On The Drying of Food Products in a Tunnel Dryer

We present a dryer model for simulating the drying of hygroscopic-porous food products in a tunnel dryer. The model employs an improved receding-front formulation by taking into consideration the material volumetric shrinkage and the variation of the heat and mass transfer coefficient during drying. Predicted results show close agreement when compared with experimental data. We report a parametric analysis using the dryer model to study the drying transient and the need to cascade the drying process so as to maximise the drying potential of the air stream.     

Analysis and Optimization of Drying of Green Bricks in a Tunnel Dryer

One way to avoid the risk of condensation in a tunnel dryer for drying of green bricks is to use higher air flow rates. However, higher air flow rates accelerate the drying rate and thus cause damages. It is necessary to determine the optimal values of the hot and outdoor air mass flow rates and optimum stack temperature to avoid the condensation. Four different temperatures, (?5, 10, 20, and 30°C) and three different relative humidities (40, 60, and 80%) for the outdoor air were selected. Optimal operation parameters and the required stack temperature to avoid condensation for different cases were computed.     

CFD 在食品干燥过程及其干燥设备设计中的应用

CFD是通过计算机数值计算和图像显示以定量描述流场的数值解,从而对物理问题进行分析研究。CFD兼有理论性和实践性的双重特点,其主要用途是对流态进行数值仿真模拟计算,能够对流态的温度场、速度场、浓度场等进行有效的指导和预测。本文综述了CFD数值模拟在食品干燥过程中以及干燥设备设计中的应用和发展前景。     

Solar Drying of Peeled Longan Using a Side Loading Type Solar Tunnel Dryer: Experimental and Simulated Performance

This article presents experimental and simulated results of drying of peeled longan in a side-loading solar tunnel dryer. This new type of solar tunnel dryer consists of a flat-plate solar air heater and a drying unit with a provision for loading and unloading from windows at one side of the dryer. These are connected in series and covered with glass plates. A DC fan driven by a 15-W solar cell module supplies hot air in the drying system. To investigate the experimental performance, five full-scale experimental runs were conducted and 100 kg of peeled longan was dried in each experimental run. The drying air temperature varied from 32 to 76°C. The drying time in the solar tunnel dryer was 16 h to dry peeled longan from an initial moisture content of 84% (w.b.) to a final moisture content of 12% (w.b.), whereas it required 16 h of natural sun drying under similar conditions to reach a moisture content of 40% (w.b.). The quality of solar-dried product was also good in comparison to the high-quality product in markets in terms of color, taste, and flavor. A system of partial differential equations describing heat and moisture transfer during drying of peeled longan in this solar tunnel dryer was developed and this system of nonlinear partial differential equations was solved numerically by the finite difference method. The numerical solution was programmed in Compaq Visual FORTRAN version 6.5. The simulated results agreed well with the experimental data for solar drying. This model can be used to provide the design data and it is essential for optimal design of the dryer.     

组合干燥技术在聚碳酸酯干燥中的应用

使用气流干燥—真空回转干燥器组合方式对聚碳酸酯进行干燥,通过干燥实验,初步证明了组合干燥设备在干燥聚碳酸酯干燥上具有一定的可行性。     

WJF型流化床干燥机应用于热敏性物料的研究

介绍了WJF型流化床干燥机的结构特点、运行条件。实例应用表明，对于热敏性物料WJF型流化床干燥机比常规的其它方法更容易达到预期的效果。初始湿含量65％、最终湿含量8％、产品温度要求低于60℃的纤维素饲料酶，经过流化床干燥机干燥后，产品品质符合要求。     

