Modelling the mass transfer during convective, microwave and combined microwave-convective drying of solid slabs and cylinders

In the present work, the mass transfer characteristics, namely moisture diffusivity and moisture transfer coefficient, of slab and cylindrical potato samples were evaluated by adopting the analytical model proposed by Dincer and Dost [Drying Technology, 13,1/2 (1995), 425]. As part of the experimental work, the moisture contents of slab and cylindrical potato samples dried under convective, microwave and combined convective–microwave conditions (air temperature, 30–60 °C; air velocity, 1–2 ms⁻¹; microwave output power, 30–650 W) were measured. From the data collected the drying coefficients and lag factors were calculated and incorporated into the model. The results showed a reasonably good agreement between the values predicted from the correlation and the experimental observations; average error between experimental and predicted results less that 10%. The experimental system exhibited mass transfer Bi numbers in the range 0.038–11.4, thus indicating the presence of finite internal and external resistances. Moisture diffusivity values in the range 0.13×10⁻⁸ to 24.22×10⁻⁸ m²s⁻¹ were calculated.     

柠檬热风干燥特性及数学模型

以新鲜柠檬为原料,研究其热风干燥特性及数学模型。以柠檬片厚度、热风温度、热风风速为因素,分析其对柠檬热风干燥特性的影响,建立柠檬热风干燥的干燥特性曲线、干燥速率曲线,并利用SAS8.0软件对试验数据进行拟合,构建柠檬热风干燥数学模型。结果表明:热风温度、柠檬片厚度对柠檬热风干燥的速率有较大影响,而热风风速对干燥速率的影响较小;柠檬热风干燥符合Page模型。     

Drying kinetics and effective moisture diffusivity of bamboo shoot slices undergoing microwave drying

Effect of microwave power on moisture content, moisture ratio, drying rate, drying time and effective moisture diffusivity (D_eff) of bamboo shoot was investigated using microwave drying. To study the effect of microwave power on drying, bamboo shoot samples (250 g) were dried at different power levels ranging from 140 to 350 W. To determine the kinetic parameters, drying data were fitted to various models based on the ratios of differences between initial and final moisture contents and equilibrium moisture content. Among the models proposed, Wang and Singh model gave a better fit for all drying conditions used. By increasing microwave output power, the D_eff values increased from 4.153 × 10^?10 to 22.835 × 10^?10 m² s^?1. A third‐order polynomial relationship was found to correlate the D_eff with moisture content. Further scope of this research work would include the effect of certain factors (shrinkage, case hardening, distortion of product and shape of bamboo shoot samples as an infinite slab) of practical significance to improve the model.     

Experimental study on drying of pear slices in a convective dryer

The experiments were conducted on pear slices with thickness of 5 mm at temperatures of 50, 57, 64 and 71 °C with an air velocity of 2.0 m s^?1. Prior to drying, pear slices were pretreated with citric acid solution (0.5% w/w, 1 min, 20 °C) or blanched in hot water (1 min, 85 °C). Also, the untreated samples were dried as control. The shortest drying time of pear slices was obtained with pretreatment with citric acid solution. It was observed that whole drying process of pear slices took place in a falling rate period. Four mathematical models were tested to fit drying data of pear slices. According to the statistical criteria (R², χ² and RMSE), the Midilli et al. model was found to be the best model to describe the drying behaviour of pear slices. The effective diffusivity of moisture transfer during drying process varied between 8.56 × 10^?11 and 2.25 × 10^?10 m² s^?1, while the activation energy of moisture diffusion in pear slices was found to be 34.95–41.00 kJ mol^?1.     

Prediction of moisture transfer parameters for microwave drying of lactose powder using Bi–G drying correlation

The mass transport properties characterising the drying of lactose powder were determined using a correlation proposed by Dincer and Hussain [Dincer, I. & Hussain, M. M. (2004). Development of a new Bi–Di correlation for solids drying. International Journal of Heat and Mass Transfer, 47, 653–658]. Experimental moisture content data for lactose samples dried under convective, microwave, combined convective-microwave and combined vacuum-microwave conditions were collected. The drying coefficients and lag factors were calculated from the experimental data and incorporated into the model. The Bi numbers were in the range 0.185–439, and moisture diffusivities and diffusion coefficients in the range from 0.135 × 10⁻⁹ to 102 × 10⁻⁹ and from 0.194 × 10⁻⁶ to 118 × 10⁻⁶ m/s, respectively. The predicted moisture profiles showed adequate agreement with the experimental observations, with the average error between experimental and predicted results being ±13.7%.     

白萝卜薄层热风干燥特性及其数学模型

以新鲜白萝卜为原料,研究在不同的热风温度、热风风速和切片厚度条件下,白萝卜的热风干燥特性。通过试验数据拟合,比较7种数学模型在白萝卜热风干燥过程中的适用性。结果表明:白萝卜热风干燥以降速过程为主,无明显的恒速阶段。干燥温度、切片厚度对白萝卜的干燥速率影响较大,风速影响较小。干燥温度越高、切片厚度越薄、风速越快,干燥用时越短。通过比较各模型的相关系数(R~2)、卡方值(χ~2)和均方根误差(RMSE),结果显示Page模型的拟合效果最好,该模型的R~2为0.997 6、χ~2为2.615×10~(-4)、RMSE为0.014 6。且用模型外的试验数据进行验证,也表现出较好的拟合度。白萝卜的有效水分扩散系数(Deff)为7.560×10~(-10)~2.130×10~(-9),随着干燥温度、风速和切片厚度的增加而增大。白萝卜的干燥活化能为26.34kJ/mol。此外,还对白萝卜片干燥前后的色差进行了测定和分析,结果表明:在50~80℃时,随着温度的增加,干燥成品的L~*值逐渐降低,而b~*、a~*以及总色差ΔE~*值呈升高的趋势。     

Heat and mass transfer in combined convective and far-infrared drying of fruit leather

Combined convective and far-infrared drying is a challenging assignment due to complex relationship between heat and mass transfer. In this paper, heat and mass transfer of fruit leather drying with combination of hot air and far-infrared has been carried out. The heat and the mass transfer coefficients were analyzed by heat–mass analogy. It could be found that the ratio between heat and mass transfer coefficients for the combination technique could not be obtained from the heat–mass analogy classical model and a modification is needed. The modified correlations for predicting ratio of heat and mass transfer coefficients and the heat transfer coefficient in term of heat transfer Nusselt number are developed. The model could fit the experimental data quite well within ±10% deviation.     

