Range Of Superficial Gas Velocities Research Articles

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 to well explain the global gas volume fraction measurements in the airlift riser for a wide range of superficial gas velocity (0.6≤JG≤10cms−1). 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.

Experimental study of the bubble size distribution in a pseudo-2D bubble column

This work presents an experimental study of the bubble size distribution of a bubbly flow using digital image analysis (DIA). In order to facilitate the image measurement technique a pseudo-2D bubble column is chosen for the experiments. To obtain well-defined inlet conditions a gas sparger, consisting of 20 needles, is used. By employing DIA, the bubble size distribution (BSD) has been measured for a range of superficial gas velocities. The resulting BSD's are expressed in terms of a probability density function (PDF). For low superficial gas velocities of 5 and 10mm/s the PDF has a unimodal shape, while for higher superficial gas velocities of 15 and 20mm/s the PDF has a bimodal shape. The effects of coalescence and break-up of bubbles are visible by evaluating the changes of the resulting BSDs for increasing superficial gas velocity. A comparison of gas hold-ups is made between the calculated BSD and the liquid expansion height. This comparison shows how well the BSD obtained with DIA describes the actual gas hold-up in the column.

Particle granular temperature of Geldart A, A/B and B particles in dense gas-fluidized beds

Extensive studies have been devoted to the boundary of Geldart A and B particles due to its great practical relevance, however, consensus on the hydrodynamic characteristics when the A/B boundary is crossed is still lacking. On the basis of extensive discrete particle simulations, it seems that there is no dramatic change with respect to the nature of granular temperature as a function of superficial gas velocity when the A/B boundary is crossed, suggesting a smooth transition from Geldart A particles to Geldart B particles, which is in agreement with some of the experimental studies (Geldart, D., 1973. Powder Technol. 7, 285–292; Grace, J.R., 1986. Can. J. Chem. Eng. 64, 353–363; Loezos et al., 2002. Chem. Eng. Sci. 57, 5123–5141), but disagrees with acoustic shot noise (ASN) studies (Cody et al., 1996. Powder Technol. 87, 211-232; and Cody et al., 2008. Powder Technol. 182, 146-170). Finally, we show that the disagreement between the results obtained from present DPM studies and ASN studies may possibly be attributed to the short range of superficial gas velocity in ASN measurement and/or the effect of the reactor size. In view of the various possible sources of uncertainty related to both DPM method and ASN technique, lot of works are needed before making a definite conclusion on the nature of hydrodynamic characteristics of dense gas-fluidized beds when the boundary of Geldart A/B particles was crossed.

Effects of Drag-Reducing Polymers on Stratified and Slug Gas–Liquid Flows in a Horizontal Pipe

This paper presents experiments in which the effect of drag-reducing polymers (DRP) on the frictional pressure drop of stratified flow and slug frequency of a slug flow of air and water flow in 2.54 cm diameter horizontal pipe is explained. The effect of DRP on the liquid holdup is also identified. The reduction in the frictional pressure drop by DRP in the stratified flow happened only at certain operational conditions. The reduction in the slug frequency also happened at certain operational condition, and the reduction is pronounced at certain range of superficial liquid and gas velocities. Liquid holdup for air–water flows was less than that for air–water with DRP.

Effect of Void Fraction on Pressure Drop in Upward Vertical Two-Phase Gas-Liquid Pipe Flow

Experimental data for the void fraction and two-phase frictional pressure drop from various sources has been compiled and analyzed. The experimental data revealed that at the lower range of superficial gas velocity and void fraction, the variations of the two-phase frictional pressure drop with superficial gas velocity and void fraction are relatively flat. However, as the superficial gas velocity and void fraction increase to higher values, the frictional pressure drop became significantly sensitive to the two parameters. In a situation when the two-phase pressure drop is sensitive to the variation of the void fraction, it is then that the proper and accurate characterization of the void fraction becomes significant. From the experimental data, regions where the pressure drop is sensitive to the variation of the void fraction are identified and evaluated.

