Total Gas Holdup Research Articles

Novel Venturi injector reactor design and application in ammonia nitrogen wastewater treatment

The Venturi injector reactor has a robust and durable design and is highly efficient and reliable. The flow field control in the Venturi reactor is an effective way to enhance gas−liquid mass transfer. In this study, a novel Venturi injector reactor with an expandable self-priming air inlet and a second water inlet in the diffusion section was designed by combining particle image velocimetry (PIV) experiments with a computational fluid dynamics simulation. This novel design increased the gas holdup, generated smaller and more uniform bubbles, and enhanced mass transfer. Furthermore, we applied the novel Venturi device to the biological degradation of ammonia nitrogen wastewater. The total gas holdup in the novel Venturi was higher than in the conventional Venturi injector reactor by 0.0056 on average. The minimum bubble diameter was approximately 0.71 mm. Moreover, the ammonia nitrogen removal efficiency of the novel Venturi equipment with the second inlet was 2 and 1.1 times higher than that of conventional aeration and conventional Venturi device, respectively. This study provides a theoretical basis and practical significance for improving the performance and efficient degradation of the ammonia nitrogen concentration in Venturi injector reactors.

Investigation of Hydrodynamic Parameters in an Airlift Photobioreactor on CO2 Biofixation by Spirulina sp.

The rise of CO2 concentration on Earth is a major environmental problem that causes global warming. To solve this issue, carbon capture and sequestration technologies are becoming more and more popular. Among them, cyanobacteria can efficiently sequestrate CO2, which is an eco-friendly and cost-effective way of reducing carbon dioxide, and algal biomass can be harvested as valuable products. In this study, the hydrodynamic parameters of an airlift photobioreactor such as gas holdup, mean bubble diameter and liquid circulation velocity were measured to investigate CO2 biofixation by Spirulina sp. The total gas holdup was found to increase linearly with the increase in the gas velocity from 0.185 to 1.936 cm/s. The mean bubble velocities in distilled water only and in the cyanobacterial culture on the first and sixth days of cultivation were 109.97, 87.98, and 65.89 cm/s, respectively. It was found that shear stress at gas velocities greater than 0.857 cm/s led to cyanobacterial death. After 7 days of batch culture, the maximum dry cell weight reached 1.62 g/L at the gas velocity of 0.524 cm/s, whereas the highest carbon dioxide removal efficiency by Spirulina sp. was 55.48% at a gas velocity of 0.185 cm/s, demonstrating that hydrodynamic parameters applied in this study were suitable to grow Spirulina sp. in the airlift photobioreactor and remove CO2.

Small bubbles formation and contribution to the overall gas holdup in large diameter columns of very high viscosity oil

A limited number of studies are available in literature on the small bubbles which create from gas-very high viscosity oils interaction and its contribution to the gas holdup in the system. The rate of small bubble formation has an important impact on heat and mass transfer in many chemical and industrial processes. The work presented in the current paper provides unique information on the formation of bubbles of millimetre diameter in high viscosity oil. A column of 290 mm diameter and Silicon oil of 330 Pa.s viscosity, were employed besides Electrical Capacitance Tomography and a high-resolution camera to investigate the characteristics of the small bubbles. Mechanism of bubble generation, effect of gas injection time and flowrate were studied. The average void fraction, total gas-liquid height, overall Probability Density Function (PDF) profile, small bubbles volume fractions and diameter were measured. Small bubbles generate from the eruption of large bubbles, at gas injection nozzles, coalescence of large bubbles, and at liquid bridges at transition to churn flow regime. Properties and concentration of the small bubbles are controlled by the location of the bubble generation, gas flowrate, and gas injection time. Small bubbles contribute by 6.6- 30% to the total gas holdup. Bubble diameter increased from 0.68 mm to 0.75 mm and decreased from 1.1 mm to 0.75 mm at the top and the bottom sections of the column respectively after 60 minof gas injection.

