Role Of Mass Transfer Research Articles

Impact of mass transport on microstructure of Granny Smith apple parenchyma during osmotic dehydration

AbstractThe chemical components of foods are organized in microscopic structures that can modify mass transfer. The study of material penetration in foods can be made using X‐ray microanalysis. In order to study the microstructural behaviour when fruit was osmotically dehydrated, a glucose molecule modified with chloral (1,2‐O‐2′,2′,2′‐trichloroethylidene‐α‐D‐glucofuranose) was used as an osmotic standard with chlorine detectable by energy‐dispersive X‐ray analysis. Parenchyma of Granny Smith apple is a simple tissue, so it was chosen as reference. The results show that both the apoplastic and symplastic routes played an active role in mass transfer. The osmotic agent penetrated both the symplast and apoplast well into the depths of the fruit, although greater deterioration of cells was observed near the surface. Copyright © 2003 Society of Chemical Industry

Solid-state synthesis and martensitic transformations in thin films

This paper presents experimental data on solid-phase synthesis in double-layer thin-film systems. The rule of the first phase formed at the interface of film reagents at elevated temperatures of annealing is formulated for solid-phase reactions determined by martensite transformations. The temperature at which synthesis is initiated in NinTi, CdnAu, and AlnNi films coincides with the temperature of the back martensite transition in NiTi, AuCd, and AlNi alloys so that martensite phases are formed in the reaction products. The first phase rule was also verified for solid-phase synthesis in CdnAg, NinMn, FeMn, and AunMn systems. In thin films, low-energy processes initiate solid-phase reactions associated with martensite transformations, mass transfer of reagents being up to 200 nm. The martensite model of atomic transfer through the reaction product during solid-phase synthesis in thin films is considered. Martensite shifts can play the dominant role in mass transfer of reagents through the reaction product. It is assumed that the first phase initially formed at the interface of film reagents is formed irrespective of the mode of solidphase synthesis initiation.

Insight into sub-critical wet oxidation of phenol

Low molecular weight organic acids, especially acetic acid, are often formed during wet oxidation. The role of mass transfer in the formation of acetic acid in wet oxidation was studied. For this purpose, phenol was considered as a model substrate. Acetic acid formed was maximum when mass transfer limitations existed. The waste may contain free radical initiators or scavengers. Therefore, the model compound hydroquinone was used as a free radical generator. It accelerated degradation but acetic acid formation increased. On the other hand, t-butanol as a free radical scavenger suppressed both phenol degradation and acetic acid formation. The strongly alkaline pH resulted in more acetic acid formation, though rates of phenol degradation increased. Wet oxidation of phenol was also studied in the presence of the homogeneous catalysts cupric sulfate and ferrous sulfate. The effects of these catalysts on the rates of oxidation and the acetic acid formation were studied. The amount of acetic acid formed in the presence of ferrous sulfate was relatively higher.

Experimental and Theoretical Studies on Hydrogenation in Multiphase Fixed-Bed Reactors

Multiphase fixed-bed reactors have complex hydrodynamic and mass transfer characteristics. The modeling and scale-up are therefore difficult. The present work focuses on the role of mass transfer on the effective reaction rate. The catalytic 1-octene hydrogenation was taken as a model reaction. The reaction rate in the trickle-bed reactor is by a factor of 20 smaller than (theoretically) in the absence of any mass transfer limitations. For high octene concentrations (> 10 %), the effective reaction rate is limited by the H2 consumption, above all by the gas/liquid and liquid/solid mass transfer. For lower octene concentrations the reaction is zero order with respect to H2 and only depends on the octene consumption, i.e., on the interplay of chemical reaction, L/S and intraparticle mass transfer of octene.

Mass transfer of elements and structural-phase changes in heterogeneous thin-film systems under high-power ion beam treatment

Doping of material surface layers though mixing of multi-element thin films with the surface layer of the sample is a promising technological process currently being developed. This work reports the results of experimental investigation of mixing and structural-phase changes in heterogeneous Al–C and Al–Si systems under a high-power pulsed ion beam (HPIB) of carbon (70%) and hydrogen (30%). We measured the distribution of the element concentrations with respect to depth in the sample and phase composition of the heterogeneous systems before and after HPIB treatment. A comparison between the experimental and theoretical results shows that the key role in mass transfer for each element is the existence of concentration gradients for radiation defects.

