DIFFUSION COEFFICIENTS FOR PREDICTING ROUGH RICE RE-WETTING BEHAVIOR

ABSTRACT

Thin-layer re-wetting experiments were conducted with medium grain rough rice in the temperature range of 17.8 to 45^°C and for relative humidities between 56 and 89.3%, with initial moisture contents in the range of 10.26 to 12.71% dry-basis to determine the diffusion coefficient of rough rice. Recent efforts to characterize the re-wetting characteristics of rough rice are summarized. New equation for temperature dependent liquid diffusion coefficients for medium grain rough rice are presented. The diffusion coefficient in re-wetting was lower than the drying. The results presented here, over typically five day re-wetting, will be useful in studying the longer term moisture transfer process occurring during ventilated storage.     

THE TWO-THIRDS POWER MODEL FOR PREDICTING DIFFUSION COEFFICIENTS IN LIQUIDS

Most of the models found in the literature for predicting diffusion coefficients in liquids take the viscosity of the solution as inversely proportional to the diffusivity. A model (previously derived for sucrose-water system)which considers the viscosity raised to the two-third power, is evaluated here using published data for the Benzene—Cylohexane, Ethanol-Water and Acetone-Chloroform systems. These results, and results for the sucrose-water system are compared with the experimental values, and with those obtained using the Hartley—Crank equation. Better agreement with the experimental values at low and in the middle of the concentration range was found when diffusivities were calculated using the model proposed than when using the Hartley-Crank equation.     

SOLUTIONS OF DIFFUSION EQUATION WITH CONSTANT DIFFUSION AND SURFACE EMISSION COEFFICIENTS

Closed-form solutions of the non-steady state diffusion equation with constant transport coefficients are presented. The diffusion coefficient is assumed to be finite, but the surface emission coefficient can be either finite or infinite. Mathematical conditions are established for the transport coefficients to be constant. When these conditions are met, the transport coefficients can then be easily evaluated. Diffusion test data can be compared against these conditions to determine whether or not the transport coefficients are constant. Desorption test data of northern red oak indicate that initial moisture content in wood and equilibrium moisture content in the environment are closely related to the constancy of the transport coefficients.     

SOLUTIONS OF DIFFUSION EQUATION WITH CONSTANT DIFFUSION AND SURFACE EMISSION COEFFICIENTS

ABSTRACT

Closed-form solutions of the non-steady state diffusion equation with constant transport coefficients are presented. The diffusion coefficient is assumed to be finite, but the surface emission coefficient can be either finite or infinite. Mathematical conditions are established for the transport coefficients to be constant. When these conditions are met, the transport coefficients can then be easily evaluated. Diffusion test data can be compared against these conditions to determine whether or not the transport coefficients are constant. Desorption test data of northern red oak indicate that initial moisture content in wood and equilibrium moisture content in the environment are closely related to the constancy of the transport coefficients.     

MODELING OF ROUGH RICE DRYING IN THIN VERTICAL COLUMNS

ABSTRACT

The solution of classical diffusion equation based on the assumption of average moisture diffusion coefficient did not adequately represent natural convection drying of rough rice in thin vertical columns exposed on both sides to hot air. Instantaneous moisture diffusivity coefficients determined from experimental drying curves decreased continuously with an increase in exposure duration and were linearly related to moisture ratio. The proponionality constant which was called apparent moisture diffusion coefficient was distinctly related to air temperature, relative humidity, and initial moisture content of rough rice. The modified moisture diffusion model using the instantaneous moisture diffusion coefficient was found to best represent the moisture removal from bulk rough rice.     

