Study of Air‐Liquid Flow Patterns in Hydrocyclone Enhanced by Air Bubbles

In order to improve the oil‐water separation efficiency of a hydrocyclone, a new process utilizing air bubbles has been developed to enhance separation performance. Using the two‐component phase Doppler particle analyzer (PDPA) technique, the velocities of two phases, air and liquid, and air bubble diameter were measured in a hydrocyclone. The air‐liquid mixing pump can produce 15 to 60 μm‐diameter air bubbles in water. There is an optimum air‐liquid ratio for oil‐water separation of a hydrocyclone enhanced by air bubbles. An air core occurs in the hydrocyclone when the air‐liquid ratio is more than 1 %. The velocities of air bubbles have a similar flow pattern to the water phase. The axial and tangential velocity differences of the air bubbles at different air‐liquid ratio are greater near the wall and near the core of the hydrocyclone. The measured results show that the size distribution of the air bubbles produced by the air‐liquid mixing pump is beneficial to the process where air bubbles capture oil droplets in the hydrocyclone. These studies are helpful to understand the separation mechanism of a hydrocyclone enhanced by air bubbles.     

脱气除油旋流系统流场分布及分离特性

目前我国大部分油田已进入高含水阶段，且采出液中常带有大量伴生气，采用旋流分离法实现采出液高效分离对于简化陆上油田地面处理工艺及提升海上平台经济环保的采出液处理技术具有重要意义。脱气除油旋流系统由GLCC型气液分离器和油滴重构旋流器串联组成，设计的目的是在保证高效脱气的同时进一步改善对小油滴的去除效果，实现油气水三相高效分离。本文基于计算流体动力学（computational fluid dynamics，CFD）方法，利用种群平衡模型（population balance model，PBM）模拟油滴破碎与聚结，对脱气除油旋流系统内部流场特性及粒度分布进行模拟分析，并采用数值模拟与试验相结合的研究方法，对比分析不同含气体积分数和气相出口分流比对脱气除油旋流系统分离性能的影响规律。结果表明：含气体积分数与气相出口分流比对脱气除油旋流系统分离效率的交互作用较显著，在研究范围内综合脱气效率和除油效率可以得到，含气10%、20%、30%的最佳分流比分别为25%、30%、35%，数值模拟结果与试验结果吻合良好，验证了数值模拟方法的可靠性。另外，含气体积分数越大，油滴重构旋流器的油滴分层重构现象越明显，油滴重构旋流器可以在一定程度上改善含气情况下油水分离效果差的现象，脱气除油旋流系统对油气水三相分离的适用性较好。     

Gas–liquid mass transfer and bubble size distribution in a multi-Cyclone separator

This article discusses the use of a multi-cyclone separator, which is a simplified form of a degassing hydrocyclone, in the separation of sweeping nitrogen bubbles and dissolved oxygen from water. The motion of the nitrogen bubbles and mass transfer of dissolved oxygen is discussed. It was observed that the Sauter mean diameter and gas volume in the swirling flow region as well as the total gas holdup increased as the volumetric ratio of gas to liquid flow increased. Almost all bubbles were found to exit through the gas outlet, indicating optimum performance of the bubble-separation process. The multi-cyclone separator was found to achieve good performance for the mass transfer of oxygen from water to nitrogen. This work is important in predicting the destination of bubbles and dissolved gas in swirling flow. © 2018 American Institute of Chemical Engineers AIChE J, 65: 215–223, 2019     

Effects of surface active agents on hydrodynamics and mass transfer characteristics in a split-cylinder airlift bioreactor with packed bed

The effects of three types of surface active agents (containing SDS, HCTBr and Tween 40) with various concentrations (0–5 ppm) on the hydrodynamic and oxygen mass transfer characteristics in a split-cylinder airlift bioreactor with and without packing were investigated. It was observed that in the surfactant solutions, surface tension of the liquid decreased and smaller bubbles were produced in comparison with pure water. So, surfactants presence strongly enhanced mixing time and gas hold-up although oxygen mass transfer coefficient and the liquid circulation velocity reduced. Furthermore, the packing installation enhanced the overall gas–liquid volumetric mass transfer coefficient by increasing flow turbulency and Reynolds number compared to an unpacked column. The packing increased gas hold-up and decreased bubbles size and liquid circulation velocity.     

