HEAT AND MASS TRANSFER RELATIONS FOR CROP DRYING

ABSTRACT

In present communication, an attempt has been made to study the heat and mass transfer for crops namely wheat and gram. The removal of moisture from the crop was determined under simulated conditions and it was used to develop heat and mass transfer relation using linear as well as multiple regression analysis. The observations were recorded for relative humidity, temperature of crop and air and for moisture evaporated. Experimental error in terms of per cent uncertainty was calculated for recorded data which is in the range of 20 - 30 per cent for forced convection and natural cooling, respectively.     

Effect of Pressure on Moisture Transfer During Moisture Adsorption

ABSTRACT

A set of coupled heat, mass, and pressure transfer equations was used to describe the moisture adsorption process in gmn kernels. The finite element method was used to solve the system of equations. The technique was applied to analyze the temperature, moisture, and pressure distribution in a barley kernel during soaking with steep water. The temperature and moisture distributions with (heat, mass, and pressure transfer model) and without (heat and mass uansfer) the effect of pressure were simulated for assumed conditions. The results obtained from the heat, mass, and pressure transfer model show a marked difference from the results obtained from the heat and mass transfer model. This indicated that a pressure gradient exists during the transfer Process, causing additional moisture movement due to filtration effect. Hence. the pressure tern cannot be assumed constant during the moisture adsorption process. The simulated temperature, moisture and pressure profiles and gradients can be used for determining the optimum time required for solking kernels with sleep water to produce barley malt.     

膜过程中热质耦合传递分析

研究了透过致密无孔膜的传热传质过程,考察了传热传质的相互作用,建立了膜过程中热质耦合传递的数学模型,并以湿空气透过薄膜分离过程为例,分析了温差及浓度差的变化对传热传质过程的影响,发现温差及浓度差的变化会引起热阻及湿阻的变化,从而进一步影响热流量和传质通量,所以对传热传质过程有加成作用。     

空气-水/吸湿溶液热湿传递特性分析（Ⅱ）传质特性分区

选取典型状态点计算了空气与水或溶液进口状态相对位置不同时的热湿处理结果,依据传质效果的不同及全热换热与传质方向的异同,将空气与溶液的热湿传递过程在焓湿图上划分为A～D区,空气与水的过程分为A～C区,分析了各分区中的热湿传递特点及获得较优传质效果的流型,确定了各分区的界线。利用可及处理区域和热湿传递的分区结果分析和优化设计空气的加湿过程和溶液的再生过程,指出应将热量投入给水或溶液来获得更好的加湿效果或再生效果。     

SOME PHYSICO-THERMAL PROPERTIES OF RAW RICE BRAN RELATED TO DRYING TECHNOLOGY

Results of the studies on physico-thermal properties of raw rice bran show that the bulk density of rice bran decreased from 284.69 to 238.81 kg/m3 with the increase of moisture content from 6.3 to 16.5 per cent (d.b.). About 74 per cent of the raw rice bran particles fall below the size of 0.30 mm only. The thermal conductivity and the specific heat of raw rice bran increased linearly from 0.065 to 0.102 Kcal/m hr0c and from 0.3685 to 0.490 Kcal/kg 0C respectively for the same increase in moisture content. The experimental data are always found to be good fit to the linear relationships developed between thermal conductivity and moisture content as well as specific heat and moisture content.     

SOME PHYSICO-THERMAL PROPERTIES OF RAW RICE BRAN RELATED TO DRYING TECHNOLOGY

Results of the studies on physico-thermal properties of raw rice bran show that the bulk density of rice bran decreased from 284.69 to 238.81 kg/m3 with the increase of moisture content from 6.3 to 16.5 per cent (d.b.). About 74 per cent of the raw rice bran particles fall below the size of 0.30 mm only. The thermal conductivity and the specific heat of raw rice bran increased linearly from 0.065 to 0.102 Kcal/m hr0c and from 0.3685 to 0.490 Kcal/kg 0C respectively for the same increase in moisture content. The experimental data are always found to be good fit to the linear relationships developed between thermal conductivity and moisture content as well as specific heat and moisture content.     

