Continuous Flow Microwave Research Articles

A High-Performance Microwave Heating Device Based on a Coaxial Structure

Continuous-flow microwave heating stands out for its ability to rapidly and uniformly heat substances, making it widely applicable in chemical production. However, in practical applications, the permittivity of the heated liquid changes dramatically as the reaction progresses, affecting the efficiency and uniformity of continuous-flow heating. Herein, this work presents a novel microwave heating device based on a coaxial structure for high-performance heating. Our approach commenced with the development of a multiphysical field model, incorporating spiraled polytetrafluoroethylene (PTFE) as a water channel and the coaxial waveguide as a container. The analysis shows that the uniform distribution of the sectional electric field of electromagnetic waves in the TEM mode within the coaxial structure can enhance heating uniformity. Then, a continuous-flow microwave heating system for different liquid loads was established, and experimental measurements were conducted. The heating efficiency for all loads exceeded 90%, which basely matched the simulation results, validating the accuracy of the model. Finally, the heating efficiency and uniformity under different permittivity loads were analyzed, as well as the impact of channel radius on heating efficiency. The device exhibits high heating efficiency under different loads, with uniform radial electric field distribution and stable heating uniformity. This continuous-flow microwave device is suitable for chemical research and production because of its high adaptability to the large dynamic range of permittivity, contributing to the promotion of microwave energy applications in the chemical industry.

Effect of Dual Heat Treatment on the Oxygen Reduction Performance of Nanoparticles of Pt on Carbon Catalysts Prepared by Continuous Flow Microwave Technology

Effect of Dual Heat Treatment on the Oxygen Reduction Performance of Nanoparticles of Pt on Carbon Catalysts Prepared by Continuous Flow Microwave Technology

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.

Batch Preparation of PtCo/C for High-Performance Oxygen Reduction Reaction Based on a Continuous-Flow Microwave Pipeline Reactor

Batch Preparation of PtCo/C for High-Performance Oxygen Reduction Reaction Based on a Continuous-Flow Microwave Pipeline Reactor

Modeling Microwave Heating of Molten Salt for Thermal Storage Systems

With the aim of affordably storing energy produced by photovoltaic and wind power plants, Power-to-heat-to-power (or Carnot batteries) are proposed as an outstanding system capable of transforming this energy into heat through existing commercial thermal storage systems for thermosolar plants, and again produce electricity by connecting these storage systems to available power blocks that can come from dismantled carbon plants or any other such as gas plants, or even nuclear. On this basis, microwave heating is studied as feasible to store energy in molten solar salt: 60%w NaNO3, 40%w KNO3. This study is expected to provide some key points for the design of microwave systems for molten solar salt, analysing different simulated cases with numerical modeling: continuous flow microwave heating in an elliptical cavity with different flow rates – 1 l/min and 1.6 l/min – and different positions of the fluid carrier tube, and in a microwave oven with helical tubes.

Continuous-flow microwave heating system with high efficiency and uniformity for liquid food

Continuous-flow microwave heating has good potential in liquid food processing. Efficiency and uniformity of continuous-flow heating are critical to processing results. During continuous-flow heating, the dielectric constant of the liquid varies with increasing temperature, and the efficiency and uniformity of continuous-flow heating are compromised under significant dynamic variations in the dielectric constant of the liquid. To address these issues, this paper proposes a continuous-flow microwave heating system based on a coaxial inner conductor structure. First, a multiphysics model was developed, the parameter of the continuous-flow heating system is optimised so that the load effectively absorbs electromagnetic waves, and the heating uniformity can be enhanced due to the uniform cross-sectional electric field distribution of TEM mode in the coaxial structure. Second, experiments were performed to validate the proposed model, and the results were in good agreement with the simulation. The heating efficiency under varying dielectric constants (from 10 to 80) and loss angle tangents (from 0.1 to 0.5) of the liquid was above 90%. Then, the proposed system is compared with multimode cavity and system without waveguide coaxial conversion structure, and the proposed system is more efficient for liquid with different dielectric constants. Next, a section of metal bar was added to the center of the glass tube and the influence of the metal bar on heating effect was analysed. With the addition of the metal bar, the efficiency of the system in heating a low-dielectric-constant liquid could be effectively improved, and the heating uniformity could be increased by more than 30%. Finally, the proposed system was applied to heat liquid food and the results indicated that all of liquid food materials' efficiency larger than 90%. The continuous-flow microwave system provides the possibility for large-scale industrial production.

