Energy absorption performances of silicon carbide particles during microwave heating process

Highlights

• Energy absorption performances of microwave heating of SiC particles were analyzed.

• Effects of SiC loading, microwave power, and reactor volume were investigated.

• A maximum energy absorption efficiency of 37.38% was observed.

• The best performance was at 30 g SiC, 350 w power, and 50 mL volume.

Abstract

In this study, energy absorption performances of SiC particles during microwave heating process were analyzed, and effects of SiC loading (20, 25, 30, 35, and 40 g), microwave power (300, 350, 400, 450, and 500 W) and reactor volume (50, 100, 150, 200, and 250 mL) were investigated. The results show that when the SiC is heated to 200 °C and loading increases from 20 g to 40 g, average heating rate decreases and then increases in the range of 0.36 ∼ 0.86 °C/s whereas energy absorption efficiency increases and then decreases in the range of 1.49% ∼ 15.57%. When microwave power increases from 300 W to 500 W, average heating rate increases from 0.89 °C/s to 7.08 °C/s whereas energy absorption efficiency increases and then decreases in the range of 4.71% ∼ 37.38%. When reactor volume increases from 50 mL to 250 mL, average heating rate decreases from 3.37 °C/s to 0.65 °C/s whereas energy absorption efficiency increases and then decreases in the range of 5.70% ∼ 35.38%. The maximum energy absorption efficiency of 37.38% was observed at the SiC loading of 30 g, microwave power of 350 W, and reactor volume of 50 mL.

Introduction

Microwave heating has gradually become a novel heating method favored by researchers because of its unique heating characteristics (i.e., high heating rate, pure volume heating, etc.) [1,2]. For example, thermochemical conversion of biomass, plastics, and the other materials using microwave as energy source has the advantages of fast reaction rate and high heating value of products for process intensification [3,4]. On the other hand, conventional materials (coal, biomass, plastics, etc.) have low microwave absorption efficiency due to their poor dielectric properties, which leads to the problem of low energy utilization efficiency in the whole microwave thermochemical conversion process [5]. Aguilera et al. [6] developed a model for the reaction kinetics and reactor system to compare the conventional and microwave heating in a quantitative way. The reactor and kinetic model can be used to predict the behavior of the complex chemical system. Leveneur et al. [7] studied the epoxidation of vegetable oil in an isothermal batch reactor system equipped with microwave or conventional heating, where the results showed that microwave irradiation can accelerate the epoxidation velocity when the continuous phase is aqueous. Aguilera et al. [8] developed a kinetic model for the entire epoxidation process and a reactor model for the three-phase system aqueous phase-oil phase-solid catalyst. When the molecules of some substances are polar molecules, one end of the molecule will be positively charged and the other end will be negatively charged. Under the irradiation of microwave electric field, the positively charged sides of polar molecules are consistent with the direction of electric field, while the negatively charged sides are opposite to the direction of electric field [9]. The direction of the electric field in the microwave field changes continuously at high frequency, which leads to molecular oscillation. When the frequency change of microwave electric field is consistent with the intrinsic frequency of molecules, the rotation of molecules will always be accelerated by the electric field, and the microwave energy will be obtained to increase the temperature of objects, which is called the phenomenon of microwave resonance [10]. Usually, microwave absorbers are needed for microwave-assisted heating, while our knowledge of relevant heating process is still far from satisfactory [11,12]. Thus, further investigations are necessary to clarify the underlined mechanisms for microwave heating, especially the issues of energy absorption.

