Experimental and modeling study of the kinetics of methane hydrate formation and dissociation

In this work, several experiments were conducted at isobaric and isothermal condition in a CSTR reactor to study the kinetics of methane hydrate formation and dissociation. Experiments were performed at five temperatures and three pressure levels (corresponding to equilibrium pressure). Methane hydrate formation and dissociation rates were modeled using mass transfer limited kinetic models and mass transfer coefficients for both formation and dissociation were calculated. Comparison of results, shows that mass transfer coefficients for methane hydrate dissociation are one order greater than formation conditions. Mass transfer coefficients were correlated by polynomials as relations of pressure and temperature. The results and the method can be applied for prediction of methane production from naturally occurring methane hydrate deposits.     

Determination of effective diffusion coefficient and interfacial mass transfer coefficient of bovine serum albumin (BSA) adsorption into porous polyethylene membrane by microscope FTIR-mapping study

Here, a new method for simultaneous determination of diffusion coefficient D and interfacial mass transfer coefficient (or convective mass transfer coefficient) k was proposed for bovine serum albumin (BSA) adsorption into porous polymeric membranes. The experimental data for BSA concentration at different membrane depth and different time were determined from FTIR-mapping measurements. Then the diffusion coefficient D and interfacial mass transfer coefficient k were estimated from the calculated dimensionless concentration data at different time and membrane depth by a trial-and-error method based on the diffusion equation initiated in this paper. The diffusion coefficient D and interfacial mass transfer coefficient k evaluated in this manner are respectively: and . The theoretical concentration values calculated from the determined parameters were compared with experimental reading from FTIR mappings, which showed a good agreement between them, especially for the case of a relatively long-time adsorption.     

The dissolution of a stationary spherical bubble beneath a flat plate

The dissolution of a single stationary bubble held in place by a horizontal plate is commonly observed experimentally. For several decades the standard approach to the analysis of such dissolution data has been to apply a correction factor of ln(2)=0.69 to the Epstein-Plesset equation for an isolated bubble. In this paper, the transport equations for a stationary bubble touching a plate are solved numerically for the common case where the flow field caused by the change in system volume as the bubble dissolves can be neglected. It is found that the total bubble lifetime is not well characterised by the use of the ln(2) factor. However, in most experimental situations, the initial stages of bubble dissolution are not captured. For low gas solubilities the use of a correction factor of 0.69 to the Epstein-Plesset equation is appropriate once the initial transients have dissipated. The correction factor varies from 0.69 to 0.77 across the full range of situations described in this paper. The mathematical model is validated by comparison to experimental data.     

Experimental and Theoretical Investigation of Mass Transfer during Leaching of Starch and Protein from Potato

In this article, mass transfer during leaching of starch and protein from potato using water as the solvent was investigated. Leaching experiments were carried out by immersion of slab-shaped potato samples in distilled water at 30°C, 45°C, and 55°C, and amounts of starch and protein loss, and moisture gain by the samples were evaluated at different process duration times. A two-parameter model was proposed for prediction of the kinetics of mass transfer and also estimation of the final equilibrium values of solutes loss and moisture gain by the samples. Effective diffusivities of solutes and moisture were estimated by fitting the experimental data of solute loss and moisture gain to the analytical solution of Fick's second law of diffusion. Moisture, starch, and protein distributions into the potatoes were predicted as a function of time and location into the samples. Mean relative errors (MREs) between the predicted concentrations by the proposed models and experimental data were in the ranges of 0.037–0.104 and 0.063–0.205 for the two-parameter model and the analytical solution, respectively. Estimated equilibrium solute losses were between 0.922 and 1.549 (g/100 g fresh fruit) for protein and between 10.893 and 11.848 (g/100 g fresh fruit) for starch. Also equilibrium water gain (WG) was in the range of 35.842–46.296 (g/100 g fresh fruit). Effective diffusivities were estimated in the range of 1.334–1.817 × 10^?10 m²/s for moisture and 1.070–1.893 × 10^?10 m²/s for solutes.     

