Modeling of Sludge Behavior in a Steam Dryer

Steam dryers are in widespread use for sewage sludge treatment. The effectiveness of the steam dryer could be improved by increasing the drying rate, which is influenced by the movement of the sludge in the dryer. Therefore, it is necessary to understand the movement of sludge and water evaporation in the dryer. In this study, tracers were used to elucidate the behavior of sludge in the dryer. The experiment confirmed that sludge did not simply flow in one direction; instead, part of the sludge flowed in the reverse direction (back-mixing). A tanks-in-series model analysis was performed to analyze the back-mixing phenomenon. This analysis provided insight into the behavior of sludge in the dryer, and the drying rate of each segment was determined.     

DRYING AND DEWATERING R&D IN JAPAN

Presently, it has become an urgent and important concern worldwide to reduce the volume of waste and rationally use the waste for electric power generation. Sludge is a dominant part in the family of wastes. Drying of sludge, not only for volume reduction but also for further utilization, is becoming an essential and integral process. On the other hand, as the requirement of new materials production, for example, fine particles of micron or sub-micron scale for advanced materials, some new drying technologies and techniques have emerged. The market-pull may be the direct and active impetus to drying research and development (R&D). In this paper, drying technologies for sludges, fine particles, coatings as well as advanced dewatering are discussed to reflect the “hot” points of drying and dewatering R&D in Japan. And some novel dryers are also presented.     

DRYING AND DEWATERING R&D IN JAPAN

Presently, it has become an urgent and important concern worldwide to reduce the volume of waste and rationally use the waste for electric power generation. Sludge is a dominant part in the family of wastes. Drying of sludge, not only for volume reduction but also for further utilization, is becoming an essential and integral process. On the other hand, as the requirement of new materials production, for example, fine particles of micron or sub-micron scale for advanced materials, some new drying technologies and techniques have emerged. The market-pull may be the direct and active impetus to drying research and development (R&D). In this paper, drying technologies for sludges, fine particles, coatings as well as advanced dewatering are discussed to reflect the “hot” points of drying and dewatering R&D in Japan. And some novel dryers are also presented.     

Application of Computational Fluid Dynamics for Simulation of Drying Processes: A Review

In recent years, computational fluid dynamics (CFD) has been used increasingly to improve process design capabilities in many industrial applications, including industrial drying processes. Drying of food and beverage products, industrial and municipal wastewater sludge, and other manufacturing and environmental products is done regularly in order to enhance the quality and life span of these products and to facilitate their use, storage, and transportation. With recent advancements in mathematical techniques and computer hardware, CFD has been found to be successful in predicting the drying phenomenon in various types of industrial dryers, which utilize all forms of drying operations including spray, freeze, and thermal drying techniques. The CFD solutions are being used to optimize and develop equipment and processing strategies in the drying industry, replacing expensive and time-consuming experimentations. However, a comprehensive review on the application of CFD for the design, study, and evaluation of industrial dryers is not yet available. A comprehensive review of the current literature on the use of CFD models in both industrial and lab-scale drying applications is presented in this article. The use of Eulerian-Eulerian and Eulerian-Lagrangian models in the study of the drying kinetics for gas–solid multiphase flow systems is fully discussed. Merits and disadvantages of using various CFD models in the design of industrial dryers are illustrated and the scope of their applicability is also discussed.     

Refractance Window Dehydration Technology: A Novel Contact Drying Method

Refractance Window® (RW) system is a novel drying method for converting liquid foods and other related biomaterials into powders, flakes, or sheets with added value. In this system, purees or juices prepared from fruits, vegetables, or herbs dry in short times, typically 3-5 min, resulting in products with excellent color, vitamin, and antioxidant retention. The RW drying systems are simple and relatively inexpensive when compared with freeze drying, which usually needs large installations to be economical. In RW drying systems thermal energy is transferred from hot water to a film of puree or juice spread thinly on a plastic conveyor belt. These drying systems operate at atmospheric pressure and are used for commercial production of scrambled egg mix, avocado powder, high carotenoid-containing algae, herbal extracts and human nutrition supplements, and food ingredients, as well as dried fruits and vegetables. This article presents the principle of Refractance Window® drying and highlights some results that show its potential and how it compares with other dryers for processing fruits, vegetables, and other heat-sensitive products.     

