Bubble column with internals: Effects on hydrodynamics and local heat transfer

Local heat transfer and column hydrodynamics are investigated in a bubble column in presence of internals of different configurations. The liquid phase used is tap water and the gas phase is oil-free compressed air. The gas velocity is varied over a wide range of 0.03–0.35 m/s. The heat transfer variations are measured with a fast response probe capable of capturing bubble dynamics as well as detect local flow direction and deduce local liquid velocity. Measurements obtained in presence of internals are compared with those without internals to elucidate the effects of different internals design. Comparisons are based on average values and temporal variations obtained with the fast response probe. The average gas holdup, local liquid velocity and bubble fractions holdups obtained with and without internals are also compared to further point out the differences. The observed differences are discussed based on the insights provided by these comparisons. The results obtained show influence of internals design on column hydrodynamics which need to be considered for their proper design and modeling.     

微涡流技术在造气污水处理中的应用

介绍了微涡流技术的原理及工艺流程.造气污水采用微涡流澄清技术,在药剂和涡漩的作用下,迅速絮凝、沉降,从而达到深度净化,改善水质,最后实现污水闭路循环、保护环境和提高水资源利用率的目的.     

壁面粗糙度对通道流动特性的影响

从管道壁面附近的流动结构着手,指出摩阻偏差系数不仅取决于相对粗糙度,还取决于粗糙元疏密度、流动再附长度等具体几何、水力特征,粗糙度对流阻的影响能否忽略不能单纯以相对粗糙度是否小于5%为标准。采用流道收缩效应的处理方法,理论地分析这些特征对偏差系数的影响,揭示出偏差系数随粗糙元密度与Re的增加而增加,粗糙元分布很疏或者Re很小时粗糙度的影响可以忽略的规律。     

CFD studies coupling hydrodynamics and solid‐liquid mass transfer in slug flow for matter removal from tube walls

Organic matter deposition on internal surfaces constitutes a drawback that impairs the efficiency of several industrial processes. To overcome this problem, sparging a train of bubbles could be useful since its presence strongly increases the wall shear stress. A detailed numerical mass‐transfer study between a finite soluble wall and the liquid around a rising Taylor bubble was performed, simultaneously solving velocity and concentration fields. The bubble passage throws solute backward and is responsible for radial dispersion. There is also an increase in the transfer rate with enhancements between 10 and 20% (depending on liquid average velocity and bubble length) compared to single‐phase flow. Mass‐transfer coefficients along the different hydrodynamic regions around the bubble nose, liquid film, and wake were characterized and their values compared with those from literature. The results suggest a promising potential of bubble train flow to enhance organic matter removal from walls in biological systems. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2420–2439, 2017     

The hydrodynamics of trickling flow in packed beds operating at high pressures. The relative permeability concept

One of the largest experimental databases of measured pressure drops and liquid holdups in high-pressure trickle-bed reactors is presented in order to evaluate the currently existing models for the prediction of hydrodynamic parameters of cocurrent two-phase flow through packed beds. Our findings support the conclusions of Carbonell (Oil & Gas Science and Technology—Revue de l'IFP 55 (2000) 417) based on theoretical analysis of existing models that only the relative permeability model and the fluid–fluid interaction model are based on solid hydrodynamic principles, which are able to predict the hydrodynamic parameters within the experimental error. Special emphasis has been given to the relative permeability model to demonstrate its practicalness in describing the complex phenomena existing within the two-phase flow through porous media.     

Effects of hydrodynamics and Lagrangian transport on chemically reacting bubble flows

The objective of this study is to investigate the dynamics of flows occurring in the wakes of rising bubbles of different shapes and sizes. Different wake dynamics can result in qualitatively different mixing characteristics. In the case of fast gas-liquid reaction networks, reactions occur almost exclusively in the bubble wake. Thus, wake mixing can have a strong impact on the reaction yield and selectivity. Dynamic numerical simulations were performed to study the flow of liquid around bubbles of different shapes. The obtained velocity and pressure fields were used to investigate the liquid-phase mixing in the flow for each case. As a strong connection between mixing and chaos is known to exist, Lagrangian tracking of passive tracer particles was used to identify chaotic fluid transport in the flows. Chaotic dynamics lead to folding and stretching of fluid elements, which results in very effective mixing. To quantify mixing, stretching fields were computed for each flow case. Finally, different liquid-phase chemical reaction networks were tested to illustrate the effects of mixing on chemical reaction yields and selectivities.     

