Kinetics and mechanism of the formation of CO2 hydrate

In-situ Raman study on kinetics behaviors of hydrated bubble in thickening

Natural gas leakage by means of bubbles in cold seep abundantly existed on the ocean floor, causing the change of ocean ecology and the increase of atmospheric temperature. Fortunately, hydrated bubbles as a way of methane sequestration can reduce the effect on the ocean ecology and the escape of gas bubbles from the ocean floor, and are getting attention. To know the growth mode and efficiency of gas hydrate sequestration on bubble, the thickening growth kinetics of hydrated bubble was studied in present work. In-situ Raman spectroscopy was used to analyze the evolution of gas pores and mass transfer channels in the sI CH₄, sI CH₄-C₂H₆ and sII CH₄-C₂H₆ hydrate films on the hydrated bubble by the peak area ratio of Raman spectra. Three types of Raman spectra (a-, b-, and c-type), three texture structures of film (Large gas pore; Small gas pore; No gas pore) and two hydrate thickening patterns (filling of new hydrate within large gas pores; covering growth on the original hydrate lattice) were provided in the thickening of hydrated bubble. Results showed that the thickening of the hydrated bubble was a multi-stages growth, i.e., quick growth (stage I), slow growth (stage II), and no growth (stage III). The texture structures and the type and size of gas pore in hydrated bubble were critical for the kinetics growth rate of hydrated bubble in thickening. Especially, the theory of heterogeneous growth of hydrated bubble was proposed to apply the hydrate growth at the interface of two or multi- bubbles, accelerating the efficiency of carbon sequestration as the hydrated bubble. This study will provide a better theoretical basis for understanding the behaviors and efficiency of hydrated carbon sequestration on the surface of bubbles resulting from the gas leakage in the hydrate exploitation or the natural cold seep. SYNOPSIS: Hydrated bubble strongly modulates the emission of a potent greenhouse gas from the deep sea.

Hydrate Phase Transition Kinetic Modeling for Nature and Industry–Where Are We and Where Do We Go?

Hydrate problems in industry have historically motivated modeling of hydrates and hydrate phase transition dynamics, and much knowledge has been gained during the last fifty years of research. The interest in natural gas hydrate as energy source is increasing rapidly. Parallel to this, there is also a high focus on fluxes of methane from the oceans. A limited portion of the fluxes of methane comes directly from natural gas hydrates but a much larger portion of the fluxes involves hydrate mounds as a dynamic seal that slows down leakage fluxes. In this work we review some of the historical trends in kinetic modeling of hydrate formation and discussion. We also discuss a possible future development over to classical thermodynamics and residual thermodynamics as a platform for all phases, including water phases. This opens up for consistent thermodynamics in which Gibbs free energy for all phases are comparable in terms of stability, and also consistent calculation of enthalpies and entropies. Examples are used to demonstrate various stability limits and how various routes to hydrate formation lead to different hydrates. A reworked Classical Nucleation Theory (CNT) is utilized to illustrate that nucleation of hydrate is, as expected from physics, a nano-scale process in time and space. Induction times, or time for onset of massive growth, on the other hand, are frequently delayed by hydrate film transport barriers that slow down contact between gas and liquid water. It is actually demonstrated that the reworked CNT model is able to predict experimental induction times.

Carbon dioxide hydrate in a microfluidic device: Phase boundary and crystallization kinetics measurements with micro-Raman spectroscopy

Various emerging carbon capture technologies depend on being able to reliably and consistently grow carbon dioxide hydrate, particularly in packed media. However, there are limited kinetic data for carbon dioxide hydrates at this length scale. In this work, carbon dioxide hydrate propagation rates and conversion were evaluated in a high pressure silicon microfluidic device. The carbon dioxide phase boundary was first measured in the microfluidic device, which showed little deviation from bulk predictions. Additionally, measuring the phase boundary takes on the order of hours compared to weeks or longer for larger scale experimental setups. Next, propagation rates of carbon dioxide hydrate were measured in the channels at low subcoolings (<2 K from phase boundary) and moderate pressures (200-500 psi). Growth was dominated by mass transfer limitations until a critical pressure was reached, and reaction kinetics limited growth upon further increases in pressure. Additionally, hydrate conversion was estimated from Raman spectroscopy in the microfluidics channels. A maximum value of 47% conversion was reached within 1 h of a constant flow experiment, nearly 4% of the time required for similar results in a large scale system. The rapid reaction times and high throughput allowed by high pressure microfluidics provide a new way for carbon dioxide gas hydrate to be characterized.

