Mass Transfer Direction Research Articles

Mass Transfer Coefficient in Liquid−Liquid Extraction and the Influence of Aqueous Phase pH

The influence of aqueous phase pH on the rate of liquid−liquid extraction was investigated, using single drop experiments for a range of drop size and in either of the mass transfer directions. The chemical system of toluene−acetone−water was employed with the advantage of high accuracy and repeatability when analyzed with gas chromatography. The pH of continuous aqueous phase was adjusted within 5.5−8, a range in which most industrial waters are lain. An extraction column was used to introduce the drops into the aqueous phase. Results have demonstrated the impact of pH on the mass transfer performance giving a significant reduction in mass transfer with increasing pH and nearly independent of the kind of accompanied cation for two utilized bases. This matter can be attributed to the adsorption of hydroxyl ions from the aqueous phase which causes small drops to be more prone to this effect. The rate of mass transfer is greater for the dispersed to continuous phase direction under similar conditions.

Measurement of Liquid Concentration Fields Near Interface with Cocurrent Gas-Liquid Flow Absorption Using Holographic Interferometry

Real-time laser holographic interferometry was applied to measure liquid concentrations of CO 2 in the vicinity of gas-liquid free interface under the conditions of cocurrent gas-liquid flow for absorption of CO 2 by ethanol. The influences of the Reynolds number on the measurable interface concentration and on the film thickness were discussed. The results show that CO 2 concentration decreases exponentially along the mass transfer direction, and the concentration gradient increases as Reynolds number of either liquid or gas increases. CO 2 concentrations fluctuate slightly along the direction of flow; on the whole, there is an increase in CO 2 concentration. The investigation also demonstrated that film thickness decreases with the increase of Reynolds number of either of the two phases. Sherwood number representing the mass transfer coefficient was finally correlated as a function of the hydrodynamic parameters and the physical properties.

Effect of mass transfer on the film drainage between colliding drops

The influence of mass transfer on the drainage behaviour of the thin liquid film between two drops immersed in another liquid colliding at constant approach velocity has been studied experimentally. The liquid–liquid system used is glycerol in silicone oil. The transferred solute is acetone and the volume concentration difference across the interface ranges from 1 to 5%. The film thickness evolution has been measured using a laser interferometry technique. The direction of mass transfer (from the drops towards the film phase and inversely) has been investigated and the results compared to the case with no mass transfer [E. Klaseboer, J.P. Chevaillier, C. Gourdon, O. Masbernat, J. Colloid Interface Sci. 229 (2000) 274–285]. When the solute transfers from the drops towards the continuous phase, the drainage rate is significantly higher than in the case with no mass transfer. This result is interpreted as a consequence of the mass transfer induced surface mobility in the film region (the so-called Marangoni effect) due to localized surface tension differences. This effect has been demonstrated by the visualization of the flow patterns in the drops and in the film phase (using a particle tracer technique). In this case, the slope of the film height as a function of time seems to be independent of the approach velocity condition imposed on the drop and appears to be controlled by the interfacial tension gradient. In the opposite case, when the solute transfers from the continuous phase towards the drops, the film drainage rate is lowered with respect to the case of no mass transfer, goes to zero or even changes its sign depending on the mass transfer intensity. The results also show that in the range of solute concentration studied, the effect of mass transfer on the film drainage process takes place at large distances compared to the scales at which lubrication theory is valid.

Comparative Investigations on the Effects of Contamination and Mass Transfer Direction in Liquid−Liquid Extraction

The influence of different factors on the overall mass transfer coefficient of liquid−liquid extraction was investigated, employing single-drop experiments with mass transfer in either direction. The chemical system of toluene−acetone−water was used with the advantage of high accuracy and repeatability when using gas chromatography for analysis. Three different types of surfactants were used as contaminant agents, and their effectiveness in retarding the mass transfer appeared in the order cationic < anionic < nonionic. Addition of contaminants did not change the trend of variation with drop size for the concentration range investigated in this study; however, a number of small drops demonstrated rigid behavior at high contaminant concentrations. The rate of mass transfer is greater in the dispersed to continuous phase direction. An empirical correlation was developed here in conjunction with a combined model which describes the variations of the mass transfer coefficient. The variation of physical properties was also taken into account.

