Experiments and modelling of a draft tube airlift reactor operated at high gas throughputs

One-dimensional modelling of global hydrodynamics and mass transfer is developed for an annulus sparged draft tube airlift reactor operating at high gas throughputs. In a first part, a specific closure law for the mean slip velocity of bubbles in the riser is proposed according for, in one hand, the collective effects on bubble rise velocity and, in the other hand, the size of the liquid recirculation in the airlift riser. This global hydrodynamics model is found towel explain the global gas volume fraction measurements in the airlift riser for a wide range of superficial gas velocity (0.6 ≤ Jg ≥10 cm sˉ¹). In a second part, mass transfer in the airlift has been studied by using the gassing-out method and a dual-tip optical probe to measure the bubble size distributions. As for bubble columns, in such airlift, the volumetric mass transfer coefficient appears to be quite proportional to the gas superficial velocity. Finally, as in Colombet et al. (2011), mass transfer at the bubble scale seems to be weakly influenced by an increase of gas volume fraction

Experiments and modelling of a draft tube airlift reactor operated at high gas throughputs

One-dimensional modelling of global hydrodynamics and mass transfer is developed for an annulus sparged draft tube airlift reactor operating at high gas throughputs. In a first part, a specific closure law for the mean slip velocity of bubbles in the riser is proposed according for, in one hand, the collective effects on bubble rise velocity and, in the other hand, the size of the liquid recirculation in the airlift riser. This global hydrodynamics model is found towel explain the global gas volume fraction measurements in the airlift riser for a wide range of superficial gas velocity (0.6 ≤ Jg ≥10 cm sˉ¹). In a second part, mass transfer in the airlift has been studied by using the gassing-out method and a dual-tip optical probe to measure the bubble size distributions. As for bubble columns, in such airlift, the volumetric mass transfer coefficient appears to be quite proportional to the gas superficial velocity. Finally, as in Colombet et al. (2011), mass transfer at the bubble scale seems to be weakly influenced by an increase of gas volume fraction

Prediction of gas–liquid flow in an annular gap bubble column using a bi-dispersed Eulerian model

We present and discuss numerical results from simulations of the air–water flow in an annular gap bubble column of 0.24 m internal diameter, at air superficial velocities ranging from 0.004 m/s to 0.225 m/s, covering the homogeneous and heterogeneous flow regimes. A bi-dispersed Eulerian model is implemented to account for both the stabilizing and destabilizing effects of small and large bubbles. Sensitivity studies on the mesh element size, time step size and number of outer iterations per time step are performed and most appropriate simulation parameters and mesh are used to predict the gas holdup curve. Comparison with two mono-dispersed models is provided to emphasize the necessity of a bi-dispersed approach for the accurate prediction of the homogeneous flow regime, given the poly-dispersed nature of the flow investigated. Two different approaches for the characterization of the small and large bubbles groups are also discussed. We found that the relative amount of small bubbles is an important input parameter for the present model and can be provided using available empirical correlations or experimental data. The results obtained from the simulations also demonstrated the necessity of a population balance model able to capture the bubbles coalescence and breakup phenomena for the correct prediction of the heterogeneous flow regime

Numerical simulation of bubble columns by integration of bubble cell model into the population balance framework

Includes bibliographical references.Bubble column reactors are widely used in the chemicals industry including pharmaceuticals, waste water treatment, flotation etc. The reason for their wide application can be attributed to the excellent rates of heat and mass transfer that are achieved between the dispersed and continuous phases in such reactors. Although these types of contactors possess the properties that make them attractive for many applications, there still remain significant challenges pertaining to their design, scale-up and optimization. These challenges are due to the hydrodynamics being complex to simulate. In most cases the current models fail to capture the dynamic features of a multiphase flow. In addition, since most of the developed models are empirical, and thus beyond the operating conditions in which they were developed, their accuracy can no longer be retained. As a result there is a necessity to develop eneric models which can predict hydrodynamics, heat and mass transfer over a wide range of operating conditions. With regard to simulating these systems, Computational Fluid Dynamics (CFD) has been used in various studies to predict mass and heat transfer characteristics, velocity gradients etc (Martín et al., 2009; Guha et al., 2008; Olmos et al., 2001; Sanyal et al., 1999; Sokolichin et al., 1997).The efficient means for solving CFD are needed to allow for investigation of more complex systems. In addition, most models report constant bubble particle size which is a limitation as this can only be applicable in the homogenous flow regime where there is no complex interaction between the continuous and dispersed phase (Krishna et al., 2000; Sokolichin & Eigenberger., 1994). The efficient means for solving CFD intimated above is addressed in the current study by using Bubble Cell Model (BCM). BCM is an algebraic model that predicts velocity, concentration and thermal gradients in the vicinity of a single bubble and is a computationally efficient approach The objective of this study is to integrate the BCM into the Population Balance Model (PBM) framework and thus predict overall mass transfer rate, overall intrinsic heat transfer coefficient, bubble size distribution and overall gas hold-up. The experimental determination of heat transfer coefficient is normally a difficult task, and in the current study the mass transfer results were used to predict heat transfer coefficient by applying the analogy that exists between heat and mass transfer. In applying the analogy, the need to determine the heat transfer coefficient experimentally or numerically was obviated. The findings indicate that at the BCM Renumbers (Max Re= 270), there is less bubble-bubble and eddy-bubble interactions and thus there is no difference between the inlet and final size distributions. However upon increasing Re number to higher values, there is a pronounced difference between the inlet and final size distributions and therefore it is important to extend BCM to higher Re numbers. The integration of BCM into the PBM framework was validated against experimental correlations reported in the literature. In the model validation, the predicted parameters showed a close agreement to the correlations with overall gas hold-up having an error of ±0.6 %, interfacial area ±3.36 % and heat transfer coefficient ±15.4 %. A speed test was also performed to evaluate whether the current model is quicker as compared to other models. Using MATLAB 2011, it took 15.82 seconds for the current model to predict the parameters of interest by integration of BCM into the PBM framework. When using the same grid points in CFD to get the converged numerical solutions for the prediction of mass transfer coefficient, the computational time was found to be 1.46 minutes. It is now possible to predict the intrinsic mass transfer coefficient using this method and the added advantage is that it allows for the decoupling of mass transfer mechanisms, thus allowing for more detailed designs.The decoupling of mass transfer mechanisms in this context refers to the separate determination of the intrinsic mass transfer coefficient and interfacial area

