High-pressure Sapphire Cell Research Articles

Avoiding costly LNG plant freeze-out-induced shutdowns: Measurement and modelling for neopentane solubility at LNG conditions

In this work, a visual high pressure sapphire cell was employed to measure the solid-fluid equilibrium (SFE) of methane + neopentane binary systems at temperatures down to 90 K and pressures up to 11 MPa. First, the conditions of solid-liquid-vapour equilibrium (SLVE) and solid-liquid equilibrium (SLE) were measured for synthetic mixtures of neopentane in methane. The SFE conditions were then investigated at a constant neopentane mole fraction of 2036 ppm. The SFE data showed a variety of phase transitions starting at higher temperatures (SVE conditions) where neopentane solid formed. Further cooling at pressures below 2 MPa resulted in a reduction of the amount of solid neopentane present, which eventually melted entirely after passing through a solid-liquid retrograde boundary. A model implemented in the software tool ThermoFAST was tuned to SFE experimental data obtained in this work and taken from the literature, which led to an optimized binary interaction parameter, capable of describing the experimental data within an R.M.S deviation of 2.7 K. Overall, the data measured suggest that neopentane is quite soluble in methane (0.10 mol fraction) at typical LNG conditions, with limited freeze-out risk during production given the small concentrations of neopentane typically present in natural gas.

Solid-fluid equilibrium measurements of benzene in methane and implications for freeze-out at LNG conditions

Information about the solubility of benzene in light hydrocarbons is particularly important for the prediction of freeze-out risk in LNG production. Engineering models developed to predict this risk need to be tested against high quality experimental data covering a range of conditions to assess their validity. A visual high pressure sapphire cell, housed in a specialized cryogenic environmental chamber, was employed to measure the melting temperature of methane + benzene binary systems at temperatures from 120 K, pressures up to 22 MPa, and benzene concentrations ranging from 120 to 1012 parts per million (ppm) by mole. The results obtained were compared with literature data and the predictions of the thermodynamic model implemented in the software package ThermoFAST. These comparisons reveal that the literature data are in fact consistent with each other, and with the measurements and predictions made in this work, within their experimental scatter. ThermoFAST was able to represent the melting temperatures obtained for benzene concentrations of 1012 and 199 ppm with r.m.s deviations of 0.7 and 3.4 K, respectively. At 120 ppm and 6.3 MPa, the measured solid-liquid equilibrium (SLE) temperature deviated from the ThermoFAST prediction by less than 2 K. However, at the higher temperature conditions representative of solid vapour equilibrium (SVE), the data measured for mixtures with concentrations at 199 and 750 ppm benzene deviated from the model predictions by up to 5 K.

The effect of regenerated MEG on hydrate inhibition performance over multiple regeneration cycles

Mono-ethylene glycol (MEG) is a favorable gas hydrate inhibitor mainly due to its recoverability through MEG regeneration facilities, and thus reducing costs. However, it is not clear how the hydrate inhibition performance of MEG is affected by multiple regeneration cycles. In this study, MEG samples that were regenerated and reclaimed over multiple cycles using an innovative bench-scale MEG pilot plant which can simulate field-like MEG operations, were assessed on their hydrate inhibition performance. The cycled MEG samples were carefully analyzed in the laboratory for their composition, and each sample was tested in a high-pressure sapphire cell for methane hydrate inhibition performance. The study found a directly proportional relationship between the number of cycles and the shift in hydrate equilibrium phase boundary. A maximum equilibrium shift of 2.21 °C was recorded for a 20 wt% MEG/deionized water sample that had experienced 9 regeneration cycles compared to pure MEG. The analysis suggests that the shift in hydrate equilibrium phase boundary was due to thermal degradation of MEG within the regeneration and reclamation units due to the presence of acetic acid. The study found that even though the operation was below MEG degradation temperature range, repeated heating of MEG may have caused its degradation. Additionally, the phase equilibria are empirically modeled as a function of the number of cycles to aid MEG end-users. Application of the model to experimental results provided accurate outcomes, and had an average relative difference of 1.24% when determining equilibrium temperatures.

