On the formation of Taylor bubbles in small tubes

The formation of Taylor bubbles and resulting bubble lengths were studied in a ID vertical tube for air-water and air-octane systems. In the co-flow tube/nozzle arrangement two nozzle sizes were used as gas inlets. Superficial velocities varied between 0.001- for the liquid and 0.002- for the gas. Three different mechanisms of initial bubble formation were observed. Of the three mechanisms, mechanism 3 is periodic (with period consisting of a bubble and a liquid slug), reproducible and can be simply modelled. After initial bubble formation further modifications may occur in the formed bubble size by coalescence or pairing. Bubble pairing is encouraged by smaller nozzles and liquid flow rates, while coalescence is observed only for cases where non-Taylor bubbles form initially.Two simple models have been proposed, the first predicts the size of the Taylor bubbles formed by mechanism 3 while the second attempts to predict the condition for bubble pairing to occur. Reasonable agreement with experimental results validates the predictions of the first model for a strong dependence of the volume of Taylor bubbles formed on the gas and liquid flow rates, a moderate dependence on nozzle diameter and a weak dependence (if at all) on the surface tension of the liquid used. Mismatch with the experimental results is caused (at least in part) by the experimental setup where there was no perfect axial alignment of the gas inlet. The experiments also suffered from problems at the outlet at low flow rates where smooth bubble disengagement could not be ensured for long Taylor bubbles. The second model for pairing predicts its occurrence for concentric tube/nozzle arrangements as a function of flow rates and channel diameters. The model over-predicted the range of liquid flow rates at which pairing was observed experimentally, but it captured the form of the boundary between different bubble volume modification mechanisms when represented on superficial velocity graphs.     

The onset of instability in the flow induced by an impulsively started rotating cylinder

The onset of initial instability in a developing Couette flow following the impulsive starting of an inner rotating cylinder is analyzed using linear theory. It is well known that there is a critical Taylor number Ta_c at which Taylor vortices first appear between two concentric cylinders. For Ta>Ta_c Taylor-like vortices occur at a certain elapsed time. In the present study, the critical time t_c to represent the onset of this initiating instability, which then grows as toroidal vortices, is analyzed using propagation theory. For this purpose a self-similar transformation is forced through scaling analysis. The resulting stability criteria compare well with the available experimental data for vortices in water. The new measures represent the onset of the fastest growing instability and also suggest the detection time for the manifestation of secondary flow in the primary time-dependent Couette flow.     

Liquid phase mixing in turbulent bed contactor

Axial mixing of the liquid phase in turbulent bed contactor (TBC) is studied through residence time distribution (RTD) experiments over a large range of variables such as flow rate of gas and liquid phases, static bed heights, diameter and density of particles and number of stages in presence of downcomer using air water system. Since all the liquid exits only through the downcomer, it enables the correct estimation of exit concentration of the tracer. The experimental RTD curves are satisfactorily compared with the axial dispersion model. The Peclet numbers evaluated by axial dispersion model and the Peclet numbers reported in the literature are used to propose a unified correlation in terms of operating and geometric parameters. Correlation is also developed for predicting the axial dispersion coefficient. It was observed in the present study that almost plug flow conditions can be achieved in multistage TBC.     

Mechanical Entrainment in W/O/W Emulsion Liquid Membrane

Abstract

An experimental study of mechanical entrainment in W/O/W emulsions is conducted. W/O/W emulsions are stirred for various stirring times under the conditions that mechanical entrainment solely occurs, and changes in volume of the W/O emulsions and size distribution of the internal water droplets are measured. The rate of change in number of the water droplets entrained is found to be proportional to the volume fraction of W/O emulsions. Based on this result, a new model for mechanical entrainment is developed. The calculated change in W/O emulsion volume with time agrees with the observed ones except in the region near phase inversion. Then, phase inversion is discussed.     

Estimation of liquid phase mixing due to solids in a turbulent bed contactor

The mixing in two-phase gas-liquid and three-phase gas-liquid-solid system (turbulent bed contactor) is evaluated through residence time distribution (RTD) studies in terms of Peclet number. RTD experiments are conducted for various gas and liquid velocities, and number of stages for two- and three-phase systems. Since the mean residence time is very short in both the systems, a mixed flow tank with exponential decay RTD is used in series. After deconvolution, the RTD of the system is obtained. The experimental RTD curves are satisfactorily compared with the axial dispersion model and Peclet numbers are evaluated for all the experiments. The axial dispersion coefficients are calculated from Peclet numbers. With this study, it is thought that liquid phase mixing may be controlled by changing the quantity of solid particles in the bed.     