Heating/Drying Using Particulate Medium : A Review Part II. Equipment

ABSTRACT

This paper presentS an overview of particulate medium dryers developed to evaluate the drying of grains using heated particulate media. Components of particulate medium heating and drying equipments as well as the devices employed to conduct the materials through the machine are described. How these components accomplish the basic processes involved in conduction/particulate medium heating is also explained.

To characterize the performance of these dryers based on criteria developed for farm level dryers was not possible because of the different methods employed by researchers and the various conditions the equipment were subjected to. So in order to assess the operating characteristics of each design. the dryers were compared based on their ability to remove moisture and their thermal and drying efficiency. Comparison was made on the methods utilised by each design to accomplish heating of the medium, mixing the medium with grain, the separation of the medium from grain, and the means of recirculating the medium.

Several studies conducted to evaluate the different parameters which influence drying using heated particulates are discussed. The granular media used in heating the grain and the corresponding types of grains used in the tests are also presented.     

SLURRY DRYING IN GAS-PARTICLE CONTACTORS: FLUID-DYNAMICS AND CAPACITY ANALYSIS

The fluid-dynamic characteristics and evaporation capacities of several substrates (pure water, a mixture of water and carbohydrates, and apple pulps with maltodextrines) for conical spouted bed, fluidized bed and pneumatic dryers was compared.

For fluidized and spouted beds, the presence of fluids increased the fluid-dynamics parameters over the values when only inert particles were present in the beds. In both cases it was proposed correlations to predict those effects. In the spouted bed drier, increasing the load of inert particles resulted in a slight increase of the water evaporation capacity, while in the fluidized bed drier the effect was more marked. For fluidized and spouted beds, under similar operation conditions, the maximums of the suspension drying capacities are lower than the values of water evaporation, but for pneumatic drying this effect was the opposite. In general terms, the maximal capacities of fluidized bed dryers are higher than those observed in the spouted bed dryers.     

Heating/Drying Using Particulate Medium : A Review Part II. Equipment

This paper presentS an overview of particulate medium dryers developed to evaluate the drying of grains using heated particulate media. Components of particulate medium heating and drying equipments as well as the devices employed to conduct the materials through the machine are described. How these components accomplish the basic processes involved in conduction/particulate medium heating is also explained.

To characterize the performance of these dryers based on criteria developed for farm level dryers was not possible because of the different methods employed by researchers and the various conditions the equipment were subjected to. So in order to assess the operating characteristics of each design. the dryers were compared based on their ability to remove moisture and their thermal and drying efficiency. Comparison was made on the methods utilised by each design to accomplish heating of the medium, mixing the medium with grain, the separation of the medium from grain, and the means of recirculating the medium.

Several studies conducted to evaluate the different parameters which influence drying using heated particulates are discussed. The granular media used in heating the grain and the corresponding types of grains used in the tests are also presented.     

Comparison Between Drying of Timber in a Solar Dryer and in an Electrically-Heated Kiln

ABSTRACT

Solar dryers have been considered for timber drying in a number of countries because of the expected savings in drying costs. From a review of past works on solar, natural, and conventional drying it was observed that while solar dryers were able to dry timber faster compared to natural drying, the difference was only marginal in some instances. The drying rates are expected to be dependent upon ambient conditions in which the dryera are operated. Solar dryers would operate more efficiently in countries with low humidity than in tropical regions. Thus the thermal performance and also the economics of solar dryer is country dependent. In the present paper, a comparison of the drying rates obtained with a solar dryer is made with that obtained with an electrically operated drying kiln.     