Dehydration and rehydration of mesolite: An in situ FTIR study

The dehydration and rehydration processes of mesolite belonging to NAT group of zeolites were investigated using in situ Fourier Transform Infrared spectroscopy (FTIR). The thermal induced variations of the water molecule bending (ν₂), the stretching (ν₃ and ν₁) modes and the corresponding second order modes in the wavenumber region 4000–8000 cm⁻¹ were followed as indicative of the dehydration process. Observed spectral variations were well correlated with thermogravimetric studies and indicate that the mesolite dehydrates in three stages (470; 510 and 650 K). Anomalous spectral variations of 4600 cm⁻¹ at first stage indicate that the dehydration is triggered by the expulsion of water coordinating AlO₄ tetrahedron. Partial rehydration (up to 85%) at second stage indicates disordering of natrolite and scolecite layers. During the third stage mesolite completely dehydrates, causing the destruction of framework.     

The measurement of the diffusion coefficient and the sorption isotherm of water in paint films

An experimental technique together with a numerical model is proposed with which the diffusion coefficient and the sorption isotherm of water in paint can be measured. Inside a closed vessel, paint films are on stainless-steel plates. Water is present as water vapour in the air and in the paint. After blowing dry or wet air through the vessel for some time, the situation moves to a new equilibrium. The relative humidity of the air inside the vessel is measured as a function of time. From fitting the theoretical/numerical model against the experimental values, follow the diffusion coefficient and the sorption isotherm of water in the paint. The results show large scattering. When the independently measured sorption isotherm is used as an input parameter in the model, the fitting procedure gives much smaller scattering for the diffusion coefficient.     

Determination of diffusion coefficient from transitory uptake or release kinetics: Incidence of a recirculation loop

The determination of the diffusion coefficient of a solute in a solid matrix (polymers, gels, adsorbents) is often determined based on batch kinetics experiments performed when a given amount of matrix is put into a stirred cell containing a defined volume of diluting fluid (gas or liquid). There is, however, a strong trend to develop smaller and smaller volume set-ups (due to, for instance, the cost of either the solute or the matrix in biological or pharmaceutical fields). Furthermore, a continuous monitoring of the solute concentration in the release (or exhaustion) medium is obviously attractive and frequently applied, in place of a sequential manual sampling protocol. This option demands, however, most often a continuous recirculation loop to be implemented on the cell volume, so that a continuous detector (such as spectrophotometric, conductimetric or refractive index) can be used. The objective of this work is to analyze the influence of such a recirculation loop, which may affect significantly the hydrodynamics (fluid residence time distribution) as well as the dynamics (delayed signal) of the experimental kinetics, in order to evaluate the impact on the diffusion coefficient determination. Numerical simulations covering a broad range of situations have been performed, and an experimental validation on a system consisting of alginate hydrogel beads as a model matrix and pullulan molecules (molecular weight ranging between 730 and 880 000) as model solute is reported. Practical guidelines are finally proposed in order to estimate, through ab initio shortcut methods, the error induced by the recirculation loop, based on explicit experimental parameters (cell/loop volume, recirculation loop residence time, solute diffusion time constant).     

Measurements and analysis of oxygen bubble distributions in LiCl–KCl molten salt

Transparent system experimental studies have been performed to provide measurement and analysis of oxygen bubble distributions at different sparging rates in LiCl–KCl molten salt at 500 °C using a high-speed digital camera and an oxygen sensor. The results reveal that bubble sizes and rise velocities increased with an increase in oxygen sparging rate. The bubbles observed were ellipsoidal in shape, and an equivalent diameter based on the ellipsoid volume was calculated, ranging from 0.00263 m to 0.00407 m. Results also show that the bubble equivalent diameters are normally distributed. A Fanning friction factor correlation was used to predict a bubble's rise velocity. The oxygen mass transfer coefficients were calculated using the oxygenation model; these values were on the order of 10⁻⁴ m/s and followed a decreasing trend corresponding to an increasing bubble size and sparging rate. The diffusivities were calculated based on two different approaches—one based on physics of the bubbles and the other on systematic properties. The diffusivity values calculated from bubble physics are 1.65 × 10⁻⁹ m²/s to 8.40 × 10⁻⁹ m²/s, which are within the range suggested by literature for gases in liquids of a similar viscosity.     

