Effects of magnetic fields on HCFC-141b refrigerant gas hydrate formation

Gas hydrates are crystalline compounds formed (usually above 0℃) by water reacting with some gases or volatile liquids (hydrate former). Guest molecules, such as gas or volatile liquid molecules, are enclosed firmly inside the host cavities and act with water molecules in weak van der Waals force. Gas hydrate usually includes natural gas hydrate, refrigerant gas hydrate and CO2 gas hydrate. Refrigerant hydrates can be formed above 0℃, and their crystallization is similar to the ordinary ice…     

Influences of different types of magnetic fields on HCFC-141b gas hydrate formation processes

Gas hydrates are crystalline compounds formedwhen gas molecules or volatile liquid molecules comein contact with water molecules through weak van derWaals force at favourable pressure and temperature.Refrigerant gas hydrates can be effectively formed atappropriate temperature (5—12℃) with a high reac-tion heat (320—380 kJ/kg). Because of their particularthermodynamic properties, refrigerant gas hydrate,especially low pressure refrigerant gas hydrate, hasbeen considered as one of the most pr…     

Natural gas hydrate shell model in gas-slurry pipeline flow

A hydrate shell model coupled with one-dimensional two-fluid pipe flow model was established to study the flow characteristics of gas-hydrate slurry flow system. The hydrate shell model was developed with kinetic limitations and mass transfer limitations, and it was solved by Euler method. The analysis of influence factors was performed. It was found that the diffusion coefficient was a key parameter in hydrate forming process. Considering the hydrate kinetics model and the contacting area between gas and water, the hydrate shell model was more close to its practical situations.     

Kinetics of hydrate formation using gas bubble suspended in water

An innovative experimental technique, which was devised to study the effects of temperature and pressure on the rate of hydrate formation at the surface of a gas bubble suspended in a stagnant water phase, was adapted in this work. Under such conditions, the hydrate-growth process is free from dynamic mass transfer factors. The rate of hydrate formation of methane and carbon dioxide has been systematically studied. The measured hydrate-growth data were correlated by using the molar Gibbs free energy as driving force. In the course of the experiments, some interesting surface phenomena were observed.     

Measurement and modeling of the effective thermal conductivity for porous methane hydrate samples

The effective thermal conductivities of gas-saturated porous methane hydrates were measured by a single-sided transient plane source (TPS) technique and simulated by a generalized fractal model of porous media that based on self-similarity.The density of porous hydrate,measured by the volume of the sample in the experimental system,was used to evaluate the porosity of methane hydrate samples.The fractal model was based on Sierpinski carpet,a thermal-electrical analogy technique and one-dimensional heat flow assumption.Both the experimental and computational results show the effective thermal conductivity of methane hydrate decreases with the porosity increase.The porosity of 0.3 can reduce the thermal conductivity of the methane hydrate by 25%.By analysis of the experimental data and the simulative result,the optimized thermal conductivity of the zero-porosity methane hydrate is about 0.7 W m-1K-1.     

Numerical studies of hydrate dissociation and gas production behavior in porous media during depressurization process

In this study,a numerical model is developed to investigate the hydrate dissociation and gas production in porous media by depressurization.A series of simulation runs are conducted to study the impacts of permeability characteristics,including permeability reduction exponent,absolute permeability,hydrate accumulation habits and hydrate saturation,sand average grain size and irreducible water saturation.The effects of the distribution of hydrate in porous media are examined by adapting conceptual models of hydrate accumulation habits into simulations to govern the evolution of permeability with hydrate decomposition,which is also compared with the conventional reservoir permeability model,i.e.Corey model.The simulations show that the hydrate dissociation rate increases with the decrease of permeability reduction exponent,hydrate saturation and the sand average grain size.Compared with the conceptual models of hydrate accumulation habits,our simulations indicate that Corey model overpredicts the gas production and the performance of hydrate coating models is superior to that of hydrate filling models in gas production,which behavior does follow by the order of capillary coating>pore coating>pore filling>capillary filling.From the analysis of t1/2,some interesting results are suggested as follows:(1) there is a "switch" value(the"switch"absolute permeability) for laboratory-scale hydrate dissociation in porous media,the absolute permeability has almost no influence on the gas production behavior when the permeability exceeds the "switch" value.In this study,the "switch" value of absolute permeability can be estimated to be between 10 and 50 md.(2) An optimum value of initial effective water saturation Sw,e exists where hydrate dissociation rate reaches the maximum and the optimum value largely coincides with the value of irreducible water saturation S wr,e.For the case of Sw,Swr,e,there are different control mechanisms dominating the process of hydrate dissociation and gas production.     

