Continuous Microwave System Research Articles

Enhancing the efficiency of Brown 24 pigment production through continuous microwave heating in conveyor belt and rotary kiln systems: A design and optimization study

In energy intensive industries, the continuous production with microwave technology presents several challenges in achieving high energy efficiency and heating uniformity. In the ceramic pigments sector, the Brown 24 is the ideal candidate for this study due to its susceptibility to thermal runaways and its high temperature to reach total conversion, higher than 1000K. By another hand, most literature related to this field focus on static systems while ignoring the continuous ones which are required by the industry. A coupled model that integrates thermal, electromagnetic and chemical phenomena within an energy system was implemented in COMSOL Multiphysics. Additionally, a MATLAB controller was employed to dynamically adjust the cavity length through a moving plunger, which maximizes the electrical efficiency. The required power is also managed to guarantee a total chemical conversion of the material. The proposed optimization methodology reduces computational costs, and it is applicable to any continuous microwave system processing moving solid materials. In this work, two microwave configurations were optimized. The first one, based on a conveyor belt, achieved a global efficiency close to 70%. While the second one, based on a rotary kiln, achieved a global efficiency of 85% and a production rate of 4.66kg/h, significantly outperforming a previous study by factors of 1.57 and 2.06, respectively. These findings show the potential for substantial improvements in continuous microwave systems.

Numerical solution-based algorithm assisted development of a continuous flow microwave reactor

Continuous flow microwave heating is a sustainable technology favored in the chemical and food processing sectors due to its short residence time and relatively low environmental footprint. To address the complexity of fluid dynamics and electromagnetic field interactions, nuanced evaluation methods beyond conventional reflection coefficient analysis are necessary to predict its efficiency and uniformity. In this study, a numerical solution-based algorithm is utilized to propose synchrony and cumulative index metrics for performance analysis. Although identical S-parameter systems demonstrate approximate electromagnetic power loss density in the fluid region, significant disparities in heating performance are observed, evidenced by outlet temperatures ranging between 108.10 °C and 147.09 °C, and covariances ranging from 0.07 to 0.46. The power loss density is corrected based on velocity to introduce the synchrony index, which is further calculated into the cumulative index. This approach provides accurate predictions of efficiency and uniformity, informing system designs that align with specific performance objectives. To achieve the best uniformity in simulated two-part systems, a combination of 63 % GIM system and 37 % EBH-2 system in terms of power input was found to be optimal. However, when considering temperature dependent dielectric properties, the optimal combination was adjusted to 60 % GIM system and 40 % EBH-4 system. The numerical solution-based algorithm facilitates optimization of complex continuous flow microwave systems, providing invaluable insights into the time–space electromagnetic response mechanism of fluids.

A computational study for the effects of sample movement and cavity geometry in industrial scale continuous microwave systems during heating and thawing processes

Microwave (MW) applications in food processing are used to reduce process time and increase process efficiency. While 915 MHz frequency is dominant for industrial processes, 2450 MHz systems are also observed. Attained temperature uniformity is specific concern with significant effect of the non-uniform electromagnetic field distribution due to the cavity geometry and design. Therefore, the objective of this study was to determine the cavity geometry effect with applied frequency and to present industrial scale continuous process system with a computational approach. A computational model was developed for heating and thawing processes, and experimental validation was completed in a cylindrical MW cavity. Sample rotational movement and cavity geometry effects (conventional rectangular versus cylindrical, ellipsoidal and triangular) were determined, and industrial scale system designs were presented with frequency effect. Temperature change and electromagnetic field distribution were also introduced for improved design. The results demonstrated improved designs for industrial processes.

Designing system cavity geometry and optimizing process variables for continuous flow microwave processing

Microwave application is a significant industrial food process applied from thawing to drying and pasteurization – sterilization. While its unique feature is the volumetric heating, the distinct drawback of a microwave process, as a contradiction, is the temperature non-uniformity due to the limited penetration depth. In addition to the effects of sample geometry and physical properties, cavity design is another significant factor for temperature uniformity especially in the continuous flow industrial systems, but this approach did not receive the required attention to prevent this noted contradiction. Therefore, the objective of this study was to demonstrate optimization studies for cavity geometry and process design in industrial scale continuous flow microwave systems. For this purpose, a previously (experimentally) validated mathematical model was used for optimization studies in industrial scale continuous flow systems. These systems had cylindrical (915MHz frequency) and rectangular (2450MHz frequency) cavity geometries. The results demonstrated that the cavity geometry had a significant effect for increasing the process efficiency while the pipe orientation within the system affected the electric field distribution and resulting temperature uniformity. This study also confirmed a different view to the industrial scale continuous flow microwave process for improving the system design and optimizing the process variables.

