INVISCID LIQUID CIRCULATION IN BUBBLE COLUMNS

For circulation in axi-symmetric (cylindrical) bubble columns, the recently developed mathematical model^25,26 has been used along with the criterion of minimum circulation strength to determine the height of each circulation cell in a tall column. This is then used to derive a theoretical expression, first of its kind, for gas hold-up inside a bubble column. The predictions of this equation as well as the equation derived here for axial liquid velocity at column axis have been compared with available data and the comparison is found to be excellent for both the variables. An explicit relation is derived for the average liquid circulation velocity. The model is also used to derive an expression for liquid axial dispersion coefficient which compares almost exactly with Deckwer et al.'s⁴ correlation.

For circulation in two-dimensional bubble columns a new mathematical model is developed. The predictions of bubble envelope shape and bubble envelope area compare well with published data. The predictions of number of circulation cells in the horizontal direction also compare well with published data.     

ON THE LIQUID FLOW REVERSAL IN BUBBLE COLUMNS

The maximum range of the radial position within which liquid flow reversal can be expected to occur in zero net liquid flow bubble columns is predicted. It is shown that existing models, that employ this position as an input parameter for predicting the liquid velocity profile, are intrinsically valid only when the flow reversal dimensiontess radius is confined to ihe narrow range of 0.644-0.707. It is demonstrated that radial positions outside this range are unacceptable on physical grounds. Guidelines for evaluating the appropriate location of the flow reversal point for typical bubble column operating conditions are proposed.     

PROFILE OF LIQUID FLOW IN BUBBLE COLUMNS

Equations for the liquid velocity profile and the average gas hold-up in bubble columns including cocurrent flow are proposed. It is shown that the inversion point of liquid flow can be used as the characteristic parameter for calculating the liquid flow profiles and gas hold-up. A tracer method was developed to measure the inversion point of liquid flow in bubble column reactors. For water as the liquid phase this inversion point was found at a distance from the column axis of 0·70 0·73 times the column radius. Besides, bubble velocities and bubble diameters in water and methanol-water solutions were determined, using a 5-point conductivity microprobe. It was found that in dilute solutions of methanol the bubble velocity is lower than in pure water. With increasing superficial gas velocity, the bubble velocity steadily increases in pure water, whereas in methanol-water solution it first decreases and, after reaching a minimum, increases too.     

DISPERSION OF GAS AND LIQUID IN BUBBLE COLUMNS OF HOMOGENEOUS BUBBLE FLOW REGIME

Mean gas holdup, lateral distribution of gas holdup and axial mixing of gas and liquid were measured in bubble columns of 12 and 19cm i.d. The lateral distribution of gas holdup was strongly dependent on the flow regimes in the column. The axial mixing of liquid in the homogeneous bubble flow regime was much smaller than that in the heterogeneous bubble flow regime, and was not expressed by existing correlations. The axial mixing of liquid in the homogeneous bubble flow and the intermediate flow regime was simulated with a flow model based on the lateral distribution of buoyancy force and the effective viscosity. The axial mixing of gas was larger than that of liquid.     

CIRCULATION IN LARGE SCALE JET BUBBLE COLUMNS

A hydrodynamic study in a gas-liquid jet bubble column was undertaken in a column with a 122 cm diameter cylindrical section and a conical bottom section approximately 180cm in height. Due to the jetting action in the cone, the circulation patterns are different from those in cylindrical bubble columns. In order to examine this difference in flow pattern, circulation velocity measurements were undertaken (for V₅ = 0.39-2.73 cm/s and V, = 0-0.044 cm/s) using the Pavlov tube technique. These measurements should be helpful in understanding other design parameters (such as mixing, phase distribution, transport coefficients, etc.). Pressure drops were also measured at nine axial taps and using these values, sectional average gas holdup values were calculated. The study was centered on the conical section; the cylindrical bubble column was assumed to be the limiting case for the interpretation of the data. Using the experimental observations, the height of the jet effective region was approximated by two different methods. A simple empirical correlation to find the centerline velocity is proposed.     

