One‐dimensional productivity assessment for on‐field methane hydrate production using CO2/N2 mixture gas

The direct recovery of methane from gas hydrate‐bearing sediments is demonstrated, where a gaseous mixture of CO₂ + N₂ is used to trigger a replacement reaction in complex phase surroundings. A one‐dimensional high‐pressure reactor (8 m) was designed to test the actual aspects of the replacement reaction occurring in natural gas hydrate (NGH) reservoir conditions. NGH can be converted into CO₂ hydrate by a “replacement mechanism,” which serves double duty as a means of both sustainable energy source extraction and greenhouse gas sequestration. The replacement efficiency controlling totally recovered CH₄ amount is inversely proportional to CO₂ + N₂ injection rate which directly affecting solid ‐ gas contact time. Qualitative/quantitative analysis on compositional profiles at each port reveals that the length more than 5.6 m is required to show noticeable recovery rate for NGH production. These outcomes are expected to establish the optimized key process variables for near future field production tests. © 2014 American Institute of Chemical Engineers AIChE J, 61: 1004–1014, 2015     

Experimental study on CO2 hydrate formation in the presence of TiO2, SiO2, MWNTs nanoparticles

ABSTRACT

A series of experiments on CO₂ hydrate formation were carried out in the presence of titanium dioxide (TiO₂), silicon dioxide (SiO₂), multi-walled carbon nanotubes (MWNTs) nanoparticles. The effects of these nanoparticles on induction time, final gas consumption, and gas storage capacity have been investigated at the temperature of 274.15 K and the initial pressure of 5.0 MPa.g. The induction time of CO₂ hydrate formation was remarkably shortened to 12.5 min in the presence of 0.005 wt% MWNTs nanoparticles. The high thermal conductivity and heat capacity of MWNTs nanoparticles presented better heat transfer, and large surface area provided more suitable sites for heterogeneous nucleation of CO₂ hydrate.     

Improving the gas recovery and separation efficiency of a hydrate-based gas separation

Efficient gas recovery and separation in a hydrate-based system was investigated for a model gaseous mixture of R22 and nitrogen. The formed hydrate settled in the recovery vessel; excess water was recycled and the hydrate was subsequently decomposed by releasing pressure from the vessel. The gas uptake rate of R22 gas from the vapor phase and the gas recovery rate from the hydrate were determined from hydrate formation and decomposition, respectively. The gas recovery rate of R22 gas gradually increased with time. On the contrary, the nitrogen gas recovery rate was a maximum in the initial stage of hydrate decomposition. A high separation factor (S.F.) was achieved by first separating the N₂-rich gas generated during initial hydrate decomposition. An efficient hydrate-based gas separation and recovery process is proposed.     

The downhole hydrocyclone separator for purifying natural gas hydrate:structure design,optimization, and performance

ABSTRACT

The exploitation efficiency of natural gas hydrate is highly affected by sand production. In this paper, hydrocyclone purification separator was designed. A combination of single factor with computational fluid dynamics (CFD) was used to optimize the structure parameters. The performance of optimized hydrocyclone was also investigated. It shows that the sand separation efficiency increases from 90.4% to 98.7%, and the natural gas hydrate separation efficiency increases from 89.5% to 97.8%. Furthermore, the cut size of sand and natural gas hydrate are as low as 3 and 2 µm, respectively, and separation efficiency remains above 80% under inlet parameters. It can provide some reference for the design and manufacture of the in situ purification separator for gas hydrate mixture slurry.     

Gas hydrate formation from hydrogen/carbon dioxide and nitrogen/carbon dioxide gas mixtures

Gas hydrates from CO₂/N₂ and CO₂/H₂ gas mixtures were formed in a semi-batch stirred vessel at constant pressure and temperature of 273.7 K. These mixtures are of interest to CO₂ separation and recovery from flue gas and fuel gas, respectively. During hydrate formation the gas uptake was determined and the composition changes in the gas phase were obtained by gas chromatography. The rate of hydrate growth from CO₂/H₂ mixtures was found to be the fastest. In both mixtures CO₂ was found to be preferentially incorporated into the hydrate phase. The observed fractionation effect is desirable and provides the basis for CO₂ capture from flue gas or fuel gas mixtures. The separation from fuel gas is also a source of H₂. The impact of tetrahydrofuran (THF) on hydrate formation from the CO₂/N₂ mixture was also observed. THF is known to substantially reduce the equilibrium formation conditions enabling hydrate formation at much lower pressures. THF was found to reduce the induction time and the rate of hydrate growth.     

