Simulation of oscillatory baffled column: CFD and population balance

In the present work, an attempt has been made to combine population balance and a CFD approach for simulating the flow in oscillatory baffled column (OBC). Three-dimensional Euler-Euler two-fluid simulations are carried out for the experimental data of Oliveira and Ni [2001. Gas hold-up and bubble diameter in a gassed oscillatory baffled column. Chemical Engineering Science 56, 6143-6148]. The experimental data include the average hold-up profile and bubble size distribution in the OBC. All the non-drag forces (turbulent dispersion force, lift force) and the drag force are incorporated in the model. The coalescence and breakage effects of the gas bubbles are modeled according to the coalescence by the random collision driven by turbulence and wake entrainment while for bubble breakage by the impact of turbulent eddies. Predicted liquid velocity and averaged gas hold-up are compared with the experimental data. The profile of the mean bubble diameter in the column and its variation with the superficial gas velocity is studied. Bubble size distribution obtained by the model is compared with the experimental data.     

Numerical simulation of multiphase flow in bubble column reactors. Influence of bubble coalescence and break-up

Population balance equations have been combined to a classical hydrodynamic Euler/Euler simulation to investigate the operation of a cylindrical bubble column. The MUSIG (mutiple-size-group) model implemented in the CFX 4.3 commercial software has been used. Hydrodynamic experimental variables, i.e. local axial liquid velocity and local gas hold-up, have been compared to the corresponding calculated values, showing a quite good agreement, except for the gas hold-up when the column is no more operating in the homogeneous regime. Bubble sizes have been investigated, showing that two domains of superficial gas velocities can be distinguished. In the first domain, coalescence occurs predominantly, Sauter diameter increases with the superficial gas velocity, bubble size distribution is narrow and Sauter diameter is continuously evolving along the column axis. In the second domain, break-up becomes more intensive and compensates coalescence, bubble size distribution becomes wider, since more small bubbles are formed, an equilibrium Sauter diameter appears when the superficial gas velocity increases. Furthermore an equilibrium Sauter diameter appears along the column axis, and it can be noticed that this phenomenon appears lower in the column when the gas flow rate is increased. In these two domains the characteristics of the bubbles are typical of those of the homogeneous and transition regimes.     

Experimental and Numerical Study of Gas-Liquid Flow in a Sectionalized External-Loop Airlift Reactor

The external loop airlift reactor(EL-ALR) is widely used for gas-liquid reactions. It's advantage of good heat and mass transfer rates compared to conventional bubble column reactors. In the case of fermentation application where a medium is highly viscous and coalescing in nature, internal in riser helps in the improvement of the interfacial area as well as in the reduction of liquid-phase back mixing. The computational fluid dynamic(CFD) as a tool is used to design and scale-up of sectionalized external loop airlift reactor. The present work deals with computational fluid dynamics(CFD) techniques and experimental measurement of a gas hold-up, liquid circulation velocity, liquid axial velocity, Sauter mean bubble diameter over a broad range of superficial gas velocity 0.0024≤U_G≤0.0168 m·s~(-1). The correlation has been made for bubble size distribution with specific power consumption for different plate configurations. The effects of an internal on different mass transfer models have been completed to assess their suitability.The predicted local mass transfer coefficient has been found higher in the sectionalized external loop airlift reactor than the conventional EL-ALR.     

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.     

Fluid dynamics and mass transfer in a bubble column with suspended particles

Slurry bubble columns are widely used in biotechnology. Therefore, the effects of solid particles on fluidization characteristics, gas hold-up and volumetric liquid-side mass transfer coefficient were measured in a slurry bubble column (i.d. 0.14 m). The density and diameter of the suspended particles were similar to those applied in biotechnology with immobilized bacteria. Based on models of turbulence and of liquid circulation induced by rising gas bubbles, equations for critical gas velocity, gas hold-up and volumetric liquid-side mass transfer coefficient were obtained by dimensional analysis.     

Radial Profiles of Gas Bubble Behavior in a Gas‐Liquid Bubble Column Reactor under Elevated Pressures

The effects of operating conditions on radial variation of gas holdups, bubble swarm rising velocity, and Sauter mean diameter were investigated in a bubble column reactor under elevated pressures using a conductivity probe method. Air served as gas phase and tap water as liquid phase with varying gas velocity and pressure. All three parameters increased constantly with higher superficial gas velocity. Maximum holdup, velocity, and Sauter mean diameter were found at the center of the cross section. Two different cases for Sauter mean diameter distribution were observed. The gas holdups increase continuously with higher system pressure, but decrease for bubble swarm rising velocity and Sauter mean diameter. According to experimental results, an empirical correlation of the gas holdup profiles is presented.     

