Increase In Mass Transfer Rate Research Articles

Confined fluidised bed porosity in the light of bubbling-bed models

Abstract Based on hydrodynamic data, Kato-Wen and Kunii-Levenspiel bubbling-bed model parameters, supplemented with assumptions characteristic for tested confined fluidised bed, were analysed. The calculated bubble diameters and the bed composition proved essential influence of inter-particle space of packed compacted component onto fluidisation character. The usability of the conducted model analysis was also confirmed. Finally, it can be concluded that Kunii-Levenspiel and Kato- Wen models with characteristic assumptions (for the tested bed) can be applied for calculation of the confined fluidised bed layer porosity. Discrepancies of ε f value, determined on the basis of the above mentioned bubbling-bed models do not exceed 8% of the error. The model parameters obtained from the matching the model relations to experimental data εf = f(u0) allow an analysis of the fluidisation character as well as gas velocity regime and the fluidised bed structural composition identification. A description of the regime of the process in which confined fluidised bed is characterised with an increase of mass and heat transfer rate is also possible using relation (17) derived in the present study.

Modelling of naphthalene sorption in shallow beach aquifers

The effect of tide-induced water table fluctuations on the sorption of naphthalene in beach aquifers is investigated. In particular, the cases of equilibrium and rate-limited sorption of naphthalene to aquifer solids are studied by examining the partition coefficient and rate of mass transfer from mobile to immobile regions. Results showed that the tidal effect on contaminant concentrations was not only dependent on the spill location but also on the partition coefficient and the rate of mass transfer. A difference of 13% of the initial spilled concentration was observed between the transient- and steady-head boundaries when the sorption partition coefficient between the contaminant and the sorbent was small (1 × 10−4 m3/kg). However, when the partition coefficient increased to 5 × 10−3 m3/kg, these differences became negligible. This suggests that for contaminants with higher hydrophobicity, a steady head may be used to simulate a seaward boundary, thereby reducing the extensive computations associated with the transient-head boundary. Likewise, small rates of mass transfer from mobile to immobile regions have produced different plume sizes in cases of transient- and steady-head boundaries. These differences in plume size disappeared upon an increase in mass transfer rate.

Shear and dilational interfacial rheology of surfactant-stabilized droplets

A new measurement method is suggested that is capable of probing the shear and dilational interfacial rheological responses of small droplets, those of size comparable to real emulsion applications. Freely suspended aqueous droplets containing surfactant and non-surface-active tracer particles are transported through a rectangular microchannel by the plane Poiseuille flow of the continuous oil phase. Optical microscopy and high-speed imaging record the shape and internal circulation dynamics of the droplets. Measured circulation velocities are coupled with theoretical descriptions of the droplet dynamics in order to determine the viscous (Boussinesq) and elastic (Marangoni) interfacial effects. A new Marangoni-induced stagnation point is identified theoretically and observed experimentally. Particle velocimetry at only two points (including gradients) in the droplet is sufficient to determine the amplitudes of the dilational and shear responses. We investigate the sensitivity for measuring interfacial properties and compare results from droplets stabilized by a small-molecule surfactant (butanol) and those stabilized by relatively large block copolymer molecules. Future increased availability of shear and dilational interfacial rheological properties is anticipated to lead to improved rules of thumb for emulsion preparation, stabilization, and general practice.

Effect of flow oscillations on mass transfer in electrodialysis with bipolar membrane

► Investigated the effect of oscillation on electrodialysis with bipolar membrane. ► Mass transfer resistance is mainly at the cation/anion ion exchange membranes. ► Diffusion resistance at bipolar membrane is insignificant. ► Close to 160% increase in limiting current density was achieved using oscillations. The potential of applying oscillatory flows as a method for enhancing the performance of electrodialysis with bipolar membrane has been investigated using the production of lactic acid from sodium lactate solution as an example. Up to 160% increase to the limiting current density was achieved using such intensification approach. The enhancement was found to increase with increasing the oscillation frequency and amplitude but decreased with the cell steady flow rate, likely due to flow reversal inhibition. The enhancement mechanism was mainly attributed to the increase in mass transfer rate at the cationic membrane fluid–surface interface due to the oscillatory shear and the eddies generated at the rough surface of the net spacers during flow reversals. This however, did not have any measurable effect on the mass transfer at the bipolar membrane, indicating negligible mass transfer resistance there. A correlation is developed for predicting the percentage limiting current density enhancement with an average error of 11%.

