Gas-liquid Downflow Research Articles

Stratified flow distribution during gas–liquid downflow in the mesodomain

The paper discusses the formation and characteristics of stratified air–water downflow in the mesodomain. The stratified pattern, observed at low phase velocities, is characterized by air and water flowing side by side and both wetting the conduit wall, similar to stratified distributions in horizontal conduits. Such flow segregation without the influence of gravity is counterintuitive and no detailed investigation on the formation and flow physics of this distribution is reported till date. We have performed extensive experiments in glass conduits of 2.5–12.5 mm diameter where the two phases are introduced through T and Y junctions with different included angles between the entry arms. Our experiments reveal that stratified flows are formed up to a critical angle ϕc subtended by the edges of the circumferential liquid film and beyond the critical wetting angle, the flow pattern is annular with liquid completely wetting the conduit wall. We further note ϕc to depend on liquid properties, included angle of Y-entry, conduit diameter, and phase flow rates. Based on experimental observations, we propose a simplistic analysis to relate the liquid properties, conduit dimension, and Y-entry included angle and the liquid and gas flow rates to describe the formation of stratified and annular flow at the junction. The analysis also estimates the in situ void fraction during stratified gas–liquid downflow.

The Effect of Gas Bubbles on the Flow Structure and Turbulence in a Downward Two-Phase Flow in a Vertical Pipe

This paper presents the experimental and numerical results of modeling the local structure of gas-liquid downflow in a vertical pipe. The measurements were carried out using the PIV/PLIF system “Polis-PIV,” developed at the Institute of Thermophysics SB RAS. The numerical model is based on the use of a two-fluid Eulerian approach. The effect of changes in the diameter of air bubbles on the local flow structure and wall friction in a two-phase bubbly flow is studied.

Assessment of the Resilience against Liquid Maldistribution of Monolith Packings under Offshore Floating Conditions

The study provides insights for adapting commercially available packings to work on floating vessels in the marine context aiming at preserving performances similar to land-based packed-bed reactors. Specifically, liquid distribution for cocurrent gas-liquid downflow (Ug = 0.0065-0.131 m/s and Ul = 0.001-0.0078 m/s) inside monolith packings embarked on a hexapod emulator was compared against glass-bead containing trickle bed and open-cell foam packings using wire-mesh sensors under vertical, inclined and rolling column conditions. Sensitivity of the tested internals with regard to liquid maldistribution was diagnosed and analyzed in terms of a maldistribution factor for the vertical/inclined and rolling columns. Liquid maldistribution was found to prevail at high gas and low liquid superficial velocities for the tested three packings regardless of column conditions. Monolith packed beds demonstrated superior robustness to counteract liquid maldistribution induced by bed static inclinations and roll motion...

Dynamic Liquid–Solid Mass Transfer in Solid Foam Packed Reactors at Trickle and Pulse Flow

The effective liquid–solid mass transfer (LSMT) has been investigated in solid foam packed reactors under cocurrent gas–liquid downflow. Based on a comprehensive literature analysis, the limiting current technique was adapted successfully to solid foam packings enabling measurements of dynamic mass transfer coefficients at different axial packing positions. The flexible reactor setup was used to analyze the appropriateness of various setup configurations. The LSMT coefficients are presented for two different solid foam pore densities for a wide range of gas and liquid velocities covering trickling and pulsing flow regime. To illustrate the difference in the LSMT dynamics in terms of electric current compared to the liquid holdup at pulse flow, a compartment model is developed. Excellent agreement is achieved between simulations and measurement data. Eventually, the effective mass transfer data are compared with conventional random and structured packings.

Void fraction and pressure drop in gas-liquid downflow through narrow vertical conduits-experiments and analysis

The present paper investigates void fraction and pressure drop characteristics of vertical air-water downflow through millichannels (0.83≤Eotvos Number≤20.6). Experiments have revealed the hydrodynamics to be a function of tube diameter and phase velocities with the functional form being different for different flow patterns. Accordingly, separate mechanistic models have been proposed to predict the aforementioned parameters for bubbly, slug, falling film and annular flow distributions. The proposed analysis have been validated with experimental results of the present study and those reported in literature.

