Ind. Eng. Chem. Res. 2009, 48, 7457–7465
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Clathrate Hydrates: From Laboratory Science to Engineering Practice Amadeu K. Sum,* Carolyn A. Koh, and E. Dendy Sloan Center for Hydrate Research, Department of Chemical Engineering, Colorado School of Mines, Golden, Colorado 80401
Clathrate hydrates have steadily emerged as an important field in the areas of flow assurance, energy storage and resource, and environment. To better understand the role of hydrates in all of these areas, knowledge developed in laboratory experiments must be effectively transferred to address the challenges related to hydrate formation, dissociation, agglomeration, and stability. This paper highlights the recent hydrate literature focusing on the thermodynamics, kinetics, structural properties, particle properties, rheological properties, and molecular mechanisms of formation. The foundation for continued understanding and development of hydrates in engineering practice will rely on laboratory measurements utilizing traditional and innovative tools capable of probing time-dependent and time-independent properties. Introduction Clathrate hydrates, which are crystalline inclusion compounds formed from a network of hydrogen-bonded water molecules, are nonstoichiometric structures that include a large number of different types of small (10 mol %), THF has an inhibiting role. It is also interesting to compare these results with previous studies that reported on the promoting effect of methanol at low concentrations (0-5 wt %) and the inhibiting effect at high concentrations,49 although methanol is not known to form hydrates. Just recently, ethanol was also found to promote sII hydrates when formed with methane.50 Similar to methanol, ethanol has been usually identified as a hydrate thermodynamic inhibitor. With the increasing interest of hydrates in sediments, studies for hydrate phase equilibria in porous media have received considerable attention. These studies are particularly important because the properties of hydrates in sediments may not exhibit the same bulk properties of hydrate and sediments separately. One such study demonstrated the change in thermodynamics of hydrates in porous silica gels with pore diameters in the range of 6-30 nm.51 Hydrate phase equilibria for various gases (CH4, C2H6, CO2, and a natural gas mixture) measured in the silica gels showed that the hydrate equilibrium boundaries shift to lower temperature and higher pressures (Lw-H-C line on Figure 2) as the pore size decreased. For the silica gels with pores of 6 nm, the shifts were substantial, ranging from ∼8 bar to 18 bar. It was suggested that this “inhibiting” effect with decreasing pore size results from the geometrical constraints and potentially ordering of the water structure in the confined spaces. In another study on hydrate equilibrium in porous medium (quartz powder with a grain size of 90-100 µm),52 the phase boundaries for methane hydrates above and below the melting point of water were determined to be unaltered by the confined quartz structure. Hydrates are also being applied as a storage and transportation medium for gases, a process that is gaining considerable attention in Japan.9,37 Thermodynamics studies to address the stability of hydrates considered for this process have been investigated by Yasuda and Ohmura,53 under conditions in which hydrates coexist with ice (i.e., below 273.15 K). Phase
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equilibria measurements for hydrates with ice are less abundant than those above the melting point of water, partly because of the difficulty in conducting these experiments. Yasuda and Ohmura have reported extensive datasets for hydrate phase equilibria with ice for methane, ethane, propane, and carbon dioxide.53 Data such as these are important to improve the reliability of the thermodynamic prediction programs. Within the scope of thermodynamics and kinetics, the subject of hydrate metastability has generated much discussion about the hydrate structures.54-58 It has been established that the initially formed hydrate may not always correspond to the expected equilibrium structure.55 Moreover, strong evidence also supports the fact that structural transitions may occur with changes in the gas composition, conditioning of the hydrate samples, or introduction of kinetic inhibitors.56 These findings prompt us to reconsider the once-assumed equilibrium structures and reinforce the need to measure the hydrate phase to confirm the structures being formed. Nucleation and Growth The kinetics of hydrate formation, in terms of nucleation and growth, is much less understood than the thermodynamics. The challenges in hydrate nucleation and growth are related to massand heat-transfer effects. For example, the concentration of methane in liquid water is more than 1000 times lower than the amount in hydrates, while, conversely, the concentration of water vapor in methane is very low; this alone explains why hydrate nucleation and growth are most likely to occur at the gas/water interface. The nucleation and growth of hydrates is often associated with a delay or induction time (metastability) from the time the system is thermodynamically favorable to form hydrates. Because the nucleation of hydrates is a stochastic process, a system may be in a metastable state from seconds to hours/days, depending on the mixing conditions, composition, apparatus geometry, etc.31,59-61 It is also well-understood that the degree of subcooling (from the equilibrium temperature) or overpressurization (from equilibrium pressure) greatly affects the induction time for hydrate formation (shorter inductions times are often observed for greater subcooling or overpressurization). A recent comprehensive review examines the current knowledge on hydrate formation kinetics, with discussions on the use and limitations of proposed kinetic models.62 As concluded in that review, further improvements to model the hydrate formation kinetics will be required to account for massand heat-transfer effects, particle size, and agglomeration. Indeed, these contributions to the hydrate formation process are currently being incorporated in the CSMHyK transient hydrate model described in the following paragraph. Hydrate kinetics plays a large role in the flow assurance of oil and gas lines. A significant amount of research is devoted to understanding the kinetics, as well as the mass and heattransfer processes, that govern the formation and growth of hydrates in pipelines. Ultimately, a robust model is needed to describe how, when, and where hydrates may form in pipelines to prevent their aggregation and eventual blockage of flow lines. Over the past decade, significant progress has been achieved in developing such a model, which is currently implemented in CSMHyK in conjunction with OLGA, a multiphase flow simulator by the SPT Group. CSMHyK is based on a conceptual model for hydrate formation (developed from an integrated experimental program) in multiphase flow conditions, which in an oil-dominated system, is conceptually illustrated in Figure 3. Initially, water droplets are emulsified and dispersed in the continuous oil phase. As the system enters hydrate formation
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Figure 3. Conceptual picture for CSMHyK on the formation of hydrates in pipelines in multiphase flow. Also shown is an illustration of the conversion of a water droplet into a hydrate particle.
conditions, a hydrate shell forms around the water droplets from the dissolved gas in the oil. The growth of the hydrate shell in this shrinking-core model is controlled by the intrinsic growth kinetics, as well as the mass and heat transfer of water and gas at the interface. An overview and demonstration of the capabilities of CSMHyK are summarized elsewhere.13,63 To develop a model for hydrate formation (Figure 3), such as that in CSMHyK, one must understand the properties and characteristics for hydrate nucleation and growth. These are obtained from several different laboratory experiments aimed at providing physical insight and parameters for the transport and thermodynamics input to the program. Such findings have been made possible from a collection of studies to obtain hydrate particle sizes, shell thickness, particle adhesion forces, particle interfacial tension, rate of hydrate shell growth, diffusion rates, and heat- and mass-transfer limitations. The following experiments have been used to measure these properties: • Focused Beam Reflectance Measurement (FBRM) and Particle Video Microscope (PVM): A combined setup used to determine particle sizes with and without hydrates. Several comprehensive studies on the methods and applications of the FBRM and PVM to determine particle size and hydrate formation have been reported.64-66 Studies from the FBRM and PVM have been essential in the development of a correlation to predict the water droplet size, as a function of the fluid properties and mixing.67 • Differential Scanning Calorimetry (DSC): A versatile instrument capable of providing data on hydrate growth, diffusion rates, and hydrate agglomeration. Several studies have discussed the many uses of DSC for hydrates, including nucleation, dissociation, agglomeration, and emulsion stability.68-70 • Micromechanical Force (MMF) Apparatus: A simple setup composed of an inverted microscope with moveable cantilevers that allows adhesive force measurements between hydrate particles and surfaces (stickiness of particles).71 Studies utilizing the MMF have been instrumental in understanding the agglomeration of particles. • Raman Spectrometry: A robust technique giving microscopic properties of hydrate samples that can also be used to infer macroscopic processes. For example, Raman spectroscopy has been used to identify the structure and monitor the kinetics of hydrate formation (signal associated with enclathration of guest into the hydrate structure),56,72,73 as well as to measure the permeation and diffusion of water and guest through a hydrate film.63 • Nuclear Magnetic Resonance (NMR) Spectrometry: A powerful spectroscopic method used to study the microscopic environment. NMR has been widely used in the identification of the chemical environment for enclosed guest molecules in
