6th International Conference on Gas Hydrates


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   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.      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 ?l  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 ? 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.    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. 2nd 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.      


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