Formation of Ice, Tetrahydrofuran Hydrate, and Methane/Propane ...

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Formation of Ice, Tetrahydrofuran Hydrate, and Methane/Propane Mixed Gas Hydrates in Strong Monovalent Salt Solutions Barbara Sowa,†,‡ Xue Hua Zhang,§ Patrick G. Hartley,† Dave E. Dunstan,‡ Karen A. Kozielski,∥ and Nobuo Maeda*,† †

CSIRO Materials Science & Engineering, Ian Wark Laboratory, Bayview Avenue, Clayton, Victoria 3168, Australia Department of Chemical and Biomolecular Engineering, School of Engineering, University of Melbourne, Melbourne, Victoria 3010, Australia § School of Civil, Environmental and Chemical Engineering, RMIT University, Melbourne, Victoria 3001, Australia ∥ CSIRO Earth Science and Resource Engineering, Ian Wark Laboratory, Bayview Avenue, Clayton, Victoria 3168, Australia ‡

ABSTRACT: 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 samples. The electrolytes used in this work are lithium chloride (LiCl), lithium bromide (LiBr), lithium iodide (LiI), sodium chloride (NaCl), sodium bromide (NaBr), sodium iodide (NaI), potassium chloride (KCl), potassium bromide (KBr), and potassium iodide (KI). We found that (1) some salts may act as kinetic hydrate promoters at low concentrations; (2) the width of the probability distributions (stochasticity) of natural gas hydrate formation in these salt solutions was significantly narrower than that in pure water. To gain further insight, we extended the study of the solutions of the same nine salts to the formation of ice and model tetrahydrofuran (THF) hydrate for comparison.

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pigging,6 the separation and dehydration processes,18−20 and chemical methods such as thermodynamic and kinetic inhibition.21,22 The most common known chemicals22 which can prevent gas hydrate formation are thermodynamic hydrate inhibitors (THIs). Typical THIs are methanol (MeOH), monoethylene glycol (MEG), and organic and inorganic salt solutions.21 In thermodynamic inhibition, the hydrate phase boundary is shifted to a lower temperature and higher pressure by reducing the water activity. Large amounts of MeOH and MEG (10−50 wt % of the water phase) are required for effective inhibition in some fields.21,22 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.21 However, it has been reported that some THIs, such as methanol,23 ethylene glycol,24 and propanol25 when doped at low concentrations, could enhance the rate of gas hydrate formation (i.e., act as kinetic promoters). The range of hypotheses presented to account for these observations are related to the heat capacity, heat transfer, and/or methanol acting as a “help gas” where methanol molecules strongly bond

INTRODUCTION Gas hydrates have been a subject of great interest mainly because of their relevance in energy production.1 Natural gas hydrates are considered as the largest source of hydrocarbons on earth2 and as a possible source of energy for the future.3,4 Other applications that have attracted the attention of researchers are related to the storage and transportation of natural gas5−7 and hydrogen,8,9 gas separation processes including carbon dioxide sequestration,10,11 and water desalination.12,13 Gas hydrates may form when water molecules enclose small guest gas molecules (such as methane, ethane, propane, carbon dioxide, hydrogen) in a hydrogen-bonded network under elevated pressures (>2 MPa) and low temperatures (18.2 MΩ) was used for the preparation of all samples. The electrolytes used in this work are lithium chloride (LiCl), lithium bromide (LiBr), lithium iodide (LiI), sodium chloride (NaCl), sodium bromide (NaBr), sodium iodide (NaI), potassium chloride (KCl), potassium bromide (KBr), and potassium iodide (KI). All salts were supplied by Sigma-Aldrich and purified by crystallization from respective solutions made from Milli-Q water. All testing solutions were then freshly prepared prior to the measurements. Table 1 contains the information about the solubility of the salt solutions we used in the temperature range of the measurements. The solubility of salts in water decreases as temperature decreases. To avoid precipitation of salt from the solution during a cooling ramp, we limited our study to the concentration range below 3 M. 6878

