Praveen Linga

Two-Stage Clathrate Hydrate/Membrane Process for Precombustion Capture of Carbon Dioxide and Hydrogen

A hybrid process for the capture of CO2 and H2 from a treated fuel gas mixture is presented. It consists of two hydrate crystallization stages operating at 273.7 K and 3.8 and 3.5 MPa, respectively. The CO2-lean stream from the first stage is directed to a membrane separation unit whereas the CO2-rich one is directed to the second hydrate stage. These operating pressures at the crystalli- zation stages are possible by adding 2.5% by mole propane. Propane enables the reduction in the hydrate formation pressure and thus reduces the cost associated with the compression of the fuel gas. The two hydrate stages would operate at 7.5 and 3.5 MPa without adding propane. This work provides the relevant kinetic data, as well as the separation efficiency and recoveries achieved.

A new porous material to enhance the kinetics of clathrate process: application to precombustion carbon dioxide capture.

In this work, the performance of a new porous medium, polyurethane (PU) foam in a fixed bed reactor for carbon dioxide separation from fuel gas mixture using the hydrate based gas separation process is evaluated. The kinetics of hydrate formation in the presence of 2.5 mol % propane as thermodynamic promoter was investigated at 4.5, 5.5, and 6.0 MPa and 274.2 K. Significantly higher gas consumption and water conversion to hydrate was achieved when PU foam was employed. PU foam as a porous medium can help convert 54% of water to hydrate in two hours of hydrate formation. In addition the induction times were very low (<3.67 min at 6.0 MPa). A normalized rate of hydrate formation of 64.48 (±3.82) mol x min(-1) x m(-3) was obtained at 6.0 MPa and 274.2 K. Based on a morphological study, the mechanism of hydrate formation from water dispersed in interstitial pore space of the porous medium is presented. Finally, we propose a four step operation of the hydrate based gas separation process to scale up.

Thermodynamic and kinetic verification of tetra-n-butyl ammonium nitrate (TBANO3) as a promoter for the clathrate process applicable to precombustion carbon dioxide capture.

In this study, tetra-n-butyl ammonium nitrate (TBANO3) is evaluated as a promoter for precombustion capture of CO2 via hydrate formation. New hydrate phase equilibrium data for fuel gas (CO2/H2) mixture in presence of TBANO3 of various concentrations of 0.5, 1.0, 2.0, 3.0, and 3.7 mol % was determined and presented. Heat of hydrate dissociation was calculated using Clausius-Clapeyron equation and as the concentration of TBANO3 increases, the heat of hydrate dissociation also increases. Kinetic performance of TBANO3 as a promoter at different concentrations was evaluated at 6.0 MPa and 274.2 K. Based on induction time, gas uptake, separation factor, hydrate phase CO2 composition, and rate of hydrate growth, 1.0 mol % TBANO3 solution was found to be the optimum concentration at the experimental conditions of 6.0 MPa and 274.2 K for gas hydrate formation. A 93.0 mol % CO2 rich stream can be produced with a gas uptake of 0.0132 mol of gas/mol of water after one stage of hydrate formation in the presence of 1.0 mol % TBANO3 solution. Solubility measurements and microscopic images of kinetic measurements provide further insights to understand the reason for 1.0 mol % TBANO3 to be the optimum concentration.

Mechanism of methane hydrate formation in the presence of hollow silica

Methane hydrates are studied extensively as a prospective medium for storing and transporting natural gas due to their inherent advantages, including high volumetric energy storage density, being environmentally benign and extremely safe method compared to conventional compression and liquefaction methods. Enhanced formation kinetics of methane hydrates has been reported in hollow silica due to the increased gas/liquid contact surface area available for efficient conversion of water to hydrates. This work elucidates the mechanism of methane hydrate formation in light weight hollow silica. Hollow silica-to-water ratio was varied and its effect on the methane hydrate formation/dissociation morphology was observed. There exists a critical hollow silica-to-water ratio (1 : 6) beyond which the hydrates preferentially crystallize on the top of the bed by drawing water from the interstitial pores, whereas below this ratio the hydrate formation occurs within the bed between inter-particular spaces of hollow silica. Due to the very low bulk density, a small fraction of hollow silica was observed to be displaced from the bed during the hydrate formation above the critical hollow silica to water ratio.

Pre and Post Combustion Capture of Carbon Dioxide via Hydrate Formation

One of the new approaches for capturing carbon dioxide from treated flue gases (post- combustion capture) is based on gas hydrate crystallization. The carbon dioxide content of the gas hydrate crystals is different than that of the flue gas. This provides the basis for the separation or capture of the CO2. Moreover, when a gas mixture of CO2 and H2 forms gas hydrates the CO2 which forms hydrate at lower pressure prefers to partition in the hydrate phase. This provides the basis for the separation of CO2 (pre- combustion capture). The present study illustrates the concept and provides basic thermodynamic and kinetic data supporting process development.

