Hydrodynamic Behavior in a Tapered Bubble Column

IntroductionBubble column( BC) and slurry bubble column( SBC) reactors have emerged as one of the mostpromising devices in chemical,biochemical and en-vironmental engineering operations because of theirsimple construction,isothermal conditions,highheat and mass transfer rates,and on- line catalystaddition and withdrawal[1_ 4] .The utilization of ta-pered slurry bubble column( TSBC) reactorshas re-cently received much attention ofsome scholars andscientists who are engaged in biochemical rea…     

鼓泡浆液反应器的应用开发研究Ⅰ.气含率和固相浓度分布特性

研究了以粉体氧化铝水合物为固相的鼓泡浆液反应器的平均气含率和固体浓度分布特性。考察了表观气速、体系温度、静液高度、固相浓度及气体分布板的开孔率等对气含率的影响和不同性质氧化铝水合物在塔中的悬浮和轴向浓度分布情况。结果表明气速增大或开孔率较大时气含率增大，但固相浓度大小对气含率没有影响。水合氧化铝固体粉末在鼓泡塔中的浓度分布特性与固体的堆积密度和吸水性能有关，吸水率大堆积密度小的拟薄水铝石在低气速条件下就可完全均匀悬浮。以上结果为用气液固三相鼓泡反应器制备晶粒大小均匀的拟薄水铝石提供了可能性。     

Kinetics of the phase separation of a low-viscosity liquid supersaturated with gas at the intermediate and later stages

The separation kinetics of a low-viscosity liquid supersaturated with gas is studied. It is shown that the number of bubbles per unit volume at the intermediate stage of the process remains virtually constant, whereas the amount of the gas per unit volume of the solution significantly decreases, almost attaining the equilibrium value. At the later stage, small subcritical bubbles disappear due to gas transfer to large supercritical bubbles; as a result, the number of bubbles in the virtually equilibrium (as to the dissolved gas) liquid decreases. For all the stages, the kinetics of the changes in the bubble distribution function and in the amount of gas per unit volume of the solution is determined.Translated from Kolloidnyi Zhurnal, Vol. 67, No. 1, 2005, pp. 94–105.Original Russian Text Copyright © 2005 by Slezov, Abyzov, Slezova.     

Non-invasive imaging of shallow bubble columns using electrical capacitance tomography

This paper deals with application of non-invasive electrical capacitance tomography to study the hydrodynamics of shallow bed bubble columns. Two bubble columns with different height to diameter ratio were used. Air–kerosene system that represents dielectric two-phase mixture was investigated. The ECT provided good measurement of the gas holdup at different gas velocities compared to the classical pressure measurements. The ECT was able to provide the gas hold up and the bubble velocities distribution across the column diameter at different gas velocities. The study revealed that spatial gas holdup and bubble velocity distributions are sharp with parabolic shape in the small bubble column (H_D/D_C = 5). However, in the large bubble column (H_D/D_C = 4) the gas holdup and bubble velocity profiles were flatter indicating improvement in the mixing homogeneity and leading to well-mixed reactor. 3D graphical visualization of the flow regimes and transition points were also examined using the ECT. In the small bubble column flow regimes were heterogeneous to slugs flow especially at high flow rate, resulted in downward flow near the walls and imperfect mixing.     

Imaging of copper oxygenation reactions in a bubble flow

Reactions of gases with liquids play a crucial role in the production of many bulk chemicals. Often, the gas is bubbled into the chosen reactor. Most of the processes at the gas–liquid interface of the bubbles and in their tails are not fully understood and warrant further investigation. For this purpose, NMR imaging or Magnetic Resonance Imaging has been applied to visualize some of the processes in the bubble tail. To generate sufficient contrast, a magnetogenic gas–liquid reaction associated with a change of magnetic state, from diamagnetic to paramagnetic, was employed. In this work, a copper(I)‐based compound was oxidized to copper(II) to exploit relaxation contrast. To match the speed of the rising bubbles to the acquisition time of the spin‐echo imaging sequence, polyethylene glycol was added to increase the viscosity of the reacting solution. Images of the oxygen ingress into a static solution as well as of oxygen bubbles rising in the solution are presented. In both cases, changes in magnetism were observed, which reported the hydrodynamic processes.     