Transient gas–liquid mass transfer model for thin liquid films on structured solid packings

This paper describes a model for gas–liquid mass transfer through thin liquid films present on structured packings for gas–liquid operations under dispersed gas flow regime. The model has been derived for two cases: the absorption (or desorption) of a gaseous component into the liquid film and the transfer of the gaseous component through the liquid film to the packing surface where an infinitely fast reaction takes place. These cases have been solved for three bubble geometries: rectangular, cylindrical, and spherical. For Fourier numbers below 0.3, the model corresponds to Higbie’s penetration theory for both cases. The Sherwood numbers for cylindrical and spherical bubbles are 20% and 35% higher, respectively, than for rectangular bubbles. In case of absorption and Fourier numbers exceeding 3, the effect of bubble geometry becomes more pronounced. The Sherwood numbers for cylindrical and spherical bubbles now are 55% and 100% higher, respectively, than for rectangular bubbles. In case of an infinitely fast reaction at the packing surface, the Sherwood number corresponds to Whitman’s film theory (Sh=1$Sh=1$) for all bubble geometries. In this paper also practical approximations to the derived Sherwood numbers are presented. The approximations for both cases and all bubble geometries describe all the model data within an error of 4%. The application of the model has been demonstrated for three examples: (1) gas–liquid mass transfer for a structured packing; (2) gas–liquid mass transfer in a microchannel operated with annular flow; (3) gas–liquid mass transfer in a microchannel with Taylor flow.     

基于旋转分相单元的电容式气液两相流含液率测量

为提高电容器对含水率测量的能力,设计与其相匹配的旋转分相单元,以实现气液两相流含水率的起旋分相式电容测量。在电容传感器的测量单元基础上,增加旋转分相单元,利用气液两相流体力学理论对其结构进行设计优化,并根据实际应用条件确定分离圈数,保证气液分相效率。旋转分相单元克服了气液两相流流动形态的多样性,将体积含水率信息转换成液膜厚度信息及截面含水率信息进行测量。经实验验证,Lockhart-Martinelli截面含气率模型与实验中电容器测量截面含气率具有良好的一致性,其能够实现对体积含水率20%以内的气液两相流有效测量,体积含水率的绝对误差在±2%以内。     

Effect of spent grains on flow regime transition in bubble column

It is well known that two main flow regimes are present in bubble columns, being the evaluation of transition between homogeneous and heterogeneous regimes of crucial importance for reactor design. For air–water systems, several models have been satisfactorily proposed to explain this phenomenon. However when gas–liquid–solids systems are considered, solid particles influence on regime transition is not yet clear, in spite of the amount of research developed over the past years.The objective of this work is to evaluate the effect of a specific solid phase – spent grains – on homogeneous regime stability and regime transition. Spent grains are cellulose-based particles that have been used to immobilize cells on biotechnology process. These particles are wettable and have a density close to water and its influence on bubble column reactors is particularly important in order to establish the limits were both regimes prevail.A cylindrical Plexiglax BC of 18 L volume was used with air, water and spent grains at different concentrations (0–20% (wt._{WET BASIS}/vol.)) as gas, liquid and solid phases. Regime transition was determined according to the drift-flux and slip speed concept.It was found that at studied concentrations of spent grains, critical gas hold-up decreases as solids concentration increases. At the highest solids concentration and lowest gas flow rates no fluidization of the solid phase was observed. It is believed that the critical hold-up decrease was mainly due to bubble coalescence, as larger bubbles were observed when heterogeneous regime was present. This coalescence may be caused by the non-uniform distribution of solid phase on the column and the interaction of spent grains with bubbles in the liquid–gas interface     

Investigations on hydrodynamics and mass transfer in gas–liquid stirred reactor using computational fluid dynamics

In this work, the hydrodynamics and mass transfer in a gas–liquid dual turbine stirred tank reactor are investigated using multiphase computational fluid dynamics coupled with population balance method (CFD–PBM). A steady state method of multiple frame of reference (MFR) approach is used to model the impeller and tank regions. The population balance for bubbles is considered using both homogeneous and inhomogeneous polydispersed flow (MUSIG) equations to account for bubble size distribution due to breakup and coalescence of bubbles. The gas–liquid mass transfer is implemented simultaneously along with the hydrodynamic simulation and the mass transfer coefficient is obtained theoretically using the equation based on the various approaches like penetration theory, slip velocity, eddy cell model and rigid based model. The CFD model predictions of local hydrodynamic parameters such as gas holdup, Sauter mean bubble diameter and interfacial area as well as averaged quantities of hydrodynamic and mass transfer parameters for different mass transfer theoretical models are compared with the reported experimental data of [Alves et al., 2002a] and [Alves et al., 2002b] . The predicted hydrodynamic and mass transfer parameters are in reasonable agreement with the experimental data.     

