Molecular Insights into the Effect of Nitrogen Bubbles on the Formation of Tetrahydrofuran Hydrates

In this work, a molecular dynamics simulation was conducted to study the microscopic mechanism of how nitrogen bubbles affect the formation of THF hydrates at the molecular level. The results obtained reveal that the nitrogen bubble can promote the formation of THF hydrates. In the system with a nitrogen bubble, more THF-filled cages were generated, and the crystal structure was more orderly. The promotion of nitrogen bubbles on hydrate crystallization comes from the dissolution of nitrogen molecules. Some of dissolved nitrogen molecules can be enclosed in small hydrate cages near the nitrogen bubble, which can serve as stable sites for hydrate crystal growth, resulting in the fact that THF-filled cages connected with N₂-filled cages are much more stable and have a long lifetime. The results in this work can help to understand the promotion effect of micro- and nano-air bubbles on the crystallization of THF hydrates.     

The influence of pressure on the stability of clathrate hydrates of hydrogen and tetrahydrofuran

Changes in the Gibbs energy of hydration of molecular hydrogen and tetrahydrofuran (THF) at pressures of 0.1, 6.0, and 12.0 MPa over the temperature range 230–300 K were studied by the molecular dynamics method. The Gibbs energy of hydrogen in water-tetrahydrofuran-hydrogen solutions passed minima over the temperature range 235–265 K, which were indicative of a comparatively stable clathrate hydrate state. The Gibbs energy of the hydrogen molecule at the local minimum at 262 K was ∼4.5 kJ/mol; at atmospheric pressure and room temperature, it was ∼2 kJ/mol. An analysis of the radial distribution function and the coordination number of the THF molecule showed that, at 240–257 K, a clathrate hydrate of THF with the structure close to clathrate sII was predominantly formed.     

Experimental study on flow characteristics of tetrahydrofuran hydrate slurry in pipelines

Tetrahydrofuran (THF) was selected as the substitute to study the flow behaviors and the mechanism of the hydrates blockage in pipelines. The slurrylike hydrates and slushlike hydrates are observed with the formation of hydrates in pipeline. There is a critical hydrate volume concentration of 50.6% for THF slurries and pipeline will be free of hydrate blockage while the hydrate volume concentration is lower than the critical volume concentration; otherwise, pipeline will be easy to be blocked. Fully turbulent flow occurs and friction factors tend to be constant when the velocity reaches 1.5 m/s. And then, constant values of friction factors that depend on the volume concentrations in the slurry were regressed to estimate the pressure drops of THF hydrate slurry at large mean velocity. Finally, a safe region, defined according to the critical hydrate volume concentration, was proposed for THF hydrate slurry, which may provide some insight for further studying the natural gas hydrate slurries and judge whether the pipeline can be run safely or not.     

NMR/MRI study of clathrate hydrate mechanisms

Clathrate hydrates are of great importance in many aspects. However, hydrate formation and dissociation mechanisms, essential to all hydrate applications, are still not well understood due to the limitations of experimental techniques capable of providing dynamic and structural information on a molecular level. NMR has been shown to be a powerful tool to noninvasively measure molecular level dynamic information. In this work, we measured nuclear magnetic resonance (NMR) spin lattice relaxation times (T1's) of tetrahydrofuran (THF) in liquid deuterium oxide (D2O) during THF hydrate formation and dissociation. At the same time, we also used magnetic resonance imaging (MRI) to monitor hydrate formation and dissociation patterns. The results showed that solid hydrate significantly influences coexisting fluid structure. Molecular evidence of residual structure was identified. Hydrate formation and dissociation mechanisms were proposed based on the NMR/MRI observations.     

Applying the Z method to estimate temperatures of melting in structure II clathrate hydrates

Equilibrium melting temperatures for structure II THF hydrate and argon/xenon (Ar/Xe) binary hydrate have been calculated using molecular dynamics using two melting techniques, namely the Z method [Belonoshko et al., Phys. Rev. B, 2006, 73, 012201] (applied for the first time to complex molecular solids) and direct phase coexistence simulations. The two methods give results in moderate agreement: calculations with the Z method give T(fus) to be 250.7 K (0.77 katm) for THF and 244.3 K (1.86 katm) for Ar/Xe hydrate respectively; the corresponding direct phase coexistence calculations give T(fus) in the range 235-240 K (0.77 katm) for THF and 240-252.5 K (1.86 katm) for Ar/Xe hydrate. The Z method was found to define the key thermodynamic states with high precision, although required long simulation times with these multicomponent molecular systems to ensure the complete melting required by the method. In contrast, the direct phase coexistence method did bracket the equilibrium temperature with little difficulty, but small thermodynamic driving forces close to phase equilibrium generated long-lived fluctuations, that obscured the precise value of phase coexistence conditions within the bracketed range.     

