Microwave Drying Process Research Articles

Modelling of moving drying process and analysis of drying characteristics for germinated brown rice under continuous microwave drying

To understand the complex continuous microwave drying of germinated brown rice (GBR) and clarify the drying characteristics, the heat and mass transfer of GBR under continuous microwave drying was investigated numerically and experimentally. A three-dimensional model of coupled multi-physics fields involving the transmission of microwave field, heat transfer and mass (moisture) transfer was developed to characterise the drying process of GBR in a continuous microwave dryer. The implementation strategy based on discrete-combined approach was proposed to achieve the simulation of continuous microwave drying of moving materials with the mutual cooperation of the computer simulation software and independently developed program code. For the continuous microwave drying of GBR, the relatively uniform distribution of electric field strength applied to the grain layer depended on the reasonable arrangement pattern of magnetrons and suitable microwave power output. The movement of materials can effectively reduce excessive absorption of microwave energy by the grain layer, and achieve uniform distribution of temperature and moisture content and high drying uniformity assisted by the synergistic effects of microwave heating, moisture evaporation and ventilation convection. The developed model and simulation strategy may provide guidance for understanding and analysis of continuous microwave drying process of granular materials such as GBR.

Modeling and kinetics study of microwave heat drying of low grade manganese ore

During the industrial-scale smelting process of manganese ore, blast may occur due to the high moisture content of the ore, and drying pretreatment of the manganese ore is needed in the aspects of safety. In the present paper, microwave drying experiments were conducted under different particle size distributions and different microwave power conditions to explore the basic theory of microwave drying characteristics and kinetics of manganese ore, and the experimental data were fitted and analyzed by using thin-layer drying dynamics model. Results indicated that with the increase of the particle size and the microwave power of the manganese ore, the microwave drying rate increased, and accompanied with a promoting on the drying efficiency. For describing the microwave drying process of manganese ore, diffusion approach model was adopted among the commonly used thin-layer drying kinetic models. Based on Fick's second law, it can be seen that the surface diffusion coefficient increased from 4.27 × 10−13 m2/s to 8.24 × 10−8 m2/s with the increase of particle size from a particle size range of 0.012–0.095 mm to a particle size range of 4.2–5.0 mm. Clearly, the particle size has a significant influence on microwave drying efficiency.

Volatilization characteristics of selenium during conventional and microwave drying of coal slime: an emerging contaminant in mining industry.

The priority feature of coal drying which accounts for its industrial use is moisture removal. Though much improvement is made in thermal-drying process, volatilization of harmful elements during coal slime drying is mostly ignored which has manifold environmental implications. In view of this, the present study attempted to investigate the moisture and selenium (Se) volatilization proportion during coal slime drying by using electric blast oven (conventional) and microwave oven (microwave) drying procedures. X-ray photoelectron spectroscopy (XPS) technique was employed to investigate the chemical changes of raw and dried coal. The results indicated that the microwave-drying process eliminates the moisture in line format, whereas, moisture content was removed slowly (initial stage) during conventional drying and then rapidly (final stage) with increasing drying period. It was confirmed that the microwave drying took less drying time to remove water and make the coal dried compared with the conventional drying. Volatilization proportion of Se to the atmosphere was 38.81 and 61.13% during conventional and microwave drying, respectively. Findings of XPS analysis demonstrated an increase in C-C and C-H content, while a decrease in C-O fraction on the coal surface, after execution of drying procedures. Microwave drying appeared not only energy efficient but also proved an optimized method for removing oxygen-functional groups than conventional drying methods. Peak fittings of XPS spectra gave indication of several oxidation forms including selenide, selenate, and oxide species in coal slime after drying. It was concluded that Se volatilization including speciation characteristics should be considered an indicator of efficiency for coal drying process from environmental safety perspective.

LQR Control of Moisture Distribution in Microwave Drying Process Based on a Finite Element Model of Parabolic PDEs

The microwave drying process is a widely used technology in the drying of porous dielectric materials. Designing a controller for moisture distribution in this process can improve product quality and reduce energy consumption and production time. In this paper, a model-based controller for moisture distribution in an industrial microwave drying process is developed. The moisture and temperature in this process are described by a pair of partial differential equations (PDEs) and have both temporal and spatial variations. In this view, using a semi-discrete finite element approximation, the coupled system of PDEs is transformed into a system of ordinary differential equations (ODEs). Based on the discretized ODEs, a linear quadratic regulator (LQR) controller is designed to determine the power levels of multiple microwave sources in this process to reach and maintain the desired moisture level. Numerical simulations are carried out in three different drying scenarios. The results show that the proposed controller achieves a very good performance in tracking the desired moisture level.

