Contribution of external resistance to mass transfer during pasta drying

The contribution of external resistance to mass transfer during pasta drying was quantified for a range of pasta properties, extrusion conditions, and drying temperatures. Comparison of drying models showed that neglecting external resistance to mass transfer significantly decreases the accuracy of models during the early stages of drying. A nonlinear uncertainty analysis revealed that failing to consider external resistance to mass transfer results in an underestimation of the effective diffusion coefficient. The Akaike information criterion confirms that external resistance to mass transfer should not be neglected for small pasta or pasta dried at high drying temperatures.     

胡萝卜对流干燥过程理论分析——临界水分质量比

收缩和非收缩条件下,对胡萝卜对流干燥特征进行了理论分析。发现临界水分质量比是试样几何形状、尺寸大小以及热空气温度、速度、相对湿度的函数。在收缩条件下,只有薄片状试样对流干燥过程包含恒速干燥阶段和降速干燥阶段;而圆柱状和球状试样干燥速率从干燥起始就随试样水分质量比的下降降低。数值实验结果表明,根据物料临界水分质量比很难确定对流干燥过程处于外部干燥条件控制的第1干燥阶段还是物料内部条件因素控制的第2干燥阶段。临界水分质量比不是表明物料干燥特性的特征参数。     

Fundamental Research on Drying Process in Institute of Engineering Thermophysics, CAS

Some results of our fundamental research on drying processes are summed up in this article. It consists of three parts: (1) Multistage fluidized bed drying, including particle flowing characteristics, heat and mass transfer between particles and drying medium, drying characteristics of drying materials; (2) Impinging stream drying, the flowing and drying characteristics of a vertical impinging stream dryer, one-stage and multistage semi-circular impinging stream dryer and combined vertical and semi-circular impinging stream dryer are discussed; (3) The effects of rapid transient heat and mass transfer on drying processes, such as time and space scales for nonFourier or nonFickian and influence of extreme heat and mass transfer are also discussed.     

Drying of Suspensions in a Fluidized Bed of Inert Particles Under Reduced Pressure

A drying method in which a heat-sensitive material is immersed in a fluidized bed under reduced pressure was used to continuously obtain dispersed, dry, fine powders of that material from a dilute suspension or solution at a low temperature with a high drying rate. The mass velocities of the drying gas, sample flow rate, and chamber pressure were varied, and the effects of these variations on the corresponding drying characteristics were examined.

The fluidization state of a fluidized bed of inert particles strongly affects the drying rate in the bed and has a greater effect than the chamber pressure on the corresponding drying characteristics. In other words, it is important to maintain a vigorous fluidization state to achieve a high drying rate. Although the maximum drying rate is independent of the chamber pressure, it can be achieved at a low mass velocity of the drying gas under reduced pressure. That is, at a low chamber pressure a high drying rate can be achieved, even at a relatively low mass velocity of the drying gas. The bed temperature at the time of drying is strongly influenced by the drying rate and decreases linearly with an increase in the maximum drying rate when the sample flow rate is equal to the drying rate.     

SIMULATION OF SUPERHEATED STEAM DRYING CONSIDERING INITIAL STEAM CONDENSATION

A heat and mass transfer model was proposed for the superheated steam drying, focusing on phenomena which occur during the initial stage of drying, i.e., condensation of superheated steam on material surfaces and subsequent shift from condensation to evaporation leading to the beginning of the actual drying (Reverse Process Model). Next, drying equations considering the reverse process were formulated for a shrinking/swelling infinite flat plate to calculate moisture content and temperature distributions in a material, changes in mass of a material with time, and a characteristic drying curve. Then, the influence of the initial thickness of a material and the heat transfer coefficient were investigated. In addition, calculation results were compared with experimental ones with regard to the change in mass of material with time and the characteristic drying curve, and good conformance was obtained for the initial stage of drying.     

Drying Research and Development in China

The developments in fundamental research on drying, drying technology, dryer manufacture from 1995 to 2001 in China were reviewed. The progress and current situation in drying fundamental research in China, such as heat and mass transfer in wet porous media, non-Fick and non-Fourier effects on rapid transient heat and mass transfer during drying, numerical simulations of a drying process and so on, were reviewed. The states of manufacturing of various dryers in China were investigated. Some studies on novel drying processes, such as pulse combustion drying, biomaterial drying, electro-dewatering technique etc., were also presented.     

