Increase In Mass Transfer Rate Research Articles

Combined effect of γ-irradiation and osmotic pretreatment on mass transfer during dehydration

The combined effect of γ-irradiation (3.0–9.0 kGy) and osmotic pretreatment (10 and 50°B) on dehydration kinetics was studied. The exposure to irradiation pretreatment resulted in an increase in cell wall permeabilization, leading to softening of tissue, which in turn resulted in faster dehydration. The effective diffusion coefficient of water in case of potato during dehydration, considering Fickian diffusion, increased from 2.38 × 10 −9 to 4.14 × 10 −9 m 2/s at 9.0 kGy, whereas the osmotic pretreatment resulted in lower dehydration rates due to infusion of solute from osmotic solution, which resulted in lower mass transfer rates. Even though the effective diffusion coefficients decreased from 2.38 × 10 −9 to 1.55 × 10 −9 m 2/s (up to 50°B), the osmotic pretreatment improves nutritional, sensorial and functional properties of food without changing its integrity, apart from partial removal of water present in food. The combining of γ-irradiation treatment with osmotic pretreatment resulted in increased mass transfer rates during air dehydration, offering a feasible solution for satisfactorily enhancing the mass transfer rates.

Mass Transfer From a Rotating Disk to a Bingham Fluid

In the present investigation, an analytical numerical solution is presented for the mass transfer from a rotating disk to a Bingham fluid for the case of laminar boundary layer flow. The analytical approach includes the coupled effects of steady disk rotation and non-Newtonian fluid properties on the mass transfer rate. A dimensionless expression for the wall mass transfer rate based on the Sherwood number, Sh, is obtained in terms of the system parameters (Reynolds number, Rep, and Schmidt number, Scp) which depend on the dimensionless yield stress or Bingham number, By. The analytical relation indicates that an increase in By (up to the limit By⩽1) leads to a slight increase in the wall mass transfer rate, and thereafter, for By>1, the mass transfer rate is reduced.

Calculation of flow fields in two and three-phase bubble columns considering mass transfer

The dimension of bubble column reactors is often based on empirical correlations. Very popular is the axial dispersion model. However, the applicability of these models is limited to the experimental conditions for which the dispersion coefficients are measured, because backmixing depends strongly on the columns dimension and the flow regime. This paper presents a numerical method for the calculation of the three-dimensional flow fields in bubble columns based on a multi-fluid model. Therefore, the local bubble size distribution is considered by a transport equation for the mean bubble volume, which is obtained from the population balance equation. For comparison with experimental results, the axial dispersion coefficients in the liquid and gas phase are calculated from the instationary, three-dimensional concentration fields of a tracer. The model is then extended to include mass transfer between the gas and liquid phase. Increasing mass transfer rates significantly influence the flow pattern. For several applications, a dispersed solid phase is added. For the calculation of three-phase gas–liquid–solid flow, the solid phase is considered numerically by an additional Eulerian phase.

Coronal mass transfer in interbinary loops

We consider the transfer of coronal mass between the members of a detached binary system along interconnecting rigid magnetic loops. We show that the asymmetry in the gravitational potential drives a flow from the star closer to fill its Roche lobe towards the other star. The loop base pressures, flow velocities and mass fluxes are not externally imposed but are a result of the heating rate along the loop. As the loop heating increases, the base pressures and mass transfer rates increase and the Mach number decreases. If the heating is not uniformly distributed along the loop, but is instead concentrated near one of the stars, then the mass transfer rate either increases or it decreases, and even changes direction, depending on which star the heating is concentrated on. We also discuss the effect of choosing different boundary conditions. Our model predicts mass transfer rates in reasonable agreement with existing observational estimates, but fails to explain the presence of chromospheric material detected between the stars. Additionally, the differential emission measure profile of the loop models is calculated and it is found to be higher in the loop leg close to the secondary star.

