Porous Mass Transfer Research Articles

Investigating the Effects of Operation Variables on All-Vanadium Redox Flow Batteries Through an Advanced Unit-Cell Model

Next-generation redox flow batteries will benefit from the progress of macroscopic continuum models that enable the optimization of new architectures without the need of expensive fabrication and experimentation. Despite previous attempts, there is still need for robust and thoroughly validated models. Here, a steady-state two-dimensional unit-cell model of an all-vanadium redox flow battery is presented. The model integrates state-of-the-art descriptions of the fundamental physical phenomena, along with new features such as local mass transfer coefficients for each active species, precise sulfuric acid dissociation kinetics, and experimental data of the electrochemical parameters and electrolyte properties. The model is validated at different states of charge and flow rates using polarization, conductivity and open circuit voltage measurements. Then, the contribution of operating conditions on battery performance is studied by analyzing its separate effect on the various phenomena that affect cell performance, such as local pore mass transfer limitations, parasitic hydrogen evolution reactions, crossover and self-discharge fluxes. The resulting model is a reliable tool that can be used to assess the relevance of these coupled phenomena that take place simultaneously within the reaction cell. This important information is critical to optimize cell components, reactor design and to select optimal operating conditions.

Study on Ion Transport Mechanism of Zinc-Nickel Single-Flow Battery with Different Porous Electrode Structures based on Lattice Boltzmann Method

Since the microstructure of porous electrode is very important to the performance of zinc-nickel single-flow battery, this paper reconstructed the microstructure of porous nickel oxide electrode by quartet structure generation set (QSGS) method. The flow mass transfer and electrochemical reaction in porous electrode were simulated by lattice Boltzmann method (LBM). The effects of different porous electrode structures (porosity, particle size and electrode thickness) on local ion concentration distribution and charging performance are studied from the perspective of seepage and mass transfer in pores. It is found that the ion concentration in the electrode presents an uneven distribution due to the randomness of the particle size and distribution of active substances. The uneven distribution of OH − concentration caused the difference of charging depth in the direction of electrode thickness, and the uneven distribution of H + concentration caused the difference of charging depth in the radial direction of particles. Under different pore structures, the decrease of porosity and particle size can increase the diffusion rates of OH − and H +, and then promote the electrochemical reaction rate, improve the charging speed of the battery, and improve the performance of the battery. The larger electrode thickness will increase the OH − diffusion resistance in the electrode, which is not conducive to the diffusion of OH − and reduce the electrochemical reaction rate, thus affecting the diffusion of H +, increasing the concentration polarization and affecting the charging efficiency of the battery. The uneven distribution of OH − concentration caused the difference of charging depth in the direction of electrode thickness, while the uneven distribution of H + concentration caused the difference of charging depth in the radial direction of particles. Under different pore structures, the decrease of porosity and particle size can increase the diffusion rate of OH − and solid phase H +, and then promote the electrochemical reaction rate and accelerate the charging speed. The larger electrode thickness increases the OH − diffusion resistance in the electrode, which is not conducive to OH − diffusion, and then affects H + diffusion and increases concentration polarization.

Effective adsorption of crystal violet dye on sugarcane bagasse–bentonite/sodium alginate composite aerogel: Characterisation, experiments, and advanced modelling

A novel sugarcane bagasse–bentonite/sodium alginate (SCB–Ben/SA) composite aerogel was prepared as an effective green adsorbent via blending–cross-linking for the removal of crystal violet (CV) dye from aqueous solution. The equilibrium adsorption capacity of SCB–Ben/SA for CV was 839.9 mg/g at 0.8 mg/mL adsorbent dosage, temperature 303 K, pH 4, and 800 mg/L initial CV concentration, and presented a removal rate of 85%. Adsorption kinetics studied using the novel pseudo-first-order and pseudo-second-order (PFO–PSO) combined model showed that CV adsorption by SCB–Ben/SA was a complex process and did not include a single mechanism, but involved both chemical and physical adsorption reactions; however, chemisorption was the dominant kinetic mechanism. The dynamic characteristics of CV adsorption process achieved through the liquid film around SCB–Ben/SA and the pores inside SCB–Ben/SA was described satisfactorily and conveniently using the novel film–pore mass transfer (FPMT) model. A multilayer adsorption model was applied to further understand the CV adsorption behaviour of SCB–Ben/SA. The number of linked CV molecules per functional group varied from 0.53 to 0.63 for SCB–Ben/SA. The CV molecules were adsorbed on SCB–Ben/SA via mixed orientation. The adsorption isotherms and thermodynamics revealed that the multilayer adsorption process was endothermic and spontaneous. Overall, SCB–Ben/SA was found to be an efficient, low-cost, green, and recyclable adsorbent for organic dye removal.

