Micro-level instability of bubble flows in packings

Bubbly liquid flow through fixed beds and its transition to dispersed-bubble flow is analyzed. The system is described as a compressible fluid filtrating through porous media. The system peculiarities—the isothermal variation of perfect gas in the spherical bubbles, weak compressibility of the liquid, etc.—are accounted for in the state equation for bubbly liquid. Dimensionless criteria were found determining the bubbly-liquid flow in packings at the macro (bed) scale as well as criterion for its stability at the micro (pore) scale. The modeling results are compared with the experimental data.     

Loading,draining and hold-up in periodically operated trickle-bed reactors

Periodic interruption of liquid flow in co-current trickle beds appears to be an attractive mode of operation. For modelling these intermittent-flow reactors, loading and draining times must be known. Experiments were undertaken using beds of activated carbon with water and air as the fluid phases. Loading time was taken as the time to water breakthrough. The gas flow was continuous while the time between the end of drainage and the start of filling was varied to simulate different periods. Drainage experiments followed the liquid flow leaving the bed as a function time. Liquid hold-ups were determined after the filling and draining measurements. Variables considered were particle size, gas and liquid velocity. Loading closely follows the plug flow model; drainage shows tailing but does not follow literature models. Static and dynamic hold-ups at zero gas flow agree with literature correlations for the larger particle size used. A gas velocity effect on both static and dynamic hold-up was observed.     

An investigation of liquid maldistribution in trickle beds

The influence of liquid maldistribution at the top of the packing on flow characteristics in packed beds of gas and liquid cocurrent downflow (trickle beds) is experimentally investigated. Particular attention is paid to the effect of gas and liquid flow rates on flow development. Tests are made in the trickling and pulsing flow regimes. A uniform, a half-blocked and a quarter-blocked liquid distributor is tested. Packings of various sizes and shapes are employed. Data are presented on pressure drop and liquid holdup as well as trickling to pulsing flow transition. Diagnosis of radial and axial liquid distribution is made by means of conductance probes. The effects of liquid foaming, bed pre-wetting, top-bed material, and blockage midway the bed on liquid distribution are also examined. Overall, liquid waves in the pulsing flow regime have a beneficial effect, promoting uniform liquid distribution in the bed cross section.     

Numerical and Experimental Study of Catalyst Loading and Body Effects on a Gas‐Liquid Trickle‐Flow Bed

The influence of tortuosity and fluid volume fractions on trickle‐flow bed performance was analyzed. Hydrodynamics of the gas‐liquid downward flow through trickle beds, filled with industrial trilobe catalysts, were investigated experimentally and numerically. The pressure drop and liquid holdup were measured at different gas and liquid velocities and in two different loading methods, namely, sock and dense catalyst loading. The effect of sharp corners on hydrodynamic parameters was considered in a bed with rectangular cross section. The reactor was simulated, considering a three‐phase model, appropriate porosity function, and interfacial forces based on the Eulerian‐Eulerian approach. Computational fluid dynamics (CFD) simulation results for pressure drop and liquid holdup agreed well with experimental data. Finally, the velocity distribution in two types of loading and the effect of bed geometry in CFD results demonstrated that pressure drop and liquid holdup were reduced compared to a cylindrical one due to high voidage at sharp corners.     

Hydrodynamics of gas–liquid flow in micropacked beds: Pressure drop,liquid holdup,and two‐phase model

Hydrodynamics of gas–liquid two‐phase flow in micropacked beds are studied with a new experimental setup. The pressure drop, residence time distribution, and liquid holdup are measured with gas and liquid flow rates varying from 4 to 14 sccm and 0.1 to 1 mL/min, respectively. Key parameters are identified to control the experimentally observed hydrodynamics, including transient start‐up procedure, gas and liquid superficial velocities, particle and packed bed diameters, and physical properties of the liquids. Contrary to conventional large packed beds, our results demonstrate that in these microsystems, capillary forces have a large effect on pressure drop and liquid holdup, while gravity can be neglected. A mathematical model describes the hydrodynamics in the micropacked beds by considering the contribution of capillary forces, and its predictions are in good agreement with experimental data. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4694–4704, 2017     

Two-phase flow hydrodynamic study in micro-packed beds – Effect of bed geometry and particle size

Microscopic visualizations nearby the wall region of micro-fixed beds and hydrodynamic measurements during gas–liquid two-phase flows were carried out with an aim to investigate the effect of particle size and capillary tube shape on the bed pressure drop, flow regime transition, hysteresis and bed transient response to flow-rate step perturbations. Visualizations through inverted microscopy revealed that a decrease in particle size leads to early inception of a high interaction flow regime whereas changing capillary shape from circular to square had no effect on flow regime changeover. The effect of particle size on the wetting pattern hysteresis in square micro-packed beds was also investigated in both imbibition and drainage paths. It was found that wetting pattern hysteresis decreases with a decrease in particle size. Finally, the transient behavior of micro-fixed beds of circular and square geometries packed with particles of two different sizes were studied by monitoring the bed pressure drop variations upon step changes in liquid flow rate at iso-G conditions. Larger particle sizes and square geometry showed shorter transient times as compared to smaller particle sizes and circular geometry.     

