6th International Conference on Gas Hydrates


You don't seem to have a PDF reader installed, try download the pdf

Item Metadata


5447.pdf [ 105.92kB ]
JSON: 1.0041046.json
JSON-LD: 1.0041046+ld.json
RDF/XML (Pretty): 1.0041046.xml
RDF/JSON: 1.0041046+rdf.json
Turtle: 1.0041046+rdf-turtle.txt
N-Triples: 1.0041046+rdf-ntriples.txt

Full Text

  NUCLEATION OF CLATHRATES FROM SUPERCOOLED THF/WATER MIXTURES SHOWS THAT NO MEMORY EFFECT EXISTS.   P.W. Wilson and A.D.J. Haymet Scripps Institution of Oceanography UC San Diego, USA  K.A. Kozielski, P. Hartley & N.C. Becker CSIRO Molecular & Health Technologies, Ian Wark Laboratory, Clayton, Victoria 3150, Australia.   ABSTRACT The liquid-to-crystal nucleation temperature is measured for clathrate-forming mixtures of tetrahydrofuran and water using both an automatic lag time apparatus (ALTA) and a ball screening apparatus. Our results are conclusive evidence that no so-called “memory effect” exists.  Either the solid form melts fully or it does not.  If it does not, then no supercooling is possible on the next cooling down of that sample, and if it does then the second cooling run and freezing on a sample is just as likely to have a colder nucleation temperature as a hotter one.  Keywords: memory effect, ALTA, tetrahydrofuran, supercooling   INTRODUCTION For many years researchers have described studies in which they measured the induction times for hydrate formation and claim to find markedly shorter induction times for systems that have previously formed hydrates (see for example, [1,2,3]).  Most workers have suggested that water that has been structured, by belonging to a solid hydrate, is then able to regain that structure more easily during subsequent cooling, even after dissociation of the hydrate during melting.  The structure has been attributed to partial hydrate cages.  This phenomenon has been reported with CO2 hydrates (eg. [4]), hydrocarbon systems (eg. [5]) and tetrahydrofuran systems (eg. [6]). An overview has recently been published by Buchanan et al. [5].  The consensus seems to have been that if a hydrate solution is heated sufficiently then the memory effect can be destroyed.  Similarly if the solution is held for long enough at a temperature above the equilibrium melting point the effect can be destroyed.  This implies immediately to us that in all previously reported cases the system simply was not at equilibrium before the cooling process had begun again.  METHODS We use a stoichiometric mixture of THF and water to examine both the induction time and the actual nucleation temperature required to form hydrates and show that previous hydrate formation does not affect subsequent formation ability in a system warmed beyond the equilibrium melting point and fully melted. 1. Screening Apparatus Experiments  - induction time at constant supercooling A screening apparatus was built, similar to that described by Sloan [8]. The apparatus has a motor to rotate test tubes in iced water.  Prior to hydrate formation Teflon bars are able to move inside the tubes as they are inverted.  The time at which the bars were no longer able to move freely along the length of the test tubes was taken as the time for hydrate formation.  The tubes contain samples from the same stock solution and are held at a constant supercooling until they freeze (taken as Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008. the induction time, for each tube.)  Once all tubes have frozen they are melted and the process repeated.  The protocol is shown in Figure 1.  Figure 1.  The protocol for the ball screening apparatus measurements of induction times.  Thot is the temperature used to melt the samples after freezing, Tcold is the level of supercooling used and Tf is the melting point of that THF solution.  Results for the induction time measurements are represented schematically in Figure 2.  Clearly there is no trend for run 2 to be shorter than run 1.   1 2 3 4 5 6 7 8 9 10 T ube 1 T ube 2 T ube3 T ube 4 T ube 5 T ube 6 0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00 900.00 T im e  ( s e c o n d s ) R un N um ber T ube 1 T ube 2 T ube3 T ube 4 T ube 5 T ube 6    Figure 2.  