International Conference on Gas Hydrates (ICGH) (6th : 2008)


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  MICROMECHANICAL ADHESION FORCE MEASUREMENTS BETWEEN CYCLOPENTANE HYDRATE PARTICLES   Laura E. Dieker, Craig J. Taylor*, Carolyn A. Koh, and E. Dendy Sloan Jr. Center for Hydrate Research Department of Chemical Engineering Colorado School of Mines 1600 Illinois Street, Golden, Colorado 80401 UNITED STATES  *Currently at Shell Global Solutions  ABSTRACT Cyclopentane hydrate interparticle adhesion force measurements were performed in pure cyclopentane liquid using a micromechanical force apparatus.  Cyclopentane hydrate adhesion force measurements were compared to those of cyclic ethers, tetrahydrofuran and ethylene oxide, which were suspected to be cyclic ether-lean and thus contain a second ice phase.  This additional ice phase led to an over-prediction of the hydrate interparticle forces by the capillary bridge theory.  The adhesion forces obtained for cyclopentane hydrate at atmospheric pressure over a temperature range from 274-279 K were lower than those obtained for the cyclic ethers at similar subcoolings from the formation temperature of the hydrate.  The measured cyclopentane interparticle adhesion forces increased linearly with increasing temperature, and are on the same order of magnitude as those predicted by the Camargo and Palermo rheology model.  Keywords: particle adhesion force; micromechanical testing; capillary bridging; tetrahydrofuran clathrate hydrate; cyclopentane clathrate hydrate   NOMENCLATURE  F    Force [N] k    Spring Constant [N/m] R* Harmonic mean radius [m] R1  Radius of particle 1 [m] R2  Radius of particle 2 [m] x    Displacement [m]   INTRODUCTION  Hydrates are solid, ice-like compounds which encapsulate guest molecules, stabilizing the species at low temperatures and high pressures. Naturally occurring hydrates in the continental margin and permafrost regions have the potential to be a source of energy in the near future. [1]  Hydrates form in offshore flowlines, in some cases completely halting production until the hydrates are depressurized and removed.  Hydrate plugs are the major flow assurance concern for the deepwater petroleum industry. [1]  The agglomeration of hydrate particles is the critical step to forming hydrate plugs in oil and gas pipelines.  Understanding the adhesive force between individual hydrate particles is one key aspect in the characterization of agglomeration behavior in oil and gas pipelines.  A micromechanical apparatus was developed to measure the adhesive forces between hydrate particles to gain insight into the agglomeration tendencies of hydrate particles.  [2]    Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008. MATERIALS AND METHODS  The 2 nd  generation micromechanical apparatus used in this study improved the accuracy and repeatability in measuring the forces between hydrate particles [3].  The micromechanical apparatus consisted of an inverted light Carl Zeiss Axiovert S100 microscope equipped with digital recording equipment.  The glass fiber cantilevers were located within an aluminum cooling cell. The left glass fiber cantilever was positioned within the viewing area of the microscope and remains stationary during the experiment.  The right glass fiber cantilever was connected to a high precision, remote-operated Eppendorf Patchman 5173 micromanipulator.  To form the cyclopentane (CyC5) hydrate particles, a quenching method was used.  A dropper was used to place a drop of deionized water on the end of one of the glass fiber cantilevers.  Immediately the droplet was quenched in liquid nitrogen.  Then the glass fiber cantilever was placed inside the aluminum cooling cell containing pure CyC5 liquid (99% pure Sigma Aldrich).  This method was repeated for the other glass fiber cantilever.  The cell temperature was then raised above the ice point to provide a driving force for hydrate formation. It is important to note that the temperature was raised above the ice formation temperature to eliminate the possibility of ice contamination of the hydrate particle.  The cyclic ether hydrates were formed in a similar manner as cyclopentane hydrate, with the major exception being that a drop of stoichiometric cyclic ether and deionized water was placed on the end of each cantilever instead of pure deionized water.  The cyclic ether experiments were performed in a n-decane solution saturated with the cyclic ether being studied.  A schematic of the cantilever movements inside the cell is shown in Figure 5.1.  The displacement of the particles was captured by digital video microscopy and analyzed using Image J [4].  The adhesion force between the hydrate particles is determined using Hooke’s Law.  F= kx   (1)  The spring constant of the stationary cantilever is indirectly calibrated using a tungsten wire of known spring constant.  The tungsten wire is directly calibrated using a Denver Instruments TB- 215-D analytical laboratory scale. [5, 6]   Figure 1.  Schematic of micromechanical apparatus experimental technique.  (A)  Particles were brought into contact at a known preload force.   (B)  Particles were slowly pulled apart. (C)  Particles have broken apart.  The displacement was measured to determine adhesion force.  The hydrate particles were located inside an aluminum cooling cell, submerged in liquid.  The microcapillary tubes were moved using micromanipulators. [3]  Forty pull-off measurements were performed for each experiment to obtain an adhesion force distribution.  Each force measurement was normalized by the harmonic mean radius (R*) of the particle pair, defined as  1/R* =1/2(1/R1 +1/R2).                                      (2)  EXPERIMENTAL RESULTS  Structure Comparison Study  A hydrate structure comparison study was performed using cyclic ether hydrates, THF sII hydrate and ethylene oxide (EtO) sI hydrate.  