The multiple roles of interfacial tension in fluid phase equilibria and fluid–solid interactions

The tension at the interfaces separating the three phases of matter is a unique property in that it can reveal a great deal of information about the phases in contact, including the direction and extent of mass transfer of components, their proximity to equilibrium, the nature of fluids distribution relative to one another, the contact angle, and the spreading and adhesion behavior of liquids on solid surfaces. In this paper we examine, with supporting experimental data, the multitude of roles played by interfacial tension in establishing (1) the phase behavior characteristics of solubility, miscibility, and the associated mass transfer mechanisms in multicomponent fluid systems, (2) the nature of fluids distribution in gas–oil–water systems in porous solid substrates and (3) the spreading and adhesion characteristics in solid–liquid–liquid systems through dynamic contact angles.

Humidity effect and its influence on the seasonal distribution of precipitation δ18O in monsoon regions

The humidity effect, namely the markedly positive correlation between the stable isotopic ratio in precipitation and the dew-point deficit ΔTd in the atmosphere, is put forward firstly and the relationships between the δ18O in precipitation and ΔTd are analyzed for the Urumqi and Kunming stations, which have completely different climatic characteristics. Although the seasonal variations in δ18O and ΔTd exhibit differences between the two stations, their humidity effect is notable. The correlation coefficient and its confidence level of the humidity effect are higher than those of the amount effect at Kunming, showing the marked influence of the humidity conditions in the atmosphere on stable isotopes in precipitation. Using a kinetic model for stable isotopic fractionation, and according to the seasonal distribution of mean monthly temperature at 500 hPa at Kunming, the variations of the δ18O in condensate in cloud are simulated. A very good agreement between the seasonal variations of the simulated mean δ18O and the mean monthly temperature at 500 hPa is obtained, showing that the oxygen stable isotope in condensate of cloud experiences a temperature effect. Such a result is markedly different from the amount effect at the ground. Based on the simulations of seasonal variations of δ18O in falling raindrops, it can be found that, in the dry season from November to April, the increasing trend with falling distance of δ18O in falling raindrops corresponds remarkably to the great ΔTd, showing a strong evaporation enrichment function in falling raindrops; however, in the wet season from May to October, the δ18O in falling raindrops displays an unapparent increase corresponding to the small ΔTd, except in May. By comparing the simulated mean δ18O at the ground with the actual monthly δ18O in precipitation, we see distinctly that the two monthly δ18O variations agree very well. On average, the δ18O values are relatively lower because of the highly moist air, heavy rainfall, small ΔTd and weak evaporation enrichment function of stable isotopes in the falling raindrops, under the influence of vapor from the oceans; but they are relatively higher because of the dry air, light rainfall, great ΔTd and strong evaporation enrichment function in falling raindrops, under the control of the continental air mass. Therefore, the δ18O in precipitation at Kunming can be used to indicate the humidity situation in the atmosphere to a certain degree, and thus indicate the intensity of the precipitation and the strength of the monsoon indirectly. The humidity effect changes not only the magnitude of the stable isotopic ratio in precipitation but also its seasonal distribution due to its influence on the strength of the evaporation enrichment of stable isotopes in falling raindrops and the direction of the net mass transfer of stable isotopes between the atmosphere and the raindrops. Consequently, it is inferred that the humidity effect is probably one of the foremost causes generating the amount effect.

Amounts and Distribution of Phases in Sulfide Plus Oxide Scales on Iron

Pure iron was exposed at 800°C to flowing, catalyzed-gas mixtures of N2/CO2/CO/SO2 adjusted to control the partial pressures of SO2, S2 and O2. The equilibrium gas compositions were such that iron oxide was thermodynamically stable with respect to sulfide. The reaction product scale was invariably a mixture of oxide plus sulfide, and grew according to parabolic kinetics at high PSO2 values and by linear kinetics in dilute gases. In both cases the reactant gas species was SO2, not molecular oxygen or sulfur. The relative amounts of sulfide and oxide corresponded to stoichiometric reaction of SO2 at high PSO2 values, but not in dilute gases. At low PSO2 values, the relationship between scale-sulfide volume fraction and PSO2 corresponded to two independent scale-SO2 reactions leading to oxide and sulfide growth. The two-phase mixture was lamellar, with platelets oriented approximately parallel to the mass-transfer direction. An inverse relationship between lamellar spacing and linear scaling rate is interpreted as evidence of a cooperative (cellular) growth mechanism.

