J.M. van Baten

Influence of scale on the hydrodynamics of bubble columns operating in the churn-turbulent regime: Experiments vs Eulerian simulations

The radial distribution of the liquid velocities, along with the liquid-phase axial dispersion coefficients, have been measured for the air–water system in bubble columns of 0.174, 0.38 and 0.63 m diameter. The experimental results emphasise the significant influence of the column diameter on the hydrodynamics, especially in the churn-turbulent regime. Computational fluid dynamics (CFD) is used to model the influence of column diameter on the hydrodynamics. The bubble column is considered to be made up of three phases: (1) liquid, (2) “small” bubbles and (3) “large” bubbles and the Eulerian description is used for each of these phases. Interactions between the gas phases and the liquid are taken into account in terms of momentum exchange, or drag, coefficients, which differ for these two gas phases. The drag coefficient between the small bubbles is estimated using the Harmathy correlation (A.I.Ch.E. Journal 6 (1960) 281–288). The drag relation for interactions between the large bubbles and the liquid, is developed from analysis of an extensive data base on large bubble swarm velocities measured in columns of 0.051, 0.1, 0.174, 0.19, 0.38 and 0.63 m diameter using a variety of liquids (water, paraffin oil, tetradecane). The interactions between the large and small bubble phases are ignored. The turbulence in the liquid phase is described using the k–e model. The three-phase description of bubble columns was implemented within the Eulerian framework of a commercial code CFX 4.1c of AEA Technology, Harwell, UK. Comparison of the experimental measurements with the Eulerian simulations show good agreement and it is concluded that the three-phase Eulerian simulation approach developed here could be a powerful design and scale-up tool.

Rise velocity of a swarm of large gas bubbles in liquids

This paper develops a procedure for estimation of the rise velocity of a swarm of large gas bubbles in a bubble column operating in the churn-turbulent flow regime. The large bubble swarm velocity is estimated by introducing two correction factors into the classical Davies–Taylor (1950) relation for rise of a single spherical cap bubble in a liquid Vb=0.71gdb(SF)(AF). The scale correction factor (SF) accounts for the influence of the column diameter. This correction is given by the Collins relation (J. Fluid Mech., 28, 97–112, 1967) and is a function of the ratio of the bubble diameter db to the column diameter DT. Volume-of-fluid simulations confirm the validity of the Davies–Taylor–Collins relations for a variety of liquid properties. The acceleration factor (AF) accounts for the increase in the rise velocity of a bubble because of its interaction with the wake of a bubble preceding it. By analysis of video recordings of the interactions between two bubbles, both in-line and off-line, it is found that the acceleration factor AF increases linearly as the vertical distance of separation between the two bubbles decreases. Increasing liquid viscosity reduces this wake acceleration effect. With the aid of an extensive data set on the large bubble swarm velocity in columns of 0.051, 0.1, 0.174, 0.19, 0.38 and 0.63 m in diameter a correlation is developed for the acceleration factor. The large bubble swarm velocity is found to be three to six times higher than that of a single isolated bubble.

Design and scale up of a bubble column slurry reactor for Fischer–Tropsch synthesis

Abstract We develop a strategy for scaling up a bubble column slurry reactor, which is used for example for carrying out the Fischer–Tropsch synthesis reaction. The strategy involves development of a proper description for the large bubble swarm velocity in highly concentrated paraffin-oil slurries in columns of varying diameters. The developed relationship is incorporated into an Eulerian simulation code which is then used to predict the hydrodynamic parameters (hold-up, velocity distribution, etc.) for reactors of commercial scale.

Scaling up bubble column reactors with the aid of CFD

Bubble column reactors, used widely in industry, often have large column diameters (up to 6 m) and are operated at high superficial gas velocities (in the range of 0.1 to 0.4 ms −1 ) in the churn-turbulent flow regime. Experimental work on bubble column hydrodynamics is usually carried out on a scale smaller than 0.3 m, at superficial gas velocities lower than 0.25 ms −1 . The extrapolation of data obtained in such laboratory scale units to the commercial scale reactors requires a systematic approach based on the understanding of the scaling principles of bubble dynamics and of the behaviour of two-phase dispersions in large scale columns. We discuss a multi-tiered approach to bubble column reactor scale up, relying on a combination of experiments, backed by Computational Fluid Dynamics (CFD) simulations for physical understanding. This approach consists of the following steps: (a) description of single bubble morphology and rise dynamics (in this case both experiments and Volume-of-Fluid (VOF) simulations are used); (b) modelling of bubble-bubble interactions, with experiments and VOF simulations as aids; (c) description of behaviour of bubble swarms and the development of the proper interfacial momentum exchange relations between the bubbles and the liquid; and (d) CFD simulations in the Eulerian framework for extrapolation of laboratory scale information to large-scale commercial reactors.

