Laboratory Scale Bubble Column Research Articles

Effect of temperature on the oxygen production capacity and growth of scenedesmus almeriensis

The optimal temperature and irradiance to maximise oxygen production were 39.3 °C and 512.5 μmol photons·m−2·s−1, respectively. These values were obtained by photorespirometry, which is a quick method to measure the photosynthetic and respiration rates of microalgae at a laboratory scale. With these conditions, the global oxygen production rate of S. almeriensis was 246.23 mgoxygen·gbiomass−1·h−1. When the culture temperature was controlled at 39.3 °C for 1 h per day, the daily oxygen production capacity of S. almeriensis increased from 3129.5 mgoxygen·gbiomass−1 to 3778.5 mgoxygen·gbiomass−1. However, keeping the temperature at 39.3 °C for a longer time caused a damage to the photosynthetic apparatus. This was validated using laboratory-scale bubble columns. The damage was reversible when heating the cells for <2 h, but keeping the temperature of the culture at 39.3 °C for 3 h led to an irreversible damage and a 6 % decrease in the photosynthetic performance. Controlling the overheating of microalgal cultures is crucial to maximise growth. In addition, the duration of the exposure to high temperatures should also be included into growth and taken in consideration.

Experimental Study of the Conversion of Polyethylene and Polypropylene to Non-Condensable Gases Using a Lab-Scale Bubble Column Reactor with Molten Metal Catalysis

Experimental Study of the Conversion of Polyethylene and Polypropylene to Non-Condensable Gases Using a Lab-Scale Bubble Column Reactor with Molten Metal Catalysis

Statistical Analysis of Bubble Parameters from a Model Bubble Column with and without Counter-Current Flow

Bubble columns are widely used in numerous industrial processes because of their advantages in operation, design, and maintenance compared to other multiphase reactor types. In contrast to their simple design, the generated flow conditions inside a bubble column reactor are quite complex, especially in continuous mode with counter-current liquid flow. For the design and optimization of such reactors, precise numerical simulations and modelling are needed. These simulations and models have to be validated with experimental data. For this reason, experiments were carried out in a laboratory-scale bubble column using shadow imaging and particle image velocimetry (PIV) techniques with and without counter-current liquid flow. In the experiments, two types of gases—relatively poorly soluble air and well-soluble CO2—were used and the bubbles were generated with three different capillary diameters. With changing gas and liquid flow rates, overall, 108 different flow conditions were investigated. In addition to the liquid flow fields captured by PIV, shadow imaging data were also statistically evaluated in the measurement volume and bubble parameters such as bubble diameter, velocity, aspect ratio, bubble motion direction, and inclination. The bubble slip velocity was calculated from the measured liquid and bubble velocities. The analysis of these parameters shows that the counter-current liquid flow has a noticeable influence on the bubble parameters, especially on the bubble velocity and motion direction. In the case of CO2 bubbles, remarkable bubble shrinkage was observed with counter-current liquid flow due to the enhanced mass transfer. The results obtained for bubble aspect ratio are compared to known correlations from the literature. The comprehensive and extensive bubble data obtained in this study will now be used as a source for the development of correlations needed in the validation of numerical simulations and models. The data are available from the authors on request.

Bubble dynamics under the influence of the Marangoni force induced by a stratified field of contamination

The Marangoni effect assumes significance in bubbly flows when temperature or concentration gradients exist in the domain. This study investigated the hydrodynamics of single bubbles under the influence of the Marangoni force induced by stratified fields of dissolved sugar, providing a numerical framework for examining these phenomena. A laboratory-scale bubble column and high-speed imaging were utilized to analyze the bubble behavior. The OpenFOAM-based geometric volume of the fluid solver was extended by incorporating the solutocapillary Marangoni effect, and a passive scalar transport equation for the sugar concentration was solved. The results revealed that small bubbles entering regions with elevated sugar concentrations experienced deceleration, transitioning into linear paths, while those departing from regions with high sugar concentrations exhibited fluctuations and meandering. Furthermore, the concentration gradient leads larger bubbles to meander throughout the entire column, without a notable increase in their velocity. The intensity of these behaviors is governed by the magnitude of the Marangoni force. The findings provide a better understanding of single bubble hydrodynamics in complex environments.

