Transport of carbon dioxide in offshore pipelines involves high pressures and low temperatures, which may lead to formation of hydrate from the residual dissolved water and carbon dioxide. While thermodynamics is able to tell us whether the hydrate phase will be stable, the question of whether its formation will actually occur under given pipeline conditions does not have a straightforward answer. In this work, we have made use of water properties obtained from molecular simulations to examine the thermodynamics of hydrate formation from water dissolved in carbon dioxide. This paper proposes a method that allows estimation of absolute thermodynamic properties and thus makes it possible to compare free energy changes due to several possible phase transitions and determine the most probable transition. This information can be used directly to choose the optimum hydrate prevention strategy. We have found that hydrate formation from a carbon dioxide solution will be thermodynamically viable at water concentration exceeding a certain level; a conclusion also supported by several previous studies. We have also extended the quantitative analysis of the thermodynamics and the kinetics of formation through a modified version of phase field theory (PFT). The work presents the way to obtain parameters required for the practical implementation of the PFT in the case of hydrate formation, as well as outlines the estimation of thermodynamic properties for systems unable to reach true equilibrium.

In this work, nonequilibrium thermodynamics and phase field theory (PFT) has been applied to study the kinetics of phase transitions associated with CO 2 injection into systems containing CH 4 hydrate, free CH 4 gas, and varying amounts of liquid water. The CH 4 hydrate was converted into either pure CO 2 or mixed CO 2 ACH 4 hydrate to investigate the impact of two primary mechanisms governing the relevant phase transitions: solid-state mass transport through hydrate and heat transfer away from the newly formed CO 2 hydrate. Experimentally proven dependence of kinetic conversion rate on the amount of available free pore water was investigated and successfully reproduced in our model systems. It was found that rate of conversion was directly proportional to the amount of liquid water initially surrounding the hydrate. When all of the liquid has been converted into either CO 2 or mixed CO 2 ACH 4 hydrate, a much slower solid-state mass transport becomes the dominant mechanism. V C 2015 American Institute of Chemical Engineers AIChE J, 61: [3944][3945][3946][3947][3948][3949][3950][3951][3952][3953][3954][3955][3956][3957] 2015

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The upper limit of water content permitted in a natural gas stream during its pipeline transport without a risk of hydrate formation is a complex issue. We propose a novel absolute-thermodynamic scheme for investigation of different routes to hydrate formation, with ideal gas used as reference state. This makes comparison between different hydrate formation routes transparent and consistent in free energy changes and associated enthalpy change. Pipeline inner walls are typically initially covered by rust. Natural gas hydrate can form directly from water dissolved in natural gas; the other two alternatives will involve water either condensing out to create a liquid water phase, or adsorbing on solid surfaces. Hydrate former guests CO2 and H2S exhibit substantial solubility in water and adsorb together with water onto rust surfaces (taken to be hematite in our study). Natural gas from the North Sea is typically lean in H2S, with separated gas streams containing up to 10% CO2. In contrast with transport of dense CO2, methane will dominate the density dependence at densities below 300 kg/m3. The maximum tolerated water fraction was not found to be sensitive to sour gas concentrations below 10%, with the results clearly more sensitive to H2S than CO2.

Deciding on the upper bound of water content permissible in a stream of dense carbon dioxide under pipeline transport conditions without facing the risks of hydrate formation is a complex issue. In this work, we outline and analyze ten primary routes of hydrate formation inside a rusty pipeline, with hydrogen sulfide, methane, argon, and nitrogen as additional impurities. A comprehensive treatment of equilibrium absolute thermodynamics as applied to multiple hydrate phase transitions is provided. We also discuss in detail the implications of the Gibbs phase rule that make it necessary to consider non-equilibrium thermodynamics. The analysis of hydrate formation risk has been revised for the dominant routes, including the one traditionally considered in industrial practice and hydrate calculators. The application of absolute thermodynamics with parameters derived from atomistic simulations leads to several important conclusions regarding the impact of hydrogen sulfide. When present at studied concentrations below 5 mol%, the presence of hydrogen sulfide will only support the carbon-dioxide-dominated hydrate formation on the phase interface between liquid water and hydrate formers entering from the carbon dioxide phase. This is in contrast to a homogeneous hydrate nucleation and growth inside the aqueous solution bulk. Our case studies indicate that hydrogen sulfide at higher than 0.1 mol% concentration in carbon dioxide can lead to growth of multiple hydrate phases immediately adjacent to the adsorbed water layers. We conclude that hydrate formation via water adsorption on rusty pipeline walls will be the dominant contributor to the hydrate formation risk, with initial concentration of hydrogen sulfide being the critical factor.