Impact of water film thickness on kinetic rate of mixed hydrate formation during injection of CO2 into CH4 hydrate

In this work, nonequilibrium thermodynamics and phase field theory (PFT) has been applied to study the kinetics of phase transitions associated with CO₂ injection into systems containing CH₄ hydrate, free CH₄ gas, and varying amounts of liquid water. The CH₄ hydrate was converted into either pure CO₂ or mixed CO₂?CH₄ hydrate to investigate the impact of two primary mechanisms governing the relevant phase transitions: solid‐state mass transport through hydrate and heat transfer away from the newly formed CO₂ hydrate. Experimentally proven dependence of kinetic conversion rate on the amount of available free pore water was investigated and successfully reproduced in our model systems. It was found that rate of conversion was directly proportional to the amount of liquid water initially surrounding the hydrate. When all of the liquid has been converted into either CO₂ or mixed CO₂?CH₄ hydrate, a much slower solid‐state mass transport becomes the dominant mechanism. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3944–3957, 2015     

Modeling of CO2‐Hydrate Formation at the Gas‐Water Interface in Sand Sediment

Sub‐seabed geological storage of CO₂ in the form of gas hydrate is attractive because clathrate hydrate stably exists at low temperature and high pressure, even if a fault occurs by diastrophism like a big earthquake. For the effective design of the storage system it is necessary to model the formation of CO₂‐hydrate. Here, it is assumed that the formation of gas hydrate on the interface between gas and water consists of two stages: gas diffusion through the CO₂‐hydrate film and consequent CO₂‐hydrate formation on the interface, between film and water. Also proposed is the presence of a fresh reaction interface, which is part of the interface between the gas and aqueous phases and not covered with CO₂‐hydrate. Parameters necessary to model the hydrate formation in sand sediment are derived by comparing the results of the present numerical simulations and the measurements in the literature.     

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     

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 the Influence of Hydrogel on CO2 Hydrate Formation

The influence of two differently cross‐linked polyacrylate particles on CO₂ hydrate formation was investigated. A series of up‐scaling experiments from small (high‐pressure differential scanning calorimetry, HP‐DSC) over medium (glass reactor) to large scale (HP‐reactor) was carried out. It was found out that there is a low influence on the induction time, which is an essential key parameter of the hydrate formation. The results show the same trends: with a low degree of cross‐linker used in low concentration CO₂ hydrate formation could be enhanced.     

Enhanced rate of gas hydrate formation in a fixed bed column filled with sand compared to a stirred vessel

The performance of two gas/liquid contact modes was evaluated in relation to the rate of gas hydrate formation. Hydrate formation experiments were conducted for several gas mixtures relevant to natural gas hydrate formation in the earth (CH₄, CH₄/C₃H₈, CH₄/C₂H₆ and CH₄/C₂H₆/C₃H₈) and two CO₂ capture and storage (CO₂, CO₂/H₂/C₃H₈). One set of experiments was conducted in a bed of silica sand, saturated with water (fixed fed column) while the other experiment was conducted in a stirred vessel for each gas/gas mixture. Both sets of experiments were conducted at a constant temperature. The rate of hydrate formation is customarily correlated with the rate of gas consumption. The results show that the rate of hydrate formation in the fixed bed column is significantly greater and thereby resulted in a higher percent of water conversion to hydrate in lesser reaction time for all the systems studied.     

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.     

Effects of Graphene Oxide Nanosheets and Al2O3 Nanoparticles on CO2 Uptake in Semi‐clathrate Hydrates

Gas hydrate/clathrate hydrate formation is an innovative method to trap CO₂ into hydrate cages under appropriate thermodynamic and/or kinetic conditions. Due to their excellent surface properties, nanoparticles can be utilized as hydrate kinetic promoters. Here, the kinetics of the CO₂ + tetra‐n‐butyl ammonium bromide (TBAB) semi‐clathrate hydrates system in the presence of two distinct nanofluid suspensions containing graphene oxide (GO) nanosheets and Al₂O₃ nanoparticles is evaluated. The results reveal that the kinetics of hydrate formation is inhibited by increasing the weight fraction of TBAB in aqueous solution. GO and Al₂O₃ are the most effective kinetic promoters for hydrates of (CO₂ + TBAB). Furthermore, the aqueous solutions of TBAB + GO or Al₂O₃ noticeably increase the storage capacity compared to TBAB aqueous solution systems.     