The effect of n-dodecane addition on oxygen transfer in stirred bioreactors for Saccharomyces cerevisiae broths

The previous works on simulated and P. shermanii broths were continued and developed for Saccharomyces cerevisiae broths. The obtained results indicated the considerable increase of k?a in the presence of n-dodecane as an oxygen-vector, but the magnitude of this effect had to be correlated with the biomass characteristics, especially with hydrophobicity. Due to the higher affinity of yeast cells for hydrocarbon droplets, increase of the oxygen mass transfer rate was lower than that recorded for simulated broths or bacterial suspensions.

Mass transfer in tidally unstable compact binaries

The 2001 outburst of WZ Sagittae has shown the most compelling evidence yet for an enhancement of the mass-transfer rate from the donor star during a dwarf nova outburst in the form of hotspot brightening. I show that, even in this extreme case, the brightening can be attributed to tidal heating near the interaction point of an accretion stream with the expanding edge of an eccentric accretion disc, with no need at all for an increase in the mass-transfer rate. Furthermore, I confirm previous suggestions that an increase in mass-transfer rate through the stream damps any eccentricity in an accretion disc and suppresses the appearance of superhumps, in contradiction to observations. Tidal heating is expected to be most significant in systems with small mass ratios. It follows that systems like WZ Sagittae – which has a tiny mass ratio – are those most likely to show a brightening in the hotspot region.

Liquid-phase mass transfer of magnetic ion exchangers in magnetically influenced fluidized beds: II. AC fields

The application of an alternating magnetic field to a fluidized bed of ferromagnetic particles leads to intensive magnetic stirring and strongly improves mass transfer. Liquid/solid mass transfer was studied with respect to the applied frequency and liquid flow rate. In the system investigated, magnetic stirring resulted in up to 4-fold increased mass transfer rates as compared to the same particles operated in a common fluidized bed without the influence of a magnetic field. The mass transfer rates showed a dependence on the square root of the rotational Reynolds number.

Effect of Hydrodynamic Environment on Tablets Dissolution Rate

The main objective of this research was to develop an experimental method to apply well‐defined flow fields to solid dosage forms, to study the rate process underlying tablet dissolution, and to better understand the role of external hydrodynamic condition on mass transfer rate and film thickness during dissolution. Two drugs models, Theophylline (class 1) and Naproxen (class 2), were selected and formulated into conventional tablets containing 105 mg Theophylline or 300 mg Naproxen using the wet granulation method. Tablets were tested for dissolution using both the basket and paddle methods at different rotational speed of 25, 50, 75, and 100 rpm. In general, the paddle method gave higher dissolution rates than the basket method and as the velocity of rotation was increased, drug release was also increased. Six different paddles and a tablet holder were designed and used to test dissolution rate. The rate of dissolution was dependent on the tablet surface area exposed to the dissolution medium, and on the shape, diameter, and area of the paddles used. Theophylline tablets showed increased mass transfer rate and decreased film thickness as basket rotation speed was increased. At 25 rpm, the mass transfer coefficient was 0.684 × 10− 4 cm/sec and film thickness was 12.003 × 10− 2 cm; at 100 rpm, the mass transfer coefficient was 3.884 × 10− 4 cm/sec and film thickness was 2.114 × 10− 2 cm. Paddle values tested at the same speed showed higher mass transfer coefficient and lower film thickness for Theophylline and Naproxen tablets. P values obtained by modification of the Stokes‐Einstein equation showed that diffusion is the rate‐limiting step to drug release and not mass transfer. This study demonstrated that hydrodynamic condition, type of dissolution testing used, and design of the paddles have an effect on dissolution rate, mass transfer rate, and the film thickness underlying the dissolution process.

Studies on ionic mass transfer in the presence of spiral coil turbulence promoters in batch fluidized beds

The enhancements in mass transfer coefficients due to the presence of spiral coil promoters in batch fluidized beds are presented in this paper. The variables studied are the flow rate of the electrolyte, the geometric parameters of the promoter-coil diameter ( d c), pitch of the coil ( p) and diameter of the coil wire ( d w), void fraction, and particle diameter ( d p). It is found that the mass transfer coefficient increased with increased flow rate and particle diameter. The increased pitch of the coil has decreased the mass transfer coefficient. The increase in coil diameter and diameter of the coil wire have increased mass transfer rates.