In situ injection of THF to trigger gas hydrate crystallization: Application to the evaluation of a kinetic hydrate promoter

This paper investigates an original method to efficiently trigger gas hydrate crystallization. This method consists of an in situ injection of a small amount of THF into an aqueous phase in contact with a gas-hydrate-former phase at pressure and temperature conditions inside the hydrate metastable zone. In the presence of a CO₂–CH₄ gas mixture, our results show that the THF injection induces immediate crystallization of a first hydrate containing THF. This triggers the formation of the CO₂–CH₄ binary hydrate as proven by the pressure and temperature reached at equilibrium. This experimental method, which “cancels out” the stochasticity of the hydrate crystallization, was used to evaluate the effect of the anionic surfactant SDS at different concentrations, on the formation kinetics of the CO₂–CH₄ hydrate. The results are discussed and compared with those published in a recent article (Ricaurte et al., 2013), where THF was not injected but present in the aqueous phase from the beginning and at much higher concentrations.     

Quantitative analysis of CO2 hydrate formation in porous media by proton NMR

Gas hydrate is a nonstoichiometric crystal compound formed from water and gas. Most nonvisual studies on gas hydrate are unable to detect how much water is converted to hydrates, and thus, the hydrate stoichiometry calculations are inaccurate. This study investigated the CO₂ hydrate formation process in porous media directly and quantitatively. The characteristics of the time-variable consumption of hydrate formation indicated a two-stage formation, hydrate enclathration and continuous occupancy. The enclathration stage occurred in the first 20 min of the formation when considerable heat is released. The continuous occupancy stage lasted longer than the hydrate enclathration because the empty cages in previously formed hydrates would also be occupied. The higher formation pressures can accelerate water consumption and increase cage occupancy. The compositions of completely formed CO₂ hydrates at 2.7, 3.0, and 3.3 MPa and 275.15 K were determined as CO₂·6.90H₂O, CO₂·6.70H₂O, and CO₂·6.49H₂O, respectively.     

Methanol decomposition to synthesis gas over supported Pd catalysts prepared from synthetic anionic clays

Supported Pd or Rh catalysts were prepared by the solid-phase crystallization method starting from hydrotalcite anionic clay minerals based on [Mg₆Al₂(OH)₁₆CO ₂ ²⁻ ]·4H₂O as the precursors. The precursors were prepared by a coprecipitation method from the raw materials containing Pd²⁺ and various trivalent metal ions which can replace each site of Mg²⁺ and Al³⁺ in the hydrotalcite. Rh³⁺ was also used for preparing the catalyst as comparison. The precursors were then thermally decomposed and reduced to form supported Pd or Rh catalysts and used for the methanol decomposition to synthesis gas. Among the precursors tested, use of Mg–Cr hydrotalcite containing Pd²⁺ resulted in the formation of efficient Pd supported catalysts for the production of synthesis gas by selective decomposition of methanol at low temperature. Although Pd²⁺ cannot well replace the Mg²⁺ site in the hydrotalcite, the Pd supported catalyst (Pd/Mg–Cr) prepared by the solid-phase crystallization method formed highly dispersed Pd metal particles and showed much higher activity than that prepared by the conventional impregnation method. When the precursor was prepared under mild conditions, more fine particles of Pd metal were formed over the catalyst, resulting in high activity. It is likely that the high activity may be due to the highly dispersed and stable Pd metal particles assisted by the role of Cr as the co-catalyst. This revised version was published online in November 2006 with corrections to the Cover Date.     

Experimental analysis of the efficiency on charge/discharge cycles in natural gas storage by adsorption

The use of vessels filled with activated carbon to store and transport natural gas (NG) at moderate pressures (about 3.5 MPa) and ambient temperature (about 298 K) has been studied as a potential alternative to compressed natural gas at high pressures (ca. 20 MPa). The present study provides an experimental investigation of charge and discharge cycles of natural gas in a prototype storage vessel filled with activated carbon and analyses the effect of the gas composition on the adsorption capacity. The adsorption properties were evaluated by measuring isotherms for each component of NG in a magnetic suspension balance. The selectivities of the main constituents of natural gas in relation to methane were determined and the influence of the pressure on the selectivity was also observed. Although NG is composed mainly of methane (ca. 90% vol.), our experimental results indicate that the preferential adsorption of the heavier hydrocarbons and CO₂ should be properly taken into account for the evaluation of the behavior of adsorbed natural gas systems along several charge and discharge cycles.     

