Effect of surfactant on the formation and dissociation kinetic behavior of methane hydrate

The effects of anionic surfactant sodium dodecyl sulfate (SDS) on the formation/dissociation kinetic behaviors of methane hydrate have been studied experimentally, with an emphasis put on dissociation kinetic behavior below ice point. The experimental results on hydrate formation show that the formation rates of methane hydrate could be speeded up by adding SDS to water and a critical SDS concentration of 650 ppm corresponding to a maximum storage capacity of 170V/V is determined. The SDS concentrations are fixed at this value in preparing hydrate samples for all dissociation tests. The dissociation experiments have been performed in two ways, at atmospheric pressure where the dissociation rates are determined by measuring the accumulative evolved gas volume, and in a closed system where the dissociation rates are determined by measuring the increasing system pressure profiles. For comparison, the dissociation tests with respect to two different cases, with and without the presence of SDS, are done in parallel. The results from tests in the first way show that the presence of SDS increases the dissociation rate of methane hydrate in whole temperature region below ice point. The results for the second way are somewhat different. The presence of SDS increases the dissociation rate and meta-stable system pressure in temperature region lower than . But when temperature is equal to or higher than , SDS speeds up the dissociation process only in beginning period, it turns to suppress the dissociation of methane hydrate several hours later and leads to a lower meta-stable system pressure compared with the case of without SDS. The experiments in closed system also demonstrate that the dissociating system approaches a meta-stable state with a pressure much lower than equilibrium dissociation pressure.     

Methane hydrates bearing synthetic sediments—Experimental and numerical approaches of the dissociation

The production of methane gas from methane hydrate bearing sediments may reach an industrial scale in the next decades owing to the huge energy reserve it represents.However the dissociation of methane hydrate in a porous medium is still poorly understood and controlled: the melting of methane hydrate involves fluids flows and heat transfer through a porous medium whose properties evolve as the hydrate phase disappears, and is replaced (or not) by an ice phase. Mass and heat transfers can be coupled in a complex way, firstly because of the permeability changes, and secondly due to material conduction changes. In our work, mass and heat transfers have been studied both experimentally and numerically.A 2D numerical model is proposed where heat and mass transfers govern the dissociation of methane hydrate. This model has been used to design an experimental device. Experiments have been obtained and finally the model has been validated.The experimental set-up consists of five cylindrical sand packs having the same diameter but different lengths. Each experiment starts by crystallizing a hydrate phase in a porous medium. Then the hydrate is dissociated by controlling the pressure at one boundary. The kinetic of dissociation is monitored by collecting gases in ballast. Simulations and experiments demonstrate that the dissociation limiting step switches from thermal transfer to mass transfer depending on the initial permeability and conductivity of the porous medium.     

Reaction rate constant of methane clathrate formation

Particle size distribution measurements were performed during the growth stage of methane hydrate formation in a semi-batch stirred tank crystallizer. Experiments were carried out at temperatures between 275.1 and 279.2 K and pressures ranging from 3873 to 5593 kPa. The reaction rate constant of methane hydrate formation was determined using the model of Bergeron and Servio (AIChE J 2008;54:2964). The experimental reaction rate constant was found to increase with temperature, following an Arrhenius-type relationship, from 8.3 × 10⁻⁸ m/s to 6.15 × 10⁻⁷ m/s over the 4° range investigated, resulting in an activation energy of 323 kJ/mol. An increase in pressure of approximately 600 kPa did not have any effect on the reaction rate constant. Population balances, based on the measured critical nuclei diameter and that predicted by homogeneous nucleation theory, were also used for comparison purposes. The initial number of hydrate particles was calculated using the mole fraction of methane in the bulk liquid phase and compared to that predicted by an energy balance.     