基于气固流态化原理的油页岩干燥动力学

为了考察气固流化床干燥器能否使油页岩含水质量分数达到要求,以柳树河油页岩颗粒为原料,研究进口气体温度和颗粒直径对油页岩干燥性能的影响,采用薄层干燥模型,对油页岩干燥实验数据进行模拟,确定油页岩干燥方程和干燥速率方程,建立油页岩干燥速率特征常数和有效扩散系数之间的关联式。研究结果表明:薄层干燥模型中修正Page模型Ⅰ适合描述油页岩的干燥过程;油页岩在流化床内干燥过程主要发生在降速干燥阶段,进口气体温度越高,油页岩颗粒直径越小,所需干燥时间越短,进口气体温度为350℃时,使2.4 mm油页岩含水质量分数低于5%,所需干燥时间为2.5 min。     

Fundamental Research on Drying Process in Institute of Engineering Thermophysics,CAS

Abstract

Some results of our fundamental research on drying processes are summed up in this article. It consists of three parts: (1) Multistage fluidized bed drying, including particle flowing characteristics, heat and mass transfer between particles and drying medium, drying characteristics of drying materials; (2) Impinging stream drying, the flowing and drying characteristics of a vertical impinging stream dryer, one-stage and multistage semi-circular impinging stream dryer and combined vertical and semi-circular impinging stream dryer are discussed; (3) The effects of rapid transient heat and mass transfer on drying processes, such as time and space scales for nonFourier or nonFickian and influence of extreme heat and mass transfer are also discussed.     

SIMULATION OF SUPERHEATED STEAM DRYING CONSIDERING INITIAL STEAM CONDENSATION

A heat and mass transfer model was proposed for the superheated steam drying, focusing on phenomena which occur during the initial stage of drying, i.e., condensation of superheated steam on material surfaces and subsequent shift from condensation to evaporation leading to the beginning of the actual drying (Reverse Process Model). Next, drying equations considering the reverse process were formulated for a shrinking/swelling infinite flat plate to calculate moisture content and temperature distributions in a material, changes in mass of a material with time, and a characteristic drying curve. Then, the influence of the initial thickness of a material and the heat transfer coefficient were investigated. In addition, calculation results were compared with experimental ones with regard to the change in mass of material with time and the characteristic drying curve, and good conformance was obtained for the initial stage of drying.     

Thermophysical Characteristics of Potatoes,Vegetables and Fruits

ABSTRACT

The main objective in this work is to study and deduce a governing equation for net mass transfer in moist air and turbulent flow. Development of simple and reliable steady state models for turbulent moist air-drying has been considered to be quite well covered in literature. However, the lack of necessary background information concerning classical drying models is now being rectified through research carried out with new approaches, which are initiated by advancement in laboratory equipment.

The known and trusted models are combined with coupled momentum, heat and mass transfer equations creating a reliable governing mass transfer equation for use in turbulent moist air drying processes, i.e. the advanced drying model (ADM). The ADM is a relatively user friendly and robust model, and it is well-suited for identifying transfer coefficients from boundary layer measurements, for example in modem high intensity paper drying machines.

The advanced drying model is analysed and verified with the specially designed experimental apparatus described in this article. The deduced mass transfer equation is then presented and experimentally verified to clarify why the use of Stefan's diffusion equation should be avoided when calculating high drying intensities in turbulent flow.

Finally, when applied to a wide drying range, the classical drying models require parameters which have been experimentally verified. Therefore, a comprehensive knowledge of governing mass transfer mechanisms will also reduce the large number of necessary drying experiments. The advanced drying model, which includes variable physical properties and transport coefficients, allows the simulation of many geometrical shapes and drying configurations and therefore provides a tool for optimising drying processes in a new manner.     

Assessment of the initial moisture content on soybean drying kinetics and transport properties

Several researchers have developed studies to obtain a mathematical model able to describe grain drying kinetics. However, most of these studies neglect the effect of grain initial moisture content on drying curves. In this study, we assessed the dependence of drying curves and mass transfer coefficients on this initial moisture, air temperature, and its velocity by measuring grain mass losses within time on a tray dryer. Mathematical models were adjusted and results indicated that initial grain moisture content has significant influence on drying curves and mass transfer coefficients.     

An Experimental Study of Wheat Drying in Thin Layer and Mathematical Simulation of a Fixed-Bed Convective Dryer

Drying of wheat (Algerian cultivar: Hadba03) in thin layers was studied and mass flux phenomenon was used to characterize the thin-layer drying process. Thin-layer drying of wheat was determined for drying air temperature range of 40–60°C, relative humidity of drying air from 10 to 30%, air velocity of 0.7 m/s, and initial grain moisture from 26 to 31% (dry basis). Equilibrium moisture content of wheat was determined using desorption isotherms obtained from the thin-layer drying data. An equilibrium model for a stationary deep bed with drying air moving vertically upward was developed using mass and energy balance between grain and drying air in the bed and drying air characteristics obtained from thin-layer drying experiments. The developed model was validated by drying wheat in a laboratory dryer using different drying air temperatures and initial moisture contents.     

The Role of Nonuniformity in Convective Heat and Mass Transfer through Porous Media,Part 1

Through-air drying is commonly used in the drying of high-quality tissue and towel products. A representative elementary volume method was used to model the fluid flow and heat and mass transfer during through drying in heterogeneous porous biobased materials such as tissue and towel products. Results of flow both upstream and downstream of a modeled porous sheet allowed visualization of the effects of mixing at the top and bottom of the porous medium. The effect of initial nonuniformity on fluid flow and convective heat and mass transfer in heterogeneous porous media was studied. The effect of material nonhomogeneity and associated transport properties on moisture content of the porous material as a function of drying time was studied. Modeling results indicate that for the first time it is possible to simulate the effect of nonuniformity on fluid flow and convective heat and mass transfer in porous media during through-air drying of paper. Moisture and structural nonuniformity contributing to nonuniformity in air flow might contribute significantly to drying nonuniformity. Depending on the moisture regimes and degree of saturation of the convective medium, heat and mass transfer coefficients may have varying effects on the overall drying.     

