Solidification by Convective Drying of Particle Layer Wetted with Dilute Agar Gel

Solidified porous slab is formed through convective drying of glass particle layer wetted with aqueous dilute agar gel. Measured critical mean moisture content increases with increasing initial moisture or agar content. The agar gel moves in viscous flow caused by capillary pressure during drying. A new drying model based on the receding evaporation plane model is proposed. Drying period is divided into surface and internal evaporation periods. Wet slab consists of dried and wet zones during the internal evaporation period, while the wet slab consists of wet zone only during the surface evaporation period. In the new model, the evaporation rate from the wet zone in the internal drying period is estimated with the linear driving force (LDF) approximation in the field of adsorption engineering. Critical moisture content, that is, mean moisture content between the surface and internal periods, is estimated with a mass balance on the surface. Simulated results by the new drying model with reasonable fitting parameters agree very well with measured drying data.     

Measurement of Absorbed Power of Glass Particle Layer on Moisture Content and Drying Rate by Combined Convective and Microwave Drying

Dependency of absorbed power by microwave on the local moisture content in a glass particle layer was measured with a new method; that is, heating the wet layer. The heating experiment was performed using a laboratory-scale combined convective and microwave heater/dryer that was manufactured by modifying a domestic microwave oven at 2.45 GHz. The measured result was strongly dependent on the local moisture content and showed a maximum and a minimum within the measured range of the moisture content. This dependency can be explained by the assumption that moisture in the wet layer behaves as a mass of the free water. The combined drying rate of the wet layer measured with the heater/dryer was simulated with both the power dependency and the experimental convective-only drying rate. Power dependency on temperature is as important as the moisture content in the simulation. Simulated results agree very well with experimental ones.     

The Spray Drying of Food Flavors

Abstract

Spray drying is the most commonly used technique for the production of dry flavorings. In spray drying, an aqueous infeed material (water, carrier, and flavor) is atomized into a stream of hot air. The atomized articles dry very rapidly, trapping volatile flavor constituents inside the droplets. The powder is recovered via cyclone collectors. Flavor retention is quite satisfactory if dryer operating parameters are properly chosen. Flavor retention is maximized by using a high infeed solids level, high viscosity infeed, optimum inlet (160–210°C) and high exit (>100°C) air temperatures, and high molecular weight flavor molecules. The shelf life of oxidizable flavor compounds is strongly influenced by the flavor carrier.     

Simulation of Convective Drying of Wet Porous Materials

Abstract

A simulation model for convective drying of wet porous materials was developed. For the simulation, we measured the moisture diffusivities within them and applied a modified Dubinin-Astakhov equation to the moisture sorption data for a membrane filter. The simulation results not accounting for internal mass transfer resistance were quite different from the experimental ones. The drying characteristics calculated by a shrinking core model with effective moisture diffusivity represented a much lower drying rate and much higher temperatures, respectively, than the experimental ones. This meant that we must consider the plural moisture transport mechanisms within the samples. Therefore, we calculated the drying rate and temperatures with an apparent overall mass transfer coefficient damping with a decrease in the moisture content. The results accounting for the hygroscopic effects broadly agreed with the experimental ones by the evaluation.     

Simulation of Convective Drying of Wet Porous Materials

A simulation model for convective drying of wet porous materials was developed. For the simulation, we measured the moisture diffusivities within them and applied a modified Dubinin-Astakhov equation to the moisture sorption data for a membrane filter. The simulation results not accounting for internal mass transfer resistance were quite different from the experimental ones. The drying characteristics calculated by a shrinking core model with effective moisture diffusivity represented a much lower drying rate and much higher temperatures, respectively, than the experimental ones. This meant that we must consider the plural moisture transport mechanisms within the samples. Therefore, we calculated the drying rate and temperatures with an apparent overall mass transfer coefficient damping with a decrease in the moisture content. The results accounting for the hygroscopic effects broadly agreed with the experimental ones by the evaluation.     