Energy absorption during microwave heating process has the overwhelming role in determining overall energy efficiency of the studied system, where heating rate and energy absorption efficiency are the main focuses [13,14]. Jia et al. [15] explored the energy absorption characteristics of diesel particles during microwave heating, and analyzed the effects of microwave power and microwave action time on the energy utilization efficiency. Their results showed that the obtained energy utilization percentage of microwave was between 5.65% and 14.64%. Nigar et al. [16] explored microwave energy utilization of NaY zeolite in a fixed bed. Their results showed that after the zeolite was heated to 180 °C within 5 min, the energy absorption efficiency was 10.3%. Wang et al. [17] investigated the energy absorption efficiency and its influencing factors in a typical microwave heating process based on experimental measurements. Their results demonstrated that feed shape, microwave power, and furnace size had significant impacts on the energy utilization efficiencies. It was pointed out that microwave absorption efficiencies of the three studied materials (water, glycerol, and paraffin oil) were about only 0.9%. Jiang et al. [18] investigated the microwave heating performances of whey protein, and the effects of microwave power, residence time, and the other factors on the energy absorption efficiencies were explored in detail. The results show that microwave power is the key factor affecting energy utilization. Zhang et al. [19] investigated the microwave heating characteristics of asphalt pavement based on experiments, and they also studied the influences of particle size and particle density on the heating efficiencies. The results showed that proper particle size of 10 mm can effectively improve the microwave absorption efficiency. Zhu et al. [20] developed a model to numerically explore the effect of microwave heating on the heating and drying characteristics of oil shale, and the effect of intermittent microwave heating on energy consumption in the water evaporation process was studied. The results showed that longer intermittent time led to more water evaporation and improved the energy utilization efficiency. Tsai et al. [21] compared microwave heating and traditional heating on the drying process of white shrimp. The results showed that microwave heating had higher energy absorption efficiencies and better chemical qualities than traditional heating. Martysiak - Zurowska et al. [22] compared microwave heating and traditional heating on the heating efficiency of breast milk. The results revealed that the breast milk heated by microwave had higher biological activity and the process had less energy loss. Hou et al. [23] investigated the characteristics and quality of flaxseed under microwave heating. The results showed that the qualities of flaxseed obtained at 8 min microwave heating and 30 min traditional heating were quite similar, and microwave heating had higher heating efficiency. It is clearly seen that microwave is beneficial to the heating processes and results, and the energy absorption performances are highly affected by key factors such as microwave absorber, microwave power, heating time, etc.

As an important part of microwave heating system, microwave heating cavity mainly has two types: monomode and multi-mode. Monomode cavity has high quality factor and large energy storage, but its volume limitation is too large to meet the needs of industrial market. At present, most microwave heating cavities used in the industrial field are multi-mode cavities, and multiple electric field modes coexist and overlap each other [24]. They have the advantages of huge power capacity, uniform electric field distribution, strong material handling capacity, etc. There are many factors that determine the heating characteristics of multimode cavity, mainly including microwave frequency, structure size of multimode cavity, number and distribution of microwave feeders, load characteristics and location, etc. The microwave oven used in this paper was a multi-mode microwave oven. Cherbański et al. [25] investigated the desorption kinetics during microwave swing regeneration (MSR) and temperature swing regeneration processes (TSR), and their results showed that MSR runs faster even when the adsorbent temperature is much lower than the gas temperature in TSR. Cherbanski et al. [26] investigated the microwave induced natural convection in water through both experimental and simulation methods. The results showed that the heating rate in the first stage is higher than that in the second stage. Gangurde et al. [27] pointed out the challenges in noncontact high temperature measurements during the microwave-assisted catalytic reaction process. Gangurde et al. [28] synthesized the ruthenium-doped strontium titanate perovskite catalysts through microwave assisted hydrothermal method, and the results showed that the heating time for 220 °C decreased from 24 h to 1h. Sturm et al. [29] investigated the parametric sensitivity of heat generation by resonant microwave fields, where the high sensitivity of the overall heating rate to geometrical and medium parametric variations was addressed. Sturm et al. [30] proposed the design principles of microwave applicators for small-scale process equipment, and they also pointed out that load size, heating uniformity and desired frequency mutually constrain one another.

Among the various absorbers used in microwave heating, silicon carbide (SiC) is one of the most promising candidates, and the intensification performances have been extensively studied. Tamang et al. [31] numerically simulated the energy absorption characteristics of SiC under microwave irradiation, and the influences of SiC particle size, microwave heating power, and the other parameters on the heating characteristics were investigated. The results showed that the temperature of SiC increased with the increases in microwave power and SiC loading. Aïssa et al. [32] investigated the electromagnetic energy absorption potential and microwave heating capacity of SiC thin films in the 1–16 GHz frequency range. The results showed that the SiC thin films reached 2000 K within 100 s, while the energy absorption efficiency was less than 20%. Jiang et al. [33] investigated the co-pyrolysis characteristics of biomass and soap stock in a downdraft gasifier using SiC as microwave-assisted absorber, and the effects of SiC loading on the energy conversion characteristics were detailed. The results showed that SiC-assisted microwave heating was very complex. As a precursor to improve the energy conversion characteristics and heating performances of SiC-assisted microwave heating, it is highly necessary to study the energy absorption characteristics of pure SiC particles during microwave heating. However, to the best of our knowledge, there is no report on the energy absorption characteristics of SiC particles during microwave heating process.

The main objective of this work study was to close this gap through demonstrating the energy absorption characteristics of SiC particles during microwave heating process. In the followed sections, the physical properties of SiC particles were described, and then the heating performances of SiC particles during microwave heating process were detailed. The effects of SiC loading, microwave power, and reactor volume on the energy absorption performances were also investigated.