SCALE-UP OF LAYER DRYERS: A UNIFIED APPROACH

Abstract

The concept of Specific Drying Rate is introduced for analysis of experimental drying data for layer dryers and as a convenient scale-up factor. In conjunction with data obtained from a batch laboratory simulation the method is applicable to most types of continuous layer dryers. Three illustrations of the use of this approach for scale-up are presented including a continuous convection heated rotary shelf dryer, an indirectly heated paddle dryer, and an indirectly heated plate dryer with heating zones.     

Air-Drying of Seeds: A Review

Beginning with harvesting, seeds are usually subjected to a series of processes which include drying for immediate or future use. Seed quality can be influenced by several factors during drying. This article presents a review of the seed air-drying process, including mathematical models based on differential equations derived from mass and energy balances for seeds and air in fixed and moving bed dryers. The article concludes with an overview of several potential drying technologies that can be applied to seeds.     

湿污泥颗粒的流化床干燥实验及模型

在鼓泡流化床内以河砂为干燥介质,对单颗粒湿污泥的流态化干燥特性进行实验研究,得到了流化床温度、污泥初始水分、污泥粒径及流化速度对干燥速率的影响规律：流化床温度及污泥粒径对干燥速率的影响都呈指数规律;污泥的水分越大,干燥速率越大;在鼓泡流化床流化速度达到2倍临界流化速度以上时,充分流化,流化速度再增大(2~5倍临界流化速度)对干燥速率没有明显影响. 在基本的扩散传质理论的基础上,利用实验数据回归得到湿污泥在鼓泡流化床内干燥的半经验模型,为流化床污泥干燥器的设计提供了基础数据和依据.     

Modelling Diffusion-Limited Drying Behaviour in a Batch Fluidized Bed Dryer

Fluidized bed dryers are often used to extract water from granular materials. When the drying process is mainly limited by the resistance against water transport inside the particle the drying behaviour is said to be diffusion-limited. In the literature there are several models that predict this drying process with very diverging results. In this study a model is set up to arrive at a better prediction for this drying process. The heat and mass transfer in the granular material and the drying air is described. The resulting equations are solved numerically. The model must be extended to incorporate the heat capacity of the dryer.     

OPTIMUM OPERATING CONDITIONS IN DRYING FOODSTUFFS WITH SUPERHEATED STEAM

It is inferred from experimental data that in drying foodstuffs with superheated steam, the initial drying rate has a direct effect on the rate at which the overall drying takes place. That is, the faster the initial drying rate, the shorter the overall drying time. This criterion is very convenient because at the beginning, water moistens the sample external surface so evaporation does not depend on internal sample characteristics, but only on external convective heat and mass transfer rates. Mass and energy balance equations are solved and the result converted into a general initial drying rate equation, in which all dryer characteristics are grouped into one dimensionless parameter. The initial drying rate equation is mathematically maximized and the optimum working conditions determined. The result shows that initial drying rate always increase with increases of either the superheated steam temperature or velocity, but once these two variables are fixed, there exists at least one “optimum” pressure at which the initial drying rate is a maximum. Finally, the initial drying rate and optimum condition equations are applied to three model dryers, a dryer for a flat sheet, a fixed bed dryer and a rotary dryer. In each case, numeric values are computed and plotted as drying rate versus pressure curves, in which the optimum drying rate is also included. Also presented is a chart to compare the optimum pressures as functions of temperature and steam velocity for the three dryers.     

A Study for Development of Real-Scale Thermal Filter Press Dewatering (TFPD)

