Optimization of Radiant-Convective Drying of a Porous Medium by Design of Experiment Methodology

The major aim of this study was to improve tools to optimize the thermal drying of a porous medium. This article was devoted to the study of the radiant-convective drying of a material (cellular concrete) while respecting some thermal constraints. A numerical program had been previously developed and experimentally validated to model the thermal and hygroscopic behavior of the product. Design of experiments methodology (DOE) was used to obtain a behavioral reduced model that allows the operating optimums to be calculated quickly. Three characteristics of a drying operation were analyzed using response surface methodology: the drying time, the total energy used in the process, and the specific moisture extraction rate. The effects of all the drying parameters (initial moisture content, infrared power, air temperature, velocity, and humidity) were studied. The optimal conditions were determined for convective and infrared-convective drying.     

Study and Optimization of the Drying Process of a Ceramic Abrasive Composite

This study presents a methodology to evaluate the drying mechanism of grinding wheels production, along the depth of a specimen, during the drying thermal cycle. The thermal and moisture gradients along the specimens were determined. The adopted experimental methodology allows obtaining precise moisture contents along the cross section of a component. Results showed that two steps of drying are present and that the drying kinetics is affected by the compaction pressure, which in turn alters the open porosity level. A critical compression pressure (17–23 MPa) was determined for the change on the drying characteristics, for the tested material.     

Drying of Wheat Based on the Non-Equilibrium Thermodynamics: A Numerical Study

A new mathematical model to describe simultaneous heat and mass (liquid and vapor) transfer and shrinkage during drying of capillary-porous bodies with particular reference to prolate spheroid solid is presented. As an application, the methodology was used to predict drying of soft red winter wheat (Arthur). The mathematical model was based on the nonequilibrium thermodynamics considering variable transport coefficients and convective boundary conditions at the surface of the solid. All the partial differential equations presented in the model have been written in prolate spheroidal coordinates and solved numerically by a finite-volume method using implicit fully formulation. Results of the drying and heating kinetics and moisture content and temperature distributions in a wheat kernel during drying process are presented and analyzed. The methodology allows verification of the heat, liquid, and vapor fluxes, taking into account the thermal and hydrical gradients inside the grain.     

A Simple Data Processing Approach for Drying Kinetics Experiments

A simple mathematical approach is proposed to be applied to drying kinetics raw data processing. The data collected in a drying experiment of powder cork under constant air drying conditions served as case study to present the methodology. Two functions (linear and third-degree polynomial) were used to fit solid moisture content in the constant drying rate and the falling rate periods. The drying rate curve was obtained by differentiation and the time at which the drying rate period's transition occurs was determined iteratively until virtually continuous functions were achieved. The critical moisture content was easily identified and two falling drying rate periods were detected.

The powder cork moisture decrease was also used to test several semiempirical models available in the literature. The Logarithmic, Midilli, and Page Modified I models were the ones that revealed the best correlations performance. When the methodology proposed was applied using these models, the critical moisture content was underpredicted.

The effective moisture diffusivity and the activation energy were also obtained for powder cork after the proposed mathematical approach has been applied on the raw data obtained in experiments performed at different air drying temperatures.     

A STUDY OF TEMPERATURE AND SAMPLE DIMENSION IN THE DRYING OF POTATOES

It is common knowledge that sample size and drying temperature are two of the most important factors affecting the drying rate and quality of food. However, what is less commonly known are how these two factors interact relative to each other and their effects on the drying behaviour of potato. These effects were evaluated under controlled drying conditions using a thermogravimetric analyser. The least-squares method was used to fit a model to the experimental data and the response variables (slope of the first drying stage, transition moisture content and slope of the second drying stage) were determined systematically. Central composite design was applied for response surface modelling of the factors and regression methodology was used to analyse the results. The results showed that both temperature and sample dimension have significant (P=0.01) effect on drying behaviour of potato. Sample dimension significantly affected the transition moisture content, whilst both temperature and sample dimension significantly affected the slopes of the first and second stage of drying.     