Process intensification of gas–liquid downflow and upflow packed beds by a new low‐shear rotating reactor concept

In the present work, a new low‐shear rotating reactor concept was introduced for process intensification of heterogeneous catalytic reactions in cocurrent gas–liquid downflow and upflow packed‐bed reactors. To properly assess potential advantages of this new reactor concept, exhaustive hydrodynamic experiments were carried out using embedded low‐intrusive wire mesh sensors. The effect of the rotational velocity on liquid flow patterns in the bed cross‐section, liquid saturation, pressure drop, and regime transition was investigated. Furthermore, liquid residence time and Péclet number estimated by a stimulus‐response technique and a macro‐mixing model were presented and discussed with respect to the prevailing flow patterns. The results revealed that the column rotation induces different flow patterns in the cross‐section of the packed bed operating in a concurrent downflow or upflow mode. Moreover, the new reactor concept exhibits a more flexible adjustment of pressure drop, liquid saturation, liquid residence time, and back‐mixing at constant flow rates. © 2016 American Institute of Chemical Engineers AIChE J, 63: 283–294, 2017

Characterisation of gas-liquid two-phase flow in minichannels with co-flowing fluid injection inside the channel, part I: unified mapping of flow regimes

Present research highlights the potential of apparatuses with integrated minichannel packings to intensify gas-liquid-solid contacting. Especially an operation of these devices within the Taylor flow regime gained extraordinary attention due to its excellent heat and mass transfer and the segmented flow characteristics. However, criteria for flow regime transitions are mainly developed from water-similar fluids and are contradictory which hinders uniform flow regime prediction.This work presents a systematic analysis of adiabatic gas-liquid downflow in a square minichannel of 1.0mm hydraulic diameter. In the mixing zone located within the flow channel, gas was injected into the co-flowing liquid by so-called capillary injectors with variable inner diameter (0.184, 0.317, 0.490mm). Experiments were conducted using water, water-glycerol, and water-ethanol mixtures to cover a broad range of material properties. The gas and liquid superficial velocities were varied between 9.81·10-4…2.72m/s and 1.7·10−4…0.80m/s, respectively. Taylor flow, Taylor-annular flow, annular flow, churn flow, and bubbly flow were observed. Using the Pi-theorem, 8 significant dimensionless groups dictating the flow transition were identified, namely uG, s/uL, s, ReG, ReL, WeG, WeL, Θ*, dIn, CI/dh, and dOu, CI/dh. Based on more than 1500 experimental data, criteria for the regime transitions of Taylor flow are provided. The derived flow regime map shows good agreement for all applied liquids and for the two larger injector geometries.

Extrudate Trilobe Catalysts and Loading Effects on Pressure Drop and Dynamic Liquid Holdup in Porous Media of Trickle Bed Reactors

The hydrodynamics of concurrent gas-liquid downflow through a porous media of fixed bed reactor has been studied experimentally in a range of trickling flow rates. A pilot bed is packed with industrial spherical and extrudate trilobe catalysts. The industrial trilobe catalysts are packed in a bed using two different methods: random close or dense packing and random sock packing. The experiments are performed for single phase in the cases of wet and dry packed beds and for two-phase flow conditions. The comparisons of pressure drops as well as liquid holdup are carried out for the above three different porous media, random close, dense packing and random sock packing. It is shown that the pressure drop of the dense loaded bed is higher than that of spherical particles which have approximately the same porosity. The results also revealed that the bed porosity, shape and contact points of the loaded catalyst have significant effects on the dynamic liquid holdup of the TBRs. Finally, a new correlation was developed for dynamic liquid holdup and pressure drop calculation for trilobe dense and sock catalyst beds and beds which are loaded with spherical particles.

