Drying of Porous Material During the Constant and the Falling Rate Period: A Critical Review of Existing Hypotheses

During drying of porous material a so-called “falling rate” period is observed, where the drying rate decreases as the moisture content decreases. This behavior is usually described by the well known “shrinking core model.” This model, however, contradicts experimental findings and violates basic laws of multiphase mass transfer in porous media as well. A new model, named “wet surface model,” is suggested which eliminates those discrepancies.     

Characterization of the First Falling Rate Period During Drying of a Porous Material

This study deals with the evolution of the surface state and its influence on the drying of porous media. Surface temperature and saturation values are obtained using optical metrology. Analysis of the experimental results allows discussion of the apparition of constant drying rate period and to characterize the transition to the falling rate period. A mathematical model is developed to account for these observations and experimental results for some physical properties of a model material. It allows determination of the internal profile of moisture and the penetration of the drying front during the falling rate period.     

Characterization of the First Falling Rate Period During Drying of a Porous Material

Abstract

This study deals with the evolution of the surface state and its influence on the drying of porous media. Surface temperature and saturation values are obtained using optical metrology. Analysis of the experimental results allows discussion of the apparition of constant drying rate period and to characterize the transition to the falling rate period. A mathematical model is developed to account for these observations and experimental results for some physical properties of a model material. It allows determination of the internal profile of moisture and the penetration of the drying front during the falling rate period.     

Water Vapor Emission From Rigid Mesoporous Materials during the Constant Drying Rate Period

It has long been thought that the evaporation rate from mesoporous materials during the constant drying rate period (CDRP) is equal to that of a free-water surface, due to the presence of a liquid film covering the surface of the material. In this article we review several early articles and demonstrate that the experimental scrutiny this hypothesis has received is insufficient. Further, we report a set of evaporative drying experiments on eight building materials whose results also do not confirm such hypothesis. Indeed, the drying rate during the CDRP is not equal either among the tested materials or between these and the free-water surfaces. To explain the differences in drying rate, we have looked at the influence of surface texture and porosity. We have concluded that surface texture, which could increase the effective surface area of the materials, did not have a relevant effect on the CDRP drying rate. However, we have found a good correlation between the CDRP drying rate and capillary porosity. This is consistent with the hypothesis that drying occurs at the pore level during the CDRP. Further, it contradicts the suggestion that there is a film of water covering the surface of the materials during this period.     

A New Variable Diffusion Drying Model for the Second Falling Rate Period of Paddy Dried in a Rapid Bin Dryer

The objectives of this article is to propose a new drying model for the second falling rate period known as the variable diffusion controlled period that follows after the first falling rate period and to propose a new method to determine the second critical moisture content that separates these two periods. Experimental work on paddy drying at minimum fluidization velocity was carried out in a rapid bin dryer. The effects of operating temperatures (60-120°C) and bed depths (2-6 cm) on the paddy drying characteristics were investigated. It was found that the normalized drying rate of paddy was proportional to the normalized moisture content in the first falling rate period but in the second falling rate period, the normalized drying rate of the material varies exponentially with the normalized moisture content. The different relationship between the normalized drying rate and the normalized moisture content in the first and second falling rate periods indicate that two different mechanism of moisture transport are at work. The new exponential model of the second falling rate period and the linear model of the first falling rate period were found to fit the experimental data very well. Derivation from variable diffusion equation shows that the linear model is the result of constant diffusion coefficient whereas the new exponential model is the result of linear diffusion coefficient. This also implies that the first falling rate period is a constant diffusion controlled period and the second falling rate period is a variable diffusion controlled period. In addition, drying kinetics data of a drying process that fits the exponential model over a very slow drying period will show that the drying process is under the effect of a linear diffusion coefficient. It was also found that the proposed new method to determine the second critical moisture content that distinguishes between the first and second falling rate periods by using a sudden change in the value of the drying rate gradient to a much lower value at that point is more rigorous and yet simpler than the method of determining the specific location of the receding drying boundary since it is based on the behavior of the actual drying kinetic data.     

Ultrasonic Convective Drying Kinetics of Clipfish During the Initial Drying Period

A hybrid drying system for high-intensity airborne ultrasound applied in convective drying was investigated for the drying of salted codfish (clipfish). Convective drying with ultrasonic assistance at 10, 20, and 30°C was compared to the same process without ultrasound. The Weibull model was used to model and investigate the drying behavior, and the effective diffusion in Fick's law was determined. The ultrasound decreased the drying time more at lower drying temperatures. The drying time was reduced by over 90% at a drying temperature of 10°C. For an industrial drying process at a temperature of 20°C, the drying time was reduced by 32.2%. The ultrasonic, convective drying of clipfish at a temperature of 20°C was faster than the same process without ultrasound at 30°C. The investigations showed a thermal effect for all products when ultrasound was applied. The specific moisture extraction ratio (SMER) in the investigated system was improved by 0.2 kg_water kWh^?1. The heat transfer coefficient in the system used was increased by 32.6% for a heating process in a separate investigation, whereas for a cooling process no increased heat transfer coefficient was determined. The thermal effect might (at least partially) explain the faster drying of ultrasonic-assisted convective drying. The results obtained demonstrate the potential of airborne ultrasound in convective drying with regard to drying time, energy consumption, and product quality. Documentation of the thermal effect should be included in future R&D on this topic.     

