Measurement and simulation of the contact drying of sewage sludge in a Nara-type paddle dryer

Contact drying experiments were carried out in a Nara-type paddle dryer to study the drying kinetics of sewage sludge in the presence of air. In order to have a better understanding of the sewage sludge drying mechanism, a penetration model developed by Tsotsas and Schlünder is used to simulate the drying kinetics of the pasty, lumpy and granular phase which the sludge experiences during the drying process. The pasty phase is assumed to be a saturated particulate phase, and the granular phase is considered as a mono-dispersed particulate phase whose diameter was experimentally determined. In the lumpy phase, a sludge wall sticking still to the paddle-shaft surface was formed, and the heat transfer resistance from the sludge wall is considered during the model calculation. The influence of the drying parameters (temperature, stirrer speed and air flowrate) on the drying process is investigated experimentally and theoretically. The result indicates that the penetration theory is able to describe the sludge drying kinetics of the three distinct phases. Experimental and calculated drying kinetics are in satisfactory agreement for different drying parameters.     

Experimental and Theoretical Study of Agitated Contact Drying of Sewage Sludge under Partial Vacuum Conditions

Agitated contact drying experiments were carried out in a cylindrical lab-scale paddle dryer to study the drying kinetics of sewage sludge under partial vacuum conditions. To explore the drying mechanisms, the penetration model for vacuum contact drying of agitated particulate materials, developed by Schlünder and Mollekopf, was applied to simulate the experimental results. The influence of the drying parameters (system pressure, heating temperature, stirrer speed, and dryer load) on the drying kinetics was studied both experimentally and theoretically. The results indicate that pressure and temperature significantly influence the drying rates of sewage sludge, whereas stirring speed and dryer load have no obvious effects on drying rates in the examined range. The experimental and simulated drying rates were in acceptable agreement as long as the temperature difference between dryer and sludge was small. A crust is likely to form on the dryer surface, however, when the temperature difference becomes large; this crust leads to markedly lower drying rates than the calculated ones. Furthermore, a comparison between partial vacuum contact drying and atmospheric contact drying is presented. The results indicate that the mass transfer resistance increases with rising drying potential in the examined experimental range.     

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

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.     

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.     

MVR空心桨叶干燥污泥的特性及动力学

基于空心桨叶干燥机建立了一套机械蒸汽再压缩式热泵干燥系统,将相同质量的污泥(100±0.1)kg,在真空压力约为95kPa、压缩机出口蒸汽温度为95~115℃进行恒温干燥实验,从而得到污泥干燥过程中含水率以及干燥速率等变化曲线,压缩机出口蒸汽温度为95~115℃,污泥临界湿含量从0.22增加到0.34。引入5种常用的污泥干燥模型,利用Origin软件对实验数据进行分段拟合分析,得到污泥加速阶段和降速阶段的干燥动力学模型MR=exp(-ktⁿ)。以分段函数形式表示的干燥方程分别为:MR=exp[-(2.78×10^-4T-0.01896)t^1.596],(加速阶段);MR=0.894-0.564(1.737×10^-4T-8.05×10^-3)t(恒速阶段);MR=exp[-(2.26×10^-4T-0.01365)t^1.984](降速阶段)。对实验得到的干燥模型进行验证,计算得到各干燥阶段污泥湿含量平均相对误差,并将实验所得干燥曲线和干燥速率曲线与模型计算值进行比较,可以看出分段处理能较好的描述污泥干燥规律。     

HEAT, MOMENTUM, AND MASS TRANSFER MEASUREMENTS IN INDIRECT AGITATED SLUDGE DRYER

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.     

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.     

