Impact of gravity on the bubble‐to‐pulse transition in packed beds

A model based on two‐phase volume‐averaged equations of motion is proposed to examine the gravity dependence of the bubble‐to‐pulse transition in gas‐liquid cocurrent down‐flow through packed beds. As input, the model uses experimental correlations for the frictional pressure drop under both normal gravity conditions and in the limit of vanishing gravity, as well as correlations for the liquid‐gas interfacial area per unit volume of bed in normal gravity. In accordance with experimental observations, the model shows that, for a given liquid flow, the transition to the pulse regime occurs at lower gas‐flow rates as the gravity level or the Bond number is decreased. Predicted transition boundaries agree reasonably well with observations under both reduced and normal gravity. The model also predicts a decrease in frictional pressure drop and an increase in total liquid holdup with decreasing gravity levels. © 2013 American Institute of Chemical Engineers AIChE J 60: 778–793, 2014     

3‐D Simulations of an Internal Airlift Loop Reactor using a Steady Two‐Fluid Model

Three‐dimensional (3‐D) simulations of an internal airlift loop reactor in a cylindrical reference frame are presented, which are based on a two‐fluid model with a revised k‐? turbulence model for two‐phase bubbly flow. A steady state formulation is used with the purpose of time saving for cases with superficial gas velocity values as high as 0.12 m/s. Special 3‐D treatment of the boundary conditions at the axis is undertaken to allow asymmetric gas‐liquid flow. The simulation results are compared to the experimental data on average gas holdup, average liquid velocity in the riser and the downcomer, and good agreement is observed. The turbulent dispersion in the present two‐fluid model has a strong effect on the gas holdup distribution and wall‐peaking behavior is predicted. The CFD code developed has the potential to be applied as a tool for scaling up loop reactors.     

Hydrodynamic Model for Horizontal Two‐phase Flow Through Porous Media

A unidirectional, two‐fluid model based on the volume‐average mass and momentum balance equations was developed for the prediction of two‐phase pressure drop and external liquid hold‐up in horizontally positioned packed beds experiencing stratified, annular and dispersed bubble flow regimes. The so‐called slit model drag force closures were used for the stratified and annular flow regimes. In the case of dispersed bubble flow regime, the liquid‐solid interaction force was formulated on the basis of the Kozeny‐Carman equation by taking into account the presence of bubbles in reducing the available volume for the flowing liquid. The gas‐liquid interaction force was evaluated by using the respective solutions of drag coefficient for an isolated bubble in viscous and turbulent flows. The proposed drag force expressions for the different flow patterns occurring in the bed associated with the two‐fluid model resulted in a predictive method requiring no adjustable parameter to describe the hydrodynamics for horizontal two‐phase flow in packed beds.     

Flow pattern transition in gas‐liquid downflow through narrow vertical tubes

The present report studies on the flow pattern transitions during vertical air water downflow through millichannels (0.83 ≤ Eötvös no. ≤ 20.63). Four basic flow patterns namely falling film flow, slug flow, bubbly flow, and annular flow are observed in the range of experimental conditions studied and their range of existence has been noted to vary with tube diameter and phase velocities. Based on experimental observations, phenomenological models are proposed to predict the transition boundaries between adjacent patterns. These have been validated with experimental flow pattern maps from the present experiments. Thus the study formalizes procedure for developing a generalized flow pattern map for gas‐liquid downflow in narrow tubes. © 2016 American Institute of Chemical Engineers AIChE J, 63: 792–800, 2017     

Gas‐Liquid Mass Transfer in Fibrous Bed Reactor with Counter‐Current Liquid Recycle

In this work, the gas‐liquid mass transfer in a lab‐scale fibrous bed reactor with liquid recycle was studied. The volumetric gas‐liquid mass transfer coefficient, k_La, is determined over a range of the superficial liquid velocity (0.0042–0.0126 m.s^–1), gas velocity (0.006–0.021 m.s^–1), surface tension (35–72 mN/m), and viscosity (1–6 mPa.s). Increasing fluid velocities and viscosity, and decreasing interfacial tension, the volumetric oxygen transfer coefficient increased. In contrast to the case of co‐current flow, the effect of gas superficial velocity was found to be more significant than the liquid superficial velocity. This behavior is explained by variation of the coalescing gas fraction and the reduction in bubble size. A correlation for k_La is proposed. The predicted values deviate within ± 15 % from the experimental values, thus, implying that the equation can be used to predict gas‐liquid mass transfer rates in fibrous bed recycle bioreactors.     

