Homogeneous Flow Regime Research Articles

Hydrodynamics of air–kerosene bubble column under elevated pressure in homogeneous flow regime

A multiphase computational fluid dynamics (CFD) model coupled with the population balance equation (PBE) was developed in a homogeneous air–kerosene bubble column under elevated pressure ( P ). The specific pressure drop (Δ P / L ), gas holdup ( α G ), and Sauter mean diameter ( d 32 ) were experimentally measured in the bubble column with 1.8 m height and 0.1 m inner diameter, which was operated at a superficial gas velocity of 12.3 mm·s −1 , and P = 1–35 bar (1 bar = 10 5 Pa). A modified drag coefficient model was proposed to consider the effect of bubble swarm and pressure on hydrodynamics of the bubble column. The Luo breakage model was modified to account for liquid density, viscosity, surface tension and gas density. The Δ P / L , α G , and d 32 obtained from the CFD model were compared with experimental data, and the gas density-dependent parameters of the CFD model were identified. With increasing P from 1 to 35 bar, the α G varied from 5.4% to 7.2% and the d 32 decreased from 2.3 to 1.5 mm. The CFD-PBE model is applicable to predict hydrodynamics of pressurized bubble columns for gas–organic liquid in the homogeneous regime.

Experimental Investigations on the Inner Flow Behavior of Centrifugal Pumps under Inlet Air-Water Two-Phase Conditions

Centrifugal pumps are widely used and are known to be sensitive to inlet air-water two-phase flow conditions. The pump performance degradation mainly depends on the changes in the two-phase flow behavior inside the pump. In the present paper, experimental overall pump performance tests were performed for two different rotational speeds and several inlet air void fractions (αi) up to pump shut-off condition. Visualizations were also performed on the flow patterns of a whole impeller passage and the volute tongue area to physically understand pump performance degradation. The results showed that liquid flow modification does not follow head modification as described by affinity laws, which are only valid for homogeneous bubbly flow regimes. Three-dimensional effects were more pronounced when inlet void fraction increased up to 3%. Bubbly flow with low mean velocities were observed close to the volute tongue for all αi, and returned back to the impeller blade passages. The starting point of pump break down was related to a strong inward reverse flow that occurred in the vicinity of the shroud gap between the impeller and volute tongue area.

Employing the Method of Characteristics to Obtain the Solution of Spectral Evolution of Turbulent Kinetic Energy Density Equation in an Isotropic Flow

This study aims to review the physical theory and parametrizations associated to Turbulent Kinetic Energy Density Function (STKE). The bibliographic references bring a broad view of the physical problem, mathematical techniques and modeling of turbulent kinetic energy dynamics in the convective boundary layer. A simplified model based on the dynamical equation for the STKE, in an isotropic and homogeneous turbulent flow regime, is done by formulating and considering the isotropic inertial energy transfer and viscous dissipation terms. This model is described by the Cauchy Problem and solved employing the Method of Characteristics. Therefore, a discussion on Linear First Order Partial Differential Equation, its existence, and uniqueness of solution has been presented. The spectral function solution obtained from its associated characteristic curves and initial condition (Method of Characteristics) reproduces the main features of a modeled physical system. In addition, this modeling allows us to obtain the scaling parameters, which are frequently employed in parameterizations for turbulent dispersion.

Modeling and experiment of gas desorption of bubble column with an external loop in the heterogeneous flow regime

This work introduces an external loop to a bubble column and presents enhanced gas exchange in the heterogeneous flow regime. Gas exchange experiments under the same amount of gas input were carried out with varying gas velocity to understand the difference of bubble characteristic between the bubble column (BCR) and the bubble column with an external loop (BCR-EL). The observation of rise and descending velocity of bubbles in the BCR-EL showed that the fraction of bubbles passing the downcomer continuously increases with the incremental superficial gas velocity. A gas molecule in the liquid phase is desorbed by another gas molecule, and this gas exchange was assumed to be a phenomenon that a reactant in the liquid phase is converted to a product. To test the validity of the assumed gas exchange as a reaction in the experiments with the BCR-EL, a modeling study was performed using Fisher-Tropsch synthesis. It prevailed that the syngas conversion was higher in the BCR at the homogeneous flow regime, while the BCR-EL at the heterogeneous flow regime (above 0.08 m/s) had higher syngas conversion than the BCR due to higher gas recycle to the downcomer.