A relationship between diffusion and charge transfer parameters in a binary melt formed during contact melting

Theoretical and experimental data on the mass and charge transfer, charge transfer during contact melting, and hydrodynamic mixing of inert labels introduced into a contact between two dissimilar metals are analyzed and an integral criterion for the charge and mass transfer direction in this system is formulated. According to this criterion, which is valid in all the known binary systems of this type, (Ω1–Ω2)(D1–D2)]=−z1*–z2*, where Ωi, D i , and z i * are the atomic volumes, partial diffusion coefficients, and effective charges of light (i=1) and heavy (i=2) components.

Enhancement Of Single Liquid Drop MassTransfer Coefficients In A Perforated PlateColumn

Mass transfer coefficients (MTCs) have been measured for single solvent drops rising through non flowing and flowing water in a short section of perforated plate extraction colunin and have been compared with the coefficients measured for free rise conditions in an elnpty column for different systems. Single drop mass tra~isfer coefficients were determined as a function of drop size, mass transfer direction, distance between plates and down flow of the continuous phase. The degree of enha~icement of the dispersed phase film coefficie~it due to the presence of plates has been ascertained. From the data obtained, it is co~icluded that only a moderate enhancement due to the presence of down flow is achieved for single liquid drop with a slightly increase in the drop size with increasing downflow and the coefficients for mass transfer directioii from the dispersed phase into the continuous phase (d + c) are slightly higher than that for the opposite direction. (c+ d).

Study of drop coalescence behavior for liquid–liquid extraction operation

Using a video camera, a time course of coalescence was observed for toluene droplets formed at the top of adjacent nozzles in water. Several parameters which may control the coalescence have been investigated, such as concentration of acetone added to toluene, direction of mass transfer, time for attaining contact of the droplets, and flow rate of the continuous phase. The average coalescence times decrease with acetone concentration when acetone is transferred from the dispersed phase towards the continuous phase, and they increase with the time required for attaining contact. At high acetone concentration, oscillation of droplets appears, which restrains the coalescence. The average coalescence time increases when the acetone is transferred from the continuous phase towards the dispersed phase. An attempt has been made to predict coalescence times at low solute concentration (no oscillation) by assuming that the difference of interfacial tension causes the film drainage and leads to the drop coalescence. The agreement between the calculated and experimental values is not sufficiently quantitative but reasonable. For lower solute concentration, calculated coalescence times depend strongly on the time for attaining contact, which is in agreement with our experimental observation.

Mass transfer performance in a coalescence–dispersion pulsed sieve plate extraction column

In order to improve the mass transfer characteristics, a coalescence–dispersion sieve plate was designed. Mass transfer experiments were carried out in a coalescence–dispersion pulsed sieve extraction (PSE) column of 150 mm in diameter with a working system of 30% TBP (in kerosene)–nitric acid–water. Mass transfer direction was from the continuous phase to the dispersed phase, namely from the aqueous phase to the organic phase. The mass transfer characteristics were evaluated by a dynamic tracer method. Three parameters, the ‘true’ height of the transfer unit, the height of the dispersion unit and the apparent height of the transfer unit, were estimated from the dynamic response curve. Compared with that in a standard PSE column, the mass transfer performance in the coalescence–dispersion PSE column was greatly improved.

An overall rate-based stage model for cross flow distillation columns

In this work a nonequilibrium stage model for cross flow distillation columns is developed. Concentration gradients on trays are integrated analytically for simple flow patterns on the basis of an overall rate approach. Empirical mixing corrections for bulk and interface concentrations and the assumption of constant interface properties of other nonequilibrium stage (NEQ) models are completely avoided. Simulations were carried out for two distillation cases of non-ideal mixtures. The importance of flow modelling on separation is shown and compared with mass transfer cross effects. Calculated separations for distillation trays, over a wide range of operating conditions, suggest that the proposed model provides a better approximation of concentration profiles, compared with the multicomponent efficiency (MEF) and the NEQ stage model. It is also shown that the efficiency-based model may predict incorrect mass transfer directions for trace components; this behaviour is caused by neglecting the convective contribution in the calculation of multicomponent mass transfer rates.

Flow of fluids with substantially different mobilities through a porous medium in the presence of phase transitions: Boundary layer phenomena, spatial phase structures, and instability of the flow

A model of flow through a porous medium with phase transitions which permits an efficient qualitative investigation is proposed for two fluids with sharply different (high-contrast) mobilities. It is shown that the model problem of flow toward a unit sink is singularly perturbed and can be solved using analytic asymptotic matching methods. The nature of the singularity is associated with violation of the condition of the flow contrast in certain zones. The solution can be unstable depending on the direction of interphase mass transfer and the zone in which the process takes place.