Capillary reactor for cyclohexane oxidation with oxygen

Cyclohexane oxidation is the first step in the currently used technology for production of Nylon-6 and Nylon-6,6 which employs a two-stage process. In the first stage 80% selectivity to two main products, cyclohexanol and cyclohexanone (KA oil) is obtained at 4–8% cyclohexane conversion in staged bubble columns or stirred tanks. There have been reports that increased oxygen concentration in the gas phase or pure oxygen is beneficial to cyclohexane oxidation and this was confirmed in our previous study (Jevtic et al., 2009). To fully utilize this advantage here, we present a novel, safer capillary reactor for cyclohexane oxidation with pure oxygen. The discrepancy between the experimental and modeling results was attributed to lower than expected mass transfer achieved in the capillary. With a better design for gas–liquid mixing and contacting this type of a reactor could potentially become attractive for gas–liquid reactions of similar nature.     

注气对油水分离水力旋流器流场影响模拟分析

采用雷诺应力湍流模型、混合模型和离散相模型对注气型油水分离水力旋流器进行数值模拟,得到其内部流场的速度分布和油滴粒子运动轨迹,分析对比了注气前后进口流量、分流比和充气量对分离效率的影响,数值计算结果与文献实验值进行了比较。结果表明,充气后流场速度增加,油滴粒子逃逸时间缩短,旋流器分离效率提高5%~10%,在一定条件下气浮对旋流分离起到强化作用。     

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.     

γ-CT measurement and CFD simulation of cross section gas holdup distribution in a gas–liquid stirred standard Rushton tank

Cross section gas holdup distributions at 3/4 dimensionless static liquid height in a gas–liquid stirred standard Rushton tank were measured using ¹³⁷Cs γ-CT scan measuring technology at larger gas flow rates and higher impeller rotating speeds. The obtained CT scan images and digital distribution curves of gas holdup with dimensionless radius based on the CT images could explain the fluctuation changes of gas holdup distribution. The dense area of gas holdup distribution appeared in the upper space of impeller blades. Gas holdup increased both with gas flow rate and impeller rotating speed, but gas flow rate had more influence on gas holdup than impeller rotating speed. The Eulerian–Eulerian two-fluid model coupling with the bubbles' coalescence and break-up models, and the drag coefficient model were established to make CFD simulation of gas holdup distributions for the gas–liquid stirred Rushton tank under different gas flow rates and impeller rotating speeds.     

Numerical Simulation of Oil‐water Hydrocyclone Using Reynolds‐Stress Model for Eulerian Multiphase Flows

A three‐dimensional oil‐water turbulent flow and oil separation process in a double‐cone liquid‐liquid hydrocyclone (LLHC) is numerically simulated using FLUENT software. The Euler‐Euler approach and Reynolds‐stress model are combined and adopted in this simulation to handle the challenging situation of anisotropic turbulent two‐phase flow with a higher volumetric ratio (over 10%) in the dispersed phase. It is visualized well in the simulation how separation, aggregation and shift of oil and water proceed in the LLHC. The oil separation efficiency is determined based on flow field and phase concentration distribution. The simulation is verified by comparing predicted and measured separation efficiency in the LLHC.     

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.     

The effect of surface tension on phase distribution of two-phase flow in a micro-T-junction

An experimental investigation has been conducted to study the effect of surface tension on phase distribution of gas–liquid two-phase ?ow through a T-junction with diameter 0.5 mm. It is found that the decrease in liquid surface tension makes the liquid taken off reduce when inlet flow pattern is slug flow, slug-annular and annular flow. These results highlight that phase distribution is remarkably influenced by surface tension in micro-T-junctions. To be specific, the surface tension contributes positively to liquid taking off. High surface tension seems to make the liquid capture more kinetic energy transported from the gas and dissipate it in form of vortexes. It is suggested that phase distribution in micro-T-junctions can be partly controlled by adjusting liquid surface tension.     