污泥间接干燥过程的实验研究

污泥间接干燥是以热传导为传热方式的干燥过程。采用楔形桨叶式干燥机进行污泥干燥实验,研究了污泥的间接干燥过程,测得了污泥在干燥机不同位置的温度及其相应的含水率,验证了污泥的间接干燥过程遵循预热阶段、恒速干燥阶段和降速干燥阶段的一般规律,获得了不同转速下污泥干燥过程的平均传热系数,且平均传热系数随着搅拌转速的提高而增大。     

用于污泥干化的新型桨叶式干燥机的设计

针对污泥的特性及其干燥机理,对现有桨叶式干燥机的结构进行改进设计,使现有的桨叶式干燥机成为一种比较理想的污泥干化设备。     

Identification of the Limiting Mechanism in Contact Drying of Agitated Sewage Sludge

In spite of a great number of industrial applications, the thermal design of contact dryers for sewage sludge remains empirical. To improve the understanding of drying mechanisms, the penetration theory developed by Schlünder and coworkers for mono- and multidispersed packing is used to represent the experimental results from a laboratory-scale dryer. For granular packing, the only adjustment parameter of the model is the mixing number, which characterizes the dryer and its stirrer. For pasty-like materials, the pasty phase is assumed to be a saturated particulate phase. As the calculation of the effective properties calculation is cumbersome for a multi-granular packing, the particulate phase is considered as a monodispersed packing, whose dimension is unknown. To identify the two adjustment parameters, the mixing number was quantified from experiments performed on activated alumina balls, for which physical and thermal characteristics are known, and then the characteristic dimension of the sludge was determined by adjustment of experimental drying kinetics measured in a batch agitated dryer. According to this model, drying is exclusively controlled by the contact resistance between the wall and the biggest particles contained in the dewatered sludge. The model permits to find most of the tendencies experimentally observed for different operating conditions.     

DRYING OF PULP AND PAPER SLUDGE IN A PULSED FLUID BED DRYER

Hydrodynamics and drying kinetics for the pulp and paper primary sludge dried in a pulsed fluid bed dryer with relocated air stream are presented. Batch experiments have indicated that drying of disintegrated sludge to the required 12% moisture content takes place during the first drying period at practically constant material temperature close to the wet bulb temperature with respect to the inlet air conditions. Equations were developed for pressure drop, minimum pulsed-fluidization velocity, dynamic bed height, and volumetric mass transfer coefficient. Continuous experiments under drying conditions determined from the average residence time concept have confirmed that transportation of disintegrated sludge along the dryer follows the plug flow model.     

Modeling of Sewage Sludge Flow in a Continuous Paddle Dryer

A model based on the theory of Markov chains has been developed to represent the residence time distribution (RTD) of municipal sewage sludge in a continuous paddle dryer. The flow of dry solids is described by a chain of n perfectly mixed cells, n corresponding to the number of paddles attached to the shaft. The transition probabilities between the cells are governed by two parameters: the parameter of internal recirculation, R, and the solids hold-up, Hu. In the absence of available correlation, both parameters are identified by fitting the model to experimental RTD data. The model demonstrates its ability to describe the sludge flow in a continuous lab-scale paddle dryer. A sensitivity analysis highlights that R is critical for the treatment uniformity while Hu controls the mean residence time and thus the final moisture content.     

DRYING OF PULP AND PAPER SLUDGE IN A PULSED FLUID BED DRYER

ABSTRACT

Hydrodynamics and drying kinetics for the pulp and paper primary sludge dried in a pulsed fluid bed dryer with relocated air stream are presented. Batch experiments have indicated that drying of disintegrated sludge to the required 12% moisture content takes place during the first drying period at practically constant material temperature close to the wet bulb temperature with respect to the inlet air conditions. Equations were developed for pressure drop, minimum pulsed-fluidization velocity, dynamic bed height, and volumetric mass transfer coefficient. Continuous experiments under drying conditions determined from the average residence time concept have confirmed that transportation of disintegrated sludge along the dryer follows the plug flow model.

     

Numerical analysis and differential push-flow structure optimization of vacuum disc drying process

The drying characteristics of sludge in a uniaxial vacuum disc dryer are numerically studied in the present study, and the results showed that the drying process of sludge includes three stages: the viscous stage, the sticky stage, and the granular stage. The influence of main parameters such as sludge feeding rate, heat source temperature, and rotating speed of the shaft on the drying process of sludge in the uniaxial vacuum disc dryer is analyzed. In order to optimize the drying efficiency of the dryer, a differential push-flow structure is proposed in this study, which provides different propulsion effects for the sludge in different drying stages. The differential push-flow is achieved by changing the number and angle of the push-flow blades on the discs. Compared with the dryer before optimization, the dryer with optimized differential push-flow structure has better performance.     

