Increasing Microwave Power Research Articles

QUALITY, PERFORMANCE ANALYSIS, MASS TRANSFER PARAMETERS AND MODELING OF DRYING KINETICS OF SOYBEAN

Abstract Different aspects of soybean drying such as energy and exergy analyses, quality, mass transfer parameters, and modeling of drying kinetics were investigated in a microwave dryer. Results showed that energy and exergy efficiency increased with increasing microwave power, while values of energy efficiency (33.70 to 66.0%) were higher than exergy efficiency (23.38 - 48.30%). Specific energy consumption and energy loss varied 4.93 to 9.11 MJ/kg water and 5.04 to 8.89 MJ/kg water, respectively. Approximately 8.94 to 20.07% of the total energy input is consumed by increasing of the product temperature. The values of improvement potential changed between 1.31- 5.35 MJ/kg water. Bulk density, degree of shrinkage and rehydration ratio varied from 726.6 to 762.8 kg/m3, 0.888 to 0.910, and 0.618 - 0.799, respectively. Parameters total color change (14.68 - 19. 89) and hue angle (88.07 to 91.73o) increased with increasing microwave power. Effective diffusivity and mass transfer coefficient varied from 1.99×10−9 to 12.25×10−9 m2/s and 2.71×10−6 to 19.98×10−6 m/s, respectively. The activation energy was found to be 4.98 W/g for a diffusion model and 5.33 W/g for a mass transfer model. Among the models, the Page model was found to best describe the drying behavior of soybean.

Simulation of microwave thin layer drying process by a new theoretical model

Various methodologies have been proposed in literature on modeling microwave drying process. However, in these methodologies moisture diffusion is normally considered in the presence of intensive microwave energy. In the present study, a new theoretical model was developed to simulate microwave drying of thin layer particulate solids, based on the consideration that moisture diffusion along material layer could be ignored due to rapid evaporation under intensive microwave energy. The model was solved numerically by using finite difference method and validated against experimental data. Results indicated good agreement between the model and experimental data, thus providing confidence in the modeling approach. For the system investigated in this study, it was demonstrated that an 80% reduction in drying time was achieved with approximately fivefold increase in microwave power (109–543W). Furthermore, it was also demonstrated that the drying rate was the maximum corresponding to the optimal layer thickness in microwave thin layer drying process. Qualitative analysis explained the optimal thickness phenomenon using principles of heat and mass transfer. Finally, the validated model was used to predict moisture and temperature distributions along the entire material layer.

Effect of Microwave Treatment on the Reactivity of Synthetic Coal Char for NO Reduction

Synthetic coal char (SCC) was made from nanocarbon powder, sucrose, deionized water, and ethanol in appropriate proportions and then pyrolyzed at 800 or 900 °C. SCC samples modified by different microwave power were investigated for reduction of NO in a microwave–electrothermal reactor system. The results showed that with increasing microwave power, the reactivity of SCC with NO first increased and then decreased. The optimum microwave power was 640 W for SCC pyrolyzed at 800 °C for 20 min (800 °C SCC) and 480 W for SCC pyrolyzed at 900 °C for 20 min (900 °C SCC). In addition, the higher the pretreatment temperature, the better the effect of microwave modification. To investigate the mechanism of modification of SCC by microwaves, mercury intrusion analysis, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy were performed on 800 °C SCC samples. The changes in the pore structure, graphitization degree, and oxygen functional groups were determined. With increasing microwave power, ...

Drying kinetics of onion (Allium cepa L.) slices with convective and microwave drying

Onion slices were dried using two different drying techniques, convective and microwave drying. Convective drying treatments were carried out at different temperatures (50, 60 and 70 °C). Three different microwave output powers 328, 447 and 557 W were used in microwave drying. In convective drying, effective moisture diffusivity was estimated to be between 3.49 × 10−8 and 9.44 × 10−8 m2 s−1 within the temperature range studied. The effect of temperature on the diffusivity was described by the Arrhenius equation with an activation energy of 45.60 kJ mol−1. At increasing microwave power values, the effective moisture diffusivity values ranged from 2.59 × 10−7 and 5.08 × 10−8 m2 s−1. The activation energy for microwave drying of samples was calculated using an exponential expression based on Arrhenius equation. Among of the models proposed, Page’s model gave a better fit for all drying conditions used.