Estimation of the overall mass flux in inclined standpipes by means of pressure fluctuation measurements

The mass and heat balances in circulating fluidized bed systems strongly depend on the overall solids flux recirculated, making it necessary to estimate or measure this flux. Many methods have been reported in literature to continuously monitor the solids flux, with the curve solids flux Gs vs. pressure drop in the riser ΔPriser as one of the most used approaches. In this work, we show that in a circulating fluidized bed equipped with an inclined standpipe the whole mass flux is well correlated with the standard deviation of the pressure fluctuations in the standpipe. Contrary to the correlation Gs vs. ΔPriser, our correlation results to be virtually independent of the superficial gas velocity in the riser in the range 3.1–7.5m/s. This makes it applicable for the on-line monitoring of the solids flux in a broad range of superficial gas velocities. Moreover, it has been shown that from the data Gs vs. standard deviation it is possible to estimate the maximum solids flux achievable by the unit. From this information, we can know at which fraction of the maximum mass flux the circulating fluidized bed is operating.

Influence of acetaminophen on gas hold-up, liquid circulation velocity and mass transfer coefficient in a split-cylinder airlift bioreactor

In this study, hydrodynamic parameters and mass transfer characteristics were measured at four acetaminophen concentrations (1000–4000mg/l) in water over the range of superficial gas velocity of 0.2–1cm/s at 25°C in a split-cylinder airlift bioreactor. The gas hold-up and overall oxygen transfer coefficient for all the fluids increased with increasing the gas velocity. The specific interfacial area increased and liquid-side oxygen transfer coefficient decreased with increasing gas velocity. Furthermore, the gas hold-up increased and bubble diameters decreased with acetaminophen addition. The overall oxygen transfer coefficient and gas hold-up increased and bubble diameter decreased with increasing acetaminophen concentration. The overall oxygen transfer coefficient increased due to increasing the specific interfacial area (with surface tension and bubble size reduction). Two correlations based on the dimensionless numbers were developed to find Sherwood number and gas hold-up.

The characteristics of gas–solid flow and wall heat transfer in a fluidized bed reactor

Numerical study using computational fluid dynamics has been carried out to investigate the heat transfer characteristics of a laboratory fluidized bed reactor. The fluidized bed reactor of vTI (Johann Heinrich von Thunen-Institute)-Institute of Wood Technology and Wood Biology is modeled. For the simulation of multiphase flow and thermal fields, an Eulerian–Eulerian approach is applied. The flow and thermal characteristics of the reactor are fully investigated for the wide range of superficial gas velocities and two different particle diameters. In particular, the contributions of the gas bubble and emulsion phase flows on the wall heat transfer are scrutinized. From the predicted results, it is fully elucidated that particular near-wall bubble motions mainly govern the wall heat transfer.

Experimental constraints on the outgassing dynamics of basaltic magmas

The dynamics of separated two‐phase flow of basaltic magmas in cylindrical conduits has been explored combining large‐scale experiments and theoretical studies. Experiments consisted of the continuous injection of air into water or glucose syrup in a 0.24 m diameter, 6.5 m long bubble column. The model calculates vesicularity and pressure gradient for a range of gas superficial velocities (volume flow rates/pipe area, 10−2–102 m/s), conduit diameters (100–2 m), and magma viscosities (3–300 Pa s). The model is calibrated with the experimental results to extrapolate key flow parameters such as Co (distribution parameter) and Froude number, which control the maximum vesicularity of the magma in the column, and the gas rise speed of gas slugs. It predicts that magma vesicularity increases with increasing gas volume flow rate and decreases with increasing conduit diameter, until a threshold value (45 vol.%), which characterizes churn and annular flow regimes. Transition to annular flow regimes is expected to occur at minimum gas volume flow rates of 103–104 m3/s. The vertical pressure gradient decreases with increasing gas flow rates and is controlled by magma vesicularity (in bubbly flows) or the length and spacing of gas slugs. This study also shows that until conditions for separated flow are met, increases in magma viscosity favor stability of slug flow over bubbly flow but suggests coexistence between gas slugs and small bubbles, which contribute to a small fraction of the total gas outflux. Gas flow promotes effective convection of the liquid, favoring magma homogeneity and stable conditions.

