Function Of Superficial Gas Velocity Research Articles

Prediction of Small Bubble Holdups in Bubble Columns Operated with Various Organic Liquids at Both Ambient and Elevated Pressures and Temperatures

This article focuses on the prediction of the small bubble holdups (assuming the existence of two major bubble classes) in two bubble columns (0.289 m in ID and 0.102 m in ID), operated with organic liquids under various conditions (including high temperature and pressure). A new correction factor has been established in the existing model for the prediction of the gas holdups in the homogeneous regime. The correction parameter is a single function of the Eötvös number (gravitational forces to surface tension forces), which characterizes the bubble shape. In addition, the behavior of small bubble holdups in 1-butanol (selected as a frequently researched alcohol) aerated with nitrogen, in a smaller BC (0.102 m in ID), at various operating pressures, is presented and discussed. The ratio of small bubble holdup to overall gas holdup, as a function of superficial gas velocity and operating pressure, has been investigated. All small bubble holdups in this work have been measured by means of the dynamic gas disengagement technique.

Determination of absorption mass transfer coefficients of methane into drilling fluid base oils at elevated pressures

Absorption mass transfer coefficients (kla) of methane into two widely used drilling fluids base oils-internal olefins and diesel were determined at elevated pressure. A laboratory-scale bubble column (0.0254 m diameter and 0.71 m static liquid height) was used to perform the experiments. Ranges of superficial gas velocity (0.0045–0.0491 m/s) and system pressure (0.69–2.07 MPa) were covered. Based on the results, the effects of the varying parameters on kla have been discussed. Based on the observations, an empirical model was developed for the volumetric mass transfer coefficient as a function of superficial gas velocity and density. This correlation fitted well (R2 > 0.98) with the measured kLa values for the investigated range of operating conditions. This study gives new quantitative insights into the absorption kinetics of methane into two drilling fluid base oils, which are important for improving gas influx management with non-aqueous drilling fluids.

Prediction of Fluctuation, Expansion Ratios and Gas Hold Up in a Three Stage Gas-solid Fluidized Bed – A Statistical Approach

Multistage fluidized bed has shown greater performance than the single stage in terms adsorption efficiency, recovery efficiency etc. Thus, experiments have been carried out in a three stage gas-solid fluidized bed. The hydrodynamic characteristicssuch as fluctuation ratio, expansion ratio and gas holdups have been studied. The dependency of these quantities with the system variables like particle size, initial static bed height and superficial gas velocity in each stage of a three-stage fluidized bed have been analyzed. It has been found that both fluctuation and expansion ratios are inverse functions of particle size and static bed height whereas a direct function of superficial gas velocity. However the gas holdup is a direct function of all the three operating parameter.It has also been observed that bed fluctuation and expansion goes on decreasing from lower bed to the upper bed. Further correlations have been developed for all the three stages for fluctuation ratio, expansion ratio and gas holdup using statistical analysis. It has been found that the experimental values of the responses agree well with the values predicted from the models.

Reliable identification of the first transition velocity in various bubble columns based on accurate sophisticated methods

In bubble columns (BCs), the reliable identification of the first transition velocity Utrans-1, marking the end of the homogeneous regime, is an essential prerequisite for the successful BC design since this parameter marks the onset of bubble coalescence. In this work, a new database with 13 experimental Utrans-1 values at various operating conditions is reported. They are identified precisely by different sophisticated methods, which are considered most suitable and powerful for the analysis of the specific experimental data used. The BCs have different column diameters (and bed aspect ratios) and they are equipped with different perforated plate (PP) gas distributors. For Utrans-1 identification, eight different time-dependent signals recorded in air-deionized water, air-tap water, nitrogen-tap water, nitrogen-ethanol (96%), air-therminol and air-benzonitrile systems have been analyzed. The hidden information in the time series has been quantified by reliable identification parameters such as degree of randomness of the signal, Kolmogorov, information and reconstruction entropies and degree of disorder of the signal. Their profiles as a function of superficial gas velocity have been used for flow regime identification.In the case of column diameters between 0.1 m and 0.46 m and bed aspect ratios beyond 5, a simple empirical rule-of-thumb has been established: when the open area (OA) of the PP gas distributors is ≤1.0%, the Utrans-1 value should occur at around 0.03 m/s. On the other hand, when the OA is greater than 1.0%, then the Utrans-1 value is identifiable at 0.04 m/s. It has been found that this new rule yields better predictions than the most popular empirical correlation of Reilly et al. (1994).

