Enhanced Mass Transfer Rates Research Articles

Enhanced mass transfer rate of methane in aqueous phase via methyl-functionalized SBA-15

Methyl-functionalized mesoporous silica SBA-15, synthesized from tetraethyl-orthosilicate (TEOS) and methyltriethoxysilane (MTES), were employed as additives for the enhancement of volumetric mass transfer coefficient (kLa) of methane in aqueous solution. The surface functional group, highly ordered structure, and exceptionally high surface area exerted great influence on mass transfer rate of methane in aqueous solution. Especially, the fraction of surface functional groups controlled by adjusting the ratio of TEOS to MTES was found to be a crucial factor. Mesoporous characteristics of synthesized materials were investigated via BET surface area analysis, small angle X-ray diffraction, and FT-IR spectroscopy. At an optimal molar ratio (20:1) of TEOS and MTES, an 88% enhancement of kLa for methane in water was observed at 30°C.

Mass Transfer Rate Enhancement for CO2 Separation by Ionic Liquids: Effect of Film Thickness

Ionic liquids (ILs) are promising in CO2 separation, while the film thickness is particularly critical for gas transport in these viscous and expensive liquids. In this work, the influence of IL-film thickness on CO2 absorption/desorption of two different IL immobilized sorbents was investigated, in which the results from the thermogravimetric analyzer were further used to estimate the scale of IL-film thickness. It is found that the IL-film in nanoscale is a prerequisite for efficient CO2 absorption/desorption; the equilibrium time can be 10-times different, and the rate constant can be 100-times different for microscale and nanoscale IL-films. This is the first time to quantitatively reveal the influence of IL-film thickness and find out its scale for a significant rate enhancement in the CO2 absorption/desorption by IL immobilized sorbents.

Enhanced Mass Transfer Rates in Nanofluids: Experiments and Modeling

Enhancement in carbon dioxide absorption in water has been studied using SiO2 and TiO2 nanoparticles using the capillary tube apparatus for which previous results on Fe3O4 nanoparticles were reported earlier. Enhancements of up to 165% in the mass transfer coefficients were observed at fairly low volume fractions of the particles. A model which accounts for the effect of particles in terms of a superimposed convection has been proposed to explain the observed effects of particle size, hold-up, and material density. The model provides a good fit to the data from wetted wall column and capillary tube experiment for Fe3O4 from the previous literature, as well as for the data from this work.

Ni/MgO-MgAl2O4 Catalysts with Bimodal Pore Structure for Steam-CO2-Reforming of Methane.

The bead type MgO-MgAl2O4 catalyst supports with bimodal pore structures were fabricated via an extrusion molding of gels derived from the precursor mixture of mesoporous MgO particles and aluminum magnesium hydroxide, followed by heat treatment. To investigate the effect of macro pore structures on the catalytic activity of the Ni/MgO-MgAl2O4 catalysts in the steam and carbon dioxide reforming of methane (SCR), two kinds of the catalysts with largely different macro pore volumes and sizes but nearly the same meso pore volume and size were compared. The bimodal catalyst with a large macro pore size and volume exhibited a highly enhanced CO2 conversion from 22.3 to 37.1% but a slightly reduced CH4 conversion from 95.3 to 92.1% at the same feed ratio. The SCR results show that the large macro pores can lead to a highly enhanced mass transfer rate of CO2 absorption into the pore channels of the magnesium alumina spinel.

Cation diffusion in the electrical double layer enhances the mass transfer rates for Sr2+, Co2+ and Zn2+ in compacted illite

Enhanced mass transfer rates have been frequently observed in diffusion studies with alkaline and earth alkaline elements in compacted clay minerals and clay rocks. Whether this phenomenon – often termed surface diffusion – is also relevant for more strongly sorbing species is an open question. We therefore investigated the diffusion of Sr2+, Co2+ and Zn2+ in compacted illite with respect to variations of the concentration of the background electrolyte, pH and carbonate. New experimental techniques were developed in order to avoid artefacts stemming from the confinement of the clay sample. A distinct dependence of the effective diffusion coefficients on the concentration of the background electrolyte was observed for all three elements. A similar correlation was found for the sorption distribution ratio (Rd) derived from tracer breakthrough in the case of Sr2+, while this dependence was much weaker for Co2+ and Zn2+. Model calculations using Phreeqc resulted in a good agreement with the experimental data when it was assumed that the cationic species, present in the electrical double layer (EDL) of the charged clay surface, are mobile. Species bound to the specific surface complexation sites at the clay edges were assumed to be immobile. An assessment of the mobility of the type of cationic elements studied here in argillaceous media thus requires an analysis of their distribution among specifically sorbed surface species and species in the EDL. The normal approach of deriving unknown effective diffusion coefficients from reference values of an uncharged water tracer may significantly underestimate the mobility of metal cations in argillaceous media.