5A分子筛粉末对四氢呋喃水合物的生成及分解过程的影响

在低于0 ℃和常压下, 将粉碎并筛分后的成型5A分子筛粉末加入四氢呋喃-水(二者质量比为19:81)体系中, 用显微镜观察5A分子筛粉末的存在对四氢呋喃水合物生成和分解过程的影响. 结果表明, 5A分子筛粉末能够促进四氢呋喃水合物的生成. 5A分子筛粉末存在下, 四氢呋喃水合物生成方式主要表现为两种, 脉状生成和块状生成; 同时5A分子筛粉末能够提高四氢呋喃水合物结晶所需要的温度, 降低四氢呋喃水合物的分解温度; 而且5A分子筛粉末粒径的大小及分布对四氢呋喃水合物生成及分解的特性也有很大影响.     

Kinetic hydrate inhibitor performance of new copolymer poly(N-vinyl-2-pyrrolidone-co-2-vinyl pyridine)s with TBAB

In oil and gas field, the application of kinetic hydrate inhibitors (KHIs) independently has remained problematic in high subcooling and high water-cut situation. One feasible method to resolve this problem is the combined use of KHIs and some synergists, which would enhance KHIs’ inhibitory effect on both hydrate nucleation and hydrate crystal growth. In this study, a novel kind of KHI copolymer poly(N-vinyl-2-pyrrolidone-co-2-vinyl pyridine)s (HGs) is used in conjunction with TBAB to show its high performance on hydrate inhibition. The performance of HGs with different monomer ratios in structure II tetrahydrofuran (THF) hydrate is investigated using kinetic hydrate inhibitor evaluation apparatus by step-cooling method and isothermal cooling method. With the combined gas hydrate inhibitor at the concentration of 1.0 wt%, the induction time of 19 wt% THF solution could be prolonged to 8.5 h at a high subcooling of 6℃. Finally, the mechanism of HGs inhibiting the formation of gas hydrate is proposed.     

Tetrahydrofuran hydrate decomposition characteristics in porous media

Many tetrahydrofuran (THF) hydrate properties are similar to those of gas hydrates. In the present work THF hydrate dissociation in four types of porous media is studied. THF solution was cooled to 275.15 K with formation of the hydrate under ambient pressure, and then it dissociated under ambient conditions. THF hydrate dissociation experiments in each porous medium were conducted three times. Magnetic resonance imaging (MRI) was used to obtain images. Decomposition time, THF hydrate saturation and MRI mean intensity (MI) were measured and analyzed. The experimental results showed that the hydrate decomposition time in BZ-4 and BZ-3 was similar and longer than that in BZ-02. In each dissociation process, the hydrate decomposition time of the second and third cycles was shorter than that of the first cycle in BZ-4, BZ-3, and BZ-02. The relationship between THF hydrate saturation and time is almost linear.     

Accelerated nucleation of tetrahydrofuran(THF)hydrate in presence of ZIF-61

Clathrate hydrate can be used in energy gas storage and transportation,CO 2 capture and cool storage etc.However,these technologies are difficult to be used due to the low formation rate and long induction time of hydrate formation.In this paper,ZIF-61(zeolite imidazolate framework,ZIF) was first used in hydrate formation to stimulate hydrate nucleation.As an additive of clathrate hydrate,ZIF-61 promoted obviously the acceleration of tetrahydrofuran(THF) hydrate nucleation.It shortened the induction time of THF hydrate formation from 2-5 h to 0.3-1 h mainly due to the template function of ZIF-61 by which the nucleation of THF hydrate has been promoted.     

Study on thermal properties of TBAB–THF hydrate mixture for cold storage by DSC

In order to study the thermal properties of new type environment-friendly binary hydrate for cold storage in air-conditioning system, tests have been carried out by DSC comprehensively on the phase-change temperature and fusion heat of TBAB hydrate, THF hydrate, and TBAB–THF hydrate mixture. The results show a good trend that TBAB–THF hydrate has the superiority for more proper phase-change temperature and increased fusion heat. A broader and more developed view is that adding appropriate amount of hydrate with lower phase-change temperature to hydrate with higher one can make the hydrate mixture more suitable for cold storage (especially for 278–281 K); some hydrates with lower phase-change temperature can even make the fusion heat of mixture hydrate increased greatly. Several new environmental working pairs for binary gas hydrates have been listed to help to promote the application.     