Continuous-Flow Microwave Reactor for High-Performance Heating of a Dynamic Chemical Reaction System

Continuous-Flow Microwave Reactor for High-Performance Heating of a Dynamic Chemical Reaction System

Continuous flow microwave processing of liquid whole egg: Pasteurization and functional characteristics evaluation

The objective of this study was to assess the impact of continuous flow microwave processing on the pasteurization efficiency and quality attributes of liquid whole egg (LWE). The heating section was accomplished using microwave, while the holding section utilized a water bath, and the process parameters were determined by a developed numerical model. The inactivation kinetics of Escherichia coli, Staphylococcus aureus, and Salmonella Enteritidis were evaluated at three temperatures (56, 60 and 64 °C) and two power-density (8 and 16 W/mL with different flow rates). A 5-log10 reduction was achieved and the Weibull model fitting the survival data well, which contributed to the optimization of microwave pasteurization process. Additionally, microwave promoted the transitions of secondary structures from β-sheet to α-helix and stabilised the system by enhancing the zeta potential value, further improving the emulsifying and foaming properties. The major proteins in LWE were not found to undergo degradation according to the results obtained from SDS-PAGE analysis. The results of this study suggested that continuous flow microwave pasteurization of LWE exhibits potential as a viable alternative to conventional pasteurization methods. Industrial relevanceThe adoption of high-efficiency physical processing methods is imperative for the sustainable and low-carbon advancement of the food industry. This study confirmed the feasibility of continuous flow microwave pasteurization as an alternative to conventional methods for liquid whole egg, addressing key limitations encountered in traditional pasteurization approaches, and the findings offer practical insights for industrial-scale production. Additionally, the development and application of a continuous flow microwave heating system for pasteurizing liquid whole egg provide valuable knowledge for the utilization of microwave treatment in processing high-viscosity and heat-sensitive fluid foods.

Multiphysics modeling of polyphenol oxidase and peroxidase inactivation in continuous-flow microwave thermal processing of coconut water

Multiphysics modeling of polyphenol oxidase and peroxidase inactivation in continuous-flow microwave thermal processing of coconut water

Continuous-Flow Microwave Heating Inactivation Kinetics of α-Amylase from Bacillus subtilis and a Comparison with Conventional Heating Conditions

The inactivation kinetics of an α-amylase enzymatic time-temperature integrator (TTI) from Bacillus subtilis (BAA) under continuous-flow microwave (MW) and conventional heating conditions were evaluated and compared in this study. The TTI dispersed in a buffer solution (pH 5.0 to 6.9) at 20 °C initially, and it was continuously circulated through two helical coils connected in a series for heating. The two coils were positioned in two domestic microwave ovens (2450 MHz and 1000 W nominal capacity each) and connected by a short tube. The sample flow rates were adjusted to result in a specific exit temperature in the range of 65 to 80 °C. A short fully insulated helical coil at the exit of the second oven was used as a holding tube. Test samples were drawn either at the exit of the second MW oven or immediately after the holding tube. The decimal reduction times obtained under conventional batch heating conditions decreased from 66 to 24 s as the temperature changed from 70 to 75 °C at pH 5.0 while they decreased from 8 to 5 s under MW in the same temperature range, but at pH 6.0, they increased both under conventional and microwave heating conditions (138 to 120 s and 89 to 61 s, respectively). The D-values under conventional thermal holding were four–eight times higher than under a continuous-flow MW heating condition. By varying the pH, the D-values could be modified to suit the validation of appropriate processing conditions.

A New Continuous Flow Microwave Radiation Process Design for Non-Isocyanate Polyurethane (NIPU).

Non-isocyanate Polyurethane (NIPU) has been known to result from a thermal-ring-opening reaction between bis-cyclic carbonate (BCC) compounds and polyamines. BCC can be obtained from carbon dioxide capture using an epoxidized compound. Microwave radiation has been found to be an alternative process to conventional heating for synthesizing NIPU on a laboratory scale. The microwave radiation process is far more efficient (>1000 times faster) than using a conventional heating reactor. Now, a flow tube reactor has been designed for a continuous and recirculating microwave radiation system for scaling up NIPU. Furthermore, the TOE (Turn Over Energy) of the microwave for a lab batch (24.61 g) reactor was 24.38 kJ/g. This decreased to 8.89 kJ/g with an increase in reaction size of up to 300 times with this new continuous microwave radiation system. This proves that synthesizing NIPU with this newly-designed continuous and recirculating microwave radiation process is not only a reliable energy-saving method, but is also convenient for scale-up, making it a green process.