Metal–Organic Framework 199 Film as a Novel Adsorbent of Thin-Film Extraction

In this study, metal–organic framework (MOF)-199 was in situ grown on the surface of stainless steel mesh by a facile one-step hydrothermal synthesis strategy. A piece of the MOF-199-coated film was used as adsorbent in thin-film extraction. With octahedral shape, accessible macropores, and high surface area-to-volume, the MOF-199 coating exhibited excellent extraction ability and enhanced mass transfer rate toward six studied aldehydes. The proposed thin-film extraction-high-performance liquid chromatography method was applied successfully for the determination of aldehyde in human exhaled breath condensates of lung cancer patients and healthy people. The experimental conditions were optimized systematically. Under the optimal conditions, a wide linear application range was obtained with linear correlations (r) above 0.995; the limits of detection were from 1.3 to 20.3 nmol L−1. Reasonable recoveries were achieved between 74 and 120 %. The method possesses the potential advantages of simple setup, convenient operation, fast analysis and factory sensitivity, and little matrix interference. It supplies us with a new possibility for non-invasive monitoring of trace aldehyde metabolites in complex biological samples.

The Effect of Nanoparticles on the Mass Transfer in Liquid–Liquid Extraction

This article investigates the effect of nanoparticles on mass transfer in the liquid–liquid extraction for the chemical system of n-butanol–succinic acid–water. For this purpose, nanofluids containing various concentrations of ZnO, carbon nanotubes (CNT), and TiO2 nanoparticles in water, as base fluid, were prepared. To examine the flow mode effect on mass transfer rate, different fluid modes including dropping and jetting were employed in the process. Results show that mass transfer rate enhancement depends on the kinds and the concentration of nanoparticles and the modes of flow. It was observed that after adding nanoparticles, the mass transfer rate significantly increases up to two-fold for ZnO nanoparticles. Furthermore, the results indicate that under the circumstances in which the mass flow rate is high enough, the effect of nanoparticles on the mass transfer phenomenon is too slight.

CO2 absorption in nanofluids in a randomly packed column equipped with magnetic field

Magnetic nanofluids have been used for absorption of CO2 in a packed column in the presence of magnetic field. Adding nanoparticles into the solvent enhances mass transfer characteristics. The optimum concentrations of Fe3O4/water and NiO/water nanofluids are 0.005, and 0.01 % respectively, and the maximum enhancement of mass transfer rate in comparison with pure water is 12 and 9.5 % respectively. The magnetic field showed positive effect on the CO2 absorption performance.

A Numerical Study on the Ventilation Coefficients of Falling Hailstones

Abstract The ventilation coefficients that represent the enhancement of mass transfer rate due to the falling motion of spherical hailstones in an atmosphere of 460 hPa and 248 K are computed by numerically solving the unsteady Navier–Stokes equation for airflow past hailstones and the convective diffusion equation for water vapor diffusion around the falling hailstones. The diameters of the hailstones investigated are from 1 to 10 cm, corresponding to Reynolds number from 5935 to 177 148. The calculated ventilation coefficients vary approximately linearly with the hailstone diameter, from about 19 for a 1-cm hailstone to about 208 for a 10-cm hailstone. Empirical formulas for ventilation coefficient variation with hailstone diameter as well as Reynolds and Schmidt numbers are given. Implications of these ventilation coefficients are discussed.

An experimental investigation on the onset from bubbling to turbulent fluidization regime in micro-structured fluidized beds

Membrane-assisted fluidized beds have been investigated for a number of industrially important applications. Recently, micro-structured membrane-assisted fluidized beds have been proposed and particular for efficient hydrogen production in particular for maximising the volumetric production rate by maximising the installed specific membrane areas. Another important advantage of micro-structuring became evident from computational studies. CFD calculations have shown that micro-structured fluidized beds can be operated in the turbulent fluidization regime at much lower superficial gas velocities compared with larger fluidized bed [1]. This work focuses on an experimental validation of the transition velocity in micro-structured fluidized beds. The influence of the reactor scale and the particle diameter on the transition gas velocity from the bubbling to the turbulent fluidization regime in micro-structure fluidized beds was measured in pseudo 2D column using different experimental techniques: in particular, the pressure drop fluctuation measured by pressure sensors and the local solid holdup fluctuation measured by digital image analysis on images recorded by high speed camera. It was found that smaller reactors filled with particles of smaller diameter can be operated in the turbulent fluidization at a relatively lower gas velocity compared with larger reactors. Interestingly, the different measurement techniques result in somewhat different transition velocities, i.e. the pressure drop fluctuation method generated higher critical velocities than the local solids' holdup fluctuation method. This difference is discussed in this paper. The experimental results confirmed that micro-structured fluidized beds can indeed be operated in the turbulent fluidization regime with lower fluidization velocities thus anticipating increased energy efficiencies and enhanced mass transfer rates.

