The Effect of Path Diffusion on The Effective Moisture Diffuslvlty in Carrot Slabs

ABSTRACT

In order to evaluate the effect of path diffusion on the average moisture diffusivity in carrot. drying curves for different shaves (slices and cylinders) and temperatures of 50, 60 and 70°C were ohtained takine into consideration the use of an average leneth of carrot sample (slice thickness or the cylinder radio). The. results showed significant differences betuecn radial and axial average diffusivities. Significant differences were also observed between core and annular diffusivity. The experimenta1 drying curves did not show enough evidence on the effect of drying temperature on the average moisture diffusivity.     

A Method of Determination of Concentration Dependent Effective Moisture Diffusivity

ABSTRACT

A method based on Fourier series solution to Fick's diffusion equation has been proposed to evaluate effective diffusivity (D) as a function of moisture content in agricultural materials undergoing shrinkage during drying process. The shrinkage kinetics of the particulate was used to correlate its instantaneous size (spherical equivalent diameter) as a function of material moisture content A computer program was used to evaluate D based on shrinkage kinetics and experimental drying data and relate it to moisture content. The method was used to obtain moisture diffusivity data for thin layer drying of grape and corn.     

冷冻处理氢氧化铝污泥干燥过程水分有效扩散系数

实验测定了有无进行冷冻处理的工业铝厂氢氧化铝污泥在恒温降速干燥阶段的干燥速率曲线 ,并分别给出了它们在降速干燥过程中水分的有效扩散系数及其经验关联式 ,最后对二者进行了比较和分析。显然 ,冷冻后水分有效扩散系数比冷冻前大约增加了 4 3倍 ,系由于冷冻使污泥粒子粗大化和多孔化所造成的结果。     

Estimation of Effective Moisture Diffusivity of Okra for Microwave Drying

The effect of microwave output power and sample amount on effective moisture diffusivity were investigated using microwave drying technique on round okra (Hibiscus esculentus L.). The various microwave output powers ranging from 180 to 900 W were used for the determination of effective moisture diffusivity for constant sample amount of 100 g okra. To examine the effect of sample amount on effective moisture diffusivity, the samples in the range of 25-100 g were dried at constant microwave output power of 360 W. By increasing the microwave output powers and decreasing the sample amounts, the effective moisture diffusivity values ranged from 20.52 × 10^-10 to 86.17 × 10^-10 and 34.87 × 10^-10 to 11.91 × 10^-9 m²/s^-1, respectively. The modeling studies were performed to illustrate the relationship between the ratio of the microwave output power to sample amount and effective moisture diffusivity. The relationship between drying constant and effective moisture diffusivity was also estimated.     

Estimation of Effective Moisture Diffusivity of Okra for Microwave Drying

The effect of microwave output power and sample amount on effective moisture diffusivity were investigated using microwave drying technique on round okra (Hibiscus esculentus L.). The various microwave output powers ranging from 180 to 900 W were used for the determination of effective moisture diffusivity for constant sample amount of 100 g okra. To examine the effect of sample amount on effective moisture diffusivity, the samples in the range of 25–100 g were dried at constant microwave output power of 360 W. By increasing the microwave output powers and decreasing the sample amounts, the effective moisture diffusivity values ranged from 20.52 × 10^?10 to 86.17 × 10^?10 and 34.87 × 10^?10 to 11.91 × 10^?9 m²/s^?1, respectively. The modeling studies were performed to illustrate the relationship between the ratio of the microwave output power to sample amount and effective moisture diffusivity. The relationship between drying constant and effective moisture diffusivity was also estimated.     

Influence of Dry Matter Content and Drying Conditions on Effective Diffusion Coefficient of Onion (Allium cepa,L.)

ABSTRACT

Demand for fresh and dehydrated onions has increased considerably over the last two decades. To meet this challenge new varieties possesing superior field characteristics have been developed whose dehydration characteristics differ markedly. In this work the influence of dry matter content in the raw onion and drying conditions on the effective diffusion coefficient is studied. In the falling rate period, the diffusion activation energy was found to be 33.9 kJ/mol, and the D_σ parameter (the effective diffusivity at high moisture contents) was found to be 5.0736×10^-5 m²/s. On the basis of measurements with seven different onion varieties (with raw dry matter content between 19.5 and 23.5 kg dry matter/kg fresh product) an exponential relationship was found between the effective diffusivity and dry matter content of the raw onion.     

An Analytical Model for Moisture Diffusion in Solid Objects During Drying

ABSTRACT

An analytical model is proposed to determine the moisture diffusivities in geometrical solid objects (namely, infinite slab, infinite cylinder, sphere) subjected to drying applications. The analysis for moisture diffusion is carried out on the basis of two imponant criteria: 0.l?Bi?100 and Bi>100. New correlations for the mots of the characteristic equations corresponding these objects are developed. For a drying application, using the similarity between cooling and drying profiles, drying coefficient and lag factor are obtained. The model is then verified using available data from literature.Results show that the presented model can be used to determine the moisture diffusion coefficients in such solid objects in a simple and accurate manner in a variety of drying applications.     