Mass transfer in the homogeneous flow regime of a bubble column

A model for the evaluation of the mass transfer coefficient in bubble column reactors in the homogeneous regime is proposed. This approach combines three-dimensional two-phase flow simulations using OpenFOAM with mass transfer models in order to evaluate the average mass transfer coefficient in the reactor. The accuracy and limitations of the simulations are discussed based on comparisons with existing experimental data and correlations from the literature. Radial gas holdup and velocity profiles at different operating conditions are in reasonable agreement at low gas velocities. A sensitivity analysis was performed to determine the best interfacial force model among several possibilities. Schiller-Naumann’s drag force model appeared to be the most appropriate, a swarm correction has a limited impact. Overall, the predicted global mass transfer coefficients fell within the range of values obtained from existing correlations in the literature. The level of accuracy of the proposed approach is, however, difficult to estimate as the predictions from the correlations in the literature varied over a 5-fold range. Despite the limitations of the proposed model (e.g., low gas velocities), the approach could be used to estimate the mass transfer in columns with internal packings, baffles or inserts, for which no correlation exists in the literature.

Influence of the bubble size distribution on the bubble column flow regime

The role of the bubble size dependent lateral lift force on the flow regime is experimentally investigated in a tall bubble column. In the experiments, only the initial bubble size distribution was modified by using a gas sparger with different injection needles and varying the flow through specific injection needles. The gas flow rate was kept unchanged in all experiments. Depending on the bubble size distribution homogeneous, transitional or heterogeneous flow regimes were observed. The flow regime is in good agreement with the predictions from the stability criterion obtained by a previously conducted linear stability analysis for poly-dispersed flow. It was found that as a rule of thumb the following criterion can be used.If the majority of the gas volume is transported by bubbles smaller than the critical diameter at which the lift force changes its sign, the flow is stabilized leading to a homogeneous flow regime. If most of the gas volume is transported by bubbles larger than that diameter the flow is de-stabilized leading a heterogeneous flow regime. If the fraction of gas transported by small and large bubbles is about the same, the initial conditions remain dominant throughout the column height.

Hydrodynamics and simulation of air–water homogeneous bubble column under elevated pressure

AbstractA gas–liquid Eulerian computational fluid dynamics (CFD) model coupled with a population balance equation (PBE) was presented to investigate hydrodynamics of an air–water bubble column (1.8 m in height and 0.1 m in inner diameter) under elevated pressure in terms of pressure drop, gas holdup, mean bubble size, and bubble surface area. The CFD‐PBE model was modified with three pressure correction factors to predict both the total gas holdup and the mean bubble size in the homogeneous bubbly flow regime. The three correction factors were optimized compared to experimental data. Increasing the pressure led to increasing the density, reducing the bubble size, and increasing the gas holdup. The bubble size distribution moved toward a smaller bubble size, as the pressure increased. The modified CFD‐PBE model validated with experimental data and empirical models represented well hydrodynamics of the bubble column at P = 0.1, 1.5, and 3.5 MPa.

