Population balance modeling of bubble size distributions in a direct-contact evaporator using a sparger model (original) (raw)
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Experimental study on bubble size distributions in a direct-contact evaporator
Brazilian Journal of Chemical Engineering, 2004
Experimental bubble size distributions and bubble mean diameters were obtained by means of a photographic technique for a direct-contact evaporator operating in the quasi-steady-state regime. Four gas superficial velocities and three different spargers were analysed for the air-water system. In order to assure the statistical significance of the determined size distributions, a minimum number of 450 bubbles was analysed for each experimental condition. Some runs were also conducted with an aqueous solution of sucrose to study the solute effect on bubble size distribution. For the lowest gas superficial velocity considered, at which the homogeneous bubbling regime is observed, the size distribution was log-normal and depended on the orifice diameter in the sparger. As the gas superficial velocity was increased, the size distribution progressively acquired a bimodal shape, regardless of the sparger employed. The presence of sucrose in the continuous phase led to coalescence hindrance.
Chemical Engineering Science, 2005
A recently developed model for coupled heat and mass transfer in binary systems during the formation and ascension of superheated bubbles was extended to a multicomponent system comprising N volatile species. The model allows variable properties and bubble radius changes, assuming diffusive mass fluxes to be properly described by Fick's law. Experimental direct-contact evaporation tests were conducted with ethyl acetate aqueous solutions to provide data for assessing the developed model. In addition, the model was tested against available literature data for an air-stripper. In both cases, a good agreement between simulation and experimental results was verified.
Modeling of the Dispersed-Phase Size Distribution in Bubble Columns
Industrial & Engineering Chemistry Research, 2002
A population balance model formulation for predicting the size distribution of the dispersed gas phase in bubble column reactors is presented. Extended source term parametrizations for the breakup and coalescence mechanisms are included in a computational fluid dynamics (CFD) model and tested on a fairly simple flow formulation corresponding to locally obtained experimental data. Most CFD codes are based on the assumption of a monodisperse bubble size distribution resulting in an inaccurate prediction of the holdup. Comparing the results obtained by the breakup model versions against experimental data indicates that the extended model provides an improved description of the bubble size distribution, holdup , and thus also the volume-averaged gas-phase velocity.
Bubble size distribution in the sparger region of bubble columns
Chemical Engineering …, 2002
It is well known that the gas distributor can play an important role on the evolution of the bubble size distribution (BSD) in gasliquid reactors, strippers and absorbers. Therefore, the main subject of the present work was to study the influence of sparger design and process parameters ...
CFD simulation of bubble columns incorporating population balance modeling
2008
A computational fluid dynamics (CFD)-code has been developed using finite volume method in Eulerian framework for the simulation of axisymmetric steady state flows in bubble columns. The population balance equation for bubble number density has been included in the CFD code. The fixed pivot method of Kumar and Ramkrishna [1996. On the solution of population balance equations by discretization-I. A fixed pivot technique. Chemical Engineering Science 51, 1311-1332] has been used to discretize the population balance equation. The turbulence in the liquid phase has been modeled by a k. model. The novel feature of the framework is that it includes the size-specific bubble velocities obtained by assuming mechanical equilibrium for each bubble and hence it is a generalized multi-fluid model. With appropriate closures for the drag and lift forces, it allows for different velocities for bubbles of different sizes and hence the proper spatial distributions of bubbles are predicted. Accordingly the proper distributions of gas holdup , liquid circulation velocities and turbulence intensities in the column are predicted. A survey of the literature shows that the algebraic manipulations of either bubble coalescence or break-up rate were mainly guided by the need to obtain the equilibrium bubble size distributions in the column. The model of Prince and Blanch [1990. Bubble coalescence and break-up in air-sparged bubble columns. A.I.Ch.E. Journal 36, 1485-1499] is known to overpredict the bubble collision frequencies in bubble columns. It has been modified to incorporate the effect of gas phase dispersion number. The predictions of the model are in good agreement with the experimental data of Bhole et al. [2006. Laser Doppler anemometer measurements in bubble column: effect of sparger. Industrial & Engineering Chemistry Research 45, 9201-9207] obtained using Laser Doppler anemometry. Comparison of simulation results with the experimental measurements of Sanyal et al. [1999. Numerical simulation of gas-liquid dynamics in cylindrical bubble column reactors. Chemical Engineering Science 54, 5071-5083] and Olmos et al. [2001. Numerical simulation of multiphase flow in bubble column reactors: influence of bubble coalescence and breakup. Chemical Engineering Science 56, 6359-6365] also show a good agreement for liquid velocity and gas holdup profiles.
