Morphology and growth rate of carbon dioxide hydrate on the interface between liquid carbon dioxide and humic acid solutions were studied in this work. It was found that after the growth of the hydrate film at the interface, further growth of hydrate due to the suction of water in the capillary system formed between the wall of the cuvette and the end boundary of the hydrate layer occurs. Most probably, substantial effects on the formation of this capillary system may be caused by variations in reactor wall properties, for example, hydrophobic-hydrophilic balance, roughness, etc. We found, that the rate of CO2 hydrate film growth on the surface of the humic acid aqueous solution is 4-fold to lower in comparison with the growth rate on the surface of pure water. We suppose that this is caused by the adsorption of humic acid associates on the surface of hydrate particles and, as a consequence, by the deceleration of the diffusion of dissolved carbon dioxide to the growing hydrate particle.

Currently, gas hydrates are considered to be a potential resource of natural gas in the future. Vast quantities of gas hydrates deposits are located in the deep sea or in the permafrost zone. Among the other potential methods for extracting natural gas from these deposits is the injection of CO2 into a hydrate-bearing reservoir. This method assumes that carbon dioxide will replace methane molecules in the clathrate framework, so this method may be able to provide long-term burial for greenhouse gas CO2 along with the getting of burning gas. Understanding the kinetics of this process might be crucial for assessing the feasibility of implementation of this technology, but there is currently no clear understanding about the applicability of standard equations for heterogeneous reactions to describe the kinetics of this process. In this paper, we present some recent kinetic data on CH4/CO2 exchange at temperature below the ice melting point, focusing on the composition of hydrates. We also propose theoretical explanations for the obtained data.

The experimental data on hydrogen solubility in methane gas hydrate and new data on phase equilibria in the hydrogen - methane - water system are presented in this work. The solubility data were obtained for hydrogen partial pressures from 0.8 to 5.4 MPa. It was found that hydrogen solubility in methane and carbon dioxide hydrates is almost the same and the dependence of hydrogen content on hydrogen partial pressure can be fitted with the equation ln(C-H2) = -4.332(54) + 0.919(62)-ln(P-H2), where CH2 is the hydrogen content in the hydrate (wt%) and P-H2 is the hydrogen partial pressure (MPa). The data obtained in this work were compared with available data on hydrogen solubility in sI and sII hydrates. Analysis of data on the solubility of hydrogen in sII double hydrates with different thermodynamic promoters (tetrahydrofuran, cyclopentane, propane, tetrathiophene, furan) taken from various literature sources also shows a relatively small scatter with respect to the straight line approximating them. In general, the fraction of free small 5(12) cages filled with hydrogen decreases in the following order: sI methane hydrate > sI CO2 hydrate > sII hydrates.(c) 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

It is known that injection of carbon dioxide into the petroleum reservoir(CO 2 flooding) is one of the effective methods for enhanced oil recovery. CO 2 flooding may be complicated by formation of CO 2 hydrate plugs. It makes topical investigation of CO 2 hydrate formation in the system gaseous CO 2 –oil–water. In this work, the growth rates of carbon dioxide hydrate films at the water–oil as well as the water–gas interface are studied in the pressure range of 2.30–3.04 MPa and at temperatures between –5.4 and 5.0°C. It is found that the growth rate for the water–oil interface is 3.5 times lower than that for the water–gas interface with carbon dioxide. It is hypothesised that the observed decrease in the growth rate is related to the mechanical resistance of the oil components adsorbed on the interface to the growth of the hydrate film. The growth rate of the film has been shown to depend on the experimental procedure,most likely due to the different initial concentrations of carbon dioxide in the aqueous solutions.

In this work, the self-preservation of small (tens of mu m) methane hydrate particles dispersed in oils was studied. The results of this work show that stirring in itself has no effect on the self-preservation of hydrate particles in oil suspensions. The particles undergo self-preservation under both static and stirred conditions. However stirring can lead to redistribution of oil components which may result in the disappearance of self-preservation. Accordingly to developed analytical model, it was shown that the effectiveness of self-preservation is determined by the value of the diffusion coefficient of methane in the pores of the ice crust. It was shown that for the samples with weak self-preservation, the diffusion coefficient of methane in the pores of the ice layer is close to 10(-15) m(2)/s, while for the samples with effective self-preservation this value is an order of magnitude lower. Thus, the pore structure of the ice layer formed on the hydrate particles completely determines whether the self-preservation effect is manifested in a single experiment. The oil environment can allow for the formation of dense ice layers that can provide effective self-preservation. Our data show that the process of the appearance of ice layer and of evolution of its pore system on the surface of hydrate particles suspended in oils and thus the hydrates decomposition rates is of a stochastic nature. Therefore, the efficiency of self-preservation may differ significantly under similar external conditions.