Decomposition of nitrotoluenes in wastewater by sonoelectrochemical and sonoelectro-Fenton oxidation

Oxidative degradation of dinitrotoluene (DNT) and 2,4,6-trinitrotoluene (TNT) in wastewater was conducted using electrochemical and electro-Fenton processes respectively, combined with ultrasonic irradiation, wherein a synergistic effect is observed. Experiments were carried out to elucidate the influence of various operating variables on the sonoelectrolytic behavior, such as electrode potential, sonoelectrolytic temperature, acidity of wastewater, oxygen dosage, and dosage of ferrous ions. It deserves to note that the nitrotoluene contaminants could be completely decomposed by sonoelectro-Fenton method, wherein hydrogen peroxide was in situ generated from cathodic reduction of oxygen, supplied partially by anodic oxidation of water. During the sonoelectrolytic process, in spite of existence of degassing phenomenon, the high yield of hydrogen peroxide was produced due to the significantly enhanced mass transfer rate of oxygen toward the cathode, caused by ultrasonic irradiation. Because higher removal efficiency of DNTs and TNT obtained at ambient conditions, it is believed that the sonoelectrolytic method is potentially applied to dispose wastewater from toluene nitration processes.

Enhancement of Mass Transfer Rate and Diminution of the Power Consumption of Copper Cementation Using Electromagnetic Field

Mass transfer rate is the main controlling parameter in the cementation process. Due to the economical aspects, iron is the most popular sacrificing metal. Accordingly, during the cementation process, electromagnetic field can be utilized to improve the motion of Fe ions in the aqueous solution. In the present study, the rate of copper (II) cementation from copper sulfate solutions on a rotating iron cylinder subjected to a magnetic field was studied to demonstrate the effect of electromagnetic field on the rate of cementation. The application of electromagnetic field (EMF) strongly enhances the rate of cementation process as 88% of copper was removed after 15 minutes when EMF was used in a direction parallel to the iron cylinder. The results showed that without EMF, the rate of mass transfer was correlated by the equation Sh = 1.18Re 0.94 Sc 0.33 and by equation Sh = 2.29Re 0.95 Sc 0.33 with applying the EMF. This work illustrates that the enhancement of copper recovery in presence of EMF is due to the induced motion of Fe +n in the solution which is limited in the range of cylinder rotation speed of 300–900 rpm. Moreover, the experimental results indicated that applying the EMF leads to reduce the power consumption for the recovery by 36% per kg of copper.

Pore‐forming technology and performance of MEA for direct methanol fuel cells

AbstractBACKGROUND: The commercialization of DMFCs is seriously restricted by its relatively low power density. Lots of work has been concentrated on catalysts with high activity, the optimization of flow path design, development of new kinds of proton exchange membrane and modification of Nafion membrane. Meanwhile, very few reports have involved the structure optimization of the membrane electrode assembly (MEA). To improve the performance of direct methanol fuel cells (DMFCs), the catalyst layer (CL) structures of anode and cathode were optimized by utilizing ammonium carbonate as pore forming agent.RESULTS: The polarization curves showed that in catalyst slurry the optimal content of ammonium carbonate was 50 wt%, and the DMFC performance was enhanced from 75.65 mW cm−2 to 167.42 mW cm−2 at 55 °C and 0.2 MPa O2. Electrochemical impedance spectroscopy and electrochemical active surface area (EASA) testing revealed that the improved performance of optimized MEAs could be mainly attributed to the increasing EASA and the enhanced mass transfer rate of CLs. But poor methanol crossover limited the performance enhancement of MEAs with porous anodes.CONCLUSION: With regard to improving cell performance, this pore‐forming technology is better applied to the cathode catalyst layer to improve its structure rather than the anode catalyst layer. © 2012 Society of Chemical Industry