Hydrodynamics of a high pressure three‐phase fluidized bed subject to foaming

The effects of pressure and surfactants on the phase holdups and flow regime transition velocities of gas–liquid–solid fluidized beds were investigated. The effect of pressure on the bed phase holdups is significant and more pronounced at larger gas flow rates where pressure has a greater effect on the equilibrium bubble size. The addition of a surfactant leads to an increase in the gas holdup and a lowering of the solids and liquid holdups. The presence of a surfactant with a liquid flow results in shearing of the bubbles across the gas–liquid distributor, limiting the effect of pressure. Finally, for all conditions, gas holdups in the freeboard region were greater than in the bed.     

Gas-liquid mass transfer coefficients for cocurrent upflow in packed beds — effect of packing shape at low flow rates

Gas-liquid mass transfer coefficients were determined by oxygen desorption for cocurrent upflow in packed beds. Data are included for seven different catalyst supports, four of which have not appeared previously in the literature. Very low flow rates (liquid: 0.9–6.0 mm/s, gas; 1.5–31 mm/s) of interest in pilot-scale catalytic hydrogenation, were investigated. The results correlated well with liquid and gas flow rates for each packing. In addition, the data for all packings were correlated reasonably well by the energy dissipated in the liquid phase.     

Gas-liquid mass transfer coefficients for cocurrent upflow in packed beds — Effect of packing shape at low flow rates

Gas-liquid mass transfer coefficients were determined by oxygen desorption for cocurrent upflow in packed beds. Data are included for seven different catalyst supports, four of which have not appeared previously in the literature. Very low flow rates (liquid: 0.9–6.0 mm/s, gas; 1.5–31 mm/s) of interest in pilot-scale catalytic hydrogenation, were investigated. The results correlated well with liquid and gas flow rates for each packing. In addition, the data for all packings were correlated reasonably well by the energy dissipated in the liquid phase.     

Multiple hydrodynamic states in trickle flow: Quantifying the extent of pressure drop, liquid holdup and gas-liquid mass transfer variation

It is well established that pressure drop and liquid holdup under trickle flow conditions are functions of the flow history. However, the extent of possible variation of these and other critical hydrodynamic parameters has not been fully quantified. In this study, specifically defined prewetting procedures are used as limiting cases for hydrodynamic hysteresis. These are:

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Non-prewetted.

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Levec prewetted: the bed is flooded and drained and after residual holdup stabilisation the gas and liquid flows are introduced.

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Kan_L prewetted: the bed is operated in the pulse flow regime (by increasing liquid velocity) after which liquid flow rate is reduced to the desired set point (all at the desired gas flow rate).

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Kan_G prewetted: the bed is operated in the pulse flow regime (by increasing gas velocity) after which gas flow rate is reduced to the desired set point (all at the desired liquid flow rate).

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Super prewetted: the bed is flooded and gas and liquid flows are introduced once draining commences.

It is shown that the upper limiting case for pressure drop is the Kan_L mode of operation. The lower limiting cases are the non-prewetted and Levec prewetted modes (these coincide). Pressure drop may vary by as much as 700% even for prewetted beds. Liquid holdup is different in all five prewetting modes. The upper limiting case is the Kan_G mode of operation, while the lower limiting case is the non-prewetted mode (Kan_G holdup is approximately 160% that of non-prewetted mode holdup at ). At low gas velocities the Kan_L holdup can be 400% of that of the non-prewetted beds. Importantly, the lower limiting case for prewetted beds is the Levec mode. Holdup in the Kan_G mode may be as much as 130% of the holdup in the Levec mode (at ).The effect of hydrodynamic multiplicity of the volumetric mass transfer coefficient is measured by the desorption of oxygen from water into nitrogen. In this case the different prewetting procedures result in three distinct regions, the upper region being the Kan and Super prewetted beds, the intermediate region being the Levec prewetted bed and the lower region being the dry bed. Mass transfer coefficients in the upper region can be as much as 600% of that of the lower region and 250% of that of the intermediate region. Evidently, prewetting (and even pulsing flow prewetting) does not guarantee that the bed is operating at the maximum values of pressure drop, holdup and mass transfer coefficient. Evidence of operation in between the limiting cases is presented. These non-limiting cases can be reached in multiple ways.     

