Gas-liquid Mass Transfer Rate Research Articles

Mass transfer and bubble hydrodynamics in stirred tank with multiple properties fluid via a CFD‐PBM method

AbstractThe performance of a stirred bioreactor was evaluated in this study in terms of the bubble hydrodynamics and the mass transfer efficiency, using a non‐viscous Newtonian fluid of water, a viscous non‐Newtonian fluid of xanthan, and a viscous non‐Newtonian fluids of xanthan with dispersed soybean powder, respectively. The computational fluid dynamics–population balance model (CFD‐PBM) method coupled with the viscosity model and the mass transfer model is established to simulate the gas–liquid mass transfer process and bubble size distribution in the stirred bioreactor. The results demonstrate that the rheological properties of the fluid play an important role in determining the gas holdup, the mass transfer efficiency, and the bubble size distribution. Viscosity of the fluid exhibits a negative impact on gas–liquid mass transfer rate and gas holdup. Moreover, by properly adjusting the operating conditions such as the stirrer speed, it is possible to modulate the gas dispersion and mass transfer rate in the reactor.

Simulation of a Multistaggered Baffle Scrubber to Enhance the Efficiency of Simultaneous Desulfurization and Denitrification.

In this study, we conduct simulation research on simultaneous desulfurization and denitrification in a multistaggered baffle spray scrubber. By employing two-phase flow simulations within the Euler-Lagrange framework and calculating the gas-liquid mass transfer rate with user-defined functions, we comprehensively analyzed the effects of various operational parameters. Initially, we validated our simulation model by comparing the simulation results with experimental data. Under conditions of a 0.2 mm droplet diameter, a liquid-to-gas ratio (L/G) of 12 L/m3, and a gas flow rate of 5 CMM using a full cone nozzle, the simulation indicated a desulfurization efficiency of 99.90 versus 99.84% obtained experimentally and a denitrification efficiency of 92.01 versus 90.67% obtained experimentally. This comparison confirmed the reliability of the simulation model. Our findings indicate that a droplet size of 2 mm is optimal, enhancing the desulfurization efficiency from 99.90 to 99.98% and the denitrification efficiency from 92.01 to 99.76%. However, when the droplet size exceeds 2 mm, efficiencies marginally decrease. Increasing the liquid-to-gas ratio to 16 L/m3 further improves desulfurization and denitrification efficiencies to 99.98 and 99.80%, respectively. In contrast, higher inlet flue gas flow rates reduce these efficiencies, with a decline observed from 100% to as low as 93.90% for denitrification with 2 mm droplets. Additionally, the use of a swirl cone nozzle, compared to full or hollow cone nozzles, better disperses droplets, enhancing the gas-liquid contact and achieving efficiencies of 99.99% for desulfurization and 99.81% for denitrification with 2 mm droplets. These insights are valuable for optimizing operational conditions in industrial-scale spray scrubbers, significantly contributing to mitigating the environmental impacts of industrial emissions.

Biogas upgrading by biotrickling filter: Effects of temperature and packing materials

Biological hydrogen-methane conversion technology can combine Power-to-Gas technology with biogas engineering and showed great potential in breaking through the limitations of both hydrogen storage and CO2 removal for biogas upgrading. However, the low gas–liquid mass transfer rate of hydrogen greatly limited its conversion efficiency and actual application. This study used a biotrickling filter (BTF) to address this limiting factor, and the effects of temperature and packing materials on hydrogen-methane conversion were investigated. Results showed that higher temperature was beneficial for hydrogen-methane conversion, and the highest conversion efficiency of 8.3 L/Lw·d was obtained at 55 °C with the archaeal community dominated by Methanothermobacter (99.97 %). The reactor using activated carbon as packing material showed the best hydrogen-methane conversion efficiency of 91.9 %, with the most methanogens fixed on the surface. Due to the electronic conductor property of activated carbon, the microbial communities of the biofilm were also different from the other two packing materials. Finally, the H2/CO2 ratio of the gas inflow was optimized, and the most suitable result was 2.5:1 (H2/CO2, v/v), which was far below the previously reported results and indicated higher efficiency in carbon dioxide removal. This study provided a promising way for biogas upgrading.