Modelling of alcohol and water diffusion in fuel cell membranes—Experimental validation by means of in situ Raman spectroscopy

In this work a multi-component transport model has been set up to describe the diffusion driven mass transport of water and methanol in fuel cell membranes. For a membrane in contact with liquid methanol and water on one side and conditioned air on the other, the corresponding differential equations and boundary conditions were derived in a polymer-related coordinate system taking into account the polymers three-dimensional swelling. Phase equilibrium parameters and unknown diffusion coefficients for Nafion^® 117 were obtained by comparing the simulation results to water and methanol concentration profiles measured with confocal Raman spectroscopy. The influence of methanol concentration, temperature and air flow rate was predicted by the model with a maximum relative mean deviation between measurement and simulation of 8.6% for methanol and 3.4% for water.     

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.     

Dissolution of a solid sphere in an unbounded, stagnant liquid

An exact analytical solution is obtained for the transient dissolution of solid spheres in a diffusion-controlled environment. This result provides a useful reference point for drug testing in humans. The dimensionless solution is expressed in terms of a single parameter, which accounts for solubility, bulk flow, and stagnant fluid composition. A simple, explicit and exact expression was found to predict time-to-complete dissolution (TCD). An approximate solution was also found which tracks the exact case for low solubility conditions.     

Analysis of the Shrinkage Effect on Mass Transfer During Convective Drying of Sawdust/Sludge Mixtures

Convective drying of wastewater sludges and sawdust/sludge mixtures was studied. The first part of this work was an experimental study performed in a cross-flow convective dryer using 500 g of wet material extruded through a disk with circular dies of 12 mm. The results showed that the sawdust addition has a positive impact on the drying process from a mass ratio of 2/8, on a dry basis, with observed drying rates higher than the original sludge. The second part of this work consisted of developing a drying model in order to identify the internal diffusion coefficient and convective mass transfer coefficient from the experimental data. A comparison was made between fitted drying curves, well represented by the Newton's model, and the analytical solutions of the diffusion equation applied to a finite cylinder. Variations of dimensional characteristics, such as the volume and exchange surface of the sample bed, were obtained by X-ray tomography. This technique allowed us to confirm that shrinkage, which is an important phenomenon occurring during sludge and sawdust/sludge mixture drying, must be taken into account. The results showed that both the internal diffusion coefficient and convective mass transfer coefficient were affected by mixing and sawdust addition. The internal diffusion coefficient changed from 7.77 × 10^?9 m²/s for the original sludge to 7.01 × 10^?9 m²/s for the mixed sludge and then increased to 8.35 × 10^?9 m²/s for the mixture of a mass ratio of 4/6. The convective mass transfer coefficient changed from 9.70 × 10^?8 m/s for the original sludge to 8.67 × 10^?8 m/s for the mixed sludge and then increased to 12.09 × 10^?8 m/s for the mixture of a mass ratio of 4/6. These results confirmed that sawdust addition was beneficial to the sludge drying process as the mass transfer efficiency between the air and material increased. Reinforcing the texture of sludge by adding sawdust can increase the drying rate and decrease the drying time, and then the heat energy supply will be reduced significantly. The study also showed that neglecting shrinkage phenomenon resulted in an overestimation for the internal diffusion coefficient for the convective drying of sludges and sawdust/sludge mixtures.     

Ozone Mass Transfer in Stirred Vessel

A method is proposed and a model is developed which are capable of providing a correlation of the mass transfer coefficient k_La, with stirrer speed and gas superficial velocity. The method can be adopted for deriving a correlation which can be profitably used for ozone gas–liquid reaction both for assessing the absorption regime and for the simulation of oxidation processes which evolve according to slow reaction regime.     

