Increase Of Gas Superficial Velocity Research Articles

Gas-liquid mass transfer in microbubble column reactors

Microbubble column reactors (MBCRs) present great potential in gas-liquid reactions due to their superior mass transfer efficiency. In this work, an automated platform was built to investigate the effect of gas/liquid superficial velocity, and liquid viscosity on mass transfer performance. Compared to conventional bubble column reactors, MBCRs exhibit significantly enhanced mass transfer efficiency. The findings show that kLa initially increases, followed by a subsequent decrease with the increase of gas superficial velocity. This behavior is attributed to transitioning from a homogeneous to a heterogeneous regime as gas superficial velocity increases. The liquid viscosity has a dual effect on kLa within a homogeneous regime. However, the kL in MBCRs is smaller than that in conventional bubble column reactors due to the movement of large bubbles enhancing liquid turbulence. Moreover, a dimensionless correlation was developed integrating the Weber number to predict the mass transfer coefficient.

Understanding two‐phase flow behaviour: CFD Assessment of silicone oil–air and water–air in an intermediate vertical pipe

AbstractThe study utilized computational fluid dynamics (CFD) simulations employing the volume of fluid (VOF) model to analyze silicone oil–air flows in a vertical pipe with a diameter of 67 mm. Various structured meshes ensured grid independence, and the k‐ε realizable model addressed turbulence. The analysis characterized bubbly to annular flows, involving the evaluation of flow patterns, radial void fractions, void fraction time series, probability density functions (PDFs), power spectral densities (PSDs), and mean void fractions. Results indicated a transition from bubbly to annular flow with increasing gas velocities and notable changes in radial void fraction profiles. Void fraction exhibited significant variations with distinct flow patterns at constant liquid velocity. PDFs identified flow regimes, and PSDs revealed frequency patterns. CFD results were validated against experiments, demonstrating good agreement. The validated CFD model was utilized to investigate radial gas velocities and pressure drops, revealing a shift from uniform velocity distributions to irregular patterns and a decrease in total pressure drop with an increase in gas superficial velocity. The model was also applied to a water‐air system to explore two‐phase flow behaviour. The impact of superficial gas velocity on flow patterns and radial void fractions was studied through numerical analysis and compared with the silicone oil‐air system. Results showed that at extreme gas velocities (0.06 and 5.53 m/s), silicone oil exhibited bubbly and annular flows, while water displayed cap bubbly and churn flows. The significant variation in radial void fraction at these velocities emphasized the impact of fluid properties.

Hydrodynamics and model of gas‐liquid flow in microbubble column reactors

AbstractMicrobubble column reactors (MBCRs) have the significant advantages of high gas holdup, large gas‐liquid interfacial area, and excellent mass transfer performance, making them attractive for applications such as hydrogenation and oxidation. However, there is still a lack of comprehensive understanding of the gas‐liquid flow behavior in MBCRs. In this work, the effects of gas and liquid superficial velocities on flow regimes, residence time distribution, and gas holdup are investigated. The gas holdup increases linearly with the increase of gas superficial velocity, even in heterogeneous regimes, ranging from 0% to 60% at gas superficial velocities of 0–25 mm/s. Additionally, the gas‐liquid interfacial areas in MBCRs are 1–2 orders of magnitude larger than those in traditional bubble column reactors. A prediction model for gas holdup is established, and the predicted values are in good agreement with the experimental values.

Experimental study of wavy-annular flow in a rectangular microchannel using LIF method

This article aims at studying gas-liquid flow in a rectangular microchannel with a high aspect ratio (200 × 2045 μm). Liquid and gas phases were 95% ethanol and nitrogen mixture. Experimental flow characteristics are obtained using high-speed visualization and laser-induced fluorescence (LIF) methods. Using the LIF method for wavy-annular flow, the average film thickness, liquid film distribution, and liquid film width were measured. The dependences of the liquid film width and the average film thickness on gas superficial velocity are presented in graphical form and analyzed. An increase in gas superficial velocity causes growth of the liquid film width and thickness of the liquid film, which indicates the process of liquid transfer from the menisci area to the liquid film. For different liquid velocities and the same gas superficial velocities, close values of averaged liquid film thickness were observed for flow with 2D waves and 3D waves on liquid film.

