Theory of Isotope Separation by Electrochemical Parametric Pumping

Abstract

Electrochemical parametric pumping is a novel separation process in which cycles of reversible electrochemical processes on high surface area electrodes are conducted in synchronization with cycles of solution flow through the separating column. In the present work, isotope separation by electrochemical parametric pumping is studied theoretically. The proposed model is based on the similarity between the parametric pumping and the countercurrent processes and on the division of the separating column to “cells” when dispersion processes are neglected. Steady-state isotopic concentration gradients are calculated and process optimization is performed with respect to system parameters such as the fraction of the isotopes bound to the electrode, the solution displacement volume, and the fraction of the isotopes withdrawn as product. The model may also be applicable to other types of parametric pumping.     

Effect of Feed Concentration in Equilibrium Parametric Pumps

Abstract

The effect of high initial feed concentration in batch equilibrium parametric pumping was experimentally investigated at different bed temperatures and cycle times. The system studied was benzene-n-hexane over a silica gel bed. It was observed that the initial feed concentration which leads to the best separation is dictated by the shape of the equilibrium isotherms of the system. An increase in the temperature difference between hot and cold cycles was shown to improve the separation because it led to a more favorable equilibrium relationship. Experiments also indicated that a long enough cycle time must be selected for true equilibrium to be established within the system, otherwise maximum separation cannot be obtained. In mathematical modeling studies the effect of nonlineariry of equilibrium isotherms at high feed concentrations was shown to be very effective for predictions of the model.     

Desorption of cadmium from algal biosorbent

Desorption of metal-laden new biosorbent material was studied using batch and column equilibrium elution processes. Equilibrium screening tests of cadmium desorption established a solution of HC1 as the most appropriate eluant at approximately pH 1.0. The desorption of Cd by protons was indicated to be a reversible exchange with a stoichiometric coefficient of 1.24. The solid to liquid ratio (biosorbent mass to elutant volume) is described as a key parameter in determination of elution efficiency, affecting simultaneously the pH at desorption equilibrium, the concentration of cadmium released, and the concentration ratio of the overall metal recovery process. When the pH is maintained constant, the solid to liquid ratio has little influence on metal recovery but still controls the concentration ratio. Recycling a small amount of eluant through a desorption column with metal-laden biosorbent material resulted in very high solid-to-liquid ratios (up to 130 g/L) leading to high value of 70 for the metal concentration ratio of the sorption/desorption process. No loss of cadmium biosorbent properties was observed in three consecutive metal uptake/desorption cycles.     

Dual-Temperature Ion-Exchange Separation of Copper and Zinc by Different Techniques

Abstract

Separation of copper from zinc-containing acidic solutions has been advanced by different techniques of dual-temperature ionexchange fractionation on iminodiacetic resin Amberlite IRC 718. Cycling-zone adsorption and parametric pumping (using single-column as well as double-column set-ups) experiments have been carried out in fixed-bed ion-exchange column. Measurement of equilibrium characteristics of the resin-solution system has shown that separation results from the variation of the ions sorbability with temperature. It has been found that temperature variation leads to reverse of selectivity between copper and hydrogen ions. Cycling-zone adsorption technique has allowed the copper content to be decreased up to 4 times in the resin phase and increased up to 1.7 times in the effluent when carrying out the process at 80 and 20 °C, respectively. The copper concentration has been decreased 4.4 fold after 9 treatment cycles of solution aliquot (15 BV) by single-column mode of the parametric pumping method. Two steps of the fractionation process (loading and elution) have been performed by continuous mode of operation in two counter-current columns. Continuous separation is preferred over fixed-bed column techniques. The steady-state sorption fronts have been formed on both stages of the process due to the reverse of selectivity mentioned above.     

