Improving the dewetability characteristics of hydrophobic fine particles by air bubble entrapments

The present study deals with the role of surface hydrophobicity and air bubble entrapment on filtration / dewatering of fine coal particles (− 0.5 mm). The experimental results showed that hydrophobizing reagents increased the contact angle (or hydrophobicity) and decreased the filtrate surface tension, cake formation time and moisture content of the fine particles. In addition to these, when the air bubbles were introduced to the hydrophobic fine coal particles in water, the cake formation time and moisture content of the filter cake were further reduced, which may be attributed to the fact that micro/nano size air bubbles were entrapped on the surface of the fine particles in a slurry and increased the dewetting characteristics of the fine particles. As a result, this process may lower the amount of moisture trapped during the dewatering fine particles and increase the efficiency of the filters.     

Contact angle and vacuum floatability of ultrafine size particles

The contact angle of ultrafine size particles has been evaluated using 1 µm monosize SiO₂ particles of various degrees of wettability. The contact angle was determined by film flotation and Zisman plots. Chlorotrimethylsilane (CTS) was used to methylate the SiO₂ particle surface and establish the level of surface wettability. Also, the vacuum floatability of the methylated ultrafine SiO₂ particles was assessed to correlate it to the contact angle. This vacuum floatability was very low below 40º and increased monotonically above this contact angle value because of favorable bubble nucleation and a greater stability of the bubbles on the hydrophobic surface. Free energy of bubble nucleation on the hydrophobic surfaces has been estimated and correlated to the vacuum floatability of the ultrafine particles.     

Gas–liquid flows in medium and large vertical pipes

Gas–liquid bubbly flows with wide range of bubble sizes are commonly encountered in many industrial gas–liquid flow systems. To assess the performances of two population balance approaches – Average Bubble Number Density (ABND) and Inhomogeneous MUlti-SIze-Group (MUSIG) models – in tracking the changes of gas volume fraction and bubble size distribution under complex flow conditions, numerical studies have been performed to validate predictions from both models against experimental data of Lucas et al. (2005) and Prasser et al. (2007) measured in the Forschungszentrum Dresden-Rossendorf FZD facility. These experiments have been strategically chosen because of flow conditions yielding opposite trend of bubble size evolution, which provided the means of carrying out a thorough examination of existing bubble coalescence and break-up kernels. In general, predictions of both models were in good agreement with experimental data. The encouraging results demonstrated the capability of both models in capturing the dynamical changes of bubbles size due to bubble interactions and the transition from “wall peak” to “core peak” gas volume fraction profiles caused by the presence of small and large bubbles. Predictions of the inhomogeneous MUSIG model appeared marginally superior to those of ABND model. Nevertheless, through the comparison of axial gas volume fraction and Sauter mean bubble diameter profiles, ABND model may be considered an alternative approach for industrial applications of gas–liquid flow systems.     

Behavior of a single coherent gas bubble chain and surrounding liquid jet flow structure

The motion of a single nitrogen gas bubble chain and the structure of water flow field surrounding the chain were experimentally studied. We developed a bubble generator that can control both the bubble diameter and the generation frequency independently. Experimental conditions of bubble Reynolds number and bubble distance divided by bubble diameter were from 300 to 650 and from 6.5 to 300, respectively. We discuss the interaction effects on the motion of each bubble rising in a chain, as compared to the effects of a single rising bubble. The bubble trajectories and the surrounding water flow fields in the state of bubbles rising in a chain were investigated using a high-speed digital video camera and an analog single-lens-reflex camera. We observed two important physical phenomena. First, bubbles passed through a nearly identical path in the case of low frequency of bubble production. On the contrary, at a height of approximately 50 mm from the nozzle, the bubbles in the case of high frequency deviated and scattered from this path due to bubble-bubble interaction. Second, with higher bubble production frequency, coherent bubble chain and the characteristic structure of the surrounding water flow called “liquid jet” were observed near the nozzle. The direction of liquid jet flow differed from the bubble trajectory. We theoretically investigated the relation of coherent bubble chain and liquid jet by applying the conservation of liquid momentum.     