Swelling and contraction driven mass transfer processes during osmotic dehydration of uncharged hydrogels

The processes of swelling or contraction act as driving forces and thus play important roles in mass transfer during osmotic dehydration of a hydrophilic gel, such as polyacrylamide (PAAm). These driving forces seem to be determined by the interaction between the low molecular solute and the gel polymeric matrix. In a solution of sugars or sodium chloride, the gel undergoes an initial rapid mass loss until it reaches a turning point where the mass transfer process is reversed and the gel swells to a significantly larger extent than in water. However, in solutions of relatively high concentration of sodium citrate or sodium acetate, the gel undergoes only a contraction process and is incapable of regaining any of its lost mass. Empirically, the swelling process follows first order kinetics, which seems to become slower the larger the solution viscosity.

The role of mass transfer in the flow-induced corrosion of equipments employing decaying swirl flow

The effect of decaying Swirl flow on the rate of diffusion controlled corrosion of a pipe was studied by using the diffusion controlled dissolution of copper in acidified dichromate. Variables studied were flow rate, physical properties of the solution and length of the tube. The data were correlated by the equation Sh=4.14 Sc 0.33 Re 0.56 L/d −0.38. The effect of polyox WSR-301 drag reducing polymer on the rate of diffusion controlled corrosion of the pipe was also studied, the polymer was found to decrease the rate of corrosion by an amount ranging from 3 to 26% depending on the operating conditions.

Experimental and Theoretical Studies on Hydrogenation in Multiphase Fixed Bed Reactors

Multiphase fixed-bed reactors have complex hydrodynamic and mass transfer characteristics. The modeling and scale-up are therefore difficult. The present work focuses on the role of mass transfer on the effective reaction rate. The catalytic 1-octene hydrogenation was taken as a model reaction. The reaction rate in the trickle-bed reactor is by a factor of 20 smaller than (theoretically) in the absence of any mass transfer limitations. For high octene concentrations (>10%), the effective reaction rate is limited by the H 2 consumption, above all by the gas/liquid and liquid/solid mass transfer. For lower octene concentrations the reaction is zero order with respect to H 2 and only depends on the octene consumption, i.e., on the interplay of chemical reaction, L/S and intraparticle mass transfer of octene.

EVAPORATION OF A SALT WATER DROP WITH CRYSTALLIZATION

Simultaneous measurements for weight loss and temperature history of an evaporating pendant salt-solution drop into dry air were made for a wide range of air flow rates, ambient temperatures, and initial salt concentrations. The effect of crystallization of salt on the rates of evaporation of an aqueous sodium chloride and potassium chloride solution drop into dry air was examined. The presence of salt crystals was first observed near the front stagnation point, which took a major role in mass transfer of a single drop, and the formation of crystals occurred before the average salt concentration reached the equilibrium value. The dimensionless gas-phase diffusion fluxes of water vapor were affected by the vapor-pressure depression and the decrease in effective surface area caused by crystal growth of inorganic salt. A new simulation method for evaporation of an aqueous salt-solution drop into dry air during crystallization of salt was proposed, and the results were in better agreement with the observed data.

The role of mass transfer in solution photocatalysis at a supported titanium dioxide surface

The kinetics of Cl- production from the UV-photocatalysed degradation of aqueous 4-chlorophenol (CP), by a thin TiO2 (Degussa P25) film, in both aerated and oxygenated solutions, have been determined by the channel flow method with electrochemical detection. The experimental approach allows surface kinetics and mass transport effects to be readily resolved. For typical irradiation intensities of (0.7–2.0)×1017 quanta cm-2 s-1, the results obtained with dilute CP solutions (⩽0.5 mM), in particular, clearly demonstrate that there is a range of practically important conditions where mass transport plays a role in controlling the kinetics of the process. When these effects are considered, the surface kinetics are consistent with the Langmuir–Hinshelwood model.

The role of mass transfer in describing the dynamics of mesoscale convective systems

The role of mass transfer in describing the dynamics of mesoscale convective systems

The role of mass transfer in describing the dynamics of mesoscale convective systems

AbstractWhen deep convection occurs in a rotating stratified fluid, potential vorticity (PV) anomalies form in response to the change in the mass field. This geostrophic adjustment process, and its importance in shaping the dynamics of long‐lived convective systems, is studied using a high‐resolution numerical model to simulate idealized precipitating and non‐precipitating convection.The first series of experiments considers the partitioning of energy between the balanced flow associated with the adjustment (EB), inertia‐gravity waves and turbulent dissipation and how this varies with convected mass (Mc). It is found, for two‐dimensional non‐precipitating convective plumes with a Coriolis parameter equal to 10−4 s−1, that, and, for the plumes which transport the largest quantities of mass, up to 60% of the energy is retained in the balanced flows – a much higher figure than previous estimates. For three‐dimensional (3–D) non‐precipitating plumes it is shown that EB α Mc2. It is suggested that these results indicate that the geostrophic adjustment to mass transfer is an important process in larger convective systems, such as Mesoscale Convective Systems, and that PV anomalies associated with such a system would be much stronger than the net anomaly associated with an ensemble of thunderstorms giving the same upward mass transfer. It is found, for the 3‐D runs, that inertia‐gravity waves comprise only 10% of the total energy released.A further series of experiments are carried out in which the effects of precipitation driven downdraughts are included. The downdraughts do not have the effect of simply reversing the mass transfer of the updraught, and the combination of the updraught and downdraught results in a complex PV anomaly structure with a mid‐level cyclonic vortex. Similar conclusions to those for the non‐precipitating convection case appear to follow for more realistic precipitating systems.

