Microwave Heating Technique Research Articles

Investigation on the decomposition of titanium slag using sodium carbonate for preparing rutile TiO2

This paper aims to provide fundamental knowledge to prepare rutile TiO2 from titanium slag (71.66% of TiO2, 7.70% of Total Fe) with the assistance of sodium carbonate roasting and microwave heating. The decomposition behaviors of the as-received titanium slag were investigated with experimental comparing groups, namely one group using conventional heating technique and another group using microwave heating technique, respectively. To understand the fundamental knowledge on the decomposition behaviour of titanium slag, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), and laser particle size analyzer (LPS) were responsible for the evaluating of the products. Additionally, the influence of sodium carbonate on the phase transformation process was studied. The results indicate that the insoluble Na2Fe2Ti6O16 phase was detected in the conventional roasted product with a low Na2CO3/slag ratio, while this phenomenon was absent in the microwave roasted product. With the increasing amounts of Na2CO3, the particles of the final produced TiO2 have a transformation from irregular shape to rod-like shape. Compared to the products produced by conventional heating techniques, the rod size of the products using the microwave heating technique is more refined. The noticed structure indicated that the microwave heating technique improves the crystallinity growth at a lower Na2CO3/slag ratio. The FT-IR results revealed that products' adsorption ability was enhanced with an increase in Na2CO3/slag ratio and the application of microwave heating techniques. The LPS exhibited that the increase of Na2CO3/slag ratio restrained the particle size of rutile TiO2, which was more evident for the microwave roasted product.

Investigations on a controlled microwave heating technique for efficient depressurization in methane hydrate reservoirs

Numerical investigations on a novel technique of controlled microwave heating with depressurization, applied to gas recovery from methane hydrate reservoirs are performed. An in house, multi-phase, multi-component fluid flow solver with microwave radiation is developed and validated with benchmark results in literature. The microwave heating in the reservoir is operated and controlled based on the rise and fall of temperature in the well-bore proximity. Results show that the proposed microwave control technique significantly improves the gas recovery and energy output of combined depressurization and microwave heating. The performance parameters such as total gas recovery and energy efficiency ratio of the controlled microwave heating technique are compared with pure depressurization, depressurization with continuous heating, and pure heating techniques. The proposed control method provides high overall energy efficiency than all other production techniques. Based on the pareto optimization strategy using fuzzy membership function, an optimum well-bore temperature bound for control of microwave operation is determined. The sensitivity study of the process inputs reveals that at the optimum temperature bound, the energy efficiency ratio of the controlled microwave heating becomes independent of the microwave power. The depressurization pressure differential has a linear impact on the energy efficiency ratio of the controlled microwave heating, while a microwave with 915 MHz frequency is seen to provide the maximum energy efficiency ratio.

Fe-N-C Self-Doped Sewage Sludge-Derived Biocarbon With Catalytic Activity For The Oxygen Reduction Reaction

The development of numerous biomass-derived carbon catalysis toward the oxygen reduction reaction (ORR) have been reported in the last decade. The catalytic activity for the ORR improves with heteroatom doping and is notably high when Fe-N-C type bonds are present. In this work, we developed a novel method to obtain Fe-N-C self-doped sewage sludge-derived biocarbons (SSBs) by intermittent microwave heating (IMH) technique. The effect of IMH on the structural and textural properties of SSBs, and their correlation with the catalytic activity for ORR was studied. The most active biocarbon was labeled as SSF, which was obtained by two-step process: chemical activation using KOH and then it was functionalized with methanol by IMH. Therefore, the obtained SSF micro-mesoporous biocarbon had high surface area (1127 m2 g-1) and quaternary-N and thiophene-S species. In addition, Fe-N-C bonds were detected on the surface of SSF. On the other hand, it was observed that IMH treatment improved the electrochemical parameters such as the on-set potential (E0), current density (j), electron transfer number (n) and %HO2 - production. In this regard, SSF shows the best performance toward the ORR (E0= 0.89 V vs RHE, j= -3.01 mA cm-2, n= 3.4 and %HO2 - = 25.85). These results indicate that sewage sludge could be used as raw material for the manufacture of cathodes, and that the treatment by IMH seems a promising eco-friendly alternative.

