A whole single cherry tomato was dried in a forced convective micro-dryer. The experiments were c... more A whole single cherry tomato was dried in a forced convective micro-dryer. The experiments were carried out at constant air velocity and humidity and temperatures of 50, 60, 70 °C. In order to study the effect of the skin, two sets of experiments were performed using a tomato with and without skin (easily removed). Shorter drying times were obtained when increasing drying temperatures as well as when removing sample skin. X-ray microtomography, a non-destructive 3D imaging technique was used to follow shrinkage of the samples. This phenomenon was introduced in the modelling part of this study. Analytical solutions of the Fick'law were used to determine the diffusion coefficient at the three temperatures studied, and then the activation energy was obtained through fitting the Arrhenius equation. The skin effect was clearly evidenced by showing that the mass transfer parameter values of an original tomato with skin were largely smaller than the one without skin. Indeed, the moisture effective diffusivity ranged from 2.56×10-11 to 7.67×10-11 m 2 ·s-1 with activation energy of 50430 J·mol-1 for tomato with skin and ranged from 4.59×10-10 m 2 ·s-1 to 6.73×10-10 m 2 ·s-1 with activation energy of 17640 J.mol-1 for tomato without skin.
ABSTRACT The objective of this work is to give the fundamental information that should be known a... more ABSTRACT The objective of this work is to give the fundamental information that should be known about wastewater sludge drying. Three methods are mainly applied: convective drying, conductive drying and solar drying, each one presenting different characteristics. When applying convective drying three phases are distinguished: adaptation phase, constant drying rate phase and falling drying rate phase. Experimental works show that several parameters influence the drying kinetic during this process, such as the origin of the sludge and operating conditions. Imaging techniques allow observing three phenomena that happen during convective drying: shrinkage, cracks and skin formation. When applying conductive drying and considering the torque variations, the product passes through: pasty phase, lumpy phase and granular phase. The results show no regular shape of the drying kinetic with high values of the drying rate and the heat transfer coefficient during the first phase. A special focus is given into the sticky phase which reduces performances of the dryer. The third applied drying method is solar drying, which depends wholly on climatic conditions, such solar radiations and air temperature. Besides, for this method no regular shape of the drying kinetic can be observed, with high drying rate values during favorable conditions and low drying rate values during unfavorable conditions. The presented studies dealing with solar drying of wastewater sludge are limited to the variations of the different air temperatures registered inside and outside the drying chamber with the product temperature and their humidity with study of the pathogen reduction. Finally, some innovative developed methods are exposed in this review, such as the use of frying and super-heated steam.
This work is part of a project aiming to develop a renewable fuel for gasification purposes, thro... more This work is part of a project aiming to develop a renewable fuel for gasification purposes, through convective drying of sludge/wood mixtures. The first step consists of characterizing the behavior of sawdust/sludge mixtures during the application of convective drying. The influence of the mixing step (no mixing versus 30 s at 40 rpm), as well as the sawdust : sludge ratio (1:9, 2:8, 3:7, and 4:6 on a dry basis) and the effect of drying temperature (50, 80, and 110°C) were investigated. In this study, X-ray tomography, a noninvasive imaging technique, is used to assess changes in the volume, void, and exchange surface at the beginning and the end of the drying process. Results first confirm the importance of the mixing step on the drying behavior: the drying rate of the mixed sludge is lower than that of the original sludge. Nevertheless, the addition of sawdust is shown to have a positive impact on the drying process from a mass ratio of 2:8, with drying rates higher than that of ...
In this study, Constant drying rate phase (CDRP) of spearmint and bay leaves are determined durin... more In this study, Constant drying rate phase (CDRP) of spearmint and bay leaves are determined during application of forced convective drying. The applied conditions are: 30, 40 and 50°C for the air temperature, the relative humidity and the air velocity vary respectively from 20 to 60% and from 0.5 to 1.5 m/s. Two methods are used to define the CDRP based on the two parameters of external drying conditions; this is the difference between the wet and dry bulb temperature of air and the drying air velocity; data empirical and a neural network approach (ANN), the expressions of CDRP are determined and the coefficients characterizing the product are identified with experimental data, an artificial neural network model, trained by levenberg-Marquardt algorithm, is developed to predict the values of the CDRP. The results of the two approached are statistically compared by calculation of the correlation coefficient R2 and the fit standard error (FitStdErr). The artificial neural network mode...
