Higher Gas Holdup Research Articles

Parametric analysis of internal gas separation within an ebullated bed reactor

Ebullated bed reactors are commonly found within resid hydroprocessors used to thermally crack and catalytically hydrogenate atmosphere and vacuum tower residue in heavy oil upgrading. These units have historically experienced very high solids-free gas holdups above 30%, displacing heavy feed and limiting product throughput, with significant focus placed on the effects of the internal recycle geometry on column performance. A fully functional 3D CFD framework for simulating the gas separation region is presented here, capable of capturing tangential and rotational fluid motion and transient gas separation dynamics. The model is applied to a first-generation recycle cup separator design and is used to explore operational parameters (fluid velocity, bubble size/holdup, recycle rate, and foam generation) and the choice of momentum coupling strategy on gas entrainment and separation efficiency. The results provide a framework for comparison and evaluation of future designs as well as fundamental insight in the separation dynamics in the freeboard region of this reactor, with efforts ongoing to compare and contrast fundamental operating modes of multiple separator designs.

Influence of impeller diameter on overall gas dispersion properties in a sparged multi-impeller stirred tank

The impeller configuration with a six parabolic blade disk turbine below two down-pumping hydrofoil propellers, identified as PDT+2CBY, was used in this study. The effect of the impeller diameter D, ranging from 0.30T to 0.40T (T as the tank diameter), on gas dispersion in a stirred tank of 0.48m diameter was investigated by experimental and CFD simulation methods. Power consumption and total gas holdup were measured for the same impeller configuration PDT+2CBY with four different D/T. Results show that with D/T increases from 0.30 to 0.40, the relative power demand (RPD) in a gas–liquid system decreases slightly. At low superficial gas velocity VS of 0.0078m·s−1, the gas holdup increases evidently with the increase of D/T. However, at high superficial gas velocity, the system with D/T=0.33 gets a good balance between the gas recirculation and liquid shearing rate, which resulted in the highest gas holdup among four different D/T. CFD simulation based on the two-fluid model along with the Population Balance Model (PBM) was used to investigate the effect of impeller diameter on the gas dispersion. The power consumption and total gas holdup predicted by CFD simulation were in reasonable agreement with the experimental data.

Ebullated bed fluid dynamics relevant to industrial hydroprocessing

This study investigates the overall fluid dynamics of an ebullated bed operating at high gas holdup conditions to provide relevant observations for industrial residue hydroprocessors. Scaling approaches for three-phase fluidized beds were compared specifically for the scale-down of the industrially observed high gas holdup conditions. Five dimensionless groups, a binary approach for bubble coalescence behaviour in multi-component liquids, and geometric considerations are proposed to achieve dynamic similitude. Experiments were carried out in a 101.6mm diameter co-current gas–liquid–solid fluidized bed operating at 0.1 and 6.5MPa with liquids that do (e.g., 0.5wt% aqueous ethanol) and do not (e.g., tap water) significantly inhibit bubble coalescence. A comparison of the overall phase holdups for two sizes of cylindrical particles (dSV of 1.6 and 3.9mm) at matching dimensionless groups provided a preliminary verification of the proposed scaling approach when bubble coalescence was sufficiently and consistently inhibited. The impacts of increased liquid viscosity (e.g., greater vacuum distillation tower residue feed fraction), varying superficial gas velocity (e.g., inlet gas flow rate and gas entrainment in the liquid recycle line), and varying superficial liquid velocity (e.g., liquid recycle pump speed) were experimentally studied due to their relevance for industrial ebullated bed hydroprocessors. For the studied operating conditions, gas holdups in the ebullated bed were much greater when bubble coalescence was sufficiently inhibited (up to approx. 45vol%) compared to a coalescing system at equivalent conditions (up to approx. 25 and 35vol% for 0.1 and 6.5MPa, respectively). When increasing the liquid viscosity of the 0.5wt% aqueous ethanol, a fraction of the gas was entrained in the liquid recirculation, increasing gas holdups and exhibiting operational similarities to industrial hydroprocessors. After adjusting the gas and liquid flow rates based on the gas recirculation, the increased liquid viscosity mainly reduced the solid holdups while gas holdups were approximately unchanged. Enhanced bubble break-up or bubble coalescence due to particles resulted in an overestimation or underestimation, respectively, of the freeboard gas holdups from a solids-free estimate based on the bed region phase holdups, where this model could not capture the complex bubble–particle interactions. Experimental results at high gas holdups conditions were used to correlate the bed and freeboard phase holdups based on the proposed dimensionless groups.