Gas dispersion and solid suspension in a three-phase stirred tank with triple impellers

The hydrodynamics is still not fully understood in the three-phase stirred tank equipped with multi-impeller due to the intensive interaction between phases. In this work, the solid critical suspension speed ( N JSG ), relative power demand (RPD) and overall gas holdup ( ε G ) were measured in an air–water–glass beads stirred tank equipped with multi-impeller, which consists of a parabolic blade disk turbine below two down-pumping hydrofoils. Results show that either the N JSG or the specific power consumption increases when increasing the volumetric solid concentration or superficial gas velocity. RPD changes less than 10% when solid volumetric concentration ranges from 0 to 15%. ε G decreases with the increase of solid concentration, and increases with the increase of both superficial gas velocity and the total specific power consumption. The quantitative correlations of N JSG , RPD and ε G were regressed as the function of superficial gas velocity, specific power consumption, Froude number and gas flow number, in order to provide the reference in the design of such three-phase stirred tank with similar multi-impellers. • Just-suspension speed, RPD and gas holdup with triple impeller were measured. • Superficial gas velocity significantly affects the suspension speed. • Overall gas holdup is evidently affected by the solid.

Bubble characterization and gas–liquid interfacial area in two phase gas–liquid system in bubble column at low Reynolds number and high temperature and pressure

Bubbles hydrodynamic in gas–liquid contactor, including bubble size distribution, bubble size and gas–liquid interfacial area, was evaluated as a function of superficial gas velocity, superficial liquid velocity, temperature, pressure and different gases (N2 and He) and liquids (water and ethanol/water mixture) phases. The results showed that with the increase of superficial gas velocity, the bubble size distribution shifted from smaller- to larger-size bubble and the Sauter mean diameter, the gas holdup and the interfacial area generally increased due to the increase of coalescence. The effect of superficial liquid velocity on bubble characteristics was not significant. Pressure and temperature showed slight influence on gas holdup and interfacial area. The bubble characteristics were not significantly influenced by the type of gas phase, but mainly affected by the liquid composition. Correlations to predict Sauter mean bubble diameter and the gas holdup are developed using Kanaris correlation and in good agreement with experimental results.

Heat transfer analysis of 5kWth circulating fluidized bed reactor for solar gasification using concentrated Xe light radiation

A combined numerical and experimental investigation of hydrodynamics and heat transfer of internally circulating fluidized bed reactor for solar gasification of coal coke to produce syngas is reported. As the main objective of this study is to study the thermal performance of the reactor, chemically inert quartz particles have been used. A numerical model has been developed by the combined approach of computational fluid dynamics and discrete element method (CFD-DEM). The radiation transfer equation has been solved by the discrete ordinate (DO) model. The experimental data have been used for model validation. The thermal performance of the reactor and the spout-annulus flow have been predicted by the model and other hydrodynamic parameters such as internal solid volume fraction, velocity and temperature of the particles are analyzed as a function of superficial gas velocity (gas flow rate) at different initial conditions.

Reliable prediction of heat transfer coefficient in three-phase bubble column reactor via adaptive neuro-fuzzy inference system and regularization network

In the present article, generalization performances of regularization network (RN) and optimize adaptive neuro-fuzzy inference system (ANFIS) are compared with a conventional software for prediction of heat transfer coefficient (HTC) as a function of superficial gas velocity (5–25 cm/s) and solid fraction (0–40 wt%) at different axial and radial locations. The networks were trained by resorting several sets of experimental data collected from a specific system of air/hydrocarbon liquid phase/silica particle in a slurry bubble column reactor (SBCR). A special convection HTC measurement probe was manufactured and positioned in an axial distance of 40 and 130 cm above the sparger at center and near the wall of SBCR. The simulation results show that both in-house RN and optimized ANFIS due to powerful noise filtering capabilities provide superior performances compared to the conventional software of MATLAB ANFIS and ANN toolbox. For the case of 40 and 130 cm axial distance from center of sparger, at constant superficial gas velocity of 25 cm/s, adding 40 wt% silica particles to liquid phase leads to about 66% and 69% increasing in HTC respectively. The HTC in the column center for all the cases studied are about 9–14% larger than those near the wall region.