The Effect of Sparger Geometry on Gas Bubble Flow Behaviors Using Electrical Resistance Tomography

By electrical resistance tomography （ERT） the cross sectional profiles of gas hold-up in a φ56mm bubble column are obtained with four designs of gas sparger. The effect of sparger geometry on the bubble distribution is revealed by applying a sensitivity conjugated gradients reconstruction method （SCG）. Experimental results show that overall hold-up obtained by ERT is generally in good agreement with those measured with the pressure transducer and the ERT system produces informative evidence that the radial profiles of hold-up is very similar to the sparger design in the lower section of bubble column. Meanwhile, the rise velocity of bubble swarm and the Sauter mean bubble size are evaluated using ERT based on dynamic gas disengagement theory. The experimental results are in good agreement with correlations and conventional estimation obtained using pressure transmitter methods.     

Mass transfer with chemical reaction in liquid foam reactors

Mass transfer studies were conducted in a stable liquid foam reactor under various operating conditions to evaluate gas holdup, effective interfacial area, liquid-phase mass transfer coefficient and a modified interfacial mass transfer coefficient to include the surface-active agents employed. Gas holdup and effective interfacial area were evaluated experimentally. The interfacial mass transfer coefficient was evaluated semitheoretically, by considering the interfacial region as a separate phase and using the experimental data developed for mass transfer accompanied by a fast first-order chemical reaction. The liquid-phase mass transfer coefficient was also evaluated semitheoretically, using Danckwert's theory for the liquid phase and the experimental data on mass transfer accompanied by a slow pseudofirst-order chemical reaction. An experimental unit was set up to provide a stable flowing foam column, simulating the foam reactor. Mass transfer rates were studied for superfacial gas velocities in the range from 1.5 × 10⁻² m/s to 5 × 10⁻² m/s, giving gas residence times in the range from 20 to 55 seconds. A cationic and nonionic surface-active agent and three different wire mesh sizes, giving bubble size distributions in the range from 2.2 to 5.4 mm Sauter mean diameters, were employed. It is observed that gas holdup is insensitive to the type of surface-active agent; it is however, dependent on wire mesh size and gas velocity. The bubble diameter and, hence, the interfacial area are found to be insensitive to gas velocity in the range studied; they are, however, strong functions of wire mesh size. The liquid-phase mass transfer coefficient increases with increase in gas velocity. The surface-active agent introduces additional resistance to mass transfer in both reaction cases, this being the controlling one in the case of the fast reaction. A comparison with conventional packed bed contactors indicates the mass transfer rates to be about 8 times lower for the foam reactor, for the fast reaction case; for slow reactions, the foam reactor has mass transfer rates approximately 2-4 times higher than those for conventional packed bed contactors.     

Hydrodynamic studies with foaming and non-newtonian solutions in bubble columns

Experiments were conducted in 0.05 m ID and 0.23 m ID by 3 m tall bubble columns to study the effect of surfactants and viscosity of liquid medium on gas hold-up and Sauter mean bubble diameter. The addition of n-butanol (0.5 and 1 wt.%) to water leads to the formation of foam and consequently produces higher gas hold-ups. The foam could be eliminated completely with the addition of a sufficient quantity (0.5 wt.%) of carboxymethyl cellulose to the aqueous alcohol solution. In the absence of foam, gas hold-ups were similar to those obtained with pure liquids. Sauter mean bubble diameters, obtained using the dynamic gas disengagement technique, increase with viscosity of liquid medium.     

Numerical simulation of the gas–liquid flow in a laboratory scale bubble column: Influence of bubble size distribution and non-drag forces

In the present work, a computational model based on an Eulerian–Eulerian approach was used for the simulation of the transient two-phase flow in a rectangular partially aerated bubble column. Superficial gas velocities (U_G) ranging from 0.24 to 2.30 cm/s were used throughout both the experiments and the simulations. The calculated results were verified by comparing them with experimental data including measurements of gas hold-up, plume oscillation period (POP) and Sauter mean bubble diameter. The study shows the effect of mesh refinement, time-step and physical model selection, the latter regarding the role of bubble size distribution and non-drag forces, on the computational results. According to the results presented here, the representation of bubble populations using multiple size groups (MUSIG model) instead of a single group improves the prediction of the experimental parameters under study. Additionally, the results obtained after including the virtual mass force term do not differ considerably from those obtained including only the drag force. On the contrary, as a consequence of introducing the lift force term into the model, the gas hold-up is overestimated and a non-symmetric bubble plume oscillation appears, a fact that is not experimentally observed.     