Hydrogen and carbon dioxide adsorption with tetra‐n‐butyl ammonium semi‐clathrate hydrates for gas separations

Gas adsorption rates of H₂, CO₂, and H₂‐CO₂ gas mixture (H₂/CO₂ = 3.4) with tetra‐n‐butyl ammonium salt (bromide, chloride, and fluoride) semi‐clathrate hydrate particles were measured at 269 K to assess their properties for gas separation. Equilibrium gas occupancies in the S‐cages of the particles were in order of (high to low) for hexagonal structure‐I, tetragonal structure‐I, and superlattice of cubic structure‐I structures with the maximum fractional occupancy by CO₂ being about 40%. The CO₂ diffusion rate depended on the anion size of the salt, which is attributed to distortion of the S‐cage that is close to the molecular size of CO₂. Simulations of semi‐clathrate hydrate particles with theory showed that H₂/CO₂ selectivities could be as high as 36 (3.0 mol% TBAF) and that selectivities for an ideal membrane (3.3 mol% TBAF) could be >100 (269 K, 0.3–4.5 MPa). Semi‐clathrate hydrates have wide application as separation media for gas mixtures. © 2014 American Institute of Chemical Engineers AIChE J, 61: 992–1003, 2015     

Replacement of CH4 in Hydrate in Porous Sediments with Liquid CO2 Injection

The dynamics of the replacement of CH₄ in hydrate in porous sediments with liquid CO₂ was investigated using a self‐developed experimental apparatus at different temperatures and initial pressures. The pressure increases steadily as the replacement reaction processes. The amount of the replaced CH₄ is almost the same as that of the CO₂ forming hydrate in the early stage and gradually becomes somewhat less in the later stage. The initial pressure has minor effects on the replacement rate, and temperature reduction causes a lower replacement rate. The experimental results suggest that the replacement rate is not related to the region of the temperature‐pressure conditions but is mainly affected by the fugacity differences of CH₄ hydrate decomposition and CO₂ hydrate formation.     

Development of formation and growth models of CO2 hydrate film

This study aims to develop models to estimate the CO₂ hydrate film formation and growth for different temperature and flow velocity conditions. First, the CO₂ hydrate film thickness at the initial stage of its formation is experimentally measured under different temperature and flow velocity conditions using laser interferometry. Based on the results, the CO₂ hydrate film thickness was found to decrease with increasing temperature and flow velocity. Next, the CO₂ hydrate film formation model and growth model are developed, and the models are verified using the present experimental data. Finally, the long term growth of CO₂ hydrate film thickness is estimated by the proposed growth model of CO₂ hydrate film thickness. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4078–4089, 2016     

Phase and kinetic behavior of the mixed methane and carbon dioxide hydrates

Large amounts of CH₄ are stored as hydrates on continental margins and permafrost regions. If the CH₄ hydrates could be converted into CO₂ hydrate, they would serve double duty as CH₄ sources and CO₂ storage sites in the deep ocean sediments. As preliminary investigations, both the phase behavior of CH₄ hydrates and kinetic behavior of CO₂ hydrate were measured at versatile conditions that can simulate actual marine sediments. When measuring three-phase equilibria (H-LW-V) containing CH₄ hydrate, we also closely examined pore and electrolyte effects of clay and NaCl on hydrate formation. These two effects inhibited hydrate nucleation and thus made the hydrate equilibrium line shift to a higher pressure region. In addition, the kinetic data of CO₂ hydrate in the mixtures containing clay and NaCl were determined at 2.0 MPa and 274.15 K. Clay mineral accelerated an initial formation rate of CO₂ hydrate by inducing nucleation as initiator, but total amount of formed CO₂, of course, decreased due to the capillary effect of clay pores. Also, the addition of NaCl in sample mixtures made both initial formation rate and total amount of CO₂ consumption decrease.     

