Bubble size and radial gas hold‐up distributions in a slurry bubble column using ultrafast electron beam X‐ray tomography

Gas hold‐up and bubble size distribution in a slurry bubble column (SBC) were measured using the advanced noninvasive ultrafast electron beam X‐ray tomography technique. Experiments have been performed in a cylindrical column (D_T = 0.07 m) with air and water as the gas and liquid phase and spherical glass particles (d_P = 100 μm) as solids. The effects of solid concentration (0 ≤ C_s ≤ 0.36) and superficial gas velocity (0.02 ≤ U_G ≤ 0.05 m/s) on the flow structure, radial gas hold‐up profile and approximate bubble size distribution at different column heights in a SBC were studied. Bubble coalescence regime was observed with addition of solid particles; however, at higher solid concentrations, larger bubble slugs were found to break‐up. The approximate bubble size distribution and radial gas hold‐up was found to be dependent on U_G and C_s. The average bubble diameter calculated from the approximate bubble size distribution was increasing with increase of U_G. The average gas hold‐up was calculated as a function of U_G and agrees satisfactorily with previously published findings. The average gas hold‐up was also predicted as a function of C_s and agrees well for low C_s and disagrees for high C_s with findings of previous literature. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1709–1722, 2013     

Predictive models for the viscosities of multicomponent liquid n‐Alkane and regular solutions

The pseudo‐binary model developed by Wu and Asfour (1992) has been used to modify some existing viscosity predictive models to that they can predict the viscosities of multicomponent liquid n‐alkane and regular mixtures. The McAllister three‐body interaction model and the Grunberg‐Nissan viscosity equation were both employed, after modification, to successfully predict the viscosity of multicomponent liquid n‐alkane and regular solutions. The modified Generalized Corresponding States Principel (MGCSP) reported by Wu and Asfour (1992), for only predicting the viscosities of n‐alkane mixtures, has been extended to predit the viscosities of multicomponent regular solutions. The predictive capabilities of several predictive models were analyzed. The obtained results showed that the pseudo‐binary McAllister model predicts the viscosity data better than the other existing predictive models.     

Particle dispersion and fluctuations in viscous liquid–solid fluidised Beds

Particle dispersion and fluctuations were investigated in a liquid–solid fluidised bed with viscous liquid media by employing the relaxation method based on the stochastic model. The bed expansion or contraction during the transient state was analysed by means of the histogram of pressure drop variation with elapsed time. The relaxation behaviour of the liquid–solid fluidised bed appeared to be heterogeneous followed by homogeneous expansion or contraction. Effects of liquid velocity (U_L), particle size (d_P), liquid viscosity (µ_L) and liquid holdup or bed porosity (ε_L) on the fluctuating frequency (F) and dispersion coefficient (D_P) of fluidised solid particles were examined. The values of F and D_P increased with an increase in d_P but decreased with µ_L and exhibited local maxima with variations of U_L and ε_L. The flow pattern of solid particles could be changed from uniform or pseudo‐homogeneous to turbulent random behaviour with an increase in the liquid velocity and liquid holdup or bed porosity. © 2012 Canadian Society for Chemical Engineering     

Mass Transfer Characteristics of Syngas Components in Slurry System at Industrial Conditions

The gas‐liquid mass transfer behavior of syngas components, H₂ and CO, has been studied in a three‐phase bubble column reactor at industrial conditions. The influences of the main operating conditions, such as temperature, pressure, superficial gas velocity and solid concentration, have been studied systematically. The volumetric liquid‐side mass transfer coefficient k_La is obtained by measuring the dissolution rate of H₂ and CO. The gas holdup and the bubble size distribution in the reactor are measured by an optical fiber technique, the specific gas‐liquid interfacial area aand the liquid‐side mass transfer coefficient k_L are calculated based on the experimental measurements. Empirical correlations are proposed to predict k_L and a values for H₂ and CO in liquid paraffin/solid particles slurry bubble column reactors.     

