Large Mass Transfer Coefficients Research Articles

(Invited) Probing the Oxygen Reduction and Hydrazine Oxidation Reactions Using Transient Amperometric Techniques

The kinetics of electrochemical reactions affected by mass transfer is often investigated by rotating disc electrode (RDE) and rotating ring disc electrodes. However, these hydrodynamic techniques require electrodes that can be shaped into a disk or catalysts that can be drop casted on the surface of a disk electrode. Instead, transient amperometric techniques such as chronoamperometry and sampled current voltammetry (SCV) hold potential to investigate the kinetics of reactions without such requirements.In SCV, a series of chronoamperograms are recorded for a range of target potentials, and the current sampled at a fixed time or over a range of time scales. Sampled current voltammograms are obtained by plotting the selected currents vs the target potentials. 1-3 As we demonstrated recently with ferri/ferrocyanide ions and with oxygen reduction reaction (ORR) on Pt polycrystalline disk electrode, proper selection of the sampling times leads to the same mass transfer coefficients as in the RDE, and so the obtained SCV are equivalent to the linear sweep voltammograms recorded with a RDE. 4 Moreover, the kinetically controlled current can be extracted from the current transients, by plotting the inverse of the current vs the square root of time and by extrapolating to infinitely short times (large mass transfer coefficients). Tafel analysis can then be conducted. 4 In this talk, we will first present how these transient amperometric techniques were used to investigate the oxygen reduction on Au (001) thin films with different thicknesses grown on insulating MgO (001) substrates. 5 It will be shown that SCV is sensitive to the structure and to themosaicity of the thin films, and thus suitable to establish reactivity structure relationships. Then, we will show some of our recent work on the hydrazine oxidation on Au and Pt polycrystalline surfaces. 6 References [1] A. J. V and L. R. Faulkner, Electrochemical methods: Fundamentals and Applications; Wiley, 2001.[2] S.C. Perry and G. Denuault, Phys. Chem. Chem. Phys. 17, 30005 (2015).[3] L. Mignard, M. Denoual, O. Lavastre, D. Floner and F. Geneste, J. Electroanal. Chem., 689, 83 (2013).[4] C. O. Soares, O. Rodríguez, G. Buvat, G., M. Duca, S. Garbarino, D. Guay, G. Denuault and A. C. Tavares, Electrochim. Acta, 362, 136946 (2020).[5] C. O. Soares, G. Buvat, Y. G. Hernández, S. Garbarino, M. Duca, A. Ruediger, G. Denuault, A. C. Tavares and D. Guay, ACS Catal 12, 1664 (2022).[6] D. Kashyap, C. O. Soares, A. C. Tavares, G. Denuault, D. Guay, in preparation. Acknowledgements This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC, Strategic Partnership program). Cybelle Oliveira Soares acknowledges the FRQNT funding by means of the PBEEE merit scholarship and an internship received from the Engineered Nickel Catalysts for Electrochemical Clean Energy project administered from Queen’s University and supported by Grant No RGPNM 477963-2015 under the NSERC Discovery Frontiers Program.

Liquid–Liquid Microfluidic Flows for Ultrafast 5-Hydroxymethyl Furfural Extraction

We study the effects of liquid–liquid flow patterns on the extraction of 5-hydroxymethyl furfural from the aqueous phase into an organic phase (methyl isobutyl ketone, ethyl acetate, and 2-pentanol) at ultrashort residence times of 0.2–20 s in a coiled capillary microchannel. Slug, droplet, slug-droplet, dispersed, parallel, annular, and irregular two-phase flow patterns are generated. We evaluate the extraction efficiency and the volumetric mass transfer coefficient via experiments and computational fluid dynamics (CFD). The CFD predictions are in quantitative agreement with the experimental data. The convective and diffusive contributions to the mass transfer of different flow patterns are analyzed. For all solvent pairs, the extraction efficiency varies irregularly with increasing flow rate due to changes in the mass transfer mechanism caused by a transition between flow regimes. Overall, we demonstrate >90% extraction efficiencies with high volumetric mass transfer coefficients ranging from 0.006 to 2.17 s–1. Furthermore, a merit index is defined to assess the trade-off between the fast mass transfer and the pressure drop in biphasic microchannels. In general, the mass transfer rate increases significantly at high flow rates at the expense of a high-pressure drop. The ethyl acetate/water irregular flow pattern is optimal due to a large throughput, extraction efficiency, and mass transfer coefficient, with a minimal pressure drop.