Determination of absorption mass transfer coefficients of methane into drilling fluid base oils at elevated pressures

Absorption mass transfer coefficients (kla) of methane into two widely used drilling fluids base oils-internal olefins and diesel were determined at elevated pressure. A laboratory-scale bubble column (0.0254 m diameter and 0.71 m static liquid height) was used to perform the experiments. Ranges of superficial gas velocity (0.0045–0.0491 m/s) and system pressure (0.69–2.07 MPa) were covered. Based on the results, the effects of the varying parameters on kla have been discussed. Based on the observations, an empirical model was developed for the volumetric mass transfer coefficient as a function of superficial gas velocity and density. This correlation fitted well (R2 > 0.98) with the measured kLa values for the investigated range of operating conditions. This study gives new quantitative insights into the absorption kinetics of methane into two drilling fluid base oils, which are important for improving gas influx management with non-aqueous drilling fluids.

Improving gas dispersion and decreasing bubble size in saline water with oscillatory air supply

The present paper describes gas dispersion in an air-sparged laboratory scale bubble column containing aqueous solutions of single electrolytes (NaCl, CaCl2, MgCl2, MgSO4, and AlCl3) and mixed electrolytes (NaCl-CaCl2 and NaCl-CaCl2-MgCl2-AlCl3) at different ionic strengths and air flow rates, with oscillatory air supply and conventional steady air supply, respectively. The gas dispersion was investigated by a light-scattering technique (turbidity measurement) and bubble size measurement. The oscillatory air supply was generated from steady air flow using a fast-switching solenoid valve and was characterized by measuring the pressure of the output air of the solenoid valve as a function of time. It was found that for each electrolyte solution tested, oscillatory air supply at a certain frequency and amplitude could give better gas dispersion than steady air supply, and the degree of improvement of gas dispersion varied with the electrolyte type and concentration and air flow rate. At each air supply mode tested, either the intensity of scattered light or the Sauter mean diameter of bubbles would scale with the ionic strength, following a decreasing trend. A cooperative effect of electrolyte addition and oscillatory air supply on gas dispersion was generally found. The combined effect of electrolyte addition and oscillatory air supply on bubble size was significant, being up to almost a 2/3 decrease in the Sauter mean diameter. The present work highlights the importance of flow condition for controlling bubble generation and coalescence in saline water and has significant implications for seawater flotation of minerals.

Process engineering strategy for large scale outdoor cultivation of Tetradesmus obliquus CT02 coupled with pH guided CO2 feeding

A novel CO2 tolerant microalga Tetradesmus obliquus CT02, was previously evaluated to be a suitable bio refinery platform for synthesis of bioactive molecules, biodiesel, and biofertilizer. In the present study, a process engineering strategy was developed targeting improved growth performance of the strain at large scale under fluctuating outdoor environmental conditions. The strategy relies on maintaining pH of the culture at its optimal value via cascade control with CO2 feeding. The strategy was developed at laboratory scale bubble column photobioreactor under diurnal variation of simulated sunlight intensity and was further validated through growth performance of the strain under outdoor conditions in a 100L airlift bioreactor. Under laboratory condition, 53.3% and 85.16% improvement in biomass concentration (1.87gL-1) and productivity (114.8mgL-1 day-1) was achieved as compared to the uncontrolled pH, respectively. The strategy demonstrated a significant improvement in biomass concentration and productivity by 225.7% and 121.6% respectively, compared to the pH uncontrolled batch, even under outdoor fluctuating environmental condition.

Statistical Analysis Of Bubble Parameters From A Model Bubble Column With And Without Countercurrent Flow

Bubble columns are widely used in numerous industrial processes, because of their advantages in operation, design and maintenance compared to other multiphase reactor types. In contrast to their simple design, the generated flow conditions inside a bubble column reactor are quite complex, especially in continuous mode with counter-current liquid flow. For the design and optimization of such reactors precise numerical simulations and modelling is needed. These simulations and models have to be validated with experimental data. For this reason experiments were carried out in a laboratory-scale bubble column with shadow imaging technique with and without countercurrent liquid flow. In the experiments two types of gases –relatively badly soluble air and good soluble CO2 – are applied and the bubbles were generated with three different capillary diameters. With changing gas and liquid flow rates overall 108 different flow conditions were investigated. The gathered data has been statistically evaluated over the whole 1 m measurement area and bubble parameters have been obtained such as bubble diameter, velocity, aspect ratio, bubble motion direction and inclination. The analysis of these parameters shows, that the countercurrent liquid flow has a noticeable influence on the bubble parameters, especially on the bubble velocity and motion direction. In the case of CO 2 bubbles remarkable bubble shrinkage has been observed at counter-current liquid flow due to the enhanced mass transfer. The comprehensive and extensive bubble data obtained in this study will now be used as a source for the validation of numerical simulations and models.

