DRYING OF PARTICULATE SOLIDS: DETERMINATION OF THE CHARACTERISTIC CURVE OF BROWN COAL.

ABSTRACT

Variation on the rate of drying of brown coal powder as a function of environmental and intrinsic properties has been studied in a gravimetric laboratory-scale drier.

Raw data have been treated to correlate the constant rate of drying and the equilibrium moisture content as a function of flow rate and temperature of the gas phase, particle size and surface additives. The characteristic drying curve occurs when the reduced rate of drying is plotted as a function of a dimensionless moisture potetial.     

APPLICATION OF THE DISCRETE ELEMENT MODELLING IN AIR DRYING OF PARTICULATE SOLIDS

The Discrete Element Method (DEM) has been widely used as a mathematical tool for the study of flow characteristics involving particulate solids. One distinct advantage of this fast developing technique is the ability to compute trajectories of discrete particles. This provides the opportunity to evaluate the interactions between particle, fluid and boundary at the microscopic level using local gas parameters and properties, which is difficult to achieve using a continuum model. To date, most of these applications focus on the flow behaviour. This paper provides an overview of the application of DEM in gas-solids flow systems and discusses further development of this technique in the application of drying particulate solids. A number of sub-models, including momentum, energy and mass transfer, have been evaluated to describe the various transport phenomena. A numerical model has been developed to calculate the heat transfer in a gas-solids pneumatic transport line. This implementation has shown advantages of this method over conventional continuum approaches. Future application of this technique in drying technology is possible but experimental validation is crucial.     

A STOCHASTIC APPROACH TO THE DRYING OF SOLIDS IN A MULTI-STAGE FLUIDIZED BED

The residence times of solids flowing through a fluidized bed dryer exhibit dispersion about the mean. In this paper, expressions for the probability density functions of solids moisture content in the various stages of a multi-stage dryer are derived. A simple recurrence relationship for the moments of the distribution is also presented. The analysis is applied to the drying of cereal grains, and it is shown that the degree of drying increases with the number of stages in the dryer. Probability density functions of the moisture content are presented.     

DRYING OF SOLIDS

Solar drying of prunes arranged in thin layers in a multi-shelf type drier is simulated. The weather conditions are those found during harvest in humid pampa region (hot-temperate and humid climate). A mathematical model based on Luikov's theory of drying and including an equation that describes the material shrinkage is employed. The airstream humidification is taken into account in order to analyse the influences of shelves spacing and drier length in the final product characteristics. It is found that the relative moisture content heterogeneity is a useful design criterion. The influence of weather conditions in the drying time and the drying capacity are evaluated.     

DETERMINATION OF THE EVAPORATION OF WATER DURING WOOD DRYING BY MEANS OF HEAT FLUX MEASUREMENT

ABSTRACT

Since the only measured value that is derived from the wood for controlling the kiln drying process is the mean moisture content, it is essential to develop new techniques for the measurement of additional process parameters. When the drying rate, which could be such an additional parameter, is seen in conjunction with other process variables, conclusions on the instantaneous drying behaviour of the wood are possible. A simple and practical way for determining the drying rate is based on the relationship between the drying rate and the heat-flux for evaporation. A measurement of the heat-flux by means of a heat-flux sensor allows the calculation of the drying rate.     

A CONTRIBUTION TO THE CHARACTERISTIC DRYING CURVE CONCEPT: APPLICATION TO THE DRYING OF PLASTER

ABSTRACT

Implicit equations relating the dimensionless drying rate to the reduced moisture content for porous non-hygroscopic slabs have been determined. This study is based on the receding evaporative front model and on the assumption of a parabolic moisture content profile in the diffusional zone of the wet region. The characteristic functions derived depend on the relative position of the receding evaporative front. Two cases were considered : the drying of thick products and the drying of thin products. It has been observed that the relationships corresponding to the C.D.C obtained for thick products depend on the initial. On the other hand, it was observed that experimental CDC. of plaster slabs determined for many thicknesses and various plaster textures depend strongly on these two parameters. Furthermore, the model presented gives good results only for thin products.     