Removal of remazol yellow from aqueous solutions by unmodified and stabilized iron modified clay

The adsorption behavior of remazol yellow from aqueous solutions onto montmorillonite KSF and iron modified montmorillonite KSF samples was investigated as a function of initial concentration of remazol yellow, contact time and pH. The regeneration of the dye saturated material was also considered.The sorption showed that the montmorillonite KSF removed about 10% more dye than the iron modified montmorillonite KSF but the standard deviations of the experiments were very high for the unmodified material. The kinetic experiments showed that the equilibrium time was reached in less than an hour for both materials and the experimental data were adjusted to the pseudo first order model (Lagergren), which is based on a surface reaction.The experimental data could be adjusted to the Langmuir, Freundlich and Langmuir–Freundlich models, but according to the characteristics of the adsorbents only the two last models should be considered because the adsorbents are heterogeneous. It was also observed that the sorption capacities for the iron modified montmorillonite were not affected by pH from 2 to 12.The results on the desorption processes showed that the best regenerator agent was Fenton's reagent, the sorption capacities were a little higher for iron modified montmorillonite KSF than for montmorillonite KSF in the first 7 sorption–desorption cycles and then the sorption of the dye increases almost 3 times in the 8–10 sorption–desorption cycles.     

Parametric studies on the performance of cation exchange for the ammonium removal

Ion exchange performance to remove ammonium in drinking water was studied experimentally in batch and continuous operation systems under the various conditions. Data were collected using commercially available strongacid cationexchange resins of Na+ and H+ types. The performance was evaluated using equilibrium concentrations for the batch system or the effluent concentration histories for the continuous column system as a function of time or the solution volume passed through the experimental column until resins were exhausted. With high temperature or low initial feed concentration, ammonium removal characteristics of the batch system increase. At the solution concentrations of 0.5, 1.0, and 2.0 mg/L of NH4+-N and the temperatures of 15, 25, and 35 ‡C, the selectivity coefficients of resin were determined between 1.38 and 1.43 for Na+ type resin, and 3.22 and 3.47 for H+ type resin. The selectivity coefficient was correlated as a function of temperature using Kraus-Raridon equation. The breakthrough curves obtained from the continuous column operation give some results; i) with small column diameter or large column height, ii) with low initial feed concentration, iii) with low volumetric flow rate, or iv) with high solution temperature, the ammonium removal for the typical macroporous type resin increase. The results of this study could be scaled up and used as a design tool for the waterpurification systems of the drinking water treatment processes. This paper was presented at The 5th International Symposium on Separation Technology-Korea and Japan held at Seoul between August 19 and 21, 1999.     

Determination of adsorption operating conditions in dynamic mode on basis of batch study: Application for Dimethylphthalate elimination on activated carbon prepared from Arundo donax

This study aims to determine the operating conditions to implement the Dimethylphthalate removal using an activated carbon prepared from Arundo donax, carbonized at 358?°C during 13?min. To achieve this objective, the study is conducted in batch and dynamic mode. Several kinetic models are applied, namely pseudo-first order, pseudo-second order, intragranular, and Bangham models. The pseudo-second-order model fits the data perfectly, the estimated regression coefficients >0.999. The intragranular diffusion takes place in two stages. The two-parameter (Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich) and three-parameter models (Redlich-Peterson, Sips, and Toth) are applied to model the equilibrium isotherms. The nonlinear regression methodology based on the error functions (hybrid fractional error function, Marquardt’s percent standard deviation, average relative error, Sum of the absolute errors) is applied. The HYBRID fits properly the data showing that the Temkin model gives the best fitting (R²_adj = 0.992), and the Dubinin-Radushkevich model the worst (R²_adj = 0.793). The thermodynamic study shows that the adsorption occurs according to a physical process. The DMP desorption is more effective with a 5% NaOH solution. In dynamic mode, the runs are conducted in fixed bed column. The effect of the bed height, the DMP initial concentration, and the flow rate on the breakthrough curves is investigated, then these breakthrough curves are modeled using the Thomas and Bed Depth Service Time models. The regeneration of the exhausted Arundo donax activated carbon is performed in a column, after 5 cycles, the breakthrough time decreases from 65.3?to 26.8?h, however, the exhaustion time varies less rapidly.     

