Gas Hold-up Time Research Articles

A transient flow reactor for rapid gas switching at atmospheric pressure.

Herein, we present a design for a transient flow reactor system with high detection sensitivity and minimal dead volume, such that it is capable of sub-second switching of the gas stream flowing through a catalytic bed. We demonstrate the reactor's capabilities for step transient, pulse, and stream oscillation experiments using the model system of CO oxidation over Pd catalysts, and we find that we are able to precisely model step transients for CO oxidation using a pseudo-homogenous-packed bed reactor model. The design principles leading to minimal gas hold-up time and increased sensitivity that are described in this paper can be implemented into existing flow reactor designs with minimal cost, providing a readily accessible alternative to the existing transient instrumentation.

Prediction of intensity of dispersion in surfactant solutions in vertical sparged bubble column

AbstractDispersion of liquid in bubble column is an important process in chemical and allied industries. In the present work, the dispersion characteristics of liquid phase in a bubble column is studied in three different surfactant solutions. A comparative analysis on the dispersion characteristics of liquid phase in different surfactants has been carried out. The gas holdup and mixing time are also analysed. The effect of different physiochemical properties and operating variables on the dispersion coefficient and gas holdup is examined. A novel model for the estimation of dispersion due to bubble motion has been developed which incorporates the effect of gas holdup, liquid viscosity and circulation velocity. The contribution of dispersion coefficient due to the bubble motion in overall dispersion is also examined. The value of velocity distribution parameter in three different surfactant solutions is also compared. Generalised correlations for gas holdup, dispersion coefficient and mixing time are also developed based on the physicochemical properties of liquid. This research may be useful for the selection and application of these three surfactants in industries. This article will also help researchers for designing multiphase flow system where role of dispersion due to the motion of bubbles is significant.

Investigation of the Gas-Liquid Flow in an Agitated Vessel Equipped with an ASI Impeller by Using Tomography Method

In this study, the hydrodynamics of the gas dispersion in an aerated vessel equipped with a newly designed impeller (ASI impeller), a combination of the pitched blade turbine and the Scaba impeller was investigated. The performance of the ASI impeller was compared with the performances of the pitched blade turbine and the Rushton impeller. The non-invasive Electrical Resistance Tomography (ERT) technique was used to assess the gas dispersion inside the mixing vessel. The effects of the volumetric gas flow, and impeller type and speed were investigated on power drawn, gas holdup, and mixing time. An analysis of the experimental data indicated that the ASI impeller exhibited a minimal effect of the gassing on the power drawn compared to the pitch blade and the Rushton turbine. Measured data on gassed power consumption indicated that the ASI impeller was about 45% more efficient than the Rushton turbine and 20% more than the downward PBT under the general operating conditions in bioreactors for gas flow rate about 1 vvm. Also, this impeller showed a higher amount of gas hold up and lower mixing time compared to the two other studied impellers. A dimensionless correlation for the relative power consumption as a function of flow number and Froud number was developed for the aerated agitated tank. Also, a dimensionless correlation was introduced to compute the overall gas hold up as a function of specific power consumption and superficial gas velocity for the gas-liquid agitated system. The overall conclusion from this study demonstrated that the axial-radial ASI impeller is a good energy-efficient impeller for the aeration system and bioreactors based on the results from the investigation of the data in terms of the power consumption, gas holdup, and mixing time.

