High Pressure Automated Lag Time Research Articles

Nucleation Curve of Carbon Dioxide Hydrate from a Linear Cooling Ramp Method.

Formation of gas hydrate is a first-order phase transition that starts with nucleation. Understanding of nucleation is of interest to many in chemical and petroleum industries, as nucleation, while beneficial in many chemical processes, is detrimentalin flow assurance of oil and natural gas pipelines. A primary difficulty in the investigation of gas hydrate nucleation has been the inability of researchers to compare nucleation rates of gas hydrates across various systems of different scales and complexities, which in turn has been limiting the ability of researchers to study the nucleation process itself. In this study, a first-generation high-pressure automated lag time apparatus (HP-ALTA MkI) was used to determine the nucleation curve of structure I (sI)-forming carbon dioxide hydrate. The instrument subjected a quiescent water sample of well-defined dimensions to a large number of linear cooling ramps under isobaric conditions, and detected and recorded carbon dioxide hydrate formation temperature distributions. A survival curve was constructed from the measured ensemble, and a nucleation curve was derived from the survival curve using the empirical model-independent method we had previously reported. The nucleation rate of carbon dioxide hydrate was found to be significantly greater than that of pure methane hydrate or that of natural gas hydrate over the entire range of subcooling investigated. We provide a new physical interpretation of an experimentally determined nucleation curve and, by doing so, solve one of the outstanding puzzles of the HP-ALTA technology.

Scaling laws for nucleation rates of gas hydrate

Formation of gas hydrates is a first-order phase transition that starts with nucleation. Nucleation rate is central to nucleation phenomena of any system as it defines the rate at which critical nuclei form in a unit volume or other measures of system size. For gas hydrates, a primary difficulty in the investigation of gas hydrate nucleation has been the inability of researchers to compare nucleation rates of gas hydrates across various systems of different scales and complexity, as it is not even clear which measure(s) of system size is most appropriate to use in a given circumstance. This inadequacy in turn has been limiting the capacity of researchers to investigate the nucleation process itself. Here we first derived general scaling laws of nucleation rates of gas hydrate systems and then applied it to compare the nucleation rate of methane hydrate determined by a High Pressure Automated Lag Time Apparatus (HP-ALTA) to that of a flow loop. The ratio of the nucleation rates of two systems (of the same guest gas and at the same driving force) scales linearly with the ratio of the system sizes (i.e., the ratio of most probable induction times scales linearly with the inverse of the ratio of the system sizes). Comparison of five conceivable measures of the system size for the scaling suggests that the total length of the three-phase-lines in the system is the most appropriate measure for scaling of the nucleation rate of methane hydrate.

Nucleation curve of carbon dioxide hydrate

The first generation High Pressure Automated Lag Time Apparatus (HP-ALTA MkI) was used to measure the nucleation curves of Structure I (sI) – forming carbon dioxide hydrate. The instrument applied a large number of constant cooling ramps to a quiescent water sample contained in a glass sample cell under isobaric conditions and recorded maximum achievable subcooling distributions. A survival curves was constructed from the measured data and a nucleation curve was derived from the survival curve using the model-independent method we had recently reported. It was found that the nucleation rate of carbon dioxide hydrate was significantly greater than that of pure methane hydrate or natural gas hydrates.

Nucleation curves of methane hydrate from constant cooling ramp methods

A High Pressure Automated Lag Time Apparatus (HP-ALTA) was used to measure the nucleation curves of Structure I (sI) – forming methane hydrate. The instrument applied a large number of constant cooling ramps to a quiescent water sample contained in a glass sample cell under isobaric conditions and recorded maximum achievable subcooling distributions. Survival curves were constructed from the measured data and nucleation curves were derived from the survival curves using the model-independent method we had recently devised. The convergence of nucleation rates with the number of experimental runs was observed after approximately 400 runs which suggests that sampling of 400 nucleation events is sufficient for the unambiguous determination of the nucleation curves. The impact of the experimental cooling rates and the approximations used in the derivation of the nucleation curves was also assessed in details. Importantly, the derived nucleation curves continuously covered over a range of 15 K. The obtained nucleation curves were then compared to the nucleation rates of methane hydrate measured at several subcoolings by Makogon and analyzed by Kashchiev and Firoozabadi. Our nucleation curves yielded nucleation rates that were broadly similar to but somewhat lower than those of Makogon and Kashchiev and Firoozabadi at the relevant subcoolings.

