Microwave Power Density Research Articles

Development of a green innovative semi-industrial scale pilot combined microwave heating and centrifugal force to extract essential oils and phenolic compounds from orange peels

A new process for extraction of essential oils from orange peels (Citrus sinensis L.) using microwave and centrifugal (MW/C) force was studied. MW/C is a combination of microwave heating to explode the internal cells of biological material, and centrifugation to intensify diffusion, collection and separation of compounds of interest. MW/C is performed at atmospheric pressure with an injection of water vapor. A detailed study concerning optimization of different operating parameters like microwave power density and centrifugation was performed. MW/C has been compared to the hydrodistillation method. In addition of essential oils collection, in the aqueous phase several phenolic compounds can be obtained and valorized. Influence of the two main parameters was discussed first on essential oils and, in second part, on phenolic compounds recovery. The new combined MW/C process provided reduction in the extraction time compared to hydrodistillation method, with an advantage to collect in a same experiment volatile and nonvolatile compounds with high quality. Industrial relevanceThis work was carried out thanks to the investment of IFTS France, an international technical center worldwide known in the field of solid-fluid separations. Considering the huge amount of plant extracts this new combined system using microwave and centrifugal (MW/C) force may be suitable for scaling up. The results of this investigation on pilot reactors pave the way for the design of an industrial device.

Ultrasound-assisted electrochemical treatment for phenolic wastewater.

With the rapid development of industry, especially the rapid rise of the chemical industry, the problem of water pollution is becoming more and more serious. Among them, the discharge of organic pollutants represented by phenolic substances has always been at the forefront. In this paper, ultrasound-assisted electrochemical treatment for phenolic wastewater is investigated. The effects of ultrasonic frequency, current, pH value and the amount of fly ash-loaded titanium TiO2-Fe3+ particles on phenol removal from phenol-containing wastewater are investigated. The experimental results demonstrate that the removal rate of phenol in phenol-containing wastewater is the best when ultrasonic frequency is 45kHz, power is 200W, the current is 1.2 A, pH is 5 and the dosage of fly ash-loaded titanium TiO2-Fe3+ particles is 3g. In addition, microwave-assisted-Fenton reagent treatment for phenol wastewater is investigated. The effects of Fenton reagent dosage, initial pH value, microwave power density and radiation time on phenol degradation rate are investigated. The results show that microwave can accelerate the reaction rate, reduce the number of metal ions, save the process cost and reduce the difficulty of post-treatment. Finally, the research status of phenol wastewater treatment technology at the present stage is reviewed, and the future development direction is discussed.

Simulation of Microwave Assisted Finish Drying and Inactivation of Escherichia Coli, Bacillus Cereus and Staphylococcus Aureus of Onion Powder

The onion powder with initial moisture content of 10% (db) in form of thin infinite layer of thickness 5mm was dried using a laboratory microwave oven at 50W/g microwave power density to final moisture content of 2% (db).The temperature, moisture change pattern of the experimental process was monitored. Computer simulation of the predicted temperature pattern with drying time was obtained by using the mathematical model governed by energy balance equation and using data of the thermo-physical properties of the onion powder obtained either experimentally or through literature review. The usage of high microwave power to mass ratio results in raised temperature with reduced drying time. The decrease in drying rate is due to initial moisture content. Effective moisture diffusivity increases with decreasing moisture content due to the application of high power density. Lethality values for Escherichia coli, Bacillus cereus spores and Staphylococcus aureus, which are more prominent in onion powder, were then evaluated from the simulated temperature curve. DT and z values were used to estimate the sterilization values for each individual microorganism. It was observed that significant sterilization of E.coli and S.aureus start after 35 s at a minimum temperature of 89oC with 0.047 and 0.044 log reduction while, for B.cereus it starts after 75 s at 117oC with 0.431 log reduction. The maximum temperature of 117ᵒC was limited due to unacceptable sensory quality of end product.