Estimation of Moisture Content Distribution in Porous Foam Using Microwave Tomography With Neural Networks

The use of microwave tomography (MWT) in an industrial drying process is demonstrated in this feasibility study with synthetic measurement data. The studied imaging modality is applied to estimate the moisture content distribution in a polymer foam during the microwave drying process. Such moisture information is crucial in developing control strategies for controlling the microwave power for selective heating. In practice, a reconstruction time less than one second is desired for the input response to the controller. Thus, to solve the estimation problem related to MWT, a neural network based approach is applied to fulfill the requirement for a real-time reconstruction. In this work, a database containing different moisture content distribution scenarios and corresponding electromagnetic wave responses are build and used to train the machine learning algorithm. The performance of the trained network is tested with two additional datasets.

PROBLEMATICS OF CONSTRUCTION OF MATHEMATICAL MODELS OF THE PROCESS OF MICROWAVE DRYING OF SOY

The article reviews various techniques for mathematical modeling of the soybean drying process. The purpose of this paper is to review, analyze and compare existing approaches to mathematically describe the processes of moisture movement during microwave drying of soybeans. The only way to provide mathematical modeling of the processes occurring in the soybean grain is a combination of the transformation of microwave energy into heat, the heating of dipole water molecules existing in the seed, the processes of moving moisture from the center to the grain shell and evaporation of moisture. Today, microwave drying is one of the promising areas for research, because this method is much more efficient than traditional post-harvest grain drying methods and, at the same time, poorly understood. Factors on which the process of microwave drying depends on are the speed of movement of moisture in the micropores of the product, the initial temperature and humidity of the raw material, the placement of the main magnetron relative to the object of drying and the speed of movement of raw materials in the drying chamber. The object of the study in this article was to simplify the mathematical model of soybean seeds. To intensify the process of microwave drying, it is advisable to use a combined effect, such as blowing, which increases the intensity and quality of the process of microwave dehumidification. In addition, in this case, the simultaneous use of two or more factors of influence, which allows for rapid processing of raw materials. This determines the relevance and prospects of this study. The developed criteria criterion is substantiated and described, which allows to simplify the complex calculation of microwave energy transfer processes, sample heating and moisture velocity in it. The basic approaches to the construction of a mathematical model of the soybean drying process are considered, and their expediency is determined. The analysis of various methods of mathematical description of the drying process is carried out, the main disadvantages of these methods are identified. It is established that drying soybean seeds in the microwave field is one of the most promising. A mathematical model for a combined soybean drying process with a combination of microwave drying methods and air blowing is proposed.

Electromagnetic wave absorbing performance of aspirin powder during 2450 MHz microwave drying

The absorbing performance of pharmaceutical powders has an important effect on the efficiency of microwave drying. In this study, in order to achieve optimal microwave absorption, the reflection loss (RL) of aspirin powder is calculated based on the theory of electromagnetic wave transmission line under different temperatures, moisture contents, microwave incident angles and polarization modes. In addition, the effect of the dielectric-assisted absorbing layer (SiC ceramics) on the microwave absorbing performance of aspirin powder (weak absorbing materials) with low moisture content is analyzed. The results show that the RL curve of aspirin powder fluctuates with increasing thickness; several absorption peaks appear, and the absorption peaks shift to smaller thicknesses as the temperature and moisture content increase. The RL of aspirin powder in the transverse electric polarization mode decreases monotonically as the incident angles increase. The effect of Brewster’s angle causes the RL of aspirin powder in the transverse magnetic polarization mode to first decrease and then increase with the increasing incident angle, and an absorption peak occurs. However, the RLs of both polarization modes are close to zero at 90°. The use of SiC ceramics as the susceptor can reduce the RL of the double-layer absorber and widen the aspirin powder thickness interval for an RL below −20 dB. This research can help to optimize the microwave drying process parameters of pharmaceutical powders and the design of microwave drying equipment.