New technique for simultaneous measurement of the local solid and gas holdup by using optical fiber probes in the slurry bubble column

Design and scale-up of the slurry bubble columns, as popular multiphase reactors, is complicated because of the complexity of the hydrodynamics of these types of reactors. In fact, to have a good design, precise information about the local hydrodynamic parameters are required. In this study, a new method is developed for simultaneous measuring of the local solid, gas and liquid holdup in the slurry bubble column. Optical fiber probes were used to measure phase distribution inside the bubble column. Our specific calibration procedure enables probes to measure solid concentration in the slurry phase as well. In order to examine the proposed measurement method, a cylindrical plexiglass column with 270 cm height and 29.2 cm diameter was utilized as the slurry column which contained water, glass beads, and air. The mean diameter of the glass beads is 35 µm and the experiments were done with 1% and 3% solid loading (volume solid/volume slurry). To cover both homogeneous and heterogeneous flow regime in the bubble column, air with a wide range of superficial gas velocity (0–22 cm/s) was injected into the slurry phase. The obtained results showed that the axial distribution of the solid particles is not uniform along the column, and a local minimum concentration observed in the middle of the column. In the homogeneous regime, the local solid concentration increased by raising the superficial gas velocity while solid particle distribution was independent of the gas velocity in the heterogeneous flow regime.

Flow regimes in slurry bubble column: Effect of column height and particle concentration

This is an experimental study focussed on the flow regimes and gas holdup behaviour in a lab-scale slurry bubble column (vertical plexiglass cylinder, 0.14 m dia, total height 2 m). The gas distributor was a fine perforated brass plate (0.5 mm dia orifices, free plate area 0.19%), producing both homogeneous (HoR) and heterogeneous (HeR) flow regimes, and the transition between them (TrR). The three-phase gas-liquid-solid mixture was composed of local air, tap water and fine porous silica particles (size 100 μm). The particles were characterized by several methods for their individual and collective properties (optical and electron microscopy, granulometry, porosimetry, effective density and viscosity, granular rheology and bulk properties, effects on liquid conductivity and surface tension). The gas holdup e was measured by bed expansion method, using free layer height evaluated visually or by digital image analysis. Three experimental parameters were tested: gas input q, initial slurry layer height H and concentration of solid particles c. It was found that increasing both H and c reduces gas holdup and destabilizes HoR and enhances its breakdown. At low solid load, the three flow regimes exist for all H tested. At higher solid load, only one regime is observed, the turbulent Pure HeR (PHeR). The experimental data were described with suitable models, based on simple physical concepts used for gas-liquid systems, which were successfully applied also to gas-liquid-solid systems. All four flow regimes observed (HoR, HeR, TrR, PHeR) were fitted with few easy formulas, at the price of few adjustable parameters with clear physical meaning.

Computational fluid-dynamic modeling of the mono-dispersed homogeneous flow regime in bubble columns

Two-phase bubble columns are equipment used to bring one or several gases into contact with a liquid phase. Despite the simple system design, bubble columns are characterized by complex fluid dynamic phenomena at different scales; for this reason, their correct design, operation and scale-up rely on the precise estimation of global and local fluid dynamics properties. In this respect, multi-phase Computational Fluid Dynamics (CFD), in the Eulerian multi-fluid framework, is particularly useful to study the fluid dynamics in multi-phase reactors. Within this approach, the accurate prediction of the fluid dynamics depends on the correct modeling of (a) the momentum exchange between the phases, (b) the effects of the dispersed phase on the turbulence of the continuous phase, and (c) the bubble coalescence and break-up phenomena. Furthermore, the global and the local fluid dynamic properties are related to the prevailing flow regime, i.e., the homogeneous flow regime and the heterogeneous flow regime. This paper mainly focuses on the homogeneous flow regime, which can be classified as “pseudo-homogeneous” or “mono-dispersed homogeneous”, depending on the prevailing bubble size distribution. The numerical modeling of the “pseudo-homogeneous” flow regime has been discussed in our previous papers (i.e., modeling closures and suitable boundary conditions); conversely, this paper contributes to the existing discussion on the modeling closures by investigating the “mono-dispersed homogeneous” flow regime in “small-scale” and “large-scale” bubble columns. To this end, two test cases have been considered: (a) a “small-scale” bubble column (a test case taken from the previous literature); (b) a large-scale bubble column (a test case experimentally studied within this paper by image analysis, optical probe and gas holdup techniques). In particular, this paper studies the effects of the interfacial forces and bubble induced turbulence modeling within the Eulerian two-fluid approach. Three-dimensional transient simulations have been performed and the numerical results were compared with experimental data (both local and global fluid dynamics parameters). The results have been critically analyzed and the reasons for the discrepancies between the numerical results and the experimental data have been identified and may serve as a basis for future studies. Likewise, recommendations on suitable closures as well as guidelines for future studies have been provided. In conclusion, this paper extends the validation of a previously proposed set of closure relations (validated for the “pseudo-homogeneous” flow regime in a “large-scale” annular gap bubble column) to the “mono-dispersed homogeneous” flow regime in “small-scale” and large-scale bubble columns.