CFD Simulation of Scale Influence on the Hydrodynamics of an Internal Loop Airlift Reactor

In this work, the overall gas hold-up in the riser and down-comer of three internal airlift reactors with a working volume of 10.5, 32 and 200 l at the range of temperatures 18℃ - 21℃, under atmospheric pressure was simulated using Com-putational Fluid Dynamics (CFD). The range of superficial gas velocity was 0.5 - 3 cm/s. The three reactors geometrically were similar to each other. CFD simulation and experimental data showed that the gas hold-up in the riser and down-comer increased by increasing the reactor scale. It was concluded that the simulated data were in good agreement with the experimental ones obtained from the literature.

High Pressure Experiments and Simulations in Cocurrent Bubble Columns

Abstract Bubble column is a multiphase reactor in which gas is passes through the liquid in two phase flow. When liquid is stationary the operation is called as batch mode and co-current when both gas and liquid are moving in the same direction. The gas hold-up increases with increase in pressure for both batch & co-current column. This increment in gas holdup is very sharp initially, but soon it becomes insignificant. The increase in liquid velocity decreases the gas hold-up at all pressures. As we increase the pressure, the effect of liquid velocity is less. Three dimensional Euler-Euler two-phase fluid model has been used to simulate two-phase up-flow in bubble column (15cm diameter) using ANSYS 12.1. These experiments and simulations were operated over a range of superficial gas velocities (1 to10 cm/s) at ambient conditions as well as high pressures. The liquid velocity range was 0 to 16 cm/s. The turbulence in the liquid phase has been modeled using the standard k-ɛ model. The interactions between the two phases are described through Schiller Neumann drag coefficient formulation. The objectives are to find the effect of pressure and liquid velocity over gas holdup and to validate the CFD simulations with experimental data. Quantitatively good agreements are obtained between experimental data for hold-up and simulation values. Radial gas holdup profiles and axial liquid velocity profiles were also obtained for the simulation.

Hydrodynamics and mass transfer characteristics in an internal loop airlift reactor with different spargers

The effect of the sparger structure on the hydrodynamics and mass transfer characteristics of an internal loop airlift reactor was investigated. Three spargers with different diameters and numbers of orifices were tested. Two different flow regimes, i.e., homogenous flow and heterogeneous flow, were identified within the experimental range of superficial gas velocities. It is found that the sparger structure has more significant effect on hydrodynamic parameters in the heterogeneous flow regime than in the homogenous flow regime. The largest gas holdup and downcomer liquid velocity were obtained with the 4-orifice nozzle, followed by the O-ring distributor and the 2-orifice nozzle. The sparger structure has less effect on the oxygen transfer coefficient k L , while it strongly affects the specific interfacial area a. The 4-orifice nozzle improved the volumetric mass transfer efficiency for that it generated a smaller mean bubble diameter and therefore a larger specific interfacial area than other spargers. Based on the material conservation and pressure balance principles, a hydrodynamic model was established to predict the liquid velocity in the downcomer, as well as the liquid velocity and the cross sectional area of the up-flow in the riser. Empirical correlations were proposed for different spargers, which well predicted the gas holdup and the volumetric mass transfer coefficient.

Heat Transfer and Fluid Flow Characteristics of Nonboiling Two-Phase Flow in Microchannels

In this study, heat transfer and fluid flow characteristics of nonboiling two-phase flow in microchannels were experimentally investigated. The effects of channel diameter (140, 222, 334, and 506 μm) on the Nusselt number and the pressure drop were considered. Air and water were used as the test fluids. Results were presented for the Nusselt number and the pressure drop over a wide range of gas superficial velocity (1.24–40.1 m/s), liquid superficial velocity (0.57–2.13 m/s), and wall heat flux (0.34–0.95 MW/m2). The results showed that the Nusselt number increased with increasing gas flow rate for the large channels of 506 and 334 μm, while the Nusselt number decreased with increasing gas flow for the small channels of 222 and 140 μm. Based on these experimental results, a new correlation for the forced convection Nusselt number was developed. In addition, the two-phase friction multiplier is shown to decrease as channel diameter decreases due to the influence of viscous and surface tension forces.

