Baharak Sajjadi

Influence of ultrasound power on acoustic streaming and micro-bubbles formations in a low frequency sono-reactor: Mathematical and 3D computational simulation

This paper aims at investigating the influence of ultrasound power amplitude on liquid behaviour in a low-frequency (24 kHz) sono-reactor. Three types of analysis were employed: (i) mechanical analysis of micro-bubbles formation and their activities/characteristics using mathematical modelling. (ii) Numerical analysis of acoustic streaming, fluid flow pattern, volume fraction of micro-bubbles and turbulence using 3D CFD simulation. (iii) Practical analysis of fluid flow pattern and acoustic streaming under ultrasound irradiation using Particle Image Velocimetry (PIV). In mathematical modelling, a lone micro bubble generated under power ultrasound irradiation was mechanistically analysed. Its characteristics were illustrated as a function of bubble radius, internal temperature and pressure (hot spot conditions) and oscillation (pulsation) velocity. The results showed that ultrasound power significantly affected the conditions of hotspots and bubbles oscillation velocity. From the CFD results, it was observed that the total volume of the micro-bubbles increased by about 4.95% with each 100 W-increase in power amplitude. Furthermore, velocity of acoustic streaming increased from 29 to 119 cm/s as power increased, which was in good agreement with the PIV analysis.

Review on Applicable breakup/coalescence models in turbulent liquid-liquid flows

Abstract Liquid-liquid flows are common in process industries, particularly in turbulent systems. These systems are usually characterized by the diameter of the dispersed phase and are governed by external forces, deformation, breakup, and coalescence processes. In this review, the common methods and equations used to predict these phenomena will be discussed. First, deformation models in both laminar and turbulent flows containing single and multi-drop are considered. Then, the breakup process and models for these mechanisms are investigated. The coalescence process and collisions that may result in coalescence are also investigated. Coalescence efficiency is another factor that will be introduced in this review. Finally, daughter droplet size distribution is investigated considering both phenomenological and statistical models.

A Comparative Fluid Flow Characterisation in a Low Frequency/High Power Sonoreactor and Mechanical Stirred Vessel

This study aims at analysing the jet-like acoustic streaming generated under low-frequency and high-power ultrasound irradiation and comparing it with fluid streaming generated by traditional mechanical mixing. The main characteristics of fluid flow, which include radial, axial and tangential terms of velocity and their effects on fluid flow pattern, pressure distribution, axial mixing time and turbulence intensity were considered at different power inputs. Both 3D CFD simulation and Particle Image Velocimetry (PIV) were used in this study. The CFD results indicated that the jet-like acoustic streaming reached the velocity magnitude of 145 cm/s at 400 W, which reduced the mixing time to 1.38 s. However, the minimum mixing time of 3.18 s corresponding to the impeller rotational speed of 800 RPM was observed for mechanical stirring. A uniform axial flow pattern was generated under ultrasound irradiation whereas the tangential flow pattern was more prominent in the stirred vessel. Besides, the highest turbulence was observed in the vicinity of the ultrasound transducer and impeller with the values of 138% and 82% for the ultrasonicator and stirred vessel, respectively. The predicted fluid flow pattern under ultrasound irradiation was in a reasonable agreement with that obtained from PIV, with a reasonable accuracy.

Mechanistic analysis of cavitation assisted transesterification on biodiesel characteristics

The influence of sonoluminescence transesterification on biodiesel physicochemical properties was investigated and the results were compared to those of traditional mechanical stirring. This study was conducted to identify the mechanistic features of ultrasonication by coupling statistical analysis of the experiments into the simulation of cavitation bubble. Different combinations of operational variables were employed for alkali-catalysis transesterification of palm oil. The experimental results showed that transesterification with ultrasound irradiation could change the biodiesel density by about 0.3kg/m(3); the viscosity by 0.12mm(2)/s; the pour point by about 1-2°C and the flash point by 5°C compared to the traditional method. Furthermore, 93.84% of yield with alcohol to oil molar ratio of 6:1 could be achieved through ultrasound assisted transesterification within only 20min. However, only 89.09% of reaction yield was obtained by traditional macro mixing/heating under the same condition. Based on the simulated oscillation velocity value, the cavitation phenomenon significantly contributed to generation of fine micro emulsion and was able to overcome mass transfer restriction. It was found that the sonoluminescence bubbles reached the temperature of 758-713K, pressure of 235.5-159.55bar, oscillation velocity of 3.5-6.5cm/s, and equilibrium radius of 17.9-13.7 times greater than its initial size under the ambient temperature of 50-64°C at the moment of collapse. This showed that the sonoluminescence bubbles were in the condition in which the decomposition phenomena were activated and the reaction rate was accelerated together with a change in the biodiesel properties.