Exergetic sustainability evaluation of horse manure biomass valorization by microwave pyrolysis

Microwave heating technique has gained widespread popularity in pyrolyzing biomass feedstocks due to its several unique features over the conventional furnace heating method. Accordingly, the present work aimed at finding the optimal operating conditions of a microwave reactor applied to pyrolyze horse manure from the exergetic, exergoeconomic, and exergoenvironmental viewpoints. The effects of two process variables, i.e., reaction temperature and catalyst:biomass ratio, were thoroughly investigated on the considered exergy-based methods. Three more key exergetic indicators “viz. exergy efficiency of the process, unitary exergoeconomic cost of the product”, and unitary exergoenvironmental impact of the product were taken into account to select the most efficient, productive, and sustainable process conditions. The reaction temperature significantly affected the exergy efficiency of the process. Interestingly, the exergy dissipation due to mass loss and thermodynamic irreversibilities accounted for over 50% of the total input exergy of the process. The quantity of activated carbon catalyst is a determinant factor in exergoeconomic and exergoenvironmental analyses. The optimum operating conditions were the reaction temperature of 350 °C and the catalyst:biomass ratio of 1:2. Under these conditions, exergy efficiency of the process, unitary exergoeconomic cost of the product, and unitary exergoenvironmental impact of the product were 49.2%, 487 USD/GJ, and 151 mPts/GJ, respectively. Overall, it could be concluded that the applied exergetic approaches could be appealing complements to the chemical evaluation for analyzing and optimizing various bioenergy systems.

The evolution of phases in FeNiCoCrCuBx high entropy alloys produced through microwave sintering and vacuum arc melting

Microwave heating and sintering techniques are applied to various production lines and material systems to improved their microstructure and mechanical properties in comparison to conventional means of production. These techniques also consume less power and energy compared to conventional heating methods. In this study, the production of high entropy alloys (HEA) by arc melting was carried out with specimens made from compacted and sintered elemental powders; the sintering process of alloy powders prior to remelting prevents certain problems such as porosity and uneven mixing that may occur during casting. We investigated the effects of conventional and microwave sintering processes prior to remelting and casting on structure and properties of FeNiCoCrCuBx HEA. Our results show that microwave sintering changes the size and shape of phases and microstructure of the alloy by affecting the liquid-phase separation mechanism. Three-point bending strength and ductility of alloys produced by microwave sintering were superior to conventional sintering.

Glass Foam from Flat Glass Waste Produced by the Microwave Irradiation Technique.

A glass foam with good thermal insulation characteristics (apparent density of 0.38 g/cm3, porosity of 81.9% and thermal conductivity of 0.089 W/m·K), high compressive strength (3.9 MPa) and a satisfactory microstructural homogeneity with pore size between 0.6–1.0 mm was obtained by sintering at 927 °C of flat glass waste, a glass waste usually not used in the manufacture of glass foam. The manufacturing recipe has been improved by the simultaneous use of two microwave susceptible foaming agents (SiC and Si3N4) and the addition of coal fly ash and an oxygen-supplying agent (MnO2). The originality of the work was the simultaneous use of the two foaming agents and also the application of the technique of predominantly direct microwave heating, compared to the conventional heating methods commonly used in the manufacture of glass foam. The remarkable energy efficiency of the microwave heating technique led to high average heating rates without affecting the structural homogeneity and very low specific energy consumption.