A STUDY OF TEMPERATURE AND SAMPLE DIMENSION IN THE DRYING OF POTATOES

Abstract

It is common knowledge that sample size and drying temperature are two of the most important factors affecting the drying rate and quality of food. However, what is less commonly known are how these two factors interact relative to each other and their effects on the drying behaviour of potato. These effects were evaluated under controlled drying conditions using a thermogravimetric analyser. The least-squares method was used to fit a model to the experimental data and the response variables (slope of the first drying stage, transition moisture content and slope of the second drying stage) were determined systematically. Central composite design was applied for response surface modelling of the factors and regression methodology was used to analyse the results. The results showed that both temperature and sample dimension have significant (P=0.01) effect on drying behaviour of potato. Sample dimension significantly affected the transition moisture content, whilst both temperature and sample dimension significantly affected the slopes of the first and second stage of drying.     

Modelling and Simulation of a Direct Contact Rotary Dryer

A mathematical model able to predict solid and drying gas temperature and moisture content axial profiles along a direct contact rotary dryer was developed. The study was focused on the drying kinetics based on phenomenological models. Two different drying mechanisms in the decreasing drying rate period were tested: proponional to the unbound moisture content and moisture diffusion inside the particle. Experimental data collected in a pilot-scale direct contact rotary dryer was used to validate the model. Soya and fish meals were used as drying material.     

Optimizing Drying Conditions for the Microwave Vacuum Drying of Enzymes

The aim of this study was the methodological use of experimental planning for the optimization of microwave vacuum drying of enzymes using α-amylase as a model. A factorial in star designwas used to optimize the microwave vacuum–drying process, and the variables were power output and vacuum pressure. The material dehydrated by this technique was analyzed with regard to its enzymatic activity, water activity, and moisture content. Response surface methodology was used to estimate the main effects of vacuum pressure and power on the enzymatic and water activities. The experimental in star design revealed that microwave vacuum drying is influenced mainly by power. The dehydrated product showed high enzymatic activity and low water activity.     

Modelling and Simulation of a Direct Contact Rotary Dryer

ABSTRACT

A mathematical model able to predict solid and drying gas temperature and moisture content axial profiles along a direct contact rotary dryer was developed. The study was focused on the drying kinetics based on phenomenological models. Two different drying mechanisms in the decreasing drying rate period were tested: proponional to the unbound moisture content and moisture diffusion inside the particle. Experimental data collected in a pilot-scale direct contact rotary dryer was used to validate the model. Soya and fish meals were used as drying material.     

薄膜蒸发技术在低聚糖生产中的应用

目前低聚糖的干燥技术主要有喷雾干燥以及冷冻干燥,存在操作温度过高、成本较高、不能处理大批量物料等缺点。为了解决这些问题,本文提出了以刮膜式薄膜蒸发器对低聚糖提取液进行单步干燥操作。实验分析了设备的处理性能,并利用响应面法拟合得到了温度、压强、进料速度与单位面积蒸发速率之间关系的数据模型。模型可以很好地符合实际结果。在温度、压强、进料速度分别设定为70.0℃、1.0kPa、0.50L/h的条件下,最终得到的产物含水率为2.1%。这表明,工艺路线以及预测方法是可行的,工艺保证了物料较好的流动性,达到了较高的分离效率,得到了质量较高的最终产品。     

Modeling and Stress Analysis During Drying of a Deformable and Saturated Porous Medium

This article addresses how to express the behaviors that develop stresses within a porous media during convective drying processes. The work is focused on the coupling of the thermal (temperature distribution), hygroscopic (moisture, humidity), and mechanical (strains and stresses) aspects shown during the drying process of a saturated porous medium. Natural clay plate samples were used as a model material. Using two different mechanical behaviors (elastic and viscoelastic), the strain–stress equations were studied and discussed through the simulation results. Obtaining almost the same parameters of the main modeling variables (temperature, liquid pressure, and moisture content), a significant difference was observed between the results obtained for the stresses assuming the two behaviors, particularly depending on the viscoelastic parameters deduced from an experimental study. The simulation highlights a response of the medium supposed viscoelastic different to that of elastic case in intensity and response time.     

MICROWAVE VACUUM DRYING OF BANANA SLICES

Vacuum drying of banana slices was studied in a domestic microwave oven. The results show that banana temperature rises uniformly and rapidly to the saturation water vapor temperature corresponding to the vacuum used then rises slowly until most of the free moisture is lost. The thermal and drying efficiencies were found to drop from almost 100% at the beginning of the drying (high moisture content) to as low as 40% and 30% respectively at the end of drying. Both efficiencies were found to increase with the use of vacuum, especially at low moisture content.     