Dielectric properties and microwave and infrared‐microwave combination drying characteristics of banana and kiwifruit

AbstractThe scope of this study was to investigate the effects of initial moisture content, different drying methods and different microwave power on quality and dielectric properties of banana (Musa cavendishii) and kiwifruit (Actinidia deliciosa) and it was also aimed to examine the correlation between dielectric properties and microwave and microwave‐infrared combination (MW‐IR) drying characteristics of banana and kiwifruit. Samples with different initial moisture contents were dried by using different microwave powers (180 W, 270 W, 360 W for banana) and (350 W, 420 W, 490 W for kiwifruit). For combination drying of banana, the application parameters were adjusted as 360 W for microwave power and 600 W for upper and lower halogen lamps (600 W) and for kiwifruit those were adjusted as 420 W and 600 W, respectively. The drying characteristics investigated throughout the study were moisture content and color values (L*, a*, and b*). Dielectric properties of fresh and processed samples were also detected. It was observed that an increase in microwave power resulted in an increase in rate of drying and a decrease in processing time. It was also found that combining infrared with microwaves resulted in higher rates of moisture loss from the product. Dielectric properties of banana and kiwifruit samples decreased with increasing microwave power and initial moisture content. When microwave and combination dried samples were compared with conventionally dried ones, products with lower final moisture content and higher quality could be produced with a time saving of approximately 98%.Practical applicationsConventional drying like hot‐air drying takes a long time even at high temperatures which results in a decrement of the dried product quality. To overcome such a problem, as well as to obtain a product with better nutritional and sensory properties, the process could be speeded up by the application of microwave energy. In this study, both microwave and microwave‐infrared combination drying were investigated. The most important property of combination drying is the ability to combine the time saving advantages of microwave with surface moisture removal advantage of infrared heating. There is a lack of knowledge of literature about an appropriate drying method to preserve the best quality of banana and kiwifruit samples, thus in this study the basic scope is developing an efficient, time‐saving drying method to improve quality characteristics of banana and kiwifruit samples.

Effects of the components in rice flour on thermal radical generation under microwave irradiation

The relationships between radical generation under microwave irradiation and the components of various types of rice flour were investigated. Electron paramagnetic resonance (EPR) spectroscopy was used to characterize the radicals found in rice flour samples. The EPR spectra revealed that several types of radical (carbon-centered, tyrosyl and semiquinone) were localized in the starch and protein fractions of the rice flour. The signal intensity of the free radicals was observed to increase exponentially with increasing microwave power and residence time. The rice bran samples exhibited the greatest free radical signal intensity, followed by the brown rice samples and the white rice samples. This finding was consistent for both the native and the microwaved samples. The ratio of rice starch to rice protein also played an important role in the generation of radicals.

Comparison between the effect of microwave irradiation and conventional heat treatments on the magnetic properties of chalcopyrite and pyrite

The effect of microwave radiation and conventional heat treatments of chalcopyrite and pyrite on their magnetic properties was investigated. Magnetic susceptibility of chalcopyrite increased significantly by increasing microwave power and treatment time. Pyrite magnetism was affected only at 500W microwave power. On the other hand, magnetic susceptibility of both chalcopyrite and pyrite increased significantly by increasing heating temperature. X-ray diffraction and XPS characterization showed that the formation of more magnetic phases after chalcopyrite and pyrite treatment is the reason behind their increased magnetism. Dry magnetic separation of individual treated minerals at several magnetic field strengths from 0.5Tesla to 2Teslas was conducted. The results suggest that selective magnetic separation of chalcopyrite from pyrite is possible only in case of microwave treatment due to its selective effect in favor of chalcopyrite at lower power levels compared to pyrite.

Mathematical analysis of compound release during microwave assisted retting of flax stems

Microwave-assisted retting was conducted at various power levels (1, 1.5 and 2 W g −1 ) on pre-soaked flax stems (12, 24 and 36 h). The retted flax stems were dried and the fibres were separated. The amount of cellulose, hemicellulose and lignin presented in the flax fibres was established by NIR (near infrared) spectroscopy. Based on the rate of change of cellulose, hemicellulose and lignin at various levels of treatments, a kinetic model was developed and the model was validated by analysing the compositions of hemp fibres obtained from pre-soaked hemp stems at various microwave power levels. The rate of change of cellulose percentage in the model fitted with the observed values of cellulose percentage with an average R 2 value of 0.87 and an average RMSE (root-mean-square error) value of 0.0130. But in hemicellulose, the R 2 value was 0.936 and average RMSE value was 0.0135, and for lignin, R 2 value was 0.92 and RMSE value of 0.0181. The rate coefficient for all the treatments was increasing within the treatment limit, which indicated the increased reaction rate with an increase in microwave power. Validation of the model was successfully conducted by analysing the components of hemp fibres at various levels of microwave powers.