Influence of low-frequency vibrations on mass transfer in pores

The work is devoted to the numerical investigation of mass transfer in a narrow extended channel at small low-frequency fluid vibrations. These questions are of interest for technological processes associated with impregnation and filtration. Three different series of mass transfer problems are considered. In the first series a low velocity flow with some diffusing component was supplied through the initial section of a long thin rectangular channel. In the second series the speed fluctuation near the zero point was considered, i.e. there was no stationary component of the fluid velocity in the channel. In the third series of calculations the mass transfer was determined for the case when the dissolving substance was absorbed on the opposite wall of the channel. Calculations of mass transfer in capillary channels showed that with longitudinal oscillations of the flow in the channel, the amplitude of the velocity oscillations has the main influence while the frequencies play practically no role.

Quaternary ammonium-functionalized magnetic chitosan microspheres as an effective green adsorbent to remove high-molecular-weight invert sugar alkaline degradation products (HISADPs)

• We elucidated the structural properties of HISADPs. • QAMCM was creatively synthesized and systematically characterized. • In vitro cytotoxicity tests were conducted to evaluate the food safety of QAMCM. • QAMCM adsorbed HISADPs well and exhibited good regenerability. • PFO–PSO and FPMT models described the adsorption mechanisms well. Quaternary ammonium-functionalized magnetic chitosan microspheres (QAMCM) were fabricated for the removal of high-molecular-weight invert sugar alkaline degradation products (HISADPs), the main class of colorants in beet juice. The equilibrium adsorption capacity of QAMCM for HISADP was 98.03 mg/g at 0.6 mg/mL QAMCM dosage, 333 K temperature, pH 7, and 60 mg/L initial HISADP concentration, and the corresponding removal rate reached 98.03%. The adsorption kinetics were studied by using a pseudo-first-order and pseudo-second-order (PFO–PSO) combined model. Results revealed that HISADP adsorption by QAMCM is a complex process because of the involvement of both chemisorption and physisorption instead of only one mechanism. However, chemisorption (electrostatic attraction and electron sharing) is the dominant kinetic mechanism. Analysis of the mass transfer mechanisms showed that the dynamic characters of the adsorption capacities of the liquid film around QAMCM and the pores of QAMCM could be well described using a film–pore mass transfer model, which could be used to provide insights into the adsorption mass transfer behaviors of the system. Additionally, HISADPs with smaller sizes were preferentially adsorbed by QAMCM compared with larger ones. Research on the isotherms and thermodynamics showed that HISADP adsorption by QAMCM occurred via an endothermic multi-layer process and was spontaneous. Overall, QAMCM was found to be an efficient, green, and recyclable decolorization adsorbent for beet juice.

Unusual van Deemter plots of optical isomers on a chiral brush-type liquid chromatography column

Unusual dynamic behavior of the enantiomers of 9-bromo-11b-(tert-butyl)-2,3,6,11b-tetrahydrooxazolo[3′,2′:1,5]pyrrolo[3,4-b]quinoline-5,11-dione (I) was observed on a Nautilus-R column packed with silica grafted with antibiotic ristocetin. It consisted in (i) antibatic behavior of the van Deemter plots of the enantiomers and (ii) high and strongly enantiomer dependent values of the A- and B-terms of the van Deemter equation. Although rare, such a pattern has been found earlier in chiral chromatography, with all reported cases limited to brush-type chiral stationary phases. Adsorption dynamics in this system was studied by means of the moment method and the peak parking technique; hydrodynamic properties of the column were explored by using unretained tracers. It was shown that the peculiar shape of the van Deemter curves for the enantiomers of I is conditioned by imperfect packing of the stationary phase, which result in high transcolumn eddy dispersion, and by slow adsorption/desorption kinetics. It was proven that the whole void volume of the column available to an eluent is not accessible to the studied analyte because it cannot penetrate the space between neighboring grafted ligands. Its mass transfer in pores is also affected by the fact that the stagnant layer of the binary water-acetonitrile mobile phase differs in composition from the bulk liquid due to preferential adsorption of water that influences the apparent molecular diffusivity of solutes. An effect of the structures of analyte and chiral selector on the adsorption kinetics is also briefly discussed.

