Diffusion of Saccharides and Sugar Alcohol Sorbitol in Chitosan Membranes and Beads

Diffusion coefficients of different sugars and the sugar alcohol sorbitol have been determined by two different experimental setups: mass transfer through a flat membrane in a diffusion cell and effusion experiments with beads in a batch stirred vessel. Diffusion experiments show highly consistent data with low uncertainties and high regression coefficients, while effusion experiments suffer from larger experimental uncertainties. For the diffusion through a chitosan membrane at 25 °C, typically one‐third of the free molecular diffusion coefficient is found, whereas at 40 °C only one‐quarter of it is reached. The influence of molecular size, structure, and temperature on the transport properties is discussed in detail.     

Concentration‐Dependent Diffusion Coefficients for Fructose in Highly Permeable Chitosan Polymers

The concentration‐dependent diffusion coefficient of fructose in precipitated chitosan membranes was determined using diffusion cells. Experiments showed that boundary layers on both sides of the membrane could not be neglected when the diffusion coefficient in chitosan is close to the free diffusion coefficient. Hence, the influence of the boundary layers was covered by measuring membranes with different thickness. The diffusion coefficient in chitosan at infinite dilution was found to be around the free diffusion coefficient. With increasing concentration, the diffusion coefficient in chitosan decreases faster than the free diffusion coefficient. Thus, the product of diffusion coefficient and concentration difference shows a maximum implying an optimum concentration for mass transfer.     

Reaction kinetics and mass transfer studies of biomass char gasification with CO2

Gasification kinetics of wheat straw char with CO₂ was investigated using Thermogravimetric apparatus (TGA). The main objective was to identify the diffusional and surface reaction phenomena that may occur during biomass char gasification experiments with CO₂. The effects of temperature (750–900 °C) and particle size (<60–925 μm) on gasification rate of char-CO₂ reaction were determined. The 50% conversion (r₅₀) rate showed that the reactivity increases with temperature, and it decreases as the particle size increases. The important diffusional parameters such as effective diffusivity, effectiveness factor and Thiele modulus were calculated based on the experimental data and the results showed that the impact of physical factors is prominent at high temperatures and large particle sizes. It was found that char gasification within the temperature range studied followed the chemically controlled reaction regime and the influence of pore diffusion was negligible for fine powder particles.     

Kinetic Study of Heterogeneously Catalyzed Glucose Oxidation in a Stirred Cell

Liquid phase oxidation of glucose to gluconic acid using Bi promoted Pd catalysts was studied in a stirred cell. Gas‐liquid mass transfer limitations are observed at lower but not at higher rotational speeds. The conversion and the deactivation of the Pd catalyst depend on the O₂ concentration in the liquid phase. With decreasing glucose concentration, the reaction rate decreases leading to higher oxygen concentration in the liquid, which deactivates the catalyst due to over‐oxidation. Severe mass transfer limitations even at low Pd loadings could be attributed to intraparticle or liquid‐to‐solid mass transfer.     

Heterogeneously Catalyzed Epimerization of Menthol Stereoisomers – An Instructive Example to Account for Diffusion Limitations in Complex Reaction Networks

The heterogeneously catalyzed epimerization of menthol stereoisomers is an important step in the synthesis of (–)‐menthol by the Haarmann and Reimer process. For an accurate design of a technical reactor, the intrinsic kinetics as well as pore diffusion limitations have to be taken into account. The epimerization consists of a complex reaction network and so a simple approach using effectiveness factors based on Thiele moduli is not possible. In this work, a method is presented to simultaneously calculate the change in concentrations in the bulk phase as well as within the porous particles with time (fixed bed reactor) or local position (batch reactor). A commercially available numerical software was used to solve the differential equations.     

Comparison of Concentration Profiles for Boundary Layers in Absorption with Chemical Reaction Processes

An analysis of five different systems of absorption‐with‐chemical‐reaction in gas‐liquid reactors, commonly encountered in various industrial processes, is presented. To analyze the interphase mass transfer from gas to liquid, the rate limiting parameters and the concentrations at the gas‐liquid interface were determined on the basis of pertinent theories. The calculations presented, are based on the Whitman theory for gas and liquid phase mass transfer coefficients and Henry equilibrium constants. The necessary diffusion coefficients were calculated from existing correlations, and the corresponding chemical reaction rate constants were obtained from the literature, assuming pseudo first order chemical reaction. The process parameters required (pressure, temperature, and the gas‐liquid contact time) were within the values that occur in industrial processes. The results presented, are the concentration profiles in the boundary layers for the systems studied, calculated and graphically presented, together with the gas and liquid film thicknesses and Hatta numbers, obtained from calculations for the liquid phase mass transfer. The results may contribute to a better understanding of the absorption‐with‐chemical‐reaction processes in industrial plants, thus lowering the operational costs of these processes and alleviating the ecological problems of existing technologies.     

