Holdup Distribution Research Articles

Modeling Flow-Guided Sieve Tray Hydraulics Using Computational Fluid Dynamics

A computational fluid dynamics (CFD) model was used to predict the hydraulics and flow patterns of flow-guided sieve tray, which combined common sieve holes with flow-guided holes. The model considered the three-dimensional two-phase flow of gas and liquid, and each phase was treated as interpenetrating continua that had separate transport equations. Interaction between the two phases was considered via an interphase momentum transfer term, which was incorporated into the CFD model. On the basis of experimental measurement, the new correlations of clear liquid height and gas holdup were developed. The velocity distribution, clear liquid height, and foam height were predicted and found to be in good agreement with the experimental data. Distribution of clear liquid height and liquid holdup proved that flow-guided holes can promote the liquid flow and decrease the clear liquid height. The CFD method is a useful tool for optimizing the novel flow-guided sieve tray.

CFD Prediction of Gas-Liquid Flow in an Aerated Stirred Vessel Using the Population Balance Model

Abstract The main topic of this study is the experimental measurement and mathematical modelling of global gas hold-up and bubble size distribution in an aerated stirred vessel using the population balance method. The air-water system consisted of a mixing tank of diameter T = 0.29 m, which was equipped with a six-bladed Rushton turbine. Calculations were performed with CFD software CFX 14.5. Turbulent quantities were predicted using the standard k-ε turbulence model. Coalescence and breakup of bubbles were modelled using the homogeneous MUSIG method with 24 bubble size groups. To achieve a better prediction of the turbulent quantities, simulations were performed with much finer meshes than those that have been adopted so far for bubble size distribution modelling. Several different drag coefficient correlations were implemented in the solver, and their influence on the results was studied. Turbulent drag correction to reduce the bubble slip velocity proved to be essential to achieve agreement of the simulated gas distribution with experiments. To model the disintegration of bubbles, the widely adopted breakup model by Luo & Svendsen was used. However, its applicability was questioned.

Impact of Surfactant Chemistry on Bubble Column Systems

AbstractThe impact of surfactant addition on the overall holdup, drag coefficient, and bubble size distribution (BSD) in a bubble column operating at industrially relevant superficial velocities was analyzed. A range of surfactants was examined, including alcohols, sugars, and antifoaming agents. It was ascertained that the effect of a surfactant on the holdup and BSD could be related to the chemical structure of the compound, specifically its hydrophilic/hydrophobic nature as characterized by the octanol/water partition coefficient. Addition of 2‐propanol to an air/water system induces a behavior similar to the fermentation media used in aerobic bioprocesses, meaning that such a system can be applied as a meaningful physical analogue in pilot‐scale experimentation.

Effect of Aeration Conditions on the Flow Field in the Submerged Membrane Bioreactor

A full size 3D numerical simulation of gas liquid two-phase flow in a submerged membrane bioreactor was carried out. The standard k-ε turbulence model and Euler multiphase flow model of fluent were used. The effect of changed aeration conditions in the reactor on the gas holdup and gas-liquid velocity distribution in the reactor was studied. The simulation results were shown that, at the same aeration rate, the liquid and gas velocities of 1mm hole aerated at the membrane surface increased faster than 2 mm and 3mm aeration holes; At the same aeration hole , with the increase of aeration rate，the liquid and gas velocities at the membrane surface increased; At the 1mm aeration hole and 5.5m3/h aeration rate, the vortex area was larger and gas holdup was higher, so that gas and liquid were contacting well and the membrane surface scouring effect was better; The Simulation also shown that local gas holdup was lower at close to the wall at the bottom of the reactor, this was not conducive to the growth of microorganisms in the activated sludge, the need to further optimize the structure of aeration and reactor.

Three-dimensional three-phase model for simulation of hydrodynamics, oxygen mass transfer, carbon oxidation, nitrification and denitrification in an oxidation ditch

A three-dimensional three-phase fluid model, supplemented by laboratory data, was developed to simulate the hydrodynamics, oxygen mass transfer, carbon oxidation, nitrification and denitrification processes in an oxidation ditch. The model provided detailed phase information on the liquid flow field, gas hold-up distribution and sludge sedimentation. The three-phase model described water-gas, water-sludge and gas–sludge interactions. Activated sludge was taken to be in a pseudo-solid phase, comprising an initially separated solid phase that was transported and later underwent biological reactions with the surrounding liquidmedia. Floc parameters were modified to improve the sludge viscosity, sludge density, oxygen mass transfer rate, and carbon substrate uptake due to adsorption onto the activated sludge. The validation test results were in very satisfactory agreement with laboratory data on the behavior of activated sludge in an oxidation ditch. By coupling species transport and biological process models, reasonable predictions are made of: (1) the biochemical kinetics of dissolved oxygen, chemical oxygen demand (COD) and nitrogen variation, and (2) the physical kinematics of sludge sedimentation.