The repeatability and uniformity of 3D fluidized beds

Hydrodynamic characterization of fluidized beds is vital to many industrial processes, including the production of biofuels, pharmaceuticals, and numerous petroleum products. Local time-average phase fractions can be quantified in these multiphase three-dimensional systems using X-ray computed tomography (CT). This paper describes repeatability and uniformity measurements in a 3D 15.2 cm diameter cold flow fluidized bed using CT imaging to quantify gas holdup. Two different bed materials and a range of superficial gas velocities are investigated. Five tests over different days are completed to show repeatability for each test condition. Highly repeatable local time-average gas holdup is recorded at various locations within the bed for each material over all flow conditions. Uniformity is determined by analyzing the local time-average gas holdup at 12 azimuthal locations for both materials and various flow conditions. Uniform axi-symmetric fluidization is observed for each material and U g when the bed height is greater than h = 0.25D, where the effects of individual aeration holes dissipate.

Dynamic modeling of gas phase propylene homopolymerization in fluidized bed reactors

A new model with comprehensive kinetics for propylene homopolymerization in fluidized bed reactors was developed to investigate the effect of mixing, operating conditions, kinetic and hydrodynamic parameters on the reactor performance as well as polymer properties. Presence of the particles in the bubbles and the excess gas in the emulsion phase was considered to improve the two-phase model, thus, considering the polymerization reaction to take place in both the bubble and emulsion phases. It was shown that in the practical range of superficial gas velocity and catalyst feed rate, the ratio of produced polymer in the bubble phase to the total production rate is roughly between 10% and 13%, which is a substantial amount and cannot be ignored. Simulation studies were carried out to compare the results of the improved two-phase, conventional well-mixed and constant bubble size models. The improved two-phase and well mixed models predicted a narrower and safer window at the same running conditions compared with the constant bubble size model. The improved two-phase model showed close dynamic behavior to the conventional models at the beginning of polymerization, but starts to diverge with the evolution of time.

Gas Fraction and Bubble Dynamics in Structured Slurry Bubble Columns

With the aim of structuring the flow and reducing backmixing in slurry bubble columns, we investigate the effect of needle spargers in three-phase systems. We apply optical probes in a 2D and 3D column to measure the bubble dynamics and gas fraction. Experimental results for air-water-glass beads show that an increase in solids volume fraction (1) decreases the gas fraction, (2) shifts the transition point from homogeneous to heterogeneous flow to a lower gas velocity, (3) increases the mean bubble velocity by 60-100% in the range of superficial gas velocities from 0.02 to 0.1 m/s, and (4) has only a limited effect on the bubble size distribution. Fitting our experimental data to the Richardson and Zaki and Garnier models shows that we cannot use the model coefficients they report in their papers. However, fitting our data results in realistic values for the terminal bubble velocity in a swarm. By using needle spargers, giving a very uniform gas supply, a first step toward structuring the hydrodynamics of slurry bubble columns has been taken.

CFD simulation of hydrodynamics and mass transfer in an internal airlift loop reactor using a steady two-fluid model

Steady state simulation of the hydrodynamics and mass transfer in bubbly flow in an axisymmetric internal airlift loop reactor (IALR) with the special treatment of interphase decoupling and outlet boundary conditions is performed, which is based on a Favre averaging two-fluid model for multiphase flow. Different models of mass transfer coefficient are compared and validated with the literature data. A model for describing the mass transfer for a wide range of superficial gas velocity is chosen and the predicted mass transfer coefficient agrees well with the experimental data in the literature. The numerical procedure can be used as a tool for the design and scale-up of IALRs.

