High Gas-liquid Mass Transfer Research Articles

The tubular baffled reactor and its potential for the biological methanation of carbon dioxide

The biological Power to Methane process (PtM) is gaining ground as an answer to the long-term renewable energy storage problem. Methane is an efficient hydrogen carrier, has an established worldwide transport infrastructure and can serve as a link between renewable power generation and a circular carbon economy. One of the defining factors regarding the scalability of the PtM process is the design of the reactor as it can determine the production rate/energy expenditure ratio. The tubular baffled reactor, a popular reactor design within the chemical industry has been assessed in the present study as a biomethanation reactor for the first time. The experiments were conducted with mixed cultures and the results point to high gas-liquid mass transfer capabilities as indicated by the methanation rates achieved (>90 % CH4 at 270 L/L/d mixed gas input rate). The gas/liquid flow ratio appears to have a stronger effect on methanation than the gas residence time. The working length of the reactor determines the pressure drop experienced by the culture, with higher pressure drops showing a negative correlation to methanogenesis.

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.

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.

A modelling approach to explore the optimum bubble size for micro-nanobubble aeration

Bubble aeration has been widely applied in water/wastewater treatment, however its low gas utilization rate results in high energy consumption. Application of micro-nanobubbles (MNB) has emerged as a process with the potential to significantly increase gas utilisation due to their high relative surface area and high gas-liquid mass transfer efficiency. In this study, we demonstrate through calibrated models that MNB of an optimum bubble size can shrink and burst at or below the water surface enabling (1) all encapsulated gas to thoroughly dissolve in water, and (2) the bursting of nanobubbles to potentially generate free radicals. Through the understanding of MNB dimensional characteristics and bubble behaviour in water, a dynamic model that integrated force balance (i.e. buoyancy force, gravity, drag force, Basset force and virtual mass force), and mass transfer was developed to describe the rising velocity and radius variation of MNB along its upward trajectory. Unlike for conventional millimetre-sized bubbles, intensive gas dissolution of MNBs led to radius reduction for small bubbles, while a large initial radius triggers bubble swelling. The initial water depth was also crucial, where greater depth could drive the potential for bubble shrinkage so that they were more liable to contract. For example, the optimum bubble size of air (42–194 μm) and oxygen (127–470 μm) MNB that could achieve complete gas transfer (100% gas utilisation) for a range of specific water depths (0.5–10 m) were calculated. The modelling results for microbubbles (10–530 μm) were well validated by the experimental data (R2>0.85). However, the validation of the modelling results for nanobubble (<1 μm) aeration requires further study due to a lack of available empirical data. In this study, the proposed model and analysis provided new insights into understanding bubble dynamics in water and offered fundamental guidance for practitioners looking to upgrade bubble aeration system.

Selective absorption process intensification of ammonia from melamine exhaust gas using a counter airflow shear-rotating packed bed

The molar ratio of ammonia to carbon dioxide (NH3/CO2) in melamine exhaust can reach up to (2.0–2.4):1, and its direct emission will aggravate the greenhouse effect and waste of resources. Therefore, there is an urgent need to separate and recycle ammonia from the gas mixture in a more economical and efficient way. As an effective process intensification equipment, the counter airflow shear-rotating packed bed (CAS-RPB) has the advantages of high gas-liquid mass transfer efficiency, milliseconds of residence time and enhanced gas turbulence，which could enhance the absorption rate of ammonia in the gas mixture, inhibit the absorption of carbon dioxide caused by chemical absorption, and realize the selective absorption of ammonia.In this study，the CAS-RPB was adopted as an absorption device and the water was used as absorbent, the effects of the gas-liquid (G/L) ratio and high gravity factor (β) on absorption process were investigated. It was found that under suitable operating conditions, NH3/CO2 was close to 1:1 or 2:1 after absorption, which meet the requirement for the co-production of ammonium hydrogen carbonate or urea, the selectivity of ammonia was above 94 %, and the ammonia concentration in the absorption liquid were not less than 8.87 wt%. To compared with Cross-flow rotating packed bed (RPB), the absorption efficiency, selectivity, and gas volumetric mass transfer coefficient of ammonia are increased by 3.34 %, 8.83 %, and 7.02 % in CAS-RPB, respectively. The results show that the CAS-RPB can significantly enhance the selective absorption process of ammonia from melamine exhaust, therefore ammonia and carbon dioxide with potential high-added value can be obtained and utilized, and the zero-emission of ammonia and carbon dioxide can be realized.

