Hydrate Composition Research Articles

Implementation of Exp-6 potential function in thermodynamic modeling of clathrate hydrates phase equilibria

Accurate modeling of clathrate hydrates phase behaviors is of significant importance to petroleum industry. It is well known that accurate consideration of the cavity force field can lead to more accurate predictions of incipient conditions of clathrate hydrates. Intermolecular repulsive force is the dominant force in cavity space that is a determinative factor in taking the guest–host interactions into account. As the result, further modifications in potential function part of van der Waals and Platteeuw (vdWP) solid solution theory could improve the model accuracy. In this study, the influence of repulsive force between guest–host molecules in hydrate cavities was investigated through replacing Kihara potential function by Exp-6 potential function. The vdWP model equations were derived based on new potential function. The pair potential parameters were regressed using equilibrium dissociation conditions data of simple hydrates of natural gas constituents. The determination performances of Kihara and Exp-6 potential functions were compared in aspects of incipient hydrate dissociation conditions and hydrate composition of binary hydrate formers and mixtures of natural gas systems. The results show a promising improvement in vdWP model in case of binary hydrate systems, while the accuracy of the model does not change dramatically in case of natural gas hydrate system. This is due to cancellation of changes in Gibbs energy of occupation for different guest species and the natural gas composition.

Interactions between Reduced Graphene Oxide with Monomers of (Calcium) Silicate Hydrates: A First-Principles Study.

Graphene is a two-dimensional material, with exceptional mechanical, electrical, and thermal properties. Graphene-based materials are, therefore, excellent candidates for use in nanocomposites. We investigated reduced graphene oxide (rGO), which is produced easily by oxidizing and exfoliating graphite in calcium silicate hydrate (CSHs) composites, for use in cementitious materials. The density functional theory was used to study the binding of moieties, on the rGO surface (e.g., hydroxyl-OH/rGO and epoxide/rGO groups), to CSH units, such as silicate tetrahedra, calcium ions, and OH groups. The simulations indicate complex interactions between OH/rGO and silicate tetrahedra, involving condensation reactions and selective repairing of the rGO lattice to reform pristine graphene. The condensation reactions even occurred in the presence of calcium ions and hydroxyl groups. In contrast, rGO/CSH interactions remained close to the initial structural models of the epoxy rGO surface. The simulations indicate that specific CSHs, containing rGO with different interfacial topologies, can be manufactured using coatings of either epoxide or hydroxyl groups. The results fill a knowledge gap, by establishing a connection between the chemical compositions of CSH units and rGO, and confirm that a wet chemical method can be used to produce pristine graphene by removing hydroxyl defects from rGO.

Hydrate-based separation of the CO2 + H2 mixtures. Phase equilibria with isopropanol aqueous solutions and hydrogen solubility in CO2 hydrate

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 H2 pressure up to 4.5 MPa was experimentally determined. Hydrate composition was found to be CO2·(0.01X)H2·6.5H2O at H2 pressure of X MPa. The equilibrium conditions of hydrates formation in the systems of H2O – CO2 – H2 and H2O – 2-propanol – CO2 – H2 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 СО2, only sI СО2 hydrate occurred in the studied systems. The obtained dependencies will be useful in analyzing the process of СО2 + Н2 gas mixtures separation by the hydrate-based method.

Raman Spectroscopy Study on Ternary Model Coal Mine Methane Hydrates.

The microfeatures of coal mine methane (CMM) hydrates, synthesized with three gas samples (CH4/C2H6/N2, G1 = 43 : 47 : 10, G2 = 60 : 30 : 10, and G3 = 74 : 16 : 10) in a self-made transparent high-pressure cell at 275.15 K and 5 MPa were investigated using Raman spectroscopy. As a discriminator, the vibrational band frequencies in the C–C regions of the recorded hydrate Raman spectra for C2H6 show that G1∼G3 hydrates are structure I. The three principal parameters used to study the microfeatures of the model CMM hydrates, including cavity occupancies, hydrate guest compositions, and hydration numbers, were calculated. The large cavity occupancies for C2H6 constantly decrease from 85.12 to 79.32%, while the small cavity occupancies for CH4 have a continuous increase from 73.75 to 96.42%. However, CH4 competes with C2H6 on entering the large cavities for their large cavity occupancies of 12.79–17.31%. The cavity occupancies of N2 are less than 1.2%. The hydrate composition calculations show that the molar fractions of C2H6 are the maximum. The hydration numbers range from 6.221 to 6.00. Based on the hydrate guest compositions and hydration numbers, the molecular formulas of the three CMM hydrates are presented.