Molecular dynamics study on growth of carbon dioxide and methane hydrate from a seed crystal

In the current work, molecular dynamics simulation is employed to understand the intrinsic growth of carbon dioxide and methane hydrate starting from a seed crystal of methane and carbon dioxide respectively. This comparison was carried out because it has relevance to the recovery of methane gas from natural gas hydrate reservoirs by simultaneously sequestering a greenhouse gas like CO₂. The seed crystal of carbon dioxide and methane hydrate was allowed to grow from a super-saturated mixture of carbon dioxide or methane molecules in water respectively. Two different concentrations (1:6 and 1:8.5) of CO₂/CH₄ molecules per water molecule were chosen based on gas–water composition in hydrate phase. The molecular level growth as a function of time was investigated by all atomistic molecular dynamics simulation under suitable temperature and pressure range which was well above the hydrate stability zone to ensure significantly faster growth kinetics. The concentration of CO₂ molecules in water played a significant role in growth kinetics, and it was observed that maximizing the CO₂ concentration in the aqueous phase may not result in faster growth of CO₂ hydrate. On the contrary, methane hydrate growth was independent of methane molecule concentration in the aqueous phase. We have validated our results by performing experimental work on carbon dioxide hydrate where it was seen that under conditions appropriate for liquid CO₂, the growth for carbon dioxide hydrate was very slow in the beginning.     

Quantitative analysis of gas hydrates using Raman spectroscopy

A calibration protocol to quantify the compositional information of gas hydrates using Raman spectroscopy is proposed. Structure I pure CH₄‐, CO₂‐ and C₂H₆‐hydrates in their deuterated and hydrogenated forms with known cage occupancies were investigated by Raman spectroscopy. Raman scattering cross sections of CH₄ in the large and small cages were found to be very similar, but not identical. Some C₂H₆ bands of C₂H₆‐hydrate were tentatively reassigned or newly reported and assigned. Our results show that the relative cross sections of guest vibrational modes in the deuterated hydrate are in agreement with those in the hydrogenated hydrate, whereas they are considerably different from those in fluid phase. Using our Raman quantification factors, the relative cage occupancies can now be determined more reliably in CH₄‐hydrates. Moreover, with additional assumptions, the absolute cage occupancies, the bulk guest composition and hydration number of pure or mixed gas hydrates become accessible by Raman spectroscopy. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2155–2167, 2013     

Efficient Cycle Recovery of Hydrogen from a Low Concentration Pyrolysis Gas Stream by Pressure Swing Adsorption – An Experimental Evaluation

Breakthrough curves, cycle mass balances, and cycle bed productivities (mg H₂ per gram of adsorbent) on three dual adsorbent amounts (g) of 2,892, 1,963, and 1,013 respectively each filling 200 cm, 135 cm, and 70 cm of a 5.0 cm internal diameter stainless steel pipe were performed. The approximate optimum (sludge pyrolysis) synthesis gas with composition in volume % of 45% H₂/35% CO/20% CH₄ was used as the feed gas with molecular sieve 5 Å and activated carbon as adsorbents. Impurity breakthroughs occurred at ～14.9, 12.3, and 5.0 minutes respectively for % cycle recoveries of 72.2, 65.0, and 60.2 using 2,892, 1,962, and 1,013 g of adsorbent respectively. Our results indicated that basing % recycle recovery on cycle bed productivity can enable efficient hydrogen recovery with savings on adsorbent amount. An optimum cycle bed productivity of 2.3 mg H₂/g of adsorbent corresponded to a cycle recovery of 66.2% for 2,300 g of adsorbent used. Only 1.7 mg H₂/g of adsorbent was obtained for a cycle recovery of 72.2% requiring up to 2,800 g of adsorbent. This makes economic sense in the pressure swing adsorption separation of hydrogen from traditionally low hydrogen concentration biomass sources.     

