Measurements of gas hydrate formation probability distributions on a quasi-free water droplet Nobuo Maeda Citation: Review of Scientific Instruments 85, 065115 (2014); doi: 10.1063/1.4884794 View online: http://dx.doi.org/10.1063/1.4884794 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Charge of water droplets in non-polar oils J. Appl. Phys. 114, 144903 (2013); 10.1063/1.4824180 Communication: Length scale dependent oil-water energy fluctuations J. Chem. Phys. 135, 201102 (2011); 10.1063/1.3664604 The mayonnaise droplet Chaos 19, 041105 (2009); 10.1063/1.3202626 Effects of compressional waves on the response of quartz crystal microbalance in contact with silicone oil droplets J. Appl. Phys. 105, 114909 (2009); 10.1063/1.3133144 Monodispersed polygonal water droplets in microchannel Appl. Phys. Lett. 92, 213109 (2008); 10.1063/1.2937076
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 065115 (2014)
Measurements of gas hydrate formation probability distributions on a quasi-free water droplet Nobuo Maedaa) CSIRO Materials Science & Engineering, Ian Wark Laboratory, Bayview Avenue, Clayton, VIC 3168, Australia
(Received 23 April 2014; accepted 11 June 2014; published online 26 June 2014) 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. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4884794] INTRODUCTION
Gas hydrates are clathrate compounds in which hydrogen bonded network of water molecules enclose typically nonpolar guest molecules, such as methane and other light hydrocarbons that comprise natural gases.1 Formation of gas hydrate is desirable for some applications, such as gas storage and desalination, while it is undesirable in other applications such as multiphase pipeline transport of hydrocarbons in natural gas pipelines.1, 2 High Pressure Automated Lag Time Apparatus (HPALTA) is an instrument that is designed to measure gas hydrate formation probability distributions at elevated gas pressures.3 The physical principles of the operations are based on those of the ambient pressure version of the instrument (ALTA) developed earlier.4 ALTA has been used for the study of formation probability distributions of ice4 and tetrahydrofuran (THF) hydrate that forms under atmospheric pressure.5 HP-ALTA has been applied for the study of the effect of guest gas pressures, cooling rate and gas compositions.6 The instrument can also be used for the relative efficacy ranking of kinetic hydrate inhibitors (KHIs) and quantification of the socalled memory effect.7 In short, HP-ALTA is most useful in comparing the gas hydrate formation probability distributions of a sample of interest to those of a control sample. In an HP-ALTA, it was observed that gas hydrate formation originated somewhere near the edges of the sample cell and gas hydrate films subsequently grew across the water– guest gas interface.3, 8 It is likely that gas hydrate nucleation took place at the three-phase-line where the sample cell, the sample water, and the guest gas met. Ideally speaking, it would be highly desirable to be able to measure such gas hydrate formation probability distributions of a single water droplet or mist that is freely levitating in a guest gas. However, this is technically difficult. So a) [email protected]
0034-6748/2014/85(6)/065115/5/$30.00
here we settle for the next best option, which is to let a water droplet sit on top of a denser, immiscible and wall-wetting hydrophobic liquid to avoid contact of a water droplet with solid walls. Below we report the development of an upgraded HP-ALTA which enabled gas hydrate formation probability distributions on such a single quasi-free water droplet.
DESCRIPTION OF THE INSTRUMENT
Basic principles of the instrument are essentially the same as those of the original version of HP-ALTA3 (which we will refer to hereafter as HP-ALTA MkI). Figure 1 shows the schematic illustration of the new version (which we will refer to hereafter as HP-ALTA MkII). Several modifications were made to accommodate a flat-bottom sample cell in the instrument and to make the assembly and the operation easier than MkI. In particular, the use of an original sample cell (“boat”) is unsuitable for the purpose of the measurements of gas hydrate formation probability distributions on a quasi-free water droplet, because we have no control over the position of the water droplet within the sample cell (i.e., whether it stays within the experimental window or not). The HP-ALTA MkII chamber is made of stainless steel and its dimensions are 40 mm × 30 mm × 45 mm. Compared with MkI (50 mm × 50 mm × 20 mm), the heat capacity is similar but the thickness of the thinnest dimension is greater, to accommodate the new flat-bottom sample cell. This factor restricts the maximum cooling rate achievable within the cooling power of the Peltier devices to a smaller value compared with MkI (see below). The flat-bottom sample cell is made of glass and its dimensions are 9 mm outer diameter, about 6.8 mm inner diameter and about 15 mm high. The flat-bottom sample cell can contain up to 500 μl of liquid, in contrast to a glass “boat” used in MkI which can only contain up to 150 μl (though in practice we do not fully fill up the sample cell in either
85, 065115-1
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065115-2
Nobuo Maeda
FIG. 1. Schematic description of the next generation High Pressure Automated Lag Time Apparatus (HP-ALTA MkII).