Flow behavior of wakes in a three-phase slurry bubble column with viscous liquid medium

Flow behavior of wakes has been investigated in a three-phase slurry bubble column of 0.102 m ID and 1.5 m in height. The dependence of wake characteristics such as rising velocity, frequency, holdup and equivalent size on the operating variables was examined by employing an electric resistivity probe method. The gas velocity, liquid viscosity and solid content in the slurry phase were chosen as independent parameters. The rising velocity of wake region increased with an increase in the gas velocity (4.0–12.0 cm/s), liquid viscosity (1.0–50.0 mPa·s) or solid content (0–25 wt%) in the slurry phase. The frequency and holdup of wake phase increased with increasing gas velocity, but decreased with increasing liquid viscosity or solid content in the slurry phase. The equivalent size of wake phase increased with increasing gas velocity, liquid viscosity or solid content in the slurry phase. The wake properties and holdup were well correlated with operation variables within these experimental conditions.     

并行微通道内气液相分配规律

微反应器的集成放大对于微化工技术的工业应用具有重要意义。利用高速摄像仪对4个并行微通道内气液两相流动状况及相分配规律进行了研究,考察了气液两相流量及液相黏度对两相分布均匀性的影响。实验所用液相为含0.3%表面活性剂十二烷基硫酸钠（SDS）的蒸馏水-甘油溶液,气相为氮气（N₂）。实验观察到了6种典型的两相流型。对各支通道均为弹状流情况下气泡长度和气泡速度的分布规律进行了研究。在一定气相流率下,各支通道气泡长度的相对标准偏差随液相流率的增大而增大,气泡速度的相对标准偏差值随液相流率的增大先升高到一定值然后逐渐减小。气相分配不均匀性随液相流率和黏度的增大而增大,液相分配不均匀性随液相黏度的增大而减小,气相流率的变化对于两相分布影响不明显。研究结果有助于并行微通道的结构设计与优化,以实现更为均匀的气液两相流动分配。     

A CFD-PBM coupled model under entire turbulent spectrum for simulating a bubble column with highly viscous media

To account for the effect of liquid viscosity, the bubble breakup model considering turbulent eddy collision based on the inertial subrange turbulent spectrum was extended to the entire turbulent spectrum that included the energy-containing, inertial, and energy-dissipation subranges. The computational fluid dynamics-population balance model coupled model was modified to include this extended bubble breakup model for simulations of a bubble column. The effect of turbulent energy spectrum on the bubble breakup and hydrodynamic behaviors was studied in a bubble column under different liquid viscosities. The results showed that when the liquid viscosity was <80 mPa s, the bubble breakup and hydrodynamics were almost independent on the turbulent spectrum. At liquid viscosity >80 mPa s, the bubble breakup rate and gas holdup were significantly under-predicted when the inertial turbulent spectrum was used, and when using the entire turbulent spectrum the predictions were more consistent with experimental data.     

Multiple effects of operating variables on the bubble properties in three-phase slurry bubble columns

Flow properties of gas phase reactants such as size, rising velocity and frequency were investigated in simulated three-phase slurry bubble column reactors. Effects of gas velocity, reactor pressure, liquid viscosity, solid content in the slurry phase and column diameter on the flow properties of a gas reactant were determined. The multiple effects of operating variables on the bubble properties were well visualized by means of contour maps. The effects of operating variables on the flow properties of bubbles changed with changing column diameter of the reactor. The size, rising velocity and frequency of reactant gas bubbles were well correlated in terms of operating variables including column diameter of the reactor. This work was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

Gas hold‐up in bubble columns: Operation with concentrated slurries versus high viscosity liquid