The Limits of Fine and Coarse Particle Flotation

The flotation behaviour of quartz particles was studied over the particle size range from 0.5 µm to 1000 µm and for advancing water contact angles between 0° and 83°. Flotation was performed in a column and in a Rushton turbine cell. Particle contact angle threshold values, below which the particles could not be floated, were identified for the particle size range 0.5–1000 µm, under different hydrodynamic conditions. The flotation response of the particles, either in a column or in a mechanically agitated cell with a similar bubble size, was comparable. Turbulence plays a role, as does bubble‐particle aggregate velocity and bubble size. The stability of the bubble‐particle aggregate controls the maximum floatable particle size of coarse particles. For fine particles, the flotation limit is dictated by the energy required to rupture the intervening liquid film between the particle and bubble. Flotation of very fine and large particles is facilitated with small bubbles and high contact angles. These results greatly extend our earlier observations and theoretical predictions.     

Water electrolysis under microgravity: Part II. Description of gas bubble evolution phenomena

Water electrolysis is carried out in both alkaline (25 and 2 wt.% KOH) and acidic (0.1N H₂SO₄) solutions for 8 s under a microgravity (μ-G) environment realized in a drop shaft. The effects of gravitational strength on gas bubble evolution behavior are analyzed in consideration of various factors (bubble movement, bubble assembly and single bubble). Under a μ-G environment, a collection of fine gas bubbles forms a froth layer in alkaline solutions, whereas bubbles frequently coalesce in acidic solution. Moreover, H₂ gas bubbles in alkaline jump from a cathode surface and O₂ bubbles often coalesce on an anode. In acidic solution both H₂ and O₂ bubbles frequently coalesce on electrode surfaces. Such gas bubble movements are reflected in the coalescence number and bubble residence time. A single bubble is characterized by the bubble size and the dynamic contact angle between a gas bubble and a Pt electrode, however, these factors are not essentially influenced by the gravitational strength.     

装配水平筛板的气升式反应器中气液传质特性的数值模拟

利用Turbulent–Lehr组合模型对装配水平筛板的气升式反应器进行了计算流体力学(CFD)模拟,研究水平筛板对气含率、气泡直径、体积传质系数(kLa)和气液流速的影响。结果表明,筛板对气相的囤积作用和对液相的阻碍作用增加了反应器的整体气含率。筛板对气相的二次均布作用减弱了筛板和液面之间区域的气泡聚并过程,筛板筛孔对气泡的破碎作用产生了大量小于初始直径的气泡,增加了气泡比表面积(a);筛板对液相的阻碍作用提高了筛板附近的气–液相流动速度差,从而提高了该区域的液膜传质系数(kL),强化了反应器内的气液传质效果。     

基于气泡动力学分段调控浸润性强化核态沸腾

基于多孔或微结构表面润湿性改性的核态沸腾强化传热,已得到广泛研究。利用CFD-VOF数值模拟方法,针对单晶硅微柱表面单气泡的生长及脱离过程,进行表面浸润性分段调控,实现气泡沸腾换热的全程强化。分别调控初始接触角为48°、60°、90°和110°后,同一时刻 （t = 0.152 ms） 变接触角为20°,对比研究分段调控浸润性对气泡动力学过程与表面换热性能的影响。结果表明：疏水性可提高气泡生长速率,增强微柱表面对气泡的黏附力,促进气泡在微结构缝隙内的横向铺展;t = 0.150 ms时接触角为110° 表面上气泡与底面接触面积增加1.3倍,微层蒸发功率增加1.2倍。需要指出的是,毛细效应随颗粒粒径变化趋势受到多孔介质复杂孔隙结构特征的影响。在当前粒径范围内,认为其具有正相关关系,但在更大范围内的对应关系,还需要在未来进一步深入揭示。     

