Ultrasound- and microwave-assisted intermittent drying of red beetroot

Abstract

A drying technique using a combination of high power airborne ultrasound, microwaves and hot air was applied to investigate the effect of intermittent drying on the process kinetics and several quality indicators of red beetroot. An innovative hybrid dryer was used in drying experiments. Six sets of drying programs were carried out. Ultrasound and microwaves were applied in convective drying continuously (hybrid processes) or periodically (hybrid intermittent processes). The drying processes were assessed in terms of drying time, drying rate, and energy consumption. Moreover, the total color change, retention of natural dye (betanin), water activity, texture, and microstructure of dry product were examined. The drying kinetics was well-fitted with the use of the Midilli-Kucuk model. It was found that hybrid intermittent drying reduces the total drying time and energy consumption, enhances both the drying rate and product quality. Furthermore, it was demonstrated that the hybrid intermittent drying can serve as an alternative to traditional hot air drying that could produce a more porous, nice color, and crispy vegetable product.     

Ultrasonic-Assisted Convective Drying of Apple Slices

A promising approach for the application of ultrasound to assist in convective food drying was developed and tested in this study. The application of ultrasound is based on the transmission of ultrasonic energy as a combination of airborne contacts and through a series of solid contacts between the ultrasound element and the product tray as the ultrasonic vibration transmitting surface. A computer-based ultrasonic drying setup was built to allow continuous recording of the process variables in real time and enabled simulation of dehydration to be accomplished under controlled conditions over a range of drying parameters. Apple slices were dried using the drying setup to study the influence of ultrasound in combination with conventional hot air drying on drying kinetics and product quality.

The results from this work indicate that ultrasound can simultaneously be applied to accelerate the processing time (i.e., reduce energy consumption and increase production throughput) in conventional hot air drying without compromising product quality. It appears that the magnitude of influence of ultrasound to enhance the air-drying process depends on the process variables employed. In particular, the ability of ultrasound to improve the efficiency of the convective drying process appears to be maximized when using low temperature and high ultrasonic power level. This finding maybe very useful when there is a need to effectively dehydrate heat-sensitive products or when shorter drying times are required in order to achieve better retention of the functional and nutritional properties of the product.     

冷冻干燥过程强化中冷冻阶段优化的研究进展

冷冻干燥产品质量高,但时间长、能耗高。本文综述了冷冻干燥过程强化中冷冻阶段的优化方法,控制冷冻速率、调节冰晶成核和退火处理可以获得大而均匀的冰晶从而提高升华干燥阶段速率,但物料内部比表面积的减小会降低解吸干燥阶段速率,这类常规的冷冻阶段优化方法对弱吸湿性的物料有一定的强化效果。有机溶剂具有较高的蒸气压,作为共溶剂时可以增加传质推动力,但较低的有机溶剂残留量要求阻碍了其进一步应用。“初始非饱和多孔介质冷冻干燥”的技术思想是将液体物料首先制备成具有一定初始孔隙的冷冻物料,然后再进行冷冻干燥。物料具有的初始孔隙为水蒸气的迁移提供了便捷的通道,而且纤薄的固体基质也有利于结合水的解吸,可以同时强化升华干燥阶段和解吸干燥阶段。该技术思想是过程低消耗和产品高质量的完美结合,为解决冷冻干燥过程速率低的问题提供了新的方案。     

吸湿多孔介质冷冻干燥传质传热模型

本研究在作者提出的吸附—解吸平衡关系的基础上,建立了一个全新的考虑吸湿效应的多孔介质冷冻干燥数学模型。模型用有限差分法进行求解,并带有一个移动边界,以模拟介电材料辅助的微波冷冻干燥过程。介电材料选用碳化硅（SiC）,原料液为脱脂奶。模拟结果表明：介电材料能够有效强化微波冷冻干燥过程。在典型操作条件下,介电材料辅助的微波冷冻干燥所用的时间比普通微波冷冻干燥减少33.1%。当料液中固体含量较低或者固体产品的损耗因子较小时,介电材料对微波加热的效果不明显。基于冰饱和度、温度和水蒸气浓度的分布,本文分析了干燥过程中的传质传热机理,并对干燥速率控制因素进行了讨论。     

Diffuse interface model of the freeze-drying process of individually frozen products