Mechanistic understanding of fouling of protein A chromatography resin

This paper aims to provide a thorough understanding of how fouling of Protein A resin takes place. Binding and mass transport properties of widely used agarose-based Protein A resin, MabSelect SuRe™, have been examined to understand the mechanism of resin fouling. There could be various factors that impact resin fouling. These include product/impurity build-up due to components in the feed material and ligand degradation due to the use of harsh buffers. To unravel their contributions, cycling studies were performed with and without product loading. The results presented in this paper provide a lucid understanding of the causative factors that limit Protein A chromatographic resin lifetime. The capacity fall for protein A resin at the end of 100th cycle due to use of feed material was found to be five times greater than that without using feed material. Compared to the fresh resin, the cycled resin samples shows 24% reduction in particle porosity and 51% reduction in pore mass transfer coefficient. Transmission electron microscopy (TEM) was used to qualitatively monitor accumulation of foulants on the cycled resin. Fouled resin sample contained a dense residue in the interior and exterior of resin particle both as a film at the bead surface and as granules. The surface activation energy increased five times in the case of fouled resin sample. The major event in fouling was identified as the non-specific adsorption of the feed material components on resin, signaling that pore diffusion is the rate limiting step. It is anticipated that these findings will assist in development of a more robust and economical downstream manufacturing process for monoclonal antibody purification.

Electrodeposition of Noble Metals into Ordered Macropores in p-Type Silicon

The electrodeposition of noble metals, i.e., platinum, palladium, and gold, into macroporous p-type silicon was examined. For platinum and palladium, the electrodeposition proceeded preferentially from the pore bottom to the opening when sodium chloride was used as a supporting electrolyte. When sodium sulfate was used as a supporting electrolyte, the electrodeposition mainly proceeded at pore openings, leading to plugging. For gold electrodeposition, a condition for achieving pore filling from the bottom was not found in either the NaCl or solutions. Pore depth was another key factor to achieve continuous filling by electrodeposition. As the depth of pores became deeper, the electrodeposition proceeded preferentially from the bottom. The effect of mass transfer in pores was also investigated by changing the concentration of metal ions and applied potential.

Behavior of direct reduced iron and hot briquetted iron in the upper blast furnace shaft: Part I. Fundamentals of kinetics and mechanism of oxidation

It is considered that the use of prereduced ferrous materials and sources of metallic iron such as direct reduced iron (DRI) or hot briquetted iron (HBI) improves the productivity of the blast furnace (BF). However, oxidation of DRI/HBI can occur in the upper zone of the BF, which may increase the content of the reducing gases but may not decrease the coke rate substantially. The behavior of DRI and HBI was investigated by measuring the rate of oxidation of the materials in CO2 gas in a temperature range of 400 °C to 900 °C. In addition, the microstructure of “as-received” and oxidized materials was examined. The iron oxide phases formed due to oxidation were determined using X-ray diffraction (XRD) and a vibrating sample magnetometer. The results of isothermal experiments indicated that the kinetics of oxidation of metallic iron is slow at 400 °C. In DRI samples, the initial rate is controlled by the limited mixed control of chemical kinetics at the iron/iron oxide interface and pore mass transfer, whereas gas diffusion in pores is the rate governing step during the final stages of oxidation. The oxidation of wustite from iron is found to be faster than the oxidation of the former to magnetite. The structure of DRI after oxidation resembled a “reverse topochemical-oxide on the surface metal in the center” structure at 600 °C to 700 °C. The final iron oxide phase formed in DRI after oxidation was magnetite and not hematite. The oxidation of HBI was limited to the surface of the samples at lower temperatures; at 900 °C, moderate oxidation was observed and a topochemical iron oxide layer was formed.

Analysis of the elimination process of polymerisation inhibitors from styrene by means of adsorption

AbstractThis work is focused on the analysis, modelling and scale‐up of a process of styrene purification. The first step in synthetic rubber production is to eliminate 4‐tert‐butylcatechol (TBC) which is added to styrene to prevent homopolymerisation during transport and storage. This process is carried out on an industrial scale by means of adsorption onto activated alumina. Equilibrium experiments at 10 °C correlated to the Langmuir–Freundlich equation: with a weighted standard deviation of 3.38%. Fixed bed column experiments were carried out on a laboratory scale to obtain breakthrough curves. A mathematical model that considers film and pore mass transfer resistances described satisfactorily the experimental results with a value of Dp = 3.96 × 10−9 m2 s−1 which was obtained from correlation of experimental data to simulated curves. Finally, a pilot plant was built and operated in order to verify the validity of the mathematical model and parameters obtained previously.© 2002 Society of Chemical Industry

Breakthrough performance of stacks of dye-cellulosic fabric in affinity chromatography of lysozyme.