The effect of biological and polymeric inhibitors on methane gas hydrate growth kinetics

Kinetic experiments were performed on a methane-water system in the presence of Type-I Antifreeze Proteins (AFPs) in order to elucidate their effectiveness as kinetic hydrate inhibitors, specifically their effect on the hydrate growth period. The results were compared to experiments done with a classical polymeric kinetic hydrate inhibitor, N-vinylpryrrolidone-co-N-vinylcaprolactam [poly(VP/VC)] at the same pressure, temperature and weight percent conditions. As well, a series of experiments was conducted on poly(VP/VC) to examine the effect of concentration on hydrate growth inhibition. Experiments were performed at temperatures between 275.15 and 279.15 K and pressures between 5800 and 7200 kPa. The effect of the polymer on the hydrate growth profile was examined as well as the effect of temperature and pressure on the performance of the polymer and the protein.     

Memory effect on the pressure-temperature condition and induction time of gas hydrate nucleation

The focus of this study is to investigate the influence of memory effect and the relation of its existence with the dissociation temperature, using gas hydrate formation and dissociation experiments. This is beneficial because memory effect is considered as an effective approach to promote the thermodynamic and dynamic conditions of gas hydrate nucleation. Seven experimental systems (twenty tests in total) were performed in a 1 L pressure cell. Three types of hydrate morphology, namely massive, whiskery and jelly crystals were present in the experiments. The pressures and temperatures at the time when visual hydrate crystals appeared were measured. Furthermore, the influence of memory effect was quantified in terms of pressure-temperature-time (p-T-t) relations. The results revealed that memory effect could promote the thermodynamic conditions and shorten the induction time when the dissociation temperature was not higher than 25 ℃. In this study, the nucleation superpressure and induction time decrease gradually with time of tests, when the earlier and the later tests are compared. It is assumed that the residual structure of hydrate dissociation, as the source of the memory effect, provides a site for mass transfer between host and guest molecules. Therefore, a driving force is created between the residual structures and its surrounding bulk phase to promote the hydrate nucleation. However, when the dissociation temperature was higher than 25 ℃, the memory effect vanished. These findings provide references for the application of memory effect in hydrate-based technology.     

Experimental and theoretical investigation of simple gas hydrate formation with or without presence of kinetic inhibitors in a flow mini-loop apparatus

Gas hydrates are ice-like crystalline compounds, which form through a combination of water and suitably sized guest molecules under low temperature and elevated pressure conditions. These solid compounds give rise to problems in the natural gas oil industry because they can plug pipelines and process equipment. Low dosage hydrate inhibitors are a recently developed hydrate control technology, which can be more cost-effective than traditional practices such as methanol and glycols. The kinetics of hydrate growth has been modeled by numerous authors who have measured the gas consumption rate during hydrate formation in batch agitator reactors.     

Intensification of the performance of kinetic inhibitors in the presence of polyethylene oxide and polypropylene oxide for simple gas hydrate formation in a flow mini-loop apparatus

The main objective of the present work is enhancement of the performance of gas hydrate kinetic inhibitors in the presence of polyethylene oxide (PEO) and polypropylene oxide (PPO) for simple gas hydrate formation in a flow mini-loop apparatus. PEO and PPO are high molecular weight polymers that are not kinetic inhibitors by their self. For this investigation, a laboratory flow mini-loop apparatus was set up to measure the induction time and rate of gas hydrate formation when a hydrate-forming substance (such as C1, C3, CO₂ and i-C4) is contacted with water containing dissolved inhibitor in presence or absence of PEO or PPO under suitable temperature and pressure conditions. In each experiment, water containing inhibitors blend saturated with pure gas is circulated up to a required pressure. Pressure is maintained at a constant value during experimental runs by means of required gas make-up. The effect of PEO and PPO on induction time and gas consumption during hydrate formation is investigated in the presence or absence of PVP (polyvinylpyrrolidone) and l-tyrosine as kinetic inhibitors. Results were shown that the induction time is prolonged in the presence of PEO or PPO compared to the inhibitor only. Inclusion of PPO into a kinetic hydrate inhibitor solution shows a higher enhancement in its inhibiting performance compare to PEO. Thus, the induction time for simple gas hydrate formation in presence of kinetic hydrate inhibitor with PPO is higher, compare to kinetic hydrate inhibitor with PEO.     