Biogas purification by adsorption of hydrogen sulphide on NaX and Ag-exchanged NaX zeolites

Biogas is a cheap renewable and eco-friendly alternative energy to fossil fuels. However, biogas is made up of a mixture of gases including hydrogen sulphide (H2S), a corrosive and toxic gas. The current study was carried out to purify biogas by adsorbing H2S on NaX and Ag-exchanged NaX zeolites. Pure crystalline NaX zeolites were synthesized by microwave heating technique. NaX zeolites were then modified following Ag-exchange with various aqueous solutions of AgNO3 (11.5%, 34.1%, 71.6%, 76.7%, and 78.8%) at 40 °C for 2–12 h. Compositional and structural analyses of NaX and Ag-exchanged NaX zeolites were done using powder X-ray diffraction (XRD), elemental analysis, nitrogen adsorption porosimetry, and atomic absorption spectrometry. Anaerobic digestion (AD) was carried out by feeding the digesters with a mixture of 600 mL of dairy manure and 4.0 g L−1 NaX and Ag-exchanged NaX zeolites. An almost complete exchange of Na+ cations by Ag+ cations was observed. The Ag-exchanged zeolites exhibited type I isotherm features with no modification to the shape. A well-dispersed Ag+ ion in the zeolite framework was observed. More H2S was efficiently removed in Ag-exchanged NaX zeolites digesters when compared to that of NaX zeolite. Methane yield was significantly increased from 167.31 (control) to 215.33 (78.8% Ag/NaX) mL g−1 after 48 days. Ag-exchanged NaX zeolites in the current study is a promising adsorbent for H2S removal in biogas.

Optimisation of heating uniformity for milk pasteurisation using microwave coaxial slot applicator system

Microwave heating is a novel solution for milk pasteurisation but the non-uniformity of thermal distribution is still a significant problem that hampers its application especially for milk pasteurisation. This paper reports on the isotherm heating of a coaxial slot antenna applicator with a microwave signal generation using a fabricated mono-mode microwave system designed to improve the temperature uniformity of batch milk pasteurisation. In this paper, applicator simulation, design, characterisation, and fabrication are described in detail. The fabricated microwave pasteurisation system was implemented using a digitally controlled water-cooled magnetron with a waveguide circulator, a three-screw waveguide tuner, a water-based waveguide terminator, and a rectangular to coaxial adapter to channel the output. The impedance matching and power calibration routines of the fabricated microwave pasteurisation system are explained and analysed in detail. Three microwave powers, namely 100 W, 125 W and 150 W were used to test and characterise the pasteurisation performance of the system. For each microwave power the thermal profile and the temperature distribution of the 100 ml milk samples in a glass beaker were simulated and measured. The pasteurisation process was evaluated based on the aerobic plate count (APC) test. Results showed 90% improvement in the temperature uniformity as compared with an existing batch microwave pasteurisation method. The APC test showed pasteurisation efficiency was 99.999% and total microbial elimination occurred after 7 min, 6 min, and 5 min of heating using 100 W, 125 W, and 150 W power respectively. • Microwave pasteurisation will preserve the taste, colour, and nutrition of cow's milk. • Mono-mode microwave heating saves energy costs and reduces heating time. • Uniformity in heating will guarantee the quality of milk pasteurization. • Dielectric loss factor determines heating efficiency of microwave heating technique. • Dielectric loss factor is a temperature and frequency dependent parameter.

Electronic transport properties of NASICON structured NaFe2(PO4)(SO4)2: A potential cathode material for Na-ion batteries, synthesized using ultrasound-assisted, indirect microwave heating technique

Low-temperature synthesis and reduced reaction time for fabrication of nanocrystalline, NaFe2PO4(SO4)2, a NASICON structured potential cathode material for the sodium-ion battery, is reported here. For this, we have used a novel ultrasound-assisted wet chemical approach with further assistance from efficient microwave heating. We were able to synthesize this NASICON compound at 450 °C in 6h using indirect microwave heating. Using broadband dielectric spectroscopy, we measured its complex permittivity and conductivity. A non-Debye type relaxation process was observed for this compound, with conducting charge carriers and polaron hopping primarily responsible for overall dielectric losses in the sample.

Mechanistic Insights into the Nonconventional Sol-Gel Synthesis of MAX Phase M2GeC (M = V, Cr).