Intensification of aqueous mineralisation of CO2 with CaO containing industrial wastes in a modified airlift reactor

CO2 mineralisation is considered the safest storage medium for mitigating climate change. One of the challenges with the industrial demonstration of the mineralisation technology is reactor design. In the current study we demonstrate the use of draft tube based reactor for the mineralisation of CO2. The draft tube reactors show better performance as compared to the absence of draft tube at near ambient pressure for CO2 at different concentrations. The draft tube has been modified to increase the gas-liquid mass transfer rate by adding multi-orifice plates on top as well as increasing the solid precipitation in the reactor with a spiral flow arrangement at the bottom of the draft tube. At higher CO2 concentrations and near ambient pressures, the draft tube with modifications shows 2.5 fold enhancement in the overall rate as compared to draft tube without modifications. Additionally, at lower CO2 concentrations (i.e. near flue gas concentrations), the draft tube with modifications has better conversion capacity as compared to draft tube without modifications. This enhancement is due to the increased gas holdup and separation of solids in the draft tube with modifications. The precipitates obtained in the draft tube reactors show nanoparticles arranged in coral or mosaic arrangements as compared to the reactor without draft tube.

Efficient C(sp3)-H Carbonylation of Light and Heavy Hydrocarbons with Carbon Monoxide via Hydrogen Atom Transfer Photocatalysis in Flow.

Despite their abundance in organic molecules, considerable limitations still exist in synthetic methods that target the direct C-H functionalization at sp3-hybridized carbon atoms. This is even more the case for light alkanes, which bear some of the strongest C-H bonds known in Nature, requiring extreme activation conditions that are not tolerant to most organic molecules. To bypass these issues, synthetic chemists rely on prefunctionalized alkyl halides or organometallic coupling partners. However, new synthetic methods that target regioselectively C-H bonds in a variety of different organic scaffolds would be of great added value, not only for the late-stage functionalization of biologically active molecules but also for the catalytic upgrading of cheap and abundant hydrocarbon feedstocks. Here, we describe a general, mild and scalable protocol which enables the direct C(sp3)-H carbonylation of saturated hydrocarbons, including natural products and light alkanes, using photocatalytic hydrogen atom transfer (HAT) and gaseous carbon monoxide (CO). Flow technology was deemed crucial to enable high gas-liquid mass transfer rates and fast reaction kinetics, needed to outpace deleterious reaction pathways, but also to leverage a scalable and safe process.

Insight into the enhancement effect of amino functionalized carbon nanotubes on the H2S removal performance of nanofluid system

By constructing nanofluid system, trace functionalized nanoparticles can significantly enhance the absorption performance of basic liquid. In this work, amino functionalized carbon nanotubes (ACNTs) and carbon nanotubes (CNTs) were introduced into alkaline deep eutectic solvents to build nanofluid systems and used for the dynamic absorption of H2S. The experiment results showed that the introduction of nanoparticles can significantly enhance the H2S removal performance of original liquid. When performing H2S removal experiments, the optimal mass concentrations of ACNTs versus CNTs were 0.05 % and 0.01 %, respectively. The characterization showed that the surface morphology and structure of the nanoparticles unchanged significantly during the absorption-regeneration process. A double mixed gradientless gas-liquid reactor was used to explore the gas-liquid absorption kinetics characteristics of the nanofluid system. It was found that the gas-liquid mass transfer rate increased significantly after the addition of nanoparticles. The highest total mass transfer coefficient of the nanofluid system of ACNTs was increased to more than 400 % of the value before the addition of nanoparticles. The analysis showed that the shuttle effect and hydrodynamic effect of nanoparticles play important role in the process of enhancing gas-liquid absorption, and the amino functionalization enhanced the shuttle effect of nanoparticles significantly.