Fluid flow and mass transfer in a sinusoidal wavy‐walled tube at moderate Reynolds numbers

AbstractFluid flow and mass transfer characteristics in an axisymmetric sinusoidal wavy‐walled tube are experimentally investigated in the Reynolds number range of 50 to 1000. Attention is paid to the transitional flow, which is observed in the Reynolds number range of 160 to 200. In the laminar flow regime, wall shear stress and mass transfer rate increase with the slopes of 1 and 1/3, respectively, whereas in the turbulent flow regime they increase with the slopes of 3/2 and 3/5, respectively. In the transitional flow regime they increase dramatically, with a sharp slope. It is found that in this flow regime, laminar‐like motion and turbulent‐like motion alternatively take place at different time intervals. This is quite different from the flow instability for the wavy‐walled channel, where Tollmien‐Schlichting waves are observed. The flow instability in the wavy‐walled tube in the transitional flow regime is considered to be responsible for a significant increase in the wall shear stress and mass transfer rate. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(7): 650–661, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.10121

THE USE OF CENTRIFUGAL FIELDS IN ELECTROCHEMICAL ENERGY CONVERSION

Intense centrifugal fields can be exploited to increase the electrochemical reaction rates in energy conversion devices, such as batteries and fuel cells, to improve their overall performance characteristics. This paper describes how “spin-enhanced natural circulation,” created under high centrifugal acceleration, can be used as a means for promoting an increase in mass transfer and reaction rates in three-dimensional porous electrodes. Some relevant scaling laws and new theoretical insights will be discussed to identify the main governing parameters in the design of centrifugal electrochemical energy conversion systems. Recent findings obtained during the development and testing of a small experimental prototype centrifugal zinc-air battery are presented to indicate possible performance trends and future prospects for this novel approach.

Nature and characteristics of pulsing flow in trickle-bed reactors

Pulsing flow is well known for its advantages in terms of an increase in mass and heat transfer rates, complete catalyst wetting and a decrease in axial dispersion compared to trickle flow. The operation of a trickle-bed reactor in the pulsing flow regime is favorable in terms of a capacity increase and the elimination of hot spots. Extending the knowledge on the hydrodynamic nature and characteristics of pulsing flow stands at the basis of further exploitation of the effects of this flow regime on reactor performance. An analysis of the hydrodynamics of pulsing flow reveals that pulse properties as liquid holdup, velocity and duration, are invariant to the superficial liquid velocity at a constant gas flow rate. The pulse frequency, however, increases with increasing superficial liquid velocity. The relative contribution of the pulses and the parts of the bed in between pulses to an average measured property can thus be obtained. By applying this concept it is shown that the linear liquid velocity inside the pulses varies between 0.1 and 0.2 m s −1 . The linear liquid velocity in between pulses, however, is invariant to gas and liquid flow rates and packing properties and equal to about 0.05 m s −1 . This suggests that a linear liquid velocity of about 0.05 m s −1 is the maximum velocity possible in the bed to maintain the trickle flow regime. All liquid in excess is transported as pulses. The liquid holdup in the parts of the bed in between pulses equals the liquid holdup at the transition to pulsing flow at all gas flow rates. The same trend holds for the linear liquid velocity in between pulses. Pulsing flow then is a hybrid of two transition states. The pulses reside at the transition to bubble flow, while the parts of the bed in between pulses reside at the transition to trickle flow. The enhanced particle-liquid heat transfer coefficient inside the pulses is mainly the result of the high linear liquid velocity inside the pulses. Particle-liquid heat transfer rates in between pulses are constant due to the constant linear liquid velocity.

Fluid Flow and Mass Transfer in a Sinusoidal Wavy-Walled Tube at Moderate Reynolds Numbers.

Fluid flow and mass transfer characteristics in an axisymmetric sinusoidal wavy-walled tube are investigated experimentally in the Reynolds number range from 50 to 1 000. The attention is paid to the transitional flow, which is observed in the Reynolds number range of 160 to 200. In the laminar flow regime wall shear stress and mass transfer rate increase with the slope of 1 and 1/3, respectively, whereas in the turbulent flow regime they increase with the slope of 3/2 and 3/5, respectively. In the transitional flow regime they increase dramatically with a sharp slope. It is found that lamiar-like motion and turbulent-like motion alternatively take place with different time intervals in this flow regime. This is quite different from the flow instability for the wavy-walled channel, in which the Tollmien-Schlichting waves are observed. The flow instability in the wavy-walled tube is considered to be responsible for a significant increment of the wall shear stress and mass transfer rate in the transitional flow regime.

