Experimental study and modeling of fluidized bed coating and agglomeration

This work deals with the fluidized bed coating and agglomeration of solid particles. The effect of particle size on coating criteria was investigated using sand particles as the coating support and aqueous solutions containing NaCl as coating liquid. The results showed that both growth rate and efficiency increase with decreasing the particle size. The growth was mainly governed by layering for particles larger than 200 μm, whereas for finer particles it occurred by agglomeration. As the particle size became less than 90 μm, the coating operation led to uncontrolled growth and bed quenching. However, the coating of the same particles was successfully achieved by adding some coarser particles. In addition, a mathematical model based on the population balance concept, taking into account the simultaneous growth by layering and agglomeration, was established to predict the time evolution of the particle size distribution. The comparison between experimental and calculated data permitted the establishment of a law for the size dependency of the agglomeration kernel.     

Determination of Mass Transfer Parameters by Means of Chemical Absorption

The paper presents an explicit equation for the enhancement factor of a fast irreversible second‐order reaction. The equation makes it possible to determine the effective interfacial area and the liquid‐phase mass‐transfer coefficient of this reaction regime. With the help of a new plot as described in this paper, the Danckwerts plot, and the method for determining the interfacial area by means of a pseudo‐first‐order reaction the paper discusses a novel method for characterizing the reaction regime of experimental data.     

Size control of polystyrene beads by multistage seeded emulsion polymerization

Micron‐sized monodisperse crosslinked polystyrene (PS) beads have been prepared by a multistage emulsion polymerization using styrene monomer, divinylbenzene crosslinking agent, and potassium persulfate initiator in the absence of emulsifier. In the first stage of the reaction, the lower the reaction temperature, the larger the bead size obtained. In the later stages of the reaction, the particle size is increased with the initiator concentration and monomer content. Particle nucleation of the preexisting polymer seed of 0.7–0.8 μm in diameter is prepared at 60°C, then the monodisperse crosslinked PS beads > 2 μm are synthesized up to the third stage of the reaction. As the particle size grows, the number of free radicals in the growing particles increases, and the conversion of the next stage is continuously increased. The reaction mechanism is suggested that the continuous polymerization be conducted due to the diffusion of monomer into the preexisting particles to induce spherical beads in the later stages of the reaction. Otherwise, phase separation or the formation of protrusion by the capture of free radicals will be taking place. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 2259–2269, 1999     

旋转床填料中的传质及其模型化

用氮气解吸水中溶解氧,发现了三个在前人研究中未曾提及的重要现象：（１）保持填料外径而扩大内径,平均体积传质系数增大;（２）比表面积大小并不是传质好坏的决定因素;（３）转子填料与旋转床机壳间空腔中液滴表面的传质量,对整个装置中的传质总量,有不可忽略的贡献。据此,建立了基于液体在填料内微粒化,传质同时在填料和液滴表面进行的传质模型。模型计算结果较好地与实验结果吻合,并对上述三个现象作出了合理的解释。     

Mass Transfer Modeling of SO2 into Large Water Drops

This article concerns the modeling of the SO₂ absorption and desorption by falling drops through air with low to high concentration of sulfur dioxide. In the liquid phase, a model based on local scales, interfacial liquid friction velocity and drop size diameter is used. In the continuous gas phase a more classical model is applied. Data obtained by the modeling of the SO₂ absorption and desorption by a single water drop are compared to published experimental results and a fairly good correspondence was found. On the other hand, the model shows that at high gas concentration (> 1 %) the internal resistance to diffusion dominates, while at low gas concentration (< 1000 ppb) the external resistance to diffusion dominates.     

Convective Heat and Mass Transfer Modeling in Gas‐Fluidized Beds

This review examines selected mechanistic and empirical models reported in the literature to predict convective heat and mass transfer coefficients in gas‐fluidized beds. The role of hydrodynamics in heat and mass transfer is briefly outlined before embarking on the modeling approaches. Both bed to wall and interphase heat transfer, are considered. In bed to wall heat transfer, the main focus of the review is the modeling of particle convective components, based on surface renewal. The concepts of transient and local heat transfer models are also discussed briefly. In the case of mass transfer, only interphase transfer is considered. Emphasis is placed on models based on combustion where mass transfer is seen to occur from a few active particles contained in a fluidized bed of inert particles.     

Modeling the Heat and Mass Transfer in Microwave Drying of White Oak

A 2D comprehensive heat and mass transfer model was developed to simulate the free liquid, vapor, and bound water movement in microwave drying of white oak specimens. The experimental and model results showed that, for white oak, moisture movement was easily impeded and high gradient of internal vapor pressure occurred. The internal vapor pressure was affected by sample dimension (length and thickness). At the same input power density, the internal pressure generated in the core increased with the sample length and thickness. However, as compared with sample length, sample thickness has less effect on the pressure gradient because of the high ratio of permeability between longitudinal and transverse directions.     

