Kinetics of Trifluoromethane Clathrate Hydrate Formation from CHF3

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Kinetics of Trifluoromethane Clathrate Hydrate Formation from CHF Gas and Ice Particles 3

Jaruwan Amtawong, Suvrajit Sengupta, Michael T Nguyen, Nicole C Carrejo, Jin Guo, Everly B. Fleischer, Rachel W. Martin, and Kenneth C. Janda J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b08730 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 17, 2017

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The Journal of Physical Chemistry

Figure 1: Schematic of the aaparatus 65x54mm (600 x 600 DPI)

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Figure 2. (a) Pressure versus time curves for a reference run and experimental runs at an initial flow rate of 30 mmol/h and four temperatures: 268 K, 263 K, 258 K, and 253 K. (b) Uptake rate versus time. (c) Percent yield versus time. 82x136mm (300 x 300 DPI)


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Figure 3. Percent yield versus time curves of experiments at 268 and 253 K, run over 55 hours. 82x49mm (300 x 300 DPI)



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Figure 4. (a) Pressure versus time curves for a reference run and experimental runs at an initial flow rate of 15 mmol/h and four temperatures: 268 K, 263 K, 258 K, and 253 K. (b) Uptake rate versus time, and (c) Percent yield versus time. 82x125mm (300 x 300 DPI)


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Figure 5. (a) Pressure versus time curves for a reference run and experimental runs at an initial flow rate of 48 mmol/h and four temperatures: 268 K, 263 K, 258 K, and 253 K. (b) Uptake rate versus time, and (c) Percent yield versus time. 82x125mm (300 x 300 DPI)



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Figure 6. Percent yield curves for experiments at 258 K with three different flow rates: 15, 30, and 48 mmol/h. 82x49mm (300 x 300 DPI)


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Figure 7. Arrhenius plots for (a) 15 mmol/h, (b) 30 mmol/h, and (c) 48 mmol/h flow rate experiments. 82x136mm (300 x 300 DPI)



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Kinetics of Trifluoromethane Clathrate Hydrate Formation from CHF3 Gas and Ice Particles Jaruwan Amtawonga, Suvrajit Sengupta, Michael T. Nguyenb, Nicole C. Carrejoc, Jin Guo, Everly B. Fleischer, Rachel W. Martin, Kenneth C. Janda* Department of Chemistry, University of California, Irvine, California 92697, United States Email: [email protected]. Tel.: (949) 824-6022.


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Abstract. We report the formation kinetics of trifluoromethane clathrate hydrate (CH) from 75 μm) were transferred under liquid nitrogen to a precooled reaction cell (310 mL) which is then submerged in a pre-cooled bath and evacuated to ~0.05 MPa. The system is equilibrated to a preset temperature. The trifluoromethane is added via a needle valve adjusted to give the desired gas flow rate into the reaction cell. Gas is


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continuously added until the cell pressure reaches the final pressure fixed by the gas regulator. As the pressure in the reaction cell increases, the gas flow rate into the cell slows down because the pressure drop across the needle valve decreases. The actual gas flow into the cell as a function of the pressure drop across the needle valve is calibrated by gas flow into a cell that contains sand instead of ice. To characterize the gas flow rate into the reaction cell in a particular run, we use the amount of gas delivered during the first hour of the experiment. Refrigerant grade trifluoromethane is used as purchased from Airgas. The actual mass of ice particles used in any experiment was determined by measuring the mass of water left in the reaction vessel after decomposing the hydrate sample. All numerical analysis is based on this mass. The Peng-Robinson equation of state, PV  ZnRT

(1)

where, Z is the compressibility factor as a function of temperature and pressure, is used to calculate the number of moles of gas (n) in the cell at any given instant, from the measured pressure (P) and temperature (T), and the known volume (V) of the cell (adjusted for the volume of ice). The uptake rate of trifluoromethane by ice

dnice can be calculated from: dt

dnm dncell dnice   dt dt dt

(2)

where,

dnm dncell is the rate of change of the number of moles in the gas phase, is the flow rate dt dt

the gas phase calculated from the reference run. The percent yield of the experiment is calculated by integrating the uptake rate as a function of time.


