A kinetic study of microwave and fluidized-bed drying of a Chinese lignite

The drying kinetics of Chinese lignite in nitrogen fluidized-bed, superheated steam fluidized-bed and microwave were investigated. The changes in the mass as a function of drying time were measured under various drying conditions. The variations of moisture ratio with time were used to test ten different thin-layer empirical drying models given in the literature. In studying the consistency of all the models, some statistical tests, such as χ², residual sum of squares (RSS) and F-value were also used as well as coefficient of determination R². In nitrogen fluidized-bed and superheated steam fluidized-bed, the Midilli–Kucuk model best described the lignite drying process. Drying data in microwave were best described by the Page model, indicative of a difference in kinetics between the two drying methods. This difference was attributed to different heat transfer mechanisms under conventional and microwave drying conditions. The effects of drying parameters in nitrogen fluidized-bed, superheated steam fluidized-bed and microwave drying on the constants and coefficients of the selected models were studied by multiple regression analysis. The apparent diffusion coefficient of moisture in samples was obtained from the kinetics data and the apparent activation energies under nitrogen fluidized-bed, superheated steam fluidized-bed and microwave drying were found to be rather similar.     

Utilization of coal-flotation wastes at OAO TsOF Belovskaya

Flotation wastes at OAO TsOF Belovskaya are studied with a view to their disposal in systems for the combustion of high-ash coal. Analysis indicates that, if the flotation wastes are mixed with regular coal, they may be used as fuel in low-temperature fluidized-bed furnaces. The introduction of such furnaces in the drying department at OAO TsOF Belovskaya is recommended, for use in the drying of coal concentrates.     

Heat and mass transfer model of dielectric-material-assisted microwave freeze-drying of skim milk with hygroscopic effect

A new porous media mathematical model for freeze-drying was developed based on the adsorption-desorption relationship proposed in this paper. A finite difference solution was obtained from a moving boundary problem for the dielectric-material-assisted microwave freeze-drying process. Silicon carbide (SiC) was selected as the dielectric material; and frozen skim milk was used as the aqueous solution to be dried. Simulation results showed that the dielectric material can significantly enhance the microwave freeze-drying process. The drying time was 33.1% shorter than that of ordinary microwave freeze-drying under typical operating conditions. When the solid content of the solution to be freeze-dried was very low, or the solid product had a very small loss factor, microwave heating was less effective without the assistance of dielectric material. The mechanisms of heat and mass transfer during drying were analyzed based on profiles of ice saturation, temperature and vapor concentration. Drying rate-controlling factors were discussed. A comparison was made between the model predictions and the reported experimental data.     

Modeling and simulation of heat and mass transfer during drying of solids with hemispherical shell geometry

A heat and mass transfer model was proposed to describe the moisture and temperature evolution during drying of solid products with hemispherical shell geometry (HSG). The dimensionless form of the model was numerically solved for both several drying conditions and values of a geometrical factor related with the inner radius of the HSG to obtain their moisture and temperature profiles. In addition, average drying kinetics were calculated from the volume integration of local moisture values. A theoretical and numerical approach was used to develop a mass transfer analogy between the proposed HSG and a simpler flat slab-shaped product. These analogies provide simple mathematical expressions for drying process simulation and estimation of diffusion coefficients in solids with the proposed geometry, and may be applicable to other mass and heat transfer operations. Furthermore, the presented procedure may be used to develop similar expressions in other non-traditional or dissection geometries.     

Heat and mass transfer in apple cubes in a microwave-assisted fluidized bed drier

In the present study, a pilot scale microwave assisted fluidized-bed dryer was designed and used to dry apple cubes. A model was developed to describe heat and mass transfer in apple cubes during drying in a combined microwave-assisted fluidized-bed dryer. A numerical solution based on the finite difference method was used to develop the model for moisture distribution and temperature variation of samples. The model was validated using experimental data, including average moisture content, center and surface temperature at various air temperatures and microwave power densities.     

A GENERAL MODEL FOR MOVING BOUNDARY PROBLEMS -APPLICATION TO DRYING OF POROUS MEDIA

This work presents a general model to describe momentum, heat and mass transfer for moving boundary problems. The equations are obtained supposing an instantaneous superposition of a moving volume with velocity V_s (Lagrangean reference frame) over a stationary volume in the stream velocity v (Eulerian reference frame)The set of equations for multicomponent single-phase systems is applied to porous media (multi-phase systems) using the volume-averaging method. Depending on the assumptions about the behaviour of the system, it is possible to obtain the different models proposed in the literature, showing the generality of the model proposed in this work. Numerical results were compared to experimental data of Kaolin drying during the shrinking stage. These results showed a good agreement.     