Vacuum ultraviolet emission from hydrogen microwave plasmas driven by surface waves

The vacuum ultraviolet (VUV) radiation emitted by hydrogen surface-wave-driven plasmas operating at microwave frequency (2.45 GHz) and low-pressure conditions (0.1–2 mbar) was investigated, in particular the influence of microwave power and gas pressure on the intensity of the emissions. The strong emission of Lyman H2 and Werner H2 molecular bands in the 80–125 nm spectral range was detected, while the most intense atomic emissions observed correspond to Lyman-α and Lyman-β lines at 121.6 nm and 102.6 nm respectively. An increase of the atomic lines and molecular bands intensities with increasing microwave power at pressure 0.1 mbar was observed. At 2 mbar the VUV spectra are entirely dominated by molecular bands. Theoretical predictions, as obtained from a collisional-radiative model, were validated by the experimental results.

Anomalous non-resonant microwave absorption in SmFeAs(O,F) polycrystalline sample

Here we present the non-resonant microwave absorption (NRMA) studies on SmFeAsO0.88F0.12 polycrystalline sample measured at 6.06K with the magnetic field swept from −250G to +250G at a frequency of 9.45GHz. It was observed that the (NRMA) line shape evolves as a function of microwave power. Again, the signal intensity increases from 22.83 µW to 0.710mW where it reaches a maximum and quite remarkably it changed from ‘normal’ absorption to ‘anomalous’ absorption at 2.247mW, then the intensity decreases with further increase of microwave power. The crossover from ‘normal’ to ‘anomalous’ NRMA absorption and its dependence on microwave power is a new phenomenon in iron pnictides superconductors and we have attributed this anomaly to come from non-hysteretic Josephson junction.

Linear polarization study of microwave-radiation-induced magnetoresistance oscillations: Comparison of power dependence to theory

We present an experimental study of the microwave power and the linear polarization angle dependence of the microwave-induced magnetoresistance oscillations in the high-mobility GaAs/AlGaAs two-dimensional electron system. Experimental results show the sinusoidal dependence of the oscillatory magnetoresistance extrema as a function of the polarization angle. Yet, as the microwave power increases, the angular dependence includes additional harmonic content, and it begins to resemble the absolute value of the cosine function. We present a theory to explain such peculiar behavior.

Chemical deterioration and discoloration of semi-dried tilapia processed by sun drying and microwave drying

ABSTRACTDrying methods can affect the chemical deterioration and discoloration of fish muscle. This study aimed to monitor the effect of microwave drying at different effective powers in comparison with sun drying on the physicochemical changes of semidried tilapia. Lipid deterioration, protein oxidation and discoloration of semidried tilapia (Oreochromis niloticus) muscle processed by sun drying and microwave drying were investigated. Results of the study revealed that the pH of microwaved tilapia was higher than that of the sun-dried tilapia (p < 0.05) and the pH increased with increasing microwave power (p < 0.05). The sun drying possessed higher degree of lipid oxidation as indicated by thiobarbituric acid-reactive substances (TBARS) with a greater level of lipolysis when compared to microwave drying (p < 0.05). Higher protein carbonyl content was in microwaved tilapia compared to sun-dried one (p < 0.05) and this content increased with increasing microwave power (p < 0.05). Sun drying resulted in a brown discoloration correlated with the highest metmyoglobin formation (p < 0.05), whereas microwave drying, particularly with high power, caused a green discoloration of semidried tilapia as revealed by a negative a* value with a significantly decreased redness index (p < 0.05). Therefore, lipid deterioration, protein oxidation and discoloration of semidried tilapia muscle were strongly influenced by drying methods.