Ultrasound and Microstructures—A Promising Combination?

Short diffusion paths and high specific interfacial areas in microstructured devices can increase mass transfer rates and thus accelerate multiphase reactions. This effect can be intensified by the application of ultrasound. Herein, we report on the design and testing of a novel versatile setup for a continuous ultrasound-supported multiphase process in microstructured devices on a preparative scale. The ultrasonic energy is introduced indirectly into the microstructured device through pressurized water as transfer medium. First, we monitored the influence of ultrasound on the slug flow of a liquid/liquid two-phase system in a channel with a high-speed camera. To quantify the influence of ultrasound, the hydrolysis of p-nitrophenyl acetate was utilized as a model reaction. Microstructured devices with varying channel diameter, shape, and material were applied with and without ultrasonication at flow rates in the mL min(-1) range. The continuous procedures were then compared and evaluated by performing a simplified life cycle assessment.

Encapsulated ionic liquids (ENILs): from continuous to discrete liquid phase

Encapsulated ionic liquid (ENIL) material was developed, consisting of ionic liquid (IL) introduced into carbon submicrocapsules. ENILs contain >85% w/w of IL but discretized in submicroscopic encapsulated drops, drastically increasing the surface contact area with respect to the neat fluid. ENIL materials were here tested for gas separation processes, obtaining a drastic increase in mass transfer rate.

Comparative analyses of diffusion coefficients for different extraction processes from thyme

This work was aimed to analyze kinetics and mass transfer phenomena for different extraction processes from thyme (Thymus vulgaris L.) leaves. Different extraction processes with ethanol were studied: Soxhlet extraction and ultrasound-assisted batch extraction on the laboratory scale as well as pilot plant batch extraction with mixing. The extraction processes with ethanol were compared to the process of supercritical carbon dioxide extraction performed at 10 MPa and 40?C. Experimental data were analyzed by mathematical model derived from the Fick?s second law to determine and compare diffusion coefficients in the periods of constant and decreasing extraction rate. In the fast extraction period, values of diffusion coefficients were one to three orders of magnitude higher compared to those determined for the period of slow extraction. The highest diffusion coefficient was reported for the fast extraction period of supercritical fluid extraction. In the case of extraction processes with ethanol, ultrasound, stirring and extraction temperature increase enhanced mass transfer rate in the washing phase. On the other hand, ultrasound contributed the most to the increase of mass transfer rate in the period of slow extraction.

DASCH ON KU Cyg: A ∼ 5 YEAR DUST ACCRETION EVENT IN ∼ 1900

KU Cyg is an eclipsing binary consisting of a F-type star accreting through a large accretion disk from a K5III red giant. Here we present the discovery of a 5-yr dip around 1900 found from its 100 yr DASCH (Digital Access to a Sky Century at Harvard) light curve. It showed a ~0.5 mag slow fading from 1899 to 1903, and brightened back around 1904 on a relatively shorter timescale. The light curve shape of the 1899-1904 fading-brightening event differs from the dust production and dispersion process observed in R Coronae Borealis (RCB) stars, which usually has a faster fading and slower recovery, and for KU Cyg is probably related to the accretion disk surrounding the F star. The slow fading in KU Cyg is probably caused by increases in dust extinction in the disk, and the subsequent quick brightening may be due to the evaporation of dust transported inwards through the disk. The extinction excess which caused the fading may arise from increased mass transfer rate in the system, or from dust clump ejections from the K giant.