Drying characteristics of sewage sludge

To obtain the drying rate of sewage sludge for use in design of a conductive indirect-heating dryer with agitation, the drying characteristics of sewage sludge from three different wastewater treatment plants were investigated with a thermogravimetric analyzer (TGA) in isothermal conditions. Temperature and sample mass were considered as experimental variables. The drying mechanism agreed well with the shrinking core model dominated by the kinetic rate. The activation energy of drying was 17.30 kJ/g mol. A rate equation was proposed for drying of sewage sludge.     

Three-dimensional modelling of pneumatic drying process

Steady-state three-dimensional calculations of heat and mass transfer in vertical pneumatic dryer were performed. The theoretical model of the drying process is based on two-phase Eulerian-Lagrangian approach for gas-particles flow and incorporates advanced drying kinetics for wet particles. The model was utilized for simulation of the drying process of wet PVC and silica particles in a large-scale vertical pneumatic dryer. The influence of wall thermal boundary conditions was investigated by assuming either known value of the wall temperature or adiabatic flow in the dryer. Analyzing the predicted particle drying kinetics, an uneven product quality was predicted due to non-uniform drying conditions in the central and peripheral zones of the pneumatic dryer. Moreover, for the case of non-insulated chamber walls such quality unevenness was estimated to be substantially greater than for the case with thermally insulated drying chamber. The examination of the predicted temperature profiles within the silica and PVC wet particles showed that the latter is subjected to higher temperature gradients potentially resulting in the greater rate of thermally-degraded final product.     

ANALYSIS OF THE DRYING OF RESIDUAL SLUDGE: FROM THE EXPERIMENT TO THE SIMULATION OF A BELT DRYER

The drying of residual sludge is a current environmental problem not sufficiently described in the literature, hence research investigations on this dewatering process are required. This paper proposes a modelling of a conveyor dryer belt. However the study of residual sludges dewatering in order to design or simulate drying equipment requires some experimental investigations. An analysis in terms of drying kinetics is proposed in this paper. Specific experiments were also needed in order to better describe some particular aspects such as crusting, shrinkage, and particle size impact. According to these data some analytical expressions have been derived and integrated in a belt dryer model. The set of air operating conditions (T, RH, v) was reduced to a single parameter, the drying potential. The results of the simulations show that a conveyor belt dryer is not well adapted to activated sludge. This is essentially due to a crust phenomenon. For a PVC industrial sludge this kind of dryer is more efficient and allows drying with reasonable residence times. Several simulations were run in order to point out the relevant parameters of the process.     

桨叶式干燥机热干燥处理制革污泥的排放特性

进行了使用桨叶式干燥机进行污泥热干燥处理的实验研究,分析在不同的温度下污泥的干燥效果、气体排放和冷凝水产生情况。实验结果表明,污泥在干燥过程中会释放出氨气、挥发性有机酸（甲酸、丙酸）和烷烃（庚烷）等挥发性有机污染物,且升高温度能够降低干燥机出料含水率,并能够通过影响水解及脱羧反应从而增加干燥过程排放的气体量及冷凝水的COD值。通过对比冷凝前后的气体排放量,表明冷凝操作有利于降低二次污染。     

污泥干燥机在污泥干化焚烧工艺中的应用

污泥干化焚烧工艺综合考虑了污泥处置的可靠性、先进性、适用性、安全性与经济性等原则,将是现阶段我国一线城市污泥治理的主流工艺路线。本文通过对不同种类的污泥干燥机进行综合性能比较,推荐了作为污泥干化焚烧工艺的主流干燥机。国产干燥机在经过对国外同类设备的消化吸收后,今后将在国内污泥干化领域占据一席之地。