哈密瓜切片热风干燥特性及数学模型

目的:提高规模化生产的哈密瓜品质,缩短干燥周期。方法:以不同漂烫时间(0.5,1.0,1.5,2.0,2.5 min)、浸渍液(0.1%,0.2%,0.3%,0.4%,0.5%柠檬酸溶液)预处理哈密瓜切片,并分别研究不同热风温度(35,45,55,65,75℃)、热风速度(0.5,1.0,1.5,2.0,2.5 m/s)和切片厚度(2,4,6,8,10 mm)条件下的哈密瓜切片热风干燥特性和水分扩散系数,拟合不同薄层干燥数学模型。结果:0.4%柠檬酸预处理后得到品质最优的干制产品,热风温度和切片厚度对切片干燥影响较为显著,哈密瓜切片无恒速干燥阶段,有效水分扩散系数为1.1348×10^-7~4.9080×10^-7 m²/s,活化能为28.15 kJ/mol。结论:哈密瓜切片的最佳热风干燥工艺为热风温度55℃、热风速度2.0 m/s、切片厚度6 mm,Page模型具有最高的R²值和最小的均方根误差,更适于评估和预测哈密瓜热风干燥的水分去除规律。     

A new solution approach for simultaneous heat and mass transfer during convective drying of mango

The present investigation is contemplated to fill a gap in analytical modelling of coupled heat and mass transfer during convective drying process. A transport model to describe the temperature and moisture evolutions of mango slab is established. The main innovation introduced in this study is represented with the procedure of temperature and moisture predictions. Mango fruit dehydration can be easily simulated with implementation of the present advanced analytical technique at different operating conditions. Moreover, the temperature and moisture history of mango slice are presented for varying values of the drying air factors counting temperature, velocity, relative humidity and initial food temperature. This work confirms that notable time can be saved without sacrificing accuracy by applying proposed model. This method is expected to be useful for fast and accurate drying simulation. The agreement between published experimental results and model predictions is remarkable and an accurate simulation of experimental drying curves is obtained.     

Modelling of air drying of fresh and blanched sweet potato slices

Effects of blanching, drying temperatures (50–80 °C) and thickness (5, 10 and 15 mm) on drying characteristics of sweet potato slices were investigated. Lewis, Henderson and Pabis, Modified Page and Page models were tested with the drying patterns. Page and Modified Page models best described the drying curves. Moisture ratio vs. drying time profiles of the models showed high correlation coefficient (R² = 0.9864–0.9967), and low root mean squared error (RMSE = 0.0018–0.0130) and chi‐squared (χ² = 3.446 × 10^–6–1.03 × 10^–2). Drying of sweet potato was predominantly in the falling rate period. The temperature dependence of the diffusion coefficient (D_eff) was described by Arrhenius relationship. D_eff increased with increasing thickness and air temperature. D_eff of fresh and blanched sweet potato slices varied between 6.36 × 10^–11–1.78 × 10^–9 and 1.25 × 10^–10–9.75 × 10^–9 m² s^–1, respectively. Activation energy for moisture diffusion of the slices ranged between 11.1 and 30.4 kJ mol^–1.     

虎杖冷冻、微波及冷冻微波联合干燥工艺研究

实验以虎杖为试材,分别对冷冻干燥、微波干燥及冷冻微波联合干燥进行实验,研究不同干燥工艺参数对干燥速率及干燥后产品品质的影响,并从微观结构角度研究并验证了虎杖干燥的转换点。结果表明:冷冻干燥的最佳工艺参数为:虎杖片厚度5 mm、真空度40 Pa、加热盘温度40℃;微波干燥的最佳温度为60℃;虎杖片冷冻微波联合干燥的最佳转换点为含水率25%,冷冻干燥至转换点再进行微波干燥至终点,所获得的虎杖片药效成分(虎杖苷、大黄素、白藜芦醇、大黄素甲醚)、体积保留率、复水比、色差与冷冻干燥产品接近;冷冻微波联合干燥工艺比冷冻干燥时间可缩短一半。      

微波泡沫干燥条件对树莓果浆干燥特性影响

为了研究微波泡沫干燥树莓果浆传热传质特性,以树莓为原料,采用中心组合实验设计方法,研究干燥条件对物料温度、含水率以及介电特性的影响规律,分析微波泡沫干燥过程中传热传质过程。结果表明:微波泡沫干燥树莓果浆过程中,干燥初期,介电常数与介电损耗因子均增大,使得物料吸收的微波能增大,果浆温度由室温上升至70℃,含水率无明显变化;干燥中期,介电常数与介电损耗因子先增大后减小,因此物料吸收的微波能呈先增大后减小趋势,果浆含水率由90%降至40%,温度变化不明显;干燥后期,介电常数与介电损耗因子均减小,物料吸收的微波能减少,果浆含水率缓慢降至15%左右,温度继续升高。微波泡沫干燥方法可有效提高高粘度果浆的干燥速度。