Cocurrent Gas-liquid Flow Research Articles

On the asymptotic stability of advection-diffusion equations of mass transport in a bubble column bioreactor

This study presents an asymptotic stability analysis of a model of a bioreactor converting carbon monoxide (CO) gas into ethanol through a C. autoethanogenum biocatalyst. The configuration is a bubble column reactor with co-current gas-liquid flows where gas feed is introduced by a gas distributor placed at the bottom of the column. A pure culture of C. autoethanogenum is subsequently injected at the bottom of the column; therein, cells are dispersed in the liquid and consume the dissolved gas and release by-products such as ethanol and acetic acid. Cellular growth and byproduct secretion are affected by spatially varying dissolved gas concentrations due to advection-diffusion mass transports which are induced by the effect of the injection pressure and gravitational force. The model accounts for four species representing the biomass, the CO substrate in the liquid phase, and two by-products - ethanol and acetic acid. Substrate dynamics is described by an advection-diffusion equation.We investigate the asymptotic stability of the biomass dynamics that is a requirement for the system’s controllability, i.e. for the possibility to steer a dynamical system from an arbitrary initial state to an arbitrary final state using a set of controls. The concept of stability of the controls is extremely relevant to controllability since almost every workable control system is designed to be stable. If a control system is not stable, it is usually of no use in practice in industrial processes. In the case of a bioreactor, the control is the biomass and controllability is the possibility of modulating through this control the ethanol production. We present a test for asymptotic stability, based on the analysis of the properties of the dynamic function defining its role as storage function.

Precise Evaluation of Gas–Liquid Two-Phase Flow Pattern in a Narrow Rectangular Channel with Stereology Method

The drive to increase the efficiency of processes based on two-phase flow demands the better precision and selection of boundary conditions in the process’ control. The two-phase flow pattern affects the phenomena of momentum, heat, and mass transfer. It becomes necessary to shift from its qualitative to quantitative evaluation. The description of the stationary structure has long been used in structural studies applied to metals and alloys. The description of a gas–liquid two-phase mixture is difficult because it changes in time and space. This paper presents a study of the precise determination of two-phase flow patterns based on stereological parameters analysis. The research area is shown against the flow map proposed by other researchers. The experiment was taken in the thin clear channel with dimensions of W = 50 × H = 1200 × T = 5 mm. The test method is based on the visualization of a two-phase air–water adiabatic flow pattern in the rectangular channel where superficial air velocities ranging from 0.006 to 0.044 m/s and the superficial water velocity ranged from 0.011 to 1.111 m/s. A high-speed camera was used for visualization. Images were analyzed with the use of stereological techniques. The study included the classification of structures according to generally accepted two-phase flow regime nomenclature for upwards co-current gas–liquid flow in a vertical rectangular channel. The result of the research was the determination of the stereological parameters’ changes with reference to the two-phase mixture flow hydrodynamics. The results were presented as waveform fluctuations in the values of stereological factors such as the volume fraction VV, interfacial surface SV, number of objects NV, mean chord l′m and the free distance λ. The description of how these parameters change with changes in phase fluxes is also presented. These waveforms help to distinguish the transient flow regimes, which allow for the automatic adjustment of the process stability. The authors found templates of the stereological parameters’ dependencies for flow pattern recognition. The research demonstrates wide possibilities of stereological methods’ application for the analysis of the two-phase gas–liquid process. The stereological model of two-phase pattern control enables the identification of process disorders.

Experimental study of droplet detachment from liquid film surface by a co-current flow inside the nozzle stagnation chamber

The paper presents the results of an experimental study on the interaction of near-wall liquid film with the co-current gas flow in the convergent channel. Conditions for transition (co-current gas flow velocity and liquid flow rate) of near-wall liquid film from annular flow mode to annular-dispersed flow accompanied by a detachment of droplets from the interface are elucidated.

