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

NATURAL GAS HYDRATE FORMATION AND GROWTH ON SUSPENDED WATER DROPLET Zhong, Dong-Liang; Liu, Dao-Ping; Wu, Zhi-Min; Zhang, Liang 2008-07-31

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   NATURAL GAS HYDRATE FORMATION AND GROWTH ON SUSPENDED WATER DROPLET   Dong-Liang Zhong*, Dao-Ping Liu, Zhi-Min Wu, Liang Zhang College of Power Engineering University of Shanghai for Science and Technology 516 Jungong Road, Yangpu District, Shanghai, 200093 CHINA   ABSTRACT  The experimental formation of natural gas hydrate on pendant water droplet exposed to natural gas was conducted and visually observed under the pressures from 3.86MPa to 6.05MPa. The temperature  was set  at  274.75K  and 273.35K. The diameter  of the  pendant  water  droplet  was around 4mm. The nucleation and growth of hydrate film on the pendant water drop exhibited a generalized  trend.  The  film  initially  generated  at  the  boundary  between  the  water  drop  and suspension  tube,  and  afterwards  grew  laterally  and  longitudinally  on  the  surface  of  the  water drop. The phenomenon of the two layers of hydrate films growing on the pendant water drop distinguished from the experiments on the sessile water drop. The effect of the driving force that resulted from the overpressure from the three equilibrium pressure on the hydrate nucleation and growth  was  investigated.  It  was  found  that  the  elevation  of  the  driving  force  reduced  the nucleation time and shortened the process of the hydrate growth on the pendant water drop. The crystals  on  the  hydrate  shell  became  coarser  with  the  increase  of  the  driving  force.  The mechanism for the hydrate film formation and growth on static pedant water droplet included four stages,  such  as  nucleation,  generation  of  the  hydrate  film,  growth  of  the  hydrate  film,  and hydration below the hydrate shell.  Key words: gas hydrate, formation, crystallization, water droplet, morphology  NOMENCLATURE  INTRODUCTION Gas  hydrate  is  a  sort  of  crystal  compound consisting  with  host  molecules  and  guest molecules. Host molecules form the cage structure to  encapsulate  guest  molecules.  Water  molecules are  the  hosts  in  natural  gas  hydrate,  which  was assembled  by  hydrogen  bonds,  and  formed polyhedral  cavities  to  accommodate  the  guest molecules  like  CH4,  C2H6,  CO2,  etc.  [1].  The knowledge on gas hydrates plays an important part in  the  fields  of  storage  and  transportation  of natural  gas  in  the  form  of  hydrates  [2,  3], exploration  of  natural  gas  hydrate  under  the seafloor  and  at  the  bottom  of  the  permafrost, *Corresponding author, Phone: +86-21-55270305 Fax: +86-21-55270305 E-mail: azhongdl@hotmail.com Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008),  Vancouver, British Columbia, CANADA, July 6 -10, 2008.   avoiding the plug in the oil and gas pipelines, and sequestration of carbon dioxide, etc. Researches  on  the  crystal  morphology  could provide  valuable  information  on  the  mechanistic aspects  of  the  hydrate  crystal  nucleation,  growth, and  dissociation.  Servio  and  Englezos  [4] conducted the morphology study of the structure-H hydrate  formation  on  water  droplets,  which showed  that  elevated  driving  force  resulted  in smaller induction times than those obtained under a low driving force. Studies of the morphology of methane and carbon dioxide hydrates formed form water  droplets  were  also  conducted  by  Englezos [5].  It  stated  that  under  the  high  pressure  water droplets  quickly  became  jagged  and  exhibited many  needlelike  or  hairlike  crystals  extruding from the droplets, whereas under the low pressure the surface of the water dorplet was smooth. Ohmura [6] described the visual observation of the formation  and  growth  of  structure-H  hydrate crystals on a water drop that was partially exposed to methane and partially immersed in a pool of a LMGS.  