Effect of a Nitrite/Nitrate-Based Accelerator on the Strength Development and Hydrate Formation in Cold-Weather Cementitious Materials.

Recently, there has been increased use of calcium-nitrite and calcium-nitrate as the main components of chloride- and alkali-free anti-freezing agents to promote concrete hydration in cold weather concreting. As the amount of nitrite/nitrate-based accelerators increases, the hydration of tricalcium aluminate (C3A phase) and tricalcium silicate (C3S phase) in cement is accelerated, thereby improving the early strength of cement and effectively preventing initial frost damage. Nitrite/nitrate-based accelerators are used in larger amounts than usual in low temperature areas below −10 °C. However, the correlation between the hydration process and strength development in concrete containing considerable nitrite/nitrate-based accelerators remains to be clearly identified. In this study, the hydrate composition (via X-ray diffraction and nuclear magnetic resonance), pore structures (via mercury intrusion porosimetry), and crystal form (via scanning electron microscopy) were determined, and investigations were performed to elucidate the effect of nitrite/nitrate-based accelerators on the initial strength development and hydrate formation of cement. Nitrite/nitrate-AFm (aluminate-ferret-monosulfate; AFm) was produced in addition to ettringite at the initial stage of hydration of cement by adding a nitrite/nitrate-based accelerator. The amount of the hydrates was attributed to an increase in the absolute amounts of NO2− and NO3− ions reacting with Al2O3 in the tricalcium aluminate (C3A phase). Further, by effectively filling the pores, it greatly contributed to the enhancement of the strength of the hardened cement product, and the degree of the contribution tended to increase with the amount of addition. On the other hand, in addition to the occurrence of cracks due to the release of a large amount of heat of hydration, the amount of expansion and contraction may increase, and it is considered necessary to adjust the amount used for each concrete work.

The Relationship between Thermal Characteristics and Microstructure/Composition of Carbon Dioxide Hydrate in the Presence of Cyclopentane

Hydrate-based carbon dioxide (CO2) separation and capture is a new technology for achieving CO2 emission reduction. However, it is still not commercially applied for the ambiguity of microscopic hydrate formation mechanism. In a constant volume experiment of hydrate formation, there are two or more pressure platforms, indicating that there might be two or more different hydrates formation in succession. In order to reveal the relationship between the microscopic process and the gas consumption in the process of hydrate formation, hydrate composition and formation mechanism of cyclopentane-CO2 (CP-CO2) system was investigated using a differential scanning calorimeter (DSC) and Raman spectroscopy. The results indicated CO2-CP binary hydrate and CO2 hydrate are formed successively, and they coexist in the final hydrate. CP-CO2 binary hydrates forms preferentially, and as crystal seeds, inducing the formation of CO2 hydrates. The two hydrates formation processes cause the two pressure-drops. The results provide a scientific basis for increasing the gas consumption in different stages of gas hydrate formation in the presence of hydrate formation promoter.

Kinetics and products composition of catalytic hydration of ethylene oxide in the presence of layered double hydroxides

Kinetics and products composition of catalytic hydration of ethylene oxide in the presence of layered double hydroxides

Структура, морфология и состав природных газовых гидратов отобранных на грязевом вулкане Кедр-1 (оз. Байкал)

Изучена структура, морфология и состав газа для природных газовых гидратов, отобранных на грязевом вулкане Кедр-1 (оз. Байкал). Показано, что все эти гидраты имеют кубическую структуру II, а связанный газ содержит около 14 % этана (остальное метан). Один из образцов представлял собой пористый монолитный слой гидрата с прилегающими к нему сверху и снизу слоями гранул гидрата. И в слое, и в гранулах гидрат имел одну и ту же структуру и состав. Насколько нам известно, ранее такие особенности морфологии природных гидратов не описаны. Обсуждаются возможные пути формирования подобных объектов.