Nucleation curves of methane – propane mixed gas hydrates in hydrocarbon oil

The second generation High Pressure Automated Lag Time Apparatus (HP–ALTA MkII) was used to measure the nucleation curves of Structure II (sII)–forming methane–propane mixed gas hydrates on the surface of a quasi–free water droplet suspended in involatile hydrocarbon oil, squalane. The measured nucleation curves were then compared to the previously obtained nucleation curves of the same guest gas hydrate on a quasi–free water droplet supported by perfluorodecalin. The comparison of the two sets of data sets showed that the nucleation rates of the two systems were broadly similar to each other. A simple master nucleation curve was derived for each system as a function of the system subcooling. The model-independent empirical equation that relates the nucleation rate to the system subcooling was Jwater-guest (s−1m−2)=2.38×10−39ΔT26.746 for a quasi-free water droplet suspended in squalane and Jwater-guest (s−1m−2)=7.70×10−11ΔT7.931 for a quasi-free water droplet supported by perfluorodecalin. The nucleation rates predicted by these empirical equations are then compared to the nucleation rates of other relevant systems in the literature. The model-independent analysis method for the derivation of nucleation curves we have developed can be applied to constant cooling ramp data obtained by any experimental techniques.

Nucleation curves of methane–propane mixed gas hydrates in the presence of a stainless steel wall

Nucleation phenomena are important to many applications of gas hydrates and yet the nucleation curves of gas hydrate systems have rarely been measured, presumably due to the experimental difficulties involved. We measured the nucleation curves of Structure II (sII) forming methane–propane mixed gas hydrates in the presence of a stainless steel (SS) wall using the second generation High Pressure Automated Lag Time Apparatus (HP-ALTA MkII). The instrument can apply a large number of linear cooling ramps to a small volume of water under isobaric conditions and record maximum achievable subcooling distributions. The survival curves with respect to methane–propane mixed gas hydrate formation were constructed from the recorded data and then converted to nucleation curves using the novel protocol we had recently reported. The nucleation rates per unit length of the three-phase-line, where the guest gas, water and the stainless steel wall met, were derived as functions of system subcooling. We also extended the measurements of the nucleation curves of the same guest gas hydrate in the presence of a glass wall to lower cooling rates than previously studied, so that the nucleation curves of these two systems could be compared at the same cooling rates. Comparison of the nucleation curves of these two systems showed that the nucleation rates per unit length of the three-phase-line were broadly similar to each other. The same instrument and experimental protocols can be utilized for the derivation of nucleation curves of a guest gas of a different composition and/or in the presence of another solid wall.

Statistical Study of the Memory Effect in Model Natural Gas Hydrate Systems.

A high pressure automated lag time apparatus (HP-ALTA) was used for the investigation of the controversial memory effect in methane-propane mixed gas hydrates. The instrument can apply a large number of linear cooling ramps to a small volume of sample water under an isobaric condition of up to 15 MPa and record the maximum achievable subcooling for each cooling ramp. Over a hundred nucleation events were recorded for each of the several superheating temperatures used for the dissociation of the gas hydrate in a sample. In total, four different sample cells were used, and the effect of heating time was also studied for two of the four sample cells. A difference between two stochastic nucleation probability distributions was systematically and unambiguously quantified in terms of the most probable difference in the maximum achievable subcoolings. The protocol offers by far the most statistically robust method of quantification of the magnitude of the memory effect in each sample. From the analysis of several thousands of nucleation events, the following conclusions were made: (1) Even though the nucleation phenomena were intrinsically stochastic, a clear bias was observed which supported the existence of the memory effect. In particular, a reduction in the most probable subcooling of at least 4 K was required for positive identification of the memory effect for one of the sample cells. (2) The reduction increased as the superheating temperature was lowered. (3) The magnitude of the memory effect varied substantially among the sample cells used. (4) No significant effect of the heating time was observed in the range studied.

Nucleation Probability Distributions of Methane–Propane Mixed Gas Hydrates in Salt Solutions and Urea

We studied nucleation probability distributions of a model natural gas (a mixture of 90% methane and 10% propane; C1/C3) hydrate in aqueous solutions of salts using the combinations of seven anions (F–, Cl–, Br–, I–, SCN–, NO3–, SO42–) and seven cations (NH4+, K+, Na+, Mg2+, Ca2+, Mn2+, Al3+) at a broad range of concentrations. A high pressure automated lag time apparatus (HP-ALTA) was used for the study. HP-ALTA can apply a large number (>100) of linear cooling ramps under isobaric conditions to the sample and construct nucleation probability distributions of C1/C3 mixed gas hydrate for each sample. We found that (1) ions can promote or inhibit gas hydrate formation at concentrations below 1 M, (2) this promotion or inhibition effect of salts did not depend on the valency of the ions involved for the range of salts studied, (3) the width of the nucleation probability distributions (stochasticity) of gas hydrate decreased in the presence of ions, (4) this decrease in the distribution width was largely ind...