Multicomponent Effects in Liquid-Liquid Extraction

One of the reasons problems arise in the design of extraction equipment is an unsatisfactory knowledge of the mechanisms responsible for the transport of components from the bulk of one phase across the interfacial area into the bulk of the other contacting phase. An investigation was carried out on mechanisms relevant for mass transfer, the importance of multicomponent effects in liquid-liquid extraction and also which models are suitable for describing mass transfer. For this, experiments were performed in a countercurrent spray column with the system ethylacetate-acetone-water. The saturation of the feed phases, the extraction factor and the direction of mass transfer were varied. When phases fed into the column are saturated, both phases change their concentration along the binodal line. When the phases are initially unsaturated, both extraction trajectories approach the binodal line. Comparing the experimental concentration profiles with simulation results, it is shown, that in the area of undersaturation, extraction trajectories can be described only if the complete matrix of multicomponent diffusion coefficients is known and the effects of diffusive coupling are taken into account. When mass transfer coefficients are calculated at bulk concentrations, the achieved results are closer to the measured extraction trajectories than in the case where they are calculated depending on interfacial concentrations.

A Simplified Correlation of Mass Transfer in a Pulsed Sieve Plate Extraction Column

In a pulsed sieve plate extraction column (PSEC) with a 150 mm diameter, a dynamic process and a conventional concentration profile measurement process were used to evaluate the mass transfer and axial mixing coefficients for the 30% TBP (in kerosene)/nitric acid/water system. The mass transfer direction was from the continuous phase to the dispersed phase. Experimental results indicated that the much simpler dynamic process could be reliably used in place of the concentration profile measurement process. A simple equation, (HTU) OC =KU C /Φ(1-Φ) n , was derived to calculate the true height of transfer unit (HTU) OC (independent of column diameter) from the experimental mass transfer data and hydrodynamic characteristics in PSEC with different diameters while the mass transfer rate of the extraction system is controlled by the drop side resistance. The derivation of calculated values from experimental data was up to ±20%. The equation was tested in several other differential type extraction columns. It was shown that this equation could also be used to predict the true height of transfer unit.

Electrode erosion and coating properties in pulsed air arc deosition of WC-based hard alloys

WC-based hard alloys were deposited from a slab source anode on to low carbon steel and pure Ti substrates using a pulsed air arc. The depositions were conducted with a series of pulses with current amplitudes of 75 A, 300 A, and 500 A, a pulse duration of 150 μs, and a pulse repetition rate of 100 Hz. The influence of the electrode material, discharge energy and deposition time on the electrode erosion rate, mass transfer direction, and the wear resistance and friction coefficient of the coatings was investigated. Anodic and cathodic mass change characteristics depended on both the anode and cathode materials. Using a steel cathode and a WC anode, depositions of the anode material always formed on the cathode. For the same WC anode but a Ti cathode, coatings only formed locally in the craters of the eroded cathode. The anodic mass loss when using the steel cathodes was about a factor of 3 larger than when using the Ti cathodes. The microhardness of the coated substrates was larger by factors of 5–12 than the uncoated Ti and low carbon steel substrates. The coatings increased the wear resistance of the steel and Ti substrates by factors of 4.3 and 1.4, respectively. The poorer performance of the coated Ti substrates is attributed to the intensive erosion of the Ti cathodes (compared to the steel cathodes) and the discontinuous nature of the coatings (i.e. only in the erosion caters).

The Tertiary dynamics of the northern Eastern Alps (Austria): changing palaeostresses in a collisional plate boundary

