PREDICTION OF GAS-PHASE HOLDUP IN A BUBBLE COLUMN

Hydrodynamic properties of bubble columns play a significant role in many chemical and biochemical processes. Recent theoretical and experimental work conducted by Krishna et al. (1991, 1994), and Wilkinson et al. (1992) have been examined in conjunction with a bubble column and data for the air-water system operating at ambient conditions. The bubble column is 0.108 m in internal diameter, has a 1.70 m tall test section, and is equipped with a perforated plate distributor having 91 holes of 0.8 mm diameter. The data are taken for five values of the slumped water column height in range from 0.79 to 1.15 m, and for superficial air velocities up to about 0.4 m/s.

The data accord to the qualitative aspects of Krishna et al. model but lead to different values of the bubble swarm rise velocity, and superficial transition air velocity characterizing the transition from homogeneous bubbly flow regime to heterogeneous churn-turbulent flow regime. The quantitative reproduction by the model expressions of these recent works of the experimental data is poor. This may be partly attributed to the geometry of the column, diameter and distributor design.

The qualitative features of Krishna et al. model for the two regimes are confirmed by the present data. For quantitative predictions of gas-phase holdup, a new model is proposed in which the large bubble flow in the churn-turbulent regime is formulated following the drift-flux theory. The proposed theory and experimental data are in good agreement.     

SO2 SCRUBBING IN A TAPERED BUBBLE COLUMN SCRUBBER

SO₂ emissions from various sources are found to occur in various concentrations and quantities. Abatement of SO₂ emission, therefore, assumes significant importance over the decades. Wet scrubbers offer great advantage over other devices for gas cleaning. That is the reason that compliance with SO₂ standards will in many cases result in the installation of scrubbing devices. This article presents results of a study on the scrubbing of SO₂ (initial concentration ranging between 400 and 1780 ppm) in a tapered bubble column scrubber using water and dilute sodium alkali. Preliminary studies reveal that the tapered bubble column is capable of generating higher fractional gas holdup than a standard bubble column under similar situations. Moreover, the tapered bubble column has generated bubbles with less power consumption than the existing columns under comparable hydrodynamical conditions. Experimental results indicate that almost 100% SO₂ removal (i.e., zero penetration) can be achieved in the scrubber developed in alkali scrubbing at an optimum Q_L/Q_G ratio of 3.0 m³/1000 ACM. The selection of any gas-cleaning device is based on the performance of the system. In view of this, empirical and semi-empirical correlations are put forward for the prediction of the performance of the scrubber in terms of different pertinent variables of the system for water as well as alkali scrubbing. Experimental results fit extremely well with the correlations. The removal efficiency achievable in the present tapered bubble column scrubber has been found to be higher than that of a single-stage standard bubble column with some modification. The present tapered bubble column is, therefore, hydrodynamically, energetically, and efficiency-wise much better than a standard bubble column.     

GAS-LIQUID MASS TRANSFER IN JET BUBBLE COLUMN

The jet bubble column consists of a conical entrance section which expands to a cylindrical column. Gas and liquid are co-currently introduced at the bottom of the column by a small diameter inlet pipe which acts like an ejector. The kinetic energy of the gas and liquid jet together with the conical geometry at the lower section of the column cause the formation and dispersion of small bubbles.

Gas-liquid mass transfer in the jet bubble column (61 cm diameter) was measured by a dynamic response technique, in which a step change was made in the gas phase oxygen concentration and the aqueous dissolved oxygen concentration response was measured at various axial and radial locations. It was found that a continuous stirred tank reactor model could be used to evaluate experimental results. The volumetric mass transfer coefficient in this type of system was found to increase with increasing gas flow rate and was about 1.5 times larger than the values obtained at similar conditions in conventional bubble columns. Preliminary measurements and calculations indicate negligible effects of liquid velocity and bed height on the mass transfer coefficient.     