热泵干燥箱的干燥过程

通过对热泵干燥箱的热力学分析和对热泵干燥箱干燥过程的试验研究，认为热泵干燥箱干燥物料具有高效节能、干燥温度低和对周围环境无污染等优点。试验结果表明热泵干燥箱在干燥过程中干燥室内的各点温度和相对湿度在干燥０１～２．５ｈ之间相差最大，最大温差为１３．５℃，这时热泵干燥箱的除水量也最大为６．１ｋｇ水／ｈ。     

CONVECTWE DRYING MODEL OF SOUTHERN PINE

ABSTRACT

A transient one dimensional first principles model is developed for the drying of a porous material (wood is used as an example) that includes both heat and mass transfer. Heat transfer by conduction and convection, mass transfer by binary gas diffusion, pressure-driven bulk flow in the gas and liquid, and diffusion of bound water are included in the analysis. The diffusive mass transfer terms are modeled using a Fickian approach, while the bulk flow is modeled assuming Darcian flow. Depending on the state (pendular or funicular) of the moisture in the wood, appropriate terms are considered in the development of the governing mass equations. The results provide distributions within the material of each moisture phase (vapor, liquid, and bound), temperature, and total pressure. Information regarding the drying rate and evaporation rate is also presented. Average distributions are obtained as a function of time, and compared with experimental data from the literature. It is observed that the total pressure within the material can be considerably above one atmosphere during the drying process.     

INFLUENCE OF THE DRYING MEDIUM PARAMETERS ON DRYING INDUCED STRESSES

ABSTRACT

A thermomechanical model of drying of capillary-porous materials whose material constants depend on moisture content and temperature is presented in the paper. The finite element method is used for the solution of two-dimensional problem of convective drying of a prismatic bar. The moisture distributions, temperature distributions, drying induced strains and stresses for various drying medium parameters are determined. The effect of these parameters on moisture distribution and in particular on drying induced stresses is discussed.     

AN ASSESSMENT OF THE USE OF CHARACTERISTIC DRYING CURVES FOR THE HIGH-TEMPERATURE DRYING OF SOFTWOOD TIMBER

ABSTRACT

An explicit assessment has been carried out, using the experimental data of Pang (1994), of the applicability of the concept of a characteristic drying curve to the drying of Pinus radiata softwood timber. This concept has been used recently by Pang and Keey (1994) and Nijdam and Key (1996) when investigating the expected drying behaviour of a complete stack of timber. The concept appears to be applicable over the range of wet-bulb depressions which are common inside the stacks of timber, supporting its use in kiln-wide analysis of drying behaviour.     

ENERGY SAVING IN INDUSTRIAL DRYING PLANTS BY PARTIAL RECOVERY OF THE LATENT HEAT OF THE EXHAUST AIR

ABSTRACT

Energy saving in industrial drying by recovering part of the latent heat of the vapor exiting the drier is considered in this paper. This heat recovery is accomplished through a two-stage heat exchanger system, wherein the first stage brings the primary circuit to vapor saturation conditions, while the second stage acts as a condenser. Optimal heat recovery is assured by intelligent, fully automated process control.     

DRYING OF CARROTS. I. DRYING MODELS.

Three models of different complexity are proposed to describe the falling rate period of the carrot drying process with shrinkage. A moving or fixed boundary problem as well as a constant or local moisture and temperature dependent effective diffusivity are considered. The moving boundary problem is solved by an explicit finite difference method. Heat transfer coefficient and effective diffusivity identification were carried out. The results of the heat transfer coefficient show a good agreement with other sources. Using experimental data and the models. describing the heat and mass transfer three different expressions for the effective diffusivity are established. Two of them are only temperature dependent considering or not particle shrinkage. The third one takes into account temperature and local moisture as well as shrinkage.