Synthesis and Evaluation of a New Kind of Kinetic Hydrate Inhibitor

A new kind of kinetic hydrate inhibitor (KHI) named KL-1 as the ramification of poly (N-vinyl caprolactam) (PVCap), was synthesized successfully by use of precipitation polymerization. The hydrate inhibition performance of KL-1 was assessed in a high pressure sapphire cell, and the onset time of hydrate formation and maximum subcooling were determined by the visual observation method and compared with the commercial KHIs, including Inhibex 501 and VC-713. Meanwhile, the synergic effect between ethanol and KL-1 developed was also studied in this work. The experimental results show that the onset time of KL-1 measured increases with the increase of the dosage and decrease of subcooling. Compared with the system without kinetic hydrate inhibitor, the morphological behavior of hydrate crystals in the systems containing KL-1 is different, and the hydrate crystals only grow to the gas phase with the hydrate formation. Additionally, based on the measurement of inhibition time, the inhibition performance of KL-1 is superior to Inhibex 501 and VC-713, and shows higher maximum subcooling at the similar conditions. Finally, we also demonstrated that ethanol can be used as synergist to improve the performance of KL-1 remarkably at suitable dosage.

SOLUBILITY AND MISCIBILITY OF REFRIGERANTS R407C AND R410A WITH SYNTHETIC COMPRESSOR OILS

In this paper, solubility and low temperature miscibility of refrigerants R407C and R410A in four different commercial Polyolester (POE) lubricants, produced by the same company but with different ISO standard viscosity grade, are measured with the aim to evaluate the possible correlation of solubility and miscibility with standard viscosity of the lubricant. Vapor pressure (solubility) is measured using calibrated, constant volume cells at oil mass fractions 30, 50, 70, 80, and 90% over a temperature range from -20 to 100 °C, and pressures up to 5 MPa. The measurements of low temperature miscibility limit (lower critical solution temperature LCST) were made using high-pressure sapphire cell. LCST was directly obtained by visual observation of a «milky haze» followed by a phase separation. Miscibility data were obtained at oil mass fractions from 5 to 50% over a temperature range from -60 to 0 °C. A set of simple equations was derived to describe the experimental results. Analysis of the obtained results showed that the solubility of refrigerant/lubricant mixtures depends weakly on the viscosity grade of the oil. At the same time, the miscibility gap (LCST) differ much bigger for various refrigerant/lubricant mixtures. The lower viscosity grade of the oil, the lower LCST of the refrigerant/lubricant mixture. LCSTs for the mixtures of R407C with the lubricants with the viscosity grades 32 and 220 differ by 30-45 °С.

Synergic Kinetic Inhibition Effect of EMIM-Cl + PVP on CO2 Hydrate Formation

Kinetic hydrate inhibitors (KHIs) are employed to mitigate gas hydrate formation in pipelines at low concentrations. Ionic liquids and conventional available KHIs (polymers) perform poorly at high subcooling and extended shut-in conditions. Thus, it's necessary to understand the inhibition mechanism and performance of mix kinetic inhibitors for gas hydrate mitigation in oil and gas transmission lines. In this work, the synergic kinetic inhibition effect of 1-Ethyl-3-methy-limidazolium chloride (EMIM-Cl) + Polyvinylpyrrolidone (PVP) on the induction time and gas uptake during carbon dioxide (CO2) gas hydrate formation is reported. The study was performed in a high pressure sapphire cell at a pressure and temperature of 36 Bar and 1 oC respectively using an isochoric constant cooling method. Result suggests that, the induction time of deionized water + carbon dioxide hydrate increased over 100% in the presence of pure EMIM-Cl and PVP, with PVP slightly higher than EMIM-Cl comparatively. Surprisingly, a significant reductions in CO2 hydrate inhibition performance is observed in the presence of EMIM-Cl + PVP studied synergic systems compared to their inhibition in pure state.