ENERGY DISSIPATION DISTRIBUTION AND MIXING IN A TORUS REACTOR

Previous works devoted to the development of the swirl flow generated by rotation of the marine screw impeller in the loop toroidal reactor show that the flow structure is characterized by the existence of a secondary flow. The deformation of the axial velocity profile, due to the bends curvature and the rotation of the agitator, modify considerably the friction and the transfer near the wall. The objective of this work is to study the toroidal flow and to characterize the wall turbulence. The wall shear stress distribution in the torus were performed by using an electrochemical method. The measurements of the wall shear stress were carried out in the inner and outer walls. The evolution of the wall velocity gradient with the longitudinal distance depends on the location in the torus. These results allow to obtain information about the friction factor, which is compared with the one measured directly by using piezometric connections. The experimental determination of the axial dispersion coefficient is obtained by using the dispersion plug flow model with a total recirculation. Finally, a relationship between the turbulent characteristics and the mixing parameters has been established.     

PHASE DISTRIBUTION AND MIXING IN A JET BUBBLE COLUMN

This paper describes the results of an experimental study to evaluate phase holdups and RTD for a jet bubble column. The experimental data were obtained in a 61 cm diameter jet bubble column with a conical inlet. Air and water were used as a two-phase system. The ranges of gas and liquid velocities examined were 0 to 9 cm/sec and 0 to 0·6 cm/sec respectively, both based on the cylinder diameter. The experimental data indicate that in the conical section of the column, the gas holdup first decreases with an increase in distance away from the cone inlet, achieves a minimum and then increases until it reaches a somewhat constant value within the cylinder. Gas holdup varies radially with the maximum at the center and the minimum near the wall. Radially-averaged gas holdup increased with gas velocity and remained essentially unchanged with liquid velocity. The RTD measurements were correlated by a two-dimensional dispersion model. The axial dispersion coefficient increased linearly from the cone inlet to the cylinder. It also increased with the gas velocity. The radial dispersion coefficients were considerably smaller than the axial dispersion coefficients.     

SO2 SCRUBBING IN A TAPERED BUBBLE COLUMN SCRUBBER

SO₂ emissions from various sources are found to occur in various concentrations and quantities. Abatement of SO₂ emission, therefore, assumes significant importance over the decades. Wet scrubbers offer great advantage over other devices for gas cleaning. That is the reason that compliance with SO₂ standards will in many cases result in the installation of scrubbing devices. This article presents results of a study on the scrubbing of SO₂ (initial concentration ranging between 400 and 1780 ppm) in a tapered bubble column scrubber using water and dilute sodium alkali. Preliminary studies reveal that the tapered bubble column is capable of generating higher fractional gas holdup than a standard bubble column under similar situations. Moreover, the tapered bubble column has generated bubbles with less power consumption than the existing columns under comparable hydrodynamical conditions. Experimental results indicate that almost 100% SO₂ removal (i.e., zero penetration) can be achieved in the scrubber developed in alkali scrubbing at an optimum Q_L/Q_G ratio of 3.0 m³/1000 ACM. The selection of any gas-cleaning device is based on the performance of the system. In view of this, empirical and semi-empirical correlations are put forward for the prediction of the performance of the scrubber in terms of different pertinent variables of the system for water as well as alkali scrubbing. Experimental results fit extremely well with the correlations. The removal efficiency achievable in the present tapered bubble column scrubber has been found to be higher than that of a single-stage standard bubble column with some modification. The present tapered bubble column is, therefore, hydrodynamically, energetically, and efficiency-wise much better than a standard bubble column.     

Hydrodynamics of a Tapered Bubble Column

The hydrodynamic behavior of a single‐stage tapered bubble column using an air‐water two‐phase system has been studied. The experimental results indicate that the holdup increases with increasing superficial gas velocity and bubble slip velocity, while it remains constant with increasing superficial liquid velocity. The gas flow rate has a subtle effect on pressure drop owing to the dynamic pressure recovery stemming from the increase in flow area in the axial direction. The results further suggest that the tapered bubble column shows higher holdup with lower energy dissipation than the conventional bubble columns under similar hydrodynamic conditions. The experimental values of the holdup are in excellent agreement with the well‐known Akita and Yoshida correlation available in the existing literature. Also, the performance of the tapered system has been shown to be much better than that of conventional columns under similar conditions in water/alkaline scrubbing of fly ash and SO₂ either alone or in combination.     

气液两相单孔鼓泡流体动力学行为混沌预测

应用混沌预测方法 ,对气液两相单孔鼓泡系统的压力波动时间序列进行了短期预测 .结果表明 ,混沌预测方法是预测气液两相鼓泡系统压力波动等流体动力学行为的有效新途径 .     