Effects of floating contactors on oxygen transfer and hydrodynamics in a bubble column

This study investigates the adoption of floating contactors to promote the rate of oxygen transfer from non-uniform air bubbles to liquid in a bubble column with continuous operation. The volumetric oxygen transfer coefficient and axial dispersion coefficient of a liquid phase have been analyzed based on the axial dispersion model. Attention was focused on the effects of the volume fraction of the floating contactors on the volumetric oxygen transfer coefficient, axial dispersion coefficient of a liquid phase, and gas phase hold-up in the bubble column. The results have shown that the volumetric oxygen transfer coefficient and gas phase hold-up can increase by up to 25% and 13%, respectively, while the axial dispersion coefficient of a liquid phase decreases by up to 30% by adding floating contactors in the column.     

微通道壁面浸润性对气-液两相流的影响规律研究

通道壁面浸润性对微通道内的气-液两相流具有重要影响。利用等离子体辅助接枝改性,将甲基丙烯酰乙基磺基甜菜碱（SBMA）及1H, 1H, 2H, 2H-全氟癸基三乙氧基硅烷接枝在聚甲基丙烯酸甲酯（PMMA）材料表面,得到了10°、40°、70°和110°四种接触角的微通道,并考察了浸润性对流型、气泡长度和压降的影响。结果表明,随接触角增大,气泡截断位置下移,膨胀阶段缩短,挤压阶段变长;低流量时,气泡长度随接触角增加而增大,高流量时则减小;建立了与材料表面水接触角相关的气泡尺寸预测关联式,与Garstecki经典预测关联式相比,预测精度更高;θ<90°时,接触角增加,压降减小;θ>90°时,三相接触线使流动阻力和压降增加。     

Effects of viscosity and relaxation time on the hydrodynamics of gas–liquid systems

Effects of liquid properties on the hydrodynamics of gas–liquid systems were investigated in lab-scale bubble column (BC) and internal loop airlift (ILA). Alginate solutions, a glycerol solution and a Boger fluid were adopted to separately address the effects of viscosity and of surface tension for Newtonian fluids, and the effects of relaxation time for non-Newtonian fluid characterized by approximately constant viscosity (low shear thinning). Hydrodynamic regimes were characterized in terms of overall gas holdup, gas–liquid mass transfer coefficient, drift-flux and liquid circulation velocity. The superficial gas velocities at the transition between hydrodynamic regimes (homogenous regime–vortical-spiral regime–heterogeneous regime) as a function the liquid viscosity was characterized by a maximum. The same behavior was observed for the maximum stable gas holdup and gas–liquid mass transfer coefficient in BC. Viscosity enhances homogeneous regime stability for μ<4.25 mPa s, in BC, and μ<7.68 mPa s, in ILA. For non-Newtonian fluids the transition velocity increases with liquid elasticity. The stabilization mechanism related to the relaxation time of Boger fluids has been discussed.     

连结基对甜菜碱型双子表面活性剂表面活性和泡沫性能的影响

以3-氯-2-羟基丙烷磺酸钠、油酸酰胺丙基二甲基叔胺和溴代烷为原料合成了一系列甜菜碱型双子表面活性剂C_s-BGS(C为连结基中亚甲基,s=2,4,6)。采用FT-IR和~1H NMR对产物进行了结构表征。分别使用吊片法和电导率法测定产物在25℃水溶液中的临界胶束浓度(cmc)。结果表明,随着连结基长度的增加,其平衡表面张力(γcmc)及cmc增加。C2-BGS的cmc最低,降低表面张力的效率(pc_(20))较高,在空气-水界面处有强烈的吸附,并且泡沫性能相对较好。此外,通过pc_(20)、极限占有面积(A_(min))、标准吸附自由能(ΔG~0_(ads))和标准胶束化自由能(ΔG~0_(mic))估算C_s-BGS的吸附和胶束化行为。结果表明,C_s-BGS的这些性质受连结基长度影响很大,且促进吸附行为的能力更强。     

A two-phase Eulerian approach using relative permeability concept for modeling of hydrodynamics in trickle-bed reactors at elevated pressure