ANALYSIS OF MASS TRANSFER IN OSMOTIC DEHYDRATION BASED ON PROFILES OF CONCENTRATION, SHRINKAGE, TRANSMEMBRANE FLUX AND BULK FLOW VELOCITY IN THE DOMAIN OF TIME AND SPACE

ABSTRACTAn analysis of mass transfer mechanisms in osmotic dehydration was carried out by examining the profiles of concentration, shrinkage, transmembrane flux and bulk flow velocity in the domain of time and space. The profiles were obtained through simulations using a computer simulation model that described mass transfer in osmotic dehydration based on cellular structure. It was shown that bulk flow was the main resistance for solute penetration, and the magnitude of bulk flow was affected not only by the transmembrane flux, but by internal cellular structure changes. Since bulk flow often played an important role in mass transfer during osmotic dehydration, precautions should be taken when interpreting mass transfer in osmotic dehydration using a simple pure diffusion theory.

Role of mass transfer on hydrogen evolution in aqueous media

The process of hydrogen evolution during alkaline electrolysis of aqueous solutions is governed by mass transfer, growth of hydrogen bubbles and removal of hydrogen from the cathode. Two mechanisms are decisive for hydrogen removal: (i) hydrogen dissolved in the solution is carried off from the cathode surface by diffusion and convection, and (ii) gas bubbles are transported by a two- phase flow. The paper describes experiments to determine the local concentration of dissolved hydrogen and the void fraction of hydrogen bubbles in aqueous solutions. Measurements were performed in a flow channel by varying the height of the cathode (40–400mm), the current density (up to 6250Am−2) and the mean velocity of the electrolyte (up to 0.95 m s−1). Two operating regimes of the electrolyser are found. At high current densities a back flow is observed leading to an increase in the electrolyte resistance. Traces of dissolved oxygen are detected at high current densities. At low current densities the two-phase flow is confined to a thin layer along the cathode surface, the concentration of dissolved hydrogen being small.

A mass transfer study of the diffusion controlled dissolution of cubical cavities under natural convection conditions

The role of mass transfer in the rate of diffusion controlled dissolution of cubical cavities was studied using an electrochemical technique involving the measurement of the limiting current of the anodic dissolution of copper cavities in H 3PO 4. Cavity side length and H 3PO 4 concentration were varied to provide (Sc.Gr) range of 8.7 × 10 9 − 3.9 × 10 11. Under these conditions the mass transfer coefficients of cavity dissolution were correlated to other parameters by the equation Sh = 2.742 ( Sc.Gr) 0.22 The equation can be used in practice to predict the rate of diffusion controlled processes which might take place inside the cavity such as electropolishing and electrochemical machining. By comparing the experimental rate of mass transfer with the value calculated by adding the rates of mass transfer at different surfaces of the cavity a remarkable agreement was found denoting little convective interaction under the experimental conditions studied.

P-T-X conditions of late Hercynian fluid penetration and the origin of granite-hosted gold quartz veins in northwestern Iberia: A multidisciplinary study of fluid inclusions and their chemistry

Quartz veins hosted by late Hercynian granites and their host rocks occur across the Variscides of the northern Iberian peninsula, and locally display Au-As mineralization. Four separate occurrences at Corcoesto, Tomino, Penedono, and Pino have been investigated to determine P-T-X conditions of formation and likely fluid sources. Special attention has been paid to fluid chemistry using a multidisciplinary investigation of fluid inclusion gases by Raman spectroscopy (individual fluid inclusion analysis) and mass spectrometry (bulk analysis), and ion chemistry using microthermometry and bulk leachate chemical analysis. Two major changes in the chemical and physical environment have been identified: (1) a progressive change in the bulk chemical composition from early CO 2-rich, C-H-O-(N) fluids, equilibrated with graphitic metamorphic host rocks, to late H 2O-dominated fluids, inferred from their halogen signature to result from an influx of meteoric or upper crustal fluids affecting the basement at the end of Variscan orogenesis, and (2) changes in the P-T conditions from early stage sulphide deposition in quartz veins, at ca. 450 °C and 150-300 MPa, towards epithermal conditions, ca. 260–310 °C and <75 MPa, at the stage of gold mineralization. Several chemical trends are shown by the fluid inclusions: (1) dilution of the early volatile-rich fluids, (2) a break of graphite buffering activity demonstrated by the CH 4 content increase in the volatile fraction of the latest As mineralizing fluids, and (3) increasing contribution of a relatively oxidizing fluid enriched in sulphate and bromide during the latest stages of fluid percolation (Au stage). These latest fluid stages are interpreted as indicative of extended fluid penetration downward in the crust enhanced by late brittle deformation and decompression, and played an important role in mass transfer at the end of the Hercynian orogeny, especially in transport of metals. Gold ores have formed mostly in granites because main fluid pathways developed in the main structurally active zones which favoured the emplacement of the granites. However, there is no evidence of genetic link between gold ores and their enclosing granites.

