Microwave Heating Pretreatment Research Articles

Mechanisms and advancements in microwave-enhanced CO2 mineralization of lightweight porous concrete

Lightweight porous concrete (LPC) typically exhibits low early strength, which can be addressed by enhancing early strength through carbon dioxide (CO2) mineralization curing while achieving permanent CO2 sequestration. The efficacy of this process relies on maintaining optimal moisture levels. This study investigates the influence of moisture content on the performance of solid waste-based lightweight porous concrete blocks (RSFAC) subjected to microwave heating pretreatment. Moisture levels were adjusted to 10%, 20%, 30%, and 40% (initial water-to-cement ratio of 45.4%) before undergoing mineralization curing within an environment set at a temperature of 20 °C, relative humidity 75%, and a CO2 concentration of 40% for a duration of 0.5 h. The impact of moisture content on RSFAC's compressive strength, carbon sequestration rate, mineralization products, and microstructure was evaluated. Results indicate that compared to conventional oven pretreatment methods requiring nearly 10 h, microwave pretreatment technology achieves equivalent water content within just 50 min, significantly reducing the time needed for mineralization pretreatment. RSFAC, when pretreated with microwave heating, outperforms untreated samples (Ref-0 and Ref-1) in terms of compressive strength and carbon sequestration efficiency. Optimal performance was observed at a moisture content level of 30%, with the highest recorded compressive strength achieved at 1, 3, and 7 d respectively. At the curing age of 7 d, the compressive strength reached up to 2.32 MPa while the carbon sequestration rate reached approximately 11.61%. Furthermore, aragonite and calcite (tetrahedral and hexahedral), without poorly crystalline spherulites, were identified as the main crystalline CaCO3 generated by RSFAC under this specific moisture content condition; pores larger than 50 nm served as primary reaction sites for RSFAC mineralization curing, whereas there was a significant increase in pore proportion with sizes less than 20 nm after filling with mineralization products.

Effect of different pretreatment techniques on quality characteristics, chemical composition, antioxidant capacity and flavor of cold-pressed rapeseed oil

This study investigated the effects of different pretreatment techniques (microwave heating, infrared heating, oven heating and enzymatic methods) on the quality of cold-pressed rapeseed oil (CPRO) with commercially available first-grade pressed rapeseed oil (CFO) as a control. The results showed that CPRO is more nutritionally valuable than CFO while meeting the quality standard without refining. Microwave heating pretreatment's cold-pressed rapeseed oil (MHO) exhibited a higher oil yield (70.83 ± 0.40 %), a higher trace active ingredient content (tocopherol 53.79 ± 0.19 mg/100 g, phytosterol 726.98 ± 4.14 mg/100 g, and polyphenol 87.09 ± 0.08 mg GAE/100 g), and a stronger antioxidant capacity. Correlation analysis and multiple linear regression analysis indicated that antioxidant capacity was mainly related to the content of α-tocopherol and polyphenols. Microwave heating promoted the degradation of the Maillard reaction and glucosinolates, significantly improving the flavor of CPRO. Therefore, microwave heating is a promising pretreatment technology for the production of high-quality CPRO.

Thermal pretreatment of poplar sawdust at 100 °C in water or with microwave heating impacts the pyrolysis behaviors

Drying at 100 °C is a common practice for processing many biomasses for their further thermochemical conversion via the routes such as pyrolysis. The drying temperature of 100 °C is low, but might not be too low to cause any impact on pyrolytic behaviors of a biomass feedstock. In this study, varied scenarios of thermal pretreatment of poplar sawdust (oven-drying, boiling in water and microwave heating) at 100 °C were screened for probing their potential influence on the pyrolytic behaviors at 500 and 750 °C. Comparing with oven-drying, boiling in water (cooking) and microwave heating at 100 °C could remove some oxygen-containing species, increasing carbon content of the pretreated sawdust and promoting carbonization in pyrolysis. This enhanced formation of biochar, heavy π-conjugated organics in bio-oil, but suppressed generation of gas. The cooking or microwave heating pretreatment also promoted aromatization to form the biochar of higher thermal stability, higher crystallinity, and higher aromatic degree, due to the enhanced deoxygenation reactions in the pyrolysis. The in-situ IR characterization suggested that microwave pretreatment promoted generation of unsaturated ketones and also the species with olefinic C = C, favoring their further aromatization. The cooking pretreatment impacted transformation routes of the oxygen-containing organics, thus modifying pyrolysis behaviors of the pretreated sawdust.

