Kinetic simulation of methane hydrate formation and dissociation in porous media

The vast amount of hydrocarbon gas deposited in the earth's crust as gas hydrates has significant implications for future energy supply and global climate. A 3-D simulator for methane hydrate formation and dissociation in porous media is developed for designing and interpreting laboratory and field hydrate experiments. Four components (hydrate, methane, water and salt) and five phases (hydrate, gas, aqueous-phase, ice and salt precipitate) are considered in the simulator. The intrinsic kinetics of hydrate formation or dissociation is considered using the Kim-Bishnoi model. Water freezing and ice melting are tracked with primary variable switch method (PVSM) by assuming equilibrium phase transition. Mass transport, including two-phase flow and molecular diffusions, and heat transfer involved in formation or dissociation of hydrates are included in the governing equations, which are discretized with finite volume difference method and are solved in a fully implicit manner. The developed simulator is used here to study the formation and the dissociation of hydrates in laboratory-scale core samples. In hydrate formation from the system of gas and ice (G+I) and in hydrate dissociation systems where ice appears, the equilibrium between aqueous-phase and ice (A-I) is found to have a “blocking” effect on heat transfer when salt is absent from the system. Increase of initial temperature (at constant outlet pressure), introduction of salt component into the system, decrease of outlet pressure, and increase of boundary heat transfer coefficient can lead to faster hydrate dissociation.     

Effect of hydrate formation/dissociation on emulsion stability using DSC and visual techniques

A key factor in hydrate risk management for an oil-dominated system is the stability of the emulsified water with gas hydrate formation. We show via differential scanning calorimetry (DSC) that gas hydrate formation and dissociation has a destabilizing effect on water-in-oil (W/O) emulsions, and can lead to a free water phase through agglomeration and coalescence of dissociated hydrate particles. High asphaltene content crude oils are shown to resist hydrate destabilization of the emulsion. Span80 was successfully used as an analog to asphaltene surface activity. Based on our experimental results, a new conceptual hydrate-induced destabilization model is proposed.     

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.     

天然气水合物热分解Stefan相变数学模拟研究

天然气水合物热分解过程是一个移动边界的Stefan相变问题。在单相连续水合物控制体的守恒积分热传导模型基础上,建立了考虑尖锐移动边界的天然气水合物热分解控制体的界面耦合Stefan能量守恒条件。利用Boltzmann相似变量求得了一半无限大天然气水合物热分解Stefan相变模型的Neumann解,对超越方程进行了单调性证明,确定了Stefan模型解的唯一性。通过实例分析验证了超越方程的单调性和Stefan模型解的唯一性,MATLAB编程实现了水合物体热分解相变过程温度、分解前缘规律性,参数敏感性拟合研究表明,随着温度的升高,λ（超越方程的解）和x_f（界面位置）逐渐增大,x_d（穿越深度）和t_d（穿透时间）逐渐减小。     

Measurements of methane hydrate heat of dissociation using high pressure differential scanning calorimetry

The methane hydrate heat of decomposition was directly measured up to 20 MPa and 292 K using a high pressure differential scanning calorimeter (DSC). The methane hydrate sample was formed ex-situ using granular ice particles and subsequently transferred into the DSC cell under liquid nitrogen. The ice and water impurities in the hydrate sample were reduced by converting any dissociated hydrate into methane hydrate inside the DSC cell before performing the thermal properties measurements. The methane hydrate sample was dissociated by raising the temperature (0.5-1.0 K/min) above the hydrate equilibrium temperature at a constant pressure. The measured methane hydrate heat of dissociation (H→W+G), ΔH_d, remained constant at 54.44±1.45 kJ/mol gas (504.07±13.48 J/gm water or 438.54± 13.78 J/gm hydrate) for pressures up to 20 MPa. The measured ΔH_d is in agreement with the Clapeyron equation predictions at high pressures; however, the Clausius-Clapeyron equation predictions do not agree with the heat of dissociation data at high pressures. In conclusion, it is recommended that the Clapeyron equation should be used for hydrate heat of dissociation estimations at high pressures.     