Prediction of induction time for natural gas components during gas hydrate formation in the presence of kinetic hydrate inhibitors in a flow mini‐loop apparatus

The objective of this work is the prediction of induction time (t_i) for simple gas hydrate formation in the presence or absence of kinetic hydrate inhibitors at various conditions based on the Kashchiev and Firoozabadi model in a flow mini‐loop apparatus. For this purpose, the t_i model is developed for simple gas hydrate formation in batch system for natural gas components during hydrate formation in a flow mini‐loop apparatus. A laboratory flow mini‐loop apparatus is designed and built up to measure the t_i for simple gas hydrate formation when a hydrate former (such as C₁, C₃, CO₂ and i‐C₄) is contacted with water in the absence or presence of dissolved inhibitor, such as poly vinylpyrrolidone, PVCap and L ‐tyrosine. 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 average absolute deviation (AAD) of the predicted t_i values from the corresponding experimental data are found to be about 11% and 9.4% for gas hydrate formation t_i in the presence or absence of kinetic hydrate inhibitors, respectively. © 2012 Canadian Society for Chemical Engineering     

The status of exploitation techniques of natural gas hydrate

Natural gas hydrate (NGH) has been widely considered as an alternative form of energy with huge potential, due to its tremendous reserves, cleanness and high energy density. Several countries involving Japan, Canada, India and China have launched national projects on the exploration and exploitation of gas hydrate resources. At the beginning of this century, an early trial production of hydrate resources was carried out in Mallik permafrost region, Canada. Japan has conducted the first field test from marine hydrates in 2013, followed by another trial in 2017. China also made its first trial production from marine hydrate sediments in 2017. Yet the low production efficiency, ice/hydrate regeneration, and sand problems are still commonly encountered; the worldwide progress is far before commercialization. Up to now, many gas production techniques have been proposed, and a few of them have been adopted in the field production tests. Nevertheless, hardly any method appears really promising; each of them shows limitations at certain conditions. Therefore, further efforts should be made on the economic efficiency as well as sustainability and environmental impacts. In this paper, the investigations on NGH exploitation techniques are comprehensively reviewed, involving depressurization, thermal stimulation, chemical inhibitor injection, CO₂–CH₄ exchange, their combinations, and some novel techniques. The behavior of each method and its further potential in the field test are discussed. The advantages and limitations of laboratory studies are also analyzed. The work could give some guidance in the future formulation of exploitation scheme and evaluation of gas production behavior from hydrate reservoirs.     

Techno-economic analysis of a biological desulfurization process for a landfill gas in Korea

ABSTRACT

We report techno-economic analysis of a biological desulfurization process to remove H₂S and to produce sulfur from a landfill gas (2000 m³ h^?1) produced in Korea. With a process simulation model developed using Aspen Plus®, parametric assessment to determine the effect of various operating parameters such as a NaOH flow rate, a NaOH concentration, and a recycle ratio has been carried out. Based on results from process simulation, economic analysis was conducted to evaluate feasibility of this technology in Korea through a cash flow diagram, net present value (NPV), and discounted payback period (DPBP). It was demonstrated that DPBP of 6.9 years and NPV of 0.39 MM$ were obtained with a 10% discount rate.     

Annealing temperature controlled crystallization mechanism and properties of gallium oxide film in forming gas atmosphere

Gallium oxide (Ga₂O₃) films had been fabricated on Al₂O₃(0001) substrate by employing pulsed laser deposition (PLD) and annealed at different temperatures under forming gas (FG) atmosphere (95% N₂ + 5% H₂). The influence of annealing temperature on the structural, optical, chemical composition, and surface morphological properties of the Ga₂O₃ thin films was investigated comprehensively. The annealing processes with hydrogen gas play a crucial role in the characteristics of Ga₂O₃ thin films. A crystallization mechanism of Ga₂O₃ films controlled by annealing temperature has been proposed firstly and analyzed systematically, which contains three kinds of competitive mechanism, namely the thermal enhanced crystallization, the enhanced H₂ dissociative adsorption on Ga₂O₃ surfaces, and the high-temperature decomposition of Ga₂O₃. Both Ga⁺ and Ga³⁺ oxidation valence states were presented in all samples, which indicated lattice oxygen deficiency in Ga₂O₃ films. The variation of the non-lattice oxygen proportion of Ga₂O₃ films related to the crystallization mechanism firstly increased and then decreased with the increase of annealing temperature. The detailed crystallization mechanism of PLD-Ga₂O₃ films annealed in FG offers a guideline and references for the further fabrication of high-quality Ga₂O₃ films and their applications in high-performance devices.     