Maximizing bubble segregation at high liquid fluxes

This study is concerned with a common class of problem involving two phase separation of a dispersed gas flow from a continuous liquid flow under extreme processing conditions. Relatively fine spherical bubbles of order 500μm were generated in the presence of a surfactant under a high shear rate within a rectangular, multi-channeled, cuboidal downcomer. Liquid fluxes, as high as 176cm/s through each channel of the downcomer, sheared bubbles from a sintered surface mounted flush to the channel wall before disengaging the downcomer flow into a vertical vessel. Both high feed fluxes, up to 15cm/s, and high gas fluxes, up to 5.5cm/s, ensured a high gas holdup beneath the downcomer and the hindered rising of the bubbles. Enhanced bubble–liquid segregation was achieved using an arrangement of parallel inclined channels incorporated below the main vertical chamber. This novel device, referred to as the Reflux Flotation Cell, prevented the entrainment of bubbles to the underflow, and significantly reduced the liquid flux to overflow, even in the absence of a conventional froth zone. Extreme upward bubble surface fluxes of up to 600s−1 were achieved, while counter-current downward liquid fluxes reached 14.4cm/s, arguably four times the bubble terminal rise velocity. Hence successful phase separation was achieved while operating well beyond the so-called flooding condition arising from extreme levels of gas and feed fluxes. This hydrodynamic arrangement should find application in increasing surfactant extraction rates in foam fractionation and ion flotation, gas absorption, and even particulate flotation.

Bubble characteristics measured using a monofibre optical probe in a bubble column and freeboard region under high gas holdup conditions

Local bubble characteristics, including gas holdups, bubble rise velocities, and chord lengths, were measured using a monofibre optical probe manufactured to withstand elevated pressures. Previous studies have validated the use of single tip probes for simultaneous measurement of local bubble properties at atmospheric conditions; however no study has been currently reported for these probes at elevated pressures. Experiments were conducted in a 101.6mm diameter column operating at pressures up to 9.0MPa. Surfactant addition and operating pressure were studied to simulate high gas holdups observed in many industrial reactors containing liquid mixtures with surface-active compounds. Experiments were hence completed using two liquid phases: tap water and a 0.5wt% aqueous ethanol solution. Liquid and gas superficial velocities were varied between 0–90mm/s and 0–150mm/s, respectively. Radial profiles at atmospheric conditions validated the probe measurements in water. Local holdups, rise velocities and chord lengths were adequately measured in water up to 9.0MPa. The probe struggled in the aqueous ethanol solution due to its physical constraints (i.e., tip diameter and sensing length) when compared to the significant bubble size reduction (chord lengths below 0.5mm). Comparisons with fluidized bed freeboard measurements demonstrated that flow through the bed enhanced bubble breakup for a coalescing system, but had a negligible impact with the added surfactant.

Hydrodynamic comparison of spherical and cylindrical particles in a gas–liquid–solid fluidized bed at elevated pressure and high gas holdup conditions

