Laurent Fraikin

Analysis of the Shrinkage Effect on Mass Transfer During Convective Drying of Sawdust/Sludge Mixtures

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 Newtons 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.

Investigation on Convective Drying of Mixtures of Sewage Sludge and Sawdust in a Fixed Bed

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.

Modeling and Simulation of Heat and Mass Transfer During Convective Drying of Wastewater Sludge with Introduction of Shrinkage Phenomena

Wastewater sludge is dried in a convective dryer using air temperatures varying from 80°C to 200°C, velocities changing from 1 m · s−1 to 2 m · s−1, and humidities ranging from . The convective dryer is equipped with a camera and an infrared pyrometer to follow respectively the external surface and the temperature of the product. The experimental results show that drying kinetic can be divided into three phases: two short first phases, called adaptation and constant drying phases, and a long third phase, called falling drying rate phase. As the moisture content decreases, the camera confirms simultaneous shrinkage effect with the volume reduction of the product of about 30–45% of the initial volume. Moreover, an increase of the product temperature towards air temperature was measured with the infrared pyrometer. In a second step of this study, the experimental results are modeled and simulated using heat and mass balances applied to the product and the heated air. The drying curve is rightly expressed with fourth-degree polynomial model with a correlation coefficient that approximates the unity and with low calculated errors. An outstanding determination of the heat transfer coefficient has permitted calculating the product temperature with good agreement with experimental results. The heat transfer coefficient expressed by means of Nusselt number is presented as a function of Reynolds and Prandlt numbers, changeable with air and product characteristics taking into account shrinkage effect. Moreover, as the applied air temperatures are sufficiently high, transfer by radiation is not neglected and is introduced in the mathematical model.

Quality enhancement of dehydrated products through the modification of solar tunnel dryer for continuous operation in rural communities

The aim o f th is study is the experimental characterization of the behaviour o f a mo rtar during convective drying. We focalise on mortar that has a rate water-cement of 0.5. The drying tests are developed in a micro-convective dryer that can use samples weighing fro m 0 to 8g. The advantage of these experiments is to reproduce the natural conditions that can be found during the treatment of the mortar-at mosphere problems. The response of the drying curve or the drying kinetic depends on the applied drying conditions. So, the temperature of the air varies fro m 60℃ to 130℃, the velocity of the air is changed fro m 2 m.s -1 to 5 m. s -1 and the relative hu midity is kept less than 1.7%. The co mparison between the experimental results and the proposed analytical solutions of the equation of diffusion represented by Ficks law, applied for a finite shape, allo ws determination o f the values of the d iffusion coefficient. It has a value of 1.71×10 -10 m 2 .s -1 at 60℃, 13.69×10 -10 m 2 .s -1 at 90℃ and 16.27×10 -10 m 2 .s -1 at 130℃. Calculation of the activation energy and the D0 constant are also possible.

Convective drying behavior of sawdust-sludge mixtures in a fixed bed

ABSTRACT This work aims at comparing the temporal evolution of 3D characteristics of the sample bed as well as the drying mechanism of sewage sludge and sawdust-sludge mixtures during the drying process. The first step are characterizing the drying behavior of sawdust-sludge mixtures during the convective drying. The influences of the mixing step (no mixing against 30 s at 40 r/min) and of the mass fractions of sawdust (10%, 20%, 30%, and 40% on a dry basis) have been investigated. X-ray tomography, a non-invasive imaging technique, was used to assess changes in the volume and total exchange surface during the drying process. Drying experiment results show that sawdust addition has a positive impact on the drying process from a mass fraction of sawdust of 20%. The X-ray tomography experiment results show that the volume and total exchange surface of the sample bed both increase with increasing amounts of sawdust during the entire drying process. Moreover, a linear decrease of both the total exchange surface and volume with the decreased normalized moisture content is observed for sludges, while a plateau is reached after a linear phase for sawdust-sludge mixtures. As expected for sludges, the drying rate decreases nearly linearly with the decreased total exchange surface during the entire drying process. However, for the sawdust-sludge mixtures, at first the drying rate decreases nearly linearly with the decreased total exchange surface, and then the total exchange surface does not change but the drying rate still decreases until it reaches zero. These promising results open the way to new management ways, especially for very pasty sludge.