Thermal diffusion kinetics of cesium in ceramic microcell UO2 fuels for accident-tolerant fuel

This work investigates the diffusion kinetics of cesium in ceramic microcell UO₂ fuels at temperatures ranging from 650 to 1200 °C to predict its release behavior under actual fuel circumstances. Diffusion couples containing ceramic microcell UO₂ pellets with silica-based cell wall compositions (with and without Al) were prepared and subjected to a heat treatment with abundant gaseous cesium source. From the experiment, kinetic data were obtained in the form of an Arrhenius equation: the addition of Al lowers the degree of cesium diffusivity. The favorable contribution of Al may stem from the formation of the cesium compound as a solid form of Cs-Al-Si-O. The findings here provide a diffusion model describing diffusion mechanism of cesium in the cell wall. Also, this study suggest an evaluation method that can be used in UO₂ fuels with trapping agents from a kinetic standpoint.     

Experimental study of mass transfer limited reaction—Part II: Existence of cross-over phenomenon

In Part II, we extend the inverse methodology which is discussed in Part I to various experiments performed in a tubular reactor to infer mass transfer coefficients. Mass transfer coefficients, inferred from the concentrations of the reactants which are extracted from the absorbance of the reaction mixture using multicomponent spectrum analysis, are then used to solve the convection-diffusion-reaction equations for the concentrations of the reactants in the bulk phase and in the dispersed phase, to explore the possibility of cross-over for a mass transfer limited reaction. Experiments are performed incorporating the asymmetry in the transport rates of the premixed reactants, which is the potential reason for the existence of cross-over. The different mass transfer coefficients of the two premixed reactants indeed indicate a switch in the concentration of the reactants in the dispersed phase, which is termed as “cross-over”. The experimental results are further analysed by validating the theoretical criterion proposed by Mchedlov-Petrossyan et al. (2003, Chem. Eng. Sci. 58, 3005-3023 & 2691-2703) to obtain the parametric space for the existence of cross-over, in order to optimize the length of the tubular reactor.     

Molecular diffusion in the laminar sub-layer during turbulent flow in a smooth tube

A model is proposed that can predict the fully developed mass transfer coefficient for a turbulent flowing fluid in a smooth tube. Model results are compared to experimental data over a range of Reynolds number from 7750 to 50,200 and for Schmidt numbers of 850, 900, and 2400. Also, the model prediction is compared to an existing correlation that was derived from experimental data. In all cases the model prediction is in excellent agreement with the published experimental data and is an improvement over the existing correlation.     

Experimental data and modeling of rice bran oil extraction kinetics using ethanol as solvent

The extraction kinetics of rice bran oil (RBO), free fatty acids (FFA), and oryzanol using ethanol (0 and 6.3 mass % of water) at 40°C–70°C were investigated. High extraction temperatures increased the yields of RBO and oryzanol by increasing the diffusivity of the solvent, regardless of its water content. Two models that permitted the estimation of mass transfer and diffusion coefficients were fitted to the oil extraction data with low average relative deviations (≤5.92%). The diffusion coefficient (1.93–7.46 × 10^–10 m²?s^–1) increased with increasing temperature and decreasing hydration of the solvent.     

并流降膜式多管湿壁塔的质量传递特性

在内径为0.1 m,由38根内径为8 mm,外径为10 mm,高120 mm陶瓷管组成的多管湿壁塔中,采用SO2/NaOH和CO_2/H_2O系统,研究了20℃的气液传质速率。实验结果表明,液相雷诺数在Re_L在55~165之间,气相雷诺数Re_G在360~2 000之间,液相舍伍德数分别与Re_G的0.662次方和Re_L的0.851次方成正比;气相舍伍德数分别与Re_G的1.103Re_L的0.226次方成正比。