浮选柱气泡发生器的研究

通过对新型浮选柱气泡发生器的研究,目的在于对工业生产浮柱中出现的具体问题提供解决的途径,并对静态混合器的研究提供可靠的理论依据。     

SOLIDS DISTRIBUTION IN A BATCH BUBBLE COLUMN

This paper presents some new experimental data for axial solid distribution in batch slurry reactors. The effects of liquid and solid properties on the solid dispersion behavior in 0.076 m and 0.305 m diameter slurry bubble columns are outlined. The effects of gas distributor, column internals and particle size distribution on the solids concentation profiles are also examined. Critical gas velocity for complete suspension of solids are measured and compared with those predicted from the available literature correlations. It is shown that when the particles are completely suspended, the Peclet number obtained from the sedimentation-dispersion model depends very significantly on the assumed boundary conditions. The effect of gas velocity on the particle Peclet number depends on the nature of the liquid-solid system.     

PREDICTION OF PRESSURE DROP IN A VENTURI SCRUBBER: EFFECT OF LIQUID FILM AND RE-ENTRAINMENT

A mathematical model has been developed for prediction of pressure drop in a Venturi scrubber. This model includes the effect of the amount of liquid film and re-entrainment of liquid droplets from liquid film. The result of the present model is compared with experimental data of Viswanathan et al. (1985) Viswanathan, S., Gnyp, W. A. and Pierre, St., C. 1985. Annular flow pressure drop model for pease-Anthony-type Venturi scrubbers. AIChE. J., 31(12): 1947–1958.  [Google Scholar] as well as with the other models (Viswanathan et al., 1985 Viswanathan, S., Gnyp, W. A. and Pierre, St., C. 1985. Annular flow pressure drop model for pease-Anthony-type Venturi scrubbers. AIChE. J., 31(12): 1947–1958.  [Google Scholar]; Boll, 1973 Boll, R. H. 1973. Particle collection and pressure drop in Venturi scrubbers. Ind. Eng. Chem. Fundam., 12: 40[Crossref], [Web of Science ®] , [Google Scholar]). Results of this study indicate that at high liquid to gas ratios prediction of pressure drop can be improved by considering re-entrainment and liquid film effects. Also the effects of gas velocity and liquid to gas ratio were investigated on the rate of droplet re-entrainment and pressure drop.     

PREDICTION OF PRESSURE DROP IN A VENTURI SCRUBBER: EFFECT OF LIQUID FILM AND RE-ENTRAINMENT

A mathematical model has been developed for prediction of pressure drop in a Venturi scrubber. This model includes the effect of the amount of liquid film and re-entrainment of liquid droplets from liquid film. The result of the present model is compared with experimental data of Viswanathan et al. (1985) as well as with the other models (Viswanathan et al., 1985; Boll, 1973). Results of this study indicate that at high liquid to gas ratios prediction of pressure drop can be improved by considering re-entrainment and liquid film effects. Also the effects of gas velocity and liquid to gas ratio were investigated on the rate of droplet re-entrainment and pressure drop.     

HYDRODYNAMIC STUDIES IN FISCHER-TROPSCH DERIVED WAXES IN A BUBBLE COLUMN

Gas hold-up and Sauter mean bubble diameter measurements were made in a 0.051 m diameter by 3 m long glass bubble column in the system, nitrogen-molten wax, with three different waxes (paraffin wax FT-300, Sasol's Arge wax and Mobil's reactor wax). Paraffin wax has a tendency to foam and gas hold-up is a strong function of gas distributor type, temperature and start-up procedure, whereas the reactor waxes do not foam and are much less affected by these variables, In experiments at 265°C with a 1.85 mm single hole orifice plate distributor the gas hold-ups were nearly the same for all three waxes. However, significant differences in Sauter mean bubble diameters were obtained in experiments with different waxes; FT-300 wax produced the smallest Sauter mean bubble diameters whereas Mobil's reactor wax produced the largest bubbles. Addition of 1-octadecanol and octadecanoic acid (up to 10wt%) to the FT-300 paraffin wax caused an increase in gas hold-up and a delay in the foam break-up in runs at 265°C with the 1.85 mm orifice plate distributor.     