带惰性载热固体煤气化流化床的气化行为

引言煤气化是生产清洁燃料或原料的重要工业方法之一．在一般流化床或移动床煤气化反应器中,碳与水蒸气的气化和碳与氧的燃烧反应同时发生．如果希望使用这类气化器生产高热值或富氢的煤气,则需要在高温和高压下提供富氧气体,而这种方法还存在很多实践上的问题．最近日本鹿儿岛大学开发了一种如图1所示的新型煤气化器,可以很方便地通过催化煤气化产生富含H_2和CO的煤气．在这种新型气化器中,气化和燃烧是分别在两个不同反应室内进行的,气化室中所需的热能由来自燃烧室的惰性固体载热体提供.作为这种新型煤气化反应器研究的一部分,…     

Thesis Summary: Study of a New Atmospheric Freeze Drying System Incorporating a Vortex Tube and Multimode Heat Input

Atmospheric freeze drying (AFD) in a vibro-fluidized bed dryer coupled with an adsorbent and multimode heat input is proposed for dehydration of food products. An experimental setup was designed and built to permit simultaneous application of convection, conduction and radiation heat input to the drying material above its freezing point to ensure sublimation using a vortex tube to produce low temperature dry air. Comparison with AFD using fixed bed, fluidized bed dryer, traditional vacuum freeze drying and heat pump drying were carried out to investigate the viability of this new system. A two-layer moving boundary model was developed to simulate the drying kinetics and temperature scenario of thin slab product. Fairly good agreement was found between the predicted values and the experimental data. Finally a three-dimensional (3D) CFD simulation for a vortex tube is carried out to capture the highly swirling compressible flow behavior and to gain basic understanding of temperature separation process. An experimental setup was built to validate the simulation results.     

多孔介质对流干燥机理及其模型

在对现有的多孔介质对流干燥传热、传质模型归类分析的基础上,从介质内部热湿迁移机制出发,建立了能较完善、较准确地描述多孔介质在恒速段及降速段热质传递规律的“三耦合-六场量”混合理论模型.同时针对干燥问题数值模拟中的移动边界问题,提出了一种迭代修正的思想,并发展了相应的数值计算方法.对砖的干燥模拟计算结果表明,本文的模型较其他模型具有更好的精确性.     

Multiphase modeling of intermittent drying using the spatial reaction engineering approach (S-REA)

Several schemes of energy minimization of drying process including intermittent drying have been attempted. Intermittent drying is conducted by applying different heat inputs in each drying period. An effective and physically meaningful drying model is useful for process design and product technology. The lumped reaction engineering approach (L-REA) has been shown previously to be accurate to model the intermittent drying In L-REA, the REA (reaction engineering approach) is used to describe the global drying rate. In this study, the REA is used to model the local evaporation/condensation rate and combined with the mechanistic drying models to yield the spatial reaction engineering approach (S-REA), a non-equilibrium multiphase drying model. The accuracy of the S-REA to model the intermittent drying under time-varying drying air temperature is evaluated here. In order to incorporate the effect of time-varying drying air temperature, the equilibrium activation energy and boundary condition of heat balance implement the corresponding drying settings in each drying period. The results of modeling using the S-REA match well with the experimental data. The S-REA can yield the spatial profiles of moisture content, concentration of water vapor, temperature and local evaporation/condensation rate so that better understanding of transport phenomena of intermittent drying can be obtained. It is argued here that the REA can describe the local evaporation rate under time-varying external conditions well. The S-REA is an effective non-equilibrium multiphase approach for modeling of intermittent drying process.     

吸湿多孔介质冷冻干燥传质传热模型

本研究在作者提出的吸附—解吸平衡关系的基础上,建立了一个全新的考虑吸湿效应的多孔介质冷冻干燥数学模型。模型用有限差分法进行求解,并带有一个移动边界,以模拟介电材料辅助的微波冷冻干燥过程。介电材料选用碳化硅（SiC）,原料液为脱脂奶。模拟结果表明：介电材料能够有效强化微波冷冻干燥过程。在典型操作条件下,介电材料辅助的微波冷冻干燥所用的时间比普通微波冷冻干燥减少33.1%。当料液中固体含量较低或者固体产品的损耗因子较小时,介电材料对微波加热的效果不明显。基于冰饱和度、温度和水蒸气浓度的分布,本文分析了干燥过程中的传质传热机理,并对干燥速率控制因素进行了讨论。     

新式流化床在PVC树脂干燥上的应用

介绍了株洲化工厂从日本引进的单级内热式流化床干燥工艺和设备参数，对不同干燥方法的优、缺点作了比较，并对内热式流化床为什么节能效果显著作了对比分析。     

MODELLING AND PRACTICAL OPERATION RESULTS OF A DRYER FOR WOOD CHIPS

A model of drying of fixed and moving bed in cross-flow is presented. A single particle model described as a multiple moving boundary problem applicable for an icy wood particle is proposed. The simultaneous drying and melting are described by shrinking cores and the effect of thermal storage is taken into account approximately. The results with both the single particle and the bed model compare favourably with reported laboratory measurements. The drying of a deep fixed bed is a two-stage process. A constant rate of drying period with saturated outlet air at constant temperature is followed by a decreasing rate of drying with increasing air outlet temperature and decreasing degree of saturation. The bed modelling is performed to support the development work of a boiler plant using a dryer. Practical operation of dryers within the boiler plants is discussed.     