The paper studies the dissociation and combustion of a layer of methane hydrate powder at a forced air flow over the upper surface of the layer (the air velocity is directed parallel to the upper surface of the layer). The influence of the layer thickness and air velocity on the combustion of gas hydrate is investigated. The calculated curves for the effect of the heat transfer coefficient, external convection and vapor concentration on the combustion temperature are obtained. The layer thickness and the air velocity significantly affect the dissociation rate of methane hydrate.

Abstract. We have developed an efficient and environmentally friendly technology based on a combination of mechanical treatment and chemical immobilization to demercurize mercury-containing waste.

A comparative investigation of methane hydrate formation from 0.1 wt % solutions of an alkaline extract of humic acids (HA) and sodium dodecyl sulfate (SDS) has been carried out. It was demonstrated that the hydrate grows as a voluminous loose mass squeezed onto the reactor walls in both cases. It turns that about 75% of water transforms into hydrate within 15-20 min. Thus, natural HA can act as kinetic hydrate promoters. At the same time, SDS increases the hydrate nucleation rate compared to pure water under the studied conditions (12.5-13.0 MPa and -5 degrees C), while HA retards the hydrate nucleation. Visual observation of the hydrate growth in the solutions of HA and SDS allowed us to propose a new mechanism of hydrate growth in the form of a porous loose mass squeezed upward over the reactor walls. The mechanism relates to overgrowing of the hydrate film formed at the gas-solution interface into the solution volume. This results in squeezing some part of the solution onto the reactor walls. Simultaneous contact of the gas, solution, and hydrate facilitates the rapid transformation of the squeezing solution into a loose hydrate mass. Then this mass soaks up the solution due to the capillary forces.

The gas hydrates' ability to preferentially bind one of the components of a gas mixture into a hydrate state makes it possible to consider hydrate-based technology as promising for the separation of gas mixtures. When a hydrate is obtained from a gas mixture, mixed hydrates with a complex composition inevitably occur. Issue of their composition determination stays apart. This a rather difficult task, which is complicated by the dissolution of small molecules such as hydrogen in the hydrate phase. This, in turn, impedes the analysis of the data obtained. In this work, the solubility of hydrogen in carbon dioxide hydrate in the range of 269.7-275.7 K and at partial H-2 pressure up to 4.5 MPa was experimentally determined. Hydrate composition was found to be CO2 center dot(0.01X)H-2 center dot 6.5H(2)O at H-2 pressure of X MPa. The equilibrium conditions of hydrates formation in the systems of H2O - CO2 - H-2 and H2O - 2-propanol - CO2 - H-2 were also determined in a wide range of hydrogen concentrations. Hydrogen seems to be an indifferent diluent gas regarding CO2 hydrate equilibrium pressure. The compositions of the equilibrium phases have been determined as well. It was shown that isopropanol does not form a double hydrate with C circle(2), only sI C circle(2) hydrate occurred in the studied systems. The obtained dependencies will be useful in analyzing the process of C circle(2) + H-2 gas mixtures separation by the hydrate-based method. (C) 2021 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Gas hydrates and wax are the major flow assurance problems for the transportation of produced hydrocarbons through pipelines. However, in most research works both these two problems are studied separately. Although simultaneous precipitation or deposition of these compounds in pipelines can lead to different mitigation/prevention strategies, the investigations in which both these problems are considered simultaneously appeared only recently. There is no information in the literature on the texture/composition and features of decomposition process of mixed wax/hydrate plugs. At the same time, this information could be useful to understand how to treat the problem of formation of these plugs. In this work, three wax/gas hydrate plugs were collected at quasi-static conditions from a water-in-oil emulsion to study their texture, composition and the features of decomposition process. Powder X-ray diffraction and IR (infrared spectroscopy) analyses showed that the plugs consisted of wax and gas hydrate. Thermovolumetric and DSC (Differential Scanning Calorimetry) experiments showed that the main part of gas hydrate in the plugs at the ambient pressure started to decompose at about 268 K. This temperature was higher than the equilibrium temperature of carbon dioxide hydrate at this pressure, indicating that the gas hydrate in the plugs could be effectively preserved at temperatures below the ice melting point (273.2 K). It was found through observation of the hydrate decomposition process in the plugs under the microscope that the gas in the samples released in small bubbles, while the hydrate particles were not visible at this magnification, indicating that the hydrate was indeed highly dispersed in the samples. A residual wax was jelly-like after decomposition of hydrate in all the cases. Rheological experiments showed that the plugs residues after decomposition of the hydrates had higher yield points and viscosities than the initial waxy crude oil originally used for the experiments.

