Influence Of Superficial Gas Velocity Research Articles

Hydrodynamic study on the riser of chemical looping hydrogen production system with multiple fluidized bed reactors

Chemical looping technology is an available way for hydrogen production with low penalty energy for improving gas purity and CO2 is captured simultaneously. Understanding the operation and flow behavior is crucial for development of chemical looping hydrogen production. Experimental investigations of the system with dual interconnected fluidized bed reactors were carried out and presented in this work. The results indicated the system reached stable operation with current design. Influence of superficial gas velocity on the pressure distribution was examined and the corresponding mass inventory in the reactor was also analyzed. It showed that the relative pressure decreases with the increase of riser fluidization velocity and more particles will be transported to the fuel reactor from riser. The pressure curve fluctuated in sinusoidal shape as expected. It was better to make a large fuel reactor which holds most of solids and contributed to the stability. A tanks-in-sires model was utilized for analyzing the pressure evolution in the riser, which was less time consumed compared with computational fluid dynamic method. It showed the obvious phase shift of pressure drop along height. Also, the force balance of a particles was analyzed for predicting the mass inventory of riser and this method overcame the insufficient of measurement technology. The influences of solid circulating rate and gas velocity were obtained and be investigated further. This is a feasible way for quick prediction of flow behavior in the riser for both the design and operation under a certain operating condition.

Comparative analysis on gas–solid drag models in MFIX-DEM simulations of bubbling fluidized bed

In this study, the open-source software MFIX-DEM simulations of a bubbling fluidized bed (BFB) are applied to assess nine drag models according to experimental and direct numerical simulation (DNS) results. The influence of superficial gas velocity on gas–solid flow is also examined. The results show that according to the distribution of time-averaged particle axial velocity in y direction, except for Wen–Yu and Tenneti–Garg–Subramaniam (TGS), other drag models are consistent with the experimental and DNS results. For the TGS drag model, the layer-by-layer movement of particles is observed, which indicates the particle velocity is not correctly predicted. The time domain and frequency domain analysis results of pressure drop of each drag model are similar. It is recommended to use the drag model derived from DNS or fine grid computational fluid dynamics–discrete element method (CFD-DEM) data first for CFD-DEM simulations. For the investigated BFB, the superficial gas velocity less than 0.9 m·s−1 should be adopted to obtain normal hydrodynamics.

Bubble size distribution and gas holdup in bubble columns employing non‐Newtonian liquids: A CFD study

AbstractThe hydrodynamics involved with the rise of air bubbles in shear‐thinning non‐Newtonian liquids in bubble columns was investigated using computational fluid dynamics (CFD). In bubble columns, the bubble size distribution (BSD) and gas holdup significantly affect the mass transfer rates and the reactor design. Consequently, the influence of superficial gas velocity, flow index, consistency index, sparger bubble size, and yield stress were analyzed on a local and global scale. The bubble sizes and the coalescence and breakage phenomena were incorporated using the homogeneous discrete method of the population balance model (PBM). The bubbles underwent coalescence during ascent and exhibited bimodal distribution. The radial homogeneity of the gas phase increased axially. Moreover, the zones of low liquid dynamic viscosity produced zones of a high gas holdup. Though the dual effect of viscosity on gas holdup was non‐existent, the gas holdup surged after 35.1 mPa s at the mid‐zone and remained constant thereafter. The noteworthy differences in simulation results put further emphasis upon the importance of sparger bubble size in CFD modelling. A decrease in the overall gas holdup and the axial air velocity with yield stress was observed. CFD simulation proved capable of providing results in reasonable agreement with experiments.

Numerical study of the hydrodynamics and mass transfer in the external loop airlift reactor

The objective of this study was to investigate the hydrodynamics and the gas-liquid mass transfer coefficient of an external-loop airlift reactor (ELAR). The ELAR was operated in three cases: different inlet velocities of fluids, different alcohols solutions (water, 0.5% methanol, 0.5% ethanol, 0.5% propanol and 0.5% butanol) and different concentration of methanol in solutions (0%, 0.5%, 1%, 2% and 5%). The influence of superficial gas velocity and various diluted alcohol solutions on hydrodynamics and the gas-liquid mass transfer coefficient of the ELAR was studied. Experimentally, the gas hold-up, liquid velocities and volumetric mass transfer coefficient values in the riser and the downcomer were obtained from the literature source. A computational fluid dynamics (CFD) model was developed, based on two-phase flow, investigating different liquids regarding surface tension, assuming the ideal gas flow, applying the finite volume method and Eulerian-Eulerian model. The volumetric mass transfer coefficient was determined using the CFD and artificial neural network model. The effects of liquid parameters and gas velocity on the characteristics of the gas-liquid mass transfer were simulated. These models were compared with the appropriate experimental results. The CFD model successfully simulates the influence of different alcohols regarding the number of C-atoms on hydrodynamics and mass transfer.