Hydrate  crystals  first  formed  on  the surface of the water droplet, and then floated up to the apex of the drop. The crystals accumulated on the apex then formed a cap or shell covering the upper area of the drop. The shell exhibited a coarse polycrystalline surface texture.   Ju Dong Lee [7] performed the morphology study of  gas  hydrate  formation  and  decomposition  on water  droplets  by  using  89.4%  methane-10.6% ethane mixture, and 90.1% methane-9.9% propane mixture. All droplets nucleated simultaneously and the droplet size and shape had no visible effect on induction  time  and  the  morphology  of  crystal growth  for  the  methane-ethane  mixture.  The surface  of  the  hydrate  crystals  from methane-propane had a hairy-like appearance, and then changed to a smooth surface.   Enlightened by the water spray hydrate formation [8-10],  we  are  paying  our  attention  to  the morphology of hydrate formation on single water droplet.  According  to  our  previous  studies  on water  spray  hydrate  formation  [11-13],  it  was found that it is essential to explore the mechanism of nucleation and growth of gas hydrates on water droplets in the water spray reactor. However, in the above experiments water droplets were placed on a stage  or  platform,  which  could  not  provide sufficient evidences for the study of gas  hydrates formation  on  falling  water  droplets  in  the  water spray  reactor.  Therefore,  in  this  paper  an experimental  apparatus  for  hydrate  formation  on pendant  water  droplet  was  presented,  and  the morphology  of  natural  gas  hydrate  formation  on the  liquid  droplet  was  investigated  with  the microscopic digital imaging system, in the hope of being  able  to  discover  the  mechanism  of nucleation  and  growth  of  gas  hydrates  on  pedant water  droplets,  and  provide  some  valuable information  for  the  research  on  the  water  spray hydrate formation.  EXPERIMENTAL Apparatus Up  to  now,  it  is  difficult  to  conduct  the investigation of hydrate formation on falling water droplets, so an experimental apparatus to study the hydrate formation on the still pedant water droplet is designed and constructed, creating a preliminary surrounding  for  the  research  on  the  hydration behavior of the water droplets in the water spray reactor.   The  schematic  of  the  experimental  apparatus  is shown in Figure 1. The apparatus is composed of a stainless  steel  crystallizer  with  a  cooling  water jacket.  The  25  vol%  ethanol  solution  circulates continuously  between  the  low  constant temperature trough and the water jacket to cool the crystallizer  chamber  to  a  predetermined temperature.  The  crystallizer  is  a  horizontal cylinder  with  an  internal  diameter  of  45mm,  a length of 150mm and a volume of 240ml, and the maximum allowed pressure is 10.0MPa. One side of  the  crystallizer  is  equipped  with  a  piece  of optical glass for the microscopic digital camera to observe  the  water  droplet.  The  optical  glass  is fixed  on  the  crystallizer  by  a  flange.  The  water droplet  is  squeezed  into  the  chamber  of  the crystallizer  by  an  injection  pump.  The  dimension of the water droplet can be adjusted slightly by the injection pump, however, if the droplet exceeds its critical size it would drop onto the surface of the crystallizer.  In  order  to  observe  the  hydrate formation on water droplets with different size, the pipes  to  hang  the  water  droplet  come  with  three different diameters, which are 1.0mm, 1.5mm, and 2.0mm,  respectively.  