Theoretical study of the self-preservation effect in CF4 gas hydrates

This work studies thermodynamic properties of CF4 gas hydrates using the statistical thermodynamic model of inclusion compounds, developed in our laboratory and closely associated with Quasiharmonic Lattice Dynamic (QLD) and Molecular Dynamic Simulation (MD). Thermal expansion coefficient of CF4 gas hydrate is calculated at different hydrate composition. The thermal expansion coefficient is shown to be lesser than the thermal expansion coefficient of CH4 hydrates, but larger than ice Ih, that still indicates the presence of the self-preservation effect. The thermal expansion coefficient is very sensitive to the composition of the hydrate and can vary in a wide range. The same is true for the hydrate lattice parameter as well; it is shown that at certain conditions it may be even less than for hypothetical empty hydrate structure. The self-preservation effect may be applied for many aspects related to CF4 compound, especially for storage and transportation purposes in producing semiconductor electronics.

Characterizing the variability of natural gas hydrate composition from a selected site of the Western Black Sea, off Romania

Natural Gas Hydrates (NGH) collected during the Ghass cruise 2015 in the Western Black Sea onboard the R/V Pourquoi pas? are characterized by a suite of techniques. Gas Chromatography and Raman spectroscopy are used for the identification of the nature of the gas source, the hydrate structure and spatial variability of cage occupancies. The nature and source of hydrate forming gases primarily reveal a high methane content (99.6 mol%) and small amount of nitrogen (>0.29 mol%) and CO2 (0.056 mol%). Isotopic analyses from the hydrate-bound methane and recently published results from Pape et al. (2020) clearly indicate a microbial source of gas supplying the hydrate deposit generated by the reduction of carbon. For the first time, Raman imaging spectroscopy was applied on NGH recovered in the Western Black Sea. The results show a heterogeneous distribution of the encapsulated guest molecules (CH4, N2 and H2S), which is associated with a spatial variability of the guest-gas composition at the micron-scale. Some portions of the 2D-Raman images clearly exhibit a relative N2-enrichment (with a concentration exceeding 6 mol% N2 at some positions), while H2S shows a rather minor contribution on all the spectral maps investigated. A correlation is then established between the composition of the gas in the NGH and its impact on the CH4 cage occupancy, with a ratio of θLC/θSC (large cage/small cage) between ~ 0.5 and 1.26 depending on the positions analyzed. The departure from the expected ratio in pure methane hydrate is attributed to the preferential encasement of N2 in the large cage of the NGH structure. In addition, the occurrence of carotenoids identified in sediment-rich zones show a minor impact on the CH4 cage occupancies. The results are discussed within the context of natural gas resource estimates in NGH to emphasize how the measured cage occupancies may impact the volumetric conversion factor commonly used with other geologic parameters to determine the resource endowment and global volume of methane. The small-scale heterogeneities revealed by the 2D-Raman images point out the importance to better understand stages of hydrates formation in methane-rich seafloor environment.

Mechanical and fracture properties of calcium silicate hydrate and calcium hydroxide composite from reactive molecular dynamics simulations

This paper firstly investigated the mechanical and fracture properties of calcium silicate hydrate and calcium hydroxide composite by reactive molecular dynamics simulations. The results show that the calcium hydroxide has the highest tensile strength and Young’s modulus, just followed by calcium silicate hydrate and CSH-CH composite. The obtained fracture toughness of CSH, CH and CSH-CH composite are consistent with experimental results, showing the fracture toughness is independent of size. The failure mechanisms of CSH-CH composite under tensile loading are revealed in terms of bond-breakage. The results can provide necessary parameter inputs for multi-scale mechanical and fracture study of cement paste.