High-pressure Sapphire Cell for Phase Equilibria Measurements of CO<sub>2</sub>/Organic/Water Systems

The high pressure sapphire cell apparatus was constructed to visually determine the composition of multiphase systems without physical sampling. Specifically, the sapphire cell enables visual data collection from multiple loadings to solve a set of material balances to precisely determine phase composition. Ternary phase diagrams can then be established to determine the proportion of each component in each phase at a given condition. In principle, any ternary system can be studied although ternary systems (gas-liquid-liquid) are the specific examples discussed herein. For instance, the ternary THF-Water-CO2 system was studied at 25 and 40 °C and is described herein. Of key importance, this technique does not require sampling. Circumventing the possible disturbance of the system equilibrium upon sampling, inherent measurement errors, and technical difficulties of physically sampling under pressure is a significant benefit of this technique. Perhaps as important, the sapphire cell also enables the direct visual observation of the phase behavior. In fact, as the CO2 pressure is increased, the homogeneous THF-Water solution phase splits at about 2 MPa. With this technique, it was possible to easily and clearly observe the cloud point and determine the composition of the newly formed phases as a function of pressure. The data acquired with the sapphire cell technique can be used for many applications. In our case, we measured swelling and composition for tunable solvents, like gas-expanded liquids, gas-expanded ionic liquids and Organic Aqueous Tunable Systems (OATS)(1-4). For the latest system, OATS, the high-pressure sapphire cell enabled the study of (1) phase behavior as a function of pressure and temperature, (2) composition of each phase (gas-liquid-liquid) as a function of pressure and temperature and (3) catalyst partitioning in the two liquid phases as a function of pressure and composition. Finally, the sapphire cell is an especially effective tool to gather accurate and reproducible measurements in a timely fashion.

Trihexyl(tetradecyl)phosphonium bromide: Liquid density, surface tension and solubility of carbondioxide

Vapour–liquid equilibria of ionic liquid – carbon dioxide systems, as well as thermo-physical properties of the system components are very important to design and optimize various separation and reaction processes. In this work the solubility of carbon dioxide (CO2) in trihexyl(tetradecyl)phosphonium bromide([THTDP][Br]) was measured using a high-pressure sapphire cell, in pressure range of 8–22MPa and at two temperatures, 313.2K and 323.2K. The thermophysical properties, namely liquid density and surface tension of the ionic liquid were determined in temperature range of 293.2–343.2K.The densities predicted by Gardas and Coutinho model showed good agreement with the experimental data obtained in this work. The critical temperature of [THTDP][Br] was estimated using the Eötvos correlation. Moreover, these experimental and calculated data gave an opportunity to apply the Peng–Robinson equation of state (PR-EoS) in order to predict/correlate the behaviour of the studied system, ([THTDP][Br]+CO2) with satisfactory results.

High-pressure solubilities of carbon dioxide in ionic liquids based on bis(trifluoromethylsulfonyl)imide and chloride

In this work, the solubility of carbon dioxide (CO2) in seven different ionic liquids (ILs) was measured using a high-pressure sapphire cell, in the pressure range of 8–22MPa and at two temperatures, 313.2K and 323.2K. In order to discuss the influence of the cation, ILs based on bis(trifluoromethylsulfonyl)imide, [NTf2] were chosen. They were coupled with 1-butyl-3-methylimidazolium, [C4mim]+, 1-decyl-3-methylimidazolium, [C10mim]+, 1-butyl-1-methylpyrrolidinium, [Pyrr4,1]+, butyltrimethylammonium, [N4,1,1,1]+, methyltrioctylammonium, [N1,8,8,8]+ and trihexyltetradecylphosphonium, [P6,6,6,14]+cation. The phosphonium based cation was also combined with chloride, [Cl]−, giving an opportunity to discuss the influence of an anion as well. The solubility data obtained in this work were used to evaluate the importance of explicitly including association for describing such systems. For that purpose, we compared the solubility modelling results from two cubic equations of state (Peng–Robinson and Soave–Redlich–Kwong) with those from the Cubic Plus Association Equation of State (CPA EoS) using different association schemes for the ionic liquids. We found that for such systems there is no major advantage in considering ILs as associating components.