Scale-up and On-line Monitoring of Gas-solid Systems Using Advanced and Non-invasive Measurement Techniques

Industry relieson gas-solid systems for numerous processes. Flow dynamics play an important role in achieving the desired results. The present study proposes, validates and demonstrates a novel mechanistic scale-up approach based on maintaining similar radial profile or cross sectional distribution of gas holdup in two different gas-solid systems in order to achieve hydrodynamics similarity using advanced measurement techniques. This new methodology for scale-up and design has been implemented on gas-solid spouted bed which has been used for drying, granulation and coating. The development can be extrapolated to other gas-solid systems encountered in phosphate processes.

Sinking/Floating Particles Solid Suspension Characteristics in Stirred Tank Based on CFD Simulation

Solid-liquid suspension characteristics for sinking and floating particles in a dual-impeller stirred tank are simulated numerically by using computational fluid dynamics(CFD). The multi-reference frame(MRF) method for treatment of impeller rotation and Euler-Euler two-fluid model based on the kinetics of granular are used in the simulation. Flow field and solid holdup distribution for sinking and floating particles in the agitated tank are predicted, and the effects of operating conditions on solid-liquid suspension are investigated. The numerical simulated liquid velocity distribution and flow pattern are good agreement with experimental data in literature. The simulated results indicated that solid-liquid suspension of sinking particles is quite different from that of floating particles. For sinking particles, solid holdup decreases along axial height as a whole, and the highest solid holdup is found in the centre of the bottom region. Local solid holdup in the centre of circulation loops, behind the blades and baffles was relatively low. Nevertheless, solid holdup distribution for floating particles is contrary to that of sinking particles as above-mentioned. The increase of impeller speed can cause local solid holdup in the centre of circulation loops decrease for sinking particle, while contrarily for floating particles. Decreasing solid content or smaller particles size is beneficial to the uniformity of solid-liquid suspension for the two types of particles. With increasing of particles content or particles size, sinking particles and floating particles are more prone to aggregate in the centre of bottom region and surface region, respectively.

Scrubber characterization and performance using hydrocarbons at elevated pressures

Separation efficiencies for a scrubber containing only an inlet vane was investigated. In addition holdup distribution, pressure drop, separation efficiencies and droplet separation characteristics of two wire mesh pads mounted in a scrubber were investigated experimentally at 20,50 and 85 bars using N2/Exxsol and a synthetic Natural gas mix. The liquid holdup was recorded at five locations for the two pads. A new method was developed to allow for accurate determination of flooding point in wire mesh pads. The flooding point was determined at four inlet stream liquid volume fractions at a given pressure and fluid system. In addition droplet size measurements were performed above the wire mesh pads. The results show unexpected phenomena occurring when moving to higher pressures using liquids with low surface tensions.

Numerical studies of solid–solid mixing behaviors in a downer reactor for coal pyrolysis

A downer reactor is ideal for fast pyrolysis during coal gasification as solids holdup distributions and residence time distribution in the reactor are fairly uniform and residence time is short. Because pyrolysis is a rapid reaction, in the downer solid–solid mixing is critical to promote pyrolysis reaction. Thus, solid–solid mixing in the downer was studied through numerical modeling in this study. An Eulerian–Lagrangian method was applied and implemented by combining Computational Fluid Dynamics and Discrete Element Method (CFD–DEM). The heat carrying media of silica sand was modeled to flow from the top of the downer while coal particles were introduced through two lateral nozzles in normal and tangential arrangements. Numerical results showed that tangential arrangement had lower solids holdups while higher particle velocities than normal arrangement. Mixing among binary solid particles was poor at the entrance of downer. The extent of mixing increased rapidly and approached an almost constant value in the downstream regions of the downer. High air velocity and small solid particle size could lead to better mixing for both normal and tangential arrangements. But tangential arrangement had better mixing of binary solid particles than normal arrangement, and when the particle sizes were reduced from 2 to 1mm, the influence of nozzle arrangement was reduced.