Diffusion of One‐ and Two‐Component Fluids into Polymeric Membranes

The starting point of this paper is a model of the non‐Fickian diffusion of a simple fluid into a polymeric medium that has been introduced in El Afif and Grmela (2002). The model is extended to a mixture consisting of two simple fluids and one polymeric medium. The effects of the polymeric medium on diffusion are included in the formulations through the use of relative momenta playing the role of internal variables. In ternary mixtures, new phenomena arise due to cross‐coupling effects. As a consequence, the single diffusion Deborah number in binary mixtures becomes a 2 × 2 matrix in ternary mixtures. Two specific examples for which experimental data are available are investigated in detail.     

High Diffusion Permeability of Anion-Exchange Membranes for Ammonium Chloride: Experiment and Modeling

It is known that ammonium has a higher permeability through anion exchange and bipolar membranes compared to K⁺ cation that has the same mobility in water. However, the mechanism of this high permeability is not clear enough. In this study, we develop a mathematical model based on the Nernst–Planck and Poisson’s equations for the diffusion of ammonium chloride through an anion-exchange membrane; proton-exchange reactions between ammonium, water and ammonia are taken into account. It is assumed that ammonium, chloride and OH⁻ ions can only pass through membrane hydrophilic pores, while ammonia can also dissolve in membrane matrix fragments not containing water and diffuse through these fragments. It is found that due to the Donnan exclusion of H⁺ ions as coions, the pH in the membrane internal solution increases when approaching the membrane side facing distilled water. Consequently, there is a change in the principal nitrogen-atom carrier in the membrane: in the part close to the side facing the feed NH₄Cl solution (pH < 8.8), it is the NH₄⁺ cation, and in the part close to distilled water, NH₃ molecules. The concentration of NH₄⁺ reaches almost zero at a point close to the middle of the membrane cross-section, which approximately halves the effective thickness of the diffusion layer for the transport of this ion. When NH₃ takes over the nitrogen transport, it only needs to pass through the other half of the membrane. Leaving the membrane, it captures an H+ ion from water, and the released OH⁻ moves towards the membrane side facing the feed solution to meet the NH₄⁺ ions. The comparison of the simulation with experiment shows a satisfactory agreement.     

Effect of hydrothermal treatment and ammonium ion incorporation in platinum-mordenite catalysis for n-hexane hydroconversion

Hydroconversion of n-hexane was studied on catalysts containing 0.25% Pt supported on H-mordenite (H-M) and NH₄-M. The H-M containing catalysts were Pt/H-M, Pt on steamed H-M (Pt/St H-M) and steamed Pt/H-M (StPt/H-M), whereas the NH₄-M containing catalysts were Pt/NH₄-M and StPt/NH₄-M. Steam-treatment of H-M containing catalysts enhanced the hydroconversion activity, whereas such treatment decreased the activity of the Pt/NH₄-M catalyst. The diffusion resistance parameter, i.e., the Thiele modulus, Φ_L, estimated for the reaction on the catalysts under study was found to increase with the increase in the catalytic activity, and both were found to decrease in the order:

Pt dispersion in the zeolite was not comparable with the catalytic activities of the H-M containing catalysts. The higher activity of the Pt/NH₄-M catalyst could be attributed to a higher Pt dispersion in the zeolitic channels, higher strength of the acid sites (determined by temperature programmed desorption (TPD) of ammonia) and higher diffusion limitation of the reactant in the catalyst pores.     

Microgels for the Intensification of Liquid-Liquid Extraction Processes – Feasibility and Advantages

Microgels are cross-linked polymers with a high application potential in liquid-liquid systems due to their surface activity and switchable stabilization properties. A process concept utilizing microgels in extraction processes is presented. The microgels are located at the drop surface and prevent coalescence, enabling monodisperse operation. At the column top, the stabilization is switched off by temperature shift. The switchable stabilization and the absence of additional mass transfer resistance are crucial requirements for the concept and tested experimentally. The results provide the basis for the model-based evaluation of the process performance, revealing a broader operating window, capacity increase at equal height equivalent of theoretical stage (HETS) for high loads, and reduced HETS for small loads.     

