Bubble Column Research Articles

Double‐sensor conductivity probe applied in a flotation system

AbstractThis study proposed a new approach for measuring bubble size distribution, bubble mean diameter, Sauter mean bubble diameter, and gas holdup using a double‐sensor conductivity probe in an air/water two‐phase system bubble column. The results for the two‐phase system were compared and calibrated using analyses from bubble images taken by a digital camera from the side of the column wall. Good agreement was observed between the two techniques. The same double‐sensor conductivity was used in an air/water/solids three‐phase system. The conductivity probe captured the change in bubble dynamic behaviour inside the pulp phase; however, the presence of the solids made it more challenging to measure. As a result, the VisioFroth commercial package, using images taken from the top of the froth layer, could be used in conjunction with the double‐sensor conductivity probe to show the dynamic evolution of mineralized bubbles from the pulp zone to the froth zone in a flotation process.

Simultaneous utilization of CO2 and nitrate wastes from compact recirculating aquaculture system for improving algal biomass (Scenedesmus armatus) production

Tilapia cultivation in an air-tight culture tank of a recirculating aquaculture system (RAS) was conducted for 8 days to produce CO2 for growing Scenedesmus armatus in bubble column photobioreactors. The average CO2 concentrations in the headspace of the culture tank were determined at 925 ± 73.4 and 1619 ± 197.7 ppm for fish stocking densities of 3 and 10 kg/m3, respectively. These CO2 concentrations were higher than CO2 concentrations at the tank inlet (496 ± 13.9 ppm) exposed to the atmosphere. Using the captured CO2 and nitrate from the effluent of a commercial tilapia farm for cultivating S. armatus yielded higher algal biomass concentration (621 ± 41.2 mg/L) than using ambient air (410 ± 30.1 mg/L). Insufficient nitrate in RAS effluent is the limiting factor for CO2 utilization and algal biomass production. In conclusion, this study demonstrates the possibility of repurposing CO2 from RAS to enhance algal cultivation.

Simulation Study on Hydrodynamic Performance of Slurry Bubble Column with Vertical Tube Internals

The gas-liquid-solid mixing characteristics in slurry bubble column were studied using the CFD-PBM model under different cross section coverage ratios of tubes. The research shows that with the increase of the proportion of the cross section area of the vertical tube, the flow field in the slurry bubble column tends to be stable, the liquid phase flow rate in the column becomes faster, and the large-scale liquid circulation strengthens, resulting in the decrease of gas holdup. As the proportion of cross section area in the vertical column increases, the distribution of gas-solid holdup becomes more uneven in the radial direction. However, increasing the number of vertical tubes can improve the axial distribution of solid particles in the reactor while reducing the size of bubbles in the column. Therefore, it can be concluded that in order to achieve effective gas, liquid, and solid mixing in slurry bubble column, the ratio of cross section area of vertical internals is crucial under given conditions.

Evaluation of the controlled hydrodynamic cavitation as gas mass transfer system for ex-situ biological hydrogen methanation

The present work represents the first study focused on controlled hydrodynamic cavitation applied as gas transfer system to supply the methanogenic archaea with exogenous H2 and CO2. Starting from a generic mixed inoculum sampled from a thermophilic full-scale anaerobic digester, a bubble column bioreactor was coupled with a rotating hydrodynamic cavitator and fed with different H2/CO2 loading rates. Process efficiency and long-term effects on process stability and microbial population were evaluated. Gas sparging through the controlled hydrodynamic cavitation device is feasible under the operating conditions tested, resulting in almost 100% efficiency in H2 utilization and recording a CH4 volumetric content more than 99% in the gas leaving the reactor, without any gas recirculation from the headspace. The experimental trials lasted about 160 days and the behavior of the bioreactor showed a substantial stability over the time. Metagenomic and FISH analyses were carried out at the end of the experimental trials, revealing a remarkable increase of hydrogenotrophic methanogens species, related to the selection-effect of H2 on community composition. The findings provide previously unidentified insights into long-term effect on process stability and microbial community diversity in the biological hydrogenotrophic methanation process coupled with a gas–liquid mass transfer system based on controlled hydrodynamic cavitation technology.

Effects of bubble cutting dynamic behaviors on microalgal growth in bubble column photobioreactor with a novel aeration device.