Microbubble generation in a co-flow device operated in a new regime

A new regime of operation of PDMS-based flow-focusing microfluidic devices is presented. We show that monodisperse microbubbles with diameters below one-tenth of the channel width (here w = 50 μm) can be produced in low viscosity liquids thanks to a strong pressure gradient in the entrance region of the channel. In this new regime bubbles are generated at the tip of a long and stable gas ligament whose diameter, which can be varied by tuning appropriately the gas and liquid flow rates, is substantially smaller than the channel width. Through this procedure the volume of the bubbles formed at the tip of the gas ligament can be varied by more than two orders of magnitude. The experimental results for the bubble diameter d(b) as function of the control parameters are accounted for by a scaling theory, which predicts d(b)/w ∝ (μ(g)/μ(l))(1/12)(Q(g)/Q(l))(5/12), where μ(g) and μ(l) indicate, respectively, the gas and liquid viscosities and Q(g) and Q(l) are the gas and liquid flow rates. As a particularly important application of our results we produce monodisperse bubbles with the appropriate diameter for therapeutic applications (d(b) ? 5 μm) and a production rate exceeding 10(5) Hz.     

Characteristic analysis of cavitation bubbles in carbon dioxide fluid

After analysing the characteristics of bubble cavitation in high-pressure carbon dioxide (CO₂) fluid, cavitation conditions and some correlative physical characteristics are investigated. The results show that the ultrasonic intensity of liquid carbon dioxide to make cavitation occur is affected by the initial radius of the bubbles, hydrostatic pressure, temperature and vapour pressure within the bubbles in liquid CO₂. At the low frequency of ultrasound, the phase-speed of the liquid CO₂ gradually approaches the sound speed of the pure liquid when void fraction increases. At high frequency, the phase-speed is nearly equal to the sound speed in the liquid under different void fractions. The attenuation of ultrasound in liquid carbon dioxide reaches a maximum near the resonance frequency and then decreases when frequency either increases or decreases. At the resonance frequency, the phase-speed and the attenuation increase when the void fraction increases.     

鼓泡浆态反应器中浸没表面的传热研究

在内径98mm的鼓泡浆态反应器内，考察了工艺参数对浸没表面与浆液间的传热系数的影响。浆态反应器轴向装有一个外径20mm，长120mm的测量传热膜系数用的铜制元件。为了模拟浆态FT合成反应系统，三相系统由N2、液体石蜡和石英砂（平均粒径53μm、110μm、180μm）或63μm以下的Fe2O3组成。工艺参数变化范围如下：表观气速0.005m/s～0.08m/s, 温度353K～453K, 压力0.1MPa～0.8MPa，固体的质量分数0～20%,初始液位高度625mm～1240mm。本研究使用单孔板、多孔板、烧结金属板三种气体分布器类型。结合实验数据，应用最小二乘法求得各个参数值，得到的无因次传热系数关联式为St=0.179(ReFr)－0.25Pr－0.66，相关指数0.98，最大偏差18％。该关联式可应用于气－液和粒径小于100μm的气－液－固体系。     

Polymer nanospheres formed by a microfluidic technique with Evans blue dye

In this work, the V‐shaped microfluidic junction (VMJ) device technique with gas/liquid interface was used to prepare textured polymer nanospheres from bubble bursting for drug delivery. The polymer/dye solution, N₂ gas, and a volatile liquid, perfluorohexane (PFH) were simultaneously fed using the tubes into the VMJ device. A high‐pressure injection of N₂ gas into the VMJ interacts with PFH and ethanol leading to the preparation of a microbubble system. Once bubbles are ejected from the VMJ outlet, nanospheres calve from the parent bubble. The collection temperature and the N₂ gas pressure play a key role in the mechanism by which nanospheres are formed. In addition, the volatile liquid, PFH, is described as a significant surface modifier. The influence of the N₂ gas pressure, collection temperature, and the volatile liquid flow rates on nanospheres size distribution and surface roughness were investigated using scanning electron microscopy. The results revealed that the N₂ gas pressure and collection temperature are crucial in tailoring the size distribution of the nanospheres and that the nanospheres textured with PFH had significantly rougher surface. Nanospheres coated with Evans blue dye were prepared, and those collected at high temperature exhibited a very different dye release profile compared with those collected at lower temperatures. Copyright © 2015 John Wiley & Sons, Ltd.     