A computation-driven, energy-efficient and hybrid of microwave and conventional drying process for fast gooseberry candy production

The health benefits of Indian gooseberry have resulted in a tremendous growth in its consumption in the form of tasty candy and marmalade prepared by drying process. The candy production process is limited by high drying time and plant size requirements as most of the processing production plants are equipped with conventional drying methods. Hence, the paper presents computational model defining relation between electric field pattern with heating and moisture diffusion in the product simultaneously. The boundary value applies on Maxwell’s equation solution as defined by gooseberry electrical properties. The solution to heat and mass transfer equation offers thermodynamic properties and profiles of gooseberry by microwave heating. The optimised process of moisture diffusion to the surface of gooseberry, followed by conventional heating, dries the sample uniformly and speeds up the process. The established relation offers a valid model for defining time for microwave and conventional heating of fresh gooseberry that takes it as an input and dries gooseberry candy as an output, thereby reducing the time of drying from 17 hours to 0.5 hour. The electromagnetic and thermodynamic results are obtained using simulation and are followed by experimental verification.

Іноваційні способи енергопідведення у процесах сушіння термолабільної сировини

Обумовлено причини зростання обсягів сушіння рослинної сировини. Визначено актуальність пошуку та дослідження нових способів енергопідведення в процесах сушіння рослинної сировини. Показано основні переваги мікрохвильового сушіння порівняно з традиційними конвенційними технологіями сушіння. Визначено основні недоліки та обмеження мікрохвильового сушіння. Показано зв'язок теми дослідження з напрямами наукової роботи кафедри процесів, обладнання та енергетичного менеджменту. Обрано робочу гіпотезу про взаємодію мікрохвильового поля з вологою в капілярно–пористих структурах. Визначено відмінності в процесі енергопідведення при мікрохвильовому та конвекційному сушінні. Визначено основні конструктивні обмеження для дизайну мікрохвильових сушильних апаратів. Описано обсяг та зміст проведених раніше досліджень. Приведено узагальнений аналіз результатів дослідження мікрохвильового сушіння для різних видів рослинної сировини. Обґрунтовано вибір апаратурної схеми установки для сушіння рослинної сировини в потоці. Визначено основні конструктивні рішення та обмеження, що визначають конструкцію мікрохвильової сушильної установки для рослинних матеріалів. Описано конструкцію та принцип дії дослідної стрічкової мікрохвильової сушильної установки створеної для проведення дослідження. Описано порядок проведення експериментального мікрохвильового сушіння насіння соняшника. Визначено основні залежності процесу мікрохвильового сушіння соняшника. Досліджено залежність між швидкістю вологовидалення та потужністю електромагнітного підведення енергії а також тривалістю сушіння. Визначено основні переваги комбінованого сушіння з використанням мікрохвильового та інфрачервоного випромінювання. Описано конструкцію та принцип дії установки для дослідження процесу вологовидалення при інфрачервоному сушіння. Визначено залежності кінетики процесу від величини енергопідведення та тривалості енергетичного впливу. Приведено аналіз отриманих результатів. Надано обґрунтування для наступного етапу дослідження.

Effect of Temperature and Microwave Power Levels on Microwave Drying Kinetics of Zhaotong Lignite

Microwave drying is a promising and effective way to drying and upgrading lignite. The influence of temperature (100–140 °C) and microwave power levels (500–800 W) on thin-layer drying characteristics of Zhaotong lignite under microwave irradiation were investigated. Fourteen thin-layer drying models were used to analyze the microwave drying process while six thin-layer drying models were used to analyze the hot-air drying process. The microwave drying processes at all temperature (100–140 °C) or low microwave power levels (500–700 W) exhibited four periods: a warm-up period, a short constant period, the first and second falling rate period, while one falling rate period was found during hot-air drying. The effective diffusion coefficient of lignite were calculated and it increases with increasing temperature and microwave power levels. During microwave drying, the two-term exponential model is the most suitable model for all applied conditions, while the Modified Page model is the most suitable model to describe the hot-air drying experiments. The apparent activation energy were determined from Arrhenius equation and the values for the first and second falling rate period are 3.349 and 20.808 kJ·mol−1 at different temperatures, while they are 13.455 and 19.580 W·g−1 at different microwave power levels. This implies the apparent activation energy is higher during the second falling rate period, which suggest that the dewatering of absorbed water is more difficult than capillary water. The value of apparent activation energy in hot-air drying is between the first and second falling rate period of microwave drying. Results indicate that microwave drying is more suitable to dewatering free water and capillary water of lignite.