dx.doi.org/10.1021/ef501701y | Energy Fuels 2014, 28, 6877−6888

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of ice and THF hydrates in the salt solutions was carried out using an atmospheric pressure automated lag time apparatus (ALTA-A),33,34 which is virtually equivalent to HP-ALTA with the “bulk transmittance configuration” that is disconnected from the high pressure gas line (Figure 1). To perform the measurements with ALTA-A, about 200 μL of salt solution was placed in a custom-made cylindrical glass tube (NMR type tube), which was inserted into a vertical bore. The glass sample cell was 50 mm in length and 4 mm in width. The distribution of ice/gas hydrate formation temperature, Tf, during a linear cooling ramp was measured using the cooling rate of 0.025K/s for C1/C3 mixed gas hydrate at 7 MPa and 0.05 K/s for ice and THF hydrate at atmospheric pressure (0.1 MPa). ALTA-A has a smaller heat capacity than HP-ALTA and allows the use of faster cooling rates. We note that we have previously shown that Tf distribution is highly insensitive to the cooling rate in this range.30 The sample was cooled until the ice/gas hydrate formation was detected, at which point the instrument reheated the sample to a predetermined temperature for dissociation. The dissociation temperature was set at 310 K for C1/C3 mixed gas hydrates and at 295 K for the ice and THF samples. These heating conditions were used to fully dissociate the studied systems and avoid the memory effect. The sample was held at that temperature for 300 to 500 s. The cycle of this cooling/detection/heating was repeated 100 times for C1/C3 mixed gas hydrates formation and 50 times for ice/THF hydrate formation. It was reported that silver iodide (AgI) provides good heterogeneous nucleation sites for ice formation between 266 and 253 K.35−37 Given that ice provides good heterogeneous nucleation sites for gas hydrate formation,36−38 we also examined the effects of AgI on the formation probability distributions of C1/C3 mixed gas hydrates. Our assumption here is that (1) AgI seeds ice nucleation35−37 and (2) ice seeds gas hydrate nucleation,39−41 so (3) perhaps AgI may seed gas hydrate nucleation. AgI was obtained from AJAX and used without purification. In these control experiments, AgI was introduced to the system using two different methods: (1) an appropriately sized AgI crystal was placed at the edge of the “boat” so that the crystal was out of the optical path and a section of the crystal was protruding through the water/gas interface; (2) fine AgI powders were scattered over the water surface. We note that AgI is almost insoluble in water (solubility of 3 × 10−6 g/L of water at 20 °C).42

Figure 1. Schematic diagram of the preferable position of the sample for detection of ice/natural gas hydrate formation by using an atmospheric pressure automated lag time apparatus (ALTA-A) and a high pressure automated lag time apparatus (HP-ALTA), respectively.

Figure 2. Example of the typical C1/C3 gas hydrates formation probability distributions in the solutions of sodium chloride, lithium chloride, and lithium bromide at the C1/C3 mixed gas pressure of 7 MPa.The integrated CPDF method calculates the most probable Tf or ΔT value of a given sample. The method is to calculate the integrated area enclosed by the probability axis and the CPDF (the gray areas on the graphs). 6879


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Figure 3. Subcooling, ΔT/K, (open symbols, CPDF method) as a function of concentration of studied monovalent salt solutions for C1/C3 gas hydrates formation. The black solid line represents the ΔT of C1/C3 gas hydrates in pure water. The black dashed lines represent “error bars” of pure water. The “error bars” indicate the full width of experimentally measured distribution as well as “stochasticity”.

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C1/C3 Mixed Gas Hydrate Formation Probability Distributions. We first show the probability distribution of C1/C3 mixed gas hydrate formation. We studied nine different combinations of three monovalent anions (Cl−, Br−, I−) and three monovalent cations (Li+, Na+, K+) at a broad range of concentrations between 10−5 and 3M. The solubility of salts decreases during a cooling ramp (Table 1). At concentrations lower than 10−5 M, the results showed that the Tf values were virtually indistinguishable from those of pure water. Figure 3 depicted the subcooling of C1/C3 mixed gas hydrate in various monovalent electrolytes. The black horizontal solid line represents the integrated ΔT (subcooling; CPDF method) of C1/C3 mixed gas hydrates in pure water. The black horizontal dashed lines indicate error bars of pure water as well as the width of the probability distribution. Highly nonlinear concentration dependence was observed in the range of 10−4 to 1 M. The exact nature of the nonlinear behavior was found to be quite dependent on the type of ions involved. Both inhibition and promotion of C1/C3 mixed gas hydrate formation may occur, with respect to that in pure water. We observed that Tf shifts to higher temperature formation in iodide solutions (LiI, KI) at all concentrations below 1 M (Figure 3). The lowest subcooling, ΔT, (the greatest promotion) was observed for lithium iodide and potassium iodide solutions. It appears that iodide salt solutions behave more like a promoter than an inhibitor at concentrations below 1 M. In contrast, sodium chloride was the only salt that exhibited inhibition effect at all concentrations studied. This finding is in a broad agreement with the very recent study on CO2 hydrate formation in a much larger batch reactor.27 It should be stressed that the “error bars” shown in Figure 3 are NOT traditional standard deviations of the Gaussian