Drying of foods is a complicated process involving simultaneous coupled heat and mass transfer phenomena which occur inside the material being dried (Chiang and Petersen, 1987). Several models are found in the literature, representing mass and energy transfer which take place during food drying (King, 1968; Sokhansanj and Gustafson, 1980). Usually, approximate solutions are obtained with these     

微波辅助干燥技术的应用进展

微波辅助干燥是一种比微波干燥更有优势、应用前景更广的干燥技术，它结合了微波干燥和传统干燥技术的优点。本文主要阐述了微波辅助干燥的原理、特点以及在国内外的研究和应用现状。同时对该技术在陶瓷坯体干燥中的应用进行了分析。     

A REVIEW OF THE FIELD DRYING OF GRASS

This paper reviews the data and models used to simulate the field drying of grass in swaths. The main factors influencing the drying rate are solar radiation, vapor pressure, rain, the swath structure and the thin layer drying rate of the grass. Most models consider these factors to some extent. The simulation of swath drying is well developed, with some models considering the heat and mass transfer processes in the swath. But the modelling of swath structure and the effect of rain have received less attention. The models agree on the effect, on the drying rate, of all weather factors except wind speed. Some models predict that more wind is beneficial, others say the opposite. This problem has not been resolved experimentally.     

THE THERMO-IMAGE EXPERIMENTAL STUDY FOR THE OPTIMIZATION OF SEEDS DRYING

Using the method of combining the two subjects of engineering thermophysics and physiology, this paper carried out the thermo-image experiments of drying Chinese cabbage seeds at both dynamic and static states in the infrared radiation vibrofluidized dryer that was made by us. Meanwhile the seed physiology experiments were also conducted. The results show that the critical drying temperature of seeds is the function of their drying time and moisture content. It is higher when the initial moisture is lower, or the drying time is shorter. This provided the theoretical basis for the optimization of heat and mass transfer of seeds drying.     

THE THERMO-IMAGE EXPERIMENTAL STUDY FOR THE OPTIMIZATION OF SEEDS DRYING

Abstract

Using the method of combining the two subjects of engineering thermophysics and physiology, this paper carried out the thermo-image experiments of drying Chinese cabbage seeds at both dynamic and static states in the infrared radiation vibrofluidized dryer that was made by us. Meanwhile the seed physiology experiments were also conducted. The results show that the critical drying temperature of seeds is the function of their drying time and moisture content. It is higher when the initial moisture is lower, or the drying time is shorter. This provided the theoretical basis for the optimization of heat and mass transfer of seeds drying.     

HIGH-INTENSITY DRYING OF PAPER

High-intensity contact drying denotes drying under suf- ficiently intensive heating conditions that, following a brief warmup period, the mist paper web operates at internal tem- peratures in excess of the ambient boiling point. A simplified, two-zone analytical model is first presented. The paper is depicted as having a dry layer, of ever-increasing thickness, adjacent to the hot surface. Heat conduction through this layer (the rate-limiting step) causes evaporation at the interface with the “wet zone.” The vapor is then considered to flow through the wet zone into the ambient. Results of bench-scale experiments are discussed. Drying rates as much as twenty times conventional rates are indicated. The drying rate increases with hot surface/boiling point tem- perature difference and applied mechanical pressure. The instsn- taneous drying rate decreases continuously after a brief warmup period. The vapor pressure at the hot surfacelpaper interface rises quickly to a peak value, followed by a continuously- diminishing level. The heat flux shows a rapid rise to boiling- like conditions, followed by a drop to a range compatible with measured drying rates. The temperature of the open side of the sheet is constant during drying, after its initial rise to the boiling point. The experimental results are compatible with pre- dictions of the model.     

食品冷冻干燥过程的模型及影响因素

在合理简化的基础上,得到了只需较少参数的冷冻干燥过程一维数学模型。通过对胡萝卜和土豆食品的冷冻干燥实验,研究了加热方式、加热搁板温度和干燥室真空度对干燥时间的影响,模型的数值解与实验值吻合良好。     

DRYING OF PARTICULATE SOLIDS: DETERMINATION OF THE CHARACTERISTIC CURVE OF BROWN COAL.

Variation on the rate of drying of brown coal powder as a function of environmental and intrinsic properties has been studied in a gravimetric laboratory-scale drier.

Raw data have been treated to correlate the constant rate of drying and the equilibrium moisture content as a function of flow rate and temperature of the gas phase, particle size and surface additives. The characteristic drying curve occurs when the reduced rate of drying is plotted as a function of a dimensionless moisture potetial.