A novel magnesium hydroxide sulfate hydrate whisker-reinforced magnesium silicate hydrate composites

Magnesium hydroxide sulfate hydrate (MHSH) whiskers are used to reinforce magnesium silicate hydrate (M-S-H) cement mortars as novel microfibrous materials because of their similar pH. The microstructure, mechanical performance, and reinforcement mechanism were investigated, and the results showed that the addition of between 1 and 5 wt% MHSH whiskers improved the compressive and flexural strengths of M-S-H cement mortars. The optimal compressive and flexural strengths were obtained at MHSH whisker contents between 3 and 4 wt%. The MHSH whiskers had a limited effect on the toughness of M-S-H cement, and mortars reinforced with MHSH whiskers exhibited brittle failure due to the small size of MHSH whiskers and low fiber bridging traction. Scanning electron microscopy (SEM) revealed that the microscale reinforcement mechanism of MHSH whiskers involved whisker pullout, crack deflection, whisker-cement coalition pullout, and whisker fracture. These mechanisms helped dissipate energy and optimize the stress distribution and transfer, which were crucial to improving the flexural strength. The SEM images revealed the rough and grooved surfaces of MHSH whiskers, and X-ray photoelectron spectroscopy (XPS) showed the presence of polar functional groups on the surface which resulted in the adhesion of M-S-H gel on MHSH whiskers due to good interfacial bonding. The mercury intrusion porosimetry (MIP) results indicated that the addition of MHSH whiskers reduced the porosity of M-S-H cement mortars, which also contributed to the increased compressive strength.

Predictions on CH4 recovery factors using the CO2 replacement method to develop natural gas hydrate resources

Injecting CO2 into the hydrate reservoirs to replace CH4 is an effective method for the natural gas hydrate (NGH) exploitation and CO2 sequestration. The accurate prediction of the CH4 recovery factor under different pressures, temperatures, and CO2 injection amounts are essential for the design of the CO2 replacement process.However, conventional multiphase flash algorithms are not stable for CO2-containing hydrate system calculations because of inappropriate initial estimations on the hydrate phase fraction and composition. This paper proposed a new multiphase flash algorithm by developing a new initialization method for the hydrate phase fraction and composition as well as the solution procedure. A total of 61 sets of experimental data at pressures from 8 MPa and 16 MPa are applied to validate the new method. Results demonstrate that the new method successes in all the 61 cases, while the software PVTsim and CSMGem do not converge for 16 and 20 cases, respectively. Also, the average relative deviation between the calculated phase compositions and experimental values is less than 18 %, which represents a comparable accuracy to the above two software. Moreover, case studies are designed to research the variations of the CH4 recovery factor with the injected CO2 concentration in the hydrate reservoir. The most economical injected CO2 concentrations are discussed at specified reservoir conditions.

Do intravenous fluid substitutions influence methotrexate clearance? An unanticipated impact of an intravenous sodium bicarbonate drug shortage.

National drug shortages of essential medications for childhood cancer have increasingly posed a challenge in the treatment of patients. The efficacy of standardized supportive care practices to avoid treatment-related toxicities may be limited during these drug shortages. High-dose methotrexate (HDMTX) plays a critical role in modern treatment protocols for acute lymphoblastic leukemia and requires stringent supportive care measures to mitigate toxicity. As the result of a national intravenous (IV) sodium bicarbonate shortage, institutional standard HDMTX supportive care guidelines had to be modified. We describe the unanticipated consequences on HDMTX clearance. We performed a retrospective chart review assessing the impact of alternative compositions of IV fluids on the mean 24-h methotrexate levels (Cpss ) of 25 patients receiving 76 total HDMTX infusions at Texas Children's Hospital Cancer Center from March to October 2017. During the sodium bicarbonate drug shortage, all patients received IV hydration consisting of either dextrose 5%, 0.45% normal saline (D5 ½ NS-Group A) or dextrose 5%, 0.2% normal saline (D5 ¼ NS-Group B). Patients receiving a higher total sodium dose demonstrated significantly lower Cpss (25.36 ± 16.6 μMol) compared to patients receiving less sodium (53.9 ± 37.9 μMol; P<.001). Our report shows that in the setting of IV sodium bicarbonate shortage, the composition of hydration IV fluids may affect methotrexate clearance. Patient who received a higher sodium load had a lower 24-h methotrexate level. This demonstrates the potential for unanticipated outcomes resulting from national drug shortages.

