Patricia Arlabosse

Heat, momentum, and mass transfer measurements in indirect agitated sludge dryer

ABSTRACT Whereas indirect agitated drying has been extensively studied for granular materials, little is known in the case of pasty products. We describe an experimental set up specially designed for the investigation of drying kinetics, of heat transfer coefficient evolution, and of the mechanical torque necessary for stirring. This device was applied to municipal sewage sludge. Preliminary experiments were performed to investigate the influence of ageing of sludge on the drying kinetics. It appears that ageing does have no effect except for the first two days. The influences of the wall temperature, the stirrer speed, the dryer load and the location of the stirrer against the heated wall were studied. Three different rheological behaviors were observed during sludge drying. In particular, the sludge goes through a “glue” phase, and high levels of mechanical strain are recorded when the compact mass begins to break up. A critical stirrer speed is found within the range 40–60 rpm. To shorten the drying time, it seems better to adjust the wall temperature in accordance with the moisture content of the sludge.

Comparison Between Static and Dynamic Methods for Sorption Isotherm Measurements

Abstract Sorption isotherms of Microcrystalline Cellulose (MCC), pharmaceutical granules, PolyEtherBlocAmide (PEBA) membrane and sewage sludges were measured at various temperatures using three different experimental gravimetric methods: the saturated salt method, the Dynamic Vapor System (DVS) apparatus and a simultaneous gravimetric and calorimetric measurement techniques using a Setaram TGA-DSC111. A comparison between the salts method and DVS is proposed at ambient temperature for the MCC, the pharmaceutical granules and the PEBA membrane. The comparison between the static method and the DVS shows that the results are consistent as long as the apparent moisture content in the material is high. When the apparent diffusion coefficient becomes low, the difficulty to reach the thermodynamic equilibrium appears in the saturated salts method. Then, DVS and TGA-DSC apparatus were used to investigate the desorption isotherms for the MCC between the ambient temperature and 90°C. Finally, an application on biological products is proposed: the desorption isotherms for two sewage sludges coming from the same wastewater treatment plant were determined with the DVS and the TGA-DSC equipments at 45°C. The accuracy of the TGA-DSC is good as long as the apparent diffusion coefficient is above 10−9 m2/s. When this threshold value is overstepped, the desorption isotherms are overestimated.

Chemometrics as a tool for the analysis of evolved gas during the thermal treatment of sewage sludge using coupled TG-FTIR

The thermal decomposition of sewage sludge has been investigated using coupled TG–FTIR for long time experiment (10 h). The exploitation of the resulted data from FTIR is performed by the SIMPLe-to-use interactive self-modelling mixture analysis (SIMPLISMA) method and allows to identify some of the evolved gases and to obtain their relative concentration profiles versus time without prior knowledge of constituents. As shown, this method can work properly for mixture with overlapped bands but some compounds remain “invisible” to FTIR analysis. More of that for long time experiment, it is possible to extract a spectrometer baseline contribution, which contributes to minimise noise and time variation.

Heat and mass transfer during fry-drying of sewage sludge

Deep-frying, which consists of immersing a wet material in a large volume of hot oil, presents a process easily adaptable to dry rather than cook materials. A suitable material for drying is sewage sludge, which may be dried using recycled cooking oils (RCO) as frying oil. One advantage is that this prepares both materials for convenient disposal by incineration. This study examines fry-drying of municipal sewage sludge using recycled cooking oil. The transport processes occurring during fry-drying were monitored through sample weight, temperature, and image analysis. Due to the thicker and wetter samples than the common fried foods, high residual moisture is observed in the sludge when the boiling front has reached the geometric center of the sample, suggesting that the operation is heat transfer controlled only during the first half of the process followed by the addition of other mechanisms that allow complete drying of the sample. A series of mechanisms comprising four stages (i.e., initial heating accompanied by a surface boiling onset, film vapor regime, transitional nucleate boiling, and bound water removal) is proposed. In order to study the effect of the operating conditions on the fry-drying kinetics, different oil temperatures (from 120 to 180°C), diameter (D = 15 to 25 mm), and initial moisture content of the sample (4.8 and 5.6 kg water·kg−1 total dry solids) were investigated.

Drying of municipal sewage sludge: from a laboratory scale batch indirect dryer to the paddle dryer

Among available technologies, disk and paddle dryers are often encountered in France to process municipal sewage sludge but their thermal design is still a question of know how rather than scientific knowledge. From experiments performed on an industrial Naratherm® paddle dryer and the literature, a simple model has been developed. The dryer is divided into two parts depending on the sludge consistency. A constant evaporating flow rate is assumed in the pasty zone whereas its linear decrease with the moisture content is taken into account in the granular zone. To compute this model, some of the sludge properties have to be characterized with a set of laboratory scale experimental techniques. The agreement with the experimental data is good.