Most commercial trickle-bed reactors (TBRs) employed in hydroprocessing and other industrially relevant operations normally operate at elevated pressures. Two-phase pressure drop and liquid holdup are two foremost important hydrodynamic parameters to consider for analysis and design of a TBR, including those operating at higher pressures. Even after several decades of research efforts directed towards the development of TBR technology, know-how about the hydrodynamics of two-phase flow in a TBR especially operating at high-pressure conditions has been inadequate. In this study, an effort has been made to assess the complex hydrodynamics of high-pressure TBR through the development of a Computational Fluid Dynamics (CFD) based model to predict pressure drop and liquid saturation. A two-phase Eulerian CFD model envisaging the flow field as porous region has been utilized for evaluating these hydrodynamic parameters. Different combinations of relative permeability correlations in the closure terms have been exercised to realize the best fit. The comparisons between model predictions and numerous experimental data, collected from different independent sources under a varied set of operating conditions, lead to the favourable implementation of this less computationally intensive, yet first-principle based CFD model to forecast the two-phase hydrodynamics for high-pressure TBRs.     

Effects of gas concentration on hydrodynamics of gas absorption in a microchannel

In this article, the effects of gas concentration on hydrodynamics of gas–liquid two-phase flow with mass transfer during gas absorption in a microchannel are investigated, by using 2-amino-2-methyl-1-propanol (AMP) solution to absorb mixtures of CO₂ and N₂ with various volume fractions. The concentration of CO₂ not only affects the driving force of gas–liquid mass transfer, but also affects the pressure drop of gas–liquid two-phase flow. The average linear velocity of the liquid phase is estimated by introducing the void fraction, which accurately characterizes the difference in the bubble velocity versus the liquid velocity. On this basis, the pressure drop model of gas–liquid two-phase flow with mass transfer in a microchannel is established. Through the pressure drop model, the influence mechanism of CO₂ concentration on the pressure drop during gas absorption in a microchannel is revealed.     

Effects of roughness elements on laminar flow and heat transfer in microchannels

A model of laminar flow and heat transfer in rough microchannels is developed and analyzed numerically to compare the effect of roughness elements on the thermal and hydrodynamic characteristics. In this model, the rough surfaces are configured with triangular, rectangular and semicircular roughness elements, respectively. Here, the effects of the Reynolds number, roughness height, and roughness element spacing on pressure drop and heat transfer in rough microchannels are all investigated and discussed. The results indicate that the global heat transfer performance is improved by the roughness elements at the expense of pressure head when compared to the smooth channel. Differing from the smooth microchannels, both the Poiseuille number and average Nusselt number of rough microchannels are no longer constant with Reynolds numbers and are larger than the classical value. Especially, the difference from the effects of three types of roughness elements is identified. With the increasing roughness height, the flow over surfaces with semicircular and triangular roughness elements induces stronger recirculation and flow separation. This contributes to heat transfer enhancement but also increases the pressure drop. However, the influence of the rectangular roughness element height is weaker than that of semicircular and triangular elements. In addition, the effects of the spacing of roughness elements on the Poiseuille number and average Nusselt number are in decreasing order for semicircular, triangular and rectangular roughness elements, respectively.     

Transparent UV-cured clay/UV-based nanocomposite coatings on wood substrates: surface roughness and effect of relative humidity on optical properties

The esthetic durability of coatings on wood surfaces of components constituting wood furniture used in bathrooms is generally affected by high humidity. In this study, surfaces of yellow birch wood (Betula alleghaniensis Britton) were protected with three different types of transparent UV-cured multilayer coatings (MCs), namely MC1, MC2, and MC3. Each MC consisted of three layers: primer, sealer, and topcoat. MC1, MC2, and MC3 contained, respectively, 0, 1, and 3 wt% of nanoclay (NC) in the topcoat, while no nanoparticle was added in the primer and sealer. The surface roughness of coated wood surfaces was measured before accelerated aging and optical properties (color and gloss) were investigated before, during, and after accelerated aging. Statistical results have shown that: (1) all coated wood samples have a similar surface roughness and (2) NC in the topcoat does not have a significant effect on initial color, whereas its effect on initial gloss is significant. There is a significant effect on relative humidity (RH) on color changes, but not between the different types of MCs. With respect to gloss, a lowering of gloss retention with the increase in aging time and RH has been observed for all coatings on wood surfaces. Significant differences appear only at high RH between: MC1 vs MC3 and MC2 vs MC3.     