The breakthrough performance of stacks of dye-cellulosic fabric in affinity chromatography of lysozyme was investigated in batch and flow experiments. Breakthrough curves were significantly affected by flow rate and were not dependent on the feed solution concentration. System dispersion curves could not explain the flow-rate dependence. Breakthrough curves were analyzed by coupling the kinetic model for pore mass transfer as the only controlling resistance and a system dispersion model. From the analysis, pore film mass transfer resistance was found to be the leading rate-limiting factor when the residence time in the column is greater than 5 min. The model was used to predict the operating and design parameters needed to obtain sharp breakthrough curves. Selectivity studies using lysozyme and bovine serum albumin mixtures showed a high system selectivity for lysozyme.

Mathematical modelling of styrene drying by adsorption onto activated alumina

This work is focused on the analysis and modelling of styrene drying, raw material in the manufacture of synthetic rubber, by means of adsorption onto activated alumina. Equilibrium experiments, carried out under isothermal conditions at 10°C, correlated to the equation q (kg/kg)=2.659×10 −4 C (mg/kg). Fixed bed column experiments were performed working with different flow rates and using different bed lengths in order to obtain experimental breakthrough curves. A mathematical model that considers the influence of both film and pore mass transfer resistances described satisfactorily well the experimental results. Finally a value of D p =6.101×10 −9 m 2/s was obtained from correlation of experimental data to simulated curves and using the minimum weighted standard deviation as optimisation criterion.

Theoretical optimization of operating parameters in non-ideal displacement chromatography

A mathematical model was developed for the simulation of non-ideal displacement chromatography. The model incorporates finite mass transport to the solid adsorbent by using a linear driving force approximation with a coupled external film and internal pore mass transfer coefficient. Equilibrium adsorption at the fluid—solid interface is described using competitive langmuirian adsorption isotherms. A finite difference numerical technique was employed to approximate the system of coupled, non-linear partial differential equations. The model was used to simulate the effluent concentration profiles under various displacement chromatographic conditions. The effects of axial dispersion and finite mass transport were examined by varying the Peclet and Stanton numbers, respectively. Slow mass transfer rates were shown to have a dispersive effect on the shock waves generated in displacement chromatography, resulting in greater zone overlap. Constant pattern formation was observed under non-ideal conditions. The throughput obtained in displacement chromatography was examined as a function of feed load, flow velocity, and displacer concentration. For non-ideal systems, the throughput was shown to exhibit a maximum at unique values of these operating parameters. The effects of particle diameter and solute diffusivity on the throughput were also examined. Model predictions indicate that the use of large particles could be detrimental to the performance of displacement systems when high velocities are employed. For macromolecular separations by displacement chromatography, small particles are required regardless of the linear velocity. The model presented here is a useful tool for the optimization and scale-up of displacement chromatographic processes.

Absorption of benzene by open‐cell polyurethane foams

AbstractThe rate of absorption of benzene by open‐cell polyurethane foams of varying pore size (30–85 pores per inch) has been shown to be dependent on the size of the foam samples. Mass transport from the bulk vapor to the matrix surface appears to be a significant resistance when compared with the rate of diffusion in the matrix itself. Even though these foams have a large permeability to air at low pressures, pore diffusion appears to be more significant than bulk flow in describing the absorption process, resulting in absorption behavior which is more characterstic of closed‐cell foams. A dual resistance model of the obsorption process, has been used to estimate matrix diffusivities and pore mass transfer coefficients. Although the model was inadequate in some regards to describe completely the absorption process, the significance of the unexpected pore diffusion resistance to mass transfer was quantified. The dependence of pore diffusion on foam size reflected the qualitative interpretation that was apparent from the absorption curves. While the reason for this anomalous behavior remains unknown, open‐cell foams cannot be considered simply as a high‐surface‐area thin‐walled form of the matrix material in describing the absorption process; the effect of foam size must also be considered.