Method for Thermal Design of Paddle Dryers: Application to Municipal Sewage Sludge

Abstract An experimental methodology was developed to improve the energy design of paddle dryers for sewage sludges. A laboratory batch dryer with a vertical agitator has been especially designed and instrumented to determine the heat flux densities. To determine the drying kinetic and the evaporation rates, the specific heat and the total heat of desorption of the sludge were measured using calorimetric devices. The evaporating flow rates recorded during the pasty and granular phases were then used in a simple model of the paddle dryer. The good agreement between the computed drying kinetics and the experimental ones measured in an industrial paddle dryer shows that the batch dryer can be used to classify the sludges according to their capacity to be dried in a paddle dryer.

Thermally assisted mechanical dewatering (TAMD) of suspensions of fine particles : Analysis of the influence of the operating conditions using the response surface methodology

Thermally assisted mechanical dewatering (TAMD) is a new process for energy-efficient liquid/solids separation which enhances conventional-device efficiency. The main idea of this process is to supply a flow of heat in mechanical dewatering processes to favour the reduction of the liquid content. This is not a new idea but the proposed combination, especially the chosen operating conditions (temperature <100 degrees C and pressure <3000 kPa) constitutes an original approach and a significant energy saving since the liquid is kept in liquid state. Response surface methodology was used to evaluate the effects of the processing parameters of TAMD on the final dry solids content, which is a fundamental dewatering parameter and an excellent indicator of the extent of TAMD. In this study, a two-factor central composite rotatable design was used to establish the optimum conditions for the TAMD of suspensions of fine particles. Significant regression models, describing changes on final dry solids content with respect to independent variables, were established with regression coefficients (usually called determination coefficients), R(2), greater than 80%. Experiments were carried out on a laboratory filtration/compression cell, firstly on different compressible materials: synthetic mineral suspensions such as talc and synthetic organic suspensions such as cellulose, and then on industrial materials, such as bentonite sludge provided by Soletanche Bachy Company. Experiment showed that the extent of TAMD for a given material is particularly dependent on their physical and chemical properties but also on processing parameters.

Method for the design of a contact dryer-application to sludge treatment in thin film boiling

Abstract A method, based on the analysis of heat transfer, is developed for the design of contact dryers in thin film boiling for sludge treatment. An experimental device was built in order to estimate the heat flux density exchanged between a hot metallic plate and the drying sample. This specific device is made of a thick copper plate heated at a temperature above 100°C, on which the sludge is coated. The contact temperature and the heat flux density are estimated using a temperature sensor inside the copper block, and an inverse conduction method (Becks sequential function specification method). The corresponding drying curve is deduced from an energy balance. Sludge drying was studied on an electrically heated laboratory continuous drum dryer. A simple steady state model of the drum dryer was developed by taking into account the combined effects of internal and external transfers. This model is validated by comparison with drying curves obtained from sampling and simulation. In this case, the relativ...

Overview of pool boiling heat transfer studies in variable gravity

Boiling studies under microgravity conditions require a perfect control of all the involved parameters: gravity level, g-jitter, the time of experimentation, the heater size and geometry, the size of the boiling vessel, the fluid properties… In this paper, an attempt is made to expose the main works achieved over the past fifty years on boiling in variable gravity level. More particularly, the experimental conditions during these different experiments as well as a comparison of the different results and conclusions proposed by the authors are presented. According to this analysis, all the experimental conditions differ notably and no clear conclusion can be drawn about boiling phenomena under microgravity conditions. Nevertheless, it seems that the gravity level has little influence on the boiling heat transfer. Many questions are still open and further experiments are expected to yield improvements in the understanding of the basic boiling physics.

The experimental study of the periodic instability of thermocapillary convection around an air bubble

This paper concerns an experimental study on thermocapillary convection at the interface between an air bubble and a silicone oil with a low Prandtl number. The silicone oil layer is heated from above and cooled from below in order to obtain a vertical temperature gradient. When the temperature gradient is increased and a critical value is exceeded, oscillations appear in the liquid. This oscillatory thermocapillary convection is analyzed versus the temperature gradient and the bubble aspect ratio. A non intrusive optical measurement technique based on shadow technique is developed. Three oscillations modes are observed in a vertical section of experiment cell. The systematic study shows the influence of the temperature gradient and of the bubble size on their associated characteristic frequencies. The values obtained are close to 1 Hz.