Investigating bubble dynamics in a bubble column containing shear thinning liquid using a dual-tip probe

Bubble dynamics was characterized experimentally in non-Newtonian shear thinning fluids with different viscosities by a dual-tip resistivity probe. The experiments were carried out in a 9 cm diameter column while the superficial gas velocity varied between 0.5 and 7 cm/s. Air, injected through a diffuser, was the dispersed phase and various solutions of carboxy-methyl cellulose were used as the continuous liquid phase. An algorithm was developed to detect each individual contact of bubbles with the probe needles. It was shown that increasing the viscosity produces larger bubbles which make the homogeneous flow regime instable at liquid viscosities greater than 7 cP. It was found that the axial profiles of bubble chord length and velocity are affected by both shear thinning behavior of the solutions and the viscosity of the solution. The shear thinning behavior causes a descending trend of axial distribution of bubble rise velocity. Moreover, gas holdup reaches a maximum in the middle of the column in solutions studied. The probability distribution of bubble chord length was well fitted by a log-normal distribution. However, the bubble rise velocity distribution is Gaussian in the homogeneous regime at low gas flow rates. Correlations are given for evaluating bubble chord length and bubble rise velocity.

A study on the influence of the tube layout on sub-channel hydrodynamics in a bubble column with internals

In this work, the hydrodynamics of a bubble column with vertical heat exchanger internals in a narrow bubble column of Di=0.1m inner diameter with a clear liquid height of Lc=1.1m was comprehensively studied. We applied ultrafast X-ray tomography to obtain hydrodynamic parameters, such as gas holdup, bubble size distribution, bubble number flux and flow patterns at hitherto inaccessible positions within the sub-channels of the tube bundles. To investigate the influence of the tube bundle pattern, square and triangular pitches were considered. Tubes of do=8 and 13 mm outer diameter were installed to study the effect of tube size, while maintaining approx. Ac=25% coverage of the cross-sectional area, which is typical for, e.g., Fischer-Tropsch processes operated in bubble column reactors. The superficial gas velocity was varied from ug=2 to 20 cm s−1 to cover homogeneous and heterogeneous flow regimes. Internals’ type and tube diameter were found to crucially influence the gas holdup distribution across the column diameter, which is known to generate liquid circulation, to shape the gas velocity profile and to cause intensive bubble interactions. The higher flow resistance induced by triangular tube configurations and dense tube patterns, which is attributed to the smaller hydraulic diameter of the respective configurations, forces bubbles to preferably rise near the column wall. Within the tube bundle, the radial holdup profiles show a pronounced non-parabolic trend, indicating zones of reverse liquid flow directions between the internal tubes. Furthermore, the gas-liquid flow morphology within various sub-channels was analyzed revealing a slug-like flow formation at superficial gas velocities larger than 10 cm s−1.