Some Considerations In Simulation Of Superheated Steam Drying Of Softwood Lumber

ABSTRACT

A mathematical model for high-temperature drying of softwood lumber with moist air has been modified and extended to simulate wood drying with superheated steam. In the simulation, differences between the two types of drying are considered, these include: external heat and mass transfer processes and calculation of equilibrium moisture content. The external mass transfer coefficient in the perheated steam drying was found to be much higher than that in the moist air drying, however, the heat ransfer coefficients for these two cases were of the same order. The predicted drying curves and wood temperatures from the superheated steam drying model were compared with experimental data and there was close agreement. Further studies will apply the model to development of commercial drying schedules for wood drying with superheated steam.     

压缩空气溶液除湿中不同除湿剂除湿性能比较

在压缩空气溶液除湿实验平台上,分别以LiBr和LiCl水溶液作为除湿剂,实验研究了两种溶液在压缩空气溶液除湿系统中的除湿性能。以溶液表面水蒸气分压力作为比较基准,压缩空气出口含湿量和除湿量作为除湿性能的评价指标,对二者的除湿能力进行比较分析。同时基于压缩空气溶液除湿器传热传质模型,结合实验数据,研究了LiBr、LiCl溶液与压缩空气间的传质系数大小以及变化规律。结果表明：在相同的处理工况下,采用LiCl溶液对压缩空气进行除湿能得到更低的空气出口含湿量和更高的除湿量,LiCl溶液除湿过程的传质系数也高于LiBr溶液,即在压缩空气溶液除湿系统中LiCl溶液具有更优的除湿能力和传质性能。     

高初始含湿多孔介质喷雾干燥过程的热质耦合传递

多孔介质是大量干燥过程的主体,由于实际多孔介质干燥过程的复杂性,建立通用的干燥过程传热传质模型十分困难。通过分析喷雾干燥过程中高初始含湿多孔介质与干燥介质之间的传热传质机理以及各因素对传热传质的影响,根据马歇尔方程探讨了干燥介质与料雾之间的水蒸汽分压差在干燥过程中的变化情况,反映了多孔湿介质在喷雾干燥操作中的传热传质过程的几种特性,为确定实际生产中喷雾干燥器的操作条件指明了新的出路。     

石粉对自密实混凝土收缩性能的影响

为了研究石粉对自密实混凝土收缩性能的影响,设计了不同水胶比和石粉质量分数的试验,得到了水胶比为0.30、0.34、0.37,石粉质量分数为0%、10%、20%时自密实混凝土28 d龄期内自收缩和干燥收缩的变化规律。结果表明:自收缩随着水胶比的减小而增大,随着石粉质量分数的增加而增大;水胶比在0.30~0.37时,其对干燥收缩的影响不大;适量的石粉对干燥收缩几乎无影响,当石粉质量分数达到20%时,干燥收缩明显增大。在试验的基础上提出了考虑水胶比和石粉质量分数影响的自密实混凝土自收缩与干燥收缩预测模型。     

BOUND MOISTURE REMOVAL: HEAT AND MASS TRANSFER IN CONTACT DRYING

Coupled heat and mass transfer in short-term contact of the moist material and the heating surface (the physical model of drying with agitation) is examined. Technological characteristics of the drying process: heating rate and drying rate, heat transfer coefficient, etc. have been determined based on solutions of the diffusion and diffusion-filtration heat and mass transfer. The usage of non-field method of determination of mass and heat fluxes on the phase interface allows calculation of the drying equipment efficiency without preliminary determination of the fields of required quantities. The results may be used for estimation of the influence of drying conditions and material properties on the moisture removal process.     

循环热风低温干燥系统湿空气冷凝特性分析

基于能量梯级利用热力系统耦合理论,集成了一种适合热敏性农副产品烘干的新型空气干燥循环系统,系统可得到热敏性干燥产品,同时回收湿空气冷凝废热用于有机朗肯循环(ORC)系统对外做功。对关键部件湿空气冷凝器建立传热传质数学模型并经实验验证,考察了关键操作参数对系统脱水速率及节能效果的影响。结果表明,湿空气湿度是影响该系统凝水和节能的最关键参数,该系统凝水及节能特性均随湿空气湿度提高而改善;当干燥箱出口湿空气含湿量温度一定时,新型空气干燥循环凝水量主要受到干燥箱出口空气流量的影响,系统的凝水量和换热量均随湿空气质量流量增加先增加后降低,在0.10~0.15 kg/s出现极大值;系统净输出功随ORC底循环蒸发温度提高显著增加。本系统下的热敏性农副产品烘干建议选择低空气流速、低烘干温度,推荐的ORC底循环蒸发温度为313~323 K。