Mass Transfer Steps Research Articles

Performance predictions in the scale-up of a liquid—liquid CSTR for indirect electro-oxidation of aromatic hydrocarbons

A theoretical study of a CSTR forming part of an indirect electrochemical process for oxidizing a toluene such as methylanisol (MA) by an acidic aqueous solution containing a metallic cation at a high valence such as cerium(IV) is proposed in the aim of selecting the set of operating conditions for the synthesis of the corresponding aldehyde. The chemical process begins by the mass transfer step of the substrate into the aqueous phase and continues by the purely homogeneous oxidation steps in the aqueous phase. The alcohol, aldehyde and acid formed are simultaneously extracted by the substrate in excess. The model developed is based on the film theory and allows for the effects on the process selectivity of the main parameters such as temperature, rotation speed, concentration and flow rate of the input oxidant solution, and the volume ratio of the phases. The data concerning the chemical kinetics, the mass transfer and the phase equilibrium, which are used in the computations have been experimentally determined in the case of methylanisol. The method enables the calculation of the operating parameter values in view of obtaining selectively the aldehyde rather than the alcohol or the acid and, what is more, for a cerium conversion higher than 80%.

Kinetic Studies of Extraction and Stripping of Copper with Dibenzoylmethane

Abstract Kinetic studies of the extraction of copper with dibenzoylmethane and the stripping of copper were carried out in a stirred transfer cell, along with a study of the extraction equilibrium of copper with the extractant at 303 K. The extraction and stripping rates were analyzed by a model of the interfacial reaction between the extractant and the adsorbed 1:1 copper chelate complex accompanied by the mass transfer steps of the species. Furthermore, the extraction rate was simulated by using the equilibrium and reaction rate constants obtained from the analysis.

Model calculations for mass transfer with mixing in ladle metallurgy

It is well known that in secondary metallurgy during gas stirring, zones with differing degrees of homogeneity can occur in the liquid steel. By means of model calculations it will be demonstrated under which conditions mixing has to be considered for mass‐transfer phenomena, as for example desulphurization. Under unfavourable conditions, i.e. large regions with low flow velocities – so‐called dead zones – and low values for the exchange volume flow between these regions and the bulk volume, the equalization of concentration will become the rate‐determining step for the total mass transfer. In order to avoid or minimize dead zones, the general flow pattern in the ladle has to be optimized.

Deposition Mechanism of Tungsten Silicide Films by Low Pressure CVD

ABSTRACTWSix thin films were deposited on SiO2/Si substrates by Low Pressure Chemical Vapor Deposition (LPCVD) using WF6 and SiH4 gases. The deposition mechanism has been studied by measuring the thickness, resistivity and composition of the films by varying deposition temperature and gas flow rate at a constant total reactant gas pressure. Below 300°C, the surface chemical reaction was the rate-limiting process and the deposition rate increased exponentially with temperature having a thermal activation energy of 3.2 kcal/mol. Meanwhile, above 300°C the reaction was governed by the mass transfer step in the gas. The deposition rate in this range is insensitive to the deposition temperature but shows dependence of the flow rate of reactant gases. AES and RBS analyses were performed to determine the stoichiometry of WSix thin film. The Si content in film gradually increased as the deposition temperature increased. The resistivity of as-deposited WSix film has dependence on both deposition temperature and Si/W ratio, and exponentially increased with a moderate slope. However, temperature insensitive behavior of resistivity appeared in the mass transfer controlled region. Such resistivity changes with temperature were discussed with the Si/W ratio and the microstructure of films.

Transport of mass and heat in porous adsorbents

The kinetics of sorption in most commercial adsorbents (e.g. charcoals or zeolites) results from the interaction of several distinct steps of mass transfer. In order to understand and analyse the compound kinetics it is expedient to design experiments so that the rate of individual steps can be observed independently. Otherwise, the rate of diffusion in micropores or in cavities of zeolite crystals is likely to be obscured by resistance to mass transfer in intercrystalline voids of the sample or in the macropores of a pellet. Temperature excursion in the solid sorbent owing to exothermicity of sorption can be controlled through direct contact of the solid sorbent with a heat sink of sufficient capacity. Results obtained for hydrocarbons in zeolite ZSM-5 are briefly reported.