气—液鼓泡塔中流动域和气含率的分布

利用压力传感器测定了气、液两相鼓泡塔内不同轴、径向位置上压强的脉动信号,分析了压强的脉动特征,计算了各种操作条件下塔内不同轴向位置截面上的平均气含率,发现在不同操作条件下,塔内可能出现不同的流动域分布,即全塔均匀鼓泡域、全塔过渡域、全塔湍动鼓泡域以及均匀鼓泡域,过渡域和湍动鼓泡域中的相邻2种或3种流域同时出现在塔内不同高度上。提出了不同轴向位置流域转变的判据。并发现,同一截面的不同径向位置基本处于同一流域内。同时研究了鼓泡塔内气含率在轴向上的分布规律,给出了不同流动域内气含率的关系式。     

HYDRODYNAMICS, MIXING AND MASS TRANSFER IN A LARGE DIAMETER JET BUBBLE COLUMN

Experimental measurements for the axial and radial variations in gas holdup, axial and radial dispersion coefficients, volumetric gas-liquid mass transfer coefficient and liquid phase circulation velocity in a cone of a large diameter (122 cm) jet bubble column are presented. Two diameters of the inlet nozzle, namely 10.16 cm and 15.24 cm, three superficial gas velocities (based on cylinder diameter), 3 cm/sec, 6 cm/sec and 8 cm/sec and two superficial liquid velocities, 0.3 cm/sec and 0.6 cm/sec, are examined. The experimental data are obtained for two different bed heights.

The experimental data showed significant axial and radial variations in the gas holdup. The volumetric average gas holdup was higher at higher gas velocity and larger nozzle diameter and somewhat higher at lower liquid velocity. The axial dispersion was high while the radial dispersion was low. The volumetric gas-liquid mass transfer coefficient was larger at higher gas velocity and larger nozzle diameter. The liquid recirculation begins only at the upper end of the cone. In general, experimental data indicate that a jet bubble column provides a high degree of mixing and transport rates.     

PRESSURE DROP AND MASS TRANSFER COEFFICIENTS IN CONCURRENT SCREEN PLATE BUBBLE COLUMN

Pressure drop and oxygen desorption from water and four other aqueous CMC solutions were determined in a 5 cm diameter multistage bubble column with and without plate reciprocation. The plates were made from stainless steel wire screens of porosities greater than 0.62. The column pressure loss was found to increase with plate agitation, phase velocities and screen mesh number. A modified Reynolds number was proposed to permit a reasonable prediction of the pressure loss, based on the model of Noh and Baird (1984). At a specific power consumption, the present volumetric mass transfer coefficients arc considerably larger than those reported in earlier studies with sieve plates. The coefficients were correlated with specific power input, phase flow velocities and system physical properties.     

导流管型鼓泡塔中带泡沫非牛顿型液体的气含率

在塔径０１１２ｍ的三组不同直径导流管装置中，采用液体溢出法测定了带泡沫的非牛顿型液体的气含率分析了表观气速Ｖｇ０和管径比（ＤＲ／Ｄ０）对气含率的影响     

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).     

EFFECTS OF TURBULENT SHEAR STRESS ON THE GROWTH OF SUSPENDED PLANT CELLS IN A BUBBLE COLUMN REACTOR

In this paper,effects of increasing the height of column,bubble size and gas flow rate onthe growth of suspended Catharanthus roseus plant cells in a 1.5 litre bubble column were studied.Statistical characteristics of turbulence such as auto-correlation function,and auto-power spectral den-sity function of the nuctuating liquid velocity were measured in a bubble column employing pseudo-cell-granulated agar.Calculations of turbulent shear stress were carried out.From these studies it is sug-gested that in plant cell suspension culture sparged with large bubbles.turbulent shear stress is prob-ably one of the most important reasons for the inhibited cell growth and cell damage.Larger bubblesor higher gas flow rates lead to the increase in shear stress and are more detrimental to plant cells.     

气泡塔生物反应器中传热特性的研究

用热探针测定了气泡塔中不同工况的给热系数;推导了一个既可用于牛顿流体;又可用于非牛顿流体的计算气泡塔中给热系数的一般理论模型,其计算值与本文的实验数据、理论分析取得了一致的结果。本工作对分析生物反应器的传热特性有重要意义。