Propane hydrate nucleation: Experimental investigation and correlation

In this work the nucleation kinetics of propane gas hydrate has been investigated experimentally using a stirred batch reactor. The experiments have been performed isothermally recording the pressure as a function of time. Experiments were conducted at different stirring rates, but in the same supersaturation region. The experiments showed that the gas dissolution rate rather than the induction time of propane hydrate is influenced by a change in agitation. This was especially valid at high stirring rates when the water surface was severely disturbed.Addition of polyvinylpyrrolidone (PVP) to the aqueous phase was found to reduce the gas dissolution rate slightly. However the induction times were prolonged quite substantially upon addition of PVP.The induction time data were correlated using a newly developed induction time model based on crystallization theory also capable of taking into account the presence of additives. In most cases reasonable agreement between the data and the model could be obtained. The results revealed that especially the effective surface energy between propane hydrate and water is likely to change when the stirring rate varies from very high to low. The prolongation of induction times according to the model is likely to be due to a change in the nuclei-substrate contact angle.     

Measurements of methane hydrate heat of dissociation using high pressure differential scanning calorimetry

The methane hydrate heat of decomposition was directly measured up to 20 MPa and 292 K using a high pressure differential scanning calorimeter (DSC). The methane hydrate sample was formed ex-situ using granular ice particles and subsequently transferred into the DSC cell under liquid nitrogen. The ice and water impurities in the hydrate sample were reduced by converting any dissociated hydrate into methane hydrate inside the DSC cell before performing the thermal properties measurements. The methane hydrate sample was dissociated by raising the temperature (0.5-1.0 K/min) above the hydrate equilibrium temperature at a constant pressure. The measured methane hydrate heat of dissociation (H→W+G), ΔH_d, remained constant at 54.44±1.45 kJ/mol gas (504.07±13.48 J/gm water or 438.54± 13.78 J/gm hydrate) for pressures up to 20 MPa. The measured ΔH_d is in agreement with the Clapeyron equation predictions at high pressures; however, the Clausius-Clapeyron equation predictions do not agree with the heat of dissociation data at high pressures. In conclusion, it is recommended that the Clapeyron equation should be used for hydrate heat of dissociation estimations at high pressures.     

Large pressure depression of methane hydrate by adding 1,1-dimethylcyclohexane

The structure-H hydrate of 1,1-dimethylcyclohexane (DMCH) helped by methane has been investigated in a temperature range of 274.6-289.3 K and pressure range up to 6.7 MPa. The present results suggest that 1,1-DMCH is a suitable additive which makes a mild-pressure handling of natural-gas hydrate possible.     

Surfactant effects on hydrate formation in an unstirred gas/liquid system: An experimental study using methane and sodium alkyl sulfates

This paper reports an experimental study on the effects of surfactant additives on the formation of a clathrate hydrate in a quiescent methane/liquid-water system, which was initially composed of a 300-cm³ aqueous phase and an ∼640-cm³ methane-gas phase, then successively provided with methane such that the system pressure was held constant. The surfactants used in the present study were three sodium alkyl sulfates appreciably different in the alkyl chain length—they were sodium dodecyl sulfate (abbreviated as SDS), sodium tetradecyl sulfate (abbreviated as STS) and sodium hexadecyl sulfate (abbreviated as SHS). For each surfactant added to water up to, at most, 1.82-3.75 times the solubility, we performed visual observations of hydrate formation simultaneously with the measurements of methane uptake due to the hydrate formation. The qualitative hydrate-formation behavior thus observed was almost the same irrespective of the species as well as the initial concentration of the surfactant used; i.e., thick, highly porous hydrate layers were formed and grew on the horizontal gas/liquid interface and also on the test-chamber wall above the level of the gas/liquid interface. In each experimental operation, hydrate formation continued for a limited time (from ∼6 to ) and then practically ceased, leaving only a small proportion (typically 15% or less) of the aqueous solution unconverted into hydrate crystals. The variations in the time-averaged rate of hydrate formation (as measured by the rate of methane uptake) and the final water-to-hydrate conversion ratio with the initial concentration of each surfactant were investigated. Moreover, we examined the promotion of hydrate formation with the aid of a water-cooled cold plate, a steel-made flat-plate-type heat sink, vertically dipped into the aqueous phase across the gas/liquid interface.     

