Microwave Processing System Research Articles

Simulation and experimental study of a K-band extended interaction oscillator for microwave processing systems.

High-power microwave sources have been widely applied for material processing in scientific research and manufacturing. The development of stable, high-frequency, high-power microwave sources is essential for achieving efficient microwave processing. This study proposes using a square doubly reentrant coupled-cavity as the slow-wave resonant structure in a K-band extended interaction oscillator (EIO). This design allows for ease of fabrication and high-power capability. The EIO is designed to operate in single 0-mode. The simulation results show that the competing π/5-mode can be effectively suppressed by properly choosing the width and location of the output coupler. The simulation and experiments successfully demonstrate stable, single-mode, tunable, high-performance operation of the EIO. The experimental measurements show a maximum output power of 1.776 kW (18.56% electronic efficiency), and a wave frequency of 24.324GHz at a beam voltage of 17.4kV and beam current of 550 mA. The EIO microwave source is suitable for interdisciplinary applications that require higher heating rates and greater uniformity.

Exploring the microwave non‐thermal effects on the fatty acid composition of Atlantic salmon ( Salmo salar ) during pasteurization using the same time–temperature profiles method

The non-thermal effects of microwave fields on the fatty acid composition of pasteurized Atlantic salmon were investigated using the same time–temperature profiles method. Two pasteurization processes with same time–temperature profiles were designed using a microwave processing system and a water bath. Then, the fatty acid composition of the processed salmon fillets was analyzed using the gas chromatography/mass spectrometry (GC/MS) method. Results showed that the time–temperature profiles during microwave and water bath processing were well suited. With same time–temperature profiles, the fatty acid compositions of salmon fillets processed by microwave and water bath were obviously different. Furthermore, microwave process brought better fatty acid quality in terms of higher values of n-3 PUFA proportion, PUFA:SFA ratio, and lower n-6 PUFA proportion. This demonstrated that microwave fields did have non-thermal effects on the fatty acid composition of salmon with 3.5 min 8 kW microwave radiation and a final temperature of 90℃. Practical applications Microwave processing could provide volumetric heating for solid foods to improve product quality by achieving high-temperature short time process. The microbial safety of microwave thermal processing has been approved by the Food and Drug Administration of United States. Its industrial application is being accelerated. However, besides thermal effects, there also exists non-thermal effects during microwave processing due to the interaction between alternating electromagnetic waves and food molecules. Thus, the influence of microwave non-thermal effects on nutritional quality and chemical safety of foods are attracting great concerns both for consumers and researchers. It is essential to provide theoretical data concerning microwave non-thermal effects to sustain its industrial application. This study investigated the influence of microwave non-thermal effects on fatty acids composition of salmon fillets using same time–temperature profiles method. The results provide important information for quality evaluation of microwave processed foods, which promotes the application of microwave processing in food industry.

Comprehensive investigation into quality of pasteurized Oncorhynchus keta Walbaum fillets and non-thermal effects of microwave

Microwave processing is a novel technology in shortening food processing time and improving product quality. These advantages are achieved through the thermal and possible non-thermal effects of microwave. The main objective of the study was to explore the non-thermal effects of microwave electromagnetic (EM) fields on quality traits of pasteurized Oncorhynchus keta Walbaum fillets. A single-mode microwave processing system processed the fillets with a pasteurization thermal lethality value of F90 = 10 min. Corresponding pasteurization process with synchronous heating was designed to match the time-temperature profile of microwave processing. Retort pasteurization was used as a conventional process to compare the fillet quality performance with microwave pasteurization. With the pasteurization thermal treatment level, the proportion of six free amino acids was significantly different (P < 0.05) between the microwave pasteurized fillets and synchronous heating pasteurized fillets. The result could be attributed to the non-thermal effects of microwave EM fields. Compared with the retort pasteurization, the microwave process performed better in overall quality. Results suggested that microwave pasteurization exerted non-thermal effects in individual quality traits and caused less damage to fillet quality than the retort process in overall quality.

Non-thermal effects of microwave processing on inactivation of Clostridium Sporogenes inoculated in salmon fillets

Microwave processing has been a promising technology in improving quality and ensuring microbial safety of food products, while non-thermal effects of microwave is in controversy. The objective of this study was to investigate the non-thermal effects of microwave electromagnetic (EM) fields on Clostridium Sporogenes (surrogate of Clostridium botulinum) inoculated in salmon fillets using same time-temperature profiles method. The inoculated salmon fillets were processed by a single-mode pilot microwave processing system with different radiation time (2, 3 and 4 min) and holding time (0, 2 and 4 min). Corresponding water bath treatments were designed for each microwave processing to obtain matching time-temperature profiles. Results showed that the same time-temperature profiles method was successfully achieved to evaluate the microwave non-thermal effects in salmon fillets. With equal or lower thermal lethality value, microwave processing brought significantly higher (P < 0.05) reduction values than water bath treatment. This determined the non-thermal effects of microwave EM fields. And the non-thermal effects increased with longer microwave radiation time and was intensified by higher temperature. However, the following holding section will weaken the additional inactivation efficiency of microwave non-thermal effects. Results suggested that microwave processing could provide better bacteria control with lower thermal processing level.

&lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$\Delta-\Sigma$ &lt;/tex-math&gt; &lt;/inline-formula&gt; Noise-Shaping in 2-D Space-Time for Wideband Antenna Array Receivers

This paper proposes multi-dimensional signal processing methods to extend $\Delta-\Sigma $ modulation into the two-dimensional (2-D) space, time case. The proposed 2-D noise-shaping methods employ lossless discrete integrators (LDIs) to reduce the spectral overlap of the regions of support (ROSs) of array signals with that of both thermal and quantization noise in microwave and mm-wave array processing systems. Spatio–temporal low-pass filtering can then be used to improve the overall noise figure, linearity, and resolution of the resulting $N$ -port receiver. A proof-of-concept 65-port receiver that uses first-order $\Delta-\Sigma $ within both the low-noise amplifier and analog-to-digital converter (ADC) has been designed in the UMC 65-nm CMOS process. Detailed circuit simulations with wideband RF inputs at a center frequency of 4 GHz confirm that the proposed 2-D noise-shaping approach provides significant improvements in noise, linearity, and resolution. Specifically, noise figure (NF) improves by 1.5 dB, IIP3 improves by 15 dB, and the effective number of bits (ENOB) improves by 3.1 bits. Experimental results from a low-frequency board-level implementation of a 64-port ADC with first-order noise shaping are also presented.

High performance and high power circularly polarized horn antenna for K-band microwave processing systems.

A single-ridged K-band circularly polarized horn antenna offering excellent performance has been developed by improving the polarization conversion and manufacturing complexity. The numerical and experimental results are consistent showing the return loss of this antenna to be less than -20 dB and the axial ratio at the boresight direction to be less than 0.7 dB in the frequency range from 23.5 GHz to 24.5 GHz. In addition, the gain of this antenna is higher than 20 dB. The newly designed circularly polarized horn antenna has a simple structure and outperforms many existing circular polarization devices in high-power operations.

A novel method for stoichiometric reduction of (U3O8,PuO2) and its controlled oxidation using microwave

We report a process for stoichiometric reduction of U3O8 and (U3O8,PuO2) mixed oxide powders using an indigenously developed 2.4 GHz microwave processing system. The process parameters were optimized by interpreting reduction kinetic curves at a temperature which is 150 °C lower than the conventional reduction. The process improved the sinterability of the powder which was evaluated in terms of average particle size, BET specific surface area and bulk density. A quick method for controlled oxidation of the reduced powder to incorporate a controlled amount of hyper-stoichiometry was demonstrated by modifying the same reduction process. The percent reduction was measured experimentally using O:(U + Pu) ratio. The X-ray diffraction analysis confirmed the various phases present. The process is novel considering shorter processing cycle, lower temperature processing, improved powder properties, energy efficiency and cost effectiveness.

Microwave dielectric characterization of Saudi Arabian date palm biomass during pyrolysis and at industrial frequencies

The knowledge of the dielectric property is essential to design and develop microwave technology for processing biomass materials. This paper focuses on the measurement of dielectric properties of three different Saudi Arabian date palm biomass (seed, leaf and stem) from room temperature to ∼800°C and at six different frequencies (397–2986MHz) using a cavity perturbation method. The result showed that all date palm biomass samples were poor microwave absorbers from room temperature till 500°C. The dielectric constant and loss factor dropped slightly in the drying region due to removal of moisture and was almost constant in pyrolysis region. However, a significant increase in dielectric constant and loss factor occurred in temperature between 500 and 750°C, except for leaf biomass. Penetration depth and tangent loss had an opposite behavior with the rise in temperature. In conclusion, the dielectric properties might not only depend on the temperature and frequency but also on the physical and chemical characteristics of the biomass material. For instance, it was interesting to know that date leaf showed a sharp decrease in loss factor from temperature 475°C, which was opposite to that observed in case of date seed and stem. The data can be very useful in designing, modeling and simulation of microwave processing system for date palm biomass.