INVESTIGATING SLIM DISK SOLUTIONS FOR HLX-1 IN ESO 243-49

The hyper luminous X-ray source HLX-1 in the galaxy ESO 243-49, currently the best intermediate mass black hole candidate, displays spectral transitions similar to those observed in Galactic black hole binaries, but with a luminosity 100-1000 times higher. We investigated the X-ray properties of this unique source fitting multi-epoch data collected by Swift, XMM-Newton & Chandra with a disk model computing spectra for a wide range of sub- and super-Eddington accretion rates assuming a non-spinning black hole and a face-on disk (i = 0 deg). Under these assumptions we find that the black hole in HLX-1 is in the intermediate mass range (~2 x 10^4 M_odot) and the accretion flow is in the sub-Eddington regime. The disk radiation efficiency is eta = 0.11 +/-0.03. We also show that the source does follow the L_X ~ T^4 relation for our mass estimate. At the outburst peaks, the source radiates near the Eddington limit. The accretion rate then stays constant around 4 x 10^(-4) M_odot yr^(-1) for several days and then decreases exponentially. Such "plateaus" in the accretion rate could be evidence that enhanced mass transfer rate is the driving outburst mechanism in HLX-1. We also report on the new outburst observed in August 2011 by the Swift-X-ray Telescope. The time of this new outburst further strengthens the ~1 year recurrence timescale.

Sorption and diffusion of compressed carbon dioxide in polycaprolactone for the development of porous scaffolds

In this work different phenomena related to sorption of carbon dioxide in polycaprolactone (PCL) have been investigated systematically. The use of compressed carbon dioxide is discussed for obtaining porous scaffolds from this biocompatible polymer. In order to determine the plasticization effect of carbon dioxide on the degree of foaming it is necessary to discuss sorption data with respect to morphological features of the polymer at conditions nearby the melting point. The amount of carbon dioxide dissolved and the kinetics of the sorption process are found to depend strongly on temperature and pressure. The solubility takes values of up to 25wt.% being favoured by a melting and glass transition temperature depression which can be observed along with an enhanced mass transfer rate. In general, CO2 sorption in PCL increases linearly with pressure. When decompressing, microfoaming occurs which enhances the rate of gas release. Changes in morphology and crystallinity occur as a consequence of the pressure treatment. Compared to the melting temperature at atmospheric pressure there is a dramatic reduction observed under pressure where melting occurs already at a temperature below 40°C. Even after pressure-treatment, there is a remaining change in melting temperature and crystallinity observed. Optimum conditions for obtaining adequate porous scaffolds of PCL are a relatively slow decompression after treatment at 17MPa and 35°C.

Mass transfer enhancement by the alkaline hydrolysis of ethyl acetate in a single droplet system

Transient mass transfer behavior in a single droplet system was investigated in the presence of a chemical reaction: alkaline hydrolysis (by NaOH) of ethyl acetate. The transfer of ethyl acetate occurred from the continuous (cyclohexanol) to the dispersed (water) phase. A computational fluid dynamics (CFD) simulation was conducted for a numerical analysis of the system. The physical mass transfer was simulated showing a good agreement with the experimental result whereas the simulation significantly underestimated the experimental result in the presence of the hydrolysis reaction of ethyl acetate. An interfacial turbulence induced by the reaction product was suggested to explain the deviation between the experimental and the simulation. Effective diffusivity was used for simulating the more enhanced mass transfer rate in the system. The concentration change inside a droplet with time could be calculated with a reasonable accuracy by the introduction of the effective diffusivity in the simulation model.

Optimization of bioactive compounds extraction from jabuticaba (Myrciaria cauliflora) skins assisted by high pressure CO2

Important bioactive compounds from Brazilian jabuticaba skins were effectively extracted by High Pressure Carbon Dioxide Assisted Extraction (HPCDAE). Statistical design was used to optimize the extraction variables: extraction pressure (65–135 bar), temperature (40–80 °C) and volume ratio of solid–liquid mixture/pressurized CO2 (RS‐L/CO2(%) (20–80%). The analysis performed to predict the optimum values for the extraction variables, in order to obtain the condition that result in an extract with high anthocyanin (2.2 ± 0.3 mg cyanidin-3-glucoside/g dry skins) and phenolic compound contents (13 ± 1 mg gallic acid equivalents/g dry skins), gave as best conditions, 117 bar extraction pressure, 80 °C extraction temperature and 20% volume ratio of solid–liquid mixture/pressurized CO2 (RS‐L/CO2(%)). Compared to Pressurized Liquid Extraction (PLE) and to control experiment the experimental results obtained using optimum HPCD Assisted-Extraction conditions were much more effective and faster in extracting total anthocyanins and phenolic compounds. Industrial relevance: Industrially, there is an increasing demand for faster extraction procedures with reduced organic solvent consumption to lower pollution burden. HPCD Assisted-Extraction combines the advantages of enhanced mass transfer rates increasing secondary metabolite diffusion from the vegetable matrix into the environmentally friendly solvent extraction. High pressure extraction methods, such as HPCD Assisted-Extraction, have other advantages that should be considered, such as the fact that native enzymes, which degrade phenolic compounds, are inhibited by extraction pressure increasing and CO2 addition, and that processed vegetable materials do not require additional sterilization steps.