Modeling Microwave Vacuum Drying Kinetics and Moisture Diffusivity of Carrot Slices

Carrot slices of 3.5 mm thickness were dried in a laboratory microwave vacuum dryer at five different microwave power density levels of 2, 4.66, 7.33, 10, and 12.66 W/g and at three vacuum chamber pressure levels of 6.66, 19.98, and 33.3 kPa to 4–6% d.b. moisture content. Inside the dryer the sample holding plate was rotated with the speed of 4 rpm for uniform microwaves application. The drying rates were increased with the increase in microwave power density at all pressure levels and the Page model was found to be the most suitable model to predict the drying behavior of carrot slices at all process conditions. The Page model drying rate constant (k, min^?1) showed high correlation with microwave power density at constant pressure by a power law equation and showed a logarithmic relationship with the microwave power density and pressure. Similar to the drying rate constant, the average moisture diffusivity at constant pressure was found to be function of microwave power density by power law equation as well as was also dependent on the power density and pressure by a logarithmic relationship.     

Microwave Heat Treatment of Spinach: Drying Kinetics and Effective Moisture Diffusivity

The effect of microwave drying technique on moisture content, moisture ratio, drying rate, drying time, effective moisture diffusivity, and porosity of spinach (Spinacia oleracea L.) were investigated. By increasing the microwave output powers (180–900 W) and the sample amounts (25–100 g), the drying time decreased from 18 to 3.5 min and increased from 7.7 to 25 min, respectively. To determine the kinetic parameters, the drying data were fitted to various models based on the ratios of the differences between the initial and final moisture contents and equilibrium moisture content versus drying time. Among of the models proposed, Page's model gave a better fit for all drying conditions applied. The activation energy was calculated using an exponential expression based on Arrhenius equation. The relationship between the drying rate constant and effective moisture diffusivity was also estimated and gave a linear relationship.     

Microwave Heat Treatment of Spinach: Drying Kinetics and Effective Moisture Diffusivity

The effect of microwave drying technique on moisture content, moisture ratio, drying rate, drying time, effective moisture diffusivity, and porosity of spinach (Spinacia oleracea L.) were investigated. By increasing the microwave output powers (180-900 W) and the sample amounts (25-100 g), the drying time decreased from 18 to 3.5 min and increased from 7.7 to 25 min, respectively. To determine the kinetic parameters, the drying data were fitted to various models based on the ratios of the differences between the initial and final moisture contents and equilibrium moisture content versus drying time. Among of the models proposed, Page's model gave a better fit for all drying conditions applied. The activation energy was calculated using an exponential expression based on Arrhenius equation. The relationship between the drying rate constant and effective moisture diffusivity was also estimated and gave a linear relationship.     

Modeling Microwave Vacuum Drying Kinetics and Moisture Diffusivity of Carrot Slices

Carrot slices of 3.5 mm thickness were dried in a laboratory microwave vacuum dryer at five different microwave power density levels of 2, 4.66, 7.33, 10, and 12.66 W/g and at three vacuum chamber pressure levels of 6.66, 19.98, and 33.3 kPa to 4-6% d.b. moisture content. Inside the dryer the sample holding plate was rotated with the speed of 4 rpm for uniform microwaves application. The drying rates were increased with the increase in microwave power density at all pressure levels and the Page model was found to be the most suitable model to predict the drying behavior of carrot slices at all process conditions. The Page model drying rate constant (k, min^-1) showed high correlation with microwave power density at constant pressure by a power law equation and showed a logarithmic relationship with the microwave power density and pressure. Similar to the drying rate constant, the average moisture diffusivity at constant pressure was found to be function of microwave power density by power law equation as well as was also dependent on the power density and pressure by a logarithmic relationship.     

The Effect of Stress Whitening on Moisture Diffusion in Thermosetting Polymers

The effects of stress whitening on the moisture diffusion rate and concentration in a polymer adhesive containing a secondary phase were investigated. This was accomplished by performing an absorption test on both stress whitened and virgin samples of the bulk adhesive and comparing the rate and amount of moisture diffusion in each. The presence of stress whitening in samples was not only observed visually, but also confirmed analytically using the “Bilinear RAMOD-2” equation. Experimental results reveal that visibly-present stress whitening resulting from fracture does indeed affect the rate and amount of moisture absorption in a polymer adhesive. Consequently, a diffusion model representing two different regions, stress-whitened and non-stress-whitened, is proposed for path of diffusion in polymer adhesives.     

The Effect of Stress Whitening on Moisture Diffusion in Thermosetting Polymers

The effects of stress whitening on the moisture diffusion rate and concentration in a polymer adhesive containing a secondary phase were investigated. This was accomplished by performing an absorption test on both stress whitened and virgin samples of the bulk adhesive and comparing the rate and amount of moisture diffusion in each. The presence of stress whitening in samples was not only observed visually, but also confirmed analytically using the “Bilinear RAMOD-2” equation. Experimental results reveal that visibly-present stress whitening resulting from fracture does indeed affect the rate and amount of moisture absorption in a polymer adhesive. Consequently, a diffusion model representing two different regions, stress-whitened and non-stress-whitened, is proposed for path of diffusion in polymer adhesives.     