Study of interface gas-liquid flow characteristics in a rectangular microchannel for wavy-annular flow

The aim of this work is an experimental study of a gas-liquid flow in a rectangular slit microchannel with a cross-section of 200 × 2045 μm. Ethanol/water (95/5) mixture and nitrogen are used as working liquid and gas, accordingly. The external T-mixer is used for obtaining of wavy-annular flow pattern. The experimental data on interfacial waves and their characteristics in the meniscus area on the short side of the microchannel are obtained using high-speed visualization for a wide range of gas and liquid superficial velocities. Images are processed using the Python libraries to define the average liquid layer thickness and maximum amplitude of waves. An increase of gas superficial velocity causes decreasing in the average liquid layer thickness and maximal amplitude of the liquid layer thickness. The waves on the liquid layer surface (maximal amplitude) can be three times larger than the average liquid layer thickness for presented liquid and gas velocities. With increasing gas superficial velocities more liquid displace from the meniscus area to the liquid film on the wide side of the microchannel.

Investigating the effect of pressure on a vertical two‐phase upward flow with a high viscosity liquid

AbstractThis article presents void fraction and pressure gradient data for sulfur hexafluoride (SF6) with gas densities of 28 and 45 kg/m3 and oil (with viscosity 35 times that for water) in a 127 mm diameter pipe. The superficial velocities of gas ranged from 0.1 to 3 m/s and those for liquid from 0.1 to 1 m/s, respectively. Measurements of void fraction data were recorded using a capacitance wire mesh sensor (WMS) system, which permits the 3D visualization of the flow patterns. All the data were obtained with a data acquisition frequency of 1,000 Hz. A differential pressure transducer was used to measure the pressure drops along the length of the pipe. The WMS provide time and cross‐sectionally resolved data on void fraction and from an analysis of its output, flow patterns were identified using the characteristic signatures of probability density function (PDF) plot of time series of void fraction. The PDF plots showed the single peak shapes associated with bubbly and churn flows but not the twin‐peaked shape usually seen in slug flows. This confirms previous work in larger diameter pipes but with less viscous liquids. For the bubble and churn flows investigated, the pressure gradient was observed to decrease with an increase in gas superficial velocity. Nevertheless, there was an insignificant observed effect of pressure on void fraction below certain transitional flow rates, the effect however became significant beyond these values. In the present work, wisps appear to be smaller, which might be due to the different fluid properties of the working fluids employed. In addition, wisps are easily revealed as long as there is a transition between churn and annular flows regardless of the pressure. Experimental data on void fraction and pressure gradient are compared against existing data. Reasonably good agreements were observed from the results of the comparison.

On slug frequency in concurrent high viscosity liquid and gas flow

Slug frequency is an important input parameter in most mechanistic and semi-mechanistic models used for estimating slug flow characteristics such as pressure gradient, slug length and slug holdup. It is also essential in the design and sizing of transport and/or production pipelines, process flow control, design and operation of process equipment, etc. This study experimentally investigates slug frequency in two horizontal pipelines with internal diameters (ID) of 0.0254 and 0.0764 m. Test liquids used in the study were mineral oil (with viscosity ranging from 1.0 to 4.0 Poise) and water while compressed air is the gas phase. Effects of liquid viscosity, gas superficial velocity, liquid superficial velocity, pipe internal diameter, pipe length and pressure gradient are discussed. Results indicate that slug frequency increased with increase in oil viscosity; decreased with increase in pipe diameter; and reduces along the length of the pipe. For gas Reynolds number, ReSG≤2500, slug frequency increased with increase in gas superficial velocity, when ReSG>2500, slug frequency decreased with increase in gas superficial velocity. A new slug frequency correlation that accounts for liquid viscosity and gas superficial velocity, hitherto not included in other correlations was proposed. A comparative analysis of the proposed models and existing slug frequency correlation on a high viscosity databank showed that the proposed correlation gave the best prediction with an average absolute percentage error (AAPE) of 19.91%.

