regenerated by flushing with air at 200°C for 30 min. Desorbed products are fully oxidized over the Pt/Al2O3 catalyst above 275°C.

Methods of Determination for Effective Diffusion Coefficient During Convective Drying of Clay Products

© 2012 Vasic et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Methods of Determination for Effective Diffusion Coefficient During Convective Drying of Clay Products

Hydrodynamics and Mass Transfer in Heterogeneous Systems

Research of transport phenomena in liquid – particles systems, in past years, had a more theoretical then practical importance (Coudrec, 1985; Lee et al., 1997; Schmidt et al., 1999). For industrial use, especially with fast development of bio and water cleaning processes, better knowing of these systems become more important. An industrial application of these systems requires determination of transfer characteristics, especially mass transfer. Frequently mass transfer is the rate determining step in the whole process. However, in the real systems, it is not always easy to differentiate the limitation due to the mass transfers from that due to the hydrodynamic. Mass transfer in liquid-solid packed and fluidized beds has been widely investigated in terms of particle–fluid mass transfer by dissolution, by electrochemical and by ion-exchange methods (Damronglerd et al. 1975; Koloini et al. 1976; Dwivedi & Upadhyay, 1977; Chun & Couderc, 1980; Kumar & Upadhyay, 1981; Rahman & Streat, 1981; Yutani et al. 1987). Some of the results of mass transfer in packed and fluidized beds have been obtained as the transfer between an immersed surface and the liquid (Riba et al. 1979; Boskovic et al. 1994; Boskovic-Vragolovic, et al., 1996&2005). Liquid fluidization of particulate solids has a history which predates the now more commonly applied gas fluidization. The broad range of operations to which liquid fluidization has found applications are: classification of particles by size and density, a special case being sink-and-float separation by density; backwashing of granular filters and washing of soils; crystal growth; leaching and washing; adsorption and ion exchange; electrolysis with both inert and electrically conducting fluidized particles; liquid-fluidized bed heat exchangers and thermal energy storage; and bioreactors. Fluidized-bed bioreactors, which have received much attention during the past thirty years, are usually characterized by the catalytic use of enzymes or microbial cells that are immobilized by attachment, entrapment, encapsulation or self-aggregation. The most common application of such bioreactors is probably in wastewater treatment and, as in the case of the other operations mentioned above, liquid fluidization must in each case be weighed against competing schemes for achieving the same objective before it is adopted commercially (Epstein, 2003). In contrast to fluidized beds data, there are no published data on mass transfer in vertical and horizontal hydrotransport of particles.