HP-ALTA to avoid spill during pressurization and/or allow for the room for thermal expansion). For operation, the sample cell (see below for preparation) was placed in the Mk II sample chamber above the bottom window on a support block and in the path of a light beam. The support block was made of Teflon and had the dimensions of 9 mm outer diameter, 10 mm high. A support block with a through-hole at the centre of either 5 mm or 4 mm diameter was used. A photodiode detector positioned above the top window measured the intensity of the transmittance beam. Solid formation in the sample caused scattering of the light beam and resulted in a sudden drop in transmittance. The time and the temperature at which this event occurred were recorded by a PC and marked the end of an experimental run. At the end of each run, the sample was heated to a prespecified temperature for pre-specified time before the next cooling cycle commenced. The Mk II chamber was connected to a high-pressure gas line via a Swagelok connection and pressurized using a pneumatically driven gas booster pump (Haskel Australasia Pty Ltd, Queensland, Australia, model number AG-62). The maximum experimental pressure was limited to 15 MPa by the pressure rating of the sodalime glass windows (NAR engineering, Leeming WA). The actual gas pressure was measured and monitored using a capacitance-diaphragm type pressure transducer (Model A-10, WIKA). Heating and cooling of the sample were achieved using two Peltier devices (Peltier-CP1.4-127-06L-RTV, Melcor). A pair of heat sinks absorbed excess heat generated by the Peltier devices, which is then dissipated by a refrigerated bath (Model WCR-P12, All-Lab Scientific). The system temperature was measured at a reference point located about 5 mm away from the sample in the Mk II chamber using a resistance thermometer (PT100 HEL705, Honeywell). The measured temperature was then related to the sample temperature using a pre-calibrated table. The calibration table which relates the thermometer reading to the actual sample temperature inside the Mk II chamber was derived from control experiments using a thermometer immersed in ethanol in the sample cell within the Mk II sample chamber (but not under pressure). A second series of control experiments was used to ascertain the current required to ensure that the Peltier device maintained the temperature of the sample cell at a specified value.
Rev. Sci. Instrum. 85, 065115 (2014)
A window heater (K005020C5-0009B, Watlow Australia, VIC, Australia) was placed around each high pressure window to prevent condensation of sample water, especially during the dissociation cycle. The temperature at which solid formation was detected in each run is denoted as Tf and the thermodynamic hydrate equilibrium dissociation temperature as Teq , which was calculated using CSMGem.1 The methane–propane mixed gas had the composition of 90 mol.% methane 10 mol.% propane and forms Structure II (sII) gas hydrates. The gas was obtained from BOC Limited. Like MkI, HP-ALTA MkII can be operated in one of the two modes:9 (1) the constant cooling mode in which a sample is subjected to a linear cooling ramp until the instrument detects the formation of gas hydrate or ice. Following the detection of gas hydrate formation/freezing, the sample is reheated to a specified temperature for specified time for dissociation/melting. Then the next cycle of linear cooling begins. (2) The induction time mode in which a sample is cooled to a given target temperature and held there for a prolonged time until gas hydrate formation is detected. In the first mode, the Tf distributions and the corresponding lag time distributions can be recorded. The “lag time” during a cooling ramp is proportional to subcooling, T ≡ Teq − Tf . The first mode is usually the less time consuming mode of operation. The stochasticity is also less than the second mode because the linear cooling ramp subjects the sample to an increasing driving force for the formation of hydrate/ice and effectively forces the hydrate/ice to form. When we want to preclude ice formation, we use the second mode of operation. In the second mode, the induction time distribution at the target temperature can be recorded. Here we define the “induction time” as the time elapsed at the target temperature prior to gas hydrate formation. An induction time distribution is highly stochastic and so there were occasions when the induction time was very long. In these cases we usually set a maximum waiting time before abandoning the run and commencing the next cycle (for the example study shown below, the maximum waiting time was set to 10 000 s). For these runs that reached the maximum waiting time without hydrate formation, we could only know that the “real” induction time exceeded 10 000 s, and we present such results as “induction time” = 10 000 s in the example shown below. Likewise, there were times when the gas hydrate formed on the way to the target temperature. For these runs, we regarded the “induction time” as zero in the example results shown below. We can readily convert our definition of “induction time” to, e.g., total time the sample spent in the subcooling region prior to hydrate formation or to “total temperature-time” (i.e., the integrated area under the subcooling temperature profile prior to hydrate formation), so we only present our definition of “induction time” in the example results below. PREPARATION OF THE SAMPLE CELLS
The container was made of glass and the wall was coated with 1H,1H,2H,2H-Perfluorodecyltriethoxysilane (PFDTES;
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065115-3
Nobuo Maeda
Rev. Sci. Instrum. 85, 065115 (2014)
FIG. 2. The glass sample cell for HP-ALTA MkII. The inner wall of the sample cell is coated with PFDTES. (a) The lower phase in the sample cell is perfluorodecalin. The upper phase (the droplet) is water. Perfluorodecalin wets the PFDTES coating, so a meniscus forms around the wall. Water wets neither PFDTES nor perfluorodecalin. (b) We added a water soluble dye to the water droplet to enhance the visual contrast between the two liquid phases. (c) Even though the perfluorodecalin meniscus renders the centre of the container the lowest point, water was sometimes observed to migrate toward one side of the wall. Inside the HP-ALTA MkII, the sample cell sits on top of a Teflon support which has a through hole. Both the sample cell and the Teflon support sit on top of the bottom window of the HP-ALTA MkII. The top window is placed directly above the bottom window so that light can pass through the water droplet from the light source to the photo detector.