Research on falling film dehumidification performance of microencapsulated phase change materials slurry

In the process of liquid desiccant dehumidification, the temperature-rise of the desiccant solution caused by moisture absorption will seriously affect the dehumidification capacity and performance. It is expected that the temperature rise of solution can be restrained by adding microencapsulated phase change materials (MicroPCMs) into the liquid desiccant solution. In this paper, based on computational fluid dynamics (CFD) method, two-dimensional models of flat plate and corrugated plate falling film dehumidification of the microencapsulated phase change material slurry (MPCMS) were developed and validated. The dynamic boundary method was used to determine the interfacial mass transfer coefficient. The influences of several factors on the dehumidification performance, including the concentration of MicroPCMs, the inlet temperature of the MPCMS and the falling film plate configuration, were investigated. The results show that, adding MicroPCMs into the liquid desiccant solution is beneficial to the dehumidification performance improvement. The inlet temperature of MPCMS is a very critical variable. When the inlet temperature of the slurry is in the phase transformation temperature range of MicroPCMs, with the adding of the microcapsule, the temperature rise in dehumidification process can be restrained, and the moisture removal performance can be improved significantly. When the MPCMS inlet temperature is at the phase transformation onset point, and the concentration of MicroPCMs is 5%, the water condensation rate is 14.2% higher than that of the base solution. Moreover, it’s not that the more microcapsules added, the larger mass transfer coefficient is. There is an optimum concentration of MicroPCMs to achieve the maximum interfacial mass transfer coefficient. The corrugated plate configuration is more conducive to the dehumidification enhancement by the MPCMS.

Ethylene/ethane absorption with AgNO3 solutions in ultrasonic microreactors

Ultrasound is an effective method to intensify gas-liquid processes as the oscillation energy is easily focused at the gas-liquid interface, which can induce strong interface oscillation and acoustic streaming. The combination of ultrasound and microreactor provides ideal control over the ultrasound field, making the energy efficiently utilized. In this study, characteristics of chemical absorption of C2H4 from the mixture of C2H6/C2H4 into AgNO3 solutions under Taylor flow are investigated in a high-power ultrasonic microreactor. The effect of ultrasound on the bubble size reduction, absorption and mass transfer coefficient is presented and discussed. It is demonstrated that very large mass transfer coefficient with kLa and kL in the range of 7–42 s−1 and 0.00169–0.0225 m s−1 respectively are obtained, presenting significant intensification compared to absorption without ultrasound in the same reactor.

Characterization of the droplet formation phase for the H2O[sbnd]LiBr absorber: An analytical and experimental analysis

The poor heat and mass transfer occurring in the H2OLiBr absorber is one of the main limitations towards reducing the size of absorption chillers. Previous research shows that, as droplets form at the base of the absorber pipes, considerably large mass transfer coefficients can be obtained. However, modeling this phase of droplet formation has not been fully explored, as most of the research focuses on the film flow process preceding the droplet formation. The present study focuses on two aspects. Firstly, the dynamics of the droplet formation is investigated, with a focus on the effect of the solid surface shape on the droplet formation. A model to describe the droplet profile geometry was developed using the Euler-Lagrange equation and validated against experimental tests. Several pin geometries were tested and the results have shown that a 120° rhomboidal geometry is more suitable to increase the liquid-vapor interface area, while lowering the risk of droplet coalescence. Secondly, an analytical heat and mass transfer model based on the Fourier Series method has been developed to study the influence of pin size on the absorption process in an adiabatic absorber. The results show that the optimum width of the 120° rhomboidal pin is found at 6 mm, which maximizes the water absorbed during the droplet formation phase, without excessive use of material. The common assumption that treats the forming droplet as a half sphere fails to capture changes in the pin-droplet interaction which adversely affects the model accuracy. The proposed model shows that for pin widths smaller than 6 mm, the absorption process is impaired by the lower surface area exposed to the water vapor, resulting in up to 67% less mass absorbed obtainable and a decrease in the cooling power obtainable.In more practical terms, a rated 3 kW of cooling power is used as a case study with a 6 mm pin which reduces the absorber volume by 30% with respect to an absorber with 3 mm wide pins. This shows that by treating the droplet formation phase with a more physically-sound geometrical domain can improve the accuracy of the absorption models and can be easily implemented with any programming language to help in the design optimization of the absorber.