Gas holdup at dynamic equilibrium region of a bubble column: Effect of bubble generator performance

Gas holdup along with the superficial gas velocity was systematically investigated in a lab-scale bubble column. The experiments were conducted in a bubble column with a height of 6 m at superficial gas velocity of 0.01–0.13 m/s. Experimental results show that dynamic equilibrium region was achieved at a distance of 2800 mm above the bubble generator. Small initial bubble size will lead to higher gas holdup and slump trend. To overcome the trade-off between sufficient gas holdup and slump trend, a reasonable gas holdup without slump can be obtained by controlling the liquid mass flow in the nozzle. Based on previous empirical correlations and experiment data, a semi empirical correlation of gas holdup was established consider the flow regime transition and bubble generator performance. The semi empirical correlation in this work gives a fairly accurate prediction while several empirical coefficients were used.

Binary Solvent Swap Processing in a Bubble Column in Batch and Continuous Modes.

A lab-scale bubble column was investigated as an alternative means to achieve a low-temperature binary solvent swap of solutions containing pharmaceutical materials at atmospheric pressure, for batch and continuous configurations. The rate of solvent evaporation was predicted by first-principles vapor–liquid equilibrium (VLE) thermodynamic modeling and compared to experimentally achieved results. For batch configurations, evaporation rates of up to 5 g/min were achieved at gas flow rates up to 2.5 L/min (0.21 m/s superficial velocity) and temperatures up to 50 °C. This achieved 99 mol % purity of the desired solvent within three “put and take” evaporations from a 50:50 starting mixture. The evaporation rate profiles for the duration of the experiments were calculated, and the changing concentration profile was predicted within satisfactory error margins of <5%. Continuous process modeling explored a multistage equilibrium configuration and could predict the approach to attaining steady-state operation for various operating conditions. All rates of evaporation and resulting changes in solution concentration were measured, and direct comparison of model predictions fell within instrumentation error margins, as previously. This underlined the capability of the model to provide accurate representations of predicted evaporation rates and binary solution concentration changes during operation.

On Inter-bubble distances and bubble clustering in bubbly Flows: An experimental study

• Positional relationships of bubbles were studied experimentally in the dense swarms. • Dimensionless inter-bubble distances were extracted from X-ray measurement data. • The distance of nearest neighbor bubbles is close to that of FCC structure. • Vertical clustering is observed for Eo ≥ 3.82. • Clustering orientation was found to be independent of the gas holdup in this study. Swarm effects in bubble columns result from the interaction of the bubbles at higher gas holdup. The flow around individual bubbles in a swarm influences the movement of the following bubbles and this has a feedback on the flow regime on a larger scale. That is, hydrodynamics that can no longer be described with models for single rising bubbles. The experimental investigation of the multiscale hydrodynamics in three-dimensional bubble columns was so far limited by the available measurement techniques. In this study, ultrafast X-ray tomography (UFXCT) was applied on bubbly flows in a lab scale bubble column to determine inter-bubble distances and clustering characteristics in the vicinity of individual bubbles for different gas holdups and bubble diameters. Applying the pair correlation function reveals a pronounced vertical clustering for large bubbles with Eo ≥ 3.82, whereas no clustering has been observed for bubbles of lower Eo . It was found that clustering orientation mainly depends on deformability of the bubbles but not on gas holdup.

Optimisation of Scenedesmus almeriensis production using pig slurry as the sole nutrient source

Pig slurry was used as the sole nutrient source to produce biomass of the microalga Scenedesmus almeriensis using laboratory-scale bubble column photobioreactors. No differences in terms of biomass productivity were observed between fresh and digested pig slurry. The optimum dilution of the slurry to enable the processing of the largest amount of pig slurry per litre was 5%, which led to a biomass productivity rate of 0.68 g·L−1·day−1, comparable to that of the standard growth medium formulated using pure chemicals (0.70 g·L−1·day−1). The inlet N-NH4+ concentration was approximately 180 mg·L−1, with inhibitory effects being observed at concentrations higher than 200 mg·L−1. The N-NH4+, N-NO3−, P-PO43−, and chemical oxygen demand removal rates were 34.1, 0.5, 2.1, and 519.3 mg·L−1·day−1. The observed turbidity of the media did not affect microalgal growth at the studied dilutions. Ozonation effectively reduced the chemical oxygen demand concentration of the media, but no effect on microalgal growth was observed. Overall, pig slurry was an effective nutrient source for the production of S. almeriensis, and this strain was shown to be robust with a N-NH4+ tolerance up to 200 mg·L−1 and the potential use in the bioremediation of agro-industrial wastes.