Surfactant effects on hydrate formation in an unstirred gas/liquid system: Amendments to the previous study using HFC-32 and sodium dodecyl sulfate

This paper reports a set of experimental data of clathrate-hydrate formation from HFC-32 (difluoromethane) gas in contact with an aqueous solution of sodium dodecyl sulfate (SDS). This supersedes the corresponding data that we previously reported in this journal [Watanabe et al., 2005. Surfactant effects on hydrate formation in an unstirred gas/liquid system: an experimental study using HFC-32 and sodium dodecyl sulfate. Chemical Engineering Science 60, 4846-4857] with the new data reported herein, because of a suspicion of hydrate plugging occurring in the gas-feed line of our experimental system used to obtain the previous data. The new data show much higher levels in both the hydrate formation rate and the final water-to-hydrate conversion ratio as compared to the previous data. Neither the hydrate formation rate nor the water-to-hydrate conversion ratio exhibited a significant change with the SDS concentration in the aqueous phase over the range from 1000 to 4000 ppm.     

Icosahedron cage occupancy of structure-H hydrate helped by Xe -1,1-, cis -1,2-, trans-1,2-, and cis-1,4-dimethylcyclohexanes

Four mixtures of 1,1-, cis-1,2-, trans-1,2-, and cis-1,4-dimethylcyclohexanes (hereafter abbreviated DMCH) including H₂O and Xe have been investigated in a temperature range over 274.5 K and a pressure range up to 2.7 MPa. The 1,1-DMCH and cis-1,2-DMCH generate the structure-H hydrate in the temperature range up to 295.2 and 280.2 K, respectively. Especially, very large depression of equilibrium pressure has been observed in the structure-H 1,1-DMCH hydrate system. On the other hand, neither trans-1,2-DMCH nor cis-1,4-DMCH generates the structure-H hydrate in the present temperature range. It is an important finding that the cis-1,4-DMCH does not generate the structure-H hydrate in the presence of Xe, while the mixture of cis-1,4-DMCH and methane generates the structure-H hydrate.     

Experimental study and computational fluid dynamics modeling of deposition of hydrate particles in a pipeline with turbulent water flow

The present paper describes a study of Freon R11 hydrate deposition in a turbulent flow of water. Eulerian–Eulerian CFD model was built in order to study the process numerically, the model was validated with experimental data from the multiphase flow loop. Different mechanisms for particulate stress were studied in the work in terms of their performance results, compared to experimental data. The model considered an expression for variable hydrate particle size which was validated in a series of population balance numerical experiments.     

Storage capacity of hydrogen in tetrahydrofuran hydrate

The storage capacity of hydrogen in tetrahydrofuran hydrate was investigated by means of pressure-volume-temperature (p-V-T) measurement and Raman spectroscopic analysis. We carried out two measurement strategies using Raman spectroscopic analysis. One was isothermal pressure-swing absorption using tetrahydrofuran hydrate at 277.15 K, and the other was the preparation of a single crystal of hydrogen+tetrahydrofuran mixed gas hydrate from compressed hydrogen and tetrahydrofuran aqueous solutions along the stability boundary. The storage amount of hydrogen at 277.15 K obtained from the p-V-T measurement is about 1.6 mol (hydrogen)/mol (tetrahydrofuran) (about 0.8 mass%) at 70 MPa, and isothermal Raman spectroscopic measurement reveals that it reaches the maximum value of 2.0 mol (hydrogen)/mol (tetrahydrofuran) at about 85 MPa. These results agree well with those for a single crystal of hydrogen+tetrahydrofuran hydrate.     

Thermal analysis of failed-batch palm oil by differential scanning calorimetry

Thermal behavior of palm oil samples drawn from the batch crystallizers that failed during crystallization and of a control oil that was drawn from a batch that produced good crystallization were analyzed by differential scanning calorimetry under constant heating and cooling conditions. Four polymorphs—β’₂, α, β’₁, and β₁—were observed, and their temperatures were tabulated. A rapid and sudden surge of heat demand was observed for samples from failed crystallizers. Less supercooling values were obtained from the control oil compared to the higher values for samples from failed crystallizers. In crystallization thermograms, a sharp high-temperature exotherm (high-T peak) and a broad low-temperature exotherm (low-T peak) were observed. Low-T peaks were found almost invariably stationary at −5.1 to −5.6°C, and high-T peaks varied depending on the saturation level of the oil. A new peak, sandwiched between the high-T and low-T peaks, was observed for the control oil.     