Growth kinetics of fluorapatite deposition on synthetic hydroxyapatite seed crystals

Crystal growth kinetics of fluorapatite on hydroxyapatite seeds were studied over a range of supersaturations and solution fluoride concentrations at pH 4.5. The kinetics of this system are complicated by two additional phenomena: (i) Fluoride is exchanged for surface hydroxyl ions. (ii) At low supersaturations, growth of the crystals becomes intrinsically more difficult as growth proceeds. The kinetics were compared with classical growth models assuming linear, parabolic, and exponential dependencies of growth rate on the solution ion activity product, on the basis of the stoichiometry of the growing phase. These models were found inadequate because the experimental data, which included a wide range of nonstoichiometric solutions, clearly showed that growth is not a single valued function of ion activity product. After modifications to more appropriately account for the implied mass transfer steps, the resulting models were in much better qualitative agreement with the experimental data.

Dynamics of continuous precipitation under non-stoichiometric conditions

Continuous precipitation from ionic solution is investigated here under conditions of non-stoichiometric operation, with a two-step model for growth. Prior studies of crystallization or precipitation have all been carried out for stoichiometric solutions. It is shown that, when supersaturation is generated by a chemical reaction such as double decomposition, and the mass transfer step for growth offers a significant resistance, maximum productivity is achieved at non-stoichiometric reactant concentrations. The effect is particularly pronounced at high supersaturations which are often attained in reaction—crystallization, and two- to three-fold improvements in production may be obtained compared to operation with stoichiometric reactant feed streams. The stability under non-stoichiometric conditions is also studied and it is found that over a certain range of the ratio of reactant concentrations the system can be unstable. Under certain conditions instability is also found to arise as the mass transfer resistance for crystal growth is reduced, indicating that low agitation speeds may facilitate stable operation. As in prior studies of crystallizer dynamics instability is found to manifest itself in the form of limit cycle oscillations.

Concentration of copper(II) ion by liquid membrane with bathocuproine.

The distribution ratios of copper(II) ion between an aqueous solution containing a copper salt in the presence of nitrate, chloride or sulfate and an organic phase containing 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (bathocuproine) in 1,2-dichloroethane (DCE) were measured as a fundamental study for establishing the liquid membrane system.The difference in distribution ratio between nitrate and sulfate used in large excess was applied to drive the uphill transport of copper ion. The liquid membrane process consisted of the DCE solvent containing bathocuproine as a transport carrier and the two aqueous copper ion solutions containing a large excess of nitrate and sulfate ions. In this process, reproducible rate data were obtained by using a liquid-film holder in which the liquid membrane was maintained at constant pressure between two sheets of hydrophilic filter to prevent its leakage. The transport rate was examined by the quasi-steady model based on the three mass transfer steps involving a series of formation and dissociation of the copper-bathocuproine ion-pair with nitrate anion that take place instantaneously and reversibly at both interfaces of the liquid membrane. As a result, it became clear that copper ion was concentrated by the extraction through one hydrophilic filter, the diffusion of ion-pair across the liquid membrane and the back-extraction through the opposite filter accompanying the pseudo-first order equilibrium reaction for the complexation of copper with sulfate ion.

Use of ascorbic acid to inhibit nitrosation: kinetic and mass transfer considerations for an in vitro system.