CFD simulations of sieve tray hydrodynamics

A Computational Fluid Dynamics (CFD) model is developed for describing the hydrodynamics of sieve trays. The gas and liquid phases are modelled in the Eulerian framework as two interpenetrating phases. The interphase momentum exchange (drag) coefficient is estimated using the Bennett et al. correlation as a basis. Several three-dimensional transient simulations were carried out for a rectangular tray (5 mm holes, 0.22 m× 0.39 m cross section) with varying superficial gas velocity, weir height and liquid weir loads. The simulations were carried out using a commercial code CFX 4.2 of AEA Technology, Harwell, UK and run on a Silicon Graphics Power Challenge workstation with six R10000 200 MHz processors used in parallel. The clear liquid height determined from these simulations is in reasonable agreement with experimental measurements carried out for air-water in a rectangular tray of the same dimensions. It is concluded that CFD can be a powerful tool for sieve tray design.

Wall effects on the rise of single gas bubbles in liquids

Abstract We report the results of an extensive experimental investigation on the velocity of rise of air bubbles in the size range db = 3 – 80 mm in water. Measurements were made in columns with inside diameters DT = 0.01, 0.02, 0.03, 0.051, 0.1, 0.174 and 0.63 m. The column diameter was found to have a significant effect on the rise velocity of the bubbles. When the ratio of the bubble diameter to the column diameter, db/DT, is smaller than 0.125 the influence of the column diameter on the rise velocity is negligible and the rise velocity is described quite accurately by the Mendelson equation. With increasing db/DT there is a significant reduction of the rise velocity, i.e. there is a significant “wall effect”. The wall effect for spherical cap bubbles rising in inviscid flow, obtained for bubble diameters larger than 0.017 m, is described adequately by the Collins relation. The wall effect for bubbles smaller than 0.017 m is described by an empirical relation suggested by Clift, Grace and Weber.

Simulating the motion of gas bubbles in a liquid

Understanding the motion of gas bubbles in a liquid is a problem of both scientific and engineering importance. About 500 years ago, Leonardo da Vinci summarized his observations on the motion of air bubbles in a liquid: “The air that submerged itself with the water⃛ returns to the air, penetrating the water in sinuous movement, changing its substance into a great number of forms⃛ it never spreads itself out from its path except to the extent to which it avoids the water which covers it”. We have attempted to simulate the motion of single gas bubbles in a liquid using the volume-of-fluid (VOF) technique, which allows us to describe the complex bubble dynamics using only the fluid phase properties as inputs.

Hydrodynamics of internal air-lift reactors: experiments versus CFD simulations

Abstract The hydrodynamics of two configurations of internal airlift reactors, both with a riser diameter of 0.1 m, operating with an air–water system, have been experimentally investigated for a range of superficial gas velocities. The experimental results are compared with a model using Computational fluid dynamics (CFD) with Eulerian descriptions of the gas and liquid phases. Interactions between the bubbles and the liquid are taken into account by means of a momentum exchange, or drag, coefficient based on a literature correlation. The turbulence in the liquid phase is described using the k – e model. The CFD model shows excellent agreement with the measured data on gas holdup, liquid velocity in the downcomer and in the riser. The developed CFD model has the potential of being applied as a tool for scaling up.

Eulerian simulations for determination of the axial dispersion of liquid and gas phases in bubble columns operating in the churn-turbulent regime

Fully three-dimensional (3D) transient simulations using computational fluid dynamics (CFD) have been carried out for bubble columns operating in the churn-turbulent flow regime. The bubble column is considered to be made up of three phases: (1) liquid, (2) small bubbles and (3) large bubbles and the Eulerian description is used for each of these phases. Interactions between both bubble populations and the liquid are taken into account in terms of momentum exchange, or drag, coefficients, which differ for the small and large bubbles. Water and Tellus oil, with a viscosity 75 times that of water, were used as liquid phase and air as gaseous phase. The transient tracer responses in the gas and liquid phases were monitored at three different stations in the column and the results analysed in terms of a one-dimensional axial dispersion model. The 3D simulation results for radial distribution of liquid velocity (V L (r)). centre-line liquid velocity (V L (0)), axial dispersion coefficients of the liquid (D ax,L ) and gas (D ax,G ) phases, for columns of 0.174, 0.38 and 0.63m in diameter were compared with experimental data generated in the laboratories and also literature correlations. There is good agreement between the values of V L (r), V L (0) and D ax,L from 3D simulations with measured experimental data. The axial dispersion coefficient of the small bubble population was almost the same as that of D ax,L , whereas the dispersion of the large bubbles is significantly lower in magnitude. It is concluded that 3D transient Eulerian simulations are potent tools for investigating the gas and liquid residence time distributions and have potential use as scale-up tools.

Radial and axial dispersion of the liquid phase within a KATAPAK-S® structure: experiments vs. CFD simulations

The radial, and axial, liquid-phase dispersion within the catalytically packed criss-crossing sandwich structures of KATAPAK-S has been studied experimentally with the use of computational fluid dynamics (CFD). The KATAPAK-S structure has excellent radial dispersion characteristics. The radial dispersion coefficient in such structures is about one order of magnitude higher than that for conventional packed (trickle) beds. The CFD simulations of the radial dispersion are in good agreement with experiments. At high-liquid loads, there is liquid flow outside the wire gauze envelopes, leading to enhanced axial dispersion. The axial dispersion coefficient of the liquid phase of KATAPAK-S is of the same order of magnitude as the radial dispersion coefficient.