Inhibition of Methane Hydrates Using Biodegradable Additives.

Prevention of hydrate plugs during transportation of oil and natural gas in the pipeline network is challenging. Certain additives are often introduced into the process to eliminate/delay plug formation. Dominantly synthetic inhibitors are deployed in large volumes (∼20 to 30% by volume) to counter the problem and are highly expensive and, in some circumstances, toxic. The search for novel additives that are eco-friendly and act as inhibitors is in demand. The present study reports the thermodynamic inhibition (THI) capacity of some vastly available natural biopowders, such as Azadirachta indica (neem), Piper betel (betel), and Nelumbo nucifera (Indian lotus) in low dosage (0.5 wt %), on methane hydrate (MH) formation. Since the gas flow is dynamic, experiments are conducted in stirred geometry by varying the speed range from 0 to 1000 rotations per minute (rpm). All of the studies are performed in the isochoric method procedure. The biopowders act as efficient thermodynamic hydrate inhibitors. Once the nucleation triggers, they act as kinetic hydrate promoters. Since sodium dodecyl sulfate (SDS) is an excellent kinetic hydrate promoter in both stirred and nonstirred geometries, the obtained results are compared with the SDS system. Hydrate nucleation is triggered at higher subcooling (∼8 to 10 K) in the presence of water-soluble bioextracts. The neem leaf extracts showed a ∼30% lower hydrate conversion than SDS in identical experimental conditions. Two-stage hydrate nucleation occurred at higher stirring speeds, and the hydrate conversion is inferior (∼6%) between the primary and secondary stages. The addition of biopowder extracts is useful in controlling hydrate formation. A small quantity of biopowders provides higher inhibition and reduces synthetic chemicals used in real-time applications.

Enhanced methane gas storage in the form of hydrates: role of the confined water molecules in silica powders.

Methane hydrates are promising materials for storage and transportation of natural gas; however, the slow kinetics and inefficient water to hydrate conversions impede its broad scale utilisation. The purpose of the present study is to demonstrate rapid (2-3 h) and efficient methane hydrate conversions by utilising the water molecules confined in the intra- and inter-granular space of silica powders. All the experiments were conducted with amorphous silica (10 g) powders of 2-30 µm; 10-20 nm grain size, to mimic the hydrate formations in fine sand and clay dominated environments under moderate methane pressure (7-8 MPa). Encasing of methane molecules in hydrate cages was confirmed by Raman spectroscopic (ex situ) and thermodynamic phase boundary measurements. The present studies reveal that the water to hydrate conversion is relatively slower in 10-20 nm grain size silica, although the nucleation event is rapid in both silicas. The process of hydrate conversion is vastly diffusion-controlled, and this was distinctly observed during the hydrate growth in nanosize silica.

Self-preservation and Stability of Methane Hydrates in the Presence of NaCl.

Gas hydrate, a solid transformed from an ensemble of water and gaseous molecules under suitable thermodynamic conditions, is present in marine and permafrost strata. The ability of methane hydrates to exist outside of its standard stability zone is vital in many aspects, such as its utility in gas storage and transportation, hydrate-related climate changes and gas reservoirs on the planet. A systematic study on the stability of methane hydrates divulges that the gas uptake decreased by about 10% by increasing the NaCl content to 5.0 wt%. The hydrate formation kinetic is relatively slower in a system with higher NaCl. The self-preservation temperature window for hydrate systems with NaCl 1.5, 3.0 and 5.0 wt% dramatically shifted to a lower temperature (252 K), while it remained around 270 K for NaCl 0.0 and 0.5 wt%. Based on powder x-ray diffraction and micro-Raman spectroscopic studies, the presence of hydrohalite (NaCl·2H2O) phase was identified along with the usual hydrate and ice phases. The eutectic melting of this mixture is responsible for shifting the hydrate stability to 252 K. A systematic lattice expansion of cubic phase infers the interaction between NaCl and water molecules of hydrate cages.

Clathrate Hydrates of Greenhouse Gases in the Presence of Natural Amino Acids: Storage, Transportation and Separation Applications.

Storage of greenhouse gases in the form of gas hydrates is attractive and is being pursued rigorously in recent times. However, slow formation rate and inefficient water to hydrate conversion are the main hindering factors. In this report, we examine the role of two amino acids (0.5 wt%), l-methionine (l-met) and l-phenylalanine (l-phe) on the formation of gas hydrates using methane (CH4), carbon dioxide (CO2) and their mixtures as guest molecules. Experiments are conducted under non-stirred and isochoric configurations. The hydrate conversion efficiency of both amino acids is identical for hydrates formed with CH4 and mixture of (CO2+CH4). However, the hydrate conversion is significantly less in CO2 hydrates in l-phe system. Addition of amino acids to the water dramatically improved the kinetics of hydrate formation and 90% of maximum gas uptake in hydrate phase occurred in less than an hour. The water to hydrate conversion is also very efficient (>85%) in the presence of amino acids. Therefore, the amino acids containing systems are suitable for storing both CH4 and CO2 gases. The gas hydrates were characterised using powder x-ray diffraction (XRD) and Raman spectroscopic measurements. These measurements indicate the formation of sI hydrates and encasing of gas molecules as guests.