Facile fabrication of novel analcime/sodium aluminum silicate hydrate and zeolite Y/faujasite mesoporous nanocomposites for efficient removal of Cu(II) and Pb(II) ions from aqueous media

In this work, novel mesoporous sodium aluminum silicate hydrate/analcime and zeolite Y/faujasite nanocomposites were fabricated utilizing the hydrothermal method using glutamic acid and L-arginine as organic structure-directing agents. In aqueous solutions, the fabricated nanocomposites showed high efficiency in removing Pb(II) and Cu(II) ions. Also, the nanocomposites were successfully utilized for the removal of the aforementioned ions from wastewater, which was collected from an electroplating factory in the 10th of Ramadan City, Egypt. The characterization of the fabricated nanocomposites was carried utilizing Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), and Brunauer-Emmet-Teller surface area (BET) instruments. The BET surface area, total pore volume, and average pore diameter of analcime/ sodium aluminum silicate hydrate composite were 12.65 m2/g, 9.049E-2 cc/g, and 28.60 nm, respectively. Also, the BET surface area, total pore volume, and average pore diameter of zeolite Y/faujasite composite were 19.17 m2/g, 5.103E-2 cc/g, and 106.50 nm, respectively. Moreover, the majority of pores of the fabricated nanocomposites belong to the mesoporous type. The Langmuir isotherm and pseudo-second-order model predominated the adsorption processes. The thermodynamic parameters confirmed the spontaneous, exothermic, and chemisorptions nature of adsorption processes. The maximum adsorption capacities of zeolite Y/faujasite and sodium aluminum silicate hydrate/analcime nanocomposites toward copper ions were 105.82 and 77.52 mg/g, respectively. Also, the maximum adsorption capacities of zeolite Y/faujasite and sodium aluminum silicate hydrate/analcime nanocomposites toward lead ions were 83.26 and 58.86 mg/g, respectively.

A holistic model for cement paste and concrete chloride diffusion coefficient

Strength and durability are pivotal for designing sustainable concrete structures. For reinforced concrete, chloride diffusion coefficient is a measure for durability. Phenomenological multi-scale models are proposed for quantifying chloride effective diffusion coefficient of bulk cement paste (DClbcp) and concrete (DClc). DClbcp model accounts for w/cm, and cement composition and degree of hydration. DClc model comprises of DClbcp, tortuosity factor and ITZ, interfacial transition zone, factor sub-models. DClbcp and DClc models results are statistically equal with 95% confidence to the experimental values and possess high accuracy and precision for a wide range of w/cm, cement content and composition, aggregate content and age. DClbcp and DClc models' results have an overall error of 12% and 11%, respectively. The errors in DClc model results are less than the standard test method repeatability and the coefficient of variation of the reproducibility. DClc model standard error, COV and R2 values are 0.36 × 10−12 m2/s, 26% and 0.94.

Kinetics, compositions and structures of carbon dioxide/hydrogen hydrate formation in the presence of cyclopentane

Cyclopentane(CP) as an efficient promoter has not been evaluated for capturing CO2 form fuel gas via forming hydrate. The work systematically studies the kinetics and CO2 recovery of gas (40.0 mol% CO2 + 60.0 mol% H2) hydrate formation in the presence of CP at 276.15 K and 6.0 MPa. Meanwhile, X-ray diffraction and Raman spectrum were adopted to determine the hydrate structure and composition. The results showed that there were two steps for hydrate formation in the system, Step 1: Pure CP hydrates formation. Step 2: Gas molecules trapped in hydrate cages to form CP/gas (CO2, H2) mixed hydrates. Besides, the CO2 recovery of up to 98.8% was obtained at gas-liquid ratio of 5/6 and CP volume concentration of 23.4 vol%, indicating that almost all CO2 could be removed from fuel gas by one-stage of hydrate formation. In addition, not only CP/gas (CO2, H2) mixed hydrates (sII) but also pure CO2 hydrates (sI) are observed in the system. Strikingly, the work confirms that some of H2 molecules are also trapped in the hydrate phase, and its amount increased as the CP volume concentration increase. From the view point of cost-effective and H2-saving, the gas-liquid ratio of 5/6, CP volume concentration of 3.0 vol% and precooling liquid are the optimum conditions for HBGS.