Mutual transformation of the water binding state and moisture diffusion characteristics of Chinese yams during microwave freeze drying

The moisture distribution changes in Chinese yam slices subjected to microwave freeze drying (MFD) were analyzed using low-field nuclear magnetic resonance (LF-NMR). During this process, the mutual transformation of the water binding state and moisture diffusion characteristics of Chinese yam were analyzed together with changes in the effective water diffusion coefficient, water content, drying rate and pore network structure. Notably, moisture migrated from a high to a low degree of freedom during the MFD process. The effective water diffusion coefficient and water migration velocity increased with increasing microwave power density, and the extent of the conversion of unbound to bound water also gradually increased. During the early stage of drying, moisture diffusion was mainly dominated by liquid phase diffusion. At later stages of drying, moisture diffusion shifted gradually from liquid phase diffusion to gas phase diffusion and was eventually dominated by the latter.

Decoupling thermal and non-thermal effects of the microwaves for lignocellulosic biomass pretreatment

In order to develop an efficient biorefinery concept based on the sustainable use of biomass, pretreatments are a major issue as they consume a considerable amount of energy, among others. Competitive and sustainable bioconversion technologies are required and microwave (MW) treatment has emerged as an interesting option during the recent years. Nevertheless, MW action mode on lignocellulosic biomass is not yet fully understood. With some exceptions, experimental investigations dealing with MW pretreatment do not address this important aspect nor energy and power level effects on biomass conversion.To study the action mode of MW on lignocellulosic biomass and its energy absorption behavior, MW assisted pretreatments of three types of grass biomass of industrial interest were performed at several solid to liquid (S/L) ratios, i.e. 1/21 or 1/9, with different solvents (water, ethanol, acid water or alkaline water) and different MW power densities (2.38 or 4.76 W/g). Incident, reflected and absorbed MW power levels and the reaction temperatures were monitored on-line. Thermal and non-thermal MW effects were studied by comparing experiments performed with a glass reactor and an in-house-made jacketed glass reactor designed to avoid the heating of sample. Under the tested conditions, only thermal MW effects have been detected. In an attempt to optimize the bio-solubilization of biomass components, microwave pretreatment with increased pressure and incident power density (7.5 W/g) was also tested. The cell wall was significantly impacted with a solubilization of 33% of hemicelluloses, which opens the way to industrial applications.To our knowledge, this is the first reported experimental study that addresses the non-thermal MW effects on biomass by decoupling heating effect from irradiation.

Optimization of Microwave Coupled Hot Air Drying for Chinese Yam Using Response Surface Methodology

The effect of microwave coupled hot air drying on rehydration ratio (RR) and total sugar content (TSC) of Chinese yam was investigated. Single factor test and response surface methodology were used for process parameter optimization with hot air temperature, hot air velocity, slice thickness, and microwave power density as variables and RR and TSC of dried products as responses. The effect of variables on RR followed the order: slice thickness > hot air temperature > microwave power density > hot air velocity. The effect of variables on TSC followed the order: slice thickness > microwave power density > hot air velocity > hot air temperature. The optimized process parameters were hot air velocity of 2.5 m/s, hot air temperature of 61.7 °C, slice thickness of 8.5 mm, and microwave power density of 5.9 W/g. Under the optimal conditions, the predicted values of RR and TSC were 1.90 g/g and 5.74 g/100 g, respectively, which is very close to corresponding actual values (1.83 g/g and 5.72 g/100 g). The desirability of 0.913 further validated the effectiveness of the model. The findings from this work may apply to other agricultural products.

Microwave absorption and reflection properties of a composite dielectric absorber

Microwave assisted technologies and its application are widely used in different research fields. One of the main difficulty of this area is to design a treatment chamber. The aim of this paper is to measure power absorption and calculate average surface microwave power density of composite dielectric materials, based on plant particles. Obtained microwave-absorbing characteristics of composite plant material can be used in microwave assisted processes and high-performance microwave treatment chambers design to facilitate the research in microwave assisted drying and extraction. The research includes experiments with different positioning of the composite materials from reflective metal screen inside the chamber-waveguide.