Hydrodynamic study of two phase flow of column flotation using electrical resistance tomography and pressure probe techniques

Gas dispersion characteristics are very important to design and selection of flotation process for the fine particle separation. Although the separation of flotation process heavily depends on hydrophobicity of given solid particles, but their rate of attachment and detachment depends on local gas dispersion characteristics. In this work, gas dispersion characteristics in a 100 mm laboratory column flotation have been investigated by exploring the flow behavior in terms of the local and mean gas hold-up, bubble rise velocity and bubble size distribution across the column. This experimental data has been used to demonstrate the validation of the two-fluid CFD model predictions in the column flotation. The column hydrodynamics are estimated by using high speed dual planar Electrical Resistance Tomography (ERT) system and pressure transducers. In all the experiments, water is used as the liquid phase and air (bubbles) as the disperse phase. Using the ERT system, measurement of two phase distributions are examined for a wide range of design and operating conditions of the column including different spargers and frother dosage, where the flow changes from homogenous to transition bubbly flow. It is confirmed by ERT that the gas-holdup increases with an increase in the sparger porosity, air superficial velocity, liquid height and liquid feed flow rate. To test the reliability of ERT measurements, simultaneously a set of pressure transducers is used to obtain the sectional average gas holdup and validated. It is observed that bubble density occupancy is more at center region of the column and gas holdup value increases with the increase of gas superficial velocity. Dynamic gas disengagement technique is utilized to measure bubble rise velocity and sauter mean bubble diameter. At end this ERT data has been used to validate two-fluid CFD model (modified with suitable drag and lift forces via user defined functions) predictions in the column and found a close agreement.

Experimental study of gas–solid overall heat transfer coefficient in vertical tank for sinter waste heat recovery

This work experimentally investigates the gas–solid heat transfer behavior in vertical tank for sinter waste heat recovery. With the purpose of predicting the heat transfer Nusselt number, a homemade experimental setup is built to obtain the temperatures of cooling air and sinter particles over time. Sinter particles of average diameter d = 18, 27 and 36 mm with a range of sphericity of 0.68 ≤ Φ ≤ 0.89 are used in the experiments. The experiments are performed in the range of gas superficial velocity of 1.04~1.75 m/s and the sinter temperature in bed layer from 100 to 750 °C. The influences of gas superficial velocity and sinter particle diameter on the gas–solid overall heat transfer coefficient are analyzed in detail for the covered test cases. The heat transfer Nusselt number is obtained according to the regression analysis of experimental data, and the reliability of the heat transfer Nusselt number is also verified. It is found that the gas–solid overall heat transfer coefficient increases with the increase of gas superficial velocity, and decreases with increasing sinter particle diameter. A small increase in gas–solid overall heat transfer coefficient with the temperature of sinter particles is also observed in the experiments. Compared with the previous literature correlations, the experimental correlation for gas–solid heat transfer in the form of Nusselt number best fits the experimental data. The mean deviation between the experimental data of Nusselt number obtained from this work and the values calculated by the experimental heat transfer correlation is 4.37%, showing good prediction.