Concentration of sodium chloride in aqueous solution by chlorodifluoromethane gas hydrate

The decomposition temperature and pressure (quadruple point) of chlorodifluoromethane (R22) gas hydrate in aqueous sodium chloride (NaCl) solution was measured at different NaCl concentrations in the solution as a phase diagram. The operative concentration curve of NaCl was obtained as a function of temperature. The maximum decomposition temperature of R22 hydrate was about 290 K at 0.9 MPa pressure, and it decreased as the NaCl concentration increased in the solution. R22 hydrate caused supercooling, and the supercooling occurrence temperature was much lower than the decomposition temperature. The ultrasonic charge reduced the supercooling of hydration effectively even though the ultrasonic charge did not change the decomposition temperature at all. The concentration experimental results from the several NaCl solutions having different NaCl concentrations were in good agreement with the theoretical operative concentration curve for NaCl.     

Is subcooling the right driving force for testing low-dosage hydrate inhibitors?

The degree of subcooling is usually used as the driving force for hydrate formation; however, it does not encompass the effect of pressure. A comprehensive driving force for hydrate formation is a function of pressure, temperature, and gas composition; however, its calculation is not as simple as that of subcooling. In this work, by application of the two latest driving force expressions for hydrate formation, the relationships between subcooling and the true driving force at different conditions for pure gas-water and natural gas-water systems are analysed. The effect of pressure on the induction time in the presence and absence of a kinetic inhibitor have been tested at similar degrees of subcooling.The results show that for pure gas-water systems subcooling is proportional to the driving force, with a good approximation over a wide pressure range at isothermal conditions. However, for multicomponent systems (e.g., natural gases), the driving force is more than that suggested by subcooling at some pressures. Changes of driving force with pressure at a constant degree of subcooling for the above systems have been presented. The results show that the pressure has no significant effect on the driving force (at a constant degree of subcooling) above a certain pressure range. The experimental results show that in a natural gas-water system at constant degree of subcooling the induction time is not significantly affected by pressure. However, in the presence of the kinetic inhibitor tested in this study, high-pressure conditions decreased the induction time.     

Studies on some alkylamide surfactant gas hydrate anti-agglomerants

Low dosage hydrate inhibitors (LDHIs) are a recently developed hydrate control technology, which can be more cost-effective than traditional practices such as the use of thermodynamic inhibitors e.g., methanol and glycols. Two classes of LDHI called kinetic inhibitors (KHIs) and anti-agglomerants (AAs) are already being successfully used in the field. This paper describes efforts to develop new classes of AA surfactant with one or two alkylamide groups in the polar head. The goal was to find an AA that was as good as commercial quaternary AAs, which would be economically competitive and more environmentally friendly. The chemistry and environmental properties of the new surfactants are described along with experiments to determine their performance carried out in high-pressure sapphire cells and a wheel loop. The results indicate positive performance for some products but not as good as a commercial quaternary ammonium-based surfactant AA. The best surfactants had one or two carbonylpyrrolidine or isopropylamide groups in the head. The performance of the best AAs was found to be dependent on the hydrocarbon phase and salinity of the water phase.     

Kinetics of crystal nucleation of poly(L-lactic acid)

The kinetics of crystal nucleation of poly(L-lactic acid) (PLLA) has been analyzed by fast scanning chip calorimetry in a wide temperature range between 313 and 383 K, that is, between temperatures about 30 K below and 40 K above the glass transition temperature. The relaxed melt was rapidly cooled to the analysis temperature and subsequently aged between 10⁻¹ and 10⁴ s, to permit formation of nuclei. The number of formed crystal nuclei has been probed by analysis of the crystallization rate at 393 K. The nucleation rate is maximal at 370–375 K and decreases monotonously with decreasing temperature in the analyzed temperature range. The observation of a monomodal dependence of the nucleation rate on temperature points to predominance of a single nucleation mechanism in the analyzed temperature range, regardless nucleation occurs in the glassy or devitrified amorphous phase. The collected data suggest that nuclei formation at ambient temperature requires a waiting time longer than about 10⁸ s. The performed study is considered as a quantitative completion of nucleation-rate data available for PLLA only at temperatures higher than 360 K, suggesting that the nucleation mechanism is independent on temperature in the analyzed temperature range between 313 and 383 K.     

Polymorphism of milk fat studied by differential scanning calorimetry and real-time X-ray powder diffraction

The crystallization behavior of milk fat was investigated by varying the cooling rate and by isothermal solidification at various temperatures while monitoring the formation of crystals by differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD). Three different polymorphic crystal forms were observed in milk fat: γ, α, and β′. The β-form, occasionally observed in previous studies, was not found. The kind of polymorph formed during crystallization of milk fat from its melted state was dependent on the cooling rate and the final temperature. Moreover, transitions between the different polymorphic forms were shown to occur upon storing or heating the milk fat. The characteristic DSC heating curve of milk fat is interpreted on the basis of the XRD measurements, and appears to be a combined effect of selective crystallization of triglycerides and polymorphism.     