Device Performance Improvement of Double‐Pass Concentric Circular Mass Exchangers under Uniform Wall Fluxes

A double‐pass concentric circular mass exchanger under uniform wall fluxes is produced by inserting a permeable barrier into a circular tube to improve the device performance. The mathematical formulation was developed theoretically for such double‐pass, forced‐convection, mass‐transfer problems which are referred to as conjugated Graetz problems. The analytical solutions are obtained by the linear superposition of an asymptotic solution and a homogeneous solution which are linear in the axial direction and solved with the use of an eigenfunction expansion in a power series. The analytical results show that the mass‐transfer rate of a double‐pass mass exchanger can be improved compared to that of a single‐pass mass exchanger by suitably adjusting the permeable barrier position. Moreover, the ratio of mass‐transfer efficiency improvement and power consumption increment is also shown to make good economic sense.     

Computational modeling of inhibitor release and transport from multifunctional organic coatings

A computational code that was originally designed to model crevice corrosion was extended to multifunctional coatings on Al alloys exposed to thin layers of electrolytes. The model is able to calculate the transient distributions of potential, current density, and all chemical species concentration, enabling the dynamic simulation of inhibitor release, inhibitor transport, and sacrificial cathodic protection. The model has been applied to both inhibitor release from and aggressive anion capture by hydrotalcites (HTs) pigments in epoxy primer coatings applied to AA2024-T3. Computational studies were carried out to investigate the effects of HT/vanadate (HT/V) epoxy coating system parameters including scratch size, inhibitor release rate, Cl⁻ gettering rate (GR), cathodic kinetics on the bare AA2024-T3, and solution layer thickness on system performance. The analyses of the computational results have quantified the important factors controlling successful corrosion inhibition by inhibitor release from coatings. The pH-dependence of the steady state inhibitor release rate was found to be the most important parameter controlling system performance. Cl⁻ gettering can also reduce the aggressiveness of solution at long times, especially when considered in conjunction with inhibitor release. However, the ion exchange capacity required poses stiff design challenges involving the loading of the ion exchanger into the resin and the service conditions. The effectiveness of inhibition decreased significantly for the larger scratch sizes. The cathodic kinetics within the scratch play an important role in determining the ability of a given inhibitor to function effectively. When the scratch is the cathode in the galvanic couple with the substrate under the coating, inhibition was more effective. For the conditions simulated here, the net effect of a decreased solution layer thickness is to increase the protection ability of the system. The increase in the inhibitor concentration overcomes the decrease in the pH at the anode.     

Immobilization of polyphenol oxidase on carboxymethylcellulose hydrogel beads: preparation and characterization

Carboxymethylcellulose (CMC) beads were prepared by a liquid curing method in the presence of trivalent ferric ions, and epicholorohydrin was covalently attached to the CMC beads. Polyphenol oxidase (PPO) was then covalently immobilized onto CMC beads. The enzyme loading was 603 µg g⁻¹ bead and the retained activity of the immobilized enzyme was found to be 44%. The K_m values were 0.65 and 0.87 mM for the free and the immobilized enzyme, and the V_max values were found to be 1890 and 760 U mg⁻¹ for the free and the immobilized enzyme, respectively. The optimum pH was 6.5 for the free and 7.0 for the immobilized enzyme. The optimum reaction temperature for the free enzyme was 40 °C and for the immobilized enzyme was 45 °C. Immobilization onto CMC hydrogel beads made PPO more stable to heat and storage, implying that the covalent immobilization imparted higher conformational stability to the enzyme. © 2000 Society of Chemical Industry     

Computational Simulation of Mass Transfer in Molecular Separation Using Microporous Polymeric Membranes

Computational simulation was conducted to predict mass‐transfer phenomenon in the molecular separation of solute using microporous membrane contactors. Both diffusional and convectional mass‐transfer mechanisms were considered. The membrane system was a hollow‐fiber contactor in which an aqueous phase containing an organic compound was contacted with an organic phase for extractive separation of the solute. Benzoic acid was used as the solute. The main focus was on understanding the mass transfer of solute in the process. The concentration equation for the solute was solved numerically using a computational fluid dynamics approach. It was found that the model can predict the solute transport from the aqueous phase to the organic phase and can be used as a predictive model for process understanding.     