Ascorbic acid and ascorbate ion (denoted together as ASC) inhibit nitrosation by competing for the nitrosating agents formed from nitrite (e.g. N2O3, NO+ and NOSCN). ASC is oxidized irreversibly by this reaction and the nitrite equivalents are reduced to nitric oxide (NO). In open, aerobic systems the effective stoichiometry of the reaction between ASC and nitrite is not fixed, but is determined by a competition between the physical removal of NO (and NO2) from the system and the oxidation of NO by dissolved O2. The oxidation of NO reconverts it to a nitrosating agent which may react again with the remaining ASC. To determine the conditions under which ASC is most effective as a nitrosation inhibitor, we examined the kinetics of the reactions between nitrite and ASC and between nitrite and proline. These reactions were studied in open shaker flasks as functions of pH, anion composition (SCN- and Cl-), temperature, and gas-liquid mass transfer rate. At lower mass transfer rates, at lower pH and/or in the presence of SCN- or Cl-, relatively more ASC was consumed by a given amount of nitrite. Increased temperature caused more or less ASC to be consumed by a given amount of nitrite, depending on the conditions. A mathematical model of the reactions and mass transfer steps was developed which describes each of these features. The model predicts the variable stoichiometry of the reaction between nitrite and ASC in open, aerobic systems, and clarifies the mechanisms by which ASC inhibits nitrosation.

Analysis of affinity separations: I: Predicting the performance of affinity adsorbers

The analysis of affinity separations has lagged behind the rapid increase in new applications. The purpose of this work is to outline an approach to analysis that will aid the design and optimization of large-scale affinity separations. This paper is an introduction to the theory of fixed bed adsorption as it applies to affinity chromatography. This theory provides analytical tools for evaluating and predicting industrial-scale affinity column performance, based on data from laboratory-scale experiments. Design equations are included for continuous separations on a rotating annular chromatograph and for batch operation of the adsorption and wash steps. The design equations are derived from a general model by introducing appropriate simplifying assumptions for rate-limiting mass transfer and sorption steps.

Kinetics of copper extraction with N-8-quinolyl-p-dodecylbenzenesulfonamide.

A kinetic study of copper extraction with reagent-grade N-8-quinolyl-p-dodecylbenzenesulfonamide was carried out by using a stirred transfer cell, along with studies of the distribution and interfacial equilibria of the extractant between the organic and aqueous solutions at 303K. The rate of copper extraction was analyzed by a model of the interfacial reaction between the adsorbed extractant (HR) and the adsorbed copper complex (CuR+) accompanied by the mass transfer steps of each species. The experimental results show good agreement with the results calculated by use of this interfacial reaction model, concerning the initial extraction rate and the extent of copper extraction over a wide range of experimental conditions.

Reduction of lead sulfide with COCO 2N 2 gas mixtures in the presence of lime

The kinetics of reduction of lead sulfide with COCO 2N 2 gas mixtures in the presence of lime were investigated in the temperature range 848–963°C. Several gas compositions were tested at each temperature. Potential soot formation was overcome by preheating the reducing gas mixture. Excess lime was added to curtail PbS-loss through vaporization; for each mole of PbS there were present four moles of CaO in the mixture. The influence of ternary eutectic, (K, Li, Na) 2CO 3, catalyst on the reduction kinetics was examined. It became clear that the reduction reaction is not amenable to catalysis. The presence of catalyst proved to be somewhat of an hindrance to the reduction conceivably through pore-blockage and pore-flooding by the molten eutectic. The experimental data was first correlated by a shrinkingcore type model to derive apparent rate constant data. Further analysis of this data revealed that significant mass-transfer effects exist. A more detailed model involving simultaneous mass transfer, reduction and vapourization steps was formulated. The data for the uncatalyzed reduction was interpreted by means of this model and rate constant data were derived.

