Average Mass Transfer Coefficient Research Articles

Research on the influence of aeration manners on the mass transfer characteristics of bubbles in multi‐column airlift photobioreactor

AbstractMass transfer of bubbles is important in microalgae cultivation. In this study, aiming to improve the microalgae culturing efficiency, computational fluid dynamics (CFD) was adopted to study the influence of the aeration manners on mass transfer of bubbles within ascending columns in multi‐column airlift photobioreactor (PBR) and the hydrodynamic conditions within the PBR under different aeration manners. In addition, the bubble generation time, the detachment diameter, and the average volumetric mass transfer coefficient of bubbles in ascending columns were analyzed. Furthermore, the experimental results were compared with the simulation results obtained from microalgae cultivation. The results showed that the whole aeration manner yields the lowest mixing strength and mass transfer efficiency of bubbles at an aeration rate of 0.2 vvm. Conversely, both the mixing strength of the liquid and mass transfer coefficient of bubbles were enhanced under the half and alternate aeration manners. However, the results demonstrated that the distribution of the flow field was not uniform under the half aeration manner and there were obvious high‐speed and low‐speed zones. In contrast, the flow field distribution in the PBR was more uniform under the alternate aeration manner, which was suitable for microalgae cultivation at high concentration. This study effectively enhanced the mixing strength and CO2 transfer rate in the PBR.

Liquid–liquid mass transfer and pressure drop behavior in micro‐packed beds with different shaped packings

AbstractIn this work, the liquid–liquid two‐phase pressure drop and mass transfer characteristics in micro‐packed beds with different shaped packing are investigated. The experimental results show that the liquid–liquid mass transfer performance of micro‐packed bed is significantly affected by both apparent flow rates and phase ratio. By changing the characteristic size of the micro‐packed bed, the experimental results show that the average volume mass transfer coefficient has no obvious correlation with the length of the bed, which indicates that the flow and dispersion behavior along the length of the micro‐packed bed is basically stable. By considering dispersion strengthening, vortex strengthening and size effect, a semi‐empirical model for predicting liquid–liquid mass transfer performance in a micro‐packed bed is established for the first time. The model is in good agreement with the experimental results and has strong applicability to various shaped packings. In addition, by considering the contribution of inertia force, viscous force and interfacial tension to energy dissipation, a semi‐empirical model of liquid–liquid two‐phase pressure drop in a micro‐packed bed is established. The pressure drop prediction model can also be applied to different shaped packings.

Experimental evaluation of performance, drying and thermal parameters of guava slabs dried in a forced convection indirect solar dryer without and with thermal energy storage

The forced convection indirect solar dryer (FISD) provides better quality food products for lower duration than open sun drying (OSD). The drying experiments on guava slices were executed on an FISD without a thermal energy storage (TES) (model-1) and with TES (model-2) to determine the impact of TES on drying. The guava slices attained a final moisture content in 14 and 9 sunshine hours in model-1 and model-2, respectively. The drying models appropriate to describe guava slices in model-1, model-2 and OSD were Page and modified Page models. The drying rate in model-1 and model-2 was found to be 0.3936 and 0.5728 kg/h, while the effective diffusivity was 7.98 × 10−9 and 8.63 × 10−9 m2/s (average), respectively. The average heat and mass transfer coefficients were 4.842 W/m2K and 4.20 × 10−3 m/s in model-1 & 5.394 W/m2K and 4.68 × 10−3 m/s in model-2. The average drying efficiency and specific moisture extraction rate were 6.84 % and 0.597 kg/kWh in model-1 & 9.77 % and 0.972 kg/kWh in model-2. The specific energy consumption and activation energy were decreased by 38.57 % and 5.25 % in model-2 compared to model-1. The performance, drying and thermal parameters were found to be better in model-2.