Hydrolysis of Mixed Sugarcane Bagasse and Rice Husk Using Cellulase Enzyme for Reducing Sugar Production

Reducing sugar can be produced from lignocellulosic raw materials. The content of polysaccharides such as cellulose, hemicellulose, and starch will be broken down into simpler carbohydrates. This study used a mixture of sugarcane bagasse and rice husks as lignocellulosic raw materials. The lignin content in the raw material must be removed through delignification or pretreatment so that enzymes can access cellulose and hemicellulose. This study used a physics-chemical pretreatment method, in which lignocellulosic material soak in 3% NaOH then heated with microwave and boiling water. The next process is enzymatic hydrolysis with variations of cellulase enzymes activity 0.434, 0.871, 2.61, and 3.49 FPU/g mixture of bagasse and rice husks. The cellulase enzyme used in this study was also derived from the fungus Trichoderma viride. Analysis of the sugar concentration resulting from hydrolysis used the DNS method with the 3.5-dinitrosalicylic acid reagent. The concentration of sugar from hydrolysis using a variety of enzymes with microwave heating pretreatment and boiling water pretreatment obtained the highest results which were the same at the addition of enzyme activity 3.49 FPU/g substrate at 24 hours, namely 4.077 g/L and 15.18 g/L. The optimum time for enzymatic hydrolysis is 12 hours and optimum enzyme activity is the addition of enzyme activity 2.61 FPU/g. The average concentration of sugar hydrolyzed by the addition of Trichoderma viride solution in pretreatment using microwave heating was 0.7611 g/L with a yield of 21.01 mg sugar/g substrate and with pretreatment in boiling water obtained 0.8679 g/L with a yield of 23.95 mg sugar/g substrate.

Numerical Modelling of Microwave Heating Assisted Rock Fracture

This paper presents a numerical study on the effects of microwave irradiation on the mechanical properties of hard rock. More specifically, the weakening effect of microwave heating induced damage on the uniaxial compressive and tensile strength of granite-like rock is numerically evaluated. Rock fracture is modelled by means of a damage-viscoplasticity model with separate damage variables for tensile and compressive failure types. We develop a global solution strategy where the electromagnetic problem is solved first separately in COMSOL multiphysics software, and then provided into a staggered implicit solution method for the thermo-mechanical problem. The thermal and mechanical parts of the problem are considered as uncoupled due to the dominance of the microwave-induced heat source. The model performance is tested in 2D finite element simulations of heterogeneous numerical rock specimens subjected first to heating in a microwave oven and then to uniaxial compression and tension tests. According to the results, the compressive and tensile strength of rock can be significantly reduced by microwave heating pretreatment.

Ultrasound and microwave pretreatments promote methane production potential and energy conversion during anaerobic digestion of lipid and food wastes