Investigations on hydrodynamics and mass transfer in gas–liquid stirred reactor using computational fluid dynamics

In this work, the hydrodynamics and mass transfer in a gas–liquid dual turbine stirred tank reactor are investigated using multiphase computational fluid dynamics coupled with population balance method (CFD–PBM). A steady state method of multiple frame of reference (MFR) approach is used to model the impeller and tank regions. The population balance for bubbles is considered using both homogeneous and inhomogeneous polydispersed flow (MUSIG) equations to account for bubble size distribution due to breakup and coalescence of bubbles. The gas–liquid mass transfer is implemented simultaneously along with the hydrodynamic simulation and the mass transfer coefficient is obtained theoretically using the equation based on the various approaches like penetration theory, slip velocity, eddy cell model and rigid based model. The CFD model predictions of local hydrodynamic parameters such as gas holdup, Sauter mean bubble diameter and interfacial area as well as averaged quantities of hydrodynamic and mass transfer parameters for different mass transfer theoretical models are compared with the reported experimental data of [Alves et al., 2002a] and [Alves et al., 2002b] . The predicted hydrodynamic and mass transfer parameters are in reasonable agreement with the experimental data.     

Solid effects on hydrodynamics and heat transfer in an external loop airlift reactor

The effect of a solid presence on global hydrodynamic parameters and heat transfer in an external loop airlift reactor has been experimentally investigated. Results obtained in both two- and three-phase flow are presented in this study. Two different external loop airlift reactor sizes have been used and local hydrodynamic characteristics including local gas hold-up and bubble velocity have been obtained in two-phase flow. Optical and ultrasound probes have been used to obtain this information, respectively. It was found that an increase of solid hold-up leads to a decrease of liquid velocity and heat transfer coefficient. Measured in a two- and three-phase reactor using a horizontal-heating probe, a correlation of the average gas hold-up and heat transfer coefficient is proposed. Correlation parameters are identified in homogeneous and heterogeneous flow regimes, which have been derived from the gas slip velocity concept. The experimental liquid velocity and gas hold-up in the riser have been represented in a satisfactory way by a hydrodynamic model, either in the absence or in the presence of solid particles.     

COMPREHENSIVE STUDY OF THE GAS HOLD UP AND BUBBLE SIZE DISTRIBUTION IN HIGHLY VISCOUS LIQUIDS II. CMC SOLUTIONS

Following properties of short bubble columns employing CMC solutions (1.0, 1.4 and 2.0%) and perforated plates (0.5, 1.0 and 3.0 mm hole diameters) were determined: relative mean gas hold up, E_G, bubble size distribution, Sauter bubble diameter, d_s, and the specific geometrical bubble surface areas, a' and a' due to the “intermediate to large bubbles.” The a' values were compared with the corresponding volumetric mass transfer coefficients, k_La's and the mass transfer coefficients, k_L, were estimated. The properties of these systems were investigated as function of the superficial gas velocity, W_SG, CMC concentration and aerator type.     

Gas holdup,bubble size distribution,and mass transfer in an airlift reactor with ceramic membrane and perforated plate distributor

The airlift reactor is one of the most commonly used gas–liquid two-phase reactors in chemical and biological processes. The objective of this study is to generate different-sized bubbles in an internal loop airlift reactor and characterize the behaviours of the bubbly flows. The bubble size, gas holdup, liquid circulation velocity, and the volumetric mass transfer coefficient of gas–liquid two-phase co-current flow in an internal loop airlift reactor equipped with a ceramic membrane module (CMM) and a perforated-plate distributor (PPD) are measured. Experimental results show that CMM can generate small bubbles with Sauter mean diameter d₃₂ less than 2.5 mm. As the liquid inlet velocity increases, the bubble size decreases and the gas holdup increases. In contrast, PPD can generate large bubbles with 4 mm < d₃₂ < 10 mm. The bubble size and liquid circulation velocity increase as the superficial gas velocity increases. Multiscale bubbles with 0.5 mm < d₃₂ < 10 mm can be generated by the CMM and PPD together. The volumetric mass transfer coefficient k_La of the multiscale bubbles is 0.033–0.062 s⁻¹, while that of small bubbles is 0.011–0.057 s⁻¹. Under the same flow rate of oxygen, the k_La of the multiscale bubbles increases by up to 160% in comparison to that of the small bubbles. Finally, empirical correlations for k_La are obtained.     

搅拌槽中气泡大小分布规律的研究

运用改进后的光电毛细管探头技术测量了搅拌槽中较宽操作条件范围内的气泡大小分布,由湍流理论分析了叶轮区和循环区影响气泡大小的主要因素,得到了气泡大小空间分布的初步规律及不同区域内的气泡平均直径与操作条件之间的定量关系.     