Study on the spatial and temporal distribution of the bed density in an air dense medium fluidized bed (ADMFB) based on the electrical capacitance tomography (ECT) measurement system

In this study, the ECT system was used to study the temporal and spatial distribution characteristics of bed density in an ADMFB under the influence of superficial gas velocity and static bed height. The results show that, In the axial distribution, the closer to the top of the bed at low gas velocity(<11.5 cm/s), the higher the density, and the closer to the top at high gas velocity (> 13 cm/s), the lower the density. In the radial distribution, the closer to the central axis, the lower the density. With the decrease of static bed height, the radial uniformity becomes worse. The optimal gas velocity is 12.5 cm/s, under which the bed spatial distribution is the most uniform, and the ecart probable moyen of spherical particles is 0.06. The separation density is smaller than the bed density and is close to the central axis density.

Influence of superficial gas velocity on heat transfer in a two-phase system with additive heater surfaces

The problem of removing high heat fluxes from a powerful microelectronic element (chip, microprocessor) is one of the most difficult tasks. One of the promising methods for cooling is the use of two-phase systems in a minichannel with a stratified flow regime. Additive technologies allowing the creation of structured surfaces are of increasing interest. The study of heat transfer using structures on the surface in the form of cylindrical protrusions in comparison with a smooth surface with characteristic size (diameter and height) equal to 300 μm is presented in this work.

Measurement of bed-to-tube surface heat transfer coefficient to a vertically immersed u-tube in bubbling loop seal of a CFB boiler

Experimental measurements of bed-to-tube surface heat transfer coefficient were conducted in a 12 MWth circulating fluidized bed boiler/gasifier where a vertical U-tube was immersed into the bed. The aim was to study the influence of superficial gas velocity, solid flux of bed material, direction of the tube relative to the circulating particle flow, choice of fluidization gas, bed temperature and to compare the results with predicted values based on heat transfer correlations.The experiments displayed high heat transfer coefficients of around 550 W/(m2K) with sand and with ilmenite approximately 850 W/(m2K) when operating with air at a superficial gas velocity of 0.25 m/s. Replacing air with steam and recirculated flue gas as fluidization gas increased the heat transfer by more than 40% with both steam and flue gas. Having the U-tube oriented parallel to the circulating particle flow reduced the heat transfer by around 8% compared to a perpendicular orientation.

3D CFD Simulation of Gas Hold-up and Mass Transfer in a Modified Airlift Reactor with Net Draft Tube

Abstract This paper documents CFD simulations of the gas hold-up (ɛg) and volumetric mass transfer coefficient (KLa) for three kinds of airlift reactors (ALRs) namely, conventional ALR, ALR with net draft tube (ALR-ndt), and packed-bed ALR with net draft tube (PBALR-ndt). The 3D two-fluid Eulerian-Eulerian model was adopted to predict the influence of superficial gas velocity on ɛg and KLa. The simulation results were consistent with the trends described previously in the experimental work regarding ɛg and KLa values and a good agreement was obtained (absolute error less than 20 %). Based on the simulation results, axial flow is the dominant flow in the conventional ALR while in ALR-ndt and PBALR-ndt radial flow streamlines are appeared in the reactors due to the presence of concentric net draft tube which improves their performance. The effective role of the net draft tube is proven since consequence of generation of small bubbles by passing through net draft tube is the entrainment of a larger percentage of gas bubbles from the riser into the downcomer which results in improvement of gas holdup and the KLa values. An exponential correlation is used for relating gas hold-up and mass transfer coefficient. Higher power obtained for ALR-ndt and PBALR-ndt (n ≈ 1.22) compared to ALR (n = 0.95) was indicative of high sensitivity of KLa value to gas hold-up in these reactors due to presence of the concentric net draft tube. The CFD modeling is considered to be an invaluable tool allowing us to analyze and visualize the impact of fluidic forces on hydrodynamic properties and consequently, reactor performance.