Mass transfer coefficient prediction method for CFD modeling of riser reactors

In gas-solid reactors, particularly circulating fluidized beds (CFB) and riser it is becoming increasingly more important to be able to predict the conversion and yield of reactant species given the ever rising cost of the reactants and the ever decreasing acceptable level of effluent contaminants. As such, the development and use of predictive models for the reactors are necessary for most processes today. These models need to take into account the interphase mass transfer. Unless equipped with specific experimentally based empirical correlations for the reactor system under consideration, the modeler is required to go to the open literature to obtain correlations for the mass transfer coefficient between the solid and gas phases. This is a difficult task at present since these literature values differ by up to 7 orders of magnitude as found in the literature dating back to 1949. The wide variation in the prediction of mass transfer coefficients in the existing literature is credited to flow regime differences that can be identified in the cited literature upon careful inspection.Applying a new theory developed by Breault and Guenther [1] that takes into account the local hydrodynamics, a predictive methodology is presented that allows the local mass transfer coefficient to be calculated based upon local properties in a CFD simulation. A sample calculation is presented and compared with the data used in developing the theory as well as correlation for the literature.     

Mass Transfer Mechanisms in the Fixed‐Bed Ion‐exchange Process for Dilute Colloidal Silica Manufacture

The ion exchange behavior of the H⁺ form C‐100 Purolite resin for the production of colloidal silica from dilute sodium silicate solutions has been investigated. The exchange isotherm has been found to be almost irreversible. The effective resin diffusion coefficient has been found to be 2.84 × 10^–10 m² s^–1 using a shrinking core model for the batch uptake experiments. Fixed bed experiments for different column heights and feed flow rates were performed. Numerical solution of the governing equations showed that the process is initially controlled by film diffusion and consequently by resin diffusion. Axial dispersion had to be taken into account. A simple power law correlation has been determined that relates the fluid Peclet number to the Reynolds number. It is presumed that some sort of resin ‘deactivation’ due to intraparticle microgel formation is responsible for the sluggish end part of the breakthrough curves.     

Synthesis of highly loaded and well‐controlled magnetic beads via emulsion polymerization

Superparamagnetic iron oxide (SPIO) incorporated polystyrene beads of uniform size distribution (～170 nm) and high magnetic content (～40%) are synthesized by emulsion styrene polymerization in the presence of functionalized SPIO. The role of surface functionality on the polymerization process and SPIO incorporation is investigated by carrying out styrene emulsion polymerizations with different functionalities on SPIO. A unique combination of oleic acid and (2‐acetoacetoxy) ethyl methacrylate as surface ligands for SPIO is used to attain the best magnetic beads. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Mass transport and surface reactions in microfluidic systems

We provide analysis of different regimes of diffusion and laminar flow convection combined with bimolecular surface reactions relevant to biochemical assays performed in microfluidic devices. Analytic solutions for concentration fields are compared to predictions from two-dimensional finite element simulations for the various operation regimes. The analytic and numerical results extend the transport models beyond the models commonly used to interpret results from surface plasmon resonance (SPR) experiments. Particular emphasis is placed on the characterization of transport in shallow microfluidic channels in which the fully developed transport regime dominates rather than the mass transfer boundary layer transport typically encountered in SPR. Under fast reaction and diffusion conditions, the surfaces saturate following moving front kinetics similar to that observed in chromatographic columns. Two key parameters relevant to on-chip biochemical assays and microfluidic sensors are studied and compiled: the capture fraction of the bulk analyte at the surface and the saturation time scale of the reactive surfaces. The physical processes in the different regimes are illustrated with data from the relevant microfluidics literature.     

Controlled release study of carbaryl insecticide from calcium alginate and nickel alginate hydrogel beads

In this study, controlled release formulations for reducing environmental impact of pesticides have been produced by encapsulating as a model pesticide carbaryl (Carb) in the alginate beads. The various hydrogel bead formulations were prepared by the ionotropic crosslinking of sodium alginate (NaAlg) with calcium and nickel ions. The surface morphology of prepared beads was characterized with scanning electron microscopy (SEM). SEM confirmed the spherical nature and surface morphology of the particles. Bead characteristics, such as carbaryl entrapment efficiency, particle size, equilibrium swelling degree, and carbaryl release kinetics, were determined. The effects of the bead preparation conditions such as crosslinker concentration and type, carbaryl/sodium alginate (Carb/NaAlg) ratio and percentage of NaAlg on the carbaryl release from the calcium alginate (Ca‐Alg) and nickel alginate (Ni‐Alg) beads were investigated in distilled water at 25°C. It was observed that carbaryl release from the Ca‐Alg beads was slower than that of Ni‐Alg beads. The release results indicated that carbaryl release from both of the Ca‐Alg and Ni‐Alg beads decreases with the increasing crosslinker concentration, Carb/NaAlg ratio and percentage of NaAlg. The highest carbaryl release was found to be 100% for the Ni‐Alg beads at 3 days whereas the lowest carbaryl release was found to be 67% for the Ca‐Alg beads at 21 days. The swelling measurements of the beads were also in consistent with the carbaryl release results. The carbaryl release from most of the bead formulations followed Case II transport. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007