Study on the Effect of Structural Parameters of Volume Control Tank on Gas–Liquid Mass Transfer

The volume control tank (VCT) is an important facility in the primary circuit of nuclear power plants. During the normal operation of nuclear power plants, the mass transfer between the gas and liquid phases occurs in the VCT at all times. It is driven by submerged jets, which may cause potential risks to the operational safety of nuclear power plants. It is necessary to conduct an in-depth study to gain a deeper understanding of the gas–liquid mass transfer behavior in the VCT. In this paper, a new gas–liquid mass transfer model is developed that combines a surface divergence model with a CFD model to accurately simulate the mass transfer process of the gas phase into the liquid phase. The simulation data were verified by the experimental results. The deviation between the simulation results and experimental results is less than 6.55%. Based on this model, a simulation study was carried out for the effect of structural parameters of the VCT on gas–liquid mass transfer. The results show that the double-vortex structure above the jet inlet, the surface jet at the gas–liquid interface, and the vortex at the end of the jet are the three factors dominating the gas–liquid mass transfer in the VCT. The gas–liquid mass transfer can be influenced by the jet diameter since the jet diameter has a remarkable effect on the Kolmogorov scale and the macroscopic flow field structure. Moreover, both the Kolmogorov scale and the macroscopic flow field structure can be affected by the jet height. However, these two effects cancel each other out. Thus, the influence of the jet height on the gas–liquid mass transfer rate is negligible.

Hydrogenation of Nitrobenzene in Trickle Bed Reactor over Ni/Sio2 Catalyst

Trickle bed reactor was used to study the hydrogenation of nitrobenzene over Ni/SiO2 catalyst. The catalyst was prepared using the Highly Dispersed Catalyst (HDC) technique. Porous silica particles (capped cylinders, 6x5.5 mm) were used as catalyst support. The catalyst was characterized by TPR, BET surface area and pore volume, X-ray diffraction, and Raman Spectra. The trickle bed reactor was packed with catalyst and diluted with fine glass beads in order to decrease the external effects such as mass transfer, heat transfer and wall effect. The catalyst bed dilution was found to double the liquid holdup, which increased the catalyst wetting and hence, the gas-liquid mass transfer rate. The main product of the hydrogenation reaction of nitrobenzene was aniline. Reaction operating conditions, i.e., temperature, liquid flow rate, and initial feed concentration were investigated to find their influences on the conversion and rate of nitrobenzene hydrogenation. Under normal conditions without bed dilution, the system was mass transfer controlled. In the diluted reactor, on the other hand, the resistance of mass transfer was nearly absent and the system became under surface kinetic control. The catalyst showed significant deactivation during the reaction period due to the adsorption of intermediate amine products on the surface of the catalyst. The kinetic study revealed that the reaction is zero order with respect to nitrobenzene concentration for the range of concentration between 0.58 to 1.17 mol/L while it was of positive order for the initial concentration less than 0.58 mol/L

Progress and development of syngas fermentation processes toward commercial bioethanol production

AbstractSyngas is created through the thermochemical conversion of biomass using gasification or pyrolysis and from CO‐rich off‐gases obtained from industries such as steel mills. The Wood–Ljungdahl metabolic pathway, or one of its variations, is used by acetogenic bacteria to convert syngas components (CO, H2, and CO2) to alcohols and other compounds. Many factors affect how well syngas is fermented, including the bacteria species used, syngas composition, medium components, bioreactor type, operational parameters used and the gas–liquid mass transfer rate. These parameters impact carbon and electron flow in the bacteria, influencing the distribution, concentration and metabolic end‐product yield, which determines process feasibility. This article focuses on gas composition, microorganisms, gas–liquid mass transfer fermentation strategies, medium design and commercialization activities to develop the syngas fermentation processes. © 2023 Society of Industrial Chemistry and John Wiley & Sons Ltd.

Task-specific polymeric membranes to achieve high gas-liquid mass transfer

In the current study, Polyimide (P84)-based polymeric membranes were fabricated and used as spargers in the bubble column reactor (BCR) to get a high gas-liquid mass transfer (GL-MT) rate of oxygen in water. Different polymeric membranes were fabricated by incorporating polyvinyl pyrrolidone (PVP) as a porogen and a Zeolitic Imidazolate Framework (ZIF-8) to induce high porosity and hydrophobicity in the membranes. The GL-MT efficiency of membranes was evaluated by measuring the overall volumetric mass transfer coefficient (kLa) of oxygen in air. The kLa of O2 (in air) was measured by supplying the gas through a fixed membrane surface area of 11.94 cm2 at a fixed gas flow rate of 3L/min under atmospheric pressure. The results revealed that adding porogen and ZIF-8 increased the porosity of the membranes compared to the pure polymeric membranes. In comparison, the ZIF-8 (3 wt%) based membrane showed the highest porosity (80%), hydrophobicity (95° contact angle) and kLa of oxygen in air (241.2 h−1) with 78% saturation in only 60 s. ZIF-8 based membranes showed the potential to increase the amount of dissolved oxygen in BCR by reducing the bubble size, increasing the number of bubbles, and improving the hydrophobicity. The study showed that ZIF-8 based membrane diffusers are expected to produce high GL-MT in microbial syngas fermentation. To the best of our knowledge, this is the first study on the fabrication and application of polymeric membranes for GL-MT applications. Further research should be conducted under real fermentation conditions to assess the practicality of the system to support substrate utilization, microbial growth, and product formation.