Pre-sodiation strategy for superior sodium storage batteries

The irreversible consumption of sodium in the initial several cycles greatly led to the attenuation of capacity, which caused the low initial coulombic efficiency (ICE) and obvious poor cycle stability. Pre-sodiation can effectively improve the electrochemical performance by compensating the capacity loss in the initial cycle. Here, carbon-coated sodium-pretreated iron disulfide (NaFeS₂@C) has been synthesized through conventional chemical method and used in sodium metal battery as a cathode material. The calculated density of states (DOS) of NaFeS₂@C is higher, which implies enhanced electron mobility and improved cycle reversibility. Because of the highly reversible conversion reaction and the compensation of irreversible capacity loss during the initial cycle, the Na/NaFeS₂@C battery achieves ultra-high initial coulombic efficiency (96.7%) and remarkable capacity (751 mA·h·g^-1 at 0.1 A·g^-1). In addition, highly reversible electrochemical reactions and ultra-thin NaF-rich solid electrolyte interphase (SEI) also benefit for the electrochemical performance, even at high current density of 100 A·g^-1, it still exhibits a reversible capacity of 136 mA·h·g^-1, and 343 mA·h·g^-1 after 2500 cycles at 5.0 A·g^-1. This work aims to bring up new insights to improve the ICE and stability of sodium metal batteries.     

The atmospheric corrosion of zinc: The effects of salt concentration, droplet size and droplet shape

The influence of droplet characteristics on the atmospheric corrosion of zinc was investigated using a multi-microelectrode approach. Arrays of close-packed zinc wires, 500 μm in diameter, were coupled such that the net anodic and cathodic current flowing through each electrode interface was measured as a function of time. Droplets in the range of 1–10 μL placed onto the array showed that electrochemical fields are immediately established and can remain unaltered for significant periods of time. The total charge passed was found to be almost identical for 0.6 M NaCl droplets of varying size when compensating for the electrolyte–zinc contact area, however significant differences in current exchange were observed when varying electrolyte concentration.In contrast to studies on other metals, the highest rates of zinc corrosion, as measured by the microelectrode array, did not occur during drying cycles. It was observed that the initial wetting and subsequent holding at high relative humidity led to the highest measured currents. Corrosion damage during drying cycles, however, appears to be enhanced for large droplets with high contact angle and increased electrolyte concentration. The importance of these observations to the modeling of atmospheric corrosion has been discussed and future developments of the technique are outlined.     

On the Origins of Flow-Rate Dependence of Elution Volume in Gel Permeation Chromatography

Abstract

The peak elution volume of a narrow polystyrene fraction in a porous silica column is a function of the solvent flow rate through the column. The explanation for this effect offered by Gudzinowicz and Alden and others is that the equilibrium assumption, usually made in size exclusion Chromatography, is no longer valid at high flow rates. An alternative explanation based on a flow-rate-dependent equilibrium distribution coefficient (ratio of intrapore polymer concentration to extrapore polymer concentration) is offered here. This view is supported by experimental results for the equilibrium flow-rate-dependent column retention measured previously in these columns.     

Film-pore diffusion models—analytical and numerical solutions

The sorption of acid dyes from aqueous effluents onto activated carbon has been studied. The effects of initial dye concentration and activated carbon mass on the rate of Acid Blue 80 and Acid Yellow 117 removal have been investigated. Three mass transport models based on film and pore diffusion control have been applied to model the experimental concentration decay curves. The models are compared on the basis of the solid-phase loading capacity using various assumptions since the assignment of an appropriate solid-phase loading has been the subject of several papers on this topic and no comparisons have been provided on the effectiveness of each approach. The equilibrium solid-phase concentration is assumed: (i) incorporating a time-dependent solid-phase concentration Y_e,t, (ii) equal to the intersection point of the equilibrium isotherm and the operating line and (iii) the point on the equilibrium isotherm where the liquid-phase concentration equals the initial concentration in the film-pore diffusion model.     

Modeling of the evolution with length of bubble size distributions in bubble columns

Many of the existing methods, for the determination of the specific interfacial area in bubble columns, consider the column in a dynamic equilibrium between bubble coalescence and breaking-up. The aim of this work is to study if this consideration can be considered true for low superficial gas velocities. Two existing models have been chosen from literature in order to predict the break-up [Wang, T., Wang, J., Jin, Y., 2003. A novel theoretical breakup kernel function for bubbles/droplets in a turbulent flow. Chemical Engineering Science 58, 4629-4637] and the coalescence [Lehr, F., Millies, M., Mewes, D., 2002. Bubble size distributions and flow fields in bubble columns. A.I.Ch.E. Journal 48, 2426] rates. In order to confirm the validity of the models, predictions were compared with experimental results obtained by image analysis. Several simulations were performed for different superficial gas velocities and initial bubble size distributions. The column length needed to reach dynamic equilibrium was calculated for each simulation. The results show that the necessary length to reach the dynamic equilibrium does not depend on the shape of the initial distribution, but essentially on its Sauter mean diameter. The necessary length to reach the dynamic equilibrium is very important for low superficial gas velocities. The assumption that the entire column is in dynamic equilibrium is in general not valid. Therefore, the initial Sauter mean diameter and the total column length are important parameters for the determination of the specific interfacial area.     