STUDY OF CAPTURING EMITTED CO2 IN THE FORM OF HYDRATES IN A TUBULAR REACTOR

Stabilizing atmospheric CO₂ concentration requires the development of novel methods for capturing it in the form of permanent reservoirs. Among the proposed methods is CO₂ storage in the form of hydrate. In this study a method was established for CO₂ conversion to hydrate. This method can be applied to bioethanol plants, which produce CO₂ as a by-product of ethanol fermentation. In this regard, a tubular recirculating flow reactor was developed for the study of CO₂ hydrate formation. The experiments were carried out at 279 K and 3.5–5 MPa to determine the rate of CO₂ hydrate formation. Further, a model was developed for prediction of the rate of hydrate formation based on the mass transfer, crystallization, and thermodynamic concepts. The predicted hydrate formation rate was compared to the experimental data in order to validate the model prediction. The predicted results were in good agreement with the experimental data at different operating conditions.     

Study on the influence of SDS and THF on hydrate-based gas separation performance

Hydrate additives can be used to mitigate hydrate formation conditions, promote hydrate growth rate and improve separation efficiency. CO₂ + N₂ and CO₂ + CH₄ systems with presence of sodium dodecyl sulfate (SDS) or tetrahydrofuran (THF) are studied to analyze the effect of hydrate additives on gas separation performance. The experiment results show that CO₂ can be selectively enriched in the hydrate phase. SDS can speed up the hydrate growth rate by facilitating gas molecules solubilization. When SDS concentration increases, split and loss fraction increase initially and then decrease slightly, resulting in a decreased separation factor. The optimum concentration of SDS exists at the range of 100–300 ppm. As THF can be easily encaged in hydrate cavities, hydrate formation condition can be mitigated greatly with its existence. Additionally, THF can also strengthen hydrate formation. The THF effect on separation performance is related to feed gas components. CO₂ occupies the small cavities of type II hydrate prior to N₂. But the competitiveness of CO₂ and CH₄ to occupy cavities are quite fair. The variations of split fraction, loss fraction and separation factor depend on the concentration of THF added. The work in this paper has a positive role in flue gas CO₂ capture and natural gas de-acidification.     

Evolved gas analysis during sintering of barium titanate

The gases evolved during the sintering of BaTiO₃ have been examined with a combined dilatometer and mass spectrometer (CDMS) apparatus. The CDMS acquires multiple mass/charge ratios in real time while simultaneously recording dilatometry data. To identify the chemical composition of the numerous recorded mass/charge ratios, cracking patterns, isotopic abundances and decomposition reactions from model compounds (BaCO₃, BaSO₄) were used. Three primary regions of gas evolution were identified. During the heating ramp and into the hold period at 1350°C, CO₂ appears, and below approximately 500°C, this may arise from adsorbed or surface CO₂. Sulphur dioxide was also observed, and its evolution occurred directly after the majority of the CO₂ appeared and immediately preceded the onset of sintering. Above 1200°C, CO₂ is the primary species observed in the gas phase. The implications of the high temperature chemistry on sintering and on microstructural development are discussed.     

Crystallization and phase transition of tobermorite synthesized by hydrothermal reaction from dicalcium silicate

In this paper, tobermorite was hydrothermally synthesized from the dicalcium silicate (C₂S) in sodium silicate solution, and the crystallization and phase transition process were investigated in detail using XRD, Raman spectra, and SEM. The tobermorite is difficult to synthesize when the temperature is lower than 160°C because it gets converted into xonotlite (without Na₂O) or pectolite (with Na₂O) when the temperature is higher than 180°C. The crystallization process of tobermorite shows “S” trend with the increase in time, which can be divided into three stages: the nucleation stage, rapid crystal growth stage, and perfect crystal forming stage. During the crystallization, 90% of the crystallization of tobermorite is completed in the stage of rapid crystal growth. Raman spectra and SEM analysis show that with the increase in hydrothermal time, the C₂S of monomer (Q⁰) is first converted into the calcium silicate hydrate of sheet (Q² and Q³), and then continues to convert into tobermorite of chain (Q²).