Temperature-controlled bacteria biofilm adhesion and formation for CO/CO2 bioconversion to ethanol by grafting N-isopropylacrylamide@SiC

Biofilm with high biomass density and high gas–liquid mass transfer efficiency is one of the most efficient bacterial culture modes for ethanol fermentation using CO/ CO2 gas. However, it is very difficult and requires a long time for bacteria to form a stable biofilm due to the weak adhesion ability between bacterial cells and the substrate. To synergistically enhance bacterial adhesion during biofilm formation and biofilm detachment after fermentation to reuse the substrate, a substrate surface with controllable properties is expected. Thus, in this study, a temperature responsive substrate was prepared by grafting a thermosensitive hydrogel of N-isopropylacrylamide onto the surface of a silicon carbide (SiC) substrate to accelerate the formation of biofilm. The hydrophobicity of the modified substrate surface changed from 52.6° to 89.4° as the temperature increased from 25 °C to 37 °C (the optimal cell culture temperature). The adhesion free energy between the bacterial biofilm and the substrate changed from −4.11 MJ m−2 to −41.7 MJ m−2, indicating a stronger adsorption capacity. Moreover, the surface zeta potential increased from −7.7 mV to +7.6 mV, forming an electrostatic attraction between the cell (-27 mV) instead of electrostatic repulsion force. All the above making the bacteria easier adhere and forming biofilm faster. Compared with that on the original SiC substrate, the bacterial biofilm-forming rate was increased by 6.2 times, which shortened the stable biofilm-forming time to 4 days. The mean ethanol productivity was 0.54 g/L d-1, 116 % higher than that on the original SiC substrate. Therefore, controlling bacterial adhesion on the temperature responsive substrate was an efficient way to enhance ethanol production by CO/CO2 fermentation.

Efficient Depolymerization of Lignin to Biobased Chemicals Using a Two-Step Approach Involving Ozonation in a Continuous Flow Microreactor Followed by Catalytic Hydrotreatment

Lignin is a promising feedstock for the replacement of conventional carbon sources for the production of chemicals and fuels. In this paper, results are reported for the depolymerization of various residual lignins in the absence of a catalyst by utilizing ozone. Reactions were performed in a microreactor setup ensuring high gas-liquid mass transfer rates, a low inventory of ozone, and straightforward scale-up possibilities. The ozonation is demonstrated using a representative model compound (vanillin) and various lignins (pyrolytic and organosolv) dissolved in methanol (2.5 wt %). Experiments were performed under ambient conditions, at gas-liquid flow ratios ranging from 30 to 90 and short residence times on the order of 12-24 s. Analyses of the products after methanol removal revealed the presence of (di)carboxylic acids, methyl esters, and acetals. Extensive depolymerization was achieved (i.e., up to 30% for pyrolytic lignin and 70% for organosolv lignins). Furthermore, a two-step approach in which the ozonated lignin is further hydrotreated (350-400 degrees C, 100 bar H-2, 4 h, Pd/C as catalyst) showed a substantial increase in depolymerization efficiency, yielding a 2.5-fold increased monomer yield in the product oil compared to a hydrotreatment step only.

NOx removal in a rotating packed bed: Oxidation and enhanced absorption process optimization

In view of the continuously stringent standard of nitrogen oxides (NOx) emissions, developing new technologies for NOx removal or updating the existing NOx removal system is very urgent. This work introduces a novel technology for NOx removal in an RPB reactor by the combination of oxidation and enhanced absorption. Experimental results show that a rotating packed bed (RPB) reactor, having high gas-liquid mass transfer efficiency, is a more appropriate choice for NOx removal. Then a series system combining firstly a stirred tank reactor (STR) and secondly an RPB reactor, has been developed in order to oxidize more NO to NO2 that is very soluble and enhances the subsequent absorption in the RPB reactor. The higher oxidation degree (~99%) was achieved with a sodium chlorite concentration of 0.10 mol L−1 and a liquid volume of 5.0 L in the STR. An absorbent containing sulfonated cobalt phthalocyanine (CoPcS) was employed in the RPB reactor to meet the demand of very efficient denitrification for a long-term running. Compared to the traditional urea/sodium chlorite composite absorbent in a single RPB system, the total NOx removal efficiency increased by approximately 30% through the absorbent containing CoPcS in a series system under the optimized conditions.

Application research of micro and nano bubbles in water pollution control

The article describes the characteristics of micro-nano bubbles in the water for a long time, large specific surface area, high gas-liquid mass transfer efficiency, high interface potential and free radical generation. The generation of micro-nano bubbles is introduced, such as dissolved gas release method and dispersion. Air method, electrolysis method, etc. It mainly summarizes the research and application of micro-nano bubbles in surface water treatment, groundwater remediation, industrial wastewater treatment and enhanced ozone treatment, as well as practical applications in ship operation, metal surface degreasing and fruit and vegetable growth.

Reaction kinetics of non-catalyzed jet aeration oxidation of magnesium sulfite

Magnesium sulfite oxidation is a key factor influencing the desulfurization efficiency and effluent quality of magnesium desulfurization processes. Non-catalyzed jet aeration is a promising method for the oxidation of magnesium sulfite because of its high gas-liquid mass transfer efficiency. In this work, the reaction kinetics of the jet aeration oxidation of magnesium sulfite were investigated by the two-film theory. The experiments of clean water aeration showed that the oxygen transfer coefficient of jet aeration increased with the liquid flow rate. The results of jet aeration oxidation of magnesium sulfite revealed that the oxidation process could be divided into two stages according to the concentration of magnesium sulfite, i.e., the saturated stage controlled by the oxygen transfer and the unsaturated reaction stage controlled by the oxygen transfer and the sulfite concentration. In the saturated reaction stage, the oxidation rate was found increasing linearly with the increase of the oxygen mass transfer coefficient. The oxidation reaction occurred in the fast reaction regime. In the unsaturated reaction stage, the oxidation reaction rate was 0.62 order in sulfite ion and zero-order in oxygen, whereas the reaction rate constant was found increasing linearly with the oxygen transfer coefficient. The oxidation reaction transited from moderate to slow reaction regime. Results from this work can serve as a useful reference for designing highly efficient jet aeration systems for oxidation of magnesium sulfite.