Co-deposition of diamond and β-SiC by microwave plasma CVD in H2-CH4-SiH4 gas mixtures

The co-deposition of diamond and silicon carbide (SiC) was realized in a microwave plasma using H2-CH4-SiH4 mixtures at enhanced microwave power density of up to 150 W/cm3, and SiC/diamond composite films on (100) Si substrates were produced. The growth parameters were selected to ensure almost equal deposition rates of ~0.3 μm/h for both phases. Composite films were formed either directly on Si surface, or on a buffer (nucleation) layer of SiC or diamond, preliminary deposited on the Si substrate. The film structure has been characterized by SEM, XRD, Raman and PL spectroscopy. It was confirmed that the silicon carbide phase in the composite is the cubic polytype β-SiC (3C-SiC) with the presence of only a small fraction of the hexagonal polytype, while the diamond is in well-faceted microcrystalline form. Diamond grains are Si-doped as was evidenced by the strong photoluminescence of silicon-vacancy color centers. Also, minor inclusions of amorphous sp2 carbon and of trans-polyacetylene (t-PA) were detected in the β-SiC phase. The overall crystalline quality of the composite is high, as evidenced by narrow Raman peaks both for diamond and β-SiC. Our results show, that with an appropriate choice of growth parameters, particularly SiH4/CH4 ratio, high-quality films of Si-doped diamond, silicon carbide, SiC-diamond composite, and combinations of these structures can be synthesized in a single process. Prime novelty statementThe co-deposition of diamond and SiC was realized by microwave plasma CVD using H2-CH4-SiH4 mixtures at enhanced microwave power density; β-SiC/diamond composite films were produced with high crystalline quality on (100) Si substrates.

Drying characteristics and modeling of apple slices during microwave intermittent drying

AbstractDrying characteristics and quality of apple slices were studied using microwave intermittent drying at different microwave power density (2.2, 3.5, 4.8, and 6.1 W/g), sample thickness (3, 5, 7, and 9 mm) and heating time in each cycle (4, 5, 6, and 7 s). Results showed that the approximate constant rate drying periods and falling rate drying periods were found in all the curves. The drying time decreased with increasing microwave power density and heating time in each cycle, while increased significantly with the growth of sample thickness. Weibull distribution model could fit the drying curves well in all conditions (R2 > 0.99) with the scale parameter values changed from 45.88 to 206.08 min and the shape parameters values varied from 1.514 to 1.69. The moisture effective diffusivity calculated by Weibull model ranged from 0.3217 × 10−9 to 1.435 × 10−9 m2·s−1. The flavor, texture and overall acceptability values displayed a decreasing tendency with the increase of microwave power density, while increased with the growth of thickness. And the shrinkage rates of apple slices showed a decline tendency with the increase of above parameters.Practical applicationsApple is a kind of popular and important fruits with high nutrient content, and it has been widely planted in China. Due to its high moisture content, fresh apples will subject to various deteriorative reactions during storage and thus lead to serious post‐harvest losses. Microwave drying technology has been widely used in dehydration of agricultural products in China because of the significant heating efficiency. However, the continuous microwave heating may easily cause the overheating of raw materials and undesirable changes in quality. Microwave intermittent drying (MID) was proposed on the basis of conventional microwave drying and regarded as an efficient processing method by solving the above issues. Currently this technology has been successfully applied to ginkgo fruits, yam and banana slices. Analyzing the effects of drying parameters on MID kinetics and quality of apple slices will have important significance for optimizing the processing efficiency and consumer acceptability of dried products.