Assessment of mass transfer in the stagnant liquid regions in trickle-bed reactors

A critical revision has been carried out of experimental investigations to quantify the size of the liquid stagnant region in fixed beds with co-current gas–liquid down-flow, and its ability to exchange mass with the dynamic region and with the particle surface. The revision has exposed that a high level of uncertainty arise to estimate, particularly, the mass exchange rate from the available evidence. Therefore, a second purpose is to introduce a geometric model to represent the stagnant region for spherical particles from which, assuming that diffusion is the only mass transport mechanism, it becomes possible to estimate the mass exchange rates with the dynamic region and with the particle surface. Finally, it was discussed why to expect that the geometric model is able to provide a lower bound to mass exchange rate with the dynamic region and a tight lower bound to mass exchange rate with the catalyst surface.

Purification Processes –biodegradation of Vinyl Acetate from Waste Air in a Trickle-bed Bioreactor (tbb).

The removal of vinyl acetate from waste gas in co-current gas-liquid downflow trickle – bed bioreactor inoculated with Pseudomonas fluorescens PCM 2123 strain was studied experimentally. Due to the lack of literature data, it was necessary to experimentally determine a rate of the partial stages of the process, which are: the rate of mass transfer process in the gas and liquid phases and the rate of biological reaction. The results of the experiments carried out in TBB were compared with the values obtained from mathematical model. The good compatibility of the calculated and experimental data was obtained.

Trickle bed mechanistic model for (non-)Newtonian power-law foaming liquids

Physical models for the description of the hydrodynamics of trickle-bed reactors for foaming Newtonian and power-law liquids are virtually inexistent in the literature. An attempt is made in this work to develop a hydrodynamic model for cocurrent gas–liquid downflow involving foaming (non-)Newtonian power-law liquids through packed beds at non-ambient temperatures and moderate pressures. The model was an extension of the well-trodden slit model variants proposed for (non-)Newtonian power-law coalescing-liquid systems. Its validation using recently published experimental data [Aydin, B., and Larachi, F., 2008. Trickle bed hydrodynamics for (non-)Newtonian foaming liquids in non-ambient conditions. Chem. Eng. J. 143, 236–243.] allowed a quantitative representation of foaming flow hydrodynamics in terms of the two-phase pressure gradient and the liquid holdup up to 90 °C and 0.7 MPa. The model, requiring no arbitrary adjustable parameters, in addition related foam stability to quantitative descriptors such as the liquid-film-borne bubble holdup and diameter, and free (or headspace)-gas holdup. The model also exhibited logical and fully interpretable trends with regard to foam stability, reactor temperature and pressure, liquid and gas superficial velocities.

Liquid Layer Characteristics in Stratified Gas-Liquid Downflow: A Study of Transition to Wavy Flow

The purpose of this work was to study the transition from the smooth to the wavy stratified flow regime for various pipe inclination angles and liquid physical properties. The accurate characterization of both the structure of the gas-liquid interface and the flow field inside the liquid layer can improve our physical understanding of the mechanisms involved in the evolution of waves in stratified gas-liquid flow. To study the influence of liquid properties on the mechanisms promoting wave formation, several liquids are used (i.e., water, Tween®, and aqueous-glycerin solutions). The experiments are conducted in a 24 mm i.d. pipe for various inclination angles (1–9°) with respect to the horizontal position. Liquid layer thickness time records are acquired using a parallel-wire conductance technique from which mean layer thickness, rms, and power spectra of the fluctuations as well as wave celerities are calculated. Measurements of the axial velocity component in the liquid layer using laser-Doppler anemometry are also reported. Statistical analysis of such local liquid velocity data in conjunction with the liquid layer characteristics reveals a strong interplay between wave evolution at the interface and the flow field development inside the liquid layer. Finally, results of numerical calculations using a CFD code are obtained to facilitate data interpretation.