Effect of gas sparger design on bubble column hydrodynamics using pure and binary liquid phases

It is known that the fluid dynamics and transport phenomena in bubble columns depend mainly on the bubble column design (i.e., the column diameter, aspect ratio, and gas sparger openings) and the liquid phase properties. In this communication, we contribute to present-day discussion through an experimental study concerning the combined effects of the gas sparger design and liquid phase properties on both the gas holdup and the main flow regime transition. The experimental study concerning gas holdup measurements was conducted in a large-diameter and large-scale bubble column (with a height of 5.3 m and inner diameter of 0.24 m) operated in the batch mode. Air was used as the dispersed phase (using gas superficial velocities in the range 0.004–0.20 m/s), and various water–monoethylene glycol (MEG) solutions were employed as binary liquid phases. The water–MEG solutions tested have viscosities between 0.9 mPa·s and 7.97 mPa·s, densities between 997.086 kg/m3 and 1094.801 kg/m3, and surface tension between 0.0715 N/m and 0.0502 N/m. Two gas spargers were tested: (a) a spider sparger (“coarse gas sparger”) and (b) a needle sparger (“fine gas sparger”). The former produced a poly-dispersed homogeneous flow regime resulting in a concave gas holdup curve, whereas the latter produced a mono-dispersed homogeneous flow regime resulting in an S-shaped gas holdup curve. It was observed that the mono-dispersed bubble size distribution stabilized the homogeneous flow regime. The addition of MEG produced different effects depending on the gas sparger design. The addition of MEG in the “coarse gas sparger” configuration produced what is usually referred to as “dual effect of viscosity”: depending on the MEG concentration, the homogeneous flow regime was stabilized/destabilized, and thus, the gas holdup increased/decreased. Conversely, the addition of MEG in the “fine gas sparger” changed the shape of the gas holdup curve from an S-shape to concave, thus rendering it similar to the ones produced by “coarse gas sparger”. We speculate that viscous solutions reduce the influence of the inlet conditions in large-diameter and large-scale bubble columns; this is a matter of future research.

Numerical simulation of two-phase flow in porous media using a wavelet based phase-field method

An understanding of the transport and dynamics of two fluids in porous media, as well as the bubbly flow regime, is important for many engineering applications, such as enhanced oil recovery (EOR) method, drilling technology, multiphase production system, etc. In this respect, the dependence of capillary stresses on the excess free-energy of a thin interfacial layer formed by two immiscible fluids is not fully clear, particularly in porous media. Of particular interests are the closure models for interphase forces which often hinder the reliable prediction of the homogeneous flow regime. This article presents a multiphase Computational Fluid Dynamics (CFD) study of bubbles in homogeneous porous media to model the flow of oil and gas, and investigates a closure model that is based on the Allen-Cahn phase-field method, where the capillary stress is derived from the excess free-energy. The governing dynamics is simulated with the volume averaged Navier-Stokes equations extended for multiphase flow in porous media. The equations have been discretized by a wavelet transform method to accurately capture the topological change of the fluid-fluid interface. To validate the closure model for interphase forces, the results of the present phase-field method have been compared with that from experiments, as well as from reference numerical models. An excellent agreement among the results from present phase-field simulations, experiments, and some reference numerical simulations has been observed. The terminal velocity of the rising gas bubble in a liquid saturated porous medium, as well as in a pure liquid has been investigated. The bubble rising velocity in both cases have been compared with respect to the theoretical and experimental results. The study illustrates how the bubble dynamics in porous media depend on the excess free energy of a thin interfacial layer formed by two immiscible fluids.

The effect of aspect ratio in counter-current gas-liquid bubble columns: Experimental results and gas holdup correlations