CFD analysis of solid particles properties effect in three-phase flotation column

Abstract A Computational Fluid Dynamics (CFD) model of flotation column was developed and validated against published experimental data. The model is based on an Eulerian multi-fluid formulation with k – ɛ turbulence model and includes three phases: gas bubbles, liquid and solid particle suspended in the liquid. The model is developed by writing FORTRAN subroutine and incorporating then into the commercial CFD code AVL FIRE, v.2014. The gas holdup in flotation column was predicted as a function of superficial gas velocity and solids concentration. The effect of particle type, density, wettability and concentration on gas holdup and bubble hydrodynamics (i.e. bubble size distribution, and attached particle density) were studied. The rate of removal of particles from the pulp zone was obtained with kinetic equations for the three sub-processes: collision, attachment, and detachment involving number density of bubbles and particle concentrations, which were also calculated in this model. As confirmed by the comparisons with available data, the modelling methodology proposed in this work represents the physics of flotation column consistently, since the CFD model correctly predicts the experimental effects of gas flow rate and solid concentration on gas holdup within the range of ±20%. It was found that the addition of hydrophobic particles to the air/water mixture promotes bubble coalescence and, therefore, reduces the gas holdup, while the addition of hydrophilic particles suppresses bubble coalescence and increases the gas holdup. It was also found that the increase of gas flow rate leads to an increase in the attached particle density due to the increase of the concentration number of bubbles that were available for the attachment process. Further, the increase of hydrophobic particles concentration led to an increase in the attached particle density.

Hydrodynamics of Pilot Plant Scale Airlift Reactor in Presence of Alcohols

Abstract The overall gas hold-up of a pilot plant scale internal loop airlift reactor was studied in presence of different alcohols in varied concentration. It has been observed that these simple alcohols can enhance overall gas hold-up of the airlift assembly possibly due to formation of protective thin layer over smaller gas bubbles thereby reducing chance of their coalescence. As the alcohols can be used as food source for the micro organisms present in the system, this green and environment friendly process have potential to replace usage of hazardous surfactants often used for enhancing overall hold-up in order to obtain desired mass transfer characteristics. An empirical relationship encompassing overall gas hold-up of the reactor as a function of superficial gas velocity and alcohol concentration is also developed.

Dry cleaning of fine lignite in a vibrated gas-fluidized bed: Segregation characteristics

The segregation characteristics of fine lignite (−3+1mm) in a vibrated gas-fluidized bed were examined. The pressure drop, ash contents and mass of a certain size fraction in lignite particles as a function of height through the bed, ash separation index, corresponding bed snapshots, and bubble size were investigated by varying superficial gas velocity. The effects of vibration parameters (frequency, amplitude, and vibration intensity) on segregation performance were also determined. Both density segregation and size segregation occurred as a function of superficial gas velocity. With increasing superficial gas velocity, the particle segregation changed to mixing state. An optimum air velocity existed to maximize segregation when fluidization velocities were between the minimum fluidization velocities of flotsam (clean coal) and jetsam (gangue). Bubble-driven flotsam–jetsam mechanism was responsible for the transition from segregation to mixing. The comparative ash segregation index and pressure drop characteristics between the vibro-fluidized bed and fluidized bed also confirmed this mechanism and suggested that application vibration achieved a high segregation performance at a low fluidization velocity because of reduced bubble sizes. The use of dimensionless vibration number (vibration intensity) alone to characterize segregation behavior could lead to misleading conclusions. Frequency and amplitude had strong effects on segregation performance.