An improved Clapeyron model for predicting liquid water–hydrate–liquid hydrate former phase equilibria

Determination of pressure–temperature phase diagram for the liquid water–hydrate–liquid hydrate former region is a challenge considering this diagram at these conditions is a strong function of temperature. The Clapeyron model is traditionally used for this purpose. However, the conventional Clapeyron model does not take into account the effect of pressure on the hydrate molar volume as well as the heat of dissolution of hydrate former in water and therefore cannot predict satisfactorily the gas hydrate phase diagram. In this work, the conventional Clapeyron model is extended to take into account the aforementioned factors as well as the change in the slope of the pressure–temperature phase diagram when increasing the temperature. Three hydrate formers, i.e. carbon dioxide, hydrogen sulfide and ethane, are studied. It is found that the effect of these factors on the determination of the gas hydrate phase diagram is not negligible.     

Experimental investigation and modeling on the dissociation conditions of methane hydrate in clayey silt cores

As the majority of global natural gas hydrate reserve, the dissociation conditions of hydrate in clayey silts are of great significance for its efficient production. In this work, the dissociation conditions of methane hydrate in clayey silt cores were experimentally measured by step-heating method at the temperature range of 280.76–289.55 K and pressure range of 8.11–15.03 MPa, respectively. Various cores including quartz powder, montmorillonite, and South China Sea sediments at the water content range of 20%–33% were used for investigation. The results showed that the dissociation temperatures of methane hydrate in clayey silt cores depressed compared to bulk hydrate. The grain size, salinity, and lithology of clayey silt cores significantly affect the dissociation conditions of hydrate. In comparison to grain size, salinity, and lithology had a more significant influence on the equilibrium temperature depression. The dissociation temperature depression of methane hydrate was considered as a consequence of the water activity depression which is caused by the effect of capillary, salt, or clay. A water activity meter was used to measure the water activity in clayey silt cores. The influence of salt component and mineral characteristics on the water activity was investigated. By combining the measured water activity data with the Chen-Guo model, a novel water activity measurement (WAM) method for the hydrate dissociation conditions prediction was proposed. With the maximum deviation less than 12%, the predicted results are in good agreement with the experimental data. It demonstrated that the WAM method could effectively predict the dissociation conditions of methane hydrate in clayey silts with convenience and accuracy.     

Experimental determination of methane hydrate dissociation curve up to 55 MPa by using a small amount of surfactant as hydrate promoter

Methane hydrate equilibrium has been studied upon continuous heating of the water-hydrate-gas system within the temperature range of 275-300 K. This temperature range corresponds to equilibrium pressures of 3.15-55 MPa. The hydrate formation/dissociation experiments were carried out in a high-pressure reactor under isochoric conditions and with no agitation. A small amount of surfactant (0.02 wt% sodium dodecyl sulfate, SDS) was added to water to promote hydrate formation. It was demonstrated that SDS did not have any influence on the gas hydrate equilibrium, but increased drastically both the hydrate formation rate and the amount of water converted into hydrate, when compared with the experiments without surfactant. To understand and clarify the influence of SDS on hydrate formation, macroscopic observations of hydrate growth were carried out using gas propane as hydrate former in a fully transparent reactor. We observed that 10^-3 wt% SDS (230 times less than the Critical Micellar Concentration of SDS) were sufficient to prevent hydrate particles from agglomerating and forming a rigid hydrate film at the liquid-gas interface. In the presence of SDS, hydrates grew mainly on the reactor walls as a porous structure, which sucked the solution due to capillary forces. Hydrates grew with a high rate until about 97 wt% of the water present in the reactor was transformed into hydrate.Our data on methane hydrate equilibrium both confirm already published literature data and complement them within the pressure range of 20-55 MPa.     

Experimental Study of Methane Hydrate Decomposition Kinetics

Experimental investigations of methane hydrate dissociation kinetics were performed. The test rig consists of a stirred reactor equipped with particle size analysis. The observed dissociation rates were found to be about one order of magnitude faster than previously reported by others. A mass transfer control of the dissociation process is proposed to dominate in the proximity of a dispersed hydrate‐liquid interface. The results are relevant for the design of processes employing dispersed gas hydrates in chemical engineering and the production of natural gas from dispersed deposits.     

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.     