In  present  study,  only  the pipe with inner diameter of 2.0mm has been used. Hydrate  forming  gas  from  the  gas  reservoir  is supplied  into  the  crystallizer  after  passing  the pressure relief valve. The pressure in crystallizer is regulated and maintained by a pressure regulator at the outlet of the gas reservoir. The temperature in the  crystallizer  is  measured  by  a  Pt100  thermal resistance  (WPK-263s,  Shanghai  Automation Instrumentation Co., Ltd., Shanghai, China) with a measuring  range  of  223.15-283.15K,  and  a  0.2% accuracy,  a  pressure  transducer  (P45,  Banna Electronics Inc., USA) which is used to report the pressure of the gas in the crystallizer with a range of  0-10.0MPa,  and  an  accuracy  of  0.5%.  The microscopic  digital  imaging  system  consists  of  a CCD  camera  (1.3  mega  pixel,  Nikon  digital camera), a monocular microscope (with a length of 168.0mm  and  a  maximum  working  distance  of 95.0mm)  and  the  image  acquisition  software (Shanghai MicroImage Technology Ltd., Shanghai, China). A computer is used to record the output of the  microscopic  digital  imaging  system  and monitor the experimental process.   Figure 1 Schematic of experimental apparatus  Experimental procedure The experimental procedure involves cleaning and drying  the  inside  of  crystallizer.  The  chamber  of the crystallizer is flushed three times with natural gas at 1.5MPa, so as to remove any residual air in the crystallizer, and then close the valves. Charge the chamber with natural gas to the experimental pressure after the crystallizer has been cooled to a predetermined  temperature,  squeeze  a  water droplet into the chamber, and keep it suspended at the  center  of  the  crystallizer.  Start  to  record  the experimental  time,  observe  the  experimental process  with  the  microscopic  digital  imaging system, continuously take pictures of the pendant water  drop,  and  conduct  data  acquisition  at  the same time. In this work, the deionized and distilled water  is  used  to  produce  water  droplets,  and natural gas from the western China is used as the hydrate forming gas, and its composition is listed in Table 1. Table  1  Composition  of  natural  gas  from  the western China Analyzed items Volume percentage (? ) CH4  88.017 C2H6  5.453 C3H8  1.658 n-C4H8  0.274 i-C4H10  0.237 n-C5H12  0.057 i-C5H12  0.075 C6H14  0.05 N2  2.796 CO2  1.382 H2S  0.001  RESULTS AND DISCUSSION The experiments that investigate the formation of natural  gas  hydrates  under  various  experimental conditions are listed in Table 2. Both the nucleation time  and  the  opaque  time  are  shown  in Table  2. Nucleation time in this paper is defined as the time when  the  first  crystal  has  been  observed  by  the microscopic  digital  system  since  the  experiment started,  while  the  opaque  time  is  defined  as  the time  when  the  surface  of  the  water  droplet  has appeared to be covered by the hydrate film since the  crystals  were  observed  by  the  microscopic digital system. Experiments  1-2  were performed  at  274.75K and 3.86MPa, and Experiments 3-11 were carried out at  the  temperature  of  273.35K  and  the  pressure between  4.78MPa  and  6.05MPa.  According  to Sloan [14, 15], the formation conditions for natural gas hydrate are 1.2MPa at 274.75K and 1.01MPa at 273.35K. Therefore, the overpressure above the equilibrium hydrate formation pressure is 2.66MPa in Experiments 1-2, 3.77MPa in Experiments 3-5, 4.76MPa  in  Experiments  6-8,  and  5.04MPa  in Experiments 9-11, indicating that the driving force (deviation of experimental pressure from the three phase equilibrium pressure at a given temperature) increases with the elevation of the system pressure. Table 2 Experiments with observed nucleation times exp.  T (K)  P (MPa)   diamet er (mm)  nucleation time (min) opaque time (min) 1  274.75  3.86  3.6  184  a 2  274.75  3.86  3.6  145  a 3  273.35  4.78  4.1  167  63 4  273.35  4.78  4.1  185  71 5  273.35  4.78  4.0  136  54 6  273.35  5.77  4.0  141  47 7  273.35  5.77  4.0  160  55 8  273.35  5.77  3.9  162  49 9  273.35  6.05  3.9  147  31 10  273.35  6.05  4.0  158  46 11  273.35   6.05  4.0  133  27 a The water droplet didn?t become opaque with in 20 hours. Morphology  of  hydrate  formation  on  pendant water droplet The  morphological  observation  on  the  formation of  natural  gas  hydrates  was  performed  in  each experiment, and photographs were taken to record the formation process and formation phenomenon. Figure  2  exhibited  the  process  of  Experiment  1. The pendant water droplet was 3.6mm in diameter, about  184  minutes  after  the  beginning  of  the experiment, hydrate nuclei appeared randomly on the surface of the water droplet, see the  scattered white  spots  in  Figure  2  (a).  