Modeling of Banana Convective Drying by the Drying Characteristic Curve (DCC) Method

Experimental convective drying tests of banana have been carried out for different air conditions to show the influence of air temperature, absolute humidity and speed on the drying rate. The analysis of the drying rate evolution as a function of product water content enables the identification of fourth drying phases: temperature rising (phase 1), exponentially decreasing drying rate (phase 2), linearly decreasing drying rate (phase 3) and very low drying rate (phase 4). The temperature rising phase 1 being very short and the last phase 4 being not reached during typical drying, the drying characteristic curve (DCC) has been represented by two different mathematical functions fitting phases 2 and 3. Their parameters have been determined by minimization of the quadratic errors between experimental and theoretical curves. It leads to a unique curve (the DCC) representing all air drying conditions the integration of which enables the calculation of the product water content with a maximum error of 0.09 between experimental and simulated values.     

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.     

High-Frequency Heating Controlled by Convective Hot Air: Toward a Solution for On-Line Drying of Softwoods

This article explores the potential of high-frequency heating combined with convective hot and moist air to dry softwood boards as fast as possible while maintaining a reasonable product quality. High-frequency drying experiments were performed in a multipurpose laboratory prototype. This device and the data logging equipment are briefly described in this article. The results obtained at different HF powers on Abies grandis board are presented and discussed. Our results prove that it is possible to dry a 50-mm-thick board from 150 to 5% in about 10 h while maintaining a good final quality by using air flow at 90°C for both dry-bulb and dew-point temperatures and an average HF power of 77 kW·m^?3.     

High-Frequency Heating Controlled by Convective Hot Air: Toward a Solution for On-Line Drying of Softwoods

This article explores the potential of high-frequency heating combined with convective hot and moist air to dry softwood boards as fast as possible while maintaining a reasonable product quality. High-frequency drying experiments were performed in a multipurpose laboratory prototype. This device and the data logging equipment are briefly described in this article. The results obtained at different HF powers on Abies grandis board are presented and discussed. Our results prove that it is possible to dry a 50-mm-thick board from 150 to 5% in about 10 h while maintaining a good final quality by using air flow at 90°C for both dry-bulb and dew-point temperatures and an average HF power of 77 kW·m^-3.     

Measurement of Shrinkage and Cracks Associated to Convective Drying of Soft Materials by X-ray Microtomography

Traditionally, the measurement of shrinkage occurring during drying is performed by destructive or poorly accurate techniques such as volume displacement methods. Cracks detection and quantification are realised either by destructive techniques or sophisticated but expensive nondestructive ones (NMR imaging). X-ray microtomography in combination with image analysis provides an accurate, nondestructive and easy to use technique to determine simultaneously shrinkage and crack extent. Results reported in this article concern drying of wastewater sludges whose management will become a real challenge in the years to come. These results show a clear relation between drying kinetics and crack development. This could be related to the development of internal diffusional limitations inducing moisture gradients and mechanical stresses leading to cracks formation.     

Measurement of Shrinkage and Cracks Associated to Convective Drying of Soft Materials by X-ray Microtomography

Abstract

Traditionally, the measurement of shrinkage occurring during drying is performed by destructive or poorly accurate techniques such as volume displacement methods. Cracks detection and quantification are realised either by destructive techniques or sophisticated but expensive nondestructive ones (NMR imaging). X-ray microtomography in combination with image analysis provides an accurate, nondestructive and easy to use technique to determine simultaneously shrinkage and crack extent. Results reported in this article concern drying of wastewater sludges whose management will become a real challenge in the years to come. These results show a clear relation between drying kinetics and crack development. This could be related to the development of internal diffusional limitations inducing moisture gradients and mechanical stresses leading to cracks formation.     