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.

Hydrodynamic characterisation of an airlift reactor for domestic wastewater treatment

This study evaluates the hydrodynamic and mass-transfer behaviour of a 20-L airlift photobioreactor, employed for secondary wastewater treatment of domestic effluent. Gas hold-up (e), as well as the volumetric mass transfer coefficient (k L a) was assessed as a function of superficial gas velocity (Ug) for both bi- and triphasic systems. In the experimental range of Ug investigated, the maximum k L a and e obtained were in the order of 0.0074 s −1 and 0.035 for water/ air system and 0.0097 s −1 and 0.026 for triphasic system respectively. The results indicate that this PBR is a good candidate for wastewater treatment, with lower power costs than other types of biological reactors.

Average shear rate in three pneumatic bioreactors

A method proposed in recent literature was applied to evaluate the average shear rate (γav) in three pneumatic bioreactors of 5-dm3 working volume: bubble column, split airlift, and concentric-tube airlift. The volumetric oxygen transfer coefficient (k(L) a) is the appropriate characteristic parameter to assess the average shear rate (γav) in this methodology. Correlations for γav as a function of superficial gas velocity in the riser region (U(GR)) and rheological fluid properties (consistency index, K, and flow index, n) were obtained for each model of pneumatic bioreactor studied. The γav values estimated by the proposed methodology lay within the range of values calculated by classical correlations. The proposed correlations were utilized to predict the γav during the Streptomyces clavuligerus cultivations carried out at the same specific air flow rate (3.5 vvm) in the different types of pneumatic bioreactors. The lowest values of γav related to the highest values of consistency index (K) were found for the bubble column bioreactor, and the highest values of γav related to the lowest values of K were found for the concentric-tube airlift bioreactor. Intermediate values were found for the split airlift bioreactor. The results showed that high γav values affect the structural health of the mycelia by the rupture of the hipha.

Hydrodynamic Considerations in an External Loop Airlift Reactor with a Modified Downcomer

Gas holdup and superficial liquid velocity in the downcomer and riser are studied for an external loop airlift reactor with a downcomer-to-riser area ratio of 1:16. Two downcomer configurations are investigated over a range of superficial gas velocities (0.5 ≤ UG ≤ 20 cm/s) using three aeration plate open area ratios (A = 0.62, 0.99, and 2.22%). These results are compared to a bubble column operated with similar operating conditions. Gas holdup in both the riser and downcomer are found to increase with increasing superficial gas velocity. Results show that riser gas holdup varies slightly with downcomer configuration, while a considerable variation is observed for downcomer gas holdup. The superficial liquid velocity varies considerably for the two downcomer configurations and is a function of superficial gas velocity and flow conditions in the downcomer. Observed variations are independent of aeration plate open area ratio.

Hydrodynamic and mass transfer characterization of a flat-panel airlift photobioreactor with high light path

This work evaluates the volumetric mass transfer coefficient ( k L a), the gas hold-up ( ɛ) and the mixing time ( t m) as a function of superficial gas velocity ( U G) in a flat-panel photobioreactor (PBR) with high light path. CO 2 utilization efficiency and volumetric power consumption ( P/ V) were also evaluated. A 50 L working volume photobioreactor was developed, 0.67 m in length, 0.57 m in height and 0.15 m in width (light path). The height-width ratio was 3.8, which is lower than reported in most PBRs. Initially, experiments were performed with air and tap water (biphasic system) and, subsequently, using a Spirulina sp. culture (triphasic system: air, culture medium, cells). Minimum and maximum superficial gas velocity values were 5 × 10 −5 and 8.4 × 10 −3 m s −1, respectively. Maximum values for k L a and ɛ were 20.34 h −1 (0.0057 s −1) and 0.033 in the biphasic system, and 31.27 h −1 (0.0087 s −1) and 0.065 in the triphasic system. CO 2 utilization efficiency was 30.57%. Results indicate that the hydrodynamic and mass transfer characteristics of this photobioreactor are more efficient than those reported elsewhere for tubular and other flat-plate PBRs, which opens the possibility of using PBRs with higher light paths than yet proposed.