To alleviate difficult degradation of lipids and its inhibition on anaerobic digestion of food waste, ultrasound and microwave heating were used to pretreat lipid and food wastes for promoting methane production. The soluble COD (10,130 mg/L) of lipid waste pretreated with ultrasound of 50,000 kJ/kg-total volatile solids (TVS) was higher than that (1910 mg/L) with microwave heating of the same energy input. This was because the thermal and mechanical effects of the ultrasound pretreatment were stronger than the thermal and solid dissolution effects of the microwave heating pretreatment. The methane yield of lipid waste pretreated with ultrasound during anaerobic digestion increased by 43.3%–927.97 mL/g-TVS, higher than that of 738.63 mL/g-TVS with microwave heating. Fourier transform infrared spectrometer (FTIR) and Scanning electronic microscopy (SEM) analysis demonstrated that the residual lipids were reduced and microorganism coating was relieved after anaerobic digestion of lipid waste pretreated with ultrasound. The energy conversion efficiency of lipid waste pretreated with ultrasound to methane product was 69.89%, which was higher than that of 58.98% with microwave heating. Ultrasound efficiently degraded lipid waste and released more energy to be converted into methane.

Integrating microwave heating pre-treatment to improve torrefaction of food waste

The food waste (FW) disposal is one of the main issues of concern, as it affects environmental and sustainable development in Malaysia. FW should be utilized to produce more valuable products such as energy. However, FW is characterized by its low high heating value (HHV) and high moisture content and which lower the energy efficiency during energy production process. To overcome this problem, torrefaction assisted by microwave heating pretreatment is used to upgrade FW properties to produce solid biofuel which is called as biochar. The objective of this study is to investigate the feasibility of microwave (MW) heating power at 240 W for 5 min to enhance the quality of torrefied FW. The pre-treated FW is then torrefied at three different temperatures (280, 300 and 320 °C) for 30, 45 and 60 min. The torrefied FW was then characterized in terms of ultimate and proximate analysis, HHV, mass and energy yield as well as its functional groups. The optimum condition of torrefaction in this study was 320°C and 30 min with HHV of 23.82 MJ/kg, 95.54% of energy yield and 84.28% of mass yield. This suggests that torrefied FW with MW heating pre-treatment has the potential to be used as an energy source.

Extraction of fresh banana waste juice as non-cellulosic and non-food renewable feedstock for direct lipase production

The bulk availability of banana waste in the world has made a move towards the development of alternative novel renewable sugars in present study as the pressed juice from banana frond and pseudostem was found to contain high level of renewable sugars such as glucose, sucrose and fructose. By using a simple sugarcane press, the glucose content in the banana frond juice was 16.6 g/L, which accounts for 55% of the total fermentable sugars. From the results, the lipase production reached ∼200 U/mL in medium comprising banana frond juice and banana pseudostem juice, comparable to basal medium with glucose (206.3 U/mL) as carbon source in basal media. As banana pseudostem possesses high content of starch, microwave heating pretreatment showed 36% enhancement of glucose content (10.4 g/L) in juice as compared to direct pressed juice (7.78 g/L). This result indicates that banana waste juice can be used as an alternative fermentable carbon source for lipase production and has potential as a fermentable carbon source.

Assessment of microwave-assisted alkali pretreatment for the production of sugars from banana fruit peel waste

ABSTRACTThe current investigation deals with the application of microwave energy to facilitate alkali (NaOH) pretreatment on banana peel waste (BPW) to achieve optimal reducing sugar. We studied the effect of microwave power (100 to 600 W) and irradiating time (1 to 6 min) on composition and reducing sugar content of BPW. The enzymatic hydrolysis of microwave-assisted alkali-pretreated BPW at 600 W for 2 min gave the maximum reducing sugar (0.561 g/g) of dry biomass waste. Atomic force microscopy revealed that the surface of all BPW was changed noticeably when microwave-assisted alkali pretreatment was employed. X-ray diffraction study revealed a 5-fold increment in the degree of crystallinity in optimal microwave alkali-treated samples (54.45%) as compared to the untreated sample (10.55%). Overall, the microwave-assisted alkali pretreatment removed a significant amount of hemicelluloses and lignin and led to a high amount of sugar production, suggesting microwave heating pretreatment as a potential alternative to conventional methods to achieve maximal reducing sugars.