High pressure rheology of gas hydrate formed from multiphase systems using modified Couette rheometer

Conventional rheometers with concentric cylinder geometries do not enhance mixing in situ and thus are not suitable for rheological studies of multiphase systems under high pressure such as gas hydrates. In this study, we demonstrate the use of modified Couette concentric cylinder geometries for high pressure rheological studies during the formation and dissociation of methane hydrate formed from pure water and water-decane systems. Conventional concentric cylinder Couette geometry did not produce any hydrates in situ and thus failed to measure rheological properties during hydrate formation. The modified Couette geometries proposed in this work observed to provide enhanced mixing in situ, thus forming gas hydrate from the gas-water-decane system. This study also nullifies the use of separate external high pressure cell for such measurements. The modified geometry was observed to measure gas hydrate viscosity from an initial condition of 0.001 Pa s to about 25 Pa s. The proposed geometries also possess the capability to measure dynamic viscoelastic properties of hydrate slurries at the end of experiments. The modified geometries could also capture and mimic the viscosity profile during the hydrate dissociation as reported in the literature. The present study acts as a precursor for enhancing our understanding on the rheology of gas hydrate formed from various systems containing promoters and inhibitors in the context of flow assurance.

HYDRATE PROCESSES FOR CO2 CAPTURE AND SCALE UP USING A NEW APPARATUS.

One of the new approaches for capturing carbon dioxide from treated flue gas (post-combustion capture) and fuel gas (pre-combustion capture) is based on gas hydrate crystallization. The presence of small amount of tetrahydrofuran (THF) substantially reduces the hydrate formation pressure from a flue (CO2/N2) gas mixture and offers the possibility to capture CO2 at medium pressures [1]. A conceptual flow sheet for a medium pressure hydrate process for pre-combustion capture from a fuel gas (CO2/H2) was also developed and presented. In order to test the hydrate-based separation processes for pre and post combustion capture of CO2 at a larger scale a new apparatus that can operate with different gas/water contact modes is set up and presented.

CH4 Hydrate Formation between Silica and Graphite Surfaces: Insights from Microsecond Molecular Dynamics Simulations

Microsecond simulations have been performed to investigate CH4 hydrate formation from gas/water two-phase systems between silica and graphite surfaces, respectively. The hydrophilic silica and hydrophobic graphite surfaces exhibit substantially different effects on CH4 hydrate formation. The graphite surface adsorbs CH4 molecules to form a nanobubble with a flat or negative curvature, resulting in a low aqueous CH4 concentration, and hydrate nucleation does not occur during 2.5 μs simulation. Moreover, an ordered interfacial water bilayer forms between the nanobubble and graphite surface thus preventing their direct contact. In contrast, the hydroxylated-silica surface prefers to be hydrated by water, with a cylindrical nanobubble formed in the solution, leading to a high aqueous CH4 concentration and hydrate nucleation in the bulk region; during hydrate growth, the nanobubble is gradually covered by hydrate solid and separated from the water phase, hence slowing growth. The silanol groups on the silica surface can form strong hydrogen bonds with water, and hydrate cages need to match the arrangements of silanols to form more hydrogen bonds. At the end of the simulation, the hydrate solid is separated from the silica surface by liquid water, with only several cages forming hydrogen bonds with the silica surface, mainly due to the low CH4 aqueous concentrations near the surface. To further explore hydrate formation between graphite surfaces, CH4/water homogeneous solution systems are also simulated. CH4 molecules in the solution are adsorbed onto graphite and hydrate nucleation occurs in the bulk region. During hydrate growth, the adsorbed CH4 molecules are gradually converted into hydrate solid. It is found that the hydrate-like ordering of interfacial water induced by graphite promotes the contact between hydrate solid and graphite. We reveal that the ability of silanol groups on silica to form strong hydrogen bonds to stabilize incipient hydrate solid, as well as the ability of graphite to adsorb CH4 molecules and induce hydrate-like ordering of the interfacial water, are the key factors to affect CH4 hydrate formation between silica and graphite surfaces.

KINETICS OF HYDRATE FORMATION AND DECOMPOSITION OF METHANE IN SILICA SAND.

Kinetics of hydrate formation and decomposition of methane hydrate formed in silica sand particles were studied in detail at three temperatures of 7.0, 4.0 and 1.0°C, respectively. A new apparatus was setup to study the decomposition behavior of the methane hydrate formed in the bed of silica sand particles. Six thermocouples are placed in different locations to study the temperature profiles during hydrate formation and decomposition experiments. Gas uptake measurement curves for the formation experiments and the gas release measurement curves for the decomposition experiment were determined from the experimental data. Percent conversion of water to hydrates was significantly higher for the experiments conducted at 4.0 and 1.0°C compared to 7.0°C. Recovery of methane occurred in two stages during the decomposition experiments carried out with a thermal stimulation approach at constant pressure. Methane recovery in the range of 95 to 98% was achieved.