Effect of tube diameter on two‐phase flow patterns in mini tubes

The effect of tube diameter on two‐phase flow patterns was investigated in circular tubes with inner diameters of 0.6, 1.2, 1.7, 2.6, and 3.4 mm using air and water. The gas and liquid superficial velocity ranges were 0.01–50 and 0.01–3 m/s, respectively. The gas and liquid flow rates were measured and the two‐phase flow pattern images were recorded using high‐speed CMOS camera. The flow patterns observed were dispersed bubbly, bubbly, slug, slug‐annular, wavy‐annular, stratified, and annular flows. These flow patterns were not observed in all the test diameters, but were found to be unique to particular tube diameters, confirming the effect of tube diameter on the flow pattern. The data obtained were compared to existing experimental data and flow regime transition maps which show generally reasonable overall agreement at the larger diameters, but significant differences were observed with the smaller diameter tubes.     

Atomization of Liquids by Two‐phase Gas‐liquid Flow through a Plain‐orifice Nozzle: Flow Regimes inside the Nozzle

Liquids or suspensions are divided into sprays of small droplets by atomization of two‐phase gas‐liquid mixtures. In this way either an equal distribution of the droplets or the generation of large surface areas of the liquid phase are accomplished, leading to increased heat‐ and mass‐transfer. The spatial and time dependency of the mean droplet diameter is a function of the total pressure upstream of the nozzle, the volumetric flow rate of the liquid and the gas, as well as on the flow regime in the nozzle. Thus the radial and axial profile of the void fraction inside the nozzle are measured with an electrical measurement technique. In addition, the flow in the nozzle is imaged by a high‐speed camera. Three flow regimes are identified. These are bubbly flow, plug flow and annular flow. A continuous flow of the emitting spray is observed for bubbly flow and annular flow only. The distribution of the dispersed bubble phase is given by ratio of the isothermic compression energy needed to pressurize the gas mass flow rate from atmospheric pressure up to the total pressure in front of the nozzle, and the potential energy of the supplied liquid mass flow rate.     

High‐throughput microporous tube‐in‐tube microreactor as novel gas–liquid contactor: Mass transfer study

High‐throughput microporous tube‐in‐tube microchannel reactor (MTMCR) was first designed and developed as a novel gas–liquid contactor. Experimentally measured k_Lα in MTMCR is at least one or two orders of magnitude higher than those in the conventional gas–liquid contactors. A high throughput of 500 L/h for gas and 43.31 L/h for liquid is over 60 times higher than that of T‐type microchannel. An increase of the gas or liquid flow rate, as well as a reduction of the micropore size and annular channel width of MTMCR, could greatly intensify the gas–liquid mass transfer. The interfacial area, α, in MTMCR was measured to be as high as 2.2 × 10⁵ m²/m³, which is much higher than those of microchannels (3400–9000 m²/m³) and traditional contactors (50–2050 m²/m³). The artificial neural network model was proposed for predicting α, revealing only an average absolute relative error of <5%. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Sensitivity Study on Modeling an Internal Airlift Loop Reactor Using a Steady 2D Two‐Fluid Model

The sensitivity study of bubbly flow in an internal airlift loop reactor is presented using a steady Reynolds averaging two‐fluid model. Comparative evaluation of different drag formulations, drag coefficient correlations, turbulence effect on the drag coefficient, outlet slip velocity, and bubble size is performed and the respective influence to the simulation results is highlighted. It is found that a complicated drag formulation may not result in reliable predictions. All the drag coefficient correlations underpredict the gas holdup if the influence of turbulence on the drag coefficient is not well incorporated. Fortunately, the global hydrodynamics is not sensitive to the outflow slip velocity for a wide range, so a steady two‐fluid model can be used to simulate the bubbly flow when the flow field is fully developed. The correct estimation of bubble size with properly selected correlations play an important role in successful simulation of gas‐liquid bubbly flow in airlift loop reactors.     

Numerische Simulation disperser Gas/Flüssig‐Strömungen in Blasensäulen bei hohen Gasgehalten mit OpenFOAM® Teil 1 – Grundlagen der Modellierung

Various chemical products are synthesized in processes using gas/liquid reactors with bubbly flows. Hence, there is significant interest in a more efficient process design as well as in process intensification with a strong focus on this reactor class. However, the design of industrial gas/liquid reactors requires more detailed information about the flow structures and characteristics of two‐ or multiphase systems. The basic models for two‐fluid model simulations of dispersed gas/liquid flows in bubble columns at high gas fractions are presented..     