the hydrate, giving information on the hydrate structure and occupancy of the cages,74-77 as well as kinetic data on growth and decomposition.78-80 NMR has also been successfully applied to monitor hydrate formation in water droplets formed in oil emulsions, thus overcoming the difficulty of observing through the oil samples and obtaining kinetic data in such complex mixtures.81-86 • Scanning Electron Microscopy (SEM): A powerful imaging tool to study the microscopic and mesoscopic structure of hydrates. For example, revealing SEM micrographs of methane hydrates showed that porous structures are often formed and are highly dependent on how the hydrate samples are formed.87,88 Recent results have shown that hydrate film formation at the interface is mass-transfer-limited,89 in contrast to the heattransfer-limited model proposed by Mori.90 These findings are consistent with other reported studies in which the hydrate formation rate was significantly increased, with a reduced induction time, from samples in which water was dispersed in finely powdered silica gel;91 those measurements removed the mass-transfer limitation by increasing the surface area of contact between the water and gas. Hydrate growth can also be largely affected by the morphology of the hydrate formed, that is, depending on the conditions (temperature, pressure, agitation) and composition, the hydrate particle/film may be either made of fine or coarse crystals. These observations have been well-documented by several studies,92-94 which generally indicate coarser structures (smaller crystals) at higher subcoolings and more-uniform structures (larger crystals) at lower subcoolings. Kinetic Inhibition One approach that is used by energy companies to prevent hydrate blockage of oil and gas lines is the introduction of lowdosage kinetic hydrate inhibitors (KHIs). This concept originated over 20 years ago with the goal of replacing costly inhibition of flow lines using methanol or glycols, which are thermodynamic inhibitors. The advantages of KHIs are that they are applied in small amounts without disturbing the pipeline flow and they do not require additional separation units to extract them from the fluid, as opposed to methanol or glycols. The working principle for KHIs is that they delay the hydrate formation without changing the thermodynamics by acting as a water or surface modifier. It is interesting to note that recent studies have indicated that certain KHIs (poly(vinyl caprolactam), abbreviated hereafter as PVCap) actually do impact the thermodynamics of hydrates by increasing the dissociation temperature.95-97 Those measurements provide some evidence that the KHIs are intimately interacting with the hydrate crystal surface, resulting in greater stability upon dissociation. Much still is unknown on the molecular mechanism by which KHIs operate, and this is one of the reasons limited advancements have been accomplished in the development of more-effective KHIs. The most-successful KHIs, to date, are still based on the first family of chemicals that was identified more than a decade ago, such as poly(vinyl pyrrolidone) (PVP) and PVCap.98,99 These are soluble polymeric compounds with pendant rings structures, which have been suggested to be key structures with regard to the way the KHIs interact with the hydrate structures. Currently, chemical companies that are serving the energy industry have their proprietary KHIs and continue to research potentially better compounds for the inhibition of hydrates. A very insightful perspective on KHIs was given by Klomp from Shell Global Solutions,100 which is one of the pioneering
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companies on the development of KHIs. In that paper, Klomp detailed the challenges for a more widespread use of KHIs as well as his view on the need for further fundamental understanding of the basic mechanism for how KHIs work. As noted by Klomp,100 it is well-recognized that experiments with KHIs suffer from reproducibility and transferability. Experiments with KHIs target the delay or induction time for hydrate formation. Often, many replica cells are considered and a statistical average of the results is reported, as a measure of the effectiveness of KHIs. Adding to the difficulty in assessing KHIs, especially when comparing results from different research laboratories, is the different types of experimental setups used, ranging from low- and high-pressure rocking cells, to high-pressure DSC, to stirred cells. High-pressure DSC has been suggested as a particularly well-suited simple and quick test for KHIs;101 this method allows for convenient sampling of a large number of hydrate nucleation events. KHIs that are denoted as green hydrate inhibitors102 have also been proposed. These “bio-KHIs” have been inspired by natural living systems, and they are being considered as stricter environmental regulations on the use and disposal of chemicals are posed to the oil and gas industry. These inhibitors originated from creatures sustaining frigid temperatures; their defense mechanism to prevent ice formation under these conditions can be associated with ice-structuring proteins,103,104 such as those found in ocean pouts and meal worms. The research on these green