97%, Sigma-Aldrich). We followed the protocol described by Choi et al.10 A glass cell was first cleaned either using freshly prepared (hot) piranha solution (mixture of 70% H2 SO4 and 30% H2 O2 ) or freshly prepared (hot)10 wt.% NaOH and then thoroughly rinsed with large quantity of milli-Q water. The rinsed cell was dried in an oven at 130 ◦ C for several hours. Then the cleaned cell was immersed in 10 mM PFDTES solution in hexadecane. Care was taken to minimize the exposure of PFDTES to ambient moisture. After several days (>1 week) of soaking, the cell was rinsed first with hexadecane, then with 1:1 hexadecane/ethanol mixture, and finally with ethanol. Each rinsing process was assisted by immersing the vial that contained the solution/cell in a sonication bath for 5 min. Following the final rinse with ethanol, the cell was dried with a flow of nitrogen gas and then in an oven at 130 ◦ C for a few hours. The contact angle of water on the PFDTES coating was about 100◦ and did not depend on the choice of the cleaning agent (piranha solution or 10 wt.% NaOH), but a shorter immersion time resulted in a lesser quality (a lower contact angle) PFDTES coating, in agreement with an earlier study.10 Either perfluorooctane (98%, Sigma-Aldrich) or perfluorodecalin (95%, Sigma-Aldrich) was placed at the bottom of the PFDTES-coated container. Perfluorodecalin is our preferred choice among the commercially available perfluorocarbon oils. The time-consuming nature of the HP-ALTA measurements, and the heating period after each cooling ramp in particular, cause a volatile liquid to evaporate from the sample cell and condense elsewhere within the high pressure chamber/gas line. Perfluorodecalin is significantly less volatile than perfluorooctane (boiling points of 142 ◦ C versus 103–106 ◦ C, respectively, from MSDS) and for this reason more suitable for a long experiment with a large number of runs. Since perfluorocarbon oils wet PFDTES, a meniscus formed around the inner wall of the container rendering the surface of the perfluorocarbon oil at the centre of the cell the
lowest point (gravity wise). A water droplet (de-ionised water from a Milli-Q unit with 18.2 M resistivity) was placed on top of the perfluorocarbon oil at the centre of the cell (Figure 2(a)). We also show a photo when a water soluble dye was used for improved contrast in Figure 2(b). Water wets neither perfluorocarbon oil nor PFDTES, and the water droplet was sometimes observed to move toward one side of the sample cell (Figure 2(c)). This phenomenon could be related to the refractive index of glass (1.47), perfluorocarbon oil (about 1.25), and water (1.33). This combination suggests that the van der Waals forces of a glass-perfluorocarbon oilwater system is attractive.11, 12 We note that perfluorocarbon oil clearly preferentially wets PFDTES over water (the contact angle of water on PFDTES was about 100◦ whereas the contact angle of perfluorodecalin on PFTDES was very small; cosθ close to 1), so it is likely that a thin film of perfluorocarbon oil separates the water droplet and the glass cell wall. However, we cannot rule out the possibility that the PFDTES coating had some defects and consequently the water droplet may come into contact with the solid wall (especially during pressurization). TYPICAL RESULTS
Figure 3(a) shows an example probability distribution for methane–propane mixed gas hydrate formation on a quasifree water droplet during linear cooling rate ramps (the first mode of operation). The water droplet was supported by perfluorodecalin in the atmosphere of 9.1 MPa of methane– propane mixed gas. Teq for the methane–propane mixed gas hydrate at this pressure was calculated to be 295 K using CSMGem. The chronological histogram of lag times is also shown (Figure 3(b)). The cooling rate used here was 0.005 K/s. The limitation of MkI that it cannot discern the formation of gas hydrate below 273 K from that of ice is still present
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065115-4
Nobuo Maeda
FIG. 3. (a) An example probability distribution for methane–propane mixed gas hydrate formation on a quasi-free water droplet during linear cooling rate ramps. The water droplet was supported by Perfluorodecalin in the atmosphere of 9.1 MPa of methane–propane mixed gas. The chronological histogram of lag times is also shown (b). The cooling rate used here was 0.005 K/s. Because of the larger heat capacity of the sample cell the cooling rate was limited to slower rates compared with the earlier version of HP-ALTA.
in HP-ALTA MkII. A control experiment using a quasi-free water droplet at 0.1 MPa (ambient pressure) showed that ice formation took place in the range 259–264 K. Even though elevated pressures are known to shift Teq of ice to lower temperatures,13 the data shown in Figure 3(a) may still contain some ice formation at the lower end. Figure 4(a) shows an example induction time distribution for methane–propane mixed gas hydrate formation on a quasifree water droplet (the second mode of operation). The water droplet was supported by perfluorodecalin in the atmosphere of 8.0 MPa of methane–propane mixed gas. The holding temperature was 273 K and the cooling rate of 0.005 K/s was used for the approach to the target holding temperature. This holding temperature corresponds to the subcooling of T = 21 K. The chronological histogram of induction times is also shown (Figure 4(b)). Here we define the “induction time” as the time elapsed at 273 K prior to methane–propane mixed gas hydrate formation. We counted the induction time as zero when the gas hydrate formed during the approach to 273 K. The maximum observed induction time was limited to 10 000 s per run in this experiment (we plotted the induction time as 10 000 s for longer runs in Figure 4(b)). Induction time is, by definition, an inverse of nucleation rate. Therefore, we think our definition of induction time (measured at a constant subcooling temperature) is appropriate. As mentioned above, we can convert our definition of induction time to, e.g., (1) total time the sample spent in
Rev. Sci. Instrum. 85, 065115 (2014)
FIG. 4. (a) An example induction time distribution for methane–propane mixed gas hydrate formation on a quasi-free water droplet. The water droplet was supported by perfluorodecalin in the atmosphere of 8.0 MPa of methane– propane mixed gas. The holding temperature was 273 K and the cooling rate of 0.005 K/s was used for the approach to the target holding temperature. The chronological histogram of induction times is also shown (b). Here we define the “induction time” as the time elapsed at 273 K prior to methane– propane mixed gas hydrate formation. We counted the induction time as zero when the gas hydrate formed during the approach to 273 K. The maximum observed induction time was limited to 10 000 s per run in this experiment (we plotted the induction time as 10 000 s for longer runs).