Real-time monitoring of organic apple (var. Gala) during hot-air drying using near-infrared spectroscopy

Dried apple (Malus domestica B.) shows a growing trend to its worldwide consumption as raw material used to produce organic snacks, integral breakfast foods, chips, etc. Apple is often dried by conventional methods (e.g. hot-air drying, freeze-drying, etc.), which are usually uncontrolled and then prone to product quality deterioration. Thus, to allow the production of high-value end products, there is a need for developing new drying systems. In this study, it was investigated the feasibility of an implementation of NIR spectroscopy in smart drying systems to non-destructively detect and monitor physicochemical changes in organic apples wedges during 8-h hot-air drying at 60 °C. Moreover, the impact of microwave heating pre-treatment (at 850 W for 45 s) as enzyme inactivators on model performances was also evaluated. Partial least squares (PLS) regression models were successfully developed to monitor changes in water activity (R2 = 0.97–0.98), moisture content (R2 = 0.97–0.98), SSC (R2 = 0.96–0.97) and chroma (R2 = 0.77–0.86) during drying. Classification analysis was performed for the development of discriminant models able to recognise dehydration phases of apple wedges on the basis of their spectral profile. The classification models were computed using K-means and Partial Least Squares Discriminant Analysis (PLS-DA) algorithms in sequence. The performance of each PLS-DA model was defined based on its accuracy, sensitivity and specificity rates. All of the selected models provided a very-good (>0.90) or excellent (>0.95) sensitivity and specificity rates for the predefined drying phases. Feature selection procedures allowed to obtain both regression and classification models with performances very similar to models computed from the full spectrum. Results suggest that effect of microwave heating on both water loss and microstructure of apple tissue was pronounced, mainly affecting the features selection procedure in terms of number of features and selected wavelengths.

Efficient sugar production from sugarcane bagasse by microwave assisted acid and alkali pretreatment

Sugarcane bagasse represents one of the best potential feedstocks for the production of second generation bioethanol. The most efficient method to produce fermentable sugars is by enzymatic hydrolysis, assisted by thermochemical pretreatments. Previous research was focused on conventional heating pretreatment and the pretreated biomass residue characteristics. In this work, microwave energy is applied to facilitate sodium hydroxide (NaOH) and sulphuric acid (H2SO4) pretreatments on sugarcane bagasse and the efficiency of sugar production was evaluated on the soluble sugars released during pretreatment. The results show that microwave assisted pretreatment was more efficient than conventional heating pretreatment and it gave rise to 4 times higher reducing sugar release by using 5.7 times less pretreatment time. It is highlighted that enrichment of xylose and glucose can be tuned by changing pretreatment media (NaOH/H2SO4) and holding time. SEM study shows significant delignification effect of NaOH pretreatment, suggesting a possible improved enzymatic hydrolysis process. However, severe acid conditions should be avoided (long holding time or high acid concentration) under microwave heating conditions. It led to biomass carbonization, reducing sugar production and forming ‘humins’. Overall, in comparison with conventional pretreatment, microwave assisted pretreatment removed significant amount of hemicellulose and lignin and led to high amount of sugar production during pretreatment process, suggesting microwave heating pretreatment is an effective and efficient pretreatment method.

Assessment of a microwave-assisted recycling process for the recovery of high-quality aggregates from concrete waste

This study presents an innovative method for concrete waste up-cycling based on concrete weakening through microwave heating before impact crushing. Two series of tests were conducted in order to assess the influence of the aggregate properties (size distribution, mineralogical nature) and the influence of the operating conditions of the microwave heating pretreatment on concrete fragmentation; and thus to evaluate the feasibility and the robustness of this process. Experiments were carried out on lab-made, cylindrical concrete specimens and on no-slump concrete waste with a multimode cavity microwave equipment (2.45GHz, 6kW) and an impact crusher. Results showed that microwave heating always induced an embrittlement of concrete samples which resulted in lower fracture energy, higher fragmentation of samples and higher liberation of aggregates (i.e. free of cement paste). A microwave-assisted comminution process is therefore an effective recycling technique for the recovery of high-quality aggregates from concrete waste.