Heat transfer between a packed bed and a larger immersed spherical particle

In this paper, heat transfer coefficients between the hot packed beds of particles and the larger cold immersed sphere were experimentally determined. The packed beds consisted of mono-sized spherical glass particles of dp=1.2, 1.94 and 2.98mm. The aluminum test spheres of Dp=6, 12 and 20mm with K-type (Ni/Al) thermocouples inserted in them were immersed into the bed. The temperatures of the test spheres were recorded until the thermal equilibrium was reached. From these recordings, heat transfer coefficients were determined. The experiments were performed in the range of gas superficial velocity of ∼0.3–0.8m/s and the bed temperature from 90 to 320°C. It was found that the measured heat transfer coefficients increased with the increase in gas superficial velocity, while only a slight increase with temperature was observed for test spheres Dp=6 and 12mm in the investigated temperature interval. The heat transfer coefficients were generally larger for smaller test spheres, while they did not show significant dependence on the size of the bed particles.The literature correlations for heat transfer coefficients in packed beds in form of Nusselt number and heat transfer factor jH were compared to experimental data from this work. The correlations proposed by Collier et al. (2004) [18] and Handley and Heggs (1968) [2] fit our experimental data best. The mean error between the experimentally determined heat transfer coefficients and the ones calculated from Collier et al. (2004) [18] correlation was 12.7%.

Experimental study of the hydrodynamic behaviour of slug flow in a vertical riser

This paper presents an investigation of the hydrodynamics of slug flow in a vertical 67mm internal diameter riser. The slug flow regime was generated using a multiphase air–silicone oil mixture over a range of gas (0.42<USG<1.35m/s) and liquid (0.05<USL<0.38m/s) superficial velocities. Electrical capacitance tomography (ECT) was used to determine: the velocities of the Taylor bubbles and liquid slugs, the slug frequencies, the lengths of Taylor bubbles and the liquid slugs, the void fractions within the Taylor bubbles and liquid slugs and the liquid film thicknesses. A differential pressure transducer was used to measure the pressure drops along the length of the riser. It was found that the translational velocity of a Taylor bubble (the structure velocity) was strongly dependent on the mixture superficial velocity. As the gas superficial velocity, was increased, the void fraction and the lengths of the liquid slugs and the Taylor bubbles were observed to increase. The increase in gas superficial velocity causes an increase in the frictional pressure drop within the pipe, whilst the total pressure drop (which is a sum of the hydrostatic and frictional pressure drop) along the length of the riser decreases. In addition, the frequencies of the liquid slugs were observed to increase as the liquid superficial velocity increases, but to be weakly dependant on the gas superficial velocity. The manual counting method for the determination of slug frequency was found to be in good agreement with the power spectral density (PSD) computed method.

A Fractal Model for the Maximum Droplet Diameter in Gas-Liquid Mist Flow

Distribution characteristics of liquid droplet size are described using the fractal theory for liquid droplet size distribution in gas-liquid mist flow. Thereby, the fractal expression of the maximum droplet diameter is derived. The fractal model for maximum droplet diameter is obtained based on the internal relationship between maximum droplet diameter and the droplet fractal dimension, which is obtained by analyzing the balance between total droplet surface energy and total gas turbulent kinetic energy. Fractal model predictions of maximum droplet diameter agree with the experimental data. Maximum droplet diameter and droplet fractal dimension are both found to be related to the superficial velocity of gas and liquid. Maximum droplet diameter decreases with an increase in gas superficial velocity but increases with an increase in liquid superficial velocity. Droplet fractal dimension increases with an increase in gas superficial velocity but decreases with an increase in liquid superficial velocity. These are all consistent with the physical facts.

Fluidization characteristics of a mixture of gasifier solid residues, switchgrass and inert material