Ultra-rapid microwave sintering employing thermal instability and resonant absorption

Abstract

Enhanced deposition rate of polycrystalline CVD diamond at high microwave power densities

We report on growth of polycrystalline diamond (PCD) by chemical vapor deposition (CVD) in a microwave plasma reactor under high absorbed microwave power density (MWPD) and high pressures, in conditions more typical for single crystal diamond epitaxy. The growth rates as high as 30–36 μm/h have been achieved in CH4-H2 mixtures at pressure of 320 Torr and MWPD of ~700 W/cm3 even at relatively low (3%) CH4 concentration, using PCD substrates with diameter of 5–20 mm and different textures. The structure and quality of the produced PCD thick films were assessed with SEM, Raman and X-ray diffraction. The plasma shapes and spatial profiles of species (excited atomic hydrogen and C2 dimer) were characterized with spatially resolved optical emission spectroscopy (OES) at moderate and high MWPD. The maximum rotational gas temperature Tg significantly enhanced to 3700 K in the latter case (675 W/cm3) compared with Tg ≈ 3100 K for lower pressure and MWPD (100 Torr, 160 W/cm3), being in agreement with the obtained high growth rates at high pressures. Analysis of numerous literature data on PCD growth rate vs substrate size in MPCVD reactors reveals a clear trend of enhanced growth rate with diminishing of the substrate dimension, our findings further confirming this tendency indirectly related to a change in absorbed power density.

Atmospheric-Pressure Microwave Plasma Torch for CVD Technology of Diamond Synthesis

An electrodeless microwave jet plasma source is considered, and its various applications in the technology of chemical vapor deposition of diamond films and dimension increasing of small diamond single crystals synthesized at high pressures and temperatures are discussed. The plasma jet is ignited in an atmospheric-pressure gas (argon) flow with hydrogen and methane additives. The operation of the microwave jet reactor is described, and the plasma characteristics measured using emission spectroscopy are presented. The brightly glowing atmospheric-pressure plasma jet is ignited and stably burns at a microwave power of ≤1 kW supplied from a microwave oven magnetron. The specific microwave power density absorbed by the compact plasma jet (≤104 W/cm3) is comparable with that absorbed by a dc arc. The growth rate of the polycrystalline diamond layer amounts to 40 µm/h. The process of film deposition on the substrate can be controlled by scanning the substrate surface with the jet.

Study of microwave discharge at high power density conditions in diamond chemical vapor deposition reactor by optical emission spectroscopy

The paper presents results of measurements of spatial distributions of emission intensity of various lines of argon, atomic hydrogen, C2 and CH molecules in a wide pressure range from 40 to 500 Torr in diamond chemical vapor deposition (CVD) reactor. Microwave discharge plasma was maintained in reactor in continuous mode both in hydrogen and in a mixture of hydrogen-methane with a small addition of argon for diagnostic purposes. Intensity distribution of argon emission was characterized by a pronounced maximum near the substrate in reactor, which indicates that microwave power density (MWPD) absorbed in the plasma is also concentrated mainly near the substrate. Maximum emission intensity of hydrogen atoms is located at some distance from the substrate, and maxima of spatial distributions of emission intensity of carbon-containing species C2 and CH are shifted farther away from the substrate. This leads to the fact that the shape and size of discharge, which are observed visually or with help of a camera, significantly differ from the distribution of MWPD. There is a difference in the shape of spatial distribution of argon lines emission (750.4 nm and 811.5 nm), that can be explained by a change in the mechanism for quenching the excited levels of argon in different regions of discharge. This imposes some restrictions on the use of argon for measuring characteristics of microwave discharge.

Study on quality attributes and drying kinetics of instant parboiled rice fortified with turmeric using hot air and microwave-assisted hot air drying