High-efficiency continuous-flow microwave heating system based on metal-ring resonant structure

Continuous-flow microwave heating has been widely applied in liquid food processing, and the heating efficiency relies heavily on the dielectric property of loads with fixed dimensions in a certain cavity. Specifically, rectangular waveguides are widely applied, and straight pipes are usually inserted through the center of rectangular waveguides. However, maintaining high efficiency is always challenging since the dielectric property will vary with temperature. Therefore, a simple and cheap resonant structure is proposed in this paper to maintain high heating efficiency for loads with a large range of relative permittivity in both modeling and experiment. First, the straight pipe surrounded by metal rings in a rectangular waveguide with a short circuit terminal was introduced to concentrate the electric field in the pipe. Then, a multiphysics model was established to calculate the energy efficiency of various load permittivity. The position of the short circuit terminal and the dimensions of the metal ring were further optimized. Finally, energy efficiency experiments of different aqueous ethanol solutions and liquid foods and point temperature rise experiments of continuous-flow water were carried out, and the results showed that the proposed structure can be used to maintain high energy efficiency over large dielectric dynamic ranges compared with the conventional microwave heating system based on rectangular waveguides. Moreover, the effect of different parameters, the pipe permittivity, the pipe wall thickness and the metal ring gap, on the heating efficiency was discussed to show the robustness of the proposed structure.

Continuous-Flow Microwave Milk Sterilisation System Based on a Coaxial Slot Radiator.

Microwave continuous-flow liquid food sterilisation, in which the liquid is mainly heated by microwaves, has the advantages of fast sterilisation speed, energy saving, comprehensive elimination, and less nutrient loss. Circular pipes are commonly used in microwave continuous-flow liquid heating processing. However, with circular pipes, which are widely used in the industry, the heating is uneven owing to the phenomenon of tube focusing when adopting external radiation. In this study, a novel microwave continuous-flow milk sterilisation system based on a coaxial slot radiator is proposed. First, the coaxial slot radiator was designed to realise efficient radiation through the establishment of multi-physics model. The structure of the system was then optimised by comparing the heating efficiency and uniformity of simulation results. The effect of microwave coaxial slot radiator rotation on heating uniformity was simulated and the results show that the heating uniformity is improved obviously. Experimental equipment was set up to verify the results of the simulation. The experimental results are consistent with the simulation results. Finally, the sensitivity analysis of the system is performed to confirm that, when the dielectric properties and types of liquid food change, the heating of the proposed microwave continuous-flow system remains efficient and uniform.

ИНТЕНСИФИКАЦИЯ ПРОЦЕССА ИЗВЛЕЧЕНИЯ ЖИРА ИЗ ЖИРОСОДЕРЖАЩЕГО СЫРЬЯ ДИЭЛЕКТРИЧЕСКИМ НАГРЕВОМ

The article substantiates the effective modes of heat treatment of fat-containing raw materials in a radiohermatic ultrahigh-frequency installation with a metal-dielectric resonator.
 The raw material for the study was the internal fat of cattle, sheep, chickens, their dielectric characteristics were systematized according to I.A. Rogov.
 The purpose of this work is to develop a continuous-flow microwave installation with air-cooled magnetrons and a resonator in the form of a truncated cone with an exponential generator, which contributes to increasing thermal efficiency and maintaining high electric field strength and radio tightness without a shielding housing when heating disinfected fat from fat-containing raw materials. The electrophysical parameters of the fat-containing raw material are analyzed, including the change in dielectric parameters from the frequency of the electromagnetic field (EMF), fat content, density. The intensity of the electric field is matched with the heating rate, the specific power of the generator. A continuous-flow microwave installation with a resonator in the form of a truncated cone with an exponential generator made of non-ferromagnetic material and a ceramic concave mirror base, inside of which a dielectric screw with a decreasing pitch and a diameter of a screw is located, has been developed. A concave ceramic mirror is installed coaxially on the large base of the resonator. A dielectric screw is inscribed along the resonator. Two sluice gates with non-magnetic bent blades are installed on the resonator. The melted fat flows down through the perforation on the surface of the resonator. Magnetrons with waveguides are installed with a shift of 120 degrees along the perimeter of the resonator base. The general properties of a resonator with dielectric inclusions (ceramic mirror, fluoro-formation auger) are analyzed. In such a metal-dielectric resonator, the reflection of the field from the non-magnetic walls of the resonator and critical sections can be combined with full internal reflection in the dielectric.
 The exponential generator of the resonator and the ceramic mirror contribute to an increase in the concentration of the electric field strength in it to 2 kV/cm when placing three magnetrons with a shift of 120 degrees along the perimeter of the large base. With a specific power of the generator of 2 W/g, the productivity of the microwave installation for heat treatment of fat-containing raw materials is 18-20 kg/h.; With a frequency of rotation of the electric screw drive of 1 rpm, 0.5-0.6 kg of raw materials is heated in 120 seconds. The specific energy costs compared to the prototype (0.2-0.25 kWh/kg) is reduced to 0.14-0.15 kWh/kg.