MEASUREMENT OF COEFFICIENTS FOB SIMULTANEOUS HEAT AND MASS TRANSFER IN FOOD PRODUCTS

Two experimental devices were designed and built to determine four coefficients K_T K_M D_M D (or δ_T = D_T / D_M occurring in simultaneous heat and mass transfer equations, where,K _T and D_M are thermal conductivity and moisture diffcusivity respectively, D_T ( or δ _T is temperature gradient Induced moisture migration coefficient and K_M is moisture gradient Induced heat transfer coefficient. Three food materials, i.e. potato, bread dough and bread, were tested. From this study, it was found that the value of 5 was higher for low density food materials, such as bread, than for high density materials, such as potato. The coefficient & measures moisture migration contribution due to temperature gradient within the material. The average values of δ _T for potato, bread dough and bread were 0.0014, 0.0059 and 0.0127 per °C, respectively. The contribution of temperature gradient to the overall moisture migration is negligible In high density materials. However, this contribution may be important in the moisture migratlon analysls for low density materials. The moisture gradient induced heat transfer coefficient % as found to be negligible for the materials tested in this study     

Development and Thermal Modeling of a New Construction Biomass Dryer

A new dryer construction has been developed for drying biomass basic materials for energy purposes (straw, sawdust, and other light granular materials). As a result of its simple design, the machine is perfectly suitable for reducing moisture by heat transfer. Compared to the machine types applied thus far, this new construction provides a better ratio of dryer floor space and drying distance and, as a result, a longer drying time. A differential equation system for convection drying was applied for the thermal modeling of the equipment; the mathematical model produced on the basis there of is suitable for examining the heat and mass transfer processes within the dryer. In the course of experimental measurements performed in an industrial size apparatus, the temperature and moisture content of the drying gas were recorded with the initial and final moisture content and surface temperature of the product. The measured values were compared to the results of calculations using the mathematical model.     

Liquefaction of wood by rapid hydropyrolysis

Poplar wood, when heated rapidly (70-90 s) in water to 350°C and quenched, yields oil in 55 per cent mass yield. Approximately 75 per cent of the wood carbon and hydrogen is retained in the oil. The oil contains 25 per cent oxygen (wood contains 45 per cent). Extended reaction significantly lowers yields which are the same in the presence of both nitrogen and hydrogen atomsopheres. Nickel carbonate addition lowers yields significantly. Experiments have been limited to wood particles less than 35 mesh (0.73 mm grid) but heat transfer studies are being extended to larger particles.     

纸张干燥中湿分扩散系数的估算

纸张干燥过程涉及到多孔介质的热质传递,如何确定质量扩散系数是所建立的多孔物料湿分扩散模型能否进行数值计算的关键。按Liukov公式将湿分扩散系数视为含湿质量分数的非线性函数,在恒温下进行纸张干燥实验,通过比较湿分蒸发质量的测量值与理论计算值,采用多变量寻优的方法对多孔介质一维情况下的湿分扩散系数进行估算,得到了实验条件下的纸张湿分扩散系数的计算公式。并进一步确定了纸张中的湿分扩散系数与含湿质量分数和温度之间的函数关系式。     

基于亲水/憎水复合膜的全热交换器换热换湿性能

膜全热交换器由于可以同时回收空调排风中的潜热和显热而受到重视。研究了基于PVAL/PVDF复合透湿膜的全热交换器的透热透湿性能,实验测定了新风与排风之间的显热交换能力和水蒸气交换能力,并建立了基于亲水/憎水复合膜的逆流膜全热交换器传热传质计算模型,实验与理论结果吻合较好。结果表明,该复合膜全热交换器的总传热系数为20～35 W·m-2·℃-1,总传质系数为（1.5～3.5）×10-3 m·s-1。     

Finite Element Analysis of a Moving Boundary Value Problem

A finite element formulation and the solution of a set of nonlinear coupled heat and mass transfer equations for a two-phase system with a moving evaporation interface is presented. The interface condition takes into account the moisture transfer balance at the moving boundary. The finite element results were compared with existing results for a single phase system for model validation. In the two-phase system, the movement of evaporation front has an appreciable effect on the temperature and moisture distribution inside the porous medium during drying. The effect of the nondimensional heat of vapourization parameter γ on the evaporation front, temperature and moisture distribution in porous medium was studied. The higher the value of γ, the slower is the movement of the evaporation front. The temperature decreased and the moisture content increased as the nondimensional vapourization parameter γ increased. This model has potential applications in studying the heat and mass transfer characteristics in food and biomaterials.     

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.     

Free Heat and Mass Transfer in a Porous Enclosure with Side Vents

Free heat and mass transfer during drying in a porous enclosure with free vents has been investigated numerically. Enclosed moist air interacts with the surrounding air through freely vented ports situated on both sides perpendicular to the heated wall. Air, heat, and moisture transport structures are visualized respectively by streamlines, heat lines, and mass lines. Effects of thermal Rayleigh number, Darcy number, vent location, and enclosure inclination on the convective heat/moisture transfer rate and volume flow rate across this enclosure are discussed. For each case, partially enclosed fluid flow undergoes different phases, increasing with buoyancy ratio; that is, heat transfer–driven flow, heat- and moisture-aided flow, and moisture transfer–dominated flow. Numerical results demonstrate that the convective heat and moisture transport patterns and transport rates greatly depend on thermal Rayleigh number, properties of porous medium, and enclosure inclination. Practices for enhancing heat and moisture transfer have been suggested for drying processes.     