The 1998 Outburst of XTE J1550−564: A Model Based on Multiwavelength Observations

The 1998 September outburst of the black hole X-ray binary XTE J1550-564 was monitored at X-ray, optical, and radio wavelengths. We divide the outburst sequence into five phases and discuss their multiwavelength properties. The outburst starts with a hard X-ray spike, while the soft X-ray flux rises with a longer timescale. We suggest that the onset of the outburst is determined by an increased mass transfer rate from the companion star, but the outburst morphology is determined by the distribution of specific angular momentum in the accreting matter. The companion in XTE J1550-564 is likely to be an active magnetic star, with a surface field strong enough to influence the dynamics of mass transfer. We suggest that its magnetic field can create a magnetic bag capable of confining gas inside the Roche lobe of the primary. The impulsive rise in the hard X-rays is explained by the inflow of material with low angular momentum onto the black hole, on a free-fall timescale, when the magnetic confinement breaks down. At the same time, high angular momentum matter, transferred via ordinary Roche lobe overflow, is responsible for the formation of a disk.

The induction of pulses in trickle-bed reactors by cycling the liquid feed

The operation of a trickle-bed reactor in the pulsing flow regime is well known for its advantages in terms of an increase in mass and heat transfer rates. However, fairly high gas and liquid flow rates necessitate the operation in the pulsing flow regime, resulting in relatively short contact times between the phases. By means of the periodic operation of a trickle-bed reactor it is possible to obtain pulsing flow at average throughputs of liquid usually associated with trickle flow during steady-state operation. This feed strategy to force pulse initiation is termed liquid-induced pulsing flow. The advantages associated with pulsing flow may then be utilized to improve reactor performance in terms of an increase in capacity and the elimination of hot spots, while interfacial contact times are comparable to trickle flow. An additional advantage of liquid-induced pulsing flow is the possibility to tune the pulse frequency and therefore the time constant of the pulses. During the periodic operation of a trickle bed, continuity shock waves are initiated in the column due to the step-change in liquid flow rate. This results in the division of the column into a region of high liquid holdup and a region of low liquid holdup. At high enough gas flow rates, the inception of pulses takes place in the liquid-rich region. Analysis of the performed experiments indicates that besides gas and liquid flow rates, an additional criterion for pulse inception is the available length for disturbances to grow into pulses. For self-generated pulsing flow this results in the upward movement of the position of the point of pulse inception with increasing gas flow rate. With liquid-induced pulsing flow this means that higher gas flow rates are necessary to induce pulses as the length of the liquid-rich region decreases. For both self-generated and liquid-induced pulsing flow this relationship between the gas flow rate and the available length for pulse formation is identical.

Laser-Induced Fluorescence Studies of Oxygen Transfer Across Unsheared Flat and Wavy Air−Water Interfaces

Oxygen transfer across unsheared flat and wavy air−water interfaces is studied using a laser-induced fluorescence (LIF) technique. The LIF technique measures the dissolved oxygen (DO) concentration through oxygen quenching of the fluorescence from dissolved pyrene butyric acid (PBA). Fluorescence intensities in a two-dimensional plane in the water near the interface are imaged (instantaneously) with a high-resolution CCD camera. A series of image-processing and image-analysis steps corrects the intensities for variations in the fluorescence attributed to factors other than oxygen quenching and transforms the corrected fluorescence intensities into spatial measurements of the DO concentration. Near-interface vertical oxygen concentration profiles and two-dimensional concentration fields result. Estimates were obtained for the concentration boundary layer thickness, δc, and the mass transfer coefficient, kl, along flat and wavy interfaces. A severalfold increase in mass transfer rates is observed for an uns...