Concentrating Model Solutions and Fruit Juices Using CO2 Hydrate Technology and Its Quantitative Effect on Phenols, Carotenoids, Vitamin C and Betanin

Fruits have an important economic impact in the context of plant-based food production. The consumption of fruit juices, mostly produced from concentrates, is particularly noteworthy. Conventional concentration methods do not always enable a sustainable and gentle concentration. The innovative gas hydrate technology addresses this point with its energy-saving, gentle character, and high concentration potential. In this study, the concentration of fruit juices and model solutions using CO2 hydrate technology was investigated. To find a suitable operating point for hydrate formation in the used bubble column, the hydrate formation in a water–sucrose model solution was evaluated at different pressure and temperature combinations (1, 3, 5 °C and 32.5, 37.5, 40 bar). The degrees of concentration indicate that the bubble column reactor operates best at 37.5 bar and 3 °C. To investigate the gentle processing character of the hydrate technology, its quantitative effects on vitamin C, betanin, polyphenols, and carotenoids were analyzed in the produced concentrates and hydrates via HPLC and UV/VIS spectrophotometry. The results for fruit juices and model solutions imply that all examined substances are accumulated in the concentrate, while only small amounts remain in the hydrate. These amounts can be related to an inefficient separation process.

Methane–propane hydrate formation and memory effect study with a reaction kinetics model

Although natural gas hydrates and hydrate exploration have been extensively studied for decades, the reaction kinetics and nucleation mechanism of hydrate formation is not fully understood. In its early stage, gas hydrate formation can be assumed to be an autocatalytic kinetic reaction with nucleation and initial growth. In this work, a reaction kinetics model has been established to form structure II methane–propane hydrate in an isochoric reactor. The computational model consists of six pseudo-elementary reactions for three dynamic processes: (1) gas dissolution into the bulk liquid, (2) a slow buildup of hydrate precursors for nucleation onset, and (3) rapid and autocatalytic hydrate growth after onset. The model was programmed using FORTRAN, with initiating parameters and rate constants that were derived or obtained from data fitted using experimental results. The simulations indicate that the length of nucleation induction is determined largely by an accumulation of oligomeric hydrate precursors up to a threshold value. The slow accumulation of precursors is the rate-limiting step for the overall hydrate formation, and its conversion into hydrate particles is critical for the rapid, autocatalytic reaction. By applying this model, the memory effect for hydrate nucleation was studied by assigning varied initial amounts of precursor or hydrate species in the simulations. The presence of pre-existing precursors or hydrate particles could facilitate the nucleation stage with a reduced induction time, and without affecting hydrate growth. The computational model with the performed simulations provides insight into the reaction kinetics and nucleation mechanism of hydrate formation.

Gas storage in "dry water" and "dry gel" clathrates

"Dry water" (DW) is a free-flowing powder prepared by mixing water, hydrophobic silica particles, and air at high speeds. We demonstrated recently that DW can be used to dramatically enhance methane uptake rates in methane gas hydrate (MGH). Here, we expand on our initial work, demonstrating that DW can be used to increase the kinetics of formation of gas clathrates for gases other than methane, such as CO(2) and Kr. We also show that the stability of the system toward coalescence can be increased via the inclusion of a gelling agent to form a "dry gel", thus dramatically improving the recyclability of the material. For example, the addition of gellan gum allows effective reuse over at least eight clathration cycles without the need for reblending. DW and its "dry gel" modification may represent a potential platform for recyclable gas storage or gas separation on a practicable time scale in a static, unmixed system.