Methane hydrate formation and an inward growing shell model in water-in-oil dispersions

Significant factors controlling gas hydrate growth in water and water-in-oil dispersions have been tested. In particular, the influence of shear rate, presence of oil, and thermodynamic driving force (represented by pressure supersaturation) on hydrate growth rates is included. Formation rates in water show some discrepancy compared to previous work, which is likely caused by differences in the apparatus geometries. A model is proposed for growth of hydrate in oil, in which a hydrate shell forms on a water droplet, followed by additional conversion of the water core to hydrate.     

The dual action of N2 on morphology regulation and mass-transfer acceleration of CO2 hydrate film

The morphology characteristics of CH₄, CO₂,and CO₂+N₂ hydrate film forming on the suspending gas bubbles are studied using microscopic visual method at supercooling conditions from 1.0 to 3.0 K. The hydrate film vertical growth rate and thickness along the planar gas-water interface are measured to study the hydrate formation kinetics and mass transfer process. Adding N₂ in the gas mixture plays the same role as lowering the supercooling conditions, both retarding the crystal nucleation and growth rates, which results in larger single crystal size and rough hydrate morphology. N₂ in the gas mixture helps to delay the secondary nucleation on the hydrate film, which is beneficial to maintain the porethroat structure and enhance the mass transfer. The vertical growth rate of hydrate film mainly depends on the supercooling conditions and gas compositions but has weak dependence on the experimental temperature and pressure. Under the same gas composition condition, the final film thickness shows a linear relationship with the supercooling conditions. The mass transfer coefficient of CH₄ molecules in hydrates ranges from 4.54 × 10^-8to 7.54 × 10^-8mol·cm^-2·s^-1·MPa^-1. The maximum mass transfer coefficient for CO₂+N₂ hydrate occurs at the composition of 60% CO₂+40% N₂, which is 3.98 × 10^-8mol·cm^-2·s^-1·MPa^-1.     

MASS TRANSFER EFFECTS ON THE ETCH RATE OF GAAS IN FLOW SYSTEMS

In industrial wet etching reactors, the fluid contacts the substrate surface as a spray of flowing stream, thus introducing mass-transfer resistances to the reaction rate. The etching of gallium arsenide in H₂O₂-NH₄OH-H₂O solutions was studied using an open-channel flow reactor to simulate the industrial conditions. The etch rate was always lower than that obtained under kinetic control, and the dependence of etch rate on H₂O₂ concentration shifted closer to first order. From the calculation of the ratio of rate constant to mass-transfer coefficient, the reaction-rate and mass-transfer resistances were both significant in this system. When the mass-transfer coefficient was calculated from equations for flow past a flat plate, the prediction of etch rate was good, particularly when the starting length for velocity boundary layer development ahead of concentration boundary layer development was taken into account. Another approach for the calculation of mass-transfer coefficient, based on the assumptions for flow between parallel plates, best represented the relative insensitivity of etch rate to fluid velocity.     

MASS TRANSFER EFFECTS ON THE ETCH RATE OF GAAS IN FLOW SYSTEMS

In industrial wet etching reactors, the fluid contacts the substrate surface as a spray of flowing stream, thus introducing mass-transfer resistances to the reaction rate. The etching of gallium arsenide in H₂O₂-NH₄OH-H₂O solutions was studied using an open-channel flow reactor to simulate the industrial conditions. The etch rate was always lower than that obtained under kinetic control, and the dependence of etch rate on H₂O₂ concentration shifted closer to first order. From the calculation of the ratio of rate constant to mass-transfer coefficient, the reaction-rate and mass-transfer resistances were both significant in this system. When the mass-transfer coefficient was calculated from equations for flow past a flat plate, the prediction of etch rate was good, particularly when the starting length for velocity boundary layer development ahead of concentration boundary layer development was taken into account. Another approach for the calculation of mass-transfer coefficient, based on the assumptions for flow between parallel plates, best represented the relative insensitivity of etch rate to fluid velocity.     