Experimental study on dehumidification/regeneration of liquid desiccant: LiBr solution

Abstract The growing demand for air conditioning has caused a significant increase in demand for energy resources. The traditional commercial, non natural working fluids, like CFC, HCFC and HFC result in both ozone depletion and global warming emission of CO2. The use of hygroscopic salts in direct contact with moist air provides an attractive alternative to conventional cooling systems. The liquid desiccant can substitute the dangerous fluids. The main operations in Liquid Desiccant Cooling System (LDCS) are dehumidification and regeneration. This paper presents an experimental study of dehumidification/regeneration processes using LiBr as liquid desiccant in direct contact with the air at different operating conditions. An analysis of the mass transfer is made in order to prove is this material is the best liquid desiccant for LDCS. Experimental results showed the effect of air conditions on mass transfer. It can be seen that LiBr is able to absorb moisture and can be regenerated at low temperature.

On Water Transfer and Hydraulic Connection Layer During the Convective Drying of Rigid Porous Material

The convective drying of a natural porous material, limestone, is investigated in this study, with both experimental and numerical approaches. The first experimental campaign, which focuses on the influence of samples’ slenderness, suggests the presence of a hydraulic connection layer between the porous water and the external environment, in spite of the very fine pore structure of the material. This hydraulic transfer enables the fast water evaporation at the beginning of the drying test, when external conditions drive the kinetics. Furthermore, the results show that this layer does not exceed 30 mm deep from the external surface, given the drying conditions of the test. A second experimental campaign aims to analyse, by mean of an X-ray tomography tool, the internal water content during the drying. It confirms that water transfer takes place within the limestone in two distinct stages. The first stage being faster than the second one with a homogeneous desaturation along the sample, it is consistent with the hypothesis of the hydraulic connection layer. Finally, the finite element modelling makes possible to identify the main mechanisms of water transfer, namely liquid convection and vapour diffusion.

USING OF AN EXPERIMENTAL DESIGN TO CHARACTERIZE THE CONVECTIVE DRYING BEHAVIOR OF DIFFERENT SLUDGES

Drying appears as a major step prior to valorization of sludge from a wastewater treatment plant. This study uses an experimental design on different sludges to highlight the drying behavior according to drying conditions, storage, and extrusion. This research is performed with an industrial view, but on a single cylinder sample, and focuses on five factors: maximum drying flux, time to reach 95% of dry matter, final volume, critical moisture, and slowing-down coefficient.

Influence of sawdust addition on drying of wastewater sludges: Comparison of structural characteristics

ABSTRACT This work aims to investigate the structural characteristics of wastewater sludges and sludge–wood mixtures, and how the addition of sawdust affects them in a positive way for the drying process. The study was performed for a fixed bed at the pilot scale, with sludges from three different wastewater treatment plants from the Liège province in Belgium, namely, Oupeye, Grosses Battes, and Embourg. X-ray tomography was chosen to evaluate variations in the structural characteristics before and after drying, which included volume, void fraction, and exchange surface of the bed. Results first confirm that the drying rate is raised after the sawdust addition operation for all three sludges, but the increase amount is smaller while the moisture of the sludge is high. With sawdust, the average drying rates increase by factors of 1.475 (Oupeye), 1.342 (Grosses Battes), and 1.162 (Embourg), respectively. It also increases initial volume as well as void fraction and surface exchange, however after drying this addition leads to less shrinkage and void fraction, and higher exchange surface. The initial void fraction increases by 7.25 (Oupeye), 8.30 (Grosses Battes), and 14.01% (Embourg), but the final void fraction decreases 1.56 (Oupeye), 10.83 (Grosses Battes), and 25.61% (Embourg). With high initial moisture content, we observed lower porosity and higher shrinkage.

Enhanced sludge dewatering and drying comparison of two linear polyelectrolytes co-conditioning with polyaluminum chloride

AbstractThis paper investigated the influence of polyaluminum chloride (PAX-14) co-conditioning with two linear polyelectrolytes on the dewatering and drying performances of urban residual sludge. Experiments were carried out on activated sludge samples collected after thickening from the Grosses Battes wastewater treatment plant. Dual conditioning implying PAX-14 coagulant and two cationic polymers, only differing by their molecular weight, were tested. The first part of the study was an experimental study of sludge dewatering improvement in terms of dry solids content of the dewatered cake and the evaluation of specific resistance to filtration (SRF) determined using a filtration–compression cell. The results showed that dual conditioning including PAX-14 addition has a positive impact on the dewatering process, based on the increase of cake dryness limit after PAX-14 addition. It was also found that the use of filtration theory based on Carman’s equation to determine the SRF parameter presents some lim...