A New Approach to Contact Drying Modelling

This paper deals with contact drying modelling. The more general models of granular material contact dryers are based on assumptions proposed by Schlunder and al. (1.21. In this paper, a sensitivity study of the models is presented in order to find and to explain the influence of the different arameters.

A new approach, based on the same Schlunder's assumptions, but with less restrictive hypothesis, which can be applied to drying in the presence of an inert gas as well as vacuum drying and which take into account phenomena which were ignored untill now such as the local grain dehydration kinetics or the vapour diffusion inside the bed has been developped.

This new model has been compared uith some experimental results found in the litterature. A good agreement between the calculated and the experimental results has been observed. Moreover, this model is able to justify some assumptions made by Schlunder, which have not been so untill now.     

Evaluation of Textile Bobbins Drying Processes: Experimental and Modelling Studies

Drying of textile bobbins was carried out on two kinds of laboratory-scale dryers: in a cross circulation drying rig, atmospheric pressure heating air is used, while in the through circulation drying apparatus, the heating air is pressurized before the drying process takes place. In both cases, a bobbin was fitted with seven thermocouples, placed in a plane normal to the bobbin axis but at different radial distances, in order to provide temperature data during the drying process. In cross circulation drying, plots of temperature against time indicated the presence of two moving fronts, one starting from inside the bobbin, another from the outside, while in through circulation drying, similar plots showed the presence of an evaporation front, moving from inside the bobbin to the outside. In both canes, numerical models were developed to simulate the drying processes.     

Evaluation of Textile Bobbins Drying Processes: Experimental and Modelling Studies

ABSTRACT

Drying of textile bobbins was carried out on two kinds of laboratory-scale dryers: in a cross circulation drying rig, atmospheric pressure heating air is used, while in the through circulation drying apparatus, the heating air is pressurized before the drying process takes place. In both cases, a bobbin was fitted with seven thermocouples, placed in a plane normal to the bobbin axis but at different radial distances, in order to provide temperature data during the drying process. In cross circulation drying, plots of temperature against time indicated the presence of two moving fronts, one starting from inside the bobbin, another from the outside, while in through circulation drying, similar plots showed the presence of an evaporation front, moving from inside the bobbin to the outside. In both canes, numerical models were developed to simulate the drying processes.     

A New Drying Approach to Enhance Quality of Konjac Glucomannan Extracted from Amorphophallus muelleri

A new drying approach to enhance physical quality of konjac glucomannan extracted from Amorphophallus muelleri was studied. It consisted of a comparison between microwave vacuum drying and conventional hot-air drying. The effects of microwave vacuum drying and hot-air drying on drying kinetics and physical and structural properties such as color, bulk density, particle density, porosity, viscosity, and morphology of konjac glucomannan (KGM) flour were investigated. It can be concluded that microwave vacuum drying significantly reduced drying time and increased porosity of dehydrated products which have a positive effect on the viscosity of the KGM solution. In contrast, it decreased bulk density and particle density when compared with conventional hot-air drying. Microstructure observations revealed the presence of large cavities in the granules of microwave-vacuum-dried KGM samples. However, there was a slight change in color of the microwave-vacuum-dried samples, resulting in a lower whiteness index than the whiteness index of hot-air-dried samples. Hence, microwave vacuum drying was found to have a number of advantages over the conventional hot-air drying method. Therefore, microwave vacuum drying has potential to become a useful drying method for the industrial production of KGM flour.     

Baker's Yeast Behavior during Vacuum Agitated Contact Drying

Drying is considered as an intensive operation that consumes large quantities of energy. Usually, baker's yeast is obtained using freeze drying or fluidized‐bed drying, both of which are considered as expensive technologies. So, exploring other techniques such as contact drying could limit this disadvantage. In addition, no work dealing with contact drying of baker's yeast has been accomplished yet. Therefore, here, the behavior of baker's yeast during vacuum agitated contact drying is presented. The results show that the drying process can be divided into three phases: the pasty phase, the lumpy phase, and the granular phase. The influence of the drying parameters, such as the temperature, the impeller velocity, and the initial mass, was also studied. It was found that the wall temperature and the impeller velocity have a positive effect on the drying kinetics, as their increase allows a reduction in the drying time. Nevertheless, an increase in the pressure level or the initial mass of the product caused the drying time to increase.     

接触角的研究现状及其在凝胶干燥中的作用简

接触角是固体表面润湿性能的重要参数之一。根据接触角可以获得固体表面的固-液、固-汽界面的许多信息。本文简要介绍了接触角在凝胶干燥中的作用,并对接触角的相关理论研究、实验测量方法以及分子动力学模拟研究进行综合评述。     

Experimental Study and Modeling of Crystalline Powders Vacuum Contact Drying with Intermittent Stirring

Modified literature models were successfully applied to simulate the evolution of the average solvent content (ethanol) of potassium chloride bed during vacuum contact drying with intermittent stirring. Our new modeling approach incorporated the following modifications: the introduction of a jacketed vessel heat transfer coefficient and an accumulation term for the heating wall temperature; the application of alternate static bed and stirred bed conditions; and modeling of all the three drying phases, namely the constant rate phase, the transition phase, and the falling rate phase. Moreover, several validation experiments were carried out with different operating conditions to identify the values of the unknown model parameter. The optimal stirring conditions were investigated by calculating total drying times for different sequences of stirring and no stirring periods.     

