The investigation of gas holdup distribution in a two-phase bubble column using ultrasonic computed tomography

In this study, time-averaged gas holdup distributions were investigated in a 16 cm diameter bubble column for two-phase dispersed system of air–water and air–glycerol solution of 10 wt% by using ultrasonic computed tomography (UCT). A quantitative result of UCT – as a coupling of the ultrasonic transmission method and the iterative filtered backprojection (IFBP) image reconstruction technique – is presented. The UCT results are in a good agreement with those by the bed expansion method. A higher gas holdup in the air–glycerol 10 wt% solution than in the air–water system was observed. The distribution of gas holdup in the column with an attached baffle is also investigated by UCT.     

Experimental and numerical study of gas hold-up in surface aerated stirred tanks

Although the distribution of gas hold-up in stirred tanks is a key factor to their design and operation, systematic experimental data on local gas hold-up of surface-aerated stirred tanks are not available in open literature. In this work, turbulent two-phase flow in a surface aeration stirred tank with a diameter of 0.380 m was investigated experimentally and numerically. The gas hold-up was measured with a conductance probe at various operating conditions. A surface baffle to improve the efficiency of surface aeration of a Rushton disk turbine was designed and tested. The experimental data suggest that the gas hold-up distribution in the surface aeration tank is very non-uniform, and the surface baffle improves the aeration rate particularly at a high agitation speed. A three-dimensional in-house computational fluid dynamic (CFD) two-fluid model with the standard k?A_p turbulence model was used to predict the gas-liquid flow, and the impeller region was handled using the improved inner-outer iterative procedure. Based on Kolmogoroff's theory of isotropic turbulence, a constitutive equation for surface aeration strength was proposed. The numerical prediction, in combination with the measurements, gives insight to the surface aeration performance of stirred tanks. It was found that the simulation reasonably predicted the gas hold-up distribution in the upper tank, but underestimated it in the region below the stirrer.     

A novel X-ray computed tomography method for fast measurement of multiphase flow

A new approach based on an improved genetic algorithm (GA) was proposed to implement the image reconstruction when using X-ray computed tomography (XCT) for the application of fast measurement of multiphase flow dynamics. Instead of directly using a traditional XCT, we pursued to develop a different discrete tomography (DT) method, aiming to achieve a high resolution in time during the measurements with only limited projection data. The proposed method assumed that the interested multiphase flow can be simplified as having distinct dense and dilute phases so that the local phase concentration can be binary-coded, e.g., 0 or 1 in a gas bubbling system. The mathematical problem under these circumstances is strongly ill-posed, and thus tackled with an optimization approach, i.e., a GA incorporated with the underlying physics as some constraints. The numerical simulations mimicking the physical measurements demonstrated the feasibility of the new approach, namely GA-XCT, especially with high robustness to the noise. Experiments were performed to simulate a transient measurement on the gas bubbles in water, with a portable X-ray tube and a 2D plane detector as the hardware and a static object rotating in between. The results further provided the validation of the GA-XCT being superior to the conventional algorithm, e.g., filtered back-projection (FBP) technique, in dealing with the tomography of multiphase system with binary local density field.     

Simultaneous measurement of gas hold-up and mass transfer coefficient by tracer dynamic technique in “Emulsair” reactor with an emulsion-venturi distributor

Hydrodynamics and gas-liquid mass transfer have been investigated in an “Emulsair” reactor with cocurrent downflow of gas and liquid. This consists of a cylindrical tank with conical bottom topped by an emulsion-venturi as the gas-liquid distributor in which the gas is self-aspired by action of the kinetic energy of the liquid recirculation. An original tracer dynamic technique using the CO₂-N₂/water system that enables the simultaneous measurement of overall gas hold-up and overall volumetric mass transfer coefficient has been developed and validated using conventional techniques, such as volume expansion for gas hold-up and dynamic oxygenation for mass transfer. It has been shown that gas hold-up and K_La_L can be deduced from the moments of experimental response curves using a CO₂ pulse in the gas feed. Experimental results have proved that hydrodynamics and mass transfer in the Emulsair reactor are strongly influenced by the flow regime in the divergent. Two different regimes have been observed depending on the liquid recirculation flow rate: annular and homogeneous bubbling flows. In both regimes, self-aspired gas flow rate, gas hold-up and K_La_L have been reported to increase with the liquid flow rate. The operation effectiveness, estimated from the gas-to-liquid flow rate ratio, has been shown to pass through a maximum around 0.59 as a function of liquid recirculation. A comparison with the literature proved that this maximum is higher than those observed for other kinds of gas-liquid reactors equipped with a venturi. Correlations for mass transfer estimation have been derived and are in agreement with the literature.     

