Comprehensive experimental investigation of counter-current bubble column hydrodynamics: Holdup, flow regime transition, bubble size distributions and local flow properties

Radon transport carried by geogas: prediction model

This paper provides an overview and information on radon migration in the crust. In the past several decades, numerous studies on radon migration have been published. However, there is no there is no comprehensive review of large-scale radon transport in the earth crust. A literature review was conducted to present the research on the mechanism of radon migration, geogas theory, investigation of multiphase flow, and modeling method of fractures. Molecular diffusion was long considered the primary mechanism for radon migration in the crust. However, a molecular diffusion mechanism cannot explain the understanding of anomalous radon concentrations. In contrast with early views, the process of radon migration and redistribution within the Earth may be determined by geogas (mainly CO2 and CH4). Microbubbles rising in fractured rocks may be a rapid and efficient way of radon migration, as reported by recent studies. All these hypotheses on the mechanisms of geogas migration are summarized into a theoretical framework, defined as "geogas theory." According to geogas theory, fractures are the principal channel of gas migration. The development of the discrete fracture network (DFN) method is expected to supply a new tool for fracture modeling. It is hoped that this paper will contribute to a deeper understanding of radon migration and fracture modeling.

Low energy harvesting of hydrophobic microalgae (Tribonema sp.) by electro-flotation without coagulation

Microalgae have great potential for biofuel production and wastewater treatment, but the high cost of harvesting hinders their practical application. In this study, economical harvesting of hydrophobic microalgae by electro-flotation without coagulation was assessed. The harvesting performance of this method for selected species of freshwater microalgae with different degrees of hydrophobicity (Tribonema sp., highly hydrophobic; Scenedesmus sp., moderately hydrophobic; and Pandorina sp., hydrophilic) were compared. It was found that microalgal hydrophobicity played a critical role in electro-flotation. Under the same condition (current 0.3 A, velocity gradient 200 s^-1, biomass concentration 1 g/L), Tribonema sp. could be effectively harvested (96.2 ± 0.4%) after 20 min of electro-flotation, while the harvesting efficiency decreased significantly with Scenedesmus sp. (70.1 ± 5.2%, 20 min) and Pandorina sp. (<10%, 1 h). The influences of current, electrolysis time, mixing intensity (velocity gradient) and biomass concentration on Tribonema sp. (hydrophobic) harvesting were further investigated. Increasing the current within a certain range (0.1 A-0.4 A) was beneficial to harvesting, while it's further increase decreased floating velocity, which was similar to the effect of the velocity gradient. Under the optimal condition, the harvesting efficiency of Tribonema sp. was 96.3% and the energy consumption (0.19 kWh/kg biomass) was much lower than other harvesting techniques, indicating that electro-flotation is a time-saving and economical approach for hydrophobic microalgae harvesting.

A Non-Invasive Method for Measuring Bubble Column Hydrodynamics Based on an Image Analysis Technique

Bubble size and its distribution are the important parameters which have a direct impact on mass transfer in bubble column reactors. For this, a new robust image processing technique was presented for investigating hydrodynamic aspects and bubble behavior in real chemical or biochemical processes. The experiments were performed in a small-scale bubble column. The study was conducted for the wide range of clear liquid heights and superficial gas velocities. However, a major challenge in image analysis techniques is identification of overlapping or cluster bubbles. This problem can be overcome with the help of the proposed algorithm. In this respect, large numbers of videos were recorded using a high-speed camera. Based on detailed experiments, the gas–liquid dispersion area was divided into different zones. A foam region width was found as inversely proportional to the clear liquid height. An entry region width was found as directly proportional to the clear liquid height. Hydrodynamic parameters, including gas holdup, bubble size distribution, and Sauter mean bubble diameter were evaluated and compared for different operating conditions. The gas holdup was calculated from both height measurement and pixel intensity methods, and it was found to be indirectly proportional to clear liquid height. Bubble sizes affect the bubble column performance; therefore, bubbles are tracked to calculate the bubble size distribution. Experimental results proved that the proposed scheme is robust.

Shapes and Rise Velocities of Single Bubbles in a Confined Annular Channel: Experiments and Numerical Simulations

Shapes and rise velocities of single air bubbles rising through stagnant water confined inside an annular channel were investigated by means of experiments and numerical simulations. Fast video imaging and image processing were used for the experiments, whilst the numerical simulations were carried out using the volume of fluid method and the open-source package OpenFOAM. The confinement of the annular channel did not affect the qualitative behavior of the bubbles, which exhibited a wobbling rise dynamic similar to that observed in bubbles rising through unconfined liquids. The effect of the confinement on the shape and rise velocity was evident; the bubbles were less deformed and rose slower in comparison with bubbles rising through unconfined liquids. The present data and numerical simulations, as well as the data collected from the literature for use here, indicate that the size, shape, and rise velocity of single bubbles are closely linked together, and prediction methods that fail to recognize this perform poorly. This study and the limited evidence documented in the literature indicate that the confinement effects observed in non-circular channels of complex shape are more complicated than those observed with circular tubes, and much less well understood.

Prediction of Bubble Size Distributions in Large-Scale Bubble Columns Using a Population Balance Model

A precise estimation of the bubble size distribution (BSD) is required to understand the fluid dynamics in gas-liquid bubble columns at the “bubble scale,” evaluate the heat and mass transfer rate, and support scale-up approaches. In this paper, we have formulated a population balance model, and we have validated it against a previously published experimental dataset. The experimental dataset consists of BSDs obtained in the “pseudo-homogeneous” flow regime, in a large-diameter and large-scale bubble column. The aim of the population balance model is to predict the BSD in the developed region of the bubble column using as input the BSD at the sparger. The proposed approach has been able to estimate the BSD correctly and is a promising approach for future studies and to estimate bubble size in large-scale gas–liquid bubble columns.