It  was  observed  that these tiny nuclei were sinking towards the bottom of  the  water  droplet  owing  to  the  effect  of  the gravity. At the same time, they were unceasingly revolving  around  the  central  axis  of  the  water droplet.  It  was  analyzed  that  the  heat  convection between the gas phase and the water droplet forced the  small  crystal  particles  to  revolve.  The  nuclei gradually  accumulated  at  the  bottom  of  the  drop and  grew.  Over  a  short  period  of  time,  star-like crystals appeared on the edge of the steel pipe and at  the  bottom  of  the  water  droplet,  as  shown  in Figure 2 (b). With the advance of the experiment, these star-like crystals kept moving round the axis of the water droplet and growing, and new nuclei also appeared. Figure 2 (d) showed that a complete star-like  crystal  had  eight  branches,  but  most crystals were incomplete and continually growing. About  170  minutes  after  the  initial  formation  of hydrate crystals, the star-like crystals had grown to a certain size that limited their movement. It was observed that some of these star-like crystals had not formed the shape with eight branches yet, seen in  Figure  2  (e).  As  indicated  in  Figure  2  (f),  315 minutes  after  the  initial  formation  of  hydrate crystals,  the  growth  of  star-like  crystals  nearly ceased, and no new nuclei were  generated either, crystals like snowflakes were sparsely distributed on  the  surface  of  the  water  droplet.  The observation of the experiment process was carried out for 20 hours, but no progress was found 315 minutes  after  the  initial  formation  of  hydrate crystals.  Therefore,  20  hours  later  we  terminated Experiment 1.      (a) 0 second  (b) 30 seconds  (c) 100 minutes      (d) 150 minutes  (e) 170 minutes  (f) 315 minutes Figure 2 Gas hydrates formation on a suspended water droplet exposed to natural gas. Times given below each picture are the time from the appearance of hydrate nuclei. (Experiment 1, T=274.75K, P=3.86MPa).  Photographs  that  exhibited  the  process  of Experiment 3 were presented in Figure 3. It should be  noted  that  Experiment  3  was  carried  out  at 273.35K  and  4.78MPa,  and  the  diameter  of  the suspended  water  droplet  was  4.1mm,  as  marked with  the  solid  line  in  Figure  3  (a).  At  the  167 minutes of the experiment, the transparency of the water  droplet  waned,  the  surface  of  the  water droplet  became  foggy  (Figure  3  (a)).  60  seconds later, hydrate crystals were spotted at the interface of the water droplet and the steel tube, as indicated in Figure 3 (b). And then, the crystals grew towards the bottom of the water droplet from the edge of the suspension tube, and the area that was covered by hydrate crystals turned to be opaque, referred to Figure  3  (c).  In  Figure  3  (e),  it  exhibited  a phenomenon that was different from the previous observation  on  sessile  water  drop  presented  by other  researchers.  A  cloudy  hydrate  film  of 0.02mm spread quickly to cover the surface of the water  droplet  in  16  seconds,  the  surface  of  this film  was  super  smooth.  Subsequently,  coarse crystals  that  originally  formed  at  the  interface between the water droplet and tube edge covered the cloudy film, with the thickness of 0.08mm. In the end, the surface of water droplet was enclosed by hydrate crystals from upside to downside, and hydration continued inside the water droplet, but, it was  observed  that  the  surface  remained  smooth without any change, shown in Figure 3 (f).   