Experiments were carried out to validate the use of spheres in lieu of cylinders when investigating the global hydrodynamic features of a co-current gas–liquid–solid fluidized bed. Two sizes of glass spheres with diameters of 4 and 1.5mm were compared to aluminum cylinders with equivalent volume/surface area ratios (i.e., matching Sauter mean diameters). Lengths/diameters of the larger and smaller cylinders were 7.5/3.2mm and 3.1/1.2mm, respectively, which resulted in equal particle sphericity of 0.8 for both sizes. The particle properties of the larger particles led to the inertial settling flow regime (ReLT∞>500) in water while the smaller particles were in the intermediate regime (0.2<ReLT∞<500). High gas holdup conditions were obtained by increasing the system pressure to 6.5MPa and/or adding a surfactant. Atmospheric conditions were also studied for comparison. Experiments were conducted in a 101.6mm diameter column with tap water or a 0.5wt.% aqueous ethanol solution as the liquid phase while the gas phase was a combination of air and nitrogen. Global phase holdups measured from the dynamic pressure profiles characterized the hydrodynamic behavior of the fluidized bed and studied the impact of particle shape. Standard deviations of the mean holdups aided the comparison and also examined the fluctuations of the bed interface. Liquid–solid fluidized bed experiments demonstrated that equivalent Sauter mean diameters resulted in comparable bed porosities. Gas–liquid–solid fluidized bed dynamics of equivalent size spherical and cylindrical particles were similar in the dispersed bubble flow regime whereas differences were observed in the presence of larger coalescing bubbles.

Experimental investigation and modeling of flotation column for treatment of oily wastewater

A unique cyclonic static microbubble flotation column was developed for oily wastewater separation. The separation efficiency was found to be highly dependent on gas holdup and bubble size distribution. By changing the circulation pressure, gas flow rate, frother concentration, the effect of operation parameters on gas holdup and oil removal efficiency were attained. A mathematical modeling between the kinetic constant and the gas holdup was established for oily water separation process. The results show that higher gas holdup and smaller microbubble sizes are beneficial to improve oil removal efficiency.

Numerical Simulation of Hydrodynamics in Slurry Bubble Column Reactor

Gas dispersion and solid suspension in industrial size slurry bubble column reactors for producing sodium dichromate are simulated numerically by using of computational fluid dynamics (CFD). The Eulerian multi-fluid model and standard k-ε turbulence model are used to describe the flow behavior in bubble columns. The simulated results show that gas is easy to flow toward the centre of the bubble column and in relatively high local gas holdup there. Installing gas-re-distributors in the bubble column is favorable for gas dispersion. Solid suspension in the bubble columns under the operating condition we investigated is relatively uniform.

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.

Experimental study on gas–liquid–liquid macro-mixing in a stirred tank

In this paper, experimental data on the mixing time of the continuous phase and power consumption of gas–liquid–liquid dispersions in a mechanically agitated baffled tank are presented. The electrical conductivity method is taken for the measurement of mixing time and the shaft-torque method for power consumption measurement. Tap water is used as the continuous phase, and kerosene and air as the dispersed ones. The effects of probe/tracer injection position, agitation speed, type of impeller, clearance of impeller off tank bottom, oil volume fraction, gas holdup and physical properties of the dispersed liquids on the macro-mixing of the gas–liquid–liquid system have been investigated. The phenomenon of gas–liquid–liquid macro-mixing in a stirred tank is largely similar to that of liquid–liquid and gas–liquid stirred tanks. Our experiments indicate that the gas–liquid–liquid macro-mixing can be enhanced at higher gas holdups while damped at low gas holdups. Contrary to gas effect, the dispersed oil phase at low holdups increases the macro-mixing intensity but at higher holdups decreases the macro-mixing intensity of the continuous phase. The experimental results show that axial impellers are more energy efficient for gas–liquid–liquid macro-mixing than radial impellers. A simple correlation is developed for predicting the mixing time in gas–liquid–liquid three-phase systems and satisfactory agreement with experimental data is observed.