Nucleation curves of model natural gas hydrates on a quasi‐free water droplet

Heterogeneous nucleation probability distributions of gas hydrates on a water droplet that was supported by inert and immiscible perfluorocarbon oil, perfluorodecalin is studied. The guest gas used was a mixture of 90 mol % methane and 10 mol % propane. The probability distribution was measured using a high pressure automated lag time apparatus under the guest gas pressure range of 6.7–12.5 MPa and the cooling rate range of 0.002–0.02 K/s. Nucleation curves were derived for unit area of water surface. The nucleation rate per unit area of water surface that was contained in a glass sample cell, which differed significantly from that on a quasi‐free water droplet, is also derived. It is concluded that the nucleation curves in the presence of a solid wall should be normalized to the unit length of the three‐phase line at which water, guest gas, and the solid wall meet. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2611–2617, 2015

Formation of Ice, Tetrahydrofuran Hydrate, and Methane/Propane Mixed Gas Hydrates in Strong Monovalent Salt Solutions

Electrolytes can thermodynamically inhibit clathrate hydrate formation by lowering the activity of water in the surrounding liquid phase, causing the hydrates to form at lower temperatures and higher pressures compared to their formation in pure water. However, it has been reported that some thermodynamic hydrate inhibitors (THIs), when doped at low concentrations, could enhance the rate of gas hydrate formation. We here report a systematic study of model natural gas (a mixture of 90% methane and 10% propane) hydrate formation in strong monovalent salt solutions in a broad range of concentrations, using a high pressure automated lag time apparatus (HP-ALTA). HP-ALTA can apply a large number (>100) of cooling ramps to a sample and construct probability distributions of gas hydrate formation for each sample. The probabilistic interpretation of data enables us to mitigate the stochastic variation inherent in the nucleation probability distributions and facilitates meaningful comparison among different sample...

Measurements of gas hydrate formation probability distributions on a quasi-free water droplet.

A High Pressure Automated Lag Time Apparatus (HP-ALTA) can measure gas hydrate formation probability distributions from water in a glass sample cell. In an HP-ALTA gas hydrate formation originates near the edges of the sample cell and gas hydrate films subsequently grow across the water-guest gas interface. It would ideally be desirable to be able to measure gas hydrate formation probability distributions of a single water droplet or mist that is freely levitating in a guest gas, but this is technically challenging. The next best option is to let a water droplet sit on top of a denser, immiscible, inert, and wall-wetting hydrophobic liquid to avoid contact of a water droplet with the solid walls. Here we report the development of a second generation HP-ALTA which can measure gas hydrate formation probability distributions of a water droplet which sits on a perfluorocarbon oil in a container that is coated with 1H,1H,2H,2H-Perfluorodecyltriethoxysilane. It was found that the gas hydrate formation probability distributions of such a quasi-free water droplet were significantly lower than those of water in a glass sample cell.

Quantitative kinetic inhibitor comparisons and memory effect measurements from hydrate formation probability distributions

In contrast with conventional experiments for ranking the performance of kinetic hydrate inhibitors (KHIs), the high pressure automated lag time apparatus (HP-ALTA) is not limited by the stochasticity of hydrate formation because it can make large numbers of formation measurements rapidly. An improved procedure for analysing HP-ALTA data is presented here in which formation probability density histograms with uniform temperature bin widths are constructed. Operations such as numerical integration and subtraction can then be applied to entire formation distributions without the need to regress the data to model analytic functions. This approach enables apparatus effects on the measured formation distributions to be quantified and mitigated. Using analogue natural gas mixtures, a demonstration of the improved method is reported here for six KHIs that were also tested using a conventional rocking cell. The cumulative formation probability distribution functions determined by numerical integration of the histograms were used to quantitatively (1) assess the performance of the KHIs, (2) study the memory effect in pure water, and (3) assess its impact on the performance of each KHI. The rankings of the KHIs obtained from the various performance metrics accessible through the HP-ALTA data were generally consistent with the rocking cell rankings, with the average absolute deviation in net subcooling for the two apparatus being 2 to 3K. In the quiescent HP-ALTA samples, the three worst performing KHIs still had expected subcoolings of about 14K unless the memory effect was present, in which case the expected net subcoolings of the three worst performing KHIs ranged from 0–6K. In contrast the memory effect reduced the expected subcoolings of the three best performing KHIs by only 1–2K. Using the new method of analysis, hydrate formation distributions measured with the HP-ALTA can also be used to identify regions of good and poor performance for a given KHI, which might aid in the determination of improved inhibition strategies or assist the development of improved hydrate inhibitors.