多孔泡沫吸气板调控沸腾气泡动力学实验研究

利用不同的改性方法对多孔泡沫铜表面进行浸润性改性处理,并制备了一系列具有不同疏水性的吸气板,系统研究了悬挂吸气板对沸腾单气泡生长、脱离阶段的影响。结果表明,不同吸气高度和吸气浸润性均对沸腾气泡动力学特性具有重要影响。当吸气高度小于气泡最大生长高度时,吸气板可实现强制气泡脱离并有效减小气泡最大脱离直径。当吸气板表面浸润角为150°时,吸气效果最佳,最短可将脱离阶段缩短至3 ms,最大瞬时吸气速率可达5.46 mm³/ms,为浸润角是100°吸气板吸气速率的3.96倍。耦合控制吸气板的吸气高度、浸润性和换热表面的微结构可共同实现对气泡生长阶段的提前干预和对脱离阶段的强制促进,真正实现沸腾动力学的全程强化。     

Analogous description of the hydrodynamics of gas-solid fluidized beds and bubble columns

In this paper we stress analogies in the hydrodynamic behaviour of gassolid fluidized beds and bubble columns. Using published experimental data, it is demonstrated that the analogous hydrodynamic-behaviour is not only qualitative but also quantitative in nature. Specifically, we show the following.(1) The gas holdup in the homogeneous regimes of bubble columns and fluidized beds can be modelled in a unified way using V_slip = υ_∞(1 − ϵ_d)ⁿ⁻¹, where V_slip refers to the slip velocity between the dispersed (bubbles or particles) and continuous phases and ϵ_d the dispersed phase holdup. The Richardson-Zaki exponent n decreases with increasing gas density.(2) The transition from homogeneous to heterogeneous flow regimes in gasliquid bubble columns and gassolid fluid beds is delayed by increasing system pressure. Extrapolation of the influence of increased gas density allows us to consider liquidliquid dispersions and liquidsolid fluid beds as limiting cases.(3) In the heterogeneous flow regime of operation the classic two-phase theory of fluidized beds can be applied with profit to also describe the hydrodynamics of gasliquid bubble columns provided that the “dilute” phase is identified with the fast-rising large bubbles and the “dense” phase is identified with the liquid phase containing entrained “small” bubbles. Tentative analogies can also be drawn for the interphase mass transfer processes.(4) The “dense” phase backmixing can be modelled in a unified manner.(5) The two-phase theory can be extended to describe slurry reactors.It is argued that, because of cross-fertilization of concepts and information, appreciation of analogies can be invaluable tool in scaling up.     

Adhesion of Streptococcus sanguis to porous and non-porous substrates with well-defined surface energies

Adhesion of the oral bacterium Streptococcus sanguis was investigated on two series of surfaces, one solid, non-porous and one porous with a pore diameter centered around 0.2 μm. Each series consisted of four substrates with varying surface free energies: pure aluminum oxide, poly(ethylene glycol) (PEG)-modified aluminum oxide, and two types of silanized aluminum oxide. The surface composition was analyzed by ESCA and the surface free energy and acid-base character were determined by contact angle measurements using water, formamide, and diiodomethane according to van Oss and Good. Contact angle measurements were conducted on non-porous substrates which had been subjected to the same silanization procedure as the porous ones. Untreated aluminum oxide and PEG-modified aluminum oxide were both hydrophilic and gave water contact angles of less than 10°. One silanization procedure gave a surface of intermediate hydrophobicity, with a γ^tot of 47.5 mN/m; the other gave a strongly hydrophobic surface of γ^tot = 27.4 mN/m. Adhesion of S. sanguis was measured by using radiolabelled bacteria. It was demonstrated that more bacteria adhered to the porous than to the non-porous substrate and that the PEG treatment, as well as the two silanization procedures, resulted in a decrease in bacterial adhesion, in relation both to the controls treated with aqueous buffer and to surfaces treated with fresh saliva.     