Hydrate Induction Time vs Run Number (19% THF/deionised Water – Teflon coated stirrer bars)    2. Automatic Lag Time Apparatus (ALTA) Experiments – nucleation temperature at constant ramping The automatic lag time apparatus has been described in detail elsewhere [9,10,11].  The 200 μl sample is linearly cooled until nucleation and freezing occurs and then warmed up to 25 °C, as shown in Figure 3. The process is repeated many hundreds of times.           Figure 3. The protocol for the ALT- apparatus measurements of nucleation temperature.  Thot is the temperature used to melt the samples after freezing, Tf is the melting point of that THF solution and α is the cooling rate.  Results for the ALTA experiments are represented schematically in Figure 4 and again they show clearly that no correlation exists between runs 1 and 2 in 14 separate solutions.  -40.00 -30.00 -20.00 -10.00 0.00 10.00 20.00 30.00 0 1 2 3 4 5 6 Tube N um ber %  D iff er en ce Tube 1 Tube 2 Tube 3 Tube 4 Tube 5  Figure 4.  Difference in nucleation temperatures between Run 1 and Run 2 for 14 samples using the ALTA (19% THF/deionised Water). CONCLUSION The data presented in Figures 2 and 4 are consistent with the hypothesis that hydrate nucleation is a stochastic process. Nothing is changed between each run, yet there is no apparent pattern to the nucleation induction times. Several short induction times may be followed with an exceptionally long one or vice versa.  ACKNOWLEDGEMENTS We gratefully acknowledge the support of Chevron Texaco and BP for partially funding the ALTA.  REFERENCES [1] Parent, J. S. and P. R. Bishnoi. Investigations into the nucleation behavious of methane gas hydrates.  1996. Chem. Eng. Commun., 144, 51 – 64. [2] Rodger, P. M. Methane hydrate: melting and memory. 2000. Annals of New York Academy of Sciences. 912, 474-482. [3] J. A. Ripmeester, Hydrate research - from correlations to a knowledge-based discipline: the importance of structure.   Annals of the New York Academy of Sciences. 2000. 912,  1, 1- 16. [4] Takeya, S., Hori, A., Hondoh, T. and T. Uchida. Freezing – memory effect of water on nucleation of CO2  hydrate crystals. 2000. J. Phys. Chem. B, 104, 17, 4164-4168. [5] Buchanan P., Soper, A.K., Thompson, H., Westacott, R.E., Creek, J.L., Hobson, G. and C. A. Koh.  Search for memory effects in methane hydrate: structure of water before hydrate formation and after hydrate decomposition. 2005. J. Chem. Phys.  123, 16, 164507. [6] Zeng, H., Wilson, L. D., Walker, V. K. and J. A. Ripmeester. Effect of antifreeze proteins on the nucleation, growth, and the memory effect during tetrahydrofuran clathrate hydrate formation.  2006, J. Am. Chem. Soc., 9, 128, 2850-. [7] Ohmura, R., Ogawa, M., Yasuja, K., and Y. H. Mori. Statistical study of clathrate hydrate nucleation in a water/hydrochlorofluourcarbon system: search for the nature of the “memory effect”. 2003. J. Phys. Chem. B, 107, 5289-5293. [8] Sloan, E. D. Clathrate Hydrates of natural gases. 2 nd  ed. (Marcel Dekker M.Y. 1998). [9] Heneghan, A. F., Wilson, P. W., Wang, G. and A. D. J. Haymet. Liquid-to-Crystal nucleation: automated lag-time apparatus to study supercooled liquids. J. Chem. Phys. 2001. 115, 7599-7603. [10] Heneghan, A. F., Wilson, P. W. and A. D. J. Haymet. 2002. Statistics of heterogeneous nucleation of supercooled water, and the effect of an added catalyst. Proc. Natl. Acad. Sci., 99, 9631- 9634. [11] Wilson, P. W., Lester, D. and A. D. J. Haymet. Heterogeneous nucleation of clathrates from supercooled tetrahydrofuran (THF) / water mixtures, and the effect of an added catalyst. Chemical Sciences Engineering. 2005. 60, 2937-2941.     


Citation Scheme:


Usage Statistics

Country Views Downloads
China 5 24
Japan 3 0
Australia 2 0
United States 2 0
Netherlands 1 0
City Views Downloads
Beijing 4 3
Tokyo 3 0
Unknown 3 0
Somers 1 0
Amsterdam 1 0
Jinan 1 0

{[{ mDataHeader[type] }]} {[{ month[type] }]} {[{ tData[type] }]}


Share to:


Related Items