Mass Transfer Constants Research Articles

Bioadsorption and ion exchange of Cr3+ and Pb2+ solutions with algae

Heavy metals can be removed from effluents and recovered using physico-chemical mechanisms as biosorption processes. In this work “Arribada” seaweed biomass was employed to assess its biosorptive capacity for the chromium (Cr3+) and lead (Pb2+) cations that usually are present in waste waters of plating industries. Equilibrium and kinetic experiments were conducted in a mixed reactor on a batch basis. Biosorption equilibrium and fluid-solid mass transfer constants data were analyzed through the concept of ion exchange sorption isotherm. The respective equilibrium exchange constants (K eqCr=173.42, K eqPb=58.86) and volumetric mass transfer coefficients ((k mCr a)′=1.13×10−3 s−1, (k mPb a)′=0.89×10−3 s−1) were employed for the dynamic analysis of Cr and Pb sorption in a fixed-bed flow-through sorption column. The breakthrough curves obtained for both metals were compared with the predicted values by the heterogeneous model (K eqCr=171.29, K eqPb=60.14; k mCr a=7.81×10−2 s−1, k mPb a=2.43×10−2 s−1), taking into account the mass transfer process. The results suggest that these algae may be employed in a metal removal/recovery process at low cost.

Efficiency Analysis of Greenhouse Gas Sequestration during Miscible CO2 Injection in Fractured Oil Reservoirs

During CO2 injection into naturally fractured oil reservoirs for enhanced oil recovery, the great portion of oil is recovered by matrix-fracture interaction. Diffusive mass transfer between matrix and fracture controls this process if CO2 is miscible with matrix oil. Oil expelled from matrix is replaced by CO2, and the matrix could be potentially a good storage medium for the long-term. For the cooptimization of the oil recovery and CO2 storage, i.e., maximizing the oil recovery while maximizing the amount of CO2 stored, we propose an efficiency analysis using a dimensionless term defined as the global effectiveness factor. The Biot number and Thiele modulus were incorporated in the development of the global effectiveness factor. Diffusion coefficients and the rate of mass-transfer constants were obtained from our previous finite element modeling study. We first defined and derived the dimensionless groups to be used in the efficiency analysis and then formulated a relationship between the dimensionless groups and the efficiency indicators, i.e., the ratios of total solute (oil) produced to total solvent injected and total solvent stored to total solvent injected. It was shown that the efficiency of the process can be represented by a dimensionless group that consists of well-known dimensionless numbers such as the Reynolds number, the Peclet number, the Sherwood number, and the global effectiveness factor.

Mass Transfer and Adsorption Kinetics of Phenolic Compounds onto Activated Carbon Prepared from Rice Husk

Activated carbon from an agriculture by-product (rice husk) was prepared by carbonization at 500°C (50°C/15 min) followed by activation at 850°C for 1 h. The adsorption of phenolic compounds onto this activated carbon was studied using batch adsorber methods. The experimental results showed that the prepared activated carbon removed phenolic compounds effectively from aqueous solution.Analysis of the contact time data gave an indication of the mechanism. The external mass-transfer constant, kf, involved in the adsorption process was determined using different initial adsorbate concentrations (C0) and adsorbent masses (M). The following relationships were obtained: kf = 0.0023(M)-0.55and kf = 0.0006(C0)0.17. A macropore rate parameter for intraparticle diffusion, kp, was defined and used to correlate the experimental data via the following expression: kp = 0.183(C0)0.48.A twin-resistance mass-transfer model based on external mass transfer and pore diffusion was applied successfully to the system under inve...

