微波辐射下CaSO4生成反应过程中的热点特性

在微波辐射化学反应过程中,常常会因为存在热点而导致爆炸或反应物烧毁等现象发生。本文以微波辐射下的硫酸钙生成反应为例,利用有限元方法求解Maxwell方程组、流体力学方程以及热传导方程,研究了微波辐射反应过程中热点的变化情况,并用实验验证了计算结果的正确性。研究结果表明：在加热过程中,随反应的进行,热点的位置是在不断变化的,且热点常出现在物质分相界面附近。     

Prediction of hot spots in protein interfaces using a random forest model with hybrid features

Prediction of hot spots in protein interfaces provides crucial information for the research on protein-protein interaction and drug design. Existing machine learning methods generally judge whether a given residue is likely to be a hot spot by extracting features only from the target residue. However, hot spots usually form a small cluster of residues which are tightly packed together at the center of protein interface. With this in mind, we present a novel method to extract hybrid features which incorporate a wide range of information of the target residue and its spatially neighboring residues, i.e. the nearest contact residue in the other face (mirror-contact residue) and the nearest contact residue in the same face (intra-contact residue). We provide a novel random forest (RF) model to effectively integrate these hybrid features for predicting hot spots in protein interfaces. Our method can achieve accuracy (ACC) of 82.4% and Matthew's correlation coefficient (MCC) of 0.482 in Alanine Scanning Energetics Database, and ACC of 77.6% and MCC of 0.429 in Binding Interface Database. In a comparison study, performance of our RF model exceeds other existing methods, such as Robetta, FOLDEF, KFC, KFC2, MINERVA and HotPoint. Of our hybrid features, three physicochemical features of target residues (mass, polarizability and isoelectric point), the relative side-chain accessible surface area and the average depth index of mirror-contact residues are found to be the main discriminative features in hot spots prediction. We also confirm that hot spots tend to form large contact surface areas between two interacting proteins. Source data and code are available at: http://www.aporc.org/doc/wiki/HotSpot.     

微波辐射下CaSO4生成反应过程中的热点特性

在微波辐射化学反应过程中，常常会因为存在热点而导致爆炸或反应物烧毁等现象发生。本文以微波辐射下的硫酸钙生成反应为例，利用有限元方法求解Maxwell方程组、流体力学方程以及热传导方程，研究了微波辐射反应过程中热点的变化情况，并用实验验证了计算结果的正确性。研究结果表明：在加热过程中，随反应的进行，热点的位置是在不断变化的，且热点常出现在物质分相界面附近。     

Local structural instability and formation of hot spots in materials under mechanical loading

We have studied the formation of hot spot in materials under high-speed loading using the method of molecular dynamics. It is shown that the appearance of hot spots is associated with release of the elastic energy accumulated in the deficient region. The evolution of a hot spot is followed by intense energy release and by structural rearrangements near its location. The results obtained are of importance for understanding of the effects of mechanical activation of the components in solid-phase chemical reactions. Institute of Strength Physics and Material Science, Siberian Division, Russian Academy of Sciences, Tomsk 634048. Translated from Fizika Goreniya i Vzryva, Vol. 33, No. 2, pp. 143–146, March–April, 1997.     

Experimental and theoretical study of microwave enhanced catalytic hydrodesulfurization of thiophene in a continuous-flow reactor

Hydrodesulfurization (HDS) of thiophene, as a gasoline model oil, over an industrial Ni-Mo/Al₂O₃ catalyst was investigated in a continuous system under microwave irradiation. The HDS efficiency was much higher (5%–14%) under microwave irradiation than conventional heating. It was proved that the reaction was enhanced by both microwave thermal and non-thermal effects. Microwave selective heating caused hot spots inside the catalyst, thus improved the reaction rate. From the analysis of the non-thermal effect, the molecular collisions were significantly increased under microwave irradiation. However, instead of being reduced, the apparent activation energy increased. This may be due to the microwave treatment hindering the adsorption though upright S-bind (η¹) and enhancing the parallel adsorption (η⁵), both adsorptions were considered to favor to the direct desulfurization route and the hydrogenation route respectively. Therefore, the HDS process was considered to proceed along the hydrogenation route under microwave irradiation.     

Carbon Dioxide Reforming of Methane with Pt Catalysts Using Microwave Dielectric Heating

Microwave heating was applied to the catalytic reforming reaction of methane with carbon dioxide over platinum catalysts. It was found that CO₂ and CH₄ conversions and the product selectivity (H₂/CO) were generally higher under microwave conditions than that obtained with conventional heating at the same measured temperature. The effect of microwave heating was attributed to the formation of hot spots with higher temperature than that measured in the bulk catalyst bed.     