Transient large-scale two-phase flow structures in a 3D bubble column reactor

This paper analyses the local and time-dependent behavior of large-scale structures responsible for liquid circulation in gas-water flow of a 3-D cylindrical bubble column of high aspect ratio with a multiple orifice for uniform aeration. The large-scale flow structures play an important role in the mixing and the mass transfer while coherent structures dominate hydrodynamic characteristics of the turbulent flow field. A three-dimensional Euler-Euler large eddy simulation (LES) model was used to calculate large-scale structures and their interaction with bubbles at inlet superficial gas velocities of UG= 6 and 8.4 cm/s where vortical-spiral and turbulent flow regimes occur. The two-phase model gives good agreements with experimental measurements. We use a conditional sampling procedure of liquid velocity and gas hold-up time series to identify and educe the development of coherent flow structures which consists in a pair of counter-rotating vortices convected in a staggered pattern along the column in both the vortical-spiral and central plume regions. On average, the detected instantaneous events for each template account for about 12–15% of the data recorded and may appear simultaneously. These events produce important fluctuations in the axial liquid velocity and gas void fraction. The sampling procedure yielded the averaged topology of the three-dimensional large-scale structures which was visualized using isosurfaces of the vorticity for different gas flow rates. The structures have spiral tube-shaped topology rotating along the column near the walls with a pair of counter-rotating cells sustained through the flow. This work provides deep insights into turbulent flow field in gas-liquid bubble column by LES and pattern recognition.

Study of the hydrodynamics and mass transfer in a rectangular air-lift bioreactor

Despite the increasing importance of air-lift reactors, very little published information is available on how the ratio of aerated to non-aerated cross-sectional areas (Ad/Ar) of the reactor, and the aerating system affect the oxygen mass transfer and other performance characteristics. The influence of the ratio of cross-sectional areas of downcomer to riser (Ad/Ar), and the type of gas-distributor on volumetric oxygen mass-transfer coefficient, gas holdup, and liquid circulation velocity were investigated in a 114L rectangular air-lift reactor with motile baffle, separating aerated from non-aerated compartments. The results indicated that the oxygen mass transfer characteristics of rectangular air-lift reactor are better at Ad/Ar of 2.0; the gas holdup and liquid circulation time showed maximum values at the same ratio. A new empirical model for the effects of the aeration rate and Ad/Ar ratio was proposed to account for the observed changes of volumetric oxygen mass transfer coefficient. The model took into account the different mixing patterns in the reactor for both cases: Ad/Ar ≤1 and Ad/Ar>1 using a wide range of these ratios.

Statistical Validation of the Mixing Length Concept in Bubble Columns Operated in the Transition Flow Regime

A new statistical parameter was de ned in order to determine the range of applicability of the mixing length concept in two bubble columns. Gas holdup time series (60,000 points) were measured in two bubble columns (0.15 and 0.4 m in i.d.) by a conductivity wire-mesh sensors. The new statistical parameter Φ was de ned as the ratio of the mean value of the signal to three times the average absolute deviation (3AAD) and it was correlated to the mixing length. It was found that this new correlation was not valid in all hydrodynamic regimes. It is applicable only in the transition ow regime. Such a clari cation has not been provided in the literature so far. In the narrow bubble column, the results were also conrmed on the basis of the Kolmogorov entropy and another statistical parameter. The new parameter Φ was also used for ow regime identi cation. In the narrow bubble column, the rst transition velocity Utrans was identi ed at 0.034 m/s; whereas, the second Utrans occurred at 0.112 m/s. In the large bubble column, the two Utrans values were identi ed at 0.045 and 0.101 m/s, respectively.

Hydrodynamics of a self-agitated draft tube airlift reactor

The main hydrodynamic characteristics of a novel-constructed, self-agitated draft tube airlift reactor (DT-ALR) were investigated. Ten impellers, driven only by the means of gas throughput and induced liquid circulation, were inserted in the draft tube. The insertion of impellers caused bubble breakup and reduction of both mean bubble size and coalescence, even under the conditions of high gas throughputs. Although the impellers induced energy losses, the resistance to the flow was relatively lower due to their rotation, unlike the internals used in other research reported in the literature. In comparison to the conventional configuration of a DT-ALR, it was found that the presence of impellers led to significant changes in hydrodynamics: riser gas holdup and mixing time increased, while overall gas holdup and liquid velocity in the downcomer decreased.