Analysis of bubble behaviors in bubble columns using electrical resistance tomography

The distribution of gas holdup, the rise velocity of gas bubble swarm and the Sauter mean bubble size are estimated with a small diameter laboratory scale bubble column using electrical resistance tomography (ERT). The theory of gas disengagement based on ERT methods has been developed for estimations of bubble size and bubble rise velocity. The gas holdups of large bubble swarm and small bubble swarm, the distribution of both bubble size are derived through the analysis of gas disengagement based on the differences of the rise velocity of bubble swarm at the cross-section imaged by electrical resistance tomography. Experimental results are in very good agreement with correlations and conventional estimation obtained using pressure transmitter methods. The proposed methodology can be also used as an analysis tool for quantifying and optimizing the performance of other types of complex reaction systems.     

THE EFFECT OF SOLUTES ON BUBBLE SIZE IN AIR-WATER DISPERSIONS

Bubble size distributions in a bubble column of 200 mm diameter were measured by means of a photoelectric probe. The gas-liquid dispersion was generated by distributing air with a porous plate into aqueous solutions of electrolytes and of organic compounds (alcohols, glycols, ketones, carboxylic acids, saccharose, carboxymethyl-cellulose, detergents). For all these solutes there exists a rather narrow concentration range in which the change from quick coalescence in pure water to coalescence suppression takes place. Though rough relationships between concentrations for coalescence suppression and molecular properties can be found (ionic strength for electrolytes, number of carbon atoms in a homologous series of organic compounds), exact equations based on a theory of coalescence cannot be given.

The bubble size distributions obtained in the experiments have also been used to show the impact of coalescence phenomena on gas-liquid mass transfer. For this purpose, surface areas were calculated from measured gas hold-up and mean bubble diameters (Sauter diameters). A comparison of these data with the trends for volumetric mass transfer coefficients reported in the literature for n-alcohols (C₁-C₂i) showed good agreement, despite the fact that the mass transfer data had been obtained in a quite different gas-liquid contacting device (stirred vessel).     

THE EFFECT OF SOLUTES ON BUBBLE SIZE IN AIR-WATER DISPERSIONS

Bubble size distributions in a bubble column of 200 mm diameter were measured by means of a photoelectric probe. The gas-liquid dispersion was generated by distributing air with a porous plate into aqueous solutions of electrolytes and of organic compounds (alcohols, glycols, ketones, carboxylic acids, saccharose, carboxymethyl-cellulose, detergents). For all these solutes there exists a rather narrow concentration range in which the change from quick coalescence in pure water to coalescence suppression takes place. Though rough relationships between concentrations for coalescence suppression and molecular properties can be found (ionic strength for electrolytes, number of carbon atoms in a homologous series of organic compounds), exact equations based on a theory of coalescence cannot be given.

The bubble size distributions obtained in the experiments have also been used to show the impact of coalescence phenomena on gas-liquid mass transfer. For this purpose, surface areas were calculated from measured gas hold-up and mean bubble diameters (Sauter diameters). A comparison of these data with the trends for volumetric mass transfer coefficients reported in the literature for n-alcohols (C₁-C₂i) showed good agreement, despite the fact that the mass transfer data had been obtained in a quite different gas-liquid contacting device (stirred vessel).     

Interaction between partitioning porous plate and rising bubbles in a trayed bubble column

In a trayed bubble column, the structure of the partitioning plate plays an important role on the bubble behavior. This study examined the effect of the opening ratio and pore size of the plate on the bubble break-up frequency and bubble size distribution. The sieve tray was used as the partitioning plate. The opening ratio was closely related to gas cap development. The stagnation of bubble flow and a gas cap were observed with an opening ratio less than 48.5%. The gas cap increased with decreasing opening ratio and increasing superficial gas velocity. The main effect of the sieve tray could be categorized into the additional drag force and bubble break-up depending on the sieve pore size. When the sieve pore size was smaller than the Sauter diameter of the bubble swarm, the movement of rising bubbles was interrupted by the drag force applied by the surrounding mesh lines. On the other hand, when the sieve pore size was larger than the Sauter diameter, the bubbles were affected by the additional bubble break-up. After the bubbles penetrated the sieve tray, the bubble size distribution shifted to a smaller one and the Sauter diameter decreased.     

Gas hold-up and mass transfer in a bubble column with viscoelastic fluids

The influence of viscoelasticity on gas hold-up and the volumetric mass transfer coefficient in a bubble column is discussed and examined experimentally. It was found that the gas hold-up increased due to an increase in the number of entrapped very small bubbles formed because of the elasticity of the liquid. On the other hand, a decrease in volumetric mass transfer coefficient due to the fluid elasticity was observed. Qualitative effects of an antifoam agent and a draft tube on the performance of the bubble column with viscoelastic fluids were also examined.