Rate Of Gas Induction Research Articles

Effect of turbulent dispersion on hydrodynamic characteristics in a liquid jet ejector

Gas induction using a liquid jet is an extensively studied phenomenon. Many studies have been published detailing various hydrodynamic and mass transport aspects of gas induction in ejectors. Conversely, multiple studies have been published detailing jet dynamics, jet break up, nozzle geometry, effects of turbulence etc. In the modelling of the jet ejector systems CFD framework is used as the flow of gas is coupled with the liquid flow and the flow parameters like volume fraction, pressure drop/profile and velocity profiles both local and average are highly interdependent. Hence, it becomes important to capture all the flow physics in the simulations in order to understand the hydrodynamics. In the previous models the effect of turbulent dispersion was not included which led to incorrect calculation of phase profile, and other related parametric values. In the present work, the turbulence effects and its impact on the jet dynamics and gas induction rate in an ejector have been studied. Capturing the correct phase as well as velocity profile is imperative for the extension of the model to mass transfer and reactions. Lastly, other hydrodynamic properties like pressure drop, phase hold up and pressure profile help better the design correlations for jet ejectors.

Estimation of gas induction in jet loop reactors: Influence of nozzle designs

Jet loop reactors are used widely for conducting gas liquid reactions because of the high mass transfer achieved in the gas–liquid ejector. Studies have shown that the mass transfer has a very strong correlation to the amount of gas induced in the ejector, and hence it is important to understand gas induction to enhance the performance of any gas–liquid nozzle. In this work, we used a single phase CFD model of the ejector with one adjustable parameter for estimating gas induction rates. After establishing that the model adequately describes the experimental data, the model was used for a quick evaluation of ejector geometries. Influence of key geometric parameters of gas–liquid ejectors like nozzle diameter, mixing tube length, distance between the nozzle outlet and mixing tube, suction chamber geometry and diffuser angle was investigated. It was found that dependence of gas induction on geometric parameters like distance between nozzle—mixing tube, suction chamber geometry, diffuser angle was either weak or had a clear maxima at or beyond a certain value of the geometric parameter. Other parameters like mixing tube length and nozzle diameter have a more complex impact on gas induction. The presented approach and results will be useful for quantifying influence of nozzle designs on gas induction rate in jet loop reactors.

Prediction of the Induced Gas Flow Rate from a Self‐Inducing Impeller with CFD

AbstractThe complex task of describing computationally two‐phase turbulent flows in aerated stirred‐tank reactors was overcome by proposing that the gas flow rate in the hollow impeller can be estimated from single‐phase flow simulations of the liquid phase in the reactor: the pressure at the impeller surface obtained from liquid phase simulations can be related to the gas induction rate. A commercial lab‐scale reactor with a radial six‐bladed hollow impeller was chosen for the study. To validate the presented methodology, the induced gas flow rate was measured experimentally from the tracking of the position of bubbles in a dynamic sequence of flow images. Notwithstanding the simplifications assumed in the presented CFD methodology, good agreement has been obtained between numerical results and experiments.

A 3D CFD simulation of a self inducing Pitched Blade Turbine Downflow

For many chemical and industrial processes, the understanding of mixing, self-induction and flow structure in a stirred tank is very important. In this work, we simulated numerically, using the Fluent software, the phenomena taking place in a stirred tank equipped with a self-inducing turbine, the Pitched Blade Turbine Downflow (PBTD). To refine our study, we studied the cases of two different blade inclinations, i.e., PBTD45° and PBTD60°. The same rotational speed of the blades was considered for each simulation, namely N=480rpm. A numerical description of the flow structure generated by the self-inducing turbine is given. The results were compared with experimental data available in the literature. The gas induction rate, the power number and the power provided by the turbine are in good agreement with these experimental studies. Finally the last stage of this work was devoted to a comparative study of each turbine performance. The CFD results given by our work show that the PBTD60° is more efficient than the PBTD45°.

Hydrodynamic and Mass Transfer Characteristics of Single and Multiple Impeller Hollow Self-Inducing Reactors

In the present work, hydrodynamics and mass transfer characteristics of a gas−liquid stirred tank provided with hollow self-inducing impellers were investigated. Critical impeller speed for the onset of gas induction, gas induction rate, power consumption, and mass transfer coefficient (kLa) were measured using two types of self-inducing impellers, viz., hollow pitched-blade downflow turbine and double-disk impeller. Experiments were carried out in 100 and 800 L tanks. Mass transfer coefficient was measured with steady-state hydrazine oxidation technique with air−water as the gas−liquid system. The impeller speed was varied from 2.5 to 14 rps, and power input per unit volume was varied from 0.5 to 10 kW/m3. Further, the performance of a multi-impeller hollow self-inducing system has been investigated. Four different hollow impellers in combination with three axial flow impellers have been studied in an 800 L tank using three impeller diameters and three liquid submergences. Suitable correlations have been proposed that also take into account all the published kLa data in the literature.