It is generally admitted that the gas holdup is independent of the column dimensions and gas sparger design if three criteria are satisfied: the diameter of the bubble column is larger than 0.15m, gas sparger openings are larger than 1–2mm and the aspect ratio is larger than 5. This paper contributes to the existing discussion; in particular, the effect of the aspect ratio (within the range 1–15) in a counter-current gas-liquid bubble column has been experimentally studied and a new gas holdup correlation to estimate the influence of aspect ratio, operation mode and working fluid on the gas holdup has been proposed. The bubble column, equipped with a spider gas sparger, is 5.3m in height, has an inner diameter of 0.24m; gas superficial velocities in the range of 0.004–0.23m/s have been considered, and, for the runs with water moving counter-currently to the gas phase, the liquid has been recirculated at a superficial velocity of −0.0846m/s. Filtered air has been used as the gaseous phase in all the experiments, while the liquid phase has included tap water and different aqueous solutions of sodium chloride as electrolyte. Gas holdup measurements have been used to investigate the flow regime transitions and the global bubble column hydrodynamics. The counter-current mode has turned out to increase the gas holdup and destabilize the homogeneous flow regime; the presence of electrolytes has resulted in increasing the gas holdup and stabilizing the homogeneous flow regime; the aspect ratio, up to a critical value, has turned out to decrease the gas holdup and destabilize the homogeneous flow regime. The critical value of the aspect ratio ranged between 5 and 10, depending on the bubble column operation (i.e., batch or counter-current modes) and liquid phase properties. Since no correlation has been found in the literature that can correctly predict the gas holdup under the investigated conditions, a new scheme of gas holdup correlation has been proposed. Starting from considerations concerning the flow regime transition, corrective parameters are included in the gas holdup correlation to account for the effect of the changes introduced by the aspect ratio, operation mode and working fluid. The proposed correlation has been found to predict fairly well the present experimental data as well as previously published gas holdup data.

Simultaneous local determination of mass transfer and residence time distributions in organic multiphase systems

A new method for simultaneous local determination of mass transfer coefficients and residence time distributions in continuously operated bubble columns has been developed. Since for organic liquids not many experimental data sets are available in respect to mass transfer, measurements were carried out with the organic gas/liquid multiphase system consisting of cumene (1-methylethylbenzene) and carbon dioxide. The concentration of carbon dioxide in the gas phase was adjusted by mixing with nitrogen. To measure concentrations of compounds in the liquid phase, attenuated total reflectance Fourier transform infrared spectroscopy (ATR FTIR) was applied: five ATR probes integrating silver-halide optical fibers with a length of 1.5m were integrated in a bubble column of 128.5cm height and a diameter of 10cm. The probes were distributed over the height of the reactor and could be shifted within the reactor to measure radial profiles of the mass transfer coefficients in steady state operation mode at different positions. The superficial liquid velocity of the system was adjusted in the range of 0.7–1.4cms−1. The used superficial gas velocities were between 1.5cms−1 and 2.1cms−1 to ensure a homogeneous flow regime. Furthermore, tracer experiments with benzaldehyde for the investigation of the residence time distributions were performed. From the FTIR spectral data, CO2 and benzaldehyde tracer concentration data were determined simultaneously by conducting multivariate chemometric models. The corresponding mass transfer coefficients were calculated using a compartmental method for stationary flow. The residence time distributions were calculated from the benzoic aldehyde concentration data. Both parameters could be determined successfully. For the mass transfer coefficient of carbon dioxide into cumene, an interesting profile could be observed, which arose from several influences like areas of different bubble densities, caused by bubble breakage and coalescence, superficial velocities of gas and liquid as well as the applied type of sparger.

The effect of liquid phase properties on bubble column fluid dynamics: Gas holdup, flow regime transition, bubble size distributions and shapes, interfacial areas and foaming phenomena