The hydrodynamics of bubble columns with concentrated slurries of paraffin oil (density, ρ_L = 790 kg/m³; viscosity, μ_L = 0.0029 Pa·s; surface tension, σ = 0.028 N·m¹) containing silica particles (mean particle diameter d_p = 38 μm) has been studied in columns of three different diameters, 0.1, 0.19 and 0.38 m. With increasing particle concentration, the total gas hold‐up decreases significantly. This decrease is primarily caused by the destruction of the small bubble population. The hold‐up of large bubbles is practically independent of the slurry concentration. The measured gas hold‐up with the 36% v paraffin oil slurry shows remarkable agreement with the corresponding data obtained with Tellus oil (ρ_L = 862 kg/m³; μ_L = 0.075 Pa·s; σ = 0.028 N·m^?1) as the liquid phase. Dynamic gas disengagement experiments confirm that the gas dispersion in Tellus oil also consists predominantly of large bubbles. The large bubble hold‐up is found to decrease significantly with increasing column diameter. A model is developed for estimation of the large bubble gas hold‐up by introduction of an wake‐acceleration factor into the Davies‐Taylor‐Collins relation (Collins, 1967), describing the influence of the column diameter on the rise velocity of an isolated spherical cap bubble.     

On the leakage flow around gas bubbles in slug flow in a microchannel

The leakage flow is that liquid does not push gas bubbles and leaks through the channel corners. This leakage flow was confirmed by tracking particles moving in the liquid film with a double light path method and was quantified by tracking the gas–liquid interface movement. The results show that leakage flow varies during bubble formation process. The average net leakage flow Q_net‐leak in a bubble formation cycle at T‐junction can be as large as 62.4% of the feeding liquid flow rate, depending on the liquid properties. Q_net‐leak for regular flow at main channel is much smaller, ranging from about 0 to 30% of the feeding liquid flow rate. The difference between the two leakage flows would lead to an increase in liquid slug length after generation. Finally, the effects of parameters such as phase flow rates, surface tension, and viscosity were investigated. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3964–3972, 2015     

Experimental Study on Bubble Rising and Descending Velocity Distribution in a Slurry Bubble Column Reactor

To determine bubble rising and descending velocity simultaneously, a BVW‐2 four‐channel conductivity probe bubble parameters apparatus and its analysis are used in gas‐liquid and gas‐liquid‐solid bubble columns. The column is 100 mm in internal diameter and 1500 mm in height. The solid particles used are glass beads with an average diameter of 17.82 μm, representing typical particle size for catalytic slurry reactors. The effects of superficial gas velocity (1.0 cm/s ≤ U_g ≤ 6.4 cm/s), solid holdup (0 % ≤ ?_s ≤ 30 %), and radial location (r/R = 0, 0.4, and 0.7) on bubble velocity distributions are determined. It is found that increasing U_g can increase the velocity of bubbles but do not exert much influence on bubble velocity distribution. Solid holdup mainly affects the distribution of bubble velocity while the radial direction affects bubble velocity distribution only slightly. The ratio of descending bubbles to rising bubbles increases from the bubble column center to the wall. It can be proved experimentally that large bubbles do not always rise faster than small bubbles at higher U_g (for example 6.4 cm/s).     

Mechanism of bubble formation in step-emulsification devices

Step-emulsification device is an emerging technology for bubble generation, which is convenient for numbering-up. However, the mechanism of bubble formation in such devices remains unclear. In this article, the mechanism of bubble formation in step-emulsification devices is investigated by using a high-speed camera. First, the evolution of gas–liquid interface with time during bubble formation is observed. Second, the variation of characteristic parameters of bubbles, such as bubble's volume V_B and generation frequency f, with gas and liquid flow rates is described under various liquid viscosities and step widths. Meanwhile, the coupling law of interface evolution with characteristic parameters of bubbles is explored. Finally, the basic guideline for generation of monodispersed bubbles in step-emulsification is put forward: low liquid viscosity. Connecting the interface evolution with the characteristic parameters, predictive models for bubble's volume, and generation frequency in step-emulsification are proposed, respectively.     