Improving the design of industrial microwave processing systems through prediction of the dielectric properties of complex multi-layered materials

Rigorous design of industrial microwave processing systems requires in-depth knowledge of the dielectric properties of the materials to be processed. These values are not easy to measure, particularly when a material is multi-layered containing multiple phases, when one phase has a much higher loss than the other and the application is based on selective heating. This paper demonstrates the ability of the Clausius–Mossotti (CM) model to predict the dielectric constant of multi-layered materials. Furthermore, mixing rules and graphical extrapolation techniques were used to further evidence our conclusions and to estimate the loss factor. The material used for this study was vermiculite, a layered alumina-silicate mineral containing up to 10 % of an interlayer hydrated phase. It was measured at different bulk densities at two distinct microwave frequencies, namely 934 and 2143 MHz. The CM model, based on the ionic polarisability of the bulk material, gives only a prediction of the dielectric constant for experimental data with a deviation of <5 % at microwave frequencies. The complex refractive index model, Landau, Lifshitz and Looyenga, Goldschmidt, Böttcher and Bruggeman–Hanai model equations are then shown to give a strong estimation of both dielectric constant and loss factor of the solid material compared to that of the measured powder with a deviation of <1 %. Results obtained from this work provide a basis for the design of further electromagnetic processing systems for multi-layered materials consisting of both high loss and low loss components.

Microwave and thermal curing of an epoxy resin for microelectronic applications

Microwave curing of thermosetting polymers has a number of advantages to natural or thermal oven curing and is considered a cost-effective alternative. Here we present a detailed study of a commercially available epoxy resin, EO1080. Samples that are thermally cured are compared to curing using a recently developed modular microwave processing system. For commercial purposes it is crucial to demonstrate that microwave curing does not adversely affect the thermal and chemical properties of the material. Therefore, the kinetics of cure and various post cure properties of the resin are investigated. Attenuated Total Reflectance Fourier-Transform Infrared (ATR-FTIR) analysis shows no significant difference between the conventionally and microwave cured samples. Differential scanning calorimetry (DSC) is used to monitor the kinetics of the curing reaction, as well as determine the thermal and ageing properties of the material. As expected, the rate of curing is higher when using microwave energy and we attempt to quantify differences compared to conventional thermal curing. No change in glass transition temperature (Tg) is observed. For the first time, enthalpy relaxation measurements performed on conventional and microwave cured samples are reported and these indicate similar ageing properties at any given temperature under Tg.

Development and evaluation of a continuous microwave processing system for hydrocarbon removal from solids

A continuous conveyor-belt processing concept using microwave heating was developed and evaluated. Four hydrocarbon-contaminated soils were used as model feedstocks, and the degree of organic removal was assessed against the power and energy input to the process. The findings of this study at scale (150kg/h) are in direct agreement with data obtained in batch laboratory scale experiments, and show that microwave heating processes are fundamentally scalable. It is shown that there is a trade-off between the efficiency of organic removal and the power distribution, and applying the power in a single stage was found to be 20–30% more energy efficient but the overall degree of organic removal is limited to 60%. 75% removal was possible using two processing steps in series, but the organic removal is ultimately limited by the amount of power that can be safely and reliably delivered to the process material. The concept presented in this work is feasible when 75% organic removal is sufficient, and could form a viable industrial-scale process based on the findings of this study.

Integrated microwave processing system for the extraction of organophosphorus pesticides in fresh vegetables

A simple and efficient integrated microwave processing system (IMPS) was firstly assembled and validated for the extraction of organophosphorus pesticides in fresh vegetables. Two processes under microwave irradiation, dynamic microwave-assisted extraction (DMAE) and microwave-accelerated solvent elution (MASE), were integrated for simplifying the pretreatment of the sample. Extraction, separation, enrichment and elution were finished in a simple step. The organophosphorus pesticides were extracted from the fresh vegetables into hexane with DMAE, and then the extract was directly introduced into the enrichment column packed with active carbon fiber (ACF). Subsequently, the organophosphorus pesticides trapped on the ACF were eluted with ethyl acetate under microwave irradiation. No further filtration or cleanup was required before analysis of the eluate by gas chromatography–mass spectrometry. Some experimental parameters affecting extraction efficiency were investigated and optimized, such as microwave output power, kind and volume of extraction solvent, extraction time, amount of sorbent, elution microwave power, kind and volume of elution solvent, elution solvent flow rate. Under the optimized conditions, the recoveries were in the range of 71.5–105.2%, and the relative standard deviations were lower than 11.6%. The experiment results prove that the present method is a simple and effective sample preparation method for the determination of pesticides in solid samples.