Abstract Effective fluidization of materials present in the reactor bed is critical for optimizing reaction conditions in a fluidized-bed gasifier. An improper fluidization leads to inefficient conversion due to many reasons such as low heat and mass transfers, ineffective gas–solid phase reactions, and uneven reactor temperature in autothermal gasification. The objective of this study was to investigate effect of reactor bed composition, i.e. a mixture of gasifier solid residues (GSR), switchgrass, and inert material, on fluidization using a 0.25 m i.d. transparent column. In this cold-flow study, the amount of inert material, i.e. silica sand, in the bed was held at 20 kg. The switchgrass in the mixture ranged from 0.17 to 5% of the sand quantity while the GSR ranged from 5 to 35% of the switchgrass. The particle geometric sizes by mass of sand, GSR and switchgrass were 348 ± 1.6 μm, 80 ± 2.6 μm, and 10.3 ± 1.7 mm, respectively. For all conditions, with an increase in gas superficial velocity, i.e. ratio of volumetric gas flow and bed cross-sectional area, the pressure drop across the bed increased reaching a maximum level at the minimum fluidization condition. Results showed that when the bed consisted of only GSR and sand, with an increase in the GSR from 5% to 35%, the gas superficial velocity at minimum fluidization condition, called minimum fluidization velocity ( U mf ), decreased significantly ( p mf ) remained constant. When the bed consisted of GSR, switchgrass and sand, there were significant effects ( p U mf and dP mf . Fluidization improved with an increase in GSR up to 35% in the mixture. Overall, both U mf and dP mf increased with an increase in levels of GSR (5 to 35%) and switchgrass (0.17 to 3%) in the mixture. Fluidization characteristics were found to be strongly dependent upon mixture's effective properties, which were determined using properties of all mixture components. Correlations available in literature were used to predict U mf using effective properties of tertiary mixture with GSR, switchgrass and sand. Prediction of U mf from all selected correlations did not match well with the experimental data for the entire range of tertiary mixture compositions. Fluidization of bed materials sustained up to 3% level of switchgrass. However, segregation of bed materials and in-bed channelization caused ineffective fluidization at 5% level of switchgrass in the mixture.

Characteristics of the Interfacial Wave Velocity on the Liquid Film of Annular Flow for Gas-Oil-Water Three-Phase Flow in a Horizontal Pipe

The experiments were conducted in a horizontal multiphase flow test loop (50mm inner diameter, 40m long) and the cross-correlation technology was used for the study of the characteristics of the interfacial wave velocity about two types of annular flow regimes (AN║DO/W and AN║DW/O) for gas-oil-water three-phase flow. The results show that the interfacial wave velocity on the liquid film of AN║DO/W flow pattern and AN║DW/O flow pattern all increases with the increase of gas superficial velocity and liquid superficial velocity on the condition of fixed ratio of oil and water flow rates, but the difference is that the increase is a linear monotonic increase for AN║DO/W flow pattern and a non-linear increase for AN║DW/O flow pattern, and the liquid superficial velocity makes a larger contribution than the gas superficial velocity. The interfacial wave velocity also increases with the increase of input water cut in liquid at different gas superficial velocities under the conditions of liquid superficial velocity fixed.

Korelasi Empiris Perpindahan Kalor Dua Fase (Air Udara) Aliran Kantung Gas dalam Pipa Horisontal Sirkular

This research aimed at determining the heat transfer correlation of a heated non-boiling two phase flow consisting of air and water in a horizontal round pipe. The test section was a 24 mm ID cooper pipe with a length 1 m, and the heat transfer data a uniform wall heat flux boundary condition ranging from about 5968,17 to 10503,97 W/m2. From this research, it can be concluded that heat transfer coefficient of two-phase flow of bubble flow is increase as increase of gas superficial velocity and liquid superfisial velocity. The comparison deviation standart between heat transfer coefficient of two-phase flow experimental and heat transfer coefficient of two-phase flow correlation that proposed is 34%, but this deviation is still in limited value for experimental research.

Counter-current gas−liquid flow between vertical corrugated plates

The paper is devoted to a theoretical analysis of the counter-current gas−liquid film flow between vertical corrugated plates. We use the Navier−Stokes equations in their full statement to describe the liquid phase hydrodynamics. For the gas phase equations, we use the Benjamin−Miles approach where the wavy liquid/gas interface is a small disturbance for the turbulent gas and where we can linearize the gas phase governing equations. We consider both the steady state and the two-periodical traveling solutions of the counter-current gas/liquid flow between the corrugated plates. The changes in the liquid film hydrodynamics with the increase in gas superficial velocity are the main interest of the investigation. What is the flooding mechanism in the case of flow between the corrugated plates and does the gas superficial velocity for the flooding depend on the wall corrugation parameters?