The Drying of Sludge in a Cyclone Dryer Using Computational Fluid Dynamics

A control volume-based technique implemented in FLUENT (ANSYS Inc., Canonsburg, PA) computational fluid dynamics (CFD) package was applied along with the kinetic theory of granular flow (KTGF) to simulate the flow pattern and heat and mass transfer processes for sludge material in a large-scale cyclone dryer. The drying characteristics of sludge at the dryer inlet were obtained from a previous study on the drying of sludge in a large-scale pneumatic dryer. User-defined subroutines were added to extend FLUENT's capability to account for mixture properties and to simulate the constant and falling rate drying periods. The convective heat and mass transfer coefficients were modeled using published correlations for Nusselt and Sherwood numbers. Sensitivity analysis was conducted to determine the effect of gas-phase velocity and temperature on the final product outcome. Numerical predictions for the multiphase flow hydrodynamics showed a highly diluted region in the dryer core and a higher concentration of particles close to the wall region, an indication of nonuniform distribution of particles at a cross-sectional area. The numerical predictions for the hydrodynamic profiles qualitatively depicted the flow behavior natural to these designs. The work demonstrated the successful application of CFD in the design stage of a combined pneumatic-cyclone dryer model.     

Modelling Diffusion-Limited Drying Behaviour in a Batch Fluidized Bed Dryer

ABSTRACT

Fluidized bed dryers are often used to extract water from granular materials. When the drying process is mainly limited by the resistance against water transport inside the particle the drying behaviour is said to be diffusion-limited. In the literature there are several models that predict this drying process with very diverging results. In this study a model is set up to arrive at a better prediction for this drying process. The heat and mass transfer in the granular material and the drying air is described. The resulting equations are solved numerically. The model must be extended to incorporate the heat capacity of the dryer.     

Modelling Diffusion-Limited Drying Behaviour in a Batch Fluidized Bed Dryer

Fluidized bed dryers are often used to extract water from granular materials. When the drying process is mainly limited by the resistance against water transport inside the particle the drying behaviour is said to be diffusion-limited. In the literature there are several models that predict this drying process with very diverging results. In this study a model is set up to arrive at a better prediction for this drying process. The heat and mass transfer in the granular material and the drying air is described. The resulting equations are solved numerically. The model must be extended to incorporate the heat capacity of the dryer.     

Experiments on Hot-Air Drying of Wheat in a Semi-Technical Mixed-Flow Dryer

Although mixed-flow grain dryers are widely used, there is still a need to optimize the process control as well as the dryer apparatus. Fluctuations of the grain moisture content at the dryer entrance are still a major problem resulting in quality and economic losses due to under- or overdrying. Therefore, a mathematical model for heat and mass transfer in a mixed-flow dryer has been developed. Practical drying experiments were carried out at a semi-technical dryer test station that was operated quasi-continuous. The measurements reveal the complexity of the mixed-flow drying process. First predicted results are in satisfactory agreement with data.     

氟硅酸钠干燥采用振动流化床干燥器

磷肥工业中副产的氟硅酸钠大多采用气流干燥。介绍采用振动流化床干燥的工艺流程,风量、风压的配置以及操作要点,并指出存在8问题和处理方法。     

Comparison Between Drying of Timber in a Solar Dryer and in an Electrically-Heated Kiln

ABSTRACT

Solar dryers have been considered for timber drying in a number of countries because of the expected savings in drying costs. From a review of past works on solar, natural, and conventional drying it was observed that while solar dryers were able to dry timber faster compared to natural drying, the difference was only marginal in some instances. The drying rates are expected to be dependent upon ambient conditions in which the dryera are operated. Solar dryers would operate more efficiently in countries with low humidity than in tropical regions. Thus the thermal performance and also the economics of solar dryer is country dependent. In the present paper, a comparison of the drying rates obtained with a solar dryer is made with that obtained with an electrically operated drying kiln.     

GZQ型振动流化床干燥机在双氰胺干燥上的应用

介绍了ＧＺＱ振动流化床干燥机的性能、优点及该机在大氰胺干燥应用中取得的成效。     

Experiments on Hot-Air Drying of Wheat in a Semi-Technical Mixed-Flow Dryer

Although mixed-flow grain dryers are widely used, there is still a need to optimize the process control as well as the dryer apparatus. Fluctuations of the grain moisture content at the dryer entrance are still a major problem resulting in quality and economic losses due to under- or overdrying. Therefore, a mathematical model for heat and mass transfer in a mixed-flow dryer has been developed. Practical drying experiments were carried out at a semi-technical dryer test station that was operated quasi-continuous. The measurements reveal the complexity of the mixed-flow drying process. First predicted results are in satisfactory agreement with data.     