X-ray computed tomography of a gas-sparged stirred-tank reactor

X-ray computed tomography (CT) is used to explore the differences in gas dispersion in a stirred-tank reactor (STR) for different operating conditions by varying the impeller speed and gas flow rate. X-ray imaging has been carried out in a 0.21 m ID acrylic STR equipped with a nylon Rushton-type impeller. From the CT images, major differences in local gas holdup are observed for different operating conditions. Completely dispersed conditions have a relatively uniform holdup profile while flooded conditions show a high gas holdup near the impeller shaft. The high resolution of the X-ray system allowed fine details such as recirculation regions behind the baffles to be visualized.     

The influence of gas inlet design on gas hold-up values for water and various solutions in a loop-type air-lift fermenter

Gas hold-up was measured in an external loop air-lift fermenter for air in water, in sodium chloride and sodium sulphate solutions (concentrations between 0.025 and 1.0 kmol m⁻³), in glucose solutions (0.025 to 0.25 kmol m⁻³), sodium carboxymethyl cellulose (CMC) solutions (0.1, 0.2 and 0.5 mass %) and in suspensions of cellulose powder in water. Gas superficial velocities were up to 0.03ms⁻¹. Two types of gas spargers were used: one had six holes each of 1.6mm diameter, the other was of porous metal, which produced very small bubbles. Porous metal spargers with different diameters were also used, to alter the fraction of the cross-sectional area of the riser which was aerated (the ‘fractional aerated area’). The use of dimensionless gas velocities, obtained by dividing the superficial gas velocities by a characteristic velocity for each of the non-Newtonian liquids and solid suspensions, made possible a general correlation between gas hold-up, dimensionless gas velocity and the fractional aerated area, in bubbly flow conditions. The characteristic velocities for the electrolyte solutions could be related to the concentrations of the electrolytes. No general correlation could be obtained for gas slug-flow conditions, although theoretically-based relationships represent the results for each liquid. Gas slug-flow was observed for the non-Newtonian CMC solutions for most of the gas rates used at the lower concentrations; only at the highest concentration was bubbly flow found over a wide range of gas rates.     

Electrical resistance tomography-assisted analysis of dispersed phase hold-up in a gas-inducing mechanically stirred vessel

Tomographic analysis of the hydrodynamic attributes of the gas–liquid–solid mixing in a 1-l capacity stirred-tank equipped with a 4-blade gas-entrainment impeller has been used to obtain the dispersed phase hold-up distribution as a function of stirring speed (impeller Reynolds Number, Re_I) and solid particle loading. Although the liquid phase stirring was turbulent, both gas and solid flows went through different hydrodynamic regimes and experienced radial hold-up gradient over the range of impeller speed employed. Global solid phase hold-up profile exhibited a sigmoid-shape with respect to the impeller Reynolds number indicative of three solid suspension regimes across the stirring range (1.0≤Re_I≤6.25×10⁴) investigated. The solid phase hold-up distribution was adequately captured by, ε_s=ε_s,max[1−exp(−τ_sppRe_I)]^γ with ε_s,max and γ dependent on solid loading. An analogous expression was also obtained for the radial solid phase hold-up distribution and has enabled the proposition of a criterion for existence of radial transport gradient in gas-induced stirred tanks (GIST). Additionally, correlations for estimating the mixing time and power number for gas-induced mechanical agitators also gave good agreement with the empirical data.     

Comparison of ML–EM algorithm and ART for reconstruction of gas hold-up profile in a bubble column

Algebraic reconstruction technique (ART) and maximum likelihood–expectation maximization (ML–EM) algorithm have been applied for image reconstruction using gamma-ray tomography. This methodology can be of immense help in establishing the hydrodynamics of several multiphase systems such as two-phase and three-phase bubble column reactors. The effect of various image processing parameters such as initial guess, grid size, stopping criteria and gamma-ray measurement parameters like beam configuration, number of projection, number of views on the quality of reconstructed image has been studied in present work. It has been observed that ML–EM algorithm shows more precise and faster results as compared to ART and it serve as a preferential tool in image reconstruction. These techniques were then used in the estimation of gas hold-up profile in a two-phase aqueous system. Average gas hold-up values in bubble column based on reconstructed local hold-up values based on the above two techniques were found to be in good agreement with the experimental data within ±10% accuracy, however, ML–EM algorithm may be preferred due to better capability of incorporating the modalities of data collection.     

The influence of electrolytes on gas hold-up and regime transition in bubble columns

Experiments were conducted in a 0.12-m-in-diameter bubble column to investigate the effect of electrolytes on gas hold-up (ε) and on the regime transition point in bubble columns. Air was used as the dispersed phase and aqueous solutions of three different salts (NaCl, Na₂SO₄ and NaI), as well as double-distilled water, were utilised as the continuous phase, varying the gas superficial velocity (u_G) in the range 0-0.26 m/s. The ε×u_G curves were a function of both the chemical nature and the concentration of the electrolytes. However, similar ε×u_G profiles were obtained regardless of the electrolyte for a given ratio between the concentration in the solution and the critical concentration of the electrolyte for bubble coalescence. This ratio therefore presents itself as a promising modelling parameter to account for the chemical nature of electrolytes. The gas hold-up data were employed to compute the regime transition point according to two different methods, evidencing its non-linear dependence on the concentration of electrolytes in the liquid.     