This study investigated the quality and drying kinetics of instant parboiled rice fortified with turmeric (IPRFT) by using hot air (HA) and microwave-assisted hot air (MWHA) drying. The cooked long grain parboiled rice (LGPR) fortified with turmeric was dried with HA at temperatures of 65, 80, 95, and 110 °C. The microwave power density of 0.588 Wg−1 was incorporated for drying with MWHA. Drying was performed until the dried IPRFT reached 16% (d.b.) of moisture content. The quality of the dried IPRFT was evaluated in terms of color, total phenolics content (TPC), total antioxidant capacity (TAC), rehydration ratio, volume expansion ratio, texture and microstructure. The results showed that the incorporation of microwave power with HA drying helped to reduce the drying time by 50% compared to conventional HA drying. A prediction of the moisture ratio by using the Page model provided the best R2 and RMSE in drying kinetics. The drying conditions had small effects on the color, TPC, TAC, and microstructure of the dried IPFRT. The rehydration ratio, volume expansion ratio and texture of the rehydrated IPFRT showed minimal variations from changes in the drying conditions. The TPC and TAC of the dried IPRFT clearly increased compared to the TPC and TAC of the initial LGPR.

Modelling study of dielectric properties of seed to improve mathematical modelling for microwave-assisted hot-air drying

Dielectric properties of canola seeds, corn, paddy rice, soybean and wheat were studied to determine the dependence of dielectric properties on frequency, moisture content and density of powdered seeds. In this context, several tests were done to find the relationship between the dielectric properties of the seeds at different frequencies (0.6–3 GHz) and at different moisture contents. Experimental data were used to parameterize standard linear models to predict the dielectric constant and loss factor. The resulting models were used in the prediction of heat and mass transfer parameters in a microwave (MW) assisted hot-air dryer at operating conditions of air temperature of 65 °C, a MW power density of 0.3 W g−1, and for an exposure time of 30 min. The predicted results were in good agreement with experimental results obtained under the studied drying conditions. Germination tests were performed before and after each drying test to study the effects of the adopted drying conditions on the selected seeds. The results showed that the germination rate decreased for soybean but not for corn seed. This may be due to the hardness of the corn seed compared to that of soybean and also due to the grain density, which is higher for corn than of soybean.

Microwave power adjusting during potato slice drying process using machine vision

In this study, machine vision was used for measuring area shrinkage of potato slices during thin layer drying process and then an artificial neural network (ANN) and linear models were investigated to predict moisture content (MC) of potato slices based on area shrinkage. Then an algorithm for adjusting the microwave power with respect to the predicted MC during the drying process was developed. A drying setup including imaging unit, lightning unit, infrared temperature sensor, image processing algorithm and microwave power adjusting program based on MC was developed. The experiments with two microwave power modes (variable and constant) have been done. The developed image processing had ability to separate connected potato samples and measured the shrinkage of center sample. The consequences expressed that the ANN with 1-3-1 structure had better results than linear model and could predict the MC based on shrinkage with 0.0966 RMSE and 96.87 R values on test data set. Also evaluating the developed ANN model with new experiments data set revealed that it could predicted the MC with 0.094 RMSE and 96 R values and resulted it has great accuracy and reliability. The real-time evaluating the image processing algorithm and ANN model with another new experiments indicated that the developed method has good promising ability for adjusting the microwave power and preventing the increased of microwave power density during the potato chips drying process.

Microwave airflow drying of pecans at variable microwave power

AbstractIn this study, the dielectric properties of two varieties of pecan nuts at different moistures (10–30% w.b.), temperatures (25–85°C) were determined and the effect of microwave (MW) power density (0.8–2.6 W/g) and exposure time (30–300 s) on moisture content of pecan were approximated by response surface analysis. According to a comprehensive assessment of these results, phased varying power design for MW‐assisted air drying (PPMAD) was appropriate designed to dry pecan nuts. The result indicated that the penetration depths of MW at 2,455 MHz to pecan nuts was distributed in the range of 4.05–10.45 cm in terms of different moisture content and temperature. The moderate output power density (1.7 W/g) is an optimized parameter during the primary stage of MW drying. In PPMAD scheme, 1.7 W/g was delivered in first 150 s, followed by cooling 30 s, and 2.6 W/g was applied in the final 120 s. The results showed that the hybrid drying of PPMAD was efficacious, leading to water losses of 19% in pecan and 16% in hickory without affecting their major compositions and sensory qualities. Further preservation showed that the oxidation degree of PPMAD‐dried nuts was lower than that of the control after 90 days of storage.Practical applicationsMicrowave hybrid drying has been reported to process food and provides a significantly increased shelf‐life for fruits and vegetables, such as apples, carrots, oranges, tomatoes, and spinach. There are no literatures focused on the parameter optimization of microwave drying on pecan and hickory during exposure. The present study provide an optimal method of microwave hybrid using a phased varying power design for MW‐assisted air drying which supply a foundation for large‐scale industrial applications of microwave drying.

