Gas Hold-Up and Mass Transfer in a Vessel with an Unsteady Rotating Concave Blade Impeller

The steady mixing of gas-liquid systems is used where a large development of the interfacial area is required. However, the presence of gas in the liquid reduces the efficiency of mass transfer by reducing the mixing power, due to the creation of gas formations behind the impeller blades and the reduction in density. The efficiency of mass transfer can be increased by using a concave blade impeller or unsteady mixing. Mass transfer efficiency studies for these impellers and unsteady mixing are limited. This paper presents an analysis of the influence of the impeller construction on the gas hold-up and volumetric mass transfer coefficient kLa. Impellers with a different number of concave blades, and with alternatively arranged concave blades, were analyzed. The obtained results were compared with the standard flat blade turbine. The obtained results indicate that the arrangement of the concave blades has the greatest effect on reducing the gas hold-up and kLa. Higher values were obtained for the four-bladed and six-bladed impellers. A comparison of the gas hold-up rate for the unsteady and steady mixing has shown that for steady mixing greater gas hold-up is achieved. The volumetric mass transfer coefficient for unsteady mixing is also greater compared to steady mixing, indicating greater efficiency in mass transfer.

Effects of geometric parameters on volumetric mass transfer coefficient of non-Newtonian fluids in stirred tanks

Scaling up stirred tanks is a significant challenge because of the research gaps between laboratory and industrial-scale setups. It is necessary to understand the effects of scale-up on the mass transfer in stirred tanks, and this requires meticulous experimental analysis. The present study investigates the effects of tank size and aspect ratio ( H L T ${H}_{L}}{T}$ ) on the volumetric mass transfer coefficients of shear-thinning fluids. The experiments were conducted in three stirred tanks of different sizes (laboratory and pilot scale) and geometries (standard and nonstandard). H L T ${H}_{L}}{T}$ was 1 for the standard tanks and 3.5 for the nonstandard stirred tanks. Three sizes of stirred tanks were used: 11 L with H L T ${H}_{L}}{T}$ of 1, 40 L with H L T ${H}_{L}}{T}$ of 3.5, and 47 L with H L T ${H}_{L}}{T}$ of 1. Impeller stirring speeds and gas flow rates were in the range of 800–900 rev min−1 and 8–10 L min−1, respectively. The volumetric mass transfer coefficient was estimated based on the dissolved oxygen concentration in the fluids, and the effects of rheology and operating conditions on the volumetric mass transfer coefficient were observed. The volumetric mass transfer coefficient decreased as tank size increased and increased with an increase in operating conditions, but these effects were also clearly influenced by fluid rheology. The impacts of scale-up and operating conditions on the volumetric mass transfer coefficient decreased as liquid viscosity increased.

Gas–liquid mass transfer characteristics of aviation fuel scrubbing in an aircraft fuel tank

Dissolved oxygen evolving from aviation fuel leads to an increase in the oxygen concentration in an inert aircraft fuel tank ullage that may increase the flammability of the tank. Aviation fuel scrubbing with nitrogen-enriched air (NEA) can largely reduce the amount of dissolved oxygen and counteract the adverse effect of oxygen evolution. The gas–liquid mass transfer characteristics of aviation fuel scrubbing are investigated using the computational fluid dynamics method, which is verified experimentally. The effects of the NEA bubble diameter, NEA superficial velocity and fuel load on oxygen transfer between NEA and aviation fuel are discussed. Findings from this work indicate that the descent rate of the average dissolved oxygen concentration, gas holdup distribution and volumetric mass transfer coefficient increase with increasing NEA superficial velocity but decrease with increasing bubble diameter and fuel load. When the bubble diameter varies from 1 to 4 mm, the maximum change of descent rate of dissolved oxygen concentration is 18.46%, the gas holdup is 8.73%, the oxygen volumetric mass transfer coefficient is 81.45%. When the NEA superficial velocities varies from 0.04 to 0.10 m/s, the maximum change of descent rate of dissolved oxygen concentration is 146.77%, the gas holdup is 77.14%, the oxygen volumetric mass transfer coefficient is 175.38%. When the fuel load varies from 35 to 80%, the maximum change of descent rate of dissolved oxygen concentration is 21.15%, the gas holdup is 49.54%, the oxygen volumetric mass transfer coefficient is 44.57%. These results provide a better understanding of the gas and liquid mass transfer characteristics of aviation fuel scrubbing in aircraft fuel tanks and can promote the optimal design of fuel scrubbing inerting systems.