Tuning methane content in gas hydrates via thermodynamic modeling and molecular dynamics simulation

Storage and transportation of natural gas as gas hydrate (“gas-to-solids technology”) is a promising alternative to the established liquefied natural gas (LNG) or compressed natural gas (CNG) technologies. Gas hydrates offer a relatively high gas storage capacity and mild temperature and pressure conditions for formation. Simulations based on the van der Waals–Platteeuw model and molecular dynamics (MD) are employed in this study to relate the methane gas content/occupancy in different hydrate systems with the hydrate stability conditions including temperature, pressure, and secondary clathrate stabilizing guests. Methane is chosen as a model system for natural gas. It was found that the addition of about 1% propane suffices to increase the structure II (sII) methane hydrate stability without excessively compromising methane storage capacity in hydrate. When tetrahydrofuran (THF) is used as the stabilizing agent in sII hydrate at concentration between 1% and 3%, a reasonably high methane content in hydrate can be maintained (∼85–100, v/v) without dealing with pressures more than 5 MPa and close to room temperature.     

Rapid hydrogen hydrate growth from non-stoichiometric tuning mixtures during liquid nitrogen quenching

In this study the rapid growth of sII H(2) hydrate within 20 min of post formation quenching towards liquid nitrogen (LN(2)) temperature is presented. Initially at 72 MPa and 258 K, hydrate samples would cool to the conditions of ~60 MPa and ~90 K after quenching. Although within the stability region for H(2) hydrate, new hydrate growth only occurred under LN(2) quenching of the samples when preformed hydrate "seeds" of THF + H(2) were in the presence of unconverted ice. The characterization of hydrate seeds and the post-quenched samples was performed with confocal Raman spectroscopy. These results suggest that quenching to LN(2) temperature, a common preservation technique for ex situ hydrate analysis, can lead to rapid unintended hydrate growth. Specifically, guest such as H(2) that may otherwise need sufficiently long induction periods to nucleate, may still experience rapid growth through an increased kinetic effect from a preformed hydrate template.     

Micro-Raman investigations of mixed gas hydrates

We report laser Raman spectroscopic measurements on mixed hydrates (clathrates), with guest molecules tetrahydrofuran (THF) and methane (CH₄), at ambient pressure and at temperatures from 175 to 280 K. Gas hydrates were synthesized with different concentrations of THF ranging from 5.88 to 1.46 mol%. In all cases THF molecules occupied the large cages of sII hydrate. The present studies demonstrate formation of sII clathrates with CH₄ molecules occupying unfilled cages for concentrations of THF ranging from 5.88 to 2.95 mol%. The Raman spectral signature of hydrates with 1.46 mol% THF are distinctly different; hydrate growth was non-uniform and structural transformation occurred from sII to sI prior to clathrate melting.     

Kinetic Analysis of Methane Hydrate Formation with Butterfly Turbine Impellers

Heat generation during gas hydrate formation is an important problem because it reduces the amount of water and gas that become gas hydrates. In this research work, we present a new design of an impeller to be used for hydrate formation and to overcome this concern by following the hydrodynamic literature. CH₄ hydrate formation experiments were performed in a 5.7 L continuously stirred tank reactor using a butterfly turbine (BT) impeller with no baffle (NB), full baffle (FB), half baffle (HB), and surface baffle (SB) under mixed flow conditions. Four experiments were conducted separately using single and dual impellers. In addition to the estimated induction time, the rate of hydrate formation, hydrate productivity and hydrate formation rate, constant for a maximum of 3 h, were calculated. The induction time was less for both single and dual-impeller experiments that used full baffle for less than 3 min and more than 1 h for all other experiments. In an experiment with a single impeller, a surface baffle yielded higher hydrate growth with a value of 42 × 10⁻⁸ mol/s, while in an experiment with dual impellers, a half baffle generated higher hydrate growth with a value of 28.8 × 10⁻⁸ mol/s. Both single and dual impellers achieved the highest values for the hydrate formation rates that were constant in the full-baffle experiments.     

Multiscale approach to CO2 hydrate formation in aqueous solution: phase field theory and molecular dynamics. Nucleation and growth

A phase field theory with model parameters evaluated from atomistic simulations/experiments is applied to predict the nucleation and growth rates of solid CO(2) hydrate in aqueous solutions under conditions typical to underwater natural gas hydrate reservoirs. It is shown that under practical conditions a homogeneous nucleation of the hydrate phase can be ruled out. The growth rate of CO(2) hydrate dendrites has been determined from phase field simulations as a function of composition while using a physical interface thickness (0.85+/-0.07 nm) evaluated from molecular dynamics simulations. The growth rate extrapolated to realistic supersaturations is about three orders of magnitude larger than the respective experimental observation. A possible origin of the discrepancy is discussed. It is suggested that a kinetic barrier reflecting the difficulties in building the complex crystal structure is the most probable source of the deviations.     