Microwave‐induced polar/nonpolar mixture separation performance in a film evaporation process

Selective microwave heating and irradiation reportedly have a significant impact on the relative volatility of polar/nonpolar mixtures. However, the use of this feature for a separation process has not yet been reported. In this article, a microwave‐induced film evaporation process based on a nonequilibrium process was developed to separate polar/nonpolar mixtures. The influencing mechanism of microwaves on the separation of a polar/nonpolar system was studied by comparing the experimental results in the presence/absence of microwave irradiation. The influences of several operating parameters, such as the initial composition, temperature, residence time, and microwave power density, were explored. The results clearly demonstrate that microwave irradiation can improve the polar component vaporization and shift the vapor phase composition. The results also indicate that the differences in the dielectric properties play a significant role in the effect of microwave irradiation on a film evaporation separation process. © 2018 American Institute of Chemical Engineers AIChE J, 65: 745–754, 2019

Ultra-rapid microwave sintering

The results of microwave sintering of powder materials in the regimes with high heating rates and zero hold time at maximum temperature are reported. Microwave processing of compacted samples based on Al2O3, Y2O3, MgAl2O4, and Yb:(LaY)2O3 ceramics was carried out using a 6 kW/24 GHz gyrotron system. The volumetric absorption of intense microwave radiation resulted in a very rapid densification with the duration of the high-temperature stage of sintering on the order of one to several minutes. In the rapid microwave heating regimes the effective high-frequency conductivity of the materials increased sharply at a certain threshold temperature as a result of the overheating instability, also known as thermal runaway. This suggests that the ultra-rapid sintering occurs via grain-boundary softening and formation of transient liquid phases. The indications of the presence of such phases were observed in the microstructure of the sintered samples. The absorbed microwave power density required for the transition to the ultra-rapid sintering is on the order of 10…100 W/cm3 for a broad class of the materials. The obtained results suggest that ultra-rapid microwave sintering proceeds via essentially the same mechanism as the so-called flash sintering that occurs in the presence of dc or low-frequency ac electric field.

Aspergillus niger inactivation in microwave rotary drum drying of whole garlic bulbs and effect on quality of dried garlic powder

Microwave rotary drum drying of whole garlic bulbs was investigated for the Aspergillus niger inactivation and moisture removal. The Weibull and Bigelow models were applied to microbial inactivation data. Garlic bulbs with initial moisture content in the range 1.95–2.14 g water g−1 dry matter were dried up to 0.06 g water g−1 dry matter. The microwave power density (PD) was varied from 1.03 to 2.67 Wg−1 at 1.5 and 2.0 pulsation ratios (PRs). Effect of PD and PR on A. niger inactivation, product temperature, moisture diffusivity, moisture ratio, drying rate, color, and sensory parameters was studied. Page model was found to be a better fit for microwave rotary drying characteristics of whole garlic bulbs. Microwave rotary drum drying resulted in the average log reduction of A. niger between 1.12 and 1.60. Weibull model predicted A. niger inactivation better than the Bigelow model as it considered the nonlinearity associated with a microbial population in the sterilization process. Garlic powder prepared at 2.0 PR and 1.85 Wg−1 PD was chosen as the best process based on sensory score. The cracking and peeling of garlic cloves were observed during microwave rotary drum drying. The SEM images confirmed the increase in the pore size of the microwave treated garlic sample than the untreated garlic which might be the reason for cracking and loosening of peel in garlic.