The correct design, operation and scale-up of bubble columns rely on the knowledge of the fluid dynamics at the “bubble-scale” and at the “reactor-scale”. This paper contributes to the existing discussion on the multi-scale fluid dynamics of bubble columns, and provides a complete dataset concerning the influence of the liquid phase properties on bubble column fluid dynamics at “bubble-scale” and at the “reactor-scale”. The bubble column is large-diameter and large-scale (5.3m in height, inner diameter of 0.24m), it was operated with superficial gas velocities in the range of 0.004–0.20m/s, and it was tested for different aspect ratios (in the range 5–12.5). Air was used as the dispersed phase and various fluids (tap water, aqueous solutions of NaCl, a water-ethanol mixture and solutions of water-monoethylene glycol of different concentrations) were employed as liquid phases. The gas holdup measurements were used to investigate the global fluid dynamics, the flow regime transition and the foaming phenomena. Subsequently, the image analysis was used to characterize the bubble size distributions and shapes in the homogeneous flow regime. Finally, the gas holdup and image analysis data were used to estimate the interfacial areas. The experimental data were extensively compared with previous studies and correlations and were used to propose novel correlations to estimate the bubble shapes and the interfacial areas, for the different liquid phases tested. It was found that a change in the liquid phase properties affects the bubble interfacial properties at the “bubble-scale”, thus changing the prevailing bubble size distribution because of the reduced/promoted coalescence phenomena. The variation in the prevailing bubble size distribution affects the “reactor-scale” (gas holdup and flow regime transition) and a lift-force-based approach is used to analyze the relationship between the “bubble-scale” and the “reactor-scale”. When the prevailing bubble size distribution shifts towards larger bubbles, because of the promoted coalescence (i.e., high viscous liquid phases), the lift force pushes the larger bubbles towards the center of the bubble column, inducing “coalescence-induced bubbles” and, consequently, destabilizing the homogeneous flow regime and decreasing the gas holdup. Conversely, when the prevailing bubble size distribution shifts towards smaller bubbles, because of the reduced coalescence (i.e., organic and inorganic active compounds, and low viscous liquid phases), the lift force pushes the small bubbles towards the wall, inducing cluster of bubbles and, consequently, stabilizing the homogeneous flow regime and increasing the gas holdup. In addition, foaming phenomena were observed for organic active compounds, in the case of high aspect ratios only: this observation suggests that the validity of the scale-up criteria is not ensured for all binary systems and future studies will be devoted to provide insights in the scale-up criteria.

The effect of riser end geometry on gas-solid hydrodynamics in a CFB riser operating in the core annular and dilute homogeneous flow regimes

Riser hydrodynamics are a function of the flow rates of gas and solids as well as the exit geometry, particularly when operated above the upper transport velocity. This work compares the exit voidage for multiple geometries and two different solids: Geldart group A glass beads and Geldart group B coke. Geometries were changed by modifying the volume of an abrupt T-shaped exit above the lateral riser exit. This was accomplished by positioning a plunger at various heights above the exit from zero to 0.38m. A dimensionless expression used to predict smooth exit voidage was modified to account for the effect of the depth of the blind-T. The new correlation contains the solids-gas load ratio, solids-to-gas density ratio, bed-to-particle diameter ratio, gas Reynolds Number, as well as a term for the exit geometry. This study also found that there was a minimum riser roof height above the blind-T exit beyond which the riser exit voidage was not affected by the exit geometry. A correlation for this minimum riser roof height has also been developed in this study. This study covered riser superficial gas velocities of 4.35 to 7.7m/s and solids circulation rates of 1.3 to 11.5kg/s.

Gas holdup and flow regime transition in spider-sparger bubble column: effect of liquid phase properties

This paper discusses the effects of the liquid velocity and the liquid phase properties on the gas holdup and the flow regime transition in a large-diameter and large-scale counter-current two-phase bubble column. In particular, we compared and analysed the experimental data obtained in our previous experimental studies. The bubble column is 5.3 m in height, has an inner diameter of 0.24 m, it was operated with gas superficial velocities in the range of 0.004–0.20 m/s and, in the counter-current mode, the liquid was recirculated up to a superficial velocity of -0.09 m/s. Air was used as the dispersed phase and various fluids (tap water, aqueous solutions of sodium chloride, ethanol and monoethylene glycol) were employed as liquid phases. The experimental dataset consist in gas holdup measurements and was used to investigate the global fluid dynamics and the flow regime transition between the homogeneous flow regime and the transition flow regime. We found that the liquid velocity and the liquid phase properties significantly affect the gas holdup and the flow regime transition. In this respect, a possible relationship (based on the lift force) between the flow regime transition and the gas holdup was proposed.

Singlet oxygen oxidations in homogeneous continuous flow using a gas–liquid membrane reactor

Safe supply of oxygen through a permeable membrane was used to carry out singlet oxygen chemistry in homogeneous flow regime.