Mixing and mass transfer in a pilot scale U-loop bioreactor.

A system capable of handling a large volumetric gas fraction while providing a high gas to liquid mass transfer is a necessity if the metanotrophic bacterium Methylococcus capsulatus is to be used in single cell protein (SCP) production. In this study, mixing time and mass transfer coefficients were determined in a 0.15 m3 forced flow U-loop fermenter of a novel construction. The effect on the impeller drawn power when a gas was introduced into the system was also studied. Mixing time decreased and mass transfer increased with increasing volumetric liquid flow rate and specific power input. This happened also for a large volume fraction of the gas, which was shown to have only minor effect on the power drawn from the pump impeller. Very large mass transfer coefficients, considerably higher than those obtainable in an STR and previous tubular loop reactors, could be achieved in the U-loop fermenter equipped with static mixers at modest volumetric liquid and gas flow rates. Biotechnol. Bioeng. 2017;114: 344-354. © 2016 Wiley Periodicals, Inc.

On flow structures associated with large wall mass transfer coefficients in orifice flows

In this study, the turbulent flow field is simultaneously measured by stereoscopic particle image velocimetry (SPIV) and the wall mass transfer coefficient using the limiting diffusion current technique (LDCT). SPIV measurements of the flow field behind an orifice in a round pipe are performed in the cross and longitudinal sections. One-point linear and quadratic stochastic estimations are employed to calculate the conditional average of the flow field associated with the large wall mass transfer coefficient. The results suggest that the counter-rotating vortex pairs play an important role in the large wall mass transfer coefficient. The flow structures associated with a large wall mass transfer coefficient in the cross sections are found to be similar to the flow structures enhancing the wall heat transfer obtained by a theoretical analysis and numerical study in literature.

Pushing the limits of intensified CO2 post-combustion capture by gas–liquid absorption through a membrane contactor

The possibility to maximize the intensification (i.e., volume reduction) of post-combustion carbon dioxide capture by chemical absorption in a monoethanolamine (MEA) solution using a membrane contactor module is reported. The influence of MEA concentration (20–90wt%) and temperature (293–333K) on the CO2 capture ratio achieved by a PTFE hollow fiber membrane contactor has been investigated with a CO2/N2 (15/85vol%) gas mixture and the experimental results modeled. It is shown, through a systematic parametric analysis, that the intensification factor of the absorption unit, defined as the ratio of the CO2 volumetric absorption capacity of the membrane contactor and of the packed column (taken at 1mol CO2m−3s−1), can be largely increased when a concentrated (70wt%) MEA solution at 333K is used. A volume reduction up to a factor 9, together with a lower energy penalty due to pressure drop effects (kWh per ton of captured CO2) is potentially obtained compared to packed column performances, when a large enough membrane mass transfer coefficient is achieved (i.e., larger than 10−4ms−1). The remaining challenges which are required in order to develop this level of performances at industrial scale based on the existing membrane contactor materials are discussed.