Continuous Heating Microwave System Based on Mie Resonances

Nowadays microwave heating is a part of many people’s daily life, whereas industrial microwave heating is not as widespread. The permittivity of many liquids is highly temperature dependent, which makes it difficult to control the heating process when using microwaves. Presently, we propose two continuous heating microwave systems for heating, pasteurization and sterilization of liquids. Both designs utilize resonances in cylindrical water-filled cavities of different sizes so that high absorption through the whole heating process is achieved. The proposed systems can heat water from 0 to 100 °C, and their effectiveness is significantly higher than that of a single water cavity.

Effect of rotation on temperature uniformity of microwave processed low - high viscosity liquids: A computational study with experimental validation

Microwave processing of liquid foods is a recent trend with volumetric heating effect compared to conventional processing. A certain temperature non-uniformity is, however, still observed due to non-uniform electric field distribution. This is especially significant in high viscous products due to the difficulty in creating natural convection mixing. Enabling rotational effects to liquid might be an approach to obtain a uniform temperature distribution. Therefore, the objectives were to develop a mathematical model for microwave processing of liquid foods, experimentally validate it and demonstrate rotation on temperature uniformity. A multi-physics finite element program was used for model development, and experimental studies were carried out in a batch system at 2.5 rpm. Validated model was then used to demonstrate the improved temperature uniformity in low - high viscous liquids with rotation (0 to 20 rpm). Considering the recent trend in the use of continuous microwave system for the industrial production, this study is expected to give a certain sight for rotational effects on microwave processing. The experimentally validated model is now to be expanded for further design and optimization studies for industrial scale continuous systems.

Experimental continuous sludge microwave system to enhance dehydration ability and hydrogen production from anaerobic digestion of sludge

Dehydrating large amounts of sludge produced by sewage treatment plants is difficult. Microwave pretreatment can effectively and significantly improve the dewaterability and hydrogen production of sludge subjected to anaerobic digestion. The aim of this study was to investigate the effects of different microwave conditions on hydrogen production from anaerobic digestion and dewaterability of sludge. Based on an analysis of the electric field distribution, a spiral reactor was designed and a continuous microwave system was built to conduct intermittent and continuous experiments under different conditions. Settling Volume, Capillary Suction Time, particle size, and moisture content of the sludge were measured. The results show that sludge pretreatment in continuous experiments has equally remarkable dehydration performance as in intermittent experiments; the minimum moisture content was 77.29% in the intermittent experiment under a microwave power of 300W and an exposure time of 60sec, and that in the continuous experiment was 77.56% under a microwave power of 400W and an exposure time of 60sec. The peak measured by Differential Scanning Calorimeter appeared earliest under a microwave power of 600W and an exposure time of 180sec. The heat flux at the peak was 4.343W/g, which is relatively small. This indicates that microwave pretreatment induced desirable effects. The maximum yield of hydrogen production was 7.967% under the conditions of microwave power of 500W, exposure time of 120sec, and water bath at 55°C. This research provides a theoretical and experimental basis for the development of a continuous microwave sludge-conditioning system.

Lethality and quality evaluation of in-packaged ready-to-eat cooked Jasmine rice subjected to industrial continuous microwave pasteurization

ABSTRACTThe use of the pasteurization unit as a measure of the lethal effect of heating processes and overall quality of ready-to-eat Jasmine rice was determined in order to compare the conventional and microwave technology. Cooked Jasmine rice was packed in a polypropylene plastic cup and sealed with a lidding film, then subjected to a continuous microwave system (eight 800 W; 2450 MHz) and conventional steamer. The pasteurization unit, log reduction, microorganism count, color, instrumental textural property, and sensory attributes were investigated over a 30-day storage at 8°C. The pasteurization process time was reduced from 420 s under the conventional process to 216 s under the microwave process for a 5-log reduction of L. monocytogenes. The microwave heating showed greater effectiveness for shelf life, extending the product from 7 days (conventional) to 30 days. Cooked rice after heating by microwave was whiter and showed lower hardness than the conventional heated sample. Throughout storage, sensory attributes of the pasteurized product heated by microwave were acceptable by panelists. The addition of 15% trehalose or 1% soybean oil into the rice before cooking decreased the hardness of the cooked rice during storage. This study successfully demonstrated that the continuous microwave pasteurization process, compared to the conventional process, required less process time for ready-to-eat cooked Jasmine rice and extended the shelf life while providing better product quality.

