@conference {43382, title = {Molecular dynamics simulations of ch4 clathrate hydrate dissociation adjacent to hydrated silica surfaces}, booktitle = {7th International Conference on Gas Hydrates}, year = {2011}, month = {JUL}, abstract = {

We use molecular dynamics simulations to study the decomposition of structure I CH4 clathrate hydrate exposed to water. Simulations are performed for a hydrate + water system and two related cases with the hydrate adjacent to hydrate silica layers. The simulations are in the NVE adiabatic ensemble paying careful attention to the non-isothermal nature of the hydrate dissociation process. The effects of mass and energy transfer on the hydrate dissociation rate are studied. The effect of the silica surface on the dissociation rate and mechanism are studied. It is observed that the hydrate layer decomposes more quickly in simulations where it is in contact with silica. The implications on decomposition of the hydrate in hydrate reservoirs adjacent to sand sediments are discussed.

}, author = {Bagherzadeh, A. and Englezos, Peter and Alavi, S. and Kumar, Rajnish and Ripmeester, John A.} } @article {43381, title = {Multiple H2 occupancy of cages of clathrate hydrate under mild condition}, journal = {Journal of the American Chemical Society}, volume = {134}, year = {2012}, month = {MAY}, pages = {9160 - 9162}, abstract = {Experiments were carried out by reacting H(2) gas with N(2) hydrate at a temperature of 243 K and a pressure of 15 MPa. The characterizations of the reaction products indicated that multiple H(2) molecules can be loaded into both large and small cages of structure II clathrate hydrates. The realization of multiple H(2) occupancy of hydrate cages under moderate conditions not only brings new insights into hydrogen clathrates but also refreshes the perspective of clathrate hydrates as hydrogen storage media.}, doi = {10.1021/ja303222u}, author = {Lu, Hailong and Wang, Jianwei and Liu, Changling and Ratcliffe, Christopher I. and Becker, Udo and Kumar, Rajnish and Ripmeester, John A.} } @article { ISI:000324153700020, title = {Medium pressure hydrate based gas separation (HBGS) process for pre-combustion capture of carbon dioxide employing a novel fixed bed reactor}, journal = {International Journal of Greenhouse Gas Control}, volume = {17}, year = {2013}, month = {SEP}, pages = {206-214}, publisher = {ELSEVIER SCI LTD}, address = {THE BOULEVARD, LANGFORD LANE, KIDLINGTON, OXFORD OX5 1GB, OXON, ENGLAND}, abstract = {

This work presents an effective medium pressure hydrate based gas separation (HBGS) process for pre-combustion carbon dioxide capture in a novel fixed bed column. 2.5 mol\% propane was added to the fuel gas mixture as an additive to decrease the operating pressure of the HBGS process. Hydrate formation kinetics was investigated at three different pressures (4.5, 5.5 and 6.0 MPa respectively) and at 274.15 K. The performance of silica sand and silica gel as a medium was evaluated. In silica sand bed, multiple nucleation events were observed. In silica gel bed, the gas uptake and water conversion to hydrates was significantly low at any given driving force than that obtained in silica sand bed. Experiments at different water saturation levels (50, 75 and 100\%) in silica sand bed were investigated at 6.0 MPa and 274.15 K. It was found that at 50\% water saturation, gas consumed for hydrate formation and water conversion to hydrates was almost three times that at 100\% saturation. Water to hydrate conversions of up to 64.3\% was achieved after 4 h of hydrate formation for the 50\% water saturated silica sand bed. Our study presents an opportunity to scale up the HBGS process for CO2 capture with enhanced kinetics by employing a fixed bed reactor configuration. Decomposition experiments at a driving force of Delta T of 10 K and 23K were carried out to recover the gas consumed for hydrate formation and it was found that Delta T of 23 K was sufficient to recover the hydrated gas. (C) 2013 Elsevier Ltd. All rights reserved.

}, keywords = {Carbon dioxide capture, Gas hydrates, Global warming, Pre-combustion, Separation}, issn = {1750-5836}, doi = {10.1016/j.ijggc.2013.05.010}, author = {Babu, Ponnivalavan and Kumar, Rajnish and Linga, Praveen} } @article { ISI:000318468400029, title = {Morphology of carbon dioxide-hydrogen-cyclopentane hydrates with or without sodium dodecyl sulfate}, journal = {Crystal Growth \& Design}, volume = {13}, number = {5}, year = {2013}, month = {MAY}, pages = {2047-2059}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