乳状液膜分离Zn^2＋的界面传质阻力及传质模型

用Ｌｗｅｉｓ恒界面传质池对乳状液膜分离Ｚｎ＾２＋的传质阻力进行实验研究，结果表明由于表面活性剂单分子层的形成，使界面传质阻力占整个传质阻力的８５％。在此基础上所建立的既考虑界面传质阻力又考虑膜破碎的液膜传质模型能使用理论计算值与实验测定值符合较好。     

壳聚糖凝胶球的制备及对银离子的吸附

以壳聚糖(CTS)为基材、硝酸银为印迹物,提出了一种新的CTS水凝胶球的制备方法.采用Ag+吸附容量作为评价指标,对凝胶球的制备条件进行了优化,并进行了吸附试验.结果显示,当CTS用量为质量分数2.5%、交联剂用量为体积分数3.0%、凝固浴三乙醇胺的质量分数为10.0%、制备温度为25℃,热处理温度为60℃,洗脱剂硫代...     

Mathematical modeling of the coupled transport and electrochemical reactions in solid oxide steam electrolyzer for hydrogen production

A mathematical model was developed to simulate the coupled transport/electrochemical reaction phenomena in a solid oxide steam electrolyzer (SOSE) at the micro-scale level. Ohm's law, dusty gas model (DGM), Darcy's law, and the generalized Butler Volmer equation were employed to determine the transport of electronic/ionic charges and gas species as well as the electrochemical reactions. Parametric analyses were performed to investigate the effects of operating parameters and micro-structural parameters on SOSE potential. The results substantiated the fact that SOSE potential could be effectively decreased by increasing the operating temperature. In addition, higher steam molar fraction would enhance the operation of SOSE with lower potential. The effect of particle sizes on SOSE potential was studied with due consideration on the SOSE activation and concentration overpotentials. Optimal particle sizes that could minimize the SOSE potential were obtained. It was also found that decreasing electrode porosity could monotonically decrease the SOSE potential. Besides, optimal values of volumetric fraction of electronic particles were found to minimize electrode total overpotentials. In order to optimize electrode microstructure to minimize SOSE electricity consumption, the concept of “functionally graded materials (FGM)” was introduced to lower the SOSE potential. The advanced design of particle size graded SOSE was found effective for minimizing electrical energy consumption resulting in efficient SOSE hydrogen production. The micro-scale model was capable of predicting SOSE hydrogen production performance and would be a useful tool for design optimization.     

Influence of Chitosan and Chitosan Derivatives on Hydrodynamics and Mass Transfer in a Bubble Contactor

The behavior of chitosan and two kinds of chitosan derivatives in carbon dioxide absorption in a bubble column contactor is analyzed. The effects of absorption type (physical or chemical), polymer type, concentration, and liquid‐phase physical properties on hydrodynamics (bubble size, gas holdup, and specific interfacial area) and mass transfer (absorption rate and mass transfer coefficient) are evaluated.     

Revisiting the concepts of competition between reaction and diffusion in poisoned catalysts

The competition between diffusion and first‐order irreversible reaction in poisoned catalysts is revisited. Two cases are considered for isothermal slab catalysts: uniform and shell‐progressive (or pore‐mouth) poisoning. Analytical concentration profiles are derived, and the implications on catalyst performance are evaluated in different regimes (chemical‐ or diffusion‐controlled) for different levels of poisoning. It was found that depending on the poisoning mechanism, the activity decay can be more or less pronounced. Being of particular concern is the pore‐mouth poisoning at high Thiele modulus, conditions under which catalyst performance is drastically affected. The reagent concentration profiles allowed the explanation of the phenomena occurring at the particle scale, in particular the effectiveness factors, observed reaction rates, and poisoning factors' dependence on the Thiele modulus and fraction of the poisoned catalyst; it was found that such relationships are dependent on the mechanism of deactivation. © 2012 Canadian Society for Chemical Engineering     

3D Modeling and Simulation of Mass Transfer in Vapor Transport through Porous Membranes

A 3D mathematical model is developed to predict the transport of water vapor through porous membranes. The model is based on solving the continuity, momentum as well as energy equations for water in the membrane contactor. The model's equations are numerically solved using the finite element method to obtain the concentration and temperature distributions of water in the membrane contactor. The model findings were in good agreement with experimental data. The proposed 3D model proved to be appropriate for predicting the performance of a membrane evaporator. Simulations were carried out in order to study the influence of different operating parameters and membrane structure on the membrane evaporation effectiveness. The results of simulation indicate that the gas velocity is a favorable parameter in the membrane evaporation process due to its tendency to keep the process far from the thermodynamic equilibrium.     