inhibitors is still nascent but potentially promising. Rheology The rheology of hydrates is an area in which only a few groups (IFP, Ecole des Mines de Saint-E´tienne, CSM) have made significant contributions. The difficulty in studying the rheology of hydrates involves the lack of being able to reproducibly control the formation of hydrates (crystalline solid structures) to the point that the flow properties of a fluid or slurry can be measured. Rheology studies on hydrates primarily have been applied toward flow assurance to understand the flow properties of hydrate slurries that may be found in oil and gas flow lines.105,106 In oil-dominated systems in pipelines, the initial formation of hydrates results in hydrate particles dispersed in the oil phase. These hydrate particles may agglomerate to form large aggregates, which, in turn, alter the effective viscosity of the fluid. The rheology of hydrates in this scenario is an important component in the flow assurance efforts to develop a comprehensive model that will describe the behavior of hydrates in a multiphase flow pipeline. Camargo and Palermo107 and Fidel-Dufour et al.105 were the first to propose a formal description of the rheology of hydrates particles dispersed in oils. Their work developed a framework from which further improvements are proposed. The model by Camargo and Palermo described an effective viscosity for hydrates in oils from their effective volume. The results of their model were shown to acceptably describe the flow behavior of hydrate slurries formed in a high-pressure-type rheometer and flow loop. More recently, Palermo and co-workers have investigated the agglomeration of ice and suspended hydrate particles to understand their flow characteristics under different flow conditions.108 These experiments were also coupled with NMR measurements to obtain kinetic data on the formation process. An understanding of the rheology of hydrate slurries has also proved to be important in the development of refrigeration systems.109,110 Two recent studies have detailed their efforts in rheological measurements to determine flow properties of
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hydrate slurries, in terms of the viscosity and yield stress. As described in those studies, the use of hydrate slurries as part of the working refrigeration system must overcome the agglomeration of hydrate particles, which can cause plugging of the lines, much in the same way as a flow assurance problem in oil and gas pipelines. Much remains to be learned about the rheology of hydrates. Current research in this area presents significant experimental challenges, but rheology is essential in the understanding of hydrates slurries, especially in the context of flow assurance in which hydrate agglomeration results in changes to the flow characteristics in pipelines. Efforts in this area are exerted to obtain sufficient experimental evidence on the rheological properties of hydrates that can be used to develop a formation and agglomeration model, which, combined with current transport and kinetic models, will allow better prediction of hydrate plugging in oil and gas flow lines. Molecular Mechanisms The understanding of hydrates at the molecular level has been greatly advanced by microscopic methods, such as NMR and Raman spectroscopy, and molecular modeling. Although these two approaches yield information at different length and time scales, they can also be complementary to each other: spectroscopic techniques sample average properties, and molecular modeling samples selected ensembles that are manifested as average properties. Molecular modeling is a useful method that gives a detailed description of the molecular processes that are observed at the macroscopic level. In the study of hydrates, molecular modeling (molecular simulation and quantum chemical calculations) has been successfully applied to give further insight into a variety of phenomena that are not well understood simply from the experimental evidence. The most-active areas of molecular modeling of hydrates are nucleation and growth, dissociation, structure, and stability. Molecular dynamics simulations have been widely used to understand the properties of hydrates at the molecular level. Although there are limitations in the simulations that can be performed, that is, system sizes on the order of few nanometers and time scales in the hundreds of nanosecond range, a wealth of information can still be obtained and related to experimental results. Many studies have been reported with different perspectives on the mechanism for hydrate nucleation and growth from molecular dynamics simulations.111-115 However, to date, even though many different ideas and disparate evidence have been collected, only limited insight has been gained toward an actual understanding of the mechanism for hydrate nucleation and growth. Part of the difficulty in the study of hydrates with molecular simulations is the same as those found when forming hydrates in the laboratory, namely, the randomness to initiate a homogeneous nucleation event and the metastability of the system. Most molecular dynamics studies on hydrates have considered times of