Pressure drop of slug flow in microchannels with increasing void fraction: experiment and modeling

Pressure drop in a gas-liquid slug flow through a long microchannel of rectangular cross-section was investigated. Pressure measurements in a lengthy (～0.8 m) microchannel determined the pressure gradient to be constant in a flow where gas bubbles progressively expanded and the flow velocity increased due to a significant pressure drop. Most of the earlier studies of slug flow in microchannels considered systems where the expansion of the gas bubbles was negligible in the channel. In contrast, we investigated systems where the volume of the gas phase increased significantly due to a large pressure drop (up to 1811 kPa) along the channel. This expansion of the gas phase led to a significant increase in the void fraction, causing considerable flow acceleration. The pressure drop in the microchannel was studied for three gas-liquid systems; water-nitrogen, dodecane-nitrogen, and pentadecane-nitrogen. Inside the microchannel, local pressure was measured using a series of embedded membranes acting as pressure sensors. Our investigation of the pressure drop showed a linear trend over a wide range of void fractions and flow conditions in the two-phase flow. The lengths and the velocities of the liquid slugs and the gas bubbles were also studied along the microchannel by employing a video imaging technique. Furthermore, a model describing the gas-liquid slug flow in a long microchannel was developed to calculate the pressure drop under conditions similar to the experiments. An excellent agreement between the developed model and the experimental data was obtained.     

Inter-bubble gas diffusion in liquid foam

Ostwald ripening (disproportionation) of gas–liquid foam is a process in which large bubbles grow at the expense of small bubbles because pressure differences drive inter-bubble gas diffusion. The rather overlooked theory of Robert Lemlich, for relatively wet foam, is revisited and its analogy with Ostwald ripening of metals grains discussed and this is put into context with recent advances. Lemlich's Theory assumes that the rate of disproportionation is governed by gas mass transfer, whereas recent work has suggested that it may be mechanically resisted by contraction/expansion of gas–liquid interfaces. In addition, the exact generalisation of the von Neumann Theory of grain growth to three dimensions is made quantitatively relevant to dry foams. Developments in tomographies and other experimental techniques have the potential to enable this model to be verified.     

加温加压条件下气液固三相鼓泡塔体积传质系数的测定

气液固三相鼓泡塔反应器被广泛地应用于化工、石油化工、煤化工、化工冶金、环境工程等领域,如石油馏分的加氢、煤的脱硫与脱氮、对二甲苯氧化、费托合成、甲醇合成、二甲醚合成以及含有机物废水的湿式氧化等工业过程[1.2].     

A model for the pressure drop in gas—liquid cocurrent downflow through packed beds

A model for pressure drop is proposed for gas—liquid flow through packed beds on the basis of the observed absence of radial pressure gradients and taking into consideration the structure of the bed and the physical properties of the fluids. The model divides the total voitage of the bed into internal and external voidage and appropriately distributes the total liquid holdup into internal and external holdup.Over 2500 experimental data, from the present study as well as those reported in literature, are correlated by the model with an r.m.s. deviation of less than ±9%. The significant parameters affecting the two-phase pressure drop are found to be the bed porosity, the Reynolds number, and the product of the Eötvos and the Morton numbers.     