Mixed convective boundary layer flow over a vertical wedge embedded in a porous medium saturated with a nanofluid: entire regime

A boundary layer analysis is presented for the mixed convection past a vertical wedge in a porous medium saturated with a nanofluid. The entire regime of the mixed convection is included since the mixed convection parameter of ξ varies from zero (pure free convection) to one (pure forced convection). The transformed non-linear system of equations is solved by using an implicit infinite difference method. Numerical results for friction factor, surface heat transfer rate and mass transfer rate have been presented for parametric variations of the buoyancy ratio parameter Nr, Brownian motion parameter Nb, thermophoresis parameter Nt and Lewis number Le. The dependency of the friction factor, surface heat transfer rate (Nusselt number) and mass transfer rate (Sherwood number) on these parameters has been discussed. The results indicate that as Nr and Nt increase, the heat transfer rate (Nusselt number) and mass transfer rate (Sherwood number) decrease. As Nb increases, the surface mass transfer rates increase whereas the surface heat transfer rate decreases. As Le increases, the heat transfer rate decreases whereas the mass transfer rate increases. As the wedge angle increases, the heat and mass transfer rates increase.

Acousto-spinodal decomposition of compressible polymer solutions: Early stage analysis

The structure and dynamics of early stage kinetics of pressure-induced phase separation of compressible polymer solutions via spinodal decomposition is analyzed using a linear Euler-Cahn-Hilliard model and the modified Sanchez Lacombe equation of state. The integrated density wave and Cahn-Hilliard equations combine the kinetic and structural characteristics of spinodal decomposition with density waves arising from pressure-induced couplings. When mass transfer rate is slower that acoustic waves, concentration gradients generate density waves that cycle back into the spinodal decomposition dynamics, resulting in oscillatory demixing. The wave attenuation increases with increasing mass transfer rates eventually leading to nonoscillatory spinodal demixing. The novel aspects of acousto-spinodal decomposition arise from the coexistence of stable oscillatory density dynamics and the unstable monotonic concentration dynamics. Scaling laws for structure and dynamics indicate deviations from incompressible behavior, with a significant slowing down of demixing due to couplings with density waves. Partial structure factors for density and density-concentration reflect the oscillatory nature of acousto-spinodal modes at lower wave vectors, while the single maximum at a constant wave vector reflects the presence of a dominant mode in the linear regime. The computed total structure factor is in qualitative agreement with experimental data for a similar polymer solution.

Design and Simulation of a New Reactor for the Production of High‐Viscosity Polymer Melts

AbstractSummary: Innovative reactor internals consisting of a number of vertically stacked perforated plates have been proposed to increase mass transfer rates of volatiles from highly viscous polymer melts. A mathematical model comprising mass and enthalpy balances for the polycondensation of polyamide 6 has been used to evaluate the new reactor design. According to the simulation, the alternating arrangement of devolatilization and residence zones leads to an effective removal of reaction water from the polymer melt. Compared to the VK‐tube reactor, which is most commonly used to produce high quality PA‐6, the new reactor design produces high quality polymer with a high degree of polymerization within a considerably shorter residence time. The application of the proposed reactor internals is not limited to PA‐6 production, since the removal of volatiles is of the same major importance during all reversible polycondensation reactions, e.g. the production of PET.

Hydrogen Recovery from Organic Wastes: Reactor Development and Improvement

The paper reviewed hydrogen production biotechnology on reactor development and design aspects. Biological hydrogen-producing reactor as acid-producing phase of two-phase anaerobic organism treatment system plays an important role in the following aspects: Reactor was developed as the follow ideas: a. CSTR-type anaerobic fermentation reactor is selected to reduce the substrate concentration in reactor and increase target product operational yield and selectivity in the reactor; b. mixture liquid in reaction area is stirred by the stirrer to reach a turbulent state in order to reduce interfacial layer thickness and temperature gradient in a floc unit particle and increase mass transfer rate; c. H2 in the particle and Liquid phase is accelerated to release to prevent accumulated H2 from bringing feedback inhibition to organism metabolism, and H2/CO2conversion to acetic acid; d. A sector turbine agitator with hoisting capacity and mixing power is selected to facilitate sludge to flow back through a effluence seam.