Review on Gas-liquid Mixing Analysis in Multiscale Stirred Vessel Using CFD

Abstract This review aims to establish common approaches and equations used in computational fluid dynamics (CFD) analysis for gas-liquid mixing operations and investigate their strengths and weaknesses. The review concluded that with a sufficient computing strength, Eulerian-Lagrangian approaches can simulate detailed flow structures for dispersed multiphase flow with high spatial resolution. Turbulence is an important factor in fluid dynamics, and literature confirmed that k-ε is the most widely used turbulence model. However, it suffers from some inherent shortcomings that stemmed from the assumption of isotropy of turbulence and homogenous mixing, which is suitable for very high Reynolds number in unbaffled stirred vessels. In CFD simulations for gas-liquid systems in stirred vessels, bubble size distribution is the most important parameter; hence, different techniques for formulation of bubble size equations have been investigated. These techniques involve source and sink terms for coalescence or breakup and provide a framework in which the population balance method together with the coalescence and breakup models can be unified into three-dimensional CFD calculations. Different discretization schemes and solution algorithms were also reviewed to confirm that third-order solutions provide the least erroneous simulation results.

Investigation of mass transfer intensification under power ultrasound irradiation using 3D computational simulation: A comparative analysis

This paper aims at investigating the influence of acoustic streaming induced by low-frequency (24kHz) ultrasound irradiation on mass transfer in a two-phase system. The main objective is to discuss the possible mass transfer improvements under ultrasound irradiation. Three analyses were conducted: i) experimental analysis of mass transfer under ultrasound irradiation; ii) comparative analysis between the results of the ultrasound assisted mass transfer with that obtained from mechanically stirring; and iii) computational analysis of the systems using 3D CFD simulation. In the experimental part, the interactive effects of liquid rheological properties, ultrasound power and superficial gas velocity on mass transfer were investigated in two different sonicators. The results were then compared with that of mechanical stirring. In the computational part, the results were illustrated as a function of acoustic streaming behaviour, fluid flow pattern, gas/liquid volume fraction and turbulence in the two-phase system and finally the mass transfer coefficient was specified. It was found that additional turbulence created by ultrasound played the most important role on intensifying the mass transfer phenomena compared to that in stirred vessel. Furthermore, long residence time which depends on geometrical parameters is another key for mass transfer. The results obtained in the present study would help researchers understand the role of ultrasound as an energy source and acoustic streaming as one of the most important of ultrasound waves on intensifying gas-liquid mass transfer in a two-phase system and can be a breakthrough in the design procedure as no similar studies were found in the existing literature.

Solid-liquid mixing analysis in stirred vessels

Abstract This review evaluates computational fluid dynamic applications to analyze solid suspension quality in stirred vessels. Most researchers typically employ either Eulerian-Eulerian or Eulerian-Lagrangian approach to investigate multiphase flow in stirred vessels. With sufficient computational resources, the E-L approach simulates flow structures with higher spatial resolution for dispersed multiphase flows. Common turbulence models such as the two-equation eddy-viscosity models (k-ε), Reynolds stress model, direct numerical simulation, and large eddy simulation are described and compared for their respective limitations and advantages. Literature confirms that k-ε is the most widely used turbulence model, but it suffers from some inherent shortcomings due to assumption of isotropy of turbulence and homogenous mixing. Subsequently, the importance of different forces concerning solid particle flotation is concluded. Studies on dilute systems take into account only drag and turbulence forces while other forces have always been ignored. The simulations of off-bottom solid suspension, solid drawdown, solid cloud height, solid concentration distribution, and particle collision are considered for studies involving solid suspension. Different models and methods applied to investigate the abovementioned phenomena are also discussed in this review.