Abstract

Vacuum freeze-drying (VFD) is a dehydration method based on the sublimation of the liquid phase contained in a certain product, previously frozen, at low pressure and temperature. Since it is a time and energy consuming process, it is crucial to select the best processing conditions to minimize drying duration, thus reducing the energy requirement. Additionally, product temperature must be monitored since it plays an important role in preserving product quality. The aim of this study was to develop a Diffuse Interface Model (DIM) for in-silico simulation of the freeze-drying process of individually frozen products. Due to the geometrical features of the samples, and to the role of radiation in the heat transfer to the product, the usual one-dimensional approach is inappropriate. Using a DIM, each cell of the computational domain can be described as a porous solid matrix filled by ice and vapor with a time-varying composition, thus allowing the use of a fixed computational grid and making the computation effort less demanding in comparison to moving interface-based models. Drying of eggplant cubic samples was considered as case study: model parameters were estimated by fitting the experimentally measured product temperature and drying time to the calculated ones. The model was proven to be reliable in providing an accurate estimate of both the drying time and the product temperature. Therefore, it can be used for off-line process design and optimization, minimizing the experimental effort required to design and optimize the process.     

Optimal quality control of baker's yeast drying in large scale batch fluidized bed

Optimal quality control of drying process of baker's yeast in large scale batch fluidized bed dryer is presented using neural network based models and modified genetic algorithm (GA). The objective of this study is to determine optimal conditions to maximize product quality while minimizing energy consumption. For this purpose, the drying process and quality models based on neural network with delay units are combined for predicting the dry matter, product temperature, change in dry matter and the quality loss while minimizing energy consumption and this model is then used for optimal quality control. A stochastic method based optimization structure is designed in order to solve the optimization problem whose the objective function is discontinuous, non-differentiable, complex and highly non-linear. The results obtained by optimal quality control based on modified GA showed that the performance of the existing industrial scale drying process was improved. The constructed optimal quality control structure is very convenient for the production process applications and may be applied without too much modification.     

Optimal control of convective drying of saturated porous materials

Numerical simulations of optimal control applied to saturated capillary‐porous materials subjected to convective drying are presented. The optimization process is concerned with such drying parameters as drying rate, energy consumption, and product quality. The thermo‐hydro‐mechanical model of drying is developed to describe the kinetics of drying and to determine the drying‐induced stresses which are responsible for damage of dried products. The effective and the admissible stresses are defined and used to formulate the Huber‐von Mises–Hencky strength criterion enabling assessment of possible material damage. The method of genetic algorithm is used for operation with drying conditions in such a way as to ensure minimum energy consumption and to get the effective stress less than the strength of dried material, and thus, to preserve a good quality of dried products at possibly high drying rate. Numerically simulated optimal drying processes are illustrated on the examples of finite dimensions of kaolin‐clay cylinders subjected to convective drying. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4846–4857, 2013     

Effect of Particle Structure on Quality Retention of Bio-Products During Thermal Drying

To study the influence of particle structure on quality retention of hioproducts during thermal drying, the porous particles formed of albumin and solid carriers were dried in a vibm-fluidized bed dryn at different inlet air temperatures and different initial bulk porosities. Equations to predict temperature and moishrre content of panicles as well as the kinetics of biomass degradation were developed. The particle bulk porosity was incorporated into concentration-dependent moisture diffusivity model to estimate the erect of particle structure on product quality. The analysis of both calculated and experimental results indicates that the more porous structure promotes moisture diffusion, increases drying rate and finally improves the quality retention of bio-products. An extensive literature survey on quality retention issues during thermal drylng has been done.     

Effect of Particle Structure on Quality Retention of Bio-Products During Thermal Drying

ABSTRACT

To study the influence of particle structure on quality retention of hioproducts during thermal drying, the porous particles formed of albumin and solid carriers were dried in a vibm-fluidized bed dryn at different inlet air temperatures and different initial bulk porosities. Equations to predict temperature and moishrre content of panicles as well as the kinetics of biomass degradation were developed. The particle bulk porosity was incorporated into concentration-dependent moisture diffusivity model to estimate the erect of particle structure on product quality. The analysis of both calculated and experimental results indicates that the more porous structure promotes moisture diffusion, increases drying rate and finally improves the quality retention of bio-products. An extensive literature survey on quality retention issues during thermal drylng has been done.     

Strain–Stress Formation During Stationary and Intermittent Drying of Deformable Media

The goal of this work is to study the effect of different drying conditions on the induced stresses within deformable media, the drying kinetics, and the energy consumption. A comparison between stationary and intermittent drying with periodically changing air temperature was performed. A theoretical formulation of the coupled heat, mass, and momentum transfers in saturated porous media was established. The model is based on the averaging theory. The thermo-hydro-mechanical coupling was closed using the effective stress theory of Terzaghi. In this approach, the viscoelastic behavior of the medium was considered. A bi-dimensional-shaped bentonite sample was used for numerical tests. The evolution of drying kinetics and stresses within the material during drying at constant and intermittent conditions was presented. It was observed that a non-stationary drying with smaller period applied at the end of the constant drying rate phase has the best effects on the product quality and energy gain without considerably extending the drying time.     