MEASUREMENT OF COEFFICIENTS FOB SIMULTANEOUS HEAT AND MASS TRANSFER IN FOOD PRODUCTS

ABSTRACT

Two experimental devices were designed and built to determine four coefficients K_T K_M D_M D (or δ_T = D_T / D_M occurring in simultaneous heat and mass transfer equations, where,K _T and D_M are thermal conductivity and moisture diffcusivity respectively, D_T ( or δ _T is temperature gradient Induced moisture migration coefficient and K_M is moisture gradient Induced heat transfer coefficient. Three food materials, i.e. potato, bread dough and bread, were tested. From this study, it was found that the value of 5 was higher for low density food materials, such as bread, than for high density materials, such as potato. The coefficient & measures moisture migration contribution due to temperature gradient within the material. The average values of δ _T for potato, bread dough and bread were 0.0014, 0.0059 and 0.0127 per °C, respectively. The contribution of temperature gradient to the overall moisture migration is negligible In high density materials. However, this contribution may be important in the moisture migratlon analysls for low density materials. The moisture gradient induced heat transfer coefficient % as found to be negligible for the materials tested in this study     

Modelling the Bulk Cooling of Alfalfa Pellets

Cooling of alfalfa pellets after being made is one o f the unit operations in the pelleting of alfalfa. Using the thermal properties and drying diffusion coeficient together with other properties reported in refereed journals. a combined heat and mass transfer model was developed for the cooling of alfalfa pellets in deep beds. The model utilized the distributed heat and mass transfer equations to describe the temperature and moisture of the pellets. The distributed model interacts with the cooling air through a convective boundary condition for the temperature and a time-varying exponential surface condition for the moisture. Coupling o f the heat and mass transfer processes was carried out at the surface of the pellet using evaporative surface condition. The model was validated with field experimental data from a double-deck crossflow cooler. The simulated pellet and air temperatures were within 5⁰C of the collected field data while predicted moisture from the model was within 0.3% o f the experimental data.     

Characterization of isotherms and thin-layer drying of red kidney beans,Part II: Three-dimensional finite element models to estimate transient mass and heat transfer coefficients and water diffusivity

Three-dimensional finite element models with consideration of shrinkage and irregular shape were developed to estimate the relationships among the transient heat and mass transfer coefficients, the transient water diffusivity, and the temperature and moisture content of the red kidney beans being dried under different drying conditions. An equation was developed to calculate the transient mass transfer coefficient using the measured time–moisture content data. This calculated transient mass transfer coefficient was further used to calculate the transient heat transfer coefficient. To verify the predicted temperature on the surface of the red kidney beans, surface temperature was measured using a handhold infrared thermometer. These measured temperature and time–moisture content data were used to determine the transient water diffusivity using the least square method when the red kidney bean kernel experienced a shrinkage during drying. Strong relationship among the transient heat and mass transfer coefficients, the water diffusivity, and the ratio of the transient heat and mass transfer coefficients was revealed. This relationship could be used to predict temperature and moisture content of the red kidney beans during the entire drying period. The Lewis number?=?27, and the ratio of the transient heat over mass transfer coefficients was 10765?J?m^?3?k^?1 at 30 and 40°C, and 10729?J?m^?3?k^?1 at 50°C. Shrinkage did not significantly influence the value of the estimated transient water diffusivity.     

Experimental Study of the Heat and Mass Transfer During Drying in a Fluidized Bed Dryer

Drying curves obtained in a pilot-scale fluidized bed dryer using biological source solids (sawdust, soya and fish meal) were used to estimate the parameters involved in heat and mass transfer phenomenas: heat transfer coefficient and moisture diffusivity coefficient. Parameters involved in mass transfer were estimated from drying models based on diffusional mechanisms and others that in addition consider internal and external resistance to the mass transfer. The estimate ef ective diffusivity coefficient was between 2x10-11 to lx10 (m2/s) for the considered products. Heat transfer coefficient was estimated from drying data points in the constant drying rate period when the external resistance to the mass transfer controls the process.     

Heat and Mass Transfer in an Agitated Contact-Convective Heated Dryer

For the investigation of the drying process of a pharmaceutical fermentation waste and for determining specific heat and mass transfer coefficients an agitated contact-convective heated dryer was constructed. This dryer is also suitable for drying of other granular solids with high moisture content. Hence we investigated the drying of a by-product from bio-ethanol production, as well. The pilot-plant agitated dryer makes possible continuous measurement and data-acquisition. Data-acquisition of heated wall temperature, inlet and outlet air temperatures and humidity, mass reduction of the material makes possible the determination of transfer coefficients by the heat and mass balance of the dryer. The measured heat and mass transfer coefficients serve as proper input parameters for the simulation calculations.