CORRELATION OF DIFFUSION COEFFICIENTS FOR LIQUIDS AND DENSE FLUIDS WITH TEMPERATURE AND PRESSURE

On the basis of the free volume theory and activation energy concept,a fundamental equation whichtakes into account the effects of temperature and pressure has been developed.By introducing differentexpressions for the free volume and activation energy,several equations for fluid diffusion coefficients were derivedaccordingly.With the van der Waals free volume and intermal energy formula,a three-parameter model for fluiddiffusion coeffficients at moderate pressure was obtained.The grand average absolute deviation percent of 345data points (44 systems)for self-and infinite dilute inter-diffusivities is 2.32,against the results of the model ofCohen and Turnbull,4.13.In particular,by means of the modified Carnahan-Starling free volume equation,afour-parameter model with average abosolute deviation percent 2.64(30 systems,644 data points)for theestimation of dense fluid inter-and self-diffusivities at high pressures and in supercritical conditions was derived.The derived model is superior to the method of L     

由干湿球温度计算相对湿度的简捷法

通过对相对湿度与干湿球温度间数据的分析,推导出了由干湿球温度计算相对湿度的两个数学公式,计算中不需查阅水蒸气数据表和试差,准确度较高,适于工程计算。在相对湿度≥４０％,湿球温度０～８０℃,风速＞５ｍ／ｓ的空气—水蒸气系统,平均相对误差为１．７％和１．１％。     

拟稳态法测定气相扩散系数

根据拟稳态一维扩散原理,建立了气相扩散系数的测定装置。在该装置上,测定了8个体系的气相扩散系数,其中CH_2Cl_2-空气,CH_2Cl_2-H_2,CH_3CH_2Br-空气,CH_3Cl_2Br-H_2,CHCl_3-H_2等5个体系的气相扩散系数实验值,迄今尚未见文献报道。实验结果表明,本测试装置稳定性好,实验误差小,实验装置体积小,测试过程较为简便。     

THE STUDY OF INTERRUPTED DRYING TECHNIQUE FOR ROUGH RICE

Abstract

The purpose of this research was to study the optimum strategy of the interrupted drying technique for rough rice. The effects of interrupted drying on drying rates, cracking enhancement ratios, and the performance of a commercial circulating rice dryer were evaluated. The results indicated that the required interrupted period of grains with moisture of ranging from 18 to 20% was 2 around 3 hours. The interrupted drying technique can significantly increase the drying rate. The dryer performance e.g. fuel-consumption rate, drying rate, and drying time were all affected by interrupting the drying process, as expected.     

THE STUDY OF INTERRUPTED DRYING TECHNIQUE FOR ROUGH RICE

The purpose of this research was to study the optimum strategy of the interrupted drying technique for rough rice. The effects of interrupted drying on drying rates, cracking enhancement ratios, and the performance of a commercial circulating rice dryer were evaluated. The results indicated that the required interrupted period of grains with moisture of ranging from 18 to 20% was 2 around 3 hours. The interrupted drying technique can significantly increase the drying rate. The dryer performance e.g. fuel-consumption rate, drying rate, and drying time were all affected by interrupting the drying process, as expected.     

A DIFFUSION MODEL WITH A MOISTURE DEPENDENT DIFFUSION COEFFICIENT FOR PARBOILED RICE

A mathematical model describing moisture migration by diffusion in a solid sphere with variable diffusion coefficient is proposed. An analytical expression for dependence of the diffusion coefficient with moisture content was derived based on the assumption that the activation energy for diffusion varies linearly with the desorption energy.

The expression for moisture dependence of diffusion coefficient was used to simulate drying of parboiled rice in the temperature range 50-90°C. The mathematical model shows good agreement between observed and predicted drying rate curves.     

醇类在超临界CO_2中无限稀释扩散系数的测定和关联

建立了测定超临界流体中无限稀释扩散系数的实验装置。在313.2～343.2K、8.1～16.0MPa内测定了苯及文献中尚未报道的7种低级脂肪醇在超临界CO_2中的无限稀释扩散系数,共144个数据点。应用本装置测定的数据及文献报道的各类体系的数据(计63个体系,817个点),考察了现有的几种模型及本文提出的简化的自由体积模型。     

脂肪族氨基酸在水溶液中扩散系数的测定与关联

用金属膜池法测定了30℃下一些脂肪族氨基酸在水溶液中可溶解范围内微分扩散系数,采用KCl标定膜池常数,并用已有可靠扩散系数值的蔗糖水溶液验证了装置的可靠性.给出了微分扩散系数与浓度及相应的密度和黏度关系式.并考核和外推了一个半经验模型,关联和计算了本实验的数据,结果与实验值吻合得很好.     