Continuous Flow Electroless Plating Enhances Optical Features of Au Films and Nanoparticles

A novel continuous flow (CF) electroless (EL) plating method deposits gold (Au) films which exhibit increased continuity and uniformity relative to batch immersion as measured by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Transmission UV−vis (T-UV) spectra exhibit lower peak heights (PH) and narrower full-width-half-maxima (fwhm), consistent with microscopy measures. Nanoparticles (NPs) obtained by thermal transformation of CF-EL Au film are larger and monodisperse relative to I-EL NPs. Their extinction spectra exhibit higher PH due to enhanced localized surface plasmon resonance (LSPR) and narrower fwhm than NP from I-EL films. Au deposition during CF-EL plating is calculated to be 78.3% higher than during I-EL plating for each 30-s interval. Optical features of T-UV [PH, fwhm, and resonant extinction maxima (λmax)] increase in proportion to time-integrated mass transport coefficient and suggest that increased mass transfer rate and steady higher average local Au concentration ...

Analysis of Laminar Falling Film Condensation Over a Vertical Plate With an Accelerating Vapor Flow

Laminar falling film condensations over a vertical plate with an accelerating vapor flow is analyzed in this work in the presence of condensate suction or slip effects at the plate surface. The following assumptions are made: (i) laminar condensate flow having constant properties, (ii) pure vapor with a uniform saturation temperature in the vapor region, and (iii) the shear stress at the liquid/vapor interface is negligible. The appropriate fundamental governing partial differential equations for the condensate and vapor flows (continuity, momentum, and energy equations) for the above case are identified, nondimensionalized, and transformed using nonsimilarity transformation. The transformed equations were solved using numerical, iterative, and implicit finite-difference methods. It is shown that the freestream striking angle has insignificant influence on the condensation mass and heat transfer rates, except when slip condition is present and at relatively small Grl/Re2 values. Moreover, it is shown that increasing the values of the dimensionless suction parameter (VS) results to an increase in dimensionless mass of condensate (Γ(L)/(μl Re)) and Nusselt number (Nu(L)/Re1/2). Thus, it results in an increase in condensation mass and heat transfer rates. Finally, it is found that the condensation and heat transfer rates increase as Jakob number, slip parameter, and saturation temperature increase. Finally, the results of this work not only enrich the literature of condensation but also provide additional methods for saving thermal energy.

Comparison of agarose and dextran-grafted agarose strong ion exchangers for the separation of protein aggregates

The control of aggregate levels in recombinant protein based drugs is a primary concern during process development and manufacture. In recent years, a novel class of dextran-grafted ion exchange matrices has gained popularity for process scale protein purification due to increased mass transfer rates and higher dynamic binding capacity compared to conventional matrices. Using bovine serum albumin and a monoclonal antibody as model proteins, we studied Sepharose FF and Sepharose XL ion exchangers for the separation of protein aggregates. Experimental results comparing linear gradient elution, stepwise elution, and flow-through chromatography for aggregate separation are described. Differences in performance for the various ion exchangers are discussed and modeled. Strategies for the optimization of protein aggregate separation are provided.

Influence of airflow and liquid properties on the mass transfer coefficient of ammonia in aqueous solutions

The influences of airflow and liquid properties on the mass transfer coefficient of ammonia from aqueous solutions were investigated. In the laboratory experiments using a small-scale model, four ventilation rates with three ventilation control strategies were used to estimate the influence of airflow. To evaluate the effects of different model scales and liquid properties, six inlet air velocities with three turbulence intensities and four pH values were studied using a small wind tunnel. In the scale model experiments, the results showed that the mass transfer coefficient increased as the ventilation rate increased and the turbulence intensity had a significant effect on the mass transfer coefficient at low air velocities. To represent the comprehensive influence of airflow on the mass transfer coefficient, a statistical model was developed based on surface air velocity, turbulence intensity and ventilation rate (coefficient of determination R 2 =0.98). It was found that the mass transfer coefficient for ammonia was much more sensitive to the variations in air velocity at lower rather than at higher air velocities. Similar responses were obtained for both turbulence intensity and ventilation rate. Comparative results using different ammonia mass transfer coefficient equations for a constant inlet opening area strategy indicated that it is not sufficient to express the mass transfer coefficient simply in terms of the air velocity and the ventilation rate. In the wind tunnel experiments, the mass transfer coefficient increased with increasing turbulence intensity. However, the mass transfer coefficient increased as the air velocity increased with lower velocities but at higher velocities ammonia emission did not increase with air velocity. It was found that increasing the pH of the solution not only increased mass transfer rates but also increased the peak value of the air velocity that induced the maximum emission of ammonia. The different correlations that were established between the mass transfer coefficient and air velocity in both the scale models and the wind tunnel may have been caused by the use of different model scales. The scale of turbulence found in the scale model cannot be reproduced in the wind tunnel. Therefore, full-scale investigations are necessary to characterise emissions affected by turbulence scale.