Industrial scale microwave processing of tomato juice using a novel continuous microwave system

This study evaluated the effect of an industrial scale continuous flow microwave volumetric heating system in comparison to conventional commercial scale pasteurisation for the processing of tomato juice in terms of physicochemical properties, microbial characteristics and antioxidant capacity. The effect against oxidative stress in Caco-2 cells, after in vitro digestion was also investigated. Physicochemical and colour characteristics of juices were very similar between technologies and during storage. Both conventional and microwave pasteurisation inactivated microorganisms and kept them in low levels throughout storage. ABTS+ values, but not ORAC, were higher for the microwave pasteurised juice at day 0 however no significant differences between juices were observed during storage. Juice processed with the microwave system showed an increased cytoprotective effect against H2O2 induced oxidation in Caco-2 cells. Organoleptic analysis revealed that the two tomato juices were very similar. The continuous microwave volumetric heating system appears to be a viable alternative to conventional pasteurisation.

Continuous combined microwave and hot air treatment of apples for fruit fly (Bactrocera tryoni and B. jarvisi) disinfestation

Apples at 24 ± 2 °C were heated in a pilot scale hot air assisted (40 °C) continuous pentagonal microwave system, to evaluate the effectiveness of this treatment on insect mortality (variety Mutsu) and fruit quality (variety Granny Smith). An average temperature of 53.4 ± 1.3 °C at core, bottom and flesh of the apple was recorded at the end of the treatment. One hundred percent mortality of the most tolerant stage of Queensland fruit fly (Bactrocera tryoni, Froggatt) and Jarvis's fruit fly (Bactrocera jarvisi, Tryon), were observed when the Mortality value (M52, equivalent time of isothermal treatment at 52 °C) at the slowest heating point applicable for each experiment was ≥ 50 min and ≥ 37 min, respectively. showed that microwave heat treatment is effective for insect disinfestation without any adverse impact on total soluble solids, flesh or peel firmness of the treated apples. The treated apples recorded a significantly higher pH and lower ion leakage than the untreated apples after 3 or 4 weeks. Therefore, the microwave heat treatment has the potential to be developed as an alternative chemical free quarantine treatment against economically significant insect pests. Hot air assisted microwave heating of fruits and vegetables, is more cost effective compared to vapour heat treatment and ionising radiation for disinfestation of insects. Microwave treatment is environmentally friendly compared to fumigation and chemical treatments. Hot air assisted microwave disinfestation can be performed at farms or centralised pack houses since the capital cost would be comparatively lower than vapour heat or ionising radiation treatments.

Improvement of heating uniformity in packaged acidified vegetables pasteurized with a 915 MHz continuous microwave system

Continuous microwave processing to produce shelf-stable acidified vegetables with moderate to high salt contents poses challenges in pasteurization due to reduced microwave penetration depths and non-uniform heating. Cups of sweetpotato, red bell pepper, and broccoli acidified to pH 3.8 with citric acid solution containing 0–1% NaCl were placed on a conveyor belt and passed through a microwave tunnel operating at 915 MHz and 4 kW with a 4 min residence time. The time–temperature profiles of vegetable pieces at 5 locations in the package were measured using fiber optic temperature sensors. Addition of 1% NaCl to the cover solution lowered microwave penetration into vegetable pieces and decreased the mean temperature in cups of acidified vegetables from 84 to 73 °C. Soaking blanched vegetables for 24 h in a solution with NaCl and citric acid prior to processing improved microwave heating. Heating was non-uniform in all packages with a cold spot of approximately 60 °C at a point in the container farthest from the incident microwaves. More uniform heating was achieved by implementation of a two-stage rotation apparatus to rotate vegetable cups 180° during processing. Rotating the cups resulted in more uniform heating and a temperature of 77 °C at the cold spot. This is above the industrial standard of 74 °C for in-pack pasteurization of acidified vegetables. The effective treatment involved blanching, soaking for 24 h in a NaCl and citric acid solution, and 180° rotation. This work has contributed to a better understanding of the influence of salt addition and distribution during dielectric heating of acidified vegetables using a 915 MHz continuous microwave system.