Six Tertiary deviatoric palaeostress tensor groups from 165 stations in the Calcareous Alps describe the upper crustal dynamics of the leading edge of the Adriatic plate during protracted continental collision with the European lower plate. Palaeostress changes are correlated to the kinematic evolution of the plate boundary and to independently derived plate kinematic data of the Alpine-Carpathian-Pannonian area. Each palaeostress direction is defined by 32 to 89 individual tensors which constrain: (1) Late Eocene to ?Oligocene NW-directed compression during thrusting and dextral shearing on WNW-striking faults; NW-directed compression along the northern margin of the Adriatic plate is correlated to its N-directed translation combined with a counterclockwise rotation. (2a) ?Late Eocene to Early Miocene N-directed compression during N-directed thrusting of Penninic and Helvetic units and the Molasse; (2b) Early to Middle Miocene N-directed strike-slip compression during the onset of eastward lateral extrusion; the change from NW- to N-directed compression is interpreted to result from strain partitioning at the Periadriatic fault which has formed the new Adriatic plate boundary since the Oligocene. Dextral shearing on this fault accommodated continued anticlockwise rotation and decoupled the northern Eastern Alps from the rotational component. N-directed compression in the Calcareous Alps paralleled the N-directed translation of the Adriatic plate. (3) Middle Miocene NE-directed compression of thrust- and strike-slip type during lateral extrusion; NE-directed compression in the Calcareous Alps resulted from the drag of the eastward extruding central Eastern Alps. Sinistral shear stress was transmitted across the ENE-trending Salzach-Ennstal fault. NE-oriented σ 1-axes include 45° with the E-W-striking zone of distributed sinistral shear in the Calcareous Alps. (4) Middle Miocene E-directed extension associated with orogen-parallel normal faulting of the central Eastern Alps; E-directed extension paralleled the direction of mass transfer towards the Pannonian Basin during lateral extrusion. Normal faulting was induced by reduced lateral confinement due to removal of material east of the Alps. On the large scale, eastward motion was enabled by the E-directed retreat of the Eastern Carpathian subduction zone. (5) Late Miocene E-directed compression; orogen parallel compression obstructed lateral extrusion. The stress change from extension to E-directed compression was caused by the end of subduction retreat in the Eastern Carpathians due to dynamic changes in the subduction zone. (6) N-directed extension; Late- to post-Miocene extension paralleled recent topographic slopes in the foothill of the Eastern Alps. Palaeostresses changes correspond to distinct phases in the kinematic evolution of the northern Eastern Alps and to the systematic formation and reactivation of major fault zones. WNW-striking faults changed deformation styles from dextral shear to dextral transpression, reverse faulting and finally to sinistral shear. The Early to Middle Miocene Salzach-Ennstal fault which formed the northern boundary of the extruded central Eastern Alps changed from sinistral transpression to sinistral shear, sinistral transtension and finally to dextral shear.

The linear stability in systems with intensive mass transfer—I. Gas (liquid)-solid

A linear analysis of the stability of the flow in a laminar boundary layer under conditions of intensive interphase mass transfer, where high mass fluxes through the phase boundary induce secondary flows, is suggested. These secondary flows have an effect of ‘injection’ or ‘suction’ in the boundary layer depending on the direction of the intensive interphase mass transfer, and they have a variable velocity along the length of the phase surface. They are also proportional to the local mass flux through the phase boundary. The critical Reynolds numbers are obtained at different intensities of non-linear mass transfer in the laminar boundary layer. The influence of the direction of the intensive interphase mass transfer on the hydrodynamic stability is shown as well. In the cases shown intensive interphase mass transfer is directed toward the phase boundary (suction) and the increase of its intensity leads to an increase in the stability of the flow. In the opposite case (injection), turbulization results at considerably smaller values of the Reynolds number.

The linear stability in systems with intensive mass transfer—II. gas-liquid

A linear analysis of the stability of the flow in a laminar boundary layer under conditions of intensive interphase mass transfer, when high mass fluxes through the gas-liquid phase boundary induce secondary flows, is suggested. Essential interaction between flows in gas and liquid is considered in the case of a movable liquid surface. The critical Reynolds numbers are obtained at different intensities of nonlinear mass transfer in a laminar boundary layer in the gas. The influence of the direction of the intensive interphase mass transfer on the hydrodynamic stability is analogous to the one in the case of motionless interface. The motion of the interface leads to a decrease of velocity gradients, which is the cause for increase in stability of the gas flow. Flow is stable at large Reynolds number in the liquid phase.

The linear stability in systems with intensive mass transfer—III. liquid-liquid

A linear analysis of the stability of the flow in a laminar boundary layer under conditions of intensive interphase mass transfer between two liquids, when high mass fluxes through the phase boundary induce secondary flows, is suggested. The first liquid is in motion over the second one (in rest). Hydrodynamic stability in the two phases is considered. The critical Reynolds numbers in the first place are obtained at different intensities of non-linear mass transfer in the laminar boundary layer. The influence of the direction of the intensive interphase mass transfer on the hydrodynamic stability is analogous to the cases where phase boundary is motionless, but depends on the distribution of the diffusive resistance in the two phases. The motion of the interface is considerably more intensive than the one in the gas-liquid system, which leads to an increase in the stability of the flow to a large degree. The flow is stable at a large Reynolds number in the second phase.