It was analyzed that the stainless steel suspension tube  directly  passed  through  the  cooling  water jacket, so the heat transfer on it was much faster than the heat convection between the reactor wall and the gas phase. Therefore, it was believed that the  top  of  the  water  droplet  that  was  in  contact with the suspension tube was slightly cooler than the  lower  portion.  In  addition,  the  interface between the water droplet and the tube was not as smooth as the surface of the water droplet, so the crystallization  first  occurred  at  the  boundary between the water droplet and the suspension tube.      (a) 0 second  (b) 60 seconds  (c) 3 minutes      (d) 10 minutes  (e) 20 minutes  (f) 70 minutes Figure 3 Sequential photographs of hydrates formation on a suspended water droplet exposed to natural gas. Times  given  below  each  picture  are  the  time  from  the  appearance  of  hydrate  nuclei.  (Experiment  3, T=273.35K, P=4.78MPa).  Figure 4 recorded the process of Experiment 6 that was performed at the temperature of 273.35K and the pressure of 5.77MPa. As measured in Figure 4 (a),  the  initial  diameter  of  the  water  droplet  was 3.98mm.  About  141minutes  after  the  start  of  the experiment,  nucleation  occurred  at  the  surface  of the  water  droplet,  and  the  transparent  surface  of the  water  droplet  turned  to  be  foggy,  see  the central zone of the water drop in Figure 4 (a). 60 seconds later, laminar hydrate crystals appeared in the  area  near  the  edge  of  the  tube,  which  was floating on the upper surface of the water droplet (Figure 4 (b)). Figure 4 (c) showed that clearances among  hydrate  plates  disappeared,  and  the dispersed hydrate plates had grown into a complete film  cap  covering  the  upper  water  droplet.  20 minutes  later,  the  whole  surface  of  the  water droplet  was  wrapped  by  the  hydrate  film.  It  was indicated that the laminar hydrates not only grew towards the bottom of the water droplet, but also grew laterally, which was consistent with the result presented  by  Servio  [5].  Figure  4  (d)  exhibited  a clear  boundary  between  the  films  located  on  the upper  and  lower  part  of  the  water  droplet.  The upper  hydrate  film  of  0.2mm  was  ice-like  and coarse,  but  the  lower  one  was  cloudy  and  thin, with  the  thickness  of  0.04mm.  As  interpreted  in Figure 3 (e), it was the phenomenon of the growth of two layers of hydrate films distinguished from observations  on  sessile  water  drops  by  other researchers.  About  47  minutes  later,  the  hydrate film  on  the  entire  surface  of  the  water  droplet turned to be identical, but it was not smooth any more.  Embryos  of  hydrate  were  observed  to project  from  the  surface  of  the  hydrate  shell,  as indicated in Figure 4 (e).   It was believed that natural gas permeated through the porous hydrate crystals to the interior interface between  the  hydrate  film  and  the  water  droplet, and  continued  to  form  hydrates.  The capillarity-driven water molecules [16] went out of the porous hydrate layer to the outer surface of the hydrate  film  and  formed  hydrates.  Figure  4  (f) presented  that  small  hydrate  embryos  had  grown into branches, the volume of the water drop shrank, and the surface near the large branches depressed, indicating that the transport of the water molecules to  the  gas/hydrate  interface  developed  and  the hydration at the hydrate/water interface continued. The appearance of such branches generated under a  high  driving  force  was  different  from  the needlelike  crystals  extruding  from  the  droplets  at high pressure [5, 7].       (a) 0 second  (b) 60 seconds  (c) 3 minutes      (d) 20 minutes  (e)47 minutes  (f) 60 minutes Figure 4 Photographs of hydrates formation on a suspended water droplet exposed to natural gas. Times given below each picture are the time from the appearance of hydrate nuclei. (Experiment 6, T= 73.35K, P=5.77MPa).  