GAS-SOLID FLOW BEHAVIOR IN A PRESSURIZED HIGH-FLUX CIRCULATING FLUIDIZED BED RISER

Gas-solid flow behavior was investigated in a high-flux circulating fluidized bed (HFCFB), whose riser is 0.068 m ID and 5.2 m high. Experiments were carried out at different pressures (0.1–0.5 MPa), solids mass fluxes (122–1015 kg/m2 · s), and standard state superficial gas velocities (6–40 m/s). Geldart group B particles 137 µm in diameter and 2490 kg/m3 in density were used as bed materials. Information on pressure drop, apparent solids holdup, average slip velocity, and solid-to-air mass flow ratio were systematically tested at elevated pressures. The results showed that increase of the operating pressure led to higher total riser pressure drop and solids holdup, lower slip velocity, more homogeneous pressure drop, and longitudinal solids holdup distribution. As a result, a high-density condition with Geldart group B particles, i.e., solids holdup in the riser over 10%, could be successfully formed in the HFCFB at elevated pressure. Similar to the trend at atmosphere, the dimensionless slip velocity at pressurized condition increased with increasing solids holdup for any given superficial gas velocity, and the total riser pressure drop increased approximately linearly with solids-to-air mass flow ratio at the given operating pressure.

Micro-structured fluidized bed membrane reactors: Solids circulation and densified zones distribution

The present paper reports an experimental investigation on the hydrodynamics of a novel membrane assisted micro-structured fluidized bed (MAmFB) operated in bubbling or turbulent flow regimes. The effects of gas addition and gas extraction through flat porous membranes confining the fluidized bed on the bubble size distribution, solids holdup distribution and solid circulation patterns have been evaluated by the combination of two non-invasive techniques, viz. Particle Image Velocimetry (PIV) and Digital Image Analysis (DIA). The experimental results show that the micro-structured fluidized bed membrane reactor improves the solid circulation compared with bigger size membrane reactors where the extraction of gas result in parts of the bed that are completely defluidized and stagnant at the membrane walls. However, also in case of small reactors great care has to be paid to the gas extraction velocity relative to the fluidization velocity. All the results indicate that the amount of densified zones (zones where solids lumps with a local solids hold-up close to the minimum solids packing have a much lower velocity than the rest of the emulsion phase) can be reduced drastically by working in the turbulent fluidization regime with a relatively low gas extraction velocity. This study indicates that actual state-of-the-art membranes can be used in the turbulent regime without the formation of densified zones, thus avoiding additional mass transfer resistances (concentration polarization).

Multiphase hydrodynamics and distribution characteristics in a monolith bed measured by optical fiber probe

The optical fiber probe has been for the first time applied to investigate the hydrodynamics and gas‐phase distribution at high gas/liquid ratios in a two‐phase flow monolith bed with 0.048 m diameter and 400 cpsi. Local hydrodynamic parameters including gas holdup, bubble frequency, bubble velocity, and bubble length in single channels were measured by 16 inserted single‐point optical fiber probes within the bed under a nozzle as the liquid distributor. The following findings are reported. (1) The optical fiber probe can be used as an efficient and convenient technique for measuring local hydrodynamic parameters inside the channels of a monolith bed; (2) within the range of high gas/liquid ratios under which experiments were conducted, churn flow regime occurred. In this regime, the monolith bed radial distribution of gas holdup, bubble frequency, bubble velocity, and bubble length is nonuniform in nature. © 2013 American Institute of Chemical Engineers AIChE J 60: 740–748, 2014

Development of a CFD Model of Bubble Column Bioreactors: Part One – A Detailed Experimental Study

AbstractThe holdup and bubble size distribution (BSD) in bubble columns using both air/water and an industrially relevant air/fermentation media system are investigated. It was found that the BSD in the air/fermentation media system was quite narrow and did not change with height. In contrast, the BSD in the air/water system varied considerably with height depending on the sparger design used. Holdup measurements were also performed for different superficial velocities. The holdup in the air/fermentation media system was greater than that for the air/water system, a result attributed to the presence of surface‐active compounds in the fermentation media.

Effects of hydrophilic particles on bubbly flow in slurry bubble column

To investigate the effects of hydrophilic particles on slurry bubble flows in a bubble column, distributions of the local gas holdup and the bubble frequency are measured using an electric conductivity probe. Particles are made of silica and their diameter is 100μm. The particle volumetric concentration CS is varied from 0 to 0.40. The measured data imply that the presence of particles promotes bubble coalescence. The film drainage time for two coalescing bubbles in a quasi two-dimensional bubble flow in a small vessel is also measured to quantitatively evaluate the particle effect on coalescence. A particle-effect multiplier is introduced into a coalescence efficiency model by taking into account the data of film drainage time and is implemented into a multi-fluid model. The main conclusions obtained are as follows: (1) the local gas holdup and bubble frequency in slurry bubble flows decrease with increasing the particle concentration, (2) the hydrophilic particles enhance bubble coalescence and the enhancement saturates at CS≃0.45, (3) the particle effect on coalescence is well accounted for by introducing the particle-effect multiplier to the film drainage time, and (4) the multi-fluid model can give good predictions for the distribution of the local gas holdup in the slurry bubble column.