Intensification of the rate of heat and mass transfer at the wall of a stirred tank reactor by integrating a helical coil turbulence promoter with the tank wall

ABSTRACT The influence of a helical coil turbulence promoter on liquid-solid mass transfer rates in a stirred tank was investigated using a technique that involved measuring the rate of diffusion-controlled copper dissolution in acidified dichromate. The study examined the effects of helical coil tube diameter (d), helical coil pitch (p), impeller rotation speed, impeller geometry (axial and radial), the presence of baffles, and solution physical properties. The presence of the helical coil near the reactor wall significantly enhanced mass transfer rates, with a factor ranging from 1.01 to 2.48. This improvement was accompanied by a corresponding increase in the volumetric mass transfer coefficient (kA), ranging from 2.68 to 4.55, depending on operating conditions. Further analysis revealed that the average mass transfer coefficient increased with decreasing coil tube diameter, while coil pitch had a minimal impact. Additionally, radial flow turbines demonstrated superior mass transfer performance compared to axial flow turbines. The presence of baffles also promoted higher mass transfer rates compared to unbaffled reactors. The experimental data were successfully correlated using dimensionless correlations, emphasizing the significance of these correlations in scaleup and design optimization of catalytic stirred tank reactors with efficient cooling systems.

Flow characteristics analysis and performance evaluation of a novel rotary proton exchange membrane fuel cell

This paper proposes a new type of rotary proton exchange membrane fuel cell (R-PEMFC) and its simulation model is established and verified based on experimental data. Then the effects of different rotational angular speeds (RASs) on reactant flow characteristics and electrochemical reactions are investigated. Also, the key quantitative indexes, such as average value, uniformity index and effective mass transfer coefficient (EMTC) are adopted, while combining the polarization curve and net power to estimate the comprehensive R-PEMFC performance. The results indicate that the R-PEMFC effectively ameliorates uneven distribution and improves the transmission capability and output performance. Compared with the static state (0 rpm), the EMTC and maximum power density are enhanced by 65.86 % and 8.51 % respectively at the RAS of 1080 rpm, and the net power density can reach 5316 W·m−2.

The electrodiffusional theory for the wall shear stress measurement by two-strip probe

This article deals with the derivation of the fundamental theory describing the mass transport on the active surface of a two-strip mass transfer probe for an arbitrary direction of fluid flow. The existence of such a general theory is a critical point for the application of the electrodiffusion method using a two-strip probe. Considering the different possible sizes of the probe segments, the analytical formulas for the average mass transfer coefficients are derived and presented in dimensionless forms for both segments. The correctness of the derived analytical expressions is verified by numerical solution of the convection-diffusion transport equation. A methodology for possible experimental data treatment is also proposed. It is based on the evaluation of two current signals collected from the segments of a two-strip probe. From the derived equations, it is possible to determine the magnitude and direction of the wall shear rate vector for both frontal and reverse flow regimes. From the analysis of the current ratio predictions for different probe geometries, an optimal probe configuration is found with respect to the sensitivity of the flow direction measurement.

Performance assessment of passive indirect solar dryer comparing without and with heat storage unit by investigating the drying kinetics of carrot

The present study examined the drying kinetics of carrots and the effectiveness of passive indirect solar dryer without (setup-I) and with (setup-II) thermal energy storage (TES). A module containing paraffin wax as a TES unit was incorporated after the test was completed in setup-I to create setup-II. The sample slices were dried in setup-I and setup-II at a drying rate of 0.5 and 0.59 kg/h, respectively. In setup-I, the sample dried from 9.13 to 0.478 dry basis (db) in 16 h, whereas, setup-II took 15 h. The thermal efficiency of the solar heater and actual heat supply for the same were 56.84 & 60.79% and 705.64 & 730.45 W, respectively. Similarly, the average heat and mass transfer coefficients (h and hm) were 6.32 & 7.08 W/m2 K and 0.0055 & 0.0062 m/s, and average effective diffusivity (De) for the same setups were 6.7 × 10−9 and 7.24 × 10−9 m2/s, respectively. Furthermore, the specific energy consumption and specific moisture extraction rate for the same were 3.5 & 0.28 kWh/kg and 0.29 & 3.62 kg/kWh, respectively. Setups I and II had an average drying efficiency of 7.5 and 10.25%, respectively. The activation energy was evaluated for both setups. The De, h and hm were correlated with moisture content for both setups. In addition to helping the drying process to be completed in a day, setup-II performed better in all the evaluated parameters than setup-I. To ensure the reliability of the results, an experimental uncertainty analysis was conducted.