Synthesis and characterization of sol-gel derived TiO2 thin films: Effect of different pretreatment process

Titanium dioxide (TiO2) thin films have been successfully synthesized deposited on glass substrates by the sol-gel dip-coating method through different pretreating processes, including heated at 100, 500°C, via freeze drying, microwave heating for 10 min and subsequently annealed at 500°C for 2 h. The as-synthesized TiO2 films were characterized using X-ray diffraction (XRD), atomic force microscopy (AFM) and ultravioletvisible (UV-vis) absorption spectra analysis technology. The preparation of the precursor sols and TiO2 films were described in detail. Effects of 100, 500°C, freeze drying and microwave heating pretreatment on crystalline structure, surface morphology, roughness, particle size, optical property and electronic transition of TiO2 thin films have been primarily investigated. The XRD results demonstrate that the TiO2 films were well-crystallized and consisted of anatase TiO2 phase only with (101) plane. The average crystalline size is only about 15 nm at 100°C pretreatment and the absorption edge shifts to shorter wavelength comparing with that at 500°C, freeze drying and microwave heating pretreatment. Pretreatment process is important during the preparation of thin films and has obviously effect on the structure and optical property of TiO2 films due to the different heating mechanisms.

An investigation on pretreatments for inactivation of lipase in naked oat kernels using microwave heating

Abstract To investigate the primary factors affecting the efficiency of microwave heating pretreatments in inactivating lipase in naked oat kernels, the effects of temperature, duration of heating, moisture content of oat kernels, tempering time and packaging conditions on the inactivation of lipase activities were examined. Microwave heat treatment for 45 s was found to inactivate 98–99% lipase in oat kernels (20 g in polyethylene package) with 11.1% moisture content, thereby stabilizing oat kernels against triglyceride lipolysis. The steam quantity played a crucial role in lipase inactivation. A trend was found: the higher the quantity of steam evaporated, the lower the residual lipase activities remained in whole kernels. Pretreatments, such as vacuum packaging, increasing the moisture content of oat kernels to a range of 17.0–25.0% from 11.1%, and conducting heat treatments as soon as the kernel moisture contents were adjusted, were found to be beneficial in accelerating steam evaporation; the efficiency of microwave heating on lipase inactivation was thereby increased.

Immunohistochemical Identification of T- and B- Lymphocytes in Formalin-Fixed, Paraffin-Embedded Rat Sections.

We investigated the possibility of immunohistochemically identifying T- and B-lymphocytes in formalin-fixed, paraffin-embedded rat tissues as a basis for future pathological study. Spleens were removed from 8-week-old male Wister rats, and were fixed in 20% neutral-buffered formalin for 7, 30 or 90 days. After fixation, the specimens were embedded in ordinary paraffin by routine method. Paraffin sections were prepared and stained immunohistochemically using anti-CD3 (clone: G4.18) and -CD45RA (clone: OX-33) antibodies. Anti-CD3 and -CD45RA antibodies failed to stain without microwave heating pretreatment. The immunoreactivity of CD3 and CD45RA were both increased by microwave heating pretreatment. Consistent and specific immunostaining of CD3 was obtained throughout all formalin-fixation periods after 40 min or more of microwave heating pretreatment. CD45RA was clearly detected in the 7-day fixed tissues after 30 or 40 min of pretreatment. In the 30- and 90-day fixed tissues, identification of CD45RA was difficult at all lengths of microwave heating pretreatment. These findings suggest that CD3 and CD45RA are detectable in routine formalin-fixed, paraffin-embedded rat tissues although the immunoreactivity is influenced by formalin-fixation period, and that the present procedure, combining appropriate antibodies and pretreatment of microwave heating, is useful for retrospective analyses of rat lymphoid subtypes.