Study of Phase Distribution of a Liquid-Solid Circulating Fluidized Bed Reactor Using Abductive Network Modeling Approach

Abstract This communication deals with the Abductive Network modeling approach to investigate the phase holdup distributions of a liquid–solid circulating fluidized bed (LSCFB) system. The Abductive Network model is developed/trained using experimental data collected from a pilot scale LSCFB reactor involving 500-μm size glass beads and water as solid and liquid phases, respectively. The trained Abductive Network model successfully predicted experimental phase holdups of the LSCFB riser under different operating parameters. It is observed that the model predicted cross-sectional average of solids holdups in the axial directions and radial flow structure are well agreement with the experimental values. The statistical performance indicators including the mean absolute error (~4.67%) and the correlation coefficient (0.992) also show favorable indications of the suitability of Abductive Network modeling approach in predicting the solids holdup of the LSCFB system.

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.

On the measurement of local gas hold-up, interfacial area and bubble size distribution in gas–liquid contactors via light sheet and image analysis: Imaging technique and experimental results

In this work a novel experimental technique for measuring local gas hold-up, interfacial area and bubble size distribution, in gas–liquid systems is proposed. The technique is based on advanced Image Processing coupled with experimental set-ups typically available for Particle Image Velocimetry. A fluorescent dye dissolved in the liquid phase allows to identify in-plane bubbles among all visible bubbles in the images. To this end, a suitable algorithm is proposed. The raw data so obtained are processed by previously developed statistical methods that result in a reliable reconstruction of actual dispersion properties.The technique is applied to the case of a gas-dispersed mechanically agitated vessel, and the data obtained are presented and discussed.

Numerical simulation of liquid–liquid turbulent flow in a stirred tank with an explicit algebraic stress model

A two-phase explicit algebraic stress model (EASM) is employed to simulate liquid–liquid turbulent flow in stirred tanks based on an Eulerian–Eulerian approach. This model is validated by comparing with the reported experimental data and other predicted results by the k–ɛ model. Mean and fluctuating velocities and the distribution of the dispersed phase holdups are predicted and discussed. The results show that the EASM can give a reasonable description of the liquid–liquid flow in stirred tanks. However, the present two-phase EASM shows moderate improvement comparing with the k–ɛ model.

CFD modeling of hydrodynamic characteristics of a gas–liquid two-phase stirred tank

A three-dimensional CFD model was developed in this work to simulate hydrodynamic characteristics of a gas–liquid two-phase stirred tank with two six-bladed turbines and four baffles, coupling of the Multiple Size Group model to determine bubble size distribution. Important hydrodynamic parameters of the multi-phase system such as volume-averaged overall and time-averaged local gas holdups and axial liquid velocities along time and transversal courses were simulated and analyzed in detail, under varied operating conditions (inlet air flow rate and impeller rotation speed). Model predictions of local transient gas holdup and liquid velocity distributions on vertical and horizontal sections of the tank were also carried out. The overall flow patterns were discussed in detail to assess the mixing. Bubble size distributions were further predicted to reveal the unique properties of gas phase. Experimental measurements of overall gas holdups and local axial liquid velocities were used to validate the developed model.

CFD-PBE simulation of gas-phase hydrodynamics in a gas-liquid-solid combined loop reactor

The computational fluid dynamics (CFD)-population balance equations (PBE) coupled model is employed to investigate the hydrodynamics in a gas-slurry internal loop reactor with external slurry circulation. The predicted radial profiles of local gas holdup and bubble diameter are in good agreement with the corresponding experimental data. The spatio-temporal velocity profile of the gas phase reveals that the upward movement of gas is slowed down and the residence time of gas is prolonged by the downward momentum of the slurry. Introduction of the external slurry can greatly improve the uniformity of gas holdup distribution in the reactor, especially in the downcomer-tube action region. Moreover, the interaction between the downward slurry and upward gas can lead to small bubble size and high interfacial area as well as good mass and heat transfer. The above results suggest the function of external slurry circulation for the internal loop reactor and would be helpful for optimizing the design and scale up of reactors.