Despite their intriguing properties, MAX phases to date remain a class of materials overwhelmingly synthesized and studied with conventional approaches that date back to their discovery. With an ever-increasing demand for new and better materials and areas of application, developing new synthesis techniques must be at the forefront of our scientific efforts and cannot be overlooked. Sol-gel chemistry, while being a very traditional approach (especially for oxides), has so far hardly been leveraged within the MAX phase community. As a newly emerging technique to access nonoxide compounds, such as MAX phases, it offers a variety of advantages over classical solid-state chemistry, namely, milder reaction conditions and greater processibility (as previously shown for Cr2GaC). Here, the sol-gel synthesis of the two MAX phase members V2GeC and Cr2GeC, in combination with both conventional and nonconventional (microwave) heating techniques, is presented. In all instances, high yields were achieved, with only minor impurities remaining in the product. This expansion of the method to other members (apart from Cr2GaC) is a critical milestone in proving the technique's viability. Additionally, using simultaneous calorimetry and mass spectrometry, first insights into the underlying carbothermal reduction reaction are presented. Understanding the chemistry and formation mechanism will help broaden the sol-gel-based synthesis technique and increase its applicability.

Crystal structure of a three-dimensional neodymium(III) coordination polymer, [Nd2(H2O)6(glutarato)(SO4)2] n.

A three-dimensional coordination polymer, poly[hexa-aqua-(μ4-glutarato)bis(μ3-sulfato)-dineodymium(III)], [Nd2(glutarato)(SO4)2(H2O)6] n (glutarato2- = C5H6O4 2-), 1, consisting of cationic {Nd2(H2O)6(SO4)2} n 2n+ layers linked by bridging glutarate ligands, was synthesized by the microwave-heating technique within few minutes. The crystal structure of 1 consists of two crystallographically independent TPRS-{NdIIIO9} (TPRS is tricapped trigonal-prismatic geometry) units that form an edge-sharing dinuclear cluster inter-connected to neighbouring dimers by the μ3-SO4 2- anions, yielding a cationic two-dimensional {Nd2(H2O)6(SO4)2} n 2n+ sheet. Adjacent cationic layers are then linked via the μ4-glutarato2- ligands into a three-dimensional coordination network. Strong O-H⋯O hydrogen bonds are the predominant inter-action in the crystal structure.

Microwave Heating as an Innovative Road Maintenance Technology: Aging Effect on Binder and Feasibility Evaluation

The microwave heating/healing technique is regarded as a green maintenance approach for asphalt pavements thanks to its promising environmental and economic benefits. However, the main concern about this technology is represented by the possible aging effect generated on bituminous binders. Currently, there is a significant lack of studies dealing with this topic. Based on these premises, the main purpose of this study is to appraise the feasibility of implementing microwave-based maintenance operations considering the associated aging effect. The assessment of fatigue life after cyclic microwave heating (MH) based on a linear amplitude sweep (LAS) test and the changes in the chemical groups detected through Fourier transform infrared (FTIR) spectroscopy document the aging phenomenon. The results indicate that the microwave aging degree on bituminous binder is nonlinear with MH cycles. The microwave radiation causes a distinct aging impact on binders during the first 10 cycles, then the values become constant. Furthermore, a feasibility analysis of MH technology is developed, encompassing four main multidisciplinary aspects: evaluation of microwave aging degree, working mechanism of MH equipment, safety assessment, and economic and ecological considerations. Despite the associated aging issue, the MH method is an efficient technology, considering its various advantages (i.e., rapidity of execution, uniform and non-pollutant treatment, and deep penetration). Meanwhile, the use of steel slag as a microwave absorber bolsters the sustainability of MH technology. This study provides a new perspective to evaluate the microwave heating technique in road engineering comprising the generated aging effect. Practice-oriented recommendations are also formulated regarding the safe implementation of MH technical operations.