Prediction of pressure drop in a modified gas–liquid downflow bubble column

A model for pressure drop in the ejector induced downflow bubble column based on mechanical energy balance within the framework of dynamic interaction of the phases has been formulated. The model includes the effect of bubble formation and form drag at interface on the pressure drop. It provides a functional form of equation for correlating pressure drop. The theoretical model proposed in the present study appears to predict the pressure drop satisfactorily for gas–liquid dispersed flow in the concurrent gas–liquid downflow bubble column.

Effect of surfactant additives on co-current gas–liquid downflow

The influence of a surfactant additive on the interfacial structure and on the transition from the smooth to the wavy stratified flow regime in inclined pipes is investigated for various gas–liquid flow rates. The effect of a surfactant on the gas–liquid stratified downflow patterns is studied using a dilute aqueous solution of a non-ionic surfactant ( Tween ® 80 ) , which is found to have a dramatic effect on two-phase flow characteristics. A conventional solution having a surface tension similar to that of the surfactant solution is also employed and this verifies that the atypical behavior of the surfactant solution is ascribed to its special chemical structure and not to its low surface tension. Liquid layer thickness time records are acquired using a parallel-wire conductance technique from which the statistics of the layer thickness, as well as wave celerities, are calculated. Laser doppler anemometry (LDA) is employed to investigate the flow structure in the thin liquid layer both with and without interfacial shear induced by a co-current gas flow. A differential pressure transducer is used for pressure drop measurements in the liquid phase. The interpretation of the results verifies that the “ delayed” appearance of the interfacial waves should be associated with the turbulence retardation within the liquid layer and it facilitates the clarification of the mechanisms in which the surfactant interacts with the turbulent structures and causes significant drag reduction.

Prediction of two‐phase pressure drop and liquid holdup in co‐current gas–liquid downflow of air–Newtonian systems through packed beds

AbstractThe dependency of pressure drop and liquid holdup on phase velocities, geometry of the column and packing materials as well as on the physical properties have been analyzed. Our experimental data (825 data points obtained using four liquid systems and three different particles) along with those of the available literature (776 data point from five different sources) were used for the analysis. The applicability and the limitations of the literature correlations were evaluated using the available data. Based on the analysis, new correlations for the estimation of pressure drop and liquid holdup, valid for low and high interaction regimes have been developed using the available data, with a wide range of variables. Copyright © 2005 Society of Chemical Industry

Co-current stratified gas–liquid downflow—Influence of the liquid flow field on interfacial structure

The transition from smooth to wavy stratified flow is studied for various pipe inclination angles with the aim to contribute to the realistic modeling and simulation of long distance two-phase pipe flow. The influence of the liquid flow field on interfacial structure is studied through local axial velocity measurements in the liquid phase in conjunction with other liquid layer characterization experiments. Observations based on fast-video recordings, are used to identify the main patterns of wave evolution. Liquid layer thickness time records are acquired using a parallel wire conductance technique from which mean layer thickness, RMS and power spectra of the fluctuations, as well as wave celerities are calculated. Laser Doppler Anemometry ( LDA) is employed to investigate the flow structure in the thin liquid layer both with and without interfacial shear induced by a co-current gas flow. The results reveal the influence of the liquid flow field development on the interfacial structure. In particular, the new data of axial velocity profiles and liquid layer characteristics suggest that the onset of the interfacial waves is strongly affected by the liquid flow structure, possibly by the laminar to turbulent transition within the layer.

Liquid holdup and pressure drop in the gas–liquid cocurrent downflow packed-bed reactor under elevated pressures

An experimental investigation of the residence time distribution, liquid holdup, and pressure drop in a gas–liquid downflow packed bed reactor with porous particles operated under elevated pressures is presented. The effects of the two-phase flow rates and reactor pressures on the external liquid holdup and pressure drop are discussed. A mechanistic model, which accounts for the interaction between the gas and liquid phases by incorporating the shear and velocity slip factors between phases, is employed to predict the external liquid holdup and pressure drop for the experimentally covered flow regime. The involved parameters, such as shear and velocity slip factors and Ergun single-phase flow bed constants, are calculated from the correlations developed via neural network regression. The model's predictions and the experimental observations at elevated pressure are compared.