The Effect of CO2 Partial Pressure on CH4 Recovery in CH4-CO2 Swap with Simulated IGCC Syngas

To investigate the influence of CO2 partial pressure on efficiency of CH4-CO2 swap from natural gas hydrates (NGHs), the replacement of CH4 from natural gas hydrate (NGH) is carried out with simulated Integrated Gasification Combined Cycle (IGCC) syngas under different pressures, and the gas chromatography (GC), in-situ Raman, and powder X-ray diffraction (PXRD) are employed to analyze the hydrate compositions and hydrate structures. The results show that with the P-T (pressure and temperature) condition shifting from that above the hydrate equilibrium curve of IGCC syngas to that below the hydrate equilibrium curve of IGCC syngas, the rate of CH4 recovery drastically rises from 32% to 71%. The presence of water can be clearly observed when P-T condition is above the hydrate equilibrium curve of IGCC syngas; however the presence of water only occurs at the interface between gas phase and hydrate phase. No H2 is found to present in the final hydrate phase at the end of process of CH4-CO2 swap with IGCC syngas.

Poorly crystallized hydroxyapatite and calcium silicate hydrate composites: Synthesis, characterization and soaking in simulated body fluid

Development of the composites based on substances with a high bioactivity potential is of current interest to medical materials science because composites' properties can be modified by varying the components ratios. Weakly crystallized calcium phosphates and calcium silicate hydrates, possessing higher chemical reactivity than their crystalline forms, are promising salts to be studied as biomaterials' constituents. In this study, precipitation from aqueous solutions was employed to obtain apatite and calcium silicate hydrate composites. Synthesized solids were examined without high-temperature thermal pretreatment using X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), laser diffraction, physical adsorption (BET method) and thermogravimetric analysis coupled with the mass spectrometry (TGA/MS). Compositions containing poorly crystallized carbonate-substituted hydroxyapatite, calcium silicate hydrate and calcite as an admixture were obtained by varying concentrations of the reagents. Soaking of the composite ceramics formed from freshly precipitated synthetic solids in simulated body fluid (SBF) at 37 °C for 14 days lead to formation of the surface layer of amorphous calcium phosphate. As the calcium silicate content in the composites increases, the coating's density and thickness also increase, because silicate ions act as active sites in the process of new phase nucleation. Furthermore, prismatic calcite crystals were identified to form on the lower side of ceramics. This may be caused by an increasing of local supersaturation with respect to calcium carbonate in small-volume confinement conditions during the experiment.

Effect of Long-Term Storage of Silicate Brick in Water on its Strength

The article is devoted to the study of long-term storage of silicate brick in stationary water conditions on its strength and phase composition. The relevance of these studies is due to the fact that there are a lot of opinions that silicate materials have low water resistance which significantly limits their application range and reduces their popularity among builders. It has been found that after nine years storage in water and followed drying the silicate brick’s strength is practically equal to the initial strength. The data on the phase composition of newgrowths in silicate brick newly made in different production periods and the phase composition of silicate hydrates of bricks water conditioned are presented in the article. Comparison of the X-ray and differential thermal analyzes results of silicate bricks samples after 9 years of water conditioning and samples of newly made silicate bricks showed that highly basic calcium silicate hydrates predominate in a newly made sample, and in a 9-year-old sample silicate hydrates are mainly represented by tobermorite, while the total content of the СSH phase is the same. The content of calcium hydroxide in a newly made sample is higher but this may be due to the completeness of Ca(OH)2 binding in a particular sample but not to its leaching. The fact that leaching of Ca(OH)2 from silicate brick in still water does not occur also can be proved because the water of brick samples storage does not turn pink when phenolphthalein is added. These results allow us to conclude that long-term storage of silicate products in still water does not have any effect on the deterioration of silicate bricks.