Carbon Dioxide Adsorption in Brazilian Coals

Carbon dioxide (CO2) is one of the most important greenhouse gases. In the period between 1980 and 1998, CO2 emissions increased more than 21% and projections suggest that the emissions will continue to increase globally by 2.2% between 2000 and 2020 and 3.3% in the developed countries. The sequestration of CO2 in deep unminable coal beds is one of the more promising of several methods of geological sequestration that are currently being investigated. CO2 can adsorb onto coal, and there are several studies demonstrating that CO2 dissolves in coals and swells them. At very low pressures (P < 1 bar), CO2 dissolution does not seem to be a problem; however, high pressures are necessary for CO2 sequestration (P > 50 bar). In this study, we evaluated the kinetics and equilibrium of sorption of CO2 on Brazilian coals at low pressures. The adsorption equilibrium isotherm at room temperature (30 °C) was measured through the static method. The results showed that the Freundlich model or the Langmuir model is suitable to describe the equilibrium experimental results. The CO2 adsorption capacity of Brazilian coals are in the range of 0.089−0.186 mmol CO2/g, which are typical values for coals with high ash content. The dynamics of adsorption in a fixed-bed column that contains granular coal (particle sizes of 0.8, 2.4, and 4.8 mm) showed that the adsorption rate is fast and a mathematical model was developed to describe the CO2 dynamics of the adsorption in a fixed-bed column. The linear driving force (LDF) was used to describe the rate of adsorption and the mass-transfer constants of the LDF model (Ks) are in the range of 1.0−2.0 min-1.

Hydrodynamics and mass transfer in a packed column: Case of toluene absorption with a viscous absorbent

Volatile organic compounds (VOCs) cause nuisance to humans and the environment. Recent legislation encourages industrialists to set up equipment for treating their VOC-loaded gaseous effluents. This piece of research studies the absorption process, using a viscous organic absorbent ( di(2-ethylhexyl) adipate = DEHA ) to treat a toluene-loaded vent gas, in terms of hydrodynamics and mass transfer. It is shown that DEHA does not lead to an excessive pressure drop. Correlations predicting hydrodynamic parameters from previous literature are summarised and tested against experimental results. It is shown that acceptable prediction accuracy can be achieved for counter-current pressure drop and liquid hold-up. Treatment efficiency for the toluene-loaded vent gas is shown to be very good. Calculation of mass transfer constants ( k L a ) enables to test literature correlations against the experimental results. The mass transfer is supposed to be limited by the liquid-side resistance. Our experimental results showed that the k L a of the system depends on the liquid velocity but also on the gas velocity. This behaviour has also been observed by the few authors who have used viscous fluids in their experiments, but is contrary to all the authors who have work on low-viscosity fluids. It is therefore clear that the influence of viscosity on the phenomenon is considerable. Not one current correlation is currently accurate in the case of a viscous absorbent.

Effect of Temperature on Chiral Separation of Ketamine Enantiomers by High‐Performance Liquid Chromatography

A semipreparative column packed with microcrystalline cellulose triacetate (MCTA) was used to separate ketamine enantiomers by high‐performance liquid chromatography (HPLC). The effect of temperature on the kinetics of mass transfer and equilibrium constants were evaluated by the moment analysis at 303, 313, and 323 K. Total porosity, bed porosity, and equilibrium constants were measured by the first moment. Pore diffusion and axial dispersion coefficients were evaluated by the second moment. It was observed that band broadening decreased with increasing temperature. The equilibrium constants were found to be greater than unity and decreasing with increasing temperature. The pore diffusion coefficients increased with increasing temperature and the main contribution to the band broadening was attributed to low mass transfer kinetics. The pore diffusion coefficients controlled the mass‐transfer process in MCTA column. These results may be used to the determination of operating conditions and computational simulation of a chromatographic separation in simulated moving bed unity.

Growth of boric acid in aqueous solution by using the rotating disc technique

The effects of hydrodynamic conditions and surface quality on the growth rate of compressed polycrystalline disc of boric acid have been measured in a rotating disc crystallizer under well-established conditions of supersaturation. A new mass transfer equation based on an assumption that crystal growth depends mainly on two consecutive steps, diffusion and surface reaction, was proposed to describe the growth rate of crystals in rotating disc system. It was found that the mass transfer constant, K, and surface reaction constant, k r , increased while overall growth rate order, g, and reaction rate order, r, decreased with increasing the solid-solution velocity and surface smoothness of disc. For both conditions, it was determined that the integration step mainly controls the growth rate of boric acid. The kinetic parameters ( K, k r and r) were calculated for different conditions by trial and error method, under no assumption.