CFD modeling of gas–liquid mass transfer process in a rotating packed bed

Process intensification by rotating packed bed (RPB) has attracted wide attention in the recent years. In particular, its high gas–liquid mass transfer efficiency is proved by plentiful experimental data. However, due to the complex structure of packing in the RPB, it is extremely difficult to acquire detailed information about mass transfer process inside the reactor by experiments. Therefore, this study firstly employed computational fluid dynamics (CFD) modeling technique to analyze mass transfer process in a rotating packed bed (RPB) by adding user defined function (UDF) programming to Fluent solver in order to expand its abilities to RPB. The simulation results were compared with previous correlation data on liquid holdup and the calculated values of mass transfer process were matched with experimental values of vacuum deaeration process in RPB. The results revealed liquid flow and mass transfer process inside the reactor and were also in agreement with the experimental data. Additionally, three optimum designs of RPB (kinds of packing, size of rotors and blades added in packing) were developed to improve the mass transfer efficiency. The results show high removal efficiency in the small Δd packing and the rotor with larger inner and outer diameter, and the packing with blades can improve mass transfer efficiency compared to the conventional one. The CFD technique was generally found to be an important and effective tool for analyzing and optimizing RPBs.

Hydrodynamics and gas–liquid mass transfer in a horizontal rotating foam stirrer reactor

This paper describes a new multiphase reactor, the horizontal rotating foam stirrer reactor, which uses a donut-shaped foam block mounted on a horizontal shaft functioning as a stirrer and as a catalyst support. The effect of different operational conditions such as stirring speed, reactor length, foam porosity, foam thickness and the presence of baffles on the gas–liquid mass transfer and the gas–liquid flow distribution is discussed for the systems water/air and glycerol/air. The rate of gas–liquid mass transfer is measured spectrometrically while the hydrodynamics of the reactor is studied by γ-ray tomography. For a partially filled reactor, three flow states could be distinguished: the trickle state, the slosh state and the ring state. In the trickle state the liquid flows in a thin stream over the foam while in the slosh state the liquid is pushed upward by the stirrer and sprayed, leading to the formation of fine liquid droplets and fine gas bubbles. The transition between the trickle state and the slosh state occurs at 200rpm. This is drastically affected by the liquid viscosity and in some extent by the reactor length and the foam thickness. When the stirring speed is constant, the ring state, which results in a cylindrical liquid layer on the inside wall, appears with increasing the liquid content in the reactor (above 70%). Due to a large gas–liquid interface in the slosh state, a high gas–liquid mass transfer is achieved. kGLaGL values up to 0.32s−1 are found. This is comparable to gas–liquid mass transfer rates in slurry reactors. However, in case of the foam stirrer a higher power input per liquid volume is needed in order to achieve the minimum stirrer speed required for complete dispersion of the gas. It is shown that mass transfer coefficients decreased with increasing viscosity, while the centrifugal force revealed to be effective in enhancing mass transfer in a viscous media. Conclusions on the optimal reactor configuration are drawn for the application in the fine chemical industry.

SHORT COMMUNICATION: Ambient gas exchange-enabled stirred culture bottle: A stirred tank reactor alternative to shake flasks for high gas-liquid mass transfer

Through a gas vent riser tube or a gas intake downer tube, spent gas exhaust and ambient substrate gas intake in a stirred bottle culture are an-order-of-magnitude enhanced without line gas sparging when compared to similarly sized shake flask cultures. In addition to realistic impeller mixing, the novel in situ ambient gas pumping design, tested in a 500 mL wide-mouth bottle, produced an E. coli growth at 90% working volume equal to or better than that of a reciprocating 250 mL Erlenmeyer flask at 20% working volume through increased gas ventilation and gas hold-up.

Salting‐out crystallisation using NH3 in a laboratory‐scale gas lift reactor

AbstractDespite the advantages offered by gas lift reactors with respect to high gas–liquid mass transfer with a low energy input, their acceptance in the crystallisation field has been slow, especially on an industrial scale. Potassium chloride, potassium sulfate, and magnesium chloride hexammoniate (MgCl2·6NH3) were crystallised in a laboratory‐scale gas lift crystalliser by salting‐out using ammonia gas. Ammonia mass transfer rates were calculated for each test. To achieve maximum crystal growth, the gas rate in these reactors needs to be optimised for each crystallising system to achieve sufficient gas mass transfer as well as providing sufficient crystal slurry agitation.