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Figure 1. Schematic of the apparatus. III. Results and Data Analysis III.A. Temperature Dependence of the Reaction Rate. A typical set of data, including a reference run and four runs at 253, 258, 263, and 268 K, is shown in Figure 2a. These data are for runs in which the initial gas flow rate into the reaction cell is about 30 mmol/h. Because there is no clathrate hydrate formation in the reference run (top blue curve), the pressure rises monotonically and reaches the maximum pressure of 0.46 MPa. This calibration allows us to determine the gas flow rate into the cell as a function of cell pressure. For each experiment with ice, the pressure initially rises monotonically because there is no hydrate formation at low pressure. Once the initiation pressure is reached, more gas is absorbed into the ice (due to hydrate formation) than leaked into the reaction cell, causing a decrease in pressure. We refer to this reaction regime as stage I, in analogy to the shrinking core model.21,22 For the highest temperature (268 K), the cell pressure reaches 0.37 MPa before the reaction starts. For the lowest temperature (253 K), the onset of reaction is at 0.25 MPa, which occurs nearly an hour sooner than for 268 K. Intermediate temperatures yield results between these two limits. For each temperature, upon initiation, the cell pressure drops to a minimum, at which the reaction rate equals


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the flow rate into the cell, and then gradually increases toward the limiting value set by the gauge of the gas source. Empirically, we refer to the region for which the reaction rate is about equal to the gas flow rate as stage II, and the long-term, slower reaction rate as stage III in analogy to the shrinking core model. We note that our definition of the three reaction stages is purely empirical, as the transition between stages in the shrinking core model is somewhat ill defined. For the data set reported in Figure 2, the duration of stage II increases slightly with temperature. The experiments performed here are too time consuming to repeat them often enough to determine accurate error bars for the results. However, each run was repeated two or more times to ensure that the data is reproducible. The reproducibility of the methods has been more completely examined in Refs. 15-17. In the data tables, we report the results to two significant figures, and they are reliable to 5% of the stated value. As described previously,20 the raw data is converted to uptake rate versus time and percent yield versus time, shown in Figures 2b and 2c, respectively. Figure 2b shows that the peak uptake rate is also inversely proportional to temperature, even though it occurs at a lower cell pressure for lower temperatures. For instance, the maximum uptake rate at 253 K is over two times higher than that of the 268 K run, even though the pressures in the cell are 0.25 and 0.37 MPa, respectively. The thermodynamic driving force for the enclathration process might be understood from the degree of undercooling from the ice-hydrate-vapor equilibrium line. At 0.25 MPa and 0.37 MPa the ice-hydrate vapor equilibrium is approximately at 267 K and 272 K respectively.23 Thus, in the 253 K experiment, the degree of undercooling at the initiation of enclathration is about 14 K, while for the 268 K experiment it is about 4 K. The higher uptake rates at lower temperatures also translate to higher percent yields for stages I and II of the reaction: e.g. 40% for 268 K and 50% for 253 K. Our experimental technique is not sensitive enough to measure the percent yield after


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many days since the errors in integrating the slow reaction rate over long times will be cumulative. A summary of all data from this study is presented in Table 1.

Figure 2. (a) Pressure versus time curves for a reference run and experimental runs at an initial flow rate of 30 mmol/h and four temperatures: 268 K, 263 K, 258 K, and 253 K. (b) Uptake rate versus time. (c) Percent yield versus time.


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Table 1. Kinetic Data Reported for Trifluoromethane Clathrate Hydrate Formation. temperature (K)

particle size (μm)

flow rate (mmol/h)

initiation pressure (MPa)

maximum uptake rate (μmol/s)

percent yielda

stage III rateb (%/104 s)

duration of stages I and II (s/104)

268 K

Kinetics of Trifluoromethane Clathrate Hydrate Formation from CHF3

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