Growth characteristics and the mass transfer mechanism of single bubble on a photoelectrode at different electrolyte concentrations.

In the photoelectrochemical water splitting reaction, the bubble attached to the working electrode is an essential factor affecting the reaction resistance, current density and gas-liquid mass transfer. An experimental measurement system based on an electrochemical workstation synchronously coupled with a high-speed microscopic camera was proposed and used to systematically study the growth kinetics and mass transfer mechanism of single oxygen bubbles at different electrolyte concentrations (Na2SO4, 0.1-2.0 M) on the TiO2 photoanode surface. Under constant voltage and constant current control conditions, when the electrolyte concentration increases, the bubble detachment diameter and the bubble detachment frequency gradually decrease. The bubble coverage equation expressed in terms of gas evolution efficiency is proposed and is associated with the photocurrent and bubble radius. The average bubble coverage and average gas evolution efficiency decrease when the electrolyte concentration is increased. According to the Sherwood dimensionless number, various mass transfer coefficients during bubble growth were calculated. The results show that the average total mass transfer coefficient is positively correlated with the change trend of the electrolyte concentration, and the mass transfer coefficient of single-phase natural convection is one order of magnitude larger than the mass transfer coefficient of bubble-induced convection. Finally, a conclusion on the transient mass transfer process in the bubble evolution process was obtained, that is, the mass transfer coefficient of single-phase natural convection and the total mass transfer coefficient remain high during the first growth stage, and gradually decrease during the second growth stage. Therefore, regulating the electrolyte concentration can effectively promote the gas-liquid mass transfer in the photoelectrochemical water splitting reaction.

Kinetics of bitumen extraction from oil sands using organic solvents

AbstractThe kinetic study of bitumen extraction from oil sands is significant for efficiency improvement and reactor design in industrial applications. Here, an improved pseudo‐first‐order kinetic model was established with the assumption of a constant Sherwood number, which took into account the mass transfer area. Five organic solvents were used for the extraction kinetics of the prepared bitumen microbeads. The improved pseudo‐first‐order kinetic model exhibited excellent fitting effects. Petroleum ether/dichloromethane was proved to be the best solvent. The results also showed an exponential decrease in the diameter of the bitumen microbeads and a linear increase in mass transfer coefficients with time for all the solvents. The kinetics of bitumen extraction from oil sands using different solvents were predicted according to the corresponding average mass transfer coefficients. The experimental results confirmed that the improved kinetic model was capable of describing the mass transfer process of bitumen extraction from oil sands.

Carbon dioxide capture in a HiGee reactor with packing featuring controllable cross-sectional area

A rotating packed bed (RPB), as one of the typical HiGee reactors, has achieved extensive praise in CO2 capture fields. In this work, the novel mesh packings with controllable cross-sectional areas (CSAs) were applied to intensify the CO2 absorption process in an RPB, as well as combining with the blended aqueous amine solutions including diethylenetriamine (DETA) and piperazine (PZ). The gas pressure drop and CO2 absorption performance of various packings were investigated. Different operating conditions including rotational speed (400–2400 r/min), liquid flow rate (10–35 L/h), gas flow rate (900–2400 L/h), and CO2 molar fraction in gas inlet (2–12%) were considered. The results exhibited that the rise in gas pressure drop of concentric packing with constant CSA (CONC-9) relative to conventional coiled packing with increasing CSA (COII-9) was less than 10% on average. Meanwhile, the CO2 absorption efficiency of CONC-9 exceeded 17.1% on average than that of COII-9. For further comparison, the ratio of mass transfer coefficient to gas pressure drop in the wet RPB (KGa/△PW) was analyzed. The KGa/△PW of CONC-9 was 48.6% larger on average than that of COII-9. The average mass transfer coefficient and height of mass transfer unit of CONC-9 reached up to 16.7 kmol∙kPa−1∙m−3∙h−1 and 0.013 m, respectively. Overall, it is a viable way to adopt the packing with constant CSA to intensify the CO2 absorption in RPBs.

