微波加热在毛条烘干中的应用研究

介绍了应用微波加热烘干毛条的基本原理,研究了微波干燥设备的烘干效率、节能效果及烘干效果,结果表明,微波加热可以应用于纺织印染行业,为微波加热在纺织印染行业的推广应用提供了研究基础。通过生产应用,在计算微波加热毛条烘干工艺参数时,微波烘干机的干燥效率可取1.1kg/kW﹒h,微波干燥后,毛条含水率均匀,颜色前后色差无变化,纤维的强度和手感得到改善,微波干燥节能效果,比射频烘干节能约20%。     

IMPROVING ENERGY EFFICIENCY IN COMBINED MICROWAVE-CONVECITVE DRYING

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.     

Assessing the moisture migration during microwave drying of coal using low-field nuclear magnetic resonance

Microwave drying of coal has the advantage of volumetric and selective heating, which saves energy and time. However, comprehensive assessment of the moisture migration during microwave drying is lacking. In this study, the low-field nuclear magnetic resonance (LF-NMR) was used for moisture characterization during microwave drying. Moisture migration occurs first in larger pores, whereas the smaller pores appear to remain water-saturated. As the microwave power increases, the drying capacity and drying rate both increase because high-power microwave can open the blind pores to promote moisture seepage. Furthermore, moisture migration during microwave drying is usually directional (i.e., along the bedding plane). The LF-NMR method provides a framework for the determination of moisture migration of coals.     

微波辐射干燥氟化铝的工艺研究

介绍了微波干燥的原理和主要特点，研究了利用微波辐射干燥氟化铝工艺，探讨了微波功率、微波辐射时间、物料质量、物料厚度对氟化铝脱水率的影响。实验的最佳条件为微波功率2．0kW、干燥时间80s、物料质量400g、物料厚度2．0cm。实验结果表明，利用微波辐射干燥，氟化铝含水质量分数小于1％，且干燥时间短、能耗低，实验为工业化提供了必要的参考数据。     

SIMULATION OF FLUIDIZED-BED DRYING OF CARROT WITH MICROWAVE HEATING

A mathematical model of coupled heat and mass transfer was applied to batch fluidized-bed drying with microwave heating of a heat sensitive material—carrot. Four kinds of microwave heating with intermittent variation were examined. The numerical results show that different microwave heating patterns can affect the fluidized bed drying significantly. Changing the microwave input pattern from uniform to intermittent mode can prevent material from overheating under the same power density. Supplying more microwave energy at the beginning of drying can increase the utilization of microwave energy while keeping temperature low within the particle. For a particle diameter of 4 mm, fluidization velocity of 2 m/s, inlet airflow temperature of 70°C and the bed area factor of 80, the drying time are 750 and 1000 s, respectively, for the two good operating conditions with on/off periods of 125/375 s and 375/375 s. The cumulative microwave energy absorbed by particles at the end of drying is 1415 and 2300 kJ/kg (dry basis), respectively.     

SIMULATION OF FLUIDIZED-BED DRYING OF CARROT WITH MICROWAVE HEATING

ABSTRACT

A mathematical model of coupled heat and mass transfer was applied to batch fluidized-bed drying with microwave heating of a heat sensitive material—carrot. Four kinds of microwave heating with intermittent variation were examined. The numerical results show that different microwave heating patterns can affect the fluidized bed drying significantly. Changing the microwave input pattern from uniform to intermittent mode can prevent material from overheating under the same power density. Supplying more microwave energy at the beginning of drying can increase the utilization of microwave energy while keeping temperature low within the particle. For a particle diameter of 4 mm, fluidization velocity of 2 m/s, inlet airflow temperature of 70°C and the bed area factor of 80, the drying time are 750 and 1000 s, respectively, for the two good operating conditions with on/off periods of 125/375 s and 375/375 s. The cumulative microwave energy absorbed by particles at the end of drying is 1415 and 2300 kJ/kg (dry basis), respectively.     

柳树河油页岩微波干燥特性及干燥模型

干燥预处理对油页岩的利用具有重要的意义,微波干燥是一种快速、高效和节能的干燥方式。在家用微波炉改造基础上搭建了微波干燥实验台,研究了不同微波功率对柳树河油页岩微波干燥特性的影响。并采用单项扩散模型等13个薄膜干燥模型等对微波干燥分别进行动力学分析,结果表明：微波干燥所需的时间远小于传统干燥所需的时间;微波干燥速率要远大于传统干燥干燥速率;油页岩微波干燥过程适用于双项扩散的半经验模型,油页岩微波干燥机理为湿分（液体或蒸气）双项扩散控制过程;300、400和550 W三个功率干燥时能耗相差不大。为了以较少的能耗达到相同的干燥效果,对于柳树河油页岩,以功率550 W干燥为佳。     

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.     

Microwave Drying of Wood Strands

Characteristics of microwave drying of wood strands with different initial moisture contents and geometries were investigated using a commercial small microwave oven under different power inputs. Temperature and moisture changes along with the drying efficiency were examined at different drying scenarios. Extractives were analyzed using gas chromatography/mass spectrometry (GC/MS). The results showed that the microwave drying process consisted of three distinct periods (warm-up period, evaporation period, and heating-up period) during which the temperature, moisture change, and drying efficiency could vary. Most of the extractives were remnant during microwave drying. It was observed that with proper selections of power input, weight of drying material, and drying time, microwave drying could increase the drying rate, save up to 50% of energy consumption, and decrease volatile organic compound (VOC) emissions when compared with the conventional drying method.     