Effects of the spacer length of gemini surfactants on the ordered pore of silica

A family of designed gemini surfactants C₁₄H₂₉(CH₃)₂N⁺-(CH₂)_S–N⁺(CH₃)₂C₁₄H₂₉·2Br⁻ (designated as C_14-S-14, S = 4, 6, 8, 10) was utilized as structure-directing agent to prepare ordered porous MCM-41 silica. The samples were characterized by small angle X-ray diffraction, high-resolution transmission electron microscopy and N₂ adsorption/desorption analysis. The results showed that the obtained materials possessed 2D-hexagonal periodic structure (space group 2D-p6mm), but the pore diameter obviously decreased as the length of the spacer group of the gemini surfactants increased. Two types mesoporosity, that is, framework-confined mesoporosity (primary pores) and voids between particles (textural mesoporosity) existed in the samples. In addition, the self-assembly ability of C_14-S-14 was stronger than that of tetradecyltrimethylammonium bromide (TTAB, which was regarded as the corresponding monomer of gemini surfactant C_14-S-14) in controlling the orderly pore structure and pore size of the porous materials.     

微流道粗糙壁面对流动阻力和控制的影响

研究了3种粗糙单元(三角形、矩形、三角形与矩形交错混合)壁面对微流道内流动和摩擦阻力的影响。利用二维模型模拟了量纲一泊肃叶数与粗糙单元的几何特性和分布的关系。结果发现:在模拟条件下,粗糙壁面摩擦阻力(泊肃叶数)总是大于光滑壁面的摩擦阻力;壁面粗糙单元的高度对摩擦阻力的影响较大,可有近乎指数型的增长,粗糙单元的长度和间隔的影响居中,而雷诺数的影响较小。说明壁面粗糙度可有效改变槽道内的流动状态,增大混合效率。     

Effects of cross-linking and spacer groups on beta-cyclodextrin bonded liquid chromatographic separation

Mesoporous glass beads and 5 μm silica particles were modified with beta-cyclodextrin ( Β-CD) by means of directly bonding, linking spacer group, and cross-linking agent. The selectivities of the Β-CD modified silica particles were measured by simple column chromatography to separate a model mixture of 1-and 2-hydroxy-naphthalene (naphthols) and ortho, meta, and para-xylene. In the packed column chromatography experiments, two major controlling factors (inclusion complex formation effect and steric hindrance effect of the analytes) on the separations were observed. The elusion orders of the Β-CD directly bonded glass beads were meta-, para-, ortho-xylene and 1-naphthol, 2-naphthol. The phenomena of Β-CD pore blocking and narrowing by spacer groups and cross-linking agent were observed. The spacer group and cross-linking agent decreased inclusion complex formation of 2-naphthol. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

A comparison of melt fracture initiation conditions in capillaries and slits

An experimental study was carried out with the objective of comparing the critical conditions for the onset of gross extrudate distortion, usually called melt fracture, in capillaries and slits. Narrow and broad molecular weight distribution polystyrenes as well as low- and high-density polyethylenes were used. The onset of melt fracture was observed to take place at higher shear stresses in slits than in capillaries. It is argued that the flow-average value of the recoverable shear strain should be used as the criterion for the initiation of melt fracture.     

提高二氧化钛孔道中水分子扩散性的分子模拟

众所周知,二氧化钛(TiO₂)材料在光催化材料、生物材料、电化学和工业催化等多个领域表现出优异的性能,因而其在解决能源和环境危机中被寄予厚望。在上述TiO₂材料的众多应用中往往都离不开水分子与TiO₂材料的接触。TiO₂表面结构,特别是水分子在TiO₂表面上形成的微结构往往对于TiO₂材料的应用起到关键作用。另一方面,大比表面积的TiO₂材料表现出性能上的优势,因此制备具有纳米孔道结构的TiO₂材料成为研究趋势。结合上述两点,水分子受限于TiO₂纳米孔道中的行为成为研究的热点之一。以往的研究结果表明,水分子在TiO₂表面具有强吸附作用,而该作用会影响到水分子在TiO₂孔道中的流动性。本文意在通过分子模拟的研究,找到提高水分子TiO₂孔道中扩散性的方法,并解释流动性增加的原因,为实验研究工作者们提供指导和解释。通过调整多项模拟参数,结果表明对于实验工作更为可靠的方法是用碳层覆盖TiO₂的表面。这样可以在不改变原有实验条件的基础上大大提高水分子在TiO₂孔道中的扩散性。