Numerical study of the hydrodynamics and mass transfer in the external loop airlift reactor

The objective of this study was to investigate the hydrodynamics and the gas-liquid mass transfer coefficient of an external-loop airlift reactor (ELAR). The ELAR was operated in three cases: different inlet velocities of fluids, different alcohols solutions (water, 0.5% methanol, 0.5% ethanol, 0.5% propanol and 0.5% butanol) and different concentration of methanol in solutions (0%, 0.5%, 1%, 2% and 5%). The influence of superficial gas velocity and various diluted alcohol solutions on hydrodynamics and gas-liquid mass transfer coefficient of the ELAR was studied. Experimentally, the gas hold-up, liquid velocities and volumetric mass transfer coefficient values in the riser and the downcomer were obtained from the literature source. A computational fluid dynamics (CFD) model was developed, based on two-phase flow, investigating different liquids regarding surface tension, assuming the ideal gas flow, applying the finite volume method and Eulerian-Eulerian model. The volumetric mass transfer coefficient was determined using CFD model, as well as artificial neural network model. The effects of liquid parameters and gas velocity on the characteristics of the gas-liquid mass transfer were simulated. These models were compared with appropriate experimental results. CFD model successfully succeed to simulate the influence of different alcohols regarding the number of C-atoms on hydrodynamics and mass transfer.

Study on Mass Transfer Characteristics of Hydrate-based Gas Absorber

Hydrated-based gas separation is a method capable of selectively separating and recovering greenhouse gases. Although a conventional hydrate-based gas separation apparatus is a batch or a semi-batch system, continuous operation is preferable to increase the throughput of gas without changing the apparatus volume. Recently, we proposed a flow type apparatus to allow continuous operation of hydrate formation (absorption) and subsequent decomposition (desorption). The aim of this study is to investigate the mass transfer characteristics of the continuous apparatus using the HFC134a-N2 mixed gas system. The volumetric mass transfer coefficient was calculated especially during a steady state of gas absorption. Besides, we compared mass transfer performance between the hydrate-based gas absorber and a conventional bubble column. Sodium dodecyl sulfate was used as a hydrate dispersant. In the flow type apparatus, the gas-liquid contact was good and the hydrate slurry state was observed during hydrate formation. In the surfactant solution, the volumetric mass transfer coefficient increased in comparison with that in water. The volumetric mass transfer coefficient with hydrate was higher than that of the bubble column. These results suggest that hydrate formation improves gas absorption performance.

Sizes of the actively aerated zone and methods of the jet aerator calculation

The possibilities of using jet aeration for biological wastewater treatment in small facilities were considered in the article. In shallow depth reservoirs, studies were carried out to change the sizes of the actively aerated zone with a decrease in the height of the liquid layer in the reservoir. The research was conducted for jet aerators with nozzles of the "keyway" type holes. It was established that a decrease in the relative depth of the reservoir makes it possible to increase the share of the beneficial use of the reservoir volume. This suggests that jet aeration will be most effective for reservoirs of relatively shallow depth. Based on the research results, a method for calculating a jet aerator was proposed, which makes it possible to evaluate the necessary parameters of the aerator based on the determination of the volumetric mass transfer coefficient.

Research on Status and Outlook of Using Different Solvents for CO2 Capture in a Rotating Packed Bed

This study investigates the removal efficiency of carbon dioxide by aqueous absorbents containing monoethanolamine (MEA), piperazine(PZ), diethylenetriamine(DETA) and ionic liquids in a rotating packed bed. The performance of an absorbent was assessed in terms of an overall volumetric mass transfer coefficient and regeneration heat duty. The CO2 removal efficiency in a rotating packed bed was observed to be more suitable than that in a packed column, suggesting a potential of a rotating packed bed can replace a traditional packed column to the usage of reduction of the greenhouse gas CO2 from the exhausted gas. The mixture containing PZ and DETA exhibited a high CO2 removal efficiency among these absorbents. Besides, DETA has a lower regeneration heat duty than MEA, which means the finest mixture for industrial CO2 capture system will be the combination of PZ, DETA, and ionic liquids, instead of traditional alkanolamines, MEA.