Effect of a dispersed immiscible liquid phase on the hydrodynamics of a bubble column and ebullated bed

Secondary undesired reactions in ebullated bed resid hydroprocessors can generate an additional dispersed liquid phase, referred as mesophase, which is denser and more viscous than the continuous liquid phase and affects the operation and transport phenomena of the fluidized bed. This study investigates the effect of a dispersed immiscible liquid phase on the overall phase holdups, bubble properties, and fluidization behavior in a bubble column and ebullated bed. The experimental system consisted of biodiesel as the continuous liquid phase, glycerol as the dispersed liquid phase, 1.3 mm diameter glass beads, and nitrogen. The addition of dispersed glycerol reduced the gas holdups in the bubble column for the studied gas and liquid superficial velocities. Dynamic gas disengagement profiles reveal a rise in the large bubble population and reductions to the small and micro bubble holdups when increasing the glycerol concentration. Liquid–liquid–solid bed expansions at various liquid flowrates confirm particle agglomeration in the presence of a more viscous dispersed liquid phase. Overall phase holdups in a gas–liquid–liquid–solid ebullated bed were obtained while varying the gas and liquid flowrates as well as the glycerol concentration. A coalesced bubble flow regime was observed in the bed region without glycerol whereas the addition of glycerol resulted in the dispersed bubble flow regime due to particle clustering and a greater apparent particle size. The resulting bubble flow regime increased the bed and freeboard region gas holdups due to enhanced bubble break-up. Observations of the fluidized bed behavior following the addition of the dispersed glycerol are also discussed.     

Gas–liquid and liquid–liquid mass transfer in microstructured reactors

This article is a comprehensive overview of gas–liquid and liquid–liquid mass transfer in microstructured reactors (MSR). MSR are known to offer high heat and mass transfer rates for two phase systems due to high surface to volume ratio as compared to conventional reactors. The reactions with fast kinetics controlled by mass transfer have been successfully intensified using MSR. The first part of the review deals with the methods of mass transfer characterization. Further, different dimensionless parameters used to analyze mass transfer in MSR are discussed. The literature data with different flow regimes and proposed empirical correlations for both gas–liquid and liquid–liquid systems is also presented. The conventional mass transfer models such as penetration and film theory are analyzed. Finally, the important issues of mass transfer in MSR are summed up.     

Modeling of turbulent gas–liquid bubbly flows using stochastic Lagrangian model and lattice-Boltzmann scheme

In this paper we present detailed, three-dimensional and time-resolved simulations of turbulent gas–liquid bubbly flows. The continuous phase is modeled using a lattice-Boltzmann (LB) scheme. The scheme solves the large-scale motions of the turbulent flow using the filtered conservation equations, where the Smagorinsky model has been used to account for the effects of the sub-filter scales. A Lagrangian approach has been used for the dispersed, bubbly phase. That is we update the equations of motion of individual bubbles. It is shown that the incorporation of the sub-filter scale fluid fluctuations along the bubble trajectory improves the predictions. Collisions between bubbles are described by the stochastic inter-particle collision model based on kinetic theory developed by Sommerfeld (2001). It has been found that the collision model not only dramatically decreases computing time compared to the direct collision method, but also provides an excellent computational efficiency on parallel platforms. Furthermore, it was found that the presented modeling technique provides very good agreement with experimental data for mean and fluctuating velocity components.     

Multi-scale analysis of gas–liquid interaction and CFD simulation of gas–liquid flow in bubble columns

The ratio of effective drag coefficient to bubble diameter is of critical importance for CFD simulation of gas–liquid flow in bubble columns. In this study, a novel model is proposed to calculate the ratio on the basis of the Dual-Bubble-Size (DBS) model. The motivation of the study is that a stability condition reflecting the compromise between different dominant mechanisms can serve for a closure in addition to mass and momentum conservative constraints, and the interphase momentum transfer should be related to different paths of energy dissipation. With the DBS model, we can first offer a physical interpretation on macro-scale regime transition via the shift of global minimum point of micro-scale energy dissipation from one potential trough to the other. Then the proposed drag model is integrated into a CFD simulation. Prior to this integration, we investigate the respective effects of bubble diameter and correction factor and found that the effect of bubble diameter is limited, whereas the correction factor due to the bubble swarm effect is eminent and appropriate correction factor has to be selected for different correlations of standard drag efficient to be in accord with experiments. By contrast, the DBS drag model can well predict the radial gas holdup distribution, the total gas holdup as well as the two-phase flow field without the need to adjust model parameters, showing its great potential and advantage in understanding the complex nature of multi-scale structure of gas–liquid flow in bubble columns.     

螺旋构件强化排液的泡沫分离塔用于牛血清蛋白分离

引言 泡沫分离技术又称泡沫吸附分离技术[1],是以气泡作为分离介质,利用被分离组分在气液两相界面吸附性质的差异进行浓缩溶液中表面活性组分.在泡沫分离过程中,通过空气分布器在泡沫塔液相中产生气泡,气泡沿着轴向向上流动,被分离表面活性组分吸附在气泡的气液两相界面上,当吸附接近平衡后,气泡离开液相,在液相上方形成泡沫相.