Effect of reactor geometry on aqueous ammonia-based carbon dioxide capture in bubble column reactors

Capture of carbon dioxide (CO2) from combustion flue gas by aqueous ammonia has been demonstrated as an effective and promising technology. To examine the effect of reactor geometry on ammonia-based CO2 capture performance, a series of bubble column reactors with same volume of working liquid but varied geometry (liquid height-to-diameter ratios H/D=0.93, 2.04 and 3.98) were designed to investigate CO2 capture efficiency with ammonia under different reaction conditions. It is found that reactor geometry characterized by liquid height-to-diameter ratio plays an important role in CO2 capture efficiency as it is associated with gas holdup time in reactors. CO2 capture efficiency increases as liquid height-to-diameter ratio increases. Under the same experimental conditions, the maximum CO2 capture efficiency can increase by up to 7%. Besides, CO2 capture efficiency has positive correlation with the ammonia concentration and reaction temperature whereas has negative correlation with the CO2 inlet concentration and flue gas flowrate.

Determination and evaluation of gas holdup time with the quadratic equation model and comparison with nonlinear models for isothermal gas chromatography

Gas holdup time (tM) is a basic parameter in isothermal gas chromatography (GC). Determination and evaluation of tM and retention behaviors of n-alkanes under isothermal GC conditions have been extensively studied since the 1950s, but still remains unresolved. The difference equation (DE) model [J. Chromatogr. A 1260: 215–223] reveals retention behaviors of n-alkanes excluding tM, while the quadratic equation (QE) model [J. Chromatogr. A 1260: 224–231] including tM is suitable for applications. In the present study, tM values were calculated with the QE model, which is referred to as tMT, evaluated and compared with other three typical nonlinear models. The QE model gives an accurate estimation of tM in isothermal GC. The tMT values are highly accurate, stable, and easy to calculate and use. There is only one tMT value at each GC condition. The proper classification of tM values can clarify their disagreement and facilitate GC retention data standardization for which tMT values are promising reference tM values.

A new accurate quadratic equation model for isothermal gas chromatography and its comparison with the linear model

The gas holdup time (tM) is a dominant parameter in gas chromatographic retention models. The difference equation (DE) model proposed by Wu et al. (J. Chromatogr. A 2012, http://dx.doi.org/10.1016/j.chroma.2012.07.077) excluded tM. In the present paper, we propose that the relationship between the adjusted retention time t′RZ and carbon number z of n-alkanes follows a quadratic equation (QE) when an accurate tM is obtained. This QE model is the same as or better than the DE model for an accurate expression of the retention behavior of n-alkanes and model applications. The QE model covers a larger range of n-alkanes with better curve fittings than the linear model. The accuracy of the QE model was approximately 2–6 times better than the DE model and 18–540 times better than the LE model. Standard deviations of the QE model were approximately 2–3 times smaller than those of the DE model.

Difference equation model for isothermal gas chromatography expresses retention behavior of homologues of n-alkanes excluding the influence of holdup time

A difference equation (DE) model is developed using the methylene retention increment (Δtz) of n-alkanes to avoid the influence of gas holdup time (tM). The effects of the equation orders (1st–5th) on the accuracy of a curve fitting show that a linear equation (LE) is less satisfactory and it is not necessary to use a complicated cubic or higher order equation. The relationship between the logarithm of Δtz and the carbon number (z) of the n-alkanes under isothermal conditions closely follows the quadratic equation for C3–C30n-alkanes at column temperatures of 24–260°C. The first and second order forward differences of the expression (Δlog Δtz and Δ2log Δtz, respectively) are linear and constant, respectively, which validates the DE model. This DE model lays a necessary foundation for further developing a retention model to accurately describe the relationship between the adjusted retention time and z of n-alkanes.

CFD-DPM Modeling of Gas-Liquid Flow in a Stirred Vessel

Gas-liquid flow in a stirred vessel was simulated numerically with computational fluid dynamics(CFD). Gas was treated as discrete phase and described by discrete phase model (DPM), while the liquid was considered as a continuum and solved under Euler reference frame. The liquid velocity, gas holdup and gas residence time distribution in the stirred vessel were predicted. The simulation results show that gas dispersion in the stirred vessel is very non-uniformity and high gas holdup is found in the centre of the stirred vessel and vortexes while relatively low in bottom region and region between two impellers. Liquid velocity has great influence on bubble residence time and size distributions.