Development of a high pressure electrical conductivity probe for experimental studies of gas hydrates in electrolytes

We previously reported the development of a high pressure automated lag time apparatus and a high pressure video cell for experimental study of nucleation and growth of gas hydrates. Here we report the development of a high pressure electrical conductivity probe that monitors the electrical conductivity of an electrolyte that is subjected to a linear cooling ramp at elevated hydrate-forming-gas pressures. The electrical conductivity steadily decreases as the linear cooling progresses because of the increasing viscosity of the electrolyte and the consequently decreasing mobility of the ions. The onset of the formation of methane-propane mixed gas hydrate films at the gas-electrolyte interface is marked by an upward spike in the electrical conductivity. The physical mechanisms behind this phenomenon remain to be elucidated. Continued cooling of the electrolyte to, and subsequent holding of, the temperature at 273 K eventually results in decreased electrical conductivity. This conductivity signal can be used for the detection of the onset of the formation of gas hydrates in optically opaque samples that contain electrolytes.

Statistical Analysis of Supercooling in Fuel Gas Hydrate Systems

The recently developed high pressure automated lag time apparatus (HP-ALTA) was applied to the study of the formation and growth of interfacial gas hydrate films for three gases; methane (C1), 90% methane/10% propane gas mix (C1/C3), and synthetic gas mix (SGM). The effects of gas pressure and cooling rate were studied for each gas. Some degree of supercooling was observed in all cases. The probability distributions of formation temperature (Tf) were often found to be bimodal, due to the formation of either gas hydrate or ice in sequential experimental runs. The width of the Tf distribution of ice was about 3–4 K. In contrast, the width of the Tf distribution of gas hydrates was about 20 K which reflects the importance of mass transfer (gas diffusion) processes in nucleation. Differences in hydrate and ice nucleation probability distributions were observed for different gases, reflecting differences in both thermodynamic equilibrium phase behavior and hydrate formation mechanisms. For all gases studied, Tf generally increased with increasing gas pressure. A minimum threshold pressure for hydrate formation was observed, with magnitude decreasing in the order C1 > SGM > C1/C3. The effect of cooling rate on gas hydrate nucleation probability was also studied. The median of the distribution of Tf (Tf50) was found to decrease with an increased cooling rate, consistent with the increases in effective induction time as samples were cooled more slowly. Our results clearly highlight the value in collecting large data sets which can be used to assemble probability distributions when studying intrinsically stochastic processes such as gas hydrate nucleation and growth.

Development of a high pressure automated lag time apparatus for experimental studies and statistical analyses of nucleation and growth of gas hydrates

Nucleation in a supercooled or a supersaturated medium is a stochastic event, and hence statistical analyses are required for the understanding and prediction of such events. The development of reliable statistical methods for quantifying nucleation probability is highly desirable for applications where control of nucleation is required. The nucleation of gas hydrates in supercooled conditions is one such application. We describe the design and development of a high pressure automated lag time apparatus (HP-ALTA) for the statistical study of gas hydrate nucleation and growth at elevated gas pressures. The apparatus allows a small volume (≈150 μl) of water to be cooled at a controlled rate in a pressurized gas atmosphere, and the temperature of gas hydrate nucleation, T(f), to be detected. The instrument then raises the sample temperature under controlled conditions to facilitate dissociation of the gas hydrate before repeating the cooling-nucleation cycle again. This process of forming and dissociating gas hydrates can be automatically repeated for a statistically significant (>100) number of nucleation events. The HP-ALTA can be operated in two modes, one for the detection of hydrate in the bulk of the sample, under a stirring action, and the other for the detection of the formation of hydrate films across the water-gas interface of a quiescent sample. The technique can be applied to the study of several parameters, such as gas pressure, cooling rate and gas composition, on the gas hydrate nucleation probability distribution for supercooled water samples.