On the Role of Mass Transfer in X-Ray Emission of RS CVN Systems

We explore the possible role of mass transfer in generating x rays in RS CVn binary systems. We have used system properties from the published literature. We also present results of our ROSAT PSPC pointed observations of RT Lacertae and SS Boötis. We find the evidence that mass transfer is responsible for significant x-ray emission to be suggestive, but not conclusive. Fundamental stellar and orbital parameters and distances must be determined to good precision for more RS CVn systems before progress on this question can be made.

Role of mass transfer and kinetics in the hydrogenation of rapeseed oil on a supported palladium catalyst

The partial hydrogenation of a rapeseed oil in the presence of a supported palladium catalyst was studied. In the temperature range 60–120°C, at which kinetic data were collected, we observed that diffusion limitations are always operative, in particular intraparticle diffusion limitations. The contribution of the mass transfer to the reaction rates was carefully evaluated by determining the mass transfer parameters according to independent routes. In spite of the masking effect of diffusion, kinetic laws suitable to describe the hydrogenation of trienic, dienic and monoenic compounds were identified. A Langmuir-Hinshelwood kinetic equation describes well the behaviour of the unsaturated molecules in the reaction, while the hydrogen reaction order was indirectly shown to be zero. A 3/2/1 ratio between the adsorption constants of trienic, dienic and monoenic compounds was assumed, corresponding to the different probability of a double bond interaction with the palladium surface. The kinetic constants obtained by fitting experimental data gave quite similar results for the polyunsaturated fatty acids and very different for the monounsaturated ones, revealing an intervention of two distinct reaction mechanisms. In the case of polyunsaturated fatty acids, double bond migration with the formation of conjugated dienes would occur before hydrogenation. Conjugated dienes are more quickly hydrogenated than isolated or single double bonds. In a second part of the paper, a more general kinetic model is developed based on a complex scheme of simultaneous and consecutive reactions where hydrogenation reactions occur together with cis-trans isomerizations. A kinetic equation with the corresponding parameters for the cis-trans isomerization of the C 18 monoenic species was determined from experimental data. Subsequently, this equation was used with the same parameters in the general kinetic model to describe the behaviour of all the other cis-trans isomerization reactions. This assumption has turned out to be reasonable. The reliability of the more general kinetic model, of the mass transfer and kinetic equations for hydrogenation processes, of the kinetic equation for cis-trans isomerizations, and of the parameters appearing in the model was verified on the basis of all the experimental data collected. A brief discussion on the factors influencing the catalyst selectivity concludes the paper.

The role of mass transfer in the electrolytic reduction of hexavalent chromium at gas evolving rotating cylinder electrodes

The rate of electrolytic reduction of hexavalent chromium from acidic solution at a hydrogen-evolving rotating cylinder lead cathode was studied under conditions of different current densities, Cr6+ concentrations and rotation speeds. The rate of the reaction was found to follow a first order rate equation. The specific reaction rate constant was found to increase with increasing rotation speed until a limiting value was reached with further increase in rotation speed. Mechanistic study of the reaction has shown that at relatively low rotation speeds the reduction of Cr6+ is partially diffusion controlled, at higher speeds the reaction becomes chemically controlled. The limiting specific reaction rate constant was related to the operating current density by the equationK=0.044i 1.385. The current efficiency of Cr6+-reduction was measured as a function of current density, initial Cr6+ concentration and rotation speed. Possible practical applications are discussed.

Reactions of nitrochlorobenzenes with sodium sulfide: change in selectivity with phase-transfer catalysts

o-Nitro- and p-nitrochlorobenzene were reduced to the corresponding chloroanilines with aqueous solutions of sodium sulfide both in the presence and absence of a phase-transfer catalyst. The use of phase-transfer catalysts, with solid sodium sulfide, was found to change the course of the reaction completely to dinitrodiphenyl sulfide. The role of mass transfer in the system was assessed with m-nitrochlorobenzene as the organic substrate.