Introduction: Carbon sequestration by microalgae is an effective approach for achieving carbon neutrality owing to its high carbon capture efficiency and environmental friendliness. To improve microalgae CO2 fixation efficiency, various methods to enhance CO2 transfer at the gas-liquid interface have resulted in high energy consumption. Methods: In this study, a novel aeration device with bubble cutting slices was installed in a photobioreactor for CO2 supply, which could precisely separate bubbles into sizes on the way to rising after departure, achieving CO2 transfer enhancement without extra energy consumption. Subsequently, the bubble cutting dynamic behaviors in the photobioreactor were studied, and the effects of thickness, hydrophilicity, and arrangement of cutting slices on microalgal growth were analyzed. Results: It was found that bubble cutting caused the maximum dry weight and biomass productivity of microalgae to improve by 6.99% and 33.33%, respectively, compared with those of the bioreactor without cutting units, owing to a 27.97% and 46.88% decrease in bubble size and rising velocity, respectively, and an 84.55% prolongation of bubble residence time. Discussion: Parallel cut slices with a thickness and spacing of less than 3mm successfully cut the bubbles. The hydrophobic slice surface prevented daughter bubble departure and prolonged the bubble residence time, impeding microalgae growth owing to bubble coalescence with subsequent bubbles. The optimal cutting slice parameters and culture conditions for microalgal growth were 1mm slice thickness, less than 1mm slice spacing, 5% inlet CO2 concentration, and 70mL/min gas flow rate.

Carbon Dioxide Absorption Using Different Solvents (MEA, NaOH, KOH and Mg(OH)2) in Bubble Column Reactor

The aim of this research is to reduce emissions by capturing carbon dioxide in a solution using an absorption method. The absorption capacity, absorption rate, carbon dioxide removal efficiency, and overall mass transfer coefficient of MEA (Monoethanolamin) and alkaline solvents (NaOH, KOH, Mg(OH)2) were investigated using a bubble column gas absorption reactor with counter current flow. The effects of operational parameters such as solvent concentration (0.01, 0.05, and 0.25M) and solvent type were studied. As a result of the study, it was determined that Mg(OH)2 was less effective in capturing CO2 than KOH, NaOH, and MEA. For all solvent types, the total mass transfer coefficient, absorption rate, and CO2 removal efficiency were increased with the increase in the concentration of solvent. The solvent concentration is increased from 0.01 M to 0.25 M to obtain the highest KGa values for MEA, NaOH, and KOH, 3.75 1/min for MEA, 3.70 1/min for NaOH, and 3.93 1/min for KOH.The MEA, NaOH, and KOH absorption rates were maximum at 0.25 M solvent concentrations as 0.19x103 mol/Ls. The maximum CO2 removal efficiencies for MEA, NaOH, and KOH at 0.25 M solvent concentration are greater than 60%. The highest absorption capacity, 0.576 mol CO2/mol MEA, was obtained at a solvent concentration of 0.01M MEA.

New dual modality technique of gamma-ray densitometry (GRD) and optical fiber probe (OFP) to investigate line-averaged diameter profiles of gas, liquid, and solid holdups along the height of a slurry bubble column

A new hybrid measurement technique was developed and employed to study line-averaged three-phase holdup distribution in a Slurry Bubble Column Reactor (SBCR) using gamma-ray densitometry (GRD) and point optical fiber probe (OFP) techniques. The OFP technique was used to measure the local gas holdup at 45 nodes in the cross-section area of the SBCR at three different axial levels. Thus, the GRD technique measured the spectrum of the SBCR for two different orientations simultaneously with the OFP technique at the same operating conditions to determine the solid holdup distribution radially and axially. The line-averaged solid holdup for 25% solids loading on the higher measurement level at the column’s center decreased compared to the lower level by 45.7%, 43.6%, and 41.7% using superficial gas velocities 0.2, 0.3, and 0.4 m/s, respectively. These results are highly remarkable as it often assumed that the solids exhibit a uniform distribution inside the slurry phase.

