IMPROVING ENERGY EFFICIENCY IN COMBINED MICROWAVE-CONVECITVE DRYING

ABSTRACT

Convective-microwave drying has been recognized as a convenient option for drying of valuable materials. However, the advantages of the method must be carefully evaluated in order to establish the limits for full scale operating conditions because, as demonstrated in this work, the reduction in the absolute drying time as a function of the applied microwave power is devalued by an increase in the unabsorbed microwave energy, decreasing the overall energy efficiency of the drying process.

Experimental study of the energy efficiency of combined microwave-convective drying of agar gel and Gelidium seaweeds was carried out in a laboratory scale microwave-convective dryer by continuously following the absorption of microwave energy by the sample as well as the change of the sample mass during the drying process, under different operating conditions. Several drying strategies based on the on-off application of the microwave power were proposed and evaluated from the point of view of both drying kinetics and energy efficiency in order to program and rationalize the use of microwave energy in the combined microwave-convective drying process.     

Analysis of Energy Consumption in Drying Process of Non-Hygroscopic Porous Packed Bed Using a Combined Multi-Feed Microwave-Convective Air and Continuous Belt System (CMCB)

In this study, the analysis of energy consumption during the drying of non-hygroscopic porous packed bed by combined multi-feed microwave-convective air and continuous belt system (CMCB) was investigated experimentally. By using a combined multi-feed microwave-convective air and continuous belt system drier, the microwave power was generated by means of 12 compressed air-cooled magnetrons of 800 W each that give a maximum of 9.6 kW. The power setting could be adjusted individually in 800 W steps. Hot air with the maximum working temperature of 240°C was generated using 24 units of electric heater where the total power capacity is 10.8 kW. Most importantly, this work focused on the investigation of drying phenomena under industrialized microwave processing. In this analysis, the effects of the drying time, hot air temperature, porous structure (F-Bed and C-Bed), and location of magnetrons on overall drying kinetics and energy consumption were evaluated in detail. The results showed that the overall drying and energy consumption depend upon the porous structure, hot air temperature, and location of magnetrons. Furthermore, using the continuous microwave application technique had several advantages over the conventional method, such as shorter processing times, volumetric dissipation of energy throughout a product, and less energy consumption. The results presented here provided fundamental understanding for the drying process using a combined multi-feed microwave-convective air and continuous belt system in industrial size.     

Microwave-Convective and Microwave-Vacuum Drying of Cranberries: A Comparative Study

Abstract

Two drying methods of cranberries (microwave-vacuum and microwave-convective) are reviewed, and their advantages and disadvantages regarding the quality of dried product and the process performance are presented. Mechanically and osmotically pretreated berries were subjected to drying and quality evaluation. Quality parameters are color (in Hunter L*a*b* coordinates), textural characteristics (toughness and Young's modulus), and organoleptic properties (color, texture, taste, and overall appearance). Special emphasis was given to the energy performance of the process, monitoring of the real-time temperature profile, and the total microwave power-on time. Two microwave power densities are assessed, as well as different microwave power-on/power-off cycling periods. In almost all observed parameters, microwave-vacuum drying exhibited enhanced characteristics when compared to microwave-convective drying. Drying performance results (defined as mass of evaporated water per unit of supplied energy) showed that microwave-vacuum drying is more energy-efficient than microwave-convective. Tasting panel results exhibited slight preference in all parameters for microwave-convective dried samples.     

Quality Evaluation of Microwave-Dried Packham's Triumph Pear

Abstract

The objectives of this work were to carry out the microwave and microwave-convective drying of pear, in order to preserve the quality by reducing the drying time of the fruit; to compare the kinetic with convective drying mode data and to fit the comparative data to models from literature. The effect of osmotic dehydration and the variation in the microwave power were analyzed at constant values of air temperature and velocity. The experimental data fit well the proposed kinetic model. The processed pear was evaluated with respect to water content, quantity of calcium, phosphorus and potassium, water activity and apparent density. The best results were found for the samples osmotically dehydrated and dried in the microwave-convective dryer. The osmotic dehydration and the short drying times were important to maintain sample characteristics.     

Characteristics of Chard Leaves during Microwave, Convective, and Combined Microwave-Convective Drying

Chard leaves (Beta vulgaris L. var. cicla), which weighs 25 g with a moisture of 9.35 (db), were dried using three different drying methods, microwave, convective, and combined microwave-convective. Drying continued until leaf moisture fell down to 0.1 (db). Drying periods lasted 5-9.5, 22-195, and 1.5-7.5 min for microwave, convective, and combined microwave-convective drying, respectively, depending on the drying level. In this study, measured values were compared with predicted values obtained from Page's semi-empirical equation. Optimum drying period, color, and energy consumption were obtained for combined microwave and convective drying. The optimum combination level was 500 W microwave applications at 75°C.     