Agitation of a gas-solid-liquid system in a vessel with high-speed impeller and vertical tubular coil

AbstractExperimental results of gas hold-up, power consumption and residence time of gas bubbles in a gas-solid-liquid system produced in an agitated vessel equipped with a high-speed impeller and a vertical tubular coil are presented in this paper. Critical agitator speed, needed for the dispersion of gas bubbles and solid particles in liquid were also identified. The studies were carried out in an agitated vessel of the inner diameter D = 0.634 m and the working liquid volume of about 0.2 m3. A tubular coil of the diameter of 0.7D, consisting of 24 vertical tubes of the diameter of 0.016D, was located inside the flat-bottomed vessel. The agitated vessel was equipped with a Rushton turbine with six blades or an A 315 impeller with four blades. Both impellers had diameter, d, equal to 0.33D. The vessel was filled with liquid up to the height H = D. In this study, air and particles of sea sand with the mean diameter of 335 μm and the concentration of up to 3.0 mass % were dispersed in distilled water as the liquid phase. The measurements were carried out within the turbulent regime of the fluid flow in the agitated vessel. Results of the measurements were processed graphically and mathematically. Lower values of the critical agitator speed, n JSG, needed for simultaneous dispersion of gas bubbles and particles with the solids concentration from 0.5 mass % to 2 mass %, were obtained for the vessel equipped with the A 315 impeller. Higher values of the specific power consumption were reached for the vessel with the Rushton turbine. Higher values of the gas hold-up and residence time of the gas bubbles in the fluid were obtained for the system equipped with the Rushton turbine. Results of the gas hold-up as a function of the specific power consumption, superficial gas velocity and solids concentration were approximated with good accuracy using Eq. (5).

Experimental analysis of the hydrodynamics of a three-phase system in a vessel with two impellers

AbstractResults of experimental analysis concerning gas hold-up and average residence time of gas bubbles in a three-phase gas-solid-liquid system produced in a baffled, double-impeller vessel are presented. Measurements were carried out in a vessel with the internal diameter of 0.288 m. Two different double-impeller configurations were used for agitation: Rushton turbine (lower) — A 315 (upper) and Rushton turbine (lower) — HE 3 (upper). Upper impellers differed in the fluid pumping mode. Coalescing and non-coalescing systems were tested. Liquid phases were distilled water (coalescing system) and aqueous solutions of NaCl (non-coalescing systems). The ability of gas bubbles to coalesce in the liquid was described using parameter Y. Dispersed phases were air and particles of sea sand. The experiments were conducted at seven different gas flow rates and two particle loadings. Effects of the ability of gas bubbles to coalesce (liquid phase properties), operating parameters (superficial gas velocity, impeller speed, solids loadings), and of the type of the impeller configuration on the investigated parameters were determined. The results were approximated mathematically. For both impeller configurations tested, significantly higher gas hold-up values were obtained in the non-coalescing gas-solid-liquid systems compared to the coalescing one. Out of the tested impeller systems, the RT-A 315 configuration proved to have better performance ensuring good gas dispersion in the liquid in the three-phase systems.

Computational fluid dynamics modeling of gas dispersion in multi impeller bioreactor

In the present study, experiments have been carried out to identify various flow regimes in a dual Rushton turbines stirred bioreactor for different gas flow rates and impeller speeds. The hydrodynamic parameters like fractional gas hold-up, power consumption and mixing time have been measured. A two fluid model along with MUSIG model to handle polydispersed gas flow has been implemented to predict the various flow regimes and hydrodynamic parameters in the dual turbines stirred bioreactor. The computational model has been mapped on commercial solver ANSYS CFX. The flow regimes predicted by numerical simulations are validated with the experimental results. The present model has successfully captured the flow regimes as observed during experiments. The measured gross flow characteristics like fractional gas hold-up, and mixing time have been compared with numerical simulations. Also the effect of gas flow rate and impeller speed on gas hold-up and power consumption have been investigated.

