Microwave-Assisted Chemistry: a Closer Look at Heating Efficiency

Traditional microwave heating requires the heated object to be placed in a metal cavity, which limits its flexibility. Moreover, the microwave heating efficiency is affected by the shape, size, and permittivity of the load. This paper proposes a direct-insertion microwave heating device based on a dielectric wedge. It offers flexible and efficient heating for different loads. Firstly, a dielectric wedge with a gradient refractive index was designed, which can convert electromagnetic waves into surface waves propagating along the dielectric. Secondly, efficient heating of materials with permittivity ranging from 10 to 80, regardless of their shape, is achieved by inserting the dielectric wedge into the load. Microwave heating efficiencies above 90 % were consistently observed in heating tests with beverages, mashed potatoes, and minced fish of varying shapes. Finally, A high-power continuous flow heating scheme was introduced, utilizing the device as a heating module and scaling up by adding more modules to boost microwave power. Multi-physics simulations confirmed the device's efficiency and versatility in microwave continuous flow heating, with minimal energy interference between microwave sources.

The contamination of soils by mineral oils presents a significant environmental challenge, particularly due to the widespread use of petroleum products in various industrial sectors. This study investigates the efficiency of microwave heating compared to conventional resistance heating for the thermal desorption of mineral oil from contaminated soils. The experimental results demonstrated that microwave heating offers superior performance in terms of pollutant removal efficiency, energy consumption, and the preservation of soil’s physical and chemical properties. This study further conducted a kinetic analysis of the desorption process, revealing that microwave heating follows a first-order kinetic model and requires lower activation energy than conventional methods. The findings suggest that microwave-assisted thermal desorption is a highly effective and energy-efficient technology for soil remediation, providing a potential alternative to conventional techniques. This research offers valuable insights into optimizing thermal desorption processes for environmental remediation, with implications for broader applications in soil treatment.

Utilizing microwave irradiation, a fundamentally different method of the energy transfer, to the chemical process units can potentially be advantageous compared to the conventional heating, inter alia due to the selective nature of interaction of the microwaves with the matter. This doctoral dissertation addresses some of the aspects associated with the use of the microwave technology in a polyesterification plant. In this context, application of the microwave irradiation to a model polyestrerification reaction, using different types of the microwave applicators was studied. Influence of the microwave irradiation as an alternative form of heat for two selected separation techniques, which can potentially be used in a downstream recovery section in the polyesterification process (for the water purification and recovery unreacted dihydroxy alcohols), namely pervaporation and adsorption-desorption processes, were studied. The microwave-activated processes were compared with the conventionally heated ones focusing on possible reduction in the production time and energy consumption and increase in product quality. Study related to heating of the individual reagents of the polyesterification process in a multimode microwave oven showed that the heating time is several times shorter than with the conventional heating at the expense of a higher electric energy consumption. The results obtained for a model reaction system of adipic acid and neopentyl glycol under the conventional and microwave heating using the multimode microwave cavity did not show significant differences in terms of conversion and the end-product properties. Optimized (lower) usage of the microwave power during the process and effective scale up possibility compared to the multimode cavity were presented by applying the Internal Transmission Line (INTLI) microwave reactor to the studied reactor system (polyesterification reaction of maleic anhydride, phthalic anhydride and 1,2-propylene glycol). The INTLI allows for irradiation of the liquid phase from the inside of the reactor, thereby enabling better coupling of the microwave energy with the liquid mixture. For the selected process conditions, the total energy consumption of the INTLI reactor was up to a factor two lower. Comparison of desorption kinetics and desorption efficiencies for a model polar and non-polar molecules from 13X molecular sieves for two techniques, the microwave swing regeneration (MSR) and the temperature swing regeneration (TSR) showed that microwaves can help to overcome the heat transfer limitations by direct heating of the adsorbent. Significantly faster desorption processes were enabled under the microwave heating with the enhancement being more pronounced in the event of the polar compound. The MSR process runs faster even when the adsorbent temperature is quite lower than the gas temperature in TSR. It was verified that the microwaves do not affect the adsorption capacity of the molecular sieves after several consecutive adsorption-desorption cycles. Membrane pervaporation for dewatering of water/ethanol mixtures, using a hydrophilic membrane, were conducted under the microwave and conventional heating in a multimode microwave oven and a convection oven, respectively. Observations were made that at the conditions with higher water content in the feed, the water flux through the membrane was higher under the conventional heating. On contrary, with lower water in the feed, the opposite trend was found; the water flux through the membrane was higher under the microwave heating. Enhancement in the permeate flux under microwaves, compared to conventional heating, at higher ethanol concentrations in the feed can be explained on the basis of stronger coupling of microwaves with ethanol than water. Contrary to water, the dielectric loss of ethanol increases with increasing temperature; therefore, microwave dissipation is preponderant in the hot areas and can easily lead to local heating and the spatial temperature gradients.