Influence of pore structure and impregnation–drying conditions on the solid distribution in porous support materials

Deposition of solids within porous materials from a drying solution is an important phenomenon in numerous natural and industrial processes. A profound knowledge about influences of different parameters on the solid distribution in the material is required for an effective targeted impregnation process. Experimental investigations and simulations are used to study the influence of pore structure, drying conditions, and solute concentration on the solid distribution in porous support materials after impregnation and drying. It is found that low drying rates lead to strong solid accumulation at the material surface, whereas high drying rates reduce the solute transport to the surface and result in more uniform solid distributions. A small pore diameter and distribution width reduce solute migration during drying and lead to uniform solid distributions without being influenced by the drying conditions. A higher initial concentration of the impregnation solution causes pronounced surface accumulation, while low initial solute concentrations result in more uniform distributions. Fundamental effects during drying are captured in an existing pore network model by adaption of experimental pore structures and impregnation–drying conditions, resulting in a good general agreement of experiments with simulations.     

Mathematical Modeling of Drying of Liquid/Solid Slurries in Steady State One-Dimensional Flow

A mathematical model of simultaneous mass, heat and momentum transfer for two-phase flow of a gas and a solid/liquid slurry was developed. The model was applied to calculation of the drying process of coal-water slurry droplets in a gas medium in a steady one-dimensional flow. The model was based on the well-known two-stage drying process for slurry droplets. After the first period of drying, in which the evaporation rate is controlled by the gas phase resistance, the evaporating liquid diffuses through the porous shell (crust) and then, by convection, into the gas medium. Inside the dry external crust of the drop, a wet central core forms, which shrinks as evaporation proceeds. The temperature of the slurry droplet rises. The process ends when the temperature of the dry outer crust reaches the coal ignition temperature in the case of combustion or when the moisture of the particle reaches the final required moisture. The developed model was based on one-dimensional balance equations of mass, energy and momentum for the liquid/solid and gas phases. The system of governing equations was represented by first-order differential equations and solved simultaneously. The numerical solution of the governing equations was obtained using Gear's method. The model permitted calculation     

Heat and mass transfer model of dielectric-material-assisted microwave freeze-drying of skim milk with hygroscopic effect

A new porous media mathematical model for freeze-drying was developed based on the adsorption-desorption relationship proposed in this paper. A finite difference solution was obtained from a moving boundary problem for the dielectric-material-assisted microwave freeze-drying process. Silicon carbide (SiC) was selected as the dielectric material; and frozen skim milk was used as the aqueous solution to be dried. Simulation results showed that the dielectric material can significantly enhance the microwave freeze-drying process. The drying time was 33.1% shorter than that of ordinary microwave freeze-drying under typical operating conditions. When the solid content of the solution to be freeze-dried was very low, or the solid product had a very small loss factor, microwave heating was less effective without the assistance of dielectric material. The mechanisms of heat and mass transfer during drying were analyzed based on profiles of ice saturation, temperature and vapor concentration. Drying rate-controlling factors were discussed. A comparison was made between the model predictions and the reported experimental data.     

Whey Drying on Porous Carriers

Whey is an undurable product. treated very often as a waste which pollutes the natural environment. Whey which is a valuable source of protein, lactose, vitamins and mineral salts should be utilized completely. The present paper is a proposal of whey drying on porous carriers. It is proved experimentally that the proposed drying method guarantees good product quality.     

Combined microwave–hot-air drying of burdock slices with feedback temperature control at surface and core

During combined microwave–hot-air drying, the surface and the core temperatures of the sample have great influence on the process. To investigate the influence systematically, drying system with feedback control strategy of the two temperatures was proposed. Then various pairs of the two temperatures were applied in the drying mode 1. However, it was found difficulty to achieve both short drying time and high product quality with fixed temperature pair, because the interaction between microwave and the sample changes as the moisture content decreases in the drying process. Different temperature pairs were applied during the three drying stages in drying mode 2, so that better product can be obtained in shorter drying time. To further improve the product quality, the drying rate was controlled by a feedback loop within a desired range in drying mode 3. The change of drying rate was realized by adjusting the two temperatures continuously. To omit the weighing scale, a feedforward control strategy for the drying rate was put forward in drying mode 4, where the temperatures were controlled along with preset lines. The results showed that the product quality and the drying time were similar to those in drying mode 3.     