Studies in the vaporization of mercury in irrigated packed beds

Mass transfer coefficients have been measured for the vaporization of mercury flowing countercurrent to air in irrigated packed beds of spheres and Raschig rings. The measured coefficients increased with gas and liquid flow rates, and were correlated in terms of gas Reynolds number and liquid rate. The mass transfer data for liquid metal irrigation were lower than published data for wetting aqueous systems, due to the non-wetting nature of liquid metals. The lower mass transfer coefficients are believed to be attributed to a lower interfacial area for the non-wetting flow of liquid metals, although direct experimental proof was not obtained. The present results are in agreement with data for zinc absorption in molten lead in packed bed (Warner, 1959) when correlated in terms of the relative velocity and total liquid holdup. The results suggest that for liquid metal irrigated beds, the total hold-up is effective in gas phase transfer processes.     

Modeling of gas–liquid flow in a rotating packed bed using an Eulerian multi-fluid approach

Rotating packed beds (RPBs) are ideal candidates for CO₂ removal from offshore natural gas due to their good mass transfer performance and significant volume savings. This article proposes an Eulerian multi-fluid approach to simulate the gas–liquid flow in RPBs. Three new multiphase drag force models are constructed based on single-phase drag force models for wire mesh packings. Based on the Eulerian multi-fluid approach, a new RPB simulation framework is developed. The predicted results using the new simulation framework with the new drag force models are compared with the experimental data. When using the Kołodziej model and the modified Kołodziej model, the predicted overall liquid holdup shows good agreement with the experimental data with errors less than 20%. In addition, the pressure drop predicted by these three models are reasonable compared with the experimental data. This work lays a foundation for RPB simulation of gas–liquid flow using Eulerian multi-fluid approach.     

Role of gas phase in the deposition dynamics of fine particles in trickle-bed reactors

Experiments were conducted to study the role of gas velocity in the capture of fine particles from non-aqueous suspensions circulated in co-current down-flow trickle flow reactors. The rate of filtration and pressure drop in the trickle bed were investigated using surfactant-stabilized kaolin-containing kerosene suspensions. It was determined that the filter coefficient was sensitive to liquid holdup and specific deposit. The initial collection efficiencies were compared with predictions based on existing theories. Agreement was generally not good with the exception for the limit of low superficial gas velocity. A general correlation establishing the relationship between the filtration rate and the liquid holdup in trickle beds was proposed to reconcile the experimental data with existing filtration theories.     

THE INVERSE FLUIDIZATION AIRLIFT BIOREACTOR, PART I: HYDRODYNAMIC STUDIES

The inverse fiuidization airlift bioreactor offers a simple solution to the problem of handling shear sensitive cultures and/or systems requiring biofilm growth control. This unit combines in a single vessel the hydrodynamic behavior of concentric tube airlift aerators or contactors and liquid-solid fluidized beds. The effects of the diameter of the concentric tubes, liquid level, bed properties and gas flow rate on gas holdup, bed expansion and liquid circulation velocity were investigated in this study. The Zuber and Findlay relation (1965) gives satisfactory results for gas holdup. The inverse bed expansion can be predicted by the Richardson and Zaki correlation (1953). The liquid circulation velocity can be calculated using a mechanical energy balance     

RADIAL DISPERSION AND BUBBLE CHARACTERISTICS IN THREE-PHASE FLUIDIZED BEDS

The effects of gas and liquid velocities, liquid viscosity and particle size on the radial dispersion coefficient of liquid phase (D_r) and the bubble properties in three-phase fluidized beds have been determined. A new flow regime map based on the drift flux theory in three-phase fluidized beds has been proposed.

In three-phase fluidized beds, D, increases with increasing gas velocity in the bubble coalescing and in the slug flow regimes, but it decreases in the bubble disintegrating regime. The coefficient exhibits a maximum value in the bed of small particles with increasing liquid velocity at lower gas velocities. However, it increases with increasing liquid velocity at higher gas velocities. In two and three-phase fluidized beds of larger particles (6,8 mm), D_r exhibits a maximum value with an increase in liquid viscosity at lower gas velocities, but it increases at higher gas velocities. The mean bubble chord length and its rising velocity increase with increasing gas velocity and liquid viscosity. However, the bubble chord length decreases with an increase in liquid velocity and it exhibits a maximum value with increasing particle size in the bed. The radial dispersion coefficients in the bubble coalescing and disintegrating regimes of three-phase fluidized beds in terms of the Peclet number in the present and previous studies have been well represented by the correlations based on the concept of isotropic turbulence theory.     