Storage of Methane Gas in the Form of Clathrates in the Presence of Natural Bioadditives.

Methane (CH4) and carbon dioxide (CO2), the important greenhouse gases, are capable of forming clathrate hydrates under some suitable thermodynamic conditions. The gas storage capacity of these materials is high, and therefore they are often useful in gas storage applications. Certain expensive and toxic chemicals are employed to accelerate/decelerate the process. In this study, we report rapid (∼30-50 min) and effective (∼80%) methane hydrate conversion in the presence of three naturally occurring additives such as dry powders from Nelumbo nucifera (Indian lotus), Piper betle (betel), and Azadirachta indica (neem), at lower concentrations (0.5 wt %). Obtained results were carefully compared with the well-known kinetic promoter (sodium dodecyl sulfate). All the biomaterials are equally good kinetic promoters for methane hydrates, although the required subcooling is significantly large. However, no hydrate formation is observed with CO2 gas.

Structural stability of methane hydrates in porous medium: Raman spectroscopic study.

Thermal and temporal stability of the methane hydrates (MH) at ambient pressure, synthesised in a spherical silica (solid and hollow, with average diameter of 70 µm) matrix, are investigated by the Raman spectroscopy. Identical Raman spectroscopic spectral features for all the synthesized hydrate samples indicate structural resemblance irrespective of matrix. It is observed that the growth of hydrates in hollow silica matrix is homogeneous, while that with solid grain is highly heterogeneous. Temporal and thermal stability of MH depends on the silica matrix. Appearance of the Raman signatures characteristic of MH, in hollow silica, indicates that the hydrates are stable over several hours (upon preserving at 153 K and 0.1 MPa) and until ∼273 K at 0.1 MPa. However, MH in solid silica matrix is highly unstable under similar P, T conditions and they are readily dissociated within 2 h. The thermal stability of these samples at 0.1 MPa is also significantly lower.

Increasing hydrogen storage capacity using tetrahydrofuran.

Hydrogen hydrates with tetrahydrofuran (THF) as a promoter molecule are investigated to probe critical unresolved observations regarding cage occupancy and storage capacity. We adopted a new preparation method, mixing solid powdered THF with ice and pressurizing with hydrogen at 70 MPa and 255 +/- 2 K (these formation conditions are insufficient to form pure hydrogen hydrates). All results from Raman microprobe spectroscopy, powder X-ray diffraction, and gas volumetric analysis show a strong dependence of hydrogen storage capacity on THF composition. Contrary to numerous recent reports that claim it is impossible to store H(2) in large cages with promoters, this work shows that, below a THF mole fraction of 0.01, H(2) molecules can occupy the large cages of the THF+H(2) structure II hydrate. As a result, by manipulating the promoter THF content, the hydrogen storage capacity was increased to approximately 3.4 wt % in the THF+H(2) hydrate system. This study shows the tuning effect may be used and developed for future science and practical applications.

Structural transformations of sVI tert-butylamine hydrates to sII binary hydrates with methane.

Binary clathrate hydrates with methane (CH(4), 4.36 A) and tert-butylamine (t-BuNH(2), 6.72 A) as guest molecules were synthesized at different molar concentrations of t-BuNH(2) (1.00-9.31 mol %) with methane at 7.0 MPa and 250 K, and were characterized by powder X-ray diffraction (PXRD) and Raman microscopy. A structural transformation from sVI to sII of t-BuNH(2) hydrate was clearly observed on pressurizing with methane. The PXRD showed sII signatures and the remnant sVI signatures were insignificant, implying the metastable nature of sVI binary hydrates. Raman spectroscopic data on these binary hydrates suggest that the methane molecules occupy the small cages and vacant large cages. The methane storage capacity in this system was nearly doubled to approximately 6.86 wt % for 5.56 mol % > t-BuNH(2) > 1.0 mol %.

Hydrogen storage in double clathrates with tert-butylamine.

The first proof-of-concept of the formation of a double tert-butylamine (t-BuNH(2)) + hydrogen (H(2)) clathrate hydrate has been demonstrated. Binary clathrate hydrates with different molar concentrations of the large guest t-BuNH(2) (0.98-9.31 mol %) were synthesized at 13.8 MPa and 250 K, and characterized by powder X-ray diffraction and Raman microscopy. A structural transformation from sVI to sII of t-BuNH(2) hydrate was clearly observed under hydrogen pressures. Raman spectroscopic data suggested that the hydrogen molecules occupied the small cages and had similar occupancy to hydrogen in the double tetrahydrofuran (THF) + H(2) clathrate hydrate. The hydrogen storage capacity in this system was approximately 0.7 H(2) wt % at the molar concentration of t-BuNH(2) close to the sII stoichiometry.