An electrolytic bubble column with an external hollow fiber membrane gas–liquid contactor for effective microbial electrosynthesis of acetate from CO2

Currently, almost all microbial electrosynthesis (MES) reactors were biofilm-driven whether the cathode electron transfer was direct or indirect. It has been proven that cathode biofilm formation is time-consuming, and the current density achieved on these biofilms was relatively low. However, the main challenge for the non-biofilm-driven MES is how to maintain a good Columbic efficiency (CE) at high current densities. Here, we report a novel electro-H2 bubble column reactor with an external hollow fiber membrane gas–liquid contactor to tackle the challenge. The electrolyzer was placed at the bottom of the reactor to produce H2 micro-bubbles (mean diameter 169 ± 7 µm) at a current density of 156 A/m2. The bubble column was put on the top of the cathodic chamber of the electrolyzer to extend the H2 retention time, while the hollow fiber membrane gas–liquid contactor in the external loop was used to recover the unused gas. Consequently, the homoacetogens in reactor could efficiently convert the electrolytically-produced H2 and the externally-supplied CO2 into acetate. The volume of the catholyte (5.5 L) in this reactor is one order of magnitude higher than all reported MES reactors. Batch tests with enriched homoacetogens demonstrated a maximum acetate production rate (1.15 g/Lcatholyte/d, 898 g/m2cathode/d), acetate titer (34.5 g/L) could be achieved in this setup, and the average (maximum) CE of the electron to acetate efficiency was 64% (92%). Besides, dissolved H2 measurement and bacteria community analysis indicated the high acetate titers were likely caused by the H2 oversaturation situation and good substrate/product mass transfer rather than the selection of certain types of homoacetogens. The electro-H2 bubble reactor seems to be a promising MES setup for scale-up and practical application.

Micro-interface enhanced mass transfer sodium carbonate absorption carbon dioxide reaction

Micro-interface intensified reactor (MIR) can be applied in series/parallel in the absorption of CO2 in industrial gases by Na2CO3 due to the ability to produce large numbers of stable microbubbles. This work focuses on the variation pattern of mass transfer characteristics parameters of the reaction gas in Na2CO3 solution under the influence of different solution properties and operating parameters in the reaction of CO2 absorption by Na2CO3. The mass transfer characteristics parameters include bubble Sauter mean diameter, gas holdup, interfacial area, liquid side mass transfer coefficient, and liquid side volume mass transfer coefficient kLa. The solution properties and operating parameters include Na2CO3 concentration (0.05–2.0 mol·L−1), superficial gas velocity (0.00221–0.01989 m·s−1), superficial liquid velocity (0.00332–0.02984 m·s−1), and ionic strength (1.42456–1.59588 mol·kg−1). And volumetric mass transfer coefficients kLa and superficial reaction rates r of the MIR and the bubble column reactor are compared in the reaction of sodium carbonate absorption of carbon dioxide, and the former shows a greater improvement under different solution properties and operating parameters. The enhanced role of MIR in mass transfer in non-homogeneous reactions is verified and the feasibility of industrial practical applications of MIR is demonstrated.

Cavitation bubble structures below a soft boundary in an ultrasonic field

We studied the layer structure of bubbles just below water/air and water/EPE (Expand aple poly ephylene) interfaces using high-speed photography. The layer structure was generated by floating spherical clusters, the source bubbles of which were identified to come from the attachment of bubble nuclei at the interface, the floating of bubbles in the bulk liquid, or bubbles generated on the surface of the ultrasonic transducer. The boundary shape affected the layer structure, which assumed a similar profile below the water/EPE interface. We developed a simplified model composed of a bubble column and bubble chain to describe interface impacts and the interaction of bubbles in a typical branching structure. We found that the resonant frequency of the bubbles is smaller than that of an isolated single bubble. Moreover, the primary acoustic field plays an important role in the generation of the structure. A higher acoustic frequency and pressure were found to shorten the distance between the structure and the interface. A hat-like layer structure of bubbles was more likely to exist in the low-frequency (28 and 40 kHz) intense inertial cavitation field, in which bubbles oscillate violently. By contrast, structures composed of discrete spherical clusters were more likely to form in the relatively weak cavitation field at 80 kHz, in which stable and inertial cavitation coexisted. The theoretical predictions were in good agreement with the experimental observations.

P3HB from CH4 using methanotrophs: aspects of bioreactor, fermentation process and modelling for cost-effective biopolymer production.