Ozonation of diclofenac in a laboratory scale bubble column: Intermediates, mechanism, and mass transfer study

Diclofenac (DCF), a non-steroidal analgesic drug, has been recurrently detected in surface and groundwater during the past two decades. Traces of DCF pass through the conventional wastewater treatment facilities without any alteration due to its resistance and high stability. This study aims the complete degradation and partial mineralization of DCF by an ozonation process. The findings of the study suggest that increasing pH of the system and ozone supply rate, and generation of hydroxyl radicals in situ enhance the degradation process. In contrast, the increasing initial concentration of DCF slows down the process. The pseudo-first-order reaction rate constants were in the range of 0.0742 – –0.0979 min−1. A model has been developed to predict the behavior of the rate constant with various operating variables. High volumetric mass transfer coefficients (i.e., 1.150–2.717 × 103 s−1) was observed during the mass transfer study of ozone in water. A probable mechanism for ozonation is suggested along with the structures of major intermediates formed during the oxidation. Distinct orange pigments appeared during the reaction, which confirmed the production of DCF-2, 5-iminoquinone. Di-chloro aniline, 5-hydroxy DCF. The carboxylic acids were mainly detected as the metabolites. The treatment cost was estimated to be USD 0.80 for treating 1 m3 of the solution containing 50 mg dm−3 DCF. In addition, the effect of other water matrices on DCF degradation was also investigated and reported.

Advanced analysis of bubble columns: Comparison of Euler/Lagrange simulations and experiments under CO2 chemisorption conditions

The chemical absorption reaction of CO2 in a circular bubble column reactor was simulated with an Euler/Lagrange CFD method and results were compared with data from a laboratory scale bubble column. The experimental data comprises gas holdup and bubble size distributions obtained with ultrafast X-ray tomography and hydroxide ion conversion obtained with a wire-mesh sensor. Large Eddy Simulation (LES) is used for calculating the fluid flow and modelling the turbulence in the continuous phase, considering the effect of sub-grid-scale (SGS) turbulence on bubble motion, and also SGS turbulence modification by bubbles. Lagrangian bubble tracking is conducted considering all relevant forces, bubble oscillation via stochastic generation of eccentricity and motion angle and consequently non-steady mass transfer via a dynamic Sherwood number. The comparison of experimental and simulation results shows very good agreement for the given data.

Hydrodynamics in bubble columns with helically-finned tube Internals: Experiments and CFD-PBM simulation

Experiments and CFD-PBM simulation were performed in this study to investigate the effects of helically-finned tube (HFT) internals on the gas-liquid flow in a laboratory-scale bubble column. Hydrodynamic parameters, including gas holdup, bubble size and flow field as well as turbulence properties, were compared for an empty column and other two columns with bare-tube (BT) or HFT internals. We found that bare rods promote the stability of two-phase flow, whereas helical fins destabilize the flow. Meanwhile, the spatial distribution of gas holdup or bubble size becomes more uniform in the presence of HFT internals, whereas liquid velocity or turbulent dissipation rate evidently decreases. The fin-induced spiral movement, flow resistance and bubble accumulation are the mechanisms underlying these experimental observations. Integration of rod and helical fin may be promising for process intensification in bubble columns.

Design, Commissioning, and Performance Assessment of a Lab-Scale Bubble Column Reactor for Photosynthetic Biogas Upgrading with Spirulina platensis.

The two-step bubble column-photobioreactor photosynthetic biogas upgrading system can enable simultaneous production of biomethane and value-added products from microalgae. However, due to the influence of a large number of variables, including downstream processes and the presence of microalgae, no unanimity has been reached regarding the performance of bubble column reactors in photosynthetic biogas upgrading. To investigate this further, the present work documents in detail, the design and commissioning of a lab-scale bubble column reactor capable of treating up to 16.3 L/h of biogas while being scalable. The performance of the bubble column was assessed at a pH of 9.35 with different algal densities of Spirulina platensis at 20 °C in the presence of light (3–5 klux or 40.5–67.5 μmol m–2 s–1). A liquid/gas flow (L/G) ratio of 0.5 allowed consistent CO2 removal of over 98% irrespective of the algal density or its photosynthetic activity. For lower concentrations of algae, the volumetric O2 concentration in the upgraded biomethane varied between 0.05 and 0.52%, thus providing grid quality biomethane. However, for higher algal concentrations, increased oxygen content in the upgraded biomethane due to both enhanced O2 stripping and the photosynthetic activity of the microalgae as well as clogging and foaming posed severe operational challenges.