Effect of Ultrasound Pretreatment on Vacuum Drying of Chinese Catalpa Wood

Ultrasound pretreatment of wood prior to drying was examined as a method to increase the effective water diffusivity, reduce drying time, and improve product quality of Chinese Catalpa wood. Pretreatment tests were carried out at three pretreatment durations, three absolute pressure levels, and three ultrasonic intensities. All specimens were then dried at 60°C and the absolute pressure level of 0.02 MPa to determine the effects of pretreatment parameters on vacuum drying characteristics. A microscopic analysis was carried out to visualize the formation of microchannels and view any other changes to wood tissue structure that occurred. Results showed that ultrasound pretreatment prior to vacuum drying enhances the effective water diffusivity; the higher the ultrasound power level, the longer the pretreatment time, and the lower the absolute pressure, the shorter is the drying time. Ultrasound creates micro channels within the tissue of wood during pretreatment. However, the pretreatment time should not be too long when the ultrasound is high.     

Reverse Methodology to Identify Moisture Diffusivity During Air-Drying of Foodstuffs

The effective moisture diffusion is a key parameter for the simulation of the drying process of food products. A conventional procedure includes an identification step performed on thin-layer drying kinetics followed by a validation step on deep-bed drying kinetics. Unfortunately, this approach requires several experiments at different air temperatures while only using moisture data is accounted for identification purposes. For this reason, a reverse approach was developed. Results showed that identification on a single deep-bed kinetic allows for valid simulations of thin-layer kinetics on a wide range of temperatures. In addition, two correlations for moisture diffusivity are tested.     

生物材料有效水分扩散系数的模型与测定

对振动流化床干燥过程中含有白蛋白的多孔混合颗粒的有效水分扩散系数进行了实验观察,建立了基于费克第二定律的数值模型。此模型可通过颗粒水分含量、体积孔隙率和温度预测有效水分扩散系数。结果表明,颗粒水分含量和孔隙结构特性对水分扩散系数有显著影响。在热力干燥过程中,特别是在干燥气流速度较高的情况下,一些生物材料的外部传质阻力很小以致水分传递只受物料内部的水分扩散控制,此时不应该忽略颗粒水分含量和孔隙结构对有效水分扩散系数的影响。     

Modeling of Moisture Diffusion in Microwave Drying of Hardwood

A one-dimensional mathematical model was developed to predict temperature and moisture content profiles in red maple (Acer rubrum L.) and white oak (Quercus alba) during microwave drying. The model was solved using the finite element analysis with MATLAB software. The predictions for temperature and moisture content agreed favorably well with the experimental data. The diffusion coefficients of the red maple and the white oak in microwave drying conditions were calculated and analyzed. Equations of the diffusion coefficient in longitudinal and transverse directions based on input microwave power level are presented in this article. In microwave drying of hardwood, the red maple was heated more efficiently than the white oak because of higher absorbing efficiency of the microwave power.     

Modeling of Moisture Diffusion in Microwave Drying of Hardwood

A one-dimensional mathematical model was developed to predict temperature and moisture content profiles in red maple (Acer rubrum L.) and white oak (Quercus alba) during microwave drying. The model was solved using the finite element analysis with MATLAB software. The predictions for temperature and moisture content agreed favorably well with the experimental data. The diffusion coefficients of the red maple and the white oak in microwave drying conditions were calculated and analyzed. Equations of the diffusion coefficient in longitudinal and transverse directions based on input microwave power level are presented in this article. In microwave drying of hardwood, the red maple was heated more efficiently than the white oak because of higher absorbing efficiency of the microwave power.     

Reducing wood drying time by application of ultrasound pretreatment

Industrial wood is typically dried artificially after felling. The drying process is often very lengthy and consumes a large amount of energy, however, and inappropriate drying schedule can cause defects in the wood. Pretreatments such as ultrasound show the potential to reduce wood drying time without impacting quality. This study investigated the influence of ultrasound pretreatment on vacuum drying behavior, hydroxyl content, and microstructural properties of fast-growing wood samples. Pretreatments were performed at ultrasound intensity of 10 W cm^?2 and frequencies of 28 and 40 kHz, respectively, for 30, 60, and 90 min, respectively, then pretreated samples were dried at 40°C temperature under absolute pressure of 0.08 MPa. Results showed that ultrasound pretreatment did shorten the wood vacuum drying time, increase effective water diffusivity, open water channels, decompose wood extractives, and decrease hydroxyl content. In effect, ultrasound pretreatment can be applied to successful reduce wood drying time.     

温度和相对湿度对褐煤干燥动力学特性的影响

为确定褐煤干燥最佳工艺参数,在恒温恒湿热风干燥实验台上进行了温度为50～90℃,相对湿度为10％～30％条件下的褐煤颗粒热风干燥实验.将实验数据与9种经验、半经验模型进行拟合,利用相关系数、方差、均方根误差3种统计学参数对不同模型进行评价.结果表明:Page模型相比其他模型更适合用于描述干燥过程中褐煤颗粒水分随时间的变...