Microwave Irradiated Eco-friendly Synthesis of Pyrimidine Derivatives as Potential Antitubercular Agents

The microwave irradiation method is applied for the efficient synthesis of pyrimidine derivatives. The synthetic protocol involves Knoevenagel condensation followed by Michael addition reaction and cyclization of equimolar quantities of aromatic aldehydes, ethyl cyanoacetate and guanidine in the presence of ethanolic NaOH solution to produce corresponding pyrimidine derivatives. The reaction mixture was allowed to reflux under microwave radiation at power level-2 for 7-12 min. The microwave heating technique offers a cleaner reaction with a shorter reaction time and improved product yield as compared to conventional synthesis. The newly synthesized compounds were characterized by their FT-IR, 1H NMR and LC-MS spectral data. All the synthesized pyrimidine derivatives were evaluated in vitro for their antitubercular activity in vitro by using the luciferase reporter phage (LRP) assay method. The antimycobacterial activity was determined in terms of the percent reduction in the relative light unit (RLU). The test compounds exhibited promising antitubercular activity against Mycobacterium tuberculosis H37Rv and clinical isolates, S, H, R and E resistant M. tuberculosis in comparison with the standard drug (isoniazid).

Hydroxyapatite reinforced surface modification of SS-316L by microwave processing

The current study was focused to modify the surface layer of stainless steel (SS-316L) for enhancing its bioactivity by reinforcing hydroxyapatite (HAp) powder through exploiting the principle of hybrid Microwave Heating (MH) technique. The modified substrates were subjected to post heat-treatment at 400 °C and 700 °C for 1 h. The microstructural analysis of the modified composite layer revealed the existence of HAp particles along with some reaction induced products primarily in the iron-based austenite dendritic matrix. The heat-treated substrates showed a higher microhardness value than that of as-deposited substrates due to densification of the modified layer after heat-treatment. The porosity, surface imperfections and flaws were reduced after heat treatment. The scanning electron microscopy (SEM) images taken over the modified and unmodified SS-316L after immersion test in simulated body fluid showed rapid apatite forming ability on the modified substrates. The apatite growth ability of the modified substrates was reduced after heat treatment performed at 700 °C due to a reduction in the amorphous phase and porosity contents.

UNCONVENTIONAL TECHNIQUE USED TO MANUFACTURE POROUS HIGH-STRENGTH GLASS FOAM

The paper presents experimental results obtained in the manufacturing process of porous high-strength glass foam for thermal insulation in buildings made of glass waste and kaolin clay as raw materials and dolomite as a foaming agent. The paper’s originality is the use of the unconventional microwave heating technique. The best product manufactured by this technique was that sintered at 1050 ºC using 18 wt.% kaolin clay and 3.5 wt.% dolomite. The product characteristics were: the apparent density of 0.66 g·cm-3, the thermal conductivity of 0.155 W/m·K, the compressive strength of 5.3 MPa and a microstructural homogeneity with pore size between 0.20-0.50 mm.

Sequestration of methylene blue dye from aqueous solution using microwave assisted dragon fruit peel as adsorbent

Activated carbon (AC) was prepared from dragon fruit peel (DFP) for methylene blue (MB) dye removal from an aqueous system. The physicochemical activation technique involving chemical treatment using potassium hydroxide, CO2 gasification, and microwave heating techniques were used. Brauner–Emmet–Teller surface area and pore volume of the prepared from DFP activated carbon (DFPAC) were 725.80 m2/g and 0.388 cm 3/g, respectively. SEM images revealed that the prepared DFPAC poses an heterogeneous surface. The FTIR spectra revealed several important functional groups (including the hydroxyl OH, alkane CH, and alkene C≡C) on the surface of DFPAC. Proximate and elemental analysis showed remarkable increase in fixed carbon contents and elemental carbon from 11.38 to 70.11% and 25.6 to 73.36% for DFP and DFPAC, respectively. The MB-DFPAC adsorption system was best explained by Freundlich isotherm with a adsorption capacity of approximately 233 mg/g. The kinetic study showed that the pseudo-second order model best described the adsorption of MB-DFPAC systems. Thermodynamic investigations established an exothermic and spontaneous nature of MB-DFPAC systems with a decreased randomness at the solid–liquid interface, and the mechanism followed physisorption. DFPAC proved to be an excellent adsorbent for Methylene blue dye removal.

Full-Scale Use of Microwave Heating in Construction of Longitudinal Joints and Crack Healing in Asphalt Pavements.