P3HB (poly-β-hydroxybutyrate), an energy-storage compound of several microorganisms, can be used as bioplastics material. P3HB is completely biodegradable under aerobic and aerobic conditions, also in the marine environment. The intracellular agglomeration of P3HB was examined employing a methanotrophic consortium. Supplanting fossil, non-degradable polymers by P3HB can significantly reduce the environmental impact of plastics. Utilizing inexpensive carbon sources like CH4 (natural gas, biogas) is a fundamental methodology to make P3HB production less costly, and to avoid the use of primary agricultural products such as sugar or starch. Biomass growth in polyhydroxyalkanoates (PHA) in general and in Poly (3-hydroxybutyrate) manufacture in specific could be a foremost point, so here the authors focus on natural gas as a proper carbon source and on the selection of bioreactors to produceP3HB, and in future further PHA, from that substrate. CH4 can also be obtained from biomass, e.g., biogas, syngas methanation or power-to-gas (synthetic natural gas, SNG). Simulation software can be utilized for examination, optimizing and scale-up of the process as shown in this paper. The fermentation systems continuously stirred tank reactor (CSTR), forced-liquid vertical loop bioreactor (VTLB), forced-liquid horizontal tubular loop bioreactor (HTLB), airlift (AL) fermenter and bubble column (BC) fermenter were compared for their methane conversion, kLa value, productivity, advantages and disadvantages. Methane is compared to methanol and other feedstocks. It was discovered that under optimum processing circumstances and using Methylocystis hirsuta, the cells accumulated 51.6% cell dry mass of P3HB in the VTLB setup.

Evaluation of nano-scaled zero valent iron (nZVI) effects on continuous syngas biomethanation under the thermophilic condition

The nano-scaled zero valent iron (nZVI) particles were applied to strengthen the syngas biomethanation under the thermophilic condition in a continuous bubble column reactor with gas circulation. The CH4 productivity was increased by 6.80% from 71.20 mmol∙Lr−1∙day−1 to the highest 76.04 mmol∙Lr−1∙day−1 at the nZVI concentration of 2.5 g/L. The measurement of iron concentration and the observation of the iron nanoparticles distribution indicate that nZVI can act as an electron conduit to enhance more efficient direct interspecies electron transfer by physically close contact between microorganisms, instead of biological corrosion. Further analysis of metabolic products shows that the nZVI addition can stimulate the EPS secretion, and the direct electron transfer relying on nZVI particles tends to replace other transfer modes. Microbial community analysis reveals that the Bacteria Bacteroidia and Firmicutes, and Archaea Methanothermobacter are the potential dominating enriched syntrophic partners. The expression of functional genes involved in methane production was also found to increase. On the other hand, the nZVI accumulation can lead to the albefaction and inactivation of partial sludge granules due to its toxicity. The negative effects of nZVI at high concentration are also more pronounced. This work shows the feasibility of improving continuous syngas biomethanation by strengthening interspecific association and accelerating electron transfer.

One-dimensional kinetic model with heat transfer and axial dispersion of molten-metal bubble column reactors for hydrogen production via methane pyrolysis

A one-dimensional (1D) molten-metal-based bubble-column reactor (MMBCR) model coupled with heat transfer and axial dispersion was developed for non-oxidative CH4 pyrolysis to produce low-carbon H2. The MMBCR model included mass, momentum, and energy balances, accounting for catalytic and non-catalytic reaction kinetics, and hydrodynamic parameters such as the gas holdup (αG), gas velocity (uG), bubble size (db), and specific interfacial surface area of bubbles (as). The 1D MMBCR model was compared with other 1D reactor models and bench-scale experimental data for CH4 conversion (XM). The MMBCR model agreed well with the experimental data. When heat transfer and axial dispersion were considered for an industrial-scale MMBCR to produce 10,000 Nm3/h, the reactor length increased to 2.4 times higher than that estimated by the model without axial dispersion to meet 80% XM. The MMBCR model has the potential to design industrial-scale MMBCRs and optimize operating conditions.

