Methane recovery from low-concentration coalbed gas via 1,3-dioxolane primary hydrate particles

Deposition of hydrate solids is one of important parts to understand the hydrate plugging mechanism in the flowlines. In this study, hydrate segregation and deposition of three different gas-dominated systems, which are water only, decane as a condensate with water, and poly-vinyl caprolactam as a kinetic inhibition polymer included systems, was investigated with measuring the torque of stirring liquid and hydrate phases. The torque – hydrate conversion curves provided some information to classify the hydrate formation process into several specific regions and to characterize each region. The present work suggests that measuring torque is one of useful laboratory scale measurements to investigate the hydrate deposition mechanism and the block properties. Introduction Gas Hydrates are ice-like nonstoichiometric crystalline solids which contain plenty of gas guest molecules in their hydrogen-bonded host water cavities [1]. In nature, huge amounts of gas hydrates containing natural gases are deposited in deep sea sediments or permafrost [2], thus methane recovery from the hydrate deposits has been one of major issues for future energy resources [3–5]. On the other hands, the formation of gas hydrates in the pipelines has been a serious concern in the oil and gas industry because it can cause blockages of flowlines leading to severe operational and safety problems [2,6]. For flow assurance of offshore flowlines transporting hydrocarbons or gases, injecting thermodynamic hydrate inhibitors (THIs) such as methanol or ethylene glycol has been most commonly used in order to prevent gas hydrate formation, as the hydrate equilibrium curve is shifted toward higher pressure and lower temperature than operation conditions [2]. However, the industry continues to explore deeper and colder region of undersea for fresh oil and gas sources and these conventional methods are facing difficulties such as larger injection and higher cost of THIs. Recent trends for flow assurance, thus, have moved from preventing hydrate formation using THIs toward risk management, that is, to allow hydrate formation in pipelines but delaying their nucleation or agglomeration to form a blockage, using kinetic hydrate inhibitors (KHIs) or anti-agglomerants (AAs) [2]. In order to prevent the plugging of hydrates in pipelines, deposition mechanism of hydrate should be understood. From laboratory scale measurements to field modeling, numerous studies for aggregation and agglomeration have been reported and their scenarios for different systems were suggested and developed [7]. To measure the cohesion adhesion force of hydrate particles and to observe hydrate particle aggregation using probe are typical attempts of laboratory experiments for understanding the agglomeration mechanism [8–11]. Torque measurement of fluid stirred in the reactor could be also one of useful methods to understand physical properties of agglomerated hydrate block in the laboratory scale measurements. However, just a few studies measured the torque to confirm whether or not hydrate blockage was formed in their system [12,13]. Here, this study focuses on the hydrate deposition of gas-dominated system containing decane as a condensate or poly-vinyl caprolactam (PVCap) as a kinetic inhibition polymer. The torque of stirring fluid was recorded during the hydrate formation and the deposition of hydrate solid was visually observed. This approach can provide a better understanding on the roles of a condensate and a kinetic inhibition polymer for plugging phenomena. Experiments A synthetic natural gas which was composed of 90 mol% CH4, 6 mol% C2H6, 3 mol% C3H8, and 1 mol% n-C4H10 was supplied by Special Gas (Korea). Decane was purchased from SIGMAALDRICH. Deionized water and PVCap (MW≈5000, purity 98.0 wt%) was used without further purification.

Chord length distributions measurements during crystallization and agglomeration of gas hydrate in a water-in-oil emulsion: Simulation and experimentation

Observed effects of hydrophobic fumed silica nanoparticles (of average primary particle size 7 nm) on the rheological behavior of hydrate-forming emulsions are presented. Liquid cyclopentane (CP) is the hydrate former. The hydrate slurry is prepared in a Couette geometry at atmospheric pressure from a water-in-oil emulsion with the phases density matched to avoid segregation. Hydrates are formed upon quenching to a low temperature at a fixed shear rate. Dispersed water droplets convert to hydrate particles, leading to an effective viscosity increase by orders of magnitude. The hydrate inhibition by silica nanoparticles at the water–oil interface, forming a Pickering type of emulsion, is characterized using the onset time of steep viscosity rise after seeding with small hydrate particles; this is termed the critical time. Seeding eliminates stochasticity associated with nucleation of the hydrate. The critical time is increased when the interface is covered with silica nanoparticles. For a particle concentration range of 0.05–0.5% (by weight based on total oil mass) at the interface, the hydrate crystallization process is delayed by 5 h in comparison to the particle-free case for a 20 vol % water-in-oil emulsion at T = −2 °C and shear rate of γ̇ = 100 s–1. The final hydrate slurry viscosity was the same as observed in the slurry with no particles. At particle concentrations greater than 1 wt %, the viscosity increased abruptly and ultimately jammed the rheometer during hydrate formation. A hypothesis is presented to explain this latter behavior and indicates some of the limitations of this method of inhibition by nanoparticles. A discussion of factors which may complicate application of the method in the field is provided.

Potassium carbonate is a promising salt for thermochemical heat storage. For an application mm-sized salt hydrate particles are manufactured to be loaded inside a reactor. The step towards larger particles is necessary to prevent a large pressure drop over the reactor bed during hydration/dehydration in a given air flow. Therefore, in this work a systematic study on the hydration kinetics of mm-sized disc shaped salt hydrate (K2CO3) particles is presented for the first time. The effect of density, primary particle size and driving force on the hydration kinetics was evaluated using a 1D diffusion model. The main conclusions are that the hydration kinetics of mm-sized salt hydrate particles is diffusion limited and that the particle density (porosity) and tortuosity are the main parameters controlling its performance. On the contrary, the primary powder size did not affect the particle performance in any way. It is shown that the calculated transport mechanism is unaffected by changes in driving force whereas the power output decreases with decreasing driving force. Lastly shape and size optimization is discussed which can possibly improve the hydration kinetics of salt hydrate particles in view of thermochemical heat storage. Since the particle hydration is expected to be similar for various other salts, the model from this work offers opportunities to predict and optimize particles made from different salts as well.