Experimental Study and Modeling of Crystalline Powders Vacuum Contact Drying with Intermittent Stirring

Modified literature models were successfully applied to simulate the evolution of the average solvent content (ethanol) of potassium chloride bed during vacuum contact drying with intermittent stirring. Our new modeling approach incorporated the following modifications: the introduction of a jacketed vessel heat transfer coefficient and an accumulation term for the heating wall temperature; the application of alternate static bed and stirred bed conditions; and modeling of all the three drying phases, namely the constant rate phase, the transition phase, and the falling rate phase. Moreover, several validation experiments were carried out with different operating conditions to identify the values of the unknown model parameter. The optimal stirring conditions were investigated by calculating total drying times for different sequences of stirring and no stirring periods.     

A Quasi-stationary Approach to Drying Kinetics of Fluidized Particulate Materials

A modified quasi-stationary method has been proposed to describe drying kinetics of particulate materials dried in convective dryers with active hydrodynamic regimes. Both our own results and literature data were used. These include sliced celery, cranberry, diced carrot, wheat and polystyrene granules dried in four types of dryers: pulsed fluid bed dryer, fluid bed dryer with a mixer, spouted bed dryer with a draft tube, and vortex dryer. The method was validated by comparing experimental data with results of modeling in terms of a reduced moisture content and material temperature. A new form of the generalized drying curve has been proposed with the reduced time and the index of hydrodynamic intensity as parameters. The equations developed can be used to calculate the total drying time and determine the temporal variation of the moisture content and material temperature.     

干燥过程模型化

为了更好地了解和分析各类干燥过程，优化设计，操作和控制，干燥过程数学模型与模拟优化已成为主要研究方向。本文扼要介绍干燥过程数学模型化方法与内容，举例列出了气流干燥过程六为量常微分方程组。     

Evaluation of Parameter Values For a High-Temperature Drying Simulation Model Using Direct Drying Experiments

ABSTRACT

An existing drying simulation model for softwood heartwood is revised lrom a theoretical point of view and is completed by extracting all the essential material parameters from moisture and temperature profiles measured in direct drying experiments. Some additional experimental results are presented to validate the qualitative characteristics of the model. The resulting computational model shows good agreement with experiments in the drying temperature range 45-120°C. Special attention is paid to the evolution of moisture gradient during drying, as the next step of the simulation project will be using the result as an input data for a computational stress model.     

化工烘干系统节能途径的探索

从用烟道气直接烘干物料入手,探索燃料燃烧燃尽,烟道气除尘,干燥设备节能方法,废气余热回收,从而提高干燥效率,为化工烘干节能途径提供一些借鉴。     

A Mortality Model for Penicillium bilaiae Subjected to Convective Air Drying

Drying of microbial cells for storage purposes is a widely practised technology. A new statistical model is proposed for survivability of microbial cells during convective air drying, where mortality is predicted by hazard functions and acceleration factors applied to temperature, moisture content and drying rate variables. The model is based on experimental survivability data generated from drying Penicillium bilaiae conidia at temperatures from 20 to 60°C and air relative humidities from 3 to 75%. Experimental data using “one at a time” variable manipulation was used to obtain five model parameters. Despite several simplifications to reduce the number of variable cross‐dependencies, the model was successfully validated using combined stress trials with maximum deviations of ±15%.     

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.     

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.     

A Preliminary Study on Strategies for Optimal Fluid-Bed Drying

ABSTRACT

The degradation of product quality during convective drying depends on the temperature and water concentration history of the panicles and drying time. For improving product quality in combination with acceptable operation costs, optimal control of the operation variables is required.

In this preliminary study the relevance of dynamic optimal operations for batch-wise fluid-bed drying is explored by simulation. Optimal trajectories of operation variables were calculated for a pilot-plant installation by using a model which concerned the drying history of the panicles and the product quality (inactivation of biological active components). The applied objective function was based on an economic criterion combining product quality and operation costs.

Although the advances for the chosen pilot-plant application are moderate, in future studies the potentials and relevance of dynamic optimal operations for drying will be quantified for installations on industrial scale.     

Multi-Scale Analysis of Vacuum Contact Drying

The modeling of the unit operation of vacuum contact drying is approached as a multi-scale problem. At the particle assembly length scale, effective transport properties (thermal conductivity, relative gas- and liquid-phase permeability) have been determined computationally by simulations on reconstructed porous media and verified by direct measurements. A distributed-parameter model of vacuum contact drying including liquid and vapor flow and differential energy balance has been formulated and used for the calculation of drying time as function of vacuum level, temperature, vessel diameter, and batch size at the unit operation length scale. Drying curves for a model system of sodium carbonate-isopropanol have been measured experimentally and compared with the model predictions. A very good agreement has been found.     

Multi-Scale Analysis of Vacuum Contact Drying

The modeling of the unit operation of vacuum contact drying is approached as a multi-scale problem. At the particle assembly length scale, effective transport properties (thermal conductivity, relative gas- and liquid-phase permeability) have been determined computationally by simulations on reconstructed porous media and verified by direct measurements. A distributed-parameter model of vacuum contact drying including liquid and vapor flow and differential energy balance has been formulated and used for the calculation of drying time as function of vacuum level, temperature, vessel diameter, and batch size at the unit operation length scale. Drying curves for a model system of sodium carbonate–isopropanol have been measured experimentally and compared with the model predictions. A very good agreement has been found.