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.

Effects of bubble coalescence and breakup models on the simulation of bubble columns

This work investigates the effects of coalescence and breakup models on the radial distribution of gas holdup and local bubble size distribution in a laboratory scale bubble column operated at 0.11 m/s, 0.14 m/s, 0.19 m/s and 0.23 m/s. The results quantitatively show the influences of bubble coalescence due to various mechanisms, coalescence efficiency calculated by different models, modification coefficients for coalescence rates, critical energy required for breakup and bubble breakage caused by large scale eddies and viscous shear on numerical simulations. Furthermore, qualitative analysis is performed to analyze the effects of these factors. The performance of various models is evaluated (e.g. Han’s breakup model and Das’s coalescence model). Some optimized combinations of coalescence and breakup models that can be used to accurately predict the local gas holdup and bubble size distribution in laboratory scale bubble columns operated at heterogeneous regime are proposed.

Parallelization of a stochastic Euler-Lagrange model applied to large scale dense bubbly flows

A parallel and scalable stochastic Direct Simulation Monte Carlo (DSMC) method applied to large-scale dense bubbly flows is reported in this paper. The DSMC method is applied to speed up the bubble-bubble collision handling relative to the Discrete Bubble Model proposed by Darmana et al. (2006) [1]. The DSMC algorithm has been modified and extended to account for bubble-bubble interactions arising due to uncorrelated and correlated bubble velocities. The algorithm is fully coupled with an in-house CFD code and parallelized using the MPI framework. The model is verified and validated on multiple cores with different test cases, ranging from impinging particle streams to laboratory-scale bubble columns. The parallel performance is shown using two different large scale systems: with an uniform and a non-uniform distribution of bubbles. The hydrodynamics of a pilot-scale bubble column is analyzed and the effect of the column scale is reported via the comparison of bubble columns at three different scales.

On the force competition in bubble columns: A numerical study

Computational Fluid Dynamics (CFD) is nowadays fully accepted in engineering practice as a potential tool for the prediction of flows, conversion rates and process efficiency. The Euler/Lagrange is one of the available approaches for describing dispersed multiphase systems, for example in bubble columns. Hence, the flow field within the bubble column is computed using large eddy simulations (LES) considering full coupling with the bubble phase through momentum source terms as well as sub-grid-scale (SGS) turbulence modification by modelling bubble induced turbulence (BIT). Despite the required point-particle approximation, necessary for computing large-scale technical and industrial systems, bubble dynamics was included in the numerical modelling, through shape (eccentricity) and trajectory oscillations. The performance of the proposed bubble dynamics model is analysed in detail and it is shown that without a bubble dynamics model the bubble fluctuating velocities cannot be predicted correctly. In order to predict the bubble motion accurately it is necessary to account for all relevant acting forces in the calculations, namely, gravity/buoyancy, drag, transverse lift, added (virtual) mass, fluid inertia (part of the pressure term), Basset (history term) force and possibly modifications of these forces due to the presence of walls. Based on the state-of-the-art, instantaneous resistance coefficients for all these forces were elaborated and extended for allowing consideration of the bubble dynamic behaviour and deformation through the eccentricity. The proposed resistance coefficients do not consider any swarm effects and are therefore only applicable to bubbly systems with volume fractions of less than 5%. For the beginning clean systems with mobile bubbles are considered only which are moving in low-viscous systems such as a water-air two phase flows with small bubble Morton numbers. Most important, a lift force coefficient which is valid for such low viscous systems is applied based on recent studies. Consequently, a comprehensive and complete model is introduced for describing bubble motion in a point-particle approximation including bubble dynamics (eccentricity) and therefore using composite formulations especially for drag and transverse lift, and in addition accounting for added mass with bubble deformation and wall effects as well as the Basset force. For the regime of deformable bubbles, the resulting instantaneous resistance coefficients are extracted from bubble column simulations and compared with the mean resistance coefficients. Moreover, the local importance of all considered forces on the motion of bubbles within the bubble column was evaluated in detail in terms of bubble size, column height, radial position and Stokes number without and with bubble dynamics model. The importance of these forces and the correspondent effects on the velocity and volume fraction profiles are explored for a laboratory-scale bubble column. The influence of the bubble dynamics model and the considered interfacial forces on the predicted hydrodynamics of the laboratory bubble column is analysed for validation considering two cases with different bubble size distributions with sizes smaller than about 5 mm. Furthermore, the effect of Basset term on particle dispersion is highlighted in a turbulent pipe flow test case.