Reduction of Devolatilization Rate of Fuel During Bubbling Fluidized Bed Combustion Using Porous Bed Material

Plastic waste combustion in bubbling fluidized bed combustors (BFBC) is characterized by the rapid devolatilization of the fuel. Noncombusted hydrocarbons are often formed, which have been reported to promote the formation of dioxins. In this work, porous bed material was employed instead of commonly used non‐porous sand to reduce the devolatilization rate. We measured (1) the heat transfer coefficient between an immersed object (brass sphere) and the bed and (2) the time required for the devolatilization of a plastic pellet after dropping it into the bed at 943 K. For porous particles we found a 30 % lower heat transfer coefficient, delayed onset of devolatilization and prolonged devolatilization time, compared with quartz sand. Therefore, porous particles were found to be effective in suppressing the rapid devolatilization of plastic waste.     

Mechanism of the First Falling Rate Period: An Alternative Approach by Image Analysis

An alternative experimental approach using the image luminosity of a drying surface photographed by a bore scope was conducted in order to elucidate the drying mechanism for the first falling rate period including the critical moisture content. The observed luminosity was in fair agreement with the luminosity predicted from the model of the configuration of water, indicating that the configuration of water plays a critical role for the drying mechanism, especially for the first falling rate period and the critical moisture content. The parameters of the configuration of water estimated in this work could be effective for prediction of drying rates along with other transport properties.     

Modeling of Diffusion in Capillary Porous Materials During the Drying Process

ABSTRACT

This paper presents a model of heterogenous diffusion in capillary porous materials during the process of drying. The governing heat and mass transfer equations have been established using the liquid as well as vapor flow. Two models have been presented. Model 1 does not consider the heat conduction while the model 2 has been established by considering the conduction. The developed models and the numerical solutions of the resulting differential equations can take into account the moisture and temperature dependent thermophysical properties of the product. All equations have been established in spherical coordinates but the programme written for the purpose of calculations can be used for other geometries also. Numerical calculations have been performed for gas concrete and tiles using model 1, while model 2 has been used for gas concrete only because of the lack of data for thermophysical properties of the tile. For gas concrete it was seen that conduction has only marginal effect on the drying process and the numerical predictions of the drying process were reasonably accurate.     

Modeling of Diffusion in Capillary Porous Materials During the Drying Process

This paper presents a model of heterogenous diffusion in capillary porous materials during the process of drying. The governing heat and mass transfer equations have been established using the liquid as well as vapor flow. Two models have been presented. Model 1 does not consider the heat conduction while the model 2 has been established by considering the conduction. The developed models and the numerical solutions of the resulting differential equations can take into account the moisture and temperature dependent thermophysical properties of the product. All equations have been established in spherical coordinates but the programme written for the purpose of calculations can be used for other geometries also. Numerical calculations have been performed for gas concrete and tiles using model 1, while model 2 has been used for gas concrete only because of the lack of data for thermophysical properties of the tile. For gas concrete it was seen that conduction has only marginal effect on the drying process and the numerical predictions of the drying process were reasonably accurate.     

On the Characteristic Drying Rate Approach to Correlating Experimental Results of the Drying of Moist Porous Materials

Schlunder's model of a surface, is the only fundamental attempt to explain the constant drying rate phenomena, for materials with pore sizes of a few microns. Here, a direct comparison between this approach and the conventional mass transfer (CMT) theory has been made, by extending the approach to incorporate cell sizes of food, plant materials and to low water contents. Contrast to CMT, the extended Schlunder's model analysis suggests that the boundary layer thickness have an effect on the relative drying rate. Some new experimental results are also presented to support one viewpoint regarding the characteristic drying rate method.     

Two Dimensional Heat and Mass Transfer During Convective Drying of Porous Media

In this paper, we study numerically two-dimensional heat and mass transfer during convective drying in porous media. The set of macroscopic equations is very comprehensive and takes into account the effect of gaseous pressure. A numerical code has been established. This code has allowed us to determine t.he space-time evolution of the temperature, the total pressure and the moisture content.     