Analysis and Optimization of Ultrasound-Assisted Alkaline Palm Oil Transesterification by RSM and ANN-GA

In this study, the effects of ultrasound irradiation on transesterification process and characteristics of the synthesized biodiesel were investigated. The study was divided into two parts. In the first part, response surface methodology (RSM) and Central Composite Design (CCD) were employed to design experiments, develop the regression model, and evaluate individual and interactive impacts of five independent operational variables. The obtained results were then predicted by an optimized artificial neural network-genetic algorithm (ANN-GA) algorithm. The estimated results were compared with the experimental results. In the second part of the work, the impact of ultrasound irradiation on the main characteristics of the synthesized biodiesel was investigated. The analysis of the operating conditions indicated that reaction temperature and MeOH:oil molar ratio were the most important variables on reaction yield. The experimental results showed that there was a change in the main properties of the synthesized palm oil biodiesel with the density changed by about 0.3 kg/m3, viscosity by 0.12 mm2/s, pour/cloud point by 1–2°C, and flash point by 5°C, depending on different combinations of operational parameters. Besides, the numerical optimization technique was employed to optimize process variables in order to obtain the maximum FAME content (reaction yield) along with the best properties using both RSM and ANN-GA techniques. The maximum reaction yields of 95.2% and 95.1% were predicted by the RSM and ANN-GA models, respectively, at the optimum conditions. The conditions predicted by RSM and ANN-GA proved to be feasible for modeling and optimizing transesterfication yield with an accuracy of 99.18% and 99.14% and biodiesel properties of 98.61% and 98.28%, respectively.

A comprehensive review on physical activation of biochar for energy and environmental applications

Abstract Biochar is a solid by-product of thermochemical conversion of biomass to bio-oil and syngas. It has a carbonaceous skeleton, a small amount of heteroatom functional groups, mineral matter, and water. Biochar’s unique physicochemical structures lead to many valuable properties of important technological applications, including its sorption capacity. Indeed, biochar’s wide range of applications include carbon sequestration, reduction in greenhouse gas emissions, waste management, renewable energy generation, soil amendment, and environmental remediation. Aside from these applications, new scientific insights and technological concepts have continued to emerge in the last decade. Consequently, a systematic update of current knowledge regarding the complex nature of biochar, the scientific and technological impacts, and operational costs of different activation strategies are highly desirable for transforming biochar applications into industrial scales. This communication presents a comprehensive review of physical activation/modification strategies and their effects on the physicochemical properties of biochar and its applications in environment-related fields. Physical activation applied to the activation of biochar is discussed under three different categories: I) gaseous modification by steam, carbon dioxide, air, or ozone; II) thermal modification by conventional heating and microwave irradiation; and III) recently developed modification methods using ultrasound waves, plasma, and electrochemical methods. The activation results are discussed in terms of different physicochemical properties of biochar, such as surface area; micropore, mesopore, and total pore volume; surface functionality; burn-off; ash content; organic compound content; polarity; and aromaticity index. Due to the rapid increase in the application of biochar as adsorbents, the synergistic and antagonistic effects of activation processes on the desired application are also covered.

Chemical activation of biochar for energy and environmental applications: a comprehensive review

Abstract Biochar (BC) generated from thermal and hydrothermal cracking of biomass is a carbon-rich product with the microporous structure. The graphene-like structure of BC contains different chemical functional groups (e.g. phenolic, carboxylic, carbonylic, etc.), making it a very attractive tool for wastewater treatment, CO2 capture, toxic gas adsorption, soil amendment, supercapacitors, catalytic applications, etc. However, the carbonaceous and mineral structure of BC has a potential to accept more favorable functional groups and discard undesirable groups through different chemical processes. The current review aims at providing a comprehensive overview on different chemical modification mechanisms and exploring their effects on BC physicochemical properties, functionalities, and applications. To reach these objectives, the processes of oxidation (using either acidic or alkaline oxidizing agents), amination, sulfonation, metal oxide impregnation, and magnetization are investigated and compared. The nature of precursor materials, modification preparatory/conditions, and post-modification processes as the key factors which influence the final product properties are considered in detail; however, the focus is dedicated to the most common methods and those with technological importance.