Analysis of catalytic reaction systems under microwaves to save energy

Effects of microwaves on catalytic reaction systems are analyzed theoretically in this work. Use of microwaves is encouraged to save energy. The effects of microwave heating are analyzed theoretically by assuming that the catalyst pellet is homogeneous. The temperature and concentration profiles within the catalyst pellet were obtained by numerical simulations for the cases of microwave heating and conventional heating. In the modeling the catalyst pellet is regarded as a continuum. When a chemical reaction was conducted in a heterogeneous medium with microwave heating, the reaction rate and the yield were found to be increased compared to conventional heating under the same reaction conditions. This is due to hot spots generated by selective heating of the catalyst pellet, resulting in an increased reaction rate.     

Drying Enhancement of Clay Slab by Microwave Heating

The effect of internal heating by microwave on the drying behavior of a slab was studied. A wet sample of kaolin pressed into a slab was subjected in microwave irradiation of 2.45 GHz. The absorption of microwave energy into a wet slab can be expressed by a function of the moisture content and the pathway length, which is a similar form to Lambert-Beer's law. The drying behavior was compared among three modes: microwave irradiation, hot air heating and radiation heating in an oven. Microwave heating with a constant power resulted in breaking the sample when the internal temperature achieves at 373 K. However, if the power was controlled to maintain the temperature less than the boiling point of water, the drying succeeded without any crack generation until the completion with a significantly faster drying rate than in convective heating or in the oven. It is also noted that the transient behavior of the temperature is quite different from the conventional drying.     

Rapid decomposition of a cationic azo‐initiator under microwave irradiation

Microwave‐assisted decomposition of 2,2′‐azobis(2‐methylpropionamidine) dihydrochloride (or V50), a cationic azo‐initiator, and emulsion polymerizations were performed and compared to reactions with conventional heating. Higher decomposition rate constants were found in microwave reactions. This was ascribed to the development of hot spots inside the reaction mixture, promoted by microwave irradiation. In polymerization reactions, microwave‐promoted acceleration of the reactions. The effect of high power irradiation was also studied, using a pulsed method in which samples were repeatedly heated at constant power (500 or 1400 W), and cooled down by immersing in an ice bath. Rapid reactions were obtained, because a great amount of energy could be applied within short time intervals. The power level had no effect on the decomposed percentages and on the monomer conversion, and only affected the irradiation time. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Ignition of a system of hot spots in the presence of heat transfer through the side surface

An asymptotic analysis is used to study the ignition of a reactive cylindrical body in which, initially, there is a periodic system of hot spots. The effects of the heat transfer through the side surface of the cylinder and the adjacency of hot spots on the ignition regimes are studied. The solution shows that the critical ignition conditions depend considerably on the heat-transfer regime. The adjacency of the hot spots leads to finite changes in the parameters only if they are sufficiently close to each other. The results obtained for a heat-insulated side surface are compared with known solutions. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 2, pp. 17–21, March–April, 2000.     

Stationary and traveling hot spots in the catalytic combustion of hydrogen in monoliths

In the course of catalytic combustion of hydrogen (1-5% H₂ in air) in monolith reactors, strongly localized stationary and traveling hot spots arise in response to a sudden and persistent rise of gas flow velocity. Such hot spots may occur, e.g. in a catalytic converter following the acceleration of a car or in a catalytic combustor as a result of a load increase. This phenomenon is illustrated by simulations using a two-phase reactor model. The temperature overshoot of the adiabatic limit is typically of the order of the adiabatic temperature rise itself.The following mechanism underlies this behavior. Light fuel is supplied to the catalytic wall by fast diffusion (in the direction perpendicular to flow), while the heat released by reaction is removed from the wall by the slower, mixture-averaged heat conduction. This leads to accumulation of heat at the catalytic surface that eventually saturates at high temperatures. The hot spots may exhibit intricate dynamics, propagating downstream or upstream, or they may remain stationary. The direction of propagation depends on the relative strength of convective downstream and conductive upstream contributions to the overall displacement of reaction fronts. Generally, the hot spot tends to drift downstream at low flow velocities, remain stationary at intermediate flow velocities, and drift upstream at high flow velocities.     