Nucleation,growth, and fractal dimensions of icosahedral and polyicosahedral clusters

In this paper, the concept offractal geometry was applied to the growth of icosahedral and polyicosahedral clusters. Fractal dimensions were calculated for high-frequency icosahedral casters, vertex-sharing polyicosahedral clusters, and linked polyicosahedral clusters. These cluster growth pathways were compared with the fractal growth mechanisms of colloidal particles. Close similarities were found despite the tremendous differences in particle sizes and the forces governing their nucleation and growth processes.Dedicated to Prof. Lawrence F. Dahl on his 65th birthday; based in part on a lecture presented by BKT at the Dahl symposium held at the University of Wisconsin, Madison, Wisconsin on September 17, 1994.     

Gas hydrate fast nucleation from melting ice and quiescent growth along vertical heat transfer tube

Gas hydrates, or clathrate hydrates, are ice-likecrystal, composed of host lattice (cavities) formed byhydrogen-bonded water molecules, and other guestmolecules called guest molecules. The guest mole-cules act with host lattice in weak van der Waals force…     

Experimental studies of the formation and dissociation of methane hydrate in loess

In order to study the nature of gas hydrate in porous media, the formation and dissociation processes of methane hydrate in loess were investigated. Five cooling rates were applied to form methane hydrate. The nucleation times of methane hydrate formation at each cooling rate were measured for comparison. The experimental results show that cooling rate is a significant factor affecting the nucleation of methane hydrate and gas conversion. Under the same initial conditions, the faster the cooling rate, the shorter the nucleation time, and the lower the methane gas conversion. Five dissociating temperatures were applied to conduct the dissociation experiment of methane hydrate formed in loess. The experimental results indicated that the temperature evidently controlled the dissociation of methane hydrate in loess and the higher the dissociating temperature, the faster the dissociating rates of methane hydrate.     

Influence of Gas Supply Changes on the Formation Process of Complex Mixed Gas Hydrates

Natural gas hydrate occurrences contain predominantly methane; however, there are increasing reports of complex mixed gas hydrates and coexisting hydrate phases. Changes in the feed gas composition due to the preferred incorporation of certain components into the hydrate phase and an inadequate gas supply is often assumed to be the cause of coexisting hydrate phases. This could also be the case for the gas hydrate system in Qilian Mountain permafrost (QMP), which is mainly controlled by pores and fractures with complex gas compositions. This study is dedicated to the experimental investigations on the formation process of mixed gas hydrates based on the reservoir conditions in QMP. Hydrates were synthesized from water and a gas mixture under different gas supply conditions to study the effects on the hydrate formation process. In situ Raman spectroscopic measurements and microscopic observations were applied to record changes in both gas and hydrate phase over the whole formation process. The results demonstrated the effects of gas flow on the composition of the resulting hydrate phase, indicating a competitive enclathration of guest molecules into the hydrate lattice depending on their properties. Another observation was that despite significant changes in the gas composition, no coexisting hydrate phases were formed.     

Comparison of the water change characteristics between the formation and dissociation of methane hydrate and the freezing and thawing of ice in sand

Hydrate formation and dissociation processes are always accompanied by water migration in porous media, which is similar to the ice. In our study, a novel pF-meter sensor which could detect the changes of water content inside sand was first applied to hydrate formation and dissociation processes. It also can study the water change characteristics in the core scale of a partially saturated silica sand sample and compare the differences of water changes between the processes of formation and dissociation of methane hydrate and freezing and thawing of ice. The experimental results showed that the water changes in the processes of formation and dissociation of methane hydrate were basically similar to that of the freezing and thawing of ice in sand. When methane hydrate or ice was formed, water changes showed the decrease in water content on the whole and the pF values rose following the formation processes. However, there were very obvious differences between the ice thawing and hydrate dissociation.     