Continuous flow microwave heating and sterilization for liquid food

Abstract Continuous flow microwave sterilization for liquid food has advantages of a short time and high retention rate of nutrients. However, uneven microwave heating is the critical factor restricting the industrialization of microwave sterilization. This paper reviews the up-to-date research on the continuous flow microwave heating and the continuous flow microwave sterilization system for liquid food. The causes of the non-uniformity of continuous flow microwave heating are thoroughly discussed and the methods of improving the uniformity are proposed. Finally, the recommendations for future research of continuous flow microwave sterilization for liquid food are presented.

Continuous flow microwave processing of peanut butter: A (hypothetical) computational process design study with experimental validation

The increase in foodborne pathogen cases in low moisture foods has raised a significant food safety concern for food industry. One of the most significant outbreaks related with these food products was observed in 2008–2009 due to Salmonella problem in peanut butter. This issue was recently observed again for a multinational case. Challenges of conventional thermal processing due to the lower thermal conductivity – diffusivity value and higher viscosity, microwave (MW) heating might be an efficient processing while the non-uniform heating is a challenging drawback. Therefore, the objective of this study was to develop a mathematical model for MW processing of peanut butter and present industrial scale designs for continuous flow processing. For this purpose, the mathematical model was first experimentally validated using a lab-scale system and validated with experimental data. Following this, continuous flow MW process design studies were carried out with different holding tube configurations to obtain a temperature increase to enable decontamination and improve the temperature uniformity. These design simulations provided a better understanding to enhance the process efficiency of high viscosity low moisture peanut butter samples and for design and optimization of continuous flow MW processing.

Design of continuous-flow microwave reactor based on a leaky waveguide

Using microwave heating for biodiesel production has several advantages, including an accelerated reaction rate and improved product quality. However, large-scale microwave-assisted biodiesel production still suffers from low efficiency and uneven heating, which limit its industrial applicability. To address these issues, in this study, a reactor based on leaky waveguide is proposed for continuous microwave-assisted biodiesel production. Firstly, a radiator based on a leaky waveguide is designed to realize a uniform distribution of microwave energy along the axial and longitudinal directions. Then, a multi-physics model based on electromagnetism, heat transfer in fluids, chemical engineering, and laminar flow is developed to simulate the continuous microwave-assisted biodiesel production process. Compared to the traditional bottom-fed and cylinder-fed reactors, the leaky-waveguide reactor has the potential to improve the heating uniformity and energy efficiency, and the microwave energy utilization efficiency can more than 95 % and the value of the outlet COV can be reduced to 1.51 × 10−3. Experiments were carried out to validate the proposed model, and the experimental results are in good agreement with the simulated results. Finally, the effects of the pitch circle radius, radius, and number of turns (screw pitch) of the helical tube used for the microwave-assisted biodiesel production process were analyzed. The radius and number of turns (screw pitch) have a greater effect on the system than pitch circle radius. This continuous microwave-assisted biodiesel production reactor provides a possibility for large-scale industrial production and can also be widely applied to the chemical engineering with pressure continuous flow reactor and food industry.