To date, there are no reliable and simple calculation models that simulate the kinetics of dissociation at negative temperatures. One of the important problems is to reduce the cost of storage and transportation of natural gas hydrates. The paper presents experimental data on the effect of layer thickness and temperature on the dissociation kinetics, as well as the effect of the external air velocity. The model enables effective modeling of the dissociation kinetics both outside the self-preservation region and in the annealing temperature window. The experiments were carried out in the presence of non-stationary and non-isothermal dissociation. The thickness of the gas hydrate layer significantly affects the dissociation rate. The effect of thickness persists over a wide range of air velocities. The inhomogeneity of the temperature field inside the powder layer increases with increasing layer thickness, resulting in the appearance of two self-preservation sites on the dissociation curve.

The combustion and dissociation of the double hydrate of propane-methane have been studied in terms of several key parameters: the velocity of the forced air flow U-0, heat flux, temperature difference, and geometry of the work area. Simple expressions relating the dissociation rate with the specified key parameters have been obtained. The ratio of dissociation rates J(1)/J(2) was determined, where the dissociation rates J(1) and J(2) correspond to the experiment with and without combustion, respectively. At U-0 = 0 m/s, the ratio J(1)/J(2) equals 8-9, and in the presence of forced gas flow J(1)/J(2) = 11-12. Forced convection increases this ratio. Approximate correlations have been obtained for assessing the time of combustion beginning and the duration of the gas hydrate burning. The dissociation rate is nonlinearly related to the velocity U-0. Two characteristic modes of gas hydrate dissociation are distinguished. Measurements of velocity fields obtained using the Particle Tracking Velocimetry (PTV) method show that the interaction of forced and free convection flows leads to a decrease in the maximum resultant velocity. Due to fuel excess over the oxidizer (violation of stoichiometric ratio), there are periodic emissions of gas bubbles, leading to incomplete combustion of the fuel. To improve the efficiency of combustion, it is advisable to use velocities U-0 = 1.2-3 m/s. A further increase in U-0 leads to the extinguishing of the flame.

Nowadays the biological methods based on utilization of bacteria for different purposes in oilfields are becoming more and more popular because these methods potentially may become a cheaper alternative to chemical treatments. Gas hydrates formation in pipelines is the one of the main flow assurance problems. In previous works it has been shown that some biodegraded oils can have components that may act as natural Anti-Agglomerants. These natural Anti-Agglomerants can prevent hydrate particles from agglomeration which allow the particles to flow downstream along with oil in form of slurry. But there is lack of understanding of how products of oil biodegradation affect the kinetic aspects of hydrate nucleation, formation and decomposition. This information might be useful for development of new approached of treatment of the hydrate related issues in oil pipelines. Impact of the changes in component composition of crude oil caused by biodegradation process on the kinetics of gas hydrates nucleation, formation and decomposition has been studied with different methods. It was demonstrated that the changes of chemical composition of crude oil caused by biodegradation can have a strong impact on the hydrate nucleation rates at static conditions. Almost twofold decrease of hydrate nucleation rate was observed in the experiments with the water-in-oil emulsion for the biodegraded oils compared to one obtained with emulsion in the original crude oil. At the same time, no significant effect of biodegradation on the hydrate film growth rates at static conditions was observed. Similar tangential and normal hydrate film growth rates were obtained in experiments with both crude oil and with biodegraded oil. Initial growth rates of hydrate in stirring emulsions were an order of magnitude higher for water emulsions in the biodegraded oil compared to ones for the original crude oil. The studies of the decomposition rates of hydrates at sub-zero temperatures and at dynamic conditions showed that the products of vital activity of bacteria promoted self-preservation of hydrates dispersed in the biodegraded oil.