Effects of thermal treatment on the CO2 sorption of triblockcopolymers derived from polyimide and poly(methylmethacrylate)

ABA‐type triblock copolymers were synthesized using 4,4‐(hexafluoroisopropylidene) diphthalic anhydride‐2,3,5,6‐tetramethyl‐1,4‐phenylenediamine (6FDA‐TeMPD) and poly(methyl methacrylate) (PMMA). The films were characterized by determining the effects of different content ratios and thermal decomposition of PMMA block on CO₂ sorption properties. TGA results showed that a thermal labile block can be completely decomposed under a previously reported thermal condition. SEM results presented that the asperity was micro‐phase separation caused by the PMMA block content rate. Numerous pores with sizes of approximately 10 to 50 nm were detected on Block(28/72) and Block(10/90). The isotherms of all films fitted the dual‐mode sorption model, and CO₂ sorption decreased with increased PMMA content rate. Infinite‐dilution CO₂ solubility depended on the Langmuir's site of each polymer because S_H0/S₀ of PI and Block(PI/PMMA) varied from 0.84 to 0.92 CO₂ affinity was increased by thermal treatment as indicated by the higher b and S₀ values of thermally treated films than those of nontreated films. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42208.     

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.     

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.     

Pressure variation due to heat shock of CO2hydrate desserts

CO₂ hydrate desserts are carbonated frozen desserts in which the CO₂ is trapped in a crystalline water‐carbon dioxide structure called a CO₂ clathrate hydrate. The CO₂ concentration of the dessert enables strong perception of carbonation, but CO₂ hydrate dissociation during heat shock can cause high package pressures during storage and distribution. In this work, a model is developed for package pressure as a function of temperature, CO₂ content, package volume, dessert mass, and recipe. The model is validated by comparison with an experimental measurement of the pressure and mass of a CO₂ hydrate dessert subjected to heat shock. It is shown that during heat shock a sealed package can reach pressures greater than the ice‐CO₂ hydrate equilibrium pressure. At pressures above the ice‐CO₂ hydrate equilibrium pressure, the fraction of water crystallized in the dessert can be increased, potentially mitigating heat shock damage. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Polymorphism and spherulite morphology of poly(1,4‐butylene adipate)/organically‐modified layered double hydroxide nanocomposites

A novel biodegradable poly(1,4‐butylene adipate)/organically‐modified layered double hydroxide (PBA/m‐LDH) nanocomposites are synthesized using the solution mixing process. The m‐LDH is originally prepared with magnesium nitrate hexahydrote (Mg(NO₃)₂ 6H₂O), aluminum nitrate‐9‐hydrate (Al(NO₃)₃ 9H₂O), oleic acid, and sorbitol by a novel one‐step co‐precipitation method to intercalate the organo‐modifier of oleic acid and sorbitol into the interlayer of LDH. The structure and morphology of the PBA/m‐LDH nanocomposites are characterized using X‐ray diffraction and transmission electron microscopy (TEM). It has been shown that the m‐LDH is exfoliated and well distributed in PBA matrix. The effect of m‐LDH on the polymorphic crystal and morphology of PBA at various crystallization temperatures (T_cs) would be investigated using WAXD and POM. Both data indicate that the addition of m‐LDH can change the starting formation temperature of α‐form crystals. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42526.     

Mass‐transfer rate enhancement for CO2 separation by ionic liquids: Theoretical study on the mechanism

To promote the development of ionic liquid (IL) immobilized sorbents and supported IL membranes (SILMs) for CO₂ separation, the kinetics of CO₂ absorption/desorption in IL immobilized sorbents was studied using a novel method based on nonequilibrium thermodynamics. It shows that the apparent chemical‐potential‐based mass‐transfer coefficients of CO₂ were in three regions with three‐order difference in magnitude for the IL‐film thicknesses in microscale, 100 nm‐scale, and 10 nm‐scale. Using a diffusion‐reaction theory, it is found that by tailoring the IL‐film thickness from microscale to nanoscale, the process was altered from diffusion‐control to reaction‐control, revealing the inherent mechanism for the dramatic rate enhancement. The extension to SILMs shows that the significant improvement of CO₂ flux can be obtained theoretically for the membranes with nanoscale IL‐films, which makes it feasible to implement CO₂ separation by ILs with low investment cost. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4437–4444, 2015     

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.