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.     

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.     

Dominant design variables in pyrolysis of biomass particles of different geometries in thermally thick regime

In this study, a simultaneous chemical kinetics and heat transfer model is used to predict the effects of the most important physical and thermal properties (thermal conductivity, heat transfer coefficient, emissivity, reactor temperature and heat of reaction number) of the feedstock on the convective-radiant pyrolysis of biomass fuels for different geometries such as slab, cylinder and sphere. The pyrolysis rate is simulated by a kinetic scheme involving two parallel primary reactions and a third secondary reaction between volatile and gaseous products and the char. Finite difference pure implicit scheme utilizing the Tri-Diagonal Matrix Algorithm (TDMA) is employed for solving heat transfer model equation. Runge-Kutta fourth order method is used for solving the chemical kinetics model equations. Simulations are carried out for different geometries considering the equivalent radius ranging from to , and the temperature ranging from to .For conversion in the thermally thick regime (intra-particle heat transfer control), it is found that variations in the properties mainly affect the activity of primary reactions. The highest sensitivity is associated with reactor temperature and emissivity. Applications of these findings in reactor design and operation are discussed. The results obtained using the model used in the present study are in excellent agreement with many reported experimental studies, much better than the agreement with earlier models reported in the literature.     

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.     

Notes on the dissolution rate of gas hydrates in undersaturated water

An elementary model for the dissolution of pure hydrate in undersaturated water is proposed that combines intrinsic decomposition within a desorption film and the subsequent diffusion of the released hydrate guest species into bulk water. Applying the proposed approach to recently published measurements of the decomposition rates of methane (CH₄) and carbon dioxide (CO₂) hydrates in deep seawater suggests that the concentration of the hydrate guest species at the interface between desorption film and diffusive boundary layer may be much lower than ambient solubility. Calculations, however, fail to account for the observed proportionality of decomposition rate with solubility for both CH₄ and CO₂ hydrates. This may indicate a limitation in the range of applicability of published formulas for intrinsic hydrate decomposition rates.     

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.     

Modelling of phenolic resin polymerisation

The kinetics of the polymerisation of resol-type phenolic resins has been modelled taking into account the phenol and formaldehyde equilibria. The kinetic parameters were obtained by adjusting the experimental evolution of phenol, formaldehyde and initial addition products during synthesis. The influence of the type and amount of catalyst, the initial pH, the initial formaldehyde-to-phenol molar ratio and the condensation temperature, on the kinetics rate constants was quantified.     

LNG管道内气液相变流动传热理论的研究现状及趋势

针对液化天然气管道内流动传热理论的研究现状及趋势进行相关调研,结果表明管道内液化天然气的相变问题十分严峻,严重影响着管道的运输能力和运输安全性。为缓解这些问题的发生,主要从液化天然气的管道运输特点、液化天然气相变机理与特性以及液化天然气流型与传热三个方面进行分析比较,指出了存在的问题以及未来的发展前景,对液化天然气管道内相变流动传热的研究提供理论基础。     

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.     

Phase behavior and structure transition of the mixed methane and nitrogen hydrates

Pure methane and nitrogen form structure I and II hydrate, respectively, and therefore the structure type of mixed gas hydrate was found to largely depend on their relative gas composition. In addition to the structural difference of size and shape, each hydrate structure shows different capacity to store the guest molecules. In this study, we investigated phase and structural behaviors according to the composition of methane+nitrogen gas mixture. Three-phase (H-Lw-V) equilibria of solid hydrate, water-rich liquid and vapor phase containing 25.24 mol%, 28.51 mol%, 31.23 mol% and 40.39 mol% of methane were determined at various temperatures (in the range from 273.30 K to 285.05 K) and pressures (from 8.325 MPa to 20.700 MPa). 13C solid-state NMR spectroscopy and powder XRD method were performed to identify the formed structure of hydrate samples. The experimental results showed that gas hydrate of the methane+nitrogen mixture changes its structure from sI to sII between 25.24 mol% and 28.51 mol% of methane concentration. These results of phase behavior and structure identification for the mixed gas hydrates are expected to be very helpful in evaluating the feasibility of exploitation of methane gas from natural gas hydrate and the separation process using gas hydrate as a storage-media     

TRANSIENT HEAT CONDUCTION EXTERIOR TO A CYLINDRICAL SURFACE

An analytical expression is obtained for the temperature surrounding a cylindrical surface of finite length. The boundary condition is that of constant heat flux and the method of solution involves Laplace and Fourier transforms.     