Hollow self-inducing impellers for gas–liquid–solid dispersion: Experimental and computational study

Abstract In this work, experimental and computational fluid dynamic studies have been carried out for hollow self-inducing three-phase stirred tank systems. The effect of different impeller designs (hollow self-inducing pitched blade down flow turbines of different blade angles (PBTD30, PBTD45, PBTD60), and self-inducing modified double disc (MDD) impeller) and operating conditions such as solid loading (0–7 wt. %) and solid particle size (125–700 μm) have been investigated on critical impeller speed for solid suspension ( N CS ), gas induction rate ( Q G ) and overall gas hold-up ( ∈ G ). Computational fluid dynamics model based on the Eulerian multi-fluid approach has been employed along with the standard k - ɛ turbulence model. A multiple reference frame (MRF) approach was used to model the impeller rotation. In this study, the multiphase flow has been simulated using a commercial CFD code, Fluent v6.2.16. A good agreement was found between the CFD predictions and the experimental values of N CS .

Hollow self-inducing impellers: Flow visualization and CFD simulation

The experimental fluid dynamics (EFD) as well as computational fluid dynamics (CFD) studies were performed for hollow self-inducing agitator system. The system is described in detail elsewhere [Deshmukh, N.A., Patil, S.S., Joshi, J.B., 2006. Gas induction characteristics of hollow self-inducing impeller. Transactions of the Institution of Chemical Engineers 84(A2), 124–132]. The low pressure regions on the impeller blade were determined by single phase CFD simulations. These results were validated with the experimentally measured power number ( N P ) . Then with the help of single phase CFD results, the orifices were drilled on the impeller blade in low pressure regions. Gas induction rate ( Q G ) , power consumption ( P ) and overall fractional gas hold-up ( ϵ G ) were measured experimentally. The comparison of gas induction rates for both the hole locations has been presented. The two phase CFD simulations yielded satisfactory predictions and proved to be most promising for the design of efficient gas-inducing system.

Gas−Liquid Mass Transfer in Benchscale Stirred TanksFluid Properties and Critical Impeller Speed for Gas Induction

This work is concerned with the gas−liquid mass transfer in stirred tanks with gas-inducing impellers. Experiments were performed to determine the critical impeller speed for the onset of gas induction and the mass-transfer group kLa, using water, n-hexadecane, toluene, acetone, and sunflower oil at various temperatures as liquids and hydrogen and nitrogen as gases. The use of different liquids and gases allowed the influence of fluid properties on the mass transfer to be studied. Special emphasis is on reducing the experimental effort in downscaling, i.e., the use of benchscale autoclaves to mimic industrial conditions. In benchscale tanks, the capillary pressure is significant, with respect to the static liquid pressure at the impeller, and must be taken into account to describe the critical impeller speed for gas induction. Downscaling implies relatively low mass-transfer rates, which can be achieved by operating close to the critical impeller speed. The critical impeller speed must be included in models for the mass-transfer rate. Our data set of kLa values was best described by a dimensionless correlation, in which a Froude group (based on the stirrer speed N and the critical stirrer speed Ncr) was used to account for the gas-induction rate, together with the Reynolds number and Schmidt number, to account for turbulence intensity and fluid properties.

Gas Induction Characteristics of Hollow Self-Inducing Impeller

The mechanism of self induction has been investigated for the case of hollow pitched blade downflow turbines. The pressure difference between the orifice and the reactor head space results into self induction of gas. The dependence of pressure difference as well as the rates of gas induction on various parameters like impeller speed, blade angle, orifice location and the number of open orifices was investigated. Gas side and liquid side resistances were experimentally measured for blade angles of 30°, 45° and 60°. An attempt has been made to propose a predictive procedure for the rate of gas induction on the basis of pressure driving force as well as the gas and the liquid side resistances. The gas induction rates for wide range of blade angle (viz. PBTD 30°, 45° and 60°), various hole combinations (viz. 1; 1,2 and 1,2,3,4,5,6) and for submergence of S = 2T/3 and for 1,2,3,4,5,6 holes at S = T/3 submergences have been predicted with this procedure. The predictions are in good agreement with the experimental results.

Mechanism of Gas Induction in a Self-Inducting Impeller

The gas induction characteristics of a stator-rotor-type self-inducing impeller were investigated in the cases without and with gas line resistance. To understand the hydrodynamics near the impeller, and to characterize the mechanism of gas induction, the local liquid velocity was measured with a laser Doppler anemometer. The effect of impeller speed, impeller submergence, and gas line resistance on the volumetric flow rate of liquid, as well as the rate of gas induction, was investigated. The dominant role of local gas holdup was identified and the mechanism of gas induction was established, with the help of an energy balance.