High-Efficiency Continuous-Flow Microwave Heating System Based on Asymmetric Propagation Waveguide

Continuous-flow microwave heating is widely used in chemical engineering and other fields. However, during microwave heating, the load’s electrical permittivity change will affect microwave heating efficiency. In this article, a WR430 asymmetric propagation waveguide is proposed based on gradient-index metasurfaces (GIMs), which can achieve high-efficiency continuous-flow heating with a large range of permittivity. First, asymmetric microwave propagation is realized in a WR430 3-D waveguide model, and the influence of metasurfaces index, thickness, and discrete element size is analyzed. Second, the asymmetric propagation waveguide is realized by fluted medium based on the permittivity equivalent method. The results show that the microwave inside the waveguide can only propagate along one direction at 2.4–2.6 GHz. Finally, the proposed waveguide is applied to a continuous-flow heating system. Compared with the single-mode cavity, multimode cavity, and waveguide water load, the proposed method can improve heating robustness and efficiency with different loads. Experiments show that the microwave absorption efficiency is stable and greater than 90% in the proposed system when the permittivity of the load changes drastically. The proposed system has a simple structure and strong applicability, which can replace the three-stub tuner and get rid of the circulator and water load for continuous-flow heating.

Creating Cu(I)-decorated defective UiO-66(Zr) framework with high CO adsorption capacity and selectivity

• Defective UiO-66 was rapidly fabricated by continuous-flow microwave synthesis. • The surface area and pore width of UiO-66 (Zr) were improved by defects. • The most defective 25Cu(I)@UiO-66 had the highest CO uptake capacity of 2.44 mmol/g. • Defective Cu(I)@UiO exhibited improved CO selectivity. • Cu(I)@UiO-66(Zr) showed good renewability and oxygen-resistant ability. Developing a highly effective material for CO storage and separation is a challenge. In this study, we first used microwave-assisted continuous-flow synthesis to create defective UiO-66(Zr) frameworks with improved surface area and pore structures, which is highly efficient for large-scale production. The defect concentration of UiO-66(Zr) was rapidly and effectively controlled within a short reaction time of 10 min. Different amounts of Cu 2+ were loaded onto defective UiO-66 samples, followed by reducing Cu 2+ to Cu + to obtain Cu(I)@UiO-66 adsorbents (Cu(I)@UiO). X-ray diffraction (XRD), N 2 -adsorption, FT-IR, SEM, TEM, thermogravimetric analysis (TGA), and XPS analyses were used to characterize the physicochemical properties of the prepared adsorbent materials. Gas adsorption experiments revealed that Cu(I)-decorated defective UiO-66 exhibited high CO adsorption capacity due to the π complexation between Cu + and CO. The CO adsorbing amount and CO selectivity onto the Cu(I)@UiO-66 samples were improved by adjusting the defect concentration and Cu(I) load on the UiO-66 host. The breakthrough experiment confirms that this defective Cu(I)@UiO adsorbent can efficiently separate CO/N 2 mixture under dynamic mixture flow conditions. Furthermore, after several cyclic adsorption–desorption experiments, the defective Cu(I)@UiO-66 is highly regenerable and has a good oxygen-resistant ability. The study describes a simple and scalable method for producing π-complexation adsorbent for CO adsorption.

Microwave-assisted preparation of polyphosphoric acid in a continuous-flow reactor

Abstract Polyphosphoric acid (PPA) is widely used in inorganic salt production, petrochemical industry, electronic material preparation and other manufacturing industries. Conventional preparation methods of PPA has disadvantages of pollution, high energy consumption and long production time. To address this problem, microwave continuous-flow preparation may be a desirable way due to its advantages of environmentally-friendly, rapidity and high efficiency. Therefore, to explore the process of preparing PPA by microwave continuous-flow method, a continuous-flow microwave reactor was designed for the dehydration process of orthophosphoric acid to prepare PPA in this paper. The microwave-assisted dehydration process was studied in comparison with the conventional dehydration process and the “closed” microwave-assisted dehydration process in terms of energy efficiency, process times and treatment capacity. The effect of input microwave power, reduced pressure and inlet flow velocity of orthophosphoric acid on the performance of the dehydration process was studied. The results showed that the influence of the microwave power on the temperature rise process during dehydration is greater than that of the reduced pressure. Moreover, the inlet flow rate has a great impact on the treatment capacity and product quality of the dehydration process. Bedsides, the comparison with the other two methods showed that microwave heating can effectively shorten the dehydration time, and the continuous-flow treatment can effectively improve the treatment capacity of microwave heating. The perspectives of the process scale-up by continuous-flow microwave heating method is also discussed.