Soybean and rice bran oil extraction in a continuous microwave system: From laboratory- to pilot-scale

The performance of a custom-designed, multimode 2450 MHz laboratory-scale, batch type-converted to continuous microwave-assisted extraction (CMAE) system was investigated and the optimization results obtained were used to develop and test a pilot-scale 5 kW, 915 MHz focused cavity CMAE system. Oil was extracted from soy flour and rice bran at various time–temperature combinations with an ethanol: feedstock ratio of 3:1. Both processes were optimized to minimize extraction time while maximizing the quantity of oil extracted. Extraction yields were compared to conventional (CE) and Soxhlet extraction. Yields in the laboratory-scale system for soybean oil were highest at the highest temperature (73 °C) and longest exposure time (21 min), while for rice bran oil, the highest yields were obtained at 73 °C and 17 min. Using the pilot-scale extraction system, greater than 93.0% of total recoverable oil was extracted from both feedstock at all tested flow rates and extraction times, more than the oil recovered by the laboratory-scale unit. Time of extraction and flow rate did not have a significant influence on extraction yield at tested temperatures for the pilot-scale process for soybeans. For rice bran, extraction yield decreased slightly with increasing flow rate (from 0.6 to 1.0 l/min) and extraction time also influenced oil yields. Analysis of oil quality indices (IV, AV, FFA content, wax and phospholipids) extracted by both CMAE processes indicate that the oil meets prescribed biodiesel feedstock quality standards, further justifying use of microwaves as a rapid tool for oil extraction.

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.

The effects of irradiation intensity on the microwave-enhanced advanced oxidation process

The microwave enhanced advanced oxidation process (MW/H2O2-AOP) was adopted in successfully treating a multitude of organic slurries. The factor of ramp rate was incorporated in this study to investigate its correlations with different nutrients release into solution from waste activated sludge. Besides the ramp rate, the amount of time the samples were held at the treatment temperature to a total of 10 minutes aided in the isolation of held temperature from ramp rates, since different ramp rates will yield different reaction times. It was found that with the exception of orthophosphate and chemical oxygen demand release, ammonia and volatile fatty acids depended highly on the ramp rates. All constituents, chemical oxygen demand, orthophosphate, ammonia and volatile fatty acids, solubilized better with longer reaction times coupled with the highest ramp rate. Besides treatment time and ramp rates, treatment temperature and hydrogen peroxide dosage also play important roles in the release of various nutrients into solution by the MW/H2O2-AOP. The results from this study can aid in design parameters for the scaling up of a continuous microwave system for the MW/H2O2-AOP.

Removal of ammonia nitrogen in wastewater by microwave radiation: A pilot-scale study

A large removal of ammonia nitrogen in wastewater has been achieved by microwave (MW) radiation in our previous bench-scale study. This study developed a continuous pilot-scale MW system to remove ammonia nitrogen in real wastewater. A typical high concentration of ammonia nitrogen contaminated wastewater, the coke-plant wastewater from a Coke company, was treated. The output power of the microwave reactor was 4.8 kW and the handling capacity of the reactor was about 5 m 3 per day. The ammonia removal efficiencies under four operating conditions, including ambient temperature, wastewater flow rate, aeration conditions and initial concentration were evaluated in the pilot-scale experiments. The ammonia removal could reach about 80% for the real coke-plant wastewater with ammonia nitrogen concentrations of 2400–11000 mg/L. The running cost of the MW technique was a little lower than the conventional steam-stripping method. The continuous microwave system showed the potential as an effective method for ammonia nitrogen removal in coke-plant water treatment. It is proposed that this process is suitable for the treatment of toxic wastewater containing high concentrations of ammonia nitrogen.

Efficiency of Artemia Cysts Removal as a Model Invasive Spore Using a Continuous Microwave System with Heat Recovery

A continuous microwave system to treat ballast water inoculated with Artemia salina cysts as a model invasive spore was tested for its efficacy in inactivating the cysts present. The system was tested at two different flow rates (1 and 2 L x min(-1)) and two different power levels (2.5 and 4.5 kW). Temperature profiles indicate that the system could deliver heating loads in excess of 100 degrees C in a uniform and near-instantaneous manner when using a heat recovery system. Except for a power and flow rate combination of 2.5 kW and 2 L x min(-1), complete inactivation of the cysts was observed at all combinations at holding times below 100 s. The microwave treatment was better or equal to the control treatment in inactivating the cysts. Use of heat exchangers increased the power conversion efficiency and the overall efficiency of the treatment system. Cost economics analysis indicates that in the present form of development microwave treatment costs are higher than the existing ballast water treatment methods. Overall, tests results indicated that microwave treatment of ballast water is a promising method that can be used in conjunction with other methods to form an efficient treatment system that can prevent introduction of potentially invasive spore forming species in non-native waters.