Flow regimes characteristics of industrial-scale center-rising airlift reactor

• Multiphase fluids in industrial-scale CRALR exhibit unique flow characteristics. • Δ P - u g curves increase stepwise with gradually rising in distributor hole area. • Pressure accumulation and release in large distributor form gas-liquid oscillations. • Increase of the CRALR diameter induces the bubble stream to rotate in advance. • Limit of gas holdup is determined by equipment parameters. Airlift reactors are frequently preferred in biochemical engineering, and related investigations are almost performed using bench-scale reactors. However, in industrial-scale reactors, multiphase fluid usually exhibits unique mixing and transfer behaviors. Therefore, we studied the flow regimes characteristics in an industrial-scale center-rising airlift reactor (CRALR) with a diameter of 1.2 m. The results show that the available jet hole area of the gas distributor gradually increases along with the superficial gas velocity in the plateau range of the total pressure drop. Since the rise in initial liquid height enlarges the available jet hole area, the flow regime can be transformed from a pure heterogeneous regime to other regimes. Meanwhile, the total gas holdup plunges within the transition regime range, which can be interpreted by the gas–liquid oscillations in the gas distributor with a large free gas volume. When the oscillations cannot maintain the small bubble ratio, the rapid transition from a homogeneous regime to heterogeneous regime is affected. Compared with previous results, the bubble stream rotates in advance in the homogeneous regime of the industrial-scale CRALR. The rotational structure helps to reduce the flow resistance and improves the radial bubble distribution. Furthermore, the total gas holdup curves under various operating conditions converge into one curve at which a limit, determined by equipment parameters, is ultimately reached in the course of increasing the superficial gas velocity. It turns out that the fluids have similar hydrodynamic behaviors in flow fields of different scales when the limit is reached.

EFFECTS OF INITIAL LIQUID HEIGHT AND LIQUID VISCOSITY ON TOTAL GAS HOLDUP IN A BUBBLE COLUMN

EFFECTS OF INITIAL LIQUID HEIGHT AND LIQUID VISCOSITY ON TOTAL GAS HOLDUP IN A BUBBLE COLUMN

Effect of pressure on the hydrodynamics of a pilot‐scale bubble column operating with low and moderate viscosity Newtonian liquids

AbstractIn a pilot‐scale bubble column operating with low and moderate viscosity hydrocarbons, the effect of pressure on the total and axial gas holdup as well as the regime transition velocity was investigated. Experiments were performed for two air‐Ketrul D100 and air‐Hydroseal G250 HL gas‐liquid hydrocarbon systems. It was found that increasing pressure increased gas holdup at the heterogeneous regime, and this effect was more pronounced for the low‐viscosity liquid. Moreover, increasing pressure further stabilized the homogeneous regime, and again, this stabilizing effect was more significant for the low‐viscosity liquid. Also, as a response to the pressure increasing, the axial gas holdup became more uniform in the case of the low‐viscosity liquid and less uniform in the case of the moderate‐viscosity liquid.

Hydrodynamics and Mass Transfer in a Concentric Internal Jet-Loop Airlift Bioreactor Equipped with a Deflector

The gas–liquid hydrodynamics and mass transfer were studied in a concentric tube internal jet-loop airlift reactor with a conical bottom. Comparing with a standard design, the gas separator was equipped with an adjustable deflector placed above the riser. The effect of riser superficial gas velocity uSGR on the total gas holdup εGT, homogenization time tH, and overall volumetric liquid-phase mass transfer coefficient kLa was investigated in a laboratory bioreactor, of 300 mm in inner diameter, in a two-phase air–water system and three-phase air–water–PVC–particle system with the volumetric solid fraction of 1% for various deflector clearances. The airlift was operated in the range of riser superficial gas velocity from 0.011 to 0.045 m/s. For the gas–liquid system, when reducing the deflector clearance, the total gas holdup decreased, the homogenization time increased twice compared to the highest deflector clearance tested, and the overall volumetric mass transfer coefficient slightly increased by 10–17%. The presence of a solid phase shortened the homogenization time, especially for lower uSGR and deflector clearance, and reduced the mass transfer coefficient by 15–35%. Compared to the gas–liquid system, the noticeable effect of deflector clearance was found for the kLa coefficient, which was found approx. 20–29% higher for the lowest tested deflector clearance.

Investigation of a GL-EMMS gradual drag model by comparative simulations of bubble columns

The mesoscale model for gas–liquid flow based on the energy minimization multi-scale method, which is denoted by the GL-EMMS model, demonstrates its capability of predicting the regime transition in bubble columns. Since the dominant mechanisms and stability conditions are important for this mesoscale model, it is desirable to reveal more details through the computational fluid dynamics (CFD) work. This work proposes a gradual drag correlation based on the GL-EMMS model, and then incorporates it into the Euler–Euler (E–E) and Euler–Lagrange (E–L) frameworks respectively. Cases with different gas distributors are simulated via the opensource platform of OpenFOAM. A simplified meshing treatment of the inlet boundary is adopted and has proven effective to reducing the computing load. The E–E simulation predicts the total gas holdup and the radial distribution of liquid velocity well, but obvious deviation is observed in the radial distribution of gas holdup, especially at lower superficial gas velocities for the single-orifice aeration. Nonetheless, this shortage can be overcome saliently by the E–L method. These verified simulations enable further possible iterative investigation between the GL-EMMS model and the CFD work, and also inspire some meaningful simulating strategy for bubble columns.