Thermodynamic measurement and modeling of hydrate dissociation for CO2/refrigerant + sucrose/fructose/glucose solutions

Hydrate dissociation conditions were studied for the CO₂/refrigerant + sucrose/fructose/glucose solution systems as a continuation of previous work into alternate separation technologies for the sugar manufacturing industries. Experimental data were measured following the isochoric pressure method for the CO₂ + sucrose/fructose solution systems. The refrigerants studied for the modeling purpose were R410a, R507, R134a, and R22 using literature data. The pressure and temperature ranges for the experimental data measured here were (1.80–4.10) MPa and (276.6–282.6) K, respectively, with solutions measured in the composition range between 0 to 0.40 mass fraction sucrose and fructose. Several models following the Van der Waals–Platteeuw solid solution theory were developed to predict the hydrate dissociation conditions of CO₂/fluorinated refrigerant in the presence of sucrose/fructose/glucose solutions. The modeling results provide a satisfactory representation of the experimental data, with AARD(P) % model errors in the overall range between 0.03% and 4.40%.     

Simple predictive relations, fugacities, and enthalpies of dissociation for single guest clathrate hydrates in porous media

We present two sets of explicit relations for the equilibrium fugacities of single-guest gas hydrates. These relations can be applied whether the hydrate is dissociated under bulk conditions, or in porous media. The first set of presented relations explicitly shows the dependence of the equilibrium fugacity and the enthalpy of dissociation on classical statistical thermodynamic parameters. The second set of relations for the fugacity and enthalpy represent a very simple empirical form which can be used to calculate these quantities, without having to resort to the use of the full statistical thermodynamic model. It is hoped that these relations will prove useful in engineering or computational endeavors where the speed and/or ease of their use may be advantageous.     

Heat transfer and kinetics in the pyrolysis of shrinking biomass particle

The impact of shrinkage on pyrolysis of biomass particles is studied employing a kinetic model coupled with heat transfer model using a practically significant kinetic scheme consisting of physically measurable parameters. The numerical model is used to examine the impact of shrinkage on particle size, pyrolysis time, product yields, specific heat capacity and Biot number considering cylindrical geometry. Finite difference pure implicit scheme utilizing tri-diagonal matrix algorithm (TDMA) is employed for solving heat transfer model equation. Runge-Kutta fourth-order method is used for chemical kinetics model equations. Simulations are carried out for radius ranging from 0.0000125 to , temperature ranging from 303 to and shrinkage factors ranging from 0.0 to 1.0. The results obtained using the model used in the present study are in excellent agreement with many experimental studies, much better than the agreement with the earlier models reported in the literature. Shrinkage affects both the pyrolysis time and the product yield in thermally thick regime. However, it is found that shrinkage has negligible affect on pyrolysis in the thermally thin regime. The impact of shrinkage reflects on pyrolysis in several ways. It includes reduction of the residence time of gases within the particle, cooling of the char layer due to higher mass flux rates of pyrolysis products and thinning the pyrolysis reaction region.     

Measurement and prediction of gas hydrate and hydrated salt equilibria in aqueous ethylene glycol and electrolyte solutions

A recently developed method in modelling electrolyte solutions is extended to include phase behaviour of aqueous solutions containing hydrated salts (e.g., calcium chloride) and organic hydrate inhibitors (e.g., ethylene glycol). A novel salt precipitation model applicable to various hydrated salts is presented. The precipitation model takes into account various precipitates of hydrated salts over a wide range of temperature (i.e., -20-120 ^°C). Due to lack of the required experimental data in the literature, new experimental data have been generated. These data, which have been used in determining the binary interaction parameters between salts and organic inhibitors, include; freezing point depression, boiling point elevation, and salt solubility in the aqueous solutions containing salts and organic inhibitors. The extended thermodynamic model is capable of predicting complex vapour-liquid-solid equilibria (VLSE) for aqueous electrolytes and/or organic inhibitor solutions over a wide range of pressure, temperature and inhibitor concentration.In addition, in order to establish the effect of a combination of salts and organic inhibitors on the locus of incipient hydrate-liquid water-vapour (H-L_W-V) curve, reliable equilibrium data have been generated for one quaternary system, methane/water/calcium chloride/ ethylene glycol at pressures up to 50 MPa. These data along with various independent literature data are used to validate the predictive capabilities of the model for phase behaviour and hydrate equilibria. Good agreement between experimental data and predictions is observed, demonstrating the reliability of the developed model.     