A SIMPLE EXPERIMENTAL TECHNIQUE TO MEASURE GAS PHASE DISPERSION IN BUBBLE COLUMNS

A new technique to measure gas phase dispersion in bubble columns is presented. This technique is fairly simple to implement, accurate and inexpensive as compared to conventional methods used in the literature. Nitrogen is used as a tracer and a step change is effected by switching from air to the tracer, nitrogen. The gas phase concentration of oxygen is monitored by means of a polarographic oxygen sensor. The system studied is air-water and the experimental results compare well with the literature data. The technique presents a simple way to measure gas phase axial dispersion in bubble columns and can also be applied to other types of reactors     

A SIMPLE EXPERIMENTAL TECHNIQUE TO MEASURE GAS PHASE DISPERSION IN BUBBLE COLUMNS

A new technique to measure gas phase dispersion in bubble columns is presented. This technique is fairly simple to implement, accurate and inexpensive as compared to conventional methods used in the literature. Nitrogen is used as a tracer and a step change is effected by switching from air to the tracer, nitrogen. The gas phase concentration of oxygen is monitored by means of a polarographic oxygen sensor. The system studied is air-water and the experimental results compare well with the literature data. The technique presents a simple way to measure gas phase axial dispersion in bubble columns and can also be applied to other types of reactors     

MORE ON MIXING OF VISCOUS LIQUIDS IN BUBBLE COLUMNS*

Four aspects of liquid-phase mixing in semi-batch bubble columns operating with viscous and non-Newtonian liquids were studied: the influence of the central plume on mixing rate, the velocity profiles, the gas hold-up, and the back-mixing. Three regimes of the bubble-induced mixing were identified and associated with the mode of the central plume. One of the modes, due to a moderate gas rate, was found to lead to optimal mixing. Also the gas hold-up and the degree of back-mixing were associated with the central plume mode. A model of the velocity profile proposed earlier was now modified and extended to apply also to the turbulent regime.     

THEORETICAL PREDICTION OF GAS HOLDUP IN BUBBLE COLUMNS WITH NEWTONIAN AND NON-NEWTONIAN LIQUIDS

A simple model based on an energy balance which takes into account the friction losses at the gas-liquid interface and the slip velocity of single bubble is used to simulate the gas holdup in bubble columns containing Newtonian and non-Newtonian liquids which circulate in both laminar and turbulent flows. Experimental data available from the literature for bubble columns up to 7 m height and 1 m diameter with water and glycerol as Newtonian liquids and different solutions of CMC in a wide range of concentrations as non-Newtonian liquids are simulated with good agreement despite the simplifications made to describe the gas liquid flow regimes. Most of the differences between experimental and calculated gas holdup are justified on the basis of the simplifying assumptions.     

LIQUID PHASE BACKMIXING IN BUBBLE COLUMN REACTORS—A NEW CORRELATION

Bubble column reactors are widely used in many industrial applications due to their simplicity of operation. Although simple to operate, bubble columns are difficult to scale-up due to the uncertainties in the estimation of some non-adjustable design parameters. One of these design parameters is the liquid phase backmixing.

The present work proposes a new correlation to estimate the liquid phase backmixing in bubble column reactors. The correlation is based on principles originally developed for flow through porous media and uses experimental data obtained over a wide range of operating conditions. This correlation is simple to use and requires parameters which are easily available or can be measured on a small scale apparatus. The proposed correlation shows a significant improvement over available literature correlations and is applicable to three phase systems as well.     

气液及气液固三相浆态鼓泡塔的气相轴向返混系数

气液及气液固三相浆态鼓泡塔的气相轴向返混系数宋同贵，赵玉龙，苏晓丽，张碧江（中国科学院山西煤炭化学研究所，煤转化国家重点实验室，太原０３０００１）关键词：气液鼓泡塔，浆态鼓泡塔，气相轴向返混１前言在鼓泡塔反应器的设计、放大及数学模拟中，气相轴向返混系...     