Effect of Osmotic Dehydration with Pulsed Vacuum on Hot-Air Drying Kinetics and Quality Attributes of Cherry Tomatoes

The effects of osmotic dehydration (OD) with or without pulsed vacuum (PV) on hot-air drying kinetics and quality attributes of cherry tomatoes were investigated. Both OD and PVOD pre-treatments were performed for 3 h at 50°C in 50 and 70^o Brix sucrose solutions with a solution-to-fruit mass ratio of 4:1, and PVOD was applied for 15 min before OD at atmospheric pressure. Samples were further dried at air temperature of 70°C. Effective moisture diffusivity (D _eff) of osmotically dehydrated samples increased gradually while the D_eff curve of fresh samples had a plateau stage during hot air drying. Lower glass transition temperature, T_g, values of osmotically dehydrated samples indicated that they needed a lower storage temperature. Both OD and PVOD pre-treatments had advantages in shortening drying cycles and improving quality of products. Compared with air drying, osmo-air drying decreased the total drying time, color change, and hardness of dried samples by 32.26%, 18.11%, and 88.21%, respectively, and increased volume ratio and vitamin C retention rate by 72.31% and 125.82%. As compared with OD, PVOD decreased color change and hardness by 28.48% and 45.17%, increased volume ratio and vitamin C retention rate by 27.41% and 17.77%, but there was no significant difference shown in drying time. Therefore, osmotic pre-treatment can shorten the total dehydration time, and improve the general quality of dried cherry tomatoes.     

微波干燥法测定过磷酸钙中的水分

提出用微波干燥法测定过磷酸钙中水分的含量,测定的精密度较高,操作简便,条件容易控制,分析速度快,RSD=(n=6)为1.23%。经多次试验证明,采用本法测定结果满意。     

不同干燥方式制备白果粉的性能比较

天然产物白果会因干燥方式的不同,而使得干燥白果粉性能有所差别。本研究分别采用喷雾干燥(SP)与真空冷冻干燥(FD)对白果浆进行干燥处理。然后对两种干燥白果粉的性能(含水量、颗粒度、色泽、蛋白质质量分数)进行对比来比较各自的优越性。喷雾干燥采用参数:风量120 m³/h,进料浓度8%(质量分数),进气温度205℃,出气温度100℃,雾化器转速32000 r/min。真空冷冻干燥先在-40℃下预冻4h后,再在真空度300 Pa下-50℃低温干燥48 h。结果发现含水量SP相似文献   

Development of SAPGD—A Simulation Software Regarding Grain Drying

Properties of air are interrelated in a drying process. Equations of psychrometrics and fluid dynamics developed by previous researchers enable the development of simulation models, which become an indispensable tool for the study of drying technology. A Windows based software, entitled SAPGD (Simulation of Air Properties and Grain Drying), written in MATLAB, was developed to simulate the equations/models related to the following four aspects:

1. The equilibrium moisture content of grains under the diverse conditions of atmosphere.
2. The pressure drop to airflow of various grains.
3. The relationship between drying time and moisture content under the model of thin layer drying of grains.
4. The relationships among drying time, moisture content, grain depth, and grain temperature under the logarithmic model of deep bed drying of grains.

     

呼伦贝尔褐煤等温干燥过程

以水分含量高达40 %的呼伦贝尔褐煤煤粉为实验物料,利用热重法研究了等温条件下的干燥特性,对比分析了温度、颗粒粒度对干燥过程的影响。实验结果表明,呼伦贝尔褐煤干燥过程具有明显的恒速干燥阶段,其特征与干燥温度和自由水含量密切相关。获得了呼伦贝尔褐煤煤粉在不同温度和粒度条件下的临界含水量(MC_cr)和平衡含水量(MC_eq),发现随着颗粒粒度的减小,褐煤煤粉干燥速率略有增大,但当粒度较小时,粒度因素对干燥过程的影响不明显。MC_cr与MC_eq均随温度的升高而减小,而随着颗粒粒度的减小,MC_cr略有降低,MC_eq略有增大。实验结果可以为褐煤制粉干燥工艺条件的选择和优化提供依据。     

Comparison of Drying Rate Curves of Porous Solids in Superheated Steam to Those in Air