The residence time distribution and mixing of the gas phase in the riser of a circulating fluidized bed

CFBs are increasingly used for both gas-catalytic and gas–solid reactions. The conversion is a function of the gas hydrodynamics, subject of the present research. Available literature on the gas mixing in the riser of a CFB shows contradictory results: some investigators neglected back-mixing of gas, whereas others report a considerable amount of back-mixing in CFB risers. The present paper reports experimental findings obtained in a 0.1 m I.D. riser, for a wide range of combined superficial gas velocity (U) and solid circulation flux (G). The gas flow mode (plug vs. mixed) is strongly affected by the operating conditions, however with a dominant mode within a specific (U, G)-range. Sand was used as bed material. The superficial gas velocity was varied from 5.5 to 8.3 m/s, the solids circulation flux was between 40 and 170 kg/m 2 s. A tracer pulse response technique was used with a pulse of propane injected at the bottom and detected at the riser exit. The cumulative response curves, F(t), define (i) an average residence time (t 50) obtained for F(t) = 0.5; and (ii) the slope of the curves (a steeper one corresponding with more pronounced plug flow) and expressed in terms of a span, σ. These parameters (t 50 and σ) define the gas flow mode. A quantitative comparison of experimental results with literature RTD-models is inconclusive although the occurrence of both mixed flow and plug flow is evident, and (U, G)-dependent. The experimental results are expressed in empirical design equations, and the comparison of predicted and experimental results is fair: low values of σ determine the plug flow regimes, whereas back-mixing is more pronounced at higher value of σ. Experiments with similar systems might favor plug flow or mixing as function of the combined (U, G)-values. The introduction of the RTD-function in reaction rate equations can improve the prediction of the gas-conversion in a riser-reactor.

Gas holdup, bubble behavior and mass transfer in a 5 m high internal-loop airlift reactor with non-Newtonian fluid

Gas holdup, bubble behavior, interfacial area and gas–liquid mass transfer in a 5 m internal-loop airlift reactor with non-Newtonian fluid were studied in the superficial gas velocity ( U g) range of 2–12 cm/s. Air and aqueous CMC solutions of 0–0.45 wt% were used as the gas and liquid phases, respectively. It was found that increased U g or CMC concentration led to a wider bubble size distribution and an increase in the bubble Sauter diameter. The volumetric mass transfer coefficient increased with an increase in U g and a decrease in CMC concentration. In the air–water system, k l a/ α g was found to be independent of U g and was 0.2 1/s, and a constant liquid-side mass transfer coefficient ( k l) was found in the heterogeneous regime. However, in the air–CMC solution system, the influences of the superficial gas velocity and liquid viscosity were much more complicated: k l a/ α g was not constant and was affected by the superficial gas velocity and CMC concentrations; the interfacial area increased with an increase in U g and a decrease in CMC concentration; k l increased more significantly with increasing U g, and no obvious trend was found for the influence of CMC concentration on k l.

Study of Liquid Weeping at a Tapered Orifice in a Bubble Column Reactor by a Computer-Aided Image Analysis Algorithm

Understanding the phenomenon of liquid weeping requires knowledge of the bubbling and weeping cycle. Hence, an image analysis algorithm is developed and subsequently coupled with high-speed imaging techniques to examine the different stages in the bubbling and weeping cycle automatically. The bubbling and weeping cycle is studied under various operating conditions, using the developed algorithm, coupled with high-speed imaging techniques and the pressure fluctuation measurement. The effect of orifice angle (where the orifice angle is a measure of the orifice taperness) on the liquid weeping rate is evaluated over a range of superficial orifice gas velocities and justified with the aid of the developed algorithm, coupled with high-speed imaging techniques. A gas distributor plate with negative angle orifices is recommended for improved operation of industrial bubble column reactors over the range of superficial orifice gas velocities considered in this study, because it substantially decreases the liquid w...