HYDRODYNAMICS OF BUBBLE COLUMNS WITH TWO IMMISCIBLE LIQUID PHASES

An experimental study of hydrodynamic parameters of bubble columns with the presence of two immiscible liquid phases, water and kerosene, was performed. The solid used consisted of glass beads with a narrow size distribution. The analysis was based on the determination of global gas holdups and phases distribution along the length of the column in the semibatch mode of operation. The results show thai the presence of two immiscible liquid phases significantly reduces the gas holdup in the bubble column as compared to the results obtained with pure aqueous and organic liquid phases. The distribution of solid phase in a slurry bubble column is drastically affected by the presence of immiscible liquid phases, and exhibits a qualitatively different behavior when compared lo systems with one liquid phase. The addition of small amounts of aqueous phase to a slurry bubble column in which the liquid phase is kerosene results in the sedimentation of the solid. As the amount of aqueous phase added is increased, the solid fluidizes yielding almost flat concentration profiles.     

PRESSURE DROPS IN PACKED BUBBLE COLUMNS

Pressure drops were measured for air-water system in a cocurrent, upflow bubble column containing each of four types of packing: Raschig rings, Intalox saddle, open-end screen cylinder and solid cylinder. The ranges of variables studied vary from 0 to0.09m/s for gas flow rate, 0 to 0.094 m/s for liquid flow rate and 0.475 to 0.976 for bed porosity. The experimental data well support a single-parameter model developed on the basis of the separated flow concept of Murdock (1962) and Lin (1982).     

EFFECT OF OPERATING CONDITIONS ON GAS HOLDUP IN SLURRY BUBBLE COLUMNS WITH A FOAMING LIQUID

This work presents experimental data on gas holdup in slurry bubble columns with a foaming liquid. The effects of solids concentration, solid particle size, superficial phase velocities and column dimensions on the gas holdup are analyzed. At low superficial gas velocities (less than 4cm/s), for which the liquid does not foam, the presence of solids with small particle size does not affect the gas holdup whereas solids with large particle size induce foam formation and thus their presence increases the gas holdup. In the foaming regime, an increase of solids concentration decreases the gas holdup. The operating mode has a strong effect on the gas holdup: the semi-batch operating mode (stagnant liquid-solid suspension) increases the ability of the liquid to foam with respect to the continuous mode. Regarding the effect of column dimensions, the results presented show that the height of the bubble column does not affect at an appreciable extent the gas holdup in the range 6 < LID < 12. At high gas velocities (greater than 6 cm/s) the gas holdups obtained in a 30 cm-internal diameter column are the same as those measured in a 10 cm-internal diameter column.     

LIQUID CIRCULATION PATTERNS AND THEIR EFFECT ON GAS HOLD-UP AND AXIAL MIXING IN BUBBLE COLUMNS

Liquid velocity profiles in a bubble column were measured with the aid of a hot-film anemometer. Two different types of profiles were detected that were characterized by the formation of a boundary layer at the column wall. Under slow flow conditions, the boundary layer is large and controls the liquid velocity profile, but in turbulent flow the boundary layer is very small and has no significant effect on the velocity profile. Microscopic and macroscopic balances were used to predict both the liquid velocity profile and the average liquid velocity.

The effect of the liquid velocity profile and average velocity on axial dispersion and gas-hold-up are analyzed and design procedures are recommended.     

LIQUID CIRCULATION PATTERNS AND THEIR EFFECT ON GAS HOLD-UP AND AXIAL MIXING IN BUBBLE COLUMNS

Liquid velocity profiles in a bubble column were measured with the aid of a hot-film anemometer. Two different types of profiles were detected that were characterized by the formation of a boundary layer at the column wall. Under slow flow conditions, the boundary layer is large and controls the liquid velocity profile, but in turbulent flow the boundary layer is very small and has no significant effect on the velocity profile. Microscopic and macroscopic balances were used to predict both the liquid velocity profile and the average liquid velocity.