Bioconversion of CO2 and potential of gas fermentation for mainstream applications: Critical advances and engineering challenges

AbstractThe undesirable consequences of climate change are attributed to the ever‐increasing emissions of greenhouse gases, especially carbon dioxide. For the sustainable development of human society, there is an urgent need to develop novel techniques for efficient conversion and utilization of carbon dioxide. Hence, carbon capture and storage, artificial bioconversion of carbon dioxide are topics of great interest for current researchers across the globe. Here, we elucidate the different techniques of carbon fixation, which fall under the broad categories of natural fixation pathways, thermo‐catalytic conversion techniques, and synthetic carbon fixation pathways. Based on a comparative analysis, gas fermentation is a promising method for the microbial conversion of CO2‐containing gases into fuels and chemicals. However, for mainstream applications, in‐depth reviews of different reactor configurations and the various engineering aspects related to process development are still lacking. We have analyzed the published literature relating to artificial bioconversion of carbon dioxide and focused on the factors like equipment selection and reactor design that can assist in scale‐up of gas fermentation technology. The review provides an in‐depth understanding of engineering aspects related to stirred tank and bubble column reactors, focusing on the technical design parameters and discussing the conditions for optimal operation and performance during commercial gas fermentation processes.

Natural Gas Pyrolysis in a Liquid Metal Bubble Column Reaction System—Part II: Pyrolysis Experiments and Discussion

This contribution presents the results of continued investigations on the production of hydrogen by means of pyrolysis in a liquid metal bubble column reactor, as developed at the Karlsruhe Institute of Technology in recent years. Part I of this contribution described the motivation and the methodology of this study, as well as a significant scale-up, and discussed its results for pure methane pyrolysis. Here in part II, two additional experimental campaigns with methane–ethane mixtures (MEMs) and high-calorific natural gas (nGH) will be presented and discussed for the first time, using the up-scaled liquid metal bubble column reactor. It could be proven that an MEM as the feed gas led to an increase in methane conversion at low temperatures, which is consistent with the literature data. The nGH pyrolysis confirms this trend and also results in a significant rise in methane conversion compared to pure methane pyrolysis. Furthermore, the nGH pyrolysis leads to an increased methane conversion even at higher temperatures compared to MEM pyrolysis. Additionally, both MEM and nGH pyrolysis also showed a shift in the formation of by-products toward lower temperatures.

Experimental and modeling study of bubble absorption-based deep dehumidification using the ionic liquid: Parametric analysis on heat and mass transfer

Conventional liquid desiccant dehumidification is good at humidity control with energy conservation and stable operation, but is limited by corrosion and crystallization in deep dehumidification. Thus, it is necessary to broaden the humidity adjustment range of liquid desiccant deep dehumidification as well as avoid corrosion. Ionic liquids (ILs) with deep dehumidification potential (e.g., extremely low vapor pressure and non-corrosion) are a promising alternative to traditional salt solutions. Meanwhile, the bubble absorption-based mode can obtain a huge gas–liquid specific interfacial area, which is expected to solve the low wettability and insufficient gas–liquid transfer efficiency in traditional falling-film/packed dehumidifiers using IL. In this work, the bubble absorption-based deep dehumidification using IL (BADD-IL) is investigated. Meanwhile, the heat and mass transfer mathematical model of BADD-IL is established, and a bubble column dehumidifier using the novel IL is experimentally examined under different conditions. The results indicated that the deep dehumidification performance is strongly dependent on operating parameters, and the minimum outlet air humidity ratio is around 1.2 g/kgda. In particular, it is most sensitive to inlet air humidity ratio, followed by liquid height and superficial velocity, and the effect of solution temperature is the least. Notably, the influential mechanism of superficial velocity and solution temperature on volumetric transfer coefficient is different. The increase of superficial velocity promotes both gas–liquid specific interfacial area and heat and mass transfer coefficient. For the IL with lower temperature, its higher heat and mass transfer coefficient makes up for the poor specific interfacial area caused by high viscosity. Additionally, the Lewis factor of bubble column dehumidifier using the novel IL is basically less than 0.6, which is important for the device design and scale-up. This study clarifies the heat and mass transfer mechanism of BADD-IL, and provides guidance for the development of ILs deep dehumidification technology.