Electropolymerized flavin adenine dinucleotide as an advanced NADH transducer.

Electropolymerizing the prosthetic group (flavin adenine dinucleotide, FAD) responsible in the active sites of dehydrogenases for NAD(+)|NADH regeneration, we succeeded in mimicking enzyme activity. Poly(FAD) characterized by an additional polymer-type redox reaction has been discovered as a highly effective electrocatalyst for NADH oxidation: operating at the lowest potentials reported for NADH transducers (0.00 V, pH 7.4), poly(FAD) is characterized by the electrochemical rate constant of 1.8 +/- 0.6 x 10(-3) cm s(-1), which is at the level of the NADH mass-transfer constant. Flow injection analysis of NADH with the poly(FAD)-modified wall-jet electrode as a detector has been characterized by a linear calibration range prolonged down to 5 x 10(-7) M and a sensitivity of 0.08 A M(-1) cm(-2), which taking into account the dispersion coefficient ( approximately 3), is at the diffusion-limiting value. In contrast to the low molecular weight mediators able to exhibit similar electrocatalytic properties, poly(FAD)-modified electrodes are characterized by the dramatically improved stability and, thus, can be considered as the most advantageous NADH transducers for analytical chemistry.

Sorption and degradation of steroid hormones in soils during transport: column studies and model evaluation.

Natural and synthetic analogues of steroid hormones and their metabolites have emerged as contaminants of concern. Characterizing sorption and degradation processes is essential to assess the environmental distribution, persistence, and ecological significance of steroid hormones in terrestrial and aquatic systems. We examined the fate and transport of testosterone and 17beta-estradiol by conducting a series of fast-flow-velocity transport experiments under pulse-type and flow-interruption boundary conditions in columns packed with a surface soil, freshwater sediment, and two sands. Flow-interruption experiments provided independent estimates of degradation coefficients for the parent hormones and their metabolites, while pulse-input type experiments were used to identify transport mechanisms for hormones by employing forward modeling approaches. Estimated degradation rate coefficients (k) for the hormones from flow-interruption experiments ranged from 0.003 to 0.015 h(-1) for testosterone and from 0.0003 to 0.075 h(-1) for estradiol, similar to those observed in batch studies. Degradation rate coefficients for the two primary metabolites were 1-2 orders of magnitude larger than those for the parent chemicals. Estimated k values decreased with column life as a result of nutrient depletion. Large sorption by soils of the parent and metabolites (log Koc approximately 2.77-3.69) did not appear to hinder degradation; k values were an order of magnitude smaller than the estimated sorption mass-transfer constants. Differences in hormone breakthrough curves from a single-pulse displacement and those predicted using independently estimated parameters suggest that modeling hormone degradation as a simple first-order kinetic process may be sufficient, but not accurate.

Kinetics of goldcyanide extraction using ion-exchange resins containing piperazine functionality

The search for suitable resins for goldcyanide recovery from cyanide solutions prompted the synthesis of resins incorporating new functionalities that solve many of the existing problems. A new type of ion-exchange resin (PS-PIP) incorporating a piperazine group, linked onto a styrene–divinylbenzene macroporous network, was developed for goldcyanide solutions. Kinetics measurements were made on the extraction of goldcyanide (Au(CN) 2 −) with the ion-exchange resin (PS-PIP). The extraction process is accompanied by fast ion-exchange reaction and is characterised by a sharp moving boundary between the reacted shell and the shrinking unreacted core within the resin. Analyses of the respective rate data in accordance with two theoretical models used to explain the metal extraction kinetics showed that the process is controlled by the rate of diffusion of the ions penetrating the reacted layer at goldcyanide concentrations in the range 10–40 mg l −1 and controlled by the rate of diffusion of the ions across the liquid film surrounding the resin particle at goldcyanide concentrations lower than <2 mg l −1. Particle diffusion coefficients and mass transfer constants across the liquid film were determined from the graphical representation of the proposed models.