Hydrodynamics, mass transfer and rheological studies of gibberellic acid production in an airlift bioreactor

Although a lot of research has been done into modelling microbial processes, the applicability of these concepts to problems specific for bioreactor design and optimization of process conditions is limited. This is partly due to the tendency to separate the two essential factors of bioreactor modelling, i.e. physical transport processes and microbial kinetics. The deficiencies of these models become especially evident in industrial production processes where O2 supply is likely to become the limiting factor, e.g. production of gibberellic acid and other organic acids. Hydrodynamics, mass transfer and rheology of gibberellic acid production by Gibberella fujikuroi in an airlift bioreactor is presented in this work. Important hydrodynamic parameters such as gas holdup, liquid velocity in the riser and in the downcomer, and mixing time were determined and correlated with superficial gas velocity in the riser. Mass transfer was studied evaluating the volumetric mass transfer coefficient, which was determined as a function of superficial gas velocity in the riser and as a function of fermentation time. Culture medium rheology was studied through fermentation time and allowed to explain the volumetric mass transfer coefficient behaviour. Rheological behaviour was explained in terms of changes in the morphology of the fungus. Finally, rheological studies let us obtain correlations for gas holdup and volumetric mass transfer coefficient estimation using the superficial gas velocity in the riser and the culture medium apparent viscosity.

Measurement of overall solids mass flux in a gas–solid Circulating Fluidized Bed

It is important to know the overall solids flux and solids Cycle Time Distribution (CTD) to properly characterize Circulating Fluidized Bed (CFB) systems. Although conceptually simple, estimating the solids mass flux in these closed loop systems, either via experiments or by using a model, is a non-trivial problem. In the present work, a non-invasive in situ technique is applied, which is sensitive, and can be used on a large-scale circulating flow systems. The technique is based on estimating the solids velocity and volume fraction distribution in a section (downcomer) of the CFB loop. Solids velocity is obtained by tracking a single radioactive particle as tracer, using a two detector setup, and the volume fraction distribution is obtained by γ-ray densitometry measurements. In the downcomer section of a 2-in.-diameter column, the solids flow was found to be close to a moving packed bed condition, with minimal random fluctuations in the solids volume fraction. Hence, only a negligible error is incurred by using the cross-sectionally averaged solids velocity and holdup values for the estimation of the solids mass flow. A calibration curve was established for the overall solids mass flux as a function of superficial gas velocity, and the mass flux values compared with those obtained from a timing and weighing method were within 4%.

Quantification of solids flow in a gas–solid riser: single radioactive particle tracking

Solids in risers of circulating fluidized beds (CFB) exhibit local backflow and recirculation. Measurement of the concentration-time response to an impulse injection of tracer, even at two elevations cannot determine the residence time distribution (RTD) of solids uniquely. Hence, evaluation of RTD in risers from conventional tracer responses is difficult and often not possible. In addition, estimating the solids circulation rate in these closed loop systems, is a non-trivial problem. In this work, a single radioactive particle in the CFB loop is tracked during its multiple visits to the riser and, by invoking ergodicity, solids circulation rate, accurate solids RTD and additional information on the solids flow pattern in the riser are estimated. A calibration curve was established for the overall solids mass flux as a function of superficial gas velocity. A second peak in the probability density function (PDF) of the solids RTD curve in the riser was observed for operating conditions in the fast-fluidization regime.

Soot removal from diesel engine exhaust using a rotating fluidized bed filter

A horizontal rotating fluidized bed filter was used to capture soot from diesel engine exhaust, achieving mass filtration efficiencies exceeding 80%. In the packed bed mode, the filtration efficiency increased with increasing gas flow rate at all rotating speeds. A mathematical model developed for this radial in-flow configuration was used to predict filtration efficiency as a function of superficial gas velocity. The resulting theoretical curve provided an upper bound for the data, exhibiting the same trend at higher rotating speeds. At low rotating speed, the efficiency after fluidization decreased with increasing gas velocity due to bubble bypassing.