Figure  5  exhibited  the  crystal  formation  in Experiment  9  which  was  conducted  at  the temperature  of  273.35K  and  the  pressure  of 6.05MPa. The solid line in Figure 5 (a) indicated that the diameter of the water droplet was 3.9mm. About  147  minutes  after  the  beginning  of  the experiment,  tiny  hydrate  nuclei  were  observed  at the  interface  between  the  water  droplet  and  the edge of the suspension tube, as shown in the top right  cornier  in  Figure  5  (a).  Meanwhile,  the transparent  surface  turned  to  be  cloudy,  small hydrate  particles  were  floating  on  the  surface  as well. 60 seconds later, hydrate film extended from the top of the water droplet to the lower point, as shown in Figure 5 (b). Compared to Figure 4 (b), it was  found  that  the  crystal  growth  in  this experimental  round  was  much  faster,  for  nearly half  of  the  water  droplet  was  covered  by  the hydrate  film  in  60  seconds,  rather  than  sparse crystals floated on the surface of the water droplet in  Figure  4  (b).  Figure  5  (c)  also  showed  the phenomenon  of  the  growth  of  two  layers  of hydrate  film.  As  measured,  the  thickness  of  the cloudy  film  and  the  coarse  hydrate  film  was 0.04mm  and  0.24mm,  respectively.  About  30 minutes later, the entire surface of the water drop was wrapped by the coarse hydrate film (Figure 5 (d)),  and  it  was  observed  that  crystals  of  the hydrate shell was not as fine as that in Figure 3 and Figure 4, the drop was 4.4mm in diameter. With the development  of  the  hydration,  the  volume  of  the droplet  diminished  (Figure  5  (e)),  the  diameter reduced  to  4.2mm,  and  hydrate  branches  were observed  extruding  to  the  gas  phase  from  the surface of the hydrate film. A clear collapse of the surface at the bottom of the droplet was observed as well. Figure 5 (f) presented the further growth of the hydrate branches towards the gas phase, which grew  up  like  trees  covering  the  surface  of  the droplet, and the collapse was more clear than that in Figure 5 (e).         (a) 0 second  (b) 60 seconds  (c) 3 minutes      (d) 30 minutes  (e)35minutes  (f) 60 minutes Figure 5 Photographs of hydrates formation on a suspended water droplet exposed to natural gas. Times given below each picture are the time from the appearance of hydrate nuclei. (Experiment 9, T=273.35K, P=6.05MPa).  Effects of driving force on nucleation time It should be noted that each experiment in Table 2 experienced  crystal  nucleation.  For  example,  in Experiments1-2,  the  average  value  of  nucleation time  was  164.5  minutes.  Under  the  condition  of 273.35K  and  4.78MPa  the  nucleation  time  was 162.7  minutes,  which  is  an  average  value  of Experiments 3-5. The average nucleation time was 154.3  minutes  for  Experiments  6-8  and  146 minutes  for  Experiments  9-11.  It  was  found  that under  the  same  experimental  conditions  the nucleation time was a bit different (Table 2), which indicated  that  the  nucleation  of  hydrates  has  a stochastic  factor  [7].  However,  although  the amount  of  the  nucleation  time  in  different experiment  groups  (Experiments3-5, Experiments6-8,  and  Experiments9-11)  varied from  133  minutes  to  185  minutes,  the  average nucleation  time  in  each  experiment  group exhibited  a  trend  that  the  nucleation  time decreased under a higher driving force. Effects  of  driving  force  on  the  formation  of hydrate film According  to  the  above  description  on  the morphology  of  crystallization  in  different experiments,  it  was  discovered  that  the  driving force played an important role in the formation of gas hydrate on the pedant water drop. Among the four  experiment  groups,  the  driving  force  in Experiment 1 was lowest, and in Experiment 9 was highest. From the morphologies of all experiments, it can be seen that with the elevation of the driving force  the  crystallization  became  faster,  and  the crystals  of  the  hydrate  film  became  coarser.  