Asphalt pavement construction technology is an industry branch that undergoes constant development. Analyzing the directions of the development, one can divide it into two mainstreams: the development of roadworks equipment and the development of roadworks technology. Microwave heating technique has been mentioned in the road industry from the early ‘70s, but research records from practical full-scale use are very rare. This article presents the evaluation of the possible use of microwave heating technique during a particular aspect of the construction process, namely, the formation of longitudinal joints and the potential repair process of the cracked asphalt pavement. Research results showed that joints constructed using microwave-assisted heating performed the same or even better with regards to tensile characteristics comparing to other techniques. Also, the highest level of compaction was reached among the other tested techniques applied to the wearing course level. The second part of the research experiment showed the large potential of the microwave crack healing technique. The asphalt pavement was healed on its full depth of 10 cm with the single healing operation applied. Although some limitations may occur in the practical use of microwave heating, the test results suggest that it is a very promising technique and should be further developed (for, e.g., shielding concerns, electricity supply). The microwave heating technique is powered with electricity, which is important when there is a constant need for further reductions of CO2 emissions. It can be reached in parallel with clean energy or clean electricity sources.

Characterization and microwave healing properties of different asphalt mixtures suffered freeze-thaw damage

Asphalt materials display the inherent ability to heal micro-cracks and the microwave heating technique can help accelerate this process thus promoting the creation of a durable asphalt pavement. The objective of this study is to investigate the difference in healing potential between two representative types of asphalt mixtures, namely dense asphalt concrete (AC-13) and open-graded friction course concrete (OGFC-13), suffered freeze-thaw (F-T) damage. In particular, OGFC concrete is especially susceptible to water damage due to its high percentage of air voids. The effect of gradation and F-T damage on the healing performance of asphalt mixtures is appraised by means of three-point bending test, freeze-thaw splitting test and X-ray computed tomography test. Results demonstrate that the gradation exerts a clear influence on the microwave healing behavior and microwave absorber content. In addition, the mechanical strength of mixtures characterized by F-T damage can be partially restored with microwave heating treatment: the healing efficiency of OGFC asphalt specimen after three cycle F-T cycles is still satisfactory. As probed with X-ray computed tomography, the proportion of micropores increases, while the amount of mesopores and macropores decrease after the microwave treatment. However, thermal heating technics are not suitable for further healing when the structural deformation of the OGFC sample occurs due to the F-T damaging process. The research outcomes can contribute to understand the microwave healing behavior for different asphalt mixture types and characterize the healing mechanism of mixtures suffered F-T damage.

New Manufacturing Method of Glass Foam by Cold Expansion of Glass Waste


 
 
 
 Abstract 
 An innovation cold manufacturing method of glass foams is presented in the paper. Traditional foaming agents used in conventional expansion processes of glass waste at high temperature were substituted with aluminium powder in aqueous solution of calcium hydroxide, which releases hydrogen forming gas bubbles in the viscous sludge and then, by solidification, a porous structure typical for the glass foam. The manufactured foam is adequate for using as a thermal insulation material for inner wall of buildings, having the apparent density of 0.31 g·cm-3, the thermal conductivity of 0.070 W/m·K and the compressive strength of 1.32 MPa. The process originality is the use of recycled aluminum waste, melted by an own microwave heating technique and sprayed with nitrogen jets. The process effectiveness is remarkable in economical and energy terms.
 
 
 

Extraction, Characterization, and Applications of Pectins from Plant By-Products

Currently, pectins are widely used in the cosmetic, pharmaceutical, and food industries, mainly as texturizing, emulsifying, stabilizing, and gelling agents. Pectins are polysaccharides composed of a large linear segment of α-(1,4) linked d-galactopyranosyluronic acids interrupted by β-(1,2)-linked l-rhamnoses and ramified by short chains composed of neutral hexoses and pentoses. The characteristics and applications of pectins are strongly influenced by their structures depending on plant species and tissues but also extraction methods. The aim of this review is therefore to highlight the structures of pectins and the various methods used to extract them, including conventional ones but also microwave heating, ultrasonic treatment, and dielectric barrier discharge techniques, assessing physico-chemical parameters which have significant effects on pectin characteristics and applications as techno-functional and bioactive agents.