Predicting gas-liquid mass transfer rates in reactors using a bubble parcel model

A parcel-based approach for predicting mass transfer in gassed bioreactors is presented. Predictions from this parcel model, which handles bubble break-up and coalescence implicitly, are compared directly to measured data and predictions from full-fidelity discrete bubble models. Guidance for determining when this coarse-graining approach is acceptable, as a function of the vessel operating conditions, is presented. When the approach is appropriate, the parcel-based model can present order-of-magnitude reductions in computational runtime with minimal loss in physical fidelity.

Newly isolated Enterobacter cloacae sp. HN01 and Klebsiella pneumoniae sp. HN02 collaborate with self-secreted biosurfactant to improve solubility and bioavailability for the biodegradation of hydrophobic and toxic gaseous para-xylene.

The gas-liquid mass transfer rate of hydrophobic volatile organic compounds (VOCs) is the limiting step in a biological treatment system. The present study aimed to utilize self-producing biosurfactants to enhance the bioavailability of hydrophobic gaseous VOCs. Two novel gram-negative rod-shaped bacteria, Enterobacter cloacae strain HN01 and Klebsiella pneumoniae strain HN02 were successfully isolated from sewage sludge by using blood agar and methylene blue agar plates. The two strains can use para-xylene (PX), a hydrophobic VOC model, as the only carbon source for biosurfactant production. Both strains can produce glycolipid biosurfactants, as confirmed by the emulsification index, Nuclear magnetic resonance, and Fourier transform infrared spectroscopy. Results indicated that PX can be completely decomposed at an initial concentration of 15.50mgL-1, pH value of 7.0, and temperature of 30°C within 36h. The Yano model is suitable for the prediction of the growth kinetics of strains over the entire PX concentration range. Gas chromatography/mass spectrometry analysis indicated that PX was converted into four and four intermediates in the presence of the strains HN01 and HN02, respectively, and the possible mechanisms were proposed. The results can be used in purifying industrial hydrophobic gaseous VOCs and improving the bioavailability of VOCs with self-produced biosurfactants.

Bioreactor modelling for syngas fermentation: Kinetic characterization

A dynamic-kinetic model for gas fermentation of C. carboxidivorans is presented. The model described the acid/alcohol production in gas-fed stirred tank reactors (GFSTR) and in batch pressurized reactors. The model took into account gas–liquid mass transfer rate, carbon monoxide uptake rate and three conversion paths: cell growth associated with acid/alcohol production, alcohol production by acid conversion and direct alcohol production not associated to cell growth. The assessment of kinetic parameters/yields was carried out by regression of experimental data retrieved from the literature. The model was then used to study the effects of several process conditions on the fermentation performances. The sensitivity was analysed with respect to three operating variables (agitation speed, gas feeding flow-rate and bioreactor filling ratio) and also to the kinetic parameters/yields. The proposed model successfully reproduced the experimental data: the R2 of the investigated variable concentrations ranged between 0.769 and 0.885. Sensitivity analysis pointed out that: i) the model was significantly affected by parameters related to the growth kinetic whereas it was not impacted by parameters related to direct alcohol production; ii) fermentation performances could be improved tuning the operating conditions, in particular, the decrease of the mass transfer rate could increase the cell/metabolite production.

