Rajnish Kumar

Clathrate hydrates have a variety of applications ranging from natural gas storage, carbon dioxide sequestration, sea water desalination, gas separation etc. But the applications have still not reached the industrial stage and commercialization, although the concept has been demonstrated. In contrast, scale-up and demonstration of technologies are often done without proper understating of the process at the molecular level. One could always tune the process for better performance if proper knowledge of the process is built by understanding the same at the molecular level. Gas hydrate is essentially a crystallisation phenomenon, and since this is a multiple component and multi-phase system, the mechanism of nucleation and growth of hydrate has to be understood clearly. There are various theories behind the mechanism of hydrate nucleation. The theories talk about the stochastic nature and the steps leading to hydrate formation. Apart from that, there are variety of factors that affect hydrate decomposition. Many of these aspects become challenging to understand using meso and macro scale experiments. This is where Molecular Dynamics simulations plays a significant role. This review provides a molecular understanding of existing theories about nucleation & growth; further, factors affecting decomposition and the analysis techniques that are used to quantify such mechanism are also discussed.

This study is focused on the valorization of heterogeneous municipal solid waste collected from the landfill using hydrothermal liquefaction process using glycerol as a co-solvent. The effects of temperature (300–350 °C) and residence time (15–45 min) on the yields and quality of the product fractions were investigated at 8 wt% solid loading. The yield of bio-crude significantly increased from 15.2 wt% with water as the solvent, to 58 wt% with water-glycerol (1:1 v/v) as the solvent possessing an energy content of 35.6 MJ/kg at 350 °C, 30 min. The quality of the bio-crude obtained using glycerol was comparable to that using tetralin as a hydrogen donor co-solvent. Phenolic compounds and cyclooxygenates were the major compounds in the bio-crude, and aliphatic hydrocarbons increased with residence time. Maximum energy recovery of 95% was achieved in the products with an energy consumption ratio of 0.43 for the bio-crude signifying the energetic feasibility of the process.

Sequestration of greenhouse gases, like CO2, in methane hydrates reduces both environmental pollution and increases fuel recovery of methane simultaneously. This approach is thermodynamically feasible as a result of the higher stability of carbon dioxide hydrates than methane hydrates. However, a few bottlenecks, like low permeability and the replacement kinetics of CO2 beyond the surface layers of the hydrate, limit this process. Therefore, the objective of this study is to understand this phenomenon at a molecular level by conducting long-scale molecular dynamics simulations for a microsecond. It then tries to verify if the defects caused in the structure with a tertiary molecule, like ammonia, can enhance the process by carrying CO2 beyond the surface layers. Additionally, the impact of ammonia on the methane recovery for the same concentrations of CO2 is also studied. Analysis of properties like hydrogen bonding and changes in the number of cages is conducted, which indicates encouraging results for such an enhancement upon optimizing the concentrations of ammonia.

While oleaginous yeasts have been extensively investigated for single-cell oil production, less attention has been paid towards valorization of the erstwhile-discarded de-oiled biomass. The current study demonstrates the valorization potential of the de-oiled biomass through production of bio-crude by hydrothermal liquefaction process. The performance of the process was enhanced by using glycerol as co-solvent and also by optimizing the processing conditions. Glycerol addition enhanced the bio-crude yield up to 50 wt% with an improved energy content of 32 MJ/kg. The major organic groups found in the bio-crude are acids, esters and phenolics. Typical energy recovery achieved in bio-crude was in the range of 43–62%. Thus, strategic valorization of yeast into green crude with net energy gain, as evident from the energy balance, implies the process is energetically favourable.

Clathrate (gas) hydrates as materials have received great interest due to their high-density gas storage potential and separation applications for their ability to preferentially separate a targeted component such as CO2 from waste streams. Among the three clathrate hydrate structures, only sH (structure H) hydrates require a large molecule such as neohexane (NH) or tert-butyl methyl ether (TBME) as well as a "help-gas"molecule such as methane (CH4) to form a stable structure. However, attempts to use CO2 as a help-gas came up with mixed results where it appeared that the sH hydrate formed was stable only at temperatures below the ice point, whereas the compound formed with CH4 was considerably more stable. sH hydrates have considerable potential for gas separation, for instance, of CH4-CO2 mixtures. Thus, in this work, several compositions were tested for their hydrate forming ability. The large cage guests tested were NH and TBME, the help-gas mixtures were a CO2-rich mixture (76% CO2 and 24% CH4), and a CO2 lean mixture (24% CO2 and 76% CH4). The phase behavior of hydrates formed from the various combinations was tested by measuring the endo- and exotherms associated with hydrate formation and decomposition in a high-pressure differential calorimeter. The different phases indicated from the DSC results were identified by employing an in situ high-pressure cell on a powder X-ray diffractometer. Powder patterns were recorded to identify the crystal phase arising from nucleation and possible re-crystallization events after annealing. It was confirmed that the CO2 lean mixture (24% CO2 and 76% CH4) forms structure H (sH) hydrate, while the CO2-rich mixture (76% CO2 and 24% CH4) forms structure I (sI) hydrate presenting a structure transition pattern across the gas mixtures investigated. Further, it was observed that the CO2 lean mixture, which forms sH hydrate, also starts as sI hydrate and gradually converts to the thermodynamically stable sH hydrate. This study, in essence, helps to understand the preference of CH4 and CO2 for three different types of cages in sH hydrates and presents a design framework for a suitable gas separation mechanism for this gas mixture of interest.