Application of EPR spectroscopy to examine free radicals evolution during storage of the thermally sterilized Ungentum ophthalmicum

Free radicals formed during thermal sterilization of the Ungentum ophthalmicum were examined by an X-band EPR spectroscopy. The influence of storage time (15 min; 1, 2 and 3 days after heating) on free radical properties and concentrations in this sample was determined. Thermal sterilization was done according to the pharmaceutical norms. The first-derivative EPR spectra with g-values about 2 were measured with magnetic modulation of 100 kHz in the range of microwave power 2.2–70 mW. The changes of amplitudes (A) and linewidths (ΔBpp) with microwave powers were evaluated. Free radicals in concentration ∼1017 spin/g were formed during heating of the tested Ungentum. Free radical concentration decreased with increase in storage time, and reached values ∼1017 spin/g after 3 days from sterilization. The tested U. ophthalmicum should not be sterilized at a temperature of 160 °C because of the free radicals formation, or it should be used 3 days after heating, when free radicals were considerably quenched. Free radical properties remain unchanged during storage of the Ungentum. The EPR lines of the U. ophthalmicum were homogeneously broadened and their linewidths (ΔBpp) increased with increase in microwave power. EPR spectroscopy is useful to examine free radicals to optimize sterilization process and storage conditions of ophthalmologic samples.

Energetic and exergetic performance analysis and modeling of drying kinetics of kiwi slices.

This work focused on the effects of the moisture content, slices thickness and microwave power on aspects of energy and exergy, drying kinetics, moisture diffusivity, activation energy, and modeling of the thin layer drying of kiwi slices. Results showed that energy and exergy efficiency increased with increasing microwave power and decreasing slice thickness while values of energy efficiency (15.15-32.27%) were higher than exergy efficiency (11.35-24.68%). Also, these parameters decreased with a decrease in moisture content. Specific energy consumption varied from 7.79 to 10.02, 8.59 to 10.77 and 9.57 to16.20 to MJ/kg water evaporated for 3, 6 and 9mm, respectively. The values of exergy loss were found to be in the range of 5.90 and 14.39MJ/kg water and decreased as the microwave power increased and slice thickness decreased. Effective diffusivity increased with decreasing moisture content and increasing microwave power and slice thickness. Average effective moisture diffusivity of kiwi slices changes between 1.47×10(-9) and 39.29×10(-9)m(2)/s within the given variables range. Activation energy (17.96-21.38W/g) showed a significant dependence on the moisture content. Although the Midilli model showed the best fit, Page's model was selected, since it had almost a similar performance but the model is simpler with two parameters instead of four.

Growth of large-scale heteroepitaxial 3C-SiC films and nanosheets on silicon substrates by microwave plasma enhanced CVD at higher powers

Abstract The controllable growth of heteroepitaxial cubic SiC (3C-SiC) films and two dimensional (2D) nanosheets arrays has been successfully achieved using a 915 MHz microwave plasma enhanced chemical vapor (MPCVD) deposited technique using tetramethylsilane (TMS) and hydrogen as the resource gases. A comprehensive analysis of the surface morphology of the 3C-SiC films was performed by tuning microwave power, TMS flow rate and gas pressure. With increasing microwave power, the morphology of the SiC crystals evolves from randomly oriented nanocrystals to well-oriented pyramid shaped crystals. The rocking curve results shows that the 3C-SiC film deposited at microwave power of 9 kW and gas pressure of 50 mbar remains epitaxial feature with increasing of TMS gas flow rate. As a result, uniform heteroepitaxial 3C-SiC film was deposited on 4-in. silicon wafer at low deposition temperature (~ 860 °C). The structure of SiC film along the radial direction was measured by X-ray diffraction (XRD) results. X-ray photoelectron spectroscopy (XPS) spectra indicate the chemical information of SiC film surface and the depth profiles, which prove the uniform of elements distribution. Moreover, with increasing gas pressure, the film morphology transforms from the heteroepitaxial SiC film into 2D nanosheets, which is due to the increased ion current.

Effect of Microwave Irradiation on the Physicochemical and Digestive Properties of Lotus Seed Starch.

The objective of this study is to investigate the effect of microwave irradiation on the physicochemical and digestive properties of lotus seed starch. The physicochemical properties of lotus seed starch were characterized by light microscopy, (1)H NMR, FT-IR spectroscopy, and HPSEC-MALLS-RI. The starch-water interaction and crystalline region increased due to the changed water distribution of starch granules and the increase of the double-helix structure. The swelling power, amylose leaching, molecular properties, and radius of gyration reduced with the increasing microwave power, which further affected the sensitivity of lotus seed starch to enzymatic degradation. Furthermore, the resistant starch and slowly digestible starch increased with the increasing microwave irradiation, which further resulted in their decreasing hydrolysis index and glycemic index. The digestive properties of lotus seed starch were mainly influenced by the reduced branching degree of amylopectin and the strong amylose-amylose interaction.