International Journal of Multiphase Flow, 2011
In a packed-bed reactor a comparative study of bubble breakup and coalescence models has been investigated to study bubble size distributions as a function of the axial location. The bubble size distributions are obtained by solving population balance equations that describe gas-liquid interactions. Each combination of bubble breakup and coalescence models is examined under two inlet flow conditions: (1) predominant bubble breakup flow and (2) predominant bubble coalescence flow. The resulting bubble size distributions, breakup and coalescence rates estimated by individual models, are qualitatively compared to each other. The change of bubble size distributions along the axial direction is also described with medians. The medians resulting from CFD analyses are compared against the experimental data. Since the predictions estimated by CFD analyses with the existing bubble breakup and coalescence models do not agree with the experimental data, a new bubble breakup and coalescence model that takes account of the geometry effects is required to describe gas-liquid interactions in a packed-bed reactor.
Chemical Engineering Research and Design, 2005
T he continuous coalescence and redispersion process of gas bubbles is an important phenomenon that has great significance on bubble size, gas holdup and gas -liquid mass transfer in gas -liquid reactors. The physical laws that determine the bubble coalescence rate are not yet well understood. Therefore, there is a great need for more detailed information about the coalescence process. The coalescence models can be utilized, for example, in multiphase CFD simulations. A combination of CFD and population balances with coalescence and break-up models is a promising topic, which enables a more detailed calculation of local hydrodynamics, gas holdup and gas -liquid mass transfer. The measurement of persistence time, defined as a time that a bubble remains attached at free gas -liquid surface, was used in this work to predict the coalescence properties in a bubble column. The persistence time was evaluated for different liquids and different bubble sizes using highspeed video camera. The applied method for measuring the persistence time is very simple and fast. The measured persistence times range from 10 ms for de-ionized water up to 15 s for water isopropanol solution. In addition, bubble size distributions were photometrically determined in a bubble column at several height positions using different liquids and operational conditions. The measured persistence times were applied in a simplified bubble column simulation model together with population balances and the measured and predicted bubble sizes were compared. A good agreement with calculated and measured size distributions was observed.
Chemical Engineering Science, 2007
New experimental data concerning the gas holdup in bubble columns equipped with porous sparger were acquired. The effect of liquid properties and sparger characteristic (i.e., pore size, dimensions) on gas holdup at the pseudo-homogeneous regime has been studied and a correlation regarding the prediction of the transition point from the pseudo-homogeneous to the heterogeneous regime has been proposed and found to be in good agreement with available data. Moreover, a previously proposed correlation [Mouza, A.A., Dalakoglou, G.K., Paras, S.V., 2005. Effect of liquid properties on the performance of bubble column reactors with fine pore spargers. Chemical Engineering Science 60(5), 1465-1475], for the prediction of gas holdup at the homogeneous regime for this type of equipment, has been modified to take into account the effect of the mean pore diameter and it is also found to be in good agreement with published data. ᭧
Flow Measurement and Instrumentation, 2018
Bubble Column Reactors (BCRs) are suitable for multiphase fluid systems due to their low energy consumption, ease of fabrication and relatively high degree of mixing. When predicting the properties of the bubble population, it was shown in our previous study that the global Bubble Number Density Distribution (BNDD) could be determined through the coupling of Electrical Resistance Tomography (ERT) to the Dynamic Gas Disengagement (DGD) process. However, due to the low spatial resolution associated with the ERT, the local BNDD, bubble coalescence and breakage rates of the fluid dispersion, and the BCR hydrodynamic parameters could not be predicted. In the present paper, these limitations are addressed by hybridizing the steady-state Population Balance Model (PBM) with the DGD results. In the first stage of the hybridization, the PBM having the source terms of bubble coalescence and breakup rates was solved to simulate the bubble evolution process in a BCR of height 1.56 m and diameter 0.29 m designed with a porous tubular gas distributor. The PBM solution was utilized as an initial condition for a modelling of the unsteady state DGD process as an advection transport of mean bubble sizes under the influence of bubble expansion rates. The unsteady state PBM that models the DGD process was solved by the method of characteristics to obtain an analytical solution of the local transient disengaging gas volume. Accordingly, the inlet mean and standard deviation of bubble sizes parameters, the coalescence and breakage rates, and the local flux gradient parameter during DGD were adjusted to ensure the modelled local static and dynamic gas holdups during DGD process agree with the corresponding experimental measurements. Validation was achieved through comparison of the macro-properties that can be determined from the resolved BNDD. Such properties include the axial distributions of Sauter Mean Bubble Diameter (SMBD), gas holdups, and the volumetric gas-liquid mass transfer rates of the steady-state bubble population swarming. The comparison of predicted hydrodynamic parameters with two literature results yielded R-squares of 0.88 and 0.82, Chi-squares of 0.86 and 1.08 and f-statistics of 35.96 and 27.24 indicating good agreement. It is therefore proposed that hybridization of the PBM and DGD process yielded a high spatial prediction of the Size and Axial Distribution (SAD) of bubble dispersion and the bubble coalescence and breakage rates even though ERT offers a relatively low spatial resolution.