Microwave drying of nickel-containing residue: dielectric properties, kinetics, and energy aspects

Abstract Nickel-containing residue (NCR) is a hazardous solid waste from battery production lines. Recently, the interest in recovering valuable metals from NCR has increased because sustainable utilization of resources is more and more valued. Drying is a key part of the recovery process. In this study, we measured the dielectric properties of the NCR for different moisture contents and temperatures using the cavity perturbation method at 2.45 GHz. The microwave absorption characteristics of NCR had a positive correlation with the moisture content, while it was less efficient in the 20-180°C temperature range. Then found that the microwave drying data at different microwave powers (400-700 W) and for different sample weights (60-120 g) have a better fit with the Midilli-Kucuk model. The activation energy (Ea) was received as 9.76 W/g using an exponential expression based on the Arrhenius equation. Finally, the energy consumption reduce 110 W·h/kg than that of drying with a single microwave power by optimizing the microwave drying process.

Optimization of the microwave drying process for potato chips based on the measurement of dielectric properties

A microwave drying system with functions of automatic power and temperature control was developed to dry potato chips. The mass and moisture content of the sample are available online. The dielectric properties of the samples were measured during microwave drying, during which the microwave absorption capacity of the samples was analyzed. Afterward, a reasonable power and temperature control scheme was formulated. The enhanced microwave drying technology of potato chips is found by comparing product quality under different schemes. Results show that the change in the dielectric properties of materials during microwave drying is closely related to moisture and temperature variations. A three-stage different temperature control with predefined variable power profiles resulted in the best product quality.

The Use of Microwave Drying Process to the Granular Materials

The use of electro thermal technologies based on microwave energy represents an important step in the development of new innovative solutions. The numerical modelling allows to study the influence of the high frequency electromagnetic and thermal field on the dielectric materials during the drying process before achieving practical installation. So, when is developed the experimental model will be already known some of the phenomena that characterize the system, being eliminated a number of unknown issues. This paper describes experiments conducted to gather data on production parameters in order to improve the stored corn seed quality. The interpretation and dissemination of results triggers the description of "recipes" for drying corn seeds. The described method is flexible and can be applied to near any agricultural seeds in further researches.

Combined conventional thermal and microwave drying process for typical Chinese lignite

A combined conventional and microwave drying process for Ximeng lignite was investigated in this paper. Samples were firstly dehydrated by hot air to achieve a conversion moisture content, and then were managed to final moisture content by microwave drying. Results showed that the drying rate was significantly improved in the decreasing rate period by microwave drying, approximately 3–5 times faster than that of conventional drying. Finally, the orthogonal test was used to clarify the effects of each factor on the energy consumption, and the magnitude ranking order was conversion moisture content > microwave power > hot air temperature.

Microwave drying process of corns based on double-porous model

ABSTRACTCurrent microwave drying of food product models have generally neglected mass transfer physics, which have resulted in higher predicted temperatures within the food matrix. It is necessary to include mass transfer physics in modeling microwave drying of food products, especially when they are dry for longer durations. In the actual drying process, most of the dry material is a double-porous media formed by the mutual accumulation of porous media, and there is no systematic theory to analyze the drying process of double-porous media. This work presents a new theoretical model based on the ractive pellet bed to achieve the simulation of microwave drying in situation of stacked corns. Compared with the applicable to porous model and analyzed the difference, finally through the change of temperature and moisture was measured by experiments to verify the rationality and accuracy of the double-porous model.

Microwave drying of CuCl residue from hydrometallurgical zinc recovery process

ABSTRACTCuCl residue is a hazardous waste from the hydrometallurgical zinc recovery plant. It is very difficult to recycle due to complicated composition, highly corrosive nature, and refractory characteristics. Recently, a new process of microwave drying and roasting was successfully developed to produce both CuO solid and HCl gas products from the CuCl residue. In this paper, the dielectric property of the CuCl residue was measured under different moisture contents. The microwave absorption characteristics of the CuCl residue improved considerably as its moisture content increased. Laboratory-scale drying tests were conducted to evaluate the kinetics of microwave drying and to optimize the operation parameters. The efficiency of microwave drying was further demonstrated in industrial-scale level by microwave dryer developed in in-house. Finally, the drying mechanism of microwave heating of the sticky material was discussed and several measurements were optimized/implemented to intensify the drying process.