Comparison of glycation in conventionally and microwave-heated ovalbumin by high resolution mass spectrometry

Microwave heating mechanism and Self-healing performance of scrap tire pyrolysis carbon black modified bitumen

This paper proposed a method based on transformation optics, implicit function, and level set methods to solve the challenge of multi-physics simulation of a microwave heating cavity with two different motion modes. A 3D computation model with a rotating turntable, a lifting support rod, and a sample is proposed as a detailed demonstration. Based on the theory of transformation optics, the rotating turntable is surrounded by two circles with a time-varying, inhomogeneous and anisotropy medium, and the electric field in the moving region is rotated by controlling the two mediums. The implicit function and level set methods compute the lifting motion by setting the properties of the lifting region as a function of space and time. The correctness of the proposed method is verified by comparing the proposed method’s results with the discrete position’s results, and then its accuracy is further verified by experiment. Subsequently, compared with the implicit function and level set methods only, the proposed method is more accurate. Finally, the effects of lifting motion, rotating motion and lifting motion (i.e., spiral motion) on microwave heating uniformity and heating efficiency were analyzed, respectively.

The study was conducted during January to June, 2019 under college of Agricultural Engineering, Department of Post Harvest Process and Food Engineering, JNKVV, Jabalpur, M.P. (482002), India. The main objective of the research was to determine how paneer cube size influenced drying time and to study the effect of the combination of microwave heating tempering time and hot air drying on drying rate of paneer cube. A paneer sample was prepared using a National Dairy Development Board-developed industrial paneer manufacturing procedure (NDDB). Response surface methodology was implemented in this study to optimize microwave heating and ambient tempering for accelerated drying of milk paneer. Optimization factors included paneer cube size (5–25 mm), microwave power level (150–750W), heating time (2–10 min), and tempering time (1–5 min), while responses included moisture content (dry basis %) and bulk density (g cc-1). Optimum parameter for highly acceptable product with (13.8 mm) cube size at the microwave power level (670.27 W), heating time (7.2 min), tempering time (4.621 min) presented the Moisture content (22.15%) and bulk density (0.36 g cc-1) by using Design expert 13. The experimental values under the microwave drying condition matched the outcome predicted by analysis of variance. It shows that the model is highly fit and that response surface methodology (RSM) was successful in maximizing the microwave power level, which has a significant and advantageous impact on drying rate.

Enhanced oxidative desulfurization of liquid model fuel under microwave irradiation over W2N@C catalyst nanoarchitectonics

Over the last few decades, the microwave heating process has experienced more common and routine use for both home and industrial applications. In industrial field, microwave heating has been used for many applications, including food processing and preservation, bleaching, pasteurization, and sterilization (Decareau, 1985; Farag et al., 2001; Knutson et al., 1987). Numerous advantages boosted the use of microwave heating making it in many cases a technique preferred to conventional heating. These advantages include precise timing, rapidity, and energy saving. The principle of microwave heating is based on the interaction of electromagnetic waves with the molecular constituents of food. Such interaction leads to heat generation in the entire volume at nearly the same rate due to internal thermal dissipation of the vibrational energy of the molecules in the food (Decareau, 1985; Kamel & Stauffer, 1993). On the contrary, conventional heating generates heat at the contact surface first, and then the heat diffuses inward. The effects of microwave heating and conventional heating on the food components are therefore expected to be completely different. Since processed foods by microwaves are heated as a result of molecular excitation (Stein, 1972), many researchers have been concerned with the evaluation of the effect of microwaves on food constituents, nutrient retention and the change of flavours and colours of heated food (Finot, 1995; Mudgett, 1982). Microwave heating of roasted seeds and beans shows a better retention of flavour and antioxidant compounds without any significant chemical changes of the lipids (Behera et al., 2004; H. Yoshida & Kajimoto, 1989, 1994). With respect to lipid components, microwave heating was studied to verify eventual heat induced effects on different oils and fats (Farag, 1994; Hiromi Yoshida et al., 1990; H. Yoshida et al., 1992). For this purpose, peroxide value, carbonyl value and conjugated diene and triene levels were assessed. Extra virgin olive oil that comes from the first pressing of the olive, without using heat or chemicals, contains natural antioxidants such as tocopherols, carotenoids, sterols, and phenolic compounds (Boskou, 1996). It should be mentioned that carotenoids play a significant role as antioxidants by scavenging free radicals, and as singlet oxygen quenchers (Burton & Ingold, 1984; Di Mascio et al., 1989). Since oil and fat have low specific heat constants and heat quickly (Jowitt, 1983), nowadays microwave frying of food has been