Detailed modeling of hydrodynamics mass transfer and chemical reactions in a bubble column using a discrete bubble model

A 3D discrete bubble model is adopted to investigate complex behavior involving hydrodynamics, mass transfer and chemical reactions in a gas¿liquid bubble column reactor. In this model a continuum description is adopted for the liquid phase and additionally each individual bubble is tracked in a Lagrangian framework, while accounting for bubble¿bubble and bubble¿wall interactions via an encounter model. The mass transfer rate is calculated for each individual bubble using a surface renewal model accounting for the instantaneous and local properties of the liquid phase in its vicinity. The distributions in space of chemical species residing in the liquid phase are computed from the coupled species balances considering the mass transfer from bubbles and reactions between the species. The model has been applied to simulate chemisorption of CO2 bubbles in NaOH solutions. Our results show that apart from hydrodynamics behavior, the model is able to predict the bubble size distribution as well as temporal and spatial variations of each chemical species involved

3D CFD Simulation of a Bubble Column with Internals: Validation of Interfacial Forces and Internal Effects for Local Gas Holdup Predictions

CFD Models (Turbulent Models and Interfacial Forces) Incorporated with the Population Balance Model (PBM) Have Been Validated, Azimuthally, with the Gamma-Ray-Computed Tomography (CT) Results to Address the Effect of the Presence of Internals with Different Arrangements and Diameters. the Superficial Gas Velocity Applied Was Varied from 0.05 to 0.45 M/s. the Results Exhibit the Capability to Predict the Hydrodynamics of the Bubble Column, Further Incorporating the Population Balance Model and Promoting the Prediction of Simulation in High Superficial Gas Velocity. the Effect of Internals Revealed that the Gas Holdup Was Significantly Enhanced in the Bubble Column\u27s Wall Region, While the Gas Holdup Was Increased Remarkably in the Center and the Wall Regions of the Bubble Column Equipped by Internals of 1 In. Diameter More Than in Internals of 0.5 In. However, Internals with a Hexagonal Arrangement Increase the Gas Holdup in the Central Region and Less in the Wall Than in the Circular Arrangement

The effects of low aspect ratio and heat exchanging internals on the bubble properties and flow regime in a pilot-plant bubble/slurry bubble column for Fischer-Tropsch synthesis

Fischer-Tropsch synthesis (F-T) is a process utilized to convert the syngas mixture of CO and H2 to synthetic fuel and chemicals that executed commercially by using the bubble/slurry bubble column reactor. The experimental results reveal that the investigated parameters, in terms the presence of internals, and reducing the aspect ratio and the solids loading, increase the local gas holdup, interfacial area, bubble passage frequency, and decrease the bubble rise velocity, bubble chord length. Meanwhile, the aspect ratio H/D = 4, and 5 provide enough height to established the fully developed flow regime. As a result of the variation in the bubble properties that in turn reflected on the flow regime transition, therefore, the presence of internals and decreasing the aspect ratio delay the transition from the transition flow regime to churn turbulent flow regime. The validated CFD codes, using Eulerian-Eulerian approach incorporated with the population balance model PBM, exhibit the capability to simulate the bubble column in bubbly and turbulent flow regimes. However, results revealed that the presence of internals enhanced the gas holdup significantly in the wall region of the column. The gas holdup radial profiles in the presence of internals in different configurations provide a uniform gas holdup profile. While the results of the effect of internals diameter exhibit that the gas holdup was increased remarkably in the center and the wall regions of the bubble column equipped by internals of 1-inch diameter more than in using internals of 0.5-inch. However, the effect of internals configurations reported that the internals with hexagonal arrangement increases the gas holdup in the center region more than the circular arrangement, and less in the wall region comparing with the circular arrangement --Abstract, page iv