In this study, effects of cyclopentane (CP) and sodium dodecyl sulfate (SDS) on the hydrate formation morphology were investigated. A gas mixture of 40.0 mol \% carbon dioxide and 60.0 mol \% hydrogen was used in an unstirred system with subcooling as the driving force. Experimental pressure is at 6.0 MPa and experimental temperatures used are at 275.65 and 277.65 K (Delta T = 15.15 and 13.15 K). Formation of hydrates started at the cyclopentane-liquid water interface. Cloud-like, equiaxed skewed dendritic, equiaxed orthogonal dendritic, long dendritic, and cactus-like crystals could be observed for the experiments in the absence of surfactants. Rapid hydrate formation was observed for the experiments with 0.9 mL CP with or without the presence of surfactants compared to the experiments with 0.45 mL CP system at the same experimental conditions. The addition of SDS had led to a change in the hydrate crystal morphology, forming fiber-like crystals from the hydrate layer. Hydrates had also shown affinity to metal surfaces and tend to grow rapidly because of better heat transfer capacity. Gas uptake measurements were found to correlate well with the morphological observations. On the basis of the morphological observations, the mechanism of the CO2-H-2-CP system in an unstirred system is presented.

}, issn = {1528-7483}, doi = {10.1021/cg400118p}, author = {Lim, Yu-An and Babu, Ponnivalavan and Kumar, Rajnish and Linga, Praveen} } @article { ISI:000359067400013, title = {Methane hydrate formation in a test sediment of sand and clay at various levels of water saturation}, journal = {Canadian Journal of Chemistry}, volume = {93}, number = {8, 1, SI}, year = {2015}, month = {AUG}, pages = {874-881}, publisher = {CANADIAN SCIENCE PUBLISHING, NRC RESEARCH PRESS}, address = {65 AURIGA DR, SUITE 203, OTTAWA, ON K2E 7W6, CANADA}, abstract = {

Kinetics of methane hydrate formation with different ratios of silica sand and clay and different water saturations were studied. At suitable temperature and methane gas pressure, water in the void spaces of silica sand packing and intercalated area of clay were converted into hydrate. It was observed that the rate of hydrate formation increases with higher void space in the packing, and addition of clay in test sediment decreases water to hydrate conversion as well as rate of hydrate formation. Maximum water to hydrate conversion of 60.0\% was achieved in pure silica sand bed at 75\% water saturation. Presence of fine clay particles is expected to reduce the void spaces and thus may hinder effective mass transfer of hydrate forming gases in the bed. However, it is also possible that the bentonite clay used in this work may actually inhibit hydrate growth. Additional experiments in stirred tank reactor were carried out to understand the inhibiting effect of bentonite clay for hydrate formation.

}, keywords = {clay, formation kinetics, methane hydrate, Silica sand, water saturation}, issn = {0008-4042}, doi = {10.1139/cjc-2014-0537}, author = {Kumar, Asheesh and Sakpal, Tushar and Roy, Sudip and Kumar, Rajnish} } @article { ISI:000351933600106, title = {Molecular dynamics study of model SI clathrate hydrates: the effect of guest size and guest-water interaction on decomposition kinetics}, journal = {Physical Chemistry Chemical Physics}, volume = {17}, number = {14}, year = {2015}, month = {FEB}, pages = {9509-9518}, publisher = {ROYAL SOC CHEMISTRY}, address = {THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND}, abstract = {

One of the options suggested for methane recovery from natural gas hydrates is molecular replacement of methane by suitable guests like CO2 and N-2. This approach has been found to be feasible through many experimental and molecular dynamics simulation studies. However, the long term stability of the resultant hydrate needs to be evaluated; the decomposition rate of these hydrates is expected to depend on the interaction between these guest and water molecules. In this work, molecular dynamics simulation has been performed to illustrate the effect of guest molecules with different sizes and interaction strengths with water on structure I (SI) hydrate decomposition and hence the stability. The van der Waals interaction between water of hydrate cages and guest molecules is defined by Lennard Jones potential parameters. A wide range of parameter spaces has been scanned by changing the guest molecules in the SI hydrate, which acts as a model gas for occupying the small and large cages of the SI hydrate. All atomistic simulation results show that the stability of the hydrate is sensitive to the size and interaction of the guest molecules with hydrate water. The increase in the interaction of guest molecules with water stabilizes the hydrate, which in turn shows a slower rate of hydrate decomposition. Similarly guest molecules with a reasonably small (similar to Helium) or large size increase the decomposition rate. The results were also analyzed by calculating the structural order parameter to understand the dynamics of crystal structure and correlated with the release rate of guest molecules from the solid hydrate phase. The results have been explained based on the calculation of potential energies felt by guest molecules in amorphous water, hydrate bulk and hydrate-water interface regions.