鼓泡塔反应器气液两相流CFD数值模拟

对圆柱形鼓泡塔反应器内的气液两相流动进行了三维瞬态数值模拟,模拟的表观气速范围为0.02～0.30 m&#8226;s^-1; 模拟采用了双流体模型,并耦合了气泡界面密度单方程模型预测气泡尺寸,该模型考虑了气泡聚并与破碎对气泡尺寸的影响。液相湍流采用考虑气相影响的修正k-ε模型,两相间的动量传输仅考虑曳力作用。模拟获得了轴向气/液相速度分布、气含率分布、湍流动能分布以及气泡表面面积密度等,对部分模拟结果与实验值进行了定量比较,结果表明模拟结果与实验结果吻合较好。     

CFD–DEM simulations of wet particles fluidization with a new evolution model for liquid bridge

A new model for liquid-bridge evolution with consideration of particle dynamics, is proposed to improve Computational Fluid Dynamics-Discrete Element Method (CFD–DEM) simulations of wet particles fluidization under high liquid loading and viscosity. A liquid bridge is allowed to form and remains stable only when the normal relative velocity of two particles is lower than a critical value v _nc. A large v _nc leads to an increase of liquid-bridge or cohesive force. The model can be reduced to the conventional liquid-bridge model in literature when v _nc = 0 or ∞. With the new model, the prediction of bubble properties including bubble center, aspect ratio, and volume agrees well with the experimental data in literature. In particular, under high liquid loading, bubble disintegration due to particle agglomerating is reasonably captured. The simulations demonstrate the advantage of the new model that can extend the liquid-bridge models and CFD–DEM for high liquid loading and viscosity.     

Gas‐Liquid Mass Transfer in a Slurry Bubble Column at High Slurry Concentrations and High Gas Velocities

The volumetric mass transfer coefficient k_La in a 0.1 m‐diameter bubble column was studied for an air‐slurry system. A C₉‐C₁₁ n‐paraffin oil was employed as the liquid phase with fine alumina catalyst carrier particles used as the solid phase. The n‐paraffin oil had properties similar to those of the liquid phase in a commercial Fischer‐Tropsch reactor under reaction conditions. The superficial gas velocity U_G was varied in the range of 0.01 to 0.8 m/s, spanning both the homogeneous and heterogeneous flow regimes. The slurry concentration ?_S ranged from 0 to 0.5. The experimental results obtained show that the gas hold‐up ?_G decreases with an increase in slurry concentration, with this decrease being most significant when ?_S < 0.2. k_La/?_G was found to be practically independent of the superficial gas velocity when U_G > 0.1 m/s is taking on values predominantly between 0.4 and 0.6 s^–1 when ?_S = 0.1 to 0.4, and 0.29 s^–1, when ?_S = 0.5. This study provides a practical means for estimating the volumetric mass transfer coefficient k_La in an industrial‐size bubble column slurry reactor, with a particular focus on the Fischer‐Tropsch process as well as high gas velocities and high slurry concentrations.     

Pressure drop in two phase cocurrent upward flow in packed beds: Air/non-newtonian liquid systems

New data on the two phase pressure drop for the concurrent upflow of air-liquid (Newtonian and non-Newtonian) mixtures through packed beds of spherical and non-spherical particles are presented. The results for single phase flows and for the air-Newtonian liquid mixtures have been used both to gauge the overall accuracy of the present experimental methods and to evaluate the validity of the predictive expressions available in the literature. The two phase pressure drop has been measured as a function of the liquid and gas flow rates, column diameter and the power law model constants. Depending upon a suitable combination of the gas and liquid fluxes and the power law index, the two phase pressure drop may be less than its value for the flow of liquid alone. A simple expression is proposed which correlates the present set of experiments (nearly 500 data points) with satisfactory levels of accuracy over the following ranges of conditions: 0.54 ≤ n ≤ 1; 0.001 ≤ Re_L^* ≤ 50; 3.7 ≤ Re_G ≤ 177 and 0.9 ≤χ (Lockhart-Martinelli parameter) ≤ 104.     