Characteristics of Chard Leaves during Microwave,Convective, and Combined Microwave-Convective Drying

Chard leaves (Beta vulgaris L. var. cicla), which weighs 25 g with a moisture of 9.35 (db), were dried using three different drying methods, microwave, convective, and combined microwave-convective. Drying continued until leaf moisture fell down to 0.1 (db). Drying periods lasted 5–9.5, 22–195, and 1.5–7.5 min for microwave, convective, and combined microwave-convective drying, respectively, depending on the drying level. In this study, measured values were compared with predicted values obtained from Page's semi-empirical equation. Optimum drying period, color, and energy consumption were obtained for combined microwave and convective drying. The optimum combination level was 500 W microwave applications at 75°C.     

Analysis of Energy Consumption in Drying Process of Biomaterials Using a Combined Unsymmetrical Double-Feed Microwave and Vacuum System (CUMV)—Case Study: Tea Leaves

An energy analysis of drying of biomaterials (tea leaves) was conducted on a combined unsymmetrical double-feed microwave and vacuum system (CUMV) to optimize the operating conditions and quality of the products. Tea leaves were dried from an initial moisture content of 172 to 7% (db). An energy consumption model based on the first law of thermodynamics was developed to evaluate energy efficiency. The influences of microwave power, vacuum pressure, and microwave operation modes on energy consumption were investigated in detail. The results showed that energy consumption as well as energy efficiency were strongly dependent on vacuum pressure and microwave power. Energy consumption and color parameters of the tea leaves were compared at different drying conditions. In particular, the experiments were carried out at different microwave powers (800 and 1,600 W) at a frequency of 2,450 MHz and different vacuum pressures (535 and 385 torr) to investigate the effects of these factors on the microwave–vacuum drying.     

湿法炼锌泥的微波干燥实验

目前回转窑干燥物料的方法主要存在干燥时间长、环境污染严重、效率低等缺点,为了解决这一问题,开展了微波干燥湿法炼锌泥的实验研究,采用实验室自制的微波干燥设备,考察微波功率、干燥时间以及物料质量对样品脱水率的影响。研究结果表明:在一定条件下,脱水率随干燥时间延长而增加,随着微波功率的增加而提高,随物料质量增加而降低。实验在微波功率600W、物料质量40g、微波加热时间3min时,湿法炼锌泥的脱水率达到99.57%;FT-IR分析可知,干燥后波数1625cm^-1和3300cm^-1附近水分子的伸缩振动峰基本消失,说明微波干燥后物料基本不含水分;与常规干燥进行比较,微波干燥湿法炼锌泥具有干燥时间短、脱水效率高、清洁无污染的优点,是一种高效节能的新型干燥方法。     

Energetic Analysis of Drying Biological Materials with High Moisture Content by Using Microwave Energy

One of the most important parameters in the drying process is the efficiency of energy transfer. Although it is well known that the absorbed component of the referred energy in the dehydration process under microwave conditions is theoretically lower—especially during the falling rate period of drying—by an order of magnitude than that of convective drying, the real efficiency of energy transfer in the microwave field depends on numerous—partly unknown—factors. In this study, the most significant relations between the energy transfer efficiency and the drying parameters were investigated. During the experiments, two power levels were used to study the effects of domain variables in apple, potato, and onion dehydration processes. The results showed that variations in microwave power level played an important role in overall drying kinetics.

Based on the recorded data, we determined the typical energy efficiency factors for microwave drying of apple, potato, and onion. The specific energy consumption was calculated to compare the energy efficiency of a single measurement series. It was evident that by applying higher power levels, the energy utilization was more efficient. Furthermore, we theoretically proved and experimentally showed the nonthermal effects of the microwave energy.     

Microwave-convective drying characteristics of pharmaceutical powders

Common pharmaceutical excipients and active ingredients, wetted with specific solvents, were dried in a combined microwave-convective system (2.45 GHz, 90 W). The drying curves showed a constant drying rate period, followed by two falling rate periods. Cross-flow air velocity had little effect on solvent evaporation rate in the initial stages; however, an increase in drying rate was observed during the falling rate period. Increasing air temperature increased the drying rate throughout the entire process, with reductions in drying time of up to 78% being observed. Average and maximum sample temperatures were found to decrease on addition of air-flow, the extent of which was dependent on material and operating conditions.     