Processing‐interphase‐property relationship in fiber‐reinforced thermosetting‐matrix composites

Fabrication of thermosetting‐matrix composites is based on a critical step of cure, which involves applying a predefined temperature cycle to a fiber‐resin mixture. Several temperature‐dependent mass transport processes occur in the vicinity of the reinforcement fiber, leading to the formation of an interphase region with different chemical and physical properties from the bulk resin. The cure cycles applied on the macroscopic boundaries of the composite govern the microscopic cure kinetics near the fiber surface, which in turn determines the interphase and composite properties. A predictive approach to directly linking the cure cycles and final composite properties is not presently available and is established for the first time in this paper. A multiscale thermochemical model is developed to predict the concentration profile evolution with time near fiber surfaces at various locations across the composite thickness. The concentration profiles at the gelation time are mapped to modulus profiles within the interphase region, and a finite element analysis is used to determine the overall composite modulus in terms of the constituent interphase, fiber, and matrix properties. Relevant numerical results are presented for the first time where the composite modulus is directly linked to the cure cycle and interphase formation parameters without assumed structures or properties of the interphase. The results provide useful information for selecting material components and cure cycles parameters to achieve desired interphase and composite properties. POLYM. COMPOS., 26:193–208, 2005. © 2005 Society of Plastics Engineers     

Analysis of reactive distillation using the esterification of acetic acid as an example

The uncatalyzed esterification of acetic acid is described in the literature as a typical example of reactive distillation. Many rigorous models were validated using this esterification as an example. Process proposals for the production of pure ethyl acetate from ethanol and acetic acid have been determined using short-cut methods with the assumption of chemical equilibrium only. In this publication, the limitations of this esterification are clarified, using a rigorous model that was developed. The reasons why reactive distillation appears to be unfavorable for this esterification are explained. It is, however, theoretically possible to obtain ethyl acetate in high purity with different variants of the process. Different process variants are examined in this work. Construction variables that are important for the design of reactive columns, such as the number of reactive separation stages and the holdup in the column, are analyzed. Furthermore, the influence of variables dependent on the component system, such as the phase equilibrium of the reactive system and the reaction kinetics on the conversion in the column, are described. It can be shown that the short-cut methods published so far for reactive distillation, which assume chemical equilibrium, are inadequate.     

Air stripping of hydrocarbon-contaminated soils: Investigation of mass transfer effects

Bench-scale experiments were carried out to simulate air stripping of soil contaminated with a semi-volatile hydrocarbon, n-octane. The experiments were conducted in an 8-cm diameter glass column, packed with two types of packings: glass beads to represent non-adsorbing materials and soil particles to represent adsorbing materials. Effects of gas superficial velocity, particle size and soil organic matter on the column outlet concentration and temperature profile were studied. Nitrogen adsorption/desorption experiments were performed to establish the desorption characteristics of the soil sample. Additionally, a mathematical model is presented which treats the interphase contaminant transport as a mass transfer rate-limited process. The predictions from the mathematical model are shown to be in good agreement with the experimental results. Most importantly, the numerical results show that, contrary to the common assumption of local equilibrium, the interphase contaminant transport (from the sorbed to the vapour phase and/or from the liquid to the vapour phase) is mass transfer controlled.     

CFD-PBM simulation of the column flotation unit of FCMC: Importance of gas–liquid interphase forces models

The cyclonic micro-bubble flotation column (FCMC) is an efficient flotation device for the separation of fine minerals, but its mechanisms are rarely studied using computational fluid dynamics (CFD). This paper reports the air–water two-phase computational fluid dynamics-population balance model (CFD-PBM) simulations for the column flotation unit of an FCMC. The shear stress transport (SST) k-ω model with curvature correction (CC) is used to simulate turbulence effects. Then, the interphase forces models considering bubble size distribution are selected according to the experimental data in a bubble column, which is in analogy to the column flotation unit of the FCMC. Finally, the optimal combination of interphase forces models (i.e., the Ishii–Zuber drag force model, the Hosokawa–Frank wall lubrication force model, and the Lopez de Bertodano turbulent dispersion force model) is applied to simulate an FCMC with a superficial gas velocity of 0.0144 m/s. The results show that the CFD-PBM simulation can achieve a relative error of 9.09% for gas volume fraction and −5.45% for bubble rising velocity, indicating the reliability of the selected combination of interphase forces models.     