Heterogeneous Oxidation of Ferrous Iron for Treatment of Mine Drainage

Heterogeneous oxidation of ferrous iron in AMD was studied at bench-top in the laboratory, at two passive abiotic treatment systems for AMD, in an aerated ditch to which ferric oxide sludge had been added, and in batch and recirculated sludge systems that were designed for abiotic heterogeneous oxidation of AMD at pH between 5 and 6.8. Heterogeneous oxidation accounted for between 60 and 99% of total oxidation of Fe(II) for the conditions that were tested. In bench-scale lab tests, the heterogeneous rate constant was k2 = 3.1x10 -8 (mg/L) -1 sec -1 for 19.3 to 23.5°C using synthetic mine drainage solutions and Eact was 180 kJ/mol for k2 (Ames 1998). In field tests at passive treatment facilities, average k2 was 2.2x10 -8 (mg/L) -1 sec -1 for 11.7 to 19.2°C (Roscoe 1999). In large- scale tests with recirculated ferric oxide sludge, ferrous iron removal rates were above 1000 g/m 2 /d for pH as low as 5.4 and k2 was 1.7 x 10 -8 (mg/L) -1 s -1 at 15°C (Dietz & Dempsey, 2001). The k2 values were consistent with values previously reported by Tamura & Nagayama (1976) and by Sung and Morgan (1980), but both of those studies were conducted at higher pH and for low ferric oxide and sulfate concentrations. Sung & Morgan concluded that heterogeneous oxidation became more important as the pH rose above 7, but this was due to their dependence on fresh precipitation of a relatively low concentration of ferric oxide. Our work showed that heterogeneous oxidation is particularly useful for AMD treatment at pH 5 to 6.8. Heterogeneous oxidation at moderately acidic pH also resulted in relatively dense sludge. These kinetic parameters and mass transfer constants for O2 and for CO2 provide a basis for design of treatment processes that can operate at high areal capacity (greater than 1000 g/m 2 /d) for net alkaline AMD waters while producing sludge that exceeds 15% (w/w) after settling.

Uncertainty in air stripping tower design: implications of the air-to-water ratio

Environmental engineers are frequently faced with uncertainty in making design decisions because the true value of many process parameters is unknown. In this study, the design of countercurrent air stripping towers was modeled using fuzzy numbers, taking into account uncertainties in mass transfer and Henry's constant. It was found that, in addition to cost, the risk of failure is an important design consideration for stripping tower design. A significant over-design is both cost-effective and results in less risk of design failure. The air-to-water ratio that yielded the least risk of failure switched from low to high as the removal efficiency of the tower increased. An important result is that at lower removal efficiencies, tower design and operation is most sensitive to uncertainties in mass transfer and at higher removal efficiencies, tower design and operation is most sensitive to uncertainties in Henry's constant . The implication is that low air-to-water ratios are best when the regulatory target removal efficiency is low and/or when the uncertainty in the value of the contaminant's Henry's constant is larger than the uncertainty in the mass transfer coefficient value. Otherwise a high air-to-water ratio results in the least risk of process failure.

Determination of Kinetic and Energetic Parameters of Chemical Reactions in a Heterogeneous Liquid/Liquid System

A global method is proposed for estimating the kinetic and energetic parameters of chemical reactions taking place in the continuous phase of a heterogeneous liquid/liquid system. This method, based on energy and mass balances, uses the thermal power freed or absorbed by the reaction mass. It is only applicable to systems with sufficient thermal effects. Knowledge of the mass flow exchanged between the dispersed and continuous phases is needed to establish the mass balance. It is determined thanks to a model based on the double film theory. The model was applied to the saponification of pure ethyl acetate in an aqueous reaction mass. It was possible to determine simultaneously the velocity constant and the reaction enthalpy, as well as the apparent overall mass transfer constant.