The effect of the liquid velocity profile and average velocity on axial dispersion and gas-hold-up are analyzed and design procedures are recommended.     

BUBBLE FORMATION IN A TRANSVERSE HORIZONTAL LIQUID FLOW

Over a wide range of liquid velocities, gas emitted from an orifice forms a cylinder or jet of gas which disrupts by varicose instability to generate bubbles. Data is presented which extends the previous use of this model by Silberman (1957).

At liquid velocities low enough to be comparable with bubble rise velocity, bubbles are formed discretely at the orifice. Mathematical models indicate that the value of hydrodynamic mass that is effective in the horizontal direction in delaying bubble release lies between 0 and 0.5 times the displacement. The value of 0.5 is a useful predictive guide up to the point where jetting occurs and the instability model predicts a smaller bubble.     

BUBBLE MOTION IN A THREE-PHASE LIQUID FLUIDIZED BED

This paper presents results for the rise velocities of air bubbles in liquids and liquid-solid fluidized beds. The bubble sizes ranged from approximately 0.03 to 0.45 cm radius. Tap water and distilled water were used as the fluidizing liquids. The solid phase consisted of low density alginate gel beads of mean radius 0.04 cm. The gel beads were translucent which permitted observation of bubbles inside the bed even at large solids volume fractions. Experiments were conducted for solids volume fractions ranging from 15% to 52% and in clear liquids. The goal of the experiments was to determine rise velocities of bubbles and to develop and evaluate correlations of bubble rise velocity based on bubble size, solids volume fraction and liquid properties. It was determined that, for moderate solids fractions (ranging from 28% to 45% solids), a semi-empirical correlation that treated the fluidized bed as a pure liquid with a higher viscosity than the liquid phase could be used to represent the data. The Thomas effective viscosity model was used to predict the viscosity. Provided that one restricts attention to a water fluidized bed, a second empirical correlation can be used to represent the data over a broader range of solids fractions.     

CHARACTERIZATION OF GAS-LIQUID FLOWS IN RECTANGULAR BUBBLE COLUMNS USING CONDUCTIVITY PROBES

Unsteady gas-liquid flows in bubble columns are comprised of various flow processes occurring with varying length and time scales and govern mixing and transport processes. In the present work, we have characterized dynamic and time-averaged properties of gas-liquid flows in rectangular bubble columns using conductivity probes. The development of a single-tip conductivity probe, data processing methodology, and photographic validation procedure is discussed in detail. The effect of superficial gas velocity and aerated liquid height-to-width (H/W) ratio on voidage fluctuations and time-averaged gas holdup was investigated. The experimental data presented here can help in understanding the dynamics of various flow processes and validating computational fluid dynamics based models.     

AN EXPERIMENTAL STUDY OF GAS HOLDUP IN TWO-PHASE BUBBLE COLUMNS WITH FOAMING LIQUIDS

Gas-liquid upward flow experiments have been performed in two bubble columns of different diameters (0.10 and 0.29 m,) using air as gas phase and several liquids: water, aqueous solutions of ethanol and glycerine, kerosene, and a solution of a surfactant in kerosene. The main goal of the study is the analysis of foaming systems, including the comparison of their behavior with respect to non-foaming systems. The gas holdup was determined experimentally as a function of the gas and liquid superficial velocities in bubbling, churn-turbulent and foaming regimes. It was found that, for foaming systems, semi-batch operation enhances foam formation, yielding higher holdups than those obtained in continuous operation at very low liquid velocities. Opposite to what is observed in non-foaming systems, the liquid superficial velocity affects the gas holdup appreciably in foaming systems. An increase in column diameter results in a decrease in gas holdup for all the systems studied. In aqueous foaming systems, this trend is more drastic since foam is inhibited as the column diameter increases.