The Comparison of Two Models of Convective Drying of Shrinking Materials Using Apple Tissue as an Example

Modeling of drying of capillary-porous materials is a mathematically complex problem. It takes into consideration simultaneous heat and mass transfer inside the material with physicochemical properties changing during the drying process. Modeling of the process mentioned above consists of describing the heat and mass transfer balances by means of differential equations. Moisture diffusion coefficient as a function of moisture content and temperature of the material is a crucial parameter that controls the process. An additional problem occurs when moving boundary of the shrinking material is taken into account. In the present work, the identification of diffusion coefficient as a function of moisture content and temperature on the basis of two different models is shown. The two models include the Pakowski model (defined in the stationary coordinates) and the Kechaou model (defined in moving coordinates). Experimental data necessary to verify the models were obtained on the basis of series of tests for different boundary conditions performed on an apple tissue. During the drying process, samples of apple undergo significant volumetric shrinkage. In this article, the comparison of the two models describing the convective drying process of shrinking material is presented together with the comparison of the identified moisture diffusion coefficient.     

The Comparison of Two Models of Convective Drying of Shrinking Materials Using Apple Tissue as an Example

Modeling of drying of capillary-porous materials is a mathematically complex problem. It takes into consideration simultaneous heat and mass transfer inside the material with physicochemical properties changing during the drying process. Modeling of the process mentioned above consists of describing the heat and mass transfer balances by means of differential equations. Moisture diffusion coefficient as a function of moisture content and temperature of the material is a crucial parameter that controls the process. An additional problem occurs when moving boundary of the shrinking material is taken into account. In the present work, the identification of diffusion coefficient as a function of moisture content and temperature on the basis of two different models is shown. The two models include the Pakowski model (defined in the stationary coordinates) and the Kechaou model (defined in moving coordinates). Experimental data necessary to verify the models were obtained on the basis of series of tests for different boundary conditions performed on an apple tissue. During the drying process, samples of apple undergo significant volumetric shrinkage. In this article, the comparison of the two models describing the convective drying process of shrinking material is presented together with the comparison of the identified moisture diffusion coefficient.     

MODELLING AND PRACTICAL OPERATION RESULTS OF A DRYER FOR WOOD CHIPS

ABSTRACT

A model of drying of fixed and moving bed in cross-flow is presented. A single particle model described as a multiple moving boundary problem applicable for an icy wood particle is proposed. The simultaneous drying and melting are described by shrinking cores and the effect of thermal storage is taken into account approximately. The results with both the single particle and the bed model compare favourably with reported laboratory measurements. The drying of a deep fixed bed is a two-stage process. A constant rate of drying period with saturated outlet air at constant temperature is followed by a decreasing rate of drying with increasing air outlet temperature and decreasing degree of saturation. The bed modelling is performed to support the development work of a boiler plant using a dryer. Practical operation of dryers within the boiler plants is discussed.     

Predictive model for countercurrent coal gasifiers

The paper deals with a model able to predict the stationary operating conditions of a moving bed gasifier with upflow of gas. The model was used to simulate coal gasifiers but the proposed approach can also be useful for biomass gasifiers. The model considers the different phenomena that occur along the reactor: drying and pyrolysis take place at the top, while the behaviour of the lower part is dominated by the combustion and gasification reactions between the coal char and the oxidizing gas. The model is structured into two levels: particle level and reactor level. At the particle level, the strong gradients of temperature in the single pellet caused by external heating were taken into account. Literature kinetic models developed with isothermal particles were employed. The calculated results of fixed carbon yield and gas were compared with experimental results obtained in our laboratory.At the whole apparatus level, the reactor was subdivided into two zones: pyrolysis (upper) zone and gasification/combustion (lower) zone. The upper zone was modelled by coupling convective heating of the particles, temperature gradients inside the particles and kinetics. In the lower zone transport of heat by radiation was taken into consideration. The governing equations were solved as a boundary condition problem, leading to a stable and reliable solution. The model was validated by comparison with some literature data of full scale gasifiers.     

Thermodynamic and economic analysis on a two-stage predrying lignite-fueled power plant

In this study, an improved configuration of lignite-fueled power plant integrated with a two-stage predrying system was proposed. The predrying system mainly consists of two fluidized-bed dryers and an additional feed water heater. Lignite is dried successively in the exhaust gas dryer and steam dryer. With boiler exhaust gas being the heat source of the first stage dryer, waste heat of a fraction of the boiler exhaust gas can be used. The exhaust gas of the second stage dryer was considered to be water vapor and its latent heat can be recovered by the additional feed water heater. The thermodynamic and economic analysis show that with the lignite drying degree being 0.1, 0.2, and 0.3?kg/kg, the power generation efficiency of the proposed power plant is 1.45, 2.12, and 2.81% higher than that of the conventional lignite power plant, respectively. Moreover, the annual net economic benefit will be 1.34, 2.03, and 1.60 M$/a during the lifetime of the drying system. The annual net economic benefit is not necessarily higher with higher power generation efficiency.     

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     

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