Gas hydrates have been the nucleus of research from a sustainable engineering standpoint, considering their unique applications in a broad spectrum of scientific contexts. One such application is the sequestration of gaseous CO2 as solid hydrates under the seabed. Low temperature and high pressure are prevalent below the seabed, making it a thermodynamically feasible process. Furthermore, improved CO2 hydrate kinetics will facilitate technological development for carbon capture, storage, and sequestration. This study focuses on comprehending the CO2 hydrate kinetics with organic aliphatic amines, particularly methylamine, amylamine, and decylamine. Additives were tested in concentrations of 0.1, 1, and 5 wt % to meticulously comprehend their impact. A 300 mL stirred tank reactor was used for the investigations at 3.5 MPa and 274.55 K with pure water, which are the typical temperature and pressure conditions that one encounters in shallow subsea sediments. All additives showed considerable promotion in induction time, assuring faster CO2 hydrate nucleation. In addition, decylamine resulted in faster uptake of CO2 in our experiments compared to the other two additives. Hydrate dissociation studies up to 293.15 K were performed to assess the effect of the considered additives on CO2 hydrate dissociation. The decylamine system also delayed the gas release rate, showing better stability than the pure water system. This study also proposes a suitable well design for enhanced subsea CO2 sequestration as solid hydrates.

Storing natural gas in the form of clathrate hydrates (termed as Solidified Natural Gas, SNG) is highly advantageous as it is non-explosive, environmentally benign and offers compact mode of natural gas storage with high volumetric storage capacity. In this work, we demonstrate rapid methane storage in saline water (1.1 mol% NaCl solution) and seawater via clathrate hydrates aided by 5.56 mol% THF in a simple unstirred tank reactor. We report extremely fast hydrate formation kinetics with methane uptake of 89.2 (±2.4) v/v in 13.8 (±2.4) minutes with saline water and 86.3 (±4.3) v/v in 15.1 (±0.8) minutes with natural seawater (t90). This uptake corresponds to an yield of 77.6 (±2.2)% for saline water and 75.0 (±3.4)% for natural seawater system respectively for the stated hydrate growth time. Further, molecular insights of the mixed hydrate formation in presence of NaCl is derived through high-pressure calorimetry, in-situ Raman, and powder X-ray diffraction analysis. Finally, we demonstrate the stability of the hydrate pellet formed employing direct seawater in presence of THF for two weeks. The direct use of natural seawater makes the SNG technology highly attractive to store/transport methane for large-scale storage needs and for low capacity natural gas production facilities like biogas manufacturing plants.

A fundamental study on hydrate formation from an equimolar CO2-CH4 gas mixture has been carried out with two focal points: accelerating the kinetics of hydrate formation and enhancing the gas separation efficiency of the process. To this effect, the impact of inducing different hydrate structures from the same gas mixture by introducing suitable additives into the system has been investigated, and experiments are being carried out in a horizontal packed bed reactor at two different initial pressures, 3.5 and 5.0 MPa, to study the effect of driving forces on the kinetics of hydrate formation and the separation efficiency of the process. sH hydrate former cyclooctane (Cyclo-O) induces rapid nucleation of hydrate and also yields significant gas uptake in hydrates, 29.55% higher compared to the water system. This may be attributed to the simultaneous formation of sH and sI hydrates when Cyclo-O is present in the system. It was observed that the environmentally benign hydrophobic amino acid tryptophan in low concentration (1 wt %) can effectively accelerate the kinetics of hydrate formation, with 90% water to hydrate conversion being obtained within the first 30 min of hydrate formation. Further, the use of Cyclo-O and tryptophan together shows a synergistic effect, resulting in the highest gas uptake among all the systems studied. Although the problem of slow kinetics of hydrate formation from CO2-CH4 gas mixtures has been satisfactorily solved through this work, there are still significant strides that need to be made toward improving the separation efficiency of the process. The formation of the mixed hydrate is unable to return a satisfactorily high efficiency for gas separation.

Methane storage in mixed hydrates is advantageous due to faster kinetics and added stability. However, capacity needs to be improved. Here we study mixed hydrates of methane (CH4) and tetrahydrofuran (THF), in the presence of sodium dodecyl sulfate (SDS) as a kinetic promoter for hydrate formation. We report the co-existence of pure methane (sI) and mixed CH4-THF hydrates (sII) in the presence of SDS; however, in the absence of SDS, co-existence of pure THF (sII) and mixed CH4-THF hydrates (sII) was observed. Thus the presence of SDS preferentially promotes the enclathration of methane over that of THF. Furthermore, through in situ Raman spectrometry, complemented by high-pressure differential scanning calorimeter, we present temperature-dependent methane occupancy in small and large cages of sI and sII hydrates. Our findings offer new insights for enhancing the methane storage capacity in more stable sII hydrate configuration for large-scale methane storage via solidified natural gas technology. Chemistry; Chemical Engineering; Separation Science