Kinetic parameter estimation and modelling of sucrose esters synthesis without solvent

In this paper, a kinetic model for the synthesis of the sucrose esters without solvent is developed. This synthesis is a heterogeneous reaction between the sucrose (solid) and the methyl palmitate (liquid) in the presence of a solid catalyst. We have established the kinetic model by steps, by first considering a homogeneous model and then by taking into account the heterogeneity of the medium. Thus, we have assumed additional steps in the model corresponding to the activation of the solid sucrose by the catalyst and autocatalytic steps. This activation parameter is associated to the viscosity of the medium. As a consequence of these observations, the study of the sensibility and the accuracy of the parameters allowed us to simplify the model.     

水合物降压分解的实验及数值模拟

在天然气水合物一维分解模拟系统上进行模拟多孔介质中水合物降压分解的实验。在考虑天然气水合物分解动力学机理以及流体渗流模型的基础上;建立天然气水合物降压分解的数学模型。利用模型对降压实验进行拟合;获得了多孔介质内水合物分解本征速度常数数量级为10 mol·m^-2·Pa^-1·s^-1;比文献中测定的纯水中水合物分解本征速度常数低3个数量级。对模型进行了参数分析;发现对于实验室规模的一维系统;分解动力学过程控制整个分解过程;而对于现场规模的水合物藏;整个开采过程受水合物藏流动特性的控制;而受水合物分解过程的影响较小。     

Storage composites for the optimisation of solar water heating systems

Composite materials based on expanded natural graphite (CENG) and various phase change materials (PCM) have been developed for low temperature solar applications (323–373 K). The integration of such composite materials directly into the solar collector could allow new storage functionality. A numerical model has been developed to describe the materials behaviour. Composites properties are presented and discussed.     

Experimental measurements and thermodynamic modelling of hydrate phase equilibrium conditions for CF4+TBAB aqueous solutions

Abstract

In this study, CF₄ hydrate dissociation conditions in the presence of different TBAB aqueous solutions with the concentrations of 0.05, 0.10, 0.20, and 0.3 mass fractions were experimentally measured. Measurements were performed using a high pressure equilibrium cell with an approximate inner volume of 40?cm³. Hydrate measurements were performed in the temperature and pressure ranges of (273.8–285.6) K and (1.03–11.57) MPa, respectively. The results show that TBAB aqueous solutions with the concentration of 0.05, 0.10, and 0.20 (mass fractions) do not have a major promotion effect on the CF₄ hydrate phase equilibrium. However, the aqueous solution with 0.30 mass fraction TBAB exhibited a significant promotion effect on CF₄ hydrate formation. The model of Chen and Guo (1998) and Joshi et al. (2012) were used to estimate the hydrate dissociation conditions. The errors between the experimental and calculated data resulted in acceptable absolute relative deviations (ARD%) below 0.1%.     

Simulation and theory of the impact of two-dimensional elastic disks

The impact of a two-dimensional elastic disk with a wall is numerically studied. It is clarified that the coefficient of restitution (COR) decreases with the impact velocity. The result is not consistent with the recent quasi-static theory of inelastic collisions even for very slow impact. This suggests that the elastic model cannot be used in the quasi-static limit. A new quasi-static theory of impacts is proposed, in which the effect of thermal diffusion is dominant. The abrupt decrease of COR has been found due to the plastic deformation of the disk, which is assisted by the initial internal motion.     

全可视化反应釜内二氧化碳水合物的生成分解特征

二氧化碳水合物封存技术已成为目前碳封存研究的热点。该技术中对于二氧化碳水合物的生成分解特征及其影响因素的研究是当前的重点和难点。本文设计了高压全透明双反应釜实验平台,以高纯度二氧化碳和去离子水作为研究对象,在17℃、7MPa的初始温压条件下,进行了二氧化碳水合物的初次和二次生成分解实验,并设置对照组对搅拌的影响进行了研究,而后与甲烷在相同条件下的实验进行对比。实验结果表明,搅拌会促进二氧化碳水合物的生成,在400r/min的转速条件下,缩短诱导时间可达40%,增大压降速率可达15%,形成更多且更致密厚实的水合物,并延缓了分解;多次生成可以减少水合物的诱导时间,但对于水合物生成的总量几乎没有影响。与甲烷水合物相比,二氧化碳水合物生成的量大且更难以分解,实验结果有利于二氧化碳的海洋水合物封存技术的开发应用。