Real-scale thermal filter press dewatering equipment (plate size: 630 mm × 630 mm) was installed and operated at a waterworks for one year in an attempt to achieve sludge reduction. During the period, the dewaterability was evaluated according to the seasonal sludge properties in order to compare the dewaterability of thermal dewatering and mechanical dewatering, as well as to determine the economics of thermal dewatering. According to the results, the winter season sludge showed a 36% decrease in water content and a two-thirds reduction in dewatering velocity compared to the summertime sludge. In addition, the dewatered cakes of the thermal filter press dewatering equipment showed a lower specific cake resistance and water content in the dewatered cakes than the mechanical filter press dewatering equipment, indicating superior dewaterability. This was attributed to the easier removal of the filtrate remaining in the capillary tubes due to thermal dewatering. The energy consumption for thermal dewatering was 300 kJ/dry solids (DS) kg. A comparison of the sludge dryers indicated that it is possible to produce dewatered cakes that consume less energy and can be recycled. According to the performance evaluation results, the real-scale thermal filter press dewatering equipment had high adaptability to the changes in seasonal sludge, showing excellent dewaterability compared to the mechanical filter press dewatering equipment, and was economical due to the lower energy consumption.     

Mathematical Model Based on DSMC Method for Particulate Drying in a Coaxial Impinging Stream Dryer

An impinging stream dryer (ISD) belongs to a unique class of dryers that has proved to be an excellent alternative to flash dryers for removing surface moisture of particulate materials due to the collision of streams and particles in the dryer. However, the performance analysis of such devices, from a viewpoint of mathematical modeling, has not been investigated extensively. In this study, a mathematical model based on the direct simulation Monte Carlo (DSMC) method is proposed to describe the drying process of particulate materials in a coaxial ISD. The collisions between particles and the heat exchange between impacting particles are included in the present mathematical model. The predicted results were in good agreement with the experimental data, which indicates the validity of the present model. The drying process and the effects of various parameters, including the feeding mode and impinging distance, on the drying performance of the dryer were then numerically investigated and discussed.     

Modelling Diffusion-Limited Drying Behaviour in a Batch Fluidized Bed Dryer

ABSTRACT

Fluidized bed dryers are often used to extract water from granular materials. When the drying process is mainly limited by the resistance against water transport inside the particle the drying behaviour is said to be diffusion-limited. In the literature there are several models that predict this drying process with very diverging results. In this study a model is set up to arrive at a better prediction for this drying process. The heat and mass transfer in the granular material and the drying air is described. The resulting equations are solved numerically. The model must be extended to incorporate the heat capacity of the dryer.     

Correlation between dewatering and hydro-textural characteristics of sewage sludge during drying

The aim of this work consists in analysing the dewatering and the shrinkage of a residual urban sludge during convective drying with the objective to find correlations between dewatering and hydro-textural characteristics of the sludge. Laboratory drying experiments are performed in immobile atmosphere, at a temperature of 30 °C, with various relative humidities. Kinetics curves of dewatering and deformation are obtained. A coupled analysis of dewatering and induced volume shrinkage is proposed. This analysis allows: (i) to distinguish the respective parts played by the intrinsic characteristics of the sludge and the process parameter and (ii) to find correlations between the hydro-textural characteristics of the sludge, and its drying and shrinkage aptitudes. Attention is given to the fact that the process parameter, i.e. relative humidity, controls the first constant rate period of the dewatering and that the hydro-textural state of the sludge determine the transitions between the different dewatering periods. The relationships between the hydro-textural characteristics and the dewatering and shrinkage aptitude allow predicting the sludge behaviour during drying according to its characteristics, which is essential information in the choice of the drying strategy.     

MODELING AND SIMULATION OF CROSSFLOW MOVING BED GRAIN DRYERS

ABSTRACT

The drying of grain in dryers of a crossflow moving bed type was theoretically and experimentally studied. Two different dryer configurations were analyzed, a dryer with central air distribution and another with multiple air duels. Experimental information was obtained in pilot-size dryers. A mathematical model to simulate the process was developed. Hindered drying was accounted for by using the concept of relative drying rate. An adjustable factor, specific to the dryers, was used to account for the uncertainties of the contact area and the transfer coefficients encountered in the literature. Agreement between experimental results and simulations was fairly good. Simulations showed that distance between inlet air and outlet devices, air to solid flow ratio and dryer height to cross section ratio have great influence on the process. The mathematical model may be a useful tool for process exploration and optimization of this type of dryers.