ABSTRACT

Drying of porous solids was experimentally investigated in superheated steam as well as in air. Drying rate curves, including critical moisture contents, in steam at subatmospheric pressure, were compared to those for air at atmospheric pressure; moreover, they were compared to those for steam at atmospheric pressure as well. The former comparison was carried out under conditions of sample temperatures of 41.8–42.5°C (which were nearly equal to saturation temperatures of 42.1–42.2°C at pressures of 8.23–8.30 kPa) for the constant rate period in steam and the corresponding sample temperatures of 42.0–45.0°C (which were close to the wet-bulb temperatures) for the constant rate period in air. There were distinct differences between normalized drying rate curves, including critical moisture contents in steam and in air at the above similar sample temperatures for materials of baked clay, firebrick, and cemented glass balloons over the minimum value of 8.3 × 10^?3 µm and up to the maximum value of 1.2 × 10² µm in cumulative pore-size distributions: longer constant rate periods and lower critical moisture contents in steam than in air, and higher drying rates in steam than in air for the falling rate period. Moreover, the latter comparison of the drying rates in steam at subatmospheric pressure to those in steam at atmospheric pressure revealed that the differences in normalized drying rates between subatmospheric pressure and atmospheric pressure were small for both materials under mild external conditions. These findings were common to the baked clay, firebrick, and cemented glass balloons over a wide range of pore-size distributions studied in the present work, as well as sintered coarse glass beads as previously reported.     

Comparison of Drying Rate Curves of Porous Solids in Superheated Steam to Those in Air

Drying of porous solids was experimentally investigated in superheated steam as well as in air. Drying rate curves, including critical moisture contents, in steam at subatmospheric pressure, were compared to those for air at atmospheric pressure; moreover, they were compared to those for steam at atmospheric pressure as well. The former comparison was carried out under conditions of sample temperatures of 41.8-42.5°C (which were nearly equal to saturation temperatures of 42.1-42.2°C at pressures of 8.23-8.30 kPa) for the constant rate period in steam and the corresponding sample temperatures of 42.0-45.0°C (which were close to the wet-bulb temperatures) for the constant rate period in air. There were distinct differences between normalized drying rate curves, including critical moisture contents in steam and in air at the above similar sample temperatures for materials of baked clay, firebrick, and cemented glass balloons over the minimum value of 8.3 × 10^-3 µm and up to the maximum value of 1.2 × 10² µm in cumulative pore-size distributions: longer constant rate periods and lower critical moisture contents in steam than in air, and higher drying rates in steam than in air for the falling rate period. Moreover, the latter comparison of the drying rates in steam at subatmospheric pressure to those in steam at atmospheric pressure revealed that the differences in normalized drying rates between subatmospheric pressure and atmospheric pressure were small for both materials under mild external conditions. These findings were common to the baked clay, firebrick, and cemented glass balloons over a wide range of pore-size distributions studied in the present work, as well as sintered coarse glass beads as previously reported.     

Formation of Particle Layer Within Coated Slurry Characterized by Thickness Variation

An understanding of the conversion process from slurry to particle layer on a substrate is required for the precise control of the particle alignment and the material distribution in the coated slurry. In this work, variation of coated slurry thickness during drying was applied to evaluate drying and particle layer formation simultaneously. The slurry used consisted of micron-sized silica or poly (methyl methacrylate) particles and an aqueous solution of poly-vinyl alcohol (PVA) possessing differing degrees of hydrolysis. During the drying process, initially the thickness of the coated slurry was observed to decrease at a constant rate in the concentration stage, and subsequently it began to show large fluctuations due to the emergence of particles on the drying surface in the packing stage. In the final fixing stage, the fluctuation of film thickness was restricted because particles were immobilized by highly viscous concentrated PVA or by PVA molecule bridging. Based on the variation and fluctuation of film thickness, we introduced two characteristic dimensionless time ratios: (a) void fraction in a packed particle layer at the end of concentration stage; and (b) the time required to fix particle position after the end of the packing stage. We concluded that the dispersed state and settling velocity of the particle determines the space between particles in a loose packing layer, and we found that the distribution of polymers in a particle layer has a strong influence on the mobility of particles in a tightly packed layer.