CFD simulation of bubble column hydrodynamics with a novel drag model based on EMMS approach

Drag model is of critical importance for CFD simulation of flow characteristics in bubble columns. In this study, a novel drag model termed DBS-Local for the ratio of effective drag coefficient to bubble diameter is derived from the Dual-Bubble-Size (DBS) model based on Energy-Minimization Multi-Scale (EMMS). A thorough comparison demonstrates that without adjustable parameters, both the DBS-Local model and the DBS-Global model are capable of improving the prediction of total and local gas holdup as well as the flow characteristics in different flow regimes. By contrast, other drag models either overestimate or underestimate the total or local gas holdup in certain flow regimes. Moreover, the centerline liquid velocity predicted by DBS-Local model shows best agreement with experiments, whereas the other models, especially the Ishii-Zuber model, over-estimate liquid velocity. This study demonstrates the significance of modeling the bubble-swarm effect based on EMMS approach.

Characterizing regime transitions in a bubble column with internals

AbstractEffects of internals on flow regimes are systematically investigated in a lab‐scale bubble column (BC) through six approaches: total gas holdup, drift‐flux, standard deviation, fractal analysis, chaos analysis and wavelet analysis. With increasing internals‐covered cross‐sectional area (CSA), these methods give various value of the first transitional superficial gas velocity, as different scales of structures are detected to characterize regime transition. While internals are found to have marginal effect on total gas holdup, significant change in gas‐holdup structure is discovered, showing a decrease in large‐bubbles holdup and an increase in small‐bubbles holdup in transitional and churn‐turbulent regimes. We propose a novel approach based on wavelet analysis to demarcate the boundaries among micro‐, meso‐, and macro‐scales. The energy fraction of these representative scales identifies two transitional velocities, suggesting that internals advance the advent of first transition and delay the second one. The operating window of transitional regime is, therefore, remarkably extended.

Experiments and CFD simulation of mass transfer and hydrodynamics in a cylindrical bubble column

Flow visualization, Particle Image Velocimetry (PIV) and dissolved oxygen charging experiments are carried out in a small-scale cylindrical air-water bubble column at different superficial gas velocities. Void fraction profile, total gas holdup, liquid phase velocity field and volumetric mass transfer coefficient are obtained. Details of each experiment are given, with a new procedure for obtaining a 2D void fraction profile from flow visualization by image processing and time averaging which could provide useful information for CFD model validation. The experimental results are analyzed and compared with simulations where good agreement is found for both local and global flow patterns. The averaged governing equations and constitutive relations used in the simulation are discussed briefly, with emphasis on the proper comparison between simulations and experiments based on the derivation of the two-fluid model. By conducting three sets of experiments related to phase distribution, velocity field and mass transfer in the same apparatus, a validated CFD model for multiphase mass transfer is established and can be used in the future design of a multi-phase reactor.

Experimental analysis of gas phase dynamics in a lab scale bubble column operated with deionized water and NaOH solution under uniform bubbly flow conditions

In this study we present an investigation on the impact of sparger configuration and fluid properties on the local gas phase dynamics for up to 17% total gas holdup. Experiments in a bubble column with 100 mm inner diameter were conducted with deionized water and NaOH solution of up to cNaOH = 32 mmol·l−1. Ultrafast X-ray computed tomography (UFXCT) was applied to obtain local hydrodynamic parameters, such as gas holdup distribution, bubble sizes and average bubble rise velocities. Radial gas holdup profiles show wall peaking, which is attributed to the uniform bubbly flow regime generated by the needle sparger used in this study. The addition of NaOH leads to similar gas holdup values but significant changes on local bubbly dynamics, such as bubble size distribution and axial gas phase velocity. The Sauter mean diameter was found to decrease with increasing concentration of NaOH, whereby the decrease is larger at higher gas flow rate.