Theoretical Analysis of Reaction and Diffusion Processes in a Biofuel Cell Electrode

A mathematical model for reaction diffusion processes in a biofuel cell electrode is discussed. This model is based on reaction diffusion equations containing a non‐linear term related to the rate of the enzyme reaction. Theoretical treatment of a reaction and diffusion processes in a biofuel cell electrode, for the steady and non‐steady state condition is discussed. Approximate analytical expressions for the steady and non‐steady state current density at the electrode surface are calculated by using the new approach to homotopy perturbation method and complex inversion formula. An analytical expression for the steady state current density is compared with numerical results and found to be excellent in agreement. A novel graphical procedure for estimating the Michaelis‐Menten constants and turnover rate solely from the current‐potential curve is suggested. Influence of the controllable parameters such as diffusion of the mediator, Michaelis‐Menten constant for substrate, second‐order rate constant, thickness of the film, turnover rate and initial substrate concentration on the current density are discussed.     

Study of Carbon Dioxide Transport in a Samaria Aerogel Catalyst by High Field Diffusion NMR

Pulsed field gradient (PFG) NMR employing a high magnetic field of 17.6 T was used to study self‐diffusion of carbon dioxide in alumina stabilized samaria aerogel, a promising porous catalyst for gas‐phase reactions. Such rare‐earth aerogels exhibit high porosity and surface area with active sites directly integrated into the pore framework. In the reported diffusion NMR studies, application of a high magnetic field was essential for obtaining sufficiently high signal‐to‐noise ratios under conditions of relatively low CO₂ densities in the primarily mesoporous catalyst particles. The diffusion studies were performed with the catalyst that was formed into the following two types of samples: macroscopic monoliths and beds of particles with sizes around 200 μm. The sorbate diffusivity inside the monolith was compared with the corresponding diffusivity in the bed under conditions of fast exchange of CO₂ between the particles and the interparticle voids of the bed. The two‐domain exchange model proposed by Kärger for zeolites was used to describe the latter diffusivity. The reported results are expected to be useful for elucidating an influence of possible transport limitations under reaction conditions in aerogel catalysts.     

The Combined Diffusion and Adsorption Concept for Prediction of Nanoparticles Transport through Dermal Layers Based on Experiments in Membranes

The non-invasive introduction of active substances into the human body is a top challenge for researchers in medicine, pharmacology, and cosmetology. Development of nanotechnology and possibilities of creating more and more complex drug carriers on a nanoscale give a more realistic prospect of meeting this challenge. However, in the absence of sufficient knowledge of the mechanisms of such systems’ transport through the human skin structure, it is necessary to look deeper into these issues. There are several models describing nanoparticles transport through the skin, but they are mainly based on diffusion process analysis. In this work, a model was proposed to predict nanoparticles transport through the skin, based on the combined diffusion and adsorption concept. This approach was based on experimental studies of silver and copper nanoparticles’ diffusion process through different filtration membrane layers. Dependence of the degree of adsorption on the surface parameter was described using modified Langmuir equation. Then, these considerations were related to the structure of the stratum corneum, which made it possible to predict the changes in the mass of penetrating nanoparticles as a function of transport path length. A discussion of the presented model, depending on such parameters as nanoparticle size, skin cell thickness, or viscosity of the “intercellular cement”, was also performed.     

Thermodynamics and mass transfer kinetics of phenol in reversed phase liquid chromatography

The thermodynamics and the mass transfer kinetics of the chromatographic system made of phenol, in a water-acetonitrile mobile phase, on a C18 RPLC column, were studied in the temperature range from 21 to and the interstitial velocity range of 0.021 to 1.27 cm/s. The equilibrium isotherm was accurately approximated by a multilayer model assuming lateral interactions between adsorbed molecules. The parameters of the kinetics of the phenol mass transfer in this column were measured by the method of moments. These data were analyzed using the available models and correlations. It was proven that the parameters of the mass transfer kinetics measured under linear conditions could be successfully used for the prediction of the concentration profiles obtained under overloaded conditions.