Extraction of penicillin G from simulated media by an emulsion liquid membrane process

The extraction of penicillin G from simulated media was performed by water/oil/water (w/o/w) emulsion liquid membranes (ELMs) and studied under various operational conditions in a batch system. The degree of extraction achieved was between 80% and 95% under specific conditions. A concentration of greater than nine times the initial concentration of penicillin G in the external phase was obtained in the internal phase. The pH of the internal aqueous solution, containing a basic salt, was theoretically calculated on the basis of the amount of penicillin G transported into the internal phase. The calculated results agreed with the experimental data well and were used to select a suitable type and concentration of a basic salt in the internal phase to give a pH within the range 5 to 8 where penicillin G was stable after the termination of extraction. The extraction of penicillin G was successfully performed by the ELM process with sodium carbonate in the internal phase.     

乳状液膜用表面活性剂研究进展

综述了液膜用表面活性剂筛选、研制及其性能研究进展,进一步强调了研制性能优良的液膜专用表面活性剂对液膜技术工业化的重要意义。     

Effect of gas expansion on the velocity of individual Taylor bubbles rising in vertical columns with water: Experimental studies at atmospheric pressure and under vacuum

This study was designed to determine the effect of gas expansion on the velocity of Taylor bubbles rising individually in a vertical column of water. This experimental study was conducted at atmospheric pressure or under vacuum (33.3 and ) using three different acrylic columns with internal diameters of 0.022, 0.032, and 0.052 m, and more than 4.0 m high. A non-intrusive optical method was used to measure velocity and length of Taylor bubbles at five different locations along the columns. The operating conditions used correspond to inertial controlled regime.In experiments performed under vacuum, there is considerable gas expansion during the rise of Taylor bubbles, particularly when they approach the liquid free surface where the pressure drop (due to the hydrostatic pressure) is of the order of magnitude of the absolute pressure. The liquid ahead of the bubble is displaced upward by an amount proportional to the gas expansion resulting in increased bubble velocity. The calculated Reynolds number suggests a laminar regime in the liquid ahead of the bubble. However, the experimentally determined velocity coefficient C for each column was much smaller than 2, which would be expected for laminar flow. The value of C obtained ranges from 1.13±0.09, for the narrowest column, to 1.40±0.24, for the widest column. This suggests that a fully developed laminar flow in the liquid ahead of the bubble is never achieved due to continuous bubble expansion at a variable rate, regardless of column height.The velocity coefficient C can be used to calculate the contribution of liquid motion to bubble velocity. Subtracting this contribution from the measured bubble velocity defines a constant value which is nearly identical to the bubble rise velocity measured in the same column operated as a constant volume system (two ends closed) where gas expansion is absent.     

乳状液膜分离技术中的交流高电压破乳器

<正>近20年来,国内外有关乳状液膜研究的结果表明,该技术实现工业化的关键在于设计并制造出具有最佳破乳效果的电破乳器。已有的关于乳状液膜电破乳技术的报道大都限于小型实验室装置,尚无成熟的定量设计方法。本文将结合高电压破乳器的平板电容模型,给出一种比较简单的设计方法。 2 交流高电压破乳器的平板电容等效模型     

环境化工中的气/液膜接触分离过程及其特性

阐述了新型膜接触器由于避免了传统分离设备的一系列缺点和不足,具有操作范围宽,分离效率高,气液两相的流速可独立控制,可直接线性放大和结构紧凑等众多优点,日益成为分离科学研究的热点。并着眼于3种气/液界面膜接触分离过程(膜吸收、膜蒸馏、膜结构填料替代)在废气、废水等环境治理与化工分离方面的研究,从膜接触器结构、膜材料、传质、分离效率等方面详细分析和探讨了三种膜过程的分离特点,并对其作用原理、操作参数及分离性能进行了综合比较。     

气/液膜接触分离过程及其膜材料研究

新型气液膜接触器是很多具有共同特点膜过程的总称,它使用高分子膜将两流体分隔开,膜孔为两流体提供有效传质的场所,与传统分离器相比具有许多独特的优点,日益成为分离科学研究的热点。简述膜接触器在不同气/液分离体系中的应用,针对膜吸收、膜蒸馏、膜结构填料等三种典型的气/液界面膜接触分离过程,从结构、膜材料、传质、分离效率等方面详细进行了分析和比较。着重介绍了近年来相关领域膜材料学的研究进展,如高分子膜材料、成膜方法以及膜材料的改性方法(等离子体改性法、紫外辐照法和表面涂覆改性法等)等。