Effect on surface charges of bubbles and fine particles on air flotation process

The adhesion of fine particles onto bubbles in flotation was studied on the basis of surface charge measurements of the bubbles and particles. The surface charges of the bubbles were measured by the use of a micro-electropheresis apparatus devised in our previous study and the mechanism of the bubble charging was studied under various experimental conditions. In distilled water, the bubbles were negatively charged and the iso-electric point appeared at pH = 2.5. The surface charges of the bubbles in the surfactant solution were determined by the surfactant molecules adsorbed at the surface and depend strongly on the values of pH. The flotation efficiency of latex particles (0.923 μm) was found to be strongly influenced by the surface charges of both the particles and the bubbles. The force between the particle and the bubble was estimated from the observation of the particle attachment to the bubble surface, and a simple equation including the effects of the hydrodynamic and surface charge interactions was proposed to determine the floatability limit.     

Effect of insoluble amine on bubble surfaces on particle–bubble attachment in potash flotation

This study shows that in the potash flotation system, in which long‐chain amine collector appears in the form of colloidal particles, the hydrophobicity of sylvite surfaces critically depends on whether the amine is placed onto the solid surface or onto the surface of a bubble. The transportation of amine collector on the surface of bubbles was found to be beneficial for the bubble/KCl particle attachment.     

静止非牛顿流体中气泡生成过程的传质

气泡生成过程中气液传质是气液接触设备的设计、优化的重要参考指标。以二氧化碳气泡在羧甲基纤维素钠(carboxymethyl cellulose,CMC)溶液中生成过程中的传质为研究对象,分别考察了气速、CMC溶液浓度、针头直径对气泡生成过程气液传质的影响,采用具有CCD显微相机的动态接触角分析仪测量了气泡形状、表面积和体积的变化,进而获得气泡生成过程气液传质系数k_l。实验结果表明,CMC浓度从0.2%增加到0.8%,黏度逐渐增大,传质系数随CMC浓度的增大而增大;针头直径从2.5 mm增加到4 mm,传质系数也随之增大;气速从1 ml·min^-1增加到9 ml·min^-1,传质系数也逐渐增大。     

气体入口角度和截面宽高比对微通道内泰勒气泡行为的影响

微通道的几何参数对于气泡或者液滴在其中的形成过程有着显著的影响。采用流体体积法(VOF法)研究了不同气体入口角度以及不同通道截面宽高比对微通道内气液两相流流动状况的影响。在所研究的操作范围内,各个微通道内的两相流流型均为泰勒流,并且在大多数情况下气泡长度分布均匀。在通道截面宽高比为0.5～2条件下,60°的气体入口角度有利于产生较短的气泡;如果通道截面宽高比达到4或8时,45°的气体入口角度更有利于形成较短的气泡。此外,随着通道截面宽高比的增大,通道内气泡的量纲1长度也随之增大,气泡长度分布的均匀性也逐渐变差。当通道截面宽高比增大到8时气泡长度分布变得很不均匀。     

Effect of gas expansion on the velocity of individual Taylor bubbles rising in vertical columns with water: Experimental studies at atmospheric pressure and under vacuum

This study was designed to determine the effect of gas expansion on the velocity of Taylor bubbles rising individually in a vertical column of water. This experimental study was conducted at atmospheric pressure or under vacuum (33.3 and ) using three different acrylic columns with internal diameters of 0.022, 0.032, and 0.052 m, and more than 4.0 m high. A non-intrusive optical method was used to measure velocity and length of Taylor bubbles at five different locations along the columns. The operating conditions used correspond to inertial controlled regime.In experiments performed under vacuum, there is considerable gas expansion during the rise of Taylor bubbles, particularly when they approach the liquid free surface where the pressure drop (due to the hydrostatic pressure) is of the order of magnitude of the absolute pressure. The liquid ahead of the bubble is displaced upward by an amount proportional to the gas expansion resulting in increased bubble velocity. The calculated Reynolds number suggests a laminar regime in the liquid ahead of the bubble. However, the experimentally determined velocity coefficient C for each column was much smaller than 2, which would be expected for laminar flow. The value of C obtained ranges from 1.13±0.09, for the narrowest column, to 1.40±0.24, for the widest column. This suggests that a fully developed laminar flow in the liquid ahead of the bubble is never achieved due to continuous bubble expansion at a variable rate, regardless of column height.The velocity coefficient C can be used to calculate the contribution of liquid motion to bubble velocity. Subtracting this contribution from the measured bubble velocity defines a constant value which is nearly identical to the bubble rise velocity measured in the same column operated as a constant volume system (two ends closed) where gas expansion is absent.     