Computational modeling of methane hydrate dissociation in a sandstone core

Hydrate dissociation in a porous sandstone core was studied using a computer modeling approach. It was assumed that the hydrate was dispersed in the pores of the core. Using FLUENT^TM code, an axisymmetric model of the core was developed and solved for multiphase flows during the hydrate dissociation. The core model contained three separate phases: methane hydrate, methane gas, and liquid water. At the start of simulation, the valve at one end of the core was opened exposing the core to low pressure; hydrate began to dissociate and methane gas and water began to flow. The depressurization was controlled by adjusting the pressure of the outlet valve.A comprehensive Users’ Defined Subroutine (UDS) for analysis of hydrate dissociation process into the FLUENT code was developed. The new UDS uses the kinetic model introduced by Kim et al. [Kim. H.C., Bishnoi, P.R., Heidemann, R.A., Rizvi, S.S.H., 1987. Kinetics of methane hydrate decomposition. Chemical Engineering Science 42, 1645-1653.] and can model multiple zones dissociation and multiphase flows. Variations of relative permeability of the core were included using Corey's model. The new model allows for variation of the porosity with hydrate saturation.For different core temperatures and various outlet valve pressures, the spatial and temporal variations of temperature, pressure, and flow fields in the core were simulated. The time evolutions of methane gas and water flow rate at the outlet were also evaluated. It was shown that the rate of hydrate dissociation in a core was a sensitive function of surrounding environment temperature, outlet pressure condition, and permeability.     

天然气水合物生成与流动特性研究进展

随着石油天然气工业不断向深海、极地等极端开采环境发展,天然气水合物已经成为油气开采和输送安全的主要威胁之一。在水合物生成和分解机理、浆液流变特性以及流动压降特性三个方面分别综述了目前的相关研究进展,同时对天然气水合物今后的研究方向提出了几条建议。     

聚酯（PET）反应过程研究：Ⅵ预缩聚反应过程的数学模拟

通过对预缩聚反应器流动模型分析，结合缩聚反应化学平衡，反应动力学和反应器内传质的研究，建立了适用于工业装置的预缩聚反应过程的数学模型     

聚酯（PET）反应过程研究：Ⅶ后缩聚反应过程研究和数学模拟

对聚酯的后缩聚反应过程进行了分析，把该过程分解为反应动力学和反应器内各种传质；对后缩聚反应进行了实验研究。建立了适用于工业装置的后缩聚反应过程的数学模型     

Intensification of nitrous acid oxidation

An alternative solution to the reduction of a discharge of residual nitric oxide and nitrogen dioxide into atmosphere has been proposed. Instead of using methane or ammonia for SCR or gas absorption into alkali solutions, which are the most popular treatment methods of tail gases, now the use of powerful oxidant—ozone capable of transforming nitrous acid and nitric oxides into nitrogen of the highest oxidation level—could be employed for this purpose. As the intensive oxidation and ozonation of nitrous acid is the heterogeneous gas-liquid process, the solubility of oxygen and ozone in HNO₂/HNO₃ aqueous solution was necessary to be determined. Variations of reaction rates depending on temperature, ozone dose and nitrous and nitric acid concentrations were studied experimentally. The kinetic model of the reactions, 2HNO₂+O₂→2HNO₃ and HNO₂+O₃→O₂+HNO₃, were proposed and the kinetic parameters (rate constants and activation energies) were estimated on the basis of experimental data in semi-batch laboratory gas-liquid contactor with the liquid phase drawn from an absorption column in the nitric acid plant. The determined kinetic parameters were then used in designing and modeling of the oxidation of nitrous acid using ozone-oxygen mixture in a continuous bubble column. The model consists of mass transfer kinetic equations and material balance equations for the gas and liquid phases. The co-current flow of gas and liquid phases and the complex kinetics of chemical reaction in the liquid phase were taken into account. The variation of the following process conditions, flow rate, compositions of the gas and liquid phases, temperature, and pressure in the bubble column of different diameters and heights, were studied in numerical solutions of the proposed model.     