用于污泥干化的新型桨叶式干燥机的设计

针对污泥的特性及其干燥机理,对现有桨叶式干燥机的结构进行改进设计,使现有的桨叶式干燥机成为一种比较理想的污泥干化设备。     

Drying Efficiency of Indonesian Lignite in a Batch-Circulating Fluidized Bed Dryer

This research explores the production of low-moisture, high-rank coal using a batch-type, laboratory-scale, circulating fluidized bed to dry low-rank Indonesian coal with a high moisture content of 35 wt%. The operation was performed using air as a fluidization gas in a riser (a 4-m-tall pipe with an inner diameter of 0.04 m) at a gas velocity ranging from 2.0 to 2.7 m/s and a riser temperature of 80 to 150°C. The electric heaters were installed in the upper part of a downcomer to prevent the condensation of the evaporated moist- ure. The drying rate of the coal was investigated in terms of the inlet gas temperature, the gas velocity, and the drying time in order to determine the optimum operating conditions. Changes in the moist- ure content of the coal, before and after the experiments, were char- acterized by a proximate analysis, an ultimate analysis, the higher heating value (HHV), the lower heating value (LHV), a particle size analysis, and by the equilibrium moisture content. The results show that 70 to 80 wt% (wet basis, wb) of the total moisture can be reduced when the gas velocity of the riser is 2.0 m/s and the gas temperature is 150°C. In experiments, a simple mathematical model based on the heat and mass balances and a thin-layer drying model were simul- taneously used to predict the drying behavior of coal under the given operating conditions. The results of the model are similar to those of the experiment.     

Modeling of Coriander Seeds Drying in an Impingement Dryer

The aim of this work is to develop a mathematical model to estimate the batch drying curve of coriander seeds in an impingement dryer and to study the axial movement of a seed in a transparent prototype impingement dryer. The apparatus is a horizontal acrylic transparent cylinder with a slight slope to induce the axial and rotational movement of particles. Gas enters tangentially downwards through a narrow slot arranged all along the dryer, flows in a counterclockwise circular motion in the chamber—in crossflow with respect to the solids—and is discharged through an upper lengthwise expansion chamber. As a result of gas drag, the particles advance in a rotational-helicoidal motion between feed and discharge.

Velocity and temperature profiles for gas in 2D turbulent flow were simulated using commercial software from Fluent Inc.^{[ 1} Quispe , J.F. ; Canales , E.R. ; Bórquez , R.M. Simulation of turbulent flows in an impingement dryer by an extended κ-ε model . Computer Methods in Applied Mechanics and Engineering 2000 , 190 , 625 – 637 .[Crossref] , [Google Scholar], ⁴ Soto , V. ; Bórquez , R.M. Simulation of superheated steam turbulence flows and heat transfer in an impingement dryer . Drying Technology 2003 , 21 ( 2 ), 311 – 328 .[Taylor & Francis Online], [Web of Science ®] , [Google Scholar] ^] Maximum velocities are shown to be located close the walls; most of the gas is recirculated, and the rest is exhausted. It is assumed that particle trajectories also follow a circular motion near the walls, as observed in the transparent reproduction of the dryer operating with ambient air for small batch of solids and/or a single particle. Air velocities along this trajectory are estimated from the simulated flow field. Particle motion, heating, and drying along this path are described by unsteady momentum, heat, and mass balances when subjected to gas drag and gravity forces.

With respect to the axial trajectory of a coriander seed, for an inlet air velocity of 20 m/s at the slot the average experimental time for a complete circular cycle is 0.18 s and the simulated time is 0.21 s, whereas average experimental residence time is 1.53 s and the simulated time is 0.94 s. Differences between experimental results and simulations are due to air instability, leading to nonhomogeneous air velocity profiles along the equipment. The mathematical model is based on the assumption that air velocity profiles are homogeneous. Experimental observations indicate that the particle does not move along the equipment but sometimes moves backward (or erratically) or spins out advancing, due to an uneven air speed profile, and impacts against the wall. Finally, the drying model gives results that adjust to the batch experimental data, taking into account the deviations found with respect to the axial trajectory from a seed. This is because the model was devised exactly to predict the conduct of the system in batch operation for a particle bed, obtaining results that show the macrocospic response of the equipment (velocity and average temperature of the air). As it happens in this type of phenomenon, the drying rate in the constant period is a function of the adimensional Reynolds number.     