Effect of Surfactants on the Film Drainage

Drainage of a partially mobile thin liquid film between two deformed and nondeformed gas bubbles with different radii is studied. The lubrication approximation is used to obtain the influence of soluble and insoluble surfactants on the velocity of film thinning in the case of quasi-steady state approach. The material properties of the interfaces (surface viscosity, Gibbs elasticity, surface diffusivity, and/or bulk diffusivity) are taken into account. In the case of deformed bubbles the influence of the meniscus is illustrated assuming simple approximated shape for the local film thickness. Simple analytical solutions for large and small values of the interfacial viscosity, and for deformed and nondeformed bubbles, are derived. The correctness of the boundary conditions used in the literature is discussed. The numerical analysis of the governing equation shows the region of transition from partially mobile to immobile interfaces. Quantitative explanation of the following effects is proposed: (i) increase of the mobility due to increasing bulk and surface diffusivities; (ii) role of the surface viscosity, comparable to that of the Gibbs elasticity; and (iii) significant influence of the meniscus on the film drainage due to the increased hydrodynamic resistance. Copyright 1999 Academic Press.     

Pore-Scale Studies on the Stability of Microfoam and the Effect of Parameters on Its Bubble Size

This paper presents a new method to prepare microfoam with excellent stability and high by using a sandpack foam generator. The micromorphology of microfoam were analyzed, and average bubble diameter and uniformity of microfoam were studied by microscope. The stability of xanthan gum-stabilized microfoam and common microfoam at the pore scale was also compared. The results showed that a highly uniform microfoam ranging in size from 10 to 100 µm in diameter with a variable coefficient less than 10% was successfully prepared. The bubble size of the microfoam could be controlled by solution viscosity, gas and liquid flow rate, temperature, and backpressure. The bubble size of microfoam decreased and became uniform with the increase of solution viscosity, total flow rate, and backpressure. The bubble size increased slightly and became non-uniform with the increase of temperature, while the concentration of foaming agent had little effect on the bubble size when above 5000 mg/L. The xanthan gum in the solution increased the viscosity and thickness of liquid membrane, so xanthan gum-stabilized microfoam maintained better stability within microconfined media than common microfoam under condition of 160 g/L salinity, 90°C, and 6 MPa backpressure.     

“Bubble Bunch” phenomenon in operation of a bubble column

While studying the operation of a rectangular bubble column in laboratory scale, it was observed that under certain circumstances tiny bubbles attach to larger bubbles without causing them to coalesce. In other words, bubbles with large diameters (d > 5 mm) swept tiny bubbles (d < 1 mm) in their way to the top of the column resulting in grapelike clusters of bubbles. This phenomenon was named “bubble bunch” by us and its effect on total gas holdup and interfacial area was discussed. Although it was found to have almost no affect on gas holdup, bubble bunch can increase the interfacial area up to 10% more than what is anticipated in the literature. The process of film thinning was modeled for this phenomenon and coalescence efficiency was calculated as a function of interfacial tension.      

Bubble entrainment with drops

The study reported here was undertaken because recent research on nucleate boiling has implicated vapor entrainment by drops as a mechanism for vapor bubble nucleation. The mechanism has been called secondary nucleation. The purpose of this research was to determine the behavior of entrained air bubbles when a drop of liquid strikes a liquid surface. A liquid drop striking the surface of a pool of the same liquid was found usually to entrain large numbers of small air bubbles. Some of these bubbles are frequently carried rapidly deep into the pool by a vortex ring but many can be deposited in a trail or left floating on the surface. Air bubble entrainment was observed with water and several organic liquids and some differences were noted. Drops with diameters from 200 μm to 4 mm were studied. Sometimes hundreds of bubbles were entrained some with diameters up to 100 μm. These results lend support to the secondary nucleation hypothesis and indicate further research on vapor bubble entrainment under conditions more typical of boiling would be appropriate.     