Modelling of mass transfer in gas-liquid stirred tanks agitated by Rushton turbine and CD-6 impeller: a scale-up study

A combined computational fluid dynamics (CFD) and population balance model (PBM) approach has been applied to the simulation of gas-liquid stirred tanks agitated by (i) a Rushton turbine or (ii) a CD-6 impeller, operating at aeration numbers from 0.017 to 0.038. The multiphase simulations were realised via an Eulerian-Eulerian two-fluid model and the drag coefficient of spherical and distorted bubbles was modelled using the Ishii-Zuber equations. The effect of the void fraction on the drag coefficient was modelled using the correlation by Behzadi et al. (2004). The local bubble size distribution was obtained by solving the PBM using the quadrature method of moments (QMOM). The local kLa was estimated using both the Higbie penetration theory and the surface renewal model. The predicted gas-liquid hydrodynamics, local bubble sizes and dissolved oxygen concentration were in good agreement with experimental measurements reported in the literature. A slight improvement in the prediction of the aerated power number was obtained using the non-uniform bubble size distribution resulting from the coupled CFD-PBM simulation. Evaluation of the prospective scale-up approaches indicates a higher probability of maintaining a similar level of mass transfer in a larger tanks by keeping the Pg/V and VVM constant. Considering its predictive capability, the method outlined in this work can provide a useful scale-up evaluation of gas-liquid stirred tanks

MODELING TURBULENT GAS-LIQUID BUBBLY FLOW IN A VERTICAL PIPE

Bubbly gas-liquid turbulent flow occurs in various industrial applications, for example oil and gas production, petrochemical plants, nuclear reactors, etc. The analysis of bubbly gas-liquid turbulent flow remains a challenging task due to complexities such as the dispersed gas phase effects on the continuous liquid phase turbulence, interphase momentum exchange, and redistribution of the gas volume fraction due to bubble coalescence and breakup. The focus of this thesis is to develop a computational model to address these challenges. The model developed in this thesis uses a state-of-the-art two-fluid method, which minimizes computational resources and is based on the Reynolds-Averaged Navier-Stokes (RANS) equations. The predictions obtained for bubbly upward flow in a vertical pipe were validated against the available experimental data. The first part of this thesis, chapter 2, documents a one-dimensional Eulerian-Eulerian two-fluid model for mono-disperse bubbly gas-liquid flow. The main challenge is the prediction of the gas volume fraction profile, based on the radial force balance of the non-drag forces for the gas phase. The shape of the volume fraction profile across the pipe changes depending on the bubble size. The volume fraction profile exhibits a peak value near the wall and at the centre line of the pipe for smaller and larger bubbles, respectively, which is consistent with experimental measurements. For the model tested, the turbulence kinetic energy was observed to increase for larger size bubbles compared to the smaller size bubbles. The second part of the thesis, chapter 3, reports a thorough investigation of the effect of bubbles on the liquid phase turbulence, referred to as turbulence modulation. The presence of bubbles in the flow can either enhance or attenuate the liquid phase turbulence. For the same flow conditions, the effect of the turbulence modulation shows both enhancement and suppression for the turbulence kinetic energy in different locations in the pipe. A budget analysis of the turbulence transport equations was used to provide insight on the relative importance of the turbulence modulation and to identify the region where it plays a significant role. The turbulence modulation was often found to have an insignificant effect on the prediction for the mean flow variables. The third part of the thesis, chapter 4, describes a numerical study of poly-disperse gas-liquid flow, which contains bubbles of different diameter. For a poly-disperse distribution of gas bubbles, the model must account for the consequences of bubbles either breaking up or coalescing with each other. To explore their effect, an inhomogeneous multiple size group (iMUSIG) approach with a bubble coalescence and breakup model was implemented. The developed model was shown to correctly redistribute the gas volume fraction among the bubble groups based on the coalescence and breakup processes. The turbulence modulation for the poly-disperse flow was found to be larger than for the mono-disperse case, which indicates one additional effect of a poly-disperse distribution of gas bubbles. Overall, this thesis research implemented a two-fluid model that is able to capture important features of bubbly gas-liquid flow for both mono-disperse and poly-disperse cases. Some significant features of the model are: the use of a radial force balance for the gas volume fraction evaluation; a turbulence modulation contribution based on source terms in the turbulence transport equations; and incorporating the effect of coalescence and breakup processes and the resultant exchange of gas volume fraction among different bubble groups. As such, the thesis documents an improved predictive model for RANS simulations of bubbly gas-liquid flow in industrial applications