the subcooling region prior to hydrate formation or (2) “total temperature-time” (i.e., the integrated area under the subcooling temperature profile prior to hydrate formation). However, we note that there are advantages and disadvantages in each method of analysis. The total time the sample spent in the subcooling region prior to hydrate formation simply shifts our definition of induction time by the time spent on the approach to the target temperature. But the driving force for hydrate formation, which is proportional to the subcooling,14 is constantly increasing during a cooling ramp. Clearly, 1 s spent at 273 K is more “effective” for hydrate formation than the same time spent at a higher temperature. In short, they should not carry the same weight. Likewise, calculation of “total temperature-time” (the integrated area under the subcooling temperature profile prior to hydrate formation) does not solve the problem. This approach implicitly assumes equal weighting of subcooling temperature and induction time (i.e., induction time and subcooling temperature contribute equally to the integrated area). However, this is clearly not the case. To take a rather extreme example for the sake of highlighting the case, subjecting a
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065115-5
Nobuo Maeda
FIG. 5. An example probability distribution for methane–propane mixed gas hydrate formation using a glass sample cell of the original version of HPALTA at a comparable guest gas pressure of 8.7 MPa (blue squares). The data obtained using HP-ALTA MkII, shown in Figure 3(a), are also displayed here for comparison (black circles). The gas pressure of 8.7 MPa was slightly lower than the measurement in HP-ALTA MkII of 9.1 MPa, and the cooling rate used was 0.025 K/s, 5 times as fast as the 0.005 K/s used in HP-ALTA MkII.
sample to a subcooling of 1 K for 100 s is not equivalent to subjecting the same sample to a subcooling of 100 K for 1 s.
Rev. Sci. Instrum. 85, 065115 (2014)
cooling rates should cause the gas hydrate formation probability distributions to shift to deeper subcoolings,6 we may conclude that the quasi-free water droplet in HP-ALTA MkII can be supercooled to deeper temperatures than the water sample in a glass sample cell in MkI under the same experimental conditions. To further support this idea, we also show another pair of examples in Figure 6. Here both HP-ALTA MkI and MkII data were obtained using the same cooling rate of 0.01 K/s and under the same methane–propane mixed gas pressure of 12.0 MPa. Like the results shown in Figure 5, the quasifree water droplet in HP-ALTA MkII could be supercooled to significantly deeper temperatures than the water sample in a glass sample cell in MkI. We hypothesize that smaller number of heterogeneous nucleation sites for a quasi-free water droplet is responsible for the deeper subcoolings achieved. ACKNOWLEDGMENTS
This work was supported by the Australian Research Council Future Fellowship (FT0991892) and CSIRO’s Energy Flagship. 1 E.
COMPARISON TO THE EARLIER RESULTS OBTAINED USING THE ORIGINAL VERSION OF HP-ALTA
We show the probability distribution for methane– propane mixed gas hydrate formation measured using HPALTA MkI at comparable guest gas pressures in Figure 5, together with the data in Figure 3(a). The gas pressure was 8.7 MPa, slightly lower than the measurement in HP-ALTA MkII of 9.1 MPa, and the cooling rate used was 0.025 K/s, 5 times as fast as the 0.005 K/s used in HP-ALTA MkII. Given that both the slightly lower gas pressure and the use of faster
FIG. 6. Comparisons of probability distributions for methane–propane mixed gas hydrate formation from a quasi-free water droplet measured using HP-ALTA MkII (red) and from water in a glass sample cell in the original version of HP-ALTA (blue). Here the guest gas pressure was 12.0 MPa and the cooling rate used was 0.01 K/s in each case.