Effects of EOR chemicals and superficial gas velocity on bubble size and gas holdup of a bubble column

Chemical Enhanced Oil Recovery (EOR) can boost oil extraction in offshore operations, however one of the main concerns regarding its application is how the efficiency of flotation units for treating produced water is affected. The present work thus focuses on investigating the impact of EOR chemicals on the physical properties of EOR effluents and how this can affect flotation performance parameters such as bubble size and gas holdup. Design of experiments has been used to assess the influence of polymer, surfactant and sodium chloride concentrations on bubble size and gas holdup of a laboratorial bubble column. The influence of superficial gas velocity has also been assessed together with chemicals concentrations, yet at low levels in order to avoid clusters, swarms and foam. The characterization of the synthetic effluent containing polymer, surfactant and sodium chloride has indicated that the fluid behaves as a non-Newtonian fluid, what makes separation processes in flotation cells challenging. Results showed that polymer concentration of 2000 mg/L can lead to significant increases in fluid viscosity, promote a growth of more than 40% in bubble size and only increases gas holdup when surfactant is present at high concentration. Therefore, polymers are expected to be detrimental to produced water treatment. Surfactants decrease both fluid surface tension and bubble size, increasing gas holdup. For the range studied, superficial gas velocity favors gas holdup and sodium chloride concentration seems to weakly influence bubble size and gas holdup. This work highlights the fact that changes in physical properties of produced water do modify bubble size distribution and gas holdup and this must therefore be taken into account when flotation-like systems are designed to deal with EOR effluents.

GAS HOLD UP IN THE CULTIVATION OF A PETROLEUM-DEGRADING BACTERIAL CONSORTIUM

The hydrodynamic behavior of bubble column bioreactors (BCB) is strongly dependent on the bubbly flow regime. Therefore, the influence of superficial gas velocity (Ug) and Sauter mean diameter (d32) on gas hold up (eg) was evaluated for the cultivation of a petroleum-degrading consortium. Hydrodynamic parameters were determined by photographic techniques. Also, the biomass cultivation was quantified by suspended solids formation (SS). Our findings indicated that the d32 increased at high Ug values (0.91.2 cm s-1), but decreased with the presence of Tween 20 surfactant (0-0.15 mL L-1) in the model medium. An enhancement in the eg was observed at high Ug values (1.0-1.3 cm s-1). Interestingly, eg values ranging from 0.02 to 0.024 reported a high concentration level of SS (8-10 g L-1) during the hexadecane degradation. Particularly, eg value of 0.024 was a convenient level to cultivate the consortium resulting in changes in bacterial population distribution, due to oxygen and hydrocarbon bioavailability. According our results, the eg is proposed as key factor related to mass transfer phenomena and agitation on the cultivation of petroleum-degrading consortium.

Molten Corium–Concrete Interaction: Investigation of convective heat transfer in a pool with gas sparging

During a severe accident in a Pressurized Water Reactor, corium may fall in the reactor pit and interact with concrete. The CLARA facility, financed by EDF, IRSN, GDF-Suez and CEA, has been designed to understand the heat transfer phenomena involved in the Molten Concrete–Corium Interaction (MCCI). The facility is composed of a pool of simulant liquids, percolated by air and internally heated by direct electrical current. It is in contact with several heat exchangers in order to maintain walls at a constant temperature and to measure local heat losses. The experiments performed aimed at studying the influence on heat transfer of liquid viscosity (varying from 1 to 10,000mPa.s by dissolution of hydroxyethyl cellulose in the water), air injection superficial velocities (ranging from 0 to 10cm/s) on both horizontal and vertical walls, and pool length.After a literature review on the existing heat transfer experiments and correlations for two-phase (liquid–gas) flow, the CLARA experiments are described. Then, the results obtained are presented: influence of superficial gas velocity and liquid properties (especially viscosity) on heat transfer, evaluation of ratio between heat transfer on vertical and horizontal interface, influence of the pool dimensions. Finally, the analysis and interpretation of CLARA experiments are given: comparison to the literature data, development of correlations concerning the heat transfer along the vertical and horizontal plate, comparison to results obtained with real materials. The results show that the presence of bubbles enhances heat exchange while an increasing viscosity lowers the heat transfer. The MCCI isotropic trends are different from those observed with prototypical materials suggesting that phenomena occurring at the corium–concrete interface are more complex than the ones present in CLARA experiments.

Influence of Superficial Gas Velocity on the Dynamic Characteristics Parameters of CFB

The effects of superficial gas velocity (Ug) on the dynamic characteristics parameters (electrical conductivity, local phase holdup and minimum fluidization velocity (Ugmf)) of CFB were examined by the conductivity probe method. Experimental results show effects of Ug on electrical conductivity, local phase holdups and Ugmf are rather obvious. In radial direction, the further to gas distribution board, with Ug increased bubble distribution was more uniform, radial distribution of electrical conductivity was more uniform. When Ug increases, electrical conductivity increases, distribution of it is uneven. From test hole 2 upward, electrical conductivity decreases in different axial direction. With the increase of Ug, local gas phase (g) also increases gradually, local solid phase (s) also increases gradually, and at different region, its amplitude is different. When value of Ug is small, fluidized bed belongs to the fixed bed, and with the increase of Ug, glass particles suspend slowly to fluidized state. Simultaneously, Ugmf is greatly affected by particle content and particle size.