OPTIMUM DESIGN OF STATOR-ROTOR ASSEMBLY IN GAS INDUCING TYPE MECHANICALLY AGITATED REACTORS

The stator-rotor combination was optimised in gas inducing type of agitated contactors. The rate of gas induction and the fractional gas hold-up were measured in a 1 m i.d. vessel. Tap water was used as the liquid phase, and air was used as the gas phase. The impeller speed was varied between 1.5 to 6.5 rev/s. Pitched bladed downflow turbine having six blades was used as impeller. The effect of blade angle and blade width was investigated. The stator was optimized in terms of diffuser width, number of stator vanes and the stator-impeller gap. The role of stator air-ring and stator vanes was also investigated. The effect of errors in the fabrication on the performance of the contactor was studied. Ways have been proposed to overcome the deterioration of performance.

Design of Gas-Inducing Reactors

A gas-inducing impeller enables efficient recycling of gas from the headspace into the liquid. Historically, these impellers were used for the first time in froth flotation machines. The various designs of gas-inducing impellers (including those used in froth flotation) could be classified into three categories, depending on the flow pattern coming into and leaving the impeller zone. These are denoted as type 11, type 12, and type 22 systems. The critical impeller speed for the onset of gas induction (NCG) is governed by a balance between the velocity head generated by the impeller and the hydrostatic head above the impeller. A number of correlations (for types 11 and 22) are based on this balance (Bernoulli's equation). The rate of gas induction (QG) for the type 11 system can be accurately determined by equating the pressure difference (between the impeller zone and the headspace) generated by the impeller and the pressure drop required for the flow of gas. For type 22 systems, the correlations for QG a...

Gas-inducing impeller design and performance characteristics

A theoretical and experimental study on the design and performance characteristics of gas-inducing impellers is presented. In particular, the model developed by Evans et al. (1991, A.I.Ch.E. Spring National Meeting, Houston, TX, Paper 33e) is critically reviewed and, as a result, improvements to the kinetic energy pressure loss analysis and to the initial conditions are proposed. In addition, the model is successfully extended to account for multiple gas outlet orifice on each blade. Experimental measurements of the power consumption, rate of gas induction, mass transfer coefficient and detached bubble size for a partially optimised, 0.154 m diameter, six-bladed concave gas-inducing impeller are presented. A significant increase in the induced gas rate is observed by adding more outlet orifices to each blade. The principal advantage of using multiple orifices is that similar size bubbles are produced, compared to a single orifice, but larger interfacial areas are generated; the aerated power input is only slightly reduced from its ungassed value. Mass transfer coefficients, k L a, of the order of 0.02 s −1 are attainable for a single outlet orifice on each blade; k L a is significantly increased by using multiple orifices. The dimensionless bubble size distributions, d d gm independent of the impeller speed over the range 4–8 rps, and can be successfully represented by a log-normal distribution.

Hydrodynamics of a Stirred Vessel Equipped with a Gas-Inducing Impeller

Hydrodynamic characteristics of a gas-liquid reactor equipped with a gas-inducing impeller were investigated. Four different impeller designs and two different ring spargers were used. The effects of superficial gas velocity, liquid submergence, and the interimpeller clearance on the critical impeller speed for gas induction, the rate of gas induction, and the fractional gas hold-up were investigated. The critical impeller speed was found to increase with an increase in the superficial velocity of the sparged gas. It was also affected by the lower impeller design and the sparger size. The rate of gas induction was found to decrease with an increase in the superficial gas velocity. It also depends on the lower impeller design and the sparger size. The liquid submergence had an optimum value at which the rate of gas induction was a maximum. The data obtained could be successfully correlated on the basis of mathematical models developed earlier.

Optimization of impeller design for gas inducing type mechanically agitated contactors

AbstractRates of gas induction and fractional gas hold‐up were measured in a 0.57 m i.d. gas inducing type mechanically agitated contactor. Several designs of the impeller were employed for optimizing the rate of gas induction with respect to power consumption. The effect of blade width blade, angle, blade thickness, blade curvature, number of blades, etc. on gas induction, has been investigated in detail. An optimum design has been proposed. Hysteresis in the gas induction rate and power consumption was observed for a wider impeller projecting out of stator.