}, issn = {1463-9076}, doi = {10.1039/c5cp00678c}, author = {Das, Subhadip and Baghel, Vikesh Singh and Roy, Sudip and Kumar, Rajnish} } @article {43990, title = {Molecular dynamics simulation and experimental study on the growth of methane hydrate in presence of methanol and sodium chloride}, journal = {Energy Procedia}, volume = {105}, year = {2017}, month = {MAY}, pages = {5026-5033}, type = {Article}, abstract = {The plugging of processing and transportation lines by gas hydrate formation is a challenging problem for safe exploitation of oil and gas. The existence of water soluble third component (like methanol and sodium chloride) in the aqueous phase influence the gas hydrate formation thermodynamically also possibly affects the kinetics of hydrate growth. Inorganic salt and organic molecule (alcohols) at high concentration in the aqueous phase have been used as thermodynamic inhibitors to effectively prevent the hydrate formation. This study utilizes molecular dynamics as well as an experimental method to investigate the mechanism of the hydrate formation and the effect of additives. The MD simulation showed that at moderate temperature and pressure, a low concentration (1 wt \%) of methanol and NaCl enhances methane hydrate growth kinetics. Significant numbers of methanol molecules were observed inside the gas hydrate cages whereas Na$^{+}$/Cl$^{-}$ ions leach out during hydrate formation.}, doi = {10.1016/j.egypro.2017.03.1008}, author = {Choudhary, Nilesh and Kushwaha, Omkar Singh and Bhattacharjee, Gaurav and Chakrabarty, Suman and Kumar, Rajnish} } @article {44391, title = {Molecular view of CO2 capture by polyethylenimine: role of structural and dynamical heterogeneity}, journal = {Langmuir}, volume = {34}, year = {2018}, month = {MAY}, pages = {5138-5148}, abstract = {The molecular thermodynamics and kinetics of CO2 sorption in Polyethylenimine (PEI) melt have been investigated systematically using GCMC and MD simulations. We elucidate presence of significant structural and dynamic heterogeneity associated with the overall absorption process. CO2 adsorption in a PEI membrane shows a distinct two-stage process of a rapid CO2 adsorption at the interfaces (hundreds of picoseconds) followed by a significantly slower diffusion limited release toward the interior bulk regions of PEI melt (hundreds of nanoseconds to microseconds). The spatial heterogeneity of local structural features of the PEI chains lead to significantly heterogeneous absorption characterized by clustering and trapping of CO2 molecules that then lead to subdiffusive motion of CO2. In the complex interplay of interaction and entropy, the latter emerges out to be the major determining factor with significantly higher solubility of CO2 near the interfaces despite having lower density of binding amine groups. Regions having higher free-volume (entropically favorable) viz. interfaces, pores and loops demonstrate higher CO2 capture ability. Various local structural features of PEI conformations, for example, inter- and intrachain loops, pores of different radii, and di- or tricoordinated pores are explored for their effects on the varying CO2 adsorption abilities.}, doi = {10.1021/acs.langmuir.8b00204}, author = {Sharma, Pragati and Chakrabarty, Suman and Roy, Sudip and Kumar, Rajnish} } @article {46444, title = {Methane recovery from marine gas hydrates: a bench scale study in presence of low dosage benign additives}, journal = {Applied Energy}, volume = {253}, year = {2019}, month = {NOV }, pages = {113566}, type = {Article; Proceedings Paper}, abstract = {

De-pressurization is one approach which has been found to be economically feasible for methane recovery from marine hydrates. Hydrate dissociation being an endothermic process suggests that de-pressurization alone would not be sufficient and some additional stimulation would be required for sustained production from one such reservoir. Thermal stimulation may overcome the challenge posed by the endothermic dissociation process; however, economically it may not be ideal. A possible way out is to use thermal stimulation, but at relatively low temperatures as compared to conventional practice. This would be economical and can be accomplished in the presence of small doses of additives mixed in with the water stream used for thermal stimulation. In the present study, a number of benign additives were identified which when used in low concentrations enhance the kinetics of methane hydrate dissociation compared to pure water. Additives were first shortlisted from a wide potential pool using quantum mechanical calculations. These additives were later tested for their efficacy in stirred tank reactor to quickly identify the best additives for the job and few selected additives were then studied in a larger bench scale setup (fixed bed configuration) where they were injected in the form of an additive-water stream to dissociate already formed hydrates. Factors such as toxicity of the additive, fluidity of additive-water stream, foam formation on mixing of additive with water, etc. were also taken into account. An energy and efficiency analysis revealed that reported additives enhance the energy ratio and thermal efficiency of the process as compared to pure water stimulation.