Microwave Drying of Wood Strands

Abstract

Characteristics of microwave drying of wood strands with different initial moisture contents and geometries were investigated using a commercial small microwave oven under different power inputs. Temperature and moisture changes along with the drying efficiency were examined at different drying scenarios. Extractives were analyzed using gas chromatography/mass spectrometry (GC/MS). The results showed that the microwave drying process consisted of three distinct periods (warm-up period, evaporation period, and heating-up period) during which the temperature, moisture change, and drying efficiency could vary. Most of the extractives were remnant during microwave drying. It was observed that with proper selections of power input, weight of drying material, and drying time, microwave drying could increase the drying rate, save up to 50% of energy consumption, and decrease volatile organic compound (VOC) emissions when compared with the conventional drying method.     

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.     

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.     

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.     

Modularer Trocknungsprozessor zur Anwendung kombinierter Trocknungsverfahren

A modular drying processor for the development of energy and time efficient combined drying processes was constructed. It allows the serial and/or parallel application of different drying methods such as freeze‐, hot‐air‐, infrared‐, contact‐, microwave‐ or vacuum‐drying without interruption between the processes. Depending on the drying method precise product structures could be achieved by using the example of carrot cubes. The serial combination of freeze‐ and microwave vacuum drying shortened the process time about 40 % while highest product quality was obtained.     

Microwave Drying of Multi-component Powder Systems

The rate of uptake of microwave technology within the pharmaceutical sector has been slow due to stringent regulatory procedures and unsuccessful microwave trials. However, microwave systems offer significant energy and time savings compared with conventional dryers. Common pharmaceutical excipients, lactose, stearic acid, and active ingredients, Aspirin, Paracetamol, were wetted with water and dried within an experimental atmospheric microwave drying system (2.45 GHz, 90 W). The drying curves obtained showed a constant drying rate, followed by two falling rate periods. Two- and three-component mixtures were dried under the same conditions. The drying characteristics were found to lie between those of the pure components. Drying kinetics of samples wetted with a range of solvents (methanol, ethanol, acetone), and two-component solvent mixtures, were examined. The relative evaporation rates of the solvents showed some dependence on their dielectric properties; latent heat of vaporisation and boiling point having greater influence on drying characteristics. Clearly this research provides information vital to achieve increased acceptance of this technology within the pharmaceutical industry.     

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.     

Trends in Microwave-Assisted Freeze Drying of Foods

Microwave-assisted freeze drying (MFD) can be accomplished in two distinct ways: freeze drying assisted concurrently with microwave application (MFD-1) and freeze drying and assisted microwave/vacuum microwave drying in two consecutive separate drying stages (MFD-2). MFD is a rapid dehydration technique that can be applied to certain foods, particularly to seafoods, solid soup, and fruits and vegetables. MFD involves much less drying time and energy consumption than conventional freeze-drying methods. Currently, this technology has been successfully used to dry many food materials and has potential in the food industry. Increasing concerns over product quality, energy savings, and production costs have motivated researchers and the industry to adopt MFD technologies. The advantages of MFD include shorter drying time, energy savings, improved product quality, and flexibility in producing a wide variety of dried products. However, current applications are limited to small categories of foods due to high startup costs and relatively complex technology compared to conventional freeze drying. This article presents a concise review of recent progress in MFD R&D and makes recommendations for future research to bridge the gap between laboratory research and industrial applications.     

Microwave Drying of Multi-component Powder Systems

Abstract

The rate of uptake of microwave technology within the pharmaceutical sector has been slow due to stringent regulatory procedures and unsuccessful microwave trials. However, microwave systems offer significant energy and time savings compared with conventional dryers. Common pharmaceutical excipients, lactose, stearic acid, and active ingredients, Aspirin, Paracetamol, were wetted with water and dried within an experimental atmospheric microwave drying system (2.45 GHz, 90 W). The drying curves obtained showed a constant drying rate, followed by two falling rate periods. Two- and three-component mixtures were dried under the same conditions. The drying characteristics were found to lie between those of the pure components. Drying kinetics of samples wetted with a range of solvents (methanol, ethanol, acetone), and two-component solvent mixtures, were examined. The relative evaporation rates of the solvents showed some dependence on their dielectric properties; latent heat of vaporisation and boiling point having greater influence on drying characteristics. Clearly this research provides information vital to achieve increased acceptance of this technology within the pharmaceutical industry.     

Microwave Drying Kinetics of Okra

In this work, the effects of power level and sample mass on moisture content, moisture ratio, drying rate, and drying time of Turkey okra (Hibiscus esculenta L.) were investigated using microwave drying technique. Various microwave power levels ranging from to 180 to 900 W were used for drying of 100 g of okra. To investigate the effect of sample mass on drying, the samples in the range of 25 to 100 g were dried at microwave power level of 360 W. To determine the kinetic parameters, the drying data were fitted to various models based on the ratios of the differences between the initial and final moisture contents and equilibrium moisture content. Among of the models proposed, Page's model gave a better fit for all drying conditions used. The activation energy for microwave drying of okra was calculated using an exponential expression based on Arrhenius equation and was found to be 5.54 W/g.     

耐火浇注料对流与微波干燥的实验研究

进行了耐火浇注料热重分析,热风和微波干燥实验研究,得到了浇注料热风和微波干燥特性曲线。实验结果表明,与热风干燥比较,微波加热极大地降低了耐火浇注料干燥时间、提高了干燥效率和能源效率高,在耐火浇注料干燥过程中有着广阔的应用前景。