Kinetic studies of enzymatic hydrolysis of insoluble cellulose: Analysis of the initial rates

A study was conducted on the kinetics of enzymatic hydrolysis of pure insoluble cellulose using unpurified culture filtrate Trichoderma reesei, with the emphasis on the initial reaction period. The initial hydrolysis rate and extent of enzyme (soluble protein)adsorption, either apparent or initial, were evaluated under various experimental conditions. It has been found that the various mass-transfer steps do not control the overall hydrolysis rate and that the hydrolysis rate is mainly controlled by the surface reaction step promoted by the adsorbed enzyme. It has also been found that the initial hydrolysis rate strongly depends on the initial extent of soluble protein adsorption and the effectiveness of the adsorbed soluble protein to promote the hydrolysis. The initial extent of soluble protein adsorption, in turn, is related to the initial cellulose concentration, enzyme concentration, and specific surface area of cellulose, whereas the effectiveness of the initially adsorbed soluble protein to promote the derived to interrelate these parameters without resorting to the Michaelis-Menten kinetics. The present result appear to imply that the role of enzyme-substrate complex formation should not be ignored in deriving a mechanistic kinetic model for enzymatic hydrolysis of cellulose.

Electrochemical hydrogenation of aromatic hydrocarbons Discrimination between ECE and disproportionation mechanisms by double potential step chronoamperometry

The electrochemical hydrogenation of anthracene and naphthalene is investigated with the aim of determining if the second electron transfer following the protonation of the anion radical occurs predominantly at the electrode (ECE) or in the solution (DISP). It is shown that double potential step chronoamperometry is a particularly sensitive method for discriminating between the two mechanisms. Indeed, a characteristic hump appears on the anodic trace in the case of ECE and not of DISP. Application of the method to the reduction of the two hydrocarbons in DMF in the presence of phenol confirms that anthracene hydrogenation follows a DISP mechanism. This is also the case for naphthalene which was previously thought to undergo an ECE-type reduction. The rates of protonation of the anion radical are derived from the ratio of the anodic to cathodic current intensities. Thus is confirmed by the statement that, in organic reactions, ECE mechanisms do not occur under conditions where they could be directly characterized by electrochemical kinetic techniques, i.e. when the system can be made at least partially reversible. The grounds and conditions of validity of this rule are discussed. For very irreversible systems, irreversibility arising from the rapidity of the chemical step and/or slowness of mass transfer, ECE reaction pathways may be followed. Indirect approaches for their characterization in such conditions are discussed.

Electrochemical hydrogenation of aromatic hydrocarbons: Discrimination between ECE and disproportionation mechanisms by double potential step chronoamperometry

The electrochemical hydrogenation of anthracene and naphthalene is investigated with the aim of determining if the second electron transfer following the protonation of the anion radical occurs predominantly at the electrode (ECE) or in the solution (DISP). It is shown that double potential step chronoamperometry is a particularly sensitive method for discriminating between the two mechanisms. Indeed, a characteristic hump appears on the anodic trace in the case of ECE and not of DISP. Application of the method to the reduction of the two hydrocarbons in DMF in the presence of phenol confirms that anthracene hydrogenation follows a DISP mechanism. This is also the case for naphthalene which was previously thought to undergo an ECE-type reduction. The rates of protonation of the anion radical are derived from the ratio of the anodic to cathodic current intensities. Thus is confirmed by the statement that, in organic reactions, ECE mechanisms do not occur under conditions where they could be directly characterized by electrochemical kinetic techniques, i.e. when the system can be made at least partially reversible. The grounds and conditions of validity of this rule are discussed. For very irreversible systems, irreversibility arising from the rapidity of the chemical step and/or slowness of mass transfer, ECE reaction pathways may be followed. Indirect approaches for their characterization in such conditions are discussed.