ABSTRACT Full text available: http://www.tandfonline.com/eprint/yEWfQMcQIVbCyhqbyIEQ/full Convect... more ABSTRACT Full text available: http://www.tandfonline.com/eprint/yEWfQMcQIVbCyhqbyIEQ/full Convective drying of wastewater sludges and sawdust/sludge mixtures was studied. The first part of this work was an experimental study performed in a cross-flow convective dryer using 500 g of wet material extruded through a disk with circular dies of 12 mm. The results showed that the sawdust addition has a positive impact on the drying process from a mass ratio of 2/8, on a dry basis, with observed drying rates higher than the original sludge. The second part of this work consisted of developing a drying model in order to identify the internal diffusion coefficient and convective mass transfer coefficient from the experimental data. A comparison was made between fitted drying curves, well represented by the Newton's model, and the analytical solutions of the diffusion equation applied to a finite cylinder. Variations of dimensional characteristics, such as the volume and exchange surface of the sample bed, were obtained by X-ray tomography. This technique allowed us to confirm that shrinkage, which is an important phenomenon occurring during sludge and sawdust/sludge mixture drying, must be taken into account. The results showed that both the internal diffusion coefficient and convective mass transfer coefficient were affected by mixing and sawdust addition. The internal diffusion coefficient changed from 7.77 × 10−9 m2/s for the original sludge to 7.01 × 10−9 m2/s for the mixed sludge and then increased to 8.35 × 10−9 m2/s for the mixture of a mass ratio of 4/6. The convective mass transfer coefficient changed from 9.70 × 10−8 m/s for the original sludge to 8.67 × 10−8 m/s for the mixed sludge and then increased to 12.09 × 10−8 m/s for the mixture of a mass ratio of 4/6. These results confirmed that sawdust addition was beneficial to the sludge drying process as the mass transfer efficiency between the air and material increased. Reinforcing the texture of sludge by adding sawdust can increase the drying rate and decrease the drying time, and then the heat energy supply will be reduced significantly. The study also showed that neglecting shrinkage phenomenon resulted in an overestimation for the internal diffusion coefficient for the convective drying of sludges and sawdust/sludge mixtures.
ABSTRACT This work is part of a project aiming to develop a renewable fuel for gasification purpo... more ABSTRACT This work is part of a project aiming to develop a renewable fuel for gasification purposes, through convective drying of sludge/wood mixtures. The first step consists of characterizing the behavior of sawdust/sludge mixtures during the application of convective drying. The influence of the mixing step (no mixing versus 30 s at 40 rpm), as well as the sawdust : sludge ratio (1:9, 2:8, 3:7, and 4:6 on a dry basis) and the effect of drying temperature (50, 80, and 110°C) were investigated. In this study, X-ray tomography, a noninvasive imaging technique, is used to assess changes in the volume, void, and exchange surface at the beginning and the end of the drying process. Results first confirm the importance of the mixing step on the drying behavior: the drying rate of the mixed sludge is lower than that of the original sludge. Nevertheless, the addition of sawdust is shown to have a positive impact on the drying process from a mass ratio of 2:8, with drying rates higher than that of the original sludge. With increasing amounts of sawdust, the initial and final bed volumes, initial and final total exchange surfaces, and initial void fraction increase linearly, but the bed volume shrinkage and final void fraction decrease linearly.
A whole single cherry tomato was dried in a forced convective micro-dryer. The experiments were c... more A whole single cherry tomato was dried in a forced convective micro-dryer. The experiments were carried out at constant air velocity and humidity and temperatures of 50, 60, 70 °C. In order to study the effect of the skin, two sets of experiments were performed using a tomato with and without skin (easily removed). Shorter drying times were obtained when increasing drying temperatures as well as when removing sample skin. X-ray microtomography, a non-destructive 3D imaging technique was used to follow shrinkage of the samples. This phenomenon was introduced in the modelling part of this study. Analytical solutions of the Fick'law were used to determine the diffusion coefficient at the three temperatures studied, and then the activation energy was obtained through fitting the Arrhenius equation. The skin effect was clearly evidenced by showing that the mass transfer parameter values of an original tomato with skin were largely smaller than the one without skin. Indeed, the moisture effective diffusivity ranged from 2.56×10-11 to 7.67×10-11 m 2 ·s-1 with activation energy of 50430 J·mol-1 for tomato with skin and ranged from 4.59×10-10 m 2 ·s-1 to 6.73×10-10 m 2 ·s-1 with activation energy of 17640 J.mol-1 for tomato without skin.
ABSTRACT The objective of this work is to give the fundamental information that should be known a... more ABSTRACT The objective of this work is to give the fundamental information that should be known about wastewater sludge drying. Three methods are mainly applied: convective drying, conductive drying and solar drying, each one presenting different characteristics. When applying convective drying three phases are distinguished: adaptation phase, constant drying rate phase and falling drying rate phase. Experimental works show that several parameters influence the drying kinetic during this process, such as the origin of the sludge and operating conditions. Imaging techniques allow observing three phenomena that happen during convective drying: shrinkage, cracks and skin formation. When applying conductive drying and considering the torque variations, the product passes through: pasty phase, lumpy phase and granular phase. The results show no regular shape of the drying kinetic with high values of the drying rate and the heat transfer coefficient during the first phase. A special focus is given into the sticky phase which reduces performances of the dryer. The third applied drying method is solar drying, which depends wholly on climatic conditions, such solar radiations and air temperature. Besides, for this method no regular shape of the drying kinetic can be observed, with high drying rate values during favorable conditions and low drying rate values during unfavorable conditions. The presented studies dealing with solar drying of wastewater sludge are limited to the variations of the different air temperatures registered inside and outside the drying chamber with the product temperature and their humidity with study of the pathogen reduction. Finally, some innovative developed methods are exposed in this review, such as the use of frying and super-heated steam.