Practical benefits for the separation process of drying that can be realized from novel multiscale modeling procedures that utilize the scientific information and results obtained from molecular dynamics modeling and simulation studies

Novel multiscale modeling procedures are constructed and presented that use the scientific information and results determined from microscopic molecular dynamics (MD) modeling and simulation studies to calculate local effective values for the parameters that characterize the heat and mass transfer mechanisms of dynamic macroscopic continuum models (Euler physics of continua) that are used in practice to describe and predict the dynamic behavior of large scale in time and space (e.g., industrial scale), separation (e.g., drying; adsorption), and chemical and biochemical reaction engineering (e.g., chemical catalysis; biocatalysis; immobilized cell bioreactor systems) processes involving porous media whose pore structure is formed either by a solid rigid matter or by a solid soft matter. Furthermore, the results determined from MD modeling and simulation studies with regard to the energies of interaction between the molecules of the different species of the porous media during the time evolution (time varying) of the drying process can be used to design a time optimally controlled heat input system that could appropriately and accurately supply at any time during drying the amount of heat necessary to provide a desired drying rate with respect to both free and bound water and to satisfy the constraints that safeguard the quality properties of the product.     

Mathematical Modeling of Drying of Liquid/Solid Slurries in Steady State One-Dimensional Flow

ABSTRACT

A mathematical model of simultaneous mass, heat and momentum transfer for two-phase flow of a gas and a solid/liquid slurry was developed. The model was applied to calculation of the drying process of coal-water slurry droplets in a gas medium in a steady one-dimensional flow. The model was based on the well-known two-stage drying process for slurry droplets. After the first period of drying, in which the evaporation rate is controlled by the gas phase resistance, the evaporating liquid diffuses through the porous shell (crust) and then, by convection, into the gas medium. Inside the dry external crust of the drop, a wet central core forms, which shrinks as evaporation proceeds. The temperature of the slurry droplet rises. The process ends when the temperature of the dry outer crust reaches the coal ignition temperature in the case of combustion or when the moisture of the particle reaches the final required moisture. The developed model was based on one-dimensional balance equations of mass, energy and momentum for the liquid/solid and gas phases. The system of governing equations was represented by first-order differential equations and solved simultaneously. The numerical solution of the governing equations was obtained using Gear's method. The model permitted calculation     

流化床氛围下多孔物料干燥传热传质的数值模拟

用有限差分法数值求解一个热、质传递耦合模型,理论研究多孔物料流化床干燥过程。方程离散采用全隐格式的控制容积方法,三对角矩阵法（TDMA）用来求解线性方程组。选用球形的苹果丁作为多孔物料。在典型操作条件下,通过分析温度、饱和度和压力的分布侧形,讨论了物料内部的热、质传递机理。在对比条件下,考察了气体入口温度、气速和床面积因子对干燥过程的影响。结果表明：干燥过程受气、固相间的耦合传热传质的影响十分明显,干燥时间随气体入口温度和气速的提高而减少;随床面积因子的增大而增加。     

Whey Drying on Porous Carriers

ABSTRACT

Whey is an undurable product. treated very often as a waste which pollutes the natural environment. Whey which is a valuable source of protein, lactose, vitamins and mineral salts should be utilized completely. The present paper is a proposal of whey drying on porous carriers. It is proved experimentally that the proposed drying method guarantees good product quality.     

Dimensionless Formulation of Convective Heat Transfer in Fry‐Drying of Sewage Sludge

Fry‐drying is an alternative for heat and mass transfer intensification. The process reuses waste oil as a heating medium for drying by contact with the wet sludge. At the end of the process, a stable derived fuel is obtained, a granular solid composed of the dried indigenous sewage solid and the impregnated oil. The fry‐dried sludge is storable and transportable without any pathogen elements. Knowledge about heat and mass transfer rates during the frying process is essential in order to assess the quality of the final product such as calorific value, oil uptake, porosity changes, etc. The heat transfer properties including transfer by free convection between the solid and the frying oil are fundamental for the process design and manufacturing of the fry‐dried product. The convective heat coefficient by temperature measurement and overall energy balance calculation is determined. The heat flux is calculated from the fry‐drying kinetics including moisture loss and oil intake kinetics. Various hydrodynamic regimes for convective heat transfer during the frying process are discussed (non‐boiling, boiling, and low‐boiling regime). A dimensionless formulation for estimating the convective transfer is proposed.