Process characteristics for a gas—liquid system agitated in a vessel equipped with a turbine impeller and tubular baffles

Results of the experimental research on gas hold-up, power consumption for liquid phase and gas—liquid systems, and on residence time of the gas bubbles are presented in the paper for an agitated vessel with a turbine impeller. Distilled water or aqueous solutions of NaCl were used as the liquid phase. Air was dispersed into liquid as the gas phase. The studies were carried out in an agitated vessel of the inner diameter D = 0.634 m. Tubular baffles of the diameter of 0.7D, consisting of 24 vertical tubes of the diameter of 0.016D, were located inside a flat-bottomed tank. Turbines with six blades and the pitch of 90°, 60°, or 45°, respectively, were used for agitation. Measurements were carried out in the range of good dispersion of gas bubbles in the liquid within the turbulent regime of the liquid flow. Effects of the gas bubbles capability to coalesce on the gas hold-up, residence time of the gas bubbles, and power consumption were analyzed. Results of the power consumption (P _G-L /P _o = f ₁(Kg, Fr)) and gas hold-up (φ= f ₂(Kg, We, Y)) were approximated mathematically, using Eqs. (5) and (6), respectively. In Eq. (6), parameter Y was introduced in order to describe the influence of the bubbles capability to coalesce on the gas hold-up. The results of the study show that power consumption does not depend on the capability of bubbles to coalesce, but the pitch of the turbine impeller affects the power characteristics in such a physical system significantly. However, the residence time of the gas phase in agitated liquid depends on the pitch of the impeller blade and on the capability of bubbles to coalesce.     

Nanobubbles at the interface between water and a hydrophobic solid

A very thin layer (5-80 nm) of gas phase, consisting of discrete bubbles with only about 40 000 molecules, is quite stable at the interface between a hydrophobic solid and water. We prepare this gas phase from either ambient air or from CO(2)(g) through a solvent exchange method reported previously. In this work, we examine the interface using attenuated total internal reflection infrared spectroscopy. The presence of rotational fine structure in the spectrum of CO(2) and D(2)O proves that molecules are present in the gas phase at the interface. The air bubbles are stable for more than 4 days, whereas the CO(2) bubbles are only stable for 1-2 h. We determine the average gas pressure inside the CO(2) bubbles from the IR spectrum in two ways: from the width of the rotational fine structure (P(gas) < 2 atm) and from the intensity in the IR spectrum (P(gas) = 1.1 +/- 0.4 atm). The small difference in gas pressure between the bubbles and the ambient (1 atm) is consistent with the long lifetime. The dimensions and curvature of a set of individual bubbles was determined by atomic force microscopy. The pressures of individual bubbles calculated from the measured curvature using the Laplace equation fall into the range P(gas) = 1.0-1.7 atm, which is concordant with the average pressure measured from the IR spectrum. We believe that the difference in stability of the CO(2) bubbles and the air bubbles is due to a combination of the much lower pressure of CO(2) in the atmosphere and the greater solubility of CO(2) in water, compared to N(2) and O(2). As expected, smaller bubbles have a shorter average lifetime than larger bubbles, and the average pressure and the curvature of individual bubbles decreases with time. Surface plasmon resonance measurements provide supporting evidence that the film is in the gas state: the thin film has a lower refractive index than water, and there are few common contaminants that satisfy this condition. Interfacial gas bubbles are not ubiquitous on hydrophobic solids: bubble-free and bubble-decorated hydrophobic interfaces can be routinely prepared.     

Experimental Investigation on Flow Characteristics of Aqueous Foams through the Jet Device and Horizontal Pipe

Aqueous foam is regarded as a versatile medium in numerous scientific and engineering applications due to its high viscosity and low density. The objective of this study is to investigate the flow characteristics of aqueous foams through the jet device and horizontal pipe. The pressure distribution and foam production capacity are measured at different operating conditions. Experimental results show that the pressure fluctuations reduce significantly by increasing the foam liquid concentration, especially in the downstream of jet device. The bubble flow turns into homogeneous foams gradually when the concentration increases from 0.025% to 0.35%, while the foam behaviors take little change at a higher concentration, and the foamability reaches a limit. Subjected to the large pressure difference produced between the top and bottom of horizontal pipe, aqueous foams undergo a gas–liquid separation at a high terminal pressure, resulting in bubbles at the top and liquid at the bottom. Therefore, the terminal pressure should be kept less than a critical value to hold a good foam pattern. Based on the above contributions, it is believed that the study laid an important foundation for the widespread application of foam technology.