Effects of nonequilibrium quasiparticles in a thin-film superconducting microwave resonator under optical illumination

We have illuminated a thin-film superconducting Al lumped-element microwave resonator with 780 nm light and observed the resonator quality factor and resonance frequency as a function of illumination and microwave power in the 20 to 300 mK temperature range. The optically induced microwave loss increases with increasing illumination but decreases with increasing microwave power. Although this behavior may suggest the presence of optically activated two-level systems, we find that the loss is better explained by the presence of nonequilibrium quasiparticles generated by the illumination and excited by the microwave drive. We model the system by assuming that the illumination creates an effective source of phonons with energy higher than double the superconducting gap and solve the coupled quasiparticle-phonon rate equations. We fit the simulation results to our measurements and find good agreement with the observed dependence of the resonator quality factor and frequency shift on temperature, microwave power, and optical illumination. Examination of the model reveals approaches to reducing optically induced loss and improving the relaxation time of superconducting quantum devices.

Effects of microwave pretreatment and transglutaminase crosslinking on the gelation properties of soybean protein isolate and wheat gluten mixtures.

The integration of soybean protein isolate (SPI) with wheat gluten (WG) crosslinked via microbial transglutaminase (MTGase) may enhance the formation of ϵ-(γ-glutamyl)lysine covalent bonds, because SPI is rich in lysine and WG contains more glutamine. Microwave pretreatment may accelerate enzymatic reactions. In this study, we aimed to elucidate the effects of microwave pretreatment on the gelation properties of SPI and WG crosslinked with MTGase. Interestingly, the gel strength, water-holding capacity (WHC) and storage modulus (G') values of MTGase-induced SPI/WG gels were dramatically improved with increasing microwave power. Moreover, the MTGase crosslinking reaction promoted the formation of disulfide bonds, markedly reducing the free SH group and soluble protein content of gels. Fourier transform infrared spectroscopic analysis of SPI/WG gels showed that microwave pretreatment increased the proportion of α-helices and β-turns and decreased the proportion of β-sheets. Results from scanning electron microscopy indicated that the MTGase-induced SPI/WG gels had denser and more homogeneous microstructures after microwave pretreatment. The effect of microwave pretreatment is useful in advancing gelation characters of MTGase-induced SPI/WG gels and provides the possibility for expanding the application of food protein. © 2015 Society of Chemical Industry.

Kinetics and Quality of Microwave-Assisted Drying of Mango (Mangifera indica).

The effect of microwave-assisted convective air-drying on the drying kinetics and quality of mango was evaluated. Both microwave power and pretreatment time were significant factors but the effect of power was more profound. Increase in microwave power and pretreatment time had a positive effect on drying time. The nonenzymatic browning index of the fresh samples increased from 0.29 to 0.60 while the ascorbic acid content decreased with increase in microwave power and time from 3.84 mg/100g to 1.67 mg/100g. The effective moisture diffusivity varied from 1.45 × 10−9 to 2.13 × 10−9 m2/s for microwave power range of 300-600 W for 2 to 4 minutes of pretreatment. The Arrhenius type power-dependent activation energy was found to be in the range of 8.58–17.48 W/mm. The fitting of commonly used drying models to the drying data showed the Midilli et al. model as the best. Microwave power of 300 W and pretreatment time of 4 minutes emerged as the optimum conditions prior to air-drying at 7°C. At this ideal condition, the energy savings as a result of microwave application was approximately 30%. Therefore, microwave-assisted drying should be considered for improved heat and mass transfer processes during drying to produce dried mangoes with better quality.

Spatial distribution of the electron component parameters in the nitrogen plasma of a low-pressure electrode microwave Discharge

Spatial distributions of charged particle concentration, electron temperature, and DC potential in an electrode microwave discharge in nitrogen at a pressure of 1 Torr have been measured using the double electric probe method. It has been shown that, near the electrode/antenna, the charged particle concentration exceeds a critical value. The concentration and heterogeneity of the discharge increase with increasing microwave power.