Dielectric and Thermal Properties of Sweet Sorghum Biomass

Sweet sorghum has recently received a lot of attention as a potential biomass feedstock in the United States. This study investigated the dielectric properties (dielectric constant s’ and dielectric loss factor s’) of sweet sorghum biomass as a function of frequency (around 915 and 2450 MHz, respectively) and water content-WH2O (important for microwave drying and processing operations). Fundamental thermal properties needed in process design calculations (specific heat, thermal conductivity, thermal diffusivity) were also determined using a thermal conductivity probe method. Dielectric constant decreases with frequency at both frequency ranges. At 915 MHz, ε’ ranged from 2.73 at wH2O = 49.5% to 1.1 at wH2O = 0%, whereas at 2450 MHz, ε’ ranged from 2.67 at WH2O = 55.4% to 1.03 at WH2O = 0%. The dielectric loss factor ranged from 0.44 at wH2O = 49.5% (at 915 MHz) and 0.7 at wH2O = 55.4% (at 2450 MHz) to near zero at WH2O = 0% for both frequencies. Thermal conductivity, specific heat, and thermal diffusivity were determined to be, respectively, 0.13±0.003 W•m-1•K-1, 2321.2±43.12 J•kg-1 • K-1, and 0.37±0.01 mm2«s”’. These results can be used for design of microwave processing systems for sweet sorghum biomass, as well as for process design calculations involving various heat-based unit operations.

Evaluating Applicator Designs for Heating Nanoparticle Flow Chemistries Using Single-Mode Microwave Energy

A single-mode microwave system can provide an electro-magnetic field with very sharp peaks and high power densities which can heat chemical process fluids rapidly on a much shorter time scale than by convective means. This type of microwave heating provides the opportunity to reduce thermal gradients within process fluids which are important, for example, in the control of reactive precipitation processes used to produce colloidal nanomaterials. Here, the effects of flow applicator design on the thermal gradients and energy efficiency of the microwave-assisted heating is considered as a function of flow rate. A heat efficiency model is developed to simulate the single-mode microwave heating of a continuous flow process fluid over a variety of flow rates and powers. To validate the model, an experimental setup is developed involving a three kW 2450 MHz microwave processing system. The system was used to heat salt water over a range of concentrations while temperature change across a microwave heating zone was measured using in-situ fiber optic probes. Experimental results were found to be in good agreement with the model yielding an average error for all cases of 4.8% and an average error for cases with salinity of 17.3%. The model is used to evaluate the effects of applicator design on thermal gradients and energy efficiency with changes in flow rate.

Frequency-reconfigurable TM010-mode reentrant cylindrical cavity for microwave material processing

This paper proposes a frequency-reconfigurable microwave reactor based on the reentrant cylindrical resonant cavity. It works with the TM010-associated mode and is able to resonate at every frequency in the range of 500–950 MHz, i.e., 62% tunable relative bandwidth. Considering the practical requirements of microwave processing, especially for the single processing capacity and the wide frequency bandwidth, the theory and design formula for the existing reentrant cylindrical resonant cavity are no longer applicable. Consequently, a new method to control the resonant frequency is proposed. The corresponding approximate design formulas are derived to show the relationship between cavity’s dimension and its electric properties (resonant frequency and quality factor). Their correctness is validated by the full-wave simulation. Furthermore, an effective feed method is developed to achieve good matching over a wideband frequency band. Within a volume of 502 mm3, relatively homogeneous -field can be generated at every operation frequency with the strength more than 105 V/m, while the single processed capacity keeps unchanged. Most of the microwave energy is concentrated within the processed region to achieve a high efficiency. This microwave reactor presents an excellent candidate in the application of a frequency-reconfigurable microwave processing system to recognize the optimal absorbent frequency for a special microwave reaction.

Dielectric Characterization of the Seeds of Invasive Chinese Tallow Tree

Biofuels have the potential to replace a significant portion of the transportation needs of the USA and the world. Low-cost lipid feedstock (i.e. tallow tree seeds) can be used for production of biodiesel, but these seeds need to be dried. Microwave drying is utilized to dry various seeds and grain, but for this process to be most efficient the dielectric properties of the materials need to be known. This study presents, for the first time, the dielectric properties of the seeds of the Chinese tallow tree. The dielectric constant and dielectric loss factor were determined using a modified free-space method for different moisture contents at frequency ranges centered around 915 and 2450 MHz respectively. The dielectric constant decreased with frequency in those respective ranges, and increased linearly with increasing moisture content, from 1.96 at 0% m.c. to 2.3 at 7% m.c. at 915 MHz, and from 2.08 at 0% m.c. to 2.42 at 7.6% m.c. at 2450 MHz. The dielectric loss factor generally decreased with frequency in both frequency ranges, and increased linearly with moisture content from a low of 0.16 at 0% m.c. to 0.21 at 7.56% at 915 MHz, respectively from a low of 0.13 at 0% m.c. to 0.2 at 7.6% m.c. at 2450 MHz. These results can be used to design microwave processing operations and systems for drying of tallow tree seeds, as well as for non-destructive determination of theses seeds’ moisture content.