CFD simulation of effects of the configuration of gas distributors on gas–liquid flow and mixing in a bubble column

Abstract Numerical simulations of gas–liquid flow in a cylindrical bubble column of 400 mm in diameter at the superficial gas velocity U g = 0.10 m s - 1 were conducted to investigate effects of the configuration of gas distributors on hydrodynamic behaviour, gas hold-up and mixing characteristics. Eight different gas distributors were adopted in the simulation. The simulation results clearly show that the configuration of gas distributor have an important impact on liquid velocity and local gas hold-up in the vicinity of the gas distributor. Comparisons of the overall gas holdup and mixing time among different gas distributors have demonstrated that none of the adopted gas distributors was able to produce the highest interfacial area and also yield the shortest mixing time. The CFD modelling results reveal that an increase in the number of gas sparging pipes used in gas distributors is beneficial in improving the gas hold-up but is disadvantageous in reducing bubble size due to a decrease in turbulent kinetic dissipation. It has been demonstrated from the simulations that the appearance of asymmetrical flow patterns in the bubble column and the adoption of smaller gas sparging pipes for gas distributors are effective in improving the mixing characteristics.

Gas holdup and mixing characteristics of a novel forced circulation loop reactor

Gas holdup and mixing time were characterized in a forced circulation internal loop draft-tube reactor (unaerated aspect ratio ≈ 6, downcomerto-riser cross-sectional area ratio = 0.493) as functions of the forced liquid superficial velocity in the riser (ULr) and the gas superficial velocity in the riser (UGr). Data were obtained in air–water system. The operation ranges were 0 ≤ ULr ≤ 0.051 m/s and 7.8 × 10 −3 ≤ UGr ≤ 3.9 × 10 −2 m/s for the liquid and gas velocities, respectively. Under forced flow conditions the reactor always operated in the bubble flow regimen, but operation as an airlift reactor (i.e. no forced flow of liquid) produced a heterogeneous churn-turbulent flow regimen. Forced flow of liquid enhanced gas holdup in comparison with the airlift mode of operation. Mixing time generally declined with increased flow rates of gas and liquid. The ability to maintain a bubble flow regimen through forced flow of liquid allowed the reactor to attain gas holdup values of >0.12 that are difficult to achieve in air–water in conventional bubble columns and airlift reactors. © 2007 Elsevier B.V. All rights reserved.

A Mechanistic Model to Estimate Power Consumption and Mixing Time in Aluminium Industries

An attempt to produce simple equations for evaluating gas hold-up, power consumption and mixing time in stirred tank reactors used in aluminium industries have been made. This simplified model, based on mass balances and continuity equations and taking into account both geometrical parameters and physical properties of the working fluids, allows the reduction of the number of adjustable parameters necessary to predict the effect of the gas flow-rate on power consumption and mixing time. Notwithstanding the simplified form of the model, it allows the prediction of, with good accuracy and few adjustable parameters, both the power consumption and the mixing time of non-standard stirred tank reactors.

00/00709 Effect of gas holdup and liquid mean residence time on coal liquefaction

00/00709 Effect of gas holdup and liquid mean residence time on coal liquefaction

High liquid holdup airlift tower loop reactor: I. Riser hydrodynamic characteristics

The hydrodynamics of high liquid holdup airlift reactors have not been widely studied. In this work we study the riser of a reactor of this type, focussing on the influence of the gas flow rate and the submergence ratio on various hydrodynamic parameters: liquid circulation velocity, gas holdup, and gas residence time in the liquid. A notable difference in reactor behavior is observed with a submergence ratio equal to unity and with a ratio lower than unity. Correlations for the parameters studied in different conditions, as several submergence ratios, are proposed, these are extrapolations of other equations proposed in the literature for airlift reactors. The reactor studied in this work can be thought of as an element of a great pool formed by multiple elements of similar flux type. The studied parameters are shown to be very important for subsequent hydrodynamic and mass transfer modeling of the reactor.