Investigation of supercritical CO2 mass transfer in porous media using X-ray micro-computed tomography

Understanding the mass transfer of CO2 into formation brine both qualitatively and quantitatively is important for improving the security of geologic carbon sequestration. In this study, quasi-dynamic X-ray micro-computed tomographic (MCT) imaging was used to track the time-evolution of supercritical CO2 (scCO2) clusters in a sandstone throughout brine injection. A cluster-matching workflow enabled the identification of depletion, merging, and snap-off of the scCO2 clusters, and subsequently the mass transfer coefficient of individual scCO2 clusters was found to range between 3.0×10−5 and 3.5×10−4 mm/s. The macroscopic average mass transfer coefficient was estimated as 1.4×10−4 mm/s. For application to geologic carbon sequestration, these values give an indication of the range of mass transfer coefficients that may be expected for similar state and flow conditions. With the macroscopic average mass transfer coefficient evaluated, we back-calculated the in-situ CO2 concentration field for brine, which provides quantitative insight of the distribution of dissolved CO2 in the sample. Despite slow injection rate (Ca = 10−7), mobilization of small scCO2 clusters was also observed, and was attributed to the combined effect of incomplete dissolution of snapped-off clusters and the reduction in the fluid–fluid interfacial tension (IFT) due to the high local CO2 concentration in brine accompanying scCO2 dissolution. This highlights the coupling of dissolution and mobilization processes and demonstrates the need to understand these interlinked dynamics to improve CO2 storage in geological formations.

Convective and evaporative heat transfer coefficients during drying of ivy gourd under natural and forced convection solar dryer.

In the present work, a study on convective heat, mass transfer coefficients and evaporative heat transfer coefficient of the thin layer drying process of ivy gourd is performed. The experiment was conducted in three drying modes such as natural, forced convection solar dryer and open sun drying. The hourly data for the rate of moisture removal, sample temperature, relative humidity inside and outside the solar and ambient air temperature for complete drying have been recorded. The drying air temperature varied from 55, 65, 70 and 75°C, and the air velocity was 1, 1.5 and 2m/s. All the drying experiments had shown a falling rate period. The data obtained from experimentation have been used to evaluate the experimental constant values of C and n by simple regression analysis. Based on the values of "C" and "n", convective and evaporative heat transfer coefficients for ivy gourd were determined. The average convective heat and mass transfer coefficients varied between 2.64 and 8.30W/m2 °C and 0.0025 to 0.0076m/s for temperature ranges, at the different air velocities, respectively. The average evaporative heat transfer coefficient for ivy gourd varied from 181.89 to 421.84W/m2 °C. It was observed that convective and evaporative heat transfer coefficients increase with the increase in drying air temperature. The rate of increment of evaporative heat transfer coefficient is higher than the convective heat transfer coefficient. The intensity of heat and mass transfer during solar drying depends on the drying air temperature and velocity.

Study of hydrodynamics and mass transfer in the bench-scale membrane testing devices

Computational fluid dynamics (CFD) was used to study hydrodynamics and mass transfer in a conventional circular bench-scale membrane testing device, and improved geometry of the device was proposed. The conventional and modified geometries were fabricated for experimental validation of the simulation results. It was observed that the modified cell outperformed the conventional cell in terms of distribution of mass transfer coefficient and shear stress on the membrane surface. Further, the ranges of average shear stress and mass transfer coefficient in the modified membrane testing cell for crossflow rates between 25 and 60 L/h overlaped with the corresponding ranges observed in a spacer-filled membrane channel. Hence, the modified geometry is a better choice for membrane testing and can also be extended to study the performance of antifouling membranes.

CO2 Absorption from Biogas Using Piperazine-Promoted 2-Amino-2-methyl-1-propanol: Process Performance in a Packed Column

In this work, CO2 absorption from simulated biogas is investigated using different blends of a PZ + AMP solution in an absorption system at CO2 partial pressures ranging between 20 and 110 kPa. The collected data were presented as CO2 removal profiles along the packed column and were evaluated in terms of CO2 removal efficiency (%) and average overall volumetric mass transfer coefficient in the gas phase (KGav¯). An increased PZ concentration in the AMP solution was found to significantly increase the CO2 removal efficiency and KGav¯ values. It was observed that, when conducted at different CO2 partial pressures, gas and liquid flow rates, and chemical concentrations, the Lamine/GCO2 ratio strongly influenced the process behaviour in the packed column. Additionally, the optimal inlet liquid temperature was observed to be 35 ± 2 °C in this study.