Characteristics of Pressure Gradient Fluctuation for Oil-Gas-Water Three-Phase Flow Based on Flow Pattern

Experiments were conducted in a horizontal multiphase flow test loop (50mm inner diameter, 40m long) to study the flow patterns for oil-gas-water three-phase flow and the pressure gradient fluctuation based on flow patterns. Using new methods of definition, 12 types of flow patterns were obtained and phase distribution characteristics of each pattern were analyzed. A new flow pattern (SW║IN) was firstly found in this work. Characteristics of the pressure gradient based on 7 flow patterns were carefully discussed. It was found that the pressure gradient increased with the increase of gas superficial velocity and oil-water mixture velocity. However, characteristics of the pressure gradient became complex with the increase of input water cut. The influence of flow structure of oil-water two-phase should be fully considered.

The effect of flow pattern on split of two-phase flow through a micro-T-junction

An experiment has been carried out to study the phase split of water–nitrogen two-phase flow through a horizontal T-junction with a square cross section (500 μm × 500 μm), focusing on the effect of flow pattern. By comparing the results of slug, slug–annular and annular flows, it is shown that phase split characteristic of micro-T-junction highly depends on inlet flow pattern. When the inlet flow is a slug flow, it takes on gas rich in side branch. But when the inlet flow shifts to an annular flow, the side branch is rich in liquid. For an slug–annular flow, the curves show transitional characteristics. The effect of superficial velocities on the phase split of each flow pattern is considered as following: the liquid taken off decreases with an increase of liquid superficial velocity and increases with an increase of gas superficial velocity. The results have been compared to that of a 500 μm T-junction with a circular cross section. It is found that the square T-junction shows more liquid taken off at slug flow and slug–annular flow. While, at annular flow, the liquid taken off increases slower at high gas taken off in square T-junction due to cross section effect.

Flow Pattern and Pressure Fluctuation of Severe Slugging in Pipeline-riser System

During the exploitation of offshore oil and gas, it is easy to form severe slugging which can cause great harm in the riser connecting wellheads and offshore platform preprocessing system. The flow pattern and pressure fluctuation of severe slugging were studied in an experimental simulation system with inner diameter of 0.051 m. It is found that severe slugging can be divided into three severe slugging regimes: regime I at low gas and liquid flow rates with large pressure fluctuation, intermittent flow of liquid and gas in the riser, and apparent cutoff of liquid phase, regime II at high gas flow rate with non-periodic fluctuation and discontinuous liquid outflow and no gas cutoff, regime III at high liquid flow rate with degenerative pressure fluctuation in form of relatively stable bubbly or plug flow. The results indicate that severe slugging still occurs when the declination angle of pipeline is 0°, and there are mainly two kinds of regimes: regime I and regime II. As the angle increases, the formation ranges of regime I and regime III increase slightly while that of regime II is not affected. With the increase of gas superficial velocity and liquid superficial velocity, the pressure fluctuation at the bottom of riser increases initially and then decreases. The maximum value of pressure fluctuation occurs at the transition boundary of regimes I and II.

Phase splitting of a slug–annular flow at a horizontal micro-T-junction

The behavior of air–water two-phase flow at a horizontal micro-T-junction with main and side branch of 500 μm diameter was studied in this work. When the inlet flow pattern was a slug–annular flow, the impact of varying of upstream gas and liquid superficial velocity on phase distribution of two-phase flow in the T-junction was investigated in detail. It is found that the liquid taken off decreases as an increase in liquid superficial velocity, while it increases as an increase in gas superficial velocity. Liquid and gas superficial velocity have stronger impact on phase distribution when the inlet gas velocity is high. When the present data are compared to those from larger diameter T-junctions at similar inlet superficial velocities, it is found that decreasing the diameter of T-junctions increases the fraction of liquid taken off.