Numerical simulation of bubble characteristics in bubble columns with different liquid viscosities and surface tensions using a CFD-PBM coupled model

Most liquids in gas–liquid phase reactions are organic solvents; hence, it is particularly important to study the effect of liquid properties on fluid dynamic parameters in bubble columns. In this study, the correction of surface tension and viscosity were used to determine the influence of different liquid properties on the hydrodynamic parameters of bubble columns. These were implemented based on the minimum energy model proposed by Li (Li et al., 1999; Li and Kwauk, 2003; Xu and Li, 1998), our previous work (Zhang et al., 2018), the drag model of atmospheric pressure from Xiao et al. (2017) and that from Yan et al. (2019). According to previous studies (Syed et al., 2017), because liquid properties significantly influence the bubble size distribution in a bubble column, the density correction term and gas holdup of large bubbles were utilized to optimize the coalescence coefficient (Ce). Computational fluid dynamics coupled with the population balance model were used to simulate the effects of liquid viscosities and liquid surface tensions on the hydrodynamic parameters of the bubble column. The simulation results showed that when the liquid viscosity or surface tension increases, there is a higher probability of coalescence between two bubbles, resulting in the formation of a large bubble. The collision of large bubbles with turbulent eddies can increase the energy required to break up large bubbles into small ones; large bubbles in a bubble column are more stable, and the gas holdup of large bubbles increase. Moreover, the gas holdup of small bubbles, total gas holdup in the bubble column, and contact area between gas and liquid in the bubble column decrease, which leads to the decrease of the mass and heat transfers between the two phases.

A recirculation cell approach for hydrodynamic and mass transfer modeling in bubble columns with and without internals

An advanced recirculation cell model is proposed, which describes fluid dynamics and mass transfer in bubble columns with and without internals. The new model incorporates the cell approach of Shimizu et al. [Chem. Eng. J.2000, 78, 21–28.] with latest breakup and coalescence kernels. Additionally, the gas flow structure is divided according to the two-bubble class assignment with fast-rising large bubbles in the column center and descending small bubbles near the wall following the liquid circulation pattern within the column’s cross-section. The effect of internals is considered dividing the column further into ‘sub-columns’ derived from the internals’ radial profile, which physically refines the liquid circulation pattern. The model was validated with experimental data of Möller et al. [Chem. Eng. Sci.2018, 179, 265–283; Chem. Eng. Res. Des.2018, 134; Chem. Eng. Sci. 2019] for narrow (0.1 m diameter) and pilot-scale (0.39 m diameter) columns, respectively, with and without internals operated up to the well-developed churn-turbulent flow regime. Predictions for bubble size distribution, total gas holdup, Sauter mean diameter as well as interfacial area and volumetric mass transfer coefficients agree well with the experiments.

Hydrodynamics and simulation of air–water homogeneous bubble column under elevated pressure

AbstractA gas–liquid Eulerian computational fluid dynamics (CFD) model coupled with a population balance equation (PBE) was presented to investigate hydrodynamics of an air–water bubble column (1.8 m in height and 0.1 m in inner diameter) under elevated pressure in terms of pressure drop, gas holdup, mean bubble size, and bubble surface area. The CFD‐PBE model was modified with three pressure correction factors to predict both the total gas holdup and the mean bubble size in the homogeneous bubbly flow regime. The three correction factors were optimized compared to experimental data. Increasing the pressure led to increasing the density, reducing the bubble size, and increasing the gas holdup. The bubble size distribution moved toward a smaller bubble size, as the pressure increased. The modified CFD‐PBE model validated with experimental data and empirical models represented well hydrodynamics of the bubble column at P = 0.1, 1.5, and 3.5 MPa.

Numerical and experimental analyses of a stirred vessel for a large volumetric flow rate of sparged air

Abstract Computational fluid dynamics (CFD) and experimental analyses of some of the basic characteristics of air sparging in a tall stirred vessel equipped with a three-stage impeller are presented. The impeller was assembled from a radial ABT impeller as the lower, a turbine 6PBT45 as the middle and an axial Scaba-type 3SHP1 impeller as the upper. All the impellers were of the same diameter, i.e., 225 mm, while the vessel diameter was 450 mm. The impeller's rotational speed was 178 r·min−1. The aeration regime was established with an air volumetric flow rate of 28.3 m3·h−1. To the best of our knowledge, this study is the first to consider the very high gassing rate by means of CFD in a tank stirred by three-stage axial/radial impellers. The numerical simulation was performed using the ANSYS Fluent (R17.2, 2016) code for solving the governing equations of fluid dynamics in single- and multi-phase systems. While discussing the bubble size distribution, a discrete population balance model (PBM) was used. Adopting CFD, the stirring power and the total void fraction (the total gas holdup) were calculated. The results were in good agreement with the measured values using a laboratory experimental device.