Two-dimensional two-phase thermal model for passive direct methanol fuel cells

A two-dimensional two-phase thermal model is presented for direct methanol fuel cells (DMFC), in which the fuel and oxidant are fed in a passive manner. The inherently coupled heat and mass transport, along with the electrochemical reactions occurring in the passive DMFC is modeled based on the unsaturated flow theory in porous media. The model is solved numerically using a home-written computer code to investigate the effects of various operating and geometric design parameters, including methanol concentration as well as the open ratio and channel and rib width of the current collectors, on cell performance. The numerical results show that the cell performance increases with increasing methanol concentration from 1.0 to 4.0 M, due primarily to the increased operating temperature resulting from the exothermic reaction between the permeated methanol and oxygen on the cathode and the increased mass transfer rate of methanol. It is also shown that the cell performance upgrades with increasing the open ratio and with decreasing the rib width as the result of the increased mass transfer rate on both the anode and cathode.

Development of High Efficient Oxygen Supply Method by Using Contacting Water-Liquid film with Air

Surface transfer and bubble transfer both contribute significantly to oxygen transfer in a conventional aeration system. However, bubble transfer plays the primary role of oxygen transfer. In the present study, the mere surface transfer is replaced by directly contacting liquid film designed elegantly with air. Liquid film aeration system is successfully developed on a laboratory scale. The experimental results from single-pass aeration mode show that the dissolved oxygen concentration in deoxygenate water reaches up to 95 % or higher, as compared with the saturation concentrations of the dissolved oxygen while using this novel aeration system. Meanwhile, an increase in oxygen mass transfer rate by 30% can be achieved in comparison with conventional aeration system. Moreover, the volumetric mass transfer coefficient for liquid film-forming apparatus (KLal) is found to be as much as 5 times higher than that for water surface in a conventional aeration test.

Comparative study on the effects of n-dodecane addition on oxygen transfer in stirred bioreactors for simulated, bacterial and yeasts broths

The efficiency of oxygen transfer into the fermentation broths could be enhanced by adding in broths oxygen-vectors, such as hydrocarbons or fluorocarbons, without increasing the energy consumption for mixing or aeration. The experimental results obtained for simulated, Propionibacterium shermanii and Saccharomyces cerevisiae broths indicated the considerable increase of k L a and ( k L a) v/( k L a) 0 by adding n-dodecane, but the magnitude of this effect must be correlated with broths and biomass characteristics, especially with the cells affinity for oxygen-vector droplets. Thus, due to the higher affinity of yeasts cells for hydrocarbon droplets, the increase of oxygen mass transfer rate was lower than that recorded for simulated or bacterial broths. By means of the experimental data, some mathematical correlations describing the influences of the main parameters (apparent viscosity, concentration of biomass, specific power input, superficial air velocity) on k L a in presence of n-dodecane have been proposed for each considered fermentation systems. These equations have the general expression of k L a = α ( P a / V ) β v s γ η a δ or k L a = α ( P a / V ) β v s γ C x δ , the coefficients ( β, γ and δ values being dependent on n-dodecane concentration, and offer a good agreement with the experiment, the average deviations being between ±9.6 and ±10.8%.

Effect of mechanical vibrations on the reextraction kinetics in the toluene-HNO3-H2O system

The kinetics of reextraction through a vibrating interface in systems containing di-(2-ethylhexyl)phosphoric acid (D2EHPA) is studied. When mechanical vibrations are excited in the interface layer, the mass transfer rate increases. It is established that mechanical vibrations promote reextraction by means of three factors: an increase in the interphase surface area, a decrease in the diffusion layer thickness, and the process of dispersion. It is demonstrated that an increase in the amplitude of vibrations results in an increase in the effective mass transfer coefficient, with its dependence on the squared amplitude being linear over the range covered. It was established how extractant adsorption, a phenomenon that influences the mass transfer of D2EHPA and nitric acid, influences the process.