Water delivery capacity of a vacuum airlift – Application to water recycling in aquaculture systems

A study was undertaken to measure the water flow (Qw) delivered by a vacuum airlift designed for recirculating aquaculture systems (RAS) in fresh (<1‰ of salinity) and sea water (35‰ of salinity). The vacuum airlift consists of two concentric tubes connected at their top to a depression chamber. The water rises in the inner tube as a result of air being injected in its lower section and flows back through the external downcomer tube. The vacuum airlift was adjusted at three different lengths: 2, 4 or 6m and water discharge could be lifted from 0 to 30cm. Air flow rate (Qg) varied from 0 to 80Lmin−1. Different types of air injectors were tested, delivering different bubble sizes (0.1–5mm) depending on porosity and functioning at low or high injection pressure. Results show an increase in water flow when pipe length and air flow were increased and lift height reduced. Water flow also depended on the type of water and ranged from 0 to 35 m3h−1 (0–580Lmin−1) for fresh water and only from 0 to 20 m3h−1 (0–330Lmin−1) for sea water (for a 6m high vacuum airlift). This difference was attributed to the smaller bubble diameter and higher gas holdup (ɛg) observed in sea water (0–20%) compared to fresh water (0–10%). When bubbles were present in the downcomer tube, they created a resistance to flow (counter-current airlift) that slowed down liquid velocity and thus water flow. Increasing the vacuum made it possible to use low air injection pressures and high injection depths. Vacuum also increased bubble size and airflow (20Lmin−1 at atmospheric pressure to 60Lmin−1 at 0.3barA) and thus water flow rates. With RAS, the presence of fish feed in water rapidly increased water flow delivered by the airlift because of changes of water quality and gas holdup. When working with low head RAS (under 0.3m), vacuum airlift could save up to 50% of the energy required for centrifugal pumps. An empirical predictive model was developed and calibrated. Simulation shows a good correlation between predicted values and measurements (R2=0.96).

Effect of slurry rheology on gas dispersion in a pilot-scale mechanical flotation cell

This paper investigates the effect of slurry rheology on gas dispersion in a 100 l pilot-scale Batequip mechanical flotation cell. The study is conducted using Kaolin, Bindura nickel and Platreef slurries. All three ores display typical non-Newtonian rheological behaviour. The slurry yield stress and viscosity increase exponentially with solids concentration. Bubble size and gas hold-up vary from 0.60 to 1.10 mm and 2% to 15%, respectively. At low/moderate solids concentrations, bubble size and gas holdup display characteristic trends, as noted in numerous literature studies. At high solids concentrations, both bubble size and gas holdup decrease significantly, which is an unexpected finding. This is attributed to the formation of a ‘cavern’ of slurry around the impeller, due to the very high slurry yield stresses. This ‘cavern’ results in the generation of small bubbles in the impeller zone, but poor dispersion of these bubbles throughout the cell, resulting in low gas hold-ups.

Characterization of two phase flows in chemical engineering reactors

Most industrial processes like fermentation, hydrogenation, oxidation, water treatment, petrochemical, nuclear and aerospace involve intimate contact between continuous phase and dispersed phase. Bubble columns and external loop airlifts are commonly used in these operations. Although these reactors are widely used, and extensive research has been carried out there exists no perfect model to characterize the local hydrodynamics and mass transfer. Computational fluid dynamics has also evolved recently trying to model the flow and transfer within these reactors but a lot of results are conflicting. Besides, there is a need to validate these results with experimental work. This work is dedicated to the experimental methods of measuring the local parameters such as the gas hold-up, the bubble velocities, the liquid velocity, bubble sizes. Two intrusive methods are used in this study, hot film anemometry for measurements of liquid phase and bi-optic probe for the gas phase. Although intrusive methods may interfere with fluid flow, presently they are the most adapted for real industrial processes with opaque equipment walls and high gas hold-ups compared to non-intrusive methods such as imagery. In this study, an external loop airlift of 16.5 l capacity is used. Local variables are measured in the riser of an external loop airlift reactor in air/water medium. The results are presented in the form of the radial profiles of void fraction, bubble diameter, liquid velocities at superficial gas velocities ranging from 0.03 to 0.11 m/s. The axial variations of the same parameters are also investigated. The results suggest that the bi-optical probe and hot film anemometer can reliably predict flow characteristics in high gas hold up contactors.

