J. Ellenberger

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.

A unified approach to the scale-up of gas-solid fluidized bed and gas-liquid bubble column reactors

The aim of this work is to develop a unified approach to the scale-up of gas—solid (G—S) fluid bed and gas—liquid (G—L) bubble column reactors. The unified approach relies on analogies in the hydrodynamic behavior of G—S and G—L systems in both the homogeneous and heterogeneous flow regimes. The homogeneous G—S fluidization regime is to be identified with homogeneous G—L bubbly flow while the heterogeneous G—S fluidization regime is to be identified with the churnturbulent regime for G—L bubble columns. In the heterogeneous flow regime of operation, the classic two-phase theory developed for G—S fluidized beds can be applied with profit to describe also the hydrodynamics of G—L bubble columns provided the “dilute” phase is identified with the fast-rising large bubbles, and the “dense” phase is identified with the liquid phase containing entrained “small” bubbles. Quantitative analogies in the hydrodynamic behavior of G—S and G—L systems are demonstrated by the use of extensive experimental data obtained in five columns of three different diameters (2 × 0.1 m, 1 × 0.19 m and 2 × 0.38 m). The total expanded bed height in the experiments varied in the range 0.5–3.5 m. About 4,000 dynamic gas disengagement experiments were carried out with various systems to determine the total gas hold-up and the hold-ups of the “dilute” and “dense” phases. The gas phases used in the experiments were air, helium, argon and sulfur hexafluoride. Fluidized cracking catalyst (FCC) was used as the solid phase in fluid bed experiments. In bubble column operations the liquid phase used was water, paraffin oil or tetradecane. Sintered plates were used for gas distribution in all the columns. The gas hold-up in the “dense” phase, ɛdf was found to be practically independent of the scale of operation. The hold-up of the fast-rising “dilute” phase, ɛb, on the other hand was found to be a significant function of the column diameter,DT, and of the total dispersion height,H. The “dilute” phase gas hold-up can be modeled for both G—S and G—L systems using a theory allowing for bubble growth in the region above distributor. The bubbles are assumed to grow in diameter up to a distanceh*, at which the bubbles reach their equilibrium size. The equilibration heighth* was found to increase with the superficial gas velocity through the dilute phase,U —Udf). For air—FCC, the value ofh* varies in the range 0.4–1.2 m. For bubble columns the values ofh* are significantly smaller and lie in the range 0–0.5 m. Increasing gas density increases the total gas voidage in both G—S and G—L systems but has no significant effect on the hold-up of the dilute phase. In G—L bubble columns, the liquid properties affect the total gas hold-up but have only a minor influence on the dilute phase hold-up. The unified model to describe the bubble hydrodynamics in G—S fluid beds and G—L bubble columns is a useful tool in scaling-up these two reactor types, because of the possibilities of cross-fertilization of design data.

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.

Counter-current operation of structured catalytically packed distillation columns: pressure drop, holdup and mixing

Structured packed columns, in which the catalyst particles are enclosed within wire gauze envelopes (“sandwiches”) are promising reactor configurations for reactive distillation and hydroconversions. By allowing preferential channels for the gas and liquid phases, counter-current operation is achieved even for millimeter sized catalyst particles without the problem of flooding. This paper reports the results of a comprehensive experimental study of the hydrodynamics of structured packed columns of 0.1 and 0.24 m diameter. The pressure drop is found to increase sharply when the superficial liquid velocity exceeds a certain threshold value. This threshold corresponds to the situation in which a maximum flow of liquid in the packed channels is realized and the excess liquid flows through the “open” channels. The liquid flow in the open channel causes a sharp rise in the pressure drop. A model is developed to describe the holdup of liquid in the open channels. With increasing liquid flow rate, the pressure drop is found to increase exponentially with the liquid holdup within the open channels. Liquid phase residence time distribution studies lead to the conclusion that there is a good exchange of the liquid phase inside and outside the packed channels. The residence time distribution can be described by an axial dispersion model. Compared with a trickle bed reactor, the results of this study show that a structured packed column has a much larger operating window at a much lower pressure drop.

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.

Flow regime transition in bubble columns

The various factors influencing the regime transition point in gas-liquid bubble columns are examined. Increasing gas density delays regime transition. This phenomenon is described in a qualitative way by the correlations of Reilly [1] and Wilkinson [2] of which the Reilly correlation is found to be more accurate. However, both correlations are unable to account for the influence of the addition of small quantities of surface tension reducing agents. The Reilly and Wilkinson correlations are also not adequate to describe the influence of the addition of catalyst particles on the transition point for a bubble column slurry reactor.

Reactor development for conversion of natural gas to liquid fuels: A scale up strategy relying on hydrodynamic analogies.

Abstract Gas-solid fluidized beds and bubble column slurry reactors are very commonly encountered in processes for conversion of natural gas to liquid fuels and light olefins. This paper proposes a unified design and scale-up strategy for such “fluidized” multiphase reactors. The approaches uses hydrodynamic analogies as a basis. These analogies have been investigated in a variety of gas-solid and gas-liquid systems in several columns varying in diameters between 0.05 m and 0.63 m. It is argued that the appreciation of the hydrodynamic analogies will allow cross-fertilisation of concept and design data, thereby reducing process development costs.

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.