For example,  seen  from  Table  2,  the  time  for  the hydrate  film  to  cover  the  entire  water  drop  (i.e., opaque time) in Experiment 3, 6, and 9 was 63min, 47min, and 31min, respectively. It was proved that the  crystallization  accelerated  under  the  high driving  force.  It  should  be  noted  that  in Experiment 1 the surface of the water drop wasn? t wrapped by the hydrate film, which was ascribed to  the  termination  of  the  mass  transfer  under  the low driving force. In addition, in Experiments 3-11 the hydrate film on the water drop thickened with the  rise  in  the  experiment  pressure,  for  instance, the  thickness  of  the  coarse  hydrate  film  in Experiment 3, 6 and 9 was 0.08mm, 0.2mm, and 0.24mm, respectively. It should be noted that these thickness values were almost obtained over 20min since  the  nucleation  appeared.  Therefore,  it  was indicated  that  the  elevation  of  the  driving  force promoted the formation of the hydrate film on the pedant water drop.  Mechanism  of  hydrate  formation  on  static pedant water droplet Through the observations and analysis of hydrate formation  on  the  static  pedant  water  drop  under different experiment conditions, the mechanism of hydrate  formation  on  static  pedant  water  droplet was acquired. As long as the formation condition for  the  hydrate  film  was  satisfied,  such  as  the Experiments  3-11  in  this  paper,  the  crystals formation  and  growth  on  the  pendant  water  drop would  undergo  four  stages.  In  the  first  stage, hydrate  nuclei  appeared  randomly  on  the  surface of the water drop and the surface became cloudy (the time equals the nucleation time); in the second stage,  hydrate  crystals  initially  appeared  at  the boundary  of  water  drop  and  suspension  tube,  in this paper the time was in the range of 20 seconds to  60  seconds  after  the  nucleation.  In  the  third stage,  two  hydrate  films  that  generated  at  the boundary  between  the  water  drop  and  the suspension  tube  grew  laterally  and  longitudinally on the surface of the water drop. At first, the foggy hydrate  film  with  the thickness  of  about  0.04mm quickly covered the water drop in 16 seconds or so. Then  the  coarse  hydrate  film  covered  the  thin hydrate  film  and  enclosed  the  water  drop completely in about 1 minute. The thickness of the coarse  hydrate  film  increased  with  the  elevation the driving force. In present work, it varied from 0.08mm  to  0.24mm.  In  the  fourth  stage,  hydrate formation  continued  below  the  hydrate  layer, which  lasted  till  the  end  of  the  experiment.  The appearance  of  the  hydrate  layer  exhibited differently according to the degree of the driving force. For instance, in Experiment 3 (T=273.35K, P=4.78MPa) the surface of the hydrate layer was smooth,  while  in  Experiment  6  (T=273.35K, P=5.77MPa)  and  Experiment  9  (T=273.35K, P=6.05MPa)  hydrate  branches  extruded  from  the surface of the hydrate layer to the gas phase.    Recommendation  for  hydrate  formation relevant to industrial system As  the  hydration  formation  on  pendant  water droplet  were  only  investigated  at  different pressures  in  this  work,  recommendations  for  the industrial  formation  were  proposed  based  on  this study. A higher driving force (i.e., a higher system pressure) is favorable to the industrial application, as the experiment results in this paper proved that a higher driving force could avoid the termination of the hydrate film formation on the pendant water droplet,  and  could  shorten  the  process  of  the hydrate formation. It is very useful to the industrial water spray formation of gas hydrates in the future. The  studies  of  the  effects  of  other  parameters (subcooling degree, the size of the water drop, etc.) on the hydrate formation on pendant water droplet are underway.  