Gas-liquid mass transfer coefficient of methane in bubble column reactor

Biological conversion of methane gas has been attracting considerable recent interest. However, methanotropic bioreactor is limited by low solubility of methane gas in aqueous solution. Although a large mass transfer coefficient of methane in water could possibly overcome this limitation, no dissolved methane probe in aqueous environment is commercially available. We have developed a reactor enabling the measurement of aqueous phase methane concentration and mass transfer coefficient (k L a). The feasibility of the new reactor was demonstrated by measuring k L a values as a function of spinning rate of impeller and flow rate of methane gas. Especially, at spinning rate of 300 rpm and flow rate of 3.0 L/min, a large k L a value of 102.9 h−1 was obtained.

Absorption of steam bubbles in lithium bromide solution

Absorption heat transformers are thermodynamic cycles that are capable of recycling waste heat energy by increasing its temperature. One of the most important unit operations in a heat transformer is the exothermic absorption of water vapour into a solution of choice at a higher temperature. Bubble columns are potentially an efficient means of achieving this. An experimental analysis is conducted which examines the absorption of single steam bubbles into a concentrated aqueous lithium bromide solution. The bubbles are tracked using a high speed camera, and their rate of absorption is modelled using a simple ordinary differential equation model. Accurate model predictions are obtained when oscillating bubble Nusselt and the Sherwood number correlations are utilised. The proposed model is capable of describing 96% of the observed experimental variability. Very large mass transfer coefficients of approximately 0.0012m/s are obtained, which is higher than any previously reported values used in heat transformer absorber design.

Factors affecting flux performance of forward osmosis systems

The performance of a forward osmosis (FO) system may be characterised by the assessment parameters: FO–RO flux ratio (Jw/Jw(RO)), apparent FO water permeability (Jw/(πds−πml)), and the newly developed flux efficiency factor (Jw,ob/Jw,re). The former two parameters offer information on extent of internal concentration polarisation and driving force utilisation, respectively. The Jw,ob/Jw,re factor has practical relevance, and reveals the inevitable trade-off between flux and recovery (φ) for a FO system. The derived Jw,ob/Jw,re factors corresponded well to experimental observations. High water permeability, low salt-to-water permeability ratio, and large mass transfer coefficient improve the performance of a FO system, but these may also be influenced by operational and fouling effects, such as draw solute transmission, fouling resistance and cake-enhanced concentration polarisation. It was shown that membrane properties also play a significant role in fouling behaviour. Fouling amelioration factors include aeration and osmotic backwash. A thin-film composite membrane showed potential for FO application with favourable intrinsic transport parameters. It was demonstrated that a FO system could achieve stable water production with both relatively high flux efficiency (Jw,ob/Jw,re=0.8) and high recovery (φ=95.8%), which attested to the technology potential.

Mass Transfer Coefficient Studies in Bubble Column Reactor

Stirred bubble column are widely used in chemical and allied processes industries. Stirred bubble column reactor promotes significant liquid hold-up and long liquid residence time. It is used when a large mass transfer area and high mass transfer coefficient in both phases are desired. These columns can operate continuously with a concurrent and counter current phase flow where high interfacial areas between phases are possible with low investment cost. An experimental work was undertaken to focus the effect of various parameters like Height to Diameter ratio (H/D), Gas flow rate and Speed of the stirrer (N) on mass transfer coefficient in stirred bubble column reactor. For this purpose, experiments were conducted in 0.14 m i.d column having 2 m height. The column with ring sparger having 67% active area is used as sparger for dispersing the dispersed phase into the continuous phase. For the optimized sparger plate, the effect of various parameters on mass transfer coefficient was studied based on CO2 –absorption Technique.