Void fraction distribution in a bisectional bubble column reactor

This article presents the distribution of the local void fraction (α) in a mock‐up bisectional bubble column with a diameter of 0.63 m. Owing to the lack of such data in the literature, α was investigated experimentally using a microresistivity (R) probe and a phase discrimination procedure based on the probe signal. The two‐phase mixture that consisted of air and tap water was measured at 342 nodes in the vertical half‐section plane of the column. Relatively small differences between the volume‐integrated local values of α and the measured total gas holdups showed reasonably good agreement under all conditions. Experimental data were used for validation of a bubble column numerical model for a low hydrodynamic regime with commercial computational fluid dynamics (CFD) code. The difference between the CFD calculated total gas holdup and the experimentally measured mean value is 8.8%, while some differences in the local void fraction distributions were found in the lower part of the column. © 2019 American Institute of Chemical Engineers AIChE J, 65: 1186–1197, 2019

Two-Bubble Class Approach Based on Measured Bubble Size Distribution for Bubble Columns with and without Internals

The complex flow patterns in bubble columns can be phenomenologically described by the two-bubble class approach. For the first time, this approach is applied to bubble columns with dense internals. Internals of square and triangular pitch tube patterns of two tube sizes (8 × 10–3 and 13 × 10–3 m) with flat and U-tube bottom design and cross-sectional occupation of ∼25% were inserted in a bubble column of 0.1 m diameter and 2 m height. Contrary to the well-known gas disengagement technique, dual-plane ultrafast X-ray computed tomography data have been used for the bubble class allocation. Experiments were performed at superficial gas velocities ranging from 0.02 to 0.20 m s–1 to cover homogeneous and heterogeneous flow conditions. The contributions of small and large bubble classes on total gas holdup, flow structure and bubble rise velocities were determined. Furthermore, the regime transition onset was determined based on the two-bubble class approach. Eventually, new correlations for regime transition,...

Modeling of co-current and counter-current bubble columns with an extended EMMS approach

Bubble columns can be operated in semi-batch, co-current or counter-current mode. This study attempts to extend the dual-bubble-size (DBS) model, viz., the energy-minimization multi-scale (EMMS) model for gas–liquid systems, to co-current and counter-current bubble columns. With the increase of superficial liquid velocity, the predicted total gas holdup and regime transition are in accordance with experiments. A DBS drag model was then developed and incorporated into the CFD simulation. The total gas holdup and local gas holdup radial distribution in both the co-current and counter-current bubble columns are well estimated. The findings in the present work demonstrate that developing closure laws through the stability condition is of great significance in modeling complicated multi-scale flow in bubble columns.

A CFD based empirical model for assessing gas holdup in bubble columns

ABSTRACTBubble columns are widely used in many industrial applications. Gas holdup knowledge is an essential element in bubble column design and process optimization. Despite the simple structure of bubble columns, the behaviour of the bubbly flow is still not well understood owing to the complexity of gas–liquid interactions and coupling between the phases. Variation of gas density and initial liquid height are the most likely reasons for gas holdup changes in bubble columns. These parameters affect bubble breakup and coalescence as well as the rise velocity of small bubbles, which causes change to bubble size distribution that finally affect the gas holdup. In this work, through the application of computational fluid dynamics (CFD) and by investigating the effect of gas density and the initial liquid height on the bubbly flow condition, a new correlation for deducing the total gas holdup is developed. To achieve this, a wide range of bubbly flows are modelled, and gas holdup values are determined for different heights and operating pressures. Ranges of the normalized pressure (P/P0), superficial Reynolds number (Resup), and the aspect ratio of the bubble column (H0/D) were 1 ≤ P/P0 ≤ 15, 3000 ≤ Resup ≤ 57000, and1.5 ≤ H0/D ≤ 8, respectively. The proposed correlation has been found to predict the experimental as well as CFD gas holdup data fairly well. The results of this research provide a fast and accurate method for predicting gas holdup in bubble columns that work either in atmospheric or high‐pressure conditions.