Modelling the increased gas capacity of self-inducing impellers

In many gas—liquid mixing processes occuring in stirred vessels, the rate of reaction is limited by mass transfer between the gas and liquid phases, e.g. acetic acid fermentation. An important aspect in the design of such devices is to increase the rate of reaction by improving the mass transfer performance. This study examines the effect of increasing the number of gas outlet orifices on each blade on the gas induction rate and mass transfer performance of a concave-bladed self-inducing impeller. Increasing the number of orifices is found to significantly increase the rate of gas induction, however, this effect diminishes as the number of orifices increases. Volumes of liquid per unit volume of gas per minute (VVMs) of up to 0.6 can be achieved using all four orifices on each blade at an impeller speed of 10 rps; this may be compared to a value of 0.25 obtained under similar conditions for a single orifice. Furthermore, this method of increasing the gas capacity is not at the expense of generating bigger bubbles, and thus large gas—liquid interfacial areas are maintained. This is reflected in experimental measurements of the mass transfer coefficient; the value of k L a for a given specific power input approximately doubles in moving from one to four orifices per blade. Mass transfer coefficients of up to 0.055s −1 (at a specific power input of 1.7 kW/m 3) are attainable for the coalescing air—water system. An existing model for gas-inducing impeller design (Evans., 1991a) is extended to include the effect of multiple orifices. The revised model gives acceptable predictions of the induced gas rate, to within 20% of the experimental data, over the full range of measurements.

Gas Inducing Type Mechanically Agitated Contactors

The rate of gas induction was measured in 0.57, 1.0, and 1.5 m i.d. gas inducing types of mechanically agitated contactors (GIMAC). The ratio of impeller diameter to vessel diameter was varied in the range 0.13 < D/T < 0.59. The effect of impeller submergence from the top and impeller clearance from the bottom was investigated in detail. The effect of impeller speed was studied in the range 0.13 < N < 13.5 r/s. A model has been developed for the critical speed for gas induction, and for the rate of gas induction with the description of forced vortex structure in the impeller region. A unified theme has been proposed for the understanding of phenomena such as the critical speed for gas induction and the rate of gas induction. The physical significance of model parameters has been interpreted on the basis of flow information in the impeller region.

Effect of vessel scaleup on the hydrodynamics of a self-aerating concave blade impeller

A theoretical model has been developed to predict the pressure at the gas outlet of a gas-inducing impeller and to determine the minimum speed at which gas induction occurs, for a given impeller design and submersion depth. The model also predicts the flow rate of gas induced at higher impeller speeds, by balancing the pressure reduction due to flow over the blades, against pressure losses associated with the flow of gas and bubble formation. Pressure measurements for an impeller moving in single-phase (prior to gas induction) were used to determine independently the model constants for a concave impeller. These constants were identified as a blade slip factor and a pressure coefficient and were shown to be independent of Reynolds number (in the turbulent regime) and constant over the range of submersion depths used in practice. The model predictions for the minimum induction speed were in good agreement with experiments for a concave impeller at a number of angles of attack and in various vessel geometries. Gas flow rate measurements at higher impeller speeds indicated that the pressure driving force was a function of the detached bubble radius, and providing this dependence was known, the model could be used to predict the rate of gas induction.

IMPROVEMENTS IN GAS INDUCING IMPELLER DESIGN

Abstract Rate of gas induction, static pressure, mixing time and power consumption have been measured in 0.57 m i.d. vessel. Different types of impellers namely shrouded disc turbine, shrouded curved blade turbine and pitched blade turbine were used. The impeller diameter was varied from 0.15–0.25 m and the impeller speed was varied from 3 to 20 r/s. The pitched blade turbine was found to give 30–60 per cent higher rates of gas induction as compared with the best design reported in the literature. The mixing time was found to be lower by a similar magnitude. Moreover in the case of pitched blade turbine it was found that the gas was getting induced radially as well as axially. This eliminates the necessity of the diffuser and hence reducing the complexities in the mechanical structure.

Mass transfer and hydrodynamic characteristics of gas inducing type of agitated contactors

AbstractPower consumption, rate of gas induction (Q), fractional gas hold‐up and mass transfer characteristics of 0.41, 0.57 and 1.0 m i.d. gas inducing type of agitated contactors were studied. The power consumption was measured by a torque table. The values of effective interfacial area (a), and liquid side mass transfer coefficient (kLa) were obtained by using chemical methods. Two types of impellers (pipe and flattened cylindrical) were employed. The impeller speed was varied from 3 to 11.7 r/s.The effects of impeller diameter, baffle position, orifice area on the impeller, number of impeller blades, position of impeller from the bottom and liquid submergence on the values of Q and a were investigated. Further, the effect of liquid viscosity and surface tension on the values of Q was studied.A satisfactory agreement was found between the observed and the predicted value of the minimum impeller speed at which gas induction occurs. The performance of the gas inducing type of agitated contactor has been compared with the conventional mechanically agitated contactor.