Pedotransfer functions for solute transport parameters of Portuguese soils

SummaryThe purpose of this study is to quantify solute transport parameters of fine‐textured soils in an irrigation district in southern Portugal and to investigate their prediction from basic soil properties and unsaturated hydraulic parameters. Solute displacement experiments were carried out on 24 undisturbed soil samples by applying a 0.05 m KCl pulse during steady flow. The chloride breakthrough curves (BTCs) were asymmetric, with early breakthrough and considerable tailing characteristic of non‐equilibrium transport. The retardation factor (R), dispersion coefficient (D), partitioning coefficient (β), and mass transfer coefficient (ω) were estimated by optimizing the solution of the non‐equilibrium convection–dispersion equation (CDE) to the breakthrough data. The solution could adequately describe the observed data as proved by a median of 0.972 for the coefficient of determination (r2) and a median for the mean squared error (MSE) of 5.1 × 10−6. The median value for R of 0.587 suggests that Cl– was excluded from a substantial part of the liquid phase. The value for β was typically less than 0.5, but the non‐equilibrium effects were mitigated by a large mass transfer coefficient (ω > 1). Pedotransfer functions (PTFs) were developed with regression and neural network analyses to predict R, D, β and ω from basic soil properties and unsaturated hydraulic parameters. Fairly accurate predictions could be obtained for logD (r2 ≈ 0.9) and β (r2 ≈ 0.8). Prediction for R and logω were relatively poor (r2 ≈ 0.5). The artificial neural networks were all somewhat more accurate than the regression equations. The networks are also more suitable for predicting transport parameters because they require only three input variables, whereas the regression equations contain many predictor variables.

Countercurrent absorption of CO2in a real flue gas into aqueous alkanolamine solutions in a wetted wall column

AbstractThe countercurrent absorption of CO2from the flue gas into aqueous alkanolamines has been studied using wetted wall column equipment connected to the stack of a power plant. The absorption rates of CO2into aqueous alkanolamines were measured at 40°C, 10kPa partial pressure, and 2m3/h of gas flow rate. From the rate of absorption and the absorption equilibrium results, the secondary amines have as large overall mass transfer coefficients as monoethanolamine. However, the coefficients of the hindered and tertiary amines are small even though the absorbed amounts of CO2at equilibrium are large. The CO2concentration profiles in a wetted wall column were estimated, and these results are due to the absorption process in connection with the degree of carbamate formation.

Modeling Quasi-Steady NAPL Dissolution in Fractured Permeable Media

This work explores the rate-limited dissolution of entrapped nonaqueous phase liquids (NAPLs) in a fractured “permeable” formation through the use of mathematical models, developed on the basis of a simple conceptual framework. The feasibility of pump-and-treat operations in such formations is assessed through characterization of dimensionless system parameters governing NAPL dissolution and solute transport. These dimensionless parameters include the ratio of permeable block to fracture flow (mobility number), the system dimensionless mass transfer coefficient, and the number of contaminated fracture sections. A closed form analytical solution is developed for the simple case of a single contaminated fracture section. For domains with a large number of contaminated fracture sections, a numerical approach is presented that combines a finite difference scheme for simulation of solute transport in the fracture network together with an analytical solution for solute transport in the permeable block flow. The most favorable conditions for pump-and-treat systems are shown to be systems with large mobility numbers and large mass transfer coefficients. In systems with intermediate and small values of these parameters, effluent solute concentrations may be significantly below equilibrium values, reducing the effectiveness of pump-and-treat remediation. It is also shown that an equivalent continuum approach can be used to model rate-limited NAPL dissolution and transport in the simplified fractured permeable formation. The effective mass transfer coefficient for the representative continuum, however, is nonlinearly related to the local mass transfer coefficient.

Convective mass transfer and pressure loss characteristics of staggered short pin-fin arrays

The effect of fin shape on the mass transfer and pressure loss of a ten-row staggered short pin-fin array in fully developed approaching flow is investigated experimentally using three different fin shapes. The Reynolds number, based on approach-velocity and fin diameter, ranges from 3000 to 18000. The fin shape affects the row-by-row variation of Sherwood number. The results indicate that the stepped-diameter circular fin arrays have not only a larger mass transfer coefficient, but also a smaller pressure loss compared to a uniform-diameter circular fin array. In stepped-diameter circular fin arrays, the effect of step-length-to-total-length ratio on Sherwood number is small, but the total mass transfer rate and pressure loss change significantly with this ratio, owing to differences in mass transfer area and free flow area, respectively.