The CO 2 capture performance of a high-intensity vortex spray scrubber

The present study focuses on the enhancement of CO 2 capture efficiency using a high-intensity vortex spray scrubber by imparting swirl to the gas flow, which has the ability to augment the rates of heat and mass transfer. Experimental investigations into the reactive absorption of CO 2 from a mixture of air–CO 2 into an aqueous solution of NaOH in a laboratory-scale counter-current spray scrubber have been carried out. The mass transfer characteristics, in terms of the overall gas phase mass transfer coefficient ( K g a) were investigated for both the swirling and the non-swirling (axial) gas flows through the scrubber in order to quantify the effect of swirl. The effects of the gas/liquid flow rates, flow arrangements, scrubber height and spray nozzle type on the CO 2 capture performance were examined. For both the axial and the swirling flows, the K g a increases initially with increasing gas flow rate up to a certain limit, beyond which it becomes essentially constant, whereas the K g a increases continuously with the liquid flow rate within the measured range. The counter-current gas–droplets flow provides higher mass transfer rates compared with those in co-current flow. The K g a deceases with the increase in the tower height. The spray nozzle producing finer droplets provides enhanced mass transfer rates. It is found that imparting swirl in the gas flow enhances the K g a up to around 49% compared with that in axial flows.

Preparation of poly-lactic acid nano/microparticles using supercritical anti-solvent with enhanced mass transfer

Supercritical anti-solvent precipitation with enhanced mass transfer (SAS-EM) was applied for the production of micro and sub-microparticles of poly-lactic acid (PLA). SAS-EM technique uses an ultrasonic vibrating surface to enhance mass transfer rate between supercritical CO 2 and solvent. Without applying ultrasonic power, which is same as SAS process, PLA particles with average diameters ranging between 1 μm and 3 μm were obtained. Using SAS-EM with the power supply of 200 W, spherical PLA particles smaller than 1 μm were obtained. The particle size was able to be controlled in the range of 0.4 μm–1.0 μm, by adjusting the power supply of ultrasonic field, the system pressure and temperature.

Sonoelectro-Fenton process: A novel hybrid technique for the destruction of organic pollutants in water

Here we present the results obtained by applying a novel hybrid advanced oxidation technique, which we have called sonoelectro-Fenton (sono-EF) process, for the degradation of organic pollutants in aqueous medium. Its high performance arises from the coupling between ultrasound irradiation and the in situ electrogeneration of Fenton’s reagent. An undivided electrolytic cell with a Pt anode and a three-dimensional carbon-felt cathode has been used to carry out the very effective electro-Fenton (EF) process at constant current, which allows the production of great amounts of hydroxyl radicals ( OH). The synergistic action of sonication in the sono-EF process has been studied at low and high frequency within a range of output power. We demonstrate that the destruction of the herbicides 4,6-dinitro- o-cresol (DNOC) and 2,4-dichlorophenoxyacetic acid (2,4-D) is significantly accelerated, whereas no improvement is observed for the degradation of the dye azobenzene (AB), suggesting that the nature of the organic structure plays an important role. A pseudo-first order kinetics is found in all cases. Similar results are reached for sono-EF at low and high frequency, but the lowest ultrasounds power (i.e., 20 W) is shown to be much better. We conclude that the improvement yielded by sono-EF is due to various contributions, which in order of relevance are: (i) the enhanced mass transfer rate of both reactants (Fe 3+ and O 2) towards the cathode for the electrochemical generation of Fenton’s reagent (Fe 2+ + H 2O 2) and its transfer into the solution in order to enhance Fenton’s reaction kinetics thus increasing the OH production rate and accelerating the destruction process, (ii) the additional generation of OH by sonolysis, and (iii) pyrolysis of organics due to cavitation generated by ultrasound irradiation.