REMOVAL OF ACETIC ACID WITH AMINE EXTRACTANTS FROM FERMENTATION BROTH USING HYDROPHOBIC HOLLOW-FIBER MEMBRANE CONTACTOR

The organic solvent with a high partition coefficient toward acid extraction must be selected to increase the efficiency of extraction of acetic acid in aqueous phase. Amine extractant plays a major role in the reactive extraction process at the interface between aqueous phase/organic phases. Therefore, it has been investigated that an effect of addition of various amines such as di-ethylamine (DEA), tri-ethylamine (TEA), and tri-n-octylamine (TOA) to organic phase could increase the extraction performance due to increasing partition coefficient. The effect of temperature with TOA content of 0 to 40 wt-% on extraction process of acetic acid was observed. The removal efficiency of acetic acid in aqueous phase increased with temperature. However, the maximum value of extraction performance was different according to experimental conditions, such as the choice of organic solvent, temperature, and TOA content. In the case of methyl isobutyl ketone (MIBK)/TOA system and chloroform/ TOA used in organic phases, the maximum value of removal of acetic acid showed 64 wt-% and 53 wt-% at 40 wt-% TOA content and 25°C, respectively. In 2-octanol/TOA system, the maximum removal efficiency of acetic acid was 67 wt-% at 30 wt-% TOA and 50°C. On the basis of the energy efficiency, the optimal condition of organic phase for the extraction of acetic acid in aqueous phase was determined to be 40 wt-% MIBK/TOA at 25°C. Using the membrane extraction process with a hollow-fiber membrane contactor, the overall mass transfer coefficient was calculated by Wilson-plot method under the assumption of constant partition coefficient. These values show a significant increase in mass transfer rate with increasing the TOA content. Additionally, an improvement of extraction performance was established by multiplying the diffusion coefficient and partition coefficient of solutes.

On the late spectral types of cataclysmic variable secondaries

We investigate why the spectral type of most cataclysmic variable (CV) secondaries is significantly later than that of a ZAMS star with the same mean density. Using improved stellar input physics, tested against observations of low-mass stars at the bottom of the main sequence, we calculate the secular evolution of CVs with low-mass donors. We consider sequences with different mass transfer rates and with a different degree of nuclear evolution of the donor prior to mass transfer. Systems near the upper edge of the gap ($P \sim 3 - 6$ h) can be reproduced by models with a wide range of mass transfer rates from $1.5 \times 10^{-9} \msolyr$ to $10^{-8} \msolyr$. Evolutionary sequences with a small transfer rate and donors that are substantially evolved off the ZAMS (central hydrogen content $0.05-0.5$) reproduce CVs with late spectral types above $P \simgr$ 6 h. Systems with the most discrepant (late) spectral type should have the smallest donor mass at any given $P$. Consistency with the period gap suggests that the mass transfer rate increases with decreasing donor mass for evolved sequences above the period gap. In this case, a single-parameter family of sequences with varying $\xc$ and increasing mass transfer rate reproduces the full range of observed spectral types. This would imply that CVs with such evolved secondaries dominate the CV population.

Synergistic effect of high hydrostatic pressure pretreatment and osmotic stress on mass transfer during osmotic dehydration

During osmotic removal of water from foods, the osmotic dehydration front moves from the surface of the food in contact with the surrounding osmotic solution to the centre, which results in disintegration of cells due to osmotic stress. When the food is pretreated with high hydrostatic pressure (HHP), it also results in cell permeabilisation. The cell permeabilisation index ( Z p, as measured by an electrophysical measurement based on electrical impedance analysis) after high pressure treatment increases with time. Osmotic dehydration of HHP-treated foods is faster than that of untreated foods. The state of the cell membrane during osmotic dehydration of high-pressure-pretreated samples can change from being partially to totally permeable, which leads to significant changes in the tissue architecture resulting in increased mass transfer rates during osmotic dehydration as compared to untreated samples.

Simpler criteria for supersaturation in gas–liquid reactions

A process involving simultaneous absorption and desorption with or without chemical reaction often involves the possibility of supersaturation in the vicinity of the gas–liquid interface. Simple mathematical criteria have been developed which can predict the possibility of such supersaturation which may cause additional turbulence near the interface and result in increased mass transfer rates.