Simulation of Temperature and Moisture Content Profiles in a Pinus radiata Board during High-Temperature Drying

A two-dimensional drying model has been simulated for industrial drying schedules to assess sensitivity of the model parameters. The predicted drying times and rates were similar to the results of experimental data. The influence of wood properties on the predicted drying rate was assessed. It was found that the effects of specific heat capacity and thermal conductivity on predicted overall drying rate of board were small. However, the effect of changes in the diffusion coefficient on drying rate below fiber saturation point (moisture content at 30%) was found to be significant.     

Simulation of Temperature and Moisture Content Profiles in a Pinus radiata Board during High-Temperature Drying

A two-dimensional drying model has been simulated for industrial drying schedules to assess sensitivity of the model parameters. The predicted drying times and rates were similar to the results of experimental data. The influence of wood properties on the predicted drying rate was assessed. It was found that the effects of specific heat capacity and thermal conductivity on predicted overall drying rate of board were small. However, the effect of changes in the diffusion coefficient on drying rate below fiber saturation point (moisture content at 30%) was found to be significant.     

Effective Moisture Diffusivity Estimation from Drying Data. A Comparison Between Various Methods of Analysis

Moisture diffusivity is the most crucial property in drying calculations. Literature data are scarce due to the variation of both experimental measurement techniques and methods of analysis. The effect of using different methods of analysis on the same experimental drying data is examined in this work. Detailed and simplified mathematical models, incorporating moisture diffusivity as model parameter, are applied. It is proved, that significant differences in the calculated values of moisture diffusivity result when different models are used, and probably these differences explain the variation in literature data. Thus, the adoption of a standardised methodology will be of great importance in moisture diffusivity evaluation.

The above findings resulted from the application of four alternative models on the drying data of three common food materials, potato, carrot and apple. A typical pilot plant scale dryer with controlled drying air conditions was used for the experiments. The moisture content dependence of the diffusion coefficient was proved significant at the last drying stage, while the temperature dependence followed the well known Arrhenius relation. The effects of considering external mass transfer and volume shrinkage during drying, were also investigated.     

Breakage and Mass Transfer Models During Drying of Rough Rice

A breakage model was investigated for thin-layer drying of rough rice. The breakage model developed can predict the percentage of head rice (E/E_o) as an exponential function of the grain moisture. Experimental data of the rice moisture content during drying were fitted with a theoretical model of the drying process to obtain the parameters. Experimental data of percentage head rice and moisture content were fitted to obtain the parameters of breakage model. Both functions were used together to obtain a drying-breakage model. This model allows us to predict the drying time required to achieve a rice moisture content desired and to estimate the head rice yield percentage for this moisture content.     

Computer model of drying behaviour of ceramic green bodies with particular reference to moisture content dependent properties

A numerical model was developed to predict the drying behavior of ceramic green bodies. Resolution of the simultaneous heat and mass transfer equations involved finite elements and the Backward Euler method. Based on experimental data, the model uses equivalent moisture diffusivity, water activity, thermal conductivity and heat capacity as input parameters which depend on moisture content. In particular, the equivalent moisture diffusivity is a key parameter controlling water transport from the body interior to the surface. A simple method was used to estimate the effect of shrinkage on drying rate during the initial drying stage. Predictions of the internal moisture distribution, drying rate and surface temperature as a function of time gave good agreement to experiment for green bodies of alumina paste. External conditions of convection coefficient and relative humidity are shown to sensitively control drying rate and surface temperature evolution during the constant rate period.     

Texturing of Chicken Breast Meat as an Innovative Way to Intensify Drying: Use of a Coupled Washing/Diffusion CWD Phenomenological Model to Enhance Kinetics and Functional Properties

In the present study, instant controlled pressure drop (DIC) was used as a novel texturing pretreatment of fresh chicken breast meat, before hot airflow drying, in order to modify the texture of the meat and obtain a controlled expansion. Three different drying techniques were compared: conventional hot airflow drying, freeze drying, and swell drying (a combination of DIC and hot airflow drying). DIC was carried out under various conditions in order to achieve multi-criteria optimization. A two-parameter, five-level response surface methodology was used to optimize the operating parameters of saturated steam pressure and thermal holding time, starting with the meat's initial water content. Saturated steam pressure had a significant impact on the expansion ratio. The higher the expansion ratio, the better the different functional qualities. Microstructural changes reduced drying time, and increased rehydration kinetics and water holding capacity (WHC). The starting accessibility δW_s and the internal effective water diffusivity D_eff were studied for both drying and rehydration processes in accordance with the coupled washing/diffusion (CWD) phenomenological model.