This structure comparison study was undertaken to determine the magnitude of difference between adhesive forces of structure I (sI) and structure II (sII) hydrates.  Since most pipeline hydrates are sII, understanding the differences between sI and sII hydrate adhesive forces is important to determine the necessity of using mixed gas systems (75-98 mol% methane in ethane gas mixtures form sII hydrate) for laboratory measurements. EtO and THF were chosen because both are cyclic ethers forming sI and sII hydrate, respectively, at atmospheric pressure, and were formed by the quenching method.  The THF and EtO hydrate adhesion force results were plotted as a function of subcooling below the eutectic temperature, as shown on Figure 2.  Using this temperature normalization, the adhesion strength of the two cyclic ether hydrates was very similar.  The eutectic temperature is the temperature at which solid ice melts into liquid water in the presence of the cyclic ether on the cyclic ether-water phase diagram and may be seen in Figure 3 (-1ºC for THF and -2ºC for Ethylene oxide hydrate, not shown).  Figure 4 is the cyclic ether-water phase diagram for THF.  EtO and THF are highly volatile and soluble in n-decane.  When the cyclic ether is depleted by hydrate formation during the course of the experiment, ice would be expected to appear as a second particle phase. 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0 2 4 6 8 10 12 Subcooling from the Eutectic Temperature (Teutectic- Texperimental) A d h e s io n  F o rc e /R a d iu s * (N /m ) THF EO Taylor (2006) THF  Figure 2.  Comparision of adhesion strength of EtO and  THF hydrate, plotted as a function of cooling below eutectic temperature, including data measured by Taylor [3]. -3 -2 -1 0 1 2 3 4 5 6 0 1 2 3 4 5 6 7 mol % THF T  ( ºC ) I + Lw H +Lw Ice + Liquid Hydrate + Liquid Hydrate + Ice Liquid Hydrate + Liquid Stoichiometric THF/water composition:  5.56 mol% THF  Figure 3. THF/water phase diagram, reproduced from Dyadin et al. [5].  Due to a significant melting occurrence at the eutectic temperature (presumably ice melting into liquid water) and the known volatility of cyclic ethers, it was determined that ice was appearing with hydrate particles as a second phase.  It was this ice contamination that was elevating the hydrate adhesive forces due to the increased water available to form the capillary bridge between the particles [7].  In order to produce more realistic hydrate adhesive forces, cyclopentane hydrate adhesive force measurements were pursued.  Cyclopentane hydrate  Cyclopentane hydrate interparticle adhesion measurements in pure cyclopentane bulk fluid were characterized from 274-279K.  Cyclopentane hydrate is a good model system because it allows for measurements to be taken above the ice point to 7.5 ºC and does not require a bulk fluid mixture. In addition, cyclopentane hydrate is stable at atmospheric pressure, is a sII hydrate (most pipeline hydrates are sII), and is insoluble in water, similar to methane hydrate.  The results of the cyclopentane adhesive force measurements are shown in Figure 4.  The forces measured for cyclopentane hydrate in pure cyclopentane fluid were on the same order as predicted in the Camargo and Palermo rheological model [8] and a preliminary cyclopentane measurement performed by Taylor [6].  0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01 0 1 2 3 4 5 6 7 8 Temperature (ºC) A d h e s iv e  F o rc e /R a d iu s * (N /m )  Figure 4. Cyclopentane hydrate adhesion measurements in pure cyclopentane bulk fluid  CONCLUSION  Cyclopentane hydrate adhesion force measurements were measured at temperatures from 274-279 K.  Cyclic ethers (tetrahydrofuran and ethylene oxide) were suspected to have evaporated so that a second ice phase occurred. Camargo and Palermo Force of Adhesion The presumed ice phase led to an over-prediction of the hydrate interparticle forces due to an increased liquid surrounding the hydrate surface. This increased liquid phase caused a larger capillary bridge between the particles [7].  The adhesion forces were measured for cyclopentane hydrate at atmospheric pressure over a temperature range from 274-279 K.  The cyclopentane interparticle adhesion forces measurements obtained increased linearly with increasing temperature and are on the same order of magnitude as those predicted by the Camargo and Palermo rheology model [8].  Acknowledgments  The authors would like to acknowledge the Colorado School of Mines Hydrate Consortium for their funding and support of this work:  BP, Champion, Chevron, ConocoPhillips, ExxonMobil, Halliburton, Petrobras, Schlumberger, Shell, and StatoilHydro.  REFERENCES 1. E.D. Sloan, Clathrate Hydrates of Natural Gases (second ed.), Marcel Dekker, New York (1998). 2. S.O. Yang, D.M. Kleehammer, Z. Huo, E.D. Sloan Jr., K.T. Miller, Temperature Dependence of Particle-Particle Adherence Forces in Ice and Clathrate Hydrates, J. Colloid Interface Sci. 277 (2004) 335. 3. C.J. Taylor, L.E. Dieker, K.T. Miller, C.A.Koh, E.D. Sloan, Micromechanical adhesion force measurements between tetrahydrofuran hydrate particles, J. Colloid Interface Sci. (2006) doi:10.1016/j.jcis.2006.10.078. 4. W.S. Rasband, ImageJ. 1997-2006, U. S. National Institutes of Health: Bethesda, Maryland, USA, 1992. 5. Dyadin, Y.A., Kuznetzov, P.N., Yakovlev, I.I., Pyrinova, A.V., Kokl.Chem, 208, 9 (1973) 6. Taylor C.J. Adhesion Force between Hydrate Particles and Macroscopic investigation of Hydrate Film Growth at the Hydrocarbon/Water Interface. Thesis, Colorado School of Mines, Golden, CO, 2006. 7.  J.N. Israelachvili, Intermolecular and Surface Forces, second ed., Academic Press, London, 1992. 8. Camargo, R., Palermo T. (2002).  Rheological Properties of Hydrate Suspensions in an Asphaltenic Crude Oil.  Proceedings of the Fourth International Conference on Gas Hydrates, Yokohama, May 19-23, 2002. 


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