Effect of Phase Change Materials on the Performance of Natural Convection Indirect Type Solar Dryer during Drying Ivy Gourd

The effect of integrating thermal energy storage (TES) unit with a natural convection indirect type solar dryer was investigated by drying ivy gourd in the dryer without TES (setup I) and with TES (setup II) provisions. Paraffin wax was filled in the concentric tubes of polycarbonate and finned aluminum to make setup II by updating setup I. The experimental data from both setups were used to estimate the drying parameters. The average collector's efficiencies were 62.6% and 63.3%. Drying efficiencies were 6.6% and 13.1% for the setup I and II, respectively. The average moisture diffusivity, mass transfer and heat transfer coefficients were 7.1 × 10−9 and 8.1 × 10−9 m2/s, 0.0033 and 0.0041 m/s and 3.9 and 4.7 W/m2K for the setup I and II, respectively. The enhancements of drying efficiency, effective diffusivity, mass and heat transfer coefficients were 98.3%, 14.2%, 24.2% and 22.1%, respectively, in setup II compared to setup I. The activation energy, average specific energy consumption, and specific moisture extraction rate were evaluated. The drying kinetics were correlated with time and moisture variations. It took 18 and 16 h to reduce the moisture content of the sample from 15.32 (db) to 0.144 (db) for the setup I and II, respectively.

Computational Modeling of Drug Dissolution in the Human Stomach.

A computational model of drug dissolution in the human stomach is developed to investigate the interaction between gastric flow and orally administrated drug in the form of a solid tablet. The stomach model is derived from the anatomical imaging data and the motion and dissolution of the drug in the stomach are modeled via fluid-structure interaction combined with mass transport simulations. The effects of gastric motility and the associated fluid dynamics on the dissolution characteristics are investigated. Two different pill densities are considered to study the effects of the gastric flow as well as the gravitational force on the motion of the pill. The average mass transfer coefficient and the spatial distributions of the dissolved drug concentration are analyzed in detail. The results show that the retropulsive jet and recirculating flow in the antrum generated by the antral contraction wave play an important role in the motion of the pill as well as the transport and mixing of the dissolved drug concentration. It is also found that the gastric flow can increase the dissolution mass flux, especially when there is substantial relative motion between the gastric flow and the pill.

BUILDING OF ALLOYED SURFACE LAYER DURING ELECTRIC SPARK TREATMENT OF STEEL 35 BY THE RHENIUM

He article presents the influence of the modes of electric spark placement of the alloyed layer on the surface of steel 35 during electric spark alloying using rhenium anode material. The use of this method allowed material scientists to develop a number of new technologies for creating protective alloyed layers of surfaces made of conductive materials with specified functional properties. (Research purpose) The research purpose is studying the effect of electric spark alloying modes during the layering of alloyed layers on steel 35 using rhenium anode material, to study the kinetics of the electro-mass transfer of the anode material to the surface depending on the energy in the pulse and its duration. (Materials and methods) Rhenium Re (99.99 percent) was used as the anode material, the cathode was steel 35. The next equipment was used: BIG-1, Elitron-22-AM with AG-2 unit, IMEIL installation; EVO-50 XVP S. ZEISS SEM, RigakuMiniFlex-II diffractometer (Japan), Cu-Ka radiation, ICDD database; scales VLO-200, VLR-220; micro hardness tester PMT-3M. The samples were cleaned in an ultrasonic bath. (Results and discussion) The data of the total change in the erosion of the anode, the gain of the cathode, the coefficient of electric mass transfer in various modes were obtained. The highest average mass transfer coefficient was observed with electric spark alloying with the highest energy values of 1.8 (62.2 percent) and 0.32 joules (37.4 percent) at the lowest frequencies of 20 and 80 hertz. The ratio of the values of energy and depth of the alloyed layer shows a tendency to decrease the depth with increasing energy and increase with increasing frequency. (Conclusions) The article describes the results of the study of the effect of the modes of electrospray alloying of 35 rhenium steel on the kinetics of mass transfer, elemental, phase composition.