Modeling of Combined Microwave and Convective Drying of Wood: Prediction of Mechanical Behavior via Internal Gas Pressure

The impact of microwave drying on the quality of dried wood remains unclear. Particular attention should be paid in order to optimize the combined microwave and convective drying process. In this study, a comprehensive internal heat and mass transfer model was developed and numerically implemented in order to simulate and understand the physical phenomena occurring inside Jack pine wood during a combined microwave and convective drying process. The model was validated on the basis of the average moisture content curves for drying scenarios at various microwave power levels. According to the simulations results, an increase in microwave power significantly decreases the drying time of Jack pine wood and increases its internal gas pressure, which increases the risk of cracking. However, compared to purely conventional convective drying, combined microwave and convective drying at medium microwave power and air temperature significantly reduces the drying time and maintains the internal gas pressure at reasonable values. At these conditions, the risk of cracking will be diminished. This last result was checked via experimental measurements of the sample strength dried at different microwave power levels. From this study, we can consider that for Jack pine wood, combined microwave and convective drying is a more efficient technology compared to classical convective drying.     

Thin layer drying kinetics,characteristics and modeling of plaster of paris

Drying behavior of thin slabs of plaster of paris was investigated in microwave, convective and microwave assisted convective drier. The process parameters involved in the study include the microwave output power, air temperature used for drying, the initial moisture content of the sample, thickness and geometry of the sample. A typical drying profile, with no constant rate period followed by a falling rate period was obtained for all the three methodologies used. The mechanism of moisture movement within the sample during the falling period, as expressed by effective moisture diffusivity, was between 1.166 × 10⁻⁸ and 2.6927 × 10⁻⁸ (m² s⁻¹) for the least microwave output power of 180 W. The specific energy consumption and the drying efficiency involved in the microwave process, estimated from the energy supplied to the drier and the total time was found to be 0.4557 MJ kg⁻¹ and 82.56% for a sample thickness of 0.013 m. The microstructure of the fractured surfaces and qualitative phase analysis of microwave activated plaster of paris were extensively investigated by Scanning Electron Microscopy, Fourier Transform Infra-Red spectroscopy and X-Ray Diffraction techniques. The experimental moisture loss data with time was fitted to nine semiempirical thin layer equations. The suitability of the models was validated by comparing it with the predicted moisture ratio for any given set of operating conditions. The statistical analyses of the available literature correlations, for their applicability to the data of the present study showed greater deviation attributing their restricted range of applicability of individual correlations. As a result, a third order polynomial equation was developed from this study.     

Drying Characteristics and Kinetics of Vacuum Microwave–Dried Potato Slices

Vacuum microwave dehydration characteristics and drying kinetics of potato slices were evaluated using a laboratory-scale dryer. The test materials were placed on a rotating tray to allow uniform exposure to the microwave field. The effect of drying conditions on the drying characteristics was investigated. As expected, higher drying rates were obtained with higher microwave power and lower vacuum levels. The effect of vacuum pressure on drying rate was not as significant as that of microwave power. The Henderson & Pabis model provided better simulation of the drying curves for potato slices. Empirical models are presented for the drying rate constant as a function of the microwave power and vacuum pressure for potato slices under the range of operating conditions tested.     

Primary energy demand and energy costs of fixed-bed drying using the example of chamomile flowers

Abstract

Energy use efficiency in the drying of medicinal and aromatic plants is largely determined by weather conditions and process parameters. While the former are beyond control, the latter can be optimized. In order to achieve such optimization, different energy supply variants, based on typical operating conditions of batch-type grate drying in Thuringia, Germany, were analyzed. It was found that partial air recirculation and integration of heat pumps allow substantial savings in primary energy. However, under the constraint of German energy prices, significant savings in energy costs can only be achieved if combined heat and power generation systems are applied at the same time.     

THE EFFECT OF PROCESS CONDITIONS ON THE DRYING KINETICS AND REHYDRATION CHARACTERISTICS OF SOME MW-VACUUM DEHYDRATED FRUITS

The effect of drying conditions, namely, microwave power, vacuum pressure, as well as sample geometry, on the drying kinetics and rehydration characteristics of five microwave vacuum dehydrated fruits (apple, avocado, mushroom, pear and strawberry) was studied. The investigation involved a wide range of microwave power, vacuum pressure and sample size levels. The drying rate and the rehydration ratio, a measure of rehydration characteristics, were found to depend on the drying conditions. An empirical mass transfer model, involving a characteristic parameter for each process (drying and rehydration( as a function of process variables, namely, microwave power, vacuum pressure and sample size, was also tested with the data obtained in a microwave oven equipped with vacuum apparatus. Furthermore, a comparison of rehydration properties of microwave vacuum and conventionally dehydrated products, was conducted.     