Mass and heat transfer at a helical coil imbedded in a fixed bed of cylinders with upward cocurrent gas-liquid flow

ABSTRACT Rates of mass and heat transfer (by analogy) at the surface of a helical coil imbedded in a fixed bed of inert cylindrical packing were measured under upward two-phase (gas-liquid) flow by the electrochemical technique. The effects of geometrical and hydrodynamic parameters were considered. The performance of cylindrical packing was compared with that of Raschig rings and spheres. The analogy between heat and mass transfer was discussed and the importance of the present study for the design and operation of heterogeneous reactors that need rapid cooling in case of highly exothermic reactions was highlighted.

Three-dimensional numerical simulation of gas-liquid falling film flow characteristics on the airside of finned-tube heat exchanger with a typical large fin pitch

The closed-type heat-source tower (CTHST) heat pump system is a more promising alternative to the conventional air-source heat pump (ASHP) unit compared with the open-type heat-source tower (OTHST) heat pump system. However, the heat transfer efficiency of the CTHST is significantly lower than that of the OTHST, resulting from its indirect heat transfer way, liquid film break-up and high gas phase pressure drop. To provide a theoretical basis for the heat-mass transfer enhancement and structural optimization design of the high-performance CTHST, a three-dimensional (3D) model taking gas-liquid interfacial friction force into consideration was established in the present paper to study counter-current and co-current gas-liquid falling film flow characteristics on the airside of the finned-tube heat exchanger (FTHE) with a typical large fin pitch used in the CTHST. The predicting results of liquid film thickness based on the established model show good agreement with those of experiments in the reference, with a maximum deviation less than 5%, meaning that this model is reliable. Then the effects of contact angle, air flow direction, air Reg and liquid film Rel on liquid film characteristics were studied. The prediction correlation of average liquid film thickness was presented. The pressure drop characteristics of gas phase were analyzed in the co-current gas-liquid flow under three different liquid film Rel. Meanwhile, the change law of interfacial friction force with air Reg was revealed. The numerical results show that the critical mass flow rate of complete film flow on the finned-tube surface with contact angle of 30° is 0.06 kg/(m⋅s). The average liquid film thickness on the surface of the FTHE is obviously higher than that on the vertical plate. There is a close and positive quadratic curve relation between average interfacial friction force and air Reg. To obtain a complete film flow and small air pressure drop, the air Reg should be less than 2190.7 and 3286.0 in the counter-current and co-current gas-liquid falling film flow, respectively.

On the life and habits of gas-core slugs: characterisation of an intermittent horizontal two-phase flow

Slugs are intermittent structures observed in horizontal co-current gas-liquid two-phase flow. Published data examining horizontal flow indicate the existence of intermittent sub-flow patterns. There is limited work performed on the study of this sub-categorisation. This paper presents a method, exploiting non-intrusive Electrical Capacitance Tomography (ECT) measurements, to assess and to characterise the spatial and temporal features of horizontal gas-oil flow. A logic-based algorithm is proposed to automatically identify and classify flow regimes. This algorithm uses the spatial distribution of the phases, based on measurements of the intrinsic dielectric properties of the fluids. The measurements were performed at the NEL UK national standard multiphase flow loop using a high-speed dual-plane ECT sensor array. The ECT measurements identified intermittent liquid structures with a sustained gas-core in the central region of the pipe cross-section with outer features analogous to those of liquid slugs. Assessment of the frequency and time span of the periodic structures showed that gas-core slugs are, typically, shorter than full slug structures but are more frequent for the same given time period. For both flow structures, the mixture velocity was a determinant factor, with gas-core slugs dominating the flow at higher velocities. The experimental work led to the modification of existing flow pattern transition models. The modified flow regime maps suggested in this study aim to widen the applicability of the results by enabling the prediction of gas-core slug flow for similar experimental and operating conditions. The present work comprises the initial steps towards automatic flow regime identification in multiphase flow metering. Automatic flow regime recognition has the potential to lead to better process control and improved accuracy in production allocation.