Suitability of microwave drying for mealworms (Tenebrio molitor) as alternative to freeze drying: Impact on nutritional quality and colour

Freeze drying represents the current practice to stabilize mealworms, even though it is an energy demanding technique. Therefore, it was examined in the present study whether microwave drying could be a proper alternative. To this end, the impact of both drying techniques on the proximate composition, vitamin B12 content, fatty acid profile, oxidation status and colour parameters of mealworms was investigated. Furthermore, the influence of the application of vacuum during microwave drying was studied. The different drying technologies resulted in small differences in the proximate composition, while the vitamin B12 content was only reduced by microwave drying. The fat fraction of freeze dried mealworms showed a higher oxidation status than the fat of microwave dried mealworms. Application of a vacuum during the microwave drying process did not appear to offer advantages. This research shows that for mealworms microwave drying can be a proper alternative to freeze drying.

The application of microwave drying process for rambutan (Nephelium lappaceum L.) seed

ABSTRACTThis study aims to investigate the drying characteristics of rambutan seed in a microwave oven. The experiments were conducted at three power levels of 250, 600 and 1000 W and three drying times of 8, 9 and 10 h with three replications. Changes in these operational variables were conducted to highlight their effects on the drying characteristics observed through their drying profiles and final moisture contents. To estimate the drying behaviour, three thin layer equations were used to fit experimental data. Results showed that the drying time and final moisture content were decreased with an increase in the microwave power. Page model was selected as the best mathematical model to describe the drying process. Moisture diffusivity obtained varied from 4.10 × 10−9 to 9.64 × 10−9 m2 s−1 and the activation energy varied between 22.10 to 110.59 g.W−1 at different powers and drying times.

Artificial Neural Network Modeling of Drying Kinetics and Color Changes of Ginkgo Biloba Seeds during Microwave Drying Process

Ginkgo biloba seeds were dried in microwave drier under different microwave powers (200, 280, 460, and 640 W) to determinate the drying kinetics and color changes during drying process. Drying curves of all samples showed a long constant rate period and falling rate period along with a short heating period. The effective moisture diffusivities were found to be 3.318 × 10−9 to 1.073 × 10−8 m2/s within the range of microwave output levels and activation energy was 4.111 W/g. The L⁎ and b⁎ values of seeds decreased with drying time. However, a⁎ value decreased firstly and then increased with the increase of drying time. Artificial neural network (ANN) modeling was employed to predict the moisture ratio and color parameters (L⁎, a⁎, and b⁎). The ANN model was trained for finite iteration calculation with Levenberg-Marquardt algorithm as the training function and tansig-purelin as the network transfer function. Results showed that the ANN methodology could precisely predict experimental data with high correlation coefficient (0.9056–0.9834) and low mean square error (0.0014–2.2044). In addition, the established ANN models can be used for online prediction of moisture content and color changes of ginkgo biloba seeds during microwave drying process.

Volatilization of mercury in coal during conventional and microwave drying and its potential guidance for environmental protection

During the thermal drying process of coal, the sufficient attention has been paid to the changes in the properties of coal before and after the drying process. However, the volatility characteristics of harmful trace elements in coal during the drying process are always neglected to date, whereas the volatilization of harmful trace elements not only has significant effects on the surrounding atmosphere environment but also influence the following utilization of dried coal. In this study, the volatility characteristics of both the moisture and mercury in the coal sample during conventional (electric blast oven) and microwave (microwave oven) drying processes were investigated. In addition, the changes in the surface properties of coal before and after the drying process were studied using X-ray photoelectron spectroscopy (XPS) analysis. The results show that the moisture content of coal decreases in non-line format (decreases slowly at the first and then quickly according to the drying time) in the conventional drying process, while it decreases in line format in the microwave drying. The microwave drying process needs much shorter time required to obtain the dried coal of a certain moisture content than the conventional drying process. Under the same volatilization rate of moisture, the volatilization rate of mercury in the microwave drying process is always greater than that in the conventional drying process. Nearly half or more (49.73% and 59.26%, respectively) of the mercury in the coal slime evaporates into the atmosphere after the drying processes. XPS results show the content of CC and CH groups on the coal surface increased, but the CO content decreased after the drying processes and the microwave drying process has a stronger ability to remove oxygen-containing functional groups than the conventional drying process.