Microwave effect on kinetics of paper cups pyrolysis

Not only are microwaves notorious in food heating, but they exhibit interesting properties in different domains including chemical engineering. Their ability to concentrate heat transfer inside dielectric materials enhances process efficiency and permits high heating rates. Nonetheless, their effect on reactions is still controversial. While some researchers believe in non-thermal effects due to the efficient conversion of microwave energy to enhance reactions without heat dissipation, others assert that microwave frequencies are not high enough to excite molecular bonds. In this study, paper cups pyrolysis is achieved in electrical and microwave TGA using two heating modes. The effect of microwaves on the kinetics of paper cups pyrolysis is shown to depend on the heating regime: at a moderate heating rate, microwave pyrolysis started at a lower temperature, while the pyrolysis in the electrical and microwave TGA have similar kinetic parameters at high heating conditions. This difference is linked with reaction mechanisms. At moderate heating conditions, cellulose decomposes first to an intermediate compound then to final products. The intermediate has a short reaction time and interacts with the microwave. Hence, hot spots at the molecular scale are generated in a short amount of time below the detectable limits of existing temperature measurements media. At a high heating rate, the decomposition of cellulose is direct and no effect is observed.     

Alkaline extraction of xylan from wood using microwave and conventional heating

This study aims to investigate the effect of microwave heating versus conventional heating for the alkaline hydrolysis of xylan from birch wood to understand the effect of the heating process on the dissolution of wood, the yield of xylan, and the degree of polymerization of the isolated xylan. The results indicate that the rate of wood dissolution is significantly higher (0.020/s) during microwave extraction than the conventional extraction (0.001/s). Wood solubilization, after an initial rapid removal of damaged fibers, is linear with time for both conventional and microwave extraction, with microwave showing a rate 20 times faster. The yield of xylan reaches a limit of about 60% for both processes but then declines slowly as thermal degradation become significant. Microwave heating provides 60% yield in 1/10th the time of the conventional process. This is found to be associated with the rapid temperature rise, and also with local “hot spots” generated during microwave treatment. The results indicated that xylan degradation was significant above 95°C. The nature of the isolated xylan was different for the two heating methods: the xylan isolated using microwave extraction for 20 min exhibits higher molecular weight (i.e., a greater degree of polymerization, about 150) than the xylan isolated using conventional extraction for the same duration (degree of polymerization, about 124) demonstrating the effectiveness of microwave heating for extraction of xylan from wood. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41330.     

Microwave power absorption profile of detergent surfactant agglomerates during microwave heating

The microwave energy absorption behavior of the linear alkylbenzene sulfonate (LAS) surfactant detergent agglomerates was studied while considering changes in the physical properties of the samples. Microwave heating was used to change the internal structure of the agglomerates to make a reduced density (fluffy) product. The absorption of energy within LAS samples indicated fluctuating trends as microwave heating progressed. This was associated with the dielectric properties of the material which are strongly dependent upon the nature (“free” or “bound”) and quantity of water present in them at any instant which changes during heating. Therefore, complete profiles of the energy absorbed by the samples have been recorded to determine their actual power absorption behavior/total energy consumption over time. The bulk density of the agglomerates decreased significantly when exposed to microwave fields. Hot air drying can be combined with microwave heating to reduce the total heating time. It has been observed that the pre-heating of the samples reduces the total heating time and microwave energy requirement. This is due to the temperature-dependent decomposition of hydrates releasing more “free” water.     

Drying and quality characteristics of tilapia fish fillets dried with hot air-microwave heating

The aim of this work was to study the effect of hot air-microwave heating on the drying and quality characteristics of fresh tilapia fish fillets. Experimental drying curves were obtained at three microwave powers (200, 400 and 600 W) after hot air drying at two air temperatures (40 and 50 °C) and a constant air velocity of 1.5 m/s. Some quality indicators such as shrinkage, rehydration and recovery properties were investigated. Results showed that an increase in microwave power resulted in a decrease in final moisture content when drying for the same period of time. The higher the hot air temperature, the higher the dehydration rate was. The shrinkage ratio and rehydration ratio increased as the microwave power and air temperature increased. However, the recovery ratio decreased as the microwave power and air temperature increased. Lower hot air temperature and microwave power are beneficial to keep the quality of tilapia fillets.     