Influence of microwave irradiation on a polyesterification reaction

For the desorption of methane, thermal stimulation is an alternative to develop coalbed methane (CBM) when it is difficult to reduce the formation pressure. Microwave heating is a promising thermal method to increase the gas recovery of CBM especially for the CBM reservoir with high heterogeneity and low water content. The current study aims to establish a fully coupled numerical model to understand the enhanced gas recovery (EGR) mechanism of CBM under microwave heating. In the study, the CBM reservoir model was developed first. Then a mathematical model considering electromagnetic excitation, heat transfer, mass transfer, Langmuir adsorption, and fluid flow was built. Moreover, some important parameters were set as temperature-dependent to achieve the coupling effects among the multiphysics. Based on the above two models, a fully coupled electromagnetic-thermal-hydraulic-mechanical model was solved by the finite element, so that the distributions of electric field, reservoir temperature, methane concentration were able to be investigated. Finally, a sensitivity analysis including water content, microwave power and microwave heating mode was done based on the heating efficiency and EGR. Under microwave heating, the electric field distributes near the microwave heater with the maximum electric intensity as 1.07×103 V/m. The high electric intensity and low thermal conductivity easily enables microwaves to generate the required temperature region within CBM reservoir, so that 200 W power was applied to continuous heat the formation. Under 1 day, the maximum temperature of CBM reservoir increased to 81 °C, enabling the desorption of methane. Moreover, heating efficiency is controlled by the dielectric properties as well as electric field intensity of the CBM reservoir, although the existence of water content increases the dielectric constant within the CBM reservoir. In addition, by setting the temperature-dependent properties, microwave heating shows the ability to induce the pore volume changes by generating thermal stress, so that the porosity and permeability of CBM reservoir near the heater increase from 0.15 to 0.24 and from 0.36 mD to 1.47 mD, respectively. Based on the above positive effects of microwave heating, the CBM recovery could be significantly enhanced. Finally, in order to transfer the heat deeper into the reservoir, the feasibility of stepwise microwave heating mode has been successfully proven based on the temperature distribution within the CBM reservoir. In the study, microwave has showed great potential in enhancing the CBM recovery resulting from its high heating efficiency and pore induction effect. The results presented in this paper can provide comprehensive guidance for the optimization of microwave heating parameters.

Decoupling microwave heating efficiency from food property variations: A reduced-gradient index metamaterial strategy for superior efficiency and uniformity

1.46 - Microwave and Radiofrequency Processing of Plant-Related Food Products

An efficient synthesis of various N-substituted 3-aminoindazoles using Buchwald-Hartwig C-N coupling reaction is described. Several parameters were varied, including the nature of the halogen atom and the protecting group of the starting materials, as well as the effects of the catalyst system, base, solvent, and reaction time. The efficiency of microwave versus conventional heating was also compared to test the outcome of the reaction. Thus, by applying this recent knowledge about metal-catalyzed aminations, an alternative for the direct synthesis of primary 3-aminoindazoles has been provided.

To study the impact of heating on Jasmine brown rice using microwaves compared to that using conventional method. Morphological, physicochemical properties of uncooked and cooked brown rice samples in native (BR), germinated (GBR) and germinated‐parboiled (PGBR) forms were investigated. It was found that the uncooked rice products had similar macro‐composition (amylose, fat and protein). The microwaves heated the rice four times faster than the conventional way. The cooked grains had large, long, narrow, canal‐like holes on the surface and inner portions. The conventional heating filled the grains with smaller, round holes. Both heating methods created B‐ and V‐type crystals with relative crystallinity of 7–8% at 2θ = 0°–35°, an endothermic peak at 42–61°C with enthalpy (ΔH) of 3–4 J/g, and texture with low hardness and stickiness. However, microwave additionally caused higher pasting temperature and viscosity values, and lower digestibility with higher RS at the expense of RDS and SDS. In terms of the rice forms, PGBR was sensitive to changes in the endothermic peak and digestibility. BR was to changes in the crystallinity, hardness and stickiness. GBR was to pasting properties. Microwave and conventional heating are good methods to diversely modify the properties of rice in various forms. However, diversity was noticed only in some properties.Practical ApplicationsThe cooked rice properties are affected by a combination of heating mechanisms and rice pre‐treatments. In comparison to the conventional heating mechanism of electronic rice cooker (ERC), the dipole/polar rotation mechanism of microwave oven (MW) induced higher values of porousness, pasting parameters and resistant starch content. The high porousness of the cooked rice supported migration of gas and liquid through such matrices suggesting rapid rate of drying, diffusion, etc. Powder from such cooked rice with higher pasting viscosity and retrogradation suggested a use in products required high viscous gel formation and then retrograded during storage such as bakery products for sugar‐blood‐control‐needed consumers. The digestibility results suggested that PGBR cooked by MW contained highest resistant starch which is the most beneficial to the specific consumer group. Hardness and stickiness of all rice types were equally acceptable as indifferently affected by both heating methods.