Theory of trickle‐bed magnetohydrodynamics under magnetic‐field gradients

AbstractExternal inhomogeneous magnetic fields exert a body force (magnetization force) on electrically nonconducting and magnetically permeable fluids. The force acts on both paramagnetic and diamagnetic fluids and can be used to compensate for or to amplify the gravitational body force, unlike the Lorentz braking force that manifests only for electrically conducting liquids, such as liquid metals. The ability to influence two‐phase flows through porous media by the application of external inhomogeneous magnetic fields is of interest in the operation of trickle‐bed reactors for catalytic process intensification, particularly in oxidation catalysis, where the O2 paramagnetic properties may be propitious to macrogravity operation for enhancing liquid holdup and wetting efficiency. An isothermal 1‐D two‐fluid magnetohydrodynamics model based on volume‐average mass and momentum balance equations to describe the gas‐liquid downflow under a spatially uniform magnetic‐field gradient. The slit model approximation was used for the derivation of drag force closures intervening in the momentum equations. The evolution of the trickle‐bed magnetohydrodynamics was theoretically investigated in terms of liquid holdup and pressure drop under four gas‐liquid combinations. Advantages of this novel approach of process intensification were rationalized in terms of catalytic reactions.

Bubble flow in trickle beds: investigations using resistive sensors

The local hydrodynamics of co-current gas–liquid down-flow through porous media are investigated in a quasi two-dimensional regular arrangement by means of a network of resistive sensors. The investigations are focused on a liquid continuous flow regime, the dispersed bubble flow, where the gas is divided into elongated bubbles. Due to the variation of the local flow channel orientation and the local void fraction, the average bubble velocity strongly depends on the local geometry. The flow is more coherent in vertical constrictions, compared to all other types of sites, this is probably due to bubble stagnation in the flow channel enlargements. At a given liquid superficial velocities and for sufficiently high superficial gas velocities, the average bubble size is independent of the gas flow rate; it is of the order of magnitude of the volume of the enlargements of the porous medium. The maximum bubble size is about three times its average size, corresponding thus to the coalescence of three average sized bubbles.

Multiphase catalytic reactors: a perspective on current knowledge and future trends

ABSTRACT Conventional and emerging processes that require the application of multiphase reactors are reviewed with an emphasis on catalytic processes. In the past, catalyst discovery and development preceded and drove the selection and development of an appropriate multiphase reactor type. This sequential approach is increasingly being replaced by a parallel approach to catalyst and reactor selection. Either approach requires quantitative models for the flow patterns, phase contacting, and transport in various multiphase reactor types. This review focuses on these physical parameters for various multiphase reactors. First, fixed-bed reactors are reviewed for gas-phase catalyzed processes with an emphasis on unsteady state operation. Fixed-bed reactors with two-phase flow are treated next. The similarities and differences are outlined between trickle beds with cocurrent gas–liquid downflow, trickle-beds with countercurrent gas–liquid flow, and packed-bubble columns where gas and liquid are contacted in cocurrent upflow. The advantages of cyclic operation are also outlined. This is followed by a discussion on conventional reactors with mobile catalysts, such as slurry bubble columns, ebullated beds, and agitated reactors. Several unconventional reactor types are reviewed also, such as monoliths for two-phase flow processing, membrane reactors, reactors with circulating solids, rotating packed beds, catalytic distillation, and moving-bed chromatographic reactors. Numerous references are cited throughout the review, and the state-of-the-art is also summarized. Measurements and experimental characterization methods for multiphase systems as well as the role of computational fluid dynamics are not covered in a comprehensive manner due to other recent reviews in these areas. While it is evident that numerous studies have been conducted to elucidate the behavior of multiphase reactors, a key conclusion is that the current level of understanding can be improved further by the increased use of fundamentals.