Solution homopolymerizations of n-butyl acrylate and styrene mediated using 2,2,5-trimethyl-4-tert-butyl-3-azahexane-3-oxyl (TITNO)

Use of Styryl-TITNO (the styrene alkoxyamine of 2,2,5-trimethyl-4-tert-butyl-3-azahexane-3-oxyl) as mediator for nitroxide-mediated polymerization (NMP) of n-butyl acrylate (BA) and styrene has been investigated at temperatures ≤110 °C. Very good control of molecular weight and molecular weight dispersity with no measurable loss of active chains, and no evidence of tails in the molecular weight distribution as conversion increases, was observed at 90 °C for BA and at 70 °C for styrene. The alkoxyamine dissociation equilibrium constant values determined for polyBA-TITNO (8.5 × 10⁻¹¹ mol L⁻¹ at 90 °C) and polystyrene-TITNO (3.1 × 10⁻⁹ mol L⁻¹ at 70 °C) are consistent with those required for control when using more established nitroxides that require higher temperatures to achieve these values. The lower optimum polymerization temperatures with Styryl-TITNO as mediator provide new opportunities for NMP and are especially significant for styrene since this appears to eliminate completely the complications from thermal initiation.     

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.     

HEAT DISSIPATION IN MICROELECTRONIC SYSTEMS USING PHASE CHANGE MATERIALS WITH NATURAL CONVECTION

The present study is an initial investigation of the use of phase change materials with natural convection for the dissipation of thermal energy in a model of an integrated circuit (IC) acting as a heat source. A 1.91cm diameter long copper cylinder filled with a phase change material (PCM), P-116 Sunoco wax, was used as the heat exchange structure.

The effectiveness of the PCM cylinder was found to be a strong function of the geometrical arrangement. With the cylinder of PCM positioned on top of the heating cartridge (which simulated the IC heat source), the gravitationally induced natural convection currents are shown to play a significant role in dissipating the heat generated by the cartridge. In an inverted position the effects of natural convection currents were not apparent. The effective thermal conductivity of the cylinder containing the melted wax was at least an order of magnitude higher when positioned above the heating cartridge than when positioned below. Pictures of the melting process were taken for three different experimental configurations, and these substantiate the existence of the natural convection currents in the melt.     

A Kind of Nanofluid Consisting of Surface-Functionalized Nanoparticles

A method of surface functionalization of silica nanoparticles was used to prepare a kind of stable nanofluid. The functionalization was achieved by grafting silanes directly to the surface of silica nanoparticles in silica solutions (both a commercial solution and a self-made silica solution were used). The functionalized nanoparticles were used to make nanofluids, in which well-dispersed nanoparticles can keep good stability. One of the unique characteristics of the nanofluids is that no deposition layer forms on the heated surface after a pool boiling process. The nanofluids have applicable prospect in thermal engineering fields with the phase-change heat transfer.     

CONJUGATE HEAT TRANSFER WITH TURBULENT FLOW IN A CIRCULAR TUBE

A steady heat transfer problem has been analyzed as a conjugate problem with turbulent flow in a circular tube. The three kinds of thermal boundary conditions considered here are specified as constant temperature, constant heat flux and constant heat transfer coefficient at the outer surface of the wall.

From the results of numerical calculation for Prandtl numbers in the range 0.01 ≤ Pr ≤ 10 and for Reynolds numbers in the range 10⁴ ≤ Re ≤ 10⁵, it was confirmed that the dimensionless parameter R_c could have significant effects on the heat transfer and the temperature field in the fluid adjacent to the wall.     

Simulation and Prediction of Methyl‐mercaptan Removal by Chemical Scrubbing with Hydrogen Peroxide

A model has been developed in order to predict the behavior of methylmercaptan chemical scrubbing using hydrogen peroxide. New constants concerning reactions involving methylmercaptan have been proposed based on mass transfer and kinetic equations for the dissociation of methylmercaptan by the hydroxide and the reaction with perhydroxyl ions. Once this step has been achieved, the model is used to characterize the influences of the different chemical scrubbing operating parameters, i.e., the gas‐liquid contact time, the oxidant concentration, and the pH. As expected, an increase in the hydrogen peroxide concentration, the contact time, or the pH leads to an increase in the methylmercaptan removal level. However, the most interesting aspect of the model lies in the possibility of defining a new or limiting value for each parameter, below which the process is not efficient enough to be industrially implemented.