Simulation of bubble column reactors using CFD coupled with a population balance model

Bubble columns are widely used in chemical and biochemical processes due to their excellent mass and heat transfer characteristics and simple construction. However, their fundamental hydrodynamic behaviors, which are essential for reactor scale-up and design, are still not fully understood. To develop design tools for engineering purposes, much research has been carried out in the area of computational fluid dynamics (CFD) modeling and simulation of gas-liquid flows. Due to the importance of the bubble behavior, the bubble size distribution must be considered in the CFD models. The population balance model (PBM) is an effective approach to predict the bubble size distribution, and great efforts have been made in recent years to couple the PBM into CFD simulations. This article gives a selective review of the modeling and simulation of bubble column reactors using CFD coupled with PBM. Bubble breakup and coalescence models due to different mechanisms are discussed. It is shown that the CFD-PBM coupled model with proper bubble breakup and coalescence models and interphase force formulations has the ability of predicting the complex hydrodynamics in different flow regimes and, thus, provides a unified description of both the homogeneous and heterogeneous regimes. Further study is needed to improve the models of bubble coalescence and breakup, turbulence modification in high gas holdup, and interphase forces of bubble swarms.

Mass Transfer Studies in a Novel Perforated Plate Bubble Column

This investigation reports the experimental and theoretical results carried out to evaluate the volumetric mass transfer coefficient (kLa) in a novel hybrid rotating and reciprocating perforated plate bubble column. Countercurrent condition is performed. kLa is studied by the absorption of oxygen from air into deoxygenated water at room temperature (27 ± 1°C). Effects of agitation level, superficial gas velocity, superficial liquid velocity and plate spacing on kLa were analyzed and found to be significant. With an increase in agitation level at a constant superficial gas and liquid velocities, the breakage process of gas bubbles starts to be more pronounced and intensive oxygen mass transfer occurs. Hence, kLa increases sharply. kLa increases with an increase in superficial gas velocity, due to higher gas holdup and the enhanced breakup of bubbles. Similarly, kLa increases with an increase in superficial liquid velocity and the effect is found to be significant. When plate spacing is decreased (by increasing the number of plates), it is observed that the kLa increases at higher superficial gas velocity and agitation level. Correlation is developed for the determination of kLa and found to concur with experimental results. This correlation can be used for the determination of kLa for this hybrid column with 95% accuracy within the range of variables investigated in this present study.

Gas holdup in a two‐phase reversed flow jet loop reactor

AbstractExperimental investigations have been carried out in Reversed Flow Jet Loop Reactor (RFJLR) to study the influence of liquid flow rate, gas flow rate, immersion height of two‐fluid nozzle in reactor and nozzle diameter on gas holdup without circulation, that is, gas–liquid mixture in draft tube only (Egd) and gas holdup with circulation loop (Eg). Also critical liquid flow rate required for transition from draft tube to circulation loop has been determined. Gas holdup was measured by isolation valve technique. Gas holdup in draft tube and circulation loop increased with increase in liquid flow rate and gas flow rate. It is observed that the increased flow rate is required for achieving a particular value of gas holdup with larger nozzle diameter. Nozzle at the top edge of draft tube have higher gas holdup as compared to other positions. It has been noted that, no significant recirculation of gas bubbles into the top of draft tube from annulus section has been observed till a particular liquid flow rate is reached. A plot of gas holdup with no circulation and with circulation mode determines minimum liquid flow rate required to achieve complete circulation loop. Critical liquid flow rate required to achieve complete circulation loop increases with increase in gas flow rate and is minimum at lowest immersion height of two‐fluid nozzle.

Flow regimes in a two phase reversed flow jet loop bioreactor

Reversed flow jet loop bioreactors (RFJLB) have been used extensively for 2 or 3 phase biochemical reactions. From visual observations and gas holdup data, 3 distinct flow regimes are identified in RFJLB, namely: (1) Bubble free regime (BFR), where bubbles are observed in the draft tube only; (2) Transition regime (TR), where bubbles are observed in both the draft tube and the annulus, but without circulation; and (3) Complete bubble circulation regime (CBCR), where bubbles circulate in both the draft tube and annulus. CBCR is the most desirable regime, since the reactor operation in this regime gives a higher gas holdup and mass transfer rate than in the other two regimes. In the present study, the hydrodynamic behavior of RFJLB was investigated under various operational and geometrical conditions, such as gas and liquid velocity and nozzle configuration. Factors affecting the critical liquid circulation velocity (CLCV) above which the CBCR is established were identified and evaluated quantitatively.