D. Sloan and C. A. Koh, Clathrate Hydrates of Natural Gases, 3rd ed. (CRC Press, Boca Raton, 2008). 2 P. Englezos, “Clathrate hydrates,” Ind. Eng. Chem. Res. 32(7), 1251–1274 (1993). 3 N. Maeda, D. Wells, N. C. Becker, P. G. Hartley, P. W. Wilson, A. D. J. Haymet, and K. A. Kozielski, “Development of a high pressure automated lag time apparatus for experimental studies and statistical analyses of nucleation and growth of gas hydrates,” Rev. Sci. Instrum. 82(6), 065109 (2011). 4 A. F. Heneghan and A. D. J. Haymet, “Liquid-to-crystal nucleation: A new generation lag-time apparatus,” J. Chem. Phys. 117(11), 5319–5327 (2002). 5 P. W. Wilson, D. Lester, and A. D. J. Haymet, “Heterogeneous nucleation of clathrates from supercooled tetrahydrofuran (THF)/water mixtures, and the effect of an added catalyst,” Chem. Eng. Sci. 60(11), 2937–2941 (2005). 6 N. Maeda, D. Wells, P. G. Hartley, and K. A. Kozielski, “Statistical analysis of supercooling in fuel gas hydrate systems,” Energy Fuels 26(3), 1820– 1827 (2012). 7 E. F. May, R. Wu, M. A. Kelland, Z. M. Aman, K. A. Kozielski, P. G. Hartley, and N. Maeda, “Quantitative kinetic inhibitor comparisons and memory effect measurements from hydrate formation probability distributions,” Chem. Eng. Sci. 107, 1–12 (2014). 8 R. Wu, K. A. Kozielski, P. G. Hartley, E. F. May, J. Boxall, and N. Maeda, “Methane-propane mixed gas hydrate film growth on the surface of water and Luvicap EG solutions,” Energy Fuels 27(5), 2548–2554 (2013). 9 R. Wu, K. A. Kozielski, P. G. Hartley, E. F. May, J. Boxall, and N. Maeda, “Probability distributions of gas hydrate formation,” AIChE J. 59(7), 2640– 2646 (2013). 10 J. Choi, M. Kawaguchi, and T. Kato, “Self-assembled monolayer formation on magnetic hard disk surface and friction measurements,” J. Appl. Phys. 91(10), 7574–7576 (2002). 11 J. Mahanty and B. W. Ninham, Dispersion Forces (Academic Press, London, 1976). 12 J. N. Israelachvili, Intermolecular and Surface Forces, 2nd ed. (Academic Press, San Diego, 1991). 13 CRC Handbook of Chemistry and Physics, 80th ed., edited by D. R. Lide (CRC press, Boca Raton, 1999). 14 D. Kashchiev and A. Firoozabadi, “Driving force for crystallization of gas hydrates,” J. Cryst. Growth 241(1–2), 220–230 (2002).
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This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 202.28.191.34 On: Fri, 19 Dec 2014 19:07:35
REVIEW OF SCIENTIFIC INSTRUMENTS 85, 065115 (2014)
Measurements of gas hydrate formation probability distributions on a quasi-free water droplet Nobuo Maedaa) CSIRO Materials Science & Engineering, Ian Wark Laboratory, Bayview Avenue, Clayton, VIC 3168, Australia
(Received 23 April 2014; accepted 11 June 2014; published online 26 June 2014) 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. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4884794] INTRODUCTION
Gas hydrates are clathrate compounds in which hydrogen bonded network of water molecules enclose typically nonpolar guest molecules, such as methane and other light hydrocarbons that comprise natural gases.1 Formation of gas hydrate is desirable for some applications, such as gas storage and desalination, while it is undesirable in other applications such as multiphase pipeline transport of hydrocarbons in natural gas pipelines.1, 2 High Pressure Automated Lag Time Apparatus (HPALTA) is an instrument that is designed to measure gas hydrate formation probability distributions at elevated gas pressures.3 The physical principles of the operations are based on those of the ambient pressure version of the instrument (ALTA) developed earlier.4 ALTA has been used for the study of formation probability distributions of ice4 and tetrahydrofuran (THF) hydrate that forms under atmospheric pressure.5 HP-ALTA has been applied for the study of the effect of guest gas pressures, cooling rate and gas compositions.6 The instrument can also be used for the relative efficacy ranking of kinetic hydrate inhibitors (KHIs) and quantification of the socalled memory effect.7 In short, HP-ALTA is most useful in comparing the gas hydrate formation probability distributions of a sample of interest to those of a control sample. In an HP-ALTA, it was observed that gas hydrate formation originated somewhere near the edges of the sample cell and gas hydrate films subsequently grew across the water– guest gas interface.3, 8 It is likely that gas hydrate nucleation took place at the three-phase-line where the sample cell, the sample water, and the guest gas met. Ideally speaking, it would be highly desirable to be able to measure such gas hydrate formation probability distributions of a single water droplet or mist that is freely levitating in a guest gas. However, this is technically difficult. So a) [email protected]
0034-6748/2014/85(6)/065115/5/$30.00
here we settle for the next best option, which is to let a water droplet sit on top of a denser, immiscible and wall-wetting hydrophobic liquid to avoid contact of a water droplet with solid walls. Below we report the development of an upgraded HP-ALTA which enabled gas hydrate formation probability distributions on such a single quasi-free water droplet.
DESCRIPTION OF THE INSTRUMENT
Basic principles of the instrument are essentially the same as those of the original version of HP-ALTA3 (which we will refer to hereafter as HP-ALTA MkI). Figure 1 shows the schematic illustration of the new version (which we will refer to hereafter as HP-ALTA MkII). Several modifications were made to accommodate a flat-bottom sample cell in the instrument and to make the assembly and the operation easier than MkI. In particular, the use of an original sample cell (“boat”) is unsuitable for the purpose of the measurements of gas hydrate formation probability distributions on a quasi-free water droplet, because we have no control over the position of the water droplet within the sample cell (i.e., whether it stays within the experimental window or not). The HP-ALTA MkII chamber is made of stainless steel and its dimensions are 40 mm × 30 mm × 45 mm. Compared with MkI (50 mm × 50 mm × 20 mm), the heat capacity is similar but the thickness of the thinnest dimension is greater, to accommodate the new flat-bottom sample cell. This factor restricts the maximum cooling rate achievable within the cooling power of the Peltier devices to a smaller value compared with MkI (see below). The flat-bottom sample cell is made of glass and its dimensions are 9 mm outer diameter, about 6.8 mm inner diameter and about 15 mm high. The flat-bottom sample cell can contain up to 500 μl of liquid, in contrast to a glass “boat” used in MkI which can only contain up to 150 μl (though in practice we do not fully fill up the sample cell in either
85, 065115-1
© 2014 AIP Publishing LLC
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP: 202.28.191.34 On: Fri, 19 Dec 2014 19:07:35
065115-2
Nobuo Maeda
FIG. 1. Schematic description of the next generation High Pressure Automated Lag Time Apparatus (HP-ALTA MkII).
