Hydrodynamics Of Bubble Column Reactors Research Articles

CFD modeling of bubble column: Influence of physico-chemical properties of the gas/liquid phases properties on bubble formation

Abstract Computational fluid dynamics (CFD) has been widely used to study the hydrodynamics of bubble column reactors. However, the interaction between the different phases, which are in fact intimately linked, and the effect of their physical properties on the dynamics of the bubbles are still not well understood. In this study, the population balance equation (PBE) is coupled with CFD model to investigate the effect of liquid and gas phases physico-chemical properties on bubbles formation and the hydrodynamic characteristics in bubble column reactors. The coupling is realized using AVL FIRE v.2017 software, and the predicted results are validated against published experimental data. User subroutines are written using FORTRAN to incorporate the scalar transport equation source term for bubble break-up and coalescence. The predicted results were in reasonable agreement with experimental observations and available literature results, since the model has been able to predict the effect of gas flow rate on the gas holdup in bubble column within the range of ±7%. The simulations showed that the average gas holdup increase with the increase in superficial gas velocity and gas phase density, and decrease with the increase in liquid phase density. It was also found that Sauter mean bubble diameter increases with the increase in liquid density and decreases with the increase in gas density. Finally, the bubble rise velocity increased when water was used as a continuous phase. On the other hand, the increase in gas density causes a decrease in the bubble rise velocity.

Local hydrodynamic parameters of bubble column reactors operating with non‐Newtonian liquids: Experiments and models development

The effects of liquid phase rheology on the local hydrodynamics of bubble column reactors operating with non‐Newtonian liquids are investigated. Local bubble properties, including bubble frequency, bubble chord length, and bubble rise velocity, are measured by placing two in‐house made optical fiber probes at various locations within a bubble column reactor operating with different non‐Newtonian liquids. It was found that the presence of elasticity can noticeably increase the bubble frequency but decreases the bubble chord length and its rise velocity. The radial profiles of bubble frequency, bubble chord length, and bubble rise velocity are shown to be relatively flat at low superficial gas velocity while they become parabolic at high superficial gas velocity. Moreover, the bubble size and gas holdup are correlated with respect to dimensionless groups by considering the ratio between dynamic moduli of viscoelastic liquids. The novel proposed correlations are capable of predicting the experimental data of bubble size and gas holdup within a mean absolute percentage error of 9.3% and 10%, respectively. © 2015 American Institute of Chemical Engineers AIChE J, 62: 1382–1396, 2016

Development of support vector regression (SVR)-based correlation for prediction of overall gas hold-up in bubble column reactors for various gas–liquid systems

The objective of this study was to develop a unified data-driven correlation for the overall gas hold-up for various gas–liquid systems using support vector regression (SVR)-based modeling technique. Over the years, researchers have amply quantified the hydrodynamics of bubble column reactors in terms of the overall gas hold-up. In this work, about 1810 experimental points were collected from 40 open sources spanning the years 1965–2007. The model for overall gas hold-up was established as a function of several parameters which include superficial gas velocity, superficial liquid velocity, gas density, molecular weight of gas, sparger type, sparger hole diameter, number of sparger holes, liquid viscosity, liquid density, liquid surface tension, operating temperature, operating pressure and column diameter of the gas–liquid system. For understanding the hold-up behavior, the data used for training the model was grouped into various gas–liquid systems viz., air–water, gas–aqueous viscous liquids, gas–organic liquids, gas–aqueous electrolyte solutions and gas–liquid systems operated over a wide range of pressure. A generalized model established using SVR was evaluated for its performance for various gas–liquid systems. Statistical analysis showed that the proposed generalized SVR-based correlation for overall gas hold-up has prediction accuracy of 97% with average absolute relative error (% AARE) of 12.11%. A comparison of this correlation with the selected system specific correlations in the literature showed that the developed SVR-based correlation significantly gives enhanced prediction of overall gas hold-up.