Study of Parameters Affecting Thermal Drying Process. (Dept. M)

Parameters affecting drying process has been studied experimentally as it associated with heat and mass transfer between porous bed to be dried and air which flows through the porous bed. To perform this study, a test rig is designed and constructed as it consists of vertical circular duct packed with wet porous bed which is made from clay balls of 20 mm in diameter. Drying process occurs as a result of forcing hot air vertically to pass through the wet porous bed, evaporation of moisture is existed in the bed due to the simultaneous heat and mass transfer. The studied operating parameters are inlet air temperature, inlet air mass flux, porous bed depth and porosity of balls. The average heat and mass transfer coefficients are estimated for flow of hot air through the wet porous bed. The results indicated that, the drying rate is directly proportional to inlet air temperature, mass flux of air, dimensionless bed depth and ball porosity. So, Drying process can be achieved in lower time as increasing both air mass flux and or inlet air temperature and decreasing bed depth. Also, heat transfer coefficient and mass transfer coefficient increase with increasing inlet air temperature, inlet air mass flux, and ball porosity, but they decrease with increasing bed depth. Comparison between the obtained experimental results and the previous work gave the same trend.

Direct measurement of the mass transport coefficient of water in silica-gel using the zero length column technique

In all silica-gel adsorption processes driven by low-grade heat, the kinetics of adsorption of water on silica-gel is very important in order to optimize design and becomes an essential factor in ultra-low grade heat applications. A new approach for the determination of the mass transfer coefficient of water in commercial silica-gel is proposed and demonstrated with measurements on a single particle using the zero length column technique. Under equilibrium conditions the methodology offers the key advantage to acquire equilibrium isotherms with thousands of points in less than one day. This allows to obtain the relationship between vapour concentration and equilibrium adsorbed amount through numerical interpolation. At higher flowrates, the system operates under kinetic control therefore allowing the determination of the mass transfer coefficient. The novel approach calculates all the elements needed to obtain the mass transfer coefficient from the measured signals without the need to use a specific model. Adsorption and desorption experiments were carried out at different flowrates and three different temperatures. The kinetic responses can be used to determine an average mass transfer coefficient, which is consistent with literature values, but a complex behaviour is observed with surface diffusion as the main contribution to the transport process.

Microbubbles intensification and mechanism of wet air oxidation process of MDEA-containing wastewater

ABSTRACT In order to the intensification of gas–liquid mass transfer of MDEA-containing wastewater during wet air oxidation (WAO) process, the microbubbles and millimetre bubbles were applied by fine-pore sparger (5 and 20–30 μm) and single pore sparger (6.35 mm), respectively. Effect of the superficial gas velocity on the average microbubble size, gas holdup and oxygen mass transfer coefficient (KLa) of MDEA-containing wastewater at the ambient conditions was studied. The results showed that the microbubbles (less than 1 mm) were beneficial to enhance mass transfer process and had a higher dissolved oxygen concentration during WAO process of MDEA-containing wastewater owing to higher gas holdup and larger oxygen mass transfer coefficient. The COD removal ratio was 66% at low superficial gas velocity (ug = 0.3 cm/s) in WAO process by microbubbles, while it achieved at high superficial gas velocity (ug = 3.0 cm/s) by millimetre bubbles. The critical oxygen mass transfer coefficient KLa was 0.183 min−1 of MDEA-containing wastewater by 20–30 and 5 μm fine pore sparger, which was 2∼5 times more than that of single pore sparger (<0.1 min−1). The microbubbles could improve dissolved oxygen concentration and enhance the formation of hydroxyl radical at short time with atmospheric pressure. During the WAO process, the MDEA would be converted into intermediates including formic acid, acetic acid, ammonium, nitrite and nitrate. The WAO process with microbubbles could significantly improve the gas–liquid mass transfer performance at low superficial gas velocity and greatly reduce air consumption for MDEA-containing wastewater.