Local time-averaged gas holdup in a fluidized bed with side air injection using X-ray computed tomography

Local time-averaged gas holdup in a 10.2 cm diameter fluidized bed is determined using X-ray computed tomography (CT) over a range of superficial gas velocities (U_g), side air injection flow rates (Q_side), and fluidized bed material. Without side air injection, only small variations in the local time-averaged gas holdup are observed for beds composed of glass beads, ground walnut shell, or ground corncob. With the introduction of side air injection, which simulates the immediate volatilization of biomass in a fluidized bed gasifier, a distinctive plume is observed along the reactor wall above the side injection port. The plume gradually expands toward the center of the bed as height increases; the expansion is found to increase with increasing Q_side. As U_g increases, fluidization becomes more uniform and the effect of the side air injection on the fluidization hydrodynamics is less pronounced. Additionally, increasing U_g increases overall gas holdup and bed expansion. Of the three bed materials examined, ground corncob fluidization is the least affected by side air injection and shows the highest overall gas holdup while glass bead fluidization is much more affected by side air injection and has the lowest overall gas holdup. This study demonstrates the usefulness of X-ray CT in noninvasively visualizing detailed internal features of fluidized beds. These results will be used in future studies to validate computational fluid dynamics (CFD) models of fluidized beds.     

A reactor network model for predicting NOx emissions in gas turbines

The numerical prediction of NOx emissions from gas turbines is addressed in this paper. Generated from Computational Fluid Dynamics (CFD), a Reactor Network (RN) is defined to model the NOx formation with a detailed chemistry. An optimized procedure is proposed to split the reactive flow field into homogeneous zones considered as Perfectly Stirred Reactors (PSR). Once connected together, they result in a Chemical Reactor Network (CRN) that yields a detailed composition regarding species and temperature in the combustion chamber. Sensitivity studies are then performed to estimate the influence of air humidity and gas turbine load on NOx predictions. The NOx emissions predicted are in good agreement with the measured data in terms of levels and trends for the case studied (a gas turbine flame tube fed with natural gas and functioning at a pressure of 15 bar). Finally, the RN methodology has shown to be efficient estimating accurately NOx emissions with a short response time (few minutes) and small CPU requirements.     

Solid hold-up measurement in a jet-impactor assisted fluidized bed using gamma-ray densitometry

A jet-impactor assisted-fluidized bed (JIAFB) for continuous de-agglomeration of nanopowder agglomerates was presented in previous work. Therein, a jet caused agglomerates to impinge up an impactor, where they would break. However, efficient impactor positioning will be dictated by particle momentum: the product of solid concentration and velocity must be highest. Herein, the variation of solid hold-up was measured in a fluidized bed of Fe₂O₃ nanoparticles using gamma-ray densitometry. Behaviour was compared under minimum fluidization and bubbling regimes, over a wide range of jet velocities (0–200 m s⁻¹). A new line-decomposition approach allowed mapping local solid distribution across seven axial and five radial positions, tangibly demonstrating how increasing the gas velocity enhanced the fluidization quality by increasing axial solid diffusivity. Conversely, increasing jet velocity locally decreased solid hold-up in the jet-affected zone, and brought about inhomogeneities. With this information in hand, jet-to-impactor distance was optimized and validated experimentally.     

Liquid phase mixing and gas hold-up in a multistage-agitated contactor with co-current upflow of air/viscous fluids

A hydrodynamic study is carried out in a multistage-agitated contactor with cocurrent upflow of air/viscous fluids. The liquid phase backmixing is characterized through a residence time distribution study in various viscous fluids (up to 20 mPa s). The liquid Reynolds number with respect to the impeller applied in the investigation ranges from 1000 to 50 000. A cascade of stirred tanks with back flow model satisfactorily fits all experimental residence time distribution curves in the present study. It is found that the increase in the liquid viscosity reduces the backmixing in the liquid phase due to its dampening effect on turbulence. The presence of the gas phase helps in reducing the backmixing by the straightening effect and entrainment effect in a co-current operation manner. The gas hold-up measured in the present study is comparable to the literature data. A new type of correlation taking into account the influence of the gas phase is proposed to predict the backmixing in a multistage-agitated contactor.     