Evaluation of ex-situ hydrogen biomethanation at mesophilic and thermophilic temperatures

The biogas is seen as a key renewable resource in achieving the targets of countries renewable energy strategies. However, due to the high amount of CO 2 concentration in biogas production, the energy level of biogas decreases, and the use of biogas as an alternative energy source is limited. The hydrogenotrophic methanogenesis process is one of the key technology that capable to convert CO 2 to CH 4 with H 2 biomethanation. In this study the ex-situ hydrogenotrophic methanogenesis under mesophilic and thermophilic conditions and kinetic parameters such as H 2 utilization rates, CH 4 yields and H 2 gas-liquid mass transfer rates were evaluated. The highest CH 4 concentrations were obtained as of 79 and 81% from the mesophilic and thermophilic reactors at H 2 :CO 2 ratio of 4:1. Also, the efficiency of hydrogen utilization ( η H 2 ) increased to 92% and the CH 4 yield ( Y C H 4 ) reached to 0.28 L CH4 /L H2 in thermophilic reactors. • Biological methanation of CO 2 to CH 4 using H 2 as the electron source was investigated. • Thermophilic conditions were quite effective at CH 4 production. • The highest CH 4 production was obtained at 81% at the H 2 :CO 2 ratio of 4:1. • The H 2 gas-liquid mass transfer was higher at a 4:1 stoichiometric ratio.

Assessing the syngas biomethanation in anaerobic sludge digestion under different syngas loading rates and homogenisation

Gasification offers digestate conversion into syngas, which can be metabolised to CH4 by a mixed anaerobic consortium. However, low gas-liquid mass transfer and CO presence can limit further scale-up. The present study assessed syngas biomethanation (55% H2, 45% CO) in a thermophilic lab-scale continuous anaerobic digester processing sewage sludge. Different syngas loading rates and mechanical stirring compared to pneumatic mixing were investigated. Results showed that anaerobic culture acclimation for CO utilisation is unnecessary. Moreover, the mechanical stirring replacement for the pneumatic system enhanced the gas-liquid mass transfer rate. Hence, the biological conversion of CO and H2 was improved by 30%. Nevertheless, increased pH to 8.6 resulted in acetic acid accumulation (>7.5 g/L). These findings improve syngas biomethanation efficiency and suggest including a proper pH control before further scale-up.

Experimental study on the absorption enhancement of CO2 by MDEA‐MEA based nanofluids

AbstractNanofluids promote the application of nanotechnology in the mass transfer process, which can effectively reduce mass transfer resistance and increase the gas–liquid mass transfer rate. A bubbling absorption system was used to explore the CO2 absorption performance of methyldiethanolamine (MDEA) and monoethanolamine (MEA) blended nanofluids. TiO2 nanoparticles and sodium polyacrylate (ASAP) surfactant were added to MDEA‐MEA aqueous solutions to prepare nanofluids. The effects of MDEA concentration, MEA concentration, blended amine ratio, and nanoparticle solid content on CO2 capture performance were investigated, respectively. The study found that, compared with single‐component MEA/MDEA solutions, nanofluids have a more significant enhancement in CO2 absorption of blended amines. The increase of MEA concentration in blended amine has a negative effect on the enhanced CO2 absorption of TiO2 nanoparticles. The enhancement factor of MDEA‐MEA nanofluids at each solid content is higher than that of two single‐component nanofluids. The best solid content of TiO2 nanoparticles in the MDEA‐MEA system is 0.6 g/L, while the enhancement factor E in 25% MDEA + 5% MEA nanofluids can reach up to 1.36. Finally, a new empirical formula was proposed to predict the enhancement factor and optimum solid content of TiO2‐MEA/MDEA nanofluids.

Improving CO2 mass transfer in microalgal cultures using an oscillatory flow reactor with smooth periodic constrictions

