On The Drying of Food Products in a Tunnel Dryer

ABSTRACT

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。     

Investigation of Air Flow Distribution in Small-Scale Food Products Dryer

Experimental and numerical investigations showed that the air flow velocity distribution between the shelves with identical inlet cross sections is nonuniform; in the gaps between the shelves located in the bottom part of the dryer the velocity is rather high and in the gaps between the shelves located in the middle and at the top of the dryer the velocity is low. A nonuniform air flow distribution causes unequal drying of the product; therefore, the position of shelves with the products must be periodically changed; that is, operation of the dryer is not optimal. It was revealed during numerical investigations that the use of variable inlet cross sections between the shelves gives more uniform air flow distribution. Numerically obtained air flow velocity distribution results between the shelves with the use of variable inlet cross sections were also justified experimentally.     

Drying of Paddy in a Column Dryer

The variation of moisture content of paddy is experimentally studied in a column dryer. It is seen that the drying of the paddy in bulk in such a dryer can be represented by the drying of a single spherical particle.     

Drying of Paddy in a Column Dryer

The variation of moisture content of paddy is experimentally studied in a column dryer. It is seen that the drying of the paddy in bulk in such a dryer can be represented by the drying of a single spherical particle.     

藕粉在旋流干燥器中的干燥动力学研究

实验研究了藕粉在旋流干燥器中不同操作参数下(进风温度50～80℃、气速1.5～3.5 m/s、进料速率0.001 7～0.002 7 kg/s)的干燥速率,建立了藕粉在旋流干燥器中的干燥动力学模型:U=a+bX+cX~2+dX~3,并进行了模型验证。结果表明该经验模型在一定因素水平范围内具有比较好的精度,研究结果对旋流干燥工艺参数的选择和旋流干燥器的设计有一定的参考价值。     

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.     

Combined Crystallization and Drying in a Pilot-Scale Spray Dryer

Combined crystallization and drying of lactose solutions was performed in a pilot-scale spray dryer over a wide range of operating conditions. The effect of different parameters, including temperature, moisture content, atomizing air flow rate, liquid feed rate, main drying air flow rate, and particle size, on the degree of crystallinity of the spray-dried powders was analyzed. Water-induced crystallization (WIC) and modulated differential scanning calorimetry (MDSC) were used to assess the effect of these parameters on the degree of crystallinity of the spray-dried powders. The particles were characterized in terms of the final moisture content using WIC and distinctive differences in the peak heights, which are indicative of the particle crystallinity, were found for spray-dried particles using different drying conditions, supporting the results from MDSC. MDSC showed that decreasing the inlet air temperature by 40°C increased the degree of crystallinity in the particles threefold from 22 to 72%. A decrease in the inlet air temperature may decrease the particle temperature, resulting in wetter particles, and a lower temperature meant a longer particle drying time and allowed the particles to rearrange themselves into a more crystalline form. Up to 72% crystallinity is achievable in a pilot-scale spray dryer by suitable adjustment of the operating conditions. The results suggest differences in the rate of crystallization and particle size between small and pilot-scale spray dryers.     

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。     

Drying of Sodium Acetate in a Pulsed Fluid Bed Dryer

Sodium acetate crystals obtained from the reaction of acetic acid with sodium hydroxide are usually dried in rotary or fluidized beds. In this study, a batch pulsed fluid bed dryer with a 0.18 m² cross‐sectional area was used in an attempt to reduce energy consumption and increase productivity. Drying curves of sodium acetate were determined for different conditions: inlet air temperature of 65 and 80 °C and pulsation frequency of 0 rpm (conventional fluidized bed), 500 and 900 rpm (pulsed fluid bed). A 2² factorial design was used to analyze the results. The intermittent flow helped to break agglomerates and provided better contact between particles and the gas. Drying rates were higher under pulsed fluidization when compared to conventional fluidization. Conventional fluidized bed drying consumed 2.5 times more energy at 80 °C. The influence of temperature on the drying rate was also evident.     

Drying of Diced Carrot in a Combined Microwave-Fluidized Bed Dryer

Drying of diced carrot (∼90% wet basis) was carried out in a laboratory microwave fluidized-bed dryer (MFB) and in a standard fluidized-bed dryer (FB). It was found that the drying time in the MFB dryer is 2-5 times shorter than in the FB dryer. Drying efficiency (DE) is a function of moisture content, microwave power and temperature of drying agent. Higher values of DE are obtained for MFB dryer. For both drying systems the water removal was proceeding in two-stage falling rate period (except short initial term).     