Fuzzy neural network for mapping CFD data of air-water flow in chemical reactors: Computational simulation

ANFIS (Adaptive Neuro-Fuzzy Inference Systems) artificial intelligence is used to enhance the computational fluid dynamic (CFD) modeling of an air-water bubble column reactor for the first time. Air temperature and velocity are 500 K and 0.006 m/s, respective in the process. The water temperature is considered at 295 K. This turbulent kinetic energy prediction is performed to measure the mixing flow within the reactor. The ANFIS projection is examined to see what impact the number of membership functions and input has. The findings showed that the most intelligent output is calculated by inputting two numbers and using a membership function with a value of 10. With respect to that, under this condition the ANFIS is capable of estimating the turbulent kinetic energy with more nodes on the domain while skipping mesh refinement in the CFD domain. The hybrid model was indicated to be robust in predicting the performance of liquid-phase reactor.

Modelling of industrial-scale bioreactors using the particle lifeline approach

A key factor in improving the performance of large-scale bioreactors is understanding the conditions experienced by the cells inside the reactor. This can be challenging due to the practical difficulties involved, hence there is increasing use of simulation to quantify the environmental conditions found in large-scale bioreactors. In this work we have used the particle lifeline approach to quantify the effect of the reactor design on the conditions experienced by two very commonly used industrial organisms (Escherichia coli and Saccharomyces cerevisiae). It was found that the cells in the stirred tank reactor tended to experience longer fluctuations of both starvation and overflow metabolism when compared with those in the bubble column, this behaviour being caused by differences in mixing between the two reactor designs. It was found that a significant (60%) fraction of the population in the stirred tank reactors experienced starvation conditions for a large fraction (>70%) of the time, with exposure to such conditions being likely to affect the cellular metabolism. Results from this work provide a detailed insight into the conditions experienced inside industrial-scale bioreactors operated at realistic conditions. Such data can be leveraged to optimise large-scale reactor designs as well as for the development of scale-down systems.

Hydrodynamic performance and structural optimization of a slurry bubble column with vertical tube internals

The slurry bubble column, as a multiphase reactor, is widely used in chemical, biological, petroleum refining, environmental, and other fields. However, most published studies of the hydrodynamic performance of slurry bubble columns have been carried out on empty columns without internal components, and recent studies on columns with vertical heat exchange tubes have been limited. In this study, the hydrodynamic performance of a laboratory-scale slurry bubble column with vertical tube bundles was studied, and a new vertical tube structure with fins was proposed. The effect of the new structure on the hydrodynamic performance of the laboratory-scale slurry bubble column was studied via a combination of experimental and numerical simulations. The results of these simulations showed that the vertical heat exchange tube structure with fins was helpful for improving fluid flow, exerted a certain fragmentation effect on large bubbles, and could strengthen mass transfer between gas and liquid, thereby improving the hydrodynamic performance.

Experimental evaluation of black liquor carbonation for carbon dioxide capture

Carbon dioxide is one of the main if not the most potent greenhouse gases responsible for climate change. Scientists put great efforts to tackle this problem and carbon dioxide capture seems to be a promising solution. The present study proposes a novel method of carbon dioxide capture using black liquor, a side stream from the paper and pulp industry. Its content in sodium hydroxide makes it an attractive candidate for carbon dioxide capture via carbonation. The black liquor was prepared from oat husks, a non-woody biomass, using the soda-pulping process. To estimate its carbon dioxide absorption capacity, a mixture of nitrogen and carbon dioxide (70:30%) was sparged into a bubble column reactor and computational fluid dynamics simulations of this setup were used to evaluate the mixing process. The formation of carbonate and bicarbonate ions throughout the carbonation process was followed using a Fourier-Transform Infrared (FTIR) probe and a pH meter. The absorption capacity was measured from the weight increase of the reactor. It was found to be around 30 g of carbon dioxide/L of black liquor. The carbonate and bicarbonate species in black liquor before and after carbonation were further characterized with 13C Nuclear Magnetic Resonance (NMR), X-ray Diffraction (XRD), Scanning Electron Microscope (SEM) and optical microscopy. Using industrial side-streams might enable an economically feasible process without the need for production of virgin absorbents or their recovery. Furthermore, this capturing process, which is performed at atmospheric conditions might reduce the overall energy consumption. The results demonstrated that black liquor could be an attractive absorbent for carbon dioxide, paving the way for a circular and resource-efficient economy.