Effects of superficial gas velocity and fluid property on the hydrodynamic performance of an airlift column with alcohol solution

In the present study, the influence of superficial gas velocity and fluid properties on gas holdup and liquid circulation velocity in a three-phase external loop airlift column using polystyrene (0.0036 m diameter and 1025.55 kg/m3 density) and nylon-6 (0.0035 m diameter and 1084.24 kg/m3 density) particles with aqueous solutions of alcohols (isoamyl alcohol and propanol) as liquids were investigated. The column was constructed using acrylic tube of inner diameter 0.084m and 2.6m in height. The gas holdup in the riser increased with increase in superficial gas velocity for air-alcohol-solid system. The presence of alcohol surfactants increased the gas holdup in the riser. It was also found that an increase in the superficial gas velocity increased the liquid circulation velocity for air-alcohol-solid system. Correlations were proposed for the prediction of gas holdup and liquid circulation velocity. Keywords: External loop airlift bioreactor, three-phase, effect of additives, hydrodynamics

A novel three‐phase airlift reactor without circulation of solids

AbstractA novel configuration of a three‐phase internal loop airlift reactor is proposed. The draft is divided in two sections: (top) gas–liquid contact section; (bottom) liquid–solid contact section. Solids particles are fluidized in the bottom section by liquid circulation. The main advantage compared with classic airlift or three‐phase fluidized bed is the reduced stress on particles as interference with bubbles is prevented. Experiments with silica sand (325 µm diameter) were carried out to characterise the hydrodynamics of the airlift. The influence of superficial gas velocity, overall solids hold‐up and sparger height were assessed. A theoretical analysis was proposed to derive simple design criteria.

Characterization of dynamics of gas–liquid flows in rectangular bubble columns

AbstractGas–liquid flow in bubble columns is inherently unsteady. The unsteady fluid dynamics influences mixing and other transport processes occurring in bubble column reactors. In this work, we have characterized dynamics of gas–liquid flows in rectangular bubble columns using wall pressure and voidage fluctuations. The low‐frequency oscillations caused by meandering bubble plume were characterized using the plume oscillation period. Experiments were carried out to study the influence of superficial gas velocity, sparger configuration, and liquid height‐to‐width (H/W) ratio on the low‐frequency oscillations. The dimensional analysis based on analogy between buoyancy‐driven thermal and bubbly flows was carried out to relate low‐frequency oscillations to various design and operating parameters. This analysis was able to correlate present as well as previously published experimental data. We have further demonstrated the importance of establishing such a quantitative relationship for validation of computational fluid dynamics (CFD)–based models by carrying out CFD simulations of gas–liquid flow in rectangular bubble columns. The importance of bubble‐scale information obtained using conductivity probes in validating Eulerian–Lagrangian models is also demonstrated. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2394–2407, 2004

Mixing in Bubble Column Reactors: Role of Unsteady Flow Structures

AbstractGas‐liquid flows in bubble column reactors are inherently unsteady. In the present work, we have characterized dynamics of gas‐liquid flow in rectangular bubble columns and studied the role of unsteady flow structures in liquid phase mixing using experiments and CFD simulations. The need for considering the unsteady nature of gas‐liquid flows was demonstrated by using mixing simulations carried out with transient and time‐averaged flow. A new methodology of using a computationally amenable 'multiple snapshots' approach, which can adequately account for dynamics of underlying flow, is proposed. CFD simulations were carried out to study the influence of superficial gas velocity and height‐to‐width ratio on mixing time. The predictions were compared with mixing time measured using conductivity probes. The present work gives useful insights into the mixing process in inherently unsteady flows.

Liquid flow and phase holdup—measurement and CFD modeling for two-and three-phase bubble columns

Bubble columns are an important class of contacting devices in chemical industry and biotechnology. Their simple setup makes them ideal reactors for two- and three-phase operations such as fermentations or heterogeneous catalysis. Still, design and operation of these reactors is subject to widely empirical scale-up strategies. With recent advances in the development of measurement techniques, a more detailed approach to the development of optimized reactors for specific operations should become possible. This report is based on detailed measurements of local dispersed phase holdups in a pilot plant-sized bubble column operated at high superficial gas velocities and solid holdups. It deals with the influence of superficial gas velocity, solid loading and sparger geometry on measured and computed liquid flow velocities and holdup distributions. Liquid velocity measurements have been performed using the electrodiffusion method, modeling calculations have been carried out using the computational fluid dynamics (CFD) code CFX-4.3. Measurement results presented here give an insight into the development of liquid circulation and fluctuating velocity distribution depending on superficial gas velocity, solid loading and sparger geometry. CFD results implementing a multi-fluid model with k– ε turbulence and special momentum exchange terms for direct gas–solid interactions show that, even on standard PC workstations, this kind of computations can deliver qualitatively reasonable agreement with measurements.