The Effect of the Time Length of Pressure Changing Steps on Concentration in the Gas Phase in the Pressure Swing Adsorption Process

Abstract To investigate the effect of the time length of pressure changing steps theoretically, mathematical expressions for the gas-phase concentration of pressure changing steps in the pressure swing adsorption process were derived under the simple initial conditions and compared with numerical solutions using the orthogonal collocation method. At this time, the linear driving force model was used. We examined whether the time length of the pressure changing steps influenced the concentration in the gas phase for various mass transfer coefficients. The time length did not affect the concentration in the gas phase for the two extreme cases, i.e., a very large mass transfer coefficient or a very small mass transfer coefficient, but the effect of time length was large for a small mass transfer coefficient. A detailed discussion of the pressure-changing steps is included.

Calculation of mass transfer coefficients for dolomitization models

Reversible mass transfer associated with alteration of carbonate rocks can be modeled in terms of a pore fluid migrating across a temperature field slowly enough that chemical equilibrium is maintained. These types of models yield both the spatial distribution of the alteration and the absolute amounts of minerals dissolved and precipitated. In order to compute the total mass transfer involved, it is necessary to obtain mass transfer coefficients for each mineral. This paper presents detailed examples of mass transfer in closed systems for the system calcite-dolomite-solution over the temperature range 0–200°C. It is shown that the mass transfer coefficients for the reversible process can be obtained from thermodynamic data and elementary solubility calculations. The connection between these reversible coefficients and the more familiar irreversible mass transfer coefficients is shown to be a simple derivative relation. Calculations are presented for two cases: calcite-ordered dolomite solution and calcite-disordered dolomite solution in both pure water and brines with Ca/Mg ratios similar to seawater. The large mass transfer coefficients obtained for the brines relative tot he pure water is attributed to the fact that the presence of both calcite and dolomite in a system fixes the Ca/Mg cation ratio in the solution as a function of temperature. Dolomitization occurring in nature likely involves both reversible and irreversible processes and accurate interpretation of field data requires the ability to distinguish between these two cases.

A model of ultrafiltration and glucose mass transfer kinetics in peritoneal dialysis.

We have investigated the variation with dwell time of dialysate volume and glucose concentration during continuous ambulatory peritoneal dialysis using a one-pool model. No assumption was made regarding the ultrafiltration rate that was calculated by the model. Results show that the volume ultrafiltered during dwell time is an increasing function of peritoneal membrane hydraulic permeability and a decreasing function of glucose mass transfer coefficient (MTC). For large MTC and low initial glucose concentration there is reabsorption of dialysate into the blood at large dwell times. For a 6 h dwell time, glycerol (92 daltons) is a more effective osmotic agent than glucose (198 daltons) at the same weight concentration. These results are in quantitative agreement with published clinical studies.

20 Absorption of nitrogen oxides into water

Absorption of NO 2 and NONO 2 mixture into weak alkaline solutions are studied. Emphasis is put on the concentration level ranging from 5000 down to 5 ppm. The absorption rates and the stoichiometric relations are determined by simultaneously measuring the gas and liquid phase compositions. Flat-liquid-level absorber and bubbling type absorber are employed for testing the effects of mass transfer coefficient. The absorption rates of NO 2 depend on the concentration of NO 2 in second order at the concentrations above around 800 ppm. At the lower concentrations, the rate dependency changes to 1 1/2 order in NO 2 and, in the case of rather large mass transfer coefficients, it further changes to 1'st order. The absorption rates from NO and NO 2 mixtures change in 3/4 order with respect to the product of NO and NO 2. A model is constructed by postulating the equilibrations between NO, NO 2, N 2O 3 and N 2O 4 and the hydration reactions of N 2O 3 and N 2O 4 in the liquid film. The computational results based on the model agree well with the all present experiments as well as those in literature. The parameters required for predicting the absorption, including the solubilities of NO 2 and N 2O 4, the hydration rates of N 2O 3 and N 2O 4 and the equilibrium constants, have been established.