Hydrodynamics of gas-liquid co-current flow through a thin sheet of highly porous open cell solid foam

An experimental study of the hydrodynamics of a gas-liquid co-current flow inside a thin sheet of Open Cell Solid Foam (OCSF) confined in a plate reactor is proposed. Image processing is used to quantify the flow patterns when varying the gas and liquid flow rates and buoyancy (horizontal and vertical upflow). The fraction of joint flow and interface motion areas, respectively εm and εi, are defined as relevant quantities to characterize the different flow regimes and phases interaction. Two distinct regimes are observed for the vertical flow: the low (LA) and high (HA) activity regimes characterized by a low or high phases interaction. In the horizontal configuration, an additional regime is observed and is called pseudo-trickling (PT) regime due to its similarity with the trickling regime observed in vertical downflow packed beds. The transition between LA and HA regimes is compared to the regime transition in trickle beds.

Experimental study of secondary flows above rough and flat interfaces in horizontal gas-liquid pipe flow

We perform an experimental analysis of stratified, co-current gas-liquid flow in a straight pipe, with the aim of investigating secondary flow structures in the gas-phase. Planar PIV (particle image velocimetry) of the pipe centerline as well as stereoscopic PIV of the pipe cross section is performed, considering cases where the interface is either rough (wavy) or flat. Results demonstrate that the interface morphology determines the direction of the secondary flow patterns. Over a flat interface, the secondary flow cells are directed up along the pipe centerline and down along the pipe walls. When the interface is rough (wavy), the direction of the secondary flow cells is reversed. Normalized by the peak axial velocity, the vertical velocity profile in the pipe centerline is seen to collapse to a common curve when the interface is flat, while results indicate that the normalized strength of the secondary flows increase with increasing surface roughness. An assessment of the Reynolds stresses in the cross section is performed, and the direction of the secondary flow cells are found to be well predicted by the dominant gradients of the radial and circumferential Reynolds stresses (τrr and τθθ) in the radial and circumferential direction respectively.

Linear and nonlinear instabilities of a co-current gas-liquid flow between two inclined plates analyzed using the Navier–Stokes equations

The paper is devoted to a theoretical analysis of a co-current gas-liquid flow between two inclined plates. As a first step, we linearized the Navier-Stokes equations in both phases and carried out a linear stability analysis of the basic steady-state solution over a wide variation of the liquid Reynolds number and the gas superficial velocity. We obtained two modes of the unstable disturbances and computed the wavelength and phase velocity of their neutral and the fastest growing disturbances varying the liquid and gas Reynolds number. The first mode corresponds to the Kapitza's waves at small values of the gas superficial velocity. The second mode of the unstable disturbances corresponds to the transition to a turbulent flow in the gas phase. We found that the co-current gas velocity destabilize the film flow at all values of the inclination angle and distance between the plates considered in the paper. The range of the wavelength of the unstable disturbances essentially increases with the gas velocity increasing. As a second step, we have performed the systematic study of nonlinear wave regimes. We found that the gas flow affects significantly the wave characteristics decreasing the amplitude and increasing the phase velocity. The complex multi-fold and multi-sheet surface, found on the plane of parameters (wavelength and the liquid Reynolds number) for the gravitational film flow, exists at all velocities of the co-current gas flow. We carried out investigation of the “optimal” waves for several folders and presented comparison with the regimes observed in experiments. Using of the strict equations without any additional assumptions is an important feature of this paper.

Numerical investigation of surface wettability on gas – Liquid flow hydrodynamics in microchannel: Application to trickle bed reactors

Two-phase gas-liquid flows are very important in trickle bed catalytic reactors. To investigate the effect of catalyst wettability on the two-phase flow hydrodynamics in the bed, microchannels are considered in order to simulate the pore scale phenomena. Since the surface area to volume ratio is a key factor at the microscale, the role of surface interaction becomes dominant. In this work, the channel hydrophobicity is varied in order to study the effect of the wetting properties of a rectangular horizontal micro-channel on the gas-liquid two-phase flow behavior. Air and water are injected through a Y junction microchannel where water is injected in the middle and air from side inlets, to investigate the air/water two-phase flow behavior in the straight section. The surface wettability of two types of catalysts is experimentally measured and used as input to the numerical computational model. The microchannel is of 300 μm, 100 μm and 1 cm in width, height and length, respectively. The CFD simulation is done on COMSOL Multiphysics, using the Level Set method. The level-set technique is used to represent a discrete fluid interface between the two phases. Navier Stokes equations and conservation of mass, coupled to the level set method keep track of the interface between the air and water in the flow field, including the interfacial interactions with the microchannel walls. The developed numerical model is used to characterize the hydrodynamics of cocurrent gas—liquid flow through the microchannel. The model was validated against published experimental data and good agreements are obtained. The prediction results demonstrate that the wetting properties of the microchannel affect drastically the two-phase flow hydrodynamic behaviors. We identified eight different flow patterns in the hydrophobic microchannel, whereas, in the case of hydrophilic microchannel, only two flow patterns were observed. All of these results were in good agreement with published experimental ones. The developed numerical model was thus used to predict trickle bed flow hydrodynamics at the microscale.