CO<SUB>2</SUB>-CO混合ガスによる溶銅の脱酸速度

Deoxidation rate of liquid copper whose oxygen concentration was above 0.25 mass% was measured by employing an agitated reactor under various experimental conditions (reaction temperature=1357∼1538 K, impeller tip speed=1∼33.3 s−1, gas flow rate=2.2∼31.7 (10−6 m3(NTP)/s), mol fraction of CO in the gas phase=0.2∼1.0).Results obtained are as follows:(1) The resistance of mass transfer in the liquid phase is small and neglected with respect to the overall resistance of the deoxidation.(2) The chemical reaction is the rate-controlling step at a high gas flow rate, and the rate (−r) is expressed by an equation, −r=k′pCO, where pCO is the partial pressure of CO and k′ is an apparent rate constant of the chemical reaction. The apparent activation energy and the frequency factor are 9830 J/mol and 2.7 (10−6 mol/m2·s·Pa), respectively.(3) Both steps of mass transfer in the gas phase and the chemical reaction are rate-controlling at a low gas flow rate. An empirical correlation on mass-transfer coefficient (kG) obtained by substracting the resistance of chemical reaction from that of the over-all reaction is Sh=0.01Re2.0Sc0.5 (Sh=kGd⁄D, Re=duρ⁄μ, Sc=μ⁄ρD, d=equivalent diameter, u=gas velocity in the nozzle, ρ=density of gas, μ=viscosity of gas, and D=diffusion coefficient of the CO2-CO system).(4) The rate of deoxidation under the experimental conditions can be estimated from the over-all rate equation and empirical equations for k′ and kG.(5) It was pointed out that the experiments under the conditions of lower oxygen concentration and higher temperature than those of the present study are necessary to discuss the mechanism of chemical reaction.

Permeability of the n-alkyl p-aminobenzoate esters across the isolated corneal membrane of the rabbit

The permeabilities of the rabbit corneal membrane to an homologous series of compounds (the n-alkyl p-aminobenzoate esters) have been determined by an in vitro technique, and a parabolic relationship between corneal membrane permeability and structure (n-alkyl chain length) is reported. This observation is interpreted as resulting from a change-over in the rate-limiting mass transfer step in the transport process, and it is described that the corneal membrane must be treated as a complex membrane, rather than a homogeneous membrane, to resolve the observed permeability behavior.

The effect of mass transfer on apparent ascorbic acid autoxidation kinetics

AbstractPrevious studies of copper‐ion catalyzed ascorbic acid autoxidation have indicated zero‐order, first‐order, or Michaelis‐Menten dependence of rate on ascorbic acid or copper concentrations and first‐ or half‐order dependence on oxygen concentration. A simple mathematical model of gas‐liquid mass transfer with liquid phase reaction accounts for the various behaviors. Use of an experimental technique that eliminates the mass transfer step shows that the oxidation rate dependence is first order in copper, half order in oxygen, and Michaelis‐Menten in ascorbic acid concentrations.

Kinetics of hydrogenation of rapeseed oil: I. Influence of transport steps in kinetic study

AbstractThe influence of mass transfer steps in the kinetic study of rapeseed oil hydrogenation in a laboratory reactor was estimated. Hydrogenations were carried out at 140–200 C and at 0.3–10 atm hydrogen pressure in the presence of 0.087% of commercial nickel‐on‐kieselguhr catalyst, corresponding to 0.05% of nickel. Despite intense mixing conditions on the macroscale of the bulk oil, corresponding to a reaction rate independent of further increase of the stirrer rate, the concentration differences across the liquid film surrounding the bubbles and the catalyst particles could not be neglected. The pore transport of triglycerides and hydrogen molecules was found not to be a slow step in most hydrogenations.

Thermal decomposition of limestone in a fluidized bed

Particles of limestone of 16 to 28 and 60 to 100 mesh sizes were decomposed in a fluidized bed. A mathematical model for the thermal decomposition was proposed comprising the thermal decomposition at the interface within particles and the related heat and mass transfer steps. It was assumed in this model that the particles are completely mixed within the fluidized bed and that gas is in upward plug flow. Fractional decomposition of limestone particles and the bed temperature during thermal decomposition calculated from this model coincide very well with the experimental results. It was further revealed that the overall reaction rate of 60 to 100 mesh size particles is virtually determined by the rate of heat transfer from the reactor wall to the fluidized bed, and that both rates of interfacial reaction and heat transfer from the wall to the bed contribute to the overall decomposition rate of 16 to 28 mesh size particles.