HP-ALTA to avoid spill during pressurization and/or allow for the room for thermal expansion). For operation, the sample cell (see below for preparation) was placed in the Mk II sample chamber above the bottom window on a support block and in the path of a light beam. The support block was made of Teflon and had the dimensions of 9 mm outer diameter, 10 mm high. A support block with a through-hole at the centre of either 5 mm or 4 mm diameter was used. A photodiode detector positioned above the top window measured the intensity of the transmittance beam. Solid formation in the sample caused scattering of the light beam and resulted in a sudden drop in transmittance. The time and the temperature at which this event occurred were recorded by a PC and marked the end of an experimental run. At the end of each run, the sample was heated to a prespecified temperature for pre-specified time before the next cooling cycle commenced. The Mk II chamber was connected to a high-pressure gas line via a Swagelok connection and pressurized using a pneumatically driven gas booster pump (Haskel Australasia Pty Ltd, Queensland, Australia, model number AG-62). The maximum experimental pressure was limited to 15 MPa by the pressure rating of the sodalime glass windows (NAR engineering, Leeming WA). The actual gas pressure was measured and monitored using a capacitance-diaphragm type pressure transducer (Model A-10, WIKA). Heating and cooling of the sample were achieved using two Peltier devices (Peltier-CP1.4-127-06L-RTV, Melcor). A pair of heat sinks absorbed excess heat generated by the Peltier devices, which is then dissipated by a refrigerated bath (Model WCR-P12, All-Lab Scientific). The system temperature was measured at a reference point located about 5 mm away from the sample in the Mk II chamber using a resistance thermometer (PT100 HEL705, Honeywell). The measured temperature was then related to the sample temperature using a pre-calibrated table. The calibration table which relates the thermometer reading to the actual sample temperature inside the Mk II chamber was derived from control experiments using a thermometer immersed in ethanol in the sample cell within the Mk II sample chamber (but not under pressure). A second series of control experiments was used to ascertain the current required to ensure that the Peltier device maintained the temperature of the sample cell at a specified value.
Rev. Sci. Instrum. 85, 065115 (2014)
A window heater (K005020C5-0009B, Watlow Australia, VIC, Australia) was placed around each high pressure window to prevent condensation of sample water, especially during the dissociation cycle. The temperature at which solid formation was detected in each run is denoted as Tf and the thermodynamic hydrate equilibrium dissociation temperature as Teq , which was calculated using CSMGem.1 The methane–propane mixed gas had the composition of 90 mol.% methane 10 mol.% propane and forms Structure II (sII) gas hydrates. The gas was obtained from BOC Limited. Like MkI, HP-ALTA MkII can be operated in one of the two modes:9 (1) the constant cooling mode in which a sample is subjected to a linear cooling ramp until the instrument detects the formation of gas hydrate or ice. Following the detection of gas hydrate formation/freezing, the sample is reheated to a specified temperature for specified time for dissociation/melting. Then the next cycle of linear cooling begins. (2) The induction time mode in which a sample is cooled to a given target temperature and held there for a prolonged time until gas hydrate formation is detected. In the first mode, the Tf distributions and the corresponding lag time distributions can be recorded. The “lag time” during a cooling ramp is proportional to subcooling, T ≡ Teq − Tf . The first mode is usually the less time consuming mode of operation. The stochasticity is also less than the second mode because the linear cooling ramp subjects the sample to an increasing driving force for the formation of hydrate/ice and effectively forces the hydrate/ice to form. When we want to preclude ice formation, we use the second mode of operation. In the second mode, the induction time distribution at the target temperature can be recorded. Here we define the “induction time” as the time elapsed at the target temperature prior to gas hydrate formation. An induction time distribution is highly stochastic and so there were occasions when the induction time was very long. In these cases we usually set a maximum waiting time before abandoning the run and commencing the next cycle (for the example study shown below, the maximum waiting time was set to 10 000 s). For these runs that reached the maximum waiting time without hydrate formation, we could only know that the “real” induction time exceeded 10 000 s, and we present such results as “induction time” = 10 000 s in the example shown below. Likewise, there were times when the gas hydrate formed on the way to the target temperature. For these runs, we regarded the “induction time” as zero in the example results shown below. We can readily convert our definition of “induction time” to, e.g., total time the sample spent in the subcooling region prior to hydrate formation or to “total temperature-time” (i.e., the integrated area under the subcooling temperature profile prior to hydrate formation), so we only present our definition of “induction time” in the example results below. PREPARATION OF THE SAMPLE CELLS
The container was made of glass and the wall was coated with 1H,1H,2H,2H-Perfluorodecyltriethoxysilane (PFDTES;
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065115-3
Nobuo Maeda
Rev. Sci. Instrum. 85, 065115 (2014)
FIG. 2. The glass sample cell for HP-ALTA MkII. The inner wall of the sample cell is coated with PFDTES. (a) The lower phase in the sample cell is perfluorodecalin. The upper phase (the droplet) is water. Perfluorodecalin wets the PFDTES coating, so a meniscus forms around the wall. Water wets neither PFDTES nor perfluorodecalin. (b) We added a water soluble dye to the water droplet to enhance the visual contrast between the two liquid phases. (c) Even though the perfluorodecalin meniscus renders the centre of the container the lowest point, water was sometimes observed to migrate toward one side of the wall. Inside the HP-ALTA MkII, the sample cell sits on top of a Teflon support which has a through hole. Both the sample cell and the Teflon support sit on top of the bottom window of the HP-ALTA MkII. The top window is placed directly above the bottom window so that light can pass through the water droplet from the light source to the photo detector.