A STUDY ON THE SEPARATION OF DIFFUSION AND SURFACE EMISSION COEFFICIENTS IN WOOD

ABSTRACT

For a more accurate simulation of the moisture sorption process in wood, the precise separation of surface emission and diffusion coefficients is essential. In this paper, a non-linear simplex technique that was developed to obtain the optimum pair of the surface emission coefficient he, and the diffusion coefficient D from a single sorption curve is presented and tested with experimental data. An easy to use computer program was employed to carry out the evaluation. Data analysis showed that the new method resulted in a more statistically accurate calculation of b_c and D than currently existing methods.     

CORRELATIONS FOR THE EQUILIBRIUM MOISTURE CONTENT OF SOI.IDS

Water adsorption and desorption isotherms at various temperatures have been generated for a variety of particulate materials (ion exchange resin, silica gel, wheat and cellulose materials). Most of thc data were obraincd with the SPS Equilibrium Moisture Content rig, using a dynamic equilibration method. Equilibration over saturated salt solutions at 2°C (static method) was also used for some of the data. Seven semi-empitical correlations, available in the literature, and one developed by SPS have been used to relate the equilibrium moisture content of the material to the relative humidity and temperature of the air. These equations enable the equilibrium moisture content to be predicted under various conditions of relative humidity and temperature from a small number of tests. For each of the six materials tested at least one of the correlations gave a good fit. However, there was no correlation which predicted the equilibrium moisnrre content well for every one of the six materials.     

PREDICTING APPARENT SHERWOOD NUMBERS FOR FLUIDIZED BEDS

ABSTRACT

Mass transfer data of bubbling fluidized beds have been reevaluated with a new model which is completely predictive. The model bases on a two-phase approach with active bypass, formally plug flow for the suspension gas and a consideration of backmixing in the main kinetic coefficient, i.e. in the apparent particle-to-fluid Sherwood number. A good agreement with experimental results of various authors with a broad range of Reynolds numbers and particle diameters is demonstrated.     

自由界面气液传质系数的旋涡作用模型

在单涡模型的基础上 ,根据各向同性湍流的能谱函数 ,考虑不同尺度的旋涡对气液传质的影响 ,建立了气液自由界面传质系数模型 .模型计算结果与管道流及搅拌反应釜的实验结果吻合较好.     

CONCURRENT FLCW RICE DRYING

ABSTRACT

Long- and medium-grain rice were dried in a commercial multi-stage concurrent-flow dryer. Drying air temperatures varied fran 82°C to 177°C. Over six points of moisture were removed in one dryer pass without affecting the rice head-yield. Energy consumption of the dryers was half that of conventional rice dryers. Simulation played a major role in the design of the mUlti-stage concurrent-flow rice dryers.     

THE DRYING OF SMALL WOOD PARTICLES

Small particles of Douglas fir of characteristic size 0.2 to 1cm have been dried by flowing the wood particles in vitiated air in a long vertical tube. Drying rate, dM/dt, was determined by measuring the increasing humidity of the transporting gas flow, and by measuring the absolute velocity of the particles by a radioactive isotope tagging method. Sawdusts of three sizes and flakes were examined. Moisture content (dry basis) was varied from approximately 2 to 125%. Most of the results were obtained for gas temperatures between 350 and 550 K.

Upon injection into the flowing hot gases, the wood particles initially dried rapidly as they encountered high relative temperature and velocity, and released their surface moisture. Following· this transient regime, the particles dried more gradually accord-     

有机物在超临界CO2中无限稀释扩散系数的分子动力学模拟

根据物质的实验临界常数或与物质最基本分子结构相关的Joback或Lydersen临界常数估算法得到物质的临界常数,结合Lennard-Jones流体的对比状态方程,按照对应态原理得到了38种有机物分子的新的Lennard-Jones位能参数。由这些位能参数结合简单的组合规则,首次对一些高附加值的复杂实用有机物在313.15　K、16MPa的超临界CO