THE ENHANCEMENT OF GAS ABSORPTION BY A HETEROGENEOUSLY CATALYSED CHEMICAL REACTION IN A STAGNANT LIQUID: TEMPERATURE RISE AND EFFECTIVENESS OF THE CATALYST

In the present paper a model has been developed to calculate the enhancement efficiency e of the gas absorption and the effectiveness factor η of the catalyst for a chemically reacting system consisting of a sedimented catalyst particles-containing layer of thickness γ_k adhering to the surface of a gas bubble situated in a stagnant liquid. This model predicts that for a heterogeneously catalysed first-order chemical reaction an increase in the thickness γ_k of the catalyst particles-containing layer will result in an increase of ε and a decrease of η. It has been shown that the optimum value (γ_k)_opt of this layer can be obtained from the relation (γ_k) = (D_AB/k₁₃). Further, a model is derived to calculate the steady-state temperature profile in the catalyst particles-containing layer adhering to the gas bubble surface. Both models have been applied to experimental results published in some of our previous papers. These experimental results concern the enhancement of the gas-absorption rate from a single hydrogen-containing gas bubble to a stagnant aqueous hydroxylaminephosphate-containing solution when a layer of sedimented Pd/C catalyst particles was adhered to the surface of the gas bubble. The model and the experimental results mentioned may approximately be considered as representative of slurry reactors in which finely divided catalyst particles show considerable adhesion to rising gas bubbles.     

Local hydrodynamic properties of gas phase in an internal-loop airlift reactor

The local hydrodynamic properties of the gas phase in an internal-loop airlift reactor were investigated in this study. The hydrodynamic properties including gas holdup, bubble velocity and bubble chord length were measured by dual electrical resistivity probes. The chord length distribution was then transformed to the bubble size distribution by modeling the bubbles as ellipsoids. It was found that the gas holdup increased with decreasing bubble velocity. In addition, most bubbles tended to rise along the riser central axis. Thus, the gas holdup in the axis was higher. The bubble size, bubble velocity and gas holdup were relatively constant in the axial direction of the riser except in the zones near the gas sparger and the gas–liquid separator. The bubble velocity became slower when the bubbles approached the gas–liquid separator. Moreover, the bubble size and bubble velocity for the three-phase system were relatively insensitive to the radial direction compared to those for the two-phase system. It was also found in this study that the bubble rise velocity and bubble size for the three-phase system were lower than that for the two-phase system. However, the gas holdup for the three-phase system were higher than that for the two-phase system due to bubble breakage caused by the solid particles.     

Measurement of sliding velocity and induction time of a single micro‐bubble under an inclined collector surface

In this study, interactions between a gas bubble and a flat solid surface were investigated by determining two dynamic parameters, bubble sliding velocity underneath an inclined solid surface and induction time of the gas bubble attaching to the solid surface in aqueous solutions. A single micro‐bubble was allowed to move vertically toward an inclined solid surface. After reaching its terminal velocity, the bubble approaches the inclined solid surface and slides underneath it. Complete trajectory of the bubble movement was monitored and recorded by a high‐speed CCD video imaging system. Various types of gas bubbles (CO₂, air, H₂, and O₂) and solid surfaces such as bitumen‐coated Teflon, hydrophobized and hydrophilic silica were used in sliding velocity and induction time measurements. The effect of water chemistry (industrial process water and de‐ionized water) and surface heterogeneity on bubble sliding velocity and induction time was investigated. The results showed that the sliding velocity of micro‐bubbles under an inclined solid surface is a strong function of water chemistry, gas type, temperature and hydrophobicity of the solid surface. This study provides relevant information on bubble–solid interactions that would assist in the understanding of bubble–solid attachment under diverse conditions.