Hydrodynamic Modelling of Internal Loop Airlift Reactor Applying Drift‐Flux Model in Bubbly Flow Regime

A new model for the liquid circulation rates in airlift reactor (ALR) is presented. The model is based on the energy balance for the flow loop (riser, turn riser‐downcomer, downcomer, and turn downcomer‐riser) coupled with a drift flux theory of two‐phase flow gas‐liquid system, considering a bubbly flow regime. The predicted values of the liquid circulation rates by the developed model are compared with experimental results performed in a 22 dm³ internal loop airlift reactor and with the results obtained in the literatures. The proposed model predicted the experimental results very well. Slip velocity relationship based on the drift flux model was proposed; including the gas holdup, bubble size and the liquid physical properties. The predicted slip velocity was similar to that obtained from the literature. The study revealed that appropriate arrangements of internal bioreactor parts can positively influence the liquid circulation velocity at the same energy consumption. The proposed models are useful in the design; scale up and characterization of the internal loop airlift reactors, and provides a direct method of predicting hydrodynamic behaviour in gas‐liquid airlift reactors.     

A Comparison of the Mixing Characteristics in Single‐ and Two‐Phase Grid‐Generated Turbulent Flow Systems

The mixing process is studied in grid‐generated turbulent flow for single‐ and bubbly two‐phase flow systems. Concentration and mixing characteristics in the liquid phase are measured with the aid of a PLIF/PLIF arrangement. A nearly isotropic turbulent flow field is generated at the center of the vertical pipe by using a honeycomb, three grids and a contraction. In two‐phase flow experiments, air bubbles were injected into the flow from a rectangular grid, with mesh size M = 6 mm, which is placed midway between two circular grids each with a mesh size of M = 2 mm. For single‐phase flow, the normalized mean concentration cross‐stream profiles have rather similar Gaussian shapes, and the cross‐stream profiles of the normalized root‐mean‐square (RMS) values of concentration were found to be quite similar. Cross‐stream profiles of the mean concentration, for bubbly two‐phase flow, were also found to be quite similar, but they did not have the Gaussian shape of the profiles for single‐phase flow. Almost self‐similar behavior was also found for the RMS values of the concentration in two‐phase systems. The turbulent diffusion coefficient in the liquid phase was also calculated. At the center of the plume, the flow was found to have a periodic coherent structure, probably of vortex shedding character. Observations showed that the period of oscillation is higher in the case of two‐phase flow than in single‐phase flow.     

Liquid holdup in concentric annuli during cocurrent gas‐liquid upflow

In the recent paper, an in‐depth investigation of liquid holdup during air‐water upflow through concentric annuli has been reported. The liquid holdup has been determined experimentally for the bubbly, slug and churn flow regimes. The drift flux model has been adopted for the theoretical estimation of holdup in the bubbly, dispersed bubbly and slug flow regimes. The pronounced effect of flow regime on this parameter as observed from experiments has been incorporated in the model by adopting different values of U₀, n and C₀. The asymmetry of the Taylor bubbles has been incorporated in the slug flow regime. The theoretical predictions exhibit a good agreement with the experimental data of the present work and that available in literature (Caetano et al., 1989b). The Hughmark's correlation is observed to correlate the churn flow data of the present work reasonably well.     

Horizontal air–water flow in a square cross‐section channel: Pseudo‐slug flow regime

New experimental data for air–water flow in a horizontal square cross‐section channel (H = 24.25 mm) is presented, including data on liquid hold‐up, gas and liquid velocities, and wave velocities and frequencies. For the majority of gas and liquid flow rates studied, the regime observed was pseudo‐slug. Using visualization studies it was possible to identify wavy‐stratified and pseudo‐slug flows. For the pseudo‐slug regime new correlations were obtained for liquid hold‐up, for gas and liquid velocities as a function of the ratio between gas and liquid mass flow rates, and for the frequency of roll‐waves as a function of gas and liquid mass flow rates.     

Numerical Simulation of the Two‐Phase Fluid Field in a Gas‐Around‐Liquid Spray Nozzle

A new gas‐around‐liquid spray nozzle (GLSN) was designed, and the two‐phase flow fluid field in this nozzle was simulated numerically. Flow characteristics under different structural parameters were obtained by changing the L/D ratio of the premixing chamber, incident angle, and inlet pressures. Increasing the L/D ratio and incident angle improved flow characteristics such as atomization flow, outlet velocity, and turbulence intensity. The nozzle performed optimally at an L/D ratio of 0.5 and incident angle of 60°. The atomization flow decreased with higher gas pressure and increased with higher liquid pressure. The outlet velocity mainly depended on the inlet gas pressure, not on the inlet liquid pressure. These results provide an indication for optimum structures and parameters of the GLSN.     