External mass transfer in a laser sintered structured reactor for continuous hydrogenation of alkynes

This work presents a study on the continuous operation of a structured reactor for alkyne hydrogenation in the field of Process Intensification. The reactor consists of a laser sintered metal structure characterized by a regular geometry, coated with a layer of ZnO/Al2O3 and impregnated with palladium nanoparticles. The partial hydrogenation of 2-methyl-3-butyn-2-ol with co-current gas-liquid upward flow was used as the test reaction system. A plug flow reactor model was applied to study the mass transfer phenomena under the reacting conditions. The reaction kinetics with the Pd/ZnO-based catalyst were simplified using a power rate law expression. The results in terms of the overall mass transfer coefficient Kov were modelled with a predictive Sherwood number correlation whose parameters were estimated by means of an optimization procedure. The structured reactor shows an overall mass transfer coefficient ranging between 0.2 and 1.2 s−1 depending on the operating conditions. The model is able to predict the impact of temperature (333–363 K), pressure (3.0–7.0 bar), gas velocity (0.005–0.024 m s−1) and liquid velocity (0.025–0.085 m s−1) on the overall mass transfer coefficient with a maximum deviation of 15%.

Cross-flow structured packing for the process intensification of post-combustion carbon dioxide capture

We introduce novel insights into a cross-flow arrangement of structured packings specifically for post-combustion carbon dioxide capture. Gas-liquid dynamics are investigated numerically, with the liquid flowing under the action of the gravity and the gas driven by a horizontal pressure gradient crossing the liquid phase. An elementary packing cell consists of two connected channels: one depicting a co-current gas-liquid flow and the other depicting a counter-current two-phase flow. While flow reversal of the liquid phase can occur in the counter-flow channel at high gas flow rates, the overall flooding point is significantly delayed in comparison to a counter-current flow arrangement traditionally used for structured packings. Varying the gas flow rate and the tilting angle of the elementary cell, a detailed numerical analysis of the flow repartition between channels, the pressure drop, the gas and liquid velocities, and the onset of flooding is presented. The pressure drop is found to be smaller when tilting the cell with respect to the initial scenario at 45°. Flow reversal instead is delayed when lowering the tilting angle, that is when the cell is tilted anti-clockwise. We also reveal the presence of long waves at the edge of the cell at low tilting angles. Finally, data of the wet pressure drop in the cross-flow cell are compared with different commercially available types of packing arranged in a conventional vertical counter-flow configuration, such as several versions of the Sulzer Mellapak™.

Do huge waves exist in horizontal gas-liquid pipe flow?

Huge waves are periodic interfacial structures which are observed in vertical co-current gas-liquid two-phase flow under churn and the transition between churn and annular flows. Published data examining vertical gas-liquid flow indicate that a huge wave has either a continuous gas core surrounded by a large-scale interfacial wave or a core with a highly-agitated mixture of gas and liquid.Employing a Wire-Mesh Sensor (WMS), the spatio/temporal investigation of high flow rate horizontal air-water flow divulged some recurrent liquid structures (one may call pseudo-slugs) analogous to huge waves of (vertical) churn flow. In both cases, the blow-through (penetration of gas into the liquid structure) was the most manifest feature.Different qualitative and quantitative methods were employed to compare the behavior of pseudo-slug to churn flow. The quantitative measures included Probability Density Function analysis (PDF), distribution coefficient in drift flux model, structural velocity, core average velocity, interfacial friction factor, and slippage number. Both flow regimes demonstrated similar behavior.