Design and Implementation of a Continuous Microwave Heating System for Ballast Water Treatment

A continuous microwave system to treat ballast water inoculated with different invasive species was designed and installed atthe Louisiana State University Agricultural Center. The effectiveness of the system to deliver the required heating loads to inactivate the organisms present was studied. The targeted organisms were microalgae (Nannochloropsis oculata), zooplankton at two different growth stages (newly hatched brine shrimp-Artemia nauplii and adult Artemia), and oyster larvae (Crassosstrea virginica). The system was tested at two different flow rates (1 and 2 liters per min) and power levels (2.5 and 4.5 kW). Temperature profiles indicate that, depending on the species present and the growth stage, the maximum temperature increase will vary from 11.8 to 64.9 degrees C. The continuous microwave heating system delivered uniform and near-instantaneous heating at the outlet proving its effectiveness. The power absorbed and power efficiency varied for the species present. More than 80% power utilization efficiency was obtained at all flow rate and microwave power combinations for microalgae, Artemia nauplii and adults. Test results indicated that microwave treatment can be an effective tool for ballast water treatment, and current high treatment costs notwithstanding, this technique can be added as supplemental technology to the palette of existing treatment methods.

EFFECT OF PROCESSING PARAMETERS ON THE TEMPERATURE AND MOISTURE CONTENT OF MICROWAVE‐BLANCHED PEANUTS

ABSTRACT Peanut blanching consists of heat application followed by abrasive removal of the seed coat. The use of a continuous microwave system for the blanching of peanuts has been proposed as a means of reducing processing time and energy costs compared to the traditional hot‐air, multizone oven. The purpose of this research was to characterize effective processing parameters for microwave blanching. The factors examined were the time of exposure in the microwave, use of increased airflow in the microwave applicator during processing and the initial moisture content (MC) of the peanuts. Processing treatments were differentiated by energy absorbed during processing, average and maximum internal temperature, loss in MC and blanchability. High blanchability resulted from higher process temperatures and greater loss in MC. Treatments exceeding 110C resulting in a final MC of 5.5% or below yielded blanchability percentages greater than the 85% industry standard. The time required to generate sufficient heat to dry peanuts for acceptable blanchability is greatly reduced by the use of microwave technology.

A Continuous Microwave System For Prevention of Invasive Species During De-Ballasting Operation-Death Kinetics

A continuous microwave heating system was tested for its effectiveness at removing potentially invasive organisms during deballasting operations. Four different organisms, namely Nan-nochloropsis oculata (microalgae), Artemia nauplii, Artemia adults and Crassosstrea virginica (oyster larvae) normally found in ballast water were investigated in a controlled study to quantify their survival after continuous microwave heating of synthetic ballast water. The experiments were performed in the microwave system using a 2 x 2 factorial design with power (2.5 and 4.5 kW) and flow rate (1.0 and 2.0 Ipm) and the organisms subsequently subjected to different holding times. The control treatment was performed in a water bath using the same temperatures and holding times as in the case of the microwave treatment. Overall, the results obtained indicated that the microwave system was more effective in eliminating the organisms when compared with the control treatment. In most cases there were no survivors present after the microwave treatment at holding times above 100 s, and temperatures as low as 50°C particularly for oyster larvae and Artemia adults. The results are promising, indicating that this technology has the potential to be an effective tool in controlling/preventing the introduction of invasive species into native environments.

High power continuous wave microwave system at 3.7 GHz

The lower hybrid current drive (LHCD) system is an important system in superconducting steady state tokamak (SST-1). It is used to drive and maintain the plasma current for 1000 s with a duty cycle of 17%. The LHCD system is being designed to launch 1 MW of radio frequency (rf) power at 3.7 GHz. The rf source is comprised of two high power klystron amplifiers, each capable of delivering 500 kW rf power. In this article, the results obtained during installation and commissioning of these klystrons are presented. Two klystrons (model TH2103D) have been successfully installed and commissioned on dummy loads, delivering ∼200 kW power for more than 1000 s. The maximum output power that could be obtained is limited due to the available direct current (dc) power supply. The test system is comprised of a TH2103D klystron, a low power rf (3.7 GHz/25 W) source, two high power four port circulators, two high power dual directional couplers, two arc detector systems, and two dummy water loads. To avoid rf breakdown in the rf components of the transmission line, the system has been pressurized with dry air to 3 bar. To energize and operate the klystron, a high voltage dc power supply, a magnet power supply, an ion pump power supply, a −65 kV floating anode modulator power supply, and a filament power supply are used. An arc detector unit has been installed to detect and initiate action within a few microseconds to protect the klystron, waveguides, and other rf passive components during arcing. To protect the klystron in the event of an arc, a fast responding (&amp;lt;10 μs), rail gap based pressurized crowbar unit has been used. The entire system is water cooled to avoid excess temperature rise during high power continuous wave operation of the klystron and other rf components. The tube requires initial conditioning. Thereafter, the output rf power is studied as a function of beam parameters such as cathode voltage and beam current.