Measurements of horizontal three‐phase solid‐liquid‐gas slug flow: Influence of hydrate‐like particles on hydrodynamics

Gas hydrate formation is a main flow assurance concern in oil and gas production. Understanding the effects of the introduction of solid particles in the slug flow is essential to improve the efficiency and safety of multiphase production. The purpose of the present work is the experimental characterization of solid‐liquid‐gas slug flow with the presence of dispersed hydrate‐like particles. Experimental tests were carried out with inert polyethylene particles of 0.5‐mm diameter with density similar to gas hydrates (938 kg/m³). The test section comprised a 26‐mm ID, 9‐m length horizontal duct of transparent Plexiglas. High Speed Imaging and resistivity sensors was used to analyze the slug flow unit cell behavior due to the introduction of the solid particles and to measure the unit cell translational velocity, the slug flow frequency, the bubble and slug lengths, and the phase fractions. Two distinct concentrations of solid particles were tested (6 and 8 g/dm³). © 2018 American Institute of Chemical Engineers AIChE J, 64: 2864–2880, 2018     

Co‐current descending two‐phase flows in inclined packed beds: Experiments versus simulations

The effect of inclination angle of a packed bed on its corresponding gas–liquid flow segregation and liquid saturation spatial distribution was measured in co‐current descending gas–liquid flows for varying inclinations and fluid velocities, and simulated using a two‐phase Eulerian computational fluid dynamics framework (CFD) adapted from trickle‐bed vertical configuration and based on the porous media concept. The model predictions were validated with our own experimental data obtained using electrical capacitance tomography. This preliminary attempt to forecast the hydrodynamics in inclined packed bed geometries recommends for the formulation of appropriate drag force closures which should be integrated in the CFD model for improved quantitative estimation.     

Heat Transfer from Immersed Vertical Cylinders in Gas‐Liquid and Gas‐Liquid‐Solid Fluidized Beds

The heat transfer coefficient, h, was measured using a cylindrical heater vertically immersed in liquid‐solid and gas‐liquid‐solid fluidized beds. The gas used was air and the liquids used were water and 0.7 and 1.5 wt‐% carboxymethylcellulose (CMC) aqueous solutions. The fluidized particles were sieved glass beads with 0.25, 0.5, 1.1, 2.6, and 5.2 mm average diameters. We tried to obtain unified dimensionless correlations for the cylinder surface‐to‐liquid heat transfer coefficients in the liquid‐solid and gas‐liquid‐solid fluidized beds. In the first approach, the heat transfer coefficients were successfully correlated in a unified formula in terms of a modified j_H‐factor and the modified liquid Reynolds number considering the effect of spatial expansion for the fluidized bed within an error of 36.1 %. In the second approach, the heat transfer coefficients were also correlated in a unified formula in terms of the dimensionless quantities, Nu/Pr^1/3, and the specific power group including energy dissipation rate per unit mass of liquid, E^1/3D^4/3/ν_l, within a smaller error of 24.7 %. It is also confirmed that a good analogy exists between the surface‐to‐liquid heat transfer and mass transfer on the immersed cylinder in the liquid‐solid and gas‐liquid‐solid fluidization systems.     

Liquid‐liquid two‐phase flow patterns in a new helical microchannel device

Flow patterns of liquid‐liquid two‐phase fluids in a new helical microchannel device were presented in this paper. Three conventional systems were considered: kerosene‐water, n‐butyl acetate‐water, and butanol‐water. Six different flow patterns, slug flow, continuous parallel flow, discontinuous deformation parallel flow, discontinuous deformation parallel‐droplet flow, droplet‐slug flow, and filiform‐droplet flow, were observed. The influence of interfacial tension, microchannel structure, and rotation rate on two‐phase flow patterns were studied, and a universal flow pattern map was presented and discussed. The systems without mass transfer (0.1 g/g (10 %) tri‐n‐butyl phosphate (TBP)‐water, 0.2 g/g (20 %) TBP‐water, and 0.8 g/g (80 %) TBP‐water) and the system with mass transfer (0.8 g/g (80 %) TBP‐0.62 g/g (62 %) H₃PO₄) were used to verify the validity of the proposed universal flow pattern map in predicting flow patterns. The results showed that the former compared with the latter can be predicted more accurately by the universal flow pattern map.     

Design methodology for barrier‐based two phase flow distributor

The barrier‐based distributor is a multiphase flow distributor for a multichannel microreactor which assures flow uniformity and prevents channeling between the two phases. For N number of reaction channels, the barrier‐based distributor consists of a gas manifold, a liquid manifold, N barrier channels for the gas, N barrier channels for the liquid, and N mixers for mixing the phases before the reaction channels. The flow distribution is studied numerically using a method based on the hydraulic resistive networks (RN). The single phase hydraulic RN model (Commenge et al., 2002;48:345–358) is extended for two phases gas‐liquid Taylor flow. For Re_GL <30, the accuracy for the model was above 90%. The developed‐model was used to study the effects of fabrication tolerance and barrier channel dimensions. A design methodology has been proposed as an algorithm to determine the required hydraulic resistance in the barrier channels and their dimensions. This methodology is demonstrated using a numerical example. © 2012 American Institute of Chemical Engineers AIChE J, 2012