Removal of Organic Impurities from Polymer Pellets

An investigation into the behavior of removing low molecular organics from polymer pellets in a countercurrent extraction column is presented. A mathematical model is developed, describing it in terms of intraparticle diffusion, adsorption, interphase equilibrium, convection and dispersion. The corresponding parameters are determined using nonlinear regression. The resulting partial differential equations are solved using a numerical method. The validity of the model is verified by means of experimental data.     

Parametric studies on the performance of anion exchange for nitrate removal

Ion exchange performance to remove nitrate in surface and underground water was studied experimentally in batch and continuous operation systems under various conditions. Data were collected by using commercially available strong-base anion-exchange resins of C1 and OH types. Equilibrium curves, obtained through the batch system and plotted as the concentration ratio versus run time, were used to evaluate the effects of temperature, resin type, and initial feed concentration on the equilibrium characteristics of nitrate. The selectivity coefficients of the resins were correlated as a function of temperature by using the Kraus-Raridon equation. Breakthrough curves, obtained through the continuous column system and plotted as the ratio of effluent to influent concentration versus solution volume passed through the experimental column, gave detailed results about the effects of the system parameters, such as temperature, resin type, feed concentration, volumetric flow rate, column diameter and height on the performance of the anion exchange to remove nitrate. The results of this study could be scaled up and used as a design tool for a water-purification system of real ground water and surface water treatment processes.     

Numerical Investigation of the Effect of Impeller Design Parameters on the Performance of a Multiphase Baffle‐Stirred Reactor

The turbulent gas‐liquid flow field in an industrial 100‐m³ stirred tank was calculated by using computational fluid dynamics based on the finite‐volume method. Turbulent effects were modeled with the shear stress transport model, and gas‐liquid bubbly flow was modeled with the Eulerian‐Eulerian approach using the Grace correlation for the drag force interphase momentum transfer. The relative motion between the rotating impeller and the stationary baffled tank was considered by using a multiple frames of reference algorithm. The effects of Rushton and pitched‐blade impeller design parameters such as blade geometry, location, and pumping direction on the mixing performance were investigated. It was found that a combination of Rushton turbines with up‐pumping pitched‐blade turbines provides the best mixing performance in terms of gas holdup and interfacial area density. The approach outlined in this work is useful for performance optimization of biotechnology reactors, as typically found in fermentation processes.     

Separation of butyric acid in fixed bed column with solvent impregnated resin containing ammonium ionic liquid

Batch equilibrium and fixed bed column extraction experiments for the extraction of butyric acid (BA) into solvent impregnated resin (SIR) have been done. Microporous Amberlite XAD-1180N was impregnated with an ammonium ionic liquid (IL) trialkylmethylammonium bis(2,4,4-trimethylpentyl)phosphinate. The BA extraction capacity isotherm has not a Langmuir type shape and no finite capacity was observed. The loading of the impregnated IL with the extracted BA at 37 °C is in agreement with the loading from L/L extraction equilibrium of BA at 25 °C. Capacity of freshly prepared SIR particles is superior to classical porous ion-exchangers. Both the temperature and the superficial velocity in column influence the shape of the breakthrough-curve in fixed bed extraction of BA using SIR. Sharpening of the breakthrough curve was observed with the increasing temperature and decreasing superficial velocity. Stripping with water is not efficient for the regeneration of the loaded column after extraction because of low concentration of the product acid in the obtained effluent. Higher BA concentration was achieved by stripping with 0.15 kmol m⁻³ and 0.075 kmol m⁻³ NaOH solutions. The combination of initial water stripping coupled with consecutive stripping by alkali can be beneficial for decreasing the consumption of chemicals and further processing of the product. After two extraction/stripping cycles, a stable capacity was achieved and sustained for 14 cycles, showing the possibility of long-term application of the prepared SIR in real technology.