Hydrodynamics of Bubble Column Reactors at High Gas Velocity: Experiments and Computational Fluid Dynamics (CFD) Simulations

This paper focuses on the modeling of flow and mixing in a bubble column reactor operated at high gas velocities (up to 0.40 m/s). A dual-tip conductivity probe was used to measure local void properties such as local time-averaged gas holdup, chord length distribution, bubble velocity distribution, and interfacial area. Chord length distribution was converted to bubble size distribution, using the backward transformation method. Liquid-phase mixing time measurements were conducted using a conductivity probe. A computational fluid dynamics (CFD) model was developed to simulate the unsteady gas−liquid flow in a bubble column using commercial code FLUENT 6.2. The time-averaged flow properties predicted by CFD simulations were compared with the experimental data. The role of unsteady flow structures in mixing was studied. The “multiple snapshots” approach was used to simulate the mixing time using CFD. The mixing times that were predicted for all superficial gas velocities compared favorably to the measured values. This study of the hydrodynamic behavior of a bubble column at higher gas velocity provides a basis for understanding and simulating solid suspension (or solid mixing) in slurry bubble column reactors.

Experimental and numerical studies of void fraction distribution in rectangular bubble columns

Bubbly flow is encountered in a wide variety of industrial applications ranging from flows in nuclear reactors to process flows in chemical reactors. The presence of a second phase, recirculating flow, instabilities of the gas plume and turbulence, complicate the hydrodynamics of bubble column reactors. This paper describes experimental and numerical results obtained in a rectangular bubble column 0.1 m wide and 0.02 m in depth. The bubble column was operated in the dispersed bubbly flow regime with gas superficial velocities up to 0.02 m/s. Images obtained from a high speed camera were used to observe the general flow pattern and have been processed to calculate bubble velocities, bubble turbulence parameters and bubble size distributions. Gas disengagement technique was used to obtain the volume averaged gas fraction over a range of superficial gas velocities. A wire mesh sensor was applied, to measure the local volume fraction at two different height positions. Numerical calculations were performed with an Eulerian–Eulerian two-fluid model approach using the commercial code CFX. The paper details the effect of various two-fluid model interfacial momentum transfer terms on the numerical results. The inclusion of a lift force was found to be necessary to obtain a global circulation pattern and local void distribution that was consistent with the experimental measurements. The nature of the drag force formulation was found to have significant effect on the quantitative volume averaged void fraction predictions.

Numerical description of flow regime transitions in bubble column reactors by a multiple gas phase model

The study of the hydrodynamics of bubble column reactors reveals that three main flow regimes can be distinguished. The present study is an original application of standard numerical multiphase models aiming at a better prediction of the transitions and a better description of these regimes. This approach consists of a two-step calculation with, in a first time, the determination of bubble size distributions using coalescence and break-up laws, and in a second time, the implementation of this distribution in a multiple gas phase Euler–Euler model. Calculations are compared with experimental data: liquid velocities and global gas hold-ups. They were obtained in a cylindrical bubble column in which local or uniform aerations were performed. The comparison shows that our approach allows a good prediction of both flow regime transitions and flow characteristics such as hydrodynamics or turbulence. Furthermore, two bubble-induced turbulence models consisting, respectively, of an added viscosity and of two turbulence source terms added in the k– ε transport equations have been compared.

Influence of scale on the hydrodynamics of bubble column reactors: an experimental study in columns of 0.1, 0.4 and [formula omitted] diameters

Measurements of liquid velocities and liquid mixing have been performed in three different bubble columns of 0.15, 0.4 and 1 m diameter with water and air, at superficial gas velocity ranging from 0.05 to 0.20 m/ s with high aspect ratios ( H D / D>4). Liquid velocities are determined with a Pavlov tube calibrated up to 25% of gas holdup. Axial dispersion coefficient is determined using a new method which allows accounting for the upflow and downflow regions. The experimental results allow selecting reliable correlations of the literature. The Ueyama and Miyauchi model is successfully compared with the experiments and a new correlation for the kinematic viscosity is proposed.