Kinetics studies on heavy metal ions extraction by Amberlite XAD2 impregnated resins containing a bifunctional organophosphorous extractant

Kinetics measurements on the extraction of divalent metal ions (Zn, Cu and Cd) with impregnated resins, prepared by adsorption of a bifunctional extractant, O-methyl-dihexyl-phosphine-oxide O′-hexyl-2-ethyl phosphoric acid (HL) into polymeric macroporous supports of Amberlite XAD2 (XAD2-HL), were made. The extraction process is accompanied by fast chemical reaction and is characterized by a sharp moving boundary between the reacted shell and the shrinking unreacted core within the impregnated resin. Analyses of the respective rate data in accordance with two theoretical models used to explain the metal extraction kinetics showed that process is controlled by the rate of diffusion of the ions penetrating the reacted layer at high metal ion concentration (1×10 −2 mol l −1) and controlled by the rate of diffusion of the ions across the liquid film surrounding the resin particle at low metal ion concentration (1×10 −4 mol l −1). Particle diffusion coefficients and mass transfer constants across the liquid film were determined from the graphical representation of the proposed models.

KINETIC STUDIES ON HEAVY METAL IONS REMOVAL BY IMPREGNATED RESINS CONTAINING DI-(2,4,4-TRYMETHYLPENTYL) PHOSPHINIC ACID

ABSTRACT Kinetics measurements on the extraction of divalent metal ions (Zn, Cu and Cd) with impregnated resins prepared by adsorption of di(2,4,4-tri-methylpentyl)phosphinic acid (DTMPPA=HL) onto a macroporous polymeric support of Amberlite XAD2 were made. The extraction process is accompanied by fast chemical reaction and is characterized by a sharp moving boundary between the reacted shell and the shrinking unreacted core within the impregnated resin. Analyses of the respective rate data in accordance with two theoretical models used to explain the metal extraction kinetics showed that process is controlled by the rate of difilision of the ions penetrating the reacted layer at high metal ion concentration (1×10−2 mol.L−1) and controlled by the rate of diffusion of the ions across the liquid film surrounding the resin particle at low metal ion concentration (1×10−4 mol.L−1). Particle diffusion coefficients and mass transfer constants across the liquid film were determined from the graphical representation of the proposed models.

Mechanism and Kinetics for the Dissolution of Apatitic Materials in Acid Solutions

Abstract - This work concerns the study of the digestion step in the production process of phosphoric acid. Some qualitative experiments indicate that the difference between the pH at the surface of the phosphate and that in the bulk of the solution is negligible and that the dissolution is controlled by diffusion of products away from the phosphate particle. In further experiments, to isolate the dissolution phenomenon from the formation of calcium sulfate, the sulfuric acid normally used industrially is replaced by hydrochloric acid. The phosphate material used in our experiments is a model apatitic material: synthetic hydroxyapatite (HAP). The dissolution of calcium hydroxyapatite was studied with increasing amounts of calcium and phosphate at different temperatures. A simple method was developed for this observation based on the time required for complete dissolution of the HAP powder. The results confirm that the dissolution is controlled by a diffusional process through an interface of calcium and phosphate ions released from the solid surface. A kinetic model for the dissolution of apatitic materials is proposed which assumes a shrinking particle behaviour controlled by diffusion of calcium ions. The experimental results are fitted to this model to determine the mass transfer constant for HAP dissolution in acid solutions. The activation energy of the reaction is about 14kJ/mol. This study was carried on in conditions similar to the industrial ones for the production of phosphoric acid by the dihydrate-process

Critical micelle concentrations and stirring are rate limiting in the loss of lipid mass during membrane degradation by phospholipase A2