Continuous energy efficient exfoliation of vermiculite through microwave heating

Fundamental knowledge of the dielectric properties of vermiculite, its interaction with electromagnetic fields and bulk materials handling technology was used to design and construct a continuous high throughput microwave processing system operating at 2.45GHz. The design methodology used to engineer the system is reported. Experimental demonstration of the effects of microwave energy on the product bulk density is provided for the three most industrially relevant grades of material and it is shown as to how the performance of the system is significantly better than conventional exfoliation technology, in terms of produced bulk densities, energy efficiencies, temperature of the product and plant footprint.

Oil extraction from Scenedesmus obliquus using a continuous microwave system – design, optimization, and quality characterization

A 1.2 kW, 2450 MHz resonant continuous microwave processing system was designed and optimized for oil extraction from green algae (Scenedesmus obliquus). Algae-water suspension (1:1 w/w) was heated to 80 and 95°C, and subjected to extraction for up to 30 min. Maximum oil yield was achieved at 95°C and 30 min. The microwave system extracted 76-77% of total recoverable oil at 20-30 min and 95°C, compared to only 43-47% for water bath control. Extraction time and temperature had significant influence (p<0.0001) on extraction yield. Oil analysis indicated that microwaves extracted oil containing higher percentages of unsaturated and essential fatty acids (indicating higher quality). This study validates for the first time the efficiency of a continuous microwave system for extraction of lipids from algae. Higher oil yields, faster extraction rates and superior oil quality demonstrate this system's feasibility for oil extraction from a variety of feedstock.

Non-Uniformity of Surface Temperatures of Grain after Microwave Treatment in an Industrial Microwave Dryer

In this study, temperature rise and non-uniformity of heating of grain with different moisture contents after microwave treatment were investigated. The temperature anomalies after microwave treatment were measured for barley and wheat at four moisture levels (12, 15, 18, and 21% wet basis) and for canola at five moisture levels (8, 12, 15, 18, and 21% wet basis). Fifty grams of grain samples were heated in a laboratory scale, continuous-type, industrial microwave dryer (2450 MHz) at five power levels (100, 200, 300, 400, and 500 W) and two exposure times (28 and 56 s). Grain samples were thermally imaged using an infrared thermal camera as soon as they came out from the microwave chamber. Average, maximum, and minimum temperatures were extracted from each thermal image and the difference between maximum and minimum temperatures (ΔT) was calculated. The grain type had significant effect on the surface temperatures after microwave treatment. The surface temperatures increased with microwave powers and exposure times but decreased with moisture content. The average surface temperatures after microwave treatment were between 72.5 and 117.5°C, 65.9 and 97.5°C, and 73.4 and 108.8°C for barley, canola and wheat, respectively, when the applied microwave power was 500 W. At the same power level, the maximum surface temperature was between 100.3 and 140.0°C, 77.8 and 117.7°C, and 98.3 and 130.9°C for barley, canola, and wheat, respectively. Non-uniform heating patterns were observed for all three grain types at all moisture contents, power levels, and exposure times. The ΔT was in the range of 7.2 to 78.9°C, 3.4 to 59.2°C, and 9.7 to 72.8°C for barley, canola, and wheat, respectively. The location of hot and cold spots may vary in different dryers based on the position of magnetron and other components, but almost similar non-uniform heating pattern is expected in all microwave dryers. Therefore, this non-uniformity must be taken into consideration while developing microwave processing systems for grains.

Microwave Innovation for Industrial Composite Fabrication—The HEPHAISTOS Technology

Summary form only given, as follows. A novel automated 2.45 GHz microwave processing system HEPHAISTOS (High Electromagnetic Power Heating Autoclave InSeT Oven System) for small and medium sized structural parts of CFRP (carbon fibre reinforced plastics) has been developed at IHM. The current system integrates advantageously the basic processing steps as tooling, tempering of the resin, the impregnation of the fibres as well as finally the curing of the laminate. The main hardware contribution of the development is realized for a specific modular applicator containment providing an excellent homogeneous electromagnetic field distribution, which is applied as an upgrade tool for conventional autoclave systems. The fabrication process can be performed pressurized or at standard conditions. The process is measured remotely by sensors and can be fully automized.