微波有机合成及在混凝土减水剂制备中的应用研究进展

微波辐射作为反应活化的新型能源,已成为有机化学中普遍应用的先进技术。本文从过热、热点和选择性加热以及高极化场下的非热效应4方面阐述微波有机合成机理,结合国内外应用案例进行理论验证。重点从反应动力学角度分析了微波辐射对提高混凝土减水剂品质上的作用效果,总结了近几年微波辅助合成减水剂的研究进展,旨在推动微波技术在水泥混凝土外加剂领域中的实际应用。最后,提出微波辐射通过在电磁场作用下极性分子的快速振动实现,其热效应对体系起深层加热作用,非热效应改变反应动力学,但对作用的反应体系具有"选择性",应着重分析。微波辅助合成减水剂具有显著优势,可增大反应速率常数且存在非热效应,应加深研究及进一步推广。     

Energy efficiency of a scaled-up microwave-assisted transesterification for biodiesel production

We propose a scalable and energy-efficient microwave-assisted chemical reactor for biodiesel production, which is composed of a partially modified conventional 10-L stainless steel vessel and a microwave coupler to enable an optimized microwave injection of 99% power efficiency. The microwave power applied via a waveguide can be directly injected into the reaction vessel using a coupling rod clamped to a pressured microwave window, giving convenience of scale-up of the reactor volume because a conventional microwave transparent vessel like glass is not need. Microwave-assisted transesterification of triglycerides with potassium hydroxide catalyst achieved an accelerated conversion of 95% in 5 min. The precisely measured microwave energy consumption was only 87% of the calculated heat requirement for both the reactant and the vessel. Computer simulation studies indicated that the cause of the energy efficiency for microwave heating was the relatively low temperature of the vessel due to a reverse temperature gradient, in contrast to those done with conventional hot wall heating.     

Fully coupled electromagnetic‐thermal‐mechanical comparative simulation of direct vs hybrid microwave sintering of 3Y‐ZrO2

Direct and hybrid microwave sintering of 3Y‐ZrO₂ are comparatively studied at frequency of 2.45 GHz. Using the continuum theory of sintering, a fully coupled electromagnetic‐thermal‐mechanical (EMTM) finite element simulation is carried out to predict powder samples deformation during their microwave processing. Direct and hybrid heating configurations are computationally tested using advanced heat transfer simulation tools including the surface to surface thermal radiation boundary conditions and a numeric proportional‐integral‐derivative regulation (PID). The developed modeling framework shows a good agreement of the calculation results with the known experimental data on the microwave sintering of 3Y‐ZrO₂ in terms of the densification kinetics. It is shown that the direct heating configuration renders highly hot spot effects resulting in nonhomogenous densification causing processed specimen's final shape distortions. Compared with the direct heating, the hybrid heating configuration provides a reduction of the thermal inhomogeneity along with a densification homogenization. As a result of the hybrid heating, the total densification of the specimen is attained without specimen distortions. It is also shown that the reduction of the sample size has a stabilization effect on the temperature and relative density spatial distributions.     

Sintering of Partially Stabilized Zirconia by Microwave Heating Using ZnO–MnO2–Al2O3 Plates in a Domestic Microwave Oven

Partially stabilized zirconia (PSZ) powders were fully densified by microwave heating using a domestic microwave oven. Pressed powder compacts of PSZ were sandwiched between two ZnO–MnO₂–Al₂O₃ ceramic plates and put into the microwave oven. In the first step, PSZ green pellets were heated by self-heating of ZnO–MnO₂–Al₂O₃ ceramics (1000°C). In the second step, the heated PSZ pellets absorbed microwave energy and self-heated up to a higher temperature (1250°C), leading to densification. The density of PSZ obtained by heating in the microwave oven for 16 min was 5.7 g/cm³, which was approximately equal to the density of bodies sintered at 1300°C for 4 h or 1400°C for 16 min by the conventional method. The average grain size of the sample obtained by this method was larger than the average grain size of samples sintered by the conventional method with a similar heating process.     

Extraction of Bioactive Compounds from Leaves of Lawsonia inermis by Green Pressurized Fluids

Supercritical carbon dioxide and microwave-assisted hydrothermal methods were investigated for extraction of bioactive components of leaves of Lawsonia inermis (henna), a world renowned source of natural hair dye component called lawsone (2-hydroxy-1,4-naphthoquinone). Results showed that the use of supercritical carbon dioxide was selective to extraction of non-polar compounds such as essential oils, and the recovery of lawsone was low. However, using hot and compressed water as a solvent, a maximum recovery of lawsone of about 76.2% could be obtained at 120°C and extraction time of 30 min. At higher temperatures and longer treatment time, the yield and recovery decreased due most likely to the degradation of the target compounds at severe conditions. Moreover, the highest composition of lawsone close to 24% was obtained at microwave power of 600 W in irradiation time of 1 min. Mild hydrothermal conditions assisted with microwave heating are recommended in order to obtain high recovery of lawsone. Direct microwave heating of the polar components and solvent could lead to rapid extraction of the target compounds.