Experiments on the dissociation of a mixed gas hydrate in various combustion methods are performed. The simultaneous influence of two determining parameters (the powder layer thickness and the external air velocity) on the efficiency of dissociation is studied. It has been shown that for the mixed hydrate, the dissociation rate under induction heating is 10–15 times higher than during the burning of a thick layer of powder, when the combustion is realized above the layer surface. The minimum temperature required for the initiation of combustion for different combustion methods was studied. As the height of the sample layer increases, the rate of dissociation decreases. The emissions of NOx and CO for the composite hydrate are higher than for methane hydrate at the same temperature in a muffle furnace. A comparison of harmful emissions during the combustion of gas hydrates with various types of coal fuels is presented. NOx concentration as a result of the combustion of gas hydrates is tens of times lower than when burning coal fuels. Increasing the temperature in the muffle furnace reduces the concentration of combustion products of gas hydrates.

Impact of different parameters on the decomposition rates of hydrate at negative temperatures with depressurisation method has been studied. In previous work it was demonstrated that decomposition rates of hydrates suspensions in oil at static conditions can be limited by the self-preservation phenomenon. Our experimental research shows that appearance of self-preservation depends on many parameters such as components composition of oil, presence of shear in the system while decomposing hydrates and probably on the history of the sample.

The technique of receiving of nano-silica nanopowders, consisting of nanoparticles with the known size, form, internal and surface structure and physicochemical properties was developed. The degree of hydrophilicity of those powders was determined. The method of the surface modification (hydrophobization) of the received nanoparticles was carried out and was optimized. Replacement of surface OH-groups on the CH3-group was produced by blowing SiCl2(CH3)2 vapor over the modified powders surface. The carbon content of the modified powders reached 3.2 %. The comparison of the hydrophobization process for the nanopowders received by two different origins was carried out. It was shown that the surface modification of Aerosil powders is faster than Tarkosil ones at similar hydrophilicity degree. Also hydrophobiс Aerosil contains more amount of carbon was cleared (up to 5,9 %). The complete replacement of OH-groups on the organic groups on the surface of powders was showed by IR spectroscopy. The representation of the organic groups by several combinations of fragments, rather than just the CH3 was showed

The structure of clathrate hydrates with tetraisoamylammonium polyacrylate salt incorporated as guest has been studied in this work. Also, quantitative studies on the stability changes of the clathrate hydrates with different degrees of cross-linking of the guest polymer (varied from 0 to 3%) have been conducted. A single crystal X-ray diffraction study of a crystal of the hydrate with linear (uncross-linked) tetraisoamylammonium polyacrylate as guest reveals a hexagonal structure (space group P6m2, a = 12.15 A, c =12.58 A at 100 K) with 39 host framework water molecules per one guest monomeric unit. Powder X-ray diffraction analyses confirm the identity of the above crystal structure of the hydrate with linear guest polymer and the crystal structure of the hydrates with cross-linked guest (hexagonal, a = 12.25 A, c =12.72 A at 276 K). In order to quantitatively determine the stability differences of the hydrates with the included guests having various degrees of cross-linking of the anionic chain, a series of differential scanning calorimetry measurements of the fusion enthalpy of the hydrate samples has been carried out. On the basis of the results obtained, a structural model describing the decrease in the stability of the clathrate hydrates with tetraisoamylammonium polyacrylate guest as a function of the degree of cross-linking of the guest polymer has been suggested.

Decomposition curves of gas hydrates formed in the ethane–hydrogen–water system were studied in the pressure interval 2–250 MPa. Gas hydrates synthesized at low (up to 5 MPa) pressures were also studied with use of X-ray powder diffraction and Raman spectroscopy. It was shown that ethane–hydrogen mixtures with hydrogen contents 0–30 mol.% form cubic structure I gas hydrates. Higher hydrogen concentration most probably results in appearance of another hydrate phase. We speculate that the gas mixtures with the hydrogen content above 60 mol.% form cubic structure II double hydrate of hydrogen and ethane at temperatures below ≈280 K and pressures above 25 MPa.

The decomposition temperatures of the hydrates of carboxylic cationites in tetraisoamylammonium (TiAA) and tetrabutylammonium (TBA) forms increase considerably in the presence of hydrogen (at 50 MPa, Delta T approximate to 6.5 degrees C for TiAA and 5.0 degrees C for TBA; at 150 MPa, Delta T approximate to 8.1 degrees C for TiAA and 6.2 degrees C for TBA).