Large Bubble Sizes and Rise Velocities in a Bubble Column Slurry Reactor

The results are reported of an experimental study of the gas holdup, ?_G, large bubble diameter, d_Lb, and large bubble rise velocity, V_Lb, in a 0.1 m wide, 0.02 m deep and 0.95 m high rectangular slurry bubble column operated at ambient temperature and pressure conditions. The superficial gas velocity U was varied in the range of 0–0.2 m/s, spanning both the homogeneous and heterogeneous flow regimes. Air was used as the gas phase. The liquid phase used was C₉‐C₁₁ paraffin oil containing varying volume fractions (?_S = 0, 0.05, 0.10, 0.15, 0.20 and 0.25) of porous catalyst (alumina catalyst support, 10 % < 10 μm; 50 % < 16 μm; 90 % < 39 μm). With increasing slurry concentrations, ?_G is significantly reduced due to enhanced bubble coalescence and for high slurry concentrations the “small” bubbles are significantly reduced in number. By the use of video imaging techniques, it was shown that the large bubble diameter is practically independent of the gas velocity for ?_S > 0.05 and U > 0.1 m/s. The measured large bubble rise velocity V_Lb agrees with the predictions of a modified Davis‐Taylor relationship.     

Viscosity and rheological behavior of microbubbles in capillary tubes

The viscosity of microbubbles has been measured in capillary tubes. Experiments were conducted in tubes of different diameters and lengths, with a constant microbubble concentration. The effects of bubble void fraction and size distribution on the viscosity of microbubbles were also investigated. Microbubbles demonstrate shear‐thinning non‐Newtonian behavior. The viscosity of microbubbles decreases with a decrease in tube diameter and bubble void fraction, and with an increase in tube length. Although viscosity changes with tube dimensions, the flow index (n^′) is only influenced by the microbubble void fraction. It is also found that bubble size distribution in the range (1–12 µm) used in this study does not affect the viscosity of microbubbles. The data were then used to develop a correlation to predict viscosity of microbubbles, which represents the experimental viscosity data with an absolute average relative deviation less than 1.3%. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2660–2669, 2014     

Flow regime transition identification in three phase co‐current bubble columns

Bubble columns have wide applications in absorption, bio‐reactions, catalytic slurry reactions, coal liquefaction; and are simple to operate, have less operating costs; provide good heat and mass transfer. Experiments have been performed for identifying transition regime in a 15 cm diameter bubble column with liquid phase as water and air as the gas phase. Glass beads of mean diameter 35 µm have been used as solid phase. The superficial gas velocity is in the range 0 ≤ U_g ≤ 16.3 cm/s and superficial liquid velocity in the range of 0 ≤ U_l ≤ 12.26 cm/s. Solid loading up to 9% (w/v) has been used. Pressure signals have been measured using differential pressure transducers (DPTs) at four different axial locations. Classical analysis (Wallis approach and Zuber–Findlay approach), Statistical analysis and Fractal analysis have been used for regime transition identification. Statistical analysis and Fractal analysis have shown almost the same transition points for all the liquid and gas velocities. Effect of solid concentration, liquid velocity and gas velocity over transition regime has also been studied. As the solid concentration is increased it has insignificant effect over transition regime for lower values (<1%), while transition values decrease for higher solid concentration (>1%). © 2012 Canadian Society for Chemical Engineering     

Free coating of viscous and viscoelastic liquids onto a partially submerged rotating roll

Experiments are performed on the dynamics of formation of a liquid coating picked up by a cylindrical roll rotating partially submerged in a free bath. Data on coating thickness for Newtonian fluids are found to correlate according to T = H(ρg/μU)^1/2 = 0.56 for fluids whose viscosities range from 0.11 to 33 poise. Data for strongly non-Newtonian and Viscoelastic fluids (polyacrylamide solutions) can be forced to fit this correlation by defining an “equivalent coating viscosity.” It is clear that this defines a pseudo viscosity, because the “equivalent coating viscosity” is observed to increase with increasing roll speed. This suggests that strongly Viscoelastic fluids respond to the rapid deformation suddenly imposed in the dynamic meniscus near the pickup point in a distinctly elastic manner that alters the flow through the meniscus.