The role of diffusional resistance on crystal growth: Interpretation of dissolution and growth rate data

Experimental results on crystal growth and dissolutions kinetics were taken from literature to test two theoretical models describing the crystal growth from solutions. One of the models, designated by two-step model (TSM), is being used for many years to describe the relation between crystal growth rates with supersaturation, temperature and hydrodynamic conditions. The other model was presented in a previous work and will be called concurrent-step model (CSM). The chosen literature data allowed calculating the mass transfer coefficients during crystal growth, for different hydrodynamic conditions. The obtained results are interpreted taking into consideration well-established mass transfer theories. According to the TSM, the measured crystal growth kinetics can only be explained by means of an unrealistic variation of the mass transfer coefficient with the relative crystal-solution velocity. Conversely, mass transfer coefficients obtained by the CSM were confirmed by appropriate semi-theoretical correlations, both in their order of magnitude and in their behaviour. In addition, crystal growth and dissolution experiments of sucrose were carried out at in a batch crystallizer for different agitation speeds. The resulting kinetics are used to test the CSM in a system that is significantly different from the inorganic salts used in the analysed literature works. As predicted by this model, the existence of an adsorbed layer in the crystal surrounding is likely to have affected the solute molecular diffusivity in the medium. Based on this premise, the results obtained with sucrose are well described by the CSM.     

Microscopic observations of clathrate-hydrate films formed at liquid/liquid interfaces. II. Film thickness in steady-water flow

A series of experiments were carried out to measure, with the aid of direct observations through a high-resolution optical microscope, the local thickness of a ring-shaped hydrate film formed over the surface of each discoid drop of HCFC-141b (CH₃CCl₂F) held stationary in a narrow liquid-water channel confined by two transparent, parallel plates, while the experimental system was controlled at a temperature below the hydrate/liquid-water/liquid-HCFC-141b equilibrium temperature, T_tri, at atmospheric pressure. It was revealed that the thickness may be significantly different from place to place over the same hydrate film and that the thickness at each location decreases with an increase in the water flow rate and with a temperature rise. To clarify the dependency of the local film thickness thus measured on the convective mass transfer of the hydrate-guest substance from the film surface to the water flow, we performed numerical simulations of the convective mass transfer and also chromatographic measurements of the solubility of HCFC-141b in liquid water so that we could predict the local mass transfer coefficient and the mass flux of the hydrate guest, HCFC-141b, at any location along the hydrate-film/liquid-water interface. Plotting the film-thickness data obtained at each temperature level against relevant predictions of the mass transfer coefficient or the mass flux, we found the film thickness to be nearly inversely proportional to the mass transfer coefficient or the mass flux. This finding supports the idea that the film thickness is determined by the balance between the rate of hydrate-crystal dissociation induced by the aforementioned convective mass transfer and the rate of hydrate-crystal formation depending on the liquid-water permeation into the hydrate film.     

Modeling of Drying Curves of Silica Nanofluid Droplets Dried in an Acoustic Levitator Using the Reaction Engineering Approach (REA) Model

The use of nanoparticles has become of great interest in different industrial applications. The spray drying of nanofluids forms nanostructured grains, preserving the nanoparticle properties. In this work, individual droplets of silica nanofluids were dried in an acoustic levitator. Tests were carried out under different experimental conditions to study the influence of the variables on the drying process. The drying curves were experimentally obtained and an REA model was used to obtain the theoretical curves and the correlations for the activation energy. The critical moisture content theoretically obtained was used to predict the grain diameter.     

Adsorption of chlorinated hydrocarbons from aqueous solutions by wetted and non-wetted synthetic sorbents: dynamics

In the present investigation the dynamics of the adsorption of several chlorinated hydrocarbons onto wetted and non-wetted synthetic sorbents was studied. A single particle model was developed to describe the adsorption behavior. The values of the mass transfer coefficient, needed to describe the experimental results, were found to be considerably less than those predicted from theory. The difference between measured and expected mass transfer coefficient was found to increase with non-ideality of the solute and to depend on type of sorbent. Apparently, the rate of mass transfer does not only depend on hydrodynamics but also on possible interaction between solute, solvent and sorbent.