Gas–liquid dispersion in a fibrous fixed bed biofilm reactor at growth and non-growth conditions

There is limited data on gas dispersion characteristics of fixed bed biofilm reactors under growth and non-growth conditions. In this paper, the gas–liquid dispersion of a bubble bed packed with a fibrous structured packing for biofilm application is studied. The reactor is operated with Pseudomonas putida aimed at aniline degradation in wastewater. Gas hold-up and bubble size distribution are determined. Running gas–liquid reaction conditions as well as non-reactive flow gas hold-up and bubble size distribution in the presence of surface-active and viscous components were measured. The properties of the gas dispersion proved to be stabilized by the fibrous bed presence and showed improvement of the dispersion parameter by the packing. Gas hold-up was found to increase monotonously with the rise of gas superficial velocity and viscosity and with surface tension fall. Liquid superficial velocity showed marginal effect. Apart from showing high gas hold-up and low bubble size due to surface-active and viscous dissolved elements, the biochemical reaction did not pose any significant additional effect. In agreement with the expected lack of bubble coalescence and break-up in the highly ionic solution practiced, the population size distribution and average bubble size were found to vary with the major operation factors opposite to their gas hold-up contribution. Gas hold-up was correlated with the specific bubble-to-channel size ratio and further with the variables considered. An empirical equation is proposed that relates gas hold-up with all studied variables. Assuming geometric similarity of the prototype and the real vessels, the equation as well as its corresponding range of fluid velocities can be used for bioreactor design and scale-up. The results concerning the gas hold-up are shown to be comparable with previous studies of mesh wire packing.

Numerical study of homogeneous bubbly flow: Influence of the inlet conditions to the hydrodynamic behavior

This paper discusses the role of the gas injection pattern on the large scale structures in a homogeneous pseudo-2D bubble column operated at relatively high gas hold-ups up to 8%. Seven cases with different inlet configuration have been studied experimentally by Harteveld et al. Each of these cases has been simulated using a (parallel) Euler–Lagrange model developed by Darmana et al. The presence of coherent structures for both uniform and non-uniform gas injection is studied. Furthermore, the influence of the gas injection pattern on the dynamics is investigated, while the statistical (average and fluctuating) quantities are compared with the PIV/PTV and LDA measurement data of Harteveld et al. The results show that the model resembles the observed experimental flow structures to a large extent.

Microbubble suspension as a carrier of oxygen and acclimated bacteria for phenanthrene biodegradation

The applicability of microbubble suspension made of saponin as a biodegradation-enhancing carrier where oxygen and pollutant-degraders are limited was studied. The saponin-microbubble suspension was used to deliver phenanthrene-degrading bacteria, inorganic nutrients, and oxygen. Bench-scale study was carried out to determine the physical properties of the microbubble suspension and to verify whether the delivered bacteria and oxygen were effectively used to degrade phenanthrene. A concentration of 2 g saponin/L H 2O generated stable microbubble suspension with a long half-drainage time and a high gas hold-up, and the addition of phenanthrene-degraders and inorganic salts to the saponin solution did not affect such properties. The flow of the microbubble suspension through a heterogeneous sand/clay-packed column occurred in two phases, with the liquid front advancing faster and the retarded gas front. The retarded gas front provided oxygen with bacteria, which enables phenanthrene biodegradation. Approximately 30% of the spiked phenanthrene was degraded in 21 days when one pore volume of 2.0 g/L saponin-microbubble suspension was applied whereas no phenanthrene decrease was observed following the application of the same saponin solution without microbubble generation. The decrease mainly occurred at the lower part of the column where the supply of oxygen by the microbubble was concentrated.