Assessment of hydrate kinetic inhibitors with visual observations

The hydrate inhibition effect of three kinetic inhibitors (inhibex 301, 501, and 713) was assessed from (CH 4 + C 2H 6 + C 3H 8) gas mixture + brine systems using a high pressure sapphire cell. The onset time of hydrate formation was determined by visual observation method and pressure drop profile method, respectively. The experimental results demonstrated that the onset time was able to be determined by the visual observation method all the time while the pressure drop profile method failed to detect the onset time clearly and correctly at lower temperatures. In some cases, the initial appearance of hydrate crystals cannot induce a clear break in the pressure–time relationship curve. The onset time measured by the visual observation method is usually shorter than or at least the same as that determined by the pressure drop profile method. The inhibiting effect on the growth of hydrate crystals can be shown by the difference of the onset time obtained by the two methods. The maximum tolerated subcooling of each inhibitor was also investigated based on the onset time. It was found that inhibex 301 behaves as the best inhibitor that can tolerate the maximum subcooling of 8.3 K at 0.5 wt% and 10.6 K at 1.0 wt%, respectively. The maximum subcooling for inhibex 501 is 6.8 K at 0.5 wt% and 6.6 K at 1.0 wt%, respectively. Inhibex 713 has relatively poor inhibiting effect among the three inhibitors with the maximum subcooling of less than 3.5 K at 0.5 wt% and 5.1 K at 1.0 wt%, respectively.

Near-criticality in dilute binary mixtures: Distribution of azulene between coexisting liquid and vapor carbon dioxide

The equilibrium distribution of dilute solutes between vapor and liquid coexisting phases of near-critical solvents exhibits a simple dependence on the solvent’s liquid density which extends over a wide temperature range; however, theory predicts this dependence only as an asymptotic limit. In order to test quantitatively the extension of this behavior and compare it with the value predicted from the asymptotic relationship, a novel high-pressure apparatus with a sampling manifold, which avoids perturbing the system during sampling, was used to measure the equilibrium concentrations of azulene in coexisting liquid and vapor carbon dioxide contained in a high pressure sapphire cell. The density and composition of the two phases under equilibrium were determined between 283 and 301 K. The linearity of RT ln KD(KD being the distribution equilibrium constant) in the solvent’s liquid density was verified over the entire experimental temperature range, and the value of the slope was close to that corresponding to the asymptotic description as calculated with the hydrostatic hypernetted chain theory. An explanation for this observation lies in the weak temperature dependence of the Krichevskii function for this system; however a full quantitative description of the observed phenomenon remains a challenge, which should be provided by crossover theory.

Experimental and Modeling Studies on the Hydrate Formation of a Methane + Nitrogen Gas Mixture in the Presence of Aqueous Electrolyte Solutions

A new apparatus was designed and built for measuring the gas hydrate formation/dissociation conditions. The equilibrium cell is a 78 cm3 variable-volume high-pressure sapphire cell, which made the sharp observation possible. The hydrate formation pressures for a methane + nitrogen gas mixture in the presence of pure water and aqueous electrolyte solutions (containing single salt and mixed salts) were measured in the temperature range of 268.1−285.8 K and the pressure range of 2.05−12.68 MPa. The generalized hydrate model proposed recently by Zuo and Guo was used to predict the hydrate formation conditions, and excellent results were obtained for aqueous phases containing a single salt and binary mixed salts.