CONCLUSION The observations of natural gas hydrate formation on  pendant  water  droplet  were  carried  out  at different  pressures  ranging  from  3.86MPa  to 6.05MPa. The  experiment  temperature  was  set  at 274.75K  and  273.35K.  It  was  found  that  the driving  force  had  an  important  effect  on  the hydrate  nucleation  and  growth,  and  the  crystal morphology  on  the  pedant  water  droplet.  The process of the hydrate nucleation and growth was shortened  under  a  higher  driving  force,  and  the hydrate  layer  on  the  pendant  water  drop  became coarser  under  a  higher  driving  force.  The mechanism  for  the  hydrate  formation  on  static pedant water droplet was discovered based on the realization  of  the  hydrate  film  formation.  It included four stages, i.e. nucleation, generation of the hydrate film, growth of the hydrate film, and hydration below the hydrate shell.  REFERENCES [1] Makogon YF. Hydrates of natural gas. Tulsa, Oklahoma:  PennWell  publishing  company; 1981. [2] Gudmundsson FH. Transport of natural gas as frozen  hydrate.  Proceedings  of  the  Fifth International  Offshore  and  Polar  Engineering Conference. The Hague, The Netherlands 1995; (6): 11-16. [3]  Gudmundsson  JS.  Natural  gas  hydrate-an alternative to liquefied natural gas. Petroleum Review 1996; (5): 232-235. [4]  Servio  P,  Englezos  P.  Morphology  study  of Structure  H  hydrate  formation  from  water droplets. Crystal Growth & Design 2003; 3(1): 61-66. [5] Servio P, Englezos P. Morphology of Methane and  Carbon  Dioxide  Hydrate  Formed  from Water  Droplets.  Environmental  and  Energy Engineering 2003; 49(1): 269-276. [6]  Ohmura  R,  et  al.  Formation  and  growth  of Structure-H hydrate crystals on a water drop in contact  with  methane  gas  and  large-molecule guest-substance  liquid.  In:  Proceedings  of the Fifth International Conference on Gas Hydrate 2005. [7] Lee JD, Susilo R, Englezos P. Methane-ethane and  methane-propane  hydrate  formation  and decomposition  on  water  droplets.  Chemical Engineering Science 2005; 60: 4203-4212. [8]  Tsuji  H,  Ohmura  R,  Mori  YH.  Forming structure-H hydrate using water spraying in methane gas: effects of chemical species of large-molecule  guest  substances.  Energy  & Fuels 2004; 18: 418-424. [9]    Ohmura R, Kashiwazaki S, Shiota S, Tsuji H, Mori  YH.  Structure-I  and  Structure-H hydrate  formation  using  water  spraying. Enery & Fuels 2002; 16:1141-1147. [10]    Tsuji H, Kobayashi T, Ohmura R, Mori YH. Hydrate  formation  by  water  spraying  in  a Methane+  Ethane+  Propane  gas  mixture: an  attempt  at  promoting  hydrate  formation utilization  large-molecule  guest  substances for  structure-H  hydrate.  Energy  &  Fuels 2005; 19: 869-876. [11] Xie YM, Liu DP, Fan Y, Liu N. Experimental research  on  natural  gas  hydrate  storage process  using  water  spraying.  Journal  of   University  of  Shanghai  for  Science  and Technology 2007; 29(4): 368-372. [12] Yang QF, Liu DP, Xie YM, Hu HH, Xu XY, Pan  YX.  The  experiment  system  of  natural gas  hydrate  formation  by  water  spray. Chemical Engineering of Oil and Gas 2006; 35(4): 256-259. [13]  Yang  QF,  Liu  DP,  Pan  YX,  Hu  HH. Experimental study on influential factors of induction  time  of  natural  gas  hydrate. Journal  of  Oil  and  Gas  Technology  2007; 29(2): 82-86. [14]  Sloan  ED.  Clathrate  Hydrates  of  Natural Gases.  Second  ed.  New  York:  Marcel Dekker; 1998. [15]  Sloan  ED.  Clathrate  hydrate  measurements: microscopic,  mesoscopic,  and  macroscopic. Journal of Chemical Thermodynamics 2003; 35: 41-53. [16]  Mori  YH,  Mochizuki  T.  Mass  transport across clathrate hydrate film ?  a capillary permeation  model.  Chemical  Engineering Science 1997; 52(20): 3613-3616. 

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