}, keywords = {Bench scale, kinetics, Marine hydrate, methane hydrate, Methane recovery, Natural gas hydrate, Process}, issn = {0306-2619}, doi = {10.1016/j.apenergy.2019.113566}, author = {Bhattacharjee, Gaurav and Choudhary, Nilesh and Barmecha, Vivek and Kushwaha, Omkar S. and Pande, Nawal K. and Chugh, Parivesh and Roy, Sudip and Kumar, Rajnish} } @article {47168, title = {Molecular dynamics study on growth of carbon dioxide and methane hydrate from a seed crystal}, journal = {Chinese Journal of Chemical Engineering}, volume = {27}, year = {2019}, month = {SEP}, pages = {2074-2080}, type = {Article}, abstract = {

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. (C) 2019 The Chemical Industry and Engineering Society of China, and Chemical Industry Press Co., Ltd. All rights reserved.

}, keywords = {Cage dynamics, CH4 recovery, CO2 sequestration, F-4 order parameter, kinetics, Natural gas hydrate, Thermodynamics}, issn = {1004-9541}, doi = {10.1016/j.cjche.2019.02.006}, author = {Nakate, Prajakta and Ghosh, Bappa and Das, Subhadip and Roy, Sudip and Kumar, Rajnish} } @article {46627, title = {Macro and molecular level insights on gas hydrate growth in the presence of hofmeister salts}, journal = {Industrial \& Engineering Chemistry Research}, volume = {59}, year = {2020}, month = {NOV }, pages = {20591-20600}, type = {Article}, abstract = {

The effect of few monovalent salts (NaCl, NH4Cl, and GdmCl) as additives, according to the Hofrneister series on the growth of methane gas hydrates, has been studied using experiments as well as molecular dynamics (MD) simulation. Further, the Hofmeister effects on hydrate crystallization have been correlated with the methanol as an additive, which is a known thermodynamic hydrate inhibitor for hydrate growth. One of the previous studies (discussed later in this article) available in the literature concludes that methane hydrate formation from ice might show enhanced kinetics in the presence of salts; this behavior is contrary to the general usage of such salts as hydrate inhibitors. This conclusion may not necessarily be true for experiments done with liquid water, and therefore, this work explores the behavior of these salts in a lab-scale setup. In addition, current work reports detailed MD simulation studies to gain insight into the mechanism of hydrate formation in the presence of Hofmeister series salts at two different concentrations of 1 and 10 wt \% in water and compare the results with hydrate formation in the methanol-water system. Our study suggests that the presence of these additives at low concentrations (1 wt \%) does enhance the hydrate growth kinetics. However, at higher concentrations (10 wt \%), inhibition by these additives persisted, and prolonged nucleation, as well as retarded growth, was observed.

}, issn = {0888-5885}, doi = {10.1021/acs.iecr.0c04389}, author = {Choudhary, Nilesh and Kushwaha, Omkar Singh and Bhattacharjee, Gaurav and Chakrabarty, Suman and Kumar, Rajnish} } @article {46618, title = {Morphology and dynamics of self-assembled structures in mixed surfactant systems (SDS plus CAPB) in the context of methane hydrate growth}, journal = {Journal of Molecular Liquids}, volume = {319}, year = {2020}, month = {DEC }, pages = {114296}, type = {Article}, abstract = {

Presence of small dosages of surfactants in the aqueous phase has been reported to enhance the rate of gas hydrate (clathrate) formation. In this work, using extensive atomistic molecular dynamics (MD) simulations, we have investigated how SDS (surfactant) aggregates in the presence/absence of CAPB (co-surfactant) at ambient conditions (temperature 298 K and pressure 1 bar) and at hydrate forming conditions (temperature 275 K and pressure 50 bar) resulting in altered growth kinetics of methane hydrate. We observe that SDS forms aggregates of different sizes and shapes depending on the thermodynamic condition starting from random distributions of the surfactants. In the presence of the CAPB co-surfactant, tightly packed mixed aggregates are formed. Using various structural order parameters, we demonstrate that shape of the aggregates deviates from spherical as well as cylindrical symmetry. During the aggregation process in presence of methane, the methane molecules get absorbed into the aggregates, enhance the aggregation kinetics and provide structural flexibility to the aggregates. This result is partly in agreement with previous experimental observations that SDS (with or without a co-surfactant) may form micelle-like structures under hydrate forming conditions and that methane gets absorbed by these aggregates leading to enhanced solubility of methane in the aqueous phase. Further, we have investigated the dynamics of shape fluctuations of the aggregates and observed that several distinct relaxation timescales exist in these heterogenous systems. (C) 2020 Elsevier B.V. All rights reserved.

}, issn = {0167-7322}, doi = {10.1016/j.molliq.2020.114296}, author = {Hande, Vrushali and Choudhary, Nilesh and Chakrabarty, Suman and Kumar, Rajnish} }