Analysing Gas-Liquid Flow in PEM Electrolyser Micro-Channels

PEM water electrolysis is a key component for closing the loop of the renewable energy eco-system. These high response water electrolysers are particular suitable for fluctuating power sources. Conventional PEM water electrolysers are typically operated at a current density of around 1 A/cm2 and are fairly expensive. One means of increasing the hydrogen yield to cost ratio of such systems, is to increase the operating current density. However, at high current densities, management of heat transfer and fluid flow in the anode GDL and channel becomes crucial. This entails that further understanding of the gas-liquid flow in both the porous media and the channel is necessary for insuring proper oxygen, water and heat management of the electrolysis cell. In this work, the vertical upward gas-liquid flow pattern in a 0.5×1×94 mm micro-channel is both numerically and experimentally analysed. A sheet of titanium felt is used as a permeable wall for permeation of air to the column of water similar to the phenomenon encountered in Oxygen Evolution Reaction (OER). The transparent setup is operated in situ and the gas-liquid flow regimes are identified using a high-speed camera. The picture shows how the transparent cell is made which consists of a channel for the inlet air and a channel for the water-bubble flow. The transparent material is Plexiglas that is sealed with a sheet of silicon. The conventional co-current gas-liquid two-phase flow patterns, such as bubbly flow, slug flow and annular flow, are observed in the present micro-channel. The phenomenon is also analysed numerically in 3D, unsteady, euler/euler multiphase method using the commercial software ANSYS FLUENT 17 and the results show good agreement with the experimental data. Influence of each multiphase flow regime is described in the study as well as the recommendation for improving the performance. A well management of the multiphase flow regime along the whole micro-channel length can assure a proper distribution of water inside the titanium felt. Figure 1

Effect of gas-liquid flow pattern and microbial diversity analysis of a pilot-scale biotrickling filter for anoxic biogas desulfurization

Hydrogen sulfide removal from biogas was studied under anoxic conditions in a pilot-scale biotrickling filter operated under counter- and co-current gas-liquid flow patterns. The best performance was found under counter-current conditions (maximum elimination capacity of 140 gS m−3 h−1). Nevertheless, switching conditions between co- and counter-current flow lead to a favorable redistribution of biomass and elemental sulfur along the bed height. Moreover, elemental sulfur was oxidized to sulfate when the feeding biogas was disconnected and the supply of nitrate (electron acceptor) was maintained. Removal of elemental sulfur was important to prevent clogging in the packed bed and, thereby, to increase the lifespan of the packed bed between maintenance episodes. The larger elemental sulfur removal rate during shutdowns was 59.1 gS m−3 h−1. Tag-encoded FLX amplicon pyrosequencing was used to study the diversity of bacteria under co-current flow pattern with liquid recirculation and counter-current mode with a single-pass flow of the liquid phase. The main desulfurizing bacteria were Sedimenticola while significant role of heterotrophic, opportunistic species was envisaged. Remarkable differences between communities were found when a single-pass flow of industrial water was fed to the biotrickling filter.

Pore-network modeling of trickle bed reactors: Pressure drop analysis

A pore network model (PNM) has been developed to simulate gas–liquid trickle flows inside fixed beds of spherical particles. The geometry has been previously built from X-ray micro-tomography experiments, and the flow in the throats between pores is modeled as a pure viscous Poiseuille two-phase flow. The flow distribution between pores and throats is obtained by solving mass and momentum balance equations. As a first application of this simple but powerful meso-scale model, a focus is proposed on the ability of PNM to estimate pressure drop and liquid saturation in co-current gas–liquid flows. PNM results arecompared to the classical 1D pressure drop models of Attou et al. (1999), Holub et al. (1992) and Larachi et al. (1991). Agreement and discrepancies are discussed, and, finally, it has been found that the actual PNM approach produces realistic pressure drops as far as inertial contributions to friction are negligible. Concerning liquid saturation, the PNM only estimates its value in the throats between pores. As a consequence, liquid saturations are overestimated, but they can be easily corrected by an ad hoc empirical model.