97%, Sigma-Aldrich). We followed the protocol described by Choi et al.10 A glass cell was first cleaned either using freshly prepared (hot) piranha solution (mixture of 70% H2 SO4 and 30% H2 O2 ) or freshly prepared (hot)10 wt.% NaOH and then thoroughly rinsed with large quantity of milli-Q water. The rinsed cell was dried in an oven at 130 ◦ C for several hours. Then the cleaned cell was immersed in 10 mM PFDTES solution in hexadecane. Care was taken to minimize the exposure of PFDTES to ambient moisture. After several days (>1 week) of soaking, the cell was rinsed first with hexadecane, then with 1:1 hexadecane/ethanol mixture, and finally with ethanol. Each rinsing process was assisted by immersing the vial that contained the solution/cell in a sonication bath for 5 min. Following the final rinse with ethanol, the cell was dried with a flow of nitrogen gas and then in an oven at 130 ◦ C for a few hours. The contact angle of water on the PFDTES coating was about 100◦ and did not depend on the choice of the cleaning agent (piranha solution or 10 wt.% NaOH), but a shorter immersion time resulted in a lesser quality (a lower contact angle) PFDTES coating, in agreement with an earlier study.10 Either perfluorooctane (98%, Sigma-Aldrich) or perfluorodecalin (95%, Sigma-Aldrich) was placed at the bottom of the PFDTES-coated container. Perfluorodecalin is our preferred choice among the commercially available perfluorocarbon oils. The time-consuming nature of the HP-ALTA measurements, and the heating period after each cooling ramp in particular, cause a volatile liquid to evaporate from the sample cell and condense elsewhere within the high pressure chamber/gas line. Perfluorodecalin is significantly less volatile than perfluorooctane (boiling points of 142 ◦ C versus 103–106 ◦ C, respectively, from MSDS) and for this reason more suitable for a long experiment with a large number of runs. Since perfluorocarbon oils wet PFDTES, a meniscus formed around the inner wall of the container rendering the surface of the perfluorocarbon oil at the centre of the cell the
lowest point (gravity wise). A water droplet (de-ionised water from a Milli-Q unit with 18.2 M resistivity) was placed on top of the perfluorocarbon oil at the centre of the cell (Figure 2(a)). We also show a photo when a water soluble dye was used for improved contrast in Figure 2(b). Water wets neither perfluorocarbon oil nor PFDTES, and the water droplet was sometimes observed to move toward one side of the sample cell (Figure 2(c)). This phenomenon could be related to the refractive index of glass (1.47), perfluorocarbon oil (about 1.25), and water (1.33). This combination suggests that the van der Waals forces of a glass-perfluorocarbon oilwater system is attractive.11, 12 We note that perfluorocarbon oil clearly preferentially wets PFDTES over water (the contact angle of water on PFDTES was about 100◦ whereas the contact angle of perfluorodecalin on PFTDES was very small; cosθ close to 1), so it is likely that a thin film of perfluorocarbon oil separates the water droplet and the glass cell wall. However, we cannot rule out the possibility that the PFDTES coating had some defects and consequently the water droplet may come into contact with the solid wall (especially during pressurization). TYPICAL RESULTS
Figure 3(a) shows an example probability distribution for methane–propane mixed gas hydrate formation on a quasifree water droplet during linear cooling rate ramps (the first mode of operation). The water droplet was supported by perfluorodecalin in the atmosphere of 9.1 MPa of methane– propane mixed gas. Teq for the methane–propane mixed gas hydrate at this pressure was calculated to be 295 K using CSMGem. The chronological histogram of lag times is also shown (Figure 3(b)). The cooling rate used here was 0.005 K/s. The limitation of MkI that it cannot discern the formation of gas hydrate below 273 K from that of ice is still present
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065115-4
Nobuo Maeda
FIG. 3. (a) An example probability distribution for methane–propane mixed gas hydrate formation on a quasi-free water droplet during linear cooling rate ramps. The water droplet was supported by Perfluorodecalin in the atmosphere of 9.1 MPa of methane–propane mixed gas. The chronological histogram of lag times is also shown (b). The cooling rate used here was 0.005 K/s. Because of the larger heat capacity of the sample cell the cooling rate was limited to slower rates compared with the earlier version of HP-ALTA.