This work is part of a project aiming to develop a renewable fuel for gasification purposes, thro... more This work is part of a project aiming to develop a renewable fuel for gasification purposes, through convective drying of sludge/wood mixtures. The first step consists of characterizing the behavior of sawdust/sludge mixtures during the application of convective drying. The influence of the mixing step (no mixing versus 30 s at 40 rpm), as well as the sawdust : sludge ratio (1:9, 2:8, 3:7, and 4:6 on a dry basis) and the effect of drying temperature (50, 80, and 110°C) were investigated. In this study, X-ray tomography, a noninvasive imaging technique, is used to assess changes in the volume, void, and exchange surface at the beginning and the end of the drying process. Results first confirm the importance of the mixing step on the drying behavior: the drying rate of the mixed sludge is lower than that of the original sludge. Nevertheless, the addition of sawdust is shown to have a positive impact on the drying process from a mass ratio of 2:8, with drying rates higher than that of ...
In this study, Constant drying rate phase (CDRP) of spearmint and bay leaves are determined durin... more In this study, Constant drying rate phase (CDRP) of spearmint and bay leaves are determined during application of forced convective drying. The applied conditions are: 30, 40 and 50°C for the air temperature, the relative humidity and the air velocity vary respectively from 20 to 60% and from 0.5 to 1.5 m/s. Two methods are used to define the CDRP based on the two parameters of external drying conditions; this is the difference between the wet and dry bulb temperature of air and the drying air velocity; data empirical and a neural network approach (ANN), the expressions of CDRP are determined and the coefficients characterizing the product are identified with experimental data, an artificial neural network model, trained by levenberg-Marquardt algorithm, is developed to predict the values of the CDRP. The results of the two approached are statistically compared by calculation of the correlation coefficient R2 and the fit standard error (FitStdErr). The artificial neural network mode...
ABSTRACT Full text available: http://www.tandfonline.com/eprint/yEWfQMcQIVbCyhqbyIEQ/full Convect... more ABSTRACT Full text available: http://www.tandfonline.com/eprint/yEWfQMcQIVbCyhqbyIEQ/full Convective drying of wastewater sludges and sawdust/sludge mixtures was studied. The first part of this work was an experimental study performed in a cross-flow convective dryer using 500 g of wet material extruded through a disk with circular dies of 12 mm. The results showed that the sawdust addition has a positive impact on the drying process from a mass ratio of 2/8, on a dry basis, with observed drying rates higher than the original sludge. The second part of this work consisted of developing a drying model in order to identify the internal diffusion coefficient and convective mass transfer coefficient from the experimental data. A comparison was made between fitted drying curves, well represented by the Newton's model, and the analytical solutions of the diffusion equation applied to a finite cylinder. Variations of dimensional characteristics, such as the volume and exchange surface of the sample bed, were obtained by X-ray tomography. This technique allowed us to confirm that shrinkage, which is an important phenomenon occurring during sludge and sawdust/sludge mixture drying, must be taken into account. The results showed that both the internal diffusion coefficient and convective mass transfer coefficient were affected by mixing and sawdust addition. The internal diffusion coefficient changed from 7.77 × 10−9 m2/s for the original sludge to 7.01 × 10−9 m2/s for the mixed sludge and then increased to 8.35 × 10−9 m2/s for the mixture of a mass ratio of 4/6. The convective mass transfer coefficient changed from 9.70 × 10−8 m/s for the original sludge to 8.67 × 10−8 m/s for the mixed sludge and then increased to 12.09 × 10−8 m/s for the mixture of a mass ratio of 4/6. These results confirmed that sawdust addition was beneficial to the sludge drying process as the mass transfer efficiency between the air and material increased. Reinforcing the texture of sludge by adding sawdust can increase the drying rate and decrease the drying time, and then the heat energy supply will be reduced significantly. The study also showed that neglecting shrinkage phenomenon resulted in an overestimation for the internal diffusion coefficient for the convective drying of sludges and sawdust/sludge mixtures.
ABSTRACT This work is part of a project aiming to develop a renewable fuel for gasification purpo... more ABSTRACT This work is part of a project aiming to develop a renewable fuel for gasification purposes, through convective drying of sludge/wood mixtures. The first step consists of characterizing the behavior of sawdust/sludge mixtures during the application of convective drying. The influence of the mixing step (no mixing versus 30 s at 40 rpm), as well as the sawdust : sludge ratio (1:9, 2:8, 3:7, and 4:6 on a dry basis) and the effect of drying temperature (50, 80, and 110°C) were investigated. In this study, X-ray tomography, a noninvasive imaging technique, is used to assess changes in the volume, void, and exchange surface at the beginning and the end of the drying process. Results first confirm the importance of the mixing step on the drying behavior: the drying rate of the mixed sludge is lower than that of the original sludge. Nevertheless, the addition of sawdust is shown to have a positive impact on the drying process from a mass ratio of 2:8, with drying rates higher than that of the original sludge. With increasing amounts of sawdust, the initial and final bed volumes, initial and final total exchange surfaces, and initial void fraction increase linearly, but the bed volume shrinkage and final void fraction decrease linearly.
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