In situ High-pressure NMR Observations on the Formation and Dissociation of Methane Hydrate

Gas hydrates are nonstoichionmetric ice-like solid. Three types of well-known structures, sI hydrate, sII hydrate, and sH hydrate, can be formed depending on the size of the guest molecules1. Most previous work focused on the equilibrium aspects of hydrat…     

Numerical Simulation on the Dissociation,Formation, and Recovery of Gas Hydrates on Microscale Approach

Investigations into the structures of gas hydrates, the mechanisms of formation, and dissociation with modern instruments on the experimental aspects, including Raman, X-ray, XRD, X-CT, MRI, and pore networks, and numerical analyses, including CFD, LBM, and MD, were carried out. The gas hydrate characteristics for dissociation and formation are multi-phase and multi-component complexes. Therefore, it was important to carry out a comprehensive investigation to improve the concept of mechanisms involved in microscale porous media, emphasizing micro-modeling experiments, 3D imaging, and pore network modeling. This article reviewed the studies, carried out to date, regarding conditions surrounding hydrate dissociation, hydrate formation, and hydrate recovery, especially at the pore-scale phase in numerical simulations. The purpose of visualizing pores in microscale sediments is to obtain a robust analysis to apply the gas hydrate exploitation technique. The observed parameters, including temperature, pressure, concentration, porosity, saturation rate, and permeability, etc., present an interrelationship, to achieve an accurate production process method and recovery of gas hydrates.     

Coordination polyhedron growth mechanism model and growth habit of crystals

A new growth mechanism model, coordination polyhedron growth mechanism model, is introduced from the angle of the coordination of anion and cation to each other at the interface. It is pointed out that the force driving the growth unit to enter the crystal lattice is the electrostatic attraction force between ions, whose relative size can be approximately measured by the electrostatic bond strength (EBS) that reaches a nearest neighbor anion (or cation) in the parent phase from a cation (or anion) at the interface. The growth habits of NaCI, ZnS, CaF2 and Csl crystals are discussed, and a new growth habit rule is proposed as follows. When the growth rate of a crystal is determined by the step generation rate, the growth habit of this crystal is related to the coordination number of the ion with the smallest coordination rate at the interface of various crystal faces. The smaller the coordination number of the ion at the interface, the faster the growth rate of corresponding crystal face. When the growth     

Effect of various types of equations of state for prediction of simple gas hydrate formation with or without the presence of kinetic inhibitors in a flow mini-loop apparatus

This paper compares the effects of using various types of equations of state (PR,¹ SRK,² ER,³ PT⁴ and VPT⁵) on the calculated driving force and rate of gas consumption based on the Kashchiev and Firoozabadi model for simple gas hydrate formation for methane, carbon dioxide, propane and iso-butane with experimental data points obtained in a flow mini-loop apparatus with or without the presence of kinetic inhibitors at various pressures and specified temperatures. For this purpose, a laboratory flow mini-loop apparatus was set up to measure gas consumption rate when a hydrate forming substance (such as C₁, C₃, CO₂ and i-C₄) is contacted with water in the presence or absence of dissolved inhibitor under suitable temperature and pressure conditions. In each experiment, a water blend saturated with pure gas is circulated up to a required pressure. Pressure is maintained at a constant value during experimental runs by means of the required gas make-up. The total average absolute deviation was found to be 15.4%, 16.3%, 15.8%, 17.8% and 17.4% for the PR, ER, SRK, PT and VPT equations of state for calculating gas consumption in simple gas hydrate formation with or without the presence of kinetic hydrate inhibitors, respectively. Comparison results between the calculated and experimental data points of gas consumption were obtained in flow loop indicate that the PR and ER equations of state have lower errors than the SRK, VPT and PT equations of state for this model.     

A simple model of growth and product formation in cell suspensions ofCatharanthus roseus G. Don

This paper describes the application of a simple log-linear model to suspension cultures of the species Catharanthus roseus G. Don. Data obtained experimentally have been interpolated by a cubic spline to give data points of regular time period. A logarithmic transformation has been applied and a linear model fitted to these transformed variables. The results show that there is a reasonable fit to the experimental data cited. Using this model a one-step ahead prediction is made for a series of untried experimental variables, and the results generated are compared with the experimental data subsequently obtained.