in HP-ALTA MkII. A control experiment using a quasi-free water droplet at 0.1 MPa (ambient pressure) showed that ice formation took place in the range 259–264 K. Even though elevated pressures are known to shift Teq of ice to lower temperatures,13 the data shown in Figure 3(a) may still contain some ice formation at the lower end. Figure 4(a) shows an example induction time distribution for methane–propane mixed gas hydrate formation on a quasifree water droplet (the second mode of operation). The water droplet was supported by perfluorodecalin in the atmosphere of 8.0 MPa of methane–propane mixed gas. The holding temperature was 273 K and the cooling rate of 0.005 K/s was used for the approach to the target holding temperature. This holding temperature corresponds to the subcooling of T = 21 K. The chronological histogram of induction times is also shown (Figure 4(b)). Here we define the “induction time” as the time elapsed at 273 K prior to methane–propane mixed gas hydrate formation. We counted the induction time as zero when the gas hydrate formed during the approach to 273 K. The maximum observed induction time was limited to 10 000 s per run in this experiment (we plotted the induction time as 10 000 s for longer runs in Figure 4(b)). Induction time is, by definition, an inverse of nucleation rate. Therefore, we think our definition of induction time (measured at a constant subcooling temperature) is appropriate. As mentioned above, we can convert our definition of induction time to, e.g., (1) total time the sample spent in
Rev. Sci. Instrum. 85, 065115 (2014)
FIG. 4. (a) An example induction time distribution for methane–propane mixed gas hydrate formation on a quasi-free water droplet. The water droplet was supported by perfluorodecalin in the atmosphere of 8.0 MPa of methane– propane mixed gas. The holding temperature was 273 K and the cooling rate of 0.005 K/s was used for the approach to the target holding temperature. The chronological histogram of induction times is also shown (b). Here we define the “induction time” as the time elapsed at 273 K prior to methane– propane mixed gas hydrate formation. We counted the induction time as zero when the gas hydrate formed during the approach to 273 K. The maximum observed induction time was limited to 10 000 s per run in this experiment (we plotted the induction time as 10 000 s for longer runs).
the subcooling region prior to hydrate formation or (2) “total temperature-time” (i.e., the integrated area under the subcooling temperature profile prior to hydrate formation). However, we note that there are advantages and disadvantages in each method of analysis. The total time the sample spent in the subcooling region prior to hydrate formation simply shifts our definition of induction time by the time spent on the approach to the target temperature. But the driving force for hydrate formation, which is proportional to the subcooling,14 is constantly increasing during a cooling ramp. Clearly, 1 s spent at 273 K is more “effective” for hydrate formation than the same time spent at a higher temperature. In short, they should not carry the same weight. Likewise, calculation of “total temperature-time” (the integrated area under the subcooling temperature profile prior to hydrate formation) does not solve the problem. This approach implicitly assumes equal weighting of subcooling temperature and induction time (i.e., induction time and subcooling temperature contribute equally to the integrated area). However, this is clearly not the case. To take a rather extreme example for the sake of highlighting the case, subjecting a
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065115-5
Nobuo Maeda
FIG. 5. An example probability distribution for methane–propane mixed gas hydrate formation using a glass sample cell of the original version of HPALTA at a comparable guest gas pressure of 8.7 MPa (blue squares). The data obtained using HP-ALTA MkII, shown in Figure 3(a), are also displayed here for comparison (black circles). The gas pressure of 8.7 MPa was slightly lower than the measurement in HP-ALTA MkII of 9.1 MPa, and the cooling rate used was 0.025 K/s, 5 times as fast as the 0.005 K/s used in HP-ALTA MkII.
sample to a subcooling of 1 K for 100 s is not equivalent to subjecting the same sample to a subcooling of 100 K for 1 s.
Rev. Sci. Instrum. 85, 065115 (2014)
cooling rates should cause the gas hydrate formation probability distributions to shift to deeper subcoolings,6 we may conclude that the quasi-free water droplet in HP-ALTA MkII can be supercooled to deeper temperatures than the water sample in a glass sample cell in MkI under the same experimental conditions. To further support this idea, we also show another pair of examples in Figure 6. Here both HP-ALTA MkI and MkII data were obtained using the same cooling rate of 0.01 K/s and under the same methane–propane mixed gas pressure of 12.0 MPa. Like the results shown in Figure 5, the quasifree water droplet in HP-ALTA MkII could be supercooled to significantly deeper temperatures than the water sample in a glass sample cell in MkI. We hypothesize that smaller number of heterogeneous nucleation sites for a quasi-free water droplet is responsible for the deeper subcoolings achieved. ACKNOWLEDGMENTS
This work was supported by the Australian Research Council Future Fellowship (FT0991892) and CSIRO’s Energy Flagship. 1 E.
COMPARISON TO THE EARLIER RESULTS OBTAINED USING THE ORIGINAL VERSION OF HP-ALTA
We show the probability distribution for methane– propane mixed gas hydrate formation measured using HPALTA MkI at comparable guest gas pressures in Figure 5, together with the data in Figure 3(a). The gas pressure was 8.7 MPa, slightly lower than the measurement in HP-ALTA MkII of 9.1 MPa, and the cooling rate used was 0.025 K/s, 5 times as fast as the 0.005 K/s used in HP-ALTA MkII. Given that both the slightly lower gas pressure and the use of faster
FIG. 6. Comparisons of probability distributions for methane–propane mixed gas hydrate formation from a quasi-free water droplet measured using HP-ALTA MkII (red) and from water in a glass sample cell in the original version of HP-ALTA (blue). Here the guest gas pressure was 12.0 MPa and the cooling rate used was 0.01 K/s in each case.
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