Injury Mechanisms of Lactic Acid Bacteria Starter Cultures During Spray Drying: A Review

Lactic acid bacteria (LAB) starter cultures can be processed by many drying techniques, among which spray drying has great potential. However, injuries of LAB during spray drying lead to a low survival rate. The microbial injuries of LAB during spray drying are specifically induced by dehydration inactivation, thermal inactivation, and balance between the two. To reduce the drying temperature, new drying techniques based on spray drying were tried; that is, low-temperature vacuum–spray drying and spray–freeze drying.     

Modeling and Stress Analysis During Drying of a Deformable and Saturated Porous Medium

This article addresses how to express the behaviors that develop stresses within a porous media during convective drying processes. The work is focused on the coupling of the thermal (temperature distribution), hygroscopic (moisture, humidity), and mechanical (strains and stresses) aspects shown during the drying process of a saturated porous medium. Natural clay plate samples were used as a model material. Using two different mechanical behaviors (elastic and viscoelastic), the strain–stress equations were studied and discussed through the simulation results. Obtaining almost the same parameters of the main modeling variables (temperature, liquid pressure, and moisture content), a significant difference was observed between the results obtained for the stresses assuming the two behaviors, particularly depending on the viscoelastic parameters deduced from an experimental study. The simulation highlights a response of the medium supposed viscoelastic different to that of elastic case in intensity and response time.     

Drying of a Porous Material in a Pulsed Fluid Bed Dryer: The Influences of Temperature,Frequency of Pulsation,and Air Flow Rate

In this work, a four-section pulsed fluid bed apparatus with a 0.18 m² cross-section area was used to investigate the influence of pulsed-fluidization variables on the drying process of molecular sieves, a test material that was chosen because it presents an initial constant drying rate period. A two-level factorial design was developed to evaluate the influences of the inlet gas temperature—40 and 70°C—the frequency of pulsation—250 and 900 rpm—and the air flow rate—500 and 600 m³(STP)/h—on the drying rate. In addition, a comparison was made between the drying rates achieved with conventional and pulsed fluidization. Results showed that all the investigated variables affect the drying rate. Moreover, drying rates with conventional fluidization are considerably higher, which shows that one must expect a lower drying rate when pulsation is used in a drying process controlled by the external evaporation. Concerning fluid dynamics, this work also analyzed the influence of the frequency of pulsation on the pressure drop across the bed. The higher the frequency, the higher the pressure drop. That result can be explained by the reduction of channeling.     

Microwave-Assisted Drying: A Review of the State-of-the-Art

Offering advantages of energy-saving rapid drying rates, short processing times, deep penetration of the microwave energy, instantaneous and precise electronic control, and clean heating processes, microwave-assisted drying (MWD) has become a popular method that is currently used for many materials and processes. This article presents a systematic and comprehensive review of experimental and theoretical studies regarding the kinetic mechanisms of MWD. Factors affecting, methods for measuring, and applications of the dielectric property are discussed. From the experimental perspective, laboratory- and commercial-scale MWD systems are elaborated, including the equipment used and the stability, safety, and regulation of MWD systems. Theoretical investigations of thermal and nonthermal equilibrium models and moving-load computational models are discussed. Finally, some future trends in the research and development of MWD systems are suggested.     

Drying Rate Curves of Porous Solids in Steam and in Air under Low-Pressure Conditions

Drying of porous solids such as sintered glass beads, baked clay, and cemented glass balloons in both steam and air streams was investigated under low-pressure conditions. There was no significant difference between the normalized observed drying rate curves in air at low pressures of 0.71-1.19 kPa and those in steam at low pressures of 0.97-0.99 kPa. However, lower critical moisture contents and higher drying rates in superheated steam at subatmospheric pressures of 8.27-8.33 kPa were observed compared to those in steam at pressures in the range of 0.97-0.99 kPa. Moreover, two models were validated for the prediction of drying rate curves of sintered glass beads at subatmospheric and low pressures. The patterns of the drying rate curves, which depend on the drying medium and its pressure, were common to these materials.     

Drying Rate Curves of Porous Solids in Steam and in Air under Low-Pressure Conditions

Drying of porous solids such as sintered glass beads, baked clay, and cemented glass balloons in both steam and air streams was investigated under low-pressure conditions. There was no significant difference between the normalized observed drying rate curves in air at low pressures of 0.71–1.19 kPa and those in steam at low pressures of 0.97–0.99 kPa. However, lower critical moisture contents and higher drying rates in superheated steam at subatmospheric pressures of 8.27–8.33 kPa were observed compared to those in steam at pressures in the range of 0.97–0.99 kPa. Moreover, two models were validated for the prediction of drying rate curves of sintered glass beads at subatmospheric and low pressures. The patterns of the drying rate curves, which depend on the drying medium and its pressure, were common to these materials.