响应曲面法漂白紫胶微波-真空干燥工艺研究

为提高漂白紫胶产品质量及生产效率,采用微波-真空干燥漂白紫胶研究了不同压强、微波功率和干燥时间对漂白紫胶产品颜色指数和含水率的影响,用响应曲面法（RSM）设计试验并建立了该工艺条件的拟合方程。结果表明,拟合方程的拟合度较高,所建立的数学模型可以用于描述漂白紫胶的微波-真空干燥。漂白紫胶微波-真空干燥的最优条件：干燥压强为3.0 kPa,微波功率为795 W,干燥时间为30 min。在优化条件下进行了验证实验,得到了颜色指数为0.9、含水率为0.028 9kg水/kg干料的产品。     

Development of a Microwave–Vacuum-Based Dehydration Technique for Fresh and Microwave–Osmotic (MWODS) Pretreated Whole Cranberries (Vaccinium macrocarpon)

A bench scale microwave–vacuum (MWV) dryer was developed using a modified consumer-grade microwave oven. MWV dehydration was first tested as a standalone method on whole frozen–thawed berries. Subsequently, a new combination drying technique was developed employing microwave osmotic dehydration under continuous-flow medium-spray (MWODS) conditions together with MWV as a secondary drying operation. Fresh (frozen–thawed) and MWODS pretreated berries were dried under a range of MWV treatments employing continuous and decreasing microwave power settings (duty cycles). Initial microwave power density for all treatments was approximately 10.2 W/g and magnetron power-on and power-off times varied from 3 to 15 s and 27 to 15 s, respectively. Drying times to 20% (db) were recorded and energy consumption was calculated according to the total magnetron power-on time where overall it was found that drying times and energy consumption decreased with increasing MWV process intensity, where drying times for all MWV treatments were significantly shorter than those of conventional air drying. Drying kinetics were fit using two models (exponential and Page's empirical model), where Page's model better fit the experimental data. The quality of the berries was monitored visually through evidence of scorching in order to screen treatments and establish upper limits of treatment intensity for further studies.     

THE EFFECT OF PROCESS CONDITIONS ON THE DRYING KINETICS AND REHYDRATION CHARACTERISTICS OF SOME MW-VACUUM DEHYDRATED FRUITS

ABSTRACT

The effect of drying conditions, namely, microwave power, vacuum pressure, as well as sample geometry, on the drying kinetics and rehydration characteristics of five microwave vacuum dehydrated fruits (apple, avocado, mushroom, pear and strawberry) was studied. The investigation involved a wide range of microwave power, vacuum pressure and sample size levels. The drying rate and the rehydration ratio, a measure of rehydration characteristics, were found to depend on the drying conditions. An empirical mass transfer model, involving a characteristic parameter for each process (drying and rehydration( as a function of process variables, namely, microwave power, vacuum pressure and sample size, was also tested with the data obtained in a microwave oven equipped with vacuum apparatus. Furthermore, a comparison of rehydration properties of microwave vacuum and conventionally dehydrated products, was conducted.     

Energy Analysis of Poly-hydroxybutirate (PHB) Drying Using a Combined Microwave/Rotating Pulsed Fluidized Bed (MW/RPFB) Dryer

The aim of this study was to investigate the drying kinetics and energy efficiency of a combined microwave/rotating pulsed fluidized bed (MW/RPFB) dryer to dry particles of poly-hydroxybutirate (PHB). In order to analyze the effects of microwaves and hot air conditions on the drying kinetics of PHB and energy efficiency, an experimental design technique was applied. Drying energy efficiencies were in the range of 12–30%, and an estimated value of 10.5% was found for pure convective drying of PHB in an RPFB with the same final moisture content.     

Drying of a Slip Casting for Tableware Product Using Microwave Continuous Belt Dryer

To demonstrate the potential of microwave drying in the ceramic industry, microwave demolding of tableware product by a continuous microwave belt drier has been investigated. This study focuses on the investigation of the effects of the irradiation time, sample size, microwave power and location of magnetron on overall drying kinetics. The results show that microwave drying has several advantages over the conventional method such as shorter processing time, volumetric dissipation of energy throughout a product, high energy efficiency, reduced amount of mold usage, and offering product quality. Further quantitative validation of experimental data could be very useful, especially in providing information for processing high-performance microwave drying for developing the ceramic industry in Thailand.