Effect of small amount of ultra high molecular weight component on the crystallization behaviors of bimodal high density polyethylene

In order to clarify the effect of high molecular weight component on the crystallization of bimodal high density polyethylene (HDPE), a commercial PE-100 pipe resin was blended with small loading of ultra high molecular weight polyethylene (UHMWPE). The isothermal crystallization kinetics and crystal morphology of HDPE/UHMWPE composites were studied by differential scanning calorimetry (DSC) and polarized optical microscopy (POM), respectively. The presence of UHMWPE results in elevated initial crystallization temperature of HDPE and an accelerating effect on isothermal crystallization. Analysis of growth rate using Lauritzen-Hoffman model shows that the fold surface free energy (σ_e) of polymer chains in HDPE/UHMWPE composites was lower than that in neat HDPE. Morphological development during isothermal crystallization shows that UHMWPE can obviously promote the nucleation rate of HDPE. It should be reasonable to conclude that UHMWPE appeared as an effective nucleating agent in HDPE matrix. Rheological measurements were also performed and it is shown that HDPE/UHMWPE composites are easy to process and own higher melt viscosity at low shear rate. Combining with their faster solidification, gravity-induced sag in practical pipe production is expected to be effectively avoided.     

Fundamental mechanisms and phenomena of clathrate hydrate nucleation

Insights into the mechanism of hydrate nucleation are of great significance for the development of hydrate-based technologies, hydrate relevant flow assurance, and the exploration of in situ natural gas hydrates. Compared with the thermodynamics of hydrate formation, understanding the nucleation mechanism is challenging and has drawn substantial attention in recent decades. In this paper, we attempt to give a comprehensive review of the recent progress of studies of clathrate hydrate nucleation. First, the existing hypotheses on the hydrate nucleation mechanism are introduced and discussed. Then, we summarize recent experimental studies on induction time, a key parameter evaluating the velocity of the nucleation process. Subsequently, the memory effect is particularly discussed, followed by the suggestion of several promising research perspectives.     

Kinetics of rapeseed oil oxidation by pressure differential scanning calorimetry measurements

Samples of rapeseed oils were oxidized at isothermal conditions in the cell of a pressure differential scanning calorimeter (PDSC). The experiments were carried out under 1400 kPa oxygen pressure at a temperature in the range of 113—144 °C. From resulting PDSC exotherms the times to reach the peak maximum (τ_max) were determined and used for the assessment of the oxidative stabilities of the samples. As PDSC exotherms were obtained at different temperatures, equations for extrapolation of the τ_max values were proposed and activation energies as well as specific rate constants for oxidation were calculated.     

Surfactant effects on hydrate formation in an unstirred gas/liquid system: An experimental study using HFC-32 and sodium dodecyl sulfate

This paper deals with the effects of a surfactant additive on the formation of a clathrate hydrate in a quiescent guest-gas/liquid-water system. The paper first presents our strong suspicion against the existing hypothesis that the surfactant-micelle formation in the liquid-water phase promotes the hydrate formation. It is pointed out that the Krafft point for sodium dodecyl sulfate (SDS), a popular anionic surfactant often used in previous hydrate-forming experiments, is presumably higher than the system temperatures set in these experiments and hence that no micelles may have formed in these experiments. The paper then describes our experimental observations of the hydrate formation from a hydrofluorocarbon gas, HFC-32 (CH₂F₂), to show how the hydrate formation behavior is affected by the addition of SDS to the water when brought into contact with HFC-32. In each experiment, HFC-32 gas was continuously supplied to a rectangular chamber partially filled with a quiescent pool of water (pure water or an aqueous SDS solution) to compensate for the gas consumption due to the hydrate formation, thereby maintaining a constant pressure inside the chamber. The present experiments featured the following characteristics: (a) detailed visual observations along horizontal axes through large side windows in the test chamber, and (b) surface tension measurements of the aqueous SDS solutions with the aid of a pendant-drop device inserted in the same chamber to determine the SDS-in-water solubility, which seems to have been misunderstood as the critical micelle concentration (CMC) in some previous studies, under the hydrate-forming conditions. The former revealed that the addition of SDS to the pool-forming water results in the formation of thick, highly porous hydrate layers not only on the liquid-pool surface but also on the chamber walls above the level of the pool surface, leaving the bulk of the liquid pool free from hydrate crystals. The latter led to an important finding that the SDS concentration at which the rate of the hydrate formation peaks is slightly lower than the solubility (the false CMC). An excessive addition of SDS beyond the solubility was found to cause a decrease in the rate of hydrate formation but an increase in the final level of the water-to-hydrate conversion.