RESULTS The main advantage of the automated lag time apparatus used here (ALTA-A and HP-ALTA) is the ability to carry out a large number (more than 100) of experimental runs in a relatively short period of time. The formation temperature of gas hydrate or ice, Tf, of each run was recorded. The collection of data was then used to construct survival curves (S-Curves) and/or cumulative probability distribution functions (CPDF) of gas hydrate formation.28,30,31 The S-Curve and CPDF are complementary; the sum of formation probability and the survival probability is always 1. The supercooled temperature (subcooling, ΔT) can then be defined as the difference between the thermodynamic equilibrium dissociation temperature at the experimental pressure (7 MPa in this work), Teq, and the formation temperature of gas hydrates, Tf, for each run (ΔT ≡ Teq − Tf). Teq is calculated using CSM Gem.43−45 We note that the calculated curve refers to the pure water hydrate baseline to which the net promotion and/or inhibition effects of a salt can be compared. CPDF can then be expressed in terms of Tf or ΔT and can be used to quantify the effectiveness of hydrate inhibitors. In addition, the shape of the CPDF (for example, unimodal or bimodal) can provide additional information as to whether more than one process is taking place.30,31 Figure 2 shows an example of typical CPDFs for C1/C3 mixed gas hydrate formation in the solutions of sodium chloride, lithium chloride, and lithium bromide at the C1/C3 mixed gas pressure of 7 MPa. The integrated CPDF method31 calculates the most probable Tf or ΔT value of a given sample. The method is to calculate the integrated area enclosed by the probability axis and the CPDF (the gray areas on the graphs, Figure 2). 6880


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Figure 4. Subcooling, ΔT/K, as a function of concentration of studied monovalent salt solutions for THF hydrates formation. The black solid line represents the ΔT of THF hydrates in pure water. The black dashed lines represent “error bars” of pure water. The “error bars” indicate the full width of experimentally measured distribution as well as “stochasticity”.

Figure 5. Subcooling, ΔT/K, as a function of concentration of studied monovalent salt solutions for ice formation. The black solid line represents the ΔT of ice formation in pure water. The black dashed lines represent “error bars” of pure water. The “error bars” indicate the full width of experimentally measured distribution as well as “stochasticity”.

Distribution which only covers the middle 68% of the entire distribution. Rather, the “error bars” shown here cover the full

width of the experimentally measured distribution. They must comprise both the random errors ubiquitous in any measure6881


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ments as well as genuine “stochasticity” that is inherent in the heterogeneous nucleation of gas hydrates. There is no a priori method to decouple the genuine stochasticity from the ubiquitous random error with certainty (nevertheless, we later offer a hypothesis which may enable such decoupling). We note that each probability distribution is such that the vast majority of runs are concentrated around the median (or the most probable value which is usually very close to the median). However, due to the presence of occasional “odd” points, the “stochasticity” appears large. In one of our earlier papers, we discussed the nature of the “stochasticity” in detail.30 In short, a probability distribution is usually asymmetric; a short “stochasticity bar” corresponds to densely populated data points, and a long “stochasticity bar” corresponds to scarcely populated data points. Thus, even when the two long “stochasticity bars” overlap, the distance between the medians is still meaningful. We refer to the scatter in the data as “stochasticity” hereafter. Even though the width of the distribution is obviously affected by the presence of a few stochastic formation events that are far away from the great majority of the formation events, it is important to emphasize that the most probable Tf value (or the median of the distribution which did not exactly coincide but is close to the most probable Tf value) is very insensitive to the presence or the absence of such an event or two. Comparison of C1/C3 Mixed Gas Hydrate Formation Probability Distributions to the Formation Probability Distributions of Ice and THF Model Hydrates. To gain further insight into the nonlinear dependence of Tf on salt concentrations observed for C1/C3 mixed gas hydrate formation, we extended the study of the solutions of the same nine salts to the formation of ice and model THF hydrate. Figures 4 and 5 show that a nonlinear behavior was also observed for ice and THF hydrate formation at concentrations below 1 M, respectively. Given that these results were obtained using a different instrument (ALTA-A instead of HP-ALTA), which employs different sample cells (a glass tube in ALTA-A vs a glass “boat” in HP-ALTA), we may conclude that the nonlinear behavior and the mixed promotion/inhibition effects observed for the salt solutions had arisen from the presence of the salts, not from some mysterious artifact(s). Even though nonlinear behavior was observed for all the 3 systems, there were a few clear differences. First, the promotion effect in dilute (