Adsorption-Inhibition of Clathrate Hydrates by Self-Assembled Nanostructures

The mechanism by which safranine O (SFO), an ice growth inhibitor, halts the growth of single crystal tetrahydrofuran (THF) clathrate hydrates was explored using microfluidics coupled with cold stages and fluorescence microscopy. THF hydrates grown in SFO solutions exhibited morphology changes and were shaped as truncated octahedrons or hexagons. Fluorescence microscopy and microfluidics demonstrated that SFO binds to the surface of THF hydrates on specific crystal planes. Cryo-TEM experiments of aqueous solutions containing millimolar concentrations of SFO exhibited the formation of bilayered lamellae with an average thickness of 4.2±0.2 nm covering several μm². Altogether, these results indicate that SFO forms supramolecular lamellae in solution, which might bind to the surface of the hydrate and inhibit further growth. As an ice and hydrate inhibitor, SFO may bind to the surface of these crystals via ordered water molecules near its amine and methyl groups, similar to some antifreeze proteins.     

A DFT study of the generation of interstellar species XN (X = Cl, and Br) activated by molecular sieve clusters

A cluster DFT (B3LYP) calculation study of ClN₃ and BrN₃ dissociation pathways catalyzed by molecular sieve is reported. A three-tetrahedral molecular sieve cluster (Al(OHSiH₃)(OSiH₃)(OH)₂, called T3) has been chosen in this study. Triplet-state products ClN and BrN are more easily produced with T3 molecular sieve cluster. Although the potential energy surfaces with T3 molecular sieve cluster are similar with those without T3 molecular sieve cluster, the dissociation process of ClN₃ and BrN₃ with T3 molecular sieve cluster becomes easier, which can be attributed to smaller energy gaps from the ground state reactant to the intersystem crossing point.     

Molecular hydrogen occupancy in binary THF-H2 clathrate hydrates by high resolution neutron diffraction

We have determined the time-space average filling of hydrogen molecules in a binary tetrahydrofuran (THF)-d(8) + D(2) sII clathrate hydrate using high resolution neutron diffraction. The filling of hydrogen in the lattice of a THF-d(8) clathrate hydrate occurred upon pressurization. The hydrogen molecules were localized in the small dodecahedral cavities at 20 K, with nuclear density from the hydrogen approximately spherically distributed and centered in the small cavity. With a formation pressure of 70 MPa, molecular hydrogen was found to only singly occupy the sII small cavity. This result helps explain discrepancies about the hydrogen occupancy in the THF binary hydrate system.     

Pressure-amorphized cubic structure II clathrate hydrate: crystallization in slow motion

A range of techniques has so far been employed for producing amorphous aqueous solutions. In case of aqueous tetrahydrofuran (THF) this comprises hyperquenching of liquid droplets, vapour co-deposition and pressure-induced amorphization of the crystalline cubic structure II clathrate. All of these samples are thermally labile and crystallize at temperatures above 110 K. We here outline a variant of the pressure-amorphization protocol developed by Suzuki [Phys. Rev. B, 2004, 70, 172108], which results in a highly crystallization resistant amorphous THF hydrate. The hydrate produced according to our protocol (annealing to 180 K at 1.8 GPa rather than to 150 K at 1.5 GPa) does not transform to the cubic structure II THF clathrate even at 150 K. We track the reason for this higher stability to the presence of crystalline remnants when following the Suzuki protocol, which are removed when using our protocol involving higher pressures and an annealing step. These crystalline remnants later serve as crystallization seeds lowering the thermal stability of the amorphous sample. Our protocol thus makes a purely amorphous THF hydrate available to the research community. We use powder X-ray diffraction to study the process of nucleation and slow crystal growth in the temperature range 160-200 K and find that the local cage structure and periodicity of the fully crystalline hydrate develops even at the earliest stages of crystallization, when the "clathrate crystal" has a size of about two unit cells.     

Thermodynamic predictions of various tetrahydrofuran and hydrogen clathrate hydrates

Tetrahydrofuran (THF) is one of the most widely used analogues for gas hydrates as well as a commonly used additive for reducing the formation pressure of a given hydrate process. Hydrates are also currently being investigated as storage materials for hydrogen as well as materials for hydrogen separations. Here we present a thermodynamic model, based on the CSMGem framework, that accurately captures the phase behavior of various hydrates containing THF and hydrogen. The model uses previously regressed parameters for components other than THF and H₂, and can reproduce hydrate formation conditions for a number of hydrates containing THF and/or hydrogen (simple THF, THF + CH₄, THF + N₂, THF + CO₂, THF + H₂, CH₄ + H₂, C₂H₆ + H₂ and C₃H₈ + H₂). The incorporation of THF and H₂ within this model framework will serve as a valuable tool for hydrate scenarios involving either of these components.