Effect of surface tension gradient on gas hold-up enhancement in aqueous solutions of electrolytes

The effect of addition of electrolytes on gas hold-up of air/water system was investigated experimentally in a laboratory scale bubble column. The experiments were carried out with four electrolytes, namely, NaCl, MgSO₄·7H₂O, Na₂SO₄ and CaCl₂·2H₂O and the concentrations of the solutions were varied from 0 to 0.3 mol/l. Enhancement of gas hold-up was observed for all four electrolytes at concentrations less than 0.1 mol/l. With the increase in concentration, the gas hold-up showed two different trends; in Na₂SO₄ and CaCl₂·2H₂O solutions, gas hold-up formed a sharp peak after the enhancement and leveled off at a value somewhat higher than that in water, whereas in NaCl and MgSO₄·7H₂O solutions, gas hold-up leveled off immediately after the enhancement without forming any peak. Experiments were also conducted to measure the surface tensions of the solutions with special focus in the low concentration region. A strong relation between the gas hold-up enhancement and the change of surface tension with the addition of electrolyte was found. It was also observed that the concentration at which maximum value of C(dσ/dC)² i.e. (concentration × surface tension gradient with respect to concentration²) is obtained corresponds to the concentration at which maximum gas hold-up enhancement occurs.     

Gas–liquid mass transfer in a new three-phase stirred airlift reactor

The gas–liquid mass transfer performances of a novel three-phase reactor involving both airlift and mechanical stirring have been tested using aqueous solutions of glucose. Stirring in addition to classical airlift leads to an importance increase of k_La. The absolute increase depends mainly on the stirrer speed are not on the gas velocity. A slight effect of the solid loading with a maximum at about 2% (w/v) was observed. Two correlations that show the influence of physical parameters are proposed for both water and glucose solutions. © 1998 SCI     

磁力搅拌反应器中微观混合测量的研究

微观混合是流体分子尺度上的混合,对化学反应的进行有重要的影响。为研究化学反应中常用的磁力搅拌器中微观混合的进行,使用碘化物-碘酸盐体系的平行一竞争反应对其中的微观混合进行了研究。通过测量溶液吸光度计算出离集指数,对不同条件下的微观混合进行分析。研究发现,提高磁力搅拌的电机转速有利于微观混合的进行,粘度的增大不利于微观混合的进行。在烧杯中心加料的微观混合效果好于远离中心处加料。在加料的摩尔流速相同时,较低浓度的加料有利于微观混合。研究得出了有利于反应器中微观混合进行的条件。     

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.     

Prediction of gas hold-up and liquid velocity in airlift reactors using two-phase flow friction coefficients

The overall friction coefficient (K_f) of airlift reactors was estimated using equivalent lengths (L_eq) and friction factors ( f ). The friction factor was calculated taking into account the riser liquid velocity profile corresponding to the two-phase flow and using classical one-phase equations. A previously described model was used to obtain simultaneously both gas hold-up and liquid circulation velocity. The model simulates experimental data obtained in a wide range of configurations of internal (2 and 30 dm³ volume) and external (from 8 to 600 dm³ volume) airlift reactors with Newtonian (water and alcohol solutions) and non-Newtonian (carboxymethylcellulose (CMC) solutions) systems. Com-parison with other models from the literature yielded similar results.     

Investigation of dynamic liquid distribution and hold-up in structured packings using ultrafast electron beam X-ray tomography

Dynamic cross-sectional liquid distribution and hold-up in a DN80 separation column filled with structured packings was studied using an ultrafast electron-beam X-ray tomograph with high temporal resolution of 2000 images per second. The modality allows visualisation and characterisation of the counter-current flow before and at the flooding point representing the upper operation limit. Two packings of the same type (Montz B1-MN) with different specific surface area were used to investigate the influence of the packing geometry on the spatial liquid distribution. The system studied was water/air at different gas and liquid loads. The results of the tomographic imaging and corresponding post-processing routines were validated by comparison with conventional draining measurements.     

Rapid measurement of gas diffusivity in liquids using a tube-in-tube reactor with an unsteady-state strategy

In this work, an unsteady-state strategy for rapid measurement of gas diffusivity in liquid is proposed, which has a quick perturbation of the liquid flow rate in inner tube for obtaining the change of gas flow rate across the membrane with time. The strategy has taken full advantages of the tube-in-tube reactor that possesses a high permeability Teflon AF-2400 membrane to accelerate the diffusion rate of gas to liquid without direct contact between the two phases. With a developed mathematical model fitting the recorded variation of gas flow rate with time, the gas diffusivity in liquid can be determined within 0.5–3 min compared with the conventional methods of 4–14 hr. In addition, the strategy is demonstrated with several gas–liquid systems (O₂-DMSO, CO₂-[Emim] [NTf₂] and CO₂-[Bmim] [BF₄]) with varied viscosities and temperatures, showing a good agreement with literature values with less than 10% deviation.