Hydrodynamics of power-law fluids in trickle-flow reactors: Mechanistic model, experimental verification and simulations

A fully predictive one-dimensional mechanistic model was developed for describing the hydrodynamics of power-law fluids in trickle-bed reactors. The model is a generalization of the slit approach to the case of non-Newtonian fluids obeying Ostwald-deWaele rheological behavior. Without recourse to adjustable parameters, the proposed model enabled prediction of the experimental values of (i) total two-phase total pressure drop and total liquid holdup in the trickle flow regime, (ii) frictional pressure drop in single-phase flows through packed beds, and (iii) total liquid holdup in gravity driven liquid downflow and stagnant gas through packed beds. Parametric simulations guided by knowledge of the behavior of highly viscous Newtonian liquids in trickle beds highlighted the capability of the model in the simulation and design of trickle flow operation using power-law fluids.     

返混对气-固反应特性测试和活化能表征的影响

采用冷态研究和高温反应相结合的方法,探究了气-固反应过程中返混对反应特性和动力学的影响。首先利用脉冲示踪法考察微型流化床内的气体停留时间分布规律和气体返混特性,揭示床内径D、表观气速U_g、介质颗粒粒径d_p对床内气体返混程度的影响,并分析气体流动偏离平推流的程度,得到其最大程度接近平推流的操作范围。进而,选取活性焦燃烧这一典型气-固反应,分析微型流化床反应分析仪中不同程度的返混状态下等温燃烧的反应行为和活化能演变,再现了D、U_g、d_p对反应测试结果的影响,即：随着床径减小及表观气速和介质颗粒粒径的增大,反应器内产物气体经历的返混程度减小,使得产物气体近平推流的输出并被即时检测,获得更好揭示反应本质特性的反应活化能。活化能数值随返混程度的减少而增大,且更易达到稳定反应状态。     

Influence of an electrochemically generated gas phase on pressure drop in a three-dimensional electrode and an inert bed

The influence of an electrochemically generated gas phase on the pressure drop in a three-dimensional electrode and an inert bed has been investigated. The electrode was composed of silvered glass particles, whereas the inert bed was composed of glass particles of the same diameter. During water electrolysis smaller gas bubbles are generated than by introducing gas into the system by means of a fluid distributor. Electrochemically generated bubbles with the liquid phase circulate upward through the electrode and the inert bed. The pressure drop for a monophase fluid was measured, as well as the change of pressure drop with time in the electrode and the inert bed for two-phase flow of fluid through an electrochemical cell. The steady and unsteady periods of the change of pressure drop have been discussed. Experiments were carried out at different electrolyte velocities and for different current densities. Higher electrolyte velocities cause an increase in the pressure drop in both beds. Also, increased current density causes an increase in gas evolution intensity at the electrode thereby increasing the pressure drop in both beds. A mathematical model describing the change of pressure drop with time has been proposed. The proposed model showed good agreement with experimental results as well as the results from the literature.     

气-液-固三相逆流化床热量传递研究

在内径0.152 m,高2.5 m的气-液-固三相逆流化床中系统研究了热量传递特性特性。获得了气体和液体速度及聚乙烯和聚丙烯颗粒密度对内置加热器与床层间热量传递系数的影响规律。研究结果表明密度相对高的聚乙烯颗粒的逆流化床的热量传递系数比密度相对低的聚丙烯颗粒的逆流化床的热量传递系数大;随着气体速度的增加,热量传递系数增加。然而,随着液体速度增加,热量传递系数具有最大值。在热量传递系数达到最大值时对应的液体速度随着颗粒密度或气体速度的增加而降低。     

高速气固流化床局部瞬态行为混沌分析

在内径 1 86mm、高 9m的提升管中 ,通过改变操作气速获得湍动流化床、快速流化床和稀相输送典型的操作状态 ,使用光纤密度探头在轴向不同截面的不同径向位置处测得密度脉动时间序列 ,以反映不同流型下 ,不同浓、稀相区的瞬态动力学行为 .利用确定性混沌理论分析了局部密度脉动信息 ,以Kolmogorov熵定量地表征系统的动力学特征 .实验结果表明 ,Kolmogorov熵随流型的变化能正确地反映相应流型的特点 ,可用来描述不同流型下的流动结构 .在忽略壁面附近行为的条件下 ,对 3种流型、不同的浓稀相区 ,局部熵值随局部固含率变化可分为两个区 :当局部固含率高于 0 0 5时 ,熵几乎不随局部固含率的变化而变化 ;当局部固含率低于 0 0 5时 ,熵随局部固含率的减小而急剧增大 .这说明系统的混沌特征与局部固含率关系密切 ,局部固含率对系统局部动态行为起控制作用 .