Crystallization and Drying of Milk Powder in a Multiple-Stage Fluidized Bed Dryer

The rationale of this study has been to use fluidized beds to crystallize amorphous spray-dried skim milk powders with multiple stages of processing at different temperatures and humidities with the aim of rapidly making mostly crystalline powders. This paper discusses the performance of a multiple-stage fluidized bed dryer, and a combination of crystallization of lactose in spray drying at high humidity (lactose nuclei formation) and subsequent fluidized bed drying. Two different combinations of spray dryer and multi-stage fluidized-bed dryer have been suggested to crystallize lactose in skim milk powder. The results show significant improvements in the crystallinity of the powders. Moisture sorption test and X-ray diffraction analysis were used to assess the crystallinity of the powders. The processed powders that were crystallized in a humid-loop spray drying combined with a two-stage fluidized-bed dryer/crystallizer showed 92% improvement in lower amorphicity by processing at different stages of 70°C, 50% RH and 80°C, 50% RH for 15 minutes. The conventionally spray-dried powders that were crystallized in a three-stage fluidized-bed dryer/crystallizer showed 87% improvement in lower amorphicity (less moisture sorption) by processing at different stages of 60°C, 50% RH; 70°C, 40% RH; and 80°C, 40% RH for 20 minutes. The multiple-stage fluidized bed system showed distinctive potential to crystallize lactose significantly in skim milk powder using an industrial-feasible process.     

Modeling of Coriander Seeds Drying in an Impingement Dryer

The aim of this work is to develop a mathematical model to estimate the batch drying curve of coriander seeds in an impingement dryer and to study the axial movement of a seed in a transparent prototype impingement dryer. The apparatus is a horizontal acrylic transparent cylinder with a slight slope to induce the axial and rotational movement of particles. Gas enters tangentially downwards through a narrow slot arranged all along the dryer, flows in a counterclockwise circular motion in the chamber—in crossflow with respect to the solids—and is discharged through an upper lengthwise expansion chamber. As a result of gas drag, the particles advance in a rotational-helicoidal motion between feed and discharge.

Velocity and temperature profiles for gas in 2D turbulent flow were simulated using commercial software from Fluent Inc.^[1,^4] Maximum velocities are shown to be located close the walls; most of the gas is recirculated, and the rest is exhausted. It is assumed that particle trajectories also follow a circular motion near the walls, as observed in the transparent reproduction of the dryer operating with ambient air for small batch of solids and/or a single particle. Air velocities along this trajectory are estimated from the simulated flow field. Particle motion, heating, and drying along this path are described by unsteady momentum, heat, and mass balances when subjected to gas drag and gravity forces.

With respect to the axial trajectory of a coriander seed, for an inlet air velocity of 20 m/s at the slot the average experimental time for a complete circular cycle is 0.18 s and the simulated time is 0.21 s, whereas average experimental residence time is 1.53 s and the simulated time is 0.94 s. Differences between experimental results and simulations are due to air instability, leading to nonhomogeneous air velocity profiles along the equipment. The mathematical model is based on the assumption that air velocity profiles are homogeneous. Experimental observations indicate that the particle does not move along the equipment but sometimes moves backward (or erratically) or spins out advancing, due to an uneven air speed profile, and impacts against the wall. Finally, the drying model gives results that adjust to the batch experimental data, taking into account the deviations found with respect to the axial trajectory from a seed. This is because the model was devised exactly to predict the conduct of the system in batch operation for a particle bed, obtaining results that show the macrocospic response of the equipment (velocity and average temperature of the air). As it happens in this type of phenomenon, the drying rate in the constant period is a function of the adimensional Reynolds number.