Visualization and quantitative analysis of dispersive mixing by a helical static mixer in upward co-current gas–liquid flow

Abstract Growing interest towards process intensification in chemical industries over the last decade promotes static mixers as an attractive contactor alternative to the bubble column and mechanically stirred vessels. In the present work, the dispersion provided by a helical static mixer in a vertical pipe at turbulent gas–liquid flow conditions was studied using ultrafast electron beam X-ray tomography. Dispersive mixing was quantified upstream and downstream of three, six and nine helical static mixer elements in terms of bubble size distribution and gas holdup. Bubble breakup within the mixer elements was visualized and quantified via bubble size distribution and interfacial area density. Moreover, the power dissipation per unit mass of liquid was calculated for different number of static mixer elements and slip. The results provide a new insight into the development of the gas phase distribution within helical static mixers and a sound basis for design improvement, optimization of flow conditions and CFD validation.

Mass transfer and liquid hold-up determination in structured packing by CFD

Mass transfer and liquid hold-up in structured packing geometry are investigated using the volume of fluid method. Numerical simulations of two-dimensional co-current gas–liquid flow on structured packing with interfacial mass transfer are performed. The volume of fluid method is used to capture the gas–liquid interface motion. The mass transfer is computed by solving the concentration equation with an adapted modeling of the solubility (Haroun et al., 2010b). The liquid hold-up and the mass transfer are studied as function of liquid flow rate and structured packing geometry. Results show how the liquid flow rate and the complex geometry affect the liquid film flow topology and the interfacial mass transfer. For a specified packing geometry, it is demonstrated that for low liquid flow rate, the liquid film remains uniform and follow closely the profile of the structured wall. For uniform liquid film flow along packing wall, it is found that the liquid hold-up is in good agreement with the model proposed by Billet and Schultes (1999) and Raynal and Royon-Lebeaud (2007). When increasing the liquid flow rate, the liquid film does not follow the shape of the structured wall anymore, a static hold-up (recirculation zone) form in the cavities and grows as the Reynolds number increases until covering most of the packing cavities. The present work gives the liquid hold-up evolution for each liquid film flow regime according to the Reynolds number and the dimensionless amplitude of the corrugation. Concerning the liquid side mass transfer, it is found that the liquid side mass transfer is well predicted by the Higbie (1935) theory provided that adequate velocity and length scales are considered for exposure time determination. The exposure time of fluid element at the interface corresponds to the ratio between the curvilinear distance between two periodic corrugation contact point and the interface velocity. An exposure time model is proposed taking into the account physical and geometric parameters.

Appraisal of a horizontal two-phase flow photobioreactor for industrial production of delicate microalgae species

A closed tubular horizontal photobioreactor with the ability to produce industrially delicate microalgal species was assessed and validated. A co-current gas–liquid flow was chosen for mass transfer along 1,200 m piping which delimited a culture volume of 4,700 L. Mixing was obtained by means of hydrodynamic events accompanying this type of flow, including waves. The constant cross section of the piping allowed the circulation of spherical bodies to wipe the inner wall and avoid the establishment of a biofilm. Two similar reactors were constructed in December 2009 and December 2010, and operated to supply the dermo-beauty care industry with active biomass of three delicate species of microalgae, including the chlorophyte Neochloris oleoabundans and the rhodophyte Porphyridium cruentum. Daily observed production of dry biomass varied between 0.2 and 1.7 kg, depending on the species, the sun exposure and the period of the year. The ability of this production system to reduce the cost of producing algal biomass through scaling-up is discussed.

Flow regime transition for cocurrent gas–liquid flow in micro-channels

Experimental investigations of gas–liquid two-phase flow regimes in micro-channels were carried out in this work. Four distinctive flow patterns of slug flow, slug-annular flow, annular flow and parallel stratified flow were captured by a digital video recording (DVR) system and the transitions among different flow regimes were studied. The effect of fluid properties and wetting properties of the channel wall on the flow regime and flow pattern transition were studied as well. Novel empirical correlations for predicting flow pattern transitions during the steady gas–liquid flow in micro-channels have been developed with the fluid properties and the wetting properties of micro-channels incorporated, whereas the traditional flow criteria were based on gas and liquid superficial velocities only. The predictions of the present empirical models agree well with the experimental data.