Contact-Sorption Drying of Penicillium bilaii in a Fluidized Bed Dryer

A fluidized bed dryer and drying system have been designed, constructed and operated to produce a powdered formulation of the fungus Penicillium bilaii. The dryer includes an air-shear atomizer and rotatable air vibrator to produce particles which are evenly coated with P. bilaii. The drying system included a humidifier, oven, vacuum pump and instrumentation to ensure precise control of operating conditions. Instant skim milk powder was found to be the best particle source for fixing the spores due to its solubility in water, its highly porous nature and because its moisture isotherms were similar to those of the spores. The dryer was operated at 35°C to maximize the drying rate while still preventing thermal death of the spores. The fluidizing air was best provided at an RH of 30%, thereby producing skim milk/spore particles with just the right amount of moisture to ensure long term storage viability. Storage under refrigeration conditions showed little long term decline in spore viability after 3 months of tests. Room temperature storage demonstrated a small decline rate in spore viability with little significant change if held at room temperature for a few days. © 1997 SCI.     

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.     

Assessment of a Two-Stage Zeolite Dryer for Energy-Efficient Drying

Multistage adsorption drying with zeolite is experimentally evaluated for a single- and a two-stage dryer. For a 1:1 ratio between air flows for drying and regeneration, the energy efficiency for a single-stage system 50–54% and for the two-stage system 63%. Calculations with a calibrated model show that the two-stage system achieves a 4:1 ratio between the air flows an efficiency of 85%, 12% above that of a single-stage system. A sensitivity analysis shows the influence of operational conditions on the energy efficiency. Options to realize multistage adsorption dryer systems and to control such systems as well as the economic aspects are discussed.     

在惰性气体环境中热泵干燥食品

在传统的食品干燥技术中,干燥介质是空气,干燥能力由变化的温度控制。通常,干燥过程在高温下进行,空气中氧的存在影响食品的营养物质。想要在低温条件下提高干燥能力,可以通过热泵来改善干燥环境。热泵是闭环系统,冷凝器产生的热控制干燥温度,蒸发器除去干燥室的水分,干燥介质在低温低湿下运行。由于整个系统在闭路下操作,为了去除空气中氧对食品的影响,采用氮气、二氧化碳等惰性气体代替空气作为干燥介质。本文主要研究：采用氮气、二氧化碳和空气干燥一些热敏性水果,如番石榴和木瓜,探讨对食品营养物质和颜色的影响,并与冷冻干燥、真空干燥的结果进行对比。     

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.     

Modeling and Optimization of a Tunnel Crape Dryer

A mathematical model of a tunnel dryer for the dehydration of grapes is presented and applied to the determination of optimal operating conditions of the dryer. The dryer is of semi-batch structure, operating with trucks and trays. The cycle period is determined by meeting appropriate quality specifications for the final product. The nominal conditions were evaluated bv suitably minimizing. the total fuel demand, expressed as fuel consumption to production capacity, under some constraints regarding the production rate of the dryer and the maximum permissible air temperature. An nominal air humidity value was evaluated suggesting a minimum cycle period value for the production capacity and fuel demand. The nominal conditions required operation of the dryer on the maximum permissible air temperature. The optimum operation was evaluated by maximizing the total profit resulting from the operation of the dryer. The optimization variables were temperature and humidity of the drying air stream. A charteristic case study of industrial grape was included to illustrate the effectiveness of the proposed approach.     

Modeling and Optimization of a Tunnel Crape Dryer

ABSTRACT

A mathematical model of a tunnel dryer for the dehydration of grapes is presented and applied to the determination of optimal operating conditions of the dryer. The dryer is of semi-batch structure, operating with trucks and trays. The cycle period is determined by meeting appropriate quality specifications for the final product. The nominal conditions were evaluated bv suitably minimizing. the total fuel demand, expressed as fuel consumption to production capacity, under some constraints regarding the production rate of the dryer and the maximum permissible air temperature. An nominal air humidity value was evaluated suggesting a minimum cycle period value for the production capacity and fuel demand. The nominal conditions required operation of the dryer on the maximum permissible air temperature. The optimum operation was evaluated by maximizing the total profit resulting from the operation of the dryer. The optimization variables were temperature and humidity of the drying air stream. A charteristic case study of industrial grape was included to illustrate the effectiveness of the proposed approach.