Although very promising, the use of microalgae for CO2 capture still faces some limitations. Due to the low solubility of this gas in the liquid medium, poor CO2 utilisation efficiencies by microalgae are very common, resulting in low CO2 fixation rates and CO2 losses to the atmosphere. Considering this limitation, several researchers have focused on improving photobioreactor (PBR) design to increase CO2 gas-liquid mass transfer rates. However, these systems typically rely on high fluid turbulence and power consumption due to CO2 gas compression. As an alternative, this study proposes using an oscillatory flow reactor with smooth periodic constrictions (OFR-SPC). So far, this reactor design has been successfully applied to improve the mixing efficiency of several chemical processes. However, there is no evidence of its use for microalgal cultivation. To evaluate its potential for microalgal biomass production, a detailed characterisation of CO2 mass transfer was performed. CO2 mass transfer and bubbles’ dynamics were evaluated under different operational conditions: (i) oscillation amplitude and frequency; (ii) aeration rate; (iii) CO2 concentration in the inlet gas; and (iv) biomass concentration. This study made it possible to understand the effect of each process variable on the individual values of liquid-side mass transfer coefficient, kL, and specific interfacial area, a. According to the results, CO2 gas-liquid mass transfer was enhanced by increasing the oscillatory movement, aeration rate and CO2 concentration. Moreover, this study demonstrated that promising CO2 mass transfer rates can be achieved without significantly increasing power consumption and fluid turbulence.

The Role of Mn Doping on Ce-Based γ-Al2O3 Catalysts for Phenol Degradation

In this article, Mn doping on Ce-based γ-Al2O3 (i.e., Mn-Ce-γ-Al2O3) was synthesized and then employed as catalysts to study the removal of phenol from aqueous solutions through catalytic ozonation process. The results showed that the phenol removal was remarkably increased to 97.50% if Mn-Ce-γ-Al2O3 was adopted as the catalyst. In addition, the kinetic rate of the reaction was increased, approximately, about 1.49 times when using Ce-γ-Al2O3 catalyst with Mn doping. Moreover, X-ray diffraction, transmission electron microscopy, Brunner-Emmet-Teller measurements, and X-ray photoelectron spectroscopy techniques were used to comprehensively characterize the physicochemical properties of the synthesized Mn-Ce-γ-Al2O3 catalyst, including the phases, morphologies, textural properties, and dispersity of the surface active species. More uniform channel mesoporous structures were obtained after Mn doping on Ce-γ-Al2O3, which could enhance the gas-liquid mass transfer rate, surface hydroxyl groups, Ce3+/Ce4+ ratio, and the formation of Mn-Ce solid solution. In this study, the effects of Mn doping on Ce-based γ-Al2O3 on phenol degradation were investigated and the removal mechanisms were suggested for a better understanding of the improved removal efficiency.

Integrating H2 injection and reactor mixing for low-cost H2 gas-liquid mass transfer in full-scale in situ biomethanation

Abstract In situ biomethanation combines Power-to-Gas and biogas upgrading by converting electricity-derived H2 and biogas-CO2 to CH4 in existing biogas reactors. Development of industrially relevant H2 injection technologies that facilitate high H2 gas-liquid mass transfer rates is key to advance the technology to commercial application. This study shows the potential of using a venturi-based biogas reactor mixing system for H2 injection into a full-scale biogas reactor by assessing the system’s H2 gas-liquid mass transfer capacity in short-term H2 injection experiments (1 h). H2 conversion rates were limited by H2 gas-liquid mass transfer at all experimental conditions and governed by H2 injection and biogas production rates. The highest experimental H2 conversion rate and H2 conversion efficiency amounted to 24.7 L H2 · m−3 · h-1 and 49 %, respectively. A predictive model suggests that the injection system can facilitate H2 conversion rates to a level equivalent to the stoichiometric conversion of 24–33% of the internally produced CO2 to CH4 in the full-scale reactor. The studied H2 injection technology hence provides a potential solution for low-cost partial in situ biogas upgrading, by using already installed mixing-capacity to facilitate H2 gas-liquid mass transfer.

Simulating of Microbial Growth Scale Up in a Stirred Tank Bioreactor for Aerobic Processes using Computational Fluid Dynamics

In this paper, the effects of some empirical scale up criteria (Reynold's number and Impeller power per bioreactor volume) on gas-liquid mass transfer and microbial growth rates are analyzed using computational fluid dynamics simulations in a stirred tank bioreactor. The effects of turbulence, fluid flow and bubbles dynamics were simulated by using a two equation turbulence model, a multiple reference frame and a population balance, respectively. The impeller power per bioreactor volume scale up criterion showed better microbial growth rates than Reynold's number scale up criterion. However, high cellular damage due to turbulence in animal cell culturing are still challenges in large scale up bioreactor prototyping.