Gas Hydrate Formation Kinetics

Gas hydrates are non-stoichiometric crystals formed by the enclosure of molecules like methane, carbon dioxide and hydrogen sulfide inside cages formed by hydrogen-bonded water molecules. There are more than 100 compounds (guests) that can combine with water (host) and form hydrates. Formation of gas hydrates is a problem in oil and gas operations because it causes plugging of the pipelines and other facilities. On the other hand natural methane hydrate exists in vast quantities in the earth s crust and is regarded as a future energy resource. 

A mechanistic model for the kinetics of gas hydrate formation was proposed by Englezos et al. (1987). The model contains one adjustable parameter for each gas hydrate forming substance. The parameters for methane and ethane were determined from experimental data in a semi-batch agitated gas-liquid vessel. During a typical experiment in such a vessel one monitors the rate of methane or ethane gas consumption, the temperature and the pressure. Gas hydrate formation is a crystallization process but the fact that it occurs from a gas-liquid system under pressure makes it difficult to measure and monitor in situ the particle size and particle size distribution as well as the concentration of the methane or ethane in the water phase. 

The experiments were conducted at four different temperatures for each gas. At each temperature experiments were performed at different pressures. A total of 14 and 11 experiments were performed for methane and ethane respectively. Based on crystallization theory, and the two film theory for gas-liquid mass transfer Englezos et al. (1987) formulated five differential equations to describe the kinetics of hydrate formation in the vessel and the associate mass transfer rates. The governing ODEs are given next. 

The first equation gives the rate of gas consumption as moles of gas (n) versus time. This is the only state variable that is measured. The initial number of moles, nO is known. The intrinsic rate constant, K is the only unknown model parameter and it enters the first model equation through the Hatta number y. The Hatta number is given by the following equation 

The other state variables are the fugacity of dissolved methane in the bulk of the liquid water phase (fb) and the zero, first and second moment of the particle size distribution (p0, Pi, l )- The initial value for the fugacity, fb° is equal to the three phase equilibrium fugacity feq. The initial number of particles, p , or nuclei initially formed was calculated from a mass balance of the amount of gas consumed at the turbidity point. The explanation of the other variables and parameters as well as the initial conditions are described in detail in the reference. The equations are given to illustrate the nature of this parameter estimation problem with five ODEs, one kinetic parameter (K ) and only one measured state variable. 

---

Nerheim, A.R., Investigation of Gas Hydrate Formation Kinetics by Laser Light Scattering, D. Ing. Thesis, Norwegian Institute of Technology, Trondheim, Norway (1993). 

As a method of promotion, propane improves conditions of gas hydrate formation kinetics observably from air aspect. The method reduces the cost of curing gas storage and transportation, and promotes the industrialization development of gas hydration solidification separation and storage and transportation. 

Englezos, P., Kalogerakis, N., Dholababhai, P.D. and Bishnoi, P.R., 1987b. Kinetics of gas hydrate formation from mixtures of methane and ethane. Chemical Engineering Science, 42(11), 2659-2666. 

Kinetic inhibitors exhibit unusual effects on hydrate formation with implications to processing (Lee and Englezos, 2006). Gas hydrate formation experiments were conducted... 

Long, J.P., Gas Hydrate Formation Mechanism and Kinetic Inhibition, Ph.D. Thesis, Colorado School of Mines, Golden, CO (1994). 

Our approach for studies of gas hydrate formation and decomposition in sedimentary pore space consists of two steps. The first one is devoted to the hydrate accumulation kinetics in pore space of frozen soils to obtain frozen hydrate-saturated samples. The second one concentrates on the pore hydrate dissociation kinetics in frozen soils under non-equilibrium conditions. 

The mineral composition of the soil will also influence the kinetics of gas hydrates dissociation in frozen soils. Our results show, that gas hydrate formations in pore space of samples with montmorillonite particles dissociate less markedly as compared to the samples with kaolinite admixture. This influence may be explained by microstructural specificities of pore hydrate saturated samples but undoubtedly requires additional micro-morphological studies for a full understanding. 

The gas consumption rate during the methane hydrate formation tends to be different for each porous medium, therefore suggesting different reaction kinetics (Figure la). The gas hydrate formation is initially fast and then slows down. It suggests an initial fast surface reaction with free water and gas access. Subsequently, the transformation from water into gas hydrates is slowing down limited by the gas and water transfer. Finally, no further... 

Bishnoi, P.R. Dholabhai, P.D. Mahadev, K.N. Solid deposition in hydrocarbon systems kinetics and thermodynamics of gas hydrate formation. Task 2, Gas hydrate equilibrium studies. Report for GRI, Contract Number 5091-260-2138. 

Combined the characteristics of gas hydrate, this paper introduces the hydrate formation rate calculation model, in order to analyze the influences of the kinetics of gas hydrate formation process quantitative, such as formula (1). 

Because in situ Raman spectra can be recorded, this provides a particularly convenient and useful tool for following the kinetics of gas hydrate formation and decomposition. For example, such studies we e performed during methane hydrate formation. " The evolution of two peaks at 2905 and 2915 cm on hydrate formation from a single peak (at 2911 cm for methane dissolved in water) can be monitored as a function of time/temper-ature/pressure. The effect of adding various chemicals that retard the hydrate formation process was also studied (Refs. [22-24] see Fig. 3). 

N. Daraboina, Ch. Malmos, N. von Solms, 2013a. Synergistic kinetic inhibition of natural gas hydrate formation. Fuel 108, 749-757. 

A controversy exists regarding the early stages of formation of gas hydrates. The mechanism proposed by Sloan and Fleyfel [384,613,1637] for the kinetics of hydrate formation is composed of... 

---