In phospholipid membranes attacked by phospholipase A(2) (PLA(2)), accumulation of degradation products influences the binding affinity as well as the catalytic activity of PLA(2). Such accumulation in its turn depends on the rate of membrane degradation and the efflux of degradation products from the membrane, the latter being influenced by the stirring conditions in the system. This complicated process was investigated with a new ellipsometric technique for in situ measurement of membrane mass in a well-defined flow system. Planar phospholipid bilayers were formed on rotating silicon discs in buffer solution. After the addition of 0.05-100 ng/ml of PLA(2) (from Naja mocambique mocambique) to the buffer, mass desorption could be measured with a precision of 3-5 ng/cm(2), that is, about 1% of the surface mass of a single bilayer. Using radiolabeled phospholipids and thin-layer chromatography, it was verified that only the degradation products desorb from the membrane, which was confirmed by the desorption of mixtures of phospholipids, lysophospholipids, and fatty acids. The rotating disc allows the exact calculation of the mass transfer constant for transport-limited exchange of lipid between fluid and disc surface, as a function of rotation rate. By using the mass transfer constant, the critical micelle concentrations, and the mole fractions of products, desorption kinetics could be fully described. The amount of degraded phospholipid could be continuously monitored as the sum of the product mass still present in the membrane, as inferred from the desorption rate, and the mass already lost from the surface. It is concluded that ellipsometry is a suitable tool for studying the effects of PLA(2) on membranes.

Rate‐limited sorption and nonequilibrium transport of organic chemicals in low organic carbon aquifer materials

The rate‐limited sorption and nonequilibrium transport of several hydrophobic organic chemicals in three low‐organic carbon (&lt;0.025%) aquifer materials was investigated. Results of miscible displacement experiments performed at two pore water velocities and with very low solution‐phase concentrations (30–60 μg L−1) were analyzed using a first‐order mass transfer nonequilibrium model, as well as a model employing the local equilibrium assumption. Results of the analyses revealed sorption to be significantly rate limited, possibly by a diffusion‐limited mechanism. The impact of rate‐limited sorption on transport was dependent upon pore water velocity. The experiments performed at a faster velocity (∼1 cm/h) could be successfully simulated only with the nonequilibrium model, whereas the equilibrium model was adequate for the slower‐velocity (∼0.2 cm/h) experiments. Comparison of experimental results to those reported in the literature revealed that time scale has a significant impact on the degree of nonequilibrium observed in, and on the values of rate constants determined from, experiments. Regression equations were developed for the two kinetics‐associated parameters contained in the nonequilibrium model: first‐order mass transfer constant and fraction of instantaneous sorption.

Mass transfer in the system cellulose nitrate-ethyl acetate

A qualitative model of mass transfer was formulated, explaining the appearance of anomalies on kinetic curves of sorption in complex microheterogeneous nitrocellulose (NC)-solvent systems. The model is based on the assumption that these systems are of fractal nature. Consequently it is possible to apply the Kohlrausch equation for describing the kinetic curves of desorption. Traditional criteria for assessing the mechanism of mass transfer in NC-solvent systems, as well as the determination of effective mass transfer constants in the glassy and highly elastic states of these systems by the classical sorption method are shown to be inadequate.

A Diffusive Sampler for Gaseous Chlorine Utilizing an Aqueous Sulfamic Acid Collection Medium and Specific Ion Electrode Analysis

Abstract A diffusive sampler for chlorine gas which uses microporous membranes and a liquid collection medium was developed. The device is constructed from a 37-mm, polystyrene aerosol filter cassette and uses two Teflon® filters in series, 11 mm apart, as diffusion membranes. The inner filter is in contact with approximately 7 ml of aqueous 0.1 percent sulfamic acid solution. The outer filter acts as a wind screen. Collected chlorine is determined with a residual chlorine-specific ion electrode. The mass transfer constant of the sampler for chlorine is 9.38 μg/ppm-hour at 25°C, with the mass transfer rate being linear up to at least 20 ppm-hour of exposure dose. The lower limit of detection for the monitor is estimated to be 0.1 ppm-hours, less than 3 percent of the current Occupational Safety and Health Administration Permissible Exposure Limit of 0.5 ppm for 8 hours of exposure. Sampler response becomes essentially constant at wind speeds above 12.7 cm/s. The response is independent of humidity while the collection rate decreases with temperature at a rate of 0.05 μg/ppm-hour per degree Celsius. The simplicity and performance of the device make it particularly suited to large-scale surveys of exposure to chlorine in the workplace.